A short loop on the ALK-2 and ALK-4 activin receptors regulates signaling specificity but cannot account for all their effects on early Xenopus development.
Activin, a member of the transforming growth factor beta (TGF-beta) superfamily, signals through a heteromeric complex of type I and type II serine-threonine kinase receptors. The two activin type I receptors previously identified, ALK-2 (ActR-I) and ALK-4 (ActR-IB), have distinct effects on gene expression, differentiation and morphogenesis in the Xenopus animal cap assay. ALK-4 reproduces the effects of activin treatment including the dose-dependent induction of progressively more dorso-anterior mesodermal and endodermal markers, whereas ALK-2 induces only ventral mesodermal markers and counteracts the effects of ALK-4. To identify regions of the receptors that determine signaling specificity we have generated chimeras of the constitutively active ALK-2 and ALK-4 receptors (termed ALK-2* and ALK-4*). The effects of these chimeric receptors on gene expression and morphogenetic movements implicate the loop between kinase subdomains IV and V in mediating the strong dorsal gene-inducing properties of ALK-4*; when the seven amino acids comprising this loop are transferred from ALK-4* to ALK-2*, the resulting chimeric receptor is capable of inducing the expression of dorsal-specific genes. In contrast, when the equivalent region of ALK-2* is transferred to the ALK-4* backbone it cannot effectively counteract the dorsalizing effects of ALK-4*, suggesting that other regions of type I receptors are also involved in determining signal specificity. (+info)
Angiogenesis defects and mesenchymal apoptosis in mice lacking SMAD5.
The transforming growth factor-beta (TGF-beta) signals are mediated by a family of at least nine SMAD proteins, of which SMAD5 is thought to relay signals of the bone morphogenetic protein (BMP) pathway. To investigate the role of SMAD5 during vertebrate development and tumorigenesis, we disrupted the Smad5 gene by homologous recombination. We showed that Smad5 was expressed predominantly in mesenchyme and somites during embryogenesis, and in many tissues of the adult. Mice homozygous for the mutation died between days 10.5 and 11.5 of gestation due to defects in angiogenesis. The mutant yolk sacs lacked normal vasculature and had irregularly distributed blood cells, although they contained hematopoietic precursors capable of erythroid differentiation. Smad5 mutant embryos had enlarged blood vessels surrounded by decreased numbers of vascular smooth muscle cells, suffered massive apoptosis of mesenchymal cells, and were unable to direct angiogenesis in vitro. These data suggest that SMAD5 may regulate endothelium-mesenchyme interactions during angiogenesis and that it is essential for mesenchymal survival. (+info)
Smad5 knockout mice die at mid-gestation due to multiple embryonic and extraembryonic defects.
Smad5 has been implicated as a downstream signal mediator for several bone morphogenetic proteins (BMPs). To understand the in vivo function of Smad5, we generated mice deficient in Smad5 using embryonic stem (ES) cell technology. Homozygous mutant embryos die between E9.5 and E11.5, and display variable phenotypes. Morphological defects are first detected at E8.0 in the developing amnion, gut and heart (the latter defect being similar to BMP-2 knockout mice). At later stages, mutant embryos fail to undergo proper turning, have craniofacial and neural tube abnormalities, and are edematous. In addition, several extraembryonic lesions are observed. After E9.0, the yolk sacs of the mutants contain red blood cells but lack a well-organized vasculature, which is reminiscent of BMP-4, TGF-beta1 and TGF-beta type II receptor knockout mice. In addition, the allantois of many Smad5 mutants is fused to the chorion, but is not well-elongated. A unique feature of the Smad5 mutant embryos is that ectopic vasculogenesis and hematopoiesis is observed in the amnion, likely due to mislocation of allantois tissue. Despite the expression of Smad5 from gastrulation onwards, and in contrast to knockouts of Smad2 and Smad4, Smad5 only becomes essential later in extraembryonic and embryonic development. (+info)
The smad5 mutation somitabun blocks Bmp2b signaling during early dorsoventral patterning of the zebrafish embryo.
Signaling by members of the TGFbeta superfamily is thought to be transduced by Smad proteins. Here, we describe a zebrafish mutant in smad5, designated somitabun (sbn). The dominant maternal and zygotic effect of the sbntc24 mutation is caused by a change in a single amino acid in the L3 loop of Smad5 protein which transforms Smad5 into an antimorphic version, inhibiting wild-type Smad5 and related Smad proteins. sbn mutant embryos are strongly dorsalized, similarly to mutants in Bmp2b, its putative upstream signal. Double mutant analyses and RNA injection experiments show that sbn and bmp2b interact and that sbn acts downstream of Bmp2b signaling to mediate Bmp2b autoregulation during early dorsoventral (D-V) pattern formation. Comparison of early marker gene expression patterns, chimera analyses and rescue experiments involving temporally controlled misexpression of bmp or smad in mutant embryos reveal three phases of D-V patterning: an early sbn- and bmp2b-independent phase when a coarse initial D-V pattern is set up, an intermediate sbn- and bmp2b-dependent phase during which the putative morphogenetic Bmp2/4 gradient is established, and a later sbn-independent phase during gastrulation when the Bmp2/4 gradient is interpreted and cell fates are specified. (+info)
Increased smad expression and activation are associated with apoptosis in normal and malignant prostate after castration.
Transforming growth factor (TGF)-beta1 is induced in the prostate after castration and has been implicated in apoptosis of epithelial cells during involution. TGF-beta1-mediated receptor activation induces phosphorylation of Smad2 and Smad3, which form complexes with Smad4, that translocate to the nucleus to regulate transcription of target genes. Smad6 and Smad7 antagonize the action of signal-transducing Smads. We have examined the immunohistochemical expression of different Smad molecules in the epithelium of rat ventral prostate before and after castration, in androgen-sensitive Dunning R3327 PAP prostatic tumor cells from untreated and castrated rats, and after treatment with estrogen. In the ventral prostate, a significant increase of phosphorylated Smad2 (P-Smad2) was observed after castration. In prostatic tumor cells we observed an increased expression of Smad2 and P-Smad2 after treatment. The levels of Smad3 and, in particular, Smad4 were enhanced in the normal ventral prostate, as well as in the tumors after castration. Interestingly, Smad6 and Smad7 expression was also up-regulated in cells with increased Smad2 activation. The staining for Smad2, P-Smad2, Smad3, Smad4, and Smad7 was nuclear in some cells and was present in areas with a large number of apoptotic cells identified by various morphological criteria, formation of apoptotic bodies and, in adjacent sections, by terminal deoxynucleotidyl transferase-mediated nick end labeling assay. Our results suggest that the signal transduction pathway for TGF-beta, leading to apoptosis, is activated in the normal prostate after castration and in the tumor model after castration, without or with estrogen treatment. (+info)
Screening SMAD1, SMAD2, SMAD3, and SMAD5 for germline mutations in juvenile polyposis syndrome.
BACKGROUND AND AIMS: Juvenile polyps occur in several Mendelian disorders, whether in association with gastrointestinal cancer alone (juvenile polyposis syndrome, JPS) or as part of known syndromes (Cowden, Gorlin, and Bannayan-Zonana) in association with developmental abnormalities, dysmorphic features, or extraintestinal tumours. Recently, some JPS families were shown to harbour germline mutations in the SMAD4 (DPC4) gene, providing further evidence for the importance of the TGFbeta signalling pathway in colorectal cancer. There remains, however, considerable, unexplained genetic heterogeneity in JPS. Other members of the SMAD family are excellent candidates for JPS, especially SMAD2 (which, like SMAD4, is mutated somatically in colorectal cancers), SMAD3 (which causes colorectal cancer when "knocked out" in mice), SMAD5, and SMAD1. METHODS: SMAD1, SMAD2, SMAD3, and SMAD5 were screened for germline mutations in 30 patients with JPS and without SMAD4 mutations. RESULTS: No mutations were found in any of these genes. A G-A C89Y polymorphism with possible effects on protein function was found in SMAD3, but the frequencies of the G and A alleles did not differ between patients with JPS and controls. CONCLUSIONS: It remains to be determined whether or not this polymorphism is involved in a minor predisposition to colorectal or other carcinomas. SMAD4 may be the only member of the SMAD family which causes JPS when mutant in the germline. The other genes underlying JPS remain to be identified. (+info)
Characterization of zebrafish smad1, smad2 and smad5: the amino-terminus of smad1 and smad5 is required for specific function in the embryo.
Members of the TGFbeta superfamily of signalling molecules play important roles in mesendoderm induction and dorsoventral patterning of the vertebrate embryo. We cloned three intracellular mediators of TGFbeta signalling, smad1, 2 and 5, from the zebrafish. The three smad genes are expressed ubiquitously at the onset of gastrulation. The pattern of expression becomes progressively restricted during somitogenesis suggesting that at later stages not only the distribution of the TGFbeta signal but also that of the intracellular smad signal transducer determine the regionally restricted effects of TGFbeta signalling. Forced expression of smad1 leads to an expansion of blood cells resembling the phenotype of moderately ventralized zebrafish mutants. In contrast to Smad1, neither Smad2 nor Smad5 caused a detectable effect when expressed as full-length molecules suggesting that these latter two Smads are more dependent on activation by the cognate TGFbeta ligands. N-terminal truncated Smad2 dorsalized embryos, in agreement with a role downstream of dorsalizing TGFbeta members such as Nodals. In contrast to the C-terminal MH2 domain of Smad2, the C-terminal region of Smad1 and Smad5 lead to pleiotropic effects in embryos giving rize to both dorsalized and ventralized characteristics in injected embryos. Analysis of truncated zebrafish Smad1 in Xenopus embryos supports the notion that the C-terminal domain of smad1 is both a hypomorph and antimorph which can act as activator or inhibitor depending on the region of expression in the embryo. These results indicate a specific function of the MH1 domain of Smad1 and 5 for activity of the molecules. (+info)
Roles of bone morphogenetic protein type I receptors and Smad proteins in osteoblast and chondroblast differentiation.
The biological effects of type I serine/threonine kinase receptors and Smad proteins were examined using an adenovirus-based vector system. Constitutively active forms of bone morphogenetic protein (BMP) type I receptors (BMPR-IA and BMPR-IB; BMPR-I group) and those of activin receptor-like kinase (ALK)-1 and ALK-2 (ALK-1 group) induced alkaline phosphatase activity in C2C12 cells. Receptor-regulated Smads (R-Smads) that act in the BMP pathways, such as Smad1 and Smad5, also induced the alkaline phosphatase activity in C2C12 cells. BMP-6 dramatically enhanced alkaline phosphatase activity induced by Smad1 or Smad5, probably because of the nuclear translocation of R-Smads triggered by the ligand. Inhibitory Smads, i.e., Smad6 and Smad7, repressed the alkaline phosphatase activity induced by BMP-6 or the type I receptors. Chondrogenic differentiation of ATDC5 cells was induced by the receptors of the BMPR-I group but not by those of the ALK-1 group. However, kinase-inactive forms of the receptors of the ALK-1 and BMPR-I groups blocked chondrogenic differentiation. Although R-Smads failed to induce cartilage nodule formation, inhibitory Smads blocked it. Osteoblast differentiation induced by BMPs is thus mediated mainly via the Smad-signaling pathway, whereas chondrogenic differentiation may be transmitted by Smad-dependent and independent pathways. (+info)