Expression of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors and PACAP in human fetal retina

Investigation of PACAP38 and PAC1 Receptor Expression in Human Retinoblastoma and the Effect of PACAP38 Administration on Human Y-79 Retinoblastoma Cells

Retinoblastoma represents the most prevalent malignant neoplasm affecting the eyes in childhood. The clear-cut origin of retinoblastoma has not yet been determined; however, based on experiments, it has been suggested that RB1 loss in cone photoreceptors causes retinoblastoma. Pituitary adenylate-cyclase activating polypeptide (PACAP) is a pleiotropic neuropeptide which has been shown to be affected in certain tumorous transformations, such as breast, lung, kidney, pancreatic, colon, and endocrine cancers. This study aimed to investigate potential changes in both PACAP38 and PAC1 receptor (PAC1R) expression in human retinoblastoma and the effect of PACAP38 administration on the survival of a human retinoblastoma cell line (Y-79). We analyzed human enucleation specimens removed because of retinoblastoma for PACAP38 and PAC1R immunostaining and the effect of PACAP38 on the survival of the Y-79 cell line. We described for the first time that human retinoblastoma cells from patients showed only perinuclear, dot-like immunopositivity for both PACAP38 and PAC1R, irrespective of laterality, genetic background, or histopathological features. Nanomolar (100 nM and 500 nM) PACAP38 concentrations had no effect on the viability of Y-79 cells, while micromolar (2 µM and 6 µM) PACAP38 significantly decreased tumor cell viability. These findings, along with general observations from animal studies showing that PACAP38 has strong anti-apoptotic, anti-inflammatory, and antioxidant effects on ocular tissues, together suggest that PACAP38 and its analogs are promising candidates in retinoblastoma therapy.

Distribution of PACAP and PAC1 Receptor in the Human Eye

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with widespread distribution and diverse biological functions. Several studies show that PACAP has strong cytoprotective effects mediated mostly through its specific PAC1 receptor (PAC1-R) and it plays important roles in several pathological conditions. Its distribution and altered expression are known in various human tissues, but there is no descriptive data about PACAP and its receptors in the human eyebulb. Since PACAP38 is the dominant form of the naturally occurring PACAP, our aim was to investigate the distribution of PACAP38-like immunoreactivity in the human eye and to describe the presence of PAC1-R. Semiquantitative evaluation was performed after routine histology and immunohistochemical labeling on human eye sections. Our results showed high level of immunopositivity in the corneal epithelium and endothelium. Within the vascular layer, the iris and the ciliary body had strong immunopositivity for both PACAP and PAC1-R. Several layers of the retina showed immunoreactivity for PACAP and PAC1-R, but the ganglion cell layer had a special pattern in the immunolabeling. Labeling was observed in the neuropil within the optic nerve in both cases and glial cells displayed immunoreactivity for PAC1-R. In summary, our study indicates the widespread occurrence of PACAP and its specific receptor in the human eye, implying that the results from in vitro and animal studies have translational value and most probably are also present in the human eye.

PACAP for Retinal Health: Model for Cellular Aging and Rescue

Retinal aging is the result of accumulating molecular and cellular damage with a manifest decline in visual functions. Somatostatin (SST) and pituitary adenylate cyclase-activating polypeptide (PACAP) have been implicated in neuroprotection through regulating disparate aspects of neuronal activity (survival, proliferation and renewal). The aim of the present study was to validate a transgenic model for SST-expressing amacrine cells and to investigate the chronic effect of PACAP on the aging of SSTergic and dopaminergic cells of the retina. SST-tdTomato transgenic mice that were 6, 12 and 18 months old were treated intravitreally with 100 pmol of PACAP every 3 months. The density of SST and dopaminergic amacrine cells was assessed in whole-mounted retinas. Cells displaying the transgenic red fluorescence were identified as SST-immunopositive amacrine cells. By comparing the three age groups. PACAP treatment was shown to induce a moderate elevation of cell densities in both the SST and dopaminergic cell populations in the 12- and 18-month-old animals. By contrast, the control untreated and saline-treated retinas showed a minor cell loss. In conclusion, we report a reliable transgenic model for examining SSTergic amacrine cells. The fundamental novelty of this study is that PACAP could increase the cell density in matured retinal tissue, anticipating new therapeutic potential in age-related pathological processes.

The Neuroprotective Peptide PACAP1-38 Contributes to Horizontal Cell Development in Postnatal Rat Retina

Purpose

PACAP1-38, a member of the secretin/glucagon superfamily, is expressed in the developing retina with documented neuroprotective effects. However, its function in retinal cell differentiation has yet to be elucidated. Our goals, therefore, were to identify PAC1 expressing cells morphologically, investigate the PACAP1-38 action functionally, and establish PACAP1-38 regulated events developmentally during the first postnatal week in rat retina.

Methods

P1 retinal sections or whole mounts of Wistar rats were used to reveal PAC1 and calbindin immunoreactive structures. P1, P3, or P7 pups were injected intravitreally with 100 pmol PACAP1-38. Tissues were harvested 24 hours post-treatment, then processed for calbindin immunohistochemistry to determine horizontal cell number, or 6, 12, 24 hours post-treatment for real-time PCR and immunoblots to detect PCNA expression. To localize proliferating cells, anti-PCNA antibody was applied.

Results

We showed various PAC1 expressing cells in RPE, NBL, and GCL in P1 retina including calbindin positive horizontal cells. We found that PACAP1-38 induced a marked cell number increase at P3 and P7 and showed upregulated cell proliferation as its mechanism; however, it was ineffective at P1. PACAP1-38 induced proliferative cells localized in the NBL, and double-marker studies demonstrated that the induced proliferative cells were horizontal cells.

Conclusions

PACAP1-38 appears to act in retinal differentiation by inducing mitosis selectively in a time and cell specific manner through PAC1. The control of horizontal cell proliferation raises the novel possibilities that (1) PACAP1-38 may be a major player in retinal patterning and (2) PACAP signaling may be critical in retinoblastoma.

Effects of PACAP on Dry Eye Symptoms, and Possible Use for Therapeutic Application

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 27- or 38-amino acid neuropeptide, which belongs to the vasoactive intestinal polypeptide/glucagon/secretin family of peptides. PACAP and its three receptor subtypes are expressed in neural tissues and in the eye, including the retina, cornea, and lacrimal gland. PACAP is known to exert pleiotropic effects on the central nervous system and in eye tissues where it plays important roles in protecting against dry eye. This review provides an overview of current knowledge regarding dry eye symptoms in aged animals and humans and the protective effects, mechanisms of action. In addition, we also refer to the development of a new preventive/therapeutic method by PACAP of dry eye patients.

Pituitary Adenylate Cyclase Activating Polypeptide (PACAP1-38) Exerts Both Pro and Anti-Apoptotic Effects on Postnatal Retinal Development in Rat

PACAP1-38, a ubiquitous and multifunctional regulator has been in the focus of neurotoxicity research due to its impressive neuroprotective potential. Although the literature extensively demonstrated its repressive effect on the apoptotic machinery in neurodegenerative models, there is a striking absence of analysis on its role in normal development. We performed quantitative analyses on caspase activity in developing retina upon 100, 50, 25 or 1 pmol intravitreal PACAP1-38 injection from postnatal day 1 (P1) through P7 in Wistar rats. Retinas were harvested at 6, 12, 18, 24 or 48 h post-injection. Apoptotic activity was revealed using fluorescent caspase 3/7 enzyme assay, western blots and TUNEL assay. Unexpectedly, we found that 100 pmol PACAP1-38 increased the activity of caspase 3/7 at P1 and P5 whereas it had no effect at P7. At P3, as a biphasic effect, PACAP1-38 repressed active caspase 3/7 at 18 h post-injection while increased their activity in 24 h post-injection. Amounts, smaller than 100 pmol, could not inhibit apoptosis whereas 50, 25 or 1 pmol PACAP1-38 could evoke significant elevation in caspase 3/7 activity. TUNEL-positive cells appeared in the proximal part of inner nuclear as well as ganglion cell layers in response to PACAP1-38 treatment. The fundamental novelty of these results is that PACAP1-38 induces apoptosis during early postnatal retinogenesis. The dose as well as stage-dependent response suggests that PACAP1-38 has a Janus face in apoptosis regulation. It not only inhibits development-related apoptosis, but as a long-term effect, facilitates it.

Pleiotropic and retinoprotective functions of PACAP

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 27- or 38-amino acid neuropeptide, which belongs to the vasoactive intestinal polypeptide/glucagon/secretin family. PACAP and its three receptor subtypes are expressed in neural tissues of the eye, including the retina, cornea and lacrimal gland, and PACAP is known to exert pleiotropic effects throughout the central nervous system. This review provides an overview of current knowledge regarding the cell protective effects, mechanisms of action and therapeutic potential of PACAP in response to several types of eye injury.

Morphology and function of three VIP-expressing amacrine cell types in the mouse retina

Amacrine cells (ACs) are the most diverse class of neurons in the retina. The variety of signals provided by ACs allows the retina to encode a wide range of visual features. Of the 30-50 AC types in mammalian species, few have been studied in detail. Here, we combine genetic and viral strategies to identify and to characterize morphologically three vasoactive intestinal polypeptide-expressing GABAergic AC types (VIP1-, VIP2-, and VIP3-ACs) in mice. Somata of VIP1- and VIP2-ACs reside in the inner nuclear layer and somata of VIP3-ACs in the ganglion cell layer, and they show asymmetric distributions along the dorsoventral axis of the retina. Neurite arbors of VIP-ACs differ in size (VIP1-ACs ≈ VIP3-ACs > VIP2-ACs) and stratify in distinct sublaminae of the inner plexiform layer. To analyze light responses and underlying synaptic inputs, we target VIP-ACs under 2-photon guidance for patch-clamp recordings. VIP1-ACs depolarize strongly to light increments (ON) over a wide range of stimulus sizes but show size-selective responses to light decrements (OFF), depolarizing to small and hyperpolarizing to large stimuli. The switch in polarity of OFF responses is caused by pre- and postsynaptic surround inhibition. VIP2- and VIP3-ACs both show small depolarizations to ON stimuli and large hyperpolarizations to OFF stimuli but differ in their spatial response profiles. Depolarizations are caused by ON excitation outweighing ON inhibition, whereas hyperpolarizations result from pre- and postsynaptic OFF-ON crossover inhibition. VIP1-, VIP2-, and VIP3-ACs thus differ in response polarity and spatial tuning and contribute to the diversity of inhibitory and neuromodulatory signals in the retina.

Glutamatergic neurotransmission from melanopsin retinal ganglion cells is required for neonatal photoaversion but not adult pupillary light reflex

Melanopsin-expressing retinal ganglion cells (mRGCs) in the eye play an important role in many light-activated non-image-forming functions including neonatal photoaversion and the adult pupillary light reflex (PLR). MRGCs rely on glutamate and possibly PACAP (pituitary adenylate cyclase-activating polypeptide) to relay visual signals to the brain. However, the role of these neurotransmitters for individual non-image-forming responses remains poorly understood. To clarify the role of glutamatergic signaling from mRGCs in neonatal aversion to light and in adult PLR, we conditionally deleted vesicular glutamate transporter (VGLUT2) selectively from mRGCs in mice. We found that deletion of VGLUT2 in mRGCs abolished negative phototaxis and light-induced distress vocalizations in neonatal mice, underscoring a necessary role for glutamatergic signaling. In adult mice, loss of VGLUT2 in mRGCs resulted in a slow and an incomplete PLR. We conclude that glutamatergic neurotransmission from mRGCs is required for neonatal photoaversion but is complemented by another non-glutamatergic signaling mechanism for the pupillary light reflex in adult mice. We speculate that this complementary signaling might be due to PACAP neurotransmission from mRGCs.

Pituitary adenylate cyclase activating polypeptide in the retina: focus on the retinoprotective effects

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide that has been shown to exert protective effects against different neuronal injuries, such as traumatic brain and spinal cord injury, models of neurodegenerative diseases, and cerebral ischemia. PACAP and its receptors are present in the retina. In this study, we summarize the current knowledge on retinal PACAP with focus on the retinoprotective effects. Results of histological, immunohistochemical, and molecular biological analysis are reviewed. In vitro, PACAP shows protection against glutamate, thapsigargin, anisomycin, and anoxia. In vivo, the protective effects of intravitreal PACAP treatment have been shown in the following models of retinal degeneration in rats: excitotoxic injury induced by glutamate and kainate, ischemic injury, degeneration caused by UV-A light, optic nerve transection, and streptozotocin-induced diabetic retinopathy. Studying the molecular mechanism has revealed that PACAP acts by activating antiapoptotic and inhibiting proapoptotic signaling pathways in the retina in vivo. These studies strongly suggest that PACAP is an excellent candidate retinoprotective agent that could be a potential therapeutic substance in various retinal diseases.

Effect of Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) on IL-6, IL-8, and MCP-1 Expression in Human Retinal Pigment Epithelial Cell Line

PURPOSE

To examine pituitary adenylate cyclase-activating polypeptide (PACAP) receptors (PAC1, VPAC1, and VPAC2) mRNA and the effect of PACAP on interleukin-6 (IL-6), interleukin-8 (IL-8), and monocyte chemotactic protein-1 (MCP-1) expression in human retinal pigment epithelial cell line (ARPE-19) stimulated with interleukin-1beta (IL-1beta).

METHODS

Expression of PACAP receptor mRNA was examined by reverse transcription polymerase chain reaction (RT-PCR). PACAP and IL-1beta were added to serum-free medium. IL-6, IL-8, and MCP-1 mRNA were measured by real-time PCR. IL-6, IL-8, and MCP-1 protein concentrations were measured using enzyme-linked immunosorbent assay. Nuclear factor kappaB (NF-kappaB) translocation was examined by immunofluorescence.

RESULTS

PAC1 and VCAP1 receptors mRNA were expressed in unstimulated cells. VCAP2 mRNA was expressed in cells stimulated with IL-1beta. IL-1beta stimulated IL-6, IL-8, and MCP-1 mRNA expression and protein levels. PACAP (10(- 7) to 10(- 6) M) inhibited IL-1beta -stimulated IL-6, IL-8, and MCP-1 mRNA and protein levels. Immunofluorescence of NF-kappaB in the nucleus was dense 30 min after stimulation with IL-1beta, and it was decreased by PACAP.

CONCLUSIONS

ARPE-19 cells had PACAP receptors mRNA. PACAP inhibited IL-6, IL-8, and MCP-1 expression and protein secretion. Possibly, the effect on cytokines may be via suppression of NF-kappaB translocation.

The neuroprotective effects of PACAP in monosodium glutamate-induced retinal lesion involve inhibition of proapoptotic signaling pathways

Pituitary adenylate cyclase activating polypeptide (PACAP) and its receptors are present in the retina and exert several distinct functions. PACAP has well-known neuroprotective effects in neuronal cultures in vitro and against different insults in vivo. Recently we have shown that PACAP is neuroprotective against monosodium glutamate (MSG)-induced retinal degeneration. In the present study we investigated the possible signal transduction pathways involved in the protective effect of intravitreal PACAP administration against apoptotic retinal degeneration induced by neonatal MSG treatment. MSG induced activation of proapoptotic signaling proteins and reduced the levels of antiapoptotic molecules in neonatal retinas. Co-treatment with PACAP attenuated the MSG-induced activation of caspase-3 and JNK, inhibited the MSG-induced cytosolic translocation of apoptosis inducing factor (AIF) and cytochrome c, and increased the level of phospho-Bad. Furthermore, PACAP treatment alone decreased cytosolic AIF and cytochrome c levels, while PACAP6-38 increased cytochrome c release, caspase-3 and JNK activity and decreased phospho-Bad activity. In summary, our results show that PACAP treatment attenuated the MSG-induced changes in apoptotic signaling molecules in vivo and suggest that also endogenously present PACAP has neuroprotective effects. These results may have further clinical implications in reducing glutamate-induced excitotoxicity in several ophthalmic diseases.

Degree of damage compensation by various PACAP treatments in monosodium glutamate-induced retinal degeneration

Pituitary adenylate cyclase activating polypeptide (PACAP) has been shown to be neuroprotective in retinal ischemia and monosodium L-glutamate (MSG)-induced retinal degeneration. Here we describe how different MSG treatments (1x and 3x application) cause retinal damage and finally lead to the destruction of the entire inner retina and how PACAP attenuates this effect. Newborn rats from both sexes were injected subcutaneously with 2 mg/g bodyweight MSG on postnatal days 1, 5 and 9. The left eye was left intact while we injected 5 microl PACAP38 solution (100 pmol) into the vitreous of the right eye with a Hamilton syringe at the time of (i) the first, (ii) the first two or (iii) all three MSG injections. Histological analysis has shown that the above described MSG treatment caused the entire inner plexiform layer (IPL) to degenerate, and the inner nuclear (INL) and ganglion cell layers (GCL) seemed fused. One time PACAP38 treatment at the first MSG application did not change the degenerative capacity of MSG. However, if animals received PACAP38 into the vitreous of the eye at the first 2 or all 3 times, a substantial protective effect could be observed. The IPL remained well discernible, the INL retained 2-3 cell rows and the number of cells in the GCL was substantially higher than in the MSG-treated retinas, and was not significantly different from that observed in the control tissue. We conclude that (i) 2 or 3 times PACAP treatment attenuates retinal degeneration; (ii) one PACAP treatment does not provide protection against repeated excitotoxic insults, and (iii) repeated application of PACAP under these experimental conditions may lead to a primed state in which further neurotoxic insults are ineffective.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Pituitary adenylate cyclase-activating polypeptide in the brain, spinal cord and sensory organs of the zebrafish, Danio rerio, during development

The distribution of Pituitary adenylate cyclase-activating polypeptide (PACAP) was investigated in the brain, pituitary and sensory organs of the zebrafish, Danio rerio, during development, in juvenile and adult specimens, using the immunofluorescence method. In 24 h post fertilization (hpf) embryos, PACAP immunoreactive cells appeared in the rostral telencephalon, dorsal diencephalon, caudal and medial rhombencephalon, spinal cord and retina. At 48 hpf stage, positive cells were present in the dorsal diencephalon, medial rhombencephalon, spinal cord, retina and olfactory placode (Op). At 72 hpf stage, additional immunoreactive elements appeared in the medial telencephalon, hypothalamus, mesencephalic tegmentum, retina and otic sensory epithelium (Ose). At day 5, new immunoreactive cells were found in the anterior rhombencephalon and pituitary pars distalis. At day 13, positive cells were mainly concentrated in the mesencephalic tegmentum and spinal cord. In the telencephalon, diencephalon, rhombencephalon and pituitary, the distribution of positive cells was similar to that previously reported. At 1 month stage, positive cells were detected in the hypothalamus, nucleus of the medial longitudinal fascicle (nMlf), rhombencephalic griseum centrale (Gc) and pituitary pars distalis. At 2-3 month stages, immunoreactive elements were found in several hypothalamic nuclei, in the mesencephalic nucleus isthmi, cerebellum and pituitary. In adults, PACAP immunoreactivity was confined to a few brain regions and the pituitary. PACAP immunoreactivity was transiently expressed in several regions suggesting that the peptide may have a role in the control of cells differentiation and proliferation during zebrafish ontogeny. The finding of positive fibers in the pituitary from day 5 onward indicates that PACAP may function from this stage as a hypophysiotropic peptide.

Distribution of the VPAC2 receptor in peripheral tissues of the mouse

The neuropeptide vasoactive intestinal peptide (VIP) exerts its actions through two structurally related G protein-coupled receptors (VPAC(1) and VPAC(2)). Pituitary adenylate cyclase-activating polypeptide (PACAP) is also a potent agonist of VPAC(1) and VPAC(2) receptors as well as of a third, PACAP-specific receptor (PAC(1)). We report here the distribution of the VPAC(2) receptor in peripheral tissues of the mouse, determined by receptor autoradiography using [(125)I]VIP and the selective VPAC(2) receptor agonist [(125)I]Ro25-1553 in wild-type and VPAC(2) receptor-null mice. In addition, displacement experiments with the VPAC(2)-selective agonist Ro25-1553 and the VPAC(1)-selective agonist [K(15),R(16),L(27)]VIP(1-7)/GRF(8-27) were performed using the universal radioligand [(125)I]VIP. The VPAC(2) receptor is found predominantly in smooth muscle (in blood vessels and in the smooth muscle layers of the gastrointestinal and reproductive systems), the basal part of the mucosal epithelium in the colon, lung, the vasculature of the kidney, adrenal medulla, and retina. Unexpectedly, the receptor was also present in thyroid follicular cells and acinar cells of the pancreas, tissues that have not been found to express the receptor in other species, and in very large amounts in the lung. Our data suggest novel functions of the VPAC(2) receptor and additional potential therapeutic uses of drugs acting at the receptor (including the treatment of erectile dysfunction), but our results also indicate that caution should be exercised in using the mouse as an animal model for the evaluation of VIP analogs intended for diagnostic or therapeutic use in man.

PAC1 receptor activation by PACAP-38 mediates Ca2+ release from a cAMP-dependent pool in human fetal adrenal gland chromaffin cells

Previous studies have shown that human fetal adrenal gland from 17- to 20-week-old fetuses expressed pituitary adenylate cyclase-activating polypeptide (PACAP) receptors, which were localized on chromaffin cells. The aim of the present study was to identify PACAP receptor isoforms and to determine whether PACAP can affect intracellular calcium concentration ([Ca(2+)](i)) and catecholamine secretion. Using primary cultures and specific stimulation of chromaffin cells, we demonstrate that PACAP-38 induced an increase in [Ca(2+)](i) that was blocked by PACAP (6-38), was independent of external Ca(2+), and originated from thapsigargin-insensitive internal stores. The PACAP-triggered Ca(2+) increase was not affected by inhibition of PLC beta (preincubation with U-73122) or by pretreatment of cells with Xestospongin C, indicating that the inositol 1,4,5-triphosphate-sensitive stores were not mobilized. However, forskolin (FSK), which raises cytosolic cAMP, induced an increase in Ca(2+) similar to that recorded with PACAP-38. Blockage of PKA by H-89 or (R(p))-cAMPS suppressed both PACAP-38 and FSK calcium responses. The effect of PACAP-38 was also abolished by emptying the caffeine/ryanodine-sensitive Ca(2+) stores. Furthermore, treatment of cells with orthovanadate (100 microm) impaired Ca(2+) reloading of PACAP-sensitive stores indicating that PACAP-38 can mobilize Ca(2+) from secretory vesicles. Moreover, PACAP induced catecholamine secretion by chromaffin cells. It is concluded that PACAP-38, through the PAC(1) receptor, acts as a neurotransmitter in human fetal chromaffin cells inducing catecholamine secretion, through nonclassical, recently described, ryanodine/caffeine-sensitive pools, involving a cAMP- and PKA-dependent phosphorylation mechanism.

Developmental changes in the expression of growth hormone-releasing hormone and pituitary adenylate cyclase-activating polypeptide in zebrafish

Growth hormone-releasing hormone (GHRH) and pituitary adenylate cyclase-activating polypeptide (PACAP) are structurally and functionally related members of the glucagon superfamily, a group of hormones important in development, growth, and metabolism. Our objectives were to determine the developmental expression pattern of the ghrh-pacap1 gene using the zebrafish model. The temporal and spatial expression pattern of the ghrh-pacap1 gene was examined by RT-PCR and in situ hybridization. In zebrafish, the ghrh-pacap1 mRNA transcript was expressed throughout development beginning at the transition between the blastula and gastrula periods. During midgastrulation, alternative splicing resulted in the generation of a novel transcript lacking the cryptic peptide. During the segmentation period, expression was localized to the neural tube, developing eye, and neural crest; strong expression was found in the developing cerebellum. Later in development, expression was localized in the hatching gland and developing pharyngeal arches. The temporal and spatial expression pattern of the ghrh-pacap1 transcript suggests that these hormones may modulate patterning during development.

Mouse pituitary adenylate cyclase-activating polypeptide (PACAP): gene, expression and novel splicing

PACAP is a conserved neuropeptide present in all vertebrates. In the mouse, the PACAP gene and various mRNAs have been isolated. To further characterize the mouse PACAP gene (Adcyap1), we have confirmed its sequence, identified its chromosomal location on distal chromosome 17 and used RT-PCR and 5'RACE in various tissues to identify eight PACAP mRNAs, four of which are novel. Three of these novel transcripts have alternate 5'UTRs, whereas the fourth is altered within the coding region. This is the first report of alternate splicing within the coding region of the PACAP gene. The expression pattern of PACAP in the mouse during embryonic development and adulthood is known. Here, expression of PACAP in the mouse at four postnatal stages in 12 tissues is assessed. We identify continuous expression of PACAP in the brain and eye and stage-specific expression in the gonads and thymus. The complex splicing of PACAP transcripts may regulate the tissue-and stage-specific expression.

Pituitary adenylyl cyclase-activating polypeptide prevents induced cell death in retinal tissue through activation of cyclic AMP-dependent protein kinase

Multiple neuroactive substances are secreted by neurons and/or glial cells and modulate the sensitivity to cell death. In the developing retina, it has been shown that increased intracellular levels of cAMP protect cells from degeneration. We tested the hypothesis that the neuroactive peptide pituitary adenylyl cyclase-activating polypeptide (PACAP) has neuroprotective effects upon the developing rat retina. PACAP38 prevented anisomycin-induced cell death in the neuroblastic layer (NBL) of retinal explants, and complete inhibition of induced cell death was obtained with 1 nm. A similar protective effect was observed with PACAP27 and with the specific PAC1 receptor agonist maxadilan but not with glucagon. Photoreceptor cell death induced by thapsigargin was also prevented by PACAP38. The neuroprotective effect of PACAP38 upon the NBL could be reverted by the competitive PACAP receptor antagonist PACAP6-38 and by the specific PAC1 receptor antagonist Maxd.4. Molecular and immunohistochemical analysis demonstrated PAC1 receptors, and treatment with PACAP38 induced phospho-cAMP-response element-binding protein immunoreactivity in the anisomycin-sensitive undifferentiated postmitotic cells within the NBL. PACAP38 produced an increase in cAMP but not inositol triphosphate, and treatment with the cAMP-dependent protein kinase inhibitor R(p)-cAMPS blocked the protective effect of PACAP38. The results indicate that activation of PAC1 receptors by PACAP38 modulates cell death in the developing retina through the intracellular cAMP/cAMP-dependent protein kinase pathway.

Developmental expression, alternative splicing and gene copy number for the pituitary adenylate cyclase-activating polypeptide (PACAP) and growth hormone-releasing hormone (GRF) gene in rainbow trout

Both growth hormone-releasing hormone (GRF) and pituitary adenylate cyclase-activating polypeptide (PACAP) are encoded on the same gene in fish, but not in mammals. Our objective was to examine the onset and pattern of expression for the grf/pacap gene and to determine whether there is more than one gene in rainbow trout. The results show that grf/pacap mRNA is first expressed at 4 days after fertilization and continues through to hatching. Alternative splicing at all developmental stages produces a full-length transcript and one lacking exon four, which encodes GRF. Thus, independent regulation of the hormones occurs throughout development. Southern analysis shows that two grf/pacap genes exist in trout, but only one gene is responsible for the two identified transcripts. Overexpression of the grf/pacap gene in transgenic fish was attempted, but did not succeed. We conclude that the early and continued expression of grf/pacap mRNA in trout embryos and regulation of the neuropeptide ratio suggests they have a role in early brain development apart from their later role in releasing pituitary hormones.

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.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

Pituitary adenylate cyclase-activating polypeptide (PACAP-27) enhances tyrosine hydroxylase activity in the nucleus accumbens of the rat

Pituitary adenylate cyclase-activating polypeptide (PACAP-27) was incubated in a tyrosine hydroxylase (TyrOH) assay with a homogenate preparation of the nucleus accumbens of the rat. TyrOH activity was determined in vitro by measuring the production of L-dopa with HPLC-ECD. Only in the presence of adenosine nucleotides (ATP, App(NH)p) PACAP-27 increased TyrOH activity with a EC(50)of 100 nM. Since the PACAP-27 effect on TyrOH was abolished when homogenate or pellet of the nucleus accumbens were coincubated with CHAPS, the peptide effect appears to be receptor mediated. TyrOH activation produced by PACAP-27 increased in the presence of the phosphodiesterase inhibitor papaverine indicating the involvement of cAMP. The marked effect of the non-hydrolysable adenosine nucleotide App(NH)p also supports a cAMP-dependent TyrOH activation not related to ADP or an ADP-dependent mechanism. This report's data suggest that PACAP-27 activates TyrOH in the rat nucleus accumbens through receptor-mediated cAMP formation. The exact receptor type present in the nucleus accumbens has yet not been specified.

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.

Identification and functional properties of the pituitary adenylate cyclase activating peptide (PAC1) receptor in human benign hyperplastic prostate

Pituitary adenylate cyclase activating peptide (PACAP) is a novel neuropeptide with regulatory and trophic functions that is related to vasoactive intestinal peptide (VIP). Here we investigate the expression of specific PACAP receptors (PAC1) and common VIP/PACAP receptors (VPAC1 and VPAC2) in the human hyperplastic prostate by immunological methods. The PAC1 receptor corresponded to a 60-KDa protein whereas the already known VPAC1 and VPAC2 receptors possessed molecular masses of 58 and 68 KDa, respectively. The heterogeneity of VIP/PACAP receptors in this tissue was confirmed by radioligand binding studies using [125I]PACAP-27 by means of stoichiometric and pharmacological experiments. At least two classes of PACAP binding sites showing different affinities could be resolved, with Kd values of 0.81 and 51.4 nM, respectively. The order of potency in displacing [125I]PACAP-27 binding was PACAP-27 approximately equal to PACAP-38 > VIP. PACAP-27 and VIP stimulated similarly adenylate cyclase activity, presumably through common VIP/PACAP receptors. The PAC1 receptor was not coupled to activation of either adenylate cyclase, nitric oxide synthase, or phospholipase C. It appears to be a novel subtype of PAC1 receptor because PACAP-27 (but not PACAP-38 or VIP) led to increased phosphoinositide synthesis, an interesting feature because phosphoinositides are involved via receptor mechanisms in the regulation of cell proliferation.

Attenuation by PACAP of glutamate-induced neurotoxicity in cultured retinal neurons

The effects of pituitary adenylate cyclase activating polypeptides (PACAPs: PACAP27, PACAP38) on glutamate-induced neurotoxicity were examined using cultured retinal neurons obtained from 3- to 5-day old Wistar rats. Cell viability was evaluated by double staining with fluorescein diacetate and propidium iodide. Effects of PACAPs on the increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in retinal neurons was investigated using the Ca(2+) image analyzing system with fura-2. The cAMP contents and the mitogen-activated protein (MAP) kinase activity in retinal cultures were measured by radioimmunoassay. Concomitant application of PACAPs (10 nM-1 microM) with glutamate (1 mM) for 10 min inhibited the delayed death of retinal neurons, which was observed 24 h after glutamate (1 mM) treatment in a dose-dependent manner. Protection by PACAPs (100 nM) against glutamate-induced neurotoxicity was antagonized by PACAP6-38 (1 microM), a PACAP antagonist, and H-89 (1 microM), a protein kinase A (PKA) inhibitor. However, PACAPs did not affect the glutamate-induced increase in [Ca(2+)](i), but PACAPs (1-100 nM) increased the cAMP levels in a dose-dependent manner. In addition, activation of MAP kinase by PACAP38 (1 microM) was inhibited by simultaneous application with H-89 (1 microM). These findings suggest that PACAPs attenuate glutamate-induced delayed neurotoxicity in cultured retinal neurons by activating MAP kinase through the activation of cAMP-stimulated PKA.