FoxD1 protein interacts with Wnt and BMP signaling to differentially pattern mesoderm and neural tissue.

The foxd1 gene (previously known as Brain Factor 2/BF2) is expressed during early Xenopus laevis development. At gastrula stages, foxd1 is expressed in dorsal mesoderm regions fated for muscle and notochord, while at neurula stages, foxd1 is expressed in the forebrain region. Previous studies in the neural plate showed that FoxD1 protein acts as transcriptional repressor downstream of BMP antagonism, neuralizing the embryo to control anterior neural cell fates. FoxD1 mesoderm function was not rigorously analyzed, but ectopic FoxD1 levels increased muscle marker expression in embryos. Using a FoxD1-specific antisense morpholino oligonucleotide, we knocked down endogenous FoxD1 protein activity in developing Xenopus embryos. In this present study, we show that FoxD1 is crucial for dorsal mesoderm formation. Analogous to neural tissue, FoxD1 acts downstream of BMP antagonism to induce dorsal mesoderm cell fates, such as muscle and notochord. FoxD1 is sensitive to its local signaling environment, having differential transcription factor activity in the presence or absence of Wnt or BMP signaling. FoxD1 induces posterior neural tissue in the presence of Wnt or BMP activities, but its activity is restricted to "normal" anterior neural tissue induction when BMP and Wnt activities are repressed. In dorsal mesoderm, FoxD1 interacts with Wnt signaling and BMP antagonism to induce muscle and notochord, while simultaneously repressing more anterior and ventral mesoderm cell fates. FoxD1 protein has multiple activities that are masked or released in the different germ layers as a function of the local signaling environment.

PTK7 modulates Wnt signaling activity via LRP6.

Protein tyrosine kinase 7 (PTK7) is a transmembrane protein expressed in the developing Xenopus neural plate. PTK7 regulates vertebrate planar cell polarity (PCP), controlling mesodermal and neural convergent-extension (CE) cell movements, neural crest migration and neural tube closure in vertebrate embryos. Besides CE phenotypes, we now show that PTK7 protein knockdown also inhibits Wnt/ß-catenin activity. Canonical Wnt signaling caudalizes the neural plate via direct transcriptional activation of the meis3 TALE-class homeobox gene, which subsequently induces neural CE. PTK7 controls meis3 gene expression to specify posterior tissue and downstream PCP activity. Furthermore, PTK7 morphants phenocopy embryos depleted for Wnt3a, LRP6 and Meis3 proteins. PTK7 protein depletion inhibits embryonic Wnt/ß-catenin signaling by strongly reducing LRP6 protein levels. LRP6 protein positively modulates Wnt/ß-catenin, but negatively modulates Wnt/PCP activities. The maintenance of high LRP6 protein levels by PTK7 triggers PCP inhibition. PTK7 and LRP6 proteins physically interact, suggesting that PTK7 stabilization of LRP6 protein reciprocally regulates both canonical and noncanonical Wnt activities in the embryo. We suggest a novel role for PTK7 protein as a modulator of LRP6 that negatively regulates Wnt/PCP activity.

Focal adhesion kinase protein regulates Wnt3a gene expression to control cell fate specification in the developing neural plate.

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase protein localized to regions called focal adhesions, which are contact points between cells and the extracellular matrix. FAK protein acts as a scaffold to transfer adhesion-dependent and growth factor signals into the cell. Increased FAK expression is linked to aggressive metastatic and invasive tumors. However, little is known about its normal embryonic function. FAK protein knockdown during early Xenopus laevis development anteriorizes the embryo. Morphant embryos express increased levels of anterior neural markers, with reciprocally reduced posterior neural marker expression. Posterior neural plate folding and convergence-extension is also inhibited. This anteriorized phenotype resembles that of embryos knocked down zygotically for canonical Wnt signaling. FAK and Wnt3a genes are both expressed in the neural plate, and Wnt3a expression is FAK dependent. Ectopic Wnt expression rescues this FAK morphant anteriorized phenotype. Wnt3a thus acts downstream of FAK to balance anterior-posterior cell fate specification in the developing neural plate. Wnt3a gene expression is also FAK dependent in human breast cancer cells, suggesting that this FAK-Wnt linkage is highly conserved. This unique observation connects the FAK- and Wnt-signaling pathways, both of which act to promote cancer when aberrantly activated in mammalian cells.