Active hydrodynamic theory of chromatin dynamics

The organization of chromatin inside the cell nucleus is crucial for the proper functioning of many nuclear processes. In differentiated cells, chromatin is spatially segregated into euchromatin and heterochromatin. The former is loosely packed and transcriptionally active, while the latter is compacted and mostly consists of transcriptionally silent genes. We describe a hydrodynamic model of chromatin and nucleoplasm at micron scales. The chromatin is modeled as a viscous, compressible fluid, as informed by microrheology experiments and their response at long time. Heterochromatin is distinguished by the presence of contractile stresses due to HP1 crosslinking. This stress induces density instabilities and large-scale flows of the chromatin fluid and nucleoplasm. Simulations reveal coarsening of heterochromatic components, which in an open system lead to a finite-size droplet, whereas in a confining domain we observe a redistribution at the boundaries, resembling a wetting phenomenon. Hence these mechanical processes may play an important role in the spatial organization of heterochromatin which is usually enriched near the nuclear periphery.