Exogenous BMP7 corrects plasma iron overload and bone loss in Bmp6-/- mice

Abstract

Purpose

Iron overload accelerates bone loss in mice lacking the bone morphogenetic protein 6 (Bmp6) gene, which is the key endogenous regulator of hepcidin, iron homeostasis gene. We investigated involvement of other BMPs in preventing haemochromatosis and subsequent osteopenia in Bmp6-/- mice.

Methods

Iron-treated wild-type (WT) and Bmp6-/- mice were analysed for hepcidin messenger RNA (mRNA) and tissue and blood BMP levels by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR), immunohistochemistry, Western blot, enzyme-linked immunosorbent assay (ELISA) and proximity extension assay. BMPs labeled with technetium-99m were used in pharmacokinetic studies.

Results

In WT mice, 4 h following iron challenge, liver Bmp6 and hepcidin expression were increased, while expression of other Bmps was not affected. In parallel, we provided the first evidence that BMP6 circulates in WT mice and that iron increased the BMP6 serum level and the specific liver uptake of 99mTc-BMP6. In Bmp6-/- mice, iron challenge led to blunted activation of liver Smad signaling and hepcidin expression with a delay of 24 h, associated with increased Bmp5 and Bmp7 expression and increased Bmp2, 4, 5 and 9 expression in the duodenum. Liver Bmp7 expression and increased circulating BMP9 eventually contributed to the late hepcidin response. This was further supported by exogenous BMP7 therapy resulting in an effective hepcidin expression followed by a rapid normalisation of plasma iron values and restored osteopenia in Bmp6-/- mice.

Conclusion

In Bmp6-/- mice, iron activated endogenous compensatory mechanisms of other BMPs that were not sufficient for preventing hemochromatosis and bone loss. Administration of exogenous BMP7 was effective in correcting the plasma iron level and bone loss, indicating that BMP6 is an essential but not exclusive in vivo regulator of iron homeostasis.

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Abbreviations

BMP:

Bone morphogenetic protein

Hamp:

Hepcidin

HJV:

Hemojuvelin

pSmad 1/5/8:

Phosphorylated Smad1, Smad5 and Smad8 protein

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Acknowledgments

We thank Djurdja Car and Mirjana M. Renic for expert animal care.

Conflict of Interest

JLB and HYL have ownership interest in a start-up company Ferrumax Pharmaceuticals, which has licensed technology from the Massachusetts General Hospital based on their work.

Financial Support

This study was supported by grants from the Ministry of Science and Technology of the Republic of Croatia. No conflict of interest for any authors. JLB was supported in part by NIH grant RO1 DK087727.

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Correspondence to Slobodan Vukicevic.

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Pauk, M., Grgurevic, L., Brkljacic, J. et al. Exogenous BMP7 corrects plasma iron overload and bone loss in Bmp6-/- mice. International Orthopaedics (SICOT) 39, 161–172 (2015). https://doi.org/10.1007/s00264-014-2550-4

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Keywords

  • Iron
  • Haemochromatosis
  • Hepcidin
  • Bone morphogenetic protein