Rap2 is required for Wnt/beta-catenin signaling pathway in Xenopus early development

The Wnt/beta-catenin signaling pathway is critical for the establishment of organizer and embryonic body axis in Xenopus development. Here, we present evidence that Xenopus Rap2, a member of Ras GTPase family, is implicated in Wnt/beta-catenin signaling during the dorsoventral axis specification. Ectopic expression of XRap2 can lead to neural induction without mesoderm differentiation. XRap2 dorsalizes ventral tissues, inducing axis duplication, organizer-specific gene expression and convergent extension movements. Knockdown of XRap2 causes ventralized phenotypes including shortened body axis and defective dorsoanterior patterning, which are associated with aberrant Wnt signaling. In line with this, XRap2 depletion inhibits beta-catenin stabilization and the induction of ectopic dorsal axis and Wnt-responsive genes caused by XWnt8, Dsh or beta-catenin, but has no effect on the signaling activities of a stabilized beta-catenin. Its knockdown also disrupts the vesicular localization of Dsh, thereby inhibiting Dsh-mediated beta-catenin stabilization and the membrane recruitment and phosphorylation of Dsh by frizzled signaling. Taking together, we suggest that XRap2 is involved in Wnt/beta-catenin signaling as a modulator of the subcellular localization of Dsh.     

The structure of the Arabidopsis thaliana SOS3: molecular mechanism of sensing calcium for salt stress response

The Arabidopsis thaliana SOS3 gene encodes a calcium sensor that is required for plant salt tolerance. The SOS3 protein binds to and activates the self-inhibited SOS2 protein kinase, which mediates the expression and activities of various transporters important for ion homeostasis under salt stress. SOS3 belongs to a unique family of calcium-binding proteins that contain two pairs of EF hand motifs with four putative metal-binding sites. We report the crystal structure of a dimeric SOS3 protein in complex with calcium, and with calcium and manganese. Analytical ultracentrifugation experiments and circular dichroism measurements show that calcium binding is responsible for the dimerization of SOS3. This leads to a change in the global shape and surface properties of the protein that may be sufficient to transmit the Ca(2+) signal elicited during salt stress.     

Intramolecular signaling pathways revealed by modeling anisotropic thermal diffusion

A variety of experimental evidence suggests that rapid, long-range propagation of conformational changes through the core of proteins plays a vital role in allosteric communication. Here, we describe a non-equilibrium molecular dynamics simulation method, anisotropic thermal diffusion (ATD), which allowed us to observe a dominant intramolecular signaling pathway in PSD-95, a member of the PDZ domain protein family. The observed pathway is in good accordance with a pathway previously inferred using a multiple sequence analysis of 276 PDZ domain proteins. In comparison with conventional solution molecular dynamics methods, the ATD method provides greatly enhanced signal-to-noise, allowing long-distance correlations to be observed clearly. The ATD method requires neither a large number of homologous proteins, nor extremely long simulation times to obtain a complete signaling pathway within a protein. Therefore, the ATD method should prove to be a powerful and general complement to experimental efforts to understand the physical basis of intramolecular signaling.     

Amplification and oscillations in the FAK/Src kinase system during integrin signaling

Integrin signaling is a major pathway of cell adhesion to extracellular matrices that regulates many physiological cell behaviors such as cell proliferation, migration or differentiation and is implied in pathologies such as tumor invasion. In this paper, we focused on the molecular system formed by the two kinases FAK (focal adhesion kinase) and Src, which undergo auto- and co-activation during early steps of integrin signaling. The system is modelled using classical kinetic equations and yields a set of three nonlinear ordinary differential equations describing the dynamics of the different phosphorylation forms of FAK. Analytical and numerical analysis of these equations show that this system may in certain cases amplify incoming signals from the integrins. A quantitative condition is obtained, which indicates that the total FAK charge in the system acts as a critical mass that must be exceeded for amplification to be effective. Furthermore, we show that when FAK activity is lower than Src activity, spontaneous oscillations of FAK phosphorylation forms may appear. The oscillatory behavior is studied using bifurcation and stability diagrams. We finally discuss the significance of this behavior with respect to recent experimental results evidencing FAK dynamics.     

TGF-beta control of cell proliferation

This article focuses on recent findings that the type V TGF-beta receptor (TbetaR-V), which co-expresses with other TGF-beta receptors (TbetaR-I, TbetaR-II, and TbetaR-III) in all normal cell types studied, is involved in growth inhibition by IGFBP-3 and TGF-beta and that TGF-beta activity is regulated by two distinct endocytic pathways (clathrin- and caveolar/lipid-raft-mediated). TGF-beta is a potent growth inhibitor for most cell types, including epithelial and endothelial cells. The signaling by which TGF-beta controls cell proliferation is not well understood. Many lines of evidence indicate that other signaling pathways, in addition to the prominent TbetaR-I/TbetaR-II/Smad2/3/4 signaling cascade, are required for mediating TGF-beta-induced growth inhibition. Recent studies revealed that TbetaR-V, which is identical to LRP-1, mediates IGF-independent growth inhibition by IGFBP-3 and mediates TGF-beta-induced growth inhibition in concert with TbetaR-I and TbetaR-II. In addition, IRS proteins and a Ser/Thr-specific protein phosphatase(s) are involved in the TbetaR-V-mediated growth inhibitory signaling cascade. The TbetaR-V signaling cascade appears to cross-talk with the TbetaR-I/TbetaR-II, insulin receptor (IR), IGF-I receptor (IGF-IR), integrin and c-Met signaling cascades. Attenuation or loss of the TbetaR-V signaling cascade may enable carcinoma cells to escape from TGF-beta growth control and may contribute to the aggressiveness and invasiveness of these cells via promoting epithelial-to-mesenchymal transdifferentiation (EMT). Finally, the ratio of TGF-beta binding to TbetaR-II and TbetaR-I is a signal controlling TGF-beta partitioning between two distinct endocytosis pathways and resultant TGF-beta responsiveness. These recent studies have provided new insights into the molecular mechanisms underlying TGF-beta-induced cellular growth inhibition, cross-talk between the TbetaR-V and other signaling cascades, the signal that controls TGF-beta responsiveness and the role of TbetaR-V in tumorigenesis.     

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.