XIAP-DEPENDENT CASPASE REGULATION IN DEGENERATING AXONS

NICOLAS UNSAIN ; JULIA HIGGINS; KRISTEN PARKER; MICHELE ZEINIEH; PHILIP A BARKER

Würzburg

Congreso; NGF 2012; 2012

University of Würzburg

Axonal loss that occurs in dorsal root sensory neurons deprived of NGF is an active process involving caspase activity. Nikolaev et al. (2009) showed that axonal degeneration in DRG sensory neurons withdrawn from NGF requires BAX-dependent activation of caspase-6 and showed that activation of caspase-3 was limited to the cell body and was not observed in axons. Schoenmann et al. (2010) reported that both intact and cleaved caspase-3 are present in axons maintained in NGF but found that NGF-deprivation did not increase axonal caspase-3 cleavage. We generated axons using the Twiss culturing method (2001) and determined levels of caspase-3 and its activation status, first through immunoblotting using antibodies directed against full-length or cleaved caspase-3. This showed that axons maintained in NGF contain abundant procaspase-3 and revealed that NGF-deprivation results in robust caspase-3 cleavage. We then examined the roles of the inhibitor of apoptosis proteins (IAPs). We found that two members of the IAP family, cIAP1 and XIAP, are abundant in DRG sensory axons maintained in NGF but interestingly, levels of XIAP were strongly reduced in NGF-deprived axons. In the IAP family, only XIAP can act as a direct inhibitor of caspase enzymatic activity and we therefore examined caspase-3 activity in axons of wild-type and XIAP-/- DRGs subjected to NGF withdrawal. These studies showed that caspase-3 levels and activity were sharply increased in XIAP-/- axons, consistent with the hypothesis that XIAP normally functions to restrain caspase-3 activity in intact as well as in degenerating axons. Taken together, these studies show that caspase-3 activation is a prominent feature within degenerating axons and identifies a novel physiological role for XIAP in axonal caspase regulation. *These authors contributed equally to this work.