Molecular mechanisms underlying Juvenile hormone (JH) signaling pathway

Juvenile hormone (JH) is an important insect hormone that controls diverse biological processes in postembryonic development and adult reproduction. JH exerts its effects through the nuclear receptor Methoprene-tolerant (MET). MET is a transcription factor of the basic helix-loop-helix (bHLH)/Per-Arnt-Sim (PAS) family. In the presence of JH, MET forms a heterodimer with its DNA-binding partner Taiman (TAI). The MET-TAI complex directly binds to the regulatory regions of some JH target genes and regulates their transcription. However many questions remain unanswered regarding the JH-regulated gene expression. The work in this report aims to determine the role of protein kinase C (PKC) in JH signaling in adult mosquitoes and to find the direct target genes of Krüppel homolog 1 (Kr-h1), a zinc finger transcription factor encoded by a JH early response gene.

We discovered that PKC is an essential component of a membrane-initiated JH signaling pathway. PKC was activated by JH in a phospholipase C (PLC)-dependent manner. Inhibition of PKC activity dramatically decreased the JH-induced gene expression. RNAi experiment indicated that several PKC isoforms were involved in the JH action in adult female mosquitoes. We showed that PKC modulated the transactivation activity of MET by enhancing the binding of MET and TAI to the promoters of JH target genes.

Kr-h1 is rapidly upregulated by JH in newly emerged mosquitoes. RNAi-mediated depletion of AaKr-h1 caused a substantial decrease in oviposited eggs, indicating that this protein plays an essential role in mosquito reproduction. We combined chromatin immunoprecipitation (ChIP) with cloning of the generated DNA and have identified chromatin binding sites of AaKr-h1 in Aedes aegypti. After adult emergence, binding of AaKr-h1 to its in vivo targets increased with the JH-induced increase in AaKr-h1. Interestingly, depletion of AaKr-h1 in newly emerged mosquitoes led to considerable upregulation of some AaKr-h1 target genes but downregulation of other target genes. The results suggest that AaKr-h1 acts downstream of AaMET to regulate gene expression in response to JH and that AaKr-h1 can activate or repress the expression of individual target gene.