Saccharomyces cerevisiae is an eukaryotic model organism that has been used for space biology research. Microgravity is a tool to study yeast mechanobiology by removing the gravitational force on the cells. Yeast cells possess mechanosensors that can sense mechanical forces. The cells transduce a mechanical stimulus into a specific cellular response by activating intracellular signaling pathways that can ultimately lead to an altered function. Microgravity is “sensed” by yeast cells as a stress condition and several mitogen-activated protein kinases (MAPK) signaling pathways are activated, including the cell wall integrity (CWI)/protein kinase C (PKC), the high osmolarity glycerol (HOG) and the target of rapamycin (TOR) pathways.
One of the indicators of morphological changes is an increase at random bud scar profile. Microgravity influences on the growth rate of yeast cells have been observed. The colony growth rate of the agar invasive S. cerevisiae Σ1278b strain was reduced as well as its agar invasiveness. Post-flight growth experiments of a brewer’s top yeast strain showed an increase in G2/M and a decrease in Sub-G1 cell population; an increased viability, a decreased lipid peroxidation level, increased glycogen content, and changes in carbohydrate metabolic enzyme activities were also observed. Using the S. cerevisiae BY4741 deletion collection, genes that provide a survival advantage in space, were identified in a batch growth experiment; no difference in growth rate was observed. Freeze-dried strains showed significant changes in the cell wall thickness.
Spaceflight unique gene expression changes were observed in stress response element (STRE) genes with transcription regulation involving Sfp1 (which is involved in the TOR pathway) and Msn4. Some of the components of the ribosome biogenesis (which is under the control of Sfp1) as well as components of the proteasome were down regulated in microgravity. Recent results indicate that microgravity imposes a “microgravity” stress on the cells, which has the characteristics of an osmotic stress. Cellular energy is directed towards protective measures such as cell wall biosynthesis (CWI pathway activation) and the production of compounds (glycerol, trehalose) that increase the osmotolerancy (HOG pathway).