In vitro beneficial activation of microglial cells by mechanically-injured astrocytes enhances the synthesis and secretion of BDNF through p38MAPK.

It has long been promulgated that microglial cells serve beneficial roles in the central nervous system (CNS). The beneficial role of microglial cells is considered to be linked with microglial activation and consequent up-regulation of various trophic factors. However, what triggers microglial activation and consequent elevated level of trophic factors, especially brain-derived neurotrophic factor (BDNF), following traumatic CNS injury has become a crucial but elusive issue. Furthermore, an effort still remains in understanding of the cellular and molecular mechanisms underlying the endogenous neuroprotection of activated microglial cells. In this study, we demonstrated that mechanically-injured astrocyte conditioned medium (ACM) could provoke beneficial activation of microglial cells and thus promote the transcription, synthesis and release of BDNF in cultured microglial cells. The microglia-derived BDNF can exerted a demonstrable biological role in promoting neurite outgrowth and intimate terminal contacts of dorsal root ganglion (DRG) neurons co-cultured with microglial cells. Moreover, ACM induced remarkable p38MAPK phosphorylation in cultured microglial cells that preceded the burst of BDNF. Activating p38-MAPK by anisomycin resulted in salutary effects similar to those seen with ACM, whereas specific inhibition of the p38MAPK by SB203580 abrogated all the positive effects of ACM, including BDNF promotion and subsequent neurite outgrowth of DRG neurite outgrowth of DRG neurons and their intimate terminal contacts with microglial cells. Together, our results indicated that the neuroprotection of the microglial source is mainly caused by micro-environmental soluble molecules released from injured astrocytes, and ACM-induced BDNF production and release from microglial cells may be mediated through p38-MAPK signaling pathway. Therefore, these findings may lay a foundation to further investigations on the microglial beneficial activation role in the repair of traumatic CNS injury and neurodegenerative diseases.