Differential requirements for β-catenin during mouse development.

Embryogenesis relies on the precise interplay of signaling cascades to activate tissue-specific differentiation programs. An important player in these morphogenetic processes is β-catenin, which is a central component of adherens junctions and canonical Wnt signaling. Lack of β-catenin is lethal before gastrulation, but mice heterozygous for β-catenin (Ctnnb1) develop as wild type. Here, we confine β-catenin amounts below the heterozygous expression level to study the functional consequences for development. We generate embryonic stem (ES) cells and embryos expressing β-catenin only from the ubiquitously active ROSA26 promoter and thereby limit β-catenin expression to ~12.5% (ROSA26(β/+)) or ~25% (ROSA26(β/β)) of wild-type levels. ROSA26(β/+) is sufficient to maintain ES cell morphology and pluripotent characteristics, but is insufficient to activate canonical target genes upon Wnt stimulation. This Wnt signaling deficiency is incompletely restored in ROSA26(β/β) ES cells. We conclude that even very low β-catenin levels are able to sustain cell adhesion, but not Wnt signaling. During development, ROSA26(β/β) as well as ROSA26(β/+) partially rescues the knockout phenotype, yet proper gastrulation is absent. These embryos differentiate according to the neural default hypothesis, indicating that gastrulation depends on high β-catenin levels. Strikingly, if ROSA26(β/+) or ROSA26(β/β) is first activated after gastrulation, subsequent development correlates with the dosage of β-catenin. Moreover, molecular evidence indicates that the amount of β-catenin controls the induction of specific Wnt target genes. In conclusion, by restricting its expression we determine the level of β-catenin required for adhesion or pluripotency and during different morphogenetic events.