The purpose of this work was to evaluate the protein and mRNA expression levels of multiple cytoskeletal proteins in the cardiac and lung tissue of mice that were euthanized onboard the United States Orbital Segment of the International Space Station 37 days after the start of the SpaceX-4 mission (September 2014, USA). The results showed no changes in the cytoskeletal protein content in the cardiac and lung tissue of the mice, but there were significant changes in the mRNA expression levels of the associated genes, which may be due to an increase in total genome methylation. The mRNA expression levels of DNA methylases, the cytosine demethylases Tet1 and Tet3, histone acetylase and histone deacetylase did not change, and the mRNA expression level of cytosine demethylase Tet2 was significantly decreased.

To analyze the effect of gravity on the structure of germinal tissues, we examined tissues of the testes and duct deferens of mice that were exposed to space flight conditions for 21–24 days (experiment Rodent Research-4, SpaceX-10 mission, February 2017, USA). We evaluated the levels of cytoskeletal proteins, sperm-specific proteins, and epigenetic events; in particular, we evaluated levels of 5-hydroxymethylcytosine and of enzymes that regulate DNA methylation/demethylation. We did not detect changes in the levels of cytoskeletal proteins, sperm-specific proteins, DNA-methylases, DNA demethylases, DNA acetylases, or histone deacetylases. However, there were changes at the gene expression level. In particular, there was an increase in the demethylase Tet2 and a decrease in the histone deacetylase Hdac1 . These gene expression changes may be of key importance during the early period of readaptation since they could lead to an increase in the expression of target genes.

Under microgravity, the gene expression levels vary in different types of cells; however, the reasons for this have not been sufficiently studied. The aim of this work was to evaluate the methylation of CpG islands in the promoter regions of the genes encoding some cytoskeletal proteins, the total methylation and 5 hmC levels, and the levels of enzymes that regulate these processes in the testes, heart, and lungs in mice after a 30-day microgravity modeling by antiorthostatic suspension and after a subsequent 12-hour recovery as well as in the corresponding control group and identical groups treated with essential phospholipids. The obtained results indicate that under modeling microgravity in the examined tissues a decrease of cytoskeletal gene expression (mainly in the heart and lungs tissues) correlated with an increase in the CpG islands methylation and an increase of the expression (mainly in the testes tissue) – with a decrease of the CpG-methylation, despite of the fact that in the examined tissues took place a decrease of the content methylases and demethylases. But the deacetylase HDAC1 content increased in the heart and lungs tissues and decreased in the testes, letting us suggest its participation in the regulation of the methylation level under microgravity conditions.