Caspase-3 and Calpains become active during (and play a role in) injury-induced axonal degeneration but are not inhibited during NAD+-mediated protection

NICOLÁS UNSAIN; AARON D JOHNSTONE; PHIL A BARKER

Huerta Grande

Congreso; SAN XXIX Anual Meeting; 2014

Sociedad Argentina de Investigación en Neurociencias (SAN)

After nerve injury, the distal portion of severed axons undergoes a degenerative process known as Wallerian degeneration (WD) that leads to axonal fragmentation. Treatment of axons with nicotinamide adenine dinucleotide (NAD+) protects against WD, indicating that the axonal degeneration induced in a regulated process. Our understanding of the molecular mechanism underlying this protection is still rudimentary.Caspases are aspartate-directed proteases that play critical roles in the regulation and execution of apoptotic cell death during development and tissue maintenance. Initial studies suggested that axonal degeneration proceeded independently of caspases, but recent genetic loss-of-function studies have shown that caspases play a crucial role in developmental axonal pruning (Unsain et al., 2013. Cell Reports). On the other hand, calpains are calcium dependent proteases that play important roles in the execution of necrotic cell death. The precise role of these proteases in injury-induced axonal degeneration is a matter of current debate.In this study, we use pure samples of axons undergoing degeneration (Unsain et al., 2014, in press. JoVE) to unveil a novel interplay among caspase-3, calpains and the NAD+-sensitive system. We observed that caspase-3 becomes activated in axons early during WD, together with calpains. Calpains, in turn, cleave the N-terminus of caspase-3 and facilitate its activation. Caspase-3 or calpain inhibition greatly delays the detachment of axonal debris from the substrate. We further show that in NAD+-protected axons caspase-3 or calpain activities are normal, suggesting that NAD+-mediated protection does not rely on the inhibition of these proteases. These results show an intriguing relationship between caspases, calpains and the mechanism underlying NAD+-mediated protection.