Electrofusion: a biophysical modification of cell membrane and a mechanism in exocytosis.

The molecular bases of the exocytosis process remain poorly known. Many proteins have been recognized to play key roles in the machinery. Their functions are well characterized in the specificity of the docking processes. Forces involved in the merging of the two partners must take into account the physics of membrane interfaces. The target membrane and the vesicle are both electrically charged interfaces. Strong electrostatic fields are triggered when they are brought in close neighborhood. These fields are high enough to induce an electropermeabilisation process. It is now well known that when applied on a cell, an external field induces a modulation of the transmembrane potential difference. When high enough the transmembrane potential may induce a membrane destabilisation. This results in a free exchange of polar molecules across well defined parts of the cell surface. Furthermore, when permeabilization is present on two cells, if those parts of the cell surfaces are brought in close contact, membrane merging occurs spontaneously. Cell fusion results from this membrane coalescence. The similarity with what is taking place in exocytosis is striking. The present review describes the state-of-the-art in the knowledge on electrofusion. It is emphasized that it results from electropermeabilisation and not from a direct effect of the external field. A local destabilisation of the vesicle membrane results from electrostatic interactions while keeping unaffected its viability. Such processes appear relevant for what takes place during exocytosis.