Regulation of Axon Degeneration after Injury and in Development by the Endogenous Calpain Inhibitor Calpastatin

• Physical injury triggers depletion of the calpain inhibitor calpastatin within axons
• Axonal calpastatin inhibits Wallerian degeneration in vitro and in vivo
• Caspase-dependent calpastatin depletion is required for death of NGF-deprived axons
• Calpastatin levels control developmental pruning of retinal axons in vivo

SummaryAxon degeneration is widespread both in neurodegenerative disease and in normal neural development, but the molecular pathways regulating these degenerative processes and the extent to which they are distinct or overlapping remain incompletely understood. We report that calpastatin, an inhibitor of calcium-activated proteases of the calpain family, functions as a key endogenous regulator of axon degeneration. Calpastatin depletion was observed in degenerating axons after physical injury, and maintaining calpastatin inhibited degeneration of transected axons in vitro and in the optic nerve in vivo. Calpastatin depletion also occurred in a caspase-dependent manner in trophic factor-deprived sensory axons and was required for this in vitro model of developmental degeneration. In vivo, calpastatin regulated the normal pruning of retinal ganglion cell axons in their target field. These findings identify calpastatin as a key checkpoint for axonal survival after injury and during development, and demonstrate downstream convergence of these distinct pathways of axon degeneration.