The sky is no longer the limit for science, with a constellation of studies routinely being performed in the space and microgravity field. In fact, studies of biological systems in variable gravity conditions have increasingly garnered interest, with astronauts working at the International Space Station, and future space missions to the Moon and Mars. Beyond spaceflight, the implications of micro- and partial-gravity extend to all sorts of movement restrictions, such as bedrest, joint immobilization, and support withdrawal. As researchers, scientists, and engineers from across the field come together for the 2024 American Society for Gravitational and Space Research (ASGSR) conference, this publication will launch into the latest insights on microgravity and muscle health.
Featured image (with figures adapted from ©Issertine et al (2024), licensed under CC BY 4.0 DEED, and photo “1965. James McDivitt, Ed White, Extravehicular Activity (EVA), Gemini 4 [Spacewalk]” by Jeremy Bishop on Unsplash) depicting an astronaut spacewalking, along with muscle resistance to a fatigue protocol conducted in adult Wistar rats that underwent hindlimb suspension. A) Change in the area under the curve (AUC) during the fatigue protocol, B) change in fatigue peak force, C) change in the time to decrease force production by 50%, and D) change in fatigue force drop. N=10 for males, n=11 for females.
Adaptation to full weight-bearing following disuse in rats: The impact of biological sex on musculoskeletal recovery
Extended time in microgravity leads to severe musculoskeletal deconditioning, including muscle atrophy, bone loss, and cardiovascular changes in astronauts, despite countermeasures like exercise. As space missions like Artemis aim to establish a long-term human presence on the Moon and Mars, astronauts will face significant challenges in re-adapting to Earth’s gravity. While prior research has shown bone loss in astronauts during space missions, sex-based differences in musculoskeletal recovery are not well understood. To address this, Issertine et al (2024) compared how male and female rats recovered from unloading-induced musculoskeletal deconditioning.
Adult Wistar rats underwent hindlimb suspension (HLS) for 14 days, followed by 7 days of full weight-bearing recovery. Grip strength was measured using a 50N digital grip force meter, while hindlimb function was assessed using Aurora Scientific’s 1305A 3-in-1 Whole Animal System. Both the maximal torque and area under the curve (AUC) were measured during nerve-stimulated plantarflexion and dorsiflexion, with muscle fatigue additionally tested in the plantarflexors. Skeletal assessments of bone mineral density were performed using in vivo peripheral quantitative computed tomography (pQCT), and histological analyses of muscle cross-sectional area and estrous cycle monitoring were specifically performed in female rats.
Upon analysis, Issertine et al (2024) found that HLS led to significant body weight loss in both male and female rats. While females fully recovered during 7 days of reloading, males did not. Force production measurements revealed that rear paw grip strength decreased after HLS in both sexes but improved with reloading, with females showing a greater recovery. During plantarflexion and dorsiflexion functional measurements, males exhibited reduced force after reloading, while females exhibited an increase. Interestingly, fatigability assessments additionally revealed that muscle endurance was less impacted in females than in males. The results suggest that sex-based differences in muscle recovery and force production after disuse are significant, with females showing greater recovery capacity. Together, these findings underscore the importance of considering sex differences in muscle rehabilitation and recovery studies.
Mechanical and signaling responses of unloaded rat soleus muscle to chronically elevated β-myosin activity
Muscle unloading, due to conditions like bed rest, immobilization, or spaceflight, leads to significant declines in muscle mass, strength, and function, particularly in the soleus muscle. While countermeasures such as exercise and neuromuscular stimulation are commonly explored, these methods may not be feasible in all clinical or physical conditions, highlighting a critical need for alternative approaches to muscle atrophy prevention. Previous studies have shown that spontaneous muscle contractions increase during disuse, potentially activating anabolic signaling pathways; however, pharmacological strategies to enhance this response have not been extensively studied. Therefore, Sergeeva et al (2024) sought to investigate the effects of Omecamtiv Mecarbil (OM), a drug that potentiates myosin function, on muscle recovery during unloading.
Male Wistar rats were divided into four groups – the controls (C), controls with OM administration for 10 days (C + OM), hindlimb suspension for 14 days (H), and hindlimb suspension for 14 days with administration of OM for 10 days (H + OM). Mechanical measurements were performed on the soleus muscles of the rats using Aurora Scientific’s 809C-IV in-vitro Mouse Apparatus as well as the 305C-LR Dual-Mode Muscle Lever. Data processing was then performed using the 615A Dynamic Muscle Control and Analysis Software. To analyze the anabolic pathway, protein extraction and Western blotting was performed, as well as real-time PCR for gene expression, MyHC immunostaining for muscle fiber type assessment, and nitric oxide detection using EPR spectroscopy.
Intriguingly, OM treatment in hindlimb-suspended rats did not significantly mitigate the mechanical or biochemical effects of muscle atrophy. While both suspended groups (“H” and “H + OM”) showed similar reductions in muscle mass, fiber cross-sectional area (CSA), and muscle force production, OM did not prevent these changes. Although the OM treatment helped maintain protein synthesis rates at control levels, it did not prevent reductions in ribosomal RNA content and muscle fiber type transformation. Additionally, OM administration altered the phosphorylation status of several key proteins involved in muscle protein synthesis and breakdown, including p70S6K, GSK-3β, and FOXO3, but did not fully protect the muscles from atrophy-induced alterations in these pathways. Overall, the findings show that while OM does not prevent hindlimb suspension-induced muscle atrophy, it could possibly increase protein synthesis by affecting the energetic state of the muscle fiber and activating mechanosensory signaling pathways.
Sex differences in muscle health in simulated micro- and partial-gravity environments in rats
Skeletal muscle strength is crucial for astronaut health during space missions, yet current exercise interventions on the International Space Station (ISS) do not fully mitigate muscle loss, especially with the space constraints for future missions like Artemis. Research has shown conflicting results regarding how biological sex affects muscle loss in microgravity or disuse models, but a consensus is strikingly lacking. A key gap in the field is the unclear role of sex steroid hormones, such as testosterone and estradiol, in modulating these sex differences during atrophy. To investigate this, Rosa-Caldwell et al (2024) explored how biological sex and sex steroid hormones influence muscle atrophy progression under long-term exposure to simulated microgravity or partial gravity environments in rats.
The study was conducted using male and female rats exposed to simulated microgravity (0g) and partial gravity (0.4g, resembling Martian gravity) for 28 days, with some rats undergoing gonadectomy (ovariectomy or castration). At 12 weeks of age, the rats underwent either castration/ovariectomy (CAST/OVX) or sham (SHAM) surgeries. After a 2-week recovery period, they were divided into either simulated micro-gravity (0g), simulated partial-gravity (40% of bodyweight, Martian gravity, 0.4g) or Earth gravity control (1g) conditions. In order to assess the strength of the mice, both grip strength and hindlimb muscle function were performed. Aurora Scientific’s 1305A 3-in-1 Whole Animal System was then used to assess maximal tetanic measurements in the dorsiflexors and plantarflexors. In addition, histological analyses and pQCT were performed to quantity muscle area and examine muscle fiber composition.
The findings revealed that females lost more body weight than males under simulated microgravity (0g) and partial gravity (0.4g) conditions, particularly in gonadectomized rats, with females exhibiting greater muscle mass and cross-sectional area loss compared to males. Although both sexes experienced similar reductions in hindlimb grip strength at 0g, females showed less grip strength loss than males after gonadectomy. Females also exhibited more significant losses in dorsiflexion and plantarflexion power, with a notable decrease in plantarflexion power at 0g, especially in gonadectomized rats. At 0.4g, although dorsiflexion power loss was similar between sexes, females still tended to have greater losses in both dorsiflexion and plantarflexion power under OVX/CAST conditions. In addition, females had significantly greater losses in muscle fiber area at 0.4g, particularly in myosin heavy chain type I fibers. In concert, these results highlight that females may be more susceptible to muscle atrophy and strength loss in spaceflight-like conditions, emphasizing the need for sex-specific interventions in future space missions.
Conclusions
These studies by Issertine et al (2024), Sergeeva et al (2024), and Rosa-Caldwell et al (2024) demonstrate how unloading and variable gravity conditions can not only alter muscle health and function, but be exacerbated by sex-based differences. Taken together, their findings underscore the importance of studying sex-based differences and interventions in the field. Furthermore, these uncovered differences can aid the development of musculoskeletal loss mitigation strategies and ultimately promote healthy muscle function both in space and on Earth.
Gravitating towards spaceflight studies?
Checkout out our ‘Out of This World Research’ interview with Brock University’s Dr. Val Fajardo and the lab’s post mission insights into the impact of spaceflight on soleus muscle!
