Sex differences in skeletal muscle atrophy induced by UBR5 suppression: a role for nuclear hormone receptor turnover

Introduction: The N-degron pathway contributes to proteolysis by targeting N-terminal residues of destabilized proteins via E3 ligases that contain a UBR-box domain (e.g. UBR5). Emerging evidence suggests that UBR5 may regulate nuclear hormone receptor (NHR) turnover via ubiquitination. We sought to investigate the effect of UBR5 knockdown (KD) on skeletal muscle metabolism and NHR turnover in male and female mice. Methodology: Adult female and male C57BL/6 mice were electroporated with a UBR5 RNAi plasmid in the tibialis anterior (TA) muscles with the contralateral leg serving as an empty vector (EV) control for a period of 7, 30 and 60 days. TA muscles were collected, weighed, and processed for biochemical and histochemical analyses. Western blotting was performed to determine levels of acetyl-lysine content, markers of mitochondrial dynamics (e.g. Drp1) and NHR content (e.g. androgen receptor). Metabolomic analyses were performed to determine changes in BCAA, TCA and glycolytic metabolism. Bulk RNA Sequencing analyses were performed at the 7-day time point on male and female transfected muscles to assess for early changes in the skeletal muscle transcriptome with UBR5 suppression. Results: UBR5 KD induced muscle atrophy in male and female mice at 30 days. Only male mice displayed further loss of muscle mass at 60 days, whereas atrophy in female RNAi muscle was no different between 30 and 60 days. At 30 days, increased BCAA catabolism was observed in male mice only. UBR5 KD induced an increased protein abundance in markers for mitochondrial dynamics in female, but not male mice. At 7 days post electroporation, we observed increased protein abundance in the glucocorticoid and androgen receptors with UBR5 KD in male mice only. Female mice displayed increased protein levels for the glucocorticoid and estrogen receptors with UBR5 KD. We observed 64 and 34 genes to be differentially expressed with UBR5 KD in male and female mice respectively. Interestingly, only 6 genes were commonly upregulated in male and female tissues, such as a transcriptional repressor (Nacc2), and regulators of redox balance (Nqo1) and fatty acid oxidation (Acod1). Conclusions: Loss of skeletal muscle mass with UBR5 suppression occurred in male and female mice, yet sex-specific differences in BCAA metabolism, mitochondrial dynamics and NHR abundance were evident. Future studies will seek to explore the interaction between UBR5 and NHRs in androgen deprivation models such as menopause and hypogonadism. D.C.H. was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under Award Numbers K01AR077684 and R03AR083980. Additional support was provided by a pilot project award to D.C.H. from the COBRE Center for Cellular Metabolism Research (NIGMS P20GM139763). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.