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Neuron–glial cell cooperation
C H A P T E R
2 Neuron–glial cell cooperation Constance Hammond, Myrian Cayre, Aude Panatier, Elena Avignone O U T L I N E 2.1 Astrocytes form a vast cellular network or syncytium between neurons, blood vessels and the surface of the brain 2.2 Oligodendrocytes form the myelin sheaths of axons in the central nervous system and allow the clustering of Na+ channels at nodes of Ranvier
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There are roughly twice as many glial cells as there are neurons in the central nervous system. They occupy the space between neurons and neuronal processes and separate neurons from blood vessels. As a result, the extracellular space between the plasma membranes of different cells is narrow, of the order of 15–20 nm. Virchow (1846) was the first to propose the existence of non-neuronal tissue in the central nervous system. He named it ‘neuroglia’ (neural putty), because it appeared to stick the neurons together. Following this, among others, Deiters (1865) and Golgi (1885) identified glial cells and distinguished them from neurons. There are several categories of glial cells. Depending on their anatomical position they are classed as follows:
2.3 Microglial cells are the macrophages of the central nervous system
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2.4 Schwann cells are the glial cells of the peripheral nervous system; they form the myelin sheath of axons or encapsulate neurons
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Further reading
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Glial cells, excluding microglia, have an ectodermal origin. Those of the central nervous system derive from the germinal neural epithelium (neural tube), while peripheral glia (Schwann cells) are derived from the neural crest. Microglia, in contrast, have a mesodermal origin. While glial cells were considered for a long time only as a supporting tissue, each category has in fact its own roles which are essential for the integrity and processing of the nervous system. Glial cells have morphological as well as functional and metabolic characteristics that distinguish them from neurons, in particular, they do not generate or conduct action potentials. We will explain in this chapter the roles of astrocytes, oligodendrocytes, microglia and Schwann cells.
• Central glia are found in the central nervous system, and comprise two groups: macroglia and microglia. This first group comprises astroglia, NG2-glial cells, and oligodendrocytes. Astroglia regroups radial glia (principally during embryonic and developmental stages), ependymal cells which form the epithelial surface covering the walls of the cerebral ventricles and of the central canal of the spinal cord, tanycytes, pituicytes, ependymocytes, choroid plexus cells, retinal pigment epithelial cells and astrocytes. • Peripheral glia comprise non-myelinating perisynaptic Schwann cells, Schwann cells, olfactory ensheathing cells, satellite glial cells and enteric glia.
2.1 ASTROCYTES FORM A VAST CELLULAR NETWORK OR SYNCYTIUM BETWEEN NEURONS, BLOOD VESSELS AND THE SURFACE OF THE BRAIN In 1895, Michael Lenhossék introduced the term ‘astrocyte’ because these cells may have a ‘star’ appearance. Astrocytes lie between vessels and neurons, suggesting that they play important roles in adult brain metabolism and synaptic transmission. Astrocytes send out processes that end on the walls of blood vessels or beneath the pial surface of the brain and spinal cord. Two kinds
Cellular and Molecular Neurophysiology. DOI: 10.1016/B978-0-12-397032-9.00002-9 Copyright © 2015 Elsevier Ltd. All rights reserved
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2. Neuron–glial cell cooperation
FIGURE 2.1 Fibrillary astrocyte. Micrograph of a fibrillary astrocyte stained with a Golgi stain observed through an optical microscope. The processes of this astrocyte make contact with a blood vessel: these are the terminal end feet. Photograph by Olivier Robain.
of astrocytes are recognized: fibrillary astrocytes in the white matter (Figure 2.1) with numerous radial processes, which are infrequently branched, and protoplasmic astrocytes, in the gray matter.
2.1.1 Astrocytes are star-shaped cells characterized by the presence of glial filaments in their cytoplasm The principal characteristics of astrocytes are the glial filaments (≈8–10 mm diameter) and glycogen granules present in the cytoplasm of their somata and main processes. A glial filament commonly used to identify astrocytes is the glial fibrillary acidic protein (GFAP). In physiological conditions, it reveals only around 13% of the morphology of the cell and its expression varies with age and brain region. Protoplasmic astrocytes are characterized by spongiform morphology, with one to several stubby main processes, from which emerge several ramifications with an irregular shape. Each astrocyte extends at least one process, called end feet, in contact with the wall of blood vessels (Figure 2.2), and a high number of processes, which are in close apposition with the two neuronal elements of the synapse. In contrast to fibrillary astrocytes,
protoplasmic astrocytes have their own territory, called domain, in which they contact around 120 000 synapses. Despite their poor overlap (only 5% at the edge), these glial cells are coupled via gap junctions to form a syncytium. These gap junctions are permeable to small molecules (less than 1000 kDa) including Ca2+ ions.
2.1.2 Astrocytes form an active bridge between neurons and the blood–brain barrier Astrocytes play an important role in the development, maintenance and regulation of the blood–brain barrier. The blood–brain barrier is a physical barrier isolating the brain cells from the blood. This is formed, in most regions of the central nervous system, by vascular endothelial cells joined together by tight junctions. It is surrounded by basal lamina and astrocytic end feet, which are not part of the blood–brain barrier. The blood–brain barrier supplies brain cells with essential nutrients and allows the efflux of large molecules and brain metabolites to maintain brain homeostasis. The permeability of the blood–brain barrier is regulated by the expression of transporters and receptors mediating transcytosis, at the level of the plasma membrane
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.1 FORMATION OF VAST CELLULAR NETWORK OR SYNCYTIUM
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FIGURE 2.2 Protoplasmic astrocytes are well positioned between neurons and vessels and organized in domains. (a) Diagram representing the position of the astrocyte in between blood vessels and synapses. (b) Diagram of the covering formed by astrocyte end feet around a capillary. (c) Tripartite synapse model showing that astrocytes detect the synaptic signal (1) and in turn regulate its efficacy (2). Parts (a, c) drawing by Aude Panatier. Part (b) from Goldstein G, Betz L (1986) La barrière qui protège le cerveau. Pour la Science, November, 84–94, with permission.
of endothelial cells. Then, the exchange between endothelial cells and astrocytes is due to the presence of receptors, transporters and channels in end feet plasma membrane. Depending on the metabolic demand, astrocytes control the local blood supply. To this end, astrocytes, together with neurons, microvessels, endothelial cells and probably microglia and oligodendrocytes, are organized into neurovascular units, a key element for the neurovascular coupling.
2.1.3 Astrocytes regulate the ionic composition of the extracellular fluid Proper function of the brain depends on the regulation of the composition of extracellular and intracellular ions. Indeed, survival, function, excitability and communication of brain neurons are based on ionic equilibrium.
In 1965, Leif Hertz proposed that astrocytes play a key role in K+ homeostasis. During action potential propagation, and more precisely during the hyperpolarizing phase, there are large fluxes of potassium ions into the extracellular space through voltage-activated potassium channels (see Sections 4.3 and 5.3). Extracellular K+ concentration needs to be tightly regulated to a concentration of 3 mM: higher concentration would induce neuron depolarization leading to an increase of neuronal excitability followed by a decrease of action potential propagation. To limit this side effect, astrocytes uptake local K+ ions through potassium channels and transporters (Kir4.1, Na+/K+ pump and Na+/K+/Cl− co-transporter). K+ ions are then redistributed in neighboring astrocytes through gap junctions, a mechanism called ‘potassium spatial buffering’. Beside their contribution in potassium clearance, astrocytes play a major role in Ca2+, Cl−, pH as well as water homeostasis.
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2.1.4 Astrocytes regulate the efficacy of synaptic transmission As mentioned above, astrocytes extend processes that are in close apposition with synapses. Astrocytes are intimate partners of neurons during synaptic transmission. First, astrocytes are required for the synthesis of transmitters and particularly glutamate and GABA. Thanks to the presence of glutamine synthetase in astrocytes (see Figure 10.13), glutamine is synthesized from glutamate. Glutamine is then uptaken by neurons and transformed back into glutamate. Secondly, astrocytes play a fundamental role in removing neurotransmitters released in the synaptic cleft, avoiding steady high concentrations of transmitters that would interfere with synaptic transmission and induce toxicity triggered by long-lasting activation of receptors (particularly glutamate receptors). Most transmitters are transported into cells from the extracellular space by specialized carrier molecules in the cell membrane, called transporters (but acetylcholine is hydrolyzed; see Figure 6.12). Although both neurons and glia express such carrier proteins, uptake into astrocytes is of particular importance, especially for glutamate. Indeed, glutamate is primarily uptaken by astrocytes through the high-affinity glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST). Finally, although initially considered passive and nonexcitable, these glial cells display a form of non-electrical excitability based on intracellular calcium variations. Indeed, they express several receptors through which they can detect the synaptic signal. Following their activation, astrocytes regulate the efficacy of synaptic transmission by releasing active substances called gliotransmitters, such as purines, d-serine and glutamate. Thus, from numerous works performed worldwide emerged the concept of the ‘tripartite synapse’ proposing that astrocytes are functional components of synapses.
Na+ channels at nodes of Ranvier and therefore to allow the fast propagation of Na+ action potentials (see Section 5.4). Oligodendrocytes also have other less characterized functions such as trophic support of long axons and rapid regulation of action potential propagation through axons.
2.2.1 Processes of interfascicular oligodendrocytes electrically isolate segments of central axons by forming the lipid-rich myelin sheath The cell bodies of interfascicular oligodendrocytes are situated between bundles of axons Interfascicular, or myelinating, oligodendrocytes have small spherical or polyhedral cell bodies of diameter 6–8 mm and few processes. They are called interfascicular because their cell bodies are aligned between bundles (fascicles) of axons. They are distinguished from astrocytes by the sites of termination of their processes: oligodendrocyte processes enwrap axons and make no contact with blood vessels. Observed by electron microscopy, the nucleus and perikaryon of oligodendrocytes appear dark (Figure 2.3), there are no glial filaments, but there are many microtubules in the somatic and dendritic cytoplasm.
2.2 OLIGODENDROCYTES FORM THE MYELIN SHEATHS OF AXONS IN THE CENTRAL NERVOUS SYSTEM AND ALLOW THE CLUSTERING OF Na+ CHANNELS AT NODES OF RANVIER Two types of oligodendrocytes are recognized: interfascicular or myelinating oligodendrocytes, found in the white matter where they make the sheaths of myelinated axons; and satellite oligodendrocytes which surround neuronal somata in the gray matter. We will deal with the former type in detail. By forming the myelin sheath, their major role is to electrically isolate segments of axons, induce the formation of clusters of
FIGURE 2.3 Myelinating oligodendrocyte. Electron micrograph of an oligodendrocyte. The cell body and one of its processes enwrapping several axons can be seen. Section taken through the spinal cord. Photograph by Olivier Robain.
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.2 CLUSTERING OF Na+ CHANNELS AT NODES OF RANVIER
Because of this, oligodendrocyte processes may be confused with fine dendrites, and it is by the absence of chemical synapses that the mature oligodendrocyte processes are identified. Oligodendrocytes can be identified by immunohistochemistry. This is done using an anti-galactoceramide immune serum (anti-gal-C), galactoceramide being a glycolipid found exclusively in the membrane of processes of myelinizing oligodendrocytes.
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The myelin sheath is a compact roll of the plasmalemma of an oligodendrocyte process: this glial membrane is rich in lipids Myelinated axons are surrounded by a succession of myelin segments, each about 1 mm long. The covered regions of axons alternate with short exposed lengths where the axonal membrane (axolemma) is not covered. These unmyelinated regions (of the order of a micron) are called nodes of Ranvier (Figures 2.4 and 2.5a).
FIGURE 2.4 Diagram and photomicrographs of myelinating oligodendrocytes and their numerous processes. (a) Each oligodendrocyte process forms a segment of myelin around a different axon in the central nervous system. Two myelin segments are represented, one partially unrolled, the other completely unrolled. (b) Maturation of oligodendrocytes grown on axons of dorsal root ganglionic (DRG) neurons in culture. DRG axons are immunostained for neurofilament (red), and cell nuclei are labeled with DAPI (blue). Upper part: oligodendrocyte progenitor cells (revealed by NG2 labeling in green) plated onto DRG axons (red) after 2 days in co-culture. They extend many processes which contact multiple axons. Lower part: after 7 days in co-culture, oligodendrocyte precursor cells have differentiated into myelinating oligodendrocytes which form multiple segments of compact myelin associated with the axons (myelin is shown in green by myelin basic protein, a component of the myelin sheath, immunolabeling). Scale bar = 15 mm. Part (a) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission. Part (b) from Lee PR, Fields RD. (2009) Regulation of myelin genes implicated in psychiatric disorders by functional activity in axons. Front. Neuroanat. 3, 4, with permission.
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A myelinated segment comprises the length of axon covered by an oligodendrocyte. One oligodendrocyte can form 20–70 myelin segments around different axons (Figure 2.4b). Thus the degeneration or dysfunction of a single oligodendrocyte leads to the disappearance of myelin segments on several different axons. Origin of oligodendrocytes and myelination process Myelination represents a crucial stage in the ontogenesis of the nervous system. In the human at birth, myelinization is only just beginning and, in some regions, is not complete even by the end of the second year of life. Oligodendrocytes are derived from oligodendrocyte progenitor cells (OPCs). These progenitors (sometimes called NG2 cells because they express the neural/glial antigen 2 proteoglycan (NG2) on their surface) represent a large and enigmatic population of glial cells. Because of their ability to generate all neural cell types in vitro (neurons, astrocytes and oligodendrocytes), NG2 cells have been proposed to be multipotent stem cells; however, studies using genetic lineage tracing with inducible Cre mouse lines demonstrate that NG2 cells only generate oligodendrocytes in vivo. Nevertheless, oligodendrocyte progenitor cells present remarkable properties: (i) they persist throughout life and represent approximately 5% of all cells in the CNS; (ii) they are the main population of slowly dividing cells in the postnatal brain; (iii) they are activated after insult of the central nervous system: they start proliferating actively, migrate toward the lesion and, finally, differentiate into oligodendrocytes thus contributing to spontaneous remyelination; and (iv) they are coupled to axons through synapses that use GABA and glutamate as neurotransmitters (half of these cells can even fire action potentials). Remarkably, synapses of oligodendrocyte progenitor cells disappear as soon as the cell differentiates into a mature oligodendrocyte. In fact, this terminal differentiation of oligodendrocyte progenitor cells is promoted by neuronal activity. Indeed, optogenetic stimulation of premotor cortex in awake mice stimulates proliferation of oligodendrocyte precursor cells and promotes myelination of axons within the deep layers of the premotor cortex and subcortical white matter. Oligodendrocytes do not myelinate all axons in the CNS. Myelination starts with the initial turn of myelin around the axon which is rapidly formed (Figure 2.4). Myelin is then slowly deposited over a period which, in humans, can reach several months. Myelinization is responsible for a large part of the increase in weight of the central nervous system following the end of neurogenesis. In order to form the compact spiral of myelin membrane, the oligodendrocyte process must roll itself around the axon many times (up to 40 turns) (Figure 2.5). It is the terminal portion of the process, called the inner loop,
situated at the interior of the roll, which progressively spirals around the axon. This movement necessitates the sliding of myelin sheets which are not firmly attached. During this period, the oligodendrocyte synthesizes several times its own weight of myelin membrane each day. Within the spiral, the cytoplasm disappears entirely (except at the internal and external loops). The internal leaflets of the plasma membranes can thus adhere to each other. This adhesion is so intimate that the internal leaflets virtually fuse, forming the period, or major, dense line of thickness of 3 nm (Figure 2.5b). The extracellular space between the different turns of membrane also disappears, and the external leaflets also stick to each other. This apposition is, however, less close and a small space remains between the external leaflets. The apposed external leaflets form the minor, or interperiod, dense line (Figure 2.5b). Thus, a cross-section of a myelinated axon observed by electron microscopy shows alternating dark and light lines forming a spiral around the axon. The major dense line terminates where the internal leaflets separate to enclose the cytoplasm within the external loop. The interperiod dense line disappears at the surface of the sheath at the end of the spiral (Figure 2.5b). In the central nervous system, there is no basal lamina around myelin segments, so myelin segments of adjacent axons may adhere to each other forming an interperiod dense line. The g ratio, determined by the axon diameter divided by axon plus myelin diameter, is often used as a myelination index. Myelin Myelin consists of a compact spiral (without intracellular or extracellular space) of glial plasma membrane of a very particular composition. Lipids make up about 70% of the dry weight of myelin and proteins only 30%. Compared with the membranes of other cells, this represents an inversion of the lipid:protein ratio. This lipid-rich membrane is highly enriched in glycosphingolipids and cholesterol. The major glycosphingolipids in myelin are galactosylceramide and its sulfated derivative sulfatide (20% of lipid dry weight). There is also an unusually high proportion of ethanolamine phosphoglycerides in the plasmalogen form, which accounts for one-third of the phospholipids. In myelin, a number of structural classes of proteins are present. Some proteins are extremely hydrophobic membrane-embedded polypeptides, some integral membrane proteins have a single transmembrane domain and clearly define extra- and intracellular domains, and some of the myelin proteins are cytosolic; however, they are often intimately associated with the myelin membrane. Myelin basic protein (MBP) and the proteolipid proteins (PLP/DM20) are the two major myelin proteins in the CNS. Myelin basic proteins are found on the
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.2 CLUSTERING OF Na+ CHANNELS AT NODES OF RANVIER
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FIGURE 2.5 Myelin sheath of central axons. (a) Three-dimensional diagram of the myelin sheath of an axon in the central nervous system (CNS). The sheath is formed by a succession of compact rolls of glial processes from different oligodendrocytes. (b) Cross-section through a myelin sheath. The dark lines, or major dense lines, and clear bands (in the middle of which are found the interperiod lines) visible with electron microscopy are accounted for by the manner in which the myelin membrane surrounds the axon, and by the composition of the membrane. The dark lines represent the adhesion of the internal leaflets of the myelin membrane while the interperiod lines represent the adhesion of the external leaflets. The lines are formed by membrane proteins while the clear bands are formed by the lipid bilayer. (c,d) Electron micrographs of myelinated axons in sciatic (c) and optic (d) nerves. Structural details of myelin ensheathment at the paranodal level (longitudinal section, c). Crosssection of myelinated axons (d). Numerous microtubules and mitochondria are present in the axons. The innermost layer of myelin sheath is often non-compacted. MVBs: multivesicular bodies. PNJ: paranodal junction. Part (a) drawing from Bunge MB, Bunge RP, Ris H (1961) Ultrastructural study of remyelination in an experimental lesion in adult cat spinal cord. J. Biophys. Biochem. Cytol. 10, 67–94, with permission of Rockerfeller University Press. Part (b) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission. Parts (c,d) from Nave KA (2010) Myelination and the trophic support of long axons. Nat. Rev. Neurosci. 11, 275–283, with permission.
cytoplasmic side and play a role in the adhesion of the internal leaflets of the specialized oligodendroglial plasma membrane. Proteolipid proteins are integral membrane proteins. Though they are in high abundance (they represent around 50% of the total myelin protein in the central nervous system), their exact biological role has not yet been elucidated.
We have seen that myelin has an inverted lipid:protein ratio, while the cell body membrane of the oligodendrocyte has a ratio comparable to that of other cell membranes. As the myelin of the oligodendrocyte process is in continuity with the plasma membrane of the cell body, it is necessary to postulate gradients in the composition of lipids and proteins (in opposite directions
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to each other) between the cell body and the various processes. During the active phase of myelination, each oligodendrocyte must produce as much as 5–50 000 mm2 of myelin membrane surface area per day. Interestingly, myelination does not stop with the end of development but is maintained throughout life. Using transgenic mice with inducible Cre recombinase system to label adult oligodendrocyte progenitor cells and trace their progeny, it was demonstrated that almost 30% of total oligodendrocytes of the corpus callosum in an 8-month-old animal were generated during adulthood. The function of this constant production of myelinating oligodendrocytes and of myelin remodeling in the adult brain remains poorly understood but, together with the activity-dependent control of myelination, it highlights the remarkable plasticity of myelin. Nodes of Ranvier In the central nervous system, the nodes of Ranvier, regions between myelin segments, are relatively long (several microns) compared with those in the peripheral nervous system. Here the axolemma is exposed and an accumulation of dense material is seen on the cytoplasmic side. The myelin sheath does not terminate abruptly. Successive layers of myelin membrane terminate at regularly spaced intervals along the axon, the internal layers (close to the axon) terminating first. This staggered termination of the different layers of myelin constitutes the paranodal region (Figure 2.5).
2.2.2 Myelination enables rapid conduction of action potentials for two reasons Isolation of internode axonal segments The high lipid content and compact structure of the myelin sheath help make it impermeable to hydrophilic substances such as ions. It prevents transmembrane ion fluxes and acts as a good electrical insulator between the intracellular (i.e. intra-axonal) and extracellular media. Between the nodes of Ranvier the axon therefore behaves as an insulated cable. This permits rapid, saltatory conduction of action potentials along the axon (see Section 5.4). Formation of Ranvier nodes with a high density of Na+ channels Na+ channels are clustered in very high density within the nodal gap whereas voltage-dependent K+ channels are segregated in juxta-paranodal regions, beneath overlying myelin (Figure 2.5). To test whether oligodendrocyte contact with axon influences Na+ channel distribution, nodes of Ranvier in the brain of hypomyelinating mouse Shiverer are examined. Shiverer mice have oligodendrocytes that ensheath axons but do not form
compact myelin and axoglial junctions. In these mutant mice, there are far fewer Na+ channel clusters than in control littermates and aberrant locations of Na+ channels are observed. If Na+ channel clustering depended only on the presence of oligodendrocytes and were independent of myelin and oligodendroglial contact, one would expect to find normal Na+ channel distribution along ‘shiverer’ axons. Contribution of myelination to signal transmission efficiency Myelination of axons by glial cells has appeared quite late in evolution. This innovation in ancient jawed vertebrates allowed speeding up nerve conduction more than 20-fold and the appearance of efficient signal transmission in large animals. Today, numerous correlative evidences suggest myelin involvement in cognition or in the development of fine motor skills.
2.2.3 Myelin-independent functions of oligodendrocytes Facilitation of conduction velocity by depolarization of oligodendrocytes Myelin increases the velocity of action potentials along axons but oligodendrocytes also contribute to optimize signal transmission via myelin-independent functions. Like neurons and astrocytes, oligodendrocytes express a large variety of ion channels and neurotransmitter receptors. They can thus participate in K+ buffering, monitor neuronal activity, be depolarized and directly increase conduction velocity along the axon. The mechanisms involved in oligodendrocyte depolarization-induced conduction velocity are not completely deciphered but may involve structural changes leading to increased insulation in the paranodal and intermodal regions. Since one oligodendrocyte myelinates multiple axons, oligodendrocytes could also contribute to signal synchronization. Oligodendrocytes support axonal integrity Axonal support is probably a common and ancestral function of all axon-ensheathing glial cells. Paradoxically, because myelin completely insulates the axon, it also deprives it from metabolic support by the extracellular milieu. Thus, oligodendrocytes need to overcome this by supplying trophic and metabolic support to axons, thereby preserving axonal integrity. Diseases affecting oligodendrocytes often lead to axon degeneration. Part of this neuroprotection effect is independent of myelin’s function. Loss of myelin proteins such as PLP or CNP, which are dispensable for myelination, leads to progressive axonal degeneration. Mechanisms underlying glial cell support are not completely understood yet.
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.3 MICROGLIAL CELLS ARE THE MACROPHAGES OF THE CENTRAL NERVOUS SYSTEM
2.2.4 Impact of myelin defects Oligodendrocytes show the highest metabolic rates among all brain cells in order to produce but also to maintain very high volume of membranes which can represent up to 100 times the weight of the cell. This metabolic requirement makes them highly sensitive to pathological conditions such as ischemia, trauma and inflammation. The consequences of massive oligodendrocyte death and demyelination have long been known from diseases such as multiple sclerosis, with functional loss depending of the areas of the CNS where demyelination occurs (blindness, incontinency, motor deficit…). More recently, it has been discovered that more subtle myelin abnormalities can contribute to a wide range of neuropsychatric disorders including depression and schizophrenia. White matter abnormalities also represent an early feature of neurodegenerative pathologies such as Huntington and Alzheimer diseases (far before neurofibrillary tangles and clinical symptoms) and might also contribute to the pathogenesis of the disease.
2.3 MICROGLIAL CELLS ARE THE MACROPHAGES OF THE CENTRAL NERVOUS SYSTEM Microglial cells are unique since they are at the interface between the immune and nervous systems, which confers them remarkable properties. Microglia are equipped by a plethora of receptors allowing them to detect signals deriving from different origins, such as an immune challenge or activity from other brain cells, and readily react to them. As macrophages, they are professional phagocytes, but they can also release several mediators which can influence neuronal activity as well as the fate of surrounding cells. After a decade long debate about their origin, we know today that these cells arise early during development from progenitors in the embryonic yolk sac (YS) that seed the brain rudiment. During development, microglial cells colonize the entire CNS and they differentiate to assume a mature phenotype adapted to their function in the adult brain.
2.3.1 Microglial cells contribute to brain physiology Microglia are highly dynamic cells In the adult CNS, microglial density and morphology change among different CNS regions, likely to better fulfill their function. Their density varies from about 12% of total cells in substantia nigra to ≈5% in corpus callosum. Despite this variability, their distribution in space in a specific region is rather uniform with minimal overlap. In white matter, their morphology is characterized
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by an elongated soma and processes preferentially oriented along fiber tracts, while in the circumventricular organs, a region with a leaky blood–brain barrier, they are compact with a few short processes. In contrast, in gray matter, they present a small cell body with highly branched processes. This typical appearance, which is quite different from a classical macrophage, has been associated with microglial ‘resting’ state and, until recently, this state was considered quiescent, i.e. functionally dormant. However, recent in vivo imaging studies employing mice with EGFP-expressing microglia demonstrated that they constantly extend and retract their fine cellular processes at a rate of few micrometers per minute to scan the brain parenchyma. During movements, a single microglia maintains its cell body position as well as its overall size and symmetry of its territory, and avoids contacts with other microglial processes. Thus, microglia are able to monitor their extracellular space and cellular neighborhood, always ready to intervene and change their activity status if abnormal changes are detected such as infections, lesions and neurodegeneration, critical variation in neuronal activity… Microglia sense and control neuronal activity During their scanning movements microglial processes rapidly contact other CNS cells, like neurons, perivascular astroglial and other glial cells. In particular, they contact post- and presynaptic elements of synapses. Interestingly, the frequency of these contacts is regulated by the neuronal activity. Indeed, it is decreased by sensorial deprivation, pharmacological block of action potential, as well as a decrease of body temperature. Thus, microglial cells can sense and react to neuronal activity. In turn, they can locally influence neuronal excitability by releasing several factors such as cytokines, neurotrophic factors, and neurotransmitters. A study in the larvae of zebra fish recently showed that an increase of neuronal firing attracts microglia through the release of ATP, and this contact decreases the activity of the targeted neuron. Microglia control adult neurogenesis Another important role of microglia is the regulation of the adult neurogenesis. Two neurogenic niches, in the ventricles and in the dentate gyrus of the hippocampus (see Chapter 19), produce new neurons throughout life. However, only a few newborn cells are incorporated into the circuitry and the majority of them are presumed to die at immature neuronal stage. Microglia can control proliferation, migration and differentiation of stem cells, but the factors regulating different steps of neurogenesis are still unknown. Furthermore, they are responsible for the phagocytosis of apoptotic cells preventing them from undergoing secondary necrosis, with the consequent spillage of cellular contents leading to an inflammatory response, detrimental for neurogenesis.
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FIGURE 2.6 Scheme of the different functions of microglia. Microglia (green) constantly move their processes to scan the brain parenchyma. During their movements they contact synapses and neuronal dendrites (orange), as well other brain cells. They can control brain activity and surrounding cells’ fate by releasing several factors. They phagocytose cells and neuronal debris, but also synaptic elements and newborn cells (orange), thus they participate in sculpting the neuronal circuits. Drawing by E. Avignone.
2.3.2 Microglia exert their activity through physical interaction and release of soluble factors We can classify the main actions of microglia into two broad categories: physical interactions and release of soluble factors (Figure 2.6). These actions are triggered by changes in the environment which modify microglia properties. Microglia with physical interactions sculpt neuronal circuits and isolate brain injuries Three types of physical interactions are described: phagocytosis, synapse stripping, and formation of a barrier around a damaged area. Phagocytosis is a major feature of microglial cells. This phenomenon is particular important during pathologies involving cell death, where cellular debris has to be removed, as well as during development. In immature circuits, microglia phagocyte presynaptic terminals and dendritic spines (synapse pruning). They thus contribute to circuit formation. Synapse stripping occurs when microglia processes form a barrier between the pre- and postsynaptic elements at the synaptic cleft. It is observed during lesions where there is an injury of innervating cells and a loss of synaptic boutons, as after facial nerve injury. As previously seen, microglia constantly move their processes to scan the brain parenchyma. As soon as a problem is detected they can extend their processes in
a few minutes towards sites of injury or substances that may represent a danger signal, like ATP or nitric oxide. These substances could be released by necrotic cells or by neurodegenerating axons. The ensemble of microglial processes then forms a barrier around the injured area in a time course from minutes to hours, thus limiting the spread of the damage of an acute lesion to the surrounding healthy tissue. Microglia release soluble factors to control neuronal survival and activity Pioneer studies in culture identified several soluble factors released by microglia, including a multitude of growth factors, cytokines, and neurotransmitters. Many released factors are known to affect neuronal activity, either directly or through other cells such as astrocytes. However, direct evidences of neuronal activity modulation by microglia in physiological conditions are still lacking. Indeed, most of these studies have been performed in culture, where microglia properties do not faithfully reflect those in the normal, non-pathological brain. Microglia change their properties in pathological conditions to respond to injuries Microglial cells are extremely sensitive to changes in their local environment. As a response to homeostasis perturbation, they adapt their phenotype in a multifaceted process called activation. Activation develops over
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.4 THE MYELIN SHEATH OF AXONS OR ENCAPSULATE NEURONS
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FIGURE 2.7 Microglia change properties after activation. The images show an example of morphological changes of microglia 48 hours after activation induced by status epilepticus. In control conditions (a) microglial cells have a small body with long and ramified processes. (b) In contrast, activated microglial cells have larger body with shorter and thicker processes. From Menteyne A, Levavasseur F, Audinat E, Avignone E (2009) Predominant functional expression of Kv1.3 by activated microglia of the hippocampus after status epilepticus. PLoS One 4, e6770, with permission.
periods ranging from hours to days, and it includes changes in morphology (Figure 2.7), electrical membrane properties and expression of a variety of markers. Activation triggers migration, proliferation and release of a variety of mediators, such as pro- and anti-inflammatory cytokines and neurotrophines. Activation is not an all-or-none process. It is variable and adaptive and it depends on the nature of triggering factors. Thus, microglia can exist in a variety of activated states. The heterogeneity of activation phenotypes and functions caused a debate on the beneficial versus detrimental roles of microglia in pathology, since it can exacerbate damage or, on the contrary, protect neurons from degeneration. A major challenge today is to characterize several aspects of microglia activation in specific diseases, and develop new drugs that block only harmful facets of activation.
2.4 SCHWANN CELLS ARE THE GLIAL CELLS OF THE PERIPHERAL NERVOUS SYSTEM; THEY FORM THE MYELIN SHEATH OF AXONS OR ENCAPSULATE NEURONS There are three types of Schwann cell: • those forming the myelin sheath of peripheral myelinated axons (myelinating Schwann cells);
• those encapsulating non-myelinated peripheral axons (non-myelinating Schwann cells); • those that encapsulate the bodies of ganglion cells (non-myelinating Schwann cells or satellite cells).
2.4.1 Myelinating Schwann cells make the myelin sheath of peripheral axons Along an axon, several Schwann cells form successive segments of the myelin sheath. In contrast to oligodendrocytes, it is not a process that enwraps the peripheral axon to form the segment of myelin, but the whole Schwann cell (Figure 2.8). Each Schwann cell therefore forms only one myelin segment. The composition of peripheral myelin differs from that of central myelin only in the proteins it contains. The principal protein constituents of peripheral myelin are: peripheral myelin protein 2 (P2), protein zero (P0) and myelin basic proteins (MBPs). The first two proteins are specific to peripheral myelin. MBP comprises a major part of the cytosolic protein of myelin and is present both in the CNS and peripheral nervous system (PNS). Protein zero is a glycoprotein that has adhesive properties and is located in the interperiod line. It functions, in part, as a homotypic adhesion molecule throughout the full thickness of the myelin sheath. It is a good marker for myelinating Schwann cells as it represents over 50% of total PNS myelin protein.
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FIGURE 2.8 Myelin sheath of a peripheral axon. (a) Three-dimensional diagram of the myelin sheath of an axon of the peripheral nervous system (PNS). The sheath is formed by successive rolled Schwann cells. (b) Process of myelinization. The internal loop wraps around the axon several times. During this process, the axon grows and the myelin becomes compact. Contact between the Schwann cell and axon occurs only at the paranodal and nodal regions. Elsewhere an extracellular, or periaxonal, space always remains. Part (a) drawing adapted from Maillet M (1977) Le Tissu Nerveux, Paris: Vigot, with permission. Part (b) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission.
2.4.2 Non-myelinating Schwann cells encapsulate the axons and cell bodies of peripheral neurons Non-myelinated axons are not uncovered in the peripheral nervous system as they are in the central nervous system; they are encapsulated. A single nonmyelinating Schwann cell surrounds several axons (about 5–20) for a distance of 200–500 mm in humans. In addition, spinal and cranial ganglia contain a large number of Schwann cells that do not produce myelin. These Schwann cells cover the somata of the ganglionic cells, leaving an extracellular space of about 20 nm between themselves and the surface of the covered neuron. The lipid and protein composition of the plasma membrane of non-myelinating Schwann cells is the
same as that of other eukaryotic cells (30% lipid, 70% protein). Apart from their role in the saltatory conduction of action potentials (myelinating Schwann cells), Schwann cells also play a role in maintenance of axonal integrity and survival. A remarkable property of Schwann cells is their ability to promote the regeneration of peripheral nerve cells. It has long been known that cut peripheral nerves can, within certain limits, regrow and reinnervate deafferented regions while central axons are not capable of this. The substantial regenerative capacity of the Schwann cells is driven by their ability to dedifferentiate following axonal injury, to divide and produce axonotrophic factors. This property of regeneration is due in large part to an enabling effect of Schwann cells on axon regrowth.
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2.4 THE MYELIN SHEATH OF AXONS OR ENCAPSULATE NEURONS
Further reading Azevedo, F.A., Carvalho, L.R., Grinberg, L.T., et al., 2009. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J. Comp. Neurol. 513, 532–541. Bélanger, M., Allaman, I., Magistretti, P.J., 2011. Brain energy metabolism: focus on astrocyte-neuron metabolic cooperation. Cell Metab. 14, 724–738. Bushong, E.A., Martone, M.E., Jones, Y.Z., Ellisman, M.H., 2002. Protoplasmic astrocytes in CA1 stratum radiatum occupy separate anatomical domains. J. Neurosci. 22, 183–192. Butt, A.M., Kalsi, A., 2006. Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions. J. Cell. Mol. Med. 10, 33–44. Buttermore, E.D.L., Thaxton, C.L., Bhat, M.A., 2013. Organization and maintenance of molecular domains in myelinated axons. J. Neurosci. Res. 91, 603–622. Dupree, J.L., Mason, J.L., Marcus, J.R., et al., 2005. Oligodendrocytes assist in the maintenance of sodium channel clusters independent of the myelin sheath. Neuron Glia Biol. 1, 1–14. El Waly, B., Macchi, M., Cayre, M., Durbec, P., 2014. Oligodendrogenesis in normal and pathological brain. Front. Neurosci. 8, 145. Farber, K., Kettenmann, H., 2005. Physiology of microglial cells. Brain Res. Brain Res. Rev. 48, 133–143. Fields, R.D., Araque, A., Johansen-Berg, H., et al., 2014. Glial biology in learning and cognition. Neuroscientist 20, 426-431. Freeman, M.R., Doherty, J., 2006. Glial cell biology in Drosophila and vertebrates. Trends Neurosci. 29, 82–90. Gibson, E.M.L., Purger, D., Mount, C.W., et al., 2014. Neuronal activity promotes oligodendrogenesis and adaptive myelination in the mammalian brain. Science, Apr 11. [Epub ahead of print]. Greer, J.M., Lees, M.B., 2002. Myelin proteolipid protein – the first 50 years. Int. J. Biochem. Cell Biol. 34, 211–215. Kettenmann, H., Kirchhoff, F., Verkhratsky, A., 2013. Microglia: new roles for the synaptic stripper. Neuron 77, 10–18. Montague, P., McCallion, A.S., Davies, R.W., Griffiths, I.R., 2006. Myelin-associated oligodendrocytic basic protein: a family
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of abundant CNS myelin proteins in search of a function. Dev. Neurosci. 28, 479–487. Nave, K.A., 2010. Myelination and support of axonal integrity by glia. Nature 468, 244–252. Pannasch, U., Rouach, N., 2013. Emerging role for astroglial networks in information processing: from synapse to behavior. Trends Neurosci. 36, 405–417. Parkhurst, C.N., Gan, W.-B., 2010. Microglia dynamics and function in the CNS. Curr. Opin. Neurobiol. 20, 595–600. Parpura, V., Heneka, M.T., Montana, V., et al., 2012. Glial cells in (patho)physiology. J. Neurochem. 121, 4–27. Peles, E., Salzer, J.L., 2000. Molecular domains of myelinated axons. Curr. Opin. Neurobiol. 10, 558–565. Pfeiffer, S.E., Warrington, A.E., Bansal, R., 1993. The oligodendrocyte and its many cellular processes. Trends Cell Biol. 3, 191–197. Popko, B., 2000. Myelin galactolipids: mediators of axon–glial interactions? Glia 29, 149–153. Shapiro, L., Doyle, J.P., Hensley, P., Colman, D.R., Hendrickson, W.A., 1996. Crystal structure of the extracellular domain from P0, the major structural protein of peripheral nerve myelin. Neuron 17, 435–449. Streit, W.J., 2000. Microglial response to brain injury: a brief synopsis. Toxicol. Pathol. 28, 28–30. Tremblay, M.-É., 2011. The role of microglia at synapses in the healthy CNS: novel insights from recent imaging studies. Neuron Glia Biol. 7, 67–76. Ueno, M., Yamashita, T., 2014. Bidirectional tuning of microglia in the developing brain: from neurogenesis to neural circuit formation. Curr. Opin. Neurobiol. 27C, 8–15. Vandenberg, R.J., Ryan, R.M., 2013. Mechanisms of glutamate transport. Physiol. Rev. 93, 1621–1657. Ventura, R., Harris, K.M., 1999. Three-dimensional relationships between hippocampal synapses and astrocytes. J. Neurosci. 19, 6897–6906. Yamazaki, Y.L., Hozumi, Y., Kaneko, K., Fujii, S., Goto, K., Kato, H., 2010. Oligodendrocytes: facilitating axonal conduction by more than myelination. Neuroscientist 16, 11–18.
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2 Neuron–glial cell cooperation Constance Hammond, Myrian Cayre, Aude Panatier, Elena Avignone O U T L I N E 2.1 Astrocytes form a vast cellular network or syncytium between neurons, blood vessels and the surface of the brain 2.2 Oligodendrocytes form the myelin sheaths of axons in the central nervous system and allow the clustering of Na+ channels at nodes of Ranvier
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There are roughly twice as many glial cells as there are neurons in the central nervous system. They occupy the space between neurons and neuronal processes and separate neurons from blood vessels. As a result, the extracellular space between the plasma membranes of different cells is narrow, of the order of 15–20 nm. Virchow (1846) was the first to propose the existence of non-neuronal tissue in the central nervous system. He named it ‘neuroglia’ (neural putty), because it appeared to stick the neurons together. Following this, among others, Deiters (1865) and Golgi (1885) identified glial cells and distinguished them from neurons. There are several categories of glial cells. Depending on their anatomical position they are classed as follows:
2.3 Microglial cells are the macrophages of the central nervous system
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2.4 Schwann cells are the glial cells of the peripheral nervous system; they form the myelin sheath of axons or encapsulate neurons
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Further reading
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Glial cells, excluding microglia, have an ectodermal origin. Those of the central nervous system derive from the germinal neural epithelium (neural tube), while peripheral glia (Schwann cells) are derived from the neural crest. Microglia, in contrast, have a mesodermal origin. While glial cells were considered for a long time only as a supporting tissue, each category has in fact its own roles which are essential for the integrity and processing of the nervous system. Glial cells have morphological as well as functional and metabolic characteristics that distinguish them from neurons, in particular, they do not generate or conduct action potentials. We will explain in this chapter the roles of astrocytes, oligodendrocytes, microglia and Schwann cells.
• Central glia are found in the central nervous system, and comprise two groups: macroglia and microglia. This first group comprises astroglia, NG2-glial cells, and oligodendrocytes. Astroglia regroups radial glia (principally during embryonic and developmental stages), ependymal cells which form the epithelial surface covering the walls of the cerebral ventricles and of the central canal of the spinal cord, tanycytes, pituicytes, ependymocytes, choroid plexus cells, retinal pigment epithelial cells and astrocytes. • Peripheral glia comprise non-myelinating perisynaptic Schwann cells, Schwann cells, olfactory ensheathing cells, satellite glial cells and enteric glia.
2.1 ASTROCYTES FORM A VAST CELLULAR NETWORK OR SYNCYTIUM BETWEEN NEURONS, BLOOD VESSELS AND THE SURFACE OF THE BRAIN In 1895, Michael Lenhossék introduced the term ‘astrocyte’ because these cells may have a ‘star’ appearance. Astrocytes lie between vessels and neurons, suggesting that they play important roles in adult brain metabolism and synaptic transmission. Astrocytes send out processes that end on the walls of blood vessels or beneath the pial surface of the brain and spinal cord. Two kinds
Cellular and Molecular Neurophysiology. DOI: 10.1016/B978-0-12-397032-9.00002-9 Copyright © 2015 Elsevier Ltd. All rights reserved
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FIGURE 2.1 Fibrillary astrocyte. Micrograph of a fibrillary astrocyte stained with a Golgi stain observed through an optical microscope. The processes of this astrocyte make contact with a blood vessel: these are the terminal end feet. Photograph by Olivier Robain.
of astrocytes are recognized: fibrillary astrocytes in the white matter (Figure 2.1) with numerous radial processes, which are infrequently branched, and protoplasmic astrocytes, in the gray matter.
2.1.1 Astrocytes are star-shaped cells characterized by the presence of glial filaments in their cytoplasm The principal characteristics of astrocytes are the glial filaments (≈8–10 mm diameter) and glycogen granules present in the cytoplasm of their somata and main processes. A glial filament commonly used to identify astrocytes is the glial fibrillary acidic protein (GFAP). In physiological conditions, it reveals only around 13% of the morphology of the cell and its expression varies with age and brain region. Protoplasmic astrocytes are characterized by spongiform morphology, with one to several stubby main processes, from which emerge several ramifications with an irregular shape. Each astrocyte extends at least one process, called end feet, in contact with the wall of blood vessels (Figure 2.2), and a high number of processes, which are in close apposition with the two neuronal elements of the synapse. In contrast to fibrillary astrocytes,
protoplasmic astrocytes have their own territory, called domain, in which they contact around 120 000 synapses. Despite their poor overlap (only 5% at the edge), these glial cells are coupled via gap junctions to form a syncytium. These gap junctions are permeable to small molecules (less than 1000 kDa) including Ca2+ ions.
2.1.2 Astrocytes form an active bridge between neurons and the blood–brain barrier Astrocytes play an important role in the development, maintenance and regulation of the blood–brain barrier. The blood–brain barrier is a physical barrier isolating the brain cells from the blood. This is formed, in most regions of the central nervous system, by vascular endothelial cells joined together by tight junctions. It is surrounded by basal lamina and astrocytic end feet, which are not part of the blood–brain barrier. The blood–brain barrier supplies brain cells with essential nutrients and allows the efflux of large molecules and brain metabolites to maintain brain homeostasis. The permeability of the blood–brain barrier is regulated by the expression of transporters and receptors mediating transcytosis, at the level of the plasma membrane
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2.1 FORMATION OF VAST CELLULAR NETWORK OR SYNCYTIUM
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FIGURE 2.2 Protoplasmic astrocytes are well positioned between neurons and vessels and organized in domains. (a) Diagram representing the position of the astrocyte in between blood vessels and synapses. (b) Diagram of the covering formed by astrocyte end feet around a capillary. (c) Tripartite synapse model showing that astrocytes detect the synaptic signal (1) and in turn regulate its efficacy (2). Parts (a, c) drawing by Aude Panatier. Part (b) from Goldstein G, Betz L (1986) La barrière qui protège le cerveau. Pour la Science, November, 84–94, with permission.
of endothelial cells. Then, the exchange between endothelial cells and astrocytes is due to the presence of receptors, transporters and channels in end feet plasma membrane. Depending on the metabolic demand, astrocytes control the local blood supply. To this end, astrocytes, together with neurons, microvessels, endothelial cells and probably microglia and oligodendrocytes, are organized into neurovascular units, a key element for the neurovascular coupling.
2.1.3 Astrocytes regulate the ionic composition of the extracellular fluid Proper function of the brain depends on the regulation of the composition of extracellular and intracellular ions. Indeed, survival, function, excitability and communication of brain neurons are based on ionic equilibrium.
In 1965, Leif Hertz proposed that astrocytes play a key role in K+ homeostasis. During action potential propagation, and more precisely during the hyperpolarizing phase, there are large fluxes of potassium ions into the extracellular space through voltage-activated potassium channels (see Sections 4.3 and 5.3). Extracellular K+ concentration needs to be tightly regulated to a concentration of 3 mM: higher concentration would induce neuron depolarization leading to an increase of neuronal excitability followed by a decrease of action potential propagation. To limit this side effect, astrocytes uptake local K+ ions through potassium channels and transporters (Kir4.1, Na+/K+ pump and Na+/K+/Cl− co-transporter). K+ ions are then redistributed in neighboring astrocytes through gap junctions, a mechanism called ‘potassium spatial buffering’. Beside their contribution in potassium clearance, astrocytes play a major role in Ca2+, Cl−, pH as well as water homeostasis.
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2.1.4 Astrocytes regulate the efficacy of synaptic transmission As mentioned above, astrocytes extend processes that are in close apposition with synapses. Astrocytes are intimate partners of neurons during synaptic transmission. First, astrocytes are required for the synthesis of transmitters and particularly glutamate and GABA. Thanks to the presence of glutamine synthetase in astrocytes (see Figure 10.13), glutamine is synthesized from glutamate. Glutamine is then uptaken by neurons and transformed back into glutamate. Secondly, astrocytes play a fundamental role in removing neurotransmitters released in the synaptic cleft, avoiding steady high concentrations of transmitters that would interfere with synaptic transmission and induce toxicity triggered by long-lasting activation of receptors (particularly glutamate receptors). Most transmitters are transported into cells from the extracellular space by specialized carrier molecules in the cell membrane, called transporters (but acetylcholine is hydrolyzed; see Figure 6.12). Although both neurons and glia express such carrier proteins, uptake into astrocytes is of particular importance, especially for glutamate. Indeed, glutamate is primarily uptaken by astrocytes through the high-affinity glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST). Finally, although initially considered passive and nonexcitable, these glial cells display a form of non-electrical excitability based on intracellular calcium variations. Indeed, they express several receptors through which they can detect the synaptic signal. Following their activation, astrocytes regulate the efficacy of synaptic transmission by releasing active substances called gliotransmitters, such as purines, d-serine and glutamate. Thus, from numerous works performed worldwide emerged the concept of the ‘tripartite synapse’ proposing that astrocytes are functional components of synapses.
Na+ channels at nodes of Ranvier and therefore to allow the fast propagation of Na+ action potentials (see Section 5.4). Oligodendrocytes also have other less characterized functions such as trophic support of long axons and rapid regulation of action potential propagation through axons.
2.2.1 Processes of interfascicular oligodendrocytes electrically isolate segments of central axons by forming the lipid-rich myelin sheath The cell bodies of interfascicular oligodendrocytes are situated between bundles of axons Interfascicular, or myelinating, oligodendrocytes have small spherical or polyhedral cell bodies of diameter 6–8 mm and few processes. They are called interfascicular because their cell bodies are aligned between bundles (fascicles) of axons. They are distinguished from astrocytes by the sites of termination of their processes: oligodendrocyte processes enwrap axons and make no contact with blood vessels. Observed by electron microscopy, the nucleus and perikaryon of oligodendrocytes appear dark (Figure 2.3), there are no glial filaments, but there are many microtubules in the somatic and dendritic cytoplasm.
2.2 OLIGODENDROCYTES FORM THE MYELIN SHEATHS OF AXONS IN THE CENTRAL NERVOUS SYSTEM AND ALLOW THE CLUSTERING OF Na+ CHANNELS AT NODES OF RANVIER Two types of oligodendrocytes are recognized: interfascicular or myelinating oligodendrocytes, found in the white matter where they make the sheaths of myelinated axons; and satellite oligodendrocytes which surround neuronal somata in the gray matter. We will deal with the former type in detail. By forming the myelin sheath, their major role is to electrically isolate segments of axons, induce the formation of clusters of
FIGURE 2.3 Myelinating oligodendrocyte. Electron micrograph of an oligodendrocyte. The cell body and one of its processes enwrapping several axons can be seen. Section taken through the spinal cord. Photograph by Olivier Robain.
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.2 CLUSTERING OF Na+ CHANNELS AT NODES OF RANVIER
Because of this, oligodendrocyte processes may be confused with fine dendrites, and it is by the absence of chemical synapses that the mature oligodendrocyte processes are identified. Oligodendrocytes can be identified by immunohistochemistry. This is done using an anti-galactoceramide immune serum (anti-gal-C), galactoceramide being a glycolipid found exclusively in the membrane of processes of myelinizing oligodendrocytes.
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The myelin sheath is a compact roll of the plasmalemma of an oligodendrocyte process: this glial membrane is rich in lipids Myelinated axons are surrounded by a succession of myelin segments, each about 1 mm long. The covered regions of axons alternate with short exposed lengths where the axonal membrane (axolemma) is not covered. These unmyelinated regions (of the order of a micron) are called nodes of Ranvier (Figures 2.4 and 2.5a).
FIGURE 2.4 Diagram and photomicrographs of myelinating oligodendrocytes and their numerous processes. (a) Each oligodendrocyte process forms a segment of myelin around a different axon in the central nervous system. Two myelin segments are represented, one partially unrolled, the other completely unrolled. (b) Maturation of oligodendrocytes grown on axons of dorsal root ganglionic (DRG) neurons in culture. DRG axons are immunostained for neurofilament (red), and cell nuclei are labeled with DAPI (blue). Upper part: oligodendrocyte progenitor cells (revealed by NG2 labeling in green) plated onto DRG axons (red) after 2 days in co-culture. They extend many processes which contact multiple axons. Lower part: after 7 days in co-culture, oligodendrocyte precursor cells have differentiated into myelinating oligodendrocytes which form multiple segments of compact myelin associated with the axons (myelin is shown in green by myelin basic protein, a component of the myelin sheath, immunolabeling). Scale bar = 15 mm. Part (a) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission. Part (b) from Lee PR, Fields RD. (2009) Regulation of myelin genes implicated in psychiatric disorders by functional activity in axons. Front. Neuroanat. 3, 4, with permission.
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A myelinated segment comprises the length of axon covered by an oligodendrocyte. One oligodendrocyte can form 20–70 myelin segments around different axons (Figure 2.4b). Thus the degeneration or dysfunction of a single oligodendrocyte leads to the disappearance of myelin segments on several different axons. Origin of oligodendrocytes and myelination process Myelination represents a crucial stage in the ontogenesis of the nervous system. In the human at birth, myelinization is only just beginning and, in some regions, is not complete even by the end of the second year of life. Oligodendrocytes are derived from oligodendrocyte progenitor cells (OPCs). These progenitors (sometimes called NG2 cells because they express the neural/glial antigen 2 proteoglycan (NG2) on their surface) represent a large and enigmatic population of glial cells. Because of their ability to generate all neural cell types in vitro (neurons, astrocytes and oligodendrocytes), NG2 cells have been proposed to be multipotent stem cells; however, studies using genetic lineage tracing with inducible Cre mouse lines demonstrate that NG2 cells only generate oligodendrocytes in vivo. Nevertheless, oligodendrocyte progenitor cells present remarkable properties: (i) they persist throughout life and represent approximately 5% of all cells in the CNS; (ii) they are the main population of slowly dividing cells in the postnatal brain; (iii) they are activated after insult of the central nervous system: they start proliferating actively, migrate toward the lesion and, finally, differentiate into oligodendrocytes thus contributing to spontaneous remyelination; and (iv) they are coupled to axons through synapses that use GABA and glutamate as neurotransmitters (half of these cells can even fire action potentials). Remarkably, synapses of oligodendrocyte progenitor cells disappear as soon as the cell differentiates into a mature oligodendrocyte. In fact, this terminal differentiation of oligodendrocyte progenitor cells is promoted by neuronal activity. Indeed, optogenetic stimulation of premotor cortex in awake mice stimulates proliferation of oligodendrocyte precursor cells and promotes myelination of axons within the deep layers of the premotor cortex and subcortical white matter. Oligodendrocytes do not myelinate all axons in the CNS. Myelination starts with the initial turn of myelin around the axon which is rapidly formed (Figure 2.4). Myelin is then slowly deposited over a period which, in humans, can reach several months. Myelinization is responsible for a large part of the increase in weight of the central nervous system following the end of neurogenesis. In order to form the compact spiral of myelin membrane, the oligodendrocyte process must roll itself around the axon many times (up to 40 turns) (Figure 2.5). It is the terminal portion of the process, called the inner loop,
situated at the interior of the roll, which progressively spirals around the axon. This movement necessitates the sliding of myelin sheets which are not firmly attached. During this period, the oligodendrocyte synthesizes several times its own weight of myelin membrane each day. Within the spiral, the cytoplasm disappears entirely (except at the internal and external loops). The internal leaflets of the plasma membranes can thus adhere to each other. This adhesion is so intimate that the internal leaflets virtually fuse, forming the period, or major, dense line of thickness of 3 nm (Figure 2.5b). The extracellular space between the different turns of membrane also disappears, and the external leaflets also stick to each other. This apposition is, however, less close and a small space remains between the external leaflets. The apposed external leaflets form the minor, or interperiod, dense line (Figure 2.5b). Thus, a cross-section of a myelinated axon observed by electron microscopy shows alternating dark and light lines forming a spiral around the axon. The major dense line terminates where the internal leaflets separate to enclose the cytoplasm within the external loop. The interperiod dense line disappears at the surface of the sheath at the end of the spiral (Figure 2.5b). In the central nervous system, there is no basal lamina around myelin segments, so myelin segments of adjacent axons may adhere to each other forming an interperiod dense line. The g ratio, determined by the axon diameter divided by axon plus myelin diameter, is often used as a myelination index. Myelin Myelin consists of a compact spiral (without intracellular or extracellular space) of glial plasma membrane of a very particular composition. Lipids make up about 70% of the dry weight of myelin and proteins only 30%. Compared with the membranes of other cells, this represents an inversion of the lipid:protein ratio. This lipid-rich membrane is highly enriched in glycosphingolipids and cholesterol. The major glycosphingolipids in myelin are galactosylceramide and its sulfated derivative sulfatide (20% of lipid dry weight). There is also an unusually high proportion of ethanolamine phosphoglycerides in the plasmalogen form, which accounts for one-third of the phospholipids. In myelin, a number of structural classes of proteins are present. Some proteins are extremely hydrophobic membrane-embedded polypeptides, some integral membrane proteins have a single transmembrane domain and clearly define extra- and intracellular domains, and some of the myelin proteins are cytosolic; however, they are often intimately associated with the myelin membrane. Myelin basic protein (MBP) and the proteolipid proteins (PLP/DM20) are the two major myelin proteins in the CNS. Myelin basic proteins are found on the
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FIGURE 2.5 Myelin sheath of central axons. (a) Three-dimensional diagram of the myelin sheath of an axon in the central nervous system (CNS). The sheath is formed by a succession of compact rolls of glial processes from different oligodendrocytes. (b) Cross-section through a myelin sheath. The dark lines, or major dense lines, and clear bands (in the middle of which are found the interperiod lines) visible with electron microscopy are accounted for by the manner in which the myelin membrane surrounds the axon, and by the composition of the membrane. The dark lines represent the adhesion of the internal leaflets of the myelin membrane while the interperiod lines represent the adhesion of the external leaflets. The lines are formed by membrane proteins while the clear bands are formed by the lipid bilayer. (c,d) Electron micrographs of myelinated axons in sciatic (c) and optic (d) nerves. Structural details of myelin ensheathment at the paranodal level (longitudinal section, c). Crosssection of myelinated axons (d). Numerous microtubules and mitochondria are present in the axons. The innermost layer of myelin sheath is often non-compacted. MVBs: multivesicular bodies. PNJ: paranodal junction. Part (a) drawing from Bunge MB, Bunge RP, Ris H (1961) Ultrastructural study of remyelination in an experimental lesion in adult cat spinal cord. J. Biophys. Biochem. Cytol. 10, 67–94, with permission of Rockerfeller University Press. Part (b) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission. Parts (c,d) from Nave KA (2010) Myelination and the trophic support of long axons. Nat. Rev. Neurosci. 11, 275–283, with permission.
cytoplasmic side and play a role in the adhesion of the internal leaflets of the specialized oligodendroglial plasma membrane. Proteolipid proteins are integral membrane proteins. Though they are in high abundance (they represent around 50% of the total myelin protein in the central nervous system), their exact biological role has not yet been elucidated.
We have seen that myelin has an inverted lipid:protein ratio, while the cell body membrane of the oligodendrocyte has a ratio comparable to that of other cell membranes. As the myelin of the oligodendrocyte process is in continuity with the plasma membrane of the cell body, it is necessary to postulate gradients in the composition of lipids and proteins (in opposite directions
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to each other) between the cell body and the various processes. During the active phase of myelination, each oligodendrocyte must produce as much as 5–50 000 mm2 of myelin membrane surface area per day. Interestingly, myelination does not stop with the end of development but is maintained throughout life. Using transgenic mice with inducible Cre recombinase system to label adult oligodendrocyte progenitor cells and trace their progeny, it was demonstrated that almost 30% of total oligodendrocytes of the corpus callosum in an 8-month-old animal were generated during adulthood. The function of this constant production of myelinating oligodendrocytes and of myelin remodeling in the adult brain remains poorly understood but, together with the activity-dependent control of myelination, it highlights the remarkable plasticity of myelin. Nodes of Ranvier In the central nervous system, the nodes of Ranvier, regions between myelin segments, are relatively long (several microns) compared with those in the peripheral nervous system. Here the axolemma is exposed and an accumulation of dense material is seen on the cytoplasmic side. The myelin sheath does not terminate abruptly. Successive layers of myelin membrane terminate at regularly spaced intervals along the axon, the internal layers (close to the axon) terminating first. This staggered termination of the different layers of myelin constitutes the paranodal region (Figure 2.5).
2.2.2 Myelination enables rapid conduction of action potentials for two reasons Isolation of internode axonal segments The high lipid content and compact structure of the myelin sheath help make it impermeable to hydrophilic substances such as ions. It prevents transmembrane ion fluxes and acts as a good electrical insulator between the intracellular (i.e. intra-axonal) and extracellular media. Between the nodes of Ranvier the axon therefore behaves as an insulated cable. This permits rapid, saltatory conduction of action potentials along the axon (see Section 5.4). Formation of Ranvier nodes with a high density of Na+ channels Na+ channels are clustered in very high density within the nodal gap whereas voltage-dependent K+ channels are segregated in juxta-paranodal regions, beneath overlying myelin (Figure 2.5). To test whether oligodendrocyte contact with axon influences Na+ channel distribution, nodes of Ranvier in the brain of hypomyelinating mouse Shiverer are examined. Shiverer mice have oligodendrocytes that ensheath axons but do not form
compact myelin and axoglial junctions. In these mutant mice, there are far fewer Na+ channel clusters than in control littermates and aberrant locations of Na+ channels are observed. If Na+ channel clustering depended only on the presence of oligodendrocytes and were independent of myelin and oligodendroglial contact, one would expect to find normal Na+ channel distribution along ‘shiverer’ axons. Contribution of myelination to signal transmission efficiency Myelination of axons by glial cells has appeared quite late in evolution. This innovation in ancient jawed vertebrates allowed speeding up nerve conduction more than 20-fold and the appearance of efficient signal transmission in large animals. Today, numerous correlative evidences suggest myelin involvement in cognition or in the development of fine motor skills.
2.2.3 Myelin-independent functions of oligodendrocytes Facilitation of conduction velocity by depolarization of oligodendrocytes Myelin increases the velocity of action potentials along axons but oligodendrocytes also contribute to optimize signal transmission via myelin-independent functions. Like neurons and astrocytes, oligodendrocytes express a large variety of ion channels and neurotransmitter receptors. They can thus participate in K+ buffering, monitor neuronal activity, be depolarized and directly increase conduction velocity along the axon. The mechanisms involved in oligodendrocyte depolarization-induced conduction velocity are not completely deciphered but may involve structural changes leading to increased insulation in the paranodal and intermodal regions. Since one oligodendrocyte myelinates multiple axons, oligodendrocytes could also contribute to signal synchronization. Oligodendrocytes support axonal integrity Axonal support is probably a common and ancestral function of all axon-ensheathing glial cells. Paradoxically, because myelin completely insulates the axon, it also deprives it from metabolic support by the extracellular milieu. Thus, oligodendrocytes need to overcome this by supplying trophic and metabolic support to axons, thereby preserving axonal integrity. Diseases affecting oligodendrocytes often lead to axon degeneration. Part of this neuroprotection effect is independent of myelin’s function. Loss of myelin proteins such as PLP or CNP, which are dispensable for myelination, leads to progressive axonal degeneration. Mechanisms underlying glial cell support are not completely understood yet.
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.3 MICROGLIAL CELLS ARE THE MACROPHAGES OF THE CENTRAL NERVOUS SYSTEM
2.2.4 Impact of myelin defects Oligodendrocytes show the highest metabolic rates among all brain cells in order to produce but also to maintain very high volume of membranes which can represent up to 100 times the weight of the cell. This metabolic requirement makes them highly sensitive to pathological conditions such as ischemia, trauma and inflammation. The consequences of massive oligodendrocyte death and demyelination have long been known from diseases such as multiple sclerosis, with functional loss depending of the areas of the CNS where demyelination occurs (blindness, incontinency, motor deficit…). More recently, it has been discovered that more subtle myelin abnormalities can contribute to a wide range of neuropsychatric disorders including depression and schizophrenia. White matter abnormalities also represent an early feature of neurodegenerative pathologies such as Huntington and Alzheimer diseases (far before neurofibrillary tangles and clinical symptoms) and might also contribute to the pathogenesis of the disease.
2.3 MICROGLIAL CELLS ARE THE MACROPHAGES OF THE CENTRAL NERVOUS SYSTEM Microglial cells are unique since they are at the interface between the immune and nervous systems, which confers them remarkable properties. Microglia are equipped by a plethora of receptors allowing them to detect signals deriving from different origins, such as an immune challenge or activity from other brain cells, and readily react to them. As macrophages, they are professional phagocytes, but they can also release several mediators which can influence neuronal activity as well as the fate of surrounding cells. After a decade long debate about their origin, we know today that these cells arise early during development from progenitors in the embryonic yolk sac (YS) that seed the brain rudiment. During development, microglial cells colonize the entire CNS and they differentiate to assume a mature phenotype adapted to their function in the adult brain.
2.3.1 Microglial cells contribute to brain physiology Microglia are highly dynamic cells In the adult CNS, microglial density and morphology change among different CNS regions, likely to better fulfill their function. Their density varies from about 12% of total cells in substantia nigra to ≈5% in corpus callosum. Despite this variability, their distribution in space in a specific region is rather uniform with minimal overlap. In white matter, their morphology is characterized
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by an elongated soma and processes preferentially oriented along fiber tracts, while in the circumventricular organs, a region with a leaky blood–brain barrier, they are compact with a few short processes. In contrast, in gray matter, they present a small cell body with highly branched processes. This typical appearance, which is quite different from a classical macrophage, has been associated with microglial ‘resting’ state and, until recently, this state was considered quiescent, i.e. functionally dormant. However, recent in vivo imaging studies employing mice with EGFP-expressing microglia demonstrated that they constantly extend and retract their fine cellular processes at a rate of few micrometers per minute to scan the brain parenchyma. During movements, a single microglia maintains its cell body position as well as its overall size and symmetry of its territory, and avoids contacts with other microglial processes. Thus, microglia are able to monitor their extracellular space and cellular neighborhood, always ready to intervene and change their activity status if abnormal changes are detected such as infections, lesions and neurodegeneration, critical variation in neuronal activity… Microglia sense and control neuronal activity During their scanning movements microglial processes rapidly contact other CNS cells, like neurons, perivascular astroglial and other glial cells. In particular, they contact post- and presynaptic elements of synapses. Interestingly, the frequency of these contacts is regulated by the neuronal activity. Indeed, it is decreased by sensorial deprivation, pharmacological block of action potential, as well as a decrease of body temperature. Thus, microglial cells can sense and react to neuronal activity. In turn, they can locally influence neuronal excitability by releasing several factors such as cytokines, neurotrophic factors, and neurotransmitters. A study in the larvae of zebra fish recently showed that an increase of neuronal firing attracts microglia through the release of ATP, and this contact decreases the activity of the targeted neuron. Microglia control adult neurogenesis Another important role of microglia is the regulation of the adult neurogenesis. Two neurogenic niches, in the ventricles and in the dentate gyrus of the hippocampus (see Chapter 19), produce new neurons throughout life. However, only a few newborn cells are incorporated into the circuitry and the majority of them are presumed to die at immature neuronal stage. Microglia can control proliferation, migration and differentiation of stem cells, but the factors regulating different steps of neurogenesis are still unknown. Furthermore, they are responsible for the phagocytosis of apoptotic cells preventing them from undergoing secondary necrosis, with the consequent spillage of cellular contents leading to an inflammatory response, detrimental for neurogenesis.
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2. Neuron–glial cell cooperation
FIGURE 2.6 Scheme of the different functions of microglia. Microglia (green) constantly move their processes to scan the brain parenchyma. During their movements they contact synapses and neuronal dendrites (orange), as well other brain cells. They can control brain activity and surrounding cells’ fate by releasing several factors. They phagocytose cells and neuronal debris, but also synaptic elements and newborn cells (orange), thus they participate in sculpting the neuronal circuits. Drawing by E. Avignone.
2.3.2 Microglia exert their activity through physical interaction and release of soluble factors We can classify the main actions of microglia into two broad categories: physical interactions and release of soluble factors (Figure 2.6). These actions are triggered by changes in the environment which modify microglia properties. Microglia with physical interactions sculpt neuronal circuits and isolate brain injuries Three types of physical interactions are described: phagocytosis, synapse stripping, and formation of a barrier around a damaged area. Phagocytosis is a major feature of microglial cells. This phenomenon is particular important during pathologies involving cell death, where cellular debris has to be removed, as well as during development. In immature circuits, microglia phagocyte presynaptic terminals and dendritic spines (synapse pruning). They thus contribute to circuit formation. Synapse stripping occurs when microglia processes form a barrier between the pre- and postsynaptic elements at the synaptic cleft. It is observed during lesions where there is an injury of innervating cells and a loss of synaptic boutons, as after facial nerve injury. As previously seen, microglia constantly move their processes to scan the brain parenchyma. As soon as a problem is detected they can extend their processes in
a few minutes towards sites of injury or substances that may represent a danger signal, like ATP or nitric oxide. These substances could be released by necrotic cells or by neurodegenerating axons. The ensemble of microglial processes then forms a barrier around the injured area in a time course from minutes to hours, thus limiting the spread of the damage of an acute lesion to the surrounding healthy tissue. Microglia release soluble factors to control neuronal survival and activity Pioneer studies in culture identified several soluble factors released by microglia, including a multitude of growth factors, cytokines, and neurotransmitters. Many released factors are known to affect neuronal activity, either directly or through other cells such as astrocytes. However, direct evidences of neuronal activity modulation by microglia in physiological conditions are still lacking. Indeed, most of these studies have been performed in culture, where microglia properties do not faithfully reflect those in the normal, non-pathological brain. Microglia change their properties in pathological conditions to respond to injuries Microglial cells are extremely sensitive to changes in their local environment. As a response to homeostasis perturbation, they adapt their phenotype in a multifaceted process called activation. Activation develops over
I. Neurons: Excitable and Secretory Cells that Establish Synapses
2.4 THE MYELIN SHEATH OF AXONS OR ENCAPSULATE NEURONS
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FIGURE 2.7 Microglia change properties after activation. The images show an example of morphological changes of microglia 48 hours after activation induced by status epilepticus. In control conditions (a) microglial cells have a small body with long and ramified processes. (b) In contrast, activated microglial cells have larger body with shorter and thicker processes. From Menteyne A, Levavasseur F, Audinat E, Avignone E (2009) Predominant functional expression of Kv1.3 by activated microglia of the hippocampus after status epilepticus. PLoS One 4, e6770, with permission.
periods ranging from hours to days, and it includes changes in morphology (Figure 2.7), electrical membrane properties and expression of a variety of markers. Activation triggers migration, proliferation and release of a variety of mediators, such as pro- and anti-inflammatory cytokines and neurotrophines. Activation is not an all-or-none process. It is variable and adaptive and it depends on the nature of triggering factors. Thus, microglia can exist in a variety of activated states. The heterogeneity of activation phenotypes and functions caused a debate on the beneficial versus detrimental roles of microglia in pathology, since it can exacerbate damage or, on the contrary, protect neurons from degeneration. A major challenge today is to characterize several aspects of microglia activation in specific diseases, and develop new drugs that block only harmful facets of activation.
2.4 SCHWANN CELLS ARE THE GLIAL CELLS OF THE PERIPHERAL NERVOUS SYSTEM; THEY FORM THE MYELIN SHEATH OF AXONS OR ENCAPSULATE NEURONS There are three types of Schwann cell: • those forming the myelin sheath of peripheral myelinated axons (myelinating Schwann cells);
• those encapsulating non-myelinated peripheral axons (non-myelinating Schwann cells); • those that encapsulate the bodies of ganglion cells (non-myelinating Schwann cells or satellite cells).
2.4.1 Myelinating Schwann cells make the myelin sheath of peripheral axons Along an axon, several Schwann cells form successive segments of the myelin sheath. In contrast to oligodendrocytes, it is not a process that enwraps the peripheral axon to form the segment of myelin, but the whole Schwann cell (Figure 2.8). Each Schwann cell therefore forms only one myelin segment. The composition of peripheral myelin differs from that of central myelin only in the proteins it contains. The principal protein constituents of peripheral myelin are: peripheral myelin protein 2 (P2), protein zero (P0) and myelin basic proteins (MBPs). The first two proteins are specific to peripheral myelin. MBP comprises a major part of the cytosolic protein of myelin and is present both in the CNS and peripheral nervous system (PNS). Protein zero is a glycoprotein that has adhesive properties and is located in the interperiod line. It functions, in part, as a homotypic adhesion molecule throughout the full thickness of the myelin sheath. It is a good marker for myelinating Schwann cells as it represents over 50% of total PNS myelin protein.
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2. Neuron–glial cell cooperation
FIGURE 2.8 Myelin sheath of a peripheral axon. (a) Three-dimensional diagram of the myelin sheath of an axon of the peripheral nervous system (PNS). The sheath is formed by successive rolled Schwann cells. (b) Process of myelinization. The internal loop wraps around the axon several times. During this process, the axon grows and the myelin becomes compact. Contact between the Schwann cell and axon occurs only at the paranodal and nodal regions. Elsewhere an extracellular, or periaxonal, space always remains. Part (a) drawing adapted from Maillet M (1977) Le Tissu Nerveux, Paris: Vigot, with permission. Part (b) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission.
2.4.2 Non-myelinating Schwann cells encapsulate the axons and cell bodies of peripheral neurons Non-myelinated axons are not uncovered in the peripheral nervous system as they are in the central nervous system; they are encapsulated. A single nonmyelinating Schwann cell surrounds several axons (about 5–20) for a distance of 200–500 mm in humans. In addition, spinal and cranial ganglia contain a large number of Schwann cells that do not produce myelin. These Schwann cells cover the somata of the ganglionic cells, leaving an extracellular space of about 20 nm between themselves and the surface of the covered neuron. The lipid and protein composition of the plasma membrane of non-myelinating Schwann cells is the
same as that of other eukaryotic cells (30% lipid, 70% protein). Apart from their role in the saltatory conduction of action potentials (myelinating Schwann cells), Schwann cells also play a role in maintenance of axonal integrity and survival. A remarkable property of Schwann cells is their ability to promote the regeneration of peripheral nerve cells. It has long been known that cut peripheral nerves can, within certain limits, regrow and reinnervate deafferented regions while central axons are not capable of this. The substantial regenerative capacity of the Schwann cells is driven by their ability to dedifferentiate following axonal injury, to divide and produce axonotrophic factors. This property of regeneration is due in large part to an enabling effect of Schwann cells on axon regrowth.
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2.4 THE MYELIN SHEATH OF AXONS OR ENCAPSULATE NEURONS
Further reading Azevedo, F.A., Carvalho, L.R., Grinberg, L.T., et al., 2009. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J. Comp. Neurol. 513, 532–541. Bélanger, M., Allaman, I., Magistretti, P.J., 2011. Brain energy metabolism: focus on astrocyte-neuron metabolic cooperation. Cell Metab. 14, 724–738. Bushong, E.A., Martone, M.E., Jones, Y.Z., Ellisman, M.H., 2002. Protoplasmic astrocytes in CA1 stratum radiatum occupy separate anatomical domains. J. Neurosci. 22, 183–192. Butt, A.M., Kalsi, A., 2006. Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions. J. Cell. Mol. Med. 10, 33–44. Buttermore, E.D.L., Thaxton, C.L., Bhat, M.A., 2013. Organization and maintenance of molecular domains in myelinated axons. J. Neurosci. Res. 91, 603–622. Dupree, J.L., Mason, J.L., Marcus, J.R., et al., 2005. Oligodendrocytes assist in the maintenance of sodium channel clusters independent of the myelin sheath. Neuron Glia Biol. 1, 1–14. El Waly, B., Macchi, M., Cayre, M., Durbec, P., 2014. Oligodendrogenesis in normal and pathological brain. Front. Neurosci. 8, 145. Farber, K., Kettenmann, H., 2005. Physiology of microglial cells. Brain Res. Brain Res. Rev. 48, 133–143. Fields, R.D., Araque, A., Johansen-Berg, H., et al., 2014. Glial biology in learning and cognition. Neuroscientist 20, 426-431. Freeman, M.R., Doherty, J., 2006. Glial cell biology in Drosophila and vertebrates. Trends Neurosci. 29, 82–90. Gibson, E.M.L., Purger, D., Mount, C.W., et al., 2014. Neuronal activity promotes oligodendrogenesis and adaptive myelination in the mammalian brain. Science, Apr 11. [Epub ahead of print]. Greer, J.M., Lees, M.B., 2002. Myelin proteolipid protein – the first 50 years. Int. J. Biochem. Cell Biol. 34, 211–215. Kettenmann, H., Kirchhoff, F., Verkhratsky, A., 2013. Microglia: new roles for the synaptic stripper. Neuron 77, 10–18. Montague, P., McCallion, A.S., Davies, R.W., Griffiths, I.R., 2006. Myelin-associated oligodendrocytic basic protein: a family
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of abundant CNS myelin proteins in search of a function. Dev. Neurosci. 28, 479–487. Nave, K.A., 2010. Myelination and support of axonal integrity by glia. Nature 468, 244–252. Pannasch, U., Rouach, N., 2013. Emerging role for astroglial networks in information processing: from synapse to behavior. Trends Neurosci. 36, 405–417. Parkhurst, C.N., Gan, W.-B., 2010. Microglia dynamics and function in the CNS. Curr. Opin. Neurobiol. 20, 595–600. Parpura, V., Heneka, M.T., Montana, V., et al., 2012. Glial cells in (patho)physiology. J. Neurochem. 121, 4–27. Peles, E., Salzer, J.L., 2000. Molecular domains of myelinated axons. Curr. Opin. Neurobiol. 10, 558–565. Pfeiffer, S.E., Warrington, A.E., Bansal, R., 1993. The oligodendrocyte and its many cellular processes. Trends Cell Biol. 3, 191–197. Popko, B., 2000. Myelin galactolipids: mediators of axon–glial interactions? Glia 29, 149–153. Shapiro, L., Doyle, J.P., Hensley, P., Colman, D.R., Hendrickson, W.A., 1996. Crystal structure of the extracellular domain from P0, the major structural protein of peripheral nerve myelin. Neuron 17, 435–449. Streit, W.J., 2000. Microglial response to brain injury: a brief synopsis. Toxicol. Pathol. 28, 28–30. Tremblay, M.-É., 2011. The role of microglia at synapses in the healthy CNS: novel insights from recent imaging studies. Neuron Glia Biol. 7, 67–76. Ueno, M., Yamashita, T., 2014. Bidirectional tuning of microglia in the developing brain: from neurogenesis to neural circuit formation. Curr. Opin. Neurobiol. 27C, 8–15. Vandenberg, R.J., Ryan, R.M., 2013. Mechanisms of glutamate transport. Physiol. Rev. 93, 1621–1657. Ventura, R., Harris, K.M., 1999. Three-dimensional relationships between hippocampal synapses and astrocytes. J. Neurosci. 19, 6897–6906. Yamazaki, Y.L., Hozumi, Y., Kaneko, K., Fujii, S., Goto, K., Kato, H., 2010. Oligodendrocytes: facilitating axonal conduction by more than myelination. Neuroscientist 16, 11–18.
I. Neurons: Excitable and Secretory Cells that Establish Synapses