Sox9 is a transcription factor that is essential for chondrocyte differentiation and chondrocyte-specific gene expression. However, the precise mechanism of Sox9 activation during chondrogenesis is not fully understood. To investigate this mechanism, we performed functional gene screening to identify genes that activate SOX9-dependent transcription, using full-length cDNA libraries generated from a murine chondrogenic cell line, ATDC5. Screening revealed that TRPV4 (transient receptor potential vanilloid 4), a cation channel molecule, significantly elevates SOX9-dependent reporter activity. Microarray and quantitative real time PCR analyses demonstrated that during chondrogenesis in ATDC5 and C3H10T1/2 (a murine mesenchymal stem cell line), the expression pattern of TRPV4 was similar to the expression patterns of chondrogenic marker genes, such as type II collagen and aggrecan. Activation of TRPV4 by a pharmacological activator induced SOX9-dependent reporter activity, and this effect was abolished by the addition of the TRPV antagonist ruthenium red or by using a small interfering RNA for TRPV4. The SOX9-dependent reporter activity due to TRPV4 activation was abrogated by both EGTA and a calmodulin inhibitor, suggesting that the Ca 2؉ /calmodulin signal is essential in this process. Furthermore, activation of TRPV4 in concert with insulin activity in ATDC5 cells or in concert with bone morphogenetic protein-2 in C3H10T1/2 cells promoted synthesis of sulfated glycosaminoglycan, but activation of TRPV4 had no effect alone. We showed that activation of TRPV4 increased the steady-state levels of SOX9 mRNA and protein and SOX6 mRNA. Taken together, our results suggest that TRPV4 regulates the SOX9 pathway and contributes to the process of chondrogenesis.Chondrogenesis is an important biological event for endochondral bone development, skeletogenesis, and tissue patterning (1, 2). The first step in chondrogenesis is the aggregation of mesenchymal cells into prechondrogenic condensations. These condensations start to express cartilage-specific genes and further differentiate into mature chondrocytes. In the growth plate, chondrocytes proliferate and further differentiate into hypertrophic chondrocytes. The control of chondrogenic differentiation and hypertrophy plays a pivotal role in the process. Dysregulation of either step leads to severe skeletal dysplasia in both mice and humans (3).The transcription factor Sox9 (SRY (sex-related Y)-type high mobility group box), which contains a SRY-related high mobility group box, has an essential role in the chondrocyte differentiation pathway (4, 5). Sox9 regulates the transcription of cartilage-specific extracellular matrix molecules, such as collagen type II (6), IX (7), and XI (8) and aggrecan (9). Heterozygous mutations in the SOX9 gene cause campomelic dysplasia characterized by severe chondrodysplasia (10). Sox9 heterozygous mutant mice and mice lacking SOX9 function show impaired endochondral bone formation (4, 5). Sox9 is also involved in the expression of Sox5 and So...

Genetic and cell biological studies have indicated that Indian hedgehog (Ihh) plays an important role in bone development and osteoblast differentiation. However, the molecular mechanism by which Ihh regulates osteoblast differentiation is complex and remains to be fully elucidated. In this study, we investigated the role of Ihh signaling in osteoblast differentiation using mesenchymal cells and primary osteoblasts. We observed that Ihh stimulated alkaline phosphatase (ALP) activity, osteocalcin expression, and calcification. Overexpression of Gli2-but not Gli3-induced ALP, osteocalcin expression, and calcification of these cells. In contrast, dominant-negative Gli2 markedly inhibited Ihh-dependent osteoblast differentiation. Ihh treatment or Gli2 overexpression also up-regulated the expression of Runx2, an essential transcription factor for osteoblastogenesis, and enhanced the transcriptional activity and osteogenic action of Runx2. Coimmunoprecipitation analysis demonstrated a physical interaction between Gli2 and Runx2. Moreover, Ihh or Gli2 overexpression failed to increase ALP activity in Runx2-deficient mesenchymal cells. Collectively, these results suggest that Ihh regulates osteoblast differentiation of mesenchymal cells through up-regulation of the expression and function of Runx2 by Gli2.

The Sox9 transcription factor plays an essential role in promoting chondrogenesis and regulating expression of chondrocyte extracellular-matrix genes. To identify genes that interact with Sox9 in promoting chondrocyte differentiation, we screened a cDNA library generated from the murine chondrogenic ATDC5 cell line to identify activators of the collagen, type II, α 1 (Col2a1) promoter. Here we have shown that paraspeckle regulatory protein 54-kDa nuclear RNA-binding protein (p54 nrb ) is an essential link between Sox9-regulated transcription and maturation of Sox9-target gene mRNA. We found that p54 nrb physically interacted with Sox9 and enhanced Sox9-dependent transcriptional activation of the Col2a1 promoter. In ATDC5 cells, p54 nrb colocalized with Sox9 protein in nuclear paraspeckle bodies, and knockdown of p54 nrb suppressed Sox9-dependent Col2a1 expression and promoter activity. We generated a p54 nrb mutant construct lacking RNA recognition motifs, and overexpression of mutant p54 nrb in ATDC5 cells markedly altered the appearance of paraspeckle bodies and inhibited the maturation of Col2a1 mRNA. The mutant p54 nrb inhibited chondrocyte differentiation of mesenchymal cells and mouse metatarsal explants. Furthermore, transgenic mice expressing the mutant p54 nrb in the chondrocyte lineage exhibited dwarfism associated with impairment of chondrogenesis. These data suggest that p54 nrb plays an important role in the regulation of Sox9 function and the formation of paraspeckle bodies during chondrogenesis.

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Osteocytes within the mineralized bone matrix control bone remodeling by regulating osteoblast and osteoclast activity. Osteocytes express the aging suppressor Klotho, but the functional role of this protein in skeletal homeostasis is unknown. Here we identify Klotho expression in osteocytes as a potent regulator of bone formation and bone mass. Targeted deletion of Klotho from osteocytes led to a striking increase in bone formation and bone volume coupled with enhanced osteoblast activity, in sharp contrast to what is observed in Klotho hypomorphic (kl/kl) mice. Conversely, overexpression of Klotho in cultured osteoblastic cells inhibited mineralization and osteogenic activity during osteocyte differentiation. Further, the induction of chronic kidney disease with high-turnover renal osteodystrophy led to downregulation of Klotho in bone cells. This appeared to offset the skeletal impact of osteocyte-targeted Klotho deletion. Thus, our findings establish a key role of osteocyte-expressed Klotho in regulating bone metabolism and indicate a new mechanism by which osteocytes control bone formation.

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Background: Ihh is required for chondrocyte differentiation with redundant functions on multiple differentiation steps. Results: Ihh induces collagen type X expression and promotes its transcription through Gli1/2 cooperating with Runx2/Smads on a specific promoter region. Conclusion: Ihh signaling plays an important role in Col X expression and mineralization. Significance: This is the first detailed description of the molecular mechanism by which Ihh signaling controls late chondrocyte differentiation.

SummarySox9 is an essential transcription factor for chondrogenesis by regulating the expression of chondrogenic genes. However, its regulatory mechanism is not fully understood. To address this, we attempted to identify the transcriptional partners of Sox9 by screening the cDNA library of the chondrogenic cell line ATDC5 using the collagen 21 (Col21) gene promoter fused to a luciferase reporter gene. One of the positive clones encoded the Znf219 gene. Whole mount in situ hybridization experiments indicated that Znf219 mRNA was specifically expressed in the developing limb buds where Col21 and Sox9 were strongly expressed. Znf219 markedly enhanced the transcriptional activity of Sox9 on the Col2a1 gene promoter. In addition, Znf219 is physically associated with Sox9 and is colocalized with Sox9 in the nucleus. We also found that overexpression of Znf219 profoundly increased Sox9-induced mRNA expression of Col2a1, aggrecan and Col11a2. Consistently, knockdown of Znf219 decreased the Sox9-induced mRNA expression of these genes. Furthermore, a dominant-negative mutant Znf219 inhibited Bmp2-induced chondrocyte differentiation. Our results suggest that Znf219 plays an important role in the regulation of chondrocyte differentiation as a transcriptional partner of Sox9.