Brown adipose tissue is the key depot for glucose clearance in microbiota depleted mice

Brown adipose tissue (BAT) is primarily known for its role in nonshivering thermogenesis, generating heat to maintain body temperature. Uncoupling protein 1 (UCP1) is crucial in this process, facilitating heat production by uncoupling oxidative phosphorylation. While UCP1-null mice can survive cold exposure, suggesting other mechanisms exist, BAT also plays a significant role in glucose homeostasis. Cold stimulation or BAT transplantation improves glucose tolerance and insulin sensitivity, often attributed to thermogenesis. However, the relationship between glucose uptake in BAT and thermogenesis remains unclear. Early research showed germ-free (GF) mice exhibit improved glucose homeostasis compared to conventionalized mice, suggesting a role for gut microbiota in blood glucose regulation. Subsequent studies confirmed that microbiota depletion improves glucose tolerance, but the underlying mechanisms are uncertain. Hypotheses included browning of white adipose tissue (WAT), altered hepatic gluconeogenesis, or changes in endocrine secretion. However, these studies primarily focused on WAT and liver, largely neglecting BAT's potential contribution. Conflicting results exist regarding the impact of microbiota depletion on BAT thermogenesis, with some studies suggesting impairment while others report no effect. A recent study even questioned the overall effect of microbiota depletion on glucose tolerance. This study aimed to resolve these discrepancies by (1) re-evaluating the impact of gut microbiota depletion on glucose clearance, (2) determining if the improved clearance is secondary to adaptive thermogenesis, and (3) defining BAT's contribution to glucose uptake modulation in microbiota-deficient mice.

Existing literature demonstrates a complex interplay between gut microbiota and host metabolism. Studies using antibiotic treatment or germ-free models consistently show improved glucose tolerance in microbiota-depleted mice. However, the specific tissues responsible for the enhanced glucose uptake have remained elusive. Several studies implicated the browning of WAT, hepatic gluconeogenesis, or altered endocrine secretion as potential mechanisms. The role of BAT, although recognized as a major glucose utilizer, has been largely understudied in this context. Conflicting evidence surrounds the impact of microbiota depletion on BAT thermogenesis and UCP1 expression, highlighting the need for further investigation to clarify the exact mechanisms involved.

This study employed three mouse models: antibiotic cocktail-treated (ABX) mice, specific-pathogen-free (SPF) mice, and Ucp1 knockout (KO) mice. Mice were fed low-fat diets (LFD) or high-fat diets (HFD). Glucose tolerance tests (GTT) were performed to assess glucose clearance. To determine glucose uptake in specific tissues, 13C-labeled glucose and 2-deoxyglucose (2DG) were administered, followed by tissue analysis to measure 13C enrichment. Oxygen consumption and respiratory exchange ratios (RER) were measured via indirect calorimetry to assess energy expenditure and substrate utilization. Ucp1-DTR mice were used to selectively delete Ucp1+ cells via diphtheria toxin (DT) injection to evaluate the role of UCP1 in the observed effects. Statistical analyses, including two-way ANOVA and Bonferroni correction, were used to analyze the data.

The study confirmed that antibiotic-mediated microbiota depletion improved glucose clearance in mice. Isotope tracing experiments revealed that BAT, rather than WAT or liver, exhibited significantly enhanced glucose uptake in ABX mice. Cold exposure further increased glucose uptake in BAT and heart. Importantly, deleting UCP1-expressing cells completely blocked the improved glucose clearance observed in ABX mice, demonstrating that BAT's contribution to improved glucose metabolism is independent of adaptive thermogenesis mediated by UCP1. Analysis of Ucp1-KO mice showed that microbiota depletion did not significantly affect UCP1-independent thermogenesis. Experiments with HFD-fed Ucp1-KO mice further confirmed that the improvement in glucose clearance induced by microbiota depletion persists even in the absence of UCP1, and that the effect was independent of total body weight, daily food intake, or UCP1 expression. In obese Ucp1-DTR mice, depletion of Ucp1+ cells impaired glucose uptake into BAT, underscoring BAT's key role in microbiota-depletion-induced glucose clearance improvement.

This study demonstrates that BAT is the primary site of enhanced glucose uptake in microbiota-depleted mice, independently of UCP1-mediated adaptive thermogenesis. These findings challenge the prevailing notion that the beneficial effects of microbiota depletion on glucose metabolism are solely dependent on increased thermogenesis. The results reveal a novel, UCP1-independent mechanism for glucose regulation in BAT, potentially opening new avenues for therapeutic interventions targeting BAT to improve glucose homeostasis. The observed increased glucose uptake in the cecum suggests that altered gut metabolism also plays a role in the overall process. Future studies should investigate the precise mechanisms underlying UCP1-independent glucose utilization in BAT and the potential crosstalk between the gut and BAT.

This study definitively establishes brown adipose tissue (BAT) as the key depot responsible for the improved glucose clearance observed in gut microbiota-depleted mice. This effect is independent of adaptive thermogenesis and UCP1 expression, highlighting a novel, UCP1-independent mechanism of glucose regulation in BAT. These findings provide important insights into the complex interplay between gut microbiota, BAT function, and glucose homeostasis, opening up potential avenues for therapeutic strategies targeting BAT for improved metabolic health.

Several limitations should be considered. First, although 13C glucose metabolism was measured at a single time point, further investigations are needed to determine glucose uptake at different time points and how BAT utilizes the glucose. Secondly, only 13C-2DG was used to measure glucose uptake; it's possible 2DG might not precisely reflect actual glucose distribution. Thirdly, the experiments were performed only in C57BL/6J mice, limiting the generalizability of the findings to other mouse strains or species. Future studies should address these limitations to provide a more comprehensive understanding of the gut-BAT axis in regulating glucose metabolism.