Human aquaporin 4 gating dynamics under and after nanosecond-scale static and alternating electric-field impulses: A molecular dynamics study of field effects and relaxation

Abstract

Water self-diffusion and the dipolar response of the selectivity filter within human aquaporin 4 have been studied using molecular dynamics (MD) simulations in the absence and presence of pulses of external static and alternating electric fields. The pulses were approximately 50 and 100 ns in duration and 0.0065 V/Å in (r.m.s.) intensity and were either static or else 2.45 or 100 GHz in frequency and applied both along and perpendicular to the channels. In addition, the relaxation of the aquaporin, water self-diffusion and gating dynamics following cessation of the impulses was studied. In previous work it was determined that switches in the dihedral angle of the selectivity filter led to boosting of water permeation events within the channels, in the presence of identical external static and alternating electric fields, although applied continuously. Here the application of field impulses (and subsequently, upon removal) has shown that it is the dipolar orientation of the histidine-201 residue in the selectivity filter which governs the dihedral angle, and hence influences water self-diffusion; this constitutes an appropriate order parameter. The dipolar response of this residue to the applied field leads to the adoption of four distinct states, which we modelled as time-homogeneous Markov jump processes, and may be distinguished in the potential of mean force (PMF) as a function of the dipolar orientation of histidine-201. The observations of enhanced "dipolar flipping" of H201 serve to explain increased levels of water self-diffusion within aquaporin channels during, and immediately following, field impulses, although the level of statistical certainty here is lower. Given the appreciable size of the energy barriers evident in PMFs computed directly from deterministic MD (whether in the absence or presence of external fields), metadynamics calculations were undertaken to explore the free-energy landscape of histidine-201 orientation with greater accuracy and precision. These indicate that electric fields do alter the free-energy profile of the H201 side-chain orientation, wherein a perturbation of the symmetric bimodal state evident in the zero-field case is observed. These effects are dependent on the field intensities.

Communication: Influence of nanosecond-pulsed electric fields on water and its subsequent relaxation: Dipolar effects and debunking memory

“…10 Further, Reale et al have concluded recently from NE-MD that particular electric and CW fields, once removed, can leave dipolar alignment of water-flux-controlling selectivity filters in human aquaporin-4 channels fixed in place, without reversion to zero-field configurations. 18 These latter findings of Refs. 10 and 18 of putative "irreversibility"-a "memory" effect from electric fields-might, at first glance, appear to give some tentative support to the controversial notion of residual polarisation in water from electric fields.…”

“…Enhancedsampling techniques, such as metadynamics in Ref. 18, show particular promise for addressing these open questions.…”

Quantitative Limits on Small Molecule Transport via the Electropermeome — Measuring and Modeling Single Nanosecond Perturbations

“…We point to previously published evidence for some of these structures and processes, and here we identify several components of the electropermeome that must be included in a comprehensive model.Physical structures: The first set is comprised of the physical structures formed as a consequence of the immediate interaction of the electric field with membrane biomolecular assemblies: simple lipid electropores and pores with cytoskeletal constraints 56 or obstructions 47 , electroconformationally altered membrane proteins 31, 57 , regions of lipid scrambling 58 or peroxidation 24, 25 , and polynucleotide-membrane complexes 34 .Biological processes: The electropermeome includes also the post-permeabilization processes that are part of the cellular stress and damage response: volume regulation and pressure release triggered by osmotic swelling 44 , sodium-potassium and calcium ion ATP-dependent pump activity following membrane depolarization and loss of ion concentration gradients 27, 30 , and membrane repair 59 . These processes take place in a compromised metabolic environment.…”

“…Physical structures: The first set is comprised of the physical structures formed as a consequence of the immediate interaction of the electric field with membrane biomolecular assemblies: simple lipid electropores and pores with cytoskeletal constraints 56 or obstructions 47 , electroconformationally altered membrane proteins 31, 57 , regions of lipid scrambling 58 or peroxidation 24, 25 , and polynucleotide-membrane complexes 34 .…”

Computing osmotic permeabilities of aquaporins AQP4, AQP5, and GlpF from near-equilibrium simulations

“…Refs. [17, 22, 50, 51, 57, 79, 89–91]) have been conducted on the basis of the theoretical approximation of single-file diffusion[88, 91, 92]. The accuracy of these single-file diffusion approximations has not been ascertained because the in vitro measurements of single-channel permeability have proven to be very challenging as well.…”