CDK8-Cyclin C Mediates Nutritional Regulation of Developmental Transitions through the Ecdysone Receptor in Drosophila

Abstract

The steroid hormone ecdysone and its receptor (EcR) play critical roles in orchestrating developmental transitions in arthropods. However, the mechanism by which EcR integrates nutritional and developmental cues to correctly activate transcription remains poorly understood. Here, we show that EcR-dependent transcription, and thus, developmental timing in Drosophila, is regulated by CDK8 and its regulatory partner Cyclin C (CycC), and the level of CDK8 is affected by nutrient availability. We observed that cdk8 and cycC mutants resemble EcR mutants and EcR-target genes are systematically down-regulated in both mutants. Indeed, the ability of the EcR-Ultraspiracle (USP) heterodimer to bind to polytene chromosomes and the promoters of EcR target genes is also diminished. Mass spectrometry analysis of proteins that co-immunoprecipitate with EcR and USP identified multiple Mediator subunits, including CDK8 and CycC. Consistently, CDK8-CycC interacts with EcR-USP in vivo; in particular, CDK8 and Med14 can directly interact with the AF1 domain of EcR. These results suggest that CDK8-CycC may serve as transcriptional cofactors for EcR-dependent transcription. During the larval–pupal transition, the levels of CDK8 protein positively correlate with EcR and USP levels, but inversely correlate with the activity of sterol regulatory element binding protein (SREBP), the master regulator of intracellular lipid homeostasis. Likewise, starvation of early third instar larvae precociously increases the levels of CDK8, EcR and USP, yet down-regulates SREBP activity. Conversely, refeeding the starved larvae strongly reduces CDK8 levels but increases SREBP activity. Importantly, these changes correlate with the timing for the larval–pupal transition. Taken together, these results suggest that CDK8-CycC links nutrient intake to developmental transitions (EcR activity) and fat metabolism (SREBP activity) during the larval–pupal transition.

Transcriptional regulation of insect steroid hormone biosynthesis and its role in controlling timing of molting and metamorphosis

“…) and among CDK8, CyclinC, EcR, and USP (Xie et al . ). Further protein interactome analyses would be important in the future.…”

“…1). These pathways include the insulin like peptides-TOR pathway (Colombani et al 2005;Koyama et al 2013), the TGFb/Activin-Smad pathway (Gibbens et al 2011), serotonin and its receptor 5HT-7 (Shimada-Niwa & Niwa 2014), octopamine and its receptor Octb3R (Ohhara et al 2012(Ohhara et al , 2015, and Bommo-FMRFamide that is a prothoracicostatic neuropeptide (Yamanaka et al 2007), all of which are required for determining the proper timing of ecdysteroid biosynthesis in the PG during development (Niwa & Niwa 2014b). It will be interesting to determine which ecdysteroidogenic TFs act downstream of any of these signaling cascades, and whether any ecdysteroidogenic TFs are regulated by those multiple signaling inputs.…”

“…It must be noted that regulatory mechanisms to control ecdysteroidogenic gene expression might be diversified among insect species. For example, several studies have reported that juvenile hormones (JHs) have significant effects on some ecdysteroidogenic enzyme genes and torso in the PG of B. mori (Yamanaka et al 2007;Young et al 2012;Ogihara et al 2015). This is less likely the case with the PG in D. melanogaster, considering that JHs appear not to have the typical "status quo" effect on larval development in D. melanogaster (Niwa et al 2008;Liu et al 2009;Riddiford et al 2010;Ono 2014;Wen et al 2015).…”

“…Physical interactions between ecdysteroidogenic TFs and other regulator proteins have also been recognized as another layer of mechanisms regulating ecdysteroid biosynthesis in the PG, as shown by the interaction between USP and FOXO ) and among CDK8, CyclinC, EcR, and USP (Xie et al 2015). Further protein interactome analyses would be important in the future.…”

“…Namiki et al 2005;Niwa et al 2005;Yamanaka et al 2007). Conversely, PTTH neuron-ablated animals or loss-of-function animals of torso, which encodes a PTTH receptor(Rewitz et al 2009b) exhibit drastic reduction of many ecdysteroidogenic enzyme genes in D. melanogaster(McBrayer et al 2007;Niwa et al 2010;Enya et al 2014).…”