Neurogenesis in the developing neocortex is a strictly regulated process of cell division and differentiation. Here we report that a gradual retreat of canonical Wnt signaling in the cortex from lateral-to-medial and anterior-to-posterior is a prerequisite of neurogenesis. Ectopic expression of a beta-catenin/LEF1 fusion protein maintains active canonical Wnt signaling in the developing cortex and delays the expression onset of the neurogenic factors Pax6, Ngn2 and Tbr2 and subsequent neurogenesis. Contrary to this, conditional ablation of beta-catenin accelerates expression of the same neurogenic genes. Furthermore, we show that a sustained canonical Wnt activity in the lateral cortex gives rise to cells with hippocampal characteristics in the cortical plate at the expense of the cortical fate, and to cells with dentate gyrus characteristics in the hippocampus. This suggests that the dose of canonical Wnt signaling determines cellular fate in the developing cortex and hippocampus, and that recession of Wnt signaling acts as a morphogenetic gradient regulating neurogenesis in the cortex.

Axial patterning of the embryonic brain requires a precise balance between canonical Wnt signaling, which dorsalizes the nervous system, and Sonic hedgehog (Shh), which ventralizes it. The ventral anterior homeobox (Vax) transcription factors are induced by Shh and ventralize the forebrain through a mechanism that is poorly understood. We therefore sought to delineate direct Vax target genes. Among these, we identify an extraordinarily conserved intronic region within the gene encoding Tcf7l2, a key mediator of canonical Wnt signaling. This region functions as a Vax2-activated internal promoter that drives the expression of dnTcf7l2, a truncated Tcf7l2 isoform that cannot bind β-catenin and that therefore acts as a potent dominant-negative Wnt antagonist. Vax2 concomitantly activates the expression of additional Wnt antagonists that cooperate with dnTcf7l2. Specific elimination of dnTcf7l2 in Xenopus results in headless embryos, a phenotype consistent with a fundamental role for this regulator in forebrain development.

Wnt/␤-catenin signaling regulates many processes during vertebrate development. To study transcriptional targets of canonical Wnt signaling, we used the conditional Cre/loxP system in mouse to ectopically activate ␤-catenin during central nervous system development. We show that the activation of Wnt/␤-catenin signaling in the embryonic mouse telencephalon results in the up-regulation of Sp5 gene, which encodes a member of the Sp1 transcription factor family. A proximal promoter of Sp5 gene is highly evolutionarily conserved and contains five TCF/LEF binding sites that mediate direct regulation of Sp5 expression by canonical Wnt signaling. We provide evidence that Sp5 works as a transcriptional repressor and has three independent repressor domains, called R1, R2, and R3, respectively. Furthermore, we show that the repression activity of R1 domain is mediated through direct interaction with a transcriptional corepressor mSin3a. Finally, our data strongly suggest that Sp5 has the same DNA binding specificity as Sp1 and represses Sp1 target genes such as p21. We conclude that Sp5 transcription factor mediates the downstream responses to Wnt/␤-catenin signaling by directly repressing Sp1 target genes.Wnt/␤-catenin signaling plays important roles in multiple developmental processes and has a profound effect on cell proliferation, cell polarity, and cell fate determination (1). Wnt molecules are secreted glycoproteins that work as signaling molecules. Wnt molecules bind with Frizzled receptors and low density lipoprotein receptor-related protein coreceptors at the cell surface to initiate the signaling. In the absence of Wnt/␤-catenin signaling, the level of cytoplasmic ␤-catenin, the key mediator of Wnt/␤-catenin signaling, is kept low. ␤-Catenin is recruited to a destruction complex containing the tumor suppressors adenomatous polyposis coli, axin, casein kinase 1, and glycogen synthase kinase 3␤, respectively, and is constitutively phosphorylated. The phosphorylated ␤-catenin protein is degraded by the ubiquitin pathway. Members of the TCF/LEF transcription factor family bind corepressor Groucho and repress Wnt target genes in the nucleus. The binding of Wnt molecules to the receptors and the coreceptors results in the inactivation of the kinase activity of the destruction complex. As a consequence, ␤-catenin protein is not phosphorylated, begins to accumulate in the cytoplasm, and is then translocated to the nucleus where it binds to TCF/LEF transcription factors. The binding converts TCF/LEF into an activator that initiates the transcription of Wnt target genes, including c-myc, Axin2, and Lef1 (2-4).During central nervous system (CNS) 2 development, multiple Wnt genes are expressed, including Wnt3a, Wnt7a, Wnt7b, and Wnt8b (5). Transgenic mice, which express a stabilized form of ␤-catenin in neural progenitor cells, develop enlarged brains (6). In Wnt3a mutant mice as well as in Lef1 mutant mice, the hippocampus is missing (5, 7). These reports indicate critical roles of canonical Wnt signaling in CNS developmen...