wishful thinking

EVOLUTIONARY HOMOLOGS

For information on activin type II receptor homologs see Punt

A human type II receptor for BMPs (BMPR-II) is distantly related to DAF-4, a BMP type II receptor from C. elegans. In transfected COS-1 cells, osteogenic protein (OP)-1/BMP-7 binds to BMPR-II. (BMP-4 also binds, but less efficiently). Alone, BMPR-II binds ligands only weakly, but binding is facilitated in the presence of previously identified type I receptors for BMPs. Binding of OP-1/BMP-7 to BMPR-II is also observed in nontransfected cell lines. A transcriptional activation signal is transduced by BMPR-II in the presence of type I receptors after stimulation by OP-1/BMP-7 (Rosenzweig, 1995).

The BMP type II receptor (BMPR-II), a missing component of this BMP receptor system in vertebrates, has been cloned. BMPR-II is a transmembrane serine/threonine kinase that binds BMP-2 and BMP-7 in association with multiple type I receptors, including BMPR-IA/Brk1, BMPR-IB, and ActR-I, which is also an activin type I receptor. Cloning of BMPR-II results from the strong interaction of its cytoplasmic domain with diverse transforming growth factor beta family type I receptor cytoplasmic domains in a yeast two-hybrid system. In mammalian cells, however, the interaction of BMPR-II is restricted to BMP type I receptors and is ligand dependent. BMPR-II binds BMP-2 and -7 on its own, but binding is enhanced by coexpression of type I BMP receptors. BMP-2 and BMP-7 can induce a transcriptional response when added to cells coexpressing ActR-I and BMPR-II but not to cells expressing either receptor alone. The kinase activity of both receptors is essential for signaling. Thus, despite their ability to bind to type I and II receptors separately, BMPs appear to require the cooperation of these two receptors for optimal binding and for signal transduction. The combinatorial nature of these receptors and their capacity to crosstalk with the activin receptor system may underlie the multifunctional nature of their ligands (Liu, 1995).

Growth/differentiation factor-5 (GDF-5) is a member of the bone morphogenetic protein (BMP) family, which plays an important role in bone development in vivo. Mutations in the GDF-5 gene result in brachypodism in mice and Hunter-Thompson type chondrodysplasia in human. BMPs transduce their effects through binding to two different types of serine/threonine kinase receptors: type I and type II. However, binding abilities appear to be different among the members of the BMP family. BMP-4 binds to two different type I receptors, BMP receptors type IA (BMPR-IA) and type IB (BMPR-IB), and a type II receptor, BMP receptor type II (BMPR-II). In addition to these receptors, osteogenic protein-1 (OP-1, also known as BMP-7) binds to activin type I receptor (ActR-I) as well as activin type II receptors (ActR-II and ActR-IIB). The binding and signaling properties of GDF-5 through type I and type II receptors has also been studied. GDF-5 induces alkaline phosphatase activity in a rat osteoprogenitor-like cell line, ROB-C26. 125I-GDF-5 binds to BMPR-IB and BMPR-II but not to BMPR-IA in ROB-C26 cells and other nontransfected cell lines. Analysis using COS-1 cells transfected with the receptor cDNAs reveals that GDF-5 binds to BMPR-IB but not to the other type I receptors when expressed alone. When COS-1 cells are transfected with type II receptor cDNAs, GDF-5 binds to ActR-II, ActR-IIB, and BMPR-II but not to transforming growth factor-beta type II receptor. In the presence of type II receptors, GDF-5 binds to different sets of type I receptors, but the binding is most efficient to BMPR-IB, as compared with the other type I receptors. A transcriptional activation signal is efficiently transduced by BMPR-IB in the presence of either BMPR-II or ActR-II after stimulation by GDF-5. These results suggest that BMPR-IB mediates certain signals for GDF-5 after forming the heteromeric complex with BMPR-II or ActR-II (Nishitoh, 1996).

Bone morphogenetic proteins (BMPs) are potent regulators of embryonic cell fate that are presumed to initiate signal transduction in recipient cells through multimeric, transmembrane, serine/threonine kinase complexes made up of type I and type II receptors. BMPRII was identified previously in mammals as the only type II receptor that binds BMPs, but not activin or TGFbeta, in vitro. Its Xenopus homolog, XBMPRII, is expressed maternally and zygotically in an initially unrestricted manner. Strikingly, XBMPRII transcripts then become restricted to the mesodermal precursors during gastrulation. Expression is lower in the dorsal organizer region, potentially providing a mechanism to suppress the actions of BMP4 on dorsally fated tissues. Similar to the results seen for a truncated type I BMP receptor (tBR), a dominant-negative form of XBMPRII (tBRII) can dorsalize ventral mesoderm, induce extensive secondary body axes, block mesoderm induction by BMP4 and directly neuralize ectoderm; this strongly suggests that XBMPRII mediates BMP signals in vivo. However, although both tBRII and tBR can induce partial secondary axes, marker analysis shows that tBRII-induced axes are more anteriorly extended. Coinjection of tBRII and tBR synergistically increases the incidence of secondary axis formation. A truncated activin type II receptor [(Delta)XAR1] is known to block both activin and BMP signaling in vivo. Such crossreactivity does not occur for tBRII, in that it does not affect activin signaling. The full-length activin type II receptor (XAR1) overcomes a block in BMP4 signaling imposed by tBRII, implicating XAR1 as a common component of BMP and activin signaling pathways in vivo. These data implicate XBMPRII as a type II receptor with high selectivity for BMP signaling, and therefore as a critical mediator of the effects of BMPs as mesodermal patterning agents and suppressors of neural fate during embryogenesis (Frisch, 1998).

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-beta superfamily, play a variety of roles during mouse development. BMP type II receptor (BMPR-II) is a type II serine/threonine kinase receptor that transduces signals for BMPs through heteromeric complexes with type I receptors, including activin receptor-like kinase 2 (ALK2), ALK3/BMPR-IA, and ALK6/BMPR-IB. To elucidate the function of BMPR-II in mammalian development, BMPR-II mutant mice were generated by gene targeting. Homozygous mutant embryos were arrested at the egg cylinder stage and could not be recovered at 9.5 days postcoitum. Histological analysis reveals that homozygous mutant embryos fail to form organized structure and lack mesoderm. The BMPR-II mutant embryos are morphologically very similar to the ALK3/BMPR-IA mutant embryos, suggesting that BMPR-II is important for transducing BMP signals during early mouse development. Moreover, the epiblast of the BMPR-II mutant embryo exhibits an undifferentiated character, although the expression of tissue-specific genes for the visceral endoderm is essentially normal. These results suggest that the function of BMPR-II is essential for epiblast differentiation and mesoderm induction during early mouse development (Beppu, 2000).

Bone morphogenetic protein-2 (BMP-2) induces bone formation and regeneration in adult vertebrates and regulates important developmental processes in all animals. BMP-2 is a homodimeric cysteine knot protein that, as a member of the transforming growth factor-ß superfamily, signals by oligomerizing type I and type II receptor serine-kinases in the cell membrane. The binding epitopes of BMP-2 for BMPR-IA (type I) and BMPR-II or ActR-II (type II) were characterized using BMP-2 mutant proteins for analysis of interactions with receptor ectodomains. A large epitope 1 for high-affinity BMPR-IA binding was detected spanning the interface of the BMP-2 dimer. A smaller epitope 2 for the low-affinity binding of BMPR-II was found to be assembled by determinants of a single monomer. Symmetry-related pairs of the two juxtaposed epitopes occur near the BMP-2 poles. Mutations in both epitopes yield variants with reduced biological activity in C2C12 cells; however, only epitope 2 variants behave as antagonists, partially or completely inhibiting BMP-2 activity. These findings provide a framework for the molecular description of receptor recognition and activation in the BMP/TGF-ß superfamily (Kirsch, 2000).

The identification and characterization of two distinct binding epitopes in human BMP-2 as well as the detection of antagonistic BMP-2 variants, provides new insights into the primary steps and mechanism of BMP receptor activation. Receptor-binding epitopes have not been described before for any of the closely related members of the TGF-ß superfamily that signal via type I and type II receptor serine-kinases. All TGF-ß-like proteins are dimers, usually homodimers, where the monomers have been compared with an open hand, with the central alpha-helix (alpha3) being the wrist or heel and the two aligned two-stranded ß-sheets representing the four fingers, with loop 1 and loop 2 at the tips of each pair of fingers. The N-terminal segment exists at the position of the thumb. Consequently, the epitope 1 assembled around the central alpha-helix is called in the following the 'wrist epitope' and epitope 2 located at the back of the hand near the outer finger segments is called the 'knuckle epitope' (Kirsch, 2000 and references therein).

The wrist epitope has dimensions of ~20 x 25-30 Å (500-600 Ų). This large area would be compatible with the function as a high-affinity interaction site. The knuckle epitope seems to be smaller with dimensions of 10 x 20-25 Å (200-250 Ų) in accordance with the lower affinity of the interaction with BMPR-II at this site. The wrist epitope is highly discontinuous and it comprises different elements of both monomers. In heterodimers, e.g. of BMP-4 and BMP-7 or of the inhibin/activin-type factors, this has the interesting consequence that the two symmetry-related epitopes are no longer equivalent and may therefore exhibit different receptor-binding properties. In the knuckle epitope, the binding residues are provided by one monomer only and are located in sheets ß3, ß4, ß7 and ß8 and possibly also in ß9 (E109)(Kirsch, 2000 and references therein).

The juxtaposed knuckle and wrist epitopes are only 10-15 Å apart. The distances between the two type I (~40 Å) or the two type II chains (~55 Å), that possibly are assembled at BMP-2, are much larger. This appears to be especially relevant for the receptor serine-kinases. Their small ectodomain is connected to the membrane-spanning segment by a short linker of <12 residues. In the receptor complex with BMP-2, this short distance between epitopes 1 and 2 might facilitate the interaction of the type I and type II receptor serine-kinases (Kirsch, 2000).

The occurrence of the BMP-2 antagonists detected in this study is most likely to be a consequence of an ordered sequential binding mechanism operating during receptor activation. The antagonist blocks the high-affinity type I receptor chain via its intact wrist epitope, and the disrupted knuckle epitope prevents the subsequent interaction with the low-affinity type II chain(s). The similarly low IC₅₀ of the antagonists as well as their efficient competition with BMP-2 for receptor binding could indicate that it is predominantly the type I chain(s) that adjust(s) the binding affinity of BMP-2 for the whole receptor complex. Interestingly, the velocity of complex formation and dissociation with BMPR-IA is equally critical, as revealed by the complete loss of biological activity of the respective double mutants. An intriguing finding is the dramatic loss of biological activity in variants of the knuckle epitope, considering that binding to the ectodomains of the type II receptor chain(s) is reduced only 10- to 15-fold. It is possible that the simultaneous binding of two type II chains is necessary for an efficient receptor activation, and therefore a decrease in binding affinity becomes aggravated (Kirsch, 2000 and references therein).

Germline mutations BMPR-II have been reported in patients with primary pulmonary hypertension (PPH), but the contribution of various types of mutations found in PPH to the pathogenesis of clinical phenotypes has not been elucidated. To determine the biological activities of these mutants, functional assays were performed testing their abilities to transduce BMP signals. The reported missense mutations within the extracellular and kinase domains of BMPR-II abrogate their signal-transducing abilities. BMPR-II proteins containing mutations at the conserved cysteine residues in the extracellular and kinase domains were detected in the cytoplasm, suggesting that the loss of signaling ability of certain BMPR-II mutants is due at least in part to their altered subcellular localization. In contrast, BMPR-II mutants with truncation of the cytoplasmic tail retain the ability to transduce BMP signals. The differences in biological activities among the BMPR-II mutants observed thus suggest that additional genetic and/or environmental factors may play critical roles in the pathogenesis of PPH (Nishihara, 2002).

Diverse heterozygous mutations of bone morphogenetic receptor type II (BMPR-II) underlie the inherited form of the vascular disorder primary pulmonary hypertension (PPH). As yet, the molecular detail of how such defects contribute to the pathogenesis of PPH remains unclear. BMPR-II is a member of the transforming growth factor-beta cell signalling superfamily. Ligand binding induces cell surface receptor complex formation and activates a cascade of phosphorylation events of intracellular intermediaries termed Smads, which initiate transcriptional regulation. Some 30% of PPH-causing mutations localize to exon 12, which may be spliced out, forming an isoform depleted of the unusually long BMPR-II cytoplasmic tail. To further elucidate the consequences of BMPR2 mutation, aspects of the cytoplasmic domain function were characterized by seeking intracellular binding partners. Tctex-1, a light chain of the motor complex dynein, interacts with the cytoplasmic domain of BMPR-II, and Tctex-1 is phosphorylated by BMPR-II, a function disrupted by PPH disease causing mutations within exon 12. BMPR-II and Tctex-1 co-localize to endothelium and smooth muscle within the media of pulmonary arterioles, key sites of vascular remodelling in PPH. Taken together, these data demonstrate a discrete function for the cytoplasmic domain of BMPR-II and justify further investigation of whether the interaction with and phosphorylation of Tctex-1 contributes to the pathogenesis of PPH (Machado, 2003).

BMPs constitute a family of ~20 growth factors involved in a tremendous variety of embryonic inductive processes. BMPs elicit dose-dependent effects on patterning during gastrulation and gradients of BMP activity are thought to be established through regulation of the relative concentrations of BMP receptors, ligands and antagonists. Whether later developmental events also are sensitive to reduced levels of BMP signaling was tested. A knockout mouse was generated that expresses a BMP type II receptor that lacks half of the ligand-binding domain. This altered receptor is expressed at levels comparable with the wild-type allele, but has reduced signaling capability. Unlike Bmpr2-null mice, mice homozygous for this hypomorphic receptor undergo normal gastrulation, providing genetic evidence of the dose-dependent effects of BMPs during mammalian development. Mutants, however, die at midgestation with cardiovascular and skeletal defects, demonstrating that the development of these tissues requires wild-type levels of BMP signaling. The most striking defects occur in the outflow tract of the heart, with absence of septation of the conotruncus below the valve level and interrupted aortic arch, a phenotype known in humans as persistent truncus arteriosus (type A4). In addition, semilunar valves do not form in mutants, while the atrioventricular valves appear unaffected. Abnormal septation of the heart and valve anomalies are the most frequent forms of congenital cardiac defects in humans; however, most mouse models display broad defects throughout cardiac tissues. The more restricted spectrum of cardiac anomalies in Bmpr2^deltaE2 mutants makes this strain a key murine model to understand the embryonic defects of persistent truncus arteriosus and impaired semilunar valve formation in humans (Délot, 2003).

Bone morphogenetic proteins (BMPs) regulate multiple cellular processes, including cell differentiation and migration. Their signals are transduced by the kinase receptors BMPR-I and BMPR-II, leading to Smad transcription factor activation via BMPR-I. LIM kinase (LIMK; see Drosophila LIM-kinase1) 1 is a key regulator of actin dynamics as it phosphorylates and inactivates cofilin, an actin depolymerizing factor. During a search for LIMK1-interacting proteins, clones encompassing the tail region of BMPR-II were isolated. Although the BMPR-II tail is not involved in BMP signaling via Smad proteins, mutations truncating this domain are present in patients with primary pulmonary hypertension (PPH). Further analysis revealed that the interaction between LIMK1 and BMPR-II inhibited LIMK1's ability to phosphorylate cofilin, which could then be alleviated by addition of BMP4. A BMPR-II mutant containing the smallest COOH-terminal truncation described in PPH failed to bind or inhibit LIMK1. This study identifies the first function of the BMPR-II tail domain and suggests that the deregulation of actin dynamics may contribute to the etiology of PPH (Foletta, 2003).

A four-generation pedigree of familial primary pulmonary hypertension (FPPH) with 14 alive members was collected. In the family, three of the 14 living familial members were diagnosed as FPPH. Mutations in bone morphogenetic protein receptor-II (BMPR-II) gene were screened by using sequencing analysis. A C-to-T transition at position 1471 in exon 11 of the BMPR-II gene was identified, resulting in an Arg491Trp mutation. Segregation of the mutation within the family was cofirmed and the presence of the mutations in a panel of 240 chromosomes from normal individuals was excluded. No mutations were found in BMPR-II gene in other 10 patients with sporadic primary pulmonary hypertension. The Arg491Trp mutation is located in the kinase domain and predicted to disturb the kinase activity of BMPR-II. A total of 7 familial members died between ages 8-45 years with various symptoms, indicating other genetic or environmental modifiers involved in the modification of the clinical phenotype (Zhicheng, 2004).

The growth and morphological differentiation of dendrites are critical events in the establishment of proper neuronal connectivity and neural function. One extrinsic factor, BMP7, has been shown to specifically affect dendritic morphogenesis; however, the underlying mechanism by which this occurs is unknown. This study shows that LIM kinase 1 (LIMK1), a key downstream effector of Rho GTPases, colocalizes with the BMP receptor, BMPRII, in the tips of neurites and binds to BMPRII. This interaction is required for BMP-dependent induction of the dendritic arbor in cortical neurons. Furthermore, the physical interaction of LIMK1 with BMPRII synergizes with the Rho GTPase, Cdc42, to activate LIMK1 catalytic activity. These studies thus define a Smad-independent pathway that directly links the BMP receptor to regulation of actin dynamics and provides insights into how extracellular signals modulate LIMK1 activity to permit fine spatial control over cytoskeletal remodelling during dendritogenesis (Lee-Hoeflich, 2004).

Heterozygous mutations of the bone morphogenetic protein type II receptor (BMPR-II) gene have been identified in patients with primary pulmonary hypertension. The mechanisms by which these mutations contribute to the pathogenesis of primary pulmonary hypertension are not fully elucidated. To assess the impact of a heterozygous mutation of the BMPR-II gene on the pulmonary vasculature, mice carrying a mutant BMPR-II allele lacking exons 4 and 5 (BMPR-II^+/- mice) were examined. BMPR-II^+/- mice have increased mean pulmonary arterial pressure and pulmonary vascular resistance compared with their wild-type littermates. Histological analyses revealed that the wall thickness of muscularized pulmonary arteries (<100 mum in diameter) and the number of alveolar-capillary units are greater in BMPR-II^+/- than in wild-type mice. Breathing 11% oxygen for 3 wk increases mean pulmonary arterial pressure, pulmonary vascular resistance, and hemoglobin concentration to similar levels in BMPR-II^+/- and wild-type mice, but the degree of muscularization of small pulmonary arteries and formation of alveolar-capillary units are reduced in BMPR-II^+/- mice. Theses results suggest that, in mice, mutation of one copy of the BMPR-II gene causes pulmonary hypertension but impairs the ability of the pulmonary vasculature to remodel in response to prolonged hypoxic breathing (Beppu, 2005).

Transmembrane receptors with intrinsic serine/threonine or tyrosine kinase domains regulate vital functions of cells in multicellular eukaryotes, e.g., differentiation, apoptosis, and proliferation. Bone morphogenetic protein type II receptor (BMPR-II) which has a serine/threonine kinase domain, and stem cell factor receptor (c-kit) which contains a tyrosine kinase domain form a complex in vitro and in vivo; the interaction is induced upon treatment of cells with BMP2 and SCF. Stem cell factor (SCF) modulates BMP2-dependent activation of Smad1/5/8 and phosphorylation of Erk kinase. SCF also enhances BMP2-dependent differentiation of C2C12 cells. BMPR-II is phosphorylated at Ser757 upon co-expression with and activation of c-kit. BMPR-II phosphorylation requires intact kinase activity of BMPR-II. Abrogation of the c-kit/SCF-dependent phosphorylation of BMPR-II at the Ser757 interfers with the cooperative effect of BMP2 and SCF. These data suggest that the complex formation between c-kit and BMPR-II leads to phosphorylation of BMPR-II at Ser757, which modulates BMPR-II-dependent signaling (Hassel, 2006).

A variety of mutations in BMPR2 have been identified in patients with pulmonary arterial hypertension. In this study, using a BMPR2 mutation database and BMPR-II protein sequences from eight distantly related species, the relationship was defined among evolutionary conservation, mutation frequency and mutation distribution. As a whole, BMPR2 is evolving slower than the average for mammalian protein-encoding genes. As expected, the kinase domain is evolving more slowly than the extracellular ligand-binding and C-terminal domains. A detailed map of evolutionary conservation shows that there are repeating peaks and valleys within the C-terminal domain, representing higher and lower evolutionary conservation. A strong correlation was observed between evolutionary conservation and the distribution of mutations along the gene. All except two of the nineteen missense mutations occur in absolutely conserved amino acids among the vertebrate homologs. In addition, six mutational hotspots (P<0.05) were identified by comparing the observed distribution of mutations to the pattern expected from a random multinomial distribution. Furthermore, analysis of the sequence environment surrounding the mutations reveals a specific pattern of mutagenesis. Over 22% of all single base-paired substitutions and 30% of all deletions and insertions are situated within tandem or non-tandem direct repeats of at least 5-bp and may be explained by slipped-mispairing model of mutagenesis. Also, over 59% of single base-paired substitutions versus 20% of deletions and insertions are located in perfect palindromic sequences that could produce 'hairpin-loop' secondary structures with relatively high thermodynamic stability under physiological conditions. In addition, 3.7% of single base-paired substitutions versus 30% of deletions and insertions are located either within or in close proximity to the Krawczak and Cooper consensus sequence (TG A/G A/G G/T A/C). Further study of the mechanism of mutagenesis in BMPR2 may help identify other potentially mutable sites and differentiate between deleterious mutations and harmless polymorphic variants (Wong, 2006).