The overall objective of this experiment was to examine the effect of prolonged weightlessness on the mechanical and biochemical properties of muscles, tendons, and their insertions into bone. Soft tissue injuries often occur at the sites of attachment of tendons and ligament to bone. The sensitivity of the musculoskeletal system to its mechanical environment may magnify the injury susceptibility of insertion sites during prolonged exposure to microgravity. Changes in loading across joints will lead to changes in tissue mechanical, structural, and biochemical properties. Joint unloading will result in a catabolic biologic state and subsequent tissue degeneration. Physiologic joint loading, on the other hand, will result in an anabolic biologic state and maintenance of tissue properties. The negative effect of unloading has most clearly been demonstrated in bone. Less is known about the effect of weightlessness on joint soft tissues.
Experiment animals: 16 mice in space flight and 16 mice housed at KSC as ground controls. Left and right shoulders (humerus-rotator cuff-scapula units) and ankles (Achilles tendon-calcaneous bone) were removed from each mouse approximately two hours after landing, processed, and shipped to our laboratory. qPCR was performed on rotator cuff mouse muscles, tendons, insertions, and bones. Biomechanics was performed on mouse supraspinatus tendon-to-bone samples. Histology was performed on supraspinatus tendon-humeral head and Achilles tendoncalcaneous samples.
Preliminary data from the STS131 BSP: The effect of microgravity on gene expression in the humerus:
Preliminary data demonstrated that bone resorption is increased (as indicated by increases in the ratio of RANKL and OPG gene expression) and osteoblast differentiation is decreased (as indicated by decreases in OSX gene expression). Surprisingly, adipogenesis was decreased due to the weightless environment (as indicated by decreases in PPARG gene expression) As expected, the mechanoresponsive gene COX2 was dramatically downregulated due to microgravity.
The effect of microgravity on the rotator cuff muscles:
Microgravity significantly downregulated the expression of both myogenic and adipogenic genes, MyoD and C/EBPa, and an increase in an increase in collagen type III (fibrosis marker).
The effect of microgravity on the rotator cuff tendons and their bony insertions:
Joint unloading due to a weightless environment led to similar changes, with significant increases in MMP-3 and MMP-13 mRNA expression and significant decreases in tendon cross-sectional area.