Molecular Bases of Disease
Extrahepatic deficiency of transferrin receptor 2 is associated with increased erythropoiesis independent of iron overload
Transferrin receptor 2 (TFR2) is a transmembrane protein expressed mainly in hepatocytes and in developing erythroid cells and is an important focal point in systemic iron regulation. Loss of TFR2 function results in a rare form of the iron-overload disease hereditary hemochromatosis. Although TFR2 in the liver has been shown to be important for regulating iron homeostasis in the body, TFR2’s function in erythroid progenitors remains controversial. In this report, we analyzed TFR2-deficient mice in the presence or absence of iron overload to distinguish between the effects caused by a high iron load and those caused by loss of TFR2 function.The catalytic, stem, and transmembrane portions of matriptase-2 are required for suppressing the expression of the iron-regulatory hormone hepcidin
Matriptase-2 (MT2) is a type-II transmembrane, trypsin-like serine protease that is predominantly expressed in the liver. It is a key suppressor for the expression of hepatic hepcidin, an iron-regulatory hormone that is induced via the bone morphogenetic protein signaling pathway. A current model predicts that MT2 suppresses hepcidin expression by cleaving multiple components of the induction pathway. MT2 is synthesized as a zymogen that undergoes autocleavage for activation and shedding. However, the biologically active form of MT2 and, importantly, the contributions of different MT2 domains to its function are largely unknown.Matriptase-2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway
Systemic iron homeostasis is maintained by regulation of iron absorption in the duodenum, iron recycling from erythrocytes, and iron mobilization from the liver and is controlled by the hepatic hormone hepcidin. Hepcidin expression is induced via the bone morphogenetic protein (BMP) signaling pathway that preferentially uses two type I (ALK2 and ALK3) and two type II (ActRIIA and BMPR2) BMP receptors. Hemojuvelin (HJV), HFE, and transferrin receptor-2 (TfR2) facilitate this process presumably by forming a plasma membrane complex with BMP receptors.Neogenin Facilitates the Induction of Hepcidin Expression by Hemojuvelin in the Liver
Hemojuvelin (HJV) regulates iron homeostasis by direct interaction with bone morphogenetic protein (BMP) ligands to induce hepcidin expression through the BMP signaling pathway in the liver. Crystallography studies indicate that HJV can simultaneously bind to both BMP2 and the ubiquitously expressed cell surface receptor neogenin. However, the role of the neogenin-HJV interaction in the function of HJV is unknown. Here we identify a mutation in HJV that specifically lowers its interaction with neogenin.CD81 Promotes Both the Degradation of Transferrin Receptor 2 (TfR2) and the Tfr2-mediated Maintenance of Hepcidin Expression
Background: Loss of function mutations in TfR2 cause iron overload in the body.Results: CD81, a scaffold protein, controls the level of TfR2 and links TfR2 to E3 ligase, GRAIL.Conclusion: CD81 down-regulates TfR2 and increases hepcidin levels in Hep3B cells.Significance: The association of TfR2 with CD81 controls both TfR2 trafficking and hepcidin mRNA.
