The histamine H₄-receptor, discovered by Oda et al and Nakamura et al in 2000, is expressed on various cells of the immune system (T-cells, dendritic cells) as well as on eosinophils, and is discussed to play an important role in the pathogenesis of bronchial asthma, atopic dermatitis, allergic rihinitis and pruritus [1].

The H₄-receptor (390 amino acids) belongs to the class A of the G-Protein-coupled receptors (GPCRs) [2]. There are substantial pharmacological differences between various H₄R species isoforms [3], especially regarding H₄R-agonists, both in terms of potency and quality of action. For example, histamine and a number of H₄R-agonists have considerably higher affinity for the human H₄R than for the mouse H₄R [4]. Some H₄R-agonists and antagonists show even opposite effects (agonism, antagonism) when investigated on different species. Such species-dependent differences represent a major problem in the validation of the H₄R as a drug target using translational animal models.

The aim of this project is to analyse the molecular basis of the pharmacological differences between human and mouse H₄R. Two complementary approaches will be applied: 1) molecular modeling to suggest key amino acids in species isoforms of the H₄R, 2) site-directed mutagenesis and pharmacological investigations to verify the role of the identified amino acids in receptor binding and function. The mutant H₄Rs are expressed in Sf9 insect cells. Affinities and activities of agonists and antagonists are determined in radioligand binding and functional assays (e.g. GTPase assay, [³⁵S]GTPγS assay). Moreover, functional investigations in genetically modified (mammalian) cells will be considered, for instance, by using reportergene assays.

 
Literature: 

1. Smits, R. A.; Leurs, R.; de Esch, I. J. P. Major advances in the development of histamine H4 receptor ligands. Drug Discovery Today 2009, 14, 745-753. 

2. de Esch, I. J. P.; Thurmond, R. L.; Jongejan, A.; Leurs, R. The histamine H4 receptor as a new therapeutic target for inflammation. Trends Pharmacol. Sci. 2005, 26, 462-469. 

3. Lim, H. D.; Jongejan, A.; Bakker, R. A.; Haaksma, E.; de Esch, I. J.; Leurs, R. Phenylalanine 169 in the second extracellular loop of the human histamine H4 receptor is responsible for the difference in agonist binding between human and mouse H4 receptors. J. Pharmacol. Exp. Ther. 2008, 327, 88-96. 

4. Lim, H. D.; de Graaf, C.; Jiang, W.; Sadek, P.; McGovern, P. M.; Istyastono, E. P.; Bakker, R. A.; de Esch, I. J.; Thurmond, R. L.; Leurs, R. Molecular determinants of ligand binding to H4R species variants. Mol Pharmacol 2010, 77, 734-743.