Oligodendrocyte progenitor cell fate and function in development and disease

Abstract

Differentiation of oligodendrocyte progenitor cells (OPC) into myelination capable mature oligodendrocytes is essential for proper function of the central nervous system (CNS). OPCs are tissue resident stem cells that populate all regions of the CNS and exist beyond development into adulthood. Disorders that lead to disruption of this critical cell state change cause devastating myelin diseases that are often associated with shortened lifespan. Recent findings have also provided support for a newly appreciated contribution of perturbed OPC differentiation to neurodegenerative and psychiatric diseases. These findings emphasize the need for a more complete understanding of OPC differentiation in health and disease. Here we review recent molecular and functional findings revealing new roles of OPCs. It is our hope that this review provides readers with an enticing snapshot of current OPC research and highlights the potential of controlling OPC fate and function to treat diseases of the brain.

Introduction

Oligodendrocytes in the central nervous system (CNS) produce myelin, a multilayered lipid membrane structure that wraps axons to enhance the speed of action potential propagation and provide axons with metabolic trophic support [1]. During development, and throughout adulthood, myelinating oligodendrocytes are formed through terminal differentiation of oligodendrocyte progenitor cells (OPCs). OPCs are CNS resident stem cells that originate in brain and spinal cord ventricular zones before proliferating and migrating to populate the CNS [2]. Unlike most progenitor cell populations, OPCs remain present throughout life. This has led glial biologists to wonder why have adult OPCs? Are they present solely to replace lost oligodendrocytes? Do OPCs have functions that are independent of differentiation? For decades, glial biologists have been able to grow and differentiate OPCs in culture providing direct access to study this process. This has led to a detailed understanding of molecular regulators of OPC differentiation into oligodendrocytes. Broad mechanisms that are known to regulate OPC differentiation include transcription factors, chromatin regulators, protein post translational modifications, miRNAs, and external cues including oxygen saturation and mechanotransduction. Molecular regulation of OPC differentiation by these mechanisms has been addressed in excellent reviews and we would direct readers there for more detail [3–7]. Nevertheless, there are still many questions that surround OPC biology. New molecular mechanisms that govern OPC differentiation during development continue to be discovered. Evidence that OPCs are involved in learning and memory is accumulating. New OPC disease cell states and their functions are being uncovered. Moreover, there is a growing appreciation for the role of OPCs in the pathophysiology of neurodegenerative and psychiatric diseases. In this review we provide a series of short vignettes that highlight recent work that expand our understanding of OPCs. We hope that this engenders in the reader the same excitement that we have for these dynamic cells and the role they play in the normal function and pathology of the CNS.

New regulator of OPC differentiation during development

Appropriate regulation of OPC differentiation is critical for the proper development and function of the CNS. Important recent work has shown that OPC differentiation is regulated by N⁶-methyladenosine (m⁶A) modifications on mRNA. This is the most common internal modification of mRNA, and its discovery adds an additional method of post-transcriptional gene regulation [8]. Modifications to mRNA are placed by “writers”, protein complexes like the m⁶A methyltransferase that adds m⁶A marks to mRNA [9]. “Readers” then recognize mRNA modifications and translate them to a functional outcome, for example some readers stabilize modified mRNA [10]. Finally, mRNA modifications can be removed by “erasers” [11]. Recent work from two groups has identified a role for an m⁶A “reader” and “writer” in OPC differentiation (Fig 1). These components allow for a dynamic system that can ultimately affect gene and protein levels in cells. Wu et al., showed that Proline rich coiled-coil 2A (Prrc2a) binds to m⁶A to stabilize the oligodendrocyte lineage specifying transcription factor oligodendrocyte transcription factor (Olig2), and that Prrc2a conditional deletion in the oligodendrocyte lineage decreased oligodendrocyte formation and caused hypomyelination [12]. In addition, Xu et al. showed that blocking m⁶A modification specifically in the oligodendrocyte lineage through conditional deletion of methyltransferase like 14 (Mettl14), a core component of the m⁶A methyltransferase complex, decreased OPC differentiation leading to hypomyelination [13]. Xu et al. also performed m⁶A-seq in OPCs and oligodendrocytes and found thousands of transcripts with dynamic m⁶A marks suggesting that this mRNA modification may have additional roles to play within the oligodendrocyte lineage. These findings also show that our understanding of OPC differentiation mechanisms during development is incomplete. In addition, there is not yet evidence of disease associated changes in m⁶A modified mRNA within the oligodendrocyte lineage. Determining whether perturbations in this added layer of regulatory complexity contributes to OPC dysfunction in disease and whether it can be leveraged as a therapeutic target should be a prioritized focus of future studies.

Figure 1. Advances in our understanding of OPC physiology.

m⁶A modifications on mRNA are positive regulators of OPC differentiation. Deletion of the m⁶A reader Prrc2a, or the m⁶A writer Mettl14, leads to decreased mature oligodendrocytes and hypomyelination. Learning in multiple paradigms promotes OPC differentiation and myelination that is required for consolidating those learned behaviors into memory. Image created with BioRender.com

New evidence of a role for OPCs in learning and memory

Memory and learning require the synchronization of action potentials, and this requires tuning the speed of actional potential propagation. One mechanism to tune actional potential speed is myelin plasticity, which is the addition or removal of myelinated segments along an axon [14]. In humans, magnetic resonance imaging studies show that learning a new task correlates with increased myelin in brain regions associated with that task [15]. Exciting new research has shed more light on this connection and shown that myelination is required for learning and memory (Fig 1). Steadman et al. utilized transgenic mice where tamoxifen treatment excises a transcription factor, specifically in OPCs, that is required for OPC differentiation into oligodendrocytes. These transgenic mice allowed the authors to block the myelination from new oligodendrocytes and they used this system to show that de-novo myelination is required for spatial learning and memory consolidation in the water maze behavioral assay [16]. In agreement with these findings, Pan et al. used the same transgenic approach to block oligodendrocyte formation and myelination during a contextual fear conditioning paradigm. In this paradigm mice are exposed to contextual cues followed by foot shocks which induces fear causing the mouse to freeze when exposed to those same contextual cues either 24hrs or 30 days after the initial fear training [17]. Without the ability to form new oligodendrocytes and myelin, Pan et al. showed that mice were still able to recall the fear memory and froze 24hrs after initial training but did not consolidate the fear memory and did not freeze 30 days after the initial training. What drives OPCs to differentiate into myelin forming oligodendrocytes in response to these learning and memory tasks? OPCs form synapses with neurons and respond to neuronal activity directly through these connections [18] and neuronal activity causes increased OPC proliferation and differentiation ultimately leading to increased myelination [19]. These exciting findings strengthen the connection between OPC function and learning and memory, indicating that interventions which promote OPC differentiation may be used to enhance learning and memory.

New OPC cell state in multiple sclerosis.

Multiple sclerosis (MS) is an autoimmune disorder caused by inflammatory attacks against myelin in the CNS. Traditionally OPCs have not been considered to play a role in the etiology of MS. Instead, the role of OPCs in repair of demyelinated lesions has been the focus. This has led to important work on remyelinating therapies aimed at enhancing the inherent remyelinating capabilities of OPCs to repair MS lesions with the goal of halting or even reversing the progressive neurological decline that occurs as MS advances. However, recent findings show that OPCs undergo a state change to acquire immune cell functions that contribute to disease pathogenesis and inhibit repair by impeding the ability of OPCs to differentiate and replace lost oligodendrocytes and myelin (Fig 2). Using single-cell RNAseq to analyze oligodendrocyte heterogeneity in the MS mouse model, experimental autoimmune encephalomyelitis, Falcão et al. identified oligodendrocyte lineage cells that express genes associated with immune cells including major histocompatibility complex class I and II [20]. The authors confirmed in vitro that interferon-gamma (IFNγ) can drive OPCs to an immune-like state with immune-like functions including phagocytosis and activation of T-cells via MHC-II. Importantly, Falcão et al. confirmed that oligodendrocyte lineage cells in human MS lesions express MHC-II. The presence of immune-like OPCs in human MS lesions was further confirmed by single-nuclei analysis of oligodendrocytes from patients with primary progressive MS [21]. Jäkel and Agirre et al. found a distinct population of oligodendrocyte lineage cells in human MS that also take on an immune-like state. Independent validation of these findings was published by Kirby et al., showing again that IFNγ can drive OPCs to an immune like state that includes expression of MHC-I and MHC-II, and the immunoproteasome [22]. Importantly, this work also showed that immune-like OPCs could cross-present antigen to become targets of cytotoxic CD8 T cells and that transition to this immune-like state driven by IFNγ inhibits differentiation of OPCs. These two findings provide additional mechanisms for failed OPC differentiation and remyelination in MS lesions. Therapeutics that inhibit this OPC state change may provide an additional avenue to slow lesion formation and support remyelination.

Figure 2. Advances in our understanding of OPC fate and function in disease.

A) Oligodendrocyte death, myelin damage, and OPC dysfunction occur, not only in myelin diseases like multiple sclerosis, but in neurodegenerative and psychiatric disease as well. In disease repair by endogenous OPCs is disrupted either by B) OPC cell state change to immune-like cells in multiple sclerosis, or by C) perturbed OPC differentiation in neurodegenerative and psychiatric diseases. D) This failure of OPCs to differentiate and replace lost myelin leads to axonal degeneration causing cognitive and motor deficits. Image created with BioRender.com

New appreciation for OPC dysfunction in neurodegenerative and psychiatric disease.

In addition to new roles in traditional myelin disorders, there is also a growing appreciation for the role that OPCs play in the pathogenesis of neurodegenerative diseases like Alzheimer’s disease. White matter pathology in Alzheimer’s disease patients can occur prior to the onset of cognitive decline and prior to the appearance of amyloid-ß plaques and neurofibrillary tangles of hyperphosphorylated tau that are hallmarks of the disease [23,24]. Oligodendrocytes also expression high levels of Alzheimer’s risk genes including bridging integrator 1 (BIN1) [25], and human post-mortem studies show demyelination and OPC dysfunction [23]. Two recent studies leveraging single-nuclei RNAseq to identify cell type specific gene changes in Alzheimer’s disease provide further support for oligodendrocyte lineage cells, including OPCs, playing an important role in Alzheimer’s disease pathology (Fig 2). Both studies by Mathys et al. and Grubman et al. found perturbed expression in genes associated with oligodendrocyte differentiation and myelination [26,27]. Both studies also identified leucine rich repeat and Ig domain containing 1 (LINGO1), a negative regulator of OPC differentiation [28], as a gene significantly upregulated in Alzheimer’s tissue in multiple cell types including neurons and oligodendrocytes [26,27]. These data support the intriguing hypothesis that Alzheimer’s disease pathology is, at least in part, driven by myelin loss followed by failed remyelination in old-age that leads to axonal damage and ultimately neuronal death [29]. Evidence of OPC involvement is also present for Parkinson’s and Huntington’s disease. In Parkinson’s disease single-nuclei RNAseq of the substantia nigra combined with GWAS data showed that genetic risk for Parkinson’s was associated with OPCs and oligodendrocytes [30] In Huntington’s, Osipovitch et al. injected human embryonic stem cell derived glial progenitor cells (a corollary to OPCs) from Huntington’s patients into mice that lack myelin [31]. These human glia chimeric mouse models showed that cells from Huntington’s patients had a significantly lower ability to generate myelinating oligodendrocytes than cells from healthy donors. Importantly, this deficit could be rescued by forced expression of transcription factors that drive OPC differentiation [31]. More studies that examine cell-autonomous OPC dysfunction are needed to provide mechanistic understanding of the molecular pathways that drive OPC dysfunction in neurodegenerative diseases and thereby represent promising approaches for OPC-targeted therapies.

OPC dysfunction is also suspected to contribute to psychiatric disorders (Fig 2). In schizophrenia multiple lines of evidence point to disrupted OPC differentiation in patients. These include studies that report lower myelin signal, decreased oligodendrocyte numbers, and decreased levels of oligodendrocyte genes and proteins in schizophrenia patients [32]. Two recent studies have leveraged schizophrenia patient derived induced pluripotent stem (iPSCs) to provide further evidence for cell-autonomous OPC dysfunction as a contributor to schizophrenia pathology. Using the same approach as described for Huntington’s above, Windrem et al. generated iPSCs derived glial progenitor cells from patients with childhood onset schizophrenia and healthy controls [33]. Compared to controls, these schizophrenia patient derived cells failed to myelinate the brains of myelin deficient mice [33]. In another study utilizing iPSC derived OPCs from two siblings with schizophrenia and one control sibling, de Vrij et al. similarly showed that OPCs from the schizophrenia patients had abnormal morphology, lower viability, and decreased differentiation and myelination of slice cultures from hypomyelinated mouse [34]. Evidence also supports a role for OPC dysfunction in major depressive disorder. In the brains of major depressive disorder patients there are fewer oligodendrocytes, the corpus callosum is thinned, and expression of myelin associated genes are decreased in patients with major depressive disorder [35–37]. Single-nuclei RNAseq has also been used to show cell autonomous defects in gene expression in OPCs from patients with major depressive disorder [38]. Nagy et al. showed that OPCs from major depressive disorder patients had the greatest number of differentially expressed genes compared to control, and further showed through pseudotime analysis that while major depressive disorder patient OPCs differentiate they dysregulated genes associated with apoptosis [38]. This suggests that perturbed OPC differentiation may contribute to major depressive disorder. In addition, Nagy et al. identified a population of oligodendrocyte lineage cells that had overlap in gene expression with OPCs in MS that acquire immune functions. Although more direct examination of OPCs in the pathogenesis of major depressive disorder is needed, these exciting results suggest that promoting effective OPC differentiation may be beneficial for this and other neuropsychiatric diseases.

Conclusion and perspective

Discovering therapies that promote OPC differentiation and enhance remyelination has long been a clinical goal for myelin disorders like multiple sclerosis. With increasing evidence that OPC dysfunction also contributes to neurodegenerative and psychiatric disorders, the potential for OPC targeted therapies to impact disease is even greater. OPCs generated from stem cells can be expanded in culture to generate millions of cells [39]. In addition to their regenerative potential as a cellular source for transplantation medicine, numerous labs have leveraged OPC platforms to perform large scale drug screens and identify small-molecules that enhance the transition of OPCs to oligodendrocytes [40–43]. The majority of small-molecules identified in these screens converge on a single unifying mechanism of action to drive oligodendrocyte formation by inhibiting specific enzymes in the cholesterol biosynthesis pathway [44]. This work suggests that sterol modulating medicines might provide a powerful approach to stimulate the regeneration of new oligodendrocytes from CNS resident OPCs in vivo. What remains unknown is whether such approaches will be universally regenerative across various neurological and neuropsychiatric diseases or whether disease- or context-specific screens will be required for effective therapies.

In close, OPCs are more than passive support cells waiting to engage in remyelination following loss of mature oligodendrocytes. These are dynamic cells with many critical functions that continue to be discovered. Here we have highlighted a sample of the exciting new discoveries that expand our understanding of OPC fate and function in health and disease. There are still more questions to be answered and we look forward to the many important discoveries to come.

Highlights.

• Oligodendrocyte progenitor cells are dynamic resident stem cells of the CNS with diverse functions in health and disease.

• m⁶A modifications on RNA is a newly discovered molecular regulator of OPC differentiation.

• OPCs undergo a cell state change and become immune-like in multiple sclerosis.

• Accumulating evidence for OPC dysfunction contributing to neurodegenerative and psychiatric diseases.