Evidence for the direct involvement of transmembrane region 6 of the lutropin/choriogonadotropin receptor in activating Gs

Pleiotropic functions of the transmembrane domain 6 of human melanocortin-4 receptor

The melanocortin-4 receptor (MC4R) is a critical regulator of energy homeostasis and has emerged as a premier target for obesity treatment. Numerous mutations in transmembrane domain 6 (TM6) of MC4R resulting in functional alterations have been identified in obese patients. Several mutagenesis studies also provided some data suggesting the importance of this domain in receptor function. To gain a better understanding of the structure-function relationship of the receptor, we performed alanine-scanning mutagenesis in TM6 to determine the functions of side chains. Of the 31 residues, two were important for cell surface expression, five were indispensable for α-melanocyte-stimulating hormone (α-MSH) and β-MSH binding, and six were important for signaling in the Gs-cAMP-PKA pathway. H264A, targeted normally to the plasma membrane, was undetectable by competitive binding assay and severely defective in basal and stimulated cAMP production and ERK1/2 phosphorylation. Nine mutants had decreased basal cAMP signaling. Seven mutants were constitutively active in cAMP signaling and their basal activities could be inhibited by two MC4R inverse agonists, Ipsen 5i and ML00253764. Five mutants were also constitutively active in the MAPK pathway with enhanced basal ERK1/2 phosphorylation. In summary, our study provided comprehensive data on the structure-function relationship of the TM6 of MC4R. We identified residues that are important for cell surface expression, ligand binding, cAMP generation, and residues for maintaining the WT receptor in active conformation. We also reported constitutive activation of the MAPK pathway and biased signaling. These data will be useful for rationally designing MC4R agonists and antagonists for treatment of eating disorders.

Follicle stimulating hormone receptor mutations and reproductive disorders

The follicle stimulating hormone receptor (FSHR) plays a critical role in reproductive function. In the males, FSH supports spermatogenesis, whereas in females, FSH is absolutely required for ovarian follicle growth. In females, inactivating mutations in the FSHR result in ovarian dysgenesis with amenorrhea and infertility. The few males reported with severe inactivating mutations exhibited varying spermatogenic defects, but not azoospermia. While these findings may potentially suggest that FSH action is not absolutely required for spermatogenesis, it cannot be ruled out that these individuals have some residual FSHR activity. Gain-of-function mutations in the FSHR cause spontaneous ovarian hyperstimulation syndrome in females due to the inappropriate stimulation of the mutant FSHR by human choriogonadotropin.

A cell surface inactive mutant of the human lutropin receptor (hLHR) attenuates signaling of wild-type or constitutively active receptors via heterodimerization

The D405N and Y546F mutations of the human lutropin receptor (hLHR) have previously been shown to partially attenuate hCG-stimulated cAMP synthesis despite normal cell surface expression and hCG binding affinity (Min, L. and Ascoli, M. Mol. Endocrinol. 14:1797-1810, 2000). We now show that these mutations each stabilize a resting state of the hLHR. A combined mutant D405N,Y546F is similarly expressed at the cell surface and exhibits normal ligand-binding, but is profoundly signaling impaired. Introduction of hLHR(wt) into cells stably expressing the signaling inactive D405N,Y546F resulted in the attenuation of hCG-stimulated cAMP production by hLHR(wt) even if excess Gs is co-expressed. Similarly, co-expression of D405N,Y546F with hLHR constitutively active mutants (CAMs) attenuated their constitutive activity. Quantitative bioluminescence resonance energy transfer (BRET) analyses demonstrated that D405N,Y546F formed heterodimers with both wt and CAM hLHR. In contrast hLHR(D405N,Y546F) did not heterodimerize with the melanocortin 3 receptor (MC3R) and agonist-stimulated cAMP production through the MC3R was not attenuated when these two receptors were co-expressed. Taken altogether, our data demonstrate that a signaling inactive hLHR mutant (that is trafficked normally to the plasma membrane) attenuates the signaling of the cell surface localized wt or the constitutively active hLHR due to receptor heterodimerization. Our studies, therefore, suggest a novel ramification of GPCR signaling resulting from receptor dimerization.

Structural determinants for G-protein activation and specificity in the third intracellular loop of the thyroid-stimulating hormone receptor

The selectivity of G-protein recognition is determined by the intracellular loops (ICLs) of seven-transmembrane-spanning receptors. In a previous study, we have shown that the N-terminal and central portions of ICL2 from F525 to D530 participate in dual Galphas-/Galphaq-protein activation by the thyroid-stimulating hormone receptor (TSHR). ICL3 is another major determinant for G-protein activation. Therefore, the aim of our study was to identify important amino acids within ICL3 of the TSHR to gain insight in more detail about its specific function for Galphas- and Galphaq-protein activation and selectivity. Single-alanine substitutions of residues in the N-terminal, middle, and C-terminal region of ICL3 were generated. N-terminal residues Y605 and V608 and C-terminal positions K618, K621, and I622 were identified as selectively important for Galphaq activation, whereas mutations in the center of ICL3 had no effect on TSHR signaling. Our findings provide evidence for an amino acid pattern in the N- and C-terminal part of ICL3, which is involved in Galphaq-mediated signaling. Furthermore, molecular modeling of interaction of TSHR ICL2 and 3 with Galphaq suggests three potential contact sites: TSHR C-terminal ICL3 with beta5-6 loop of Galphaq, TSHR ICL2 residues I523-R531 with beta2-3 loop and N-terminal helix of Galphaq, and TSHR ICL2/transmembrane helix (TMH) 3+ICL3/TMH6 with C-terminal tail of Galphaq.

Variable presentation of precocious puberty associated with the D564G mutation of the LHCGR gene in children with testotoxicosis

We report on a family with familial male-limited precocious puberty (FMPP) due to a D564G mutation of the LHCGR gene. Family members show a varied phenotypic expression from severe precocity unresponsive to therapy with compromise of the predicted final height in some members, to attainment of tall final stature in other members who never received medical treatment. DNA amplification and sequencing of exon 11 of the LHCGR gene was done for the three affected male members and their mother. DNA analysis revealed a D564G mutation in the third cytoplasmic loop of the LHCGR receptor. All three males had precocious puberty with elevated testosterone levels. The index case developed central precocious puberty and evidence of compromised final height while on therapy. In contrast, the untreated older siblings attained a tall final height. This report underscores the possibility that the effects of the mutant luteinizing hormone/choriogonadotropin receptor on phenotypic expression of FMPP, such as adult final height, are modified by other factors.

Repulsive Separation of the Cytoplasmic Ends of Transmembrane Helices 3 and 6 Is Linked to Receptor Activation in a Novel Thyrotropin Receptor Mutant (M626I)

Ligand-dependent activation of G protein-coupled receptors (GPCRs) involves repositioning of the juxtacytoplasmic ends of transmembrane helices TM3 and TM6. This concept, inferred from site-directed spin labeling studies, is supported by chemical cross-linking of the cytoplasmic ends of TM3 and TM6 blocking GPCR activation. Here we report a novel constitutive active mutation (M626I) in TM6 of the TSH receptor (TSHR), identified in affected members of a family with nonautoimmune hyperthyroidism. The specific constitutive activity of M626I, measured by its basal cAMP generation corrected for cell surface expression, was 13-fold higher than that of wild-type TSHR. Homology modeling of the TSHR serpentine domain based on the rhodopsin crystal structure suggests that M626 faces the side chain of I515 of TM3 near the membrane-cytoplasmic junction. Steric hindrance of the introduced isoleucine by I515 is consistent with the fact that shorter or more flexible side chains at position 626 did not increase constitutivity. Furthermore, a reciprocal mutation at position 515 (I515M), when introduced into the M626I background, acts as revertant mutation by allowing accommodation of the isoleucine sidechain at position 626 and fully restoring the constitutive activity to the level of wild-type TSHR. Thus, repulsive separation of the juxtacytoplasmic TM6 and TM3 in the M626I model conclusively demonstrates a direct link between the opening of this cytoplasmic face of the receptor structure and G protein coupling.

Gonadotropins: potential targets for preventive and therapeutic interventions in Alzheimer’s disease

Increased prevalence of Alzheimer’s disease (AD) in women has led to an interest in the role of hormonal changes in the neurodegenerative process. In particular, research has been directed towards investigating the effect of changes in sex hormone levels following reproductive senescence. Clinical trials of hormone-replacement therapy for the prevention of AD are proving contentious, and considerably more research is necessary before the benefit of the hormone replacement strategy can be ascertained. However, evidence is now emerging to support the notion that increased gonadotropin levels may confer an increased risk of AD. Gonadotropins have been implicated in the metabolism of β-amyloid, a key protein that is central to the pathogenesis of AD. Gonadotropin reduction represents a promising new target for therapeutic intervention in AD and, potentially, dementia in general. In this review, the authors discuss the therapeutic and preventive potential of gonadotropin-reducing agents in the management of AD.

Molecular mechanisms of ligand binding, signaling, and regulation within the superfamily of G-protein-coupled receptors: molecular modeling and mutagenesis approaches to receptor structure and function

The superfamily of G-protein-coupled receptors (GPCRs) could be subclassified into 7 families (A, B, large N-terminal family B-7 transmembrane helix, C, Frizzled/Smoothened, taste 2, and vomeronasal 1 receptors) among mammalian species. Cloning and functional studies of GPCRs have revealed that the superfamily of GPCRs comprises receptors for chemically diverse native ligands including (1) endogenous compounds like amines, peptides, and Wnt proteins (i.e., secreted proteins activating Frizzled receptors); (2) endogenous cell surface adhesion molecules; and (3) photons and exogenous compounds like odorants. The combined use of site-directed mutagenesis and molecular modeling approaches have provided detailed insight into molecular mechanisms of ligand binding, receptor folding, receptor activation, G-protein coupling, and regulation of GPCRs. The vast majority of family A, B, C, vomeronasal 1, and taste 2 receptors are able to transduce signals into cells through G-protein coupling. However, G-protein-independent signaling mechanisms have also been reported for many GPCRs. Specific interaction motifs in the intracellular parts of these receptors allow them to interact with scaffold proteins. Protein engineering techniques have provided information on molecular mechanisms of GPCR-accessory protein, GPCR-GPCR, and GPCR-scaffold protein interactions. Site-directed mutagenesis and molecular dynamics simulations have revealed that the inactive state conformations are stabilized by specific interhelical and intrahelical salt bridge interactions and hydrophobic-type interactions. Constitutively activating mutations or agonist binding disrupts such constraining interactions leading to receptor conformations that associates with and activate G-proteins.

The first and third intracellular loops together with the carboxy terminal tail of the δ-opioid receptor contribute toward functional interaction with Gα16

Opioid peptides exert their regulatory effects on both central and peripheral nervous systems via multiple opioid receptors that are linked to seemingly identical sets of guanine nucleotide-binding regulatory proteins (G proteins). In contrast to the mu-opioid receptor, the delta-opioid receptor can efficiently stimulate phospholipase C via G16. We used a series of mu/delta-opioid receptor chimeras to examine the involvement of intracellular receptor domains in the recognition of G16. After ascertaining that the chimeras can bind opioid ligands with high affinity and elicit inhibition of adenylyl cyclase, COS-7 cells were cotransfected with cDNAs encoding Galpha16 and a mu/delta-opioid receptor chimera and assayed for [D-Ala2,D-Leu5]enkephalin-induced stimulation of phospholipase C. Our results indicate that (i) the carboxy terminal tail of the delta-opioid receptor is necessary but insufficient for conferring coupling to Galpha16, (ii) the third inner loop together with the carboxy terminal tail of the delta-opioid receptor can provide sufficient contact domains for Galpha16, and (iii) the first inner loop of the delta-opioid receptor, in particular Leu80, as well as the fifth transmembrane domain and/or the third extracellular loop may also contribute in defining the fidelity of interaction between the delta-opioid receptor and Galpha16. These results indicate that efficient coupling of the delta-opioid receptor to Galpha16 requires the participation of most of the intracellular regions, including the first intracellular loop.

Sequence analysis reveals how G protein-coupled receptors transduce the signal to the G protein

Sequence entropy-variability plots based on alignments of very large numbers of sequences-can indicate the location in proteins of the main active site and modulator sites. In the previous article in this issue, we applied this observation to a series of well-studied proteins and concluded that it was possible to detect most of the residues with a known functional role. Here, we apply the method to rhodopsin-like G protein-coupled receptors. Our conclusion is that G protein binding is the main evolutionary constraint on these receptors, and that other ligands, such as agonists, act as modulators. The activation of the receptors can be described as a simple, two-step process, and the residues involved in signal transduction can be identified.

Structural requirements for mutational lutropin/choriogonadotropin receptor activation

Different activation mechanisms of glycoprotein hormone receptors, which are members of the G protein-coupled receptor superfamily, have been proposed. For example, the large ectodomain of glycoprotein hormone receptors may function as an inverse agonist keeping the transmembrane domain in an inactive conformation. To provide support for this hypothesis, we have generated different lutropin/choriogonadotropin receptor (LHR) constructs lacking the ectodomain. Although some ectodomain-deficient LHR constructs were targeted to the cell surface, cAMP levels remained unchanged under basal conditions and agonist application but could be increased by a mutation within the transmembrane domain 6 (D578H). Taking advantage of a constitutive activating mutation (S277N) located in the extracellular domain, we showed that the intact leucine-rich repeat-containing ectodomain is essential for constitutive activation of the LHR by mutation of the hinge region. Our findings support an activation scenario in which agonist binding or mutational alterations expose a structure within the ectodomain, which then activates the transmembrane core.

Cis- and trans-activation of hormone receptors: the LH receptor

G protein-coupled receptors (GPCRs) accommodate a wide spectrum of activators from ions to glycoprotein hormones. The mechanism of activation for this large and clinically important family of receptors is poorly understood. Although initially thought to function as monomers, there is a growing body of evidence that GPCR dimers form, and in some cases that these dimers are essential for signal transduction. Here we describe a novel mechanism of intermolecular GPCR activation, which we refer to as trans-activation, in the LH receptor, a GPCR that does not form stable dimers. The LH receptor consists of a 350-amino acid amino-terminal domain, which is responsible for high-affinity binding to human CG, followed by seven-transmembrane domains and connecting loops. This seven-transmembrane domain bundle transmits the signal from the extracellular amino terminus to intracellular G proteins and adenylyl cyclase. Here, we show that binding of hormone to one receptor can activate adenylyl cyclase through its transmembrane bundle, intramolecular activation (cis-activation), as well as trans-activation through the transmembrane bundle of an adjacent receptor, without forming a stable receptor dimer. Coexpression of a mutant receptor defective in hormone binding and another mutant defective in signal generation rescues hormone-activated cAMP production. Our observations provide new insights into the mechanism of receptor activation mechanisms and have implications for the treatment of inherited disorders of glycoprotein hormone receptors.

The lutropin/choriogonadotropin receptor, a 2002 perspective

Reproduction cannot take place without the proper functioning of the lutropin/choriogonadotropin receptor (LHR). When the LHR does not work properly, ovulation does not occur in females and Leydig cells do not develop normally in the male. Also, because the LHR is essential for sustaining the elevated levels of progesterone needed to maintain pregnancy during the first trimester, disruptions in the functions of the LHR during pregnancy have catastrophic consequences. As such, a full understanding of the biology of the LHR is essential to the survival of our species. In this review we summarize our current knowledge of the structure, functions, and regulation of this important receptor.

Study of the family of a patient with male-limited precocious puberty (MPP) due to T1193C transition in exon 11 of LH receptor gene

Molecular diagnostics of the LHR gene was conducted in a 5-year-old boy with clinical symptoms and hormonal profile typical of precocious puberty. His parents and 4 sisters were also diagnosed. Single-strand conformation polymorphism analysis under temperature gradient conditions (Multitemperature SSCP) of 3 overlapping fragments of exon 11 of LHR gene revealed a mutation in the fragment spanning nucleotides 1072 to 1804. This mutation was found in the patient, in his mother and in his 4 sisters, and was confirmed by digestion with the use of restriction enzyme Bbr Cl. Direct sequencing revealed a heterozygous T1193C transition in the DNA fragment of the patient and in one of the alleles of his mother's and sister's DNA. This mutation causes Met398Thr substitution in the second transmembrane helix and results in a constitutive activation of LH receptor. This is the second identical mutation detected in Poland and one of the 7 identified so far in the world population.

Functional segregation of the highly conserved basic motifs within the third endoloop of the human secretin receptor

In this study, a mutagenesis-based strategy was employed to assess the roles of two highly conserved motifs (KLR and RLAR) within the third endoloop of the human secretin receptor. Block deletion of KLRT and mutation of Lys323 (K(323)I) significantly reduced cAMP accumulation, and these mutations did not affect ligand interaction and receptor number expressed on the cell surface. Thus, the KLRT region at the N terminus of the third endoloop, particularly Lys323, is important for G protein coupling. For the RLAR motif, receptors with substitutions at positions 339 and 342 from Arg to Ala (R(339, 342)A), Glu (R(339, 342)E), or Ile (R(339, 342)I) as well as block deletion of the RLAR motif were all found to be defective in both secretin-binding and cAMP production. Interestingly, a single mutation at the corresponding positions of Arg339 or Arg342 responded as the wild-type human secretin receptor in all functional assays, indicating that the presence of one Arg at either position within the RLAR motif is sufficient for a normal receptor function. Immunofluorescent staining of these mutant receptors showed that these Arg residues are responsible for surface presentation and/or receptor stability.

Pleiotropic effects of substitutions of a highly conserved leucine in transmembrane helix III of the human lutropin/choriogonadotropin receptor with respect to constitutive activation and hormone responsiveness

It has been shown previously that a naturally occurring mutation of the human LH/CG receptor (hLHR), which replaces L457 in helix III with arginine, results in a receptor that constitutively elevates basal cAMP but does not respond to human CG (hCG) with further cAMP production. In the present study, substitutions of L457 with several amino acids were examined. The constitutive activation of cAMP production was observed only when L457 was replaced with a positively charged residue. Although constitutive activation of the inositol phosphate pathway could not be detected when measuring inositol phosphate production, the use of a more sensitive reporter gene assay for protein kinase C activation revealed the constitutive activation of this pathway by the R- and K-substituted mutants. Therefore, L457 of the hLHR plays a key role in stabilizing the receptor in an inactive conformation. Molecular modeling shows that the insertion of R, K, or H at position 457 triggers the receptor transition toward an active state due to the proximity of an anionic amino acid, D578, in helix VI. These substitutions cause perturbations in helix III-helix VI and helix III-helix VII interactions that culminate in the opening of a solvent-accessible site in the cytosolic domains potentially involved in Gs recognition. Interestingly, L457R was completely unresponsive and the K- and H-substituted L457 hLHR mutants were significantly blunted in their cAMP responses to hCG stimulation. Cells expressing L457R were also unresponsive to hCG with regards to increased inositol phosphate production. Other substitutions of L457 were identified, though, that selectively permit the hormonal stimulation of only one of the two signaling pathways. These results suggest a pivotal role for L457 in hormone-stimulated signal transduction by the hLHR.

Transformation of a kappa-opioid receptor antagonist to a kappa-agonist by transfer of a guanidinium group from the 5'- to 6'-position of naltrindole

The importance of the indole scaffold of GNTI 3 in directing its address (5'-guanidinium group) to associate with the Glu297 residue of the kappa-opioid receptor was investigated by the synthesis and biological evaluation of its 4'- (4a), 6'- (4b), and 7'- (4c) regioisomers. The finding that only the 5'-regioisomer (GNTI) possessed potent kappa-opioid antagonist activity and high affinity at kappa-receptors illustrates the importance of the 5'-position in orienting the guanidinium group to the proper recognition locus (Glu 297) for potent kappa-antagonist activity. The discovery that the 6'-regioisomer of GNTI was a potent kappa-agonist, together with the results of site-directed mutagenesis studies that are consistent with association between the 6'-guanidinium group and Glu297, suggest that the transition from an inactive to an active state of the kappa-receptor involves a conformational change of TM6. We propose that association of the 6'-guanidinium group of 4b with Glu297 promotes axial rotational motion of transmembrane helix VI which leads to receptor activation via a conformational change of inner loop 3.

Requirement of specific intrahelical interactions for stabilizing the inactive conformation of glycoprotein hormone receptors

Systematic analysis of structural changes induced by activating mutations has been frequently utilized to study activation mechanisms of G-protein-coupled receptors (GPCRs). In the thyrotropin receptor and the lutropin receptor (LHR), a large number of naturally occurring mutations leading to constitutive receptor activation were identified. Saturating mutagenesis studies of a highly conserved Asp in the junction of the third intracellular loop and transmembrane domain 6 suggested a participation of this anionic residue in a salt bridge stabilizing the inactive receptor conformation. However, substitution of all conserved cationic residues at the cytoplasmic receptor surface did not support this hypothesis. Asp/Glu residues are a common motif at the N-terminal ends of alpha-helices terminating and stabilizing the helical structure (helix capping). Since Asp/Glu residues in the third intracellular loop/transmembrane domain 6 junction are not only preserved in glycoprotein hormone receptors but also in other GPCRs we speculated that this residue probably participates in an N-terminal helix-capping structure. Poly-Ala stretches are known to form and stabilize alpha-helices. Herein, we show that the function of the highly conserved Asp can be mimicked by poly-Ala substitutions in the LHR and thyrotropin receptor. CD and NMR studies of peptides derived from the juxtamembrane portion of the LHR confirmed the helix extension by the poly-Ala substitution and provided further evidence for an involvement of Asp in a helix-capping structure. Our data implicate that in addition to well established interhelical interactions the inactive conformation of GPCRs is also stabilized by specific intrahelical structures.

Male pseudohermaphroditism due to inactivating luteinizing hormone receptor mutations

Human male sexual development is regulated by chorionic gonadotropin (CG) and luteinizing hormone (LH), the action of both mediated by the LH receptor (LHR). Mutations that inactivate the LHR cause Leydig cell hypoplasia (LCH), an autosomal recessive disorder. In its mild form, LCH patients present with male hypogonadism. In its severe form, patients present with male pseudohermaphroditism, with female external genitalia, and cryptorchid testis. Mullerian derivatives are absent. Histological examination of the testis shows absence of mature Leydig cells. LCH patients have elevated plasma levels of LH, normal-to-elevated levels of follicle stimulating hormone (FSH), and low levels of testosterone that do not respond to CG stimulation. Missense mutations, nonsense mutations, deletion mutations, and in-frame insertion mutation of the LHR have been identified in patients with LCH. These mutations are not localized in any particular region of the gene and cause variable degrees of receptor-activity loss. The clinical manifestation of patients with LCH with homozygous or compound heterozygous mutations can be correlated with the residual activity of their respective mutated LHRs. Homozygous inactivating mutations of the LHR in the female cause hypergonadotrophic hypogonadism with primary amenorrhea or oligoamenorrhea, cystic ovaries, and infertility.

A novel subgroup of class I G-protein-coupled receptors

Based on structural similarities of an expressed sequence tag with the platelet-activating factor (PAF) receptor a cDNA clone encoding a novel G-protein-coupled receptor (GPCR), named GPR34, was isolated from a human fetal brain cDNA library. Genomic DNA analyses revealed the receptor to be encoded by an intronless single-copy gene at Xp11. 3-11.4. The predicted 381-amino-acid protein disclosed all structural features characteristic of a member of the class I GPCR family. Except an obvious sequence homology in transmembrane domain 6, no further similarities to the PAF receptor or any other known GPCR were found. The corresponding mouse receptor DNA was isolated from a genomic P1 library displaying a 90% amino acid identity compared to the human receptor. Phylogenetic studies showed that GPR34 is preserved among vertebrates, and the existence of GPR34 subtypes was demonstrated. The receptor mRNA is abundantly expressed in human and mouse tissues. In addition to the major 2-kb transcript, a 4-kb transcript was found only in mouse liver and testis. Expression of the human GPR34 in COS-7 cells followed by Western blot studies revealed specific bands of a highly glycosylated protein between 75 and 90 kDa. A number of potential ligands including phospholipids, leukotrienes, hydroxy-eicosatetraenoic acids, nucleotides and peptides were tested in functional assays. Since none of the applied substances led to significant changes in second messenger levels (cAMP and inositol phosphates), the natural ligand and coupling profile of this novel GPCR subgroup remains unknown.

Structural basis of G protein-coupled receptor function

The vast majority of extracellular signaling molecules, like hormones and neurotransmitters, interact with a class of membranous receptors characterized by a uniform molecular architecture of seven transmembrane alpha-helices linked by extra- and intracelluar peptide loops. In a reversible manner, binding of diverse agonists to heptahelical receptors leads to activation of a limited repertoire of heterotrimeric guanine nucleotide-binding proteins (G proteins) forwarding the signal to intracellular effectors such as enzymes and ion channels. Proper functioning of a G protein-coupled receptor is based on a complex interplay of structural determinants which are ultimately responsible for receptor folding, trafficking and transmembrane signaling. Applying novel biochemical and molecular biological methods interesting insights into receptor structure/function relationships became available. These studies have a significant impact on our understanding of the molecular basis of human diseases and may eventually lead to novel therapeutic strategies.

beta-arrestin-dependent desensitization of luteinizing hormone/choriogonadotropin receptor is prevented by a synthetic peptide corresponding to the third intracellular loop of the receptor

Desensitization is a ubiquitous response of guanine nucleotide-binding protein-coupled receptors (GPCRs) characterized by the waning of effector activity despite continued presence of agonist. Binding of an arrestin to the activated, often phosphorylated GPCR triggers desensitization. We reported for the luteinizing hormone/choriogonadotropin receptor (LH/CG R) that beta-arrestin tightly bound to porcine ovarian follicular membranes mediates agonist-dependent desensitization of LH/CG R-stimulated adenylyl cyclase (AC) activity (Mukherjee, S., Palczewski, K., Gurevich, V. V., Benovic, J. L., Banga, J. P., and Hunzicker-Dunn, M. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 493-498). We now show that addition of a synthetic peptide corresponding to the entire third intracellular loop (3i) of the LH/CG R completely and specifically reverses desensitization of AC activity, with an ED50 of 10 microM but does not modulate basal, hCG-stimulated, or forskolin-stimulated AC activities. beta-Arrestin binds selectively to the 3i peptide coupled to activated Sepharose. Desensitization of LH/CG R-stimulated AC activity is rescued when the 3i peptide is preincubated with exogenous beta-arrestin. These results show that endogenous beta-arrestin participates in cell-free desensitization of agonist-dependent LH/CG R-stimulated AC activity in follicular membranes by interacting directly with the 3i loop of the receptor, thereby preventing Gs activation.

Role of the third intracellular loop for the activation of gonadotropin receptors

Hyperfunctional endocrine thyroid and testicular disorders can frequently be traced back to gainof-function mutations in glycoprotein hormone receptor genes. Deletion mutations in the third intracellular (i3) loop of the TSH receptor have recently been identified as a cause of constitutive receptor activity. To examine whether the underlying mechanism of receptor activation applies to all glycoprotein hormone receptors, we created deletion mutations in the LH and FSH receptors. In analogy to the situation with the TSH receptor, a deletion of nine amino acids resulted in constitutive activity irrespective of the location of deletions within the i3 loop of the LH receptor. In contrast, only one (delta563-566) of four different 4-amino acid deletion mutants displayed agonist-independent activity. Systematic examination of the structural requirements for this effect in the delta563-566 mutant revealed that only deletions including D564 resulted in constitutive receptor activity. Replacement of D564 by G, K, and N led to agonist-independent cAMP formation while introduction of a negatively charged E silenced constitutive receptor activity, indicating that an anionic amino acid at this position may be required to maintain an inactive receptor conformation. Insertion of A residues up- and downstream of D564 did not perturb receptor quiescence, showing that a certain degree of spatial freedom of the negatively charged amino acid within the context of the i3 loop is well tolerated. In contrast to the results obtained with the LH receptor, deletion of the corresponding D567 from the i3 loop of the FSH receptor did not cause constitutive receptor activation, highlighting significant differences in the activation mechanism of gonadotropin receptors.

Mutations that induce constitutive activation and mutations that impair signal transduction modulate the basal and/or agonist-stimulated internalization of the Lutropin/Choriogonadotropin receptor

Previous results from this laboratory suggested that the same active conformation of the lutropin/choriogonadotropin receptor (LHR) is involved in the stimulation of G proteins and in triggering the internalization of the bound agonist. We have now analyzed two naturally occurring, constitutively active mutants of the human LHR. These mutations were introduced into the rat LHR (rLHR) and are designated L435R and D556Y. Cells expressing rLHR-D556Y bind human choriogonadotropin (hCG) with normal affinity, exhibit a 25-fold increase in basal cAMP and respond to hCG with a normal increase in cAMP accumulation. This mutation does not affect the internalization of the free receptor, but it enhances the internalization of the agonist-occupied receptors approximately 3-fold. Cells expressing rLHR-L435R also bind hCG with normal affinity, exhibit a 47-fold increase in basal cAMP, and do not respond to hCG with a further increase in cAMP accumulation. This mutation enhances the internalization of the free and agonist-occupied receptors approximately 2- and approximately 17-fold, respectively. We conclude that the state of activation of the rLHR can modulate its basal and/or agonist-stimulated internalization. Since the internalization of hCG is involved in the termination of hCG actions, we suggest that the lack of responsiveness detected in cells expressing rLHR-L435R is due to the fast rate of internalization of the bound hCG. The finding that membranes expressing rLHR-L435R (a system where internalization does not occur) respond to hCG with an increase in adenylyl cyclase activity supports this suggestion.

Certain activating mutations within helix 6 of the human luteinizing hormone receptor may be explained by alterations that allow transmembrane regions to activate Gs

Male-limited gonadotropin-independent precocious puberty (MPP) is frequently associated with mutations of the human LH/CG receptor (hLHR) that result in constitutively active hLHRs. Many such activating mutations have been identified in transmembrane 6 of the hLHR, with the substitution of Asp-578 being the most frequently observed mutation. Mutagenesis of a transmembrane helix of a G protein-coupled receptor can cause local alterations in the conformation near the mutated residue, allosteric changes elsewhere in the protein, and/or changes in the interhelical packing of the receptor. Therefore, while it has been hypothesized that activation of the receptor by mutations of Asp-578 may arise via alterations in the interactions of helix 6 with other transmembrane helices and/or by allosterically altering the conformation of the third intracellular loop, it has not been possible to ascertain the role of the sixth transmembrane helix per se in activating Gs in the mutated full-length receptor. Recently, however, we have shown that a peptide KMAILIFT, corresponding to the juxtacytoplasmic portion of helix 6 of the hLHR, is capable of activating Gs. These results suggest that helix 6 itself can directly interact with Gs. Importantly, the KMAILIFT peptide did not include Asp-578, which lies just C-terminal to this sequence. We show herein that a peptide extended to include Asp-578 (KMAILIFTDFT) is a poor activator of Gs. However, if the peptide is synthesized with the aspartate replaced with either a glycine or tyrosine, substitutions that are found in some patients with MPP, these peptides have Gs-stimulating activity. Additionally, a transmembrane 6 peptide with the substitution of Ile-575 with leucine, another mutation found in MPP, mimicked the activating effects of this mutation in the full-length receptor. The ability of peptides in which Asp-578 or Ile-575 is substituted to mimic the activating effects of these mutations in the full-length receptor suggests that the sixth transmembrane helix represents a site for direct interaction with Gs. In addition to the stimulatory effects of transmembrane 6 peptides, peptides corresponding to the juxtacytoplasmic portions of the fourth, fifth, and seventh helices were also able to stimulate Gs. These results are consistent with the hypothesis that the transmembrane helices may form a pocket for interaction with Gs and that constitutive activation of the hLHR may involve the opening of the pocket formed by these helices, thus exposing Gs-binding sites on these helices.

Deletions in the third intracellular loop of the thyrotropin receptor. A new mechanism for constitutive activation

Gain-of-function mutations of the thyrotropin receptor (TSHR) gene have been invoked as one of the major causes of toxic thyroid adenomas. In a toxic thyroid nodule, we recently identified a 9-amino acid deletion (amino acid positions 613-621) within the third intracellular (i3) loop of the TSHR resulting in constitutive receptor activity. This finding exemplifies a new mechanism of TSHR activation and raises new questions concerning the function of the i3 loop. Because the i3 loop is thought to be critical for receptor/G protein interaction in many receptors, we systematically reexamined the role of the TSHR's i3 loop for G protein coupling. Thus, various deletion mutants were generated and functionally characterized. We identified an optimal deletion length responsible for constitutive activity. If the number of deleted amino acids was reduced, elevated basal cAMP accumulation was found to be concomitantly diminished. Expansion of the deletion dramatically impaired cell surface expression of the receptor. Shifting the deletion toward the N terminus of the i3 loop resulted in unaltered strong constitutive receptor activity. In contrast, translocation of the deletion toward the C terminus led to significantly reduced basal cAMP formation, most probably due to destruction of a conserved cluster of amino acids. In this study, we show for the first time that amino acid deletions within the i3 loop of a G protein-coupled receptor result in constitutive receptor activity. In the TSHR, 75% of the i3 loop generally assumed to play an essential role in G protein coupling can be deleted without rendering the mutant receptor unresponsive to thyrotropin. These findings support a novel model explaining the molecular events accompanying receptor activation by agonist.

A homozygous microdeletion in helix 7 of the luteinizing hormone receptor associated with familial testicular and ovarian resistance is due to both decreased cell surface expression and impaired effector activation by the cell surface receptor

In this report, the genomic DNA was examined from two siblings with gonadal LH resistance. A 46,XY pseudohermaphrodite presented with female external genitalia and his 46,XX sister exhibited menstrual irregularities (oligoamenorrhea) and infertility. Exons 1-11 of the LH receptor (LHR) gene were amplified by the PCR using different sets of intronic primers and were directly sequenced. Sequencing revealed that both individuals carried a deletion of nucleotides 1822-1827, resulting in the deletion of Leu-608 and Val-609 within the seventh transmembrane helix. This mutation was introduced into a recombinant human (h) LHR cDNA. Transfections of 293 cells with hLHR(wt) vs. hLHR(deltaL608,V609) revealed that very little of the mutant receptor was expressed at the cell surface. This was due to both a decrease in the total amount of receptor expressed as well as to an increased intracellular retention of the mutant receptor. In spite of the decreased cell surface expression of the mutant, sufficient amounts were present to allow for assessment of its functions. Equilibrium binding assays showed that the cell surface hLHR(deltaL608,V609) binds hCG with an affinity comparable to that of the wild-type receptor. However, the cells expressing the hLHR(deltaL608,V609) exhibit only a 1.5- to 2.4-fold stimulation of cAMP production in response to hCG. In contrast, cells expressing comparably low levels of hLHR(wt) responded to hCG with 11- to 30-fold increases of cAMP levels. Therefore, the testicular and ovarian unresponsiveness to LH in these patients appears to be due to a mutation of the hLHR gene in which Leu-608 and Val-609 are deleted. As a consequence, the majority of the mutant receptor is retained intracellularly. The small percentage of mutant receptor that is expressed at the cell surface binds hormone normally but is unable to activate Gs.