Solubilization of G Protein-Coupled Receptors

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.

Membrane Dipole Potential: An Emerging Approach to Explore Membrane Organization and Function

Biological membranes are complex organized molecular assemblies of lipids and proteins that provide cells and membrane-bound intracellular organelles their individual identities by morphological compartmentalization. Membrane dipole potential originates from the electrostatic potential difference within the membrane due to the nonrandom arrangement (orientation) of amphiphile and solvent (water) dipoles at the membrane interface. In this Feature Article, we will focus on the measurement of dipole potential using electrochromic fluorescent probes and highlight interesting applications. In addition, we will focus on ratiometric fluorescence microscopic imaging technique to measure dipole potential in cellular membranes, a technique that can be used to address novel problems in cell biology which are otherwise difficult to address using available approaches. We envision that membrane dipole potential could turn out to be a convenient tool in exploring the complex interplay between membrane lipids and proteins and could provide novel insights in membrane organization and function.

Insights into cellular signaling from membrane dynamics

Biological membranes allow morphological compartmentalization of cells and represent complex micro-heterogeneous fluids exhibiting a range of dynamics. The plasma membrane occupies a central place in cellular signaling which allows the cell to perform a variety of functions. In this review, we analyze cellular signaling in a dynamic biophysical framework guided by the "mobile receptor hypothesis". We describe a variety of examples from literature in which lateral diffusion of signaling membrane proteins acts as an important determinant in the efficiency of signaling. A major focus in our review is on membrane-embedded G protein-coupled receptors (GPCRs) which act as cellular signaling hubs for diverse cellular functions. Taken together, we describe a dynamics-based signaling paradigm with chosen examples from literature to elucidate how such a paradigm helps us understand signaling by GPCRs, maintenance of cellular polarity in yeast and infection by pathogens. We envision that with further technological advancement, it would be possible to explore cellular signaling more holistically as cells undergo development, differentiation and aging, thereby providing us a robust window into the dynamics of the cellular interior and its functional correlates.

Structure, dynamics and lipid interactions of serotonin receptors: excitements and challenges

Serotonin (5-hydroxytryptamine, 5-HT) is an intrinsically fluorescent neurotransmitter found in organisms spanning a wide evolutionary range. Serotonin exerts its diverse actions by binding to distinct cell membrane receptors which are classified into many groups. Serotonin receptors are involved in regulating a diverse array of physiological signaling pathways and belong to the family of either G protein-coupled receptors (GPCRs) or ligand-gated ion channels. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions such as sleep, mood, pain, anxiety, depression, aggression, and learning. Serotonin receptors act as drug targets for a number of diseases, particularly neuropsychiatric disorders. The signaling mechanism and efficiency of serotonin receptors depend on their amazing ability to rapidly access multiple conformational states. This conformational plasticity, necessary for the wide variety of functions displayed by serotonin receptors, is regulated by binding to various ligands. In this review, we provide a succinct overview of recent developments in generating and analyzing high-resolution structures of serotonin receptors obtained using crystallography and cryo-electron microscopy. Capturing structures of distinct conformational states is crucial for understanding the mechanism of action of these receptors, which could provide important insight for rational drug design targeting serotonin receptors. We further provide emerging information and insight from studies on interactions of membrane lipids (such as cholesterol) with serotonin receptors. We envision that a judicious combination of analysis of high-resolution structures and receptor-lipid interaction would allow a comprehensive understanding of GPCR structure, function and dynamics, thereby leading to efficient drug discovery.

A Critical Analysis of Molecular Mechanisms Underlying Membrane Cholesterol Sensitivity of GPCRs

G protein-coupled receptors (GPCRs) are the largest and a diverse family of proteins involved in signal transduction across biological membranes. GPCRs mediate a wide range of physiological processes and have emerged as major targets for the development of novel drug candidates in all clinical areas. Since GPCRs are integral membrane proteins, regulation of their organization, dynamics, and function by membrane lipids, in particular membrane cholesterol, has emerged as an exciting area of research. Cholesterol sensitivity of GPCRs could be due to direct interaction of cholesterol with the receptor (specific effect). Alternately, GPCR function could be influenced by the effect of cholesterol on membrane physical properties (general effect). In this review, we critically analyze the specific and general mechanisms of the modulation of GPCR function by membrane cholesterol, taking examples from representative GPCRs. While evidence for both the proposed mechanisms exists, there appears to be no clear-cut distinction between these two mechanisms, and a combination of these mechanisms cannot be ruled out in many cases. We conclude that classifying the mechanism underlying cholesterol sensitivity of GPCR function merely into these two mutually exclusive classes could be somewhat arbitrary. A more holistic approach could be suitable for analyzing GPCR-cholesterol interaction.

Surveying GPCR solubilisation conditions using surface plasmon resonance

Biophysical screening techniques, such as surface plasmon resonance, enable detailed kinetic analysis of ligands binding to solubilised G-protein coupled receptors. The activity of a receptor solubilised out of the membrane is crucially dependent on the environment in which it is suspended. Finding the right conditions is challenging due to the number of variables to investigate in order to determine the optimum solubilisation buffer for any given receptor. In this study we used surface plasmon resonance technology to screen a variety of solubilisation conditions including buffers and detergents for two model receptors: CXCR4 and CCR5. We tested 950 different combinations of solubilisation conditions for both receptors. The activity of both receptors was monitored by using conformation dependent monoclonal antibodies and the binding of small molecule ligands. Despite both receptors belonging to the chemokine receptor family they show some differences in their preference for solubilisation conditions that provide the highest level of binding for both the conformation dependent antibodies and small molecules. The study described here is focused not only on finding the best solubilisation conditions for each receptor, but also on factors that determine the sensitivity of the assay for each receptor. We also suggest how these data about different buffers and detergents can be used as a guide for selecting solubilisation conditions for other membrane proteins.

Structural Stringency of Cholesterol for Membrane Protein Function Utilizing Stereoisomers as Novel Tools: A Review

Cholesterol is an important lipid in the context of membrane protein function. The function of a number of membrane proteins, including G protein-coupled receptors (GPCRs) and ion channels, has been shown to be dependent on membrane cholesterol. However, the molecular mechanism underlying such regulation is still being explored. In some cases, specific interaction between cholesterol and the protein has been implicated. In other cases, the effect of cholesterol on the membrane properties has been attributed for the regulation of protein function. In this article, we have provided an overview of experimental approaches that are useful for determining the degree of structural stringency of cholesterol for membrane protein function. In the process, we have highlighted the role of immediate precursors in cholesterol biosynthetic pathway in the function of membrane proteins. Special emphasis has been given to the application of stereoisomers of cholesterol in deciphering the structural stringency required for regulation of membrane protein function. A comprehensive examination of these processes would help in understanding the molecular basis of cholesterol regulation of membrane proteins in subtle details.

Micellar dipole potential is sensitive to sphere-to-rod transition

Structural transitions involving shape changes play an important role in cellular physiology. Charged micelles offer a convenient model system in which structural transitions can be suitably induced by increasing the ionic strength of the medium. In this paper, we have explored sphere-to-rod transition in charged micelles of SDS and CTAB by monitoring micellar dipole potential using the dual wavelength ratiometric approach utilizing the potential-sensitive membrane probe di-8-ANEPPS. Our results show that micellar dipole potential is sensitive to sphere-to-rod transition in charged micelles. Micellar dipole potential exhibited increase with increasing ionic strength (salt), irrespective of the nature of micellar charge, implying considerable dipolar reorganization underlying structural transitions. We interpret the increase in dipole potential due to sphere-to-rod transition because of an increase in the population of confined (nonrandom) dipoles induced by micellar organizational change. This is due to the fact that dipole potential arises due to the nonrandom arrangement of micellar dipoles and water molecules at the micelle interface. Our results constitute one of the first reports describing drastic dipolar reorganization due to micellar shape (and size) change. We envision that dipole potential measurements could provide novel insights into micellar processes that are associated with dipolar reorganization.

Dipolar rearrangement during micellization explored using a potential-sensitive fluorescent probe

Dipole potential is the potential difference within the membrane bilayer, which originates due to the nonrandom arrangement of lipid dipoles and water molecules at the membrane interface. Although dipole potential is generally used in the context of bilayer membranes, the nonrandom arrangement of amphiphiles and water dipoles would also contribute to dipole potential in organized molecular assemblies such as micelles. In this work, we show that the process of micelle formation from monomers for a representative variety of detergents is associated with dipolar rearrangement. We monitor the dipolar reorganization upon micellization as a change in dipole potential, measured by the dual wavelength ratiometric approach utilizing the potential-sensitive membrane probe di-8-ANEPPS. We further utilized this phenomenon to estimate the critical micelle concentration (CMC) of a variety of detergents. CMC determined by this method are in overall agreement with the literature values of CMC for these detergents. To the best of our knowledge, these results constitute the first report showing dipolar reorientation during micellization. We conclude that dipole potential measurements could provide a novel approach to explore micellar organization.