Contraction and AICAR Stimulate IL-6 Vesicle Depletion From Skeletal Muscle Fibers In Vivo

Recent studies suggest that interleukin 6 (IL-6) is released from contracting skeletal muscles; however, the cellular origin, secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with enhanced green fluorescent protein (EGFP)-tagged IL-6 (IL-6-EGFP), and 5 days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP–containing vesicles and protein by 62% (P < 0.05), occurring rapidly and progressively over 25 min of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMP-activated protein kinase (AMPK) α2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6–positive vesicles that are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine and can be used to characterize the secretion kinetics of other putative myokines

NAMPT-mediated NAD+ biosynthesis is indispensable for adipose tissue plasticity and development of obesity

Objective: The ability of adipose tissue to expand and contract in response to fluctuations in nutrient availability is essential for the maintenance of whole-body metabolic homeostasis. Given the nutrient scarcity that mammals faced for millions of years, programs involved in this adipose plasticity were likely evolved to be highly efficient in promoting lipid storage. Ironically, this previously advantageous feature may now represent a metabolic liability given the caloric excess of modern society. We speculate that nicotinamide adenine dinucleotide (NAD+) biosynthesis exemplifies this concept. Indeed NAD+/NADH metabolism in fat tissue has been previously linked with obesity, yet whether it plays a causal role in diet-induced adiposity is unknown. Here we investigated how the NAD+ biosynthetic enzyme nicotinamide phosphoribosyltransferase (NAMPT) supports adipose plasticity and the pathological progression to obesity. Methods: We utilized a newly generated Nampt loss-of-function model to investigate the tissue-specific and systemic metabolic consequences of adipose NAD+ deficiency. Energy expenditure, glycemic control, tissue structure, and gene expression were assessed in the contexts of a high dietary fat burden as well as the transition back to normal chow diet. Results: Fat-specific Nampt knockout (FANKO) mice were completely resistant to high fat diet (HFD)-induced obesity. This was driven in part by reduced food intake. Furthermore, HFD-fed FANKO mice were unable to undergo healthy expansion of adipose tissue mass, and adipose depots were rendered fibrotic with markedly reduced mitochondrial respiratory capacity. Yet, surprisingly, HFD-fed FANKO mice exhibited improved glucose tolerance compared to control littermates. Removing the HFD burden largely reversed adipose fibrosis and dysfunction in FANKO animals whereas the improved glucose tolerance persisted. Conclusions: These findings indicate that adipose NAMPT plays an essential role in handling dietary lipid to modulate fat tissue plasticity, food intake, and systemic glucose homeostasis. Keywords: Adipose metabolism, Obesity, NAMPT, NAD+ synthesis, Energy homeostasis, Adipose plasticity, Glucose homeostasi

Aerobic and Resistance Exercise Training Reverses Age-Dependent Decline in NAD + Salvage Capacity in Human skeletal muscle

© 2019 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society. Aging decreases skeletal muscle mass and strength, but aerobic and resistance exercise training maintains skeletal muscle function. NAD+ is a coenzyme for ATP production and a required substrate for enzymes regulating cellular homeostasis. In skeletal muscle, NAD+ is mainly generated by the NAD+ salvage pathway in which nicotinamide phosphoribosyltransferase (NAMPT) is rate-limiting. NAMPT decreases with age in human skeletal muscle, and aerobic exercise training increases NAMPT levels in young men. However, whether distinct modes of exercise training increase NAMPT levels in both young and old people is unknown. We assessed the effects of 12 weeks of aerobic and resistance exercise training on skeletal muscle abundance of NAMPT, nicotinamide riboside kinase 2 (NRK2), and nicotinamide mononucleotide adenylyltransferase (NMNAT) 1 and 3 in young (≤35 years) and older (≥55 years) individuals. NAMPT in skeletal muscle correlated negatively with age (r2 = 0.297, P \u3c 0.001, n = 57), and VO2peak was the best predictor of NAMPT levels. Moreover, aerobic exercise training increased NAMPT abundance 12% and 28% in young and older individuals, respectively, whereas resistance exercise training increased NAMPT abundance 25% and 30% in young and in older individuals, respectively. None of the other proteins changed with exercise training. In a separate cohort of young and old people, levels of NAMPT, NRK1, and NMNAT1/2 in abdominal subcutaneous adipose tissue were not affected by either age or 6 weeks of high-intensity interval training. Collectively, exercise training reverses the age-dependent decline in skeletal muscle NAMPT abundance, and our findings highlight the value of exercise training in ameliorating age-associated deterioration of skeletal muscle function

Disrupted circadian oscillations in type 2 diabetes are linked to altered rhythmic mitochondrial metabolism in skeletal muscle

Funding: The authors are supported by grants from the AstraZeneca SciLifeLab Research Programme, Novo Nordisk Foundation (NNF14OC0011493, and NNF17OC0030088), Swedish Diabetes Foundation (DIA2018-357), Swedish Research Council (2015-00165 and 2018-02389), the Knut and Alice Wallenberg Foundation (2018-0094), the Strategic Research Programme in Diabetes at Karolinska Institutet (2009-1068), the Stockholm County Council (SLL20170159), and the Swedish Research Council for Sport Science (P2019-0140). B.M.G. was supported by fellowships from the Novo Nordisk Foundation (NNF19OC0055072), the Wenner-Gren Foundation, an Albert Renold Travel Fellowship from the European Foundation for the Study of Diabetes, and an Eric Reid Fund for Methodology from the Biochemical Society. N.J.P. and L.S.-P. were supported by an Individual Fellowship from the Marie Skłodowska-Curie Actions (European Commission: 704978 and 675610). X.Z. and K.A.E. were supported by NIH R01AR066082. N.J.P. was supported by grants from the Sigurd och Elsa Goljes Minne and Lars Hierta Memorial Foundations (Sweden). We acknowledge the Beta Cell in-vivo Imaging/Extracellular Flux Analysis core facility supported by the Strategic Research Program in Diabetes for the usage of the Seahorse flux analyzer. Additional support was received from the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen (NNF18CC0034900). The Novo Nordisk Foundation Center for Basic Metabolic Research is an independent research center at the University of Copenhagen, partially funded by an unrestricted donation from the Novo Nordisk Foundation. We acknowledge the Single-Cell Omics platform at the Novo Nordisk Foundation Center for Basic Metabolic Research for technical and computational expertise and support. Schematics are created with BioRender.com.Peer reviewedPublisher PD

The aromatic amino acid sensor GPR142 controls metabolism through balanced regulation of pancreatic and gut hormones.

OBJECTIVES: GPR142, which is highly expressed in pancreatic islets, has recently been deorphanized as a receptor for aromatic amino acids; however, its physiological role and pharmacological potential is unclear. METHODS AND RESULTS: We find that GPR142 is expressed not only in β- but also in α-cells of the islets as well as in enteroendocrine cells, and we confirm that GPR142 is a highly selective sensor of essential aromatic amino acids, in particular Trp and oligopeptides with N-terminal Trp. GPR142 knock-out mice displayed a very limited metabolic phenotype but demonstrated that L-Trp induced secretion of pancreatic and gut hormones is mediated through GPR142 but that the receptor is not required for protein-induced hormone secretion. A synthetic GPR142 agonist stimulated insulin and glucagon as well as GIP, CCK, and GLP-1 secretion. In particular, GIP secretion was sensitive to oral administration of the GPR142 agonist an effect which in contrast to the other hormones was blocked by protein load. Oral administration of the GPR142 agonist increased [3H]-2-deoxyglucose uptake in muscle and fat depots mediated through insulin action while it lowered liver glycogen conceivably mediated through glucagon, and, consequently, it did not lower total blood glucose. Nevertheless, acute administration of the GPR142 agonist strongly improved oral glucose tolerance in both lean and obese mice as well as Zucker fatty rat. Six weeks in-feed chronic treatment with the GPR142 agonist did not affect body weight in DIO mice, but increased energy expenditure and carbohydrate utilization, lowered basal glucose, and improved insulin sensitivity. CONCLUSIONS: GPR142 functions as a sensor of aromatic amino acids, controlling GIP but also CCK and GLP-1 as well as insulin and glucagon in the pancreas. GPR142 agonists could have novel interesting potential in modifying metabolism through a balanced action of gut hormones as well as both insulin and glucagon