GPR17 expressing NG2‐Glia: Oligodendrocyte progenitors serving as a reserve pool after injury

In the adult brain NG2‐glia continuously generate mature, myelinating oligodendrocytes. To which extent the differentiation process is common to all NG2‐glia and whether distinct pools are recruited for repair under physiological and pathological conditions still needs clarification. Here, we aimed at investigating the differentiation potential of adult NG2‐glia that specifically express the G‐protein coupled receptor 17 (GPR17), a membrane receptor that regulates the differentiation of these cells at postnatal stages. To this aim, we generated the first BAC transgenic GPR17‐iCreER^T2 mouse line for fate mapping studies. In these mice, under physiological conditions, GPR17⁺ cells —in contrast to GPR17^‐ NG2‐glia— did not differentiate within 3 months, a peculiarity that was overcome after cerebral damage induced by acute injury or ischemia. After these insults, GPR17⁺ NG2‐glia rapidly reacted to the damage and underwent maturation, suggesting that they represent a ‘reserve pool’ of adult progenitors maintained for repair purposes. GLIA 2016;64:287–299     

Axons and astrocytes release ATP and glutamate to evoke calcium signals in NG2‐glia

NG2‐glia are an abundant population of cells in the adult CNS that make up a novel glial cell type. Here, we have examined calcium signals in NG2‐glia identified by expression of the fluorescent protein DsRed under the control of the NG2 promoter in the white matter of the mouse optic nerve. We focused on mice aged postnatal day (P)12–16, after the main period of oligodendrocyte generation. Using fluo‐4 and fura‐2 calcium imaging in isolated intact nerves, we show that glutamate and ATP evoke Ca²⁺ signals in NG2‐glia in situ, acting on AMPA‐type glutamate receptors and P2Y₁ and P2X₇ purine receptors; NMDA evoked a weak Ca²⁺ signal in a small proportion of NG2‐glia. We show that axonal action potentials and mechanical stimulation of astrocytes effect the release of glutamate and ATP to act on NG2‐glia; ATP alone evokes robust Ca²⁺ signals, whereas glutamate did not unless AMPA receptor desensitization was blocked with cyclothiazide. We identify the precise contacts that NG2‐glia form with axons at nodes of Ranvier, and the intricate bipartite sheaths formed between the processes of NG2‐glia and astrocytes. In addition, we provide evidence that NG2‐glia express synaptophysin, indicating they have mechanisms for transmitting as well as receiving signals. This study places NG2‐glia within a neuron‐glial network, and identifies roles for glutamate and ATP in communication with astrocytes as well as axons. © 2009 Wiley‐Liss, Inc.     

Olig2‐lineage cells preferentially differentiate into oligodendrocytes but their processes degenerate at the chronic demyelinating stage of proteolipid protein‐overexpressing mouse

In chronic demyelinating lesions of the central nervous system, insufficient generation of oligodendrocytes (OLs) is not due to a lack of oligodendrocyte precursor cells (OPCs), because the accumulation of OPCs and premyelinating OLs can be observed within these lesions. Here we sought to identify the basis for the failure of OLs to achieve terminal differentiation in chronic demyelinating lesions through the utilization of plp1‐overexpressing (Plp ^tg/?) mice. These mice are characterized by progressive demyelination in young adults and chronic demyelinating lesions at more mature stages. We show that neural stem cells, which are the precursors of OL‐lineage cells, are present in the Plp ^tg/? mouse brain and that their multipotentiality and ability to self‐renew are comparable to those of wild‐type adults in culture. Lineage‐tracing experiments using a transgenic mouse line, in which an inducible Cre recombinase is knocked in at the Olig2 locus, revealed that Olig2‐lineage cells preferentially differentiated into OPCs and premyelinating OLs, but not into astrocytes, in the Plp ^tg/? mouse brain. These Olig2‐lineage cells matured to express myelin basic protein but after that their processes degenerated in the chronic demyelinating lesions of the Plp ^tg/? brain. These results indicate that in chronic demyelinated lesions more OL‐lineage cells are produced as part of the repair process, but their processes degenerate after maturation. © 2012 Wiley Periodicals, Inc.     

Roles of NG2‐glia in ischemic stroke

Recent studies have shown that a widely distributed class of glial cells, termed NG2‐glia, engages in rapid signaling with surrounding neurons through direct synaptic contacts in the developing and mature central nervous system (CNS). This unique glial cell group has a typical function of proliferating and differentiating into oligodendrocytes during early development of the brain, which is crucial to axon myelin formation. Therefore, NG2‐glia are also called oligodendrocyte precursor cells (OPCs). In vitro and in vivo studies reveal that NG2‐glia expressing receptors and ion channels demonstrate functional significance for rapid signaling with neuronal synapses and modulation of neuronal activities in both physiological and pathological conditions. Although it is well known that NG2‐glia play an important role in demyelinating diseases such as multiple sclerosis, little is known about how NG2‐glia or OPCs impact neurons and brain function following ischemic injury. This review summarizes recent progress on the roles of NG2‐glia in ischemic stroke and illustrates new approaches for targeting NG2‐glia in the brain to treat this disease.     

Extensive regenerative plasticity among adult NG2‐glia populations is exclusively based on self‐renewal

NG2‐glia are known to proliferate in the adult brain, however the extent of their mitotic and regenerative capacity and particularly their adult origin is uncertain. By employing a paradigm of mitotic blockade in conjunction with genetic fate tracing we demonstrate that intracerebroventricular mitotic blocker infusion leads to wide‐spread and complete ablation of NG2‐glial cells in the hypothalamus and other periventricular brain regions. However, despite the extensive glia loss, parenchymal NG2‐glia coverage is fully restored to pretreatment levels within two weeks. We further reveal that in response to mitotic blocker treatment, NG2‐glia bordering the ablated territories start to express the stem cell marker nestin, divide and migrate to replace the lost cells. Importantly, the migration front of repopulating NG2‐glia invariably proceeds from the distal parenchyma towards the ventricles, ruling out contributions of the subventricular zone neurogenic niche or the corresponding area of the third ventricle as source of new NG2‐glia. NG2‐CreER‐based fate tracing further substantiates that NG2‐glia which have been spared from mitotic blockade are the sole source of regenerating NG2‐glia. Collectively, our data reveals that all adult NG2‐glia retain the ability to divide and that they are capable of fully restoring parenchymal NG2‐glia coverage after wide‐spread NG2 cell loss, indicating complete self‐sufficiency in maintaining NG2‐glia population levels in the adult brain. GLIA 2013;61:1735–1747     

Activated microglia in a rat stroke model express NG2 proteoglycan in peri‐infarct tissue through the involvement of TGF‐β1

We investigated activated microglia in ischemic brain lesions from rats that had been subjected to transient middle cerebral artery occlusion. Activated microglia expressing NG2 chondroitin sulfate proteoglycan (NG2) were found only in the narrow zone (demarcation zone) that demarcated the peri‐infarct tissue and ischemic core. NG2^? activated microglia were abundantly distributed in the peri‐infarct tissue outside the demarcation zone. NG2⁺ microglia but not NG2^? microglia expressed both CD68 and a triggering receptor expressed on myeloid cells 2 (TREM‐2), suggesting that NG2⁺ microglia eliminated apoptotic neurons. In fact, NG2⁺ microglia often attached to degenerating neurons and sometimes internalized NeuN⁺ or neurofilament protein⁺ material. Kinetic studies using quantitative real‐time RT‐PCR revealed that expression of transforming growth factor‐β1 (TGF‐β1) was most evident in the ischemic core; with this marker produced mainly by macrophages located in this region. TGF‐β receptor mRNA expression peaked at 3 days post reperfusion (dpr) in the peri‐infarct tissue, including the demarcation zone. Primary cultured rat microglia also expressed the receptor mRNA. In response to TGF‐β1, primary microglia enhanced the expression of NG2 protein and TREM‐2 mRNA as well as migratory activity. A TGF‐β1 inhibitor, SB525334, abolished these effects. The present results suggest that TGF‐β1 produced in the ischemic core diffused toward the peri‐infarct tissue, driving activated microglial cells to eliminate degenerating neurons. Appropriate control of NG2⁺ microglia in the demarcation zone might be a novel target for the suppression of secondary neurodegeneration in the peri‐infarct tissue. GLIA 2014;62:185–198     

An Fgfr3‐iCreERT2 transgenic mouse line for studies of neural stem cells and astrocytes

The lack of markers for astrocytes, particularly gray matter astrocytes, significantly hinders research into their development and physiological properties. We previously reported that fibroblast growth factor receptor 3 (Fgfr3) is expressed by radial precursors in the ventricular zone of the embryonic neural tube and subsequently by differentiated astrocytes in gray and white matter. Here, we describe an Fgfr3‐iCreER^T2 phage artificial chromosome transgenic mouse line that allows efficient tamoxifen‐induced Cre recombination in Fgfr3‐expressing cells, including radial glial cells in the embryonic neural tube and both fibrous and protoplasmic astrocytes in the mature central nervous system. This mouse strain will therefore be useful for studies of normal astrocyte biology and their responses to CNS injury or disease. In addition, Fgfr3‐iCreER^T2 drives Cre recombination in all neurosphere‐forming stem cells in the adult spinal cord and at least 90% of those in the adult forebrain subventricular zone. We made use of this to show that there is continuous accumulation of all major interneuron subtypes in the olfactory bulb (OB) from postnatal day 50 (P50) until at least P230 (∼8 months of age). It therefore seems likely that adult‐born interneurons integrate into existing circuitry and perform long‐term functions in the adult OB. © 2010 Wiley‐Liss, Inc.     

Interaction of NG2+ glial progenitors and microglia/macrophages from the injured spinal cord

Spinal cord contusion produces a central lesion surrounded by a peripheral rim of residual white matter. Despite stimulation of NG2⁺ progenitor cell proliferation, the lesion remains devoid of normal glia chronically after spinal cord injury (SCI). To investigate potential cell–cell interactions of the predominant cells in the lesion at 3 days after injury, we used magnetic activated cell sorting to purify NG2⁺ progenitors and OX42⁺ microglia/macrophages from contused rat spinal cord. Purified NG2⁺ cells from the injured cord grew into spherical masses when cultured in defined medium with FGF2 plus GGF2. The purified OX42⁺ cells did not form spheroids and significantly reduced sphere growth by NG2⁺ cells in co‐cultures. Conditioned medium from these OX42⁺ cells, unlike that from normal peritoneal macrophages or astrocytes also inhibited growth of NG2⁺ cells, suggesting inhibition by secreted factors. Expression analysis of freshly purified OX42⁺ cells for a panel of six genes for secreted factors showed expression of several that could contribute to inhibition of NG2⁺ cells. Further, the pattern of expression of four of these, TNFα, TSP1, TIMP1, MMP9, in sequential coronal tissue segments from a 2 cm length of cord centered on the injury epicenter correlated with the expression of Iba1, a marker gene for OX42⁺ cells, strongly suggesting a potential regional influence by activated microglia/macrophages on NG2⁺ cells in vivo after SCI. Thus, the nonreplacement of lost glial cells in the central lesion zone may involve, at least in part, inhibitory factors produced by microglia/macrophages that are concentrated within the lesion. © 2009 Wiley‐Liss, Inc.     

Differing in vitro survival dependency of mouse and rat NG2+ oligodendroglial progenitor cells

NG2 chondroitin sulfate proteoglycan is a surface marker of oligodendroglial progenitor cells (OPCs) in various species. In contrast to well‐studied rat OPCs, however, we found that purified mouse NG2 surface positive cells (NG2⁺ cells) require additional activation of cyclic AMP (cAMP) signaling for survival in a medium containing 30% B104 neuroblastoma conditioned medium supplemented with fibroblast growth factor‐2 (B104CM+FGF2), whereas B104CM+FGF2 alone is sufficient for survival and selective proliferation of rat OPCs. After induction of in vitro differentiation, more than 90% of mouse NG2⁺ cells became O4‐positive, and a majority expressed myelin basic protein by 5 day of differentiation, which confirmed the identity of isolated mouse NG2⁺ cells as OPCs. In comparison to rat OPCs, mouse OPCs in B104CM+FGF2 were less motile, and demonstrated lower basal phosphorylation levels of ERK1/2 and cAMP response element‐binding protein (CREB) and a higher incidence of apoptosis mediated by the intrinsic pathway. Transient up‐regulation of cAMP‐CREB signaling partially inhibited apoptosis of mouse OPCs independently of the ERK pathway. This study demonstrates a difference in trophic requirements between mouse and rat OPCs, with an essential role for cAMP signaling to preserve viability of mouse OPCs. © 2009 Wiley‐Liss, Inc.     

Deficiency of NG2+ cells contributes to the susceptibility of stroke-prone spontaneously hypertensive rats

Aims: The purpose of this study is to investigate whether the NG2⁺ cells, a class of oligodendrocyte progenitor cells, is involved in the pathophysiology of stroke in stroke‐prone spontaneously hypertensive rat (SHR‐SP). Methods: SHR‐SP, SHR, Wistar‐Kyoto rats (WKY), and C57BJ/6 mice were used. Immunohistochemistry was conducted to evaluate the number of NG2⁺ cells in frozen brain sections. Demyelination was evaluated by Sudan black staining and serum level of myelin basic protein. Middle cerebral artery occlusion (MCAO) was performed to prepare experimental stroke model. Results: The number of NG2⁺ cells was significantly decreased in infarct core and increased in penumbra in WKY rats after MCAO. In brain sections of 6‐month‐old SHR‐SP, the number of NG2⁺ cells was significantly (P < 0.01) less than that in age‐matched SHR and WKY rats. However, this phenomenon was not observed in 3‐month‐old rats. Demyelination was found in 6‐month‐old SHR‐SP but not in 3‐month‐old SHR‐SP. Pharmacological treatment of cuprizone in mice induced demyelination and enlargement of cerebral infarction after MCAO. Conclusion: The decline of NG2⁺ cells may cause demyelination and contribute to the susceptibility of SHR‐SP to ischemic brain injury.     

Early embryonic NG2 glia are exclusively gliogenic and do not generate neurons in the brain

In the central nervous system, the type I transmembrane glycoprotein NG2 (nerve-glia antigen 2) is only expressed by pericytes and oligodendrocyte precursor cells (OPCs). Therefore, OPCs are also termed NG2 glia. Their fate during development has been investigated systematically in several genetically modified mouse models. Consensus exists that postnatal NG2 glia are restricted to the oligodendrocyte (OL) lineage, while, at least in the forebrain, embryonic NG2 glia could also generate astrocytes. In addition, experimental evidence for a neurogenic potential of NG2 glia in the early embryonic brain (before E16.5) has been provided. However, this observation is still controversial. Here, we took advantage of reliable transgene expression in NG2-EYFP and NG2-CreERT2 knock-in mice to study the fate of early embryonic NG2 glia. While pericytes were the main cells with robust NG2 gene activity at E12.5, only a few OPCs expressed NG2 at this early stage of embryogenesis. Subsequently, this proportion of OPCs increased from 3% (E12.5) to 11% and 25% at E14.5 and E17.5, respectively. When Cre DNA recombinase activity was induced at E12.5 and E14.5 and pups were analyzed at postnatal day 0 (P0) and P10, the vast majority of recombined cells, besides pericytes, belonged to the OL lineage cells, with few astrocytes in the ventral forebrain. In other brain regions such as brain stem, cerebellum, and olfactory bulb only OL lineage cells were detected. Therefore, we conclude that NG2 glia from early embryonic brain are restricted to a gliogenic fate and do not differentiate into neurons after birth.     

Spatial organization of NG2 glial cells and astrocytes in rat hippocampal CA1 region

Similar to astrocytes, NG2 glial cells are uniformly distributed in the central nervous system (CNS). However, little is known about the interspatial relationship, nor the functional interactions between these two star‐shaped glial subtypes. Confocal morphometric analysis showed that NG2 immunostained cells are spatially organized as domains in rat hippocampal CA1 region and that each NG2 glial domain occupies a spatial volume of ～178, 364 μm³. The processes of NG2 glia and astrocytes overlap extensively; each NG2 glial domain interlaces with the processes deriving from 5.8 ± 0.4 neighboring astrocytes, while each astrocytic domain accommodates processes stemming from 4.5 ± 0.3 abutting NG2 glia. In CA1 stratum radiatum, the cell bodies of morphologically identified glial cells often appear to make direct somatic‐somata contact, termed as doublets. We used dual patch recording and postrecording NG2/GFAP double staining to determine the glial identities of these doublets. We show that among 44 doublets, 50% were NG2 glia–astrocyte pairs, while another 38.6% and 11.4% were astrocyte–astrocyte and NG2 glia–NG2 glia pairs, respectively. In dual patch recording, neither electrical coupling nor intercellular biocytin transfer was detected in astrocyte–NG2 glia or NG2 glia–NG2 glia doublets. Altogether, although NG2 glia and astrocytes are not gap junction coupled, their cell bodies and processes are interwoven extensively. The anatomical and physiological relationships revealed in this study should facilitate future studies to understand the metabolic coupling and functional communication between NG2 glia and astrocytes. © 2013 Wiley Periodicals, Inc.     

Down‐regulation of microglia and NG2‐positive cells reaction in trimethyltin‐injured hippocampus of rats treated with myelin basic protein‐reactive T cells: Possible contribution to the neuroprotective effect of T cells

In our previous investigations, we demonstrated that CD4⁺ antimyelin basic protein (MBP) T cells protect hippocampal neurons against trimethyltin‐induced damage. We hypothesized involvement of T cells, interacting with the various glial populations activated during the neurodegeneration process. In this study, we employ immunocytochemical methods to investigate the influence of administration of T cells on the response of microglia and of NG2⁺ cells to trimethyltin (TMT)‐induced damage. Female Lewis rats were treated with anti‐MBP CD4⁺ T cells (4 million per animal, i.v) 24 hr after TMT (8 mg/kg, i.p) intoxication. TMT caused degeneration of CA4 hipppocampal neurons and evoked an abundant reaction of microglial and NG2⁺ cells in the injured region. The cells changed morphology into the activated state, and the number of OX42⁺ and NG2⁺ cells increased about 4.5‐fold and 3‐fold, respectively, relative to controls as assessed on day 21 after TMT treatment. Additionally, the cells of ameboid morphology, which expressed NG2 or microglial antigens, appeared in the zone of neurodegeneration. Furthermore, certain cells of ameboid phenotype shared both antigens. In rats treated with T cells, down‐regulation of the activation of both glial classes and reduction of formation of their ameboid forms was observed. The number of the total OX42⁺ and NG2⁺ cells decreased by 21% and 54%, respectively, and the number of their ameboid forms decreased by 46% and 73%, respectively. Our data suggest that the diminished activation of microglia and NG2⁺ cells, particularly the reduced number of their ameboid forms, may contribute to the neuroprotective effect of T cells. © 2009 Wiley‐Liss, Inc.     

Temporal control of gene recombination in astrocytes by transgenic expression of the tamoxifen-inducible DNA recombinase variant CreERT2

Inducible gene modification using the Cre/loxP system provides a valuable tool for the analysis of gene function in the active animal. GFAP-Cre transgenic mice have been developed to achieve gene recombination in astrocytes, the most abundant cells of the central nervous system, with pivotal roles during brain function and pathology. Unfortunately, these mice displayed neuronal recombination as well, since the GFAP promoter is also active in embryonic radial glia, which possess a substantial neurogenic potential. To enable the temporal control of gene deletions in astrocytes only, we generated a transgenic mouse with expression of CreERT2, a fusion protein of the DNA recombinase Cre and a mutated ligand-binding domain of the estrogen receptor, under the control of the human GFAP promoter. In offspring originating from crossbreedings of GFAP-CreERT2-transgenic mice with various Cre-sensitive reporter mice, consecutive intraperitoneal injections of tamoxifen induced genomic recombination selectively in astrocytes of almost all brain regions. In Bergmann glia, which represent the main astroglial cell population of the cerebellum, virtually all cells showed successful gene recombination. When adult mice received cortical stab wound lesions, simultaneously given tamoxifen induced substantial recombination in reactive glia adjacent to the site of injury. These transgenic GFAP-CreERT2 mice will allow the functional analysis of loxP-modified genes in astroglia of the postnatal and adult brain.     

Generation of reactive astrocytes from NG2 cells is regulated by sonic hedgehog

NG2 cells, a fourth glial cell type in the adult mammalian central nervous system, produce oligodendrocytes in the healthy nervous tissue, and display wide differentiation potential under pathological conditions, where they could give rise to reactive astrocytes. The factors that control the differentiation of NG2 cells after focal cerebral ischemia (FCI) are largely unknown. Here, we used transgenic Cspg4‐cre/Esr1/ROSA26Sortm14(CAG‐tdTomato) mice, in which tamoxifen administration triggers the expression of red fluorescent protein (tomato) specifically in NG2 cells and cells derived therefrom. Differentiation potential (in vitro and in vivo) of tomato‐positive NG2 cells from control or postischemic brains was determined using the immunohistochemistry, single cell RT‐qPCR and patch–clamp method. The ischemic injury was induced by middle cerebral artery occlusion, a model of FCI. Using genetic fate‐mapping method, we identified sonic hedgehog (Shh) as an important factor that influences differentiation of NG2 cells into astrocytes in vitro. We also manipulated Shh signaling in the adult mouse brain after FCI. Shh signaling activation significantly increased the number of astrocytes derived from NG2 cells in the glial scar around the ischemic lesion, while Shh signaling inhibition caused the opposite effect. Since Shh signaling modifications did not change the proliferation rate of NG2 cells, we can conclude that Shh has a direct influence on the differentiation of NG2 cells and therefore, on the formation and composition of a glial scar, which consequently affects the degree of the brain damage. GLIA 2016;64:1518–1531     

Decrease in newly generated oligodendrocytes leads to motor dysfunctions and changed myelin structures that can be rescued by transplanted cells

NG2‐glia in the adult brain are known to proliferate and differentiate into mature and myelinating oligodendrocytes throughout lifetime. However, the role of these newly generated oligodendrocytes in the adult brain still remains little understood. Here we took advantage of the Sox10‐iCreER^T2 x CAG‐eGFP x Esco2^fl/fl mouse line in which we can specifically ablate proliferating NG2‐glia in adult animals. Surprisingly, we observed that the generation of new oligodendrocytes in the adult brain was severely affected, although the number of NG2‐glia remained stable due to the enhanced proliferation of non‐recombined cells. This lack of oligodendrogenesis led to the elongation of the nodes of Ranvier as well as the associated paranodes, which could be locally rescued by myelinating oligodendrocytes differentiated from transplanted NG2‐glia deriving from wildtype mice. Repetitive measurements of conduction velocity in the corpus callosum of awake animals revealed a progressive deceleration specifically in the mice lacking adult oligodendrogenesis that resulted in progressive motor deficits. In summary, here we demonstrated for the first time that axon function is not only controlled by the reliable organization of myelin, but also requires a dynamic and continuous generation of new oligodendrocytes in the adult brain. GLIA 2016;64:2201–2218     

Activity‐dependent astrocyte swelling is mediated by pH‐regulating mechanisms

During neuronal activity in the mammalian brain, the K⁺ released into the synaptic space is initially buffered by the astrocytic compartment. In parallel, the extracellular space (ECS) shrinks, presumably due to astrocytic cell swelling. With the Na⁺/K⁺/2Cl^? cotransporter and the Kir4.1/AQP4 complex not required for the astrocytic cell swelling in the hippocampus, the molecular mechanisms underlying the activity‐dependent ECS shrinkage have remained unresolved. To identify these molecular mechanisms, we employed ion‐sensitive microelectrodes to measure changes in ECS, [K⁺]_o and [H⁺]_o/pH_o during electrical stimulation of rat hippocampal slices. Transporters and receptors responding directly to the K⁺ and glutamate released into the extracellular space (the K⁺/Cl^? cotransporter, KCC, glutamate transporters and G protein‐coupled receptors) did not modulate the extracellular space dynamics. The ‐transporting mechanism, which in astrocytes mainly constitutes the electrogenic Na⁺/ cotransporter 1 (NBCe1), is activated by the K⁺‐mediated depolarization of the astrocytic membrane. Inhibition of this transporter reduced the ECS shrinkage by ～25% without affecting the K⁺ transients, pointing to NBCe1 as a key contributor to the stimulus‐induced astrocytic cell swelling. Inhibition of the monocarboxylate cotransporters (MCT), like‐wise, reduced the ECS shrinkage by ～25% without compromising the K⁺ transients. Isosmotic reduction of extracellular Cl^? revealed a requirement for this ion in parts of the ECS shrinkage. Taken together, the stimulus‐evoked astrocytic cell swelling does not appear to occur as a direct effect of the K⁺ clearance, as earlier proposed, but partly via the pH‐regulating transport mechanisms activated by the K⁺‐induced astrocytic depolarization and the activity‐dependent metabolism.     

NG2‐glia crosstalk with microglia in health and disease

Neurodegenerative diseases are increasingly becoming a global problem. However, the pathological mechanisms underlying neurodegenerative diseases are not fully understood. NG2‐glia abnormalities and microglia activation are involved in the development and/or progression of neurodegenerative disorders, such as multiple sclerosis, Alzheimer''s disease, Parkinson''s disease, and cerebrovascular diseases. In this review, we summarize the present understanding of the interaction between NG2‐glia and microglia in physiological and pathological states and discuss unsolved questions concerning their fate and potential fate. First, we introduce the NG2‐glia and microglia in health and disease. Second, we formulate the interaction between NG2‐glia and microglia. NG2‐glia proliferation, migration, differentiation, and apoptosis are influenced by factors released from the microglia. On the other hand, NG2‐glia also regulate microglia actions. We conclude that NG2‐glia and microglia are important immunomodulatory cells in the brain. Understanding the interaction between NG2‐glia and microglia will help provide a novel method to modulate myelination and treat neurodegenerative disorders.