Sonic hedgehog signaling by the patched-smoothened receptor complex.
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BACKGROUND: The Hedgehog (Hh) family of secreted proteins is involved in a number of developmental processes as well as in cancer. Genetic and biochemical data suggest that the Sonic hedgehog (Shh) receptor is composed of at least two proteins: the tumor suppressor protein Patched (Ptc) and the seven-transmembrane protein Smoothened (Smo). RESULTS: Using a biochemical assay for activation of the transcription factor Gli, a downstream component of the Hh pathway, we show here that Smo functions as the signaling component of the Shh receptor, and that this activity can be blocked by Ptc. The inhibition of Smo by Ptc can be relieved by the addition of Shh. Furthermore, oncogenic forms of Smo are insensitive to Ptc repression in this assay. Mapping of the Smo domains required for binding to Ptc and for signaling revealed that the Smo-Ptc interaction involves mainly the amino terminus of Smo, and that the third intracellular loop and the seventh transmembrane domain are required for signaling. CONCLUSIONS: These data demonstrate that Smo is the signaling component of a multicomponent Hh receptor complex and that Ptc is a ligand-regulated inhibitor of Smo. Different domains of Smo are involved in Ptc binding and activation of a Gli reporter construct. The latter requires the third intracellular loop and the seventh transmembrane domain of Smo, regions often involved in coupling to G proteins. No changes in the levels of cyclic AMP or calcium associated with such pathways could be detected following receptor activation, however. (+info)
Canine preprorelaxin: nucleic acid sequence and localization within the canine placenta.
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Employing uteroplacental tissue at Day 35 of gestation, we determined the nucleic acid sequence of canine preprorelaxin using reverse transcription- and rapid amplification of cDNA ends-polymerase chain reaction. Canine preprorelaxin cDNA consisted of 534 base pairs encoding a protein of 177 amino acids with a signal peptide of 25 amino acids (aa), a B domain of 35 aa, a C domain of 93 aa, and an A domain of 24 aa. The putative receptor binding region in the N'-terminal part of the canine relaxin B domain GRDYVR contained two substitutions from the classical motif (E-->D and L-->Y). Canine preprorelaxin shared highest homology with porcine and equine preprorelaxin. Northern analysis revealed a 1-kilobase transcript present in total RNA of canine uteroplacental tissue but not of kidney tissue. Uteroplacental tissue from two bitches each at Days 30 and 35 of gestation were studied by in situ hybridization to localize relaxin mRNA. Immunohistochemistry for relaxin, cytokeratin, vimentin, and von Willebrand factor was performed on uteroplacental tissue at Day 30 of gestation. The basal cell layer at the core of the chorionic villi was devoid of relaxin mRNA and immunoreactive relaxin or vimentin but was immunopositive for cytokeratin and identified as cytotrophoblast cells. The cell layer surrounding the chorionic villi displayed specific hybridization signals for relaxin mRNA and immunoreactivity for relaxin and cytokeratin but not for vimentin, and was identified as syncytiotrophoblast. Those areas of the chorioallantoic tissue with most intense relaxin immunoreactivity were highly vascularized as demonstrated by immunoreactive von Willebrand factor expressed on vascular endothelium. The uterine glands and nonplacental uterine areas of the canine zonary girdle placenta were devoid of relaxin mRNA and relaxin. We conclude that the syncytiotrophoblast is the source of relaxin in the canine placenta. (+info)
Ontogeny of expression of a receptor for platelet-activating factor in mouse preimplantation embryos and the effects of fertilization and culture in vitro on its expression.
(3/6698)
Platelet-activating factor (PAF; 1-o-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a potent ether phospholipid. It is one of the preimplantation embryo's autocrine growth/survival factors. It may act via a G protein-linked receptor on the embryo; however, the evidence for this is conflicting. The recent description of the intracellular form of the PAF:acetlyhydrolase enzyme as having structural homology with G proteins and Ras also suggests this as a potential intracellular receptor/transducer for PAF. This study used reverse transcription-polymerase chain reaction to examine the ontogeny of expression of the genes for these proteins in the oocyte and preimplantation-stage embryo. Transcripts for the G protein-linked PAF receptor were detected in the late 2-cell-stage embryo and in all stages from the 4-cell stage to blastocysts. They were also present in unfertilized oocytes and newly fertilized zygotes but only at relatively low levels. The incidence of expression was generally low and variable in late zygotes and early 2-cell embryos. Expression past the 2-cell stage was alpha-amanitin sensitive. The results indicated that mRNA for this receptor is a maternal transcript that was degraded during the zygote-2-cell stage. New expression of the receptor transcript required activation of the zygotic genome. Fertilization of embryos in vitro caused this transcript not to be expressed in the zygote. Culture of zygotes (irrespective of their method of fertilization) caused expression from the zygotic genome to be retarded by more than 24 h. This retardation did not occur if culture commenced at the 2-cell stage. The transcripts for the subunits of intracellular PAF:acetylhydrolase were not detected in oocytes or at any stage of embryo development examined, despite their being readily detected in control tissue. This study confirms the presence of the G protein-linked PAF receptor in the 2-cell embryo and describes for the first time its normal pattern of expression during early development. The adverse effects of in vitro fertilization (IVF) and embryo culture on the expression of this transcript may be a contributing factor for the poor viability of embryos produced in this manner. The reduced expression of PAF-receptor mRNA following IVF predicts that such embryos may have a deficiency in autocrine stimulation and also suggests that supplementation of growth media with exogenous PAF would be only partially beneficial. The effect of IVF and culture may also explain the conflicting literature. (+info)
Isolation and characterization of a human homologue of the latrophilin gene from a region of 1p31.1 implicated in breast cancer.
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We have identified a region of chromosome 1p31.1 that shows high frequency loss of heterozygosity (LOH) in human breast cancer. This region forms part of a 7 Mb YAC/BAC contig. In order to identify candidate sequences, mutation of which might contribute to the development of disease, we have carried out mapping studies of ESTs localized to 1p31.1. This analysis, coupled with library screening and a modified 5' RACE-PCR strategy, resulted in the identification and characterization of a novel gene (LPHH1) which is located adjacent to the smallest region of overlapping loss (SRO) seen in tumours. The 4209 bp open reading frame of the 7 kb LPHH1 transcript encodes a peptide which shows approximately 65% identity to rat latrophilin, a G-coupled, seven span transmembrane protein, which binds alpha-latrotoxin. In the human sequence, whilst conservation of the transmembrane domain is high, the intra- and extracellular domains show two regions of variable structure, which are presumably generated by alternative splicing. Surprisingly, while expression of the rat gene is tightly restricted to neurological and perhaps some endocrine cells, the human sequence appears to be expressed very widely in all normal tissues tested. Northern and RT-PCR analysis of a panel of tumour cell lines showed that LPHH1 expression was variable, apparently elevated in some lines and absent or markedly reduced in others. Furthermore, characterization of the range of transcripts encoded in a breast tumour cell line, compared to normal breast, suggested that gene product variability was higher in the tumour. (+info)
Sphingosine 1-phosphate stimulation of the p42/p44 mitogen-activated protein kinase pathway in airway smooth muscle. Role of endothelial differentiation gene 1, c-Src tyrosine kinase and phosphoinositide 3-kinase.
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We report here that cultured airway smooth muscle cells contain transcripts of endothelial differentiation gene 1 (EDG-1), a prototypical orphan Gi-coupled receptor whose natural ligand is sphingosine 1-phosphate (S1P). This is consistent with data that showed that S1P activated both c-Src and p42/p44 mitogen-activated protein kinase (p42/p44 MAPK) in a pertussis toxin (PTX)-sensitive manner in these cells. An essential role for c-Src was confirmed by using the c-Src inhibitor, PP1, which markedly decreased p42/p44 MAPK activation. We have also shown that phosphoinositide 3-kinase (PI-3K) inhibitors (wortmannin and LY294002) decreased p42/p44 MAPK activation. An essential role for PI-3K was supported by experiments that showed that PI-3K activity was increased in Grb-2 immunoprecipitates from S1P-stimulated cells. Significantly, Grb-2 associated PI-3K activity was decreased by pretreatment of cells with PTX. Finally, we have shown that the co-stimulation of cells with platelet-derived growth factor (PDGF) and S1P (which failed to stimulate DNA synthesis) elicited a larger p42/p44 MAPK activation over a 30 min stimulation compared with each agonist alone. This was associated with a S1P-dependent increase in PDGF-stimulated DNA synthesis. These results demonstrate that S1P activates c-Src and Grb-2-PI-3K (intermediates in the p42/p44 MAPK cascade) via a PTX-sensitive mechanism. This action of S1P is consistent with the stimulation of EDG-1 receptors. S1P might also function as a co-mitogen with PDGF, producing a more robust activation of a common permissive signal transduction pathway linked to DNA synthesis. (+info)
Putative mammalian taste receptors: a class of taste-specific GPCRs with distinct topographic selectivity.
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Taste represents a major form of sensory input in the animal kingdom. In mammals, taste perception begins with the recognition of tastant molecules by unknown membrane receptors localized on the apical surface of receptor cells of the tongue and palate epithelium. We report the cloning and characterization of two novel seven-transmembrane domain proteins expressed in topographically distinct subpopulations of taste receptor cells and taste buds. These proteins are specifically localized to the taste pore and are members of a new group of G protein-coupled receptors distantly related to putative mammalian pheromone receptors. We propose that these genes encode taste receptors. (+info)
Developmental pathways: Sonic hedgehog-Patched-GLI.
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Developmental pathways are networks of genes that act coordinately to establish the body plan. Disruptions of genes in one pathway can have effects in related pathways and may result in serious dysmorphogenesis or cancer. Environmental exposures can be associated with poor pregnancy outcomes, including dysmorphic offspring or children with a variety of diseases. An important goal of environmental science should be reduction of these poor outcomes. This will require an understanding of the genes affected by specific exposures and the consequence of alterations in these genes or their products, which in turn will require an understanding of the pathways critical in development. The ligand Sonic hedgehog, the receptors Patched and Smoothened, and the GLI family of transcription factors represent one such pathway. This pathway illustrates several operating principles important in the consideration of developmental consequences of environmental exposures to toxins. (+info)
PAF binding to a single receptor in corneal epithelium plasma membrane.
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PURPOSE: To study the binding characteristics and the expression of platelet-activating factor receptors (PAF-R) in corneal epithelium to elucidate the site of action of PAF. METHODS: Binding of [3H]PAF was investigated in subcellular fractions of the epithelia of bovine corneas and in membranes from cultured rabbit corneal epithelial cells. Dose-response inhibition curves of [3H]PAF-specific binding were generated using increasing concentrations of several PAF-R antagonists. RNA from rabbit corneal epithelial cells was probed for PAF-R expression by reverse transcription-polymerase chain reaction (RT-PCR) with specifically designed degenerated primers. RESULTS: Scatchard analysis showed a high-affinity binding site in bovine and rabbit corneal epithelium. The dissociation constant (Kd) and the maximum binding sites (Bmax) in a bovine membrane preparation and similar rabbit fraction were 0.77+/-0.03 nM and 180+/-21 femtomoles/mg protein and 4.3 nM and 1.3 picomoles/mg protein, respectively. Specific PAF-binding sites were found in bovine preparations enriched in plasma membranes with a Kd = 69.6 pM and Bmax = 80 femtomoles/mg protein; no specific binding was found in nuclei or microsomal fractions. RT-PCR of rabbit corneal epithelium generated a single product of the predicted size (478 bp). The deduced amino acid sequence of the purified PCR product was 87% homologous to human PAF-R. The hetrazepines BN 50727 and BN 50730 and the PAF structural analogues CV 3988 and CV 6209 competitively inhibited [3H]PAF binding to corneal epithelium with similar potency. WEB 2086 BS was two orders of magnitude less active in antagonizing PAF binding. CONCLUSIONS: Corneal epithelium contains a single population of receptors localized in the plasma membrane. PAF antagonists exert their actions by blocking this PAF-R. The partial sequence deduced in rabbit corneal PAF-R show a higher homology to the human PAF-R. (+info)