A SMAD ubiquitin ligase targets the BMP pathway and affects embryonic pattern formation

Bone morphogenetic protein signal transduction in bone

Bone morphogenetic proteins (BMPs) promote bone formation by stimulating the proliferation and differentiation of osteoblasts. It has been suggested that non-union of the bone, and delayed healing, may be the result of decreased levels of BMP activity. Activation of BMP receptors initiates phosphorylation of the downstream effector proteins, known as receptor-regulated Smads, leading to signal transduction. Receptor-regulated Smads form a hetero-oligomeric complex with a common mediator Smad, which translocates into the nucleus and regulates target gene transcription. The BMP signalling cascade is closely regulated, with the inhibitory Smads blocking the intracellular signal cascade. Extracellular antagonists, such as noggin, inhibit binding to BMP receptors. BMP-2 and BMP-7 have demonstrated clinical utility for bone regeneration, and are commercially available through the use of recombinant DNA technology.

Degradation of RhoA by Smurf1 ubiquitin ligase

The Rho family of small GTPases plays a key role in the dynamic regulation of the actin cytoskeleton that underlies various important cellular functions such as shape changes, migration, and polarity. We found that Smurf1, a HECT domain E3 ubiquitin ligase, could specifically target RhoA but not Cdc42 or Rac1 for degradation. Smurf1 interacts with the dominant inactive form of RhoA, RhoA N19, which binds constitutively to guanine nucleotide exchange factors (GEFs) in vivo. Smurf1 also interacts directly with either nucleotide-free or GDP-bound RhoA in vitro; however, loading with GTPgammaS inhibits the interaction. RhoA is ubiquitinated by wild-type Smurf1 but not the catalytic mutant of Smurf1 (C699A) in vivo and in vitro, indicating that RhoA is a direct substrate of Smurf1. In this chapter, we summarize the systems and methods used in the analyses of Smurf1-regulated RhoA ubiquitination and degradation.

The BMP signaling and in vivo bone formation

Bone morphogenetic proteins (BMPs) are multi-functional growth factors that belong to the transforming growth factor beta (TGFbeta) superfamily. The roles of BMPs in embryonic development and cellular functions in postnatal and adult animals have been extensively studied in recent years. Signal transduction studies have revealed that Smads 1, 5 and 8 are the immediate downstream molecules of BMP receptors and play a central role in BMP signal transduction. Studies from transgenic and knockout mice and from animals and humans with naturally occurring mutations in BMPs and their signaling molecules have shown that BMP signaling plays critical roles in bone and cartilage development and postnatal bone formation. BMP activities are regulated at different molecular levels. Tissue-specific knockout of a specific BMP ligand, a subtype of BMP receptors or a specific signaling molecule is required to further determine the specific role of a BMP ligand, receptor or signaling molecule in a particular tissue.

Regulation of Smurf2 ubiquitin ligase activity by anchoring the E2 to the HECT domain

The conjugation of ubiquitin to proteins involves a cascade of activating (E1), conjugating (E2), and ubiquitin-ligating (E3) type enzymes that commonly signal protein destruction. In TGFbeta signaling the inhibitory protein Smad7 recruits Smurf2, an E3 of the C2-WW-HECT domain class, to the TGFbeta receptor complex to facilitate receptor degradation. Here, we demonstrate that the amino-terminal domain (NTD) of Smad7 stimulates Smurf activity by recruiting the E2, UbcH7, to the HECT domain. A 2.1 A resolution X-ray crystal structure of the Smurf2 HECT domain reveals that it has a suboptimal E2 binding pocket that could be optimized by mutagenesis to generate a HECT domain that functions independently of Smad7 and potently inhibits TGFbeta signaling. Thus, E2 enzyme recognition by an E3 HECT enzyme is not constitutively competent and provides a point of control for regulating the ubiquitin ligase activity through the action of auxiliary proteins.

TEB4 is a C4HC3 RING finger-containing ubiquitin ligase of the endoplasmic reticulum

In the present study, the human TEB4 is identified as a novel ER (endoplasmic reticulum)-resident ubiquitin ligase. TEB4 has homologues in many species and has a number of remarkable properties. TEB4 contains a conserved RING (really interesting new gene) finger and 13 predicted transmembrane domains. The RING finger of TEB4 and its homologues is situated at the N-terminus and has the unconventional C4HC3 configuration. The N-terminus of TEB4 is located in the cytosol. We show that the isolated TEB4 RING domain catalyses ubiquitin ligation in vitro in a reaction that is ubiquitin Lys48-specific and involves UBC7 (ubiquitin-conjugating enzyme 7). These properties are reminiscent of E3 enzymes, which are involved in ER-associated protein degradation. TEB4 is an ER degradation substrate itself, promoting its own degradation in a RING finger- and proteasome-dependent manner.

BMP7 signaling in renal development and disease

Fibrosis, and in particular tubulointerstitial fibrosis, is a common feature of almost all chronic renal diseases. Over the past several years, significant progress has been made in defining the underlying mechanisms of tubulointerstitial fibrosis. In a variety of mouse models, expression of transforming growth factor-beta is a primary causative factor which leads to increased numbers of myofibroblasts, collagen deposition and loss of tubular epithelia. More recently, another member of the transforming growth factor-beta superfamily, BMP7, was shown to counteract transforming growth factor-beta-mediated fibrosis. The activities of these secreted factors are regulated, in part, by extracellular ligand binding proteins which can enhance or suppress receptor ligand interactions.

Regulating the Regulators: Control of Protein Ubiquitination and Ubiquitin-like Modifications by Extracellular Stimuli

Like many other posttranscriptional modifications, ubiquitination and conjugation of ubiquitin-like polypeptides to target proteins are tightly regulated by extracellular stimuli. In many cases, this regulation is dependent upon protein phosphorylation. The regulatory step affected by phosphorylation could involve either recognition of the substrate by an E3 ubiquitin ligase or the actual conjugation reaction. Regulation occurs through phosphorylation of either the substrates or the E3 ligases themselves. This review focuses on recent advances in understanding how extracellular stimuli modulate the attachment of ubiquitin and ubiquitin-like peptides to target proteins.

NEDD4-2 (neural precursor cell expressed, developmentally down-regulated 4-2) negatively regulates TGF-beta (transforming growth factor-beta) signalling by inducing ubiquitin-mediated degradation of Smad2 and TGF-beta type I receptor

Inhibitory Smad, Smad7, is a potent inhibitor of TGF-beta (transforming growth factor-beta) superfamily signalling. By binding to activated type I receptors, it prevents the activation of R-Smads (receptor-regulated Smads). To identify new components of the Smad pathway, we performed yeast two-hybrid screening using Smad7 as bait, and identified NEDD4-2 (neural precursor cell expressed, developmentally down-regulated 4-2) as a direct binding partner of Smad7. NEDD4-2 is structurally similar to Smurfs (Smad ubiquitin regulatory factors) 1 and 2, which were identified previously as E3 ubiquitin ligases for R-Smads and TGF-beta superfamily receptors. NEDD4-2 functions like Smurfs 1 and 2 in that it associates with TGF-beta type I receptor via Smad7, and induces its ubiquitin-dependent degradation. Moreover, NEDD4-2 bound to TGF-beta-specific R-Smads, Smads 2 and 3, in a ligand-dependent manner, and induced degradation of Smad2, but not Smad3. However, in contrast with Smurf2, NEDD4-2 failed to induce ubiquitination of SnoN (Ski-related novel protein N), although NEDD4-2 bound to SnoN via Smad2 more strongly than Smurf2. We showed further that overexpressed NEDD4-2 prevents transcriptional activity induced by TGF-beta and BMP, whereas silencing of the NEDD4-2 gene by siRNA (small interfering RNA) resulted in enhancement of the responsiveness to TGF-beta superfamily cytokines. These data suggest that NEDD4-2 is a member of the Smurf-like C2-WW-HECT (WW is Trp-Trp and HECT is homologous to the E6-accessory protein) type E3 ubiquitin ligases, which negatively regulate TGF-beta superfamily signalling through similar, but not identical, mechanisms to those used by Smurfs.

The deubiquitinating enzyme UCH37 interacts with Smads and regulates TGF-β signalling

Disruption of components in the transforming growth factor-beta (TGF-beta) signalling cascade is a common occurrence in human cancers. TGF-beta pathway activation is accomplished via serine/threonine kinase receptors and intracellular Smad transcription factors. A key regulatory step involves specific ubiquitination by Smurfs that mediate the proteasomal degradation of Smads and/or receptors. Here, we report a novel interaction between Smads and ubiquitin C-terminal hydrolase UCH37, a deubiquitinating enzyme that could potentially reverse Smurf-mediated ubiquitination. In GST pull down experiments, UCH37 bound weakly to Smad2 and Smad3, and bound very strongly to Smad7 in a region that is distinct from the -PY- motif in Smad7 that interacts with Smurf ubiquitin ligases. Endogenous Smad7 and UCH37 formed a stable complex in U4A/JAK1 cells, and FLAG-Smad7 co-immunoprecipitated with HA-UCH37 in transfected HEK-293 cells. In addition, we show that UCH37 can deubiquitinate and stabilize the type I TGF-beta receptor. Furthermore, overexpression of UCH37 upregulates TGF-beta-dependent transcription, and this effect is reversed in cells subject to RNAi-mediated knockdown of endogenous UCH37. These findings support a new role for deubiquitinating enzymes in the control of the TGF-beta signalling pathway, and provide a novel molecular target for the design of inhibitors with therapeutic potential in cancer.

AIP4 Restricts Transforming Growth Factor-β Signaling through a Ubiquitination-independent Mechanism

Smad7 functions as an intracellular antagonist in transforming growth factor-beta (TGF-beta) signaling. In addition to interacting stably with the activated TGF-beta type I receptor (TbetaRI) to prevent phosphorylation of the receptor-regulated Smads (Smad2 and Smad3), Smad7 also induces degradation of the activated TbetaRI through association with different E3 ubiquitin ligases. Using the two-hybrid screen, we identified atrophin 1-interacting protein 4 (AIP4) as an E3 ubiquitin ligase that specifically targets Smad7 for ubiquitin-dependent degradation without affecting the turnover of the activated TbetaRI. Surprisingly, we found that despite the ability to degrade Smad7, AIP4 can inhibit TGF-beta signaling, presumably by enhancing the association of Smad7 with the activated TbetaRI. Consistent with this notion, expression of a catalytic mutant of AIP4, which is unable to induce ubiquitination and degradation of Smad7, also stabilizes the TbetaRI.Smad7 complex, resulting in inhibition of TGF-beta signaling. The ability of AIP4 to enhance the inhibitory function of Smad7 independent of its ubiquitin ligase activity reveals a new mechanism by which E3 ubiquitin ligases may function to turn off TGF-beta signaling.

Repulsive guidance molecule (RGMa), a DRAGON homologue, is a bone morphogenetic protein co-receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily of ligands, which regulate many mammalian physiologic and pathophysiologic processes. BMPs exert their effects through type I and type II serine/threonine kinase receptors and the Smad intracellular signaling pathway. Recently, the glycosylphosphatidylinositol (GPI)-anchored protein DRAGON was identified as a co-receptor for BMP signaling. Here, we investigate whether a homologue of DRAGON, repulsive guidance molecule (RGMa), is similarly involved in the BMP signaling pathway. We show that RGMa enhances BMP, but not TGF-beta, signals in a ligand-dependent manner in cell culture. The soluble extracellular domain of RGMa fused to human Fc (RGMa.Fc) forms a complex with BMP type I receptors and binds directly and selectively to radiolabeled BMP-2 and BMP-4. RGMa mediates BMP signaling through the classical BMP signaling pathway involving Smad1, 5, and 8, and it up-regulates endogenous inhibitor of differentiation (Id1) protein, an important downstream target of BMP signals. Finally, we demonstrate that BMP signaling occurs in neurons that express RGMa in vivo. These data are consistent with a role for RGMa as a BMP co-receptor.

Germ-layer specification and control of cell growth by Ectodermin, a Smad4 ubiquitin ligase

TGF-beta signaling is essential for development and proliferative homeostasis. During embryogenesis, maternal determinants act in concert with TGF-beta signals to form mesoderm and endoderm. In contrast, ectoderm specification requires the TGF-beta response to be attenuated, although the mechanisms by which this is achieved remain unknown. In a functional screen for ectoderm determinants, we have identified Ectodermin (Ecto). In Xenopus embryos, Ecto is essential for the specification of the ectoderm and acts by restricting the mesoderm-inducing activity of TGF-beta signals to the mesoderm and favoring neural induction. Ecto is a RING-type ubiquitin ligase for Smad4, a TGF-beta signal transducer. Depletion of Ecto in human cells enforces TGF-beta-induced cytostasis and, moreover, plays a causal role in limiting the antimitogenic effects of Smad4 in tumor cells. We propose that Ectodermin is a key switch in the control of TGF-beta gene responses during early embryonic development and cell proliferation.

Ubiquitin-mediated degradation a mechanism for fine-tuning TGF-beta signaling

Effects of the cytokine TGF-beta can be dampened by E3 ubiquitin ligases that target specific Smads, the TGF-beta signal transducers, for proteolytic destruction. Two papers in this issue of Cell highlight the importance of this mechanism in regulating the in vivo effects of TGF-beta. The first paper identifies and characterizes a novel Smad4 ubiquitin ligase, and the second paper redefines the role of a previously identified Smad1 ubiquitin ligase, Smurf-1 (Dupont et al., 2005; Yamashita et al., 2005).

Specific interaction between Smad1 and CHIP: a surface plasmon resonance study

The TGF-beta superfamily signaling pathway regulates many important biological processes, including cell growth, differentiation and embryonic pattern formation. Smad1, a member of this signaling pathway that functions downstream of serine/threonine kinase receptors, has ability to interact with carboxyl terminus of Hsc70-interacting protein (CHIP), which is an E3 ubiquitin ligase in other cases. It has been reported that Smurf1, a member of the Hect family E3 ubiquitin ligases, can target Smad1 to 26S proteasome for degradation. In this paper, we studied the interaction of Smad1 and CHIP by combination of surface plasmon resonance and supported monolayer approach. The specific binding of Smad1 to CHIP indicates that the degradation of Smad1 may also be mediated by CHIP, and CHIP may play an essential role in the TGF-beta signaling pathway.

Degradation of the Tumor Suppressor Smad4 by WW and HECT Domain Ubiquitin Ligases

Smad4 mediates signaling by the transforming growth factor-beta (TGF-beta) superfamily of cytokines. Smad signaling is negatively regulated by inhibitory (I) Smads and ubiquitin-mediated processes. Known mechanisms of proteasomal degradation of Smads depend on the direct interaction of specific E3 ligases with Smads. Alternatively, I-Smads elicit degradation of the TGF-beta receptor by recruiting the WW and HECT domain E3 ligases, Smurfs, WWP1, or NEDD4-2. We describe an equivalent mechanism of degradation of Smad4 by the above E3 ligases, via formation of ternary complexes between Smad4 and Smurfs, mediated by R-Smads (Smad2) or I-Smads (Smad6/7), acting as adaptors. Smurfs, which otherwise cannot directly bind to Smad4, mediated poly-ubiquitination of Smad4 in the presence of Smad6 or Smad7. Smad4 co-localized with Smad7 and Smurf1 primarily in the cytoplasm and in peripheral cell protrusions. Smad2 or Smad7 mutants defective in Smad4 interaction failed to induce Smurf1-mediated down-regulation of Smad4. A Smad4 mutant defective in Smad2 or Smad7 interaction could not be effectively down-regulated by Smurf1. We propose that Smad4 is targeted for degradation by multiple ubiquitin ligases that can simultaneously act on R-Smads and signaling receptors. Such mechanisms of down-regulation of TGF-beta signaling may be critical for proper physiological response to this pathway.

The role of bone morphogenetic proteins in endochondral bone formation

Bone morphogenetic proteins (BMPs) were originally identified as proteins capable of inducing endochondral bone formation when implanted at extraskeletal sites. BMPs have diverse biological activities during early embryogenesis and various aspects of organogenesis. BMPs bind to BMP receptors on the cell surface, and these signals are transduced intracellularly by Smad proteins. BMP signal pathways can be inhibited by both extra- and intracellular mechanisms. As for skeletal development, genetic studies suggest that BMPs are skeletal mesoderm inducers. Recent studies of tissue-specific activation and inactivation of BMP signals have revealed that BMP signals control proliferation and differentiation of chondrocytes, differentiation of osteoblasts and bone quality. These findings may contribute not only to understanding of bone biology and pathology, but also to improvement of the clinical efficacy of BMPs.

Transcriptional regulation of chondrocyte maturation: Potential involvement of transcription factors in OA pathogenesis

The principle function of articular cartilage is to provide a low friction load-bearing surface that facilitates free movement of joints. Maintenance of this surface depends on the maturational arrest of chondrocytes before terminal hypertrophic differentiation occurs [Exp. Cell Res. 216 (1995) 191; Osteoarthritis Cartilage 7 (1999) 389; J. Cell Biol. 139 (1997) 541; J. Cell Biol. 145 (1999) 783]. In contrast to endochondral ossification which involves a programmed process of chondrocyte maturation culminating in terminal hypertrophy and mineralization [Nat. Genet. 9 (1995) 15], articular chondrocytes (ACs) are constrained from completing the maturational program as evidenced by a lack of type X collagen (colX) and alkaline phosphatase expression [Arthritis Res. 3 (2001) 107; Biochem. J. 362 (2002) 473]. Also, ACs are not responsive to factors that impact the maturational process, including bone morphogenetic protein-2 (BMP-2), a potent stimulator of chondrocyte maturation [J. Orthop. Res. 14 (1996) 937]. Factors that constrain AC maturation are only relieved under unique circumstances such as in osteoarthritis (OA), where proliferation and an increase in the expression of hypertrophic hallmarks indicates that the cells have differentiated into a mature phenotype [Calcif. Tissue Int. 63 (2000) 230]. OA may thus involve the functional loss of mechanisms that arrest articular cartilage differentiation. Responsiveness to various growth or systemic factors translates into activation or repression of specific genes through transcriptional mediators. Understanding the downstream mechanisms involved in this process is of paramount importance. Thus, unraveling the molecular interplay between various factors that regulate chondrocyte maturation during OA occurrence and progression is the main focus of ongoing efforts.

TGF-beta and TNF-alpha: antagonistic cytokines controlling type I collagen gene expression

The balance between production and degradation of type I collagen plays a critical role in the development and maintenance of organ and tissue integrity. It also represents the most crucial element governing the process of tissue repair. The synthesis of type I collagen gene is highly regulated by different cytokines at the transcriptional level. Especially, transforming growth factor beta (TGF-beta), a key player in the physiopathology of tissue repair, enhances type I collagen gene expression. In contrast, tumor necrosis factor alpha (TNF-alpha), whose matrix-remodelling function is opposite to that of TGF-beta, reduces type I collagen gene expression. This review focuses on transcriptional regulation of type I collagen by TGF-beta and TNF-alpha and on the molecular mechanisms that control the antagonistic activity of TNF-alpha against TGF-beta-driven type I collagen gene expression.

Transcriptional induction of Smurf2 ubiquitin ligase by TGF-beta

Smad ubiquitination regulatory factor 2 (Smurf2), a ubiquitin ligase for Smads, plays critical roles in the regulation of transforming growth factor-beta (TGF-beta)-Smad signaling via ubiquitin-dependent degradation of Smad2 and Smad7. We found that TGF-beta stimulates Smurf2 expression. TGF-beta activated the Smurf2 promoter in a TGF-beta responsive cell lines, whereas IL-1alpha, PDGF and epidermal growth factor did not. TGF-beta-mediated Smurf2 promoter activation was inhibited by Smad7 or an activin receptor-like kinase 5 inhibitor but not by dominant negative Smad or disruption of Smad-binding elements in the promoter. Moreover, inhibition of the phosphatidil inositol 3 kinase (PI3K)/Akt pathway suppressed TGF-beta-mediated Smurf2 induction. These results suggest that TGF-beta stimulates Smurf2 expression by Smad-independent pathway such as PI3K/Akt pathway via TGF-beta receptor.

Negative regulation of TGF-beta signaling in development

The TGF-beta superfamily members have important roles in controlling patterning and tissue formation in both invertebrates and vertebrates. Two types of signal transducers, receptors and Smads, mediate the signaling to regulate expression of their target genes. Despite of the relatively simple signal transduction pathway, many modulators have been found to contribute to a tight regulation of this pathway in a variety of mechanisms. This article reviews the negative regulation of TGF-beta signaling with focus on its roles in vertebrate development.

CHIP Controls the Sensitivity of Transforming Growth Factor-β Signaling by Modulating the Basal Level of Smad3 through Ubiquitin-mediated Degradation

Transforming growth factor-beta (TGF-beta) signaling is critical in a variety of biological processes such as cell proliferation, differentiation, and apoptosis. TGF-beta signaling is mediated by a group of proteins including TGF-beta receptors and Smads. It is known that different cells can exhibit different sensitivities to TGF-beta. Several molecular mechanisms, such as the differential expression of the receptor levels, have been suggested as contributing to these differences. Here, we report evidence for a novel mechanism of regulating TGF-beta sensitivity that depends on the role of CHIP (carboxyl terminus of Hsc70-interacting protein) in regulating the basal level of Smad3 via the ubiquitin-dependent degradation pathway. First, using a luciferase assay we found that overexpression of CHIP inhibited TGF-beta signaling, whereas silencing CHIP expression by small interfering RNAs led to increased TGF-beta signaling sensitivity. Second, based on the results of cell proliferation assays and JunB expression, we found that TGF-beta signaling could be abolished by stably overexpressing CHIP. Third, in those cell lines with stably expressed CHIP, we observed that the Smad3 protein level was dramatically decreased. Finally, we demonstrated that CHIP served as a U-box dependent E3 ligase that can directly mediate ubiquitination and degradation of Smad3 and that this action of CHIP was independent of TGF-beta signaling. Taken together, these findings suggest that CHIP can modulate the sensitivity of the TGF-beta signaling by controlling the basal level of Smad3 through ubiquitin-mediated degradation.

The ARID domain protein dril1 is necessary for TGF(beta) signaling in Xenopus embryos

ARID domain proteins are members of a highly conserved family involved in chromatin remodeling and cell-fate determination. Dril1 is the founding member of the ARID family and is involved in developmental processes in both Drosophila and Caenorhabditis elegans. We describe the first embryological characterization of this gene in chordates. Dril1 mRNA expression is spatiotemporally regulated and is detected in the involuting mesoderm during gastrulation. Inhibition of dril1 by either a morpholino or an engrailed repressor-dril1 DNA binding domain fusion construct inhibits gastrulation and perturbs induction of the zygotic mesodermal marker Xbra and the organizer markers chordin, noggin, and Xlim1. Xenopus tropicalis dril1 morphants also exhibit impaired gastrulation and axial deficiencies, which can be rescued by coinjection of Xenopus laevis dril1 mRNA. Loss of dril1 inhibits the response of animal caps to activin and secondary axis induction by smad2. Dril1 depletion in animal caps prevents both the smad2-mediated induction of dorsal mesodermal and endodermal markers and the induction of ventral mesoderm by smad1. Mesoderm induction by eFGF is uninhibited in dril1 morphant caps, reflecting pathway specificity for dril1. These experiments identify dril1 as a novel regulator of TGF(beta) signaling and a vital component of mesodermal patterning and embryonic morphogenesis.

MFB-1, an F-box-type ubiquitin ligase, regulates TGF-beta signalling

TGF-beta signalling regulates cell growth, differentiation, morphogenesis and apoptosis. MAFbx/Atrogin-1 has been identified as a regulator for skeletal muscle atrophy and encodes an F-box-type E3 ubiquitin ligase. However, little is known about how MAFbx/Atrogin-1 regulates cellular signalling. Here, we identify and genetically characterize MFB-1, a MAFbx/Atrogin-1 homologue from Caenorhabditis elegans. The mfb-1 deletion mutant significantly enhanced the dauer constitutive (Daf-c) phenotype caused by mutations in the DAF-7/TGF-beta-like signalling pathway, but not the DAF-2/insulin receptor-like signalling pathway. Conversely, the Daf-c phenotypes of DAF-7 pathway mutants were partially suppressed by mfb-1 cDNA transgenes. Therefore, MFB-1 acts genetically downstream in the DAF-7 pathway. A mfb-1::GFP fusion was found to be expressed in the nervous system, hypodermis and intestine and overlapped expression of many DAF-7 pathway genes. We propose that MFB-1 is a novel F-box protein that negatively regulates dauer formation in concert with the DAF-7 signalling pathway in C. elegans.

BMP signaling in skeletal development

Development of the vertebrate skeleton, a complex biological event that includes diverse processes such as formation of mesenchymal condensations at the sites of future skeletal elements, osteoblast and chondrocyte differentiation, and three dimensional patterning, is regulated by many growth factors. Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily, play a pivotal role in the signaling network and are involved in nearly all processes associated with skeletal morphogenesis. BMP signals are transduced from the plasma membrane receptors to the nucleus through both Smad pathway and non-Smad pathways, and regulated by many extracellular and intercellular proteins that interact with BMPs or components of the BMP signaling pathways. To gain a better understanding of the molecular mechanisms underlying the role of BMP in early skeletal development, it is necessary to elucidate the BMP signaling transduction pathways in chondrocytes and osteoblasts. The major objective of this review was to summarize BMP signaling pathways in the context of craniofacial, axial, and limb development. In particular, this discourse will focus on recent advances of the role of different ligands, receptors, Smads, and BMP regulators in osteoblast and chondrocyte differentiation during embryonic development.

Ubiquitin ligase Smurf1 controls osteoblast activity and bone homeostasis by targeting MEKK2 for degradation

Bone is constantly resorbed and formed throughout life by coordinated actions of osteoclasts and osteoblasts. Here we show that Smurf1, a HECT domain ubiquitin ligase, has a specific physiological role in suppressing the osteogenic activity of osteoblasts. Smurf1-deficient mice are born normal but exhibit an age-dependent increase of bone mass. The cause of this increase can be traced to enhanced activities of osteoblasts, which become sensitized to bone morphogenesis protein (BMP) in the absence of Smurf1. However, loss of Smurf1 does not affect the canonical Smad-mediated intracellular TGFbeta or BMP signaling; instead, it leads to accumulation of phosphorylated MEKK2 and activation of the downstream JNK signaling cascade. We demonstrate that Smurf1 physically interacts with MEKK2 and promotes the ubiquitination and turnover of MEKK2. These results indicate that Smurf1 negatively regulates osteoblast activity and response to BMP through controlling MEKK2 degradation.

Utilisation de facteurs de croissance pour la réparation osseuse

Osteoformation is induced by numerous growth factors that play an important role in bone repair such as fracture healing. They may serve as therapeutic agent in the treatment of squeletal injuries in the orthopeadic and maxillo-facial fields. Among these proteins, Bone Morphogenetic Proteins (BMP) are the only known osteoinductive growth factors. Unfortunately, they are highly susceptible to proteolysis in vivo and require a suitable delivery system to potentiate their biological activity in a local, controlled and durable manner. In this aim, three options are under investigations: (i) osteoinductive materials made of appropriate carrier to release the protein in situ, (ii) in vivo gene therapy in which the gene is directly transfected in cells of the patient or (iii) ex vivo gene therapy in which cells are harvested from the patient, transfected with DNA in culture and then implanted in the defect. These different kinds of BMP delivery will be discussed.

Fine Tuning and Cross-talking of TGF-β Signal by Inhibitory Smads

Transforming Growth Factor (TGF)-beta family, including TGF-beta, bone morphorgenic protein (BMP), and activn, plays an important role in essential cellular functions such as proliferation, differentiation, apoptosis, tissue remodeling, angiognesis, immune responses, and cell adhesions. TGF-beta predominantly transmits the signals through serine/threonine receptor kinases and cytoplasmic proteins called Smads. Since the discovery of TGF-beta in the early 1980s, the dysregulation of TGF-beta /Smad signaling has been implicated in the pathogenesis of human diseases. Among signal transducers in TGF-beta/Smad signaling, inhibitory Smads (I-Smads), Smad6 and Smad7, act as major negative regulators forming autoinhibitory feedback loops and mediate the cross-talking with other signaling pathways. Expressions of I-Smads are mainly regulated on the transcriptional levels and post-translational protein degradations and their intracellular levels are tightly controlled to maintain the homeostatic balances. However, abnormal levels of I-Smads in the pathological conditions elicit the altered TGF-beta signaling in cells, eventually causing TGF-beta-related human diseases. Thus, exploring the molecular mechanisms about the regulations of I-Smads may provide the therapeutic clues for human diseases induced by the abnormal TGF-beta signaling.

Smad1 expression and function during mouse embryonic lung branching morphogenesis

Bone morphogenetic protein (BMP) 4 plays very important roles in regulating developmental processes of many organs, including lung. Smad1 is one of the BMP receptor downstream signaling proteins that transduce BMP4 ligand signaling from cell surface to nucleus. The dynamic expression patterns of Smad1 in embryonic mouse lungs were examined using immunohistochemistry. Smad1 protein was predominantly detected in peripheral airway epithelial cells of early embryonic lung tissue [embryonic day 12.5 (E12.5)], whereas Smad1 protein expression in mesenchymal cells increased during mid-late gestation. Many Smad1-positive mesenchymal cells were localized adjacent to large airway epithelial cells and endothelial cells of blood vessels, which colocalized with a molecular marker of smooth muscle cells (alpha-smooth muscle actin). The biological function of Smad1 in early lung branching morphogenesis was then studied in our established E11.5 lung explant culture model. Reduction of endogenous Smad1 expression was achieved by adding a Smad1-specific antisense DNA oligonucleotide, causing approximately 20% reduction of lung epithelial branching. Furthermore, airway epithelial cell proliferation and differentiation were also inhibited when endogenous Smad1 expression was knocked down. Therefore, these data indicate that Smad1, acting as an intracellular BMP signaling pathway component, positively regulates early mouse embryonic lung branching morphogenesis.

5-azacytidine alters TGF-beta and BMP signaling and induces maturation in articular chondrocytes

Maintenance of the articular surface depends on the function of articular chondrocytes (ACs) which produce matrix and are constrained from undergoing the maturation program seen in growth plate chondrocytes. Only during pathologic conditions, such as in osteoarthritis, are maturational constraints lost causing recapitulation of the process that occurs during endochondral ossification. With the aim of establishing a model to identify regulatory mechanisms that suppress AC hypertrophy, we examined the capability of 5-azacytidine (Aza) to have an impact on the maturational program of these cells. Primary ACs do not spontaneously express markers of maturation and are refractory to treatment by factors that normally regulate chondrocyte maturation. However, following exposure to Aza, ACs (i) were induced to express type X collagen (colX), Indian hedgehog, and alkaline phosphatase and (ii) showed altered colX and AP expression in response to bone morphogenetic protein-2 (BMP-2), transforming growth factor-beta (TGF-beta), and parathyroid hormone-related protein (PTHrP). Since Aza unmasked responsiveness of ACs to BMP-2 and TGF-beta, we examined the effect of Aza treatment on signaling via these pathways by assessing the expression of the TGF-beta Smads (2 and 3), the BMP-2 Smads (1 and 5), and the Smad2 and 3-degrading ubiquitin E3 ligase Smurf2. Aza-treated ACs displayed less Smad2 and 3 and increased Smad1, 5, and Smurf2 protein and showed a loss of TGF-beta signaling on the P3TP-luciferase reporter. Suggesting that Aza-induction of Smurf2 may be responsible for the loss of Smad2 and 3 protein via this pathway, immunoprecipitation and metabolic labeling experiments confirmed that Aza accelerated the ubiquitination and degradation of these targets. Overall, Aza-treated ACs represent a novel model for the study of mechanisms that regulate maturational potential of articular cartilage, with the data suggesting that maturation of these cells may be due to up-regulation of Smad1 and 5 coupled with a Smurf2-dependent degradation of Smad2 and 3 and loss of TGF-beta signaling.

Bone repair in the twenty-first century: biology, chemistry or engineering?

Increases in reconstructive orthopaedic surgery, such as total hip replacement and spinal fusion, resulting from advances in surgical practice and the ageing population, have lead to a demand for bone graft that far exceeds supply. Consequently, a number of synthetic bone-graft substitutes (BGSs) have been developed with mixed success and surgical acceptance. Skeletal tissue regeneration requires the interaction of three basic elements: cells, growth factors (GFs) and a permissive scaffold. This can be achieved by pre-loading a synthetic scaffold with GFs or pre-expanded cells; however, a 'simpler' approach is to design intrinsic 'osteoinductivity' into your BGS, i.e. the capability to recruit and stimulate the patient's own GFs and stem cells. Through investigation of the mechanisms controlling bone repair in BGSs, linking interactions between the local chemical and physical environment, scientists are currently developing osteoinductive materials that can stimulate bone regeneration through control of the scaffold chemistry and structure. Moreover, this body of research is providing the foundations for future generations of BGSs and bone-repair therapies and may ultimately contribute towards improving the quality of life through maintenance of the skeleton and reversal of disease states, as opposed to the mending of broken bones that we currently practice. Will we be able to grow our own bones in a bioreactor for use as autologous graft materials in the future? Could surgery be limited to accidental trauma cases, with greater restoration of function through biochemical or gene therapies? The technology and research probes necessary to this task are currently being developed with the advent of nanotechnology, genomics and proteomics: are we about to embark on a chemical revolution in medicine? This paper aims to discuss some of the current thinking on the mechanisms behind bioactivity and biocompatibility in bone and how a fuller understanding of the interactions between cells and the materials used today could bring about completely new approaches for the treatment of bone fracture and disease tomorrow.

BMP signaling in the control of skin development and hair follicle growth

Bone morphogenetic proteins (BMPs), their antagonists, and BMP receptors are involved in controlling a large number of biological functions including cell proliferation, differentiation, cell fate decision, and apoptosis in many different types of cells and tissues during embryonic development and postnatal life. BMPs exert their biological effects via using BMP-Smad and BMP-MAPK intracellular pathways. The magnitude and specificity of BMP signaling are regulated by a large number of modulators operating on several levels (extracellular, cytoplasmic, nuclear). In developing and postnatal skin, BMPs, their receptors, and BMP antagonists show stringent spatio-temporal expressions patterns to achieve proper regulation of cell proliferation and differentiation in the epidermis and in the hair follicle. Genetic studies assert an essential role for BMP signaling in the control of cell differentiation and apoptosis in developing epidermis, as well as in the regulation of key steps of hair follicle development (initiation, cell fate decision, cell lineage differentiation). In postnatal hair follicles, BMP signaling plays an important role in controlling the initiation of the growth phase and is also involved in the regulation of apoptosis-driven hair follicle involution. However, additional efforts are required to fully understand the mechanisms and targets involved in the realization of BMP effects on distinct cell population in the skin and hair follicle. Progress in this area of research will hopefully lead to the development of new therapeutic approaches for using BMPs and BMP antagonists in the treatment of skin and hair growth disorders.

Smurf2 up-regulation activates telomere-dependent senescence

Progressive telomere shortening activates replicative senescence, which prevents somatic cells from being propagated indefinitely in culture. The limitation of proliferative capacity imposed by replicative senescence is thought to contribute to both organismal aging and the prevention of tumor development. Here we report that up-regulation of Smurf2, an E3 ubiquitin ligase previously implicated in TGF-beta signaling, is a specific consequence of telomere attrition in human fibroblasts and that such up-regulation is sufficient to produce the senescence phenotype. Adventitious production of the Smurf2 protein in early passage fibroblasts at the same physiological level observed during telomere-mediated senescence resulted in proliferative arrest in a viable state, morphological and biochemical alterations characteristic of senescence, acquisition of senescence-specific alterations in gene expression, and reversal of cellular immortalization by telomerase. We show that the senescence-inducing actions of Smurf2 occur in the absence of detectable DNA damage or stress response, that Smurf2's effects require a novel function distinct from its E3 activity, that Smurf2 recruits the Rb and p53 pathways for senescence induction, and that while p21 is elevated by Smurf2, Smurf2-mediated senescence is independent of p21. Smurf2 is the first gene found to be both up-regulated by telomere attrition and sufficient to induce senescence.

Itch E3 ligase-mediated regulation of TGF-beta signaling by modulating smad2 phosphorylation

Protein ubiquitination has been implicated in the intracellular biochemical events transduced by TGF-beta receptor via different mechanisms including the degradation of Smads or their binding proteins. Here we show that loss of Itch E3 ligase in mouse embryonic fibroblasts (MEFs) results in reduced susceptibility of TGF-beta-induced cell growth arrest and decreased phosphorylation of Smad2, without apparent alteration in protein levels for Smad2, Smad4, and Smad7 in Itch-/- MEFs. Itch promotes ubiquitination of Smad2 and augments Smad2 phosphorylation that requires an intact ligase activity of Itch. Moreover, Itch facilitates complex formation between TGF-beta receptor and Smad2 and enhances TGF-beta-induced transcription. This study reveals a previously unrecognized positive TGF-beta signaling pathway via proteolysis-independent ubiquitination.

Composite mesostructures by nano-confinement

In a physically confined environment, interfacial interactions, symmetry breaking, structural frustration and confinement-induced entropy loss can play dominant roles in determining molecular organization. Here we present a systematic study of the confined assembly of silica-surfactant composite mesostructures within cylindrical nanochannels of varying diameters. Using exactly the same precursors and reaction conditions that form the two-dimensional hexagonal SBA-15 mesostructured thin film, unprecedented silica mesostructures with chiral mesopores such as single- and double-helical geometries spontaneously form inside individual alumina nanochannels. On tightening the degree of confinement, a transition is observed in the mesopore morphology from a coiled cylindrical to a spherical cage-like geometry. Self-consistent field calculations carried out to account for the observed mesostructures accord well with experiment. The mesostructures produced by confined syntheses are useful as templates for fabricating highly ordered mesostructured nanowires and nanowire arrays.

Runx2/Cbfa1 stimulates transdifferentiation of primary skeletal myoblasts into a mineralizing osteoblastic phenotype

Runx2, a transcriptional activator downstream of bone morphogenetic protein (BMP) signaling, is essential to osteoblastic differentiation and bone formation and maintenance. BMPs activate complex signaling networks, utilizing numerous signaling molecules and transcription factors to induce expression of osteoblastic markers in mesenchymal cell types. However, the role of Runx2 in this process, particularly in an environment independent of the other regulatory elements modulated by BMPs, remains poorly understood. In the present study, we used retroviral gene delivery to examine the effects of sustained Runx2 expression in primary myoblasts. Runx2 inhibited myogenesis, as demonstrated by suppression of MyoD and myogenin mRNA levels and reduced myotube formation. Additionally, Runx2-stimulated osteogenesis including osteoblastic gene expression, alkaline phosphatase activity, and biological mineral deposition. Notably, these osteogenic markers were induced to significantly greater levels than those observed in BMP-2-treated controls. These results demonstrate that direct exogenous expression of the Runx2 transcription factor, only one of numerous downstream targets of BMP signaling, is sufficient to induce transdifferentiation of myogenic cells into a mineralizing osteogenic lineage. This work underscores the potency of Runx2 as a regulator of osteogenesis and cell differentiation and provides new insights into the plasticity of committed mesenchymal cells.

The novel E3 ubiquitin ligase Tiul1 associates with TGIF to target Smad2 for degradation

Ubiquitin-dependent degradation plays an important role in the negative regulation of TGF-beta signaling. Here, we identify Tiul1 (for TGIF interacting ubiquitin ligase 1), a novel E3 ubiquitin ligase that inhibits TGF-beta signaling by targeting both the activated receptor and Smad2 for degradation. Tiul1 associates constitutively with Smad7 and induces degradation of the activated type I receptor without affecting the expression levels of Smad7. Tiul1 can also interact with Smad2 and the nuclear corepressor TGIF upon activation of TGF-beta signaling. Like Smad7, the steady-state levels of TGIF are not affected by Tiul1, but the interaction of Tiul1 with TGIF allows this ubiquitin ligase to target Smad2 for degradation. Consistent with this, overexpression of Tiul1 suppressed TGF-beta-induced growth arrest and transcriptional responses. In addition, silencing of Tiul1 or TGIF genes by siRNA resulted in suppression of the TGF-beta-dependent degradation of Smad2 and an enhancement of TGF-beta-mediated gene expression. These results reveal a new role for TGIF as a component of a ubiquitin ligase complex that mediates the degradation of Smad2 in response to TGF-beta signaling.

Ubiquitination and proteolysis of cancer-derived Smad4 mutants by SCFSkp2

Smad4/DPC4, a common signal transducer in transforming growth factor beta (TGF-beta) signaling, is frequently inactivated in human cancer. Although the ubiquitin-proteasome pathway has been established as one mechanism of inactivating Smad4 in cancer, the specific ubiquitin E3 ligase for ubiquitination-mediated proteolysis of Smad4 cancer mutants remains unclear. In this report, we identified the SCFSkp2 complex as candidate Smad4-interacting proteins in an antibody array-based screen and further elucidated the functions of SCFSkp2 in mediating the metabolic instability of cancer-derived Smad4 mutants. We found that Skp2, the F-box component of SCFSkp2, physically interacted with Smad4 at the physiological levels. Several cancer-derived unstable mutants exhibited significantly increased binding to Skp2, which led to their increased ubiquitination and accelerated proteolysis. These results suggest an important role for the SCFSkp2 complex in switching cancer mutants of Smad4 to undergo polyubiquitination-dependent degradation.

Transcriptional control of synaptic differentiation by retrograde signals

Synaptic differentiation during development is a multi-step process, which requires reciprocal communication between pre- and postsynaptic cells. Cell surface interactions can induce the assembly of synaptic specializations but maintenance and growth of synapses depend on transcriptional regulation. Transcriptional responses associated with synaptic differentiation are observed in central and peripheral neurons and depend on retrograde signals coming from the target region. Although the identity of most of the retrograde signaling pathways remains to be identified, the TGFbeta family of growth factors have emerged as one crucial signal at the neuromuscular junction. Here, we discuss evidence for transcriptional control during synaptic differentiation and the signaling pathways mediating retrograde TGFbeta signaling.

Bone morphogenetic protein 2-induced osteoblast differentiation requires Smad-mediated down-regulation of Cdk6

Because a temporal arrest in the G(1) phase of the cell cycle is thought to be a prerequisite for cell differentiation, we investigated cell cycle factors that critically influence the differentiation of mouse osteoblastic MC3T3-E1 cells induced by bone morphogenetic protein 2 (BMP-2), a potent inducer of osteoblast differentiation. Of the G(1) cell cycle factors examined, the expression of cyclin-dependent kinase 6 (Cdk6) was found to be strongly down-regulated by BMP-2/Smads signaling, mainly via transcriptional repression. The enforced expression of Cdk6 blocked BMP-2-induced osteoblast differentiation to various degrees, depending on the level of its overexpression. However, neither BMP-2 treatment nor Cdk6 overexpression significantly affected cell proliferation, suggesting that the inhibitory effect of Cdk6 on cell differentiation was exerted by a mechanism that is largely independent of its cell cycle regulation. These results indicate that Cdk6 is a critical regulator of BMP-2-induced osteoblast differentiation and that its Smads-mediated down-regulation is essential for efficient osteoblast differentiation.

Ubiquitin ligases and the immune response

Ubiquitin (Ub)-protein conjugation represents a novel means of posttranscriptional modification in a proteolysis-dependent or -independent manner. E3 Ub ligases play a key role in governing the cascade of Ub transfer reactions by recognizing and catalyzing Ub conjugation to specific protein substrates. The E3s, which can be generally classified into HECT-type and RING-type families, are involved in the regulation of many aspects of the immune system, including the development, activation, and differentiation of lymphocytes, T cell-tolerance induction, antigen presentation, immune evasion, and virus budding. E3-promoted ubiquitination affects a wide array of biological processes, such as receptor downmodulation, signal transduction, protein processing or translocation, protein-protein interaction, and gene transcription, in addition to proteasome-mediated degradation. Deficiency or mutation of some of the E3s like Cbl, Cbl-b, or Itch, causes abnormal immune responses such as autoimmunity, malignancy, and inflammation. This review discusses our current understanding of E3 Ub ligases in both innate and adaptive immunity. Such knowledge may facilitate the development of novel therapeutic approaches for immunological diseases.

Molecular recognition in bone morphogenetic protein (BMP)/receptor interaction

Bone morphogenetic proteins (BMPs) and other members of the TGF-β superfamily are secreted signalling proteins determining the development, maintenance and regeneration of tissues and organs. These dimeric proteins bind, via multiple epitopes, two types of signalling receptor chains and numerous extracellular modulator proteins that stringently control their activity. Crystal structures of free ligands and of complexes with type I and type II receptor extracellular domains and with the modulator protein Noggin reveal structural epitopes that determine the affinity and specificity of the interactions. Modelling of a ternary complex BMP/(BMPR-IAEC)2/(ActR-IIEC)2suggests a mechanism of receptor activation that does not rely on direct contacts between extracellular domains of the receptors. Mutational and interaction analyses indicate that the large hydrophobic core of the interface of BMP-2 (wrist epitope) with the type I receptor does not provide a hydrophobic hot spot for binding. Instead, main chain amide and carbonyl groups that are completely buried in the contact region represent major binding determinants. The affinity between ligand and receptor chains is probably strongly increased by two-fold interactions of the dimeric ligand and receptor chains that exist as homodimers in the membrane (avidity effects). BMP muteins with disrupted epitopes for receptor chains or modulator proteins provide clues for drug design and development.

Atrophin-1-interacting Protein 4/Human Itch Is a Ubiquitin E3 Ligase for Human Enhancer of Filamentation 1 in Transforming Growth Factor-β Signaling Pathways

Atrophin-1-interacting protein 4 (AIP4) is the human homolog of the mouse Itch protein (hItch), an E3 ligase for Notch and JunB. Human enhancer of filamentation 1 (HEF1) has been implicated in signaling pathways such as those mediated by integrin, T cell receptor, and B cell receptor and functions as a multidomain docking protein. Recent studies suggest that HEF1 is also involved in the transforming growth factor-beta (TGF-beta) signaling pathways, by interacting with Smad3, a key signal transducer downstream of the TGF-beta type I receptor. The interaction of Smad3 with HEF1 induces HEF1 proteasomal degradation, which was further enhanced by TGF-beta stimulation. The detailed molecular mechanisms of HEF1 degradation regulated by Smad3 were poorly understood. Here we report our studies that demonstrate the function of AIP4 as an ubiquitin E3 ligase for HEF1. AIP4 forms a complex with both Smad3 and HEF1 through its WW domains in a TGF-beta-independent manner and regulates HEF1 ubiquitination and degradation, which can be enhanced by TGF-beta stimulation. These findings reveal a new mechanism for Smad3-regulated proteasomal degradation events and also broaden the network of cross-talk between the TGF-beta signaling pathway and those involving HEF1 and AIP4.

Negative regulation of transforming growth factor-β (TGF-β) signaling by WW domain-containing protein 1 (WWP1)

Smad7 negatively regulates transforming growth factor (TGF)-beta superfamily signaling by binding to activated type I receptors, thereby preventing the phosphorylation of receptor-regulated Smads (R-Smads), as well as by recruiting HECT-type E3 ubiquitin ligases to degrade type I receptors through a ubiquitin-dependent mechanism. To elucidate the regulatory mechanisms of TGF-beta signaling, we searched for novel members of proteins that interact with Smad7 using a yeast two-hybrid system. One of the proteins identified was the WW domain-containing protein 1 (WWP1) that is structurally related to Smad ubiquitin regulatory factors (Smurfs), E3 ubiquitin ligases for Smads and TGF-beta superfamily receptors. Using a TGF-beta-responsive reporter in mammalian cells, we found that WWP1 inhibited transcriptional activities induced by TGF-beta. Similar to Smurfs, WWP1 associated with Smad7 and induced its nuclear export, and enhanced binding of Smad7 to TGF-beta type I receptor to cause ubiquitination and degradation of the receptor. Consistent with these results, WWP1 inhibited phosphorylation of Smad2 induced by TGF-beta. WWP1 thus negatively regulates TGF-beta signaling in cooperation with Smad7. However, unlike Smurfs, WWP1 failed to ubiquitinate R-Smads and SnoN. Importantly, WWP1 and Smurfs were expressed in distinct patterns in human tissues and carcinoma cell lines, suggesting unique pathophysiological roles of WWP1 and Smurfs.

Coordinate regulation of cell growth and differentiation by TGF-beta superfamily and Runx proteins

Runx proteins regulate various biological processes, including growth and differentiation of hematopoietic cells, lymphocytes, osteoblasts, and gastric epithelial cells. Some of the biological activities of Runx proteins are reminiscent of those of transforming growth factor (TGF)-beta superfamily cytokines. Consistent with this notion, receptor-regulated Smads (R-Smads), signal mediators of the TGF-beta superfamily cytokines, and Runx proteins have been shown to physically interact with each other. R-Smads activated by TGF-beta and Runx proteins cooperatively induce synthesis of IgA in B lymphocytes, and those activated by bone morphogenetic proteins and Runx2 induce osteoblastic differentiation. Moreover, the R-Smad-Runx signaling pathways are regulated by an E3 ubiquitin ligase Smurf1, as well as a signal transducer of interferons, STAT1. Since Runxl and Runx3 are involved in the development of some cancers including acute leukemia and gastric cancer, it will be of interest to examine in detail whether TGF-beta-specific R-Smads and Runx proteins coordinately regulate growth and differentiation of hematopoietic cells and gastric epithelial cells.

Down-regulation of Smad7 expression by ubiquitin-dependent degradation contributes to renal fibrosis in obstructive nephropathy in mice

Overexpression of transforming growth factor beta (TGF-beta) has been shown to play pathogenic roles in progression of renal fibrosis, and the severity of tubulointerstitial fibrosis correlates better with renal function than the severity of glomerulosclerosis. Smad proteins are signaling transducers downstream from TGF-beta receptors. Three families of Smad proteins have been identified: receptor-regulated Smad2 and Smad3, common partner Smad4, and inhibitory Smad7 (part of a negative-feedback loop). We investigated Smad-mediated TGF-beta signaling pathway and regulatory mechanisms of inhibitory Smad7 in unilateral ureteral obstruction (UUO) kidneys in mice, a model of progressive tubulointerstitial fibrosis. Compared with sham-operated kidneys, the level of Smad7 protein, but not mRNA, decreased progressively in UUO kidneys, whereas immunoreactivity for nuclear phosphorylated Smad2 and Smad3 and renal fibrosis were inversely increased. Furthermore, we demonstrated that both the degradation and ubiquitination activity of Smad7 protein were increased markedly in UUO kidneys compared with sham-operated ones. We also found that both Smurf1 and Smurf2 (Smad ubiquitination regulatory factors), which are E3 ubiquitin ligases for Smad7, were increased and that they interacted with Smad7 in UUO kidneys. Our results suggest that the reduction of Smad7 protein resulting from enhanced ubiquitin-dependent degradation plays a pathogenic role in progression of tubulointerstitial fibrosis.

The RING domain of PIASy is involved in the suppression of bone morphogenetic protein-signaling pathway

Bone morphogenetic proteins (BMPs) play central roles in differentiation, development, and physiologic tissue remodeling. Recently, we have demonstrated that a protein inhibitor of activated STAT, PIASy, suppresses TGF-beta signaling by interacting with Sma and MAD-related protein 3 (Smad3). In this study, we examined a PIASy-dependent inhibitory effect on BMP signaling. PIASy expression was induced by BMP-2 stimulation and suppressed BMP-2-dependent Smad activity in hepatoma cells. Furthermore, BMP-2-regulated Smads directly bound to PIASy. We also demonstrated that the RING domain of PIASy played an important role in PIASy-mediated suppression of Smad activity. We here provide evidence that the inhibitory action of PIASy on BMP-regulated Smad activity was due to direct physical interactions between Smads and PIASy through its RING domain.

Differential effects of the Ski-interacting protein (SKIP) on differentiation induced by transforming growth factor-β1 and bone morphogenetic protein-2 in C2C12 cells

The transforming growth factor-beta (TGF-beta) and bone morphogenetic proteins (BMP) are key regulatory factors that affect many critical cellular events in growth and development. Recently, we have shown that the Ski-interacting protein (SKIP) can augment TGF-beta signals. Here, we extended these studies by examining the biologic consequences of SKIP overexpression on TGF-beta1 and BMP-2 signals in C2C12 cells. C2C12 myoblasts differentiate into myotubes when the media is depleted of mitogenic factors, and TGF-beta1 inhibits this myotube formation. BMP-2 not only inhibits the myotube formation, but also induces C2C12 cells to differentiate into osteoblasts. Here, we show that SKIP-overexpressing C2C12 cells treated with TGF-beta1 or BMP-2 displayed no differences in comparison to vector control cells in their ability to form myotubes or in the expression of the myogenic markers myosin heavy chain-1 and myogenin. Unexpectedly, SKIP-overexpressing C2C12 cells treated with BMP-2 displayed suppressed expression of the induced osteoblast markers alkaline phosphatase, osteocalcin, and the transcription factor Runx2. Lastly, SKIP could repress transcription induced by BMP-2 in luciferase reporter assays done in C2C12 cells. These data show that SKIP has specific inhibitory effects on BMP-2-induced differentiation and implicate SKIP to be a novel regulator of the differentiation programming induced by TGF-beta signals.