Metformin and leucine increase satellite cells and collagen remodeling during disuse and recovery in aged muscle.

Loss of muscle mass and strength after disuse followed by impaired muscle recovery commonly occurs with aging. Metformin (MET) and leucine (LEU) individually have shown positive effects in skeletal muscle during atrophy conditions but have not been evaluated in combination nor tested as a remedy to enhance muscle recovery following disuse atrophy in aging. The purpose of this study was to determine if a dual treatment of metformin and leucine (MET + LEU) would prevent disuse-induced atrophy and/or promote muscle recovery in aged mice and if these muscle responses correspond to changes in satellite cells and collagen remodeling. Aged mice (22-24 months) underwent 14 days of hindlimb unloading (HU) followed by 7 or 14 days of reloading (7 or 14 days RL). MET, LEU, or MET + LEU was administered via drinking water and were compared to Vehicle (standard drinking water) and ambulatory baseline. We observed that during HU, MET + LEU resolved whole body grip strength and soleus muscle specific force decrements caused by HU. Gastrocnemius satellite cell abundance was increased with MET + LEU treatment but did not alter muscle size during disuse or recovery conditions. Moreover, MET + LEU treatment alleviated gastrocnemius collagen accumulation caused by HU and increased collagen turnover during 7 and 14 days RL driven by a decrease in collagen IV content. Transcriptional pathway analysis revealed that MET + LEU altered muscle hallmark pathways related to inflammation and myogenesis during HU. Together, the dual treatment of MET and LEU was able to increase muscle function, satellite cell content, and reduce collagen accumulation, thus improving muscle quality during disuse and recovery in aging.

Short-term metformin ingestion by healthy older adults improves myoblast function.

Muscle progenitor cells (MPCs) in aged muscle exhibit impaired activation into proliferating myoblasts, thereby impairing fusion and changes in secreted factors. The antihyperglycemic drug metformin, currently studied as a candidate antiaging therapy, may have potential to promote function of aged MPCs. We evaluated the impact of 2 wk of metformin ingestion on primary myoblast function measured in vitro after being extracted from muscle biopsies of older adult participants. MPCs were isolated from muscle biopsies of community-dwelling older (4 male/4 female, ∼69 yr) adult participants before (pre) and after (post) the metformin ingestion period and studied in vitro. Cells were extracted from Young participants (4 male/4 female, ∼27 yr) to serve as a "youthful" comparator. MPCs from Old subjects had lower fusion index and myoblast-endothelial cell homing compared with Young, while Old MPCs, extracted after short-term metformin ingestion, performed better at both tasks. Transcriptomic analyses of Old MPCs (vs. Young) revealed decreased histone expression and increased myogenic pathway activity, yet this phenotype was partially restored by metformin. However, metformin ingestion exacerbated pathways related to inflammation signaling. Together, this study demonstrated that 2 wk of metformin ingestion induced persistent effects on Old MPCs that improved function in vitro and altered their transcriptional signature including histone and chromatin remodeling.

Pharmacological inhibition of TLR4 ameliorates muscle and liver ceramide content after disuse in previously physically active mice.

Toll-like receptor 4 (TLR4) is a proposed mediator of ceramide accumulation, muscle atrophy, and insulin resistance in skeletal muscle. It is currently unknown whether pharmacological inhibition of TLR4, using the TLR4-specific inhibitor TAK-242 during muscle disuse, is able to prevent changes in intracellular ceramide species and consequently preserve muscle size and insulin sensitivity in physically active mice. To address this question, we subjected running wheel-conditioned C57BL/6 male mice (13 wk old; ∼10/group) to 7 days of hindlimb suspension (HS), 7 days of continued wheel running (WR), or daily injections of TAK-242 during HS (HS + TAK242) for 7 days. We measured hindlimb muscle morphology, intramuscular and liver ceramide content, HOMA-IR, mRNA proxies of ceramide turnover and lipid trafficking, and muscle fatty acid and glycerolipid content. As a result, soleus and liver ceramide abundance was greater (P < 0.05) in HS vs. WR but was reduced with TLR4 inhibition (HS + TAK-242 vs. HS). Muscle mass declined (P < 0.01) with HS (vs. WR), but TLR4 inhibition did not prevent this loss (soleus: P = 0.08; HS vs. HS + TAK-242). HOMA-IR was impaired (P < 0.01) in HS versus WR mice, but only fasting blood glucose was reduced with TLR4 inhibition (HS + TAK-242 vs HS, P < 0.05). Robust decreases in muscle Spt2 and Cd36 mRNA and muscle lipidomic trafficking may partially explain reductions in ceramides with TLR4 inhibition. In conclusion, pharmacological TLR4 inhibition in wheel-conditioned mice prevented ceramide accumulation during the early phase of hindlimb suspension (7 days) but had little effect on muscle size and insulin sensitivity.

Influence of Exercise Training on Skeletal Muscle Insulin Resistance in Aging: Spotlight on Muscle Ceramides.

: Intramuscular lipid accumulation has been associated with insulin resistance (IR), aging, diabetes, dyslipidemia, and obesity. A substantial body of evidence has implicated ceramides, a sphingolipid intermediate, as potent antagonists of insulin action that drive insulin resistance. Indeed, genetic mouse studies that lower ceramides are potently insulin sensitizing. Surprisingly less is known about how physical activity (skeletal muscle contraction) regulates ceramides, especially in light that muscle contraction regulates insulin sensitivity. The purpose of this review is to critically evaluate studies (rodent and human) concerning the relationship between skeletal muscle ceramides and IR in response to increased physical activity. Our review of the literature indicates that chronic exercise reduces ceramide levels in individuals with obesity, diabetes, or hyperlipidemia. However, metabolically healthy individuals engaged in increased physical activity can improve insulin sensitivity independent of changes in skeletal muscle ceramide content. Herein we discuss these studies and provide context regarding the technical limitations (e.g., difficulty assessing the myriad ceramide species, the challenge of obtaining information on subcellular compartmentalization, and the paucity of flux measurements) and a lack of mechanistic studies that prevent a more sophisticated assessment of the ceramide pathway during increased contractile activity that lead to divergences in skeletal muscle insulin sensitivity.

Aging impairs mouse skeletal muscle macrophage polarization and muscle-specific abundance during recovery from disuse.

Impaired recovery of aged muscle following a disuse event is an unresolved issue facing the older adult population. Although investigations in young animals have suggested that rapid regrowth of skeletal muscle following a disuse event entails a coordinated involvement of skeletal muscle macrophages, this phenomenon has not yet been thoroughly tested as an explanation for impaired muscle recovery in aging. To examine this hypothesis, young (4-5 mo) and old (24-26 mo) male mice were examined as controls following 2 wk of hindlimb unloading (HU) and following 4 (RL4) and 7 (RL7) days of reloading after HU. Muscles were harvested to assess muscle weight, myofiber-specifc cross-sectional area, and skeletal muscle macrophages via immunofluorescence. Flow cytometry was used on gastrocnemius and soleus muscle (at RL4) single-cell suspensions to immunophenotype skeletal muscle macrophages. Our data demonstrated impaired muscle regrowth in aged compared with young mice following disuse, which was characterized by divergent muscle macrophage polarization patterns and muscle-specifc macrophage abundance. During reloading, young mice exhibited the classical increase in M1-like (MHC II+CD206-) macrophages that preceeded the increase in percentage of M2-like macrophages (MHC II-CD206+); however, old mice did not demonstrate this pattern. Also, at RL4, the soleus demonstrated reduced macrophage abundance with aging. Together, these data suggest that dysregulated macrophage phenotype patterns in aged muscle during recovery from disuse may be related to impaired muscle growth. Further investigation is needed to determine whether the dysregulated macrophage response in the old during regrowth from disuse is related to a reduced ability to recruit or activate specific immune cells.

Skeletal muscle ceramides and relationship with insulin sensitivity after 2 weeks of simulated sedentary behaviour and recovery in healthy older adults.

KEY POINTS: Insulin sensitivity (as determined by a hyperinsulinaemic-euglyceamic clamp) decreased 15% after reduced activity. Despite not fully returning to baseline physical activity levels, insulin sensitivity unexpectedly, rebounded above that recorded before 2 weeks of reduced physical activity by 14% after the recovery period. Changes in insulin sensitivity in response to reduced activity were primarily driven by men but, not women. There were modest changes in ceramides (nuclear/myofibrillar fraction and serum) following reduced activity and recovery but, in the absence of major changes to body composition (i.e. fat mass), ceramides were not related to changes in inactivity-induced insulin sensitivity in healthy older adults. ABSTRACT: Older adults are at risk of physical inactivity as they encounter debilitating life events. It is not known how insulin sensitivity is affected by modest short-term physical inactivity and recovery in healthy older adults, nor how insulin sensitivity is related to changes in serum and muscle ceramide content. Healthy older adults (aged 64-82 years, five females, seven males) were assessed before (PRE), after 2 weeks of reduced physical activity (RA) and following 2 weeks of recovery (REC). Insulin sensitivity (hyperinsulinaemic-euglyceamic clamp), lean mass, muscle function, skeletal muscle subfraction, fibre-specific, and serum ceramide content and indices of skeletal muscle inflammation were assessed. Insulin sensitivity decreased by 15 ± 6% at RA (driven by men) but rebounded above PRE by 14 ± 5% at REC. Mid-plantar flexor muscle area and leg strength decreased with RA, although only muscle size returned to baseline levels following REC. Body fat did not change and only minimal changes in muscle inflammation were noted across the intervention. Serum and intramuscular ceramides (nuclear/myofibrillar fraction) were modestly increased at RA and REC. However, ceramides were not related to changes in inactivity-induced insulin sensitivity in healthy older adults. Short-term inactivity induced insulin resistance in older adults in the absence of significant changes in body composition (i.e. fat mass) are not related to changes in ceramides.

Global deletion of CCL2 has adverse impacts on recovery of skeletal muscle fiber size and function and is muscle specific.

Timely and complete recovery of muscle mass and function following a bout of physical disuse are critical components of returning to normal activities of daily living and lifestyle. Proper cross talk between the muscle tissue and myeloid cells (e.g., macrophages) throughout the recovery period from disuse atrophy plays a significant role in the complete resolution of muscle size and function. Chemokine C-C motif ligand 2 (CCL2) has a critical function of recruiting macrophages during the early phase of muscle damage. However, the importance of CCL2 has not been defined in the context of disuse and recovery. Here, we utilized a mouse model of whole body CCL2 deletion (CCL2KO) and subjected them to a period of hindlimb unloading followed by reloading to investigate the importance of CCL2 on the regrowth of muscle following disuse atrophy using ex vivo muscle tests, immunohistochemistry, and fluorescence-activated cell sorting approaches. We show mice that lack CCL2 display an incomplete recovery of gastrocnemius muscle mass, myofiber cross-sectional area, and EDL muscle contractile characteristics during the recovery from disuse atrophy. The soleus and plantaris had limited impact as a result of CCL2 deficiency suggesting a muscle-specific effect. Mice that lack CCL2 have decreased skeletal muscle collagen turnover, which may be related to defects in muscle function and stiffness. In addition, we show that the recruitment of macrophages to gastrocnemius muscle was dramatically reduced in CCL2KO mice during the recovery from disuse atrophy, which likely precipitated poor recovery of muscle size and function and aberrant collagen remodeling.NEW & NOTEWORTHY We provide evidence that the whole body loss of CCL2 in mice has adverse impacts on whole body function and skeletal muscle-specific contractile characteristics and collagen content. These defects in muscle function worsened during the recovery from disuse atrophy and corresponded with decreased recovery of muscle mass. We conclude that the absence of CCL2 decreased recruitment of proinflammatory macrophages to the muscle during the regrowth phase following disuse atrophy resulting in impaired collagen remodeling events and full resolution of muscle morphology and function.

Pharmacological modulation of adrenergic tone alters the vasodilatory response to passive leg movement in young but not in old adults.

The age-related increase in α-adrenergic tone may contribute to decreased leg vascular conductance (LVC) both at rest and during exercise in the old. However, the effect on passive leg movement (PLM)-induced LVC, a measure of vascular function, which is markedly attenuated in this population, is unknown. Thus, in eight young (25 ± 5 yr) and seven old (65 ± 7 yr) subjects, this investigation examined the impact of systemic ß-adrenergic blockade (propanalol, PROP) alone, and PROP combined with either α1-adrenergic stimulation (phenylephrine, PE) or α-adrenergic inhibition (phentolamine, PHEN), on PLM-induced vasodilation. LVC, calculated from femoral artery blood flow and pressure, was determined and PLM-induced Δ peak (LVCΔpeak) and total vasodilation (LVCAUC, area under curve) were documented. PROP decreased LVCΔpeak (PROP: 4.8 ± 1.8, Saline: 7.7 ± 2.7 mL·mmHg-1, P < 0.001) and LVCAUC (PROP: 1.1 ± 0.7, Saline: 2.4 ± 1.6 mL·mmHg-1, P = 0.002) in the young, but not in the old (LVCΔpeak, P = 0.931; LVCAUC, P = 0.999). PE reduced baseline LVC (PE: 1.6 ± 0.4, PROP: 2.3 ± 0.4 mL·min-1·mmHg-1, P < 0.01), LVCΔpeak (PE: 3.2 ± 1.3, PROP: 4.8 ± 1.8 mL·min-1·mmHg-1, P = 0.004), and LVCAUC (PE: 0.5 ± 0.4, PROP: 1.1 ± 0.7 mL·mmHg-1, P = 0.011) in the young, but not in the old (baseline LVC, P = 0.199; LVCΔpeak, P = 0.904; LVCAUC, P = 0.823). PHEN increased LVC at rest and throughout PLM in both groups (drug effect: P < 0.05), however LVCΔpeak was only improved in the young (PHEN: 6.4 ± 3.1, PROP: 4.4 ± 1.5 mL·min-1·mmHg-1, P = 0.004), and not in the old (P = 0.904). Furthermore, the magnitude of α-adrenergic modulation (PHEN - PE) of LVCΔpeak was greater in the young compared with the old (Young: 3.35 ± 2.32, Old: 0.40 ± 1.59 mL·min-1·mmHg-1, P = 0.019). Therefore, elevated α-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.NEW & NOTEWORTHY Stimulation of α1-adrenergic receptors eliminated age-related differences in passive leg movement (PLM) by decreasing PLM-induced vasodilation in the young. Systemic ß-blockade attenuated the central hemodynamic component of the PLM response in young individuals. Inhibition of α-adrenergic receptors did not improve the PLM response in older individuals, though withdrawal of α-adrenergic modulation augmented baseline and maximal vasodilation in both groups. Accordingly, α-adrenergic signaling plays a role in modulating the PLM vasodilatory response in young but not in old adults, and elevated α-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.

Muscle disuse as hindlimb unloading in early postnatal mice negatively impacts grip strength in adult mice: a pilot study.

Physical inactivity has many detrimental effects on health, yet the impact of physical inactivity in early life on muscle health in adulthood remains unknown. Early postnatal malnutrition has prolonged effects into adulthood and we propose that early postnatal (P) physical inactivity would have similar negative effects. To test this hypothesis, we exposed postnatal mice (∼P28, C57BL/6J) to 14 days of physical inactivity (shortly after weaning, from ∼P28 to P42 days of age) in the form of muscle disuse with hindlimb unloading (HU). After this early-life physical inactivity, they were allowed to normally ambulate until 5 mo of age (P140, adulthood) when they underwent 14 days of HU with and without 7-day recovery. They were then tested for physical function (grip strength) and muscles were extracted and weighed. Immunofluorescence was carried out on these muscle cross sections for analysis of myofiber cross-sectional area (fCSA), macrophage density (CD68+ cells), and extracellular matrix (ECM) area. Muscle weights and fCSA and myofiber diameter were used to quantify changes in muscle and fiber size. Compared with age-matched controls, no notable effects of early-life physical inactivity (HU) on skeletal muscle and myofiber size were observed. However, a significant reduction in adult grip strength was observed in those exposed to HU early in life. This was associated with reduced muscle macrophages and increased ECM area. Exposure to a short period of early life disuse has negative enduring effects into adulthood impacting grip strength, muscle macrophages, and muscle composition as low muscle quality.NEW & NOTEWORTHY We demonstrate that early life disuse resulted in less grip strength in adulthood. Analysis of muscle composition demonstrated no loss of whole muscle or myofiber size indicating lower muscle quality akin to premature aging. This poor muscle quality was characterized by altered muscle macrophages and extracellular matrix area. We demonstrate intriguing correlations between this loss of grip strength and muscle macrophages and also area of noncontractile tissue in the muscle.

Disuse-induced muscle fibrosis, cellular senescence, and senescence-associated secretory phenotype in older adults are alleviated during re-ambulation with metformin pre-treatment.

Muscle inflammation and fibrosis underlie disuse-related complications and may contribute to impaired muscle recovery in aging. Cellular senescence is an emerging link between inflammation, extracellular matrix (ECM) remodeling and poor muscle recovery after disuse. In rodents, metformin has been shown to prevent cellular senescence/senescent associated secretory phenotype (SASP), inflammation, and fibrosis making it a potentially practical therapeutic solution. Thus, the purpose of this study was to determine in older adults if metformin monotherapy during bed rest could reduce muscle fibrosis and cellular senescence/SASP during the re-ambulation period. A two-arm controlled trial was utilized in healthy male and female older adults (n = 20; BMI: <30, age: 60 years+) randomized into either placebo or metformin treatment during a two-week run-in and 5 days of bedrest followed by metformin withdrawal during 7 days of recovery. We found that metformin-treated individuals had less type-I myofiber atrophy during disuse, reduced pro-inflammatory transcriptional profiles, and lower muscle collagen deposition during recovery. Collagen content and myofiber size corresponded to reduced whole muscle cellular senescence and SASP markers. Moreover, metformin treatment reduced primary muscle resident fibro-adipogenic progenitors (FAPs) senescent markers and promoted a shift in fibroblast fate to be less myofibroblast-like. Together, these results suggest that metformin pre-treatment improved ECM remodeling after disuse in older adults by possibly altering cellular senescence and SASP in skeletal muscle and in FAPs.

Cardiovasomobility: an integrative understanding of how disuse impacts cardiovascular and skeletal muscle health.

Cardiovasomobility is a novel concept that encompasses the integration of cardiovascular and skeletal muscle function in health and disease with critical modification by physical activity, or lack thereof. Compelling evidence indicates that physical activity improves health while a sedentary, or inactive, lifestyle accelerates cardiovascular and skeletal muscle dysfunction and hastens disease progression. Identifying causative factors for vascular and skeletal muscle dysfunction, especially in humans, has proven difficult due to the limitations associated with cross-sectional investigations. Therefore, experimental models of physical inactivity and disuse, which mimic hospitalization, injury, and illness, provide important insight into the mechanisms and consequences of vascular and skeletal muscle dysfunction. This review provides an overview of the experimental models of disuse and inactivity and focuses on the integrated responses of the vasculature and skeletal muscle in response to disuse/inactivity. The time course and magnitude of dysfunction evoked by various models of disuse/inactivity are discussed in detail, and evidence in support of the critical roles of mitochondrial function and oxidative stress are presented. Lastly, strategies aimed at preserving vascular and skeletal muscle dysfunction during disuse/inactivity are reviewed. Within the context of cardiovasomobility, experimental manipulation of physical activity provides valuable insight into the mechanisms responsible for vascular and skeletal muscle dysfunction that limit mobility, degrade quality of life, and hasten the onset of disease.

Short-term exposure to a clinical dose of metformin increases skeletal muscle mitochondrial H2O2 emission and production in healthy, older adults: A randomized controlled trial.

BACKGROUND AND AIMS: Metformin is the most commonly prescribed medication to treat diabetes. Emerging evidence suggests that metformin could have off target effects that might help promote healthy muscle aging, but these effects have not been thoroughly studied in glucose tolerant older individuals. The purpose of this study was to investigate the short-term effects of metformin consumption on skeletal muscle mitochondrial bioenergetics in healthy older adults. METHODS: We obtained muscle biopsy samples from 16 healthy older adults previously naïve to metformin and treated with metformin (METF; 3F, 5M), or placebo (CON; 3F, 5M), for two weeks using a randomized and blinded study design. Samples were analyzed using high-resolution respirometry, immunofluorescence, and immunoblotting to assess muscle mitochondrial bioenergetics, satellite cell (SC) content, and associated protein markers. RESULTS: We found that metformin treatment did not alter maximal mitochondrial respiration rates in muscle compared to CON. In contrast, mitochondrial H2O2 emission and production were elevated in muscle samples from METF versus CON (METF emission: 2.59 ± 0.72 SE Fold, P = 0.04; METF production: 2.29 ± 0.53 SE Fold, P = 0.02). Furthermore, the change in H2O2 emission was positively correlated with the change in type 1 myofiber SC content and this was biased in METF participants (Pooled: R2 = 0.5816, P = 0.0006; METF: R2 = 0.674, P = 0.0125). CONCLUSIONS: These findings suggest that acute exposure to metformin does not impact mitochondrial respiration in aged, glucose-tolerant muscle, but rather, influences mitochondrial-free radical and SC dynamics. CLINICAL TRIAL REGISTRATION: NCT03107884, clinicaltrials.gov.

Reduced Physical Activity Alters the Leucine-Stimulated Translatome in Aged Skeletal Muscle.

Periods of inactivity experienced by older adults induce nutrient anabolic resistance creating a cascade of skeletal muscle transcriptional and translational aberrations contributing to muscle dysfunction. The purpose of this study was to identify how inactivity alters leucine-stimulated translation of molecules and pathways within the skeletal muscle of older adults. We performed ribosomal profiling alongside RNA sequencing from skeletal muscle biopsies taken from older adults (n = 8; ~72 years; 6 F/2 M) in response to a leucine bolus before (Active) and after (Reduced Activity) 2 weeks of reduced physical activity. At both visits, muscle biopsies were taken at baseline, 60 minutes (early response), and 180 minutes (late response) after leucine ingestion. Previously identified inactivity-related gene transcription changes (PFKFB3, GADD45A, NMRK2) were heightened by leucine with corresponding changes in translation. In contrast, leucine also stimulated translational efficiency of several transcripts in a manner not explained by corresponding changes in mRNA abundance ("uncoupled translation"). Inactivity eliminated this uncoupled translational response for several transcripts, and reduced the translation of most mRNAs encoding for ribosomal proteins. Ingenuity Pathway Analysis identified discordant circadian translation and transcription as a result of inactivity such as translation changes to PER2 and PER3 despite unchanged transcription. We demonstrate inactivity alters leucine-stimulated "uncoupled translation" of ribosomal proteins and circadian regulators otherwise not detectable by traditional RNA sequencing. Innovative techniques such as ribosomal profiling continues to further our understanding of how physical activity mediates translational regulation, and will set a path toward therapies that can restore optimal protein synthesis on the transcript-specific level to combat negative consequences of inactivity on aging muscle.

Impact of presymptomatic COVID-19 on vascular and skeletal muscle function: a case study.

The impact of COVID-19 has been largely described after symptom development. Although the SARS-CoV-2 virus elevates heart rate (HR) prior to symptom onset, whether this virus evokes other presymptomatic alterations is unknown. This case study details the presymptomatic impact of COVID-19 on vascular and skeletal muscle function in a young woman [24 yr, 173.5 cm, 89 kg, body mass index (BMI): 29.6 kg·m-2]. Vascular and skeletal muscle function were assessed as part of a separate study with the first and second visits separated by 2 wk. On the evening following the second visit, the participant developed a fever and a rapid antigen test confirmed a positive COVID-19 diagnosis. Compared with the first visit, the participant presented with a markedly elevated HR (∼30 beats/min) and a lower mean blood pressure (∼8 mmHg) at the second visit. Vascular function measured by brachial artery flow-mediated dilation, reactive hyperemia, and passive leg movement were all noticeably attenuated (25%-65%) as was leg blood flow during knee extension exercise. Muscle strength was diminished as was ADP-stimulated respiration (30%), assessed in vitro, whereas there was a 25% increase in the apparent Km. Lastly, an elevation in IL-10 was observed prior to symptom onset. Notably, 2.5 mo after diagnosis symptoms of fatigue and cough were still present. Together, these findings provide unique insight into the physiological responses immediately prior to onset of COVID-19 symptoms; they suggest that SARS-CoV-2 negatively impacts vascular and skeletal muscle function prior to the onset of common symptoms and may set the stage for the widespread sequelae observed following COVID-19 diagnosis.NEW & NOTEWORTHY This unique case study details the impact of SARS-CoV-2 infection on vascular and skeletal muscle function in a young predominantly presymptomatic woman. Prior to COVID-19 diagnosis, substantial reductions in vascular, skeletal muscle, and mitochondrial function were observed along with an elevation in IL-10. This integrative case study indicates that the presymptomatic impact of COVID-19 is widespread and may help elucidate the acute and long-term sequelae of this disease.

Acute Effects of Cheddar Cheese Consumption on Circulating Amino Acids and Human Skeletal Muscle.

Cheddar cheese is a protein-dense whole food and high in leucine content. However, no information is known about the acute blood amino acid kinetics and protein anabolic effects in skeletal muscle in healthy adults. Therefore, we conducted a crossover study in which men and women (n = 24; ~27 years, ~23 kg/m2) consumed cheese (20 g protein) or an isonitrogenous amount of milk. Blood and skeletal muscle biopsies were taken before and during the post absorptive period following ingestion. We evaluated circulating essential and non-essential amino acids, insulin, and free fatty acids and examined skeletal muscle anabolism by mTORC1 cellular localization, intracellular signaling, and ribosomal profiling. We found that cheese ingestion had a slower yet more sustained branched-chain amino acid circulation appearance over the postprandial period peaking at ~120 min. Cheese also modestly stimulated mTORC1 signaling and increased membrane localization. Using ribosomal profiling we found that, though both milk and cheese stimulated a muscle anabolic program associated with mTORC1 signaling that was more evident with milk, mTORC1 signaling persisted with cheese while also inducing a lower insulinogenic response. We conclude that Cheddar cheese induced a sustained blood amino acid and moderate muscle mTORC1 response yet had a lower glycemic profile compared to milk.

Neuromuscular electrical stimulation and protein during bed rest increases CD11b+ skeletal muscle macrophages but does not correspond to muscle size or insulin sensitivity.

With this cohort, we previously demonstrated preservation of thigh lean tissue with neuromuscular electrical stimulation combined with protein supplementation (NMES+PRO) treatment during bed rest in healthy older adults. Because macrophage polarization plays a significant role in the repair and maintenance of muscle size and insulin sensitivity, we hypothesized that muscle macrophages would be induced by NMES+PRO and would correspond to an increase in lean mass and an attenuated insulin resistance response altered by bed rest. Older adults (60-80 years old; body mass index < 30 kg/m2) underwent 5 days of bed rest and were randomized to either thrice daily treatment of NMES+PRO (n = 8) or CON (n = 8). Lean mass, insulin sensitivity, and markers of muscle macrophages, inflammation, and connective tissue were determined before and after bed rest. Glucose intolerance and insulin resistance occurred after bed rest but there was not a treatment effect (p > 0.10). Proinflammatory-like macrophages (CD11b+, CD206-) increased (p < 0.05) with NMES+PRO treatment and was different than CON. Minor changes in noncontractile tissue were observed. However, changes in muscle macrophages or extracellular matrix were not related to the preservation of thigh lean mass or insulin resistance. Daily NMES+PRO treatment during bed rest induced a muscle proinflammatory-like macrophage response and was unrelated to muscle size or metabolic function. This study is listed as clinical trial NCT02566590. Novelty Neuromuscular electrical stimulation combined with protein supplementation (NMES+PRO) increased proinflammatory-like macrophages and extracellular matrix content in older adults after bed rest. NMES+PRO changes in macrophages and noncontractile tissue macrophages were not related to muscle size preservation or insulin sensitivity.

Ceramide Biomarkers Predictive of Cardiovascular Disease Risk Increase in Healthy Older Adults After Bed Rest.

Acute bed rest places older adults at risk for health complications by disrupting homeostasis in many organ systems, including the cardiovascular system. Circulating ceramides are emerging biomarkers predictive of cardiovascular and metabolic health and have recently been shown to be sensitive indices of cardiovascular (CV) risk. Therefore, the purpose of this study was to characterize the time course of changes in circulating ceramides in healthy younger and older adults after 5 days of bed rest and to determine whether short-term bed rest alters CV-related circulating ceramides. We hypothesized that circulating ceramides predictive of poor cardiometabolic outcomes would increase following 5 days of bed rest. Thirty-five healthy younger and older men and women (young: n = 13, old: n = 22) underwent 5 days of controlled bed rest. Fasting blood samples collected daily during the course of bed rest were used to measure circulating ceramides, lipoproteins, adiponectin, and fibroblast growth factor 21 (FGF21) levels. The primary findings were that circulating ceramides decreased while ceramide ratios and the cardiac event risk test 1 score were increased primarily in older adults, and these findings were independent of changes in circulating lipoprotein levels. Additionally, we found that changes in circulating adiponectin, FGF21 and the 6-minute walk test (6MW) inversely correlated with CV-related circulating ceramides after bed rest. The results of this study highlight the sensitivity of circulating ceramides to detect potential CV dysfunction that may occur with acute physical disuse in aging.

Absence of MyD88 from Skeletal Muscle Protects Female Mice from Inactivity-Induced Adiposity and Insulin Resistance.

OBJECTIVE: Inactivity and inflammation are linked to obesity and insulin resistance. It was hypothesized that MyD88 (mediates inflammation) knockout from muscle (MusMyD88-/- ) would prevent, whereas miR146a-/- (MyD88 inhibitor) would exacerbate, inactivity-induced metabolic disturbances. METHODS: Cre-control, MusMyD88-/- , and miR146a-/- mice were given running wheels for 5 weeks to model an active phenotype. Afterward, half were placed into a small mouse cage (SMC) to restrict movement for 8 days. Body composition, muscle (3 H)2-deoxyglucose uptake, visceral fat histology, and tissue weight (hind limb muscles, visceral fat, and liver) were assessed. In skeletal muscle and visceral fat, RNA sequencing and mitochondrial function were performed on female MusMyD88-/- and Cre-control SMC mice. RESULTS: The SMC induced adiposity, hyperinsulinemia, and muscle insulin-stimulated glucose uptake, which was worsened in miR146a-/- mice. In females, MusMyD88-/- mice were protected. Female MusMyD88-/- mice during the SMC period (vs. Cre-control) exhibited higher Igf1 and decreased Ip6k3 and Trim63 muscle expression. Visceral fat transcript changes corresponded to improved lipid metabolism, decreased adipose expansion (Gulp1↑, Anxa2↓, Ehd1↓) and meta-inflammation (Hmox1↓), and increased beiging (Fgf10↑). Ralgapa2, negative regulator of GLUT4 translocation, and inflammation-related gene 993011J21Rik2 were decreased in both muscle and fat. CONCLUSIONS: Whole-body miR146a-/- exacerbated inactivity-induced fat gain and muscle insulin resistance, whereas MusMyD88-/- prevented insulin resistance in female mice.

Disuse-induced insulin resistance susceptibility coincides with a dysregulated skeletal muscle metabolic transcriptome.

Short-term muscle disuse is characterized by skeletal muscle insulin resistance, although this response is divergent across subjects. The mechanisms regulating inactivity-induced insulin resistance between populations that are more or less susceptible to disuse-induced insulin resistance are not known. RNA sequencing was conducted on vastus lateralis muscle biopsies from subjects before and after bed rest (n = 26) to describe the transcriptome of inactivity-induced insulin resistance. Subjects were separated into Low (n = 14) or High (n = 12) Susceptibility Groups based on the magnitude of change in insulin sensitivity after 5 days of bed rest. Both groups became insulin-resistant after bed rest, and there were no differences between groups in nonmetabolic characteristics (body mass, body mass index, fat mass, and lean mass). The High Susceptibility Group had more genes altered >1.5-fold (426 high versus 391 low) and more than twofold (73 high versus 55 low). Twenty-four genes were altered more than twofold in the High Susceptibility Group that did not change in the Low Susceptibility Group. 95 gene changes correlated with the changes in insulin sensitivity; 6 of these genes changed more than twofold in the High Susceptibility Group. Participants in the High Susceptibility Group were uniquely characterized with muscle gene responses described by a decrease in pathways responsible for lipid uptake and oxidation, decreased capacity for triglyceride export (APOB), increased lipogenesis (i.e., PFKFB3, FASN), and increased amino acid export (SLC43A1). These transcriptomic data provide a comprehensive examination of pathways and genes that may be useful biomarkers, or novel targets to offset muscle disuse-induced insulin resistance. NEW & NOTEWORTHY Short-term muscle disuse results in skeletal muscle insulin resistance through mechanisms that are not fully understood. Following a 5-day bed rest intervention, subjects were divided into High and Low Susceptibility Groups to inactivity-induced insulin resistance. This was followed by a genome-wide transcriptional analysis on muscle biopsy samples to gain insight on divergent insulin sensitivity responses. Our primary finding was that the skeletal muscle of subjects who experienced the most inactivity-induced insulin resistance (high susceptibility) was characterized by a decreased preference for lipid oxidation, increased lipogenesis, and increased amino acid export.

Age-dependent skeletal muscle transcriptome response to bed rest-induced atrophy.

Short-term muscle disuse induces significant muscle loss in older adults and in some reports may be more accelerated with aging. Identifying muscle transcriptional events in response to bed rest may help identify therapeutic targets to offset muscle loss. Therefore, we compared the muscle transcriptome between young and older adults after bed rest and identified candidate targets related to changes in muscle loss. RNA was sequenced (HiSeq, Illumina; DESeq, R) from muscle biopsies obtained from young [ n = 9; 23 yr (SD 3)] and older [ n = 18; 68 yr (SD 6)] adults before and after 5-day bed rest. Significantly altered pathways in both young and old subjects relating to mechanosensing and cell adhesion (Actin Cytoskeleton Signaling, ILK Signaling, RhoA Signaling, and Integrin Signaling) were altered (activation z score) to a greater extent in old subjects. Hepatic Fibrosis/Hepatic Stellate Cell Activation was the top regulated pathway significantly altered only in the old. Fifty-one differentially regulated genes were only altered in the young after bed rest and resembled a gene expression profile like that in the old at baseline. Inflammation and muscle wasting genes (CXCL2, GADD45A) were uniquely increased in the old after bed rest, and the macrophage gene MAFB decreased in the old and correlated with the change in leg lean mass. In summary, skeletal muscle dysregulation during bed rest in the old may be driven by alterations in molecules related to fibrosis, inflammation, and cell adhesion. This information may aid in the development of mechanistic-based therapies to combat muscle atrophy during short-term disuse. NEW & NOTEWORTHY Using RNA sequencing and bioinformatics approaches, we identified that older adult skeletal muscle was characterized by dysregulated pathways associated with fibrosis, inflammation (upregulated), and cell adhesion and mechanosensing (downregulated) pathways, with a subset of genes differentially regulated in old and young muscle after bed rest that may describe predisposition to muscle loss. Unique upregulated genes only expressed in old muscle after bed rest indicated increased inflammation and muscle wasting (CXCL2, GADD45A) and decreased MAFB correlated with the change in leg lean mass.