Prx Genes in Morphogenesis of Meckel's Cartilage

Prx1 and Prx2 are closely-related members of the paired-related family of homeobox genes and are co-expressed in a variety of sites, including craniofacial mesenchyme. Craniofacial abnormalities in Prx1-/- and Prx1-/-;Prx2-/- knock-out mice revealed essential roles of these gene products in craniofacial morphogenesis. Objective: To address the roles and functions of Prx genes in mandibular chondrogenesis. Methods: In situ hybridization and RT-PCR analyses were used to examine the expression of Prx1a, Prx1b and Prx2 during mandibular chondrogenesis in vivo and in vitro. To understand the function of Prx1a in mandibular chondrogenesis, retroviral vectors expressing Prx1a isoform were constructed (RCAS-Prx1a), characterized and used in gain of function studies in vivo and in vitro. The effects of over-expression of Prx1a on chondrogenesis were analyzed by various methods. Results: At all stages of development Prx1 and Prx2 were expressed in the undifferentiated mandibular mesenchyme and osteogenic regions. However, their expression was excluded from chondrogenic condensation, chondrocytes and chondrogenic nodules in micromass cultures. Skeletal staining at HH37 showed that embryos injected with RCAS-Prx1a at stage 20 did not display severe abnormalities in mandibular outgrowth, morphogenesis of Meckel's cartilage and mandibular bones on the injected side. Furthermore, RCAS-Prx1a had no discernable effect on in vitro chondrogenesis. The lack of noticeable abnormalities on in vivo and in vitro mandibular chondrogenesis was not related to unsuccessful infection by RCAS-Prx1a. Immunocytochemical analysis showed widespread expression of gag protein in the injected side of mandibles and in infected micromass cultures. Furthermore, RCAS-Prx1a resulted in significant increases in Tenascin-C expression, previously identified as a downstream target of Prx1. Conclusions: These observations provide evidence for nonessential roles of Prx genes in mandibular chondrogenesis and suggest that abnormalities in the Prx1-/-; Prx2-/- double knock-out mice may be indirect and/or related to changes in the cell fate. Supported by NIH grant DE08682.