Ligand-directed biased agonism at human histamine H₃ receptor isoforms across Gα_i/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 Gα_i/o-mediated (i.e. [³⁵S]GTPγS accumulation, cAMP inhibition, pERK1/2 activation, pAKT T308/S473 activation) and non Gα_i/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.