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Regular Article |
a Laboratory of Animal Physiology,
b School of Agricultural Sciences, Nagoya University, Chikusa, Nagoya, Japan 464-860
c Center for Molecular Biology and Genetics, Mie University, Tsu, Mie, Japan 514-850
d Laboratory of Animal Genetics, School of Agricultural Sciences, Nagoya University, Chikusa, Nagoya, Japan 464-860
e Department of Animal Science, McGill University, Ste Anne de Bellevue, Quebec, Canada H9X 3V
ABSTRACT
Chicken vasoactive intestinal polypeptide receptor (VIPR) cDNA was cloned by the reverse transcription-polymerase chain reaction method using primers designed on the basis of other species of VIPR cDNA. The cDNA obtained was sequenced by the dideoxy-mediated chain-termination method. Of the 2227 nucleotides that were sequenced, 84, 855, and 1338 bases represent the 5'-untranslated region (UTR), the 3'-UTR, and the open reading frame that predicts a peptide of 446 amino acids. The cDNA of the chicken VIPR shows 65% and 60% homologies to human cDNA of VIP1 and VIP2 receptors, respectively. The clone had the expected similarity to highly conserved features of the other G protein-coupled receptors (GPCRs) such as six cysteine residues that are functionally important in the VIPR subfamily. In addition, the seven potential membrane-spanning domains characteristic of the family B group III GPCR superfamily and highly conserved motif within the third cellular loop between transmembrane regions 5 and 6. Northern blot hybridization analysis in this study indicated mRNA expression of VIPRs in the various tissues of the chicken. Strong signal was detected in the brain and anterior pituitary gland. High levels of VIPR mRNA in the brain was consistent with VIP-binding experiments and with the function of VIP in the brain as a neuroendocrine factor or neurotransmitter. Expression of VIPR was detected in the anterior pituitary gland of chick embryos. The expression of VIPR mRNA in the chick anterior pituitary gland may indicate a regulatory function of VIP on prolactin (PRL) production or PRL cell proliferation during embryogenesis. Chicken VIPR shows high homology with mammalian type I VIPR but, in some part, possesses similarity of amino acid sequence. Expression of VIPR in various tissues supports diverse functions for VIP in the chicken.
anterior pituitary, mechanisms of hormone action, neuroendocrinology, pituitary hormones, signal transduction
INTRODUCTION
Vasoactive intestinal polypeptide (VIP) is an amidated 28-amino acid peptide that belongs to a family of regulatory peptides including secretin, glucagon, growth hormone releasing factor (GRF), and pituitary adenylate activating polypeptide (PACAP). Vasoactive intestinal polypeptide was originally identified in the duodenum [1] but subsequent studies have indicated that it is widely expressed throughout the body and has important regulatory effects on the circulatory, immune, reproductive, and gastrointestinal systems [2, 3]. In the central nervous system (CNS), VIP functions not only as a neurotransmitter but also as a neuroendocrine releasing factor and powerful neurotrophic factor [4].
In avian species, a major role of VIP involves the regulation of synthesis and secretion of the adenohypophyseal hormone prolactin (PRL). Immunoreactive nerve terminals containing VIP were found in the median eminence in the quail, bantam hen, turkey, and pigeon [59] and hypothalamic content of mRNA levels of VIP vary with physiological status of the hen [8, 10, 11]. In contrast to the inhibitory control mechanism for PRL involving dopamine in mammals [1214], stimulation of the avian anterior pituitary gland by VIP results in increased secretion of PRL [1518] and transcription of the gene encoding PRL [1923]. Vasoactive intestinal polypeptide also enhances PRL mRNA stability [24]. Conversely, if the release of VIP is blocked via active or passive immunization against VIP, circulating levels of PRL as well as PRL mRNA will decrease [16, 2427].
The actions of VIP are mediated via interaction with its specific receptors that are coupled to adenylyl cyclase and the production of cAMP. In mammals, VIP receptors (VIPRs) have been cloned and functionally characterized [2834]. Pharmacological evidence has indicated that there are two VIPR subtypes (VIP1 and VIP2) with different but related amino acid sequences. Each receptor is expressed in a tissue-specific manner [31, 33, 35], and the two receptor subtypes are distinguished by the effects of secretin that interact with only the type I receptor. Recently, nonmammalian VIPRs from goldfish [36] and frog [37] were cloned and characterized. Because secretin had shown agonistic effects on the goldfish VIPR, this led to the conclusion that the receptor characterized in the goldfish was a VIP type I receptor. The frog receptor cDNA showed higher homology to the type I receptor than to the type II receptor of humans. However, the frog receptor exhibited pharmacological and tissue distribution features characteristic of both type I and type II receptors in mammals. A partial turkey VIPR showing high homology to mammalian VIP type I receptor was reported [38]. These results may suggest that a single VIPR is expressed and functions in nonmammalian species.
In birds, the presence of VIP receptors in the anterior pituitary gland has been indicated in the turkey [39] and the chicken [40, 41], and stimulation by VIP increased levels of intracellular cAMP in chicken anterior pituitary cells [23]. These studies suggested that the functional VIPRs may be located in the anterior pituitary gland.
In this report, we describe the cloning of the chicken VIPR, its tissue distribution, and chromosomal localization.
MATERIALS AND METHODS
Tissue Sampling and RNA Isolation
Anterior pituitary glands, liver, kidney, pancreas, intestine, lung, brain, cerebellum, and stomach were removed from 12-mo-old White Leghorn hens. Tissue samples were snap-frozen in liquid nitrogen and stored at -80°C until RNA extraction. The total RNA was extracted using RNA isolation reagent (TRIzol; Gibco-BRL, Life Technologies, Rockville, MD) according to the method described by Chomczynski [42]. The amount of total RNA was estimated by spectrophotometry (Beckman Instruments Inc., Fullerton, CA).
Polymerase Chain Reaction Cloning of Chicken VIPR cDNA
Total RNA (1 µg) of adult anterior pituitary gland was denatured at 70°C for 10 min with random hexamer primers and reverse-transcribed with 200 units of SuperScript II (Gibco-BRL) in a 20-µl mixture. Based on the sequences of human, rat, mice, and turkey VIPR cDNA [2630], primers were designed. Location and primer sequence were indicated in Figure 1. The reverse-transcribed product was subjected to 35 cycles of PCR amplification using Taq polymerase (Takara, Shiga, Japan) in a total volume of 100 µl. The amplification profile consisted of 2 min of denaturation at 94°C for the first cycle and 30 sec per cycle thereafter, 1-min annealing at 55°C, and 1-min extension at 72°C for the first 34 cycles, and 10-min extension on the final cycle. The 5' region was cloned using a Marathon rapid amplification of cDNA ends (RACE) kit (Clontech, Palo Alto, CA). The 3' region was cloned using a Takara 3-RACE kit (Takara). Amplified PCR products were electrophoresed on 1.2% agarose gels, isolated, purified (Easy Trap; Takara), and subcloned into the pCRII plasmid vector (Invitrogen, San Diego, CA). DNA sequencing was performed on plasmids using dye-terminator chemistry on an Applied Biosystem model 373S sequencer by the dideoxy-mediated chain-termination method [43]. After preliminary sequencing, polymerase chain reaction (PCR) products were amplified from different total RNA samples using new primers (5-1-1 and 3-5, Fig. 1) and directly sequenced from both strands after purification (QIAquick PCR purification kit; Qiagen, Germany). Sequencing was conducted three times using three different PCR products. The cloned sequence was analyzed by using FASTA and Philip.
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Hybridization Analysis
Northern hybridization Aliquots of total RNA (20 µg) were denatured with formaldehyde at 65°C, electrophoresed in 1.5% agarose gels, and transferred to Hybond-N+ (Amersham Inc., U.K.) by capillary action with 10x SSC (1x SSC = 0.15 M sodium chloride, 0.015 M sodium citrate, pH 7.0). Following blotting, air-dried membranes were cross-linked using a UV cross-linker (Stratagene, La Jolla, CA).
The membranes were prehybridized and probed with 32P-labeled chicken VIPR cDNAs prepared using a BcaBest labeling kit (Takara). The hybridization was carried out at 42°C for 18 h in a solution consisting of 50% formamide, 5x Denhardt solution (0.01% Ficoll 400, 0.01% polyvinylpyrrolidone, 0.1% BSA), 0.1% SDS 5x SSPE (900 mM NaCl, 5 mM EDTA, 50 mM Na2HPO4·12H2) and sonicated salmon sperm DNA (100 µg/ml). The membranes were washed to remove nonspecific radioactivity twice for 15 min at 42°C in 0.1x SSC-0.1% SDS, for 15 min at 50°C in 0.1x SSC-0.1% SDS, for 15 min at 55°C in 0.1x SSC-0.1% SDS, and twice for 10 min at 60°C in 0.1x SSC-0.1% SDS. Membranes were exposed to x-ray film with intensifying screens (Amersham Inc., U.K.) at -80°C for 20 h. For quantification of the radioactive signal intensity, the membranes were exposed to the Image Plate and analyzed by the BAS-2000 Bio-Imaging analyzer (Fuji Photo Film Co., Ltd., Japan).
Fluorescence in situ hybridization The direct R-banding fluorescence in situ hybridization (FISH) method was used for chromosomal assignment. Preparation of R-banded chromosomes and FISH were performed as described by Matsuda et al. [44] and Matsuda and Chapman [45]. Chicken VIP receptor cDNA was labeled by nick translation with biotin 16-dUTP (Boehringer Mannheim, Mannheim, Germany) according to manufacturer's instructions. The labeled cDNA fragment was ethanol-precipitated with salmon sperm DNA and tRNA, then denatured for 10 min at 75°C in 100% formamide, and kept at 4°C. The chromosomal DNA was denatured at 70°C for 2 min in 70% formamide/2x SSC and dehydrated successively (70%, 90%, and 100%) in ethanol at 4°C. Hybridization solution (20 µl; 50% formamide, 2x SSC, 10% dextran sulfate, and 1 mg/ml BSA (Boehringer Mannheim) containing 250 ng labeled DNA was put on the denatured slides, covered with parafilm, and incubated overnight at 37°C. The slides were washed for 20 min in 50% formamide/2x SSC at 37°C, and in 2x SSC and 1x SSC for 20 min each at room temperature. After rinsing in 4x SSC, the slides were incubated under coverslips with goat anti-biotin antibodies (Vector Laboratories, Burlingame, CA) at a 1:500 dilution in 1% BSA/4x SSC for 1 h at 37°C. The slides were washed with 4x SSC, 0.1% Nonidet P-40/4x SSC, and 4x SSC for 5 min each and then stained with FluoroLink Cy2-labeled donkey anti-goat IgG (Amersham Pharmacia Biotech, U.K.) at a 1:500 dilution for 1 h at 37°C. After washing as above for 10 min each on the shaker and draining the excess liquid, they were stained with 0.5 µg/ml propidium iodide. The hybridization signals were visualized by excitation at 450490 nm (Nikon filter set B-2A) and 365 nm (UV-2A) wavelengths. Kodak Ektachrome ASA100 film was used for photomicrography.
RESULTS
PCR Cloning of Chicken VIPR cDNA
Approximately 520 base pairs (bp) of partial VIP receptor clone was primarily obtained by PCR amplification between 5-1 and 3-1 (Fig. 1). Secondary, based on the turkey VIPR sequence and the sequence of the first PCR product, PCR product between 5-2 and 3-2 was obtained. Likewise, PCR product between 5-3 and 3-3 was obtained. Finally, untranslated regions both up- and downstream were cloned by RACE.
Figure 2 shows the nucleotide sequence of the clone together with the deduced amino acid sequence. Of the 2227 nucleotides that were sequenced, 84, 855, and 1338 bases represent the 5'-untranslated region (UTR), 3'-UTR, and open reading frame, which predicts a peptide of 446 amino acids. The predicted amino acid sequence has an overall similarity with a comparable region of human (65.6%), porcine (65.8%), rat (66.4%), and turkey (98.1%) VIPR. Chicken VIPR was found to have 67.4%, 68.7%, 67.3%, and 96.8% sequence homology at the cDNA level with type I receptor of human, porcine, rat, and turkey, respectively (see Fig. 3). Figure 4 shows a Kyte-Doolittle hydrophobicity analysis of the receptor that indicates seven hydrophobic, putative membrane-spanning domains. Figure 5 shows a secondary structure model of the chicken VIPR. A phylogenetic tree was generated by comparison of the deduced amino acid sequence of the VIP-PACAP receptor (Fig. 6). Phylogenetic analysis of VIPRs indicated that the chicken VIPR was closely related to mammalian VIP type I receptors.
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Northern Blot Hybridization Analysis
Figure 7 shows the tissue distribution of the chicken VIPRs determined by Northern hybridization analysis using total RNA from various chicken tissues. Transcripts of 6.8 kb and 3.8 kb of mRNA were detected in all of the tissues examined. In most tissues the 6.8-kb band was dominant; however, in the intestine the 3.8-kb band was mainly expressed. The 6.8-kb transcript was most abundant in the anterior pituitary gland, and moderate levels were found in the lung, intestine, and brain. Weak expression was also observed in the cerebellum, hypothalamus, liver, kidney, granulosa layer of the largest follicle, and pancreas.
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Fluorescence In Situ Hybridization
The localization of fluorescent signals to chromosomes was readily made on R-banded metaphase chromosomes after identification of each macrochromosome by Hoechst G-band patterns revealed by UV excitation (Fig. 8). As shown in Figure 8, a and c, the hybridization signals of VIPR were demonstrated directly on R-banded chromosomes, and a precise regional assignment of the gene was made. Specific signals were detected on one or both sister chromatids of chromosome 2 around the region of p3.2. No significant signals on any other chromosome were detected.
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DISCUSSION
Considerable evidence has accumulated to suggest that VIP is the hypophysiotropic PRL-releasing factor in birds [1620] and measurement of levels of VIP in the portal blood closely correlate with levels of PRL assessed in the general circulation [46]. Receptors for VIP are localized on lactotroph cells, and high affinity binding of VIP to its receptor in the adenohypohysis is the first step in the signal transduction mechanism that results in the de novo synthesis and secretion of PRL [39]. The identification and cloning of the chicken VIPR in this study should prove useful for studying the mechanisms underlying the regulation of VIP and control of the expression of the PRL gene.
The chicken VIPR cDNA coded for a predicted protein of 446 amino acids. The clone had the expected similarity to highly conserved features of the other G protein-coupled receptors (GPCRs) such as six cysteine residues that are functionally important in the VIPR subfamily [47]. In addition, the seven potential membrane-spanning domains characteristic of the family B, group III GPCR superfamily [48] and highly conserved motif within the third cellular loop between transmembrane regions 5 and 6 [49]. Overall, the chicken VIPR shared a high degree of amino acid sequence similarity with other cloned VIP type I receptors (Fig. 3), and phylogenetic analysis (Fig. 6) indicated that it is more closely related to type I than type II receptors.
Northern blot hybridization analysis in this study indicated mRNA expression of VIPR in the various tissues of the chicken. Interestingly, the 3.8-kb mRNA was abundant in the intestine, whereas a 6.8-kb mRNA was abundant in the other tissues examined. In the rat, VIPR subtype I is expressed in a tissue-dependent manner, and the 5.5-kb and 2.4-kb VIP type I receptor mRNA was detected in the lung and intestine [28]. Thus, a difference in the length of VIPR mRNA may indicate the differential regulation of mRNA expression of VIPR or a different posttranscription mechanism between intestine and other tissues. Hof et al. [50] demonstrated high VIP-binding to the tectum opticum, neostriatum, hyperstriatum ventrale, and archistriatum ventrale in the pigeon [50]. In the chick embryonic brain and optic lobe, high levels of mRNA for the VIPR were observed by Northern hybridization (Fig. 7). High levels of VIPR mRNA in the brain were consistent with VIP-binding experiments and with the function of VIP in the brain as a neuroendocrine factor or neurotransmitter.
Strong mRNA signal was observed in the anterior pituitary gland by Northern hybridization analysis. High mRNA levels of VIPRs may account for the increase in PRL gene transcription in response to VIP stimulation [20]. Although only White Legohorn was assessed, results of this study support the observations from VIP binding studies. Gonzales et al. [40, 41] have suggested that VIPRs may be located mainly in the cephalic lobe of the anterior pituitary gland, and specific VIP binding varies depending on the reproductive stages of the chicken. Thus, more mRNA for VIPR is expected in the incubation phase.
In mammals, the functional pituitary receptor for VIP is considered as type II due to changes in levels during the reproductive cycle. In this study, however, no PCR products were obtained by using primers designed based on the VIP type II receptor-specific region. At 14 days of embryonic development, VIPR mRNAs were detected in the anterior pituitary gland by Northern hybridization. Messenger RNA of VIPR was also observed by Day 18 of embryogenesis. Changes in the levels of PRL mRNA before hatching were observed by our laboratory [51, 52]. The increase in the expression of PRL mRNA observed during the late stages of embryonic development is consistent with the differentiation of lactotrophs in the chick embryo [53]. Furthermore, Woods and Porter [52] also demonstrated increased responsiviness to VIP of the embryonic PRL cells during the same stages of development. During embryogenesis, PRL plays a role in osmoregulatory function [54]. Thus, the presence of VIPR mRNA in the anterior pituitary gland at Days 14 and 18 of incubation may indicate regulatory function of VIP on PRL production or PRL cell proliferation.
The chicken VIPR gene was mapped on p3.2 of chromosome 2. In humans, type I and type II receptors were mapped to p22 of chromosome 3 [55] and q36.3 of chromosome 7 [56], respectively. However, synteny was not reported in this region between chickens and humans. Analysis of conservation of syntenic groups on the p-arm of chicken chromosome 2 and comparative analysis of VIP binding and immunocytochemical analysis using antibody against chicken VIPR may clarify this issue.
In conclusion, phylogenetically, chicken VIPR is most closely related to the type I receptor. Chicken VIPR sequenced in this experiment was partial, thus a full clone of the chicken VIPR may clarify the type of VIPR in the chicken. Furthermore, expression of VIPR in the various tissues supports diverse functions for VIP in the chicken.
ACKNOWLEDGMENTS
We thank Drs. M. Tanaka (Mie University), P.J. Sharp (Roslin Institute), and M.E. El Halawani (University of Minnesota) for their kind advice and discussion. We also appreciate the Radioisotope Research Center of Nagoya University for use of the Radioanalytic Image System BAS-2000.
FOOTNOTES
1 This research was supported by grants from the Monbusho International Scientific Research Program (Joint Research; 07044190) to K.S. and by a JSPS Research Fellowship for Young Scientists awarded to N.K. (80003425). The nucleotide sequence report in this paper has been submitted to GenBank/EMBL/DDBJ Bank with the accession number AB029895. ![]()
2 Correspondence. FAX: 81 52 789 4065; g44500a{at}nucc.cc.nagoya-u.ac.jp ![]()
3 Current address: Chromosome Research Unit, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, Japan 060-0810. ![]()
Accepted: January 16, 2001.
Received: May 22, 2000.
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