Regulation of G-protein coupled receptor traffic by an evolutionary conserved hydrophobic signal

Inhibiting Intracellular α2C-Adrenoceptor Surface Translocation Using Decoy Peptides: Identification of an Essential Role of the C-Terminus in Receptor Trafficking

The G protein-coupled α2-adrenoceptor subtype C (abbreviated α2C-AR) has been implicated in peripheral vascular conditions and diseases such as cold feet-hands, Raynaud's phenomenon, and scleroderma, contributing to morbidity and mortality. Microvascular α2C-adrenoceptors are expressed in specialized smooth muscle cells and mediate constriction under physiological conditions and the occlusion of blood supply involving vasospastic episodes and tissue damage under pathological conditions. A crucial step for receptor biological activity is the cell surface trafficking of intracellular receptors, triggered by cAMP-Epac-Rap1A GTPase signaling, which involves protein-protein association with the actin-binding protein filamin-2, mediated by critical amino acid residues in the last 14 amino acids of the receptor carboxyl (C)-terminus. This study assessed the role of the C-terminus in Rap1A GTPase coupled receptor trafficking by domain-swapping studies using recombinant tagged receptors in transient co-transfections and compared with wild-type receptors using immunofluorescence microscopy. We further tested the biological relevance of the α2C-AR C-terminus, when introduced as competitor peptides, to selectively inhibit intracellular α2C-AR surface translocation in transfected as well as in microvascular smooth muscle cells expressing endogenous receptors. These studies contribute to establishing proof of principle to target intracellular α2C-adrenoceptors to reduce biological activity, which in clinical conditions can be a target for therapy.

Exploring the eukaryotic Yip and REEP/Yop superfamily of membrane-shaping adapter proteins (MSAPs): A cacophony or harmony of structure and function?

Polytopic cargo proteins are synthesized and exported along the secretory pathway from the endoplasmic reticulum (ER), through the Golgi apparatus, with eventual insertion into the plasma membrane (PM). While searching for proteins that could enhance cell surface expression of olfactory receptors, a new family of proteins termed “receptor expression-enhancing proteins” or REEPs were identified. These membrane-shaping hairpin proteins serve as adapters, interacting with intracellular transport machinery, to regulate cargo protein trafficking. However, REEPs belong to a larger family of proteins, the Yip (Ypt-interacting protein) family, conserved in yeast and higher eukaryotes. To date, eighteen mammalian Yip family members, divided into four subfamilies (Yipf, REEP, Yif, and PRAF), have been identified. Yeast research has revealed many intriguing aspects of yeast Yip function, functions that have not completely been explored with mammalian Yip family members. This review and analysis will clarify the different Yip family nomenclature that have encumbered prior comparisons between yeast, plants, and eukaryotic family members, to provide a more complete understanding of their interacting proteins, membrane topology, organelle localization, and role as regulators of cargo trafficking and localization. In addition, the biological role of membrane shaping and sensing hairpin and amphipathic helical domains of various Yip proteins and their potential cellular functions will be described. Lastly, this review will discuss the concept of Yip proteins as members of a larger superfamily of membrane-shaping adapter proteins (MSAPs), proteins that both shape membranes via membrane-sensing and hairpin insertion, and well as act as adapters for protein-protein interactions. MSAPs are defined by their localization to specific membranes, ability to alter membrane structure, interactions with other proteins via specific domains, and specific interactions/effects on cargo proteins.

Targeting trafficking as a therapeutic avenue for misfolded GPCRs leading to endocrine diseases

G protein-coupled receptors (GPCRs) are plasma membrane proteins associated with an array of functions. Mutations in these receptors lead to a number of genetic diseases, including diseases involving the endocrine system. A particular subset of loss-of-function mutant GPCRs are misfolded receptors unable to traffic to their site of function (i.e. the cell surface plasma membrane). Endocrine disorders in humans caused by GPCR misfolding include, among others, hypo- and hyper-gonadotropic hypogonadism, morbid obesity, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, X-linked nephrogenic diabetes insipidus, congenital hypothyroidism, and familial glucocorticoid resistance. Several in vitro and in vivo experimental approaches have been employed to restore function of some misfolded GPCRs linked to endocrine disfunction. The most promising approach is by employing pharmacological chaperones or pharmacoperones, which assist abnormally and incompletely folded proteins to refold correctly and adopt a more stable configuration to pass the scrutiny of the cell's quality control system, thereby correcting misrouting. This review covers the most important aspects that regulate folding and traffic of newly synthesized proteins, as well as the experimental approaches targeted to overcome protein misfolding, with special focus on GPCRs involved in endocrine diseases.

Molecular Mechanism for Ligand Recognition and Subtype Selectivity of α2C Adrenergic Receptor

Adrenergic G-protein-coupled receptors (GPCRs) mediate different cellular signaling pathways in the presence of endogenous catecholamines and play important roles in both physiological and pathological conditions. Extensive studies have been carried out to investigate the structure and function of β adrenergic receptors (βARs). However, the structure of α adrenergic receptors (αARs) remains to be determined. Here, we report the structure of the human α_2C adrenergic receptor (α_2CAR) with the non-selective antagonist, RS79948, at 2.8 Å. Our structure, mutations, modeling, and functional experiments indicate that a α_2CAR-specific D206^ECL2-R409^ECL3-Y405^6.58 network plays a role in determining α₂ adrenergic subtype selectivity. Furthermore, our results show that a specific loosened helix at the top of TM4 in α_2CAR is involved in receptor activation. Together, our structure of human α_2CAR-RS79948 provides key insight into the mechanism underlying the α₂ adrenergic receptor activation and subtype selectivity.

Intracellular Trafficking of Gonadotropin Receptors in Health and Disease

Gonadotropin receptors belong to the highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily, the so-called Rhodopsin-like family (class A), which is the largest class of GPCRs and currently a major drug target. Both the follicle-stimulating hormone receptor (FSHR) and the luteinizing hormone/chorionic gonadotropin hormone receptor (LHCGR) are mainly located in the gonads where they play key functions associated to essential reproductive functions. As any other protein, gonadotropin receptors must be properly folded into a mature tertiary conformation compatible with quaternary assembly and endoplasmic reticulum export to the cell surface plasma membrane. Several primary and secondary structural features, including presence of particular amino acid residues and short motifs and in addition, posttranslational modifications, regulate intracellular trafficking of gonadotropin receptors to the plasma membrane as well as internalization and recycling of the receptor back to the cell surface after activation by agonist. Inactivating mutations of gonadotropin receptors may derive from receptor misfolding and lead to absent or reduced plasma membrane expression of the altered receptor, thereby manifesting an array of phenotypical abnormalities mostly characterized by reproductive failure and/or abnormal or absence of development of secondary sex characteristics. In this chapter we review the structural requirements necessary for intracellular trafficking of the gonadotropin receptors, and describe how mutations in these receptors may lead to receptor misfolding and disease in humans.

Molecular determinants of the human α2C-adrenergic receptor temperature-sensitive intracellular traffic

The human α2C-adrenergic receptor (α2C-AR) is localized intracellularly at physiologic temperature. Decreasing the environmental temperature strongly stimulates the receptor transport to the cell surface. In contrast, rat and mouse α2C-AR plasma membrane levels are less sensitive to decrease in temperature, whereas the opossum α2C-AR cell surface levels are not changed in these conditions. Structural analysis demonstrated that human α2C-AR has a high number of arginine residues in the third intracellular loop and in the C-terminus, organized as putative RXR motifs. Although these motifs do not affect the receptor subcellular localization at 37°C, deletion of the arginine clusters significantly enhanced receptor plasma membrane levels at reduced temperature. We found that this exaggerated transport of the human receptor is mediated by two functional arginine clusters, one in the third intracellular loop and one in the C-terminus. This effect is mediated by interactions with COPI vesicles, but not by 14-3-3 proteins. In rat α2C-AR, the arginine cluster from the third intracellular loop is shifted to the left due to three missing residues. Reinsertion of these residues in the rat α2C-AR restored the same temperature sensitivity as in the human receptor. Proteomic and coimmunoprecipitation experiments identified pontin as a molecule having stronger interactions with human α2C-AR compared with rat α2C-AR. Inhibition of pontin activity enhanced human receptor plasma membrane levels and signaling at 37°C. Our results demonstrate that human α2C-AR has a unique temperature-sensitive traffic pattern within the G protein-coupled receptor class due to interactions with different molecular chaperones, mediated in part by strict spatial localization of specific arginine residues.

Regulation of α2B-Adrenerigc Receptor Export Trafficking by Specific Motifs

Intracellular trafficking and precise targeting to specific locations of G protein-coupled receptors (GPCRs) control the physiological functions of the receptors. Compared to the extensive efforts dedicated to understanding the events involved in the endocytic and recycling pathways, the molecular mechanisms underlying the transport of the GPCR superfamily from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane are relatively less well defined. Over the past years, we have used α(2B)-adrenergic receptor (α(2B)-AR) as a model to define the factors that control GPCR export trafficking. In this chapter, we will review specific motifs identified to mediate the export of nascent α(2B)-AR from the ER and the Golgi and discuss the possible underlying mechanisms. As these motifs are highly conserved among GPCRs, they may provide common mechanisms for export trafficking of these receptors.

Common α2A and α2C adrenergic receptor polymorphisms do not affect plasma membrane trafficking

Various naturally occurring polymorphic forms of human G protein-coupled receptors (GPCRs) have been identified and linked to diverse pathological diseases, including receptors for vasopressin type 2 (nephrogenic diabetes insipidus) and gonadotropin releasing hormone (hypogonadotropic hypogonadism). In most cases, polymorphic amino acid mutations disrupt protein folding, altering receptor function as well as plasma membrane expression. Other pathological GPCR variants have been found that do not alter receptor function, but instead affect only plasma membrane trafficking (e.g., delta opiate and histamine type 1 receptors). Thus, altered membrane trafficking with retained receptor function may be another mechanism causing polymorphic GPCR dysfunction. Two common human α2A and α2C adrenergic receptor (AR) variants have been identified (α2A N251K and α2C Δ322-325 ARs), but pharmacological analysis of ligand binding and second messenger signaling has not consistently demonstrated altered receptor function. However, possible alterations in plasma membrane trafficking have not been investigated. We utilized a systematic approach previously developed for the study of GPCR trafficking motifs and accessory proteins to assess whether these α2 AR variants affected intracellular trafficking or plasma membrane expression. By combining immunofluorescent microscopy, glycosidic processing analysis, and quantitative fluorescent-activated cell sorting (FACS), we demonstrate that neither variant receptor had altered intracellular localization, glycosylation, nor plasma membrane expression compared to wild-type α2 ARs. Therefore, pathopharmacological properties of α2A N251K and α2C Δ322-325 ARs do not appear to be due to altered receptor pharmacology or plasma membrane trafficking, but may involve interactions with other intracellular signaling cascades or proteins.

Mutations in G protein-coupled receptors that impact receptor trafficking and reproductive function

G protein coupled receptors (GPCRs) are a large superfamily of integral cell surface plasma membrane proteins that play key roles in transducing extracellular signals, including sensory stimuli, hormones, neurotransmitters, or paracrine factors into the intracellular environment through the activation of one or more heterotrimeric G proteins. Structural alterations provoked by mutations or variations in the genes coding for GPCRs may lead to misfolding, altered plasma membrane expression of the receptor protein and frequently to disease. A number of GPCRs regulate reproductive function at different levels; these receptors include the gonadotropin-releasing hormone receptor (GnRHR) and the gonadotropin receptors (follicle-stimulating hormone receptor and luteinizing hormone receptor), which regulate the function of the pituitary-gonadal axis. Loss-of-function mutations in these receptors may lead to hypogonadotropic or hypergonadotropic hypogonadism, which encompass a broad spectrum of clinical phenotypes. In this review we describe mutations that provoke misfolding and failure of these receptors to traffick from the endoplasmic reticulum to the plasma membrane. We also discuss some aspects related to the therapeutic potential of some target-specific drugs that selectively bind to and rescue function of misfolded mutant GnRHR and gonadotropin receptors, and that represent potentially valuable strategies to treat diseases caused by inactivating mutations of these receptors.

REEP1 and REEP2 proteins are preferentially expressed in neuronal and neuronal-like exocytotic tissues

The six members of the Receptor Expression Enhancing Protein (REEP) family were originally identified based on their ability to enhance heterologous expression of olfactory receptors and other difficult to express G protein-coupled receptors. Interestingly, REEP1 mutations have been linked to neurodegenerative disorders of upper and lower motor neurons, hereditary spastic paraplegia (HSP) and distal hereditary motor neuropathy type V (dHMN-V). The closely related REEP2 isoform has not demonstrated any such disease linkage. Previous research has suggested that REEP1 mRNA is ubiquitously expressed in brain, muscle, endocrine, and multiple other organs, inconsistent with the neurodegenerative phenotype observed in HSP and dHMN-V. To more fully examine REEP1 expression, we developed and characterized a new REEP1 monoclonal antibody for both immunoblotting and immunofluorescent microscopic analysis. Unlike previous RT-PCR studies, immunoblotting demonstrated that REEP1 protein was not ubiquitous; its expression was restricted to neuronal tissues (brain, spinal cord) and testes. Gene expression microarray analysis demonstrated REEP1 and REEP2 mRNA expression in superior cervical and stellate sympathetic ganglia tissue. Furthermore, expression of endogenous REEP1 was confirmed in cultured murine sympathetic ganglion neurons by RT-PCR and immunofluorescent staining, with expression occurring between Day 4 and Day 8 of culture. Lastly, we demonstrated that REEP2 protein expression was also restricted to neuronal tissues (brain and spinal cord) and tissues that exhibit neuronal-like exocytosis (testes, pituitary, and adrenal gland). In addition to sensory tissues, expression of the REEP1/REEP2 subfamily appears to be restricted to neuronal and neuronal-like exocytotic tissues, consistent with neuronally restricted symptoms of REEP1 genetic disorders.

REEPs are membrane shaping adapter proteins that modulate specific g protein-coupled receptor trafficking by affecting ER cargo capacity

Receptor expression enhancing proteins (REEPs) were identified by their ability to enhance cell surface expression of a subset of G protein-coupled receptors (GPCRs), specifically GPCRs that have proven difficult to express in heterologous cell systems. Further analysis revealed that they belong to the Yip (Ypt-interacting protein) family and that some REEP subtypes affect ER structure. Yip family comparisons have established other potential roles for REEPs, including regulation of ER-Golgi transport and processing/neuronal localization of cargo proteins. However, these other potential REEP functions and the mechanism by which they selectively enhance GPCR cell surface expression have not been clarified. By utilizing several REEP family members (REEP1, REEP2, and REEP6) and model GPCRs (α2A and α2C adrenergic receptors), we examined REEP regulation of GPCR plasma membrane expression, intracellular processing, and trafficking. Using a combination of immunolocalization and biochemical methods, we demonstrated that this REEP subset is localized primarily to ER, but not plasma membranes. Single cell analysis demonstrated that these REEPs do not specifically enhance surface expression of all GPCRs, but affect ER cargo capacity of specific GPCRs and thus their surface expression. REEP co-expression with α2 adrenergic receptors (ARs) revealed that this REEP subset interacts with and alter glycosidic processing of α2C, but not α2A ARs, demonstrating selective interaction with cargo proteins. Specifically, these REEPs enhanced expression of and interacted with minimally/non-glycosylated forms of α2C ARs. Most importantly, expression of a mutant REEP1 allele (hereditary spastic paraplegia SPG31) lacking the carboxyl terminus led to loss of this interaction. Thus specific REEP isoforms have additional intracellular functions besides altering ER structure, such as enhancing ER cargo capacity, regulating ER-Golgi processing, and interacting with select cargo proteins. Therefore, some REEPs can be further described as ER membrane shaping adapter proteins.

Auxiliary KChIP4a suppresses A-type K+ current through endoplasmic reticulum (ER) retention and promoting closed-state inactivation of Kv4 channels

In the brain and heart, auxiliary Kv channel-interacting proteins (KChIPs) co-assemble with pore-forming Kv4 α-subunits to form a native K(+) channel complex and regulate the expression and gating properties of Kv4 currents. Among the KChIP1-4 members, KChIP4a exhibits a unique N terminus that is known to suppress Kv4 function, but the underlying mechanism of Kv4 inhibition remains unknown. Using a combination of confocal imaging, surface biotinylation, and electrophysiological recordings, we identified a novel endoplasmic reticulum (ER) retention motif, consisting of six hydrophobic and aliphatic residues, 12-17 (LIVIVL), within the KChIP4a N-terminal KID, that functions to reduce surface expression of Kv4-KChIP complexes. This ER retention capacity is transferable and depends on its flanking location. In addition, adjacent to the ER retention motif, the residues 19-21 (VKL motif) directly promote closed-state inactivation of Kv4.3, thus leading to an inhibition of channel current. Taken together, our findings demonstrate that KChIP4a suppresses A-type Kv4 current via ER retention and enhancement of Kv4 closed-state inactivation.

Systematic and quantitative analysis of G protein-coupled receptor trafficking motifs

Plasma membrane expression of G protein-coupled receptors (GPCRs) is a dynamic process balancing anterograde and retrograde trafficking. Multiple interrelated cellular processes determine the final level of cell surface expression, including endoplasmic reticulum (ER) export/retention, receptor internalization, recycling, and degradation. These processes are highly regulated to achieve specific localization to subcellular domains (e.g., dendrites or basolateral membranes) and to affect receptor signaling. Analysis of potential ER trafficking motifs within GPCRs requires careful consideration of intracellular dynamics, such as protein folding, ER export and retention, and glycosylation. This chapter presents an approach and methods for qualitative and quantitative assessment of these processes to aid in accurate identification of GPCR trafficking motifs, utilizing the analysis of a hydrophobic extracellular trafficking motif in α2C adrenergic receptors as a model system.

Normal testicular function without detectable follicle-stimulating hormone. A novel mutation in the follicle-stimulating hormone receptor gene leading to apparent constitutive activity and impaired agonist-induced desensitization and internalization

Activating mutations in the follicle-stimulating hormone (FSH) receptor (FSHR) gene are rarely detected due to the absence of a clearly defined phenotype, particularly in men. We here report the biochemical features of a novel mutation in the first extracellular loop of the FSHR. The mutation (N431I) was detected in an asymptomatic man exhibiting normal spermatogenesis, suppressed serum FSH, and normal or elevated levels of biochemical markers of FSH action. Employing different experimental strategies on HEK-293 cells transiently expressing the N431I FSHR mutant, we found that the mutation led to decreased cell surface plasma membrane expression of the receptor protein, but conferred a low level of constitutive activity associated with markedly altered agonist-stimulated desensitization and internalization. These latter features may contribute and/or amplify the persistent activation of the receptor in both absence and presence of agonist and provide new insights into opportunities for adjuvant therapies based on disruption of these processes.

Controlling subcellular delivery to optimize therapeutic effect

This article focuses on drug targeting to specific cellular organelles for therapeutic purposes. Drugs can be delivered to all major organelles of the cell (cytosol, endosome/lysosome, nucleus, nucleolus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes and proteasomes) where they exert specific effects in those particular subcellular compartments. Delivery can be achieved by chemical (e.g., polymeric) or biological (e.g., signal sequences) means. Unidirectional targeting to individual organelles has proven to be immensely successful for drug therapy. Newer technologies that accommodate multiple signals (e.g., protein switch and virus-like delivery systems) mimic nature and allow for a more sophisticated approach to drug delivery. Harnessing different methods of targeting multiple organelles in a cell will lead to better drug delivery and improvements in disease therapy.

Pharmacoperones: a new therapeutic approach for diseases caused by misfolded G protein-coupled receptors

G Protein-coupled receptors (GPCRs) are cell membrane proteins that recognize specific chemical signals such as drugs and hormones and transduce these signals into cellular responses by activating G-proteins. As is the case for all newly synthesized proteins, GPCRs are subjected to conformational scrutiny at the endoplasmic reticulum prior to processing and trafficking to the cell surface membrane. Because of this stringent quality control screening mechanism, mutations that result in protein misfolding frequently lead to retention in the endoplasmic reticulum, aggregation or other misrouting and, eventually, to disease. This article reviews some patents and new therapeutic opportunities based on the misfolding and retention of otherwise functional GPCRs that represent promising approaches to correct conformational abnormalities leading to distinct disease states.

ER-bound steps in the biosynthesis of G protein-coupled receptors

The polypeptide of a G protein-coupled receptor is inserted into the membrane of the endoplasmic reticulum while being translated and this process by itself may be sufficient to establish the proper receptor fold. X-ray structures reveal a common polypeptide topology with little variation in the alignment and orientation of the seven transmembrane segments, the proximal carboxyl terminus (C-tail) and parts of the extracellular loops. These define a structural core the stability of which probably represents a major criterion for the receptor to pass endoplasmic reticulum (ER) quality control; point mutations affecting the structure of the core have an extraordinary chance of causing receptor retention. In contrast, cytoplasmic loops 2 and 3 and the distal C-tail are poorly ordered at least in the absence of an interaction partner. Similarly, the amino terminal tail of rhodopsin-related receptors (but not of receptor subtypes where ligand binding requires a stable fold of the N-tail) is unlikely to establish a stable fold. These segments can cause ER retention when mutated to inappropriately expose hydrophobic peptide patches; to prevent protein aggregation chaperone molecules attach to them thus initiating selection for ER-associated degradation. It is less clear however if there are additional mechanisms to specifically survey the transmembrane core at the level of the lipid bilayer or if insufficient packing is detected due to misalignment of the cytoplasmic or extracellular face of the receptor.

Pharmacological chaperones correct misfolded GPCRs and rescue function: protein trafficking as a therapeutic target

G-protein-coupled receptors (GPCRs) are a large superfamily of plasma membrane proteins that play central roles in transducing endocrine, neural and -sensory signals. In humans, more than 30 disorders are associated with mutations in GPCRs and these proteins are common drug development targets, with 30-50% of drugs targeting them. GPCR mutants are frequently misfolded, recognized as defective by the cellular quality control system, retained in the endoplasmic reticulum and do not traffic to the plasma membrane. The use of small molecules chaperones (pharmacological chaperones or "pharmacoperones") to rescue misfolded GPCRs has provided a new approach for treatment of human diseases caused by misfolding and misrouting. This chapter provides an overview of the molecular basis of this approach using the human gonadotropin-releasing hormone receptor (hGnRHR) as model for treatment of conformational diseases provoked by -misfolded GPCRs.

Regulation of post-Golgi traffic of G protein-coupled receptors

Anterograde trafficking of newly synthesized G protein-coupled -receptors (GPCRs) from the endoplasmic reticulum to the cell surface represents a crucial checkpoint in controlling the amount of the functional receptors at the cell surface and the strength of signaling initiated by the receptors. In contrast to the extensively studied, well-understood endocytic and recycling pathways, the molecular mechanisms underlying the cell-surface targeting of the receptors remain poorly defined. In this chapter, I will discuss current advances in understanding post-Golgi transport of GPCRs by focusing on specific motifs or sequences that may function as sorting signals regulating export from the Golgi and subsequent transport to the plasma membrane of GPCRs.

Intracellular α(2C)-adrenoceptors: storage depot, stunted development or signaling domain?

G-protein coupled receptors (GPCRs) are generally considered to function as cell surface signaling structures that respond to extracellular mediators, many of which do not readily access the cell's interior. Indeed, most GPCRs are preferentially targeted to the plasma membrane. However, some receptors, including α(2C)-Adrenoceptors, challenge conventional concepts of GPCR activity by being preferentially retained and localized within intracellular organelles. This review will address the issues associated with this unusual GPCR localization and discuss whether it represents a novel sub-cellular niche for GPCR signaling, whether these receptors are being stored for rapid deployment to the cell surface, or whether they represent immature or incomplete receptor systems.

Identification of a novel endoplasmic reticulum export motif within the eighth α-helical domain (α-H8) of the human prostacyclin receptor

The human prostacyclin receptor (hIP) undergoes agonist-dependent trafficking involving a direct interaction with Rab11a GTPase. The region of interaction was localised to a 14 residue Rab11a binding domain (RBD) within the proximal carboxyl-terminal (C)-tail domain of the hIP, consisting of Val(299)-Val(307) within the eighth helical domain (α-H8) adjacent to the palmitoylated residues at Cys(308)-Cys(311). However, the factors determining the anterograde transport of the newly synthesised hIP from the endoplasmic reticulum (ER) to the plasma membrane (PM) have not been identified. The aim of the current study was to identify the major ER export motif(s) within the hIP initially by investigating the role of Lys residues in its maturation and processing. Through site-directed and Ala-scanning mutational studies in combination with analyses of protein expression and maturation, functional analyses of ligand binding, agonist-induced intracellular signalling and confocal image analyses, it was determined that Lys(297), Arg(302) and Lys(304) located within α-H8 represent the critical determinants of a novel ER export motif of the hIP. Furthermore, while substitution of those critical residues significantly impaired maturation and processing of the hIP, replacement of the positively charged Lys with Arg residues, and vice versa, was functionally permissible. Hence, this study has identified a novel 8 residue ER export motif within the functionally important α-H8 of the hIP. This ER export motif, defined by "K/R(X)(4)K/R(X)K/R," has a strict requirement for positively charged, basic Lys/Arg residues at the 1st, 6th and 8th positions and appears to be evolutionarily conserved within IP sequences from mouse to man.

Pharmacological chaperones for misfolded gonadotropin-releasing hormone receptors

Structural alterations provoked by mutations or genetic variations in the gene sequence of G protein-coupled receptors (GPCRs) may lead to abnormal function of the receptor molecule. Frequently, this leads to disease. While some mutations lead to changes in domains involved in agonist binding, receptor activation, or coupling to effectors, others may cause misfolding and lead to retention/degradation of the protein molecule by the quality control system of the cell. Several strategies, including genetic, chemical, and pharmacological approaches, have been shown to rescue function of trafficking-defective misfolded GPCRs. Among these, pharmacological strategies offer the most promising therapeutic tool to promote proper trafficking of misfolded proteins to the plasma membrane (PM). Pharmacological chaperones or "pharmacoperones" are small compounds that permeate the PM, enter cells, and bind selectively to misfolded proteins and correct folding allowing routing of the target protein to the PM, where the receptor may bind and respond to agonist stimulation. In this review, we describe new therapeutic opportunities based on mislocalization of otherwise functional human gonadotropin-releasing hormone receptors. This particular receptor is highly sensitive to single changes in chemical charge, and its intracellular traffic is delicately balanced between expression at the PM or retention/degradation in the endoplasmic reticulum; it is, therefore, a particularly instructive model to understand both the protein routing and the molecular mechanisms, whereby pharmacoperones rescue misfolded intermediates or conformationally defective receptors.

Modulation of α(2C) adrenergic receptor temperature-sensitive trafficking by HSP90

Decreasing the temperature to 30°C is accompanied by significant enhancement of α(2C)-AR plasma membrane levels in several cell lines with fibroblast phenotype, as demonstrated by radioligand binding in intact cells. No changes were observed on the effects of low-temperature after blocking receptor internalization in α(2C)-AR transfected HEK293T cells. In contrast, two pharmacological chaperones, dimethyl sulfoxide and glycerol, increased the cell surface receptor levels at 37°C, but not at 30°C. Further, at 37°C α(2C)-AR is co-localized with endoplasmic reticulum markers, but not with the lysosomal markers. Treatment with three distinct HSP90 inhibitors, radicicol, macbecin and 17-DMAG significantly enhanced α(2C)-AR cell surface levels at 37°C, but these inhibitors had no effect at 30°C. Similar results were obtained after decreasing the HSP90 cellular levels using specific siRNA. Co-immunoprecipitation experiments demonstrated that α(2C)-AR interacts with HSP90 and this interaction is decreased at 30°C. The contractile response to endogenous α(2C)-AR stimulation in rat tail artery was also enhanced at reduced temperature. Similar to HEK293T cells, HSP90 inhibition increased the α(2C)-AR contractile effects only at 37°C. Moreover, exposure to low-temperature of vascular smooth muscle cells from rat tail artery decreased the cellular levels of HSP90, but did not change HSP70 levels. These data demonstrate that exposure to low-temperature augments the α(2C)-AR transport to the plasma membrane by releasing the inhibitory activity of HSP90 on the receptor traffic, findings which may have clinical relevance for the diagnostic and treatment of Raynaud Phenomenon.