Calpain as a novel regulator of autophagosome formation

Ubiquitously expressed mu and m-calpain proteases consist of 80-kDa catalytic subunits encoded by the Capn 1 and Capn2 genes, respectively, and a common 28-kDa regulatory subunit encoded by the calpain small 1 (Capns 1) gene. The mu- and m-calpain proteases have been implicated in both pro- or anti-apoptotic functions. We have found that Capns 1 depletion is coupled to increased sensitivity to apoptosis triggered by a number of autophagy-inducing stimuli in mammalian cells. Therefore we investigated the involvement of calpains in autophagy using MEFs derived from Capns 1 knockout mice and Capns 1 depleted human cells as model systems. We found that autophagy is impaired in Capns 1-deficient cells by immunostaining of the endogenous autophagosome marker LC3 and electron microscopy experiments. Accordingly, the enhancement of lysosomal activity and long-lived proteins degradation, normally occurring upon starvation, are also reduced. In Capns 1-depleted cells ectopic LC3 accumulates in early endosome-like vesicles that might represent a salvage pathway for protein degradation when autophogy is defective.