Researcher Spotlight: Studying Effects of Partial Gravity on Musculoskeletal Systems of Mice on a NASA ISS Mission

Mary L. Bouxsein, PhD
Professor, Dept of Orthopedic Surgery, Harvard Medical School
Director, Center for Advanced Orthopedic Studies,
Beth Israel Deaconess Medical Center | BIDMC of Boston

On March 14, 2023, 24 mice were launched on NASA’s SpaceX CRS-27 in a NASA-funded research project headed by Mary Bouxsein, PhD. A key part of the research involves measuring body composition and body mass density both pre- and post-flight utilizing the InAlyzer2 DEXA from Medikors, distributed by Micro Photonics Inc.

Figure 1: Photo of researcher doing pre-flight studies of live mice sent to the ISS on a NASA space launch on March 14, 2023. © Copyright – Bouxsein Lab, Harvard Medical School. All rights reserved.

The mice are part of a study to investigate the physiologic response to different levels of artificial gravity, induced by centrifugation, in mice on the International Space Station (ISS).  This is a collaboration between NASA and the Japan Aerospace Exploration Agency (JAXA) and there are several researchers involved in the US and Japan.

If space exploration is to include long trips, such as to Mars, or extended stays in a space station, preventing disuse attrophy from extended weightlessness will be essential for the health of the astronauts. There are terrestrial implications as well because disuse attrophy also occurs in those who spend extended times in bed rest.

We will be posting much more on this exciting project after the mice return to Earth, but a summary of a discussion with Dr. Bouxsein prior to liftoff is provided below.

How does weightlessness affect the musculoskeletal system and why are spaceflight-based studies such as this one needed?

Mechanical loading, usually generated through physical activity in the presence of gravity, is required for maintenance of the musculoskeletal system. Exposure to spaceflight or reduced mechanical loading on Earth induces marked bone loss, muscle atrophy and degradation of soft-tissue structures in both the knee (e.g., cartilage, menisci, and ligaments) and hip (e.g., cartilage) joints. Moreover, complex functions, such as gait and balance, which depend on several physiologic systems, are compromised following prolonged exposure to weightlessness. These are a major concern for astronauts during and after long-duration spaceflight, as they may be at increased risk for reduced performance, bone fractures, and both early-onset osteoporosis and arthritis.

Intermittent or continuous exposure to artificial gravity is a possible approach to mitigate declines not only in musculoskeletal function and performance, but also in many other physiologic systems. Our own prior work has established that the deficits in bone and muscle following exposure to partial gravity, simulated by partial weight bearing in mice, are directly proportional to the degree of unloading. Yet, the ability of partial gravity, induced by centrifugation, to inhibit adverse musculoskeletal changes in weightlessness remains poorly understood. Notably, given the constraints of studying centrifugation as a countermeasure on Earth, spaceflight-based studies are needed.

Can you explain the specific aims of this research project?

When astronauts are in space they do two or more hours of exercise for their musculo- and cardio-systems. These are exercises such as a bicycle or treadmill where the astronaut is essentially held down by bungee-type cords to apply downward force, and there are similar systems for resistance training, but they still lose bone and muscle.

Long space flights in small vehicles, such as to Mars, will have even more limitations. Little is known about the nature and extent of multi-system physiologic adaptation across the gravity spectrum. We want to understand how much artificial gravity would need to be provided to reduce the deleterious effects of spaceflight on the musculoskeletal system.

Thus, we have two key aims:

Aim 1:  Quantify effects of varying G-levels on neuromotor performance, as well as bone, muscle and joint structure and function in mice centrifuged aboard the ISS.

Aim 2:  Determine cell and molecular mechanisms underlying the musculoskeletal and neuromotor response to microgravity and varying G-levels induced by centrifugation.

What are the pre- and post-flight studies?

Eight-week old male C57Bl6/J mice will be exposed to different levels of partial gravity (0.25G, 0.5 G, 0.75G and 1G), induced by centrifugation, on the ISS.  Flight-based control mice will be exposed to 0G, whereas ground-based controls will be housed in an identical habitat. Comprehensive pre- and post-flight in vivo outcomes, as well as ex vivo imaging, mechanics, histology, and transcriptomic and proteomic analyses are being employed.

Pre- and post-flight in vivo testing includes bone mineral density, fat, and lean mass measured by Dual-Energy X-ray Absorptiometry (DEXA), in collaboration with Medikors and Micro Photonics, utilizing the DEXA InAlyzer2 system. These same tests will be performed post-flight.

The DEXA InAlyzer2 facilitated our pre-flight testing by allowing us to test 214 animals in two days, with good accuracy. Previous devices we had worked with could never have accomplished this.

Other testing includes gait analysis, grip strength, mid-air righting reflex (post-flight only), and body weight, along with a number of post-flight ex vivo outcomes.

Can you describe how the mice travel and live in the ISS?

The mice travel to the ISS individually housed in shoebox-sized cages, and are then transferred to a murine habitat unit (MHU). The MHU, designed by JAXA,  makes it possible to study partial gravity exposure utilizing centripetal acceleration to generate artificial gravity.

Food pellets are provided from a special feeding device, which the mice are eating from for three weeks prior to launch. They also have a special plastic water bag with a spring that compresses the bag and dispenses water.

The mice are video-checked every day by veterinarians here on Earth, and the astronauts maintain the cages weekly in space.

Are these preliminary studies that will be replicated at some point with humans?

When we do move forward from mice to research with people, we will be using some kind of long-arm centrifuge. Plans for this type of device are in place. We need to understand all the tissues being collected and how the systems respond to different degrees of mechanical loading.

Other options include investigating whether we can develop therapeutics that would make the body think it was undergoing mechanical loading.

What are the real-world benefits and goals of this research?

In any mission to Mars, astronauts will be exposed to ~ 8 to 10 months of weightlessness en route, perhaps 12 – 18 months of partial gravity exposure in the vicinity of Mars, and another ~ 8 to 10 months of weightlessness during the return to Earth. Without further advances in countermeasures, astronauts risk serious short- and long-term musculoskeletal debilitation after such a mission.

By determining a dose-response relationship to partial gravity for multiple physiologic systems and tissues, we will be gaining understanding of some issues related to skeletal health with long duration spaceflight.

On terra firma, there will hopefully be some insights that can be applied to other forms of disuse atrophy, such as that experienced by invalids who are bed-bound. Bone loss, muscle atrophy, and joint degradation are deleterious consequences due to reduced mechanical loading both in spaceflight and in conditions on Earth, such as spinal cord injury and stroke.  Indeed it has been suggested that the risks of microgravity are akin to those faced by physically inactive older adults, or those who suffer from short-term immobility due to an injury.  Knowledge about the utility of artificial gravity via centrifugation would have broad benefits in understanding how to combat musculoskeletal disability here on Earth.

Selected publications:

Burkhart K, Allaire B, Anderson DE, Lee D, Keaveny TM, Bouxsein ML. Effects of long-duration spaceflight on vertebral strength and risk of spine fracture.  J Bone Min Res 2020;35(2):269-276. doi: 10.1002/jbmr.3881. PubMed PMID: 31670861.

Mortreux M, Riveros D, Bouxsein ML, Rutkove SB. A Moderate Daily Dose of Resveratrol Mitigates Muscle Deconditioning in a Martian Gravity Analog. Front Physiol. 2019;10:899. doi: 10.3389/fphys.2019.00899. PubMed PMID: 31379604; PMCID: PMC6656861.

Mortreux M, Riveros D, Bouxsein ML, Rutkove SB. Mimicking a Space Mission to Mars Using Hindlimb Unloading and Partial Weight Bearing in Rats. J Vis Exp. 2019(146). doi: 10.3791/59327. PubMed PMID: 31009001

Burkhart K, Allaire B, Bouxsein ML. Negative Effects of Long-duration Spaceflight on Paraspinal Muscle Morphology. Spine (Phila Pa 1976). 2019;44(12):879-86. doi: 10.1097/BRS.0000000000002959. PubMed PMID: 30624302


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