CaMKK2 is not involved in contraction-stimulated AMPK activation and glucose uptake in skeletal muscle

OBJECTIVE

The AMP-activated protein kinase (AMPK) gets activated in response to energetic stress such as contractions and plays a vital role in regulating various metabolic processes such as insulin-independent glucose uptake in skeletal muscle. The main upstream kinase that activates AMPK through phosphorylation of α-AMPK Thr172 in skeletal muscle is LKB1, however some studies have suggested that Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) acts as an alternative kinase to activate AMPK. We aimed to establish whether CaMKK2 is involved in activation of AMPK and promotion of glucose uptake following contractions in skeletal muscle.

METHODS

A recently developed CaMKK2 inhibitor (SGC-CAMKK2-1) alongside a structurally related but inactive compound (SGC-CAMKK2-1N), as well as CaMKK2 knock-out (KO) mice were used. In vitro kinase inhibition selectivity and efficacy assays, as well as cellular inhibition efficacy analyses of CaMKK inhibitors (STO-609 and SGC-CAMKK2-1) were performed. Phosphorylation and activity of AMPK following contractions (ex vivo) in mouse skeletal muscles treated with/without CaMKK inhibitors or isolated from wild-type (WT)/CaMKK2 KO mice were assessed. Camkk2 mRNA in mouse tissues was measured by qPCR. CaMKK2 protein expression was assessed by immunoblotting with or without prior enrichment of calmodulin-binding proteins from skeletal muscle extracts, as well as by mass spectrometry-based proteomics of mouse skeletal muscle and C2C12 myotubes.

RESULTS

STO-609 and SGC-CAMKK2-1 were equally potent and effective in inhibiting CaMKK2 in cell-free and cell-based assays, but SGC-CAMKK2-1 was much more selective. Contraction-stimulated phosphorylation and activation of AMPK were not affected with CaMKK inhibitors or in CaMKK2 null muscles. Contraction-stimulated glucose uptake was comparable between WT and CaMKK2 KO muscle. Both CaMKK inhibitors (STO-609 and SGC-CAMKK2-1) and the inactive compound (SGC-CAMKK2-1N) significantly inhibited contraction-stimulated glucose uptake. SGC-CAMKK2-1 also inhibited glucose uptake induced by a pharmacological AMPK activator or insulin. Relatively low levels of Camkk2 mRNA were detected in mouse skeletal muscle, but neither CaMKK2 protein nor its derived peptides were detectable in mouse skeletal muscle tissue.

CONCLUSIONS

We demonstrate that pharmacological inhibition or genetic loss of CaMKK2 does not affect contraction-stimulated AMPK phosphorylation and activation, as well as glucose uptake in skeletal muscle. Previously observed inhibitory effect of STO-609 on AMPK activity and glucose uptake is likely due to off-target effects. CaMKK2 protein is either absent from adult murine skeletal muscle or below the detection limit of currently available methods.

Nampt controls skeletal muscle development by maintaining Ca2+ homeostasis and mitochondrial integrity

Objective

NAD⁺ is a co-factor and substrate for enzymes maintaining energy homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT) controls NAD⁺ synthesis, and in skeletal muscle, NAD⁺ is essential for muscle integrity. However, the underlying molecular mechanisms by which NAD⁺ synthesis affects muscle health remain poorly understood. Thus, the objective of the current study was to delineate the role of NAMPT-mediated NAD⁺ biosynthesis in skeletal muscle development and function.

Methods

To determine the role of Nampt in muscle development and function, we generated skeletal muscle-specific Nampt KO (SMNKO) mice. We performed a comprehensive phenotypic characterization of the SMNKO mice, including metabolic measurements, histological examinations, and RNA sequencing analyses of skeletal muscle from SMNKO mice and WT littermates.

Results

SMNKO mice were smaller, with phenotypic changes in skeletal muscle, including reduced fiber area and increased number of centralized nuclei. The majority of SMNKO mice died prematurely. Transcriptomic analysis identified that the gene encoding the mitochondrial permeability transition pore (mPTP) regulator Cyclophilin D (Ppif) was upregulated in skeletal muscle of SMNKO mice from 2 weeks of age, with associated increased sensitivity of mitochondria to the Ca²⁺-stimulated mPTP opening. Treatment of SMNKO mice with the Cyclophilin D inhibitor, Cyclosporine A, increased membrane integrity, decreased the number of centralized nuclei, and increased survival.

Conclusions

Our study demonstrates that NAMPT is crucial for maintaining cellular Ca²⁺ homeostasis and skeletal muscle development, which is vital for juvenile survival.

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NAD⁺ salvage capacity is important for skeletal muscle development and survival.

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Skeletal muscle-specific Nampt knockout mice exhibit a dystrophy-like phenotype.

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Nampt deletion alters Ca²⁺ homeostasis and impairs mitochondrial function.

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Low NAD⁺ levels signals mitochondrial permeability transition pore opening.

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Cyclosporin A treatment improves sarcolemma integrity and increases survival rate.

Aerobic and resistance exercise training reverses age-dependent decline in NAD+ salvage capacity in human skeletal muscle

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 < 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.

Nicotinamide riboside does not alter mitochondrial respiration, content or morphology in skeletal muscle from obese and insulin-resistant men

KEY POINTS

This is the first long-term human clinical trial to report on effects of nicotinamide riboside (NR) on skeletal muscle mitochondrial function, content and morphology. NR supplementation decreases nicotinamide phosphoribosyltransferase (NAMPT) protein abundance in skeletal muscle. NR supplementation does not affect NAD metabolite concentrations in skeletal muscle. Respiration, distribution and quantity of muscle mitochondria are unaffected by NR. NAMPT in skeletal muscle correlates positively with oxidative phosphorylation Complex I, sirtuin 3 and succinate dehydrogenase.

ABSTRACT

Preclinical evidence suggests that the nicotinamide adenine dinucleotide (NAD⁺ ) precursor nicotinamide riboside (NR) boosts NAD⁺ levels and improves diseases associated with mitochondrial dysfunction. We aimed to determine if dietary NR supplementation in middle-aged, obese, insulin-resistant men affects mitochondrial respiration, content and morphology in skeletal muscle. In a randomized, placebo-controlled clinical trial, 40 participants received 1000 mg NR or placebo twice daily for 12 weeks. Skeletal muscle biopsies were collected before and after the intervention. Mitochondrial respiratory capacity was determined by high-resolution respirometry on single muscle fibres. Protein abundance and mRNA expression were measured by Western blot and quantitative PCR analyses, respectively, and in a subset of the participants (placebo n = 8; NR n = 8) we quantified mitochondrial fractional area and mitochondrial morphology by laser scanning confocal microscopy. Protein levels of nicotinamide phosphoribosyltransferase (NAMPT), an essential NAD⁺ biosynthetic enzyme in skeletal muscle, decreased by 14% with NR. However, steady-state NAD⁺ levels as well as gene expression and protein abundance of other NAD⁺ biosynthetic enzymes remained unchanged. Neither respiratory capacity of skeletal muscle mitochondria nor abundance of mitochondrial associated proteins were affected by NR. Moreover, no changes in mitochondrial fractional area or network morphology were observed. Our data do not support the hypothesis that dietary NR supplementation has significant impact on skeletal muscle mitochondria in obese and insulin-resistant men. Future studies on the effects of NR on human skeletal muscle may include both sexes and potentially provide comparisons between young and older people.

Mitochondrial function in liver cells is resistant to perturbations in NAD+ salvage capacity

Supplementation with NAD precursors such as nicotinamide riboside (NR) has been shown to enhance mitochondrial function in the liver and to prevent hepatic lipid accumulation in high-fat diet (HFD)-fed rodents. Hepatocyte-specific knockout of the NAD⁺-synthesizing enzyme nicotinamide phosphoribosyltransferase (NAMPT) reduces liver NAD⁺ levels, but the metabolic phenotype of Nampt-deficient hepatocytes in mice is unknown. Here, we assessed Nampt's role in maintaining mitochondrial and metabolic functions in the mouse liver. Using the Cre-LoxP system, we generated hepatocyte-specific Nampt knockout (HNKO) mice, having a 50% reduction of liver NAD⁺ levels. We screened the HNKO mice for signs of metabolic dysfunction following 60% HFD feeding for 20 weeks ± NR supplementation and found that NR increases hepatic NAD⁺ levels without affecting fat mass or glucose tolerance in HNKO or WT animals. High-resolution respirometry revealed that NR supplementation of the HNKO mice did not increase state III respiration, which was observed in WT mice following NR supplementation. Mitochondrial oxygen consumption and fatty-acid oxidation were unaltered in primary HNKO hepatocytes. Mitochondria isolated from whole-HNKO livers had only a 20% reduction in NAD⁺, suggesting that the mitochondrial NAD⁺ pool is less affected by HNKO than the whole-tissue pool. When stimulated with tryptophan in the presence of [¹⁵N]glutamine, HNKO hepatocytes had a higher [¹⁵N]NAD⁺ enrichment than WT hepatocytes, indicating that HNKO mice compensate through de novo NAD⁺ synthesis. We conclude that NAMPT-deficient hepatocytes can maintain substantial NAD⁺ levels and that the Nampt knockout has only minor consequences for mitochondrial function in the mouse liver.

Human Paneth cell α-defensin-5 treatment reverses dyslipidemia and improves glucoregulatory capacity in diet-induced obese mice

Overnutrition is the principal cause of insulin resistance (IR) and dyslipidemia, which drive nonalcoholic fatty liver disease (NAFLD). Overnutrition is further linked to disrupted bowel function, microbiota alterations, and change of function in gut-lining cell populations, including Paneth cells of the small intestine. Paneth cells regulate microbial diversity through expression of antimicrobial peptides, particularly human α-defensin-5 (HD-5), and have shown repressed secretory capacity in human obesity. Mice were fed a 60% high-fat diet for 13 wk and subsequently treated with physiologically relevant amounts of HD-5 (0.001%) or vehicle for 10 wk. The glucoregulatory capacity was determined by glucose tolerance tests and measurements of corresponding insulin concentrations both before and during intervention. Gut microbiome composition was examined by 16S rRNA gene amplicon sequencing. HD-5-treated mice exhibited improved glucoregulatory capacity along with an ameliorated plasma and liver lipid profile. This was accompanied by specific decrease in jejunal inflammation and gut microbiota alterations including increased Bifidobacterium abundances, which correlated inversely with metabolic dysfunctions. This study provides proof of concept for the use of human defensins to improve host metabolism by mitigating the triad cluster of dyslipidemia, IR, and NAFLD.

Perturbations of NAD+ salvage systems impact mitochondrial function and energy homeostasis in mouse myoblasts and intact skeletal muscle

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.

Age-dependent alterations of glucose clearance and homeostasis are temporally separated and modulated by dietary fat

Diet- and age-dependent changes in glucose regulation in mice occur, but the temporal development, mechanisms and influence of dietary fat source remain to be defined. We followed metabolic changes in three groups of mice including a low-fat diet (LFD) reference group and two high-fat, high-sucrose diets based on either fish oil (FOD) or soybean oil (SOD), rich in ω3- and ω6-polyunsaturated fatty acids, respectively, to closely monitor the age-dependent development in glucose regulation in both obese (SOD-fed) and lean (LFD- and FOD-fed) mice. We assessed glucose homeostasis and glucose clearance at week 8, 12, 16, 24, 31, and 39 and performed an insulin tolerance test at week 40. We further analyzed correlations between the gut microbiota and key metabolic parameters. Interestingly, alterations in glucose homeostasis and glucose clearance were temporally separated, while 16S ribosomal gene amplicon sequencing revealed that gut microbial alterations formed correlation clusters with fat mass and either glucose homeostasis or glucose clearance, but rarely both. Importantly, effective glucose clearance was maintained in FOD- and even increased in LFD-fed mice, whereas SOD-fed mice rapidly developed impaired glucose clearance followed by a gradual improvement from week 8 to week 39. All groups had similar responses to insulin 40 weeks post diet initiation despite severe nonalcoholic steatohepatitis in SOD-fed mice. We conclude that age-related alterations in glucose regulation may occur in both lean and obese mice and are modulated by dietary fat as indicated by the sustained metabolic homeostasis observed in mice fed ω3-polyunsaturated fatty acids.

The handicap profile of the spinal cord-damaged in case file, communication documents and population survey

"Definitions" of handicap are given, with a classification of intrinsic (personal) handicaps, as well as Tables for the preparation of an individual's Handicap Profile, of use in population Surveys and in case files."

Classification of incontinence

A clinical definition of incontinence and a clinical classification of its various forms is offered for consideration. The classification is part of a wider classification of handicaps being prepared to supplement existing classifications of diseases and disorders classified according to aetiology, systems, and anatomy. Nine handicaps (locomotor, visceral, visual, communication, intellectual, emotional, invisible, and "other") are distinguished. Incontinence is classified as a "disorder of excretion, which is itself one of four divisions of visceral handicap'.