Cdk5: A Mediator of Mptp Opening
Saranya NavaneethaKrishnan1, Jesusa L. Rosales1, Ki-Young Lee1*
1Department of Cell Biology & Anatomy, Arnie Charbonneau Cancer and Alberta Children’s Hospital Research Institutes, Cumming School of Medicine, University of Calgary, Calgary, AB, Canada
Mitochondrial Ca2+ concentration ([Ca2+]mt) regulates cellular functions as diverse as bioenergetic processes and apoptosis1. Ca2+ uptake by mitochondria via the mitochondrial Ca2+ uniporter (MCU) causes an increase in respiratory rate by activating dehydrogenases and electron carriers required for ATP production2. However, prolonged accumulation of mitochondrial Ca2+ results in sustained opening of the mitochondrial permeability transition pore (mPTP) which causes cell death3. In fact, mPTP is a Ca2+-dependent and cyclosporin A-sensitive large conductance channel in the inner mitochondrial membrane (IMM). It plays a role in mitochondrial Ca2+ homeostasis and other physiological as well as pathological functions3. Thus, although mPTP serves as a mitochondrial Ca2+ extrusion channel, prolonged mPTP opening induces increased production of reactive oxygen species (ROS), mitochondrial depolarization and caspase-mediated apoptosis4,5.
Our previous studies in breast cancer cells demonstrate that Cdk5 depletion by siRNA triggers mPTP opening that leads to ROS increase, mitochondrial depolarization, mitochondrial fragmentation and intrinsic apoptosis. These events are accompanied by a rise in intracellular Ca2+ concentration ([Ca2+]i)6. These phenotypes were recapitulated in primary mouse embryonic fibroblasts (MEFs) from Cdk5−/− mice7, which exhibit perinatal mortality (i.e. 64% die in utero and newborns are dead or weak and die within 12 hours after birth)8. Treatment of Cdk5-depleted breast cancer cells6 or Cdk5−/− MEFs7 with an mPTP inhibitor, cyclosporine A (CsA) or sanglifehrin A (SfA), reverses mPTP-mediated mitochondrial dysfunction and apoptosis6, suggesting that Cdk5 acts as a modulator of mPTP opening7.
Mitochondria associate with endoplasmic reticulum (ER) via the Cdk5-containing mitochondria-associated ER membrane (MAM). A MAM-targeting motif in Cdk5 has not been reported. However, since the Cdk5 activator, p35, contains an N-terminal myristoylation signal motif that allows targeting of the Cdk5/p35 complex to cellular membranes, we speculate that Cdk5 localizes to MAMs through p35. The versatile MAMs are fundamental for Ca2+ and lipid transfer between mitochondria and ER. Approximately 5 to 20% of mitochondrial surface is physically in contact with the ER, and contact distances range from 10 to 30 nm9. As the mitochondrial Ca2+ uptake machinery has low affinity to Ca2+, MAM microdomains facilitate efficient 1,4,5-trisphosphate receptor (IP3R)-mediated Ca2+ transfer from the ER to the adjacent mitochondria10. Interestingly, loss of Cdk5 in MAMs significantly increases the number and lengths of ER-mitochondrial contact sites as well as the percentage of ER-mitochondria contact sites with a distance of <30 nm, leading to increased Ca2+ transfer from the ER to the mitochondria, elevation of [Ca2+]mt and subsequent mPTP opening7. The fact that mPTP opening due to Cdk5 loss is completely blocked by inhibition of either ER Ca2+ release with xestospongin c (XeC) or mitochondrial Ca2+ uptake with ruthenium red (RuR)7 further supports our view that mPTP opening is regulated by Cdk5. Since the regulatory function of Cdk5 is mostly attributed to its kinase activity, whether such activity is required for Cdk5 control of mPTP opening is currently under investigation.
It is interesting that Cdk5 loss induces apoptosis in breast cancer cells6 but not in Cdk5−/− MEFs7. Further analysis on how Cdk5 regulates ER-mitochondria tethering and intracellular Ca2+ signaling in normal and diseased conditions could provide clues on how loss of Cdk5 leads to mitochondria-associated apoptosis in cancer cells.
This work was supported by a grant from the NSERC (RGPIN/06270-2019) to KYL and an Alberta Cancer Foundation graduate studentship to SN.
SN drafted the manuscript. JLR and KYL critically revised the manuscript for important intellectual content and wrote the final version of the manuscript
Disclosure of conflicts of interest
The authors declare no competing financial interests.
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