Naomi X. Y. Ling

Background/Objectives: Acute exercise remodels many interconnected biochemical pathways in metabolically active tissues. This remodeling involves the activation of the energy-sensing AMP-activated protein kinase (AMPK) to maintain cellular energy homeostasis. Critical energy reserves of glycogen, primarily stored in liver and skeletal muscle and known to interact with AMPK, are utilized to help meet increased energy demands with exercise. However, the breadth of metabolic pathways regulated by acute exercise and AMPK’s interactive roles with glycogen remain incompletely understood. This study therefore aimed to map mouse liver and skeletal muscle metabolite responses to continuous acute exercise and disruption of AMPK-glycogen interactions. Methods: Liquid chromatography–mass spectrometry-based untargeted metabolomics was used to measure the relative abundance of liver and gastrocnemius muscle metabolites at rest and following an acute bout of continuous treadmill running in wild type (WT) and AMPK transgenic mice with double knock-in (DKI) mutations in the β subunit carbohydrate binding module that mediates glycogen binding. Results: Over 200 total metabolites were identified/annotated across liver and skeletal muscle, including 45 metabolites responsive to exercise (p < 0.05; FDR < 0.1). Exercise-regulated metabolites included known metabolic pathways and metabolites never associated or with only emerging evidence related to exercise (e.g., ergothioneine) and/or AMPK-glycogen interactions (N6,N6,N6-trimethyl-L-lysine, a precursor of L-carnitine). Conclusions: Liver and skeletal muscle metabolomic profiles displayed shifts between WT and DKI mice at rest, with shifts also detected following a continuous acute exercise bout. An interaction effect was also observed in skeletal muscle, suggesting differential muscle metabolite responses to acute exercise in DKI mice that may contribute to their functional impairments in metabolic control and exercise capacity versus WT. Collectively, these findings expand the molecular landscape of acute exercise and reveal liver and muscle metabolites underlying exercise-induced metabolic responses.

Recent identification of new human muscle glycogen phosphorylation sites has renewed interest in understanding human variations in the regulation of glycogen metabolism and glucose homeostasis. This paper presents a detailed method for the measurement of glycogen phosphorylase (GPh) activity in skeletal muscle. Our approach incorporates modifications to existing radiolabelling assays, optimizing specificity and sensitivity while enabling the assessment of both active and total enzyme activity levels. The utilization of radioisotope tracers and scintillation counting ensures accurate quantification of GPh activity, which we use to validate a previously published reduction in GPh activity in an Actn3 deficient mouse model. Moreover, we introduce a step-by-step guide for data acquisition, highlight the use of appropriate homogenization, discuss the need for allosteric activators/inhibitors and the importance of assay optimization to record a GPh activity assay for skeletal muscle. In conclusion, our refined method not only contributes to a deeper understanding of glycogen metabolism in muscle tissue but also provides a framework for future investigations, underscoring its role in advancing research on glycogen utilization and glucose homeostasis. NEW & NOTEWORTHY: The study optimizes the glycogen phosphorylase radiolabelled activity assay, unveiling nuances in muscle homogenization, sample dilution, and caffeine inclusion. The research introduces standardized conditions, enhancing assay reliability and reproducibility across mouse strains to reveal sex specific variations in GPh activity and underscore novel distinctions in an Actn3 deficient mouse model. These findings advance our understanding of muscle glycogen metabolism, offering a crucial tool for researchers and facilitating meaningful inter-laboratory comparisons.

Protein phosphorylation dynamically integrates environmental and cellular information to control biological processes. Identifying functional phosphorylation amongst the thousands of phosphosites regulated by a perturbation at a global scale is a major challenge. Here we introduce ‘personalized phosphoproteomics’, a combination of experimental and computational analyses to link signaling with biological function by utilizing human phenotypic variance. We measure individual subject phosphoproteome responses to interventions with corresponding phenotypes measured in parallel. Applying this approach to investigate how exercise potentiates insulin signaling in human skeletal muscle, we identify both known and previously unidentified phosphosites on proteins involved in glucose metabolism. This includes a cooperative relationship between mTOR and AMPK whereby the former directly phosphorylates the latter on S377, for which we find a role in metabolic regulation. These results establish personalized phosphoproteomics as a general approach for investigating the signal transduction underlying complex biology.

Genetic and pharmacological inhibition of mitochondrial fission induced by acute myocardial ischaemia/reperfusion injury (IRI) has been shown to reduce myocardial infarct size. The clinically used anti-hypertensive and heart failure medication, hydralazine, is known to have anti-oxidant and anti-apoptotic effects. Here, we investigated whether hydralazine confers acute cardioprotection by inhibiting Drp1-mediated mitochondrial fission. Pre-treatment with hydralazine was shown to inhibit both mitochondrial fission and mitochondrial membrane depolarisation induced by oxidative stress in HeLa cells. In mouse embryonic fibroblasts (MEFs), pre-treatment with hydralazine attenuated mitochondrial fission and cell death induced by oxidative stress, but this effect was absent in MEFs deficient in the mitochondrial fission protein, Drp1. Molecular docking and surface plasmon resonance studies demonstrated binding of hydralazine to the GTPase domain of the mitochondrial fission protein, Drp1 (KD 8.6±1.0 µM), and inhibition of Drp1 GTPase activity in a dose-dependent manner. In isolated adult murine cardiomyocytes subjected to simulated IRI, hydralazine inhibited mitochondrial fission, preserved mitochondrial fusion events, and reduced cardiomyocyte death (hydralazine 24.7±2.5% vs. control 34.1±1.5%, P=0.0012). In ex vivo perfused murine hearts subjected to acute IRI, pre-treatment with hydralazine reduced myocardial infarct size (as % left ventricle: hydralazine 29.6±6.5% vs. vehicle control 54.1±4.9%, P=0.0083), and in the murine heart subjected to in vivo IRI, the administration of hydralazine at reperfusion, decreased myocardial infarct size (as % area-at-risk: hydralazine 28.9±3.0% vs. vehicle control 58.2±3.8%, P<0.001). We show that, in addition to its antioxidant and anti-apoptotic effects, hydralazine, confers acute cardioprotection by inhibiting IRI-induced mitochondrial fission, raising the possibility of repurposing hydralazine as a novel cardioprotective therapy for improving post-infarction outcomes.

Long-chain fatty acids (LCFAs) play important roles in cellular energy metabolism, acting as both an important energy source and signalling molecules¹. LCFA-CoA esters promote their own oxidation by acting as allosteric inhibitors of acetyl-CoA carboxylase, which reduces the production of malonyl-CoA and relieves inhibition of carnitine palmitoyl-transferase 1, thereby promoting LCFA-CoA transport into the mitochondria for β-oxidation^2,3,4,5,6. Here we report a new level of regulation wherein LCFA-CoA esters per se allosterically activate AMP-activated protein kinase (AMPK) β1–containing isoforms to increase fatty acid oxidation through phosphorylation of acetyl-CoA carboxylase. Activation of AMPK by LCFA-CoA esters requires the allosteric drug and metabolite site formed between the α-subunit kinase domain and the β-subunit. β1 subunit mutations that inhibit AMPK activation by the small-molecule activator A769662, which binds to the allosteric drug and metabolite site, also inhibit activation by LCFA-CoAs. Thus, LCFA-CoA metabolites act as direct endogenous AMPK β1–selective activators and promote LCFA oxidation.

Objectives: Loss‐of‐function mutations in the gene encoding the calcium‐calmodulin (Ca²⁺‐CaM)‐dependent protein kinase kinase‐2 (CaMKK2) enzyme are linked to bipolar disorder. Recently, a de novo arginine to cysteine (R311C) mutation in CaMKK2 was identified from a whole exome sequencing study of bipolar patients and their unaffected parents. The aim of the present study was to determine the functional consequences of the R311C mutation on CaMKK2 activity and regulation by Ca²⁺‐CaM.Methods: The effects of the R311C mutation on CaMKK2 activity and Ca²⁺‐CaM activation were examined using a radiolabeled adenosine triphosphate (ATP) kinase assay. We performed immunoblot analysis to determine whether the R311C mutation impacts threonine‐85 (T85) autophosphorylation, an activating phosphorylation site on CaMKK2 that has also been implicated in bipolar disorder. We also expressed the R311C mutant in CaMKK2 knockout HAP1 cells and used immunoblot analysis and an MTS reduction assay to study its effects on Ca²⁺‐dependent downstream signaling and cell viability, respectively.Results: The R311C mutation maps to the conserved HRD motif within the catalytic loop of CaMKK2 and caused a marked reduction in kinase activity and Ca²⁺‐CaM activation. The R311C mutation virtually abolished T85 autophosphorylation in response to Ca²⁺‐CaM and exerted a dominant‐negative effect in cells as it impaired the ability of wild‐type CaMKK2 to initiate downstream signaling and maintain cell viability.Conclusions: The highly disruptive, loss‐of‐function impact of the de novo R311C mutation in human CaMKK2 provides a compelling functional rationale for being considered a potential rare monogenic cause of bipolar disorder.