Tuning cell-cell interactions through regulation and manipulation of cell surface crowding

About

Future Leaders in Bioengineering Seminar Series

Daniel S.W. Lee, Ph.D

Postdoctoral Scholar

Department of Bioengineering

University of California, Berkeley

Abstract The cell surface is the physical vehicle for intercellular interactions, including immune recognition, cell-cell fusion, and viral infection. However, it is also covered in a dense and diverse population of macromolecules, including glycosylated proteins, lipids, and even nucleic acids, that extend well beyond the membrane at a density of approximately 20,000 per square micron. Together, these components constitute the glycocalyx, a nanoscopic polymer brush through which ligands must navigate to bind receptors on the cell surface. To quantify this energetic barrier, we developed a molecular sensor for cell surface crowding that inserts into lipid bilayers and acts as a model receptor. We found that surface crowding typically results in a tenfold increase in dissociation constant for surface binding on a cell membrane compared to a lipid-coated bead, equivalent to a 2-3kBT energy penalty per antibody-sized molecule. In this talk, I will describe how we used this crowding sensor to understand how cell surface crowding acts as a tunable physical barrier to key physiological cell-cell interactions.

First, we interrogated crowding on cells undergoing differentiation and found that surface crowding can be developmentally regulated. During differentiation, both myoblast and murine thymocyte surfaces become less crowded, possibly to promote cell-cell fusion and T cell signaling respectively, which we directly demonstrate are dependent on surface crowding through exogenous perturbations. To investigate what biological processes control and regulate crowding, we developed a multiplexed version of the crowding assay and combined it with a whole-genome CRISPRi screen to measure the impact of nearly 19,000 protein-coding genes in the human genome on cell surface crowding. We identified several axes of control, including glycosylation, actin organization, gene expression, and, most surprisingly, cell surface RNAs, which we hypothesize play a role in promoting crowding across multiple cell types. These results suggest that cells can dynamically and locally tune effective receptor affinity by altering cell surface crowding, in the absence of changes in receptor expression or affinity. We aim to use these insights as the basis for engineering new immunotherapies.

Biography