Low-dose lithium supplementation and SERCA uncoupling in C2C12 cells and murine skeletal muscle.

Abstract

Adaptive thermogenesis is a cellular process that accelerates energy expenditure while increasing heat production in response to prolonged cold exposure or caloric excess. The prevalence of obesity along with its comorbidities is continually rising. Obesity is a result of energy intake superseding energy expenditure, however, a balance between energy intake versus energy expenditure is key in weight maintenance. Therefore, enhancing adaptive thermogenesis may be relevant in combatting diet-induced obesity. Skeletal muscle via sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) uncoupling and brown/beige adipose via mitochondrial uncoupling are the two sites for adaptive thermogenesis in mammals. Recent evidence has shown that glycogen synthase kinase 3 (GSK3) negatively regulates adipose-based thermogenesis by repressing uncoupling protein-1 expression in brown adipocytes. However, to our knowledge, no studies have examined whether GSK3 also negatively regulates muscle-based thermogenesis via SERCA uncoupling. The SERCA pump catalyzes the active transport of 2 Ca2+ ions into the sarcoplasmic reticulum per 1 ATP hydrolyzed under optimal conditions. Sarcolipin (SLN), an uncoupler of SERCA makes Ca2+ transport less efficient by reducing SERCA coupling ratio. The objective of this thesis was to determine whether GSK3 inhibition with low dose lithium (Li) supplementation can increase SLN expression and promote SERCA uncoupling in both C2C12 cells and in murine soleus muscle. Our results show that in C2C12 cells, 0.5mM LiCl promotes GSK3 inhibition and SERCA uncoupling via an increase in ryanodine receptor (RYR) but not SLN. In contrast, soleus muscles from chow-fed and lithium supplemented mice did not result in any notable changes in SERCA coupling ratio or the content of SERCA associated proteins. We next determined whether this would differ under an added stress of a high-fat diet. Our results show that soleus homogenates of HFD+Li supplemented mice have significant reductions in SERCA coupling ratio compared with HFD alone, which was presumably due to an increase in SERCA uncoupling proteins SLN and NNAT. Altogether these data suggest the potential role of GSK3 inhibition via low dose lithium supplementation in activating muscle-based thermogenesis, particularly under the stress of a HFD.