Robert C. Noland ¹ SUJOY GHOSH ¹ Carlos J. Crisanto ² Antonio Aleman ² McKenna K. Chaney ³ Samuel E. Velasquez ¹ Jaycob Warfel ^4*

• ¹ Pennington Biomedical Research Center, Baton Rouge, Louisiana, United States
• ² Christian Brothers University, Memphis, Tennessee, United States
• ³ Southeastern Louisiana University, Hammond, Indiana, United States
• ⁴ University of Tennessee at Martin, Martin, Tennessee, United States

Introduction: Metabolic flexibility is the ability of a system to switch between metabolic substrates. Human and murine skeletal muscle tissue and cells with decreased activity of the regulatory RNA binding protein, Human Antigen R (HuR), have decreased capacity for fat oxidation, and thus decreased metabolic flexibility. This study aimed to assess the preference for carbohydrates in mice lacking HuR in skeletal muscle.Methods: Experiments were performed on weight-matched control and HuR knockout mice of both sexes. Palmitate and pyruvate oxidation were performed in mouse muscle following the release of 14 CO2. In vivo glucose and lipid uptake were assayed in mouse tissue following nonmetabolizable 3 H-2-deoxyglucose or 14 C-bromopalmitate injection. Transcriptomics analyses were performed in skeletal muscle of all mice, followed by qPCR validation of select genes. Serum lactate and glucose were measured in mice via tail nick, and muscle glycogen was measured through colorimetric assay. Indirect calorimetry was used to measure respiratory exchange ratios.Results: Male muscle-specific HuR knockout mice show increased glucose uptake relative to controls specifically in skeletal muscle and have increased muscle glycogen content. These mice also display greater respiratory exchange ratios relative to controls. None of these differences were noted in females. Transcriptomics show far more differences between male and female mice than between control and HuR knockout mice. However, differential gene expression between male and female mice is diminished by 50% following removal of HuR. Male HuR knockout mouse skeletal muscle has increased glycolytic gene expression relative to controls but show no difference relative to females of the same genotype. Both palmitate and pyruvate oxidation are decreased in skeletal muscle of male HuR knockout mice relative to controls, and serum lactate levels are increased. No notable differences were seen in females between genotypes.The increase in markers of glucose utilization with decreased HuR activity in male mice may indicate a switch toward glycolysis as compensation for decreased fat oxidation. These results continue to highlight a sex dependence on HuR as a driver of fat oxidation in mouse skeletal muscle, while also indicating that muscle itself shows greater ambiguity between males and females following removal of HuR.