The effects of age and muscle contraction on AMPK activity and heterotrimer composition

AMPK signaling inhibits the differentiation of myofibroblasts: impact on age-related tissue fibrosis and degeneration

Disruption of the extracellular matrix (ECM) and an accumulation of fibrotic lesions within tissues are two of the distinctive hallmarks of the aging process. Tissue fibroblasts are mesenchymal cells which display an impressive plasticity in the regulation of ECM integrity and thus on tissue homeostasis. Single-cell transcriptome studies have revealed that tissue fibroblasts exhibit a remarkable heterogeneity with aging and in age-related diseases. Excessive stress and inflammatory insults induce the differentiation of fibroblasts into myofibroblasts which are fusiform contractile cells and abundantly secrete the components of the ECM and proteolytic enzymes as well as many inflammatory mediators. Detrimental stresses can also induce the transdifferentiation of certain mesenchymal and myeloid cells into myofibroblasts. Interestingly, many age-related stresses, such as oxidative and endoplasmic reticulum stresses, ECM stiffness, inflammatory mediators, telomere shortening, and several alarmins from damaged cells are potent inducers of myofibroblast differentiation. Intriguingly, there is convincing evidence that the signaling pathways stimulated by the AMP-activated protein kinase (AMPK) are potent inhibitors of myofibroblast differentiation and accordingly AMPK signaling reduces fibrotic lesions within tissues, e.g., in age-related cardiac and pulmonary fibrosis. AMPK signaling is not only an important regulator of energy metabolism but it is also able to control cell fate determination and many functions of the immune system. It is known that AMPK signaling can delay the aging process via an integrated signaling network. AMPK signaling inhibits myofibroblast differentiation, e.g., by suppressing signaling through the TGF-β, NF-κB, STAT3, and YAP/TAZ pathways. It seems that AMPK signaling can alleviate age-related tissue fibrosis and degeneration by inhibiting the differentiation of myofibroblasts.

Greater Phosphorylation of AMPK and Multiple AMPK Substrates in the Skeletal Muscle of 24-Month-Old Calorie Restricted Compared to Ad-Libitum Fed Male Rats

AMP-activated protein kinase (AMPK), a highly conserved, heterotrimeric serine/threonine kinase with critical sensory and regulatory functions, is proposed to induce antiaging actions of caloric restriction (CR). Although earlier studies assessed CR's effects on AMPK in rodent skeletal muscle, the scope of these studies was narrow with a limited focus on older animals. This study's purpose was to fill important knowledge gaps related to CR's influence on AMPK in skeletal muscle of older animals. Therefore, using epitrochlearis muscles from 24-month-old ad-libitum fed (AL) and CR (consuming 65% of AL intake for 8 weeks), male Fischer-344 × Brown Norway F1 rats, we determined: (a) AMPK Thr172 phosphorylation (a key regulatory site) by immunoblot; (b) AMPKα1 and AMPKα2 activity (representing the 2 catalytic α-subunits of AMPK), and AMPKγ3 activity (representing AMPK complexes that include the skeletal muscle-selective regulatory γ3 subunit) using enzymatic assays; (c) phosphorylation of multiple protein substrates that are linked to CR-related effects (acetyl-CoA carboxylase [ACC], that regulates lipid oxidation; Beclin-1 and ULK1 that are autophagy regulatory proteins; Raptor, mTORC1 complex protein that regulates autophagy; TBC1D1 and TBC1D4 that regulate glucose uptake) by immunoblot; and (d) ATP and AMP concentrations (key AMPK regulators) by mass spectrometry. The results revealed significant CR-associated increases in the phosphorylation of AMPKThr172 and 4 AMPK substrates (ACC, Beclin-1, TBC1D1, and TBC1D4), without significant diet-related differences in ATP or AMP concentration or AMPKα1-, AMPKα2-, or AMPKγ3-associated activity. The enhanced phosphorylation of multiple AMPK substrates provides novel mechanistic insights linking AMPK to functionally important consequences of CR.

Cardiac System during the Aging Process

The aging process is accompanied by a continuous decline of the cardiac system, disrupting the homeostatic regulation of cells, organs, and systems. Aging increases the prevalence of cardiovascular diseases, thus heart failure and mortality. Understanding the cardiac aging process is of pivotal importance once it allows us to design strategies to prevent age-related cardiac events and increasing the quality of live in the elderly. In this review we provide an overview of the cardiac aging process focus on the following topics: cardiac structural and functional modifications; cellular mechanisms of cardiac dysfunction in the aging; genetics and epigenetics in the development of cardiac diseases; and aging heart and response to the exercise.

Systems modelling predicts chronic inflammation and genomic instability prevent effective mitochondrial regulation during biological ageing

The regulation of mitochondrial turnover under conditions of stress occurs partly through the AMPK-NAD+-PGC1α-SIRT1 signalling pathway. This pathway can be affected by both genomic instability and chronic inflammation since these will result in an increased rate of NAD+ degradation through PARP1 and CD38 respectively. In this work we develop a computational model of this signalling pathway, calibrating and validating it against experimental data. The computational model is used to study mitochondrial turnover under conditions of stress and how it is affected by genomic instability, chronic inflammation and biological ageing in general. We report that the AMPK-NAD+-PGC1α-SIRT1 signalling pathway becomes less responsive with age and that this can prime for the accumulation of dysfunctional mitochondria.

Physiological Systems in Promoting Frailty

Frailty is a complex syndrome affecting a growing sector of the global population as medical developments have advanced human mortality rates across the world. Our current understanding of frailty is derived from studies conducted in the laboratory as well as the clinic, which have generated largely phenotypic information. Far fewer studies have uncovered biological underpinnings driving the onset and progression of frailty, but the stage is set to advance the field with preclinical and clinical assessment tools, multiomics approaches together with physiological and biochemical methodologies. In this article, we provide comprehensive coverage of topics regarding frailty assessment, preclinical models, interventions, and challenges as well as clinical frameworks and prevalence. We also identify central biological mechanisms that may be at play including mitochondrial dysfunction, epigenetic alterations, and oxidative stress that in turn, affect metabolism, stress responses, and endocrine and neuromuscular systems. We review the role of metabolic syndrome, insulin resistance and visceral obesity, focusing on glucose homeostasis, adenosine monophosphate-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), and nicotinamide adenine dinucleotide (NAD⁺ ) as critical players influencing the age-related loss of health. We further focus on how immunometabolic dysfunction associates with oxidative stress in promoting sarcopenia, a key contributor to slowness, weakness, and fatigue. We explore the biological mechanisms involved in stem cell exhaustion that affect regeneration and may contribute to the frailty-associated decline in resilience and adaptation to stress. Together, an overview of the interplay of aging biology with genetic, lifestyle, and environmental factors that contribute to frailty, as well as potential therapeutic targets to lower risk and slow the progression of ongoing disease is covered. © 2022 American Physiological Society. Compr Physiol 12:1-46, 2022.

Attenuation of the Counter-Regulatory Glucose Response in CVLM C1 Neurons: A Possible Explanation for Anorexia of Aging

This study aimed to determine the effect of age on CVLM C1 neuron glucoregulatory proteins in the feeding pathway. Male Sprague Dawley rats aged 3 months and 24 months old were divided into two subgroups: the treatment group with 2-deoxy-d-glucose (2DG) and the control group. Rat brains were dissected to obtain the CVLM region of the brainstem. Western blot was used to determine protein expression of tyrosine hydroxylase (TH), phosphorylated TH at Serine40 (pSer40TH), AMP-activated protein kinase (AMPK), phosphorylated AMPK (phospho AMPK), and neuropeptide Y Y5 receptors (NPY5R) in CVLM samples. Immunofluorescence was used to determine TH-, AMPK-, and NPY5R-like immunoreactivities among other brain coronal sections. Results obtained denote a decrease in basal TH phosphorylation levels and AMPK proteins and an increase in TH proteins among aged CVLM neurons. Increases in the basal immunoreactivity of TH+, AMPK+, NPY5R+, TH+/AMPK+, and TH+/NPY5R+ were also observed among old rats. Young treatment-group rats saw a decrease in TH phosphorylation and AMPK proteins following 2DG administration, while an increase in AMPK phosphorylation and a decrease in TH proteins were found among the old-treatment-group rats. These findings suggest the participation of CVLM C1 neurons in counter-regulatory responses among young and old rats. Altering protein changes in aged CVLM C1 neurons may attenuate responses to glucoprivation, thus explaining the decline in food intake among the elderly.

AMP-activated protein kinase-dependent nuclear localization of glyceraldehyde 3-phosphate dehydrogenase in senescent human diploid fibroblasts

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key glycolytic enzyme that participates in various cellular events, such as DNA repair and apoptosis. The functional diversity of GAPDH depends on its intracellular localization. Because AMP-activated protein kinase (AMPK) regulates the nuclear translocation of GAPDH in young cells and AMPK activity significantly increases during aging, we investigated whether altered AMPK activity is involved in the nuclear localization of GAPDH in senescent cells. Age-dependent nuclear translocation of GAPDH was confirmed by confocal laser scanning microscopy in human diploid fibroblasts (HDFs) and by immunohistochemical analysis in aged rat skin cells. Senescence-induced nuclear localization was reversed by lysophosphatidic acid but not by platelet-derived growth factor. The extracellular matrix from young cells also induced the nuclear export of GAPDH in senescent HDFs. An activator of AMPK, 5-Aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), increased the level of nuclear GAPDH, whereas an inhibitor of AMPK, Compound C, decreased the level of nuclear GAPDH in senescent HDFs. Transfection with AMPKα siRNA prevented nuclear translocation of GAPDH in senescent HDFs. The stimulatory effect of AICAR and serum depletion on GAPDH nuclear translocation was reduced in AMPKα1/α2-knockout mouse embryonic fibroblasts. Overall, increased AMPK activity may play a role in the senescence-associated nuclear translocation of GAPDH.

Prior AICAR induces elevated glucose uptake concomitant with greater γ3-AMPK activation and reduced membrane cholesterol in skeletal muscle from 26-month-old rats

Attenuated skeletal muscle glucose uptake (GU) has been observed with advancing age. It is important to elucidate the mechanisms linked to interventions that oppose this detrimental outcome. Earlier research using young rodents and (or) cultured myocytes reported that treatment with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; an AMP-activated protein kinase (AMPK) activator) can increase γ3-AMPK activity and reduce membrane cholesterol content, each of which has been proposed to elevate GU. However, the effect of AICAR treatment on γ3-AMPK activity and membrane cholesterol in skeletal muscle of aged animals has not been reported. Our purpose was to evaluate the effects of AICAR treatment on these potential mechanisms for enhanced glucose uptake in the skeletal muscle of aged animals. Epitrochlearis muscles from 26-27-month-old male rats were isolated and incubated ± AICAR, followed by 3 h incubation without AICAR, and then incubation with 3-O-methyl-[3 H] glucose (to assess GU ± insulin). Muscles were also analyzed for γ3-AMPK activity and membrane cholesterol content. Prior AICAR treatment led to increased γ3-AMPK activity, reduced membrane cholesterol content, and enhanced glucose uptake in skeletal muscle from aged rats. These observations revealed that two potential mechanisms for greater GU previously observed in younger animals and (or) cell models are also potentially relevant for enhanced GU by muscles from older animals.

Exercise effects on γ3-AMPK activity, Akt substrate of 160 kDa phosphorylation, and glucose uptake in muscle of normal and insulin-resistant female rats

Previous studies demonstrated that acute exercise can enhance glucose uptake (GU), γ3-AMP-activated protein kinase (AMPK) activity, and Akt substrate of 160 kDa (AS160) phosphorylation in skeletal muscles from low-fat diet (LFD)- and high-fat diet (HFD)-fed male rats. Because little is known about exercise effects on these outcomes in females, we assessed postexercise GU by muscles incubated ± insulin, delta-insulin GU (GU of muscles incubated with insulin minus GU uptake of paired muscles incubated without insulin), and muscle signaling proteins from female rats fed a LFD or a brief HFD (2 wk). Rats were sedentary (LFD-SED, HFD-SED) or swim exercised. Immediately postexercise (IPEX) or 3 h postexercise (3hPEX), epitrochlearis muscles were incubated (no insulin IPEX; ±insulin 3hPEX) to determine GU. Muscle γ3-AMPK activity (IPEX, 3hPEX) and phosphorylated AS160 (pAS160; 3hPEX) were also assessed. γ3-AMPK activity and insulin-independent GU of IPEX rats exceeded sedentary rats without diet-related differences in either outcome. At 3hPEX, both GU by insulin-stimulated muscles and delta-insulin GU exceeded their respective diet-matched sedentary controls. GU by insulin-stimulated muscles, but not delta-insulin GU for LFD-3hPEX, exceeded HFD-3hPEX. LFD-3hPEX versus LFD-SED had greater γ3-AMPK activity and greater pAS160. HFD-3hPEX exceeded HFD-SED for pAS160 but not for γ3-AMPK activity. pAS160 and γ3-AMPK at 3hPEX did not differ between diet groups. These results revealed that increased γ3-AMPK activity at 3hPEX was not essential for greater GU in insulin-stimulated muscle or greater delta-insulin GU in HFD female rats. Similarly elevated γ3-AMPK activity in LFD-IPEX versus HFD-IPEX and pAS160 in LFD-3hPEX versus HFD-3hPEX may contribute to the comparable delta-insulin GU at 3hPEX in both diet groups.NEW & NOTEWORTHY Glucose uptake (GU) and phosphorylated AS160 (pAS160) by insulin-stimulated muscles at 3 h postexercise (3hPEX) exceeded diet-matched controls in female low-fat diet-fed (LFD) or high-fat diet-fed (HFD) rats. GU with insulin for LFD-3hPEX exceeded HFD-3hPEX, whereas pAS160 was similar between these groups. γ3-AMPK immediately postexercise (IPEX) was similarly elevated in LFD and HFD, but only LFD-3hPEX had increased γ3-AMPK. These results suggest that greater γ3-AMPK at IPEX and pAS160 at 3hPEX may contribute to elevated GU with insulin, but greater γ3-AMPK at 3hPEX was dispensable for female HFD rats.

Mitochondria-localized AMPK responds to local energetics and contributes to exercise and energetic stress-induced mitophagy

Mitochondria form a complex, interconnected reticulum that is maintained through coordination among biogenesis, dynamic fission, and fusion and mitophagy, which are initiated in response to various cues to maintain energetic homeostasis. These cellular events, which make up mitochondrial quality control, act with remarkable spatial precision, but what governs such spatial specificity is poorly understood. Herein, we demonstrate that specific isoforms of the cellular bioenergetic sensor, 5' AMP-activated protein kinase (AMPKα1/α2/β2/γ1), are localized on the outer mitochondrial membrane, referred to as mitoAMPK, in various tissues in mice and humans. Activation of mitoAMPK varies across the reticulum in response to energetic stress, and inhibition of mitoAMPK activity attenuates exercise-induced mitophagy in skeletal muscle in vivo. Discovery of a mitochondrial pool of AMPK and its local importance for mitochondrial quality control underscores the complexity of sensing cellular energetics in vivo that has implications for targeting mitochondrial energetics for disease treatment.

Exercise, but Not Metformin Prevents Loss of Muscle Function Due to Doxorubicin in Mice Using an In Situ Method

Though effective in treating various types of cancer, the chemotherapeutic doxorubicin (DOX) is associated with skeletal muscle wasting and fatigue. The purpose of this study was to assess muscle function in situ following DOX administration in mice. Furthermore, pre-treatments with exercise (EX) or metformin (MET) were used in an attempt to preserve muscle function following DOX. Mice were assigned to the following groups: control, DOX, DOX + EX, or DOX + MET, and were given a single injection of DOX (15 mg/kg) or saline 3 days prior to sacrifice. Preceding the DOX injection, DOX + EX mice performed 60 min/day of running for 5 days, while DOX + MET mice received 5 daily oral doses of 500 mg/kg MET. Gastrocnemius–plantaris–soleus complex function was assessed in situ via direct stimulation of the sciatic nerve. DOX treatment increased time to half-relaxation following contractions, indicating impaired recovery (p < 0.05). Interestingly, EX prevented any increase in half-relaxation time, while MET did not. An impaired relaxation rate was associated with a reduction in SERCA1 protein content (p = 0.07) and AMPK phosphorylation (p < 0.05). There were no differences between groups in force production or mitochondrial respiration. These results suggest that EX, but not MET may be an effective strategy for the prevention of muscle fatigue following DOX administration in mice.

Lifelong Aerobic Exercise Alleviates Sarcopenia by Activating Autophagy and Inhibiting Protein Degradation via the AMPK/PGC-1α Signaling Pathway

Sarcopenia is an aging-induced syndrome characterized by a progressive reduction of skeletal muscle mass and strength. Increasing evidence has attested that appropriate and scientific exercise could induce autophagy or optimize the functional status of autophagy, which plays a critical role in senescent muscular dystrophy. As a publicly recognized strategy for extending lifespan and improving the health of the elderly, the underlying mechanisms of lifelong regular aerobic exercise for the prevention of sarcopenia have not been fully elucidated. To explore the role of lifelong aerobic exercise in the beneficial regulation of autophagic signaling pathways in senescent skeletal muscle, the natural aging mice were used as the sarcopenia model and subjected to lifelong treadmill running to evaluate corresponding parameters related to skeletal muscle atrophy and autophagic signaling pathways. Compared with the young control mice, the aged mice showed a significant reduction in skeletal muscle mass, gastrocnemius muscle weight/body weight (GMW/BW) ratio, and cross-sectional areas (CSA) of skeletal muscle fibers (p < 0.01). In contrast, lifelong aerobic exercise effectively rescued these reduced biomarkers associated with muscle atrophy. Moreover, as shown in the activated AMPK/PGC-1α signaling pathway, lifelong aerobic exercise successfully prevented the aging-induced impairment of the ubiquitin-proteasome system (UPS), excessive apoptosis, defective autophagy, and mitochondrial dysfunction. The exercise-induced autophagy suppressed the key regulatory components of the UPS, inhibited excessive apoptosis, and optimized mitochondrial quality control, thereby preventing and delaying aging-induced skeletal muscle atrophy.

Manifestations of Age on Autophagy, Mitophagy and Lysosomes in Skeletal Muscle

Sarcopenia is the loss of both muscle mass and function with age. Although the molecular underpinnings of sarcopenia are not fully understood, numerous pathways are implicated, including autophagy, in which defective cargo is selectively identified and degraded at the lysosome. The specific tagging and degradation of mitochondria is termed mitophagy, a process important for the maintenance of an organelle pool that functions efficiently in energy production and with relatively low reactive oxygen species production. Emerging data, yet insufficient, have implicated various steps in this pathway as potential contributors to the aging muscle atrophy phenotype. Included in this is the lysosome, the end-stage organelle possessing a host of proteolytic and degradative enzymes, and a function devoted to the hydrolysis and breakdown of defective molecular complexes and organelles. This review provides a summary of our current understanding of how the autophagy-lysosome system is regulated in aging muscle, highlighting specific areas where knowledge gaps exist. Characterization of the autophagy pathway with a particular focus on the lysosome will undoubtedly pave the way for the development of novel therapeutic strategies to combat age-related muscle loss.

Smooth muscle-specific HuR knockout induces defective autophagy and atherosclerosis

Human antigen R (HuR) is a widespread RNA-binding protein involved in homeostatic regulation and pathological processes in many diseases. Atherosclerosis is the leading cause of cardiovascular disease and acute cardiovascular events. However, the role of HuR in atherosclerosis remains unknown. In this study, mice with smooth muscle-specific HuR knockout (HuR^SMKO) were generated to investigate the role of HuR in atherosclerosis. HuR expression was reduced in atherosclerotic plaques. As compared with controls, HuR^SMKO mice showed increased plaque burden in the atherosclerotic model. Mechanically, HuR could bind to the mRNAs of adenosine 5′-monophosphate-activated protein kinase (AMPK) α1 and AMPKα2, thus increasing their stability and translation. HuR deficiency reduced p-AMPK and LC3II levels and increased p62 level, thereby resulting in defective autophagy. Finally, pharmacological AMPK activation induced autophagy and suppressed atherosclerosis in HuR^SMKO mice. Our findings suggest that smooth muscle HuR has a protective effect against atherosclerosis by increasing AMPK-mediated autophagy.

Overexpression of regucalcin mitigates the ageing-related changes in oxidative stress and sperm quality

Age-related changes, namely the increase in oxidative stress (OS) with the consequent sperm damage, result in decreased male fertility. Regucalcin (RGN) is a Ca²⁺-binding protein that has been shown to have beneficial effects on spermatogenesis by suppressing OS and chemical/radiation-induced damage. This work aims to evaluate whether RGN overexpression reduces the ageing-associated decline of male reproductive function. Sperm and testicular function analysis were performed in young-adult and senescent transgenic rats overexpressing RGN (Tg-RGN) comparatively with their wild-type (Wt) littermates. The gonadosomatic index (GI), tubular differentiation index and the expression levels of RGN and other proliferation regulators were evaluated. Moreover, the sperm parameters, OS analysis and immunolocalization of RGN were assessed, as well as morphometric evaluation of epididymal tubules. Both GI and sperm counts were reduced in the senescent Wt rats, but maintained in the Tg-RGN. Also, the levels of stem cell factor (SCF), c-Kit, and Akt were maintained in the testis of aged Tg-RGN rats, suggesting that the normal spermatogenic output was preserved over time in these animals, an effect not observed in Wt. Senescent Tg-RGN rats also presented lower sperm lipid peroxidation and total oxidant status relative to the Wt. Furthermore, aged Tg-RGN rats displayed higher sperm viability, higher frequency of sperm with normal morphology, and reduced incidence of head and neck/midpiece defects when compared with Wt, which may be a consequence of the lower OS levels found in the sperm of these animals. Interestingly, RGN expression increased with ageing in sperm, being mainly localized in the acrosome. Altogether, these findings indicate that the modulation of RGN levels may alleviate the age-related decline in sperm quality and testicular function.

AICAR and nicotinamide treatment synergistically augment the proliferation and attenuate senescence-associated changes in mesenchymal stromal cells

Background

Mesenchymal stromal cell (MSC) stemness capacity diminishes over prolonged in vitro culture, which negatively affects their application in regenerative medicine. To slow down the senescence of MSCs, here, we have evaluated the in vitro effects of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), an AMPK activator, and nicotinamide (NAM), an activator of sirtuin1 (SIRT1).

Methods

Human adipose-derived MSCs were cultured to passage (P) 5. Subsequently, the cells were grown in either normal medium alone (control group), the medium supplemented with AICAR (1 mM) and NAM (5 mM), or in the presence of both for 5 weeks to P10. Cell proliferation, differentiation capacity, level of apoptosis and autophagy, morphological changes, total cellular reactive oxygen species (ROS), and activity of mTORC1 and AMPK were compared among different treatment groups.

Results

MSCs treated with AICAR, NAM, or both displayed an increase in proliferation and osteogenic differentiation, which was augmented in the group receiving both. Treatment with AICAR or NAM led to decreased expression of β-galactosidase, reduced accumulation of dysfunctional lysosomes, and characteristic morphologic features of young MSCs. Furthermore, while NAM administration could significantly reduce the total cellular ROS in aged MSCs, AICAR treatment did not. Moreover, AICAR-treated cells possess a high proliferation capacity; however, they also show the highest level of cellular apoptosis. The observed effects of AICAR and NAM were in light of the attenuated mTORC1 activity and increased AMPK activity and autophagy.

Conclusions

Selective inhibition of mTORC1 by AICAR and NAM boosts autophagy, retains MSCs’ self-renewal and multi-lineage differentiation capacity, and postpones senescence-associated changes after prolonged in vitro culture. Additionally, co-administration of AICAR and NAM shows an additive or probably a synergistic effect on cellular senescence.

Exercise effects on γ3-AMPK activity, phosphorylation of Akt2 and AS160, and insulin-stimulated glucose uptake in insulin-resistant rat skeletal muscle

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle. Prior research on healthy muscle suggests that enhanced postexercise ISGU depends on elevated γ3-AMPK activity leading to greater phosphorylation of Akt substrate of 160 kDa (pAS160) on an AMPK-phosphomotif (Ser⁷⁰⁴). Phosphorylation of AS160^Ser704, in turn, may favor greater insulin-stimulated pAS160 on an Akt-phosphomotif (Thr⁶⁴²) that regulates ISGU. Accordingly, we tested if exercise-induced increases in γ3-AMPK activity and pAS160 on key regulatory sites accompany improved ISGU at 3 h postexercise (3hPEX) in insulin-resistant muscle. Rats fed a high-fat diet (HFD; 2-wk) that induces insulin resistance either performed acute swim-exercise (2 h) or were sedentary (SED). SED rats fed a low-fat diet (LFD; 2 wk) served as healthy controls. Isolated epitrochlearis muscles from 3hPEX and SED rats were analyzed for ISGU, pAS160, pAkt2 (Akt-isoform that phosphorylates pAS160^Thr642), and γ1-AMPK and γ3-AMPK activity. ISGU was lower in HFD-SED muscles versus LFD-SED, but this decrement was eliminated in the HFD-3hPEX group. γ3-AMPK activity, but not γ1-AMPK activity, was elevated in HFD-3hPEX muscles versus both SED controls. Furthermore, insulin-stimulated pAS160^Thr642, pAS160^Ser704, and pAkt2^Ser474 in HFD-3hPEX muscles were elevated above HFD-SED and equal to values in LFD-SED muscles, but insulin-independent pAS160^Ser704 was unaltered at 3hPEX. These results demonstrated, for the first time in an insulin-resistant model, that the postexercise increase in ISGU was accompanied by sustained enhancement of γ3-AMPK activation and greater pAkt2^Ser474. Our working hypothesis is that these changes along with enhanced insulin-stimulated pAS160 increase ISGU of insulin-resistant muscles to values equaling insulin-sensitive sedentary controls.NEW & NOTEWORTHY Earlier research focusing on signaling events linked to increased insulin sensitivity in muscle has rarely evaluated insulin resistant muscle after exercise. We assessed insulin resistant muscle after an exercise protocol that improved insulin-stimulated glucose uptake. Prior exercise also amplified several signaling steps expected to favor enhanced insulin-stimulated glucose uptake: increased γ3-AMP-activated protein kinase activity, greater insulin-stimulated Akt2 phosphorylation on Ser474, and elevated insulin-stimulated Akt substrate of 160 kDa phosphorylation on Ser588, Thr642, and Ser704.

Increased Adenine Nucleotide Degradation in Skeletal Muscle Atrophy

Adenine nucleotides (AdNs: ATP, ADP, AMP) are essential biological compounds that facilitate many necessary cellular processes by providing chemical energy, mediating intracellular signaling, and regulating protein metabolism and solubilization. A dramatic reduction in total AdNs is observed in atrophic skeletal muscle across numerous disease states and conditions, such as cancer, diabetes, chronic kidney disease, heart failure, COPD, sepsis, muscular dystrophy, denervation, disuse, and sarcopenia. The reduced AdNs in atrophic skeletal muscle are accompanied by increased expression/activities of AdN degrading enzymes and the accumulation of degradation products (IMP, hypoxanthine, xanthine, uric acid), suggesting that the lower AdN content is largely the result of increased nucleotide degradation. Furthermore, this characteristic decrease of AdNs suggests that increased nucleotide degradation contributes to the general pathophysiology of skeletal muscle atrophy. In view of the numerous energetic, and non-energetic, roles of AdNs in skeletal muscle, investigations into the physiological consequences of AdN degradation may provide valuable insight into the mechanisms of muscle atrophy.

The effects of resveratrol feeding and exercise training on the skeletal muscle function and transcriptome of aged rats

This study investigated the effects of resveratrol feeding and exercise training on the skeletal muscle function and transcriptome of aged rats. Male SD rats (25 months old) were divided into the control group (Old), the daily exercise training group (Trained), and the resveratrol feeding group (Resveratrol). After 6 weeks of intervention, the body mass, grip strength, and gastrocnemius muscle mass were determined, and the muscle samples were analyzed by transcriptome sequencing. The differentially expressed genes were analyzed followed by GO enrichment analysis and KEGG analysis. The Old group showed positive increases in body mass, while both the Trained and Resveratrol groups showed negative growth. No significant differences in the gastrocnemius muscle index and absolute grip strength were found among the three groups. However, the relative grip strength was higher in the Trained group than in the Old group. Only 21 differentially expressed genes were identified in the Trained group vs. the Old group, and 12 differentially expressed genes were identified in the Resveratrol group vs. the Old group. The most enriched GO terms in the Trained group vs. the Old group were mainly associated with RNA metabolic processes and transmembrane transporters, and the significantly upregulated KEGG pathways included mucin-type O-glycan biosynthesis, drug metabolism, and pyrimidine metabolism. The most enriched GO terms in the Resveratrol group vs. the Old group were primarily associated with neurotransmitter transport and synaptic vesicle, and the upregulated KEGG pathways included synaptic vesicle cycle, nicotine addiction, retinol metabolism, insulin secretion, retrograde endocannabinoid signaling, and glutamatergic synapse. Neither exercise training nor resveratrol feeding has a notable effect on skeletal muscle function and related gene expression in aged rats. However, both exercise training and resveratrol feeding have strong effects on weight loss, which is beneficial for reducing the exercise loads of the elderly.

Toxin-induced hormesis may restrain aging

Mild environmental stress might have beneficial effects in aging by activating maintenance and repair processes in cells and organs. These beneficial stress effects fit to the concept of hormesis. Prominent stressors acting in a hormetic way are physical exercises, fasting, cold and heat. This review will introduce some toxins, which have been found to induce hormetic responses in animal models of aging research. To highlight the molecular signature of these hormetic effects we will depict signaling pathways affected by low doses of toxins on cellular and organismic level. As prominent examples for signaling pathways involved in both aging processes as well as toxin responses, PI3K/Akt/mTOR- and AMPK-signal transduction will be described in more detail. Due to the striking overlap of signaling pathways mediating toxin induced responses and aging processes we propose considering the ability of low doses of toxins to slow down the rate of aging.

Postexercise improvement in glucose uptake occurs concomitant with greater γ3-AMPK activation and AS160 phosphorylation in rat skeletal muscle

A single exercise session can increase insulin-stimulated glucose uptake (GU) by skeletal muscle, concomitant with greater Akt substrate of 160 kDa (AS160) phosphorylation on Akt-phosphosites (Thr⁶⁴² and Ser⁵⁸⁸) that regulate insulin-stimulated GU. Recent research using mouse skeletal muscle suggested that ex vivo 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) or electrically stimulated contractile activity-inducing increased γ3-AMPK activity and AS160 phosphorylation on a consensus AMPK-motif (Ser⁷⁰⁴) resulted in greater AS160 Thr⁶⁴² phosphorylation and GU by insulin-stimulated muscle. Our primary goal was to determine whether in vivo exercise that increases insulin-stimulated GU in rat skeletal muscle would also increase γ3-AMPK activity and AS160 site-selective phosphorylation (Ser⁵⁸⁸, Thr⁶⁴², and Ser⁷⁰⁴) immediately postexercise (IPEX) and/or 3 h postexercise (3hPEX). Epitrochlearis muscles isolated from sedentary and exercised (2-h swim exercise; studied IPEX and 3hPEX) rats were incubated with 2-deoxyglucose to determine GU (without insulin at IPEX; without or with insulin at 3hPEX). Muscles were also assessed for γ1-AMPK activity, γ3-AMPK activity, phosphorylated AMPK (pAMPK), and phosphorylated AS160 (pAS160). IPEX versus sedentary had greater γ3-AMPK activity, pAS160 (Ser⁵⁸⁸, Thr⁶⁴², Ser⁷⁰⁴), and GU with unaltered γ1-AMPK activity. 3hPEX versus sedentary had greater γ3-AMPK activity, pAS160 Ser⁷⁰⁴, and GU with or without insulin; greater pAS160 Thr⁶⁴² only with insulin; and unaltered γ1-AMPK activity. These results using an in vivo exercise protocol that increased insulin-stimulated GU in rat skeletal muscle are consistent with the hypothesis that in vivo exercise-induced enhancement of γ3-AMPK activation and AS160 Ser⁷⁰⁴ IPEX and 3hPEX are important for greater pAS160 Thr⁶⁴² and enhanced insulin-stimulated GU by skeletal muscle.

The Role of AMPK in the Regulation of Skeletal Muscle Size, Hypertrophy, and Regeneration

AMPK (5’-adenosine monophosphate-activated protein kinase) is heavily involved in skeletal muscle metabolic control through its regulation of many downstream targets. Because of their effects on anabolic and catabolic cellular processes, AMPK plays an important role in the control of skeletal muscle development and growth. In this review, the effects of AMPK signaling, and those of its upstream activator, liver kinase B1 (LKB1), on skeletal muscle growth and atrophy are reviewed. The effect of AMPK activity on satellite cell-mediated muscle growth and regeneration after injury is also reviewed. Together, the current data indicate that AMPK does play an important role in regulating muscle mass and regeneration, with AMPKα1 playing a prominent role in stimulating anabolism and in regulating satellite cell dynamics during regeneration, and AMPKα2 playing a potentially more important role in regulating muscle degradation during atrophy.

The aging heart

As the elderly segment of the world population increases, it is critical to understand the changes in cardiac structure and function during the normal aging process. In this review, we outline the key molecular pathways and cellular processes that underlie the phenotypic changes in the heart and vasculature that accompany aging. Reduced autophagy, increased mitochondrial oxidative stress, telomere attrition, altered signaling in insulin-like growth factor, growth differentiation factor 11, and 5'- AMP-activated protein kinase pathways are among the key molecular mechanisms underlying cardiac aging. Aging promotes structural and functional changes in the atria, ventricles, valves, myocardium, pericardium, the cardiac conduction system, and the vasculature. We highlight the factors known to accelerate and attenuate the intrinsic aging of the heart and vessels in addition to potential preventive and therapeutic avenues. A greater understanding of the processes involved in cardiac aging may facilitate our ability to mitigate the escalating burden of CVD in older individuals and promote healthy cardiac aging.

Integrated study on comparative transcriptome and skeletal muscle function in aged rats

The present study aimed to reveal aging-related changes in the skeletal muscle of SD rats by comparing transcriptome analysis, integrated with muscle physiological parameters. Ten rats aged 25 months were set as the old group (OG) and ten rats aged 6 months were set as the young group (YG). After 6 weeks of feeding, the body mass, grip strength, and gastrocnemius muscle mass were determined, and the differentially expressed genes were analyzed by transcriptome sequencing, followed by GO enrichment analysis and KEGG analysis. The results showed that the muscle index and the relative grip strength were lower in OG rats than YG rats. The expressions of AMPK, UCP3, IGF-1, several ion channel associated genes and collagen family genes were down-regulated in OG rats. MGMT, one of the strength determining genes and CHRNa1, a subunit of the acetylcholine receptor were up-regulated in OG rats. The present results supply the global transcriptomic information involved in aging related skeletal muscle dysfunction in rats. The reduced expressions of AMPK, IGF-1, and CASK can explain the losses of muscle mass and function in the aged rats. In addition, the up-regulation of MGMT and CHRNa1 also contribute to muscle wasting and weakness during aging.

AMPK orchestrates an elaborate cascade protecting tissue from fibrosis and aging

Fibrosis is a common process characterized by excessive extracellular matrix (ECM) accumulation after inflammatory injury, which is also a crucial cause of aging. The process of fibrosis is involved in the pathogenesis of most diseases of the heart, liver, kidney, lung, and other organs/tissues. However, there are no effective therapies for this pathological alteration. Annually, fibrosis represents a huge financial burden for the USA and the world. 5'-AMP-activated protein kinase (AMPK) is a pivotal energy sensor that alleviates or delays the process of fibrogenesis. In this review, we first present basic background information on AMPK and fibrogenesis and describe the protective roles of AMPK in three fibrogenic phases. Second, we analyze the protective action of AMPK during fibrosis in myocardial, hepatic, renal, pulmonary, and other organs/tissues. Third, we present a comprehensive discussion of AMPK during fibrosis and draw a conclusion. This review highlights recent advances, vital for basic research and clinical drug design, in the regulation of AMPK during fibrosis.

Piperine regulates UCP1 through the AMPK pathway by generating intracellular lactate production in muscle cells

This study characterizes the human metabolic response to piperine, a curcumin extract, and the details of its underlying molecular mechanism. Using ¹H-NMR-based metabolome analysis, we showed the metabolic effect of piperine on skeletal muscle and found that piperine increased the level of intracellular lactate, an important metabolic intermediate that controls expression of several genes involved in mitochondrial activity. Piperine also induced the phosphorylation of AMP-activated protein kinase (AMPK) and its downstream target, acetyl-CoA carboxylase (ACC), while additionally stimulating glucose uptake in an AMPK dependent manner. Piperine also stimulates the p38 mitogen-activated protein kinase (p38 MAPK), an effect that was reversed by pretreatment with compound C, an AMPK inhibitor. Inhibition of p38 MAPK resulted in no piperine-induced glucose uptake. Increased level of lactate resulted in increased expression of mitochondrial uncoupling protein 1 (UCP1), which regulates energy expenditure, thermogenesis, and fat browning. Knock-down of AMPK blocked piperine-induced UCP1 up-regulation, demonstrating the required role of AMPK in this effect. Taken together, these results suggest that piperine leads to benign metabolic effects by activating the AMPK-p38 MAPK signaling pathway and UCP1 expression by activating intracellular lactate production in skeletal muscle.

Age-related changes in AMPK activation: Role for AMPK phosphatases and inhibitory phosphorylation by upstream signaling pathways

AMP-activated protein kinase (AMPK) is a fundamental regulator of energy metabolism, stress resistance, and cellular proteostasis. AMPK signaling controls an integrated signaling network which is involved in the regulation of healthspan and lifespan e.g. via FoxO, mTOR/ULK1, CRCT-1/CREB, and SIRT1 signaling pathways. Several studies have demonstrated that the activation capacity of AMPK signaling declines with aging, which impairs the maintenance of efficient cellular homeostasis and enhances the aging process. However, it seems that the aging process affects AMPK activation in a context-dependent manner since occasionally, it can also augment AMPK activation, possibly attributable to the type of insult and tissue homeostasis. Three protein phosphatases, PP1, PP2A, and PP2C, inhibit AMPK activation by dephosphorylating the Thr172 residue of AMPKα, required for AMPK activation. In addition, several upstream signaling pathways can phosphorylate Ser/Thr residues in the β/γ interaction domain of the AMPKα subunit that subsequently blocks the activation of AMPK. These inhibitory pathways include the insulin/AKT, cyclic AMP/PKA, and RAS/MEK/ERK pathways. We will examine the evidence whether the efficiency of AMPK responsiveness declines during the aging process. Next, we will review the mechanisms involved in curtailing the activation of AMPK. Finally, we will elucidate the potential age-related changes in the inhibitory regulation of AMPK signaling that might be a part of the aging process.

AMP-activated kinase α2 deficiency protects mice from denervation-induced skeletal muscle atrophy

AMP-activated protein kinase (AMPK) is a master regulator of skeletal muscle metabolic pathways. Recently, AMPK activation by AICAR has been shown to increase myofibrillar protein degradation in C2C12 myotubes via stimulating autophagy and ubiquitin proteasome system. However, the impact of AMPKα on denervation induced muscle atrophy has not been tested. In this study, we performed sciatic denervation on hind limb muscles in both wild type (WT) and AMPKα2(-/-) mice. We found that AMPKα was phosphorylated in atrophic muscles following denervation. In addition, deletion of AMPKα2 significantly attenuated denervation induced skeletal muscle wasting and protein degradation, as evidenced by preserved muscle mass and myofiber area, as well as lower levels of ubiquitinated protein, Atrogin-1 and MuRF-1 expression, and LC3-II/I ratio in tibial anterior (TA) muscles. Interestingly, the phosphorylated FoxO3a at Ser253 was significantly decreased in atrophic TA muscles, which was preserved in AMPKα2(-/-) mice. Collectively, our data support the notion that the activation of AMPKα2 contributes to the atrophic effects of denervation.

Mitochondria, muscle health, and exercise with advancing age

Skeletal muscle health is dependent on the optimal function of its mitochondria. With advancing age, decrements in numerous mitochondrial variables are evident in muscle. Part of this decline is due to reduced physical activity, whereas the remainder appears to be attributed to age-related alterations in mitochondrial synthesis and degradation. Exercise is an important strategy to stimulate mitochondrial adaptations in older individuals to foster improvements in muscle function and quality of life.

Expanding roles for AMPK in skeletal muscle plasticity

Skeletal muscle possesses a remarkable plasticity and responds to environmental and physiological challenges by changing its phenotype in terms of size, composition, and metabolic properties. Muscle fibers rapidly adapt to drastic changes in energy demands during exercise through fine-tuning of the balance between catabolic and anabolic processes. One major sensor of energy demand in exercising muscle is AMP-activated protein kinase (AMPK). Recent advances have shed new light on the relevance of AMPK both as a multitask gatekeeper and as an energy regulator in skeletal muscle. Here we summarize recent findings on the function of AMPK in skeletal muscle adaptation to contraction and highlight its role in the regulation of energy metabolism and the control of skeletal muscle regeneration post-injury.