Postdoc UT Southwestern Medical Center Allen, Texas, United States
Introduction:: Complex modeling is effective for studying in vivo biological systems at nano-scales for validating experiments, generating hypotheses, and interpreting findings. Here, we built a complex model to study clathrin-mediated endocytosis (CME), the major cellular pathway of internalizing surface-bound cargo molecules. CME plays essential roles in many diseases, including COVID, cancer, and Alzheimer’s. During CME, clathrin forms a self-assembled basket to consecutively bend the membrane to enclose the cargos into a ~100nm-wide vesicle within ~25 seconds before intracellular release. Multiple endocytic proteins (EP) besides clathrin also contribute to this process. However, little is clear regarding the detailed role of the EPs in assisting the clathrin basket in bending the membrane, mainly because of the insufficient understanding of the mechanical relation between the EPs and the cell membrane.
Materials and Methods:: To fill this gap, we developed a state-of-the-art stochastic model that incorporates the clathrin basket, the endocytic proteins, the cell membrane, and their mechanical interactions. We show, experimentally, that CALM (an adaptor protein) is required to initiate curvature. Next, we adjust the model based on the CALM result and the known flexibility of clathrin to predict two distinct states of the clathrin baskets at equilibrium: a shallow open state that defines abortive CME events and a curved, closed state that defines productive CME events. We further explore various values of a parameter that controls the tightness of the clathrin baskets and discover a phase transition from the closed to open state. We hypothesize and then experimentally confirm that curvature induced by a regulatory protein dynamin2 determines the transition from open to closed states and drives productive CME.
Results, Conclusions, and Discussions:: We conclude: 1) There is a checkpoint indicating a competition between the energy in clathrin baskets and the energy of membrane bending, resulting in two distinct fates of CME: abortive and productive, and 2) Dynamin2 powers clathrin baskets to pass the checkpoint and drive productive CME to enhance CME efficiency. In sum, our interdisciplinary study complementing quantitative experiments with a complex model serves as an example of bridging physics and cell biology at nanoscales.
