The third lecture develops the elements for understanding how adhesion, a macroscopic intercellular property, emerges from molecular interactions at the cell surface. We explore the relationship between, on the one hand, the biochemistry and biophysics of molecular-scale binding forces and, on the other, cellular adhesion forces. Initially, the lecture characterizes extracellular molecular interactions, and the structural and dynamic elements of cis- and trans- bonds between cadherins. These analyses show that it is impossible to adequately predict adhesion energy from intermolecular interaction affinity alone at the cell surface.
We then turn to the dynamic interactions with cortical actin, discussing in turn how these affect cadherin surface dynamics, intercellular adhesion forces and the macroscopic behavior of cell sorting. Thus, intercellular adhesion emerges from extracellular (between cadherins) and intracellular (with cortical actin) dynamic couplings.
Finally, the lecture adopts an evolutionary perspective on the origin of the adhesion mechanisms mediated by cadherins, insofar as they involve intra- and extracellular couplings. In so doing, we present a new hypothesis for the emergence of multicellularity.
