BMP receptor signaling is required for postnatal maintenance of articular cartilage.

Ryan B Rountree,David M Kingsley,Vincent Harley,Michael Schoor,Hao Chen,Yuji Mishina,Melissa E Marks,Lee Niswander Lee Niswander

doi:10.1371/journal.pbio.0020355

Abstract

Articular cartilage plays an essential role in health and mobility, but is frequently damaged or lost in millions of people that develop arthritis. The molecular mechanisms that create and maintain this thin layer of cartilage that covers the surface of bones in joint regions are poorly understood, in part because tools to manipulate gene expression specifically in this tissue have not been available. Here we use regulatory information from the mouse Gdf5 gene (a bone morphogenetic protein [BMP] family member) to develop new mouse lines that can be used to either activate or inactivate genes specifically in developing joints. Expression of Cre recombinase from Gdf5 bacterial artificial chromosome clones leads to specific activation or inactivation of floxed target genes in developing joints, including early joint interzones, adult articular cartilage, and the joint capsule. We have used this system to test the role of BMP receptor signaling in joint development. Mice with null mutations in Bmpr1a are known to die early in embryogenesis with multiple defects. However, combining a floxed Bmpr1a allele with the Gdf5-Cre driver bypasses this embryonic lethality, and leads to birth and postnatal development of mice missing the Bmpr1a gene in articular regions. Most joints in the body form normally in the absence of Bmpr1a receptor function. However, articular cartilage within the joints gradually wears away in receptor-deficient mice after birth in a process resembling human osteoarthritis. Gdf5-Cre mice provide a general system that can be used to test the role of genes in articular regions. BMP receptor signaling is required not only for early development and creation of multiple tissues, but also for ongoing maintenance of articular cartilage after birth. Genetic variation in the strength of BMP receptor signaling may be an important risk factor in human osteoarthritis, and treatments that mimic or augment BMP receptor signaling should be investigated as a possible therapeutic strategy for maintaining the health of joint linings.

Highlights

Thin layers of articular cartilage line the bones of synovial joints and provide a smooth, wear-resistant structure that reduces friction and absorbs impact forces (Brandt et al 1998)
A bacterial artificial chromosome (BAC) containing the growth differentiation factor 5 (Gdf5) locus was modified by homologous recombination in bacteria to insert a cassette encoding Cre-internal ribosome entry site (IRES)human placental alkaline phosphatase into the translation start site of Gdf5 (Figure 1A)
The resulting Gdf5-Cre transgenic mice were tested for transgene expression and Cre recombinase activity by crossing them to lacZ ROSA26 Cre reporter strain (R26R) reporter mice that activate the expression of lacZ after Cre-mediated removal of transcriptional stop sequences (Soriano 1999)

Summary

Introduction

Thin layers of articular cartilage line the bones of synovial joints and provide a smooth, wear-resistant structure that reduces friction and absorbs impact forces (Brandt et al 1998). A better understanding of the molecular mechanisms that create and maintain articular cartilage is crucial for discovering the causes of joint disorders and providing useful medical treatments. Joint formation begins during embryogenesis, when stripes of high cell density called interzones form across developing skeletal precursors (Haines 1947). Non-joint precursors of the skeleton typically develop into cartilage, which hypertrophies and is replaced by bone. Cells within the highdensity layers of the interzone are excluded from this process and develop into the permanent layers of articular cartilage found in the mature joint (Mitrovic 1978)

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Conclusion