Between-Subject and Within-Subject Variaton of Muscle Atrophy and Bone Loss in Response to Experimental Bed Rest

Spaceflight on the ISS changed the skeletal muscle proteome of two astronauts

Skeletal muscle undergoes atrophy and loss of force during long space missions, when astronauts are persistently exposed to altered gravity and increased ionizing radiation. We previously carried out mass spectrometry-based proteomics from skeletal muscle biopsies of two astronauts, taken before and after a mission on the International Space Station. The experiments were part of an effort to find similarities between spaceflight and bed rest, a ground-based model of unloading, focused on proteins located at the costameres. We here extend the data analysis of the astronaut dataset and show compartment-resolved changes in the mitochondrial proteome, remodeling of the extracellular matrix and of the antioxidant response. The astronauts differed in their level of onboard physical exercise, which correlated with their respective preservation of muscle mass and force at landing in previous analyses. We show that the mitochondrial proteome downregulation during spaceflight, particularly the inner membrane and matrix, was dramatic for both astronauts. The expression of autophagy regulators and reactive oxygen species scavengers, however, showed partially opposite expression trends in the two subjects, possibly correlating with their level of onboard exercise. As mitochondria are primarily affected in many different tissues during spaceflight, we hypothesize that reactive oxygen species (ROS) rather than mechanical unloading per se could be the primary cause of skeletal muscle mitochondrial damage in space. Onboard physical exercise might have a strong direct effect on the prevention of muscle atrophy through mechanotransduction and a subsidiary effect on mitochondrial quality control, possibly through upregulation of autophagy and anti-oxidant responses.

Genetic diversity modulates the physical and transcriptomic response of skeletal muscle to simulated microgravity in male mice

Developments in long-term space exploration necessitate advancements in countermeasures against microgravity-induced skeletal muscle loss. Astronaut data shows considerable variation in muscle loss in response to microgravity. Previous experiments suggest that genetic background influences the skeletal muscle response to unloading, but no in-depth analysis of genetic expression has been performed. Here, we placed eight, male, inbred founder strains of the diversity outbred mice (129S1/SvImJ, A/J, C57BL/6J, CAST/EiJ, NOD/ShiLtJ, NZO/HILtJ, PWK/PhJ, and WSB/EiJ) in simulated microgravity (SM) via hindlimb unloading for three weeks. Body weight, muscle morphology, muscle strength, protein synthesis marker expression, and RNA expression were collected. A/J and CAST/EiJ mice were most susceptible to SM-induced muscle loss, whereas NOD/ShiLtJ mice were the most protected. In response to SM, A/J and CAST/EiJ mice experienced reductions in body weight, muscle mass, muscle volume, and muscle cross-sectional area. A/J mice had the highest number of differentially expressed genes (68) and associated gene ontologies (328). Downregulation of immunological gene ontologies and genes encoding anabolic immune factors suggest that immune dysregulation contributes to the response of A/J mice to SM. Several muscle properties showed significant interactions between SM and mouse strain and a high degree of heritability. These data imply that genetic background plays a role in the degree of muscle loss in SM and that more individualized programs should be developed for astronauts to protect their skeletal muscles against microgravity on long-term missions.

Adjuvant pharmacological strategies for the musculoskeletal system during long-term space missions

Despite 2 h of daily exercise training, muscle wasting and bone loss are still present after 6-month missions to the international space station. Some crew members lose bone much faster than others. In preparation for missions to the Moon and Mars, space agencies are therefore reviewing their countermeasure portfolios. Here, we discuss the potential of current pharmacological strategies. Bone loss in space is fuelled by bone resorption. Alendronate, an oral bisphosphonate, reduced bone losses in experimental bed rest and space. However, gastrointestinal side effects precluded its further utilization in space. Zoledronate (a potent bisphosphonate), denosumab (RANKL antagonist) and romosozumab (sclerostin antagonist) are all administered via injection. They effectively suppress bone resorption and are routinely prescribed against osteoporosis. Their serious adverse effects, namely, osteonecrosis of the jaw and atypical femur fractures occur very rarely when the usage is limited to 1 or 2 years. Hence, utilization of one of these compounds may outweigh the bone risks of space travelling, in particular in those with high bone resorption rates. Muscle wasting in space is likely due to hampered muscle protein synthesis. Even though this might theoretically be countered by the synthesis-boosting effects of anabolic steroids, the practical grounds for such recommendation are currently weak. Moreover, they reveal their full potential only when combined with an anabolic exercise stimulus, for example, via strength training. It therefore seems that a combination of exercise and pharmacological countermeasures should be considered for musculoskeletal health on the way to the Moon and Mars and back.

The Characteristic Response of the Human Leukocyte Transcriptome to 60 Days of Bed Rest and to Reambulation

INTRODUCTION

We sought to isolate the microgravity effect of spaceflight from other space stressors by characterizing the leukocytes' transcriptome of participants to a 60-d bed rest study; an Earth model of microgravity.

METHODS

Twenty healthy men received a nutritional supplement or not and 10 blood samples were collected throughout three study phases: baseline data collection (BDC) (BDC-12, BDC-11), head-down tilt (HDT) bed rest (HDT1, HDT2, HDT30, HDT60), and reambulation (R1, R2, R12, R30). We measured gene expression through RNA sequencing of leukocytes, applied generalized linear models to assess differential expression followed by enrichment analysis to identify temporal changes (model 1) and to measure the impact of a nutritional supplement (model 2).

RESULTS

Baseline transcriptomes included 14,624 protein-coding transcripts and showed both high intraindividual correlations (mean Kendall coefficient, 0.91 ± 0.04) and interindividual homogeneity (0.89 ± 0.03). We identified 2415 differentially expressed protein-coding transcripts grouping into six clusters (C1-C6). At phase transitions, clusters showed either a decrease-then-increase (C3 and C5) or an increase-then-decrease (C1, C2, C6) pattern. All six clusters converged toward average expression at HDT30 and HDT60. Gene ontology terms at baseline related to immune functions while in bed rest and reambulation related to sequestration of ions, immune response, cellular stress, and mineralization. The nutritional intervention had no effect.

CONCLUSIONS

The temporal profiles of leukocytes' transcriptomes emphasized the dynamic nature of gene expression occurring during and after bed rest. Enriched biological processes among the differentially expressed genes included immune related and unrelated responses. The convergence toward no differential expression at days 30 and 60 of bed rest suggests a hypometabolic state. Current findings can guide future work on the complex responses and adaptation mechanisms to microgravity.

Plasma proteome profiling of healthy subjects undergoing bed rest reveals unloading-dependent changes linked to muscle atrophy

BACKGROUND

Inactivity and unloading induce skeletal muscle atrophy, loss of strength and detrimental metabolic effects. Bed rest is a model to study the impact of inactivity on the musculoskeletal system. It not only provides information for bed-ridden patients care, but it is also a ground-based spaceflight analogue used to mimic the challenges of long space missions for the human body. In both cases, it would be desirable to develop a panel of biomarkers to monitor muscle atrophy in a minimally invasive way at point of care to limit the onset of muscle loss in a personalized fashion.

METHODS

We applied mass spectrometry-based proteomics to measure plasma protein abundance changes in response to 10 days of bed rest in 10 young males. To validate the correlation between muscle atrophy and the significant hits emerging from our study, we analysed in parallel, with the same pipeline, a cohort of cancer patients with or without cachexia and age-matched controls. Our analysis resulted in the quantification of over 500 proteins.

RESULTS

Unloading affected plasma concentration of proteins of the complement cascade, lipid carriers and proteins derived from tissue leakage. Among the latter, teneurin-4 increased 1.6-fold in plasma at bed rest day 10 (BR10) compared with BR0 (6.E9 vs. 4.3E9, P = 0.02) and decreased to 0.6-fold the initial abundance after 2 days of recovery at normal daily activity (R + 2, 2.7E9, P = 3.3E-4); the extracellular matrix protein lumican was decreased to 0.7-fold (1.2E9 vs. 8.5E8, P = 1.5E-4) at BR10 and remained as low at R + 2. We identified six proteins distinguishing subjects developing unloading-mediated muscle atrophy (decrease of >4% of quadriceps cross-sectional area) from those largely maintaining their initial muscle mass. Among them, transthyretin, a thyroid hormone-binding protein, was significantly less abundant at BR10 in the plasma of subjects with muscle atrophy compared with those with no atrophy (1.6E10 vs. 2.6E10, P = 0.001). Haptoglobin-related protein was also significantly reduced in the serum of cancer patients with cachexia compared with that of controls.

CONCLUSIONS

Our findings highlight a combination or proteomic changes that can be explored as potential biomarkers of muscle atrophy occurring under different conditions. The panel of significant proteomic differences distinguishing atrophy-prone and atrophy-resistant subjects after 10 days of bed rest need to be tested in a larger cohort to validate their potential to predict inactivity-triggered muscle loss in humans.

Musculoskeletal research in human space flight - unmet needs for the success of crewed deep space exploration

Based on the European Space Agency (ESA) Science in Space Environment (SciSpacE) community White Paper "Human Physiology - Musculoskeletal system", this perspective highlights unmet needs and suggests new avenues for future studies in musculoskeletal research to enable crewed exploration missions. The musculoskeletal system is essential for sustaining physical function and energy metabolism, and the maintenance of health during exploration missions, and consequently mission success, will be tightly linked to musculoskeletal function. Data collection from current space missions from pre-, during-, and post-flight periods would provide important information to understand and ultimately offset musculoskeletal alterations during long-term spaceflight. In addition, understanding the kinetics of the different components of the musculoskeletal system in parallel with a detailed description of the molecular mechanisms driving these alterations appears to be the best approach to address potential musculoskeletal problems that future exploratory-mission crew will face. These research efforts should be accompanied by technical advances in molecular and phenotypic monitoring tools to provide in-flight real-time feedback.

The Contribution of Genetics to Muscle Disuse, Retraining, and Aging

Genetic background may partly explain differences in muscle responses to internal or external stimuli. Muscle disuse involves various degrees of skeletal muscle atrophy due to inactivity and mechanical unloading. Whether and to which extent genetic background impacts disuse atrophy and retraining in individuals of different ages are currently unclear. Here, we provide a brief overview of relevant literature on the contribution of genetics to muscle disuse, retraining, and aging, and offer a perspective on unanswered questions on the subject that may open new venues for research.

Imaging techniques to study diabetic bone disease

PURPOSE OF REVIEW

This review article presents the most recent research on bone fragility in individuals with diabetes from a medical imaging perspective.

RECENT FINDINGS

The widespread availability of dual-energy X-ray absorptiometry (DXA) and trabecular bone score (TBS) software has led to recent assessments of bone fragility with this texture parameter in several studies of type 2 diabetes mellitus (T2D), but in few of type 1 diabetes mellitus (T1D). Although most studies show a trend of reduced TBS values in T2D independent of areal bone mineral density (aBMD) of the lumbar spine, some studies also show the limitations of TBS in both T2D and T1D. Given the limitations of DXA to assess bone strength and investigate the etiology of bone fragility in diabetes, more investigators are incorporating three-dimensional (3D) medical imaging techniques in their studies. Recent use of 3D medical imaging to assess bone fragility in the setting of diabetes has been mostly limited to a few cross-sectional studies predominantly incorporating high-resolution peripheral quantitative computed tomography (HR-pQCT). Although HR-pQCT studies indicate higher tibial cortical porosity in subjects with T2D, results are inconsistent in T1D due to differences in study designs, sample sizes, and subject characteristics, among other factors. With respect to central CT, recent studies support a previous finding in the literature indicating femoral neck geometrical impairments in subjects with T2D and provide encouraging results for the incorporation of finite element analysis (FEA) to assess bone strength in studies of T2D. In the recent literature, there are no studies assessing bone fragility in T1D with QCT, and only two studies used pQCT reporting tibial and radial impairments in young women and children with T1D, respectively. Magnetic resonance imaging (MRI) has not been recently used in diabetic studies of bone fragility.

SUMMARY

As bone fragility in diabetes is not explained by DXA-derived aBMD and given the limitations of cross-sectional studies, it is imperative to use 3D imaging techniques for longitudinal assessments of the density, quality, and microenvironment of bone to improve our understanding of the effects of diabetes on bone and reduce the risk of fracture in this large and vulnerable population of subjects with diabetes.