Matriptase-2, A Novel Suppressor of Hepcidin.

Abstract SCI-26

Hepcidin inhibition is a process essential to increase iron release from duodenal cells and macrophages to plasma in conditions of high iron requests such as iron deficiency, hypoxia and erythropoietic expansion. Among the reported potential inhibitors of hepcidin the serine protease matriptase-2 has a strong effect in vivo both in mice and in humans. Matriptase-2 is a member of the transmembrane serine protease (TTPS) family, encoded by TMPRSS6 gene, whose expression is tissue restricted, mainly to the liver and to a lesser extent to kidney and olfactory epithelium. The role of matriptase-2 in iron metabolism was first defined in the Mask mouse, which after birth develops a type of iron deficiency anemia refractory to oral iron and a peculiar pattern of hair loss, because of inappropriate overexpression of hepcidin and impaired iron absorption, a phenotype due to the deletion of the matriptase-2 serine protease domain. An identical phenotype is reported in Tmprss6 -/- mice and a correspondent phenotype is observed in iron-refractory iron-deficiency anemia (IRIDA) patients. We have shown that matriptase-2 cleaves through a proteolytic processing the bone morphogenetic protein (BMP) co-receptor hemojuvelin from plasma membrane in vitro, indicating that the suppression of the BMP signaling is essential for hepcidin inhibition. The finding also suggests that the BMP pathway requiring hemojuvelin as coreceptor is the main iron-dependent pathway of hepcidin regulation. The hepcidin inhibitory effect is observed also in zebrafish and is abolished in the human homologue of Mask. We have investigated several missense TMPRSS6 mutants affecting different domains of the protein, reported in IRIDA patients, in comparison with wild type matriptase-2 and Mask. Transient overexpression in hepatoma cells shows that all mutants are deficient in the ability to inhibit the hepcidin promoter in a classic luciferase-based assay and that the decreased hepcidin inactivation broadly corresponds to the inability to cleave hemojuvelin from plasma membrane. Studies of the in vitro processing of the mutants indicate that determinants of the pathogenic effect others than the protease activity are the intracellular processing and the ability of self-activation of matriptase-2. These results have implications for the molecular pathogenesis of IRIDA.