Microgravity has been shown to affect the formation of cytoskeletal fibers in flight experiments; however, the mechanism of gravity sensing has not yet been elucidated. It is well known that the small GTPase RhoA regulates the cytoskeletal components. In this study, we evaluated the effect of simulated microgravity that affects actin fiber remodeling and tubulin assembly in cells by using biochemical and molecular biological methods. We found that RhoA had wide intracellular distribution in the clinorotated cells. Although the protein expression of RhoA was upregulated, the actin stress fibers and tubulin assembly was disturbed. Messenger RNA fingerprinting results show that gene expression of LARG might be suppressed under simulated microgravity. These results suggest that the Rho activation process is suppressed by simulated microgravity, thereby inducing the disorganization of these cytoskeletal fibers.

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Unloading conditions occasionally cause muscle atrophy and osteopenia in humans and animals. In general, transcriptional alternations in bone and skeletal muscle cells mediate, respectively, the bone and muscle atrophy caused by unloading. In this study, we examined comprehensive gene expression in the femur of rats exposed to spaceflight (STS-58), tail-suspension and denervation (sciatic neurectomy), to characterize unloading-induced changes in bone gene expression. Interestingly, the transcriptional responses of bone cells to spaceflight were different from those to the other conditions: (i) the expression of bone formation markers was suppressed by spaceflight and denervation, but only spaceflight significantly inhibited the expression of bone resorption markers; (ii) the numbers of the genes that exhibited more than a five-fold change in expression level in response to spaceflight, tail-suspension and denervation were 122 (up, 14 genes; down, 108 genes), 74 (up, 72 genes; down, 2 genes) and 23 (up, 20 genes; down, 3 genes), respectively; (iii) spaceflight preferentially down-regulated the expression of signal transduction and gene-expression regulatory genes, especially GTP-binding protein-signaling molecules involved in bone formation; and (iv) only spaceflight up-regulated the expression of a predicted ubiquitin ligase, F-box protein 30. Our present study identifies novel key genes with which to elucidate further the mechanism of osteopenia caused by unloading, especially spaceflight.

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Full-grown Xenopus oocytes arrest at the prophase of meiosis I. The hormone progesterone induces resumption of the meiotic cell cycle, leading to maturation of the oocytes into fertilizable eggs. We have been investigating the effects of hypergravity on Xenopus oocyte maturation. Recently we reported that hypergravity did not affect the initiation of maturation, whereas the progression of maturation was affected regarding the external appearance of the white spot. The present study examined changes in spindle morphology during Xenopus oocyte maturation. Cytological examination showed that bipolar spindle formation at meiosis II was largely inhibited and monopolar (or monoastral) spindles were observed, although oocytes formed apparently normal bipolar spindle upon the entry into meiosis I. These findings suggest that hypergravity inhibits the bipolar spindle formation of meiosis ll and hence adversely affects the process of oocyte maturation in Xenopus laevis.

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