Extracellular and cytoplasmic domains of endoglin interact with the transforming growth factor-beta receptors I and II

Endoglin is an auxiliary component of the transforming growth factor-beta (TGF-beta) receptor system, able to associate with the signaling receptor types I (TbetaRI) and II (TbetaRII) in the presence of ligand and to modulate the cellular responses to TGF-beta1. Endoglin cannot bind ligand on its own but requires the presence of the signaling receptors, supporting a critical role for the interaction between endoglin and TbetaRI or TbetaRII. This study shows that full-length endoglin interacts with both TbetaRI and TbetaRII, independently of their kinase activation state or the presence of exogenous TGF-beta1. Truncated constructs encoding either the extracellular or the cytoplasmic domains of endoglin demonstrated that the association with the signaling receptors occurs through both extracellular and cytoplasmic domains. However, a more specific mapping revealed that the endoglin/TbetaRI interaction was different from that of endoglin/TbetaRII. TbetaRII interacts with the amino acid region 437-558 of the extracellular domain of endoglin, whereas TbetaRI interacts not only with the region 437-558 but also with the protein region located between amino acid 437 and the N terminus. Both TbetaRI and TbetaRII interact with the cytoplasmic domain of endoglin, but TbetaRI only interacts when the kinase domain is inactive, whereas TbetaRII remains associated in its active and inactive forms. Upon association, TbetaRI and TbetaRII phosphorylate the endoglin cytoplasmic domain, and then TbetaRI, but not TbetaRII, kinase dissociates from the complex. Conversely, endoglin expression results in an altered phosphorylation state of TbetaRII, TbetaRI, and downstream Smad proteins as well as a modulation of TGF-beta signaling, as measured by the reporter gene expression. These results suggest that by interacting through its extracellular and cytoplasmic domains with the signaling receptors, endoglin might affect TGF-beta responses.     

Mimicry of erythropoietin and interleukin-6 signalling by an antibody/cytokine receptor chimera in murine myeloid 32D cells

We have previously designed antibody-cytokine receptor chimeras that could respond to a cognate antigen. While these chimeric receptors were functional, it has not been investigated exactly how they mimic signal transduction through corresponding wild-type receptors. In this study, we compared the growth properties and the phosphorylation status of intracellular signal transducers between the erythropoietin receptor (EpoR)- or gp130-based chimeric receptors and wild-type EpoR or EpoR-gp130 chimera, respectively. Expression plasmids, encoding V(H) or V(L) region of anti-hen egg lysozyme (HEL) antibody HyHEL-10 tethered to a pair of extracellular D2 domain of EpoR and transmembrane/cytoplasmic domains of either EpoR or gp130, were constructed, and pairs of chimeric receptor combinations (V(H)-EpoR and V(L)-EpoR, V(H)-gp130 and V(L)-gp130, V(H)-EpoR and V(L)-gp130, V(H)-gp130 and V(L)-EpoR) were expressed in an IL-3-dependent myeloid cell line, 32D. Growth assay revealed that the transfectants all grew in a HEL-dependent manner. As for phosphorylation of Stat3, Stat5, ERK and Akt, the chimeric receptors showed similar activation pattern of signalling molecules with wild-type receptors, although the chimeric receptors showed ligand-independency and a little lower maximal phosphorylation than the corresponding wild-type receptors. The results demonstrate that antibody-receptor chimeras could substantially mimic wild-type receptors.     

Construction of a fluorescein-responsive chimeric receptor with strict ligand dependency

Since many cell functions are regulated by members of the cytokine receptor superfamily, the artificial mimicry of the cytokine receptor system would be attractive for cellular engineering. We previously showed that an antibody/cytokine receptor chimera can transduce a growth signal in response to non-natural ligands, such as fluorescein-conjugated BSA. However, considerable background of cell proliferation was observed without antigen. Therefore, we redesigned chimeric receptor constructs with different combinations of domains containing anti-fluorescein single chain Fv (ScFv), extracellular D1/D2 as well as transmembrane domains of erythropoietin receptor (EpoR), and the intracellular domain of glycoprotein 130 (gp130), to obtain strictly fluorescein-dependent chimeric receptors. When interleukin-3-dependent Ba/F3 cells were transduced with retroviral vectors encoding individual chimeric receptors, the chimeras either with both D1 and D2 domains or without any EpoR extracellular domain attained a strict ligand-dependent ON/OFF regulation.     

In the absence of type III receptor, the transforming growth factor (TGF)-beta type II-B receptor requires the type I receptor to bind TGF-beta2

Transforming growth factor beta (TGF-beta) ligands exert their biological effects through type II (TbetaRII) and type I receptors (TbetaRI). Unlike TGF-beta1 and -beta3, TGF-beta2 appears to require the co-receptor betaglycan (type III receptor, TbetaRIII) for high affinity binding and signaling. Recently, the TbetaRIII null mouse was generated and revealed significant non-overlapping phenotypes with the TGF-beta2 null mouse, implying the existence of TbetaRIII independent mechanisms for TGF-beta2 signaling. Because a variant of the type II receptor, the type II-B receptor (TbetaRII-B), has been suggested to mediate TGF-beta2 signaling in the absence of TbetaRIII, we directly tested the ability of TbetaRII-B to bind TGF-beta2. Here we show that the soluble extracellular domain of the type II-B receptor (sTbetaRII-B.Fc) bound TGF-beta1 and TGF-beta3 with high affinity (K(d) values = 31.7 +/- 22.8 and 74.6 +/- 15.8 pm, respectively), but TGF-beta2 binding was undetectable at corresponding doses. Similar results were obtained for the soluble type II receptor (sTbetaRII.Fc). However, sTbetaRII.Fc or sTbetaRII-B.Fc in combination with soluble type I receptor (sTbetaRI.Fc) formed a high affinity complex that bound TGF-beta2, and this complex inhibited TGF-beta2 in a biological inhibition assay. These results show that TGF-beta2 has the potential to signal in the absence of TbetaRIII when sufficient TGF-beta2, TbetaRI, and TbetaRII or TbetaRII-B are present. Our data also support a cooperative model for receptor-ligand interactions, as has been suggested by crystallization studies of TGF-beta receptors and ligands. Our cell-free binding assay system will allow for testing of models of receptor-ligand complexes prior to actual solution of crystal structures.     

Positive and negative regulation of type II TGF-beta receptor signal transduction by autophosphorylation on multiple serine residues.

The type II transforming growth factor-beta (TGF-beta) receptor Ser/Thr kinase (TbetaRII) is responsible for the initiation of multiple TGF-beta signaling pathways, and loss of its function is associated with many types of human cancer. Here we show that TbetaRII kinase is regulated intricately by autophosphorylation on at least three serine residues. Ser213, in the membrane-proximal segment outside the kinase domain, undergoes intra-molecular autophosphorylation which is essential for the activation of TbetaRII kinase activity, activation of TbetaRI and TGF-beta-induced growth inhibition. In contrast, phosphorylation of Ser409 and Ser416, located in a segment corresponding to the substrate recognition T-loop region in a three-dimensional structural model of protein kinases, is enhanced by receptor dimerization and can occur via an intermolecular mechanism. Phosphorylation of Ser409 is essential for TbetaRII kinase signaling, while phosphorylation of Ser416 inhibits receptor function. Mutation of Ser416 to alanine results in a hyperactive receptor that is better able than wild-type to induce TbetaRI activation and subsequent cell cycle arrest. Since on a single receptor either Ser409 or Ser416, but not both simultaneously, can become autophosphorylated, our results show that TbetaRII phosphorylation is regulated intricately and affects TGF-beta receptor signal transduction both positively and negatively.     

Transforming growth factor-beta induces formation of a dithiothreitol-resistant type I/Type II receptor complex in live cells

Transforming growth factor-beta (TGF-beta) binds to and signals via two serine-threonine kinase receptors, the type I (TbetaRI) and type II (TbetaRII) receptors. We have used different and complementary techniques to study the physical nature and ligand dependence of the complex formed by TbetaRI and TbetaRII. Velocity centrifugation of endogenous receptors suggests that ligand-bound TbetaRI and TbetaRII form a heteromeric complex that is most likely a heterotetramer. Antibody-mediated immunofluorescence co-patching of epitope-tagged receptors provides the first evidence in live cells that TbetaRI. TbetaRII complex formation occurs at a low but measurable degree in the absence of ligand, increasing significantly after TGF-beta binding. In addition, we demonstrate that pretreatment of cells with dithiothreitol, which inhibits the binding of TGF-beta to TbetaRI, does not prevent formation of the TbetaRI.TbetaRII complex, but increases its sensitivity to detergent and prevents TGF-beta-activated TbetaRI from phosphorylating Smad3 in vitro. This indicates that either a specific conformation of the TbetaRI. TbetaRII complex, disrupted by dithiothreitol, or direct binding of TGF-beta to TbetaRI is required for signaling.     

A kinase subdomain of transforming growth factor-beta (TGF-beta) type I receptor determines the TGF-beta intracellular signaling specificity.

Transforming growth factor-beta (TGF-beta) signals through a heteromeric complex of related type I and type II serine/threonine kinase receptors. In Mv1Lu cells the type I receptor TbetaRI mediates TGF-beta-induced gene expression and growth inhibition, while the closely related type I receptors Tsk7L and TSR1 are inactive in these responses. Using chimeras between TbetaRI and Tsk7L or TSR1, we have defined the structural requirements for TGF-beta signaling by TbetaRI. The extracellular/transmembrane or cytoplasmic domains of TbetaRI and Tsk7L were functionally not equivalent. The juxtamembrane domain, including the GS motif, and most regions in the kinase domain can functionally substitute for each other, but the alphaC-beta4-beta5 region from kinase subdomains III to V conferred a distinct signaling ability. Replacement of this sequence in TbetaRI by the corresponding domain of Tsk7L inactivated TGF-beta signaling, whereas its introduction into Tsk7L conferred TGF-beta signaling. The differential signaling associated with this region was narrowed down to a sequence of eight amino acids, the L45 loop, which is exposed in the three-dimensional kinase structure and diverges highly between TbetaRI and Tsk7L or TSR1. Replacement of the L45 sequence in Tsk7L with that of TbetaRI conferred TGF-beta responsiveness to the Tsk7L cytoplasmic domain in Mv1Lu cells. Thus, the L45 sequence between kinase subdomains IV and V specifies TGF-beta responsiveness of the type I receptor.     

A dominant inhibitory mutant of the type II transforming growth factor beta receptor in the malignant progression of a cutaneous T-cell lymphoma.

In many cancers, inactivating mutations in both alleles of the transforming growth factor beta (TGF-beta) type 11 receptor (TbetaRII) gene occur and correlate with loss of sensitivity to TGF-beta. Here we describe a novel mechanism for loss of sensitivity to growth inhibition by TGF-beta in tumor development. Mac-1 cells, isolated from the blood of a patient with an indolent form of cutaneous T-cell lymphoma, express wild-type TbetaRII and are sensitive to TGF-beta. Mac-2A cells, clonally related to Mac-1 and isolated from a skin nodule of the same patient at a later, clinically aggressive stage of lymphoma, are resistant to TGF-beta. They express both the wild-type TbetaRII and a receptor with a single point mutation (Asp-404-Gly [D404G]) in the kinase domain (D404G-->TbetaRII); no TbetaRI or TbetaRII is found on the plasma membrane, suggesting that D404G-TbetaRII dominantly inhibits the function of the wild-type receptor by inhibiting its appearance on the plasma membrane. Indeed, inducible expression, under control of a tetracycline-regulated promoter, of D404G-TbetaRII in TGF-beta- sensitive Mac-1 cells as well as in Hep3B hepatoma cells results in resistance to TGF-beta and disappearance of cell surface TbetaRI and TbetaRII. Overexpression of wild-type TbetaRII in Mac-2A cells restores cell surface TbetaRI and TbetaRH and sensitivity to TGF-beta. The ability of the D404G-TbetaRH to dominantly inhibit function of wild-type TGF-beta receptors represents a new mechanism for loss of sensitivity to the growth-inhibitory functions of TGF-beta in tumor development.     

Active conformation of the erythropoietin receptor: random and cysteine-scanning mutagenesis of the extracellular juxtamembrane and transmembrane domains

In the absence of erythropoietin (Epo) cell surface Epo receptors (EpoR) are dimeric; dimerization is mediated mainly by the transmembrane domain. Binding of Epo changes the orientation of the two receptor subunits. This conformational change is transmitted through the juxtamembrane and transmembrane domains, leading to activation of JAK2 kinase and induction of proliferation and survival signals. To define the active EpoR conformation(s) we screened libraries of EpoRs with random mutations in the transmembrane domain and identified several point mutations that activate the EpoR in the absence of ligand, including changes of either of the first two transmembrane domain residues (Leu(226) and Ile(227)) to cysteine. Following this discovery, we performed cysteine-scanning mutagenesis in the EpoR juxtamembrane and transmembrane domains. Many mutants formed disulfide-linked receptor dimers, but only EpoR dimers linked by cysteines at positions 223, 226, or 227 activated EpoR signal transduction pathways and supported proliferation of Ba/F3 cells in the absence of cytokines. These data suggest that activation of dimeric EpoR by Epo binding is achieved by reorienting the EpoR transmembrane and the connected cytosolic domains and that certain disulfide-bonded dimers represent the activated dimeric conformation of the EpoR, constitutively activating downstream signaling. Based on our data and the previously determined structure of Epo bound to a dimer of the EpoR extracellular domain, we present a model of the active and inactive conformations of the Epo receptor.     

A functional green fluorescent protein-erythropoietin receptor despite physical separation of JAK2 binding site and tyrosine residues

Signaling through hematopoietic cytokine receptors such as the erythropoietin receptor (EpoR) depends on the activation of a receptor-bound Janus kinase (JAK) and tyrosine phosphorylation of the cytoplasmic domain. To visualize the EpoR and elucidate structural requirements coordinating signal transduction, we probed the EpoR by inserting the green fluorescent protein (GFP) at various positions. We show that insertion of GFP in proximity to the transmembrane domain, either in the extracellular or the cytoplasmic domain, results in EpoR-GFP receptors incompetent to elicit biological responses in a factor-dependent cell line or in erythroid progenitor cells. Surprisingly, a receptor harboring GFP insertion in the middle of the cytoplasmic domain, and thereby separating the JAK2 binding site from the tyrosine residues, is capable of supporting signal transduction in response to ligand binding. Comparable with the wild type EpoR, but more efficient than a C-terminal EpoR-GFP fusion, this chimeric receptor promotes the maturation of erythroid progenitor cells and is localized in punctated endosome-like structures. We conclude that the extracellular, transmembrane, and membrane-proximal segment of the cytoplasmic domain form a rigid structural entity whose precise orientation is essential for the initiation of signal transduction, whereas the cytoplasmic domain possesses flexibility in adopting an activated conformation.     

Syndecan-2 regulates transforming growth factor-beta signaling

Transforming growth factor-beta (TGF-beta) has multiple functions including increasing extracellular matrix deposition in fibrosis. It functions through a complex family of cell surface receptors that mediate downstream signaling. We report here that a transmembrane heparan sulfate proteoglycan, syndecan-2 (S2), can regulate TGF-beta signaling. S2 protein increased in the renal interstitium in diabetes and regulated TGF-beta-mediated increased matrix deposition in vitro. Transfection of renal papillary fibroblasts with S2 or a S2 construct that has a truncated cytoplasmic domain (S2DeltaS) promoted TGF-beta binding and S2 core protein ectodomain directly bound TGF-beta. Transfection with S2 increased the amounts of type I and type II TGF-beta receptors (TbetaRI and TbetaRII), whereas S2DeltaS was much less effective. In contrast, S2DeltaS dramatically increased the level of type III TGF-beta receptor (TbetaRIII), betaglycan, whereas S2 resulted in a decrease. Syndecan-2 specifically co-immunoprecipitated with betaglycan but not with TbetaRI or TbetaRII. This is a novel mechanism of control of TGF-beta action that may be important in fibrosis.     

Antigen-mediated migration of murine pro-B Ba/F3 cells via an antibody/receptor chimera

Cell migration is one of the fundamental cellular responses governing development, homeostasis and disorders of the body. Therefore, artificial control of cell migration holds great promise for the treatment of many diseases. In this study, we developed an artificial cell migration system based on chimeric receptors that can respond to an artificial ligand that is quite different from natural chemoattractants. Chimeric receptors consisting of an anti-fluorescein single-chain Fv tethered to the extracellular D2 domain of erythropoietin receptor (EpoR) and the transmembrane/cytoplasmic domains of EpoR, gp130, interleukin-2 receptor, c-Kit, c-Fms, epidermal growth factor receptor (EGFR) or insulin receptor were expressed in the murine Ba/F3 pro-B cell line. Migration assays revealed that chimeric receptors containing the cytoplasmic domain of c-Kit, c-Fms or EGFR transduced migration signals in response to fluorescein-conjugated bovine serum albumin (BSA-FL). Furthermore, based on the cell migration in response to BSA-FL, we successfully selected genetically modified cells from mixtures of gene-transduced and untransduced cells. This study represents the first demonstration of cell migration in response to an artificial ligand that is quite different from natural chemoattractants, suggesting its potential application to immunotherapies and tissue engineering.     

Albumin endocytosis in endothelial cells induces TGF-beta receptor II signaling

Vascular endothelial cells undergo albumin endocytosis using a set of albumin binding proteins. This process is important for maintaining cellular homeostasis. We showed by several criteria that the previously described 73-kDa endothelial cell surface albumin binding protein is the 75-kDa transforming growth factor (TGF)-beta receptor type II (TbetaRII). Albumin coimmunoprecipitated with TbetaRII from a membrane fraction from rat lung microvascular endothelial cells. Albumin endocytosis-negative COS-7 cells became albumin endocytosis competent when transfected with wild-type TbetaRII but not when transfected with a domain-negative kinase mutant of TbetaRII. An antibody specific for TbetaRII inhibited albumin endocytosis. A mink lung epithelial cell line, which expresses both the TGF-beta receptor type I (TbetaRI) and the TbetaRII receptor, exhibited albumin binding to the cell surface and endocytosis. In contrast, mutant L-17 and DR-26 cells lacking TbetaRI or TbetaRII, respectively, each showed a dramatic reduction in binding and endocytosis. Albumin endocytosis induced Smad2 phosphorylation and Smad4 translocation as well as increased protein expression of the inhibitory Smad, Smad7. We identified regions of significant homology between amino acid sequences of albumin and TGF-beta, suggesting a structural basis for the interaction of albumin with the TGF-beta receptors and subsequent activation of TbetaRII signaling. The observed albumin-induced internalization of TbetaRII signaling may be an important mechanism in the vessel wall for controlling TGF-beta responses in endothelial cells.     

Constitutive homo- and hetero-oligomerization of TbetaRII-B, an alternatively spliced variant of the mouse TGF-beta type II receptor

Transforming growth factor (TGF)-beta ligands signal through transmembrane type I and type II serine/threonine kinase receptors, which form heteromeric signalling complexes upon ligand binding. Type II TGF-beta receptors (TbetaRII) are reported to exist as homodimers at the cell surface, but the oligomerization pattern and dynamics of TbetaRII splice variants in live cells has not been demonstrated thus far. Using co-immunoprecipitation and bioluminescence resonance energy transfer (BRET), we demonstrate that the mouse TbetaRII receptor splice variant TbetaRII-B is capable of forming ligand-independent homodimers and heterodimers with TbetaRII. The homomeric interaction of mouse (m)TbetaRII-B isoforms, however, is less robust than the heteromeric interactions of mTbetaRII-B with wild-type TbetaRII, which indicates that these receptors may be more likely to heterodimerize when both receptors are expressed. Moreover, we demonstrate that mTbetaRII-B is a signalling receptor with ubiquitous tissue expression. Our study thus demonstrates previously unappreciated complex formation of TGF-beta type II receptors, and suggests that mTbetaRII-B can direct TGF-beta-induced signalling in vitro and in vivo.     

Growth control of hybridoma cells with an artificially induced EpoR-gp130 heterodimer

IL-6 has been known to modulate the growth of many hybridoma cells and also promote resultant antibody productivity. However, IL-6 is so expensive that the use of IL-6-dependent hybridomas for industrial antibody production is not practical. In this study, we aimed at designing antibody/gp130 and antibody/EpoR chimeras which could tightly control cell growth in response to more affordable cognate antigen. Retroviral vectors encoding V_H or V_L region of anti-hen egg lysozyme (HEL) antibody HyHEL-10 tethered to a pair of extracellular D2/transmembrane domains of erythropoietin receptor (EpoR) and cytoplasmic domains of either EpoR or gp130, were constructed, and a homodimeric or a heterodimeric pair of chimeric receptor combinations (V_H-gp130 and V_L-gp130 or V_H-gp130 and V_L-EpoR) were expressed in an IL-6-dependent hybridoma 7TD1. The chimeric receptor-derived growth signal was observed in both combinations, while some residual growth signal was observed in the absence of HEL. To reduce interchain interaction between the two receptor chains, we introduced mutations to the transmembrane domain of both chimera combinations. Consequently, the heterodimeric combination of V_H-gp130 and V_L-EpoR showed clear HEL-dependent cell growth, while the homodimeric combination of V_H-gp130 and V_L-gp130 showed reduced cell growth in the absence of HEL. This is the first report that an EpoR-gp130 cytoplasmic domain heterodimer could transduce a growth signal in hybridoma cells, indicating tight and economical growth control of hybridoma cells via our chimeric receptors.     

Antiapoptotic effects of erythropoietin in differentiated neuroblastoma SH-SY5Y cells require activation of both the STAT5 and AKT signaling pathways

The hematopoietic cytokine erythropoietin (Epo) prevents neuronal death during ischemic events in the brain and in neurodegenerative diseases, presumably through its antiapoptotic effects. To explore the role of different signaling pathways in Epo-mediated antiapoptotic effects in differentiated human neuroblastoma SH-SY5Y cells, we employed a prolactin receptor (PrlR)/erythropoietin receptor (EpoR) chimera system, in which binding of prolactin (Prl) to the extracellular domain activates EpoR signaling in the cytosol. On induction of apoptosis by staurosporine, Prl supports survival of the SH-SY5Y cells expressing the wild-type PrlR/EpoR chimera. In these cells Prl treatment strongly activates the STAT5, AKT, and MAPK signaling pathways and induces weak activation of the p65 NF-kappaB factor. Selective mutation of the eight tyrosine residues of the EpoR cytoplasmic domain results in impaired or absent activation of either STAT5 (mutation of Tyr(343)) or AKT (mutation of Tyr(479)) or both (mutation of all eight tyrosine residues). Most interestingly, Prl treatment does not prevent apoptosis in cells expressing mutant PrlR/EpoR chimeras in which either the STAT5 or the AKT signaling pathways are not activated. In contrast, ERK 1/2 is fully activated by all mutant PrlR/EpoR chimeras, comparable with the level seen with the wild-type PrlR/EpoR chimera, implying that activation of the MAPK signaling pathway per se is not sufficient for antiapoptotic activity. Therefore, the antiapoptotic effects of Epo in neuronal cells require the combinatorial activation of multiple signaling pathways, including STAT5, AKT, and potentially MAPK as well, in a manner similar to that observed in hematopoietic cells.     

Construction of antibody/insulin receptor chimera for growth induction of mammalian cells

The insulin receptor (IR) is expressed ubiquitously in various tissues, where insulin exerts various biological effects on the target cells, such as cellular metabolic changes, cell proliferation and differentiation. Therefore, mimicry of insulin signaling would be a promising strategy to realize artificial control of such cellular fates. In this study, we constructed an antibody/insulin receptor chimera that enables to utilize any antigen as the ligand in principle. We constructed chimeric receptors consisting of anti-fluorescein single chain Fv (scFv), the extracellular D2 domain of erythropoietin receptor and the transmembrane/intracellular domains of IR (scFv-IR; S-IR). The function of S-IR was evaluated in terms of growth signal transduction in murine pro-B Ba/F3 cells and murine fibroblast NIH/3T3 cells. S-IR exerted IL-3-independent cell growth in Ba/F3 cells, while NIH/3T3 cells expressing S-IR acquired growth advantage over parental NIH/3T3 cells in a low-serum condition. S-IR induced phosphorylation of S-IR itself and key signaling molecules downstream of IR. Although antigen-independent activation was significantly observed, S-IR enabled specific amplification of the gene-transduced cells.