Repression of a potassium channel by nuclear hormone receptor and TGF-β signaling modulates insulin signaling in Caenorhabditis elegans

Transforming growth factor β (tgf-β) signaling acts through smad proteins to play fundamental roles in cell proliferation,differentiation,apoptosis,and metabolism. the receptor associated smads (r-smads) interact with dna and other nuclear proteins to regulate target gene transcription. here,we demonstrate that the caenorhabditis elegans r-smad daf-8 partners with the nuclear hormone receptor nhr-69,a c. elegans ortholog of mammalian hepatocyte nuclear factor 4α hnf4α),to repress the exp-2 potassium channel gene and increase insulin secretion. we find that nhr-69 associates with daf-8 both in vivo and in vitro. functionally,daf-8 nhr-69 double mutants show defects in neuropeptide secretion and phenotypes consistent with reduced insulin signaling such as increased expression of the sod-3 and gst-10 genes and a longer life span. expression of the exp-2 gene,encoding a voltage-gated potassium channel,is synergistically increased in daf-8 nhr-69 mutants compared to single mutants and wild-type worms. in turn,exp-2 acts selectively in the asi neurons to repress the secretion of the insulin-like peptide daf-28. importantly,exp-2 mutation shortens the long life span of daf-8 nhr-69 double mutants,demonstrating that exp-2 is required downstream of daf-8 and nhr-69. finally,animals over-expressing nhr-69 specifically in daf-28-secreting asi neurons exhibit a lethargic,hypoglycemic phenotype that is rescued by exogenous glucose. we propose a model whereby daf-8/r-smad and nhr-69 negatively regulate the transcription of exp-2 to promote neuronal daf-28 secretion,thus demonstrating a physiological crosstalk between tgf-β and hnf4α-like signaling in c. elegans. nhr-69 and daf-8 dependent regulation of exp-2 and daf-28 also provides a novel molecular mechanism that contributes to the previously recognized link between insulin and tgf-β signaling in c. elegans. © 2012 park et al.