Neuronal UCP1 expression suggests a mechanism for local thermogenesis during hibernation

Anti-obesity effects of a standardized ethanol extract of Eisenia bicyclis by regulating the AMPK signaling pathway in 3T3-L1 cells and HFD-induced mice

Obesity requires treatment to mitigate the potential development of further metabolic disorders, including diabetes, hyperlipidemia, tumor growth, and non-alcoholic fatty liver disease. We investigated the anti-obesity effect of a 30% ethanol extract of Eisenia bicyclis (Kjellman) Setchell (EEB) on 3T3-L1 preadipocytes and high-fat diet (HFD)-induced obese C57BL/6 mice. Adipogenesis transcription factors including peroxisome proliferator-activated receptor (PPAR)γ, CCAAT/enhancer-binding protein-alpha (C/EBPα), and sterol regulatory element-binding protein-1 (SREBP-1) were ameliorated through the AMP-activated protein kinase (AMPK) pathway by EEB treatment in differentiated 3T3-L1 cells. EEB attenuated mitotic clonal expansion by upregulating cyclin-dependent kinase inhibitors (CDKIs) while downregulating cyclins and CDKs. In HFD-fed mice, EEB significantly decreased the total body weight, fat tissue weight, and fat in the tissue. The protein expression of PPARγ, C/EBPα, and SREBP-1 was increased in the subcutaneous fat and liver tissues, while EEB decreased the expression levels of these transcription factors. EEB also inhibited lipogenesis by downregulating acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) expression in the subcutaneous fat and liver tissues. Moreover, the phosphorylation of AMPK and ACC was downregulated in the HFD-induced mouse group, whereas the administration of EEB improved AMPK and ACC phosphorylation; thus, EEB treatment may be related to the AMPK pathway. Histological analysis showed that EEB reduced the adipocyte size and fat accumulation in subcutaneous fat and liver tissues, respectively. EEB promotes thermogenesis in brown adipose tissue and improves insulin and leptin levels and blood lipid profiles. Our results suggest that EEB could be used as a potential agent to prevent obesity.

Uncoupling protein 1-driven Cre (Ucp1-Cre) is expressed in the epithelial cells of mammary glands and various non-adipose tissues

OBJECTIVE

Uncoupling protein 1 (UCP1), a mitochondrial protein responsible for nonshivering thermogenesis in adipose tissue, serves as a distinct marker for thermogenic brown and beige adipocytes. Ucp1-Cre mice are thus widely used to genetically manipulate these thermogenic adipocytes. However, evidence suggests that UCP1 may also be expressed in non-adipocyte cell types. In this study, we investigated the presence of UCP1 expression in different mouse tissues that have not been previously reported.

METHODS

We employed Ucp1-Cre mice crossed with Cre-inducible transgenic reporter Nuclear tagging and Translating Ribosome Affinity Purification (NuTRAP) mice to investigate Ucp1-Cre expression in various tissues of adult female mice and developing embryos. Tamoxifen-inducible Ucp1-CreERT2 mice crossed with NuTRAP mice were used to assess active Ucp1 expression in adult mice. Immunostaining, RNA analysis, and single-cell/nucleus RNA-seq (sc/snRNA-seq) data analysis were performed to determine the expression of endogenous UCP1 and Ucp1-Cre-driven reporter expression. We also investigated the impact of UCP1 deficiency on mammary gland development and function using Ucp1-knockout (KO) mice.

RESULTS

Ucp1-Cre expression was observed in the mammary glands within the inguinal white adipose tissue of female Ucp1-Cre; NuTRAP mice. Ucp1-Cre was activated during embryonic development in various tissues, including mammary glands, as well as in the brain, kidneys, eyes, and ears, specifically in epithelial cells in these organs. However, Ucp1-CreERT2 showed no or only partial activation in these tissues of adult mice, indicating the potential for low or transient expression of endogenous Ucp1. While sc/snRNA-seq data suggest potential expression of UCP1 in mammary epithelial cells in adult mice and humans, Ucp1-KO female mice displayed normal mammary gland development and function.

CONCLUSIONS

Our findings reveal widespread Ucp1-Cre expression in various non-adipose tissue types, starting during early development. These results highlight the importance of exercising caution when interpreting data and devising experiments involving Ucp1-Cre mice.

The zebrafish heart harbors a thermogenic beige fat depot analog of human epicardial adipose tissue

Epicardial adipose tissue (eAT) is a metabolically active fat depot that has been associated with a wide array of cardiac homeostatic functions and cardiometabolic diseases. A full understanding of its diverse physiological and pathological roles is hindered by the dearth of animal models. Here, we show, in the heart of an ectothermic teleost, the zebrafish, the existence of a fat depot localized underneath the epicardium, originating from the epicardium and exhibiting the molecular signature of beige adipocytes. Moreover, a subset of adipocytes within this cardiac fat tissue exhibits primitive thermogenic potential. Transcriptomic profiling and cross-species analysis revealed elevated glycolytic and cardiac homeostatic gene expression with downregulated obesity and inflammatory hallmarks in the teleost eAT compared to that of lean aged humans. Our findings unveil epicardium-derived beige fat in the heart of an ectotherm considered to possess solely white adipocytes for energy storage and identify pathways that may underlie age-driven remodeling of human eAT.

Neural control of fluid homeostasis is engaged below 10°C in hibernation

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) hibernate for several months each winter without access to water,1 but the mechanisms that maintain fluid homeostasis during hibernation are poorly understood. In torpor, when body temperature (TB) reaches 4°C, squirrels decrease metabolism, slow heart rate, and reduce plasma levels of the antidiuretic hormones arginine vasopressin (AVP) and oxytocin (OXT).1 Squirrels spontaneously undergo interbout arousal (IBA) every 2 weeks, temporarily recovering an active-like metabolism and a TB of 37°C for up to 48 h.1,2 Despite the low levels of AVP and OXT during torpor, profound increases in blood pressure and heart rate during the torpor-IBA transition are not associated with massive fluid loss, suggesting the existence of a mechanism that protects against diuresis at a low TB. Here, we demonstrate that the antidiuretic hormone release pathway is activated by hypothalamic supraoptic nucleus (SON) neurons early in the torpor-arousal transition. SON neuron activity, dense-core vesicle release from the posterior pituitary, and plasma hormone levels all begin to increase before TB reaches 10°C. In vivo fiber photometry of SON neurons from hibernating squirrels, together with RNA sequencing and c-FOS immunohistochemistry, confirms that SON is electrically, transcriptionally, and translationally active to monitor blood osmolality throughout the dynamic torpor-arousal transition. Our work emphasizes the importance of the antidiuretic pathway during the torpor-arousal transition and reveals that the neurophysiological mechanism that coordinates the hormonal response to retain fluid is active at an extremely low TB, which is prohibitive for these processes in non-hibernators.

Integrated Redox-Metabolic Orchestration Sustains Life in Hibernating Ground Squirrels

Significance: The ultimate manifestations of life, birth, survival under various environmental pressures and death are based on bioenergetics. Hibernation is a unique survival strategy for many small mammals that is characterised by severe metabolic depression and transition from euthermia to hypothermia (torpor) at body temperatures close to 0°C. These manifestations of life were made possible by the remarkable "social" behavior of biomolecules during billions of years of evolution: the evolution of life with oxygen. Oxygen was necessary for energy production and the evolutionary explosion of aerobic organisms. Recent Advances: Nevertheless, reactive oxygen species, formed through oxidative metabolism, are dangerous-they can kill a cell and, on the other hand, play a plethora of fundamentally valuable roles. Therefore, the evolution of life depended on energy metabolism and redox-metabolic adaptations. The more extreme the conditions for survival are, the more sophisticated the adaptive responses of organisms become. Hibernation is a beautiful illustration of this principle. Hibernating animals use evolutionarily conserved molecular mechanisms to survive adverse environmental conditions, including reducing body temperature to ambient levels (often to ∼0°C) and severe metabolic depression. This long-built secret of life lies at the intersection of oxygen, metabolism, and bioenergetics, and hibernating organisms have learned to exploit all the underlying capacities of molecular pathways to survive. Critical Issues: Despite such drastic changes in phenotype, tissues and organs of hibernators sustain no metabolic or histological damage during hibernation or upon awakening from hibernation. This was made possible by the fascinating integration of redox-metabolic regulatory networks whose molecular mechanisms remain undisclosed to this day. Future Directions: Discovering these molecular mechanisms is not warranted only to understand hibernation in itself but to help explain complex medical conditions (hypoxia/reoxygenation, organ transplantation, diabetes, and cancer) and to even help overcome limitations associated with space travel. This is a review of integrated redox-metabolic orchestration in hibernation. Antioxid. Redox Signal. 40, 345-368.

Mitochondrial Targeting against Alzheimer's Disease: Lessons from Hibernation

Alzheimer's disease (AD) is the most common cause of dementia worldwide and yet remains without effective therapy. Amongst the many proposed causes of AD, the mitochondrial cascade hypothesis is gaining attention. Accumulating evidence shows that mitochondrial dysfunction is a driving force behind synaptic dysfunction and cognitive decline in AD patients. However, therapies targeting the mitochondria in AD have proven unsuccessful so far, and out-of-the-box options, such as hibernation-derived mitochondrial mechanisms, may provide valuable new insights. Hibernators uniquely and rapidly alternate between suppression and re-activation of the mitochondria while maintaining a sufficient energy supply and without acquiring ROS damage. Here, we briefly give an overview of mitochondrial dysfunction in AD, how it affects synaptic function, and why mitochondrial targeting in AD has remained unsuccessful so far. We then discuss mitochondria in hibernation and daily torpor in mice, covering current advancements in hibernation-derived mitochondrial targeting strategies. We conclude with new ideas on how hibernation-derived dual mitochondrial targeting of both the ATP and ROS pathways may boost mitochondrial health and induce local synaptic protein translation to increase synaptic function and plasticity. Further exploration of these mechanisms may provide more effective treatment options for AD in the future.

Transcriptional repression of beige fat innervation via a YAP/TAZ-S100B axis

Sympathetic innervation is essential for the development of functional beige fat that maintains body temperature and metabolic homeostasis, yet the molecular mechanisms controlling this innervation remain largely unknown. Here, we show that adipocyte YAP/TAZ inhibit sympathetic innervation of beige fat by transcriptional repression of neurotropic factor S100B. Adipocyte-specific loss of Yap/Taz induces S100b expression to stimulate sympathetic innervation and biogenesis of functional beige fat both in subcutaneous white adipose tissue (WAT) and browning-resistant visceral WAT. Mechanistically, YAP/TAZ compete with C/EBPβ for binding to the zinc finger-2 domain of PRDM16 to suppress S100b transcription, which is released by adrenergic-stimulated YAP/TAZ phosphorylation and inactivation. Importantly, Yap/Taz loss in adipocytes or AAV-S100B overexpression in visceral WAT restricts both age-associated and diet-induced obesity, and improves metabolic homeostasis by enhancing energy expenditure of mice. Together, our data reveal that YAP/TAZ act as a brake on the beige fat innervation by blocking PRDM16-C/EBPβ-mediated S100b expression.

Plasmodium falciparum OPA3-like protein (PfOPA3) is essential for maintenance of mitochondrial homeostasis and parasite proliferation

Metabolic pathways and proteins responsible for maintaining mitochondrial dynamics and homeostasis in the Plasmodium parasite, the causative agent of malaria, remain to be elucidated. Here, we identified and functionally characterized a novel OPA3-like domain-containing protein in P. falciparum (PfOPA3). We show that PfOPA3 is expressed in the intraerythrocytic stages of the parasite and localizes to the mitochondria. Inducible knock-down of PfOPA3 using GlmS ribozyme hindered the normal intraerythrocytic cycle of the parasites; specifically, PfOPA3-iKD disrupted parasite development as well as parasite division and segregation at schizont stages, which resulted in a drastic reduction in the number of merozoites progenies. Parasites lacking PfOPA3 show severe defects in the development of functional mitochondria; the mitochondria showed reduced activity of mtETC but not ATP synthesis, as evidenced by reduced activity of complex III of the mtETC, and increased sensitivity for drugs targeting DHODH as well as complex III, but not to the drugs targeting complex V. Further, PfOPA3 downregulation leads to reduction in the level of mitochondrial proton transport uncoupling protein (PfUCP) to compensate reduced activity of complex III and maintain proton efflux across the inner membrane. The reduced activity of DHODH, which is responsible for pyrimidine biosynthesis required for nuclear DNA synthesis, resulted in a significant reduction in parasite nuclear division and generation of progeny. In conclusion, we show that PfOPA3 is essential for the functioning of mtETC and homeostasis required for the development of functional mitochondria as well as for parasite segregation, and thus PfOPA3 is crucial for parasite survival during blood stages.

Uncoupling protein 1-driven Cre (Ucp1-Cre) is expressed in the epithelial cells of mammary glands and various non-adipose tissues

Objective

Uncoupling protein 1 (UCP1), a mitochondrial protein responsible for nonshivering thermogenesis in adipose tissue, serves as a distinct marker for thermogenic brown and beige adipocytes. Ucp1-Cre mice are thus widely used to genetically manipulate these thermogenic adipocytes. However, evidence suggests that UCP1 may also be expressed in non-adipocyte cell types. In this study, we investigated the presence of UCP1 expression in different mouse tissues that have not been previously reported.

Methods

We employed Ucp1-Cre mice crossed with Cre-inducible transgenic reporter Nuclear tagging and Translating Ribosome Affinity Purification (NuTRAP) mice, to investigate Ucp1-Cre expression in various tissues of adult female mice and developing embryos. Tamoxifen-inducible Ucp1-CreERT2 mice crossed with NuTRAP mice were used to assess active UCP1 expression. Immunostaining, RNA analysis, and single-cell/nucleus RNA-seq (sc/snRNA-seq) data analysis were performed to determine the expression of endogenous UCP1 and Ucp1-Cre-driven reporter expression. We also investigated the impact of UCP1 deficiency on mammary gland development and function using Ucp1-knockout (KO) mice.

Results

Ucp1-Cre expression was observed in the mammary glands within the inguinal white adipose tissue of female Ucp1-Cre; NuTRAP mice. However, endogenous Ucp1 was not actively expressed as Ucp1-CreERT2 failed to induce the reporter expression in the mammary glands. Ucp1-Cre was activated during embryonic development in various tissues, including mammary glands, as well as in the brain, kidneys, eyes, and ears, specifically in epithelial cells in these organs. While sc/snRNA-seq data suggest potential expression of UCP1 in mammary epithelial cells in adult mice and humans, Ucp1-KO female mice displayed normal mammary gland development and function.

Conclusions

Our findings reveal widespread Ucp1-Cre expression in various non-adipose tissue types, starting during early development. These results highlight the importance of exercising caution when interpreting data and devising experiments involving Ucp1-Cre mice.

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.

Towards understanding the neural origins of hibernation

Hibernators thrive under harsh environmental conditions instead of initiating canonical behavioral and physiological responses to promote survival. Although the physiological changes that occur during hibernation have been comprehensively researched, the role of the nervous system in this process remains relatively underexplored. In this Review, we adopt the perspective that the nervous system plays an active, essential role in facilitating and supporting hibernation. Accumulating evidence strongly suggests that the hypothalamus enters a quiescent state in which powerful drives to thermoregulate, eat and drink are suppressed. Similarly, cardiovascular and pulmonary reflexes originating in the brainstem are altered to permit the profoundly slow heart and breathing rates observed during torpor. The mechanisms underlying these changes to the hypothalamus and brainstem are not currently known, but several neuromodulatory systems have been implicated in the induction and maintenance of hibernation. The intersection of these findings with modern neuroscience approaches, such as optogenetics and in vivo calcium imaging, has opened several exciting avenues for hibernation research.

Conditional gene targeting using UCP1-Cre mice directly targets the central nervous system beyond thermogenic adipose tissues

Objective

Uncoupling protein 1 (UCP1) is a mitochondrial protein critical for adaptive thermogenesis in adipose tissues, and it is typically believed to be restricted to thermogenic adipose tissues. UCP1-Cre transgenic mice are utilized in numerous studies to provide “brown adipose-specific” conditional gene targeting. Here, we examined the distribution of Cre and UCP1 throughout the body in UCP1-Cre reporter mice.

Methods

UCP1-Cre mice crossed to Ai14-tdTomato and Ai9-tdTomato reporter mice were used to explore the tissue distribution of Cre recombinase and Ucp1 mRNA in various tissues. UCP1-Cre mice were independently infected with either a Cre-dependent PHP.eB-tdTomato virus or a Cre-dependent AAV-tdTomato virus to determine whether and where UCP1 is actively expressed in the adult central nervous system. In situ analysis of the deposited single cell RNA sequencing data was used to evaluate Ucp1 expression in the hypothalamus.

Results

As expected, Ucp1 expression was detected in both brown and inguinal adipose tissues. Ucp1 expression was also detected in the kidney, adrenal glands, thymus, and hypothalamus. Consistent with detectable Ucp1 expression, tdTomato expression was also observed in brown adipose tissue, inguinal white adipose tissue, kidney, adrenal glands, and hypothalamus of both male and female UCP1-Cre; Ai14-tdTomato and UCP1-Cre; Ai9-tdTomato mice by fluorescent imaging and qPCR. Critically, expression of tdTomato, and thus UCP1, within the central nervous system was observed in regions of the brain critical for the regulation of energy homeostasis, including the ventromedial hypothalamus (VMH).

Conclusions

TdTomato expression in UCP1-Cre; tdTomato mice is not restricted to thermogenic adipose tissues. TdTomato was also expressed in the kidneys, adrenal glands, and throughout the brain, including brain regions and cell types that are critical for multiple aspects of central regulation of energy homeostasis. Collectively, these data have important implications for the utility of UCP1-Cre mice as genetic tools to investigate gene function specifically in brown adipose tissue.

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UCP1-Cre transgenic mice express Cre in tissues beyond thermogenic adipose tissues.

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UCP1-Cre; tdTomato reporter mice show that Cre is also expressed throughout the brain, kidney, and adrenal glands.

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Ucp1 mRNA is actively expressed in the central nervous system of adult mice.

Avian red blood cell mitochondria produce more heat in winter than in autumn

Endotherms in cold regions improve heat-producing capacity when preparing for winter. We know comparatively little about how this change is fueled by seasonal adaptation in cellular respiration. Thus, we studied the changes of mitochondrial function in red blood cells in sympatric Coal (Periparus ater), Blue (Cyanistes caeruleus), and Great (Parus major) tits between autumn and winter. These species differ more than twofold in body mass and in several aspects of their foraging ecology and social dominance, which could require differential seasonal adaptation of energy expenditure. Coal and Great tits in particular upregulated the mitochondrial respiration rate and mitochondrial volume in winter. This was not directed toward ATP synthesis, instead reflecting increased uncoupling of electron transport from ATP production. Because uncoupling is exothermic, this increased heat-producing capacity at the sub-cellular level in winter. This previously unexplored the route of thermogenesis in birds should be addressed in future work.

A bioactive fraction of Pterocarpus santalinus inhibits adipogenesis and inflammation in 3T3-L1 cells via modulation of PPAR-γ/SREBP-1c and TNF-α/IL-6

Pterocarpus santalinus has huge demand owing to its commercial and medicinal value. However, there are limited research studies on its therapeutic activity against obesity and obesity-induced inflammation and underlying mechanism of action. Therefore, in the present study, chloroform bioactive fraction of P. santalinus (CFP) was isolated and evaluated for its activity against adipogenesis and adipogenesis-induced inflammation in 3T3-L1 cell culture model. LC-MS/MS analysis of CFP was performed to identify the compounds present. CFP-treated 3T3-L1 cells (50, 100 and 200 μg/ml) have significantly (p < 0.01 or < 0.05) enhanced glycerol release and adiponectin level, but reduced lipid accumulation and leptin, and MTT assay demonstrated CFP was non-toxic till a dose of 300 µg/ml at 24 and 48 h. A considerable reduction in tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) levels was witnessed in lipopolysaccharide (LPS)-induced 3T3-L1 cells with CFP treatment in dose-dependent manner. Gene expression studies demonstrated down-regulation of mRNA expression of peroxisome proliferator-activated receptor-γ (PPAR-γ), sterol regulatory element-binding protein-1c (SREBP-1c), leptin, TNF-α and IL-6 but up-regulation of adiponectin and uncoupling protein-1 (UCP-1) and the same trend was observed in protein expression also. In conclusion, it is suggested that CFP could be beneficial to treat obesity and associated inflammation.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

Cellular, Molecular, and Physiological Adaptations of Hibernation: The Solution to Environmental Challenges

Thriving in times of resource scarcity requires an incredible flexibility of behavioral, physiological, cellular, and molecular functions that must change within a relatively short time. Hibernation is a collection of physiological strategies that allows animals to inhabit inhospitable environments, where they experience extreme thermal challenges and scarcity of food and water. Many different kinds of animals employ hibernation, and there is a spectrum of hibernation phenotypes. Here, we focus on obligatory mammalian hibernators to identify the unique challenges they face and the adaptations that allow hibernators to overcome them. This includes the cellular and molecular strategies used to combat low environmental and body temperatures and lack of food and water. We discuss metabolic, neuronal, and hormonal cues that regulate hibernation and how they are thought to be coordinated by internal clocks. Last, we touch on questions that are left to be addressed in the field of hibernation research. Studies from the last century and more recent work reveal that hibernation is not simply a passive reduction in body temperature and vital parameters but rather an active process seasonally regulated at the molecular, cellular, and organismal levels.

Protein Deimination Signatures in Plasma and Plasma-EVs and Protein Deimination in the Brain Vasculature in a Rat Model of Pre-Motor Parkinson's Disease

The identification of biomarkers for early diagnosis of Parkinson’s disease (PD) is of pivotal importance for improving approaches for clinical intervention. The use of translatable animal models of pre-motor PD therefore offers optimal opportunities for novel biomarker discovery in vivo. Peptidylarginine deiminases (PADs) are a family of calcium-activated enzymes that contribute to protein misfolding through post-translational deimination of arginine to citrulline. Furthermore, PADs are an active regulator of extracellular vesicle (EV) release. Both protein deimination and extracellular vesicles (EVs) are gaining increased attention in relation to neurodegenerative diseases, including in PD, while roles in pre-motor PD have yet to be investigated. The current study aimed at identifying protein candidates of deimination in plasma and plasma-EVs in a rat model of pre-motor PD, to assess putative contributions of such post-translational changes in the early stages of disease. EV-cargo was further assessed for deiminated proteins as well as three key micro-RNAs known to contribute to inflammation and hypoxia (miR21, miR155, and miR210) and also associated with PD. Overall, there was a significant increase in circulating plasma EVs in the PD model compared with sham animals and inflammatory and hypoxia related microRNAs were significantly increased in plasma-EVs of the pre-motor PD model. A significantly higher number of protein candidates were deiminated in the pre-motor PD model plasma and plasma-EVs, compared with those in the sham animals. KEGG (Kyoto encyclopedia of genes and genomes) pathways identified for deiminated proteins in the pre-motor PD model were linked to “Alzheimer’s disease”, “PD”, “Huntington’s disease”, “prion diseases”, as well as for “oxidative phosphorylation”, “thermogenesis”, “metabolic pathways”, “Staphylococcus aureus infection”, gap junction, “platelet activation”, “apelin signalling”, “retrograde endocannabinoid signalling”, “systemic lupus erythematosus”, and “non-alcoholic fatty liver disease”. Furthermore, PD brains showed significantly increased staining for total deiminated proteins in the brain vasculature in cortex and hippocampus, as well as increased immunodetection of deiminated histone H3 in dentate gyrus and cortex. Our findings identify EVs and post-translational protein deimination as novel biomarkers in early pre-motor stages of PD.

Bone marrow adipose tissue does not express UCP1 during development or adrenergic-induced remodeling

Adipocytes within the skeleton are collectively termed bone marrow adipose tissue (BMAT). BMAT contributes to peripheral and local metabolism, however, its capacity for cell-autonomous expression of uncoupling protein 1 (UCP1), a biomarker of beige and brown adipogenesis, remains unclear. To overcome this, Ucp1-Cre was used to drive diphtheria toxin expression in cells expressing UCP1 (Ucp1Cre+/DTA+). Despite loss of brown adipose tissue, BMAT volume was not reduced in Ucp1Cre+/DTA+ mice. Comparably, in mTmG reporter mice (Ucp1Cre+/mTmG+), Ucp1-Cre expression was absent from BMAT in young (3-weeks) and mature (16-weeks) male and female mice. Further, β3-agonist stimulation failed to induce Ucp1-Cre expression in BMAT. This demonstrates that BMAT adipocytes are not UCP1-expressing beige/brown adipocytes. Thus, to identify novel and emerging roles for BMAT adipocytes in skeletal and whole-body homeostasis, we performed gene enrichment analysis of microarray data from adipose tissues of adult rabbits. Pathway analysis revealed genetic evidence for differences in BMAT including insulin resistance, decreased fatty acid metabolism, and enhanced contributions to local processes including bone mineral density through candidate genes such as osteopontin. In sum, this supports a paradigm by which BMAT adipocytes are a unique subpopulation that is specialized to support cells within the skeletal and hematopoietic niche.

Conditional knockout of kisspeptin signaling in brown adipose tissue increases metabolic rate and body temperature and lowers body weight

The peptide kisspeptin and its receptor, Kiss1r, act centrally to stimulate reproduction. Evidence indicates that kisspeptin signaling is also important for body weight (BW) and metabolism. We recently reported that Kiss1r KO mice develop obesity, along with reduced metabolism and energy expenditure, independent of estradiol levels. Outside the brain, Kiss1r is expressed in several metabolic tissues, including brown adipose tissue (BAT), but it is unknown which specific tissue is responsible for the metabolic phenotype in Kiss1r KOs. We first determined that global Kiss1r KO mice have significant alterations in body temperature and BAT thermogenic gene expression, perhaps contributing to their obesity. Next, to test whether kisspeptin signaling specifically in BAT influences BW, metabolism, or body temperature, we used Cre/lox technology to generate conditional Kiss1r knockout exclusively in BAT (BAT-Kiss1r KO). Unlike global Kiss1r KOs, BAT-Kiss1r KOs (lacking Kiss1r in just BAT) were not hypogonadal, as expected. Surprisingly, however, BAT-Kiss1r KOs of both sexes displayed significantly lower BW and adiposity than controls. This novel BAT-Kiss1r KO phenotype was of greater magnitude in females and was associated with improved glucose tolerance, increased metabolism, energy expenditure, and locomotor activity, along with increased body temperature and BAT gene expression, specifically Cox8b. Our findings suggest that the previously observed obesity and decreased metabolism in global Kiss1r KOs reflect impaired kisspeptin signaling in non-BAT tissues. However, the novel finding of increased metabolism and body temperature and lower BW in BAT-Kiss1r KOs reveal a previously unidentified role for endogenous kisspeptin signaling in BAT in modulating metabolic and thermogenic physiology.

Identification of a lipid-rich depot in the orbital cavity of the thirteen-lined ground squirrel

We discovered a previously undescribed orbital lipid depot in the thirteen-lined ground squirrel during the first ever magnetic resonance image (MRI) of this common experimental model of mammalian hibernation. In animals housed at constant ambient temperatures (5°C or 25°C, 12 h:12 h light:dark photoperiod), the volume of this depot increased in the autumn and decreased in the spring, suggesting an endogenous circannual pattern. Water-fat MRI revealed that throughout the year this depot is composed of ∼40% lipid, similar to brown adipose tissue (BAT). During arousal from torpor, thermal images showed higher surface temperatures near this depot before the rest of the head warmed, suggesting a thermoregulatory function. This depot, however, does not contain uncoupling protein 1, a BAT biomarker, or uncoupling protein 3. Histology shows blood vessels in close proximity to each other, suggesting it may serve as a vascular rete, perhaps to preferentially warm the eye and brain during arousals.

A dual Ucp1 reporter mouse model for imaging and quantitation of brown and brite fat recruitment

OBJECTIVES

Brown adipose tissue (BAT) dissipates nutritional energy as heat through uncoupling protein 1 (UCP1). The discovery of functional BAT in healthy adult humans has promoted the search for pharmacological interventions to recruit and activate brown fat as a treatment of obesity and diabetes type II. These efforts require in vivo models to compare the efficacy of novel compounds in a relevant physiological context.

METHODS

We generated a knock-in mouse line expressing firefly luciferase and near-infrared red florescent protein (iRFP713) driven by the regulatory elements of the endogenous Ucp1 gene.

RESULTS

Our detailed characterization revealed that firefly luciferase activity faithfully reports endogenous Ucp1 gene expression in response to physiological and pharmacological stimuli. The iRFP713 fluorescence signal was detected in the interscapular BAT region of cold-exposed reporter mice in an allele-dosage dependent manner. Using this reporter mouse model, we detected a higher browning capacity in female peri-ovarian white adipose tissue compared to male epididymal WAT, which we further corroborated by molecular and morphological features. In situ imaging detected a strong luciferase activity signal in a previously unappreciated adipose tissue depot adjunct to the femoral muscle, now adopted as femoral brown adipose tissue. In addition, screening cultured adipocytes by bioluminescence imaging identified the selective Salt-Inducible Kinase inhibitor, HG-9-91-01, to increase Ucp1 gene expression and mitochondrial respiration in brown and brite adipocytes.

CONCLUSIONS

In our mouse model, firefly luciferase activity serves as a bona fide reporter for dynamic regulation of Ucp1. In addition, by means of iRFP713 we are able to monitor Ucp1 expression in a non-invasive fashion.

Molecular evolution of uncoupling proteins and implications for brain function

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier superfamily and catalyze important metabolic functions at the mitochondrial inner membrane. While the thermogenic role of UCP1 in brown fat of eutherian mammals is well established, the molecular functions of UCP1 in ectothermic vertebrates and of other UCP paralogs remain less clear. Here, we critically discuss the existence of brain UCPs and their potential roles. Applying phylogenetic classification of novel UCPs, we summarize the evidence for brain UCP1 among vertebrates, the role of UCP2 in specific brain areas, and the existence of brain-specific UCPs. The phylogenetic analyses and discussion on functional data should alert the scientific community that the molecular function of so-called UCP1 homologues is by far not clarified and possibly relates to neither thermogenesis nor mitochondrial uncoupling.

Hypoxic and Cold Adaptation Insights from the Himalayan Marmot Genome

The Himalayan marmot (Marmota himalayana) is a hibernating mammal that inhabits the high-elevation regions of the Himalayan mountains. Here we present a draft genome of the Himalayan marmot, with a total assembly length of 2.47 Gb. Phylogenetic analyses showed that the Himalayan marmot diverged from the Mongolian marmot approximately 1.98 million years ago. Transcriptional changes during hibernation included genes responsible for fatty acid metabolism in liver and genes involved in complement and coagulation cascades and stem cell pluripotency pathways in brain. Two selective sweep genes, Slc25a14 and ψAamp, showed apparent genotyping differences between low- and high-altitude populations. As a processed pseudogene, ψAamp may be biologically active to influence the stability of Aamp through competitive microRNA binding. These findings shed light on the molecular and genetic basis underlying adaptation to extreme environments in the Himalayan marmot.

Somatosensory Neurons Enter a State of Altered Excitability during Hibernation

Hibernation in mammals involves prolonged periods of inactivity, hypothermia, hypometabolism, and decreased somatosensation. Peripheral somatosensory neurons play an essential role in the detection and transmission of sensory information to CNS and in the generation of adaptive responses. During hibernation, when body temperature drops to as low as 2°C, animals dramatically reduce their sensitivity to physical cues [1, 2]. It is well established that, in non-hibernators, cold exposure suppresses energy production, leading to dissipation of the ionic and electrical gradients across the plasma membrane and, in the case of neurons, inhibiting the generation of action potentials [3]. Conceivably, such cold-induced elimination of electrogenesis could be part of a general mechanism that inhibits sensory abilities in hibernators. However, when hibernators become active, the bodily functions-including the ability to sense environmental cues-return to normal within hours, suggesting the existence of mechanisms supporting basal functionality of cells during torpor and rapid restoration of activity upon arousal. We tested this by comparing properties of somatosensory neurons from active and torpid thirteen-lined ground squirrels (Ictidomys tridecemlineatus). We found that torpid neurons can compensate for cold-induced functional deficits, resulting in unaltered resting potential, input resistance, and rheobase. Torpid neurons can generate action potentials but manifest markedly altered firing patterns, partially due to decreased activity of voltage-gated sodium channels. Our results provide insights into the mechanism that preserves somatosensory neurons in a semi-active state, enabling fast restoration of sensory function upon arousal. These findings contribute to the development of strategies enabling therapeutic hypothermia and hypometabolism.

iPSCs from a Hibernator Provide a Platform for Studying Cold Adaptation and Its Potential Medical Applications

Hibernating mammals survive hypothermia (<10°C) without injury, a remarkable feat of cellular preservation that bears significance for potential medical applications. However, mechanisms imparting cold resistance, such as cytoskeleton stability, remain elusive. Using the first iPSC line from a hibernating mammal (13-lined ground squirrel), we uncovered cellular pathways critical for cold tolerance. Comparison between human and ground squirrel iPSC-derived neurons revealed differential mitochondrial and protein quality control responses to cold. In human iPSC-neurons, cold triggered mitochondrial stress, resulting in reactive oxygen species overproduction and lysosomal membrane permeabilization, contributing to microtubule destruction. Manipulations of these pathways endowed microtubule cold stability upon human iPSC-neurons and rat (a non-hibernator) retina, preserving its light responsiveness after prolonged cold exposure. Furthermore, these treatments significantly improved microtubule integrity in cold-stored kidneys, demonstrating the potential for prolonging shelf-life of organ transplants. Thus, ground squirrel iPSCs offer a unique platform for bringing cold-adaptive strategies from hibernators to humans in clinical applications. VIDEO ABSTRACT.

Insights into the neurochemical signature of the Innervation of Beige Fat

Objective

The potential for brown adipose tissue (BAT) to be targeted as a therapeutic option to combat obesity has been heightened by the discovery of a brown–like form of inducible “beige” adipose tissue in white fat which has overlapping structural and functional properties to “classical” BAT. The likelihood that both beige and brown fat are recruited functionally by neural mechanisms, taken together with the lack of a detailed understanding of the nature of changes in the nervous system when white adipose tissue (WAT) is transformed to brown, provides the impetus for this study. Here, we aim to identify whether there is a shift in the gene expression profile in neurons directly innervating inguinal white adipose tissue (iWAT) that has undergone “beiging” to a signature that is more similar to neurons projecting to BAT.

Methods

Two groups of rats, one housed at thermoneutrality (27 °C) and the other exposed to cold (8 °C) for 7 days, were killed, and their T13/L1 ganglia, stellate ganglion (T1/T2), or superior cervical ganglion (SCG, C2/3) removed. This approach yielded ganglia containing neurons that innervate either beiged white fat (8 °C for 7 days), inguinal WAT (27 °C for 7 days), BAT (both 27 °C and 8 °C for 7 days) or non-WAT (8 °C for 7 days), the latter included to isolate changes in gene expression that were more aligned with a response to cold exposure than the transformation of white to beige adipocytes. Bioinformatics analyses of RNA sequencing data was performed followed by Ingenuity Pathway Analysis (IPA) to determine differential gene expression and recruitment of biosynthetic pathways.

Results

When iWAT is “beiged” there is a significant shift in the gene expression profile of neurons in sympathetic ganglia (T13/L1) innervating this depot toward a gene neurochemical signature that is similar to the stellate ganglion projecting to BAT. Bioinformatics analyses of “beiging” related genes revealed upregulation of genes encoding neuropeptides proopiomelanocortin (POMC) and calcitonin-gene related peptide (CGRP) within ganglionic neurons. Treatment of differentiated 3T3L1 adipocytes with αMSH, one of the products cleaved from POMC, results in an elevation in lipolysis and the beiging of these cells as indicated by changes in gene expression markers of browning (Ucp1 and Ppargc1a).

Conclusion

These data indicate that, coincident with beiging, there is a shift toward a “brown-like” neurochemical signature of postganglionic neurons projecting to inguinal white fat, an increased expression of POMC, and, consistent with a causative role for this prohormone in beiging, an αMSH-mediated increase in beige gene markers in isolated adipocytes.

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RNA Seq showed shifts in neuronal gene expression following browning of white fat.

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Gene expression in ganglia projecting to white fat became brown-like with beiging.

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Bioinformatics analyses revealed neuronal gene candidates associated with beiging.

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Prominent gene candidates associated with beiging included POMC and CGRP.

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POMC cleavage product α-MSH caused beiging of cultured fat cells.

Evolution of UCP1

Brown adipose tissue (BAT), the specialized heat-producing organ found in many placental mammals including humans, may be accessible for clinical drug intervention to help combat metabolic diseases. Understanding the biology of BAT and its thermogenic uncoupling protein 1 (UCP1) will benefit from an assessment of its evolution, answering where UCP1 originated and how it has been modified and integrated into cellular energy metabolism. Here, we review topical insights regarding the molecular evolution of UCP1-also reconstructing the proximate and ultimate factors selecting for brown fat thermogenesis in placental mammals. This new thinking on "old" events will assist our understanding of how thermogenic mitochondrial uncoupling was integrated into the physiology of the brown adipocyte. Recent comparative studies examining the occurrence of UCP1 in vertebrates not only identified the ancient (pre-mammal) rise of UCP1 but also its repeated downfall during mammalian evolution as evidenced by multiple independent gene loss and/or inactivation events. Together with the comparative physiology of various species, we may be able to find conditions that favor UCP1 thermogenesis and, learning from these insights, identify molecular networks that will be useful to pharmacologically stimulate the tissue.

Water-fat MRI in a hibernator reveals seasonal growth of white and brown adipose tissue without cold exposure

Obligate hibernators, such as ground squirrels, display circannual patterns which persist even under constant laboratory conditions, suggesting that they are regulated by endogenous rhythms. Brown adipose tissue (BAT) is important for thermogenesis during periodic arousals from hibernation when core body temperature rises spontaneously from 5 to 37 °C. In most small eutherians BAT growth requires several weeks of cold exposure. We hypothesized that in the thirteen-lined ground squirrel (Ictidomys tridecemlineatus), a hibernator, BAT growth is regulated, in part, by an endogenous rhythm and we predicted that this growth would precede the hibernation season without cold exposure. We tested this prediction using repeated water-fat magnetic resonance imaging over a year, including the hibernation season. Thoracic BAT depots increased in volume from spring through autumn even though animals were housed at ~22 °C. Subsequent cold exposure (5 °C) enlarged the thoracic BAT further. The fat fraction of this tissue fell significantly during the period of peak growth, indicating relative increases in non-triglyceride components, perhaps mitochondria or vasculature. We also found that the proportion of the body consisting of white adipose tissue (WAT) increased steadily from spring through autumn, and fell throughout hibernation, mirroring changes in body mass. Unlike BAT, WAT fat fractions remained constant (near 90%) throughout the year. Future studies will evaluate the significance of photoperiod and cold exposure on the growth of these tissues. We also found tissue with a fat fraction characteristic of BAT in the head near the eyes, a potentially novel discovery that requires further confirmation.

Enhanced oxidative capacity of ground squirrel brain mitochondria during hibernation

During hibernation, thirteen-lined ground squirrels (Ictidomys tridecemlineatus) regularly cycle between bouts of torpor and interbout arousal (IBA). Most of the brain is electrically quiescent during torpor but regains activity quickly upon arousal to IBA, resulting in extreme oscillations in energy demand during hibernation. We predicted increased functional capacity of brain mitochondria during hibernation compared with spring to accommodate the variable energy demands of hibernation. To address this hypothesis, we examined mitochondrial bioenergetics in the ground squirrel brain across three time points: spring (SP), torpor (TOR), and IBA. Respiration rates of isolated brain mitochondria through complex I of the electron transport chain were more than twofold higher in TOR and IBA than in SP (P < 0.05). We also found a 10% increase in membrane potential between hibernation and spring (P < 0.05), and that proton leak was lower in TOR and IBA than in SP. Finally, there was a 30% increase in calcium loading in SP brain mitochondria compared with TOR and IBA (P < 0.01). To analyze brain mitochondrial abundance between spring and hibernation, we measured the ratio of copy number in a mitochondrial gene (ND1) vs. a nuclear gene (B2M) in frozen cerebral cortex samples. No significant differences were observed in DNA copies between SP and IBA. These data show that brain mitochondrial bioenergetics are not static across the year and suggest that brain mitochondria function more effectively during the hibernation season, allowing for rapid production of energy to meet demand when extreme physiological changes are occurring.

Increased Expression of Beige/Brown Adipose Markers from Host and Breast Cancer Cells Influence Xenograft Formation in Mice

The initiation and progression of breast cancer is a complex process that is influenced by heterogeneous cell populations within the tumor microenvironment. Although adipocytes have been shown to promote breast cancer development, adipocyte characteristics involved in this process remain poorly understood. In this study, we demonstrate enrichment of beige/brown adipose markers, contributed from the host as well as tumor cells, in the xenografts from breast cancer cell lines. In addition to uncoupling protein-1 (UCP1) that is exclusively expressed in beige/brown adipocytes, gene expression for classical brown (MYF5, EVA1, and OPLAH) as well as beige (CD137/TNFRSF9 and TBX1) adipocyte markers was also elevated in the xenografts. Enrichment of beige/brown characteristics in the xenografts was independent of the site of implantation of the breast tumor cells. Early stages of xenografts showed an expansion of a subset of mammary cancer stem cells that expressed PRDM16, a master regulator of brown adipocyte differentiation. Depletion of UCP1(+) or Myf5(+) cells significantly reduced tumor development. There was increased COX2 (MT-CO2) expression, which is known to stimulate formation of beige adipocytes in early xenografts and treatment with a COX2 inhibitor (SC236) reduced tumor growth. In contrast, treatment with factors that induce brown adipocyte differentiation in vitro led to larger tumors in vivo. A panel of xenografts derived from established breast tumor cells as well as patient tumor tissues were generated that expressed key brown adipose tissue-related markers and contained cells that morphologically resembled brown adipocytes.

IMPLICATIONS

This is the first report demonstrating that beige/brown adipocyte characteristics could play an important role in breast tumor development and suggest a potential target for therapeutic drug design.

Evolutionary adaptation to thermosensation

Organisms continuously evolve to adapt to changing environmental conditions. Chief among these are daily and seasonal temperature fluctuations. Relatively small in terms of real physical values, temperature fluctuations of just a few degrees can profoundly affect organismal functions. In vertebrates, temperature is detected by primary afferents of somatosensory neurons, which express thermo-gated ion channels. Most of our knowledge about temperature receptors comes from seminal studies in mice and rats. Recent work uncovered thermosensory mechanisms in other vertebrates, shedding light onto the diversity of thermosensory adaptations. Here, we summarize molecular mechanisms of thermosensation in different species and discuss the need to use the standard laboratory rodents and non-standard species side-by-side in order to understand fundamental principles of somatosensation.

Elena Gracheva: Ion channels run hot and cold

Gracheva studies the molecular basis and evolutionary origins of hibernation.