A deletion mutation in the third cytoplasmic loop of the mouse m1 muscarinic acetylcholine receptor unmasks cryptic G-protein binding sites

Ligand function at constitutively active receptor mutants is affected by two distinct yet interacting mechanisms

It has been proposed that mutations that induce constitutive activity in G-protein-coupled receptors (GPCRs) concomitantly enhance the ability of partial agonists to trigger second-messenger signaling. Using the cholecystokinin type 2 receptor (CCK-2R) as a model system, we have explored whether this association applies to a diverse set of activating mutations. Consistent with established principles, constitutively active CCK-2Rs resulting from amino acid substitutions within the third intracellular loop each systematically increased partial agonist activities versus corresponding wild-type values. In contrast, activating mutations within transmembrane domain segments near the extracellular loops led to an increase in efficacy of only a subset of compounds but decreased or did not change the function of others. When transmembrane domain amino acid substitutions were introduced in combination with intracellular amplifying mutations, observed changes in ligand activity were defined by the product of two discernible factors 1) systematic amplification caused by an equilibrium shift from the inactive to the active receptor conformation and 2) ligand-specific alterations in signaling, which probably result from mutation-induced changes in the putative binding pocket. These findings illustrate functional heterogeneity among GPCR mutants with ligand-independent signaling. A subgroup of activating mutations facilitates receptor isomerization to the active state and in parallel perturbs ligand receptor interactions. These mutants do not adhere to the previously proposed "hallmark criteria" of constitutive activity.

Biological activities of two porcine growth hormone-releasing hormone receptor isoforms

Binding of growth hormone-releasing hormone (GHRH) to two isoforms (G3R and G5R) of the porcine GHRH receptor was studied. Both G3R- and G5R-cDNA were isolated from a porcine anterior pituitary cDNA library and have an identical primary structure from aa 1 to 418 and a different aa sequence from aa 419 to 423. In addition, the G5R isoform contains an extra C-terminal tail of 28 aa. The G3R and G5R mRNAs arise from alternative splicing of a single precursor mRNA for GHRH receptors. A mammalian cell expression vector containing either G3R or G5R cDNA under the regulation of a strong human cytomegalovirus promoter was constructed and used to transfect a human embryonic kidney 293 cell line. Two stable transfectants (293/G3R-4 and 293/G5R-12) were isolated on the basis of high expression of the receptor mRNAs. Both G3R and G5R mRNAs were expressed at similarly high levels in 293/G3R-4 and 293/G5R-12 cells; however, GHRH binding to 293/G3R-4 cells was much greater than that observed for 293/G5R-12 cells. Basal as well as GHRH-stimulated GTPase activity and intracellular cAMP concentration are also significantly greater in 293/G3R-4 cells as compared to 293/G5R-12 cells. We conclude that the modification of GHRH receptor at the C-terminal region hindered GHRH binding to the receptor and thus attenuates its biological activities.

Distinct internalization of M2 muscarinic acetylcholine receptors confers selective and long-lasting desensitization of signaling to phospholipase C

Although M1-M4 muscarinic acetylcholine receptors (mAChRs) in HEK-293 cells internalize on agonist stimulation, only M1, M3, and M4 but not M2 mAChRs recycle to the plasma membrane. To investigate the functional consequences of this phenomenon, we compared desensitization and resensitization of M2 versus M4 mAChRs. Treatment with 1 mM carbachol for 1 h at 37 degrees C reduced numbers of cell surface M2 and M4 mAChRs by 40-50% and M2 and M4 mAChR-mediated inhibition of adenylyl cyclase, intracellular Ca2+ concentration ([Ca2+]i) increases, and phospholipase C (PLC) activation by 60-70%. Receptor-mediated inhibition of adenylyl cyclase and [Ca2+]i increases significantly resensitized within 3 h. However, M4 but not M2 mAChR-mediated PLC activation resensitized. At 16 degrees C, M2 mAChR-mediated [Ca2+]i increases and PLC stimulation desensitized to a similar extent as at 37 degrees C. However, at 16 degrees C, where M2 mAChR internalization is negligible, both M2 mAChR responses resensitized, demonstrating that M2 mAChR resensitization proceeds at the plasma membrane. Examination of M2 mAChR responses following inactivation of cell surface mAChRs by quinuclidinyl benzilate revealed substantial receptor reserve for coupling to [Ca2+]i increases but not to PLC. We conclude that M2 mAChR internalization induces long-lasting PLC desensitization predominantly because receptor loss is not compensated for by receptor recycling or receptor reserve.

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.

Review: amino acid domains involved in constitutive activation of G-protein-coupled receptors

Guanine nucleotide-binding protein-coupled receptors may attain an active conformation in the absence of agonist by spontaneous isomerization and thus yield constitutive, agonist-independent, activity. This has mainly been demonstrated for isolated membranes and recombinant wild-type receptors, and mutant receptors. They generally show remarkable increases in the sensitivity of a biological response. The location of activating mutations both within a single receptor and across receptors is widespread, with changes reported in the seven-transmembrane domains, the second and third intracellular loop. For most of these receptors, examples of ligands defined as inverse agonists have been documented. Regulation of these receptors by inverse agonists opposite to that observed by agonists, and the therapeutic potential of inverse agonists is underlined.

Interruption of G protein-coupling in CXCR2 does not alter ligand binding, but eliminates ligand-activation of GTPgamma35S binding, calcium mobilization, and chemotaxis

CXCR2 is a seven-transmembrane receptor that transduces intracellular signals in response to the chemokines IL-8, MGSA/GRO, and other ELR motif-containing CXC chemokines by coupling to heterotrimeric GTP-binding proteins. In this study, we have mutated two putative G protein-coupling regions of CXCR2 and characterized the effects of these mutations on ligand-activated signal transductions: aspartic acid 89 in the second transmembrane domain and the HRAMR sequence (BBXXB motif, found in the third intracellular loop where B indicates a basic amino acid and X represents any amino acid). The Asp89 was replaced by either asparagine (D89N) or glutamic acid (D89E). For the BBXXB motif, the first two basic amino acids were mutated to two neutral isoleucines (HR-II), or alternatively, two isoleucines were inserted between alanine and methionine (II-insert). When expressed in human embryonic kidney 293 cells, the D89E mutant was localized intracellularly with no detectable cell surface expression. In contrast, D89N, HR-II, and II-insert mutants displayed cell surface expression, with Kd values and expression levels similar to that of the wild-type transfectant. The ability of the mutants to transduce signal was assessed by ligand-stimulated GTPgamma35S binding, mobilization of intracellular free Ca2+, and chemotaxis assays. Both D89N and HR-II mutants signaled similarly to a wild-type receptor in all three assays. However, the II-insert mutant exhibited a loss of ligand-stimulated GTPgamma35S binding, calcium mobilization, and chemotaxis. Unexpectedly, this receptor underwent ligand-induced sequestration comparable to wild-type CXCR2. These data indicate that Asp89 and the basic amino acids in the third intracellular domain do not play essential roles in ligand-induced signal transduction through CXCR2. However, proper secondary structure and orientation of the third intracellular loop of CXCR2 are essential for ligand-mediated signal transduction but not for receptor sequestration.

Random mutagenesis of the cAMP chemoattractant receptor, cAR1, of Dictyostelium. Evidence for multiple states of activation

cAMP receptor 1 (cAR1) of Dictyostelium couples to the G protein G2 to mediate activation of adenylyl and guanylyl cyclases, chemotaxis, and cell aggregation. Other cAR1-dependent events, including receptor phosphorylation and influx of extracellular Ca2+, do not require G proteins. To further characterize signal transduction through cAR1, we performed random mutagenesis of the third intracellular loop (24 amino acids), since the corresponding region of other seven helix receptors has been implicated in the coupling to G proteins. Mutant receptors were expressed in car1(-) cells and were characterized for G protein-dependent and -independent signal transduction. Our results demonstrate that cAR1 is remarkably tolerant to amino acid substitutions in the third intracellular loop. Of the 21 positions where amino acid substitutions were observed, one or more replacements were found that retained full biological function. However, certain alterations resulted in receptors with reduced ability to bind cAMP and/or transduce signals. There were specific signal transduction mutants that could undergo cAMP-dependent cAR1 phosphorylation but were impaired either in coupling to G proteins, in G protein-independent Ca2+ influx, or in both pathways. In addition, there were general activation mutants that failed to restore aggregation to car1(-) cells and displayed severe defects in all signal transduction events, including the most robust response, cAMP-dependent cAR1 phosphorylation. Certain of these mutant phenotypes were obtained in a complementary study, where the entire region of cAR1 from the third to the seventh transmembrane helices was randomly mutagenized. Considered together, these studies indicate that the activation cycle of cAR1 may involve a number of distinct receptor intermediates. A model of G protein-dependent and -independent signal transduction through cAR1 is discussed.

Identification of a region required for subtype-specific agonist-induced sequestration of the m2 muscarinic acetylcholine receptor

When the m1 and m2 muscarinic acetylcholine receptors are transiently expressed in JEG-3 cells, the m2, but not the m1, receptor undergoes agonist-induced sequestration. Both receptors exhibit internalization when expressed in Y1 cells. These results suggest that the m1 and m2 receptors use distinct cellular mechanisms or pathways for agonist-induced internalization and that JEG-3 cells are deficient in the mechanism or pathway used by the m1 receptor. Transfection experiments with chimeric receptors indicate that the specificity for agonist-induced internalization for the m2 receptor lies in the carboxyl-terminal fifth of the receptor. The intracellular carboxyl-terminal tail of the m2 receptor is neither sufficient nor required for the m2-specific sequestration. Site-directed mutagenesis demonstrates that two amino acids in the carboxyl-terminal end of the third cytoplasmic loop of the m2 receptor are required for sequestration in JEG-3 cells. In addition, the sixth transmembrane domain, which is adjacent to this cytoplasmic domain, is also required. Thus, m2-specific agonist-induced sequestration requires sequences both in the carboxyl-terminal end of the third cytoplasmic loop and the adjacent transmembrane domain.

His381 of the rat CCKB receptor is essential for CCKB versus CCKA receptor antagonist selectivity

A great interest is devoted to antagonists of the cholecystokinin type B (CCKB) receptor such as L-365,260, which reduces panic attacks in humans and to antagonists of the cholecystokinin type A (CCKA) receptor, such as L-364,718 which might be efficient in mental diseases. The A/B specificity of these antagonists was proposed to be mainly dependent on the amino acid sequence of the seventh transmembrane domain (Mantamadiotis and Baldwin (1994) Biochem. Biophys. Res. Commun. 201,1382). In our study, one of these residues, His381 was replaced in the rat CCKB receptor by leucine (the corresponding residue in the CCKA receptor), phenylalanine or arginine using site-directed mutagenesis. Changing histidine for leucine or phenylalanine did not modify significantly the affinity of the CCKB receptor antagonists, L-365,260 and PD-134,308 although both compounds belong to different chemical classes, but strongly improved the affinity of the CCKA receptor antagonists tested. Interestingly, the A selectivity of these CCKA receptor antagonists was recovered by substituting His381 by arginine. Moreover, these results are discussed on the basis of a three dimentional model of the CCKB receptor. The mutated receptors possessed unchanged binding properties for agonists, suggesting that determinants confering specificity for agonists and antagonists are different.

Purification of chlorpromazine-sensitive GTPase from rat cerebral cortex

The chlorpromazine-sensitive GTPase from the cell membrane of rat cerebral cortex was purified to homogenity by using DEAE Bio-Gel A agarose, hydroxyapatite and heparin agarose chromatography. The purified chlorpromazine-sensitive GTPase was purified 370-fold to obtain a final specific activity of 40 mumol GTP hydrolyzed2min/mg protein. The purified enzyme was inhibited by chlorpromazine but not by compound 48/80. Magnesium was required for its activity instead of calcium. The purified enzyme had an apparent pH optimum of 8.0, and molecular weight was estimated to be 58,000.

Cholecystokinin and gastrin are not equally sensitive to GTP gamma S at CCKB receptors: importance of the sulphated tyrosine

We have shown that gastrin and cholecystokinin octapeptide (CCK-8) are differently coupled to G protein (GTP-binding protein) through type B cholecystokinin receptors in guinea-pig brain membranes and Jurkat cells. Indeed, the gastrin-13 binding affinity is strongly reduced by stable guanyl nucleotides, whereas CCK-8 binding is only slightly affected. In order to determine the structural requirements regulating such coupling, we have synthesized several gastrin and cholecystokinin fragments (sulphated or unsulphated) elongated at the N-terminus of the common C-terminal tetrapeptide. We investigated their interaction with CCKB receptors in guinea pig brain membranes and Jurkat cells and their involvement in the G protein coupling. Their apparent binding affinities to CCKB receptors were measured by inhibition of [125I]Bolton Hunter-CCK-8 (3-[125I]iodo-4-hydroxyphenyl)propionyl-CCK-8) binding in the presence or absence of GTP gamma S (guanosine 5'-O-(3-thio)triphosphate) or aluminum tetrafluoride (AlF4-). Activation of the G proteins by GTP gamma S or AlF4- led to a decrease in binding affinity for the gastrin related peptides, the common CCK-gastrin C-terminal forms, the cholecystokinin hexapeptide and the unsulphated cholecystokinin heptapeptide. Sulphated CCK-7, CCK-8, and cionin apparent binding affinities were not affected. These finding indicated that the sulphated tyrosine in position 7 in CCK (as counted from the C-terminus), provides the cholecystokinin selectivity for the CCKB receptor compared to gastrin. The results are discussed with the aim to better clarify the physiological relevance of gastrin and cholecystokinin toward CCKB receptors and their related intracellular events.

Interleukin-8 receptor beta. The role of the carboxyl terminus in signal transduction

Two interleukin-8 (IL-8) receptors, alpha and beta, have been identified and cloned. Both receptors are thought to transduce signals by coupling to GTP-binding proteins. The aim of this study is to determine whether the carboxyl terminus (C') of IL-8 receptor beta (IL-8R beta) is involved in signaling in response to IL-8. We have constructed a number of IL-8R beta genes that encode truncated forms of the IL-8R beta. The deletions consisted of amino acids 349-355, 336-355, 325-355, and 317-355 (termed beta 2, beta 3, beta 4, and beta 5, respectively). 293 human embryonic kidney cells were transfected with the wild type IL-8R beta (beta 1) and with these mutants. Cells transfected with the mutated receptors expressed the receptors and bound IL-8 with the same high affinity as cells transfected with the wild type receptor. The capacity of the mutated receptors to convey functional signals was evaluated by comparing the chemotaxis index of cells expressing the C'-truncated receptors to the index of cells expressing the wild type receptor. The results indicate that while cells expressing beta 1, beta 2, beta 3, and beta 4 were chemoattracted in response to IL-8, cells expressing beta 5 did not migrate in response to IL-8 stimulation. Therefore, the data suggest that amino acids 317-324 are involved in signaling by IL-8R beta.

Activating and inactivating mutations in N- and C-terminal i3 loop junctions of muscarinic acetylcholine Hm1 receptors

The N- and C-terminal junctions of the third intracellular loop (i3) of G protein-coupled receptors play a role in the coupling process. We had previously constructed two triple point alanine mutants of the i3 junction of the muscarinic Hm1 receptor, W209A/I211A/Y212A and E360A/K362A/T366A, which are defective in mediating carbachol stimulation of phosphatidylinositol (PI) turnover (Moro, O., Lameh, J., Högger, P., and Sadée, W. (1993) J. Biol. Chem. 268, 22273-22276). Each of the corresponding six single point mutations were constructed to determine residues crucial to receptor coupling. Mutants W209A and T366A were similar to or only slightly less effective than wild type Hm1 in stimulating PI turnover. In the N-terminal junction, I211A and Y212A were defective in coupling, and I211A was even more defective than the corresponding triple mutant. Therefore, the triple mutation compensated at least partially for the effect of these two single point mutations. In the C-terminal i3 loop junction, mutant K362A was again more strongly defective than the corresponding triple mutant. In contrast, mutation E360A was found to be activating, leading to elevated PI turnover in the absence of agonist and sensitization toward carbachol activation. Activating mutations in the C-terminal i3 loop junction have been reported previously for the adrenergic receptors, but E360A represents the first muscarinic receptor with substantial basal activity. The effects of the single point mutations observed in this study were not readily predictable from similar mutations from closely related G protein-coupled receptors despite sequence conservation in the i3 loop junctions. Our results caution against defining precise coupling domains in these regions by mutagenesis results.

Gs couples thyrotropin-releasing hormone receptors expressed in Xenopus oocytes to phospholipase C

Coupling of thyrotropin-releasing hormone (TRH) receptors to individual G-proteins has been studied in Xenopus oocytes injected with receptor cRNA and antisense oligonucleotides to mRNA encoding different G-protein alpha- and beta-subunits. Injection of antisenses which target mRNA sequences shared by several G-protein alpha or beta gamma polypeptides effectively blocked Ca(2+)-dependent Cl- currents induced by TRH through activation of phospholipase C. Three different alpha s-specific antisense oligonucleotides complementary to sequences located in different positions along the coding region of the alpha s protein mRNA were highly effective in inhibiting TRH-induced responses. Anti-alpha o, -alpha q, -alpha i, or -alpha z oligonucleotides were not able to modify the TRH-evoked response. In contrast, anti-alpha o, but not anti-alpha s, oligonucleotides blocked the response to serotonin in oocytes injected with serotonin 5-HT1c receptor cRNA. Cholera toxin catalyzed the [32P]ADP-ribosylation of 40-42- and 50-52-kDa proteins in GH3 cell plasma membranes. [32P]ADP-ribosylation of oocyte membranes with the toxin labeled several proteins. These include a single 50-55-kDa substrate, which is clearly diminished in membranes from anti-alpha s-injected oocytes. Amplification of oocyte RNA in a polymerase chain reaction system and sequencing of the amplified products demonstrated that anti-alpha s oligonucleotides selectively recognize the message for the Xenopus alpha s polypeptide. It is concluded that Gs, but not Go, Gq, Gi, or Gz, couples TRH receptors expressed in oocytes to activation of phospholipase C and subsequent inositol 1,4,5-trisphosphate-dependent stimulation of Ca(2+)-dependent Cl- currents.