Molecular Aspects of Histamine Receptors

Ligand-directed biased agonism at human histamine H3 receptor isoforms across Gαi/o- and β-arrestin2-mediated pathways

The histamine H3 receptor (H3R) is a neurotransmitter receptor that is primarily found in the brain, where it controls the release and synthesis of histamine, as well as the release of other neurotransmitters (e.g. dopamine, serotonin). Notably, 20 H3R isoforms are differentially expressed in the human brain as a consequence of alternative gene splicing. The hH3R-445, -415, -365 and -329 isoforms contain the prototypical GPCR (7TM) structure, yet exhibit deletions in the third intracellular loop, a structural domain that is pivotal for G protein-coupling, signaling and regulation. To date, the physiological relevance underlying the individual and combinatorial function of hH3R isoforms remains poorly understood. Nevertheless, given their significant implication in physiological processes (e.g. cognition, homeostasis) and neurological disorders (e.g. Alzheimer's and Parkinson's disease, schizophrenia), widespread targeting of hH3R isoforms by drugs may lead to on-target side effects in brain regions that are unaffected by disease. To this end, isoform- and/or pathway-selective targeting of hH3R isoforms by biased agonists could be of therapeutic relevance for the development of region- and disease-specific drugs. Hence, we have evaluated ligand biased signaling at the hH3R-445, -415, -365 and -329 isoforms across various Gαi/o-mediated (i.e. [35S]GTPγS accumulation, cAMP inhibition, pERK1/2 activation, pAKT T308/S473 activation) and non Gαi/o-mediated (i.e. β-arrestin2 recruitment) endpoints that are relevant to neurological diseases. Our findings indicate that H3R agonists display significantly altered patterns in their degree of ligand bias, in a pathway- and isoform-dependent manner, underlining the significance to investigate GPCRs with multiple isoforms to improve development of selective drugs. SUBJECT CATEGORY: Neuropharmacology.

The implications of histamine metabolism and signaling in renal function

Inflammation is an essential part of the immune response; it has been found to be central to the disruption of kidney function in acute kidney injury, diabetic nephropathy, hypertension, and other renal conditions. One of the well‐known mediators of the inflammatory response is histamine. Histamine receptors are expressed throughout different tissues, including the kidney, and their inhibition has proven to be a viable strategy for the treatment of many inflammation‐associated diseases. Here, we provide an overview of the current knowledge regarding the role of histamine and its metabolism in the kidney. Establishing the importance of histamine signaling for kidney function will enable new approaches for the treatment of kidney diseases associated with inflammation.

Covalent Inhibition of the Histamine H3 Receptor

Covalent binding of G protein-coupled receptors by small molecules is a useful approach for better understanding of the structure and function of these proteins. We designed, synthesized and characterized a series of 6 potential covalent ligands for the histamine H₃ receptor (H₃R). Starting from a 2-amino-pyrimidine scaffold, optimization of anchor moiety and warhead followed by fine-tuning of the required reactivity via scaffold hopping resulted in the isothiocyanate H₃R ligand 44. It shows high reactivity toward glutathione combined with appropriate stability in water and reacts selectively with the cysteine sidechain in a model nonapeptide equipped with nucleophilic residues. The covalent interaction of 44 with H₃R was validated with washout experiments and leads to inverse agonism on H₃R. Irreversible binder 44 (VUF15662) may serve as a useful tool compound to stabilize the inactive H₃R conformation and to study the consequences of prolonged inhibition of the H₃R.

Histamine H3 receptor ligands by hybrid virtual screening, docking, molecular dynamics simulations, and investigation of their biological effects

Histamine H₃ receptors (H₃ R), belonging to G-protein coupled receptors (GPCR) class A superfamily, are responsible for modulating the release of histamine as well as of other neurotransmitters by a negative feedback mechanism mainly in the central nervous system (CNS). These receptors have gained increased attention as therapeutic target for several CNS related neurological diseases. In the current study, we aimed to identify novel H₃ R ligands using in silico virtual screening methods. To this end, a combination of ligand- and structure-based approaches was utilized for screening of ZINC database on the homology model of human H₃ R. Structural similarity- and pharmacophore-based approaches were employed to generate compound libraries. Various molecular modeling methodologies such as molecular docking and dynamics simulation along with different drug likeness filtering criteria were applied to select anti-H₃ R ligands as promising candidate molecules based on different known parent lead compounds. In vitro binding assays of the selected molecules demonstrated three of them being active within the micromolar and submicromolar K_i range. The current integrated computational and experimental methods used in this work can provide new general insights for systematic hit identification for novel anti-H₃ R agents from large compound libraries.