Specific activation of Smad1 signaling pathways by the BMP7 type I receptor, ALK2

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

The TGF-beta superfamily of proteins regulates many different biological processes, including cell growth, differentiation and embryonic pattern formation. TGF-beta-like factors signal across cell membranes through complexes of transmembrane receptors known as type I and type II serine/threonine-kinase receptors, which in turn activate the SMAD signalling pathway. On the inside of the cell membrane, a receptor-regulated class of SMADs are phosphorylated by the type-I-receptor kinase. In this way, receptors for different factors are able to pass on specific signals along the pathway: for example, receptors for bone morphogenetic protein (BMP) target SMADs 1, 5 and 8, whereas receptors for activin and TGF-beta target SMADs 2 and 3. Phosphorylation of receptor-regulated SMADs induces their association with Smad4, the 'common-partner' SMAD, and stimulates accumulation of this complex in the nucleus, where it regulates transcriptional responses. Here we describe Smurf1, a new member of the Hect family of E3 ubiquitin ligases. Smurf1 selectively interacts with receptor-regulated SMADs specific for the BMP pathway in order to trigger their ubiquitination and degradation, and hence their inactivation. In the amphibian Xenopus laevis, Smurf1 messenger RNA is localized to the animal pole of the egg; in Xenopus embryos, ectopic Smurf1 inhibits the transmission of BMP signals and thereby affects pattern formation. Smurf1 also enhances cellular responsiveness to the Smad2 (activin/TGF-beta) pathway. Thus, targeted ubiquitination of SMADs may serve to control both embryonic development and a wide variety of cellular responses to TGF-beta signals.

Functional antagonism between activin and osteogenic protein-1 in human embryonal carcinoma cells

Activin A and osteogenic protein-1 (OP-1) exerted antagonistic effects on each other's responses on the human Tera-2 embryonal carcinoma cell line. OP-1 dose dependently inhibited activin A-induced activation of p3TP-Lux transcriptional reporter, containing part of the human plasminogen activator inhibitor-1 (PAI-1) promoter, while activin A inhibited OP-1-mediated alkaline phosphatase induction. Approximately equimolar concentrations of both growth factors resulted in 50% inhibition of the respective biological responses. Affinity cross-linking studies using 125I-activin A or 125I-OP-1 followed by receptor-immunoprecipitations revealed that both ligands bound to the activin type II receptor (ActR-II), but recruited different type I receptors. In addition, OP-1 competed with binding of 125I-activin A, and activin A competed with binding of 125I-OP-1 to ActR-II. Transient transfection studies showed that competition between activin A and OP-1 also occurred at the type I receptor (ActR-1) level; constitutively active (CA)-ActR-I inhibited CA-ActR-IB-mediated p3TP-Lux reporter induction. There was no competition between activin A and OP-1 for availability of Smad4, indicating that the concentration of this common signal transducer is not limiting for generating the observed biological responses. Overexpression of ActR-II abolished the inhibitory effect of OP-1 on activin A-induced p3TP-Lux activation and, surprisingly, led to OP-1-induced transcriptional reporter activity. Whereas the exact mechanism of competition is unclear, the role of ActR-II in the competition between activin A and OP-1 is discussed in light of the observed interference in downstream signaling by CA-ActR-I and CA-ActR-IB.

Signal transduction by bone morphogenetic protein receptors: functional roles of Smad proteins

Intracellular signals for bone morphogenetic proteins (BMPs) and other members in the transforming growth factor (TGF)-beta superfamily are mediated by Smad proteins. Receptor-regulated Smads (R-Smads) are activated by serine/threonine kinase receptors upon ligand binding. R-Smads then form hetero-oligomeric complexes with a common-mediator Smad (co-Smad) and translocate into the nucleus, where they regulate transcription of target genes. Smads 1, 5, and 8 are R-Smads activated by BMP receptors, whereas Smads 2 and 3 are activated by TGF-beta and activin receptors. Smad4 is the only co-Smad isolated in mammals, and is shared by BMP and TGF-beta/activin signaling pathways. Smads 6 and 7 are anti-Smads, which block signals by preventing the activation of R-Smads by serine/threonine kinase receptors. Anti-Smads are induced by ligand stimulation, suggesting that they constitute a negative feedback loop in the signal transduction pathways of the TGF-beta superfamily.

Localized expression of BMP and GDF mRNA in the rodent brain

Expression of BMP- and GDF-related factors within the transforming growth factor-beta (TGF-beta) superfamily was examined in the rat and mouse brain by in situ hybridization. Strong signals were obtained in neurons for GDF-1 and GDF-10. GDF-1 is expressed at postnatal day 6 in the cerebral cortex, hippocampal CA1 through CA3 neurons, while only weakly expressed by cells in the dentate gyrus. Granule cells and neurons in the polymorph layer of the dentate gyrus are GDF-1-positive, as are the majority of neurons in the cortex. GDF-10 shows a distinct pattern of expression: At P6, strong labelling was seen in the superficial layers of cortex, notably in the posterior cingulate cortex, and in CA3 and dentate gyrus. From postnatal day 21, GDF-1 expression is strong in the hippocampus, cortex, and thalamic nuclei, while GDF-10 expression becomes restricted to the granule cell layer in the dentate gyrus. In contrast, OP-1 expression is restricted throughout development to cells of the medial habenular nucleus, choroid plexus, and leptomeninges. The markedly different expression patterns of these BMPs suggest they serve separate functions in the brain.

Differential inhibition of Smad6 and Smad7 on bone morphogenetic protein- and activin-mediated growth arrest and apoptosis in B cells

Smad6 and Smad7 prevent ligand-induced activation of signal-transducing Smad proteins in the transforming growth factor-beta family. Here we demonstrate that both Smad6 and Smad7 are human bone morphogenetic protein-2 (hBMP-2)-inducible antagonists of hBMP-2-induced growth arrest and apoptosis in mouse B cell hybridoma HS-72 cells. Moreover, we confirmed that the ectopic expressions of Smad6 and Smad7 inhibited the hBMP-2-induced Smad1/Smad5 phosphorylation. We previously reported that Smad7 is an activin A-inducible antagonist of activin A-induced growth arrest and apoptosis in HS-72 cells. Interestingly, although mRNA expression of Smad6 was induced by activin A in HS-72 cells, Smad6 showed no antagonistic effect on activin A-induced growth arrest and apoptosis. Moreover, we found that the ectopic expression of Smad7, but not Smad6, inhibited the activin A-induced Smad2 phosphorylation in HS-72 cells. Thus, Smad6 and Smad7 exhibit differential inhibitory effects in bone morphogenetic protein-2- and activin A-mediated signaling in B lineage cells.

Homeobox proteins as signal transduction intermediates in regulation of NCAM expression by recombinant human bone morphogenetic protein-2 in osteoblast-like cells

The role of homeobox genes in signaling of recombinant human bone morphogenetic protein-2 (rhBMP-2) was studied in osteoblast-like cells. Expression of several homeobox genes was decreased by rhBMP-2. The finding that this regulation of homeobox gene expression by rhBMP-2 was not dependent on protein synthesis suggests that homeobox proteins can act as direct intermediates in signal transduction of BMPs. Therefore, we studied the regulation of neural cell adhesion molecule (NCAM), which has previously been described as a target gene of both rhBMP-2 and homeobox proteins. We now show that in osteoblast-like cells, rhBMP-2 inhibits NCAM expression, while HOXC6 increases its expression, both acting via the same region of the promoter. As overexpression of HOXC6 could abolish effects of rhBMP-2 on NCAM promoter activity, these data show for the first time that members of the homeobox gene family may form direct functional intermediates in the signaling mechanism of the TGF-beta superfamily.

Smad5 knockout mice die at mid-gestation due to multiple embryonic and extraembryonic defects

Smad5 has been implicated as a downstream signal mediator for several bone morphogenetic proteins (BMPs). To understand the in vivo function of Smad5, we generated mice deficient in Smad5 using embryonic stem (ES) cell technology. Homozygous mutant embryos die between E9.5 and E11.5, and display variable phenotypes. Morphological defects are first detected at E8.0 in the developing amnion, gut and heart (the latter defect being similar to BMP-2 knockout mice). At later stages, mutant embryos fail to undergo proper turning, have craniofacial and neural tube abnormalities, and are edematous. In addition, several extraembryonic lesions are observed. After E9.0, the yolk sacs of the mutants contain red blood cells but lack a well-organized vasculature, which is reminiscent of BMP-4, TGF-beta1 and TGF-beta type II receptor knockout mice. In addition, the allantois of many Smad5 mutants is fused to the chorion, but is not well-elongated. A unique feature of the Smad5 mutant embryos is that ectopic vasculogenesis and hematopoiesis is observed in the amnion, likely due to mislocation of allantois tissue. Despite the expression of Smad5 from gastrulation onwards, and in contrast to knockouts of Smad2 and Smad4, Smad5 only becomes essential later in extraembryonic and embryonic development.

A Smad transcriptional corepressor

Following TGFbeta receptor-mediated phosphorylation and association with Smad4, Smad2 moves into the nucleus, binds to target promoters in association with DNA-binding cofactors, and recruits coactivators such as p300/CBP to activate transcription. We identified the homeodomain protein TGIF as a Smad2-binding protein and a repressor of transcription. A TGFbeta-activated Smad complex can recruit TGIF and histone deacetylases (HDACs) to a Smad target promoter, repressing transcription. Thus, upon entering the nucleus, a Smad2-Smad4 complex may interact with coactivators, forming a transcriptional activation complex, or with TGIF and HDACs, forming a transcriptional repressor complex. Formation of one of these two mutually exclusive complexes is determined by the relative levels of Smad corepressors and coactivators within the cell.

TAKs, thylakoid membrane protein kinases associated with energy transduction

The phosphorylation of proteins within the eukaryotic photosynthetic membrane is thought to regulate a number of photosynthetic processes in land plants and algae. Both light quality and intensity influence protein kinase activity via the levels of reductants produced by the thylakoid electron transport chain. We have isolated a family of proteins called TAKs, Arabidopsis thylakoid membrane threonine kinases that phosphorylate the light harvesting complex proteins. TAK activity is enhanced by reductant and is associated with the photosynthetic reaction center II and the cytochrome b6f complex. TAKs are encoded by a gene family that has striking similarity to transforming growth factor beta receptors of metazoans. Thus thylakoid protein phosphorylation may be regulated by a cascade of reductant-controlled membrane-bound protein kinases.

Smad1 recognition and activation by the ALK1 group of transforming growth factor-beta family receptors

Two structural elements, the L45 loop on the kinase domain of the transforming growth factor-beta (TGF-beta) family type I receptors and the L3 loop on the MH2 domain of Smad proteins, determine the specificity of the interactions between these receptors and Smad proteins. The L45 sequence of the TGF-beta type I receptor (TbetaR-I) specifies Smad2 interaction, whereas the related L45 sequence of the bone morphogenetic protein (BMP) type I receptor (BMPR-I) specifies Smad1 interactions. Here we report that members of a third receptor group, which includes ALK1 and ALK2 from vertebrates and Saxophone from Drosophila, specifically phosphorylate and activate Smad1 even though the L45 sequence of this group is very divergent from that of BMPR-I. We investigated the structural elements that determine the specific recognition of Smad1 by ALK1 and ALK2. In addition to the receptor L45 loop and the Smad1 L3 loop, the specificity of this recognition requires the alpha-helix 1 of Smad1. The alpha-helix 1 is a conserved structural element located in the vicinity of the L3 loop on the surface of the Smad MH2 domain. Thus, Smad1 recognizes two distinct groups of receptors, the BMPR-I group and the ALK1 group, through different L45 sequences on the receptor kinase domain and a differential use of two surface structures on the Smad1 MH2 domain.

Endoglin is an accessory protein that interacts with the signaling receptor complex of multiple members of the transforming growth factor-beta superfamily

Endoglin (CD105) is a transmembrane glycoprotein that binds transforming growth factor (TGF)-beta1 and -beta3, and coprecipitates with the Ser/Thr kinase signaling receptor complex by affinity labeling of endothelial and leukemic cells. The present study shows that in addition to TGF-beta1 and -beta3, endoglin interacts with activin-A, bone morphogenetic protein (BMP)-7, and BMP-2 but requires coexpression of the respective ligand binding kinase receptor for this association. Endoglin cannot bind ligands on its own and does not alter binding to the kinase receptors. It binds TGF-beta1 and -beta3 by associating with the TGF-beta type II receptor and interacts with activin-A and BMP-7 via activin type II receptors, ActRII and ActRIIB, regardless of which type I receptor partner is coexpressed. However, endoglin binds BMP-2 by interacting with the ligand binding type I receptors, ALK3 and ALK6. The formation of heteromeric signaling complexes was not altered by the presence of endoglin, although it was coprecipitated with these complexes. Endoglin did not interact with BMP-7 through complexes containing the BMP type II receptor, demonstrating specificity of its action. Our data suggest that endoglin is an accessory protein of multiple kinase receptor complexes of the TGF-beta superfamily.

SARA, a FYVE domain protein that recruits Smad2 to the TGFbeta receptor

Smads transmit signals from transmembrane ser/thr kinase receptors to the nucleus. We now identify SARA (for Smad anchor for receptor activation), a FYVE domain protein that interacts directly with Smad2 and Smad3. SARA functions to recruit Smad2 to the TGFbeta receptor by controlling the subcellular localization of Smad2 and by interacting with the TGFbeta receptor complex. Phosphorylation of Smad2 induces dissociation from SARA with concomitant formation of Smad2/Smad4 complexes and nuclear translocation. Furthermore, mutations in SARA that cause mislocalization of Smad2 inhibit TGFbeta-dependent transcriptional responses, indicating that the regulation of Smad localization is important for TGFbeta signaling. These results thus define SARA as a component of the TGFbeta pathway that brings the Smad substrate to the receptor.