Characterization of the mechanical properties of the mouse Achilles tendon enthesis by microindentation. Effects of unloading and subsequent reloading.

The fibrocartilaginous tendon enthesis, i.e. the site where a tendon is attached to bone through a fibrocartilaginous tissue, is considered as a functionally graded interface. However, at local scale, a very limited number of studies have characterized micromechanical properties of this transitional tissue. The first goal of this work was to characterize the micromechanical properties of the mineralized part of the healthy Achilles tendon enthesis (ATE) through microindentation testing and to assess the degree of mineralization and of carbonation of mineral crystals by Raman spectroscopy. Since little is known about enthesis biological plasticity, our second objective was to examine the effects of unloading and reloading, using a mouse hindlimb-unloading model, on both the micromechanical properties and the mineral phase of the ATE. Elastic modulus, hardness, degree of mineralization, and degree of carbonation were assessed after 14 days of hindlimb suspension and again after a subsequent 6 days of reloading. The elastic modulus gradually increased along the mineralized part of the ATE from the tidemark to the subchondral bone, with the same trend being found for hardness. Whereas the degree of carbonation did not differ according to zone of measurement, the degree of mineralization increased by >70 % from tidemark to subchondral bone. Thus, the gradient in micromechanical properties is in part explained by a mineralization gradient. A 14-day unloading period did not appear to affect the gradient of micromechanical properties of the ATE, nor the degree of mineralization or carbonation. However, contrary to a short period of unloading, early return to normal mechanical load reduced the micromechanical properties gradient, regardless of carbonate-to-phosphate ratios, likely due to the more homogeneous degree of mineralization. These findings provide valuable data not only for tissue bioengineering, but also for musculoskeletal clinical studies and microgravity studies focusing on long-term space travel by astronauts.

Comprehensive assessment of physiological responses in women during the ESA dry immersion VIVALDI microgravity simulation.

Astronauts in microgravity experience multi-system deconditioning, impacting their inflight efficiency and inducing dysfunctions upon return to Earth gravity. To fill the sex gap of knowledge in the health impact of spaceflights, we simulate microgravity with a 5-day dry immersion in 18 healthy women (ClinicalTrials.gov Identifier: NCT05043974). Here we show that dry immersion rapidly induces a sedentarily-like metabolism shift mimicking the beginning of a metabolic syndrome with a drop in glucose tolerance, an increase in the atherogenic index of plasma, and an impaired lipid profile. Bone remodeling markers suggest a decreased bone formation coupled with an increased bone resorption. Fluid shifts and muscular unloading participate to a marked cardiovascular and sensorimotor deconditioning with decreased orthostatic tolerance, aerobic capacity, and postural balance. Collected datasets provide a comprehensive multi-systemic assessment of dry immersion effects in women and pave the way for future sex-based evaluations of countermeasures.

Effects of hindlimb unloading and subsequent reloading on the structure and mechanical properties of Achilles tendon-to-bone attachment.

While muscle and bone adaptations to deconditioning have been widely described, few studies have focused on the tendon enthesis. Our study examined the effects of mechanical loading on the structure and mechanical properties of the Achilles tendon enthesis. We assessed the fibrocartilage surface area, the organization of collagen, the expression of collagen II, the presence of osteoclasts, and the tensile properties of the mouse enthesis both after 14 days of hindlimb suspension (HU) and after a subsequent 6 days of reloading. Although soleus atrophy was severe after HU, calcified fibrocartilage (CFc) was a little affected. In contrast, we observed a decrease in non-calcified fibrocartilage (UFc) surface area, collagen fiber disorganization, modification of morphological characteristics of the fibrocartilage cells, and altered collagen II distribution. Compared to the control group, restoring normal loads increased both UFc surface area and expression of collagen II, and led to a crimp pattern in collagen. Reloading induced an increase in CFc surface area, probably due to the mineralization front advancing toward the tendon. Functionally, unloading resulted in decreased enthesis stiffness and a shift in site of failure from the osteochondral interface to the bone, whereas 6 days of reloading restored the original elastic properties and site of failure. In the context of spaceflight, our results suggest that care must be taken when performing countermeasure exercises both during missions and during the return to Earth.

Severe Muscle Deconditioning Triggers Early Extracellular Matrix Remodeling and Resident Stem Cell Differentiation into Adipocytes in Healthy Men.

Besides the loss of muscle mass and strength, increased intermuscular adipose tissue (IMAT) is now a well-recognized consequence of muscle deconditioning as experienced in prolonged microgravity. IMAT content may alter the muscle stem cell microenvironment. We hypothesized that extracellular matrix structure alterations and microenvironment remodeling induced by fast and severe muscle disuse could modulate fibro-adipogenic progenitor fate and behavior. We used the dry immersion (DI) model that rapidly leads to severe muscle deconditioning due to drastic hypoactivity. We randomly assigned healthy volunteers (n = 18 men) to the control group (only DI, n = 9; age = 33.8 ± 4) or to the DI + thigh cuff group (n = 9; age = 33.4 ± 7). Participants remained immersed in the supine position in a thermo-neutral water bath for 5 days. We collected vastus lateralis biopsies before (baseline) and after DI. 5 days of DI are sufficient to reduce muscle mass significantly, as indicated by the decreased myofiber cross-sectional area in vastus lateralis samples (−18% vs. baseline, p < 0.05). Early and late adipogenic differentiation transcription factors protein levels were upregulated. Platelet-derived growth Factors alpha (PDGFR⍺) protein level and PDGFR⍺-positive cells were increased after 5 days of DI. Extracellular matrix structure was prone to remodeling with an altered ECM composition with 4 major collagens, fibronectin, and Connective Tissue Growth Factor mRNA decreases (p < 0.001 vs. baseline). Wearing thigh cuffs did not have any preventive effect on the measured variable. Our results show that altered extracellular matrix structure and signaling pathways occur early during DI, a severe muscle wasting model, favoring fibro-adipogenic progenitor differentiation into adipocytes.

Ergothioneine Improves Aerobic Performance Without Any Negative Effect on Early Muscle Recovery Signaling in Response to Acute Exercise.

Physical activity is now recognized as an essential element of healthy lifestyles. However, intensive and repeated exercise practice produces a high level of stress that must be managed, particularly oxidative damage and inflammation. Many studies investigated the effect of antioxidants, but reported only few positive effects, or even muscle recovery impairment. Secondary antioxidants are frequently highlighted as a way to optimize these interactions. Ergothioneine is a potential nutritional supplement and a secondary antioxidant that activates the cellular NRF2 pathway, leading to antioxidant response gene activation. Here, we hypothesized that ergothioneine could improve performance during aerobic exercise up to exhaustion and reduce exercise-related stress without impairing early muscle recovery signaling. To test this hypothesis, 5-month-old C56B6J female mice were divided in two groups matched for maximal aerobic speed (MAS): control group (Ctrl; n = 9) and group supplemented with 70 mg ergothioneine/kg/day (ET; n = 9). After 1 week of supplementation (or not), mice performed a maximum time-to-exhaustion test by running on a treadmill at 70% of their MAS, and gastrocnemius and soleus muscles were collected 2 h after exercise. Time to exhaustion was longer in the ET than Ctrl group (+41.22%, p < 0.01). Two hours after exercise, the ET group showed higher activation of protein synthesis and satellite cells, despite their longer effort. Conversely, expression in muscles of metabolic stress and inflammation markers was decreased, as well as oxidative damage markers in the ET group. Moreover, ergothioneine did not seem to impair mitochondrial recovery. These results suggest an important effect of ergothioneine on time-to-exhaustion performance and improved muscle recovery after exercise.

Early Deconditioning of Human Skeletal Muscles and the Effects of a Thigh Cuff Countermeasure.

Muscle deconditioning is a major consequence of a wide range of conditions from spaceflight to a sedentary lifestyle, and occurs as a result of muscle inactivity, leading to a rapid decrease in muscle strength, mass, and oxidative capacity. The early changes that appear in the first days of inactivity must be studied to determine effective methods for the prevention of muscle deconditioning. To evaluate the mechanisms of muscle early changes and the vascular effect of a thigh cuff, a five-day dry immersion (DI) experiment was conducted by the French Space Agency at the MEDES Space Clinic (Rangueil, Toulouse). Eighteen healthy males were recruited and divided into a control group and a thigh cuff group, who wore a thigh cuff at 30 mmHg. All participants underwent five days of DI. Prior to and at the end of the DI, the lower limb maximal strength was measured and muscle biopsies were collected from the vastus lateralis muscle. Five days of DI resulted in muscle deconditioning in both groups. The maximal voluntary isometric contraction of knee extension decreased significantly. The muscle fiber cross-sectional area decreased significantly by 21.8%, and the protein balance seems to be impaired, as shown by the reduced activation of the mTOR pathway. Measurements of skinned muscle fibers supported these results and potential changes in oxidative capacity were highlighted by a decrease in PGC1-α levels. The use of the thigh cuff did not prevent muscle deconditioning or impact muscle function. These results suggest that the major effects of muscle deconditioning occur during the first few days of inactivity, and countermeasures against muscle deconditioning should target this time period. These results are also relevant for the understanding of muscle weakness induced by muscle diseases, aging, and patients in intensive care.

Does Physical Inactivity Induce Significant Changes in Human Gut Microbiota? New Answers Using the Dry Immersion Hypoactivity Model.

Gut microbiota, a major contributor to human health, is influenced by physical activity and diet, and displays a functional cross-talk with skeletal muscle. Conversely, few data are available on the impact of hypoactivity, although sedentary lifestyles are widespread and associated with negative health and socio-economic impacts. The study aim was to determine the effect of Dry Immersion (DI), a severe hypoactivity model, on the human gut microbiota composition. Stool samples were collected from 14 healthy men before and after 5 days of DI to determine the gut microbiota taxonomic profiles by 16S metagenomic sequencing in strictly controlled dietary conditions. The α and ß diversities indices were unchanged. However, the operational taxonomic units associated with the Clostridiales order and the Lachnospiraceae family, belonging to the Firmicutes phylum, were significantly increased after DI. Propionate, a short-chain fatty acid metabolized by skeletal muscle, was significantly reduced in post-DI stool samples. The finding that intestine bacteria are sensitive to hypoactivity raises questions about their impact and role in chronic sedentary lifestyles.

Glucose 6-P dehydrogenase delays the onset of frailty by protecting against muscle damage.

BACKGROUND: Frailty is a major age-associated syndrome leading to disability. Oxidative damage plays a significant role in the promotion of frailty. The cellular antioxidant system relies on reduced nicotinamide adenine dinucleotide phosphate (NADPH) that is highly dependent on glucose 6-P dehydrogenase (G6PD). The G6PD-overexpressing mouse (G6PD-Tg) is protected against metabolic stresses. Our aim was to examine whether this protection delays frailty. METHODS: Old wild-type (WT) and G6PD-Tg mice were evaluated longitudinally in terms of frailty. Indirect calorimetry, transcriptomic profile, and different skeletal muscle quality markers and muscle regenerative capacity were also investigated. RESULTS: The percentage of frail mice was significantly lower in the G6PD-Tg than in the WT genotype, especially in 26-month-old mice where 50% of the WT were frail vs. only 13% of the Tg ones (P < 0.001). Skeletal muscle transcriptomic analysis showed an up-regulation of respiratory chain and oxidative phosphorylation (P = 0.009) as well as glutathione metabolism (P = 0.035) pathways in the G6PD-Tg mice. Accordingly, the Tg animals exhibited an increase in reduced glutathione (34.5%, P < 0.01) and a decrease on its oxidized form (-69%, P < 0.05) and in lipid peroxidation (4-HNE: -20.5%, P < 0.05). The G6PD-Tg mice also showed reduced apoptosis (BAX/Bcl2: -25.5%, P < 0.05; and Bcl-xL: -20.5%, P < 0.05), lower levels of the intramuscular adipocyte marker FABP4 (-54.7%, P < 0.05), and increased markers of mitochondrial content (COX IV: 89.7%, P < 0.05; Grp75: 37.8%, P < 0.05) and mitochondrial OXPHOS complexes (CII: 81.25%, P < 0.01; CIII: 52.5%, P < 0.01; and CV: 37.2%, P < 0.05). Energy expenditure (-4.29%, P < 0.001) and the respiratory exchange ratio were lower (-13.4%, P < 0.0001) while the locomotor activity was higher (43.4%, P < 0.0001) in the 20-month-old Tg, indicating a major energetic advantage in these mice. Short-term exercise training in young C57BL76J mice induced a robust activation of G6PD in skeletal muscle (203.4%, P < 0.05), similar to that achieved in the G6PD-Tg mice (142.3%, P < 0.01). CONCLUSIONS: Glucose 6-P dehydrogenase deficiency can be an underestimated risk factor for several human pathologies and even frailty. By overexpressing G6PD, we provide the first molecular model of robustness. Because G6PD is regulated by pharmacological and physiological interventions like exercise, our results provide molecular bases for interventions that by increasing G6PD will delay the onset of frailty.

Nitrosative Redox Homeostasis and Antioxidant Response Defense in Disused Vastus lateralis Muscle in Long-Term Bedrest (Toulouse Cocktail Study).

Increased oxidative stress by reactive oxygen species (ROS) and reactive nitrogen species (RNS) is a major determinant of disuse-induced muscle atrophy. Muscle biopsies (thigh vastus lateralis, VL) obtained from healthy male subjects enrolled in the Toulouse Cocktail bedrest (BR) study were used to assess efficacy of an antioxidant cocktail (polyphenols, omega-3, vitamin E, and selenium) to counteract the increased redox homeostasis and enhance the antioxidant defense response by using label-free LC-MS/MS and NITRO-DIGE (nitrosated proteins), qPCR, and laser confocal microscopy. Label-free LC-MS/MS indicated that treatment prevented the redox homeostasis dysregulation and promoted structural remodeling (TPM3, MYH7, MYBPC, MYH1, MYL1, HRC, and LUM), increment of RyR1, myogenesis (CSRP3), and skeletal muscle development (MUSTN1, LMNA, AHNAK). These changes were absent in the Placebo group. Glycolysis, tricarboxylic acid cycle (TCA), oxidative phosphorylation, fatty acid beta-oxidation, and mitochondrial transmembrane transport were normalized in treated subjects. Proteins involved in protein folding were also normalized, whereas protein entailed in ion homeostasis decreased. NITRO-DIGE analysis showed significant protein nitrosylation changes for CAT, CA3, SDHA, and VDAC2 in Treatment vs. Placebo. Similarly, the nuclear factor erythroid 2-related factor 2 (Nrf-2) antioxidant response element (Nrf-2 ARE) signaling pathway showed an enhanced response in the Treatment group. Increased nitrosative redox homeostasis and decreased antioxidant defense response were found in post-BR control (Placebo, n = 10) vs. the antioxidant cocktail treated group (Treatment, n = 10). Taken together, increased nitrosative redox homeostasis and muscle deterioration during BR-driven physical inactivity were prevented, whereas decreased antioxidant nitrosative stress defense response was attenuated by Treatment suggesting positive effects of the nutritional intervention protocol in bedrest.

Acute and chronic effects of Rhaponticum carthamoides and Rhodiola rosea extracts supplementation coupled to resistance exercise on muscle protein synthesis and mechanical power in rats.

BACKGROUND: Owing to its strength-building and adaptogenic properties, Rhaponticum carthamoides (Rha) has been commonly used by elite Soviet and Russian athletes. Rhodiola rosea (Rho) is known to reduce physical and mental fatigue and improve endurance performance. However, the association of these two nutritional supplements with resistance exercise performance has never been tested. Resistance exercise is still the best way to stimulate protein synthesis and induce chronic muscle adaptations. The aim of this study was to investigate the acute and chronic effects of resistance exercise coupled with Rha and Rho supplementation on protein synthesis, muscle phenotype, and physical performance. METHODS: For the acute study, fifty-six rats were assigned to either a trained control group or one of the groups treated with specific doses of Rha and/or Rho. Each rats performed a single bout of climbing resistance exercise. The supplements were administered immediately after exercise by oral gavage. Protein synthesis was measured via puromycin incorporation. For the chronic study, forty rats were assigned to either the control group or one of the groups treated with doses adjusted from the acute study results. The rats were trained five times per week for 4 weeks with the same bout of climbing resistance exercise with additionals loads. Rha + Rho supplement was administered immediately after each training by oral gavage. RESULTS: The findings of the acute study indicated that Rha and Rha + Rho supplementation after resistance exercise stimulated protein synthesis more than resistance exercise alone (p < 0.05). After 4 weeks of training, the mean power performance was increased in the Rha + Rho and Rha-alone groups (p < 0.05) without any significant supplementation effect on muscle weight or fiber cross-sectional area. A tendency towards an increase in type I/ type II fiber ratio was observed in Rha/Rho-treated groups compared to that in the trained control group. CONCLUSION: Rhodiola and Rhaponticum supplementation after resistance exercise could synergistically improve protein synthesis, muscle phenotype and physical performance.

Evaluation of an Antioxidant and Anti-inflammatory Cocktail Against Human Hypoactivity-Induced Skeletal Muscle Deconditioning.

Understanding the molecular pathways involved in the loss of skeletal muscle mass and function induced by muscle disuse is a crucial issue in the context of spaceflight as well as in the clinical field, and development of efficient countermeasures is needed. Recent studies have reported the importance of redox balance dysregulation as a major mechanism leading to muscle wasting. Our study aimed to evaluate the effects of an antioxidant/anti-inflammatory cocktail (741 mg of polyphenols, 138 mg of vitamin E, 80 µg of selenium, and 2.1 g of omega-3) in the prevention of muscle deconditioning induced by long-term inactivity. The study consisted of 60 days of hypoactivity using the head-down bed rest (HDBR) model. Twenty healthy men were recruited; half of them received a daily antioxidant/anti-inflammatory supplementation, whereas the other half received a placebo. Muscle biopsies were collected from the vastus lateralis muscles before and after bedrest and 10 days after remobilization. After 2 months of HDBR, all subjects presented muscle deconditioning characterized by a loss of muscle strength and an atrophy of muscle fibers, which was not prevented by cocktail supplementation. Our results regarding muscle oxidative damage, mitochondrial content, and protein balance actors refuted the potential protection of the cocktail during long-term inactivity and showed a disturbance of essential signaling pathways (protein balance and mitochondriogenesis) during the remobilization period. This study demonstrated the ineffectiveness of our cocktail supplementation and underlines the complexity of redox balance mechanisms. It raises interrogations regarding the appropriate nutritional intervention to fight against muscle deconditioning.

Muscle Resting and TGF-ß Inhibitor Treatment Prevent Fatty Infiltration Following Skeletal Muscle Injury.

BACKGROUND/AIMS: Skeletal muscle injuries are the most common type of injury occurring in sports, and investigating skeletal muscle regeneration as well as understanding the related processes is an important aspect of the sports medicine field. The process of regeneration appears to be complex and precisely orchestrated, involving fibro-adipogenic progenitors (FAPs) which are a muscle-resident stem cell population that appears to play a major role in abnormal development of fibrotic tissue or intermuscular adipose tissue (IMAT). Our present study aims to investigate whether muscle resting or endurance exercise following muscle injury may change the behavior of FAPs and subsequently impact the development of fatty infiltrations and fibrosis, two hallmarks of regeneration failure. METHODS: We used the validated glycerol muscle injury model to mimic abnormal muscle regenerative conditions in mice. We challenged this specific regeneration model with hindlimb unloading or endurance exercise and, in a second set of experiments, we treated mice with decorin, a TGF-ß inhibitor. RESULTS: In this study, we demonstrated that: i) muscle resting just after injury leads to inhibition of IMAT development, ii) TNF-α mediated FAP apoptosis might be perturbed in this specific glycerol model of muscle injury, leading to IMAT development, and iii) treatment with the TGF-ß inhibitor decorin decreases IMAT development and might restores FAP apoptosis. CONCLUSION: In addition to the potential clinical relevance of decorin treatment in situations involving muscle plasticity and regeneration, this study also demonstrates that a period of muscle resting is necessary following muscle injury to achieve efficient muscle regeneration which is associated with a reduction in fatty infiltration. Unreasonably early resumption of exercise brings no gain to regeneration, further highlighting that this resting period is necessary.

A nutrient cocktail prevents lipid metabolism alterations induced by 20 days of daily steps reduction and fructose overfeeding: result from a randomized study.

Physical inactivity and sedentary behaviors are independent risk factors for numerous diseases. We examined the ability of a nutrient cocktail composed of polyphenols, omega-3 fatty acids, vitamin E, and selenium to prevent the expected metabolic alterations induced by physical inactivity and sedentary behaviors. Healthy trained men ( n = 20) (averaging ∼14,000 steps/day and engaged in sports) were randomly divided into a control group (no supplementation) and a cocktail group for a 20-day free-living intervention during which they stopped exercise and decreased their daily steps (averaging ∼3,000 steps/day). During the last 10 days, metabolic changes were further triggered by fructose overfeeding. On days 0, 10, and 20, body composition (dual energy X-ray), blood chemistry, glucose tolerance [oral glucose tolerance test (OGTT)], and substrate oxidation (indirect calorimetry) were measured. OGTT included 1% fructose labeled with (U-13C) fructose to assess liver de novo lipogenesis. Histological changes and related cellular markers were assessed from muscle biopsies collected on days 0 and 20. While the cocktail did not prevent the decrease in insulin sensitivity and its muscular correlates induced by the intervention, it fully prevented the hypertriglyceridemia, the drop in fasting HDL and total fat oxidation, and the increase in de novo lipogenesis. The cocktail further prevented the decrease in the type-IIa muscle fiber cross-sectional area and was associated with lower protein ubiquitination content. The circulating antioxidant capacity was improved by the cocktail following the OGTT. In conclusion, a cocktail of nutrient compounds from dietary origin protects against the alterations in lipid metabolism induced by physical inactivity and fructose overfeeding. NEW & NOTEWORTHY This is the first study to test the efficacy of a novel dietary nutrient cocktail on the metabolic and physiological changes occurring during 20 days of physical inactivity along with fructose overfeeding. The main findings of this study are that 1) reduction in daily steps leads to decreased insulin sensitivity and total fat oxidation, resulting in hyperlipemia and increased de novo lipogenesis and 2) a cocktail supplement prevents the alterations on lipid metabolism.

The exerkine apelin reverses age-associated sarcopenia.

Sarcopenia, the degenerative loss of skeletal muscle mass, quality and strength, lacks early diagnostic tools and new therapeutic strategies to prevent the frailty-to-disability transition often responsible for the medical institutionalization of elderly individuals. Herein we report that production of the endogenous peptide apelin, induced by muscle contraction, is reduced in an age-dependent manner in humans and rodents and is positively associated with the beneficial effects of exercise in older persons. Mice deficient in either apelin or its receptor (APLNR) presented dramatic alterations in muscle function with increasing age. Various strategies that restored apelin signaling during aging further demonstrated that this peptide considerably enhanced muscle function by triggering mitochondriogenesis, autophagy and anti-inflammatory pathways in myofibers as well as enhancing the regenerative capacity by targeting muscle stem cells. Taken together, these findings revealed positive regulatory feedback between physical activity, apelin and muscle function and identified apelin both as a tool for diagnosis of early sarcopenia and as the target of an innovative pharmacological strategy to prevent age-associated muscle weakness and restore physical autonomy.

Metabolic Inflexibility Is an Early Marker of Bed-Rest-Induced Glucose Intolerance Even When Fat Mass Is Stable.

Context: The effects of energy-balanced bed rest on metabolic flexibility have not been thoroughly examined. Objective: We investigated the effects of 21 days of bed rest, with and without whey protein supplementation, on metabolic flexibility while maintaining energy balance. We hypothesized that protein supplementation mitigates metabolic inflexibility by preventing muscle atrophy. Design and Setting: Randomized crossover longitudinal study conducted at the German Aerospace Center, Cologne, Germany. Participants and Interventions: Ten healthy men were randomly assigned to dietary countermeasure or isocaloric control diet during a 21-day bed rest. Outcome Measures: Before and at the end of the bed rest, metabolic flexibility was assessed during a meal test. Secondary outcomes were glucose tolerance by oral glucose tolerance test, body composition by dual energy X-ray absorptiometry, ectopic fat storage by magnetic resonance imaging, and inflammation and oxidative stress markers. Results: Bed rest decreased the ability to switch from fat to carbohydrate oxidation when transitioning from fasted to fed states (i.e., metabolic inflexibility), antioxidant capacity, fat-free mass (FFM), and muscle insulin sensitivity along with greater fat deposition in muscle (P < 0.05 for all). Changes in fasting insulin and inflammation were not observed. However, glucose tolerance was reduced during acute overfeeding. Protein supplementation did not prevent FFM loss and metabolic alterations. Conclusions: Physical inactivity triggers metabolic inflexibility, even when energy balance is maintained. Although reduced insulin sensitivity and increased fat deposition were observed at the muscle level, systemic glucose intolerance was detected only in response to a moderately high-fat meal. This finding supports the role of physical inactivity in metabolic inflexibility and suggests that metabolic inflexibility precedes systemic glucose intolerance.

Short-term disuse promotes fatty acid infiltration into skeletal muscle.

BACKGROUND: Many physiological and/or pathological conditions lead to muscle deconditioning, a well-described phenomenon characterized by a loss of strength and muscle power mainly due to the loss of muscle mass. Fatty infiltrations, or intermuscular adipose tissue (IMAT), are currently well-recognized components of muscle deconditioning. Despite the fact that IMAT is present in healthy human skeletal muscle, its increase and accumulation are linked to muscle dysfunction. Although IMAT development has been largely attributable to inactivity, the precise mechanisms of its establishment are still poorly understood. Because the sedentary lifestyle that accompanies age-related sarcopenia may favour IMAT development, deciphering the early processes of muscle disuse is of great importance before implementing strategies to limit IMAT deposition. METHODS: In our study, we took advantage of the dry immersion (DI) model of severe muscle inactivity to induce rapid muscle deconditioning during a short period. During the DI, healthy adult men (n = 12; age: 32 ± 5) remained strictly immersed, in a supine position, in a controlled thermo-neutral water bath. Skeletal muscle biopsies were obtained from the vastus lateralis before and after 3 days of DI. RESULTS: We showed that DI for only 3 days was able to decrease myofiber cross-sectional areas (-10.6%). Moreover, protein expression levels of two key markers commonly used to assess IMAT, perilipin, and fatty acid binding protein 4, were upregulated. We also observed an increase in the C/EBPα and PPARγ protein expression levels, indicating an increase in late adipogenic processes leading to IMAT development. While many stem cells in the muscle environment can adopt the capacity to differentiate into adipocytes, fibro-adipogenic progenitors (FAPs) represent the population that appears to play a major role in IMAT development. In our study, we showed an increase in the protein expression of PDGFRα, the specific cell surface marker of FAPs, in response to 3 days of DI. It is well recognized that an unfavourable muscle environment drives FAPs to ectopic adiposity and/or fibrosis. CONCLUSIONS: This study is the first to emphasize that during a short period of severe inactivity, muscle deconditioning is associated with IMAT development. Our study also reveals that FAPs could be the main resident muscle stem cell population implicated in ectopic adiposity development in human skeletal muscle.

Proteome-wide Adaptations of Mouse Skeletal Muscles during a Full Month in Space.

The safety of space flight is challenged by a severe loss of skeletal muscle mass, strength, and endurance that may compromise the health and performance of astronauts. The molecular mechanisms underpinning muscle atrophy and decreased performance have been studied mostly after short duration flights and are still not fully elucidated. By deciphering the muscle proteome changes elicited in mice after a full month aboard the BION-M1 biosatellite, we observed that the antigravity soleus incurred the greatest changes compared with locomotor muscles. Proteomics data notably suggested mitochondrial dysfunction, metabolic and fiber type switching toward glycolytic type II fibers, structural alterations, and calcium signaling-related defects to be the main causes for decreased muscle performance in flown mice. Alterations of the protein balance, mTOR pathway, myogenesis, and apoptosis were expected to contribute to muscle atrophy. Moreover, several signs reflecting alteration of telomere maintenance, oxidative stress, and insulin resistance were found as possible additional deleterious effects. Finally, 8 days of recovery post flight were not sufficient to restore completely flight-induced changes. Thus in-depth proteomics analysis unraveled the complex and multifactorial remodeling of skeletal muscle structure and function during long-term space flight, which should help define combined sets of countermeasures before, during, and after the flight.

Towards human exploration of space: the THESEUS review series on muscle and bone research priorities.

Without effective countermeasures, the musculoskeletal system is altered by the microgravity environment of long-duration spaceflight, resulting in atrophy of bone and muscle tissue, as well as in deficits in the function of cartilage, tendons, and vertebral disks. While inflight countermeasures implemented on the International Space Station have evidenced reduction of bone and muscle loss on low-Earth orbit missions of several months in length, important knowledge gaps must be addressed in order to develop effective strategies for managing human musculoskeletal health on exploration class missions well beyond Earth orbit. Analog environments, such as bed rest and/or isolation environments, may be employed in conjunction with large sample sizes to understand sex differences in countermeasure effectiveness, as well as interaction of exercise with pharmacologic, nutritional, immune system, sleep and psychological countermeasures. Studies of musculoskeletal biomechanics, involving both human subject and computer simulation studies, are essential to developing strategies to avoid bone fractures or other injuries to connective tissue during exercise and extravehicular activities. Animal models may be employed to understand effects of the space environment that cannot be modeled using human analog studies. These include studies of radiation effects on bone and muscle, unraveling the effects of genetics on bone and muscle loss, and characterizing the process of fracture healing in the mechanically unloaded and immuno-compromised spaceflight environment. In addition to setting the stage for evidence-based management of musculoskeletal health in long-duration space missions, the body of knowledge acquired in the process of addressing this array of scientific problems will lend insight into the understanding of terrestrial health conditions such as age-related osteoporosis and sarcopenia.

Early structural and functional signature of 3-day human skeletal muscle disuse using the dry immersion model.

KEY POINTS: Our study contributes to the characterization of muscle loss and weakness processes induced by a sedentary life style, chronic hypoactivity, clinical bed rest, immobilization and microgravity. This study, by bringing together integrated and cellular evaluation of muscle structure and function, identifies the early functional markers and biomarkers of muscle deconditioning. Three days of muscle disuse in healthy adult subjects is sufficient to significantly decrease muscle mass, tone and force, and to induce changes in function relating to a weakness in aerobic metabolism and muscle fibre denervation. The outcomes of this study should be considered in the development of an early muscle loss prevention programme and/or the development of pre-conditioning programmes required before clinical bed rest, immobilization and spaceflight travel. ABSTRACT: Microgravity and hypoactivity are associated with skeletal muscle deconditioning. The decrease of muscle mass follows an exponential decay, with major changes in the first days. The purpose of the study was to dissect out the effects of a short-term 3-day dry immersion (DI) on human quadriceps muscle function and structure. The DI model, by suppressing all support zones, accurately reproduces the effects of microgravity. Twelve healthy volunteers (32 ± 5 years) completed 3 days of DI. Muscle function was investigated through maximal voluntary contraction (MVC) tests and muscle viscoelasticity. Structural experiments were performed using MRI analysis and invasive experiments on muscle fibres. Our results indicated a significant 9.1% decrease of the normalized MVC constant (P = 0.048). Contraction and relaxation modelization kinetics reported modifications related to torque generation (kACT  = -29%; P = 0.014) and to the relaxation phase (kREL  = +34%; P = 0.040) after 3 days of DI. Muscle viscoelasticity was also altered. From day one, rectus femoris stiffness and tone decreased by, respectively, 7.3% (P = 0.002) and 10.2% (P = 0.002), and rectus femoris elasticity decreased by 31.5% (P = 0.004) after 3 days of DI. At the cellular level, 3 days of DI translated into a significant atrophy of type I muscle fibres (-10.6 ± 12.1%, P = 0.027) and an increased proportion of hybrid, type I/IIX fibre co-expression. Finally, we report an increase (6-fold; P = 0.002) in NCAM+ muscle fibres, showing an early denervation process. This study is the first to report experiments performed in Europe investigating human short-term DI-induced muscle adaptations, and contributes to deciphering the early changes and biomarkers of skeletal muscle deconditioning.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention.

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and ß-hydroxy-ß-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.