A collage of cholesterol interaction motifs in the serotonin1A receptor: An evolutionary implication for differential cholesterol interaction

Synergistic and Competitive Lipid Interactions in the Serotonin1A Receptor Microenvironment

The interaction of lipids with G-protein-coupled receptors (GPCRs) has been shown to modulate and dictate several aspects of GPCR organization and function. Diverse lipid interaction sites have been identified from structural biology, bioinformatics, and molecular dynamics studies. For example, multiple cholesterol interaction sites have been identified in the serotonin_1A receptor, along with distinct and overlapping sphingolipid interaction sites. How these lipids interact with each other and what is the resultant effect on the receptor is still not clear. In this work, we have analyzed lipid-lipid crosstalk at the receptor of the serotonin_1A receptor embedded in a membrane bilayer that mimics the neuronal membrane composition by long coarse-grain simulations. Using a set of similarity coefficients, we classified lipids that bind at the receptor together as synergistic cobinding, and those that bind individually as competitive. Our results show that certain lipids interact with the serotonin_1A receptor in synergy with each other. Not surprisingly, the ganglioside GM1 and cholesterol show a synergistic cobinding, along with the relatively uncommon GM1-phosphatidylethanolamine (PE) and cholesterol-PE synergy. In contrast, certain lipid pairs such as cholesterol and sphingomyelin appear to be in competition at several sites, despite their coexistence in lipid nanodomains. In addition, we observed intralipid competition between two lipid tails, with the receptor exhibiting increased interactions with the unsaturated lipid tails. We believe our work represents an important step in understanding the diversity of GPCR-lipid interactions and exploring synergistic cobinding and competition in natural membranes.

Lysine 101 in the CRAC Motif in Transmembrane Helix 2 Confers Cholesterol-Induced Thermal Stability to the Serotonin1A Receptor

G protein-coupled receptors (GPCRs) constitute the largest class of membrane proteins that transduce signals across the plasma membrane and orchestrate a multitude of physiological processes within cells. The serotonin_1A receptor is a crucial neurotransmitter receptor in the GPCR family involved in a multitude of neurological, behavioral and cognitive functions. We have previously shown, using a combination of experimental and simulation approaches, that membrane cholesterol acts as a key regulator of organization, dynamics, signaling and endocytosis of the serotonin_1A receptor. In addition, we showed that membrane cholesterol stabilizes the serotonin_1A receptor against thermal deactivation. In the present work, we explored the molecular basis of cholesterol-induced thermal stability of the serotonin_1A receptor. For this, we explored the possible role of the K101 residue in a cholesterol recognition/interaction amino acid consensus (CRAC) motif in transmembrane helix 2 in conferring the thermal stability of the serotonin_1A receptor. Our results show that a mutation in the K101 residue leads to loss in thermal stability of the serotonin_1A receptor imparted by cholesterol, independent of membrane cholesterol content. We envision that our results could have potential implications in structural biological advancements of GPCRs and design of thermally stabilized receptors for drug development.

Mechanisms of selective G protein-coupled receptor localization and trafficking

The trafficking of G protein-coupled receptors (GPCRs) to different membrane compartments has recently emerged as being a critical determinant of the signaling profiles of activation. GPCRs, which share many structural and functional similarities, also share many mechanisms that traffic them between compartments. This sharing raises the question of how the trafficking of individual GPCRs is selectively regulated. Here, we will discuss recent studies addressing the mechanisms that contribute to selectivity in endocytic and biosynthetic trafficking of GPCRs.

Impact of Cholesterol Concentration and Lipid Phase on Structure and Fluctuation of Amyloid Precursor Protein

Elevated levels of cellular cholesterol have been identified as one factor contributing to the onset of Alzheimer's disease (AD). Specific interaction between cholesterol and the amyloid precursor protein (APP), investigated via NMR experiments and computational studies, has been proposed to play a critical role in the processing of APP by secretases and the biogenesis of amyloid-β (Aβ) protein. We present all-atom molecular dynamics simulations of the 40-residue congener of the C-terminal domain of APP, C9916-55 (C99), in cholesterol-enriched DMPC lipid bilayers. We investigated the effect of cholesterol concentration on the conformational ensemble of wild-type C99 and C99-cholesterol associations at the low pH of endosomal environments, at which residues E22 and D23 are neutral. C99 was also characterized in liquid ordered domains for Dutch (E22Q) and Iowa (D23N) Familial AD mutants at low pH and for the wild-type sequence using protonation states characteristic of neutral pH. Our results reproduce the equilibrium constant of past NMR characterizations of the C99-cholesterol interaction but are not consistent with the C99-cholesterol binding hypothesis. We find that the lifetimes of both DMPC and cholesterol complexed with C99 display a power-law distribution of residence lifetimes. Longer-lived C99-DMPC and C99-cholesterol complexes are primarily stabilized by salt bridges and hydrogen bonds of lysine amines to phosphate and hydroxyl groups. Nevertheless, specific interfaces for C99-cholesterol association which are not present for DMPC can be identified. Changes to C99-cholesterol interfaces are found to depend on C99 tilt angle and orientation of the juxtamembrane domain of C99 containing residues E22 and D23. These observations support a more nuanced view of the C99-cholesterol interaction than has previously been suggested. We propose that cholesterol modulates the conformation and activity of C99 and other small transmembrane proteins indirectly through induction of the liquid ordered phase and directly through hydrogen bonding. This suggests a critical role for membrane heterogeneity introduced by cholesterol in modulating the structural ensemble of C99 and the production of Aβ.

Molecular evolution of a collage of cholesterol interaction motifs in transmembrane helix V of the serotonin1A receptor

The human serotonin_1A receptor is a representative member of the superfamily of G protein-coupled receptors (GPCRs) and an important drug target for neurological disorders. Using a combination of biochemical, biophysical and molecular dynamics simulation approaches, we and others have shown that membrane cholesterol modulates the organization, dynamics and function of vertebrate serotonin_1A receptors. Previous studies have shown that the cytoplasmic portion of transmembrane helix V (TM V) and the extramembraneous intracellular loop 3 are critical for G-protein coupling, phosphorylation and desensitization of the receptor. We have recently resolved a collage of putative cholesterol interaction motifs from the amino acid sequence overlapping this region. In this paper, we explore the sequence plasticity of this fragment that may have adapted to altered membrane lipidome, after vertebrates evolved from primordial invertebrates. Since invertebrates have lower levels of membrane cholesterol relative to vertebrates, we compared TM V sequence fragments from invertebrate serotonin₁ receptors with vertebrate orthologs to infer the sequence plasticity in TM V. We report that the average number of cholesterol interaction motifs in TM V for diverse phyla represents an increasing trend that could mirror vertebrate evolution from primordial invertebrates. By statistical modeling, we propose that the collage of cholesterol interaction motifs in TM V of the human serotonin_1A receptor may have evolved from rudimentary collages, reminiscent of primordial invertebrate orthologs. Taken together, we propose that a repertoire of cholesterol-philic nonsynonymous substitutions may have enhanced collage complexity in TM V during vertebrate evolution.

Cholesterol interaction motifs in G protein-coupled receptors: Slippery hot spots?

G protein-coupled receptors (GPCRs) are cell membrane associated signaling hubs that orchestrate a multitude of cellular functions upon binding to a diverse variety of extracellular ligands. Since GPCRs are integral membrane proteins with seven-transmembrane domain architecture, their function, organization and dynamics are intimately regulated by membrane lipids, such as cholesterol. Cholesterol is an extensively studied lipids in terms of its effects on GPCR structure and function. One of the possible mechanisms underlying modulation of GPCR function by cholesterol is via specific interaction of GPCRs with membrane cholesterol. These interactions of GPCRs with membrane cholesterol are often attributed to structural features of GPCRs that could facilitate their preferential association with cholesterol. In this backdrop, cholesterol interaction motifs represent putative interaction sites on GPCRs that could facilitate cholesterol-sensitive function of these receptors. In this review, we provide an overview of cholesterol interaction motifs found in GPCRs, which have been identified through a combination of crystallography, bioinformatics analysis, and functional studies. In addition, we will highlight, using specific examples, why mere presence of a cholesterol interaction motif at a given site may not directly implicate its role in interaction with membrane cholesterol. We therefore believe that experimental approaches, followed by functional analysis of cholesterol sensitivity of GPCRs, would provide a better understanding of the role played by these motifs in cholesterol-sensitive function. We envision that a comprehensive knowledge of cholesterol interaction sites in GPCRs would allow us to develop a better understanding of GPCR structure-function paradigm, and could be useful in future therapeutics. This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Analytical and Computational Methods > Computational Methods Laboratory Methods and Technologies > Macromolecular Interactions, Methods.

A Perspective: Active Role of Lipids in Neurotransmitter Dynamics

Synaptic neurotransmission is generally considered as a function of membrane-embedded receptors and ion channels in response to the neurotransmitter (NT) release and binding. This perspective aims to widen the protein-centric view by including another vital component—the synaptic membrane—in the discussion. A vast set of atomistic molecular dynamics simulations and biophysical experiments indicate that NTs are divided into membrane-binding and membrane-nonbinding categories. The binary choice takes place at the water-membrane interface and follows closely the positioning of the receptors’ binding sites in relation to the membrane. Accordingly, when a lipophilic NT is on route to a membrane-buried binding site, it adheres on the membrane and, then, travels along its plane towards the receptor. In contrast, lipophobic NTs, which are destined to bind into receptors with extracellular binding sites, prefer the water phase. This membrane-based sorting splits the neurotransmission into membrane-independent and membrane-dependent mechanisms and should make the NT binding into the receptors more efficient than random diffusion would allow. The potential implications and notable exceptions to the mechanisms are discussed here. Importantly, maintaining specific membrane lipid compositions (MLCs) at the synapses, especially regarding anionic lipids, affect the level of NT-membrane association. These effects provide a plausible link between the MLC imbalances and neurological diseases such as depression or Parkinson’s disease. Moreover, the membrane plays a vital role in other phases of the NT life cycle, including storage and release from the synaptic vesicles, transport from the synaptic cleft, as well as their synthesis and degradation.