Sam Virtue

Metabolic flexibility, the capacity to switch between carbohydrate and lipid fuels, is fundamental to metabolic health, yet its genetic regulation has not been systematically explored in vivo . To address this gap, we developed a high-throughput physiological screen in mice using the respiratory exchange ratio range (ΔRER) as quantitative index of metabolic flexibility. Screening over 800 knockout lines identified Cyb561, a vesicular cytochrome required for catecholamine biosynthesis, as an unexpected regulator of systemic substrate utilisation.Mice lacking Cyb561 exhibited impaired lipolysis, blunted brown adipose thermogenesis, and delayed lipid clearance, while maintaining preserved glucose tolerance. Under high-fat feeding or at thermoneutrality (28°C), Cyb561 deficiency exacerbated insulin resistance, revealing metabolic vulnerability. Remarkably, despite impaired lipolysis and BAT dysfunction, Cyb561 knockout mice did not develop increased adiposity. Mechanistically, we demonstrate that catecholamines transcriptionally couple lipid uptake and release in white adipose tissue, thereby sustaining lipid turnover and maintaining systemic metabolic flexibility.Together, these findings establish catecholamine tone as a key determinant of whole-body fuel switching and demonstrate that high-throughput ΔRER phenotyping offers a powerful and sensitive approach for large-scale analysis of the genetic determinants of metabolic flexibility.

Recent work has shown that the prevalence and character of metabolic diseases differs between male and female mammals. This strongly suggests that the control mechanisms that govern, for example lipid metabolism, differ between the sexes. If true, a one-size-fits-all approach to treating metabolic disease will not be effective in all patients. We tested three hypotheses to understand how the lipid metabolism of male and female mammals may differ. First, whether endogenous fatty acid biosynthesis differed between tissues in the same male and female mice. Second, whether the system-level control of lipid pathways differed between the sexes. Third, whether lipid composition differs between males and females at a population level. We found that fatty acid biosynthesis was distinct in male and female mice across tissues. Systemic control of phospholipid and triglyceride metabolism also differed between the sexes. A human population showed that both phospholipid and triglyceride metabolism differed between males and females.

Lipid metabolism is recognised as being central to growth, disease and health. Lipids, therefore, have an important place in current research on globally significant topics such as food security and biodiversity loss. However, answering questions in these important fields of research requires not only identification and measurement of lipids in a wider variety of sample types than ever before, but also hypothesis-driven analysis of the resulting ‘big data’. We present a novel pipeline that can collect data from a wide range of biological sample types, taking 1 000 000 lipid measurements per 384 well plate, and analyse the data systemically. We provide evidence of the power of the tool through proof-of-principle studies using edible fish (mackerel, bream, seabass) and colonies of Bombus terrestris. Bee colonies were found to be more like mini-ecosystems and there was evidence for considerable changes in lipid metabolism in bees through key developmental stages. This is the first report of either high throughput LCMS lipidomics or systemic analysis in individuals, colonies and ecosystems. This novel approach provides new opportunities to analyse metabolic systems at different scales at a level of detail not previously feasible, to answer research questions about societally important topics.

The in vitro oxygen microenvironment profoundly affects the capacity of cell cultures to model physiological and pathophysiological states. Cell culture is often considered to be hyperoxic, but pericellular oxygen levels, which are affected by oxygen diffusivity and consumption, are rarely reported. Here, we provide evidence that several cell types in culture actually experience local hypoxia, with important implications for cell metabolism and function. We focused initially on adipocytes, as adipose tissue hypoxia is frequently observed in obesity and precedes diminished adipocyte function. Under standard conditions, cultured adipocytes are highly glycolytic and exhibit a transcriptional profile indicative of physiological hypoxia. Increasing pericellular oxygen diverted glucose flux toward mitochondria, lowered HIF1α activity, and resulted in widespread transcriptional rewiring. Functionally, adipocytes increased adipokine secretion and sensitivity to insulin and lipolytic stimuli, recapitulating a healthier adipocyte model. The functional benefits of increasing pericellular oxygen were also observed in macrophages, hPSC-derived hepatocytes and cardiac organoids. Our findings demonstrate that oxygen is limiting in many terminally-differentiated cell types, and that considering pericellular oxygen improves the quality, reproducibility and translatability of culture models.

Mitochondrial dysfunction has been reported in obesity and insulin resistance, but primary genetic mitochondrial dysfunction is generally not associated with these, arguing against a straightforward causal relationship. A rare exception, recently identified in humans, is a syndrome of lower body adipose loss, leptin-deficient severe upper body adipose overgrowth, and insulin resistance caused by the p.Arg707Trp mutation in MFN2, encoding mitofusin 2. How the resulting selective form of mitochondrial dysfunction leads to tissue- and adipose depot-specific growth abnormalities and systemic biochemical perturbation is unknown. To address this, Mfn2^R707W/R707W knock-in mice were generated and phenotyped on chow and high fat diets. Electron microscopy revealed adipose-specific mitochondrial morphological abnormalities. Oxidative phosphorylation measured in isolated mitochondria was unperturbed, but the cellular integrated stress response was activated in adipose tissue. Fat mass and distribution, body weight, and systemic glucose and lipid metabolism were unchanged, however serum leptin and adiponectin concentrations, and their secretion from adipose explants were reduced. Pharmacological induction of the integrated stress response in wild-type adipocytes also reduced secretion of leptin and adiponectin, suggesting an explanation for the in vivo findings. These data suggest that the p.Arg707Trp MFN2 mutation selectively perturbs mitochondrial morphology and activates the integrated stress response in adipose tissue. In mice, this does not disrupt most adipocyte functions or systemic metabolism, whereas in humans it is associated with pathological adipose remodelling and metabolic disease. In both species, disproportionate effects on leptin secretion may relate to cell autonomous induction of the integrated stress response.

Cell culture, the workhorse of biomedical research, is generally considered to be hyperoxic. However, oxygen consumption by cells is underappreciated. High cellular respiration rates can rapidly deplete oxygen, resulting in local hypoxia. Increasing pericellular oxygen levels rewired the metabolism of multiple post-mitotic cell-lines, both in monolayer and organoid culture. Under standard conditions, cultured adipocytes are hypoxic and highly glycolytic. Increased oxygen availability diverted glucose flux toward mitochondria and increased lipogenesis from glucose-derived carbon. These metabolic changes were coupled to thousands of gene expression changes, and rendered adipocytes more sensitive to insulin and lipolytic stimuli. Importantly, pathway analyses revealed increasing oxygen tension made in vitro adipocytes more similar to in vivo adipose tissue. hPSC-derived hepatocytes and cardiac organoids were also functionally enhanced by increased oxygen. Our findings suggest that oxygen is limiting in many standard cell culture systems, and highlight how controlling oxygen availability can improve translatability of cell models.

Mitochondrial dysfunction has been reported in obesity and insulin resistance, but primary genetic mitochondrial dysfunction is generally not associated with these, arguing against a straightforward causal relationship. A rare exception, recently identified in humans, is a syndrome of lower body adipose loss, leptin-deficient severe upper body adipose overgrowth, and insulin resistance caused by the p.Arg707Trp mutation in MFN2 , encoding mitofusin-2. How this selective perturbation of mitochondrial function leads to tissue- and adipose depot-specific growth abnormalities and systemic biochemical perturbation is unknown. To address this, Mfn2 ^R707W/R707W knock-in mice were generated and phenotyped on chow and high fat diets. Electron microscopy revealed adipose-specific mitochondrial morphological abnormalities. Oxidative phosphorylation by isolated mitochondria was unperturbed, but the cellular integrated stress response was activated in adipose tissue. Fat mass and distribution, body weight, and systemic glucose and lipid metabolism were unchanged, however serum leptin and adiponectin concentrations, and their secretion from adipose explants were reduced. Pharmacological induction of the integrated stress response in wild-type adipocytes also reduced secretion of leptin and adiponectin, suggesting an explanation for the in vivo findings. These data suggest that the p.Arg707Trp MFN2 mutation perturbs mitochondrial morphology and activates the integrated stress response selectively in adipose tissue. In mice, this does not disrupt most adipocyte functions or systemic metabolism, whereas in humans it is associated with pathological adipose remodelling and metabolic disease. In both species, disproportionate effects on leptin secretion may relate to cell autonomous induction of the integrated stress response.

Summary: Obesity in mice and humans is associated with elevated levels of at least two hormones responsive to cellular stress, namely GDF15 and FGF21. Over-expression of each of these is associated with weight loss and beneficial metabolic changes but where they are secreted from and what they are required for physiologically in the context of overfeeding remains unclear. Here we used tissue selective knockout mouse models to establish that, like FGF21, circulating GDF15 is primarily derived from the liver, rather than adipose tissue, muscle or macrophages in high fat fed mice. Combined whole body deletion of FGF21 and GDF15 does not result in any additional weight gain in high fat fed mice but is associated with significantly greater hepatic steatosis and insulin resistance. Collectively the data suggest that activation of the integrated stress response in hepatocytes is a major driver for GDF15 and FGF21 secretion in the context of overfeeding, and that they both act to alleviate this metabolic stress.