- Email: [email protected]
Dynamics and pathology of dendritic spines
Progress in Brain Research, Vol. 147 ISSN 0079-6123 Copyright ß 2005 Elsevier BV. All rights reserved
CHAPTER 3
Dynamics and pathology of dendritic spines Shelley Halpain*, Kathy Spencer and Simone Graber The Scripps Research Institute, Department of Cell Biology and Institute for Childhood and Neglected Diseases, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
Abstract: Dendritic spines are key players in information processing in the brain. Changes in spine shape and wholesale spine turnover provide mechanisms for modifying existing synaptic connections and altering neuronal connectivity. Although neuronal cell death in acute and chronic neurodegenerative diseases is clearly an important factor in decline of cognitive or motor function, loss of dendritic spines, in the absence of cell death, may also contribute to impaired brain function in these diseases, as well as in psychiatric disorders and aging. Because spines can function in neuroprotection in vitro, advances toward a molecular understanding of spine maintenance might one day aid in the design of therapies to minimize neurological damage following excitotoxic injury. In addition, progress in defining the biochemical basis of spine development and stabilization may yield insights into mental retardation and psychiatric disorders.
Introduction
malformed or lost in many disease states, including epilepsy, stroke, schizophrenia, mental retardation, dementia, and chronic substance abuse. Here we review the structure, function, and development of spines, and highlight molecular mechanisms for their regulation. Such information may one day form the basis of new therapies for treating neurological and psychiatric diseases.
The human brain contains more than 1013 dendritic spines — small protrusions that form the postsynaptic part of glutamate-releasing CNS synapses (Nimchinsky et al., 2002). Many types of both excitatory and inhibitory neurons bear these micronsized membrane specializations. For example, spiny neuron populations include the glutamatergic pyramidal neurons of neocortex and hippocampus as well as GABAergic cerebellar Purkinje neurons and medium-sized projection neurons of the striatum. Spines contain specialized sets of molecules that not only determine the spine shape but, most importantly, enable the postsynaptic neuron to respond biochemically to glutamate or other transmembrane signals (Smart and Halpain, 2000; Hering and Sheng, 2001; Zhang and Benson, 2001). Dendritic spines therefore play critical roles in cognitive and motor function and in memory formation. They are
Spines are dynamic structures The small size of spines approaches the limits of optical resolution in light microscopy. Therefore much of the early work that characterized spines in detail was conducted using electron microscopy. Most spines in the mature brain have a club-like morphology. They are connected to their parent dendrite by thin stalks 0.04–1 mm long, and have variably shaped bulbous tips 0.5–2 mm in diameter (Harris and Kater, 1994). On mature spiny neurons, spine density ranges from 1–10 spines per micron length of dendrite. The vast majority of CNS excitatory synapses contact
*Corresponding author. E-mail: [email protected] DOI: 10.1016/S0079-6123(04)47003-4
29
30
spines, although such glutamate synapses can also form directly on the dendrite shaft (so-called ‘‘shaft synapses’’). In contrast, nearly all inhibitory synapses are shaft synapses. Based on electron microscopic images of fixed brain preparations, investigators defined distinct morphological categories of spines (e.g., ‘‘mushroom shaped,’’ ‘‘thin,’’ or ‘‘stubby’’ (Peters and KaisermanAbramof, 1970; Sorra and Harris, 2000). Beginning in the late 1990s greatly improved methods for fluorescence time-lapse imaging were applied to living neurons maintained in dissociated or organotypic culture. Such studies revealed that spine shapes are highly dynamic. Although spines themselves persist over many hours and maintain a fairly consistent neck length, the heads of many spines continually shift their size and shape on a time scale of seconds to minutes (Fischer et al., 1998; Dunaevsky et al., 2001). A common term applied to this motility is ‘‘morphing.’’ At present the exact function of this intrinsic morphing remains mysterious. It is possible that synapses have evolved mechanisms to rapidly change the biochemical compartmentalization of the spine and thereby alter its signal processing parameters. It is also possible that the motility itself has no specific function but instead reflects one or more underlying cellular mechanisms (for example, regulation of synapse adhesion or the trafficking of membrane vesicles containing glutamate receptors or other signaling molecules (Halpain, 2000). More recent studies using multiphoton microscopy have extended dendritic spine observations into living animals. Remarkably, such state-of-the-art imaging experiments in mice have revealed that spines can turn over with half-lives of a day or less, even in adult animals (Grutzendler et al., 2002). In somatosensory cortex approximately 50% of spines were observed to turn over every few days, even though the overall numbers of spines remained constant (Trachtenberg et al., 2002). However, in other parts of the adult cortex, spines were stable over several months, with only a small fraction turning over within this time frame (Grutzendler et al., 2002). As might be expected, both spine turnover and the rapid spine ‘‘morphing’’ described above are greater in developing animals than in adults. Our ability to monitor spine turnover in vivo is currently at an early stage. Nonetheless, it seems
likely that the wholesale appearance and disappearance of dendritic spines is a normal part of brain function, not only for refinement of synaptic circuits during development, but also for synaptic remodeling in the adult. It is reasonable to postulate that the relative permanence of dendritic spines varies greatly depending on brain region, age, disease state, and level of neuronal activity. Several studies have demonstrated that spine density changes under specific physiological circumstances. For example, significant cyclic changes in spine density are observed over the four-day estrus cycle in female rats (Woolley et al., 1990; Woolley, 1999), during hibernation and reawakening in ground squirrels (Popov et al., 1992), and in response to enriched environmental experience in rats (Comery et al., 1996). In addition, recent studies show that spines can appear very rapidly (within minutes) in response to physiological stimuli (Engert and Bonhoeffer, 1999; Maletic-Savatic et al., 1999) and collapse very rapidly (within minutes) in response to pathological stimuli (Halpain et al., 1998). These dynamic changes in spine shape and existence are presumably mediated by various extracellular signals, including glutamate and other neurotransmitters (Wong and Wong, 2001), neurotrophins such as brain-derived neurotrophic factor (McAllister et al., 1999), and cell-cell interactions mediated by the Eph-ephrin system (Murai et al., 2003), or cadherin-catenin complexes (Murase et al., 2002; Abe et al., 2004). As discussed below, responses to such transmembrane signals, in turn, are likely to be mediated via the actin cytoskeleton.
Spine development Spine formation occurs as a late step in neuronal morphogenesis, peaking, for example, during the third postnatal week in rodent hippocampus. This timing and order of events is preserved in cultured neurons, making neuronal cultures a suitable model system for studies of spine regulation. Synaptogenesis begins as afferent axons reach their postsynaptic targets. At this initial stage, dendrites of nearly all neurons — both spiny and nonspiny — bear numerous thin protrusions termed ‘‘filopodia’’ (Ziv and Smith, 1996; Wong and Wong, 2001). Dendritic
31
filopodia are long, narrow protrusions that lack a bulbous head and usually contain lower amounts of F-actin than spines. They are more transient in nature than dendritic spines, extending and retracting from the dendrite shaft with half-lives of about 10 min, as compared to half-lives of hours, days, or months for most mature spines (Ziv and Smith, 1996; Sorra and Harris, 2000). Gradually, over the course of several days, the numerous dendritic filopodia are replaced by spines. Spiny synapses have been proposed to arise by two mechanisms: (a) a synapse forms directly on the dendrite shaft, and a spine gradually grows out from the shaft; (b) a synapse forms on a dendritic filopodium and the filopodium gradually ‘‘converts’’ into a mature spine. Experimental evidence supports each of these mechanisms, and it seems likely that both can and do occur even within a single neuron. Although filopodia seem to be involved in the formation of at least some dendritic spines, it is also clear that not all filopodia are destined to become spines (Ziv and Smith, 1996). Indeed, filopodia are also observed on nonspiny neurons early in their synaptogenesis phase (Wong and Wong, 2001). It is likely that filopodia, with their high motility and transient nature, play key roles in establishing many kinds of synapses, because they maximize the chance encounter of pre- and postsynaptic elements during the search for synaptic partners (see Wong and Wong, 2001 for discussion). The emergence of a spine as a semi-permanent protrusion is probably not dictated by the presence or absence of a filopodium per se, but rather by the presence of molecular complexes that cluster and accumulate at the plasma membrane in response to signals from the nerve terminal (Goda and Davis, 2003). Through a process of mutual cross-talk, the pre- and postsynaptic sides coordinately orchestrate the maturation of the entire synapse, including, presumably, the spine structure itself.
Molecular composition of spines As in all cells, cytoskeletal proteins take the lead in determining the shape and stability of spines. The spine cytoskeleton is based mainly on actin filaments (F-actin), since intermediate filaments and microtubules are absent from nearly all spines
(although intermediate filament proteins and tubulin are detected in spines (Benson et al., 1994). Spines are unusually rich in F-actin compared to other parts of the neuron (Matus et al., 1982). Pharmacological manipulations that inhibit F-actin dynamics alter spine shape and motility (Fischer et al., 1998; Dunaevsky et al., 1999), and experimentally-induced spine collapse can be prevented by stabilizing actin filaments (Halpain et al., 1998). A growing roster of F-actin binding and regulatory proteins have been found in spines (Smart and Halpain, 2000; Zhang and Benson, 2000), many of which have been shown to regulate spine shape. Examples include Rho family GTPases (Nakayama et al., 2000) and proteins that regulate them, such as SPAR (spine-associated GTPase activating protein for Rap) (Pak et al., 2001) and kalirin (a guanine nucleotide exchange factor for Rac; (Penzes et al., 2003); the scaffolding protein shank (Sala et al., 2001); and the actin binding proteins drebrin (Hayashi and Shirao, 1999); cortactin (Hering and Sheng, 2003), and profilin (Ackermann and Matus, 2003). Changing the expression levels of these proteins has been shown to alter spine numbers, shape, or motility. It is likely that many such proteins will eventually be linked to diseases that affect synaptic function. Indeed, it was recently shown that the gene encoding LIM kinase-1 (LIMK-1), an actin regulatory molecule, is one of the approximately 20 genes deleted in patients with Williams syndrome, a developmental disorder characterized by cardiovascular and cognitive deficits (Hoogenraad et al., 2004). LIMK-1 responds to the small GTPase Rac by phosphorylating and thereby inactivating its target, the actin-depolymerizing factor ADF/cofilin (Sarmiere and Bamburg, 2002). LIMK-1 knock-out mice and Williams syndrome patients exhibit abnormal dendritic spines (Meng et al., 2002). Other molecules that act downstream of Rho-family GTPases have also been linked to hereditary forms of mental retardation and spine abnormalities (Ramakers, 2002). The heads of dendritic spines contain a specialized organelle called the postsynaptic density (PSD), which sits directly across the cleft from the presynaptic active zone, where synaptic vesicles are clustered. The PSD is a supramolecular assembly of
32
glutamate receptors, scaffolding molecules, and enzymes (Hering and Sheng, 2003). A significant component of the PSD is the calcium/calmodulindependent protein kinase type II (CaMKII), which may play both structural and enzymatic roles within the PSD (Kennedy, 2000). Scaffolding molecules, such as PSD-95, spinophilin, and Homer are proteins that have no enzymatic function of their own but which serve to concentrate receptors and their downstream effector molecules into the correct assembly for efficient signal transduction (Kasai et al., 2003). This protein meshwork also contributes to regulation and maintenance of spine structure. Despite their small size, dendritic spines also contain molecular machinery for many other functions, including local control of membrane trafficking, protein synthesis, post-translational processing, and protein degradation. Most dendritic spines in the cerebellum and about 30% of spines in the hippocampus (those with the largest head volumes) contain endoplasmic reticulum (ER) in the spine neck, which exists either as loose tubular structures or as an organelle called the spine apparatus that consists of stacks of membrane cisternae (Cooney et al., 2002). ER is thought to contribute not only to trafficking of membrane proteins but also to regulation of calcium dynamics within the spines. Among other functions, ER releases calcium in response to calcium influx into the spines and thereby amplifies the calcium signal (Villa et al., 1992; Sorra and Harris, 2000). In addition, many dendrites have polyribosomes just beneath the spine neck, and there is growing evidence that protein synthesis and protein degradation can occur locally within spines (Jiang and Schuman, 2002; Ehlers, 2003; Steward and Schuman, 2003). Disruption of local protein synthesis has emerged as a potential mechanism for cognitive deficits in Fragile X syndrome, because Fragile X mRNA is targeted to dendrites, and the Fragile X mental retardation protein itself regulates protein synthesis (Antar and Bassell, 2003).
Function of spines Spines clearly contain specialized collections of receptors and signaling complexes, and they are uniquely associated with excitatory CNS synapses.
However, the function of spines is not completely understood. Indeed, many types of synapses, both excitatory and inhibitory, do not occur on spiny protrusions, so clearly a spine is not a pre-requisite for a functional synapse. It is likely that the existence of the unique spine structure confers some special property to the synapse that would be lost if the spine structure collapsed. Early work addressed this issue using computer modeling and suggested that the electrical properties of synapses (for example, the passive spread of synaptic current) are different for a synapse occurring on a spine as compared to one occurring on a dendrite shaft (Pongracz, 1985). Such models predicted that changes in spine shape would modulate current spread, leading some investigators to speculate that changes in spine shape might underlie learning and memory and other forms of synaptic plasticity, such as LTP. Later studies, however, questioned whether such changes would attain sufficient magnitude to significantly impact synaptic strength (reviewed in Nimchinsky et al., 2002). A current, widely accepted idea is that spines act as individual biochemical compartments. By virtue of its narrow neck, a spine retains second messengers and other small, diffusible molecules that are generated at the site of synaptic contact. Such biochemical compartmentalization has been directly demonstrated for small diffusible dyes and also for the important ions sodium and calcium (Guthrie et al., 1991; Muller and Connor, 1991; Svoboda et al., 1996; Rose and Konnerth, 2001). For calcium in particular, local control of concentration is crucial in orchestrating physiological responses to stimuli. Biochemical compartmentalization by spines provides two advantages. First, it allows small molecules to attain high concentrations in the postsynapse prior to their diffusing away. Second, it biochemically and therefore functionally isolates individual synapses from the parent dendrite (reviewed in Nimchinsky et al., 2002). This latter concept means that molecular signals generated at one spine are restricted to that synapse, and fail to influence a neighboring synapse on the same dendrite. This property of dendritic spines could contribute to the phenomenon of ‘‘input specificity,’’ whereby a given set of nerve terminals induces changes only at those synapses that are specific to their postsynaptic contacts, and not at other synapses on the same
33
neuron that are driven by different axons (Malenka and Nicoll, 1999). The function of the spine neck in limiting calcium diffusion from spine head to adjacent dendrite has also been postulated to play a protective role in excitotoxic injury during trauma, seizures, or ischemic conditions (Segal, 1995). In such diseases the excitatory neurotransmitter glutamate is released in excess, leading to overly strong activation of glutamate receptors and injurious calcium overload in the postsynaptic neurons. Such calcium overload is a major factor in excitotoxic neuronal cell death (Choi, 1995; Arundine and Tymianski, 2004). The diffusion barrier of the spine neck could limit the rise of calcium concentration in dendrite shaft and cell body, and thereby possibly protect from cell death. Indeed, recent studies in our laboratory using a neuronal cell culture model support an excitoprotective role for spines. We find that neurons become more sensitive to an excitotoxic stimulus when spine collapse is first induced. Furthermore, cell death is attenuated when dendritic spines are stabilized by an actin stabilizing agent prior to the excitotoxic stimulus (S. Graber and S. Halpain, unpublished).
Spine abnormalities in neurological disease A wide variety of human neurological and psychiatric diseases exhibit loss of spines or abnormalities in spine shape, paralleled by impaired cognitive function. Spine loss or distortion is associated with stroke, epilepsy, trauma, dementia, brain tumors, schizophrenia, major depression, substance abuse, and normal aging (Swann et al., 2000; Glantz and Lewis, 2001; Nimchinsky et al., 2002; Fiala et al., 2002). Brain regions showing spine loss typically correlate with brain systems affected in the disease state. For example, schizophrenic patients exhibit decreased spine density selectively in prefrontal cortical pyramidal neurons (Glantz and Lewis, 2000; Glantz and Lewis, 2001). Spines are also significantly reduced in some animal models of chronic depression, an effect reversed by antidepressants (Norrholm and Ouimet, 2001). A direct link between spine loss and specific pathological events is best established for acute disorders such as seizures and stroke, which involve
excitotoxic injury to the brain. Human epileptic patients display spine loss and other dendritic abnormalities (Swann et al., 2000). This observation is supported by work in animal models. Tetrodotoxin induction of epileptic seizures in rats reduces spine density on hippocampal pyramidal cells to 20–35% of normal controls (Jiang et al., 1998). Pilocarpineor bicuculline-induced seizures also cause spine loss (Isokawa, 2000). Presumably, the excessive activation of glutamate receptors during seizures or postischemia reperfusion swamps the calcium-buffering capacity of the spines, leading to spine retraction or collapse. Excitotoxic injury to spines can be studied in cultured neurons by addition of glutamate analogs such as NMDA to the culture medium (Choi, 1995; Arundine and Tymianski, 2004). In such models spine loss appears very rapidly, within 5–10 min of the onset of the stimulus (Halpain et al., 1998). Spine loss can also be induced in culture by addition of a calcium ionophore, suggesting a crucial role for glutamate-induced calcium influx in triggering spine collapse. However, such excessive stimulation of glutamate receptors and calcium also induces excitotoxic cell death (Choi, 1995; Arundine and Tymianski, 2004). Thus, spine loss could be merely an early morphological step in the progression toward cell death. However, our laboratory recently showed that intense but sub-lethal activation of glutamate receptors in cultured neurons can induce widespread spine loss in the absence of cell death (S. Graber and S. Halpain, unpublished). This dissociation between spine loss and cell death is consistent with the idea that spine loss itself might contribute to abnormal brain function after injury, even when the neurons themselves are preserved. Chronic neurodegenerative diseases such as Alzheimer’s and Parkinson’s often exhibit dendritic changes, including spine loss, as an early step in disease progression. Emerging evidence suggests that initial changes in motor and cognitive functions in such diseases are due to subtle alterations in synaptic function, rather than reflecting neuronal loss per se. For example, it is now widely believed that the initial stages of Alzheimer’s disease actually reflect disrupted synaptic function that greatly precedes the later decline in neuronal numbers (Selkoe, 2002). Dendrites and spines also degenerate prior to cell loss in some murine cerebellar ataxias (Hadj-Sahraoui
34
et al., 2001; Hamilton et al., 1996). Early spine loss occurs without concomitant neuronal death in a mouse model of Huntington’s disease (Guidetti et al., 2001), although the human disease clearly shows cell loss at later stages. It was previously thought that, as in neurodegenerative diseases, normal aging also involved a decline in neuronal cell numbers; however, more recent studies suggest that this is not the case, and instead that synapse numbers are reduced (Morrison and Hof, 1997). Even if spine loss per se is not the underlying cause of cognitive decline in either aging or neurodegenerative disease, it seems likely that changes in dendritic spines contribute at least partially to impaired cognition. Perhaps the most compelling link between spine abnormalities and cognitive dysfunction occurs in mental retardation, where brain structure and neurochemistry are grossly normal, implying that synaptic function is somehow impaired via more subtle defects (reviewed in Kaufmann and Moser, 2000; Ramakers, 2002). Patients with nonspecific mental retardation, Down’s syndrome, and Patau’s syndrome have fewer spines and/or spines with thin, elongated necks and enlarged heads (Marin-Padilla, 1972; Purpura, 1974; Marin-Padilla, 1975; MarinPadilla, 1976; Kaufmann and Moser, 2000; Swann and Hablitz, 2000; Fiala et al., 2002). Fragile X syndrome patients have abnormal neurites, thin, elongated spines, and smaller synaptic contacts than normal (Rudelli et al., 1985; Bardoni and Mandel, 2002; Morales et al., 2002), and a mouse model of the disease, in which the gene encoding Fragile X mental retardation protein (FMRP) is deleted, similarly exhibits abnormal spine shapes (reviewed in Bear et al., 2004). Moreover, a LIMK-1 knock-out mouse, a model of Williams syndrome, a disease that results from a micro-deletion in chromosome 7 that affects this and other genes, also exhibits spine abnormalities (Meng et al., 2002). Before the intrinsic motility of spines was appreciated (see above), it was thought that in these developmental diseases spines fail to make the transition from filopodia to spines. In light of more recent observations on spine motility and morphing in adult neurons, it is possible that at least some of these diseases are caused by dysregulation of spine motility or shape maintenance, rather than initial spine production.
Researchers still do not understand the relationship between the number of spines and overall mental performance. It is perhaps nai¨ve to assume that cognitive deficits must necessarily correlate with reduced spine number. Accordingly, it is interesting to note that postnatal neurological changes following chronic placental insufficiency were reported to include increased spine density (Dieni and Rees, 2003). Together the above observations demonstrate a strong correlation between abnormal cognition and impairments in the production, elimination, or maintenance of spines. However, they do not prove that the spine abnormalities directly lead to deficits, rather than being secondary to some other event. Indeed, since it is likely that neural activity throughout life regulates both the number and shape of spines, it remains possible that spine abnormalities are the effect, rather than the cause, of cognitive deficits. Finally, much work is required to establish the precise relationship between the shape and physiological properties of dendritic spine synapses.
Molecular mechanisms in spine destabilization At present it is unclear whether the molecular pathways in excitotoxic spine loss are distinct from those regulating physiological spine turnover. Elucidation of the enzymes or other molecules that participate in spine loss may provide therapeutic targets that help to prevent spine loss or enhance spine recovery. A novel pathway for inducing spine loss in mature neurons was recently described (Pak and Sheng, 2003). Neuronal activity was shown to activate a protein kinase called serum-inducible kinase, which in turn phosphorylates the actin regulatory protein SPAR. Phosphorylated SPAR becomes subject to proteolysis through the ubiquitinmediated proteasome pathway. This elegant mechanism represents the first model that links extracellular signals to specific protein degradation in the context of dendritic spine regulation. As described above, the main cytoskeletal elements of spines are actin filaments. F-actin disassembly appears necessary for spine loss, since drugs that stabilize F-actin prevent glutamate-induced
35
spine collapse (Halpain et al., 1998). In our cell culture model we observe that spines can recover from sub-lethal stimuli, and that blockade of glutamate receptors dramatically promotes spine recovery, even when added after spine collapse has occurred (S. Graber and S. Halpain, unpublished). It is likely that in this case the synaptic connections are not broken, and that spines re-emerge at their original sites of contact with the presynaptic terminal. Therefore, under certain circumstances it may be possible to preserve network connectivity during the process of spine loss and recovery. In view of a potential neuroprotective role for spines, promotion of spine recovery may be a meaningful goal in the therapeutic setting. The mechanisms that lead to initial destabilization of F-actin in the context of glutamate-induced spine loss are not yet identified; however candidate mechanisms include specific proteolysis, as described above for SPAR, activation of actin severing enzymes, or uncapping of actin filaments leading to depolymerization. Future studies will no doubt uncover these and novel factors that contribute both to spine development and spine elimination.
Acknowledgments This work is supported by grants NS37311 and MH50861 (S.H.).
References Abe, K., Chisaka, O., Van Roy, F. and Takeichi, M. (2004) Stability of dendritic spines and synaptic contacts is controlled by alphaN-catenin. Nat. Neurosci., 7: 357–363. Ackermann, M. and Matus, A. (2003) Activity-induced targeting of profilin and stabilization of dendritic spine morphology. Nat. Neurosci., 6: 1194–1200. Antar, L.N. and Bassell, G.J. (2003) Sunrise at the synapse: the FMRP mRNP shaping the synaptic interface. Neuron, 37: 555–558. Arundine, M. and Tymianski, M. (2004) Molecular mechanisms of glutamate-dependent neurodegeneration in ischemia and traumatic brain injury. Cell Mol. Life Sci., 61: 657–668. Bardoni, B. and Mandel, J.L. (2002) Advances in understanding of fragile X pathogenesis and FMRP function, and in identification of X linked mental retardation genes. Curr. Opin. Genet. Dev., 12: 284–293.
Bear, M.F., Huber, K.M. and Warren, S.T. (2004) The mGluR theory of fragile X mental retardation. Trends Neurosci., In press. Benson, D.L., Watkins, F.H., Steward, O. and Banker, G.A. (1994) Characterization of GABAergic neurons in hippocampal cell cultures. J. Neurocytol., 23: 279–295. Choi, D.W. (1995) Calcium: still center-stage in hypoxicischemic neuronal death. Trends. Neurosci., 18: 58–60. Comery, T.A., Stamoudis, C.X., Irwin, S.A. and Greenough, W.T. (1996) Increased density of multiple-head dendritic spines on medium-sized spiny neurons of the striatum in rats reared in a complex environment. Neurobiol. Learn. Mem., 66: 93–96. Cooney, J.R., Hurlburt, J.L., Selig, D.K., Harris, K.M. and Fiala, J.C. (2002) Endosomal compartments serve multiple hippocampal dendritic spines from a widespread rather than a local store of recycling membrane. J. Neurosci., 22: 2215–2224. Dieni, S. and Rees, S. (2003) Dendritic morphology is altered in hippocampal neurons following prenatal compromise. J. Neurobiol., 55: 41–52. Dunaevsky, A., Blazeski, R., Yuste, R. and Mason, C. (2001) Spine motility with synaptic contact. Nat. Neurosci., 4: 685–686. Dunaevsky, A., Tashiro, A., Majewska, A., Mason, C. and Yuste, R. (1999) Developmental regulation of spine motility in the mammalian central nervous system. Proc. Natl. Acad. Sci. U.S.A., 96: 13438–13443. Ehlers, M.D. (2003) Activity level controls postsynaptic composition and signaling via the ubiquitin-proteasome system. Nat. Neurosci., 6: 231–242. Engert, F. and Bonhoeffer, T. (1999) Dendritic spine changes associated with hippocampal long-term synaptic plasticity. Nature, 399: 66–70. Fiala, J.C., Spacek, J. and Harris, K.M. (2002) Dendritic spine pathology: cause or consequence of neurological disorders? Brain Res. Brain Res. Rev., 39: 29–54. Fischer, M., Kaech, S., Knutti, D. and Matus, A. (1998) Rapid actin-based plasticity in dendritic spines. Neuron, 20: 847–854. Glantz, L.A. and Lewis, D.A. (2000) Decreased dendritic spine density on prefrontal cortical pyramidal neurons in schizophrenia. Arch. Gen. Psychiatry, 57: 65–73. Glantz, L.A. and Lewis, D.A. (2001) Dendritic spine density in schizophrenia and depression. Arch. Gen. Psychiatry, 58: 203. Goda, Y. and Davis, G.W. (2003) Mechanisms of synapse assembly and disassembly. Neuron, 40: 243–264. Grutzendler, J., Kasthuri, N. and Gan, W.B. (2002) Long-term dendritic spine stability in the adult cortex. Nature, 420: 812–816. Guidetti, P., Charles, V., Chen, E.Y., Reddy, P.H., Kordower, J.H., Whetsell, W.O., Jr., Schwarcz, R. and Tagle, D.A. (2001) Early degenerative changes in transgenic
36 mice expressing mutant huntingtin involve dendritic abnormalities but no impairment of mitochondrial energy production. Exp. Neurol., 169: 340–350. Guthrie, P.B., Segal, M. and Kater, S.B. (1991) Independent regulation of calcium revealed by imaging dendritic spines. Nature, 354: 76–80. Hadj-Sahraoui, N., Frederic, F., Zanjani, H., DelhayeBouchaud, N., Herrup, K. and Mariani, J. (2001) Progressive atrophy of cerebellar Purkinje cell dendrites during aging of the heterozygous staggerer mouse (Rora(+/sg)). Brain Res. Dev. Brain Res., 126: 201–209. Halpain, S. (2000) Actin and the agile spine: how and why do dendritic spines dance? Trends. Neurosci., 23: 141–146. Halpain, S., Hipolito, A. and Saffer, L. (1998) Regulation of F-actin stability in dendritic spines by glutamate receptors and calcineurin. J. Neurosci., 18: 23. Hamilton, B.A., Frankel, W.N., Kerrebrock, A.W., Hawkins, T.L., FitzHugh, W., Kusumi, K., Russell, L.B., Mueller, K.L., van, B.V., Birren, B.W., Kruglyak, L. and Lander, E.S. (1996) Disruption of the nuclear hormone receptor RORalpha in staggerer mice. Nature, 379: 736–739. Harris, K.M. and Kater, S.B. (1994) Dendritic spines: cellular specializations imparting both stability and flexibility to synaptic function. Annu. Rev. Neurosci., 17: 341–371. Hayashi, K. and Shirao, T. (1999) Change in the shape of dendritic spines caused by overexpression of drebrin in cultured cortical neurons. J. Neurosci., 19: 3918–3925. Hering, H. and Sheng, M. (2001) Dendritic spines: structure, dynamics and regulation. Nat. Rev. Neurosci., 2: 880–888. Hering, H. and Sheng, M. (2003) Activity-dependent redistribution and essential role of cortactin in dendritic spine morphogenesis. J. Neurosci., 23: 11759–11769. Hoogenraad, C.C., Akhmanova, A., Galjart, N. and De Zeeuw, C.I. (2004) LIMK1 and CLIP-115: linking cytoskeletal defects to Williams syndrome. BioEssays, 26: 141–150. Isokawa, M. (2000) Remodeling dendritic spines of dentate granule cells in temporal lobe epilepsy patients and the rat pilocarpine model. Epilepsia, 41(Suppl 6): S14–S17. Jiang, C. and Schuman, E.M. (2002) Regulation and function of local protein synthesis in neuronal dendrites. Trends Biochem. Sci., 27: 506–513. Jiang, M., Lee, C.L., Smith, K.L. and Swann, J.W. (1998) Spine loss and other persistent alterations of hippocampal pyramidal cell dendrites in a model of early-onset epilepsy. J Neurosci., 18: 8356–8368. Kasai, H., Matsuzaki, M., Noguchi, J., Yasumatsu, N. and Nakahara, H. (2003) Structure-stability-function relationships of dendritic spines. Trends Neurosci., 26: 360–368. Kaufmann, W.E. and Moser, H.W. (2000) Dendritic anomalies in disorders associated with mental retardation. Cereb. Cortex, 10: 981–991. Kennedy, M.B. (2000) Signal-processing machines at the postsynaptic density. Science, 290: 750–754.
Malenka, R.C. and Nicoll, R.A. (1999) Long-term potentiation — a decade of progress? Science, 285: 1870–1874. Maletic-Savatic, M., Malinow, R. and Svoboda, K. (1999) Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity. Science, 283: 1923–1927. Marin-Padilla, M. (1972) Structural abnormalities of the cerebral cortex in human chromosomal aberrations: a Golgi study. Brain Res., 44: 625–629. Marin-Padilla, M. (1975) Abnormal neuronal differentiation (functional maturation) in mental retardation. Birth Defects Orig. Artic. Ser., 11: 133–153. Marin-Padilla, M. (1976) Pyramidal cell abnormalities in the motor cortex of a child with Down’s syndrome. A Golgi study. J. Comp Neurol., 167: 63–81. Matus, A., Ackermann, M., Pehling, G., Byers, H.R. and Fujiwara, K. (1982) High actin concentrations in brain dendritic spines and postsynaptic densities. Proc. Natl. Acad. Sci. USA, 79: 7590–7594. McAllister, A.K., Katz, L.C. and Lo, D.C. (1999) Neurotrophins and synaptic plasticity. Annu. Rev. Neurosci., 22: 295–318. Meng, Y., Zhang, Y., Tregoubov, V., Janus, C., Cruz, L., Jackson, M., Lu, W.Y., MacDonald, J.F., Wang, J.Y., Falls, D.L. and Jia, Z. (2002) Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice. Neuron, 35: 121–133. Morales, J., Hiesinger, P.R., Schroeder, A.J., Kume, K., Verstreken, P., Jackson, F.R., Nelson, D.L. and Hassan, B.A. (2002) Drosophila fragile X protein, DFXR, regulates neuronal morphology and function in the brain. Neuron, 34: 961–972. Morrison, J.H. and Hof, P.R. (1997) Life and death of neurons in the aging brain. Science, 278: 412–419. Muller, W. and Connor, J.A. (1991) Dendritic spines as individual neuronal compartments for synaptic Ca2+ responses. Nature, 354: 73–76. Murai, K.K., Nguyen, L.N., Irie, F., Yamaguchi, Y. and Pasquale, E.B. (2003) Control of hippocampal dendritic spine morphology through ephrin- A3/EphA4 signaling. Nat. Neurosci., 6: 153–160. Murase, S., Mosser, E. and Schuman, E.M. (2002) Depolarization drives beta-Catenin into neuronal spines promoting changes in synaptic structure and function. Neuron, 35: 91–105. Nakayama, A.Y., Harms, M.B. and Luo, L. (2000) Small GTPases Rac and Rho in the maintenance of dendritic spines and branches in hippocampal pyramidal neurons. J. Neurosci., 20: 5329–5338. Nimchinsky, E.A., Sabatini, B.L. and Svoboda, K. (2002) Structure and function of dendritic spines. Annu. Rev. Physiol., 64: 313–353. Norrholm, S.D. and Ouimet, C.C. (2001) Altered dendritic spine density in animal models of depression and in response to antidepressant treatment. Synapse, 42: 151–163.
37 Pak, D.T. and Sheng, M. (2003) Targeted protein degradation and synapse remodeling by an inducible protein kinase. Science, 302: 1368–1373. Pak, D.T., Yang, S., Rudolph-Correia, S., Kim, E. and Sheng, M. (2001) Regulation of dendritic spine morphology by SPAR, a PSD-95-associated RapGAP. Neuron, 31: 289–303. Penzes, P., Beeser, A., Chernoff, J., Schiller, M.R., Eipper, B.A., Mains, R.E. and Huganir, R.L. (2003) Rapid induction of dendritic spine morphogenesis by trans-synaptic ephrinBEphB receptor activation of the Rho-GEF kalirin. Neuron, 37: 263–274. Peters, A. and Kaiserman-Abramof, I.R. (1970) The small pyramidal neuron of the rat cerebral cortex. The perikaryon, dendrites and spines. Am. J. Anat., 127: 321–355. Pongracz, F. (1985) The function of dendritic spines: a theoretical study. Neuroscience, 15: 933–946. Popov, V.I., Bocharova, L.S. and Bragin, A.G. (1992) Repeated changes of dendritic morphology in the hippocampus of ground squirrels in the course of hibernation. Neuroscience, 48: 45–51. Purpura, D.P. (1974) Dendritic spine ‘‘dysgenesis’’ and mental retardation. Science, 186: 1126–1128. Ramakers, G.J. (2002) Rho proteins, mental retardation and the cellular basis of cognition. Trends Neurosci., 25: 191–199. Rose, C.R. and Konnerth, A. (2001) NMDA receptor-mediated Na+ signals in spines and dendrites. J. Neurosci., 21: 4207–4214. Rudelli, R.D., Brown, W.T., Wisniewski, K., Jenkins, E.C., Laure-Kamionowska, M., Connell, F. and Wisniewski, H.M. (1985) Adult fragile X syndrome. Clinico-neuropathologic findings. Acta Neuropathol. (Berl.), 67: 289–295. Sala, C., Piech, V., Wilson, N.R., Passafaro, M., Liu, G. and Sheng, M. (2001) Regulation of dendritic spine morphology and synaptic function by Shank and Homer. Neuron, 31: 115–130. Sarmiere, P.D. and Bamburg, J.R. (2002) Head, neck, and spines: a role for LIMK-1 in the hippocampus. Neuron, 35: 3–5. Segal, M. (1995) Dendritic spines for neuroprotection: a hypothesis. Trends Neurosci., 18: 468–471. Selkoe, D.J. (2002) Alzheimer’s disease is a synaptic failure. Science, 298: 789–791. Smart, F. and Halpain, S. (2000) Regulation of dendritic spine stability. Hippocampus, 10: 542–554.
Sorra, K.E. and Harris, K.M. (2000) Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines. Hippocampus, 10: 501–511. Steward, O. and Schuman, E.M. (2003) Compartmentalized synthesis and degradation of proteins in neurons. Neuron, 40: 347–359. Svoboda, K., Tank, D.W. and Denk, W. (1996) Direct measurement of coupling between dendritic spines and shafts. Science, 272: 716–719. Swann, J.W., Al Noori, S., Jiang, M. and Lee, C.L. (2000) Spine loss and other dendritic abnormalities in epilepsy. Hippocampus, 10: 617–625. Swann, J.W. and Hablitz, J.J. (2000) Cellular abnormalities and synaptic plasticity in seizure disorders of the immature nervous system. Ment. Retard. Dev. Disabil. Res. Rev., 6: 258–267. Trachtenberg, J.T., Chen, B.E., Knott, G.W., Feng, G., Sanes, J.R., Welker, E. and Svoboda, K. (2002) Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex. Nature, 420: 788–794. Villa, A., Sharp, A.H., Racchetti, G., Podini, P., Bole, D.G., Dunn, W.A., Pozzan, T., Snyder, S.H. and Meldolesi, J. (1992) The endoplasmic reticulum of Purkinje neuron body and dendrites: molecular identity and specializations for Ca2+ transport. Neuroscience, 49: 467–477. Wong, W.T. and Wong, R.O. (2001) Changing specificity of neurotransmitter regulation of rapid dendritic remodeling during synaptogenesis. Nat. Neurosci., 4: 351–352. Woolley, C.S. (1999) Effects of estrogen in the CNS. Curr. Opin. Neurobiol., 9: 349–354. Woolley, C.S., Gould, E., Frankfurt, M. and McEwen, B.S. (1990) Naturally occurring fluctuation in dendritic spine density on adult hippocampal pyramidal neurons. J. Neurosci., 10: 4035–4039. Zhang, W. and Benson, D.L. (2000) Development and molecular organization of dendritic spines and their synapses. Hippocampus, 10: 512–526. Zhang, W. and Benson, D.L. (2001) Stages of synapse development defined by dependence on f-actin. J. Neurosci., 21: 5169–5181. Ziv, N.E. and Smith, S.J. (1996) Evidence for a role of dendritic filopodia in synaptogenesis and spine formation. Neuron, 17: 91–102.
CHAPTER 3
Dynamics and pathology of dendritic spines Shelley Halpain*, Kathy Spencer and Simone Graber The Scripps Research Institute, Department of Cell Biology and Institute for Childhood and Neglected Diseases, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
Abstract: Dendritic spines are key players in information processing in the brain. Changes in spine shape and wholesale spine turnover provide mechanisms for modifying existing synaptic connections and altering neuronal connectivity. Although neuronal cell death in acute and chronic neurodegenerative diseases is clearly an important factor in decline of cognitive or motor function, loss of dendritic spines, in the absence of cell death, may also contribute to impaired brain function in these diseases, as well as in psychiatric disorders and aging. Because spines can function in neuroprotection in vitro, advances toward a molecular understanding of spine maintenance might one day aid in the design of therapies to minimize neurological damage following excitotoxic injury. In addition, progress in defining the biochemical basis of spine development and stabilization may yield insights into mental retardation and psychiatric disorders.
Introduction
malformed or lost in many disease states, including epilepsy, stroke, schizophrenia, mental retardation, dementia, and chronic substance abuse. Here we review the structure, function, and development of spines, and highlight molecular mechanisms for their regulation. Such information may one day form the basis of new therapies for treating neurological and psychiatric diseases.
The human brain contains more than 1013 dendritic spines — small protrusions that form the postsynaptic part of glutamate-releasing CNS synapses (Nimchinsky et al., 2002). Many types of both excitatory and inhibitory neurons bear these micronsized membrane specializations. For example, spiny neuron populations include the glutamatergic pyramidal neurons of neocortex and hippocampus as well as GABAergic cerebellar Purkinje neurons and medium-sized projection neurons of the striatum. Spines contain specialized sets of molecules that not only determine the spine shape but, most importantly, enable the postsynaptic neuron to respond biochemically to glutamate or other transmembrane signals (Smart and Halpain, 2000; Hering and Sheng, 2001; Zhang and Benson, 2001). Dendritic spines therefore play critical roles in cognitive and motor function and in memory formation. They are
Spines are dynamic structures The small size of spines approaches the limits of optical resolution in light microscopy. Therefore much of the early work that characterized spines in detail was conducted using electron microscopy. Most spines in the mature brain have a club-like morphology. They are connected to their parent dendrite by thin stalks 0.04–1 mm long, and have variably shaped bulbous tips 0.5–2 mm in diameter (Harris and Kater, 1994). On mature spiny neurons, spine density ranges from 1–10 spines per micron length of dendrite. The vast majority of CNS excitatory synapses contact
*Corresponding author. E-mail: [email protected] DOI: 10.1016/S0079-6123(04)47003-4
29
30
spines, although such glutamate synapses can also form directly on the dendrite shaft (so-called ‘‘shaft synapses’’). In contrast, nearly all inhibitory synapses are shaft synapses. Based on electron microscopic images of fixed brain preparations, investigators defined distinct morphological categories of spines (e.g., ‘‘mushroom shaped,’’ ‘‘thin,’’ or ‘‘stubby’’ (Peters and KaisermanAbramof, 1970; Sorra and Harris, 2000). Beginning in the late 1990s greatly improved methods for fluorescence time-lapse imaging were applied to living neurons maintained in dissociated or organotypic culture. Such studies revealed that spine shapes are highly dynamic. Although spines themselves persist over many hours and maintain a fairly consistent neck length, the heads of many spines continually shift their size and shape on a time scale of seconds to minutes (Fischer et al., 1998; Dunaevsky et al., 2001). A common term applied to this motility is ‘‘morphing.’’ At present the exact function of this intrinsic morphing remains mysterious. It is possible that synapses have evolved mechanisms to rapidly change the biochemical compartmentalization of the spine and thereby alter its signal processing parameters. It is also possible that the motility itself has no specific function but instead reflects one or more underlying cellular mechanisms (for example, regulation of synapse adhesion or the trafficking of membrane vesicles containing glutamate receptors or other signaling molecules (Halpain, 2000). More recent studies using multiphoton microscopy have extended dendritic spine observations into living animals. Remarkably, such state-of-the-art imaging experiments in mice have revealed that spines can turn over with half-lives of a day or less, even in adult animals (Grutzendler et al., 2002). In somatosensory cortex approximately 50% of spines were observed to turn over every few days, even though the overall numbers of spines remained constant (Trachtenberg et al., 2002). However, in other parts of the adult cortex, spines were stable over several months, with only a small fraction turning over within this time frame (Grutzendler et al., 2002). As might be expected, both spine turnover and the rapid spine ‘‘morphing’’ described above are greater in developing animals than in adults. Our ability to monitor spine turnover in vivo is currently at an early stage. Nonetheless, it seems
likely that the wholesale appearance and disappearance of dendritic spines is a normal part of brain function, not only for refinement of synaptic circuits during development, but also for synaptic remodeling in the adult. It is reasonable to postulate that the relative permanence of dendritic spines varies greatly depending on brain region, age, disease state, and level of neuronal activity. Several studies have demonstrated that spine density changes under specific physiological circumstances. For example, significant cyclic changes in spine density are observed over the four-day estrus cycle in female rats (Woolley et al., 1990; Woolley, 1999), during hibernation and reawakening in ground squirrels (Popov et al., 1992), and in response to enriched environmental experience in rats (Comery et al., 1996). In addition, recent studies show that spines can appear very rapidly (within minutes) in response to physiological stimuli (Engert and Bonhoeffer, 1999; Maletic-Savatic et al., 1999) and collapse very rapidly (within minutes) in response to pathological stimuli (Halpain et al., 1998). These dynamic changes in spine shape and existence are presumably mediated by various extracellular signals, including glutamate and other neurotransmitters (Wong and Wong, 2001), neurotrophins such as brain-derived neurotrophic factor (McAllister et al., 1999), and cell-cell interactions mediated by the Eph-ephrin system (Murai et al., 2003), or cadherin-catenin complexes (Murase et al., 2002; Abe et al., 2004). As discussed below, responses to such transmembrane signals, in turn, are likely to be mediated via the actin cytoskeleton.
Spine development Spine formation occurs as a late step in neuronal morphogenesis, peaking, for example, during the third postnatal week in rodent hippocampus. This timing and order of events is preserved in cultured neurons, making neuronal cultures a suitable model system for studies of spine regulation. Synaptogenesis begins as afferent axons reach their postsynaptic targets. At this initial stage, dendrites of nearly all neurons — both spiny and nonspiny — bear numerous thin protrusions termed ‘‘filopodia’’ (Ziv and Smith, 1996; Wong and Wong, 2001). Dendritic
31
filopodia are long, narrow protrusions that lack a bulbous head and usually contain lower amounts of F-actin than spines. They are more transient in nature than dendritic spines, extending and retracting from the dendrite shaft with half-lives of about 10 min, as compared to half-lives of hours, days, or months for most mature spines (Ziv and Smith, 1996; Sorra and Harris, 2000). Gradually, over the course of several days, the numerous dendritic filopodia are replaced by spines. Spiny synapses have been proposed to arise by two mechanisms: (a) a synapse forms directly on the dendrite shaft, and a spine gradually grows out from the shaft; (b) a synapse forms on a dendritic filopodium and the filopodium gradually ‘‘converts’’ into a mature spine. Experimental evidence supports each of these mechanisms, and it seems likely that both can and do occur even within a single neuron. Although filopodia seem to be involved in the formation of at least some dendritic spines, it is also clear that not all filopodia are destined to become spines (Ziv and Smith, 1996). Indeed, filopodia are also observed on nonspiny neurons early in their synaptogenesis phase (Wong and Wong, 2001). It is likely that filopodia, with their high motility and transient nature, play key roles in establishing many kinds of synapses, because they maximize the chance encounter of pre- and postsynaptic elements during the search for synaptic partners (see Wong and Wong, 2001 for discussion). The emergence of a spine as a semi-permanent protrusion is probably not dictated by the presence or absence of a filopodium per se, but rather by the presence of molecular complexes that cluster and accumulate at the plasma membrane in response to signals from the nerve terminal (Goda and Davis, 2003). Through a process of mutual cross-talk, the pre- and postsynaptic sides coordinately orchestrate the maturation of the entire synapse, including, presumably, the spine structure itself.
Molecular composition of spines As in all cells, cytoskeletal proteins take the lead in determining the shape and stability of spines. The spine cytoskeleton is based mainly on actin filaments (F-actin), since intermediate filaments and microtubules are absent from nearly all spines
(although intermediate filament proteins and tubulin are detected in spines (Benson et al., 1994). Spines are unusually rich in F-actin compared to other parts of the neuron (Matus et al., 1982). Pharmacological manipulations that inhibit F-actin dynamics alter spine shape and motility (Fischer et al., 1998; Dunaevsky et al., 1999), and experimentally-induced spine collapse can be prevented by stabilizing actin filaments (Halpain et al., 1998). A growing roster of F-actin binding and regulatory proteins have been found in spines (Smart and Halpain, 2000; Zhang and Benson, 2000), many of which have been shown to regulate spine shape. Examples include Rho family GTPases (Nakayama et al., 2000) and proteins that regulate them, such as SPAR (spine-associated GTPase activating protein for Rap) (Pak et al., 2001) and kalirin (a guanine nucleotide exchange factor for Rac; (Penzes et al., 2003); the scaffolding protein shank (Sala et al., 2001); and the actin binding proteins drebrin (Hayashi and Shirao, 1999); cortactin (Hering and Sheng, 2003), and profilin (Ackermann and Matus, 2003). Changing the expression levels of these proteins has been shown to alter spine numbers, shape, or motility. It is likely that many such proteins will eventually be linked to diseases that affect synaptic function. Indeed, it was recently shown that the gene encoding LIM kinase-1 (LIMK-1), an actin regulatory molecule, is one of the approximately 20 genes deleted in patients with Williams syndrome, a developmental disorder characterized by cardiovascular and cognitive deficits (Hoogenraad et al., 2004). LIMK-1 responds to the small GTPase Rac by phosphorylating and thereby inactivating its target, the actin-depolymerizing factor ADF/cofilin (Sarmiere and Bamburg, 2002). LIMK-1 knock-out mice and Williams syndrome patients exhibit abnormal dendritic spines (Meng et al., 2002). Other molecules that act downstream of Rho-family GTPases have also been linked to hereditary forms of mental retardation and spine abnormalities (Ramakers, 2002). The heads of dendritic spines contain a specialized organelle called the postsynaptic density (PSD), which sits directly across the cleft from the presynaptic active zone, where synaptic vesicles are clustered. The PSD is a supramolecular assembly of
32
glutamate receptors, scaffolding molecules, and enzymes (Hering and Sheng, 2003). A significant component of the PSD is the calcium/calmodulindependent protein kinase type II (CaMKII), which may play both structural and enzymatic roles within the PSD (Kennedy, 2000). Scaffolding molecules, such as PSD-95, spinophilin, and Homer are proteins that have no enzymatic function of their own but which serve to concentrate receptors and their downstream effector molecules into the correct assembly for efficient signal transduction (Kasai et al., 2003). This protein meshwork also contributes to regulation and maintenance of spine structure. Despite their small size, dendritic spines also contain molecular machinery for many other functions, including local control of membrane trafficking, protein synthesis, post-translational processing, and protein degradation. Most dendritic spines in the cerebellum and about 30% of spines in the hippocampus (those with the largest head volumes) contain endoplasmic reticulum (ER) in the spine neck, which exists either as loose tubular structures or as an organelle called the spine apparatus that consists of stacks of membrane cisternae (Cooney et al., 2002). ER is thought to contribute not only to trafficking of membrane proteins but also to regulation of calcium dynamics within the spines. Among other functions, ER releases calcium in response to calcium influx into the spines and thereby amplifies the calcium signal (Villa et al., 1992; Sorra and Harris, 2000). In addition, many dendrites have polyribosomes just beneath the spine neck, and there is growing evidence that protein synthesis and protein degradation can occur locally within spines (Jiang and Schuman, 2002; Ehlers, 2003; Steward and Schuman, 2003). Disruption of local protein synthesis has emerged as a potential mechanism for cognitive deficits in Fragile X syndrome, because Fragile X mRNA is targeted to dendrites, and the Fragile X mental retardation protein itself regulates protein synthesis (Antar and Bassell, 2003).
Function of spines Spines clearly contain specialized collections of receptors and signaling complexes, and they are uniquely associated with excitatory CNS synapses.
However, the function of spines is not completely understood. Indeed, many types of synapses, both excitatory and inhibitory, do not occur on spiny protrusions, so clearly a spine is not a pre-requisite for a functional synapse. It is likely that the existence of the unique spine structure confers some special property to the synapse that would be lost if the spine structure collapsed. Early work addressed this issue using computer modeling and suggested that the electrical properties of synapses (for example, the passive spread of synaptic current) are different for a synapse occurring on a spine as compared to one occurring on a dendrite shaft (Pongracz, 1985). Such models predicted that changes in spine shape would modulate current spread, leading some investigators to speculate that changes in spine shape might underlie learning and memory and other forms of synaptic plasticity, such as LTP. Later studies, however, questioned whether such changes would attain sufficient magnitude to significantly impact synaptic strength (reviewed in Nimchinsky et al., 2002). A current, widely accepted idea is that spines act as individual biochemical compartments. By virtue of its narrow neck, a spine retains second messengers and other small, diffusible molecules that are generated at the site of synaptic contact. Such biochemical compartmentalization has been directly demonstrated for small diffusible dyes and also for the important ions sodium and calcium (Guthrie et al., 1991; Muller and Connor, 1991; Svoboda et al., 1996; Rose and Konnerth, 2001). For calcium in particular, local control of concentration is crucial in orchestrating physiological responses to stimuli. Biochemical compartmentalization by spines provides two advantages. First, it allows small molecules to attain high concentrations in the postsynapse prior to their diffusing away. Second, it biochemically and therefore functionally isolates individual synapses from the parent dendrite (reviewed in Nimchinsky et al., 2002). This latter concept means that molecular signals generated at one spine are restricted to that synapse, and fail to influence a neighboring synapse on the same dendrite. This property of dendritic spines could contribute to the phenomenon of ‘‘input specificity,’’ whereby a given set of nerve terminals induces changes only at those synapses that are specific to their postsynaptic contacts, and not at other synapses on the same
33
neuron that are driven by different axons (Malenka and Nicoll, 1999). The function of the spine neck in limiting calcium diffusion from spine head to adjacent dendrite has also been postulated to play a protective role in excitotoxic injury during trauma, seizures, or ischemic conditions (Segal, 1995). In such diseases the excitatory neurotransmitter glutamate is released in excess, leading to overly strong activation of glutamate receptors and injurious calcium overload in the postsynaptic neurons. Such calcium overload is a major factor in excitotoxic neuronal cell death (Choi, 1995; Arundine and Tymianski, 2004). The diffusion barrier of the spine neck could limit the rise of calcium concentration in dendrite shaft and cell body, and thereby possibly protect from cell death. Indeed, recent studies in our laboratory using a neuronal cell culture model support an excitoprotective role for spines. We find that neurons become more sensitive to an excitotoxic stimulus when spine collapse is first induced. Furthermore, cell death is attenuated when dendritic spines are stabilized by an actin stabilizing agent prior to the excitotoxic stimulus (S. Graber and S. Halpain, unpublished).
Spine abnormalities in neurological disease A wide variety of human neurological and psychiatric diseases exhibit loss of spines or abnormalities in spine shape, paralleled by impaired cognitive function. Spine loss or distortion is associated with stroke, epilepsy, trauma, dementia, brain tumors, schizophrenia, major depression, substance abuse, and normal aging (Swann et al., 2000; Glantz and Lewis, 2001; Nimchinsky et al., 2002; Fiala et al., 2002). Brain regions showing spine loss typically correlate with brain systems affected in the disease state. For example, schizophrenic patients exhibit decreased spine density selectively in prefrontal cortical pyramidal neurons (Glantz and Lewis, 2000; Glantz and Lewis, 2001). Spines are also significantly reduced in some animal models of chronic depression, an effect reversed by antidepressants (Norrholm and Ouimet, 2001). A direct link between spine loss and specific pathological events is best established for acute disorders such as seizures and stroke, which involve
excitotoxic injury to the brain. Human epileptic patients display spine loss and other dendritic abnormalities (Swann et al., 2000). This observation is supported by work in animal models. Tetrodotoxin induction of epileptic seizures in rats reduces spine density on hippocampal pyramidal cells to 20–35% of normal controls (Jiang et al., 1998). Pilocarpineor bicuculline-induced seizures also cause spine loss (Isokawa, 2000). Presumably, the excessive activation of glutamate receptors during seizures or postischemia reperfusion swamps the calcium-buffering capacity of the spines, leading to spine retraction or collapse. Excitotoxic injury to spines can be studied in cultured neurons by addition of glutamate analogs such as NMDA to the culture medium (Choi, 1995; Arundine and Tymianski, 2004). In such models spine loss appears very rapidly, within 5–10 min of the onset of the stimulus (Halpain et al., 1998). Spine loss can also be induced in culture by addition of a calcium ionophore, suggesting a crucial role for glutamate-induced calcium influx in triggering spine collapse. However, such excessive stimulation of glutamate receptors and calcium also induces excitotoxic cell death (Choi, 1995; Arundine and Tymianski, 2004). Thus, spine loss could be merely an early morphological step in the progression toward cell death. However, our laboratory recently showed that intense but sub-lethal activation of glutamate receptors in cultured neurons can induce widespread spine loss in the absence of cell death (S. Graber and S. Halpain, unpublished). This dissociation between spine loss and cell death is consistent with the idea that spine loss itself might contribute to abnormal brain function after injury, even when the neurons themselves are preserved. Chronic neurodegenerative diseases such as Alzheimer’s and Parkinson’s often exhibit dendritic changes, including spine loss, as an early step in disease progression. Emerging evidence suggests that initial changes in motor and cognitive functions in such diseases are due to subtle alterations in synaptic function, rather than reflecting neuronal loss per se. For example, it is now widely believed that the initial stages of Alzheimer’s disease actually reflect disrupted synaptic function that greatly precedes the later decline in neuronal numbers (Selkoe, 2002). Dendrites and spines also degenerate prior to cell loss in some murine cerebellar ataxias (Hadj-Sahraoui
34
et al., 2001; Hamilton et al., 1996). Early spine loss occurs without concomitant neuronal death in a mouse model of Huntington’s disease (Guidetti et al., 2001), although the human disease clearly shows cell loss at later stages. It was previously thought that, as in neurodegenerative diseases, normal aging also involved a decline in neuronal cell numbers; however, more recent studies suggest that this is not the case, and instead that synapse numbers are reduced (Morrison and Hof, 1997). Even if spine loss per se is not the underlying cause of cognitive decline in either aging or neurodegenerative disease, it seems likely that changes in dendritic spines contribute at least partially to impaired cognition. Perhaps the most compelling link between spine abnormalities and cognitive dysfunction occurs in mental retardation, where brain structure and neurochemistry are grossly normal, implying that synaptic function is somehow impaired via more subtle defects (reviewed in Kaufmann and Moser, 2000; Ramakers, 2002). Patients with nonspecific mental retardation, Down’s syndrome, and Patau’s syndrome have fewer spines and/or spines with thin, elongated necks and enlarged heads (Marin-Padilla, 1972; Purpura, 1974; Marin-Padilla, 1975; MarinPadilla, 1976; Kaufmann and Moser, 2000; Swann and Hablitz, 2000; Fiala et al., 2002). Fragile X syndrome patients have abnormal neurites, thin, elongated spines, and smaller synaptic contacts than normal (Rudelli et al., 1985; Bardoni and Mandel, 2002; Morales et al., 2002), and a mouse model of the disease, in which the gene encoding Fragile X mental retardation protein (FMRP) is deleted, similarly exhibits abnormal spine shapes (reviewed in Bear et al., 2004). Moreover, a LIMK-1 knock-out mouse, a model of Williams syndrome, a disease that results from a micro-deletion in chromosome 7 that affects this and other genes, also exhibits spine abnormalities (Meng et al., 2002). Before the intrinsic motility of spines was appreciated (see above), it was thought that in these developmental diseases spines fail to make the transition from filopodia to spines. In light of more recent observations on spine motility and morphing in adult neurons, it is possible that at least some of these diseases are caused by dysregulation of spine motility or shape maintenance, rather than initial spine production.
Researchers still do not understand the relationship between the number of spines and overall mental performance. It is perhaps nai¨ve to assume that cognitive deficits must necessarily correlate with reduced spine number. Accordingly, it is interesting to note that postnatal neurological changes following chronic placental insufficiency were reported to include increased spine density (Dieni and Rees, 2003). Together the above observations demonstrate a strong correlation between abnormal cognition and impairments in the production, elimination, or maintenance of spines. However, they do not prove that the spine abnormalities directly lead to deficits, rather than being secondary to some other event. Indeed, since it is likely that neural activity throughout life regulates both the number and shape of spines, it remains possible that spine abnormalities are the effect, rather than the cause, of cognitive deficits. Finally, much work is required to establish the precise relationship between the shape and physiological properties of dendritic spine synapses.
Molecular mechanisms in spine destabilization At present it is unclear whether the molecular pathways in excitotoxic spine loss are distinct from those regulating physiological spine turnover. Elucidation of the enzymes or other molecules that participate in spine loss may provide therapeutic targets that help to prevent spine loss or enhance spine recovery. A novel pathway for inducing spine loss in mature neurons was recently described (Pak and Sheng, 2003). Neuronal activity was shown to activate a protein kinase called serum-inducible kinase, which in turn phosphorylates the actin regulatory protein SPAR. Phosphorylated SPAR becomes subject to proteolysis through the ubiquitinmediated proteasome pathway. This elegant mechanism represents the first model that links extracellular signals to specific protein degradation in the context of dendritic spine regulation. As described above, the main cytoskeletal elements of spines are actin filaments. F-actin disassembly appears necessary for spine loss, since drugs that stabilize F-actin prevent glutamate-induced
35
spine collapse (Halpain et al., 1998). In our cell culture model we observe that spines can recover from sub-lethal stimuli, and that blockade of glutamate receptors dramatically promotes spine recovery, even when added after spine collapse has occurred (S. Graber and S. Halpain, unpublished). It is likely that in this case the synaptic connections are not broken, and that spines re-emerge at their original sites of contact with the presynaptic terminal. Therefore, under certain circumstances it may be possible to preserve network connectivity during the process of spine loss and recovery. In view of a potential neuroprotective role for spines, promotion of spine recovery may be a meaningful goal in the therapeutic setting. The mechanisms that lead to initial destabilization of F-actin in the context of glutamate-induced spine loss are not yet identified; however candidate mechanisms include specific proteolysis, as described above for SPAR, activation of actin severing enzymes, or uncapping of actin filaments leading to depolymerization. Future studies will no doubt uncover these and novel factors that contribute both to spine development and spine elimination.
Acknowledgments This work is supported by grants NS37311 and MH50861 (S.H.).
References Abe, K., Chisaka, O., Van Roy, F. and Takeichi, M. (2004) Stability of dendritic spines and synaptic contacts is controlled by alphaN-catenin. Nat. Neurosci., 7: 357–363. Ackermann, M. and Matus, A. (2003) Activity-induced targeting of profilin and stabilization of dendritic spine morphology. Nat. Neurosci., 6: 1194–1200. Antar, L.N. and Bassell, G.J. (2003) Sunrise at the synapse: the FMRP mRNP shaping the synaptic interface. Neuron, 37: 555–558. Arundine, M. and Tymianski, M. (2004) Molecular mechanisms of glutamate-dependent neurodegeneration in ischemia and traumatic brain injury. Cell Mol. Life Sci., 61: 657–668. Bardoni, B. and Mandel, J.L. (2002) Advances in understanding of fragile X pathogenesis and FMRP function, and in identification of X linked mental retardation genes. Curr. Opin. Genet. Dev., 12: 284–293.
Bear, M.F., Huber, K.M. and Warren, S.T. (2004) The mGluR theory of fragile X mental retardation. Trends Neurosci., In press. Benson, D.L., Watkins, F.H., Steward, O. and Banker, G.A. (1994) Characterization of GABAergic neurons in hippocampal cell cultures. J. Neurocytol., 23: 279–295. Choi, D.W. (1995) Calcium: still center-stage in hypoxicischemic neuronal death. Trends. Neurosci., 18: 58–60. Comery, T.A., Stamoudis, C.X., Irwin, S.A. and Greenough, W.T. (1996) Increased density of multiple-head dendritic spines on medium-sized spiny neurons of the striatum in rats reared in a complex environment. Neurobiol. Learn. Mem., 66: 93–96. Cooney, J.R., Hurlburt, J.L., Selig, D.K., Harris, K.M. and Fiala, J.C. (2002) Endosomal compartments serve multiple hippocampal dendritic spines from a widespread rather than a local store of recycling membrane. J. Neurosci., 22: 2215–2224. Dieni, S. and Rees, S. (2003) Dendritic morphology is altered in hippocampal neurons following prenatal compromise. J. Neurobiol., 55: 41–52. Dunaevsky, A., Blazeski, R., Yuste, R. and Mason, C. (2001) Spine motility with synaptic contact. Nat. Neurosci., 4: 685–686. Dunaevsky, A., Tashiro, A., Majewska, A., Mason, C. and Yuste, R. (1999) Developmental regulation of spine motility in the mammalian central nervous system. Proc. Natl. Acad. Sci. U.S.A., 96: 13438–13443. Ehlers, M.D. (2003) Activity level controls postsynaptic composition and signaling via the ubiquitin-proteasome system. Nat. Neurosci., 6: 231–242. Engert, F. and Bonhoeffer, T. (1999) Dendritic spine changes associated with hippocampal long-term synaptic plasticity. Nature, 399: 66–70. Fiala, J.C., Spacek, J. and Harris, K.M. (2002) Dendritic spine pathology: cause or consequence of neurological disorders? Brain Res. Brain Res. Rev., 39: 29–54. Fischer, M., Kaech, S., Knutti, D. and Matus, A. (1998) Rapid actin-based plasticity in dendritic spines. Neuron, 20: 847–854. Glantz, L.A. and Lewis, D.A. (2000) Decreased dendritic spine density on prefrontal cortical pyramidal neurons in schizophrenia. Arch. Gen. Psychiatry, 57: 65–73. Glantz, L.A. and Lewis, D.A. (2001) Dendritic spine density in schizophrenia and depression. Arch. Gen. Psychiatry, 58: 203. Goda, Y. and Davis, G.W. (2003) Mechanisms of synapse assembly and disassembly. Neuron, 40: 243–264. Grutzendler, J., Kasthuri, N. and Gan, W.B. (2002) Long-term dendritic spine stability in the adult cortex. Nature, 420: 812–816. Guidetti, P., Charles, V., Chen, E.Y., Reddy, P.H., Kordower, J.H., Whetsell, W.O., Jr., Schwarcz, R. and Tagle, D.A. (2001) Early degenerative changes in transgenic
36 mice expressing mutant huntingtin involve dendritic abnormalities but no impairment of mitochondrial energy production. Exp. Neurol., 169: 340–350. Guthrie, P.B., Segal, M. and Kater, S.B. (1991) Independent regulation of calcium revealed by imaging dendritic spines. Nature, 354: 76–80. Hadj-Sahraoui, N., Frederic, F., Zanjani, H., DelhayeBouchaud, N., Herrup, K. and Mariani, J. (2001) Progressive atrophy of cerebellar Purkinje cell dendrites during aging of the heterozygous staggerer mouse (Rora(+/sg)). Brain Res. Dev. Brain Res., 126: 201–209. Halpain, S. (2000) Actin and the agile spine: how and why do dendritic spines dance? Trends. Neurosci., 23: 141–146. Halpain, S., Hipolito, A. and Saffer, L. (1998) Regulation of F-actin stability in dendritic spines by glutamate receptors and calcineurin. J. Neurosci., 18: 23. Hamilton, B.A., Frankel, W.N., Kerrebrock, A.W., Hawkins, T.L., FitzHugh, W., Kusumi, K., Russell, L.B., Mueller, K.L., van, B.V., Birren, B.W., Kruglyak, L. and Lander, E.S. (1996) Disruption of the nuclear hormone receptor RORalpha in staggerer mice. Nature, 379: 736–739. Harris, K.M. and Kater, S.B. (1994) Dendritic spines: cellular specializations imparting both stability and flexibility to synaptic function. Annu. Rev. Neurosci., 17: 341–371. Hayashi, K. and Shirao, T. (1999) Change in the shape of dendritic spines caused by overexpression of drebrin in cultured cortical neurons. J. Neurosci., 19: 3918–3925. Hering, H. and Sheng, M. (2001) Dendritic spines: structure, dynamics and regulation. Nat. Rev. Neurosci., 2: 880–888. Hering, H. and Sheng, M. (2003) Activity-dependent redistribution and essential role of cortactin in dendritic spine morphogenesis. J. Neurosci., 23: 11759–11769. Hoogenraad, C.C., Akhmanova, A., Galjart, N. and De Zeeuw, C.I. (2004) LIMK1 and CLIP-115: linking cytoskeletal defects to Williams syndrome. BioEssays, 26: 141–150. Isokawa, M. (2000) Remodeling dendritic spines of dentate granule cells in temporal lobe epilepsy patients and the rat pilocarpine model. Epilepsia, 41(Suppl 6): S14–S17. Jiang, C. and Schuman, E.M. (2002) Regulation and function of local protein synthesis in neuronal dendrites. Trends Biochem. Sci., 27: 506–513. Jiang, M., Lee, C.L., Smith, K.L. and Swann, J.W. (1998) Spine loss and other persistent alterations of hippocampal pyramidal cell dendrites in a model of early-onset epilepsy. J Neurosci., 18: 8356–8368. Kasai, H., Matsuzaki, M., Noguchi, J., Yasumatsu, N. and Nakahara, H. (2003) Structure-stability-function relationships of dendritic spines. Trends Neurosci., 26: 360–368. Kaufmann, W.E. and Moser, H.W. (2000) Dendritic anomalies in disorders associated with mental retardation. Cereb. Cortex, 10: 981–991. Kennedy, M.B. (2000) Signal-processing machines at the postsynaptic density. Science, 290: 750–754.
Malenka, R.C. and Nicoll, R.A. (1999) Long-term potentiation — a decade of progress? Science, 285: 1870–1874. Maletic-Savatic, M., Malinow, R. and Svoboda, K. (1999) Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity. Science, 283: 1923–1927. Marin-Padilla, M. (1972) Structural abnormalities of the cerebral cortex in human chromosomal aberrations: a Golgi study. Brain Res., 44: 625–629. Marin-Padilla, M. (1975) Abnormal neuronal differentiation (functional maturation) in mental retardation. Birth Defects Orig. Artic. Ser., 11: 133–153. Marin-Padilla, M. (1976) Pyramidal cell abnormalities in the motor cortex of a child with Down’s syndrome. A Golgi study. J. Comp Neurol., 167: 63–81. Matus, A., Ackermann, M., Pehling, G., Byers, H.R. and Fujiwara, K. (1982) High actin concentrations in brain dendritic spines and postsynaptic densities. Proc. Natl. Acad. Sci. USA, 79: 7590–7594. McAllister, A.K., Katz, L.C. and Lo, D.C. (1999) Neurotrophins and synaptic plasticity. Annu. Rev. Neurosci., 22: 295–318. Meng, Y., Zhang, Y., Tregoubov, V., Janus, C., Cruz, L., Jackson, M., Lu, W.Y., MacDonald, J.F., Wang, J.Y., Falls, D.L. and Jia, Z. (2002) Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice. Neuron, 35: 121–133. Morales, J., Hiesinger, P.R., Schroeder, A.J., Kume, K., Verstreken, P., Jackson, F.R., Nelson, D.L. and Hassan, B.A. (2002) Drosophila fragile X protein, DFXR, regulates neuronal morphology and function in the brain. Neuron, 34: 961–972. Morrison, J.H. and Hof, P.R. (1997) Life and death of neurons in the aging brain. Science, 278: 412–419. Muller, W. and Connor, J.A. (1991) Dendritic spines as individual neuronal compartments for synaptic Ca2+ responses. Nature, 354: 73–76. Murai, K.K., Nguyen, L.N., Irie, F., Yamaguchi, Y. and Pasquale, E.B. (2003) Control of hippocampal dendritic spine morphology through ephrin- A3/EphA4 signaling. Nat. Neurosci., 6: 153–160. Murase, S., Mosser, E. and Schuman, E.M. (2002) Depolarization drives beta-Catenin into neuronal spines promoting changes in synaptic structure and function. Neuron, 35: 91–105. Nakayama, A.Y., Harms, M.B. and Luo, L. (2000) Small GTPases Rac and Rho in the maintenance of dendritic spines and branches in hippocampal pyramidal neurons. J. Neurosci., 20: 5329–5338. Nimchinsky, E.A., Sabatini, B.L. and Svoboda, K. (2002) Structure and function of dendritic spines. Annu. Rev. Physiol., 64: 313–353. Norrholm, S.D. and Ouimet, C.C. (2001) Altered dendritic spine density in animal models of depression and in response to antidepressant treatment. Synapse, 42: 151–163.
37 Pak, D.T. and Sheng, M. (2003) Targeted protein degradation and synapse remodeling by an inducible protein kinase. Science, 302: 1368–1373. Pak, D.T., Yang, S., Rudolph-Correia, S., Kim, E. and Sheng, M. (2001) Regulation of dendritic spine morphology by SPAR, a PSD-95-associated RapGAP. Neuron, 31: 289–303. Penzes, P., Beeser, A., Chernoff, J., Schiller, M.R., Eipper, B.A., Mains, R.E. and Huganir, R.L. (2003) Rapid induction of dendritic spine morphogenesis by trans-synaptic ephrinBEphB receptor activation of the Rho-GEF kalirin. Neuron, 37: 263–274. Peters, A. and Kaiserman-Abramof, I.R. (1970) The small pyramidal neuron of the rat cerebral cortex. The perikaryon, dendrites and spines. Am. J. Anat., 127: 321–355. Pongracz, F. (1985) The function of dendritic spines: a theoretical study. Neuroscience, 15: 933–946. Popov, V.I., Bocharova, L.S. and Bragin, A.G. (1992) Repeated changes of dendritic morphology in the hippocampus of ground squirrels in the course of hibernation. Neuroscience, 48: 45–51. Purpura, D.P. (1974) Dendritic spine ‘‘dysgenesis’’ and mental retardation. Science, 186: 1126–1128. Ramakers, G.J. (2002) Rho proteins, mental retardation and the cellular basis of cognition. Trends Neurosci., 25: 191–199. Rose, C.R. and Konnerth, A. (2001) NMDA receptor-mediated Na+ signals in spines and dendrites. J. Neurosci., 21: 4207–4214. Rudelli, R.D., Brown, W.T., Wisniewski, K., Jenkins, E.C., Laure-Kamionowska, M., Connell, F. and Wisniewski, H.M. (1985) Adult fragile X syndrome. Clinico-neuropathologic findings. Acta Neuropathol. (Berl.), 67: 289–295. Sala, C., Piech, V., Wilson, N.R., Passafaro, M., Liu, G. and Sheng, M. (2001) Regulation of dendritic spine morphology and synaptic function by Shank and Homer. Neuron, 31: 115–130. Sarmiere, P.D. and Bamburg, J.R. (2002) Head, neck, and spines: a role for LIMK-1 in the hippocampus. Neuron, 35: 3–5. Segal, M. (1995) Dendritic spines for neuroprotection: a hypothesis. Trends Neurosci., 18: 468–471. Selkoe, D.J. (2002) Alzheimer’s disease is a synaptic failure. Science, 298: 789–791. Smart, F. and Halpain, S. (2000) Regulation of dendritic spine stability. Hippocampus, 10: 542–554.
Sorra, K.E. and Harris, K.M. (2000) Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines. Hippocampus, 10: 501–511. Steward, O. and Schuman, E.M. (2003) Compartmentalized synthesis and degradation of proteins in neurons. Neuron, 40: 347–359. Svoboda, K., Tank, D.W. and Denk, W. (1996) Direct measurement of coupling between dendritic spines and shafts. Science, 272: 716–719. Swann, J.W., Al Noori, S., Jiang, M. and Lee, C.L. (2000) Spine loss and other dendritic abnormalities in epilepsy. Hippocampus, 10: 617–625. Swann, J.W. and Hablitz, J.J. (2000) Cellular abnormalities and synaptic plasticity in seizure disorders of the immature nervous system. Ment. Retard. Dev. Disabil. Res. Rev., 6: 258–267. Trachtenberg, J.T., Chen, B.E., Knott, G.W., Feng, G., Sanes, J.R., Welker, E. and Svoboda, K. (2002) Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex. Nature, 420: 788–794. Villa, A., Sharp, A.H., Racchetti, G., Podini, P., Bole, D.G., Dunn, W.A., Pozzan, T., Snyder, S.H. and Meldolesi, J. (1992) The endoplasmic reticulum of Purkinje neuron body and dendrites: molecular identity and specializations for Ca2+ transport. Neuroscience, 49: 467–477. Wong, W.T. and Wong, R.O. (2001) Changing specificity of neurotransmitter regulation of rapid dendritic remodeling during synaptogenesis. Nat. Neurosci., 4: 351–352. Woolley, C.S. (1999) Effects of estrogen in the CNS. Curr. Opin. Neurobiol., 9: 349–354. Woolley, C.S., Gould, E., Frankfurt, M. and McEwen, B.S. (1990) Naturally occurring fluctuation in dendritic spine density on adult hippocampal pyramidal neurons. J. Neurosci., 10: 4035–4039. Zhang, W. and Benson, D.L. (2000) Development and molecular organization of dendritic spines and their synapses. Hippocampus, 10: 512–526. Zhang, W. and Benson, D.L. (2001) Stages of synapse development defined by dependence on f-actin. J. Neurosci., 21: 5169–5181. Ziv, N.E. and Smith, S.J. (1996) Evidence for a role of dendritic filopodia in synaptogenesis and spine formation. Neuron, 17: 91–102.