Biological properties change as gravitational force is diminished, underscoring the relationship between physical force and biological function. Cells exposed to microgravity can indeed be profoundly affected by the physical changes that occur in this unique environment, which include the loss of gravity-dependent convection, negligible hydrodynamic shear, and lack of sedimentation. Microgravity induces a generalized inhibition of proliferation and a contemporary increase in apoptosis rate. Cells cannot be considered “blind” with respect to gravity. Spaceflight has subtle but significant effects on core cellular processes including growth control via RNA and ribosomal biogenesis, metabolism, modification, and decay pathways.
Cells may “sense” changes in the microgravitational field through (a) an indirect mechanism (mainly based on the modification of physical properties of their microenvironment); (b) the development of specialized structures for the mechanical perception and transduction of gravitational forces (like the cytoskeleton); and (c) changes in the dynamics of enzymes kinetics or protein network self-assembly. It is worth noting that the latter two processes are dramatically affected by nonequilibrium dynamics. Nonlinear dynamical processes far from equilibrium involve an appropriate combination of reaction and diffusion, and the pattern arising from those interactions is tightly influenced by even minimal changes in reactant concentrations or modification in the strength of the morphogenetic field.
Gravity seems to be an “inescapable” constraint that obliges living beings to adopt only a few configurations among many others. By “removing” the gravitational field, living structures will be free to recover more degrees of freedom, thus acquiring new phenotypes and new functions/properties. That statement raises several crucial questions. Some of these entail fundamentals of theoretical biology, as they question the gene-centered paradigm, according to which biological behavior can be explained by solely genetic mechanisms.
A common outcome in nearly all cell types exposed to microgravity is indeed the alteration of cytoskeletal elements: actin, microfilaments, and microtubules. Endothelial cells are highly sensitive to gravitational stress, as microgravity leads to changes in the production and expression of vasoactive and inflammatory mediators and adhesion molecules, which mainly results from changes in the remodelling of the cytoskeleton and the distribution of caveolae. In addition, by keeping in mind that the cytoskeleton dynamics is a fundamental player in cell proliferation and migration, it is not surprising that microgravity significantly affects the flytrap closure, a process involving not only the actin dynamics but also the ion channels and aquaporin activities.