Nicotinamide adenine dinucleotide (NAD) can be synthesized by nicotinamide phosphoribosyltransferase (NAMPT). We aimed to determine the role of NAMPT in maintaining NAD levels, mitochondrial function, and metabolic homeostasis in skeletal muscle cells. We generated stable Nampt knockdown (sh Nampt KD) C2C12 cells using a shRNA lentiviral approach. Moreover, we applied gene electrotransfer to express Cre recombinase in tibialis anterior muscle of floxed Nampt mice. In sh Nampt KD C2C12 myoblasts, Nampt and NAD levels were reduced by 70% and 50%, respectively, and maximal respiratory capacity was reduced by 25%. Moreover, anaerobic glycolytic flux increased by 55%, and 2-deoxyglucose uptake increased by 25% in sh Nampt KD cells. Treatment with the NAD precursor nicotinamide riboside restored NAD levels in sh Nampt cells and increased maximal respiratory capacity by 18% and 32% in control and sh Nampt KD cells, respectively. Expression of Cre recombinase in muscle of floxed Nampt mice reduced NAMPT and NAD levels by 38% and 43%, respectively. Glucose uptake increased by 40%, and mitochondrial complex IV respiration was compromised by 20%. Hypoxia-inducible factor (HIF)-1α-regulated genes and histone H3 lysine 9 (H3K9) acetylation, a known sirtuin 6 (SIRT6) target, were increased in shNampt KD cells. Thus, we propose that the shift toward glycolytic metabolism observed, at least in part, is mediated by the SIRT6/HIF1α axis. Our findings suggest that NAMPT plays a key role for maintaining NAD levels in skeletal muscle and that NAMPT deficiency compromises oxidative phosphorylation capacity and alters energy homeostasis in this tissue.

The G-protein-coupled receptor 39 (GPR39) is a G-protein-coupled receptor activated by Zn. We used a homology model-based approach to identify small-molecule pharmacological tool compounds for the receptor. The method focused on a putative binding site in GPR39 for synthetic ligands and knowledge of ligand binding to other receptors with similar binding pockets to select iterative series of minilibraries. These libraries were cherry-picked from all commercially available synthetic compounds. A total of only 520 compounds were tested in vitro, making this method broadly applicable for tool compound development. The compounds of the initial library were inactive when tested alone, but lead compounds were identified using Zn as an allosteric enhancer. Highly selective, highly potent Zn-independent GPR39 agonists were found in subsequent minilibraries. These agonists identified GPR39 as a novel regulator of gastric somatostatin secretion.

The ADAMTS9 rs4607103 C allele is one of the few gene variants proposed to increase the risk of type 2 diabetes through an impairment of insulin sensitivity. We show that the variant is associated with increased expression of the secreted ADAMTS9 and decreased insulin sensitivity and signaling in human skeletal muscle. In line with this, mice lacking Adamts9 selectively in skeletal muscle have improved insulin sensitivity. The molecular link between ADAMTS9 and insulin signaling was characterized further in a model where ADAMTS9 was overexpressed in skeletal muscle. This selective overexpression resulted in decreased insulin signaling presumably mediated through alterations of the integrin β1 signaling pathway and disruption of the intracellular cytoskeletal organization. Furthermore, this led to impaired mitochondrial function in mouse muscle—an observation found to be of translational character because humans carrying the ADAMTS9 risk allele have decreased expression of mitochondrial markers. Finally, we found that the link between ADAMTS9 overexpression and impaired insulin signaling could be due to accumulation of harmful lipid intermediates. Our findings contribute to the understanding of the molecular mechanisms underlying insulin resistance and type 2 diabetes and point to inhibition of ADAMTS9 as a potential novel mode of treating insulin resistance.

Aims/hypothesisA genetic risk score (GRS) consisting of 53 insulin resistance variants (GRS53) was recently demonstrated to associate with insulin resistance in adults. We speculated that the GRS53 might already associate with insulin resistance during childhood, and we therefore aimed to investigate this in populations of Danish children and adolescents. Furthermore, we aimed to address whether the GRS associates with components of the metabolic syndrome and altered body composition in children and adolescents.MethodsWe examined a total of 689 children and adolescents who were overweight or obese and 675 children and adolescents from a population-based study. Anthropometric data, dual-energy x-ray absorptiometry scans, BP, fasting plasma glucose, fasting serum insulin and fasting plasma lipid measurements were obtained, and HOMA-IR was calculated. The GRS53 was examined for association with metabolic traits in children by linear regressions using an additive genetic model.ResultsIn overweight/obese children and adolescents, the GRS53 associated with higher HOMA-IR (β = 0.109 ± 0.050 (SE); p = 2.73 × 10−2), fasting plasma glucose (β = 0.010 ± 0.005 mmol/l; p = 2.51 × 10−2) and systolic BP SD score (β = 0.026 ± 0.012; p = 3.32 × 10−2) as well as lower HDL-cholesterol (β = −0.008 ± 0.003 mmol/l; p = 1.23 × 10−3), total fat-mass percentage (β = −0.143 ± 0.054%; p = 9.15 × 10−3) and fat-mass percentage in the legs (β = −0.197 ± 0.055%; p = 4.09 × 10−4). In the population-based sample of children, the GRS53 only associated with lower HDL-cholesterol concentrations (β = −0.007 ± 0.003 mmol/l; p = 1.79 × 10−2).Conclusions/interpretationAn adult-based GRS comprising 53 insulin resistance susceptibility SNPs associates with insulin resistance, markers of the metabolic syndrome and altered fat distribution in a sample of Danish children and adolescents who were overweight or obese.Electronic supplementary materialThe online version of this article (10.1007/s00125-018-4640-0) contains peer-reviewed but unedited supplementary material, which is available to authorised users.