Opposing roles of two isoforms of the Prx1 homeobox gene in chondrogenesis.

The Prx1 homeobox gene is critical for cartilage and bone development as suggested by previous expression studies and demonstrated by gene targeting. However, neither approach assessed the individual roles of the two isoforms Prx1a and Prx1b. In this study, Western blot analysis demonstrates that, in the early stages of chondrogenesis, during mesenchymal condensation, only Prx1a is expressed. Higher level Prx1b expression is concomitant with the formation of a defined perichondrium. Prx1a overexpression in limb micro mass cultures results in an increase in the number of prechondrogenic condensations and cartilage nodules, whereas overexpression of Prx1b results in a decrease. Prx1a increases the percentage of proliferating cells in micro mass cultures and decreases apoptosis. The Prx1b isoform does not alter proliferation, but it does increase apoptosis, which is opposite of Prx1a. These results suggest that the Prx1a:Prx1b ratio and the alternative splicing mechanism that generates these two isoforms are critical in controlling chondrogenesis.