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

Distinct mechanisms of cAMP induction by constitutively activating LH receptor and wild-type LH receptor activated by hCG

Asp578Gly is the major mutation of luteinizing hormone (LH) receptors in humans. It is a dominant mutant, constitutively activates Galphas, and induces cAMP production in the absence of the cognate hormone, causing the familial male precocious puberty. The mechanism of the elevated basal cAMP level is unclear. Our data show strikingly different mechanisms between the elevated basal cAMP induced by the activating mutant and the cAMP induced by the wild-type receptor activated by human chorionic gonadotropin (hCG) binding. The study suggests an approach to attenuating the elevated basal cAMP of the activating mutant LH receptor, which could be useful for controlling the familial male precocious puberty. For the study, we used the C-terminal peptides of Galphas and Galphai2, which couple to the receptor.

Cooperative conformational changes in a G-protein-coupled receptor dimer, the leukotriene B(4) receptor BLT1

We have used an isolated receptor, the leukotriene B(4) receptor BLT1, to analyze the mechanism of receptor activation in a G-protein-coupled receptor dimer. The isolated receptor is essentially a dimer whether the agonist is present or not, provided the detergent used stabilizes the inactive dimeric assembly. We have produced a receptor mutant where Cys(97) in the third transmembrane domain has been replaced by a serine. This mutation leads to an approximately 100-fold decrease in the affinity for the agonist. 5-Hydroxytryptophan has then been introduced at position 234 in the C97A mutant sixth transmembrane domain. Agonist binding to the labeled receptor is associated with variations in the fluorescence properties of 5-hydroxytryptophan due to specific agonist-induced conformational changes. The C97A mutant labeled with 5-hydroxytryptophan has then been associated with a wild-type receptor in a dimeric complex that has been subsequently purified. The purified complex activates its G-protein partner in a similar manner as the wild-type homodimer. Due to the difference in the affinity for the agonist between the wild-type and mutant protomers in this dimer, we have been able to reach a state where one of the protomers, the mutant, is in its unliganded state, whereas the other, the wild type, is loaded with the agonist. We show that agonist binding to the wild-type receptor induces specific changes in the conformation of the unliganded protomer, as evidenced by the variations in the emission of the 5-hydroxytryptophan residue in the mutant receptor. These data provide a direct demonstration for agonist-induced cooperative conformational changes in a GPCR dimer.

Subunits of a Yeast Oligomeric G Protein-coupled Receptor Are Activated Independently by Agonist but Function in Concert to Activate G Protein Heterotrimers

G protein-coupled receptors (GPCRs) form dimeric or oligomeric complexes in vivo. However, the function of oligomerization in receptor-mediated G protein activation is unclear. Previous studies of the yeast alpha-factor receptor (STE2 gene product) have indicated that oligomerization promotes signaling. Here we have addressed the mechanism by which oligomerization facilitates G protein signaling by examining the ability of ligand binding- and G protein coupling-defective alpha-factor receptors to form complexes in vivo and to correct their signaling defects when co-expressed (trans complementation). Newly and previously identified receptor mutants indicated that ligand binding involves the exofacial end of transmembrane domain (TM) 4, whereas G protein coupling involves ic1, ic3, the C-terminal tail, and the intracellular ends of TM2 and TM3. Mutant receptors bearing substitutions in these domains formed homo-oligomeric or hetero-oligomeric complexes in vivo, as indicated by results of fluorescence resonance energy transfer experiments. Co-expression of ligand binding- and G protein coupling-defective mutant receptors did not significantly improve signaling. In contrast, co-expression of ic1 and ic3 mutations in trans but not in cis significantly increased signaling efficiency. Therefore, we suggest that subunits of the alpha-factor receptor: 1) are activated independently rather than cooperatively by agonist, and 2) function in a concerted fashion to promote G protein activation, possibly by contacting different subunits or regions of the G protein heterotrimer.

Constitutive and Agonist-dependent Self-association of the Cell Surface Human Lutropin Receptor

The human lutropin receptor (hLHR) is a G protein-coupled receptor (GPCR) that plays an essential role in reproductive physiology. The present studies were undertaken to determine whether the hLHR self-associates. We show that high molecular weight complexes of the hLHR can be co-immunoprecipitated from 293 cells transfected with differentially tagged hLHRs. These complexes are detected only in extracts from cells that have been co-transfected and not in extracts combined from cells expressing only one form of tagged hLHR, confirming the in vivo self-association of the receptor. In transiently transfected cells, in which a small percentage of cells overexpress hLHR and most of the hLHR is located intracellularly in the ER, the self-associated hLHR is composed predominantly of immature hLHR. When cells were transiently co-transfected with wild-type hLHR and a misfolded mutant of the hLHR, a physical association of the ER-localized misfolded mutant with the immature hLHR was observed, resulting in a decreased cell surface expression of the wild-type receptor. In contrast, in stably transfected cells, where the majority of cells express receptor and there is much less intracellular accumulation of hLHR, the self-associated forms of the hLHR are composed predominantly of cell surface receptor. The abundance of cell surface hLHR dimers and oligomers, as detected on SDS gels, is increased further upon human choriogonadotropin treatment of the stably transfected cells. In addition to documenting the self-association of cell surface hLHR, our results underscore the importance of the cellular distribution of recombinant GPCR as it relates to the nature of the GPCR dimerization and oligomerization.

Trans-activation of mutant follicle-stimulating hormone receptors selectively generates only one of two hormone signals

Previously, we reported that a liganded LH receptor (LHR) is capable of activating itself (cis-activation) and other nonliganded LHRs to induce cAMP (trans-activation). Trans-activation of the LHR raises two crucial questions. Is trans-activation unique to LHR or common to other G protein-coupled receptors? Does trans-activation stimulate phospholipase Cbeta as it does adenylyl cyclase? To address these questions, two types of novel FSH receptors (FSHRs) were constructed, one defective in hormone binding and the other defective in signal generation. The FSHR, a G protein-coupled receptor, comprises two major domains, the N-terminal extracellular exodomain that binds the hormone and the membrane-associated endodomain that generates the hormone signals. For signal defective receptors, the exodomain was attached to glycosyl phosphatidylinositol (ExoGPI) or the transmembrane domain of CD8 immune receptor (ExoCD). ExoGPI and ExoCD can trans-activate another nonliganded FSH. Surprisingly, the trans-activation generates a signal to activate either adenylyl cyclase or phospholipase Cbeta, but not both. These results indicate that trans-activation in these mutant receptors is selective and limited in signal generation, thus providing new approaches to investigating the generation of different hormone signals and a novel means to selectively generate a particular hormone signal. Our data also suggest that the FSHR's exodomain could not trans-activate LHR.

Dimers of class A G protein-coupled receptors function via agonist-mediated trans-activation of associated G proteins

The histamine H1 receptor and the alpha1b-adrenoreceptor are G protein-coupled receptors that elevate intracellular [Ca2+] via activation of Gq/G11. Assessed by co-immunoprecipitation and time-resolved fluorescence resonance energy transfer they both exist as homo-dimers. The addition of the G protein G11alpha to the C terminus of these receptors did not prevent dimerization. Agonists produced a large stimulation of guanosine 5'-3-O-([35S]thio)triphosphate ([35S]GTPgammaS) binding to receptor-G protein fusions containing wild type forms of both polypeptides. For both receptors this was abolished by incorporation of G208AG11alpha into the fusions. Mutation of a highly conserved leucine in intracellular loop 2 of each receptor also eliminated agonist function but not binding. Co-expression of the two non-functional but complementary fusion constructs reconstituted agonist-mediated binding of [35S]GTPgammaS in membranes of HEK293 cells and elevation of [Ca2+]i in mouse embryo fibroblasts lacking both Gq and G11. Co-expression of the histamine H1 receptor- and the alpha1b-adrenoreceptor-G11alpha fusions allowed detection of functional hetero-dimeric complexes, whereas co-expression of histamine H1 receptor-G11alpha with increasing amounts of L151Dalpha1b-adrenoreceptor resulted in decreasing levels of histamine-stimulated [35S]GTPgammaS binding. Co-expression of the alpha1b-adrenoreceptor with a fusion protein incorporating the N-terminal domain and transmembrane helix 1 of the alpha1b-adrenoreceptor and G11alpha did not result in agonist activation of the G protein but did indicate a role for transmembrane helix 1 in dimerization. These data demonstrate that dimers of these class A receptors function via trans-activation of associated G proteins.

Ligand-independent CXCR2 dimerization

Homo- and hetero-oligomerization have been reported for several G protein-coupled receptors (GPCRs). The CXCR2 is a GPCR that is activated, among the others, by the chemokines CXCL8 (interleukin-8) and CXCL2 (growth-related gene product beta) to induce cell chemotaxis. We have investigated the oligomerization of CXCR2 receptors expressed in human embryonic kidney cells and generated a series of truncated mutants to determine whether they could negatively regulate the wild-type (wt) receptor functions. CXCR2 receptor oligomerization was also studied by coimmunoprecipitation of green fluorescent protein- and V5-tagged CXCR2. Truncated CXCR2 receptors retained their ability to form oligomers only if the region between the amino acids Ala-106 and Lys-163 was present. In contrast, all of the deletion mutants analyzed were able to form heterodimers with the wt CXCR2 receptor, albeit with different efficiency, competing for wt/wt dimer formation. The truncated CXCR2 mutants were not functional and, when coexpressed with wt CXCR2, interfered with receptor functions, impairing cell signaling and chemotaxis. When CXCR2 was expressed with the AMPA-type glutamate receptor GluR1, CXCR2 dimerization was again impaired in a dose-dependent way, and receptor functions were prejudiced. In contrast, CXCR1, a chemokine receptor that shares many similarities with CXCR2, did not dimerize alone or with CXCR2 and when coexpressed with CXCR2 did not impair receptor signaling and chemotaxis. The formation of CXCR2 dimers was also confirmed in cerebellar neuron cells. Taken together, we conclude from these studies that CXCR2 functions as a dimer and that truncated receptors negatively modulate receptor activities competing for the formation of wt/wt dimers.

Yoked complexes of human choriogonadotropin and the lutropin receptor: evidence that monomeric individual subunits are inactive

Human choriogonadotropin (hCG) contains an alpha-subunit, common to other members of the glycoprotein hormone family, and a unique beta-subunit that determines hormone specificity. It is generally thought that heterodimer formation is obligatory for full hormonal activity, although other studies have indicated that individual subunits and homodimeric hCGbeta were capable of low affinity binding to the LH receptor (LHR) and subsequent activation. Previously, we constructed two yoked hormone (hCG)-LHR complexes, where the two hormone subunits and the heptahelical receptor were engineered to form single polypeptide chains, i.e. N-beta-alpha-LHR-C and N-alpha-beta-LHR-C. Expression of both complexes led to constitutive stimulation of cAMP production. In the present study, we investigated whether the human alpha-subunit and hCGbeta can act as functional agonists when covalently attached to or coexpressed with the LH receptor. Our initial results showed that hCGbeta, but not alpha, was able to activate LHR with an increase in intracellular cAMP in human embryonic kidney 293 cells but not in Chinese hamster ovary or COS-7 cells. Further examination of this apparent cell-specific agonist activity of hCGbeta revealed that low levels of endogenous alpha-subunit were expressed in human embryonic kidney 293 cells, thus enabling sufficient amounts of active heterodimer to form with the transfected hCGbeta to activate LHR. The studies in Chinese hamster ovary and COS-7 cells clearly demonstrate that, even under experimental conditions where hormone-receptor interactions are maximized, individual subunits of hCG can not act as functional agonists, at least in their monomeric form.