A C-terminally truncated human parathyroid hormone receptor is functional and activates multiple G proteins

A Comprehensive Structure-Function Map of the Intracellular Surface of the Human C5a Receptor

Within any given cell many G protein-coupled receptors are expressed in the presence of multiple G proteins, yet most receptors couple to a specific subset of G proteins to elicit their programmed response. Numerous studies demonstrate that the carboxyl-terminal five amino acids of the Galpha subunits are a major determinant of specificity, however the receptor determinants of specificity are less clear. We have used a collection of 133 functional mutants of the C5a receptor obtained in a mutagenesis screen targeting the intracellular loops and the carboxyl terminus (Matsumoto, M. L., Narzinski, K., Kiser, P. D., Nikiforovich, G. V., and Baranski, T. J. (2007) J. Biol. Chem. 282, 3105-3121) to investigate how specificity is encoded. Each mutant, originally selected for its ability to signal through a nearly full-length Galpha(i) in yeast, was tested to see whether it could activate three versions of chimeric Galpha subunits consisting of Gpa1 fused to the carboxyl-terminal five amino acids of Galpha(i), Galpha(q), or Galpha(s) in yeast. Surprisingly the carboxyl-terminal tail of the C5a receptor is the most important specificity determinant in that nearly all mutants in this region showed a gain in coupling to Galpha(q) and/or Galpha(s). More than half of the receptors mutated in the second intracellular loop also demonstrated broadened G protein coupling. Given a lack of selective advantage for this broadened signaling in the initial screen, we propose a model in which the carboxyl-terminal tail acts together with the intracellular loops to generate a specificity filter for receptor-G protein interactions that functions primarily to restrict access of incorrect G proteins to the receptor.

PKC-mediated modulation of L-type calcium channels may contribute to fat-induced insulin resistance

Increased intracellular free calcium [Ca2+]i has been noted in adipocytes, platelets, and leukocytes of subjects with insulin resistance syndrome or allied disorders. In rodent studies, measures which increase [Ca2+]i in adipocytes and skeletal muscle are associated with impaired insulin signaling, attributable at least in part to diminished ability of insulin to activate phosphoserine phosphatase-1 (PP-1). In fat-fed insulin resistant rats, pre-treatment with a drug that selectively chelates intracellular calcium eliminates about half of the decrement in insulin-stimulated glucose uptake induced by fat feeding; since this chelator does not influence the insulin sensitivity of chow-fed rats, it is reasonable to suspect that fat feeding boosts [Ca2+]i in skeletal muscle, and that this effect is partially responsible for the associated reduction in insulin sensitivity. Clinical insulin resistance is associated with increased levels of triglycerides and other fatty acid metabolites in muscle fibers; this can give rise to diacylglycerol-mediated activation of PKC, which in turn compromises insulin signaling by triggering kinase cascades that phosphorylate IRS-1 on key serine residues. Yet there is also evidence that, in skeletal muscle, PKC activity up-regulates the function of L-type calcium channels, increasing their maximal conductance while left-shifting their voltage dependence. Thus, the PKC activation associated with fat overexposure might be expected to boost basal [Ca2+]i in skeletal muscle, potentially impeding insulin-mediated activation of PP-1. This hypothesis is consistent with several clinical studies demonstrating that long-acting inhibitors of L-type calcium channels can improve insulin sensitivity in overweight hypertensives; it should be readily testable in rodent models of fat-induced insulin resistance. Since parathyroid hormone can act on adipocytes and muscle to boost [Ca2+]i, mild secondary hyperparathyroidism associated with low calcium intakes and poor vitamin D status may contribute to insulin resistance, consistent with certain clinical and epidemiological findings. Magnesium, often thought of as a mild calcium antagonist, appears to have favorable effects on insulin sensitivity and risk for diabetes, and recent evidence indicates that increases of intracellular magnesium within the physiological range can diminish calcium influx through phosphorylated L-type calcium channels. It will be of interest to determine whether calcium antagonism does indeed underlie the favorable influence of good magnesium status on insulin function. A report that chromium picolinate can induce the plasmalemmal Ca2+-ATPase in smooth muscle cells, raises the possibility that modulation of calcium transport might play a role in the insulin-sensitizing efficacy of bioactive chromium.

Isogenic human cell lines for drug discovery: regulation of target gene expression by engineered zinc-finger protein transcription factors

Isogenic cell lines differing only in the expression of the protein of interest provide the ideal platform for cell-based screening. However, related natural lines differentially expressing the therapeutic target of choice are rare. Here the authors report a strategy for drug screening employing isogenic human cell lines in which the expression of the target protein is regulated by a gene-specific engineered zinc-finger protein (ZFP) transcription factor (TF). To demonstrate this approach, a ZFP TF activator of the human parathyroid hormone receptor 1 (PTHR1) gene was identified and introduced into HEK293 cells (negative for PTHR1). Following induction of ZFP TF expression, this cell line produced functional PTHR1 protein, resulting in a robust and ligand-specific cyclic adenosine monophosphate (cAMP) response. Reciprocally, the natural expression of PTHR1 observed in SAOS2 cells was dramatically reduced by the introduction of the appropriate PTHR1-specific ZFP TF repressor. Moreover, this ZFP-driven PTHR1 repression selectively eliminated the functional cAMP response invoked by known ligands of PTHR1. These data establish ZFP TF-generated isogenic lines as a general approach for the identification of therapeutic agents specific for the target gene of interest.

Galpha protein selectivity determinant specified by a viral chemokine receptor-conserved region in the C tail of the human herpesvirus 8 g protein-coupled receptor

The viral G-protein coupled receptor (vGPCR) specified by human herpesvirus 8 (HHV-8) open reading frame 74 (ORF74) is a ligand-independent chemokine receptor that has structural and functional homologues among other characterized gammaherpesviruses and related receptors in the betaherpesviruses. Sequence comparisons of the gammaherpesvirus vGPCRs revealed a highly conserved region in the C tail, just distal to the seventh transmembrane domain. Mutagenesis of the corresponding codons of HHV-8 ORF74 was carried out to provide C-tail-altered proteins for functional analyses. By measuring receptor-activated vascular endothelial growth factor promoter induction and NF-kappaB, mitogen-activated protein kinase, and Ca(2+) signaling, we found that while some altered receptors showed general signaling deficiencies, others had distinguishable activation profiles, suggestive of selective Galpha protein coupling. This was supported by the finding that vGPCR and representative functionally altered variants, vGPCR.8 (R322W) and vGPCR.15 (M325S), were affected differently by inhibitors of Galpha(i) (pertussis toxin), protein kinase C (GF109203X), and phosphatidylinositol 3-kinase (wortmannin). Consistent with the signaling data, [(35)S]GTPgammaS incorporation assays revealed preferential coupling of vGPCR.15 to Galpha(q) and an inability of vGPCR.8 to couple functionally to Galpha(q). However, both variants, wild-type vGPCR, and a C-tail deletion version of the receptor were equally able to associate physically with Galpha(q). Combined, our data demonstrate that HHV-8 vGPCR contains discrete sites of Galpha interaction and that receptor residues in the proximal region of the cytoplasmic tail are determinants of Galpha protein coupling specificity.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

The PTH/PTHrP receptor can delay chondrocyte hypertrophy in vivo without activating phospholipase C

One G protein-coupled receptor (GPCR) can activate more than one G protein, but the physiologic importance of such activation has not been demonstrated in vivo. We have generated mice expressing exclusively a mutant form of the PTH/PTHrP receptor (DSEL) that activates adenylyl cyclase normally but not phospholipase C (PLC). DSEL mutant mice exhibit abnormalities in embryonic endochondral bone development, including delayed ossification and increased chondrocyte proliferation. Analysis of the differentiation of embryonic metatarsals in vitro shows that PTH(1-34) and forskolin inhibit, whereas active phorbol ester stimulates, hypertrophic differentiation. Thus, PLC signaling via the PTH/PTHrP receptor normally slows the proliferation and hastens the differentiation of chondrocytes, actions that oppose the dominant effects of PTH/PTHrP receptors and that involve cAMP-dependent signaling pathways.

Signal-selectivity of parathyroid hormone (PTH)/PTH-related peptide receptor-mediated regulation of differentiation in conditionally immortalized growth-plate chondrocytes

Type-1 PTH/PTH-related peptide receptors (PTH1Rs), which activate both adenylyl cyclase and phospholipase C (PLC), control endochondral bone development by regulating chondrocyte differentiation. To directly analyze PTH1R function in such cells, we isolated conditionally transformed clonal chondrocytic cell lines from tibial growth plates of neonatal mice heterozygous for PTH1R gene ablation. Among 104 cell lines isolated, messenger RNAs for PTH1R, collagen II, and collagen X were detected in 28%, 90%, and 29%, respectively. These cell lines were morphologically diverse. Some appeared large, rounded, and enveloped by abundant extracellular matrix; whereas others were smaller, flattened, and elongated. Two PTH1R-expressing clones showed similar PTH1R binding and cAMP responsiveness to PTH and PTH-related peptide but disparate morphologic features, characteristic of hypertrophic (hC1--5) or nonhypertrophic (nhC2--27) chondrocytes, respectively. hC1--5 cells expressed messenger RNAs for collagen II and X, alkaline phosphatase (ALP), and matrix GLA protein, whereas nhC2--27 cells expressed collagen II and Indian hedgehog but not collagen X or ALP. In hC1--5 cells, PTH and cAMP analog, but not phorbol ester, inhibited both ALP and mineralization. PTH1R-null hC1--5 subclones were isolated by in vitro selection and then reconstituted by stable transfection with wild-type PTH1Rs or mutant (DSEL) PTH1Rs defective in PLC activation. ALP and mineralization were inhibited similarly via both forms of the receptor. These results indicate that PLC activation is not required for PTH1R regulation of mineralization or ALP in hypertrophic chondrocytes and are consistent with a major role for cAMP in regulating differentiation of hypertrophic chondrocytes.

New members of the parathyroid hormone/parathyroid hormone receptor family: the parathyroid hormone 2 receptor and tuberoinfundibular peptide of 39 residues

The parathyroid hormone (PTH) family currently includes three peptides and three receptors. PTH regulates calcium homeostasis through bone and kidney PTH1 receptors. PTH-related peptide, probably also through PTH1 receptors, regulates skeletal, pancreatic, epidermal, and mammary gland differentiation and bladder and vascular smooth muscle relaxation and has a CNS role that is under investigation. Tuberoinfundibular peptide of 39 residues (TIP39) was recently purified from bovine hypothalamus based on selective PTH2 receptor activation. PTH2 receptor expression is greatest in the CNS, where it is concentrated in limbic, hypothalamic, and sensory areas, especially hypothalamic periventricular neurons, nerve terminals in the median eminence, superficial layers of the spinal cord dorsal horn, and the caudal part of the sensory trigeminal nucleus. It is also present in a number of endocrine cells. Thus TIP39 and PTH2 receptor-influenced functions may range from pituitary and pancreatic hormone release to pain perception. A third PTH-recognizing receptor has been found in zebrafish.

Role of the carboxyl-terminal region, di-leucine motif and cysteine residues in signalling and internalization of vasopressin V1a receptor

The structural requirements for internalization and signalling of the vasopressin V1a receptor were investigated in stably transfected HEK-293 cells. Removal of the 51 C-terminal amino acids did not affect vasopressin binding, calcium signalling, heterologous desensitization or internalization of the receptor. Deletion of 14 additional amino acids reduced vasopressin-dependent calcium increase and impaired receptor internalization. Substitution of cysteines 371-372 did not affect intracellular signalling, but decreased endocytosis by 26%. Substitution of the 361-362 leucine by alanine residues reduced by 56% V1a receptor sequestration without affecting calcium signalling. These results indicate that di-cysteine and mostly di-leucine motifs present in the C-terminal region of the V1a receptor are involved in its internalization.

Amino-terminal modifications of human parathyroid hormone (PTH) selectively alter phospholipase C signaling via the type 1 PTH receptor: implications for design of signal-specific PTH ligands

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) activate the PTH/PTHrP receptor to trigger parallel increases in adenylyl cyclase (AC) and phospholipase C (PLC). The amino (N)-terminal region of PTH-(1-34) is essential for AC activation. Ligand domains required for activation of PLC, PKC, and other effectors have been less well-defined, although some studies in rodent systems have identified a core region [hPTH-(29-32)] involved in PKC activation. To determine the critical ligand domain(s) for PLC activation, a series of truncated hPTH-(1-34) analogues were assessed using LLC-PK1 cells that stably express abundant transfected human or rat PTH/PTHrP receptors. Phospholipase C signaling and ligand-binding affinity were reduced by carboxyl (C)-terminal truncation of hPTH-(1-34) but were coordinately restored when a binding-enhancing substitution (Glu(19) --> Arg(19)) was placed within hPTH-(1-28), the shortest hPTH peptide that could fully activate both AC and PLC. Phospholipase C, but not AC, activity was reduced by substituting Gly(1) for Ser(1) in hPTH-(1-34) and was eliminated entirely by removing either residue 1 or the alpha-amino group alone. These changes did not alter binding affinity. These findings led to design of an analogue, [Gly(1),Arg(19)]hPTH-(1-28), that was markedly signal-selective, with full AC but no PLC activity. Thus, the extreme N-terminus of hPTH constitutes a critical activation domain for coupling to PLC. The C-terminal region, especially hPTH-(28-31), contributes to PLC activation through effects upon receptor binding but is not required for full PLC activation. The N-terminal determinants of AC and PLC activation in hPTH-(1-34) overlap but are not identical, as subtle modifications in this region may dissociate activation of these two effectors. The [Gly(1),Arg(19)]hPTH-(1-28) analogue, in particular, should prove useful in dissociating AC- from PLC-dependent actions of PTH.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Characterization of the carboxyl-terminal domain of the rat glucose-dependent insulinotropic polypeptide (GIP) receptor. A role for serines 426 and 427 in regulating the rate of internalization

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone involved in the regulation of insulin secretion. In non-insulin-dependent diabetes mellitus insulin responses to GIP are blunted, possibly due to altered signal transduction or reduced receptor number. Site-directed mutagenesis was used to construct truncated GIP receptors to study the importance of the carboxyl-terminal tail (CT) in binding, signaling, and receptor internalization. Receptors truncated at amino acids 425, 418, and 405, expressed in COS-7 or CHO-K1 cells, exhibited similar binding to wild type receptors. GIP-dependent cAMP production with the 405 mutant was decreased in COS-7 cells. Maximal cAMP production in CHO-K1 cells was reduced with all truncated forms. Binding was undetectable with a receptor truncated at amino acid 400; increasing tail length by adding 5 alanines restored binding and signaling. Mutants produced by alanine scanning of residues 394-401, adjacent to transmembrane domain 7, were all functional. CT truncation by 30 or more amino acids, mutation of serines 426/427, singly or combined, or complete CT serine knockout all reduced receptor internalization rate. The majority of the GIP receptor CT is therefore not required for signaling, a minimum chain length of approximately 405 amino acids is needed for receptor expression, and serines 426 and 427 are important for regulating rate of receptor internalization.

Dual signaling and ligand selectivity of the human PTH/PTHrP receptor

Parathyroid hormone (PTH) activates PTH/PTH-related peptide-related receptors (PTHRs) to stimulate both adenylyl cyclase (AC) and phospholipase C (PLC). How these parallel signals mediate specific cellular and tissue responses to PTH, such as the complex anabolic versus catabolic actions of PTH on bone, remains unsettled. Previous studies of PTHR signaling and function employed mainly rodent or other cell lines that express endogenous PTHRs and, possibly, alternate species of PTH receptors. To preclude confounding effects of such receptors, we stably expressed recombinant human PTHRs (hPTHRs) at different levels of surface density in LLC-PK1 porcine renal epithelial cells that lack endogenous PTH responsiveness. hPTH(1-34) induced concentration-dependent activation of both AC and PLC via transfected hPTHRs. Maximal intensity of each signal increased with receptor density, but more hPTHRs were required for PLC than for AC activation. Coupling to AC was saturated at receptor densities too low to detect sustained PLC activation. hPTH(3-34), found by others to be a PLC/protein kinase C (PKC)-selective peptide in rat cells, did not activate PLC via human (or rat) PTHRs under conditions (1 microM peptide, 106 hPTHRs/cell) where hPTH(1-34) stimulated PLC severalfold. Other cellular responses that require PKC activation in these cells, such as sodium-dependent phosphate transport and cAMP-independent secretion of plasminogen activator, were induced by PTH(1-34) but not by hPTH(3-34) or hPTH(7-34). We conclude that amino-truncated PTH analogs reported to activate PKC cannot directly activate phosphatidylinositol-specific PLC via the human or rat PTHR and therefore that PTH receptors may access alternate, PLC-independent pathways of PKC activation in some target cells. The relative intensity of AC and PLC signaling via the hPTHR may be strongly regulated by changes in its surface expression.

Conditionally immortalized murine osteoblasts lacking the type 1 PTH/PTHrP receptor

Osteoblasts synthesize and mineralize bone matrix and are principal target cells for parathyroid hormone (PTH). The type 1 PTH/PTH-related protein (PTHrP) receptor (PTH1R), cloned from rat osteoblastic cells, activates multiple intracellular signaling mechanisms. The specific roles of these PTH1R signals, or of responses to other types of PTH receptors that may be expressed, in regulating osteoblast function are incompletely understood. Use of established mammalian osteoblastic cell lines has led to much understanding of PTH action in bone, although such cells are of neoplastic origin or have other characteristics that compromise their validity as models of normal osteoblasts. To examine the role of the PTH1R in osteoblast biology, we have isolated a series of clonal murine calvarial osteoblastic cell lines that are only conditionally immortalized, via expression of a transgene encoding the tsA58 temperature-sensitive SV40 large T antigen, and that lack both functional alleles of the PTH1R gene. When cultured under nontransforming conditions, these cells stopped proliferating, expressed a series of characteristic osteoblastic genes (including the nonfunctional remnant of the PTH1R gene), and, after 3-4 weeks, produced mineralized bone nodules in a manner that was regulated by 1,25-dihydroxyvitamin D3 but not by PTH(1-84). Cyclic AMP measurements revealed no evidence of expression of alternate species of Gs-linked PTH receptors. Stable transfection with PTH1R cDNA reconstituted both PTH binding and adenylyl cyclase activation, increased basal osteocalcin expression, and supported PTH stimulation of c-Fos expression and matrix mineralization. These conditionally transformed, PTH1R(-/-) clonal osteoblastic cell lines should prove useful for studies of the regulation of osteoblast differentiation and function by both endogenous nonclassical species of PTH (or PTHrP) receptors and mutant signal-selective PTH1Rs.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Gonadotropin-releasing hormone receptor couples to multiple G proteins in rat gonadotrophs and in GGH3 cells: evidence from palmitoylation and overexpression of G proteins

There is evidence in several cell systems suggesting that the GnRH receptor couples to multiple G proteins. Presently there are no published studies showing GnRH receptor coupling to Gialpha, Gsalpha, and Gq/11alpha in a single cell type. To examine this possibility we measured palmitoylation of G proteins in response to GnRH receptor occupancy, since this event is a measure of G-protein activation by cognate receptors. GnRH stimulated time (0-120 min)- and dose (10(-12)-10(-6) g/ml)-dependent palmitoylation of both Gialpha and Gsalpha. Palmitoylation is G-protein activation dependent; accordingly, pertussis toxin (100 ng/ml; PTX), phorbol myristic acid (100 ng/ml), and Antide (50 nM; a GnRH antagonist) did not stimulate palmitoylation of Gialpha or Gsalpha above basal levels. However, cholera toxin (5 microgram/ml), an activator of Gsalpha, stimulated palmitoylation of Gsalpha but not Gialpha. We used a lactotrope-derived cell line expressing the GnRH receptor (GGH3) to examine whether the ability of the receptor to couple multiple G proteins is gonadotroph specific. GGH3 cells were transfected with specific cDNA coding for different G proteins, and agonist-stimulated second messenger production was assessed. Buserelin (a GnRH agonist) stimulated increased cAMP release in Gsalpha cDNA-transfected GGH3 cells, whereas in Gialpha cDNA-transfected cells, both inositol phosphate (IP) production and cAMP release were decreased in response to buserelin. Transfection of Gqalpha, G11alpha, G14alpha, and G15alpha cDNA into GGH3 cells resulted in an increased IP production in response to buserelin, indicating that GnRH receptor couples to this PTX-insensitive G-protein family. The observations presented in this study provide evidence for GnRH receptor coupling to multiple G proteins in a single cell type.

Requirement of membrane-proximal amino acids in the carboxyl-terminal tail for expression of the rat AT1a angiotensin receptor

A series of deletion mutants was created to analyze the function of the membrane-proximal region of the cytoplasmic tail of the rat type 1a (AT1a) angiotensin receptor. In transiently transfected COS-7 cells, the truncated mutant receptors showed a progressive decrease in surface expression, with no major change in binding affinity for the peptide antagonist, [Sar1,Ile8]angiotensin II. In parallel with the decrease in receptor expression, a progressive decrease in angiotensin II-induced inositol phosphate responses was observed. Alanine substitutions in the region 307-311 identified the highly conserved phenylalanine309 and adjacent lysine residues as significant determinants of AT1a receptor expression.

Coupling of D1 and D5 dopamine receptors to multiple G proteins: Implications for understanding the diversity in receptor-G protein coupling

Dopamine receptors are a subclass of the super family of G protein-coupled receptors, that transduce their effects by coupling to specific G proteins. Within the dopamine receptor family, the adenylyl cyclase stimulatory receptors include the D1 and D5 subtypes. The D1 and D5 dopamine receptors are genetically distinct, sharing >80% sequence homology within the highly conserved seven transmembrane spanning domains, but displaying only 50% overall homology at the amino acid level. When expressed in transfected GH4C1 rat pituitary cells, both D1 and D5 receptors stimulate adenylyl cyclase and have identical affinities toward dopaminergic agonists and antagonists. In order to analyze specific signaling pathways mediated by activation of either D1 or D5 receptors, we have identified the G proteins that are coupled to these receptors. Through functional analyses and competition binding studies, and from immunoprecipitation techniques, using antisera against the various alpha subunits of G proteins, we have established that both D1 and D5 receptors couple to G(s)alpha. In addition, D1 receptors are also coupled to G(o)alpha. Since G(o)alpha has been implicated in the regulation of Ca2+, K+, and Na+ channels, this finding would suggest that D1 receptors can mediate the functional activity of these ion channels. There is also evidence to indicate that D5 receptors couple to G(z)alpha, a novel G protein abundantly expressed in neurons. Thus, despite similar pharmacological properties, such differential coupling of D1 and D5 receptors to G proteins other than G(s)alpha, indicates that dopamine can transduce varied signaling responses upon the simultaneous stimulation of both these receptors.

Folding and cell surface expression of the vasopressin V2 receptor: requirement of the intracellular C-terminus

We characterized truncations of the human vasopressin V2 receptor to determine the role of the intracellular C-terminus (comprising about 44 amino acids) in receptor function and cell surface expression. In contrast to the wild-type receptor, the naturally occurring mutant R337X failed to confer specific [3H]AVP binding to transfected cells. In addition, no vasopressin-sensitive adenylyl cyclase was detectable in membrane preparations of these cells. Laser scanning microscopy revealed that c-myc epitope- or green fluorescent protein-tagged R337X mutant receptors were retained within the endoplasmic reticulum. Increasing the number of C-terminal residues (truncations after codons 348, 354 and 356) restored G protein coupling, but revealed a length-dependent reduction of cell surface expression. Replacement of positively charged residues within the C-terminus by glutamine residues also decreased cell surface expression. A chimeric V2 receptor with the C-terminus replaced by that of the beta2-adrenergic receptor did not bind [3H]AVP and was retained within the cell. These data suggest that residues in the N-terminal part of the C-terminus are necessary for correct folding and that C-terminal residues are important for efficient cell surface expression.

A cluster of basic residues in the carboxyl-terminal tail of the short metabotropic glutamate receptor 1 variants impairs their coupling to phospholipase C

Among phospholipase C-coupled metabotropic glutamate receptors (mGluRs), some have a surprisingly long carboxyl-terminal intracellular domain (mGluR1a, -5a, and -5b), and others have a short one (mGluR1b, -1c, and -1d). All mGluR1 sequences are identical up to 46 residues following the 7th transmembrane domain, followed by 313, 20, 11, and 26 specific residues in mGluR1a, mGluR1b, mGluR1c, and mGluR1d, respectively. Several functional differences have been described between the long isoforms (mGluR1a, -5a, and -5b) and the short ones (mGluR1b, -1c, and -1d). Compared with the long receptors, the short ones induce slower increases in intracellular Ca2+, are activated by higher concentration of agonists, and do not exhibit constitutive, agonist-independent activity. To identify the residues responsible for these functional properties, a series of truncated, chimeric, and mutated receptors were constructed. We found that the deletion of the last 19 carboxyl-terminal residues in mGluR1c changed its properties into those of mGluR1a. Moreover, the exchange of the long carboxyl-terminal domain of mGluR5a with that of mGluR1c generated a chimeric receptor that possessed functional properties similar to those of mGluR1c. Mutagenesis of specific residues within the 19 carboxyl-terminal residues of mGluR1c revealed the importance of a cluster of 4 basic residues in defining the specific properties of this receptor. Since this cluster is part of the sequence common to all mGluR1 variants, we conclude that the long carboxyl-terminal domain of mGluR1a suppresses the inhibitory action of this sequence element.

Parathyroid hormone-related protein and human myometrial cells: action and regulation

Parathyroid hormone-related protein (PTH-rP), like parathyroid hormone (PTH), acts on myometrial smooth muscle to cause relaxation, and PTH-rP expression has been demonstrated in the myometrium of pregnant and estrogen-treated nonpregnant rats and in human myometrium, leiomyomata and separated myometrial smooth muscle cells in culture. PTH-rP may facilitate the myometrial quiescence characteristic of the first 95% of normal pregnancy and uterine vasorelaxation. This study was conducted to explore further the function and regulation of expression of PTH-rP in human myometrium. Treatment of myometrial smooth muscle cells with analogs of PTH-rP caused an increase the intracellular levels of cAMP and treatment of myometrial cells with forskolin or dibutyryl cyclic AMP caused an increase in the levels of PTH-rP mRNAs. Tetradecanoyl phorbol acetate (TPA) and okadaic acid caused a striking increase in the levels of PTH-rP mRNAs, much greater than that evoked by forskolin, dibutyryl cAMP, or transforming growth factor-beta (TGF-beta). These findings are supportive of the conclusion that myometrial cells respond to PTH-rP with activation of adenylyl cyclase and that PTH-rP gene expression may be modulated by way of a number of distinct intracellular signaling pathways.

The second intracellular loop of the alpha2-adrenergic receptors determines subtype-specific coupling to cAMP production

The alpha2-adrenergic receptors (alpha2-ARs), which primarily couple to inhibition of cAMP production, have been reported to have a stimulating effect on adenylyl cyclase activity in certain cases. When expressed in Spodoptera frugiperda Sf9 cells the alpha2A subtype showed only inhibition of forskolin-stimulated cAMP production when activated by norepinephrine (NE), whereas the alpha2B subtype displayed a biphasic dose-response curve with inhibition at low concentrations of NE and a potentiation at higher concentrations. To further investigate the subtype-specific coupling, we expressed a set of chimeric alpha2A-/alpha2B-ARs at similar expression levels in Sf9 cells to determine the structural domain responsible for the difference between the two subtypes. When the third intracellular loops were interchanged between alpha2A and alpha2B subtypes, the coupling specificity remained unchanged, indicating that this loop does not confer selectivity toward a stimulating response. A biphasic dose-response curve, typical for the alpha2B subtype, could be seen when the second intracellular loop of the alpha2B subtype was inserted into the alpha2A subtype, suggesting that this loop is important for determining the subtype-specific coupling of alpha2-ARs to cAMP production. Site-directed mutagenesis of non-conserved amino acids in the second intracellular loop of the alpha2A subtype indicated that several residues are involved in the coupling specificity.

Multiple G-protein coupling of the dog thyrotropin receptor

We investigated, in dog thyroid membranes, the ability of the dog thyrotropin (TSH) receptor to interact with the endogenous G proteins expressed in this tissue. Activation of the receptor led to increased incorporation of the photoreactive GTP analog [alpha-(32)P]GTP azidoanilide into immunoprecipitated alpha subunits of three G protein families: G(s), G(q/11), G(i/o). This effect was not due to a general loss of receptor G protein specificity since carbamylcholine, in the same membrane preparations, only stimulated the binding of the GTP analog to the alpha subunits of G(q/11) proteins. To investigate the multiple coupling of the dog TSH receptor in intact cells, cyclic AMP accumulation, IP(3) formation and (45)Ca2+ efflux experiments were performed. When thyrocytes were pretreated with pertussis toxin (PTX), the TSH receptor-mediated accumulation of cAMP increased by approximately 45% with TSH at 1 mU/ml, suggesting that the TSH receptor coupled to both G(s) and G(i) in vivo. On the other hand, no increase in IP(3) accumulation nor Ca2+ efflux was observed in the presence of thyrotropin. These data in intact cells are thus in contradiction with those obtained in membranes, suggesting that receptor-mediated transmembrane signalling may implicate a specificity which itself may reflect a localization and organization of the different components (receptors, G proteins, ...) in the plasma membrane of intact cells. As in some cells, G(i) activates mitogenesis by hormone activated G-protein-coupled receptors, we tested its role in the stimulation by TSH of the proliferation of thyrocytes. This was not affected by PTX, suggesting that the mitogenic effect of TSH does not involve G(i)-proteins.

A new linear V1A vasopressin antagonist and its use in characterizing receptor/G protein interactions

We characterized a new iodinated, high affinity, linear V1a vasopressin antagonist, phenylacetylD-Tyr(Et)Phe-Gln-Asn-Lys-Pro-Arg-Tyr-NH2. The antagonist bound specifically to the V1a vasopressin receptor in crude rat liver membranes with an apparent Kd value of 0.168 nM. This affinity is approximately 1 order of magnitude greater than that of the natural agonist, vasopressin. The inhibitory activity of the antagonist can be demonstrated by its inability to elicit activation and uncoupling of G proteins from the receptor. Thus, after occupancy of receptor sites in rat liver membranes with labeled antagonist and detergent solubilization, the labeled receptor (approximately 60 kDa) was eluted as a stable 400-kDa complex on size-exclusion chromatography. In contrast, when the receptor sites were occupied by the agonist [3H]vasopressin, the receptor eluted as a 60-kDa peak. Coincubation of membranes with iodinated antagonist and an excess of unlabeled vasopressin caused both reduced antagonist binding and a complete shift from the 400-kDa to the 60-kDa peak. The addition of vasopressin to unliganded 400-kDa fractions resulted in a 75% increase in [35S]guanosine-5'-O-(3-thio)triphosphate binding activity, indicating that the 400-kDa fraction contains complexes between the V1a receptor and G proteins. The vasopressin-elicited increase was inhibited by antagonist. Using specific antibodies and immunoadsorption to protein A/Sepharose columns, we found that G protein isotypes G(alpha q/11), G(alpha i3), and G(alpha s), and effector enzymes PLC-beta1, PLC-gamma2 and PLA-2 were associated with the antagonist-labeled receptor in the 400-kDa fraction. Because the 400-kDa complex was found in the absence of ligand, the V1a receptor and the appropriate G proteins and effector enzymes are likely preassociated with each other and do not aggregate after antagonist addition. The association of V1a receptor with the different specific G proteins and effector enzymes is consistent with the multiple actions of vasopressin on liver cells. Antibodies directed against a portion of the carboxyl-terminal domain of the V1a receptor interacted with 60-kDa antagonist-occupied receptor but not with receptor in the 400-kDa complex. These results suggest that the carboxyl-terminal region of the receptor is sterically hindered when coupled to G proteins. The iodinated linear vasopressin antagonist therefore allows stable receptor/G protein complexes and can be an important tool (along with the antisera) for use in the study of factors that control V1a receptor/G protein coupling.

The N-terminal region of the third intracellular loop of the parathyroid hormone (PTH)/PTH-related peptide receptor is critical for coupling to cAMP and inositol phosphate/Ca2+ signal transduction pathways

Structural determinants within the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor that mediate G-protein activation of adenylate cyclase and phospholipase C are unknown. We investigated the role of the N-terminal region of the third intracellular loop of the opossum PTH/PTHrP receptor in coupling to two signal transduction pathways. We mutated residues in this region by tandem-alanine scanning and expressed these mutant receptors in COS-7 cells and/or Xenopus oocytes. All mutant receptors retained high affinity PTH binding in COS-7 cells, indistinguishable from wild-type receptors. Receptors with tandem-alanine substitutions in two N-terminal segments (377RVL379 and 381TKLR384) demonstrated impaired adenylate cyclase and phospholipase C activation. Receptor mutants with single-alanine substitutions scanning these two segments showed three different signaling defects in COS-7 cells. 1) Two mutant receptors (V378A and L379A) had reduced inositol phosphate (IP), but normal cAMP responses to PTH. 2) Mutant receptor T381A showed reduced cAMP, but wild-type IP responses to PTH. 3) Mutant receptor K382A demonstrated both markedly reduced cAMP and IP production due to PTH. In oocytes, mutants T381A and K382A showed decreased PTH-stimulated cAMP accumulation and intracellular Ca2+ mobilization. Thus, the N-terminal region of the third intracellular loop of this receptor plays a critical role in coupling to both Gs- and Gq-mediated second-messenger generation.

Regulation of HSP70 by PTH: a model of gene regulation not mediated by changes in cAMP levels

Parathyroid hormone (PTH) activates both adenylate cyclase and phospholipase C in target cells, and cloned PTH/PTH-related protein (PTHrP) receptor can mediate both responses when expressed in host cells such as LLC-PK1 renal epithelial cells. Because calcitonin (CT) is known to augment 70-kDa heat shock protein (HSP70) mRNA by an adenosine 3',5'-cyclic monophosphate (cAMP)-independent mechanism in LLC-PK1 cells, we examined regulation of HSP70 transcription by PTH in these cells. Like CT, human PTH-(1-34) [hPTH-(1-34); 10(-10) to 10(-7) M)] increased porcine HSP70 mRNA and human HSP70 promoter-chloramphenicol acetyltransferase (CAT) expression within 4 h in LLC-PK1 cells that stably express > or = 100,000 PTH/PTHrP receptors per cell. The effect of PTH on HSP70 mRNA was not mimicked by cAMP analogues, forskolin, phorbol esters, Ca2+ ionophores, or alpha-thrombin; was insensitive to pertussis toxin; and was not due to increased mRNA stability. The upregulation of HSP70 gene transcription by hPTH (and CT) was clearly observed even after deletion of the functional heat shock consensus element in the promoter region of the human HSP70/CAT reporter. Upregulation of HSP70 transcription via endogenous PTH receptors also was observed in the osteoblastic cell lines SaOS-2 and ROS 17/2.8. Regulation of HSP70 gene transcription by PTH may be a common cellular response to the hormone, which, in some cells, may not be mediated by activation of adenylate cyclase or protein kinase C.

Structurally diverse N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) inhibit the Na+/H+ exchanger NHE3 isoform by binding to the PTH/PTHRP receptor type I and activating distinct signaling pathways

N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) elicit a wide variety of biological responses in target cells, including the inhibition of Na+/H+ exchanger NHE3 activity in renal cells. This response is believed to be mediated by ligand binding to a common receptor (i.e. PTH/PTHRP receptor type I) and activation of cAMP-dependent and/or Ca2+/phospholipid-dependent protein kinases (PKA and PKC, respectively). However, the mechanism of action of these N-terminal peptides is now unclear because of recent data reporting the existence of additional receptor isoforms. Therefore, to directly examine the ligand binding and signaling characteristics of the PTH/PTHRP receptor type I and its ability to elicit a biological response, cDNAs encoding the rat type I receptor and the rat NHE3 isoform were transfected into Chinese hamster ovary (AP-1) cells that lack endogenous expression of these proteins. Competition binding assays using [125I-Tyr36]PTHRP-(1-36)-NH2 radioligand indicated that several biologically active human N-terminal PTH and PTHRP fragments (PTH-(1-34), PTH-(3-34), PTH-(28-42), PTH-(28-48), and PTHRP-(1-34)) were capable of binding to the type I receptor. Both PTH-(1-34) and PTHRP-(1-34) stimulated adenylate cyclase and PKC activities in these cells, whereas PTH-(3-34), PTH-(28-42), and PTH-(28-48) selectively enhanced only PKC activity. PTHRP-(1-16), a biologically inert fragment, was incapable of binding to this receptor and influencing either the PKA or PKC pathway. Furthermore, all the analogues with the exception of PTHRP-(1-16) inhibited NHE3 activity. Inhibition of PKC by the potent antagonist chelerythrine chloride abolished the depression of NHE3 activity by PTH-(3-34), PTH-(28-42), and PTH-(28-48) but did not alleviate the effects of PTH-(1-34). Likewise, antagonism of PKA by H-89 was unable to prevent the inhibition caused by PTH-(1-34). However, inhibition of both PKA and PKC by the nonselective protein kinase antagonist H-7 abolished the reduction of NHE3 activity by PTH-(1-34). These data indicate that discrete N-terminal analogues of PTH and PTHRP can interact with the classical PTH/PTHRP receptor type I and activate PKA and/or PKC. Activation of either signaling pathway independently leads to inhibition of NHE3.

Functional expression and signaling properties of cloned human parathyroid hormone receptor in Xenopus oocytes. Evidence for a novel signaling pathway

Expression of human parathyroid hormone receptor (hPTHR) was obtained in Xenopus oocytes. Receptor function was detected by hormone stimulation of endogenous Ca2+-activated Cl- current. This current was blocked by injected, but not by extracellular, EGTA, confirming that the hPTHR activates cytosolic Ca2+ signaling pathways. PTH responses were acutely desensitized but were regained in 6 12 h. Injection of cAMP or analogues had no effect on either responsiveness or desensitization to hPTH. The hPTH response was more sluggish than seen with serotonin 5-hydroxytryptamine (5-HT2C) receptor. In oocytes co-expressing both hPTHR and 5-HT2C receptors, homologous desensitization was seen, but cross-desensitization was not observed. Injection of inositol 1,4,5-trisphosphate (InsP3) elicited a fast inward current similar to that induced by serotonin, and complete cross-desensitization occurred between the InsP3 and 5-HT2C responses. Desensitization by hPTH did not affect responses to either InsP3 or serotonin, but cells desensitized to injected InsP3 still responded strongly to PTH. Oocytes did not respond to either cADPR or NAADP+, but NADP+ and analogues were found to be potent inhibitors of PTH signaling. We suggest that PTH cytosolic Ca2+ signaling in oocytes either involves a novel signaling system or proceeds through a Ca2+ compartment whose responsiveness is regulated in a novel way.

Inositol 1-,4-,5-trisphosphate-dependent Ca2+ signaling by the recombinant human PTH/PTHrP receptor stably expressed in a human kidney cell line

We previously reported the preparation and partial characterization of a series of human embryonic kidney cell lines (HEK-293) stably expressing various numbers of the recombinant human (h) parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (Rc). Using this expression system we examined ligand (PTH or PTHrP) binding characteristics and cyclic AMP responsiveness. We have now extended these studies to investigate the calcium signal transduction pathways activated by the hPTH/PTHrP Rc. In parental HEK-293 cells, which lack endogenous PTH/PTHrP Rc, incubation with hPTH(1-34) had no effect on cytosolic free Ca2+ concentration [Ca2+]i. In HEK-293 clone C-21, stably expressing approximately 400,000 Rc/cell, PTH stimulated an increase in [Ca2+]i by Ca2+ release from intracellular stores; PTH released Ca2+ exclusively from the IP3 sensitive Ca2+ pool. Unlike previous studies, the ability of PTH to elicit both cAMP responses and [Ca2+]i transients occurred over a wide range of Rc numbers (between 400,000 and 3000 Rc/cell); both responses were always observed at PTH concentrations in the same dose range although the magnitude of the responses decrease with Rc number. Pretreatment of C-21 cells with pertussis toxin for 24 h, which significantly enhanced PTH-stimulated cAMP accumulation, did not modulate PTH-stimulated [Ca2+]i transients. At each PTH concentration tested which resulted in increased cAMP levels, there was also an increase in [Ca2+]i transients. Treatment of C-21 cells with a battery of midregion and C-terminal PTH or PTHrP peptides showed no effect on either [Ca2+]i transients or cAMP accumulation, indicating a lack of functional interactions between these peptides and the form of the hPTH/PTHrP Rc stably expressed in these cells. Immunological analysis of G-protein expression demonstrated the presence of Gs, Gi, and Gq in all parental and transfected cells lines examined. Taken together, these data demonstrate that the hPTH/PTHrP Rc, stably expressed in HEK-293 cells, elicits responses in both the cAMP and IP3-dependent [Ca2+]i pathways and is responsive only to N-terminal PTH/PTHrP peptides.

The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G-proteins

Metabotropic glutamate receptors (mGluR) share no sequence homology with any other G-protein-coupled receptors (GPCRs). The characterization of their G-protein coupling domains will therefore help define the general rules for receptor-G-protein interaction. To this end, the intracellular domains of mGluR3 and mGluR1, receptors coupled negatively to adenylyl cyclase and positively to phospholipase C, respectively, were systematically exchanged. The ability of these chimeric receptors to induce Ca2+ signals were examined in Xenopus oocytes and HER 293 cells. The chimeric receptors that still possessed the second intracellular loop (i2) of these proteins were targeted correctly to the plasma membrane. Consistent Ca2+ signals could be recorded only with chimeric mGluR3 receptors that contains i2 and at least one other intracellular domains of mGluR3 have to be replaced by their mGluR1 equivalent to produce optimal coupling to G protein. These observations indicate that i2 of mGluR1 is a critical element in determining the transduction mechanism of this receptor. These results suggest that i2 of mGluRs may play a role similar to i3 of most other GPCRs in the specificity of coupling to the G-proteins. Moreover, as in many other GPCRs, our data revealed cooperation between the different mGluR intracellular domains to control efficient coupling to G-proteins.

A mechanism for the differential regulation of gonadotropin subunit gene expression by gonadotropin-releasing hormone

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) is released in a pulsatile fashion, with its frequency varying throughout the reproductive cycle. Varying pulse frequencies and amplitudes differentially regulate the biosynthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by pituitary gonadotropes. The mechanism by which this occurs remains a major question in reproductive physiology. Previous studies have been limited by lack of available cell lines that express the LH and FSH subunit genes and respond to GnRH. We have overcome this limitation by transfecting the rat pituitary GH3 cell line with rat GnRH receptor (GnRHR) cDNA driven by a heterologous promoter. These cells, when cotransfected with regulatory regions of the common alpha, LH beta, or FSH beta subunit gene fused to a luciferase reporter gene, respond to GnRH with an increase in luciferase activity. Using this model, we demonstrate that different cell surface densities of the GnRHR result in the differential regulation of LH and FSH subunit gene expression by GnRH. This suggests that the differential regulation of gonadotropin subunit gene expression by GnRH observed in vivo in rats may, in turn, be mediated by varying gonadotrope cell surface GnRHR concentrations. This provides a physiologic mechanism by which a single ligand can act through a single receptor to regulate differentially the production of two hormones in the same cell.

Probing the bimolecular interactions of parathyroid hormone and the human parathyroid hormone/parathyroid hormone-related protein receptor. 2. Cloning, characterization, and photoaffinity labeling of the recombinant human receptor

Parathyroid hormone (PTH) acts to regulate calcium homeostasis by interacting with a G-protein-coupled receptor that also binds PTH-related protein (PTHrP). In this report we describe the cloning, characterization, and biological activity of the cloned human (h) PTH/PTHrP receptor (Rc) and cross-linking of a benzophenone-substituted PTH analog, [Nle8,18,Lys13(epsilon-pBZ2),L-2-Nal23,Tyr34]bPTH(1-34 )NH2(K13), to cells endogenously expressing the Rc and cells transiently or stably transfected with the human Rc. A full-length cDNA clone was isolated and fully sequenced from a human kidney cDNA library. Northern blot analysis of normal human tissues revealed a limited tissue distribution: a single transcript of approximately 2.3 kb was detected in kidney, lung, placenta, and liver. In human embryonic kidney cells (HEK-293, clone C-21) stably transfected with hPTH/PTHrP Rc, a single 85-90 kDa Rc-hormone complex was formed after photolysis in the presence of K13. This covalent cross-linking reaction was specifically inhibited by excess quantities of biologically active 1-34 analogs of bovine (b) PTH or hPTHrP but not by C-terminal and midregion PTH peptides. Photoincorporation of 125I-labeled K13 into the Rc occurred with high efficiency (60-70%), approximately an order of magnitude greater than that achieved with conventional aryl azide cross-linking reagents. These results support the feasibility of our approach for specifically cross-linking a tagged PTH analog to the Rc, as a first step in the effort to identify directly the amino acid residues that constitute the Rc binding site.

Coupling of human D-1 dopamine receptors to different guanine nucleotide binding proteins. Evidence that D-1 dopamine receptors can couple to both Gs and G(o)

Coupling between D-1 dopamine receptors and G proteins in cell lines expressing human D-1 receptors and different G proteins was examined. Pertussis toxin (PTX) treatment of rat pituitary GH4C1 cells significantly reduced, but did not abolish, agonist high affinity binding sites of the D-1 dopamine receptor; in SK-N-MC neuroblastoma cells, PTX failed to have any effect on D-1 high affinity sites. Cholera toxin (CTX) treatment of GH4C1 cells reduced but did not abolish the high affinity sites of D-1 receptors, while in SK-N-MC cells, treatment with CTX abolished all the high affinity sites. Western blot analyses with specific antisera indicated that Gs alpha, Gi1 alpha, Gi3 alpha, and Gq alpha were expressed in both cell lines, while Gi2 alpha and G(o) alpha were expressed in GH4C1 but not SK-N-MC cells. Antisera NEI-805 (anti-Gs alpha) and 9072 (anti-G(o) alpha) immunoprecipitated 24 +/- 4.3 and 34.4 +/- 6.9%, respectively, of G protein-associated D-1 dopamine receptors. Antisera 3646 (anti-Gi1 alpha), 1521 (anti-Gi2 alpha), 1518 (anti-Gi3 alpha), and 0941 (anti-Gq alpha) failed to coimmunoprecipitate appreciable levels of soluble receptors. These data indicate that D-1 dopamine receptors are coupled to both Gs alpha and G(o) alpha but not to Gq alpha.

Heparin-insensitive calcium release from intracellular stores triggered by the recombinant human parathyroid hormone receptor

1. In the present study we have characterized the parathyroid hormone (PTH)-induced calcium signalling in 293 cells stably transfected with the human PTH receptor cDNA. In these cells, human PTH-1(1-38) strongly stimulates adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation (EC50 = 0.39 nM) but fails to activate phosphoinositide (PI) turnover. The latter pathway is strongly activated, however, by carbachol (CCh) acting through endogenous M3-muscarinic receptors. 2. Despite the lack of detectable inositol phosphate (IP) formation, hPTH-(1-38) elicited calcium transients (EC50 = 11.2 nM) which were comparable to the signals evoked by CCh. These signals are independent of cyclic AMP generation as cyclic AMP elevating agents did not mimic or modify the PTH response. 3. The PTH-stimulated calcium signal still occurred in calcium-free medium but was absent in cells pretreated with thapsigargin, an inhibitor of the calcium pump of the endoplasmic reticulum (ER). hPTH-(1-38) did not accelerate Mn(2+)-influx through the plasma membrane. These data indicate that PTH releases calcium from intracellular stores. 4. Using heparin, an inhibitor of the IP3-activated calcium release channel of the ER, we tested whether the formation of a low amount of IP3, escaping detection by our biochemical assay, might be the origin of the PTH-induced calcium response. However, intracellular infusion of heparin through patch pipettes in voltage clamp experiments failed to block hPTH-(1-38)-induced calcium signals, whereas it abolished the CCh response. 5. The PTH response, like the CCh response, was insensitive to micromolar concentrations of ryanodine and ruthenium red, eliminating the possibility that hPTH-(1-38) stimulates calcium-induced calcium release through ryanodine receptors.6. We conclude that the recombinant human PTH receptor stimulates calcium release from intracellular stores through a novel pathway not involving IP3- or ryanodine receptors.