Collaborative Research: Isothermal Phase Transition in Lipid Vesicles and Swell-Burst Cycles

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In this project the PI, using a combination of theory, simulations and experiments, will investigate the complexities of lipid membranes. The project combines concepts from the fields of polymer physics, membrane mechanics and bioengineering, surface and interface science, and soft condensed matter to study the organization of biological membranes. The modeling efforts will develop new and novel mathematics and new numerical schemes to solve the resulting differential equations. The current understanding of how multi-component giant unilamellar vesicles (GUVs) respond to osmotic pressure differentials is incomplete, and experimental observations indicate that a non-linear model coupling membrane dynamics with 3D fluid flow is needed to fully explain the non-linearities of the system. Developing this theoretical framework and providing insight into the underlying physics is crucial for the understanding of how membranes undergo morphological transitions. These models will explain existing experiments and also predict membrane response to different osmotic loads and membrane compositions. Understanding this process is important for better experimental design of in vitro reconstituted systems such as vesicles and also cellular systems. The work is inherently interdisciplinary, using mathematics and physics in biological systems. Both fields will benefit from this approach to studying biological phenomena; the theory will be grounded in experiments and also make predictions to design future experiments. This research will be integrated into the teaching efforts of the PIs in developing new courses at the interface of engineering and biology. The PIs will continue their efforts in enhancing diversity in the UC system while pursuing the research program. Biological membranes are inherently heterogeneous mixtures of lipids and proteins. A key characteristic of this heterogeneity is the coexistence of liquid-ordered and liquid-disordered phases. This coexistence is thought to be the key organizing principle for the formation of lipid rafts. Studying the formation and organization of the different phases in cellular system is experimentally challenging, given the complex nature of the cells. Giant unilamellar vesicles with controlled compositions, allow us to study lipid behavior in bilayer membranes and gain insight into phase behavior which is important for understanding cellular membranes. Although GUVs are used widely experimentally, our theoretical understanding of lipid phase separation remains rudimentary, since existing models focus on the line tension between preexisting domains and not on domain growth and swell-burst cycles, which are the features observed experimentally. The objectives of this project are to formulate quantitative models of isothermal phase separation and swell-burst cycle in multi-component GUVs and test model predictions experimentally. STUDY 1-DYNAMICS OF SOLUTE EFFLUX. We will use theory, simulations, and experiments to understand the factors that control pore radius, vesicle radius, and the lifetime of the pore. STUDY 2-PHASE SEPARATION IN OSMOTICALLY STRESSED VESICLES. Using a viscoelastic model of multi-component lipid membranes, we will investigate the role of governing energetics in domain growth versus true phase transitions. We will experimentally test the model predictions by tuning the osmotic pressure difference, lipid composition, and sample temperature. STUDY 3-COUPLING BETWEEN DOMAIN FORMATION AND SWELL-BURST CYCLES. In this study, we will develop the mathematical framework to model the complete dynamics of the oscillatory phase separation coupled with the swell-burst cycle observed in the preliminary experiments. This model will combine the dynamics of pore formation outlined in Study 1, with the domain growth model including membrane viscosity in Study 2. The significance of the proposed activities lies in its promise to not only elucidate the fundamental properties of mixtures of lipids reduced dimensional, bilayer configuration but also furnish design principles for designing synthetic protocellular compartments for applications spanning in vitro production of proteins, chemistry in confinement, and delivery of biomedically relevant cargo (e.g., enzymes, drugs, and imaging agents). Using a combination of theory, simulations and experiments, this work will be able to provide insight into the complexities of lipid membranes. The long-term impact of the proposed activities stems from the fact that the project combines concepts from the fields of polymer physics, membrane mechanics and bioengineering, surface and interface science, and soft condensed matter. The modeling efforts outlined here will result in new and novel mathematics and new numerical schemes to solve the resulting differential equations.