The activation of Calpains and Caspases during axonal degeneration

N. UNSAIN

Montevideo

Workshop; Neuron Connectivity: Local axonal processes and synaptic function; 2016

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 in vitrowith nicotinamide adenine dinucleotide (NAD+) protects against WD, indicating that axonal degeneration following axon transection isa regulated process. However, our understanding of the molecular mechanism underlying this protection remains 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 proceedsindependently of caspase activation, but recent genetic loss-of-function studies have shown that caspases play a crucial role in developmental axonal pruning. 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 used pure samples of axons undergoing degeneration 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 andcalpain are activated normally, suggesting that NAD+-mediated protection does not rely on the inhibition of these proteases. These results show an unsuspectedrelationship between caspases, calpains, and the mechanism underlying NAD+-mediated protection.