Micro-Domain Migration And Coarsening Dynamics In Membrane Phase Separation Of Biomimetic Phospholipids

Recently,the exploration of physical problems in cell biology has become one of the topical issues in biophysics.It is crucial to simulate and investigate the intrinsic physical mechanism of cell membrane structure and function in living activities.Phase separation in cell membranes and self-organization of lipid rafts are much attention.As a characteristic structure regulating biological functions,lipid rafts play a key role in component sorting,protein transport,and signaling processes.Based on Giant unilamellar vesicles(GUVs),the migration and evolution of the phase-separated micro-domains were observed by fluorescent dye.Through single domain tracking and domain boundary volatility quantification,the key parameters to control the growth and migration of micro-domains: membrane viscosity and line tension.Micro-domains diffusing laterally across the cell membrane perform vital function towards improving the efficiency of biomolecule signals transduction.In the first chapter of this paper,the composition and structure of cell membranes were briefly introduced.To elucidate the importance of lipid rafts and the research difficulties.The basic physical properties of the cell membrane,including phase state and viscosity,lateral mobility,lateral heterogeneity,are introduced.Subsequently,the recent progress of cell membrane lipid rafts and heterogeneity was reviewed,and the entry point for our research was proposed.The second chapter introduces the main experimental research methods.A single-domain tracking method is described on how to quantify micro-domain diffusion through motion trajectories.According to the motion of the micro-domain in the membrane environment,we explain how the micro-domain boundary.The above method provides a reliable basis for us to explore the dynamics of micro-domain growth.Lipid rafts are small biomembrane functional units,formed as the result of the lateral phase separation of phospholipids.Phospholipid phase separation plays a crucial role in the spatial organization of biomolecules in life activities.Here,we study the kinetics of multi-component phospholipid phase separation quantitatively using the single domain characterization methods including the movement tracking and radial fluctuation analyses,which provide valuable information about the physical and mechanical properties of the bulks and domains.The study is carried out in a low line tension condition similar to that in cells.Through the tracking of domains,it is found that the bulk viscosity dominates the dynamics of domain coalescence.The coalescence of domains produces strong hydrodynamic flows in low viscosity bulk,which promotes the non-Brownian motion of surrounding domains,accelerating the lateral diffusion and coalescence of the domains.However,these hydrodynamic flows reduce significantly in high viscosity bulk.The domains rely mainly on Brownian motion to diffuse in this highly viscous medium,resulting in the slow lateral diffusion and low coalescence.Furthermore,we observe a bulk-viscosity-dependent scaling relation between the domain size and coarsening time in experiments.A theoretical model of domain diffusion and coalescence is established to understand the scaling relation.If the bulk viscosity is low,the hydrodynamic flow produces a high power exponent of 1.0.And if the bulk viscosity is high,the Brownian diffusion produces a low power exponent of 0.5.In addition,we demonstrate that the bulk viscosity can be regulated through the relative content of cholesterol.In all,our study deepens the understanding of the physical mechanism underlying the formation of lipid rafts.It also provides a reference for regulation of the biomolecule distribution in cell membranes.There are often flow fields,chemical gradient fields(such as membrane potential),and temperature fields in living systems,leading to individual cells in a non-equilibrium state.However,there are few studies on the growth and stability of lipid rafts in non-equilibrium states.In Chapter 4,we focus on exploring the micro-domain formation,lateral diffusion and its asymmetry distribution under gradient temperature fields,and understanding the characteristics of micro-domains under non-equilibrium conditions.Based on GUVs,comparing the motion of micro-domains without temperature gradient,the upper and lower surface diffuses through Brownian motion,and the diffusion and migration of micro-domains are observed.The results show that both ordered and disordered phase domain(component-independent)move to the high temperature side of the vesicle.During migration,larger micro-domains to reduce line tension.When the micro-domain is in a higher temperature membrane viscosity decreases the micro-domain diffusion rate.When the temperature field is applied,the diffusion velocity of the cold surface micro-domain exceeds the diffusion speed of the hot surface micro-domain,and the viscosity decreases.However,the higher the thermal surface temperature,the smaller the line tension of the micro-domain,to ensure the stable migration to the thermal surface.Finally,the kinetic behavior of micro-domains in phase-separated vesicles is summarized to deepen the understanding of the interaction between micro-domains and cell membranes and prospects future work in this paper.