Spaceflight-related suboptimal conditions can accentuate the altered gravity response of Drosophila transcriptome.

Genome-wide transcriptional profiling shows that reducing gravity levels during Drosophila metamorphosis in the International Space Station (ISS) causes important alterations in gene expression: a large set of differentially expressed genes (DEGs) are observed compared to 1g controls. However, the preparation procedures for spaceflight and the nonideal environmental conditions on board the ISS subject the organisms to additional environmental stresses that demonstrably affect gene expression. Simulated microgravity experiments performed on the ground, under ideal conditions for the flies, using the random position machine (RPM), show much more subtle effects on gene expression. However, when the ground experiments are repeated under conditions designed to reproduce the additional environmental stresses imposed by spaceflight procedures, 79% of the DEGs detected in the ISS are reproduced by the RPM experiment. Gene ontology analysis of them shows they are genes that affect respiratory activity, developmental processes and stress-related changes. Here, we analyse the effects of microgravity on gene expression in relation to the environmental stresses imposed by spaceflight. Analysis using 'gene expression dynamics inspector' (GEDI) self-organizing maps reveals a subtle response of the transcriptome to microgravity. Remarkably, hypergravity simulation induces similar response of the transcriptome, but in the opposite direction, i.e. the genes promoted under microgravity are usually suppressed under hypergravity. These results suggest that the transcriptome is finely tuned to normal gravity and that microgravity, together with environmental constraints associated with space experiments, can have profound effects on gene expression.

Plant cell proliferation and growth are altered by microgravity conditions in spaceflight.

Seeds of Arabidopsis thaliana were sent to space and germinated in orbit. Seedlings grew for 4d and were then fixed in-flight with paraformaldehyde. The experiment was replicated on the ground in a Random Positioning Machine, an effective simulator of microgravity. In addition, samples from a different space experiment, processed in a similar way but fixed in glutaraldehyde, including a control flight experiment in a 1g centrifuge, were also used. In all cases, comparisons were performed with ground controls at 1g. Seedlings grown in microgravity were significantly longer than the ground 1g controls. The cortical root meristematic cells were analyzed to investigate the alterations in cell proliferation and cell growth. Proliferation rate was quantified by counting the number of cells per millimeter in the specific cell files, and was found to be higher in microgravity-grown samples than in the control 1g. Cell growth was appraised through the rate of ribosome biogenesis, assessed by morphological and morphometrical parameters of the nucleolus and by the levels of the nucleolar protein nucleolin. All these parameters showed a depletion of the rate of ribosome production in microgravity-grown samples versus samples grown at 1g. The results show that growth in microgravity induces alterations in essential cellular functions. Cell growth and proliferation, which are strictly associated functions under normal ground conditions, appeared divergent after gravity modification; proliferation was enhanced, whereas growth was depleted. We suggest that the cause of these changes could be an alteration in the cell cycle regulation, at the levels of checkpoints regulating cell cycle progression, leading to a shortened G2 period.