Cdx1 is essential for the initiation of HoxC8 expression during early embryogenesis

The Hox genes pattern the anterior-posterior axis in developing embryos through tightly regulated, partially overlapping, temporal and spatial expression domains. Initial regulation of Hox expression is important to establish these overlapping transcription domains. The Cdx homeodomain factors have been proposed as Hox regulators, but their precise role and mechanism during this regulatory interaction remain unclear. In Xenopus embryos, HoxC8 transcripts begin to accumulate during mid/late gastrula. Cdx1 overexpression and knockdown lead to precocious or slower HoxC8 expression, respectively. The mouse HoxC8 early enhancer when introduced into Xenopus embryos recapitulates the endogenous XHoxC8 temporal expression pattern and shows the same responsiveness to Cdx1 regulation. Three pairs of conserved Cdx binding sites were identified within the HoxC8 early enhancer. We demonstrate that Cdx1 binds directly these responsive elements during embryogenesis, as part of the mechanism for the timely activation of HoxC8 expression. We define the function and mechanism of Cdx1 regulation on HoxC8 expression and suggest the possibility that the temporal changes in Cdx activity levels during gastrulation, combined with differential DNA binding affinity, might play a role in the establishment of Hox sequential activation.