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Smad6/Smurf1 overexpression in cartilage delays chondrocyte hypertrophy and causes dwarfism with osteopenia

Mitsuru Horiki1, Takeshi Imamura, Mina Okamoto

  • 1Department of Orthopaedics, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan.

The Journal of Cell Biology
|May 5, 2004
PubMed

Insights

Smad6 overexpression in mice causes dwarfism by inhibiting bone morphogenetic protein (BMP) signaling in chondrocytes. Smurf1 supports Smad6’s in vivo function, as shown by enhanced skeletal defects in double-transgenic mice.

Area of Science:

  • Skeletal biology
  • Molecular signaling
  • Genetics

Background:

  • Smad6 and Smad ubiquitin regulatory factor 1 (Smurf1) are known inhibitors of bone morphogenetic protein (BMP) signaling pathways.
  • Their specific roles in regulating skeletal development in vivo remain largely uncharacterized.

Purpose of the Study:

  • To investigate the in vivo functions of Smad6 and Smurf1 in chondrocytes during skeletal development.
  • To elucidate the interplay between Smad6 and Smurf1 in regulating endochondral ossification.

Main Methods:

  • Generation of transgenic mice overexpressing Smad6 or Smurf1 specifically in chondrocytes.
  • Analysis of skeletal phenotypes, including dwarfism, osteopenia, and endochondral ossification.
  • Assessment of BMP signaling pathway activation (Smad1/5/8 phosphorylation) in chondrocytes.
  • In vitro organ culture of cartilage rudiments to study BMP2-induced chondrocyte hypertrophy.

Main Results:

  • Chondrocyte-specific Smad6 overexpression led to postnatal dwarfism, osteopenia, and inhibited Smad1/5/8 phosphorylation.
  • Smad6 transgenic mice exhibited delayed chondrocyte hypertrophy and reduced trabecular bone formation.
  • BMP2-induced chondrocyte hypertrophy was impaired in Smad6 transgenic cartilage.
  • Smurf1 overexpression alone did not cause detectable abnormalities.
  • Double-transgenic mice (Smad6 and Smurf1) showed more severe defects in endochondral ossification than Smad6 transgenic mice.

Conclusions:

  • Smad6 plays a critical role in regulating chondrocyte hypertrophy and endochondral ossification in vivo.
  • Smurf1 acts to support Smad6 function in vivo, enhancing its inhibitory effect on BMP signaling during skeletal development.

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