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The Institute for Enzyme Research, University of Tokushima, 3-18-15 Kuramoto, Tokushima 770-8503, Japan
(Requests for offprints should be addressed to K Tsuchida at Fujita Health University; Email: tsuchida{at}fujita-hu.ac.jp)
(Z H Liu is now at School of Basic Medical Sciences, Jilin University, 2 Xinmin Street, Changchun 130021, China)
(K Tsuchida is now at Division for Therapies Against Intractable Diseases, Institute for Comprehensive Medical Science, Fujita Health University, Toyoake, Aichi 470-1192, Japan)
(T Matsuzaki is now at Department of Biology, Faculty of Sciences, Kyushu University Graduate School, Fukuoka 812-8581, Japan)
(Y L Bao is now at The Institute of Genetics and Cytology, Northeast Normal University, 5268 Renmin St, Changchun 130024, China)
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The significance of the different ActRIIs is yet to be determined. Signal transduction via transmembrane receptors is regulated by their interaction with PDZ domain-containing molecules. PDZ domain-containing proteins play an important role in assembly of receptors and signaling molecules near submembranous regions (Fanning & Anderson 1999). To determine the specific functions of ActRIIs in regulating activin signal transduction, we searched for cytoplasmic proteins that interacted with the receptors. We identified two cytoplasmic proteins named activin receptor-interacting proteins 1 and 2 (ARIP1 and ARIP2). ARIP1 showed multiple protein–protein interacting domains including two WW domains which interacted with Smad3, and five to six PDZ domains which interacted with ActRIIs, phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and ß-catenin (Hirao et al. 1998, Shoji et al. 2000). ARIP2 possesses one PDZ domain which interacts with ActRIIA. ARIP2 also associates with Ral binding protein 1 (RalBP1), and assembles into a ternary complex that includes ARIP2, ActRII and RalBP1 (Matsuzaki et al. 2002). Overexpression of ARIP1 and ARIP2 inhibits activin-induced transcriptional activity in a dose-dependent manner (Shoji et al. 2000, Matsuzaki et al. 2002). In the present study, we report the identification of two isoforms of ARIP2, named ARIP2b and 2c. Similar to ARIP2, ARIP2b and 2c have only one PDZ domain, which specifically interacts with ActRIIA. However, even though ARIP2b and 2c were structural variants of ARIP2, they showed different functions. Overexpression of ARIP2b and 2c resulted in increased activin-induced signaling. Furthermore, we found that ARIP2b and 2c increased ActRIIA expression at the cell surface without affecting internalization of ActRIIA, whereas ARIP2 was involved in endocytosis of ActRIIA (Matsuzaki et al. 2002). Thus, multiple ARIP2 proteins can interact with ActRII to regulate its signaling and trafficking in a different manner.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Yeast two-hybrid screening was performed using a commercially available system (Matchmaker Two-Hybrid System 2; Clontech) in accordance with the manufacturer’s protocol. Approximately 4 x 106 clones of a mouse brain cDNA library were screened using the bait construct pAS-ActRIIA, which contains nucleotide sequences for the entire cytoplasmic region of mouse ActRIIA (554–1995) fused to a GAL4 DNA binding domain (Shoji et al. 2000, Tsuchida et al. 2001). One clone obtained by the yeast two-hybrid screening was named #YA-1. A mouse brain cDNA library in the lambda ZAPII vector (Stratagene, La Jolla, CA, USA) was screened using the #YA-1 cDNA as probe. ARIP2 cDNA was identified by screening as described previously (Matsuzaki et al. 2002). Additional clones encoding full-length ARIP2b and 2c sequences were identified by screening the brain library using full-length ARIP2 cDNA as hybridization probe.
DNA constructs
DNA constructs for the yeast two-hybrid screening were made using either the plasmid pAS2–1 to express the fusion protein with the GAL4 DNA-binding domain, or the plasmid pACT2 to express the fusion protein with the GAL4 activation domain. pAS-ActRIIA was made by introducing nucleotides 554–1995 of mouse ActRIIA (Mathews & Vale 1991) into pAS2–1. For the mammalian two-hybrid assay, DNA was constructed using the plasmid pBIND to express the fusion protein with the GAL4 DNA-binding domain, and the plasmid pACT to express the fusion protein with the VP16 activation domain. To make pBIND-ActRIIA, cDNA fragments encoding the entire cytoplasmic regions of ActRIIA were subcloned into pBIND. Using PCR-based mutagenesis, mutations in the COOH terminus of ActRIIA were generated by replacing appropriate nucleotides with mutated oligonucleotides (Shoji et al. 2000). Other receptor constructs have been described previously (Shoji et al. 2000). To make pACT-ARIP2b and 2c, cDNA fragments covering coding regions (nucleotides 453–758 of ARIP2b, and nucleotides 1–357 of ARIP2c) were prepared and sub-cloned into pACT. To make pACT-ARIP2c
C and pACT-ARIP2cPDZ, cDNA fragments composed of nucleotides 1–297 and 298–357 of ARIP2c respectively, were prepared by PCR and ligated into pACT. Expression constructs of ARIP2b and 2c were made by subcloning nucleotides 453–758 of ARIP2b and nucleotides 1–357 of ARIP2c into pCIneo and pcDNA3 respectively. FLAG-tagged ARIP2b and 2c were sub-cloned into pcDNA3. Full length ActRIIA cDNA was subcloned into pcDLSR
. Myc-tagged ActRIIA cDNA in pcDNA3 was made by incorporating 7 myc epitopes after the arginine residue (amino acid number 24) of ActRIIA cDNA in pcDNA3.
Yeast and mammalian two-hybrid analysis
Yeast two-hybrid assays were performed using a commercially available kit (Matchmaker Two-Hybrid System 2; Clontech) in accordance with the manufacturer’s protocol. Mammalian two-hybrid assays were performed using the CheckMate Mammalian Two-Hybrid System (Promega). In brief, Chinese hamster ovary K1 (CHO-K1) cells were co-transfected with appropriate plasmids, a cytomegalovirus promoter-driven ß-galactosidase (CMV-ß-gal), and the reporter plasmid pG5 luc, which drives the luciferase gene under the control of the GAL4-responsive promoter. Luciferase activity was measured and normalized against the level of ß-gal activity as described previously (Tsuchida et al. 1995).
Immunoprecipitation and Western blotting
Interactions of full-length ARIP2b or 2c with ActRIIA were studied. Using the calcium phosphate precipitation method, COS-7 cells or HEK 293 cells were transfected either with pcDNA3-FLAG-ARIP2b or 2c alone or co-transfected with pcDNA3-FLAG-ARIP2b (or 2c) and pcDLSR-ActRIIA. Two days after transfection, cells were harvested using lysis buffer (1% Nonidet P-40, 50 mM Tris–HCl (pH 7.5), 150 mM NaCl, 1 mM NaF, 1 mM phenylmethylsulfonyl fluoride (PMSF), 4 µg/ml leupeptin, and 1 µg/ml aprotinin). The lysate was cleared by centrifugation, and proteins were immunoprecipitated by adding a rabbit anti-ActRIIA polyclonal antibody (R&D Systems, Minneapolis, MN, USA), or anti-FLAG M2 monoclonal antibody (Sigma). Samples were then incubated with protein G-sepharose (Amersham Pharma-cia Biotech) at 4 °C for 2 h. Precipitated proteins were fractionated by SDS-PAGE, and transferred onto a polyvinylidene difluoride (PVDF) membrane. Membranes were initially incubated with an anti-FLAG antibody (1:1000 dilution), and then with a horseradish peroxidase-conjugated secondary antibody. Labeled proteins were detected by chemiluminescence (ECL-Plus; Amersham Pharmacia Biotech) (Bao et al. 2005). To study interactions of ARIP2 or ARIP2c with RalBP1, FLAG-ARIP2 (or 2c) and myc-tagged RalBP1 were co-transfected into HEK 293 cells and the lysate was immunoprecipitated with an anti-FLAG antibody and immunodetected using myc antibody (1:1000 dilution).
RNA extraction and RT-PCR
Total RNA from mouse tissues was extracted using the TRIzol reagent according to the manufacturer’s protocol (Invitrogen). Polyadenylated RNA was further purified using oligo (dT) latex beads (Takara, Tokyo, Japan). RNA samples were then reverse transcribed with oligo dT primer using SUPERSCRIPT II reverse transcriptase (Gibco-BRL). The ARIP2 primer set included the sense primer 5'-GGA GAG CAG TCA GAT ATG AAC G-3', and the antisense primer 5'-CAC GAA GAC CAA AAG AAC CTC CAA C-3', and the ARIP2c primer set included the sense primer 5'-GGA GAG CAG TCA GAT ATG AAC G-3', and the antisense primer 5'-CTA CTG TCC CAT ATC CAG GTG C-3'. PCR was performed for 28 cycles , with each cycle consisting of 94 °C for 30 s, 60 °C for 30 s and 72 °C for 30 s, followed by a final extension step at 72 °C for 8 min. Amplified PCR products were subjected to 2% agarose gel electrophoresis, and were stained using ethidium bromide for detection.
Antibody preparation and Western blotting
Glutathione S-transferase (GST) fusion proteins of full-length ARIP2b (amino acids 1–101) were used for immunization of New Zealand White rabbits. To obtain polyclonal antibodies recognizing the COOH-terminal region of ARIP2b/2c, but not ARIP2 and other PDZ proteins, crude rabbit antibodies were purified using protein A-Sepharose 4B (Amersham Biosciences), and affinity-chromatography of GST fusion proteins with N-terminal amino acids 1–82 of ARIP2b. The antibodies that recognize the COOH-terminal region of ARIP2b/ 2c, but not ARIP2, were thus obtained and used for Western blot analysis. Multiple mouse tissues were homogenized in buffer (1% Triton X-100, 50 mM Tris–HCl (pH 7.5), 150 mM NaCl, 1 mM PMSF, 4 µg/ml leupeptin, 1 µg/ml aprotinin), and centrifuged to yield supernatant fractions. Proteins were immunoprecipitated by adding an anti-ARIP2b/2c polyclonal antibody to the supernatant, and then incubated with protein G-sepharose (Amersham Pharmacia Biotech) at 4 °C for 2 h. Precipitated proteins were fractionated by SDS-PAGE using a 15% gel, and transferred onto a PVDF membrane. Membranes were initially incubated with an anti-ARIP2b/2c polyclonal antibody (diluted 1:500), and then with a horseradish peroxidase-conjugated secondary antibody. As a loading control, membranes were also probed with anti-actin antibody (Sigma).
Activin-responsive promoter analysis
The CAGA-lux construct has been described previously (Dennler et al. 1998). CAGA-lux, CMV-ß-gal, pCIneo-ARIP2b and pcDNA3-ARIP2c were introduced into HEK 293 cells or LßT2 cells using TransFast liposome reagents (Promega) in accordance with the manufacturer’s protocol. Stimulation by activin A and measurement of luciferase activity were performed as described previously (Tsuchida et al. 1995). We used activin A from WAKO Chemicals (Osaka, Japan) that is fully active.
Radioimmunoassay for follicle stimulating hormone (FSH)
LßT2 cells were grown in 12-well plates in DMEM supplemented with 10% fetal calf serum (FCS). ARIP2b cDNA was introduced into LßT2 cells using TransFast liposome reagents. Stimulation by 50 ng/ml activin A and measurement of FSH levels in conditioned media by radioimmunoassay (RIA) were performed as previously described (Graham et al. 1999). The sensitivity of the RIA was 4 ng/ml.
Cell surface 125I-activin binding and internalization assay
CHO-K1 cells grown in 6-well plates were transfected with ActRIIA cDNA with or without pcDNA3-ARIP2c using TransFast liposome reagents. To examine total (cell surface and internalized) activin binding activity of cells, transfected cells were washed three times with cold binding medium (minimum essential medium containing 20 mM HEPES-NaOH (pH 7.4) and 0.1% bovine serum albumin), and incubated in 800 µl cold binding medium containing 15 ng 125I-activin A labeled by the chloramine T method (Matsuzaki et al. 2002) for the indicated time periods on ice. Then unbound ligands were removed by washing three times with cold PBS. Cells were solubilized in 1 N NaOH, and the radioactivities of the cells were counted in a 
counter. To study internalization of ActRIIA, transfected cells were washed three times with binding medium, and incubated in 800 µl binding medium containing 15 ng 125I-activin A at 37 °C for the appropriate time periods. Following incubation, plates were placed on ice, washed three times with cold PBS, and then incubated with cold acid washing buffer (0.2 M acetic acid and 0.5 M NaCl) for 5 min on ice. Then, cells were solubilized in 1 M NaOH, and the radioactivities in the lysates were counted as above. Cell-associated radioactivity after the acid wash represented internalized activin mainly through receptor binding. Internalization was determined as the ratio of internalized 125I-activin-A/ surface bound 125I-activin-A. Nonspecific binding and internalization activities were determined in the presence of a 100-fold excess of unlabeled activin A.
Determination of ActRIIA expression at the cell surface
HEK 293 cells in 6-well dishes were transfected with myc-tagged ActRIIA cDNA and/or pcDNA3-ARIP2c expression plasmids, then cells were harvested in 200 µl PBS at various time points after transfection. Cells were then treated with 3 µl rabbit anti-myc antibody at 4 °C for 2 h to label ActRIIA at the cell surface, washed three times with PBS, and then extracted with lysis buffer as described in the Materials amd Methods section. Labeled receptors were collected with protein A sepharose, and detected by blotting with anti-ActRIIA antibody as described above.
Quantitative measurement of cell surface ActRIIA associated with ARIP2
HEK 293 cells were co-transfected with myc-tagged ActRIIA, FLAG-ARIP2 and/or FLAG-ARIP2c cDNAs. To quantitate ARIP2s that associate with ActRIIA, cell proteins were extracted with lysis buffer and immuno-precipitated with anti-myc antibody, and co-precipitated ARIP2s were detected by blotting with anti-FLAG antibody. To detect cell surface ActRIIA associated with ARIP2s, myc-tagged ActRIIA at the cell surface was labeled with myc antibody, collected by protein A sepharose and blotted with anti-FLAG antibody as described above.
Detection of recycling ActRIIA associated with ARIP2
HEK 293 cells were transfected with myc-tagged ActRIIA, FLAG-ARIP2 and FLAG-ARIP2c cDNAs. Twenty-four hours after transfection, cells were washed with PBS and exposed to 0.025% trypsin for 10 min at 4 °C. At the end of trypsin treatment, cells were washed three times with medium containing 10% FCS to stop trypsin activity. Then, cells were re-incubated in medium with 10% FCS at 37 °C, and treated either with 500 mM chloroquine or 50 mM monensin from WAKO Chemicals. Cells were collected at appropriate times. Cell surface myc-ActRIIA was labeled with myc antibody, collected with protein A sepharose and blotted with anti-FLAG antibody to detect ARIP2s as described above. To detect recycling ActRIIA associated with ARIP2s, total lysate was immunoprecipitated with myc antibody, then co-precipitated ARIP2s were detected by blotting with anti-FLAG antibody.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In the search for intracellular proteins that interact with ActRIIs, we obtained about 30 positive clones, including ARIP1 and ARIP2, with the yeast two-hybrid screening. Association of ARIP1 and ARIP2 with ActRIIA in mammalian cells was verified by co-immunoprecipitation and mammalian two-hybrid analysis (Shoji et al. 2000, Matsuzaki et al. 2002). Screening of a mouse brain cDNA library with the 462 bp coding region of ARIP2 as hybridization probe yielded full length cDNA clones of ARIP2b and 2c, and ARIP2 (Fig. 1A
). ARIP2 encoded a protein of 153 amino acids including the single PDZ domain as reported previously (Matsuzaki et al. 2002). ARIP2b cDNA encoded a protein of 101 amino acids with one PDZ domain which was identical to that of ARIP2 except for 8 amino acids at the NH2-terminal. However, the ARIP2b amino acid sequences differed from ARIP2 outside the PDZ domain (Fig. 1A,B). ARIP2c is composed of 118 amino acids, and is completely identical to ARIP2 from the NH2-terminal region through the PDZ domain, but its COOH-terminal region is identical with that of ARIP2b (Fig. 1A).
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PDZ domains recognize a class I consensus PDZ-binding motif XSX(V/I/L) (where X is any amino acid) in the COOH terminus of target proteins (Songyang et al. 1997). In agreement with the consensus PDZ-binding sequence, the four COOH-terminal amino acids of ActRIIA were ESSL. ARIP2b interacted with ActRIIA, but did not associate with ActRIIB, TGF-ß receptor type II (TGFßRII) or BMP receptor type II (BMPRII) by mammalian two hybrid analysis (Fig. 2A). Like ARIP2b, ARIP2c also interacted with ActRIIA. ARIP2c did not interact with mutated ActRIIA proteins that lacked either the COOH-terminal SSL or COOH-terminal leucine (Fig. 2B). Similar results were obtained for ARIP2b (data not shown). Thus, COOH-terminal amino acids of ActRIIA play a key role in interactions between ARIP2b and ARIP2c via the PDZ domain. Amino acid sequences of target proteins relate to specific interactions with various PDZ proteins. It is worthwhile noting that although ARIP2 interacted with both ActRIIA and ActRIIB, ARIP2b and 2c interacted with ActRIIA, but not with ActRIIB (Fig. 2A,B). These results indicate that ARIP2b or 2c interact specifically with ActRIIA among type II serine/threonine kinase receptors of the TGF-ß superfamily, and that this interaction occurs in both yeast and mammalian cells. We then studied which part of ARIP2c interacted with ActRIIA using a mammalian two-hybrid method. ARIP2cC, but not ARIP2cPDZ interacted with ActRIIA, indicating that ARIP2c interacts with ActRIIA through a PDZ domain-mediated interaction (Fig. 2C). ARIP2b, 2c and ARIP2 have an identical amino acid sequence in the PDZ domain which is important for interactions with ActRIIA. Lysates from COS-7 cells that have been co-transfected with ActRIIA, and FLAG-tagged ARIP2c were incubated with an anti-ActRIIA antibody, and co-immunoprecipitated ARIP2c protein was detected by an anti-FLAG antibody. As shown in Fig. 2D, ActRIIA and ARIP2c formed a protein complex in transfected cells. Similar interactions of ActRIIA and ARIP2b were observed (data not shown). ARIP1 interacts with Smads via WW domains, whereas ARIP2 does not interact with Smad proteins (Shoji et al. 2000, Matsuzaki et al. 2002). ARIP2 isoforms, i.e. ARIP2b and 2c, like ARIP2, did not show any interaction with Smad (data not shown).
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Northern blot analysis of poly(A)+RNA extracted from several mouse tissues using full length ARIP2 cDNA probe yielded two major bands of 4.5 kb and 1.1 kb (Matsuzaki et al. 2002). Since very short sequences in the COOH-terminal of ARIP2b/2c are different from ARIP2, and ARIP2 has an additional isoform called synaptojanin binding protein (also known as outer membrane protein of 25 kDa, OMP25) (Nemoto & DeCamilli 1999), it is difficult to detect ARIP2b/2c by Northern blotting. Therefore, we compared the tissue distribution of ARIP2 and ARIP2c mRNAs using RT-PCR. As shown in Fig. 3A, both ARIP2 and ARIP2c are widely expressed in various mouse tissues. ARIP2 mRNA was detected in multiple tissues. ARIP2c mRNA was detected in brain, liver, kidney, ovary and testis Although ARIP2 and 2c are structurally similar, their distributions differ slightly. ARIP2b mRNA was detected in heart, spleen and testes by RT-PCR, but its expression levels were much lower than those of ARIP2c mRNA (data not shown). We also studied protein expression of ARIP2b/ 2c. Antibodies that recognized ARIP2b/2c but not ARIP2 or OMP25 were prepared and used for Western blot analysis. As shown in Fig. 3B, ARIP2b/2c signal was present in heart, liver and kidneys, and to a lesser extent in brain, ovaries and lungs.
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HEK 293 cells were co-transfected with ARIP2c cDNA and the reporter plasmid CAGA-lux, and activin-induced luciferase activity was measured. CAGA-lux is a reporter plasmid that has CAGA tandem repeats, and responds well to activin and TGF-ß stimuli (Dennler et al. 1998). Overexpression of ARIP2c augmented activin-induced transcriptional activity in a dose-dependent manner (Fig. 4A). We also studied the effects of ARIP2b on FSH secretion in gonadotroph LßT2 cells, which secrete FSH in response to activin (Graham et al. 1999). LßT2 cells were transfected with ARIP2b and CAGA-lux DNAs, and activin-induced luciferase activity was measured. Overexpression of ARIP2b cDNA increased activity of CAGA promoter in a dose-dependent manner (Fig. 4B). FSH secretion by activin in the culture media was actually induced by ARIP2b cDNA transfection in LßT2 cells (Fig. 4C). ARIP2c showed a similar activity to ARIP2b.
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In a previous report, we showed that ARIP2 inhibited activin signaling (Matsuzaki et al. 2002). Since ARIP2 and ARIP2b/2c have opposite effects on activin signaling, we investigated whether ARIP2 and ARIP2b have antagonistic activities when expressed simultaneously. Expression of ARIP2b cDNA augmented activin signaling in a dose-dependent manner in HEK 293 cells (Fig. 4D
). In the presence of ARIP2, activin signaling was inhibited. When ARIP2b was co-expressed with ARIP2, the inhibitory effect of ARIP2 was blocked by ARIP2b cDNA in a dose-dependent manner. This result indicates that ARIP2 and ARIP2b have antagonistic activities on activin signaling. ARIP2c, like ARIP2b, showed antagonistic activities to ARIP2 (data not shown). Since ARIP2b and 2c are expressed in numerous tissues, they could play a role in controlling activin signaling transduction.
ARIP2b/2c neither affects endocytosis of ActRIIA nor interacts with RalBP1
In a previous study, we reported that ARIP2 regulated endocytosis of ActRIIA through the Ral/RalBP1-dependent pathway (Matsuzaki et al. 2002). To study the role of ARIP2b/2c in internalization of activin through ActRIIA, we performed an internalization experiment. Unlike ARIP2, overexpression of ARIP2c did not affect internalization of ActRIIA (Fig. 5A). In a parallel experiment, we reproduced enhanced internalization of ActRIIA by ARIP2 expression. To examine interactions of ARIP2c with RalBP1, we performed an immuno-precipitation experiment. As shown in Fig. 5B, ARIP2c did not significantly interact with RalBP1, whereas ARIP2 interacted with RalBP1. ARIP2b, like ARIP2c, did not intereact with RalBP1 (data not shown).
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To further investigate interactions between ARIP2c and ActRIIA, we co-expressed these proteins and examined the effects on surface receptor expression levels. Figure 6A shows the effects of activity of ARIP2c on the expression of ActRIIA in HEK 293 cells. In the presence of ARIP2c, the number of ActRIIA at the cell surface increased faster after the 18 h time point compared with control (Fig. 6A). In contrast, in the presence of ARIP2, cell surface levels of ActRIIA did not significantly change when compared with control (Fig. 6B).
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We next quantitated the amounts of ARIP2c and ARIP2 that associated with cell surface ActRIIA. After 6 h transfection, ARIP2 and ARIP2c began to be expressed, then increased and reached a maximal value at 18 h, and finally remained constant from 18 to 36 h after transfection (Fig. 6C, middle row). After 12 h transfection, ARIP2c co-precipitated with cell surface ActRIIA, and gradually reached a maximal amount from 18 to 36 h. In contrast, ARIP2 co-precipitated with cell surface ActRIIA at a much slower rate than ARIP2c. After 18 h transfection, co-precipitation of ARIP2 was observed, and slowly increased thereafter (Fig. 6C, top row). We also investigated total (cell surface and intracellular) ActRIIA associating with ARIP2 or ARIP2c (Fig. 6D). ARIP2c co-precipitated with total ActRIIA after 6 h transfection, then increased to reach a maximal amount and then remained constant from 18 to 36 h. In contrast, ARIP2 co-precipitated with ActRIIA after 12 h transfection, then slowly increased. We studied differences in association mechanisms of ARIP2 and ARIP2c to cell surface ActRIIA. First, we trypsinized transfected HEK 293 cells to degrade cell surface receptor binding activities, and newly synthesized ActRIIA associating with ARIP2s was quantitated at various time points by co-immunoprecipitation. As shown in Fig. 7A, after 2 h trypsin stripping, cell surface receptor-associated ARIP2 and ARIP2c were observed, and increased rapidly. Almost the same amounts of ARIP2 and ARIP2c co-precipitated with cell surface ActRIIA (Fig. 7A). Next, we used cycloheximide to block new protein synthesis after trypsin treatment, and measured the amounts of ARIP2 and ARIP2c that bound to newly appearing ActRIIA at the plasma membrane (Fig. 7B). Although almost equal amounts of ARIP2 and ARIP2c were observed in the total lysate, only small amounts of ARIP2c that co-precipitated with cell surface ActRIIA were detected (Fig. 7B). This result indicates that ARIP2c, possibly acting in a dominant-negative manner to block ARIP2, is involved in blocking endocytosis of ActRIIA by ARIP2. Then, we studied the association of ARIP2 or ARIP2c with ActRIIA using the recycling inhibitors, chloroquine or monensin. When chloroquine was used to interfere with recycling after trypsin treatment, the amounts of cell surface ARIP2 associating with ActRIIA significantly decreased, whereas the presence of chloroquine did not significantly affect the amounts of ARIP2c co-precipitating with cell surface ActRIIA (Fig. 7C, left panel). When monensin was used to prevent recycling of the internalized receptor back to the cell surface, only a small amount of ARIP2 co-precipitated with cell surface ActRIIA (Fig. 7C, right panel). Even without trypsin treatment, chroloquine treatment affected the amounts of cell surface ActRIIA associated with ARIP2, whereas it did not significantly affect the amounts of ARIP2c co-precipitating with cell surface ActRIIA (Fig. 7D). Taken together, these results indicate that ARIP2 is involved in endocytosis and recycling of ActRIIA, whereas ARIP2c, acting in a dominant-negative manner to block ARIP2, enhances ActRII levels at the cell surface.
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C. Interestingly, ARIP2C, like ARIP2b and 2c, enhances activin signaling (Matsuzaki et al. 2002). We did not detect significant interaction between ARIP2b/2c and ActRIIB. Similarly, there was no significant interaction between ARIP2b/2c and either TGFßRII or BMPRII (Fig. 2A). Analysis of interactions between ARIP2b/2c and ActRIIAs showed that ARIP2b/2c interact with the COOH terminus of ActRIIAs through the PDZ domain. The PDZ domains of ARIP2 and its isoforms are identical. The three COOH terminal amino acids of ActRIIA, consistent with the PDZ-binding motif, are critical for this interaction (Fig. 2). ARIP2 interacts with ActRIIA via its PDZ domain and with RalBP1 via its COOH-terminus, resulting in both endocytosis of activin type II receptors and an attenuated activin-induced transcriptional response (Matsuzaki et al. 2002). ARIP2b and 2c did not interact with RalBP1. Overexpression of ARIP2b or 2c augmented activin-induced transcription in HEK 293 cells (Fig. 4A), and increased activin-induced secretion of FSH from LßT2 cells (Fig. 4C). Further characterization revealed that, unlike ARIP2, ARIP2b/2c did not affect internalization of ActRII and showed antagonistic activity to ARIP2. Whether ARIP2b or 2c increase signal transduction in physiological conditions remains to be determined. Since ARIP2 and ARIP2b/2c have opposite effects on receptor localization and function, the ratio of ARIP2 to its isoforms, ARIP2b/2c, likely regulates ActRIIA activity. Our characterization of ARIP2 and its isoforms offers another example of isoforms having different functions. Involvement of Smad proteins in cellular signaling induced by the TGF-ß superfamily has been extensively studied (Heldin et al. 1997). In addition, there is accumulating evidence that multiple adaptors and scaffolding proteins interact with receptor serine/threonine kinases. ARIP1 and ARIP2 interact with ActRIIA and ActRIIB. Dok-1 shows homologies to pleckstrin and to phosphotyrosine-binding domains that associate with activin and TGF-ß receptors, and is a key element in activin-mediated apoptosis in B cells (Yamakawa et al. 2002). TRIP1 is a WD domain-containing protein that interacts with TGF-ß type II receptors (Chen et al. 1995). TRAP-1-like protein (TLP) also associates with activin and TGF-ß receptors (Felici et al. 2003). Interestingly, TLP suppresses Smad3-dependent signaling but potentiates Smad2-dependent signaling. These results indicate that adaptor proteins have a role in coordinating signal transduction by regulating protein complexes including receptor serine/threonine kinase receptors. ActRIIA is shared with activins, myostatin, nodal and BMP-7. These growth factors are known to form ligand gradients. One potential mechanism for forming morphogenetic gradients involves regulation of receptor numbers at the cell surface (McDowell & Gurdon 1999). Thus, ARIP2 and its variants may have a role in shaping morphogenetic gradients and in fine-tuning activin signaling during tissue formation (McDowell & Gurdon 1999).
Our characterization of ARIP2 and its isoforms as mediators of activin signaling has revealed a novel regulation of receptor serine/threonine kinases, and adds new insights into the mechanisms of regulation of signal transduction through ActRII by PDZ proteins.
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References |
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Received in final form 1 February 2006
Accepted 9 February 2006
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), ActRIIA (
), ActRIIA and ARIP2c (•) or ActRIIA and ARIP2 (
), treated in binding buffer with 15 ng 125I-activin A, and incubated at 37 °C for appropriate time periods. Graphs show internalization of ActRIIA calculated using the ratio of internalized activin/surface activin binding as described in Materials and Methods. Values shown are means and S.D. of triplicate determinations. (B) Interaction of RalBP1 with ARIP2 or ARIP2c. COS-7 cells were transfected with myc-RalBP1 and FLAG-ARIP2 or FLAG-ARIP2c, and then lysates were immunoprecipitated (IP) with anti-FLAG antibody, separated by SDS-PAGE, blotted onto a membrane, and probed with anti-myc antibody. In the middle row, total lysate was analyzed for RalBP1 expression. In the bottom row, total lysate was analyzed for FLAG (F)-ARIP2 or F-ARIP2c expression.
