Cdx1 Autoregulation Is Governed by a Novel Cdx1-LEF1 Transcription Complex

The Cdx1 gene product is essential for normal anterior-posterior vertebral patterning. Expression of Cdx1 is regulated by several pathways implicated in anterior-posterior patterning events, including retinoid and Wnt signaling. We have previously shown that retinoic acid plays a key role in early stages of Cdx1 expression at embryonic day 7.5 (E7.5), while both Wnt3a signaling and an autoregulatory loop, dependent on Cdx1 itself, are involved in later stages of expression (E8.5 to E9.5). This autoregulation is reflected by the ability of Cdx1 to affect expression from proximal Cdx1 promoter sequences in tissue culture. However, this region is devoid of a demonstrable Cdx response element(s). We have now found that Cdx1 and LEF1, a nuclear effector of Wnt signaling, synergize to induce expression from the Cdx1 promoter through previously documented LEF/T-cell factor response elements. We also found a direct physical interaction between the homeodomain of Cdx1 and the B box of LEF1, suggesting a basis for this synergy. Consistent with these observations, analysis of Cdx1 Wnt3a vt compound mutants demonstrated that Wnt and Cdx1 converged on Cdx1 expression and vertebral patterning in vivo. Further data suggest that Cdx-high-mobility group box interactions might be involved in a number of additional pathways. Somites are derived from segmentation of the paraxial mesoderm in the caudal embryo and subsequently differentiate into the dermamyotome and sclerotome, the latter being the anlage of the vertebrae. Many vertebrae exhibit morphological differences along the anterior-posterior axis, such as the ribs characteristic of thoracic vertebrae. These distinct morphological characteristics are indicative of patterning events which dictate vertebral identity along the anterior-posterior axis. A number of signaling molecules, such as retinoic acid, are well documented to affect vertebral anterior-posterior patterning. All such effectors typically impact on the expression of Hox genes, and a wealth of gain- and loss-of-function experiments clearly demonstrate a critical role for Hox gene products in vertebral patterning (see, e.g., references 9, 11, 13, 19, and 27). The 39 murine Hox genes are distributed in four clusters, Hoxa to Hoxd, which have likely evolved by duplication of an ancestral complex related to the HOM-C genes of Drosophila melanogaster (17, 18, 20). In the mouse, Hox expression is initiated at embryonic day 7.5 (E7.5) in the primitive streak, with transcripts subsequently expanding anteriorly in the neural tube and mesoderm to eventually reach a predetermined rostral limit (14, 45, 50). The onset and rostral limit of expression are generally related to the location of a given Hox gene within its complex, with more 3 members initiated earlier and reaching more rostral limits of expression than 5 paralogs. This results in staggered domains of Hox expression along the anterior-posterior axis, which have been suggested to comprise a Hox code (5, 23, 33).