NG2-proteoglycan-dependent contributions of oligodendrocyte progenitors and myeloid cells to myelin damage and repair

BACKGROUND

The NG2 proteoglycan is expressed by several cell types in demyelinated lesions and has important effects on the biology of these cells. Here we determine the cell-type-specific roles of NG2 in the oligodendrocyte progenitor cell (OPC) and myeloid cell contributions to demyelination and remyelination.

METHODS

We have used Cre-Lox technology to dissect the cell-type-specific contributions of NG2 to myelin damage and repair. Demyelination is induced by microinjection of 1 % lysolecithin into the spinal cord white matter of control, OPC-specific NG2-null (OPC-NG2ko), and myeloid-specific NG2-null (My-NG2ko) mice. The status of OPCs, myeloid cells, axons, and myelin is assessed by light, immunofluorescence, confocal, and electron microscopy.

RESULTS

In OPC-NG2ko mice 1 week after lysolecithin injection, the OPC mitotic index is reduced by 40 %, resulting in 25 % fewer OPCs at 1 week and a 28 % decrease in mature oligodendrocytes at 6 weeks post-injury. The initial demyelinated lesion size is not affected in OPC-NG2ko mice, but lesion repair is delayed by reduced production of oligodendrocytes. In contrast, both the initial extent of demyelination and the kinetics of lesion repair are decreased in My-NG2ko mice. Surprisingly, the OPC mitotic index at 1 week post-injury is also reduced (by 48 %) in My-NG2ko mice, leading to a 35 % decrease in OPCs at 1 week and a subsequent 34 % reduction in mature oligodendrocytes at 6 weeks post-injury. Clearance of myelin debris is also reduced by 40 % in My-NG2ko mice. Deficits in myelination detected by immunostaining for myelin basic protein are confirmed by toluidine blue staining and by electron microscopy. In addition to reduced myelin repair, fewer axons are found in 6-week lesions in both OPC-NG2ko and My-NG2ko mice, emphasizing the importance of myelination for neuron survival.

CONCLUSIONS

Reduced generation of OPCs and oligodendrocytes in OPC-NG2ko mice correlates with reduced myelin repair. Diminished demyelination in My-NG2ko mice may stem from a reduction (approximately 70 %) in myeloid cell recruitment to lesions. Reduced macrophage/microglia numbers may then result in decreased myelin repair via diminished clearance of myelin debris and reduced stimulatory effects on OPCs.

Integration and differentiation of neural stem cells after transplantation into the dysmyelinated central nervous system of adult mice

Mutant mice deficient in the myelin-associated glycoprotein (MAG) and the nonreceptor-type tyrosine kinase Fyn are characterized by a severely hypomyelinated central nervous system (CNS) and morphologically abnormal myelin sheaths. Despite this pronounced phenotype, MAG/Fyn-deficient mice have a normal longevity. In the present study, we took advantage of the normal life expectancy of this myelin mutant and grafted neural stem cells (NSCs) into the CNS of MAG/Fyn-deficient mice to study in short- and long-term experiments the fate of NSCs in adult dysmyelinated brains. Neural stem cells were isolated from spinal cords of transgenic mouse embryos ubiquitously expressing enhanced green fluorescent protein. Cells were expanded in vitro in the presence of mitogens for up to 5 weeks before they were grafted into the lateral ventricles or injected into white matter tracts. Analysis of mutant brains 3-15 weeks after intracerebroventricular transplantation of NSCs revealed only limited integration of donor cells into the host brains. However, injection of NSCs directly into white matter tracts resulted in widespread distribution of donor cells within the host tissue. Donor cells survived for at least 15 weeks in adult host brains. The majority of grafted cells populated white matter tracts and differentiated into oligodendrocytes that myelinated host axons. Results suggest that intraparenchymal transplantation of NSCs might be a strategy to reconstruct myelin in dysmyelinated adult brains.