Integration of nutritional status with germline proliferation: characterizing the roles of NHR-88 and NHR-49 in the C. elegans gonad

Abstract

As organisms cycle between feeding and fasting, they must balance nutritional input with energy expenditures such as reproduction, growth, and repair. To achieve the proper equilibrium, these processes must be tightly regulated, requiring that nutritional status be communicated to all tissues. Lipid-responsive transcription factors called nuclear receptors are key to this transmission of information. Despite a growing knowledge of nuclear receptors, one significant question that remains is how this class of proteins integrates an environmental signal into an organismal response. I have chosen to address this question using the nematode C. elegans, in which the nuclear receptor NHR-49 plays a central role. Complementing previous work demonstrating a requirement of NHR-49 for metabolic homeostasis in fed and fasted worms, I have characterized additional facets of the NHR-49-dependent fasting response, including regulation of cell cycle and autophagy-related genes. My findings have also uncovered an extreme sensitivity to NHR-49 protein levels, as demonstrated by toxicity of multiple nhr-49 rescue constructs. Since the presence of genomic regions including the 3'UTR ameliorates this toxicity, I propose that miRNAs may be involved in titration of NHR-49 levels. To elucidate this involvement, I focused my studies on the two miRNAs predicted to target nhr-49: mir-243 and mir-797. Although I was unable to define the requirement for these miRNAs in fed animals, this research lead me to the establishment of a role for both mir-243 and mir-797 in recovery from the NHR-49-regulated fasting response, adult reproductive diapause (ARD). Additionally, my work implicated another nuclear receptor, NHR-88, in ARD entry and recovery. I also found that a mutation in nhr-88 synthetically interacts with two other alleles: cyp-35a5(ok1985) and the as-yet unidentified fhc10. Together these genes regulate the size of the mitotically proliferating population in the germline, fecundity, fat metabolism, and lifespan, possibly in response to a dihomo-gamma-linolenic acid-derived ligand. This newly defined network provides a model for studying nuclear receptor-driven transmission of environmental signals throughout the organism. Finally, identification of three factors with selective ARD phenotypes make this diapause a more viable tool for studying conserved processes such as stem cell maintenance and protective autophagy during starvation.