Increased Reliance on Muscle-based Thermogenesis upon Acute Minimization of Brown Adipose Tissue Function

Sympathetic innervation of interscapular brown adipose tissue is not a predominant mediator of OT-elicited reductions of body weight gain and adiposity in male diet-induced obese rats

Recent studies indicate that central administration of oxytocin (OT) reduces body weight (BW) in high fat diet-induced obese (DIO) rodents by reducing energy intake and increasing energy expenditure (EE). Previous studies in our lab have shown that administration of OT into the fourth ventricle (4V; hindbrain) elicits weight loss and stimulates interscapular brown adipose tissue temperature (TIBAT) in DIO rats. We hypothesized that OT-elicited stimulation of sympathetic nervous system (SNS) activation of IBAT contributes to its ability to activate BAT and reduce BW in DIO rats. To test this, we determined the effect of disrupting SNS activation of IBAT on OT-elicited stimulation of TIBAT and reduction of BW in DIO rats. We first confirmed that bilateral surgical SNS denervation to IBAT was successful based on having achieved ≥ 60% reduction in IBAT norepinephrine (NE) content from DIO rats. NE content was selectively reduced in IBAT by 94.7 ± 2.7, 96.8 ± 1.8 and 85.9 ± 6.1% (P<0.05) at 1, 6 and 7-weeks post-denervation, respectively, and was unchanged in liver or inguinal white adipose tissue. We then measured the impact of bilateral surgical SNS denervation to IBAT on the ability of acute 4V OT (1, 5 μg) to stimulate TIBAT in DIO rats. We found that the high dose of 4V OT (5 μg) stimulated TIBAT similarly between sham and denervated rats (P=NS) and that the effects of 4V OT to stimulate TIBAT did not require beta-3 adrenergic receptor signaling. We subsequently measured the effect of bilateral surgical denervation of IBAT on the effect of chronic 4V OT (16 nmol/day) or vehicle infusion to reduce BW, adiposity, and energy intake in DIO rats. Chronic 4V OT reduced BW gain by -7.2 ± 9.6 g and -14.1 ± 8.8 g in sham and denervated rats (P<0.05 vs vehicle treatment), respectively, and this effect was similar between groups (P=NS). These effects were associated with reductions in adiposity and energy intake (P<0.05). Collectively, these findings support the hypothesis that sympathetic innervation of IBAT is not required for central OT to increase BAT thermogenesis and reduce BW gain and adiposity in male DIO rats.

Sympathetic Innervation of Interscapular Brown Adipose Tissue Is Not a Predominant Mediator of Oxytocin-Induced Brown Adipose Tissue Thermogenesis in Female High Fat Diet-Fed Rats

Recent studies have indicated that hindbrain [fourth ventricle (4V)] administration of the neurohypophyseal hormone, oxytocin (OT), reduces body weight, energy intake and stimulates interscapular brown adipose tissue temperature (TIBAT) in male diet-induced obese (DIO) rats. What remains unclear is whether chronic hindbrain (4V) OT can impact body weight in female high fat diet-fed (HFD) rodents and whether this involves activation of brown adipose tissue (BAT). We hypothesized that OT-elicited stimulation of sympathetic nervous system (SNS) activation of interscapular brown adipose tissue (IBAT) contributes to its ability to activate BAT and reduce body weight in female high HFD-fed rats. To test this hypothesis, we determined the effect of disrupting SNS activation of IBAT on OT-elicited stimulation of TIBAT and reduction of body weight in DIO rats. We first measured the impact of bilateral surgical SNS denervation to IBAT on the ability of acute 4V OT (0.5, 1, and 5 µg ≈ 0.5, 0.99, and 4.96 nmol) to stimulate TIBAT in female HFD-fed rats. We found that the high dose of 4V OT (5 µg ≈ 4.96 nmol) stimulated TIBAT similarly between sham rats and denervated rats (p = NS). We subsequently measured the effect of bilateral surgical denervation of IBAT on the effect of chronic 4V OT (16 nmol/day ≈ 16.1 μg/day) or vehicle infusion to reduce body weight, adiposity and energy intake in female HFD-fed rats (N = 7-8/group). Chronic 4V OT reduced body weight gain (sham: -18.0 ± 4.9 g; denervation: -15.9 ± 3.7 g) and adiposity (sham: -13.9 ± 3.7 g; denervation: -13.6 ± 2.4 g) relative to vehicle treatment (p < 0.05) and these effects were similar between groups (p = NS). These effects were attributed, in part, to reduced energy intake evident during weeks 2 (p < 0.05) and 3 (p < 0.05). To test whether these results translate to other female rodent species, we also examined the effect of chronic 4V infusion of OT on body weight and adiposity in two strains of female HFD-fed mice. Similar to what we found in the HFD-fed rat model, we also found that chronic 4V OT (16 nmol/day) infusion resulted in reduced body weight gain, adiposity and energy intake in female DIO C57BL/6J and DBA/2J mice (p < 0.05 vs. vehicle). Together, these findings suggest that (1) sympathetic innervation of IBAT is not necessary for OT-elicited increases in BAT thermogenesis and weight loss in female HFD-fed rats and (2) the effects of OT to reduce weight gain and adiposity translate to other female mouse models of diet-induced obesity (DIO).

Silencing of dopamine receptor D5 inhibits the browning of 3T3-L1 adipocytes and ATP-consuming futile cycles in C2C12 muscle cells

BACKGROUND

As a part of the catecholamines, dopamine receptors (DRs) have not been extensively studied like β3-AR in the thermogenesis process. The present study investigates the effect of DRD5 in browning events and ATP-consuming futile cycles.

METHODS

siRNA technology, qPCR, immunoblot analysis, immunofluorescence, and staining methods were used to investigate the effect of DRD5 on 3T3-L1 and C2C12 cells.

RESULTS

siDdr5 increased lipogenesis-associated effectors, and adipogenesis markers while reducing the expression of beige fat effectors. ATP-consuming futile cycle markers were also reduced following the siDrd5. On the contrary, pharmacological activation of DRD5 stimulated these effectors. Our mechanistic studies elucidated that DRD5 mediates fat browning via the cAMP-PKA-p38 MAPK signalling pathway in 3T3-L1 cells as well as the cAMP-SERCA-RyR pathway for the ATP-consuming futile cycles in both cells.

CONCLUSIONS

siDrd5 positively regulates browning and ATP-consuming futile cycles, and understanding its functions will provide insights into novel strategies to treat obesity.

Relative sarcolipin (SLN) and sarcoplasmic reticulum Ca2+ ATPase (SERCA1) transcripts levels in closely related endothermic and ectothermic scombrid fishes: Implications for molecular basis of futile calcium cycle non-shivering thermogenesis (NST)

Regional endothermy is the ability of an animal to elevate the temperature of specific regions of the body above that of the surrounding environment and has evolved independently among several fish lineages. Sarcolipin (SLN) is a small transmembrane protein that uncouples the sarcoplasmic reticulum calcium ATPase pump (SERCA1b) resulting in futile Ca2+ cycling and is thought to play a role in non-shivering thermogenesis (NST) in cold-challenged mammals and possibly some fishes. This study investigated the relative expression of sln and serca1 transcripts in three regionally-endothermic fishes (the skipjack, Katsuwonus pelamis, and yellowfin tuna, Thunnus albacares, both of which elevate the temperatures of their slow-twitch red skeletal muscle (RM) and extraocular muscles (EM), as well as the cranial endothermic swordfish, Xiphias gladius), and closely related ectothermic scombrids (the Eastern Pacific bonito, Sarda chiliensis, and Pacific chub mackerel, Scomber japonicus). Using Reverse Transcription quantitative PCR (RT-qPCR) and species-specific primers, relative sln expression trended higher in both the RM and EM for all four scombrid species compared to white muscle. In addition, relative serca1 expression was found to be higher in RM of skipjack and yellowfin tuna in comparison to white muscle. However, neither sln nor serca1 transcripts were higher in swordfish RM, EM or cranial heater tissue in comparison to white muscle. A key phosphorylation site in sarcolipin, threonine 5, is conserved in the swordfish, but is mutated to alanine or valine in tunas and the endothermic smalleye Pacific opah, Lampris incognitus, which should result in increased uncoupling of the SERCA pump. Our results support the role of potential SLN-NST in endothermic tunas and the lack thereof for swordfish.

Adaptive Induction of Nonshivering Thermogenesis in Muscle Rather Than Brown Fat Could Counteract Obesity

Warm-blooded animals such as birds and mammals are able to protect stable body temperature due to various thermogenic mechanisms. These processes can be facultative (occurring only under specific conditions, such as acute cold) and adaptive (adjusting their capacity according to long-term needs). They can represent a substantial part of overall energy expenditure and, therefore, affect energy balance. Classical mechanisms of facultative thermogenesis include shivering of skeletal muscles and (in mammals) non-shivering thermogenesis (NST) in brown adipose tissue (BAT), which depends on uncoupling protein 1 (UCP1). Existence of several alternative thermogenic mechanisms has been suggested. However, their relative contribution to overall heat production and the extent to which they are adaptive and facultative still needs to be better defined. Here we focus on comparison of NST in BAT with thermogenesis in skeletal muscles, including shivering and NST. We present indications that muscle NST may be adaptive but not facultative, unlike UCP1-dependent NST. Due to its slow regulation and low energy efficiency, reflecting in part the anatomical location, induction of muscle NST may counteract development of obesity more effectively than UCP1-dependent thermogenesis in BAT.

Sympathetic innervation of interscapular brown adipose tissue is not a predominant mediator of oxytocin-elicited reductions of body weight and adiposity in male diet-induced obese mice

Previous studies indicate that CNS administration of oxytocin (OT) reduces body weight in high fat diet-induced obese (DIO) rodents by reducing food intake and increasing energy expenditure (EE). We recently demonstrated that hindbrain (fourth ventricular [4V]) administration of OT elicits weight loss and elevates interscapular brown adipose tissue temperature (TIBAT, a surrogate measure of increased EE) in DIO mice. What remains unclear is whether OT-elicited weight loss requires increased sympathetic nervous system (SNS) outflow to IBAT. We hypothesized that OT-induced stimulation of SNS outflow to IBAT contributes to its ability to activate BAT and elicit weight loss in DIO mice. To test this hypothesis, we determined the effect of disrupting SNS activation of IBAT on the ability of 4V OT administration to increase TIBAT and elicit weight loss in DIO mice. We first determined whether bilateral surgical SNS denervation to IBAT was successful as noted by ≥ 60% reduction in IBAT norepinephrine (NE) content in DIO mice. NE content was selectively reduced in IBAT at 1-, 6- and 7-weeks post-denervation by 95.9 ± 2.0, 77.4 ± 12.7 and 93.6 ± 4.6% (P<0.05), respectively and was unchanged in inguinal white adipose tissue, pancreas or liver. We subsequently measured the effects of acute 4V OT (1, 5 µg ≈ 0.99, 4.96 nmol) on TIBAT in DIO mice following sham or bilateral surgical SNS denervation to IBAT. We found that the high dose of 4V OT (5 µg ≈ 4.96 nmol) elevated TIBAT similarly in sham mice as in denervated mice. We subsequently measured the effects of chronic 4V OT (16 nmol/day over 29 days) or vehicle infusions on body weight, adiposity and food intake in DIO mice following sham or bilateral surgical denervation of IBAT. Chronic 4V OT reduced body weight by 5.7 ± 2.23% and 6.6 ± 1.4% in sham and denervated mice (P<0.05), respectively, and this effect was similar between groups (P=NS). OT produced corresponding reductions in whole body fat mass (P<0.05). Together, these findings support the hypothesis that sympathetic innervation of IBAT is not necessary for OT-elicited increases in BAT thermogenesis and reductions of body weight and adiposity in male DIO mice.

Intramuscular administration of fractalkine modulates mitochondrial properties and promotes fast glycolytic phenotype

A newly categorized myokine called fractalkine (CX3CL1) has been associated with divergent conditions such as obesity, tissue inflammation, and exercise. CX3CL1 works through specific membrane-bound receptors (CX3CR1) found in various tissues including skeletal muscles. Studies indicate CX3CL1 induces muscles to uptake energy substrates thereby improving glucose utilization and countering diabetes. Here, we tested if the administration of purified CX3CL1 directly into mice skeletal muscles affects its histoarchitecture, mitochondrial activity, and expression of metabolic proteins. We analyzed four muscles: two upper-limb (quadriceps, hamstrings) and two lower-limb (tibialis anterior, gastrocnemius), contralateral leg muscles were taken as controls. The effects of CX3CL1 treatment on histoarchitecture, mitochondrial activity, and expression of metabolic proteins in muscles were characterized. We used histochemical staining succinate dehydrogenase (SDH)/cytochrome c oxidase (COX), myosin ATPase, alkaline phosphatase (ALP) to evaluate the mitochondrial activity, fiber types, and vascularization in the muscles, respectively. Western blotting was used to evaluate the expression of proteins associated with mitochondrial metabolism (OXPHOS), glycolysis, and vascularization. Overall, this study indicates CX3CL1 primarily modulates mitochondrial metabolism and shifts substrate preference toward glucose in the skeletal muscle. Evidence also supports that CX3CL1 stimulates the relative composition of fast fiber types, influencing selection of energy substrates in the skeletal muscle.

Mitochondrial function in skeletal muscle contributes to reproductive endothermy in tegu lizards (Salvator merianae)

AIM

In cyclic climate variations, including seasonal changes, many animals regulate their energy demands to overcome critical transitory moments, restricting their high-demand activities to phases of resource abundance, enabling rapid growth and reproduction. Tegu lizards (Salvator merianae) are ectotherms with a robust annual cycle, being active during summer, hibernating during winter, and presenting a remarkable endothermy during reproduction in spring. Here, we evaluated whether changes in mitochondrial respiratory physiology in skeletal muscle could serve as a mechanism for the increased thermogenesis observed during the tegu's reproductive endothermy.

METHODS

We performed high-resolution respirometry and calorimetry in permeabilized red and white muscle fibers, sampled during summer (activity) and spring (high activity and reproduction), in association with citrate synthase measurements.

RESULTS

During spring, the muscle fibers exhibited increased oxidative phosphorylation. They also enhanced uncoupled respiration and heat production via adenine nucleotide translocase (ANT), but not via uncoupling proteins (UCP). Citrate synthase activity was higher during the spring, suggesting greater mitochondrial density compared to the summer. These findings were consistent across both sexes and muscle types (red and white).

CONCLUSION

The current results highlight potential cellular thermogenic mechanisms in an ectothermic reptile that contribute to transient endothermy. Our study indicates that the unique feature of transitioning to endothermy through nonshivering thermogenesis during the reproductive phase may be facilitated by higher mitochondrial density, function, and uncoupling within the skeletal muscle. This knowledge contributes significant elements to the broader picture of models for the evolution of endothermy, particularly in relation to the enhancement of aerobic capacity.

Futile cycles: Emerging utility from apparent futility

Futile cycles are biological phenomena where two opposing biochemical reactions run simultaneously, resulting in a net energy loss without appreciable productivity. Such a state was presumed to be a biological aberration and thus deemed an energy-wasting "futile" cycle. However, multiple pieces of evidence suggest that biological utilities emerge from futile cycles. A few established functions of futile cycles are to control metabolic sensitivity, modulate energy homeostasis, and drive adaptive thermogenesis. Yet, the physiological regulation, implication, and pathological relevance of most futile cycles remain poorly studied. In this review, we highlight the abundance and versatility of futile cycles and propose a classification scheme. We further discuss the energetic implications of various futile cycles and their impact on basal metabolic rate, their bona fide and tentative pathophysiological implications, and putative drug interactions.

Sympathetic innervation of interscapular brown adipose tissue is not a predominant mediator of oxytocin-elicited reductions of body weight and adiposity in male diet-induced obese mice

Previous studies indicate that CNS administration of oxytocin (OT) reduces body weight in high fat diet-induced obese (DIO) rodents by reducing food intake and increasing energy expenditure (EE). We recently demonstrated that hindbrain (fourth ventricular [4V]) administration of OT elicits weight loss and elevates interscapular brown adipose tissue temperature (T IBAT , a surrogate measure of increased EE) in DIO mice. What remains unclear is whether OT-elicited weight loss requires increased sympathetic nervous system (SNS) outflow to IBAT. We hypothesized that OT-induced stimulation of SNS outflow to IBAT contributes to its ability to activate BAT and elicit weight loss in DIO mice. To test this hypothesis, we determined the effect of disrupting SNS activation of IBAT on the ability of 4V OT administration to increase T IBAT and elicit weight loss in DIO mice. We first determined whether bilateral surgical SNS denervation to IBAT was successful as noted by ≥ 60% reduction in IBAT norepinephrine (NE) content in DIO mice. NE content was selectively reduced in IBAT at 1-, 6- and 7-weeks post-denervation by 95.9±2.0, 77.4±12.7 and 93.6±4.6% ( P <0.05), respectively and was unchanged in inguinal white adipose tissue, pancreas or liver. We subsequently measured the effects of acute 4V OT (1, 5 µg ≈ 0.99, 4.96 nmol) on T IBAT in DIO mice following sham or bilateral surgical SNS denervation to IBAT. We found that the high dose of 4V OT (5 µg ≈ 4.96 nmol) elevated T IBAT similarly in sham mice as in denervated mice. We subsequently measured the effects of chronic 4V OT (16 nmol/day over 29 days) or vehicle infusions on body weight, adiposity and food intake in DIO mice following sham or bilateral surgical denervation of IBAT. Chronic 4V OT reduced body weight by 5.7±2.23% and 6.6±1.4% in sham and denervated mice ( P <0.05), respectively, and this effect was similar between groups ( P =NS). OT produced corresponding reductions in whole body fat mass ( P <0.05). Together, these findings support the hypothesis that sympathetic innervation of IBAT is not necessary for OT-elicited increases in BAT thermogenesis and reductions of body weight and adiposity in male DIO mice.

Echinacoside stimulates myogenesis and ATP-dependent thermogenesis in the skeletal muscle via the activation of D1-like dopaminergic receptors

Recent studies have shown that some natural compounds from plants prevent obesity and related disorders, including the loss of skeletal muscle mass and strength. In this study, we investigated the effect of echinacoside (ECH), a caffeic acid glycoside from the phenylpropanoid class, on myogenesis and ATP-dependent thermogenesis in the skeletal muscle and its interaction with the dopaminergic receptors 1 and 5 (DRD1 and DRD5). We applied RT-PCR, immunoblot analysis, a staining method, and an assay kit to determine the effects of ECH on diverse target genes and proteins involved in skeletal muscle myogenesis and ATP-consuming futile processes. Our study demonstrated that ECH enhanced myogenic differentiation, glucose, and fatty acid uptake, as well as lipid catabolism, and induced ATP-dependent thermogenesis in vitro and in vivo. Moreover, ECH upregulated mitochondrial biogenesis proteins, mitochondrial oxidative phosphorylation (OXPHOS) complexes, and intracellular Ca2+ signaling as well as thermogenic proteins. These findings were further elucidated by mechanistic studies which showed that ECH mediates myogenesis via the DRD1/5 in C2C12 muscle cells. In addition, ECH stimulates α1-AR-mediated ATP-dependent thermogenesis via the DRD1/5/cAMP/SLN/SERCA1a pathway in C2C12 muscle cells. To the best of our knowledge, this is the first report that demonstrates the myogenic and thermogenic potential of ECH activity through the dopaminergic receptors. Understanding the novel functions of ECH in terms of its ability to prevent skeletal muscle loss and energy expenditure via ATP-consuming futile processes could help to develop potential alternative strategies to address muscle-related diseases, including combating obesity.

Sarcolipin (sln) and Sarcoplasmic Reticulum calcium ATPase pump (serca1) expression increase in Japanese medaka (Oryzias latipes) skeletal muscle tissue following cold challenge

Endothermy is the process by which organisms maintain a constant body temperature despite dynamic environmental temperatures. There are two mechanisms organisms use to elevate body temperature: shivering thermogenesis (ST) and non-shivering thermogenesis (NST). Skeletal muscle NST is achieved through a futile Ca2+ cycling of sarcoplasmic reticulum Ca2+ ATPase (Serca1) in the presence of sarcolipin (Sln). Here we subjected Japanese medaka to a cold challenge to examine the expression of sln and serca1 transcripts from slow-twitch red and fast-twitch white muscle as environmental temperature decreased. We show a significant increase in relative sln and serca1 transcript expression in skeletal muscle tissues of cold-challenged Japanese medaka. The elevated transcripts support the role of Sln as a component of NST and support previous literature with the increase in serca1. To date, this is the first cold challenge on an ectothermic fish investigating sln transcripts. The ability of medaka to respond to a cold challenge with an increase in key calcium cycling components, specifically the calcium pump and sarcolipin suggest that teleost fish share a conserved transcriptional program in response to cold stimuli with fish species that possess the requisite anatomical adaptations to conserve metabolic heat.

In vivo heat production dynamics during a contraction-relaxation cycle in rat single skeletal muscle fibers

Skeletal muscle generates heat via contraction-dependent (shivering) and independent (nonshivering) mechanisms. While this thermogenic capacity of skeletal muscle undoubtedly contributes to the body temperature homeostasis of animals and impacts various cellular functions, the intracellular temperature and its dynamics in skeletal muscle in vivo remain elusive. We aimed to determine the intracellular temperature and its changes within skeletal muscle in vivo during contraction and following relaxation. In addition, we tested the hypothesis that sarcoplasmic reticulum Ca2+ ATPase (SERCA) generates heat and increases the myocyte temperature during a transitory Ca2+-induced contraction-relaxation cycle. The intact spinotrapezius muscle of anesthetized adult male Wistar rats (n = 18) was exteriorized and loaded with the fluorescent probe Cellular Thermoprobe for Fluorescence Ratio (49.3 μM) by microinjection over 1 s. The fluorescence ratio (i.e., 580 nm/515 nm) was measured in vivo during 1) temperature increases induced by means of an external heater, and 2) Ca2+ injection (3.9 nL, 2.0 mM). The fluorescence ratio increased as a linear function of muscle surface temperature from 25 °C to 40 °C (r2 = 0.97, P < 0.01). Ca2+ injection (3.9 nL, 2.0 mM) significantly increased myocyte intracellular temperature: An effect that was suppressed by SERCA inhibition with cyclopiazonic acid (CPA, Ca2+: 38.3 ± 1.4 °C vs Ca2++CPA: 28.3 ± 2.8 °C, P < 0.01 at 1 min following injection). While muscle shortening occurred immediately after the Ca2+ injection, the increased muscle temperature was maintained during the relaxation phase. In this investigation, we demonstrated a novel model for measuring the intracellular temperature of skeletal muscle in vivo and further that heat generation occurs concomitant principally with SERCA functioning and muscle relaxation.

Seasonal adaptation of Mangalica pigs in terms of muscle morphology and metabolism

The skeletal muscle plays an important role in maintaining body temperature, which is mediated by thermogenesis and glucose or lipid metabolism. Mangalica is a native Hungarian pig that has cold tolerance and can live in grazing environments throughout the year. We evaluated the morphological and genetic aspects of Mangalica using muscle tissues to elucidate the mechanisms underlying the tolerance to seasonal effects in grazing environments. The muscle tissues in each season were analysed using morphological evaluation and electron microscopy. The cross-sectional area of skeletal muscle cells in summer was significantly larger than that in winter. The thickness of myofibrils in summer was significantly higher than in winter. The thickness of the Z-line in winter was significantly higher than in summer. The expression of MYH4 and GLUT4 was significantly lower in winter than in summer. The result of ATPase staining indicated significantly increase the muscle fibre ratio of type 1 in winter than that in summer. These findings indicate that the muscle fibre in Mangalica shifts from fast-twitch to slow-twitch fibre, and the metabolic physiology of the muscle was adapted to the cold environment. This study demonstrates that Mangalica gained tolerance to both seasonal heat and cold stresses that are caused by significant changes in ambient temperature in a year because of changes in their muscle fibre type and metabolic function. This study may contribute to elucidating the mechanism of thermogenetic adaptation in cold and heat environments among mammals.

Characterization of differential distribution patterns between mitofusin isoforms and their interaction in developing skeletal muscles of rat

Skeletal muscle during postnatal development undergoes several structural and biochemical modifications. It is proposed that these changes are closely intertwined with the increase in load-bearing capacity of the muscle (i.e., myofibrils) and molecular machinery to support the energy demand (i.e., mitochondria). Concomitant establishment of the sarcoplasmic reticulum (SR) and mitochondrial network seems to be a major developmental adjustment of skeletal muscle leading to adult phenotype. Here, we have studied oxidativeness, vascularization, and the changes in mitofusins (Mfn) 1-Mfn 2 expression and interaction in the due course of muscle development. Toward this, we used a series of histochemical techniques to compare neonatal and adult limb muscles (Gastrocnemius and Quadriceps) of Wistar rat (Rattus norvegicus). Additionally, we probed the proximity between Mfn 1 and Mfn 2 using a highly sensitive antibody-based proximity ligation assay indicating the change in mitochondrial fusion pattern or mitochondria-SR interaction. The results show that neonatal fibers bear a uniform distribution of mitochondria while a differential pattern of distribution is seen in adults. The distribution of the blood vessels is also quite distinct in adult muscles with a well-formed capillary network but in neonates, only central blood vessels are seen. Interestingly, our Mfn 1-Mfn 2 interaction data show that this interaction is uniformly distributed throughout the neonatal fibers, while it becomes peripherally localized in fibers of adult muscles. This peripheralization of Mfn 1-Mfn 2 interaction must be an important event of muscle development and might be critical to cater to the metabolic needs of adult muscle.

Transcriptome Analysis Revealed Potential Genes of Skeletal Muscle Thermogenesis in Mashen Pigs and Large White Pigs under Cold Stress

Pigs are susceptible to cold stress due to the absence of brown fat caused by the partial deletion of uncoupling protein 1 during their evolution. Some local pig breeds in China exhibit potential cold adaptability, but research has primarily focused on fat and intestinal tissues. Skeletal muscle plays a key role in adaptive thermogenesis in mammals, yet the molecular mechanism of cold adaptation in porcine skeletal muscle remains poorly understood. This study investigated the cold adaptability of two pig breeds, Mashen pigs (MS) and Large White pigs (LW), in a four-day cold (4 °C) or normal temperature (25 °C) environment. We recorded phenotypic changes and collected blood and longissimus dorsi muscle for transcriptome sequencing. Finally, the PRSS8 gene was randomly selected for functional exploration in porcine skeletal muscle satellite cells. A decrease in body temperature and body weight in both LW and MS pigs under cold stress, accompanied by increased shivering frequency and respiratory frequency, were observed. However, the MS pigs demonstrated stable physiological homeostasis, indicating a certain level of cold adaptability. The LW pigs primarily responded to cold stress by regulating their heat production and glycolipid energy metabolism. The MS pigs exhibited a distinct response to cold stress, involving the regulation of heat production, energy metabolism pathways, and robust mitochondrial activity, as well as a stronger immune response. Furthermore, the functional exploration of PRSS8 in porcine skeletal muscle satellite cells revealed that it affected cellular energy metabolism and thermogenesis by regulating ERK phosphorylation. These findings shed light on the diverse transcriptional responses of skeletal muscle in LW and MS pigs under cold stress, offering valuable insights into the molecular mechanisms underlying cold adaptation in pigs.

Seasonal cold induces divergent structural/biochemical adaptations in different skeletal muscles of Columba livia: evidence for nonshivering thermogenesis in adult birds

Birds are endothermic homeotherms even though they lack the well-studied heat producing brown adipose tissue (BAT), found in several clades of eutherian mammals. Earlier studies in ducklings have demonstrated that skeletal muscle is the primary organ of nonshivering thermogenesis (NST) plausibly via futile calcium (Ca2+)-handling through ryanodine receptor (RyR) and sarco-endoplasmic reticulum Ca2+-ATPase (SERCA). However, recruitment of futile Ca2+-cycling in adult avian skeletal muscle has not been documented. Studies in mammals show remarkable mitochondrial remodeling concurrently with muscle NST during cold. Here, we wanted to define the mitochondrial and biochemical changes in the muscles in free-ranging adult birds and whether different skeletal muscle groups undergo similar seasonal changes. We analyzed four different muscles (pectoralis, biceps, triceps and iliotibialis) from local pigeon (Columba livia) collected during summer and winter seasons in two consecutive years. Remarkable increase in mitochondrial capacity was observed as evidenced from succinate dehydrogenase (SDH) and cytochrome c oxidase (COX) activity staining in all the muscles. Interestingly, fibers with low SDH activity exhibited greater cross-sectional area during winter in all muscles except iliotibialis and became peripherally arranged in individual fascicles of pectoralis, which might indicate increased shivering. Furthermore, gene expression analysis showed that SERCA, sarcolipin and RyR are up-regulated to different levels in the muscles analyzed indicating muscle NST via futile Ca2+-cycling is recruited to varying degrees in winter. Moreover, proteins of mitochondrial-SR-tethering and biogenesis also showed differential alterations across the muscles. These data suggest that tropical winter (∼15°C) is sufficient to induce distinct remodeling across muscles in adult bird.

EARLY DECREASED RESPIRATORY CHAIN CAPACITY IN RESUSCITATED EXPERIMENTAL SEPSIS IS A MAJOR CONTRIBUTOR TO LACTATE PRODUCTION

ABSTRACT

Background : Increased plasma lactate levels in patients with sepsis may be due to insufficient oxygen delivery, but mitochondrial dysfunction or accelerated glycolysis may also contribute. We studied the effect of the latter on muscle metabolism by using microdialysis in a sepsis model with sustained oxygen delivery and decreased energy consumption or mitochondrial blockade. Methods : Pigs were subjected to continuous Escherichia coli infusion (sepsis group, n = 12) or saline infusion (sham group, n = 4) for 3 h. Protocolized interventions were applied to normalize the oxygen delivery and blood pressure. Microdialysis catheters were used to monitor muscle metabolism (naïve). The same catheters were used to block the electron transport chain with cyanide or the Na + /K + -ATPase inhibitor, ouabain locally. Results: All pigs in the sepsis group had positive blood cultures and a Sequential Organ Failure Assessment score increase by at least 2, fulfilling the sepsis criteria. Plasma lactate was higher in the sepsis group than in the sham group ( P < 0.001), whereas muscle glucose was lower in the sepsis group ( P < 0.01). There were no changes in muscle lactate levels over time but lactate to pyruvate ratio (LPR) was elevated in the sepsis versus the sham group ( P < 0.05). Muscle lactate, LPR, and glutamate levels were higher in the sepsis group than in the sham group in the cyanide catheters ( P < 0.001, all comparisons) and did not normalize in the former group. Conclusions: In this experimental study on resuscitated sepsis, we observed increased aerobic metabolism and preserved mitochondrial function. Sepsis and electron transport chain inhibition led to increased LPR, suggesting a decreased mitochondrial reserve capacity in early sepsis.

Non-shivering thermogenesis is differentially regulated during the hibernation season in Arctic ground squirrels

Arctic ground squirrels are small mammals that experience physiological extremes during the hibernation season. Body temperature rises from 1°C to 40°C during interbout arousal and requires tight thermoregulation to maintain rheostasis. Tissues from wild-caught Arctic ground squirrels were sampled over 9 months to assess the expression of proteins key to thermogenic regulation. Animals were sacrificed while aroused, and the extensor digitorum longus, diaphragm, brown adipose tissue, and white adipose tissue were probed using Western blots to assess protein expression and blood was sampled for metabolite analysis. Significant seasonal expression patterns emerged showing differential regulation. Contrary to our prediction, white adipose tissue showed no expression of uncoupling protein 1, but utilization of uncoupling protein 1 peaked in brown adipose tissue during the winter months and began to taper after terminal arousal in the spring. The opposite was true for muscular non-shivering thermogenesis. Sarco/endoplasmic reticulum calcium ATPase 1a and 2a expressions were depressed during the late hibernation season and rebounded after terminal arousal in diaphragm tissues, but only SERCA2a was differentially expressed in the extensor digitorum longus. The uncoupler, sarcolipin, was only detected in diaphragm samples and had a decreased expression during hibernation. The differential timing of these non-shivering pathways indicated distinct functions in maintaining thermogenesis which may depend on burrow temperature, availability of endogenous resources, and other seasonal activity demands on these tissues. These results could be impacted by fiber type makeup of the muscles collected, the body weight of the animal, and the date of entrance or exit from hibernation.

Hypermetabolism in mice carrying a near-complete human chromosome 21

The consequences of aneuploidy have traditionally been studied in cell and animal models in which the extrachromosomal DNA is from the same species. Here, we explore a fundamental question concerning the impact of aneuploidy on systemic metabolism using a non-mosaic transchromosomic mouse model (TcMAC21) carrying a near-complete human chromosome 21. Independent of diets and housing temperatures, TcMAC21 mice consume more calories, are hyperactive and hypermetabolic, remain consistently lean and profoundly insulin sensitive, and have a higher body temperature. The hypermetabolism and elevated thermogenesis are likely due to a combination of increased activity level and sarcolipin overexpression in the skeletal muscle, resulting in futile sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) activity and energy dissipation. Mitochondrial respiration is also markedly increased in skeletal muscle to meet the high ATP demand created by the futile cycle and hyperactivity. This serendipitous discovery provides proof-of-concept that sarcolipin-mediated thermogenesis via uncoupling of the SERCA pump can be harnessed to promote energy expenditure and metabolic health.

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.

Distinct Transcriptional Responses of Skeletal Muscle to Short-Term Cold Exposure in Tibetan Pigs and Bama Pigs

Piglets are susceptible to cold, and piglet death caused by cold stress leads to economic losses in the pig industry in cold areas. Skeletal muscle plays a key role in adaptive thermogenesis in mammals, but the related mechanism in pigs is unclear. In this study, cold-tolerant Tibetan pigs and cold-sensitive Bama pigs were subjected to either a cold environment (4 °C) or a room temperature environment (25 °C) for 3 days. The biceps femoris (BF) and longissimus dorsi muscle (LDM) were collected for phenotypic analysis, and the BF was used for genome-wide transcriptional profiling. Our results showed that Tibetan pigs had a higher body temperature than Bama pigs upon cold stimulation. RNA-seq data indicated a stronger transcriptional response in the skeletal muscle of Tibetan pigs upon cold stimulation, as more differentially expressed genes (DEGs) were identified with the same criteria (p < 0.05 and fold change > 2). In addition, distinct pathway signaling patterns in skeletal muscle upon cold exposure were found between the breeds of pigs. Mitochondrial beta-oxidation-related genes and pathways were significantly upregulated in Tibetan pigs, indicating that Tibetan pigs may use fatty acids as the primary fuel source to protect against cold. However, the significant upregulation of inflammatory response- and glycolysis-related genes and pathways in the skeletal muscle of Bama pigs suggested that these pigs may use glucose as the primary fuel source in cold environments. Together, our study revealed the distinct transcriptional responses of skeletal muscle to cold stimulation in Tibetan pigs and Bama pigs and provided novel insights for future investigation of the cold adaptation mechanism in pigs.

Physiological and molecular mechanisms of cold-induced improvements in glucose homeostasis in humans beyond brown adipose tissue

Exposure to low ambient temperatures has previously been demonstrated to markedly improve glucose homeostasis in both rodents and humans. Although the brown adipose tissue is key in mediating these beneficial effects in rodents, its contribution appears more limited in humans. Hence, the exact tissues and underlying mechanisms that mediate cold-induced improvements in glucose homeostasis in humans remain to be fully established. In this review, we evaluated the response of the main organs involved in glucose metabolism (i.e. pancreas, liver, (white) adipose tissue, and skeletal muscle) to cold exposure and discuss their potential contribution to cold-induced improvements in glucose homeostasis in humans. We here show that cold exposure has widespread effects on metabolic organs involved in glucose regulation. Nevertheless, cold-induced improvements in glucose homeostasis appear primarily mediated via adaptations within the skeletal muscle and (presumably) white adipose tissue. Since the underlying mechanisms remain elusive, future studies should be aimed at pinpointing the exact physiological and molecular mechanisms involved in humans. Nonetheless, cold exposure holds great promise as a novel, additive lifestyle approach to improve glucose homeostasis in insulin resistant individuals. Parts of this graphical abstract were created using (modified) images from Servier Medical Art, licensed under the Creative Commons Attribution 3.0 Unported License. TG = thermogenesis, TAG = triacylglycerol, FFA = free fatty acid, SLN = sarcolipin, UCP3 = uncoupling protein 3, β2-AR = beta-2 adrenergic receptor, SNS = sympathetic nervous system.

Hypermetabolism in mice carrying a near complete human chromosome 21

The consequences of aneuploidy have traditionally been studied in cell and animal models in which the extrachromosomal DNA is from the same species. Here, we explore a fundamental question concerning the impact of aneuploidy on systemic metabolism using a non-mosaic transchromosomic mouse model (TcMAC21) carrying a near complete human chromosome 21. Independent of diets and housing temperatures, TcMAC21 mice consume more calories, are hyperactive and hypermetabolic, remain consistently lean and profoundly insulin sensitive, and have a higher body temperature. The hypermetabolism and elevated thermogenesis are due to sarcolipin overexpression in the skeletal muscle, resulting in futile sarco(endo)plasmic reticulum Ca ²⁺ ATPase (SERCA) activity and energy dissipation. Mitochondrial respiration is also markedly increased in skeletal muscle to meet the high ATP demand created by the futile cycle. This serendipitous discovery provides proof-of-concept that sarcolipin-mediated thermogenesis via uncoupling of the SERCA pump can be harnessed to promote energy expenditure and metabolic health.

Impairment of adrenergically-regulated thermogenesis in brown fat of obesity-resistant mice is compensated by non-shivering thermogenesis in skeletal muscle

OBJECTIVE

Non-shivering thermogenesis (NST) mediated by uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) can be activated via the adrenergic system in response to cold or diet, contributing to both thermal and energy homeostasis. Other mechanisms, including metabolism of skeletal muscle, may also be involved in NST. However, relative contribution of these energy dissipating pathways and their adaptability remain a matter of long-standing controversy.

METHODS

We used warm-acclimated (30 °C) mice to characterize the effect of an up to 7-day cold acclimation (6 °C; CA) on thermoregulatory thermogenesis, comparing inbred mice with a genetic background conferring resistance (A/J) or susceptibility (C57BL/6 J) to obesity.

RESULTS

Both warm-acclimated C57BL/6 J and A/J mice exhibited similar cold endurance, assessed as a capability to maintain core body temperature during acute exposure to cold, which improved in response to CA, resulting in comparable cold endurance and similar induction of UCP1 protein in BAT of mice of both genotypes. Despite this, adrenergic NST in BAT was induced only in C57BL/6 J, not in A/J mice subjected to CA. Cold tolerance phenotype of A/J mice subjected to CA was not based on increased shivering, improved insulation, or changes in physical activity. On the contrary, lipidomic, proteomic and gene expression analyses along with palmitoyl carnitine oxidation and cytochrome c oxidase activity revealed induction of lipid oxidation exclusively in skeletal muscle of A/J mice subjected to CA. These changes appear to be related to skeletal muscle NST, mediated by sarcolipin-induced uncoupling of sarco(endo)plasmic reticulum calcium ATPase pump activity and accentuated by changes in mitochondrial respiratory chain supercomplexes assembly.

CONCLUSIONS

Our results suggest that NST in skeletal muscle could be adaptively augmented in the face of insufficient adrenergic NST in BAT, depending on the genetic background of the mice. It may provide both protection from cold and resistance to obesity, more effectively than BAT.

The crosstalk between BAT thermogenesis and skeletal muscle dysfunction

Metabolic defects increase the risk of skeletal muscle diseases, and muscle impairment might worsen metabolic disruption, leading to a vicious cycle. Both brown adipose tissue (BAT) and skeletal muscle play important roles in non-shivering thermogenesis to regulate energy homeostasis. BAT regulates body temperature, systemic metabolism, and seretion of batokines that have positive or negative impacts on skeletal muscle. Conversely, muscle can secrete myokines that regulate BAT function. This review explained the crosstalk between BAT and skeletal muscle, and then discussed the batokines and highlighted their impact on skeletal muscle under physiological conditions. BAT is now considered a potential therapeutic target for obesity and diabetes treatment. Moreover, manipulation of BAT may be an attractive approach for the treatment of muscle weakness by correcting metabolic deficits. Therefore, exploring BAT as a potential treatment for sarcopenia could be a promising avenue for future research.

Recruitment of Muscle Genes as an Effect of Brown Adipose Tissue Ablation in Cold-Acclimated Brandt's Voles (Lasiopodomys brandtii)

Skeletal muscle-based nonshivering thermogenesis (NST) plays an important role in the regulation and maintenance of body temperature in birds and large mammals, which do not contain brown adipose tissue (BAT). However, the relative contribution of muscle-based NST to thermoregulation is not clearly elucidated in wild small mammals, which have evolved an obligate thermogenic organ of BAT. In this study, we investigated whether muscle would become an important site of NST when BAT function is conditionally minimized in Brandt's voles (Lasiopodomys brandtii). We surgically removed interscapular BAT (iBAT, which constitutes 52%~56% of total BAT) and exposed the voles to prolonged cold (4 °C) for 28 days. The iBAT-ablated voles were able to maintain the same levels of NST and body temperature (~37.9 °C) during the entire period of cold acclimation as sham voles. The expression of uncoupling protein 1 (UCP1) and its transcriptional regulators at both protein and mRNA levels in the iBAT of cold-acclimated voles was higher than that in the warm group. However, no difference was observed in the protein or mRNA levels of these thermogenesis-related markers except for PGC-1α in other sites of BAT (including infrascapular region, neck, and axilla) between warm and cold groups either in sham or iBAT-ablated voles. The iBAT-ablated voles showed higher UCP1 expression in white adipose tissue (WAT) than sham voles during cold acclimation. The expression of sarcolipin (SLN) and sarcoplasmic endoplasmic reticulum Ca²⁺-dependent adenosine triphosphatase (SERCA) in skeletal muscles was higher in cold than in warm, but no alteration in phospholamban (PLB) and phosphorylated-PLB (P-PLB) was observed. Additionally, there was increased in iBAT-ablated voles compared to that in the sham group in cold. Moreover, these iBAT-ablated voles underwent extensive remodeling of mitochondria and genes of key components related with mitochondrial metabolism. These data collectively indicate that recruitment of skeletal muscle-based thermogenesis may compensate for BAT impairment and suggest a functional interaction between the two forms of thermogenic processes of iBAT and skeletal muscle in wild small mammals for coping cold stress.

Structural functionality of skeletal muscle mitochondria and its correlation with metabolic diseases

The skeletal muscle is one of the largest organs in the mammalian body. Its remarkable ability to swiftly shift its substrate selection allows other organs like the brain to choose their preferred substrate first. Healthy skeletal muscle has a high level of metabolic flexibility, which is reduced in several metabolic diseases, including obesity and Type 2 diabetes (T2D). Skeletal muscle health is highly dependent on optimally functioning mitochondria that exist in a highly integrated network with the sarcoplasmic reticulum and sarcolemma. The three major mitochondrial processes: biogenesis, dynamics, and mitophagy, taken together, determine the quality of the mitochondrial network in the muscle. Since muscle health is primarily dependent on mitochondrial status, the mitochondrial processes are very tightly regulated in the skeletal muscle via transcription factors like peroxisome proliferator-activated receptor-γ coactivator-1α, peroxisome proliferator-activated receptors, estrogen-related receptors, nuclear respiratory factor, and Transcription factor A, mitochondrial. Physiological stimuli that enhance muscle energy expenditure, like cold and exercise, also promote a healthy mitochondrial phenotype and muscle health. In contrast, conditions like metabolic disorders, muscle dystrophies, and aging impair the mitochondrial phenotype, which is associated with poor muscle health. Further, exercise training is known to improve muscle health in aged individuals or during the early stages of metabolic disorders. This might suggest that conditions enhancing mitochondrial health can promote muscle health. Therefore, in this review, we take a critical overview of current knowledge about skeletal muscle mitochondria and the regulation of their quality. Also, we have discussed the molecular derailments that happen during various pathophysiological conditions and whether it is an effect or a cause.

Intrinsic challenges of neonatal adaptation in swine

The losses of piglets in commercial pig farming remain at concerning levels and need to be addressed through the implementation of new sustainable breeding and management strategies. In fact, piglets are especially at risk in the first days of life. Both genetics and the farrowing process have been shown to impact piglet vitality. In addition, knowledge of the animal-intrinsic responses in adapting to extra-uterine life is particularly important but is scarcely described in the scientific literature. In this review, the three phases that constitute neonatal adaptation in the pig are systematically presented. The first phase of early adaptation involves primarily the development of cardiorespiratory function (within the first 10 min of life) as well as thermoregulatory processes and acid-base balance (up to 24 h of life). In the second phase, homeostasis is established, and organ maturation takes place (up to 14 d post natum). The final third phase aims at the development of neurological, immunological and muscular features (up to 28 d of life). The involvement of aggravating and ameliorating factors such as dystocia, low colostrum yield and heat supply is key to the development of strategies to reduce piglet losses and increase vitality. The insights are of particular value in addressing current concerns in pig farming and to further improve animal welfare in pig production across different management types.

β-Sitosterol & quercetin enhances brain development in iodine deficient rat models

BACKGROUND

Recently thyroid hormone studies on brain growth, development and activity are regaining popularity. Thyroid hormones have long been believed to play critical role in mammalian brain growth and maturation regulating facets of neuronal cell growth, proliferation and differentiation and further signaling and glial cell differentiation. Deficiency of these hormones in mother leads to mental retardation in the subsequent offspring's.

METHODS

In this presented study, brain development of iodine deficient rat models created through deficiency in feeding, mating and further selection. Young adult female wistar rats were induced with iodine deficiency and then mated with healthy male rats. These pregnant hypothyroid induced females were treated with β-sitosterol (150 mg/kg/day) and quercetin (150 mg/kg/day) alone and in combination for whole gestation period. Analysis were dealt with the genetic and histological studies of the pups brain. PCR based RNA analysis was also carried out. Histology was done using eosin and hematoxylin.

RESULTS

Positive impacts of the β-sitosterol and quercetin on the iodine deficient brain were observed upon histological and PCR analysis. Altogether, the analysis proves that combined doses of β-sitosterol and quercetin for normal brain development in iodine deficient infants hence can be potentially applied as therapeutics in iodine deficiency circumstances.

Avian adjustments to cold and non-shivering thermogenesis: whats, wheres and hows

Avian cold adaptation is hallmarked by innovative strategies of both heat conservation and thermogenesis. While minimizing heat loss can reduce the thermogenic demands of body temperature maintenance, it cannot eliminate the requirement for thermogenesis. Shivering and non-shivering thermogenesis (NST) are the two synergistic mechanisms contributing to endothermy. Birds are of particular interest in studies of NST as they lack brown adipose tissue (BAT), the major organ of NST in mammals. Critical analysis of the existing literature on avian strategies of cold adaptation suggests that skeletal muscle is the principal site of NST. Despite recent progress, isolating the mechanisms involved in avian muscle NST has been difficult as shivering and NST co-exist with its primary locomotory function. Herein, we re-evaluate various proposed molecular bases of avian skeletal muscle NST. Experimental evidence suggests that sarco(endo)plasmic reticulum Ca²⁺ -ATPase (SERCA) and ryanodine receptor 1 (RyR1) are key in avian muscle NST, through their mediation of futile Ca²⁺ cycling and thermogenesis. More recent studies have shown that SERCA regulation by sarcolipin (SLN) facilitates muscle NST in mammals; however, its role in birds is unclear. Ca²⁺ signalling in the muscle seems to be common to contraction, shivering and NST, but elucidating its roles will require more precise measurement of local Ca²⁺ levels inside avian myofibres. The endocrine control of avian muscle NST is still poorly defined. A better understanding of the mechanistic details of avian muscle NST will provide insights into the roles of these processes in regulatory thermogenesis, which could further inform our understanding of the evolution of endothermy among vertebrates.

Calcium cycling as a mediator of thermogenic metabolism in adipose tissue

Canonical non-shivering thermogenesis (NST) in brown and beige fat relies on uncoupling protein 1 (UCP1)-mediated heat generation, although alternative mechanisms of NST have been identified, including sarcoplasmic reticulum (SR)-calcium cycling. Intracellular calcium is a crucial cell signaling molecule for which compartmentalization is tightly regulated, and the sarco-endoplasmic calcium ATPase (SERCA) actively pumps calcium from the cytosol into the SR. In this review, we discuss the capacity of SERCA-mediated calcium cycling as a significant mediator of thermogenesis in both brown and beige adipocytes. Here, we suggest two primary mechanisms of SR calcium mediated thermogenesis. The first mechanism is through direct uncoupling of the ATPase and calcium pump activity of SERCA, resulting in the energy of ATP catalysis being expended as heat in the absence of calcium transport. Regulins, a class of SR membrane proteins, act to decrease the calcium affinity of SERCA and uncouple the calcium transport function from ATPase activity, but remain largely unexplored in adipose tissue thermogenesis. A second mechanism is through futile cycling of SR calcium whereby SERCA-mediated SR calcium influx is equally offset by SR calcium efflux, resulting in ATP consumption without a net change in calcium compartmentalization. A fuller understanding of the functional and mechanistic role of calcium cycling as a mediator of adipose tissue thermogenesis and how manipulation of these pathways can be harnessed for therapeutic gain remains unexplored. Significance Statement Enhancing thermogenic metabolism in brown or beige adipose tissue may be of broad therapeutic utility to reduce obesity and metabolic syndrome. Canonical BAT-mediated thermogenesis occurs via uncoupling protein 1 (UCP1). However, UCP1-independent pathways of thermogenesis, such as sarcoplasmic (SR) calcium cycling, have also been identified, but the regulatory mechanisms and functional significance of these pathways remain largely unexplored. Thus, this mini-review discusses the state of the field with regard to calcium cycling as a thermogenic mediator in adipose tissue.

Whole-body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians

The whole-body (tachymetabolic) endothermy seen in modern birds and mammals is long held to have evolved independently in each group, a reasonable assumption when it was believed that its earliest appearances in birds and mammals arose many millions of years apart. That assumption is consistent with current acceptance that the non-shivering thermogenesis (NST) component of regulatory body heat originates differently in each group: from skeletal muscle in birds and from brown adipose tissue (BAT) in mammals. However, BAT is absent in monotremes, marsupials, and many eutherians, all whole-body endotherms. Indeed, recent research implies that BAT-driven NST originated more recently and that the biochemical processes driving muscle NST in birds, many modern mammals and the ancestors of both may be similar, deriving from controlled 'slippage' of Ca2+ from the sarcoplasmic reticulum Ca2+ -ATPase (SERCA) in skeletal muscle, similar to a process seen in some fishes. This similarity prompted our realisation that the capacity for whole-body endothermy could even have pre-dated the divergence of Amniota into Synapsida and Sauropsida, leading us to hypothesise the homology of whole-body endothermy in birds and mammals, in contrast to the current assumption of their independent (convergent) evolution. To explore the extent of similarity between muscle NST in mammals and birds we undertook a detailed review of these processes and their control in each group. We found considerable but not complete similarity between them: in extant mammals the 'slippage' is controlled by the protein sarcolipin (SLN), in birds the SLN is slightly different structurally and its role in NST is not yet proved. However, considering the multi-millions of years since the separation of synapsids and diapsids, we consider that the similarity between NST production in birds and mammals is consistent with their whole-body endothermy being homologous. If so, we should expect to find evidence for it much earlier and more widespread among extinct amniotes than is currently recognised. Accordingly, we conducted an extensive survey of the palaeontological literature using established proxies. Fossil bone histology reveals evidence of sustained rapid growth rates indicating tachymetabolism. Large body size and erect stature indicate high systemic arterial blood pressures and four-chambered hearts, characteristic of tachymetabolism. Large nutrient foramina in long bones are indicative of high bone perfusion for rapid somatic growth and for repair of microfractures caused by intense locomotion. Obligate bipedality appeared early and only in whole-body endotherms. Isotopic profiles of fossil material indicate endothermic levels of body temperature. These proxies led us to compelling evidence for the widespread occurrence of whole-body endothermy among numerous extinct synapsids and sauropsids, and very early in each clade's family tree. These results are consistent with and support our hypothesis that tachymetabolic endothermy is plesiomorphic in Amniota. A hypothetical structure for the heart of the earliest endothermic amniotes is proposed. We conclude that there is strong evidence for whole-body endothermy being ancient and widespread among amniotes and that the similarity of biochemical processes driving muscle NST in extant birds and mammals strengthens the case for its plesiomorphy.

Limited Physiological Compensation in Response to an Acute Microclimate Change in a Malagasy Bat

Rapid environmental changes are challenging for endothermic species because they have direct and immediate impacts on their physiology by affecting microclimate and fundamental resource availability. Physiological flexibility can compensate for certain ecological perturbations, but our basic understanding of how species function in a given habitat and the extent of their adaptive scope is limited. Here we studied the effect of acute, experimental microclimate change on the thermal physiology of two populations of the widespread Malagasy bat, Macronycteris commersoni. Populations of this species are found roosting under contrasting conditions, i.e., in a constant hot and humid cave or below foliage unprotected from fluctuations in ambient conditions. We exposed free-ranging individuals of each population to the respective opposite condition and thus to novel microclimate within an ecologically realistic scope while measuring metabolic rate and skin temperature. Cave bats in forest setting had a limited capacity to maintain euthermia to the point that two individuals became hypothermic when ambient temperature dropped below their commonly experienced cave temperature. Forest bats on the other hand, had difficulties to dissipate heat in the humid cave set-up. The response to heat, however, was surprisingly uniform and all bats entered torpor combined with hyperthermia at temperatures exceeding their thermoneutral zone. Thus, while we observed potential for flexible compensation of heat through “hot” torpor, both populations showed patterns suggestive of limited potential to cope with acute microclimate changes deviating from their typically occupied roosts. Our study emphasizes that intraspecific variation among populations could be misleading when assessing species’ adaptive scopes, as variation may arise from genetic adaptation, developmental plasticity or phenotypic flexibility, all of which allow for compensatory responses at differing time scales. Disentangling these mechanisms and identifying the basis of variation is vital to make accurate predictions of species’ chances for persisting in ever rapidly changing habitats and climates.

ATP-consuming futile cycles as energy dissipating mechanisms to counteract obesity

Obesity results from an imbalance in energy homeostasis, whereby excessive energy intake exceeds caloric expenditure. Energy can be dissipated out of an organism by producing heat (thermogenesis), explaining the long-standing interest in exploiting thermogenic processes to counteract obesity. Mitochondrial uncoupling is a process that expends energy by oxidizing nutrients to produce heat, instead of ATP synthesis. Energy can also be dissipated through mechanisms that do not involve mitochondrial uncoupling. Such mechanisms include futile cycles described as metabolic reactions that consume ATP to produce a product from a substrate but then converting the product back into the original substrate, releasing the energy as heat. Energy dissipation driven by cellular ATP demand can be regulated by adjusting the speed and number of futile cycles. Energy consuming futile cycles that are reviewed here are lipolysis/fatty acid re-esterification cycle, creatine/phosphocreatine cycle, and the SERCA-mediated calcium import and export cycle. Their reliance on ATP emphasizes that mitochondrial oxidative function coupled to ATP synthesis, and not just uncoupling, can play a role in thermogenic energy dissipation. Here, we review ATP consuming futile cycles, the evidence for their function in humans, and their potential employment as a strategy to dissipate energy and counteract obesity.

UCP1 Knockin Induces Lipid Dynamics and Transcriptional Programs in the Skeletal Muscles of Pigs

Uncoupling protein 1 (UCP1), the hallmark protein responsible for nonshivering thermogenesis in adipose tissue (especially brown adipose tissue) has regained researchers' attention in the context of metabolic disorders following the realization that UCP1 can be activated in adult humans and reconstituted in pigs. Both skeletal muscle and adipose tissue are highly dynamic tissues that interact at the metabolic and hormonal level in response to internal and external stress, and they coordinate in maintaining whole-body metabolic homeostasis. Here, we utilized lipidomics and transcriptomics to identify the altered lipid profiles and regulatory pathways in skeletal muscles from adipocyte-specific UCP1 knock-in (KI) pigs. UCP1 KI changed the contents of glycerophospholipids and acyl carnitines of skeletal muscles. Several metabolic regulatory pathways were more enriched in the UCP1 KI skeletal muscle. Comparison of the transcriptomes of adipose and skeletal muscle suggested that nervous system or chemokine signaling might account for the crosstalk between these two tissues in UCP1 KI pigs. Comparison of the lipid biomarkers from UCP1 KI pigs and other mammals suggested associations between UCP1 KI-induced metabolic alternations and metabolic and muscle dysfunction. Our study reveals the lipid dynamics and transcriptional programs in the skeletal muscle of UCP1 KI pigs and suggests that a network regulates metabolic homeostasis between skeletal muscle and adipose tissue.

Ca2+ leak through ryanodine receptor 1 regulates thermogenesis in resting skeletal muscle

The evolution of mammals to use skeletal muscle as a source of heat allowed them to spread to all parts of the globe. The generation of heat requires increased adenosine triphosphate (ATP) hydrolysis in the resting muscle in a regulated manner, but how this mechanism works is unknown. The results suggest that mammals increase their RyR1 Ca²⁺ leak rate to amplify a basal ATP turnover rate at the sarcoplasmic reticulum Ca²⁺ pump that is higher than that of lower vertebrates. Muscle-based thermogenesis allows regulation of body temperature that is essential for life in mammals and provides a potential pathway for manipulating body weight or temperature by altering metabolic rate.

Mammals rely on nonshivering thermogenesis (NST) from skeletal muscle so that cold temperatures can be tolerated. NST results from activity of the sarcoplasmic reticulum (SR) Ca²⁺ pump in skeletal muscle, but the mechanisms that regulate this activity are unknown. Here, we develop a single-fiber assay to investigate the role of Ca²⁺ leak through ryanodine receptor 1 (RyR1) to generate heat at the SR Ca²⁺ pump in resting muscle. By inhibiting a subpopulation of RyR1s in a single-fiber preparation via targeted delivery of ryanodine through transverse tubules, we achieve in-preparation isolation of RyR1 Ca²⁺ leak. This maneuver provided a critical increase in signal-to-noise of the SR-temperature-sensitive dye ER thermoyellow fluorescence signal from the fiber to allow detection of SR temperature changes as either RyR1 or SR Ca²⁺ pump activity was altered. We found that RyR1 Ca²⁺ leak raises cytosolic [Ca²⁺] in the local vicinity of the SR Ca²⁺ pump to amplify thermogenesis. Furthermore, gene-dose-dependent increases in RyR1 leak in RYR1 mutant mice result in progressive rises in leak-dependent heat, consistent with raised local [Ca²⁺] at the SR Ca²⁺ pump via RyR1 Ca²⁺ leak. We also show that basal RyR Ca²⁺ leak and the heat generated by the SR Ca²⁺ pump in the absence of RyR Ca²⁺ leak is greater in fibers from mice than from toads. The distinct function of RyRs and SR Ca²⁺ pump in endothermic mammals compared to ectothermic amphibians provides insights into the mechanisms by which mammalian skeletal muscle achieves thermogenesis at rest.

Augmented CCL5/CCR5 signaling in brown adipose tissue inhibits adaptive thermogenesis and worsens insulin resistance in obesity

Chemokine (C-C motif) ligand 5 (CCL5) and CCR5, one of its receptors have been reported to be highly expressed in white adipose tissue (WAT) and are associated with the progression of inflammation and the development of insulin resistance in obese humans and mice. However, the role of CCL5/CCR5 signaling in obesity-associated dysregulation of energy metabolism remains unclear. Here, we demonstrate that global CCL5/CCR5 double knockout (DKO) mice have higher cold stress-induced energy expenditure and thermogenic function in BAT than wild-type (WT) mice. DKO mice have higher cold stress-induced energy expenditure and thermogenic function in BAT than wild-type mice. KEGG pathway analysis indicated that deletion of CCL5/CCR5 further facilitated the cold-induced expression of genes related to oxidative phosphorylation and lipid metabolic pathways. In primary brown adipocytes of DKO mice, the augmentation of CL-316243-stimulated thermogenic and lipolysis responses was reversed by co-treatment with AMPKα1 and α2 siRNA. Overexpression of BAT CCL5/CCR5 genes by local lentivirus injection in WT mice suppressed cold stress-induced lipolytic processes and thermogenic activities. In contrast, knockdown of BAT CCL5/CCR5 signaling further upregulated AMPK phosphorylation as well as thermogenic and lipolysis responses to chronic adrenergic stimuli and subsequently decreased level of body weight gain. Chronic knockdown of BAT CCL5/CCR5 signaling improved HFD-induced insulin resistance in WT mice. It is suggested that obesity-induced augmentation of AT CCL5/CCR5 signaling could, at least in part, suppress energy expenditure and adaptive thermogenesis by inhibiting AMPK-mediated lipolysis and oxidative metabolism in thermogenic AT to exacerbate the development of obesity and insulin resistance.

Metabolic responses to mild cold acclimation in type 2 diabetes patients

Mild cold acclimation for 10 days has been previously shown to markedly improve insulin sensitivity in patients with type 2 diabetes. Here we show in a single-arm intervention study (Trialregister.nl ID: NL4469/NTR5711) in nine patients with type 2 diabetes that ten days of mild cold acclimation (16–17 °C) in which observable, overt shivering was prevented, does not result in improved insulin sensitivity, postprandial glucose and lipid metabolism or intrahepatic lipid content and only results in mild effects on overnight fasted fat oxidation, postprandial energy expenditure and aortic augmentation index. The lack of marked metabolic effects in this study is associated with a lack of self-reported shivering and a lack of upregulation of gene expression of muscle activation or muscle contraction pathways in skeletal muscle and suggests that some form of muscle contraction is needed for beneficial effects of mild cold acclimation.

Cold acclimation has been shown to have beneficial metabolic effects, including improved insulin sensitivity in patients with type 2 diabetes. Here the authors show that a mild cold acclimation regiment during which overt shivering was prevented did not result in improved insulin sensitivity in a small group of patients with type 2 diabetes.

Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation

The protein α-actinin-3 expressed in fast-twitch skeletal muscle fiber is absent in 1.5 billion people worldwide due to homozygosity for a nonsense polymorphism in ACTN3 (R577X). The prevalence of the 577X allele increased as modern humans moved to colder climates, suggesting a link between α-actinin-3 deficiency and improved cold tolerance. Here, we show that humans lacking α-actinin-3 (XX) are superior in maintaining core body temperature during cold-water immersion due to changes in skeletal muscle thermogenesis. Muscles of XX individuals displayed a shift toward more slow-twitch isoforms of myosin heavy chain (MyHC) and sarcoplasmic reticulum (SR) proteins, accompanied by altered neuronal muscle activation resulting in increased tone rather than overt shivering. Experiments on Actn3 knockout mice showed no alterations in brown adipose tissue (BAT) properties that could explain the improved cold tolerance in XX individuals. Thus, this study provides a mechanism for the positive selection of the ACTN3 X-allele in cold climates and supports a key thermogenic role of skeletal muscle during cold exposure in humans.

Is Upregulation of Sarcolipin Beneficial or Detrimental to Muscle Function?

Sarcolipin (SLN) is a regulator of sarco/endo plasmic reticulum Ca²⁺-ATPase (SERCA) pump and has been shown to be involved in muscle nonshivering thermogenesis (NST) and energy metabolism. Interestingly, SLN expression is significantly upregulated both during muscle development and in several disease states. However, the significance of altered SLN expression in muscle patho-physiology is not completely understood. We have previously shown that transgenic over-expression of SLN in skeletal muscle is not detrimental, and can promote oxidative metabolism and exercise capacity. In contrast, some studies have suggested that SLN upregulation in disease states is deleterious for muscle function and ablation of SLN can be beneficial. In this perspective article, we critically examine both published and some new data to determine the relevance of SLN expression to disease pathology. The new data presented in this paper show that SLN levels are induced in muscle during systemic bacterial (Salmonella) infection or lipopolysaccharides (LPS) treatment. We also present data showing that SLN expression is significantly upregulated in different types of muscular dystrophies including myotubular myopathy. These data taken together reveal that upregulation of SLN expression in muscle disease is progressive and increases with severity. Therefore, we suggest that increased SLN expression should not be viewed as the cause of the disease; rather, it is a compensatory response to meet the higher energy demand of the muscle. We interpret that higher SLN/SERCA ratio positively modulate cytosolic Ca²⁺ signaling pathways to promote mitochondrial biogenesis and oxidative metabolism to meet higher energy demand in muscle.

Contribution of thermogenic mechanisms by male and female mice lacking pituitary adenylate cyclase-activating polypeptide in response to cold acclimation

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide critical to the regulation of the stress response, including having a role in energy homeostasis. Mice lacking PACAP are cold-sensitive and have impaired adrenergic-induced thermogenesis. Interestingly, Pacap null mice can survive cold housing if acclimated slowly, similar to observations in uncoupling protein 1 (UCP1)-deficient mice. We hypothesized that Pacap null mice use alternate thermogenic pathways to compensate for impaired adaptive thermogenesis when acclimated to cold. Observations of behavior and assessment of fiber type in skeletal muscles did not show evidence of prolonged burst shivering or changes in oxidative metabolism in male or female Pacap^-/- mice during cold acclimation compared with Pacap^+/+ mice. Despite previous work that has established impaired capacity for adaptive thermogenesis in Pacap null mice, adaptive thermogenesis can be induced in mice lacking PACAP to support survival with cold housing. Interestingly, sex-specific morphological and molecular differences in adipose tissue remodeling were observed in Pacap null mice compared with controls. Thus, sexual dimorphisms are highlighted in adipose tissue remodeling and thermogenesis with cold acclimation in the absence of PACAP.NEW & NOTEWORTHY This manuscript adds to the literature of endocrine regulation of adaptive thermogenesis and energy balance. It specifically describes the role of pituitary adenylate cyclase-activating polypeptide on the regulation of brown adipose tissue via the sympathetic nervous system with a focus on compensatory mechanisms of thermogenesis. We highlight sex-specific differences in energy metabolism.

Chronic cold exposure induces mitochondrial plasticity in deer mice native to high altitudes

KEY POINTS

Small mammals native to high altitude must sustain high rates of thermogenesis to cope with cold. Skeletal muscle is a key site of shivering and non-shivering thermogenesis, but the importance of mitochondrial plasticity in cold hypoxic environments remains unresolved. We examined high-altitude deer mice, which have evolved a high capacity for aerobic thermogenesis, to determine the mechanisms of mitochondrial plasticity during chronic exposure to cold and hypoxia, alone and in combination. Cold exposure in normoxia or hypoxia increased mitochondrial leak respiration and decreased phosphorylation efficiency and OXPHOS coupling efficiency, which may serve to augment non-shivering thermogenesis. Cold also increased muscle oxidative capacity, but reduced the capacity for mitochondrial respiration via complex II relative to complexes I and II combined. High-altitude mice had a more oxidative muscle phenotype than low-altitude mice. Therefore, both plasticity and evolved changes in muscle mitochondria contribute to thermogenesis at high altitude.

ABSTRACT

Small mammals native to high altitude must sustain high rates of thermogenesis to cope with cold and hypoxic environments. Skeletal muscle is a key site of shivering and non-shivering thermogenesis, but the importance of mitochondrial plasticity in small mammals at high altitude remains unresolved. High-altitude deer mice (Peromyscus maniculatus) and low-altitude white-footed mice (P. leucopus) were born and raised in captivity, and chronically exposed as adults to warm (25°C) normoxia, warm hypoxia (12 kPa O2 ), cold (5°C) normoxia, or cold hypoxia. We then measured oxidative enzyme activities, oxidative fibre density and capillarity in the gastrocnemius, and used a comprehensive substrate titration protocol to examine the function of muscle mitochondria by high-resolution respirometry. Exposure to cold in both normoxia or hypoxia increased the activities of citrate synthase and cytochrome oxidase. In lowlanders, this was associated with increases in capillary density and the proportional abundance of oxidative muscle fibres, but in highlanders, these traits were unchanged at high levels across environments. Environment had some distinct effects on mitochondrial OXPHOS capacity between species, but the capacity of complex II relative to the combined capacity of complexes I and II was consistently reduced in both cold environments. Both cold environments also increased leak respiration and decreased phosphorylation efficiency and OXPHOS coupling efficiency in both species, which may serve to augment non-shivering thermogenesis. These cold-induced changes in mitochondrial function were overlaid upon the generally more oxidative phenotype of highlanders. Therefore, both plasticity and evolved changes in muscle mitochondria contribute to thermogenesis at high altitudes.

Brown adipose tissue lipoprotein and glucose disposal is not determined by thermogenesis in uncoupling protein 1-deficient mice

Adaptive thermogenesis is highly dependent on uncoupling protein 1 (UCP1), a protein expressed by thermogenic adipocytes present in brown adipose tissue (BAT) and white adipose tissue (WAT). Thermogenic capacity of human and mouse BAT can be measured by positron emission tomography-computed tomography quantifying the uptake of ¹⁸F-fluodeoxyglucose or lipid tracers. BAT activation is typically studied in response to cold exposure or treatment with β-3-adrenergic receptor agonists such as CL316,243 (CL). Currently, it is unknown whether cold-stimulated uptake of glucose or lipid tracers is a good surrogate marker of UCP1-mediated thermogenesis. In metabolic studies using radiolabeled tracers, we found that glucose uptake is increased in mildly cold-activated BAT of Ucp1 ^-/- versus WT mice kept at subthermoneutral temperature. Conversely, lower glucose disposal was detected after full thermogenic activation achieved by sustained cold exposure or CL treatment. In contrast, uptake of lipoprotein-derived fatty acids into chronically activated thermogenic adipose tissues was substantially increased in UCP1-deficient mice. This effect is linked to higher sympathetic tone in adipose tissues of Ucp1 ^-/- mice, as indicated by elevated levels of thermogenic genes in BAT and WAT. Thus, glucose and lipoprotein handling does not necessarily reflect UCP1-dependent thermogenic activity, but especially lipid uptake rather mirrors sympathetic activation of adipose tissues.

UCP1-independent thermogenesis

Obesity results from energy imbalance, when energy intake exceeds energy expenditure. Brown adipose tissue (BAT) drives non-shivering thermogenesis which represents a powerful mechanism of enhancing the energy expenditure side of the energy balance equation. The best understood thermogenic system in BAT that evolved to protect the body from hypothermia is based on the uncoupling of protonmotive force from oxidative phosphorylation through the actions of uncoupling protein 1 (UCP1), a key regulator of cold-mediated thermogenesis. Similarly, energy expenditure is triggered in response to caloric excess, and animals with reduced thermogenic fat function can succumb to diet-induced obesity. Thus, it was surprising when inactivation of Ucp1 did not potentiate diet-induced obesity. In recent years, it has become clear that multiple thermogenic mechanisms exist, based on ATP sinks centered on creatine, lipid, or calcium cycling, along with Fatty acid-mediated UCP1-independent leak pathways driven by the ADP/ATP carrier (AAC). With a key difference between cold- and diet-induced thermogenesis being the dynamic changes in purine nucleotide (primarily ATP) levels, ATP-dependent thermogenic pathways may play a key role in diet-induced thermogenesis. Additionally, the ubiquitous expression of AAC may facilitate increased energy expenditure in many cell types, in the face of over feeding. Interest in UCP1-independent energy expenditure has begun to showcase the therapeutic potential that lies in refining our understanding of the diversity of biochemical pathways controlling thermogenic respiration.

Linking Biochemical and Structural States of SERCA: Achievements, Challenges, and New Opportunities

Sarcoendoplasmic reticulum calcium ATPase (SERCA), a member of the P-type ATPase family of ion and lipid pumps, is responsible for the active transport of Ca²⁺ from the cytoplasm into the sarcoplasmic reticulum lumen of muscle cells, into the endoplasmic reticulum (ER) of non-muscle cells. X-ray crystallography has proven to be an invaluable tool in understanding the structural changes of SERCA, and more than 70 SERCA crystal structures representing major biochemical states (defined by bound ligand) have been deposited in the Protein Data Bank. Consequently, SERCA is one of the best characterized components of the calcium transport machinery in the cell. Emerging approaches in the field, including spectroscopy and molecular simulation, now help integrate and interpret this rich structural information to understand the conformational transitions of SERCA that occur during activation, inhibition, and regulation. In this review, we provide an overview of the crystal structures of SERCA, focusing on identifying metrics that facilitate structure-based categorization of major steps along the catalytic cycle. We examine the integration of crystallographic data with different biophysical approaches and computational methods to link biochemical and structural states of SERCA that are populated in the cell. Finally, we discuss the challenges and new opportunities in the field, including structural elucidation of functionally important and novel regulatory complexes of SERCA, understanding the structural basis of functional divergence among homologous SERCA regulators, and bridging the gap between basic and translational research directed toward therapeutic modulation of SERCA.

A hallmark of phospholamban functional divergence is located in the N-terminal phosphorylation domain

Sarcoplasmic reticulum Ca²⁺ pump (SERCA) is a critical component of the Ca²⁺ transport machinery in myocytes. There is clear evidence for regulation of SERCA activity by PLB, whose activity is modulated by phosphorylation of its N-terminal domain (residues 1–25), but there is less clear evidence for the role of this domain in PLB’s functional divergence. It is widely accepted that only sarcolipin (SLN), a protein that shares substantial homology with PLB, uncouples SERCA Ca²⁺ transport from ATP hydrolysis by inducing a structural change of its energy-transduction domain; yet, experimental evidence shows that the transmembrane domain of PLB (residues 26–52, PLB_26–52) partially uncouples SERCA in vitro. These apparently conflicting mechanisms suggest that PLB’s uncoupling activity is encoded in its transmembrane domain, and that it is controlled by the N-terminal phosphorylation domain. To test this hypothesis, we performed molecular dynamics simulations (MDS) of the binary complex between PLB_26–52 and SERCA. Comparison between PLB_26–52 and wild-type PLB (PLB_WT) showed no significant changes in the stability and orientation of the transmembrane helix, indicating that PLB_26–52 forms a native-like complex with SERCA. MDS showed that PLB_26–52 produces key intermolecular contacts and structural changes required for inhibition, in agreement with studies showing that PLB_26–52 inhibits SERCA. However, deletion of the N-terminal phosphorylation domain facilitates an order-to-disorder shift in the energy-transduction domain associated with uncoupling of SERCA, albeit weaker than that induced by SLN. This mechanistic evidence reveals that the N-terminal phosphorylation domain of PLB is a primary contributor to the functional divergence among homologous SERCA regulators.

Uncoupling of sarcoendoplasmic reticulum calcium ATPase pump activity by sarcolipin as the basis for muscle non-shivering thermogenesis

Thermogenesis in endotherms relies on both shivering and non-shivering thermogenesis (NST). The role of brown adipose tissue (BAT) in NST is well recognized, but the role of muscle-based NST has been contested. However, recent studies have provided substantial evidence for the importance of muscle-based NST in mammals. This review focuses primarily on the role of sarcoplasmic reticulum (SR) Ca²⁺-cycling in muscle NST; specifically, it will discuss recent data showing how uncoupling of sarcoendoplasmic reticulum calcium ATPase (SERCA) (inhibition of Ca²⁺ transport but not ATP hydrolysis) by sarcolipin (SLN) results in futile SERCA pump activity, increased ATP hydrolysis and heat production contributing to muscle NST. It will also critically examine how activation of muscle NST can be an important factor in regulating metabolic rate and whole-body energy homeostasis. In this regard, SLN has emerged as a powerful signalling molecule to promote mitochondrial biogenesis and oxidative metabolism in muscle. Furthermore, we will discuss the functional interplay between BAT and muscle, especially with respect to how reduced BAT function in mammals could be compensated by muscle-based NST. Based on the existing data, we argue that SLN-mediated thermogenesis is an integral part of muscle NST and that muscle NST potentially contributed to the evolution of endothermy within the vertebrate clade. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Sarcolipin Signaling Promotes Mitochondrial Biogenesis and Oxidative Metabolism in Skeletal Muscle

The major objective of this study was to understand the molecular basis of how sarcolipin uncoupling of SERCA regulates muscle oxidative metabolism. Using genetically engineered sarcolipin (SLN) mouse models and primary muscle cells, we demonstrate that SLN plays a crucial role in mitochondrial biogenesis and oxidative metabolism in muscle. Loss of SLN severely compromised muscle oxidative capacity without affecting fiber-type composition. Mice overexpressing SLN in fast-twitch glycolytic muscle reprogrammed mitochondrial phenotype, increasing fat utilization and protecting against high-fat dietinduced lipotoxicity. We show that SLN affects cytosolic Ca²⁺ transients and activates the Ca²⁺/ calmodulin-dependent protein kinase II (CamKII) and PGC1α axis to increase mitochondrial biogenesis and oxidative metabolism. These studies provide a fundamental framework for understanding the role of sarcoplasmic reticulum (SR)-Ca²⁺ cycling as an important factor in mitochondrial health and muscle metabolism. We propose that SLN can be targeted to enhance energy expenditure in muscle and prevent metabolic disease.

Maurya et al. report that sarcolipin, a regulator of the SERCA pump, promotes mitochondrial biogenesis and oxidative phenotype in muscle. Loss of SLN decreases fat oxidation, whereas overexpression of SLN in muscle provides resistance against diet-induced lipotoxicity. By increasing cytosolic Ca²⁺ transients, SLN activates the CamKII-PGC1α signaling pathway to promote mitochondrial biogenesis.