Wnt/Frizzled activation of Rho regulates vertebrate gastrulation and requires a novel Formin homology protein Daam1.

Wnt signaling via the Frizzled (Fz) receptor controls cell polarity and movement during development, but the molecular nature of Wnt/Fz polarity signal transduction remains poorly defined. Here we report that in human cells and during Xenopus embryogenesis, Wnt/Fz signaling activates the small GTPase Rho, a key regulator of cytoskeleton architecture. Wnt/Fz activation of Rho requires the cytoplasmic protein Dishevelled (Dvl) and a novel Formin homology protein Daam1. Daam1 binds to both Dvl and Rho, and mediates Wnt-induced Dvl-Rho complex formation. Inhibition or depletion of Daam1 prevents Wnt/Fz activation of Rho and of Xenopus gastrulation, but not of beta-catenin signaling. Our study illustrates a molecular pathway from Wnt/Fz signaling to Rho activation in cell polarity signal transduction.

Activation of codependent transcription factors is required for transcriptional induction of the vgf gene by nerve growth factor and Ras.

Nerve growth factor (NGF) treatment of PC12 cells leads to the elaboration of a neuronal phenotype, including the induction of neuronally expressed genes such as vgf. To study vgf transcription, we have created chimeric vgf/beta-globin genes in which vgf promoter sequences drive the expression of the beta-globin reporter gene or of a chimeric beta-globin gene fused to 3' untranslated vgf gene sequences. We have found that the level of inducibility of the latter construct by NGF resembles that of the endogenous vgf gene. Using transient transfection of the chimeric reporter genes into PC12 cells, into PC12 subclones expressing activated or dominantly interfering mutant Ras proteins, and into PC12 variants expressing specific NGF receptor/Trk mutants, we show that transcriptional regulation of the vgf promoter by NGF is mediated through a Ras-dependent signaling pathway. By mutational analysis of the vgf promoter, we have identified three promoter elements involved in mediating transcriptional induction by NGF and Ras. In addition to the cyclic AMP-responsive element (CRE), which binds to ATF-1, ATF-2, and CRE-binding protein in PC12 nuclear extracts, a novel CCAAT element and its binding proteins were identified, which, like the CRE, is necessary but not sufficient for the Ras-dependent induction of the vgf gene by NGF. We also identify a G(S)G element unusually located between the TATA box and transcriptional start site, which binds the NGF- and Ras-induced transcription factor, NGFI-A, and amplifies the transcriptional response. Integrating data from studies of vgf promoter regulation and NGF signal transduction, we present a model for vgf gene induction in which transcriptional activation is achieved through the persistent, direct activation of multiple interacting transcription factors binding to CRE and CCAAT elements, coordinated with the delayed transcription factor action at a G(S)G element resulting from the induced expression of NGFI-A.