Human regenerative health depends on constant tissue repair and regeneration by cells derived from somatic stem cell lineages, a process that is stimulated b y mechanical loading of tissues. In microgravity, many tissue regenerative processes, such as bone and muscle maintenance, as well as blood and immune cell production are impaired, and are a significant concern for health during spaceflight. Our broad hypothesis is that tissue unloading in microgravity leads to a broad regenerative deficiency affecting multiple tissues and that this is caused by an impairment of mechano-transduction signaling in somatic stem cell proliferation and differentiation. We used mouse bone and bone marrow stem cells flown in microgravity on NASA’s STS-131 mission, and embryonic stem cells on STS-131 and STS-135 Space Tissue Loss experiments to test our hypotheses and elucidate cellular and molecular mechanisms of tissue re generation as it relates to mechanical load.

Spaceflight in microgravity caused significant bone loss through osteoclastic activity and osteocytic osteolysis. Decreased mechano-transduction signaling in microgravity down-regulated MAPK and Pi3K signaling cascades and activated the cell cycle and tissue regeneration inhibitor, CDKN1A/p21, in periosteal osteoblasts, strongly suggesting a cellular and molecular mechanism for bone regenerative inhibition in microgravity. Studies of CDKN1A/p21 null mice also show significant increases in bone volume further supporting a role for p21 expression in bone regenerative arrest. Bone marrow mesenchymal stem cells from mice flown in microgravity showed down-regulation of genes promoting proliferation and differentiation of osteoblasts and osteoclasts and accumulation of their precursors. Inhibition of stem cell differentiation was also seen in embryonic stem cells, induced to differentiate in space into embryoid bodies and keratinocytes, and resulted in decreased ability to perform lineage specific functions.

The results of this dissertation highlight, for the first time, the importance of mechanical stimulation in stem cell based tissue regeneration and the profound effects of diminished stem cell regenerative health in space. Furthermore, decreased mechano- transduction signaling and up-regulation of CDKN1A/p21 provide a potential mechanism for the observed arrest of regeneration in space and may be used in the future to develop countermeasures to prevent microgravity and tissue disuse-induced degenerative conditions.