PACAP Modulates Expression of Hypoxia-Inducible Factors in Streptozotocin-Induced Diabetic Rat Retina

The impact of vitamin D on the etiopathogenesis and the progression of type 1 and type 2 diabetes in children and adults

Diabetes is a common chronic metabolic disease with complex causes and pathogenesis. As an immunomodulator, vitamin D has recently become a research hotspot in the occurrence and development of diabetes and its complications. Many studies have shown that vitamin D can reduce the occurrence of diabetes and delay the progression of diabetes complications, and vitamin D can reduce oxidative stress, inhibit iron apoptosis, promote Ca2+ influx, promote insulin secretion, and reduce insulin resistance. Therefore, the prevention and correction of vitamin D deficiency is very necessary for diabetic patients, but further research is needed to confirm what serum levels of vitamin D3 are maintained in the body. This article provides a brief review of the relationship between vitamin D and diabetes, including its acute and chronic complications.

PACAP-ADNP axis prevents outer retinal barrier breakdown and choroidal neovascularization by interfering with VEGF secreted from retinal pigmented epitelium cells

During diabetic retinopathy (DR) progression, the retina undergoes various metabolic changes, including hypoxia-signalling cascade induction in the cells of retinal pigmented epithelium (RPE). The overexpression of hypoxic inducible factors causes transcription of many target genes including vascular endothelial growth factor (VEGF). The RPE cells form the outer blood retinal barrier (oBRB), a specialized structure that regulates ions and metabolites flux into the retina to maintain a suitable quality of its extracellular microenvironment. VEGF worsens retinal condition since its secretion from the basolateral compartment of RPE cells compromises the barrier's integrity and induces choroidal neovascularization. In this work, we hypothesized that PACAP prevents the damage to oBRB and controls choroidal neovascularization through the induction of ADNP. Firstly, we demonstrated that ADNP is expressed in Streptozotocin (STZ)-induced diabetic animals. To validate our hypothesis, we cultured endothelial cells (H5V) forming vessels-like structures, in a conditioned medium (CM) derived from ARPE-19 cells exposed to hyperglycaemic/hypoxic insult, containing a known VEGF concentration. The involvement of PACAP-ADNP axis on oBRB integrity was evaluated through the measurement of trans-epithelial-electrical resistance and permeability assay performed on ARPE cell monolayer cultured in CM and by analysing the expression of two tight junction forming proteins, ZO1 and occludin. By culturing H5V in CM, we demonstrated that PACAP-ADNP axis counteracted vessels-like structures formation promoted by VEGF. In conclusion, the results suggested a primary role of PACAP/ADNP axis in preventing oBRB damage and in controlling aberrant choroidal neovascularization induced by VEGF secreted from RPE cells exposed to hyperglycaemia/hypoxic insult in DR.

The β3 adrenoceptor in proliferative retinopathies: "Cinderella" steps out of its family shadow

In the retina, hypoxic condition leads to overgrowing leaky vessels resulting in altered metabolic supply that may cause impaired visual function. Hypoxia-inducible factor-1 (HIF-1) is a central regulator of the retinal response to hypoxia by activating the transcription of numerous target genes, including vascular endothelium growth factor, which acts as a major player in retinal angiogenesis. In the present review, oxygen urge by the retina and its oxygen sensing systems including HIF-1 are discussed in respect to the role of the beta-adrenergic receptors (β-ARs) and their pharmacologic manipulation in the vascular response to hypoxia. In the β-AR family, β1- and β2-AR have long been attracting attention because their pharmacology is intensely used for human health, while β3-AR, the third and last cloned receptor is no longer increasingly emerging as an attractive target for drug discovery. Here, β3-AR, a main character in several organs including the heart, the adipose tissue and the urinary bladder, but so far a supporting actor in the retina, has been thoroughly examined in respect to its function in retinal response to hypoxia. In particular, its oxygen dependence has been taken as a key indicator of β3-AR involvement in HIF-1-mediated responses to oxygen. Hence, the possibility of β3-AR transcription by HIF-1 has been discussed from early circumstantial evidence to the recent demonstration that β3-AR acts as a novel HIF-1 target gene by playing like a putative intermediary between oxygen levels and retinal vessel proliferation. Thus, targeting β3-AR may implement the therapeutic armamentarium against neovascular pathologies of the eye.

Relevance of Peptide Homeostasis in Metabolic Retinal Degenerative Disorders: Curative Potential in Genetically Modified Mice

The mammalian retina contains approximately 30 neuropeptides that are synthetized by different neuronal cell populations, glia, and the pigmented epithelium. The presence of these neuropeptides leaves a mark on normal retinal molecular processes and physiology, and they are also crucial in fighting various pathologies (e.g., diabetic retinopathy, ischemia, age-related pathologies, glaucoma) because of their protective abilities. Retinal pathologies of different origin (metabolic, genetic) are extensively investigated by genetically manipulated in vivo mouse models that help us gain a better understanding of the molecular background of these pathomechanisms. These models offer opportunities to manipulate gene expression in different cell types to help reveal their roles in the preservation of retinal health or identify malfunction during diseases. In order to assess the current status of transgenic technologies available, we have conducted a literature survey focused on retinal disorders of metabolic origin, zooming in on the role of retinal neuropeptides in diabetic retinopathy and ischemia. First, we identified those neuropeptides that are most relevant to retinal pathologies in humans and the two clinically most relevant models, mice and rats. Then we continued our analysis with metabolic disorders, examining neuropeptide-related pathways leading to systemic or cellular damage and rescue. Last but not least, we reviewed the available literature on genetically modified mouse strains to understand how the manipulation of a single element of any given pathway (e.g., signal molecules, receptors, intracellular signaling pathways) could lead either to the worsening of disease conditions or, more frequently, to substantial improvements in retinal health. Most attention was given to studies which reported successful intervention against specific disorders. For these experiments, a detailed evaluation will be given and the possible role of converging intracellular pathways will be discussed. Using these converging intracellular pathways, curative effects of peptides could potentially be utilized in fighting metabolic retinal disorders.

A Broad Overview on Pituitary Adenylate Cyclase-Activating Polypeptide Role in the Eye: Focus on Its Repairing Effect in Cornea

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) is a neuropeptide with widespread distribution throughout the central and peripheral nervous system as well as in many other peripheral organs. It plays cytoprotective effects mediated mainly through the activation of specific receptors. PACAP is known to play pleiotropic effects on the eye, including the cornea, protecting it against different types of insult. This review firstly provides an overview of the anatomy of the cornea and summarizes data present in literature about PACAP’s role in the eye and, in particular, in the cornea, either in physiological or pathological conditions.

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.

Modulatory role of PACAP and VIP on HIFs expression in lung adenocarcinoma

Lung adenocarcinoma is the most frequent form of non-small cell lung cancer. Inside the tumor mass, uncontrolled cell proliferation generates hypoxic areas leading to activation of hypoxia-inducible factors (HIFs) responsible for neovascularization and tumor metastasis. Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are two neuropeptides widely distributed in respiratory organs. Previous studies have demonstrated that these peptides interfere with hypoxic pathways in various diseases, including tumors. However, their modulatory role in HIFs expression in lung adenocarcinomas has not yet been evaluated. In the present paper, we detected the expression profile of PACAP, VIP and related receptors in healthy and adenocarcinoma human lung tissue. To characterize peptides' modulatory effects on HIFs expression, we also exposed A549 lung adenocarcinoma cells and human normal bronchial epithelial BEAS-2B cells to microenvironmental hypoxia by treating them with deferoxamine (DFX). The results showed that PACAP and VIP significantly reduced HIF-1α and HIF-2α levels in both cell lines following hypoxic stress. The HIF-3α expression profile was related to cellular phenotype as it was lower in BEAS-2B and higher in A549 cells under low oxygen tension. In lung adenocarcinoma cells, peptide treatment restored HIF-3 α expression to control levels. These results suggest that endogenous PACAP and VIP exert controversial roles in cellular hypoxic microenvironments depending on the pathophysiological conditions of the lung tissue.

Protective Effects of PACAP in a Rat Model of Diabetic Neuropathy

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide with a widespread occurrence and diverse effects. PACAP has well-documented neuro- and cytoprotective effects, proven in numerous studies. Among others, PACAP is protective in models of diabetes-associated diseases, such as diabetic nephropathy and retinopathy. As the neuropeptide has strong neurotrophic and neuroprotective actions, we aimed at investigating the effects of PACAP in a rat model of streptozotocin-induced diabetic neuropathy, another common complication of diabetes. Rats were treated with PACAP1-38 every second day for 8 weeks starting simultaneously with the streptozotocin injection. Nerve fiber morphology was examined with electron microscopy, chronic neuronal activation in pain processing centers was studied with FosB immunohistochemistry, and functionality was assessed by determining the mechanical nociceptive threshold. PACAP treatment did not alter body weight or blood glucose levels during the 8-week observation period. However, PACAP attenuated the mechanical hyperalgesia, compared to vehicle-treated diabetic animals, and it markedly reduced the morphological signs characteristic for neuropathy: axon–myelin separation, mitochondrial fission, unmyelinated fiber atrophy, and basement membrane thickening of endoneurial vessels. Furthermore, PACAP attenuated the increase in FosB immunoreactivity in the dorsal spinal horn and periaqueductal grey matter. Our results show that PACAP is a promising therapeutic agent in diabetes-associated complications, including diabetic neuropathy.

Effect of PACAP on Hypoxia-Induced Angiogenesis and Epithelial-Mesenchymal Transition in Glioblastoma

Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts different effects in various human cancer. In glioblastoma (GBM), PACAP has been shown to interfere with the hypoxic micro-environment through the modulation of hypoxia-inducible factors via PI3K/AKT and MAPK/ERK pathways inhibition. Considering that hypoxic tumor micro-environment is strictly linked to angiogenesis and Epithelial–Mesenchymal transition (EMT), in the present study, we have investigated the ability of PACAP to regulate these events. Results have demonstrated that PACAP and its related receptor, PAC1R, are expressed in hypoxic area of human GBM colocalizing either in epithelial or mesenchymal cells. By using an in vitro model of GBM cells, we have observed that PACAP interferes with hypoxic/angiogenic pathway by reducing vascular-endothelial growth factor (VEGF) release and inhibiting formation of vessel-like structures in H5V endothelial cells cultured with GBM-conditioned medium. Moreover, PACAP treatment decreased the expression of mesenchymal markers such as vimentin, matrix metalloproteinase 2 (MMP-2) and matrix metalloproteinase 9 (MMP-9) as well as CD44 in GBM cells by affecting their invasiveness. In conclusion, our study provides new insights regarding the multimodal role of PACAP in GBM malignancy.

Photoreceptor cells and RPE contribute to the development of diabetic retinopathy

Diabetic retinopathy (DR) is a leading cause of blindness. It has long been regarded as vascular disease, but work in the past years has shown abnormalities also in the neural retina. Unfortunately, research on the vascular and neural abnormalities have remained largely separate, instead of being integrated into a comprehensive view of DR that includes both the neural and vascular components. Recent evidence suggests that the most predominant neural cell in the retina (photoreceptors) and the adjacent retinal pigment epithelium (RPE) play an important role in the development of vascular lesions characteristic of DR. This review summarizes evidence that the outer retina is altered in diabetes, and that photoreceptors and RPE contribute to retinal vascular alterations in the early stages of the retinopathy. The possible molecular mechanisms by which cells of the outer retina might contribute to retinal vascular damage in diabetes also are discussed. Diabetes-induced alterations in the outer retina represent a novel therapeutic target to inhibit DR.

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.

Diabetes Affects the Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP)-Like Immunoreactive Enteric Neurons in the Porcine Digestive Tract

Diabetic gastroenteropathy is a common complication, which develops in patients with long-term diabetes. The pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide known for its cytoprotective properties and plays an important role in neuronal development, neuromodulation and neuroprotection. The present study was designed to elucidate, for the first time, the impact of prolonged hyperglycaemia conditions on a population of PACAP-like immunoreactive neurons in selected parts of the porcine gastrointestinal tract. The experiment was conducted on 10 juvenile female pigs assigned to two experimental groups: The DM group (pigs with streptozocin-induced diabetes) and the C group (control pigs). Diabetes conditions were induced by a single intravenous injection of streptozocin. Six weeks after the induction of diabetes, all animals were euthanised and further collected, and fixed fragments of the stomach, duodenum, jejunum, ileum and descending colon were processed using the routine double-labelling immunofluorescence technique. Streptozotocin-induced hyperglycaemia caused a significant increase in the population of PACAP-containing enteric neurons in the porcine stomach, small intestines and descending colon. The recorded changes may result from the direct toxic effect of hyperglycaemia on the ENS neurons, oxidative stress or inflammatory conditions accompanying hyperglycaemia and suggest that PACAP is involved in regulatory processes of the GIT function in the course of diabetes.

Attenuation of High Glucose-Induced Damage in RPE Cells through p38 MAPK Signaling Pathway Inhibition

This study aimed to investigate the high glucose damage on human retinal pigment epithelial (RPE) cells, the role of p38 MAPK signaling pathway and how dimethyl fumarate can regulate that. We carried out in vitro studies on ARPE-19 cells exposed to physiological and high glucose (HG) conditions, to evaluate the effects of DMF on cell viability, apoptosis, and expression of inflammatory and angiogenic biomarkers such as COX-2, iNOS, IL-1β, and VEGF. Our data have demonstrated that DMF treatment attenuated HG-induced apoptosis, as confirmed by reduction of BAX/Bcl-2 ratio. Furthermore, in RPE cells exposed to HG we observed a significant increase of iNOS, COX-2, and IL-1β expression, that was reverted by DMF treatment. Moreover, DMF reduced the VEGF levels elicited by HG, inhibiting p38 MAPK signaling pathway. The present study demonstrated that DMF provides a remarkable protection against high glucose-induced damage in RPE cells through p38 MAPK inhibition and the subsequent down-regulation of VEGF levels, suggesting that DMF is a small molecule that represents a good candidate for diabetic retinopathy treatment and warrants further in vivo and clinical evaluation.

Stabilization of HIF-1α in Human Retinal Endothelial Cells Modulates Expression of miRNAs and Proangiogenic Growth Factors

Retinal hypoxia is one of the causative factors of diabetic retinopathy and is also one of the triggers of VEGF release. We hypothesized that specific dysregulated miRNAs in diabetic retinopathy could be linked to hypoxia-induced damage in human retinal endothelial cells (HRECs). We investigated in HRECs the effects of chemical (CoCl₂) hypoxia on the expression of HIF-1α, VEGF, PlGF, and of a focused set of miRNAs. We found that miR-20a-5p, miR-20b-5p, miR-27a-3p, miR-27b-3p, miR-206-3p, miR-381-3p correlated also with expression of TGFβ signaling pathway genes in HRECs, challenged with chemical hypoxic stimuli. In conclusion, our data suggest that retinal angiogenesis would be promoted, at least under HIF-1α activation, by upregulation of PlGF and other factors such as miRNAs, VEGFA, and TGFβ1.

PACAP Modulates the Autophagy Process in an In Vitro Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease of complex etiology leading to motor neuron degeneration. Many gene alterations cause this pathology, including mutation in Cu, Zn superoxide dismutase (SOD1), which leads to its gain of function. Mutant SOD1 proteins are prone to aberrant misfolding and create aggregates that impair autophagy. The hypoxic stress is strictly linked to the disease progression since it induces uncontrolled autophagy activation and the consequent high rates of cell death. Previously, we showed that pituitary adenylate cyclase-activating polypeptide (PACAP) exerts neurotrophic activity in cultured mSOD1 motor neurons exposed to serum deprivation. To date, no studies have examined whether the protective effect of PACAP on mSOD1 cells exposed to hypoxic insult is mediated through the regulation of the autophagy process. In the present study, we used the neuroblastoma-spinal cord-34 (NSC-34) cell line, stably expressing human wild type or mutant SOD1 G93A, to represent a well characterized in vitro model of a familial form of ALS. These cells were exposed to 100-µM desferrioxamine mesylate salt for 24h, to mimic the hypoxic stress affecting motor neurons during the disease progression. Our results showed that PACAP treatment significantly reduced cell death and hypoxia-induced mSOD1 accumulation by modulating the autophagy process in G93A motor neurons, as revealed by the decreased LC3II and the increased p62 levels, two autophagy indicators. These results were also confirmed by evaluating the vacuole formation detected through light chain 3 (LC3) immunofluorescence. Furthermore, the PACAP effects on autophagy seem to be mediated through the activation of the MAPK/ERK signaling pathway. Overall, our data demonstrated that PACAP exerts an ameliorative effect on the mSOD1 motor neuron viability by modulating a hypoxia-induced autophagy process through activation of MAPK/ERK signaling cascade.

Protection by vitamin D against high-glucose-induced damage in retinal pigment epithelial cells

Diabetic retinopathy (DR) is a diabetes-associated complication characterized by irreversible deterioration of the microvessels within the retina, leading subsequently to severe retinal damage and vision loss. Vitamin D (VITD), a steroid hormone, plays multiple physiological functions in cellular homeostasis. Deficiency of VITD has been suggested to be associated with DR. To study the potential protective function of VITD in DR, high-glucose-treated ARPE-19 cells and STZ-induced diabetic mice were used as in vitro and in vivo models. The protective effects of VITD were assessed based on the changes of expression of antioxidant enzymes and cytokines in high-glucose-treated retinal pigment epithelial (RPE) cells and in the retina and RPE of diabetic and VITD-treated diabetic mice. The present study demonstrated that exposure to a high level of glucose caused upregulation of pro-inflammatory cytokines and a decrease in anti-oxidant enzyme expression in both in vitro and in vivo models. VITD treatment increased cell viability, reduced reactive oxygen species (ROS) production and caspase-3/7 activities in high-glucose-treated RPE cells. Our data suggest that VITD can protect the retina and RPE from high-glucose-induced oxidative damage and inflammation.

Novel indole derivatives targeting HuR-mRNA complex to counteract high glucose damage in retinal endothelial cells

The ELAVL1 (or human antigen R - HuR) RNA binding protein stabilizes the mRNA, with an AU-rich element, of several genes such as growth factors (i.e. VEGF) and inflammatory cytokines (i.e. TNFα). We hereby carried out a virtual screening campaign in order to identify and test novel HuR-mRNA disruptors. Best-scored compounds were tested in an in-vitro model of diabetic retinopathy, namely human retinal endothelial cells (HRECs) challenged with high-glucose levels (25 mM). HuR, VEGF and TNFα protein contents were evaluated by western-blot analysis in total cell lysates. VEGF and TNFα released from HRECs were measured in cell medium by ELISA. We found that two derivatives bearing indole moiety, VP12/14 and VP12/110, modulated HuR expression and decreased VEGF and TNF-α release by HREC exposed to high glucose (HG) levels. VP12/14 and VP12/110 inhibited VEGF and TNF-α release in HRECs challenged with high glucose levels, similarly to dihydrotanshinone (DHTS), a small molecule known to interfere with HuR- TNFα mRNA binding. The present findings demonstrated that VP12/14 and VP12/110 are innovative molecules with anti-inflammatory and anti-angiogenic properties, suggesting their potential use as novel candidates for treatment of diabetic retinopathy.

Protective effect of PACAP against ultraviolet B radiation-induced human corneal endothelial cell injury

The human cornea, a sophisticated example of natural engineering, is composed in the innermost layer by endothelial cells maintaining stromal hydration and clarity. Different types of insults, including ultraviolet (UV) radiations, can lead to corneal opacity due to their degenerative and limited proliferative capability. In our previous studies, we have shown the protective effects of pituitary adenylate cyclase activating polypeptide (PACAP) in human corneal endothelial cells (HCECs), after growth factors deprivation. The aim of the present work has been to investigate the effect of this peptide on UV-B-induced HCECs injury. The results have shown that UV-B irradiations induced apoptotic cells death and consequently alteration in human corneal endothelial barrier. We found that PACAP treatment significantly increased viability, trans-endothelial electrical resistance and tight junctions expression of HCECs exposed to UV-B insult. In conclusion, data have suggested that this peptide could have protective effect to preserve the physiological state of human corneal endothelium exposed to UV-B damage.

Relationships Between Neurodegeneration and Vascular Damage in Diabetic Retinopathy

Diabetic retinopathy (DR) is a common complication of diabetes and constitutes a major cause of vision impairment and blindness in the world. DR has long been described exclusively as a microvascular disease of the eye. However, in recent years, a growing interest has been focused on the contribution of neuroretinal degeneration to the pathogenesis of the disease, and there are observations suggesting that neuronal death in the early phases of DR may favor the development of microvascular abnormalities, followed by the full manifestation of the disease. However, the mediators that are involved in the crosslink between neurodegeneration and vascular changes have not yet been identified. According to our hypothesis, vascular endothelial growth factor (VEGF) could probably be the most important connecting link between the death of retinal neurons and the occurrence of microvascular lesions. Indeed, VEGF is known to play important neuroprotective actions; therefore, in the early phases of DR, it may be released in response to neuronal suffering, and it would act as a double-edged weapon inducing both neuroprotective and vasoactive effects. If this hypothesis is correct, then any retinal stress causing neuronal damage should be accompanied by VEGF upregulation and by vascular changes. Similarly, any compound with neuroprotective properties should also induce VEGF downregulation and amelioration of the vascular lesions. In this review, we searched for a correlation between neurodegeneration and vasculopathy in animal models of retinal diseases, examining the effects of different neuroprotective substances, ranging from nutraceuticals to antioxidants to neuropeptides and others and showing that reducing neuronal suffering also prevents overexpression of VEGF and vascular complications. Taken together, the reviewed evidence highlights the crucial role played by mediators such as VEGF in the relationship between retinal neuronal damage and vascular alterations and suggests that the use of neuroprotective substances could be an efficient strategy to prevent the onset or to retard the development of DR.

The PACAP-derived peptide MPAPO facilitates corneal wound healing by promoting corneal epithelial cell proliferation and trigeminal ganglion cell axon regeneration

It is well known that the cornea plays an important role in providing protection to the eye, but it is fragile and vulnerable. To clarify the biological effects and molecular mechanisms of the pituitary adenylate cyclase activating polypeptide (PACAP)-derived peptide MPAPO (named MPAPO) to promote corneal wound healing, we applied a mechanical method to establish a corneal injury model and analyzed the repair effects of MPAPO on corneal injury. MPAPO significantly promoted corneal wound repair in C57BL/6 mice. In addition, we established injury models of epithelial cells and trigeminal ganglion cells with H₂O₂. The results show that when the concentration of MPAPO is 1 μM, it can significantly promote the repair of injured corneal epithelial cells and the regeneration of trigeminal ganglion cell axons. MPAPO repairs epithelial cells through the promotion of GSK3β phosphorylation by binding to PAC1 and activating AKT. β-catenin escapes the phosphorylation of GSK3β and enters the nucleus to promote the expression of cyclin D1, accelerate cell cycle progression and promote cell proliferation. MPAPO promotes axonal regeneration by binding to the PAC1 receptor and activating adenylate cyclase activity, followed by the cAMP activation of protein kinase A activity and the promotion of CREB phosphorylation. Phosphorylated CREB promotes Bcl₂ expression and axonal regeneration. In conclusion, our data support the role of MPAPO to facilitate corneal wound healing by promoting corneal epithelial cell proliferation and trigeminal ganglion cell axon regeneration.

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.

Neuroprotective Peptides in Retinal Disease

In the pathogenesis of many disorders, neuronal death plays a key role. It is now assumed that neurodegeneration is caused by multiple and somewhat converging/overlapping death mechanisms, and that neurons are sensitive to unique death styles. In this respect, major advances in the knowledge of different types, mechanisms, and roles of neurodegeneration are crucial to restore the neuronal functions involved in neuroprotection. Several novel concepts have emerged recently, suggesting that the modulation of the neuropeptide system may provide an entirely new set of pharmacological approaches. Neuropeptides and their receptors are expressed widely in mammalian retinas, where they exert neuromodulatory functions including the processing of visual information. In multiple models of retinal diseases, different peptidergic substances play neuroprotective actions. Herein, we describe the novel advances on the protective roles of neuropeptides in the retina. In particular, we focus on the mechanisms by which peptides affect neuronal death/survival and the vascular lesions commonly associated with retinal neurodegenerative pathologies. The goal is to highlight the therapeutic potential of neuropeptide systems as neuroprotectants in retinal diseases.

PACAP through EGFR transactivation preserves human corneal endothelial integrity

The corneal endothelium is composed of a single hexagonal-shaped cells layer adherent to the Descemet's membrane. The primary function of these cells is maintaining of tissue clarity by regulating its hydration. Trauma, aging or other pathologies cause their loss, counterbalanced by enlargement of survived cells unable to guarantee an efficient fluid pumping to and from the stroma. Regenerative medicine using human corneal endothelial cells (HCECs) isolated from peripheral corneal-scleral tissue of a donor could be an attractive solution, overcoming transplantation problems. In a previous study, we have demonstrated that HCECs treatment with pituitary adenylate cyclase-activating polypeptide (PACAP) following growth factors deprivation prevents their degeneration. However, the molecular mechanism mediating this effect has not been clarified, yet. Here, we have shown for the first time the expression of PACAP and its receptor (PAC1R) in human corneal endothelium and demonstrated that this peptide, selectively binding to PAC1R, induces epidermal growth factor receptor (EGFR) phosphorylation and the MAPK/ERK1/2 signaling pathway activation. In conclusion, our data have suggested that PACAP could represent an important trophic factor in maintaining human corneal endothelial integrity through EGFR transactivation. Therefore, PACAP, as well as epidermal growth factor and fibroblast growth factor, could co-operate to guarantee tissue physiological functioning by supporting corneal endothelial barrier integrity.

Molecular mechanisms involved in the protective effect of pituitary adenylate cyclase-activating polypeptide in an in vitro model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of upper and lower motor neurons. Based on transcriptional profiles of motor cortex samples, in a previous work, we were able to classify two subgroups of sporadic ALS (SALS) patients, named SALS1 and SALS2. A further meta-analysis study has revealed sixteen drug targets commonly deregulated in SALS2 and superoxide dismutase 1 (SOD1) G93A mice. The identified candidate drug targets included pituitary adenylate cyclase-activating polypeptide (PACAP), epidermal growth factor receptor (EGFR) and matrix metallopeptidase-2 (MMP-2). By using a motor neuron-like hybrid cell line (NSC-34) expressing human SOD1 G93A as an in vitro model of ALS, here we investigated the functional correlation among these three genes. Our results have shown that PACAP increases cell viability following serum deprivation. This effect is induced through EGFR transactivation mediated by protein kinase A stimulation. Furthermore, EGFR phosphorylation activates mitogen-activated protein kinases/extracellular signal-regulated kinases 1 and 2 survival signaling pathway and increases MMP-2 expression, significantly reduced by serum starvation. These results suggest that a deeper characterization of mechanisms involved in PACAP/EGFR/MMP-2 axis activation in G93A SOD1 mutated neurons may allow identifying new targets for ALS therapy.

Adenosine A1 receptor: A neuroprotective target in light induced retinal degeneration

Light induced retinal degeneration (LIRD) is a useful model that resembles human retinal degenerative diseases. The modulation of adenosine A1 receptor is neuroprotective in different models of retinal injury. The aim of this work was to evaluate the potential neuroprotective effect of the modulation of A1 receptor in LIRD. The eyes of rats intravitreally injected with N6-cyclopentyladenosine (CPA), an A1 agonist, which were later subjected to continuous illumination (CI) for 24 h, showed retinas with a lower number of apoptotic nuclei and a decrease of Glial Fibrillary Acidic Protein (GFAP) immunoreactive area than controls. Lower levels of activated Caspase 3 and GFAP were demonstrated by Western Blot (WB) in treated animals. Also a decrease of iNOS, TNFα and GFAP mRNA was demonstrated by RT-PCR. A decrease of Iba 1⁺/MHC-II⁺ reactive microglial cells was shown by immunohistochemistry. Electroretinograms (ERG) showed higher amplitudes of a-wave, b-wave and oscillatory potentials after CI compared to controls. Conversely, the eyes of rats intravitreally injected with dipropylcyclopentylxanthine (DPCPX), an A1 antagonist, and subjected to CI for 24 h, showed retinas with a higher number of apoptotic nuclei and an increase of GFAP immunoreactive area compared to controls. Also, higher levels of activated Caspase 3 and GFAP were demonstrated by Western Blot. The mRNA levels of iNOS, nNOS and inflammatory cytokines (IL-1β and TNFα) were not modified by DPCPX treatment. An increase of Iba 1⁺/MHC-II⁺ reactive microglial cells was shown by immunohistochemistry. ERG showed that the amplitudes of a-wave, b-wave, and oscillatory potentials after CI were similar to control values. A single pharmacological intervention prior illumination stress was able to swing retinal fate in opposite directions: CPA was neuroprotective, while DPCPX worsened retinal damage. In summary, A1 receptor agonism is a plausible neuroprotective strategy in LIRD.

PACAP and VIP regulate hypoxia-inducible factors in neuroblastoma cells exposed to hypoxia

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are two related peptides acting as neurotransmitters/neuromodulators in central and peripheral nervous system. They are also involved in cancer showing a controversial role. Particulary, they are implicated in neuroblastoma differentiation (NB). This pediatric tumor can evolve to a malignant metastatic disease or spontaneously regress towards a benign form, known as ganglioneuroblastoma/ganglioneuroma. A negative hallmark of neoplasia progression is represented by hypoxic microenvironment. Low oxygen tension induces activation of hypoxia-inducible factors (HIFs) promoting cells proliferation and metastasis formation. Moreover, HIFs trigger vascular endothelial growth factor (VEGF) release favouring high-risk NB phenotype development. In the present work, we have investigated for the first time, if PACAP and VIP interfere with NB differentiation through modulation of hypoxic/angiogenic process. To this end, we analyzed their effect in malignant undifferentiated and all-trans retinoic acid (RA) differentiated SH-SY5Y cells, representing the benign form of this tumor. Our results have suggested tha both peptides, but predominantly VIP, induce NB differentiation into benign form by regulating HIFs, VEGF and VEGFRs expression and distribution. All these data give new insight regarding PACAP/VIP regulatory role in NB progression.

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.

Trophic effect of PACAP on human corneal endothelium

Cornea's posterior surface includes endothelium maintaining stromal hydration and clarity. Due to their limited proliferative capability, the loss of endothelial cells can outcome in permanent opacity. In the last years, different studies have demonstrated the protective effect of pituitary adenylate cyclase-activating polypeptide (PACAP) in different ocular diseases. However, its role on human corneal endothelial cells (HCECs) has not been investigated, yet. Here, we have developed a culture protocol to differentiate HCECs from donor's cornea. PACAP treatment prevented damage induced by growth factors deprivation of cells grown on transwell supports as revealed by TERR measurements. Moreover, this peptide significantly increased tight junction proteins expression by conferring resistance to endothelial barrier. This effect is also related to promotion of cell viability as demonstrated by MTT assay. Furthermore, PACAP stimulated repairing of corneal endothelium lesion as shown by wound healing analysis. In conclusion, our data suggest that this peptide could represent an important trophic factor in maintaining functionality of human corneal endothelium.

Inhibition of retinal ganglion cell apoptosis: regulation of mitochondrial function by PACAP

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an endogenous peptide with neuroprotective effects on retinal neurons, but the precise mechanism underlying these effects remains unknown. Considering the abundance of mitochondria in retinal ganglion cells (RGCs), we postulate that the protective effect of PACAP is associated with the regulation of mitochondrial function. RGC-5 cells were subjected to serum deprivation for 48 hours to induce apoptosis in the presence or absence of 100 nM PACAP. As revealed with the Cell Counting Kit-8 assay, PACAP at different concentrations significantly increased the viability of RGC-5 cells. PACAP also inhibited the excessive generation of reactive oxygen species in RGC-5 cells subjected to serum deprivation. We also showed by flow cytometry that PACAP inhibited serum deprivation-induced apoptosis in RGC-5 cells. The proportions of apoptotic cells and cells with mitochondria depolarization were significantly decreased with PACAP treatment. Western blot assays demonstrated that PACAP increased the levels of Bcl-2 and inhibited the compensatory increase of PAC1. Together, these data indicate protective effects of PACAP against serum deprivation-induced apoptosis in RGCs, and that the mechanism of this action is associated with maintaining mitochondrial function.

Regulation of Excitatory Amino Acid Transmission in the Retina: Studies on Neuroprotection

Excitotoxicity occurs in neurons due to the accumulation of excitatory amino acids such as glutamate in the synaptic and extrasynaptic locations. In the retina, excessive glutamate concentrations trigger a neurotoxic cascade involving several mechanisms, including the elevation of intracellular calcium (Ca²⁺⁾ and the activation of α-amino-3-hydroxy 5-methyl-4-iso-xazole-propionic acid/kainate (AMPA/KA) and N-methyl-d-aspartate (NMDA) receptors leading to retinal degeneration. Both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are present in the mammalian retina. Indeed, due to the abundant expression of GluRs, the mammalian retina is highly susceptible to excitotoxic neurodegeneration. Excitotoxicity has been postulated to present a common downstream mechanism for several stimuli, including hypoglycemia, hypoxia, ischemia, and chronic neurodegenerative diseases. Experimental approaches to the study of neuroprotection in the retina have utilized insults that trigger hypoxia, hypoglycemia, or excitotoxicity. Using these experimental approaches, the neuroprotective potential of GluR agents, including the NMDA receptor modulators (MK801, ifenprodil, memantine); AMPA/KA receptor antagonist (CNQX); Group II and III mGluR agonists (LY354740, quisqualate); and Ca²⁺-channel blockers (diltiazem, lomerizine, verapamil, ω-conotoxin), and others (pituitary adenylate cyclase activating polypeptide, neuropeptide Y, acetylcholine receptor agonists) have been elucidated. In addition to corroborating the exocytotic role of excitatory amino acids in retinal degeneration, these studies affirm that multiple mechanism/s contribute to the prevention of damage caused by excitotoxicity in the retina. Therefore, it is feasible that several pathways are involved in protecting the retina from toxic insults in ocular neurodegenerative conditions such as glaucoma and retinal ischemia. Furthermore, these experimental models are viable tools for evaluating therapeutic candidates in ocular neuropathies.

Matrix metalloproteinase 14 modulates diabetes and Alzheimer's disease cross-talk: a meta-analysis

Diabetes mellitus is associated with dementia, but whether diabetes is associated with Alzheimer's disease remains controversial. Alzheimer's disease is characterized by amyloid beta aggregation. We hypothesized that genes, involved in amyloid beta degradation, may be altered due to diabetes and thus participate in progression of Alzheimer's disease. Expression profiling of amyloid beta-degrading enzymes in streptozotocin-induced diabetic mice and their correlation with expression of amyloid precursor protein in hippocampus of Alzheimer's disease patients were accessed. We found that matrix metalloproteinase 14 decreased in brain but not in other tissues of streptozotocin-induced diabetic mice, and was negatively correlated with expression of amyloid precursor protein in hippocampus of Alzheimer's disease patients. These findings suggested matrix metalloproteinase 14 may link insulin-deficient diabetes to Alzheimer's disease.

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.

PACAP and PAC1R are differentially expressed in motor cortex of amyotrophic lateral sclerosis patients and support survival of iPSC-derived motor neurons

Amyotrophic lateral sclerosis (ALS) is a fatal and disabling neurodegenerative disease characterized by upper and lower motor neurons depletion. In our previous work, comprehensive genomic profiling of 41 motor cortex samples enabled to discriminate controls from sporadic ALS patients, and segregated these latter into two distinct subgroups (SALS1 and SALS2), each associated with different deregulated genes. In the present study, we focused our attention on two of them, Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) and its type 1 receptor (PAC1R), and validated the results of the transcriptome experiments by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), immunohistochemistry and Western blot analysis. To assess the functional role of PACAP and PAC1R in ALS, we developed an in vitro model of human induced pluripotent stem cells (iPSC)-derived motor neurons and examined the trophic effects of exogenous PACAP following neurodegenerative stimuli. Treatment with 100 nm PACAP was able to effectively rescue iPSC-derived motor neurons from apoptosis, as shown by cell viability assay and protein dosage of the apoptotic marker (BAX). All together, these data suggest that perturbations in the PACAP-PAC1R pathway may be involved in ALS pathology and represent a potential drug target to enhance motor neuron viability.

Proteomic Analysis of Parkin Isoforms Expression in Different Rat Brain Areas

PARK2 gene's mutations are related to the familial form of juvenile Parkinsonism, also known as the autosomic recessive juvenile Parkinsonism. This gene encodes for parkin, a 465-amino acid protein. To date, a large number of parkin isoforms, generated by an alternative splicing mechanism, have been described. Currently, Gene Bank lists 27 rat PARK2 transcripts, which matches to 20 exclusive parkin alternative splice variants. Despite the existence of these isoforms, most of the studies carried out so far, have been focused only on the originally cloned parkin. In this work we have analyzed the expression profile of parkin isoforms in some rat brain areas including prefrontal cortex, hippocampus, substantia nigra and cerebellum. To discriminate among these isoforms, we detected their localization through the use of two antibodies that are able to identify different domains of the parkin canonical sequence. Our analysis has revealed that at least fourteen parkin isoforms are expressed in rat brain with a various distribution in the regions analyzed. Our study might help to elucidate the pathophysiological role of these proteins in the central nervous system.

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.

Accelerated retinal aging in PACAP knock-out mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide. PACAP and its receptors are widely distributed in the retina. A number of reports provided evidence that PACAP is neuroprotective in retinal degenerations. The current study compared retina cell type-specific differences in young (3-4months) and aged adults (14-16months), of wild-type (WT) mice and knock-out (KO) mice lacking endogenous PACAP production during the course of aging. Histological, immunocytochemical and Western blot examinations were performed. The staining for standard neurochemical markers (tyrosine hydroxylase for dopaminergic cells, calbindin 28 kDa for horizontal cells, protein kinase Cα for rod bipolar cells) of young adult PACAP KO retinas showed no substantial alterations compared to young adult WT retinas, except for the specific PACAP receptor (PAC1-R) staining. We could not detect PAC1-R immunoreactivity in bipolar and horizontal cells in young adult PACAP KO animals. Some other age-related changes were observed only in the PACAP KO mice only. These alterations included horizontal and rod bipolar cell dendritic sprouting into the photoreceptor layer and decreased ganglion cell number. Also, Müller glial cells showed elevated GFAP expression compared to the aging WT retinas. Furthermore, Western blot analyses revealed significant differences between the phosphorylation state of ERK1/2 and JNK in KO mice, indicating alterations in the MAPK signaling pathway. These results support the conclusion that endogenous PACAP contributes to protection against aging of the nervous system.

Nap Interferes with Hypoxia-Inducible Factors and VEGF Expression in Retina of Diabetic Rats

The retinal microvascular damage is a complication of diabetic retinopathy (DR). Hyperglycemia and hypoxia are responsible of aberrant vessel's proliferation. The cellular response to hypoxia is mediated through activation of hypoxia-inducible factors (HIFs). Among these, HIF-1α modulates expression of its target gene, VEGF, whose upregulation controls the angiogenic event during DR development. In a previous study, we have demonstrated that a small peptide, NAP, is able to protect retina from hyperglycemic insult. Here, we have demonstrated that its intraocular administration in a rat model of diabetic retinopathy has reduced expression of HIF-1α, HIF-2α, and VEGF by increasing HIF-3α levels. These data have been also confirmed by immunolocalization study by confocal microscopy. Although these evidences need to be further deepened to understand the molecular mechanism involved in the protective NAP action, the present data suggest that this small peptide may be effective to prevent the development of this ocular pathology.

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.

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.