BPS2026 – Synergy of acid and oxidative stress on membrane permeability explained by a pH-dependent conformational switch of oxidized phospholipids

The plasma membrane constitutes a selective diffusion barrier that permits effective cell function and communication. During bacterial infections, a defensive measure of the immune system is to phagocytose bacteria and expose them to a combination of acid and oxidative stress within phagolysosomes. This combination of stresses usually leads to the impairment of the membrane barrier function in bacteria, but the precise mechanisms are not well known. We therefore investigated the effects of acid and oxidative stress on bacterial membranes and membrane models to unravel the precise molecular mechanisms underlying this activity. In Gram-positive bacteria, we observed that acid stress led to increased potassium release and a depleted membrane potential, while oxidative stress increased permeability toward larger molecules. Both effects synergistically inhibited bacterial growth. In Gram-negative bacteria, both stresses acted synergistically on the outer membrane, which correlated with potassium release and growth inhibition, although no large pores were formed in the plasma membrane. To delve into the molecular mechanism behind these synergistic effects, we investigated how acidity might alter membrane permeability induced by oxidized phospholipids in liposomes. We observed a pH-dependent permeability change in the presence of oxidized phospholipids that possessed truncated acyl chains with terminal carboxyl groups. This behavior resulted from a pH-dependent conformational switch of the carboxylated acyl chain. At cytosolic pH, the carboxyl was ionized, promoting its presence at the membrane interface, while at low phagolysosomal pH, ionization was reduced, promoting its localization to the membrane interior, which likely led to a compromised hydrophobic barrier function. This mechanism might partly explain the synergy of acid and oxidative stress on oxidized membranes, while truncated carboxylated phospholipids could also be promising candidates for the formulation of pH-dependent liposomal drug-delivery systems.