Ya-Xiong Tao - G Protein Signaling Pathways in Health and Disease

G protein-coupled receptor; Inactivating mutation; Molecular classification; Nonsense mutation; Pharmacological chaperone; Retinitis pigmentosa; Hypogonadism; Obesity; Nephrogenic diabetes insipidus

Abbreviations

AAV adeno-associated virus

adRP autosomal dominant retinitis pigmentosa

AVPR2 arginine vasopressin receptor 2 (gene name for V2R)

DMSO dimethyl sulfoxide

ER endoplasmic reticulum

ERG electroretinogram

FSH follicle-stimulating hormone

FSHR follicle-stimulating hormone receptor

GnRHR gonadotropin-releasing hormone receptor

GPCR G protein-coupled receptor

hCG human chorionic gonadotropin

LH luteinizing hormone

LHCGR luteinizing hormone/chorionic gonadotropin receptor

MC4R melanocortin-4 receptor

NDI nephrogenic diabetes insipidus

4-PBA 4-phenylbutyrate

PC pharmacological chaperone

PTC premature termination codon

RHO rhodopsin

TM transmembrane

TMAO trimethyl-amine- N -oxide

TUDCA tauroursodeoxycholic acid

V2R V2 vasopressin receptor

WT wild type

As one of the largest and most functionally diverse family of membrane proteins, G protein-coupled receptors (GPCRs) act as signal transducers to transduce an array of both external stimuli (such as photons, odorants, and pheromones) and internal stimuli (such as ions, lipids, and hormones) into the cell. All GPCRs share the same topology consisting of seven -helical transmembrane (TM) domains connected by alternating extracellular and intracellular loops and are flanked by extracellular N-termini and intracellular C-terminal tails. Since GPCRs regulate almost all physiological functions, they have been popular therapeutic targets, with an estimate of ~ 27% of current drugs in the global market targeting GPCRs, accounting for ~$890 billion in sales from 2011 to 2015.

Since the first discovery of naturally occurring mutations in Rhodopsin ( RHO ) in 1990, Some of the most extensively studied GPCRs in this aspect are rhodopsin, V2 vasopressin receptor (V2R), melanocortin-4 receptor (MC4R), gonadotropin-releasing hormone (GnRH) receptor (GnRHR), luteinizing hormone (LH)/chorionic gonadotropin (CG) receptor (LHCGR), and follicle-stimulating hormone (FSH) receptor (FSHR). In this chapter, we seek to illustrate the molecular basis, pathophysiological mechanisms, and therapeutic approaches for inherited diseases resulted from inactivating GPCR mutations.

Rhodopsin, the dim-light photoreceptor expressed in the rod cells of the retina, is a crucial player in GPCR structural biology when its high-resolution crystal structure was first reported in 2000.

Vasopressin, a peptide hormone synthesized in the hypothalamus and released in response of increased blood osmolarity or decreased cardiac volume, plays an important role in renal water reabsorption to maintain water homeostasis. This antidiuretic effect is achieved by activation of its receptor, V2R, via the Gsadenylyl cyclasecAMP-dependent signaling, leading to translocation of water channel aquaporin-2 to the apical membrane. Mutations in the AVPR2 ( arginine vasopressin receptor 2 ) gene (encoding V2R) are the major cause for X-linked nephrogenic diabetes insipidus (NDI), a rare inherited disease characterized by failure to respond to vasopressin and to concentrate urine.

It is well established that the MC4R, a downstream mediator of leptin action in the hypothalamus, plays an important role in the regulation of energy homeostasis in both rodents and humans. As a rhodopsin-like GPCR, the MC4R primarily responds to the activation of a potent endogenous peptide agonist, -melanocyte-stimulating hormone (-MSH), resulting in increased satiety, energy expenditure, and weight loss, and to the inhibition of a natural antagonist/inverse agonist, agouti-related peptide, leading to increased food intake, energy conservation, and weight gain. Since the first 2 reports of frameshift MC4R mutations that correlate with early-onset morbid obesity,

GnRH and its receptor, GnRHR, play critical roles in the hypothalamuspituitarygonadal axis. Receptor activation by GnRH stimulates the secretion of gonadotropins, resulting in development and maturation of gonads in fetal life and after birth. Inactivating mutations in the GNRHR gene resulting in hypogonadotropic hypogonadism, a reproductive disease characterized by low levels of gonadotropins and delayed or absent sexual development, was originally reported by de Roux et al.). Hypogonadotropic hypogonadism is transmitted as a recessive trait in all published cases of inactivating GNRHR mutations to date.

The LHCGR and FSHR, collectively termed gonadotropin receptors, are pivotal mediators in reproductive physiology. As two structurally relevant rhodopsin-like GPCRs, the endogenous ligands for LHCGR (LH and CG) and FSHR (FSH) are composed of two subunits, an identical -subunit and a hormone-specific -subunit.

Therefore, loss-of-function mutations in these receptors have been implicated in several reproductive disorders, including Leydig cell hypoplasia and hypergonadotropic hypogonadism in males, as well as polycystic ovaries, amenorrhea, and infertility in females.

It is well established that GPCRs begin their life cycle from transcription into mRNA in the nucleus, followed by translation into polypeptides in the ribosome located at rough endoplasmic reticulum (ER) and inserted into the ER, where the nascent proteins are scrutinized by the stringent cell quality control system. Only correctly folded receptors are able to route onto the cell membrane after further processing in other cell compartments, especially the Golgi apparatus. At the cell surface, GPCRs bind to ligands and undergo conformational changes, resulting in downstream signal transduction by coupling and catalyzation of G proteins. Prolonged activation of GPCRs usually leads to receptor desensitization and internalization. The internalized GPCRs will either recycle back to cell membrane or degrade in the lysosome (reviewed in Ref. ).