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First published online 23 April 2008
doi: 10.1242/dev.018044

Development 135, 1947-1956 (2008)
Published by The Company of Biologists 2008
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Indian hedgehog signals independently of PTHrP to promote chondrocyte hypertrophy

Kinglun Kingston Mak1, Henry M. Kronenberg2, Pao-Tien Chuang3, Susan Mackem4 and Yingzi Yang1,*

1 Genetic Disease Research Branch, National Human Genome Research Institute, 49 Convent Drive, MSC 4472, Bethesda, MD 20892, USA.
2 Endocrine Unit, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA.
3 Cardiovascular Research Institute, University of California, San Francisco, CA 94143, USA.
4 Laboratory of Pathology, National Cancer Institute, 9000 Rockville Pike, Bethesda, MD 20892-1500, USA.

* Author for correspondence (e-mail: yingzi{at}mail.nih.gov)

Accepted 31 March 2008

Chondrocyte hypertrophy is an essential process required for endochondral bone formation. Proper regulation of chondrocyte hypertrophy is also required in postnatal cartilage homeostasis. Indian hedgehog (Ihh) and PTHrP signaling play crucial roles in regulating the onset of chondrocyte hypertrophy by forming a negative feedback loop, in which Ihh signaling regulates chondrocyte hypertrophy by controlling PTHrP expression. To understand whether there is a PTHrP-independent role of Ihh signaling in regulating chondrocyte hypertrophy, we have both activated and inactivated Ihh signaling in the absence of PTHrP during endochondral skeletal development. We found that upregulating Ihh signaling in the developing cartilage by treating PTHrP-/- limb explants with sonic hedgehog (Shh) protein in vitro, or overexpressing Ihh in the cartilage of PTHrP-/- embryos or inactivating patched 1 (Ptch1), a negative regulator of hedgehog (Hh) signaling, accelerated chondrocyte hypertrophy in the PTHrP-/- embryos. Conversely, when Hh signaling was blocked by cyclopamine or by removing Smoothened (Smo), a positive regulator of Hh signaling, chondrocyte hypertrophy was delayed in the PTHrP-/- embryo. Furthermore, we show that upregulated Hh signaling in the postnatal cartilage led to accelerated chondrocyte hypertrophy during secondary ossification, which in turn caused reduction of joint cartilage. Our results revealed a novel role of Ihh signaling in promoting chondrocyte hypertrophy independently of PTHrP, which is particularly important in postnatal cartilage development and homeostasis. In addition, we found that bone morphogenetic protein (Bmp) and Wnt/β-catenin signaling in the cartilage may both mediate the effect of upregulated Ihh signaling in promoting chondrocyte hypertrophy.

Key words: Ihh, Patched 1, Cartilage, PTHrP, Chondrocyte hypertrophy






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