The fate and function of oligodendrocyte progenitor cells after traumatic spinal cord injury.

Oligodendrocyte progenitor cells (OPCs) are the most proliferative and dispersed population of progenitor cells in the adult central nervous system, which allows these cells to rapidly respond to damage. Oligodendrocytes and myelin are lost after traumatic spinal cord injury (SCI), compromising efficient conduction and, potentially, the long-term health of axons. In response, OPCs proliferate and then differentiate into new oligodendrocytes and Schwann cells to remyelinate axons. This culminates in highly efficient remyelination following experimental SCI in which nearly all intact demyelinated axons are remyelinated in rodent models. However, myelin regeneration comprises only one role of OPCs following SCI. OPCs contribute to scar formation after SCI and restrict the regeneration of injured axons. Moreover, OPCs alter their gene expression following demyelination, express cytokines and perpetuate the immune response. Here, we review the functional contribution of myelin regeneration and other recently uncovered roles of OPCs and their progeny to repair following SCI.

Locomotor recovery following contusive spinal cord injury does not require oligodendrocyte remyelination.

Remyelination occurs after spinal cord injury (SCI) but its functional relevance is unclear. We assessed the necessity of myelin regulatory factor (Myrf) in remyelination after contusive SCI by deleting the gene from platelet-derived growth factor receptor alpha positive (PDGFRα-positive) oligodendrocyte progenitor cells (OPCs) in mice prior to SCI. While OPC proliferation and density are not altered by Myrf inducible knockout after SCI, the accumulation of new oligodendrocytes is largely prevented. This greatly inhibits myelin regeneration, resulting in a 44% reduction in myelinated axons at the lesion epicenter. However, spontaneous locomotor recovery after SCI is not altered by remyelination failure. In controls with functional MYRF, locomotor recovery precedes the onset of most oligodendrocyte myelin regeneration. Collectively, these data demonstrate that MYRF expression in PDGFRα-positive cell derived oligodendrocytes is indispensable for myelin regeneration following contusive SCI but that oligodendrocyte remyelination is not required for spontaneous recovery of stepping.

Myelinogenic Plasticity of Oligodendrocyte Precursor Cells following Spinal Cord Contusion Injury.

Spontaneous remyelination occurs after spinal cord injury (SCI), but the extent of myelin repair and identity of the cells responsible remain incompletely understood and contentious. We assessed the cellular origin of new myelin by fate mapping platelet-derived growth factor receptor α (PDGFRα), Olig2+, and P0+ cells following contusion SCI in mice. Oligodendrocyte precursor cells (OPCs; PDGFRα+) produced oligodendrocytes responsible for de novo ensheathment of ∼30% of myelinated spinal axons at injury epicenter 3 months after SCI, demonstrating that these resident cells are a major contributor to oligodendrocyte regeneration. OPCs also produced the majority of myelinating Schwann cells in the injured spinal cord; invasion of peripheral myelinating (P0+) Schwann cells made only a limited contribution. These findings reveal that PDGFRα+ cells perform diverse roles in CNS repair, as multipotential progenitors that generate both classes of myelinating cells. This endogenous repair might be exploited as a therapeutic target for CNS trauma and disease.SIGNIFICANCE STATEMENT Spinal cord injury (SCI) leads to profound functional deficits, though substantial numbers of axons often survive. One possible explanation for these deficits is loss of myelin, creating conduction block at the site of injury. SCI leads to oligodendrocyte death and demyelination, and clinical trials have tested glial transplants to promote myelin repair. However, the degree and duration of myelin loss, and the extent and mechanisms of endogenous repair, have been contentious issues. Here, we use genetic fate mapping to demonstrate that spontaneous myelin repair by endogenous oligodendrocyte precursors is much more robust than previously recognized. These findings are relevant to many types of CNS pathology, raising the possibility that CNS precursors could be manipulated to repair myelin in lieu of glial transplantation.

Myelin regulatory factor drives remyelination in multiple sclerosis.

Remyelination is limited in the majority of multiple sclerosis (MS) lesions despite the presence of oligodendrocyte precursor cells (OPCs) in most lesions. This observation has led to the view that a failure of OPCs to fully differentiate underlies remyelination failure. OPC differentiation requires intricate transcriptional regulation, which may be disrupted in chronic MS lesions. The expression of few transcription factors has been differentially compared between remyelinating lesions and lesions refractory to remyelination. In particular, the oligodendrocyte transcription factor myelin regulatory factor (MYRF) is essential for myelination during development, but its role during remyelination and expression in MS lesions is unknown. To understand the role of MYRF during remyelination, we genetically fate mapped OPCs following lysolecithin-induced demyelination of the corpus callosum in mice and determined that MYRF is expressed in new oligodendrocytes. OPC-specific Myrf deletion did not alter recruitment or proliferation of these cells after demyelination, but decreased the density of new glutathione S-transferase π positive oligodendrocytes. Subsequent remyelination in both the spinal cord and corpus callosum is highly impaired following Myrf deletion from OPCs. Individual OPC-derived oligodendrocytes, produced in response to demyelination, showed little capacity to express myelin proteins following Myrf deletion. Collectively, these data demonstrate a crucial role of MYRF in the transition of oligodendrocytes from a premyelinating to a myelinating phenotype during remyelination. In the human brain, we find that MYRF is expressed in NogoA and CNP-positive oligodendrocytes. In MS, there was both a lower density and proportion of oligodendrocyte lineage cells and NogoA+ oligodendrocytes expressing MYRF in chronically demyelinated lesions compared to remyelinated shadow plaques. The relative scarcity of oligodendrocyte lineage cells expressing MYRF in demyelinated MS lesions demonstrates, for the first time, that chronic lesions lack oligodendrocytes that express this necessary transcription factor for remyelination and supports the notion that a failure to fully differentiate underlies remyelination failure.

Myelin inhibits oligodendroglial maturation and regulates oligodendrocytic transcription factor expression.

Myelin loss is a hallmark of multiple sclerosis (MS) and promoting central nervous system myelin repair has become a major therapeutic target. Despite the presence of oligodendrocytes precursors cells (OPCs) in chronic lesions of MS, remyelination often fails. The mechanism underlying this failure of remyelination remains unknown, but it is hypothesized that environmental cues act to inhibit the maturation/differentiation of oligodendroglia, preventing remyelination. The rate of CNS remyelination is correlated to the speed of phagocytosis of myelin debris, which is present following demyelination and trauma. Thus, myelin debris could inhibit CNS remyelination. Here, we demonstrate that OPCs cultured on myelin were robustly inhibited in their maturation, as characterized by the decreased expression of immature and mature oligodendrocytes markers, the impaired production of myelin gene products, as well as their stalled morphological complexity relative to OPCs cultured on a control substrate. OPCs in contact with myelin stopped proliferating and decreased the expression of OPC markers to a comparable degree as cells grown on a control substrate. The expression of two transcription factors known to prevent OPC differentiation and maturation were increased in cells that were in contact with myelin: inhibitor of differentiation family (ID) members 2 and 4. Overexpression of ID2 and ID4 in OPCs was previously reported to decrease the percentage of cells expressing mature oligodendrocyte markers. However, knockdown of ID2 and/or ID4 in OPCs did not increase oligodendroglial maturation on or off of myelin, suggesting that contact with myelin regulates additional regulatory elements.