Takuya Nakayama

Identification of Smad7, a TGFβ-inducible antagonist of TGF-β signalling

TGF-β signals from the membrane to the nucleus through serine/threonine kinase receptors and their downstream effectors, termed SMAD proteins. The activated TGF-β receptor induces phosphorylation of two such proteins, Smad2 and Smad3 (refs 2, 3, , 5, 6), which form hetero-oligomeric complex(es) with Smad4/DPC4 (refs 5, 6, 7, 8, 9, 10) that translocate to the nucleus,,,, where they then regulate transcriptional responses,. However, the mechanisms by which the intracellular signals of TGF-β are switched off are unclear. Here we report the identification of Smad7, which is related to Smad6 (ref. 13). Transfection of Smad7 blocks responses mediated by TGF-β in mammalian cells, and injection of Smad7 RNA into Xenopus embryos blocks activin/TGF-β signalling. Smad7 associates stably with the TGF-β receptor complex, but is not phosphorylated upon TGF-β stimulation. TGFβ-mediated phosphorylation of Smad2 and Smad3 is inhibited by Smad7, indicating that the antagonistic effect of Smad7 is exerted at this important regulatory step. TGF-β rapidly induces expression of Smad7 mRNA, suggesting that Smad7 may participate in a negative feedback loop to control TGF-β responses.

Daughters against dpp modulates dpp organizing activity in Drosophila wing development

The family of TGF-β signalling molecules play inductive roles in various developmental contexts. One member of this family, Drosophila Decapentaplegic (Dpp) serves as a morphogen that patterns both the embryo, and adult,. We have now isolated a gene, Daughters against dpp ( Dad), whose transcription is induced by Dpp. Dad shares weak homology with Drosophila Mad (Mothers against dpp), a protein required for transduction of Dpp signals. In contrast to Mad or the activated Dpp receptor, whose overexpression hyperactivates the Dpp signalling pathway, overexpression of Dad blocks Dpp activity. Expression of Dad together with either Mad or the activated receptor rescues phenotypic defects induced by each protein alone. Dad can also antagonize the activity of a vertebrate homologue of Dpp, bone morphogenetic protein (BMP-4; ref. 7), as evidenced by induction of dorsal or neural fate following overexpression in Xenopus embryos. We conclude that the pattern-organizing mechanism governed by Dpp involves a negative-feedback circuit in which Dpp induces expression of its own antagonist, Dad. This feedback loop appears to be conserved in vertebrate development.

Genome evolution in the allotetraploid frog Xenopus laevis

To explore the origins and consequences of tetraploidy in the African clawed frog, we sequenced the Xenopus laevis genome and compared it to the related diploid X. tropicalis genome. We characterize the allotetraploid origin of X. laevis by partitioning its genome into two homoeologous subgenomes, marked by distinct families of ‘fossil’ transposable elements. On the basis of the activity of these elements and the age of hundreds of unitary pseudogenes, we estimate that the two diploid progenitor species diverged around 34 million years ago (Ma) and combined to form an allotetraploid around 17–18 Ma. More than 56% of all genes were retained in two homoeologous copies. Protein function, gene expression, and the amount of conserved flanking sequence all correlate with retention rates. The subgenomes have evolved asymmetrically, with one chromosome set more often preserving the ancestral state and the other experiencing more gene loss, deletion, rearrangement, and reduced gene expression.

Simple and efficient CRISPR/Cas9-mediated targeted mutagenesis in Xenopus tropicalis.

We have assessed the efficacy of the recently developed CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR‐associated) system for genome modification in the amphibian Xenopus tropicalis. As a model experiment, targeted mutations of the tyrosinase gene were verified, showing the expected albinism phenotype in injected embryos. We further tested this technology by interrupting the six3 gene, which is required for proper eye and brain formation. Expected eye and brain phenotypes were observed when inducing mutations in the six3 coding regions, as well as when deleting the gene promoter by dual targeting. We describe here a standardized protocol for genome editing using this system. This simple and fast method to edit the genome provides a powerful new reverse genetics tool for Xenopus researchers. genesis 51:835–843.

Smad6 functions as an intracellular antagonist of some TGF‐β family members during Xenopus embryogenesis

Bone morphogenetic proteins (BMPs) transmit signals via the intracellular protein Smad1, which is phosphorylated by ligand bound receptors, translocates to the nucleus, and functions to activate BMP target genes. Recently, a subclass of Smad proteins has been shown to inhibit, rather than transduce, BMP signalling, either by binding to the intracellular domain of BMP receptors, thereby preventing phosphorylation‐mediated activation of Smad1, or by binding directly to Smad1, thereby inhibiting its ability to activate gene transcription.

Can't get no SMADisfaction: Smad proteins as positive and negative regulators of TGF‐β family signals

The identification of Smad proteins as molecular components of the transforming growth factor‐β (TGF‐β) signaling cascade has enhanced our understanding of how ligand‐mediated activation of TGF‐β receptors leads to modulation of target gene transcription. Recent studies have identified a distinct, structurally related class of Smads which inhibits, rather than transduces, TGF‐β family signals. The molecular mechanism of action and the exact signaling pathways that are targeted by antagonistic Smads are not completely understood. These proteins appear to participate in autoregulatory negative feedback loops in which signaling initiated by specific TGF‐β family ligands induces the expression of an inhibitory Smad that then functions to modulate the amplitude or duration of signaling. Negative feedback circuits such as these play important roles in fine‐tuning the activity of multifunctional signaling molecules during embryonic patterning and in response to pathologic stimuli in adults. BioEssays 21:382–390, 1999.

Mutation of an upstream cleavage site in the BMP4 prodomain leads to tissue-specific loss of activity

ProBMP4 is initially cleaved at a site adjacent to the mature ligand (the S1 site) allowing for subsequent cleavage at an upstream (S2) site. Mature BMP4 synthesized from a precursor in which the S2 site cannot be cleaved remains in a complex with the prodomain that is targeted for lysosomal degradation, and is thus less active when overexpressed in Xenopus. Here we report that mice carrying a point mutation that prevents S2 processing show severe loss of BMP4 activity in some tissues, such as testes and germ cells, whereas other tissues that are sensitive to Bmp4 dosage, such as the limb, dorsal vertebrae and kidney, develop normally. In a haploinsufficient background, inability to cleave the S2 site leads to embryonic and postnatal lethality due to defects in multiple organ systems including the allantois, placental vasculature, ventral body wall, eye and heart. These data demonstrate that cleavage of the S2 site is essential for normal development and, more importantly, suggest that this site might be selectively cleaved in a tissue-specific fashion. In addition, these studies provide the first genetic evidence that BMP4 is required for dorsal vertebral fusion and closure of the ventral body wall.

Regulation of BMP/Dpp signaling during embryonic development

Abstract. Bone morphogenetic protein-4 (BMP-4) and its Drosophila ortholog, decapentaplegic (Dpp), are multifunctional developmental regulators. Both gain-of-function and loss-of-function studies demonstrate that the biological activity and signaling range of these morphogens must be strictly regulated to ensure normal embryonic patterning. BMP-4 and Dpp are produced from inactive precursors that are proteolytically cleaved, following which the active ligand is secreted into the extracellular space. Binding of BMP-4 or Dpp to its cognate receptor leads to phosphorylation of intracellular signal-transducing Smad proteins that then form hetero-oligomers, translocate to the nucleus and modulate transcription of target genes. Recent studies have shown that the BMP signal transduction cascade can be modulated at every step of this process.

Cas9-based genome editing in Xenopus tropicalis.

Xenopus tropicalis has been developed as a model organism for developmental biology, providing a system offering both modern genetics and classical embryology. Recently, the Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated (CRISPR/Cas) system for genome modification has provided an additional tool for Xenopus researchers to achieve simple and efficient targeted mutagenesis. Here, we provide insights into experimental design and procedures permitting successful application of this technique to Xenopus researchers, and offer a general strategy for performing loss-of-function assays in F0 and subsequently F1 embryos.

Dissection of inhibitory Smad proteins: both N- and C-terminal domains are necessary for full activities of Xenopus Smad6 and Smad7

Smad6 and Smad7 comprise a subclass of vertebrate Smads that antagonize, rather than transduce, TGF-beta family signaling. These Anti-Smads can block BMP signaling, as evidenced by their ability to induce a secondary dorsal axis when misexpressed ventrally in Xenopus embryos. Smad7 inhibits additional TGF-beta related pathways, and causes spina bifida when misexpressed dorsally. We have performed structure-function analyses to identify domains of Anti-Smads that are responsible for their shared and unique activities. We find that the C-terminal domain of Smad7 displays strong axis inducing activity but cannot induce spina bifida. The isolated N-terminal domain of Smad7 is inactive but restores the ability of the C-terminus to cause spina bifida when the two are co-expressed. By contrast, the N- and C-terminal domains of Smad6 have weak axis inducing activity when expressed individually, but show full activity when co-expressed. Chimeric analysis demonstrates that the C-terminal domain of Smad7, but not Smad6, can induce spina bifida when fused to the N-terminal domain of either Smad6 or Smad7. Thus, although the C-terminal domain is the primary determinant of the intrinsic activity of Xenopus Anti-Smads, the N-terminal domain is essential for full activity, is interchangeable between Smad6 and 7, and can function in trans.