XIAP-Dependent Caspase Regulation in Degenerating Axons

NICOLAS UNSAIN; JULIA HIGGINS; PHILIP A BARKER

Tahoe City

Congreso; Growing to Extremes: Cell Biology and Pathology of Axons - Keystone Symposia; 2013

Keystone Symposia on Molecular and Cellular Biology

Axonal degeneration is an important developmental event whose regulatory mechanisms are still poorly understood. In Drosophila melanogaster, developmental dendritic pruning of sensory neurons is mediated by the fly caspase DRONC which in turn is controlled by DIAP1, a member of the inhibitor-of-apoptosis (IAP) family (Kuo et al., 2006; Williams et al., 2006). Recent studies have shown that axonal degeneration of mammalian dorsal root sensory neurons (DRGs) deprived of NGF also involves caspases (Nikolaev et al., 2009; Schoenmann et al., 2010). X-linked IAP (XIAP, the mammalian equivalent of DIAP1) is the only mammalian IAP family member that directly inhibits caspases (Eckelman et al., 2006), but has not been shown to regulate caspases in a normal physiological setting. We tested the hypothesis that the caspase-IAP loop that regulates neurite degeneration in flies is phylogenetically conserved and functional in mammalian axons. We cultured DRG axons using the Twiss method and examined the caspase activation profile within pure axonal preparations. We found that axons maintained in NGF contain abundant procaspase-3, -6 and -9 and that NGF-deprivation induces the accumulation of cleaved caspase-3, and -6. We showed that cleaved caspase-3 within degenerating axons actively cleaves substrates and demonstrated that caspase-3 inhibition prevents degeneration. Turning our attention to XIAP, we found that axons derived from XIAP-/- mice show increased caspase-3 activity and accelerated degeneration when withdrawn from NGF. We also found that XIAP levels are dramatically reduced in NGF-deprived axons and that preventing XIAP loss sharply reduces caspase-3 activity and axonal degeneration. Taken together, these studies show that caspase-3 activation is a prominent feature within degenerating axons and identifies a physiological role for XIAP in axonal caspase regulation. *Contributed equally to this work. Study funded by a grant to PAB from the Canadian Institutes of Health Research (MOP123352).