Melatonin Alters Fluid Phase Coexistence in POPC/DPPC/Cholesterol Membranes

Abstract

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 (Aβ) 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 Aβ 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 Aβ. SIGNIFICANCE Melatonin has been shown to be protective against Aβ 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.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

“…Melatonin may efficiently mediate important PTMs that regulate proteins which can form physiological condensates or pathological prion-like aggregates due to its ability to protect mitochondrial and cytoplasmic ATP levels and maintain requisite RNA concentration, which not only ensure proper formation and dissolution of condensates [ 125 ] but possibly also modulate reentrant phase transitions that are important biochemical timekeeping RNA-dependent transformations where increased RNA dissolves condensates to return to an identical or macroscopically similar state before the phase transition [ 324 ]. Since prion targeting of lipid rafts [ 272 , 325 , 326 ] can affect membrane signaling [ 327 , 328 ] and lipid composition [ 329 ], the role of melatonin in the prevention of lipid peroxidation, modification of lipid hydrocarbon chain to promote phase separation in ternary membrane models [ 330 , 331 ], stabilizing lipid liquid ordered (L o ) to liquid disordered (L d ) phase separation over a range of temperatures [ 332 ], and displacing cholesterol in competitive binding to lipid molecules [ 330 ] provides additional insight into the complex relationship between melatonin and prion physiological and potential pathological conversion mediated by phase separation and associated processes.…”

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

“…In 2005, melatonin was first observed to induce phase-separation in DPPC lipid bilayers [ 318 ]; recently, melatonin has been observed to modify lipid hydrocarbon chain order to promote phase separation in ternary membrane models [ 319 ]. Due to a preference to localize at membrane interfaces [ 320 ], melatonin can form strong hydrogen bonds with membrane lipid anionic headgroups that could significantly modulate lipid acyl chain flexibility and lipid dynamics [ 318 ].…”

“…Due to a preference to localize at membrane interfaces [ 320 ], melatonin can form strong hydrogen bonds with membrane lipid anionic headgroups that could significantly modulate lipid acyl chain flexibility and lipid dynamics [ 318 ]. Melatonin is able to directly interact with cholesterol [ 321 ] and displaced cholesterol due to competitive binding to lipid molecules, increasing disorder in the L d phase to drive cholesterol into the ordered L o phase [ 319 ]. These subtle changes in lipid nanodomains can profoundly affect amyloid processing at membrane sites.…”

“…Increased unesterified cholesterol in membranes from tau fibrillation [ 721 ] may cause L c domains to become more fluid, mechanistically deforming membrane bilayer structure [ 724 ]. Melatonin has been reported to interact with cholesterol [ 321 ], binding competitively to lipid molecules to displace cholesterol [ 319 ]. L c domains that have become more fluid will transition into L o lipid raft domains [ 721 , 724 ] that could potentially potentiate the formation of enlarged lipid rafts (inflammarafts) [ 196 , 198 ].…”

“…Melatonin may regulate lipid dynamics and composition, modifying the lipid hydrocarbon chain order to promote phase separation in ternary membrane models [ 318 , 319 ], as well as preserving nanoscopic lipid raft domains by stabilizing lipid L o –L d phase separation over a range of temperatures [ 350 ]. The ability of melatonin to penetrate and re-order lipids in membranes provides insight into its neuroprotective effects against tauopathy.…”