Alternative splicing of the histamine H(3) receptor mRNA at the third cytoplasmic loop is not detectable in humans

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.

A280V polymorphism in the histamine H3 receptor as a risk factor for migraine

BACKGROUND AND AIMS

Activation of histamine H3 receptors blocks the release of peptides responsible for headache. Our objective was to investigate the association between the genotypes of A280V polymorphism in the H3 receptor and migraine risk.

METHODS

We evaluated the frequency of the genotypes of A280V, polymorphism A280V and allelic variants of H3 receptor in 147 migraine patients and 186 healthy controls using a PCR-RLFP method.

RESULTS

V allele frequency was 6.46% and 2.68% for the cases and controls, respectively (p = 0.02) (OR 2.67; 95% CI 1.20-5.93). The frequency of V/V + V/A genotypes was 12.92% in migraine patients, significantly higher when compared to the 3.22% frequency in the control group (p = 0.001) (OR 4.45; 95% CI 1.7-11.46).

CONCLUSIONS

The results of this study suggest that V-allele genotypes in the H3 receptor gene are related to migraine risk in the Mexican population. We propose the hypothesis that the V-allele genotypes in the H3 receptor gene increase the population of inactive receptors, enhancing the inhibition of the negative feedback mechanism on the H3 receptor and increasing histamine release, which correlates with migraine attacks in susceptible patients. The case-control study reinforces the role of histamine in migraine pathogenesis.

Developmental expression analysis and immunolocalization of a biogenic amine receptor in Schistosoma mansoni

A Schistosoma mansoni G-protein coupled receptor (SmGPCR) was previously cloned and shown to be activated by the biogenic amine, histamine. Here we report a first investigation of the receptor's subunit organization, tissue distribution and expression levels in different stages of the parasite. A polyclonal antibody was produced in rabbits against the recombinant third intracellular loop (il3) of SmGPCR. Western blot studies of the native receptor and recombinant protein expressed in HEK293 cells showed that SmGPCR exists both as a monomer (65 kDa) and an apparent dimer of approximately 130 kDa These species were verified by immunoprecipitation of SmGPCR from S. mansoni extracts, using antibody that was covalently attached to agarose beads. Further investigation determined that the SmGPCR dimer was resistant to treatment with various detergents, 4 M urea and 0.1 M DTT but could be made to dissociate at acidic pH, suggesting the dimer is non-covalent in nature. Confocal immunofluorescence studies revealed significant SmGPCR immunoreactivity in sporocysts, schistosomula and adult worms but not miracidia. SmGPCR was found to be most widely expressed in the schistosomula, particularly the tegument, the subtegumental musculature and the acetabulum. In the adult stage we detected SmGPCR immunofluorescence mainly in the tubercles of male worms and, to a lesser extent, the body wall musculature. Localization in sporocysts was mainly confined to the tegument and cells within parenchymal matrices. A real-time quantitative reverse-transcription PCR analysis revealed that SmGPCR is upregulated at the mRNA level in the parasitic stages compared to the free-living miracidium and cercariae, and it is particularly elevated during early sporocyst and schistosomula development. The results identify SmGPCR as an important parasite receptor with potential functions in muscle and the tegument of S. mansoni.

Cloning and characterization of the monkey histamine H3 receptor isoforms

We have recently identified three splice isoforms of the histamine H(3) receptor in multiple brain regions of cynomolgus monkey (Macaca fascicularis). Two of the novel isoforms displayed a deletion in the third intracellular loop (H(3)(413) and H(3)(410)), the third isoform H(3)(335) displayed a deletion in the i3 intracellular loop and a complete deletion of the putative fifth transmembrane domain TM5. We have confirmed by RT-PCR the expression of full-length H(3)(445) mRNA as well as H(3)(413), H(3)(410), and H(3)(335) splice isoform mRNA in multiple monkey brain regions including the frontal, parietal and occipital cortex, parahippocampal gyrus, hippocampus, amygdala, caudate nucleus, putamen, thalamus, hypothalamus, and cerebellum. The full-length isoform H(3)(445) was predominant in all of the regions tested, followed by H(3)(335), with the H(3)(413) and H(3)(410) being of low abundance. When expressed in C6 cells, H(3)(445), H(3)(413), and H(3)(410) exhibit high affinity binding to the agonist ligand [(3)H]-(N)-alpha-methylhistamine with respective pK(D) values of 9.7, 9.7, and 9.6. As expected, the H(3)(335) isoform did not display any saturable binding with [(3)H]-(N)-alpha-methylhistamine. The histamine H(3) receptor agonists histamine, (R)-alpha-methylhistamine, imetit and proxyfan were able to activate calcium mobilization responses through H(3)(445), H(3)(413) and H(3)(410) receptors when they were co-expressed with the chimeric G alpha(qi5)-protein in HEK293 cells, while no response was elicited in cells expressing the H(3)(335) isoform. The existence of multiple H(3) receptor splice isoforms across species raises the possibility that isoform specific properties including ligand affinity, signal transduction coupling, and brain localization may differentially contribute to observed in vivo effects of histamine H(3) receptor antagonists.

Alternative splicing and expression profile analysis of expressed sequence tags in domestic pig

Domestic pig (Sus scrofa domestica) is one of the most important mammals to humans. Alternative splicing is a cellular mechanism in eukaryotes that greatly increases the diversity of gene products. Expression sequence tags (ESTs) have been widely used for gene discovery, expression profile analysis, and alternative splicing detection. In this study, a total of 712,905 ESTs extracted from 101 different non-normalized EST libraries of the domestic pig were analyzed. These EST libraries cover the nervous system, digestive system, immune system, and meat production related tissues from embryo, newborn, and adult pigs, making contributions to the analysis of alternative splicing variants as well as expression profiles in various stages of tissues. A modified approach was designed to cluster and assemble large EST datasets, aiming to detect alternative splicing together with EST abundance of each splicing variant. Much efforts were made to classify alternative splicing into different types and apply different filters to each type to get more reliable results. Finally, a total of 1,223 genes with average 2.8 splicing variants were detected among 16,540 unique genes. The overview of expression profiles would change when we take alternative splicing into account.

Histamine H3 receptor antagonists: From target identification to drug leads

The successful cloning and functional expression of the histamine H(3) receptor in the late 1990 s has greatly facilitated our efforts to identify small molecule, non-imidazole based compounds to permit the evaluation of H(3) antagonists in models of CNS disorders. High-throughput screening identified several series of lead compounds, including a series of imidazopyridines, which led to JNJ-6379490, a compound with high affinity for the human H(3) receptor. Analysis of structural features common to several series of non-imidazole H(3) receptor ligands resulted in a pharmacophore model. This model led to the design of JNJ-5207852, a diamine-based H(3) antagonist with good in vitro and in vivo efficacy but with an undesirable long half-life. However, further modifications of the template provided an understanding of the effect of structural modifications on pharmacokinetic properties, ultimately affording several additional series of compounds including JNJ-10181457, a compound with an improved pharmacokinetic profile. These compounds allowed in vivo pharmacological evaluation to show that H(3) antagonists promote wakefulness, but unlike modafinil and classical psychostimultants, they do not increase locomotor activity or produce any alteration of the EEG power spectral activity in rats. H(3) antagonists also increase extracellular acetylcholine and norepinephrine but not dopamine in rat frontal cortex and show efficacy in various models of learning-memory deficit. In addition, cFos immunoreactivity studies show H(3) antagonists activate neuronal cells in restricted rat brain regions in contrast to widespread activation after modafinil or amphetamine treatment. Therefore, H(3) antagonists are promising clinical candidates for the treatment of excessive day time sleepiness and/or cognitive disorders.

Combining restriction digestion and touchdown PCR permits detection of trace isoforms of histamine H3 receptor

The conserved sequences of the mouse histamine H3 receptor at the potential alternative splice junctions suggest that the splice isoforms found in guinea pig, rat, human, and hamster may also be present in the mouse. However, the trace amount isoforms are hard to be detected by the regular PCR approach. In this paper, we report a method in which the unspliced long isoform is cut by restriction endonuclease so that the short isoforms can be amplified to detectable levels to confirm the existence of the splice isoforms of H3 receptor mRNA in the mouse. This method is applicable to the detection of trace amounts of splice isoforms that coexist with the long, more abundant isoforms.

Generation of Cell Lines for Drug Discovery Through Random Activation of Gene Expression: Application to the Human Histamine H3 Receptor

Target-based high-throughput screening (HTS) plays an integral role in drug discovery. The implementation of HTS assays generally requires high expression levels of the target protein, and this is typically accomplished using recombinant cDNA methodologies. However, the isolated gene sequences to many drug targets have intellectual property claims that restrict the ability to implement drug discovery programs. The present study describes the pharmacological characterization of the human histamine H3 receptor that was expressed using random activation of gene expression (RAGE), a technology that over-expresses proteins by up-regulating endogenous genes rather than introducing cDNA expression vectors into the cell. Saturation binding analysis using [125I]iodoproxyfan and RAGE-H3 membranes revealed a single class of binding sites with a K(D) value of 0.77 nM and a B(max) equal to 756 fmol/mg of protein. Competition binding studies showed that the rank order of potency for H3 agonists was N(alpha)-methylhistamine approximately (R)-alpha- methylhistamine > histamine and that the rank order of potency for H3 antagonists was clobenpropit > iodophenpropit > thioperamide. The same rank order of potency for H3 agonists and antagonists was observed in the functional assays as in the binding assays. The Fluorometic Imaging Plate Reader assays in RAGE-H3 cells gave high Z' values for agonist and antagonist screening, respectively. These results reveal that the human H3 receptor expressed with the RAGE technology is pharmacologically comparable to that expressed through recombinant methods. Moreover, the level of expression of the H3 receptor in the RAGE-H3 cells is suitable for HTS and secondary assays.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Selective histamine H3 receptor antagonists for treatment of cognitive deficiencies and other disorders of the central nervous system

Evidence exists to implicate the monoamine histamine in the control of arousal and cognitive functions. Antagonists of H(3) receptors are postsynaptic and presynaptic modulators of neural transmission in a variety of neuronal circuits relevant to cognition. Accumulating neuroanatomical, neurochemical, pharmacological, and behavioral data support the idea that H(3) receptor antagonists may function to improve cognitive performances in disease states (e.g., Alzheimer's disease and mild cognitive impairment states). Thus, H(3) receptor antagonists have been shown to increase performance in attention and memory tests in nonhuman experiments and prevent the degradation in performances produced by scopolamine, MK-801, or age. In contrast, agonists of the H(3) receptor generally produce cognitive impairing effects in animal models. The role of H(3) receptors in these behavioral effects is substantiated by data indicating a central origin for their effects, the selectivity of some of the H(3) receptor antagonists studied, and the pharmacological modification of effects of H(3) receptor antagonists by selective H(3) receptor agonists. Data and issues that challenge the potential role for H(3) receptor antagonists in cognitive processes are also critically reviewed. H(3) receptor antagonists may also have therapeutic value in the management of obesity, pain, sleep disorders, schizophrenia, and attention deficit hyperactivity disorder.

Cloning and pharmacological characterization of the monkey histamine H3 receptor

Species differences have been described previously for histamine H(3) receptor pharmacology. Rat selective histamine H(3) receptor ligands such as ciproxifan and A-304121 (2-amino-1-[4-[3-(4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl]-propan-1-one) show over 100-fold selectivity for the rat receptor compared to the human receptor. To date, however, the pharmacology of the cloned monkey histamine H(3) receptor has not been examined. In this study, we cloned the monkey histamine H(3) receptor gene (H(3)R) and evaluated the receptor pharmacology in binding and functional assays. The monkey histamine H(3) receptor is highly homologous to the human receptor with 438 identities in their 445 amino acid sequences, but less homologous to the rat receptor. However, unlike the human or rat, we found no evidence for additional splicing for the monkey H(3)R. Pharmacological analysis indicated that the monkey receptor exhibited similar pharmacological profiles to those of the human receptor, providing critical information for characterizing histamine H(3) receptor ligands in monkey behavioral models.

Genetic and pharmacological aspects of histamine H3 receptor heterogeneity

Histaminergic H3 receptors modulate the release of neurotransmitters within the CNS and periphery. Ligands for these receptors have potential clinical utility in a variety of disease states. However, the pharmacological characteristics of these receptors have been enigmatic for more than a decade because of the diversity of pharmacological effects observed with the limited number of heretofore-available compounds. Recent cloning of the H3 receptor has revealed interspecies differences in the protein sequences in key regions, the existence of splice variants that differ in composition between species, and potential differences in signal transduction processes between either different tissues and/or species. This review attempts to summarize these findings within the context of the molecular biological and pharmacological data accumulated to date. Also, we suggest a nomenclature strategy to reduce potential confusion that has arisen from different naming systems used by various investigators. While some facets of this genetic and pharmacological diversity help to rationalize various aspects of H3 receptor heterogeneity, there remains an insufficient repertoire of selective ligands, assays, or other measures to completely resolve all components of this diversity. The promise of newly available tools to further explore H3 receptor function may provide the insight to bring the promised clinical potential of H3 receptor ligands to realization.

Improgan antinociception does not require neuronal histamine or histamine receptors

Improgan, a chemical congener of the H(2) antagonist cimetidine, induces antinociception following intracerebroventricular (i.c.v.) administration in rodents, but the mechanism of action of this compound remains unknown. Because the chemical structure of improgan closely resembles those of histamine and certain histamine blockers, and because neuronal histamine is known to participate in pain-relieving responses, the antinociceptive actions of improgan were evaluated in mice containing null mutations in the genes for three histamine receptors (H(1), H(2), and H(3)) and also in the gene for histidine decarboxylase (the histamine biosynthetic enzyme). Similar to earlier findings in Swiss-Webster mice, improgan induced maximal, reversible, dose-related reductions in thermal nociceptive responses in ICR mice, but neither pre-improgan (baseline) nor post-improgan nociceptive latencies were changed in any of the mutant mice as compared with wild-type controls. Improgan also had weak inhibitory activity in vitro (pK(i)=4.7-4.9) on specific binding to three recently-discovered, recombinant isoforms of the rat H(3) receptor (H(3A), H(3B), and H(3C)). The present findings strongly support the hypothesis that neuronal histamine and its receptors fail to play a role in improgan-induced antinociception.

Molecular and pharmacological characterization of the mouse histamine H3 receptor

Human, guinea pig and rat histamine H(3) receptors have been investigated at both pharmacological and molecular levels in recent years. Here we report the cloning, molecular, and pharmacological characterization of the mouse histamine H(3) receptor. The amino acid sequence of the mouse histamine H(3) receptor exhibits high homology to rat, guinea pig and human histamine H(3) receptors with 98%, 95%, 94% identities, respectively. The distribution of the mRNA encoding the mouse histamine H(3) receptor was predominant in the brain as detected by Reverse Transcription-Polymerase Chain Reaction (RT-PCR) and RNase protection assay. Although several splice forms have been reported for human, guinea pig and rat histamine H(3) receptor mRNAs, we did not detect equivalent isoforms in the mouse in several tissues by either RNase protection assay or robust Polymerase Chain Reaction (PCR) amplifications. Human embryonic kidney (HEK)-293 cells transiently transfected with mouse histamine H(3) receptor cDNA and Gq(i5) exhibited increases of intracellular Ca(2+) in response to histamine and several histamine H(3) receptor agonists. COS-7 (African green monkey kidney) cells transfected with mouse histamine H(3) receptor cDNA showed high affinity binding for histamine H(3) receptor ligands in competition binding assays. The pharmacological comparison of human, guinea pig, rat and mouse histamine H(3) receptors indicated that, as expected, the mouse histamine H(3) receptor exhibited a more similar pharmacological profile to the rat histamine H(3) receptor than to either the human or the guinea pig histamine H(3) receptor. Taken together, these findings allow a further appreciation of the histamine H(3) receptor at the molecular level and provide an additional species to assist in the pharmacological assessment of histamine H(3) receptor function.

Characteristics of recombinantly expressed rat and human histamine H3 receptors

Human and rat histamine H(3) receptors were recombinantly expressed and characterized using receptor binding and a functional cAMP assay. Seven of nine agonists had similar affinities and potencies at the rat and human histamine H(3) receptor. S-alpha-methylhistamine had a significantly higher affinity and potency at the human than rat receptor, and for 4-[(1R*,2R*)-2-(5,5-dimethyl-1-hexynyl)cyclopropyl]-1H-imidazole (Perceptin) the preference was the reverse. Only two of six antagonists had similar affinities and potencies at the human and the rat histamine H(3) receptor. Ciproxifan, thioperamide and (1R*,2R*)-trans-2-imidazol-4 ylcyclopropyl) (cyclohexylmethoxy) carboxamide (GT2394) had significantly higher affinities and potencies at the rat than at the human histamine H(3) receptor, while for N-(4-chlorobenzyl)-N-(7-pyrrolodin-1-ylheptyl)guanidine (JB98064) the preference was the reverse. All antagonists also showed potent inverse agonism properties. Iodoproxyfan, Perceptin, proxyfan and GR175737, compounds previously described as histamine H(3) receptor antagonists, acted as full or partial agonists at both the rat and the human histamine H(3) receptor.

Identification of a histamine H4 receptor on human eosinophils--role in eosinophil chemotaxis

Eosinophils are recruited to sites of inflammation via the action of a number of chemical mediators, including PAF, leukotrienes, eotaxins, ECF-A and histamine. Although many of the cell-surface receptors for these mediators have been identified, histamine-driven chemotaxis has not been conclusively attributed to any of the three known histamine receptor subtypes, suggesting the possibility of a 4th histamine-responsive receptor on eosinophils. We have identified and cloned a novel G protein-coupled receptor (GPCR), termed Pfi-013, from an IL-5 stimulated eosinophil cDNA library which is homologous to the human histamine H3 receptor, both at the sequence and gene structure level. Expression data indicates that Pfi-013 is predominantly expressed in peripheral blood leukocytes, with lower expression levels in spleen, testis and colon. Ligand-binding studies using Pfi-013 expressed in HEK-293Galpha15 cells, demonstrates specific binding to histamine with a Kd of 3.28 +/- 0.76 nM and possesses a unique rank order of potency against known histaminergic compounds in a competitive ligand-binding assay (histamine > clobenpropit > iodophenpropit > thioperamide > R-alpha-methylhistamine > cimetidine > pyrilamine). We have therefore termed this receptor human histamine H4. Chemotaxis studies on isolated human eosinophils have confirmed that histamine is chemotactic and that agonists of the known histamine receptors (H1, H2, and H3) do not induce such a response. Furthermore, studies employing histamine-receptor antagonists have shown an inhibition of chemotaxis only by the H3 antagonists clobenpropit and thioperamide. Since these compounds are also antagonists of hH4 we postulate that the receptor mediating histaminergic chemotaxis is this novel histamine H4 receptor.

Species-related pharmacological heterogeneity of histamine H(3) receptors

We compared radioligand binding and functional data for histamine H(3) receptor ligands across different tissues or species to evaluate the basis for pharmacological evidence of receptor heterogeneity previously reported. Agonist binding affinities showed correlation coefficients near unity in comparing human, dog, rat, and guinea pig cerebral cortical histamine H(3) receptors. Antagonist binding affinities revealed lower correlations for human compared to dog, rat, or guinea pig, suggesting species-based pharmacological differences. The functional potencies of histamine H(3) receptor antagonists in field-stimulated guinea pig ileum were highly correlated to binding affinities for guinea pig, dog, and, to a lesser extent, rat cerebral cortex. However, antagonist binding affinity at human cerebral cortex did not correlate well with guinea pig ileum functional potency. These results suggest significant interspecies histamine H(3) receptor heterogeneity, consistent with recent receptor gene sequence data. Therefore, genetic heterogeneity, rather than peripheral and central histamine H(3) receptor diversity, is responsible for the pharmacological differences observed.