Microglial cell cycle‐associated proteins control microglial proliferation in vivo and in vitro and are regulated by GM‐CSF and density‐dependent inhibition

Hyperplasia of microglia is one characteristic of reactive gliosis, which is observed in various pathologic conditions in the central nervous system (CNS). To investigate the regulation mechanisms of microglial proliferation, the effect of granulocyte macrophage colony-stimulating factor (GM-CSF) on the expression of cell cycle-associated proteins was examined in the microglial cell line GMI-M6-3. After GM-CSF administration, cyclins D1, E, and A and cyclin-dependent kinase inhibitor p21Cip1 were increased, and another cyclin-dependent kinase inhibitor, p27Kip1, was decreased with morphologic transformation into ameboid form. By contrast, downregulation of these cyclins and p21Cip1, and strong upregulation of p27Kip1 accompanied by ramification were observed with GM-CSF deprivation. We also found that GMI-M6-3 exhibited homotypic contact inhibition of proliferation without any morphologic transformation. The increase of p27Kip1 and the decrease of cyclin A were suggested to play an important role in microglial contact inhibition. In addition, the direct effect of p27Kip1 to inhibit microglial proliferation was demonstrated both in vitro and in vivo by overexpression of p27Kip1.