Spatio-temporal expression of Pbx3 during mouse organogenesis.

Pbx3 is a member of the Pbx family of TALE (three amino acid loop extension) class homeodomain transcription factors. These transcription factors are implicated in developmental and transcriptional gene regulation in numerous cell types through their abilities to form hetero-oligomeric DNA-binding complexes. Pbx3 was found to be expressed at high levels in the developing central nervous system (CNS), including a region of the medulla oblongata which is implicated in the control of respiration. Furthermore, as reported, Pbx3-deficient mice develop to term but die within a few hours of birth from central respiratory failure. In this study, we have characterized Pbx3 expression patterns during organogenesis in numerous tissues and organ systems other than the CNS, as a first step toward understanding the potentially overlapping functions of Pbx3 with other Pbx family members during vertebrate development. We have performed in situ hybridization on whole mount and sectioned mouse embryos from gestational day (E) 9 to E16.5. During early organogenesis, until E12.5, Pbx3 expression is found mostly in the embryonic head, forelimbs, and septum transversum, unlike Pbx1 and Pbx2 expression which is more widespread. Conversely, later in organogenesis, Pbx3 expression becomes more widely detectable throughout the developing embryo. Epithelial and mesenchymal tissues, as well as the CNS, represent major sites of Pbx3 expression. The enteric nervous system also expresses high levels of Pbx3, distinctively in the cells of the ganglia of Auerbach's myenteric nerve plexus, that also express Dlx2 and Notch1. Cartilage is also a site of Pbx3 expression. Interestingly, like Pbx1, Pbx3 is highly expressed in proliferating chondrocytes but is lost as chondrocytes become hypertrophic during endochondral ossification. Finally, Pbx3 is expressed only in the forelimb buds during early limb development, while the hindlimb bud is devoid of Pbx3. This finding leads us to add Pbx3 to the sparse list of early forelimb-specific molecular markers.