Retinoic Acid Regulation of Cdx1: an Indirect Mechanism for Retinoids and Vertebral Specification

Abstract

Retinoic acid (RA) is required for diverse developmental programs, including vertebral specification. Both RA receptor disruption and excess RA result in homeotic transformations of the axial skeleton. These effects are believed to occur through altered expression of Hox genes, several of which have been demonstrated to be direct RA targets. Members of the cdx (caudal) homeobox gene family are also implicated in regulating Hox expression. Disruption of cdx1 results in vertebral homeotic transformations and alteration of Hox expression boundaries; similar homeosis is also observed in cdx2 heterozygotes. In Xenopus, gain or loss of Cdx function affects vertebral morphogenesis through a mechanism that also correlates with altered Hox expression. Taken together with the finding of putative Cdx binding motifs in several Hox promoters, these data strongly support a role for Cdx members in direct regulation of expression of at least some Hox genes. Most retinoid-responsive Hox genes have not been demonstrated to be direct RA targets, suggesting that intermediaries are involved. Based on these findings, we hypothesized that one or more cdx members may transduce the effects of RA on Hox transcription. Consistent with this, we present evidence that cdx1 is a direct RA target gene, suggesting an additional pathway for retinoid-dependent vertebral specification.

Negative autoregulation of Oct3/4 through Cdx1 promotes the onset of gastrulation

“…Activation of this regulatory module by the onset of gastrulation requires the activation and up-regulation of the Cdx genes. Multiple signals have been identified that contribute to Cdx expression including FGF, Wnt, BMP, and retinoic acid (Pillemer et al, 1998b;Houle et al, 2000;Keenan et al, 2006;Pilon et al, 2006). The surprising finding was that, initially, during late blastula, the Oct3/4 factors promote Cdx1 expression through up-regulation of FGF signaling.…”

Origins of Cdx1 regulatory elements suggest roles in vertebrate evolution

“…Mouse Cdx1 is activated by retinoic acid (RA) in both embryos (Houle et al, 2000) and P19 or F9 embryonal carcinoma cells (Beland and Lohnes, 2005;Houle et al, 2000). RA response reporter studies show that RA is present in the embryonic regions known to express Cdx1: the primitive streak of 7.5 day mouse embryos and, by 8.5 days, the developing somites (Rossant et al, 1991).…”

“…The Cdx1 upstream RARE (Houle et al, 2000) is atypical in containing only one clear consensus element.…”

“…RA activates Cdx1 by a retinoic acid response element (RARE) located upstream of the coding region (Houle et al, 2000). Mutation of this RARE results in homeotic shifts in cervical vertebrae, indicating that this element plays a functional role (Houle et al, 2003b).…”

“…The construct contains the upstream RARE (Houle et al, 2000) and also the putative intron RARE earlier identified as conserved in chicken Cdx1 (CdxA) (Gaunt et al, 2003). We first examined luciferase (luc) reporters since these are most commonly used in culture assays.…”