Association Between Ligand-Induced Conformational Changes of Integrin IIbβ3 and IIbβ3-Mediated Intracellular Ca2+ Signaling

Blood ◽  
1998 ◽  
Vol 92 (10) ◽  
pp. 3675-3683 ◽  
Author(s):  
Shigenori Honda ◽  
Yoshiaki Tomiyama ◽  
Toshiaki Aoki ◽  
Masamichi Shiraga ◽  
Yoshiyuki Kurata ◽  
...  
Keyword(s):  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Conformational Changes ◽  
Binding Sites ◽  
Critical Role ◽  
Thromboxane A2 ◽  
Fibrinogen Binding ◽  
Group I ◽  
Group Ii ◽  
Inhibition Studies

Abstract Platelet IIbβ3 is a prototypic integrin and plays a critical role in platelet aggregation. Occupancy of IIbβ3 with multivalent RGD ligands, such as fibrinogen, induces both expression of ligand-induced binding sites (LIBS) and IIbβ3 clustering, which are thought to be necessary for outside-in signaling. However, the association between LIBS expression and outside-in signaling remains elusive. In this study, we used various IIbβ3-specific peptidomimetic compounds as a monovalent ligand instead of fibrinogen and examined the association between LIBS expression and outside-in signaling such as IIbβ3-mediated intracellular Ca2+ signaling. Using a set of monoclonal antibodies (MoAbs) against LIBS, we showed that antagonists can be divided into two groups. In group I, antagonists can induce LIBS on both IIb and β3 subunits. In group II, antagonists can induce LIBS on the IIb subunit, but not on the β3 subunit. Inhibition studies suggested that group I and group II antagonists interact with distinct but mutually exclusive sites on IIbβ3. Neither group I nor group II antagonist increased intracellular Ca2+concentrations ([Ca2+]i) in nonactivated platelets. All antagonists at nanomolar concentrations abolished the increase in [Ca2+]i in 0.03 U/mL thrombin-stimulated platelets, which is dependent on both fibrinogen-binding to IIbβ3 and platelet-aggregation. However, only group I antagonists at higher concentrations dose-dependently augmented the [Ca2+]i increase, which is due to aggregation-independent thromboxane A2 production. This increase in [Ca2+]i was not observed in thrombasthenic platelets, which express no detectable IIbβ3. Thus, only the group I antagonists, albeit a monovalent ligand, can initiate IIbβ3-mediated intracellular Ca2+ signaling in the presence of thrombin stimulation. Our findings strongly suggest the association between β3LIBS expression and IIbβ3-mediated intracellular Ca2+ signaling in platelets.

Association Between Ligand-Induced Conformational Changes of Integrin IIbβ3 and IIbβ3-Mediated Intracellular Ca2+ Signaling

Blood ◽  
1998 ◽  
Vol 92 (10) ◽  
pp. 3675-3683 ◽  
Author(s):  
Shigenori Honda ◽  
Yoshiaki Tomiyama ◽  
Toshiaki Aoki ◽  
Masamichi Shiraga ◽  
Yoshiyuki Kurata ◽  
...  
Keyword(s):  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Conformational Changes ◽  
Binding Sites ◽  
Critical Role ◽  
Thromboxane A2 ◽  
Fibrinogen Binding ◽  
Group I ◽  
Group Ii ◽  
Inhibition Studies

Platelet IIbβ3 is a prototypic integrin and plays a critical role in platelet aggregation. Occupancy of IIbβ3 with multivalent RGD ligands, such as fibrinogen, induces both expression of ligand-induced binding sites (LIBS) and IIbβ3 clustering, which are thought to be necessary for outside-in signaling. However, the association between LIBS expression and outside-in signaling remains elusive. In this study, we used various IIbβ3-specific peptidomimetic compounds as a monovalent ligand instead of fibrinogen and examined the association between LIBS expression and outside-in signaling such as IIbβ3-mediated intracellular Ca2+ signaling. Using a set of monoclonal antibodies (MoAbs) against LIBS, we showed that antagonists can be divided into two groups. In group I, antagonists can induce LIBS on both IIb and β3 subunits. In group II, antagonists can induce LIBS on the IIb subunit, but not on the β3 subunit. Inhibition studies suggested that group I and group II antagonists interact with distinct but mutually exclusive sites on IIbβ3. Neither group I nor group II antagonist increased intracellular Ca2+concentrations ([Ca2+]i) in nonactivated platelets. All antagonists at nanomolar concentrations abolished the increase in [Ca2+]i in 0.03 U/mL thrombin-stimulated platelets, which is dependent on both fibrinogen-binding to IIbβ3 and platelet-aggregation. However, only group I antagonists at higher concentrations dose-dependently augmented the [Ca2+]i increase, which is due to aggregation-independent thromboxane A2 production. This increase in [Ca2+]i was not observed in thrombasthenic platelets, which express no detectable IIbβ3. Thus, only the group I antagonists, albeit a monovalent ligand, can initiate IIbβ3-mediated intracellular Ca2+ signaling in the presence of thrombin stimulation. Our findings strongly suggest the association between β3LIBS expression and IIbβ3-mediated intracellular Ca2+ signaling in platelets.

CHARACTERIZATION OF THE FIBRINOGEN BINDING SITES USING MONOCLONAL ANTIBODIES TO HOMAN PLATELET MEMBRANE GLYCOPROTEINS IIb/IIIa*

1987 ◽  
Author(s):  
C Ruan ◽  
X Du ◽  
H Wan ◽  
X Hu ◽  
X Xi ◽  
...  
Keyword(s):  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Binding Sites ◽  
Human Platelets ◽  
Prolonged Treatment ◽  
Antigenic Determinants ◽  
Membrane Glycoproteins ◽  
Fibrinogen Binding ◽  
Igg1 Subclass ◽  
Academy Of Sciences

Two new murine monoclonal antibodies, SZ-21 and SZ-22, (both IgG1 subclass), were produced by the hybridoma technique using washed human platelets as the immunogen. Both SZ-21 and SZ-22 reanted specifically with normal platelets and megakaryocytes but not with Glanzmann's thrombasthenic platelets which lack the membrane glycoprotein(GP)IIb/IIIa complex. Platelets from 10 normal donors bound 64,500±20,300(x+SD) SZ-21 molecules/platelet with KD4.4±1.5nM and 61,000±19,900 SZ-22 molecules/platelet with KD18.8±6.7nM respectively.Affinity chromatography confirmedthat SZ-21 and SZ-22 reacted with theGPIIb/IIIa complex. SZ-21 inhibited the platelet aggregation and secretioninduced by collagen, arachidonic acidand thrombin,and the second wave of aggregation in response to ADP, adrenaline and ristocetin. The fibrinogen binding to human platelets induced by ADP, arachidonic acid and PAF was also inhibited by SZ-21. But SZ-22 hado effects on platelet aggregation andfibrinogen binding. Western blot analyses indicated that SZ-21reanted with GPIIIa and that SZ-22 bound to GPIIb. When the protein was reduced by 2-mercaptoethand, SZ-22 reacted with the enchain of GPIIb, but SZ-21 lost its ability of binding to GPIIIa, suggested that the antigenic determinants of SZ-21 depended on the integrity ofthe intra αchain disulfide bond of GPIIIa. On chymotrypsin treated platelets,the epitope for SZ-21wasidentified on a 66 KD membrane-bound fragment of GPIIIa. The appearanceofthe 66 KD fragment was related withthechymotrypsin-mediated fibrinogen binding and aggregation. However the prolonged treatment with chymotrypsin reduced the platelet aggregation, which coincided with the appearance of the 66KD fragment, a product of thefurther hydrolysis of the 66KD fragment.These results suggested that the domainbetween the 66KD and 60KD fragments of GPIIIa might be essential for the maintenance of the fibrinogen binding sites.* Supported by the Science fund of the Chinese Academy of Sciences ( project No.82-172 ) and the National Science fund of China ( project No.3860713 ).

Difference of [Ca2+]i Movements in Platelets Stimulated by Thrombin and TRAP: the Involvement of αIIbβ3-Mediated TXA2 Synthesis

1998 ◽  
Vol 79 (06) ◽  
pp. 1184-1190 ◽  
Author(s):  
Yoshiaki Tomiyama ◽  
Shigenori Honda ◽  
Kayoko Senzaki ◽  
Akito Tanaka ◽  
Mitsuru Okubo ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Fibrinogen Binding ◽  
Adp Release ◽  
The Difference ◽  
Txa2 Receptor Antagonist ◽  
High Level ◽  
Inhibition Studies ◽  
Peak Increase ◽  
Second Peak ◽  
Assay Conditions

SummaryThis study investigated the difference of [Ca2+]i movement in platelets in response to thrombin and TRAP. The involvement of αIIbβ3 in this signaling was also studied. Stimulation of platelets with thrombin at 0.03 U/ml caused platelet aggregation and a two-peak increase in [Ca2+]i. The second peak of [Ca2+]i, but not the first peak was abolished by the inhibition of platelet aggregation with αIIbβ3 antagonists or by scavenging endogenous ADP with apyrase. A cyclooxygenase inhibitor, aspirin, and a TXA2 receptor antagonist, BM13505, also abolished the second peak of [Ca2+]i but not the first peak, although these regents did not inhibit aggregation. Under the same assay conditions, measurement of TXB2 demonstrated that αIIbβ3 antagonists and aspirin almost completely inhibited the production of TXB2. In contrast to thrombin-stimulation, TRAP caused only a single peak of [Ca2+]i even in the presence of platelet aggregation, and a high level of [Ca2+]i increase was needed for the induction of platelet aggregation. The inhibition of aggregation with αIIbβ3 antagonists had no effect on [Ca2+]i change and TXB2 production induced by TRAP. Inhibition studies using anti-GPIb antibodies suggested that GPIb may be involved in the thrombin response, but not in the TRAP. Our findings suggest that low dose thrombin causes a different [Ca2+]i response and TXA2 producing signal from TRAP. Endogenous ADP release and fibrinogen binding to αIIbβ3 are responsible for the synthesis of TXA2 which results in the induction of the second peak of [Ca2+]i in low thrombin- but not TRAP-stimulated platelets.

scholarly journals Inhibition of receptor binding and neutralization of bioactivity by anti-erythropoietin monoclonal antibodies

Blood ◽  
1990 ◽  
Vol 75 (4) ◽  
pp. 874-880 ◽  
Author(s):  
AD D'Andrea ◽  
PJ Szklut ◽  
HF Lodish ◽  
EM Alderman
Keyword(s):  
Monoclonal Antibodies ◽  
Receptor Binding ◽  
Sodium Dodecyl ◽  
Dependent Manner ◽  
Epo Receptor ◽  
High Affinity ◽  
Group I ◽  
Dependent Cell ◽  
Group Ii ◽  
Murine Erythroleukemia

Abstract We have generated four high affinity monoclonal antibodies (MoAbs) to recombinant human erythropoietin (EPO). All four MoAbs immunoprecipitate radioiodinated native EPO, and the concentrations of MoAbs required for maximum binding range from 10 nmol/L to 100 nmol/L. Two MoAbs, designated Group I MoAbs, bind to an epitope within the N- terminal 20 amino acids of EPO and also immunoprecipitate sodium dodecyl sulfate (SDS)-denatured EPO. Two other MoAbs (Group II MoAbs) do not immunoprecipitate SDS-denatured EPO and do not bind to any of the eight endo C fragments of EPO. We first used murine erythroleukemia (MEL) cells to test the MoAbs for inhibition of EPO-receptor binding. MEL cells, although unresponsive to EPO, express 760 high affinity receptors for EPO per cell (Kd = 0.24 nmol/L). To assay our MoAbs, MEL cells were grown as monolayers on fibronectin-coated Petri dishes and incubated at 4 degrees C with radioiodinated EPO. Group I MoAbs do not inhibit binding of radioiodinated EPO to the MEL EPO-receptor, but Group II MoAbs do inhibit binding in a dose-dependent manner. We next examined the neutralization of EPO bioactivity by our MoAbs, using EPO- dependent cell line. Only Group II MoAbs inhibit a newly developed EPO- dependent cell growth, demonstrating that inhibition of EPO-receptor binding correlates with neutralization of EPO bioactivity.

scholarly journals Inhibition of acanthamoeba actomyosin-II ATPase activity and mechanochemical function by specific monoclonal antibodies.

1984 ◽  
Vol 99 (3) ◽  
pp. 1024-1033 ◽  
Author(s):  
D P Kiehart ◽  
T D Pollard
Keyword(s):  
Monoclonal Antibodies ◽  
Atpase Activity ◽  
Conformational Changes ◽  
Binding Sites ◽  
Polyclonal Antibodies ◽  
Myosin Ii ◽  
Antigenic Site ◽  
Actin Binding ◽  
Filamentous Form ◽  
Antibody Effects

Monoclonal and polyclonal antibodies that bind to myosin-II were tested for their ability to inhibit myosin ATPase activity, actomyosin ATPase activity, and contraction of cytoplasmic extracts. Numerous antibodies specifically inhibit the actin activated Mg++-ATPase activity of myosin-II in a dose-dependent fashion, but none blocked the ATPase activity of myosin alone. Control antibodies that do not bind to myosin-II and several specific antibodies that do bind have no effect on the actomyosin-II ATPase activity. In most cases, the saturation of a single antigenic site on the myosin-II heavy chain is sufficient for maximal inhibition of function. Numerous monoclonal antibodies also block the contraction of gelled extracts of Acanthamoeba cytoplasm. No polyclonal antibodies tested inhibited ATPase activity or gel contraction. As expected, most antibodies that block actin-activated ATPase activity also block gel contraction. Exceptions were three antibodies M2.2, -15, and -17, that appear to uncouple the ATPase activity from gel contraction: they block gel contraction without influencing ATPase activity. The mechanisms of inhibition of myosin function depends on the location of the antibody-binding sites. Those inhibitory antibodies that bind to the myosin-II heads presumably block actin binding or essential conformational changes in the myosin heads. A subset of the antibodies that bind to the proximal end of the myosin-II tail inhibit actomyosin-II ATPase activity and gel contraction. Although this part of the molecule is presumably some distance from the ATP and actin-binding sites, these antibody effects suggest that structural domains in this region are directly involved with or coupled to catalysis and energy transduction. A subset of the antibodies that bind to the tip of the myosin-II tail appear to inhibit ATPase activity and contraction through their inhibition of filament formation. They provide strong evidence for a substantial enhancement of the ATPase activity of myosin molecules in filamentous form and suggest that the myosin filaments may be required for cell motility.

Role of phosphoinositide 3-kinase β in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways

2010 ◽  
Vol 429 (2) ◽  
pp. 369-377 ◽  
Author(s):  
Analia Garcia ◽  
Soochong Kim ◽  
Kamala Bhavaraju ◽  
Simone M. Schoenwaelder ◽  
Satya P. Kunapuli
Keyword(s):  
Platelet Aggregation ◽  
Critical Role ◽  
Thromboxane A2 ◽  
Inhibitory Effect ◽  
Human Platelets ◽  
P2y12 Receptor ◽  
Functional Responses ◽  
Induce Platelet Aggregation ◽  
Erk Phosphorylation

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y12 receptor stimulation and generate pro-aggregatory signals. P2Y12 receptor has been shown to play a key role in the platelet aggregation and thromboxane A2 generation caused by co-stimulation with Gq or Gz, or super-stimulation of Gi pathways. In the present study, we evaluated the role of specific PI3K isoforms α, β, γ and δ in platelet aggregation, thromboxane A2 generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A2 generation. We also show that Gq plus Gi- or Gi plus Gz-mediated platelet aggregation, ERK phosphorylation and thromboxane A2 generation in human platelets was inhibited by TGX-221, a PI3Kβ-selective inhibitor, but not by PIK75 (a PI3Kα inhibitor), AS252424 (a PI3Kγ inhibitor) or IC87114 (a PI3Kδ inhibitor). TGX-221 also showed a similar inhibitory effect on the Gi plus Gz-mediated platelet responses in platelets from P2Y1−/− mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3Kβ in Gi-mediated signalling. Taken together, our results demonstrate that PI3Kβ plays an important role in ADP-induced platelet aggregation. Moreover, PI3Kβ mediates ADP-induced thromboxane A2 generation by regulating ERK phosphorylation.

Platelet Function Studies in Normal Volunteers and Patients with Angina Pectoris

1979 ◽  
Author(s):  
J.J.C. Jonker ◽  
den G.J.H. Ottolander
Keyword(s):  
Platelet Aggregation ◽  
Angina Pectoris ◽  
Platelet Function ◽  
Normal Range ◽  
Double Blind ◽  
Group Iii ◽  
Platelet Survival ◽  
Group I ◽  
Group Ii

In 30 normal subjects (group I) and in 89 patients with angina pectoris we studied: the platelet survival time (PST), the platelet aggregation test I (PAT I) acc. to Breddin, the platelet aggregation ratio (PAR) acc. to Wu and Hoak and the Filtragometer log TA acc. to Hornstra. The patients were divided in two groups: 46 patients had already been treated for 6 months with Clofibrate (group II) and 43 patients with placebo (group III) in a double blind trial. The average PST (T½) was within the normal range (group I 99 hrs. group II 105,7 hrs.; group III 102,0 hrs.). About 20% of patients of group II and III had abnormally shortened T½. The PAT I was on average abnormal in group II and III (PAT I in group II 2,3; group III 2,7), but group II normalized after 12 months treatment (PAT I 1,85). The PAR was abnormal in group III, while group II was within the normal range (group I 0,87; group II 0,82; group III 0,69). The log TA results were abnormal in group II and III (group I 2, 45, group II 2,1; group III 2, 1), after 12 months treatment the patient group remained abnormal (group II 2,2; group III 2,1). We failed to find a correlation between the four platelet function tests, nor with these tests and basic laboratory values. The PAT I, the PAR and the Filtragometer seems to be valuable in the detection of abnormal platelet behavior in vitro, but it does not mean than an abnormal platelet survival in vivo occurs in the same individuals.

Requirement Of Fibrinogen And Calcium For Inter-Platelet Bridges In Platelet Aggregation: A Hypothesis

1981 ◽  
Author(s):  
Elizabeth Kornecki ◽  
Stefan Niewiarowski
Keyword(s):  
Platelet Aggregation ◽  
Binding Sites ◽  
Proteolytic Enzymes ◽  
Divalent Cations ◽  
Platelet Surface ◽  
Fibrinogen Binding ◽  
Platelet Aggregates ◽  
High Concentrations ◽  
Induced Aggregation ◽  
Aggregation Of Platelets

Fibrinogen and calcium are required for the aggregation of platelets stimulated by ADP or pre-treated with proteolytic enzymes. Specific platelet surface fibrinogen binding sites (receptors) are exposed after platelets are stimulated by ADP or pre-treated with Chymotrypsin or pronase. It has previously been shown in our laboratory that an intact, symmetrical fibrinogen molecule is essential for fibrinogen binding and fibrinogen-induced aggregation of both ADP-stimulated and proteolytically-treated platelets. Here we propose that the mechanism by which fibrinogen and calcium aggregate platelets is by forming inter-platelet bridges linking the fibrinogen receptors of adjacent platelets together. In support of this proposition are the following new lines of evidence: 1) The fibrinogen-induced aggregations of ADP-stfiliulated or proteolytically-treated platelets are inhibited by high concentrations of fibrinogen (Ki=2.6 and 8.5 × 10 5M, respectively). The fibrinogen binding sites on adjacent platelets, at these concentrations, would be saturated by fibrinogen and therefore no inter-platelet fibrinogen bridges could be formed to hold the platelets together. 2) ADP-stimulated or chymotrypsin-treated platelets aggregated by fibrinogen are deaggregated by Chymotrypsin or pronase and this deaggregation coincides with the loss of 125I-fibrinogen from the platelet surface. 3) Preincubation of platelets with EDTA results in inhibition of both platelet aggregation and 125I-fibrinogen binding. Following the aggregations of ADP-stimulated or of chymotrypsin-treated platelets by fibrinogen, the addition of EDTA to the platelet aggregates results in both their deaggregation and their loss of bound 125I-fibrinogen. Thus it appears that divalent cations, especially calcium, are essential for the formation of fibrinogen-linked platelet aggregates.

Urinary sodium, phosphate, and hydrogen excretion following unilateral kidney clamping

1978 ◽  
Vol 56 (3) ◽  
pp. 428-434 ◽  
Author(s):  
Gérard E. Plante ◽  
Hubert Hermkens
Keyword(s):  
Sodium Phosphate ◽  
Renal Plasma Flow ◽  
Left Kidney ◽  
Critical Role ◽  
Extracellular Volume ◽  
Urinary Sodium ◽  
Ammonium Excretion ◽  
Titratable Acid ◽  
Group I ◽  
Group Ii

The influence of extracellular volume on compensatory adaptation of sodium, phosphate, and hydrogen excretion after unilateral kidney exclusion was examined in the dog. Ammonium chloride was administered to all animals to raise urinary hydrogen excretion. Eleven dogs (group I) were studied under relative hydropenia and 11 (group II) were volume expanded with isotonic NaCl. Six animals (group III) prepared as group II were sham operated.In groups I and II, plasma bicarbonate decreased by 1.1 and 1.2 mM/ℓ after unilateral kidney clamping, while serum phosphate rose from 5.3 to 6.1 and from 3.6 to 3.9 mg%, respectively. The following parameters are those obtained from the left kidney before and after contralateral clamping. Urinary sodium remained unchanged after clamping in group I but rose from 422 to 681 μequiv./min in group II. In contrast, urinary phosphate increased from 83 to 233 and from 94 to 189 μg/min in groups I and II, respectively.Measured titratable acid and urinary ammonium excretion remained unchanged in group I. A small but significant decrement in ammonium was obtained with time in group II. Glomerular filtration and renal plasma flow were not influenced by contralateral clamping, but filtration fraction was lower in group II than in group I. No change occurred in group III.Extracellular volume appears to play a critical role in the physiological expression of contralateral natriuresis after unilateral clamping whereas compensatory phosphaturia is obtained in both hydropenic and volume-expanded animals.

scholarly journals Synergistic action of two murine monoclonal antibodies that inhibit ADP- induced platelet aggregation without blocking fibrinogen binding

Blood ◽  
1987 ◽  
Vol 69 (2) ◽  
pp. 668-676
Author(s):  
PJ Newman ◽  
RP McEver ◽  
MP Doers ◽  
TJ Kunicki
Keyword(s):  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Adenosine Diphosphate ◽  
Platelet Surface ◽  
Synergistic Inhibition ◽  
Activated Platelets ◽  
Fibrinogen Binding ◽  
Complete Restoration ◽  
Binding Event ◽  
Murine Monoclonal Antibodies

We have used two murine monoclonal antibodies, each directed against one component of the human platelet membrane glycoprotein (GP) IIb-IIIa complex, to examine further the molecular requirements for fibrinogen binding to the platelet surface and subsequent platelet-platelet cohesion (aggregation). Although neither AP3, which is directed against GPIIIa, nor Tab, which is specific for GPIIb, were individually able to inhibit adenosine diphosphate (ADP)-induced fibrinogen binding, platelet aggregation, or secretion, the combination of AP3 and Tab completely abolished platelet aggregation and the release reaction. Unexpectedly, this synergistic inhibition of platelet-platelet cohesion occurred in the presence of apparently normal fibrinogen binding. Both the number of fibrinogen molecules bound and the dissociation constant for fibrinogen binding remained essentially unchanged in the presence of these two antibodies. Inhibition of aggregation was dependent upon the divalency of both AP3 and Tab because substitution of Fab fragments of either antibody for the intact IgG resulted in a complete restoration of both aggregation and secretion. In contrast to ADP induction, thrombin-activated platelets neither aggregated nor bound fibrinogen in the presence of AP3 plus Tab but were fully capable of secretion, which illustrated the multiple mechanisms by which the platelet surface can respond to different agonists. These data demonstrate that fibrinogen binding to the platelet surface alone is not sufficient to support platelet-platelet cohesion and that an additional post-fibrinogen-binding event(s) that is inhibitable by these two monoclonal antibodies may be required.

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