Commercially available antibodies against human and murine histamine H₄-receptor lack specificity

Opportunities and challenges in the therapeutic exploitation of histamine and histamine receptor pharmacology in inflammation-driven disorders

Inflammation-driven diseases encompass a wide array of pathological conditions characterised by immune system dysregulation leading to tissue damage and dysfunction. Among the myriad of mediators involved in the regulation of inflammation, histamine has emerged as a key modulatory player. Histamine elicits its actions through four rhodopsin-like G-protein-coupled receptors (GPCRs), named chronologically in order of discovery as histamine H1, H2, H3 and H4 receptors (H1-4R). The relatively low affinity H1R and H2R play pivotal roles in mediating allergic inflammation and gastric acid secretion, respectively, whereas the high affinity H3R and H4R are primarily linked to neurotransmission and immunomodulation, respectively. Importantly, however, besides the H4R, both H1R and H2R are also crucial in driving immune responses, the H2R tending to promote yet ill-defined and unexploited suppressive, protective and/or resolving processes. The modulatory action of histamine via its receptors on inflammatory cells is described in detail. The potential therapeutic value of the most recently discovered H4R in inflammatory disorders is illustrated via a selection of preclinical models. The clinical trials with antagonists of this receptor are discussed and possible reasons for their lack of success described.

Histamine H1- and H4-receptor expression in human colon-derived cell lines

In previous studies, we demonstrated the involvement of H4R in inflammatory bowel disease (IBD) and IBD-associated colon cancer in mice and could ascribe H4R-mediated histamine function to colon epithelial cells. The transferability of obtained data to humans is however lacking. Functional expression of H4R on colon epithelial cells is a prerequisite to pursue the hypothesis of involvement of H4R in carcinogenesis. Thus, we here compared the expression of histamine receptor subtypes in a series of cell lines. Out of these, three colon-derived cell lines displaying different combinations of H1R and H4R expression were submitted to functional analyses. Human hematopoietic HMC-1, HL-60, and U937, lung-derived A549 and Calu-3, and colorectal LoVo, SW 480, Caco-2, HT-29, and HCT116 cells were included in the study. mRNA expression was quantified by RT-qPCR. For functional analyses, Caco-2, HT-29, and HCT116 cells were treated by incubation with 1 - 10 µM histamine in the presence or absence of selective histamine receptor antagonists. Calcium mobilization, cAMP accumulation, and cell proliferation were measured by fluorimetry, mass spectrometry, and real-time bioimpedance measurements, respectively. Histamine receptor expression was heterogeneous in the cell lines tested. In most cell lines, we detected H1R mRNA while H4R mRNAs were found only occasionally. The colon-derived epithelial cell lines LoVo, SW480, and HT-29 expressed H1R mRNA exclusively, while in HCT116 cells H1R and H4R mRNAs and in CaCo-2 H2R mRNA were detectable. Subsequent functional analyses in HT29, Caco-2, and HCT116 cells, however, indicated that only HT-29 responded to histamine stimulation, by means of H1R. For a detailed analysis of histamine receptor function, esp. that of H1R and H4R, in human colon-derived cell lines, the cell lines tested here are not fully convenient unless genetically modified.

Potential determinants of low circulating glucagon-like peptide 2 concentrations in Zambian children with non-responsive stunting

NEW FINDINGS

What is the central question of this study? Non-responsive stunting is characterised by a progressive decline of circulating glucagon-like peptide 2: what are the possible causes of this decline? What is the main finding and its importance? In contrast with the established loss of Paneth and goblet cells in environmental enteropathy, there was no evidence of a parallel loss of enteroendocrine cells as seen by positive tissue staining for chromogranin A. Transcriptomic and genomic analyses showed evidence of genetic transcripts that could account for some of the variability seen in circulating glucagon-like peptide 2 values.

ABSTRACT

Nutrient sensing determines digestive and hormonal responses following nutrient ingestion. We have previously reported decreased levels of glucagon-like peptide 2 (GLP-2) in children with stunting. Here we demonstrate the presence of enteroendocrine cells in stunted children and explore potential pathways that may be involved in reduced circulating levels of GLP-2. At the time of performing diagnostic endoscopies for non-responsive stunted children, intestinal biopsies were collected for immunofluorescence staining of enteroendocrine cells and transcriptomic analysis. Circulating levels of GLP-2 were also measured and correlated with transcriptomic data. An exploratory genome-wide association study (GWAS) was conducted on DNA samples (n = 158) to assess genetic contribution to GLP-2 variability. Intestinal tissue sections collected from non-responsive stunted children stained positive for chromogranin A (88/89), alongside G-protein-coupled receptors G-protein receptor 119 (75/87), free fatty acid receptor 3 (76/89) and taste 1 receptor 1 (39/45). Transcriptomic analysis found three pathways correlated with circulating GLP-2: sugar metabolism, epithelial transport, and barrier function, which likely reflect downstream events following receptor-ligand interaction. GWAS analysis revealed potential genetic contributions to GLP-2 half-life and receptor binding. Enteroendocrine cell loss was not identified in stunted Zambian children as has been observed for goblet and Paneth cells. Transcriptomic analysis suggests that GLP-2 has pleiotrophic actions on the intestinal mucosa in malnutrition, but further work is needed to dissect pathways leading to perturbations in nutrient sensing.

Advances in the molecular level understanding of G-protein coupled receptor

G-protein coupled receptors (GPCRs) represent largest family of plasma membrane-bound receptor proteins that are involved in numerous cellular and physiological functions. Many extracellular stimuli such as hormones, lipids and chemokines activate these receptors. Aberrant expression and genetic alteration in GPCR are associated with many human diseases including cancer and cardiovascular disease. GPCRs have emerged as potential therapeutic target and numerous drugs are either approved by FDA or under clinical trial. This chapter provides an update on GPCR research and its significance as a promising therapeutic target.

Effects of Cyclosporine A and Adalimumab on the expression profiles histaminergic system-associated genes and microRNAs regulating these genes in HaCaT cells

Previous studies have not completely elucidated the role of the histaminergic system in the pathogenesis of psoriasis. This study aimed to evaluate the effects of adalimumab and cyclosporine A on the expression of histaminergic system-related genes and miRNAs regulating these genes in bacterial lipopolysaccharide A (LPS)-stimulated human keratinocyte (HaCaT) cells. HaCaT cells were treated with 1 µg/mL LPS for 8 h, followed by treatment with 8 µg/mL adalimumab or 100 ng/mL cyclosporine A for 2, 8, or 24 h. Untreated cells served as controls. The cells were subjected to ribonucleic acid (RNA) extraction and microarray, quantitative real-time polymerase chain reaction, and enzyme-linked immunosorbent assay analyses. Statistical analysis was performed using the Statistica 13.0 PL (StatSoft, Cracow, Poland) and the Transcriptome Analysis Console programs (Affymetrix, Santa Clara, CA, USA) (p < 0.05). The differential expression of the following two miRNAs was not affected in LPS-stimulated cells upon treatment with cyclosporine A or adalimumab: hsa-miR-583 (downregulated expression), involved in the regulation of histamine receptor 1 - HRH1 (overexpression); has-miR-1275 (downregulated expression), involved in the regulation of histamine receptor 1 - HRH3 (overexpression) and Solute carrier family 22 member 3 - SLC23A2 (downregulated expression)). Adalimumab and cyclosporine A modulated the histaminergic system in HaCaT cells in vitro. However, further studies are needed to elucidate the underlying mechanisms.Abbreviations: (-) - downregulated in comparison to the control, (+) - overexpression in comparison to the control, ACTB - β-actine, ADA - Adenosine deaminase, ADCYAP1 - Adenylate Cyclase Activating Polypeptide 1, BMP - bone morphogenetic protein, bp - base pair, cAMP - adenosine 3' 5'-cyclic monophosphate, CBX7 - Chromobox protein homolog 7, cDNA - double-stranded complementary DNA, CSA - cyclosporine A DAG - diacylglycerol, DIAPH - Diaphanous related formin 1, DNMT - DNA methyltransferases, DRD2 - Dopamine receptor D2, EDN1 - Endothelin 1, EDNRA - Endothelin receptor type A, ELISA - Enzyme-linked immunosorbent assay, EZH2 - Enhancer of zeste homolog 2, FC - fold change, GABRB1 - Gamma-aminobutyric acid (GABA) A receptor, alpha 1, GABRB2 - Gamma-aminobutyric acid (GABA) A receptor, alpha 2, GABRB3 - Gamma-aminobutyric acid (GABA) A receptor, alpha 3, HaCaT - Human adult, low-calcium, high-temperature keratinocytes, HIS - Human Histamine, HLAs - human leukocyte antigens, HNMT - Histamine N-methyltransferase, HNMT - Histamine N-Methyltransferase, HRH1 - histamine receptor 1, HRH2 - histamine receptor 2, HRH3 - histamine receptor 3, HRH4 - histamine receptor 4, HTR6 - 5-Hydroxytryptamine Receptor 6, IGF1 - Insulin-like growth factor 1, IL10 -interleukin 10, IL12 -interleukin 12, IL6 - interleukin 6, IP3 - inositol 1,4,5-triphosphate, LPS - bacterial lipopolysaccharide A, LYN - LYN Proto-Oncogene, Src Family Tyrosine Kinase, MAPKs -mitogen-activated protein kinases, miRNA - micro RNA, MMP2 - matrix metalloproteinase-2, NHDF - Normal Human Dermal Fibroblasts, NHEK - Normal Human Epidermal Keratinocytes, OCT3 - organic cation transporter 3, PANTHER - Protein ANalysis THrough Evolutionary Relationships Classification, PBS - phosphate-buffered saline, PI3K-AKT - phosphatidylinositol 3-kinase-protein kinase B, PIP2 - phosphatidylinositol 4,5 bisphosphate, PMSF - phenylmethylsulfonyl fluoride, PSORS1- psoriasis susceptibility gene 1, qRT-PCR - quantitative Reverse Transcription Polymerase Chain Reaction, RNA - ribonucleic acid, RNAi - RNA interference, RTqPCR - Real-Time Quantitative Reverse Transcription Reaction, SLC223A2 - Solute carrier family 22 member 3, SNX -Sorting nexin, SOX9 - SRY-Box Transcription Factor 9, TGF-α - transforming growth factor α, TGF-β - transforming growth factor beta, TNF-α - tumor necrosis factor alpha, TP53 - tumor protein 5 z, VAMP2 - Vesicle associated membrane protein 2.

The Galaninergic System: A Target for Cancer Treatment

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.

Secretin Receptor as a Target in Gastrointestinal Cancer: Expression Analysis and Ligand Development

Secretin was originally discovered as a gastrointestinal peptide that stimulates fluid secretion from the pancreas and liver and delays gastric emptying. In disease, a secretin receptor (SCTR) was found to occur as a splice variant in gastrinoma and pancreatic adenocarcinoma. Overexpression of SCTR has been described for gastrinomas, carcinoid tumors of the lung and cholangiocarcinoma. SCTR therefore is considered a candidate target for molecular tumor imaging as well as for peptide receptor radioligand therapy (PRRT) in a number of oncological indications. The aim of this study was to characterize SCTR expression in esophageal and pancreatic cancer, demonstrating for the first time high SCTR overexpression in these tumor types. In total, 65 of 70 pancreatic ductal adenocarcinoma tissues stained strongly positive for SCTR in immunohistochemistry, as did most of the 151 esophageal cancer samples, with minor influence of grading in both entities. In addition, the aim of this study was to further delineate residues in human secretin that are critical for binding to and activation of human SCTR. For a potential development of short and metabolically stable analogs for clinical use, it was intended to probe the peptide for its capacity to incorporate deletions and substitutions without losing its affinity to SCTR. In a systematic approach, a library of 146 secretin variants containing single amino acid substitutions as well as truncations on either end was tested in β-arrestin2-GFP translocation and fluorescent ligand internalization assays employing high-content analysis, in cAMP assays which run in agonist and antagonist mode, and in radioligand binding. The main structural determinants of SCTR binding and activation were localized to the N-terminus, with His1, Asp3 being among the most sensitive positions, followed by Phe6, Thr7 and Leu10. Aminoterminal truncation caused a rapid decline in receptor activity and most of these variants proved to be partial agonists showing antagonistic properties. In this study, the most potent novel antagonist showed an IC50 of 309 ± 74 nM in the β-arrestin2-GFP translocation assay on human SCTR while remaining a weak partial agonist. Future studies will have to demonstrate the utility of further enhanced secretin analogues as tracers for in vivo imaging and therapy.

Histamine H4 Receptor Agonism Induces Antitumor Effects in Human T-Cell Lymphoma

The discovery of the human histamine H4 receptor (H4R) has contributed to our understanding of the role of histamine in numerous physiological and pathological conditions, including tumor development and progression. The lymph nodes of patients with malignant lymphomas have shown to contain high levels of histamine, however, less is known regarding the expression and function of the H4R in T-cell lymphoma (TCL). In this work we demonstrate the expression of H4R isoforms (mRNA and protein) in three human aggressive TCL (OCI-Ly12, Karpas 299, and HuT78). Histamine and specific H4R agonists (VUF8430 and JNJ28610244) significantly reduced cell viability in a dose-dependent manner (p < 0.05). The combined treatment with the H4R antagonist (JNJ7777120, 10 µM) reversed the effects of the H4R ligands. Importantly, we screened a drug repurposing library of 433 FDA-approved compounds (1 μM) in combination with histamine (10 μM) in Hut78 cells. Histamine produced a favorable antitumor effect with 18 of these compounds, including the histone deacetylase inhibitor panobinostat. Apoptosis, proliferation, and oxidative stress studies confirmed the antitumoral effects of the combination. We conclude that the H4R is expressed in TCL, and it is involved in histamine-mediated responses.

Autoimmunity to the Follicle-Stimulating Hormone Receptor (FSHR) and Luteinizing Hormone Receptor (LHR) in Polycystic Ovarian Syndrome

Hyperandrogenemia and ovulatory dysfunction are hallmarks of polycystic ovary syndrome (PCOS), pointing to a deranged hypothalamus-pituitary-ovarian (HPO) axis. An autoimmune etiology of PCOS is suspected in a subset of patients due to the relatively high concordance of PCOS with common autoimmune diseases. For this reason, we tested the hypothesis that natural autoantibodies (aAb) to the follicle-stimulating hormone receptor (FSHR) or luteinizing hormone receptor (LHR) are prevalent in PCOS. To this end, new luminometric assays for quantifying aAb to the FSHR (FSHR-aAb) or LHR (LHR-aAb) were developed using full-length recombinant human receptors as fusion proteins with luciferase as reporter. Prevalence of FSHR-aAb and LHR-aAb was determined in serum samples from healthy controls and PCOS patients. Steroid hormone profiles were compared between patients with and without FSHR-aAb or LHR-aAb. Signal linearity and detection ranges were characterized and both methods passed basic performance quality checks. The analysis revealed a relatively low prevalence, with 4 out of 430 samples positive for FSHR-aAb in the control versus 11 out of 550 samples in the PCOS group, i.e., 0.9% versus 2.0%, respectively. Similarly, there were only 5 samples positive for LHR-aAb in the control versus 2 samples in the PCOS group, i.e., 1.2% versus 0.4%, respectively. Samples positive for FSHR-aAb displayed steroid hormones in the typical range of PCOS patients, whereas the two samples positive for LHR-aAb showed relatively elevated free testosterone in relation to total testosterone concentrations with unclear significance. We conclude that the FSHR and LHR constitute potential autoantigens in human subjects. However, the prevalence of specific autoantibodies to these receptors is relatively low, both in control subjects and in women with PCOS. It is therefore unlikely that autoimmunity to the LHR or FSHR constitutes a frequent cause of hyperandrogenemia or ovulatory dysfunction in PCOS.

Histamine Increases Th2 Cytokine-Induced CCL18 Expression in Human M2 Macrophages

The chemokine CCL18 is produced in cells of the myelomonocytic lineage and represents one of the most highly expressed chemokines in lesional skin and serum of atopic dermatitis patients. We investigated the role of histamine in CCL18 production in human monocyte-derived M2 macrophages differentiated in the presence of M-CSF and activated with IL-4, IL-13 or with IL-10. Since expression and regulation of histamine H1 receptor (H1R), H2R and H4R by IL-4 and IL-13 on human M2 macrophages were described, we analyzed expression of the histamine receptors in response to IL-10 stimulation by quantitative RT-PCR. IL-10 upregulated H2R and downregulated H4R mRNA expression by trend in M2 macrophages. IL-10, but in a more pronounced manner, IL-4 and IL-13, also upregulated CCL18. Histamine increased the cytokine-induced upregulation of CCL18 mRNA expression by stimulating the H2R. This effect was stronger in IL-10-stimulated M2 macrophages where the upregulation of CCL18 was confirmed at the protein level by ELISA using selective histamine receptor agonist and antagonists. The histamine-induced CCL18 upregulation in IL-10-activated M2 macrophages was almost similar in cells obtained from atopic dermatitis patients compared to cells from healthy control persons. In summary, our data stress a new function of histamine showing upregulation of the Th2 cells attracting chemokine CCL18 in human, activated M2 macrophages. This may have an impact on the course of atopic dermatitis and for the development of new therapeutic interventions.

Histamine in cancer immunology and immunotherapy. Current status and new perspectives

Cancer is the second leading cause of death globally and its incidence and mortality are rapidly increasing worldwide. The dynamic interaction of immune cells and tumor cells determines the clinical outcome of cancer. Immunotherapy comes to the forefront of cancer treatments, resulting in impressive and durable responses but only in a fraction of patients. Thus, understanding the characteristics and profiles of immune cells in the tumor microenvironment (TME) is a necessary step to move forward in the design of new immunomodulatory strategies that can boost the immune system to fight cancer. Histamine produces a complex and fine-tuned regulation of the phenotype and functions of the different immune cells, participating in multiple regulatory responses of the innate and adaptive immunity. Considering the important actions of histamine-producing immune cells in the TME, in this review we first address the most important immunomodulatory roles of histamine and histamine receptors in the context of cancer development and progression. In addition, this review highlights the current progress and foundational developments in the field of cancer immunotherapy in combination with histamine and pharmacological compounds targeting histamine receptors.

Histamine-4 Receptor: Emerging Target for the Treatment of Neurological Diseases

A major challenge in the field of the biogenic amine histamine is the search for new-generation histamine receptor specific drugs. Daniel Bovet and Sir James Black received their Nobel Prizes for Medicine for their work on histamine-1 receptor (H₁R) and H₂R antagonists to treat allergies and gastrointestinal disorders. The first H₃R-targeting drug to reach the market was approved for the treatment of the neurological disorder narcolepsy in 2018. The antagonists for the most recently identified histamine receptor, H₄R, are currently under clinical evaluation for their potential therapeutic effects on inflammatory diseases such as atopic dermatitis and pruritus. In this chapter, we propose that H₄R antagonists are endowed with prominent anti-inflammatory and immune effects, including in the brain. To substantiate this proposition, we combine data from transcriptional analyses of postmortem human neurodegenerative disease brain samples, human genome-wide association studies (GWAS), and translational animal model studies. The results prompt us to suggest the potential involvement of the H₄R in various neurodegenerative diseases and how manipulating the H₄R may create new therapeutic opportunities in central nervous system diseases.

[18 F]-labeled positron emission tomography ligand for the histamine H4 receptor

We synthesized 5-[18 F]-fluoro-1H-indol-2-yl)(4-methyl-1-piperazinyl)methanone ([18 F]5) via a Suzuki approach starting from a protected pinacol borane precursor followed by acidic hydrolysis of the t-Boc protecting group. The non-optimized radiochemical yield was 5.7 ± 1.35%, radiochemical purity was over 99%, and molar activity was 100.7 ± 34.5 GBq/μmol (n = 3). [18 F]5 was stable in rat plasma for at least 4 h and was evaluated by μPET imaging and biodistribution using a unilateral quinolinic acid rat model of neuroinflammation. The time-activity curve showed that [18 F]5 entered the brain immediately after intravenous injection and then left it progressively with a very low level reached from 30 min after injection. The biodistribution study showed no difference in the accumulation of [18 F]5 between the lesioned and intact side of the brain and between control rats and animals pretreated with a saturating dose of JNJ-7777120 as a specific H4R antagonist. Hence, despite its in vitro nanomolar affinity for H4R, and its ability to cross the blood-brain barrier in rats, [18 F]5 does not appear suitable to image in vivo the receptor by PET.

The Function of the Histamine H4 Receptor in Inflammatory and Inflammation-Associated Diseases of the Gut

Histamine is a pleiotropic mediator involved in a broad spectrum of (patho)-physiological processes, one of which is the regulation of inflammation. Compounds acting on three out of the four known histamine receptors are approved for clinical use. These approved compounds comprise histamine H1-receptor (H₁R) antagonists, which are used to control allergic inflammation, antagonists at H₂R, which therapeutically decrease gastric acid release, and an antagonist at H₃R, which is indicated to treat narcolepsy. Ligands at H₄R are still being tested pre-clinically and in clinical trials of inflammatory diseases, including rheumatoid arthritis, asthma, dermatitis, and psoriasis. These trials, however, documented only moderate beneficial effects of H₄R ligands so far. Nevertheless, pre-clinically, H₄R still is subject of ongoing research, analyzing various inflammatory, allergic, and autoimmune diseases. During inflammatory reactions in gut tissues, histamine concentrations rise in affected areas, indicating its possible biological effect. Indeed, in histamine-deficient mice experimentally induced inflammation of the gut is reduced in comparison to that in histamine-competent mice. However, antagonists at H₁R, H₂R, and H₃R do not provide an effect on inflammation, supporting the idea that H₄R is responsible for the histamine effects. In the present review, we discuss the involvement of histamine and H₄R in inflammatory and inflammation-associated diseases of the gut.

Histamine-4 receptor antagonist ameliorates Parkinson-like pathology in the striatum

Growing evidence indicates that microglia activation and a neuroinflammatory trigger contribute to dopaminergic cell loss in Parkinson's disease (PD). Furthermore, increased density of histaminergic fibers and enhanced histamine levels have been observed in the substantia nigra of PD-postmortem brains. Histamine-induced microglial activation is mediated by the histamine-4 receptor (H₄R). In the current study, gene set enrichment and pathway analyses of a PD basal ganglia RNA-sequencing dataset revealed that upregulation of H₄R was in the top functional category for PD treatment targets. Interestingly, the H₄R antagonist JNJ7777120 normalized the number of nigrostriatal dopaminergic fibers and striatal dopamine levels in a rotenone-induced PD rat model. These improvements were accompanied by a reduction of α-synuclein-positive inclusions in the striatum. In addition, intracerebroventricular infusion of JNJ7777120 alleviated the morphological changes in Iba-1-positive microglia and resulted in a lower tumor necrosis factor-α release from this brain region, as well as in ameliorated apomorphine-induced rotation behaviour. Finally, JNJ7777120 also restored basal ganglia function by decreasing the levels of γ-aminobutyric acid (GABA) and the 5-hydroxyindoleactic acid to serotonin (5-HIAA/5-HT) concentration ratios in the striatum of the PD model. Our results highlight H₄R inhibition in microglia as a promising and specific therapeutic target to reduce or prevent neuroinflammation, and as such the development of PD pathology.

Mouse Colonic Epithelial Cells Functionally Express the Histamine H4 Receptor

We hypothesized that, in mice, histamine via the histamine receptor subtype 4 (H₄R) on colon epithelial cells affects epithelial barrier integrity, perturbing physiologic function of the colonic mucosa and thus aggravating the severity of colitis. To test this hypothesis, bone marrow-chimeric mice were generated from H₄R knockout (H₄R^-/-) and wild-type (WT) BALB/cJ mice and subjected to the dextrane sodium sulfate (DSS)-induced acute colitis model. Clinical symptoms and pathohistological derangements were scored. Additionally, total RNA was extracted from either mouse whole-colon homogenates or primary cell preparations enriched for epithelial cells, and gene expression was analyzed by real-time quantitative polymerase chain reaction. The impact of the H₄R on epithelial barrier function was assessed by measurement of transepithelial electrical resistence of organoid-derived two-dimensional monolayers from H₄R^-/- and WT mice using chopstick electrodes. Bone marrow-chimeric mice with genetic depletion of the H₄R in nonhematopoietic cells exhibited less severe DSS-induced acute colitis symptoms compared with WT mice, indicating a functional proinflammatory expression of H₄R in nonimmune cells of the colon. Analysis of H₄R expression revealed the presence of H₄R mRNA in colon epithelial cells. This expression could be confirmed and complemented by functional analyses in organoid-derived epithelial cell monolayers. Thus, we conclude that the H₄R is functionally expressed in mouse colon epithelial cells, potentially modulating mucosal barrier integrity and intestinal inflammatory reactions, as was demonstrated in the DSS-induced colitis model, in which presence of the H₄R on nonhematopoietic cells aggravated the inflammatory phenotype. SIGNIFICANCE STATEMENT: The histamine H₄ receptor (H₄R) is functionally expressed on mouse colon epithelial cells, thereby aggravating dextrane sodium sulfate-induced colitis in BALB/cJ mice. Histamine via the H₄R reduces transepithelial electrical resistance of colon epithelial monolayers, indicating a function of H₄R in regulation of epithelial barrier integrity.

Histamine, histamine receptors, and neuropathic pain relief

Histamine, acting via distinct histamine H₁, H₂, H₃, and H₄ receptors, regulates various physiological and pathological processes, including pain. In the last two decades, there has been a particular increase in evidence to support the involvement of H₃ receptor and H₄ receptor in the modulation of neuropathic pain, which remains challenging in terms of management. However, recent data show contrasting effects on neuropathic pain due to multiple factors that determine the pharmacological responses of histamine receptors and their underlying signal transduction properties (e.g., localization on either the presynaptic or postsynaptic neuronal membranes). This review summarizes the most recent findings on the role of histamine and the effects mediated by the four histamine receptors in response to the various stimuli associated with and promoting neuropathic pain. We particularly focus on mechanisms underlying histamine‐mediated analgesia, as we aim to clarify the analgesic potential of histamine receptor ligands in neuropathic pain.

Linked Articles

This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc

Pathophysiological Role of Histamine H4 Receptor in Cancer: Therapeutic Implications

Cancer is a leading cause of death in both developed and developing countries. Although advances in cancer research lead to improved anti-neoplastic therapies, they continue to have unfavorable outcomes, including poor response and severe toxicity. Thus, the challenge for the new therapeutic approaches is to increase anti-tumor efficacy by targeting different molecules encompassed in the tumor and its microenvironment, as well as their specific interactions. The histamine H4 receptor (H4R) is the last discovered histamine receptor subtype and it modulates important immune functions in innate and in adaptive immune responses. Several ligands have been developed and some of them are being used in clinical trials for immune disorders with promising results. When searched in The Cancer Genome Atlas (TCGA) database, human H4R gene was found to be expressed in bladder cancer, kidney cancer, breast cancer, gastrointestinal cancers, lung cancer, endometrial cancer, and skin cancer. In the present work, we aimed to briefly summarize current knowledge in H4R's pharmacology and in the clinical use of H4R ligands before focusing on recent data reporting the expression of H4R and its pathophysiological role in cancer, representing a potential molecular target for cancer therapeutics. H4R gene and protein expression in different types of cancers compared with normal tissue as well as its relationship with patient prognosis in terms of survival will be described.

GPCRomics: An Approach to Discover GPCR Drug Targets

G protein-coupled receptors (GPCRs) are targets for ∼35% of approved drugs but only ∼15% of the ∼800 human GPCRs are currently such targets. GPCRomics, the use of unbiased, hypothesis-generating methods [e.g., RNA-sequencing (RNA-seq)], with tissues and cell types to identify and quantify GPCR expression, has led to the discovery of previously unrecognized GPCRs that contribute to functional responses and pathophysiology and that may be therapeutic targets. The combination of GPCR expression data with validation studies (e.g., signaling and functional activities) provides opportunities for the discovery of disease-relevant GPCR targets and therapeutics. Here, we review insights from GPCRomic approaches, gaps in knowledge, and future directions by which GPCRomics can advance GPCR biology and the discovery of new GPCR-targeted drugs.

Histamine-4 receptor antagonist JNJ7777120 inhibits pro-inflammatory microglia and prevents the progression of Parkinson-like pathology and behaviour in a rat model

The activation of microglial cells is presumed to play a key role in the pathogenesis of Parkinson's disease (PD). The activity of microglia is regulated by the histamine-4 receptor (H₄R), thus providing a novel target that may prevent the progression of PD. However, this putative mechanism has so far not been validated. In our previous study, we found that mRNA expression of H₄R was upregulated in PD patients. In the present study, we validated this possible mechanism using the rotenone-induced PD rat model, in which mRNA expression levels of H₄R-, and microglial markers were significantly increased in the ventral midbrain. Inhibition of H₄R in rotenone-induced PD rat model by infusion of the specific H₄R antagonist JNJ7777120 into the lateral ventricle resulted in blockade of microglial activation. In addition, pharmacological targeting of H₄R in rotenone-lesioned rats resulted in reduced apomorphine-induced rotational behaviour, prevention of dopaminergic neuron degeneration and associated decreases in striatal dopamine levels. These changes were accompanied by a reduction of Lewy body-like neuropathology. Our results provide first proof of the efficacy of an H₄R antagonist in a commonly used PD rat model, and proposes the H₄R as a promising target to clinically tackle microglial activation and thereby the progression of PD.

No answer to the lack of specificity: mouse monoclonal antibody targeting the angiotensin II type 1 receptor AT1 fails to recognize its target

Numerous antibodies targeting G protein-coupled receptors (GPCRs) have been described as non-specific among the polyclonal antibodies against angiotensin II type 1 receptor (AT₁). We have tested the newly developed AT₁ receptor mouse monoclonal antibody for its specificity. Human embryonic kidney (HEK293) cells, which do not endogenously express AT₁ receptor, were transfected in order to overexpress a fluorescently labeled enhanced green fluorescent protein (EGFP)-tagged human AT₁ receptor. Western blot and immunofluorescence assays were performed to test the specificity of the Santa Cruz monoclonal antibody sc-57036. These results were compared to the ones obtained with the polyclonal sc-1173 anti-AT₁ receptor antibodies that have already been described as non-specific. While the positive controls using GFP antibodies detected the EGFP-tagged AT₁ receptor, both polyclonal and monoclonal anti-AT₁ receptor antibodies failed to specifically recognize the corresponding band by Western blot, as similar bands were revealed in either transfected or non-transfected cells. It also failed to detect AT₁ receptor in immunofluorescence experiments. The lack of target recognition of the monoclonal AT₁ receptor antibody in our experimental conditions suggests that this antibody could give misleading results such as misidentification of the protein. To our knowledge, no specific antibodies targeting AT₁ receptors have been developed so far and the field is thus in need of new technical developments.

Challenges in Developing a Biochip for Intact Histamine Using Commercial Antibodies

This study describes the development and the challenges in the development of an on-chip immunoassay for histamine using commercially available antibodies. Histamine can be used as an indicator of food freshness and quality, but it is also a relevant marker in clinical diagnostics. Due to its low molecular weight, simple structure and thus low immunogenicity production of high specificity and affinity antibodies is difficult. From six commercial anti-histamine antibodies tested, only two bound the histamine free in the solution. A fluorescent on-chip immunoassay for histamine was established with a dynamic range of 8–111 µg/mL using polyclonal anti-histamine antibody H7403 from Sigma (Mendota Heights, MN, USA). The anti-histamine antibodies described and used in published literature are thoroughly reviewed and the quality of commercial antibodies and their traceability and quality issues are highlighted and extensively discussed.

Changes in Histidine Decarboxylase, Histamine N-Methyltransferase and Histamine Receptors in Neuropsychiatric Disorders

Compared to other monoamine neurotransmitters, information on the association between the histaminergic system and neuropsychiatric disorders is scarce, resulting in a lack of histamine-related treatment for these disorders. The current chapter tries to combine information obtained from genetic studies, neuroimaging, post-mortem human brain studies and cerebrospinal fluid measurements with data from recent clinical trials on histamine receptor agonists and antagonists, with a view to determining the possible role of the histaminergic system in neuropsychiatric disorders and to pave the way for novel histamine-related therapeutic strategies.

Human basophil chemotaxis and activation are regulated via the histamine H4 receptor

BACKGROUND

IgE-mediated cross-linking of FcεRI results in the release of mediators stored in basophil granules, such as histamine and proteases, and in the de novo synthesis of sulfidoleukotrienes.

OBJECTIVE

In this study, we investigated the role of the histamine receptors, in particular that of the histamine H4 receptor (H4R), in modulating human basophil function.

METHODS

The mRNA expression of the histamine receptors was measured by real-time PCR. Migration of basophils was assessed using the modified Boyden chamber technique. The expression levels of CD63 and CD203c on the cell surface and the sulfidoleukotriene release were determined by flow cytometry and ELISA, respectively.

RESULTS

We could show that highly purified basophils express the H1R, H2R, and H4R but not the H3R mRNA. Human basophils expressed higher H4R mRNA levels as compared to the expression levels of the H1R (P < 0.01). Histamine and the H4R agonist ST-1006 initiated active migration of basophils (P < 0.001). A significant reduction in FcεRI cross-linking-mediated surface expression of CD63 and CD203c was observed on basophils after pre-incubation with histamine or the specific H4R agonist ST-1006 (P < 0.01). The synthesis and release of sulfidoleukotrienes from basophils after activation with different stimuli, by FcεRI cross-linking or by stimulation with hymenoptera venom allergens, were significantly reduced by histamine or the H4R agonist ST-1006 (P < 0.05-0.001).

CONCLUSION

These data imply that the H4R regulates IgE-dependent processes in human basophils and provides a novel function of the H4R preventing an overwhelming immune reaction by engagement of a negative feedback loop.

Strategies for immunohistochemical protein localization using antibodies: What did we learn from neurotransmitter transporters in glial cells and neurons

Immunocytochemistry and Western blotting are still major methods for protein localization, but they rely on the specificity of the antibodies. Validation of antibody specificity remains challenging mostly because ideal negative controls are often unavailable. Further, immunochemical labeling patterns are also influenced by a number of other factors such as postmortem changes, fixation procedures and blocking agents as well as the general assay conditions (e.g., buffers, temperature, etc.). Western blotting similarly depends on tissue collection and sample preparation as well as the electrophoretic separation, transfer to blotting membranes and the immunochemical probing of immobilized molecules. Publication of inaccurate information on protein distribution has downstream consequences for other researchers because the interpretation of physiological and pharmacological observations depends on information on where ion channels, receptors, enzymes or transporters are located. Despite numerous reports, some of which are strongly worded, erroneous localization data are being published. Here we describe the extent of the problem and illustrate the nature of the pitfalls with examples from studies of neurotransmitter transporters. We explain the importance of supplementing immunochemical observations with other measurements (e.g., mRNA levels and distribution, protein activity, mass spectrometry, electrophysiological recordings, etc.) and why quantitative considerations are integral parts of the quality control. Further, we propose a practical strategy for researchers who plan to embark on a localization study. We also share our thoughts about guidelines for quality control. GLIA 2016;64:2045-2064.

The histamine H4-receptor and the central and peripheral nervous system: A critical analysis of the literature

Expression and function of histamine H4R in central and peripheral nervous system have been a matter of controversy for more than a decade. The scientific discussion is often limited to a few publications postulating the presence of functional H4R on neurons of the central and peripheral nervous system, but the even larger number of reports showing negative data is often neglected. In this article, we critically review the existing literature on H4R in central and peripheral nervous system and discuss the weak points often overlooked by the community. We identified as most important problems (i) insufficient validation or quality of antibodies, (ii) missing knockout controls, (iii) uncritical interpretation of RT-PCR results instead of qPCR experiments, (iv) insufficient controls to confirm specificity of pharmacological tools, (v) uncritical reliance on results produced by a single method and (vi) uncritical reliance on results not reproduced by independent research groups. Additionally, there may be a publication as well as a citation bias favoring the awareness of positive results, but neglecting negative data. We conclude that H4R expression on neurons of the brain is not convincingly supported by the current literature, at least as long as the positive data are not reproduced by independent research groups. Expression and function of H4R on peripheral neurons or non-neuronal cells of the nervous system, specifically on microglia is an interesting alternative hypothesis that, however, requires further verification. This article is part of a Special Issue entitled 'Histamine Receptors'.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Histamine Receptor Expression in the Gastrointestinal Tract of Dogs

Histamine is an important mediator of many physiological processes including gastrointestinal function that acts via four different histamine receptors (H1R to H4R). Elevated histamine levels and increased HR messenger ribonucleic acid (mRNA) have been shown in humans with gastrointestinal disorders such as irritable bowel syndrome or allergic intestinal diseases. As there is limited knowledge concerning the distribution of histamine receptors (HR) in dogs, one aim of this study was to investigate the expression of histamine 1 receptor (H1R), histamine 2 receptor (H2R) and histamine 4 receptor (H4R) in the canine gastrointestinal tract at protein level using immunohistochemistry. Histamine 1 receptor, H2R and H4R were widely expressed throughout the canine gastrointestinal tract including epithelial, mesenchymal, neuronal and immune cells. In addition, in situ hybridisation was established for detecting canine H4R mRNA. Results showed H4R mRNA to be present in enterocytes, lamina propria immune cells and submucosal plexus in the duodenum and colon of nearly all investigated animals. The results elucidate the importance of HR in the canine gut and represent the basis for investigating their possible impact on canine inflammatory gastrointestinal disorders.

Cellular analysis of the histamine H4 receptor in human myeloid cells

The human histamine H4 receptor (H4R) is a Gαi/o-coupled receptor which is mainly expressed on hematopoietic cells. Accordingly, the receptor is implicated in the pathology of various diseases such as autoimmune disorders, bronchial asthma and pruritus. Due to complicated receptor pharmacology, the lack of a reliable antibody and limited availability of primary cells expressing the receptor the physiology of this receptor is still poorly understood. Therefore, we aimed to assess absolute receptor mRNA expression and functionality (intracellular Ca(2+) release) in various human myeloid cell types such as granulocytes, monocytes, macrophages and dendritic cells (DCs). This was put into context with the expression of the H1R and H2R. In addition, the influence of various inflammatory stimuli on H4R expression was investigated in macrophages and monocyte-derived DCs. We found that classically activated macrophages treated with pro-inflammatory stimuli down-regulated histamine receptor mRNA expression as did LPS and zymosan A matured monocyte-derived DCs. In contrast, alternatively activated macrophages (IL-4 or IL-13) upregulated H2R and H4R expression compared to controls. Consistent with existing literature, we found eosinophils to be the major source of the H4R. Since availability of primary eosinophils is limited, we developed a cell model based on the differentiated eosinophilic cell line EOL-1, in which H4R pharmacology and physiology may be studied.

Commercially available antibodies directed against α-adrenergic receptor subtypes and other G protein-coupled receptors with acceptable selectivity in flow cytometry experiments

Several previous reports suggested that many commercially available antibodies directed against G protein-coupled receptors (GPCR) lack sufficient selectivity. Accordingly, it has been proposed that receptor antibodies should be validated by at least one of several criteria, such as testing tissues or cells after knockout or silencing of the corresponding gene. Here, we tested whether 12 commercially available antibodies directed against α-adrenergic receptor (AR) subtypes (α1A/B/D, α2A/B/C), atypical chemokine receptor 3 (ACKR3), and vasopressin receptor 1A (AVPR1A) suffice these criteria. We detected in flow cytometry experiments with human vascular smooth muscle cells that the fluorescence signals from each of these antibodies were reduced by 46 ± 10 %-91 ± 2 % in cells treated with commercially available small interfering RNA (siRNA) specific for each receptor, as compared with cells that were incubated with non-targeting siRNA. The tested antibodies included anti-ACKR3 (R&D Systems, mab42273), for which specificity has previously been demonstrated. Staining with this antibody resulted in 72 ± 5 % reduction of the fluorescence signal after ACKR3 siRNA treatment. Furthermore, staining with anti-α1A-AR (Santa Cruz, sc1477) and anti-ACKR3 (Abcam, ab38089), which have previously been reported to be non-specific, resulted in 70 ± 19 % and 80 ± 4 % loss of the fluorescence signal after α1A-AR and ACKR3 siRNA treatment, respectively. Our findings demonstrate that the tested antibodies show reasonable selectivity for their receptor target under our experimental conditions. Furthermore, our observations suggest that the selectivity of GPCR antibodies depends on the method for which the antibody is employed, the species from which cells/tissues are obtained, and on the type of specimens (cell, tissue/cell homogenate, or section) tested.

Cannabinoid receptor 2 expression modulates Gβ(1)γ(2) protein interaction with the activator of G protein signalling 2/dynein light chain protein Tctex-1

The activator of G protein signalling AGS2 (Tctex-1) forms protein complexes with Gβγ, and controls cell proliferation by regulating cell cycle progression. A direct interaction of Tctex-1 with various G protein-coupled receptors has been reported. Since the carboxyl terminal portion of CB2 carries a putative Tctex-1 binding motif, we investigated the potential interplay of CB2 and Tctex-1 in the absence and presence of Gβγ. The supposed interaction of cannabinoid receptor CB2 with Tctex-1 and the influence of CB2 on the formation of Tctex-1-Gβγ-complexes were studied by co- and/or immunoprecipitation experiments in transiently transfected HEK293 cells. The analysis on Tctex-1 protein was performed in the absence and presence of the ligands JWH 133, 2-AG, and AM 630, the protein biosynthesis inhibitor cycloheximide or the protein degradation blockers MG132, NH4Cl/leupeptin or bafilomycin. Our results show that CB2 neither directly nor indirectly via Gβγ interacts with Tctex-1, but competes with Tctex-1 in binding to Gβγ. The Tctex-1-Gβγ protein interaction was disrupted by CB2 receptor expression resulting in a release of Tctex-1 from the complex, and its degradation by the proteasome and partly by lysosomes. The decrease in Tctex-1 protein levels is induced by CB2 expression "dose-dependently" and is independent of stimulation by agonist or blocking by an inverse agonist treatment. The results suggest that CB2 receptor expression independent of its activation by agonists is sufficient to competitively disrupt Gβγ-Tctex-1 complexes, and to initiate Tctex-1 degradation. These findings implicate that CB2 receptor expression modifies the stability of intracellular protein complexes by a non-canonical pathway.

Western Blotting Inaccuracies with Unverified Antibodies: Need for a Western Blotting Minimal Reporting Standard (WBMRS)

Western blotting is a commonly used technique in biological research. A major problem with Western blotting is not the method itself, but the use of poor quality antibodies as well as the use of different experimental conditions that affect the linearity and sensitivity of the Western blot. Investigation of some conditions that are commonly used and often modified in Western blotting, as well as some commercial antibodies, showed that published articles often fail to report critical parameters needed to reproduce the results. These parameters include the amount of protein loaded, the blocking solution and conditions used, the amount of primary and secondary antibodies used, the antibody incubation solutions, the detection method and the quantification method utilized. In the present study, comparison of ubiquitinated proteins in rat heart and liver samples showed different results depending on the antibody utilized. Validation of five commercial ubiquitin antibodies using purified ubiquitinated proteins, ubiquitin chains and free ubiquitin showed that these antibodies differ in their ability to detect free ubiquitin or ubiquitinated proteins. Investigating proteins modified with interferon-stimulated gene 15 (ISG15) in young and old rat hearts using six commercially available antibodies showed that most antibodies gave different semi-quantitative results, suggesting large variability among antibodies. Evidence showing the importance of the Western blot buffer and the concentration of antibody used is presented. Hence there is a critical need for comprehensive reporting of experimental conditions to improve the accuracy and reproducibility of Western blot analysis. A Western blotting minimal reporting standard (WBMRS) is suggested to improve the reproducibility of Western blot analysis.

Involvement of H1 and H2 receptors and soluble guanylate cyclase in histamine-induced relaxation of rat mesenteric collecting lymphatics

OBJECTIVE

This study investigated the roles of the H1 and H2 histamine receptors, NO synthase, and sGC cyclase in histamine-induced modulation of rat mesenteric collecting lymphatic pumping.

METHODS

Isolated rat mesenteric collecting lymphatics were treated with 1- to 100-μM histamine. Histamine receptors were blocked with either the H1 antagonist mepyramine or the H2 antagonist cimetidine. The role of NO/sGC signaling was tested using the arginine analog L-NAME, the sGC inhibitor ODQ, and SNP as a positive control.

RESULTS

Histamine applied at 100 μM decreased tone and CF of isolated rat mesenteric collecting lymphatics. Pharmacologic blockade of either H1 or H2 histamine receptors significantly inhibited the response to histamine. Pretreatment with ODQ, but not L-NAME, completely inhibited the histamine-induced decrease in tone. ODQ pretreatment also significantly inhibited SNP-induced lymphatic relaxation.

CONCLUSIONS

H1 and H2 histamine receptors are both involved in histamine-induced relaxation of rat mesenteric collecting lymphatics. NO synthesis does not appear to contribute to the histamine-induced response. However, sGC is critical for the histamine-induced decrease in tone and contributes to the drop in CF.

Selectivity of pharmacological tools: implications for use in cell physiology. A review in the theme: Cell signaling: proteins, pathways and mechanisms

Pharmacological inhibitors are frequently used to identify the receptors, receptor subtypes, and associated signaling pathways involved in physiological cell responses. Based on the effects of such inhibitors conclusions are drawn about the involvement of their assumed target or lack thereof. While such inhibitors can be useful tools for a better physiological understanding, their uncritical use can lead to incorrect conclusions. This article reviews the concept of inhibitor selectivity and its implication for cell physiology. Specifically, we discuss the implications of using inhibitor vs. activator approaches, issues of direct vs. indirect pathway modulation, implications of inverse agonism and biased signaling, and those of orthosteric vs. allosteric, competitive vs. noncompetitive, and reversible vs. irreversible inhibition. Additional problems can result from inconsistent estimates of inhibitor potency and differences in potency between cell-free systems and intact cells. These concepts are illustrated by several examples of inhibitors displaying affinity for related but distinct targets or even unrelated targets. Of note, many of the issues being addressed are also applicable to genetic inhibition strategies. The main practical conclusion following from these concepts is that investigators should be critical in the choice of inhibitor, its concentrations, and its mode of application. When this advice is adhered to, small-molecule pharmacological inhibitors can be important experimental tools in the hand of physiologists.

The histamine H4 -receptor (H4 R) regulates eosinophilic inflammation in ovalbumin-induced experimental allergic asthma in mice

Via the histamine H4 -receptor (H4 R), histamine promotes the pathogenesis of experimental allergic asthma in mice. Application of H4 R antagonists during sensitization as well as during provocation reduces the severity of the disease. However, the specific cell types functionally expressing H4 R in experimental allergic asthma have not been well characterized in vivo. In this study, we identified the cell type(s) responsible for H4 R activity in experimental asthma and related physiological mechanisms. Using H4 R-deficient mice, we studied the role of H4 R in the sensitization and effector phase. DCs lacking H4 R expression during the in vitro sensitization reaction resulted in effector T cells unable to induce an entire eosinophilic inflammation in the lung upon adoptive transfer in vivo. Recipient mice lacking H4 R expression, which were adoptively transferred with H4 R(+/+) T cells polarized in the presence of H4 R(+/+) DCs, showed reduced signs of inflammation and ameliorated lung function. Here, we provide in vivo evidence that in experimental asthma in mice the H4 R specifically regulates activation of DCs during sensitization, while in the effector phase the H4 R is active in cells involved in the activation of eosinophils, and possibly other cells. A putative therapy targeting the H4 R may be an option for asthma patients developing IL-5-dependent eosinophilia.

Histamine H₄ receptors in the gastrointestinal tract

Histamine is a well-established mediator involved in a variety of physiological and pathophysiological mechanisms and exerts its effect through activation of four histamine receptors (H1-H₄). The histamine H₄ receptor is the newest member of this histamine receptor family, and is expressed throughout the gastrointestinal tract as well as in the liver, pancreas and bile ducts. Functional studies using a combination of selective and non-selective H₄ receptor ligands have rapidly increased our knowledge of H₄ receptor involvement in gastrointestinal processes both under physiological conditions and in models of disease. Strong evidence points towards a role for H₄ receptors in the modulation of immune-mediated responses in gut inflammation such as in colitis, ischaemia/reperfusion injury, radiation-induced enteropathy and allergic gut reactions. In addition, data have emerged implicating H₄ receptors in gastrointestinal cancerogenesis, sensory signalling, and visceral pain as well as in gastric ulceration. These studies highlight the potential of H₄ receptor targeted therapy in the treatment of various gastrointestinal disorders such as inflammatory bowel disease, irritable bowel syndrome and cancer.

The human histaminergic system in neuropsychiatric disorders

Histaminergic neurons are exclusively located in the hypothalamic tuberomamillary nucleus, from where they project to many brain areas. The histaminergic system is involved in basic physiological functions, such as the sleep-wake cycle, energy and endocrine homeostasis, sensory and motor functions, cognition, and attention, which are all severely affected in neuropsychiatric disorders. Here, we present recent postmortem findings on the alterations in this system in neuropsychiatric disorders, including Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD), depression, and narcolepsy. In addition, we highlight the need to validate animal models for these diseases and also for Tourette's syndrome (TS) in relation to alterations in the histaminergic system. Moreover, we discuss the potential for, and concerns over, the use of novel histamine 3 receptor (H3R) antagonists/inverse agonists as treatment for such disorders.

Antihistamines and itch

Histamine is one of the best-characterized pruritogens in humans. It is known to play a role in pruritus associated with urticaria as well as ocular and nasal allergic reactions. Histamine mediates its effect via four receptors. Antihistamines that block the activation of the histamine H₁receptor, H₁R, have been shown to be effective therapeutics for the treatment of pruritus associated with urticaria, allergic rhinitis, and allergic conjunctivitis. However, their efficacy in other pruritic diseases such as atopic dermatitis and psoriasis is limited. The other histamine receptors may also play a role in pruritus, with the exception of the histamine H₂receptor, H₂R. Preclinical evidence indicates that local antagonism of the histamine H₃receptor, H₃R, can induce scratching perhaps via blocking inhibitory neuronal signals. The histamine H₄receptor, H₄R, has received a significant amount of attention as to its role in mediating pruritic signals. Indeed, it has now been shown that a selective H₄R antagonist can inhibit histamine-induced itch in humans. This clinical result, in conjunction with efficacy in various preclinical pruritus models, points to the therapeutic potential of H₄R antagonists for the treatment of pruritus not controlled by antihistamines that target the H₁R.

Rapid Knockout and Reporter Mouse Line Generation and Breeding Colony Establishment Using EUCOMM Conditional-Ready Embryonic Stem Cells: A Case Study

As little as a decade ago, generation of a single knockout mouse line was an expensive and time-consuming undertaking available to relatively few researchers. The International Knockout Mouse Consortium, established in 2007, has revolutionized the use of such models by creating an open-access repository of embryonic stem (ES) cells that, through sequential breeding with first FLP1 recombinase and then Cre recombinase transgenic mice, facilitates germline global or conditional deletion of almost every gene in the mouse genome. In this Case Study, we describe our experience using the repository to create mouse lines for a variety of experimental purposes. Specifically, we discuss the process of obtaining germline transmission of two European Conditional Mouse Mutagenesis Program (EUCOMM) "knockout-first" gene targeted constructs and the advantages and pitfalls of using this system. We then outline our breeding strategy and the outcomes of our efforts to generate global and conditional knockouts and reporter mice for the genes of interest. Line maintenance, removal of recombinase transgenes, and cryopreservation are also considered. Our approach led to the generation of heterozygous knockout mice within 6 months of commencing breeding to the founder mice. By describing our experiences with the EUCOMM ES cells and subsequent breeding steps, we hope to assist other researchers with the application of this valuable approach to generating versatile knockout mouse lines.

A search for functional histamine H4 receptors in the human, guinea pig and mouse brain

Histamine H4 receptors are expressed in immune cells, but their potential role in the brain is less clear. Although H4 transcripts have been identified in human and rat brain, the presence of H4 receptors on the protein level has so far not been proven since appropriate antibodies fulfilling the strict criteria for G protein-coupled receptors are missing. Here, we searched for functional H4 receptors in human, guinea pig and mouse cortex. We studied whether H4 receptor activation is associated with increased GTPγS binding and reduced noradrenaline release. The latter two effects have been previously shown for H3 receptors, which, like the H4 receptors, are coupled to G i/o protein. G protein activation was studied using (35)S-GTPγS binding in cortical membranes. The electrically induced (3)H-noradrenaline release was determined in superfused cortical slices. The H4 agonist 4-methylhistamine failed to affect (35)S-GTPγS binding and/or noradrenaline release in human, guinea pig and mouse cortex although an H 3 receptor-mediated increase in (35)S-GTPγS binding and inhibition of noradrenaline release occurred in parallel experiments. In conclusion, functional H4 receptors increasing (35)S-GTPγS binding and/or decreasing noradrenaline release are not found in human, guinea pig and mouse cortex.

The therapeutic potential of histamine receptor ligands in inflammatory bowel disease

In the intestine of patients suffering from inflammatory bowel disease concentrations of histamine are increased compared to healthy controls. Genetic ablation of histamine production in mice ameliorates the course of experimentally induced colitis. These observations and first pharmacological studies indicate a function of histamine in the pathogenesis of inflammatory bowel disease. However, a closer examination reveals that available data are highly heterogeneous, limiting the rational design of strategies addressing specific histamine receptor subtypes as possible target for pharmacological interaction. However, very recently first clinical data indicate that antagonism at the histamine receptor subtype H4 provides a beneficial effect in at least the skin. Here, we discuss the available data on histamine effects and histamine receptor subtype functions in inflammatory bowel disease with a special emphasis on the histamine H4-receptor.

GLP-1 receptor localization in monkey and human tissue: novel distribution revealed with extensively validated monoclonal antibody

Glucagon-like peptide 1 (GLP-1) analogs are increasingly being used in the treatment of type 2 diabetes. It is clear that these drugs lower blood glucose through an increase in insulin secretion and a lowering of glucagon secretion; in addition, they lower body weight and systolic blood pressure and increase heart rate. Using a new monoclonal antibody for immunohistochemistry, we detected GLP-1 receptor (GLP-1R) in important target organs in humans and monkeys. In the pancreas, GLP-1R was predominantly localized in β-cells with a markedly weaker expression in acinar cells. Pancreatic ductal epithelial cells did not express GLP-1R. In the kidney and lung, GLP-1R was exclusively expressed in smooth muscle cells in the walls of arteries and arterioles. In the heart, GLP-1R was localized in myocytes of the sinoatrial node. In the gastrointestinal tract, the highest GLP-1R expression was seen in the Brunner's gland in the duodenum, with lower level expression in parietal cells and smooth muscle cells in the muscularis externa in the stomach and in myenteric plexus neurons throughout the gut. No GLP-1R was seen in primate liver and thyroid. GLP-1R expression seen with immunohistochemistry was confirmed by functional expression using in situ ligand binding with (125)I-GLP-1. In conclusion, these results give important new insight into the molecular mode of action of GLP-1 analogs by identifying the exact cellular localization of GLP-1R.

Comparative expression study of the endo-G protein coupled receptor (GPCR) repertoire in human glioblastoma cancer stem-like cells, U87-MG cells and non malignant cells of neural origin unveils new potential therapeutic targets

Glioblastomas (GBMs) are highly aggressive, invasive brain tumors with bad prognosis and unmet medical need. These tumors are heterogeneous being constituted by a variety of cells in different states of differentiation. Among these, cells endowed with stem properties, tumor initiating/propagating properties and particularly resistant to chemo- and radiotherapies are designed as the real culprits for tumor maintenance and relapse after treatment. These cells, termed cancer stem-like cells, have been designed as prominent targets for new and more efficient cancer therapies. G-protein coupled receptors (GPCRs), a family of membrane receptors, play a prominent role in cell signaling, cell communication and crosstalk with the microenvironment. Their role in cancer has been highlighted but remains largely unexplored. Here, we report a descriptive study of the differential expression of the endo-GPCR repertoire in human glioblastoma cancer stem-like cells (GSCs), U-87 MG cells, human astrocytes and fetal neural stem cells (f-NSCs). The endo-GPCR transcriptome has been studied using Taqman Low Density Arrays. Of the 356 GPCRs investigated, 138 were retained for comparative studies between the different cell types. At the transcriptomic level, eight GPCRs were specifically expressed/overexpressed in GSCs. Seventeen GPCRs appeared specifically expressed in cells with stem properties (GSCs and f-NSCs). Results of GPCR expression at the protein level using mass spectrometry and proteomic analysis are also presented. The comparative GPCR expression study presented here gives clues for new pathways specifically used by GSCs and unveils novel potential therapeutic targets.