Abstract
We have recently shown that a 150-bp Col10a1 distal promoter (−4296 to −4147 bp) is sufficient to direct hypertrophic chondrocyte-specific reporter (LacZ) expression in vivo. More recently, through detailed sequence analysis we identified two putative tandem-repeat Runx2 binding sites within the 3′-end of this 150-bp region (TGTGGG-TGTGGC, −4187 to −4176 bp). Candidate electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation, and transfection studies demonstrate that these putative Runx2 sites bind Runx2 and mediate upregulated Col10a1/reporter activity in vitro. Transgenic studies using the 5′-sequence without Runx2 sites were not able to drive the cell-specific LacZ reporter activity, suggesting the in vivo requirement of the Runx2 sites located in the 3′-end in mediating Col10a1/reporter expression. Indeed, mutating the Runx2 sites in the context of the 150-bp promoter abolishes its capacity to drive hypertrophic chondrocyte-specific reporter expression in transgenic mice. We have also generated multiple transgenic mouse lines using only the 3′-sequence containing the Runx2 sites to drive the LacZ gene. Interestingly, no hypertrophic chondrocyte-specific blue staining was observed in these transgenic mice. Together, our data support that Runx2 directly interacts with murine Col10a1 cis-enhancer. This interaction is required but not sufficient for cell-specific Col10a1 promoter activity in vivo. Additional cooperative/repressive elements within the 5′- or 3′-sequences of this 150-bp promoter are needed to work with Runx2 together to mediate cell-specific Col10a1 expression. Further delineation of these elements/factors has the potential to identify novel therapeutic targets for multiple skeletal disorders, including osteoarthritis, that show abnormal Col10a1 expression and altered chondrocyte maturation. © 2011 American Society for Bone and Mineral Research
Highlights
Chondrocyte maturation is the terminal phase of chondrocyte differentiation, a critical stage of endochondral ossification linking both bone and cartilage formation during skeletal development
These findings have clearly demonstrated that physiological distribution of type X collagen during chondrocyte hypertrophy is essential for endochondral bone formation in skeletal development, whereas altered Col10a1 expression is observed in multiple skeletal disorders associated with abnormal chondrocyte maturation
We have previously shown that a 150-bp Col10a1 distal promoter (À4296 to À4147 bp) is sufficient to direct hypertrophic chondrocyte-specific reporter expression in vivo.[16]. To further localize the cis enhancer element and to identify its binding factors, we have performed electrophoretic mobility shift assay (EMSA) using a series of annealed DNA oligomers derived from this 150-bp promoter and the hypertrophic MCT cell nuclear extracts
Summary
Chondrocyte maturation is the terminal phase of chondrocyte differentiation, a critical stage of endochondral ossification linking both bone and cartilage formation during skeletal development. Type X collagen synthesis is observed in the cartilaginous callus, which is composed of hypertrophic and degenerative chondrocytes, suggesting increased vascularity and matrix mineralization during fracture repair.[10] As to the correlation of COL10A1 expression and chondrocyte maturation with osteoarthritis, previous studies have reported the upregulation of COL10A1 and enhanced chondrocyte hypertrophy in human osteoarthritic cartilage.[11,12] It was suggested that upon osteoarthritis progression, factors that constrain articular chondrocyte maturation are relieved These articular chondrocytes achieve a mature phenotype that is characterized by expression of hypertrophic hallmarks, including Col10a1.(13,14)
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