Computational Investigation into the Effect of SS-31 on Membrane Ion Distributions, Pore Formation and Ion Leakage in the Presence of Transmembrane Potentials

Abstract

Mitochondrial dysfunction encompasses a broad array of disorders related to aging and neurodegeneration, and often presents with altered membrane potentials and overproduction of reactive oxygen species (ROS). Szeto-Schiller (SS) peptides comprise a family of synthetic, amphipathic tetrapeptides with demonstrated efficacy against many mitochondrial disorders, and which accumulate strongly in mitochondria and bind to lipid bilayers. However, little is known regarding how SS peptides interact with lipid bilayers in the presence of a transmembrane potential. In this study, we analyzed the interactions of lead compound SS-31 (Elamipretide) with model membranes through molecular dynamics (MD) simulations. We performed equilibrium MD in single-bilayer systems to compare with experimental findings which showed that the peptide's ability to modulate membrane surface electrostatics is an important aspect of SS-31's mechanism of action. We went onto study how SS-31 influences membrane pore formation and ion leakage in the presence of transmembrane potentials. These simulations employed double-membrane systems and several computational techniques including external electric fields, explicit ion imbalances and the GROMACS computational electrophysiology module. We found that in the presence of a high membrane potential, SS-31's presence doubled the likelihood of transmembrane ion leakage compared to membrane-only systems. Most notably, in a seeming compensatory fashion, SS-31's interaction promoted anion leakage but inhibited cation leakage which ultimately preserved the cation gradient and membrane potential after leakage events. We propose a mechanism by which SS-31's mitoprotective properties could hinge on relieving dysfunctional hyperpolarization, and thereby halting damage from excessive ROS production yet still preserving the proton gradient needed for ATP production. This study could help elucidate how SS peptides correct this clinically-important complication of mitochondrial dysfunction and assist in enhancing the efficacy of future compound variants.