mPTP opening caused by Cdk5 loss is due to increased mitochondrial Ca(2+) uptake

We previously demonstrated that loss of Cdk5 in breast cancer cells promotes ROS-mediated cell death by inducing mitochondrial permeability transition pore (mPTP) opening (Oncogene 37, 1788–1804). However, the molecular mechanism by which Cdk5 loss causes mPTP opening remains to be investigated. Using primary mouse embryonic fibroblasts (MEFs) isolated from Cdk5(−/−) mouse embryos, we show that absence of Cdk5 causes a significant increase in both mPTP opening and mitochondrial Ca(2+) level. Analysis of subcellular fractions of MEFs demonstrates that Cdk5 localizes in the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) and Cdk5 loss in MAMs causes increased ER-mitochondria tethering, a process required for Ca(2+) transfer from the ER to the mitochondria. Loss of Cdk5 also causes increased ATP-mediated mitochondrial Ca(2+) uptake from the ER. Inhibition of ER Ca(2+) release or mitochondrial Ca(2+) uptake in Cdk5(−/−) MEFs prevents mPTP opening, indicating that mPTP opening in Cdk5(−/−) MEFs is due to increased Ca(2+) transfer from the ER to the mitochondria. Altogether, our findings suggest that Cdk5 in MAMs regulates mitochondrial Ca(2+) homeostasis that is disturbed upon Cdk5 loss, which leads to mPTP opening.