Gene expression changes related to bone mineralization, blood pressure and lipid metabolism in mouse kidneys after space travel.

Space travel burdens health by imposing considerable environmental stress associated with radioactivity and microgravity. In particular, gravity change predominantly impacts blood pressure and bone homeostasis, both of which are controlled mainly by the kidneys. Nuclear factor erythroid-2-related transcription factor 2 (Nrf2) plays essential roles in protecting the kidneys from various environmental stresses and injuries. To elucidate the effects of space travel on mammals in preparation for the upcoming space era, our study investigated the contribution of Nrf2 to kidney function in mice two days after their return from a 31-day stay in the International Space Station using Nrf2 knockout mice. Meaningfully, expression levels of genes regulating bone mineralization, blood pressure and lipid metabolism were found to be significantly altered in the kidneys after space travel in an Nrf2-independent manner. In particular, uridine diphosphate-glucuronosyltransferase 1A (Ugt1a) isoform genes were found to be expressed in an Nrf2-dependent manner and induced exclusively in the kidneys after return to Earth. Since spaceflight elevated the concentrations of fatty acids in the mouse plasma, we suggest that Ugt1a isoform expression in the kidneys was induced to promote glucuronidation of excessively accumulated lipids and excrete them into urine after the return from space. Thus, the kidneys were proven to play central roles in adaptation to gravity changes caused by going to and returning from space by controlling blood pressure and bone mineralization. Additionally, kidney Ugt1a isoform induction after space travel implies a significant role of the kidneys for space travelers in the excretion of excessive lipids.