Wnt signaling plays an important role in embryonic development and tumorigenesis. These biological effects are exerted by activation of the b-catenin/TCF transcription complex and consequent regulation of a set of downstream genes. TCF-binding elements have been found in the promoter regions of many TCF target genes and characterized by a highly conserved consensus sequence. Utilizing this consensus sequence, we performed an in silico screening for new TCF target genes. Through computational screening and subsequent experimental analysis, we identified a novel TCF target gene, DKK1, which has been shown to be a potent inhibitor of Wnt signaling. Our finding suggests the existence of a novel feedback loop in Wnt signaling.

In the mammalian central nervous system, neurogenesis precedes gliogenesis; neurons are primarily generated at the neural stage, whereas most glial cells are generated perinatally and postnatally. However, the signals that regulate this sequence of events remain unclear. Here we show that Wnt signaling induces neuronal and astroglial differentiation but suppresses oligodendroglial differentiation. We observed that precursor cells infected with a retrovirus encoding β β β β -catenin differentiated into neurons, while astrocytes developed from uninfected precursor cells surrounding infected cells. As neurogenesis proceeded, expression of the bone morphogenetic proteins (BMPs), BMP2, 4 and 7, progressively increased in the cells infected with the retrovirus encoding β β β β -catenin. Furthermore, treatment of cells with Noggin, a BMP antagonist, completely inhibited astroglial differentiation but partially restored oligodendroglial differentiation. These results suggest that Wnt signaling indirectly regulates gliogenesis by inducing BMPs in neuronal cells. Thus, cooperation between Wnt and BMP signaling may play a key role in determining the sequence of neurogenesis and gliogenesis.

Inhibitor of ␤-catenin and T cell factor (ICAT) inhibits Wnt signaling by interfering with the interaction between ␤-catenin and T cell factor. Here we show that ICAT ؊/؊ embryos exhibit malformation of the forebrain and craniofacial bones and lack the kidney. Analysis of the neuronal differentiation of embryonic stem cells revealed that Wnt3a redirects the fate of neural progenitors to a posterior character, whereas ICAT induces forebrain cells by inhibiting Wnt signaling. Furthermore, ICAT ؊/؊ embryonic stem cells were found to differentiate into neuronal cells possessing a posterior character. These results suggest that ICAT plays an important role in the anteriorization of neural cells by inhibiting the posteriorizing activity of Wnt signaling.W nt signaling plays a crucial role in a number of developmental processes, including body axis formation, development of the central nervous system, and axial specification in limb development (1-8). Wnt signaling stabilizes ␤-catenin, which in turn associates with T cell factor (TCF)͞lymphoid-enhancing factor family transcription factors, ultimately altering the expression of Wnt target genes. In the absence of Wnt signaling, ␤-catenin is recruited into the multiprotein complex containing adenomatous polyposis coli (APC), glycogen synthase kinase-3␤, casein kinase 1␣, and Axin or the closely related factor conductin͞Axil and subjected to proteasome-mediated degradation. Wnt signaling is further inhibited by the association of ␤-catenin with the inhibitor of ␤-catenin and TCF (ICAT) (9-12). ICAT is an 81-aa protein that interferes with the interaction between ␤-catenin and TCF. ICAT contains an amino-terminal helical domain that binds to armadillo repeats 10-12 of ␤-catenin, and a carboxy-terminal tail that competes with TCF for binding to armadillo repeats 5-10 (9, 11, 12). Overexpression of ICAT induces G 2 arrest and cell death of colorectal tumor cells mutated in APC or ␤-catenin and hepatocellular carcinoma cells mutated in Axin (10).It has been shown that Wnt signaling specifies posterior-toanterior fates within the neural plate (13-16). Inhibition of Wnt signaling is required for anterior specification; negative regulators of Wnt signaling play a crucial role in establishing a gradient of Wnt activity patterning the anterior-posterior axis. Mouse embryos lacking Dickkopf1, a secreted protein that acts as an inhibitor of the Wnt coreceptor low density lipoprotein receptor-related protein 6, lack head structures anterior to the midbrain (17). Also, mouse embryos lacking Six3 (sine oculis homeobox homolog 3), a direct negative regulator of Wnt1 expression, lack forebrain structures and exhibit posteriorization of the remaining mutant heads (18). In addition, zebrafish mutants for the negative intracellular regulators of Wnt signaling tcf3͞headless and axin͞masterblind display anterior defects (19 -21). In the present study, we show that mouse embryos lacking ICAT exhibit multiple defects including malformation of the forebrain. Furthermore, by analyzing the neuron...