Fig. 7: Illustration summarizing the regulations induced in microgravity as observed in our cell model.

The “Cell proliferation” and “DNA repair” pathways are clearly inhibited as judged by the regulation of expression of some key genes. Conversely, “Senescence”, “Ferroptosis” and “Inflammation”, three pathways with partial overlaps, are strongly upregulated. Modifications in the structure of the microtubule network are another regulation induced in microgravity. As a way to identify the molecular mechanisms underlying these regulations, the transcriptomic datasets were further analyzed, using the “Ingenuity Pathway Analysis” software (IPA), in order to identify potential key upstream regulatory pathways. This analysis identified the YAP1 pathway as the most likely to be involved in the response to microgravity, by directly participating in the regulation of the expression of numerous modulated genes. This pathway is also known to be critically involved in mechano-sensing and mechanotransduction signaling and to be capable of regulating the other two likely upstream regulators, NUPR1 and CKAP2L. NUPR1 is a transcription regulator that converts stress signals into a program of gene expression that empowers cells with resistance to the stress induced by a change in their microenvironment, while CKAP2L is a microtubule-associated protein that is therefore implicated in cell cycle progression. The use of an inhibitor of the YAP1 pathway (verteporfin) in studies carried out on Earth reproduced some, but not all, of the regulations observed in microgravity (see Supplementary Note 3). This suggests that YAP1, although clearly involved, especially in the regulation of the activated pathways, is not the only regulator of microgravity effects, and that other mechanisms have yet to be discovered. These could affect NUPR1, CKAP2L or, more directly, the microtubule network. Chromatin remodeling (presence of facultative chromatin) and changes in the microtubule network (which may be regulated by CKAP2L) are also observed in microgravity. Interestingly, these features are also considered markers of senescence, making senescence the most central element of microgravity-induced changes.