Oligodendrocyte-microglia cross-talk in the central nervous system

Abstract

Communication between the immune system and the central nervous system (CNS) is exemplified by cross-talk between glia and neurons shown to be essential for maintaining homeostasis. While microglia are actively modulated by neurons in the healthy brain, little is known about the cross-talk between oligodendrocytes and microglia. Oligodendrocytes, the myelin-forming cells in the CNS, are essential for the propagation of action potentials along axons, and additionally serve to support neurons by producing neurotrophic factors. In demyelinating diseases such as multiple sclerosis, oligodendrocytes are thought to be the victims. Here, we review evidence that oligodendrocytes also have strong immune functions, express a wide variety of innate immune receptors, and produce and respond to chemokines and cytokines that modulate immune responses in the CNS. We also review evidence that during stress events in the brain, oligodendrocytes can trigger a cascade of protective and regenerative responses, in addition to responses that elicit progressive neurodegeneration. Knowledge of the cross-talk between microglia and oligodendrocytes may continue to uncover novel pathways of immune regulation in the brain that could be further exploited to control neuroinflammation and degeneration.

Introduction

It was Pio del Rio Hortega who, in 1919, first described oligodendrocytes and microglia as separate cell types¹ that, together with the astrocytes and the ependymal cells, form the neuroglia. For a long time, glia cells were thought to play only a passive role in the brain, i.e. to support and form an optimal environment for the neurons. It is only in the last decades that neuroscientists have focused on the individual roles of these non-neuronal cells in brain homeostasis.¹

The oligodendrocyte provides multiple axons with myelin sheaths, extending processes to as many as 50 axons, and has the capacity to renew its myelin sheaths three times within 24 hr.² Myelin insulates the naked axon, a necessary step for optimal signal transduction in the central nervous system (CNS). As a consequence of their high metabolic rate oligodendrocytes are extremely vulnerable to oxidative and endoplasmic reticulum stress due to high intracellular iron stores and relatively low levels of anti-oxidative enzymes.³

Oligodendrocyte pathology is observed in several types of disorders. Primary oligodendropathies, as a result of inflammatory, toxic or genetic dysfunctions, show large white matter lesions associated with clinical symptoms (Table 1).

Table 1.

Oligodendrocyte pathology and microglia changes in neurodegenerative and white matter disorders

Disease	Oligodendrocyte	Microglia	References
Multiple sclerosis	Reduced numbers. Signs of stress and apoptosis. Expression of caspase 3, phagocytosis of apoptotic oligodendrocytes. Swelling of cells with abnormal nuclei, complement deposition, and lysis	Clusters in normal-aapearance white matter Microglia and macrophages in active and chronic active lesions. Activated microglia in areas of remyelination	4,5
Neuromyelitis optica	Loss of oligodendrocytes within the lesions. Oligodendrocyte apoptosis	Extensive macrophage infiltration and microglia activation	4,6
Vanishing white matter disease	Highly foamy oligodendrocytes. Signs of apoptosis	Activated microglia	7
Hereditary diffuse leucoencephalopathy	Myelin loss. Presence of ‘foamy’ oligodendroglial cells	Foamy microglia	8
Alzheimer's disease	Complement-activated oligodendroglia	Activated microglia	9,10
Parkinson's disease	Complement-activated oligodendroglia	Activated microglia	11
Stroke	ATP released during ischaemia activates P2X7 receptor leading to myelin damage	Activated microglia promote oligodendrocyte growth	12,13
Periventricular leucomalacia	Diffuse periventricular depletion of premyelinating oligodendrocytes	Prominent activation of microglia diffusely throughout the white matter	9
Viral infections
Measles	PML Expression of MHC class II in demyelinating lesions restricted to activated macrophages/microglia. HIV oligodendrocytes damage. Severity of myelin damage related to microglia activation	PML-activated microglia associated with myelin loss HIV-activated microglia	14
Mumps
Rubella
PML
HIV
Schizophrenia Mood disorders	Reduced and altered oligodendrocyte numbers in some regions. Structural and functional changes of the axoglial paranode	Activated microglia	9,15,16

PML, progressive multifocal leucoencephalopathy.

Such pathology is typified in vanishing white matter disorder, one of the most prevalent hereditary childhood leucoencephalopathies. Likewise, oligodendrocyte damage is seen in inflammatory disorders of the CNS, and may be immune-mediated as observed in viral infections, acute demyelinating encephalomyelitis, and chronic demyelinating disease such as multiple sclerosis (MS; Table 1). In addition to its task in maintaining myelin, oligodendrocytes also maintain axonal integrity, support axonal metabolism and aid neuronal survival.^17,18 Given these key roles in protecting neurons, oligodendrocytes are increasingly recognized as important components in the pathogenesis of neurodegenerative disorders including Alzheimer's disease, spinal cord injury, MS, Parkinson's disease and amyotrophic lateral sclerosis. More recently, the involvement of glia has also been linked to complex neuropsychiatric disorders that also result in loss of oligodendrocytes and myelin.^17,19,16 To a certain extent, activated microglia are detectable in many oligodendropathies as well as in the classical neurodegenerative disorders mentioned above.^20,21

Microglia, the macrophages of the brain and spinal cord parenchyma, are considered key players in immune regulation of the CNS. Such regulation is controlled by the production of chemokines and cytokines, as well as by the production of free radicals such as reactive oxygen species (ROS) and nitric oxide (NO), known to contribute to neuronal damage and tissue damage. However, microglia activation is also important during neurogenesis, playing a crucial role in synaptic pruning and damage restoration, by removing apoptotic cells as well as by secreting growth factors.^21,22 It is therefore clear that glia cells are not just cells that fill ‘the space not occupied by neurons’, as Virchow suggested in the late nineteenth century, but actively play a role in neuronal support and dysfunction.¹ Yet, there is little known about the interaction between these glia cells.

Here, we review evidence for cross-talk between oligodendrocytes and microglia, showing that there is a delicate balance between activated microglia being harmful to the myelin-producing cells on one hand, and on the other being necessary for their repair and genesis. Yet, oligodendrocytes in turn can control microglial activity by the production of chemokine, cytokines and chaperokines. Understanding the mechanisms behind this interaction that lead to oligodendrocyte damage could provide novel clues to the pathogenesis of a broad spectrum of neurological disorders.

Microglia-induced damage of oligodendrocytes

Microglia represent 10–20% of the glia cells and, unlike the two other glia types, are of mesodermal origin. During embryogenesis, they move from the periphery into the brain and fulfil functions similar to the other tissue macrophages, including antigen presentation, cytokine production and phagocytosis.²³ However, their unique ramified appearance under physiological conditions would not immediately typify them as macrophages. Despite this apparent resting state, microglia are dynamic and continuously scanning their environment for molecular alterations. Upon activation, the morphology of microglia changes into an amoeboid appearance, more reminiscent of macrophages.²⁴

Activated microglia produce various pro-inflammatory mediators including glutamate, matrix metalloproteinases, lipolytic enzymes, reactive oxygen and nitrogen species (O₂, H₂O₂, OH⁻, NO, ONOO⁻), excitotoxins (glutamate, quinolonic acid), chemokines and cytokines.²³ Although such production is necessary to kill invading pathogens, these mediators may also induce bystander damage to neighbouring glia and neurons. In such environments, oligodendrocytes are particularly susceptible to these microglia-derived factors because of their high metabolic activity and energy demands. They frequently respond by producing poor-quality myelin, which may contribute to the pathology observed in neurological diseases. This is exemplified by studies using the endotoxin lipopolysaccharide (LPS), a major component of the Gram-negative bacterial cell wall that induces a strong immunological response. As an example, intracerebral injection of LPS in young rats invokes activation of astrocytes and microglia and the subsequent production of a range of pro-inflammatory cytokines that ultimately lead to hypomyelination.^25,15 In vitro, microglia activated with LPS arrest oligodendrocyte progenitor cell (OPC) proliferation and induce OPC death.^26,27

Once damaged, cells may undergo several cell death pathways. Depending on that pathway, innate immune cells may become activated (e.g. via necrosis), or avoid immune activation (e.g. via apoptosis). A growing body of evidence indicates that extracellular nucleotides play important roles in glial activation in the CNS via purinergic receptors. Oligodendrocytes are also known to use purinergic receptor signalling for their development and for myelination. In addition, as a consequence of demyelination or cell death, oligodendrocytes may also release nucleotides that are recognized by ionotropic P2X and metabotropic P2Y purinergic receptors expressed on microglia. How this impacts on neurodegenerative disorders is as yet unknown. However, several mechanisms of cell death may activate microglia. For example, when the brain is exposed to hypoxic–ischaemic conditions, glutamate released by injured axons and glia cells accumulates in the white matter. Activated microglia may respond to this surge in glutamate release by increasing glutamate–cystine exchange transporter expression, necessary to exchange cysteine with glutamate.⁹ Under physiological conditions, glutamate is an important neurotransmitter in the CNS, communicating via glutamate receptors. However, over-activation of the glutamate receptors under pathological conditions due to high levels of glutamate results in Ca²⁺-mediated excitotoxicity.^9,28,29 Since oligodendrocytes express the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-d-aspartate and kainate glutamate receptor subtypes, glutamate-induced oligodendrocyte death is considered to play a major role in hypoxic–ischaemic white matter pathologies, such as stroke and perventricular leucomalacia.^9,28,30,31 Besides the direct apoptotic effects of abundant glutamate concentration in the brain, such levels also enhance the release of tumour necrosis factor-α (TNF-α) and interleukin1-β (IL-1β) by microglia, which might further contribute to myelin damage.²⁹ This is supported by in vitro studies of primary cultured rat microglia, showing that both cytokines are rapidly induced during hypoxia.³² While TNF-α is widely considered to be an important modulator of the acute phase of inflammation by initiating a cascade of other cytokines, its mode of action is largely determined by binding to either of two receptors. There are two types of receptors through which TNF-α can act, namely TNF-R type 1 and 2, both expressed by oligodendrocytes and neurons. The binding of TNF-α to TNF-R1 induces damage of the target cell by activation of the mitogen-activated protein kinase signalling pathway.^15,29,32,33 In 1-day-old Wistar rats, increased expression of TNF-R1 on oligodendrocytes after hypoxia exposure, concomitant with an increase in apoptosis of oligodendrocytes, suggests a prominent role for TNF-α in hypoxia-induced periventricular white matter damage.³² This is supported by studies in which transgenic mice that over-express TNF-α spontaneously developed chronic demyelinating inflammatory disease. In these mice, lymphocyte infiltration, astrocytosis and focal demyelination bearing close resemblance to the pathology of MS were observed.³⁴

A wide range of pro-inflammatory cytokines, including interleukins (IL) 1, 2, 3, interferons (IFNs) α, β and γ, TNF-α and lymphotoxin, released by microglia, have been detected in demyelinating MS lesions, suggesting a correlation between microglial activity and oligodendrocyte damage in MS.²³ Like TNF-α, IL-1β is expressed by activated microglia, and plays a role in acute immune response regulation. Binding of IL-1β to its receptor IL-1R1 triggers the mitogen-activated protein kinase signalling pathway. While both systemic and intracerebral injections of IL-1β in neonatal mice lead to hypomyelination and a reduction of developing oligodendrocytes, IL-1β does not induce apoptosis directly, in contrast to TNF-α. Nevertheless, IL-1β delays remyelination during disease.^15,29,32,33 Another product of microglia, and closely related to TNF-α, lymphotoxin is thought to play a more significant role in oligodendrocyte cell death by increasing intracellular ceramide concentrations, thus provoking to cell death.³⁵ IFN-γ also fulfils a dual detrimental role in oligodendrocyte damage. Similar to LPS, IFN-γ induces apoptosis of oligodendrocytes^36,37 and inhibits CNS remyelination through a process modulated by endoplasmic reticulum stress via both signal transducer and activator of transcription 1-dependent and -independent pathways. In addition, exposure of oligodendrocytes to IFN-γ in vitro increases the expression of caspases 1, 4, 7 and 8 mRNA and Fas, making them vulnerable to cell death. Such treatment also increases the expression of TNF-R1 leaving the oligodendrocytes more susceptible to the harmful effects of TNF-α.^38–40

One mechanism by which IFN-γ, TNF-α and IL-1β contribute to oligodendrocyte damage is via iNOS gene activation. The inducible isoform of the nitric oxide synthetase (NOS) family, iNOS catalyses the production of NO from l-arginine during immune responses. NO production induces cytolysis, cytostasis and inhibition of the Kreb's cycle and has been linked to oligodendrocyte death.^19,41 While iNOS is known to be activated in astrocytes and microglia, it has also been shown that iNOS is also produced in oligodendrocytes.⁴² Li and colleagues⁴³ demonstrated that the reaction product of NO and superoxide anion produced following LPS-stimulation of microglia, namely peroxynitrite, is toxic to oligodendrocytes. Another mechanism of NO-induced injury is oxidization, via ROS production. Normally, ROS homeostasis is maintained by endogenous antioxidants that either neutralize or eliminate ROS. During oxidative stress, this balance is disrupted.

In summary, many products of microglial activation may be potentially detrimental to oligodendrocytes. As mentioned before, high metabolic activity combined with low levels of anti-oxidants render oligodendrocytes extremely susceptible to oxidative damage.

Microglia in remyelination

Although macrophages and microglia may contribute to oligodendrocyte and myelin damage during inflammation, they may also play a crucial role in wound healing, regeneration and repair in the CNS. By recruitment, proliferation and maturation of OPCs into a demyelinated area, the CNS is able to restore myelin. Several lines of evidence indicate that microglia play a key role in this process. Animal model studies have shown that removal or blocking of the action of microglia negatively affects repair, suggesting an important interplay between microglia and oligodendrocytes during remyelination.^44,45

That microglia activation is strongly influenced by the environment has been exploited to generate polarized M1 and M2 phenotypes in vitro. In this way, LPS and IFN-γ are used to induce a pro-inflammatory (M1) phenotype, whereas IL-4 stimulation results in the production of anti-inflammatory mediators (M2).^36,46 However, intermediate states of activation are found in vivo, indicating that the plethora of molecules released during tissue damage and repair conditions is more complicated than suggested by a simple bipolar model of microglial differentiation.

Although TNF-α is generally considered to be a pro-inflammatory cytokine, contributing to oligodendrocyte damage, several lines of evidence indicate that TNF-α also exhibits potent immunosuppressive properties, and participates in the reparative process.⁴⁷ Deletion or suppression of TNF-α in experimental models leads to a reduction of proliferating OPCs, along with a delay in remyelination.⁴⁸ These data provide a possible explanation for the disease-exacerbating effects of anti-TNF treatment in MS. As mentioned before, the dual role of TNF-α probably relates to the existence of two different receptors. Binding to TNF-R2 but not TNF-R1 appears to promote the protective effects of TNF-α.⁴⁹ That microglia-derived cytokines are closely associated with remyelination is further exemplified by the observation that, in the absence of IL-1β produced by microglia, the level of IGF-1 is reduced, resulting in impaired differentiation of OPCs into myelinating oligodendrocytes.⁴⁹

Nevertheless, macrophages and microglia appear to play a beneficial role in remyelination. In experimental autoimmune encephalomyelitis (EAE), a widely used animal model of MS, the injection into the cerebrospinal fluid of IL-4-stimulated microglia induces oligodendrogenesis and improvement of clinical symptoms.⁵⁰ In contrast, microglia exposed to IFN-γ fail to promote survival, maturation and migration of OPCs.^51,52 This implies that promotion of a repair-supportive environment aids remyelination and could be a promising therapeutic approach in demyelinating diseases. In support of this, a genome-wide gene expression analysis of microglia during demyelination and remyelination in the mouse cuprizone model revealed the existence of a microglia phenotype that supports remyelination. Not only was this microglia phenotype present at the onset of demyelination but it expressed a cytokine and chemokine repertoire known to aid recruitment of OPCs.⁵³ To assist in generating a favourable environment, microglia are essential for removal of myelin debris generated during demyelination. When such phagocytosis and removal of debris is inhibited, myelin repair is blocked by the inhibitory effect of myelin debris on OPC differentiation. In addition, phagocytosis of apoptotic cells and myelin debris by macrophages or microglia induces the production of anti-inflammatory cytokines, including IL-10 and transforming growth factor-β (TGF-β), contributing to a favourable environment to block further tissue damage.^36,53–55 As well as anti-inflammatory effects, TGF-β also has a direct influence on the OPC by encouraging the differentiation of mature myelinating oligodendrocytes.^56,57 Furthermore, TGF-β contributes to the early phase of remyelination by triggering microglia to secrete the chemotactic factor hepatocyte growth factor, aiding the attraction of OPCs to areas of damage. Interestingly, IFN-β, prescribed as first-line treatment in MS, is also able to induce hepatocyte growth factor production by microglia.⁵²

In addition to these factors, microglia associated with remyelination secrete a broad spectrum of growth factors, promoting cell migration, proliferation and differentiation.^53,55 Oligodendrocytes, in particular OPCs, express receptors for epidermal growth factor (EGF), fibroblast growth factor (FGF), insulin-like growth factor 1 (IGF-1), platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). Although VEGF is primarily known for its regulation of angiogenesis, recent data show that VEGF plays an active role in remyelination by promoting the migration of OPCs.⁵⁸ Likewise, PDGF and FGF also have pro-migratory effects, as well as mitogenic functions.^55,58 IGF-1 encourages proliferation and differentiation of OPC. Supporting this is the finding that mice with growth hormone deficiency exhibit low levels of circulating IGF-1, concomitant with hypomyelination. Furthermore, IGF-1-deficient mice die shortly after birth as the result of severe myelin abnormalities, suggesting that IGF-1 is a key player in myelination during embryogenesis.⁵⁷ For example, over-expression of IGF-1 results in accelerated remyelination,⁴⁹ revealing the strong impact of microglial growth factors on oligodendrocyte functioning.

A subfamily of growth factors that include brain-derived neurotrophic factor and neurotrophin-3 are known to contribute to neuron and oligodendrocyte development. In rats, injection into the optic nerve of neutralizing antibodies to neurotrophin-3 reduces the numbers of OPCs and mature oligodendrocytes.⁵⁷ That the anti-inflammatory cytokine IL-10 produced by microglia induces neurotrophins clearly implicates a cross-talk between microglia and oligodendrocytes in the remyelination process.⁵⁹

In summary, these data emphasize the complexity of the interaction between oligodendrocytes and microglia, and additionally clarify that cytokines that are often considered to be pathogenic, may in fact be involved in oligodendrocyte regeneration and remyelination as well.

Oligodendrocytes as immune targets and immune regulators

From an immunological point of view, oligodendrocytes were originally thought of as inert and merely representing bystander victims of immune responses. This view has now changed in the light of accumulating evidence that oligodendrocytes actively produce a wide range of immune-regulatory factors, and express receptors for such factors, as summarized in Table 2.

Table 2.

Expression and impact of immune markers on oligodendrocytes

Immune marker	Marker expression	Impact of expression	References
MHC	MHC I HLA E (human oligodendrocytes); NK2GC and NK2GA ligand NK2GD ligands MHC II (in vitro)	Antigen presentation and target for oligodendrocyte and/or myelin-specific T cells	60,61
Complement	A range of complement factors and inhibitors expressed on oligodendrocytes have been described almost mainly restricted to cells in vitro. e.g. CD59, DAF	Complement mediated damage	62–65
Glutamate receptors	α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors, N-methyl-d-aspartate receptors, kainate receptors	Over-expression of glutamate by microglia may induce excitotoxic death of oligodendrocytes	28–31
Chemokines	CXCL10, CCL3, CCL2, CCL5 CXCR1, CXCR2, CXCR3, CXCR4	Recruitment of microglia and macrophages	66
Cytokines	TNF-family, TNF-R1, R2 TRAIL 2, TRAIL 4 IL-6, IL-8, IL-18 IL-2R, IL-4R, IL-6R, IL-7R (mouse), IL-10R, IL-11αR, IL-12R, IL-18R IFN-γR	TNF-α mediated damage and repair. Cytokine and receptor expression recruits microglia	67–69
TLR	TLR2 (rat/human) TLR3 (rat/human)	Activation by TLR ligands	70–72
Heat shock proteins	HSPB5 expressed by oligodendrocytes in MS	HSPB5 activates human microglia, in vitro inducing repair mechanism and OPC recruitment factors	73
Apoptosis	FAS receptor (FasR; CD95,) Fas ligand (FasL, CD95L) p75NTR, TRAIL	Susceptibility to Fas/Fas ligand-induced apoptosis	74–79
CD200	Expression in vivo	May aid immune regulation of microglia expressing CD200R	80

HSPB5, heat-shock protein B5; IFN, interferon; IL, interleukin; MS, multiple sclerosis; OPC, oligodendrocyte progenitor cell; TLR, Toll-like receptor; TNF, tumour necrosis factor; TRAIL, TNF-related apoptosis-inducing ligand.

That oligodendrocytes express MHC class I molecules, and so can be targeted by CD8⁺ T cells, was already described in the 1990s.^81,48 These findings were initially controversial, but it was subsequently clarified that IFN-γ treatment induces MHC class I expression on oligodendrocytes in vitro,⁸² supporting the idea that the presence of IFN-γ in demyelinating disorders such as MS may render oligodendrocytes susceptible to MHC class I-dependent cell death in disease.⁸³ Studies in vitro suggested that rat oligodendrocytes express MHC class II molecules, necessary for CD4⁺ T-cell recognition,⁸¹ but several attempts to confirm the expression of MHC-II on oligodendrocytes in vivo met with failure. Still, recent studies have revealed that oligodendrocytes express the non-classical MHC-I molecule HLA-E in response to inflammatory cytokines,⁶⁰ in addition to the classical MHC molecules. HLA-E is a ligand for NKG2A and NKG2C members of the natural killer cell receptor family NKG2, expressed on human natural killer cells and T cells. NKG2A and NKG2C have opposing working mechanisms, although both form functional complexes with CD94. Whereas NKG2A ligation induces an inhibitory signal in the effector cell, NKG2C interaction induces a stimulatory signal. Hence, CD4⁺ T-cell cytotoxicity toward oligodendrocytes could be mediated by NKG2C signalling. Supporting the in vitro studies, both NKG2C⁺ CD4 T cells and HLA-E⁺ oligodendrocytes are present in MS tissues, indicating that expression of NKG2C may mediate oligodendrocyte death and hence demyelination during disease.⁶⁰ In active MS lesions, oligodendrocytes also express ligands to NKG2D, indicating that NKG2D–NKG2D ligand interaction may also induce specific myelin cytotoxicity because oligodendrocytes are the only cells in the CNS known to express NKG2D ligands.⁶¹ Similar to HLA-E, NKG2D ligands are moderately present under normal conditions whereas expression is increased in response to environmental factors, pointing towards an active role for oligodendrocytes in regulating their own destiny. However, in disease it remains to be determined whether expression of the classical and non-classical MHC molecules induces a pathogenic response leading to T-cell-mediated damage or, alternatively may recruit T cells that aid repair and regulation.

Another class of membrane-bound proteins involved in controlling the fate of oligodendrocytes are the so-called pattern recognition receptors (PRRs). The PRRs comprise Toll-like receptors (TLRs), C-type lectins, nucleotide oligomerization domain-like receptors, retinoic acid inducible gene-I-like receptors and the pyrin and HIN domain-containing family members. Each recognizes conserved pathogen-associated molecular patterns expressed by bacteria, fungi and viruses, or damage-associated molecular patterns expressed by damaged or stressed cells and tissues. Hence, not only invading micro-organisms, but also endogenous signals such as heat-shock proteins can switch on innate responses in the CNS. While innate receptors are expressed by microglia, little is known about the endogenous stimuli of innate responses in the CNS, and even less is known about the endogenous triggers of PRRs on the CNS cells themselves. Classically, TLRs have been described as key players in innate immunity, generally assumed to generate an adverse environment. For example, the heat-shock protein 60 activates microglia through a TLR4-dependent pathway leading to axonal damage.⁸⁴ That TLRs are also present in the CNS was reported by Bsibsi and colleagues,^70,71 who also revealed the expression of functional TLR2 and TLR3 on oligodendrocytes because treatment with zymosan, a TLR2 agonist, promoted cell differentiation and maturation of primary rat oligodendrocytes whereas activation with poly-I:C, a TLR3 agonist, reduced survival of the oligodendroglia.⁷¹ In contrast, Sloan and colleagues⁷² showed that other TLR2 agonists inhibit OPC maturation in vitro, indicating that the role of TLRs in oligodendrocyte maturation and survival depends on many factors. Nevertheless, these studies underscore the cross-talk between microglia and oligodendrocytes at the levels of PRR such as pathogen-associated molecular pattern and damage-associated molecular pattern expression. Yet, the full impact of such cross-talk in neurodegenerative disorders awaits clarification.

Broad expression of immune receptors and their corresponding ligands by both microglia and oligodendrocytes provides the platform for extensive interactions between these two cell types. As discussed above, TNF-α expression determines the fate of the myelin-producing cell by binding to either TNF-R1 or TNF-R2 on oligodendrocytes.²⁹ Another member of the TNF family is TNF-related apoptosis-inducing ligand (TRAIL), known to induce apoptosis in cells expressing the receptors TRAIL-R1 and -R2, while expression of TRAIL-R3 and TRAIL-R4 inhibits this death signal.^78,79 However, TRAIL-R2 and R4 are expressed on oligodendrocytes in vitro. In culture, TRAIL induces cell death in human oligodendrocytes via ligation with TRAIL-R1,⁸⁵ although whether this is relevant to diseases such as MS is uncertain, given the low levels of TRAIL receptor expression in human brains.

Oligodendrocytes express receptors to a wide range of mediators including IL-4, IL-6, IL-7, IL-10, IL-11, IL-12 and IL-18. Expression of several of these is increased in neurological disorders.^68,69,86 In addition, several chemokine receptors such as CXCR1, CXCR2 and CXCR3 are also expressed on oligodendrocytes at increased levels in MS, stroke and amyotrophic lateral sclerosis.⁸⁷ However, much is still to be learned on their role in disease. CXCR4, a member of the CXC/α receptors, appears to be involved in OPC differentiation into myelinating oligodendrocytes. It can be bound by CXCL12 secreted by astrocytes and microglia.⁸⁸ Oligodendrocytes not only respond to signals from astrocytes or microglia but actively engage in cross-talk with these cells. One example of such cross-talk is the communication between oligodendrocytes and microglia via the membrane glycoprotein CD200. Binding of CD200 to its receptor CD200R expressed by microglia delivers an inhibitory signal, resulting in down-regulation of microglial activity. CD200-deficient mice, for example, are characterized by spontaneously activated microglia, and an increased susceptibility to EAE.⁸⁰ Expression of CD200 on oligodendrocytes⁸⁰ therefore illustrates how these cells may contribute to immune regulation within the CNS by interacting with CD200R microglia.

The strongest evidence for an active role of oligodendrocytes during inflammation comes from studies showing secretion of immune-regulatory cytokines and their impact on other immune cells. For example, during infection with Borrelia burgdorferi, the cause of Lyme neuroborreliosis, oligodendrocytes produce CCL2, IL-8 and IL-6.⁶⁷ Interleukin-6 is known to play a dual role in the CNS, being necessary for brain homeostasis, and dysregulated levels of IL-6 are associated with MS, Alzheimer's disease and schizophrenia.⁶⁷ Both IL-8 and CCL-2 are involved in the recruitment of immune cells during acute inflammation, which suggests that oligodendrocytes are able to attract inflammatory cells such as microglia. This idea is supported by Balabanov et al.,⁶⁶ who showed expression of the chemoattractants CCL2, CCL3, CCL5 and CXCL10 by primary rat oligodendrocytes in response to IFN-γ stimulation. IFN-γ production by microglia is induced by the binding of IL-18 a pro-inflammatory cytokine (or an alternative ligand)⁸⁹ to its receptor.³⁷ Interestingly, oligodendrocytes are a source of IL-18 and express the IL-18R. Such expression is observed during the active stage of MS, in which IFN-γ is also abundantly present and, as described above is known to contribute to oligodendrocyte damage.⁶⁹ This proposes yet another possibility for direct dialogue between the two glia cell types.

Interleukin-18 belongs to the same superfamily as IL-1β, which plays an intricate role in oligodendrocyte homeostasis. Both OPCs and differentiated oligodendrocytes generate endogenous IL-1β,⁹⁰ a major inducer of cyclooxygenase-2 (COX-2) in the CNS.⁹¹ COX-2 is involved in the formation of prostanoids, signalling molecules involved in immune responses and is abundantly present at inflammation sites. In active MS lesions, dying oligodendrocytes also contribute to the levels of COX-2.⁹² While COX-2 is considered to contribute to disease, emerging studies reveal that by catalysing the production of lipid resolvins it may also contribute to the immune-suppressive indoleamine 2,3-dioxygenase 1 functions.⁹³

Stress induction in oligodendrocytes

Oligodendrocyte death and myelin loss in neurodegenerative disorders may be a direct response to cytotoxic viral infections such as JC virus, which causes progressive multifocal leucoencephalopathy.²¹ Clearly, in these cases the immune response is directed against the virus as well as virally infected cells undergoing apoptosis and necrosis. However, changes to oligodendrocytes that do not necessarily induce apoptosis or necrosis may lead to immune activation in more subtle ways. For example, cells undergoing attack by viruses or exposed to ROS for example, initiate stress response pathways to resist and counteract damage and apoptosis. As shown in Fig. 1, many factors are known to induce oligodendrocyte stress, including a variety of pathophysiological processes, such as inflammation, genetic abnormalities, mitochondrial dysfunction, hypoactive N-methyl-d-aspartate receptors, as well as neuronal and axonal damage. As mentioned above, the myelin-producing cell is highly vulnerable to oxidative stress. By activating the sphingomyelinase/ceramide pathway, pro-apoptotic signalling cascades are triggered that could contribute to oligodendrocyte loss under pathological conditions such as ischaemia and MS.⁹⁴ Furthermore, metabolic disturbances, such as imbalanced levels of glucocorticoids, are known to decrease proliferation and viability of oligodendrocytes. Such changes have been suggested to underlie oligodendrocyte alterations in depression.¹⁶

Figure 1.

Potential triggers of oligodendrocyte stress, dysfunction and death in neurodegenerative disorders (see text for further details). ER, endoplasmic reticulum

The expression of an arsenal of receptors renders the myelin-producing cell vulnerable to glutamate toxicity and pro-inflammatory cytokines, both key players in brain injury. Inflammation is one of the most important contributors to oligodendrocyte stress. Already during embryogenesis, pro-inflammatory cytokines could harm the oligodendrocyte. This is supported by studies in a mouse model that mimics maternal inflammation by maternal exposure to LPS administered through intraperitoneal injection at day 15 of gestation. This model reveals that apoptosis of oligodendrocytes occurs 5 days after injection, and leads to hypomyelination 21 days after birth.¹⁵ In humans, maternal infection by for example herpes simplex virus increases the risk of the unborn child to develop several neuropsychiatric disorders, such as autism and schizophrenia, that are associated with oligodendrocyte deviations.^16,15 Several pathogens, such as B. burgdorferi (the cause of Lyme neuroborreliosis) and JC virus infect oligodendrocytes. Besides the damaging influence of pro-inflammatory cytokines released during these infections, the accumulation of pathogens also leads to destruction of the cell.^67,94

Many demyelinating animal models are based on oligodendrocyte damage induced by viral infections, such as Theiler's murine encephalomyelitis virus, or mouse hepatitis virus.⁹⁵ In addition, the cuprizone model relies on toxins causing oligodendrocyte-stress and apoptosis. Feeding the toxin cuprizone to adult mice results in an early and selective oligodendrocyte loss, which is closely followed by microglial activation.⁹⁶ While the mechanism of apoptosis induced by cuprizone is not completely understood, it is generally accepted that cuprizone-mediated copper deficiency affects the energy metabolism of the cell.⁹⁶ Myelin synthesis is a highly energetic process, for which an optimal mitochondrial function is required. Oligodendrocytes are therefore extremely vulnerable to mitochondrial dysfunction, induced by for example cuprizone intoxication, or free oxygen and nitric oxide radicals.⁹⁴

Another mechanism of oligodendrocyte-stress induction is linked to the presence of autoantibodies directed against oligodendrocyte and myelin proteins. Immunization with CNS antigens induces demyelination in the autoimmune model EAE.²¹ Among the different CNS antigens, myelin oligodendrocyte glycoprotein (MOG) is of particular interest in the fate of oligodendrocytes. The myelin protein MOG has an immunoglobulin-like extracellular domain, and is expressed on the outermost surface of the myelin sheath. Injection of monoclonal antibodies directed to MOG at the onset of EAE (clone Z12) augments clinical disease and demyelination.⁹⁷ An in vitro model of MOG-antibody-directed demyelination revealed decreased levels of the myelin basic protein following addition of MOG resulting from antibodies being added to cultured oligodendrocytes, but no real evidence for abundant cell death. However, increased levels of the stress-induced chaperokines heat-shock protein 32 and heat-shock protein B5 (αB-crystallin) were detectable in the cultured oligodendrocytes following treatment with MOG-reactive antibodies. This suggests that although several pathways may trigger apoptosis in oligodendrocytes in vivo, these cells are able to resist apoptosis by producing heat-shock proteins.⁹⁸ How such stress events reinforce the cross-talk between oligodendrocytes and microglia is discussed below.

Stressed oligodendrocytes activate microglia

A number of experimental studies have demonstrated a strong positive correlation between oligodendrocyte susceptibility to injury and the extent of CNS inflammation in experimental models of MS. In mice double heterozygous for the leukaemia inhibitory factor receptor-null mutation and a gp130 intracellular truncation, absence of leukaemia inhibitory factor signalling increases oligodendrocyte susceptibility to injury, and augments the inflammatory reaction and the severity of symptoms.⁹⁹ In addition, mice lacking pro-apoptotic genes, or over-expressing anti-apoptotic molecules exclusively in oligodendrocytes, display resistance to EAE and inflammatory demyelination.^100,77 The critical role of oligodendrocytes in CNS inflammation is further exemplified in mice with peroxisome-deficient oligodendrocytes, which develop spontaneous neuroinflammation.¹⁰¹ However, the molecular mechanisms linking oligodendrocyte stress with the regulation of early microgliosis are poorly understood.

In response to stress, oligodendrocytes produce several immune mediators known to modulate the activation state of microglia. The expression of chemoattractants, such as CXCL10, CCL2 and CCL3, during inflammation and infection suggests that oligodendrocytes actively recruit microglia to damaged tissues.^67,66 Of interest in this respect is the focus on the role of oligodendrocyte–microglia interactions in demyelinating disorders such as MS.⁵ While microglia and macrophages are the main cell type in active and chronic active MS lesions, focal microglia nodules are observed in the normal-appearing white matter of MS patients.⁵ In the absence of any signs of blood–brain barrier disruption, leucocyte infiltration, astrogliosis or demyelination, the so-called preactive lesions in normal-appearing white matter are thought to be the first stage of lesion formation,¹⁰² also present in early stages of the disease.¹⁰³ That oligodendrocytes might contribute to the formation of these preactive lesions is evidenced by their expression of the microglia-activating heat-shock protein αB-crystallin.⁷³ This small heat-shock protein, similar to other heat-shock proteins, is induced by heat or osmotic shock and other stress events, suggesting a role for stressed oligodendrocytes in the pathogenesis of MS. That αB-crystallin in oligodendrocytes is protective is suggested by its induction of a range of immune-regulatory factors,⁷³ and the fact that administration of αB-crystallin during neuroinflammation ameliorates clinical symptoms and reduces tissue damage.¹⁰⁴ Interestingly, among the many factors that are induced in microglia by αB-crystallin is activin A, recently identified as a promotor of remyelination.¹⁰⁵

In summary, these findings suggest that stress events in the CNS may trigger oligodendrocytes to respond in a variety of ways, including necrosis and apoptosis. Depending on the pathway activated, oligodendrocytes may release factors that stimulate microglia to phagocytose damaged cells, thereby removing debris and aid repair (Fig. 2). Alternatively, stressed cells may trigger protective microglial pro-repair responses, for example by production of stress proteins such as αB-crystallin.

Figure 2.

Consequences of oligodendrocyte–microglia cross-talk. Microglia, as polarized or intermediate phenotypes⁴⁵ release a plethora of factors. Such production of chemokines, cytokines, and growth factors may induce mild damage to death of oligodendrocytes (A) or, conversely, promote neuroprotection and repair including recruitment of oligodendrocyte progenitors (B). Oligodendrocytes also interact with, and respond to microglia via production of various factors and production and uptake of exosomes. Abbreviations: GluR, glutamate receptor; HSPs, heat-shock proteins; IFN, interferon; IL-1, interleukin 1β; LT, lymphotoxin; MMP2–9, matrix metalloproteinase 2–9; NO, nitric ocide; RNS, reactive nitrogen species; ROS, reactive oxygen species; TCR, T-cell receptor; tumour growth factor-β; TNF-α, tumour necrosis factor-α

Cross-talk via exosomes

Exosomes are small vesicles released from multiple cell types that contain protein, lipids or RNA species including viral RNA. The presence of exosomes in human tissues was first described in 1987, and they have since been found in urine, plasma and the CSF. The interest in exosomes has increased dramatically in the last few years, after the finding that they also contain mRNA and regulatory microRNAs that can alter the functional state of neighbouring cells after uptake. As described above, stressed oligodendrocytes may communicate with microglia in part by secretion of exosomes, small vesicles containing proteins, lipids and regulatory RNAs. Hence, exosomes play important roles in neurodegenerative disorders. That exosomes might represent a form of communication between oligodendrocytes and microglia is supported by the finding that exosomes isolated from mouse Oli-neu cells are enriched in ceramide and myelin proteins. In disease, exosomes contain TNF-R1 and cytokines, indicating that they may also contain markers that reflect a disease process. Not only are exosomes secreted, but they are also taken up by neighbouring cells, and able to deliver their contents to those recipient cells. Via this strategy, exosomes have been successfully used to attenuate neurological disease in EAE, to suppress LPS-induced inflammation in the brain, and to inhibit glioma GL26 tumour growth in the murine CNS.¹⁰⁶ Thus, exosomes are signatures of cells in health and disease, and they transfer protein and RNA to their neighbours, pointing to a likely regulatory role in human demyelinating diseases.

Conclusions

Oligodendrocytes play a central role in the pathogenesis of a wide spectrum of neurological disorders that encompass several neurodegenerative diseases, in addition to the classical demyelinating disorders. There is a growing awareness that apart from being merely myelin factories, oligodendrocytes play an important role in regulating the immune response in the CNS. The fate of these myelin-producing cells and their progenitors in part depends on the delicate interplay with microglia (Fig. 2). Neuroimmunological research from the last decades has provided strong evidence for the complexity of this cross-talk. The outcome of this communication relies on the situation (e.g. infection, stress), and environment (e.g. cytokine milieu), that influence whether microglia are harmful or protective to oligodendrocytes, and whether they aid or inhibit remyelination. More insight into the communication between microglia and oligodendrocytes should provide important novel clues into how to manipulate this interplay to promote repair and dampen inflammation. Such insights will be crucial to develop new therapeutic approaches to treat disorders in which myelin damage is inextricably associated with innate immune activation in the CNS.

Glossary

CNS

central nervous system

COX-2

cyclooxygenase-2

EAE

experimental autoimmune encephalomyelitis

EGF

epidermal growth factor

FGF

fibroblast growth factor

GluR

glutamate receptors

HGF

hepatocyte growth factor

HIV

human immunodeficiency virus

IFN

interferon

IGF-1

insulin-like growth factor 1

IL

interleukin

iNOS

inducible nitric oxide synthase

LPS

lipopolysaccharide

MOG

myelin oligodendrocyte glycoprotein

MS

multiple sclerosis

NO

nitric oxide

NOS

nitric oxide synthetase

OPC

oligodendrocyte progenitor cell

PDGF

platelet derived-growth factor

PRR

pattern recognition receptors

ROS

reactive oxygen species

TGF-β

transforming growth factor-β

TLR

toll-like receptor

TNF-α

tumour necrosis factor-α

VEGF

vascular endothelial growth factor

Conflict of interests

Johannes M. van Noort holds stock in Delta Crystallon BV, a company aimed at the commercial development of the therapeutic properties of heat-shock protein B5.

Disclosures

The authors declare that they have no competing interests.