Cloning of a splice variant of the pituitary adenylate cyclase-activating polypeptide (PACAP) type I receptor

Agnathan VIP, PACAP and their receptors: ancestral origins of today's highly diversified forms

VIP and PACAP are pleiotropic peptides belonging to the secretin superfamily of brain-gut peptides and interact specifically with three receptors (VPAC₁, PAC₁ and VPAC₂) from the class II B G protein-coupled receptor family. There is immense interest regarding their molecular evolution which is often described closely alongside gene and/or genome duplications. Despite the wide array of information available in various vertebrates and one invertebrate the tunicate, their evolutionary origins remain unresolved. Through searches of genome databases and molecular cloning techniques, the first lamprey VIP/PACAP ligands and VPAC receptors are identified from the Japanese lamprey. In addition, two VPAC receptors (VPACa/b) are identified from inshore hagfish and ligands predicted for sea lamprey. Phylogenetic analyses group these molecules into their respective PHI/VIP, PRP/PACAP and VPAC receptor families and show they resemble ancestral forms. Japanese lamprey VIP/PACAP peptides synthesized were tested with the hagfish VPAC receptors. hfVPACa transduces signal via both adenylyl cylase and phospholipase C pathways, whilst hfVPACb was only able to transduce through the calcium pathway. In contrast to the widespread distribution of VIP/PACAP ligands and receptors in many species, the agnathan PACAP and VPAC receptors were found almost exclusively in the brain. In situ hybridisation further showed their abundance throughout the brain. The range of VIP/PACAP ligands and receptors found are highly useful, providing a glimpse into the evolutionary events both at the structural and functional levels. Though representative of ancestral forms, the VIP/PACAP ligands in particular have retained high sequence conservation indicating the importance of their functions even early in vertebrate evolution. During these nascent stages, only two VPAC receptors are likely responsible for eliciting functions before evolving later into specific subtypes post-Agnatha. We also propose VIP and PACAP's first functions to predominate in the brain, evolving alongside the central nervous system, subsequently establishing peripheral functions.

VPAC and PAC receptors: From ligands to function

Vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptides (PACAPs) share 68% identity at the amino acid level and belong to the secretin peptide family. Following the initial discovery of VIP almost four decades ago a substantial amount of knowledge has been presented describing the mechanisms of action, distribution and pleiotropic functions of these related peptides. It is now known that the physiological actions of these widely distributed peptides are produced through activation of three common G-protein coupled receptors (VPAC(1), VPAC(2) and PAC(1)R) which preferentially stimulate adenylate cyclase and increase intracellular cAMP, although stimulation of other intracellular messengers, including calcium and phospholipase D, has been reported. Using a range of in vitro and in vivo approaches, including cell-based functional assays, transgenic animals and rodent models of disease, VPAC/PAC receptor activation has been associated with numerous physiological processes (e.g. control of circadian rhythms) and clinical conditions (e.g. pulmonary hypertension), which underlies on-going research efforts and makes these peptides and their cognate receptors attractive targets for the pharmaceutical industry. However, despite the considerable interest in VPAC/PAC receptors and the processes which they mediate, there is still a paucity of selective and available, non-peptide ligands, which has hindered further advances in this field both at the basic research and clinical level. This review summarises the current knowledge of VIP/PACAP and the VPAC/PAC receptors with regard to their distribution, pharmacology, signalling pathways, splice variants and finally, the utility of animal models in exploring their physiological roles.

Cloning and characterization of a PAC1 receptor hop-1 splice variant in goldfish (Carassius auratus)

In several vertebrates, it has been demonstrated that alternative splicing of PAC1 receptor (PAC1-R) transcripts can generate a number of functional receptor variants which utilize different signal transduction pathways to mediate their activities. As PACAP is a physiological growth hormone-releasing factor in fish, and PACAP and the PAC1-R are highly conserved in vertebrate evolution, it would be of interest to investigate the structure and cellular distribution, particularly in the pituitary, of PAC1-R splice variants in a fish model. Our laboratory has previously cloned a receptor cDNA corresponding to the goldfish PAC1-R-s (goldfish PAC1-R-short). In the present study, a goldfish PAC1-R-hop1 variant was characterized. Functional expression of goldfish PAC1-R-s and PAC1-R-hop1 in Chinese Hamster Ovary cells revealed that, upon stimulation by ovine PACAP38, these receptor variants exhibited similar EC50 values (8.7+/-1.5 and 8.8+/-1.9 nM, respectively) and maximal responses in activating intracellular cAMP production. The presence and expression levels of these transcripts were measured by quantitative real-time PCR in the brain, heart, pituitary and male gonad, and goldfish PAC1-R-s were found to be the predominant form. In situ hybridization of goldfish PAC1-R in the pituitary revealed its prevalent presence in the pars distalis. In summary, the present study provides information to confirm the role of PACAP in the pituitary and to elucidate the pleiotropic effects of PACAP in fish.

Neuropeptides of the pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide/growth hormone-releasing hormone/secretin family in testis

Mammalian testicular development and the maintenance of spermatogenesis are hormone-dependent processes that are controlled by the pituitary gonadotropins and testosterone. Recent studies have demonstrated the presence of many neuropeptides and their receptors in the testis, suggesting that these peptides operate as local regulators of testicular germ cell development and function. Among these testicular neuropeptides, the peptides that belong to the pituitary adenylate cyclase-activating polypeptide (PACAP) family, particularly growth hormone-releasing hormone and secretin, appear to show some unique common features in terms of intratesticular localization and the time of expression during the spermatogenic cycle. However, their precise physiologic roles and mechanisms of action remain unknown. This review analyzes the available information on the functional interactions among the testicular cells that appear to be mediated by locally produced neuropeptides, with a special emphasis on the peptides of the PACAP family.

Neuropeptides of the pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide/growth hormone-releasing hormone/secretin family in testis

Mammalian testicular development and the maintenance of spermatogenesis are hormone-dependent processes that are controlled by the pituitary gonadotropins and testosterone. Recent studies have demonstrated the presence of many neuropeptides and their receptors in the testis, suggesting that these peptides operate as local regulators of testicular germ cell development and function. Among these testicular neuropeptides, the peptides that belong to the pituitary adenylate cyclase-activating polypeptide (PACAP) family, particularly growth hormone-releasing hormone and secretin, appear to show some unique common features in terms of intratesticular localization and the time of expression during the spermatogenic cycle. However, their precise physiologic roles and mechanisms of action remain unknown. This review analyzes the available information on the functional interactions among the testicular cells that appear to be mediated by locally produced neuropeptides, with a special emphasis on the peptides of the PACAP family.

VIP as a trophic factor in the CNS and cancer cells

The effects of vasoactive intestinal peptide (VIP) on the proliferation of central nervous system (CNS) and cancer cells were investigated. VIP has important actions during CNS development. During neurogenesis, VIP stimulates the proliferation and differentiation of brain neurons. Addition of VIP to embryonic mouse spinal cord cultures increases neuronal survival and activity dependent neurotrophic factor (ADNF) secretion from astroglial cells. VIP is an integrative regulator of brain growth and development during neurogenesis and embryogenesis. Also, VIP causes increased proliferation of human breast and lung cancer cells in vitro. VIP binds with high affinity to cancer cells, elevates the cAMP and increases gene expression of c-fos, c-jun, c-myc and vascular endothelial cell growth factor. The effects of VIP on cancer cells are reversed by VIPhybrid, a synthetic VPAC(1) receptor antagonist. VIPhyb inhibits the basal growth of lung cancer cells in vitro and tumors in vivo and potentiates the ability of chemotherapeutic drugs to kill cancer cells. Due to the high density of VPAC(1) receptors in cancer cells, VIP has been radiolabeled with 123I, 18F and 99mTc to image tumors. It remains to be determined if radiolabeled VIP analogs will be useful agents for early detection of cancer in patients.

PAC1 and PACAP expression, signaling, and effect on the growth of HCT8, human colonic tumor cells

The pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1) is a heptahelical, G protein-coupled receptor that has been shown to be expressed by non-squamous lung cancer and breast cancer cell lines, and to be coupled to the growth of these tumors. We have previously shown that PACAP and its receptor, PAC1, are expressed in rat colonic tissue. In this study, we used polyclonal antibodies directed against the COOH terminal of PAC1, as well as fluorescently labeled PACAP, Fluor-PACAP, to demonstrate the expression of PAC1 on HCT8 human colonic tumor cells, using FACS analysis and confocal laser scanning microscopy. Similarly, anti-PACAP polyclonal antibodies were used to confirm the expression of PACAP hormone by this cell line. We then investigated the signal transduction properties of PAC1 in these tumor cells. PACAP-38 elevated intracellular cAMP levels in a dose-dependent manner, with a half-maximal (EC(50)) stimulation of approximately 3 nM. In addition, PACAP-38 stimulation caused an increase in cytosolic Ca(2+) concentration [Ca(2+)](i), which was partially inhibited by the PACAP antagonist, PACAP-(6-38). Finally, we studied the potential role of PACAP upon the growth of these tumor cells. We found that PACAP-38, but not VIP, increased the number of viable HCT8 cells, as measured by MTT activity. We also demonstrated that HCT8 cells expressed the Fas receptor (Fas-R/CD95), which was subsequently down-regulated upon activation with PACAP-38, further suggesting a possible role for PACAP in the growth and survival of these tumor cells. These data indicate that HCT8 human colon tumor cells express PAC1 and produce PACAP hormone. Furthermore, PAC1 activation is coupled to adenylate cyclase, increase cytosolic [Ca(2+)](i), and cellular proliferation. Therefore, PACAP is capable of increasing the number of viable cells and regulating Fas-R expression in a human colonic cancer cell line, suggesting that PACAP might play a role in the regulation of colon cancer growth and modulation of T lymphocyte anti-tumoral response via the Fas-R/Fas-L apoptotic pathway.

Differential regulation of pituitary adenylate cyclase-activating peptide receptor variants in the rat suprachiasmatic nucleus

Pituitary adenylate cyclase-activating peptide is densely distributed in the suprachiasmatic nucleus, which functions as the circadian pacemaker. A receptor for pituitary adenylate cyclase-activating peptide, denoted as PAC(1), exists in six variant forms. We used reverse transcriptase-polymerase chain reaction to identify the PAC(1) variants that are expressed in the suprachiasmatic nucleus. Dominant variant forms of PAC(1) in the suprachiasmatic nucleus were PAC(1)short, PAC(1)hip, and PAC(1)hop1. By in situ hybridization, we examined 24-h profiles of mRNAs for the identified receptor variants in the suprachiasmatic nucleus in constant darkness and during the light-dark cycle. In constant darkness there were clear circadian rhythms in PAC(1)short mRNA with a peak at circadian time 4 but no rhythmicity was observed in PAC(1)hip mRNA or PAC(1)hop1 mRNA. In light-dark cycles, on the other hand, PAC(1)hip mRNA displayed a bimodal rhythm with troughs at zeitgeber time 4 and 16 but PAC(1)hop1 mRNA stayed constant during the day. These results suggest that PAC(1) splice variants are differentially regulated in the rat suprachiasmatic nucleus.

Characterization of functional VIP/PACAP receptors in the human erythroleukemic HEL cell line

The presence of VIP/PACAP receptors was investigated on the human erythroleukemic cell line HEL. Specific binding of [125I]-PACAP or [125I]-VIP on HEL cells or membranes was very low and did not allow to perform competition curves. At 37 degrees C PACAP transiently increased cAMP levels in the presence of the non-specific phosphodiesterase inhibitor IBMX, suggesting rapid desensitization. Kinetic studies revealed that optimal conditions to measure the EC(50) of PACAP(1-27) were 10 min at 20 degrees C. Under those conditions, PACAP-related peptides increased cAMP levels with EC(50) in agreement with the pharmacological profile of the VPAC(1) receptor subtype: PACAP = VIP > [K(15), R(16,) L(27)]VIP(1-7)/GRF(8-27) = [R(16)]ChSn (two VPAC(1) agonists) >> helodermin = secretin. RO 25-1553, a selective activator of VPAC(2) receptor was inactive at 1 microM. Dose-response curves of VPAC(1) agonist molecules (PACAP, VIP, [K(15), R(16), L(27)]VIP(1-7)/GRF(8-27), [R(16)]ChSn) were shifted to the right by the VPAC(1) receptor antagonist [AcHis(1), D-Phe(2), Lys(15), Leu(17)]VIP(3-7)/GRF(8-27), with a K(i) of 3 +/- 1 nM (n = 3). The presence of VPAC(1) receptor mRNA was confirmed by RT-PCR. Preincubation with PACAP or PMA showed that VPAC(1) receptors underwent homologous and heterologous desensitization. This study provides the first evidence for the expression of functional VPAC(1) receptors undergoing rapid desensitization in HEL cells.

Molecular cloning and expression analysis of the turkey vasoactive intestinal peptide receptor

Vasoactive intestinal peptide (VIP) is a prolactin (PRL)-releasing factor whose activity in avian species is believed to be mediated by a specific VIP receptor (VIP-R). Circulating PRL levels are closely related to hypothalamic VIP immunoreactivity, hypothalamic VIP mRNA content, and hypophysial-portal blood VIP concentrations in turkeys. In the present study, a turkey VIP-R (tVIP-R) cDNA was cloned and its mRNA abundance was quantified in various tissues during different reproductive stages. The 2347-bp tVIP-R cDNA encoded a 457 amino acid protein, with a predicted Mr of 52 kDa. The full-length cDNA shares approximately 55% similarity with the mammalian VIP receptor-1. Northern blot analysis revealed that a major 2.7-kb transcript was expressed in laying hen pituitaries. Furthermore, two minor tVIP-R transcripts of 3.7 and 3.4 kb were observed. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) was performed using RNA from various turkey brain and peripheral tissues throughout the reproductive cycle. The steady-state levels of pituitary tVIP-R mRNA changed during the reproductive cycle, whereas mRNA expression in other tissues was not affected. The steady-state levels of tVIP-R mRNA were only affected in the pituitary, whereas mRNA expression in any of the other tissues examined following the immunization of turkeys against VIP were not affected.

The neuropeptide pituitary adenylate cyclase-activating polypeptide and islet function

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is ubiquitously distributed in both the central and peripheral nervous systems and exerts a variety of effects. PACAP is a neuropeptide in pancreatic islets, where it has been suggested as a parasympathetic and sensory neurotransmitter. PACAP stimulates insulin secretion in a glucose-dependent manner, by an effect executed mainly through augmenting the formation of cAMP and stimulating the uptake of calcium. Accumulating evidence in animal studies points to a physiological importance of PACAP in the regulation of the insulin response to feeding. This review summarizes the current knowledge of islet actions and mechanisms and the function of PACAP.

ADP-Ribosylation Factor-Dependent Phospholipase D Activation by VPAC Receptors and a PAC1 Receptor Splice Variant

The VPAC(1) and VPAC(2) receptors for vasoactive intestinal polypeptide and the PAC(1) receptor for pituitary adenylate cyclase-activating polypeptide are members of a subfamily of G protein-coupled receptors (GPCRs). We recently reported that phospholipase D (PLD) activation by members of the rhodopsin group of GPCRs occurs by at least two routes, one of which seems to involve the small G protein ADP-ribosylation factor (ARF) and its physical association with GPCRs. Here we report that rat VPAC and PAC(1) receptors can also stimulate PLD (albeit less potently than adenylate cyclase) in transfected cells and also in cells where they are natively expressed. PLD responses of the VPAC receptors and the hop1 spice variant of the PAC(1) receptor but not its null form are sensitive to brefeldin A (BFA), an inhibitor of GTP exchange at ARF. The presence of the hop1 cassette in the rat PAC(1) receptor facilitates PLD activation in the absence of marked changes in ligand binding, receptor internalization, and adenylate cyclase activation, with some reduction in phospholipase C activation. Both VPAC(2) and PAC(1-hop1) (but not PAC(1-null)) receptors were shown to associate with immunoprecipitates directed against native or epitope-tagged ARF. A chimeric construct of the VPAC(2) receptor body with intracellular loop 3 (i3) of the PAC(1-null) receptor mediated BFA-insensitive activation of PLD, whereas the response of the corresponding PAC(1-hop1) construct was BFA-sensitive. Motifs in i3 of the PAC(1-hop1) receptor may act as critical determinants of coupling to ARF-dependent PLD activation by contributing to the GPCR:ARF interface.

Novel alternatively spliced exon in the extracellular ligand-binding domain of the pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1R) selectively increases ligand affinity and alters signal transduction coupling during spermatogenesis

The expression of the paracrine signaling hormone pituitary adenylate cyclase-activating polypeptide (PACAP) is regulated in a cyclical fashion during the 12-day spermatogenic cycle of the adult rat testis. The precise functions of PACAP in the development of germ cells are uncertain, but cycle- and stage-specific expression may augment cAMP-regulated gene expression in germ cells and associated Sertoli cells. Here we report the existence of a heretofore unrecognized exon in the extracellular domain of the PACAP type 1 receptor (PAC1R) that is alternatively spliced during the spermatogenic cycle in the rat testis. This splice variant encodes a full-length receptor with the insertion of an additional 72 base pairs encoding 24 amino acids (exon 3a) between coding exons 3 and 4. The PAC1R(3a) mRNA is preferentially detected in seminiferous tubules and is expressed at the highest levels in round spermatids and Sertoli cells. Analyses of ligand binding and signaling functions in stably transfected HEK293 cells expressing the two receptor isoforms reveals a 6-fold increase in the affinity of the PAC1R(3a) to bind PACAP-38, and alterations in its coupling to both cAMP and inositol phosphate signaling pathways relative to the wild type PAC1R. These findings suggest that the extracellular region between coding exons 3 and 6 of PAC1R may play an important role in the regulation of the relative ligand affinities and the relative coupling to G(s) (cAMP) and G(q) (inositol phosphates) signal transduction pathways during spermatogenesis.

Mechanisms mediating pituitary adenylate cyclase-activating polypeptide depolarization of rat sympathetic neurons

The direct effects of pituitary adenylate cyclase-activating polypeptides (PACAP) on sympathetic neurons were investigated using rat superior cervical ganglion neurons. Electrophysiological and pharmacological analyses were used to evaluate PACAP modulation of sympathetic neuron membrane potentials and to investigate potential ionic and intracellular signaling mechanisms mediating the responses. More than 90% of the sympathetic neurons were depolarized by the PACAP peptides even when stimulated release was blocked, indicating that the PACAP peptides elicited primary responses in the postganglionic neurons. The response profile was consistent for activation of PACAP-selective PAC(1) receptors: nanomolar concentrations of PACAP27 and PACAP38 were required to stimulate depolarization, whereas vasoactive intestinal peptide failed to evoke any response. Furthermore, depolarizations elicited by PACAP27 were reduced by the PAC(1) receptor antagonist PACAP(6-38). Both sodium influx and inhibition of a potassium current contributed to the peptide-induced depolarizations. Activation of neither pertussis toxin- nor cholera toxin-sensitive G-proteins was required for generation of the depolarizations. cAMP and diacylglycerol production and activation of protein kinase A or protein kinase C also were not requisite for the responses. By contrast, phospholipase C (PLC)-dependent inositol 1,4,5-triphosphate (IP(3)) synthesis was crucial to the PACAP-mediated depolarizations. Although calcium release from IP(3)-sensitive stores was not required for the PACAP-induced responses, inhibition of IP(3) receptors reduced the depolarizations. Thus, among the many signal transduction pathways coupled to the PAC(1) receptor, the PACAP-induced depolarization of sympathetic neurons appears to require activation of PLC and subsequent generation of IP(3).

Molecular cloning and characterization of alternatively spliced transcripts of the turkey pituitary adenylate cyclase-activating polypeptide

Pituitary adenylate cyclase-activating polypeptide (PACAP) increases the release of growth hormone (GH) and prolactin (PRL) in mammals. However, the evolutionary and functional relationships of PACAP, GH, and PRL are not clear. To understand how PACAP is regulated in the turkey, a turkey PACAP (tPACAP) cDNA has been cloned by the combination of reverse transcription-polymerase chain reaction and the rapid amplification of cDNA 5'- and 3'-ends. The deduced amino acid sequence of tPACAP-38 and turkey PACAP-related peptide (tPRP) displayed 87-97 and 52-63% similarity when compared to a variety of known PACAP-38 and PRP sequences, respectively. Two major transcripts (1.3 and 3.0 kb) of tPACAP were detected by Northern blot analysis. The highest levels of tPACAP mRNA were shown to be expressed in the hypothalamus, the cerebellum, and the cerebrum. In contrast, most of the other tissues tested expressed relatively low steady-state levels of tPACAP mRNA. Alternative splicing of tPACAP resulted in the expression of two different isoforms. The smaller form of tPACAP was expressed in the hypothalamus during early embryonic development and decreased significantly in later stages.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Pituitary adelylate cyclase-activating peptide is an activator of vasoactive intestinal polypeptide gene transcription in human neuroblastoma cells

In many ganglia, the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) innervates nerve cell bodies containing the homologous neuropeptide vasoactive intestinal polypeptide (VIP). We therefore investigated whether PACAP affected the VIP gene expression and elucidated the molecular mechanisms using the human neuroblastoma cell line NB-1. A concentration dependent induction of the VIP mRNA level was found upon PACAP stimulation. Five nM PACAP mediated transient elevation of the VIP mRNA being evident after 2 h, the maximal 65-fold induction was reached after 6-8 h and hereafter the level decreased rapidly. In cell extracts, the concentration of immunoreactive VIP was elevated four-fold upon PACAP stimulation for 8 h, and it remained elevated during the next 40 h. In conditioned medium, a stable 20-fold VIP increase was seen after 8-24 h. Experiments with the translational inhibitor cycloheximide showed a direct effect of PACAP on the VIP mRNA level, and nuclear run-on assays revealed a three- to four-fold enhancement of the VIP gene transcription rate after PACAP stimulation. The VIP mRNA induction was abolished by transcriptional inhibition with the actinomycin D, and PACAP did not seem to mediate any changes in the VIP mRNA half-life. However, the VIP mRNA level seemed very stable during the transcriptional cessation. Reporter gene constructs were used to evaluate involvement of the VIP CRE site in the PACAP mediated induction of the VIP gene transcription. Mutation of the CRE site did not abolish the induction suggesting it to be of minor if any importance for the induction. In conclusion, the PACAP mediated induction of the VIP gene expression suggests that PACAP released from nerve terminals could influence the function of VIP'ergic neurons in target tissues.

N-terminal splice variants of the type I PACAP receptor: isolation, characterization and ligand binding/selectivity determinants

Three full-length cDNAs encoding functional splice variants of the pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1) were isolated from Y-79 retinoblastoma cells and human cerebellum. Although the third intracellular loops of the three splice variants were identical, their N-terminal extracellular domains differed. The first full-length PAC1 variant, PAC1normal (PAC1n), encoded the entire N-terminus, whereas the second variant named PAC1short (PAC1s) was deleted by 21 amino acids (residues 89-109). Finally, the third variant, named PAC1very short (PAC1vs), was deleted by 57 amino acids (residues 53-109). Using semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis, it was established that all three variants were expressed in neuronal tissues. Binding- and cAMP studies using human embryonic kidney 293 (HEK293) cells stably transfected with PAC1n, PAC1s and PAC1vs showed significant differences in the affinities and selectivities towards PACAP38, PACAP27 and VIP. PAC1n bound PACAP38 and PACAP27 with affinities in the low nanomolar range whereas VIP was bound with up to 400-fold lower affinity. PAC1vs preferentially bound PACAP38 (Ki=121 nM) and PACAP27 (Ki=129 nM) over VIP (Ki>1000 nM) but with 100-fold lower affinity than PAC1n. Surprisingly, PAC1s unselectively bound all three ligands with high affinity. These data indicate that residues 53-88 within the N-terminal domain of the PAC1 are important for high affinity ligand binding, whereas residues 89-109 determine the receptor's ligand selectivity.

Long-term enhancement of REM sleep by the pituitary adenylyl cyclase-activating polypeptide (PACAP) in the pontine reticular formation of the rat

In rats, rapid eye movement (REM) sleep can be elicited by microinjection of vasoactive intestinal polypeptide (VIP) into the oral pontine reticular nucleus (PnO). In the present study, we investigated whether this area could also be a REM-promoting target for a peptide closely related to VIP: the pituitary adenylyl cyclase-activating polypeptide (PACAP). When administered into the posterior part of the PnO, but not in nearby areas, of freely moving chronically implanted rats, PACAP-27 and PACAP-38 (0.3 and 3 pmol) induced a marked enhancement (60-85% over baseline) of REM sleep for 8 h that could be prevented by prior infusion of the antagonist PACAP-(6-27) (3 pmol) into the same site. Moreover, injections of PACAP into the centre of the posterior PnO resulted in REM sleep enhancement which could last for up to 11 consecutive days. Quantitative autoradiography using [125I]PACAP-27 revealed the presence in the PnO of specific binding sites with high affinity for PACAP-27 and PACAP-38 (IC50 = 2.4 and 3.2 nM, respectively), but very low affinity for VIP (IC50 > 1 microM). These data suggest that PACAP within the PnO may play a key role in REM sleep regulation, and provide evidence for long-term (several days) mechanisms involved in such a control. PAC1 receptors which have a much higher affinity for PACAP than for VIP might mediate this long-term action of PACAP on REM sleep.

Biological relevance of pituitary adenylate cyclase-activating polypeptide (PACAP) in the gastrointestinal tract

Since its initial discovery in 1989, pituitary adenylate cyclase activating peptide (PACAP) has been noted to distribute widely in the brain, the respiratory and the gastrointestinal system. It occurs in two bioactive molecules, PACAP-27 and the C-terminally extended PACAP-38, which evoke activity by binding to three distinct types of high-affinity, G-protein coupled membrane receptors. It is present throughout the entirety of the gut but is rare in certain areas such as the intestinal mucosa and islets of Langerhans. PACAP-induced biological effects are protean and include alterations of motility in the bowel and the gallbladder, stimulation of gastric acid and intestinal secretion, hormone/enzyme release from the exocrine and endocrine pancreas, and the induction as well as inhibition of proliferation in neuroendocrine cells and tumors. Its hepatic activity has to date not been elucidated in detail. One of the interesting features of PACAP is the species and organ dependent variation of its biological effects. Of particular note is its superior potency when compared with other neuropeptides identified in the gut, and the involvement of a number of different second messenger systems upon PACAP receptor activation.

Domains determining agonist selectivity in chimaeric VIP2 (VPAC2)/PACAP (PAC1) receptors

1 The VPAC2 and PAC1 receptors are closely related members of the Group II G protein-coupled receptor family. At the VPAC2 receptor, VIP is equipotent to PACAP-38 in stimulating cyclic AMP production, whereas at the PAC1 receptor PACAP-38 is many fold more potent than VIP. In this study, domains which confer this selectivity were investigated by constructing four chimaeric receptors in which segments of the VPAC2 receptor were exchanged with the corresponding segment from the PAC1 receptor. 2 When expressed in COS 7 cells all the chimaeric receptors bound the common ligand [125I]PACAP-27 and produced cyclic AMP in response to agonists. 3 Relative selectivity for agonists was determined primarily by the amino terminal extracellular domain of the PAC1 receptor and the VPAC2 receptor. The interchange of other domains had little effect on the potency of PACAP-38 or PACAP-27. 4 For chimaeric constructs with a PAC1 receptor amino terminal domain, the substitution of increasing portions of the VPAC2 receptor decreased the potency of VIP yet increased that of helodermin. 5 This suggests that the interaction of VIP/helodermin but not PACAP with the PAC1 receptor may be influenced (and differentially so) by additional receptor domains.

Pituitary adenylate cyclase-activating polypeptides directly stimulate sympathetic neuron neuropeptide Y release through PAC(1) receptor isoform activation of specific intracellular signaling pathways

Pituitary adenylate cyclase-activating polypeptides (PACAP) have potent regulatory and neurotrophic activities on superior cervical ganglion (SCG) sympathetic neurons with pharmacological profiles consistent for the PACAP-selective PAC(1) receptor. Multiple PAC(1) receptor isoforms are suggested to determine differential peptide potency and receptor coupling to multiple intracellular signaling pathways. The current studies examined rat SCG PAC(1) receptor splice variant expression and coupling to intracellular signaling pathways mediating PACAP-stimulated peptide release. PAC(1) receptor mRNA was localized in over 90% of SCG neurons, which correlated with the cells expressing receptor protein. The neurons expressed the PAC(1)(short)HOP1 receptor but not VIP/PACAP-nonselective VPAC(1) receptors; low VPAC(2) receptor mRNA levels were restricted to ganglionic nonneuronal cells. PACAP27 and PACAP38 potently and efficaciously stimulated both cAMP and inositol phosphate production; inhibition of phospholipase C augmented PACAP-stimulated cAMP production, but inhibition of adenylyl cyclase did not alter stimulated inositol phosphate production. Phospholipase C inhibition blunted neuron peptide release, suggesting that the phosphatidylinositol pathway was a prominent component of the secretory response. These studies demonstrate preferential sympathetic neuron expression of PACAP-selective receptor variants contributing to regulation of autonomic function.

Molecular cloning and expression of a chicken pituitary adenylate cyclase-activating polypeptide receptor

Although, since the isolation of pituitary adenylate cyclase-activating polypeptide (PACAP), a wealth of literature has been published describing its localization, binding sites, and biological activities in a variety of mammalian tissues, only very little is known about PACAP in avian species. Therefore, in order to find out the sites of actions of PACAP and to elucidate its physiological significance in birds, we identified a chicken PACAP receptor homologue of the mammalian type I receptors (PAC(1)-Rs). The chicken PACAP type I cDNA sequence was obtained using reverse transcriptase-polymerase chain reaction (RT-PCR) in combination with 3'- and 5'-RACE PCR. This cDNA encodes a 471 amino acid precursor protein, sharing 81-83% sequence identity with mammalian analogs and 76% amino acid identity with the goldfish type I PACAP receptor. Northern blot analysis of chicken brain poly(A)(+)-rich RNA revealed the presence of a 5.5 kb and 7.5 kb PAC(1) receptor transcript. RT-PCR revealed that the chicken PACAP receptor is mainly expressed in the brain and gonads. A smaller amount of the receptor mRNA was found in pituitary, adrenal gland, kidney, intestine, pancreas, lung, and heart tissue. In situ hybridization with specific antisense oligodeoxynucleotide probes showed a widespread distribution of PAC(1) receptor mRNA in the chicken brain, with the highest expression being found in the dorsal telencephalon, olfactory bulb, hypothalamus, optic tectum, and cerebellar cortex. These findings suggest that PACAP affect a variety of functions both in the brain and peripheral tissues of the chicken.

Pituitary adenylate cyclase-activating polypeptide receptors during development: expression in the rat embryo at primitive streak stage

The distribution and localization of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor the PAC1 receptor (previously called the type 1 PACAP receptor or PVR1), which binds PACAP, but not vasoactive intestinal peptide, with high affinity] were first investigated in rats with in situ hybridization for its messenger RNA, and with immunohistochemical methods during prenatal and postnatal development. The expression of PACAP receptor messenger RNA was first detected in the rat embryo at the primitive streak stage as early as embryonic day 9, and it was intensely expressed in the neural plate. PACAP receptor messenger RNA was also intensely expressed in the neuroepithelia of the mesencephalon and rhombencephalon at embryonic day 11, and expressed in the basal telencephalon, hippocampal formation neuroepithelium, cortical neuroepithelium and cerebellar neuroepithelium after embryonic day 13. It was also expressed in the olfactory bulb neuroepithelium after embryonic day 16, and in mature regions of the older embryos. In postnatal developing brains, PACAP receptor messenger RNA was intensely expressed in the olfactory bulb, hippocampal formation, cerebellum and other scattered regions. The localization of PACAP receptor-like immunoreactivity coincided well with that of the gene transcripts. We also used reverse transcription-polymerase chain reaction methods to determine the expression of the splice variants of the PACAP receptor gene. At each ontogenetic stage of the rat from embryonic day 9 to postnatal day 60, two major products were detected with reverse transcription-polymerase chain reaction, a thick band (303 base pairs) corresponding to the short splice variant of the receptor that lacks both the "hip" and "hop" cassettes, and a thin band (387 base pairs) corresponding to the splice variant that contains one cassette of "hop" or "hip". There was no evidence for the other larger splice variants. Some of the amplified products were sequenced and found to have the exact sequences of "PACAP receptor" and "PACAP receptor-hopl", which are coupled to different signal transduction pathways. These results indicate that the PACAP receptor is actively expressed in different neuroepithelia from early developmental stages and expressed in various brain regions during prenatal and postnatal development, and that the major splice variants are "PACAP receptor" and "PACAP receptor-hopl". The initial mapping of ontogenetic localization of the PACAP receptor provides the basis for a better understanding of the functions of PACAP and its receptors during the development of the brain.

Structural motifs of pituitary adenylate cyclase-activating polypeptide (PACAP) defining PAC1-receptor selectivity

Pituitary adenylate cyclase-activating polypeptide (PACAP) interacts with three types of PACAP/VIP-receptors. The PAC1-receptor accepts PACAP as a high affinity ligand but not vasoactive intestinal peptide (VIP) similarly binding to VPAC1- and VPAC2-receptors. To identify those amino acids not present in VIP defining PAC1-receptor selectivity of PACAP, radio receptor binding assays on AR4-2J cells were performed. It could be shown that PACAP(1-27) exhibited a distinct and much higher susceptibility to VIP-amino acid substitutions, compared to PACAP(1-38). Positions 4 and 5 seem to be most important for receptor binding of PACAP(1-27), whereas position 13 was identified to be crucial for maximal affinity of PACAP(1-38). PACAP(29-38) extension analogues of VIP revealed a stabilizing effect of the C-terminus of PACAP(1-38) on the optimal peptide conformation. The substitution analogues were also checked for their capacity to stimulate IP3 and cAMP formation in AR4-2J cells. Compared to PACAP(1-27) and PACAP(1-38), most analogues revealed potencies reduced congruously to their lower binding affinities. However, one of the analogues, PACAP(1-27) substituted in position 5, may represent a weak antagonist since this peptide was less potent in inducing second messengers than in label displacement. Our findings indicate that PACAP(1-27) and PACAP(1-38) differ in terms of their requirement of the amino acids in positions 4, 5, 9, 11 and 13 for maximal interaction with the PAC1-receptor.

Role of charged amino acids conserved in the vasoactive intestinal polypeptide/secretin family of receptors on the secretin receptor functionality

The secretin receptor is a member of a large family of G-protein-coupled receptors that recognize polypeptide hormone and/or neuropeptides. Charged, conserved residues might play a key role in their function, either by interacting with the ligand or by stabilizing the receptor structure. Of the four charged amino acids that are conserved in the whole secretin receptor family, D49 and R83 (in the N-terminal domain) were probably important for the secretin receptor structure: replacement of D49 by H or R and of R83 by D severely reduced both the maximal response to secretin and its potency. No functional secretin receptor could be detected after replacement of R83 by L. Mutation of D49 to E, A, or N had no effect or reduced 5-fold the potency of secretin. The highly conserved positive charges found at the extracellular ends of TM III (K194) and IV (R255) were important for the secretin receptor function, as K194 mutation to A or Q and R255 mutation to Q or D decreased the secretin's affinity 15- to 1000-fold, respectively. Six extracellular charged residues are conserved in closely related receptors but not in the whole family. K121 (TM I) and R277 (TM V) were not important for functional secretin receptor expression. D174 (TM II) was necessary to stabilize the active receptor structure: the D174N mutant receptors were unable to stimulate normally the adenylate cyclase in response to secretin, and functional D174A receptors could not be found. Mutation of R255, E259 (second extracellular loop), and E351 (third extracellular loop) to uncharged residues reduced only 10- to 100-fold the secretin potency without changing its efficacy: these residues either stabilized the active receptor conformation or formed hydrogen rather than ionic bonds with secretin. Mutation of K121 (TM I) to Q or L and of R277 (TM V) to E or Q did not affect the receptor functional properties.

Mechanisms of pituitary adenylate cyclase activating polypeptide (PACAP)-induced depolarization of sympathetic superior cervical ganglion (SCG) neurons

Our understanding of PACAP expression and regulation of sympathetic neuronal function has been augmented considerably over the last few years. Among the three major VIP/PACAP receptor subtypes, the SCG appears to express preferentially one particular variant of the PACAP-selective PACAP1 receptor coupled to multiple intracellular signaling cascades. The in situ histochemical hybridization and immunocytochemical studies of PACAP1 receptor mRNA and protein are in good agreement; nearly all of the SCG neurons express the PACAP-selective receptor, suggesting that most of the sympathetic neurons are under PACAP neuromodulation. In accord with that possibility, several independent studies have now demonstrated PACAP peptide expression in the IML sympathetic preganglionic neurons and fibers, including those projecting to the SCG, further emphasizing the significance of PACAP peptides as a preganglionic noncholinergic mediator of sympathetic function. Given the high potency of PACAP on any of a number of cellular responses, the functional relevance of PACAP peptides on SCG neurons is considerable. We have previously demonstrated the potency and efficacy of both PACAP27 and PACAP38 on sympathetic neuron neurotransmitter/neuropeptide production and secretion; the ability of these peptides to stimulate neuronal second messenger activation was also in the nanomolar range. These results are congruous with our current electrophysiological studies, which were driven to further define the dynamic sympathetic responses to PACAP. In line with the morphological studies, for example, more than 90% of the sympathetic neurons responded to PACAP. In agreement with previous neuropharmacological data, the PACAP-induced depolarizations were elicited at physiologically relevant peptide concentrations at high affinity PACAP-selective receptors. The effects were direct and the alterations in postganglionic neuronal membrane properties appeared to be mediated by several ionic mechanisms. If these studies were analogous to pieces in a puzzle to understand the effects of PACAP in sympathetic development and function, the picture of late has been more completely assembled. But several important challenges still remain. What are the signal transduction mechanisms that mediate the PACAP-induced changes in sympathetic membrane properties? How do the resulting alterations impact the acute and more long-term responses of sympathetic neurons? Does the coupling of PACAP1 receptors to intracellular signaling pathways differ during development, resulting in a transition from the neurotrophic properties of PACAP in neuroblasts to neuromodulatory roles of the peptides in postmitotic neurons? By looking at these issues in one distinct neuronal system, we enlarge our understanding and appreciation of peptides, and PACAP in particular, in the molecular and cellular events guiding neuronal development, function, and plasticity.

A critical view of the methods for characterization of the VIP/PACAP receptor subclasses

The binding properties of the three cloned VIP/PACAP receptors and their coupling to G proteins and effectors can be studied in cells expressing each recombinant protein. The data obtained in these models must be critically evaluated: the expression of a high receptor density may reveal irrelevant receptors states and coupling to non-cognate G protein, and entail a marked amplification of the response as well as distortions in the selectivity profile of full and partial agonists. These models are, however, of great interest in the design of selective agonists and antagonists for each receptor subtype. The availability of selective ligands will facilitate the identification of the receptor subtype responsible for PACAP and VIP actions in cells and tissues.

Perspectives on pituitary adenylate cyclase activating polypeptide (PACAP) in the neuroendocrine, endocrine, and nervous systems

PACAP is a pleiotropic neuropeptide that belongs to the secretin/glucagon/VIP family. PACAP functions as a hypothalamic hormone, neurotransmitter, neuromodulator, vasodilator, and neurotrophic factor. Its structure has been remarkably conserved during evolution. The PACAP receptor is G protein-coupled with seven transmembrane domains and also belongs to the VIP receptor family. PACAP, but not VIP, binds to PAC1-R, whereas PACAP and VIP bind to VPAC1-R and VPAC2-R with a similar affinity. Despite the sizable homology of the structures of PACAP and VIP and their receptors, the distribution of these peptides and receptors is quite different. At least eight subtypes of PACAP specific, or PAC1-R, result from alternate splicing. Each subtype is coupled with specific signaling pathways, and its expression is tissue or cell specific. Although PACAP fulfills most requirements for a physiological hypothalamic hypophysiotropic hormone, it does not consistently stimulate secretion of the adenohypophysial hormones, except for stimulation of IL-6 release from the FS cells of the pituitary. The major regulatory role of PACAP in pituitary cells appears to be the regulation of gene expression of pituitary hormones and/or regulatory proteins that control growth and differentiation of the pituitary glandular cells. These effects appear to be exhibited directly and indirectly through a paracrine or autocrine action. Although PACAP stimulates the release of AVP, the physiological role of neurohypophysial PACAP remains unknown. One important action of PACAP in the endocrine system is its role as a potent secretagogue for adrenaline from the adrenal medulla through activation of TH. PACAP also stimulates the release of insulin and increases [Ca2+]i from pancreatic beta-cells at an extremely small concentration. The stage-specific expression of PACAP in testicular germ cells during spermatogenesis suggests its regulatory role in the maturation of germ cells. In the ovary, PACAP is transiently expressed in the granulosa cells of the preovulatory follicles and appears to be involved in the LH-induced cellular events in the ovary, including prevention of follicular apoptosis. In the central nervous system, PACAP acts as a neurotransmitter or neuromodulator, which has been supported by IHC and electrophysiological methods. More important, PACAP is a neurotrophic factor that may play an important role during the development of the brain. In the adult brain, PACAP appears to function as a neuroprotective factor that attenuates the neuronal damage resulting from various insults.

Expression, purification, and reconstitution of receptor for pituitary adenylate cyclase-activating polypeptide. large-scale purification of a functionally active G protein-coupled receptor produced in Sf9 insect cells

Human pituitary adenylate cyclase-activating polypeptide (PACAP) receptor was expressed in Sf9 insect cells and Chinese hamster ovary (CHO) cells. The recombinant receptor in Sf9 cell membranes had low affinity for 125I-PACAP27 (Kd = 155.3 pM) and was insensitive to guanosine 5'-O-3-thiotriphosphate (GTPgammaS), whereas the receptor in CHO membranes had a high affinity (Kd = 44.4 pM) and was GTPgammaS sensitive. The receptor in Sf9 membranes was converted to a high affinity state (Kd = 20-40 pM) following solubilization with digitonin. A large quantity (2 mg from 8 liters of insect cells) of the purified PACAP receptors (Bmax = 23.9 nmol/mg of protein) were obtained in a digitonin-induced high affinity state (Kd = 17.3 pM) using biotinylated ligand affinity chromatography. The apparent molecular weight of the purified receptor (Mr = 48,000) was smaller than that of the receptor from CHO cells (Mr = 58,000) due to differences in asparagine-linked sugar chains. The purified receptor reverted to a low affinity state (Kd = 182.6 pM) upon reconstitution into lipid vesicles, however, the receptor reconstituted with Gs protein had a high affinity (Kd = 40.2 pM) and was GTPgammaS sensitive. [35S]GTPgammaS binding to the reconstituted Gs protein was enhanced by PACAP27 and PACAP38 (EC50 = 42.5 and 9.4 pM, respectively) but not by antagonist PACAP(6-38), indicating that the purified receptor was functionally active.

Functional and molecular expression of PACAP/VIP receptors in the rat retina

Receptor binding sites for pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP), positively coupled to adenylate cyclase, have been previously described in the retina of different mammalian species. In the present study, we determined the mRNA expression of PACAP/VIP receptor variants in the rat retina and investigated their coupling to phospholipase C in addition to adenylate cyclase. The two forms of PACAP, PACAP27 and PACAP38, induced a dose-dependent (1-100 nM) increase of cAMP and [3H]inositol monophosphate levels, whereas VIP stimulated, with lower potency and efficacy, cAMP formation only. Reverse transcription-PCR analysis in the rat retina detected both type-I (PACAP-R and PACAP-HOP splice variants) and type-II (VIP-I and -2) receptor-mRNAs. These data indicate that PACAP and VIP may interact with multiple receptor subtypes and activate one (VIP) or two (PACAP) signal transduction mechanisms in the rat retina.

Role of VIP, PACAP, and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are members of a family of regulatory peptides that are widely distributed in the body and share numerous biologic actions. The two peptides display a remarkable amino acid-sequence homology, and bind to a class of G protein-coupled receptors, named PACAP/VIP receptors (PVRs), whose signaling mechanism mainly involves the activation of adenylate-cyclase and phospholipase-C cascades. A large body of evidence suggests that VIP and PACAP play a role in the control of the hypothalamo--pituitary-adrenal (HPA) axis, almost exclusively acting in a paracrine manner, since their blood concentration is very low. VIP and PACAP are contained in both nerve fibers and neurons of the hypothalamus, and VIP, but not PACAP, is also synthesized in the pituitary gland. Both peptides are expressed in the adrenal gland, and especially in medullary chromaffin cells. All the components of the HPA axis are provided with PVRs. VIP and PACAP enhance pituitary ACTH secretion, VIP by eliciting the hypothalamic release of CRH and potentiating its secretagogue action, and PACAP by directly stimulating pituitary corticotropes. Through this central mechanism, VIP and PACAP may increase mineralo- and glucocorticoid secretion of the adrenal cortex. VIP but not PACAP also exerts a weak direct secretagogue action on adrenocortical cells by activating both PVRs and probably a subtype of ACTH receptors. VIP and PACAP raise aldosterone production via a paracrine indirect mechanism involving the stimulation of medullary chromaffin cells to release catecholamines, which in turn enhance the secretion of zona glomerulosa cells via a beta-adrenoceptor-mediated mechanism. PACAP appears to be able to evoke a glucocorticoid response through the activation, at least in the rat, of the intramedullary CRH/ACTH system. The relevance of these effects of VIP and PACAP under basal conditions is questionable, although there are indications that endogenous VIP is involved in the maintenance of the normal growth and steroidogenic capacity of rat adrenal cortex. However, indirect evidence suggests that these peptides might play a relevant role under paraphysiological conditions (e.g., in the mediation of HPA axis responses to cold and inflammatory stresses) or may be somehow involved in the pathogenesis of Cushing disease or some case of hyperaldosteronism associated with secreting pheochromocytomas.

Specific antibody recognition of rat pituitary adenylate cyclase activating polypeptide receptors

Pituitary adenylate cyclase activating polypeptide (PACAP) is a new member of the secretin/VIP family of peptides. The specific receptor for PACAP has been cloned in rat, human, and bovine tissues. The distribution of the transcripts of PACAP receptor genes has been studied in various tissues using in situ hybridization. However, the unavailability of a specific antibody against the PACAP receptor has hampered further study of the expression of receptor proteins. In the present study, rabbit antisera were generated against a synthetic 25-residue peptide corresponding to the C-terminal intracellular domain of the rat PACAP receptor. To validate the specificity of the antisera, CHO cells and cells stably transfected with rat PACAP receptor cDNA were prepared. Using one of these antisera, the membrane and soluble fractions of the transformants were examined by Western blot analysis. Three bands were observed in subcellular fractions from the transfected CHO cells, but no bands were found in similar preparations from the nontransfected cells. A distinct 57-kDa band, which corresponds to the size of cloned rat PACAP receptor, was detected. In addition, a less intense band, larger than 57 kDa, and a very weakly stained band, smaller than 57 kDa, were demonstrated. All of these bands disappeared or were considerably diminished when the antiserum was preabsorbed with the synthetic immunogen peptide. This suggests that these bands are PACAP receptor-related proteins. The membranes from the transfected CHO cells bound to [125I]PACAP27. The size of the ligand/protein crosslinked product approximated 60 kDa, corresponding to the combined size of the PACAP receptor and PACAP27. No additional bands were observed, indicating that the immunopositive proteins larger or smaller than 57 kDa do not bind to the ligand and are not functional. Unlabeled PACAP27 and PACAP38, but not VIP, displaced the binding, suggesting that the receptors expressed in CHO cells are specific for PACAP. Solubilized membrane fractions prepared from rat brains were used for an immunoprecipitation study with [125I]PACAP27 and [125I]VIP. The PACAP receptor antiserum recognized [125I]PACAP-, but not [125I]VIP-bound proteins in the solubilized brain membrane fractions. Immunohistochemistry using this antiserum showed a distribution of PACAP receptor-like immunoreactivities similar to the distribution of the mRNA of PACAP receptor in the rat brain. Thus, the PACAP receptor antiserum is sufficiently specific to be used as a tool for studying the expression of PACAP receptors and related proteins.

Identification of endogenous sympathetic neuron pituitary adenylate cyclase-activating polypeptide (PACAP): depolarization regulates production and secretion through induction of multiple propeptide transcripts

The vasoactive intestinal peptide/pituitary adenylate cyclase-activating polypeptide (PACAP)/secretin/glucagon family of peptides displays numerous physiological roles in autonomic nervous system development and function. The regulated endogenous production and release of PACAP peptides in sympathetic neurons of the superior cervical ganglion (SCG) was investigated. The two posttranslationally processed forms of PACAP, PACAP27 and PACAP38, were identified in rat adult, neonatal, and cultured SCG neurons. PACAP38 levels were approximately 5-10 fmol/adult SCG and approximately 2 fmol/neonatal SCG; PACAP27 levels were comparable. The authenticity of peptide immunoreactivity in these tissues was verified by coelution with synthetic PACAP in reverse-phase HPLC analysis. Reverse transcription-PCR and sequence-specific hybridization revealed PACAP mRNA in adult, neonatal, and cultured SCG neurons; in situ hybridization histochemistry and immunocytochemistry localized the PACAP peptide and proPACAP mRNA to a subset of the SCG neuronal population. Basal and stimulated release of endogenous PACAP38 from cultured sympathetic neurons was established, suggesting that these peptides may function as signaling molecules at target tissues. Chronic depolarization with 40 mM potassium stimulated the PACAP secretory rate 10- to 20-fold, with concomitant increases in cellular PACAP peptide and mRNA levels. When examined using Northern analysis, depolarizing conditions not only stimulated the 2.2 kb form of PACAP mRNA, but also induced the expression of a shortened, 0.9 kb, transcript. Further reverse-transcription PCR analysis demonstrated that this smaller transcript was not identical to the unique testicular message. These studies identify PACAP38 and PACAP27 as regulated endogenous releasable peptides contributing to the functional diversity and phenotypic plasticity of the sympathetic nervous system.

Mutational analysis of extracellular cysteine residues of rat secretin receptor shows that disulfide bridges are essential for receptor function

We attempted to express point-mutant secretin receptors where each of the 10 extracellular Cys residues was replaced by a Ser residue, in Chinese hamster ovary (CHO) cells. Six of the point-mutant receptors (C24-->S, C44-->S, C53-->S, C67-->S, C85-->S and C101-->S) could not be detected by binding or functional studies: the mutations resulted in functional inactivation of the receptor. In contrast, the four other point-mutant receptors (C11-->S, C186-->S, C193-->S and C263-->S) were able to bind poorly 125I-secretin, and to activate adenylate cyclase with high secretin EC50 values. These results suggest that cysteine residues 24, 44, 53, 67, 85 and 101 are necessary for receptor function, and that the two putative disulfide bridges formed by cysteine residues 11, 186, 193 and 263 are functionally relevant, but not essential for receptor expression. Secretin activated the adenylate cyclase through the quadruple mutant (C11,186,193,263-->S), the four triple mutants, and through double mutants C186,193-->S and C186,263-->S with a very high (microM) EC50 value, suggesting that, in the wild-type receptor, disulfide bridges are formed between C11-C186, and between C193-C263. Prior treatment with dithiothreitol resulted in a marked EC50 increase of the wild-type receptor and of those receptors with at least the two cysteine residues in positions 11 and 186, suggesting that the C11-C186 (but not the C193-C263) disulfide bridge was accessible to this reducing agent. Several results nevertheless indicated that, in mutant receptors, alternative disulfide bridges can be formed between cysteine 186 and cysteine 193 or 263, suggesting that these three residues are in close spatial proximity in the wild-type receptor.

PACAP hybrid: a new PACAP receptor antagonist

The effects of pituitary adenylate cyclase activating polypeptide (PACAP) hybrid, a synthetic antagonist, was investigated on NIH/3T3 cells containing PACAP receptor (R) splice variants (SVs). PACAPhybrid inhibited 125I-PACAP-27 binding to NIH/3T3 cells stably expressing PACAP-R basic, SV-1, SV-2 or SV-3 with an IC50 of 1000 nM. PACAPhybrid antagonized the ability of PACAP-27 to elevate cAMP regardless of the PACAP-R SV used. PACAP was more efficacious at increasing cytosolic Ca2+ in NIH/3T3 cells containing PACAP-R SV-2 than PACAP-R basic, SV-1 or SV-3. PACAPhybrid antagonized the increase in cytosolic Ca2+ caused by PACAP-27 regardless of the PACAP-R SV used. PACAP was more potent at elevating c-fos mRNA using NIH/3T3 cells transfected with PACAP-R SV-2 than PACAP-R basic, SV-1 or SV-3. PACAPhybrid antagonized the increase in c-fos mRNA caused by PACAP-27. These data suggest that PACAPhybrid is a useful PACAP receptor antagonist for PACAP-R SVs.

Pituitary adenylate cyclase-activating polypeptides, PACAP-38 and PACAP-27, regulation of sympathetic neuron catecholamine, and neuropeptide Y expression through activation of type I PACAP/VIP receptor isoforms

The current studies have implicated a prominent role for PACAP peptides in modulating the physiological function of cells derived from the sympathoadrenal lineage. Compared to VIP, both PACAP-27 and PACAP-38 demonstrated potent, efficacious, and sustained stimulatory effects on sympathetic neuronal NPY and catecholamine production. The differential effects of PACAP peptides on SCG NPY and catecholamine content and secretion coincided with previous studies that activated directly the sympathetic intracellular cyclic AMP-protein kinase A signaling pathway. These effects appear to be mediated primarily by PACAP1 receptor splice variants coupled to both adenylyl cyclase and phospholipase C in SCG neurons. The actions of PACAP peptides in the SCG shared many parallels with adrenal medullary chromaffin cells, suggesting diverse roles for the PACAP peptidergic system in sympathoadrenal cell development and function. Rather than solutions, these results pose additional questions for the future. What are the endogenous sources of PACAP that regulate sympathetic and adrenal function? Do PACAP peptides, like VIP, have dual roles and also act as sympathetic postganglionic neuromodulators? Are VIP/PACAP receptors expressed during SCG development? What regulates sympathetic PACAP1 receptor isoform expression and how are they differentially coupled to neuronal intracellular signaling cascades? What defines the tissue-specific responses to PACAP-27 and PACAP-38? While many of these questions are not easily approached, future studies of these issues will certainly illuminate the function of PACAP and PACAP receptors in the nervous and endocrine systems.

Effect of introduction of an arginine16 in VIP, PACAP and secretin on ligand affinity for the receptors

Rabbit secretin, which differs from all other mammalian secretins in having a Leu residue in position 6 (instead of Phe) and a basic residue (Arg) in position 16, had a lower affinity than porcine secretion on recombinant rat secretin receptors but had a greater affinity than porcine secretin on recombinant rat VIP1 and PACAP I receptors. Synthetic [L6] porcine secretin had a reduced potency on secretin and VIP1 receptors whereas [R16] porcine secretin had a similar binding profile as rabbit secretin. Thus, an arginine residue in position 16 reduced 3-fold the affinity of secretin for secretin receptors but increased 30-fold its affinity for the VIP1 and PACAP I receptors. The introduction of an arginine residue in position 16, instead of glutamine, in VIP and PACAP had a similar effect: [R16] VIP and [R16] PACAP had 3- to 10-fold higher affinities than VIP and PACAP for VIP1 and PACAP I receptors, and 3-fold lower affinities for the secretin receptors. The three [R16] peptides also had a reduced potency on the chimeric receptor consisting of the N-terminal part of the secretin receptor grafted on the VIP1 receptor, and an enhanced potency on the chimeric receptor consisting of the N-terminal part of VIP1 receptor grafted on the secretin receptor, indicating that position 16 of each ligand interacted with the N-terminal extracellular domain of the receptors.

Alternative splicing in the N-terminal extracellular domain of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor modulates receptor selectivity and relative potencies of PACAP-27 and PACAP-38 in phospholipase C activation

Pituitary adenylate cyclase-activating polypeptide (PACAP)-27 and PACAP-38 are neuropeptides of the vasoactive intestinal peptide/secretin/glucagon family. We previously described alternative splicing of the region encoding the third intracellular loop of the PACAP receptor generating six isoforms with differential signal transduction properties (Spengler, D., Waeber, C., Pantaloni, C., Holsboer, F., Bockaert, J., Seeburg, P. H., and Journot, L. (1993) Nature 365, 170-175). In addition, we demonstrated that the potencies of the two forms of PACAP are similar for adenylate cyclase stimulation, whereas PACAP-38 is more potent than PACAP-27 in phospholipase C activation. In the present work, we document the existence of a new splice variant of the PACAP receptor that was characterized by a 21-amino-acid deletion in the N-terminal extracellular domain. We demonstrate that this domain modulates receptor selectivity with respect to PACAP-27 and -38 binding and controls the relative potencies of the two agonists in phospholipase C stimulation.

Interaction of amino acid residues at positions 8-15 of secretin with the N-terminal domain of the secretin receptor

The ability of secretin, PACAP-(1-27)-peptide, and ten hybrid peptides to recognize and activate the rat secretin and vasoactive intestinal polypeptide (PACAP type II VIP1) receptors was tested on recombinant Chinese hamster ovary (CHO) cell lines. PACAP had a 2500-fold lower affinity than secretin for the secretin receptor, and secretin had a 300-fold lower affinity than PACAP for the VIP1 receptor. Amino acids 8, 13, and 15 of the PACAP molecule contributed significantly to the low affinity of PACAP for the secretin receptor. The amino acids at positions 5, 9, 10, 15, 16, and unidentified amino acid(s) between positions 17-20 made limited contributions to the low affinity of secretin for the VIP1 receptor. To identify the receptor region that interacts with these amino acids, we constructed chimeric receptors, which consist either of the N-terminal extracellular part of the secretin receptor and the core of the VIP1 receptor (N-Sn/VIP1r) or the N-terminal extracellular part of the VIP1 receptor and the core of the secretin receptor (N-VIP1/Snr), and tested the ability of the hybrid ligands to activate the adenylate cyclase of CHO cells expressing these chimeric receptors. The N-Sn/VIP1 receptors had a higher affinity for secretin than for PACAP. The hybrid peptide 6 that consists of the PACAP-(1-8)-Sn-(9-15)-PACAP-(16-27)-peptide sequence had a 30-fold to 200-fold higher potency than either parent peptide for the chimeric receptor, which suggests that while the N- and/or C-terminal part of the peptide interact with the transmembrane domain of the receptor, the discriminator region 9-15 recognizes the extracellular N-terminal domain of the receptor. This was confirmed by the observation that, out of all the peptides tested, hybrid 6 had the weakest potency for activation of the N-VIP1/Sn chimeric receptors.

C-terminally shortened pituitary adenylate cyclase-activating peptides (PACAP) discriminate PACAP I, PACAP II-VIP1 and PACAP II-VIP2 recombinant receptors

Pituitary adenylate cyclase-activating polypeptide (PACAP) analogues were tested for their ability to occupy the recombinant selective PACAP receptors (PACAP type I receptors) and the non-selective PACAP-vasoactive intestinal polypeptide (VIP) receptors (PACAP type II, VIP1 and PACAP type II, VIP2 receptors), stably transfected and expressed in Chinese hamster ovary cells. Their capacity to stimulate the adenylate cyclase activity was also measured. The synthetic analogues tested were peptides shortened at the carboxyl terminus by the removal of 1-4 amino acids (PACAP-26 to PACAP-23). All the peptides discriminated the 3 receptor subtypes and had the highest affinity for the VIP1 receptors, and the lowest affinity for the VIP2 receptors; PACAP-25 having the highest ability to discriminate the VIP1 and VIP2 receptors. All the peptides tested were full agonists on the PACAP I and VIP1 receptors; PACAP-25 and -26 were partial agonists on VIP2 receptors and may be appropriate tools to establish the receptor subtype involved in a given cellular response.

Expression of PACAP and its type-I receptor isoforms in the rat ovary

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the secretin/glucagon/vasoactive intestinal peptide (VIP)/growth hormone releasing hormone (GHRH) family of neuropeptides, several of which stimulate steroidogenesis in ovarian granulosa cells. PACAP receptors are of two major subtypes; the type I receptor (PACAP-I-R) has much higher affinity for PACAP than VIP, and the type II receptor (PACAP-II-R) has similar affinity for both peptides. In the rat ovary, expression of the PACAP gene was demonstrated by amplification of ovarian RNA by the reverse transcription/polymerase chain reaction (RT-PCR). In addition, hybridization of Northern blots of rat ovarian poly(A)+ RNA with a 706-nt rat hypothalamic PACAP-I-R cDNA probe revealed the presence of a 7.0 kb PACAP receptor transcript, similar to that detected in brain and hypothalamus. RT-PCR using specific primers for the PACAP-I-R gene yielded products of the expected size with RNA obtained from ovarian tissue, brain, and hypothalamus. The authenticity of the PCR products was confirmed by Southern blotting and nested PCR, which revealed at least three splice variants of the PACAP-I-R in the rat ovary. These findings demonstrate that both PACAP and PACAP-I-R isoforms are expressed in the rat ovary, where they could exert autocrine or paracrine actions on granulosa cell function.

Tissue-specific and developmental expression of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors in rat brain

The two forms of pituitary adenylate cyclase-activating polypeptide, PACAP27,and PACAP38, are novel members of the vasoactive intestinal peptide (VIP)/secretin/glucagon family of peptides. PACAP receptors that are positively coupled to adenylate cyclase and phospholipase C have been recently identified. We examined the expression of PACAP receptors in the rat cortex, hippocampus, cerebellum and hypothalamus during postnatal development. Functional studies revealed PACAP stimulation of cAMP formation in all the brain areas examined and [3H]inositol monophosphate ([3H]InsP) accumulation only in the cerebellum and hypothalamus. Throughout development, the efficacy or PACAP in stimulating cAMP formation slightly increased in the cortex and hypothalamus and decreased in the hippocampus and cerebellum; PACAP stimulation of [3H]InsP formation decreased in the cerebellum and remained steady in the hypothalamus. The effects of PACAP27 and PACAP38 on cAMP levels and inositol phospholipid hydrolysis were dose-dependent between 1 and 100 nM. In the same brain areas, treatment with VIP increased cAMP formation at doses greater than 100nM and failed to affect [3H]InsP content, thus suggesting the existence of type-1 PACAP receptors. The reverse transcription polymerase chain reaction (RT-PCR) was used to analyse the mRNA expression of type-1 PACAP receptor splice variants. PACAP receptor gene expression in the central nervous system was regulated in a developmental- and tissue-specific manner. The PACAP-R transcript was detected in all the brain areas examined whereas PACAP-R-hop mRNA ocurred only in the cerebellum and hypothalamus. The different expression profiles and functional properties of PACAP receptors in the developing rat brain suggest an involvement of PACAP in histogenesis, maturation and neurotransmission.