Cloning and functional characterization of a third pituitary adenylate cyclase-activating polypeptide receptor subtype expressed in insulin-secreting cells

Down-regulation of cytokine expression in murine lymphocytes by PACAP and VIP

Neuropeptides, such as VIP and PACAP, released or produced in the microenvironment of the primary and secondary lymphoid organs, could affect a variety of immune responses through the regulation of cytokine expression. VIP has been previously shown to inhibit IL-2, IL-4, and IL-10 production in murine lymphocytes stimulated through the TCR-associated CD3 complex. This study shows that, similar to VIP, PACAP-38 inhibits IL-2 production in T lymphocytes. Comparisons with forskolin, a known cAMP inducer, suggest that the increase in intracellular cAMP represents at least one of the transduction pathways involved in IL-2 inhibition, especially in the higher range of neuropeptide concentration. Studies of the detailed molecular mechanisms involved in the regulation of IL-2 expression indicate that reduction of de novo transcription and destabilization of the message contribute to the reduction of steady-state IL-2 mRNA levels following VIP treatment. Examination of several IL-2 transcriptional factors indicates that only NFAT is down-regulated by VIP. Neuropeptides, such as VIP and PACAP, which specifically modulate the expression of various cytokines, could play an important role in the intricate cytokine network controlling local immune responses.

PACAP/VIP receptors in pancreatic beta-cells: their roles in insulin secretion

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide belonging to the vasoactive intestinal polypeptide (VIP)/glucagon/secretin family. We have isolated a third PACAP receptor subtype, designated PACAPR-3, by molecular cloning. The cDNA encoding PACAPR-3 has been isolated from a mouse insulin-secreting beta-cell line MIN6 cDNA library. Mouse PACAPR-3 is a protein of 437 amino acids that has 50% and 51% identity with rat PACAP type I and type II receptors, respectively. We have expressed PACAPR-3 in mammalian cells and Xenopus oocytes. PACAPR-3 binds to VIP as well as PACAP-38 and -27, with a slightly higher affinity for PACAP-38, and is positively coupled to adenylate cyclase. PACAP-38, -27, and VIP evoked Ca2+ activated-Cl- currents in Xenopus oocytes. RNA blotting studies reveal that PACAPR-3 mRNA is expressed widely in tissues and cell lines, including pancreatic islets, insulin-secreting cell lines (MIN6, HIT-T15, and RINm5F), lung, brain, stomach, colon, and heart. Furthermore, insulin secretion from the MIN6 cells is stimulated significantly by PACAP-38 and VIP. The possible mechanisms of insulin secretion by PACAP and VIP are also discussed.

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.

Enhancement of glucose-induced insulin secretion in transgenic mice overexpressing human VIP gene in pancreatic beta cells

Using transgenic mice technology, it has now become possible to test directly whether VIP and PHM-27 can enhance glucose-induced insulin secretion and reduce blood glucose in vivo. By microinjecting the entire human VIP gene ligated to the rat insulin II promoter, we have established a mouse model that overproduces VIP and PHM-27 in pancreatic beta cells. VIP was secreted from transgenic islets in a glucose-dependent manner. Analyses of these VIP-transgenic mice indicated that the transgene efficiently enhances glucose-induced insulin secretion and significantly reduces blood glucose as compared with control mice. The transgene also ameliorated glucose intolerance of 70% depancreatized mice. The present results suggest that somatic cell gene therapy directed to diabetic islets by human VIP/PHM-27 gene introduction may provide a means to improve the secretory function of the diabetic islets.

Mechanisms of action of VIP and PACAP in the stimulation of insulin release

VIP and PACAP stimulate insulin release by interaction with the VIP-2/PACAP-3 receptor on the beta cell. Activation of the receptor results in Gs-mediated stimulation of adenylyl cyclase and increased cellular cyclic AMP levels. Increased cyclic AMP results in a small and transient increase in [Ca2+]i, which is likely to have only a small and transient effect on the secretion rate. Cyclic AMP also potentiates insulin secretion by an as yet unknown action at a distal site. A third action of VIP and PACAP is responsible for the continued stimulation of insulin secretion after the levels of cyclic AMP and [Ca2+]i have returned to basal values. This third pathway, which is identified at present only by its sensitivity to low concentrations of wortmannin, plays a major role in the prolonged stimulation of insulin release by VIP and PACAP.

Current status of PACAP as a regulator of insulin secretion in pancreatic islets

PACAP-27 and PACAP-38 as low as 10(-13) M stimulate insulin release from rat islets in a glucose-dependent manner. PACAP also glucose dependently increases cAMP and [Ca2+]i in rat islet beta cells. The [Ca2+]i and insulin secretory responses to PACAP exhibit a similar concentration-response relationship, exhibiting a peak at 10(-13) M. When the [Ca2+]i response is abolished by nitrendipine, a blocker of L-type Ca2+ channels, the insulin response is also inhibited. Insulinotropic peptides glucagon, GLP-1, and VIP also increase [Ca2+]i in beta cells, but only in the nanomolar concentration range. PACAP is 4 logs more potent that VIP, a peptide that exhibits 68% amino acid homology and shares the type II PACAP receptor with PACAP. Immunoreactivity for the type I PACAP receptor is demonstrated in rat islets. Furthermore, PACAP immunoreactivity is demonstrated in nerve fibers and islets in rat pancreas. Based on these findings, we can draw the following conclusions: (1) PACAP is localized in pancreatic nerve fibers and islets; (2) PACAP in the subpicomolar range stimulates insulin release from islets; (3) the stimulation of insulin release is mediated by the cAMP-dependent increase in [Ca2+]i in beta cells; (4) all the PACAP effects are glucose-dependent; (5) PACAP is the most potent insulinotropic hormone known, and (6) the type I PACAP receptor appears to mediate the action of PACAP in the subpicomolar range. Finally, we hypothesize that PACAP is a pancreatic peptide of both neural and islet origin and functions as an intrinsic potentiator of glucose-induced insulin secretion in pancreatic islets (FIG 6).

PACAP/VIP receptor subtypes, signal transducers, and effectors in pituitary cells

Rat anterior pituitary tissue expresses mRNA for PVR1 and PVR3, as well as a low level of PVR2. The PVR1 appears to be highly expressed in gonadotroph-like cells, while somatotroph-like cells apparently express the PVR3. We have recently demonstrated the expression of mRNA for both PVR2 and PVR3 in corticotroph-like AtT20 cells (FIG.3). If normal corticotrophs express the same mRNA as AtT20 cells, this may partly explain the low levels of PVR2 seen in normal pituitary tissue. Significant levels of at least two PVR1 splice variants mRNAs (PVR1s and PVR1hop) were expressed in clonal gonadotroph-like alpha T3-1 cells and normal rat anterior pituitary tissue. However, these splice variants are reported to have almost identical pharmacological characteristics in terms of binding, and the activation of AC and PLC. Further experiments are necessary to determine the functional consequences of differential splice variant expression in such cells. Interestingly, all three pituitary-cell lines studied expressed mRNA for the PVR3 (FIG.3), whereas earlier binding studies demonstrate a predominance of PACAP-preferring binding sites on normal anterior pituitary-cell membranes. In addition, it is clear that the different PVR subtypes can couple to different intracellular messenger systems. Thus it will be important to determine the expression of the different PVR subtypes in normal anterior pituitary-cell types if we are to begin to understand the regulation of pituitary-cell regulation by PACAP. Such questions form the basis of some of the ongoing studies in our laboratory.

Molecular cloning of a novel variant of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor that stimulates calcium influx by activation of L-type calcium channels

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a novel neuropeptide that produces its biological effects by interacting with G protein-coupled receptors. Molecular cloning of the PACAP receptor revealed the existence of five splice variant receptor forms differing in the third intracellular loop region, with four variants activating both adenylyl cyclase and phosphoinositide phospholipase C and one variant activating only adenylyl cyclase (Spengler, D., Waeber, C., Pantaloni, C., Holsboer, F., Bockaert, J., Seeburg, P. H., and Journot, L. (1993) Nature 365, 170-175). Here, we report cloning of a novel PACAP receptor variant, designated PACAPR TM4 (transmembrane domain IV), that differs from the previously cloned short form of the PACAP receptor (PACAPR) primarily by discrete sequences located in transmembrane domains II and IV. Reverse transcriptase-polymerase chain reaction and primer extension analyses demonstrated tissue-specific differential expression of mRNAs encoding PACAPR TM4 and splice variant forms of the PACAP receptor. PACAPR TM4 and PACAPR possess identical intracellular domains, implicated as primary determinants of G protein recognition by rhodopsin-like receptors. However, unlike the PACAPR, PACAPR TM4 does not activate either adenylyl cyclase or phosphoinositide phospholipase C in response to PACAP in either transient or stable expression systems. However, PACAP stimulates increases in [Ca2+]i in cells expressing PACAPR TM4 by activating L-type Ca2+ channels, a response not elicited by stimulation with vasoactive intestinal polypeptide. The signaling phenotype of PACAPR TM4 is characteristic of the PACAP receptor involved in regulation of insulin secretion from pancreatic beta islets, a tissue expressing transcripts for PACAPR TM4 but not for PACAPR or its longer splice variant forms. These findings are consistent with a role of PACAPR TM4 in the physiological control of insulin release by PACAP in beta-islet cells. The finding that PACAPR TM4 has a unique signaling phenotype, although it possesses intracellular domains identical to those of the PACAPR, suggests that receptor-G protein recognition by rhodopsin-like receptors can be determined by sequences other than those located in intracellular receptor domains.

Pituitary adenylate cyclase-activating peptide-immunoreactive nerve fibers in the cat eye

PURPOSE

To study the occurrence and distribution of the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) in ocular and orbital structures of the cat.

METHODS

Immunocyto-chemistry to localize PACAP and double immunostaining to detect co-localization of PACAP with other neuropeptides.

RESULTS

Numerous PACAP-immunoreactive nerve fibers were observed in the lacrimal gland, choroid and retroocular arteries. There was a sparse supply of PACAP-containing nerve fibers in the iris, ciliary body and conjunctiva. Subpopulations of PACAP-containing nerve fibers stored vasoactive intestinal peptide (VIP) or calcitonin gene-related peptide (CGRP). Around 10% of the ganglion cells in the sphenopalatine ganglion harbored PACAP immunoreactivity. In the trigeminal ganglion around 5% of the neuronal cell bodies and in the ciliary ganglion only occasional ganglion cells contained PACAP immunoreactivity. PACAP immunoreactivity co-localized with VIP in the sphenopalatine ganglion and with CGRP in the trigeminal ganglion.

CONCLUSION

PACAP-containing nerves in the eye and associated structures demonstrate a distribution pattern resembling that of VIP. Subpopulations of nerve fibers containing PACAP immunoreactivity store VIP or CGRP immunoreactivity. Neuronal PACAP in the eye and orbit may take part in regulation of smooth muscle tone, glandular secretion and sensory processing.

Distribution of the mRNA for a pituitary adenylate cyclase-activating polypeptide receptor in the rat brain: an in situ hybridization study

The distribution of the mRNA for a pituitary adenylate cyclase-activating polypeptide (PACAP) receptor (PACAP-R) was examined in the rat brain, and also in the hypophysis and pineal gland, by in situ hybridization with a specific 35S-labeled riboprobe which was generated from a rat PACAP-R cDNA clone. In the brain, expression of PACAP-R mRNA was most prominent in the periglomerular and granule cells of the olfactory bulb, granule cells of the dentate gyrus, supraoptic nucleus, and area postrema. The expression was also intense in the piriform, cingulate, and retrosplenial cortices, pyramidal cells in CA2, non-pyramidal cells in CA1-CA3, neuronal cells in the hilus of the dentate gyrus, lateral septal nucleus, intercalated amygdaloid nucleus, anterodorsal thalamic nucleus, most of the midline and intralaminar thalamic nuclei, many regions of the hypothalamus, dorsal motor nucleus of the vagus nerve, hypoglossal nucleus, and lateral reticular nucleus. No significant expression was detected in the mitral and tufted cells in the olfactory bulb, pyramidal cells in CA1 and CA3, posterior nuclear group of the thalamus, dorsal lateral geniculate nucleus, and Purkinje, Golgi, and granule cells in the cerebellar cortex. Moderate-to-weak expression was further observed in many other regions of the brain. In the cerebellar cortex, presumed Bergmann glia cells showed moderate expression. In the hypophysis, the expression was moderate in the anterior lobe, and weak to moderate in the posterior lobe; no significant expression was observed in the intermediate lobe. In the pineal gland, the expression was very weak, if any. Thus, the expression of PACAP-R was detected not only on neuronal cells but also on some particular glial cells. The present study has shown, for the first time, the exact site of PACAP-R expression in the brain and hypophysis. Although the functional significance of PACAP and PACAP-R in the brain still remains to be clarified, the present results are considered to provide some direction for future functional studies.

Differential expression of vasoactive intestinal peptide receptors 1 and 2 (VIP-R1 and VIP-R2) mRNA in murine lymphocytes

Vasoactive intestinal peptide (VIP), a neuropeptide present in the lymphoid microenvironment, modulates cytokine expression and affects T cell proliferation. Recent molecular studies identified two VIP receptors. VIP-R1 and VIP-R2, primarily in nonlymphoid cells. In this study, we investigate the expression of VIP-R1 and VIP-R2 mRNA in unstimulated and stimulated lymphocytes and thymocytes, and in various lymphocyte subpopulations. In contrast to VIP-R1 which is constitutively expressed, the expression of VIP-R2 is induced only following stimulation through the TCR-associated CD3 complex. Both CD4+ and CD8+ T cells express VIP-R1 and VIP-R2. Two T cell lines, EL-4.IL-2 and D10.G4.1 express exclusively VIP-R2. VIP induces the expression of the VIP-R2 gene in the absence of additional stimuli. Differential expression and regulation of the two VIP receptors in T lymphocytes suggests different physiological roles in mediating the immunomodulatory activities of VIP and related neuropeptides.

Murine T-lymphocytes express vasoactive intestinal peptide receptor 1 (VIP-R1) mRNA

Vasoactive intestinal peptide (VIP), a neuropeptide present in primary and secondary lymphoid organs has been previously reported to inhibit IL-2 and IL-4 production as well as the proliferation of mitogen- or antigen-stimulated T-cells. Binding studies suggested that the immunoregulatory effects of VIP are mediated through specific VIP-binding sites present on lymphocyte subpopulations. Here we report on the expression of VIP-R1 mRNA in various murine lymphocyte subpopulations. By using RT-PCR. RNase protection assay, cDNA cloning, and sequence analysis, we show that stimulated and unstimulated murine spleen cells, thymocytes. CD4+ and CD8+ T-cells express VIP-R1. The VIP-R1 fragment amplified from murine brain, thymocytes, spleen cells and CD4+ T-cells share identical nucleotide sequences, and a high degree of homology with the corresponding nonlymphoid rat and human VIP-R1 sequences. The expression of VIP-R1 in thymocytes and peripheral lymphocytes, and especially in the CD4+ T-cell subset supports the idea that VIP produced or released locally in the lymphoid microenvironment could directly affect cytokine production and proliferation of T-lymphocytes.

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.

Pituitary adenylate cyclase-activating polypeptide induces expression of corticosteroid-binding globulin in cultured fetal hepatocytes: synergy with tri-iodothyronine

The purpose of the present study was to determine whether functional receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in cultured rat fetal hepatocytes and eventually play a role in regulating gene expression of corticosteroid-binding globulin (CBG). We found PACAP38 and PACAP27 to elevate cAMP levels in hepatocytes in a dose-dependent manner, with a plateau being achieved at 10 nM and EC50 values of about 0.5-1 nM. PACAP failed to alter the turnover of inositol phosphates, whereas PACAP and VIP stimulated cAMP accumulation in an equipotent manner, suggesting the presence in these cells of type II receptor isoforms. As revealed by measurements of both CBG mRNA levels and concentrations of binding sites, long-term treatment of fetal cells with 10 nM PACAP, although resulting in partial desensitization of peptide-induced cAMP accumulation, caused a significant 3-fold elevation in CBG synthesis. This stimulatory influence of PACAP was mimicked by the cell permeant N6,2'-O-dibutyryladenosine 3',5'-phosphate (dbcAMP). Treatment of hepatocytes with tri-iodothyronine (T3) enhanced CBG expression and, most interestingly, appeared to synergize with PACAP to elicit a 2-3-fold amplification of CBG synthesis. This study thus provides first evidence for the up-regulation by PACAP and cAMP of CBG expression in fetal hepatocytes and for T3's playing a synergistic role in enhancing PACAP-induced synthesis of the binder.

Comparative effects of PACAP and VIP on pancreatic endocrine secretions and vascular resistance in rat

1. The effects of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal peptide (VIP) and secretin on pancreatic endocrine secretions and vascular resistance were investigated and compared in the isolated perfused pancreas of the rat. The PACAP/VIP receptor types involved have been characterized. 2. On insulin secretion, in the range 10(-11) to 10(-8) M, PACAP and VIP elicited a concentration-dependent biphasic response from pancreas perfused with 8.3 mM glucose; the peptides were equipotent. In contrast, secretin was ineffective in the range 10(-11) to 10(-9) M; at 10(-8) and 10(-7) M, it induced only low and transient insulin responses. On the other hand, the peptides did not modify the basal insulin release in the presence of a non stimulating glucose concentration (2.8 mM). 3. On glucagon secretion, PACAP and VIP (10(-11) to 10(-8) M) but also secretin (10(-9) to 10(-7) M) caused a concentration-dependent peak shaped response from pancreas perfused with 2.8 mM glucose; PACAP and VIP were equipotent and 20 times more potent then secretin. On the other hand, the peptides did not affect the glucagon release in the presence of 8.3 mM glucose. 4. On pancreatic vessels, in the range 10(-11) to 10(-9) M, the three peptides were equipotent in inducing a concentration-dependent sustained increase in pancreatic flow rate. On the other hand, at the high concentration of 10(-7) M PACAP but not VIP provoked a transient decrease of flow rate. 5. This study provides evidence for PACAP/VIP type II receptors mediating insulin and glucagon secretion as well as vasodilatation in rat pancreas. In addition, the different efficacies of secretin suggest that these effects are mediated by different PACAP/VIP type II receptor subtypes.

A wortmannin-sensitive signal transduction pathway is involved in the stimulation of insulin release by vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide

Vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27), and PACAP-38 stimulated insulin release with EC50 values of 0.15, 0.15, and 0.06 nM respectively, as expected for the VIP2/PACAP3 receptor subtype. Secretion was stimulated promptly and peaked at 6-10 min. At 30 min, the secretion rate was still 2-3-fold higher than the control rate. The peptides increased cyclic AMP and [Ca2+]i transiently so that at 30 min they had returned to control values. Therefore, an additional signal is required to explain the prolonged stimulation of release. The prolonged effects, but not the acute effects of VIP and PACAP on insulin release were inhibited by low concentrations of wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. While wortmannin inhibited PI 3-kinase activity in cell lysates, no activation by the peptides was seen. Therefore, the wortmannin-sensitive pathway is either dependent on basal PI 3-kinase activity, or another target for wortmanin is responsible for inhibition of the peptide-stimulated secretion. It is concluded that the acute stimulation of insulin release by VIP and PACAP is mediated by increased cyclic AMP and [Ca2+]i, whereas the sustained release is mediated by a novel wortmannin-sensitive pathway.

VIP modulation of immune cell functions

Neuropeptides have recently been shown to modulate the immune response. Vasoactive intestinal peptide (VIP) released from nerve endings and from immune cells modulates the mobility and adherence of lymphocytes and macrophages, phagocytic cell functions (phagocytosis and free radical production), the lymphocyte proliferative response, lymphokine and immunoglobulin production and the natural killer cell activity, with opposite effects in vitro on these immune cell functions. The VIP receptor heterogeneity and the different action mechanisms of VIP-mediated immunoregulation could explain, at least in part, the different VIP effects observed on lymphoid and phagocytic cells. The evidence supports the theory that VIP acts not as an inhibitor, but as a modulator of immune functions, as previously thought, and that this neuropeptide may play a relevant role in vivo.

Characterization of the VIP receptor from SUP T1 lymphoblasts

The SUP T1 lymphoblasts express an original subtype of VIP receptors characterized by a high affinity for the VIP analogue from lizard venom named helodermin, a preference for the neuropeptide PACAP-38 over PACAP-27 and VIP, and an extremely low affinity for secretin. The molecular cloning of that receptor revealed its identity with the VIP2 receptor subtype first cloned in rat and mouse tissues. The access to selective probes permits the detection of the mRNA coding for the VIP2 receptor by Northern blot, reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. These highly selective and sensitive techniques identify the cell types that are equipped to synthesize the receptor but do not prove that the receptor is indeed efficiently expressed at the cell surface. VIP2 mRNA was detected in selected areas of the brain different from that expressing the classical VIP1 receptor, in pituitary, in pineal, in pancreatic islets, in testes and ovary. It was also detected in the stomach, in the thymus and in spleen and in T lymphoblastic cell lines. A systematic screening of the immunocompetent cells must still be performed.

Regulatory effects of vasoactive intestinal peptide on cytokine production in central and peripheral lymphoid organs

Vasoactive intestinal peptide (VIP) belongs to an ever growing family of neuropeptides with immunomodulatory functions. VIP-containing nerve fibers are present in both primary and secondary lymphoid organs, frequently in close proximity to immune cells. In addition, several types of immune cells, including T lymphocytes may function as local VIP sources in the lymphoid microenvironment. VIP released from neuronal and/or non neuronal sources exerts immunomodulatory effects through direct binding to VIP receptors (VIP-Rs), which are expressed on most immune cells. The existence of lymphocytic VIP-Rs has been demonstrated initially through binding studies, and more recently, through molecular biology technology. Both VIP-R1 and VIP-R2, which express high affinity for VIP and related neuropeptides such as the pituitary adenylate cyclase activating peptide (PACAP), are present on lymphocyte subsets, and recent reports suggest that whereas VIP-R1 is expressed constitutively, VIP-R2 expression is induced upon lymphocyte activation. Although VIP affects a variety of immune functions, its primary immunomodulatory function seems to be anti-inflammatory in nature. Whereas a rapid inflammatory response is essential for the ultimate elimination of foreign antigens, its intensity and duration have to be strictly controlled to avoid extensive tissue damage. In this respect, neuropeptides with anti-inflammatory functions such as VIP or the structurally related PACAP, timely released within the lymphoid organs, could play an important physiological role in the down-regulation of the immune response. Cytokines, soluble products of immune cells, play major roles in lymphocyte development, activation, and differentiation. As most cytokines are functionally pleiotropic, redundant, and interdependent, local interactions within the cytokine-neuroendocrine network have significant impact on cytokine production and function. Therefore, the immunomodulatory activities of VIP could be mediated, at least partially, through effects on the production of cytokines. The purpose of this article is to review the existing information regarding the VIP modulation of cytokine expression in immune cells. Both VIP and PACAP downregulate the expression of IL-2 mRNA and protein in T cells activated through the T cell receptor, through reducing both the stability and the de novo transcriptional rate of the IL-2 message. Reduction in the amount of IL-2 generated by the activated CD4+ T cells impacts on both T cell proliferation and on further sequential cytokine production. This is indeed the case with IL-4, which is affected by VIP indirectly, through inhibition of IL-2. In contrast, the inhibitory effect of VIP and PACAP on IL-10 production proceeds through a direct transcriptional event. In contrast to IL-2 which functions solely as a proinflammatory cytokine, IL-4 and IL-10 act as pro- or anti-inflammatory cytokines, depending on their involvement in specific immune responses. Therefore, depending on interactions with the local cytokine network, VIP and related neuropeptides may contribute significantly to controlling the amplitude and timing of the inflammatory response to foreign antigens. Although the role of VIP and related peptides on T cell development has not been investigated yet, the presence of VIP and VIP-Rs in the thymus, and their effect on thymic cytokine production, suggests that VIP and/or PACAP released locally within the thymic environment could also affect T cell development, and therefore participate in the generation and maturation of immune cells.

Reconstitution of IKATP: an inward rectifier subunit plus the sulfonylurea receptor

A member of the inwardly rectifying potassium channel family was cloned here. The channel, called BIR (Kir6.2), was expressed in large amounts in rat pancreatic islets and glucose-responsive insulin-secreting cell lines. Coexpression with the sulfonylurea receptor SUR reconstituted an inwardly rectifying potassium conductance of 76 picosiemens that was sensitive to adenosine triphosphate (ATP) (IKATP) and was inhibited by sulfonylureas and activated by diazoxide. The data indicate that these pancreatic beta cell potassium channels are a complex composed of at least two subunits--BIR, a member of the inward rectifier potassium channel family, and SUR, a member of the ATP-binding cassette superfamily. Gene mapping data show that these two potassium channel subunit genes are clustered on human chromosome 11 at position 11p15.1.

Pituitary adenylate cyclase-activating polypeptide: occurrence and relaxant effect in female genital tract

The distribution, localization, and smooth muscle effects of pituitary adenylate cyclase-activating polypeptide (PACAP) were studied in the human female genital tract. The concentrations of PACAP-38 and PACAP-27 were measured by radioimmunoassays, and both peptides were found throughout the genital tract. The highest concentrations of PACAP-38 were detected in the ovary, the upper part of vagina, and the perineum. The concentrations of PACAP-27 were generally low, in some regions below the detection limit and in other regions 1 to 5% of the PACAP-38 concentrations. Immunocytochemistry revealed that PACAP was located in delicate varicose nerve fibers that were most abundant in the internal cervical os, where they mainly seemed to innervate blood vessels and smooth muscle cells. PACAP-38 and PACAP-27 (10(-10)-10(-6) M) caused a concentration-dependent relaxation of the spontaneous activity of the nonvascular smooth muscle strips from fallopian tube and myometrium in vitro. Likewise, both peptides (10(-10)-10(-6) M) caused relaxation of nonrepinephrine (10(-6) M)-precontracted intramyometrial arteries. No effect of the PACAP sequences, PACAP-(6-27), PACAP-(16-38), and PACAP-(18-27), on fallopian tube was observed. The findings suggest a smooth muscle regulatory role of PACAP in the human female reproductive tract.

The nucleus raphe obscurus controls pancreatic hormone secretion in the rat

Until recently, the dorsal vagal complex (DVC) was considered as the only brain stem regulatory center for the vagal control of the endocrine pancreas. Because the nucleus raphe obscurus (NRO) maintains anatomic connections via the DVC to the pancreas, a functional significance of these findings was investigated in the present study. Kainic acid and vehicle were microinjected into the right DVC and the NRO of alpha-chloralose-anesthetized rats, and plasma concentrations of rat insulin, glucagon, and glucose were determined before and 5, 15, 30, and 60 min after injections. Chemical stimulation of neurons in the DVC by kainic acid at a dose of 200 pmol evoked increases in concentrations of insulin, with a peak at 15 min, and glucagon, with a peak at 30 min. Microinjection of kainic acid into the NRO at a dose of 200 pmol, but not at a dose of 20 pmol, produced increases in plasma concentrations of insulin, with a peak at 30 min, and glucagon, with a peak at 60 min. Plasma glucose levels on microinjection of kainic acid into the NRO at a dose of 20 pmol were decreased, whereas no changes on microinjection of kainic acid at a dose of 200 pmol were observed. The effects of kainic acid on insulin and glucagon secretion in the NRO were abolished by bilateral vagotomy. The study demonstrates for the first time that the NRO can contribute to vagal control of pancreatic endocrine function, although the exact circuitry and neurotransmitters involved in this response remain unknown.

Pituitary adenylate cyclase-activating polypeptide induces the voltage-independent activation of inward membrane currents and elevation of intracellular calcium in HIT-T15 insulinoma cells

The secretion of insulin by pancreatic beta-cells is controlled by synergistic interactions of glucose and hormones of the glucagon-related peptide family, of which pituitary adenylate cyclase-activating polypeptide (PACAP) is a member. Here we show by simultaneous recording of intracellular calcium ion ([Ca2+]i) and membrane potential that both PACAP-27 and PACAP-38 depolarize HIT-T15 cells and raise [Ca2+]i. PACAP stimulation can result in membrane depolarization by two distinct mechanisms: 1) PACAP reduces the membrane conductance and increases membrane excitability; and 2) PACAP activates a pronounced inward current that is predominantly a Na+ current, blockade by La3+, and which exhibits a reversal potential of about -28 mV. Activation of this current does not require membrane depolarization, because the response is observed when cells are held under voltage clamp at -70 mV. This current may result from the cAMP-dependent activation of nonspecific cation channels because the current is also observed in response to forskolin or membrane-permeant analogs of cAMP. We also suggest that PACAP raises [Ca2+]i and stimulates insulin secretion by three distinct mechanisms: 1) depolarization activates Ca2+ influx through L-type voltage-dependent calcium channels, 2) mobilization of intracellular Ca2+ stores, and 3) entry of Ca2+ via voltage-independent Ca2+ channels. These effects of PACAP may play an important role in a neuro-entero-endocrine loop regulating insulin secretion from pancreatic beta-cells during the transition period from fasting to feeding.

Structure, expression, and chromosomal localization of the type I human vasoactive intestinal peptide receptor gene

Vasoactive intestinal peptide (VIP) and other members of the pituitary adenylyl cyclase-activating peptide (PACAP) and secretin neuroendocrine peptide family are recognized with specificity by related G protein-coupled receptors. We report here the cloning, characterization, and chromosomal location of the gene encoding the human type I VIP receptor (HVR1), also termed the type II PACAP receptor. The gene spans approximately 22 kb and is composed of 13 exons ranging from 42 to 1400 bp and 12 introns ranging from 0.3 to 6.1 kb. Primer extension analysis with poly(A)+ RNA from human HT29 colonic adenocarcinoma cells indicated that the transcription initiation site is located at position -110 upstream of the first nucleotide (+1) of the translation start codon, and 75 nt downstream of a consensus CCAAT-box motif. The G+C-rich 5' flanking region contains potential binding sites for several nuclear factors, including Sp1, AP2, ATF, interferon regulatory factor 1, NF-IL6, acute-phase response factor, and NF-kappa B. The HVR1 gene is expressed selectively in human tissues with a relative prevalence of lung > prostate > peripheral blood leukocytes, liver, brain, small intestine > colon, heart, spleen > placenta, kidney, thymus, testis. Fluorescence in situ hybridization localized the HVR1 gene to the short arm of human chromosome 3 (3p22), in a region associated with small-cell lung cancer.

EMR1, an unusual member in the family of hormone receptors with seven transmembrane segments

Proteins with seven transmembrane segments (7TM) define a superfamily of receptors (7TM receptors) sharing the same topology: an extracellular N-terminus, three extramembranous loops on either side of the plasma membrane, and a cytoplasmic C-terminal tail. Upon ligand binding, cytoplasmic portions of the activated receptor interact with heterotrimeric G-coupled proteins to induce various second messengers. A small group, recently recognized on the basis of homologous primary amino acid sequences, comprises receptors to hormones of the secretin/vasoactive intestinal peptide/glucagon family, parathyroid hormone and parathyroid hormone-related peptides, growth hormone-releasing factor, corticotropin-releasing factor, and calcitonin. A cDNA, extracted from a neuroectodermal cDNA library, was predicted to encode a new 886-amino-acid protein with three distinct domains. The C-terminal third contains the seven hydrophobic segments and characteristic residues that allow the protein to be readily aligned with the various hormone receptors in the family. Six egf-like modules, at the N-terminus of the predicted mature protein, are separated from the transmembrane segments by a serine/threonine-rich domain, a feature reminiscent of mucin-like, single-span, integral membrane glycoproteins with adhesive properties. Because of its unique characteristics, this putative egf module-containing, mucin-like hormone receptor has been named EMR1. Southern analysis of a panel of somatic cell hybrids and fluorescence in situ hybridization have assigned the EMR1 gene to human chromosome 19p13.3.

Cloning and functional characterization of a novel ATP-sensitive potassium channel ubiquitously expressed in rat tissues, including pancreatic islets, pituitary, skeletal muscle, and heart

ATP-sensitive K+ (KATP) channels play a crucial role in coupling metabolic energy to the membrane potential of cells. We have isolated a cDNA encoding a novel member (uKATP-1) of the inward rectifier K+ channel family from a rat pancreatic islet cDNA library. Rat uKATP-1 is a 424-amino acid residue protein (M(r) = 47,960). Electrophysiological studies of uKATP-1 expressed in Xenopus laevis oocytes show that uKATP-1 is a weak rectifier and is blocked with Ba2+ ions. Single-channel patch clamp study of clonal human kidney epithelial cells (HEK293) transfected with uKATP-1 cDNA reveals that uKATP-1 closes in response to 1 mM ATP and has a single channel conductance of 70 +/- 2 picosiemens (n = 6), indicating that uKATP-1 is an ATP-sensitive inward rectifier K+ channel. In addition, uKATP-1 is activated by the KATP channel opener, diazoxide. RNA blot analysis shows that uKATP-1 mRNA is expressed ubiquitously in rat tissues, including pancreatic islets, pituitary, skeletal muscle, and heart, suggesting that uKATP-1 may play a physiological role as a link between the metabolic state and membrane K+ permeability of cells in almost every normal tissue. Since uKATP-1 shares only 43-46% amino acid identity with members of previously reported inward rectifier K+ channel subfamilies, including ROMK1, IRK1, GIRK1, and cKATP-1, uKATP-1 is not an isoform of these subfamilies and, therefore, represents a new subfamily of the inward rectifier K+ channel family having two transmembrane segments.

Somatostatin receptor subtype SSTR2 mediates the inhibition of high-voltage-activated calcium channels by somatostatin and its analogue SMS 201-995

Somatostatin and its analogue SMS 201-995 inhibit high voltage-activated (HVA) Ca2+ currents in the rat insulinoma cell line RINm5F which stably express cloned human somatostatin receptor subtype 2 (hSSTR2). In contrast, neither somatostatin nor SMS 201-995 suppresses the HVA Ca2+ currents in RINm5F which stably express cloned hSSTR1. These results suggest that somatostatin-induced inhibition of HVA Ca2+ currents is mediated by a specific receptor subtype and that inhibition of calcium influx through HVA Ca2+ channels is one of the mechanisms of SMS 201-995 action on inhibitory processes of hormone secretion and cell proliferation.