In vitro toxicity of the galanin receptor 3 antagonist SNAP 37889

Galanin receptor 3 - A new pharmacological target in retina degeneration

The neuropeptide galanin receptor 3 (GALR3) is a class A G protein-coupled receptor (GPCR) broadly expressed in the nervous system, including the retina. GALR3 is involved in the modulation of immune and inflammatory responses. Tight control of these processes is critical for maintaining homeostasis in the retina and is required to sustain vision. Here, we investigated the role of GALR3 in retina pathologies triggered by bright light and P23H mutation in the rhodopsin (RHO) gene, associated with the activation of oxidative stress and inflammatory responses. We used a multiphase approach involving pharmacological inhibition of GALR3 with its antagonist SNAP-37889 and genetic depletion of GALR3 to modulate the GALR3 signaling. Our in vitro experiments in the retinal pigment epithelium-derived cells (ARPE19) susceptible to all-trans-retinal toxicity indicated that GALR3 could be involved in the cellular stress response to this phototoxic product. Indeed, blocking the GALR3 signaling in Abca4-/-/Rdh8-/- and wild-type Balb/cJ mice, sensitive to bright light-induced retina damage, protected retina health in these mice exposed to light. The retina morphology and function were substantially improved, and stress response processes were reduced in these mouse models compared to the controls. Furthermore, in P23H Rho knock-in mice, a model of retinitis pigmentosa (RP), both pharmacological inhibition and genetic ablation of GALR3 prolonged the survival of photoreceptors. These results indicate that GALR3 signaling contributes to acute light-induced and chronic RP-linked retinopathies. Together, this work provides the pharmacological knowledge base to evaluate GALR3 as a potential target for developing novel therapies to combat retinal degeneration.

The Galaninergic System: A Target for Cancer Treatment

Simple Summary

Peptidergic systems play an important role in cancer progression. The galaninergic system (the peptide galanin and its receptors: galanin 1, 2 and 3) is involved in tumorigenesis, the invasion and migration of tumor cells and angiogenesis and it has been correlated with tumor stage/subtypes, metastasis and recurrence rate in many types of cancer. Galanin exerts a dual action in tumor cells: a proliferative or an antiproliferative effect depending on the galanin receptor involved in these mechanisms. Galanin receptors could be used in certain tumors as therapeutic targets and diagnostic markers for treatment, prognosis and surgical outcome. This review shows the importance of the galaninergic system in the development of tumors and suggests future promising clinical antitumor applications using galanin agonists or antagonists.

Abstract

The aim of this review is to show the involvement of the galaninergic system in neuroendocrine (phaeochromocytomas, insulinomas, neuroblastic tumors, pituitary tumors, small-cell lung cancer) and non-neuroendocrine (gastric cancer, colorectal cancer, head and neck squamous cell carcinoma, glioma) tumors. The galaninergic system is involved in tumorigenesis, invasion/migration of tumor cells and angiogenesis, and this system has been correlated with tumor size/stage/subtypes, metastasis and recurrence rate. In the galaninergic system, epigenetic mechanisms have been related with carcinogenesis and recurrence rate. Galanin (GAL) exerts both proliferative and antiproliferative actions in tumor cells. GAL receptors (GALRs) mediate different signal transduction pathways and actions, depending on the particular G protein involved and the tumor cell type. In general, the activation of GAL₁R promoted an antiproliferative effect, whereas the activation of GAL₂R induced antiproliferative or proliferative actions. GALRs could be used in certain tumors as therapeutic targets and diagnostic markers for treatment, prognosis and surgical outcome. The current data show the importance of the galaninergic system in the development of certain tumors and suggest future potential clinical antitumor applications using GAL agonists or antagonists.

Galanin System in Human Glioma and Pituitary Adenoma

Expression of neuropeptides and their corresponding receptors has been demonstrated in different cancer types, where they can play a role in tumor cell growth, invasion, and migration. Human galanin (GAL) is a 30-amino-acid regulatory neuropeptide which acts through three G protein-coupled receptors, GAL₁-R, GAL₂-R, and GAL₃-R that differ in their signal transduction pathways. GAL and galanin receptors (GALRs) are expressed by different tumors, and direct involvement of GAL in tumorigenesis has been shown. Despite its strong expression in the central nervous system (CNS), the role of GAL in CNS tumors has not been extensively studied. To date, GAL peptide expression, GAL receptor binding and mRNA expression have been reported in glioma, meningioma, and pituitary adenoma. However, data on the cellular distribution of GALRs are sparse. The aim of the present study was to examine the expression of GAL and GALRs in different brain tumors by immunohistochemistry. Anterior pituitary gland (n = 7), pituitary adenoma (n = 9) and glioma of different WHO grades I-IV (n = 55) were analyzed for the expression of GAL and the three GALRs with antibodies recently extensively validated for specificity. While high focal GAL immunoreactivity was detected in up to 40% of cells in the anterior pituitary gland samples, only one pituitary adenoma showed focal GAL expression, at a low level. In the anterior pituitary, GAL₁-R and GAL₃-R protein expression was observed in up to 15% of cells, whereas receptor expression was not detected in pituitary adenoma. In glioma, diffuse and focal GAL staining was noticed in the majority of cases. GAL₁-R was observed in eight out of nine glioma subtypes. GAL₂-R immunoreactivity was not detected in glioma and pituitary adenoma, while GAL₃-R expression was significantly associated to high-grade glioma (WHO grade IV). Most interestingly, expression of GAL and GALRs was observed in tumor-infiltrating immune cells, including neutrophils and glioma-associated macrophages/microglia. The presence of GALRs on tumor-associated immune cells, especially macrophages, indicates that GAL signaling contributes to homeostasis of the tumor microenvironment. Thus, our data indicate that GAL signaling in tumor-supportive myeloid cells could be a novel therapeutic target.

Validation of antibody-based tools for galanin research

Antibodies are an integral biomedical tool, not only for research but also as therapeutic agents. However, progress can only be made with sensitive and specific antibodies. The regulatory (neuro)peptide galanin and its three endogenous receptors (GAL_1-3-R) are widely distributed in the central and peripheral nervous systems, and in peripheral non-neuronal tissues. The galanin system has multiple biological functions, including feeding behavior, pain processing, nerve regeneration and inflammation, to name only a few. Galanin could serve as biomarker in these processes, and therefore its receptors are potential drug targets for various diseases. For that reason, it is of paramount interest to precisely measure galanin peptide levels in tissues and to determine the cellular and subcellular localization of galanin receptors. A plethora of antibodies and antibody-based tools, including radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) kits, are commercially available to detect galanin and its receptors. However, many of them lack rigorous validation which casts doubt on their specificity. A goal of the present study was to raise awareness of the importance of validation of antibodies and antibody-based tools, with a specific focus on the galanin system. To that end, we tested and report here about commercially available antibodies against galanin and galanin receptors that appear specific to us. Furthermore, we investigated the validity of commercially available galanin ELISA kits. As the tested ELISAs failed to meet the validation requirements, we developed and validated a specific sandwich ELISA which can be used to detect full-length galanin in human plasma.

Neuropeptide and Small Transmitter Coexistence: Fundamental Studies and Relevance to Mental Illness

Neuropeptides are auxiliary messenger molecules that always co-exist in nerve cells with one or more small molecule (classic) neurotransmitters. Neuropeptides act both as transmitters and trophic factors, and play a role particularly when the nervous system is challenged, as by injury, pain or stress. Here neuropeptides and coexistence in mammals are reviewed, but with special focus on the 29/30 amino acid galanin and its three receptors GalR1, -R2 and -R3. In particular, galanin's role as a co-transmitter in both rodent and human noradrenergic locus coeruleus (LC) neurons is addressed. Extensive experimental animal data strongly suggest a role for the galanin system in depression-like behavior. The translational potential of these results was tested by studying the galanin system in postmortem human brains, first in normal brains, and then in a comparison of five regions of brains obtained from depressed people who committed suicide, and from matched controls. The distribution of galanin and the four galanin system transcripts in the normal human brain was determined, and selective and parallel changes in levels of transcripts and DNA methylation for galanin and its three receptors were assessed in depressed patients who committed suicide: upregulation of transcripts, e.g., for galanin and GalR3 in LC, paralleled by a decrease in DNA methylation, suggesting involvement of epigenetic mechanisms. It is hypothesized that, when exposed to severe stress, the noradrenergic LC neurons fire in bursts and release galanin from their soma/dendrites. Galanin then acts on somato-dendritic, inhibitory galanin autoreceptors, opening potassium channels and inhibiting firing. The purpose of these autoreceptors is to act as a 'brake' to prevent overexcitation, a brake that is also part of resilience to stress that protects against depression. Depression then arises when the inhibition is too strong and long lasting - a maladaption, allostatic load, leading to depletion of NA levels in the forebrain. It is suggested that disinhibition by a galanin antagonist may have antidepressant activity by restoring forebrain NA levels. A role of galanin in depression is also supported by a recent candidate gene study, showing that variants in genes for galanin and its three receptors confer increased risk of depression and anxiety in people who experienced childhood adversity or recent negative life events. In summary, galanin, a neuropeptide coexisting in LC neurons, may participate in the mechanism underlying resilience against a serious and common disorder, MDD. Existing and further results may lead to an increased understanding of how this illness develops, which in turn could provide a basis for its treatment.

Lack of Galanin Receptor 3 Alleviates Psoriasis by Altering Vascularization, Immune Cell Infiltration, and Cytokine Expression

The neuropeptide galanin is distributed in the central and peripheral nervous systems and in non-neuronal peripheral organs, including the skin. Galanin acts via three G protein-coupled receptors which, except galanin receptor 1, are expressed in various skin structures. The galanin system has been associated with inflammatory processes of the skin and of several other organs. Psoriasis is an inflammatory skin disease with increased neovascularization, keratinocyte hyperproliferation, a proinflammatory cytokine milieu, and immune cell infiltration. In this study, we showed that galanin receptor 3 is present in endothelial cells in human and murine dermal vessels and is co-expressed with nestin in neo-vessels of psoriatic patients. Moreover, in a murine psoriasis model, we showed that C57/BL6 mice lacking galanin receptor 3 display a milder course of psoriasis upon imiquimod treatment, leading to decreased disease severity, delayed neo-vascularization, reduced infiltration of neutrophils, and significantly lower levels of proinflammatory cytokines compared with wild-type mice. In contrast, galanin receptor 2-knockout animals did not differ significantly from wild type mice at both the macroscopic and molecular levels in their inflammatory response to imiquimod treatment. Our data indicate that galanin receptor 3, but not galanin receptor 2, plays an important role in psoriasis-like skin inflammation.

Lack of Galanin 3 Receptor Aggravates Murine Autoimmune Arthritis

Neurogenic inflammation mediated by peptidergic sensory nerves has a crucial impact on the pathogenesis of various joint diseases. Galanin is a regulatory sensory neuropeptide, which has been shown to attenuate neurogenic inflammation, modulate neutrophil activation, and be involved in the development of adjuvant arthritis, but our current understanding about its targets and physiological importance is incomplete. Among the receptors of galanin (GAL₁–₃), GAL₃ has been found to be the most abundantly expressed in the vasculature and on the surface of some immune cells. However, since there are minimal in vivo data on the role of GAL₃ in joint diseases, we analyzed its involvement in different inflammatory mechanisms of the K/BxN serum transfer-model of autoimmune arthritis employing GAL₃ gene-deficient mice. After arthritis induction, GAL₃ knockouts demonstrated increased clinical disease severity and earlier hindlimb edema than wild types. Vascular hyperpermeability determined by in vivo fluorescence imaging was also elevated compared to the wild-type controls. However, neutrophil accumulation detected by in vivo luminescence imaging or arthritic mechanical hyperalgesia was not altered by the lack of the GAL₃ receptor. Our findings suggest that GAL₃ has anti-inflammatory properties in joints by inhibiting vascular hyperpermeability and consequent edema formation.