Calpain activity and expression increased in activated glial and inflammatory cells in penumbra of spinal cord injury lesion

Following traumatic injury of the spinal cord, cells adjacent to the lesion are subject to ischemic cell death as a result of vascular disruption and secondary inflammatory responses. Proteases such as calcium-activated neutr al proteinase (calpain) have been implicated in axon and myelin destruction following injury since they degrade structural proteins in the axon-myelin unit. To examine the role of calpain in cell death following spinal cord i njury (SCI), calpain activity and translational expression were evaluated u sing Western blotting techniques. Calpain activity (as measured by specific substrate degradation) was significantly increased in and around the lesio n site as early as 4 hr following injury with continued elevation at 48 hr compared to sham controls. Likewise, calpain expression was significantly i ncreased in both the lesion site and penumbra at 4 and 48 hr after injury. Using double immunofluorescent labeling for calpain and cell-specific marke rs, this increase in calpain expression was found to be due in part to acti vated glial/inflammatory cells such as astrocytes, microglia, and infiltrat ing macrophages in these areas. Thus, since calpain degrades many myelin an d axonal structural proteins, the increased activity and expression of this enzyme may be responsible for destruction of myelinated axons adjacent to the lesion site following SCI. (C) 2000 Wiley-Liss, Inc.