Nucleocytoplasmic shuttling of Smad proteins

TRB3 interacts with SMAD3 promoting tumor cell migration and invasion

Tribbles homolog 3 (TRB3, also known as TRIB3, NIPK and SKIP3), a human homolog of Drosophila Tribbles, has been found to interact with a variety of signaling molecules to regulate diverse cellular functions. Here, we report that TRB3 is a novel SMAD3-interacting protein. Expression of exogenous TRB3 enhanced the transcriptional activity of SMAD3, whereas knocking down endogenous TRB3 reduced the transcriptional activity of SMAD3. The kinase-like domain (KD) of TRB3 was responsible for the interaction with SMAD3 and the regulation of SMAD3-mediated transcriptional activity. In addition, TGF-β1 stimulation or overexpression of SMAD3 enhanced the TRB3 promoter activity and expression, suggesting that there is a positive feedback loop between TRB3 and TGF-β–SMAD3 signaling. Mechanistically, TRB3 was found to trigger the degradation of SMAD ubiquitin regulatory factor 2 (Smurf2), which resulted in a decrease in the degradation of SMAD2 and phosphorylated SMAD3. Moreover, TRB3–SMAD3 interaction promoted the nuclear localization of SMAD3 because of the interaction of TRB3 with the MH2 domain of SMAD3. These effects of TRB3 were responsible for potentiating the SMAD3-mediated activity. Furthermore, knockdown of endogenous TRB3 expression inhibited the migration and invasion of tumor cells in vitro, which were associated with an increase in the expression of E-cadherin and a decrease in the expression of Twist-1 and Snail, two master regulators of epithelial-to-mesenchymal transition, suggesting a crucial role for TRB3 in maintaining the mesenchymal status of tumor cells. These results demonstrate that TRB3 acts as a novel SMAD3-interacting protein to participate in the positive regulation of TGF-β–SMAD-mediated cellular biological functions.

Lemur tyrosine kinase-2 signalling regulates kinesin-1 light chain-2 phosphorylation and binding of Smad2 cargo

A recent genome wide association study identified the gene encoding lemur tyrosine kinase-2 (LMTK2) as a susceptibility gene for prostate cancer. The identified genetic alteration is within intron 9 but the mechanisms by which LMTK2 may impact upon prostate cancer are not clear because the functions of LMTK2 are poorly understood. Here, we show that LMTK2 regulates a known pathway that controls phosphorylation of kinesin-1 light chain-2 (KLC2) by glycogen synthase kinase-3β (GSK3β). KLC2 phosphorylation by GSK3β induces release of cargo from KLC2. LMTK2 signals via protein phosphatase-1C (PP1C) to increase inhibitory phosphorylation of GSK3β on serine-9 that reduces KLC2 phosphorylation and promotes binding of the known KLC2 cargo Smad2. Smad2 signals to the nucleus in response to transforming growth factor-β (TGFβ) receptor stimulation and transport of Smad2 by kinesin-1 is required for this signalling. We show that siRNA loss of LMTK2 not only reduces binding of Smad2 to KLC2 but also inhibits TGFβ-induced Smad2 signalling. Thus, LMTK2 may regulate the activity of kinesin-1 motor function and Smad2 signalling.

USP15 is a deubiquitylating enzyme for receptor-activated SMADs

The TGFβ pathway is critical for embryonic development and adult tissue homeostasis. On ligand stimulation, TGFβ and BMP receptors phosphorylate receptor-activated SMADs (R-SMADs), which then associate with SMAD4 to form a transcriptional complex that regulates gene expression through specific DNA recognition. Several ubiquitin ligases serve as inhibitors of R-SMADs, yet no deubiquitylating enzyme (DUB) for these molecules has so far been identified. This has left unexplored the possibility that ubiquitylation of R-SMADs is reversible and engaged in regulating SMAD function, in addition to degradation. Here we identify USP15 as a DUB for R-SMADs. USP15 is required for TGFβ and BMP responses in mammalian cells and Xenopus embryos. At the biochemical level, USP15 primarily opposes R-SMAD monoubiquitylation, which targets the DNA-binding domains of R-SMADs and prevents promoter recognition. As such, USP15 is critical for the occupancy of endogenous target promoters by the SMAD complex. These data identify an additional layer of control by which the ubiquitin system regulates TGFβ biology.

cGMP inhibits TGF-beta signaling by sequestering Smad3 with cytosolic beta2-tubulin in pulmonary artery smooth muscle cells

Atrial natriuretic peptide (ANP) and TGF-β play counterregulatory roles in pulmonary vascular adaptation to chronic hypoxia. We have demonstrated that ANP-cyclic GMP (cGMP)-protein kinase G (PKG) signaling inhibits TGF-β signaling by blocking TGF-β-induced nuclear translocation of mothers against decapentaplegic homolog (Smad)3 in pulmonary artery smooth muscle cells (PASMC). The current study tested the novel hypothesis that activation of the ANP-cGMP-PKG pathway limits TGF-β-induced Smad3 nuclear translocation by enhancing Smad3 binding to cytosolic anchoring proteins in isolated pulmonary artery smooth muscle cells. Cells were pretreated with vehicle or cGMP and then exposed to TGF-β1 treatment. Cytosolic fractions were isolated and immunoprecipitated with a selective anti-Smad3 antibody. Differential proteomic analysis of the cytosolic Smad3-interacting proteins by two-dimensional differential in-gel electrophoresis and mass spectroscopy followed by coimmunoprecipitation and immunostaining demonstrated that Smad3 was bound to β2-tubulin in a TGF-β1/cGMP-dependent manner: binding of Smad3 to β2-tubulin was decreased by TGF-β1 and increased by cGMP treatment. A site-directed mutagenesis study demonstrated that mutating Smad3 at Thr388, but not Ser309, two potential sites of PKG-induced hyperphosphorylation, inhibited cGMP-induced Smad3 binding to β2-tubulin. Further, luciferase reporter analysis showed that muation of T388 in Smad3 abolished the inhibitory effect of cGMP on TGF-β1-induced plasminogen activator inhibitor-1 (PAI-1) transcription. In addition, disruption of β2-tubulin with the microtubule depolymerizers nocodazole and colchicine promoted Smad3 dissociation from β2-tubulin, increased both TGF-β1-induced Smad3 nuclear translocation and PAI-1 mRNA expression, and abolished the inhibitory effects of cGMP on these processes. In contrast, the microtubule stabilizers paclitaxel and epothilone B increased cytosolic Smad3 binding to β2-tubulin and enhanced the inhibitory effect of cGMP on Smad3 nuclear translocation and PAI-1 expression in response to TGF-β1. These provocative findings suggest that sequestering Smad3 by β2-tubulin in cytosol is a key mechanism by which ANP-cGMP-PKG signaling interferes with downstream signaling from TGF-β and thus protects against pulmonary arterial remodeling in response to hypoxia stress.

TGF-β/BMPs: crucial crossroad in neural autoimmune disorders

Transforming growth factor beta (TGF-β) has a crucial role in the differentiation of ectodermal cells to neural or epidermal precursors. TGF-β and bone morphogenetic protein molecules (BMPs) are involved in many developmental processes, including cell proliferation and differentiation, apoptosis, mitotic arrest and intercellular interactions during morphogenesis. Additionally, the failure of central thymic tolerance mechanisms, leading to T cells with a skewed autoreactive response, is being described as a contributor in inflammatory processes in autoimmune diseases such as multiple sclerosis. Since TGF-β and BMP proteins are crucial for the development of the neural system and the thymus, as well as for the differentiation of T cells, it is essential to further investigate their role in the pathophysiology of this disorder by using references from embryonic experimental research. Available literature in the TGF/BMP signalling cascade, mostly during embryonic development of the nervous system is being reviewed. An attempt is made to further elucidate a potential role of TGF/BMP signalling in the pathophysiology of MS. During demyelination, BMP signaling, through various molecular mechanisms, directs the development of the adult neural stem cell in the astrocyte rather than the oligodendrocyte direction, therefore inhibiting the repair process. Further understanding of the above relationships could lead to the development of potentially efficient therapies for MS in the future.

Betaglycan: a multifunctional accessory

Betaglycan is a co-receptor for the TGFβ superfamily, particularly important in establishing the potency of its ligands on their target cells. In recent years, new insights have been gained into the structure and function of betaglycan, expanding its role from that of a simple co-receptor to include additional ligand-dependent and ligand-independent roles. This review focuses on recent advances in the betaglycan field, with a particular emphasis on its newly discovered actions in mediating the trafficking of TGFβ superfamily receptors and as a determinant of the functional output of TGFβ superfamily signalling. In addition, this review encompasses a discussion of the emerging roles of the betaglycan/inhibin pathway in reproductive cancers and disease.

Emerging roles for the transforming growth factor-{beta} superfamily in regulating adiposity and energy expenditure

Members of the TGF-β superfamily regulate many aspects of development, including adipogenesis. Studies in cells and animal models have characterized the effects of superfamily signaling on adipocyte development, adiposity, and energy expenditure. Although bone morphogenetic protein (BMP) 4 is generally considered a protein that promotes the differentiation of white adipocytes, BMP7 has emerged as a selective regulator of brown adipogenesis. Conversely, TGF-β and activin A inhibit adipocyte development, a process augmented in TGF-β-treated cells by Smads 6 and 7, negative regulators of canonical TGF-β signaling. Other superfamily members have mixed effects on adipogenesis depending on cell culture conditions, the timing of expression, and the cell type, and many of these effects occur by altering the expression or activities of proteins that control the adipogenic cascade, including members of the CCAAT/enhancer binding protein family and peroxisome proliferator-activated receptor-γ. BMP7, growth differentiation factor (GDF) 8, and GDF3 are versatile in their mechanisms of action, and altering their normal expression characteristics has significant effects on adiposity in vivo. In addition to their roles in adipogenesis, activins and BMP7 regulate energy expenditure by affecting the expression of genes that contribute to mitochondrial biogenesis and function. GDF8 signals through its own receptors during adipogenesis while antagonizing BMP7, an example of a ligand from one major branch of the superfamily regulating the other. With such intricate relationships that ultimately affect adiposity, TGF-β superfamily signaling holds considerable promise as a target for treating human obesity and its comorbidities.

PARP-1 attenuates Smad-mediated transcription

The versatile cytokine transforming growth factor β (TGF-β) regulates cellular growth, differentiation, and migration during embryonic development and adult tissue homeostasis. Activation of TGF-β receptors leads to phosphorylation of Smad2 and Smad3, which oligomerize with Smad4 and accumulate in the nucleus where they recognize gene regulatory regions and orchestrate transcription. Termination of Smad-activated transcription involves Smad dephosphorylation, nuclear export, or ubiquitin-mediated degradation. In an unbiased proteomic screen, we identified poly(ADP-ribose) polymerase-1 (PARP-1) as a Smad-interacting partner. PARP-1 dissociates Smad complexes from DNA by ADP-ribosylating Smad3 and Smad4, which attenuates Smad-specific gene responses and TGF-β-induced epithelial-mesenchymal transition. Thus, our results identify ADP-ribosylation of Smad proteins by PARP-1 as a key step in controlling the strength and duration of Smad-mediated transcription.

Phosphorylation meets nuclear import: a review

Phosphorylation is the most common and pleiotropic modification in biology, which plays a vital role in regulating and finely tuning a multitude of biological pathways. Transport across the nuclear envelope is also an essential cellular function and is intimately linked to many degeneration processes that lead to disease. It is therefore not surprising that phosphorylation of cargos trafficking between the cytoplasm and nucleus is emerging as an important step to regulate nuclear availability, which directly affects gene expression, cell growth and proliferation. However, the literature on phosphorylation of nucleocytoplasmic trafficking cargos is often confusing. Phosphorylation, and its mirror process dephosphorylation, has been shown to have opposite and often contradictory effects on the ability of cargos to be transported across the nuclear envelope. Without a clear connection between attachment of a phosphate moiety and biological response, it is difficult to fully understand and predict how phosphorylation regulates nucleocytoplasmic trafficking. In this review, we will recapitulate clue findings in the field and provide some general rules on how reversible phosphorylation can affect the nuclear-cytoplasmic localization of substrates. This is only now beginning to emerge as a key regulatory step in biology.

Activin A regulates porcine follicle-stimulating hormone beta-subunit transcription via cooperative actions of SMADs and FOXL2

Activins stimulate FSH synthesis and secretion by pituitary gonadotrope cells. Activin A induction of porcine and murine FSHβ (Fshb) gene transcription in immortalized gonadotropes is dependent on homolog of Drosophila mothers against decapentaplegic (SMAD) proteins as well as the forkhead transcription factor L2 (FOXL2). Using both heterologous and homologous cell models, we demonstrate that FOXL2 functionally synergizes with SMAD3/4 to stimulate porcine Fshb promoter-reporter activity. We further show that endogenous FOXL2 and SMAD2/3 physically interact in homologous cells. We identify two composite cis-elements of adjacent FOXL2 and SMAD binding sites in the proximal porcine Fshb promoter that mediate activin A, FOXL2, and SMAD3 actions. FOXL2 can bind these elements independently of SMADs, whereas SMAD3/4 binding requires high-affinity FOXL2 binding. Conversely, FOXL2 alone is insufficient to regulate Fshb transcription and requires SMADs to induce promoter activity. Collectively, our data suggest a model in which activins stimulate formation and nuclear accumulation of SMAD3/4 complexes, which interact with FOXL2 bound to at least two proximal promoter elements. This association stabilizes SMAD3/4 binding to adjacent SMAD binding elements. SMAD-FOXL2 complexes then mediate activation of transcription through a currently unknown mechanism. Conservation of one of the two composite cis-elements suggests that this may form part of a general mechanism whereby activins regulate Fshb subunit transcription and FSH synthesis.

The dynamic roles of TGF-β in cancer

The transforming growth factor-β (TGF-β) signalling pathway plays a critical and dual role in the progression of human cancer. During the early phase of tumour progression, TGF-β acts as a tumour suppressor, exemplified by deletions or mutations in the core components of the TGF-β signalling pathway. On the contrary, TGF-β also promotes processes that support tumour progression such as tumour cell invasion, dissemination, and immune evasion. Consequently, the functional outcome of the TGF-β response is strongly context-dependent including cell, tissue, and cancer type. In this review, we describe the molecular signalling pathways employed by TGF-β in cancer and how these, when perturbed, may lead to the development of cancer. Concomitantly with our increased appreciation of the molecular mechanisms that govern TGF-β signalling, the potential to therapeutically target specific oncogenic sub-arms of the TGF-β pathway increases. Indeed, clinical trials with systemic TGF-β signalling inhibitors for treatment of cancer patients have been initiated. However, considering the important role of TGF-β in cardiovascular and many other tissues, careful screening of patients is warranted to minimize unwanted on-target side effects.

Bone morphogenetic proteins: a critical review

Bone Morphogenetic Proteins (BMPs) are potent growth factors belonging to the Transforming Growth Factor Beta superfamily. To date over 20 members have been identified in humans with varying functions during processes such as embryogenesis, skeletal formation, hematopoiesis and neurogenesis. Though their functions have been identified, less is known regarding levels of regulation at the extracellular matrix, membrane surface, and receptor activation. Further, current models of activation lack the integration of these regulatory mechanisms. This review focuses on the different levels of regulation, ranging from the release of BMPs into the extracellular components to receptor activation for different BMPs. It also highlights areas in research that is lacking or contradictory.

Palate morphogenesis: current understanding and future directions

In the past, most scientists conducted their inquiries of nature via inductivism, the patient accumulation of "pieces of information" in the pious hope that the sum of the parts would clarify the whole. Increasingly, modern biology employs the tools of bioinformatics and systems biology in attempts to reveal the "big picture." Most successful laboratories engaged in the pursuit of the secrets of embryonic development, particularly those whose research focus is craniofacial development, pursue a middle road where research efforts embrace, rather than abandon, what some have called the "pedestrian" qualities of inductivism, while increasingly employing modern data mining technologies. The secondary palate has provided an excellent paradigm that has enabled examination of a wide variety of developmental processes. Examination of cellular signal transduction, as it directs embryogenesis, has proven exceptionally revealing with regard to clarification of the "facts" of palatal ontogeny-at least the facts as we currently understand them. Herein, we review the most basic fundamentals of orofacial embryology and discuss how functioning of TGFbeta, BMP, Shh, and Wnt signal transduction pathways contributes to palatal morphogenesis. Our current understanding of palate medial edge epithelial differentiation is also examined. We conclude with a discussion of how the rapidly expanding field of epigenetics, particularly regulation of gene expression by miRNAs and DNA methylation, is critical to control of cell and tissue differentiation, and how examination of these epigenetic processes has already begun to provide a better understanding of, and greater appreciation for, the complexities of palatal morphogenesis.

Dorsomorphin and LDN-193189 inhibit BMP-mediated Smad, p38 and Akt signalling in C2C12 cells

Bone morphogenetic proteins (BMPs) are key regulators of cell fate decisions during embryogenesis and tissue homeostasis. BMPs signal through a coordinated assembly of two types of transmembrane serine/threonine kinase receptors to induce Smad1/5/8 plus non-Smad pathways, such as MAPK and Akt. The recent discovery of BMP receptor inhibitors opened new avenues to study specific BMP signalling and to delineate this effect from TGF-β and Activin signalling. Here we present comprehensive and quantitative analyses on both canonical and non-Smad mediated BMP signalling under Dorsomorphin (DM) and LDN-193189 (LDN) treatment conditions. We demonstrate for the first time, that both compounds affect not only the Smad but also the non-Smad signalling pathways induced by either BMP2, BMP6 or GDF5. The activation of p38, ERK1/2 and Akt in C2C12 cells was inhibited by DM and LDN. In addition "off-target" effects on all branches of BMP non-Smad signalling are presented. From this we conclude that the inhibition of BMP receptors by DM and more efficiently by LDN-193189 affects all known BMP induced signalling cascades.

Targeting the transforming growth factor-beta signaling pathway in human cancer

The transforming growth factor-ss (TGF-beta) signaling pathway plays a pivotal role in diverse cellular processes. TGF-beta switches its role from a tumor suppressor in normal or dysplastic cells to a tumor promoter in advanced cancers. It is widely believed that the Smad-dependent pathway is involved in TGF-beta tumor-suppressive functions, whereas activation of Smad-independent pathways, coupled with the loss of tumor-suppressor functions of TGF-beta, is important for its pro-oncogenic functions. TGF-beta signaling has been considered a useful therapeutic target. The discovery of oncogenic actions of TGF-beta has generated a great deal of enthusiasm for developing TGF-beta signaling inhibitors for the treatment of cancer. The challenge is to identify the group of patients where targeted tumors are not only refractory to TGF-beta-induced tumor suppressor functions but also responsive to the tumor-promoting effects of TGF-beta. TGF-beta pathway inhibitors, including small and large molecules, have now entered clinical trials. Preclinical studies with these inhibitors have shown promise in a variety of different tumor models. Here, we focus on the mechanisms of signaling and specific targets of the TGF-beta pathway that are critical effectors of tumor progression and invasion. This report also examines the therapeutic intervention of TGF-ss signaling in human cancers.

Inhibins: from reproductive hormones to tumor suppressors

Inhibins are peptide hormones shown originally to be produced by the gonads to regulate the secretion of follicle stimulating hormone by pituitary gonadotropes. Although gonadotropes have been regarded as the canonical inhibin target cells, in recent years extrapituitary actions of inhibins have come into the spotlight. In particular, disruptions to the local actions of inhibins in peripheral tissues might underlie certain diseases, especially cancers of the reproductive tract. This review focuses on recent advances in the inhibin field, with a particular emphasis on the determinants of inhibin availability, mechanisms of inhibin action, and the physiological relevancy of local inhibin actions in the development and progression of reproductive cancers.

Transcription factor Smad3 is required for the inhibition of adipogenesis by retinoic acid

The process of adipocyte differentiation is driven by a highly coordinated cascade of transcriptional events that results in the development of the mature adipocyte and in lipid accumulation. One of the early events of differentiation is the up-regulation of CCAAT/enhancer-binding protein beta (C/EBPbeta) expression. C/EBPbeta then acts to up-regulate the expression of adipogenic factors such as C/EBPalpha, which control the late stage of adipogenesis. Retinoic acid (RA) is a potent inhibitor of adipogenesis, and its action appears to block C/EBPbeta transcriptional potential early during differentiation. Using preadipocytes and mesenchymal stem cell models, we show that RA specifically blocks the occupancy of C/EBPbeta of the Cebpa promoter, thereby abrogating the differentiation process. RA does not act directly on C/EBPbeta but rather stimulates the expression of the transforming growth factor beta-effector protein Smad3, which can interact with C/EBPbeta via its Mad homology 1 domain and can interfere with C/EBPbeta DNA binding. The RA-induced increase in Smad3 expression results in increased cytoplasmic and nuclear Smad3, an important event as ectopic expression of Smad3 in preadipocytes in the absence of RA treatment only modestly inhibits adipogenesis and C/EBPbeta DNA binding, suggesting that Smad3 alone is not sufficient to completely recapitulate the effects of retinoic acid treatment during differentiation. However, in the absence of Smad3, RA is not able to inhibit adipocyte differentiation or to elicit a decrease in C/EBPbeta DNA occupancy suggesting that Smad3 is necessary to convey the inhibitory effects of retinoic acid during adipogenesis.

HSP72 inhibits Smad3 activation and nuclear translocation in renal epithelial-to-mesenchymal transition

Although heat shock protein 72 (HSP72) ameliorates renal tubulointerstitial fibrosis by inhibiting epithelial-to-mesenchymal transition (EMT), the underlying mechanism is unknown. Because Smad proteins transduce TGF-beta signaling from the cytosol to the nucleus and HSP72 assists in protein folding and facilitates nuclear translocation, we investigated whether HSP72 inhibits TGF-beta-induced EMT by modulating Smad expression, activation, and nuclear translocation. To evaluate the roles of distinct HSP72 structural domains in these processes, we constructed vectors that expressed wild-type HSP72 or mutants lacking either the peptide-binding domain (HSP72-DeltaPBD), which is responsible for substrate binding and refolding, or the nuclear localization signal (HSP72-DeltaNLS). Overexpression of wild-type HSP72 or HSP72-DeltaNLS inhibited TGF-beta1-induced EMT, but HSP72-DeltaPBD did not, suggesting a critical role for the PBD in this inhibition. HSP72 overexpression inhibited TGF-beta1-induced phosphorylation and nuclear translocation of Smad3 and p-Smad3, but not Smad2; these inhibitory effects required the PBD but not the NLS. Coimmunoprecipitation assays suggested a physical interaction between Smad3 and the PBD. siRNA knockdown of endogenous HSP72 enhanced both TGF-beta1-induced Smad3 phosphorylation and EMT and confirmed the interaction of HSP72 with both Smad3 and p-Smad3. In vivo, induction of HSP72 by geranylgeranylacetone suppressed Smad3 phosphorylation in renal tubular cells after unilateral ureteral obstruction. In conclusion, HSP72 inhibits EMT in renal epithelial cells primarily by exerting domain-specific effects on Smad3 activation and nuclear translocation.

Transforming Growth Factor type beta and Smad family signaling in stem cell function

Ligands of the Transforming Growth Factor type beta (TGFbeta) family exert multiple and sometimes opposite effects on most cell types in vivo depending on cellular context, which mainly includes the stage of the target cell, the local environment of this cell or niche, and the identity and the dosage of the ligand. Significant progress has been made in the molecular dissection of the regulation of the activity of the ligands and their intracellular signal transduction pathways, including via the canonical Smad pathway where Smads interact with many transcription factors. This knowledge together with results from functional studies within the embryology and stem cell research fields is giving us insight in the role of individual ligands and other components of this signaling system and where and how it regulates many properties of embryonic and adult stem/progenitor cells, which is anticipated to contribute to successful cell-based therapy in the future. We review and discuss recent progress on the effects of Nodal/Activin and Bone Morphogenetic Proteins (BMPs) and their canonical signaling in cells with stem cell properties. We focus on embryonic stem cells and their maintenance and pluripotency, and conversion into selected cell types of neuroectoderm, mesoderm and endoderm, on induced pluripotent cells and on neurogenic cells in the adult brain.

BMP signaling regulates sympathetic nervous system development through Smad4-dependent and -independent pathways

Induction of the sympathetic nervous system (SNS) from its neural crest (NC) precursors is dependent on BMP signaling from the dorsal aorta. To determine the roles of BMP signaling and the pathways involved in SNS development, we conditionally knocked out components of the BMP pathways. To determine if BMP signaling is a cell-autonomous requirement of SNS development, the Alk3 (BMP receptor IA) was deleted in the NC lineage. The loss of Alk3 does not prevent NC cell migration, but the cells die immediately after reaching the dorsal aorta. The paired homeodomain factor Phox2b, known to be essential for survival of SNS precursors, is downregulated, suggesting that Phox2b is a target of BMP signaling. To determine if Alk3 signals through the canonical BMP pathway, Smad4 was deleted in the NC lineage. Loss of Smad4 does not affect neurogenesis and ganglia formation; however, proliferation and noradrenergic differentiation are reduced. Analysis of transcription factors regulating SNS development shows that the basic helix-loop-helix factor Ascl1 is downregulated by loss of Smad4 and that Ascl1 regulates SNS proliferation but not noradrenergic differentiation. To determine if the BMP-activated Tak1 (Map3k7) pathway plays a role in SNS development, Tak1 was deleted in the NC lineage. We show that Tak1 is not involved in SNS development. Taken together, our results suggest multiple roles for BMP signaling during SNS development. The Smad4-independent pathway acts through the activation of Phox2b to regulate survival of SNS precursors, whereas the Smad4-dependent pathway controls noradrenergic differentiation and regulates proliferation by maintaining Ascl1 expression.

P68 RNA helicase is a nucleocytoplasmic shuttling protein

P68 RNA helicase is a prototypical DEAD box RNA helicase. The protein plays a very important role in early organ development and maturation. In consistence with the function of the protein in transcriptional regulation and pre-mRNA splicing, p68 was found to predominately localize in the cell nucleus. However, recent experiments demonstrate a transient cytoplasmic localization of the protein. We report here that p68 shuttles between the nucleus and the cytoplasm. The nucleocytoplasmic shuttling of p68 is mediated by two nuclear localization signal (NLS) and two nuclear exporting signal (NES) sequence elements. Our experiments reveal that p68 shuttles via a classical RanGTPase dependent pathway.

Human BAMBI cooperates with Smad7 to inhibit transforming growth factor-beta signaling

Transforming growth factor beta (TGF-beta) and related growth factors are essential regulators of embryogenesis and tissue homeostasis. The signaling pathways mediated by their receptors and Smad proteins are precisely modulated by various means. Xenopus BAMBI (bone morphogenic protein (BMP) and activin membrane-bound inhibitor) has been shown to function as a general negative regulator of TGF-beta/BMP/activin signaling. Here, we provide evidence that human BAMBI (hBAMBI), like its Xenopus homolog, inhibits TGF-beta- and BMP-mediated transcriptional responses as well as TGF-beta-induced R-Smad phosphorylation and cell growth arrest, whereas knockdown of endogenous BAMBI enhances the TGF-beta-induced reporter expression. Mechanistically, in addition to interfering with the complex formation between the type I and type II receptors, hBAMBI cooperates with Smad7 to inhibit TGF-beta signaling. hBAMBI forms a ternary complex with Smad7 and the TGF-beta type I receptor ALK5/TbetaRI and inhibits the interaction between ALK5/TbetaRI and Smad3, thus impairing Smad3 activation. These findings provide a novel insight to understand the molecular mechanism underlying the inhibitory effect of BAMBI on TGF-beta signaling.

Essential roles for early growth response transcription factor Egr-1 in tissue fibrosis and wound healing

The early growth response gene (Egr-1) codes for a zinc finger transcription factor that has important roles in the regulation of cell growth, differentiation, and survival. Aberrant Egr-1 expression is implicated in carcinogenesis, inflammation, atherosclerosis, and ischemic injury. We reported previously that normal fibroblasts stimulated by transforming growth factor-ss showed rapid and transient induction of Egr-1. Moreover, we observed that tissue expression of Egr-1 was elevated in patients with scleroderma, which suggests that Egr-1 may be involved in tissue repair and fibrosis. Here, we investigated matrix remodeling and wound healing in mice harboring gain of function or loss of function mutations of Egr-1. Using the model of bleomycin-induced scleroderma, we found that the early influx of inflammatory cells into the skin and lungs, and the subsequent development of fibrosis in these organs, were markedly attenuated in Egr-1 null mice. Furthermore, full-thickness incisional skin wound healing was impaired, and skin fibroblasts lacking Egr-1 showed reduced migration and myofibroblast transdifferentiation in vitro. In contrast, transgenic mice with fibroblast-specific Egr-1 overexpression showed exuberant tissue repair, with enhanced collagen accumulation and increased tensile strength of incisional wounds. Together, these results point to the fundamental role that Egr-1 plays in the regulation of transforming growth factor-ss-dependent physiological and pathological matrix remodeling.

The role of the nuclear transport system in cell differentiation

The eukaryotic cell nuclear transport system selectively mediates molecular trafficking to facilitate the regulation of cellular processes. The components of this system include diverse transport factors such as importins and nuclear pore components that are precisely organized to coordinate cellular events. A number of studies have demonstrated that the nuclear transport system is indispensible in many types of cellular responses. In particular, the nuclear transport machinery has been shown to be an important regulator of development, organogenesis, and tissue formation, wherein altered nuclear transport of key transcription factors can lead to disease. Importantly, precise switching between distinct forms of importin alpha is central to neural lineage specification, consistent with the hypothesis that importin expression can be a key mediator of cell differentiation.

TGF-beta signalling is regulated by Schnurri-2-dependent nuclear translocation of CLIC4 and consequent stabilization of phospho-Smad2 and 3

CLIC4 (chloride intracellular channel 4), a multifunctional protein that traffics between the cytoplasm and nucleus, interacts with Schnurri-2, a transcription factor in the bone morphogenetic protein (BMP) signalling pathway. Here we show that transforming growth factor beta (TGF-beta) promotes the expression of CLIC4 and Schnurri-2 as well as their association in the cytoplasm and their translocation to the nucleus. In the absence of CLIC4 or Schnurri-2, TGF-beta signalling is abrogated. Direct nuclear targeting of CLIC4 enhances TGF-beta signalling and removes the requirement for Schnurri-2. Nuclear CLIC4 associates with phospho (p)-Smad2 and p-Smad3, protecting them from dephosphorylation by nuclear phosphatases. An intact TGF-beta signalling pathway is essential for CLIC4-mediated growth-arrest. These results newly identify Schnurri-2 and CLIC4 as modifiers of TGF-beta signalling through their stabilization of p-Smad2 and 3 in the nucleus.