Saranya NavaneethaKrishnan1 · Vincent Law1 · Jungkwon Lee1 · Jesusa L. Rosales1 · Ki‑Young Lee1
1 Department of Cell Biology and Anatomy, Arnie Charbonneau Cancer and Alberta Children’s Hospital Research Institutes, Cumming School of Medicine, University of Calgary, Calgary, AB T2N 4N1, Canada
Loss of cyclin-dependent kinase 5 (Cdk5) in the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) increases ER–mitochondria tethering and ER Ca2+ transfer to the mitochondria, subsequently increasing mitochondrial Ca2+ concentration ([Ca2+]mt). This suggests a role for Cdk5 in regulating intracellular Ca2+ dynamics, but how Cdk5 is involved in this process remains to be explored. Using ex vivo primary mouse embryonic fibroblasts (MEFs) isolated from Cdk5−/− mouse embryos, we show here that loss of Cdk5 causes an increase in cytosolic Ca2+concentration ([Ca2+]cyt), which is not due to reduced internal Ca2+ store capacity or increased Ca2+ influx from the extracellular milieu. Instead, by stimulation with ATP that mediates release of Ca2+ from internal stores, we determined that the rise in [Ca2+]cyt in Cdk5−/− MEFs is due to increased inositol 1,4,5-trisphosphate receptor (IP3R)-mediated Ca2+ release from internal stores. Cdk5 interacts with the IP3R1 Ca2+ channel and phosphorylates it at Ser421. Such phosphorylation controls IP3R1-mediated Ca2+ release as loss of Cdk5, and thus, loss of IP3R1 Ser421 phosphorylation triggers an increase in IP3R1-mediated Ca2+ release in Cdk5−/− MEFs, resulting in elevated [Ca2+]cyt. Elevated [Ca2+]cyt in these cells further induces the production of reactive oxygen species (ROS), which upregulates the levels of Nrf2 and its targets, Prx1 and Prx2. Cdk5−/− MEFs, which have elevated [Ca2+]cyt, proliferate at a faster rate compared to wt, and Cdk5−/− embryos have increased body weight and size compared to their wt littermates. Taken together, we show that altered IP3R1-mediated Ca2+ dynamics due to Cdk5 loss correspond to accelerated cell proliferation that correlates with increased body weight and size in Cdk5−/− embryos.