Hox and Pbx Factors Control Retinoic Acid Synthesis during Hindbrain Segmentation

In vertebrate embryos, retinoic acid (RA) synthesized in the mesoderm by Raldh2 emanates to the hindbrain neuroepithelium, where it induces anteroposterior (AP)-restricted Hox expression patterns and rhombomere segmentation. However, how appropriate spatiotemporal RA activity is generated in the hindbrain is poorly understood. By analyzing Pbx1/Pbx2 and Hoxa1/Pbx1 null mice, we found that Raldh2 is itself under the transcriptional control of these factors and that the resulting RA-deficient phenotypes can be partially rescued by exogenous RA. Hoxa1-Pbx1/2-Meis2 directly binds a specific regulatory element that is required to maintain normal Raldh2 expression levels in vivo. Mesoderm-specific Xhoxa1 and Xpbx1b knockdowns in Xenopus embryos also result in Xraldh2 downregulation and hindbrain defects similar to mouse mutants, demonstrating conservation of this Hox-Pbx-dependent regulatory pathway. These findings reveal a feed-forward mechanism linking Hox-Pbx-dependent RA synthesis during early axial patterning with the establishment of spatially restricted Hox-Pbx activity in the developing hindbrain.