Recognition of phosphorylated-Smad2-containing complexes by a novel Smad interaction motif

Circulating miRNAs as Potential Biomarkers Distinguishing Relapsing-Remitting from Secondary Progressive Multiple Sclerosis. A Review

Multiple sclerosis (MS) is a debilitating neurodegenerative, highly heterogeneous disease with a variable course. The most common MS subtype is relapsing–remitting (RR), having interchanging periods of worsening and relative stabilization. After a decade, in most RR patients, it alters into the secondary progressive (SP) phase, the most debilitating one with no clear remissions, leading to progressive disability deterioration. Among the greatest challenges for clinicians is understanding disease progression molecular mechanisms, since RR is mainly characterized by inflammatory processes, while in SP, the neurodegeneration prevails. This is especially important because distinguishing RR from the SP subtype early will enable faster implementation of appropriate treatment. Currently, the MS course is not well-correlated with the biomarkers routinely used in clinical practice. Despite many studies, there are still no reliable indicators correlating with the disease stage and its activity degree. Circulating microRNAs (miRNAs) may be considered valuable molecules for the MS diagnosis and, presumably, helpful in predicting disease subtype. MiRNA expression dysregulation is commonly observed in the MS course. Moreover, knowledge of diverse miRNA panel expression between RRMS and SPMS may allow for deterring disability progression through successful treatment. Therefore, in this review, we address the current state of research on differences in miRNA panel expression between the phases.

Mutations in SKI in Shprintzen-Goldberg syndrome lead to attenuated TGF-β responses through SKI stabilization

Shprintzen-Goldberg syndrome (SGS) is a multisystemic connective tissue disorder, with considerable clinical overlap with Marfan and Loeys-Dietz syndromes. These syndromes have commonly been associated with enhanced TGF-β signaling. In SGS patients, heterozygous point mutations have been mapped to the transcriptional co-repressor SKI, which is a negative regulator of TGF-β signaling that is rapidly degraded upon ligand stimulation. The molecular consequences of these mutations, however, are not understood. Here we use a combination of structural biology, genome editing, and biochemistry to show that SGS mutations in SKI abolish its binding to phosphorylated SMAD2 and SMAD3. This results in stabilization of SKI and consequently attenuation of TGF-β responses, both in knockin cells expressing an SGS mutation and in fibroblasts from SGS patients. Thus, we reveal that SGS is associated with an attenuation of TGF-β-induced transcriptional responses, and not enhancement, which has important implications for other Marfan-related syndromes.

Recent progress in TGF-β inhibitors for cancer therapy

Transforming growth factor-β (TGF-β) is a multifunctional cytokine that is involved in proliferation, metastasis, and many other important processes in malignancy. Inhibitors targeting TGF-β have been considered by pharmaceutical companies for cancer therapy, and some of them are in clinical trial now. Unfortunately, several of these programs have recently been relinquished, and most companies that remain in the contest are progressing slowly and cautiously. This review summarizes the TGF-β signal transduction pathway, its roles in oncogenesis and fibrotic diseases, and advancements in antibodies and small-molecule inhibitors of TGF-β.

Structural basis for transcriptional coactivator recognition by SMAD2 in TGF-β signaling

Transforming growth factor-β (TGF-β) proteins regulate multiple cellular functions, including cell proliferation, apoptosis, and extracellular matrix formation. The dysregulation of TGF-β signaling causes diseases such as cancer and fibrosis, and therefore, understanding the biochemical basis of TGF-β signal transduction is important for elucidating pathogenic mechanisms in these diseases. SMAD proteins are transcription factors that mediate TGF-β signaling-dependent gene expression. The transcriptional coactivator CBP directly interacts with the MH2 domains of SMAD2 to activate SMAD complex-dependent gene expression. Here, we report the structural basis for CBP recognition by SMAD2. The crystal structures of the SMAD2 MH2 domain in complex with the SMAD2-binding region of CBP showed that CBP forms an amphiphilic helix on the hydrophobic surface of SMAD2. The expression of a mutated CBP peptide that showed increased SMAD2 binding repressed SMAD2-dependent gene expression in response to TGF-β signaling in cultured cells. Disrupting the interaction between SMAD2 and CBP may therefore be a promising strategy for suppressing SMAD-dependent gene expression.

Identification of FOXH1 mutations in patients with sporadic conotruncal heart defect

Conotruncal heart defects (CTD) are an important subtype of congenital heart disease that occur due to abnormality in the development of the cardiac outflow tract (OFT). FOXH1 is a transcription factor that participates in the morphogenesis of the right ventricle and OFT. In this study, we confirmed the expression of FOXH1 in mouse and human embryos during OFT development. We also scanned the coding exons and splicing regions of the FOXH1 gene in 605 patients with sporadic CTD and 300 unaffected controls, from which we identified seven heterozygous FOXH1 gene mutations. According to bioinformatics analysis results, they were predicted potentially deleterious at conserved amino acid sites. Western blot was used to show that all the variants decreased the expression of FOXH1 protein, while dual-luciferase reporter assay showed that six of them, with an exception of p.P35R, had enhanced abilities to modulate the expression of MEF2C, which interacts with NKX2.5 and is involved in cardiac growth. The electrophoretic mobility shift assays result showed that two mutations altered DNA-binding abilities of mutant FOXH1 proteins. Phenotype heterogeneity was found in patients with the same mutation. These results indicate that FOXH1 mutations lead to disease-causing functional changes that contribute to the occurrence of CTD.

Temporal dynamics in the formation and interpretation of Nodal and BMP morphogen gradients

One of the most powerful ideas in developmental biology has been that of the morphogen gradient. In the classical view, a signaling molecule is produced at a local source from where it diffuses, resulting in graded levels across the tissue. This gradient provides positional information, with thresholds in the level of the morphogen determining the position of different cell fates. While experimental studies have uncovered numerous potential morphogens in biological systems, it is becoming increasingly apparent that one important feature, not captured in the simple model, is the role of time in both the formation and interpretation of morphogen gradients. We will focus on two members of the transforming growth factor-β family that are known to play a vital role as morphogens in early vertebrate development: the Nodals and the bone morphogenetic proteins (BMPs). Primarily drawing on the early zebrafish embryo, we will show how recent studies have demonstrated the importance of feedback and other interactions that evolve through time, in shaping morphogen gradients. We will further show how rather than simply reading out levels of a morphogen, the duration of ligand exposure can be a crucial determinant of how cells interpret morphogens, in particular through the unfolding of downstream transcriptional events and in their interactions with other pathways.

Nuclear Transport and Accumulation of Smad Proteins Studied by Single-Molecule Microscopy

Nuclear translocation of stimulated Smad heterocomplexes is a critical step in the signal transduction of transforming growth factor β (TGF-β) from transmembrane receptors into the nucleus. Specifically, normal nuclear accumulation of Smad2/Smad4 heterocomplexes induced by TGF-β1 is involved in carcinogenesis. However, the relationship between nuclear accumulation and the nucleocytoplasmic transport kinetics of Smad proteins in the presence of TGF-β1 remains obscure. By combining a high-speed single-molecule tracking microscopy and Förster resonance energy transfer technique, we tracked the entire TGF-β1-induced process of Smad2/Smad4 heterocomplex formation, as well as their transport through nuclear pore complexes in live cells, with a high single-molecule localization precision of 2 ms and <20 nm. Our single-molecule Förster resonance energy transfer data have revealed that in TGF-β1-treated cells, Smad2/Smad4 heterocomplexes formed in the cytoplasm, imported through the nuclear pore complexes as entireties, and finally dissociated in the nucleus. Moreover, we found that basal-state Smad2 or Smad4 cannot accumulate in the nucleus without the presence of TGF-β1, mainly because both of them have an approximately twofold higher nuclear export efficiency compared to their nuclear import. Remarkably and reversely, heterocomplexes of Smad2/Smad4 induced by TGF-β1 can rapidly concentrate in the nucleus because of their almost fourfold higher nuclear import rate in comparison with their nuclear export rate. Thus, we believe that the determined TGF-β1-dependent transport configurations and efficiencies for the basal-state Smad or stimulated Smad heterocomplexes elucidate the basic molecular mechanism to understand their nuclear transport and accumulation.

Hydrophobic patches on SMAD2 and SMAD3 determine selective binding to cofactors

The transforming growth factor-β (TGF-β) superfamily of cytokines regulates various biological processes, including cell proliferation, immune responses, autophagy, and senescence. Dysregulation of TGF-β signaling causes various diseases, such as cancer and fibrosis. SMAD2 and SMAD3 are core transcription factors involved in TGF-β signaling, and they form heterotrimeric complexes with SMAD4 (SMAD2-SMAD2-SMAD4, SMAD3-SMAD3-SMAD4, and SMAD2-SMAD3-SMAD4) in response to TGF-β signaling. These heterotrimeric complexes interact with cofactors to control the expression of TGF-β-dependent genes. SMAD2 and SMAD3 may promote or repress target genes depending on whether they form complexes with other transcription factors, coactivators, or corepressors; therefore, the selection of specific cofactors is critical for the appropriate activity of these transcription factors. To reveal the structural basis by which SMAD2 and SMAD3 select cofactors, we determined the crystal structures of SMAD3 in complex with the transcription factor FOXH1 and SMAD2 in complex with the transcriptional corepressor SKI. The structures of the complexes show that the MAD homology 2 (MH2) domains of SMAD2 and SMAD3 have multiple hydrophobic patches on their surfaces. The cofactors tether to various subsets of these patches to interact with SMAD2 and SMAD3 in a cooperative or competitive manner to control the output of TGF-β signaling.

Gramine inhibits angiogenesis and induces apoptosis via modulation of TGF-β signalling in 7,12 dimethylbenz[a]anthracene (DMBA) induced hamster buccal pouch carcinoma

BACKGROUND

Transforming growth factor-β (TGF-β) and its receptors are considered as a novel target in cancer chemotherapy. Gramine, an indole alkaloid, possesses various pharmacological properties including antiproliferative and anticancer. However, the anti-angiogenic property remains unexplored.

PURPOSE

The present study was designed to evaluate the anti-angiogenic and apoptosis induction properties of gramine through inhibiting TGF-β on DMBA induced oral squamous cell carcinoma (OSCC) in the hamster buccal pouch (HBP).

METHODS

The effects of gramine on TGF-β signalling in DMBA induced carcinogenic events such as angiogenesis and apoptosis were analysed by studying the mRNA expression using RT-PCR, protein expression by western blot and histopathological analysis using haematoxylin and eosin (H & E) staining.

RESULTS

Gramine significantly inhibited phosphorylation and nuclear translocation of Smad2 and Smad4 by blocking activity of the TGFβ-RII, RI and activation of inhibitory Smad7. Gramine inhibited angiogenic markers such as MMP-2, MMP-9, HIF-1α, VEGF, and VEGF-R2 as well as increased TIMP-2 expression. Furthermore, gramine induced apoptosis in DMBA induced tumour bearing animals by up regulating the pro apoptotic proteins Bax, cytochrome C, apaf-1, caspase-9 caspase-3 and PARP.

CONCLUSION

In this study, we clearly demonstrated that gramine treatment diminishes angiogenesis and induces apoptosis in hamster buccal pouch (HBP) carcinogenesis by modulating TGF-β signals.

A Temporal Window for Signal Activation Dictates the Dimensions of a Nodal Signaling Domain

Morphogen signaling is critical for the growth and patterning of tissues in embryos and adults, but how morphogen signaling gradients are generated in tissues remains controversial. The morphogen Nodal was proposed to form a long-range signaling gradient via a reaction-diffusion system, on the basis of differential diffusion rates of Nodal and its antagonist Lefty. Here we use a specific zebrafish Nodal biosensor combined with immunofluorescence for phosphorylated Smad2 to demonstrate that endogenous Nodal is unlikely to diffuse over a long range. Instead, short-range Nodal signaling activation in a temporal window is sufficient to determine the dimensions of the Nodal signaling domain. The size of this temporal window is set by the differentially timed production of Nodal and Lefty, which arises mainly from repression of Lefty translation by the microRNA miR-430. Thus, temporal information is transformed into spatial information to define the dimensions of the Nodal signaling domain and, consequently, to specify mesendoderm.

Genome-wide view of TGFβ/Foxh1 regulation of the early mesendoderm program

Nodal/TGFβ signaling regulates diverse biological responses. By combining RNA-seq on Foxh1 and Nodal signaling loss-of-function embryos with ChIP-seq of Foxh1 and Smad2/3, we report a comprehensive genome-wide interaction between Foxh1 and Smad2/3 in mediating Nodal signaling during vertebrate mesendoderm development. This study significantly increases the total number of Nodal target genes regulated by Foxh1 and Smad2/3, and reinforces the notion that Foxh1-Smad2/3-mediated Nodal signaling directly coordinates the expression of a cohort of genes involved in the control of gene transcription, signaling pathway modulation and tissue morphogenesis during gastrulation. We also show that Foxh1 may function independently of Nodal signaling, in addition to its role as a transcription factor mediating Nodal signaling via Smad2/3. Finally, we propose an evolutionarily conserved interaction between Foxh1 and PouV, a mechanism observed in Pou5f1-mediated regulation of pluripotency in human embryonic stem and epiblast cells.

Inhibition of TGF-β signaling at the nuclear envelope: characterization of interactions between MAN1, Smad2 and Smad3, and PPM1A

Signaling by transforming growth factor-β (TGF-β) is critical for various developmental processes and culminates in the activation of the transcription factors Smad2 and Smad3. MAN1, an integral protein of the inner nuclear membrane, inhibits TGF-β signaling by binding to Smad2 and Smad3. Depletion of the gene LEMD3 encoding MAN1 leads to developmental anomalies in mice, and heterozygous loss-of-function mutations in LEMD3 in humans cause sclerosing bone dysplasia. We modeled the three-dimensional structure of the MAN1-Smad2 complex from nuclear magnetic resonance and small-angle x-ray scattering data. As predicted by this model, we found that MAN1 competed in vitro and in cells with the transcription factor FAST1 (forkhead activin signal transducer 1) for binding to Smad2. The model further predicted that MAN1 bound to activated Smad2-Smad4 or Smad3-Smad4 complexes, which was confirmed by in vitro experiments; however, in cells, MAN1 bound only to Smad2 and Smad3 and not to the Smad4-containing complexes. Overexpression of MAN1 led to dephosphorylation of Smad2 and Smad3, thus hindering their recognition by Smad4, and MAN1 bound directly in vitro to the phosphatase PPM1A, which catalyzes the dephosphorylation of Smad2/3. These results demonstrate a nuclear envelope-localized mechanism of inactivating TGF-β signaling in which MAN1 competes with transcription factors for binding to Smad2 and Smad3 and facilitates their dephosphorylation by PPM1A.

Mechanisms of activin-stimulated FSH synthesis: the story of a pig and a FOX

Activins were discovered and, in fact, named more than a quarter century ago based on their abilities to stimulate pituitary follicle-stimulating hormone (FSH) synthesis and secretion. However, it is only in the last decade that we have finally come to understand their underlying mechanisms of action in gonadotroph cells. In this minireview, we chronicle the research that led to the recent discovery of forkhead box L2 (FOXL2) as an essential mediator of activin-regulated FSH beta subunit (Fshb) transcription in vitro and in vivo.

Arkadia regulates tumor metastasis by modulation of the TGF-β pathway

TGF-β can act as a tumor suppressor at early stages of cancer progression and as a tumor promoter at later stages. The E3 ubiquitin ligase Arkadia (RNF111) is a critical component of the TGF-β signaling pathway, being required for a subset of responses, those mediated by Smad3-Smad4 complexes. It acts by mediating ligand-induced degradation of Ski and SnoN (SKIL), which are 2 potent transcriptional repressors. Here, we investigate the role of Arkadia in cancer using model systems to address both potential tumor-suppressive and tumor-promoting roles. Stable reexpression of Arkadia in lung carcinoma NCI-H460 cells, which we show contain a hemizygous nonsense mutation in the Arkadia/RNF111 gene, efficiently restored TGF-β-induced Smad3-dependent transcription, and substantially decreased the ability of these cells to grow in soft agar in vitro. However, it had no effect on tumor growth in vivo in mouse models. Moreover, loss of Arkadia in cancer cell lines and human tumors is rare, arguing against a prominent tumor-suppressive role. In contrast, we have uncovered a potent tumor-promoting function for Arkadia. Using 3 different cancer cell lines whose tumorigenic properties are driven by TGF-β signaling, we show that loss of Arkadia function, either by overexpression of dominant negative Arkadia or by siRNA-induced knockdown, substantially inhibited lung colonization in tail vein injection experiments in immunodeficient mice. Our findings indicate that Arkadia is not critical for regulating tumor growth per se, but is required for the early stages of cancer cell colonization at the sites of metastasis.

The forkhead transcription factor FoxB1 regulates the dorsal-ventral and anterior-posterior patterning of the ectoderm during early Xenopus embryogenesis

The formation of the dorsal-ventral (DV) and anterior-posterior (AP) axes, fundamental to the body plan of animals, is regulated by several groups of polypeptide growth factors including the TGF-β, FGF, and Wnt families. In order to ensure the establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated. However, the molecular mechanisms responsible for these interactions remain unclear. Here, we demonstrate that the forkhead box transcription factor FoxB1, which is upregulated by the neuralizing factor Oct-25, plays an important role in the formation of the DV and AP axes. Overexpression of FoxB1 promoted neural induction and inhibited BMP-dependent epidermal differentiation in ectodermal explants, thereby regulating the DV patterning of the ectoderm. In addition, FoxB1 was also found to promote the formation of posterior neural tissue in both ectodermal explants and whole embryos, suggesting its involvement in embryonic AP patterning. Using knockdown analysis, we found that FoxB1 is required for the formation of posterior neural tissues, acting in concert with the Wnt and FGF pathways. Consistent with this, FoxB1 suppressed the formation of anterior structures via a process requiring the function of XWnt-8 and eFGF. Interestingly, while downregulation of FoxB1 had little effect on neural induction, we found that it functionally interacted with its upstream factor Oct-25 and plays a supportive role in the induction and/or maintenance of neural tissue. Our results suggest that FoxB1 is part of a mechanism that fine-tunes, and leads to the coordinated formation of, the DV and AP axes during early development.

Studying Smad2 intranuclear diffusion dynamics by mathematical modelling of FRAP experiments

We combine Fluorescence Recovery After Photobleaching (FRAP) experiments with mathematical modelling to study the dynamics inside the nucleus of both the TGF-β-sensitive transcriptional regulator Smad2, and Green-Fluorescent Protein (GFP). We show how combining modelling with bleaching strips of different areas allows a rigorous test of whether or not a protein is moving via diffusion as a single species. As noted recently by others, it is important to consider diffusion during the bleaching process. Neglecting it can cause serious error. Also, it is possible to use the bleaching process itself to provide an extra consistency test to the models predicting the recovery. With our method we show that the dynamics of GFP are consistent with it diffusing as a single species in a uniform environment in which flow is negligible. In contrast, the dynamics of the intracellular signal transducer Smad2 are never consistent with it moving as a single species via simple diffusion in a homogeneous environment without flow. Adding TGF-β slows down the dynamics of Smad2 but even without TGF-β, the Smad2 dynamics are influenced by one or more of: association, flow, and inhomogeneity in space of the dynamics. We suggest that the dynamics inside cells of many proteins may be poorly described by simple diffusion of a single species, and that our methodology provides a general and powerful way to test this hypothesis.

Structural analysis of the Smad2-MAN1 interaction that regulates transforming growth factor-β signaling at the inner nuclear membrane

MAN1, an integral protein of the inner nuclear membrane, influences transforming growth factor-β (TGF-β) signaling by directly interacting with R-Smads. Heterozygous loss of function mutations in the gene encoding MAN1 cause sclerosing bone dysplasias and an increased level of TGF-β signaling in cells. As a first step in elucidating the mechanism of TGF-β pathway regulation by MAN1, we characterized the structure of the MAN1 C-terminal region that binds Smad2. Using nuclear magnetic resonance spectroscopy, we observed that this region is comprised of a winged helix domain, a structurally heterogeneous linker, a U2AF homology motif (UHM) domain, and a disordered C-terminus. From nuclear magnetic resonance and small-angle X-ray scattering data, we calculated a family of models for this MAN1 region. Our data indicate that the linker plays the role of an intramolecular UHM ligand motif (ULM) interacting with the UHM domain. We mapped the Smad2 binding site onto the MAN1 structure by combining GST pull-down, fluorescence, and yeast two-hybrid approaches. The linker region, the UHM domain, and the C-terminus are necessary for Smad2 binding with a micromolar affinity. Moreover, the intramolecular interaction between the linker and the UHM domain is critical for Smad2 binding. On the basis of the structural heterogeneity and binding properties of the linker, we suggest that it can interact with other UHM domains, thus regulating the MAN1-Smad2 interaction.

Foxa1 functions as a pioneer transcription factor at transposable elements to activate Afp during differentiation of embryonic stem cells

Epigenetic control of genes that are silent in embryonic stem cells, but destined for expression during differentiation, includes distinctive hallmarks, such as simultaneous activating/repressing (bivalent) modifications of chromatin and DNA hypomethylation at enhancers of gene expression. Although alpha-fetoprotein (Afp) falls into this class of genes, as it is silent in pluripotent stem cells and activated during differentiation of endoderm, we find that Afp chromatin lacks bivalent histone modifications. However, critical regulatory sites for Afp activation, overlapping Foxa1/p53/Smad-binding elements, are located within a 300-bp region lacking DNA methylation, due to transposed elements underrepresented in CpG sequences: a short interspersed transposable element and a medium reiterated sequence 1 element. Forkhead family member Foxa1 is activated by retinoic acid treatment of embryonic stem cells, binds its DNA consensus site within the short interspersed transposable/medium reiterated sequence 1 elements, and displaces linker histone H1 from silent Afp chromatin. Small interfering RNA depletion of Foxa1 showed that Foxa1 is essential in providing chromatin access to transforming growth factor beta-activated Smad2 and Smad4 and their subsequent DNA binding. Together these transcription factors establish highly acetylated chromatin and promote expression of Afp. Foxa1 acts as a pioneer transcription factor in de novo activation of Afp, by exploiting a lack of methylation at juxtaposed transposed elements, to bind and poise chromatin for intersection with transforming growth factor beta signaling during differentiation of embryonic stem cells.

TMEPAI, a transmembrane TGF-beta-inducible protein, sequesters Smad proteins from active participation in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine of key importance for controlling embryogenesis and tissue homeostasis. How TGF-beta signals are attenuated and terminated is not well understood. Here, we show that TMEPAI, a direct target gene of TGF-beta signaling, antagonizes TGF-beta signaling by interfering with TGF-beta type I receptor (TbetaRI)-induced R-Smad phosphorylation. TMEPAI can directly interact with R-Smads via a Smad interaction motif. TMEPAI competes with Smad anchor for receptor activation for R-Smad binding, thereby sequestering R-Smads from TbetaRI kinase activation. In mammalian cells, ectopic expression of TMEPAI inhibited TGF-beta-dependent regulation of plasminogen activator inhibitor-1, JunB, cyclin-dependent kinase inhibitors, and c-myc expression, whereas specific knockdown of TMEPAI expression prolonged duration of TGF-beta-induced Smad2 and Smad3 phosphorylation and concomitantly potentiated cellular responsiveness to TGF-beta. Consistently, TMEPAI inhibits activin-mediated mesoderm formation in Xenopus embryos. Therefore, TMEPAI participates in a negative feedback loop to control the duration and intensity of TGF-beta/Smad signaling.

Tgf-beta superfamily signaling in embryonic development and homeostasis

TGF-beta superfamily signaling pathways emerged with the evolution of multicellular animals, suggesting that these pathways contribute to the increased diversity and complexity required for the development and homeostasis of these organisms. In this review we begin by exploring some key developmental and disease processes requiring TGF-beta ligands to underscore the fundamental importance of these pathways before delving into the molecular mechanism of signal transduction, focusing on recent findings. Finally, we discuss how these ligands act as morphogens, how their activity and signaling range is regulated, and how they interact with other signaling pathways to achieve their specific and varied functional roles.

FoxL2 and Smad3 coordinately regulate follistatin gene transcription

Follistatin is a transcriptional target and a modulator of activin action. Through an autocrine/paracrine loop, activin controls follistatin levels and thus regulates its own bioavailability. In gonadotropic alphaT3-1 cells, activin induces follistatin transcription primarily through the action of Smad3 at an intronic Smad-binding element (SBE1). Using a proteomics approach, we searched for endogenous alphaT3-1 proteins that participate in SBE1-mediated transcription. We identified FoxL2, a member of the forkhead family, as a candidate modulator of SBE1 function. Mutations of FoxL2 are associated with the blepharophimosis/ptosis/epicanthus inversus syndrome characterized with craniofacial defects and premature ovarian failure. FoxL2 localizes to alpha-glycoprotein subunit- and follicle-stimulating hormone beta-positive cells of the adult mouse pituitary and is present in alphaT3-1 and LbetaT2 cells, but its pituitary actions remain largely unknown. We have determined that FoxL2 binds to a forkhead-binding element (FKHB) located just downstream of the SBE1 site of the follistatin gene and functions as a Smad3 partner to drive SBE1-mediated transcription in alphaT3-1 cells treated with activin. Chromatin immunoprecipitation assays confirm that endogenous FoxL2 and Smad3 are recruited to the intronic enhancer of the follistatin gene where the SBE1 and FKHB sites are located. Exogenous FoxL2 enhances SBE1-mediated transcription, and short hairpin RNA-mediated knockdown of endogenous FoxL2 protein compromises this effect in alphaT3-1 cells. FoxL2 directly associates with Smad3 but not Smad2 or Smad4. This association between Smad3 and FoxL2 is mediated by the MH2 domain of Smad3 and is dependent on an intact forkhead domain in FoxL2. Altogether, these observations highlight a novel role for FoxL2 and suggest that it may function as a transcriptional regulator and a coordinator of Smad3 targets.

TGF-beta signal transduction in chronic kidney disease

Transforming growth factor (TGF)-beta is a central stimulus of the events leading to chronic progressive kidney disease, having been implicated in the regulation of cell proliferation, hypertrophy, apoptosis and fibrogenesis. The fact that it mediates these varied events suggests that multiple mechanisms play a role in determining the outcome of TGF-beta signaling. Regulation begins with the availability and activation of TGF-beta and continues through receptor expression and localization, control of the TGF-beta family-specific Smad signaling proteins, and interaction of the Smads with multiple signaling pathways extending into the nucleus. Studies of these mechanisms in kidney cells and in whole-animal experimental models, reviewed here, are beginning to provide insight into the role of TGF-beta in the pathogenesis of renal dysfunction and its potential treatment.

An N-terminally truncated Smad2 protein can partially compensate for loss of full-length Smad2

TGFβ (transforming growth factor β) superfamily signalling is critical both for early embryonic development and later for tissue homoeostasis in adult organisms. The use of gene-disruption techniques in mice has been essential to understanding the functional roles of the components of the pathways downstream of TGFβ superfamily ligands, in particular, the receptors and the Smads that transduce signals from the plasma membrane to the nucleus. Smad2 functions downstream of TGFβ, Activin and Nodal, and a number of Smad2 mutant mice have been generated by different laboratories. Although in all cases these Smad2-deficient mice were embryonic lethal, those created by deletion of the first coding exon survived longer than those generated by replacing part of the MH (Mad homology) 1 domain or deleting all or part of the MH2 domain. Moreover, they displayed a less severe phenotype, as they were capable of transiently inducing mesoderm. In the present study, we show that embryonic fibroblasts taken from the Smad2 mutant mice created by deletion of the first coding exon express a small amount of an N-terminally truncated Smad2 protein. We show this protein results from internal initiation at Met241 and encodes the entire MH2 domain and the C-terminal part of the linker. We demonstrate that this protein is incorporated into Smad heteromeric complexes, can interact with DNA-binding transcription factors and thereby can mediate TGFβ-induced transcriptional activation from a number of TGFβ-responsive elements. We propose that this functional truncated Smad2 protein can partially compensate for the loss of full-length Smad2, thereby providing an explanation for the differing phenotypes of Smad2 mutant mice.

The pro-domain of the zebrafish Nodal-related protein Cyclops regulates its signaling activities

Nodal proteins are secreted signaling factors of the transforming growth factor β (TGFβ) family with essential roles in embryonic development in vertebrates. Mutations affecting the Nodal factors have severe consequences in mammals and fish. Furthermore, increased Nodal levels have been associated with melanoma tumor progression. Like other TGFβ-related proteins, Nodal factors consist of a pro-domain and a mature domain. The pro-domain of mouse Nodal protein stabilizes its precursor. However, the mechanisms by which the pro-domains exert their activities are unknown. Here, we characterize the zebrafish Nodal-related factor Cyclops (Cyc) and find unexpected functions for the pro-domain in regulating Cyc activity. We identified a lysosome-targeting region in the Cyc pro-domain that destabilizes the precursor and restricts Cyc activity, revealing the molecular basis for the short-range signaling activities of Cyc. We show that both the pro- and mature-domains of Cyc regulate its stability. We also characterize a mutation in the pro-domain of human NODAL (hNODAL) that underlies congenital heterotaxia. Heterologous expression of mutant hNODAL increases expression of Nodal-response genes. Our studies reveal unexpected roles for the pro-domain of the Nodal factors and provide a possible mechanism for familial heterotaxia.

Reduced NODAL signaling strength via mutation of several pathway members including FOXH1 is linked to human heart defects and holoprosencephaly

Abnormalities of embryonic patterning are hypothesized to underlie many common congenital malformations in humans including congenital heart defects (CHDs), left-right disturbances (L-R) or laterality, and holoprosencephaly (HPE). Studies in model organisms suggest that Nodal-like factors provide instructions for key aspects of body axis and germ layer patterning; however, the complex genetics of pathogenic gene variant(s) in humans are poorly understood. Here we report our studies of FOXH1, CFC1, and SMAD2 and summarize our mutational analysis of three additional components in the human NODAL-signaling pathway: NODAL, GDF1, and TDGF1. We identify functionally abnormal gene products throughout the pathway that are clearly associated with CHD, laterality, and HPE. Abnormal gene products are most commonly detected in patients within a narrow spectrum of isolated conotruncal heart defects (minimum 5%-10% of subjects), and far less commonly in isolated laterality or HPE patients (approximately 1% for each). The difference in the mutation incidence between these groups is highly significant. We show that apparent gene dosage discrepancies between humans and model organisms can be reconciled by considering a broader combination of sequence variants. Our studies confirm that (1) the genetic vulnerabilities inferred from model organisms with defects in Nodal signaling are indeed analogous to humans; (2) the molecular analysis of an entire signaling pathway is more complete and robust than that of individual genes and presages future studies by whole-genome analysis; and (3) a functional genomics approach is essential to fully appreciate the complex genetic interactions necessary to produce these effects in humans.

Foxp3 transcription-factor-dependent and -independent regulation of the regulatory T cell transcriptional signature

The CD4(+)CD25(+) lineage of regulatory T (Treg) cells plays a key role in controlling immune and autoimmune responses and is characterized by a unique transcriptional signature. The transcription factor Foxp3 had been thought to determine the Treg cell lineage, a hypothesis challenged by recent observations. We have performed a cross-sectional analysis of the Treg cell signature in Treg-like cells generated under a number of conditions, with or without Foxp3, to delineate the elements that can be ascribed to T cell activation, interleukin-2, transforming growth factor-beta (TGF-beta) signaling, or Foxp3 itself. These influences synergized to determine many of the signature's components. Much of the Treg cell signature was not ascribable to Foxp3 because it contained gene clusters that are coregulated with, but not transactivated by, Foxp3. Thus, a higher level of regulation upstream of Foxp3 determines the lineage, distinct from elements downstream of Foxp3 that are essential for its regulatory properties.

TGFbeta-SMAD signal transduction: molecular specificity and functional flexibility

Ligands of the transforming growth factor-beta (TGFbeta) superfamily of growth factors initiate signal transduction through a bewildering complexity of ligand-receptor interactions. Signalling then converges to nuclear accumulation of transcriptionally active SMAD complexes and gives rise to a plethora of specific functional responses in both embryos and adult organisms. Current research is focused on the mechanisms that regulate SMAD activity to evoke cell-type-specific and context-dependent transcriptional programmes. An equally important challenge is understanding the functional role of signal strength and duration. How are these quantitative aspects of the extracellular signal regulated? How are they then sensed and interpreted, and how do they affect responses?

SMAD3 prevents binding of NKX2.1 and FOXA1 to the SpB promoter through its MH1 and MH2 domains

Mechanisms of gene repression by transforming growth factor-beta (TGF-beta) are not well understood. TGF-beta represses transcription of pulmonary surfactant protein-B gene in lung epithelial cells. Repression is mediated by SMAD3 through interactions with NKX2.1 and FOXA1, two key transcription factors that are positive regulators of SpB transcription. In this study, we found that SMAD3 interacts through its MAD domains, MH1 and MH2 with NKX2.1 and FOXA1 proteins. The sites of interaction on NKX2.1 are located within the NH2 and COOH domains, known to be involved in transactivation function. In comparison, weaker interaction of FOXA1 winged helix, and the NH(2)-terminal domains was documented with SMAD3. Both in vitro studies and in vivo ChIP assays show that interaction of SMAD3 MH1 and MH2 domains with NKX2.1 and FOXA1 results in reduced binding of NKX2.1 and FOXA1 to their cognate DNA-binding sites, and diminished promoter occupancy within the SpB promoter. Thus, these studies reveal for the first time a mechanism of TGF-beta-induced SpB gene repression that involves interactions between specific SMAD3 domains and the corresponding functional sites on NKX2.1 and FOXA1 transcription factors.

How the Smads regulate transcription

The primary signalling pathway downstream of ligands of the transforming growth factor beta (TGF-beta) superfamily is the Smad pathway. Activated receptors phosphorylate receptor-regulated Smads, which form homomeric complexes and heteromeric complexes with Smad4. These activated Smad complexes accumulate in the nucleus, where they are directly involved in the regulation of transcription of target genes. This apparently very simple pathway is subject to complex regulation, much of which is at the level of post-translational modifications of pathway components, in particular, the Smads. The enzymes responsible may be constitutively active, may be cell type-specific or may be regulated by other signalling pathways or by the cell cycle. In this way, signals from TGF-beta superfamily ligands are integrated with signals from other growth factors and cytokines, are regulated by the cell cycle and are dependent on cell type. This may go some way to explaining the pleiotropic nature of TGF-beta superfamily responses. In this review we focus on the mechanisms whereby the Smads are modified and regulated. We then go on to discuss how the activated Smad complexes regulate transcription.

Arkadia activates Smad3/Smad4-dependent transcription by triggering signal-induced SnoN degradation

E3 ubiquitin ligases play important roles in regulating transforming growth factor beta (TGF-beta)/Smad signaling. Screening of an E3 ubiquitin ligase small interfering RNA library, using TGF-beta induction of a Smad3/Smad4-dependent luciferase reporter as a readout, revealed that Arkadia is an E3 ubiquitin ligase that is absolutely required for this TGF-beta response. Knockdown of Arkadia or overexpression of a dominant-negative mutant completely abolishes transcription from Smad3/Smad4-dependent reporters, but not from Smad1/Smad4-dependent reporters or from reporters driven by Smad2/Smad4/FoxH1 complexes. We show that Arkadia specifically activates transcription via Smad3/Smad4 binding sites by inducing degradation of the transcriptional repressor SnoN. Arkadia is essential for TGF-beta-induced SnoN degradation, but it has little effect on SnoN levels in the absence of signal. Arkadia interacts with SnoN and induces its ubiquitination irrespective of TGF-beta/Activin signaling, but SnoN is efficiently degraded only when it forms a complex with both Arkadia and phosphorylated Smad2 or Smad3. Finally, we describe an esophageal cancer cell line (SEG-1) that we show has lost Arkadia expression and is deficient for SnoN degradation. Reintroduction of wild-type Arkadia restores TGF-beta-induced Smad3/Smad4-dependent transcription and SnoN degradation in these cells, raising the possibility that loss of Arkadia function may be relevant in cancer.

Nodal signaling: developmental roles and regulation

Nodal-related ligands of the transforming growth factor-beta (TGFbeta) superfamily play central roles in patterning the early embryo during the induction of mesoderm and endoderm and the specification of left-right asymmetry. Additional roles for this pathway in the maintenance of embryonic stem cell pluripotency and in carcinogenesis have been uncovered more recently. Consistent with its crucial developmental functions, Nodal signaling is tightly regulated by diverse mechanisms including the control of ligand processing, utilization of co-receptors, expression of soluble antagonists, as well as positive- and negative-feedback activities.

Smads orchestrate specific histone modifications and chromatin remodeling to activate transcription

Smads are intracellular transducers for TGF-beta superfamily ligands, but little is known about the mechanism by which complexes of receptor-phosphorylated Smad2 and Smad4 regulate transcription. Using an in vitro transcription system, we have discovered that, unlike most transcription factors that are sufficient to recruit the basal transcription machinery and therefore activate transcription on both naked DNA and chromatin templates, the Smads only activate transcription from chromatin templates. We demonstrate that Smad2-mediated transcription requires the histone acetyltransferase, p300. Smad2-recruited p300 exhibits an altered substrate specificity, specifically acetylating nucleosomal histone H3 at lysines 9 and 18, and these modifications are also detected on an endogenous Smad2-dependent promoter in a ligand-induced manner. Furthermore, we show that endogenous Smad2 interacts with the SWI/SNF ATPase, Brg1, in a TGF-beta-dependent manner, and demonstrate that Brg1 is recruited to Smad2-dependent promoters and is specifically required for TGF-beta-induced expression of endogenous Smad2 target genes. Our data indicate that the Smads define a new class of transcription factors that absolutely require chromatin to assemble the basal transcription machinery and activate transcription.

In control of biology: of mice, men and Foxes

Forkhead proteins comprise a highly conserved family of transcription factors, named after the original forkhead gene in Drosophila. To date, over 100 forkhead genes have been identified in a large variety of species, all sharing the evolutionary conserved 'forkhead' DNA-binding domain, and the cloning and characterization of forkhead genes have continued in recent years. Forkhead transcription factors regulate the expression of countless genes downstream of important signalling pathways in most, if not all, tissues and cell types. Recent work has provided novel insights into the mechanisms that contribute to their functional diversity, including functional protein domains and interactions of forkheads with other transcription factors. Studies using loss- and gain-of-function models have elucidated the role of forkhead factors in developmental biology and cellular functions such as metabolism, cell division and cell survival. The importance of forkhead transcription factors is underlined by the developmental defects observed in mutant model organisms, and multiple human disorders and cancers which can be attributed to mutations within members of the forkhead gene family. This review provides a comprehensive overview of current knowledge on forkhead transcription factors, from structural organization and regulatory mechanisms to cellular and developmental functions in mice and humans. Finally, we will discuss how novel insights gained from involvement of 'Foxes' in the mechanisms underlying human pathology may create new opportunities for treatment strategies.

The mechanism of nuclear export of Smad3 involves exportin 4 and Ran

Transforming growth factor beta (TGF-beta) receptors phosphorylate Smad3 and induce its nuclear import so it can regulate gene transcription. Smad3 can return to the cytoplasm to propagate further cycles of signal transduction or to be degraded. We demonstrate that Smad3 is exported by a constitutive mechanism that is insensitive to leptomycin B. The Mad homology 2 (MH2) domain is responsible for Smad3 export, which requires the GTPase Ran. Inactive, GDP-locked RanT24N or nuclear microinjection of Ran GTPase activating protein 1 blocked Smad3 export. Inactivation of the Ran guanine nucleotide exchange factor RCC1 inhibited Smad3 export and led to nuclear accumulation of phosphorylated Smad3. A screen for importin/exportin family members that associate with Smad3 identified exportin 4, which binds a conserved peptide sequence in the MH2 domain of Smad3 in a Ran-dependent manner. Exportin 4 is sufficient for carrying the in vitro nuclear export of Smad3 in cooperation with Ran. Knockdown of endogenous exportin 4 completely abrogates the export of endogenous Smad3. A short peptide representing the minimal interaction domain in Smad3 effectively competes with Smad3 association to exportin 4 and blocks nuclear export of Smad3 in vivo. We thus delineate a novel nuclear export pathway for Smad3.

B cell-specific deficiency for Smad2 in vivo leads to defects in TGF-beta-directed IgA switching and changes in B cell fate

Smad2 is a member of the intracellular mediators that transduce signals from TGF-beta receptors and activin receptors. Targeted inactivation of Smad2 in mice leads to early lethality before gastrulation. It was shown previously that TGF-betaRII deficiency in vivo leads to defects in B cell homeostasis, Ag responsiveness, and IgA class switch recombination of B cells. To investigate the importance of Smad2-mediated signaling in B lymphocytes, we generated a B cell-specific inactivation of Smad2 in mice (bSmad2(-/-)). bSmad2(-/-) mice had normal B cell numbers in the spleen but showed a reduced population of marginal zone B cells. In contrast, B cells in Peyer's patches and peritoneal B-1a cells of bSmad2(-/-) mice were increased in numbers. bSmad2(-/-) mice showed a reduced number of surface-IgA(+) B cells and of IgA-secreting cells in Peyer's patches, decreased levels of IgA in serum, and, after immunization with a T cell-dependent Ag, a reduced IgA response. Class switch recombination to IgA was impaired in Smad2-deficient B cells, when stimulated in vitro with LPS in the presence of TGF-beta. The growth-inhibitory effects of TGF-beta in LPS-stimulated B cells were not affected in Smad2-deficient B cells. In summary, our data indicate a crucial role of Smad2 in mediating signals for the TGF-beta-directed class switch to IgA and the induction of IgA responses in vivo. Other B cell functions like growth-inhibitory signaling, which are known to be regulated by signals via the TGF-betaR, are not affected in Smad2-deficient B cells.

Deletion of Smad2 in mouse liver reveals novel functions in hepatocyte growth and differentiation

Smad family proteins Smad2 and Smad3 are activated by transforming growth factor beta (TGF-beta)/activin/nodal receptors and mediate transcriptional regulation. Although differential functional roles of Smad2 and Smad3 are apparent in mammalian development, the relative functional roles of Smad2 and Smad3 in postnatal systems remain unclear. We used Cre/loxP-mediated gene targeting for hepatocyte-specific deletion of Smad2 (S2HeKO) in adult mice and generated hepatocyte-selective Smad2/Smad3 double knockouts by intercrossing AlbCre/Smad2(f/f) (S2HeKO) and Smad3-deficient Smad3ex8/ex8 (S3KO) mice. All strains were viable and had normal adult liver. However, necrogenic CCL4-induced hepatocyte proliferation was significantly increased in S2HeKO compared to Ctrl and S3KO livers, and transplanted S2HeKO hepatocytes repopulated recipient liver at dramatically increased rates compared to Ctrl hepatocytes in vivo. Using primary hepatocytes, we found that TGF-beta-induced G1 arrest, apoptosis, and epithelial-to-mesenchymal transition in Ctrl and S2HeKO but not in S3KO hepatocytes. Interestingly, S2HeKO cells spontaneously acquired mesenchymal features characteristic of epithelial-to-mesenchymal transition (EMT). Collectively, these results demonstrate that Smad2 suppresses hepatocyte growth and dedifferentiation independent of TGF-beta signaling. Smad2 is not required for TGF-beta-stimulated apoptosis, EMT, and growth inhibition in hepatocytes.

Kinetic analysis of Smad nucleocytoplasmic shuttling reveals a mechanism for transforming growth factor beta-dependent nuclear accumulation of Smads

Upon transforming growth factor beta (TGF-beta) stimulation, Smads accumulate in the nucleus, where they regulate gene expression. Using fluorescence perturbation experiments on Smad2 and Smad4 fused to either enhanced green fluorescent protein or photoactivatable green fluorescent protein, we have studied the kinetics of Smad nucleocytoplasmic shuttling in a quantitative manner in vivo. We have obtained rate constants for import and export of Smad2 and show that the cytoplasmic localization of Smad2 in uninduced cells reflects its nuclear export being more rapid than import. We find that TGF-beta-induced nuclear accumulation of Smad2 is caused by a pronounced drop in the export rate of Smad2 from the nucleus, which is associated with a strong decrease in nuclear mobility of Smad2 and Smad4. TGF-beta-induced nuclear accumulation involves neither a release from cytoplasmic retention nor an increase in Smad2 import rate. Hence, TGF-beta-dependent nuclear accumulation of Smad2 is caused exclusively by selective nuclear trapping of phosphorylated, complexed Smad2. The proposed mechanism reconciles signal-dependent nuclear accumulation of Smad2 with its continuous nucleocytoplasmic cycling properties.

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.

The novel Smad-interacting protein Smicl regulates Chordinexpression in the Xenopus embryo

In this paper, we investigate the function of Smicl, a zinc-finger Smad-interacting protein that is expressed maternally in the Xenopusembryo. Inhibition of Smicl function by means of antisense morpholino oligonucleotides causes the specific downregulation of Chordin, a dorsally expressed gene encoding a secreted BMP inhibitor that is involved in mesodermal patterning and neural induction. Chordin is activated by Nodal-related signalling in an indirect manner, and we show here that Smicl is involved in a two-step process that is necessary for this activation. In the first, Smad3 (but not Smad2) activates expression of Xlim1 in a direct fashion. In the second, a complex containing Smicl and the newly induced Xlim1 induces expression of Chordin. As well as revealing the function of Smicl in the early embryo, our work yields important new insight in the regulation of Chordin and identifies functional differences between the activities of Smad2 and Smad3 in the Xenopus embryo.

Positive and negative regulation of the transforming growth factor beta/activin target gene goosecoid by the TFII-I family of transcription factors

Goosecoid (Gsc) is a homeodomain-containing transcription factor present in a wide variety of vertebrate species and known to regulate formation and patterning of embryos. Here we show that in embryonic carcinoma P19 cells, the transcription factor TFII-I forms a complex with Smad2 upon transforming growth factor beta (TGFbeta)/activin stimulation, is recruited to the distal element (DE) of the Gsc promoter, and activates Gsc transcription. Downregulation of endogenous TFII-I by small inhibitory RNA in P19 cells abolishes the TGFbeta-mediated induction of Gsc. Similarly, Xenopus embryos with endogenous TFII-I expression downregulated by injection of TFII-I-specific antisense oligonucleotides exhibit decreased Gsc expression. Unlike TFII-I, the related factor BEN (binding factor for early enhancer) is constitutively recruited to the distal element in the absence of TGFbeta/activin signaling and is replaced by the TFII-I/Smad2 complex upon TGFbeta/activin stimulation. Overexpression of BEN in P19 cells represses the TGFbeta-mediated transcriptional activation of Gsc. These results suggest a model in which TFII-I family proteins have opposing effects in the regulation of the Gsc gene in response to a TGFbeta/activin signal.

C-terminal mutants of C. elegans Smads reveal tissue-specific requirements for protein activation by TGF-beta signaling

TGF-beta signaling in the nematode Caenorhabditis elegans plays multiple roles in the development of the animal. The Sma/Mab pathway controls body size, male tail sensory ray identity and spicule formation. Three Smad genes, sma-2, sma-3 and sma-4, are all required for signal transduction, suggesting that the functional complex could be a heterotrimer. Because the C termini of Smads play important roles in receptor-mediated activation and heteromeric complex formation, we generated C-terminal mutations in the C. elegans Smad genes and tested their activities in vivo in each of their distinct developmental roles. We show that pseudophosphorylated SMA-3 is dominant negative in body size, but functional in sensory ray and spicule development. Somewhat differently, pseudophosphorylated SMA-2 is active in any tissue. The C-terminal mutants of SMA-4 function like wild type, suggesting that the SMA-4 C terminus is dispensable. Using a combination of different C-terminal mutations in SMA-2 and SMA-3, we found a complex set of requirements for Smad-phosphorylation state that are specific to each outcome. Finally, we detected a physical interaction of SMA-3 with the forkhead transcription factor LIN-31, which is enhanced by SMA-3 pseudophosphorylation and reduced in an unphosphorylatable mutant. We conclude that the tissue-specific requirements for Smad phosphorylation may result, in part, from the need to interact with tissue-specific transcription co-factors that have different affinities for phosphorylated and unphosphorylated Smad protein.

Selective inhibition of TGF-beta responsive genes by Smad-interacting peptide aptamers from FoxH1, Lef1 and CBP

Transforming growth factor beta (TGF-beta) stimulation results in the assembly of Smad-containing protein complexes that mediate activation or repression of TGF-beta responsive genes. To determine if disruption of specific Smad protein-protein interactions would selectively inhibit responses to TGF-beta or generally interfere with Smad-dependent signaling, we developed three Smad-binding peptide aptamers by introducing Smad interaction motifs from Smad-binding proteins CBP, FoxH1 and Lef1 into the scaffold protein E. coli thioredoxin A (Trx). All three classes of aptamers bound to Smads by GST pulldown assays and co-immunoprecipitation from mammalian cells. Expression of the aptamers in HepG2 cells did not generally inhibit Smad-dependent signaling as evaluated using seven TGF-beta responsive luciferase reporter genes. The Trx-xFoxH1b aptamer inhibited TGF-beta-induced expression from a reporter dependent on the Smad-FoxH1 interaction, A3-lux, by 50%. Trx-xFoxH1b also partially inhibited two reporters not dependent on a Smad-FoxH1 interaction, 3TP-lux and Twntop, and endogenous PAI-1 expression. Trx-Lef1 aptamer only inhibited expression of the Smad-Lef1 responsive reporter gene TwnTop. The Trx-CBP aptamer had no significant effect on reporter gene expression. The results suggest that Smad-binding peptide aptamers can be developed to selectively inhibit TGF-beta-induced gene expression.

Generation of novel conditional and hypomorphic alleles of the Smad2 gene

Smad2 is an intracellular mediator of the transforming growth factor beta signaling (TGFbeta) pathway. It has been previously shown that, in the mouse, ablation of functional Smad2 results in embryonic lethality due to gastrulation defects. To circumvent the early lethality and study the spatially and temporally specific functions of Smad2, we utilized the Cre-loxP system to generate a Smad2 conditional allele. Here we show that a conditional allele, Smad2(flox), was generated. In this allele, exons 9 and 10 are flanked by loxP sites and the gene is functionally wildtype. Cre-mediated recombination results in a deletion allele which phenocopies our previously reported Smad2(DeltaC) null mutation. To generate this conditional allele, we first made a targeted mutation which introduced a floxed neo cassette into intron 10. This allele (Smad2(3loxP)) functions hypomorphically when placed opposite a null allele, and unlike the other published Smad2 hypomorphic allele, can be maintained in the homozygous state.

SnoN is a cell type-specific mediator of transforming growth factor-beta responses

The transforming growth factor-beta (TGF-beta) family of secreted proteins have pleiotropic functions that are critical to normal development and homeostasis. However, the intracellular mechanisms by which the TGF-beta proteins elicit cellular responses remain incompletely understood. The Smad proteins provide a major means for the propagation of the TGF-beta signal from the cell surface to the nucleus, where the Smad proteins regulate gene expression leading to TGF-beta-dependent cellular responses including the inhibition of cell proliferation. Recent studies have suggested that a nuclear Smad-interacting protein termed SnoN, when overexpressed in cells, suppresses TGF-beta-induced Smad signaling and TGF-beta inhibition of cell proliferation. However, the physiologic function of endogenous SnoN in TGF-beta-mediated biological responses remained to be elucidated. Here, we determined the effect of genetic knock-down of SnoN by RNA interference on TGF-beta responses in mammalian cells. Unexpectedly, we found that SnoN knock-down specifically inhibited TGF-beta-induced transcription in the lung epithelial cell line Mv1Lu but not in HeLa or HaCaT cells. SnoN knock-down was also found to block TGF-beta-dependent cell cycle arrest in Mv1Lu cells. Collectively, these data indicate that rather than suppressing TGF-beta-induced responses, endogenous SnoN acts as a positive mediator of TGF-beta-induced transcription and cell cycle arrest in lung epithelial cells. Our study also shows that SnoN couples the TGF-beta signal to gene expression in a cell-specific manner.

Of Fox and Frogs: Fox (fork head/winged helix) transcription factors in Xenopus development

Transcription factors of the Fox (fork head box) family have been found in all metazoan organisms. They are characterised by an evolutionary conserved DNA-binding domain of winged helix structure. In the South African clawed frog, Xenopus laevis, more than 30 Fox genes have been found so far. This review summarises our present knowledge regarding the general structure and common features of the fork head box and will then characterise Fox genes that have been described in Xenopus. Special attention was paid to the temporal and spatial expression patterns during early embryonic development. For some of these genes, the molecular mechanisms leading to their regulation after the onset of zygotic transcription are known. We also report on functional aspects including target gene regulation, cell or tissue specification and interference with the cell cycle. Finally, Fox proteins serve as mediators of signalling pathways and they might function as checkpoint molecules for the cross-regulatory interactions of different intracellular signal transduction chains.

Casein kinase Iepsilon plays a functional role in the transforming growth factor-beta signaling pathway

The transforming growth factor-beta (TGF-beta) signaling pathway is known to be involved in a wide range of biological events, including development, cellular differentiation, apoptosis, and oncogenesis. The TGF-beta signal is mediated by ligand binding to the type II receptor, leading to the recruitment and activation of the type I receptor, and subsequent activation of a family of intracellular signal transducing proteins called Smads. Here we report a regulatory role for casein kinase Iepsilon (CKIepsilon) in the TGF-beta signaling cascade. We find that CKIepsilon binds to all Smads and the cytoplasmic domains of the type I and type II receptors both in vitro and in vivo. The interaction of CKIepsilon with the type I and type II receptors is independent of TGF-beta stimulation, whereas the CKIepsilon/Smad interaction is transiently disrupted by ligand treatment. Additionally, CKIepsilon is able to phosphorylate the receptor-activated Smads (Smads 1-3 and 5) and the type II receptor in vitro. Transcriptional reporter assays reveal that transient overexpression of wild type CKIepsilon dramatically reduces basal reporter activity but enhances TGF-beta-stimulated transcription. Furthermore, overexpression of a kinase-dead mutant of CKIepsilon inhibits both basal and ligand-induced transcription, whereas inhibition of endogenous CKI catalytic activity with IC261 blocks only TGF-beta-stimulated reporter activity. Finally, knocking down CKIepsilon protein levels results in a significant increase in basal and TGF-beta-induced transcription. These results suggest that CKIepsilon plays a ligand-dependent, differential, and dual regulatory role within the TGF-beta signaling pathway.