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

Abstract

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.

Conserved nucleocytoplasmic density homeostasis drives cellular organization across eukaryotes

“…Based on his work on sea urchin embryos and algae, he proposed the “Kern-Plasma-Relation” as a constant characteristic for a given cell type (73, 74). Here, we identify a conserved ratio of nuclear-to-cytoplasmic density, which we propose to be the biophysical driver of the repeatedly observed “Kern-Plasma-Relation” (24, 42, 47–50, 58–61, 65, 70–80). Our experimental data together with the pressure balance model provide critical quantitative evidence for a mechanism which intrinsically links nuclear size to cell size by establishing a specific N/C density ratio.…”

“…How can the addition of molecules to the nucleus make it less dense? It has been proposed that the import of nuclear proteins generates an osmotic pressure across the nuclear envelope that inflates the nucleus through water influx (45–47).…”

“…How can the addition of molecules to the nucleus make it less dense? It has been proposed that the import of nuclear proteins generates an osmotic pressure across the nuclear envelope that inflates the nucleus through water influx (45)(46)(47). (A) Nuclear transport results in the establishment of a specific nuclear proteome that is distinct from the cytoplasmic protein composition.…”

Cell size scaling laws: a unified theory

“…On the other hand, the DNA charge Q eff is constant during G1 phase while P n grows with time, so NC 2 decreases with time. The fact that the NC ratio remains almost constant during growth ([47], [48]) suggest that cells are closer to the NC 1 regime, and point at the crucial role of metabolites in setting the NC ratio (Fig.4 and S1.B). Importantly, these conclusions are overlooked in a large part of the existing literature ([31],[32],[41]) which often assumes that metabolites do not play any role on the NC ratio due to their permeability at the NE.…”

“…On the other hand, the DNA charge Q ef f is constant during G1 phase while P n grows with time, so N C 2 decreases with time. The fact that the NC ratio remains almost constant during growth ( [47], [48]) suggest that cells are closer to the N C 1 regime, and point at the crucial role of metabolites in setting the NC ratio (Fig. 4 and S1.B).…”