Increasing evidence suggests that in addition to brown adipose tissue (BAT), skeletal muscle is an important site for Nonshivering thermogenesis (NST). Although several studies have suggested that SR Ca2+ cycling may play a role in muscle thermogenesis, the molecular details were not known. Studies from our laboratory and others have shown that Sarcolipin (SLN), a regulator of SR Ca2+ ATPase (SERCA), can bind to SERCA in the presence of Ca2+ and promote uncoupling of SERCA and increase ATP hydrolysis. In the presence of SLN, SERCA becomes inefficient; transporting less than 2 mol Ca2+ per mol ATP, thereby increasing ATP hydrolysis and heat production. These studies hinted that SLN could play a role in muscle thermogenesis .To define the relevance of SLN in muscle, we took a genetic approach and recently showed that loss of SLN predisposes mice to develop hypothermia during acute cold exposure but reintroduction of SLN in the Sln-/- background fully restored muscle-based thermogenesis. In addition, when Sln-/- mice when fed on high fat diet (HFD) gained significantly more weight than WT controls, whereas WT mice fed on HFD were less obese but showed significant upregulation of SLN (3-4 fold) suggesting that SLN is recruited in diet induced thermogenesis. These novel findings, for the first time, suggest SLN is the missing link for enhancing SERCA dependent heat generation by uncoupling the pump and this mechanism is recruited to increase energy expenditure during diet overload. However the mechanistic details with regard to recruitment of muscle NST, SLN-mediated uncoupling of SERCA, and how SLN increases muscle metabolism and energetics are poorly understood. The research proposed in this application challenges current thinking that BAT alone is responsible for Nonshivering thermogenesis. It seeks to establish that muscle is an important site of NST and can replace/substitute for thermogenesis in animals where BAT is absent or nonfunctional. The revised proposal places a greater emphasis on understanding the mechanistic basis of muscle NST. The current proposal seeks to identify sub cellular mechanisms that lead to activation of SERCA/SLN based thermogenesis.
In Aim 1, we will investigate the mechanism behind activation of muscle-based NST during cold exposure and determine if SLN can replace/substitute for loss of BAT (UCP1) function in mammals.
In Aim 2, we will test the therapeutic relevance of SLN and determine if overexpression of SLN can protect against diet- induced obesity by increasing energy expenditure. Another important goal of this aim is to discover how SLN increases muscle metabolism, if this involves Ca2+ dependant signaling pathways.
In Aim 3, we will identify the structural features (binding sites and residues) of SLN/SERCA interaction and determine how SLN interaction leads to uncoupling and increased ATP hydrolysis. The experiments proposed here will collectively establish that SLN/SERCA interaction is the mechanism for skeletal muscle based NST and their relevance to Tc and whole body energy metabolism. Most importantly they will provide a mechanistic basis to show that SLN alone but not Phospholamban binding to SERCA causes uncoupling of the SERCA pump. We suggest that a better understanding of muscle thermogenesis has broader implications to our overall understanding of muscle metabolism, energy expenditure and evidently obesity in mammals including humans. Identification of the molecular mechanisms behind muscle based NST is of paramount importance to humans, since this strategy could be exploited to increase energy expenditure in muscle, thereby providing newer targets for obesity treatment.
Skeletal muscle constitutes more than 40% of body mass and recent studies have suggested that muscle is more than contractile machinery; it is a heat generating furnace and a major player in metabolic regulation. However it is not known how exactly muscle contributes to heat generation and contributes to increased energy metabolism. In our laboratory we have recently identified a novel molecule namely sarcolipin (SLN) and it is found abundantly in skeletal muscles of larger animals including human. We recently found that this protein plays an important role in heat generation in muscle during cold adaptation since mice that lack this protein developed hypothermia. Interestingly loss of SLN also predisposes mice to develop obesity suggesting that this protein could also promote increased energy expenditure in muscle. These studies will have broader implications on our overall understanding of how muscle based heat generation can affect muscle metabolism, energy expenditure and inevitably obesity in mammals including humans.
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