Functional interaction of H2-receptors and 5HT4-receptors in atrial tissues isolated from double transgenic mice and from human patients

In the past, we generated transgenic mice that overexpress the human histamine 2 (H2)-receptor (H2-TG) or that overexpress the human serotonin 4 (5-HT4)-receptor (5-HT4-TG) in the heart. Here, we crossbred these lines of mice to generate double transgenic mice that overexpress both receptors (DT). This was done to study a conceivable interaction between these receptors in the mouse heart as a model for the human heart. When in left atria, initially, force of contraction was elevated maximally with 1 µM serotonin, and subsequently, histamine was cumulatively applied; a biphasic effect of histamine was noted: the force of contraction initially decreased, maximally at 10 nM histamine, and thereafter, the force of contraction increased again at 1 µM histamine. Notably, functional interaction between 5-HT and histamine was also identified in isolated electrically stimulated trabeculae carneae from human right atrium (obtained during cardiac surgery). These functional and biochemical data together are consistent with a joint overexpression of inotropically active H2-receptors and 5-HT4-receptors in the same mouse heart. We also describe an antagonistic interaction on the force of contraction of both receptors in the mouse atrium (DT) and in the human atrial muscle strips. We speculate that via this interaction, histamine might act as a “brake” on the cardiac actions of 5-HT via inhibitory GTP-binding proteins acting on the activity of adenylyl cyclase.

VIRTUAL TARGET CONSTRUCTION FOR STRUCTURE-BASED SCREENING IN THE DISCOVERY OF HISTAMINE H2 RECEPTOR LIGANDS

Objective: This study aimed to develop validated targets to be employed in structure-based virtual screening (SBVS) to discover ligands for the human histamine H2 receptor (hHRH2). Methods: The virtual targets construction was initiated by homology modeling with the reference compound ranitidine as the ligand followed by 100 ns molecular dynamics (MD) simulations. During MD simulations, the snapshot with the lowest value of the free energy of binding was selected for further validation by re-docking simulations. All simulations were performed in YASARA-Structure. Results: The research presented here resulted in one validated target for the SBVS. Additionally, by employing a clustering module in MD simulations analysis in YASARA-Structure, more than ten different virtual targets are also available for further uses. Conclusion: The virtual targets resulted in this research offer possibilities to construct valid SBVS protocols to identify ligands for the hHRH2.

Production of a Human Histamine Receptor for NMR Spectroscopy in Aqueous Solutions

G protein-coupled receptors (GPCRs) bind a broad array of extracellular molecules and transmit intracellular signals that initiate physiological responses. The signal transduction functions of GPCRs are inherently related to their structural plasticity, which can be experimentally observed by spectroscopic techniques. Nuclear magnetic resonance (NMR) spectroscopy in particular is an especially advantageous method to study the dynamic behavior of GPCRs. The success of NMR studies critically relies on the production of functional GPCRs containing stable-isotope labeled probes, which remains a challenging endeavor for most human GPCRs. We report a protocol for the production of the human histamine H1 receptor (H1R) in the methylotrophic yeast Pichia pastoris for NMR experiments. Systematic evaluation of multiple expression parameters resulted in a ten-fold increase in the yield of expressed H1R over initial efforts in defined media. The expressed receptor could be purified to homogeneity and was found to respond to the addition of known H1R ligands. Two-dimensional transverse relaxation-optimized spectroscopy (TROSY) NMR spectra of stable-isotope labeled H1R show well-dispersed and resolved signals consistent with a properly folded protein, and 19F-NMR data register a response of the protein to differences in efficacies of bound ligands.

Over-expression of the Regulator of the Signaling of G Protein-Coupled Receptors RGS9-2 reduces the signaling elicited by the human histamine H3 receptor in HEK-293T cells

In HEK-293T cells transiently transfected with the human histamine H3 receptor (hH3R), we studied the effect of over-expressing the human RGS9-2 protein on H3R-mediated stimulation of [35S]-GTPγS binding and inhibition of forskolin-induced cAMP formation. Maximal specific binding (Bmax) of [3H]-N-methyl-histamine to cell membranes was 468 ± 12 and 442 ± 38 fmol/mg protein for HEK-293T-hH3R and HEK-293T-hH3R/hRGS9-2 cells, respectively, with dissociation constants (Kd) 2.57 nM and 3.38 nM. The H3R agonist immepip stimulated [35S]-GTPγS binding with similar potency and efficacy (Emax 146.3 ± 4.4 % and 150.0 ± 5.3 % of basal, pEC50 8.57 ± 0.26 and 9.00 ± 0.33, respectively), but was significantly less efficacious to inhibit forskolin-induced cAMP accumulation in HEK-293T-hH3R/hRGS9 cells (-19.2 ± 5.3 versus -37.7 ± 5.1 % in HEK-293T-hH3R cells) with no significant difference in potency (pEC50 9.60 ± 0.14 and 9.07 ± 0.29, respectively). These results indicate that in HEK-293T cells hRGS9-2 regulates hH3R445 signaling downstream G protein activation