The fourth lecture brings the chapter on adhesion to a close. In the light of the molecular diagram from the previous lecture, we develop the non-equilibrium character of adhesion, its origin and functional implications.
Based on the existence of intermolecular couplings in the extracellular domain and within the intracellular cortex, the lecture first presents a model of adhesion as an active process. This is illustrated by the organization of cadherins into small aggregates, revealing the contributions of couplings with the actin cortex and of endocytosis. Secondly, mechanosensation and mechanotransduction by cadherins testify to the action of contractile forces on adhesion: stabilization and reinforcement of interactions within adhesion complexes, adaptation to the mechanical environment of a cell, etc.
Energy dissipation within adhesion complexes is addressed in a second step. Although this subject is little developed in the literature, the lecture sets out the reasons for studying this central notion. Dissipative processes ensure the irreversibility of adherent interactions and their ceaseless dynamics at supramolecular and cellular scales. Dissipation relies on the joint existence of a large number of low-affinity molecular interactions, rapidly renewed and exchanged, and active processes that steer the dynamics of the interactions. The characterization of dissipation modes and associated time scales is a major challenge for research in this field.
The lecture concludes with an overview of adhesion, highlighting the common principles at work in intercellular adhesion (cadherins) and cell-substrate adhesion (integrins).
