Abstract
Antxr1/Tem8 is highly expressed in tumor endothelial cells and is a receptor for anthrax toxin. Mutation of Antxr1 causes GAPO syndrome, which is characterized by growth retardation, alopecia, pseudo-anodontia, and optic atrophy. However, the mechanism underlying the growth retardation remains to be clarified. Runx2 is essential for osteoblast differentiation and chondrocyte maturation and regulates chondrocyte proliferation through Ihh induction. In the search of Runx2 target genes in chondrocytes, we found that Antxr1 expression is upregulated by Runx2. Antxr1 was highly expressed in cartilaginous tissues and was directly regulated by Runx2. In skeletal development, the process of endochondral ossification proceeded similarly in wild-type and Antxr1–/– mice. However, the limbs of Antxr1–/– mice were shorter than those of wild-type mice from embryonic day 16.5 due to the reduced chondrocyte proliferation. Chondrocyte-specific Antxr1 transgenic mice exhibited shortened limbs, although the process of endochondral ossification proceeded as in wild-type mice. BrdU-uptake and apoptosis were both increased in chondrocytes, and the apoptosis-high regions were mineralized. These findings indicated that Antxr1, of which the expression is regulated by Runx2, plays an important role in chondrocyte proliferation and that overexpression of Antxr1 causes chondrocyte apoptosis accompanied by matrix mineralization.
Highlights
Anthrax toxin receptor 1 (Antxr1)/tumor endothelial marker 8 (Tem8) was discovered in the screening of proteins expressed in tumor endothelium at an increased level and identified as a receptor for protective antigen (PA) of anthrax toxin [1,2]
We report that Antxr1 expression is directly regulated by Runx2, that chondrocyte proliferation is reduced in Antxr1–/– mice, and that the overexpression of Antxr1 in chondrocytes increases DNA replication and apoptosis
We introduced Runx2or green fluorescent protein (GFP)-expressing adenovirus into Runx2–/– primary chondrocytes or wild-type primary chondrocytes and examined Antxr1 expression by real-time RT-PCR (Figure 1A,B)
Summary
Anthrax toxin receptor 1 (Antxr1)/tumor endothelial marker 8 (Tem8) was discovered in the screening of proteins expressed in tumor endothelium at an increased level and identified as a receptor for protective antigen (PA) of anthrax toxin [1,2]. Mutation of Antxr results in GAPO syndrome, which is an autosomal recessive disorder and characterized by growth retardation, alopecia, pseudo-anodontia, and progressive visual impairment. It is characterized by an abnormal accumulation of extracellular matrix [3,4]. Antxr1–/– mice exhibit dwarfism; excess extracellular matrix in ovaries, uterus, skin, and periodontal ligament of the incisors; and the accumulation of collagen I and VI [7,8]. Antxr1–/– mice exhibit dwarfism, the function of Antxr in skeletal development remains to be clarified
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