Localization of pituitary adenylate cyclase-activating polypeptide and its type I receptors in the rat ovary: immunohistochemistry and in situ hybridization

Discovery of PACAP and its receptors in the brain

Pituitary adenylate-cyclase-activating polypeptide (PACAP) is a 27- or 38-amino acid neuropeptide, which belongs to the vasoactive intestinal polypeptide (VIP)/glucagon/secretin family. PACAP shows particularly high homology (~ 68%) to VIP. Because of the high homology of the amino acid sequences of PACAP and VIP, these peptides share three class B-G-protein coupled receptors: the PAC1-Receptor (PAC1-R), the VPAC1-Receptor (VPAC1-R) and VPAC2-Receptor (VPAC2-R). These receptors have high homology to each other, and their high homology is utilized for these discoveries. This review provides mainly an overview of the history of the discovery of PACAP and its three receptors.

GATA augments GNRH-mediated increases in Adcyap1 gene expression in pituitary gonadotrope cells

Pituitary adenylate cyclase-activating polypeptide 1 (PACAP or ADCYAP1) regulates gonadotropin biosynthesis and secretion, both alone and in conjunction with GNRH. Initially identified as a hypothalamic-releasing factor, ADCYAP1 subsequently has been identified in pituitary gonadotropes, suggesting it may act as an autocrine-paracrine factor in this tissue. GNRH has been shown to increase pituitary Adcyap1 gene expression through the interaction of CREB and jun/fos with CRE/AP1 cis-elements in the proximal promoter. In these studies, we were interested in identifying additional transcription factors and cognate cis-elements which regulate Adcyap1 gene promoter activity and chose to focus on the GATA family of transcription factors known to be critical for both pituitary cell differentiation and gonadotropin subunit expression. By transient transfection and electrophoretic mobility shift assay analysis, we demonstrate that GATA2 and GATA4 stimulate Adcyap1 promoter activity via a GATA cis-element located at position -191 in the rat Adcyap1 gene promoter. Furthermore, we show that addition of GATA2 or GATA4 significantly augments GNRH-mediated stimulation of Adcyap1 gene promoter activity in the gonadotrope LβT2 cell line. Conversely, blunting GATA expression with specific siRNA inhibits the ability of GNRH to stimulate ADCYAP1 mRNA levels in these cells. These data demonstrate a complex interaction between GNRH and GATA on ADCYAP1 expression, providing important new insights into the regulation of gonadotrope function.

Cellular localization of PACAP and its receptors in the ovary of the spotted ray Torpedo marmorata Risso 1880 (Elasmobranchii: Torpediniformes)

The pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the glucagon-related family and occurs in two amidated forms, PACAP38 and PACAP27, with 38 and 27 amino acids, respectively. PACAP acts by binding to three different receptors, that are classified by their binding affinity for PACAP and VIP (vasoactive intestinal polypeptide): PAC(1)R (PACAP-specific receptor) exclusively binds PACAP, while VPAC(1)R (VIP/PACAP receptor, subtype 1) and VPAC(2)R (VIP/PACAP receptor, subtype 2) bind both PACAP and VIP. PACAP, first discovered in the brain, was then localized in several peripheral tissues of mammals, including the ovary. Besides mammals, PACAP and its receptors have been reported in fish too; however, less is known about the presence of PACAP in the fish ovary and the studies are limited to teleosts. The aim of our work was to study the distribution of the PACAP/PACAP-Rs system in the ovary of the cartilaginous fish Torpedo marmorata. Using in situ hybridization (ISH) and immunohistochemistry techniques, we demonstrated that PACAP and its receptors are widely represented in the Torpedo ovary in a stage-dependent manner. Moreover, our findings suggest an involvement of this peptide in the whole follicologenesis, probably influencing steroidogenesis, follicle development, and oocyte growth.

PACAP-mediated sperm-cumulus cell interaction promotes fertilization

The developing acrosome in spermatids contains pituitary adenylate cyclase-activating polypeptide (PACAP). However, the role of the acrosomal PACAP remains unclear because it has not been detected in mature spermatids and sperm. We reinvestigated whether the sperm acrosome contains PACAP. An antiserum produced against PACAP reacted to the anterior acrosome in epididymal sperm fixed under mild conditions, suggesting that PACAP acts on oocytes and/or cumulus cells at the site of fertilization. Immunolabeling and RT-PCR demonstrated the presence of PACAP type I receptor, a PACAP-specific receptor, in postovulatory cumulus cells. To investigate the role of PACAP in fertilization, we pretreated cumulus-oocyte complexes with the polypeptide. At a low concentration of sperm, the fertilization rate was significantly enhanced by PACAP in a dose-dependent manner. Sperm penetration through the oocyte investment, cumulus layer, and zona pellucida was also enhanced by PACAP. The enhancement was probably due to an enhancement in sperm motility and the zona-induced acrosome reaction, which were stimulated by a cumulus cell-releasing factor. Indeed, PACAP treatment increased the secretion of progesterone from the cumulus-oocyte complexes. These results strongly suggest that in response to PACAP, cumulus cells release a soluble factor that probably stimulates sperm motility and the acrosome reaction, thereby promoting fertilization.

Pituitary adenylate cyclase-activating polypeptide and its receptors in amphibians

Pituitary adenylate cyclase-activating polypeptide (PACAP), a novel peptide of the secretin/glucagon/vasoactive intestinal polypeptide superfamily, has been initially characterized in mammals in 1989 and, only 2 years later, its counterpart has been isolated in amphibians. A number of studies conducted in the frog Rana ridibunda have demonstrated that PACAP is widely distributed in the central nervous system (particularly in the hypothalamus and the median eminence) and in peripheral organs including the adrenal gland. The cDNAs encoding the PACAP precursor and 3 types of PACAP receptors have been cloned in amphibians and their distribution has been determined by in situ hybridization histochemistry. Ontogenetic studies have revealed that PACAP is expressed early in the brain of tadpoles, soon after hatching. In the frog Rana ridibunda, PACAP exerts a large array of biological effects in the brain, pituitary, adrenal gland, and ovary, suggesting that, in amphibians as in mammals, PACAP may act as neurotrophic factor, a neurotransmitter and a neurohormone.

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.

Pituitary adenylate cyclase-activating polypeptide precursor is processed solely by prohormone convertase 4 in the gonads

Pituitary adenylate cyclase-activating polypeptide (PACAP) is abundant not only in the brain, but also in the testis. Immunohistochemical studies have shown that PACAP-LI in rat testis is expressed stage specifically in spermatids. This suggests that testicular PACAP participates in the regulatory mechanism of spermatogenesis. Additionally, the ovary contains a relatively small amount of PACAP, conceivably involved in the regulation of folliculogenesis. PACAP is synthesized as a preprohormone and is processed by prohormone convertases, such as PC1, PC2, and PC4. PC4 is expressed only in the testis and ovary, where neither PC1 nor PC2 is expressed. However, whether PC4 is the sole endoprotease for the PACAP precursor in the gonads remains unknown. Recent studies using PC4-transgenic mice revealed that male PC4-null mice exhibited severely impaired fertility, although spermatogenesis appeared to be normal. The female PC4-null mice exhibited delayed folliculogenesis in the ovaries. To examine whether PC4 is the sole processing enzyme for the PACAP precursor in the gonads, we analyzed testicular and ovarian extracts from the PC4-null and wild-type mice for PACAP (PACAP38 and PACAP27) and its messenger RNA using reverse phase HPLC combined with specific RIAs and ribonuclease protection assay, respectively. For RIAs, three different polyclonal antisera with different recognition sites were used to identify PACAP38, PACAP27, and its precursor. Neither the testis nor the ovary from the PC4-null mice expressed PACAP38 or PACAP27, but the levels of PACAP transcripts in the testis and ovary of homozygous PC4-deficient mice were considerably elevated compared with those of the wild-type and heterozygous animals. The findings indicate that PC4 is the sole processing enzyme for the precursor of PACAP in the testis and ovary of mice. The possibility that the absence of bioactive PACAP in the testis and ovary of PC4-null mice caused severely impaired fertility in the males and delayed folliculogenesis in females warrants investigation.

Expression of pituitary adenylate cyclase activating polypeptide (PACAP) and PACAP type I A receptor mRNAs in granulosa cells of preovulatory follicles of the rat ovary

Pituitary adenylate cyclase activating polypeptide (PACAP) was isolated from ovine hypothalamus and known to stimulate the production of cAMP in anterior pituitary cells. In the recent report, the expression of PACAP was detected in preovulatory follicles, and treatment with PACAP stimulated the production of progesterone and prostaglandin E(2) through the action of AC and PLC pathways in the ovary. PACAP binds to three type receptors. Type I A receptor is coupled to adenylate cyclase (AC) and phospholipase C (PLC) pathways, while type I B and type II receptors are only coupled to AC. Thus, the present study aimed to evaluate the temporal expression of PACAP and its type I A receptor mRNAs in the rat ovary after treatment with pregnant mare's serum gonadotropin and human chorionic gonadotropin (hCG). Northern blot analysis showed that PACAP transcripts were transiently expressed from 3-9 hr after hCG treatment, reaching a maximum at 6 hr. During these time points, PACAP mRNAs were specifically and strongly expressed in granulosa cells and cumulus cells of large preovulatory follicles and interstitial glandular cells. Type I A receptor mRNAs were also transiently expressed in granulosa cells of large preovulatory follicles from 3-9 hr after hCG treatment. PACAP and its type I A receptor mRNAs were expressed in the same preovulatory follicles. These results demonstrate that PACAP acts as an autoregulator or pararegulator through type I A receptor in granulosa cells and cumulus cells of large preovulatory follicles. Thus, we suggest that PACAP may have a critical role in granulosa cells of preovulatory follicles for the preparation of ovulation.

Cellular distribution of the splice variants of the receptor for pituitary adenylate cyclase-activating polypeptide (PAC(1)-R) in the rat brain by in situ RT-PCR

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide and its specific receptor (the PAC(1) receptor) is widely distributed in the rat brain. It has been reported that alternative splicing of the region encoding the third intracellular loop of the PAC(1) receptor generates six isoforms which are differentially coupled to signal transduction pathways, but the precise distribution and localization of these splice isoforms in the brain remain to be determined. Using the initial specific primer pairs which correspond to the 'hip' or 'hop' types of receptors for the solution-phase reverse transcription-polymerase chain reaction (RT-PCR), we demonstrated that the major splice variants of the PAC(1) receptor in various regions of the rat brain are the short splice isoform 'PAC(1)-R-s' which does not contain either the 'hip' or 'hop' cassette and the another splice isoform, 'PAC(1)-R-hop', which contains the 'hop' cassette. With an innovative molecular histochemical technique, in situ RT-PCR, we determined that these two splice isoforms are both intensely expressed in the mitral cells of the olfactory bulb, the Purkinje cells of the cerebellum, the pyramidal cells of the hippocampus and neocortex, and many neurons in the nuclei of hypothalamus and thalamus as well as other regions. The initial mapping of the cell type-specific expression of these two splice variants of the PAC(1) receptor provides the basis for a better understanding of the functional significance of the PAC(1)-R and its ligand PACAP in various brain regions.

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.

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.