Calcium influx and activation of Calpain I mediate acute reactive gliosis in injured spinal cord

Buffering extracellular pH at the site of a spinal cord crush-injury may st imulate axonal regeneration in rats (1; Guth et al., Exp. Neurol. 88: 44-55 , 1985). We demonstrated in cultured astrocytes that acidic pH initiates a rapid increase in immunoreactivity for GFAP (GFAP-IR), a hallmark of reacti ve gliosis (2; Oh ct al., Glia 13: 319-322, 1995). We extended these studie s by investigating the effects of certain treatments on reactive gliosis de veloping in situ in a rat spinal cord injury model. A significant reactive gliosis was observed within 2 days of cord lesion in untreated crush or veh icle-treated, crush control animals as evidenced by increased GFAP-IR and h ypertrophy of astrocytes. By contrast, infusion of Pipes buffer (pH 7.4) in to the lesion site significantly reduced this increase. The increased GFAP- IR appeared to be linked to Ca2+ influx since infusion of a blocker of L-ty pe calcium channels, nifedipine, reduced the ensuing reactive gliosis signi ficantly. While Ca2+ modulates many signaling pathways within cells, its ef fect on reactive gliosis appeared to result from an activation of calpain I . Calpain inhibitor I, a selective inhibitor of mu-calpain, also significan tly reduced reactive gliosis. However, calpain inhibitor II, a close struct ural analog which blocks m-calpain, had no salutary effect. We suggest, the refore, that the initial reactive gliosis seen in vivo may result from the activation of a neutral, Ca(2+-)dependent protease, calpain I, through calc ium influx. (C) 1999 Academic Press.