Effect of PACAP on MAP kinases, Akt and cytokine expressions in rat retinal hypoperfusion

Pituitary Adenylate Cyclase-Activating Polypeptide Protects Corneal Epithelial Cells against UV-B-Induced Apoptosis via ROS/JNK Pathway Inhibition

PACAP is widely expressed throughout the body. It exerts a beneficial role in the eye, including the cornea. The corneal epithelium is regularly exposed to diverse types of insults, including ultraviolet B (UV-B) radiation. Previously, we showed the protective role played by PACAP in counteracting UV-B ray insults in human corneal endothelial cells; however, its involvement in corneal epithelium protection against ROS induced by UV-B radiation, and the underlying mechanisms, remain to be determined. Here, we demonstrated that the peptide treatment reduced UV-B-induced ROS generation by playing an anti-apoptotic role via JNK-signaling pathway inhibition. Overall, our results can provide guidance in the therapeutic use of PACAP for the treatment of epithelial corneal damage.

A Promising Combination: PACAP and PARP Inhibitor Have Therapeutic Potential in Models of Diabetic and Hypertensive Retinopathies

Diabetes and hypertension are complex pathologies with increasing prevalence nowadays. Their interconnected pathways are frequently manifested in retinopathies. Severe retinal consequences and their tight connections as well as their possible treatments are particularly important to retinal research. In the present, work we induced diabetes with streptozotocin in spontaneously hypertensive rats and treated them either with PACAP or olaparib and alternatively with both agents. Morphological and immunohistochemical analyses were carried out to describe cell-specific changes during pathologies and after different treatments. Diabetes and hypertension caused massive structural and cellular changes especially when they were elicited together. Hypertension was crucial in the formation of ONL and OPL damage while diabetes caused significant differences in retinal thickness, OPL thickness and in the cell number of the GCL. In diabetes, double neuroprotective treatment ameliorated changes of calbindin-positive cells, rod bipolar cells and dopaminergic amacrine cells. Double treatment was curative in hypertensive diabetic rat retinas, especially in the case of rod bipolar and parvalbumin-positive cells compared to untreated or single-treated retinas. Our results highlighted the promising therapeutic benefits of olaparib and PACAP in these severe metabolic retinal disorders.

PACAP-derived mutant peptide MPAPO protects trigeminal ganglion cell and the retina from hypoxic injury through anti-oxidative stress, anti-apoptosis, and promoting axon regeneration

The purpose of this study was to determine whether the MPAPO, derived peptide of pituitary adenylate cyclase-activating polypeptide (PACAP), would protect trigeminal ganglion cells (TGCs) and the mice retinas from a hypoxic insult. The nerve endings of the ophthalmic nerve of the trigeminal nerve are widely distributed in eye tissues. In TGCs after hypoxia exposure, we discovered that reactive oxygen species level, the contents of cytosolic cytochrome c and cleaved-caspase-3 were significantly increased, in the meanwhile, m-Calpain was activated and cytoskeleton proteins (αII-spectrin and Synapsin) were degraded, neurites of TGCs disappeared, but these effects were reversed in TGCs treated with MPAPO. The structure of the mice retinas after hypoxic exposure was disordered. Increased lipid peroxidation (LPO), decreased glutathione (GSH) levels, and decreased superoxide dismutase (SOD) activity, positive cells of terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), the disintegration of nerve fibers was examined in the retinas following a hypoxic insult. Disordered retina was attenuated with MPAPO eye drops, as well as hypoxia-induced apoptosis in the developing retina, increase in LPO, and decrease in GSH levels and SOD activity of the retina. Moreover, the disintegrated retinal nerve fibers were reassembled after MPAPO treatment. These results suggest that hypoxia induces oxidative stress, apoptosis, and neurites disruption, while MPAPO is remarkably protective against these adverse effects of hypoxia in TGCs and the developing retinas by specifically activating PAC1 receptor.

Pituitary Adenylate Cyclase Activating Polypeptide Has Inhibitory Effects on Melanoma Cell Proliferation and Migration In Vitro

Pituitary adenylate cyclase activating polypeptide (PACAP) is an endogenous neuropeptide which is distributed throughout the body. PACAP influences development of various tissues and exerts protective function during cellular stress and in some tumour formation. No evidence is available on its role in neural crest derived melanocytes and its malignant transformation into melanoma. Expression of PACAP receptors was examined in human skin samples, melanoma lesions and in a primary melanocyte cell culture. A2058 and WM35 melanoma cell lines, representing two different stages of melanoma progression, were used to investigate the effects of PACAP. PAC1 receptor was identified in melanocytes in vivo and in vitro and in melanoma cell lines as well as in melanoma lesions. PACAP administration did not alter viability but decreased proliferation of melanoma cells. With live imaging random motility, average speed, vectorial distance and maximum distance of migration of cells were reduced upon PACAP treatment. PACAP administration did not alter viability but decreased proliferation capacity of melanoma cells. On the other hand, PACAP administration decreased the migration of melanoma cell lines towards fibronectin chemoattractant in the Boyden chamber. Furthermore, the presence of the neuropeptide inhibited the invasion capability of melanoma cell lines in Matrigel chambers. In summary, we provide evidence that PACAP receptors are expressed in melanocytes and in melanoma cells. Our results also prove that various aspects of the cellular motility were inhibited by this neuropeptide. On the basis of these results, we propose PACAP signalling as a possible target in melanoma progression.

PACAP is Protective Against Cellular Stress in Retinal Pigment Epithelial Cells

The integrity of the innermost, pigment epithelial layer of the retina is crucial for the photoreceptor survival and for maintaining the outer blood–retina barrier. In several ocular degenerations, such as diabetic retinopathy or macular edema, the stress caused by various harmful stimuli (hypoxia, oxidative stress, hyperosmosis) lead to severe molecular biological changes in this layer, promoting neovascularization of the retina. Pituitary adenylate cyclase activating polypeptide (PACAP) occurs throughout the whole body, including the eye. It has numerous functions in the retina, including the previously described anti-apoptotic and anti-angiogenic effects in retinal pigment epithelial cells. The aim of this present study was to investigate the influence of PACAP on different stress factors. In accordance with previous findings, PACAP significantly ameliorated the increased Hif1-α levels in hypoxic conditions. In H2O2-induced oxidative stress PACAP had an anti-apoptotic effect, it could decrease the expression of cytochrome-c and p53, while it upregulated the concentration of three antioxidants, namely SOD2, PON2 and thioredoxin. In conclusion, we provided new information on the molecular biological background of the retinoprotective effect of PACAP.

The Protective Effects of Endogenous PACAP in Oxygen-Induced Retinopathy

Pituitary adenylate cyclase–activating polypeptide (PACAP) is a neuropeptide having trophic and protective functions in neural tissues, including the retina. Previously, we have shown that intravitreal PACAP administration can maintain retinal structure in the animal model of retinopathy of prematurity (ROP). The purpose of this study is to examine the development of ROP in PACAP-deficient and wild-type mice to reveal the function of endogenous PACAP. Wild-type and PACAP-knockout (KO) mouse pups at postnatal day (PD) 7 were maintained at 75% oxygen for 5 consecutive days then returned to room air on PD12 to develop oxygen-induced retinopathy (OIR). On PD15, animals underwent electroretinography (ERG) to assess visual function. On PD16, eyes were harvested for either immunohistochemistry to determine the percentage of the central avascular retinal area or molecular analysis to assess angiogenesis proteins by array kit and anti-apoptotic protein kinase B (Akt) change by western blot. Retinas of PACAP-deficient OIR mice showed a greater central avascular area than that of the wild types. ERG revealed significantly decreased b-wave amplitude in PACAP KO compared to their controls. Several angiogenic proteins were upregulated due to OIR, and 11 different proteins markedly increased in PACAP-deficient mice, whereas western blot analysis revealed a reduction in Akt phosphorylation, suggesting an advanced cell death in the lack of PACAP. This is the first study to examine the endogenous effect of PACAP in the OIR model. Previously, we have shown the beneficial effect of exogenous local PACAP treatment in the rat OIR model. Together with the present findings, we suggest that PACAP could be a novel retinoprotective agent in ROP.

Relation of Retinal Oxygen Measures to Electrophysiology and Survival Indicators after Permanent, Incomplete Ischemia in Rats

Studies in experimental ischemia models by permanent bilateral common carotid artery occlusion (BCCAO) have reported reduced retinal electrophysiological function, coupled with inner retinal degeneration and gliosis. In the current study, we tested the hypothesis that long-term (up to 14 days) BCCAO impairs oxygen delivery (DO₂), which affects oxygen metabolism (MO₂) and extraction fraction (OEF), electrophysiological function, morphology, and biochemical pathways. Twenty-one rats underwent BCCAO (N = 12) or sham surgery (N = 9) and were evaluated in separate groups after 3, 7, or 14 days. Electroretinography (ERG), optical coherence tomography, blood flow and vascular oxygen tension imaging, and morphological and biochemical evaluations were performed in both eyes. Reduced ERG b-wave amplitudes and delayed implicit times were reported at 3, 7, and 14 days following BCCAO. Total retinal blood flow, MO₂, and DO₂ were reduced in all BCCAO groups. OEF was increased in both 3- and 7-day groups, while no significant difference was observed in OEF at 14 days compared to the sham group. At 14 days following BCCAO, total and inner retinal layer thickness was reduced, while the outer nuclear layer thickness and gliosis were increased. There was an increase in nuclei containing fragmented DNA at 3 days following BCCAO. The compensatory elevation in OEF following BCCAO did not meet the tissue demand, resulting in the subsequent reduction of MO₂. The associations between retinal MO₂, DO₂, and retinal function were shown to be significant in the sequelae of persistent ischemia. In sum, measurements of DO₂, MO₂, and OEF may become useful for characterizing salvageable tissue in vision-threatening pathologies.

Neuroprotective Potential of Pituitary Adenylate Cyclase Activating Polypeptide in Retinal Degenerations of Metabolic Origin

Pituitary adenylate cyclase-activating polypeptide (PACAP1-38) is a highly conserved member of the secretin/glucagon/VIP family. The repressive effect of PACAP1-38 on the apoptotic machinery has been an area of active research conferring a significant neuroprotective potential onto this peptide. A remarkable number of studies suggest its importance in the etiology of neurodegenerative disorders, particularly in relation to retinal metabolic disorders. In our review, we provide short descriptions of various pathological conditions (diabetic retinopathy, excitotoxic retinal injury and ischemic retinal lesion) in which the remedial effect of PACAP has been well demonstrated in various animal models. Of all the pathological conditions, diabetic retinopathy seems to be the most intriguing as it develops in 75% of patients with type 1 and 50% of patients with type 2 diabetes, with concomitant progression to legal blindness in about 5%. Several animal models have been developed in recent years to study retinal degenerations and out of these glaucoma and age-related retina degeneration models bear human recapitulations. PACAP neuroprotection is thought to operate through enhanced cAMP production upon binding to PAC1-R. However, the underlying signaling network that leads to neuroprotection is not fully understood. We observed that (i) PACAP is not equally efficient in the above conditions; (ii) in some cases more than one signaling pathways are activated; (iii) the coupling of PAC1-R and signaling is stage dependent; and (iv) PAC1-R is not the only receptor that must be considered to interpret the effects in our experiments. These observations point to a complex signaling mechanism, that involves alternative routes besides the classical cAMP/protein kinase A pathway to evoke the outstanding neuroprotective action. Consequently, the possible contribution of the other two main receptors (VPAC1-R and VPAC2-R) will also be discussed. Finally, the potential medical use of PACAP in some retinal and ocular disorders will also be reviewed. By taking advantage of, low-cost synthesis technologies today, PACAP may serve as an alternative to the expensive treatment modelities currently available in ocular or retinal conditions.

Pituitary Adenylate Cyclase-Activating Polypeptide: 30 Years in Research Spotlight and 600 Million Years in Service

Emerging from the depths of evolution, pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors (i.e., PAC1, VPAC1, VPAC2) are present in multicellular organisms from Tunicates to humans and govern a remarkable number of physiological processes. Consequently, the clinical relevance of PACAP systems spans a multifaceted palette that includes more than 40 disorders. We aimed to present the versatility of PACAP1-38 actions with a focus on three aspects: (1) when PACAP1-38 could be a cause of a malfunction, (2) when PACAP1-38 could be the cure for a malfunction, and (3) when PACAP1-38 could either improve or impair biology. PACAP1-38 is implicated in the pathophysiology of migraine and post-traumatic stress disorder whereas an outstanding protective potential has been established in ischemia and in Alzheimer’s disease. Lastly, PACAP receptors could mediate opposing effects both in cancers and in inflammation. In the light of the above, the duration and concentrations of PACAP agents must be carefully set at any application to avoid unwanted consequences. An enormous amount of data accumulated since its discovery (1989) and the first clinical trials are dated in 2017. Thus in the field of PACAP research: “this is not the end, not even the beginning of the end, but maybe the end of the beginning.”

Protective effect of PACAP-38 on retinal pigmented epithelium in an in vitro and in vivo model of diabetic retinopathy through EGFR-dependent mechanism

Diabetic retinopathy (DR) is one of the most common microvascular complications of diabetes. In the last years, several in vivo studies have demonstrated the protective role of pituitary adenylate cyclase-activating peptide (PACAP-38) to counteract several alterations occurring during DR. Recently, different studies have demonstrated that some PACAP-38 effects are mediated by EGFR trans-activation, although no data exist regarding the link between this peptide and EGFR in DR. The aim of the present study has been to investigate whether retinal effect of PACAP-38 against high glucose damage is mediated by EGFR phosphorylation. Diabetes was induced by a single injection of streptozotocin (STZ) in rats. After 1 week, a group of animals was treated with a single intravitreal injection of 100 μM PACAP-38 or saline solution. Immunohistochemistry and western blot analysis have demonstrated that intravitreal injection of PACAP-38 induced p-EGFR over-expression in retina of diabetic rats. Several pathogenic mechanisms may contribute to diabetic retinopathy including BRB alteration. To better clarify the relationship between PACAP-38 and EGFR, we have also carried out a study on ARPE-19 cells, representing a model in vitro of outer BRB. Our results have shown that PACAP-38 treatment improved cell viability in ARPE-19 cells exposed to hyperglycemic/hypoxic insult mimicking tissue microenvironment occurring in DR. Binding to PAC1R, peptide induces EGFR phosphorylation via PKA-signaling cascade stimulation. EGFR trans-activation triggers MAPK/ERK signaling pathway involved in cell survival and proliferation. In conclusion, data have suggested that PACAP-38 acts through EGFR phosphorylation in DR and this effect particularly occurs on RPE layer.

PACAP Attenuates Optic Nerve Crush-Induced Retinal Ganglion Cell Apoptosis Via Activation of the CREB-Bcl-2 Pathway

Retinal ganglion cell (RGC) apoptosis is considered an important pathological hallmark of glaucoma. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic peptide with potent neuroprotective properties. In our previous study, we found that the expression of PACAP and its high-affinity receptor PACAP receptor type 1 (PAC1R) increased markedly after optic nerve crush (ONC), and occurred mainly in the ganglion cell layer of the retina. This suggests that the upregulation of PACAP may play a vital role in inhibiting RGC death after ONC. Therefore, in the present study, we investigate the specific effects and underlying mechanism of PACAP in RGC death after ONC. Vehicle (physiological saline) or PACAP (1 nM to 200 nM) solution was injected into the vitreous body. Seven days later, the retinas were harvested, and the surviving RGCs were retrogradely labeled with Fluoro-Gold (FG; Fluorochrome) at different concentrations of PACAP. Immunofluorescence double staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay were used to observe the effects of PACAP on RGC apoptosis. Our results showed that PACAP treatment inhibited caspase-3-mediated RGC apoptosis, promoted the phosphorylation of cAMP response element binding protein (CREB), up-regulated the expression of B-cell lymphoma 2 (Bcl-2), and ultimately improved RGC survival. These results suggest that PACAP may prevent RGC apoptosis after ONC via activation of CREB-mediated Bcl-2 transcription. The study thus contributes to a basic understanding of the mechanism by which PACAP decreased RGC apoptosis and provides a theoretical basis for future clinical application of PACAP in the treatment of glaucoma.

NAP modulates hyperglycemic-inflammatory event of diabetic retina by counteracting outer blood retinal barrier damage

Diabetic retinopathy (DR) is a common microvascular complication of diabetes. Prolonged hyperglycemia stimulates inflammatory pathway characterized by the release of some cytokines leading to the impairment of blood retinal barrier (BRB). NAP exerts a protective effect in various eye diseases, including DR. So far, the role of NAP in the modulation of inflammatory event during early phase of this pathology has not been investigated yet. In the current study, we have studied the retinal protective effect of NAP, injected into the eye, in diabetic rats. NAP treatment exerts a dual effect downregulating interleukin (IL)-1β and its related receptors and upregulating IL-1Ra expression. We have also tested the role of this peptide in human retinal epithelial cells (ARPE19) cultured on a semipermeable support and exposed to hyperglycemic-inflammatory insult, representing a in vitro model of diabetic macular edema, a clinical manifestation of DR. The results have shown that NAP prevents outer BRB impairment by upregulating the tight junctions. In conclusion, deepened characterization of NAP action mechanism on hyperglycemic-inflammatory damage may be useful to develop a new strategy to prevent retinal damage during DR.

Protective Effect of PACAP on Ischemia/Reperfusion-Induced Kidney Injury of Male and Female Rats: Gender Differences

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that exerts general cytoprotective effects, including protection in different kidney disorders. The aim of our study was to investigate the ischemia/reperfusion-induced kidney injury of male and female rats to confirm the protective effects of PACAP in the kidney and to reveal possible gender differences.Male and female Wistar rats underwent unilateral renal artery clamping followed by 24-h, 48-h, or 14-day reperfusion. PACAP was administered intravenously before arterial clamping in half of the rats. Tubular damage, cytokine expression pattern, oxidative stress marker, antioxidative status and signaling pathways were evaluated using histology, immunohistology, cytokine array, PCR, and Western blot. Tubular damage was significantly less severe in the PACAP-treated male and female rats compared to controls. Results of female animals were significantly better in both treated and untreated groups. Cytokine expression, oxidative stress marker and antioxidative status confirmed the histological results. We also revealed that PACAP counteracted the decreased PKA phosphorylation, influenced the expression of BMP2 and BMP4, and increased the expression of the protein Smad1.We conclude that PACAP is protective in ischemia/reperfusion-induced kidney injury in both sexes, but females had markedly less pronounced injury after ischemia/reperfusion, possibly also involving further protective factors, the investigation of which could have future therapeutic value in treating ischemic kidney injuries.

Involvement of vascular endothelial growth factor (VEGF) and mitogen-activated protein kinases (MAPK) in the mechanism of neuroleptic drugs

BACKGROUND

Recent evidence suggests that the mitogen activated protein kinase (MAPK)-associated signaling pathway in the frontal cortical areas demonstrates abnormal activity in cases of schizophrenia. Moreover, schizophrenia patients often display alterations in the regional cellular energy metabolism and blood flow of the brain; these are shown to parallel changes in angiogenesis primarily mediated by vascular endothelial growth factor (VEGF).

METHODS

The present study examines the differential effects of time-dependent treatment with haloperidol, olanzapine and amisulpride (20μM) on VEGF and MAPK mRNA expression and VEGF level, using the T98 cell line as an example of nerve cells. For the purposes of comparison, the effect of neuroprotective pituitary adenylate cyclase-activating polypeptide (PACAP) on the expression of VEGF mRNA and secretion were also evaluated in this cell model.

RESULTS

RT-PCR analysis revealed that all the tested neuroleptics increased VEGF mRNA expression after 72-h incubation; however, only haloperidol and olanzapine also increased the level of VEGF detected by ELISA, and they demonstrated significantly stronger effects than PACAP. Haloperidol and olanzapine, but not amisulpride, decreased MAPK14 mRNA expression in T98G cells after 72-h incubation.

CONCLUSION

The obtained results suggest that haloperidol and olanzapine can trigger the MAPK and VEGF signaling pathway, which may contribute to their neuroprotective mechanism of action.

Protective effects of PACAP in ischemia

Pituitary adenylate cyclase activating polypeptide (PACAP) is an ubiquitous peptide involved, among others, in neurodevelopment, neuromodulation, neuroprotection, neurogenic inflammation and nociception. Presence of PACAP and its specific receptor, PAC1, in the trigeminocervical complex, changes of PACAP levels in migraine patients and the migraine-inducing effect of PACAP injection strongly support the involvement of PACAP/PAC1 receptor in migraine pathogenesis. While antagonizing PAC1 receptor is a promising therapeutic target in migraine, the diverse array of PACAP's functions, including protection in ischemic events, requires that the cost-benefit of such an intervention is well investigated by taking all the beneficial effects of PACAP into account. In the present review we summarize the protective effects of PACAP in ischemia, especially in neuronal ischemic injuries, and discuss possible points to consider when developing strategies in migraine therapy interfering with the PACAP/PAC1 receptor system.

Modulation of IL-1β and VEGF expression in rat diabetic retinopathy after PACAP administration

Diabetic retinopathy (DR) is a microvascular complication of diabetes. Hyperglycemic/hypoxic microenvironment concurs to aberrant angiogenesis characterizing the pathology and activates many downstream target genes including inflammatory cytokines and vasoactive peptides, such as interleukin-1β (IL-1β) and vascular endothelial growth factor (VEGF). It has been largely demonstrated that pituitary adenylate cyclase-activating peptide (PACAP) plays a protective effect in DR. In the present study, we investigated the role of PACAP to protect retinal tissue through IL-1β and VEGF expression. Diabetes was induced in rats by streptozotocin (STZ) injection, and one week later a single intravitreal injection of 100μM PACAP was administrated. Analyses of IL-1β and VEGF levels were performed three weeks after diabetes induction. The results demonstrated that a single intraocular administration of PACAP significantly reduced the expression of IL-1β in diabetic animals. Moreover, it affects VEGF and its receptors (VEGFRs) levels and interferes with their retinal layers distribution as showed by confocal microscopy analysis. In particular, PACAP treatment downregulates VEGF and VEGFRs that are increasingly expressed in STZ-treated animals as compared to controls. These results indicate that PACAP plays an important role to attenuate the early phase of DR.

Passage through the Ocular Barriers and Beneficial Effects in Retinal Ischemia of Topical Application of PACAP1-38 in Rodents

The neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) has two active forms, PACAP1-27 and PACAP1-38. Among the well-established actions are PACAP’s neurotrophic and neuroprotective effects, which have also been proven in models of different retinopathies. The route of delivery is usually intravitreal in studies proving PACAP’s retinoprotective effects. Recently, we have shown that PACAP1-27 delivered as eye drops in benzalkonium-chloride was able to cross the ocular barriers and exert retinoprotection in ischemia. Since PACAP1-38 is the dominant form of the naturally occurring PACAP, our aim was to investigate whether the longer form is also able to cross the barriers and exert protective effects in permanent bilateral common carotid artery occlusion (BCCAO), a model of retinal hypoperfusion. Our results show that radioactive PACAP1-38 eye drops could effectively pass through the ocular barriers to reach the retina. Routine histological analysis and immunohistochemical evaluation of the Müller glial cells revealed that PACAP1-38 exerted retinoprotective effects. PACAP1-38 attenuated the damage caused by hypoperfusion, apparent in almost all retinal layers, and it decreased the glial cell overactivation. Overall, our results confirm that PACAP1-38 given in the form of eye drops is a novel protective therapeutic approach to treat retinal diseases.

PACAP and VIP Inhibit HIF-1α-Mediated VEGF Expression in a Model of Diabetic Macular Edema

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) exert a protective role against retinal injuries, including diabetic macular edema (DME). The macular damage is induced by hyperglycemia, which damages vessels supplying blood to the retina and induces hypoxia. The microenvironmental changes stimulate the expression of hypoxia-inducible factors (HIFs), which promote the choroidal endothelial cell transmigration across the retinal pigmented epithelium (RPE) into neurosensory retina, where they proliferate into new vessels under stimulation of the vascular endothelial growth factor (VEGF). In the present study, we have investigated whether PACAP and VIP prevent retinal damage by modulating the expression of HIFs, VEGF, and its receptors. In accord to our hypothesis, we have shown that both peptides are able to significantly reduce HIF-1α and increase HIF-3α expression in ARPE-19 cells exposed to hyperglycemic/hypoxic insult. This effect is also related to a reduction of VEGF and its receptors expression. Moreover, both peptides also reduce the activation of p38 mitogen-activated protein kinase (MAPK), a pro-apoptotic signaling pathway, which is activated by VEGFR-1 and 2 receptors. In conclusion, our study has further elucidated the protective role performed by PACAP and VIP, against the harmful combined effect of hyperglycemia/hypoxia characterizing the DME microenvironment. J. Cell. Physiol. 232: 1209-1215, 2017. © 2016 Wiley Periodicals, Inc.

VIP Family Members Prevent Outer Blood Retinal Barrier Damage in a Model of Diabetic Macular Edema

Diabetic macular edema (DME), characterized by an increase of thickness in the eye macular area, is due to breakdown of the blood-retinal barrier (BRB). Hypoxia plays a key role in the progression of this pathology by activating the hypoxia-inducible factors. In the last years, various studies have put their attention on the role of pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) in retinal dysfunction. However, until now, no study has investigated their protective role against the harmful combined effect of both hyperglycemia and hypoxia on outer BRB. Therefore, in the present study, we have analyzed the role of these peptides on permeability, restoration of tight junctions expression and inhibition of hyperglycemia/hypoxia-induced apoptosis, in an experimental in vitro model of outer BRB. Our results have demonstrated that the peptides' treatment have restored the integrity of outer BRB induced by cell exposure to hyperglycemia/hypoxia. Their effect is mediated through the activation of phosphoinositide 3 kinase (PI3K)/Akt and mammalian mitogen activated protein kinase/Erk kinase (MAPK/ERK) signaling pathways. In conclusion, our study further clarifies the mechanism through which PACAP and VIP perform the beneficial effect on retinal damage induced by hyperglycemic/hypoxic insult, responsible of DME progression. J. Cell. Physiol. 232: 1079-1085, 2017. © 2016 Wiley Periodicals, Inc.

The Protective Role of PAC1-Receptor Agonist Maxadilan in BCCAO-Induced Retinal Degeneration

A number of studies have proven that pituitary adenylate cyclase activating polypeptide (PACAP) is protective in neurodegenerative diseases. Permanent bilateral common carotid artery occlusion (BCCAO) causes severe degeneration in the rat retina. In our previous studies, protective effects were observed with PACAP1-38, PACAP1-27, and VIP but not with their related peptides, glucagon, or secretin in BCCAO. All three PACAP receptors (PAC1, VPAC1, VPAC2) appear in the retina. Molecular and immunohistochemical analysis demonstrated that the retinoprotective effects are most probably mainly mediated by the PAC1 receptor. The aim of the present study was to investigate the retinoprotective effects of a selective PAC1-receptor agonist maxadilan in BCCAO-induced retinopathy. Wistar rats were used in the experiment. After performing BCCAO, the right eye was treated with intravitreal maxadilan (0.1 or 1 μM), while the left eye was injected with vehicle. Sham-operated rats received the same treatment. Two weeks after the operation, retinas were processed for standard morphometric and molecular analysis. Intravitreal injection of 0.1 or 1 μM maxadilan caused significant protection in the thickness of most retinal layers and the number of cells in the GCL compared to the BCCAO-operated eyes. In addition, 1 μM maxadilan application was more effective than 0.1 μM maxadilan treatment in the ONL, INL, IPL, and the entire retina (OLM-ILM). Maxadilan treatment significantly decreased cytokine expression (CINC-1, IL-1α, and L-selectin) in ischemia. In summary, our histological and molecular analysis showed that maxadilan, a selective PAC1 receptor agonist, has a protective role in BCCAO-induced retinal degeneration, further supporting the role of PAC1 receptor conveying the retinoprotective effects of PACAP.

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.

Pituitary Adenylate Cyclase Activating Polypeptide, A Potential Therapeutic Agent for Diabetic Retinopathy in Rats: Focus on the Vertical Information Processing Pathway

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide that has been shown to exert protective effects in different neuronal injuries, such as retinal degenerations. Diabetic retinopathy (DR), the most common complication of diabetes, affects the microvasculature and neuronal architecture of the retina. We have proven earlier that PACAP is also protective in a rat model of DR. In this study, streptozotocin-induced DR was treated with intravitreal PACAP administration in order to further analyze the synaptic structure and proteins of PACAP-treated diabetic retinas, primarily in the vertical information processing pathway. Streptozotocin-treated Wistar rats received intravitreal PACAP injection three times into the right eye 2 weeks after the induction of diabetes. Morphological and molecular biological (qRT-PCR; Western blot) methods were used to analyze retinal synapses (ribbons, conventional) and related structures. Electron microscopic analysis revealed that retinal pigment epithelium, the ribbon synapses and other synaptic profiles suffered alterations in diabetes. However, in PACAP-treated diabetic retinas more bipolar ribbon synapses were found intact in the inner plexiform layer than in DR animals. The ribbon synapse was marked with C-terminal binding protein 2/Bassoon and formed horseshoe-shape ribbons, which were more retained in PACAP-treated diabetic retinas than in DR rats. These results are supported by molecular biological data. The selective degeneration of related structures such as bipolar and ganglion cells could be ameliorated by PACAP treatment. In summary, intravitreal administration of PACAP may have therapeutic potential in streptozotocin-induced DR through maintaining synapse integrity in the vertical pathway.

Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) Pathway Is Induced by Mechanical Load and Reduces the Activity of Hedgehog Signaling in Chondrogenic Micromass Cell Cultures

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurohormone exerting protective function during various stress conditions either in mature or developing tissues. Previously we proved the presence of PACAP signaling elements in chicken limb bud-derived chondrogenic cells in micromass cell cultures. Since no data can be found if PACAP signaling is playing any role during mechanical stress in any tissues, we aimed to investigate its contribution in mechanotransduction during chondrogenesis. Expressions of the mRNAs of PACAP and its major receptor, PAC1 increased, while that of other receptors, VPAC1, VPAC2 decreased upon mechanical stimulus. Mechanical load enhanced the expression of collagen type X, a marker of hypertrophic differentiation of chondrocytes and PACAP addition attenuated this elevation. Moreover, exogenous PACAP also prevented the mechanical load evoked activation of hedgehog signaling: protein levels of Sonic and Indian Hedgehogs and Gli1 transcription factor were lowered while expressions of Gli2 and Gli3 were elevated by PACAP application during mechanical load. Our results suggest that mechanical load activates PACAP signaling and exogenous PACAP acts against the hypertrophy inducing effect of mechanical load.

PACAP and VIP signaling in chondrogenesis and osteogenesis

Skeletal development is a complex process regulated by multifactorial signaling cascades that govern proper tissue specific cell differentiation and matrix production. The influence of certain regulatory peptides on cartilage or bone development can be predicted but are not widely studied. In this review, we aimed to assemble and overview those signaling pathways which are modulated by PACAP and VIP neuropeptides and are involved in cartilage and bone formation. We discuss recent experimental data suggesting broad spectrum functions of these neuropeptides in osteogenic and chondrogenic differentiation, including the canonical downstream targets of PACAP and VIP receptors, PKA or MAPK pathways, which are key regulators of chondro- and osteogenesis. Recent experimental data support the hypothesis that PACAP is a positive regulator of chondrogenesis, while VIP has been reported playing an important role in the inflammatory reactions of surrounding joint tissues. Regulatory function of PACAP and VIP in bone development has also been proved, although the source of the peptides is not obvious. Crosstalk and collateral connections of the discussed signaling mechanisms make the system complicated and may obscure the pure effects of VIP and PACAP. Chondro-protective properties of PACAP during oxidative stress observed in our experiments indicate a possible therapeutic application of this neuropeptide.

Dysregulation of neurotrophic and inflammatory systems accompanied by decreased CREB signaling in ischemic rat retina

Although permanent bilateral common carotid artery occlusion (2VO) has been demonstrated to induce retinal injury, there is still a lack of systematic research on the complex processing of retinal degeneration. In the present study, time-dependent (at three, 14, 60 days after 2VO surgery) changes of neurotrophic and inflammatory systems, as well as cAMP-responsive element binding protein (CREB) signaling, which has been previously reported to effectively regulate these two systems, were evaluated. First, a morphological study confirmed that 2VO surgery progressively induced severe inner retinal degeneration and down-regulation of synaptic proteins, PSD95 and synaptophysin. The mRNA or protein levels of neurotrophic factors (NGF, BDNF, NT-3 and GDNF) and their receptors (TrkA, TrkB and TrkC) showed marked and persistent down-regulation in the rat retina since three days after 2VO surgery, whereas the gene transcription levels of CNTF were increased and p75(NTR) mRNA levels remained unchanged. In contrast to inner retinal degeneration, retinal Müller cells displayed rapid and prolonged activation since three days after 2VO lesion, whereas the microglia cell number, and TNF-α and IL-1β levels showed a robust increase with a maximal effect at three days and returned to levels that were slightly over baseline at 14 and 60 days after 2VO lesion. Interestingly, the gene expression levels of iNOS significantly decreased in the rat retina at both three and 14 days after 2VO surgery. Finally, as we hypothesized, remarkable reduction of CREB and extracellular signal-regulated kinase (ERK) phosphorylation levels were observed in the rat retina at three days after 2VO surgery. Thus, for the first time, our study demonstrated that chronic ischemia induced long-term aberrant CREB signaling and time-dependent progressive dysregulation of neurotrophic and inflammatory systems in the retina, which may provide important clues for a better understanding of the pathogenesis of retinal ischemic damage.

Investigation of PACAP Fragments and Related Peptides in Chronic Retinal Hypoperfusion

Pituitary adenylate cyclase activating polypeptide (PACAP) has neuroprotective effects in different neuronal and retinal injuries. Retinal ischemia can be effectively modelled by permanent bilateral common carotid artery occlusion (BCCAO), which causes chronic hypoperfusion-induced degeneration in the entire rat retina. The retinoprotective effect of PACAP 1-38 and VIP is well-established in ischemic retinopathy. However, little is known about the effects of related peptides and PACAP fragments in ischemic retinopathy. The aim of the present study was to investigate the potential retinoprotective effects of different PACAP fragments (PACAP 4-13, 4-22, 6-10, 6-15, 11-15, and 20-31) and related peptides (secretin, glucagon) in BCCAO-induced ischemic retinopathy. Wistar rats (3-4 months old) were used in the experiment. After performing BCCAO, the right eyes of the animals were treated with PACAP fragments or related peptides intravitreal (100 pM), while the left eyes were injected with saline serving as control eyes. Sham-operated (without BCCAO) rats received the same treatment. Routine histology was performed 2 weeks after the surgery; cells were counted and the thickness of retinal layers was compared. Our results revealed significant neuroprotection by PACAP 1-38 but did not reveal retinoprotective effect of the PACAP fragments or related peptides. These results suggest that PACAP 1-38 has the greatest efficacy in ischemic retinopathy.

PACAP application improves functional outcome of chronic retinal ischemic injury in rats-evidence from electroretinographic measurements

Retinoprotective effects of pituitary adenylate cyclase activating polypeptide (PACAP) are well-known and have been demonstrated in various pathological conditions, such as diabetic retinopathy, excitotoxic retinal injury, UV light-induced degeneration, and ischemic retinal lesion. The neuronal degeneration observed in the different retinal layers under the above pathological conditions can be successfully decreased by PACAP; however, whether this morphological improvement is also reflected in functional amelioration remains unknown. Therefore, our purpose was to investigate the protective effect of PACAP on the rat retina after bilateral common carotid artery occlusion (BCCAO) with electroretinography (ERG) to parallel the functional data with the previous morphological and neurochemical observations. Control eyes received saline treatment while PACAP was injected into the vitreous space of the other eye immediately after the induction of ischemia. Retinal damage and protective effects of PACAP were quantified by the changes in the wave forms and amplitudes. On postoperative days 2 and 14, several parameters were assessed with special attention to the changes of b wave. The results confirm that the previously described morphological protection induced by PACAP treatment is reflected in functional improvement in ischemic retinal lesions.

Neuropeptides, trophic factors, and other substances providing morphofunctional and metabolic protection in experimental models of diabetic retinopathy

Vision is the most important sensory modality for many species, including humans. Damage to the retina results in vision loss or even blindness. One of the most serious complications of diabetes, a disease that has seen a worldwide increase in prevalence, is diabetic retinopathy. This condition stems from consequences of pathological metabolism and develops in 75% of patients with type 1 and 50% with type 2 diabetes. The development of novel protective drugs is essential. In this review we provide a description of the disease and conclude that type 1 diabetes and type 2 diabetes lead to the same retinopathy. We evaluate existing experimental models and recent developments in finding effective compounds against this disorder. In our opinion, the best models are the long-term streptozotocin-induced diabetes and Otsuka Long-Evans Tokushima Fatty and spontaneously diabetic Torii rats, while the most promising substances are topically administered somatostatin and pigment epithelium-derived factor analogs, antivasculogenic substances, and systemic antioxidants. Future drug development should focus on these.

Effect of PACAP treatment on kidney morphology and cytokine expression in rat diabetic nephropathy

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide, exerting diverse effects. One of its frequently examined functions is cell protection, which is achieved mainly via inhibiting apoptotic, inflammatory and oxidative processes. All its three receptors (PAC1, VPAC1, VPAC2) are expressed in the kidney and PACAP has been shown to have protective effects against different renal pathologies. Diabetic nephropathy is the leading cause of end stage renal disease. The aim of the present study was to investigate the possible ameliorative effect of PACAP in streptozotocin-induced diabetic nephropathy and to evaluate its anti-inflammatory effect in this model. Diabetes was induced by a single intravenous injection of streptozotocin (65 mg/kg) in male Wistar rats. PACAP-treated animals were administered ip. 20 μg PACAP every second day, while untreated animals were given vehicle. Kidneys were removed after 8-weeks survival. Besides the complex histological analysis (glomerular PAS positive area/glomerulus area, tubular damage, arteriolar hyalinosis), expression of several cytokines was evaluated by cytokine array and Luminex assay. Histological analysis revealed severe diabetic changes in kidneys of control diabetic animals (glomerular PAS-positive area expansion, tubular damage, Armanni-Ebstein phenomenon). PACAP treatment significantly diminished the damage. Diabetic kidneys showed significant cytokine activation compared to their healthy controls. PACAP was effective in downregulation of several cytokines including CINC-1, TIMP-1, LIX, MIG, s-ICAM. To conclude, PACAP is effective in ameliorating diabetic nephropathy at least partly through its well-known anti-inflammatory effect. These results raise the opportunity for the use of PACAP as a possible therapeutic or preventive method in treating the complications of diabetes.

Distribution and protective function of pituitary adenylate cyclase-activating polypeptide in the retina

Pituitary adenylate cyclase-activating polypeptide (PACAP), which is found in 27- or 38-amino acid forms, belongs to the VIP/glucagon/secretin family. PACAP and its three receptor subtypes are expressed in neural tissues, with PACAP known to exert a protective effect against several types of neural damage. The retina is considered to be part of the central nervous system, and retinopathy is a common cause of profound and intractable loss of vision. This review will examine the expression and morphological distribution of PACAP and its receptors in the retina, and will summarize the current state of knowledge regarding the protective effect of PACAP against different kinds of retinal damage, such as that identified in association with diabetes, ultraviolet light, hypoxia, optic nerve transection, and toxins. This article will also address PACAP-mediated protective pathways involving retinal glial cells.