Calpain 10: Degradation and Role in Mitochondrial Dysfunction

Calpain 10 is a Ca2+-regulated cysteine protease that is ubiquitously expressed and has been localized to the nucleus, mitochondria and cytosol. Dysregulation of calpain 10 activity has been implicated in three diseases: type 2 diabetes, renal aging and acute kidney injury.;Since dysregulation of calpain 10 activity leads to human disease, we determined the degradation pathways of both mitochondrial and cytosolic calpain 10. Using pharmacological inhibitors we showed that mitochondrial calpain 10 can become oxidized and degraded by Lon protease, while the 20S proteasome degrades cytosolic calpain 10.;In a streptozocotin-induced diabetic rat model calpain 10 is downregulated. Using this model and calpain 10 siRNA-treated rats, we determined in both models that there was mitochondrial dysfunction leading to mitophagy prior to apoptosis. This research suggests that some of the mitochondrial defects seen during chronic hyperglycemia are related to calpain 10 loss and that calpain 10 is important for proper mitochondrial function.;Previously, our laboratory created a calpain 10 specific peptide inhibitor, CYGAK, which was potent in isolated mitochondria but not efficacious in cells. Thus we created a homology model to optimize the inhibitor. This led to CYGAbuK, which exhibited increased potency over CYGAK, but was ineffective in cells. Therefore, we created a CYGAK-oleic acid heterodimer that was efficacious in cells. We detected decreased calpain activity and accumulation of calpain 10 substrates 24 hours after treatment in RPTC.;Since loss of calpain 10 in vivo causes mitochondrial dysfunction we determined if the same phenomenon occurred with calpain 10 inhibition in RPTC. Calpain 10 inhibition resulted in caspase-dependent apoptosis, decreased mitochondrial respiration, decreased mitochondrial fusion, increased mitochondrial fission and mitophagy. In summary, we determined how mitochondrial and cytosolic calpain 10 are degraded and we showed that loss of calpain 10 activity in vitro and in vivo results in mitochondrial dysfunction and increased mitophagy. Future work on why diabetes and aging cause downregulation of renal calpain 10 will lead to greater understanding of calpain 10 physiology and may provide insight to prevention and treatment of these diseases.