PMID:18434416

Citation	Mak, KK, Kronenberg, HM, Chuang, PT, Mackem, S and Yang, Y (2008) Indian hedgehog signals independently of PTHrP to promote chondrocyte hypertrophy. Development 135:1947-56
Abstract	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/beta-catenin signaling in the cartilage may both mediate the effect of upregulated Ihh signaling in promoting chondrocyte hypertrophy.
Links	PubMed Online version:10.1242/dev.018044
Keywords	Animals; Bone Morphogenetic Proteins/genetics; Bone Morphogenetic Proteins/metabolism; Bone Morphogenetic Proteins/physiology; Cell Differentiation/genetics; Cell Differentiation/physiology; Cell Enlargement; Cells, Cultured; Chondrocytes/metabolism; Chondrocytes/pathology; Forelimb/cytology; Forelimb/metabolism; Hedgehog Proteins/genetics; Hedgehog Proteins/metabolism; Hedgehog Proteins/pharmacology; Hedgehog Proteins/physiology; Immunohistochemistry; In Situ Hybridization; Mice; Mutation; Organ Culture Techniques; Parathyroid Hormone-Related Protein/genetics; Parathyroid Hormone-Related Protein/physiology; Receptors, Cell Surface/genetics; Receptors, Cell Surface/metabolism; Receptors, Cell Surface/physiology; Recombinant Proteins/metabolism; Recombinant Proteins/pharmacology; Reverse Transcriptase Polymerase Chain Reaction; Signal Transduction/drug effects; Signal Transduction/genetics; Signal Transduction/physiology; Wnt Proteins/genetics; Wnt Proteins/metabolism; Wnt Proteins/physiology; beta Catenin/genetics; beta Catenin/metabolism; beta Catenin/physiology
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