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EVOLUTIONARY HOMOLOGS part 1/3

Type I TGF-beta and activin receptors: Functions of different type I receptors

The TGFbeta family member activin induces different mesodermal cell types in a dose-dependent fashion in the Xenopus animal cap assay. High concentrations of activin induce dorsal and anterior cell types such as notochord and muscle, while low concentrations induce ventral and posterior tissues such as mesenchyme and mesothelium. Does this threshold phenomenon involve the differential effects of the two type I activin receptors ALK-2 and ALK-4? Injection of RNA encoding constitutively active forms of the receptors (here designated ALK-2* and ALK-4*) reveals that ALK-4* strongly induces the more posterior mesodermal marker Xbra (Drosophila homolog: Brachyenteron) and the dorsoanterior marker goosecoid in animal cap explants. Maximal levels of Xbra expression are attained using lower concentrations of RNA than are required for the strongest activation of goosecoid (Drosophila homolog: Goosecoid); at the highest doses of ALK-4*, levels of Xbra transcription decrease, as is seen with high concentrations of activin. By contrast, the ALK-2* receptor activates Xbra but fails to induce goosecoid to significant levels. At later stages, ALK-4* signaling induces the formation of a variety of mesodermal derivatives, including dorsal cell types, in a dose-dependent fashion; high levels also induce endoderm. By contrast, the ALK-2* receptor induces only ventral mesodermal markers. Consistent with these observations, ALK-4* is capable of inducing a secondary axis when injected into the ventral side of 32-cell stage embryos while ALK-2* cannot. Co-injection of RNAs encoding constitutively active forms of both receptors reveals that ventralizing signals from ALK-2* antagonize the dorsal mesoderm-inducing signal derived from ALK-4*, suggesting that the two receptors use distinct and interfering signaling pathways. Together, these results show that although ALK-2* and ALK-4* transduce distinct signals, the threshold responses characteristic of activin cannot be due to interactions between these two pathways; rather, thresholds can be established by ALK-4* alone. The effects of ALK-2* signaling are at odds with it behaving as an activin receptor in the early Xenopus embryo (Armes, 1997).

Zebrafish activin receptor-like kinase-8 (zALK-8) is a novel type I serine/threonine (ser/thr) kinase receptor of the transforming growth factor beta (TGF-beta) family. zALK-8 is novel, in that it contains an extracellular domain that is quite distinct from that of previously identified ALK receptors 1 through 7. Analysis of the predicted amino acid sequence of the 506 amino acid zALK-8 receptor reveals a ser/thr kinase domain characteristic of type I TGF-beta family member receptors. zALK-8, therefore, is a traditional type I ser/thr kinase receptor of the TGF-beta family, but it may exhibit novel ligand-binding activities. The developmental expression of zALK-8 mRNA was examined by wholemount in situ hybridization analysis using a probe from the 3'-untranslated sequence of zALK-8, which does not cross react with other members of the highly conserved TGF-beta receptor family. zALK-8 mRNA is present as a maternal message that is expressed ubiquitously before the start of zygotic transcription. By 16 hr postfertilization (hpf), zALK-8 mRNA is still expressed fairly evenly throughout the embryo. In 24-hpf embryos, zALK-8 mRNA is expressed predominantly in the developing eye and neural structures. By 48 hpf, zALK-8 mRNA is faintly detectable as a diffuse signal throughout the head. zALK-8 mRNA is not detectable by this method in 72-hpf or 96-hpf embryos. Northern analysis of zALK-8 mRNA in poly(A+) mRNA isolated from 6-9 hpf embryos detects a major transcript of 3.6 kb and a minor transcript of 4.3 kb. zALK-8 mRNA expression correlates well with known functions of TGF-beta family members as early axial patterning and mesoderm-inducing growth factors and as potent growth and differentiation factors in craniofacial development (Yelick, 1998).

ALK-1 is a type I serine/threonine kinase receptor for members of the TGF-beta superfamily of growth factors; its endogenous ligand is not known. Temporal and spatial expression pattern of ALK-1 mRNA was examined in mouse embryos from the one-cell zygote until 12.5 dpc using RT-PCR and in situ hybridization. ALK-1 mRNA is first detected in the embryo at 6.5 dpc. From 7.5-8.5 dpc expression was highest at sites of vasculogenesis in both the embryonic and extraembryonic part of the conceptus, in trophoblast giant cells, and in the endothelial lining of the blood vessels in the decidua. From 9.5-12.5 dpc, ALK-1 is found to be expressed in several different tissues and organs, but is highest in blood vessels, mesenchyme of the lung, submucosal layer of the stomach and intestines, and at specific sites of epithelial-mesenchymal interactions. Its expression pattern suggests that ALK-1 is a type I receptor for TGF-beta1 in the developing mouse (Roelen, 1997).

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, the structural requirements for TGF-beta signaling by TbetaRI have been defined. The extracellular/transmembrane or cytoplasmic domains of TbetaRI and Tsk7L are not functionally equivalent. The juxtamembrane domain, including the GS motif, and most regions in the kinase domain can functionally substitute for each other, but regions from kinase subdomains confer a distinct signaling ability. Replacement of these sequencez in TbetaRI by the corresponding domain of Tsk7L inactivates TGF-beta signaling, whereas its introduction into Tsk7L confers TGF-beta signaling. The differential signaling associated with this region is found in 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 confers 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 (Feng, 1997).

Members of the transforming growth factor-beta (TGF-beta) superfamily are secreted proteins that interact with cell-surface receptors to elicit signals that regulate a variety of biological processes during vertebrate embryogenesis. Alk2, also known as ActRIA, Tsk7L, and SKR1, encodes a type I TGF-beta family receptor for activins and BMP-7. Initially, Alk2 transcripts are detected in the visceral endoderm of gastrula stage mouse embryos, suggesting a signaling role in extraembryonic tissues during development. To study the role of Alk2 during mammalian development, Alk2 mutant mice were generated. After embryonic day 9.5 (E9.5), no homozygous mutants were recovered from heterozygote matings. Homozygous mutants with morphological defects were first detected at E7.0 and were smaller than controls. Morphological and molecular examination demonstrate that Alk2 mutant embryos form a primitive streak, although abnormally thickened, and are arrested in their development around the late streak stage. These gastrulation defects are rescued in chimeric embryos generated by injection of Alk2 mutant embryonic stem (ES) cells into wild-type blastocysts. This rescue of gastrulation defects is also observed in chimeric embryos generated by aggregation of Alk2 homozygous mutant ES cells with tetraploid wild-type embryos. However, at E9.5, these embryos, which are completely ES-derived, also have defects. In contrast, chimeric embryos generated by injection of wild-type ES cells into Alk2 mutant blastocysts do not show rescue of the gastrulation defects. These results suggest that signaling through this type I receptor is essential in extraembryonic tissues at the time of gastrulation for normal mesoderm formation and also suggest that subsequent Alk2 signaling is essential for normal development after gastrulation (Mishina, 1999).

The gene for activin ßA is expressed in the early odontogenic mesenchyme of all murine teeth but mutant mice show a patterning defect where incisors and mandibular molars fail to develop but maxillary molars develop normally. In order to understand why maxillary molar tooth development can proceed in the absence of activin, the role of mediators of activin signalling in tooth development was explored. Analysis of tooth development in activin receptor II and Smad2 mutants shows that a similar tooth phenotype to activin ßA mutants can be observed. In addition, a novel downstream target of activin signalling, the Iroquois-related homeobox gene, Irx1, has been identified; its expression in activin ßA mutant embryos is lost in all tooth germs, including the maxillary molars. These results strongly suggest that other TGFß molecules are not stimulating the activin signalling pathway in the absence of activin. This was confirmed by a non-genetic approach using exogenous soluble receptors to inhibit all activin signalling in tooth development. These reproduced the genetic phenotypes. Activin, thus, has an essential role in early development of incisor and mandibular molar teeth but this pathway is not required for development of maxillary molars (Ferguson, 2001).

Transforming growth factor-ß regulates the activation state of the endothelium via two opposing type I receptor/Smad pathways. Activin receptor-like kinase-1 (ALK1) induces Smad1/5 phosphorylation, leading to an increase in endothelial cell proliferation and migration, while ALK5 promotes Smad2/3 activation and inhibits both processes. ALK5 is important for TGFß/ALK1 signaling; endothelial cells lacking ALK5 are deficient in TGFß/ALK1-induced responses. More specifically, ALK5 mediates a TGFß-dependent recruitment of ALK1 into a TGFß receptor complex, and the ALK5 kinase activity is required for optimal ALK1 activation. TGFß type II receptor is also required for ALK1 activation by TGFß. Interestingly, ALK1 not only induces a biological response opposite that of ALK5 but also directly antagonizes ALK5/Smad signaling Goumans, 2003).

Type I TGF-beta and activin receptors: Interactions with ligand

Xenopus blastula cells activate different mesodermal genes as a concentration-dependent response to activin, which behaves like a morphogen. To understand how cells recognize morphogen concentration, radioactively labeled activin was bound to cells and binding was related to the choice of gene activation. The increasing occupancy of a single receptor type can cause cells to switch gene expression. Cells sense ligand concentration by the absolute number of occupied receptors per cell (100 and 300 molecules of bound activin induce Xbra and Xgsc, respectively, i.e., 2% and 6% of the total receptors) and not by a ratio of occupied to unoccupied receptors. The long duration of occupancy explains a previously described ratchet effect. These results suggest a new concept of morphogen gradient formation and interpretation that is particularly well suited to the needs of early development (Dyson, 1998).

An unexpected result of this work is that cells sense morphogen concentration and switch gene response when a remarkably small proportion of their receptors is occupied by ligand. However, this is very understandable if the interpretation of a morphogen gradient by the cells is envisioned in the following way: it is supposed that cells in one region of an embryo actively secrete morphogen for a few hours, during which time the concentration increases. After this, cells discontinue emitting morphogen, and its concentration decreases. It is believed that responsive cells monitor morphogen concentration continuously and respond by a ratchet mechanism to the highest concentration that they experience within their competent life. This proposed mechanism has at least two advantages for early development: (1) cells can bind ligand and respond rapidly to the morphogen and do not therefore need to wait for the morphogen to reach equilibrium. Through the ratchet effect, cells would always respond to the highest concentration that they experience within the few hours of their competent life even when the number of occupied receptors per cell might temporarily decrease, as during cell division. The ratchet effect would operate by the very high affinity of ligand for its receptors. (2) If cells were to respond at high occupancy but still measure the absolute numbers of occupied receptors, there could be some inconsistency of response due to titration of type I receptors. Under these conditions, the ligand type II receptor complexes first formed would see a much higher concentration of type I receptors than subsequent complexes. It would then be predicted that overexpression of Type I receptors would change response to activin concentration. The results presented in this paper show that this is not the case. It is concluded that, at the very low occupancies seen, both type I and type II receptors are in such excess that the few receptors actually used for signaling do not significantly reduce the overall pool of available receptors; therefore, increased occupancy can be directly reflected in increased signaling. The mechanism proposed can explain an apparent paradox: the ligand must be limiting in order to account for the concentration-dependent responses that are observed, however, the ligand must also be in excess to be able to create a concentration gradient in distant cells. If this were not the case, most if not all the ligand would be sequestered by cells nearest the source, as may happen in the case of Hedgehog in Drosophila. This paradox may be explained as follows: it has been shown that cells can respond when very low levels of ligand are bound (100-300 molecules). This means that only a small proportion of the ligand in the intercellular space needs to be bound by receptors within the time available. In this way, cells are able to generate a concentration-dependent response without significantly reducing the concentration of ligand around them and therefore without disturbing the gradient (Dyson, 1998).

Nodal ligands are essential for the patterning of chordate embryos. Genetic studies have revealed that Nodal and its orthologs require EGF-CFC factors for their biological effects. EGF-CFC factors are membrane proteins that attach to the extracellular surface via glycosylphosphatidylinositol (GPI) linkages. They are named for two conserved motifs common to all EGF-CFC family members: (1) a region with homology to epidermal growth factor (the EGF-like motif), and (2) a region with homology specific to the EGF-CFC family (the CFC motif). To date, seven EGF-CFC factors are known: human Cripto and human Cryptic, mouse Cripto and mouse Cryptic, Xenopus FRL-1, zebrafish one-eyed pinhead (Oep), and chick Cripto. The role of Cripto in Nodal signaling has been investigated. Cripto interacts with the type I receptor ALK4 via the conserved CFC motif in Cripto. Cripto interaction with ALK4 is necessary both for Nodal binding to the ALK4/ActR-IIB receptor complex and for Smad2 activation by Nodal. Nodal can inhibit BMP signaling by a Cripto-independent mechanism. Inhibition appears to be mediated by heterodimerization between Nodal and BMPs, indicating that antagonism between Nodal and BMPs can occur at the level of dimeric ligand production (Yeo, 2001).

While Nodal-BMP7 heterodimers have not yet been identified in vivo, the finding that Nodal's affinity for BMP7 in heterodimerization is similar to Nodal's affinity for itself, strongly suggests that this heteromeric interaction can occur in vivo in tissues in which Nodal and BMPs are expressed at similar levels. This heterodimerization provides a novel, intracellular mechanism for the patterned inhibition of BMP signals. This intracellular inhibitory mechanism is significant in that in this case a BMP-antagonizing signal would be restricted to Nodal-expressing cells and not propagated into adjacent cell layers. In this model, Nodal antagonism of BMP signaling would complement diffusible BMP antagonists to permit the generation of spatially complex patterns of BMP antagonism (Yeo, 2001).

Type I TGF-beta and activin receptors: Effects of mutation

ActRIB is a type I transmembrane serine/threonine kinase receptor that has been shown to form heteromeric complexes with the type II activin receptors to mediate activin signal. To investigate the function of ActRIB in mammalian development, both ActRIB-deficient mice and ES cell lines were generated by gene targeting. Analysis of the ActRIB-/- embryos shows that the epiblast and the extraembryonic ectoderm are disorganized, resulting in disruption and developmental arrest of the egg cylinder before gastrulation. To assess the function of ActRIB in mesoderm formation and gastrulation, chimera analysis was conducted. ActRIB-/- ES cells injected into wild-type blastocysts are able to contribute to the mesoderm in chimeric embryos, suggesting that ActRIB is not required for mesoderm formation. Primitive streak formation, however, is impaired in chimeras when ActRIB-/- cells contribute highly to the epiblast. Chimeras generated by injection of wild-type ES cells into ActRIB-/- blastocysts form relatively normal extraembryonic tissues, but the embryo proper develops poorly, probably resulting from severe gastrulation defect. These results provide genetic evidence that ActRIB functions in both epiblast and extraembryonic cells to mediate signals that are required for egg cylinder organization and gastrulation. Whereas the nodal receptor has yet to be identified through ligand binding analysis, the similar defects in primitive streak formation in nodal-/- and ActRIB-/- embryos raise the possibility that ActRIB might function as the type I receptor for nodal (Gu, 1998).

ActRIA (or ALK2), one of the type I receptors of the transforming growth factor-beta (TGF-beta) superfamily, can bind both activin and bone morphogenetic proteins (BMPs) in conjunction with the activin and BMP type II receptors, respectively. In mice, ActRIA is expressed primarily in the extraembryonic visceral endoderm before gastrulation and later in both embryonic and extraembryonic cells during gastrulation. To elucidate its function in mouse development, the transmembrane domain of ActRIA was disrupted by gene targeting. Embryos homozygous for the mutation are arrested at the early gastrulation stage, displaying abnormal visceral endoderm morphology and severe disruption of mesoderm formation. To determine in which germ layer ActRIA functions during gastrulation, reciprocal chimera analyses were performed. (1) Homozygous mutant ES cells injected into wild-type blastocysts are able to contribute to all three definitive germ layers in chimeric embryos. However, a high contribution of mutant ES cells in chimeras disrupts normal development at the early somite stage. (2) Consistent with ActRIA expression in the extraembryonic cells, wild-type ES cells fail to rescue the gastrulation defect in chimeras in which the extraembryonic ectoderm and visceral endoderm are derived from homozygous mutant blastocysts. Furthermore, expression of HNF4, a key visceral endoderm-specific transcription regulatory factor, is significantly reduced in the mutant embryos. Together, these results indicate that ActRIA in extraembryonic cells plays a major role in early gastrulation, whereas ActRIA function is also required in embryonic tissues during later development in mice (Gu, 1999).

Renal dysplasia, the most frequent cause of childhood renal failure in humans, arises from perturbations in a complex series of morphogenetic events during embryonic renal development. The molecular pathogenesis of renal dysplasia is largely undefined. While investigating the role of a BMP-dependent pathway that inhibits branching morphogenesis in vitro, a novel model of renal dysplasia was generated in a transgenic (Tg) model of ALK3 (activin-like kinase 3; BMPR1A) receptor signaling. This study reports the renal phenotype, and the discovery of molecular interactions between effectors in the BMP and WNT signaling pathways in dysplastic kidney tissue. Expression of the constitutively active ALK3 receptor ALK3^QD, in two independent transgenic lines causes renal aplasia/severe dysgenesis in 1.5% and 8.4% of hemizygous and homozygous Tg mice, respectively, and renal medullary cystic dysplasia in 49% and 74% of hemizygous and homozygous Tg mice, respectively. The dysplastic phenotype, which included a decreased number of medullary collecting ducts, increased medullary mesenchyme, collecting duct cysts and decreased cortical thickness, is apparent by E18.5. The pathogenesis of dysplasia in these mice was investigated, and a 30% decrease in branching morphogenesis was demonstrated at E13.5 before the appearance of histopathogical features of dysplasia. The formation of ß-catenin/SMAD1/SMAD4 molecular complexes was also demonstrated in dysplastic renal tissue. Increased transcriptional activity of a ß-catenin reporter gene in ALK3^QD;Tcf-gal mice demonstrates functional cooperativity between the ALK3 and ß-catenin-dependent signaling pathways in kidney tissue. Together with the results in the dysplastic mouse kidney, the findings that phospho-SMAD1 and ß-catenin are overexpressed in human fetal dysplastic renal tissue suggest that dysregulation of these signaling effectors is pathogenic in human renal dysplasia. This work provides novel insights into the role that crucial developmental signaling pathways may play during the genesis of malformed renal tissue elements (Hu, 2003).

Type I TGF-beta and activin receptors: Interactions with type II receptors

Activins and other ligands in the TGFbeta superfamily signal through a heteromeric complex of receptors. Disruption of signaling by a truncated type II activin receptor, XActRIIB (previously called XAR1), blocks mesoderm induction and promotes neuralization in Xenopus embryos. A type I activin receptor, XALK4 has been cloned and characterized. Like truncated XActRIIB, a truncated mutant (tXALK4) blocks mesoderm formation both in vitro and in vivo; moreover, an active form of the receptor induces mesoderm in a ligand-independent manner. Unlike truncated XActRIIB, however, tXALK4 does not induce neural tissue. This difference is explained by the finding that tXALK4 does not block BMP4-mediated epidermal specification, while truncated XActRIIB inhibits all BMP4 responses in embryonic explants. The neural/ectodermal fate engendered by BMP4 is likely to require another type I receptor AlK3 (BMPR1) and another type II receptor dedicated to ectodermal fate. Thus, the type I and type II activin receptors are involved in overlapping but distinct sets of embryonic signaling events (Chang, 1997).

The type II receptors serving TGFß and activin recognize these ligands free in the medium, whereas their type I receptors do not. The type I receptors recognize ligand-bound type II receptors, forming an oligomeric complex, probably a heterotetramer. The BMP receptor system is somewhat different. In this case, the type II receptors and the type I receptors both separately have low affinity for the ligand and together achieve high affinity binding. A central event in the generation of signals by type I and type II complexes is phosphorylation of the type I receptor. This is likely to be catalyzed by the type II receptor kinase. The activity of this kinase is required for phosphorylation in the cell, and it phosphorylates recombinant type I receptor in vitro. Phosphorylation occurs in a cluster of five serine and threonine residues in the GS domain, a highly conserved region next to the N-terminus of the kinase domain in all type I receptors. The type II receptors have kinase activity that does not seem to be augmented by ligand binding. The ligand may be acting as an adaptor that brings a substrate (the type I receptor) to the primary receptor kinase (Massagué, 1996).

Growth regulation of fibroblasts is important for lung development and repair of lung injury. The role of transforming growth factor-beta (TGF-beta) type II receptor in the TGF-beta-dependent proliferative response of lung fibroblasts has been investigated. TGF-beta stimulates the proliferation of adult lung fibroblasts at a low concentration (1 ng/ml), but inhibits the growth of fetal lung fibroblasts in a dose-dependent fashion (0.1-10 ng/ml). The two lung fibroblast cell lines express the TGF-beta type I receptor (T beta RI) and type II receptor (T beta RII). A truncated derivative of T beta RII that lacks the cytoplasmic serine/threonine kinase domain (T beta RII delta K) was overexpressed in lung fibroblasts. T beta RII delta K is a dominant-negative inhibitor of TGF-beta signal transduction blocking not only TGF-beta-induced mitogenic action on adult lung fibroblasts but also TGF-beta-induced growth inhibition of fetal lung fibroblasts. The results indicate that the type II receptor is indispensable for mediating both the mitogenic and antiproliferative effects of TGF-beta on lung fibroblasts (Zhao, 1996).

Transforming growth factor beta (TGF-beta) transduces signals through two related serine/threonine kinase receptors, the type I and type II receptors, which have the ability to interact with each other. In the heteromeric complex, the type II receptor is the primary determinant of ligand binding and phosphorylates the cytoplasmic domain of the type I receptor. Using a chimeric receptor strategy, it has been shown that a functional TGF-beta receptor complex requires heteromerization of both extracellular and intracellular domains of type I and type II receptors. Overexpression of two receptors carrying a heteromeric combination of cytoplasmic domains results in ligand-independent responses, further supporting the functional requirement of the two heterologous cytoplasmic domains in TGF-beta signaling. Furthermore, coexpression of only the cytoplasmic domains of both the type I and II receptors or tethering the type II to the type I cytoplasmic domain activates TGF-beta responses in a ligand-independent manner. In cotransfected COS-1 cells, both cytoplasmic domains are associated with one another. These results indicate that the cytoplasmic domains of the type I and type II TGF-beta receptors physically and functionally interact with each other in the heteromeric complex (Feng, 1996).

Activins and inhibins belong to the transforming growth factor (TGF-beta)-like superfamily and exert their effects on a broad range of cellular targets by modulating cell differentiation and proliferation. Members of this family interact with two structurally related classes of receptors (type I and type II), both containing a serine/threonine kinase domain. When expressed alone, the type II but not the type I activin receptor can bind activin. However, the presence of a type I receptor is required for signaling. For TGF-1, ligand binding to the type II receptor results in the recruitment and transphosphorylation of the type I receptor. Transient overexpression of the two types of activin receptors results in ligand-independent receptor heteromerization and activation. Nevertheless, activin addition to the transfected cells increases complex formation between the two receptors, suggesting a mechanism of action similar to that observed for the TGF-beta receptor. A stable cell line, expressing the two types of human activin receptors upon induction, was generated in the human erythroleukemia cell line K562. Activin specifically induces heteromer formation between the type I and type II receptors in a time-dependent manner. Activin signal transduction mediated through its type I and type II receptors results in an increase in the hemoglobin content of the cells and limits their proliferation. The inhibin antagonistic effects on activin-induced biological responses are mediated through a competition for the type II activin receptor but also require the presence of an inhibin-specific binding component (Lebrun, 1997).

Transforming growth factor-beta (TGFbeta) signaling requires phosphorylation of the type I receptor TbetaR-I by TbetaR-II. Although TGFbeta promotes the association of TbetaR-I with TbetaR-II, these receptor components have affinity for each other that can lead to their ligand-independent activation. The immunophilin FKBP12 (a cytosolic protein known to bind the immunosuppressants FK506 and rapamycin) binds to TbetaR-I and inhibits its signaling function. FKBP12 binding to TbetaR-I involves the rapamycin/Leu-Pro binding pocket of FKBP12 and a Leu-Pro sequence located next to the activating phosphorylation sites in TbetaR-I. Mutations in the binding sites of FKBP12 or TbetaR-I abolish the interaction between these proteins, leading to receptor activation in the absence of added ligand. FKBP12 does not inhibit TbetaR-I association with TbetaR-II; rather, it inhibits TbetaR-I phosphorylation by TbetaR-II. Rapamycin, which blocks FKBP12 binding to TbetaR-I, reverses the inhibitory effect of FKBP12 on TbetaR-I phosphorylation. By impeding the activation of TGFbeta receptor complexes formed in the absence of ligand, FKBP12 may provide a safeguard against leaky signaling resulting from the innate tendency of TbetaR-I and TbetaR-II to interact with each other (Chen, 1997).

A WD-40 repeat protein, TRIP-1, associates with the type II transforming growth factor beta (TGF-beta) receptor. Another WD-40 repeat protein, the Balpha subunit of protein phosphatase 2A, associates with the cytoplasmic domain of type I TGF-beta receptors. This association depends on the kinase activity of the type I receptor; it is increased by coexpression of the type II receptor (which is known to phosphorylate and activate the type I receptor) and allows the type I receptor to phosphorylate Balpha. Furthermore, Balpha enhances the growth inhibition activity of TGF-beta in a receptor-dependent manner. Because Balpha has been characterized as a regulator of phosphatase 2A activity, these observations suggest possible functional interactions between the TGF-beta receptor complex and the regulation of protein phosphatase 2A (Griswold-Prenner, 1998).

The effect of Balpha on the growth inhibition response of TGF-beta complements the role of Smads as effectors of TGF-beta receptor signaling. Smads function as transcriptional activators that induce the expression of various genes. Since the transcription of several genes is induced by Smads, Smads may induce growth inhibition by inducing transcription of the cdk inhibitors p15 and p21 in response to TGF-beta. Overexpression of Balpha induces growth inhibition to a level comparable to that of overexpression of Smads and, like the Smads, the effect of Balpha on growth inhibition depends on receptor activity. Furthermore, the antiproliferative effect of Balpha does not depend on Smad4, suggesting that TGF-beta receptor activation may induce two parallel pathways that lead to the antiproliferative response, one propagated by Smad proteins and the other one propagated through Balpha. Although the mechanism of the receptor-dependent growth inhibition by Balpha is not known, one possibility is that it acts through the ability of PP2A to regulate MAP kinase activity, especially since PP2A is a major enzyme involved in dephosphorylating MAP kinase. Therefore, altered PP2A activity following TGF-beta receptor activation might contribute to growth inhibition by deactivating this growth stimulatory pathway, theregy complementing the direct induction of growth inhibition by Smads. Moreover, a possible regulation of PP2A activity by TGF-beta may also directly affect the cell cycle, which would be consistent with the observed role of PP2A in cell cycle control (Griswold-Prenner, 1998 and references).

Two different chimeric TGF-beta superfamily receptors were generated: TbetaR-I/BMPR-IB, containing the extracellular domain of TGF-beta type I receptor (TbetaR-I) and the intracellular domain of bone morphogenetic protein type IB receptor (BMPR-IB), and TbetaR-II/ActR-IIB, containing the extracellular domain of TGF-beta type II receptor (TbetaR-II) and the intracellular domain of activin type IIB receptor (ActR-IIB). Upon stable transfection in mink lung epithelial cell lines, and in the presence of TGF-beta1, TbetaR-I/BMPR-IB and TbetaR-II/ActR-IIB form heteromeric complexes with wild-type TbetaR-II and TbetaR-I, respectively. TbetaR-II/ActR-IIB restores the responsiveness upon transfection in mutant cell lines lacking functional TbetaR-II with respect to TGF-beta-mediated activation of a transcriptional signal, extracellular matrix formation, growth inhibition, and Smad phosphorylation. TbetaR-I/BMPR-IB and TbetaR-II/ActR-IIB forms a functional complex in response to TGF-beta and induces phosphorylation of Smad1. However, complex formation is not enough for signal propagation, which is shown by the inability of TbetaR-I/BMPR-IB to restore responsiveness to TGF-beta in cell lines deficient in functional. The TGF-beta1-induced complex between TbetaR-II/ActR-IIB and TbetaR-I stimulates endogenous Smad2 phosphorylation, a TGF-beta-like response. This observation is in agreement with the current model for receptor activation, in which the type I receptor determines signal specificity (Persson, 1997).

Interactions of Smad proteins with TGF-beta receptors

Continued: baboon Evolutionary homologs part 2/3 | part 3/3 |

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