Hemojuvelin N-terminal mutants reach the plasma membrane but do not activate the hepcidin response

Alessia Pagani, Laura Silvestri, Antonella Nai, Clara Camaschella

Research output: Contribution to journalArticle

Abstract

Background: Hemojuvelin is a glycosylphosphatidylinositol-anchored protein, expressed in liver, skeletal muscle and heart. As a co-receptor of bone morphogenetic protein, membrane hemojuvelin positively modulates the iron regulator hepcidin. Mutations of the gene encoding for hemojuvelin cause juvenile hemochromatosis, characterized by hepcidin deficiency and severe iron overload. We have previously shown that several hemojuvelin variants do not efficiently reach the plasma membrane, whereas a few N-terminal mutants localize to the plasma membrane. Design and Methods: We studied hemojuvelin mutants of N-terminus (C80R, S85P, G99V, ΔRGD) and GDPH-consensus site for autoproteolysis (A168D, F170S, D172E) transiently expressed in HeLa cells, using electron microscopy, morphometric analysis and binding assays at different time points. Hepcidin activation by wild-type and mutant forms of hemojuvelin was assessed in Hep3B cells transfected with a hepcidin-promoter luciferase-reporter construct. Results: S85P, G99V and ΔRGD were localized to plasma membrane 36 hours after transfection, but less efficiently exported than the wild-type protein at earlier (24-30 hours) times. Morphometric analysis clearly documented delayed export and endoplasmic reticulum retention of G99V. C80R was exported without delay. GDPH variants were partially retained in the endoplasmic reticulum and Golgi apparatus, but showed impaired plasma membrane localization. In the hepcidin promoter assay only wild type hemojuvelin was able to activate hepcidin. Conclusions: The delayed export and retention in the endoplasmic reticulum of some N-terminal mutants could contribute to the pathogenesis of juvenile hemochromatosis, reducing a prompt response of bone morphogenetic protein. However, independently of their plasma membrane export, all hemojuvelin mutants tested showed no or minimal hepcidin activation.

Original languageEnglish
Pages (from-to)1466-1472
Number of pages7
JournalHaematologica
Volume93
Issue number10
DOIs
Publication statusPublished - Oct 2008

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Hepcidins
Cell Membrane
Endoplasmic Reticulum
Bone Morphogenetic Protein Receptors
Glycosylphosphatidylinositols
Bone Morphogenetic Proteins
Iron Overload
Golgi Apparatus
Luciferases
HeLa Cells
Transfection
Electron Microscopy
Skeletal Muscle
Proteins
Iron
Mutation
Membranes
Liver

Keywords

  • Hemojuvelin
  • Hepcidin
  • Juvenile hemochromatosis
  • Protein-processing and trafficking

ASJC Scopus subject areas

  • Hematology

Cite this

Hemojuvelin N-terminal mutants reach the plasma membrane but do not activate the hepcidin response. / Pagani, Alessia; Silvestri, Laura; Nai, Antonella; Camaschella, Clara.

In: Haematologica, Vol. 93, No. 10, 10.2008, p. 1466-1472.

Research output: Contribution to journalArticle

Pagani, Alessia ; Silvestri, Laura ; Nai, Antonella ; Camaschella, Clara. / Hemojuvelin N-terminal mutants reach the plasma membrane but do not activate the hepcidin response. In: Haematologica. 2008 ; Vol. 93, No. 10. pp. 1466-1472.
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N2 - Background: Hemojuvelin is a glycosylphosphatidylinositol-anchored protein, expressed in liver, skeletal muscle and heart. As a co-receptor of bone morphogenetic protein, membrane hemojuvelin positively modulates the iron regulator hepcidin. Mutations of the gene encoding for hemojuvelin cause juvenile hemochromatosis, characterized by hepcidin deficiency and severe iron overload. We have previously shown that several hemojuvelin variants do not efficiently reach the plasma membrane, whereas a few N-terminal mutants localize to the plasma membrane. Design and Methods: We studied hemojuvelin mutants of N-terminus (C80R, S85P, G99V, ΔRGD) and GDPH-consensus site for autoproteolysis (A168D, F170S, D172E) transiently expressed in HeLa cells, using electron microscopy, morphometric analysis and binding assays at different time points. Hepcidin activation by wild-type and mutant forms of hemojuvelin was assessed in Hep3B cells transfected with a hepcidin-promoter luciferase-reporter construct. Results: S85P, G99V and ΔRGD were localized to plasma membrane 36 hours after transfection, but less efficiently exported than the wild-type protein at earlier (24-30 hours) times. Morphometric analysis clearly documented delayed export and endoplasmic reticulum retention of G99V. C80R was exported without delay. GDPH variants were partially retained in the endoplasmic reticulum and Golgi apparatus, but showed impaired plasma membrane localization. In the hepcidin promoter assay only wild type hemojuvelin was able to activate hepcidin. Conclusions: The delayed export and retention in the endoplasmic reticulum of some N-terminal mutants could contribute to the pathogenesis of juvenile hemochromatosis, reducing a prompt response of bone morphogenetic protein. However, independently of their plasma membrane export, all hemojuvelin mutants tested showed no or minimal hepcidin activation.

AB - Background: Hemojuvelin is a glycosylphosphatidylinositol-anchored protein, expressed in liver, skeletal muscle and heart. As a co-receptor of bone morphogenetic protein, membrane hemojuvelin positively modulates the iron regulator hepcidin. Mutations of the gene encoding for hemojuvelin cause juvenile hemochromatosis, characterized by hepcidin deficiency and severe iron overload. We have previously shown that several hemojuvelin variants do not efficiently reach the plasma membrane, whereas a few N-terminal mutants localize to the plasma membrane. Design and Methods: We studied hemojuvelin mutants of N-terminus (C80R, S85P, G99V, ΔRGD) and GDPH-consensus site for autoproteolysis (A168D, F170S, D172E) transiently expressed in HeLa cells, using electron microscopy, morphometric analysis and binding assays at different time points. Hepcidin activation by wild-type and mutant forms of hemojuvelin was assessed in Hep3B cells transfected with a hepcidin-promoter luciferase-reporter construct. Results: S85P, G99V and ΔRGD were localized to plasma membrane 36 hours after transfection, but less efficiently exported than the wild-type protein at earlier (24-30 hours) times. Morphometric analysis clearly documented delayed export and endoplasmic reticulum retention of G99V. C80R was exported without delay. GDPH variants were partially retained in the endoplasmic reticulum and Golgi apparatus, but showed impaired plasma membrane localization. In the hepcidin promoter assay only wild type hemojuvelin was able to activate hepcidin. Conclusions: The delayed export and retention in the endoplasmic reticulum of some N-terminal mutants could contribute to the pathogenesis of juvenile hemochromatosis, reducing a prompt response of bone morphogenetic protein. However, independently of their plasma membrane export, all hemojuvelin mutants tested showed no or minimal hepcidin activation.

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