Canonical Wnt signaling plays a rate-limiting role in regulating self-renewal 
and differentiation in mouse embryonic stem cells (ESCs). We have previously 
shown that mutation in the Apc (adenomatous polyposis coli) tumor suppressor 
gene constitutively activates Wnt signaling in ESCs and inhibits their capacity 
to differentiate towards ecto-, meso-, and endodermal lineages. However, the 
underlying molecular and cellular mechanisms through which Wnt regulates lineage 
differentiation in mouse ESCs remain to date largely unknown. To this aim, we 
have derived and studied the gene expression profiles of several Apc-mutant ESC 
lines encoding for different levels of Wnt signaling activation. We found that 
down-regulation of Tcf3, a member of the Tcf/Lef family and a key player in the 
control of self-renewal and pluripotency, represents a specific and primary 
response to Wnt activation in ESCs. Accordingly, rescuing Tcf3 expression 
partially restored the neural defects observed in Apc-mutant ESCs, suggesting 
that Tcf3 down-regulation is a necessary step towards Wnt-mediated suppression 
of neural differentiation. We found that Tcf3 down-regulation in the context of 
constitutively active Wnt signaling does not result from promoter DNA 
methylation but is likely to be caused by a plethora of mechanisms at both the 
RNA and protein level as shown by the observed decrease in activating histone 
marks (H3K4me3 and H3-acetylation) and the upregulation of miR-211, a novel 
Wnt-regulated microRNA that targets Tcf3 and attenuates early neural 
differentiation in mouse ESCs. Our data show for the first time that Wnt 
signaling down-regulates Tcf3 expression, possibly at both the transcriptional 
and post-transcriptional levels, and thus highlight a novel mechanism through 
which Wnt signaling inhibits neuro-ectodermal lineage differentiation in mouse 
embryonic stem cells.