Problems associated with the use of commercial and non-commercial antibodies against the histamine H4 receptor

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.

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'.

Regulation of the Cardiovascular System by Histamine

Histamine mediates a wide range of cellular responses, including allergic and inflammatory reactions, gastric acid secretion, and neurotransmission in the central nervous system. Histamine also exerts a series of actions upon the cardiovascular system but may not normally play a significant role in regulating cardiovascular function. During tissue injury, inflammation, and allergic responses, mast cells (or non-mast cells) within the tissues can release large amounts of histamine that leads to noticeable cardiovascular effects. Owing to intensive research during several decades, the distribution, function, and pathophysiological role of cardiovascular H₁- and H₂-receptors has become recognized adequately. Besides the recognized H₁- and H₂-receptor-mediated cardiovascular responses, novel roles of H₃- and H₄-receptors in cardiovascular physiology and pathophysiology have been identified over the last decade. In this review, we describe recent advances in our understanding of cardiovascular function and dysfunction mediated by histamine receptors, including H₃- and H₄-receptors, their potential mechanisms of action, and their pathological significance.

No evidence for histamine H4 receptor in human monocytes

The histamine H4 receptor (H4R) is a classic pertussis toxin-sensitive Gi protein-coupled receptor that mediates increases in intracellular calcium concentration ([Ca(2+)]i). The presence of H4R in human eosinophils has been rigorously documented by several independent groups. It has also been suggested that H4R is expressed in human monocytes, but this suggestion hinges in part on H4R antibodies with questionable specificity. This situation prompted us to reinvestigate H4R expression in human monocytes. As positive control, we studied human embryonic kidney 293T cells stably expressing the human H4R (hH4R). In these cells, histamine (HA) and the H4R agonist UR-PI376 (2-cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[(2-phenylthio)ethyl]guanidine) induced pertussis toxin-sensitive [Ca(2+)]i increases. However, in quantitative real-time polymerase chain reaction studies we failed to detect hH4R mRNA in human monocytes and U937 promonocytes. In human monocytes, ATP and N-formyl-l-methionyl-l-leucyl-l-phenylalanine increased [Ca(2+)]i, but HA, UR-PI376, and 5-methylhistamine (a dual H4R/H2 receptor agonist) did not. In U937 promonocytes and differentiated U937 cells, HA increased [Ca(2+)]i, but this increase was mediated via HA H1 receptor. In conclusion, there is no evidence for the presence of H4R in human monocytes.

Lack of specificity shown by P2Y6 receptor antibodies

P2Y6 receptor in bladder smooth muscle responds to UDP by increasing muscle tone and augmenting bladder contractions. The exact cellular location of the receptor is however unknown. Three commercially available antibodies to P2Y6 receptor gave clean bands on Western blot which were eliminated by specific peptide competition. Two of the three also immunostained bladder smooth muscle cells while leaving adjacent interstitial cells of Cajal unstained. However, attempts to validate the specificity of these antibodies by performing the same assays on bladders from P2Y6 knockout mice were unsuccessful. In Western blots, all three antibodies bound similar proteins in both wild type and P2Y6 knockout tissue. Immunostaining of knockout tissue sections also showed no difference in staining patterns or intensity. We conclude that rigorous controls are required when using commercial reagents to this G-protein coupled receptor and perhaps to other members of the P2Y receptor family.

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.