Molecular regulation of human skeletal muscle protein synthesis in response to exercise and nutrients: a compass for overcoming age-related anabolic resistance

Combined Nutrition with Exercise: Fueling the Fight Against Sarcopenia Through a Bibliometric Analysis and Review

Objective

This bibliometric analysis and review aimed to examine the current research status and trends in the combination of nutrition and exercise training for sarcopenia. Additionally, it sought to provide researchers with future research directions in this field.

Methods

Relevant publications were obtained from the Web of Science Core Collection (WoSCC) database, covering the period from January 1995 to October 2023. The collected publications were analyzed using CiteSpace, VOSviewer, Bibliometrix, and Review Manager.

Results

Out of the 2528 retrieved publications, the United States emerged as the leading contributor in terms of publication volume. The University of Texas System was identified as the most productive institution. Luc J C van Loon emerged as the most published author in this field. Analysis of keywords revealed recent hot topics and emerging areas of interest, such as "gut microbiota" and "mechanisms". Upon further evaluation, resistance training (RT) and protein supplementation were identified as the most commonly employed and effective methods.

Conclusion

RT and protein supplementation are widely recognized as effective strategies. Future research should focus on investigating the molecular aspects of sarcopenia. Moreover, the potential therapeutic role of gut microbiota in sarcopenia requires further comprehensive investigation in human subjects to establish its correlation.

Acute ingestion of a ketone monoester, whey protein, or their co-ingestion in the overnight postabsorptive state elicit a similar stimulation of myofibrillar protein synthesis rates in young males: a double-blind randomized trial

BACKGROUND

Ketone bodies may have anabolic effects in skeletal muscle via their capacity to stimulate protein synthesis. Whether orally ingested exogenous ketones can stimulate postprandial myofibrillar protein synthesis (MyoPS) rates with and without dietary protein co-ingestion is unknown.

OBJECTIVES

This study aimed to evaluate the effects of ketone monoester intake and elevated blood β-hydroxybutyrate (β-OHB) concentration, with and without dietary protein co-ingestion, on postprandial MyoPS rates and mechanistic target of rapamycin complex 1 (mTORC1) pathway signaling.

METHODS

In a randomized, double-blind, parallel group design, 36 recreationally active healthy young males (age: 24.2 ± 4.1 y; body fat: 20.9% ± 5.8%; body mass index: 23.4 ± 2 kg/m2) received a primed continuous infusion of L-[ring-2H5]-phenylalanine and ingested one of the following: 1) the ketone monoester (R)-3-hydroxybutyl (R)-3-hydroxybutyrate (KET), 2) 10 g whey protein (PRO), or 3) the combination of both (KET+PRO). Blood and muscle biopsy samples were collected during basal and postprandial (300 min) conditions to assess β-OHB, glucose, insulin, and amino acid concentrations, MyoPS rates, and mTORC1 pathway signaling.

RESULTS

Capillary blood β-OHB concentration increased similarly during postprandial conditions in KET and KET+PRO, with both being greater than PRO from 30 to 180 min (treatment × time interaction: P < 0.001). Postprandial plasma leucine and essential amino acid (EAA) incremental area under the curve (iAUC) over 300 min was greater (treatment: both P < 0.001) in KET+PRO compared with PRO and KET. KET, PRO, and KET+PRO stimulated postprandial MyoPS rates (0-300 min) higher than basal conditions [absolute change: 0.020%/h; (95% CI: 0.013, 0.027%/h), 0.014%/h (95% CI: 0.009, 0.019%/h), 0.019%/h (95% CI: 0.014, 0.024%/h), respectively (time: P < 0.001)], with no difference between treatments (treatment: P = 0.383) or treatment × time interaction (interaction: P = 0.245). mTORC1 pathway signaling responses did not differ between treatments (all P > 0.05).

CONCLUSIONS

Acute oral intake of a ketone monoester, 10 g whey protein, or their co-ingestion in the overnight postabsorptive state elicit a similar stimulation of postprandial MyoPS rates in healthy young males. This trial was registered at clinicaltrials.gov as NCT04565444 (https://clinicaltrials.gov/study/NCT04565444).

Exercise induces tissue-specific adaptations to enhance cardiometabolic health

The risk associated with multiple cancers, cardiovascular disease, diabetes, and all-cause mortality is decreased in individuals who meet the current recommendations for physical activity. Therefore, regular exercise remains a cornerstone in the prevention and treatment of non-communicable diseases. An acute bout of exercise results in the coordinated interaction between multiple tissues to meet the increased energy demand of exercise. Over time, the associated metabolic stress of each individual exercise bout provides the basis for long-term adaptations across tissues, including the cardiovascular system, skeletal muscle, adipose tissue, liver, pancreas, gut, and brain. Therefore, regular exercise is associated with a plethora of benefits throughout the whole body, including improved cardiorespiratory fitness, physical function, and glycemic control. Overall, we summarize the exercise-induced adaptations that occur within multiple tissues and how they converge to ultimately improve cardiometabolic health.

The Plateau in Muscle Growth with Resistance Training: An Exploration of Possible Mechanisms

It is hypothesized that there is likely a finite ability for muscular adaptation. While it is difficult to distinguish between a true plateau following a long-term training period and short-term stalling in muscle growth, a plateau in muscle growth has been attributed to reaching a genetic potential, with limited discussion on what might physiologically contribute to this muscle growth plateau. The present paper explores potential physiological factors that may drive the decline in muscle growth after prolonged resistance training. Overall, with chronic training, the anabolic signaling pathways may become more refractory to loading. While measures of anabolic markers may have some predictive capabilities regarding muscle growth adaptation, they do not always demonstrate a clear connection. Catabolic processes may also constrain the ability to achieve further muscle growth, which is influenced by energy balance. Although speculative, muscle cells may also possess cell scaling mechanisms that sense and regulate their own size, along with molecular brakes that hinder growth rate over time. When considering muscle growth over the lifespan, there comes a point when the anabolic response is attenuated by aging, regardless of whether or not individuals approach their muscle growth potential. Our goal is that the current review opens avenues for future experimental studies to further elucidate potential mechanisms to explain why muscle growth may plateau.

The effects of branched-chain amino acids on muscle protein synthesis, muscle protein breakdown and associated molecular signalling responses in humans: an update

Branched-chain amino acids (BCAA: leucine, isoleucine and valine) are three of the nine indispensable amino acids, and are frequently consumed as a dietary supplement by athletes and recreationally active individuals alike. The popularity of BCAA supplements is largely predicated on the notion that they can stimulate rates of muscle protein synthesis (MPS) and suppress rates of muscle protein breakdown (MPB), the combination of which promotes a net anabolic response in skeletal muscle. To date, several studies have shown that BCAA (particularly leucine) increase the phosphorylation status of key proteins within the mechanistic target of rapamycin (mTOR) signalling pathway involved in the regulation of translation initiation in human muscle. Early research in humans demonstrated that BCAA provision reduced indices of whole-body protein breakdown and MPB; however, there was no stimulatory effect of BCAA on MPS. In contrast, recent work has demonstrated that BCAA intake can stimulate postprandial MPS rates at rest and can further increase MPS rates during recovery after a bout of resistance exercise. The purpose of this evidence-based narrative review is to critically appraise the available research pertaining to studies examining the effects of BCAA on MPS, MPB and associated molecular signalling responses in humans. Overall, BCAA can activate molecular pathways that regulate translation initiation, reduce indices of whole-body and MPB, and transiently stimulate MPS rates. However, the stimulatory effect of BCAA on MPS rates is less than the response observed following ingestion of a complete protein source providing the full complement of indispensable amino acids.

The Resistance Training Effects on Skeletal Muscle Stem Cells in Older Adult: A Systematic Review and Meta-Analysis

The objective of this systematic review and meta-analysis is to examine the effects of resistance exercise training on muscle stem cells in older adults. A database search was performed (PubMed, Scopus, Web of Science and Google Scholar) to identify controlled clinical trials in English language. The mean difference (MD) with 95% confidence intervals (CIs) and overall effect size were calculated for all comparisons. The PEDro scale was used to assess the methodological quality. Nineteen studies were included in the review. The meta-analysis found a significant effect of resistance training (RT) on muscle stem cells in the elderly (difference in means=-0.008, Z=-3.415, P=0.001). Also, muscle stem cells changes were similar in men and women (difference in means=-0.004, Z=-1.558, P=0.119) and significant changes occur in type II muscle fibers (difference in means=-0.017, Z=-7.048, P=0.000). Resistance-type exercise training significantly increased muscle stem cells content in intervention group that this result is similar in men and womenthis increase occurred more in type II muscle fibers.

Leucine ingestion promotes mTOR translocation to the periphery and enhances total and peripheral RPS6 phosphorylation in human skeletal muscle

The activation of the mechanistic target of rapamycin complex 1 (mTORC1), a master regulator of protein synthesis, by anabolic stimuli (such as muscle contraction or essential amino acids) involves its translocation to the cell periphery. Leucine is generally considered the most anabolic of amino acids for its ability to independently modulate muscle protein synthesis. However, it is currently unknown if free leucine impacts region-specific mTORC1-mediated phosphorylation events and protein-protein interactions. In this clinical trial (NCT03952884; registered May 16, 2019), we used immunofluorescence methods to investigate the role of dietary leucine on the postprandial regulation of mTORC1 and ribosomal protein S6 (RPS6), an important downstream readout of mTORC1 activity. Eight young, healthy, recreationally active males (n = 8; 23 ± 3 yrs) ingested 2 g of leucine with vastus lateralis biopsies collected at baseline, 30, 60, and 180 min postprandial. Leucine promoted mTOR translocation to the periphery (~ 18-29%; p ≤ 0.012) and enhanced mTOR localization with the lysosome (~ 16%; both p = 0.049) at 30 and 60 min post-feeding. p-RPS6^Ser240/244 staining intensity, a readout of mTORC1 activity, was significantly elevated at all postprandial timepoints in both the total fiber (~ 14-30%; p ≤ 0.032) and peripheral regions (~ 16-33%; p ≤ 0.014). Additionally, total and peripheral p-RPS6^Ser240/244 staining intensity at 60 min was positively correlated (r = 0.74, p = 0.036; r = 0.80, p = 0.016, respectively) with rates of myofibrillar protein synthesis over 180 min. The ability of leucine to activate mTORC1 in peripheral regions favors an enhanced rate of MPS, as this is the intracellular space thought to be replete with the cellular machinery that facilitates this anabolic process.

Alternative dietary protein sources to support healthy and active skeletal muscle aging

To mitigate the age-related decline in skeletal muscle quantity and quality, and the associated negative health outcomes, it has been proposed that dietary protein recommendations for older adults should be increased alongside an active lifestyle and/or structured exercise training. Concomitantly, there are growing environmental concerns associated with the production of animal-based dietary protein sources. The question therefore arises as to where this dietary protein required for meeting the protein demands of the rapidly aging global population should (or could) be obtained. Various non-animal-derived protein sources possess favorable sustainability credentials, though much less is known (compared with animal-derived proteins) about their ability to influence muscle anabolism. It is also likely that the anabolic potential of various alternative protein sources varies markedly, with the majority of options remaining to be investigated. The purpose of this review was to thoroughly assess the current evidence base for the utility of alternative protein sources (plants, fungi, insects, algae, and lab-grown "meat") to support muscle anabolism in (active) older adults. The solid existing data portfolio requires considerable expansion to encompass the strategic evaluation of the various types of dietary protein sources. Such data will ultimately be necessary to support desirable alterations and refinements in nutritional guidelines to support healthy and active aging, while concomitantly securing a sustainable food future.

The skeletal muscle fiber periphery: A nexus of mTOR-related anabolism

Skeletal muscle anabolism is driven by numerous stimuli such as growth factors, nutrients (i.e., amino acids, glucose), and mechanical stress. These stimuli are integrated by the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) signal transduction cascade. In recent years, work from our laboratory and elsewhere has sought to unravel the molecular mechanisms underpinning the mTOR-related activation of muscle protein synthesis (MPS), as well as the spatial regulation of these mechanisms within the skeletal muscle cell. These studies have suggested that the skeletal muscle fiber periphery is a region of central importance in anabolism (i.e., growth/MPS). Indeed, the fiber periphery is replete with the substrates, molecular machinery, and translational apparatus necessary to facilitate MPS. This review provides a summary of the mechanisms underpinning the mTOR-associated activation of MPS from cell, rodent, and human studies. It also presents an overview of the spatial regulation of mTORC1 in response to anabolic stimuli and outlines the factors that distinguish the periphery of the cell as a highly notable region of skeletal muscle for the induction of MPS. Future research should seek to further explore the nutrient-induced activation of mTORC1 at the periphery of skeletal muscle fibers.

Walking or body weight squat 'activity snacks' increase dietary amino acid utilization for myofibrillar protein synthesis during prolonged sitting

Interrupting prolonged sitting with intermittent exercise enhances postprandial glycemic control but has unknown effects on sensitizing skeletal muscle to dietary amino acids. We hypothesized that brief walking or body weight squats would enhance the utilization of dietary phenylalanine for myofibrillar protein synthesis (MyoPS) during prolonged sitting. Participants (7 males and 5 females; ~23y; ~25.1kg/m²; ~7300 steps/d) completed three 7.5h trials consisting of prolonged sitting (SIT) or sitting with intermittent (every 30 minutes) walking (WALK) or body weight squatting (SQUAT). Two mixed-macronutrient meals (~55:30:15% carbohydrate:fat:protein), enriched with L-[ring-²H₅]phenylalanine or L-[ring-¹³C₆]phenylalanine, were provided to mimic breakfast and lunch. Tracer incorporation into myofibrillar protein was determined from the vastus lateralis with MyoPS estimated using plasma enrichment as precursor surrogate. Phosphorylation of candidate anabolic signaling proteins were determined by immunoblotting. There was no difference between conditions (p≥0.78) in the time course or area under the curve for plasma phenylalanine enrichment. MyoPS was greater (p<0.05, weighted planned comparison) in SQUAT (0.103±0.030%/h) and WALK (0.118±0.037%/h) compared to SIT (0.080±0.032%/h). Compared to SIT, there were moderate-to-large effect sizes, respectively, for SQUAT (ES=0.75; 95% CI -0.10-1.55) and WALK (ES=1.10; 95% CI 0.20-1.91). Fold change in rpS6^Ser240/244 phosphorylation was greater in SQUAT compared to SIT (7.6±2.7 vs. 1.6±0.45 fold, p<0.05) with no difference (p≥0.21) in any other targets measured (4E-BP1^Thr37/46, eEF2^Thr56, mTOR^Ser2448, ERK1/2^{Thr202/Tyr204}). Interrupting prolonged sitting with short 'activity snacks' improves the utilization of dietary amino acids for MyoPS. The long term impact of this practical lifestyle modification for muscle mass or quality should be investigated.

Ubiquitin Ligases in Longevity and Aging Skeletal Muscle

The development and prevalence of diseases associated with aging presents a global health burden on society. One hallmark of aging is the loss of proteostasis which is caused in part by alterations to the ubiquitin-proteasome system (UPS) and lysosome-autophagy system leading to impaired function and maintenance of mass in tissues such as skeletal muscle. In the instance of skeletal muscle, the impairment of function occurs early in the aging process and is dependent on proteostatic mechanisms. The UPS plays a pivotal role in degradation of misfolded and aggregated proteins. For the purpose of this review, we will discuss the role of the UPS system in the context of age-related loss of muscle mass and function. We highlight the significant role that E3 ubiquitin ligases play in the turnover of key components (e.g., mitochondria and neuromuscular junction) essential to skeletal muscle function and the influence of aging. In addition, we will briefly discuss the contribution of the UPS system to lifespan. By understanding the UPS system as part of the proteostasis network in age-related diseases and disorders such as sarcopenia, new discoveries can be made and new interventions can be developed which will preserve muscle function and maintain quality of life with advancing age.

Challenges of rapamycin repurposing as a potential therapeutic candidate for COVID-19: implications for skeletal muscle metabolic health in older persons

The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged as the causative agent of the ongoing coronavirus disease 2019 (COVID-19) pandemic that has spread worldwide, resulting in over 6 million deaths as of March 2022. Older people have been disproportionately affected by the disease, as they have a greater risk of hospitalization, are more vulnerable to severe infection, and have higher mortality than younger patients. Although effective vaccines have been rapidly developed and administered globally, several clinical trials are ongoing to repurpose existing drugs to combat severe infection. One such drug, rapamycin, is currently under study for this purpose, given its immunosuppressant effects that are mediated by its inhibition of the mechanistic target of rapamycin (mTOR), a master regulator of cell growth. Consistent with this premise, acute rapamycin administration in young healthy humans blocks or attenuates mTOR and its downstream effectors, leading to the inhibition of muscle protein synthesis (MPS). Skeletal muscle mass declines when MPS is chronically lower than muscle protein breakdown. This is consequential for older people who are more susceptible to anabolic resistance (i.e., the blunting of MPS) due to reduced activity, sedentariness, or bed rest such as that associated with COVID-19 hospitalization, and who have also demonstrated a delayed or blunted ability to regain inactivity-induced muscle loss. The lack of studies investigating rapamycin administration on skeletal muscle in older people, and the emergence of effective antiviral medications against severe infection, may indicate the reduced relevance of drug repurposing for present or future pandemics.

An Organ-Specific Metabolite Annotation Approach for Ambient Mass Spectrometry Imaging Reveals Spatial Metabolic Alterations of a Whole Mouse Body

Rapid and accurate metabolite annotation in mass spectrometry imaging (MSI) can improve the efficiency of spatially resolved metabolomics studies and accelerate the discovery of reliable in situ disease biomarkers. To date, metabolite annotation tools in MSI generally utilize isotopic patterns, but high-throughput fragmentation-based identification and biological and technical factors that influence structure elucidation are active challenges. Here, we proposed an organ-specific, metabolite-database-driven approach to facilitate efficient and accurate MSI metabolite annotation. Using data-dependent acquisition (DDA) in liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) to generate high-coverage product ions, we identified 1620 unique metabolites from eight mouse organs (brain, liver, kidney, heart, spleen, lung, muscle, and pancreas) and serum. Following the evaluation of the adduct form difference of metabolite ions between LC-MS and airflow-assisted desorption electrospray ionization (AFADESI)-MSI and deciphering organ-specific metabolites, we constructed a metabolite database for MSI consisting of 27,407 adduct ions. An automated annotation tool, MSIannotator, was then created to conduct metabolite annotation in the MSI dataset with high efficiency and confidence. We applied this approach to profile the spatially resolved landscape of the whole mouse body and discovered that metabolites were distributed across the body in an organ-specific manner, which even spanned different mouse strains. Furthermore, the spatial metabolic alteration in diabetic mice was delineated across different organs, exhibiting that differentially expressed metabolites were mainly located in the liver, brain, and kidney, and the alanine, aspartate, and glutamate metabolism pathway was simultaneously altered in these three organs. This approach not only enables robust metabolite annotation and visualization on a body-wide level but also provides a valuable database resource for underlying organ-specific metabolic mechanisms.

Disuse-induced skeletal muscle atrophy in disease and non-disease states in humans: mechanisms, prevention, and recovery strategies

Decreased skeletal muscle contractile activity (disuse) or unloading leads to muscle mass loss, also known as muscle atrophy. The balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB) is the primary determinant of skeletal muscle mass. A reduced mechanical load on skeletal muscle is one of the main external factors leading to muscle atrophy. However, endocrine and inflammatory factors can act synergistically in catabolic states, amplifying the atrophy process and accelerating its progression. Additionally, older individuals display aging-induced anabolic resistance, which can predispose this population to more pronounced effects when exposed to periods of reduced physical activity or mechanical unloading. Different cellular mechanisms contribute to the regulation of muscle protein balance during skeletal muscle atrophy. This review summarizes the effects of muscle disuse on muscle protein balance and the molecular mechanisms involved in muscle atrophy in the absence or presence of disease. Finally, a discussion of the current literature describing efficient strategies to prevent or improve the recovery from muscle atrophy is also presented.

Pathophysiological mechanisms leading to muscle loss in chronic kidney disease

Loss of muscle proteins is a deleterious consequence of chronic kidney disease (CKD) that causes a decrease in muscle strength and function, and can lead to a reduction in quality of life and increased risk of morbidity and mortality. The effectiveness of current treatment strategies in preventing or reversing muscle protein losses is limited. The limitations largely stem from the systemic nature of diseases such as CKD, which stimulate skeletal muscle protein degradation pathways while simultaneously activating mechanisms that impair muscle protein synthesis and repair. Stimuli that initiate muscle protein loss include metabolic acidosis, insulin and IGF1 resistance, changes in hormones, cytokines, inflammatory processes and decreased appetite. A growing body of evidence suggests that signalling molecules secreted from muscle can enter the circulation and subsequently interact with recipient organs, including the kidneys, while conversely, pathological events in the kidney can adversely influence protein metabolism in skeletal muscle, demonstrating the existence of crosstalk between kidney and muscle. Together, these signals, whether direct or indirect, induce changes in the levels of regulatory and effector proteins via alterations in mRNAs, microRNAs and chromatin epigenetic responses. Advances in our understanding of the signals and processes that mediate muscle loss in CKD and other muscle wasting conditions will support the future development of therapeutic strategies to reduce muscle loss.

Skeletal muscle transcriptome response to a bout of endurance exercise in physically active and sedentary older adults

Age-related declines in cardiorespiratory fitness and physical function are mitigated by regular endurance exercise in older adults. This may be due, in part, to changes in the transcriptional program of skeletal muscle following repeated bouts of exercise. However, the impact of chronic exercise training on the transcriptional response to an acute bout of endurance exercise has not been clearly determined. Here, we characterized baseline differences in muscle transcriptome and exercise-induced response in older adults who were active/endurance trained or sedentary. RNA-sequencing was performed on vastus lateralis biopsy specimens obtained before, immediately after, and 3 h following a bout of endurance exercise (40 min of cycling at 60%-70% of heart rate reserve). Using a recently developed bioinformatics approach, we found that transcript signatures related to type I myofibers, mitochondria, and endothelial cells were higher in active/endurance-trained adults and were associated with key phenotypic features including V̇o_2peak, ATP_max, and muscle fiber proportion. Immune cell signatures were elevated in the sedentary group and linked to visceral and intermuscular adipose tissue mass. Following acute exercise, we observed distinct temporal transcriptional signatures that were largely similar among groups. Enrichment analysis revealed catabolic processes were uniquely enriched in the sedentary group at the 3-h postexercise timepoint. In summary, this study revealed key transcriptional signatures that distinguished active and sedentary adults, which were associated with difference in oxidative capacity and depot-specific adiposity. The acute response signatures were consistent with beneficial effects of endurance exercise to improve muscle health in older adults irrespective of exercise history and adiposity.NEW & NOTEWORTHY Muscle transcript signatures associated with oxidative capacity and immune cells underlie important phenotypic and clinical characteristics of older adults who are endurance trained or sedentary. Despite divergent phenotypes, the temporal transcriptional signatures in response to an acute bout of endurance exercise were largely similar among groups. These data provide new insight into the transcriptional programs of aging muscle and the beneficial effects of endurance exercise to promote healthy aging in older adults.

Exogenous Ketone Supplements in Athletic Contexts: Past, Present, and Future

The ketone bodies acetoacetate (AcAc) and β-hydroxybutyrate (βHB) have pleiotropic effects in multiple organs including brain, heart, and skeletal muscle by serving as an alternative substrate for energy provision, and by modulating inflammation, oxidative stress, catabolic processes, and gene expression. Of particular relevance to athletes are the metabolic actions of ketone bodies to alter substrate utilisation through attenuating glucose utilisation in peripheral tissues, anti-lipolytic effects on adipose tissue, and attenuation of proteolysis in skeletal muscle. There has been long-standing interest in the development of ingestible forms of ketone bodies that has recently resulted in the commercial availability of exogenous ketone supplements (EKS). These supplements in the form of ketone salts and ketone esters, in addition to ketogenic compounds such as 1,3-butanediol and medium chain triglycerides, facilitate an acute transient increase in circulating AcAc and βHB concentrations, which has been termed 'acute nutritional ketosis' or 'intermittent exogenous ketosis'. Some studies have suggested beneficial effects of EKS to endurance performance, recovery, and overreaching, although many studies have failed to observe benefits of acute nutritional ketosis on performance or recovery. The present review explores the rationale and historical development of EKS, the mechanistic basis for their proposed effects, both positive and negative, and evidence to date for their effects on exercise performance and recovery outcomes before concluding with a discussion of methodological considerations and future directions in this field.

RPS6 phosphorylation occurs to a greater extent in the periphery of human skeletal muscle fibers, near focal adhesions, after anabolic stimuli

Following anabolic stimuli (mechanical loading and/or amino acid provision) the mechanistic target of rapamycin complex 1 (mTORC1), a master regulator of protein synthesis, translocates toward the cell periphery. However, it is unknown if mTORC1-mediated phosphorylation events occur in these peripheral regions or prior to translocation (i.e. in central regions). We therefore aimed to determine the cellular location of a mTORC1-mediated phosphorylation event, RPS6^Ser240/244, in human skeletal muscle following anabolic stimuli. Fourteen young, healthy males either ingested a protein-carbohydrate beverage (0.25g/kg protein, 0.75g/kg carbohydrate) alone (n=7;23±5yrs;76.8±3.6kg;13.6±3.8%BF, FED) or following a whole-body resistance exercise bout (n=7;22±2yrs;78.1±3.6kg;12.2±4.9%BF, EXFED). Vastus lateralis muscle biopsies were obtained at rest (PRE) and 120 and 300min following anabolic stimuli. RPS6^Ser240/244 phosphorylation measured by immunofluorescent staining or immunoblot was positively correlated (r=0.76, p<0.001). Peripheral staining intensity of p-RPS6^Ser240/244 increased above PRE in both FED and EXFED at 120min (~54% and ~138% respectively, p<0.05) but was greater in EXFED at both post-stimuli time points (p<0.05). The peripheral-central ratio of p-RPS6^240/244 staining displayed a similar pattern, even when corrected for total RPS6 distribution, suggesting RPS6 phosphorylation occurs to a greater extent in the periphery of fibers. Moreover, p-RPS6^Ser240/244 intensity within paxillin-positive regions, a marker of focal adhesion complexes, was elevated at 120min irrespective of stimulus (p=0.006) before returning to PRE at 300min. These data confirm that RPS6^Ser240/244 phosphorylation occurs in the region of human muscle fibers to which mTOR translocates following anabolic stimuli and identifies focal adhesion complexes as a potential site of mTORC1 regulation in vivo.

A muscle-centric view of time-restricted feeding for older adults

PURPOSE OF REVIEW

The practice of time-restricted feeding (TRF) has received fervent interest in recent years as a strategy to mitigate obesity and metabolic disease. We sought to review the implications of TRF for skeletal muscle health and function in aging.

RECENT FINDINGS

TRF has high adherence and can promote body weight loss in older populations. Body weight reductions favor fat mass in the young, however, there is also the potential for undesirable losses in lean mass. There is currently no evidence to support TRF for skeletal muscle function and metabolism in older persons, and only tentative findings in the young. With a narrow eating window of 6-8 h and a prolonged fasting period to minimize daily insulin exposure, TRF may contradict recommended dietary practices for optimizing skeletal muscle anabolism in older people.

SUMMARY

TRF might represent a promising intervention to address obesity and its associated metabolic diseases, however, at present there is insufficient evidence for optimizing skeletal muscle mass or health in older individuals. Further research is needed to: (1) ascertain the impact of TRF on body composition, skeletal muscle anabolism, and autophagy in aging, and; (2) delineate the potentially myoprotective roles of dietary protein and exercise within the framework of TRF in older persons.

LAT1 and SNAT2 Protein Expression and Membrane Localization of LAT1 Are Not Acutely Altered by Dietary Amino Acids or Resistance Exercise Nor Positively Associated with Leucine or Phenylalanine Incorporation in Human Skeletal Muscle

The influx of essential amino acids into skeletal muscle is primarily mediated by the large neutral amino acid transporter 1 (LAT1), which is dependent on the glutamine gradient generated by the sodium-dependent neutral amino acid transporter 2 (SNAT2). The protein expression and membrane localization of LAT1 may be influenced by amino acid ingestion and/or resistance exercise, although its acute influence on dietary amino acid incorporation into skeletal muscle protein has not been investigated. In a group design, healthy males consumed a mixed carbohydrate (0.75 g·kg^-1) crystalline amino acid (0.25 g·kg^-1) beverage enriched to 25% and 30% with LAT1 substrates L-[1-¹³C]leucine (LEU) and L-[ring-²H₅]phenylalanine (PHE), respectively, at rest (FED: n = 7, 23 ± 5 y, 77 ± 4 kg) or after a bout of resistance exercise (EXFED: n = 7, 22 ± 2 y, 78 ± 11 kg). Postprandial muscle biopsies were collected at 0, 120, and 300 min to measure transporter protein expression (immunoblot), LAT1 membrane localization (immunofluorescence), and dietary amino acid incorporation into myofibrillar protein (ΔLEU and ΔPHE). Basal LAT1 and SNAT2 protein contents were correlated with each other (r = 0.55, p = 0.04) but their expression did not change across time in FED or EXFED (all, p > 0.05). Membrane localization of LAT1 did not change across time in FED or EXFED whether measured as outer 1.5 µm intensity or membrane-to-fiber ratio (all, p > 0.05). Basal SNAT2 protein expression was not correlated with ΔLEU or ΔPHE (all, p ≥ 0.05) whereas basal LAT1 expression was negatively correlated with ΔPHE in FED (r = -0.76, p = 0.04) and EXFED (r = -0.81, p = 0.03) but not ΔLEU (p > 0.05). Basal LAT1 membrane localization was not correlated with ΔLEU or ΔPHE (all, p > 0.05). Our results suggest that LAT1/SNAT2 protein expression and LAT1 membrane localization are not influenced by acute anabolic stimuli and do not positively influence the incorporation of dietary amino acids for de novo myofibrillar protein synthesis in healthy young males.

Energy deficiency impairs resistance training gains in lean mass but not strength: A meta-analysis and meta-regression

Short-term energy deficits impair anabolic hormones and muscle protein synthesis. However, the effects of prolonged energy deficits on resistance training (RT) outcomes remain unexplored. Thus, we conducted a systematic review of PubMed and SportDiscus for randomized controlled trials performing RT in an energy deficit (RT+ED) for ≥3 weeks. We first divided the literature into studies with a parallel control group without an energy deficit (RT+CON; Analysis A) and studies without RT+CON (Analysis B). Analysis A consisted of a meta-analysis comparing gains in lean mass (LM) and strength between RT+ED and RT+CON. Studies in Analysis B were matched with separate RT+CON studies for participant and intervention characteristics, and we qualitatively compared the gains in LM and strength between RT+ED and RT+CON. Finally, Analyses A and B were pooled into a meta-regression examining the relationship between the magnitude of the energy deficit and LM. Analysis A showed LM gains were impaired in RT+ED vs RT+CON (effect size (ES) = -0.57, p = 0.02), but strength gains were comparable between conditions (ES = -0.31, p = 0.28). Analysis B supports the impairment of LM in RT+ED (ES: -0.11, p = 0.03) vs RT+CON (ES: 0.20, p < 0.001) but not strength (RT+ED ES: 0.84; RT+CON ES: 0.81). Finally, our meta-regression demonstrated that an energy deficit of ~500 kcal · day^-1 prevented gains in LM. Individuals performing RT to build LM should avoid prolonged energy deficiency, and individuals performing RT to preserve LM during weight loss should avoid energy deficits >500 kcal day^-1 .

α-Hydroxyisocaproic Acid Decreases Protein Synthesis but Attenuates TNFα/IFNγ Co-Exposure-Induced Protein Degradation and Myotube Atrophy via Suppression of iNOS and IL-6 in Murine C2C12 Myotube

There is ongoing debate as to whether or not α-hydroxyisocaproic acid (HICA) positively regulates skeletal muscle protein synthesis resulting in the gain or maintenance of skeletal muscle. We investigated the effects of HICA on mouse C2C12 myotubes under normal conditions and during cachexia induced by co-exposure to TNFα and IFNγ. The phosphorylation of AMPK or ERK1/2 was significantly altered 30 min after HICA treatment under normal conditions. The basal protein synthesis rates measured by a deuterium-labeling method were significantly lowered by the HICA treatment under normal and cachexic conditions. Conversely, myotube atrophy induced by TNFα/IFNγ co-exposure was significantly improved by the HICA pretreatment, and this improvement was accompanied by the inhibition of iNOS expression and IL-6 production. Moreover, HICA also suppressed the TNFα/IFNγ co-exposure-induced secretion of 3-methylhistidine. These results demonstrated that HICA decreases basal protein synthesis under normal or cachexic conditions; however, HICA might attenuate skeletal muscle atrophy via maintaining a low level of protein degradation under cachexic conditions.

Resistance Exercise, Aging, Disuse, and Muscle Protein Metabolism

Skeletal muscle is the organ of locomotion, its optimal function is critical for athletic performance, and is also important for health due to its contribution to resting metabolic rate and as a site for glucose uptake and storage. Numerous endogenous and exogenous factors influence muscle mass. Much of what is currently known regarding muscle protein turnover is owed to the development and use of stable isotope tracers. Skeletal muscle mass is determined by the meal- and contraction-induced alterations of muscle protein synthesis and muscle protein breakdown. Increased loading as resistance training is the most potent nonpharmacological strategy by which skeletal muscle mass can be increased. Conversely, aging (sarcopenia) and muscle disuse lead to the development of anabolic resistance and contribute to the loss of skeletal muscle mass. Nascent omics-based technologies have significantly improved our understanding surrounding the regulation of skeletal muscle mass at the gene, transcript, and protein levels. Despite significant advances surrounding the mechanistic intricacies that underpin changes in skeletal muscle mass, these processes are complex, and more work is certainly needed. In this article, we provide an overview of the importance of skeletal muscle, describe the influence that resistance training, aging, and disuse exert on muscle protein turnover and the molecular regulatory processes that contribute to changes in muscle protein abundance. © 2021 American Physiological Society. Compr Physiol 11:2249-2278, 2021.

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; ~72y; 6F/2M) 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, 60min (early response), and 180min (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 (T.E.) 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 towards therapies that can restore optimal protein synthesis on the transcript specific level to combat negative consequences of inactivity on aging muscle.

Molecular Regulation of Skeletal Muscle Growth and Organelle Biosynthesis: Practical Recommendations for Exercise Training

The regulation of skeletal muscle mass and organelle homeostasis is dependent on the capacity of cells to produce proteins and to recycle cytosolic portions. In this investigation, the mechanisms involved in skeletal muscle mass regulation-especially those associated with proteosynthesis and with the production of new organelles-are presented. Thus, the critical roles of mammalian/mechanistic target of rapamycin complex 1 (mTORC1) pathway and its regulators are reviewed. In addition, the importance of ribosome biogenesis, satellite cells involvement, myonuclear accretion, and some major epigenetic modifications related to protein synthesis are discussed. Furthermore, several studies conducted on the topic of exercise training have recognized the central role of both endurance and resistance exercise to reorganize sarcomeric proteins and to improve the capacity of cells to build efficient organelles. The molecular mechanisms underlying these adaptations to exercise training are presented throughout this review and practical recommendations for exercise prescription are provided. A better understanding of the aforementioned cellular pathways is essential for both healthy and sick people to avoid inefficient prescriptions and to improve muscle function with emergent strategies (e.g., hypoxic training). Finally, current limitations in the literature and further perspectives, notably on epigenetic mechanisms, are provided to encourage additional investigations on this topic.

Antagonistic control of myofiber size and muscle protein quality control by the ubiquitin ligase UBR4 during aging

Sarcopenia is a degenerative condition that consists in age-induced atrophy and functional decline of skeletal muscle cells (myofibers). A common hypothesis is that inducing myofiber hypertrophy should also reinstate myofiber contractile function but such model has not been extensively tested. Here, we find that the levels of the ubiquitin ligase UBR4 increase in skeletal muscle with aging, and that UBR4 increases the proteolytic activity of the proteasome. Importantly, muscle-specific UBR4 loss rescues age-associated myofiber atrophy in mice. However, UBR4 loss reduces the muscle specific force and accelerates the decline in muscle protein quality that occurs with aging in mice. Similarly, hypertrophic signaling induced via muscle-specific loss of UBR4/poe and of ESCRT members (HGS/Hrs, STAM, USP8) that degrade ubiquitinated membrane proteins compromises muscle function and shortens lifespan in Drosophila by reducing protein quality control. Altogether, these findings indicate that these ubiquitin ligases antithetically regulate myofiber size and muscle protein quality control.

Diet quality and a traditional dietary pattern predict lean mass in Australian women: Longitudinal data from the Geelong Osteoporosis Study

Low muscle mass is associated with reduced independence and increased risk for falls and fractures. Identification of modifiable risk factors for low muscle mass is thus imperative. This study aimed to examine the longitudinal relationship between both diet quality and patterns and lean mass in Australian women. Data from n = 494 participants of the Geelong Osteoporosis Study's 10- and 15-year women's follow-ups were used (conducted in 2004-08 and 2011-14, respectively), and participants were aged 21-89 years. Self-reported lifestyle and demographics were collected, and food frequency questionnaire data informed the dietary exposure variables: the Australian Recommended Food Score (ARFS); the Dietary Inflammatory Index (DII); and a posteriori dietary patterns. The outcome, Skeletal Muscle Index (SMI), was calculated from DXA-derived appendicular lean mass (ALM) relative to height (ALM kg/m2). Analyses employed Generalised Estimating Equations. A higher ARFS score positively predicted SMI over 5-years, and adjustments for age and physical activity did not attenuate this relationship (B:0.044, (95%CI 0.004, 0.084) kg/m2). Following adjustment, both an anti-inflammatory diet (B:-0.034, (95%CI -0.070, -0.002) kg/m2) and a 'traditional' dietary pattern predicted higher SMI (B:0.081, (95%CI 0.004, 0.158) kg/m2). No other associations were observed. Our study reinforces the importance of diet quality for healthy, aging muscle mass. Furthermore, a less inflammatory diet and a diet comprising a wide variety of plant and animal foods may be conducive to maintenance of muscle mass in women. Further studies investigating diet quality's impact on various muscle health measures over longer time periods are warranted.

Sport Performance and Manual Therapies: A Review on the Effects on Mitochondrial, Sarcoplasmatic and Ca2+ Flux Response

The present narrative review aims to highlight the possible effects manual therapies could have on cells and mitochondria, as these effects could improve athletic performance management. To this aim, this review summarizes the relationship between mechanical stimulation, with a special focus on physical activity, and cell response based on the most recent mechanobiology findings. Mechanobiology analyzes how cells respond to mechanical stressors coming from the environment. Indeed, endogenous (e.g., blood pressure, heartbeat and gastrointestinal motility) and exogenous (e.g., physical activity and manual therapies) stimuli can induce biochemical and epigenetic modifications that alter protein synthesis with heavy consequences on cell behavior. Mechanical stress can also influence mitochondrial behavior (i.e., biogenesis, autophagy, fusion, fission and energy production), sarcoplasmic response and calcium ion (Ca²⁺) flux. Since manual therapies have been shown to affect the extracellular matrix, which represents a primary source of mechanical stress that may alter both the cytoskeleton and mitochondrial metabolism, it is conceivable manual therapies could also affect cellular and mitochondrial behavior. Lastly, by suggesting possible directions for future laboratory and clinical studies, the authors expect this review to inspire further research on how manual therapies could affect bioenergetic metabolism and, thus, athletic performance.

Low responders to endurance training exhibit impaired hypertrophy and divergent biological process responses in rat skeletal muscle

NEW FINDINGS

What is the central question of this study? The extent to which genetics determines adaptation to endurance versus resistance exercise is unclear. Previously, a divergent selective breeding rat model showed that genetic factors play a major role in the response to aerobic training. Here, we asked: do genetic factors that underpin poor adaptation to endurance training affect adaptation to functional overload? What is the main finding and its importance? Our data show that heritable factors in low responders to endurance training generated differential gene expression that was associated with impaired skeletal muscle hypertrophy. A maladaptive genotype to endurance exercise appears to dysregulate biological processes responsible for mediating exercise adaptation, irrespective of the mode of contraction stimulus.

ABSTRACT

Divergent skeletal muscle phenotypes result from chronic resistance-type versus endurance-type contraction, reflecting the principle of training specificity. Our aim was to determine whether there is a common set of genetic factors that influence skeletal muscle adaptation to divergent contractile stimuli. Female rats were obtained from a genetically heterogeneous rat population and were selectively bred from high responders to endurance training (HRT) or low responders to endurance training (LRT; n = 6/group; generation 19). Both groups underwent 14 days of synergist ablation to induce functional overload of the plantaris muscle before comparison to non-overloaded controls of the same phenotype. RNA sequencing was performed to identify Gene Ontology biological processes with differential (LRT vs. HRT) gene set enrichment. We found that running distance, determined in advance of synergist ablation, increased in response to aerobic training in HRT but not LRT (65 ± 26 vs. -6 ± 18%, mean ± SD, P < 0.0001). The hypertrophy response to functional overload was attenuated in LRT versus HRT (20.1 ± 5.6 vs. 41.6 ± 16.1%, P = 0.015). Between-group differences were observed in the magnitude of response of 96 upregulated and 101 downregulated pathways. A further 27 pathways showed contrasting upregulation or downregulation in LRT versus HRT in response to functional overload. In conclusion, low responders to aerobic endurance training were also low responders for compensatory hypertrophy, and attenuated hypertrophy was associated with differential gene set regulation. Our findings suggest that genetic factors that underpin aerobic training maladaptation might also dysregulate the transcriptional regulation of biological processes that contribute to adaptation to mechanical overload.

Protein Intake, Protein Mealtime Distribution and Seafood Consumption in Elderly Norwegians: Associations with Physical Function and Strength

Protein intake is considered important in the maintenance of muscle health in ageing. However, both the source and mealtime distribution of protein might affect the intake of protein and its effect on muscle protein synthesis. In this study, protein intake, mealtime distribution of protein, and seafood consumption were assessed in 92 older adults (aged 65+), and associations with physical performance (Short Physical Performance Battery (SPPB)), grip strength and gait speed were assessed in a multiple linear regression analysis. The participants had a mean age of 73 ± 8.9 years. Mean protein intake was 1.1 g/kg body weight. Protein intake was well distributed, with coefficient of variance between meals (CV meals) 0.6 ± 0.3. However, dinner had the highest protein intake. No associations were found between the nutrition factors and physical performance or strength; however, this result might have been caused by a ceiling effect in the chosen test batteries, as the mean score on SPPB was 10.3 ± 2.7, and 48.9% of the participants reached the top score of 12 points. Mean grip strength was 44.4 ± 9.4 kg (men) and 26.2 ± 6.8 kg (women). Mean gait speed was 1.0 ± 0.3 m/s. The interaction analysis suggests that there might be gender differences in the effect of seafood consumption on gait speed.

Influence of Fermented Red Clover Extract on Skeletal Muscle in Early Postmenopausal Women: A Double-Blinded Cross-Over Study

Objective: To investigate effects of supplementation with a fermented red clover (RC) extract on signaling proteins related to muscle protein synthesis and breakdown at rest and in response to a resistance exercise bout. Methods: Ten postmenopausal women completed a double-blinded cross-over trial with two different intervention periods performed in random order: (A) RC extract twice daily for 14 days, and (B) placebo drink twice daily for 14 days. The intervention periods were separated by a two-week washout period. After each intervention period a muscle tissue sample was obtained before and three hours after a one-legged resistance exercise bout. Muscle strength was assessed before and after each intervention period. Results: Protein expression of FOXO1 and FOXO3a, two key transcription factors involved in protein degradation, were significantly lower and HSP27, a protein involved in cell protection and prevention of protein aggregation was significantly higher following RC extract compared to placebo. No significant treatment × time interaction was observed for muscle protein expression in response to exercise. However, p-mTOR, p-p70S6k and HSP90 protein content were significantly increased in response to exercise in both groups. Conclusions: This study demonstrates that RC extract supplementation downregulates molecular markers of muscle protein degradation compared to placebo in postmenopausal women.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.

Is power training effective to produce muscle hypertrophy in older adults? A systematic review and meta-analysis

Power training has been suggested to be effective in improving strength, power, and functional capacity in older adults. However, there is still a lack of systematic investigations reporting its effectiveness for muscle hypertrophy. Thus, this study investigated the effect of power training on muscle hypertrophy and compared its magnitude with traditional moderate-velocity resistance training in older adults. A systematic search was conducted to identify clinical trials investigating the effect of power training on muscle hypertrophy (power training vs. control) and/or comparing the effect of power training versus moderate-velocity resistance training for a meta-analytical approach. Ten studies comparing power training to control conditions and 9 studies comparing power training to moderate-velocity resistance training were selected. Three studies were classified as high quality and 2 were preregistered. The meta-analysis showed that power training was superior for muscle hypertrophy compared with control condition (n = 8 studies; standardised mean difference (SMD) = 0.31; 95% confidence interval (CI) = 0.04, 0.58; p = 0.029), and resulted in similar hypertrophy compared with moderate-velocity resistance training (n = 7 studies; SMD = 0.07; 95% CI = –0.18, 0.32; p = 0.50). No significant heterogeneity was observed (p = 0.46 and 0.54, and I2 = 0% and 0%, respectively). Our data suggest that power training is effective for muscle hypertrophy in older adults, with similar effectiveness as moderate-velocity resistance training. (PROSPERO registration no.: CRD42019128951.)

Novelty It is known that power training might be superior to moderate-velocity resistance training for function improvements in older adults, but there was no meta-analysis investigating its effect on muscle hypertrophy. Power training is effective to induce muscle hypertrophy in older adults to a similar extent as moderate-velocity resistance training.

Novel Essential Amino Acid Supplements Following Resistance Exercise Induce Aminoacidemia and Enhance Anabolic Signaling Irrespective of Age: A Proof-of-Concept Trial

We investigated the effects of ingesting a leucine-enriched essential amino acid (EAA) gel alone or combined with resistance exercise (RE) versus RE alone (control) on plasma aminoacidemia and intramyocellular anabolic signaling in healthy younger (28 ± 4 years) and older (71 ± 3 years) adults. Blood samples were obtained throughout the three trials, while muscle biopsies were collected in the postabsorptive state and 2 h following RE, following the consumption of two 50 mL EAA gels (40% leucine, 15 g total EAA), and following RE with EAA (combination (COM)). Protein content and the phosphorylation status of key anabolic signaling proteins were determined via immunoblotting. Irrespective of age, during EAA and COM peak leucinemia (younger: 454 ± 32 µM and 537 ± 111 µM; older: 417 ± 99 µM and 553 ± 136 µM) occurred ~60–120 min post-ingestion (younger: 66 ± 6 min and 120 ± 60 min; older: 90 ± 13 min and 78 ± 12 min). In the pooled sample, the area under the curve for plasma leucine and the sum of branched-chain amino acids was significantly greater in EAA and COM compared with RE. For intramyocellular signaling, significant main effects were found for condition (mTOR (Ser2481), rpS6 (Ser235/236)) and age (S6K1 (Thr421/Ser424), 4E-BP1 (Thr37/46)) in age group analyses. The phosphorylation of rpS6 was of similar magnitude (~8-fold) in pooled and age group data 2 h following COM. Our findings suggest that a gel-based, leucine-enriched EAA supplement is associated with aminoacidemia and a muscle anabolic signaling response, thus representing an effective means of stimulating muscle protein anabolism in younger and older adults following EAA and COM.

Nutritional Supplements to Support Resistance Exercise in Countering the Sarcopenia of Aging

Skeletal muscle plays an indispensable role in metabolic health and physical function. A decrease in muscle mass and function with advancing age exacerbates the likelihood of mobility impairments, disease development, and early mortality. Therefore, the development of non-pharmacological interventions to counteract sarcopenia warrant significant attention. Currently, resistance training provides the most effective, low cost means by which to prevent sarcopenia progression and improve multiple aspects of overall health. Importantly, the impact of resistance training on skeletal muscle mass may be augmented by specific dietary components (i.e., protein), feeding strategies (i.e., timing, per-meal doses of specific macronutrients) and nutritional supplements (e.g., creatine, vitamin-D, omega-3 polyunsaturated fatty acids etc.). The purpose of this review is to provide an up-to-date, evidence-based account of nutritional strategies to enhance resistance training-induced adaptations in an attempt to combat age-related muscle mass loss. In addition, we provide insight on how to incorporate the aforementioned nutritional strategies that may support the growth or maintenance of skeletal muscle and subsequently extend the healthspan of older individuals.

Overexpression of the Tuberous sclerosis complex 2 (TSC2) gene inhibits goat myoblasts proliferation and differentiation in understanding the underlying mechanism of muscle development

The growth of animal skeletal muscle is mainly determined by the synthesis processes of total proteins in skeletal muscle cells, which has a significant impact on the postnatal growth of young animals. An increasing number of studies are focusing on the functions of Tuberous sclerosis complex 2 (TSC2) during the process of cell protein synthesis and growth. However, it is still unclear the effect of whether and how TSC2 on goat myoblasts proliferation and differentiation. Here, we found that TSC2 gene has opposite expression patterns in proliferation and differentiation of myoblasts. An expression vector containing goat TSC2 cDNA sequences linked with pcDNA3.1 plasmid was constructed. Myoblasts proliferation activity was significantly inhibited and cell cycle transition slowed down after the transfection of pcDNA3.1-TSC2 plasmid into goat primary myoblasts by EdU staining, CCK-8 and flow cytometry. Mechanically, we further confirmed that the overexpression TSC2 was able to down-regulate the mRNA and protein expression of mechanistic target of rapamycin (mTOR), p70 ribosomal S6 kinase 1 (p70S6K) and some cell cycle related genes. In addition, the expression of myogenic genes and myotube formation were attenuated. Collectively, all our results of the experiment demonstrate that TSC2 could regulate myoblasts cells proliferation and differentiation via the activation of the mTOR/p70S6K signaling pathway.

Muscle Mass Loss in the Older Critically Ill Population: Potential Therapeutic Strategies

Skeletal muscle plays a critical role in everyday life, and its age-associated reduction has severe health consequences. The pre-existing presence of sarcopenia, combined with anabolic resistance, protein undernutrition, and the pro-catabolic/anti-anabolic milieu induced by aging and exacerbated in critical care, may accelerate the rate at which skeletal muscle is lost in patients with critical illness. Advancements in intensive care unit (ICU)-care provision have drastically improved survival rates; therefore, attention can be redirected toward other significant issues affecting ICU patients (e.g., length of stay, days on ventilation, nosocomial disease development, etc.). Thus, strategies targeting muscle mass and function losses within an ICU setting are essential to improve patient-related outcomes. Notably, loading exercise and protein provision are the most compelling. Many older ICU patients seldom meet the recommended protein intake, and loading exercise is difficult to conduct in the ICU. Nevertheless, the incorporation of physical therapy (PT), neuromuscular electrical stimulation, and early mobilization strategies may be beneficial. Furthermore, a number of nutrition practices within the ICU have been shown to improve patient-related outcomes ((e.g., feeding strategy [i.e., oral, early enteral, or parenteral]), be hypocaloric (∼70%-80% energy requirements), and increase protein provision (∼1.2-2.5 g/kg/d)). The aim of this brief review is to discuss the dysregulation of muscle mass maintenance in an older ICU population and highlight the potential benefits of strategic nutrition practice, specifically protein, and PT within the ICU. Finally, we provide some general guidelines that may serve to counteract muscle mass loss in patients with critical illness.

Leucine-enriched amino acids maintain peripheral mTOR-Rheb localization independent of myofibrillar protein synthesis and mTORC1 signaling postexercise

Postexercise protein ingestion can elevate rates of myofibrillar protein synthesis (MyoPS), mTORC1 activity, and mTOR translocation/protein-protein interactions. However, it is unclear if leucine-enriched essential amino acids (LEAA) can similarly facilitate intracellular mTOR trafficking in humans after exercise. The purpose of this study was to determine the effect of postexercise LEAA (4 g total EAAs, 1.6 g leucine) on acute MyoPS and mTORC1 translocation and signaling. Recreationally active men performed lower-body resistance exercise (5 × 8-10 leg press and leg extension) to volitional failure. Following exercise participants consumed LEAA (n = 8) or an isocaloric carbohydrate drink (PLA; n = 10). MyoPS was measured over 1.5-4 h of recovery by oral pulse of l-[ring-²H₅]-phenylalanine. Phosphorylation of proteins in the mTORC1 pathway were analyzed via immunoblotting and mTORC1-LAMP2/WGA/Rheb colocalization via immunofluorescence microscopy. There was no difference in MyoPS between groups (LEAA = 0.098 ± 0.01%/h; PL = 0.090 ± 0.01%/h; P > 0.05). Exercise increased (P < 0.05) rpS6^Ser240/244(LEAA = 35.3-fold; PLA = 20.6-fold), mTOR^Ser2448(LEAA = 1.8-fold; PLA = 1.2-fold) and 4EBP1^Thr37/46(LEAA = 1.5-fold; PLA = 1.4-fold) phosphorylation irrespective of nutrition (P > 0.05). LAT1 and SNAT2 protein expression were not affected by exercise or nutrient ingestion. mTOR-LAMP2 colocalization was greater in LEAA preexercise and decreased following exercise and supplement ingestion (P < 0.05), yet was unchanged in PLA. mTOR-WGA (cell periphery marker) and mTOR-Rheb colocalization was greater in LEAA compared with PLA irrespective of time-point (P < 0.05). In conclusion, the postexercise consumption of 4 g of LEAA maintains mTOR in peripheral regions of muscle fibers, in closer proximity to its direct activator Rheb, during prolonged recovery independent of differences in MyoPS or mTORC1 signaling compared with PLA ingestion. This intracellular localization of mTOR may serve to "prime" the kinase for future anabolic stimuli.NEW & NOTEWORTHY This is the first study to investigate whether postexercise leucine-enriched amino acid (LEAA) ingestion elevates mTORC1 translocation and protein-protein interactions in human skeletal muscle. Here, we observed that although LEAA ingestion did not further elevate postexercise MyoPS or mTORC1 signaling compared with placebo, mTORC1 peripheral location and interaction with Rheb were maintained. This may serve to "prime" mTORC1 for subsequent anabolic stimuli.

Recent advances in understanding resistance exercise training-induced skeletal muscle hypertrophy in humans

Skeletal muscle plays a pivotal role in the maintenance of physical and metabolic health and, critically, mobility. Accordingly, strategies focused on increasing the quality and quantity of skeletal muscle are relevant, and resistance exercise is foundational to the process of functional hypertrophy. Much of our current understanding of skeletal muscle hypertrophy can be attributed to the development and utilization of stable isotopically labeled tracers. We know that resistance exercise and sufficient protein intake act synergistically and provide the most effective stimuli to enhance skeletal muscle mass; however, the molecular intricacies that underpin the tremendous response variability to resistance exercise-induced hypertrophy are complex. The purpose of this review is to discuss recent studies with the aim of shedding light on key regulatory mechanisms that dictate hypertrophic gains in skeletal muscle mass. We also aim to provide a brief up-to-date summary of the recent advances in our understanding of skeletal muscle hypertrophy in response to resistance training in humans.