Location and Orientation of Serotonin Receptor 1a Agonists in Model and Complex Lipid Membranes

Abstract

Magic angle spinning (MAS) NMR has been used to investigate the location and orientation of five serotonin receptor 1a agonists (serotonin, buspirone, quipazine, 8-OH-DPAT, and LY-163,165) in single component model lipid and brain lipid membranes. The agonist locations are probed by monitoring changes in the lipid proton chemical shifts and by MAS-assisted nuclear Overhauser enhancement spectroscopy, which indicates the orientation of the agonists with respect to the 1,2-dioleoyl-sn-glycero-3-phosphocholine lipids. In the single component bilayer, the membrane agonists are found predominantly in the top of the hydrophobic chain or in the glycerol region of the membrane. Most of the agonists orient approximately parallel to the membrane plane, with the exception of quipazine, whose piperazine ring is found in the glycerol region, whereas its benzene ring is located within the lipid hydrophobic chain. The location of the agonist in brain lipid membranes is similar to the 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayers; however, many of the agonists appear to locate close to the cholesterol in the membrane in preference to the phospholipids.Drug molecule interaction with lipid membranes is a critical factor governing its final activity, because it needs to negotiate its way through several membranes on the way to its receptor protein. In an effort to facilitate the design of drugs, water/ octanol partitioning coefficients are often used. However, this simple two-phase model falls short of adequately describing the complexity of lipid/drug interactions. A lipid membrane is a chemically very diverse environment. Lipid headgroups are often charged and highly hydrated, and the interface between the headgroup and hydrophobic core of a membrane is typified by the presence of glycerol, carbonyl groups, and lower concentrations of water. It is only the very center of the membrane that is nonpolar and excludes water (1, 2).Drug molecules tend to contain substituted aromatic groups that add further complications; steric effects exclude aromatic groups from the lipid chain region (3), whereas other attractive forces between the ring and lipid carbonyls draw the molecule to the interface between the chain and headgroup regions of the membrane (4, 5). Furthermore, any charged chemical groups will be excluded from the low dielectric environment at the center of the bilayer (2). Consequently, the mechanism by which a molecule partitions into lipid bilayers or crosses a biological membrane will be a complex interplay of many factors.The majority of drug targets are membrane proteins. Hence the interaction of a drug with the membrane is crucial for its efficacy and involves several interesting aspects, such as its location probability profile across the membrane bilayer, its orientation, and its structure. A high location probability in a particular part of a membrane and its orientation with respect to the membrane normal could well be important to how the drug is presented to the binding site of the target protein (6).Fin...

Cholesterol and Lipid Phases Influence the Interactions between Serotonin Receptor Agonists and Lipid Bilayers

“…The locations of quipazine and LY-165,163 in DPPC broadly agree with our previous studies conducted in dioleoyl-sn-glycero-3-phosphocholine (DOPC) (17); the aromatic ring at one end of the quipazine molecule has a maximum location probability in the chain region of the bilayer, whereas the other end locates in the interface region, indicating that it has a net orientation parallel to the lipids. In the case of LY-165,163, the maximum location probability is the same as reported previously in DOPC, but in DPPC, it is much more tightly distributed around the chain/glycerol regions.…”

“…Assignments of the agonist signals were performed previously (17). However, for the sake of simplicity, only two peaks for each will be used for the analysis.…”

“…A particularly well studied mixture consists of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Proton NMR and specifically magic angle spinning-assisted nuclear Overhauser enhancement spectroscopy (MAS-NOESY) experiments have proven to be an excellent tool for investigating, with atomic resolution, the location of small molecules embedded in lipid membranes (17)(18)(19)(20). However, these studies have been limited to binary mixtures because anything more complicated results in serious spectral overlap.…”

“…The work presented here follows from a study in which it was noticed that several of the agonists induced significant chemical shifts in the cholesterol signals of a brain lipid extract (17 phases, shedding light on how cholesterol and bilayer phases influence the interactions between bilayer components. Finally we have also analyzed the effect that agonist have on the phase behavior of the lipid bilayers, and we comment on how this may provide an alternative route of action for drugs.…”

“…We concentrate on quipazine and LY-165,163 ( Fig. 1) because they previously showed strong interactions with cholesterol (17).…”