Transient Cnp expression by early progenitors causes Cre-Lox-based reporter lines to map profoundly different fates

Oligodendrocytes produce amyloid-β and contribute to plaque formation alongside neurons in Alzheimer's disease model mice

Amyloid-β (Aβ) is thought to be neuronally derived in Alzheimer's disease (AD). However, transcripts of amyloid precursor protein (APP) and amyloidogenic enzymes are equally abundant in oligodendrocytes (OLs). By cell-type-specific deletion of Bace1 in a humanized knock-in AD model, APPNLGF, we demonstrate that OLs and neurons contribute to Aβ plaque burden. For rapid plaque seeding, excitatory projection neurons must provide a threshold level of Aβ. Ultimately, our findings are relevant for AD prevention and therapeutic strategies.

Downregulated expression of lactate dehydrogenase in adult oligodendrocytes and its implication for the transfer of glycolysis products to axons

Oligodendrocytes and astrocytes are metabolically coupled to neuronal compartments. Pyruvate and lactate can shuttle between glial cells and axons via monocarboxylate transporters. However, lactate can only be synthesized or used in metabolic reactions with the help of lactate dehydrogenase (LDH), a tetramer of LDHA and LDHB subunits in varying compositions. Here we show that mice with a cell type-specific disruption of both Ldha and Ldhb genes in oligodendrocytes lack a pathological phenotype that would be indicative of oligodendroglial dysfunctions or lack of axonal metabolic support. Indeed, when combining immunohistochemical, electron microscopical, and in situ hybridization analyses in adult mice, we found that the vast majority of mature oligodendrocytes lack detectable expression of LDH. Even in neurodegenerative disease models and in mice under metabolic stress LDH was not increased. In contrast, at early development and in the remyelinating brain, LDHA was readily detectable in immature oligodendrocytes. Interestingly, by immunoelectron microscopy LDHA was particularly enriched at gap junctions formed between adjacent astrocytes and at junctions between astrocytes and oligodendrocytes. Our data suggest that oligodendrocytes metabolize lactate during development and remyelination. In contrast, for metabolic support of axons mature oligodendrocytes may export their own glycolysis products as pyruvate rather than lactate. Lacking LDH, these oligodendrocytes can also "funnel" lactate through their "myelinic" channels between gap junction-coupled astrocytes and axons without metabolizing it. We suggest a working model, in which the unequal cellular distribution of LDH in white matter tracts facilitates a rapid and efficient transport of glycolysis products among glial and axonal compartments.

Antagonistic actions of PAK1 and NF2/Merlin drive myelin membrane expansion in oligodendrocytes

In the central nervous system, the formation of myelin by oligodendrocytes (OLs) relies on the switch from the polymerization of the actin cytoskeleton to its depolymerization. The molecular mechanisms that trigger this switch have yet to be elucidated. Here, we identified P21-activated kinase 1 (PAK1) as a major regulator of actin depolymerization in OLs. Our results demonstrate that PAK1 accumulates in OLs in a kinase-inhibited form, triggering actin disassembly and, consequently, myelin membrane expansion. Remarkably, proteomic analysis of PAK1 binding partners enabled the identification of NF2/Merlin as its endogenous inhibitor. Our findings indicate that Nf2 knockdown in OLs results in PAK1 activation, actin polymerization, and a reduction in OL myelin membrane expansion. This effect is rescued by treatment with a PAK1 inhibitor. We also provide evidence that the specific Pak1 loss-of-function in oligodendroglia stimulates the thickening of myelin sheaths in vivo. Overall, our data indicate that the antagonistic actions of PAK1 and NF2/Merlin on the actin cytoskeleton of the OLs are critical for proper myelin formation. These findings have broad mechanistic and therapeutic implications in demyelinating diseases and neurodevelopmental disorders.

Perinatal loss of galactosylceramidase in both oligodendrocytes and microglia is crucial for the pathogenesis of Krabbe disease in mice

Lysosomal galactosylceramidase (GALC) is expressed in all brain cells, including oligodendrocytes (OLs), microglia, and astrocytes, although the cell-specific function of GALC is largely unknown. Mutations in GALC cause Krabbe disease (KD), a fatal neurological lysosomal disorder that usually affects infants. To study how Galc ablation in each glial cell type contributes to Krabbe pathogenesis, we used conditional Galc-floxed mice. Here, we found that OL-specific Galc conditional knockout (CKO) in mice results in a phenotype that includes wasting, psychosine accumulation, and neuroinflammation. Microglia- or astrocyte-specific Galc deletion alone in mice did not show specific phenotypes. Interestingly, mice with CKO of Galc from both OLs and microglia have a more severe neuroinflammation with an increase in globoid cell accumulation than OL-specific CKO alone. Moreover, the enhanced phenotype occurred without additional accumulation of psychosine. Further studies revealed that Galc knockout (Galc-KO) microglia cocultured with Galc-KO OLs elicits globoid cell formation and the overexpression of osteopontin and monocyte chemoattractant protein-1, both proteins that are known to recruit immune cells and promote engulfment of debris and damaged cells. We conclude that OLs are the primary cells that initiate KD with an elevated psychosine level and microglia are required for the progression of neuroinflammation in a psychosine-independent manner.

The role of TSC1 and TSC2 proteins in neuronal axons

Tuberous Sclerosis Complex 1 and 2 proteins, TSC1 and TSC2 respectively, participate in a multiprotein complex with a crucial role for the proper development and function of the nervous system. This complex primarily acts as an inhibitor of the mechanistic target of rapamycin (mTOR) kinase, and mutations in either TSC1 or TSC2 cause a neurodevelopmental disorder called Tuberous Sclerosis Complex (TSC). Neurological manifestations of TSC include brain lesions, epilepsy, autism, and intellectual disability. On the cellular level, the TSC/mTOR signaling axis regulates multiple anabolic and catabolic processes, but it is not clear how these processes contribute to specific neurologic phenotypes. Hence, several studies have aimed to elucidate the role of this signaling pathway in neurons. Of particular interest are axons, as axonal defects are associated with severe neurocognitive impairments. Here, we review findings regarding the role of the TSC1/2 protein complex in axons. Specifically, we will discuss how TSC1/2 canonical and non-canonical functions contribute to the formation and integrity of axonal structure and function.

Oligodendrocyte calcium signaling promotes actin-dependent myelin sheath extension

Myelin is essential for rapid nerve signaling and is increasingly found to play important roles in learning and in diverse diseases of the CNS. Morphological parameters of myelin such as sheath length are thought to precisely tune conduction velocity, but the mechanisms controlling sheath morphology are poorly understood. Local calcium signaling has been observed in nascent myelin sheaths and can be modulated by neuronal activity. However, the role of calcium signaling in sheath formation remains incompletely understood. Here, we use genetic tools to attenuate oligodendrocyte calcium signaling during myelination in the developing mouse CNS. Surprisingly, genetic calcium attenuation does not grossly affect the number of myelinated axons or myelin thickness. Instead, calcium attenuation causes myelination defects resulting in shorter, dysmorphic sheaths. Mechanistically, calcium attenuation reduces actin filaments in oligodendrocytes, and an intact actin cytoskeleton is necessary and sufficient to achieve accurate myelin morphology. Together, our work reveals a cellular mechanism required for accurate CNS myelin formation and may provide mechanistic insight into how oligodendrocytes respond to neuronal activity to sculpt and refine myelin sheaths.

Autophagy collaborates with apoptosis pathways to control oligodendrocyte number

Oligodendrocytes are the sole myelin-producing cells in the central nervous system. Oligodendrocyte number is tightly controlled across diverse brain regions to match local axon type and number, yet the underlying mechanisms remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under physiological conditions, elicits premyelinating oligodendrocyte apoptosis during development. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy functions cell autonomously in the premyelinating oligodendrocyte to trigger cell apoptosis, and it genetically interacts with the TFEB pathway to limit oligodendrocyte number across diverse brain regions. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrogenesis.

Autophagy collaborates with apoptosis pathways to control myelination specificity and function

Oligodendrocytes are the sole myelin producing cells in the central nervous system. Oligodendrocyte numbers are tightly controlled across diverse brain regions to match local axon type and number, but the underlying mechanisms and functional significance remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under canonical settings, elicits premyelinating oligodendrocyte apoptosis during development and regulates critical aspects of nerve pulse propagation. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy acts in the TFEB-Bax/Bak pathway and elevates PUMA mRNA levels to trigger premyelinating oligodendrocyte apoptosis cell-autonomously. Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath numbers and fine-tune nerve pulse propagation. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrocyte number.

HIGHLIGHTS

Autophagy flux increases in the premyelinating and myelinating oligodendrocytesAutophagy promotes premyelinating oligodendrocyte (pre-OL) apoptosis to control myelination location and timing Autophagy acts in the TFEB-PUMA-Bax/Bak pathway and elevates PUMA mRNA levels to determine pre-OL fate Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath thickness and finetune nerve pulse propagation.

Ral GTPases are critical regulators of spinal cord myelination and homeostasis

Efficient myelination supports nerve conduction and axonal health throughout life. In the central nervous system, oligodendrocytes (OLs) carry out this demanding anabolic duty in part through biosynthetic pathways controlled by mTOR. We identify Ral GTPases as critical regulators of mouse spinal cord myelination and myelin maintenance. Ablation of Ral GTPases (RalA, RalB) in OL-lineage cells impairs timely onset and radial growth of developmental myelination, accompanied by increased endosomal/lysosomal abundance. Further examinations, including transcriptomic analyses of Ral-deficient OLs, were consistent with mTORC1-related deficits. However, deletion of the mTOR signaling-repressor Pten in Ral-deficient OL-lineage cells is unable to rescue mTORC1 activation or developmental myelination deficiencies. Induced deletion of Ral GTPases in OLs of adult mice results in late-onset myelination defects and tissue degeneration. Together, our data indicate critical roles for Ral GTPases to promote developmental spinal cord myelination, to ensure accurate mTORC1 signaling, and to protect the healthy state of myelin-axon units over time.

CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes

Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

Stage-specific control of oligodendrocyte survival and morphogenesis by TDP-43

Generation of oligodendrocytes in the adult brain enables both adaptive changes in neural circuits and regeneration of myelin sheaths destroyed by injury, disease, and normal aging. This transformation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes requires processing of distinct mRNAs at different stages of cell maturation. Although mislocalization and aggregation of the RNA-binding protein, TDP-43, occur in both neurons and glia in neurodegenerative diseases, the consequences of TDP-43 loss within different stages of the oligodendrocyte lineage are not well understood. By performing stage-specific genetic inactivation of Tardbp in vivo, we show that oligodendrocyte lineage cells are differentially sensitive to loss of TDP-43. While OPCs depend on TDP-43 for survival, with conditional deletion resulting in cascading cell loss followed by rapid regeneration to restore their density, oligodendrocytes become less sensitive to TDP-43 depletion as they mature. Deletion of TDP-43 early in the maturation process led to eventual oligodendrocyte degeneration, seizures, and premature lethality, while oligodendrocytes that experienced late deletion survived and mice exhibited a normal lifespan. At both stages, TDP-43-deficient oligodendrocytes formed fewer and thinner myelin sheaths and extended new processes that inappropriately wrapped neuronal somata and blood vessels. Transcriptional analysis revealed that in the absence of TDP-43, key proteins involved in oligodendrocyte maturation and myelination were misspliced, leading to aberrant incorporation of cryptic exons. Inducible deletion of TDP-43 from oligodendrocytes in the adult central nervous system (CNS) induced the same progressive morphological changes and mice acquired profound hindlimb weakness, suggesting that loss of TDP-43 function in oligodendrocytes may contribute to neuronal dysfunction in neurodegenerative disease.

Novel Toolboxes for the Investigation of Activity-Dependent Myelination in the Central Nervous System

Myelination is essential for signal processing within neural networks. Emerging data suggest that neuronal activity positively instructs myelin development and myelin adaptation during adulthood. However, the underlying mechanisms controlling activity-dependent myelination have not been fully elucidated. Myelination is a multi-step process that involves the proliferation and differentiation of oligodendrocyte precursor cells followed by the initial contact and ensheathment of axons by mature oligodendrocytes. Conventional end-point studies rarely capture the dynamic interaction between neurons and oligodendrocyte lineage cells spanning such a long temporal window. Given that such interactions and downstream signaling cascades are likely to occur within fine cellular processes of oligodendrocytes and their precursor cells, overcoming spatial resolution limitations represents another technical hurdle in the field. In this mini-review, we discuss how advanced genetic, cutting-edge imaging, and electrophysiological approaches enable us to investigate neuron-oligodendrocyte lineage cell interaction and myelination with both temporal and spatial precision.

Generation of Multipotential NG2 Progenitors From Mouse Embryonic Stem Cell-Derived Neural Stem Cells

Embryonic stem cells (ESC) have the potential to generate homogeneous immature cells like stem/progenitor cells, which appear to be difficult to isolate and expand from primary tissue samples. In this study, we developed a simple method to generate homogeneous immature oligodendrocyte (OL) lineage cells from mouse ESC-derived neural stem cell (NSC). NSC converted to NG2+/OLIG2+double positive progenitors (NOP) after culturing in serum-free media for a week. NOP expressed Prox1, but not Gpr17 gene, highlighting their immature phenotype. Interestingly, FACS analysis revealed that NOP expressed proteins for NG2, but not PDGFRɑ, distinguishing them from primary OL progenitor cells (OPC). Nevertheless, NOP expressed various OL lineage marker genes including Cspg4, Pdgfrα, Olig1/2, and Sox9/10, but not Plp1 genes, and, when cultured in OL differentiation conditions, initiated transcription of Gpr17 and Plp1 genes, and expression of PDGFRα proteins, implying that NOP converted into a matured OPC phenotype. Unexpectedly, NOP remained multipotential, being able to differentiate into neurons as well as astrocytes under appropriate conditions. Moreover, NOP-derived OPC myelinated axons with a lower efficiency when compared with primary OPC. Taken together, these data demonstrate that NOP are an intermediate progenitor cell distinguishable from both NSC and primary OPC. Based on this profile, NOP may be useful for modeling mechanisms influencing the earliest stages of oligogenesis, and exploring the cellular and molecular responses of the earliest OL progenitors to conditions that impair myelination in the developing nervous system.

Mek/ERK1/2-MAPK and PI3K/Akt/mTOR signaling plays both independent and cooperative roles in Schwann cell differentiation, myelination and dysmyelination

Multiple signals are involved in the regulation of developmental myelination by Schwann cells and in the maintenance of a normal myelin homeostasis throughout adult life, preserving the integrity of the axons in the PNS. Recent studies suggest that Mek/ERK1/2-MAPK and PI3K/Akt/mTOR intracellular signaling pathways play important, often overlapping roles in the regulation of myelination in the PNS. In addition, hyperactivation of these signaling pathways in Schwann cells leads to a late onset of various pathological changes in the sciatic nerves. However, it remains poorly understood whether these pathways function independently or sequentially or converge using a common mechanism to facilitate Schwann cell differentiation and myelin growth during development and in causing pathological changes in the adult animals. To address these questions, we analyzed multiple genetically modified mice using simultaneous loss- and constitutive gain-of-function approaches. We found that during development, the Mek/ERK1/2-MAPK pathway plays a primary role in Schwann cell differentiation, distinct from mTOR. However, during active myelination, ERK1/2 is dependent on mTOR signaling to drive the growth of the myelin sheath and regulate its thickness. Finally, our data suggest that peripheral nerve pathology during adulthood caused by hyperactivation of Mek/ERK1/2-MAPK or PI3K is likely to be independent or dependent on mTOR-signaling in different contexts. Thus, this study highlights the complexities in the roles played by two major intracellular signaling pathways in Schwann cells that affect their differentiation, myelination, and later PNS pathology and predicts that potential therapeutic modulation of these pathways in PNS neuropathies could be a complex process.

Potential neuron-autonomous Purkinje cell degeneration by 2',3'-cyclic nucleotide 3'-phosphodiesterase promoter/Cre-mediated autophagy impairments

Studies of neuroglial interaction largely depend on cell-specific gene knockout (KO) experiments using Cre recombinase. However, genes known as glial-specific genes have recently been reported to be expressed in neuroglial stem cells, leading to the possibility that a glia-specific Cre driver results in unwanted gene deletion in neurons, which may affect sound interpretation. 2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) is generally considered to be an oligodendrocyte (OL) marker. Accordingly, Cnp promoter-controlled Cre recombinase has been used to create OL-specific gene targeting mice. However, in this study, using Rosa26-tdTomato-reporter/Cnp-Cre mice, we found that many forebrain neurons and cerebellar Purkinje neurons belong to the lineages of Cnp-expressing neuroglial stem cells. To answer whether gene targeting by Cnp-Cre can induce neuron-autonomous defects, we conditionally deleted an essential autophagy gene, Atg7, in Cnp-Cre mice. The Cnp-Cre-mediated Atg7 KO mice showed extensive p62 inclusion in neurons, including cerebellar Purkinje neurons with extensive neurodegeneration. Furthermore, neuronal areas showing p62 inclusion in Cnp-Cre-mediated Atg7 KO mice overlapped with the neuronal lineage of Cnp-expressing neuroglial stem cells. Moreover, Cnp-Cre-mediated Atg7-KO mice did not develop critical defects in myelination. Our results demonstrate that a large population of central neurons are derived from Cnp-expressing neuroglial stem cells; thus, conditional gene targeting using the Cnp promoter, which is known to be OL-specific, can induce neuron-autonomous phenotypes.

Progenies of NG2 glia: what do we learn from transgenic mouse models ?

In the mammalian central nervous system, nerve-glia antigen 2 (NG2) glia are considered the fourth glial population in addition to astrocytes, oligodendrocytes and microglia. The fate of NG2 glia in vivo has been carefully studied in several transgenic mouse models using the Cre/loxP strategy. There is a clear agreement that NG2 glia mainly serve as progenitors for oligodendrocytes and a subpopulation of astrocytes mainly in the ventral forebrain, whereas the existence of a neurogenic potential of NG2 glia is lack of adequate evidence. This mini review summarizes the findings from recent studies regarding the fate of NG2 glia during development. We will highlight the age-and-region-dependent heterogeneity of the NG2 glia differentiation potential. We will also discuss putative reasons for inconsistent findings in various transgenic mouse lines of previous studies.

Unraveling the adult cell progeny of early postnatal progenitor cells

NG2-glia, also referred to as oligodendrocyte precursor cells or polydendrocytes, represent a large pool of proliferative neural cells in the adult brain that lie outside of the two major adult neurogenic niches. Although their roles are not fully understood, we previously reported significant clonal expansion of adult NG2-cells from embryonic pallial progenitors using the StarTrack lineage-tracing tool. To define the contribution of early postnatal progenitors to the specific NG2-glia lineage, we used NG2-StarTrack. A temporal clonal analysis of single postnatal progenitor cells revealed the production of different glial cell types in distinct areas of the dorsal cortex but not neurons. Moreover, the dispersion and size of the different NG2 derived clonal cell clusters increased with age. Indeed, clonally-related NG2-glia were located throughout the corpus callosum and the deeper layers of the cortex. In summary, our data reveal that postnatally derived NG2-glia are proliferative cells that give rise to NG2-cells and astrocytes but not neurons. These progenitors undergo clonal cell expansion and dispersion throughout the adult dorsal cortex in a manner that was related to aging and cell identity, adding new information about the ontogeny of these cells. Thus, identification of clonally-related cells from specific progenitors is important to reveal the NG2-glia heterogeneity.

Cell Fate Potential of NG2 Progenitors

Determining the origin of different glial subtypes is crucial to understand glial heterogeneity, and to enhance our knowledge of glial and progenitor cell behavior in embryos and adults. NG2-glia are homogenously distributed in a grid-like manner in both, gray and white matter of the adult brain. While some NG2-glia in the CNS are responsible for the generation of mature oligodendrocytes (OPCs), most of them do not differentiate and they can proliferate outside of adult neurogenic niches. Thus, NG2-glia constitute a heterogeneous population containing different subpopulations with distinct functions. We hypothesized that their diversity emerges from specific progenitors during development, as occurs with other glial cell subtypes. To specifically target NG2-pallial progenitors and to define the NG2-glia lineage, as well as the NG2-progenitor potential, we designed two new StarTrack strategies using the NG2 promoter. These approaches label NG2 expressing progenitor cells, permitting the cell fates of these NG2 progenitors to be tracked in vivo. StarTrack labelled cells producing different neural phenotypes in different regions depending on the age targeted, and the strategy selected. This specific genetic targeting of neural progenitors in vivo has provided new data on the heterogeneous pool of NG2 progenitors at both embryonic and postnatal ages.

Glial type specific regulation of CNS angiogenesis by HIFα-activated different signaling pathways

The mechanisms by which oligodendroglia modulate CNS angiogenesis remain elusive. Previous in vitro data suggest that oligodendroglia regulate CNS endothelial cell proliferation and blood vessel formation through hypoxia inducible factor alpha (HIFα)-activated Wnt (but not VEGF) signaling. Using in vivo genetic models, we show that HIFα in oligodendroglia is necessary and sufficient for angiogenesis independent of CNS regions. At the molecular level, HIFα stabilization in oligodendroglia does not perturb Wnt signaling but rather activates VEGF. At the functional level, genetically blocking oligodendroglia-derived VEGF but not Wnt significantly decreases oligodendroglial HIFα-regulated CNS angiogenesis. Blocking astroglia-derived Wnt signaling reduces astroglial HIFα-regulated CNS angiogenesis. Together, our in vivo data demonstrate that oligodendroglial HIFα regulates CNS angiogenesis through Wnt-independent and VEGF-dependent signaling. These findings suggest an alternative mechanistic understanding of CNS angiogenesis by postnatal glial cells and unveil a glial cell type-dependent HIFα-Wnt axis in regulating CNS vessel formation.

In the central nervous system, the maturation of glial cells is temporally and functionally coupled with that of the vascular network during postnatal development. Here the authors show that oligodendroglial HIFα regulates CNS angiogenesis through Wnt-independent and VEGF-dependent signaling, while astroglial HIFα participates through Wnt-dependent signaling.

Developmental stage-specific role of Frs adapters as mediators of FGF receptor signaling in the oligodendrocyte lineage cells

FGF signaling is important for numerous cellular processes and produces diverse cellular responses. Our recent studies using mice conditionally lacking FGF-Receptor-1 (Fgfr1) or Fgfr2 during different stages of myelinogenesis revealed that Fgfr signaling is first required embryonically for the specification of oligodendrocyte progenitors (OPCs) and then later postnatally for the growth of the myelin sheath during active myelination but not for OPC proliferation, differentiation, or ensheathment of axons. What intracellular signal transduction pathways are recruited immediately downstream of Fgfrs and mediate these distinct developmentally regulated stage-specific responses remain unclear. The adapter protein Fibroblast-Growth-Factor-Receptor-Substrate-2 (Frs2) is considered a key immediate downstream target of Fgfrs. Therefore, here, we investigated the in vivo role of Frs adapters in the oligodendrocyte lineage cells, using a novel genetic approach where mice were engineered to disrupt binding of Frs2 to Fgfr1 or Fgfr2, thus specifically uncoupling Frs2 and Fgfr signaling. In addition, we used conditional mutants with complete ablation of Frs2 and Frs3. We found that Frs2 is required for specification of OPCs in the embryonic telencephalon downstream of Fgfr1. In contrast, Frs2 is largely dispensable for transducing Fgfr2-mediated signals for the growth of the myelin sheath during postnatal myelination, implying the potential involvement of other adapters downstream of Fgfr2 for this function. Together, our data demonstrate a developmental stage-specific function of Frs2 in the oligodendrocyte lineage cells. This contextual requirement of adapter proteins, downstream of Fgfrs, could partly explain the distinct responses elicited by the activation of Fgfrs during different stages of myelinogenesis.

The Chromatin Environment Around Interneuron Genes in Oligodendrocyte Precursor Cells and Their Potential for Interneuron Reprograming

Oligodendrocyte precursor cells (OPCs), also known as NG2 glia, arise from neural progenitor cells in the embryonic ganglionic eminences that also generate inhibitory neurons. They are ubiquitously distributed in the central nervous system, remain proliferative through life, and generate oligodendrocytes in both gray and white matter. OPCs exhibit some lineage plasticity, and attempts have been made to reprogram them into neurons, with varying degrees of success. However, little is known about how epigenetic mechanisms affect the ability of OPCs to undergo fate switch and whether OPCs have a unique chromatin environment around neuronal genes that might contribute to their lineage plasticity. Our bioinformatic analysis of histone posttranslational modifications at interneuron genes in OPCs revealed that OPCs had significantly fewer bivalent and repressive histone marks at interneuron genes compared to astrocytes or fibroblasts. Conversely, OPCs had a greater degree of deposition of active histone modifications at bivalently marked interneuron genes than other cell types, and this was correlated with higher expression levels of these genes in OPCs. Furthermore, a significantly higher proportion of interneuron genes in OPCs than in other cell types lacked the histone posttranslational modifications examined. These genes had a moderately high level of expression, suggesting that the "no mark" interneuron genes could be in a transcriptionally "poised" or "transitional" state. Thus, our findings suggest that OPCs have a unique histone code at their interneuron genes that may obviate the need for erasure of repressive marks during their fate switch to inhibitory neurons.

Oligodendrocytes regulate presynaptic properties and neurotransmission through BDNF signaling in the mouse brainstem

Neuron–glia communication contributes to the fine-tuning of synaptic functions. Oligodendrocytes near synapses detect and respond to neuronal activity, but their role in synapse development and plasticity remains largely unexplored. We show that oligodendrocytes modulate neurotransmitter release at presynaptic terminals through secretion of brain-derived neurotrophic factor (BDNF). Oligodendrocyte-derived BDNF functions via presynaptic tropomyosin receptor kinase B (TrkB) to ensure fast, reliable neurotransmitter release and auditory transmission in the developing brain. In auditory brainstem slices from Bdnf^+/– mice, reduction in endogenous BDNF significantly decreased vesicular glutamate release by reducing the readily releasable pool of glutamate vesicles, without altering presynaptic Ca²⁺ channel activation or release probability. Using conditional knockout mice, cell-specific ablation of BDNF in oligodendrocytes largely recapitulated this effect, which was recovered by BDNF or TrkB agonist application. This study highlights a novel function for oligodendrocytes in synaptic transmission and their potential role in the activity-dependent refinement of presynaptic properties.

Conditional Mutagenesis in Oligodendrocyte Lineage Cells

Cell-type-specific gene targeting with the Cre/loxP system has become an indispensable technique in experimental neuroscience, particularly for the study of late-born glial cells that make myelin. A plethora of conditional mutants and Cre-expressing mouse lines is now available to the research community that allows laboratories to readily engage in in vivo analyses of oligodendrocytes and their precursor cells. This chapter summarizes concepts and strategies in targeting myelinating glial cells in mice for mutagenesis or imaging, and provides an overview of the most important Cre driver lines successfully used in this rapidly growing field.

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.

Cell-autonomous requirement of TDP-43, an ALS/FTD signature protein, for oligodendrocyte survival and myelination

TDP-43 aggregates in neurons and glia are the defining pathological hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), raising the possibility of glial damage in the disease pathogenesis. However, the normal physiological functions of TDP-43 in glia are largely unknown. To address how TDP-43 may be required for oligodendroglial functions we selectively deleted TDP-43 in mature oligodendrocytes in mice. Although mice with TDP-43 deleted in oligodendrocytes are born in an expected Mendelian ratio, they develop progressive neurological phenotypes leading to early lethality accompanied by a progressive reduction in myelination. The progressive myelin reduction is likely due to a combination of the cell-autonomous RIPK1-mediated necroptosis of mature oligodendrocytes and the TDP-43-dependent reduction in the expression of myelin genes. Strikingly, enhanced proliferation of NG2-positive oligodendrocyte precursor cells within the white matter, but not the gray matter, was able to replenish the loss of mature oligodendrocytes, indicating an intrinsic regeneration difference between the gray and white matter oligodendrocytes. By contrast, there was no loss of spinal cord motor neurons and no sign of denervation at the neuromuscular synapses. Taken together, our data demonstrate that TDP-43 is indispensable for oligodendrocyte survival and myelination, and loss of TDP-43 in oligodendrocytes exerts no apparent toxicity on motor neurons.

PTEN negatively regulates the cell lineage progression from NG2+ glial progenitor to oligodendrocyte via mTOR-independent signaling

Oligodendrocytes (OLs), the myelin-forming CNS glia, are highly vulnerable to cellular stresses, and a severe myelin loss underlies numerous CNS disorders. Expedited OL regeneration may prevent further axonal damage and facilitate functional CNS repair. Although adult OL progenitors (OPCs) are the primary players for OL regeneration, targetable OPC-specific intracellular signaling mechanisms for facilitated OL regeneration remain elusive. Here, we report that OPC-targeted PTEN inactivation in the mouse, in contrast to OL-specific manipulations, markedly promotes OL differentiation and regeneration in the mature CNS. Unexpectedly, an additional deletion of mTOR did not reverse the enhanced OL development from PTEN-deficient OPCs. Instead, ablation of GSK3β, another downstream signaling molecule that is negatively regulated by PTEN-Akt, enhanced OL development. Our results suggest that PTEN persistently suppresses OL development in an mTOR-independent manner, and at least in part, via controlling GSK3β activity. OPC-targeted PTEN-GSK3β inactivation may benefit facilitated OL regeneration and myelin repair.

Effect of glial cells on remyelination after spinal cord injury

Remyelination plays a key role in functional recovery of axons after spinal cord injury. Glial cells are the most abundant cells in the central nervous system. When spinal cord injury occurs, many glial cells at the lesion site are immediately activated, and different cells differentially affect inflammatory reactions after injury. In this review, we aim to discuss the core role of oligodendrocyte precursor cells and crosstalk with the rest of glia and their subcategories in the remyelination process. Activated astrocytes influence proliferation, differentiation, and maturation of oligodendrocyte precursor cells, while activated microglia alter remyelination by regulating the inflammatory reaction after spinal cord injury. Understanding the interaction between oligodendrocyte precursor cells and the rest of glia is necessary when designing a therapeutic plan of remyelination after spinal cord injury.

Acetyl-CoA production from pyruvate is not necessary for preservation of myelin

Oligodendrocytes and Schwann cells not only form myelin in the central and peripheral nervous system, but also provide metabolic and trophic support to the axons they ensheathe. Acetyl-CoA is potentially a key molecule in Schwann cells and oligodendrocytes because it is at the crossroads of cellular lipid biosynthesis and energy generation. The main route for acetyl-CoA production is the oxidation of pyruvate by the pyruvate dehydrogenase complex (PDC). PDC deficiency in humans results in neurodegeneration and developmental impairments in both white and gray matter structures. To address the importance of PDC in myelinating glia, we deleted Pdha1 gene specifically in oligodendrocytes and Schwann cells. Surprisingly, sciatic and optic nerve morphology and the motor performance of Pdha1^f/Y; Cnp^Cre/+ mice are undistinguishable from those of controls at 1 month of age. In addition, myelin is stably maintained for at least 10 months. However, Pdha1^f/Y; Cnp^Cre/+ mice showed reduced fiber density and signs of axonal degeneration in both sciatic and optic nerves from 6 months of age. In contrast, 10 month-old mice bearing a floxed Pdha1 gene with either P0-Cre (expressed only by Schwann cells) or NG2-Cre^ER (expressed in oligodendrocyte precursor cells) do not show any sign of axonal pathology or alterations in myelin structure or thickness. This indicates that the axonopathy is specific to the Pdha1^f/Y; Cnp^Cre/+ mice. Taken together, these results suggest that acetyl-CoA derived from pyruvate is not necessary for myelin maintenance and, thus, myelin-forming cells are not likely to contribute to the pathophysiology of PDC deficiency.

Oligodendroglial myelination requires astrocyte-derived lipids

In the vertebrate nervous system, myelination of axons for rapid impulse propagation requires the synthesis of large amounts of lipids and proteins by oligodendrocytes and Schwann cells. Myelin membranes are thought to be cell-autonomously assembled by these axon-associated glial cells. Here, we report the surprising finding that in normal brain development, a substantial fraction of the lipids incorporated into central nervous system (CNS) myelin are contributed by astrocytes. The oligodendrocyte-specific inactivation of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP), an essential coactivator of the transcription factor SREBP and thus of lipid biosynthesis, resulted in significantly retarded CNS myelination; however, myelin appeared normal at 3 months of age. Importantly, embryonic deletion of the same gene in astrocytes, or in astrocytes and oligodendrocytes, caused a persistent hypomyelination, as did deletion from astrocytes during postnatal development. Moreover, when astroglial lipid synthesis was inhibited, oligodendrocytes began incorporating circulating lipids into myelin membranes. Indeed, a lipid-enriched diet was sufficient to rescue hypomyelination in these conditional mouse mutants. We conclude that lipid synthesis by oligodendrocytes is heavily supplemented by astrocytes in vivo and that horizontal lipid flux is a major feature of normal brain development and myelination.

Oligodendrocyte precursor cells in the mouse optic nerve originate in the preoptic area

The present study aims to examine the origin of oligodendrocyte progenitor cells (OPCs) in the mouse optic nerve (ON) by labeling OPCs in the fetal forebrain. The labeling of OPCs in the ON was performed by injection of a retrovirus vector carrying the lacZ gene into the lateral ventricle, or by inducible Cre/loxP of Olig2-positive cells. The retrovirus labeling revealed that ventricular zone-derived cells of the fetal forebrain relocated to the ON and differentiated into oligodendrocytes. In addition, lineage tracing of Olig2-positive cells and whole-mount staining of PDGFRα-positive cells demonstrated that OPCs appeared by E12.5 in the preoptic area, and spread caudally to enter the ON. Our results also suggest that OPCs generated during the early stage are depleted from the ON after maturation.