Importance of adipocyte browning in the evolution of endothermy

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.

Mother's warmth from maternal genes: genomic imprinting of brown adipose tissue

Background and objectives

Brown adipose tissue (BAT) plays key roles in mammalian physiology, most notably with regard to thermoregulation in infants and juveniles. Previous studies have suggested that intragenomic conflict, in the form of genomic imprinting, mediates BAT thermogenesis, because it represents a public good for groups of siblings, or a mother with her offspring, who huddle together to conserve warmth. By this hypothesis, maternally expressed imprinted genes should promote BAT, while paternally expressed genes should repress it.

Methodology

We systematically searched the literature using two curated lists of genes imprinted in humans and/or mice, in association with evidence regarding effects of perturbation to imprinted gene expression on BAT development or activity.

Results

Overall, enhanced BAT was associated with relatively higher expression of maternally expressed imprinted genes, and relatively lower expression of paternally expressed imprinted genes; this pattern was found for 16 of the 19 genes with sufficient information for robust ascertainment (Binomial test, P < 0.005, 2-tailed).

Conclusions and implications

These results support the kinship theory of imprinting and indicate that future studies of BAT, and its roles in human health and disease, may usefully focus on effects of imprinted genes and associated genomic conflicts.

New tools suggest a middle Jurassic origin for mammalian endothermy: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time

We suggest that mammalian endothermy was established amongst Middle Jurassic crown mammals, through reviewing state-of-the-art fossil and living mammal studies. This is considerably later than the prevailing paradigm, and has important ramifications for the causes, pattern, and pace of physiological evolution amongst synapsids. Most hypotheses argue that selection for either enhanced aerobic activity, or thermoregulation was the primary driver for synapsid physiological evolution, based on a range of fossil characters that have been linked to endothermy. We argue that, rather than either alternative being the primary selective force for the entirety of endothermic evolution, these characters evolved quite independently through time, and across the mammal family tree, principally as a response to shifting environmental pressures and ecological opportunities. Our interpretations can be tested using closely linked proxies for both factors, derived from study of fossils of a range of Jurassic and Cretaceous mammaliaforms and early mammals.

Fluorescent Genetic Tools for Studying Brown Fat Development and Function in Mice

Techniques to trace and isolate brown adipocyte precursor and adipocytes during development and disease are essential to fully understand brown adipose tissue development and function. Here we report several protocols using the R26R-mTmG reporter mice in thermogenic tissues based on confocal microscopy and fluorescence based flow cytometry. These techniques may be useful to understand the influence of genetic or environmental alterations in brown adipocyte precursors and adipocyte biology.

Individual (Co)variation in Resting and Maximal Metabolic Rates in Wild Mice

AbstractBasal metabolic rate (BMR) represents the lowest level of aerobic metabolism in a resting, postabsorptive endotherm as measured within the thermoneutral zone. By contrast, maximal metabolic rate ([Formula: see text]max) reflects the upper limit of aerobic metabolism achieved during intensive exercise. As BMR and [Formula: see text]max define the boundaries of the possible levels of aerobic metabolism expressed by a normothermic individual, a key question is whether BMR and [Formula: see text]max are correlated. In the present study, we took repeated paired measurements of thermoneutral resting metabolic rate (RMR_t) and [Formula: see text]max on 165 white-footed mice (Peromyscus leucopus). Over a single summer (May-October), repeatability (R ± SE) was low but statistically significant ([Formula: see text]) for both RMR_t and [Formula: see text]max ([Formula: see text] for RMR_t; [Formula: see text] for [Formula: see text]max). Willingness to run during the forced-exercise trials was also significantly repeatable ([Formula: see text]). At the residual level (within individual), RMR_t and [Formula: see text]max tended to be positively correlated ([Formula: see text], [Formula: see text]), suggesting the presence of correlated phenotypic plasticity. By contrast, RMR_t and [Formula: see text]max were significantly negatively correlated at the among-individual level ([Formula: see text]). To the extent that variation in RMR_t reflects variation in BMR, the negative among-individual correlation does not corroborate the idea that a costly metabolic machinery is needed to support a high [Formula: see text]max. Future research should investigate the (genetic) relationship between RMR_t (and BMR) and other energetically expensive behaviors and activities to better understand how energy is allocated within individuals.

HOXA5 Participates in Brown Adipose Tissue and Epaxial Skeletal Muscle Patterning and in Brown Adipocyte Differentiation

Brown adipose tissue (BAT) plays critical thermogenic, metabolic and endocrine roles in mammals, and aberrant BAT function is associated with metabolic disorders including obesity and diabetes. The major BAT depots are clustered at the neck and forelimb levels, and arise largely within the dermomyotome of somites, from a common progenitor with skeletal muscle. However, many aspects of BAT embryonic development are not well understood. Hoxa5 patterns other tissues at the cervical and brachial levels, including skeletal, neural and respiratory structures. Here, we show that Hoxa5 also positively regulates BAT development, while negatively regulating formation of epaxial skeletal muscle. HOXA5 protein is expressed in embryonic preadipocytes and adipocytes as early as embryonic day 12.5. Hoxa5 null mutant embryos and rare, surviving adults show subtly reduced iBAT and sBAT formation, as well as aberrant marker expression, lower adipocyte density and altered lipid droplet morphology. Conversely, the epaxial muscles that arise from a common dermomyotome progenitor are expanded in Hoxa5 mutants. Conditional deletion of Hoxa5 with Myf5/Cre can reproduce both BAT and epaxial muscle phenotypes, indicating that HOXA5 is necessary within Myf5-positive cells for proper BAT and epaxial muscle development. However, recombinase-based lineage tracing shows that Hoxa5 does not act cell-autonomously to repress skeletal muscle fate. Interestingly, Hoxa5-dependent regulation of adipose-associated transcripts is conserved in lung and diaphragm, suggesting a shared molecular role for Hoxa5 in multiple tissues. Together, these findings establish a role for Hoxa5 in embryonic BAT development.

Is Endothermy an Evolutionary By-Product?

Endothermy alters the energetic relationships between organisms and their environment and thereby influences fundamental niches. Endothermy is closely tied to energy metabolism. Regulation of energy balance is indispensable for all life and regulatory pathways increase in complexity from bacteria to vertebrates. Increasing complexity of metabolic networks also increase the probability for endothermic phenotypes to appear. Adaptive arguments are problematic epistemologically because the regulatory mechanisms enabling endothermy have not evolved for the 'purpose' of endothermy and the utility of current traits is likely to have changed over evolutionary time. It is most parsimonious to view endothermy as the evolutionary by-product of energy balance regulation rather than as an adaptation and interpret its evolution in the context of metabolic networks.

The evolution of mechanisms involved in vertebrate endothermy

Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century. The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints. Many groups of vertebrates, however, are thought to possess other mechanisms of heat production, and alternative ways to regulate thermogenesis that are not always considered in the palaeontological literature. Here, we perform a review of the mechanisms involved in heat production, with a focus on cellular and molecular mechanisms, in a phylogenetic context encompassing the entire vertebrate diversity. We show that endothermy in mammals and birds is not as well defined as commonly assumed by evolutionary biologists and consists of a vast array of physiological strategies, many of which are currently unknown. We also describe strategies found in other vertebrates, which may not always be considered endothermy, but nonetheless correspond to a process of active thermogenesis. We conclude that endothermy is a highly plastic character in vertebrates and provides a guideline on terminology and occurrences of the different types of heat production in vertebrate evolution. This article is part of the theme issue 'Vertebrate palaeophysiology'.

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'.

Vertebrate palaeophysiology

Physiology is a functional branch of the biological sciences, searching for general rules by which explanatory hypotheses are tested using experimental procedures, whereas palaeontology is a historical science dealing with the study of unique events where conclusions are drawn from congruence among independent lines of evidence. Vertebrate palaeophysiology bridges these disciplines by using experimental data obtained from extant organisms to infer physiological traits of extinct ones and to reconstruct how they evolved. The goal of this theme issue is to understand functional innovations imprinted on modern vertebrate clades, and how to infer (or 'retrodict') physiological capacities in their ancient relatives a posteriori. As such, the present collection of papers deals with different aspects of a rapidly growing field to understand innovations in: phospho-calcic metabolism, acid-base homeostasis, thermometabolism, respiratory physiology, skeletal growth, palaeopathophysiology, genome size and metabolic rate, and it concludes with a historical perspective. Sometimes, the two components (physiological mechanism and palaeobiological inference) are proposed in separate papers. Other times, the two components are integrated in a single paper. In all cases, the approach was comparative, framed in a phylogenetic context, and included rigorous statistical methods that account for evolutionary patterns and processes. This article is part of the theme issue 'Vertebrate palaeophysiology'.