Volume and Surface Area Dynamics of Giant Unilamellar Vesicles

Abstract

Giant unilamellar vesicles (GUVs) have proven to be useful systems in the quest for artificially capturing the essential components of living cells. Here we elucidate the mechanisms by which GUVs respond dynamically to two common environmental stressors: osmotic swelling by depletion of external osmolytes, and area reduction by exposure to surfactant. Strikingly, both of these stressors produce a dynamic pulsatory behavior of the GUVs, characterized by repeated sequences of large pore opening and closing. We first focus on how GUV volume change drives the dynamics of lipid vesicles. We recently unraveled how hypotonic GUVs filled with sucrose solution and bathed in water, exhibit characteristic swell-burst cycles. Here we ask, what is the effect of intra-vesicular crowding on the vesicle dynamics? Our experimental observations, supported by our model results, show that GUVs encapsulating PEG and/or Dextran polymers still exhibit swell-burst cycles even at zero or negative concentration differential. Our model allows us to identify the three main phenomena leading to this intriguing behavior: the non-ideal osmotic pressure induced by the polymer solutions, the slower pore dynamics due to higher solution viscosity, and the slower diffusive flux through the pore. Second, we ask how the dynamics of GUVs is affected by surface stressors? It has been shown that lipid vesicles exposed to surfactants lose surface area through lipid solubilization, leading to two main possible outcomes: cycles of pore opening/closing, or long-lived stable pores. We determine the parameter space related to these two behaviors, and solve numerically our model, allowing us to predict the dynamics of GUVs exposed to various surfactants.