Glial cells in the mammalian central nervous system (CNS) that express the NG2 proteoglycan (NG2 cells) represent a fourth major glial cell population that is distinct from mature oligodendrocytes, astrocytes, or resting ramified microglia. They differentiate into oligodendrocytes in gray and white matter and provide an endogenous source of myelinating cells during development and repair of demyelinated lesions. While all NG2 cells express platelet-derived growth factor receptor alpha and the basic helix-loop-helix transcription factor Olig2, there are some regional differences in their fate and proliferative behavior. One example is the astrogliogenic fate of NG2 cells in the dorsal and ventral forebrain. A subpopulation of NG2 cells in the embryonic ventral forebrain generates 40% of the protoplasmic astrocytes in the region, whereas the fate of NG2 cells in the dorsal forebrain is restricted to the oligodendrocyte lineage. It has been shown that NG2 cells in the ventral and dorsal forebrain arise from distinct germinal zones that are specified by distinct transcription factors.
Specific Aim 1 will investigate whether the dorsoventral differences in the astrogliogenic fate of NG2 cells are established by the transcription factor code of the germinal zone of origin. Another example of NG2 cell heterogeneity is the difference in the rate of proliferation and oligodendrocyte differentiation between NG2 cells in the gray and white matter. NG2 cells in the white matter are known to undergo greater expansion and oligodendrocyte differentiation than those in the gray matter. Our recent slice culture experiments indicate that NG2 cells in the white matter proliferate to a greater extent in response to platelet-derived growth factor compared with their gray matter counterparts. Furthermore, proliferation of NG2 cells in the gray but not white matter is increased by activation of a family of potassium channels.
Specific Aim 2 will investigate the mechanisms that lead to differences in proliferation and oligodendrocyte differentiation of NG2 cells in gray and white matter during development and remyelination.
NG2 cells represent a glial progenitor population that is ubiquitously distributed throughout the gray and white matter of the central nervous system and provide an endogenous source of remyelinating cells for the repair of demyelinated lesions in multiple sclerosis. The proposed studies will use a combination of slice culture and mouse genetic tools to investigate the mechanism of the observed regional differences in the fate and proliferative behavior of NG2 cells. Elucidating the basis of the heterogeneity of NG2 cells will enable future studies to be directed toward promoting repair from specific subpopulations of NG2 cells.
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