Functional characterization and potential applications for enhanced green fluorescent protein- and epitope-fused human M1 muscarinic receptors

Unraveling a molecular determinant for clathrin-independent internalization of the M2 muscarinic acetylcholine receptor

Endocytosis and postendocytic sorting of G-protein-coupled receptors (GPCRs) is important for the regulation of both their cell surface density and signaling profile. Unlike the mechanisms of clathrin-dependent endocytosis (CDE), the mechanisms underlying the control of GPCR signaling by clathrin-independent endocytosis (CIE) remain largely unknown. Among the muscarinic acetylcholine receptors (mAChRs), the M4 mAChR undergoes CDE and recycling, whereas the M2 mAChR is internalized through CIE and targeted to lysosomes. Here we investigated the endocytosis and postendocytic trafficking of M2 mAChR based on a comparative analysis of the third cytoplasmic domain in M2 and M4 mAChRs. For the first time, we identified that the sequence (374)KKKPPPS(380) servers as a sorting signal for the clathrin-independent internalization of M2 mAChR. Switching (374)KKKPPPS(380) to the i3 loop of the M4 mAChR shifted the receptor into lysosomes through the CIE pathway; and therefore away from CDE and recycling. We also found another previously unidentified sequence that guides CDE of the M2 mAChR, (361)VARKIVKMTKQPA(373), which is normally masked in the presence of the downstream sequence (374)KKKPPPS(380). Taken together, our data indicate that endocytosis and postendocytic sorting of GPCRs that undergo CIE could be sequence-dependent.

Fluorescent derivatives of AC-42 to probe bitopic orthosteric/allosteric binding mechanisms on muscarinic M1 receptors

Two fluorescent derivatives of the M1 muscarinic selective agonist AC-42 were synthesized by coupling the lissamine rhodamine B fluorophore (in ortho and para positions) to AC42-NH(2). This precursor, prepared according to an original seven-step procedure, was included in the study together with the LRB fluorophore (alone or linked to an alkyl chain). All these compounds are antagonists, but examination of their ability to inhibit or modulate orthosteric [(3)H]NMS binding revealed that para-LRB-AC42 shared several properties with AC-42. Carefully designed experiments allowed para-LRB-AC42 to be used as a FRET tracer on EGFP-fused M1 receptors. Under equilibrium binding conditions, orthosteric ligands, AC-42, and the allosteric modulator gallamine behaved as competitors of para-LRB-AC42 binding whereas other allosteric compounds such as WIN 51,708 and N-desmethylclozapine were noncompetitive inhibitors. Finally, molecular modeling studies focused on putative orthosteric/allosteric bitopic poses for AC-42 and para-LRB-AC42 in a 3D model of the human M1 receptor.

Oligomeric size of the m2 muscarinic receptor in live cells as determined by quantitative fluorescence resonance energy transfer

Fluorescence resonance energy transfer (FRET), measured by fluorescence intensity-based microscopy and fluorescence lifetime imaging, has been used to estimate the size of oligomers formed by the M(2) muscarinic cholinergic receptor. The approach is based on the relationship between the apparent FRET efficiency within an oligomer of specified size (n) and the pairwise FRET efficiency between a single donor and a single acceptor (E). The M(2) receptor was fused at the N terminus to enhanced green or yellow fluorescent protein and expressed in Chinese hamster ovary cells. Emission spectra were analyzed by spectral deconvolution, and apparent efficiencies were estimated by donor-dequenching and acceptor-sensitized emission at different ratios of enhanced yellow fluorescent protein-M(2) receptor to enhanced green fluorescent protein-M(2) receptor. The data were interpreted in terms of a model that considers all combinations of donor and acceptor within a specified oligomer to obtain fitted values of E as follows: n = 2, 0.495 +/- 0.019; n = 4, 0.202 +/- 0.010; n = 6, 0.128 +/- 0.006; n = 8, 0.093 +/- 0.005. The pairwise FRET efficiency determined independently by fluorescence lifetime imaging was 0.20-0.24, identifying the M(2) receptor as a tetramer. The strategy described here yields an explicit estimate of oligomeric size on the basis of fluorescence properties alone. Its broader application could resolve the general question of whether G protein-coupled receptors exist as dimers or larger oligomers. The size of an oligomer has functional implications, and such information can be expected to contribute to an understanding of the signaling process.

Characterization of endogenous calcium responses in neuronal cell lines

An increasing number of putative therapeutic targets have been identified in recent years for the treatment of neuronal pathophysiologies including pain, epilepsy, stroke and schizophrenia. Many of these targets signal through calcium (Ca(2+)), either by directly facilitating Ca(2+) influx through an ion channel, or through activation of G proteins that couple to intracellular Ca(2+) stores or voltage-gated Ca(2+) channels. Immortalized neuronal cell lines are widely used models to study neuropharmacology. However, systematic pharmacological characterization of the receptors and ion channels expressed in these cell lines is lacking. In this study, we systematically assessed endogenous Ca(2+) signaling in response to addition of agonists at potential therapeutic targets in a range of cell lines of neuronal origin (ND7/23, SH-SY5Y, 50B11, F11 and Neuro2A cells) as well as HEK293 cells, a cell line commonly used for over-expression of receptors and ion channels. This study revealed a remarkable diversity of endogenous Ca(2+) responses in these cell lines, with one or more cell lines responding to addition of trypsin, bradykinin, ATP, nicotine, acetylcholine, histamine and neurotensin. Subtype specificity of these responses was inferred from agonist potency and the effect of receptor subtype specific antagonist. Surprisingly, HEK293 and SH-SY5Y cells responded to the largest number of agonists with potential roles in neuronal signaling. These findings have implications for the heterologous expression of neuronal receptors and ion channels in these cell lines, and highlight the potential of neuron-derived cell lines for the study of a range of endogenously expressed receptors and ion channels that signal through Ca(2+).

Pirenzepine promotes the dimerization of muscarinic M1 receptors through a three-step binding process

Ligand binding to G protein-coupled receptors is a complex process that involves sequential receptor conformational changes, ligand translocation, and possibly ligand-induced receptor oligomerization. Binding events at muscarinic acetylcholine receptors are usually interpreted from radioligand binding studies in terms of two-step ligand-induced receptor isomerization. We report here, using a combination of fluorescence approaches, on the molecular mechanisms for Bodipy-pirenzepine binding to enhanced green fluorescent protein (EGFP)-fused muscarinic M1 receptors in living cells. Real time monitoring, under steady-state conditions, of the strong fluorescence energy transfer signal elicited by this interaction permitted a fine kinetic description of the binding process. Time-resolved fluorescence measurements allowed us to identify discrete EGFP lifetime species and to follow their redistribution upon ligand binding. Fluorescence correlation spectroscopy, with EGFP brightness analysis, showed that EGFP-fused muscarinic M1 receptors predominate as monomers in the absence of ligand and dimerize upon pirenzepine binding. Finally, all these experimental data could be quantitatively reconciled into a three-step mechanism, with four identified receptor conformational states. Fast ligand binding to a peripheral receptor site initiates a sequence of conformational changes that allows the ligand to access to inner regions of the protein and drives ligand-receptor complexes toward a high affinity dimeric state.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

Fluorescence resonance energy transfer to probe human M1 muscarinic receptor structure and drug binding properties

Human M1 muscarinic receptor chimeras were designed (i) to allow detection of their interaction with the fluorescent antagonist pirenzepine labelled with Bodipy [558/568], through fluorescence resonance energy transfer, (ii) to investigate the structure of the N-terminal extracellular moiety of the receptor and (iii) to set up a fluorescence-based assay to identify new muscarinic ligands. Enhanced green (or yellow) fluorescent protein (EGFP or EYFP) was fused, through a linker, to a receptor N-terminus of variable length so that the GFP barrel was separated from the receptor first transmembrane domain by six to 33 amino-acids. Five fluorescent constructs exhibit high expression levels as well as pharmacological and functional properties superimposable on those of the native receptor. Bodipy-pirenzepine binds to the chimeras with similar kinetics and affinities, indicating a similar mode of interaction of the ligand with all of them. From the variation in energy transfer efficiencies determined for four different receptor-ligand complexes, relative donor (EGFP)-acceptor (Bodipy) distances were estimated. They suggest a compact architecture for the muscarinic M1 receptor amino-terminal domain which may fold in a manner similar to that of rhodopsin. Finally, this fluorescence-based assay, prone to miniaturization, allows reliable detection of unlabelled competitors.

Subcellular compartmentalization of activation and desensitization of responses mediated by NK2 neurokinin receptors

A functional fluorescent neurokinin NK2 receptor was constructed by joining enhanced green fluorescent protein to the amino-terminal end of the rat NK2 receptor and was expressed in human embryonic kidney cells. On cell suspensions, the binding of fluorescent Bodipy-labeled neurokinin A results in a saturatable and reversible decrease of NK2 receptor fluorescence via fluorescence resonance energy transfer. This can be quantified for nM to microM agonist concentrations and monitored in parallel with intracellular calcium responses. On single cells, receptor site occupancy and local agonist concentration can be determined in real time from the decrease in receptor fluorescence. Simultaneous measurement of intracellular calcium responses and agonist binding reveals that partial receptor site occupancy is sufficient to desensitize cellular response to a second agonist application to the same membrane area. Subsequent stimulation of a distal membrane area leads to a second response to agonist, provided that it had not been exposed to agonist during the first application. Together with persistent translocation of fluorescent protein kinase C to the membrane area exposed to agonist, the present data support that not only homologous desensitization but also heterologous desensitization of NK2 receptors is compartmentalized to discrete membrane domains.