Palmitoylation of the luteinizing hormone/human chorionic gonadotropin receptor regulates receptor interaction with the arrestin-mediated internalization pathway

Post-Translational Modifications in GPCR Internalization

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors that serve as the most important drug targets. Classically, GPCR internalization has been considered to lead to receptor desensitization. However, many studies over the past decade have reported that internalized membrane receptors can trigger distinct signal activation. The "internalized activation" provides a completely new understanding for the receptor internalization, the mechanism of physiology/pathology and novel drug targets for precision medicine. GPCR internalization undergoes a series of strict regulations, especially by post-translational modifications (PTMs). Here, this review summarizes different PTMs in GPCR internalization and analyzes their significance in GPCR internalization dynamics, internalization routes, post-internalization fates and related diseases, which will offer new insights into the regulatory mechanism of GPCR signaling and novel drug targets for precision medicine.

Luteinizing hormone signaling is involved in synchronization of Leydig cell's clock and is crucial for rhythm robustness of testosterone production†

In mammals, circadian clock regulates concentration of many reproductive hormones including testosterone. Previously, we characterized pattern of circadian transcription of core clock genes in testosterone-producing Leydig cells. Here, the potential role of luteinizing hormone receptor (LHR)-cAMP signaling in synchronization of Leydig cell's circadian clock and rhythmic testosterone production were examined. Results showed that activation of LHR-cAMP signaling in primary rat Leydig cell culture increased Star/STAR and changed expression of many clock genes (upregulated Per1/PER1, Dec1/2, and Rorb, and downregulated Bmal1 and Rev-erba/b). Inhibition of protein kinase A prevented LHR-triggered increase in transcription of Per1 and Dec1. Effect of stimulated LHR-cAMP signaling on Leydig cell's clock transcription was also confirmed in vivo, using rats treated with single hCG injection. To analyze in vivo effect of low LH-cAMP activity on rhythmical Leydig cell function, rats with experimental hypogonadotropic hypogonadism were used. Characteristics of hypogonadal rats were decreased LH and testosterone secretion without circadian fluctuation; in Leydig cells decreased arrhythmic cAMP and transcription of steroidogenic genes (Cyp11a1 and Cyp17a1) were observed, while decreased Star/STAR expression retains circadian pattern. However, expression of clock genes, despite changes in transcription levels (increased Bmal1, Per2, Cry1, Cry2, Rora, Rorb, Rev-erba/b/REV-ERBB, Dec1, Csnk1e, and decreased Npas2 and PER1) kept circadian patterns observed in control groups. Altogether, the results strengthened the hypothesis about role of LH-cAMP signaling as synchronizer of Leydig cell's clock. However, clock in Leydig cells is not sufficient to sustain rhythmicity of testosterone production in absence of rhythmic activity of LH-cAMP signaling.

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.

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.

HCG-mediated activation of mTORC1 signaling plays a crucial role in steroidogenesis in human granulosa lutein cells

Luteinizing hormone/human chorionic gonadotropin stimulates progesterone biosynthesis in the corpus luteum by activating cyclic adenosine monophosphate/protein kinase A cascade. Recent studies have shown that cyclic adenosine monophosphate-mediated activation of protein kinase A interacts with the mammalian target of rapamycin signaling pathways. Furthermore, the use of mammalian target of rapamycin inhibitors for immunosuppression in transplant patients has shown adverse effects in reproductive functions. This study examined whether the mammalian target of rapamycin pathway plays any role in luteinizing hormone-mediated regulation of progesterone production. Human granulosa lutein cells were isolated from follicular aspirates of women undergoing in vitro fertilization. Cells were cultured for 72 h and treated with human chorionic gonadotropin (50 ng/ml) for different time periods with or without pretreatment with mammalian target of rapamycin complex 1 inhibitor, rapamycin, (20 nM) for 1 h. Expression of steroidogenic enzymes, including steroidogenic acute regulatory protein, cholesterol side chain cleavage enzyme, and 3β-hydroxysteroid dehydrogenase type 1 messenger RNA, were examined by real-time polymerase chain reaction after 6 h of human chorionic gonadotropin treatment. Expressions of phospho-ribosomal protein S6 kinase and cholesterol side chain cleavage enzyme were analyzed after 15 min and 24 h of human chorionic gonadotropin treatment, respectively. Progesterone production was analyzed by an enzyme immunoassay kit after human chorionic gonadotropin (50 ng/ml) or forskolin (10 μM) treatment for 24 h. Treatment with human chorionic gonadotropin increased the expression of downstream targets of mammalian target of rapamycin complex 1, as well as cholesterol side chain cleavage enzyme, 3β-hydroxysteroid dehydrogenase type 1 and steroidogenic acute regulatory protein messenger RNAs. These increases were inhibited by rapamycin pretreatment. Increased progesterone production in response to treatment with human chorionic gonadotropin or forskolin was also blocked by rapamycin pretreatment. Our findings support a role for mammalian target of rapamycin complex 1 in regulating steroidogenesis in human granulosa lutein cells.

Role of Cysteine Residues in the Carboxyl-Terminus of the Follicle-Stimulating Hormone Receptor in Intracellular Traffic and Postendocytic Processing

Posttranslational modifications occurring during the biosynthesis of G protein-coupled receptors include glycosylation and palmitoylation at conserved cysteine residues located in the carboxyl-terminus of the receptor. In a number of these receptors, these modifications play an important role in receptor function and particularly, in intracellular trafficking. In the present study, the three cysteine residues present in the carboxyl-terminus of the human FSHR were replaced with glycine by site-directed mutagenesis. Wild-type and mutant (Cys627/629/655Gly) FSHRs were then transiently expressed in HEK-293 cells and analyzed for cell-surface plasma membrane expression, agonist-stimulated signaling and internalization, and postendocytic processing in the absence and presence of lysosome and/or proteasome inhibitors. Compared with the wild-type FSHR, the triple mutant FSHR exhibited ~70% reduction in plasma membrane expression as well as a profound attenuation in agonist-stimulated cAMP production and ERK1/2 phosphorylation. Incubation of HEK-293 cells expressing the wild-type FSHR with 2-bromopalmitate (palmitoylation inhibitor) for 6 h, decreased plasma membrane expression of the receptor by ~30%. The internalization kinetics and β-arrestin 1 and 2 recruitment were similar between the wild-type and triple mutant FSHR as disclosed by assays performed in non-equilibrium binding conditions and by confocal microscopy. Cells expressing the mutant FSHR recycled the internalized FSHR back to the plasma membrane less efficiently than those expressing the wild-type FSHR, an effect that was counteracted by proteasome but not by lysosome inhibition. These results indicate that replacement of the cysteine residues present in the carboxyl-terminus of the FSHR, impairs receptor trafficking from the endoplasmic reticulum/Golgi apparatus to the plasma membrane and its recycling from endosomes back to the cell surface following agonist-induced internalization. Since in the FSHR these cysteine residues are S-palmitoylated, the data presented emphasize on this posttranslational modification as an important factor for both upward and downward trafficking of this receptor.

Molecular cloning of two gonadotropin receptors in mummichog Fundulus heteroclitus and their gene expression during follicular development and maturation

Two cDNAs encoding gonadotropin receptors, follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) were cloned from mummichog (Fundulus heteroclitus) ovary. Deduced amino acid sequences of the mummichog FSHR (fhFSHR) and LHR (fhLHR) showed high homologies to teleost FSHRs (77-53%) and teleost LHRs (76-62%), respectively. Both the fhFSHR and fhLHR are composed of a typical structural architecture of glycoprotein hormone receptors consisting of the large N-terminal extracellular domain, the transmembrane domain containing seven cell surface membrane-spanning regions, and the intracellular domain. Functional analysis using HEK293 cells stably expressing the fhFSHR or fhLHR demonstrated that both the receptors are specifically activated by mummichog FSH or LH, respectively. Reverse transcription-polymerase chain reaction revealed that both the fhFSHR and fhLHR were expressed in the ovary, testis, and pituitary, and the fhLHR was also expressed in several extra-gonadal tissues. Real-time quantitative-PCR analysis revealed that the fhFSHR gene was abundantly expressed in developing follicles whereas expression of the fhLHR gene markedly increased in follicles of the final maturational stage. These results indicate that gonadotropin stimulation on follicles is regulated by the two distinct pathways via their cognate receptors.

Structure, function and regulation of gonadotropin receptors - a perspective

Luteinizing hormone receptor and follicle stimulating hormone receptor play a crucial role in female and male reproduction. Significant new information has emerged about the structure, mechanism of activation, and regulation of expression of these receptors. Here we provide an overview of the current information on those aspects with an in-depth discussion of the recent developments in the post-transcriptional mechanism of LH receptor expression mediated by a specific LH receptor mRNA binding protein, designated as LRBP. LRBP was identified by electrophoretic gel mobility shift assay using cytosolic fractions from ovaries in the down regulated state. LRBP was purified, its binding site on LH receptor mRNA was identified and characterized. During ligand-induced down regulation, LRBP expression is increased through the cAMP/PKA and ERK signaling pathway, is translocated to translating ribosomes, binds LH receptor mRNA and forms an untranslatable ribonucleoprotein complex. This complex is then routed to the mRNA degradation machinery resulting in diminished levels of both LHR mRNA and cell surface expression of LH receptor. The studies leading to these conclusions are presented.

Palmitoylation of human proteinase-activated receptor-2 differentially regulates receptor-triggered ERK1/2 activation, calcium signalling and endocytosis

hPAR(2) (human proteinase-activated receptor-2) is a member of the novel family of proteolytically activated GPCRs (G-protein-coupled receptors) termed PARs (proteinase-activated receptors). Previous pharmacological studies have found that activation of hPAR(2) by mast cell tryptase can be regulated by receptor N-terminal glycosylation. In order to elucidate other post-translational modifications of hPAR(2) that can regulate function, we have explored the functional role of the intracellular cysteine residue Cys(361). We have demonstrated, using autoradiography, that Cys(361) is the primary palmitoylation site of hPAR(2). The hPAR(2)C361A mutant cell line displayed greater cell-surface expression compared with the wt (wild-type)-hPAR(2)-expressing cell line. hPAR(2)C361A also showed a decreased sensitivity and efficacy (intracellular calcium signalling) towards both trypsin and SLIGKV. In stark contrast, hPAR(2)C361A triggered greater and more prolonged ERK (extracellular-signal-regulated kinase) phosphorylation compared with that of wt-hPAR(2) possibly through Gi, since pertussis toxin inhibited the ability of this receptor to activate ERK. Finally, flow cytometry was utilized to assess the rate and extent of receptor internalization following agonist challenge. hPAR(2)C361A displayed faster internalization kinetics following trypsin activation compared with wt-hPAR(2), whereas SLIGKV had a negligible effect on internalization for either receptor. In conclusion, palmitoylation plays an important role in the regulation of PAR(2) expression, agonist sensitivity, desensitization and internalization.

The role of Rab5a GTPase in endocytosis and post-endocytic trafficking of the hCG-human luteinizing hormone receptor complex

This study examined the role of Rab5a GTPase in regulating hCG-induced internalization and trafficking of the hCG-LH receptor complex in transfected 293T cells. Coexpression of wild-type Rab5a (WT) or constitutively active Rab5a (Q79L) with LHR significantly increased hCG-induced LHR internalization. Conversely, coexpression of dominant negative Rab5a (S34N) with LHR reduced internalization. Confocal microscopy showed LHR colocalizing with Rab5a (WT) and Rab5a (Q79L) in punctuate structures. Coexpression of Rab5a (WT) and Rab5a (Q79L) with LHR significantly increased colocalization of LHR in early endosomes. Conversely, dominant negative Rab5a (S34N) decreased this colocalization. While Rab5a stimulated internalization of LHR, it significantly decreased LHR recycling to the cell surface and increased degradation. Dominant negative Rab5a (S34N) increased LHR recycling and decreased degradation. These results suggest that Rab5a plays a role in LHR trafficking by facilitating internalization and fusion to early endosomes, increasing the degradation of internalized receptor resulting in a reduction in LHR recycling.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Molecular regulation of gonadotropin receptor expression: relationship to sterol metabolism

We have identified a specific LHR mRNA binding protein that selectively binds to the polypyrimidine-rich bipartite sequence in the coding region of the LHR mRNA and accelerates its degradation. This process has been shown to be one of the mechanisms that is responsible for the loss of the steady-state levels of LHR mRNA following the preovulatory LH surge or the down regulation of the receptor in response to the administration of a pharmacological dose of LH or hCG. The trans factor, designated as the LHR mRNA binding protein (LRBP), was purified and its identity was established as being mevalonate kinase, an enzyme involved in cholesterol biosynthesis. When mevalonate kinase expression was abolished by treating cultured luteal cells with 25-hydroxycholesterol, the ability to undergo LH-induced down regulation of LHR mRNA was completely abrogated. Examination of the crystal structure of mevalonate kinase coupled with mutagenesis of the critical residues in the catalytic site revealed that the catalytic site is in close proximity to the LHR mRNA binding site. Further studies revealed that mevalonate kinase causes LHR mRNA degradation by acting as a translational suppressor by forming an untranslatable ribonucleoprotein (RNP) complex which is then targeted for degradation. These studies show that LHR expression in the ovary is regulated by a post-transcriptional mechanism mediated by mevalonate kinase thereby linking LHR expression with cholesterol metabolism.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

C-terminal domains within human MT1 and MT2 melatonin receptors are involved in internalization processes

Melatonin, a molecule implicated in a variety of diseases, including cancer, often exerts its effects through G-protein-coupled melatonin receptors, MT(1) and MT(2). In this study, we sought to understand further the domains involved in the function and desensitization patterns of these receptors through site-directed mutagenesis. Two mutations were constructed in the cytoplasmic C-terminal tail of each receptor subtype: (i) a cysteine residue in the C-terminal tail was mutated to alanine, thus removing a putative palmitoylation site, and a site possibly required for normal receptor function (MT(1)C7.72A and MT(2)C7.77A) and (ii) the C-terminal tail in the MT(1) and MT(2) receptors was truncated, removing the putative phosphorylation and beta-arrestin binding sites (MT(1)Y7.64 and MT(2)Y7.64). These mutations did not alter the affinity of 2-[(125)I]-iodomelatonin binding to the MT(1) or MT(2) receptors. Using confocal microscopy, it was determined that the putative palmitoylation site (cysteine residue) did not play a role in receptor internalization; however, this residue was essential for receptor function, as determined by 3',5'-cyclic adenosine monophosphate (cAMP) accumulation assays. Truncation of the C-terminal tail of both receptors (MT(1)Y7.64 and MT(2)Y7.64) inhibited internalization as well as the cAMP response, suggesting the importance of the C-terminal tail in these receptor functions.

Functional and structural roles of conserved cysteine residues in the carboxyl-terminal domain of the follicle-stimulating hormone receptor in human embryonic kidney 293 cells

The carboxyl-terminal segment of G protein-coupled receptors has one or more conserved cysteine residues that are potential sites for palmitoylation. This posttranslational modification contributes to membrane association, internalization, and membrane targeting of proteins. In contrast to other members of the glycoprotein hormone receptor family (the LH and thyroid-stimulating hormone receptors), it is not known whether the follicle-stimulating hormone receptor (FSHR) is palmitoylated and what are the effects of abolishing its potential palmitoylation sites. In the present study, a functional analysis of the FSHR carboxyl-terminal segment cysteine residues was carried out. We constructed a series of mutant FSHRs by substituting cysteine residues with alanine, serine, or threonine individually and together at positions 629 and 655 (conserved cysteines) and 627 (nonconserved). The results showed that all three cysteine residues are palmitoylated but that only modification at Cys629 is functionally relevant. The lack of palmitoylation does not appear to greatly impair coupling to G(s) but, when absent at position 629, does significantly impair cell surface membrane expression of the partially palmitoylated receptor. All FSHR Cys mutants were capable of binding agonist with the same affinity as the wild-type receptor and internalizing on agonist stimulation. Molecular dynamics simulations at a time scale of approximately 100 nsec revealed that replacement of Cys629 resulted in structures that differed significantly from that of the wild-type receptor. Thus, deviations from wild-type conformation may potentially contribute to the severe impairment in plasma membrane expression and the modest effects on signaling exhibited by the receptors modified in this particular position.

Constitutively-active human LH receptors are self-associated and located in rafts

Several naturally occurring mutations in human luteinizing hormone receptors (LHR) at position 578 are associated with constitutive activation of the receptor. To determine whether human LHRs that signal in the absence of ligand are self-associated, fluorescence resonance energy transfer (FRET) between receptors was evaluated. Values for FRET between wild type LHR in the absence of ligand were less than 1% and increased significantly to over 11% after exposure to hCG. Constitutively active receptors exhibited 11-15% FRET efficiency in the absence of hormone and these values did not change with hCG treatment. A large fraction of constitutively active LHR-D578H receptors were also associated with so-called plasma membrane rafts. Disruption of these membrane microdomains reduced FRET efficiency but did not affect signalling through cAMP. Thus, in the absence of ligand, constitutively active receptors are self-associated and located in high buoyancy membrane fractions, both characteristics of the hormone-treated wild type receptor.

Palmitoylation of the TPbeta isoform of the human thromboxane A2 receptor. Modulation of G protein: effector coupling and modes of receptor internalization

Palmitoylation is a prevalent feature amongst G protein-coupled receptors. In this study we sought to establish whether the TPα and TPβ isoforms of the human prostanoid thromboxane (TX) A₂ receptor (TP) are palmitoylated and to assess the functional consequences thereof. Consistent with the presence of three cysteines within its unique carboxyl-terminal domain, metabolic labelling and site-directed mutagenesis confirmed that TPβ is palmitoylated at Cys³⁴⁷ and, to a lesser extent, at Cys^373,377 whereas TPα is not palmitoylated. Impairment of palmitoylation did not affect TPβ expression or its ligand affinity. Conversely, agonist-induced [Ca²⁺]_i mobilization by TPβ^C347S and the non-palmitoylated TPβ^{C347,373,377S}, but not by TPβ^C373S or TPβ^C373,377S, was significantly reduced relative to the wild type TPβ suggesting that palmitoylation at Cys³⁴⁷ is specifically required for efficient Gq/phospholipase Cβ effector coupling. Furthermore, palmitoylation at Cys^373,377 is critical for TPβ internalization with TPβ^C373S, TPβ^C373,377S and TPβ^{C347,373,377S} failing to undergo either agonist-induced or temperature-dependent tonic internalization. On the other hand, whilst TPβ^C347S underwent reduced agonist-induced internalization, it underwent tonic internalization to a similar extent as TPβ. The deficiency in agonist-induced internalization by TPβ^C347S, but not by TPβ^C373,377 nor TPβ^{C347,373,377S}, was overcome by over-expression of either β-arrestin1 or β-arrestin2. Taken together, data herein suggest that whilst palmitoylation of TPβ at Cys^373,377 is critical for both agonist- and tonic-induced internalization, palmitoylation at Cys³⁴⁷ has a role in determining which pathway is followed, be it by the β-arrestin-dependent agonist-induced pathway or by the β-arrestin-independent tonic internalization pathway.

Luteinizing hormone receptors translocate to plasma membrane microdomains after binding of human chorionic gonadotropin

Receptor-mediated signal transduction by G protein-coupled receptors can involve redistribution of plasma membrane receptors into membrane structures that are characterized by insolubility in Triton X-100 and low buoyant density in sucrose gradients. Here we describe the translocation of wild-type (wt) rat LH receptors (LHR-wt) from the bulk membrane into membrane microdomains (rafts) after the binding of human chorionic gonadotropin (hCG). In sucrose gradient ultracentrifugation of plasma membranes from cells stably expressing FLAG-tagged LHR-wt, receptors were located in high-density membrane fractions before binding of hormone and in low-density fractions after hCG treatment. Receptor translocation to low-density sucrose fractions did not occur when cells were pretreated with 1% methyl-beta-cyclodextrin, which reduces membrane cholesterol and disrupts rafts. Single-particle tracking of individual FLAG-LHR-wt receptors showed that hCG-treated receptors become confined in small compartments with a diameter of 86 +/- 36 nm, significantly smaller than 230 +/- 79 nm diameter regions accessed by the untreated receptor. Receptors were no longer confined in these small compartments after disruption of rafts by methyl-beta-cyclodextrin, a treatment that also decreased levels of cAMP in response to hCG. Finally, translocation of LHR into rafts required a functional hormone-receptor complex but did not occur after extensive receptor cross-linking that elevated cAMP levels. Thus, retention of LHR in rafts or small membrane compartments is a characteristic of functional, hormone-occupied LHR-wt. Although raft translocation was not essential for cAMP production, it may be necessary for optimizing hormone-mediated signaling.

Differential impact of intracellular carboxyl terminal domains on lipid raft localization of the murine gonadotropin-releasing hormone receptor

The mammalian type I GNRH receptor (GNRHR) is unique among G protein-coupled receptors (GPCRs) because of the absence of an intracellular C-terminus. Previously, we have found that the murine GNRHR is constitutively localized to low-density membrane microdomains termed lipid rafts. As such, association of the GNRHR with lipid rafts may reflect both a loss (C-terminus) and a gain (raft association address) of structural characteristics. To address this, we fused either the full-length C-terminus from the nonraft-associated LH receptor (LHCGR; GNRHR-LF) or a truncated (t631) LHCGR C-terminus to the GNRHR. These chimeric receptors are trafficked to the plasma membrane, bind ligand, and display increased agonist-induced receptor internalization, but they do not partition into lipid rafts. Thus, a heterologous C-terminus from a nonraft-associated GPCR redirects localization of the GNRHR to nonraft domains. In contrast to the murine GNRHR, the catfish GNRHR (cfGNRHR) possesses an intracellular C-terminus. We found that the cfGNRHR was localized to lipid rafts and that the cfGNRHR C-terminus did not alter raft localization of the mammalian receptor. Consistent with placement in different lipid microenvironments within the plasma membrane, fluorescence recovery after photobleaching revealed different lateral diffusion phenotypes of the raft-associated GNRHR and cfGNRHR versus the nonraft-associated GNRHR-LF fusion protein. We conclude that whereas an intracellular C-terminus is capable of redirecting the GNRHR to nonraft compartments, this is not a generalized feature of GPCR C-terminal tails. Thus, constitutive raft localization of the GNRHR is not simply a result of the loss of an intracellular C-terminus.

Functional interactions between the alpha1b-adrenoceptor and Galpha11 are compromised by de-palmitoylation of the G protein but not of the receptor

Both the alpha1b-adrenoceptor and Galpha11 are targets for post-translational thio-acylation that is regulated by agonist occupancy of the receptor [P.A. Stevens, J. Pediani, J.J. Carrillo, G. Milligan, J. Biol. Chem. 276 (2001) 35883]. In co-expression studies mutation of the sites of thio-acylation in the G protein or treatment of cell membranes with hydroxylamine greatly reduced agonist stimulation of guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTPgammaS) binding. In alpha1b-adrenoceptor-Galpha11 fusion proteins mutation of thio-acylation sites in receptor or G protein did not alter the binding affinity of the antagonist [3H]prazosin or the agonist phenylephrine. Although the potency of phenylephrine to stimulate binding of [35S]GTPgammaS to alpha1b-adrenoceptor-Galpha11 fusion proteins was unaffected by the thio-acylation potential of either element, the maximal effect was reduced by some 50% when the G protein but not the receptor was mutated to prevent thio-acylation. This reflected lack of thio-acylation of the G protein rather than mutation of Cys9 and Cys10 to Ser because treatment with hydroxylamine mimicked this in fusions containing the wild type G protein but was without effect in those mutated to prevent thio-acylation. Mutation to reduce binding of beta/gamma to Galpha11 markedly reduced phenylephrine stimulation of [35S]GTPgammaS binding. Combination of mutations to prevent thio-acylation and beta/gamma binding did not, however, have an additive effect on [35S]GTPgammaS binding. These results indicate that the thio-acylation status of the alpha1b-adrenoceptor does not regulate G protein activation whereas thio-acylation of Galpha11 plays a key role in activation by the receptor beyond providing membrane association and proximity.

Activation of an alpha2A-adrenoceptor-Galphao1 fusion protein dynamically regulates the palmitoylation status of the G protein but not of the receptor

Post-translational thio-acylation of a fusion protein between the alpha2A-adrenoceptor and the alpha subunit of the G protein G(o1) is both dynamic and regulated by agonist binding. Incorporation of [3H]palmitate into the fusion protein was reduced substantially in the presence of the agonist adrenaline. This was dependent on the concentration of adrenaline and correlated with occupancy of the ligand binding site. Both the receptor and G-protein elements of the fusion construct incorporated [3H]palmitate but this occurred more rapidly for the G-protein element and regulation of acylation by the agonist occurred only for the G protein. The kinetics of de-palmitoylation of the alpha2A-adrenoceptor-Galpha(o1) fusion were accelerated markedly by agonist. Again, this reflected modulation of the G protein but not of the receptor. Agonist-induced regulation of the kinetics of thio-acylation of the G protein was abolished, however, in a mutant unable to bind guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTP[S]) in response to adrenaline. Despite the dynamic nature of the post-translational acylation and its regulation by agonist, the ability of adrenaline to activate the G protein, monitored by stimulation of the binding of [35S]GTP[S] to such fusion constructs, was unaffected by the palmitoylation potential of either the receptor or G-protein element.

Gonadotropin receptors: role of post-translational modifications and post-transcriptional regulation

This review focuses on the post-translational modifications of LH and FSH receptors and recent studies on the regulation of LH receptor expression mediated by an RNA binding protein. Both LH and FSH receptors undergo extensive post-translational modifications. N-linked glycosylation occurs co-translationally and plays a role in the maturation and processing of the receptor, while palmitoylation is involved in receptor endocytosis and post-endocytic trafficking. A third type of post-translational modification is phosphorylation and its function has been reviewed. Finally, the regulation of LH receptor at the mRNA level by an RNA binding protein is discussed in the context of ovarian function.

Evidence that palmitoylation of carboxyl terminus cysteine residues of the human luteinizing hormone receptor regulates postendocytic processing

Palmitoylation is a well-conserved posttranslational modification among members of the G protein-coupled receptor superfamily. The present study examined the role of palmitoylation in endocytosis and postendocytic trafficking of the human LH receptor (LHR). Palmitoylation of the LHR was determined by incorporation of [3H]palmitic acid into wild-type (WT) or mutant receptor in which the potential palmitoylation sites, C643 and C644, were mutated to glycine residues. The WT receptor showed incorporation of [3H]palmitic acid into the mature 90-kDa form of the receptor whereas mutation of the two Cys residues abrogated this incorporation, indicating that Cys 643 and C644 are the sites of palmitoylation. The role of palmitoylation on endocytosis and postendocytic processing was examined by testing the ability of the WT and mutant receptor to undergo internalization, recycling, and lysosomal degradation. Compared with the WT receptor, the mutant receptor showed increased internalization and decreased recycling, suggesting that retention of palmitic acid residues at Cys 643 and 644 promotes LHR recycling. The role of palmitoylation on receptor recycling was substantiated by demonstrating that a different mutant, D578H LHR, which is deficient in palmitoylation, also recycled less efficiently. Furthermore, the data show that palmitoylation, not the rate of internalization, determines the efficiency of recycling. The present study shows that palmitoylation of cysteine residues 643 and 644 of the human LHR is a determinant of recycling.

Regulation of luteinizing hormone/human chorionic gonadotropin receptor expression: a perspective

The LH/hCG receptor, a member of the G protein coupled receptor family mediates the cellular actions of LH in the ovary. A considerable amount of information regarding its structure, mechanism of activation, and regulation of expression has emerged in recent years. Here we provide a brief overview of the current information on the structural organization of the receptor and the mechanism of receptor mediated signaling as well as an in-depth discussion on recent developments pertaining to the regulation of receptor expression. Specifically, we describe studies from our laboratory showing that the posttranscriptional regulation of the receptor involves an LH/hCG receptor mRNA-binding protein. We also propose a model to explain the loss of steady-state LH/hCG receptor mRNA levels during receptor down-regulation.

Multiple determinants for rapid agonist-induced internalization of a nonmammalian gonadotropin-releasing hormone receptor: a putative palmitoylation site and threonine doublet within the carboxyl-terminal tail Are critical

The chicken GnRH receptor (cGnRH-R) differs from all mammalian GnRH-Rs in possessing a cytoplasmic carboxyl-terminal tail. We have previously demonstrated that the cGnRH-R undergoes more rapid agonist-induced internalization than the mammalian GnRH-Rs and requires the carboxyl-terminal tail for this process. To investigate the structural determinants mediating this rapid internalization, a series of mutant receptors was generated, including progressive truncations of the tail and substitution of serine and threonine residues with alanine. Truncation of the carboxyl-terminal tail to position 366 and then to position 356 resulted in a progressive attenuation of the rate and total extent of receptor internalization. However, truncation between positions 356 and 346 did not alter the kinetics of internalization further, whereas a further truncation to position 337 resulted in an additional marked reduction of internalization. We show that the membrane-proximal Cys(328) and the Thr(369)Thr(370) doublet located in the distal carboxyl terminus play a critical role in mediating rapid internalization. We demonstrate that the cGnRH-R, when expressed in both COS-7 and HEK 293 cells, preferentially undergoes rapid agonist-induced internalization in a caveolae-like, dynamin-dependent manner. These conclusions are based on our observation that pretreatments with filipin and methyl-beta-cyclodextrin, agents that disrupt lipid rafts such as caveolae, and coexpression of dominant-negative dynamin-1 (K44A) and caveolin-1 (Delta 1-81) mutants, effectively inhibited rapid agonist-induced internalization. Furthermore, cGnRH-Rs appeared to be mobilized to the beta-arrestin- and clathrin-coated, vesicle-mediated endocytic pathway upon beta-arrestin overexpression.

Constitutive localization of the gonadotropin-releasing hormone (GnRH) receptor to low density membrane microdomains is necessary for GnRH signaling to ERK

Specialized membrane microdomains known as lipid rafts are thought to contribute to G-protein coupled receptor (GPCR) signaling by organizing receptors and their cognate signaling molecules into discrete membrane domains. To determine if the GnRHR, an unusual member of the GPCR superfamily, partitions into lipid rafts, homogenates of alpha T3-1 cells expressing endogenous GnRHR or Chinese hamster ovary cells expressing an epitope-tagged GnRHR were fractionated through a sucrose gradient. We found the GnRHR and c-raf kinase constitutively localized to low density fractions independent of hormone treatment. Partitioning of c-raf kinase into lipid rafts was also observed in whole mouse pituitary glands. Consistent with GnRH induced phosphorylation and activation of c-raf kinase, GnRH treatment led to a decrease in the apparent electrophoretic mobility of c-raf kinase that partitioned into lipid rafts compared with unstimulated cells. Cholesterol depletion of alpha T3-1 cells using methyl-beta-cyclodextrin disrupted GnRHR but not c-raf kinase association with rafts and shifted the receptor into higher density fractions. Cholesterol depletion also significantly attenuated GnRH but not phorbol ester-mediated activation of extracellular signal-related kinase (ERK) and c-fos gene induction. Raft localization and GnRHR signaling to ERK and c-Fos were rescued upon repletion of membrane cholesterol. Thus, the organization of the GnRHR into low density membrane microdomains appears critical in mediating GnRH induced intracellular signaling.

Phosphorylation-independent desensitization of GABA(B) receptor by GRK4

Agonist-promoted desensitization of the heterodimeric metabotropic GABA(B) receptor was investigated. Whereas no desensitization was observed in HEK293 cells heterologously expressing the receptor, GABA and the synthetic agonist baclofen induced a robust desensitization in cerebellar granule cells endogenously expressing the receptor. Taking advantage of this cell-specific desensitization phenotype, we identified GRK4 as the kinase involved in the neuronal desensitization. Transfection of small interference RNA directed against GRK4 significantly reduced GRK4 levels in cerebellar granule cells and strongly inhibited the agonist-promoted desensitization. Reciprocally, transfection of GRK4 in HEK293 cells restored agonist-promoted desensitization, confirming that this kinase is sufficient to support desensitization. Surprisingly, this desensitization occurred in the absence of ligand-induced receptor phosphorylation and could be promoted by GRK4 mutants deleted of their kinase domain. Taken together, these results suggest that GRK4 plays a central role in the agonist-promoted desensitization of GABA(B) receptor and that it does so through an atypical mechanism that challenges the generally accepted model linking the kinase activity of GRKs to their role in receptor desensitization.

Highly conserved serine in the third transmembrane helix of the luteinizing hormone/human chorionic gonadotropin receptor regulates receptor activation

The elucidation of the role of highly conserved polar amino acids in the transmembrane helices of G-protein-coupled receptors (GPCRs) is important in understanding the mechanism of receptor activation. To this end, the significance of a highly conserved serine residue in the third transmembrane alpha-helix (TM3) of the luteinizing hormone/human chorionic gonadotropin receptor (LH/hCGR) in regulating receptor activation was examined. Results showed that mutation of serine 431 to alanine (S431A) decreased the ability of the receptor to mediate cAMP production in response to hCG, suggesting that S431 stabilizes the active state of the receptor. Homology with other GPCRs suggests that S431 may participate in the coordination of a Na(+) ion. Since Na(+) has been found to stabilize the active state of the receptor in the presence of hCG, the possibility that S431 promotes receptor activation by mediating the effects of Na(+) was explored. Results showed that the regulation of hormone-induced receptor activation by S431 was independent of Na(+). A rhodopsin-based homology model of the TM region of the LH/hCGR was developed to identify other amino acids that might mediate the effects of Na(+) on receptor function. Results indicate that substitution of an Asp at position 556 with Tyr alters the ability of Na(+) to regulate receptor activation. The homology model is used to explain this result as well as to identify a mechanism through which S431 may regulate receptor signaling. Taken together, these studies provide novel insights into the mechanism of LH/hCG receptor activation.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Expression of the palmitoylation-deficient CD151 weakens the association of alpha 3 beta 1 integrin with the tetraspanin-enriched microdomains and affects integrin-dependent signaling

Transmembrane proteins of the tetraspanin superfamily are assembled in multimeric complexes on the cell surface. Spatial orientation of tetraspanins within these complexes may affect signaling functions of the associated transmembrane receptors (e.g. integrins, receptor-type tyrosine kinases). The structural determinants that control assembly of the tetraspanin complexes are unknown. We have found that various tetraspanins and the alpha(3) integrin subunit are palmitoylated. The stability and molecular composition of the palmitoylated alpha(3)beta(1)-tetraspanin complexes are not affected by adhesion. To assess the significance of palmitoylation in the function of the alpha(3)beta(1)-tetraspanin complexes we mapped the sites of palmitoylation for CD151. Mutation of six cysteines, Cys(11), Cys(15), Cys(79), Cys(80), Cys(242), and Cys(243) was necessary to completely abolish palmitoylation of CD151. The association of the palmitoylation-deficient mutant of CD151 (CD151Cys8) with CD81 and CD63 was markedly decreased, but the interaction of the alpha(3)beta(1)-CD151Cys8 complex with phosphatidylinositol 4-kinase was not affected. Ectopic expression of CD151Cys8 in Rat-1 cells impaired the interactions of the endogenous CD63 and CD81 with the alpha(3)beta(1) integrin. Although the expression of the palmitoylation-deficient CD151 does not change cell spreading on the extracellular matrix, the number of focal adhesions increased. Adhesion-induced phosphorylation of PKB/c-Akt is markedly increased in cells expressing a palmitoylation-deficient mutant, thereby providing direct evidence for the role of the tetraspanin microdomains in regulation of the integrin-dependent phosphatidylinositol 3-kinase signaling pathway. In contrast, activation of FAK and ERK1/2 were not affected by the expression of CD151Cys8. Our results demonstrate that palmitoylation of tetraspanins is critical not only for the organization of the integrin-tetraspanin microdomains but also has a specific role in modulation of adhesion-dependent signaling.

ARF6: a newly appreciated player in G protein-coupled receptor desensitization

The luteinizing hormone/choriogonadotropin hormone receptor (LH/CG R) signals to regulate ovulation, corpus luteum formation, and fetal survival during pregnancy. Agonist binding to the LH/CG R is poorly reversible, emphasizing the importance of a cellular mechanism to temper signaling by a potentially persistently active receptor. Like other G protein-coupled receptors (GPCRs), signaling by this receptor is modulated by its binding of an arrestin. We have identified ADP ribosylation factor 6 (ARF6) as a protein whose activation state is regulated by the LH/CG R and which functions to regulate the availability of plasma membrane-docked arrestin 2 to this receptor. We hypothesize that ARF6 might also serve GPCRs other than the LH/CG R to regulate the availability of arrestin 2 for receptor desensitization.

Aspartic acid 564 in the third cytoplasmic loop of the luteinizing hormone/choriogonadotropin receptor is crucial for phosphorylation-independent interaction with arrestin2

Arrestin2 binding to the active but unphosphorylated luteinizing hormone/choriogonadotropin receptor (LH/CG R) in ovarian follicles is triggered by activation of ADP-ribosylation factor 6 (ARF6) and leads to uncoupling of this receptor from cAMP signaling. We sought to determine how arrestin2 binds to LH/CG R, if binding is of high affinity, and if the receptor also binds arrestin3. Desensitization of intact LH/CG R was equally sensitive to ectopic constructs of arrestin2 that bind other G protein-coupled receptors (GPCRs) either in a phosphorylation-independent or -dependent manner. Intact LH/CG R was not desensitized by ectopic arrestin3 constructs. Surface plasmon resonance studies showed that arrestin2 bound a synthetic third intracellular (3i) LH/CG R loop peptide with picomolar affinity; arrestin3 bound with millimolar affinity. To determine whether Asp-564 in the 3i loop mimicked the phosphorylated residue of other GPCRs, human embryonic kidney (HEK) cells were transfected with wild-type (WT) and D564G LH/CG R. An agonist-stimulated ARF6-dependent arrestin2 undocking pathway to drive desensitization of WT receptor was recapitulated in HEK cell membranes, and ectopic arrestin2 promoted desensitization of WT LH/CG R. However, D564G LH/CG R in HEK cells was not desensitized, and synthetic 3i D564G peptide did not bind arrestin2. Synthetic 3i loop peptides containing D564E, D564V, or D564N also did not bind arrestin2. We conclude that the ARF6-mediated mechanism to release a pool of membrane-delimited arrestin to bind GPCRs may be a widespread mechanism to deliver arrestin to GPCRs for receptor desensitization. Unlike other GPCRs that additionally require receptor phosphorylation, LH/CG R activation is sufficient to expose a conformation in which Asp-564 in the 3i loop confers high affinity binding selectively to arrestin2.

G protein-coupled receptor kinase 4 gene variants in human essential hypertension

Essential hypertension has a heritability as high as 30-50%, but its genetic cause(s) has not been determined despite intensive investigation. The renal dopaminergic system exerts a pivotal role in maintaining fluid and electrolyte balance and participates in the pathogenesis of genetic hypertension. In genetic hypertension, the ability of dopamine and D(1)-like agonists to increase urinary sodium excretion is impaired. A defective coupling between the D(1) dopamine receptor and the G protein/effector enzyme complex in the proximal tubule of the kidney is the cause of the impaired renal dopaminergic action in genetic rodent and human essential hypertension. We now report that, in human essential hypertension, single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK4gamma, increase G protein-coupled receptor kinase (GRK) activity and cause the serine phosphorylation and uncoupling of the D(1) receptor from its G protein/effector enzyme complex in the renal proximal tubule and in transfected Chinese hamster ovary cells. Moreover, expressing GRK4gammaA142V but not the wild-type gene in transgenic mice produces hypertension and impairs the diuretic and natriuretic but not the hypotensive effects of D(1)-like agonist stimulation. These findings provide a mechanism for the D(1) receptor coupling defect in the kidney and may explain the inability of the kidney to properly excrete sodium in genetic hypertension.

G protein-coupled receptor kinase 4 gene variants in human essential hypertension

Essential hypertension has a heritability as high as 30-50%, but its genetic cause(s) has not been determined despite intensive investigation. The renal dopaminergic system exerts a pivotal role in maintaining fluid and electrolyte balance and participates in the pathogenesis of genetic hypertension. In genetic hypertension, the ability of dopamine and D(1)-like agonists to increase urinary sodium excretion is impaired. A defective coupling between the D(1) dopamine receptor and the G protein/effector enzyme complex in the proximal tubule of the kidney is the cause of the impaired renal dopaminergic action in genetic rodent and human essential hypertension. We now report that, in human essential hypertension, single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK4gamma, increase G protein-coupled receptor kinase (GRK) activity and cause the serine phosphorylation and uncoupling of the D(1) receptor from its G protein/effector enzyme complex in the renal proximal tubule and in transfected Chinese hamster ovary cells. Moreover, expressing GRK4gammaA142V but not the wild-type gene in transgenic mice produces hypertension and impairs the diuretic and natriuretic but not the hypotensive effects of D(1)-like agonist stimulation. These findings provide a mechanism for the D(1) receptor coupling defect in the kidney and may explain the inability of the kidney to properly excrete sodium in genetic hypertension.