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.

Pituitary adenylate cyclase-activating polypeptide receptor activation in the hypothalamus recruits unique signaling pathways involved in energy homeostasis

Pituitary adenylate cyclase activating polypeptide (PACAP) exerts pleiotropic effects on ventromedial nuclei (VMN) of the hypothalamus and its control of feeding and energy expenditure through the Type I PAC1 receptor. However, the endogenous role of PAC1R's in the VMN and the downstream signaling responsible for PACAP's effects on energy balance are unknown. Numerous studies have revealed that PAC1Rs are coupled to both Gas/ adenylate cyclase/protein kinase A (Gas/AC/PKA) and Gaq/phospholipase C/protein kinase C (Gaq/PLC/PKC), while also undergoing trafficking following stimulation. To determine the endogenous role PAC1R's and downstream signaling that may explain PACAP's pleiotropic effects, we used RNA interference to knockdown VMN PAC1Rs and pharmacologically inhibited PKA, PKC and PAC1R trafficking. Knocking down PAC1Rs increased meal sizes, reduced total number of meals, and induced body weight gain. Inhibition of either PKA or PKC alone in awake male Sprague Dawley rats, attenuated PACAP's hypophagic and anorectic effects during the dark phase. However, PKA or PKC inhibition potentiated PACAP's thermogenic effects during the light phase. Analysis of locomotor activity revealed that PKA inhibition augmented PACAP's locomotor effects, however, PKC inhibition had no effect. Finally, PACAP infusion in the VMN induces surface PAC1R trafficking into the cytosol which was blocked by endocytosis inhibitors. Subsequently, inhibition of PAC1R trafficking into the cytosol attenuated PACAP-induced hypophagia. These results revealed that endogenous PAC1Rs uniquely engage PKA, PKC and receptor trafficking to mediate PACAP's pleiotropic effects in VMN control of feeding and metabolism.

Potential Therapeutic Role of Pituitary Adenylate Cyclase-Activating Polypeptide for Dry Eye Disease

Dry eye disease (DED) is caused by a reduction in the volume or quality of tears. The prevalence of DED is estimated to be 100 million in the developed world. As aging is a risk factor for DED, the prevalence of DED is expected to grow at a rapid pace in aging populations, thus creating an increased need for new therapies. This review summarizes DED medications currently in clinical use. Most current medications for DED focus on stimulating tear secretion, mucin secretion, or suppressing inflammation, rather than simply replenishing the ocular surface with moisture to improve symptoms. We recently reported that the neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) induces tear secretion and suppresses corneal injury caused by a reduction in tears. Moreover, it has been reported that a PACAP in water and a 0.9% saline solution at +4 °C showed high stability and achieved 80–90% effectiveness after 2 weeks of treatment. These results reveal PACAP as a candidate DED medication. Further research on the clinical applications of PACAP in DED is necessary.

Evaluation of Serum Levels of Transient Receptor Potential Cation Channel Subfamily V Member 1, Vasoactive Intestinal Polypeptide, and Pituitary Adenylate Cyclase-Activating Polypeptide in Chronic and Episodic Migraine: The Possible Role in Migraine Transformation

Objectives: This study aimed to investigate the role of serum levels of transient receptor potential cation channel subfamily V member 1 (TRPV1), vasoacive intestinal peptide (VIP), and pituitary adenylate cyclase-activating polypeptide (PACAP) in the development and also the transformation of migraine in patients suffering from migraine.Methods: Eighty-nine participants with a mean age of 39 years were divided into 23 episodic migraine (EM), 36 chronic migraine (CM), and 30 healthy control groups. Demographic, anthropometric, and headache characteristic information, and also blood samples, was collected. Serum levels of TRPV1, VIP, and PACAP were measured using the enzyme-linked immunosorbent assay (ELISA) technique.Results: Based on our findings, the serum level of TRPV1 was significantly higher in CM compared to the control group (p < 0.05), whereas serum levels of VIP (p < 0.01) and PACAP (p < 0.05) in the EM group were significantly more than the control group. There was no significant difference between EM and CM groups.Conclusions: An elevation in the serum levels of TRVP1 among chronic migraineurs and increments in the levels of VIP and PACAP were observed among EM patients compared to healthy subjects. However, our data failed to demonstrate the probable role of these biomarkers in migraine progression, and more studies are needed to clarify the molecular mechanisms involved in migraine progression.

Gene Expression Data Mining Reveals the Involvement of GPR55 and Its Endogenous Ligands in Immune Response, Cancer, and Differentiation

G protein-coupled receptor 55 (GPR55) is a recently deorphanized lipid- and peptide-sensing receptor. Its lipidic endogenous agonists belong to lysoglycerophospholipids, with lysophosphatidylinositol (LPI) being the most studied. Peptide agonists derive from fragmentation of pituitary adenylate cyclase-activating polypeptide (PACAP). Although GPR55 and its ligands were implicated in several physiological and pathological conditions, their biological function remains unclear. Thus, the aim of the study was to conduct a large-scale re-analysis of publicly available gene expression datasets to identify physiological and pathological conditions affecting the expression of GPR55 and the production of its ligands. The study revealed that regulation of GPR55 occurs predominantly in the context of immune activation pointing towards the role of the receptor in response to pathogens and in immune cell lineage determination. Additionally, it was revealed that there is almost no overlap between the experimental conditions affecting the expression of GPR55 and those modulating agonist production. The capacity to synthesize LPI was enhanced in various types of tumors, indicating that cancer cells can hijack the motility-related activity of GPR55 to increase aggressiveness. Conditions favoring accumulation of PACAP-derived peptides were different than those for LPI and were mainly related to differentiation. This indicates a different function of the two agonist classes and possibly the existence of a signaling bias.

PACAP-PAC1 Receptor Inhibition is Effective in Models of Opioid-Induced Medication Overuse Headache

Opioids are regularly prescribed for migraine and can result in medication overuse headache and dependence. We recently showed that pituitary adenylate cyclase activating polypeptide (PACAP) is upregulated following opioid administration or in a model of chronic migraine. The goal of this study was to determine if PACAP was a link between opioid use and headache chronification. We tested the effect of PACAP-PAC1 receptor inhibition in novel models of opioid-exacerbated migraine pain and aura; and examined the co-expression between mu opioid receptor (MOR), PAC1, and PACAP in headache-associated brain and peripheral regions.To model opioid exacerbated migraine pain, mice were injected daily with morphine (10 mg/kg) or vehicle for 11 days. On days 3,5,7,9, and 11 they also received the known human migraine trigger nitroglycerin (0.1 mg/kg) or vehicle. To model opioid exacerbated aura, mice were treated with vehicle or morphine twice daily for 4 days (20 mg/kg on days 1-3, 40 mg/kg on day 4), a well-established paradigm for causing opioid-induced hyperalgesia. On day 5 they underwent cortical spreading depression, a physiological correlate of migraine aura. The effect of the PAC1 inhibitor, M65 (0.1 mg/kg), was tested in these models. Fluorescent in situ hybridization was used to investigate the expression of MOR, PAC1, and PACAP.Only mice treated with combined morphine and nitroglycerin developed chronic cephalic allodynia (n=18/group). M65 reversed this hypersensitivity (n=9/group). Morphine significantly increased the number of CSD events (n=8-9/group); and M65 decreased this exacerbation by morphine (n=8-12/group). PAC1 and/or PACAP were highly co-expressed with MOR, and varied by region (n=6/group). MOR and PACAP were co-expressed in the trigeminal ganglia, while MOR and PAC1 receptor showed near complete overlap in the trigeminal nucleus caudalis and periaqueductal gray. The cortex showed similar cellular co-expression between MOR-PACAP and MOR-PAC1.These results show that opioids facilitate the transition to chronic headache through induction of PACAPergic mechanisms. Antibodies or pharmacological agents targeting PACAP or PAC1 receptor may be particularly beneficial for the treatment of opioid-induced medication overuse headache.

Impact of Vitamin D Supplementation on Inflammatory Markers’ Levels in Obese Patients

In view of research suggesting a possible beneficial impact of vitamin D on systemic inflammatory response, the authors decided to investigate an influence of vitamin D supplementation on serum levels of certain inflammatory markers in obese patients. The current study included such biomarkers as interleukin-6 (IL-6), pituitary adenylate cyclase-activating peptide (PACAP), advanced oxidation protein products (AOPP), C-X3-C Motif Chemokine Ligand 1 (CX3CL1), monocyte chemoattractant protein-1 (MCP-1), and nitric oxide (NO). The measurements were performed with the ELISA method before and after 3-month-long supplementation of 2000 IU of vitamin D orally. The results showed that the therapy did not induce any statistically significant changes in serum levels of MCP-1, IL-6, CX3CL1, and PACAP. The supplementation was related to a significant increase in measurements of NO and AOPP levels, although the correlation analysis between vitamin D concentration after its supplementation and the concentration of the molecular parameters did not show significant relation. In conclusion, our study seems to contradict certain aspects of findings available in the literature regarding the vitamin D’s impact.

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.

Motor Neuron Diseases and Neuroprotective Peptides: A Closer Look to Neurons

Motor neurons (MNs) are specialized neurons responsible for muscle contraction that specifically degenerate in motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS), spinal and bulbar muscular atrophy (SBMA), and spinal muscular atrophy (SMA). Distinct classes of MNs degenerate at different rates in disease, with a particular class named fast-fatigable MNs (FF-MNs) degenerating first. The etiology behind the selective vulnerability of FF-MNs is still largely under investigation. Among the different strategies to target MNs, the administration of protective neuropeptides is one of the potential therapeutic interventions. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with beneficial effects in many neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, and more recently SBMA. Another neuropeptide that has a neurotrophic effect on MNs is insulin-like growth factor 1 (IGF-1), also known as somatomedin C. These two peptides are implicated in the activation of neuroprotective pathways exploitable in the amelioration of pathological outcomes related to MNDs.