Emerging Mitochondrial-Mediated Mechanisms Involved in Oligodendrocyte Development

Abstract

Oligodendrocytes are the myelinating glia of the central nervous system and are generated after oligodendrocyte progenitor cells (OPCs) transition into pre-oligodendrocytes and then into myelinating oligodendrocytes. Myelin is essential for proper signal transmission within the nervous system and axonal metabolic support. Although the intrinsic and extrinsic factors that support the differentiation, survival, integration, and subsequent myelination of appropriate axons have been well investigated, little is known about how mitochondria-related pathways such as mitochondrial dynamics, bioenergetics, and apoptosis finely-tune these developmental events. Previous findings suggest that changes to mitochondrial morphology act as an upstream regulatory mechanism of neural stem cell (NSC) fate decisions. Whether a similar mechanism is engaged during OPC differentiation has yet to be elucidated. Maintenance of mitochondrial dynamics is vital for regulating cellular bioenergetics, functional mitochondrial networks, and the ability of cells to distribute mitochondria to subcellular locations, such as the growing processes of oligodendrocytes. Myelination is an energy-consuming event, thus, understanding the interplay between mitochondrial dynamics, metabolism, and apoptosis will provide further insight into mechanisms that mediate oligodendrocyte development in healthy and disease states. Here we will provide a concise overview of oligodendrocyte development and discuss the potential contribution of mitochondrial mitochondrial-mediated mechanisms to oligodendrocyte bioenergetics and development.

Introduction

In the central nervous system, oligodendrocytes produce the insulating myelin sheath of neuronal axons. A single oligodendrocyte has the capacity to myelinate several axonal segments^1,2. Myelin is composed of a proteolipid-rich membrane that wraps around axons to provide electrical insulation and metabolic support^3,4. Segments of unmyelinated regions along myelination axons are created to allow for clustering of voltage-gated sodium channels^5,6. These unmyelinated segments are known as nodes of Ranvier, which host the molecular machinery responsible for fast action potential propagation over long distances through a process called saltatory conduction^5,7–11. The brain has a high energy requirement to account for action potential propagation and synaptic transmission. A functional mitochondrial network, in both neurons and glia, is essential for maintaining this metabolic demand^12,13. The production of myelinating oligodendrocytes requires a series of finely and highly coordinated mitochondrial signaling events. For example, during critical periods of differentiation, oligodendrocyte lineage cells undergo apoptosis if they do not have access to the appropriate survival signals or if they are not able to integrate¹⁴. Integration of healthy oligodendrocytes into the neural circuit is key for meeting the energy demands of the brain. This review will focus on mitochondrial-mediated mechanisms related to morphology, dynamics, apoptosis, and bioenergetics and their role in modulating oligodendrocyte differentiation and survival.

The developmental origin of oligodendrocytes

Neural tube patterning.

The foundation of a functional nervous system begins with a regulated process that produces a highly organized temporal and spatial order known as neural tube patterning^15,16. Specific genetic networks establish regionally-restricted progenitor domains along the anterior–posterior and dorsal–ventral axes of the neural tube^17–19. These domains contain transcription factors that synchronize the organization and fate of progenitor cells, and consequently, give rise to neuronal and glial subtype specification^17,20. Activation or repression of the transcription factors is dependent on their location along the dorsal-ventral axis and the activity of sonic hedgehog (Shh) (ventral floor plate), bone morphogenetic protein (BMP) (dorsal roof plate), and WNT (dorsal roof plate) signals^15,20–24. Following the induction of progenitor cells, they begin to expand at different rates to form the specialized parts of the nervous system such as the forebrain, midbrain, hindbrain, and spinal cord. The principles of neural tube patterning have been described extensively^15,17,20,25. However, the contribution of metabolic gradients and mitochondrial signals underlying these events remain unexplored.

The cellular environment during neural tube closure is hypoxic as angiogenesis and placental development has yet to finalize^26–28. Thus, ATP is mainly generated via glycolysis, which is responsible for synthesizing metabolic intermediates such as nucleotides, amino acids, and lipids needed for proliferation and neural development^29–31. Early forebrain development following neural tube closure is accompanied by expansion of endoplasmic reticulum, Golgi, and mitochondria^32,33. This modification in the mitochondrial landscape and morphology is needed for the metabolic shift to oxidative phosphorylation (OXPHOS) (discussed in a later section).

Oligodendrocyte production in the spinal cord and forebrain.

Most of our knowledge on oligodendrocyte development originates from chick and rodent studies³⁴. OPCs are the early progenitor cells of the oligodendrocyte lineage and are recognizable by their specific expression of the proteoglycan neural-glia antigen 2 (NG2) and platelet derived growth factor receptor alpha (PDGFRα)³⁵. OPCs are generated in sequential waves throughout the developing spinal cord and forebrain^36–41.

During spinal cord development, there are three waves of OPC production. The first wave is dependent on Shh signaling and its induction of oligodendrocyte-determining proteins Olig1 and Olig2 (Olig1/2), which are required for the development of ventral progenitor cells and the establishment of the ventral progenitor domain (pMN)^20,42. This domain gives rise to spinal motor neurons (sMNs) just prior to the production of OPCs⁴³. During this wave, OPCs arise from the pMN domain by division of radial glia in the ventricular zones (VZ) of the spinal cord at mouse embryonic day (E)12.5 and corresponding gestational week 6.5 in humans⁴⁴. Ventrally-derived OPCs proliferate rapidly and migrate in all directions of the spinal cord. The second wave of OPC production begins at E15.5 and it functions independently of Shh^34,40,41,45. It derives from Olig1/2-expressing cells in the VZ of dorsal progenitor domains (dP3–6) through a fibroblast growth factor (FGF)-dependent mechanism⁴⁵. Dorsally derived OPCs are less migratory than ventrally derived OPCs. Consequently, dorsally derived OPCs remain mainly in the dorsal half of the cord and ventrally-derived OPCs are responsible for 85–90% of the OPCs in the mouse spinal cord⁴⁶. It could be speculated that these differences in migratory capacity of the dorsally and ventrally derived OPCs could arise due to inherent metabolic signatures and energy states of these cells. The third wave of OPC production occurs in the post-natal ventricular-subventricular zone (V-SVZ)^18,37,47. OPCs in the spinal cord begin to differentiate into myelinating oligodendrocytes prior to birth at around E18.5⁴⁴.

In the developing forebrain, a similar set of sequential waves of OPC production plays out in the telencephalon in a ventral-to-dorsal arrangement. At E12.5, the initial wave commences from Nkx2.1-expressing precursors in the VZ of the ventral medial ganglionic eminence (MGE) and anterior entopeduncular area (AEP)³⁷. In the human forebrain, OPCs are detected in the VZ of the MGE at 7.5 weeks of gestation⁴⁸. This population of cells migrates to the entire embryonic telencephalon including the cerebral cortex. The following wave is generated from Gsh2-expressing precursors in the lateral ganglionic eminence (LGE) and MGE and they invade the developing cortex at E15.5³⁷. The third and final wave begins from Emx1-expressing precursors in the cortical VZ after birth and it is the primary source of OPCs in the mature neocortex³⁶. It was initially believed that MGE-derived Nkx2.1-expressing OPCs are gradually eliminated and replaced by progenitors generated in the second and final wave, but new evidence challenged this concept^34,44. Findings from Orduz et al. suggest that not all MGE-derived Nkx2.1-expressing OPCs die in the developing neocortex, and the progenitors that survive have preferential synaptic connectivity with their ontogenetically related interneurons⁴⁹. The mechanistic details of these highly coordinated survival signals are not completely understood.

The functional significance of the diverse developmental origins of OPCs has prompted further investigation by other research groups^50–52. A population of OPCs remains in the adult central nervous system in an immature or quiescent state where they are able to generate new oligodendrocytes during aging or disease^44,53. Recent findings have uncovered that oligodendrocyte lineage cells are a heterogenous population of cells with different abilities to generate myelin, but also with the capacity to perform additional functions besides myelination^52,54,55. Work from Xiao et al. revealed that OPCs play a role in fine-tuning neural circuits, independent of their role in myelination^52,56. Research on these additional functions of oligodendrocyte lineage cells could provide insight into the functional heterogeneity of OPCs as well as new mechanisms involved in oligodendrocyte cell-fate transitions. New technical advances could help reveal the metabolic bases for this functional heterogeneity of OPC subtypes.

Regulation of oligodendrocyte lineage cell survival and integration.

The process of generating myelinating oligodendrocytes is gradual, and it involves several intrinsic and extrinsic factors that determine the transition into oligodendrocyte lineage cells^57–61. OPCs have an intrinsic mechanism that limits the number of cell divisions and drives differentiation into post-mitotic oligodendrocytes^62,63. Following terminal differentiation, OPCs become post-mitotic and enter the pre-oligodendrocyte stage¹⁴. This stage is defined as a population of cells that are no longer OPCs but have not initiated myelin sheath formation⁶⁴. In the developing brain, OPCs either transition to mature oligodendrocytes or undergo apoptosis^57,65,66. Regulated cell death occurs during neurodevelopment to ensure proper refinement of the neural network^67–69. Time-lapse imaging of OPC maturation in the adult mouse cortex demonstrates that pre-oligodendrocytes undergo apoptosis if they fail to integrate into the nervous system as oligodendrocytes⁷⁰. The surviving pre-oligodendrocytes depend on extrinsic and intrinsic factors such as extracellular ligands, secreted molecules, neuronal activity, microRNAs, and transcriptional regulation to become myelinating oligodendrocytes⁷¹. Cell-to-cell interactions with astrocytes and microglia are known to promote the survival and function of oligodendrocyte lineage cells¹⁴. Oligodendrocytes begin to myelinate by engaging with nearby axons and extending microfilament-rich filipodia-like processes onto their preferred axon^46,71–74. Following this event, oligodendrocytes undergo a refinement period where sheaths either continue to elongate and stabilize or are retracted⁷³. Once the initial contact with an axonal membrane is stabilized, proteins and lipids are delivered to the growing membrane to generate compact myelin⁷⁵. In vivo time-lapse imaging in zebrafish indicates that the duration of myelin sheath generation is about 5 hours⁷⁶. A rapid in vitro protocol involving a co-culture system of rat OPCs and retinal ganglion cells suggests that oligodendrocytes have the capability to myelinate within a 6–12 hour time window⁷⁷.

Apoptosis in the developing nervous system

The BCL-2 family.

Neuronal apoptosis is highly regulated and critical for the organization of a functional neural circuit⁶⁹. Apoptosis is triggered if cells are depleted of survival factors or if there is a need for elimination of unwanted, damaged, or misplaced cells during differentiation. Disruption in the balance between cell death and survival can lead to developmental alterations⁷⁸. While the contribution of non-apoptotic cell death mechanisms to neural development are emerging, most developmental cell death proceeds via mitochondria-mediated apoptosis^79,80.

The survival of neural cells is tightly controlled by the BCL-2 family of proteins^81,82. All members of this family contain a BCL-2 homology 3 (BH3) domain, which is one of the four BH domains involved in interactions between the proteins. The BCL-2 family is subdivided into three groups based on their function: anti-apoptotic members (BCL-2, BCL-xL, BCL-W, MCL-1, BFL-1/A1), pro-apoptotic members (BAX, BAK, BOK), and pro-apoptotic BCL-2 homology 3 (BH3)-only members (BAD, BID, BIK/NBK, BIM/BOD, BMF, HRK/DP5, NOXA and PUMA/BBC3)^83–88. The effector proteins promote mitochondrial outer membrane permeabilization (MOMP)⁸². MOMP triggers the release of intermembrane space (IMS) proteins such as cytochrome c and second mitochondria-derived activator of caspase (SMAC) into the cytosol^81,89,90. In the cytosol, cytochrome c binds to the key caspase adaptor molecule, apoptotic protease-activating factor-1 (Apaf-1)⁹¹. This interaction triggers homo-oligomerization of Apaf-1 into a caspase-activating complex known as the apoptosome⁸². The apoptosome recruits and activates the initiator caspase-9, leading to cleavage and activation of effector caspases-3 and −7. Activated caspases-3 and −7 execute their function by cleaving proteins that are responsible for proper cellular function and homeostasis, leading to their degradation.

Several models have been proposed for the regulation of MOMP by BCL-2 proteins^82,92,93. A subset of BH3-only proteins known as “activators” bind to BCL-2 effector proteins, BAX and BAK, to promote their activation⁸⁴. Activation of BAX and BAK induces a series of conformational changes resulting in BAX/BAK homo-oligomerization and MOMP. The anti-apoptotic members of the BCL-2 family inhibit BAX and BAK by sequestration, which subsequently prevents oligomerization and activation⁹⁴. “Sensitizer” BH3-only proteins inactivate anti-apoptotic proteins by binding to and sequestering them as well⁸⁴.

During apoptosis in differentiated cells, activated BAX translocates from the cytosol to the outer mitochondrial membrane (OMM), where BAK is constitutively located⁹³. In human embryonic stem cells (hESCs), BAX is constitutively active and localized at the Golgi network, proposing a mechanism that alters apoptotic machinery and allows for rapid translocation to the mitochondria during critical stages of early embryogenesis^95,96. This unique regulation of the apoptotic machinery suggests a central function of mitochondrial priming during development. Considering how cells from the oligodendrocyte lineage transition through various stages of development, it can be proposed that a specialized series of mitochondrial-centric checkpoints allow for an efficient transition from OPC to myelinating oligodendrocyte.

Oligodendrocyte development is regulated by the intrinsic apoptotic pathway.

Following neural tube patterning, apoptosis regulates the number of cells that remain in the central nervous system^97,98. The BCL-2 family functions early at the onset of neurogenesis⁹⁹. MCL-1’s anti-apoptotic function has a specific temporal requirement during neurogenesis as it is essential for the survival of neural progenitor cells (NPCs) in the developing mouse brain^99,100. As differentiation continues, BCL-xL maintains survival of a subset of post-mitotic neurons in the upper layers of the cortex, but not in NPCs¹⁰¹. Thus, MCL-1 and BCL-xL have distinct and overlapping roles as MCL-1 is required for survival of NPCs at the earliest stages of neurogenesis and BCL-xL is critical for the survival of post-mitotic neurons⁹⁹.

The function of the BCL-2 family of proteins on oligodendrocyte lineage cell transitions has not been well delineated. Itoh et al. provided an extensive mRNA expression profile of the BCL-2 family of proteins during in vitro differentiation of rat oligodendrocytes¹⁰². BCLXL and MCL1 are highly expressed in oligodendrocyte lineage cells, particularly in mature oligodendrocytes¹⁰². Consistent with mRNA expression, protein expression of BCL-xL is increased during oligodendrocyte differentiation, while BCL-2 decreases from OPC to immature oligodendrocyte. mRNA and protein expression of BAX and BAK remained steady during differentiation¹⁰². Whole-cell single cell RNA sequencing (scRNAseq) demonstrates that fetal-derived oligodendrocytes express higher levels of MCL1 and BAX compared to pediatric and adult donor oligodendrocytes⁸⁰. Late OPCs and early OPCs have higher anti-apoptotic gene expression than pre-oligodendrocytes and mature oligodendrocytes, indicating a gradual switch in apoptosis sensitivity as oligodendrocytes differentiate⁸⁰. Although both studies cited above are informative as they provide insight into the dynamic expression of the BCL-2 family of proteins during development, the mechanistic and functional implications are not explored. Thus, further investigation is needed to understand the exact mechanisms by which the BCL-2 family of proteins participate in oligodendrocyte development.

Mature oligodendrocytes from BAX/BAK knockout mice are resistant to apoptosis in vitro. Their processes become irregular and fragmented, perhaps indicating functional impairment of the mature cells¹⁰³. Mice that are deficient in BAX and BAK exhibit numerous differentiated oligodendrocytes in brain regions that are typically myelinated at later developmental stages, suggesting a role of BAX and BAK in spatiotemporal control of myelination (Figure 1)⁵⁷. Brain-wide deletion of MCL-1 in neural stem cells led to progressive reduction of white matter and myelin, indicating that MCL-1 is required for oligodendrocyte function in mice¹⁰⁴. The lack of MCL-1 reduced the number of mature oligodendrocytes, however, the number of OPCs was maintained, indicating that MCL-1 regulates differentiation of OPCs (Figure 1). These findings indicate that in addition to mediating cell death, the BCL-2 family of proteins may execute a separate role in supporting function and differentiation of oligodendrocytes.

Figure 1.

OPC differentiation is regulated by mitochondrial-mediated mechanisms. A) Knockout of BAX and BAK triggers early OPC differentiation in brain regions that are ordinarily myelinated at later post-natal stages⁵⁷. MCL-1 regulates survival of mature oligodendrocytes during the OPC differentiation stage¹⁰⁴. B) Inhibition of mitochondrial respiratory chain complex IV prior to OPC differentiation impaired formation of mature oligodendrocyte processes¹⁴². Following inhibition of mitochondrial respiratory chain complex I, differentiation of OPCs was blocked¹⁴⁴. The mitochondrial contact site and cristae organizing center (MICOS) is shown at the cristae junctions.

The BCL-2 family and its role in maintaining mitochondrial dynamics.

Mitochondria are dynamic organelles that constantly undergo ultrastructural modifications modulated by fusion and fission events¹⁰⁵. Mitochondrial fusion is mediated by mitofusin 1/2 (MFN1/2) and optic atrophy 1 (OPA1), whereas fission is largely mediated by dynamin-related protein 1 (DRP1)¹⁰⁶. The balance between these two events is termed mitochondrial dynamics. These coordinated events are needed to maintain mitochondrial function and are dependent on the metabolic state and requirements of the cell. Mitochondrial fusion functions to maintain normal mitochondrial activity by supplementing mtDNA, lipids, proteins, or metabolites to damaged mitochondrial from the components of healthy mitochondria^107,108. Mitochondrial fission maintains proper distribution of mitochondria to meet the local demand for ATP, which is vital in highly polarized neural cells¹⁰⁹. Additionally, mitochondrial fission allows for symmetrical segregation of mitochondria between daughter cells during mitosis¹¹⁰.

Fission events are a hallmark of apoptosis, resulting in smaller and increased number of mitochondria¹¹¹. These DRP-1-mediated alterations to mitochondrial morphology occur early in the apoptotic pathway^112–114. DRP1 is a large cytosolic GTPase that translocates to the OMM where it uses energy from GTP hydrolysis to execute constriction and fission of the mitochondria¹¹⁵. Blocking mitochondrial fission by downregulating expression of DRP1 inhibits release of cytochrome c and subsequently delays cell death¹¹⁶. Evidence suggests that the association of DRP1 and BAX with the OMM is interdependent^114,117. Further studies show that BAX is an essential part of the mitochondrial fragmentation machinery in apoptotic cells^118,119. OPA1 also controls apoptosis through cristae remodeling and cytochrome c release^120,121. MCL-1 modulates mitochondrial dynamics through interactions with DRP-1 and OPA1 in human embryonic stem cells and human cardiomyocytes^122,123. Thus, BCL-2 protein family members have non-apoptotic functions regulating mitochondrial integrity in healthy and apoptotic cells.

Mitochondrial dynamics during differentiation.

Dysfunction of mitochondrial fission/fusion machinery and aberrant mitochondrial fragmentation is a hallmark of several neurodevelopmental disorders¹²⁴. Mitochondria in the developing mouse brain undergo morphological modifications as cells commit to a neurogenic fate. Mitochondria in embryonic NSCs have an elongated morphology that becomes fragmented in NPCs¹²⁵. As early-born neurons mature, their mitochondria gradually fuse and become elongated¹²⁶. The switch in morphology from NSCs to a committed progenitor is accompanied by a metabolic shift of cells from glycolysis to OXPHOS. These mitochondrial morphological changes that occur in NSCs act as an upstream regulatory mechanism of stem cell fate decisions. During asymmetric cell division in neurogenesis, cells that become neurons display high levels of mitochondrial fission, whereas those that undergo self-renewal as NSCs display rapid mitochondria fusion¹²⁶. Additionally, induction of mitochondria fusion following mitosis triggers most NSCs to undergo self-renewal, altering cell fate.

How mitochondrial dynamics are altered during oligodendrocyte differentiation has not been clearly characterized. Pathological examination of post-mortem brains from patients with vanishing white matter disease (VWMD), a type of leukodystrophy, demonstrated a disproportionately high number of OPCs compared to myelinating oligodendrocyte^127,128. Experimental models of VWMD have further demonstrated that OPCs have impaired differentiation capacity and mitochondrial function¹²⁹. Thus, examining the interplay between oligodendrocyte lineage cell differentiation and mitochondrial function is critical for understanding neurodevelopmental disorders such as VWMD.

Oligodendrocyte Energy Metabolism

Energetic demands of oligodendrocytes.

For myelination to occur, oligodendrocytes must generate expansive membranes that wrap around nearby axons. This process demands a large amount of energy from oligodendrocytes, which must have the capacity to generate ATP and carbon skeletons to produce lipids utilized for long-term myelination⁷². Cholesterol, phospholipids, and glycosphingolipids account for approximately 70–85% of the dry weight of the myelin membrane^130,131. Oligodendrocytes also provide metabolic support to the axons they myelinate, increasing their energetic demands^132,133. Oligodendrocytes must constantly maintain energy production to meet the needs of the growing myelin membrane through the developing nervous system.

Glucose is transported from the bloodstream into the perivascular end-feet of astrocytes by facilitated diffusion via glucose transporter 1 (GLUT1). Glucose is taken up by oligodendrocytes via GLUT1^134,135. In astrocytes, glucose is either stored as glycogen or it undergoes glycolysis to generate pyruvate. Pyruvate can either enter the mitochondria where it is oxidized to acetyl-CoA via pyruvate dehydrogenase (PDH), or it is converted to lactate by lactate dehydrogenase (LDH). Lactate is then exported to oligodendrocytes and neurons via monocarboxylate transporters¹³². Oligodendrocytes are also capable of processing glucose into pyruvate and exporting lactate to neurons and astrocytes in a similar manner. This is known as the astrocyte-oligodendrocyte-neuron-lactate-shuttle^132,136,137. Oligodendrocytes oxidize lactate for metabolic fuel and lipid synthesis¹³⁸. Additionally, the presence of lactate upregulates mouse OPC differentiation in vitro¹³⁹. In a hypoglycemic state, proliferation, maturation, and migration of primary rat OPCs is reduced and myelination is inhibited^134,140. Lactate is capable of supporting myelination during hypoglycemia^134,139. Although not fully understood, results from Yan and Rivkees, Rinholm et al., Sánchez-Abarca et al., and Ichihara et al. suggest that proper function of the astrocyte-oligodendrocyte-neuron-lactate-shuttle is essential for the differentiation of oligodendrocyte lineage cells^{134,138–140}.

Oligodendrocyte metabolic switch.

The Cox10 gene encodes a farnesyltransferase involved in the assembly of cytochrome c oxidase (COX), a key component of the mitochondrial electron transport chain (ETC) in the mitochondria¹⁴¹. Inhibition of COX in vitro alters the initiation of myelin production and triggers death of OPCs and oligodendrocytes¹⁴². Knockdown of Cox10 in vivo following completion of myelination did not alter myelin or axonal function¹⁴³. These results indicate that OPCs have a high OXPHOS rate before and during myelination, and myelinating oligodendrocytes have low OXPHOS and high glycolytic rates (Figure. 2). This switch to a metabolic state that has increased dependence on glycolysis as oligodendrocytes mature differs from neurons. Uncommitted NSCs cells exhibit a glycolytic metabolism whereas post-mitotic neurons progressively shift to mitochondrial-dependent OXPHOS, as discussed in an earlier section of this review¹²⁵. Although the mechanism that regulates the switch between energy sources is not well understood, Schoenfeld et al. reports proper function of OXPHOS is an essential component of OPC differentiation. Following treatment with rotenone, a specific inhibitor of mitochondrial complex I, OPCs were not able to differentiate and did not alter viability (Figure 1)¹⁴⁴. Inhibition of OPC differentiation was detected by low gene expression of myelin basic protein (MBP) and cyclic nucleotide phosphodiesterase (CNPase) comparison to differentiated oligodendrocytes. These results place an emphasis the mitochondrial-dependent OXPHOS requirements that OPCs maintain to become functional oligodendrocytes.

Figure 2.

A) BCL-2 family of proteins. BH3-only sensitizer proteins inhibit anti-apoptotic proteins, BCL-2, MCL-1, and BCL-xL. BH3-only activator proteins activate BAX and BAK leading to oligomerization of these pro-apoptotic proteins and mitochondrial outer membrane permeabilization (MOMP). BCL-2, MCL-1, and BCL-xL inhibit activation and oligomerization of BAX and BAK, thus preventing apoptosis. B) Mitochondrial fission and fusion proteins. Optic atrophy 1 (OPA1) and mitochondrial fusion protein 1, 2 (MFN1, MFN2) promote mitochondrial fusion. Dynamin-related protein-1 (DRP1) and mitochondrial fission 1 protein (FIS1) promote mitochondrial fission. C) Methods and associated mitochondrial-mediated mechanisms. Previously used methods to understand mitochondrial-mediated mechanisms during oligodendrocyte development. As oligodendrocytes mature, they depend less on OXPHOS and more on glycolysis^134,143,146. There are changes in the expression of pro and anti-apoptotic BCL-2 family genes across development^80,102. Regulators of mitochondrial dynamics are increased after differentiation to myelinating oligodendrocytes¹⁴⁴. ? = Mechanism is unknown at this developmental stage.

Follow-up studies investigated the overlap between OXPHOS and glycolysis and how it differs between species. Rone et al. found that human adult oligodendrocytes are less metabolically active than adult OPCs and both are less active than oligodendrocytes and OPCs collected from post-natal rats¹⁴⁵. They also demonstrated that human adult oligodendrocytes and adult OPCs generate the majority of their ATP from glycolysis, whereas the generation of ATP in oligodendrocytes and OPCs from post-natal rats is mostly dependent on OXPHOS. In this study, human OPCs and oligodendrocytes were obtained from adults that sustained surgical resections as a treatment for non- tumor-related intractable epilepsy. Species differences, the varying developmental states of the cells, and the origin of the human cells may account for the disparity between the energy utilization properties of human and rodent oligodendrocyte lineage cells. In a follow-up study, the authors found that adult rat derived oligodendrocytes generate more ATP through glycolysis than OXPHOS. Conversely, oligodendrocytes that were differentiated in vitro from post-natal rat OPCs have a greater dependence on OXPHOS for ATP production than glycolysis. The opposing developmental stages between adult oligodendrocytes and cultured oligodendrocytes may explain this difference in metabolic requirements¹⁴⁶. These properties must be further elucidated in a platform that allows for investigation of healthy human oligodendrocyte lineage cells, such as human stem cell models¹⁴⁷.

Mitochondrial morphology in oligodendrocytes.

Mitochondrial dynamics within the thin processes of oligodendrocyte lineage cells have yet to be well characterized. Investigating mitochondrial dynamics in oligodendrocyte lineage cells will provide a greater understanding of how the mitochondria promotes ATP and fatty acid metabolism as oligodendrocytes extend their plasma membrane during axon selection and stabilization of the myelin sheath. Rinholm et el. found that mitochondrial density and length is higher in the primary processes of mouse oligodendrocytes than in the myelin sheath¹⁴⁸. The average mitochondrial length in primary processes and the myelin sheath is lower than what is reported in neuronal dendrites, axons, and astrocytes¹³⁶. Electron microscopy revealed that mitochondria in the cytoplasmic compartments of the myelin sheath were small in size and have few cristae in comparison with neuronal mitochondria. These data may suggest that oligodendrocyte mitochondria perform minimal ATP synthesis via OXPHOS¹⁴⁹. Higher spatial resolution may be needed to validate these results. Microarray studies in primary rat OPCs and human oligodendrocyte cell lines demonstrate that mitochondrial fatty acid oxidation, mitochondrial electron transport, and ATP synthesis transcripts were activated as a consequence of oligodendrocyte differentiation¹⁴⁴. Interestingly, OPA1 and FIS1 were induced, indicating that mitochondria undergo morphological changes during differentiation. Genes related to apoptosis were induced as well, in agreement with findings that indicate that oligodendrocyte lineage cells become vulnerable to apoptosis during development.

Concluding remarks and future perspectives

The BCL-2 family of proteins, mitochondrial fission and fusion machinery, and changes to the metabolic state are all mitochondrial-mediated mechanisms that may contribute to the survival and function of oligodendrocytes (Figure 1 and Figure 2). The findings summarized here stem from experiments that measured gene expression. While the findings are valuable, they do not provide an explanation as to how or why these mitochondrial-mediated mechanisms regulate oligodendrocyte development. The function of mitochondrial communication with other organelles such as the endoplasmic reticulum and peroxisomes in oligodendrocyte development needs to be explored. In addition, members of the BCL-2 family of proteins can regulate the integrity and morphology of not only mitochondria but also peroxisomes, which are required for the maturation of myelinating oligodendrocytes^150,151. There is also a critical need for human stem cell models to investigate these mechanisms. The transition to the pre-oligodendrocyte stage is key for integration of mature oligodendrocytes into the nervous system. Due to the small window of time that cells remain in this stage, it has been difficult to investigate the factors that contribute to the function of this cell type. Bridging these gaps in knowledge will allow for development of therapeutics aimed to alleviate metabolic dysfunctions in diseases involving white matter dysfunction.

Significance.

Oligodendrocytes are the myelinating glia of the central nervous system, which are essential for proper signal transmission within the nervous system and axonal metabolic support. How mitochondria-related pathways such as mitochondrial dynamics, bioenergetics, and apoptosis finely-tune the differentiation and function of myelinating oligodendrocytes is not well understood. Considering that altered mitochondrial functions are associated with a wide array of diseases, including leukodystrophies, it is pivotal to examine the molecular details by which mitochondrial-mediated signaling impact oligodendrocyte development and myelination. This manuscript summarizes critical studies in this area and provides insight into the mitochondrial-related changes that take place during oligodendrocyte development. We highlight several gaps in the field of mitochondrial dynamics and bioenergetics in glia development that require further understanding.