Interactions between amyloid beta peptide and lipid membrane models with complex cis-[Ru(phen)2(3,4Apy)2]2+ as a luminescent probe

Abstract

The neurotoxicity of oligomeric species of amyloid beta (OA²) peptide has been linked to damage caused to cell membranes by pore formation through interactions with cholesterol and ganglioside lipids. Consequently, studies of compounds that act directly on OA² at lipid membrane interfaces has enlarged considerably. Recently, we have demonstrated that the luminescent cis-[Ru(phen)2(3,4-Apy)]2+ complex (RuApy, phen = 1,10-phenantroline, 3,4-Apy = 3,4-diaminopyridine) prepared in our laboratory protects PC12 cells against the toxicity of A²1-40 pore-like oligomeric species by changing the aggregation mechanism to a nontoxic pathway by forming micelle-like conformations. To continue these studies, the next question that arises is whether RuApy would have the same behavior when OA² is in contact with cell membranes. To answer this question, it is important to understand the behavior of monomeric and fibril forms of Ab in the presence of model lipid membranes, as well as its effect on membrane integrity and how RuApy affects these interactions. This is the focus of this project. As phospholipids are non-fluorescent, changes in the luminescence responses of RuApy can be used to probe the interactions of Ab with model membrane using spectroscopic techniques. Herein, with the aid of steady-state and time-resolved fluorescence spectroscopy, optical microscopy, pH-sensitive dyes and measurements using Langmuir monolayer we intend to systematically study the interaction between different fragments of Amyloid beta peptide and model lipid membranes (SUVs and SLBs) and the influence of luminescent RuApy complex. Since the RuApy complex is positively charged, we will explore the effects of zwitterionic as well as negatively charged phospholipids as well as additional membrane components like gangliosides and cholesterol. In order to explore the interactions of Ab with supported lipid bilayers (SLBs), we will use the direct luminescence responses of the RuApy complex (lem = 650 nm, tem= 200 ns). We will also indirectly probe this interaction by pH modulation sensing with organic dyes (lem = 590 nm, tem= 4-10 ns). Additionally, as the RuApy complex and organic dyes present characteristic emissive responses, we can also monitor the fluorescence signal changes of the organic dye and the RuApy phosphorescence simultaneously during their interaction with the SLB membranes, thereby enabling differentiation and independent analysis of association with the membranes. Moreover, quantitative information on binding can be obtained by making measurements as a function of Ab concentration and abstracting a Langmuir binding isotherm. Detailed information on the molecular level interactions between the monomeric and fibril forms of Ab and the lipid membranes and how RuApy affects these interactions will be explored. This should help reveal details of the potential theranostic role played by RuApy in A²/lipid membrane interactions, which are linked to Alzheimer Disease (AD) pathogenesis. (AU)