Investigation of the Interactions of the SS-31 Peptides with Cardiolipin Variants: A Potential Therapeutic for Barth Syndrome

Szeto-Schiller (SS) peptides are synthetic tetrapeptides containing an alternating aromatic-cationic motif. They are readily taken up by cells and are targeted to the mitochondrial inner membrane. Among them, SS-31 has been shown to bind to bilayers in a cardiolipin-dependent manner via electrostatic and hydrophobic interactions. As a therapeutic agent, SS-31 leads to clinical improvements in many complex diseases, likely linked to its selective interaction with the cardiolipin-rich inner membrane, protection of mitochondrial ultrastructure, curtailment of ROS, and restoration of oxidative phosphorylation. SS-31 represents a first-in-class cardiolipin-protective therapeutic; however, its molecular mechanism of action remains largely unknown. In this study we have explored the use of SS-31 as a potential therapeutic for Barth syndrome (BTHS), a multisystem disorder characterized by cardiac and skeletal myopathy, neutropenia, growth retardation, and premature death in young males. BTHS is caused by mutations in the gene that encodes the transacylase tafazzin, involved in acyl chain remodeling of cardiolipin following its biosynthesis. Dysfunctional tafazzin results in decreased levels of cardiolipin with abnormal fatty acid tail profiles, and an accumulation of the remodeling intermediate monolysocardiolipin. Using model membrane systems and in organello studies, we show that the binding affinities of SS-31 to bilayers containing cardiolipin and monolysocardiolipin are similar. Moreover, the presence of SS-31 appears to modulate the interaction of the electron carrier cytochrome c with bilayers containing cardiolipin and its variants. Our results suggest that SS-31 may act in part by altering membrane phase behavior. Our empirical results are complemented by computational approaches that address the nature of the SS-31 bilayer interaction. Our results point to the potential use of SS peptides as therapeutics for BTHS and give key mechanistic insights into the nature of their bilayer interactions.