Structural and Molecular Determinants for Isoform Bias at Human Histamine H₃ Receptor Isoforms

Abstract

The human histamine H3 receptor (hH3R) is predominantly expressed in the CNS, where it regulates the synthesis and release of histamine and other neurotransmitters. Due to its neuromodulatory role, the hH3R has been associated with various CNS disorders, including Alzheimer’s and Parkinson’s disease. Markedly, the hH3R gene undergoes extensive splicing, resulting in 20 isoforms, of which 7TM isoforms exhibit variations in the intracellular loop 3 (IL3) and/or C-terminal tail. Particularly, hH3R isoforms that display variations in IL3 (e.g., hH3R-365) are shown to differentially signal via Gαi-dependent pathways upon binding of biased agonists (e.g., immepip, proxifan, imetit). Nevertheless, the mechanisms underlying biased agonism at hH3R isoforms remain unknown. Using a structure–function relationship study with a broad range of H3R agonists, we thereby explored determinants underlying isoform bias at hH3R isoforms that exhibit variations in IL3 (i.e., hH3R-445, -415, -365, and -329) in a Gαi-dependent pathway (cAMP inhibition). Hence, we systematically characterized hH3R isoforms on isoform bias by comparing various ligand properties (i.e., structural and molecular) to the degree of isoform bias. Importantly, our study provides novel insights into the structural and molecular basis of receptor isoform bias, highlighting the importance to study GPCRs with multiple isoforms to better tailor drugs.

Pharmacological characterization of seven human histamine H3receptor isoforms

“…In contrast to human H3R-445, the shorter H3R-365 is not able to stimulate glycogen synthase kinase 3b (GSK3b) phosphorylation, while also difference signaling bias profiles were observed for 5 agonists at these two isoforms (Riddy et al, 2017), whereas analysis of a larger set of agonists on H3R-445, H3R-415, H3R-365, and H3R-329 revealed distinct agonist efficacies in Gimediated signaling between isoforms (Rahman et al, 2023).…”

“…Considering that the three shorter human isoforms showed displayed largest differences in ligand binding and/or constitutive activity as compared to reference isoform H 3 R-445, it would be interesting to investigate whether these shorter isoforms do exist in these species that are used for disease models in pre-clinical drug testing and where they are localized in the central nervous system. In contrast to human H 3 R-445, the shorter H 3 R-365 is not able to stimulate glycogen synthase kinase 3β (GSK3β) phosphorylation, while also difference signaling bias profiles were observed for 5 agonists at these two isoforms (Riddy et al, 2017), whereas analysis of a larger set of agonists on H 3 R-445, H 3 R-415, H 3 R-365, and H 3 R-329 revealed distinct agonist efficacies in G i -mediated signaling between isoforms (Rahman et al, 2023).…”

“…The copyright holder for this this version posted December 11, 2023. ; https://doi.org/10.1101/2023.12.06.570349 doi: bioRxiv preprint agonists on H3R-445, H3R-415, H3R-365, and H3R-329 revealed distinct agonist efficacies in Gimediated signaling between isoforms (Rahman et al, 2023).…”