Murine hoxd4 expression in the CNS requires multiple elements including a retinoic acid response element

We have identified a retinoic acid response element (RARE) within a neural enhancer located 3' to the Hoxd4 gene. This RARE is required for the initiation and maintenance of Hoxd4 transgene expression in neurectoderm, and for full anteriorized expression upon retinoic acid (RA) treatment. Mutations within the sequence TTTTCTG, located 2 bp downstream of the RARE, posteriorized transgene activity. However, the onset of transgene expression and its response to RA were indistinguishable from wild type. While the TTTTCTG motif resembles a CDX binding site, human CDX1 protein did not interact with this element in vitro. Three additional regions were also shown to control transgene expression in neurectoderm, establishing that multiple elements constitute the Hoxd4 neural enhancer.     

The role of a retinoic acid response element in establishing the anterior neural expression border of Hoxd4 transgenes

The zebrafish hoxd4a locus was compared to its murine ortholog, Hoxd4. The sequence of regulatory elements, including a DR5 type retinoic acid response element (RARE) required for Hoxd4 neural enhancer activity, are highly conserved. Additionally, zebrafish and mouse neural enhancers function identically in transgenic mouse embryos. We tested whether sequence conservation reflects functional importance by altering the spacing and sequence of the RARE in the Hoxd4 neural enhancer. Stabilizing receptor-DNA interactions did not anteriorize transgene expression. By contrast, conversion of the RARE from a DR5 to a DR2 type element decreased receptor-DNA stability and posteriorized expression. Hence, the setting of the Hox anterior expression border is not a simple function of the affinity of retinoid receptors for their cognate element.