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 Col2 alpha 1 at various stages of mouse embryonic development, Expression of Col2 alpha 1 colocalized with expression of Sox9 in all chondroprogenitor cells. In the sclerotomal compartment of somites the onset of Sox9 expression preceded that of Col2 alpha 1. A perfect correlation was also seen between high levels of Sox9 expression and high levels of Col2 alpha 1 expression in chondrocytic cells. However, no Sox9 expression was detected in hypertrophic chondrocytes; only low levels of Col2 alpha 1 RNA were found in the upper hypertrophic zone, Coexpression of Sox9 and Col2 alpha 1 was also seen in the notochord. At E11.5 Sox9 expression in the brain and spinal neural tube was more widespread than that of Col2 alpha 1 although at E14.5 Sox9 and Col2 alpha 1 transcripts were colocalized in discrete areas of the brain. Distinct differences between Sox9 and Col2 alpha 1 expression were observed in the otic vesicle at E11.5. At E8.5, expression of Sox9 but not of Col2 alpha 1 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 Col2 alpha 1 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 Col2 alpha 1 gene is a direct target for Sox9, these results suggest that Sox9 plays a major role in expression of Col2 alpha 1. The correlation between high expression levels of Sox9 and high expression levels of Col2 alpha 1 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 Col2 alpha 1 would be compatible with other cell specifications. (C) 1997 Wiley-Liss, Inc.