Osmotic Coupling of Mechanics and Epigenetics Regulates Chromatin Phase Organization

Cells experience epigenetic, osmotic, and mechanical perturbations that reorganize chromatin, yet how these distinct cues are integrated into a common physical response of the nucleus remains unclear. Here, we identify nuclear hydration as a central state variable that links microenvironmental perturbations to chromatin phase organization and nuclear mechanics. We develop a non-equilibrium mesoscale model that couples chromatin elasticity, water transport, and epigenetic reaction-diffusion kinetics. In this framework, chromatin behaves as a deformable polymer that segregates into coexisting water-rich euchromatic and water-poor heterochromatic phases, whose compositions and morphologies are regulated by hydration-dependent swelling, elastic strain, and histone-mark interconversion. The model predicts that epigenetic perturbations, osmotic shocks, and volume-changing mechanical loads reorganize chromatin by shifting intranuclear water content, nuclear volume, and euchromatin-heterochromatin phase balance. It further predicts that anisotropic forces impose directional strain, driving heterochromatin elongation, alignment, and lamellar transitions. Quantitative comparisons with super-resolution imaging, AFM and chromatin-accessibility measurement - across pharmacological, osmotic, and mechanical perturbations including confinement and micropatterning - support these predictions. Together, our results establish a chemo-mechano-osmotic framework in which nuclear hydration integrates biochemical and biophysical cues to regulate chromatin organization across scales.