Regulation of germline proteostasis by insulin/IGF-1 signaling
Gametogenesis involves active protein synthesis and heavily relies on proteostasis. How animals regulate germline proteostasis at the organismal level is poorly understood. Our recent work in C. elegans indicates that germline development requires coordinated activities between insulin/IGF-1 signaling and HSF-1, the transcriptional activator of many molecular chaperones in stress and physiological conditions. In this study, we show that HSF-1 is important for germline proteostasis at ambient temperature. Depletion of HSF-1 from germ cells impairs chaperone gene expression, causing protein degradation and aggregation and, consequently, declines in fecundity and gamete quality. Reduced insulin/IGF-1 signaling confers germ cells' tolerance to limited protein folding capacity and proteotoxic stress by lowering ribosome biogenesis and translation. Interestingly, regulation of germline proteostasis by insulin/IGF-1 signaling occurs non-cell-autonomously. Our data suggest that insulin/IGF-1 signaling controls the expression of the evolutionarily conserved intestinal peptide transporter PEPT-1 via its downstream transcription factor FOXO/DAF-16, therefore allowing dietary proteins to be incorporated into an amino acid pool that fuels ribosomal biogenesis and translation in the germline. We propose that this pathway plays a critical role in regulating germline protein synthesis, which must be at balance with HSF-1-dependent protein folding to achieve proteostasis in gametogenesis. Overall design: To understand the role of IIS in regulating germline proteostasis, we performed two series of transcriptomic analysis using a reduction-of-function mutant of IGF-1R/DAF-2. First, we depleted HSF-1 from the germline in the daf-2(e1370) mutant worms either starting from egg-lay for 88 hours or starting from the young adult stage for 16 hours till the worms grow to gravid adults. These experiments were to check if there are chaperone expression change and proteotoxic stress responses associated with loss of HSF-1. Auxin-inducible degron system was used for HSF-1 depletion upon auxin treatment. Mock-treatment with ethanol (EtOH) was used as the negative control.  Second, we isolated and enriched germline nuclei from both the wild-type control and the daf-2(e1370) mutant at the young adult stage, and examine the expression of proteostasis network players in the germline upon reduction of IIS.