Force-biased nuclear import sets nuclear-cytoplasmic volumetric coupling by osmosis

In eukaryotes, cytoplasmic and nuclear volumes are tightly regulated to ensure proper cell homeostasis. However, the detailed mechanisms underlying nucleus-cytoplasm volumetric coupling remain unknown. Recent evidence supports a primary role of osmotic mechanisms in determining a tight link between nuclear and cytoplasmic volume, but this hypothesis remains largely untested in mammalian cells. We approach the question in single cultured adhering human cells, by jointly measuring cytoplasmic and nuclear volumes, in real time and across cell cycles. Surprisingly, we find that cytoplasmic and nuclear volumes follow different average growth laws: while the cytoplasm grows exponentially, the nucleus grows linearly. Moreover, by combining several experimental perturbations and analyzing a mathematical model including osmotic effects and tension, we conclude that the mechanical forces exerted by the cytoskeleton on the nuclear envelope can strongly affect nucleus-cytoplasm volumetric coupling by biasing nuclear import. Our results unveil how osmo-mechanical equilibrium regulates nuclear size in mammalian cells. One-Sentence SummaryCytoskeletal forces exerted on the nuclear envelope impact on nuclear volume through modulation of force-coupled nucleo-cytoplasmic transport, affecting osmosis.