James H. Davis

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 Alzheimer’s disease (AD) the structure of neuronal membranes becomes compromised by the toxic effect of amyloid- β (A β ) 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 β 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 ² H-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- d ₃₁ in POPC- d ₃₁ and DPPC- d ₆₂ 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- d ₃₁ or DPPC- d ₆₂ was used. At 5 mol% melatonin we observed phase separation in samples with POPC- d ₃₁ , but not with DPPC- d ₆₂ . However, at 10 mol% melatonin phase separation was observed in both samples with either POPC- d ₃₁ or DPPC- d ₆₂ . 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 β . SIGNIFICANCE: Melatonin has been shown to be protective against A β pathology in animal and cellular studies. Although the mechanism of this protection is not well-understood, melatonin’s 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.

The results of full-atom molecular dynamics simulations of the transmembrane domains (TMDs) of both native, and Glu⁶⁶⁴-mutant (either protonated or unprotonated) Neu in an explicit fully hydrated dimyristoylphosphatidylcholine (DMPC) lipid bilayer are presented. For the native TMD peptide, a 10.05 ns trajectory was collected, while for the mutant TMD peptides 5.05 ns trajectories were collected for each. The peptides in all three simulations display stable predominantly α-helical hydrogen bonding throughout the trajectories. The only significant exception occurs near the C-terminal end of the native and unprotonated mutant TMDs just outside the level of the lipid headgroups, where π-helical hydrogen bonding develops, introducing a kink in the backbone structure. However, there is no indication of the formation of a π bulge within the hydrophobic region of either native or mutant peptides. Over the course of the simulation of the mutant peptide, it is found that a significant number of water molecules penetrate the hydrophobic region of the surrounding lipid molecules, effectively hydrating Glu⁶⁶⁴. If the energy cost of such water penetration is significant enough, this may be a factor in the enhanced dimerization affinity of Glu⁶⁶⁴-mutant Neu.