Pharmacological chaperones for misfolded gonadotropin-releasing hormone receptors

Gonadotropin-Releasing Hormone Receptor (GnRHR) and Hypogonadotropic Hypogonadism

Human sexual and reproductive development is regulated by the hypothalamic-pituitary-gonadal (HPG) axis, which is primarily controlled by the gonadotropin-releasing hormone (GnRH) acting on its receptor (GnRHR). Dysregulation of the axis leads to conditions such as congenital hypogonadotropic hypogonadism (CHH) and delayed puberty. The pathophysiology of GnRHR makes it a potential target for treatments in several reproductive diseases and in congenital adrenal hyperplasia. GnRHR belongs to the G protein-coupled receptor family and its GnRH ligand, when bound, activates several complex and tissue-specific signaling pathways. In the pituitary gonadotrope cells, it triggers the G protein subunit dissociation and initiates a cascade of events that lead to the production and secretion of the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) accompanied with the phospholipase C, inositol phosphate production, and protein kinase C activation. Pharmacologically, GnRHR can be modulated by synthetic analogues. Such analogues include the agonists, antagonists, and the pharmacoperones. The agonists stimulate the gonadotropin release and lead to receptor desensitization with prolonged use while the antagonists directly block the GnRHR and rapidly reduce the sex hormone production. Pharmacoperones include the most recent GnRHR therapeutic approaches that directly correct the misfolded GnRHRs, which are caused by genetic mutations and hold serious promise for CHH treatment. Understanding of the GnRHR's genomic and protein structure is crucial for the most appropriate assessing of the mutation impact. Such mutations in the GNRHR are linked to normosmic hypogonadotropic hypogonadism and lead to various clinical symptoms, including delayed puberty, infertility, and impaired sexual development. These mutations vary regarding their mode of inheritance and can be found in the homozygous, compound heterozygous, or in the digenic state. GnRHR expression extends beyond the pituitary gland, and is found in reproductive tissues such as ovaries, uterus, and prostate and non-reproductive tissues such as heart, muscles, liver and melanoma cells. This comprehensive review explores GnRHR's multifaceted role in human reproduction and its clinical implications for reproductive disorders.

ER-to-lysosome-associated degradation in a nutshell: mammalian, yeast, and plant ER-phagy as induced by misfolded proteins

Conserved catabolic pathways operate to remove aberrant polypeptides from the endoplasmic reticulum (ER), the major biosynthetic organelle of eukaryotic cells. The best known are the ER-associated degradation (ERAD) pathways that control the retrotranslocation of terminally misfolded proteins across the ER membrane for clearance by the cytoplasmic ubiquitin/proteasome system. In this review, we catalog folding-defective mammalian, yeast, and plant proteins that fail to engage ERAD machineries. We describe that they rather segregate in ER subdomains that eventually vesiculate. These ER-derived vesicles are captured by double membrane autophagosomes, engulfed by endolysosomes/vacuoles, or fused with degradative organelles to clear cells from their toxic cargo. These client-specific, mechanistically diverse ER-phagy pathways are grouped under the umbrella term of ER-to-lysosome-associated degradation for description in this essay.

Spinocerebellar ataxia 38: structure-function analysis shows ELOVL5 G230V is proteotoxic, conformationally altered and a mutational hotspot

Fatty acid elongase ELOVL5 is part of a protein family of multipass transmembrane proteins that reside in the endoplasmic reticulum where they regulate long-chain fatty acid elongation. A missense variant (c.689G>T p.Gly230Val) in ELOVL5 causes Spinocerebellar Ataxia subtype 38 (SCA38), a neurodegenerative disorder characterized by autosomal dominant inheritance, cerebellar Purkinje cell demise and adult-onset ataxia. Having previously showed aberrant accumulation of p.G230V in the Golgi complex, here we further investigated the pathogenic mechanisms triggered by p.G230V, integrating functional studies with bioinformatic analyses of protein sequence and structure. Biochemical analysis showed that p.G230V enzymatic activity was normal. In contrast, SCA38-derived fibroblasts showed reduced expression of ELOVL5, Golgi complex enlargement and increased proteasomal degradation with respect to controls. By heterologous overexpression, p.G230V was significantly more active than wild-type ELOVL5 in triggering the unfolded protein response and in decreasing viability in mouse cortical neurons. By homology modelling, we generated native and p.G230V protein structures whose superposition revealed a shift in Loop 6 in p.G230V that altered a highly conserved intramolecular disulphide bond. The conformation of this bond, connecting Loop 2 and Loop 6, appears to be elongase-specific. Alteration of this intramolecular interaction was also observed when comparing wild-type ELOVL4 and the p.W246G variant which causes SCA34. We demonstrate by sequence and structure analyses that ELOVL5 p.G230V and ELOVL4 p.W246G are position-equivalent missense variants. We conclude that SCA38 is a conformational disease and propose combined loss of function by mislocalization and gain of toxic function by ER/Golgi stress as early events in SCA38 pathogenesis.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

Effects of salicylate derivatives on localization of p.H723R allele product of SLC26A4

OBJECTIVE

Pendrin is a transmembrane protein encoded by the SLC26A4 gene that functions in maintaining ion concentrations in the endolymph of the inner ear, most likely by acting as a chloride/bicarbonate transporter. Variants in the SLC26A4 gene are responsible for sensorineural hearing loss. Although pendrin localizes to the plasma membrane, we previously identified that 8 missense allele products of SLC26A4 were retained in the intracellular region and lost their anion exchange function. We also found that 10 mM salicylate induced the translocation of 4 out of 8 allele products from the intracellular region to the plasma membrane and restored their anion exchanger activity. However, since 10 mM salicylate exhibits cytotoxicity, the use of chemical compounds with less cell toxicity is needed. In the present study, therefore, salicylate derivatives were used as the chemical compounds and their effects on the p.H723R allele products of SLC26A4 were investigated.

METHODS

HEK293 cells were transfected with the cDNA of p.H723R. Cell proliferation, viability and toxicity assays were performed to investigate the response and health of cells in culture after treatment with four types of salicylate derivatives, i.e., 2-hydroxybenzyl alcohol, 2,3-dihydroxybenzoic acid, 2'-hydroxyacetophenone and methyl salicylate. The effects of these salicylate derivatives on the localization of the p.H723R were investigated by immunofluorescence microscopy.

RESULTS

The application of 10 mM salicylate showed an increase in cell toxicity and decrease in cell viability, leading to a significant decrease in cell proliferation. In contrast, the application of 1 mM salicylate derivatives did not show any significant increase in cell toxicity and decrease in cell viability, corresponding to a logarithmic increase in cell concentration with an increase in culture time. Immunofluorescence experiments showed that the p.H723R retained in the endoplasmic reticulum (ER). Among the salicylate derivatives applied, 2-hydroxybenzyl alcohol induced the translocation of p.H723R from the ER to the plasma membrane 3 h after its application.

CONCLUSION

The results obtained showed that 2-hydroxybenzyl alcohol restored the localization of the p.H723R allele products of SLC26A4 from the ER to the plasma membrane at a concentration of 1 mM by 3 h after its administration with less cytotoxicity than 10 mM salicylate.

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 basis for the evolved instability of a human G-protein coupled receptor

Membrane proteins are prone to misfolding and degradation. This is particularly true for mammalian forms of the gonadotropin-releasing hormone receptor (GnRHR). Although they function at the plasma membrane, mammalian GnRHRs accumulate within the secretory pathway. Their apparent instability is believed to have evolved through selection for attenuated GnRHR activity. Nevertheless, the molecular basis of this adaptation remains unclear. We show that adaptation coincides with a C-terminal truncation that compromises the translocon-mediated membrane integration of its seventh transmembrane domain (TM7). We also identify a series of polar residues in mammalian GnRHRs that compromise the membrane integration of TM2 and TM6. Reverting a lipid-exposed polar residue in TM6 to an ancestral hydrophobic residue restores expression with no impact on function. Evolutionary trends suggest variations in the polarity of this residue track with reproductive phenotypes. Our findings suggest that the marginal energetics of cotranslational folding can be exploited to tune membrane protein fitness.

Integral membrane proteins are prone to misfolding, especially mammalian gonadotropin-releasing hormone receptors (GnRHRs). Chamness et al. show that the evolved instability of mammalian GnRHRs stems from adaptive modifications that disrupt translocon-mediated membrane integration, suggesting that membrane protein misfolding can be exploited to tune fitness.

Misfolded G Protein-Coupled Receptors and Endocrine Disease. Molecular Mechanisms and Therapeutic Prospects

Misfolding of G protein-coupled receptors (GPCRs) caused by mutations frequently leads to disease due to intracellular trapping of the conformationally abnormal receptor. Several endocrine diseases due to inactivating mutations in GPCRs have been described, including X-linked nephrogenic diabetes insipidus, thyroid disorders, familial hypocalciuric hypercalcemia, obesity, familial glucocorticoid deficiency [melanocortin-2 receptor, MC2R (also known as adrenocorticotropin receptor, ACTHR), and reproductive disorders. In these mutant receptors, misfolding leads to endoplasmic reticulum retention, increased intracellular degradation, and deficient trafficking of the abnormal receptor to the cell surface plasma membrane, causing inability of the receptor to interact with agonists and trigger intracellular signaling. In this review, we discuss the mechanisms whereby mutations in GPCRs involved in endocrine function in humans lead to misfolding, decreased plasma membrane expression of the receptor protein, and loss-of-function diseases, and also describe several experimental approaches employed to rescue trafficking and function of the misfolded receptors. Special attention is given to misfolded GPCRs that regulate reproductive function, given the key role played by these particular membrane receptors in sexual development and fertility, and recent reports on promising therapeutic interventions targeting trafficking of these defective proteins to rescue completely or partially their normal function.

Elagolix in the treatment of heavy menstrual bleeding associated with uterine fibroids in premenopausal women

INTRODUCTION

Uterine fibroids (UFs) are the most common benign tumor arising from myometrium of reproductive age women, with significant financial burden estimated in hundreds of billions of dollars. Unfortunately, there are limitations in available long-term treatment options. Thus, there is a large unmet need in the UF space for noninvasive therapeutics.

AREAS COVERED

Authors reviewed the literature available for elagolix; an orally bioavailable, second-generation, non-peptide gonadotropin-releasing hormone (GnRH) antagonist recently approved by the US Food and Drug Administration (FDA) in combination with estradiol/norethindrone acetate for the management of heavy menstrual bleeding associated with UFs in premenopausal women.

EXPERT OPINION

The utility of new-generation oral GnRH-antagonists, such as elagolix, relugolix and linzagolix, is offering a new potential opportunity for the future therapy of UFs: elagolix has been the most studied drug of this class for treating benign gynecological diseases, including endometriosis and UFs, for which it has been US FDA-approved in 2018 and 2020, respectively.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

The melanocortin pathway and control of appetite-progress and therapeutic implications

The initial discovery that ob/ob mice become obese because of a recessive mutation of the leptin gene has been crucial to discover the melanocortin pathway to control appetite. In the melanocortin pathway, the fed state is signaled by abundance of circulating hormones such as leptin and insulin, which bind to receptors expressed at the surface of pro-opiomelanocortin (POMC) neurons to promote processing of POMC to the mature hormone α-melanocyte-stimulating hormone (α-MSH). The α-MSH released by POMC neurons then signals to decrease energy intake by binding to melanocortin-4 receptor (MC4R) expressed by MC4R neurons to the paraventricular nucleus (PVN). Conversely, in the 'starved state' activity of agouti-related neuropeptide (AgRP) and of neuropeptide Y (NPY)-expressing neurons is increased by decreased levels of circulating leptin and insulin and by the orexigenic hormone ghrelin to promote food intake. This initial understanding of the melanocortin pathway has recently been implemented by the description of the complex neuronal circuit that controls the activity of POMC, AgRP/NPY and MC4R neurons and downstream signaling by these neurons. This review summarizes the progress done on the melanocortin pathway and describes how obesity alters this pathway to disrupt energy homeostasis. We also describe progress on how leptin and insulin receptors signal in POMC neurons, how MC4R signals and how altered expression and traffic of MC4R change the acute signaling and desensitization properties of the receptor. We also describe how the discovery of the melanocortin pathway has led to the use of melanocortin agonists to treat obesity derived from genetic disorders.

Modulation of proteostasis and protein trafficking: a therapeutic avenue for misfolded G protein-coupled receptors causing disease in humans

Proteostasis refers to the process whereby the cell maintains in equilibrium the protein content of different compartments. This system consists of a highly interconnected network intended to efficiently regulate the synthesis, folding, trafficking, and degradation of newly synthesized proteins. Molecular chaperones are key players of the proteostasis network. These proteins assist in the assembly and folding processes of newly synthesized proteins in a concerted manner to achieve a three-dimensional structure compatible with export from the endoplasmic reticulum to other cell compartments. Pharmacologic interventions intended to modulate the proteostasis network and tackle the devastating effects of conformational diseases caused by protein misfolding are under development. These include small molecules called pharmacoperones, which are highly specific toward the target protein serving as a molecular framework to cause misfolded mutant proteins to fold and adopt a stable conformation suitable for passing the scrutiny of the quality control system and reach its correct location within the cell. Here, we review the main components of the proteostasis network and how pharmacoperones may be employed to correct misfolding of two G protein-coupled receptors, the vasopressin 2 receptor and the gonadotropin-releasing hormone receptor, whose mutations lead to X-linked nephrogenic diabetes insipidus and congenital hypogonadotropic hypogonadism in humans respectively.

Gonadotropin-releasing hormone analog therapeutics

Dysregulation at any level of the hypothalamic-pituitary-gonadal (HPG) axis results in, or aggravates, a number of hormone-dependent diseases such as delayed or precocious puberty, infertility, prostatic and ovarian cancer, benign prostatic hyperplasia, polycystic ovarian syndrome, endometriosis, uterine fibroids, lean body mass, as well as metabolism and cognitive impairment. As gonadotropin-releasing-hormone (GnRH) is an essential regulator of the HPG axis, agonist and antagonist analogs are efficacious in the treatment of these conditions. GnRH analogs also play an important role in assisted reproductive therapies. This review highlights the current and future therapeutic potential of GnRH analogs and upstream regulators of GnRH secretion.

Allosteric Regulation of the Follicle-Stimulating Hormone Receptor

Follicle-stimulating hormone receptor (FSHR) belongs to the leucine-rich repeat family of the G protein-coupled receptor (LGR), which includes the glycoprotein hormone receptors luteinizing hormone receptor, thyrotropin receptor, and other LGRs 4, 5, 6, and 7. FSH is the key regulator of folliculogenesis in females and spermatogenesis in males. FSH elicits its physiological response through its cognate receptor on the cell surface. Binding of the hormone FSH to its receptor FSHR brings about conformational changes in the receptor that are transduced through the transmembrane domain to the intracellular region, where the downstream effector interaction takes place, leading to activation of the downstream signaling cascade. Identification of small molecules that could activate or antagonize FSHR provided interesting tools to study the signal transduction mechanism of the receptor. However, because of the nature of the ligand-receptor interaction of FSH-FSHR, which contains multiple sites in the extracellular binding domain, most of the small-molecule modulators of FSHR are unable to bind to the orthosteric site of the receptors. Rather they modulate receptor activation through allosteric sites in the transmembrane region. This review will discuss allosteric modulation of FSHR primarily through the discovery of small-molecule modulators, focusing on current data on the status of development and the utility of these as tools to better understand signaling mechanisms.

Selective progesterone receptor modulators for fertility preservation in women with symptomatic uterine fibroids

Uterine fibroids (UFs, AKA leiomyoma) are the most important benign neoplastic threat to women's health, with costs up to hundreds of billions of health care dollars worldwide. Uterine fibroids caused morbidities exert a tremendous health toll, impacting the quality of life of women of all ethnicities, especially women of color. Clinical presentations include heavy vaginal bleeding, pelvic pain, bulk symptoms, subfertility, and obstetric complications. Current management strategies heavily lean toward surgical procedures; nonetheless, the choice of treatment is generally subject to patient's age and her desire to preserve future fertility. Women with UF who desire to maintain future fertility potential face a dilemma because of the limited treatment choices that are currently available to help them achieve that goal. Recently, ulipristal acetate the first of the promising family of oral selective progesterone receptor modulators has been approved for UF treatment in Europe, Canada, and several other countries and is under review for possible approval in the USA. In this review article, we discuss recent advances in the management options against UF with a bend toward oral effective long-term treatment alternatives who are particularly suited for those seeking to preserve their future fertility potential. We also explore the transformative concept of primary and secondary UF prevention using these new anti-UF agents. We envision a remarkable shift in the management of UF in future years from surgical/invasive treatment to orally administrated options; clearly, this potential shift will require additional intense clinical research.

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.

Pharmacological Chaperones: Beyond Conformational Disorders

Pharmacological chaperones (PCs) are small molecules that bind to nascent protein targets to facilitate their biogenesis. The ability of PCs to assist in the folding and subsequent forward trafficking of disease-causative protein misfolding mutants has opened new avenues for the treatment of conformational diseases such as cystic fibrosis and lysosomal storage disorders. In this chapter, an overview of the use of PCs for the treatment of conformational disorders is provided. Beyond the therapeutic application of PCs for the treatment of these disorders, pharmacological chaperoning of wild-type integral membrane proteins is discussed. Central to this discussion is the notion that the endoplasmic reticulum is a reservoir of viable but inefficiently processed wild-type protein folding intermediates whose biogenesis can be facilitated by PCs to increase functional pools. To date, the potential therapeutic use of PCs to enhance the biogenesis of wild-type proteins has received little attention. Here the rationale for the development of PCs that target WT proteins is discussed. Also considered is the likelihood that some commonly used therapeutic agents may exert unrecognized pharmacological chaperoning activity on wild-type targets in patient populations.

Pharmacological Chaperones as Potential Therapeutic Strategies for Misfolded Mutant Vasopressin Receptors

Pharmacological chaperones recently opened new possibilities in G protein-coupled receptor drug discovery. Even more interestingly, some unique ligands combine pharmacological chaperoning and biased agonism properties, boosting their therapeutic interest in many human diseases resulting from G protein-coupled receptor mutation and misfolding. These compounds displaying dual characteristics would constitute a perfect treatment for congenital Nephrogenic Diabetes Insipidus, a typical conformational disease. This X-linked genetic pathology is mostly associated with inactivating mutations of the renal arginine-vasopressin V2 receptor leading to misfolding and intracellular retention of the receptor, causing the inability of patients to concentrate their urine in response to the antidiuretic hormone. Cell-permeable pharmacological chaperones have been successfully challenged to restore plasma membrane localization of many V2 receptor mutants. In addition, different classes of specific ligands such as antagonists, agonists as well as biased agonists of the V2 receptor have proven their usefulness in rescuing mutant receptor function. This is particularly relevant for small-molecule biased agonists which only trigger Gs protein activation and cyclic adenosine monophosphate production, the V2-induced signaling pathway responsible for water reabsorption. In parallel, high-throughput screening assays based on receptor trafficking rescue approaches have been developed to discover novel V2 pharmacological chaperone molecules from different chemical libraries. These new hit compounds, which still need to be pharmacologically characterized and functionally tested in vivo, represent promising candidates for the treatment of congenital Nephrogenic Diabetes Insipidus.

Pharmacoperone rescue of vasopressin 2 receptor mutants reveals unexpected constitutive activity and coupling bias

Pharmacoperones are small molecules that diffuse into cells and rescue misfolded, mistrafficked protein mutants, restoring their function. These substances act with high target specificity, serving as templates to fold (or refold) receptors, enzymes, ion channels or other proteins and enable them to pass the scrutiny of the cellular quality control system ("rescue"). In the present study we demonstrate that a rescued mutant (L83Q) of the vasopressin type 2 receptor (V2R), shows a strong bias for Gs coupling unlike the WT V2 receptor, which couples to both Gs and Gq/11. Failure of the mutant to couple to Gq/11 was not due to a limiting quantity of G-proteins since other Gq/11-coupled receptors (WT V2R, histamine receptor and muscarinic receptor) responded appropriately to their ligands. Transfection with DNA encoding Gq enabled the V2 receptor mutant to couple to this G protein, but only modestly compared with the WT receptor. Fourteen V2R mutant pharmacoperones, of multiple chemical classes, obtained from a high throughput screen of a 660,000 structure library, and one V2R peptidomimetic antagonist rescues L83Q. The rescued mutant shows similar bias with all pharmacoperones identified, suggesting that the bias is intrinsic to the mutant protein's structure, rather than due to the chemical class of the pharmacoperone. In the case of V2R mutant Y128S, rescue with a pharmacoperone revealed constitutive activity, also with bias for Gs, although both IP and cAMP were produced in response to agonist. These results suggest that particular rescued receptor mutants show functional characteristics that differ from the WT receptor; a finding that may be important to consider as pharmacoperones are developed as therapeutic agents.

Regulation of GPCR expression through an interaction with CCT7, a subunit of the CCT/TRiC complex

A direct and functional interaction between a subunit of the CCT/TCP-1 ring complex (TRiC) chaperonin complex and G protein–coupled receptor (GPCRs) is shown. Evidence is provided that distinct nascent GPCRs can undergo alternative folding pathways and that CCT/TRiC is critical in preventing aggregation of some GPCRs and in promoting their proper maturation and expression.

Mechanisms that prevent aggregation and promote folding of nascent G protein–coupled receptors (GPCRs) remain poorly understood. We identified chaperonin containing TCP-1 subunit eta (CCT7) as an interacting partner of the β-isoform of thromboxane A₂ receptor (TPβ) by yeast two-hybrid screening. CCT7 coimmunoprecipitated with overexpressed TPβ and β₂-adrenergic receptor (β₂AR) in HEK 293 cells, but also with endogenous β₂AR. CCT7 depletion by small interfering RNA reduced total and cell-surface expression of both receptors and caused redistribution of the receptors to juxtanuclear aggresomes, significantly more so for TPβ than β₂AR. Interestingly, Hsp90 coimmunoprecipitated with β₂AR but virtually not with TPβ, indicating that nascent GPCRs can adopt alternative folding pathways. In vitro pull-down assays showed that both receptors can interact directly with CCT7 through their third intracellular loops and C-termini. We demonstrate that Trp³³⁴ in the TPβ C-terminus is critical for the CCT7 interaction and plays an important role in TPβ maturation and cell-surface expression. Of note, introducing a tryptophan in the corresponding position of the TPα isoform confers the CCT7-binding and maturation properties of TPβ. We show that an interaction with a subunit of the CCT/TCP-1 ring complex (TRiC) chaperonin complex is involved in regulating aggregation of nascent GPCRs and in promoting their proper maturation and expression.

The gonadotropin-releasing hormone system: Perspectives from reproduction to cancer (Review)

Recently, an increasing amount of evidence indicates that human gonadotropin-releasing hormone (hGnRH) and its receptor (hGnRHR) are important regulatory components not only to the reproduction process but also in the regulation of some cancer cell functions such as cell proliferation, in both hormone-dependent and -independent types of tumors. The hGnRHR is a naturally misfolded protein that is retained mostly in the endoplasmic reticulum; however, this mechanism can be overcome by treatment with several pharmacoperones, therefore, increasing the amount of receptors in the cell membrane. In addition, several reports indicate that the expression level of hGnRHR in tumor cells is even lower than in pituitary or gonadotrope cells. The signal transduction pathways activated by hGnRH in both gonadotrope and different cancer cell types are described in the present review. We also discuss how the rescue of misfolded receptors in tumor cells could be a promising strategy for cancer therapy.

A molecular switch in the scaffold NHERF1 enables misfolded CFTR to evade the peripheral quality control checkpoint

The peripheral protein quality control (PPQC) checkpoint removes improperly folded proteins from the plasma membrane through a mechanism involving the E3 ubiquitin ligase CHIP (carboxyl terminus of Hsc70 interacting protein). PPQC limits the efficacy of some cystic fibrosis (CF) drugs, such as VX-809, that improve trafficking to the plasma membrane of misfolded mutants of the CF transmembrane conductance regulator (CFTR), including F508del-CFTR, which retains partial functionality. We investigated the PPQC checkpoint in lung epithelial cells with F508del-CFTR that were exposed to VX-809. The conformation of the scaffold protein NHERF1 (Na(+)/H(+) exchange regulatory factor 1) determined whether the PPQC recognized "rescued" F508del-CFTR (the portion that reached the cell surface in VX-809-treated cells). Activation of the cytoskeletal regulator Rac1 promoted an interaction between the actin-binding adaptor protein ezrin and NHERF1, triggering exposure of the second PDZ domain of NHERF1, which interacted with rescued F508del-CFTR. Because binding of F508del-CFTR to the second PDZ of NHERF1 precluded the recruitment of CHIP, the coexposure of airway cells to Rac1 activator nearly tripled the efficacy of VX-809. Interference with the NHERF1-ezrin interaction prevented the increase of efficacy of VX-809 by Rac1 activation, but the actin-binding domain of ezrin was not required for the increase in efficacy. Thus, rather than mainly directing anchoring of F508del-CFTR to the actin cytoskeleton, induction of ezrin activation by Rac1 signaling triggered a conformational change in NHERF1, which was then able to bind and stabilize misfolded CFTR at the plasma membrane. These insights into the cell surface stabilization of CFTR provide new targets to improve treatment of CF.

Regulation of nicotinic acetylcholine receptors in Alzheimer׳s disease: a possible role of chaperones

Nicotinic acetylcholine receptors (nAChRs) seem to play an integral role in the progress and/or prevention of Alzheimer׳s diseases (AD). Functional abnormalities and problems in biogenesis and trafficking of nAChRs are two major culprits in AD; on the other hand, chaperones modulate post-translational changes in nAChRs. Moreover, they indirectly regulate nAChRs by controlling AD-related proteins such as tau and amyloid beta (Aβ). In this review, we go through recent studies which are showing that chaperones modulate the expression of nAChRs in a subtype-specific manner and explain how AD progress is affected by nAChRs chaperoning.

A small molecule agonist THIQ as a novel pharmacoperone for intracellularly retained melanocortin-4 receptor mutants

Although mutations in the melanocortin-4 receptor (MC4R) gene cause severe early-onset obesity, we still do not have effective approaches to correct the defects of these mutations. Several antagonists have been identified as pharmacoperones of the MC4R whereas no agonist of the MC4R has been reported. In the present study, we investigated the effect of a small molecule agonist of the MC4R, THIQ, on the cell surface expression and signaling of ten intracellularly retained MC4R mutants using different cell lines. We showed that THIQ increased the cell surface expression of three mutants (N62S, C84R, and C271Y) and two of them (N62S and C84R) had increased signaling in HEK293 cells. Interestingly, THIQ increased the signaling of two other mutants (P78L and P260Q) without increasing their cell surface expression in HEK293 cells. In neuronal cells, THIQ exhibited a more potent effect, correcting the cell surface expression and signaling of seven mutants (N62S, I69R, P78L, C84R, W174C, P260Q, and C271Y). Other mutants were not rescued by THIQ. We also showed that THIQ did not rescue MC4R mutants defective in ligand binding or signaling or one intracellularly retained mutant of the melanocortin-3 receptor. In summary, we demonstrated that a small molecule agonist acted as a pharmacoperone of the MC4R rescuing the cell surface expression and signaling of some intracellularly retained MC4R mutants.

Using pharmacological chaperones to restore proteostasis

Normal organismal physiology depends on the maintenance of proteostasis in each cellular compartment to achieve a delicate balance between protein synthesis, folding, trafficking, and degradation while minimizing misfolding and aggregation. Defective proteostasis leads to numerous protein misfolding diseases. Pharmacological chaperones are cell-permeant small molecules that promote the proper folding and trafficking of a protein via direct binding to that protein. They stabilize their target protein in a protein-pharmacological chaperone state, increasing the natively folded protein population that can effectively engage trafficking machinery for transport to the final destination for function. Here, as regards the application of pharmacological chaperones, we focus on their capability to promote the folding and trafficking of lysosomal enzymes, G protein coupled receptors (GPCRs), and ion channels, each of which is presently an important drug target. Pharmacological chaperones hold great promise as potential therapeutics to ameliorate a variety of protein misfolding diseases.

Quality control autophagy degrades soluble ERAD-resistant conformers of the misfolded membrane protein GnRHR

Molecular chaperones triage misfolded proteins via action as substrate selectors for quality control (QC) machines that fold or degrade clients. Herein, the endoplasmic reticulum (ER)-associated Hsp40 JB12 is reported to participate in partitioning mutant conformers of gonadotropin-releasing hormone receptor (GnRHR), a G protein-coupled receptor, between ER-associated degradation (ERAD) and an ERQC autophagy pathway. ERQC autophagy degrades E90K-GnRHR because pools of its partially folded and detergent-soluble degradation intermediates are resistant to ERAD. S168R-GnRHR is globally misfolded and disposed of via ERAD, but inhibition of p97, the protein retrotranslocation motor, shunts S168R-GnRHR from ERAD to ERQC autophagy. Partially folded and grossly misfolded forms of GnRHR associate with JB12 and Hsp70. Elevation of JB12 promotes ERAD of S168R-GnRHR, with E90K-GnRHR being resistant. E90K-GnRHR elicits association of the Vps34 autophagy initiation complex with JB12. Interaction between ER-associated Hsp40s and the Vps34 complex permits the selective degradation of ERAD-resistant membrane proteins via ERQC autophagy.

Pharmacological chaperoning: a primer on mechanism and pharmacology

Approximately forty percent of diseases are attributable to protein misfolding, including those for which genetic mutation produces misfolding mutants. Intriguingly, many of these mutants are not terminally misfolded since native-like folding, and subsequent trafficking to functional locations, can be induced by target-specific, small molecules variably termed pharmacological chaperones, pharmacoperones, or pharmacochaperones (PCs). PC targets include enzymes, receptors, transporters, and ion channels, revealing the breadth of proteins that can be engaged by ligand-assisted folding. The purpose of this review is to provide an integrated primer of the diverse mechanisms and pharmacology of PCs. In this regard, we examine the structural mechanisms that underlie PC rescue of misfolding mutants, including the ability of PCs to act as surrogates for defective intramolecular interactions and, at the intermolecular level, overcome oligomerization deficiencies and dominant negative effects, as well as influence the subunit stoichiometry of heteropentameric receptors. Not surprisingly, PC-mediated structural correction of misfolding mutants normalizes interactions with molecular chaperones that participate in protein quality control and forward-trafficking. A variety of small molecules have proven to be efficacious PCs and the advantages and disadvantages of employing orthostatic antagonists, active-site inhibitors, orthostatic agonists, and allosteric modulator PCs are considered. Also examined is the possibility that several therapeutic agents may have unrecognized activity as PCs, and this chaperoning activity may mediate/contribute to therapeutic action and/or account for adverse effects. Lastly, we explore evidence that pharmacological chaperoning exploits intrinsic ligand-assisted folding mechanisms. Given the widespread applicability of PC rescue of mutants associated with protein folding disorders, both in vitro and in vivo, the therapeutic potential of PCs is vast. This is most evident in the treatment of lysosomal storage disorders, cystic fibrosis, and nephrogenic diabetes insipidus, for which proof of principle in humans has been demonstrated.

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.

Ipsen 5i is a Novel Potent Pharmacoperone for Intracellularly Retained Melanocortin-4 Receptor Mutants

Inactivating mutations of the melanocortin-4 receptor (MC4R) cause early-onset severe obesity in humans. Comprehensive functional studies show that most of the inactivating mutants of the MC4R are retained intracellularly. In the present study, we investigated whether a small molecule inverse agonist of the MC4R, Ipsen 5i, could act as a pharmacoperone and correct the cell surface expression and function of intracellularly retained mutant MC4Rs using multiple cell lines, including HEK293 and two neuronal cell lines. We showed that Ipsen 5i rescued the cell surface expression of all 11 intracellularly retained mutant MC4Rs studied herein in at least one cell line. Ipsen 5i functionally rescued seven mutants in all cell lines used. One mutant (Y157S) was functionally rescued in HEK293 cells but not in the two neuronal cell lines. Ipsen 5i increased cell surface expression of three mutants (S58C, G98R, and F261S) but did not affect signaling. Ipsen 5i had no effect on mutant MC4Rs with other defects (Δ88-92, D90N, I102S) or no defect (N274S). It also did not affect trafficking of a misrouted MC3R mutant (I335S). Cell impermeable peptide ligands of the MC4R or cell permeable small molecule ligand of δ opioid receptor could not rescue misrouted mutant MC4R. In summary, we demonstrated that Ipsen 5i was a novel potent pharmacoperone of the MC4R, correcting trafficking and signaling of a significant portion (73%) of intracellularly retained mutants. Additional studies are needed to demonstrate its in vivo efficacy.

The role of autophagy in neurodegenerative disease

Autophagy is a lysosomal degradative process used to recycle obsolete cellular constituents and eliminate damaged organelles and protein aggregates. These substrates reach lysosomes by several distinct mechanisms, including delivery within endosomes as well as autophagosomes. Completion of digestion involves dynamic interactions among compartments of the autophagic and endocytic pathways. Neurons are particularly vulnerable to disruptions of these interactions, especially as the brain ages. Not surprisingly, mutations of genes regulating autophagy cause neurodegenerative diseases across the age spectrum with exceptional frequency. In late-onset disorders such as Alzheimer's disease, amyotrophic lateral sclerosis and familial Parkinson's disease, defects arise at different stages of the autophagy pathway and have different implications for pathogenesis and therapy. This Review provides an overview of the role of autophagy in neurodegenerative disease, focusing particularly on less frequently considered lysosomal clearance mechanisms and their considerable impact on disease. Various therapeutic strategies for modulating specific stages of autophagy and the current state of drug development for this purpose are also evaluated.

Current and future applications of GnRH, kisspeptin and neurokinin B analogues

Reproductive hormones affect all stages of life from gamete production, fertilization, fetal development and parturition, neonatal development and puberty through to adulthood and senescence. The reproductive hormone cascade has, therefore, been the target for the development of numerous drugs that modulate its activity at many levels. As the central regulator of the cascade, gonadotropin-releasing hormone (GnRH) agonists and antagonists have found extensive applications in treating a wide range of hormone-dependent diseases, such as precocious puberty, prostate cancer, benign prostatic hyperplasia, endometriosis and uterine fibroids, as well as being an essential component of in vitro fertilization protocols. The neuroendocrine peptides that regulate GnRH neurons, kisspeptin and neurokinin B, have also been identified as therapeutic targets, and novel agonists and antagonists are being developed as modulators of the cascade upstream of GnRH. Here, we review the development and applications of analogues of the major neuroendocrine peptide regulators of the reproductive hormone cascade: GnRH, kisspeptin and neurokinin B.

Genetics of hyperuricemia and gout: implications for the present and future

Gout is the most common inflammatory arthropathy and occurs in the setting of elevated serum urate levels. Gout is also known to be associated with multiple comorbidities including cardiovascular disease and the metabolic syndrome. Recent advances in research have increased our understanding and improved our knowledge of the pathophysiology of gout. Genome-wide association studies have permitted the identification of several new and common genetic factors that contribute to hyperuricemia and gout. Most of these are involved with the renal urate transport system (the uric acid transportasome), generally considered the most influential regulator of serum urate homeostasis. Thus far, SCL22A12, SCL2A9, and GLUT9 have been found to have the greatest variation and most influence on serum urate levels. However, genetics are only a part of the explanation in the development of hyperuricemia and gout. As results have been mixed, the role of known urate influential genes in gout's associated comorbidities remains unclear. Regardless, GWAS findings have expanded our understanding of the pathophysiology of hyperuricemia and gout, and will likely play a role in the development of future therapies and treatment of this ancient disease.

Inside job: ligand-receptor pharmacology beneath the plasma membrane

Most drugs acting on the cell surface receptors are membrane permeable and thus able to engage their target proteins in different subcellular compartments. However, these drugs' effects on cell surface receptors have historically been studied on the plasma membrane alone. Increasing evidence suggests that small molecules may also modulate their targeted receptors through membrane trafficking or organelle-localized signaling inside the cell. These additional modes of interaction have been reported for functionally diverse ligands of GPCRs, ion channels, and transporters. Such intracellular drug-target engagements affect cell surface expression. Concurrent intracellular and cell surface signaling may also increase the complexity and therapeutic opportunities of small molecule modulation. Here we discuss examples of ligand-receptor interactions that are present in both intra- and extracellular sites, and the potential therapeutic opportunities presented by this phenomenon.

Minireview: ubiquitination-regulated G protein-coupled receptor signaling and trafficking

G protein-coupled receptors (GPCRs) are the largest and most diverse superfamily of membrane proteins and mediate most cellular responses to hormones and neurotransmitters. Posttranslational modifications are considered the main regulators of all GPCRs. In addition to phosphorylation, glycosylation, and palmitoylation, increasing evidence as reviewed here reveals that ubiquitination also regulates the magnitude and temporospatial aspects of GPCR signaling. Posttranslational protein modification by ubiquitin is a key molecular mechanism governing proteins degradation. Ubiquitination mediates the covalent conjugation of ubiquitin, a highly conserved polypeptide of 76 amino acids, to protein substrates. This process is catalyzed by 3 enzymes acting in tandem: an E1, ubiquitin-activating enzyme; an E2, ubiquitin-carrying enzyme; and an E3, ubiquitin ligase. Ubiquitination is counteracted by deubiquitinating enzymes that deconjugate ubiquitin-modified proteins and rescue the substrate from proteasomal degradation. Although ubiquitination is known to target many GPCRs for lysosomal or proteasomal degradation, emerging findings define novel roles for the basal status of ubiquitination and for rapid deubiquitination and transubiquitination controlling cell surface expression and cellular responsiveness of some GPCRs. In this review, we highlight the classical and novel roles of ubiquitin in the regulation of GPCR function, signaling, and trafficking.

Pharmacological chaperones for enzyme enhancement therapy in genetic diseases

Pharmacological chaperone therapy (PCT) is a rather new approach consisting in targeting incorrectly folded proteins by small molecules, thus, facilitating the correct folding of the protein and inducing a recovery of its functionality. Many diseases result from mutations on specific genes; this patent review focuses on those pathologies where PCT has a potential application for enzymatic enhancement. Rare diseases are the main area where PCT has been applied and the most advanced compounds are aiming to cure lysosomal storage disorders such as Fabry, Pompe or Gaucher. Until now, most compounds used as pharmacological chaperones were based on substrate-like chemical structures but recently new nonsubstrate-like and non-inhibitory compounds have been disclosed for Gaucher and Pompe diseases. This initiates a new era for pharmacological chaperones with more diverse chemical structures and binding modes. This review covers the patents relating to enzyme enhancement on pharmacological chaperone therapy. Only an update is presented for Gaucher disease, where PCT is highly applied and recently reviewed.

Psychiatric drugs bind to classical targets within early exocytotic pathways: therapeutic effects

The classical targets for antipsychotic and antidepressant drugs are G protein-coupled receptors and neurotransmitter transporters, respectively. Full therapeutic actions of these drugs require several weeks. We show how therapeutic effects may eventually accrue after existing therapeutic ligands bind to these classical targets, not on the plasma membrane but rather within endoplasmic reticulum (ER) and cis-Golgi. Consequences of such binding may include pharmacological chaperoning: the nascent drug targets are stabilized against degradation and can therefore exit the ER more readily. Another effect may be matchmaking: heterodimers and homodimers of the target form and can more readily exit the ER. Summarizing recent data for nicotinic receptors, we explain how such effects could lead to reduced ER stress and to a decreased unfolded protein response, including changes in gene activation and protein synthesis. In effects not directly related to cellular stress, escorting would allow increased ER exit and trafficking of known associated proteins, as well as other proteins such as growth factors and their receptors, producing both cell-autonomous and non-cell-autonomous effects. Axonal transport of relevant proteins may underlie the several weeks required for full therapy. In contrast, the antidepressant effects of ketamine and other N-methyl-D-aspartate receptor ligands, which occur within <2 hours, could arise from dendritically localized intracellular binding, followed by chaperoning, matchmaking, escorting, and reduced ER stress. Thus, the effects of intracellular binding extend beyond proteostasis of the targets themselves and involve pathways distinct from ion channel and G protein activation. We propose experimental tests and note pathophysiological correlates.

Ubiquitination of G protein-coupled receptors: functional implications and drug discovery

G protein-coupled receptors (GPCRs) comprise the largest and most diverse family of signaling receptors and control a vast array of physiological responses. Modulating the signaling responses of GPCRs therapeutically is important for the treatment of various diseases, and discovering new aspects of GPCR signal regulation is critical for future drug development. Post-translational modifications are integral to the regulation of GPCR function. In addition to phosphorylation, many GPCRs are reversibly modified with ubiquitin. Ubiquitin is covalently attached to lysine residues within the cytoplasmic domains of GPCRs by ubiquitin ligases and removed by ubiquitin-specific proteases. In many cases, ubiquitin functions as a sorting signal that facilitates trafficking of mammalian GPCRs from endosomes to lysosomes for degradation, but not all GPCRs use this pathway. Moreover, there are distinct types of ubiquitin conjugations that are known to serve diverse functions in controlling a wide range of cellular processes, suggesting broad roles for GPCR ubiquitination. In this review, we highlight recent studies that illustrate various roles for ubiquitin in regulation of GPCR function. Ubiquitination is known to target many GPCRs for lysosomal degradation, and current studies now indicate that basal ubiquitination, deubiquitination, and transubiquitination of certain GPCRs are important for controlling cell surface expression and cellular responsiveness. In addition, novel functions for ubiquitin in regulation of GPCR dimers and in mediating differential GPCR regulation induced by biased agonists have been reported. We will discuss the implications of these new discoveries for ubiquitin regulation of GPCR function in the context of drug development.

Chemical chaperone TUDCA preserves cone photoreceptors in a mouse model of Leber congenital amaurosis

PURPOSE

Mutations in either retinoid isomerase (RPE65) or lecithin-retinol acyltransferase (LRAT) lead to Leber congenital amaurosis (LCA). By using the Lrat(-/-) mouse model, previous studies have shown that the rapid cone degeneration in LCA was caused by endoplasmic reticulum (ER) stress induced by S-opsin aggregation. The purpose of this study is to examine the efficacy of an ER chemical chaperone, tauroursodeoxycholic acid (TUDCA), in preserving cones in the Lrat(-/-) model.

METHODS

Lrat(-/-) mice were systemically administered with TUDCA and vehicle (0.15 M NaHCO(3)) every 3 days from P9 to P28. Cone cell survival was determined by counting cone cells on flat-mounted retinas. The expression and subcellular localization of cone-specific proteins were analyzed by western blotting and immunohistochemistry, respectively.

RESULTS

TUDCA treatment reduced ER stress and apoptosis in Lrat(-/-) retina. It significantly slowed down cone degeneration in Lrat(-/-) mice, resulting in a ∼3-fold increase in cone density in the ventral and central retina as compared with the vehicle-treated mice at P28. Furthermore, TUDCA promoted the degradation of cone membrane-associated proteins by enhancing the ER-associated protein degradation pathway.

CONCLUSIONS

Systemic injection of TUDCA is effective in reducing ER stress, preventing apoptosis, and preserving cones in Lrat(-/-) mice. TUDCA has the potential to lead to the development of a new class of therapeutic drugs for treating LCA.

Rescue of misrouted GnRHR mutants reveals its constitutive activity

G protein-coupled receptors (GPCR) play central roles in almost all physiological functions, and mutations in GPCR are responsible for over 30 hereditary diseases associated with loss or gain of receptor function. Gain of function mutants are frequently described as having constitutive activity (CA), that is, they activate effectors in the absence of agonist occupancy. Although many GPCR have mutants with CA, the GnRH receptor (GnRHR) was not, until 2010, associated with any CA mutants. The explanation for the failure to observe CA appears to be that the quality control system of the cell recognizes CA mutants of GnRHR as misfolded and retains them in the endoplasmic reticulum. In the present study, we identified several human (h)GnRHR mutants with substitutions in transmembrane helix 6 (F(272)K, F(272)Q, Y(284)F, C(279)A, and C(279)S) that demonstrate varying levels of CA after being rescued by pharmacoperones from different chemical classes and/or deletion of residue K(191), a modification that increases trafficking to the plasma membrane. The movement of the mutants from the endoplasmic reticulum (unrescued) to the plasma membrane (after rescue) is supported by confocal microscopy. Judging from the receptor-stimulated inositol phosphate production, mutants F(272)K and F(272)Q, after rescue, display the largest level of CA, an amount that is comparable with agonist-stimulated activation. Because mutations in other GPCR are, like the hGnRHR, scrutinized by the quality control system, this general approach may reveal CA in receptor mutants from other systems. A computer model of the hGnRHR and these mutants was used to evaluate the conformation associated with CA.

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.