An 18-base-pair sequence in the mouse proalpha1(II) collagen gene is sufficient for expression in cartilage and binds nuclear proteins that are selectively expressed in chondrocytes

Toward understanding SOX9 function in chondrocyte differentiation

The transcription factors that trigger the determinative switch to chondrocyte differentiation in mesenchymal cells are still unknown. In humans, mutations in the gene for SOX9, a transcription factor with a DNA-binding domain similar to that of the mammalian testis-determining factor SRY, cause campomelic dysplasia, a severe dwarfism syndrome which affects all cartilage-derived structures. During mouse embryonic development, the Sox9 gene becomes active in all prechondrocytic mesenchymal condensations, and at later stages its expression is maintained at high levels in fully differentiated chondrocytes. A chondrocyte-specific enhancer in the gene for collagen type II (Col2a1), a characteristic marker of chondrocytes, is a direct target for SOX9, and ectopic expression of SOX9 in transgenic mouse embryos is sufficient to activate the endogenous Col2a1 gene in some tissues. These data suggest that SOX9 could have a major role in chondrogenesis. Studies are in progress to identify other target genes for SOX9 in chondrocytes and also other transcription factors that are believed to cooperate with SOX9 in the activation of chondrocyte-specific genes. Defining SOX9 function and the mechanisms that regulate SOX9 gene expression should contribute to a better understanding of chondrocyte differentiation.

A transcription activator with restricted tissue distribution regulates cell-specific expression of alpha1(XI) collagen

Different regulatory programs are likely to control expression of the alpha1(XI) collagen (COL11A1) gene in cartilaginous and non-cartilaginous tissues and in coordination with different collagen genes. Here, we report the identification of a cis-acting element that is required for constitutive and tissue-specific activity of the proximal COL11A1 promoter. The element binds an apparently novel activator whose expression is restricted mostly, but not exclusively, to cells of mesenchymal origin. Transient transfection experiments using wild-type and mutant constructs demonstrated the critical contribution of a 45-base pair upstream element (FP9) to promoter activity. The same functional tests and DNA binding assays narrowed down the critical portion of FP9 to a 20-base pair sequence, which consists of an imperfect palindrome with strong homology to the GATA consensus motif. Despite being able to bind GATA proteins in vitro, FP9 is actually recognized by a distinct approximately 100-kDa polypeptide (FP9C) probably belonging to the zinc-finger family of transcription factors. FP9C binding was mostly identified in nuclei from cells of mesenchymal origin, including those actively engaged in COL11A1 transcription. A positive correlation was also established between the level of FP9C binding and the degree of cell differentiation in vitro. Thus, FP9C represents an unusual example of tissue-specific and differentiation-related transcription factor with overlapping expression in hard and soft connective tissues.

Parallel expression of Sox9 and Col2a1 in cells undergoing chondrogenesis

To assess the role of the transcription factor Sox9 in cartilage formation we have compared the expression pattern of Sox9 and Col2a1 at various stages of mouse embryonic development. Expression of Col2a1 colocalized with expression of Sox9 in all chondroprogenitor cells. In the sclerotomal compartment of somites the onset of Sox9 expression preceded that of Col2a1. A perfect correlation was also seen between high levels of Sox9 expression and high levels of Col2a1 expression in chondrocytic cells. However, no Sox9 expression was detected in hypertrophic chondrocytes; only low levels of Col2a1 RNA were found in the upper hypertrophic zone. Coexpression of Sox9 and Col2a1 was also seen in the notochord. At E11.5 Sox9 expression in the brain and spinal neural tube was more widespread than that of Col2a1 although at E14.5 Sox9 and Col2a1 transcripts were colocalized in discrete areas of the brain. Distinct differences between Sox9 and Col2a1 expression were observed in the otic vesicle at E11.5. At E8.5, expression of Sox9 but not of Col2a1 was seen in the dorsal tips of the neural folds and after neural tube closure also in presumptive crest cells emigrating from the dorsal pole of the neural tube. No Col2a1 expression was detected in gonadal ridges in which high levels of Sox9 expression were detected. Together with our previous results showing that the chondrocyte-specific enhancer element of the Col2a1 gene is a direct target for Sox9, these results suggest that Sox9 plays a major role in expression of Col2a1. The correlation between high expression levels of Sox9 and high expression levels of Col2a1 in chondrocytes suggests the hypothesis that high levels of Sox9 are needed for full expression of the chondrocyte phenotype; lower levels of Sox9 such as in neuronal tissues which are also associated with lower expression levels of Col2a1 would be compatible with other cell specifications.

SOX9 directly regulates the type-II collagen gene

Mutations in human SOX9 are associated with campomelic dysplasia (CD), characterised by skeletal malformation and XY sex reversal. During chondrogenesis in the mouse, Sox9 is co-expressed with Col2a1, the gene encoding type-II collagen, the major cartilage matrix protein. Col2a1 is therefore a candidate regulatory target of SOX9. Regulatory sequences required for chondrocyte-specific expression of the type-II collagen gene have been localized to conserved sequences in the first intron in rats, mice and humans. We show here that SOX9 protein binds specifically to sequences in the first intron of human COL2A1. Mutation of these sequences abolishes SOX9 binding and chondrocyte-specific expression of a COL2A1-driven reporter gene (COL2A1-lacZ) in transgenic mice. Furthermore, ectopic expression of Sox9 trans-activates both a COL2A1-driven reporter gene and the endogenous Col2a1 gene in transgenic mice. These results demonstrate that COL2A1 expression is directly regulated by SOX9 protein in vivo and implicate abnormal regulation of COL2A1 during, chondrogenesis as a cause of the skeletal abnormalities associated with campomelic dysplasia.

SOX9 is a potent activator of the chondrocyte-specific enhancer of the pro alpha1(II) collagen gene

The identification of mutations in the SRY-related SOX9 gene in patients with campomelic dysplasia, a severe skeletal malformation syndrome, and the abundant expression of Sox9 in mouse chondroprogenitor cells and fully differentiated chondrocytes during embryonic development have suggested the hypothesis that SOX9 might play a role in chondrogenesis. Our previous experiments with the gene (Col2a1) for collagen II, an early and abundant marker of chondrocyte differentiation, identified a minimal DNA element in intron 1 which directs chondrocyte-specific expression in transgenic mice. This element is also a strong chondrocyte-specific enhancer in transient transfection experiments. We show here that Col2a1 expression is closely correlated with high levels of SOX9 RNA and protein in chondrocytes. Our experiments indicate that the minimal Col2a1 enhancer is a direct target for Sox9. Indeed, SOX9 binds to a sequence of the minimal Col2a1 enhancer that is essential for activity in chondrocytes, and SOX9 acts as a potent activator of this enhancer in cotransfection experiments in nonchondrocytic cells. Mutations in the enhancer that prevent binding of SOX9 abolish enhancer activity in chondrocytes and suppress enhancer activation by SOX9 in nonchondrocytic cells. Other SOX family members are ineffective. Expression of a truncated SOX9 protein lacking the transactivation domain but retaining DNA-binding activity interferes with enhancer activation by full-length SOX9 in fibroblasts and inhibits enhancer activity in chondrocytes. Our results strongly suggest a model whereby SOX9 is involved in the control of the cell-specific activation of COL2A1 in chondrocytes, an essential component of the differentiation program of these cells. We speculate that in campomelic dysplasia a decrease in SOX9 activity would inhibit production of collagen II, and eventually other cartilage matrix proteins, leading to major skeletal anomalies.