Hypergravity Provokes a Temporary Reduction in CD4+CD8+ Thymocyte Number and a Persistent Decrease in Medullary Thymic Epithelial Cell Frequency in Mice

Chronic Hypergravity Induces a Modification of Histone H3 Lysine 27 Trimethylation at TCRβ Locus in Murine Thymocytes

Gravity changes are major stressors encountered during spaceflight that affect the immune system. We previously evidenced that hypergravity exposure during gestation affects the TCRβ repertoire of newborn pups. To identify the mechanisms underlying this observation, we studied post-translational histone modifications. We first showed that among the four studied post-translational histone H3 modifications, only lysine 27 trimethylation (H3K27me3) is downregulated in the thymus of mice exposed to 2× g for 21 days. We then asked whether the TCRβ locus chromatin structure is altered by hypergravity exposure. ChIP studies performed on four Vβ segments of the murine double-negative SCIET27 thymic cell line, which corresponds to the last maturation stage before V(D)J recombination, revealed increases in H3K27me3 after 2× g exposure. Finally, we evaluated the implication for the EZH2 methyltransferase in the regulation of the H3K27me3 level at these Vβ segments by treating SCIET27 cells with the GSK126-specific inhibitor. These experiments showed that the downregulation of H3K27me3 contributes to the regulation of the Vβ germline transcript expression that precedes V(D)J recombination. These data show that modifications of H3K27me3 at the TCRβ locus likely contribute to an explanation of why the TCR repertoire is affected by gravity changes and imply, for the first time, EZH2 in the regulation of the TCRβ locus chromatin structure.

Impact of spaceflight on the murine thymus and mitigation by exposure to artificial gravity during spaceflight

The environment experienced during spaceflight may impact the immune system and the thymus appears to undergo atrophy during spaceflight. However, molecular aspects of this thymic atrophy remain to be elucidated. In this study, we analysed the thymi of mice on board the international space station (ISS) for approximately 1 month. Thymic size was significantly reduced after spaceflight. Notably, exposure of mice to 1 × g using centrifugation cages in the ISS significantly mitigated the reduction in thymic size. Although spaceflight caused thymic atrophy, the global thymic structure was not largely changed. However, RNA sequencing analysis of the thymus showed significantly reduced expression of cell cycle-regulating genes in two independent spaceflight samples. These reductions were partially countered by 1 × g exposure during the space flights. Thus, our data suggest that spaceflight leads to reduced proliferation of thymic cells, thereby reducing the size of the thymus, and exposure to 1 × g might alleviate the impairment of thymus homeostasis induced by spaceflight.

Effects of hypergravity on gene levels in anti-gravity muscle and bone through the vestibular system in mice

We recently reported that hypergravity with 3 g for 4 weeks affects muscle and bone through the vestibular system in mice. The purpose of this study was to investigate the effects of hypergravity with 2 g, which had no influence on circulating glucocorticoid level, on the gene levels in muscle and bone, as well as the roles of the vestibular system in those changes using vestibular lesioned (VL) mice. Hypergravity for 2 and 8 weeks or VL exerted little effects on the mRNA levels of muscle differentiation factors and myokines in the soleus muscle. Although hypergravity for 2 weeks significantly elevated alkaline phosphatase (ALP) and type I collagen mRNA levels in the tibia, VL significantly attenuated the levels of ALP mRNA enhanced by hypergravity. In conclusion, the present study suggests that a 2-g load for 2 weeks enhances osteoblast differentiation partly through the vestibular system in mice.

Long-term hindlimb unloading causes a preferential reduction of medullary thymic epithelial cells expressing autoimmune regulator (Aire)

Hindlimb unloading (HU) of rodents has been used as a ground-based model of spaceflight. In this study, we investigated the detailed impact of 14-day HU on the murine thymus. Thymic mass and cell number were significantly reduced after 14 days of hindlimb unloading, which was accompanied by an increment of plasma corticosterone. Although corticosterone reportedly causes selective apoptosis of CD4+CD8+ thymocytes (CD4+CD8+DPs) in mice treated with short-term HU, the reduction of thymocyte cellularity after the 14-day HU was not selective for CD4+CD8+DPs. In addition to the thymocyte reduction, the cellularity of thymic epithelial cells (TECs) was also reduced by the 14-day HU. Flow cytometric and RNA-sequencing analysis suggested that medullary TECs (mTECs) were preferentially reduced after HU. Moreover, immunohistochemical staining suggested that the 14-day HU caused a reduction of the mTECs expressing autoimmune regulator (Aire). Our data suggested that HU impacts both thymocytes and TECs. Consequently, these data imply that thymic T cell repertoire formation could be disturbed during spaceflight-like stress.

Effects of gravity changes on gene expression of BDNF and serotonin receptors in the mouse brain

Spaceflight entails various stressful environmental factors including microgravity. The effects of gravity changes have been studied extensively on skeletal, muscular, cardiovascular, immune and vestibular systems, but those on the nervous system are not well studied. The alteration of gravity in ground-based animal experiments is one of the approaches taken to address this issue. Here we investigated the effects of centrifugation-induced gravity changes on gene expression of brain-derived neurotrophic factor (BDNF) and serotonin receptors (5-HTRs) in the mouse brain. Exposure to 2g hypergravity for 14 days showed differential modulation of gene expression depending on regions of the brain. BDNF expression was decreased in the ventral hippocampus and hypothalamus, whereas increased in the cerebellum. 5-HT1BR expression was decreased in the cerebellum, whereas increased in the ventral hippocampus and caudate putamen. In contrast, hypergravity did not affect gene expression of 5-HT1AR, 5-HT2AR, 5-HT2CR, 5-HT4R and 5-HT7R. In addition to hypergravity, decelerating gravity change from 2g hypergravity to 1g normal gravity affected gene expression of BDNF, 5-HT1AR, 5-HT1BR, and 5-HT2AR in various regions of the brain. We also examined involvement of the vestibular organ in the effects of hypergravity. Surgical lesions of the inner ear's vestibular organ removed the effects induced by hypergravity on gene expression, which suggests that the effects of hypergravity are mediated through the vestibular organ. In summary, we showed that gravity changes induced differential modulation of gene expression of BDNF and 5-HTRs (5-HT1AR, 5-HT1BR and 5-HT2AR) in some brain regions. The modulation of gene expression may constitute molecular bases that underlie behavioral alteration induced by gravity changes.