Signaling centers in the mouse embryo


Role of Wnts during mouse development

It was shown that Wnt3 mutant embryos lack a primitive streak and therefore no mesoderm is formed during gastrulation. The anterior visceral endoderm (aVE), a cell layer underlying the future head region and located in the embryo on the side opposite to the primitive streak, is, however, properly located and correctly patterned. Moreover, a mutation in the beta-Catenin gene (Catnb), a cellular effector of the Wnt pathway, leads to a lack of mesoderm, confirming the role of Wnt in axis formation.
It has also been shown that Dkk1, a Wnt antagonist, is expressed in the aVE region of the gastrula underlying the future head. However, the analysis of the Dkk1 mutant phenotype shows that it is not required for the formation of the head at the early gastrula stage but that it is required at a later stage. In contrast, in the zebrafish, the tlc Wnt antagonist has been shown to be necessary for the initial patterning of the anterior brain. Moreover, another Wnt antagonist, Sfrp5 is also expressed in the aVE.
These gastrula organizing activities are reflected in the expression pattern and role of Wnt signals in the primitive streak and of Wnt antagonists in the aVE (reviewed in Leyns and Kemp, 2004).

As gastrulation proceeds and organs start to be formed, other Wnt genes begin to be expressed in specific domains and several have been identified to play critical roles during development. The inactivation of Wnt1, Wnt2, Wnt3a, Wnt4, Wnt5a, Wnt7a, Wnt7b and Wnt11 lead to embryonic phenotypes (see phenotype overview and references on the Wnt gene Homepage).
A serious difficulty encountered in the study of the functions of the Wnt genes is their potential redundancy since various Wnts can bind to the same Frizzled receptor and trigger the activation of the pathway. This has been shown in a study of mouse embryos double mutant for Wnt1 and Wnt3a. Therefore, the role of some Wnts or of their antagonists during mouse development may not be revealed by loss-of-function experiments.

Research: gene expression studies

To identify the Wnt genes and their antagonists that may be involved in the antero-posterior patterning of the gastrula and in the formation of the blastocyst, we analyzed the expression level of all nineteen Wnt genes and eleven potential antagonists by reverse transcription real-time PCR (qRT-PCR) in the mouse blastocyst, pre-gastrula, gastrula and neurula stages. This approach allowed us to carry out a semi-quantitative study with a high sensitivity of complete gene families involved in Wnt signaling. The most interesting profiles were further investigated by whole-mount ISH to reveal new expression domains for Wnt1, Wnt2b, Wnt3a, Wnt4, Wnt6, Wnt7b, Wnt9a, Wnt10b, Sfrp1, Sfrp5 and Dkk1.

Wnts in the mouse blastocyst

To test the possibility that Wnts are involved in the maintenance of pluripotency of blastocyst cells, we analyzed the mRNA levels of the Wnt and antagonist genes and the expression domains of the selected candidate genes by ISH on blastocyst embryos. We have shown that the highest expression level for the Wnt3a, -6, -7b, -9a and -10b genes was in the blastocyst.
We found that Wnt3a, Wnt6, Wnt7b and Wnt10b are expressed in the entire blastocyst but that Wnt6 expression is stronger in the cells surrounding the blastocoel cavity. We also found that Wnt1 and Sfrp1 are predominantly expressed in the inner cell mass of the blastocyst. The expression of the Wnt1, -3a, -6 and -7b genes, known to trigger the Wnt canonical pathway in which beta-Catenin is a key player, may provide autocrine and paracrine Wnt signals to the inner cell mass cells. Neither the gene inactivation of Wnt1 nor that of Wnt3a or Wnt7b leads to pre-implantation defects, suggesting that these Wnts may play redundant roles during patterning of the pre-implantation embryo.

Wnts in the mouse gastrula

By whole-mount ISH followed by sectioning, we found that the Wnt2b gene starts to be expressed before the appearance of the primitive streak in the epiblast and underlying visceral endoderm on one side of the embryo, the presumptive posterior side. During the formation of the primitive streak and the rest of gastrulation, Wnt2b is expressed in the primitive streak, the newly formed mesoderm and the underlying endoderm.

We also showed that at the 5.5dpc stage, the aVE expresses three secreted Wnt antagonists (Sfrp1, Sfrp5 and Dkk1) in partially overlapping domains suggesting a possible redundant function during head induction and patterning.We propose that Sfrp1, Sfrp5 and Dkk1 are responsible for the inhibition of Wnt signaling at the anterior side of the embryo therefore allowing the formation of the head region. The expression of these Wnt antagonists in overlapping domains suggests that they may be redundant which may explain the lack of phenotype in Dkk1 mutant embryos during initiation of head formation even if at later stages the Dkk1 mutation is responsible for missing head structures.


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