Structural and Molecular Determinants for Isoform Bias at Human Histamine H 3 Receptor Isoforms.

The human histamine H 3 receptor (hH 3 R) is predominantly expressed in the CNS, where it regulates the synthesis and release of histamine and other neurotransmitters. Due to its neuromodulatory role, the hH 3 R has been associated with various CNS disorders, including Alzheimer's and Parkinson's disease. Markedly, the hH 3 R 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, hH 3 R isoforms that display variations in IL3 (e.g., hH 3 R-365) are shown to differentially signal via Gα i -dependent pathways upon binding of biased agonists (e.g., immepip, proxifan, imetit). Nevertheless, the mechanisms underlying biased agonism at hH 3 R isoforms remain unknown. Using a structure-function relationship study with a broad range of H 3 R agonists, we thereby explored determinants underlying isoform bias at hH 3 R isoforms that exhibit variations in IL3 (i.e., hH 3 R-445, -415, -365, and -329) in a Gα i -dependent pathway (cAMP inhibition). Hence, we systematically characterized hH 3 R 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.