Skeletal muscle immobilisation-induced atrophy: mechanistic insights from human studies.

Periods of skeletal muscle disuse lead to rapid declines in muscle mass (atrophy), which is fundamentally underpinned by an imbalance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). The complex interplay of molecular mechanisms contributing to the altered regulation of muscle protein balance during disuse have been investigated but rarely synthesised in the context of humans. This narrative review discusses human models of muscle disuse and the ensuing inversely exponential rate of muscle atrophy. The molecular processes contributing to altered protein balance are explored, with a particular focus on growth and breakdown signalling pathways, mitochondrial adaptations and neuromuscular dysfunction. Finally, key research gaps within the disuse atrophy literature are highlighted providing future avenues to enhance our mechanistic understanding of human disuse atrophy.

Hydrogen sulfide supplementation as a potential treatment for primary mitochondrial diseases.

Primary mitochondrial diseases (PMD) are amongst the most common inborn errors of metabolism causing fatal outcomes within the first decade of life. With marked heterogeneity in both inheritance patterns and physiological manifestations, these conditions present distinct challenges for targeted drug therapy, where effective therapeutic countermeasures remain elusive within the clinic. Hydrogen sulfide (H2S)-based therapeutics may offer a new option for patient treatment, having been proposed as a conserved mitochondrial substrate and post-translational regulator across species, displaying therapeutic effects in age-related mitochondrial dysfunction and neurodegenerative models of mitochondrial disease. H2S can stimulate mitochondrial respiration at sites downstream of common PMD-defective subunits, augmenting energy production, mitochondrial function and reducing cell death. Here, we highlight the primary signalling mechanisms of H2S in mitochondria relevant for PMD and outline key cytoprotective proteins/pathways amenable to post-translational restoration via H2S-mediated persulfidation. The mechanisms proposed here, combined with the advent of potent mitochondria-targeted sulfide delivery molecules, could provide a framework for H2S as a countermeasure for PMD disease progression.

Molecular mechanisms underpinning favourable physiological adaptations to exercise prehabilitation for urological cancer surgery.

BACKGROUND: Surgery for urological cancers is associated with high complication rates and survivors commonly experience fatigue, reduced physical ability and quality of life. High-intensity interval training (HIIT) as surgical prehabilitation has been proven effective for improving the cardiorespiratory fitness (CRF) of urological cancer patients, however the mechanistic basis of this favourable adaptation is undefined. Thus, we aimed to assess the mechanisms of physiological responses to HIIT as surgical prehabilitation for urological cancer. METHODS: Nineteen male patients scheduled for major urological surgery were randomised to complete 4-weeks HIIT prehabilitation (71.6 ± 0.75 years, BMI: 27.7 ± 0.9 kg·m2) or a no-intervention control (71.8 ± 1.1 years, BMI: 26.9 ± 1.3 kg·m2). Before and after the intervention period, patients underwent m. vastus lateralis biopsies to quantify the impact of HIIT on mitochondrial oxidative phosphorylation (OXPHOS) capacity, cumulative myofibrillar muscle protein synthesis (MPS) and anabolic, catabolic and insulin-related signalling. RESULTS: OXPHOS capacity increased with HIIT, with increased expression of electron transport chain protein complexes (C)-II (p = 0.010) and III (p = 0.045); and a significant correlation between changes in C-I (r = 0.80, p = 0.003), C-IV (r = 0.75, p = 0.008) and C-V (r = 0.61, p = 0.046) and changes in CRF. Neither MPS (1.81 ± 0.12 to 2.04 ± 0.14%·day-1, p = 0.39) nor anabolic or catabolic proteins were upregulated by HIIT (p > 0.05). There was, however, an increase in phosphorylation of AS160Thr642 (p = 0.046) post-HIIT. CONCLUSIONS: A HIIT surgical prehabilitation regime, which improved the CRF of urological cancer patients, enhanced capacity for skeletal muscle OXPHOS; offering potential mechanistic explanation for this favourable adaptation. HIIT did not stimulate MPS, synonymous with the observed lack of hypertrophy. Larger trials pairing patient-centred and clinical endpoints with mechanistic investigations are required to determine the broader impacts of HIIT prehabilitation in this cohort, and to inform on future optimisation (i.e., to increase muscle mass).

Adaptability to eccentric exercise training is diminished with age in female mice.

The ability of skeletal muscle to adapt to eccentric contractions has been suggested to be blunted in older muscle. If eccentric exercise is to be a safe and efficient training mode for older adults, preclinical studies need to establish if older muscle can effectively adapt and if not, determine the molecular signatures that are causing this impairment. The purpose of this study was to quantify the extent age impacts functional adaptations of muscle and identify genetic signatures associated with adaptation (or lack thereof). The anterior crural muscles of young (4 mo) and older (28 mo) female mice performed repeated bouts of eccentric contractions in vivo (50 contractions/wk for 5 wk) and isometric torque was measured across the initial and final bouts. Transcriptomics was completed by RNA-sequencing 1 wk following the fifth bout to identify common and differentially regulated genes. When torques post eccentric contractions were compared after the first and fifth bouts, young muscle exhibited a robust ability to adapt, increasing isometric torque 20%-36%, whereas isometric torque of older muscle decreased up to 18% (P ≤ 0.047). Using differential gene expression, young and older muscles shared some common transcriptional changes in response to eccentric exercise training, whereas other transcripts appeared to be age dependent. That is, the ability to express particular genes after repeated bouts of eccentric contractions was not the same between ages. These molecular signatures may reveal, in part, why older muscles do not appear to be as adaptive to exercise training as young muscles.NEW & NOTEWORTHY The ability to adapt to exercise training may help prevent and combat sarcopenia. Here, we demonstrate young mouse muscles get stronger whereas older mouse muscles become weaker after repeated bouts of eccentric contractions, and that numerous genes were differentially expressed between age groups following training. These results highlight that molecular and functional plasticity is not fixed in skeletal muscle with advancing age, and the ability to handle or cope with physical stress may be impaired.

Enhancing European capabilities for application of multi-omics studies in biology and biomedicine space research.

Following on from the NASA twins' study, there has been a tremendous interest in the use of omics techniques in spaceflight. Individual space agencies, NASA's GeneLab, JAXA's ibSLS, and the ESA-funded Space Omics Topical Team and the International Standards for Space Omics Processing (ISSOP) groups have established several initiatives to support this growth. Here, we present recommendations from the Space Omics Topical Team to promote standard application of space omics in Europe. We focus on four main themes: i) continued participation in and coordination with international omics endeavors, ii) strengthening of the European space omics infrastructure including workforce and facilities, iii) capitalizing on the emerging opportunities in the commercial space sector, and iv) capitalizing on the emerging opportunities in human subjects research.

Comparative Analysis of Muscle Atrophy During Spaceflight, Nutritional Deficiency and Disuse in the Nematode Caenorhabditis elegans.

While spaceflight is becoming more common than before, the hazards spaceflight and space microgravity pose to the human body remain relatively unexplored. Astronauts experience muscle atrophy after spaceflight, but the exact reasons for this and solutions are unknown. Here, we take advantage of the nematode C. elegans to understand the effects of space microgravity on worm body wall muscle. We found that space microgravity induces muscle atrophy in C. elegans from two independent spaceflight missions. As a comparison to spaceflight-induced muscle atrophy, we assessed the effects of acute nutritional deprivation and muscle disuse on C. elegans muscle cells. We found that these two factors also induce muscle atrophy in the nematode. Finally, we identified clp-4, which encodes a calpain protease that promotes muscle atrophy. Mutants of clp-4 suppress starvation-induced muscle atrophy. Such comparative analyses of different factors causing muscle atrophy in C. elegans could provide a way to identify novel genetic factors regulating space microgravity-induced muscle atrophy.

Caenorhabditis elegans in microgravity: An omics perspective.

The application of omics to study Caenorhabditis elegans (C. elegans) in the context of spaceflight is increasing, illuminating the wide-ranging biological impacts of spaceflight on physiology. In this review, we highlight the application of omics, including transcriptomics, genomics, proteomics, multi-omics, and integrated omics in the study of spaceflown C. elegans, and discuss the impact, use, and future direction of this branch of research. We highlight the variety of molecular alterations that occur in response to spaceflight, most notably changes in metabolic and neuromuscular gene regulation. These transcriptional features are reproducible and evident across many spaceflown species (e.g., mice and astronauts), supporting the use of C. elegans as a model organism to study spaceflight physiology with translational capital. Integrating tissue-specific, spatial, and multi-omics approaches, which quantitatively link molecular responses to phenotypic adaptations, will facilitate the identification of candidate regulatory molecules for therapeutic intervention and thus represents the next frontiers in C. elegans space omics research.

Proteomic features of skeletal muscle adaptation to resistance exercise training as a function of age.

Resistance exercise training (RET) can counteract negative features of muscle ageing but older age associates with reduced adaptive capacity to RET. Altered muscle protein networks likely contribute to ageing RET adaptation; therefore, associated proteome-wide responses warrant exploration. We employed quantitative sarcoplasmic proteomics to compare age-related proteome and phosphoproteome responses to RET. Thigh muscle biopsies were collected from eight young (25 ± 1.1 years) and eight older (67.5 ± 2.6 years) adults before and after 20 weeks supervised RET. Muscle sarcoplasmic fractions were pooled for each condition and analysed using Isobaric Tags for Relative and Absolute Quantification (iTRAQ) labelling, tandem mass spectrometry and network-based hub protein identification. Older adults displayed impaired RET-induced adaptations in whole-body lean mass, body fat percentage and thigh lean mass (P > 0.05). iTRAQ identified 73 differentially expressed proteins with age and/or RET. Despite possible proteomic stochasticity, RET improved ageing profiles for mitochondrial function and glucose metabolism (top hub; PYK (pyruvate kinase)) but failed to correct altered ageing expression of cytoskeletal proteins (top hub; YWHAZ (14-3-3 protein zeta/delta)). These ageing RET proteomic profiles were generally unchanged or oppositely regulated post-RET in younger muscle. Similarly, RET corrected expression of 10 phosphoproteins altered in ageing, but these responses were again different vs. younger adults. Older muscle is characterised by RET-induced metabolic protein profiles that, whilst not present in younger muscle, improve untrained age-related proteomic deficits. Combined with impaired cytoskeletal adhesion responses, these results provide a proteomic framework for understanding and optimising ageing muscle RET adaptation.

Curcumin Enhances Fed-State Muscle Microvascular Perfusion but Not Leg Glucose Uptake in Older Adults.

Therapeutic interventions aimed at enhancing blood flow may combat the postprandial vascular and metabolic dysfunction that manifests with chronological ageing. We compared the effects of acute curcumin (1000 mg) coupled with an oral nutritional supplement (ONS, 7.5 g protein, 24 g carbohydrate and 6 g fat) versus a placebo and ONS (control) on cerebral and leg macrovascular blood flow, leg muscle microvascular blood flow, brachial artery endothelial function, and leg insulin and glucose responses in healthy older adults (n = 12, 50% male, 73 ± 1 year). Curcumin enhanced m. tibialis anterior microvascular blood volume (MBV) at 180 and 240 min following the ONS (baseline: 1.0 vs. 180 min: 1.08 ± 0.02, p = 0.01 vs. 240 min: 1.08 ± 0.03, p = 0.01), and MBV was significantly higher compared with the control at both time points (p < 0.05). MBV increased from baseline in the m. vastus lateralis at 240 min after the ONS in both groups (p < 0.05), and there were no significant differences between groups. Following the ONS, leg blood flow and leg vascular conductance increased, and leg vascular resistance decreased similarly in both conditions (p < 0.05). Brachial artery flow-mediated dilation and middle cerebral artery blood flow were unchanged in both conditions (p > 0.05). Similarly, the curcumin and control groups demonstrated comparable increases in glucose uptake and insulin in response to the ONS. Thus, acute curcumin supplementation enhanced ONS-induced increases in m. tibialis anterior MBV without potentiating m. vastus lateralis MBV, muscle glucose uptake, or systemic endothelial or macrovascular function in healthy older adults.

Space omics research in Europe: Contributions, geographical distribution and ESA member state funding schemes.

The European research community, via European Space Agency (ESA) spaceflight opportunities, has significantly contributed toward our current understanding of spaceflight biology. Recent molecular biology experiments include "omic" analysis, which provides a holistic and systems level understanding of the mechanisms underlying phenotypic adaptation. Despite vast interest in, and the immense quantity of biological information gained from space omics research, the knowledge of ESA-related space omics works as a collective remains poorly defined due to the recent exponential application of omics approaches in space and the limited search capabilities of pre-existing records. Thus, a review of such contributions is necessary to clarify and promote the development of space omics among ESA and ESA state members. To address this gap, in this review, we i) identified and summarized omics works led by European researchers, ii) geographically described these omics works, and iii) highlighted potential caveats in complex funding scenarios among ESA member states.

Loss of physical contact in space alters the dopamine system in C. elegans.

Progressive neuromuscular decline in microgravity is a prominent health concern preventing interplanetary human habitation. We establish functional dopamine-mediated impairments as a consistent feature across multiple spaceflight exposures and during simulated microgravity in C. elegans. Animals grown continuously in these conditions display reduced movement and body length. Loss of mechanical contact stimuli in microgravity elicits decreased endogenous dopamine and comt-4 (catechol-O-methyl transferase) expression levels. The application of exogenous dopamine reverses the movement and body length defects caused by simulated microgravity. In addition, increased physical contact made comt-4 and dopamine levels rise. It also increased muscular cytoplasmic Ca2+ firing. In dop-3 (D2-like receptor) mutants, neither decrease in movement nor in body length were observed during simulated microgravity growth. These results strongly suggest that targeting the dopamine system through manipulation of the external environment (contact stimuli) prevents muscular changes and is a realistic and viable treatment strategy to promote safe human deep-space travel.

Transcriptomic adaptation during skeletal muscle habituation to eccentric or concentric exercise training.

Eccentric (ECC) and concentric (CON) contractions induce distinct muscle remodelling patterns that manifest early during exercise training, the causes of which remain unclear. We examined molecular signatures of early contraction mode-specific muscle adaptation via transcriptome-wide network and secretome analyses during 2 weeks of ECC- versus CON-specific (downhill versus uphill running) exercise training (exercise 'habituation'). Despite habituation attenuating total numbers of exercise-induced genes, functional gene-level profiles of untrained ECC or CON were largely unaltered post-habituation. Network analysis revealed 11 ECC-specific modules, including upregulated extracellular matrix and immune profiles plus downregulated mitochondrial pathways following untrained ECC. Of 3 CON-unique modules, 2 were ribosome-related and downregulated post-habituation. Across training, 376 ECC-specific and 110 CON-specific hub genes were identified, plus 45 predicted transcription factors. Secreted factors were enriched in 3 ECC- and/or CON-responsive modules, with all 3 also being under the predicted transcriptional control of SP1 and KLF4. Of 34 candidate myokine hubs, 1 was also predicted to have elevated expression in skeletal muscle versus other tissues: THBS4, of a secretome-enriched module upregulated after untrained ECC. In conclusion, distinct untrained ECC and CON transcriptional responses are dampened after habituation without substantially shifting molecular functional profiles, providing new mechanistic candidates into contraction-mode specific muscle regulation.

Green Tea Extract Concurrent with an Oral Nutritional Supplement Acutely Enhances Muscle Microvascular Blood Flow without Altering Leg Glucose Uptake in Healthy Older Adults.

Postprandial macro- and microvascular blood flow and metabolic dysfunction manifest with advancing age, so vascular transmuting interventions are desirable. In this randomised, single-blind, placebo-controlled, crossover trial, we investigated the impact of the acute administration of green tea extract (GTE; containing ~500 mg epigallocatechin-3-gallate) versus placebo (CON), alongside an oral nutritional supplement (ONS), on muscle macro- and microvascular, cerebral macrovascular (via ultrasound) and leg glucose/insulin metabolic responses (via arterialised/venous blood samples) in twelve healthy older adults (42% male, 74 ± 1 y). GTE increased m. vastus lateralis microvascular blood volume (MBV) at 180 and 240 min after ONS (baseline: 1.0 vs. 180 min: 1.11 ± 0.02 vs. 240 min: 1.08 ± 0.04, both p < 0.005), with MBV significantly higher than CON at 180 min (p < 0.05). Neither the ONS nor the GTE impacted m. tibialis anterior perfusion (p > 0.05). Leg blood flow and vascular conductance increased, and vascular resistance decreased similarly in both conditions (p < 0.05). Small non-significant increases in brachial artery flow-mediated dilation were observed in the GTE only and middle cerebral artery blood flow did not change in response to GTE or CON (p > 0.05). Glucose uptake increased with the GTE only (0 min: 0.03 ± 0.01 vs. 35 min: 0.11 ± 0.02 mmol/min/leg, p = 0.007); however, glucose area under the curve and insulin kinetics were similar between conditions (p > 0.05). Acute GTE supplementation enhances MBV beyond the effects of an oral mixed meal, but this improved perfusion does not translate to increased leg muscle glucose uptake in healthy older adults.

Cocoa Flavanols Adjuvant to an Oral Nutritional Supplement Acutely Enhances Nutritive Flow in Skeletal Muscle without Altering Leg Glucose Uptake Kinetics in Older Adults.

Ageing is associated with postprandial muscle vascular and metabolic dysfunction, suggesting vascular modifying interventions may be of benefit. Reflecting this, we investigated the impact of acute cocoa flavanol (450-500 mg) intake (versus placebo control) on vascular (via ultrasound) and glucose/insulin metabolic responses (via arterialised/venous blood samples and ELISA) to an oral nutritional supplement (ONS) in twelve healthy older adults (50% male, 72 ± 4 years), in a crossover design study. The cocoa condition displayed significant increases in m. vastus lateralis microvascular blood volume (MBV) in response to feeding at 180 and 240-min after ONS consumption (baseline: 1.00 vs. 180 min: 1.09 ± 0.03, p = 0.05; 240 min: 1.13 ± 0.04, p = 0.002), with MBV at these timepoints significantly higher than in the control condition (p < 0.05). In addition, there was a trend (p = 0.058) for MBV in m. tibialis anterior to increase in response to ONS in the cocoa condition only. Leg blood flow and vascular conductance increased, and vascular resistance decreased in response to ONS (p < 0.05), but these responses were not different between conditions (p > 0.05). Similarly, glucose uptake and insulin increased in response to ONS (p < 0.05) comparably between conditions (p > 0.05). Thus, acute cocoa flavanol supplementation can potentiate oral feeding-induced increases in MBV in older adults, but this improvement does not relay to muscle glucose uptake.

Exploring the impact of COVID-19 on the willingness of older adults to participate in physiology research: views from past and potential volunteers.

We explored the views of older (≥65 years) past and potential volunteers in regard to participating in physiology research during the COVID-19 pandemic. Using an online questionnaire and focus groups, we found that past volunteers (n = 55) were more likely to take part in both acute (p < 0.05) and chronic (p < 0.05) physiology studies, compared with potential future volunteers (n = 57). Both cohorts demonstrated a positive attitude towards volunteering during the COVID-19 pandemic, although concern was evident. Novelty: Volunteers demonstrated a positive attitude and also concern towards participating in physiology research during COVID-19.

Transcriptomic meta-analysis of disuse muscle atrophy vs. resistance exercise-induced hypertrophy in young and older humans.

BACKGROUND: Skeletal muscle atrophy manifests across numerous diseases; however, the extent of similarities/differences in causal mechanisms between atrophying conditions in unclear. Ageing and disuse represent two of the most prevalent and costly atrophic conditions, with resistance exercise training (RET) being the most effective lifestyle countermeasure. We employed gene-level and network-level meta-analyses to contrast transcriptomic signatures of disuse and RET, plus young and older RET to establish a consensus on the molecular features of, and therapeutic targets against, muscle atrophy in conditions of high socio-economic relevance. METHODS: Integrated gene-level and network-level meta-analysis was performed on publicly available microarray data sets generated from young (18-35 years) m. vastus lateralis muscle subjected to disuse (unilateral limb immobilization or bed rest) lasting ≥7 days or RET lasting ≥3 weeks, and resistance-trained older (≥60 years) muscle. RESULTS: Disuse and RET displayed predominantly separate transcriptional responses, and transcripts altered across conditions were mostly unidirectional. However, disuse and RET induced directly inverted expression profiles for mitochondrial function and translation regulation genes, with COX4I1, ENDOG, GOT2, MRPL12, and NDUFV2, the central hub components of altered mitochondrial networks, and ZMYND11, a hub gene of altered translation regulation. A substantial number of genes (n = 140) up-regulated post-RET in younger muscle were not similarly up-regulated in older muscle, with young muscle displaying a more pronounced extracellular matrix (ECM) and immune/inflammatory gene expression response. Both young and older muscle exhibited similar RET-induced ubiquitination/RNA processing gene signatures with associated PWP1, PSMB1, and RAF1 hub genes. CONCLUSIONS: Despite limited opposing gene profiles, transcriptional signatures of disuse are not simply the converse of RET. Thus, the mechanisms of unloading cannot be derived from studying muscle loading alone and provides a molecular basis for understanding why RET fails to target all transcriptional features of disuse. Loss of RET-induced ECM mechanotransduction and inflammatory profiles might also contribute to suboptimal ageing muscle adaptations to RET. Disuse and age-dependent molecular candidates further establish a framework for understanding and treating disuse/ageing atrophy.

Effects of short-term graded dietary carbohydrate intake on intramuscular and whole body metabolism during moderate-intensity exercise.

Altering dietary carbohydrate (CHO) intake modulates fuel utilization during exercise. However, there has been no systematic evaluation of metabolic responses to graded changes in short-term (< 1 wk) dietary CHO intake. Thirteen active men performed interval running exercise combined with isocaloric diets over 3 days before evaluation of metabolic responses to 60-min running at 65% V̇O2max on three occasions. Diets contained lower [LOW, 2.40 ± 0.66 g CHO·kg-1·day-1, 21.3 ± 0.5% of energy intake (EI)], moderate (MOD, 4.98 ± 1.31 g CHO·kg-1·day-1, 46.3 ± 0.7% EI), or higher (HIGH, 6.48 ± 1.56 g CHO·kg-1·day-1, 60.5 ± 1.6% EI) CHO. Preexercise muscle glycogen content was lower in LOW [54.3 ± 26.4 mmol·kg-1 wet weight (ww)] compared with MOD (82.6 ± 18.8 mmol·kg -1 ww) and HIGH (80.4 ± 26.0 mmol·kg-1 ww, P < 0.001; MOD vs. HIGH, P = 0.85). Whole body substrate oxidation, systemic responses, and muscle substrate utilization during exercise indicated increased fat and decreased CHO metabolism in LOW [respiratory exchange ratio (RER): 0.81 ± 0.01] compared with MOD (RER 0.86 ± 0.01, P = 0.0005) and HIGH (RER: 0.88 ± 0.01, P < 0.0001; MOD vs. HIGH, P = 0.14). Higher basal muscle expression of genes encoding proteins implicated in fat utilization was observed in LOW. In conclusion, muscle glycogen availability and subsequent metabolic responses to exercise were resistant to increases in dietary CHO intake from ∼5.0 to ∼6.5 g CHO·kg-1·day-1 (46% to 61% EI), while muscle glycogen, gene expression, and metabolic responses were sensitive to more marked reductions in CHO intake (∼2.4 g CHO·kg-1·day-1, ∼21% EI).NEW & NOTEWORTHY The data presented here suggest that metabolic responses to steady-state aerobic exercise are somewhat resistant to short-term changes in dietary carbohydrate (CHO) intake within the 5-6.5 g CHO·kg-1·day-1 [46-61% energy intake (EI)] range. In contrast, reduction in short-term dietary CHO intake to ∼2.4 g CHO·kg-1·day-1 (21% EI) evoked clear changes indicative of increased fat and decreased CHO metabolism during exercise.

Spaceflight affects neuronal morphology and alters transcellular degradation of neuronal debris in adult Caenorhabditis elegans.

Extended space travel is a goal of government space agencies and private companies. However, spaceflight poses risks to human health, and the effects on the nervous system have to be better characterized. Here, we exploited the unique experimental advantages of the nematode Caenorhabditis elegans to explore how spaceflight affects adult neurons in vivo. We found that animals that lived 5 days of adulthood on the International Space Station exhibited hyperbranching in PVD and touch receptor neurons. We also found that, in the presence of a neuronal proteotoxic stress, spaceflight promotes a remarkable accumulation of neuronal-derived waste in the surrounding tissues, suggesting an impaired transcellular degradation of debris released from neurons. Our data reveal that spaceflight can significantly affect adult neuronal morphology and clearance of neuronal trash, highlighting the need to carefully assess the risks of long-duration spaceflight on the nervous system and to develop adequate countermeasures for safe space exploration.