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

Abstract

Axon 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.

A recoverable state of axon injury persists for hours after spinal cord contusion in vivo

“…To determine whether axons that are at risk of fragmentation can still be rescued by therapeutic interventions initiated after injury, we pharmacologically inhibited the activity of calpain, a calcium- dependent protease implicated in axonal fragmentation [14][15][16] Fig. 5g,h).…”

Local endocytosis triggers dendritic thinning and pruning in Drosophila sensory neurons

“…For instance, in both cases, degeneration requires Ca 2 þ influx through VGCCs into axons or dendrites 19,42 . In addition, co-activation of the two proteases calpains and caspases is required for both dendrite degeneration in C4da neurons and axon degeneration in the mouse DRG neurons 19,43 . Furthermore, in flies overexpressing the causal gene product of the slow Wallerian degeneration mouse in C4da neurons, dendrite pruning was slightly but significantly retarded 15,44 .…”

NMNAT3 is protective against the effects of neonatal cerebral hypoxia‐ischemia

“…Taken together these observations suggest that NMNATs functions upstream to the cellular cascade that triggers the depletion of ATP and subsequent activation of the calpain‐calpastatin degenerative cascade following axotomy. Although we cannot conclude that this same mechanism occurs in injured immature brain neurons in response to increases in NMNAT3, the above findings and the observation that similar results have been noted in developing injured retinal ganglion neurons28 suggest that NMNATs may involve common death signaling pathways in both, the peripheral and central nervous system. Yet, the various NMNAT isoforms may individually differ in the cellular mechanisms that they employ to protect the injured brain.…”

“…Indeed, CASTN appears to be a key suicide substrate for the degeneration of neurons as a result of ischemic cerebral injury 25. Furthermore, CASTN has been recently implicated in the inhibition of axonal degeneration by NMNATs as overexpression of cytoplasmic NMNAT1 from injured axons of dorsal root ganglia (DRG) prevented injury‐induced CASTN degradation in vitro 28. Therefore, we investigated whether NMNAT3 overexpression results in the inhibition of CASTN degradation after neonatal H‐I.…”