Sex differences are pervasive in metabolic and cardiovascular traits, yet they have often been ignored in human and animal model research. Sex differences can arise from reversible hormonal effects, ...from irreversible organizational (developmental) processes, and from gene expression differences from the X and Y chromosomes. We briefly review our current understanding of the impact of these factors in metabolic traits and disorders, with an emphasis on the recent literature.
Novel sex differences continue to be identified for metabolic and cardiovascular traits. For example, it is now clear that gut microbiota tend to differ between men and women, with potentially large implications for disease susceptibility. Also, tissue-specific gene regulation differs between men and women, contributing to differential metabolism. These new insights will open up personalized therapeutic avenues for cardiometabolic diseases.
Sex differences in body fat distribution, glucose homeostasis, insulin signaling, ectopic fat accumulation, and lipid metabolism during normal growth and in response to hormonal or nutritional imbalance are mediated partly through sex hormones and the sex chromosome complement. Most of these differences are mediated in a tissue-specific manner. Important future goals are to better understand the interactions between genetic variation and sex differences, and to bring an understanding of sex differences into clinical practice.
We studied sex differences in over 50 cardio-metabolic traits in a panel of 100 diverse inbred strains of mice. The results clearly showed that the effects of sex on both clinical phenotypes and gene ...expression depend on the genetic background. In support of this, genetic loci associated with the traits frequently showed sex specificity. For example, Lyplal1, a gene implicated in human obesity, was shown to underlie a sex-specific locus for diet-induced obesity. Global gene expression analyses of tissues across the panel implicated adipose tissue “beiging” and mitochondrial functions in the sex differences. Isolated mitochondria showed gene-by-sex interactions in oxidative functions, such that some strains (C57BL/6J) showed similar function between sexes, whereas others (DBA/2J and A/J) showed increased function in females. Reduced adipose mitochondrial function in males as compared to females was associated with increased susceptibility to obesity and insulin resistance. Gonadectomy studies indicated that gonadal hormones acting in a tissue-specific manner were responsible in part for the sex differences.
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•Sex differences in phenotype and gene expression depend upon genetic background•The gene Lypla1 impacts obesity in a sex-specific manner•Functional analyses reveal sex differences for adipose tissue beiging•Adipose mitochondrial function depends upon gene-by-sex interactions
Norheim and colleagues use the hybrid mouse diversity panel to comprehensively address the role of sex, and its interaction with genetic background, on cardio-metabolic phenotypes and gene expression. They discover a sex-specific obesity locus for Lyplal1, reveal sex-specific regulation of adipose tissue beiging, and find gene-by-sex interactions for mitochondrial function.
Elevated hepatic ceramide levels have been implicated in both insulin resistance (IR) and hepatic steatosis. To understand the factors contributing to hepatic ceramide levels in mice of both sexes, ...we have quantitated ceramides in a reference population of mice, the Hybrid Mouse Diversity Panel that has been previously characterized for a variety of metabolic syndrome traits. We observed significant positive correlations between Cer(d18:1/16:0) and IR/hepatic steatosis, consistent with previous findings, although the relationship broke down between sexes, as females were less insulin resistant, but had higher Cer(d18:1/16:0) levels than males. The sex difference was due in part to testosterone-mediated repression of ceramide synthase 6. One ceramide species, Cer(d18:1/20:0), was present at higher levels in males and was associated with IR only in males. Clear evidence of gene-by-sex and gene-by-diet interactions was observed, including sex-specific genome-wide association study results. Thus, our studies show clear differences in how hepatic ceramides are regulated between the sexes, which again suggests that the physiological roles of certain hepatic ceramides differ between the sexes.
Host genetic variations play an important role in several pathogenic diseases, and we have previously provided strong evidences that these genetic variations contribute significantly to differences ...in susceptibility and clinical outcomes of invasive Group A Streptococcus (GAS) infections, including sepsis and necrotizing soft tissue infections (NSTIs). Our initial studies with conventional mouse strains revealed that host genetic variations and sex differences play an important role in orchestrating the severity, susceptibility and outcomes of NSTIs. To understand the complex genetic architecture of NSTIs, we utilized an unbiased, forward systems genetics approach in an advanced recombinant inbred (ARI) panel of mouse strains (BXD). Through this approach, we uncovered interactions between host genetics, and other non-genetic cofactors including sex, age and body weight in determining susceptibility to NSTIs. We mapped three NSTIs-associated phenotypic traits (i.e., survival, percent weight change, and lesion size) to underlying host genetic variations by using the WebQTL tool, and identified four NSTIs-associated quantitative genetic loci (QTL) for survival on mouse chromosome (Chr) 2, for weight change on Chr 7, and for lesion size on Chr 6 and 18 respectively. These QTL harbor several polymorphic genes. Identification of multiple QTL highlighted the complexity of the host-pathogen interactions involved in NSTI pathogenesis. We then analyzed and rank-ordered host candidate genes in these QTL by using the QTLminer tool and then developed a list of 375 candidate genes on the basis of annotation data and biological relevance to NSTIs. Further differential expression analyses revealed 125 genes to be significantly differentially regulated in susceptible strains compared to their uninfected controls. Several of these genes are involved in innate immunity, inflammatory response, cell growth, development and proliferation, and apoptosis. Additional network analyses using ingenuity pathway analysis (IPA) of these 125 genes revealed interleukin-1 beta network as key network involved in modulating the differential susceptibility to GAS NSTIs.
The etiology of non-alcoholic fatty liver disease (NAFLD), the most common form of chronic liver disease, is poorly understood. To understand the causal mechanisms underlying NAFLD, we conducted a ...multi-omics, multi-tissue integrative study using the Hybrid Mouse Diversity Panel, consisting of ∼100 strains of mice with various degrees of NAFLD. We identified both tissue-specific biological processes and processes that were shared between adipose and liver tissues. We then used gene network modeling to predict candidate regulatory genes of these NAFLD processes, including Fasn, Thrsp, Pklr, and Chchd6. In vivo knockdown experiments of the candidate genes improved both steatosis and insulin resistance. Further in vitro testing demonstrated that downregulation of both Pklr and Chchd6 lowered mitochondrial respiration and led to a shift toward glycolytic metabolism, thus highlighting mitochondria dysfunction as a key mechanistic driver of NAFLD.
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•NAFLD was modeled in a population of ∼100 diverse inbred strains of mice•Network modeling was used to predict key driver genes regulating NAFLD•In vivo knockdown of these genes rescued from steatosis and insulin resistance•In vitro knockdown of Pklr or Chchd6 shifted toward glycolytic metabolism
Chella Krishnan et al. apply integrative genetics approaches to delineate “key driver” genes regulating NAFLD using multi-omics data from ∼100 mouse strains. In vivo modulation of these genes rescued animals from steatosis and insulin resistance. Follow-up bioenergetics studies highlight mitochondrial dysfunction as a key mechanistic driver of NAFLD.
To elucidate the contributions of specific lipid species to metabolic traits, we integrated global hepatic lipid data with other omics measures and genetic data from a cohort of about 100 diverse ...inbred strains of mice fed a high‐fat/high‐sucrose diet for 8 weeks. Association mapping, correlation, structure analyses, and network modeling revealed pathways and genes underlying these interactions. In particular, our studies lead to the identification of Ifi203 and Map2k6 as regulators of hepatic phosphatidylcholine homeostasis and triacylglycerol accumulation, respectively. Our analyses highlight mechanisms for how genetic variation in hepatic lipidome can be linked to physiological and molecular phenotypes, such as microbiota composition.
Synopsis
A survey of hepatic lipidome in a cohort of genetically diverse mice on high‐fat/high‐sucrose diet identifies novel modulators of liver lipid metabolism in the context of hepatic steatosis.
The association between lipids, gene expression and metabolic traits is mapped.
Network modeling identifies co‐regulated modules of hepatic lipids.
Association mapping prioritizes potential drivers of hepatic lipid metabolism.
Identification and validation of Ifi203 and Map2k6 as regulators of specific hepatic lipids.
A survey of hepatic lipidome in a cohort of genetically diverse mice on high‐fat/high‐sucrose diet identifies novel modulators of liver lipid metabolism in the context of hepatic steatosis.
Host genetic variations play an important role in several pathogenic diseases, and we previously provided strong evidence that these genetic variations contribute significantly to differences in ...susceptibility and clinical outcomes of invasive group A Streptococcus (GAS) patients, including sepsis and necrotizing soft tissue infections (NSTIs). The goal of the present study was to investigate how genetic variations and sex differences among four commonly used mouse strains contribute to variation in severity, manifestations, and outcomes of NSTIs. DBA/2J mice were more susceptible to NSTIs than C57BL/6J, BALB/c, and CD-1 mice, as exhibited by significantly greater bacteremia, excessive dissemination to the spleen, and significantly higher mortality. Differences in the sex of the mice also contributed to differences in disease severity and outcomes: DBA/2J female mice were relatively resistant compared to their male counterparts. However, DBA/2J mice exhibited minimal weight loss and developed smaller lesions than did the aforementioned strains. Moreover, at 48 h after infection, compared with C57BL/6J mice, DBA/2J mice had increased bacteremia, excessive dissemination to the spleen, and excessive concentrations of inflammatory cytokines and chemokines. These results indicate that variations in the host genetic context as well as sex play a dominant role in determining the severity of and susceptibility to GAS NSTIs.
Sympathetic activation during cold exposure increases adipocyte thermogenesis via the expression of mitochondrial protein uncoupling protein 1 (UCP1)
. The propensity of adipocytes to express UCP1 is ...under a critical influence of the adipose microenvironment and varies between sexes and among various fat depots
. Here we report that mammary gland ductal epithelial cells in the adipose niche regulate cold-induced adipocyte UCP1 expression in female mouse subcutaneous white adipose tissue (scWAT). Single-cell RNA sequencing shows that glandular luminal epithelium subtypes express transcripts that encode secretory factors controlling adipocyte UCP1 expression under cold conditions. We term these luminal epithelium secretory factors 'mammokines'. Using 3D visualization of whole-tissue immunofluorescence, we reveal sympathetic nerve-ductal contact points. We show that mammary ducts activated by sympathetic nerves limit adipocyte UCP1 expression via the mammokine lipocalin 2. In vivo and ex vivo ablation of mammary duct epithelium enhance the cold-induced adipocyte thermogenic gene programme in scWAT. Since the mammary duct network extends throughout most of the scWAT in female mice, females show markedly less scWAT UCP1 expression, fat oxidation, energy expenditure and subcutaneous fat mass loss compared with male mice, implicating sex-specific roles of mammokines in adipose thermogenesis. These results reveal a role of sympathetic nerve-activated glandular epithelium in adipocyte UCP1 expression and suggest that mammary duct luminal epithelium has an important role in controlling glandular adiposity.
Although the role of hydrophilic antioxidants in the development of hepatic insulin resistance and nonalcoholic fatty liver disease has been well studied, the role of lipophilic antioxidants remains ...poorly characterized. A known lipophilic hydrogen peroxide scavenger is bilirubin, which can be oxidized to biliverdin and then reduced back to bilirubin by cytosolic biliverdin reductase. Oxidation of bilirubin to biliverdin inside mitochondria must be followed by the export of biliverdin to the cytosol, where biliverdin is reduced back to bilirubin. Thus, the putative mitochondrial exporter of biliverdin is expected to be a major determinant of bilirubin regeneration and intracellular hydrogen peroxide scavenging. Here, we identified ABCB10 as a mitochondrial biliverdin exporter. ABCB10 reconstituted into liposomes transported biliverdin, and ABCB10 deletion caused accumulation of biliverdin inside mitochondria. Obesity with insulin resistance up-regulated hepatic ABCB10 expression in mice and elevated cytosolic and mitochondrial bilirubin content in an ABCB10-dependent manner. Revealing a maladaptive role of ABCB10-driven bilirubin synthesis, hepatic ABCB10 deletion protected diet-induced obese mice from steatosis and hyperglycemia, improving insulin-mediated suppression of glucose production and decreasing lipogenic SREBP-1c expression. Protection was concurrent with enhanced mitochondrial function and increased inactivation of PTP1B, a phosphatase disrupting insulin signaling and elevating SREBP-1c expression. Restoration of cellular bilirubin content in ABCB10 KO hepatocytes reversed the improvements in mitochondrial function and PTP1B inactivation, demonstrating that bilirubin was the maladaptive effector linked to ABCB10 function. Thus, we identified a fundamental transport process that amplifies intracellular bilirubin redox actions, which can exacerbate insulin resistance and steatosis in obesity.
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