Critical Temperatures in Plasma Membrane Vesicles are Dependent on the Cell Cycle

Abstract

Giant plasma membrane vesicles (GPMVs) isolated from RBL-2H3 cells appear uniform at physiological temperatures, contain coexisting liquid-ordered and liquid-disordered phases at low temperatures, and experience micron-sized critical fluctuations close to their critical temperature. We observe a broad distribution of critical temperatures in GPMVs isolated from a dish of seemingly identical cells even though each individual vesicle has a well defined critical temperature. In addition, we observe that the average transition temperature of GPMVs isolated from a population of cells is inversely proportional to the surface density of cells growing in the culture dish. Since it is known that cellular doubling times are reduced in more densely plated cells due to contact inhibition, we hypothesized that critical temperatures are linked to the cell cycle. To test this hypothesis, we isolated GPMVs from populations of cells synchronized using a double Thymidine block that arrests cells at the border between G1 and S phases. After Thymidine is removed from the culture media, cells proceed synchronously through one cell cycle and GPMVs are isolated at time-points corresponding to S, G2, M, and G1 phases. We find that critical temperatures are significantly elevated in the cell cycle phases that immediately preceding cell division (G2 and M) compared to the remainder of the cell cycle phases (G1 and S), consistent with our observations of higher overall transition temperatures in more rapidly dividing cells. We speculate that membrane heterogeneity arising from this critical point may play a role in cytokinesis or other processes vital for cell division. If this hypothesis is correct, it may suggest that biochemical perturbations that lower critical temperatures in GPMVs can be used to inhibit cellular proliferation, possibly providing a novel treatment strategy for some cancers.