Biophysical modeling of inward and outward membrane curvature generation - Project Summary/Abstract: Biophysical modeling of membrane curvature generation for inward and outward budding. Curvature generation of cellular membranes is important for communication between the intracellular and extracellular space. During endocytosis, cells curve their plasma membrane inward to take up cargo from the extracellular space. In contrast, during the formation of extracellular vesicles, the plasma membrane curves outwards. Both of these processes are fundamental to cellular function and defects in these processes are implicated in cancer, altered neurotransmission, inflammation, and heart disease. While many distinct proteins are involved in membrane curvature generation, they ultimately work together to generate mechanical stress at membrane surfaces. As a result, biophysical modeling has become an important tool for understanding how curvature may be generated. These models often treat the membrane as a thin lipid bilayer without detailed considerations of the interactions of the membrane with the environment. As a result, many of these models ignore the fundamental aspects of curvature generation: the lipid and protein compo- sition of the membrane, interaction of the membrane with the actin cortex, and interaction of the membrane with key components of the extracellular matrix including the glycocalyx. Research in my group in the past 5 years has made fundamental advances in our understanding of the biophysics of membrane curvature generation that support the hypothesis that interaction of the lipid bilayer with the cytoskele- ton and the glycocalyx generate differential stresses across the membrane, which ultimately controls the direc- tion of membrane bending. The magnitude of these stresses depends on the specific biochemical properties of the molecules involved. Specifically, in endocytosis, we showed that membrane composition can alter membrane tension and therefore the energy required for endocytosis. Furthermore, we showed that the organization of the cytoskeleton and linker molecules along the endocytic pit is a critical determinant of force generation during endocytosis. We also showed that detachment of the membrane from actin cortex is a critical determinant of effective outward budding for extracellular vesicle formation. These findings lead us to the following urgent questions about how cellular membranes might generate curvature for endocytosis and extracellular vesicle formation: First, how does the asymmetry due to lipid and protein composition determine the direction of membrane budding? Second, how does the interaction with the actin cytoskeleton drive membrane bending for endocytosis and extracellular vesicle formation? And finally, what role does the glycocalyx play in altering the direction of membrane bending? Building on our recent discoveries, we aim to answer these questions using computational modeling to generate experimentally verifiable predictions. These predictions will be tested by experiments conducted in the Stachowiak, Wehman, and Drubin labs. By incorporating the mechanistic details of the composition of the plasma membrane, glycocalyx, and the actin cytoskeleton, the quantitative modeling framework we propose to develop will be applicable to wide range of cellular phenomena.
