Mutations in G protein-linked receptors: novel insights on disease

Quantification of GPCR internalization by single-molecule microscopy in living cells

Receptor internalization upon ligand stimulation is a key component of a cell's response and allows a cell to correctly sense its environment. Novel fluorescent methods have enabled the direct visualization of the agonist-stimulated G-protein-coupled receptors (GPCR) trafficking in living cells. However, it is difficult to observe internalization of GPCRs in vivo due to intrinsic autofluorescence and cytosolic signals of fluorescently labeled GPCRs. This study uses the superior positional accuracy of single-molecule fluorescence microscopy to visualize in real time the internalization of Dictyostelium discoideum cAMP receptors, cAR1, genetically encoded with eYFP. This technique made it possible to follow the number of receptors in time revealing that the fraction of cytosolic receptors increases after persistent agonist stimulation and that the majority of the receptors were degraded after internalization. The observed internalization process was phosphorylation dependent, as shown with the use of a phosphorylation deficient cAR1 mutant, cm1234-eYFP, or stimulation with an antagonist, Rp-cAMPS that does not induce receptor phosphorylation. Furthermore, experiments done in mound-stage cells suggest that intrinsic, phosphorylation-induced internalization of cAR1 is necessary for Dictyostelium wild type cells to progress properly through multicellular development. To our knowledge, this observation illustrates for the first time phosphorylation-dependent internalization of single cAR1 molecules in living cells and its involvement in multicellular development. This very sensitive imaging of receptor internalization can be a useful and universal approach for pharmacological characterization of GPCRs in other cell types.

Avaliação pré-operatória com PET-18F-FDG de nódulos de tireóide com citologia indeterminada

Esse artigo tem o objetivo de discutir o papel da tomografia por emissão de pósitrons (PET) com 18F-FDG na avaliação pré-operatória de pacientes com nódulos de tireóide com citologia indeterminada. Para o cálculo da sensibilidade, foram selecionados todos os estudos com pacientes com carcinoma de tireóide. Para o cálculo da especificidade, foram selecionados apenas estudos desenhados para avaliação dos pacientes com nódulos com citologia indeterminada. O achado de captação focal na PET-18F-FDG relacionou-se com a presença de carcinoma de tireóide na maioria dos estudos. A sensibilidade do exame foi bastante alta na detecção de malignidade tireoidiana, porém sua especificidade variou de 0% a 66%, sendo de 39% em estudo brasileiro. Concluindo, os estudos indicam que a PET-18F-FDG pode reduzir o número de tireoidectomias desnecessárias em pacientes com nódulos de tireóide com citologia indeterminada. Entretanto, o percentual relativamente elevado de resultados falso-positivos, o alto custo, a baixa disponibilidade do exame em países em desenvolvimento e a pouca experiência clínica ainda limitam o uso da PET-18F-FDG com essa finalidade.

Toxic thyroid adenoma presenting as hypokalemic periodic paralysis

Toxic thyroid adenoma presenting as hypokalemic periodic paralysis is extraordinarily rare. We describe a 26-year-old Japanese man who suffered from acute and painful muscle weakness of extremity in the morning. Physical examination showed a left anterior neck mass and laboratory tests revealed hypokalemia during his paralysis, and thyrotoxicosis. Neck sonogram showed a solitary nodule in the left lobe of the thyroid. Thyroid scintigraphy revealed a hot nodule of the tumor region with suppressed uptake in the other thyroid area. The tumor was surgically removed and his paralytic attack ceased. No somatic mutation of TSH receptor was found in his thyroid adenoma and no known genetic mutations of ionic channel genes, such as calcium (CACN1S), sodium (SCN4A) and potassium (KCNE3), were found. Although thyrotoxic periodic paralysis is usually accompanied with Graves' disease, thyrotoxicosis of other conditions including Plummer's disease should be considered.

[Endocrine tumors associated to protein Gsalpha/Gi2alpha mutations]

Many oncogenic mutations promote tumor growth by inducing autonomous activity of proteins that normally transmit proliferative signal initiated by extracellular factors. G proteins are a family of guanine nucleotide binding proteins, which are structurally homologous and widely distributed in eukaryotic cells. They are composed of three different subunits (alpha, beta e gamma). The alpha subunit, which contains the guanine nucleotide-binding site, is unique to each G protein. The G proteins couple an array of seven transmembrane receptors at the cell surface with a variety of intracellular effectors, which produce second messenger molecules. A subset of endocrine tumors, such as GH- or ACTH-secreting pituitary adenomas, functioning thyroid adenomas, adrenocortical and gonadal tumors were associated with somatic activating mutations in the highly conserved codons of the Gs (Arg201 and Gln227) and Gi (Arg179 and Gln205) proteins. These findings indicated that the G proteins play a role as oncogenes, contributing with the human endocrine tumorigenesis.

Structural and functional characterization of the first intracellular loop of human thromboxane A2 receptor

The conformation of a constrained peptide mimicking the putative first intracellular domain (iLP1) of thromboxane A(2) receptor (TP) was determined by (1)H 2D NMR spectroscopy. Through completed assignments of TOCSY, DQF-COSY, and NOESY spectra, a NMR structure of the peptide showed a beta-turn in residues 56-59 and a short helical structure in the residues 63-66. It suggests that residues 63-66 may be part of the second transmembrane domain (TM), and that Arg60, in an exposed position on the outer surface of the loop, may be involved in signaling through charge contact with Gq protein. The sequence alignment of Lys residue in the same position of other prostanoid receptors mediates different G protein couplings, suggesting that the chemical properties of Arg and Lys may also affect the receptor signaling activity. These hypotheses were supported by mutagenesis studies, in which the mutant of Arg60Leu completely lost activity in increasing intracellular calcium level through Gq coupling, and the mutant of Arg60Lys retained only about 35% signaling activity. The difference between the side chain functions of Lys and Arg in effecting the signaling was discussed.

Genetics of vasopressin receptors

Membrane receptors that couple to guanine nucleotide binding protein (GPCRs) represent one of the largest families of proteins in the genome. Because of their universal distribution and multiple actions, genetic variations of GPCRs are associated with various human diseases. For instance, the clinical phenotype of congenital nephrogenic diabetes insipidus has been linked to more than 155 loss-of-function putative mutations of the arginine vasopressin (AVP) V(2) receptor, which span each and every segment of this seven-transmembrane domain receptor. These mutant receptors, which are mostly trapped in the endoplasmic reticulum, can be rescued by membrane-permeant nonpeptidic AVP receptor antagonists. An overexpression of V(1)-vascular and V(3)-pituitary AVP receptors has been observed in some endocrine tumors. The single nucleotide polymorphism of AVP receptors in the context of complex genetic traits is currently being investigated, and preliminary findings have been reported in arterial hypertension and autism.

A novel view on the mechanisms of action of insulin and other insulin superfamily peptides: involvement of adenylyl cyclase signaling system

A new signaling mechanism common to mammalian insulin, insulin-like growth factor I, relaxin and mollusc insulin-like peptide, and involving receptor-tyrosine kinase==>G(i) protein (betagamma)==>phosphatidylinositol-3-kinase==>protein kinase Czeta==>adenylyl cyclase==>protein kinase A was discovered in the muscles and some other tissues of vertebrates and invertebrates. The authors' data were used to reconsider the problem of participation of the adenylyl cyclase-cAMP system in the regulatory effects of insulin superfamily peptides. A hypothesis has been put forward according to which the adenylyl cyclase signaling mechanism producing cAMP has a triple co-ordinating role in the regulatory action of insulin superfamily peptides on the main cell processes, inducing the mitogenic and antiapoptotic effects and inhibitory influence on some metabolic effects of the peptides. It is suggested that cAMP is a key regulator responsible for choosing the transduction pathway by concerted launching of one (proliferative) program and switching off (suppression) of two others, which lead to cell death and to the predomination of anabolic processes in a cell. The original data obtained give grounds to conclude that the adenylyl cyclase signaling system is a mechanism of signal transduction not only of hormones with serpentine receptors, but also of those with receptors of the tyrosine kinase type (insulin superfamily peptides and some growth factors).

Sensitization of adenylate cyclase induced by a dopamine D2 receptor mutant: inverse agonism by D2 receptor antagonists

Mutations of residues in the third intracellular loops of several G-protein coupled receptors have been shown to confer constitutive activation. The authors investigated the effects of one such mutation in the dopamine D2 receptor. Compared to the wild type D2, the mutant D2 receptor (D2T344K) showed a substantial increase in agonist affinity with affinity for antagonists unchanged. The increased agonist affinity was unaffected by pertussis toxin treatment, indicating it is an intrinsic property of the mutant receptor. The potency of dopamine for acute inhibition of forskolin-stimulated cAMP production in stably expressing Chinese Hamster Ovary (CHO) cells was higher for the mutant than the wild type receptor. CHO cells stably expressing D2T344K displayed enhanced responses to forskolin-stimulated adenylate cyclase activity compared with cells stably expressing the wild type D2 receptor. The increased forskolin responsiveness of adenylate cyclase is similar to the sensitization previously observed with wild type D2 receptor after agonist treatment. Adenylate cyclase responsiveness of CHO cells stably expressing D2T344K receptor was not further increased by agonist treatment. Sensitization was blocked by pertussis toxin and D2 receptor antagonists haloperidol, butaclamol, and clozapine, indicating inverse agonist activity of these compounds at D2T344K. Inverse agonist activity was further demonstrated by the finding that overnight treatment with these compounds drastically increased the density of the mutant receptor but had minimal effect on the density of the wild type receptor. Taken together, these results suggest the authors have generated a constitutively active dopamine D2 receptor capable of sensitizing adenylate cyclase in the absence of agonist activation.

Three novel activating mutations in the calcium-sensing receptor responsible for autosomal dominant hypocalcemia

We report three novel activating mutations in the calcium-sensing receptor (CASR) that are responsible for autosomal dominant hypocalcemia (ADH) in three unrelated families. Each mutation involves a missense substitution resulting in a nonconservative amino acid alteration, P221L, E228Q, and Q245R. These mutations were observed in affected family members, but not in unaffected family members or in unrelated control samples. All three mutations are clustered in the extracellular domain of the CASR in a region dominated by negatively charged amino acids. Each mutant and wild-type receptor was expressed in Cos-1 cells. A luciferase reporter gene assay was utilized to detect the level of receptor activity by utilizing a protein kinase C-activated promoter to drive the production of luciferin, the reporter gene product. All three mutant receptors exhibited an increased sensitivity to calcium at all concentrations tested when compared to the wild-type receptor, supporting the hypothesis that these are activating mutations and are responsible for the ADH phenotype in these families. The data presented in this study suggest the importance of this highly negatively charged region of the extracellular domain in normal CASR function.

Cystic fibrosis transmembrane conductance regulator and the outwardly rectifying chloride channel: a relationship between two chloride channels expressed in epithelial cells

1. Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) result in the primary defect observed in patients with cystic fibrosis. 2. The CFTR is a member of the ATPase-binding cassette (ABC) transporter family but, unlike other members of this group, CFTR conducts a chloride current that is activated by cAMP. 3. In epithelial cells, the cAMP-stimulated chloride current is conducted by both CFTR and the outwardly rectifying chloride channel (ORCC). 4. The present review summarizes the current knowledge of the properties of the two channels, as well as their relationship. Because the gene encoding the ORCC has not been identified, a discussion as to possible candidates for this chloride channel is included.

Progressive osseous heteroplasia

Progressive osseous heteroplasia (POH) is a recently described genetic disorder of mesenchymal differentiation characterized by dermal ossification during infancy and progressive heterotopic ossification of cutaneous, subcutaneous, and deep connective tissues during childhood. The disorder can be distinguished from fibrodysplasia ossificans progressiva (FOP) by the presence of cutaneous ossification, the absence of congenital malformations of the skeleton, the absence of inflammatory tumorlike swellings, the asymmetric mosaic distribution of lesions, the absence of predictable regional patterns of heterotopic ossification, and the predominance of intramembranous rather than endochondral ossification. POH can be distinguished from Albright hereditary osteodystrophy (AHO) by the progression of heterotopic ossification from skin and subcutaneous tissue into skeletal muscle, the presence of normal endocrine function, and the absence of a distinctive habitus associated with AHO. Although the genetic basis of POH is unknown, inactivating mutations of the GNAS1 gene are associated with AHO. The report in this issue of the JBMR of 2 patients with combined features of POH and AHO--one with classic AHO, severe POH-like features, and reduced levels of Gsalpha protein and one with mild AHO, severe POH-like features, reduced levels of Gsalpha protein, and a mutation in GNAS1--suggests that classic POH also could be caused by GNAS1 mutations. This possibility is further supported by the identification of a patient with atypical but severe platelike osteoma cutis (POC) and a mutation in GNAS1, indicating that inactivating mutations in GNAS1 may lead to severe progressive heterotopic ossification of skeletal muscle and deep connective tissue independently of AHO characteristics. These observations suggest that POH may lie at one end of a clinical spectrum of ossification disorders mediated by abnormalities in GNAS1 expression and impaired activation of adenylyl cyclase. Analysis of patients with classic POH (with no AHO features) is necessary to determine whether the molecular basis of POH is caused by inactivating mutations in the GNAS1 gene.

Cholera toxin and extracellular Ca2+ induce adherence of non-piliated Neisseria: evidence for an important role of G-proteins and Rho in the bacteria-cell interaction

In this study, we characterize the interaction between non-piliated (P-) Neisseria gonorrhoeae and human epithelial cells. P- mutants lacking the pilus subunit protein PilE attach at low levels to cells. Although the binding may not lead to heavy inflammatory responses, the interaction between P- Neisseria and host cells most probably play a role in colonization and asymptomatic carriage of the pathogen. Here we show that the adherence of P N. gonorrhoeae is blocked by GDP-beta-S [guanosine 5'-O(thio)diphosphate], a non-hydrolyzable GTP analogue, and by C3 exotoxin, an inhibitor of the small G-protein Rho. G-protein activators such as cholera toxin, that activates Gs, and fluoroaluminate, a general G-protein activator, induced bacterial adherence. Furthermore, increase of the extracellular free [Ca2+] dramatically enhanced adherence of non-piliated Neisseria. The pharynx and the urogenital tract are natural entry sites of the pathogenic Neisseria species, and at both sites the epithelial cells can be exposed to wide variations in Ca2+ concentration. Taken together, these data show the importance of extracellular Ca2+ in the pathogenic Neisseria-host interaction, and reveal a novel function of cholera toxin, namely induction of bacterial adherence.

Molecular variations in the calcium-sensing receptor in relation to sodium balance and presence of hypertension in blacks and whites

Sodium (Na) excretion is to an extent tied to calcium (Ca) excretion; increases in Ca result in increased Na excretion. We hypothesized that molecular variation in the calcium-sensing receptor (CaSR), which imparts certain of the influences of extracellular Ca, might be related to differences in Na balance and blood pressure. We further hypothesized that such an influence by CaSR is more pronounced in blacks than in whites, as the hypertension in blacks appears to be more dependent on Na retention. Three common molecular variants in CaSR were studied. Two were more frequent in the whites (A986S, P < .0001, and G990R, P = .093), whereas Q1011E was more frequent in the blacks (P < .0001). Two distinctly separate groups were studied: (1) healthy schoolchildren in whom levels of the renin-aldosterone axis and blood pressure were measured, and (2) normotensive and hypertensive adults. Studies of association were made separately in the whites and the blacks. No association of any of the variants with Na balance (as estimated from renin and aldosterone levels) was observed. In the black schoolchildren, Q1011E showed a marginal association with a higher blood pressure (P = .093 for systolic and P = .025 for diastolic), a relationship that was considered to be nonsignificant after adjusting for multiple comparisons. Nor was there a significant association of the variants with presence or absence of hypertension. In summary, studies of two cohorts that included whites and blacks did not suggest that molecular variations in the CaSR influence either Na balance or blood pressure.

Osteogenic induction in hereditary disorders of heterotopic ossification

The formation of heterotopic bone within soft connective tissue is a common feature of at least three distinct genetic disorders of osteogenesis in humans: fibrodysplasia ossificans progressiva; progressive osseous heteroplasia; and Albright hereditary osteodystrophy. The pathobiologic characteristics of osteogenic induction, the histopathologic features of osteogenesis, the anatomic distribution of heterotopic lesions, and the developmental patterns of disease progression differ among all three conditions. The molecular and cellular basis of redirecting a mature connective tissue phenotype to form bone is a remarkable biological phenomenon with enormous implications for the control of bone regeneration, fracture healing, and disorders of osteogenesis.

Molecular basis of pituitary oncogenesis

Recent advances in the molecular biology has served to unveil the underlying genetic and epigenetic alterations in pituitary adenomas. Three nuclear transcriptional factors, AP-1, CREB, and Pit-1, which are targets of protein kinase C and A, appear to play critical roles in both neoplastic growth and hormone secretion in hormone-producing adenomas. The alteration of G proteins such as Gs and Gi2 is a direct cause of the activation of such transcriptional factors. Autocrine growth factor/cytokine loops also contribute to the augmented signal transductions. Bromocriptine and somatostatin analogs have effects to lower cellular cAMP level through inhibitory G proteins, although the mechanism leading to cellular apoptosis is unknown. On the other hand, most non-functioning adenomas may not have PKC- or PKA-mediated oncogenic mechanisms. Although the loss of Rb and p27Kip1 genes has been demonstrated as a cause of murine pituitary adenomas, the role of tumor suppressor genes for human pituitary adenomas remains elusive. However, potential candidates for the suppressor genes are now emerging. The recently cloned multiple endocrine neoplasia type I gene is one example. Alterations of c-myc/bcl-2, and ras, although rare, appear to be an important cause of the process by which adenoma cells acquire aggressive phenotypes. Further studies on the links between abnormal signal transductions and aberrant tumor suppressor genes will be needed to clarify the whole picture of pituitary oncogenesis.

Constitutive activation of stimulatory guanine nucleotide binding protein (G(S)alphaQL)-mediated signaling increases invasiveness and tumorigenicity of PC-3M prostate cancer cells

An abnormal stimulation of cAMP signaling cascade has been implicated in various human carcinomas. Since the agents activating G(S)alpha-mediated signaling pathways have been shown to increase in vitro proliferation of prostate cancer cells, present studies examined the G(S)alpha-mediated signaling in tumorigenicity and invasiveness of PC-3M prostate cancer cells. PC-3M cells were stably transfected with plasmids containing either wild type (G(S)alpha-WT) or constitutively active (gsp mutant of G(S)alpha or G(S)alpha-QL) cDNAs. The stable transfectants were then tested for: (1) colony formation in soft agar; (2) cell migration and penetration of basement matrix in an in vitro invasion assay; and (3) the ability to form tumors and metastases in nude mice. PC-3M cells expressing G(S)alpha-QL protein displayed 15-fold increase in their ability to migrate and penetrate the basement membrane as compared to parental PC-3M cells or those expressing G(S)alpha-WT. G(S)alpha-QL transfectants also displayed a dramatically greater rate of growth in soft agar, and greater tumorigenicity and metastasis forming ability when orthotopically implanted in nude mice. All mice receiving PC-3M cells produced primary tumors within 5 weeks after implantation. However, the cells expressing G(S)alpha-QL displayed a significantly faster tumor growth as assessed by prostate weight (greater than 20-fold as compared to PC-3M cells), and produced metastases in kidneys, lymph nodes, blood vessels, bowel mesentery and intestine. Interestingly, expression of G(S)alpha-WT reduced the ability of PC-3M cells to form tumors in nude mice. These results suggest that persistent activation of G(S)alpha-mediated signaling cascade can dramatically accelerate tumorigenesis and metastasizing ability of prostate cancer cells.

The cyclophilin-like domain mediates the association of Ran-binding protein 2 with subunits of the 19 S regulatory complex of the proteasome

The combination of the Ran-binding domain 4 and cyclophilin domains of Ran-binding protein 2 selectively associate with a subset of G protein-coupled receptors, red/green opsins, upon cis-trans prolyl isomerase-dependent and direct modification of opsin followed by association of the modified opsin isoform to Ran-binding domain 4. This effect enhances in vivo the production of functional receptor and generates an opsin isoform with no propensity to self-aggregate in vitro. We now show that another domain of Ran-binding protein 2, cyclophilin-like domain, specifically associates with the 112-kDa subunit, P112, and other subunits of the 19 S regulatory complex of the 26 S proteasome in the neuroretina. This association possibly mediates Ran-binding protein 2 limited proteolysis into a smaller and stable isoform. Also, the interaction of Ran-binding protein 2 with P112 regulatory subunit of the 26 S proteasome involves still another protein, a putative kinesin-like protein. Our results indicate that Ran-binding protein 2 is a key component of a macro-assembly complex selectively linking protein biogenesis with the proteasome pathway and, thus, with potential implications for the presentation of misfolded and ubiquitin-like modified proteins to this proteolytic machinery.

G alpha q cDNA sequence from human platelets

G-proteins play a major role in cell signaling and specific abnormalities in these proteins have been implicated in some diseases. Although, two human G alpha q coding sequences are published, they differ at functionally important regions. We have sequenced the human G alpha q cDNA synthesized from normal platelet mRNA by reverse transcription. Our results showed that the penultimate amino acid leucine is conserved in the human G alpha q sequence which has 94.4% identity in nucleotide sequence and 99.2% homology in amino acid sequence with murine sequence.

Alanine-261 in intracellular loop III of the human gonadotropin-releasing hormone receptor is crucial for G-protein coupling and receptor internalization

Gonadotropin-releasing hormone (GnRH) is a decapeptide that regulates reproductive function via binding to the GnRH receptor, which is a G-protein-coupled receptor (GPCR). For several members of this family, the C-terminal domain of intracellular loop III is important in ligand-mediated coupling to G-proteins; mutations in that region can lead to constitutive activity. A specific alanine residue is involved in certain GPCRs, the equivalent of which is Ala-261 in the GnRH receptor. Mutation of this residue to Leu, Ile, Lys, Glu or Phe in the human GnRH receptor did not result in constitutive activity and instead led to complete uncoupling of the receptor (failure to support GnRH-stimulated inositol phosphate production). When this residue was mutated to Gly, Pro, Ser or Val, inositol phosphate production was still supported. All the mutants retained the ability to bind ligand, and the affinity for ligand, where measured, was unchanged. These results show that Ala-261 cannot be involved in ligand binding but is critical for coupling of the receptor to its cognate G-protein. Coupling is also dependent on the size of the residue in position 261. When the amino acid side chain has a molecular mass of less than 40 Da efficient coupling is still possible, but when its molecular mass exceeds 50 Da the receptor is uncoupled. Internalization studies on the Ala261-->Lys mutant showed a marked decrease in receptor internalization compared with the wild type, indicating that coupling is necessary for effective receptor internalization in the GnRH receptor system. Activation of protein kinase C (with PMA), but not protein kinase A (with forskolin) markedly increased the internalization of the mutant receptor while having a small effect on the wild-type receptor.

Platelet signal transduction defect with Galpha subunit dysfunction and diminished Galphaq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]) was diminished in the patient's platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of alpha-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[gammaS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Galphas) and its inhibition (mediated by Galphai) by thrombin in the patient's platelet membranes were normal. Immunoblot analysis of Galpha subunits in the patient's platelet membranes showed a decrease in Galphaq (<50%) but not Galphai, Galphaz, Galpha12, and Galpha13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein alpha-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Galphaq in thrombin-induced responses.

Heterotrimeric G-proteins and development

Heterotrimeric G-proteins are well-known transducers of signaling from a populous class of heptihelical, membrane receptors to a smaller group of effector molecules that includes adenylylcyclases, cyclic GMP phosphodiesterases, phospholipases (type C beta), and various ion channels. Dramatic changes in specific G-protein subunits that coincide with commitment to highly-specialized cell types suggest a key role for these extrinsic membrane proteins in cell differentiation and development. Through analysis of the effects of gain-of-function and loss-of-function mutants, it has been possible to explore this new dimension in G-protein biology, intimately linking specific G-proteins to development. G-protein subunits are shown to be important molecular switches in the complex biological processes controlling both cellular differentiation and early development.

Turned on by Ca2+! The physiology and pathology of Ca(2+)-binding proteins in the retina

Vertebrate photoreceptor cells can signal the absorption of a single photon and then modulate their response as the intensity of the light and the intensity of the background illumination vary, and it has long been recognized that Ca2+ ions contribute to the underlying processes. Recently, several Ca(2+)-binding proteins of the EF-hand family were identified that mediate the actions of Ca2+ during the response to light. Molecular interactions between these Ca(2+)-binding proteins and their cellular targets are amenable to study owing in part to the unique features of phototransduction. In addition, two of the proteins, recoverin and guanylate cyclase activating protein (GCAP), appear to be involved in separate degenerative diseases of the retina that arise in humans and in animal models of human disease. Information obtained from these studies should also be relevant to the growing number of homologous proteins found in other neural tissues.

Co-evolution of ligand-receptor pairs in the vasopressin/oxytocin superfamily of bioactive peptides

In order to understand the molecular mechanisms that underlie the co-evolution of related yet functionally distinct peptide-receptor pairs, we study receptors for the vasopressin-related peptide Lys-conopressin in the mollusc Lymnaea stagnalis. In addition to a previously cloned Lys-conopressin receptor (LSCPR1), we have now identified a novel Lys-conopressin receptor subtype, named LSCPR2. The two receptors have a differential distribution in the reproductive organs and the brain, which suggests that they are involved in the control of distinct aspects of reproduction and mediate transmitter-like and/or modulatory effects of Lys-conopressin on different types of central neurons. In contrast to LSCPR1, LSCPR2 is maximally activated by both Lys-conopressin and Ile-conopressin, an oxytocin-like synthetic analog of Lys-conopressin. Together with a study of the phylogenetic relationships of Lys-conopressin receptors and their vertebrate counterparts, these data suggest that LSCPR2 represents an ancestral receptor to the vasopressin/oxytocin receptor family in the vertebrates. Based on our findings, we provide a theory of the molecular co-evolution of the functionally distinct ligand-receptor pairs of the vasopressin/oxytocin superfamily of bioactive peptides.

The stoichiometry of expression of protein components of the stimulatory adenylyl cyclase cascade and the regulation of information transfer

Quantitative analysis of the proteins which compromise the stimulatory arm of the adenylyl cyclase cascade indicate that the adenylyl cyclase catalytic component is usually the least highly expressed. The effects on both potency of agonist ligands and maximal output resulting from targetted alterations in expression levels of each element of this cascade are discussed.

Evidence for coupling of phosphotyrosine phosphatase to gonadotropin-releasing hormone receptor in ovarian carcinoma membrane

BACKGROUND

Gonadotropin-releasing hormone (Gn-RH) receptor (Gn-RHR) has been demonstrated in epithelial ovarian carcinoma (Imai et al., Cancer 1994; 74:2555-61). To examine whether Gn-RHR mediates direct antiproliferative effects, we attempted to determine stimulatory regulation by Gn-RH of phosphotyrosine phosphatase (PTP) activity in plasma membranes isolated from ovarian carcinoma samples.

METHODS

Surgically removed ovarian carcinomas were screened for Gn-RHR expression prior to plasma membrane isolation. The phosphotyrosine level was observed by: (1) immunoblotting of membrane extracts with antiphosphotyrosine antibodies, and (2) dephosphorylation from 32P-labeled membrane protein. Membrane PTP activity was determined using the synthetic substrate p-nitrophenyl in a spectrophotometric assay.

RESULTS

A Gn-RH analog alone, or guanosine thiotriphosphate (GTP-gamma-S) alone, caused a remarkable loss of phosphotyrosine from a 35-kD protein of the membranes; incubation with a Gn-RH analog and GTP-gamma-S produced a further dephosphorylation of this endogenous protein. The Gn-RH analog buserelin stimulated the PTP activity of the membranes in a dose-dependent manner (P < 0.01). GTP-gamma-S enhanced the stimulatory action of Gn-RH on PTP; GDP-gamma-S reversed the Gn-RH action. A similar stimulation of PTP was observed (P < 0.01) when carcinoma tissue slices were exposed to Gn-RH analog in vivo prior to assay in vitro.

CONCLUSIONS

Activation of PTP by Gn-RH stimulated the loss of phosphotyrosine from endogenous proteins through GTP-binding protein within plasma membrane isolated from Gn-RHR-expressing ovarian carcinoma. The antimitogenic action of the hormone may occur by counteracting tyrosine phosphorylation to promote cell growth.

Molecular cloning, functional expression, and signal transduction of the GIP-receptor cloned from a human insulinoma

Glucose-dependent insulinotropic polypeptide (GIP) plays an important role in the regulation of postprandial insulin secretion and proinsulin gene expression of pancreatic beta-cells. This study demonstrates the molecular cloning of a cDNA for the GIP-receptor from a human insulinoma lambda gt11 cDNA library. The cloned cDNA encoded a seven transmembrane domain protein of 466 amino acids which showed high homology (41%) to the human glucagon-like peptide 1 (GLP-1) receptor. Homology to the GIP receptor from rat or hamster was 79% and 81%, respectively. When transfected stably into fibroblast CHL-cells a high affinity receptor was expressed which coupled to the adenylate cyclase with normal basal cAMP and increasing intracellular cAMP levels under stimulation with human GIP-1-42 (EC50 = 1.29 x 10(-13) M). The receptor accepted only human GIP 1-42 (Kd = 1.93 +/- 0.2 x 10(-8) M) and porcine truncated GIP 1-30 (Kd = 1.13 +/- 0.1 x 10(-8) M) as high affinity ligands. At 1 microM, exendin-4 and (9-39)amide weakly reduced GIP-binding (25%) whereas secretin, glucagon, glucagon-like peptide-1, vasoactive intestinal polypeptide, peptide histidine-isoleucine, and pituitary adenylyl cyclase activating peptide were without effect. In transfected CHL cells, GIP-1-42 did not increase intracellular calcium. Northern analysis revealed one transcript of human GIP receptor mRNA with an apparent size of 5.5 kb. The exact understanding of GIP receptor regulation and signal transduction will aid in the understanding of the incretin hormone's failure to exert its biological action at the pancreatic B-cell in type II diabetes mellitus.

The extracellular calcium receptor

The importance of intracellular calcium in regulating cell function is well recognized. No less important, but less well understood (and probably appreciated), is the fundamental role played by extracellular calcium, Ca2+o, in the modulation of cell function. The recent cloning of Ca2+o-sensing, G-protein-coupled receptors from bovine (and human) parathyroid and rat kidney (and brain) has clearly demonstrated that Ca2+o can function as a traditional 'first messenger'. The identification of 'inactivating' and 'activating' mutations in this Ca2+o-sensing receptor in two hypercalcemic disorders and in an autosomal dominant form of hypocalcemia, respectively, has underscored the physiological relevance of this receptor in Ca2+ homeostasis in man. These advances have significantly enhanced our understanding of the molecular mechanisms involved in extracellular calcium sensing in parathyroid and kidney. Moreover, the localization of the Ca2+o-sensing receptor in tissues previously not known to have Ca2+o-sensing capability has suggested novel and potentially quite important roles for Ca2+o in regulating the function of cells not apparently directly involved in Ca2+ homeostasis.

Identification of constitutively activating mutation of the luteinising hormone receptor in a family with male limited gonadotrophin independent precocious puberty (testotoxicosis)

A family of male limited gonadotrophin independent precocious puberty was examined for activating mutation of the LH receptor. A transition of A to G in nucleotide 1733 of the human LH receptor gene was identified in all affected males and in an unaffected carrier female. The mutation was shown by identifying a new restriction site created by the mutation. This mutation appears to be a common feature of the disorder, as it has been reported previously in unrelated families. Therefore, the presence of this new restriction site can serve as a diagnostic tool in males at risk before the onset of symptoms, as well as identifying carrier females.

Increased expression of the c-fos proto-oncogene in bone from patients with fibrous dysplasia

BACKGROUND

Fibrous dysplasia is characterized by intense marrow fibrosis and increased rates of bone turnover. The lesions of fibrous dysplasia resemble those described in the long bones of transgenic mice overexpressing the c-fos proto-oncogene. Activating mutations in the alpha subunit of the stimulatory guanine-nucleotide-binding protein (GS alpha) linked to adenylate cyclase have recently been described in bone cells from patients with the McCune-Albright syndrome and fibrous dysplasia.

METHODS

We used in situ hybridization to determine the level of expression of c-fos in bone-biopsy specimens from two normal subjects, eight patients with fibrous dysplasia, and six patients with other bone disorders characterized by high rates of bone turnover. The probe used corresponded to the fourth exon of the c-fos gene.

RESULTS

High levels of c-fos expression were detected in the bone lesions from all eight patients with fibrous dysplasia. No expression of c-fos was detected in bone specimens from the normal subjects or from specimens of normal bone obtained from patients with fibrous dysplasia. The cells that expressed c-fos in the dysplastic lesions were fibroblastic and populated the marrow space. A very low level of c-fos expression was detected in the biopsy specimens from the patients with other bone diseases. One patient with polyostotic fibrous dysplasia and one patient with the McCune-Albright syndrome were tested for the previously described GS alpha gene mutations and were found to express these mutations in bone.

CONCLUSIONS

Increased expression of the c-fos proto-oncogene, presumably a consequence of increased adenylate cyclase activity, may be important in the pathogenesis of the bone lesions in patients with fibrous dysplasia.

Arg60 to Leu mutation of the human thromboxane A2 receptor in a dominantly inherited bleeding disorder

Recent advances in molecular genetics have revealed the mechanisms underlying a variety of inherited human disorders. Among them, mutations in G protein-coupled receptors have clearly demonstrated two types of abnormalities, namely loss of function and constitutive activation of the receptors. Thromboxane A2 (TXA2) receptor is a member of the family of G protein-coupled receptors and performs an essential role in hemostasis by interacting with TXA2 to induce platelet aggregation. Here we identify a single amino acid substitution (Arg60-->Leu) in the first cytoplasmic loop of the TXA2 receptor in a dominantly inherited bleeding disorder characterized by defective platelet response to TXA2. This mutation was found exclusively in affected members of two unrelated families with the disorder. The mutant receptor expressed in Chinese hamster ovary cells showed decreased agonist-induced second messenger formation despite its normal ligand binding affinities. These results suggest that the Arg60 to Leu mutation is responsible for the disorder. Moreover, dominant inheritance of the disorder suggests the possibility that the mutation produces a dominant negative TXA2 receptor.

Signal transduction of the GLP-1-receptor cloned from a human insulinoma

GLP-1 (glucagon-like peptide 1 (7-36) amide) plays an important role in the regulation of insulin secretion and proinsulin gene expression of pancreatic beta-cells. Patients with insulinoma tumors show uncontrolled insulin hypersecretion. This study demonstrates the molecular cloning of a cDNA for the GLP-1 receptor from a human insulinoma employing a lambda-gt11 cDNA library. The cloned cDNA encoded a seven transmembrane domain protein of 463 amino acids which showed high homology to the GLP-1 receptor in normal human pancreas. Four amino acid exchanges were found in comparison to a receptor sequence obtained from regular pancreatic islets. When transfected transiently into COS-7 or stably into fibroblast CHL cells a high affinity receptor was expressed which coupled to the adenylate cyclase with normal basal cAMP and increasing intracellular cAMP levels under GLP-1 stimulation. The receptor accepted GLP-1 and the non-mammalian agonist exendin-4 as high affinity ligands. In transfected COS-7 cells, GLP-1 did not influence intracellular calcium, whereas in the stably transfected fibroblasts GLP-1 transiently increased intracellular calcium to a small extent. The understanding of GLP-1 receptor regulation and signal transduction will aid in the discovery of compounds that act as agonists of the GLP-1 receptor for potential use in the treatment of diabetes and will facilitate the understanding of its expression under normal and pathophysiological conditions.