Tissue‐specific regulation of calpain and calpastatin activities by PIN1 in mice

Abstract

The peptidyl‐proline isomerase, Protein Interacting with Never In Mitosis Gene A‐1 (PIN1), facilitates cis‐trans isomerization of phospho‐serine/threonine‐proline motifs in proteins, thereby modulating their synthesis, degradation and function. We previously found that depletion of PIN1 in endothelial cells (EC) reduced calpain activity. PIN1 may regulate calpains or the endogenous inhibitor, calpastatin. Here, we hypothesized that genetic deletion of PIN1 would reduce calpain activity and increase calpain inhibitory activity in mice. Calpain activity in different tissue extracts was measured with a fluorescently quenched peptide substrate. Calpain inhibitory activity was determined by first heat‐inactivating endogenous calpain, and then assessing the ability of extracts to inhibit cleavage of the peptide substrate by exogenous porcine erythrocyte μ‐calpain. As in EC, endogenous calpain activity was reduced in extracts of lung and kidney from PIN1 knockout mice. In addition, calpain inhibitory activity was increased in these knockout tissues. Surprisingly, the opposite phenotype was observed in knockout brain extracts: endogenous calpain activity was increased, and calpain inhibitory activity was reduced. These results indicate that PIN1 regulates calpain or calpastatin in vivo, but in a tissue‐specific manner.