The link between nuclear size and cell size has been known for over a century, but its mechanism remained unclear. Neighboring papers in the archive explore how cellular motors set droplet size, how curvature dictates size in the celestial mechanics of cells, how size and compression trigger asymmetric division, and how nuclear shape changes its size. This work stands apart: it directly tests the osmotic equilibrium hypothesis, while its neighbors focus on mechanical forces and geometry.
In the yeast S. cerevisiae, high-level expression of the fluorescent protein 3xFP was induced either with a nuclear localization signal (3xFP-NLS) or with a nuclear export signal (3xFP-NES). After 6 hours, cell and nuclear volumes were measured by microscopy. To separate the effects of excluded volume and osmotic pressure, the size of cytoplasmic macromolecules in Xenopus egg extract was varied at constant protein mass. Additionally, optogenetic export of protein from the nucleus was used in HeLa cells.
Expression of 3xFP-NLS increased nuclear volume 7.7-fold and raised the N/C ratio from 6.3% to 18.8%. Expression of 3xFP-NES lowered the N/C ratio from 5.3% to 3.2% and caused nuclear deformation. In egg extract, replacing large complexes with BSA of the same mass reduced nuclear size, confirming the role of particle number rather than mass. The model estimated the number of osmotically active colloids in the cell at roughly 7.2–20.4 million, close to proteomic data. Changing the N/C ratio altered the dry mass density of the nucleus and cytoplasm and also caused redistribution of RNA polymerase II: when the nucleus enlarged, weakly expressed genes (including subtelomeric genes and retrotransposons) became activated, while compression enhanced expression of highly active genes. Nuclear compression in enlarged cells reversed the transcriptomic changes.
This work is the first to directly show that the osmotic pressure of proteins is sufficient to establish the N/C ratio, and it explains the universal link between nuclear size and cell size. Unlike neighboring studies that consider mechanical forces or geometry, this one quantitatively assesses the contribution of colloidal osmotic pressure specifically, and also demonstrates that changes in nuclear size alone cause transcriptome remodeling similar to that observed during cellular aging.
The field may move toward studying how changes in osmotic pressure and chromatin phase separation regulate gene accessibility in health and disease. It may also test the role of nuclear size in stem cell differentiation and in aging processes.
The results will influence cell biology, nuclear biophysics, aging and cancer research, as well as our understanding of transcriptional regulation through the physical parameters of the cell.
Next steps include determining exactly how osmotic pressure affects chromatin phase separation and promoter accessibility, and testing whether the mechanism is conserved in mammalian cells in vivo.
The work links the problem of nuclear size control to long-standing questions about the mechanisms of gene silencing, retrotransposon activation during aging, and coordination of cell growth with division. It offers a physical basis for understanding how changes in nuclear architecture during aging and differentiation can directly affect gene expression.
The concentration of osmotically active macromolecules in the cell is only about 0.24–0.68 mM, hundreds of times lower than the concentration of small molecules, yet it is enough to determine nuclear size.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.