Many roles for oligodendrocyte precursor cells in physiology and pathology.

Oligodendrocyte precursor cells (OPCs) are a fourth resident glial cell population in the mammalian central nervous system. They are evenly distributed throughout the gray and white matter and continue to proliferate and generate new oligodendrocytes (OLs) throughout life. They were understudied until a few decades ago when immunolabeling for NG2 and platelet-derived growth factor receptor alpha revealed cells that are distinct from mature OLs, astrocytes, neurons, and microglia. In this review, we provide a summary of the known properties of OPCs with some historical background, followed by highlights from recent studies that suggest new roles for OPCs in certain pathological conditions.

The K2P -channel TASK1 affects Oligodendroglial differentiation but not myelin restoration.

In multiple sclerosis, demyelination occurs as a consequence of chronic autoimmunity in the central nervous system causing progressive neurological impairment in patients. After a demyelinating event, new myelin sheaths are formed by adult oligodendroglial progenitor cells; a process called remyelination. However, remyelination often fails in multiple sclerosis due to insufficient recruitment and differentiation of oligodendroglial precursor cells. A pivotal role for the two-pore-domain potassium (K2P ) channel, TASK1, has already been proven for an animal model of multiple sclerosis. However, the mechanisms underlying the TASK1-mediated effects are still elusive. Here, we tested the role of TASK1 channels in oligodendroglial differentiation and remyelination after cuprizone-induced demyelination in male mice. We found TASK1 channels to be functionally expressed on primary murine and human, pluripotent stem cell-derived oligodendrocytes. Lack of TASK1 channels resulted in an increase of mature oligodendrocytes in vitro as well as a higher number of mature oligodendrocytes and accelerated developmental myelination in vivo. Mechanistically, Task1-deficient cells revealed a higher amount of phosphorylated WNK1, a kinase known to be involved in the downstream signaling of the myelination regulator LINGO-1. Furthermore, we analyzed the effect of genetic TASK1 ablation or pharmacological TASK1 inhibition on disease-related remyelination. Neither channel inhibition nor lack of TASK1 channels promoted remyelination after pathological demyelination. In summary, we conclude that functional TASK1 channels participate in the modulation of differentiating oligodendroglial cells in a previously unknown manner. However, while being involved in developmental myelination our data suggest that TASK1 channels have no major effect on remyelination.

Robust Myelination of Regenerated Axons Induced by Combined Manipulations of GPR17 and Microglia.

Myelination facilitates rapid axonal conduction, enabling efficient communication across different parts of the nervous system. Here we examined mechanisms controlling myelination after injury and during axon regeneration in the central nervous system (CNS). Previously, we discovered multiple molecular pathways and strategies that could promote robust axon regrowth after optic nerve injury. However, regenerated axons remain unmyelinated, and the underlying mechanisms are elusive. In this study, we found that, in injured optic nerves, oligodendrocyte precursor cells (OPCs) undergo transient proliferation but fail to differentiate into mature myelination-competent oligodendrocytes, reminiscent of what is observed in human progressive multiple sclerosis. Mechanistically, we showed that OPC-intrinsic GPR17 signaling and sustained activation of microglia inhibit different stages of OPC differentiation. Importantly, co-manipulation of GPR17 and microglia led to extensive myelination of regenerated axons. The regulatory mechanisms of stage-dependent OPC differentiation uncovered here suggest a translatable strategy for efficient de novo myelination after CNS injury.

Quantitative Imaging of White and Gray Matter Remyelination in the Cuprizone Demyelination Model Using the Macromolecular Proton Fraction.

Macromolecular proton fraction (MPF) has been established as a quantitative clinically-targeted MRI myelin biomarker based on recent demyelination studies. This study aimed to assess the capability of MPF to quantify remyelination using the murine cuprizone-induced reversible demyelination model. MPF was measured in vivo using the fast single-point method in three animal groups (control, cuprizone-induced demyelination, and remyelination after cuprizone withdrawal) and compared to quantitative immunohistochemistry for myelin basic protein (MBP), myelinating oligodendrocytes (CNP-positive cells), and oligodendrocyte precursor cells (OPC, NG2-positive cells) in the corpus callosum, caudate putamen, hippocampus, and cortex. In the demyelination group, MPF, MBP-stained area, and oligodendrocyte count were significantly reduced, while OPC count was significantly increased as compared to both control and remyelination groups in all anatomic structures (p < 0.05). All variables were similar in the control and remyelination groups. MPF and MBP-stained area strongly correlated in each anatomic structure (Pearson's correlation coefficients, r = 0.80-0.90, p < 0.001). MPF and MBP correlated positively with oligodendrocyte count (r = 0.70-0.84, p < 0.01 for MPF; r = 0.81-0.92, p < 0.001 for MBP) and negatively with OPC count (r = -0.69--0.77, p < 0.01 for MPF; r = -0.72--0.89, p < 0.01 for MBP). This study provides immunohistological validation of fast MPF mapping as a non-invasive tool for quantitative assessment of de- and remyelination in white and gray matter and indicates the feasibility of using MPF as a surrogate marker of reparative processes in demyelinating diseases.

Efficient Remyelination Requires DNA Methylation.

Oligodendrocyte progenitor cells (OPCs) are the principal source of new myelin in the central nervous system. A better understanding of how they mature into myelin-forming cells is of high relevance for remyelination. It has recently been demonstrated that during developmental myelination, the DNA methyltransferase 1 (DNMT1), but not DNMT3A, is critical for regulating proliferation and differentiation of OPCs into myelinating oligodendrocytes (OLs). However, it remains to be determined whether DNA methylation is also critical for the differentiation of adult OPCs during remyelination. After lysolecithin-induced demyelination in the ventrolateral spinal cord white matter of adult mice of either sex, we detected increased levels of DNA methylation and higher expression levels of the DNA methyltransferase DNMT3A and lower levels of DNMT1 in differentiating adult OLs. To functionally assess the role of DNMT1 and DNMT3 in adult OPCs, we used mice with inducible and lineage-specific ablation of Dnmt3a and/or Dnmt1 (i.e., Plp-creER(t);Dnmt3a-flox, Plp-creER(t);Dnmt1-flox, Plp-creER(t);Dnmt1-flox;Dnmt3a-flox). Upon lysolecithin injection in the spinal cord of these transgenic mice, we detected defective OPC differentiation and inefficient remyelination in the Dnmt3a null and Dnmt1/Dnmt3a null mice, but not in the Dnmt1 null mice. Taken together with previous results in the developing spinal cord, these data suggest an age-dependent role of distinct DNA methyltransferases in the oligodendrocyte lineage, with a dominant role for DNMT1 in neonatal OPCs and for DNMT3A in adult OPCs.