THE ROLE OF NG2 EXPRESSING CELLS AND POSSIBLE MECHANISM OF THEIR ACTIVATION UNDER THE BRAIN INFLAMMATION

Abstract

NG2 expressing cells, the fourth major glial cells in the central nervous system (CNS) besides astrocytes, microglia and oligodendrocytes, are generally considered as oligodendrocyte progenitor cells (OPCs). However, the facts of the substantial NG2 expressing cell numbers, even distribution in the brain parenchyma, and reactive responses in pathological conditions have cast the doubt on role of NG2 expressing cells only as OPCs. The current study explored some possible functions and mechanisms of NG2 expressing cell activation. NG2+ cells showed reactive responses to the neuroinflammation induced by focal injection of lipopolysaccharide (LPS) in the brain cortex, presenting not only increase in NG2+ cell numbers but also increase in the size of cell bodies and shorting in the length of cell processes. The appearance of NG2+/OX42+ cells suggests that some activated microglia could be induced to express NG2 proteins. Although the exact function of the constitutive NG2 cells and microglia with induced NG2 protein remains unknown, NG2 expressing cells, especially induced NG2 microglia, did not show phagocytic capacity but differential expression of IL-1ß. Further results showed that activation of NG2 expressing cells may be an event downstream to microglial activation in neuroinflammation and the expression of NG2 protein on microglia may be possibly induced by the TGF-ß1 via the Smad2/3 signalling pathway. The findings obtained could broaden the current knowledge about functions of NG2 expressing cells and gain a better understanding about the mechanism of their activation in the cellular network of the CNS, which may help to create new therapeutic targets in treatment of nerve injuries and neurological diseases.