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

The structure and biophysical properties of lipid biomembranes are important for normal function of plasma and organelle membranes, which is essential for proper functioning of living cells. In Alzheimers disease (AD) the structure of neuronal membranes becomes compromised by the toxic effect of amyloid-{beta} (A{beta}) protein which accumulates at neuron synapses, resulting in membrane perforation and dysfunction, oxidative stress and cell death. Melatonin is an important pineal gland hormone that has been shown to be protective against A{beta} toxicity in cellular and animal studies, but the molecular mechanism of this protection is not well understood. It has been shown that melatonin can 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, we consider a more complex ternary lipid mixture as our membrane model. In this study, we used 2H-NMR to investigate the effect of melatonin on lipid phase behaviour of a three-component model lipid membranes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol. We used deuterium labelled palmitoyl-d31 in POPC-d31 and DPPC-d62 separately, to probe the changes in hydrocarbon chain order as a function of temperature and varying concentrations of melatonin. We found that melatonin concentration influences phase separation in these ternary mixtures somewhat differently depending on whether POPC-d31 or DPPC-d62was used. At 5 mol% melatonin we observed phase separation in samples with POPC-d31, but not with DPPC-d62. However, 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 A{beta}. SIGNIFICANCEMelatonin has been shown to be protective against A{beta} pathology in animal and cellular studies. Although the mechanism of this protection is not well-understood, melatonins membrane-active properties may be important in this regard. In this work solid-state deuterium nuclear magnetic resonance was used to study the effect of melatonin on the POPC/DPPC/cholesterol model membranes. Specifically, we showed that melatonin modifies lipid hydrocarbon chain order to promote phase separation. This knowledge helps to explain the role of melatonin in lipid domain formation and may provide a deeper understanding of the mechanism of melatonin neuroprotection in AD.