Dev Bio SuperGroup Meeting

Ubiquitin-like protease regulates HMGS-1 SUMOylation:a novel regulation of metabolic networks in development, aging, and mitochondrial stress
Amir Sapir,  Sternberg lab

Metabolic networks are highly regulated to support the dynamic physiology of all organisms. The molecular details of this regulation, however, remain in many cases unknown.  Based on their ability to rapidly modify protein interactions, subcellular localization, and stability, we hypothesized that post translational modification pathways may be critical regulators of key metabolic network nodes. We focused on the small ubiquitin-like modifier (SUMO) pathway in C. elegans and identified a predicted SUMO protease, ULP-4, that exhibits a dynamic expression pattern and a striking cytosol-to-mitochondria translocation in various muscles and neurons during development, aging, and upon mitochondrial stress. Screening two-hybrid libraries, we found a strong association between ULP-4 and HMGS-1, the C. elegans ortholog of human 3-hydroxy-3-methylglutaryl CoA synthase proteins that play a role both in the cytosolic mevalonate pathway and in starvation-induced mitochondrial ketogenesis. Immunoprecipitation of tagged HMGS-1 protein demonstrates that HMGS-1 undergoes in vivo sumoylation and that the protein is highly sumoylated in 8-day old worms. Strikingly, HMGS-1::GFP is ectopically expressed and ectopically localized to mitochondria in smo-1 mutant background whereas HMGS-1::GFP signal is abolished in ulp-4 mutants. Like ulp-4, hmgs-1 expression is regulated developmentally and during aging and upon mitochondrial stress. ulp-4 and hmgs-1 loss resulted in age-dependent  attenuation of muscle
activity and impaired fat accumulation. Our results suggest a novel regulatory circuit in which ULP-4 dynamic expression and cytosol-to-mitochondria translocation regulate the sumoylation state of HMGS-1 during development, aging, and stress. Our study predicts that by modification of human HMGS-1 orthologs, SUMO proteases may control key metabolic pathways such as cholesterolbiogenesis and ketone body formation.

The role of Piwi in the Drosophila germline
Adrien Le Thomas, Aravin Lab

In the metazoan germline, piwi proteins and associated piwi-interacting RNAs (piRNAs) provide a defense system against the expression of transposable elements. In the cytoplasm, piRNA sequences guide piwi complexes to destroy complementary transposon transcripts by endonucleolytic cleavage. However, some piwi family members are nuclear, raising the possibility of alternative pathways for piRNA-mediated regulation of gene expression. We found that Drosophila Piwi is recruited to chromatin, colocalizing with RNA polymerase II (Pol II) on polytene chromosomes. Knockdown of Piwi in the germline increases expression of transposable elements that are targeted by piRNAs, whereas protein-coding genes remain largely unaffected. Derepression of transposons upon Piwi depletion correlates with increased occupancy of Pol II on their promoters. Expression of piRNAs that target a reporter construct results in a decrease in Pol II occupancy and an increase in repressive H3K9me3 marks and heterochromatin protein 1 (HP1) on the reporter locus. Our results indicate that Piwi identifies targets complementary to the associated piRNA and induces transcriptional repression by establishing a repressive chromatin state when correct targets are found.