Melatonin alters fluid phase co-existence in POPC/DPPC/cholesterol membranes

Abstract The structure and biophysical properties of lipid membranes are important for cellular functions in health and disease. In Alzheimer’s disease (AD) the neuronal membrane is a target for toxic amyloid-β (Aβ). Melatonin is an important pineal gland hormone that has been shown to protect against Aβ toxicity in cellular and animal studies, but the molecular mechanism of this protection is not fully understood. Melatonin is a small membrane active molecule, which has been shown to interact with model lipid membranes and alter the membrane biophysical properties, such as membrane molecular order and dynamics. This effect of melatonin has been previously studied in simple model bilayers with one or two lipid components, to make it more relavant to neuronal membranes we used a more complex ternary lipid mixture as our membrane model. In this study, we used 2H-NMR to investigate the effect of melatonin on the phase behaviour of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol lipid membranes. We used deuterium labelled POPC-d31 and DPPC-d62, separately, to probe the changes in hydrocarbon chain order as a function of temperature and melatonin concentration. We find that POPC/DPPC/cholesterol at molar proportions of 3:3:2 is close to liquid disordered/liquid ordered phase separation and that melatonin can induce phase separation in these ternary mixtures by preferentially incorporating into the disordered phase and increasing its level of disorder. At 5 mol% melatonin we observed phase separation in samples with POPC-d31, but not with DPPC-d62, while at 10 mol% melatonin phase separation was observed in both samples with either POPC-d31 or DPPC-d62. These results indicate that melatonin can have a strong effect on membrane structure and physical properties, which may provide some clues to understanding how melatonin protects against Aβ, and that choice of chain perdeuteration is an important consideration from a technical point of view.