Membrane Permeabilizing Electric Fields Disrupt Water Channel Function and Selectivity

Abstract

External MV/m electric fields can permeabilize cellular membranes, allowing influx and efflux of normally-impermeant molecules across the protective lipid bilayer. Utilization of short-duration permeabilizing electric fields (PEFs) have become widespread in the food industry, in biotechnology, and in medicine, where targeted high-voltage electric fields are used to transiently enhance the transport of water and other large molecules into and out of cells. However, despite the increasing use of PEFs in clinical and biological systems, their interactions with transmembrane proteins remain largely unknown. These limitations yield an incomplete picture of membrane electropermeabilization, where multiple components of the cell are likely perturbed (and possibly denatured) under PEFs. In order to test if PEFs affect transmembrane proteins, especially those that natively transport water, molecular dynamics simulations were carried out on human aquaporin 1 (hAQP1) water channels under cell-permeabilizing voltages. Simulations revealed significant changes to water orientation and flux through hAQP1 under PEFs, with voltage-dependent transport rates that diverge from diffusive and osmotic experiments. Surprisingly, hAQP1 was able to transport Cl anions (but not Na cations) when 1-2 volts was applied, despite its inability to do so under physiological conditions. The protein did not affect the construction of discrete lipid electropores around the periphery of the channel, or the timescales of lipid bilayer permeabilization, however a small reduction in the protein's alpha-helicity was detected. hAQP1 Cl conduction was also sensitive to the protonation state of the histidine residue located in the two-stage filter, suggesting that pH may also affect anion conduction. Taken together, these results suggest that PEFs have a direct effect on transmembrane water channels, even before membrane electropores interact with them. Further studies of additional membrane components under PEFs are necessary if optimization of therapeutic electric fields is to be significantly improved.