Multiple Pathways Governing Cdx1 Expression during Murine Development

Abstract

Cdx1 encodes a mammalian homeobox gene involved in vertebral patterning. Retinoic acid (RA) is likewise implicated in vertebral patterning. We have previously shown that Cdx1 is a direct retinoid target gene, suggesting that Cdx1 may convey some of the effects of retinoid signaling. However, RA appears to be essential for only early stages of Cdx1 expression, and therefore other factors must be involved in maintaining later stages of expression. Based on function and pattern of expression, Wnt family members, in particular Wnt3a, are candidates for regulation of expression of Cdx1. Consistent with this, we confirm prior results which demonstrated that Cdx1 can be directly regulated by Wnt signaling, and identify functional LEF/TCF response motifs essential for this response. We also find that Cdx1 expression is markedly attenuated in a stage- and tissue-specific fashion in the Wnt3a hypomorph vestigial tail, and present data demonstrating that Wnt3a and RA synergize strongly to activate Cdx1. Finally, we show that Cdx1 positively regulates its own expression. These data prompt a model whereby retinoid and Wnt signaling function directly and synergistically to initiate Cdx1 expression in the caudal embryo. Expression is then maintained, at least in part, by an autoregulatory mechanism at later stages.

Caudal-related Homeobox (Cdx) Protein-dependent Integration of Canonical Wnt Signaling on Paired-box 3 (Pax3) Neural Crest Enhancer

“…In this regard, it is interesting to note that all three Cdx genes are direct targets of Wnt⅐␤-catenin signaling in P19 cells. Although this was expected for Cdx1 and Cdx4 (27)(28)(29)(30), such an outcome was somehow surprising for Cdx2, given a previous report indicating that exogenous Wnt3a can specifically induce Cdx1 but not Cdx2 in embryo culture (27). However, this result is in agreement with more recent work suggesting that Cdx2 is also a direct target of canonical Wnt signals (39 -42, 44).…”

“…This delay being suggestive of an indirect regulation, we then directly assessed the necessity for an intermediary factor by inhibiting de novo protein synthesis with the protein synthesis inhibitor CHX. Cdx1 and Cdx4 are known direct targets of Wnt⅐␤-catenin signaling (27)(28)(29)(30). Accordingly, Cdx1 and Cdx4 induction by Wnt3a-conditioned medium is not affected by inhibition of protein synthesis (Fig.…”

“…Cdx gene expression is regulated by posteriorizing signals from Wnt, retinoic acid (RA), and fibroblast growth factor (FGF) pathways in multiple species (27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38). Among these posteriorizing signals, the evolutionarily conserved role of the canonical Wnt pathway in Cdx regulation is the best characterized.…”

“…Among these posteriorizing signals, the evolutionarily conserved role of the canonical Wnt pathway in Cdx regulation is the best characterized. Indeed, both Cdx1 and Cdx4 have been clearly identified as direct targets of the Wnt⅐␤-catenin pathway (27,29,30). Moreover, other data suggest that Cdx2 is also responsive to canonical Wnt signals, although evidence for a direct regulation is sparse (39 -45).…”

Chicken Ovalbumin Upstream Promoter-Transcription Factor Members Repress Retinoic Acid-induced Cdx1 Expression

“…By E8.5, Cdx1 expression is maintained in the caudal embryo through mechanisms involving both Wnt signaling (8) and autoregulation (9). RA, however, is absent in this region at this stage but is found in a more anterior domain in the prospective trunk (10).…”