Life Cycle of an Electropore: Field-Dependent and Field-Independent Steps in Pore Creation and Annihilation

Abstract

Electropermeabilization, an electric field-induced modification of the barrier functions of the cell membrane, is widely used in laboratories and increasingly in the clinic; but the mechanisms and physical structures associated with the electromanipulation of membrane permeability have not been definitively characterized. Indirect experimental observations of electrical conductance and small molecule transport as well as molecular dynamics simulations have led to models in which hydrophilic pores form in phospholipid bilayers with increased probability in the presence of an electric field. Presently available methods do not permit the direct, nanoscale examination of electroporated membranes that would confirm the existence of these structures. To facilitate the reconciliation of poration models with the observed properties of electropermeabilized lipid bilayers and cell membranes, we propose a scheme for characterizing the stages of electropore formation and resealing. This electropore life cycle, based on molecular dynamics simulations of phospholipid bilayers, defines a sequence of discrete steps in the electric field-driven restructuring of the membrane that leads to the formation of a head group-lined, aqueous pore and then, after the field is removed, to the dismantling of the pore and reassembly of the intact bilayer. Utilizing this scheme we can systematically analyze the interactions between the electric field and the bilayer components involved in pore initiation, construction and resealing. We find that the pore creation time depends strongly on the electric field gradient across the membrane interface and that the pore annihilation time is at least weakly dependent on the magnitude of the pore-initiating electric field and, in general, much longer than the pore creation time.