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By using an innovative ground-based analogue model, we are able to simulate a space mission including a trip to (0 g) and a stay on Mars (0.38 g) in rats. This model allows for a longitudinal assessment of the physiological changes occurring during the two hypo-gravitational stages of the mission.
Rodent ground-based models are widely used to understand the physiological consequences of space flight on the physiological system and have been routinely employed since 1979 and the development of hind limb unloading (HLU). However, the next steps in space exploration now include to travel to Mars where the gravity is 38% of Earth’s gravity. Since no human being has experienced this level of partial gravity, a sustainable ground-based model is necessary to investigate how the body, already impaired by the time spent in microgravity, would react to this partial load. Here, we used our innovative partial weight-bearing (PWB) model to mimic a short mission and stay on Mars to assess the physiological impairments in the hind limb muscles induced by two different levels of reduced gravity applied in sequential fashion. This could provide a safe, ground-based model to study the musculoskeletal adaptations to gravitational change and to establish effective countermeasures to preserve astronauts’ health and function.
Extraterrestrial targets, including the Moon and Mars, represent the future of human space exploration, but both have considerably weaker gravity than Earth. While the consequences of weightlessness on the musculoskeletal system have been extensively studied in astronauts1,2,3,4,5 and in rodents6,7,8,9, the latter thanks to the well-established hindlimb unloading (HLU) model10, very little is known about the effects of partial gravity. Martian gravity is 38% of Earth’s and this planet has become the focus of long-term exploration11; hence, it is crucial to understand the muscular alterations that may occur in this setting. To do so, we developed a partial weight bearing (PWB) system in rats12, based on previous work done in mice6,13, which was validated using both muscle and bone outcomes. However, the exploration of Mars will be preceded by a prolonged period of microgravity, which was not addressed in our previously described model12. Therefore, in this study, we altered our model to mimic a trip to Mars, comprised of a first phase of total hindlimb unloading and immediately followed by a second phase of partial weight bearing at 40% of normal loading.
Unlike most HLU models, we chose to use a pelvic harness (based on the one described by Chowdhury et al.9) rather than a tail suspension to improve animals’ comfort and to be able to move seamlessly and effortlessly from HLU to PWB in a matter of minutes. In conjunction, we used the cages and suspension devices that we previously developed and described extensively12. In addition to providing reliable/consistent data, we also previously demonstrated that the fixed attachment point of the suspension system at the center of the rod did not prevent the animals from moving, grooming, feeding, or drinking. In this article, we will describe how to unload the animals’ hind limbs (both totally and partially), verify their achieved gravity levels, as well as how to functionally assess the resulting muscular alterations using grip force and wet muscle mass. This model would be extremely useful for researchers seeking to investigate the consequences of partial gravity (either artificial or extra-terrestrial) on an already compromised musculoskeletal system, thus allowing them to investigate how organisms adapt to partial reloading, and for the development of countermeasures that could be developed to maintain health during and after human spaceflight.
All methods described here were approved by the Institutional Animal Care and Use Committee (IACUC) of Beth Israel Deaconess Medical Center under protocol number 067-2016.
NOTE: Male Wistar rats aged of 14 weeks at baseline (day 0) are used. Rats are housed individually in custom cages 24 h prior baseline to allow for acclimation.
1. Hindlimb unloading
NOTE: The pelvic harness can be put on either anesthetized or awake animals. Here, the description of the protocol is given on anesthetized animals. Wear proper personal protective equipment (PPE) to handle animals.
2. Partial weight bearing
NOTE: This step can be realized in both awake and anesthetized animals.
3. Assessment of hindlimb grip force
4. Recording of muscle wet mass
Taking advantage of the new cages that we previously designed and described in detail12, we used a stainless steel chain-based suspension device that is suitable for both hindlimb unloading (HLU, Figure 1) and partial weight-bearing (PWB, Figure 2). The critical advantage of our design is the ability to go from one type of unloading to the other in a matter of minutes while maintaining an identical environ...
This model presents the first ground-based analogue developed to investigate successive mechanical unloading levels and aims to mimic a trip to and stay on Mars.
Many steps of this protocol are critical to ensure its success and need to be closely examined. First, it is crucial to monitor the wellbeing of the animals and ensure that they are maintaining a normal behavior (i.e., performing tasks such as eating, resting, and exploring), particularly during the PWB state where they maintain a rel...
The authors have nothing to disclose.
This work was supported by the National Aeronautics and Space Administration (NASA: NNX16AL36G). Authors would like to thank Carson Semple for providing the drawings included in this manuscript.
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