Deciphering novel functions of Cyclin dependent kinase 8 using Drosophila melanogaster

Cyclin dependent kinase 8 (CDK8) is a serine/threonine protein kinase that forms a module with Cyclin C, Mediator 12 and Mediator 13. The kinase module reversibly interacts with the Mediator, a 26-subunit protein complex that regulates RNA Polymerase II-mediated gene expression. CDK8 is implicated in regulating multiple signaling pathways, and enhanced activity of CDK8 has been found in tumorigenesis. Functional analysis of CDK8 is limited across different model organisms. Using Drosophila as a genetic model organism, researchers previously found that cdk8 mutations led to lethality and heterozygous cdk8 mutants had increased fat contents during larval stages. We found a novel function of Cdk8 in regulating mitochondrial morphology when we analyzed the function of Cdk8 throughout development of Drosophila. Ubiquitous depletion of Cdk8 produced progeny with abnormal wing postures with significant reduced survival rates and climbing ability. Cdk8 plays a critical role in regulating the structure of mitochondria, with its loss causing elongated mitochondria in muscles throughout development. We used multiple approaches to demonstrate that Cdk8 promotes the phosphorylation of Dynamin-related protein 1, Drp1 at S616, a protein required for mitochondrial fission. Interestingly, PTEN-induced kinase 1 (Pink1), a mitochondrial kinase implicated in Parkinson's disease, also phosphorylates Drp1 at the same residue. Indeed, overexpression of Cdk8 significantly suppresses the phenotypes observed in flies with low levels of Pink1, including elevated levels of ROS, mitochondrial dysmorphology, and behavioral defects. Drosophila Cdk8 is the sole orthologue of vertebrate CDK8 and CDK19. Expression of human CDK19 can significantly rescue defective phenotypes caused by Cdk8 depletion in fly muscles and neurons, indicating the conserved functions between fly Cdk8 and CDK19. Interestingly, CDK19 has both cytoplasmic and nuclear localization in muscles, which is consistent with the endogenous GFP-tagged fly Cdk8. This localization supports potential cytoplasmic functions. Expressing wildtype CDK19 or cytoplasmically targeted CDK19 can rescue behavioral defects in pink1 mutant flies, which demonstrates that the rescuing ability between fly Cdk8 and CDK19 are likely to be non-nuclear. Altogether, we found that Cdk8 can modulate Pink1 mutant phenotypes by promoting Drp1-mediated fission to restore mitochondrial homeostasis. Furthermore, we found fly Cdk8 shares conserved functions with CDK19 in regulating mitochondrial morphologies.