NG2 proteoglycan-expressing microglia as multipotent neural progenitors in normal and pathologic brains

Rat primary microglia (MG) acquired a multipotent property to give rise to neuroectodermal cells through two-step culture in 10 and 70% serum-supplemented media for 5 days. Such multipotent MG, called promicroglioblasts (ProMGBs), formed cell aggregates, which generated cells with neuroectodermal phenotypes shortly after their transfer into serum-free medium. As revealed by immunohistochemistry, there were a few MG expressing NG2 chondroitin sulfate proteoglycan (NG2) in the neonatal rat brain. Primary culture from the neonatal brain contained NG2(+) MG, which appeared to be the source of NG2(+) ProMGB aggregates. The aggregates were MG marker(+)/NG2(+)/GFAP(+)/NCAM(+)/S-100 beta(-) and had alkaline phosphatase activity. The marked accumulation of NG2(+) MG was observed close to stab wounds made in the mature rat brain. The accumulated NG2(+) MG in the wound gradually decreased in number, but the cells persisted up to 150 days postlesioning. In addition, GFAP immunoreactivity increased markedly around the wound. The NG2+ MG in the wounds separated with trypsin-EDTA formed NG2(+) aggregates in 70% serum-supplemented medium and then transformed into cells with neuroectodermal phenotypes in serum-free medium. Although it is difficult to separate viable neurons from mature brains, cells from stab wounds generated process-bearing beta-tubulin III+ cells in vitro easily. These data suggest that NG2(+) MG in normal developing or pathologic brains are involved in the genesis or regeneration of the brain. (c) 2006 Wiley-Liss, Inc.