IgM-associated gut bacteria in obesity and type 2 diabetes in C57BL/6 mice and humans

Abstract: Aims/hypothesis: IgM is the primary antibody produced by B cells and we hypothesise that IgM antibodies to gut microbiota may play a role in immunometabolism in obesity and type 2 diabetes. To test our hypothesis, we used B6 mice deficient in activation-induced cytidine deaminase (Aid−/− [also known as Aicda−/−]) which secrete only IgM antibodies, and human faecal samples. Methods: We studied the immunometabolic effects and gut microbial changes in high-fat-diet-induced obesity (HFDIO) in Aid−/− B6 mice compared with wild-type mice. To determine similarities between mice and humans, human stool samples were collected from children and adolescents who were obese with normal glucose tolerance (NGT), obese with glucose intolerance (IGT), or obese and newly diagnosed with type 2 diabetes, for faecal microbiota transplant (FMT) into germ-free (GF) B6 mice and we assessed IgM-bound bacteria and immune responses. Results: Compared with wild-type mice, Aid−/− B6 mice developed exacerbated HFDIO due to abundant levels of IgM. FMT from Aid−/− B6 to GF B6 mice promoted greater weight gain in recipient mice compared with FMT using wild-type mouse faecal microbiota. Obese youth with type 2 diabetes had more IgM-bound gut bacteria. Using the stools from the obese youth with type 2 diabetes for FMT to GF B6 mice, we observed that the gut microbiota promoted body weight gain and impaired glucose tolerance in the recipient GF B6 mice. Importantly, some clinical features of these obese young individuals were mirrored in the GF B6 mice following FMT. Conclusions/interpretation: Our results suggest that IgM-bound gut microbiota may play an important role in the immuno-pathogenesis of obesity and type 2 diabetes, and provide a novel link between IgM in obesity and type 2 diabetes in both mice and humans. Data availability: The 16s rRNA sequencing datasets supporting the current study have been deposited in the NCBI SRA public repository (https://www.ncbi.nlm.nih.gov/sra; accession no. SAMN18796639). Graphical abstract

Insights from shotgun metagenomics into bacterial species and metabolic pathways associated with NAFLD in obese youth

Abstract Nonalcoholic fatty liver disease (NAFLD) is the most common form of liver disease and is often the precursor for more serious liver conditions such as nonalcoholic steatohepatitis and cirrhosis. Although the gut microbiome has been implicated in the development of NAFLD, the strong association of obesity with NAFLD and its effect on microbiome structure has made interpreting study outcomes difficult. In the present study, we examined the taxonomic and functional differences between the microbiomes of youth with obesity and with and without NAFLD. Shotgun metagenome sequencing was performed to profile the microbiomes of 36 subjects, half of whom were diagnosed with NAFLD using abdominal magnetic resonance imaging. Beta diversity analysis showed community‐wide differences between the groups (p = 0.002). Specific taxonomic differences included increased relative abundances of the species Fusicatenibacter saccharivorans (p = 0.042), Romboutsia ilealis (p = 0.046), and Actinomyces sp. ICM47 (p = 0.0009), and a decrease of Bacteroides thetaiotamicron (p = 0.0002), in the NAFLD group as compared with the non‐NAFLD group. At the phylum level, Bacteroidetes (p < 0.0001) was decreased in the NAFLD group. Functionally, branched‐chain amino acid (p = 0.01343) and aromatic amino acid (p = 0.01343) synthesis pathways had increased relative abundances in the NAFLD group along with numerous energy use pathways, including pyruvate fermentation to acetate (p = 0.01318). Conclusion: Community‐wide differences were noted based on NAFLD status, and individual bacterial species along with specific metabolic pathways were identified as potential drivers of these differences. The results of the present study support the idea that the NAFLD phenotype displays a differentiated microbial and functional signature from the obesity phenotype

978-P: Rare Variants in Melanocortin 4 Receptor Gene (MC4R) Are Associated with Increased Visceral Fat and Altered Glucose Metabolism Independent of the Effect of Obesity in Children

Melanocortin 4 receptor gene (MC4R) mutations result in early-onset obesity, but it is unclear how they affect abdominal fat distribution, intrahepatic fat, and related metabolic sequelae. 484 overweight/obese (BMI &gt;85th percentile for age, sex, and height) youth (6-21 years) were screened for functionally damaging, rare variants (minor allele frequency &lt;0.01) in the coding region of MC4R and were assigned to a Pathogenic Variant (n=10) or No Variant (n=474) group. Participants underwent MC4R sequencing, abdominal MRI, and a 180-minute oral glucose tolerance test (OGTT) with mathematical modeling of insulin kinetics and β cell function. Compared to No Variant group, the Pathogenic Variant group showed greater visceral fat (Figure 1A) , intrahepatic fat (Figure 1B) , and higher plasma glucose (Figure 1C) and insulin (Figure 1D) during the OGTT, as well as a delayed glucose peak (65.4 ± 3.71 vs. 58.8 ± 0.417 minutes; P=0.036) , insulin resistance (WBISI: 0.9± 0.163 vs. 1.82 ± 0.051; P=0.0006) , and lower insulin clearance (0.441 ± 0.065 vs. 0.6± 0.012 µU/mL/min; P=0.033) despite similar BMI z-scores (P=0.189) and body fat percentage (P=0.704) between groups. These results show that rare variants in MC4R are associated with increased visceral fat, intrahepatic fat, and insulin resistance, independent from the effect of obesity

Intrahepatic fat, irrespective of ethnicity, is associated with reduced endogenous insulin clearance and hepatic insulin resistance in obese youths: a cross-sectional and longitudinal study from the Yale Pediatric NAFLD cohort

Non-Alcoholic Fatty Liver Disease (NAFLD) has been associated with reduced endogenous insulin clearance (EIC) and hepatic insulin resistance (HIRI). These relationships, however, might be differentially affected by the ethnic background, as populations of African ancestry are typically featured with reduced intrahepatic fat content (HFF%) but impaired EIC. Therefore, we evaluated the influence of the ethnicity on the relationships between HFF%, EIC and HIRI

A Reduced Incretin Effect Mediated by the rs7903146 Variant in the TCF7L2 Gene Is an Early Marker of β-Cell Dysfunction in Obese Youth

The risk genotype for the common variant rs7903146 of the transcription factor 7-like-2 (TCF7L2) gene has been found to affect the incretin response in healthy and obese adults; however, whether a similar functional defect is also present in obese adolescents remains unexplored. Herein, we examined the functional effect of the rs7903146 variant in the TCF7L2 gene on the incretin effect and determined its translational metabolic manifestation by performing deep phenotyping of the incretin system, β-cell function relative to insulin sensitivity, the gastrointestinal-induced glucose disposal (GIGD) in obese youth with normal and impaired glucose tolerance

Altered In Vivo Lipid Fluxes and Cell Dynamics in Subcutaneous Adipose Tissues Are Associated With the Unfavorable Pattern of Fat Distribution in Obese Adolescent Girls.

Patterns of abdominal fat distribution (for example, a high vs. low visceral adipose tissue [VAT]/[VAT + subcutaneous adipose tissue (SAT)] ratio), independent of obesity, during adolescence carry a high risk for insulin resistance and type 2 diabetes. Longitudinal follow-up of a cohort of obese adolescents has recently revealed that a high ratio (high VAT/[VAT + SAT]) is a major determinant of fatty liver and metabolic impairment over time, with these effects being more pronounced in girls than in boys. To unravel the underlying metabolic alterations associated with the unfavorable VAT/(VAT + SAT) phenotype, we used the 2H2O labeling method to measure the turnover of adipose lipids and cells in the subcutaneous abdominal and gluteal/femoral adipose tissue (SAT) of weight-stable obese adolescent girls with a similar level of obesity but discordant VAT/(VAT + SAT) ratios. Girls with the unfavorable (high VAT/[VAT + SAT]) phenotype exhibited higher in vivo rates of triglyceride (TG) turnover (representing both lipolysis and synthesis at steady state), without significant differences in de novo lipogenesis in both abdominal and gluteal depots, compared with obese girls with the favorable phenotype. Moreover, mature adipocytes had higher turnover, with no difference in stromal vascular cell proliferation in both depots in the metabolically unfavorable phenotype. The higher TG turnover rates were significantly correlated with higher intrahepatic fat stores. These findings are contrary to the hypothesis that impaired capacity to deposit TGs or proliferation of new mature adipocytes are potential mechanisms for ectopic fat distribution in this setting. In summary, these results suggest that increased turnover of TGs (lipolysis) and of mature adipocytes in both abdominal and gluteal SAT may contribute to metabolic impairment and the development of fatty liver, even at this very early stage of disease

A low n-6 to n-3 polyunsaturated fatty acid ratio diet improves hyperinsulinemia by restoring insulin clearance in obese youth

Aim: A low n-6 to n-3 polyunsaturated fatty acid (PUFA) ratio diet reduces hyperinsulinemia in insulin-resistant adolescents even in the absence of change in body weight and glucose metabolism. The aim of this study was to examine the determinants and metabolic impact of the dietary-induced reduction in fasting and post-load insulin levels in obese youths. Materials and methods: Insulin secretion and clearance were assessed by measuring and modeling plasma insulin and C-peptide in 17 obese youth who underwent a 9-point, 180-min oral glucose tolerance test (OGTT) before and after a 12-week, eucaloric low n-6:n-3 PUFA ratio diet. Hepatic fat content was assessed by repeated abdominal magnetic resonance imaging (MRI). Results: Insulin clearance at fasting and during the OGTT was significantly increased after the diet, while absolute and glucose-dependent insulin secretion and model-derived parameters of β cell function were not affected. Dietary-induced changes in insulin clearance positively correlated with changes in whole-body insulin sensitivity and β cell glucose sensitivity, but not with changes in hepatic fat. Subjects with greater increases in insulin clearance showed a worse metabolic profile at enrollment, characterized by impaired insulin clearance, β cell glucose sensitivity, and glucose tolerance, and benefited the most from the diet, achieving greater improvements in glucose-stimulated hyperinsulinemia, insulin resistance, and β cell function. Conclusions: We demonstrated that a 12-week low n-6:n-3 PUFA ratio diet improves hyperinsulinemia by increasing fasting and post-load insulin clearance in obese youth, independently of weight loss, glucose concentrations and insulin secretion. This article is protected by copyright. All rights reserved

Comparison of Metabolic Response to Colonic Fermentation in Lean Youth vs Youth With Obesity

Importance: Pediatric obesity is a growing health care burden. Understanding how the metabolic phenotype of youth with obesity may modify the effect of intestinal fermentation on human metabolism is key to designing early intervention. Objective: To assess whether adiposity and insulin resistance in youth may be associated with colonic fermentation of dietary fibers and its production of acetate, gut-derived hormone secretion, and adipose tissue lipolysis. Design, setting, and participants: Cross-sectional study of youths aged 15 to 22 years with body mass index in the 25th to 75th percentile or higher than the 85th percentile for age and sex throughout the New Haven County community in Connecticut. Recruitment, studies, and data collection occurred from June 2018 to September 2021. Youths were assigned to a lean, obese insulin sensitive (OIS), or obese insulin resistant (OIR) group. Data were analyzed from April 2022 to September 2022. Exposure: Participants consumed 20 g of lactulose during a continuous 10-hour sodium d3-acetate intravenous infusion to measure the rate of appearance of acetate in plasma. Main outcomes and measures: Plasma was obtained hourly to measure acetate turnover, peptide tyrosine tyrosine (PYY), ghrelin, active glucagon-like peptide 1 (GLP-1), and free fatty acids (FFA). Results: A total of 44 youths participated in the study (median [IQR] age, 17.5 [16.0-19.3] years; 25 [56.8%] were female; 23 [52.3%] were White). Consequent to lactulose ingestion, there was a reduction of plasma FFA, an improvement of adipose tissue insulin sensitivity index, an increase in colonic acetate synthesis, and an anorexigenic response characterized by an increased plasma concentration of PYY and active GLP-1 and a reduction of ghrelin in the subgroups. Compared with the lean and OIS groups, the OIR group showed a less marked median (IQR) rate of acetate appearance (OIR: 2.00 [-0.86 to 2.69] μmol × kg-1 × min-1; lean: 5.69 [3.04 to 9.77] μmol × kg-1 × min-1; lean vs OIR P = .004; OIS: 2.63 [1.22 to 4.52] μmol × kg-1 × min-1; OIS vs OIR P = .09), a blunted median (IQR) improvement of adipose insulin sensitivity index (OIR: 0.043 [ 0.006 to 0.155]; lean: 0.277 [0.220 to 0.446]; lean vs OIR P = .002; OIS: 0.340 [0.048 to 0.491]; OIS vs OIR P = .08), and a reduced median (IQR) PYY response (OIR: 25.4 [14.8 to 36.4] pg/mL; lean: 51.3 [31.6 to 83.3] pg/mL; lean vs OIR P = .002; OIS: 54.3 [39.3 to 77.2] pg/mL; OIS vs OIR P = .011). Conclusions and relevance: In this cross-sectional study, lean, OIS, and OIR youth demonstrated different associations between colonic fermentation of indigestible dietary carbohydrates and the metabolic response, with OIR youth showing minimal metabolic modifications as compared with the other 2 groups. Trial registration: ClinicalTrials.gov Identifier: NCT03454828