Ectopic expression of the transforming growth factor beta type II receptor disrupts mesoderm organisation during mouse gastrulation.
Transforming growth factor beta (TGFbeta) regulates the cell cycle and extracellular matrix (ECM) deposition of many cells in vitro. We have analysed chimaeric mouse embryos generated from embryonic stem cells with abnormal receptor expression to study the effect of TGFbeta on these processes in vivo and the consequences for normal development. The binding receptor for TGFbeta, TbetaRII, is first detected in the embryo proper around day 8.5 in the heart. Ectopic expression of TbetaRII from the blastocyst stage onward resulted in an embryonic lethal around 9.5 dpc. Analysis of earlier stages revealed that the primitive streak of TbetaRII chimaeras failed to elongate. Furthermore, although cells passed through the streak and initially formed mesoderm, they tended to accumulate within the streak. These defects temporally and spatially paralleled the expression of the TGFbeta type I receptor, which is first expressed in the node and primitive streak. We present evidence that classical TGFbeta-induced growth inhibition was probably the cause of insufficient mesoderm being available for paraxial and axial structures. The results demonstrate that (1) TGFbeta mRNA and protein detected previously in early postimplantation embryos is present as a biologically active ligand; and (2) assuming that ectopic expression of TbetaRII results in no other changes in ES cells, the absence of TbetaRII is the principle reason why the embryo proper is unresponsive to TGFbeta ligand until after gastrulation. (+info)
Targeted disruption of SMAD3 results in impaired mucosal immunity and diminished T cell responsiveness to TGF-beta.
SMAD3 is one of the intracellular mediators that transduces signals from transforming growth factor-beta (TGF-beta) and activin receptors. We show that SMAD3 mutant mice generated by gene targeting die between 1 and 8 months due to a primary defect in immune function. Symptomatic mice exhibit thymic involution, enlarged lymph nodes, and formation of bacterial abscesses adjacent to mucosal surfaces. Mutant T cells exhibit an activated phenotype in vivo, and are not inhibited by TGF-beta1 in vitro. Mutant neutrophils are also impaired in their chemotactic response toward TGF-beta. Chronic intestinal inflammation is infrequently associated with colonic adenocarcinoma in mice older than 6 months of age. These data suggest that SMAD3 has an important role in TGF-beta-mediated regulation of T cell activation and mucosal immunity, and that the loss of these functions is responsible for chronic infection and the lethality of Smad3-null mice. (+info)
Dominant-negative Smad2 mutants inhibit activin/Vg1 signaling and disrupt axis formation in Xenopus.
Smads are central mediators of signal transduction for the TGFbeta superfamily. However, the precise functions of Smad-mediated signaling pathways in early development are unclear. Here we demonstrate a requirement for Smad2 signaling in dorsoanterior axis formation during Xenopus development. Using two point mutations of Smad2 previously identified in colorectal carcinomas, we show that Smad2 ushers Smad4 to the nucleus to form a transcriptional activation complex with the nuclear DNA-binding protein FAST-1 and that the mutant proteins interact normally with FAST-1 but fail to recruit Smad4 into the nucleus. This mechanism of inhibition specifically restricts the dominant-negative activity of these mutants to the activin/Vg1 signaling pathway without inhibiting BMPs. Furthermore, expression of these mutants in Xenopus animal caps inhibits but does not abolish activin and Vg1 induction of mesoderm and in the embryo results in a truncated dorsoanterior axis. These studies define a mechanism through which mutations in Smad2 may block TGFbeta-dependent signaling and suggest a critical role for inductive signaling mediated by the Smad2 pathway in Xenopus organizer function. (+info)
No mutations of the Smad2 gene in human sporadic gastric carcinomas.
BACKGROUND: The majority of cancer cells escape from TGF-beta-mediated growth control. However, the mechanism of resistance to the growth inhibitory effects by TGF-beta is not clear. TGF-beta signaling is initiated when the type I receptor phosphorylates the SMAD proteins, Smad2 and Smad3. Recently, mutations of Smad2 have been detected in human colon and lung cancers. Mutation of coding sequences of Smad2 in gastric carcinomas has not yet been elucidated adequately. METHODS: PCR-SSCP analysis of the entire coding region of Smad2 in 35 human sporadic gastric cancers and eight gastric cancer cell lines was performed using 11 sets of intron-based primers. RESULTS: No mutations of Smad2 were detected in any tumor or cell line. CONCLUSIONS: The results suggest that mutation of Smad2 does not play a key role in human stomach carcinogenesis. (+info)
MEKK-1, a component of the stress (stress-activated protein kinase/c-Jun N-terminal kinase) pathway, can selectively activate Smad2-mediated transcriptional activation in endothelial cells.
Smad proteins are essential components of the intracellular signaling pathways utilized by members of the transforming growth factor-beta (TGF-beta) superfamily of growth factors. Certain Smad proteins (e.g. Smad1, -2, and -3) can act as regulated transcriptional activators, a process that involves phosphorylation of these proteins by activated TGF-beta superfamily receptors. We demonstrate that the intracellular kinase mitogen-activated protein kinase kinase kinase-1 (MEKK-1), an upstream activator of the stress-activated protein kinase/c-Jun N-terminal kinase pathway, can participate in Smad2-dependent transcriptional events in cultured endothelial cells. A constitutively active form of MEKK-1 but not mitogen-activated protein kinase kinase-1 (MEK-1) or TGF-beta-activated kinase-1, two distinct intracellular kinases, can specifically activate a Gal4-Smad2 fusion protein, and this effect correlates with an increase in the phosphorylation state of the Smad2 protein. These effects do not require the presence of the C-terminal SSXS motif of Smad2 that is the site of TGF-beta type 1 receptor-mediated phosphorylation. Activation of Smad2 by active MEKK-1 results in enhanced Smad2-Smad4 interactions, nuclear localization of Smad2 and Smad4, and the stimulation of Smad protein-transcriptional coactivator interactions in endothelial cells. Overexpression of Smad7 can inhibit the MEKK-1-mediated stimulation of Smad2 transcriptional activity. A physiological level of fluid shear stress, a known activator of endogenous MEKK-1 activity in endothelial cells, can stimulate Smad2-mediated transcriptional activity. These data demonstrate a novel mechanism for activation of Smad protein-mediated signaling in endothelial cells and suggest that Smad2 may act as an integrator of diverse stimuli in these cells. (+info)
A molecular basis for Smad specificity.
Bone morphogenetic proteins (BMPs) and activins are members of the TGFbeta superfamily of growth factors, a crucial group of regulators of induction and patterning of embryonic germ layers in metazoa. In early Xenopus embryos, activin, Vgl, and nodal are potent inducers of dorsal mesoderm, whereas BMPs can ventralize mesoderm, repress neural fate, and induce blood differentiation. These characteristic responses rely on ligand-specific signaling pathways, encompassing transmembrane kinase receptors and signal transducers belonging to the Smad family. The overexpression in Xenopus embryos of BMP-activated Smad1 and of activin/Vg1/ nodal-activated Smad2 is sufficient to specifically recapitulate ligand responses. In a search for determinants of a Smad specificity code, we have identified two small regions within the conserved carboxyl-domain that are necessary and sufficient for specific Smad action. Swapping both residue clusters (C1 and C2) between Smadl and Smad2 completely switches Smad effects in vivo. Thus, Smadl with swapped Smad2 clusters responds specifically to BMP but elicits an activin response, while a Smad2 protein containing the Smadl clusters is activated by activin and elicits a BMP response. Furthermore, association between Smads and FAST-1, a mediator of mesoderm induction by activin, is dependent upon the presence of the Smad2 C1 sequence. Finally, the Smadl-specific antagonist Smad6 can inhibit a Smad2 molecule harboring Smadl C1 and C2 sequences. Thus, the C1 and C2 regions of Smads specify the association between Smads and pathway-specific partners, such as FAST-1 and Smad6, and account for activin- and BMP- specific responses in vertebrate embryogenesis. (+info)
A short amino-acid sequence in MH1 domain is responsible for functional differences between Smad2 and Smad3.
Smad proteins are essential components of the signalling cascade initiated by members of the Transforming Growth Factor-beta family. TGFbeta binding to heteromeric complexes of transmembrane Ser/Thr kinases induces Smad2 and Smad3 phosphorylation on their C terminus residues. This phosphorylation leads to oligomerization with Smad4, a common mediator of TGF-beta, activin and BMP signalling. The Smad complexes then translocate to the nucleus where they play transcription regulator roles. Even if they share 92% identity, the two TGFbeta/ restricted Smad2 and Smad3 are not functionally equivalent. As we have previously shown, Smad3 acts as a transcription factor by binding to a TGFbeta-responsive sequence termed CAGA box whereas Smad2 does not. Smad2 differs from Smad3 mainly in the N-terminal MH1 domain where it contains two additional stretches of amino acids that are lacking in Smad3. Here, we show that one of these domains corresponding to exon 3 is responsible for the absence of Smad2 transcriptional activity in CAGA box-containing promoters. Furthermore, in vitro studies indicate that this domain prevents Smad2 from binding to this DNA sequence. This suggests that Smad2 and Smad3 may have different subsets of target genes participating thus in distinct responses among TGFbeta pleiotropic effects. (+info)
A mechanism of repression of TGFbeta/ Smad signaling by oncogenic Ras.
TGFbeta can override the proliferative effects of EGF and other Ras-activating mitogens in normal epithelial cells. However, epithelial cells harboring oncogenic Ras mutations often show a loss of TGFbeta antimitogenic responses. Here we report that oncogenic Ras inhibits TGFbeta signaling in mammary and lung epithelial cells by negatively regulating the TGFbeta mediators Smad2 and Smad3. Oncogenically activated Ras inhibits the TGFbeta-induced nuclear accumulation of Smad2 and Smad3 and Smad-dependent transcription. Ras acting via Erk MAP kinases causes phosphorylation of Smad2 and Smad3 at specific sites in the region linking the DNA-binding domain and the transcriptional activation domain. These sites are separate from the TGFbeta receptor phosphorylation sites that activate Smad nuclear translocation. Mutation of these MAP kinase sites in Smad3 yields a Ras-resistant form that can rescue the growth inhibitory response to TGFbeta in Ras-transformed cells. EGF, which is weaker than oncogenic mutations at activating Ras, induces a less extensive phosphorylation and cytoplasmic retention of Smad2 and Smad3. Our results suggest a mechanism for the counterbalanced regulation of Smad2/Smad3 by TGFbeta and Ras signals in normal cells, and for the silencing of antimitogenic TGFbeta functions by hyperactive Ras in cancer cells. (+info)