Gene-by-Sex Interactions in Mitochondrial Functions and Cardio-Metabolic Traits

The mitochondrial multi-omic response to exercise training across rat tissues

Mitochondria have diverse functions critical to whole-body metabolic homeostasis. Endurance training alters mitochondrial activity, but systematic characterization of these adaptations is lacking. Here, the Molecular Transducers of Physical Activity Consortium mapped the temporal, multi-omic changes in mitochondrial analytes across 19 tissues in male and female rats trained for 1, 2, 4, or 8 weeks. Training elicited substantial changes in the adrenal gland, brown adipose, colon, heart, and skeletal muscle. The colon showed non-linear response dynamics, whereas mitochondrial pathways were downregulated in brown adipose and adrenal tissues. Protein acetylation increased in the liver, with a shift in lipid metabolism, whereas oxidative proteins increased in striated muscles. Exercise-upregulated networks were downregulated in human diabetes and cirrhosis. Knockdown of the central network protein 17-beta-hydroxysteroid dehydrogenase 10 (HSD17B10) elevated oxygen consumption, indicative of metabolic stress. We provide a multi-omic, multi-tissue, temporal atlas of the mitochondrial response to exercise training and identify candidates linked to mitochondrial dysfunction.

Differential impact of sex on regulation of skeletal muscle mitochondrial function and protein homeostasis by hypoxia-inducible factor-1α in normoxia

Hypoxia-inducible factor (HIF)-1α is continuously synthesized and degraded in normoxia. During hypoxia, HIF1α stabilization restricts cellular/mitochondrial oxygen utilization. Cellular stressors can stabilize HIF1α even during normoxia. However, less is known about HIF1α function(s) and sex-specific effects during normoxia in the basal state. Since skeletal muscle is the largest protein store in mammals and protein homeostasis has high energy demands, we determined HIF1α function at baseline during normoxia in skeletal muscle. Untargeted multiomics data analyses were followed by experimental validation in differentiated murine myotubes with loss/gain of function and skeletal muscle from mice without/with post-natal muscle-specific Hif1a deletion (Hif1amsd). Mitochondrial oxygen consumption studies using substrate, uncoupler, inhibitor, titration protocols; targeted metabolite quantification by gas chromatography-mass spectrometry; and post-mitotic senescence markers using biochemical assays were performed. Multiomics analyses showed enrichment in mitochondrial and cell cycle regulatory pathways in Hif1a deleted cells/tissue. Experimentally, mitochondrial oxidative functions and ATP content were higher with less mitochondrial free radical generation with Hif1a deletion. Deletion of Hif1a also resulted in higher concentrations of TCA cycle intermediates and HIF2α proteins in myotubes. Overall responses to Hif1amsd were similar in male and female mice, but changes in complex II function, maximum respiration, Sirt3 and HIF1β protein expression and muscle fibre diameter were sex-dependent. Adaptive responses to hypoxia are mediated by stabilization of constantly synthesized HIF1α. Despite rapid degradation, the presence of HIF1α during normoxia contributes to lower mitochondrial oxidative efficiency and greater post-mitotic senescence in skeletal muscle. In vivo responses to HIF1α in skeletal muscle were differentially impacted by sex. KEY POINTS: Hypoxia-inducible factor -1α (HIF1α), a critical transcription factor, undergoes continuous synthesis and proteolysis, enabling rapid adaptive responses to hypoxia by reducing mitochondrial oxygen consumption. In mammals, skeletal muscle is the largest protein store which is determined by a balance between protein synthesis and breakdown and is sensitive to mitochondrial oxidative function. To investigate the functional consequences of transient HIF1α expression during normoxia in the basal state, myotubes and skeletal muscle from male and female mice with HIF1α knockout were studied using complementary multiomics, biochemical and metabolite assays. HIF1α knockout altered the electron transport chain, mitochondrial oxidative function, signalling molecules for protein homeostasis, and post-mitotic senescence markers, some of which were differentially impacted by sex. The cost of rapid adaptive responses mediated by HIF1α is lower mitochondrial oxidative efficiency and post-mitotic senescence during normoxia.

Sexual dimorphism and the multi-omic response to exercise training in rat subcutaneous white adipose tissue

Subcutaneous white adipose tissue (scWAT) is a dynamic storage and secretory organ that regulates systemic homeostasis, yet the impact of endurance exercise training (ExT) and sex on its molecular landscape is not fully established. Utilizing an integrative multi-omics approach, and leveraging data generated by the Molecular Transducers of Physical Activity Consortium (MoTrPAC), we show profound sexual dimorphism in the scWAT of sedentary rats and in the dynamic response of this tissue to ExT. Specifically, the scWAT of sedentary females displays -omic signatures related to insulin signaling and adipogenesis, whereas the scWAT of sedentary males is enriched in terms related to aerobic metabolism. These sex-specific -omic signatures are preserved or amplified with ExT. Integration of multi-omic analyses with phenotypic measures identifies molecular hubs predicted to drive sexually distinct responses to training. Overall, this study underscores the powerful impact of sex on adipose tissue biology and provides a rich resource to investigate the scWAT response to ExT.

Lack of detectable sex differences in the mitochondrial function of Caenorhabditis elegans

BACKGROUND

Sex differences in mitochondrial function have been reported in multiple tissue and cell types. Additionally, sex-variable responses to stressors including environmental pollutants and drugs that cause mitochondrial toxicity have been observed. The mechanisms that establish these differences are thought to include hormonal modulation, epigenetic regulation, double dosing of X-linked genes, and the maternal inheritance of mtDNA. Understanding the drivers of sex differences in mitochondrial function and being able to model them in vitro is important for identifying toxic compounds with sex-variable effects. Additionally, understanding how sex differences in mitochondrial function compare across species may permit insight into the drivers of these differences, which is important for basic biology research. This study explored whether Caenorhabditis elegans, a model organism commonly used to study stress biology and toxicology, exhibits sex differences in mitochondrial function and toxicant susceptibility. To assess sex differences in mitochondrial function, we utilized four male enriched populations (N2 wild-type male enriched, fog-2(q71), him-5(e1490), and him-8(e1498)). We performed whole worm respirometry and determined whole worm ATP levels and mtDNA copy number. To probe whether sex differences manifest only after stress and inform the growing use of C. elegans as a mitochondrial health and toxicologic model, we also assessed susceptibility to a classic mitochondrial toxicant, rotenone.

RESULTS

We detected few to no large differences in mitochondrial function between C. elegans sexes. Though we saw no sex differences in vulnerability to rotenone, we did observe sex differences in the uptake of this lipophilic compound, which may be of interest to those utilizing C. elegans as a model organism for toxicologic studies. Additionally, we observed altered non-mitochondrial respiration in two him strains, which may be of interest to other researchers utilizing these strains.

CONCLUSIONS

Basal mitochondrial parameters in male and hermaphrodite C. elegans are similar, at least at the whole-organism level, as is toxicity associated with a mitochondrial Complex I inhibitor, rotenone. Our data highlights the limitation of using C. elegans as a model to study sex-variable mitochondrial function and toxicological responses.

Identifying latent genetic interactions in genome-wide association studies using multiple traits

The "missing" heritability of complex traits may be partly explained by genetic variants interacting with other genes or environments that are difficult to specify, observe, and detect. We propose a new kernel-based method called Latent Interaction Testing (LIT) to screen for genetic interactions that leverages pleiotropy from multiple related traits without requiring the interacting variable to be specified or observed. Using simulated data, we demonstrate that LIT increases power to detect latent genetic interactions compared to univariate methods. We then apply LIT to obesity-related traits in the UK Biobank and detect variants with interactive effects near known obesity-related genes (URL: https://CRAN.R-project.org/package=lit ).

Sex differences in energy metabolism: natural selection, mechanisms and consequences

Metabolic homeostasis operates differently in men and women. This sex asymmetry is the result of evolutionary adaptations that enable women to resist loss of energy stores and protein mass while remaining fertile in times of energy deficit. During starvation or prolonged exercise, women rely on oxidation of lipids, which are a more efficient energy source than carbohydrates, to preserve glucose for neuronal and placental function and spare proteins necessary for organ function. Carbohydrate reliance in men could be an evolutionary adaptation related to defence and hunting, as glucose, unlike lipids, can be used as a fuel for anaerobic high-exertion muscle activity. The larger subcutaneous adipose tissue depots in healthy women than in healthy men provide a mechanism for lipid storage. As female mitochondria have higher functional capacity and greater resistance to oxidative damage than male mitochondria, uniparental inheritance of female mitochondria may reduce the transmission of metabolic disorders. However, in women, starvation resistance and propensity to obesity have evolved in tandem, and the current prevalence of obesity is greater in women than in men. The combination of genetic sex, programming by developmental testosterone in males, and pubertal sex hormones defines sex-specific biological systems in adults that produce phenotypic sex differences in energy homeostasis, metabolic disease and drug responses.

The role of gonadal hormones and sex chromosomes in sex-dependent effects of early nutrition on metabolic health

Early-life conditions such as prenatal nutrition can have long-term effects on metabolic health, and these effects may differ between males and females. Understanding the biological mechanisms underlying sex differences in the response to early-life environment will improve interventions, but few such mechanisms have been identified, and there is no overall framework for understanding sex differences. Biological sex differences may be due to chromosomal sex, gonadal sex, or interactions between the two. This review describes approaches to distinguish between the roles of chromosomal and gonadal sex, and summarizes findings regarding sex differences in metabolism. The Four Core Genotypes (FCG) mouse model allows dissociation of the sex chromosome genotype from gonadal type, whereas the XY* mouse model can be used to distinguish effects of X chromosome dosage vs the presence of the Y chromosome. Gonadectomy can be used to distinguish between organizational (permanent) and activational (reversible) effects of sex hormones. Baseline sex differences in a variety of metabolic traits are influenced by both activational and organizational effects of gonadal hormones, as well as sex chromosome complement. Thus far, these approaches have not been widely applied to examine sex-dependent effects of prenatal conditions, although a number of studies have found activational effects of estradiol to be protective against the development of hypertension following early-life adversity. Genes that escape X chromosome inactivation (XCI), such as Kdm5c, contribute to baseline sex-differences in metabolism, while Ogt, another XCI escapee, leads to sex-dependent responses to prenatal maternal stress. Genome-wide approaches to the study of sex differences include mapping genetic loci influencing metabolic traits in a sex-dependent manner. Seeking enrichment for binding sites of hormone receptors among genes showing sexually-dimorphic expression can elucidate the relative roles of hormones. Using the approaches described herein to identify mechanisms underlying sex-dependent effects of early nutrition on metabolic health may enable the identification of fundamental mechanisms and potential interventions.

Direct androgen receptor control of sexually dimorphic gene expression in the mammalian kidney

Mammalian organs exhibit distinct physiology, disease susceptibility, and injury responses between the sexes. In the mouse kidney, sexually dimorphic gene activity maps predominantly to proximal tubule (PT) segments. Bulk RNA sequencing (RNA-seq) data demonstrated that sex differences were established from 4 and 8 weeks after birth under gonadal control. Hormone injection studies and genetic removal of androgen and estrogen receptors demonstrated androgen receptor (AR)-mediated regulation of gene activity in PT cells as the regulatory mechanism. Interestingly, caloric restriction feminizes the male kidney. Single-nuclear multiomic analysis identified putative cis-regulatory regions and cooperating factors mediating PT responses to AR activity in the mouse kidney. In the human kidney, a limited set of genes showed conserved sex-linked regulation, whereas analysis of the mouse liver underscored organ-specific differences in the regulation of sexually dimorphic gene expression. These findings raise interesting questions on the evolution, physiological significance, disease, and metabolic linkage of sexually dimorphic gene activity.

Deficiency of histone variant macroH2A1.1 is associated with sexually dimorphic obesity in mice

Obesity has a major socio-economic health impact. There are profound sex differences in adipose tissue deposition and obesity-related conditions. The underlying mechanisms driving sexual dimorphism in obesity and its associated metabolic disorders remain unclear. Histone variant macroH2A1.1 is a candidate epigenetic mechanism linking environmental and dietary factors to obesity. Here, we used a mouse model genetically depleted of macroH2A1.1 to investigate its potential epigenetic role in sex dimorphic obesity, metabolic disturbances and gut dysbiosis. Whole body macroH2A1 knockout (KO) mice, generated with the Cre/loxP technology, and their control littermates were fed a high fat diet containing 60% of energy derived from fat. The diet was administered for three months starting from 10 to 12 weeks of age. We evaluated the progression in body weight, the food intake, and the tolerance to glucose by means of a glucose tolerance test. Gut microbiota composition, visceral adipose and liver tissue morphology were assessed. In addition, adipogenic gene expression patterns were evaluated in the visceral adipose tissue. Female KO mice for macroH2A1.1 had a more pronounced weight gain induced by high fat diet compared to their littermates, while the increase in body weight in male mice was similar in the two genotypes. Food intake was generally increased upon KO and decreased by high fat diet in both sexes, with the exception of KO females fed a high fat diet that displayed the same food intake of their littermates. In glucose tolerance tests, glucose levels were significantly elevated upon high fat diet in female KO compared to a standard diet, while this effect was absent in male KO. There were no differences in hepatic histology. Upon a high fat diet, in female adipocyte cross-sectional area was larger in KO compared to littermates: activation of proadipogenic genes (ACACB, AGT, ANGPT2, FASN, RETN, SLC2A4) and downregulation of antiadipogenic genes (AXIN1, E2F1, EGR2, JUN, SIRT1, SIRT2, UCP1, CCND1, CDKN1A, CDKN1B, EGR2) was detected. Gut microbiota profiling showed increase in Firmicutes and a decrease in Bacteroidetes in females, but not males, macroH2A1.1 KO mice. MacroH2A1.1 KO mice display sexual dimorphism in high fat diet-induced obesity and in gut dysbiosis, and may represent a useful model to investigate epigenetic and metabolic differences associated to the development of obesity-associated pathological conditions in males and females.

Plasma Citrate Levels Are Associated with an Increased Risk of Cardiovascular Mortality in Patients with Type 2 Diabetes (Zodiac-64)

Circulating citrate may represent a proxy of mitochondrial dysfunction which plays a role in the development of vascular complications in type 2 diabetes (T2D). Here, we determined the associations between plasma citrate levels and cardiovascular (CV) mortality in T2D patients. In this prospective cohort study, 601 patients were included who participated in the Zwolle Outpatient Diabetes project Integrating Available Care (ZODIAC). Plasma citrate levels were measured by nuclear magnetic resonance spectroscopy. Cox proportional hazards regression models were used to evaluate the associations between plasma citrate and the risk of CV mortality. Over a median follow-up of 11.4 years, 119 (19.8%) of the 601 patients died from a CV cause. In multivariable Cox proportional hazards regression models, adjusting for conventional risk factors, plasma citrate was associated with an increased risk of CV mortality (the hazard ratio (HR) per 1-SD increment was 1.19 (95%CI: 1.00-1.40), p = 0.048). This association was prominent in males (n = 49 with CV mortality) (HR 1.52 (95%CI: 1.14-2.03), p = 0.005), but not in females (n = 70 with CV mortality) (HR 1.11 (95%CI: 0.90-1.37), p = 0.319) (age-adjusted Pinteraction = 0.044). In conclusion, higher plasma citrate levels are associated with an increased risk of CV mortality in patients with established T2D. Future studies are warranted to unravel the potential role of citrate-related pathways in the pathogenesis of T2D-related vascular complications.

Identifying latent genetic interactions in genome-wide association studies using multiple traits

Genome-wide association studies of complex traits frequently find that SNP-based estimates of heritability are considerably smaller than estimates from classic family-based studies. This 'missing' heritability may be partly explained by genetic variants interacting with other genes or environments that are difficult to specify, observe, and detect. To circumvent these challenges, we propose a new method to detect genetic interactions that leverages pleiotropy from multiple related traits without requiring the interacting variable to be specified or observed. Our approach, Latent Interaction Testing (LIT), uses the observation that correlated traits with shared latent genetic interactions have trait variance and covariance patterns that differ by genotype. LIT examines the relationship between trait variance/covariance patterns and genotype using a flexible kernel-based framework that is computationally scalable for biobank-sized datasets with a large number of traits. We first use simulated data to demonstrate that LIT substantially increases power to detect latent genetic interactions compared to a trait-by-trait univariate method. We then apply LIT to four obesity-related traits in the UK Biobank and detect genetic variants with interactive effects near known obesity-related genes. Overall, we show that LIT, implemented in the R package lit, uses shared information across traits to improve detection of latent genetic interactions compared to standard approaches.

Myoglobin in Brown Adipose Tissue: A Multifaceted Player in Thermogenesis

Brown adipose tissue (BAT) plays an important role in energy homeostasis by generating heat from chemical energy via uncoupled oxidative phosphorylation. Besides its high mitochondrial content and its exclusive expression of the uncoupling protein 1, another key feature of BAT is the high expression of myoglobin (MB), a heme-containing protein that typically binds oxygen, thereby facilitating the diffusion of the gas from cell membranes to mitochondria of muscle cells. In addition, MB also modulates nitric oxide (NO•) pools and can bind C16 and C18 fatty acids, which indicates a role in lipid metabolism. Recent studies in humans and mice implicated MB present in BAT in the regulation of lipid droplet morphology and fatty acid shuttling and composition, as well as mitochondrial oxidative metabolism. These functions suggest that MB plays an essential role in BAT energy metabolism and thermogenesis. In this review, we will discuss in detail the possible physiological roles played by MB in BAT thermogenesis along with the potential underlying molecular mechanisms and focus on the question of how BAT-MB expression is regulated and, in turn, how this globin regulates mitochondrial, lipid, and NO• metabolism. Finally, we present potential MB-mediated approaches to augment energy metabolism, which ultimately could help tackle different metabolic disorders.

Circulating Citrate Is Associated with Liver Fibrosis in Nonalcoholic Fatty Liver Disease and Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is associated with mitochondrial damage. Circulating mitochondrial metabolites may be elevated in NAFLD but their associations with liver damage is not known. This study aimed to assess the association of key mitochondrial metabolites with the degree of liver fibrosis in the context of NAFLD and nonalcoholic steatohepatitis (NASH). Cross-sectional analyses were performed on two cohorts of biopsy-proven NAFLD and/or NASH subjects. The association of circulating mitochondrial metabolite concentrations with liver fibrosis was assessed using linear regression analysis. In the single-center cohort of NAFLD subjects (n = 187), the mean age was 54.9 ±13.0 years, 40.1% were female and 86.1% were White. Type 2 diabetes (51.3%), hypertension (43.9%) and obesity (72.2%) were prevalent. Those with high citrate had a higher proportion of moderate/significant liver fibrosis (stage F ≥ 2) (68.4 vs. 39.6%, p = 0.001) and advanced fibrosis (stage F ≥ 3) (31.6 vs. 13.6%, p = 0.01). Citrate was associated with liver fibrosis independent of age, sex, NAFLD activity score and metabolic syndrome (per 1 SD increase: β = 0.19, 95% CI: 0.03-0.35, p = 0.02). This association was also observed in a cohort of NASH subjects (n = 176) (β = 0.21, 95% CI: 0.07-0.36, p = 0.005). The association of citrate with liver fibrosis was observed in males (p = 0.005) but not females (p = 0.41). In conclusion, circulating citrate is elevated and associated with liver fibrosis, particularly in male subjects with NAFLD and NASH. Mitochondrial function may be a target to consider for reducing the progression of liver fibrosis and NASH.

Genetic architecture of heart mitochondrial proteome influencing cardiac hypertrophy

Mitochondria play an important role in both normal heart function and disease etiology. We report analysis of common genetic variations contributing to mitochondrial and heart functions using an integrative proteomics approach in a panel of inbred mouse strains called the Hybrid Mouse Diversity Panel (HMDP). We performed a whole heart proteome study in the HMDP (72 strains, n=2-3 mice) and retrieved 848 mitochondrial proteins (quantified in ≥50 strains). High-resolution association mapping on their relative abundance levels revealed three trans-acting genetic loci on chromosomes (chr) 7, 13 and 17 that regulate distinct classes of mitochondrial proteins as well as cardiac hypertrophy. DAVID enrichment analyses of genes regulated by each of the loci revealed that the chr13 locus was highly enriched for complex-I proteins (24 proteins, P=2.2E-61), the chr17 locus for mitochondrial ribonucleoprotein complex (17 proteins, P=3.1E-25) and the chr7 locus for ubiquinone biosynthesis (3 proteins, P=6.9E-05). Follow-up high resolution regional mapping identified NDUFS4, LRPPRC and COQ7 as the candidate genes for chr13, chr17 and chr7 loci, respectively, and both experimental and statistical analyses supported their causal roles. Furthermore, a large cohort of Diversity Outbred mice was used to corroborate Lrpprc gene as a driver of mitochondrial DNA (mtDNA)-encoded gene regulation, and to show that the chr17 locus is specific to heart. Variations in all three loci were associated with heart mass in at least one of two independent heart stress models, namely, isoproterenol-induced heart failure and diet-induced obesity. These findings suggest that common variations in certain mitochondrial proteins can act in trans to influence tissue-specific mitochondrial functions and contribute to heart hypertrophy, elucidating mechanisms that may underlie genetic susceptibility to heart failure in human populations.

Sexual dimorphism in mitochondrial dysfunction and diabetes mellitus: evidence from a population-based cohort study

BACKGROUND

Pathophysiological mechanisms underlying sex-based differences in diabetes remain poorly understood. Mitochondrial metabolite methylmalonic acid (MMA) accumulation reflects mitochondrial dysfunction which is involved in sex-specific pathophysiological responses biologically. We aimed to investigate the sex-specific associations between mortality risk and MMA in adults with the presence or absence of type 2 diabetes.

METHODS

This cohort study included 24,164 adults (12,123 females and 12,041 males) from the NHANES study during 1999-2014. Both sexes were separately categorized as those with no diabetes, prediabetes, undiagnosed diabetes, and diagnosed diabetes. Circulating MMA level was measured at baseline by mass-spectrometric detection. Mortality status was ascertained from baseline until December 31, 2015.

RESULTS

During a median follow-up of 11.1 years, 3375 deaths were documented. Males had a particularly higher mortality than females in adults with diagnosed diabetes compared to differences in those with no diabetes, prediabetes and undiagnosed diabetes (sex differences in mortality rate per 1000 person-years across diabetic status: 0.62, 1.44, 5.78, and 9.77, p < 0.001). Notably, the sex-specific difference in associations between MMA and mortality was significant only in adults with diagnosed diabetes (p for interaction = 0.028), not in adults with no diabetes and prediabetes. Adjusted HRs (95%CIs) per doubling of MMA for all-cause mortality were 1.19 (1.04-1.37) in females with diagnosed diabetes versus 1.58 (1.36-1.86) in male counterparts. In addition, MMA levels had an insignificant or weak correlation with sex hormone profiles at baseline, regardless of diabetes status and sex.

CONCLUSIONS

Sex difference in mortality risk was especially significant in diagnosed type 2 diabetes. Increasing equivalent exposure to mitochondrial metabolite MMA was associated with a greater excess risk of future mortality in males with diabetes than in females.

Conserved multi-tissue transcriptomic adaptations to exercise training in humans and mice

Physical activity is associated with beneficial adaptations in human and rodent metabolism. We studied over 50 complex traits before and after exercise intervention in middle-aged men and a panel of 100 diverse strains of female mice. Candidate gene analyses in three brain regions, muscle, liver, heart, and adipose tissue of mice indicate genetic drivers of clinically relevant traits, including volitional exercise volume, muscle metabolism, adiposity, and hepatic lipids. Although ∼33% of genes differentially expressed in skeletal muscle following the exercise intervention are similar in mice and humans independent of BMI, responsiveness of adipose tissue to exercise-stimulated weight loss appears controlled by species and underlying genotype. We leveraged genetic diversity to generate prediction models of metabolic trait responsiveness to volitional activity offering a framework for advancing personalized exercise prescription. The human and mouse data are publicly available via a user-friendly Web-based application to enhance data mining and hypothesis development.

Direct androgen receptor regulation of sexually dimorphic gene expression in the mammalian kidney

Mammalian organs exhibit distinct physiology, disease susceptibility and injury responses between the sexes. In the mouse kidney, sexually dimorphic gene activity maps predominantly to proximal tubule (PT) segments. Bulk RNA-seq data demonstrated sex differences were established from 4 and 8 weeks after birth under gonadal control. Hormone injection studies and genetic removal of androgen and estrogen receptors demonstrated androgen receptor (AR) mediated regulation of gene activity in PT cells as the regulatory mechanism. Interestingly, caloric restriction feminizes the male kidney. Single-nuclear multiomic analysis identified putative cis-regulatory regions and cooperating factors mediating PT responses to AR activity in the mouse kidney. In the human kidney, a limited set of genes showed conserved sex-linked regulation while analysis of the mouse liver underscored organ-specific differences in the regulation of sexually dimorphic gene expression. These findings raise interesting questions on the evolution, physiological significance, and disease and metabolic linkage, of sexually dimorphic gene activity.

A change of heart: understanding the mechanisms regulating cardiac proliferation and metabolism before and after birth

Mammalian cardiomyocytes undergo major maturational changes in preparation for birth and postnatal life. Immature cardiomyocytes contribute to cardiac growth via proliferation and thus the heart has the capacity to regenerate. To prepare for postnatal life, structural and metabolic changes associated with increased cardiac output and function must occur. This includes exit from the cell cycle, hypertrophic growth, mitochondrial maturation and sarcomeric protein isoform switching. However, these changes come at a price: the loss of cardiac regenerative capacity such that damage to the heart in postnatal life is permanent. This is a significant barrier to the development of new treatments for cardiac repair and contributes to heart failure. The transitional period of cardiomyocyte growth is a complex and multifaceted event. In this review, we focus on studies that have investigated this critical transition period as well as novel factors that may regulate and drive this process. We also discuss the potential use of new biomarkers for the detection of myocardial infarction and, in the broader sense, cardiovascular disease.

The potential of integrating human and mouse discovery platforms to advance our understanding of cardiometabolic diseases

Cardiometabolic diseases encompass a range of interrelated conditions that arise from underlying metabolic perturbations precipitated by genetic, environmental, and lifestyle factors. While obesity, dyslipidaemia, smoking, and insulin resistance are major risk factors for cardiometabolic diseases, individuals still present in the absence of such traditional risk factors, making it difficult to determine those at greatest risk of disease. Thus, it is crucial to elucidate the genetic, environmental, and molecular underpinnings to better understand, diagnose, and treat cardiometabolic diseases. Much of this information can be garnered using systems genetics, which takes population-based approaches to investigate how genetic variance contributes to complex traits. Despite the important advances made by human genome-wide association studies (GWAS) in this space, corroboration of these findings has been hampered by limitations including the inability to control environmental influence, limited access to pertinent metabolic tissues, and often, poor classification of diseases or phenotypes. A complementary approach to human GWAS is the utilisation of model systems such as genetically diverse mouse panels to study natural genetic and phenotypic variation in a controlled environment. Here, we review mouse genetic reference panels and the opportunities they provide for the study of cardiometabolic diseases and related traits. We discuss how the post-GWAS era has prompted a shift in focus from discovery of novel genetic variants to understanding gene function. Finally, we highlight key advantages and challenges of integrating complementary genetic and multi-omics data from human and mouse populations to advance biological discovery.

Sexual dimorphism and the multi-omic response to exercise training in rat subcutaneous white adipose tissue

Subcutaneous white adipose tissue (scWAT) is a dynamic storage and secretory organ that regulates systemic homeostasis, yet the impact of endurance exercise training and sex on its molecular landscape has not been fully established. Utilizing an integrative multi-omics approach with data generated by the Molecular Transducers of Physical Activity Consortium (MoTrPAC), we identified profound sexual dimorphism in the dynamic response of rat scWAT to endurance exercise training. Despite similar cardiorespiratory improvements, only male rats reduced whole-body adiposity, scWAT adipocyte size, and total scWAT triglyceride abundance with training. Multi-omic analyses of adipose tissue integrated with phenotypic measures identified sex-specific training responses including enrichment of mTOR signaling in females, while males displayed enhanced mitochondrial ribosome biogenesis and oxidative metabolism. Overall, this study reinforces our understanding that sex impacts scWAT biology and provides a rich resource to interrogate responses of scWAT to endurance training.

Thermogenic Brown Fat in Humans: Implications in Energy Homeostasis, Obesity and Metabolic Disorders

In mammals including humans, there are two types of adipose tissue, white and brown adipose tissues (BATs). White adipose tissue is the primary site of energy storage, while BAT is a specialized tissue for non-shivering thermogenesis to dissipate energy as heat. Although BAT research has long been limited mostly in small rodents, the rediscovery of metabolically active BAT in adult humans has dramatically promoted the translational studies on BAT in health and diseases. It is now established that BAT, through its thermogenic and energy dissipating activities, plays a role in the regulation of body temperature, whole-body energy expenditure, and body fatness. Moreover, increasing evidence has demonstrated that BAT secretes various paracrine and endocrine factors, which influence other peripheral tissues and control systemic metabolic homeostasis, suggesting BAT as a metabolic regulator, other than for thermogenesis. In fact, clinical studies have revealed an association of BAT not only with metabolic disorders such as insulin resistance, diabetes, dyslipidemia, and fatty liver, but also with cardiovascular diseases including hypertension and atherosclerosis. Thus, BAT is an intriguing tissue combating obesity and related metabolic diseases. In this review, we summarize current knowledge on human BAT, focusing its patho-physiological roles in energy homeostasis, obesity and related metabolic disorders. The effects of aging and sex on BAT are also discussed.

The mitochondrial multi-omic response to exercise training across tissues

Mitochondria are adaptable organelles with diverse cellular functions critical to whole-body metabolic homeostasis. While chronic endurance exercise training is known to alter mitochondrial activity, these adaptations have not yet been systematically characterized. Here, the Molecular Transducers of Physical Activity Consortium (MoTrPAC) mapped the longitudinal, multi-omic changes in mitochondrial analytes across 19 tissues in male and female rats endurance trained for 1, 2, 4 or 8 weeks. Training elicited substantial changes in the adrenal gland, brown adipose, colon, heart and skeletal muscle, while we detected mild responses in the brain, lung, small intestine and testes. The colon response was characterized by non-linear dynamics that resulted in upregulation of mitochondrial function that was more prominent in females. Brown adipose and adrenal tissues were characterized by substantial downregulation of mitochondrial pathways. Training induced a previously unrecognized robust upregulation of mitochondrial protein abundance and acetylation in the liver, and a concomitant shift in lipid metabolism. The striated muscles demonstrated a highly coordinated response to increase oxidative capacity, with the majority of changes occurring in protein abundance and post-translational modifications. We identified exercise upregulated networks that are downregulated in human type 2 diabetes and liver cirrhosis. In both cases HSD17B10, a central dehydrogenase in multiple metabolic pathways and mitochondrial tRNA maturation, was the main hub. In summary, we provide a multi-omic, cross-tissue atlas of the mitochondrial response to training and identify candidates for prevention of disease-associated mitochondrial dysfunction.

Sex Differences in Glucose Homeostasis

Sexual dimorphism has been demonstrated to have an effect on various physiological functions. In this regard, researchers have investigated its impact on glucose homeostasis in both preclinical and clinical studies. Sex differences mainly arise from physiological factors such as sex hormones, body fat and muscle distribution, and sex chromosomes. The sexual dimorphism has also been studied in the context of diabetes. Reflecting the prevalence of the disease among the population, studies focusing on the sex difference in type 1 diabetes (T1D) are not common as the ones in type 2 diabetes (T2D). T1D is reported as the only major specific autoimmune disease that exhibits a male predominance. Clinical studies have demonstrated that impaired fasting glucose is more frequent in men whereas women more commonly exhibit impaired glucose tolerance. Understanding the sex difference in glucose homeostasis becomes more attractive when focusing on the findings that highlight sexual dimorphism on the efficacy or adverse effect profile of antidiabetic medications. Thus, in this chapter, we aimed to discuss the impact of sex on the glucose homeostasis both in health and in diabetes.

Sex differences in thermoregulation in mammals: Implications for energy homeostasis

Thermal homeostasis is a fundamental process in mammals, which allows the maintenance of a constant internal body temperature to ensure an efficient function of cells despite changes in ambient temperature. Increasing evidence has revealed the great impact of thermoregulation on energy homeostasis. Homeothermy requires a fine regulation of food intake, heat production, conservation and dissipation and energy expenditure. A great interest on this field of research has re-emerged following the discovery of thermogenic brown adipose tissue and browning of white fat in adult humans, with a potential clinical relevance on obesity and metabolic comorbidities. However, most of our knowledge comes from male animal models or men, which introduces unwanted biases on the findings. In this review, we discuss how differences in sex-dependent characteristics (anthropometry, body composition, hormonal regulation, and other sexual factors) influence numerous aspects of thermal regulation, which impact on energy homeostasis. Individuals of both sexes should be used in the experimental paradigms, considering the ovarian cycles and sexual hormonal regulation as influential factors in these studies. Only by collecting data in both sexes on molecular, functional, and clinical aspects, we will be able to establish in a rigorous way the real impact of thermoregulation on energy homeostasis, opening new avenues in the understanding and treatment of obesity and metabolic associated diseases.

Transcriptomic profiling reveals sex-specific molecular signatures of adipose endothelial cells under obesogenic conditions

Female mice display greater adipose angiogenesis and maintain healthier adipose tissue than do males upon high-fat diet feeding. Through transcriptome analysis of endothelial cells (EC) from the white adipose tissue of male and female mice high-fat-fed for 7 weeks, we found that adipose EC exhibited pronouncedly sex-distinct transcriptomes. Genes upregulated in female adipose EC were associated with proliferation, oxidative phosphorylation, and chromatin remodeling contrasting the dominant enrichment for genes related to inflammation and a senescence-associated secretory of male EC. Similar sex-biased phenotypes of adipose EC were detectable in a dataset of aged EC. The highly proliferative phenotype of female EC was observed also in culture conditions. In turn, male EC displayed greater inflammatory potential than female EC in culture, based on basal and tumor necrosis factor alpha-stimulated patterns of gene expression. Our study provides insights into molecular programs that distinguish male and female EC responses to pathophysiological conditions.

Integrative systems analysis identifies genetic and dietary modulators of bile acid homeostasis

Bile acids (BAs) are complex and incompletely understood enterohepatic-derived hormones that control whole-body metabolism. Here, we profiled postprandial BAs in the liver, feces, and plasma of 360 chow- or high-fat-diet-fed BXD male mice and demonstrated that both genetics and diet strongly influence BA abundance, composition, and correlation with metabolic traits. Through an integrated systems approach, we mapped hundreds of quantitative trait loci that modulate BAs and identified both known and unknown regulators of BA homeostasis. In particular, we discovered carboxylesterase 1c (Ces1c) as a genetic determinant of plasma tauroursodeoxycholic acid (TUDCA), a BA species with established disease-preventing actions. The association between Ces1c and plasma TUDCA was validated using data from independent mouse cohorts and a Ces1c knockout mouse model. Collectively, our data are a unique resource to dissect the physiological importance of BAs as determinants of metabolic traits, as underscored by the identification of CES1C as a master regulator of plasma TUDCA levels.

Liver-heart cross-talk mediated by coagulation factor XI protects against heart failure

Tissue-tissue communication by endocrine factors is a vital mechanism for physiologic homeostasis. A systems genetics analysis of transcriptomic and functional data from a cohort of diverse, inbred strains of mice predicted that coagulation factor XI (FXI), a liver-derived protein, protects against diastolic dysfunction, a key trait of heart failure with preserved ejection fraction. This was confirmed using gain- and loss-of-function studies, and FXI was found to activate the bone morphogenetic protein (BMP)-SMAD1/5 pathway in the heart. The proteolytic activity of FXI is required for the cleavage and activation of extracellular matrix-associated BMP7 in the heart, thus inhibiting genes involved in inflammation and fibrosis. Our results reveal a protective role of FXI in heart injury that is distinct from its role in coagulation.

Suppression of estrogen receptor beta classical genomic activity enhances systemic and adipose-specific response to chronic beta-3 adrenergic receptor (β3AR) stimulation

White adipose tissue (WAT) dysfunction independently predicts cardiometabolic disease, yet there is a lack of effective adipocyte-targeting therapeutics. B3AR agonists enhance adipocyte mitochondrial function and hold potential in this regard. Based on enhanced sensitivity to B3AR-mediated browning in estrogen receptor (ER)alpha-null mice, we hypothesized that ERβ may enhance the WAT response to the B3AR ligand, CL316,243 (CL). Methods: Male and female wild-type (WT) and ERβ DNA binding domain knock-out (ERβDBDKO) mice fed high-fat diet (HFD) to induce obesity were administered CL (1 mg/kg) daily for 2 weeks. Systemic physiological assessments of body composition (EchoMRI), bioenergetics (metabolic chambers), adipocyte mitochondrial respiration (oroboros) and glucose tolerance were performed, alongside perigonadal (PGAT), subcutaneous (SQAT) and brown adipose tissue (BAT) protein expression assessment (Western blot). Mechanisms were tested in vitro using primary adipocytes isolated from WT mice, and from Esr2-floxed mice in which ERβ was knocked down. Statistical analyses were performed using 2 × 2 analysis of variance (ANOVA) for main effects of genotype (G) and treatment (T), as well as GxT interactions; t-tests were used to determine differences between in vitro treatment conditions (SPSS V24). Results: There were no genotype differences in HFD-induced obesity or systemic rescue effects of CL, yet ERβDBDKO females were more sensitive to CL-induced increases in energy expenditure and WAT UCP1 induction (GxT, p < 0.05), which coincided with greater WAT B3AR protein content among the KO (G, p < 0.05). Among males, who were more insulin resistant to begin with (no genotype differences before treatment), tended to be more sensitive to CL-mediated reduction in insulin resistance. With sexes combined, basal WAT mitochondrial respiration trended toward being lower in the ERβDBDKO mice, but this was completely rescued by CL (p < 0.05). Confirming prior work, CL increased adipose tissue ERβ protein (T, p < 0.05, all), an effect that was enhanced in WAT and BAT the female KO (GxT, p < 0.01). In vitro experiments indicated that an inhibitor of ERβ genomic function (PHTPP) synergized with CL to further increase UCP1 mRNA (p = 0.043), whereas full ERβ protein was required for UCP1 expression (p = 0.042). Conclusion: Full ERβ activity appears requisite and stimulatory for UCP1 expression via a mechanism involving non-classical ERβ signaling. This novel discovery about the role of ERβ in adipocyte metabolism may have important clinical applications.

Chromosomal and environmental contributions to sex differences in the vulnerability to neurological and neuropsychiatric disorders: Implications for therapeutic interventions

Neurological and neuropsychiatric disorders affect men and women differently. Multiple sclerosis, Alzheimer's disease, anxiety disorders, depression, meningiomas and late-onset schizophrenia affect women more frequently than men. By contrast, Parkinson's disease, autism spectrum condition, attention-deficit hyperactivity disorder, Tourette's syndrome, amyotrophic lateral sclerosis and early-onset schizophrenia are more prevalent in men. Women have been historically under-recruited or excluded from clinical trials, and most basic research uses male rodent cells or animals as disease models, rarely studying both sexes and factoring sex as a potential source of variation, resulting in a poor understanding of the underlying biological reasons for sex and gender differences in the development of such diseases. Putative pathophysiological contributors include hormones and epigenetics regulators but additional biological and non-biological influences may be at play. We review here the evidence for the underpinning role of the sex chromosome complement, X chromosome inactivation, and environmental and epigenetic regulators in sex differences in the vulnerability to brain disease. We conclude that there is a pressing need for a better understanding of the genetic, epigenetic and environmental mechanisms sustaining sex differences in such diseases, which is critical for developing a precision medicine approach based on sex-tailored prevention and treatment.

Sex Differences in Adiposity and Cardiovascular Diseases

Body fat distribution is a well-established predictor of adverse medical outcomes, independent of overall adiposity. Studying body fat distribution sheds insights into the causes of obesity and provides valuable information about the development of various comorbidities. Compared to total adiposity, body fat distribution is more closely associated with risks of cardiovascular diseases. The present review specifically focuses on the sexual dimorphism in body fat distribution, the biological clues, as well as the genetic traits that are distinct from overall obesity. Understanding the sex determinations on body fat distribution and adiposity will aid in the improvement of the prevention and treatment of cardiovascular diseases (CVD).

Sexual Dimorphism in Brown Adipose Tissue Activation and White Adipose Tissue Browning

The present narrative review gathers the studies reported so far, addressing sex differences in the effects of cold exposure, feeding pattern and age on brown adipose tissue (BAT) thermogenesis and white adipose tissue (WAT) browning. In rodents, when exposed to decreasing temperatures, females activate thermogenesis earlier. Results obtained in humans go in the same line, although they do not provide results as solid as those obtained in rodents. Regarding the effects of overfeeding, interesting sex differences on BAT thermogenic capacity have been reported, and the greater or lower sensitivity of each sex to this dietary situation seems to be dependent on the type of feeding. In the case of energy restriction, females are more sensitive than males. In addition, sex differences have also been observed in thermogenesis changes induced by phenolic compound administration. During sexual development, an increase in BAT mass and BAT activity takes place. This phenomenon is greater in boys than in girls, probably due to its relation to muscle-mass growth. The opposite situation takes place during ageing, a lifespan period where thermogenic capacity declines, this being more acute in men than in women. Finally, the vast majority of the studies have reported a higher susceptibility to developing WAT browning amongst females. The scarcity of results highlights the need for further studies devoted to analysing this issue, in order to provide valuable information for a more personalised approach.

Diabesity in Elderly Cardiovascular Disease Patients: Mechanisms and Regulators

Cardiovascular disease (CVD) is the leading cause of death in the world. In 2019, 550 million people were suffering from CVD and 18 million of them died as a result. Most of them had associated risk factors such as high fasting glucose, which caused 134 million deaths, and obesity, which accounted for 5.02 million deaths. Diabesity, a combination of type 2 diabetes and obesity, contributes to cardiac, metabolic, inflammation and neurohumoral changes that determine cardiac dysfunction (diabesity-related cardiomyopathy). Epicardial adipose tissue (EAT) is distributed around the myocardium, promoting myocardial inflammation and fibrosis, and is associated with an increased risk of heart failure, particularly with preserved systolic function, atrial fibrillation and coronary atherosclerosis. In fact, several hypoglycaemic drugs have demonstrated a volume reduction of EAT and effects on its metabolic and inflammation profile. However, it is necessary to improve knowledge of the diabesity pathophysiologic mechanisms involved in the development and progression of cardiovascular diseases for comprehensive patient management including drugs to optimize glucometabolic control. This review presents the mechanisms of diabesity associated with cardiovascular disease and their therapeutic implications.

Sex differences in heart mitochondria regulate diastolic dysfunction

Heart failure with preserved ejection fraction (HFpEF) exhibits a sex bias, being more common in women than men, and we hypothesize that mitochondrial sex differences might underlie this bias. As part of genetic studies of heart failure in mice, we observe that heart mitochondrial DNA levels and function tend to be reduced in females as compared to males. We also observe that expression of genes encoding mitochondrial proteins are higher in males than females in human cohorts. We test our hypothesis in a panel of genetically diverse inbred strains of mice, termed the Hybrid Mouse Diversity Panel (HMDP). Indeed, we find that mitochondrial gene expression is highly correlated with diastolic function, a key trait in HFpEF. Consistent with this, studies of a "two-hit" mouse model of HFpEF confirm that mitochondrial function differs between sexes and is strongly associated with a number of HFpEF traits. By integrating data from human heart failure and the mouse HMDP cohort, we identify the mitochondrial gene Acsl6 as a genetic determinant of diastolic function. We validate its role in HFpEF using adenoviral over-expression in the heart. We conclude that sex differences in mitochondrial function underlie, in part, the sex bias in diastolic function.

Illuminating the Mechanisms Underlying Sex Differences in Cardiovascular Disease

Sex is a key risk factor for many types of cardiovascular disease. It is imperative to understand the mechanisms underlying sex differences to devise optimal preventive and therapeutic approaches for all individuals. Both biological sex (determined by sex chromosomes and gonadal hormones) and gender (social and cultural behaviors associated with femininity or masculinity) influence differences between men and women in disease susceptibility and pathology. Here, we focus on the application of experimental mouse models that elucidate the influence of 2 components of biological sex-sex chromosome complement (XX or XY) and gonad type (ovaries or testes). These models have revealed that in addition to well-known effects of gonadal hormones, sex chromosome complement influences cardiovascular risk factors, such as plasma cholesterol levels and adiposity, as well as the development of atherosclerosis and pulmonary hypertension. One mechanism by which sex chromosome dosage influences cardiometabolic traits is through sex-biased expression of X chromosome genes that escape X inactivation. These include chromatin-modifying enzymes that regulate gene expression throughout the genome. The identification of factors that determine sex-biased gene expression and cardiometabolic traits will expand our mechanistic understanding of cardiovascular disease processes and provide insight into sex differences that remain throughout the lifespan as gonadal hormone levels alter with age.

Relative contributions of sex hormones, sex chromosomes, and gonads to sex differences in tissue gene regulation

Sex differences in physiology and disease in mammals result from the effects of three classes of factors that are inherently unequal in males and females: reversible (activational) effects of gonadal hormones, permanent (organizational) effects of gonadal hormones, and cell-autonomous effects of sex chromosomes, as well as genes driven by these classes of factors. Often, these factors act together to cause sex differences in specific phenotypes, but the relative contribution of each and the interactions among them remain unclear. Here, we used the four core genotypes (FCG) mouse model with or without hormone replacement to distinguish the effects of each class of sex-biasing factors on transcriptome regulation in liver and adipose tissues. We found that the activational hormone levels have the strongest influence on gene expression, followed by the organizational gonadal sex effect, and last, sex chromosomal effect, along with interactions among the three factors. Tissue specificity was prominent, with a major impact of estradiol on adipose tissue gene regulation and of testosterone on the liver transcriptome. The networks affected by the three sex-biasing factors include development, immunity and metabolism, and tissue-specific regulators were identified for these networks. Furthermore, the genes affected by individual sex-biasing factors and interactions among factors are associated with human disease traits such as coronary artery disease, diabetes, and inflammatory bowel disease. Our study offers a tissue-specific account of the individual and interactive contributions of major sex-biasing factors to gene regulation that have broad impact on systemic metabolic, endocrine, and immune functions.

Sex differences in the intergenerational inheritance of metabolic traits

Strong evidence suggests that early-life exposures to suboptimal environmental factors, including those in utero, influence our long-term metabolic health. This has been termed developmental programming. Mounting evidence suggests that the growth and metabolism of male and female fetuses differ. Therefore, sexual dimorphism in response to pre-conception or early-life exposures could contribute to known sex differences in susceptibility to poor metabolic health in adulthood. However, until recently, many studies, especially those in animal models, focused on a single sex, or, often in the case of studies performed during intrauterine development, did not report the sex of the animal at all. In this review, we (a) summarize the evidence that male and females respond differently to a suboptimal pre-conceptional or in utero environment, (b) explore the potential biological mechanisms that underlie these differences and (c) review the consequences of these differences for long-term metabolic health, including that of subsequent generations.

Genetic variation of putative myokine signaling is dominated by biological sex and sex hormones

Skeletal muscle plays an integral role in coordinating physiological homeostasis, where signaling to other tissues via myokines allows for coordination of complex processes. Here, we aimed to leverage natural genetic correlation structure of gene expression both within and across tissues to understand how muscle interacts with metabolic tissues. Specifically, we performed a survey of genetic correlations focused on myokine gene regulation, muscle cell composition, cross-tissue signaling, and interactions with genetic sex in humans. While expression levels of a majority of myokines and cell proportions within skeletal muscle showed little relative differences between males and females, nearly all significant cross-tissue enrichments operated in a sex-specific or hormone-dependent fashion; in particular, with estradiol. These sex- and hormone-specific effects were consistent across key metabolic tissues: liver, pancreas, hypothalamus, intestine, heart, visceral, and subcutaneous adipose tissue. To characterize the role of estradiol receptor signaling on myokine expression, we generated male and female mice which lack estrogen receptor α specifically in skeletal muscle (MERKO) and integrated with human data. These analyses highlighted potential mechanisms of sex-dependent myokine signaling conserved between species, such as myostatin enriched for divergent substrate utilization pathways between sexes. Several other putative sex-dependent mechanisms of myokine signaling were uncovered, such as muscle-derived tumor necrosis factor alpha (TNFA) enriched for stronger inflammatory signaling in females compared to males and GPX3 as a male-specific link between glycolytic fiber abundance and hepatic inflammation. Collectively, we provide a population genetics framework for inferring muscle signaling to metabolic tissues in humans. We further highlight sex and estradiol receptor signaling as critical variables when assaying myokine functions and how changes in cell composition are predicted to impact other metabolic organs.

Potential Mechanisms for How Long-Term Physical Activity May Reduce Insulin Resistance

Insulin became available for the treatment of patients with diabetes 100 years ago, and soon thereafter it became evident that the biological response to its actions differed markedly between individuals. This prompted extensive research into insulin action and resistance (IR), resulting in the universally agreed fact that IR is a core finding in patients with type 2 diabetes mellitus (T2DM). T2DM is the most prevalent form of diabetes, reaching epidemic proportions worldwide. Physical activity (PA) has the potential of improving IR and is, therefore, a cornerstone in the prevention and treatment of T2DM. Whereas most research has focused on the acute effects of PA, less is known about the effects of long-term PA on IR. Here, we describe a model of potential mechanisms behind reduced IR after long-term PA to guide further mechanistic investigations and to tailor PA interventions in the therapy of T2DM. The development of such interventions requires knowledge of normal glucose metabolism, and we briefly summarize an integrated physiological perspective on IR. We then describe the effects of long-term PA on signaling molecules involved in cellular responses to insulin, tissue-specific functions, and whole-body IR.

Sex- and strain-specific effects of mitochondrial uncoupling on age-related metabolic diseases in high-fat diet-fed mice

Mild uncoupling of oxidative phosphorylation is an intrinsic property of all mitochondria and may have evolved to protect cells against the production of damaging reactive oxygen species. Therefore, compounds that enhance mitochondrial uncoupling are potentially attractive anti‐aging therapies; however, chronic ingestion is associated with a number of unwanted side effects. We have previously developed a controlled‐release mitochondrial protonophore (CRMP) that is functionally liver‐directed and promotes oxidation of hepatic triglycerides by causing a subtle sustained increase in hepatic mitochondrial inefficiency. Here, we sought to leverage the higher therapeutic index of CRMP to test whether mild mitochondrial uncoupling in a liver‐directed fashion could reduce oxidative damage and improve age‐related metabolic disease and lifespan in diet‐induced obese mice. Oral administration of CRMP (20 mg/[kg‐day] × 4 weeks) reduced hepatic lipid content, protein kinase C epsilon activation, and hepatic insulin resistance in aged (74‐week‐old) high‐fat diet (HFD)‐fed C57BL/6J male mice, independently of changes in body weight, whole‐body energy expenditure, food intake, or markers of hepatic mitochondrial biogenesis. CRMP treatment was also associated with a significant reduction in hepatic lipid peroxidation, protein carbonylation, and inflammation. Importantly, long‐term (49 weeks) hepatic mitochondrial uncoupling initiated late in life (94–104 weeks), in conjugation with HFD feeding, protected mice against neoplastic disorders, including hepatocellular carcinoma (HCC), in a strain and sex‐specific manner. Taken together, these studies illustrate the complex variation of aging and provide important proof‐of‐concept data to support further studies investigating the use of liver‐directed mitochondrial uncouplers to promote healthy aging in humans.

Systems-level analysis of insulin action in mouse strains provides insight into tissue- and pathway-specific interactions that drive insulin resistance

Skeletal muscle and adipose tissue insulin resistance are major drivers of metabolic disease. To uncover pathways involved in insulin resistance, specifically in these tissues, we leveraged the metabolic diversity of different dietary exposures and discrete inbred mouse strains. This revealed that muscle insulin resistance was driven by gene-by-environment interactions and was strongly correlated with hyperinsulinemia and decreased levels of ten key glycolytic enzymes. Remarkably, there was no relationship between muscle and adipose tissue insulin action. Adipocyte size profoundly varied across strains and diets, and this was strongly correlated with adipose tissue insulin resistance. The A/J strain, in particular, exhibited marked adipocyte insulin resistance and hypertrophy despite robust muscle insulin responsiveness, challenging the role of adipocyte hypertrophy per se in systemic insulin resistance. These data demonstrate that muscle and adipose tissue insulin resistance can occur independently and underscore the need for tissue-specific interrogation to understand metabolic disease.

Proteome-wide systems genetics identifies UFMylation as a regulator of skeletal muscle function

Improving muscle function has great potential to improve the quality of life. To identify novel regulators of skeletal muscle metabolism and function, we performed a proteomic analysis of gastrocnemius muscle from 73 genetically distinct inbred mouse strains, and integrated the data with previously acquired genomics and >300 molecular/phenotypic traits via quantitative trait loci mapping and correlation network analysis. These data identified thousands of associations between protein abundance and phenotypes and can be accessed online (https://muscle.coffeeprot.com/) to identify regulators of muscle function. We used this resource to prioritize targets for a functional genomic screen in human bioengineered skeletal muscle. This identified several negative regulators of muscle function including UFC1, an E2 ligase for protein UFMylation. We show UFMylation is up-regulated in a mouse model of amyotrophic lateral sclerosis, a disease that involves muscle atrophy. Furthermore, in vivo knockdown of UFMylation increased contraction force, implicating its role as a negative regulator of skeletal muscle function.

White Adipose Tissue Depots Respond to Chronic Beta-3 Adrenergic Receptor Activation in a Sexually Dimorphic and Depot Divergent Manner

Beta-3 adrenergic receptor activation via exercise or CL316,243 (CL) induces white adipose tissue (WAT) browning, improves glucose tolerance, and reduces visceral adiposity. Our aim was to determine if sex or adipose tissue depot differences exist in response to CL. Daily CL injections were administered to diet-induced obese male and female mice for two weeks, creating four groups: male control, male CL, female control, and female CL. These groups were compared to determine the main and interaction effects of sex (S), CL treatment (T), and WAT depot (D). Glucose tolerance, body composition, and energy intake and expenditure were assessed, along with perigonadal (PGAT) and subcutaneous (SQAT) WAT gene and protein expression. CL consistently improved glucose tolerance and body composition. Female PGAT had greater protein expression of the mitochondrial uncoupling protein 1 (UCP1), while SQAT (S, p < 0.001) was more responsive to CL in increasing UCP1 (S×T, p = 0.011) and the mitochondrial biogenesis induction protein, PPARγ coactivator 1α (PGC1α) (S×T, p = 0.026). Females also displayed greater mitochondrial OXPHOS (S, p < 0.05) and adiponectin protein content (S, p < 0.05). On the other hand, male SQAT was more responsive to CL in increasing protein levels of PGC1α (S×T, p = 0.046) and adiponectin (S, p < 0.05). In both depots and in both sexes, CL significantly increased estrogen receptor beta (ERβ) and glucose-related protein 75 (GRP75) protein content (T, p < 0.05). Thus, CL improves systemic and adipose tissue-specific metabolism in both sexes; however, sex differences exist in the WAT-specific effects of CL. Furthermore, across sexes and depots, CL affects estrogen signaling by upregulating ERβ.

Myoglobin, expressed in brown adipose tissue of mice, regulates the content and activity of mitochondria and lipid droplets

The identification of novel physiological regulators that stimulate energy expenditure through brown adipose tissue (BAT) activity in substrate catalysis is of utmost importance to understand and treat metabolic diseases. Myoglobin (MB), known to store or transport oxygen in heart and skeletal muscles, has recently been found to bind fatty acids with physiological constants in its oxygenated form (i.e., MBO2). Here, we investigated the in vivo effect of MB expression on BAT activity. In particular, we studied mitochondrial function and lipid metabolism as essential determinants of energy expenditure in this tissue. We show in a MB-null (MBko) mouse model that MB expression in BAT impacts on the activity of brown adipocytes in a twofold manner: i) by elevating mitochondrial density plus maximal respiration capacity, and through that, by stimulating BAT oxidative metabolism along with the organelles` uncoupled respiration; and ii) by influencing the free fatty acids pool towards a palmitate-enriched composition and shifting the lipid droplet (LD) equilibrium towards higher counts of smaller droplets. These metabolic changes were accompanied by the up-regulated expression of thermogenesis markers UCP1, CIDEA, CIDEC, PGC1-α and PPAR-α in the BAT of MB wildtype (MBwt) mice. Along with the emergence of the "browning" BAT morphology, MBwt mice exhibited a leaner phenotype when compared to MBko littermates at 20 weeks of age. Our data shed novel insights into MB's role in linking oxygen and lipid-based thermogenic metabolism. The findings suggest potential new strategies of targeting the MB pathway to treat metabolic disorders related to diminishing energy expenditure.

Human Brown Adipose Tissue and Metabolic Health: Potential for Therapeutic Avenues

Obesity-associated metabolic abnormalities comprise a cluster of conditions including dyslipidemia, insulin resistance, diabetes and cardiovascular diseases that has affected more than 650 million people all over the globe. Obesity results from the accumulation of white adipose tissues mainly due to the chronic imbalance of energy intake and energy expenditure. A variety of approaches to treat or prevent obesity, including lifestyle interventions, surgical weight loss procedures and pharmacological approaches to reduce energy intake and increase energy expenditure have failed to substantially decrease the prevalence of obesity. Brown adipose tissue (BAT), the primary source of thermogenesis in infants and small mammals may represent a promising therapeutic target to treat obesity by promoting energy expenditure through non-shivering thermogenesis mediated by mitochondrial uncoupling protein 1 (UCP1). Since the confirmation of functional BAT in adult humans by several groups, approximately a decade ago, and its association with a favorable metabolic phenotype, intense interest on the significance of BAT in adult human physiology and metabolic health has emerged within the scientific community to explore its therapeutic potential for the treatment of obesity and metabolic diseases. A substantially decreased BAT activity in individuals with obesity indicates a role for BAT in the setting of human obesity. On the other hand, BAT mass and its prevalence correlate with lower body mass index (BMI), decreased age and lower glucose levels, leading to a lower incidence of cardio-metabolic diseases. The increased cold exposure in adult humans with undetectable BAT was associated with decreased body fat mass and increased insulin sensitivity. A deeper understanding of the role of BAT in human metabolic health and its interrelationship with body fat distribution and deciphering proper strategies to increase energy expenditure, by either increasing functional BAT mass or inducing white adipose browning, holds the promise for possible therapeutic avenues for the treatment of obesity and associated metabolic disorders.

Sex-specific genetic regulation of adipose mitochondria and metabolic syndrome by Ndufv2

We have previously suggested a central role for mitochondria in the observed sex differences in metabolic traits. However, the mechanisms by which sex differences affect adipose mitochondrial function and metabolic syndrome are unclear. Here we show that in both mice and humans, adipose mitochondrial functions are elevated in females and are strongly associated with adiposity, insulin resistance and plasma lipids. Using a panel of diverse inbred strains of mice, we identify a genetic locus on mouse chromosome 17 that controls mitochondrial mass and function in adipose tissue in a sex- and tissue-specific manner. This locus contains Ndufv2 and regulates the expression of at least 89 mitochondrial genes in females, including oxidative phosphorylation genes and those related to mitochondrial DNA content. Overexpression studies indicate that Ndufv2 mediates these effects by regulating supercomplex assembly and elevating mitochondrial reactive oxygen species production, which generates a signal that increases mitochondrial biogenesis.

A framework for developing sex-specific engineered heart models

The convergence of tissue engineering and patient-specific stem cell biology has enabled the engineering of in vitro tissue models that allow the study of patient-tailored treatment modalities. However, sex-related disparities in health and disease, from systemic hormonal influences to cellular-level differences, are often overlooked in stem cell biology, tissue engineering and preclinical screening. The cardiovascular system, in particular, shows considerable sex-related differences, which need to be considered in cardiac tissue engineering. In this Review, we analyse sex-related properties of the heart muscle in the context of health and disease, and discuss a framework for including sex-based differences in human cardiac tissue engineering. We highlight how sex-based features can be implemented at the cellular and tissue levels, and how sex-specific cardiac models could advance the study of cardiovascular diseases. Finally, we define design criteria for sex-specific cardiac tissue engineering and provide an outlook to future research possibilities beyond the cardiovascular system.

NOTUM promotes thermogenic capacity and protects against diet-induced obesity in male mice

We recently showed that NOTUM, a liver-secreted Wnt inhibitor, can acutely promote browning of white adipose. We now report studies of chronic overexpression of NOTUM in liver indicating that it protects against diet-induced obesity and improves glucose homeostasis in mice. Adeno-associated virus (AAV) vectors were used to overexpress GFP or mouse Notum in the livers of male C57BL/6J mice and the mice were fed an obesifying diet. After 14 weeks of high fat, high sucrose diet feeding, the AAV-Notum mice exhibited decreased obesity and improved glucose tolerance compared to the AAV-GFP mice. Gene expression and immunoblotting analysis of the inguinal fat and brown fat revealed increased expression of beige/brown adipocyte markers in the AAV-Notum group, suggesting enhanced thermogenic capacity by NOTUM. A β3 adrenergic receptor agonist-stimulated lipolysis test suggested increased lipolysis capacity by NOTUM. The levels of collagen and C–C motif chemokine ligand 2 (CCL2) in the epididymal white adipose tissue of the AAV-Notum mice were significantly reduced, suggesting decreased fibrosis and inflammation, respectively. RNA sequencing analysis of inguinal white adipose of 4-week chow diet-fed mice revealed a highly significant enrichment of extracellular matrix (ECM) functional cluster among the down-regulated genes in the AAV-Notum group, suggesting a potential mechanism contributing to improved glucose homeostasis. Our in vitro studies demonstrated that recombinant human NOTUM protein blocked the inhibitory effects of WNT3A on brown adipocyte differentiation. Furthermore, NOTUM attenuated WNT3A’s effects on upregulation of TGF-β signaling and its downstream targets. Overall, our data suggest that NOTUM modulates adipose tissue function by promoting thermogenic capacity and inhibiting fibrosis through inhibition of Wnt signaling.

Characterization of the blood-brain barrier in genetically diverse laboratory mouse strains

Background

Genetic variation in a population has an influence on the manifestation of monogenic as well as multifactorial disorders, with the underlying genetic contribution dependent on several interacting variants. Common laboratory mouse strains used for modelling human disease lack the genetic variability of the human population. Therefore, outcomes of rodent studies show limited relevance to human disease. The functionality of brain vasculature is an important modifier of brain diseases. Importantly, the restrictive interface between blood and brain—the blood–brain barrier (BBB) serves as a major obstacle for the drug delivery into the central nervous system (CNS). Using genetically diverse mouse strains, we aimed to investigate the phenotypic and transcriptomic variation of the healthy BBB in different inbred mouse strains.

Methods

We investigated the heterogeneity of brain vasculature in recently wild-derived mouse strains (CAST/EiJ, WSB/EiJ, PWK/PhJ) and long-inbred mouse strains (129S1/SvImJ, A/J, C57BL/6J, DBA/2J, NOD/ShiLtJ) using different phenotypic arms. We used immunohistochemistry and confocal laser microscopy followed by quantitative image analysis to determine vascular density and pericyte coverage in two brain regions—cortex and hippocampus. Using a low molecular weight fluorescence tracer, sodium fluorescein and spectrophotometry analysis, we assessed BBB permeability in young and aged mice of selected strains. For further phenotypic characterization of endothelial cells in inbred mouse strains, we performed bulk RNA sequencing of sorted endothelial cells isolated from cortex and hippocampus.

Results

Cortical vessel density and pericyte coverage did not differ among the investigated strains, except in the cortex, where PWK/PhJ showed lower vessel density compared to NOD/ShiLtJ, and a higher pericyte coverage than DBA/2J. The vascular density in the hippocampus differed among analyzed strains but not the pericyte coverage. The staining patterns of endothelial arteriovenous zonation markers were similar in different strains. BBB permeability to a small fluorescent tracer, sodium fluorescein, was also similar in different strains, except in the hippocampus where the CAST/EiJ showed higher permeability than NOD/ShiLtJ. Transcriptomic analysis of endothelial cells revealed that sex of the animal was a major determinant of gene expression differences. In addition, the expression level of several genes implicated in endothelial function and BBB biology differed between wild-derived and long-inbred mouse strains. In aged mice of three investigated strains (DBA/2J, A/J, C57BL/6J) vascular density and pericyte coverage did not change—expect for DBA/2J, whereas vascular permeability to sodium fluorescein increased in all three strains.

Conclusions

Our analysis shows that although there were no major differences in parenchymal vascular morphology and paracellular BBB permeability for small molecular weight tracer between investigated mouse strains or sexes, transcriptomic differences of brain endothelial cells point to variation in gene expression of the intact BBB. These baseline variances might be confounding factors in pathological conditions that may lead to a differential functional outcome dependent on the sex or genetic polymorphism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00269-w.

Male but not female mice with severe osteogenesis imperfecta are partially protected from high-fat diet-induced obesity

Previously we have shown that young mice with a dominant severe form of osteogenesis imperfecta (OI), caused by mutated collagen type I, exhibit an altered glucose/insulin metabolism and energy expenditure along with elevated levels of osteocalcin, a bone-derived hormone involved in the regulation of whole-body metabolism. This study aimed to examine the long-term effects of a western diet in these OI mice. Male and female OI mice and wild type littermates (WT) were fed a high-fat diet (HFD) or a matched low-fat diet (LFD) for 26 weeks. HFD-induced obesity was observed in male and female WT and female OI mice, but not in male OI mice. HFD-fed WT and OI mice of both sexes developed hyperglycemia and glucose intolerance, but the degree of glucose intolerance was significantly lower in male and female OI mice compared to sex- and diet-matched WT mice. Indirect calorimetry revealed increased movement of male OI mice on HFD compared to LFD and, while HFD lowered energy expenditure in WT mice, energy expenditure was not changed in OI mice. Further, HFD-fed male OI mice demonstrated a diet-induced increased expression of the thermogenesis genes, Ucp1 and Pgc1α, in brown adipose tissue. On LFD, total and Gla-13 osteocalcin levels were similar in 30-week-old WT and OI mice, but on HFD, both were significantly higher in OI mice than WT. Thus, male OI mice respond to HFD with increased movement, energy expenditure, brown adipose tissue thermogenesis, and higher levels of osteocalcin, resulting in partial protection against HFD-induced obesity.