Intracellular Trafficking of Gonadotropin Receptors in Health and Disease

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.

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.

Recent advances in understanding gonadotropin signaling

Gonadotropins are glycoprotein sex hormones regulating development and reproduction and bind to specific G protein–coupled receptors expressed in the gonads. Their effects on multiple signaling cascades and intracellular events have recently been characterized using novel technological and scientific tools. The impact of allosteric modulators on gonadotropin signaling, the role of sugars linked to the hormone backbone, the detection of endosomal compartments supporting signaling modules, and the dissection of different effects mediated by these molecules are areas that have advanced significantly in the last decade. The classic view providing the exclusive activation of the cAMP/protein kinase A (PKA) and the steroidogenic pathway by these hormones has been expanded with the addition of novel signaling cascades as determined by high-resolution imaging techniques. These new findings provided new potential therapeutic applications. Despite these improvements, unanswered issues of gonadotropin physiology, such as the intrinsic pro-apoptotic potential to these hormones, the existence of receptors assembled as heteromers, and their expression in extragonadal tissues, remain to be studied. Elucidating these issues is a challenge for future research.

Novel mutations of the LHCGR gene in two families with 46,XY DSD causing Leydig cell hypoplasia I

PURPOSE

Leydig cell hypoplasia is a rare autosomal recessive 46,XY disorder of sexual development (DSD). It is caused by homozygous or compound heterozygous inactivating mutations in the human luteinizing hormone/chorionic gonadotropin hormone receptor (LHCGR) gene. In Leydig cell hypoplasia type I, patients are characterized by predominantly female external genitalia, which usually go unrecognized until the age of puberty.

METHODS

This study reports three patients descending from two unrelated families. We performed clinical, hormonal, histopathological, molecular, and bioinformatics studies for the studied cases.

RESULTS

All investigations suggested 46,XY DSD and Leydig cell hypoplasia. Molecular analysis showed two novel homozygous inactivating mutations (p.Glu148Ter and p.Leu104Pro) within the extracellular domain of the LHCGR gene.

CONCLUSION

Although the mutations of the LHCGR gene are distributed heterogeneously, without hotspot or recurrent mutations, about one fifth of the reported mutations worldwide have been detected in Arab patients. This is probably due to the high consanguinity rate in these populations, which increases the percentage of autosomal recessive disorders and the homozygous LHCGR gene mutations.

Lipid mediator Leukotriene D4-induces airway epithelial cells proliferation through EGFR/ERK1/2 pathway

BACKGROUND

Cysteinyl leukotrienes (CysLTs), the potent lipid inflammatory mediators, are elevated in many pathological conditions and implicated in various inflammatory diseases including asthma, however their role in airway epithelial cells modulation is not clearly understood. We have investigated the effects of a CysLT, Leukotriene D₄ (LTD₄) on human airway epithelial cells, and assessed its role and mode of action in these cells.

METHODOLOGY

Human small airway epithelial cells (SAECs) and A549 cells were incubated with different concentrations of LTD₄ for different time intervals. Subsequently trypan blue dye exclusion assay, MTT assay, Western blotting, RT-PCR and immunofluorescence experiments were performed to examine the effects of LTD₄ on proliferation and related molecular changes in the airway epithelial cells.

RESULTS

The treatment of human airway epithelial cells with LTD₄ resulted in a significant increase in cell proliferation and modulation in the expression of receptors, CysLT₁R and CysLT₂R in SAECs as well as A549 cells. In both types of cells, LTD₄ increased the expression levels of PCNA and c-myc, and trans-activated EGF receptor and increased the activation of ERK1/2. When treated along with epidermal growth factor (EGF), LTD₄ showed a marginal additive effect in ERK1/2 and EGFR phosphorylation compared to LTD₄ alone in both types of airway epithelial cells.

CONCLUSION

In conclusion, these results suggest that sustained presence of lipid inflammatory mediator LTD₄ could induce human airway epithelial cell proliferation through ERK1/2 phosphorylation, either directly via CysLT₁ receptor or by transactivating EGFR.

Structure-Function Relationships of the Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) plays a crucial role in reproduction. This structurally complex receptor is a member of the G-protein coupled receptor (GPCR) superfamily of membrane receptors. As with the other structurally similar glycoprotein hormone receptors (the thyroid-stimulating hormone and luteinizing hormone-chorionic gonadotropin hormone receptors), the FSHR is characterized by an extensive extracellular domain, where binding to FSH occurs, linked to the signal specificity subdomain or hinge region. This region is involved in ligand-stimulated receptor activation whereas the seven transmembrane domain is associated with receptor activation and transmission of the activation process to the intracellular loops comprised of amino acid sequences, which predicate coupling to effectors, interaction with adapter proteins, and triggering of downstream intracellular signaling. In this review, we describe the most important structural features of the FSHR intimately involved in regulation of FSHR function, including trafficking, dimerization, and oligomerization, ligand binding, agonist-stimulated activation, and signal transduction.

Pharmacoperones for Misfolded Gonadotropin Receptors

The gonadotropin receptors (luteinising hormone receptor; LHR and follicle-stimulating hormone receptor; FSHR) are G protein-coupled receptors (GPCRs) that play an important role in the endocrine control of reproduction. Thus genetic mutations that cause impaired function of these receptors have been implicated in a number of reproductive disorders. Disease-causing genetic mutations in GPCRs frequently result in intracellular retention and degradation of the nascent protein through misfolding and subsequent recognition by cellular quality control machinery. The discovery and development of novel compounds termed pharmacological chaperones (pharmacoperones) that can stabilise misfolded receptors and restore trafficking and plasma membrane expression are therefore of great interest clinically, and promising in vitro data describing the pharmacoperone rescue of a number of intracellularly retained mutant GPCRs has provided a platform for taking these compounds into in vivo trials. Thienopyrimidine small molecule allosteric gonadotropin receptor agonists (Org 42599 and Org 41841) have been demonstrated to have pharmacoperone activity. These compounds can rescue cell surface expression and in many cases, hormone responsiveness, of a range of retained mutant gonadotropin receptors. Should gonadotropin receptor selectivity of these compounds be improved, they could offer therapeutic benefit to subsets of patients suffering from reproductive disorders attributed to defective gonadotropin receptor trafficking.