Evidence that Hoxa expression domains are evolutionarily transposed in spinal ganglia, and are established by forward spreading in paraxial mesoderm

Expression of a Hoxa-7/lacZ reporter construct in transgenic mouse embryos is shifted anteriorly when the upstream enhancer is multimerized. The shift occurs in spinal ganglia, neurectoderm and in both paraxial and lateral plate mesoderms. Much of the multimer effect is inhibited by destruction of a single caudal (cdx) binding motif in the additional copies of the enhancer. These observations are in agreement with earlier enhancer multimerization analyses made for Hoxb-8 (Charité et al., 1998). Our findings therefore provide further evidence that the anterior limit of a Hox gene's expression domain is normally dependent upon and is determined by, the dosage of transcription factor(s) which bind to its enhancer element(s) and that these factors may be, or must include, the cdx proteins. We consider these findings in terms of both instructional (morphogen-like) gradient and timing models for the establishment of Hox gene expression domains. Enhancer multimerization results in an earlier onset of Hoxa-7/lacZ activity in the embryo. In neurectoderm at 8.7 days and in mesoderm at 10.5 days, the anterior boundaries of expression are located posterior to those seen at some earlier stages of development. We discuss how these findings are in keeping with a model where Hox expression boundaries become set along instructional cdx gradients, formed by cdx decay in cells moving away from the primitive streak region.

Section: Introductionmentioning

confidence: 67%

“…Three transient transgenic embryos showed similar neural expression boundaries and boundaries in paraxial mesoderm that varied from pv12 to pv13 (Gaunt et al, 1999).…”

Section: Resultsmentioning

confidence: 95%

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Additional enhancer copies, with intact cdx binding sites, anteriorize Hoxa-7/lacZ expression in mouse embryos: evidence in keeping with an instructional cdx gradient

Gaunt¹,

Cockley²,

Drage³

2004

“…Despite the attractiveness of this hypothesis because of its simplicity, Hox gene expression can "spread" among adjacent cells as a result of cell interactions and is not fixed solely according to lineage history (Deschamps and Wijgerde, 1993;Gaunt and Strachan, 1994;Gaunt et al, 1999;Forlani et al, 2003). Moreover, cell interactions across germ layers are required at much later stages to fix the precise boundaries of Hox genes, which remain plastic for a long time.…”

Section: Hox Genes and Positional Addressesmentioning

confidence: 99%

Head-tail patterning of the vertebrate embryo: one, two or many unresolved problems?

Stern

Charité²,

Deschamps³

et al. 2006

132

113

When, where and how is the head-tail axis of the embryo set up during development? These are such fundamental and intensely studied questions that one might expect them to have been answered long ago. Not so; we still understand very little about the cellular or molecular mechanisms that lead to the orderly arrangement of body elements along the head-tail axis in vertebrates. In this paper, we outline some of the major outstanding problems and controversies and try to identify some reasons why it has been so difficult to resolve this important issue

“…For preparation of construct 1 (Fig. 1B), the two PCR fragments were assembled as in the normal cdx4 gene, but with lacZ/SV40 DNA (Gaunt et al, 1999) spliced at the 3' end. Construct 2 (Fig.…”

Section: Preparation Of Cdx4/lacz Reporter Constructsmentioning

confidence: 99%

cdx4/lacZ and cdx2/lacZ protein gradients formed by decay during gastrulation in the mouse

Gaunt

Drage²,

Trubshaw³

2005

Expression of the mouse caudal genes cdx4 and cdx2 is examined by use of lacZ reporter constructs expressed in transgenic mouse embryos. During early gastrulation, up to at least 8.5 days of development, reporter mRNA distributions are apparently similar to those of endogenous cdx mRNAs. By 8.25 to 8.8 days, cdx/lacZ protein activities have become distributed as posterior-to-anterior gradients along the neural and mesoderm tissues. The gradients form by decay of activity as cells become distanced from the regressing tailbud. In situ hybridization studies indicate that the decay is primarily in cdx/lacZ protein activities rather than mRNAs. As gastrulation proceeds, the locations of the gradients regress progressively posteriorly along the growing axis. Our results indicate how cdx4 and cdx2 protein gradients might be generated by decay during normal development. The smoothness of the gradients that we detect shows that there cannot be extensive mixing of cells once they leave the tailbud to contribute to the growing axis. An enhancer element located in the first intron of the cdx4 gene is essential for correct transgene expression.