Studies about the regulation of cartilage growth and differentiation by Gdf5, main regulator of joint formation in digits

毛发-鼻-趾骨综合征（TRPS）是由TRPS 1基因突变引起的常染色体显性遗传骨骼疾病。虽然TRPS以指端骨骺变形为特征，但其发病机制尚不清楚。我们以前的研究表明，Trps 1蛋白的合成受到Gdf 5的调控，Gdf 5是关节形成的早期标志。在这项研究中，我们假设Trps 1在形成趾关节的Gdf 5信号通路中具有某些作用，并检查了Trps 1缺陷（KO）小鼠趾关节的形成。我们发现KO小鼠由于缺乏关节软骨表层，关节形成不完全。为了解决哪些蛋白质参与Gdf 5/Trps 1下游不完全形成的关节，我们通过RT-PCR检测了与关节形成相关的几种蛋白质的表达。我们发现，Prg 4，一个主要组成部分的滑液，减少在KO小鼠。Prg 4在野生型小鼠发育趾的浅层中表达，而在KO小鼠中不连续表达。使用ATDC 5细胞，软骨细胞系，我们发现，Gdf 5诱导Prg 4在ATDC 5细胞的表达和Trps 1促进Prg 4的Gdf 5诱导。这些数据表明，Gdf 5/Trps 1途径可能参与关节形成的数字通过诱导Prg 4表达在浅层。

Tricho-rhino-phalangeal syndrome (TRPS) is an autosomal dominant skeletal disorder caused by mutations of the TRPS1 gene. Althogh TRPS is characterized by deformed epiphysis of the digits, the pathological mechanism remain unknown. Our previous study showed that the Trps1 protein synthesis was regulated by Gdf5, an early marker of joint formation. In this study, we hypothesized that Trps1 has some roles in the Gdf5 signaling pathway that formed joints of the digits and examined the joint formation of the digits in the Trps1-dificient (KO) mice. We found that the joint formation was incomplete due to lack of the superficial layer of the articular cartilage in KO mice. To address which proteins are involved in the incompletely formed joints downstream of Gdf5/Trps1, we examined the expression of several proteins related to joint formation by RT-PCR. We found that Prg4, a major component of the synovial fluid, was reduced in KO mice. Prg4 was expressed in the superficial layer of the developing digits of wild type mice, whereas it was discontinuously expressed in KO mice. Using ATDC5 cells, a chondrogenic cell line, we revealed that Gdf5 induced Prg4 expression in ATDC5 cells and that Trps1 promoted the Prg4 induction by Gdf5. These data suggest that the Gdf5/Trps1 pathway may be involved in joint formation of the digits by inducing Prg4 expression in the superficial layer.