The Mediator CDK8-Cyclin C complex modulates Dpp signaling in Drosophila by stimulating Mad-dependent transcription

Abstract

Dysregulation of CDK8 (Cyclin-Dependent Kinase 8) and its regulatory partner CycC (Cyclin C), two subunits of the conserved Mediator (MED) complex, have been linked to diverse human diseases such as cancer. Thus, it is essential to understand the regulatory network modulating the CDK8-CycC complex in both normal development and tumorigenesis. To identify upstream regulators or downstream effectors of CDK8, we performed a dominant modifier genetic screen in Drosophila based on the defects in vein patterning caused by specific depletion or overexpression of CDK8 or CycC in developing wing imaginal discs. We identified 26 genomic loci whose haploinsufficiency can modify these CDK8- or CycC-specific phenotypes. Further analysis of two overlapping deficiency lines and mutant alleles led us to identify genetic interactions between the CDK8-CycC pair and the components of the Decapentaplegic (Dpp, the Drosophila homolog of TGFβ, or Transforming Growth Factor-β) signaling pathway. We observed that CDK8-CycC positively regulates transcription activated by Mad (Mothers against dpp), the primary transcription factor downstream of the Dpp/TGFβ signaling pathway. CDK8 can directly interact with Mad in vitro through the linker region between the DNA-binding MH1 (Mad homology 1) domain and the carboxy terminal MH2 (Mad homology 2) transactivation domain. Besides CDK8 and CycC, further analyses of other subunits of the MED complex have revealed six additional subunits that are required for Mad-dependent transcription in the wing discs: Med12, Med13, Med15, Med23, Med24, and Med31. Furthermore, our analyses confirmed the positive roles of CDK9 and Yorkie in regulating Mad-dependent gene expression in vivo. These results suggest that CDK8 and CycC, together with a few other subunits of the MED complex, may coordinate with other transcription cofactors in regulating Mad-dependent transcription during wing development in Drosophila.

Author summary

CDK8 and its dedicated partner CycC are conserved subunits of the Mediator complex that bridges transcription factors with RNA Polymerase II in eukaryotes. Here we explore the function and regulation of the CDK8-CycC pair in Drosophila by performing a dominant modifier genetic screen based on wing vein patterning defects caused by specific alteration of CDK8-CycC activities. We have observed that multiple components of the Dpp/TGFβ signaling pathway genetically interact with CDK8-CycC. CDK8 and CycC positively regulate gene expression activated by Mad, the key transcription factor downstream of Dpp/TGFβ signaling, and CDK8 can directly interact with the linker region of the Mad protein. We also identify additional, but not all, subunits of the Mediator complex that play positive roles in regulating Mad-dependent gene expression. Given the fundamental role of Dpp/TGFβ signaling in regulating development and its misregulation in a variety of diseases, understanding how Mad/Smad interacts with the Mediator complex may have broad implications in understanding the pathogenesis of these diseases.