Sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) uncoupling in skeletal muscle and mitochondrial uncoupling via uncoupling protein 1 (UCP1) in brown/beige adipose tissue are two mechanisms implicated in energy expenditure. Here, we investigated the effects of glycogen synthase kinase 3 (GSK3) inhibition via lithium chloride (LiCl) treatment on SERCA uncoupling in skeletal muscle and UCP1 expression in adipose. C2C12 and 3T3-L1 cells treated with LiCl had increased SERCA uncoupling and UCP1 protein levels, respectively, ultimately raising cellular respiration; however, this was only observed when LiCl treatment occurred throughout differentiation. In vivo, LiCl treatment (10 mg/kg/day) increased food intake in chow-fed diet and high-fat diet (HFD; 60% kcal)–fed male mice without increasing body mass—a result attributed to elevated daily energy expenditure. In soleus muscle, we determined that LiCl treatment promoted SERCA uncoupling via increased expression of SERCA uncouplers, sarcolipin and/or neuronatin, under chow-fed and HFD-fed conditions. We attribute these effects to the GSK3 inhibition observed with LiCl treatment as partial muscle-specific GSK3 knockdown produced similar effects. In adipose, LiCl treatment inhibited GSK3 in inguinal white adipose tissue (iWAT) but not in brown adipose tissue under chow-fed conditions, which led to an increase in UCP1 in iWAT and a beiging-like effect with a multilocular phenotype. We did not observe this beiging-like effect and increase in UCP1 in mice fed a HFD, as LiCl could not overcome the ensuing overactivation of GSK3. Nonetheless, our study establishes novel regulatory links between GSK3 and SERCA uncoupling in muscle and GSK3 and UCP1 and beiging in iWAT.

White adipose tissue (WAT) is a dynamic endocrine organ that can play a significant role in thermoregulation. WAT has the capacity to adopt structural and functional characteristics of the more metabolically active brown adipose tissue (BAT) and contribute to non-shivering thermogenesis under specific stimuli. Non-shivering thermogenesis was previously thought to be uncoupling protein 1 (UCP1)-dependent however, recent evidence suggests that UCP1-independent mechanisms of thermogenesis exist. Namely, futile creatine cycling has been identified as a contributor to WAT thermogenesis. The purpose of this study was to examine the efficacy of creatine supplementation to alter mitochondrial markers as well as adipocyte size and multilocularity in inguinal (iWAT), gonadal (gWAT), and BAT. Thirty-two male and female Sprague-Dawley rats were treated with varying doses (0 g/L, 2.5 g/L, 5 g/L, and 10 g/L) of creatine monohydrate for 8 weeks. We demonstrate that mitochondrial markers respond in a sex and depot specific manner. In iWAT, female rats displayed significant increases in COXIV, PDH-E1alpha, and cytochrome C protein content. Male rats exhibited gWAT specific increases in COXIV and PDH-E1alpha protein content. This study supports creatine supplementation as a potential method of UCP1-independant thermogenesis and highlights the importance of taking a sex-specific approach when examining the efficacy of browning therapeutics in future research.

Promotion of oxidative metabolism in skeletal muscle improves muscle performance, fatigue resistance, and may help combat the obesogenic effects of a high fat diet. Lithium is a well‐known glycogen synthase kinase (GSK3) inhibitor and we have recently shown that low‐dose lithium supplementation (10 mg kg ^‐1 day ^‐1 ) augments skeletal muscle oxidative capacity in mice. In the soleus muscle, lithium mediated GSK3 inhibition increased the expression of oxidative markers such as peroxisome proliferator‐activated receptor gamma coactivator 1‐alpha (PGC‐1a) and myosin heavy chain I (MHCI) leading to improvements in muscle contractility and fatigue resistance. Interestingly, this same dose of lithium (10 mg kg ^‐1 day ^‐1 ) has been previously shown to attenuate high fat diet induced weight gain and metabolic efficiency (weight gained per calories consumed). However, the impact of lithium supplementation on muscle oxidative phenotype after high fat feeding has yet to be investigated. In this study, 36 wild‐type male C57BL/6J mice were randomly assigned to low‐fat standard chow diet (LFD, 4% kcal fat), high‐fat diet (HFD, 60% kcal fat), or HFD with lithium chloride (HFD + Li; 10 mg kg ^‐1 day ^‐1 ) treatment groups for 12 weeks. Soleus muscles were collected at the end for Western blot analysis of total and phosphorylated GSK3 (inhibitory sites serine9 for GSK3b and serine21 for GSK3a), PGC‐1a, cytochrome c oxidase subunit IV (COXIV), and MHCI protein content. Our results showed that the level of phosphorylated GSK3a and GSK3b (relative to total GSK3) were significantly lower after the HFD (‐41 and ‐44%, respectively, p < 0.05), but in the HFD + Li group levels were similar to the LFD group. Therefore, the HFD enhanced soleus muscle GSK3 activation, and lithium supplementation was able to reduce this effect. No significant changes in MHCI content were observed. When examining PGC‐1a, we found that the HFD + Li group had protein levels that were nearly 2‐fold higher vs LFD, albeit only trending towards significance ( p = 0.06). Further, COXIV content progressively increased from LFD, to HFD, and to HFD + Li (linear trend, r ² =0.26, p = 0.002) with both HFD and HFD + Li groups having significantly higher COXIV content when compared to LFD. Altogether, our data shows that GSK3 inhibition via low dose lithium supplementation can augment mitochondrial biogenesis in the presence of a HFD. Future studies will further examine whether GSK3 inhibition via lithium can augment muscle oxidative metabolism after high fat feeding.

Neuronatin (NNAT) is a paternally imprinted gene involved in many aspects of metabolism but the underlying cellular mechanism remains unknown. Its sequence homology with phospholamban (PLN) and sarcolipin (SLN) suggest that NNAT has a putative role in regulating the enzymatic activity of sarco(endo)plasmic reticulum Ca ²⁺ ‐ATPase (SERCA) in striated muscles. Both PLN and SLN physically interact and functionally inhibit SERCA by lowering its affinity for Ca ²⁺ , but only SLN has the unique ability to uncouple SERCA‐mediated Ca ²⁺ transport from ATP hydrolysis in mouse soleus muscles. Uncoupling the SERCA pump increases energy expenditure and may be beneficial in combatting energy imbalances, such as obesity. The objective of this study was to test our hypothesis that like SLN, NNAT would also promote SERCA uncoupling. In addition, we also examined whether NNAT expression would be altered in soleus muscles obtained from C57BL/6 mice fed a high fat diet (HFD). Using human embryonic kidney (HEK) cells co‐transfected with SERCA and NNAT cDNA, we measured SERCA coupling ratio using an Indo‐1 based fluorometric Ca ²⁺ uptake assay and an enzyme‐linked spectrophotometric ATPase assay. NNAT promoted uncoupling of both SERCA1a and SERCA2a isoforms in HEK cells in a dose‐dependent manner with SERCA1a showing increased sensitivity when compared to SERCA2a. Western blotting was used to confirm HEK cell transfection as well as to assess NNAT and SERCA content in soleus muscles from mice fed a low (LFD) or HFD (60% kcal) for 12 weeks. A ~40% reduction in SERCA1a and a ~50% increase in SERCA2a protein content was observed in the soleus muscles of mice fed a HFD compared to LFD. Total NNAT content and its levels relative to SERCA (SERCA1a + SERCA2a) were reduced by ~40% and ~45%, respectively in HFD mice compared to LFD mice. Plotting muscle NNAT content (relative to SERCA) against total weight gained after 12 weeks of high fat feeding also revealed a significant inverse relationship (r ² =0.49, p =0.01), suggesting that higher NNAT content (relative to SERCA) is related to reduced weight gained from a HFD in vivo . This is the first study to demonstrate NNAT as a SERCA uncoupler and that its expression in muscle is negatively associated with HFD induced weight gain. These findings suggest that NNAT may regulate whole‐body metabolism and energy balance via SERCA uncoupling.