Heteromultimeric Gap-Junction Channel Permeance: Directional Fluxes Simulated Using a Brownian Dynamics Model

Abstract

Permeance for distinct connexin channels has been hard to predict. Heterotypic connexin combinations yield preferential directional fluxes of fluorescent molecules with small influence from particle charge, suggesting regulation through differences in pore's shape. We have simulated particles' movement across a 3D geometric pore's representation from X-ray crystallography following a Brownian Dynamics Model (BDM). A central prolate represents the pore's vestibule; a cone and a cylinder represents the pore's mouths for Cell 1 (C1) and Cell 2 (C2) respectively. Lucifer yellow molecules (e=0 or -2e-, Stokes radius of 4.9A) were represented as spheres. Particle-channel charge interactions were simulated placing a charged ring near the channel's mouth. BDM described closely particle behavior where the displacement vector (dx) was calculated using particles' diffusion matrix, net force and a random vector from Gaussian distributions after every time step (dt=10ps). Unidirectional particle fluxes were determined by counting particles that crossed the channel (particle gradient 4 to 0).Uncharged particle flux was not affected by the pore's mouth shape. When e=-2e-, particle flux depended on pore's shape (C1to2=1.6±0.04x106 particles/sec/channel [p/s/ch]; C2to1=0.75±0.03x106 p/s/ch). A negative charged ring reduced/blocked particle flux and pore-particle interactions. A positive charged ring doubled particle flux for C2to1 only, highlighting the mechanism's complexity.Particle number at steady-state (SS) during bidirectional fluxes was calculated after placing 8 particles on either side. The average number of particles per cell remained close to 4 at SS and did not vary significantly (<6%;n=15) over 6μs; the time to reach steady state for C1to2 was smaller (1.5μs and 2.1μs for C2to1). Our mathematical model explains preferential directional fluxes by differences in pore mouth's shape. Identical number of particles at SS indicates thermodynamics compliance.