The histamine H4 receptor is, like the other three histamine receptors, a member of the G protein-coupled receptor superfamily.
Tissue distribution
H4 is highly expressed in bone marrow and white blood cells and regulates neutrophil release from bone marrow and subsequent infiltration in the zymosan-induced pleurisy mouse model. It is also expressed in the colon, liver, lung, small intestine, spleen, testes, thymus, tonsils, and trachea.
Function
The Histamine H4 receptor has been shown to be involved in mediating eosinophil shape change and mast cell chemotaxis. This occurs via the βγ subunit acting at phospholipase C to cause actin polymerisation and eventually chemotaxis.
Structure
The 3D structure of the H4 receptor has not been solved yet due to the difficulties of GPCR crystallization. Some attempts have been made to develop structural models of the H4 receptor for different purposes. The first H4 receptor model was built by homology modelling based on the crystal structure of bovine rhodopsin. This model was used for the interpretation of site-directed mutagenesis data, which revealed the crucial importance of Asp94 (3.32) and Glu182 (5.46) residues in ligand binding and receptor activation.