Meghna Pant

Sarcolipin is a newly identified regulator of muscle-based thermogenesis in mammals

The role of skeletal muscle in nonshivering thermogenesis (NST) is not well understood. Here we show that sarcolipin (Sln), a newly identified regulator of the sarco/endoplasmic reticulum Ca2+-ATPase (Serca) pump, is necessary for muscle-based thermogenesis. When challenged to acute cold (4 °C), Sln−/− mice were not able to maintain their core body temperature (37 °C) and developed hypothermia. Surgical ablation of brown adipose tissue and functional knockdown of Ucp1 allowed us to highlight the role of muscle in NST. Overexpression of Sln in the Sln-null background fully restored muscle-based thermogenesis, suggesting that Sln is the basis for Serca-mediated heat production. We show that ryanodine receptor 1 (Ryr1)-mediated Ca2+ leak is an important mechanism for Serca-activated heat generation. Here we present data to suggest that Sln can continue to interact with Serca in the presence of Ca2+, which can promote uncoupling of the Serca pump and cause futile cycling. We further show that loss of Sln predisposes mice to diet-induced obesity, which suggests that Sln-mediated NST is recruited during metabolic overload. These data collectively suggest that SLN is an important mediator of muscle thermogenesis and whole-body energy metabolism.

Sarcolipin Is a Key Determinant of the Basal Metabolic Rate, and Its Overexpression Enhances Energy Expenditure and Resistance against Diet-induced Obesity

Background: Sarcolipin (SLN), a regulator of SR Ca2+ ATPase (SERCA) in muscle, can promote the uncoupling of SERCA from Ca2+ transport and increase heat production. Results: Overexpression of SLN in muscle increases energy expenditure and provides resistance against diet-induced obesity. Conclusion: SLN plays a role in whole-body metabolism. Significance: SLN can serve as novel target to increase energy expenditure in muscle. Sarcolipin (SLN) is a novel regulator of sarcoplasmic reticulum Ca2+ ATPase (SERCA) in muscle. SLN binding to SERCA uncouples Ca2+ transport from ATP hydrolysis. By this mechanism, SLN promotes the futile cycling of SERCA, contributing to muscle heat production. We recently showed that SLN plays an important role in cold- and diet-induced thermogenesis. However, the detailed mechanism of how SLN regulates muscle metabolism remains unclear. In this study, we used both SLN knockout (Sln−/−) and skeletal muscle-specific SLN overexpression (SlnOE) mice to explore energy metabolism by pair feeding (fixed calories) and high-fat diet feeding (ad libitum). Our results show that, upon pair feeding, SlnOE mice lost weight compared with the WT, but Sln−/− mice gained weight. Interestingly, when fed with a high-fat diet, SlnOE mice consumed more calories but gained less weight and maintained a normal metabolic profile in comparison with WT and Sln−/− mice. We found that oxygen consumption and fatty acid oxidation were increased markedly in SlnOE mice. There was also an increase in both mitochondrial number and size in SlnOE muscle, together with increased expression of peroxisome proliferator-activated receptor δ (PPARδ) and PPAR γ coactivator 1 α (PGC1α), key transcriptional activators of mitochondrial biogenesis and enzymes involved in oxidative metabolism. These results, taken together, establish an important role for SLN in muscle metabolism and energy expenditure. On the basis of these data we propose that SLN is a novel target for enhancing whole-body energy expenditure.

IKKα and alternative NF-κB regulate PGC-1β to promote oxidative muscle metabolism

Alternative NF-κB signaling modulates the activity of PGC-1β to promote oxidative metabolism in skeletal muscle.

Sarcolipin overexpression improves muscle energetics and reduces fatigue.

Sarcolipin (SLN) is a regulator of sarcoendoplasmic reticulum calcium ATPase in skeletal muscle. Recent studies using SLN-null mice have identified SLN as a key player in muscle thermogenesis and metabolism. In this study, we exploited a SLN overexpression (Sln(OE)) mouse model to determine whether increased SLN level affected muscle contractile properties, exercise capacity/fatigue, and metabolic rate in whole animals and isolated muscle. We found that Sln(OE) mice are more resistant to fatigue and can run significantly longer distances than wild-type (WT). Studies with isolated extensor digitorum longus (EDL) muscles showed that Sln(OE) EDL produced higher twitch force than WT. The force-frequency curves were not different between WT and Sln(OE) EDLs, but at lower frequencies the pyruvate-induced potentiation of force was significantly higher in Sln(OE) EDL. SLN overexpression did not alter the twitch and force-frequency curve in isolated soleus muscle. However, during a 10-min fatigue protocol, both EDL and soleus from Sln(OE) mice fatigued significantly less than WT muscles. Interestingly, Sln(OE) muscles showed higher carnitine palmitoyl transferase-1 protein expression, which could enhance fatty acid metabolism. In addition, lactate dehydrogenase expression was higher in Sln(OE) EDL, suggesting increased glycolytic capacity. We also found an increase in store-operated calcium entry (SOCE) in isolated flexor digitorum brevis fibers of Sln(OE) compared with WT mice. These data allow us to conclude that increased SLN expression improves skeletal muscle performance during prolonged muscle activity by increasing SOCE and muscle energetics.

Sarcolipin: A Key Thermogenic and Metabolic Regulator in Skeletal Muscle

Skeletal muscle constitutes ∼40% of body mass and has the capacity to play a major role as thermogenic, metabolic, and endocrine organ. In addition to shivering, muscle also contributes to nonshivering thermogenesis via futile sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) activity. Sarcolipin (SLN), a regulator of SERCA activity in muscle, plays an important role in regulating muscle thermogenesis and metabolism. Uncoupling of SERCA by SLN increases ATP hydrolysis and heat production, and contributes to temperature homeostasis. SLN also affects whole-body metabolism and weight gain in mice, and is upregulated in various muscle diseases including muscular dystrophy, suggesting a role for SLN during increased metabolic demand. In this review we also highlight the physiological roles of skeletal muscle beyond contraction.

Cold adaptation overrides developmental regulation of sarcolipin expression in mice skeletal muscle: SOS for muscle-based thermogenesis?

ABSTRACT Neonatal mice have a greater thermogenic need than adult mice and may require additional means of heat production, other than the established mechanism of brown adipose tissue (BAT). We and others recently discovered a novel mediator of skeletal muscle-based thermogenesis called sarcolipin (SLN) that acts by uncoupling sarcoendoplasmic reticulum Ca2+-ATPase (SERCA). In addition, we have shown that SLN expression is downregulated during neonatal development in rats. In this study we probed two questions: (1) is SLN expression developmentally regulated in neonatal mice?; and (2) if so, will cold adaptation override this? Our data show that SLN expression is higher during early neonatal stages and is gradually downregulated in fast twitch skeletal muscles. Interestingly, we demonstrate that cold acclimation of neonatal mice can prevent downregulation of SLN expression. This observation suggests that SLN-mediated thermogenesis can be recruited to a greater extent during extreme physiological need, in addition to BAT. Summary: Sarcolipin plays an important role in muscle-based thermogenesis. It is abundantly expressed in neonatal mouse muscles and cold challenge prevents its developmental downregulation, indicating higher recruitment of muscle-based thermogenesis in neonates.

Metabolic Dysfunction and Altered Mitochondrial Dynamics in the Utrophin-Dystrophin Deficient Mouse Model of Duchenne Muscular Dystrophy

The utrophin-dystrophin deficient (DKO) mouse model has been widely used to understand the progression of Duchenne muscular dystrophy (DMD). However, it is unclear as to what extent muscle pathology affects metabolism. Therefore, the present study was focused on understanding energy expenditure in the whole animal and in isolated extensor digitorum longus (EDL) muscle and to determine changes in metabolic enzymes. Our results show that the 8 week-old DKO mice consume higher oxygen relative to activity levels. Interestingly the EDL muscle from DKO mouse consumes higher oxygen per unit integral force, generates less force and performs better in the presence of pyruvate thus mimicking a slow twitch muscle. We also found that the expression of hexokinase 1 and pyruvate kinase M2 was upregulated several fold suggesting increased glycolytic flux. Additionally, there is a dramatic increase in dynamin-related protein 1 (Drp 1) and mitofusin 2 protein levels suggesting increased mitochondrial fission and fusion, a feature associated with increased energy demand and altered mitochondrial dynamics. Collectively our studies point out that the dystrophic disease has caused significant changes in muscle metabolism. To meet the increased energetic demand, upregulation of metabolic enzymes and regulators of mitochondrial fusion and fission is observed in the dystrophic muscle. A better understanding of the metabolic demands and the accompanied alterations in the dystrophic muscle can help us design improved intervention therapies along with existing drug treatments for the DMD patients.

Malignant hyperthermia: to buffer or not to buffer

Malignant hyperthermia (MH) is a clinical condition characterized by a massive rise in body temperature as a result of increased metabolism that can lead to death if not treated. MH cases reported so far suggest that skeletal muscle is the site of heat production. The majority of the heat generated during this event can potentially come from the two main ATP-consuming molecules in the skeletal muscle: myosin ATPase and sarco(endo)plasmic reticulum calcium ATPase (SERCA). Muscle as a source of endothermic heat production in a non-shivering manner has been proposed by several groups of researchers since the 1920s. Even a century later, the mechanism behind muscle-based non-shivering heat production remains elusive. In a recent issue of Journal of Physiology, Manno et al. (2013) studied the role of abnormal calcium dynamics in a mouse model of MH. We read this paper with great interest as it shed some light on the plausible mechanism behind muscle-based heat production. Most of the MH episodes result primarily due to mutations in ryanodine receptor 1 (RYR1). These RYR1 mutations make the mouse model susceptible to volatile anaesthetics and/or heat-induced calcium release, thus causing uncontrolled heat production from myosin ATPase and continuous SERCA cycling. Dantrolene, which inhibits this leak from RYR1, is currently used for the treatment of MH. To study the mechanism behind heat-induced or anaesthesia-induced MH, Chelu et al. (2006) created a mouse model susceptible to MH harbouring the Y524S mutation in RYR1 (YS mice). The Y524S mutation in mice is equivalent to the Y522S mutation in human. The mice homozygous for the mutation fail to survive but the heterozygous mice exhibit elevated temperature and whole body contractures in response to isoflurane and heat. In their paper, Manno et al. (2013) employ imaging techniques to measure cytosolic and sarcoplasmic reticulum (SR) calcium concentration [Ca2+] in the YS mutant cells. They demonstrate that although the SR release flux is similar, SR [Ca2+] buffering and SR membrane permeability are drastically altered in the MH mutant mice.