Cloning and characterization of dominant negative splice variants of the human histamine H4 receptor

The histamine H4 receptor (H4R) is the latest identified member of the histamine receptor subfamily of G protein-coupled receptors with potential functional implications in inflammatory diseases and cancer. The H4R is primarily expressed in eosinophils and mast cells and shares highest homology with the H3R. The occurrence of at least twenty different hH3R isoforms led us to investigate the possible existence of H4R splice variants. Herein we report on the cloning of the first two alternatively spliced H4R isoforms from CD34+ cord blood cell derived eosinophils and mast cells. These H4R splice variants are localized predominantly intracellular when expressed recombinantly in mammalian cells. We failed to detect any ligand binding, H4R-ligand induced signaling or constitutive activity for these H4R splice variants. However, when co-expressed with the full-length H4R (H4R(390)), the H4R splice variants have a dominant negative effect on the surface expression of the H4R(390). We detect H4R(390)-H4R splice variant hetero-oligomers by employing both biochemical (immunoprecipitation, cell-surface labeling) and biophyisical (time-resolved FRET) techniques. Messenger RNAs encoding the H4R splice variants are detected in various cell types and are expressed at similar levels as the full-length H4R(390) mRNA in e.g. premonocytes. We conclude that the herein described H4R splice variants have a dominant negative effect on H4R(390) functionality, being able to retain the H4R(390) intra-cellularly and to inactivate a population of H4R(390) presumably via hetero-oligomerization