A molecular basis for Smad specificity

Opposing Nodal/Vg1 and BMP signals mediate axial patterning in embryos of the basal chordate amphioxus

The basal chordate amphioxus resembles vertebrates in having a dorsal, hollow nerve cord, a notochord and somites. However, it lacks extensive gene duplications, and its embryos are small and gastrulate by simple invagination. Here we demonstrate that Nodal/Vg1 signaling acts from early cleavage through the gastrula stage to specify and maintain dorsal/anterior development while, starting at the early gastrula stage, BMP signaling promotes ventral/posterior identity. Knockdown and gain-of-function experiments show that these pathways act in opposition to one another. Signaling by these pathways is modulated by dorsally and/or anteriorly expressed genes including Chordin, Cerberus, and Blimp1. Overexpression and/or reporter assays in Xenopus demonstrate that the functions of these proteins are conserved between amphioxus and vertebrates. Thus, a fundamental genetic mechanism for axial patterning involving opposing Nodal and BMP signaling is present in amphioxus and probably also in the common ancestor of amphioxus and vertebrates or even earlier in deuterostome evolution.

SPECIFICITY AND VERSATILITY IN TGF-β SIGNALING THROUGH SMADS

The TGF-beta family comprises many structurally related differentiation factors that act through a heteromeric receptor complex at the cell surface and an intracellular signal transducing Smad complex. The receptor complex consists of two type II and two type I transmembrane serine/threonine kinases. Upon phosphorylation by the receptors, Smad complexes translocate into the nucleus, where they cooperate with sequence-specific transcription factors to regulate gene expression. The vertebrate genome encodes many ligands, fewer type II and type I receptors, and only a few Smads. In contrast to the perceived simplicity of the signal transduction mechanism with few Smads, the cellular responses to TGF-beta ligands are complex and context dependent. This raises the question of how the specificity of the ligand-induced signaling is achieved. We review the molecular basis for the specificity and versatility of signaling by the many ligands through this conceptually simple signal transduction mechanism.

Four types of smad4 found in the common carp,Cyprinus carpio

Smad4 is defined as the common-mediator Smad (Co-Smad) required for transducing signals for all transforming growth factor-beta (TGF-beta) superfamily members. In this study, we have isolated eight distinct Smad4 full-length cDNAs from the common carp (Cyprinus carpio). These cDNAs were classified into four types and each type consisted of two subtypes. The eight cDNAs encoded four distinct proteins ranging from 505aa to 568aa in size, with close similarities in the Mad homology 1 and 2 (MH1 and MH2, respectively), but with differences in the linker regions and the C-terminus as well as in the 5'- and 3'-untranslated regions. Genomic Southern blotting demonstrated the existence of at least six Smad4 gene loci in the carp genome, meaning that the multiple forms of the carp Smad4 cDNAs were not due to allelic variations. Reverse transcriptase polymerase chain reaction (RT-PCR)/Southern hybridizations showed different expression patterns among the four types of Smad4s. These results suggest that some of carp Smad4s have deviated from the original function of Smad4 through vertebrate evolution, and regulated the TGF-beta signaling pathway by changing the expression level in tissues.

Effect of transforming growth factor beta and bone morphogenetic proteins on rat hepatic stellate cell proliferation and trans-differentiation

AIM: To explore different roles of transforming growth factor beta (TGF-β) and bone morphogenetic proteins (BMPs) in hepatic stellate cell proliferation and trans-differentiation.

METHODS: Hepatic stellate cells were isolated from male Sprague-Dawley rats. Sub-cultured hepatic stellate cells were employed for cell proliferation assay with WST-1 reagent and Western blot analysis with antibody against smooth muscle alpha actin (SMA).

RESULTS: The results indicated that TGF-β1 significantly inhibited cell proliferation at concentration as low as 0.1 ng/mL, but both BMP-2 and BMP-4 did not affect cell proliferation at concentration as high as 10 ng/mL. The effect on hepatic stellate cell trans-differentiation was similar between TGF-β1 and BMPs. However, BMPs was more potent at trans-differentiation of hepatic stellate cells than TGF-β1. In addition, we observed that TGF-β1 transient reduced the abundance of SMA in hepatic stellate cells.

CONCLUSION: TGF-β may be more important in regulation of hepatic stellate cell proliferation while BMPs may be the major cytokines regulating hepatic stellate cell trans-differentiation.

Approaches to studying cellular signaling: a primer for morphologists

Many research projects will lead to understanding tissue and/or cell responses to extracellular influences either from soluble factors or the surrounding extracellular matrix. These types of investigations will require the understanding of signal transduction. This particular cell biological field has literally exploded with information and new technical approaches in the past 10 years. This article is directed toward investigators interested in using these new approaches to study their systems. An overview of the general principles of signal transduction events including the types of receptors and intracellular signaling events is followed by an introduction to methods for visualizing signal transduction. This is followed by an introduction to biochemical analysis and an example of combining several approaches to understanding a tissue response to extracellular matrix stimulus.

Targeted mutagenesis of Smad1 reveals an essential role in chorioallantoic fusion

The Smad family of intracellular signaling intermediates transduce signals downstream from the transforming growth factor beta (TGF-beta) family of receptor serine threonine kinases. The original member of this family, Smad1, has been shown to mediate signals from receptors for the bone morphogenetic proteins (BMPs), a large group of ligands in the TGF-beta superfamily that mediate important developmental events. We have targeted the Smad1 gene in mice and created mutants null at this locus. Smad1 mutant mice die at approximately 9.5 days postcoitum due to defects in allantois formation. In Smad1 mutant mice, the allantois fails to fuse to the chorion, resulting in a lack of placenta and failure to establish a definitive embryonic circulation. Although vasculogenesis is initiated in the mutant allantois, the vessels formed are disorganized, and VCAM-1 protein, a marker for distal allantois development, is not expressed. Smad1 null fibroblasts are still able to respond to BMP2, however, suggesting that the defect observed in the developing extraembryonic tissue is caused by a very specific loss of transcriptional activity regulated by Smad1. Our data further demonstrate that although highly similar structurally, Smad proteins are not functionally homologous.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

TGF-beta signaling by Smad proteins

Members of the transforming growth factor-beta (TGF-beta) family bind to type II and type I serine/threonine kinase receptors, which initiate intracellular signals through activation of Smad proteins. Receptor-regulated Smads (R-Smads) are anchored to the cell membrane by interaction with membrane-bound proteins, including Smad anchor for receptor activation (SARA). Upon ligand stimulation, R-Smads are phosphorylated by the receptors and form oligomeric complexes with common-partner Smads (Co-Smads). The oligomeric Smad complexes then translocate into the nucleus, where they regulate the transcription of target genes by direct binding to DNA, interaction with various DNA-binding proteins, and recruitment of transcriptional coactivators or corepressors. A third class of Smads, inhibitory Smads (I-Smads), inhibits the signals from the serine/threonine kinase receptors. Since the expression of I-Smads is induced by the TGF-beta superfamily proteins, Smads constitute an autoinhibitory signaling pathway. The functions of Smads are regulated by other signaling pathways, such as the MAP kinase pathway. Moreover, Smads interact with and modulate the functions of various transcription factors which are downstream targets of other signaling pathways. Loss of function of certain Smads is involved in tumorigenesis, e.g., pancreatic and colorectal cancers. Analyses by gene targeting revealed pivotal roles of Smads in early embryogenesis, angiogenesis, and immune functions in vivo.

The amino terminus of Smads permits transcriptional specificity

Smad proteins transduce transforming growth factor-beta signals from the cell surface to the nucleus, regulating a variety of physiologic processes. In the nucleus, Smads control gene expression by binding to both DNA and transcription factors. Individual Smads regulate distinct subsets of target genes. The key residues important for this specificity are thought to reside in the carboxyl-terminal MH2 domain. To further examine Smad specificity in vivo, we undertook structure-function studies in Xenopus laevis embryos and found that truncated Smads containing the MH2 domain activate gene transcription. A striking finding revealed by the in vivo analyses was that the functional truncated Smads all behaved identically and had lost wild-type specificity. For most Smads, wild-type activity required the presence of an MH1 domain, either in cis or in trans. Of note, even heterologous MH1 domains could restore wild-type signaling specificity to effector MH2 domains. We found a possible mechanism to account for these observations, as Smad MH1 domains altered the binding of pathway-specific transcription factors to the MH2 domain. Thus, Smad MH1 domains are important to the regulation of transcriptional specificity.

More than 95% reversal of left-right axis induced by right-sided hypodermic microinjection of activin into Xenopus neurula embryos

In recent years, genes that show left-right (L-R) asymmetric expression patterns have been identified one after another in vertebrate gastrula-neurula embryos. However, we still have little information about when the irreversible L-R specification is established in vertebrate embryos. In this report, we show that almost 100% of the embryos develop to be L-R-inverted larvae after microinjection of activin molecules into the right lateral hypodermic space of Xenopus neurula embryos. After right-side injection of 10-250 pg activin protein, both early neurulae just after gastrulation movement (stage 13-14) and late neurulae just before neural tube closure (stage 17-18) showed almost 100% reversal of the heart and gut L-R axes. At higher doses of activin, more than 90% of the L-R-inverted embryos showed L-R reversal of both heart and gut. The survival ratio of the right-injected 4-day embryos was 90% on average. In the left-injected embryos, the occurrence of L-R inversion was less than 2% as observed in normal untreated siblings (1.7%). When the same amount of activin (1-50 pg) was microinjected into both sides of neurula embryos, the incidence of L-R inversion was reduced to 58%. The injection of activin along the dorsal midline in the trunk region also randomized the visceral L-R axis. Injection of activin into the right side changed normal left-handed expression of Xnr-1 to right-handed or bilateral expression. In contrast, left-handed expression of Pitx2 was switched to the right side by right activin injection. This is the first report of a method that achieves complete inversion of the visceral L-R axis by treatment of embryos at the neurula stage. Activin not only acts on the neurulae to cancel the original L-R specification up to the late neurula stage, but also rebuilds a new L-R axis whose left side coincides with the injection side. It is suggested that the left and right halves of neurulae have equal potential for L-R differentiation.