Water movements in and out of the cell and the nucleus accross timescales from milliseconds to days

Because mammalian cells and their nuclei have semi-permeable membranes with high permeation, water will rapidly move to equilibrate osmotic and hydrostatic pressures. I will present results based on fast and precise measures of cell and nuclei volume that allowed us to study these movements of water in and out of the cell and the nuclei at short and long timescales. While the mechanism driving water movements at very short millisecond timescales remain mysterious, the seconds timescale is related to passive permeation and the minute timescales to ion fluxes. On longer timescales, the dynamics is mostly driven by changes in the non-permeable content of the cell, mostly corresponding to accumulation or degradation of macromolecules. The nucleus also has membranes and a defined volume, that is an important parameter because it determines the concentration of nuclear macromolecules. Water movements in and out of the nucleus relate on similar principles but with very significant differences that I will discuss. I will finish the talk on the consequence of these differences when considering the case of mechanically constrained cells, focusing on the case of 2D vertical confinement: in such a context, the nucleus loses a significant volume and intranuclear concentration increases to the point of producing crowding effects slowing down the movement of large particles, while the cytoplasm remains diluted enough to allow their normal diffusion. We still do not fully understand all the aspects of this phenomenon, and in particular the role of chromatin, but we observed that confined cells show compartmentalised diffusion properties of particles of tens of nanometres, called GEMs (very fast in blebs, resembling diffusion in water, normal in the cytoplasm and slow in the nucleus), while it is not the case in non-confined cells in which diffusion is similar in the cytoplasm and in the nucleoplasm. I will be happy to discuss what we think we understand, and I hope to also get some input on the aspects we still do not understand, that I will try to clearly expose to foster discussion. My plan for this talk is to try to spend about 15 minutes to discuss short timescales for the cell volume, then about 15 minutes for long timescales (growth), then 15 minutes on nuclear volume (we still do not have all the timescales for the nucleus) and finish on the open question of diffusion and crowding in confined cells.