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J. Biol. Chem., Vol. 280, Issue 37, 32122-32132, September 16, 2005
Identification of Receptors and Signaling Pathways for Orphan Bone Morphogenetic Protein/Growth Differentiation Factor Ligands Based on Genomic Analyses*From the Division of Reproductive Biology, Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, California 94305-5317
Received for publication, April 27, 2005 , and in revised form, June 27, 2005.
There are more than 30 human transforming growth factor /bone morphogenetic protein/growth differentiation factor (TGF /BMP/GDF)-related ligands known to be important during embryonic development, organogenesis, bone formation, reproduction, and other physiological processes. Although select TGF /BMP/GDF proteins were found to interact with type II and type I serine/threonine receptors to activate downstream Smad and other proteins, the receptors and signaling pathways for one-third of these TGF /BMP/GDF paralogs are still unclear. Based on a genomic analysis of the entire repertoire of TGF /BMP/GDF ligands and serine/threonine kinase receptors, we tested the ability of three orphan BMP/GDF ligands to activate a limited number of phylogenetically related receptors. We characterized the dimeric nature of recombinant GDF6 (also known as BMP13), GDF7 (also known as BMP12), and BMP10. We demonstrated their bioactivities based on the activation of Smad1/5/8-, but not Smad2/3-, responsive promoter constructs in the MC3T3 cell line. Furthermore, we showed their ability to induce the phosphorylation of Smad1, but not Smad2, in these cells. In COS7 cells transfected with the seven known type I receptors, overexpression of ALK3 or ALK6 conferred ligand signaling by GDF6, GDF7, and BMP10. In contrast, transfection of MC3T3 cells with ALK3 small hairpin RNA suppressed Smad signaling induced by all three ligands. Based on the coevolution of ligands and receptors, we also tested the role of BMPRII and ActRIIA as the type II receptor candidates for the three orphan ligands. We found that transfection of small hairpin RNA for BMPRII and ActRIIA in MC3T3 cells suppressed the signaling of GDF6, GDF7, and BMP10. Thus, the present approach provides a genomic paradigm for matching paralogous polypeptide ligands with a limited number of evolutionarily related receptors capable of activating specific downstream Smad proteins.
The transforming growth factor (TGF )2 superfamily of ligands is a multifunctional polypeptide growth factor that includes TGF proteins, activins/inhibins, bone morphogenetic proteins (BMPs), growth differentiation factors (GDFs), and others. These polypeptide ligands regulate cell proliferation, differentiation, and apoptosis that are essential for embryonic development, organogenesis, bone formation, reproduction, and other physiological processes (1). Homo- or heterodimeric TGF family ligands initiate signaling by assembling type II and type I serine/threonine kinase receptor complexes. After ligand binding, the type II receptors phosphorylate the type I receptors, which activate downstream Smad transcription factors (2). In the human genome, there are only five type II receptors, including TGFRII for TGF , ActRIIA and ActRIIB for activins and BMPs, BMPRII for several BMPs, and AMHRII for AMH (2). In contrast, there are seven type I receptors designated as activin receptor-like kinases (ALKs) (3). They include the type I receptors for TGF (ALK5 and ALK1), activins (ALK4), several BMPs/GDFs (ALK3, ALK6, and ALK2), and nodal (ALK7) (2, 4, 5). After ligand binding, activation of the type I receptors initiates two distinct downstream Smad-signaling pathways; ALK13 and -6 receptors stimulate the Smad1/5/8 pathway (6, 7), and ALK4, -5, and -7 receptors stimulate the Smad2/3 pathway (6, 8, 9). Once stimulated through phosphorylation, these receptor-activated Smads form heterodimeric complexes with the common Smad4. The resulting complexes, in turn, translocate to the nucleus to regulate gene expression.
Although the signaling pathways for many TGF
Phylogenetic AnalysisPhylogenetic analyses were performed based on multiple sequence alignment using the ClusterW algorithm (www.ch.embnet.org/software/ClustalW.html) and TreeView drawing software (taxonomy.zoology.gla.ac.uk/rod/treeview.html). For ligand analyses, the C-terminal regions containing the cystine knot starting from the first conserved cysteine residue were used. The GenBankTM accession numbers for individual human protein sequences are as follows: BMP5, P22003 [GenBank] ; BMP6, NP_001709 [GenBank] ; BMP7, P18075 [GenBank] ; BMP8a, AAP74559 [GenBank] BMP8b, AAP74560 [GenBank] BMP2, BMHU2; BMP4, NP_001193 [GenBank] ; GDF5, NP_000548 [GenBank] ; GDF6/GDF16, XP_373260; GDF7, NP_878248 [GenBank] ; BMP9, AF188285 [GenBank] ; BMP10, O95393 [GenBank] ; GDF1, AAB94786 [GenBank] GDF3, BC030959 [GenBank] ; BMP3, NM_001201 [GenBank] ; BMP3b/GDF10, NM_004962 [GenBank] ; GDF9, NP_005251 [GenBank] ; BMP15/GDF9b, NP_005439 [GenBank] ; Nodal/BMP16, NP_060525 [GenBank] ; Inhibin -A, B24248
[GenBank]
; Inhibin -B, NP_002184
[GenBank]
; Inhibin -C, NP_005529
[GenBank]
; Inhibin -E, NP_113667
[GenBank]
; BMP11/GDF11, AAC72852
[GenBank]
GDF8, AAB86694
[GenBank]
TGF 1, NP_000651
[GenBank]
; TGF 2, AAA50405
[GenBank]
TGF 3, A36169; LeftyB, O75610
[GenBank]
; Lefty A, NP_003231
[GenBank]
; Inhibin- , P05111
[GenBank]
; AMH: P03971
[GenBank]
; GDF15, NM_004864
[GenBank]
.
Reagents and HormonesDulbecco's modified Eagle's medium, Generation of Recombinant Human BMP10Human BMP10 cDNA was amplified from the human heart Marathon-Ready cDNA library (Clontech) using specific primers (forward, 5'-TTAGCGGCCGCATGGGCTCTCTGGTCCTGACACTGT-3'; Reverse, 5'-TTATCTAGACTATCTACAGCCACATTCGGAGACG-3') and Turbo Pfu polymerase (Stratagene, La Jolla, CA). Fidelity of the PCR products was confirmed by sequencing after subcloning the cDNA into the expression vector pcDNA3.1/Zeo (Invitrogen). To facilitate purification, the BMP10 construct was tagged with a polyhistidine tag (His6) at the N terminus. Recombinant BMP10 proteins were generated from 293T cells transfected with expression plasmids using Lipofectamine (Invitrogen) and maintained in Dulbecco's modified Eagle's medium/F-12 supplemented with 10% FBS. Clonal cell lines stably expressing recombinant proteins were selected under 500 µg/ml Zeocin (Invitrogen) and maintained in 200 µg/ml Zeocin. After 3 days of serum-free culture, the conditioned media were harvested, adjusted to 20 mM imidazole, and incubated with metal-chelating Sepharose (Amersham Biosciences). The bound proteins were eluted with the wash buffer containing 500 mM imidazole. Protein purity and biochemical characteristics were analyzed after electrophoresis using a 12% SDS-PAGE.
Characterization of Recombinant GDF6, GDF7, and BMP10Recombinant proteins treated under nonreducing or reducing conditions (5% Immunoblots were performed using anti-GDF6 (1 µg/ml), anti-GDF7 (0.1 µg/ml), or anti-His (1:600 dilution) monoclonal antibodies. Signals were detected by chemiluminescence using the ECL system (Amersham Biosciences). Reporter Gene Constructs and Expression PlasmidsThe reporter plasmids pGL3 (BRE)2-lux (15) and pGL3(CAGA)12-lux (16) were provided by Drs. O. Korchynskyi (University of North Carolina, Chapel Hill) and C. H. Heldin (Ludwig Institute for Cancer Research, Uppsala, Sweden), respectively. The sequences for these constructs have been reported (17). The expression plasmid for ALK4 was provided by Dr. A. Klibanski (Massachusetts General Hospital, Boston) (18) and subcloned into the pcDNA3 vector (Invitrogen). The pcDNA3 expression plasmids encoding ALK1, ALK2, ALK3, ALK5, ALK6, ALK7, Smad6, and Smad7 were from Dr. P. Ten Dijke (The Netherlands Cancer Institute, Amsterdam, The Netherlands). Small Hairpin RNA Plasmid ConstructionThe pSilencer 2.0-U6 plasmid (Ambion, Austin, TX) containing the human U6 RNA polymerase III promoter was used to construct the shRNA-encoding plasmids targeting BMPRII, ActRIIA, or ALK6 mRNAs. They were generated by inserting annealed oligonucleotides (TABLE ONE) into the pSilencer 2.0-U6 plasmid between the BamHI and HindIII restriction sites using the T4 DNA ligase (Roche Diagnostics). Each oligonucleotide contained the target sequences, the restriction sites at both ends, the linker sequences, and a stretch of T or A residues as the polymerase III terminator. The 21-nucleotide target sequences were defined from the mouse cDNA sequences by using the small interfering RNA Target Finder (Ambion). After synthesis, the sense and antisense oligonucleotides were annealed at 90 °C for 3 min and 37 °C for 1 h. In addition, the plasmid encoding mouse ALK3 shRNA was purchased from OpenBiosystems (Huntsville, AL). A single oligonucleotide (TABLE ONE), containing the target sequences, was used as the PCR template. The amplified PCR fragments were cloned between the XhoI and EcoRI sites of the pSHAG-MAGIC2 vector.
Cell Lines and TransfectionMC3T3 and P19 cells were cultured in -minimal essential medium supplemented with 10% FBS, together with 100 units/ml penicillin, 100 µg/ml streptomycin, and 2 mM L-glutamine (PSG). COS7 cells were cultured in Dulbecco's modified Eagle's medium/high glucose supplemented with 10% FBS and PSG. MC3T3 and COS7 cells were seeded at 90% confluency in 24-well plates and transiently transfected in Opti-MEM for 4 h with 1 µg (COS7 cells) or 300 ng (MC3T3 cells) of plasmid DNA per well using Lipofectamine 2000 (Invitrogen). Fifty ng of the pCMV- -galactosidase vector was routinely cotransfected to monitor transfection efficiency. After transfection, cells were treated with the appropriate ligands. To monitor luciferase activities, lysis buffer (200 µl) (Promega Corp., Madison, WI) was added to each well, and 30 µl of the supernatant was used for luciferase determination using a luminometer (Bio-Rad). Aliquots of the cell lysate were also used to measure -galactosidase activities. The reporter activity is expressed as the ratio of relative light unit/ -galactosidase activity. Data are the mean ± S.E. of triplicates from representative experiments.
Western Blotting Analysis of Smad ProteinsTo investigate Smad activation by GDF6, GDF7, and BMP10, MC3T3 cells (180,000 cells/well) were cultured overnight in 12-well plates in Receptors Expression Profiles Based on RT-PCR and Real-time PCRMessenger RNAs were extracted from MC3T3, P19, and COS cell lines using the RNeasy kit (Qiagen, Valencia, CA). cDNA was synthesized by using reverse transcriptase (Clontech). Traditional PCR was carried out using the Advantage cDNA polymerase kit (Clontech), and real-time PCR was performed using SmartCycler (Cepheid Inc., Sunnyvale, CA) according to the manufacturer's protocol. Primers used for individual genes are listed in TABLE TWO. To determine the copy number of target transcripts, ALK3, ALK6, BMPRII, and ActRIIA cDNAs were used to generate calibration curves by plotting the threshold cycle (Ct) versus the known copy number for each plasmid template. The copy numbers for target samples were determined according to the calibration curve.
Phylogenetic Relationship of TGF Family Ligands and Their Corresponding Signaling PathwaysThe TGF superfamily includes 33 related ligands, but there are only 5 type II and 7 type I serine/threonine kinase receptors in the human genome (1, 19). Although approximately one-third of these ligands are still orphans without known receptors and downstream signaling pathways, cognate receptors and downstream Smads for the remaining ligands have been characterized (Fig. 1). After interacting with type I and type II serine/threonine kinase receptors, the ligands activate two major pathways characterized by the activation of two different groups of intracellular Smad proteins. Smad1, Smad5, and Smad8 are phosphorylated by the type I receptors ALK13 or -6, whereas Smad2 and Smad3 are activated by the receptors ALK4, -5, or -7 (69). It is apparent that combinatorial uses of a limited number of type I and type II receptors led to differential activation by the large number of ligands. Because many of the BMP and GDF proteins are still orphan ligands, we hypothesized that these ligands are likely to interact with the limited number of receptors, and phylogenetically related ligands are likely to interact with the same receptors. We selected GDF6, GDF7, and BMP10 as prototypic orphan ligands, and we investigated their signaling pathways. Based on the phylogenetic analysis (Fig. 1), the three orphan ligands are closely related to BMP2 and GDF5. Thus, they could activate the Smad1/5/8 pathway through interactions with the type I receptors ALK3 and ALK6 and the type II receptors BMPRII and ActRIIA.
Characterization of Recombinant GDF6, GDF7, and BMP10Members of the TGF superfamily are synthesized as large precursor molecules that are cleaved proteolytically at an RXXR site to release a C-terminal mature peptide of 110140 amino acids in length (2, 20). This region shares conserved cysteine residues involved in the cystine knot conformation (21). In many cases, an extra cysteine residue is engaged in intermolecular disulfide bond formation necessary for the assembly of biologically active dimers. We analyzed the biochemical properties of recombinant GDF6 and GDF7 under both reducing and nonreducing conditions using anti-GDF6 (Fig. 2A, lanes 12) and anti-GDF7 (Fig. 2A, lanes 34) antibodies. Under reducing conditions, a band of 13 kDa likely represented the GDF6 monomer (Fig. 2A, lane 2). Under nonreducing conditions (Fig. 2A, lane 1), an 30-kDa band and a 13-kDa band likely represented GDF6 dimers and monomers, respectively. For GDF7, distinct dimer and monomer bands of 32 (Fig. 2A, lane 3) and 16 kDa (Fig. 2A, lane 4) were found under nonreduced and reduced conditions, respectively. These data suggested that both GDF6 and GDF7 formed disulfide-linked homodimers.
To generate recombinant BMP10, we first performed RT-PCR using a heart cDNA library to obtain the full-length human BMP10 cDNA. Human 293T cells were transfected with expression vectors containing the BMP10 cDNA tagged at its N terminus with a polyhistidine epitope His6. Following selection of permanent clones, cell lines were cultured under serum-free conditions to obtain conditioned media. Recombinant BMP10 was purified using metal chelate chromatography based on the presence of the His6 tag. Following Coomassie Blue staining, three distinct bands were found under nonreducing conditions. The 25-kDa band represented the dimer form, and the 64- and 120-kDa bands likely represented the unprocessed monomers and dimers, respectively (Fig. 2B, lane 1). These unprocessed BMP10 forms contained the uncleaved large prodomain in addition to the C-terminal mature region. Under reducing conditions, BMP10 migrated as a 12-kDa band representing the monomer form and a 60-kDa band corresponding to the unprocessed monomer (Fig. 2B, lane 2). A third weak band of 48 kDa could represent a shorter unprocessed form of BMP10 generated after cleavage of the N-terminal region of the prodomain (Fig. 2B, lane 2). In addition, under reducing conditions, the anti-His6 antibody revealed the same three bands representing the monomer (12 kDa) and the unprocessed monomer forms of 60 and 48 kDa (Fig. 2B, lane 4). After treatment with PNGase F to remove N-linked carbohydrate chains, the 60- and 48-kDa unprocessed forms of BMP10 migrated faster (Fig. 2B, lanes 3 and 5), suggesting that the N-glycosylation sites are located in the prodomain. These data suggested that the recombinant BMP10 was secreted as an N-glycosylated homodimeric proform, and after processing, the mature protein formed nonglycosylated homodimers.
Treatment with GDF6, GDF7, or BMP10 Activates the Smad1/5/8 Pathway in MC3T3 CellsWe hypothesized that GDF6, GDF7, and BMP10 shares downstream signaling molecules with other TGF ligands. We took advantage of the availability of different promoter-luciferase constructs for these hormones to elucidate their signaling pathways. The BRE promoter is known to be activated by several BMPs mediated by Smad1/5/8, whereas the CAGA promoter is activated by TGF /activin mediated by Smad2 and Smad3 (15, 16, 22). We tested the responses of the two reporter constructs in several cell lines transfected with these plasmids following treatment with the three orphan ligands. Although most cell lines tested did not respond to the ligand stimulation (data not shown), treatment of the MC3T3 cells with GDF6, GDF7, or BMP10 stimulated the luciferase reporter driven by the BRE promoter (Fig. 3A). Consistent with earlier findings (15, 22), the BRE promoter was activated by BMP2 and BMP7. Although TGF- was ineffective in activating the BRE promoter (Fig. 3A), treatment with TGF- stimulated the reporter driven by the CAGA promoter (Fig. 3B) (16). In contrast, treatment with GDF6, GDF7, or BMP10 did not activate the CAGA promoter (Fig. 3B). Similarly, treatment with BMP2 or BMP7 did not stimulate the CAGA promoter (Fig. 3B). Furthermore, the stimulatory effects of GDF6, GDF7, or BMP10 on the BRE promoter were found to be dose-dependent (Fig. 3C). The ability of GDF6, GDF7, and BMP10 to activate the Smad 1/5/8 pathway was further confirmed by immunoblotting analysis using specific antibodies against phospho-Smad1 or phospho-Smad2. As shown in Fig. 3D, treatment with GDF6, GDF7, or BMP10, similar to BMP2, induced the phosphorylation of Smad1 in MC3T3 cells. In contrast, they were ineffective in inducing phospho-Smad2 in the same cells. These findings suggest that MC3T3, a mouse pre-osteoblast cell line derived from neonatal mouse calvaria (23), likely expresses receptors necessary for GDF6, GDF7, and BMP10 signaling. Stimulation of the BRE Promoter by GDF6, GDF7, or BMP10 Is Blocked by the Inhibitory Smad6 and Smad7The transcriptional activities of all R-Smad proteins (Smad13, -5, and -8) are blocked by the inhibitory Smad7 (2427), whereas those of Smad1, -5, and -8, but not Smad2 and -3, are blocked by the inhibitory Smad6 (2729). We tested the inhibitory activities of these two inhibitory Smad proteins in MC3T3 cells treated with GDF6, GDF7, or BMP10. As shown in Fig. 4, cotransfection with plasmids encoding either Smad6 or Smad7 led to a dose-dependent suppression of the stimulation of the BRE promoter activity by GDF6, GDF7, or BMP10. In addition, a minor inhibition of the basal promoter activity was also found. Consistent with the reported role of Smad1, -5, and -8 in BMP2 signaling (30), overexpression of either Smad6 or Smad7 blocked the BMP2 stimulation of the BRE promoter in the same cells (Fig. 4). These results suggest that ligand signaling by GDF6, GDF7, or BMP10 does not involve the Smad2 and Smad3 pathway but is mediated by the BMP-responsive pathway through Smad1, -5, and -8.
Overexpression of ALK3 or ALK6 Confers GDF6 and GDF7 Responsiveness in the COS7 CellsIn preliminary tests, we found that treatment with GDF6 or GDF7 was minimally effective in activating the BRE promoter in the COS7 cells (Fig. 5A). We hypothesized that the type I receptor levels were too low in this cell line to allow signaling by the two orphan ligands. We performed overexpression tests in an attempt to gain ligand responsiveness and to identify the type I receptors involved in GDF6 and GDF7 signaling. As shown in Fig. 5A, transfection of these cells with increasing amounts of the ALK3 or the ALK6 expression plasmid led to a dose-dependent stimulation of the BRE promoter activity by GDF6 or GDF7. In contrast, overexpression of ALK1, -2, -4, -5, or -7 did not confer GDF6 or GDF7 activation of the BRE promoter. Unlike GDF6 and GDF7, COS7 cells were already responsive to BMP10 treatment, and transfection of increasing amounts of the ALK3 plasmid, but not the ALK6 plasmid, showed a small increase in the stimulation of the BRE promoter activity (Fig. 5A). In addition, overexpression of ALK1, -2, -4, -5, or -7 did not confer BMP10 activation of the BRE promoter. The increased expression of all seven ALK receptors in transfected cells was confirmed using RT-PCR (Fig. 5B). Expression of the Type I Receptor Transcripts in MC3T3 CellsTo confirm the importance of ALK3 and ALK6 in MC3T3 cells that were responsive to GDF6, GDF7, or BMP10, we performed RT-PCR using total RNA extracted from these cells to define the expression pattern of these receptors. As shown in Fig. 6, transcripts for ALK3, but not ALK6, were expressed in MC3T3. In addition, P19 cells were used as a positive control to verify the amplification of ALK6 transcript by RT-PCR (Fig. 6). The transcript levels for ALK3 and ALK6 in MC3T3 cells were confirmed by quantitative real-time PCR analysis (ALK3, 798,190 ± 134,79 copies/80,000 cells; ALK6, <2,000 copies/80,000 cells).
The Roles of Endogenous ALK3, BMPRII, and ActRIIA in Ligand Signaling by GDF6, GDF7, and BMP10Because MC3T3 cells are responsive to GDF6, GDF7, and BMP10 and express transcripts for the candidate type I receptor ALK3, we further tested the involvement of endogenous ALK3 in GDF6, GDF7, and BMP10 signaling. MC3T3 cells were transfected with plasmids encoding small hairpin (sh)RNA before testing the ability of the ligands to activate the BRE promoter. As shown in Fig. 7, transfection with increasing amounts of the ALK3 shRNA, but not the control shRNA, led to dose-dependent decreases in the BRE promoter activity stimulated by GDF6, GDF7, or BMP10. Although transfection with the ALK3 shRNA also suppressed BRE promoter activity stimulated by BMP2 in a dose-dependent manner, no suppression of the CAGA promoter activity by TGF Phylogenetic analysis of the entire repertoire of BMP/GDF ligands suggested that GDF6, GDF7, and BMP10 are closely related to BMP2, BMP7, and GDF5 (Fig. 1) and are likely to share the same type II receptors, BMPRII and ActRIIA. We first performed RT-PCR analysis to confirm the expression of BMPRII and ActRIIA in MC3T3 cells (Fig. 6). In addition, quantitative RT-PCR indicated that the BMPRII expression level was higher (836,360 ± 166,050 copies/80,000 cells) than the ActRIIA expression level (43,890 ± 7,790 copies/80,000 cells).
To test the role of endogenous BMPRII and ActRIIA in GDF6, GDF7, and BMP10 signaling, plasmids encoding BMPRII and ActRIIA shRNA were transfected into MC3T3 before testing the ability of the three ligands to activate the BRE promoter. As shown in Fig. 8, transfection of MC3T3 cells with BMPRII shRNA strongly suppressed the BRE promoter activity induced by GDF6, GDF7, or BMP10 in a dose-dependent manner, reaching a complete inhibition with 3 ng of transfected plasmid. Although ActRIIA shRNA was transfected at higher doses, GDF6 and GDF7 signaling was decreased by only 25%, and the BMP10 signaling was decreased by 50%. In addition, treatment with BMPRII shRNA or ActRIIA shRNA also suppressed BRE promoter activity stimulated by BMP2 in a dose-dependent manner (Fig. 8). However, no suppression of CAGA promoter activity induced by TGF
Based on a genomic analysis of the entire repertoire of TGF /BMP/GDF ligands and their corresponding serine/threonine kinase receptors, we tested the ability of three orphan GDF/BMP ligands to activate a limited number of phylogenetically related receptors. We characterized dimeric recombinant GDF6, GDF7, and BMP10 proteins, followed by the identification of the MC3T3 cell line responsive to these ligands based on the activation of the BMP-responsive promoter construct and the phosphorylation of the intracellular Smad1. Overexpression studies in a low responsive COS7 cell line and RNA interference analyses in MC3T3 cells further demonstrated the role of the type I receptors ALK3 and ALK6 in the mediation of GDF6, GDF7, or BMP10 signaling. We also found that overexpression of shRNA for the type II receptors BMP-RII and ActRIIA suppressed the signaling of GDF6, GDF7, and BMP10 in MC3T3 cells. Thus, the present approach provides a genomic paradigm for matching paralogous polypeptide ligands with a limited number of evolutionarily related receptors capable of activating downstream Smad proteins.
In vertebrates, the TGF superfamily consists of more than 30 related ligands, including TGF s, inhibins, BMPs, GDFs, Nodal, AMH, and Lefty (1, 2). Based on sequence comparison of their C-terminal cystine knot domains, a subfamily of closely related ligands, including GDF5, GDF6, GDF7, BMP9, and BMP10, can be identified (Fig. 1). They share 51% amino acid sequence similarity with each other. For comparison, the other subfamily containing BMP2/4/5/6/7/8 shares only 39% sequence similarity. The biologically active forms of many TGF family ligands are disulfide-linked dimers (31, 32). Here we showed that recombinant dimeric GDF6, GDF7, and BMP10 are bioactive. The three orphan ligands studied here play important roles in diverse physiological processes, including bone morphogenesis, nervous system development, and heart formation. GDF6 and GDF7 are expressed in developing cartilage, tendons, and ligaments, as well as in the nervous system (11, 13, 3336). GDF6 mutant mice showed joint defects with fusion between bones in the wrists and ankles (35). In Xenopus and Zebrafish embryos, GDF6 is expressed in the ectoderm and acts as an epidermal inducer and a neural inhibitor (37, 38). In contrast, GDF7 mutant mice were characterized by the absence of a subclass of commissural interneurons in the spinal cord and defects in the development of the seminal vesicle (11, 12, 39). A recent study also demonstrated the role of GDF7 in restricting the development of neural crest cells to sensory neurons (13). Furthermore, BMP10 expression is restricted to the developing and postnatal heart in the mouse and chicken (4042). BMP10-deficient mice are embryonic lethal at E9.510 because of cardiac dysgenesis (14). Studies using mice with defective type I and type II receptors only provided limited information on the potential receptors involved for GDF6, GDF7, and BMP10 because the phenotypes of different receptor null mice were distinct from those of mice with GDF6, GDF7, or BMP10 mutations. BMPRII and ALK3 are expressed in diverse embryonic and adult tissues, and null mice for each receptor showed early embryonic lethality (1, 3, 4346). Of interest, targeted deletion of ALK3 in the heart leads to the thinning of the myocardial wall and expansion of the trabeculated myocardium, similar to the phenotypes of BMP10 null mice (14, 47). In addition, the ActRIIA null mice showed reproductive defects, whereas the ALK6 null mice exhibited a similar phenotype to that of the GDF5 null mice, both exhibiting joint defects (1, 45, 48, 49). The present findings indicate that BMPRII and ALK3 are receptors for GDF6, GDF7, and BMP10. These data provide the basis to generate mice with tissue-specific deletion of these receptor genes in specific target tissues for future investigation.
In the present study, we found a pre-osteoblast MC3T3 cell line responsive to the three orphan ligands GDF6, GDF7, and BMP10. These ligands activated the BRE promoter through the phosphorylation of Smad1/5/8 (15, 22). As shown in Fig. 1, the three orphan ligands are closely related to BMP2, BMP7, and BMP9, reported previously to stimulate the BRE promoter (2, 15, 17, 22, 50). The essential role of the Smad1/5/8 pathway in GDF6, GDF7, and BMP10 signaling is further confirmed by the blocking effect of the inhibitory Smad6. Smad6 specifically competes with R-Smad1 for complex formation with Smad4 (28), thus preferentially inhibiting the Smad1/5/8 pathway. We also confirmed the blocking effect of Smad7, which stably interacts with all activated type I receptors to prevent R-Smad activation and downstream transcriptional modulation (24, 25). Similarly, Nakahara et al. (51) found that Smad6 and Smad7 also suppressed GDF5-mediated signaling in mouse B lineage cells.
Although GDF6 and GDF7 were expressed in the joint (3436), previous data suggested that GDF6 and GDF7 are not bone-inducing factors because, unlike BMP2, they did not induce the expression of the osteogenic marker alkaline phosphatase in the C2C12 cell lines (52, 53). However, by using the BRE promoter activation assay directly associated with the activation of the Smad pathway, our results demonstrated that GDF6 and GDF7 were able to activate the Smad1/5/8 signaling in the pre-osteblast MC3T3 cell line. These data suggest that GDF6 and GDF7 may play a role on bone cells, likely independent of the stimulation of alkaline phosphatase expression. In contrast, osteoblasts are not likely to be the BMP10 target in vivo because of the restricted expression of BMP10 in the heart. Nevertheless, our results showed its potent ability to activate the BRE promoter in the pre-osteoblast cell line MC3T3. Our findings are consistent with earlier data showing the induction of the osteogenic marker alkaline phosphatase by BMP10 in the C2C12 cell line (53). It is clear that the specificity of the paracrine actions of various BMP family members is not only dependent on the available repertoire of receptors expressed in target cells but also on the tissue-specific expression of ligands.
To identify the type I receptors for GDF6, GDF7, and BMP10, we used overexpression and RNA interference approaches. In COS7 cells, we overexpressed individually the seven type I receptors, and we identified ALK3 and ALK6 as candidate type I receptors for GDF6 and GDF7 based on the stimulation of the BRE promoter. It is interesting to note that COS7 cells were already responsive to BMP10 treatment, and only overexpression of ALK3 slightly increased BMP10 actions. These results suggested that the endogenous levels of type I and type II receptors in COS7 cells are sufficient for BMP10 signaling but inadequate for Smad1/5/8 activation by GDF6 or GDF7, raising the possibility of differences in receptor binding properties between BMP10 and GDF6/GDF7. We tested BMP10 action in diverse cell lines (P19, 293, CHO, and COS7) and found COS7 cells to be least responsive. Despite the observed responsiveness in COS7 cells, a potential role of ALK3 could still be demonstrated. Regarding the endogenous ALK proteins, our RT-PCR analyses indicated that ALK3, but not ALK6, was expressed in MC3T3 cells, suggesting that ALK3 is the type I receptor mediating GDF6, GDF7, and BMP10 signaling in this cell line. Indeed, transfection with the ALK3 shRNA suppressed GDF6, GDF7, and BMP10 stimulation of the BRE promoter. As expected, overexpression of the ALK3 shRNA also suppressed BMP2 signaling (43, 54) but not the TGF
ALK3, ALK6, and ALK2 have been shown to be the three type I receptors mediating responses by at least 10 BMP-related ligands (Fig. 1). The effects seen with various BMP family members are highly dependent on the tissue/cell type being investigated and likely reflect the available repertoire and expression levels of individual receptors and downstream effectors. Thus, a specific response to a given ligand may be determined by both the stoichiometry of ALKs in the cell and their ability to form complexes with particular type II receptors. Moreover, the specificity of hormonal responses may also depend on the levels of specific ligands and the presence of accessory receptors, thus leading to different degrees of Smad activation (58). Formation of heterodimers between ligands may also modify the affinity for a given receptor complex and induce different physiological responses (5962). Indeed, GDF7 has been shown to enhance the axon-orienting activity of BMP7 (62) likely through the formation of GDF7/BMP7 heterodimers. Because our results demonstrated that BMP7 and GDF7 homodimers share the same receptors, heterodimers formed by BMP7 and GDF7 could bind the receptor complex with different affinities to initiate differential downstream signaling. Furthermore, various receptor-interacting proteins could regulate signaling by TGF
Genomic analysis of the entire repertoire of TGF
* This work was supported by the NICHD, National Institutes of Health, CooperativeAgreement U54 HD31398 as part of the Specialized Cooperative Centers Program in Reproduction Research. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 To whom correspondence should be addressed. E-mail: aaron.hsueh{at}stanford.edu.
2 The abbreviations used are: TGF
We thank Dr. P. Ten Dijke (The Netherlands Cancer Institute, Amsterdam, Netherlands) and Dr. O. Ritvos (University of Helsinki, Finland) for providing ALK1, ALK2, ALK5, ALK6, ALK7, Smad6, and Smad7 plasmid constructs; Dr. A. Klibanski (Massachusetts General Hospital, Boston) for the pcI-ALK4 construct; Dr. C. H. Heldin (Ludwig Institute for Cancer Research, Stockholm, Sweden) for the CAGA promoter-luciferase construct and the anti-phospho-Smads antibodies; and Dr. O. Korchynskyi (University of North Carolina, Chapel Hill) for the BRE promoter-luciferase construct. We also thank C. Spencer for editorial assistance.
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