Chromatin decompaction modulates the liquid phase behavior in nuclei of living cells

The cell nucleus can be thought of as a living substrate that functions to store and process cells’ genetic material. The genetic material is not randomly stored, but instead is packed into a network of chromatin fibers, whose organization is linked to various phase-separated nuclear bodies. While the structure of chromatin has been extensively studied, it remains unclear how its biophysical organization and mechanical properties impact phase separation inside the nucleus. Here, we utilize a biomimetic optogenetic system to interrogate the role of the chromatin network on the liquid-liquid phase separation in the nucleus of a living cell. We tune the density of the chromatin network with a histone deacetylases inhibitor (HDACi) and drive phase separation by shining blue light to oligomerize the proteins in the nucleus. We demonstrate that phase separation can be strongly inhibited once the dense heterochromatin network is decompacted by HDACi. We further show that the intranuclear mechanical properties change, with a corresponding change in phase behavior, including changes in the size of the phase-separated droplets, and altered coarsening dynamics. Our findings highlight the importance of the material state of the chromatin network for liquid-liquid phase separation in the nucleus, and have implications for the biophysical regulation of biomolecular condensates.