- Email: [email protected]
Microglia: phagocyte and glia cell
The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
Cells in focus
Microglia: phagocyte and glia cell Frederik Vilhardt∗ Structural Cell Biology Unit, The Panum Institute, Copenhagen University, Building 18.4, Blegdamsvej 3A, 2200 Copenhagen N, Denmark Received 8 December 2003; received in revised form 9 June 2004; accepted 21 June 2004
Abstract Microglia are the resident immune cells of the brain, and are located within the brain parenchyme behind the blood–brain barrier. They originate from mesodermal hemapoietic precursors and are slowly turned over and replenished by proliferation in the adult central nervous system. In the healthy brain resting, ramified microglia function as supportive glia cells, and their activation status is regulated by neurons through soluble mediators and cell–cell contact. However, in response to brain pathology microglia become activated: acquisition of innate immune cell functions render microglia competent to react towards brain injury through tissue repair or induction of immune responses. In certain pathological conditions, however, microglia activation may sustain a chronic inflammation of the brain, leading to neuronal dysfunction and cell death. This might be mediated by the microglial release of extracellular toxic reactive oxygen and nitrogen species. Nevertheless, in the future microglia may potentially be harnessed for therapeutical purposes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Microglia; Cell activation; Inflammation; Superoxide
Cell facts • • • • •
Resident immune cell of the brain. Constitute approximately 20% of the total glia cell population. Originate from mesodermal precursor cells of hemapoietic lineage. Traditionally divided into resting (ramified) and activated (ameboid) microglia. Resident microglia are slowly turned over and replaced by proliferation.
1. Introduction Microglia are the resident immune cells of the central nervous system (CNS) parenchyme and belong ∗ Tel.: +45 35 32 72 99; fax: +45 35 32 72 85. E-mail address: [email protected] (F. Vilhardt).
1357-2725/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocel.2004.06.010
to the mononuclear phagocyte lineage related to other organ specific macrophage populations such as the Kupffer cells of the liver and bone osteoclasts. Microglia occur throughout the brain parenchyme where they assume a dendritic morphology with many highly ramified processes (see Fig. 1). As observed by scanning electron microscopy the microglial cell
18
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
Fig. 1. Human hippocampal microglia stained for the gp91phox subunit of the phagocyte NADPH oxidase.
surface is covered with spines (spiky protrusions), which may be a uniquely identifying trait of microglia, as they are not observed in other macrophage cell types. Ramified microglia are often referred to as resting microglia, since injury-induced microglia activation results in contraction of cellular processes and conversion into an ameboid macrophage-like morphology. Concurrently, de novo acquisition of phagocyte effector functions including phagocytosis, induction of inflammation, and antigen presentation to lymphocytes is observed (Aloisi, 2001). Being resident immune cells microglia perform functions similar to those of tissue macrophages in other organs, serving as tissue phagocytes (when required) and constituting the first line of defense against invading pathogens. On the other hand microglia are also glia cells, and the term resting should not be used in the sense of inactive, since ramified microglia in the healthy brain interact dynamically with other glial cells and neurons, thus fulfilling important neurotrophic roles (Polazzi & Contestabile, 2002; Streit, 2002).
2. Cell origin and plasticity Microglia derive from mesodermal precursor cells of hemapoietic lineage that populate the CNS in early development. Resident microglia are turned over slowly, and are replenished throughout adult life by proliferation. Even in many paradigms of brain injury where the otherwise low infiltration of monocytes/macrophages into the brain parenchyme is increased, microglia remain the predominant immune cell type in the brain, because of their capacity to
proliferate and their active migration towards sites of injury. There are some indications that microglia represent an immature cell type, which can display plasticity. Thus, cultured microglia can differentiate along both macrophage and dendritic cell (DC) lineages by stimulation with macrophage colony stimulating factor (M-CSF) and granulocyte M-CSF (GM-CSF), respectively (Fischer & Reichmann, 2001; Santambrogio et al., 2001). Although definitive proof of DC differentiation in vivo is still lacking, the generation of DCs can potentially have important functional consequences in the setting of infectious or autoimmune encephalitis, as DCs are superior to microglia with respect to stimulation of na¨ıve or allogeneic T-cell proliferation (Fischer & Reichmann, 2001).
3. Functions In the developing brain ameboid microglia phagocytose surplus cells undergoing apoptosis. But they are also actively involved in the determination of cell fate (elimination/survival) of developing neurons. For example, microglia provoke the death of developing Purkinje cells by a superoxide-dependent mechanism (Marin-Teva et al., 2004). Microglia are potentially also promotors of the migration, axonal growth, and terminal differentiation of different neuronal subsets, through the release of extracellular matrix components, soluble factors and direct cell–cell contact (Polazzi & Contestabile, 2002; Streit, 2002). In the adult brain, most phagocyte effector functions are downregulated in resting, ramified microglia, which function as supportive glia cells. Cross-talk with neurons is believed to be an important factor in guarding microglia cells in a quiescent state (Polazzi & Contestabile, 2002). For example interaction of the neuronal membrane protein CD200 with the myeloid cell receptor CD200R dampens microglial activation. Mice deficient in CD200 show morphological and molecular signs of microglia activation in the resting CNS, and the microglial response to different forms of experimental brain injury is excessive (Hoek et al., 2000). However, the resting state of microglia is abandoned when they are presented with pathological endogenous (e.g. neuronal dysfunction/death, abnormal protein aggregation, or immune cell interaction) or exogenous (infection) signals (Fig. 2). Upregulation of
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
19
Fig. 2. (1) In the healthy brain microglia support neuronal well-being, and in turn receives cues from neurons and glial cells to remain in the resting state. (2) In response to a wide array of noxious stimuli microglia undergo activation. Activation may be beneficial to the host (3) when ROS and secreted cytokines are kept at low and/or transient levels. In this instance these proinflammatory mediators are neuroprotective. However, when they surpass a certain level of host tolerance (4) these mechanisms become neurotoxic and result in neuronal dysfunction and cell death, which may further contribute to microglial activation.
multiple phagocyte effector functions and the expression of a range of molecular mediators permit microglia to respond specifically and appropriately towards the insult by tissue repair, neurotrophic support, induction of inflammation, or activation of lymphocytes. The graded process of acquisition of these functions is referred to as activation and is paralleled by both morphological transformation, going from ramified over rodlike to ameboid configuration, and discrete temporal changes in gene expression profile. The acquired functions include cell proliferation, migration, phagocytosis, upregulation of antigen-presenting cell capabilities, upregulation of innate immune cell surface receptors (pattern recognition, complement, and Fc receptors), secretion of proinflammatory mediators such as prostaglandins, TNF-␣, IL-1, and chemokines, secretion of proteases, and the generation of reactive oxygen species (ROS) and nitrogen intermediates (Aloisi, 2001). Activated microglia also have the potential to secrete anti-inflammatory compounds like IL-10 or TGF-, as well as neurotrophic substances like neurotrophins. A given insult/stimulus may elicit a specific response encompassing only a subset of these functions and molecular mediators. For example activation of microglia with IFN-␥, LPS or prion protein involves quite distinct gene expression profiles (Baker & Manuelidis, 2003). The list of functions presented above is not exhaustive, but indicates the potential of activated microglia to fulfill roles ranging from neuroprotective and proregenerative over immune surveilanceenhancing to proinflammatory and neurotoxic (Fig. 2).
Depending on injury model these diverse roles are also reflected in animal models of genetic deficiency specifically affecting microglia. Mice deficient in M-CSF (op/op mice), which is an important mitogen for microglia, have a reduced complement of microglia and the normal proliferative response of microglia towards brain injury is absent. In these mice infarct size following ischemic insult is increased, suggesting a proregenerativ function of microglia. In contrast, microglial activation and recruitment alone (i.e. in the absence of proliferation) following excitotoxin injection are sufficient to mediate neuronal death, indicating a neurotoxic role of microglia. The signals and mediators, which determine whether microglia function in a neurotrophic or neurotoxic mode in inflammatory states are currently being elucidated. Microglia activation and brain pathology in different forms of brain injury are reduced in mutant mice lacking cytokines such as IL-6, IL-1, MCP-1 or TNF-. In contrast, soluble mediators like estrogen and IFN-, and cell–cell mediated interactions, e.g. CD200-CD200R, excert an opposing effect on microglia activation and inflammation.
4. Associated pathologies In pathological conditions, microglia have in particular received attention in a class of diseases characterized by chronic inflammation of the CNS with no overt T-cell involvement. These include Alzheimer’s disease
20
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
(AD), Parkinson’s disease (PD) and HIV-associated dementia (HAD). Microglia have been associated with both initiation and perpetuation of chronic inflammation. For example, in the 1-methyl-4phenyl-1,2,3,6tertrahydropyridine (MPTP) model of PD, a single administration of the neurotoxin MPTP causes direct injury and death of dopaminergic neurons in the substantia nigra. Nevertheless, it is known that antiinflammatory agents or drugs that reduce microglial activation, inhibit disease progression, and this is also the case in AD. On the other hand intracerebral infusion of LPS, which is not neurotoxic, causes microglia activation, resulting in a delayed and progressive loss of dopaminergic neurons (Gao et al., 2002), indicating that microglial activation in some settings is sufficient to induce neuronal death. Also in some disease models, e.g. Sandhoff disease, microglia activation precedes apparent acute neurodegeneration. In the chronic inflammatory brain diseases, it is not clear what triggers nor sustains microglial activation. An emerging line of speculation is that microglial activation in dementia is secondary to ageing (and genetic/epigenetic risk factors). Oligonucleotide array analysis of ageing rodent brain demonstrates a gene expression profile indicative of an inflammatory response, including oxidative stress, as well as downregulation of neurotrophic support (Lee, Weindruch, & Prolla, 2000). Possibly, neuro-immune communication in the ageing brain is compromised, leading to unopposed microglial activation in response to insults, e.g. amyloid- aggregates in AD. Resulting neurotoxicity may in turn cause stressed or dying neurons to release substances which further aggrevate microglial activation, thereby closing a self sustaining inflammatory cycle. Which effector mechanisms are responsible for neuronal dysfunction and cell death in these neurodegenerative diseases? Deterioration of neurotrophic support because of microglial dysfunction resulting from ageing or direct insults, e.g. viral infection in HAD, is most likely one side of the story (Streit, 2002). However, evidence is emerging that oxidative stress, caused by an imbalance between ROS production and endogenous detoxification mechanisms, may be an important pathological factor. Pathological findings in animal models or post mortem tissue demonstrate oxidative damage to brain tissue, in some cases before neuronal death has taken place, placing oxidative stress upstream in the cascade of deleterious events that lead to neurodegener-
ation. Microglia express all subunits of the superoxideproducing phagocyte NADPH oxidase (see Fig. 1) (Vilhardt et al., 2002). It is therefore interesting that not only are the subunits of NADPH oxidase upregulated in inflammation, but in the MPTP mouse model (Wu et al., 2003), and in AD brain (Shimohama et al., 2000), the NADPH oxidase is assembled and therefore actively generating superoxide. Of potential relevance to HAD, the HIV-1 virulence factor Nef primes the NADPH oxidase and massively exacerbates microglial superoxide release in response to bacterial metabolites such as fMLP or LPS (Vilhardt et al., 2002). Intriguingly, loss of dopaminergic neurons in the MTMP model is prevented in animals deficient in either NADPH oxidase or inducible NO synthase (iNOS), suggesting a causal relationship between oxidative stress and neuronal death (Liberatore et al., 1999; Wu et al., 2003).
5. Can microglia be used for therapy? Although microglia have been named the culprits of inflammatory brain disease, there is nevertheless a possibility that microglia can be harnessed for theurapeutical purposes. Thus, clinical trials based on vaccination of AD patients with amyloid- have been launched (McGeer & McGeer, 2003). Vaccination clearly caused microglia to phagocytose amyloid- aggregates, causing the dissolution of senile plaques, a major histopathological finding of AD brain. However, the trial had to be halted after a few months when about 5% of the patients developed severe neuroinflammation, probably because vaccination boosted the already present neuroinflammation to levels above host tolerance (McGeer & McGeer, 2003). Another intriguing possibility is to use bone marrow- or blood-derived hemapoietic stem cells as vehicles for transport of recombinant therapeutic protein into the brain following lentiviral introduction of transgenes. Animal studies have already demonstrated that stem cells can migrate to the CNS, engraft into the brain parenchyme, and develop into differentiated parenchymal microglia.
Acknowledgement Apologies are extended to those investigators who could not be cited due to space considerations.
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
References Aloisi, F. (2001). Immune function of microglia. Glia, 36, 165–179. Baker, C. A., & Manuelidis, L. (2003). Unique inflammatory RNA profiles of microglia in Creutzfeldt-Jakob disease. Proc. Natl. Acad. Sci. U.S.A., 100, 675–679. Fischer, H. G., & Reichmann, G. (2001). Brain dendritic cells and macrophages/microglia in central nervous system inflammation. J. Immunol., 166, 2717–2726. Gao, H. M., Jiang, J., Wilson, B., Zhang, W., Hong, J. S., & Liu, B. (2002). Microglial activation-mediated delayed and progressive degeneration of rat nigral dopaminergic neurons: Relevance to Parkinson’s disease. J. Neurochem., 81, 1285–1297. Hoek, R. M., Ruuls, S. R., Murphy, C. A., Wright, G. J., Goddard, R., Zurawski, S. M., et al. (2000). Down-regulation of the macrophage lineage through interaction with OX2 (CD200). Science, 290, 1768–1771. Lee, C. K., Weindruch, R., & Prolla, T. A. (2000). Gene-expression profile of the ageing brain in mice. Nat. Genet., 25, 294–297. Liberatore, G. T., Jackson-Lewis, V., Vukosavic, S., Mandir, A. S., Vila, M., McAuliffe, W. G., et al. (1999). Inducible nitric oxide synthase stimulates dopaminergic neurodegeneration in the MPTP model of Parkinson disease. Nat. Med., 5, 1403–1409. Marin-Teva, J. L., Dusart, I., Colin, C., Gervais, A., van Rooijen, N., & Mallat, M. (2004). Microglia promote the death of developing Purkinje cells. Neuron, 41, 535–547.
21
McGeer, P. L., & McGeer, E. (2003). Is there a future for vaccination as a treatment for Alzheimer’s disease? Neurobiol. Aging, 24, 391–395. Polazzi, E., & Contestabile, A. (2002). Reciprocal interactions between microglia and neurons: From survival to neuropathology. Rev. Neurosci., 13, 221–242. Santambrogio, L., Belyanskaya, S. L., Fischer, F. R., Cipriani, B., Brosnan, C. F., Ricciardi-Castagnoli, P., et al. (2001). Developmental plasticity of CNS microglia. Proc. Natl. Acad. Sci. U.S.A., 98, 6295–6300. Shimohama, S., Tanino, H., Kawakami, N., Okamura, N., Kodama, H., Yamaguchi, T., et al. (2000). Activation of NADPH oxidase in Alzheimer’s disease brains. Biochem. Biophys. Res. Commun., 273, 5–9. Streit, W. G. (2002). Microglia as neuroprotective, immunocompetent cells of the CNS. Glia, 40, 133–139. Vilhardt, F., Plastre, O., Sawada, M., Suzuki, K., Wiznerowicz, M., Kiyokawa, E., et al. (2002). The HIV-1 Nef protein and phagocyte NADPH oxidase activation. J. Biol. Chem., 277, 42136– 42143. Wu, D. C., Teismann, P., Tieu, K., Vila, M., Jackson-Lewis, V., Ischiropoulos, H., et al. (2003). NADPH oxidase mediates oxidative stress in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson’s disease. Proc. Natl. Acad. Sci. U.S.A., 100, 6145–6150.
Cells in focus
Microglia: phagocyte and glia cell Frederik Vilhardt∗ Structural Cell Biology Unit, The Panum Institute, Copenhagen University, Building 18.4, Blegdamsvej 3A, 2200 Copenhagen N, Denmark Received 8 December 2003; received in revised form 9 June 2004; accepted 21 June 2004
Abstract Microglia are the resident immune cells of the brain, and are located within the brain parenchyme behind the blood–brain barrier. They originate from mesodermal hemapoietic precursors and are slowly turned over and replenished by proliferation in the adult central nervous system. In the healthy brain resting, ramified microglia function as supportive glia cells, and their activation status is regulated by neurons through soluble mediators and cell–cell contact. However, in response to brain pathology microglia become activated: acquisition of innate immune cell functions render microglia competent to react towards brain injury through tissue repair or induction of immune responses. In certain pathological conditions, however, microglia activation may sustain a chronic inflammation of the brain, leading to neuronal dysfunction and cell death. This might be mediated by the microglial release of extracellular toxic reactive oxygen and nitrogen species. Nevertheless, in the future microglia may potentially be harnessed for therapeutical purposes. © 2004 Elsevier Ltd. All rights reserved. Keywords: Microglia; Cell activation; Inflammation; Superoxide
Cell facts • • • • •
Resident immune cell of the brain. Constitute approximately 20% of the total glia cell population. Originate from mesodermal precursor cells of hemapoietic lineage. Traditionally divided into resting (ramified) and activated (ameboid) microglia. Resident microglia are slowly turned over and replaced by proliferation.
1. Introduction Microglia are the resident immune cells of the central nervous system (CNS) parenchyme and belong ∗ Tel.: +45 35 32 72 99; fax: +45 35 32 72 85. E-mail address: [email protected] (F. Vilhardt).
1357-2725/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocel.2004.06.010
to the mononuclear phagocyte lineage related to other organ specific macrophage populations such as the Kupffer cells of the liver and bone osteoclasts. Microglia occur throughout the brain parenchyme where they assume a dendritic morphology with many highly ramified processes (see Fig. 1). As observed by scanning electron microscopy the microglial cell
18
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
Fig. 1. Human hippocampal microglia stained for the gp91phox subunit of the phagocyte NADPH oxidase.
surface is covered with spines (spiky protrusions), which may be a uniquely identifying trait of microglia, as they are not observed in other macrophage cell types. Ramified microglia are often referred to as resting microglia, since injury-induced microglia activation results in contraction of cellular processes and conversion into an ameboid macrophage-like morphology. Concurrently, de novo acquisition of phagocyte effector functions including phagocytosis, induction of inflammation, and antigen presentation to lymphocytes is observed (Aloisi, 2001). Being resident immune cells microglia perform functions similar to those of tissue macrophages in other organs, serving as tissue phagocytes (when required) and constituting the first line of defense against invading pathogens. On the other hand microglia are also glia cells, and the term resting should not be used in the sense of inactive, since ramified microglia in the healthy brain interact dynamically with other glial cells and neurons, thus fulfilling important neurotrophic roles (Polazzi & Contestabile, 2002; Streit, 2002).
2. Cell origin and plasticity Microglia derive from mesodermal precursor cells of hemapoietic lineage that populate the CNS in early development. Resident microglia are turned over slowly, and are replenished throughout adult life by proliferation. Even in many paradigms of brain injury where the otherwise low infiltration of monocytes/macrophages into the brain parenchyme is increased, microglia remain the predominant immune cell type in the brain, because of their capacity to
proliferate and their active migration towards sites of injury. There are some indications that microglia represent an immature cell type, which can display plasticity. Thus, cultured microglia can differentiate along both macrophage and dendritic cell (DC) lineages by stimulation with macrophage colony stimulating factor (M-CSF) and granulocyte M-CSF (GM-CSF), respectively (Fischer & Reichmann, 2001; Santambrogio et al., 2001). Although definitive proof of DC differentiation in vivo is still lacking, the generation of DCs can potentially have important functional consequences in the setting of infectious or autoimmune encephalitis, as DCs are superior to microglia with respect to stimulation of na¨ıve or allogeneic T-cell proliferation (Fischer & Reichmann, 2001).
3. Functions In the developing brain ameboid microglia phagocytose surplus cells undergoing apoptosis. But they are also actively involved in the determination of cell fate (elimination/survival) of developing neurons. For example, microglia provoke the death of developing Purkinje cells by a superoxide-dependent mechanism (Marin-Teva et al., 2004). Microglia are potentially also promotors of the migration, axonal growth, and terminal differentiation of different neuronal subsets, through the release of extracellular matrix components, soluble factors and direct cell–cell contact (Polazzi & Contestabile, 2002; Streit, 2002). In the adult brain, most phagocyte effector functions are downregulated in resting, ramified microglia, which function as supportive glia cells. Cross-talk with neurons is believed to be an important factor in guarding microglia cells in a quiescent state (Polazzi & Contestabile, 2002). For example interaction of the neuronal membrane protein CD200 with the myeloid cell receptor CD200R dampens microglial activation. Mice deficient in CD200 show morphological and molecular signs of microglia activation in the resting CNS, and the microglial response to different forms of experimental brain injury is excessive (Hoek et al., 2000). However, the resting state of microglia is abandoned when they are presented with pathological endogenous (e.g. neuronal dysfunction/death, abnormal protein aggregation, or immune cell interaction) or exogenous (infection) signals (Fig. 2). Upregulation of
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
19
Fig. 2. (1) In the healthy brain microglia support neuronal well-being, and in turn receives cues from neurons and glial cells to remain in the resting state. (2) In response to a wide array of noxious stimuli microglia undergo activation. Activation may be beneficial to the host (3) when ROS and secreted cytokines are kept at low and/or transient levels. In this instance these proinflammatory mediators are neuroprotective. However, when they surpass a certain level of host tolerance (4) these mechanisms become neurotoxic and result in neuronal dysfunction and cell death, which may further contribute to microglial activation.
multiple phagocyte effector functions and the expression of a range of molecular mediators permit microglia to respond specifically and appropriately towards the insult by tissue repair, neurotrophic support, induction of inflammation, or activation of lymphocytes. The graded process of acquisition of these functions is referred to as activation and is paralleled by both morphological transformation, going from ramified over rodlike to ameboid configuration, and discrete temporal changes in gene expression profile. The acquired functions include cell proliferation, migration, phagocytosis, upregulation of antigen-presenting cell capabilities, upregulation of innate immune cell surface receptors (pattern recognition, complement, and Fc receptors), secretion of proinflammatory mediators such as prostaglandins, TNF-␣, IL-1, and chemokines, secretion of proteases, and the generation of reactive oxygen species (ROS) and nitrogen intermediates (Aloisi, 2001). Activated microglia also have the potential to secrete anti-inflammatory compounds like IL-10 or TGF-, as well as neurotrophic substances like neurotrophins. A given insult/stimulus may elicit a specific response encompassing only a subset of these functions and molecular mediators. For example activation of microglia with IFN-␥, LPS or prion protein involves quite distinct gene expression profiles (Baker & Manuelidis, 2003). The list of functions presented above is not exhaustive, but indicates the potential of activated microglia to fulfill roles ranging from neuroprotective and proregenerative over immune surveilanceenhancing to proinflammatory and neurotoxic (Fig. 2).
Depending on injury model these diverse roles are also reflected in animal models of genetic deficiency specifically affecting microglia. Mice deficient in M-CSF (op/op mice), which is an important mitogen for microglia, have a reduced complement of microglia and the normal proliferative response of microglia towards brain injury is absent. In these mice infarct size following ischemic insult is increased, suggesting a proregenerativ function of microglia. In contrast, microglial activation and recruitment alone (i.e. in the absence of proliferation) following excitotoxin injection are sufficient to mediate neuronal death, indicating a neurotoxic role of microglia. The signals and mediators, which determine whether microglia function in a neurotrophic or neurotoxic mode in inflammatory states are currently being elucidated. Microglia activation and brain pathology in different forms of brain injury are reduced in mutant mice lacking cytokines such as IL-6, IL-1, MCP-1 or TNF-. In contrast, soluble mediators like estrogen and IFN-, and cell–cell mediated interactions, e.g. CD200-CD200R, excert an opposing effect on microglia activation and inflammation.
4. Associated pathologies In pathological conditions, microglia have in particular received attention in a class of diseases characterized by chronic inflammation of the CNS with no overt T-cell involvement. These include Alzheimer’s disease
20
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
(AD), Parkinson’s disease (PD) and HIV-associated dementia (HAD). Microglia have been associated with both initiation and perpetuation of chronic inflammation. For example, in the 1-methyl-4phenyl-1,2,3,6tertrahydropyridine (MPTP) model of PD, a single administration of the neurotoxin MPTP causes direct injury and death of dopaminergic neurons in the substantia nigra. Nevertheless, it is known that antiinflammatory agents or drugs that reduce microglial activation, inhibit disease progression, and this is also the case in AD. On the other hand intracerebral infusion of LPS, which is not neurotoxic, causes microglia activation, resulting in a delayed and progressive loss of dopaminergic neurons (Gao et al., 2002), indicating that microglial activation in some settings is sufficient to induce neuronal death. Also in some disease models, e.g. Sandhoff disease, microglia activation precedes apparent acute neurodegeneration. In the chronic inflammatory brain diseases, it is not clear what triggers nor sustains microglial activation. An emerging line of speculation is that microglial activation in dementia is secondary to ageing (and genetic/epigenetic risk factors). Oligonucleotide array analysis of ageing rodent brain demonstrates a gene expression profile indicative of an inflammatory response, including oxidative stress, as well as downregulation of neurotrophic support (Lee, Weindruch, & Prolla, 2000). Possibly, neuro-immune communication in the ageing brain is compromised, leading to unopposed microglial activation in response to insults, e.g. amyloid- aggregates in AD. Resulting neurotoxicity may in turn cause stressed or dying neurons to release substances which further aggrevate microglial activation, thereby closing a self sustaining inflammatory cycle. Which effector mechanisms are responsible for neuronal dysfunction and cell death in these neurodegenerative diseases? Deterioration of neurotrophic support because of microglial dysfunction resulting from ageing or direct insults, e.g. viral infection in HAD, is most likely one side of the story (Streit, 2002). However, evidence is emerging that oxidative stress, caused by an imbalance between ROS production and endogenous detoxification mechanisms, may be an important pathological factor. Pathological findings in animal models or post mortem tissue demonstrate oxidative damage to brain tissue, in some cases before neuronal death has taken place, placing oxidative stress upstream in the cascade of deleterious events that lead to neurodegener-
ation. Microglia express all subunits of the superoxideproducing phagocyte NADPH oxidase (see Fig. 1) (Vilhardt et al., 2002). It is therefore interesting that not only are the subunits of NADPH oxidase upregulated in inflammation, but in the MPTP mouse model (Wu et al., 2003), and in AD brain (Shimohama et al., 2000), the NADPH oxidase is assembled and therefore actively generating superoxide. Of potential relevance to HAD, the HIV-1 virulence factor Nef primes the NADPH oxidase and massively exacerbates microglial superoxide release in response to bacterial metabolites such as fMLP or LPS (Vilhardt et al., 2002). Intriguingly, loss of dopaminergic neurons in the MTMP model is prevented in animals deficient in either NADPH oxidase or inducible NO synthase (iNOS), suggesting a causal relationship between oxidative stress and neuronal death (Liberatore et al., 1999; Wu et al., 2003).
5. Can microglia be used for therapy? Although microglia have been named the culprits of inflammatory brain disease, there is nevertheless a possibility that microglia can be harnessed for theurapeutical purposes. Thus, clinical trials based on vaccination of AD patients with amyloid- have been launched (McGeer & McGeer, 2003). Vaccination clearly caused microglia to phagocytose amyloid- aggregates, causing the dissolution of senile plaques, a major histopathological finding of AD brain. However, the trial had to be halted after a few months when about 5% of the patients developed severe neuroinflammation, probably because vaccination boosted the already present neuroinflammation to levels above host tolerance (McGeer & McGeer, 2003). Another intriguing possibility is to use bone marrow- or blood-derived hemapoietic stem cells as vehicles for transport of recombinant therapeutic protein into the brain following lentiviral introduction of transgenes. Animal studies have already demonstrated that stem cells can migrate to the CNS, engraft into the brain parenchyme, and develop into differentiated parenchymal microglia.
Acknowledgement Apologies are extended to those investigators who could not be cited due to space considerations.
F. Vilhardt / The International Journal of Biochemistry & Cell Biology 37 (2005) 17–21
References Aloisi, F. (2001). Immune function of microglia. Glia, 36, 165–179. Baker, C. A., & Manuelidis, L. (2003). Unique inflammatory RNA profiles of microglia in Creutzfeldt-Jakob disease. Proc. Natl. Acad. Sci. U.S.A., 100, 675–679. Fischer, H. G., & Reichmann, G. (2001). Brain dendritic cells and macrophages/microglia in central nervous system inflammation. J. Immunol., 166, 2717–2726. Gao, H. M., Jiang, J., Wilson, B., Zhang, W., Hong, J. S., & Liu, B. (2002). Microglial activation-mediated delayed and progressive degeneration of rat nigral dopaminergic neurons: Relevance to Parkinson’s disease. J. Neurochem., 81, 1285–1297. Hoek, R. M., Ruuls, S. R., Murphy, C. A., Wright, G. J., Goddard, R., Zurawski, S. M., et al. (2000). Down-regulation of the macrophage lineage through interaction with OX2 (CD200). Science, 290, 1768–1771. Lee, C. K., Weindruch, R., & Prolla, T. A. (2000). Gene-expression profile of the ageing brain in mice. Nat. Genet., 25, 294–297. Liberatore, G. T., Jackson-Lewis, V., Vukosavic, S., Mandir, A. S., Vila, M., McAuliffe, W. G., et al. (1999). Inducible nitric oxide synthase stimulates dopaminergic neurodegeneration in the MPTP model of Parkinson disease. Nat. Med., 5, 1403–1409. Marin-Teva, J. L., Dusart, I., Colin, C., Gervais, A., van Rooijen, N., & Mallat, M. (2004). Microglia promote the death of developing Purkinje cells. Neuron, 41, 535–547.
21
McGeer, P. L., & McGeer, E. (2003). Is there a future for vaccination as a treatment for Alzheimer’s disease? Neurobiol. Aging, 24, 391–395. Polazzi, E., & Contestabile, A. (2002). Reciprocal interactions between microglia and neurons: From survival to neuropathology. Rev. Neurosci., 13, 221–242. Santambrogio, L., Belyanskaya, S. L., Fischer, F. R., Cipriani, B., Brosnan, C. F., Ricciardi-Castagnoli, P., et al. (2001). Developmental plasticity of CNS microglia. Proc. Natl. Acad. Sci. U.S.A., 98, 6295–6300. Shimohama, S., Tanino, H., Kawakami, N., Okamura, N., Kodama, H., Yamaguchi, T., et al. (2000). Activation of NADPH oxidase in Alzheimer’s disease brains. Biochem. Biophys. Res. Commun., 273, 5–9. Streit, W. G. (2002). Microglia as neuroprotective, immunocompetent cells of the CNS. Glia, 40, 133–139. Vilhardt, F., Plastre, O., Sawada, M., Suzuki, K., Wiznerowicz, M., Kiyokawa, E., et al. (2002). The HIV-1 Nef protein and phagocyte NADPH oxidase activation. J. Biol. Chem., 277, 42136– 42143. Wu, D. C., Teismann, P., Tieu, K., Vila, M., Jackson-Lewis, V., Ischiropoulos, H., et al. (2003). NADPH oxidase mediates oxidative stress in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson’s disease. Proc. Natl. Acad. Sci. U.S.A., 100, 6145–6150.