Genomic organization of the rat pituitary adenylate cyclase-activating polypeptide receptor gene. Alternative splicing within the 5'-untranslated region

Allostery and Biased Agonism at Class B G Protein-Coupled Receptors

Class B G protein-coupled receptors (GPCRs) respond to paracrine or endocrine peptide hormones involved in control of bone homeostasis, glucose regulation, satiety, and gastro-intestinal function, as well as pain transmission. These receptors are targets for existing drugs that treat osteoporosis, hypercalcaemia, Paget's disease, type II diabetes, and obesity and are being actively pursued as targets for numerous other diseases. Exploitation of class B receptors has been limited by difficulties with small molecule drug discovery and development and an under appreciation of factors governing optimal therapeutic efficacy. Recently, there has been increasing awareness of novel attributes of GPCR function that offer new opportunity for drug development. These include the presence of allosteric binding sites on the receptor that can be exploited as drug binding pockets and the ability of individual drugs to enrich subpopulations of receptor conformations to selectively control signaling, a phenomenon termed biased agonism. In this review, current knowledge of biased signaling and small molecule allostery within class B GPCRs is discussed, highlighting areas that have progressed significantly over the past decade, in addition to those that remain largely unexplored with respect to these phenomena.

Alternative Splicing of the Pituitary Adenylate Cyclase-Activating Polypeptide Receptor PAC1: Mechanisms of Fine Tuning of Brain Activity

Alternative splicing of the precursor mRNA encoding for the neuropeptide receptor PAC1/ADCYAP1R1 generates multiple protein products that exhibit pleiotropic activities. Recent studies in mammals and zebrafish have implicated some of these splice isoforms in control of both cellular and body homeostasis. Here, we review the regulation of PAC1 splice variants and their underlying signal transduction and physiological processes in the nervous system.

Growth hormone-releasing hormone and pituitary adenylate cyclase-activating polypeptide in the reproductive system

Growth hormone-releasing hormone (GHRH) and pituitary adenylate cyclase-activating polypeptide (PACAP) are both members of the glucagon superfamily that, with gonadotropins, act at central and peripheral levels as paracrine and autocrine coregulators of reproductive function. GHRH and PACAP are ancient peptides. Their original forms (both 27 amino acids long) were encoded by a single ancestral gene, several duplications of which led to the genes that encode the neuropeptides of the glucagon superfamily. In the male and female reproductive tracts, GHRH and PACAP interact with a subset of G protein-coupled receptors that are structurally similar to the PACAP receptor and variants of the vasoactive intestinal peptide receptor, and share several biological actions. These are related mainly to the modulation of cAMP-dependent and other signal transduction pathways in several cells of the pituitary-gonadal axis. The recent discovery that antagonists of GHRH and PACAP suppress the growth of human cancer cell lines that are derived from reproductive tissues indicates the potential importance of these peptides as local regulators of cell division, cell cycle arrest, differentiation and cell death.

Expression of VPAC2 receptor and PAC1 receptor splice variants in the trigeminal ganglion of the adult rat

PACAP and VIP are members of the VIP/secretin/glucagon family of peptides with neurotransmitter, neuroprotective, and neurotrophic functions. PACAP and VIP are known to be upregulated in primary sensory neurons following nerve injury, implying that these neuropeptides could be mediators of sensory transmission in neuropathic pain states. Nerve injury at the level of the trigeminal root is thought to be the prime cause of trigeminal neuralgia. Since cross-excitation (a chemically-mediated form of nonsynaptic transmission) within the TG is postulated to play a central role in trigeminal neuralgia, we studied the expression of PACAP and VIP receptors in the TG by RT PCR and immunocytochemistry. Of the three known receptors (PAC1, VPAC1 and VPAC2), RT PCR revealed the presence of mRNA for VPAC2 and several splice variants of the PAC1 receptor. Immunocytochemistry showed PAC1 and VPAC2 to be present in small-diameter TG neurons. Thus, PACAP and VIP are potential mediators of cross-excitation in the TG.

Novel splice variants of type I pituitary adenylate cyclase-activating polypeptide receptor in frog exhibit altered adenylate cyclase stimulation and differential relative abundance

Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts its various effects through activation of two types of G protein-coupled receptors, a receptor with high affinity for PACAP named PAC1-R and two receptors exhibiting similar affinity for both PACAP and vasoactive intestinal polypeptide named VPAC1-R and VPAC2-R. Here, we report the characterization of PAC1-R and novel splice variants in the frog Rana ridibunda. The frog PAC1-R has 78% homology with human PAC1-R and is highly expressed in the central nervous system. Two splice variants of the frog receptor that display additional amino acid cassettes in the third intracellular loop were characterized. PAC1-R25 carries a 25-amino acid insertion that matches the hop cassette of the mammalian receptor, whereas PAC1-R41 carries a cassette with no homology to any mammalian PAC1-R variant. A third splice variant of PAC1-R, exhibiting a completely different intracellular C-terminal domain, named PAC1-Rmc has also been identified. Determination of cAMP formation in cells transfected with the cloned receptors showed that PACAP activated PAC1-R, PAC1-R25, and PAC1-R41 with similar potency. In contrast, PACAP failed to stimulate adenylate cyclase in cells transfected with PAC1-Rmc. Fusion of PAC1-R or PAC1-Rmc with the green fluorescent protein revealed that both receptors are expressed and targeted to the plasma membrane in transfected cells. The different PAC1-R variants are highly expressed in the frog brain and spinal cord and to a lesser extent in peripheral tissues, where only certain isoforms could be detected. The present data indicate that in frog, PACAP may act through different PAC1-R splice variants that differ in their G(s) protein coupling and their abundance in various tissues.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is involved in melatonin release via the specific receptor PACAP-r1, but not in the circadian oscillator, in chick pineal cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates melatonin release from pineal cells and modulates glutamatergic regulation of the suprachiasmatic circadian clock in rodents. We investigated whether PACAP is involved in melatonin release and the circadian oscillation system in chick pineal cells, and if so, whether its effects are mediated by the PACAP-specific receptor (PACAP-r1) or the vasoactive intestinal polypeptide (VIP) receptor. Chick pineal cells were maintained for 4 days under a 12-h light/dark cycle, and thereafter in constant darkness. In the dose-range 10(-10) to 10(-6) M, PACAP increased melatonin release dose-dependently during the 12-h light period on day 3 of culture, and the degree of stimulation was greater than that produced by VIP. VIP receptor antagonists only slightly inhibited PACAP-stimulated melatonin release. Simultaneous addition of VIP and PACAP produced almost additive melatonin release. Under constant dark conditions, 6-h pulses of PACAP started at zeitgeber times (ZT) 15, 21, 3 and 9 h in separate groups of pineal cells did not cause any phase shift in their melatonin rhythm. In addition, PACAP did not affect the light-induced phase advance (ZT 15 h) and delay (ZT 9 h) in melatonin rhythms. The expression of mRNA for the PACAP-r1 (including its splicing variant with a hop cassette) was observed in chick pineal cells. These results suggest that PACAP participates in melatonin release, but not in the circadian oscillator system, via the specific receptor PACAP-r1 in chick pineal cells.

Bradykinin and angiotensin II-induced [Ca2+]i rise in cultured rat pituitary folliculo-stellate cells

Folliculo-stellate cells of the anterior pituitary are thought to modulate pituitary hormone secretion through a paracrine mechanism. Angiotensin II and pituitary adenylate cyclase-activating polypeptide (PACAP) have previously been shown to increase the intracellular Ca2+ concentration ([Ca2+]i) of these cells. In the present study, we examined the effects of various peptides such as bradykinin, angiotensin II, endothelin-1, PACAP, galanin and neurotensin by Ca2+-imaging of folliculo-stellate cells in primary culture. Bradykinin and angiotensin II increased [Ca2+]i in folliculo-stellate cells. Both responses were completely suppressed by thapsigargin and were significantly suppressed by the phospholipase C inhibitor, U-73122. Ryanodine did not significantly modify the responses. A B2 antagonist and angiotensin II receptor antagonist inhibited the response induced by bradykinin and angiotensin II, respectively. Endothelin-1 and PACAP increased [Ca2+]i in fewer than 50% of folliculo-stellate cells but galanin and neurotensin did not influence [Ca2+]i in any of the folliculo-stellate cells tested. These results indicate that bradykinin and angiotensin II increase [Ca2+]i in folliculo-stellate cells by activating phospholipase C through B2 receptor and AT1 receptor, respectively, and that endothelin-1 and PACAP also increase [Ca2+]i in some folliculo-stellate cells.

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.

Vasoactive intestinal peptide mediation of development and functions of T lymphocytes

The first phase in investigating neural regulation of immunity has delineated anatomical connections, shared mediators and receptors for mediators with distinctive effects, and the immune functional consequences of altering relevant neural activities. Vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase-activating peptide (PACAP) are represented prominently in immune organs. They have potent novel effects on many aspects of immunity, are derived from and serve as autacoids in some sets of immune cells, and they participate in both physiological and pathological immune responses. The present phase of neuroimmune research has begun to elucidate the genetic determinants of expression and functions of neuromediators in immunity. Our evolving understanding of the novel mechanisms for adaptation and specificity in the VIP/PACAP neuroimmune network suggests the importance of immunoselective transcriptional control of expression of VIP/PACAP receptors in T cells, a dominant role for numerous cytokines, and the critical involvement of small subsets of VIP-/PACAP-responsive thymocytes and T cells.

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.

Progesterone receptor activation mediates LH-induced type-I pituitary adenylate cyclase activating polypeptide receptor (PAC(1)) gene expression in rat granulosa cells

We have previously reported that the pituitary adenylate cyclase activating polypeptide (PACAP) gene is regulated in ovarian granulosa cells by the autocrine and/or paracrine interaction between progesterone and its nuclear receptor progesterone receptor (PR). To initiate studies on the functional significance of the progesterone-induced PACAP production in luteinizing granulosa cells, we sought to determine the expression and hormonal regulation of PACAP receptors in the rat ovary. The relative mRNA levels of three known PACAP receptor subtypes (PAC(1), VPAC(1), and VPAC(2)) were determined in ovaries of immature rats treated with gonadotropins, by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) assays. Results show that all PAC(1), VPAC(1), and VPAC(2) transcripts are expressed at a detectable level in immature rat ovaries. Importantly, the ovarian level of PAC(1), but not VPAC(1) or VPAC(2), mRNA notably changes during gonadotropin challenges. Ovarian PAC(1) mRNA expression decreases during the pregnant mare's serum gonadotropin (PMSG)-induced follicular phase but substantially increases during the human chorionic gonadotropin (hCG)-induced periovulatory period. Because the hCG-induced increase in ovarian PAC(1) mRNA expression is attributable to the hormone-induced PAC(1) mRNA expression in granulosa cells of the preovulatory follicles, we next examined whether hCG regulates PAC(1) mRNA expression by directly acting on granulosa cells. When granulosa cells isolated from PMSG (40 h)-primed immature rats were challenged with hCG (or forskolin), PAC(1), but not VPAC(1) or VPAC(2), mRNA expression significantly increased within 6 h. Because the LH-induced PAC(1) mRNA expression (6 h) proceeds PR activation (3 h) in granulosa cells as the LH-induced PACAP mRNA expression (6 h) does, we further determined the cause-effect relationship among LH, PR activation and PAC(1) receptor gene expression, by examining the effect of PR antagonist, ZK98299, on the ability of LH to increase PAC(1) mRNA levels in luteinizing granulosa cells. Results show that ZK98299 inhibited the stimulatory effect of hCG (or forskolin) on PAC(1) mRNA expression, at the level of all known splice variants of PAC(1) mRNA in granulosa cells. In summary, our results demonstrating that PR activation is critical for the LH-induced PAC(1) gene expression in luteinizing granulosa cells suggest that PR activation regulates the finely tuned expression of the PACAP/PACAP receptor genes in luteinizing granulosa cells and thus dictates the timing of the autocrine and/or paracrine function of PACAP in preovulatory follicles.

Novel alternative splicing and nuclear localization of human RGS12 gene products

RGS proteins are GTPase-activating proteins for certain Galpha subunits, accelerating the shutoff mechanism of G protein signaling, and also may interact with receptors and effectors to modulate G protein signaling. Here, we report identification of 12 distinct transcripts of human RGS12 that arise by unusually complex splicing of the RGS12 gene, which spans 70 kilobase pairs of genomic DNA and contains 16 exons. These transcripts arise by both cis- and trans-splicing mechanisms, are expressed in a tissue-specific manner, and encode proteins ranging in size from 356 to 1447 amino acids. Both 5'- and 3'-splicing of two primary RGS12 transcripts occur to generate RGS12 mRNAs encoding proteins with four distinct N-terminal domains, three distinct C-terminal domains, and a common internal region where the semiconserved RGS domain is located. Confocal microscopy and subcellular fractionation of COS-7 cells expressing RGS12 proteins with three different N termini (brain (B), peripheral (P), and trans-spliced (TS)) and a shared short (S) C-terminal domain demonstrated exclusive nuclear localization of these proteins and an influence of the N-terminal region on the pattern of intranuclear distribution. Both native RGS12TS-S in HEK-293T cells and ectopically expressed RGS12TS-S localized to discrete nuclear foci (dots), a characteristic of various tumor suppressor proteins. Subnuclear localization of RGS12TS-S into nuclear dots was cell cycle-dependent. Native RGS12TS-S associated with the metaphase chromosome during mitosis, and ectopically expressed RGS12TS-S induced formation of abnormally shaped and multiple nuclei in COS-7 cells. Expression of RGS12 proteins with long and intermediate C-terminal domains was not observed in COS-7 cells, suggesting that 3'-splicing of RGS12 transcripts may influence the expression or stability of the encoded proteins. These results document extraordinary structural complexity in the RGS12 family and the role of alternative splicing and cell cycle-dependent mechanisms in expression and subnuclear targeting of RGS12 proteins.

Structural and functional characterizations of the 5'-flanking region of the mouse glucagon receptor gene: comparison with the rat gene

A putative proximal promoter was defined previously for the mouse glucagon receptor (GR) gene. In the present study, a distal promoter was characterized upstream from a novel non-coding exon revealed by the 5'-rapid amplification of cDNA ends from mouse liver tissue. The 5'-flanking region of the mouse GR gene was cloned up to 6 kb and the structural organization was compared to the 5' untranslated region of the rat gene cloned up to 7 kb. The novel exon, separated by an intron of 3.8 kb from the first coding exon, displayed a high homology (80%) with the most distal of the two untranslated exons found in the 5' region of the rat GR gene. The mouse distal promoter region, extending up to -1 kb from the novel exon, displayed 85% identity with the rat promoter. Both contain a highly GC-rich sequence with five putative binding sites for Sp1, but no consensus TATA or CAAT elements. To evaluate basal promoter activities, 5'-flanking sequences of mouse or rat GR genes were fused to a luciferase reporter gene and transiently expressed in a mouse and in a rat cell line, respectively or in rat hepatocytes. Both mouse and rat distal promoter regions directed a high level of reporter gene activity. Deletion of the Sp1 binding sites region or mutation of the second proximal Sp1 sequence markedly reduced the distal promoter activity of the reporter gene. The mouse proximal promoter activity was 2- to 3-fold less than the distal promoter, for which no functional counterpart was observed in the similar region of the rat gene.

Breeding stock-specific variation in peptidylglycine alpha-amidating monooxygenase messenger ribonucleic acid splicing in rat pituitary

Peptidylglycine alpha-amidating monooxygenase (PAM) is a bifunctional enzyme that catalyzes the carboxyl-terminal amidation of glycine-extended peptides in a two-step reaction involving a monooxygenase and a lyase. Several forms of PAM messenger RNA result from alternative splicing of the single copy PAM gene. The presence of alternately spliced exon A between the two enzymatic domains allows endoproteolytic cleavage to occur in selected tissues, generating soluble monooxygenase and membrane lyase from integral membrane PAM. While using an exon A antiserum, we made the unexpected observation that Charles River Sprague Dawley rats expressed forms of PAM containing exon A in their pituitaries, whereas Harlan Sprague Dawley rats did not. Forms of PAM containing exon A were expressed in the atrium and hypothalamus of both types of Sprague Dawley rat, although in different proportions. PAM transmembrane domain splicing also differed between rat breeders, and full-length PAM-1 was not prevalent in the anterior pituitary of either type of rat. Despite striking differences in PAM splicing, no differences in levels of monooxygenase or lyase activity were observed in tissue or serum samples. The splicing patterns of other alternatively spliced genes, pituitary adenylate cyclase-activating polypeptide receptor type 1 and cardiac troponin T, did not vary with rat breeder. Strain-specific variations in the splicing of transcripts such as PAM must be taken into account in analyzing the resultant proteins, and knowledge of these differences should identify variations with functional significance.

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.

Glucose regulation of the expression of the glucagon receptor gene

The glucagon receptor gene is a member of a gene family, the expression of which is strongly upregulated by glucose. This review deals with the structure of both the glucagon receptor gene and its promoter. Attention is focused on the glucose regulatory element that we discovered in the promoter of this gene. Regulation by glucose of genes implicated in glucose homeostasis represents one mechanism contributing to the control of fuel utilization. Its deficiency or imbalance could potentially lead to or participate in pathological situations such as diabetes mellitus. On the other hand, the regulatory element of the glucagon receptor gene promoter could be used as a tool for the glucose-regulated expression of other genes. Indeed, an analysis of the glucagon receptor gene promoter demonstrated that only a short fragment of the genomic DNA, easy to subclone, contains all required elements for activation by glucose. Its potential use for gene therapy is also considered, therefore, in this report.

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.

Structure of the human VIPR2 gene for vasoactive intestinal peptide receptor type 2

The VPAC(2) (vasoactive intestinal peptide (VIP)(2)) receptor is a seven-transmembrane spanning G protein-coupled receptor which responds similarly to VIP and pituitary adenylate cyclase activating polypeptide (PACAP) in stimulating cAMP production. Recently, we reported the localisation of the human VPAC(2) receptor gene (VIPR2) to chromosome 7q36.3 (Mackay, M. et al. (1996) Genomics 37, 345-353). Here, we describe the characterisation of the VIPR2 gene structure and promoter region. The VIPR2 gene is encoded by 13 exons, the initiator codon of the 438 amino acid open reading frame is located in exon 1 and the termination signal and a poly-adenylation signal sequence are located in exon 13. The 5' untranslated region extends 187 bp upstream of the initiator codon and is extremely GC-rich (80%). The poly-adenylation signal is located 2416 bp downstream of the stop codon. Intron sizes range from 68 bp (intron 11) to 45 kb (intron 4) and the human gene spans 117 kb.

The rat growth hormone-releasing hormone receptor gene: structure, regulation, and generation of receptor isoforms with different signaling properties

The interaction of GHRH with membrane-bound receptors on somatotroph cells of the anterior pituitary is an important step in the regulation of GH synthesis and secretion. The identification of a G protein-coupled receptor for GHRH has made it possible to investigate the pathway by which GHRH regulates pituitary somatotroph cell function. To initiate an analysis of the mechanisms regulating expression and function of the GHRH receptor, the structure of the gene and its promoter region were analyzed. The coding sequence of the rat GHRH receptor gene is contained within 14 exons spanning approximately 15 kb of genomic DNA. Four transcription start sites are located within 286 bp upstream of the initiation codon. The 5' flanking region of the GHRH receptor gene acts as a functional promoter in rat pituitary tumor GH3 cells, and basal promoter activity is enhanced in GH3 and COS7 cells by cotransfection of an expression construct encoding the pituitary-specific transcription factor Pit-1. The rat GHRH receptor gene is subject to at least 1 alternative RNA processing event that generates 2 receptor isoforms differing by 41 amino acids within the third intracellular loop (IL) of the protein. The short isoform of the GHRH receptor is predominant in pituitary cells. The MtT/S pituitary tumor cell line was found to express the GHRH receptor, and different populations of these cells produce predominantly the long or short isoforms of the receptor messenger RNA, suggesting that the alternative splicing can be regulated. Functional analysis of the two GHRH receptor isoforms demonstrates that both bind GHRH, but only the short isoform signals through a cAMP-mediated pathway. Neither receptor isoform is able to stimulate calcium mobilization from internal stores after GHRH treatment. Our findings indicate that the pituitary-specific transcription factor Pit-1 is involved in the somatotroph-specific expression of the GHRH receptor gene and that functionally distinct receptor proteins are generated by an alternative RNA processing mechanism.

Identification of a glucose response element in the promoter of the rat glucagon receptor gene

We cloned the 5' upstream region of the rat glucagon receptor gene, demonstrating that the 5' noncoding domain of the glucagon receptor mRNA contained two untranslated exons of 131 and 166 nucleotides (nt), respectively, separated by two introns of 0.6 and 3.2 kilobase pairs. We also observed an alternative splicing involving the 166-base pair exon. Cloning of up to 2 kilobase pairs of the newly identified genomic domain and transfection of various constructs driving a reporter gene, in pancreatic islet cell line INS-1, uncovered a strong glucose regulation of the promoter activity of plasmids containing up to nucleotide -868, or more, upstream from the transcriptional start point. This promoter activity displayed threshold-like behavior, with low activity of the promoter below 5 mM glucose, and maximal activation as of 10 mM glucose. This glucose regulation was mapped to a highly palindromic 19-nucleotide region between nt -545 and -527. Indeed, deletion or mutation of this sequence abolished the glucose regulation. This domain contained two palindromic "E-boxes" CACGTG and CAGCTG separated by 3 nt, a feature similar to the "L4 box" found in the pyruvate kinase L gene promoter. This is the first description of a G protein-coupled receptor gene promoter regulated by glucose.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

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.

Selective expression of vasoactive intestinal peptide (VIP)2/pituitary adenylate cyclase-activating polypeptide (PACAP)3 receptors in rabbit and guinea pig gastric and tenia coli smooth muscle cells

In both functional and radioligand binding studies of gastric smooth muscle from rabbit and guinea pig, vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) show equal potency indicating that the receptor type is either a VIP1/PACAP2 or a VIP2/PACAP3 receptor. We have characterized the VIP/PACAP receptor expressed in freshly dispersed and cultured gastric and tenia coli smooth muscle cells of rabbit and guinea pig by reverse transcriptase-polymerase chain reaction (RT-PCR), Northern analysis, and cloning of the first extracellular domain. Specific primers based on cDNA sequences for rat VIP1/PACAP2, VIP2/PACAP3 and PACAP1 receptors were designed spanning the first extracellular domain. A 275 base pair product corresponding to VIP2/PACAP3 receptor was amplified by RT-PCR in muscle cells from both species. No RT-PCR product was obtained with primers for VIP1/PACAP2 and PACAP1 receptors. The deduced amino acid sequences showed 90% similarity in rabbit and 77% in guinea pig to the sequence in rat. The location of the aspartate, tryptophan and glycine residues and all six N-terminal cysteines required for VIP binding were conserved. The sequence in guinea pig tenia coli differed from that in guinea pig stomach by two amino acid residues, Phe40 and Phe41. Northern analysis revealed a single 3.9 kilobase (kb) mRNA corresponding to VIP2/PACAP3 receptors in rabbit and a 2.1 kb mRNA in guinea pig gastric and tenia coli muscle cells. We conclude that only VIP2/PACAP3 receptors are expressed in smooth muscle cells of rabbit and guinea pig. The two amino acid difference in the sequence obtained from guinea pig tenia coli may reflect the distinct binding and functional properties of this tissue.

Pharmacological, molecular and functional characterization of vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide receptors in the rat pineal gland

Melatonin secretion from the mammalian pineal gland is strongly stimulated by noradrenaline and also by vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP). Three types of receptors for VIP and PACAP have been characterized so far: VIP1/PACAP receptors and VIP2/PACAP receptors, which possess similar high affinities for VIP and PACAP, and PACAP1 receptors which exhibit a 100-1000-fold higher affinity for PACAP. The aim of the present study was to characterize the receptor subtype(s) mediating the stimulatory effects of VIP and PACAP on melatonin synthesis in the rat pineal gland. Autoradiographic studies showed that PACAP and VIP were equally potent in displacing binding of radioiodinated PACAP27 from pineal sections. Amplification of pineal complementary DNAs by polymerase chain reaction using specific primers for the different receptor subtypes revealed that all three receptor messenger RNAs are expressed and that VIP1/PACAP receptor messenger RNA was predominant over VIP2/PACAP receptor messenger RNA. In vitro, VIP and PACAP stimulated melatonin synthesis with similar high potency and the effect of the two peptides were not additive. The selective VIP1/PACAP receptor agonists [R16]chicken secretin (1-25) and [K15, R16, L27]VIP(1-7)/growth hormone releasing factor(8-27) were significantly more potent than the selective VIP2/PACAP receptor agonist RO 25-1553 in stimulating melatonin secretion. The stimulatory effects of VIP and PACAP were similarly inhibited by the VIP1/PACAP antagonist [acetyl-His1, D-Phe2, K15, R16, L27]VIP(3-7)/growth hormone releasing factor(8-27). These data strongly suggest that VIP and PACAP exert a stimulatory effect on melatonin synthesis mainly through activation of a pineal VIP1/PACAP receptor subtype.

Transcriptional regulation of the glycoprotein hormone alpha-subunit gene by pituitary adenylate cyclase-activating polypeptide (PACAP) in alphaT3-1 cells

We showed previously that pituitary adenylate cyclase-activating polypeptide (PACAP) increases glycoprotein hormone alpha-subunit gene expression and secretion in alphaT3-1 cells. We have now used 5'-flanking deletion and clustered point mutations of the mouse alpha-subunit promoter fused to the luciferase (LUC) reporter gene in transient transfection assays to further characterize the cell signaling pathways and sequences involved in responsiveness to PACAP. PACAP stimulated LUC activity at a lower concentration than VIP, supporting the notion that PACAP acts through type 1 receptors. The effect of PACAP on LUC activity was observed by 2 h, peaked at 4-12 h, and persisted until at least 20 h. alphaT3-1 cells were transfected with mouse alpha-LUC constructs truncated at -507, -424, -288, -205, -146, and -133, and treated with PACAP, a cell-permeable cAMP analog (8Br-cAMP), phorbol myristate acetate (PMA), or control medium. Transcriptional activation by PACAP was highest with the -288 and -205 mouse alpha-LUC vectors (7-8-fold stimulation) and decreased significantly with truncation of the 5'-flanking region to -146 or -133. The pattern of alpha-subunit stimulation by cAMP closely paralleled that of PACAP. With PMA, stepwise decrements in LUC activity were observed between -507 and -424 and, especially, -424 and -288, and there was no further loss of activity with deletion to -205, -146, or -133. Clustered point mutations in the pituitary glycoprotein hormone basal element (-337 to -330) or the gonadotropin-releasing hormone response element (GnRH-RE)(-406 to -399) of the -507 to +46 mouse alpha-promoter significantly (P < 0.05) increased and decreased, respectively, PACAP's effect on transcriptional activity. These results indicate that there are several regions of the mouse alpha-subunit promoter that mediate responsiveness to PACAP. The co-localization of PACAP and cAMP responsiveness as well as the results of studies involving specific inhibitors of protein kinase A (H-89) or protein kinase C (PKC) (bisindolylmaleimide) suggests that the action of PACAP on alpha-subunit transcription is mediated primarily by the protein kinase A (PKA) pathway.

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.