INTERDEPENDENCE OF THE MUSCLOSKELETAL SYSTEM DURING CHANGES INFLUENCED BY SIMULATED MICROGRAVITY, SIMULATED SPACE RADIATION, AND MECHANICAL LOADING

Open Access
- Author:
- Krause, Andrew Robert
- Graduate Program:
- Anatomy
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 04, 2017
- Committee Members:
- Henry Donahue, Dissertation Advisor/Co-Advisor
Charles H Lang, Committee Chair/Co-Chair
Patricia Mclaughlin, Committee Member
Gregory Stephen Lewis, Committee Member
Gail Doreen Thomas, Outside Member - Keywords:
- Hind Limb Suspension
Space Radiation
Unloading
Mechanical Loading
Sclerostin
HLS
Bone
Muscle
Compression - Abstract:
- Astronauts experience both bone and muscle loss resulting from the unloading conditions and radiation associated with spaceflight. Other patients are also exposed to unloading from neurological disease, extended bed-rest, weight-bearing limitations associated with fractures, and aging. Together, osteopenia and sarcopenia result in decreased function, increased injury risk and higher morbidity rates once patients begin to re-ambulate and muscle and bone are reloaded. While countermeasures exist for bone loss or muscle loss in isolation, the developing concept that muscle and bone are intricately linked more than anatomically shows a clear demand for a better understanding of the bone-muscle inter-relationship. This may result in more effective treatments being developed against catabolic conditions, such as those experienced during spaceflight, for both tissues. The relationship between bone and muscle is not well understood, but a growing field of research suggests it is more complicated than the shared physical interaction. The objective of this dissertation is to characterize bone-muscle interactions and determine their interdependence during exposure to stressors analogous to space travel, notably the catabolic environment of unloading, the increased exposure to space radiation, and during re-ambulation following a period of decreasing structure and function. To address these objectives, we utilized a murine model of ground based simulated microgravity referred to as hind limb suspension (HLS), in conjunction with simulated space radiation, and a non-invasive model of tibia compression loading. Our central hypothesis is that unloading induced sarcopenia is at least partially dependent on unloading induced osteopenia, and unloading induced osteopenia is at least partially dependent on unloading induced sarcopenia; similarly, recovery of muscle during reloading is at least partially dependent on bone, and recovery of bone during reloading is at least partially dependent on muscle. To test our central hypothesis we examined three specific aims: 1) Determine if protecting against bone loss during HLS would also protect against muscle loss utilizing a bone SOST deficient mouse model; 2) Characterize the musculoskeletal changes in response to HLS in conjunction simulated space radiation; and 3) Determine the timeline of musculoskeletal changes during a re-ambulatory period with additional compressive loading to facilitate bone recovery. We conducted specific aim 1 by utilizing a C57BL/6 background mouse globally deficient in the SOST gene, resulting in a Sclerostin deficiency. Tissues were collected from knockout (KO) and wildtype (WT) animals after undergoing 2 weeks of HLS unloading or regularly loaded ground control conditions. Results showed that SOST KO mice displayed a baseline phenotype with larger bone, less lean body mass and more fat body mass compared to WT mice. WT mice showed significant deleterious effects on bone, while SOST KO mice were protected from bone loss during HLS. However, the protective effect on bone did not transfer to muscle. SOST KO mice were sarcopenic at baseline compared to controls, and were not protected from further muscle loss during HLS. Interestingly, the decrease in protein synthesis provided by HLS was prevented in SOST KO mice, but this did not translate to the preservation of muscle mass. Mechanical testing showed greater failure strength and stiffness in SOST KO mice compared to WT, and no change in mechanical properties as a result of HLS in either group. These results show that while Sclerostin deficiency is effective in mitigating bone loss, it does not protect against muscle loss and may have a detrimental effect on body composition. This specific aim highlights a disconnect between muscle and bone during unloading. Specific aim 2 examined the HLS model with the addition of simulated space radiation, conditions that more closely resemble the catabolic variables experienced by astronauts during spaceflight. C57BL/6 mice were exposed to proton radiation, and proton in combination with high (H) atomic number (Z) and energy (E) (HZE) (O16) radiation during HLS. Tissues collected after 2 weeks of unloading showed that HLS had substantial effects on both bone and muscle. 50 cGy of proton radiation had no additional effect on bone or muscle of mice that underwent HLS. However, 50 cGy proton in combination with 10 cGy O16 radiation did alter bone outcomes. For example, additional trabecular bone loss was observed for bone volume fraction (-20%), trabecular number (-12%), trabecular separation (+12%), connectivity density (-29%), and tissue mineral density (-13%) in mice receiving proton+HZE radiation compared to either the HLS alone or HLS+proton groups. In contrast, while protein synthesis and muscle mass were consistently lower in all three HLS groups compared to ground control (GC) groups (~40%), they were not affected further by radiation. These results indicate that radiation in representative amounts to 400 days of space flight can sensitize bone, but not muscle, to the catabolic effects on unloading. Therefore, specific aim 2 provides another example of a disconnect between bone and muscle during unloading and space radiation. Specific aim 3 examined the time course of bone and muscle recovery during a re-ambulatory period following a period of unloading. Adult C57BL/6 mice underwent 2 weeks of HLS unloading before being returned to regularly loaded conditions. During reloading, the right limb of mice that had undergone HLS received additional mechanical compression loading to facilitate bone recovery, while the left limb served as a contralateral control and was not loaded. Tissues were collected at the conclusion of HLS (day 14), and during re-ambulation time-points (day 28, and day 56). Our data indicated that 2 weeks of HLS causes catabolic changes to both bone and muscle. For trabecular bone, these losses were partially recovered with re-ambulation alone, but were fully recovered with the addition of acute mechanical compression loading. For example, trabecular bone volume fraction (BV/TV) showed a -50% decrease from baseline after HLS which was partially recovered (-37% from baseline) in the left limb and fully recovered to GC values in the right limb that was mechanically loaded. For cortical bone, we showed a delayed response to HLS observed during the re-ambulation period that has not been previously reported; these data indicated the catabolic insult of HLS on cortical bone is not detectable immediately following unloading. Furthermore, the catabolic changes in some cortical outcomes were mitigated by acute compression loading. In contrast, muscle mass recovery was not facilitated in the mechanically loaded limb and showed a decline with loading at the later day 56 time point. Specific aim 3 was a third example of an apparent disconnect between bone and muscle during a period of recovery. To conclude, our results show that while Sclerostin deficiency is an effective method of preventing bone loss, it did not protect against muscle loss. We also showed that simulated space radiation and HLS unloading cause a cumulative loss for some bone outcomes, while muscle appears less sensitive to the additional catabolic insult of radiation. Finally, we described the time course of bone and muscle recovery following unloading. These results indicate that muscle recovers more quickly than bone during re-ambulation, similar to catabolic observations preceding bone loss during an unloading period. Furthermore, limbs receiving mechanical loading displayed facilitated bone recovery compared to the non-loaded limb, but aided recovery was not detectable in muscle. Data from specific aim 1, 2 and 3 interpreted together indicate a disconnect between bone and muscle in these examples of simulated spaceflight environments as well as re-ambulation.