Adaptation of the proximal femur to skeletal reloading after long-duration spaceflight

Protective alleles and precision healthcare in crewed spaceflight

Common and rare alleles are now being annotated across millions of human genomes, and omics technologies are increasingly being used to develop health and treatment recommendations. However, these alleles have not yet been systematically characterized relative to aerospace medicine. Here, we review published alleles naturally found in human cohorts that have a likely protective effect, which is linked to decreased cancer risk and improved bone, muscular, and cardiovascular health. Although some technical and ethical challenges remain, research into these protective mechanisms could translate into improved nutrition, exercise, and health recommendations for crew members during deep space missions.

Unloading-Induced Skeletal Interoception Alters Hypothalamic Signaling to Promote Bone Loss and Fat Metabolism

Microgravity is the primary factor that affects human physiology in spaceflight, particularly bone loss and disturbances of the central nervous system. However, little is known about the cellular and molecular mechanisms of these effects. Here, it is reported that in mice hindlimb unloading stimulates expression of neuropeptide Y (NPY) and tyrosine hydroxylase (TH) in the hypothalamus, resulting in bone loss and altered fat metabolism. Enhanced expression of TH and NPY in the hypothalamus occurs downstream of a reduced prostaglandin E2 (PGE2)-mediated ascending interoceptive signaling of the skeletal interoception. Sympathetic antagonist propranolol or deletion of Adrb2 in osteocytes rescue bone loss in the unloading model. Moreover, depletion of TH+ sympathetic nerves or inhibition of norepinephrine release ameliorated bone resorption. Stereotactic inhibition of NPY expression in the hypothalamic neurons reduces the food intake with altered energy expenditure with a limited effect on bone, indicating hypothalamic neuroendocrine factor NPY in the facilitation of bone formation by sympathetic TH activity. These findings suggest that reduced PGE2-mediated interoceptive signaling in response to microgravity or unloading has impacts on the skeletal and central nervous systems that are reciprocally regulated.

Anti-Siglec-15 Antibody Prevents Marked Bone Loss after Acute Spinal Cord Injury-Induced Immobilization in Rats

Rapid and extensive sublesional bone loss after spinal cord injury (SCI) is a difficult medical problem that has been refractory to available interventions except the antiresorptive agent denosumab (DMAB). While DMAB has shown some efficacy in inhibiting bone loss, its concurrent inhibition of bone formation limits its use. Sialic acid-binding immunoglobulin-like lectin (Siglec)-15 is expressed on the cell surface of mature osteoclasts. Anti-Siglec-15 antibody (Ab) has been shown to inhibit osteoclast maturation and bone resorption while maintaining osteoblast activity, which is distinct from current antiresorptive agents that inhibit the activity of both osteoclasts and osteoblasts. The goal of the present study is to test a Siglec-15 Ab (NP159) as a new treatment option to prevent bone loss in an acute SCI model. To this end, 4-month-old male Wistar rats underwent complete spinal cord transection and were treated with either vehicle or NP159 at 20 mg/kg once every 2 weeks for 8 weeks. SCI results in significant decreases in bone mineral density (BMD, -18.7%), trabecular bone volume (-43.1%), trabecular connectivity (-59.7%), and bone stiffness (-76.3%) at the distal femur. Treatment with NP159 almost completely prevents the aforementioned deterioration of bone after SCI. Blood and histomorphometric analyses revealed that NP159 is able to greatly inhibit bone resorption while maintaining bone formation after acute SCI. In ex vivo cultures of bone marrow cells, NP159 reduces osteoclastogenesis while increasing osteoblastogenesis. In summary, treatment with NP159 almost fully prevents sublesional loss of BMD and metaphysis trabecular bone volume and preserves bone strength in a rat model of acute SCI. Because of its unique ability to reduce osteoclastogenesis and bone resorption while promoting osteoblastogenesis to maintain bone formation, Siglec-15 Ab may hold greater promise as a therapeutic agent, compared with the exclusively antiresorptive or anabolic agents that are currently used, in mitigating the striking bone loss that occurs after SCI or other conditions associated with severe immobilization. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Predicting Bone Adaptation in Astronauts during and after Spaceflight

A method was previously developed to identify participant-specific parameters in a model of trabecular bone adaptation from longitudinal computed tomography (CT) imaging. In this study, we use these numerical methods to estimate changes in astronaut bone health during the distinct phases of spaceflight and recovery on Earth. Astronauts (N = 16) received high-resolution peripheral quantitative CT (HR-pQCT) scans of their distal tibia prior to launch (L), upon their return from an approximately six-month stay on the international space station (R+0), and after six (R+6) and 12 (R+12) months of recovery. To model trabecular bone adaptation, we determined participant-specific parameters at each time interval and estimated their bone structure at R+0, R+6, and R+12. To assess the fit of our model to this population, we compared static and dynamic bone morphometry as well as the Dice coefficient and symmetric distance at each measurement. In general, modeled and observed static morphometry were highly correlated (R2> 0.94) and statistically different (p < 0.0001) but with errors close to HR-pQCT precision limits. Dynamic morphometry, which captures rates of bone adaptation, was poorly estimated by our model (p < 0.0001). The Dice coefficient and symmetric distance indicated a reasonable local fit between observed and predicted bone volumes. This work applies a general and versatile computational framework to test bone adaptation models. Future work can explore and test increasingly sophisticated models (e.g., those including load or physiological factors) on a participant-specific basis.

Multi-scale numerical simulation on mechano-transduction of osteocytes in different gravity fields

A three-dimensional model for osteocyte was established to research the mechanisms of mechano-transduction and amplification of primary cilium and osteocyte process in every gravity field. The results showed that significant stress concentration was observed in the area of physical connection between TES and the osteocyte process, where the fluid shear stress (FSS) was around two orders of magnitude higher than that in other areas. Due to the significant amplification effect of the TES structure on mechanical stimulation, making osteocyte process the "optimal mechanical receptor". In microgravity, the mechanical signal conduction ability of the osteocyte decreased significantly.. HighlightsAt the micro-nano scale, a 3D model for single bone lacunae-osteocyte system is established.The stress amplification mechanism of the transverse element is verified.Compared with the primary cilium, osteocyte process is the 'optimal mechanical receptor'.In microgravity, the mechanical signal conduction ability of osteocyte system decreased.

Intervention Effects of Deer-Tendon Collagen Hydrolysates on Osteoporosis In Vitro and In Vivo

Deer tendon, a deer processing byproduct, is an excellent protein source for the preparation of peptides for improving osteoporosis by its high protein content and high nutritional value. The optimal process of collagen acid extraction was implemented and the results showed that the acid concentration was 7%, the material-liquid ratio was 1:25, and the soaking time was 48 h. DTCHs could promote MC3T3-E1 cell proliferation and increase alkaline phosphatase activities in vitro. In addition, compared with the model group, the DTCHs treatment groups with an oral dosage of 350, 750, and 1500 mg/kg rat/day could significantly improve the shape, weight, bone mechanics, and alkaline phosphatase activities of tail-suspended mice. Bone microstructure and mineralization also recovered significantly in vivo. This result is expected to provide the structural and biological information for DTCHs-based functional foods.

Reambulation following hindlimb unloading attenuates disuse-induced changes in murine fracture healing

Patients with bone and muscle loss from prolonged disuse have higher risk of falls and subsequent fragility fractures. In addition, fracture patients with continued disuse and/or delayed physical rehabilitation have worse clinical outcomes compared to individuals with immediate weight-bearing activity following diaphyseal fracture. However, the effects of prior disuse followed by physical reambulation on fracture healing cellular processes and adjacent bone and skeletal muscle recovery post-injury remains poorly defined. To bridge this knowledge gap and inform future treatment and rehabilitation strategies for fractures, a preclinical model of fracture healing with a history of prior unloading with and without reambulation was employed. First, skeletally mature male and female C57BL/6J mice (18 weeks) underwent hindlimb unloading by tail suspension (HLU) for 3 weeks to induce significant bone and muscle loss modeling enhanced bone fragility. Next, mice had their right femur fractured by open surgical dissection (stabilized with 24-gauge pin). The, mice were randomly assigned to continued HLU or allowed normal weight-bearing reambulation (HLU + R). Mice given normal cage activity throughout the experiment served as healthy age-matched controls. All mice were sacrificed 4-days (DPF4) or 14-days (DPF14) following fracture to assess healing and uninjured hindlimb musculoskeletal properties (6-10 mice per treatment/biological sex). We found that continued disuse following fracture lead to severely diminished uninjured hindlimb skeletal muscle mass (gastrocnemius and soleus) and femoral bone volume adjacent to the fracture site compared to healthy age-matched controls across mouse sexes. Furthermore, HLU led to significantly decreased periosteal expansion (DPF4) and osteochondral tissue formation by DPF14, and trends in increased osteoclastogenesis (DPF14) and decreased woven bone vascular area (DPF14). In contrast, immediate reambulation for 2 weeks after fracture, even following a period of prolonged disuse, was able to increase hindlimb skeletal tissue mass and increase osteochondral tissue formation, albeit not to healthy control levels, in both mouse sexes. Furthermore, reambulation attenuated osteoclast formation seen in woven bone tissue undergoing disuse. Our results suggest that weight-bearing skeletal loading in both sexes immediately following fracture may improve callus healing and prevent further fall risk by stimulating skeletal muscle anabolism and decreasing callus resorption compared to minimal or delayed rehabilitation regimens.

Preventing Disused Bone Loss through Inhibition of Advanced Glycation End Products

Bone loss occurs in astronauts during long-term space flight, but the mechanisms are still unclear. We previously showed that advanced glycation end products (AGEs) were involved in microgravity-induced osteoporosis. Here, we investigated the improvement effects of blocking AGEs formation on microgravity-induced bone loss by using the AGEs formation inhibitor, irbesartan. To achieve this objective, we used a tail-suspended (TS) rat model to simulate microgravity and treated the TS rats with 50 mg/kg/day irbesartan, as well as the fluorochrome biomarkers injected into rats to label dynamic bone formation. To assess the accumulation of AGEs, pentosidine (PEN), non-enzymatic cross-links (NE-xLR), and fluorescent AGEs (fAGEs) were identified in the bone; 8-hydroxydeoxyguanosine (8-OHdG) was analyzed for the reactive oxygen species (ROS) level in the bone. Meanwhile, bone mechanical properties, bone microstructure, and dynamic bone histomorphometry were tested for bone quality assessment, and Osterix and TRAP were immunofluorescences stained for the activities of osteoblastic and osteoclastic cells. Results showed AGEs increased significantly and 8-OHdG expression in bone showed an upward trend in TS rat hindlimbs. The bone quality (bone microstructure and mechanical properties) and bone formation process (dynamic bone formation and osteoblastic cells activities) were inhibited after tail-suspension, and showed a correlation with AGEs, suggesting the elevated AGEs contributed to the disused bone loss. After being treated with irbesartan, the increased AGEs and 8-OHdG expression were significantly inhibited, suggesting irbesartan may reduce ROS to inhibit dicarbonyl compounds, thus suppressing AGEs production after tail-suspension. The inhibition of AGEs can partially alter the bone remodeling process and improve bone quality. Both AGEs accumulation and bone alterations almost occurred in trabecular bone but not in cortical bone, suggesting AGEs effects on bone remodeling under microgravity are dependent on the biological milieu.

Prediction of osteoporotic degradation of tibia human bone at trabecular scale

A theoretical numerical model is proposed to predict patient dependent osteoporotic bone degradation. The model parameters are identified through a particle swarm optimization algorithm and based on individual patient high resolution peripherical quantitative computer tomography (HRpQCT) scan data. The degradation model is based on cellular activity initiated by the elastic strain energy developed in the bone microstructure through patient's body weight. The macro (organ scale) and meso (trabecular scale) scale analyses are carried out and predicted bone volume fraction and microstructure evolution are compared with in-vivo experimental bone degradation for four elderly women over a period of 10 years. A significant correlation (r > 0.9) is observed between the model predictions and in-vivo experiments in all cases with an average deviation error of 1.46%. The model can easily be extended to other patients and provide good predictions for different population categories such as ethnicity, gender, age, etc.

Glucose- and glutamine-dependent bioenergetics sensitize bone mechanoresponse after unloading by modulating osteocyte calcium dynamics

Disuse osteoporosis is a metabolic bone disease resulting from skeletal unloading (e.g., during extended bed rest, limb immobilization, and spaceflight), and the slow and insufficient bone recovery during reambulation remains an unresolved medical challenge. Here, we demonstrated that loading-induced increase in bone architecture/strength was suppressed in skeletons previously exposed to unloading. This reduction in bone mechanosensitivity was directly associated with attenuated osteocytic Ca2+ oscillatory dynamics. The unloading-induced compromised osteocytic Ca2+ response to reloading resulted from the HIF-1α/PDK1 axis-mediated increase in glycolysis, and a subsequent reduction in ATP synthesis. HIF-1α also transcriptionally induced substantial glutaminase 2 expression and thereby glutamine addiction in osteocytes. Inhibition of glycolysis by blockade of PDK1 or glutamine supplementation restored the mechanosensitivity in those skeletons with previous unloading by fueling the tricarboxylic acid cycle and rescuing subsequent Ca2+ oscillations in osteocytes. Thus, we provide mechanistic insight into disuse-induced deterioration of bone mechanosensitivity and a promising therapeutic approach to accelerate bone recovery after long-duration disuse.

Differential bone adaptation to mechanical unloading and reloading in young, old, and osteocyte deficient mice

Mechanical unloading causes rapid loss of bone structure and strength, which gradually recovers after resuming normal loading. However, it is not well established how this adaptation to unloading and reloading changes with age. Clinically, elderly patients are more prone to musculoskeletal injury and longer periods of bedrest, therefore it is important to understand how periods of disuse will affect overall skeletal health of aged subjects. Bone also undergoes an age-related decrease in osteocyte density, which may impair mechanoresponsiveness. In this study, we examined bone adaptation during unloading and subsequent reloading in mice. Specifically, we examined the differences in bone adaptation between young mice (3-month-old), old mice (18-month-old), and transgenic mice that exhibit diminished osteocyte density at a young age (3-month-old BCL-2 transgenic mice). Mice underwent 14 days of hindlimb unloading followed by up to 14 days of reloading. We analyzed trabecular and cortical bone structure in the femur, mechanical properties of the femoral cortical diaphysis, osteocyte density and cell death in cortical bone, and serum levels of inflammatory cytokines. We found that young mice lost ~10% cortical bone volume and 27-42% trabecular bone volume during unloading and early reloading, with modest recovery of metaphyseal trabecular bone and near total recovery of epiphyseal trabecular bone, but no recovery of cortical bone after 14 days of reloading. Old mice lost 12-14% cortical bone volume and 35-50% trabecular bone volume during unloading and early reloading but had diminished recovery of trabecular bone during reloading and no recovery of cortical bone. In BCL-2 transgenic mice, no cortical bone loss was observed during unloading or reloading, but 28-31% trabecular bone loss occurred during unloading and early reloading, with little to no recovery during reloading. No significant differences in circulating inflammatory cytokine levels were observed due to unloading and reloading in any of the experimental groups. These results illustrate important differences in bone adaptation in older and osteocyte deficient mice, suggesting a possible period of vulnerability in skeletal health in older subjects during and following a period of disuse that may affect skeletal health in elderly patients.

Multi-system responses to altered gravity and spaceflight: Insights from Drosophila melanogaster

NASA is planning to resume human-crewed lunar missions and lay the foundation for human exploration to Mars. However, our knowledge of the overall effects of long-duration spaceflight on human physiology is limited. During spaceflight, astronauts are exposed to multiple risk factors, including gravitational changes, ionizing radiation, physiological stress, and altered circadian lighting. These factors contribute to pathophysiological responses that target different organ systems in the body. This review discusses the advancements in gravitational biology using Drosophila melanogaster, one of the first organisms to be launched into space. As a well-established spaceflight model organism, fruit flies have yielded significant information, including neurobehavioral, aging, immune, cardiovascular, developmental, and multi-omics changes across tissues and developmental stages, as detailed in this review.

Bone deconditioning during partial weight-bearing in rodents - A systematic review and meta-analysis

Space agencies are preparing to send humans to the Moon (16% Earth's gravity) and Mars (38% Earth's gravity), however, there is limited evidence regarding the effects of hypogravity on the skeletal system. A novel rodent partial weight-bearing (PWB) model may provide insight into how human bone responds to hypogravity. The aim of this study was to perform a systematic review investigating the effect of PWB on the structure and function of rodent bone. Five online databases were searched with the following inclusion criteria: population (rodents), intervention (PWB for ≥1-week), control (full weight-bearing), outcomes (bone structure/function), and study design (animal intervention). Of the 2,993 studies identified, eight were included. The main findings were that partial weight-bearing exposure for 21-28 days at 20%, 40%, and 70% of full loading causes: (1) loss of bone mineral density, (2) loss of trabecular bone volume, thickness, number, and increased separation, (3) loss of cortical area and thickness, and 4) reduced bone stiffness and strength. These findings predominately relate the tibia/femur of young/mature female mice, however, their deconditioning response appeared similar, but not identical, to male rats. A dose-response trend was frequently observed between the magnitude of deconditioning and PWB level. The deconditioning patterns in PWB resembled those in rodents and humans exposed to microgravity and microgravity analogs. The present findings suggest that countermeasures against bone deconditioning may be required for humans exploring the Lunar and Martian surfaces.

The Impacts of COVID-19 on Musculoskeletal Health

PURPOSE OF REVIEW

Although COVID-19 was originally characterized as a respiratory disease, recent findings have shown lingering side effects in those who have recovered, and much is still unknown about the long-term consequences of the illness. Thus, the potential of unearthing multi-system dysfunction is high, with current data revealing significant impacts on musculoskeletal health.

RECENT FINDINGS

Multiple animal models of COVID-19 infection have revealed significant post-infection bone loss at several different skeletal sites. While how this loss occurred is unknown, this current review discusses the primary bone loss studies, and examines the possible mechanisms of action including: direct infection of bone marrow macrophages or hematopoietic progenitors, a proinflammatory response as a result of the COVID-19 induced cytokine storm, and/or a result of hypoxia and oxidative stress. This review will further examine how therapeutics used to treat COVID-19 affect the skeletal system. Finally, this review will examine the possible consequence that delayed care and limited healthcare accessibility has on musculoskeletal-related patient outcomes. It is important to investigate the potential impact COVID-19 infection has on musculoskeletal health.

Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight

Determining the extent of bone recovery after prolonged spaceflight is important for understanding risks to astronaut long-term skeletal health. We examined bone strength, density, and microarchitecture in seventeen astronauts (14 males; mean 47 years) using high-resolution peripheral quantitative computed tomography (HR-pQCT; 61 μm). We imaged the tibia and radius before spaceflight, at return to Earth, and after 6- and 12-months recovery and assessed biomarkers of bone turnover and exercise. Twelve months after flight, group median tibia bone strength (F.Load), total, cortical, and trabecular bone mineral density (BMD), trabecular bone volume fraction and thickness remained − 0.9% to − 2.1% reduced compared with pre-flight (p ≤ 0.001). Astronauts on longer missions (> 6-months) had poorer bone recovery. For example, F.Load recovered by 12-months post-flight in astronauts on shorter (< 6-months; − 0.4% median deficit) but not longer (− 3.9%) missions. Similar disparities were noted for total, trabecular, and cortical BMD. Altogether, nine of 17 astronauts did not fully recover tibia total BMD after 12-months. Astronauts with incomplete recovery had higher biomarkers of bone turnover compared with astronauts whose bone recovered. Study findings suggest incomplete recovery of bone strength, density, and trabecular microarchitecture at the weight-bearing tibia, commensurate with a decade or more of terrestrial age-related bone loss.

Basolateral Amygdala Mediates Central Mechanosensory Feedback of Musculoskeletal System

Musculoskeletal diseases, such as osteoporosis and sarcopenia, are tremendous and growing public health concerns. Considering the intimate functional relationship between muscle and bone throughout development, growth, and aging, muscle provides the primary source of skeletal loading through contraction force. However, significant gaps exist in our knowledge regarding the role of muscle in bone homeostasis and little is known regarding the mechanism through which the central nervous system responds and regulates unloading-induced bone loss. Here, we showed that the basolateral amygdala (BLA) and medial part of the central nucleus (CeM) are anatomically connected with the musculoskeletal system. Unloading-induced bone loss is accompanied by a decrease in serum semaphorin 3A (Sema3A) levels as well as sensory denervation. In vivo fiber photometry recordings indicated that the mechanical signal is integrated by the BLA and CeM within 24 h and subsequently regulates bone remodeling. Moreover, chemogenetic activation of BLA^CaMKII neurons mitigates severe bone loss caused by mechanical unloading via increased serum levels of Sema3A and sensory innervation. These results indicate that the BLA integrates the mechanosensory signals rapidly and mediates the systemic hormonal secretion of Sema3A to maintain bone homeostasis.

Soluble RANKL exaggerates hindlimb suspension-induced osteopenia but not muscle protein balance

We examined the hypothesis that exaggerating unloading-induced bone loss using a combination of hindlimb suspension (HLS) and exogenous injections of receptor activator of nuclear factor-κB ligand (RANKL) also exaggerates gastrocnemius and quadriceps muscle loss. Forty, male C57Bl/6J mice (16 weeks) were subjected to HLS or normal ambulation (ground control, GC) for 14 days. Mice received three intraperitoneal injections of either human recombinant soluble RANKL or phosphate-buffered saline as control (n = 10/group) at 24 h intervals starting on Day 1 of HLS. GC + RANKL and HLS mice exhibited similar decreases in trabecular bone volume and density in both proximal tibias and distal femurs. However, RANKL affected trabecular number, separation, and connectivity density, while HLS decreased trabecular thickness. The combination of RANKL and HLS exacerbated these changes. Similarly, GC + RANKL and HLS mice saw comparable decreases in cortical bone volume, thickness, and strength in femur midshafts, and combination treatment exacerbated these changes. Plasma concentrations of P1NP were increased in both groups receiving RANKL, while CTX concentrations were unchanged. HLS decreased gastrocnemius weight and was associated with a reduction in global protein synthesis, and no change in proteasome activity. This change was correlated with a decrease in S6K1 and S6 phosphorylation, but no change in 4E-BP1 phosphorylation. Injection of RANKL did not alter gastrocnemius or quadriceps muscle protein metabolism in GC or HLS mice. Our results suggest that injection of soluble RANKL exacerbates unloading-induced bone loss, but not unloading-induced gastrocnemius or quadriceps muscle loss.

Update on the effects of microgravity on the musculoskeletal system

With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

Effects of Spaceflight on Musculoskeletal Health: A Systematic Review and Meta-analysis, Considerations for Interplanetary Travel

Background

If interplanetary travel is to be successful over the coming decades, it is essential that countermeasures to minimize deterioration of the musculoskeletal system are as effective as possible, given the increased duration of spaceflight associated with such missions. The aim of this review, therefore, is to determine the magnitude of deconditioning of the musculoskeletal system during prolonged spaceflight and recommend possible methods to enhance the existing countermeasures.

Methods

A literature search was conducted using PubMed, Ovid and Scopus databases. 5541 studies were identified prior to the removal of duplicates and the application of the following inclusion criteria: (1) group means and standard deviations for pre- and post-spaceflight for measures of strength, muscle mass or bone density were reported (or provided by the corresponding author when requested via e-mail), (2) exercise-based countermeasures were included, (3) the population of the studies were human, (4) muscle function was assessed and (5) spaceflight rather than simulated spaceflight was used. The methodological quality of the included studies was evaluated using a modified Physiotherapy Evidence Database (PEDro) scale for quality, with publication bias assessed using a failsafe N (Rosenthal method), and consistency of studies analysed using I² as a test of heterogeneity. Secondary analysis of studies included Hedges’ g effect sizes, and between-study differences were estimated using a random-effects model.

Results

A total of 11 studies were included in the meta-analyses. Heterogeneity of the completed meta-analyses was conducted revealing homogeneity for bone mineral density (BMD) and spinal muscle size (Tau² < 0.001; I² = 0.00%, p > 0.05), although a high level of heterogeneity was noted for lower body force production (Tau² = 1.546; I² = 76.03%, p < 0.001) and lower body muscle mass (Tau² = 1.386; I² = 74.38%, p < 0.001). The estimated variance (≤ -0.306) for each of the meta-analyses was significant (p ≤ 0.033), for BMD (− 0.48 to − 0.53, p < 0.001), lower body force production (− 1.75, p < 0.001) and lower body muscle size (− 1.98, p < 0.001). Spaceflight results in small reductions in BMD of the femur (Hedges g = − 0.49 [− 0.69 to – 0.28]), trochanter (Hedges g = − 0.53 [− 0.77 to – 0.29]), and lumbo-pelvic region (Hedges g = − 0.48 [− 0.73 to – 0.23]), but large decreases in lower limb force production (Hedges g = − 1.75 [− 2.50 to – 0.99]) and lower limb muscle size (Hedges g = − 1.98 [− 2.72 to – 1.23]).

Conclusions

Current exercise countermeasures result in small reductions in BMD during long-duration spaceflight. In contrast, such exercise protocols do not alleviate the reductions in muscle function or muscle size, which may be attributable to the low to moderate loads reported by crewmembers and the interference effect associated with concurrent training. It is recommended that higher-load resistance exercise and the use of high-intensity interval training should be investigated, to determine if such modifications to the reported training practices result in more effective countermeasures to the deleterious effect of long-duration spaceflight on the muscular system.

Gene-metabolite networks associated with impediment of bone fracture repair in spaceflight

Adverse effects of spaceflight on musculoskeletal health increase the risk of bone injury and impairment of fracture healing. Its yet elusive molecular comprehension warrants immediate attention, since space travel is becoming more frequent. Here we examined the effects of spaceflight on bone fracture healing using a 2 mm femoral segmental bone defect (SBD) model. Forty, 9-week-old, male C57BL/6J mice were randomized into 4 groups: 1) Sham surgery on Ground (G-Sham); 2) Sham surgery housed in Spaceflight (FLT-Sham); 3) SBD surgery on Ground (G-Surgery); and 4) SBD surgery housed in Spaceflight (FLT-Surgery). Surgery procedures occurred 4 days prior to launch; post-launch, the spaceflight mice were house in the rodent habitats on the International Space Station (ISS) for approximately 4 weeks before euthanasia. Mice remaining on the Earth were subjected to identical housing and experimental conditions. The right femur from half of the spaceflight and ground groups was investigated by micro-computed tomography (µCT). In the remaining mice, the callus regions from surgery groups and corresponding femoral segments in sham mice were probed by global transcriptomic and metabolomic assays. µCT confirmed escalated bone loss in FLT-Sham compared to G-Sham mice. Comparing to their respective on-ground counterparts, the morbidity gene-network signal was inhibited in sham spaceflight mice but activated in the spaceflight callus. µCT analyses of spaceflight callus revealed increased trabecular spacing and decreased trabecular connectivity. Activated apoptotic signals in spaceflight callus were synchronized with inhibited cell migration signals that potentially hindered the wound site to recruit growth factors. A major pro-apoptotic and anti-migration gene network, namely the RANK-NFκB axis, emerged as the central node in spaceflight callus. Concluding, spaceflight suppressed a unique biomolecular mechanism in callus tissue to facilitate a failed regeneration, which merits a customized intervention strategy.

Spaceflight and hind limb unloading induces an arthritic phenotype in knee articular cartilage and menisci of rodents

Reduced knee weight-bearing from prescription or sedentary lifestyles are associated with cartilage degradation; effects on the meniscus are unclear. Rodents exposed to spaceflight or hind limb unloading (HLU) represent unique opportunities to evaluate this question. This study evaluated arthritic changes in the medial knee compartment that bears the highest loads across the knee after actual and simulated spaceflight, and recovery with subsequent full weight-bearing. Cartilage and meniscal degradation in mice were measured via microCT, histology, and proteomics and/or biochemically after: (1) ~ 35 days on the International Space Station (ISS); (2) 13-days aboard the Space Shuttle Atlantis; or (3) 30 days of HLU, followed by a 49-day weight-bearing readaptation with/without exercise. Cartilage degradation post-ISS and HLU occurred at similar spatial locations, the tibial-femoral cartilage-cartilage contact point, with meniscal volume decline. Cartilage and meniscal glycosaminoglycan content were decreased in unloaded mice, with elevated catabolic enzymes (e.g., matrix metalloproteinases), and elevated oxidative stress and catabolic molecular pathway responses in menisci. After the 13-day Shuttle flight, meniscal degradation was observed. During readaptation, recovery of cartilage volume and thickness occurred with exercise. Reduced weight-bearing from either spaceflight or HLU induced an arthritic phenotype in cartilage and menisci, and exercise promoted recovery.

Static magnetic field of 0.2-0.4 T promotes the recovery of hindlimb unloading-induced bone loss in mice

PURPOSE

Bone loss is one of the most serious medical problem associated with prolonged weightlessness in long-term spaceflight mission. Skeletal reloading after prolonged spaceflight have indicated incomplete recovery of lost bone, which may lead to an increased risk of fractures in astronauts when returning to Earth. Substantial studies have revealed the capacity of static magnetic fields (SMFs) on treating various bone disorders, whereas it is unknown whether SMFs have the potential regulatory effects on bone quality in unloaded mice during unloading. This study was conducted to investigate the potential effects of whole-body SMF exposure with 0.2-0.4 T on the recovery of unloading-induced bone loss.

MATERIALS AND METHODS

Eight-week-old male C57BL/6J mice were subjected to hindlimb unloading (HLU) for 4 weeks, following the mice were reloaded for 4 weeks under geomagnetic field (GMF) and SMF of 0.2-0.4 T. Bone quality indexes, including bone mineral density (BMD) and bone mineral content (BMC), bone microarchitecture, and bone mechanical properties were examined by the measurement of dual energy X-ray absorptiometry (DEXA), micro-computed tomography (Micro-CT), and 3-point bending. Bone turnover was evaluated by bone histomorphometric and serum biochemical assay.

RESULTS

We found that SMF exposure for 4 weeks significantly promoted the recovery in HLU-induced decrease of BMD and BMC, deterioration of bone microarchitecture, and reduction of bone strength. The results from bone turnover determination revealed that SMF exposure for 4 weeks induced lower osteoclast number of trabecular bone and serum TRAP-5b levels in reloaded mice, whereas SMF showed no significant alteration in skeletal osteoblast number and serum osteocalcin levels.

CONCLUSIONS

Together, our findings suggest that SMF of 0.2-0.4 T facilitated the recovery of unloading-induced bone loss by inhibiting the increase of bone resorption in reloaded mice, and indicate that SMF might become a promising biophysical countermeasure for maintaining bone health in astronauts after landing.

Electroacupuncture stimulation at BL20, BL23 and SP6 prevents hind limb unloading-induced osteoporosis in rats

BACKGROUND

Bone loss induced by microgravity is a serious problem in space flight. However, the effects of acupuncture stimulation on osteoporosis induced by microgravity have not been studied. With the goal of developing an effective countermeasure, our aim was to evaluate the effects of electroacupuncture (EA) stimulation at BL20, BL23, and SP6 on osteoporosis induced by simulated microgravity in rats.

METHODS

Thirty male Wistar rats (aged 10 weeks) were randomly divided into three groups: healthy control group (CON, n = 10), hind limb unloading by tail-suspension group (T-S, n = 10), and EA treatment group (TRE, n = 10). Rats in the T-S and TRE groups were subjected to tail-suspension at -30° for 30 days, while the CON group experienced freedom of activity. In this period, the TRE group received EA treatment at BL20, BL23, and SP6 for 30 min every other day, which continued for 30 days. The microarchitecture of the proximal tibia and the biomechanical features of the femur in the rats were analyzed. In addition, the levels of serum biomarkers bone alkaline phosphatase (BALP) and osteocalcin (BGP) were measured.

RESULTS

Compared with the CON group, the value of bone volume/total volume (BV/TV) and trabecular number (Tb.N) of the tibias in the TRE group remarkably decreased (p < 0.01). However, these changes were markedly less than those of the T-S group after 4 weeks of EA treatment (p < 0.05). Moreover, the serum concentration of BGP in the TRE group was also significantly higher than that of the T-S group (p < 0.05).

CONCLUSIONS

These findings indicate that EA stimulation at BL20, BL23, and SP6 retards osteoporosis induced by hind limb unloading in rats.

Effects of spaceflight on cancellous and cortical bone in proximal femur in growing rats

Mechanical loading of the skeleton during normal weight bearing plays an important role in bone accrual and turnover balance. We recently evaluated bone microarchitecture in the femoral head in 5.6-week-old male Sprague Dawley rats subjected to a 4-day spaceflight aboard STS-41. Compared to weight bearing ground controls, cancellous bone volume fraction was dramatically lower in animals subjected to microgravity. The effects of spaceflight on the rat skeleton are potentially influenced by factors such as age, duration of flight, strain and sex. To test the generalizability of our initial observation, we evaluated archived proximal femora from two additional spaceflight missions: a 10-day mission (STS-57) with 7.5-week-old male Fisher 344 rats, and a 14-day mission (STS-62) with 12-week-old ovariectomized (ovx) female Fisher 344 rats. Cancellous microarchitecture and cortical thickness were assessed using x-ray microtomography/microcomputed tomography. In male rats, cancellous bone volume fraction (bone volume/tissue volume) was lower in flight animals compared to flight controls, but differences were not significant compared to baseline. In ovx female rats, cancellous bone volume fraction was lower in flight animals compared to flight controls and baseline, indicating net bone loss. Cortical thickness did not differ among groups in either experiment. In summary, findings from three separate studies support the conclusion that spaceflight results in cancellous osteopenia in femoral head of growing rats.

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Spaceflight resulted in cancellous osteopenia in femoral head of growing rats.

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Osteopenia was observed in female ovariectomized and male Fisher 344 rats.

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The femoral head should be evaluated in future spaceflight experiments.

The individual and combined effects of spaceflight radiation and microgravity on biologic systems and functional outcomes

Both microgravity and radiation exposure in the spaceflight environment have been identified as hazards to astronaut health and performance. Substantial study has been focused on understanding the biology and risks associated with prolonged exposure to microgravity, and the hazards presented by radiation from galactic cosmic rays (GCR) and solar particle events (SPEs) outside of low earth orbit (LEO). To date, the majority of the ground-based analogues (e.g., rodent or cell culture studies) that investigate the biology of and risks associated with spaceflight hazards will focus on an individual hazard in isolation. However, astronauts will face these challenges simultaneously Combined hazard studies are necessary for understanding the risks astronauts face as they travel outside of LEO, and are also critical for countermeasure development. The focus of this review is to describe biologic and functional outcomes from ground-based analogue models for microgravity and radiation, specifically highlighting the combined effects of radiation and reduced weight-bearing from rodent ground-based tail suspension via hind limb unloading (HLU) and partial weight-bearing (PWB) models, although in vitro and spaceflight results are discussed as appropriate. The review focuses on the skeletal, ocular, central nervous system (CNS), cardiovascular, and stem cells responses.

Implications of Altered Endosome and Lysosome Biology in Space Environments

Space exploration poses multiple challenges for mankind, not only on a technical level but also to the entire physiology of the space traveller. The human system must adapt to several environmental stressors, microgravity being one of them. Lysosomes are ubiquitous to every cell and essential for their homeostasis, playing significant roles in the regulation of autophagy, immunity, and adaptation of the organism to changes in their environment, to name a few. Dysfunction of the lysosomal system leads to age-related diseases, for example bone loss, reduced immune response or cancer. As these conditions have been shown to be accelerated following exposure to microgravity, this review elucidates the lysosomal response to real and simulated microgravity. Microgravity activates the endo-lysosomal system, with resulting impacts on bone loss, muscle atrophy and stem cell differentiation. The investigation of lysosomal adaptation to microgravity can be beneficial in the search for new biomarkers or therapeutic approaches to several disease pathologies on earth as well as the potential to mitigate pathophysiology during spaceflight.

Population analysis of space travelers

Although many space missions have been completed in the last 60 years, space exploration is still technologically and medically challenging. While large-scale medical studies are impossible in space travelers, meta-analysis allows combining data from small crews that participated in space missions over several decades. Our primary objective was to examine space-travelers' sociodemographic characteristics and spaceflight activities, and their changes with time from the first spaceflight. Our secondary objective was to evaluate the publication practices to assess data availability for health-related meta-analytic studies. Based on state-funded space agencies used as primary sources, and third-party websites used as secondary sources, 565 humans (501 males/64 females) have currently completed spaceflight. The average age of space-travelers increased from 34±4 in the 1960s to 45±4 in the 2010s. While the duration of space missions has increased consistently, the number of missions per year varied in correlation with technological events. Using papers identified in the systematic review of bone health in astronauts, we examined the changes in reporting practices with time. The reported sample size varied from 1 to 58 people, in total providing data for 148 individuals. Data confidentiality significantly improved with time; however, the corresponding decrease in the availability of individual parameters did not allow stratification even by age, sex, and mission duration. Thus, space travelers represent a diverse population suitable for comparative studies, however, it is important to develop reporting practices that ensure consistent, transparent, and ethical presentation of outcomes to support meta-analyses that are critical for understanding the scope of spaceflight-induced health issues.

Irisin prevents microgravity-induced impairment of osteoblast differentiation in vitro during the space flight CRS-14 mission

Understanding molecular mechanisms responsible for bone cells unbalance in microgravity would allow the development of better countermeasures for astronauts, and eventually advancing terrestrial osteoporosis treatments. We conduct a unique investigation by using a controlled 3D in vitro cell model to mimic the bone microenvironment in microgravity aboard the SpaceX Dragon cargo ferry to the ISS. Osteoblasts (OBs), osteoclasts (OCs), and endothelial cells (ECs), seeded on Skelite discs, were cultured w/ or w/o rec-Irisin and exposed to 14 days of microgravity in the eOSTEO hardware. Gene expression analysis was assessed, and results were compared to ground controls treated within identical payloads. Our results show that the microgravity-induced downregulation of mRNA levels of genes encoding for OB key transcription factors (Atf4 -75%, P < .01; RunX2 -87%, P < .001, Osterix -95%, P < .05 vs ground) and proteins (Collagen I -84%, P < .05; Osteoprotegerin -94%, P < .05) were prevented by irisin. Despite it was not effective in preventing Trap and Cathepsin K mRNA increase, irisin induced a 2.8-fold increase of Osteoprotegerin (P < .05) that might act for reducing osteoclastogenesis in microgravity. Our results provide evidence that irisin supports OB differentiation and activity in microgravity and it might represent a countermeasure to prevent bone loss in astronauts.

Disorder of Iron Metabolism Inhibits the Recovery of Unloading-Induced Bone Loss in Hypomagnetic Field

Exposure of humans and animals to microgravity in spaceflight results in various deleterious effects on bone health. In addition to microgravity, the hypomagnetic field (HyMF) is also an extreme environment in space, such as on the Moon and Mars; magnetic intensity is far weaker than the geomagnetic field (GMF) on Earth. Recently, we showed that HyMF promoted additional bone loss in hindlimb unloading-induced bone loss, and the underlying mechanism probably involved an increase of body iron storage. Numerous studies have indicated that bone loss induced by mechanical unloading can be largely restored after skeletal reloading in GMF conditions. However, it is unknown whether this bone deficit can return to a healthy state under HyMF condition. Therefore, the purpose of this study is to examine the effects of HyMF on the recovery of microgravity-induced bone loss, and illustrates the changes of body iron storage in this process. Our results showed that there was lower bone mineral content (BMC) in the HyMF reloading group compared to the GMF reloading group. Reloaded mice in the HyMF condition had a worse microstructure of femur than in the GMF condition. Femoral mechanical properties, including elastic modulus, stiffness, and ultimate stress, were poorer and toughness was higher in the HyMF group compared with the GMF group. Simultaneously, more iron content in serum, the tibia, liver, and spleen was found under HyMF reloading than GMF reloading. The iron chelator deferoxamine mesylate (DFO) decreased the iron content in the bone, liver, and spleen, and significantly relieved unloading-induced bone loss under HyMF reloading. These results showed that HyMF inhibits the recovery of microgravity-induced bone loss, probably by suppressing the elevated iron levels' return to physiological level. © 2019 American Society for Bone and Mineral Research.

Antioxidant Strategy to Prevent Simulated Microgravity-Induced Effects on Bone Osteoblasts

The effects induced by microgravity on human body functions have been widely described, in particular those on skeletal muscle and bone tissues. This study aims to implement information on the possible countermeasures necessary to neutralize the oxidative imbalance induced by microgravity on osteoblastic cells. Using the model of murine MC3T3-E1 osteoblast cells, cellular morphology, proliferation, and metabolism were investigated during exposure to simulated microgravity on a random positioning machine in the absence or presence of an antioxidant—the 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox). Our results confirm that simulated microgravity-induced morphological and metabolic alterations characterized by increased levels of reactive oxygen species and a slowdown of the proliferative rate. Interestingly, the use of Trolox inhibited the simulated microgravity-induced effects. Indeed, the antioxidant-neutralizing oxidants preserved cell cytoskeletal architecture and restored cell proliferation rate and metabolism. The use of appropriate antioxidant countermeasures could prevent the modifications and damage induced by microgravity on osteoblastic cells and consequently on bone homeostasis.

Use of Quantitative Computed Tomography to Assess for Clinically-relevant Skeletal Effects of Prolonged Spaceflight on Astronaut Hips

INTRODUCTION

In 2010, experts in osteoporosis and bone densitometry were convened by the Space Life Sciences Directorate at NASA Johnson Space Center to identify a skeletal outcome in astronauts after spaceflight that would require a clinical response to address fracture risk. After reviewing astronaut data, experts expressed concern over discordant patterns in loss and recovery of bone mineral density (BMD) after spaceflight as monitored by dual-energy X-ray absorptiometry (DXA) and quantitative computed tomography (QCT). The pilot study described herein demonstrates the use of QCT to evaluate absence of recovery in hip trabecular BMD by QCT as an indicator of a clinically actionable response.

METHODOLOGY

QCT and DXA scans of both hips were performed on 10 astronauts: once preflight and twice postflight about 1 wk and 1 yr after return. If trabecular BMD had not returned to baseline (i.e., within QCT measurement error) in 1 or both hips 1 yr after flight, then another QCT hip scan was obtained at 2 yr after flight.

RESULTS

Areal BMD by DXA recovered in 9 of 10 astronauts at 1 yr postflight while incomplete recovery of trabecular BMD by QCT was evident in 5 of 10 astronauts and persisted in 4 of the 5 astronauts 2 yr postflight.

CONCLUSION

As an adjunct to DXA, QCT is needed to detect changes to hip trabecular BMD after spaceflight and to confirm complete recovery. Incomplete recovery at 2 yr should trigger the need for further evaluation and possible intervention to mitigate premature fragility and fractures in astronauts following long-duration spaceflight.

Connexin 43 Channels in Osteocytes Regulate Bone Responses to Mechanical Unloading

Connexin (Cx) 43 forms gap junctions and hemichannels that mediate communication between osteocytes and adjacent cells or the extracellular environment in bone, respectively. To investigate the role of each channel type in response to mechanical unloading, two transgenic mouse models overexpressing dominant-negative Cx43 predominantly in osteocytes driven by a 10 kb dentin matrix protein 1 (Dmp1) promoter were generated. The R76W mutation resulted in gap junction inhibition and enhancement of hemichannels, whereas the Δ130-136 mutation inhibited both gap junctions and hemichannels. Both mutations led to cortical bone loss with increased endocortical osteoclast activity during unloading. Increased periosteal osteoclasts with decreased apoptotic osteocytes were observed only in R76W mice. These findings indicated that inhibiting osteocytic Cx43 channels promotes bone loss induced by unloading, mainly in the cortical area; moreover, hemichannels protect osteocytes against apoptosis and promote periosteal bone remodeling, whereas gap junctions modulate endocortical osteoclast activity in response to unloading.

Selected discoveries from human research in space that are relevant to human health on Earth

A substantial amount of life-sciences research has been performed in space since the beginning of human spaceflight. Investigations into bone loss, for example, are well known; other areas, such as neurovestibular function, were expected to be problematic even before humans ventured into space. Much of this research has been applied research, with a primary goal of maintaining the health and performance of astronauts in space, as opposed to research to obtain fundamental understanding or to translate to medical care on Earth. Some people—scientists and concerned citizens—have questioned the broader scientific value of this research, with the claim that the only reason to perform human research in space is to keep humans healthy in space. Here, we present examples that demonstrate that, although this research was focused on applied goals for spaceflight participants, the results of these studies are of fundamental scientific and biomedical importance. We will focus on results from bone physiology, cardiovascular and pulmonary systems, and neurovestibular studies. In these cases, findings from spaceflight research have provided a foundation for enhancing healthcare terrestrially and have increased our knowledge of basic physiological processes.

Effects of Long-Duration Spaceflight on Vertebral Strength and Risk of Spine Fracture

Although the negative impact of long-duration spaceflight on spine BMD has been reported, its impact on vertebral strength and risk of vertebral fracture remains unknown. This study examined 17 crewmembers with long-duration service on the International Space Station in whom computed tomography (CT) scans of the lumbar spine (L₁ and L₂ ) were collected preflight, immediately postflight and 1 to 4 years after return to Earth. We assessed vertebral strength via CT-based finite element analysis (CT-FEA) and spinal loading during different activities via subject-specific musculoskeletal models. Six months of spaceflight reduced vertebral strength by 6.1% (-2.3%, -8.7%) (median [interquartile range]) compared to preflight (p < 0.05), with 65% of subjects experiencing deficits of greater than 5%, and strengths were not recovered up to 4 years after the mission. This decline in vertebral strength exceeded (p < 0.05) the 2.2% (-1.3%, -6.0%) decline in lumbar spine DXA-BMD. Further, the subject-specific changes in vertebral strength were not correlated with the changes in DXA-BMD. Although spinal loading increased slightly postflight, the ratio of vertebral compressive load to vertebral strength for typical daily activities remained well below a value of 1.0, indicating a low risk of vertebral fracture despite the loss in vertebral strength. However, for more strenuous activity, the postflight load-to-strength ratios ranged from 0.3 to 0.7, indicating a moderate risk of vertebral fracture in some individuals. Our findings suggest persistent deficits in vertebral strength following long-duration spaceflight, and although risk of vertebral fracture remains low for typical activities, the risk of vertebral fracture is notable in some crewmembers for strenuous exercise requiring maximal effort. © 2019 American Society for Bone and Mineral Research.

A human mission to Mars: Predicting the bone mineral density loss of astronauts

A round-trip human mission to Mars is anticipated to last roughly three years. Spaceflight conditions are known to cause loss of bone mineral density (BMD) in astronauts, increasing bone fracture risk. There is an urgent need to understand BMD progression as a function of spaceflight time to minimize associated health implications and ensure mission success. Here we introduce a nonlinear mathematical model of BMD loss for candidate human missions to Mars: (i) Opposition class trajectory (400-600 days), and (ii) Conjunction class trajectory (1000-1200 days). Using femoral neck BMD data (N = 69) from astronauts after 132-day and 228-day spaceflight and the World Health Organization's fracture risk recommendation, we predicted post-mission risk and associated osteopathology. Our model predicts 62% opposition class astronauts and 100% conjunction class astronauts will develop osteopenia, with 33% being at risk for osteoporosis. This model can help in implementing countermeasure strategies and inform space agencies' choice of crew candidates.

Therapeutic Effects of Low-Intensity Pulsed Ultrasound on Osteoporosis in Ovariectomized Rats: Intensity-Dependent Study

This study investigated the effects of low-intensity pulsed ultrasound (LIPUS) of different spatial-average-temporal-average intensity (I_SATA) ranging from 15-150 mW/cm² on the treatment of osteoporosis in ovariectomized rats. Healthy 3-mo-old female Sprague-Dawley rats were randomly divided into nine groups (n = 12 per group): sham-ovariectomy (OVX) control group, OVX control group and OVX groups treated with LIPUS at seven different intensities (I_SATA: 15, 30, 50, 75, 100, 125 and 150 mW/cm², respectively). LIPUS was applied to bilateral femurs 12 wk post-OVX for 20 min/d for 6 wk. Micro-computed tomography, biomechanical tests, serum biochemical analysis and grip strength tests were performed to evaluate the therapeutic effects of LIPUS at different intensities. Results revealed that LIPUS intensity yielded strong correlations with bone mineral density and bone microstructure (R² = 0.57-0.83) and bone mechanical strength (R² = 0.80-0.97), and that the intensity of 150 mW/cm², instead of the 30 mW/cm² widely used in bone fracture healing, was most effective in maintaining bone mass among all the LIPUS signals between 15 and 150 mW/cm². This suggests that higher ultrasound intensity (i.e., 150 mW/cm²) may be more effective than lower intensity in mitigation of osteopenia and osteoporosis.

Protective Effects of Controlled Mechanical Loading of Bone in C57BL6/J Mice Subject to Disuse

Prolonged reduction in weightbearing causes bone loss. Disuse of bone is associated with recovery from common musculoskeletal injury and trauma, bed rest resulting from various medical conditions, and spaceflight. The hindlimb‐suspension rodent model is popular for simulating unloading and disuse. We hypothesized that controlled mechanical loading of the tibia would protect against bone loss occurring from concurrent disuse. Additionally, we hypothesized that areas of high mechanical peak strains (midshaft) would provide more protection than areas of lower strain (distal shaft). Adult C57BL6/J mice were suspended for 3 weeks, with one limb subjected to tibial compression four times per week. μCT imaging was completed at days 0, 11, and 21, in addition to serum analysis. Significant bone loss caused by hindlimb suspension was detected in trabecular bone by day 11 and worsened by day 21 (p < 0.05). Bone loss was also detected in cortical thickness and area fraction by day 21. However, four short bouts per week of compressive loading protected the loaded limb from much of this bone loss. At day 21, we observed a 50% loss in trabecular bone volume/total volume and a 6% loss in midshaft cortical thickness in unloaded limbs, but only 15% and 2% corresponding losses in contralateral loaded limbs (p = 0.001 and p = 0.02). Many bone geometry parameters of the loaded limbs of suspended animals did not significantly differ from non‐suspended control limbs. Conversely, this protective effect of loading was not detected in cortical bone at the lower‐strained distal shaft. Analysis of bone metabolism markers suggested that the benefits of loading occurred through increased formation instead of decreased resorption. This study uniquely isolates the role of externally applied mechanical loading of the mouse tibia, in the absence of muscle stimulation, in protecting bone from concurrent disuse‐related loss, and demonstrates that limited bouts of loading may be highly effective during prolonged disuse. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

The NATO project: nanoparticle-based countermeasures for microgravity-induced osteoporosis

Recent advances in nanotechnology applied to medicine and regenerative medicine have an enormous and unexploited potential for future space and terrestrial medical applications. The Nanoparticles and Osteoporosis (NATO) project aimed to develop innovative countermeasures for secondary osteoporosis affecting astronauts after prolonged periods in space microgravity. Calcium- and Strontium-containing hydroxyapatite nanoparticles (nCa-HAP and nSr-HAP, respectively) were previously developed and chemically characterized. This study constitutes the first investigation of the effect of the exogenous addition of nCa-HAP and nSr-HAP on bone remodeling in gravity (1 g), Random Positioning Machine (RPM) and onboard International Space Station (ISS) using human bone marrow mesenchymal stem cells (hBMMSCs). In 1 g conditions, nSr-HAP accelerated and improved the commitment of cells to differentiate towards osteoblasts, as shown by the augmented alkaline phosphatase (ALP) activity and the up-regulation of the expression of bone marker genes, supporting the increased extracellular bone matrix deposition and mineralization. The nSr-HAP treatment exerted a protective effect on the microgravity-induced reduction of ALP activity in RPM samples, and a promoting effect on the deposition of hydroxyapatite crystals in either ISS or 1 g samples. The results indicate the exogenous addition of nSr-HAP could be potentially used to deliver Sr to bone tissue and promote its regeneration, as component of bone substitute synthetic materials and additive for pharmaceutical preparation or food supplementary for systemic distribution.

Resistive exercise in astronauts on prolonged spaceflights provides partial protection against spaceflight-induced bone loss

Bone loss in astronauts during spaceflight may be a risk factor for osteoporosis, fractures and renal stone formation. We previously reported that the bisphosphonate alendronate, combined with exercise that included an Advanced Resistive Exercise Device (ARED), can prevent or attenuate group mean declines in areal bone mineral density (aBMD) measured soon after ~ 6-month spaceflights aboard the International Space Station (ISS). It is unclear however if the beneficial effects on postflight aBMD were due to individual or combined effects of alendronate and ARED. Hence, 10 additional ISS astronauts were recruited who used the ARED (ARED group) without drug administration using similar measurements in the previous study, i.e., densitometry, biochemical assays and analysis of finite element (FE) models. In addition densitometry data (DXA and QCT only) were compared to published data from crewmembers (n = 14-18) flown prior to in-flight access to the ARED (Pre-ARED). Group mean changes from preflight (± SD %) were used to evaluate effects of countermeasures as sequentially modified on the ISS (i.e., Pre-ARED vs. ARED; ARED vs. Bis+ARED). Spaceflight durations were not significantly different between groups. Postflight bone density measurements were significantly reduced from preflight in the Pre-ARED group. As previously reported, combined Bis+ARED prevented declines in all DXA and QCT hip densitometry and in estimates of FE hip strengths; increased the aBMD of lumbar spine; and prevented elevations in urinary markers for bone resorption during spaceflight. ARED without alendronate partially attenuated declines in bone mass but did not suppress biomarkers for bone resorption or prevent trabecular bone loss. Resistive exercise in the ARED group did not prevent declines in hip trabecular vBMD, but prevented reductions in cortical vBMD of the femoral neck, in FE estimate of hip strength for non-linear stance (NLS) and in aBMD of the femoral neck. We conclude that a bisphosphonate, when combined with resistive exercise, enhances the preservation of bone mass because of the added suppression of bone resorption in trabecular bone compartment not evident with ARED alone.

Comparisons of Resistance Training and "Cardio" Exercise Modalities as Countermeasures to Microgravity-Induced Physical Deconditioning: New Perspectives and Lessons Learned From Terrestrial Studies

Prolonged periods in microgravity (μG) environments result in deconditioning of numerous physiological systems, particularly muscle at molecular, single fiber, and whole muscle levels. This deconditioning leads to loss of strength and cardiorespiratory fitness. Loading muscle produces mechanical tension with resultant mechanotransduction initiating molecular signaling that stimulates adaptations in muscle. Exercise can reverse deconditioning resultant from phases of detraining, de-loading, or immobilization. On Earth, applications of loading using exercise models are common, as well as in μG settings as countermeasures to deconditioning. The primary modalities include, but are not limited to, aerobic training (or "cardio") and resistance training, and have historically been dichotomized; the former primarily thought to improve cardiorespiratory fitness, and the latter primarily improving strength and muscle size. However, recent work questions this dichotomy, suggesting adaptations to loading through exercise are affected by intensity of effort independent of modality. Furthermore, similar adaptations may occur where sufficient intensity of effort is used. Traditional countermeasures for μG-induced deconditioning have focused upon engineering-based solutions to enable application of traditional models of exercise. Yet, contemporary developments in understanding of the applications, and subsequent adaptations, to exercise induced muscular loading in terrestrial settings have advanced such in recent years that it may be appropriate to revisit the evidence to inform how exercise can used in μG. With the planned decommissioning of the International Space Station as early as 2024 and future goals of manned moon and Mars missions, efficiency of resources must be prioritized. Engineering-based solutions to apply exercise modalities inevitably present issues relating to devices mass, size, energy use, heat production, and ultimately cost. It is necessary to identify exercise countermeasures to combat deconditioning while limiting these issues. As such, this brief narrative review considers recent developments in our understanding of skeletal muscle adaptation to loading through exercise from studies conducted in terrestrial settings, and their applications in μG environments. We consider the role of intensity of effort, comparisons of exercise modalities, the need for concurrent exercise approaches, and other issues often not considered in terrestrial exercise studies but are of concern in μG environments (i.e., O2 consumption, CO2 production, and energy costs of exercise).

Overview of the NASA 70-day Bed Rest Study

PURPOSE

The purpose of this article was to provide an overview of the National Aeronautics and Space Administration (NASA) 70-day Bed Rest Study. The integrated complement of investigations and the standardized bed rest environment that served as the platform for this study complement are described. Outcomes of the studies will not be presented here but will be reported in separate publications.

METHODS

A set of studies running in an integrated fashion along the entire period (pre-, in-, and post-bed rest) and using the same subjects is referred in this article as "the campaign" or "complement." NASA selected eight individual studies to participate in the 70-d bed rest campaign. These studies were integrated to increase efficiency in the utilization of resources and to share common measures among the investigations. In addition to the individual studies addressing specific aims, a battery of standardized measures was included. Standard measures target a wide range of physiologic systems and represent some of the testing routinely done on astronauts. Bed rest subjects underwent rigorous medical and psychological screening. Standardized conditions included 70 d of bed rest in a 6° head-down tilt position. Subjects' vital signs, body weight, and fluid intake and output were measured daily. A standardized diet was provided to ensure consistent nutritional intake across subjects. Exercising subjects were prescribed individualized aerobic and resistance training 6 d·wk performed in a horizontal body position. Subjects in the testosterone supplementation countermeasure group received testosterone enanthate injections at 2-wk intervals during bed rest.

CONCLUSION

Long-duration head-down tilt bed rest provided a suitable platform for examining physiologic effects of spaceflight and testing countermeasures in a ground-based model. Integrating studies into a complement is an effective way to support multiple investigations while minimizing the number of subjects to answer many research questions.

Sex and Laterality Differences in Medullary Humerus Morphology

Percutaneous osseointegrated (OI) prosthetic limb attachment holds promise for transhumeral amputees. Understanding humeral medullary morphology is necessary for informed design of upper extremity OI systems, and is beneficial to the field of megaprosthetic reconstruction of the distal humerus where diaphyseal fixation is desired. The purpose of this study was to quantify the sex and laterality differences in humerus morphology, specifically over the diaphysis. Three‐dimensional surface reconstructions of 58 pairs of cadaveric humeri (43 male, 15 female) were generated from CT data. Measures describing periosteal and medullary morphology were collected relative to an anatomic coordinate system. Sex and laterality differences in biomechanical length (BML) were observed (P ≤ 0.001 and 0.022, respectively). Head radius was larger in males than females (P ≤ 0.001). Retroversion was increased in right humeri relative to left (P ≤ 0.001). Canal orientation exhibited a conformational shift from anteversion to retroversion distally at approximately 65% BML. Right humeri exhibited larger medullary diameters than left in the 1st and 2nd principal directions (P ≤ 0.024). Males displayed larger diameter medullary canals proximally (P ≤ 0.029) and an increased rate of divergence of the endosteal cortex in the proximal diaphysis (P ≤ 0.009). Females exhibited higher canal aspect ratios at mid‐shaft (P ≤ 0.014) and lower mean cortical thickness (P ≤ 0.001). Human humeral diaphysis morphology exhibits sex and laterality differences, which are dependent on position along the diaphysis. Understanding humeral morphology is necessary to achieve adequate primary stability and bone apposition in design of endoprosthetic stems for percutaneous OI implants, and distal humerus replacement. Anat Rec, 302:1709–1717, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association for Anatomy

Effects of loading rate on the of mechanical behavior of the femur in falling condition

The Surgeon General estimates that by 2020, half of all Americans could have weak bones due to bone loss. Osteoporosis causes more than 1.5 million fractures every year. Identifying effective interventions based on individual patient characteristics remains a major challenge. Proximal femur fractures are common and devastating events for individuals with osteoporosis. Since fracture is primarily a mechanical event, noninvasive predictions of fracture strength and location would be useful both for identifying at-risk individuals and evaluating treatment effects. However, bone fracture prediction is complicated due to the complex microstructure and nanostructure of bone. Bone is a highly heterogeneous material with rate-dependent mechanical behavior and large inter-individual variation. In this study, we designed two mechanical test procedures to understand the mechanical response of bone under impact and quasi-static load tests. The boundary conditions of the tests were designed in a way to simulate a fall to the side. The present study consists of three main parts: cadaver testing, quantitative image analysis, and finite element (FE) modeling. We obtained ten human femur bones and used high-resolution CT to quantify the microstructure and density of each sample. Specimen-specific FE models were created to evaluate the ability of various failure criteria to predict experimental fracture. Afterward, the samples were tested and their failure patterns were recorded. The fractured samples were rescanned to analyze the fractured surfaces. Our experimental results show that the loading necessary to fracture the femur samples is much higher in the impact tests. However, the toughening mechanisms are more pronounced in quasi-static tests. We found that FE model formulations were able to accurately predict femur stiffness and strength for quasi-static and impact conditions separately, but that no single formulation could account for the rate-dependent outcomes.

Forecasting post-flight hip fracture probability using probabilistic modeling

A probabilistic model predicts hip fracture probability for post-flight male astronauts during lateral fall scenarios from various heights. A biomechanical representation of the hip provides impact load. Correlations relate spaceflight bone mineral density (BMD) loss and post-flight BMD recovery to bone strength. Translations convert fracture risk index, the ratio of applied load to bone strength, to fracture probability. Parameter distributions capture uncertainty and Monte Carlo simulations provide probability outcomes. The fracture probability for a 1 m fall 0 days post-flight is 15% greater than preflight and remains 6% greater than pre-flight at 365 days post-flight. Probability quantification provides insight into how spaceflight induced BMD loss affects fracture probability. A bone loss rate reflecting improved exercise countermeasures and dietary intake further reduces the post-flight fracture probability to 6% greater than preflight at 0 days post-flight and 2% greater at 365 days post-flight. Quantification informs assessments of countermeasure effectiveness. When preflight BMD is one standard deviation below mean astronaut preflight BMD, fracture probability at 0 days post-flight is 34% greater than the preflight fracture probability calculated with mean BMD and 28% greater at 365 days post-flight. Quantification aids review of astronaut BMD fitness for duty standards. Increases in post-flight fracture probability are associated with an estimated 18% reduction in post-flight bone strength. Therefore, a 0.82 deconditioning coefficient modifies force application limits for crew vehicles.

Impact of muscle atrophy on bone metabolism and bone strength: implications for muscle-bone crosstalk with aging and disuse

Bone fractures in older adults are often preceded by a loss of muscle mass and strength. Likewise, bone loss with prolonged bed rest, spinal cord injury, or with exposure to microgravity is also preceded by a rapid loss of muscle mass. Recent studies using animal models in the setting of hindlimb unloading or botulinum toxin (Botox) injection also reveal that muscle loss can induce bone loss. Moreover, muscle-derived factors such as irisin and leptin can inhibit bone loss with unloading, and knockout of catabolic factors in muscle such as the ubiquitin ligase Murf1 or the myokine myostatin can reduce osteoclastogenesis. These findings suggest that therapies targeting muscle in the setting of disuse atrophy may potentially attenuate bone loss, primarily by reducing bone resorption. These potential therapies not only include pharmacological approaches but also interventions such as whole-body vibration coupled with resistance exercise and functional electric stimulation of muscle.

Effect of Constraint Loading on the Lower Limb Muscle Forces in Weightless Treadmill Exercise

Long exposure to the microgravity will lead to muscle atrophy and bone loss. Treadmill exercise could mitigate the musculoskeletal decline. But muscle atrophy remains inevitable. The constraint loading applied on astronauts could affect the muscle force and its atrophy severity. However, the quantitative correlation between constraint loading mode and muscle forces remains unclear. This study aimed to characterize the influence of constraint loading mode on the lower limb muscle forces in weightless treadmill exercise. The muscle forces in the full gait cycle were calculated with the inverse dynamic model of human musculoskeletal system. The calculated muscle forces at gravity were validated with the EMG data. Muscle forces increased at weightlessness compared with those at the earth's gravity. The increasing percentage from high to low is as follows: biceps femoris, gastrocnemius, soleus, vastus, and rectus femoris, which was in agreement with the muscle atrophy observed in astronauts. The constraint loading mode had an impact on the muscle forces in treadmill exercise and thus could be manipulated to enhance the effect of the muscle training in spaceflight. The findings could provide biomechanical basis for the optimization of treadmill constraint system and training program and improve the countermeasure efficiency in spaceflight.

Combination of hindlimb suspension and immobilization by casting exaggerates sarcopenia by stimulating autophagy but does not worsen osteopenia

Astronauts in space experience a unique environment that causes the concomitant loss of bone and muscle. However, the interaction between these tissues and how osteopenia and sarcopenia affect each other is unclear. We explored this relationship by exaggerating unloading-induced muscle loss using a unilateral casting model in conjunction with hindlimb suspension (HLS). Five-month-old, male C57Bl/6J mice subjected to HLS for 2 weeks displayed a significant decrease in gastrocnemius and quadriceps weight (-9-10%), with a two-fold greater decrease in muscle mass observed in the HLS + casted limb. However, muscle from casted limbs had a higher rate of protein synthesis (+16%), compared to HLS alone, with coordinated increases in S6K1 (+50%) and 4E-BP1 (+110%) phosphorylation. Increased protein content for surrogate markers of autophagy, including LC3-II (+75%), Atg7 (+10%), and Atg5-12 complex (+20%) was only detected in muscle from the casted limb. In proximal tibias, HLS resulted in significant decreases in bone volume fraction (-24% vs -8%), trabecular number (-6% vs +0.3%), trabecular thickness (-10% vs -2%), and trabecular spacing (+8.4% vs +2%) compared to ground controls. There was no further bone loss in casted limbs compared to HLS alone. In tibia midshafts, HLS resulted in decreased total area (-2% vs +1%) and increased bone mineral density (+1% vs -0.3%) compared to ground controls. Cortical bone from casted limbs showed an increase in cortical thickness (+9% vs +2%) and cortical area/total area (+1% vs -0.6%) compared to HLS alone. Our results suggest that casting exacerbates unloading-induced muscle loss via activation of autophagy. Casting did not exacerbate bone loss suggesting that the unloading-induced loss of muscle and bone can be temporally dissociated and the effect of reduced muscle activity plays a relatively minor role compared to reduced load bearing on trabecular bone structure.