Rats lacking Ucp1 present a novel translational tool for the investigation of thermogenic adaptation during cold challenge.

AIM: Valuable studies have tested the role of UCP1 on body temperature maintenance in mice, and we sought to knockout Ucp1 in rats (Ucp1-/- ) to provide insight into thermogenic mechanisms in larger mammals. METHODS: We used CRISPR/Cas9 technology to create Ucp1-/- rats. Body weight and adiposity were measured, and rats were subjected to indirect calorimetry. Rats were maintained at room temperature or exposed to 4°C for either 24 h or 14 days. Analyses of brown and white adipose tissue and skeletal muscle were conducted via histology, western blot comparison of oxidative phosphorylation proteins, and qPCR to compare mitochondrial DNA levels and mRNA expression profiles. RNA-seq was performed in skeletal muscle. RESULTS: Ucp1-/- rats withstood 4°C for 14 days, but core temperature steadily declined. All rats lost body weight after 14 days at 4°C, but controls increased food intake more robustly than Ucp1-/- rats. Brown adipose tissue showed signs of decreased activity in Ucp1-/- rats, while mitochondrial lipid metabolism markers in white adipose tissue and skeletal muscle were increased. Ucp1-/- rats displayed more visible shivering and energy expenditure than controls at 4°C. Skeletal muscle transcriptomics showed more differences between genotypes at 23°C than at 4°C. CONCLUSION: Room temperature presented sufficient cold stress to rats lacking UCP1 to activate compensatory thermogenic mechanisms in skeletal muscle, which were only activated in control rats following exposure to 4°C. These results provide novel insight into thermogenic responses to UCP1 deficiency; and highlight Ucp1-/- rats as an attractive translational model for the study of thermogenesis.

Female Mice Are Protected from Metabolic Decline Associated with Lack of Skeletal Muscle HuR.

Male mice lacking HuR in skeletal muscle (HuRm-/-) have been shown to have decreased gastrocnemius lipid oxidation and increased adiposity and insulin resistance. The same consequences have not been documented in female HuRm-/- mice. Here we examine this sexually dimorphic phenotype. HuRm-/- mice have an increased fat mass to lean mass ratio (FM/LM) relative to controls where food intake is similar. Increased body weight for male mice correlates with increased blood glucose during glucose tolerance tests (GTT), suggesting increased fat mass in male HuRm-/- mice as a driver of decreased glucose clearance. However, HuRm-/- female mice show decreased blood glucose levels during GTT relative to controls. HuRm-/- mice display decreased palmitate oxidation in skeletal muscle relative to controls. This difference is more robust for male HuRm-/- mice and more exaggerated for both sexes at high dietary fat. A high-fat diet stimulates expression of Pgc1α in HuRm-/- male skeletal muscle, but not in females. However, the lipid oxidation Pparα pathway remains decreased in HuRm-/- male mice relative to controls regardless of diet. This pathway is only decreased in female HuRm-/- mice fed high fat diet. A decreased capacity for lipid oxidation in skeletal muscle in the absence of HuR may thus be linked to decreased glucose clearance in male but not female mice.

The RNA binding protein HuR influences skeletal muscle metabolic flexibility in rodents and humans.

BACKGROUND: Metabolic flexibility can be assessed by changes in respiratory exchange ratio (RER) following feeding. Though metabolic flexibility (difference in RER between fasted and fed state) is often impaired in individuals with obesity or type 2 diabetes, the cellular processes contributing to this impairment are unclear. MATERIALS AND METHODS: From several clinical studies we identified the 16 most and 14 least metabolically flexible male and female subjects out of >100 participants based on differences between 24-hour and sleep RER measured in a whole-room indirect calorimeter. Global skeletal muscle gene expression profiles revealed that, in metabolically flexible subjects, transcripts regulated by the RNA binding protein, HuR, are enriched. We generated and characterized mice with a skeletal muscle-specific knockout of the HuR encoding gene, Elavl1 (HuRm-/-). RESULTS: Male, but not female, HuRm-/- mice exhibit metabolic inflexibility, with mild obesity, impaired glucose tolerance, impaired fat oxidation and decreased in vitro palmitate oxidation compared to HuRfl/fl littermates. Expression levels of genes involved in mitochondrial fatty acid oxidation and oxidative phosphorylation are decreased in both mouse and human muscle when HuR is inhibited. CONCLUSIONS: HuR inhibition results in impaired metabolic flexibility and decreased lipid oxidation, suggesting a role for HuR as an important regulator of skeletal muscle metabolism.