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Background Venipuncture is a common procedure in the neonatal intensive care unit (NICU) and causes significant pain for neonates. Aim To evaluate the effect of maternal voice on pain caused by venipuncture (including peripheral venipuncture and femoral venipuncture) in neonates hospitalized in the NICU. Design Experimental, randomized controlled study. Setting The study was conducted in the NICU of two hospitals in China from November 2017 to January 2019. Methods One hundred and sixteen neonates were randomly assigned to the maternal voice or routine care groups. The maternal voice group received recorded maternal voice intervention before, during, and after venipuncture. Three phases of procedures were videotaped. Neonatal Infant Acute Pain Assessment Scale (NIAPAS) was assessed by the same evaluator at different phases. Results The study showed that NIAPAS scores, behavioral indicator scores, and physiological indicator scores in the maternal voice group were significantly lower compared with those in the routine care group. Conclusion Recorded maternal voice can improve pain caused by venipuncture in neonates. These are simple, rapid, and cost-effective methods that nurses can implement during venipuncture in neonates.

Ibrutinib is highly efficacious and used at 420 mg/d for treatment of chronic lymphocytic leukemia (CLL). We previously demonstrated a decline in Bruton’s tyrosine kinase (BTK) protein levels in CLL cells after 1 cycle of ibrutinib, suggesting ibrutinib dose could be lowered after the first cycle without loss of biological effect. To test this postulate, a pilot study (NCT02801578) was designed to systematically reduce ibrutinib dosing within the same patient with CLL over the course of three 28-day cycles. After an initial cycle of 420 mg/d, the dose was reduced to 280 mg/d in cycle 2, and then to 140 mg/d in cycle 3. Eleven patients began study treatment, and 9 completed the 3 cycles. Plasma and intracellular pharmacokinetics (PK), BTK occupancy, and pharmacodynamic (PD) response at different doses of ibrutinib were compared. Plasma and intracellular levels of ibrutinib were dose-dependent, and even the lowest dose was sufficient to occupy, on average, more than 95% of BTK protein. In concert, BTK downstream signaling inhibition was maintained with 140 mg/d ibrutinib in cycle 3, and there were comparable reductions in total and phospho-BTK (Tyr223) protein levels across 3 cycles. Reductions of plasma chemokine CCL3 and CCL4 levels, considered to be biomarkers of ibrutinib response, were similar during the 3 cycles. These PK/PD data demonstrate that after 1 cycle of ibrutinib at the standard 420 mg/d dose, the dose can be reduced without losing biological activity. Clinical efficacy of lower doses needs to be systematically evaluated. Such dose reductions would lower drug cost, lessen untoward toxicity, and facilitate rationale-based combinations. This trial was registered at www.clinicaltrials.gov as #NCT02801578.

Objectives The objective of this study was to describe the occurrence of neonatal procedural pain and explore the factors that influence the frequency of painful procedures. Design A descriptive prospective epidemiologic study. Setting NICU at a general hospital in China. Methods A demographic and diagnosis or illness information questionnaire and an occurrence of procedural pain questionnaire specifically designed for this study were used to record the current status of neonatal procedural pain. The neonatal infant pain scale (NIPS) was used to measure pain intensity. A multiple linear regression model was used to explore the factors influencing the frequency of painful procedures. Results One hundred and twenty neonates experienced a total of 16,840 painful procedures. Each neonate was exposed to a median (IQR) of 66.5(27,154.75) painful procedures during hospitalization and a median (IQR) of 13(11, 19) painful procedures. All 27 different procedures were considered painful, and 70.37% (19/27) of these procedures caused severe pain. Overall, the mean NIPS score of the 27 procedures was 5.04 ± 1.52 with a range from 0 to 7. Respiratory support, age, and length of hospital stay were factors influencing the frequency of painful procedures. Conclusions NICU neonates experience pain at a high frequency and intensity during hospitalization. Respiratory support, age, and length of hospital stay were factors influencing the frequency of painful procedures. Strategies are needed to bridge the gap between practice and the evidence-based guidelines.

Paraneoplastic thrombocytosis has been reported in different types of solid tumors, including ovarian epithelial cancer, and found to be associated with a worse outcome. Although the effect of cancer on increasing platelet counts is well documented, the effect of cancer on platelet functions is not well-known. We compared in vitro aggregation response of platelets isolated from 34 patients with ovarian cancer to those of platelets from 19 patients with benign ovarian tumors. Aggregation studies were conducted in a light transmission aggregometer, using both a high and a low dose of ADP and collagen. We evaluated platelet preactivation by measuring the plasma concentration of β-thromboglobulin (β-TG) and platelet factor-4 (PF-4) as markers of platelet α granule secretion, using ELISA. We found that ovarian cancer is not associated with an enhanced aggregation response of platelets to ADP or collagen, and plasma concentration of β-TG and PF-4 is not higher in patients with ovarian cancer compared to those in patients with benign ovarian tumors.

We examined the hypothesis that filamin A binding to the cytoplasmic tail of platelet glycoprotein Ibα (GpIbα) is regulated by pathologic shear stress and modulates von Willebrand factor (VWF)–induced platelet activation. To begin, we examined filamin binding to GpIbα in Chinese hamster ovary cells coexpressing mutant human GpIb-IX and wild-type human filamin A. We observed that many different deletions and truncations N-terminal to GpIbα's cytoplasmic domain residue 594 disrupted filamin A binding, but that binding was unaffected by 14 different point mutations in hydrophilic residues between amino acids 557 and 593. To try to narrow GpIbα's filamin A–binding domain, we next measured the effect of several cytoplasmic domain peptides on human filamin A binding to a GST-GpIbα cytoplasmic domain fusion protein. One peptide (residues 557-575; designated “A4 peptide”) inhibited filamin A binding to the GST-GpIbα cytoplasmic domain fusion protein and competed with GpIbα for binding to filamin A. When the A4 peptide was delivered to intact human platelets using a carrier peptide, we observed the dose-dependent inhibition of VWF-induced platelet aggregation in response to both ristocetin and shear stress. The effect of the A4 peptide on shear-induced platelet aggregation was accompanied by the attenuation of shear-induced filamin A binding to GpIbα and diminished shear-dependent protein tyrosine phosphorylation. These results suggest that shear-dependent VWF-induced platelet activation affects filamin A binding to GpIb-IX-V, and that filamin A binding to the cytoplasmic tail of GpIbα regulates proaggregatory tyrosine kinase signaling.

Pathologically elevated shear stress triggers aspirin-insensitive platelet thrombosis. Signaling mechanisms involved in shear-induced platelet thrombosis are not well understood. To investigate these, we examined the hypothesis that functionally important platelet phosphatidylinositol 3-kinase (PI3-K) activity is stimulated by an in vitro shear stress of 120 dynes/cm(2) (shear rate of 6,000 sec(-1)). Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) production was examined in washed human platelets subjected to pathological shear stress in a cone-plate viscometer. PIP(3) production peaks 30 s after shear begins and is initiated by von Willebrand factor (VWF) binding to the glycoprotein (Gp) Ib-IX-V complex. Inhibiting PI3-K with wortmannin or 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) results in the inhibition of shear-induced platelet aggregation. In resting platelets, class IA PI3-K associates with the tyrosine kinase Syk. Within 30 s of beginning shear, PI3-K-associated Syk becomes tyrosine phosphorylated. Inhibiting Syk activation with piceatannol results in the inhibition of PIP(3) production and aggregation. Selective blockade of the P2Y(12) receptor results in the inhibition of Syk phosphorylation, PIP(3) production, and aggregation. These results indicate that shear-induced VWF binding to platelet GpIb-IX-V stimulates functionally important PI3-K activity. PI3-K activation is signaled by rapid feedback amplification that involves P2Y(12) receptor-mediated activation of Syk.

Several complement proteins interact with hemostatic factors. We discovered that von Willebrand factor (VWF) acts as a cofactor for factor I–mediated cleavage of complement C3b, thereby shutting down complement activation. The complement regulatory function of VWF multimers depends on their size. Smaller VWF multimers enhance cleavage of C3b but large and ultra-large VWF (ULVWF) multimers have no effect on C3b cleavage and permit default complement activation. We conclude that normal plasma VWF multimers prevent complement activation and steer the complement pathway toward generation of inactivated C3b (iC3b). ULVWF multimers, as are present in patients with thrombotic microangiopathy, lack an inhibitory effect on complement and permit complement activation.

The extracellular domain of glycoprotein (Gp) Ibalpha serves as the von Willebrand factor (vWf) receptor that triggers shear stress-dependent platelet aggregation. Its intracellular domain associates with actin-binding protein-280 (filamin 1a) that binds directly to filamentous actin, thereby linking the membrane skeleton to GpIbalpha. We examined the functional significance of GpIbalpha interactions with actin during platelet aggregation in response to 120 dyn/cm(2) shear stress. Lysates of resting and sheared platelets were centrifuged at approximately 13,000xg for 15 min, and GpIbalpha was immunoprecipitated from the lysate supernatant. GpIbalpha and coimmunoprecipitated proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotted with antibodies specific for GpIbalpha and actin. We observed a significant increase in the amounts of actin coimmunoprecipitating with GpIbalpha as platelets aggregated in response to shear stress. Actin/GpIbalpha interactions reached a maximum after 90 s of shear stress. Monoclonal antibody (mAb) blockade of vWf binding to GpIbalpha inhibited shear stress-induced platelet aggregation and actin associating with GpIbalpha. Pretreatment of platelets with cytochalasin D resulted in the inhibition of actin binding to GpIbalpha in sheared platelets and in an increase in the rate and magnitude of platelet disaggregation. These data indicate that shear stress causes changes in the association between GpIbalpha and the actin-based membrane skeleton. The increased interaction between GpIbalpha and the actin-based membrane skeleton results from shear-induced vWf binding to GpIbalpha and is mechanoprotective in that it maintains shear-induced aggregation of activated platelets.

Shear stress causes the platelet glycoprotein (Gp) Ib/IX/V to bind to von Willebrand factor, resulting in platelet adhesion. GpIb/IX/V also functions to stimulate transmembranous signaling, leading to platelet activation and the expression of a ligand-receptive GpIIb-IIIa complex. The highly conserved cytoplasmic domain of GpIbalpha binds directly to a dimeric 14-3-3 adapter protein zeta isoform. To explore structural determinants of GpIb/IX/V binding to 14-3-3zeta, the authors examined 14-3-3zeta interactions with GpIbalpha and GpIbbeta in heterologous cells and platelets. Truncations of GpIbalpha at amino acid 542 or 594, or deletions of residues 542 through 590, inhibited binding of 14-3-3zeta. Deletion of GpIbalpha from Trp(570) to Ser(590) eliminated 14-3-3zeta binding, and deletion of the sequence from Arg(542)-Trp(570) enhanced binding of 14-3-3zeta to GpIbalpha. All GpIbalpha mutations that eliminated GpIbalpha binding to the GST-14-3-3zeta fusion protein also eliminated GpIbbeta binding to the fusion protein. Forskolin treatment of Chinese hamster ovary cells expressing wild-type GpIbalpha/beta/IX resulted in the phosphorylation of GpIbbeta associated with enhanced binding of GpIbbeta to GST-14-3-3zeta fusion protein and increased 14-3-3zeta coimmunoprecipitated with GpIbalpha. When intact human platelets aggregated in response to 90 dynes/cm(2) shear stress, 14-3-3zeta disassociated from GpIbalpha. Prostacyclin treatment of platelets inhibited shear stress-induced aggregation and the release of 14-3-3zeta from GpIbalpha. These data demonstrate that amino acid residues in the cytoskeletal interaction domains of GpIbalpha regulate 14-3-3zeta binding to GpIbalpha/beta/IX, and suggest that protein kinase A-dependent phosphorylation of GpIbbeta enhances 14-3-3zeta binding to the GpIb/IX/V complex in human platelets. (Blood. 2000;95:551-557)

The glycoprotein (Gp) Ib/IX complex contains three transmembranous leucine-rich repeat polypeptides (GpIb, GpIbβ, and GpIX) that form the platelet von Willebrand factor (vWF) receptor. GpIb/IX functions to effect platelet adhesion, activation, and aggregation under conditions of high shear stress. GpIb/IX is expressed late in the ontogeny of megakaryocytes, the precursor cell that releases platelets when it reaches its terminal stage of differentiation. Because signal pathways can be reused at different stages of development by integration with different effector pathways and because cellular adhesion through other receptor families often modulates cell growth, the hypothesis that GpIb/IX regulates cell growth was investigated. The surface expression of recombinant GpIb decreases the proliferation of transduced CHO cells. GpIb causes growth arrest in the G1 phase of the cell cycle associated with the induction of the cyclin-dependent kinase inhibitor p21. G1 arrest induced by recombinant GpIb in heterologous cells requires signaling through the 14-3-3ζ binding domain of GpIb and is partially dependent on its engagement by the extracellular ligand vWF. Growth arrest induced by the expression of recombinant GpIb/IX is followed by apoptosis of the transduced cells. The endogenous expression of GpIb in human hematopoietic cells is associated with decreased proliferation. These results suggest that the expression of the GpIb/IX complex regulates megakaryocyte growth.

Interaction of fibroblast growth factor receptors (FGFR) sufficient for a trans-phosphorylation event in which one intracellular domain is substrate for the other is essential for signal transduction. By analysis of the direct interaction of recombinant constructions co-expressed in baculoviral-infected insect cells, we identified a 17-amino acid sequence that is required for the stable interaction between ectodomains of FGFR. The sequence 160ERSPHRPILQAGLPANK176 (Glu160-Lys176) connects immunoglobulin modules II and III. In insect cells, the interaction between Glu160-Lys176 domains occurs independently of intact heparin or FGF binding domains. The sequence is not required for the binding of heparin or FGF-1, but is essential for mitogenic activity of the FGFR kinase in mammalian cells. The results support a model in which the homeo-interaction between Glu160-Lys176 in the ectodomain contributes to the interaction between intracellular domains in mammalian cell membranes (Kan, M., Wang, F., Kan, M., To, B., Gabriel, J. L., and McKeehan, W. L. (1996) J. Biol. Chem. 271, 26143-26148). We propose that the Glu160-Lys176 domain plays a pivotal role in restriction of the interaction between kinases by pericellular matrix heparan sulfate proteoglycan and divalent cations. Restrictions are overcome by FGF or constitutively by diverse gain of function mutations which cause skeletal and craniofacial abnormalities.

Objective— Ultralarge von Willebrand factor (vWF) strings are secreted by, and anchored to, stimulated human endothelial cells. A disintegrin and metalloprotease with thrombospondin domains-type 13 cleaves the ultralarge vWF strings into large soluble vWF multimers. Normal plasma contains a nonproteolytic reducing activity that subsequently rapidly diminishes the size of the large soluble vWF multimers. Approach and Results— The vWF reductase activity was isolated from normal cryoprecipitate-poor plasma by chromatography and identified as the complement regulatory protein, factor H (FH), by mass spectroscopy, SDS-PAGE, and monospecific anti-FH antibody. Removal of FH from partially purified vWF reductase by immunoabsorption eliminated the reducing activity, and the activity was recovered in the eluates. Recombinant human FH reduced large soluble vWF multimers in a free thiol-dependent reaction that was not inhibited by a variety of protease inhibitors. Conclusions— FH contributes to the reduction of large soluble vWF multimers.

Complement factor H (fH) is a plasma protein that regulates activation of the alternative pathway, and mutations in fH are associated with a rare form of thrombotic microangiopathy (TMA), known as atypical hemolytic uremic syndrome (aHUS). A more common TMA is thrombotic thrombocytopenic purpura, which is caused by the lack of normal ADAMTS-13-mediated cleavage of von Willebrand factor (VWF). We investigated whether fH interacts with VWF and affects cleavage of VWF. We found that factor H binds to VWF in plasma, to plasma-purified VWF, and to recombinant A1 and A2 domains of VWF as detected by co-immunoprecipitation (co-IP) and surface plasmon resonance assays. Factor H enhanced ADAMTS-13-mediated cleavage of recombinant VWF-A2 as determined by quantifying the cleavage products using Western-blotting, enhanced cleavage of a commercially available fragment of VWF-A2 (FRETS-VWF73) as determined by fluorometric assay, and enhanced cleavage of ultralarge (UL) VWF under flow conditions as determined by cleavage of VWF-platelet strings attached to histamine stimulated endothelial cells. Using recombinant full-length and truncated fH molecules, we found that the presence of the C-terminal half of fH molecule is important for binding to VWF-A2 and for enhancing cleavage of the A2 domain by ADAMTS-13. We conclude that factor H binds to VWF and may modulate cleavage of VWF by ADAMTS-13.

Complement dysregulation leads to atypical hemolytic uremic syndrome (aHUS), while ADAMTS13 deficiency causes thrombotic thrombocytopenic purpura. We investigated whether genetic variations in the ADAMTS13 gene partially explain the reduced activity known to occur in some patients with aHUS. We measured complement activity and ADAMTS13 function, and completed mutation screening of multiple complement genes and ADAMTS13 in a large cohort of aHUS patients. In over 50% of patients we identified complement gene mutations. Surprisingly, 80% of patients also carried at least 1 nonsynonymous change in ADAMTS13, and in 38% of patients, multiple ADAMTS13 variations were found. Six of the 9 amino acid substitutions in ADAMTS13 were common single nucleotide polymorphisms; however, 3 variants-A747V, V832M, and R1096H- were rare, with minor allele frequencies of 0.0094%, 0.5%, and 0.32%, respectively. Reduced complement and ADAMTS13 activity (60% of normal activity) were found in over 60% and 50% of patients, respectively. We concluded that partial ADAMTS13 deficiency is a common finding in aHUS patients and that genetic screening and functional tests of ADAMTS13 should be considered in these patients.

The thrombotic microangiopathies (TMA) are a group of disorders defined by the presence of microangiopathic hemolytic anemia and thrombocytopenia. The most common of these is thrombotic thrombocytopenic purpura (TTP), which is a systemic disorder of microvascular thromboses due to deficiency of ADAMTS-13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). A less common TMA is the atypical hemolytic uremic syndrome (aHUS), which is a renal vascular TMA caused by complement dysregulation. Despite overlapping clinical and pathologic manifestations, TTP and aHUS have distinct etiologies. TTP is often caused by a deficiency of ADAMTS-13 that is the result of gene mutations or acquired autoantibodies (Tsai, 2006). Atypical HUS is caused by defects of regulation and/or excessive activation of the alternative complement pathway (Kavanagh & Goodship, 2010). The mechanism by which complement dysregulation contributes to aHUS is not precisely defined, although complement-mediated glomerular endothelial injury and enhanced complement-mediated platelet activation are probably involved (Stahl et al, 2008). Similarly, triggers and co-factors directing systemic platelet deposition in TTP are not completely understood. Evidence that complement activation might play a role in TTP (Noris et al, 1999; Ruiz-Torres et al, 2005; Reti et al, 2012) raises the possibility of a cross-talk between ADAMTS-13/ultra-large von Willebrand factor (ULVWF) and the complement system. We studied plasma samples of 81 patients diagnosed with TMA according to clinical criteria for functional abnormalities in both ADAMTS-13 and complement regulation. Citrated platelet-poor plasma samples were obtained for testing before the initial plasma infusion or exchange procedures. All patients had microangiopathic hemolytic anemia and thrombocytopenia without an alternative cause, and treated with either plasma infusion or plasma exchanges. None of our patients had acute renal failure. Samples for analysis of DNA were not obtained/stored from this group of patients. All human subject studies were conducted according to the approved institutional review board protocols in the Rice University and University of Texas M.D. Anderson Cancer Center. ADAMTS-13 activity was measured by: (1) the rate of cleavage of a substrate that contains 73 amino acids of the A2 domain of VWF with fluorescence resonance energy transfer (FRET) tags on either side of the cleavage site for ADAMTS-13 (FRETS-VWF73), according to the manufacturer’s protocol (GTi Diagnostics); and (2) cleavage of urea-treated ULVWF multimers (obtained from human umbilical vein endothelial cell supernatant) by citrated patient plasma, followed by VWF multimeric analysis using SDS-1% agarose electrophoresis and Western-blotting with anti-VWF antibody. This is a modification of the method described by Furlan, et al. (Furlan et al, 1998). The presence or absence of ADAMTS-13 inhibitors was determined by measuring cleavage of urea-treated ULVWF multimers before and after mixing normal citrated plasma with an equal volume of patient citrated plasma (Furlan et al, 1998). Complement activity was measured by the hemolysis of sheep erythrocytes after incubation with human serum or plasma according to modified techniques from Sanchez-Corral, et al. (Sanchez-Corral et al, 2004). Factor H-depleted plasma causes complement-induced lysis of sheep erythrocytes with the visually apparent release of hemoglobin. Pooled normal plasma or serum caused 7% and 8% hemolysis of sheep erythrocytes, respectively. Optimal dilution of plasma or serum for the assay was determined to be between 4/100 to 6/100, and optimal incubation time was 10 min. Sixty percent (49/81) of TMA patients had severe ADAMTS-13 deficiency (less than 10% activity). Eighty percent (65/81) of our patients’ plasma samples caused little to no hemolysis of sheep erythrocytes (median of 10%; range of 0–15%). In contrast, 20% (16/81) of the patients’ samples showed significant hemolysis (median of 60% hemolysis; range of 23–89%) (Figure 1). Sixteen percent (8/49) of plasma samples from TTP patients with severe ADAMTS-13 deficiency caused increased hemolysis. Only one of the 8 patients with concurrent excessive complement-induced hemolysis and severe ADAMTS-13 deficiency had detectable antibody (in low titer) against ADAMTS-13 (Table 1). Twenty-five percent (8/32) of plasma samples from patients who did not have severe ADAMTS-13 deficiency also caused increased hemolysis. Figure 1 Sheep erythrocyte hemolysis assay in plasma samples from patients with thrombotic microangiopathy Table 1 Patients with a clinical diagnosis of TTP and decreased complement regulation. Severe deficiency of functional ADAMTS-13 is associated with TTP; however, many patients with a TTP-like syndrome have normal ADAMTS-13 levels, as did 40% (32/81) of the patients in our study. There are several reports of patients with reduced ADAMTS-13 function who either did not develop TTP, or did so later in life (Noris et al, 2005). These observations raise the possibility of the presence of additional factors besides ADAMTS-13 deficiency involved in the pathophysiology of TTP (Ruiz-Torres et al, 2005; Reti et al, 2012; Noris et al, 2005; Chapin et al, 2012). Activation of the complement system in both familial (Noris et al, 1999) and acquired TTP (Reti et al, 2012) has been reported, based on the lower concentration of C3 and elevated levels of complement activation products (C3a and sC5b-9) in the sera of patients with acute TTP, and deposition of C3 and C5b-9 on endothelial cell exposed to TTP sera (Ruiz-Torres et al, 2005). We studied activity of the alternative complement pathway in 81 patients with the clinical diagnosis of TTP-like TMA requiring plasma infusion and/or plasma exchange. Some patients with severe ADAMTS-13 deficiency (8/49; 16%) or TTP-like TMA (8/32; 25%), had elevated plasma complement activity. We did not detect an increased titer of ADAMTS-13 inhibitor in ADAMTS-13 deficient TTP patients with complement dysregulation, and the majority of these patients (5/8; 68%) had a history of familial or recurrent TTP (Table 1). Our data suggest that the complement system may be an important co-factor involved in the pathogenesis of TMA. Excessive alternative pathway activity occurred in a significant number of TTP patients indicating that concurrent defects in ADAMTS-13 and complement regulation may occur more frequently than previously reported (Noris et al, 2005; Chapin et al, 2012). In addition, our findings indicate that excessive alternative pathway activity can be associated with a TTP-like TMA in some patients who do not have severe deficiencies of ADAMTS-13. Further genetic studies of patients with the clinical diagnosis of TTP may be informative.

A significant number of patients with ovarian cancer develop venous thromboembolism that is associated with a worse prognosis. The etiology of an increased frequency of venous thrombosis in cancer patients is not clear, and various hypotheses, including the presence of hyperreactive platelets, have been postulated. Hyperreactive platelets have a lower threshold for aggregation, and hence there is a higher number of degranulated platelets in circulation and a higher concentration of platelet granular contents in plasma. We compared ADP- and collagen-induced platelet aggregation in patients with ovarian cancer to those in patients with benign ovarian tumors. To reduce the effect of confounding factors such as surgery or chemotherapy, all blood samples were collected prior to any surgical interventions or chemotherapy. To detect hyperreactivity of platelets, we used both low and high doses of platelet agonists. To evaluate platelet preactivation at baseline, we measured the plasma concentration of b-thromboglobulin (b-TG) and platelet factor-4 (PF-4) as markers of platelet α granule secretion. All studies were approved by the Institutional Review Boards of the University of Texas, M.D. Anderson Cancer Center. Whole blood samples were collected from 34 patients with ovarian cancer and 19 patients with benign ovarian tumors into sodium citrate anticoagulant and processed within 2 hours after collection. Platelet rich plasma (PRP) was prepared by 15 min of 850 rpm centrifugation at room temperature. Aggregation studies were conducted in a light transmission aggregometer (Bio/Data Corporation), using ADP (at 2mM and 20 mM) and collagen (at 19 mg/ml and 190 mg/ml). Platelet poor plasma (PPP) was prepared by centrifuging PRP samples at 2500 rpm for 20 min at room temperature. All PPP samples were stored at -80ºC and the quantity of b-TG and PF-4 was determined by ELISA. We found that platelets isolated from ovarian cancer patients showed aggregation responses similar to platelets from patients with benign ovarian tumors (Figure 1). There was a statistically significant difference in the high-dose collagen-induced platelet aggregation between cancer and benign tumor samples, with increased aggregation of platelets from patients with benign tumors (79 ± 4.9%) in comparison to aggregation of platelets from cancer platelets (69 ± 6.3%). To investigate preactivation of platelets in cancer patients, we measured b-TG and PF-4 in PPP samples. We did not detect a higher concentration of b-TG and PF-4 in cancer PPP samples (Figure 2). In fact, PPP from cancer patients had a lower concentration of both α granule constituents. In the case of PF-4, the difference was statistically significant (9.8 ± 1.5 ng/ml for cancer patients versus 11.7 ± 1.7 ng/ml for patients with non-malignant tumors). We conclude that platelets from ovarian cancer patients are not hyperreactive and are not degranulated or preactivated. Links between ovarian cancer, venous thromboembolism and platelets may be absent or may involve non-hemostatic platelet functions. Figure 1. Figure 1. Figure 2. Figure 2. Disclosures No relevant conflicts of interest to declare.

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced transmembranously by an integrin's ligand-bound extracellular domain through its β-subunit's cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin αIIbβ3is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on αIIbβ3, we examined αIIbβ3/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain, and Syk coimmunoprecipitate with αIIbβ3in resting platelets and that 120 dyn/cm2shear stress leads to their disassociation from αIIbβ3. Shear-induced disassociation of α-actinin and myosin heavy chain from the β3tail is unaffected by blocking von Willebrand factor (VWF) binding to glycoprotein (Gp) Ib-IX-V but abolished by blocking VWF binding to αIIbβ3. Syk's disassociation from β3is inhibited when VWF binding to either GpIb-IX-V or αIIbβ3is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine-phosphorylated adhesion and degranulation-promoting adapter protein are inhibited by blocking ligand binding to αIIbβ3but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary cells expressing αIIbβ3with β3truncated of its cytoskeletal binding domains demonstrate diminished shear-dependent adhesion and cohesion. These results support the hypothesis that shear stress directly modulates αIIbβ3function and suggest that shear-induced αIIbβ3-mediated signaling contributes to the regulation of platelet aggregation by directing the release of constraining cytoskeletal elements from the β3-tail.

Platelets are mediators of physiologic hemostasis and pathologic thrombosis. They operate within distinctive vascular and rheologic microenvironments, and their participation in hemostasis or thrombosis is directed by distinct variables operating within the microenvironment. Thrombosis is not simply too much hemostasis: there is good evidence that triggering mechanisms of platelet aggregation under low shear stress conditions are different from those operating under high shear stress conditions. Such differences are hypothesized to exist in vivo and to separate mechanisms of microvascular hemostasis from mechanisms of arterial thrombosis, such as those involved in myocardial, cerebral and peripheral vascular ischemia and infarction. This separation forms the conceptual basis for the hypothesis that lesion-specific antithrombotic agents might some day be invented that inhibit arterial thrombosis without causing bleeding that arises from impaired hemostasis. The focus of much of the work in this field has been platelet aggregation initiated by shear-dependent von Willebrand factor binding to the platelet glycoprotein Ib-IX-V complex. It is hypothesized that by elucidating molecular mechanisms of platelet activation operating under pathologically elevated shear stresses, targets of lesion-specific therapies will one day be identified for use in clinical syndromes of arterial thrombosis.

The glycoprotein (Gp) Ib-IX-V complex is essential for platelet-mediated hemostasis and thrombosis. The cytoplasmic domain of its largest polypeptide subunit GpIbalpha possesses a binding region for filamin A, which links GpIb-IX-V to the platelet cytoskeleton. There is evidence that filamin A binding to GpIbalpha directs the surface expression of GpIb-IX. To investigate the mechanism of this effect, we examined GpIbalpha biosynthesis in Chinese hamster ovary (CHO) cells stably co-expressing wild-type or mutant GpIbalpha with GpIbbeta, GpIX with and without filamin A. We observed that surface GpIbalpha expression is enhanced in CHO cells co-expressing human filamin A. In comparison with cells expressing only GpIbalpha, GpIbbeta, and GpIX (CHO-GpIbalpha/betaIX), lysates from CHO-GpIbalpha/betaIX + filamin A-expressing cells showed greater amounts of immature, incompletely O-glycosylated and fully mature GpIbalpha, but lesser amounts of the approximately 15-kDa C-terminal peptide released when the extracellular domain of GpIbalpha is cleaved by proteases. When filamin A binding is eliminated by truncation of GpIbalpha at C-terminal residue 557 or by a deletion between amino acids 560-570, the decreased synthesis of mature GpIbalpha is accompanied by decreased immature GpIbalpha and by an increased immunodetectable C-terminal peptide. The synthesis of mature GpIbalpha in CHO-GpIbalpha/betaIX cells is eliminated by brefeldin A (which inhibits transport out of the endoplasmic reticulum (ER)) and restored by lactacystin (which inhibits proteasomal degradation). These results suggest that GpIbalpha binds to filamin A within the ER and that filamin A binding directs post-ER trafficking of GpIbalpha to the cell surface.

Shear-induced platelet responses are triggered by VWF binding to the platelet GpIb-IX complex, and there is evidence that this ligand-receptor coupling stimulates transmembranous signaling through the cytoplasmic tail of glycoprotein (Gp) Ib alpha. To investigate the mechanism by which signaling is effected, new molecular interactions involving GpIb-IX that develop in response to pathological shearing stress were examined in intact human platelets. Exposure to shear, but not alpha-thrombin, results in the co-immunoprecipitation of the actin cross-linking protein alpha-actinin with the GpIb-IX complex. Blockers of VWF binding to GpIb alpha or actin polymerization inhibit the association of alpha-actinin with the GpIb-IX complex, but the association of alpha-actinin with the GpIb-IX complex is not affected by inhibiting VWF binding to platelet integrin alpha IIb beta 3 (GpIIb-IIIa). alpha-Actinin becomes tyrosine phosphorylated in response to pathological shear stress, and phosphorylated alpha-actinin associates with GpIb-IX. In resting platelets, class IA heterodimeric phosphatidylinositol 3-kinase (PI 3-K) and protein kinase N (PKN) associate with nonphosphorylated alpha-actinin. Shear stress causes PI 3-K to disassociate from alpha-actinin, while it stimulates PKN binding to alpha-actinin. These results demonstrate that shear-induced VWF binding to GpIb alpha causes enhanced binding of cytoskeletal alpha-actinin to GpIb-IX and suggest that alpha-actinin, perhaps through tyrosine phosphorylation, serves as an adapter for a signaling complex that could regulate VWF-induced platelet aggregation.

Platelets have been called “ancient granulocytes” because they are capable of recognizing and destroying microbial invaders through their release of a variety of anti-microbial proteins. Experimental evidence indicates that this may involve direct platelet-microbial interactions or occur indirectly through platelet-leukocyte interactions, neutrophil histone-platelet interactions and/or platelet-directed neutrophil extracellular trap formation. To test the hypothesis that platelets contribute to innate lung immunity, we examined the clinical course, lung histopathology and pathogen burden of mice -/+ antibody-mediated platelet depletion who were subjected to aerosolized bacteria (Streptococcus pneumoniae or Pseudomonas aeruginosa). Mice (C57BL/6, 10 weeks) were injected by tail vein with a purified rat monoclonal antibody directed against mouse GpIbα (or control antibody) resulting in platelet depletion to ~ 20% at 3 hours that persisted for 48 hours. 30 minutes after GpIbα antibody injection mice were placed into a closed system for 60 minutes where they were exposed to 10 ml nebulized S. pneumoniae (2-6 x 1011 CFU/ml) or P. aeruginosa (1-4 x 1010 CFU/ml). All platelet depleted animals infected with S. pneumoniae died by day 4 after infection, whereas all platelet replete mice survived the infection. Infection with P. aeruginosa resulted in death of all animals, although the platelet replete mice lived longer than the platelet depleted mice. No platelet depleted sham-infected animals died. The lungs of animals were harvested, fixed and prepared for histopathology with hematoxylin and eosin staining; or harvested, homogenized and plated for bacterial CFU measurements. Whole lungs of platelet-depleted and infected mice were congested. Histopathology showed increased red cells in the venules of platelet-depleted S. pneumoniae-infected mice but no difference in air-space hemorrhage in comparison with platelet-depleted sham-infected lungs and platelet-replete S. pneumoniae-infected lungs. Platelet depletion resulted in ~ 5-fold increase in lung S. pneumoniae CFUs and ~ 10-fold increase in P. aeruginosa CFUs. These results indicate that platelets provide defense against pneumonia. The mechanism of this effect is not known. Experiments are ongoing to determine if it is a consequence of platelet-dependent hemostasis, platelet-leukocyte interactions and/or direct platelet-microbial interactions leading to the release of bactericidal compounds. Disclosures Kroll: Boerhinger-Ingelheim: Membership on an entity's Board of Directors or advisory committees; Aplagon Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Complement, besides its involvement in eliminating microbes, participates in such diverse processes as synapse maturation, clearance of immune complexes, angiogenesis and tissue regeneration. Delicate balance between complement activation and regulation contributes to complement’s role in physiology. Any trigger that tips this balance can induce self-attack. Atypical hemolytic uremic syndrome (aHUS) is a systemic disease characterized by non-immune hemolytic anemia, thrombocytopenia, and renal impairment. In over 50% of cases, aHUS is known to be caused by uncontrolled activation of the complements. Von Willebrand factor (VWF) is a large multimeric glycoprotein that plays an important role in stopping the escape of blood from vessels following vascular injury. VWF and VWF cleavage enzyme ADAMTS13 gene defects have been identified in patients with aHUS, raising the possibility that VWF could have contributed to complement regulation. To examine the role of VWF in complement activation, we investigated whether VWF functions as a cofactor for FI-mediated C3b cleavage through in vitro assay. We found that C3b binds to VWF. In the presence of plasma-purified VWF (pVWF), FI cleaves C3b to 68kD and 43kD degradation products (iC3b) (Figure (A)). VWF alone, or FI alone, did not have any effect on C3b cleavage. C3b, not C3 or iC3b, was the mainly substrate for FI/VWF proteolysis. Increasing VWF concentration or prolonging the incubation time with VWF enhanced FI-mediated C3b cleavage. To remove the possibility that another plasma protein co-purified with pVWF that affects our results, we used recombinant VWF dimers purified from human embryonic kidney (HEK) 293 cell expressing VWF-Dpro cDNA, and detected a similar cofactor activity (Figure (B)).To investigate whether the size of VWF multimers have any effect on C3b cleavage, we compared the cofactor activity of pVWF, recombinant VWF dimers, and ULVWF multimers. While pVWF and dimer VWF enhanced C3b cleavage by FI, ULVWF did not have any effect on C3b cleavage (Figure (B)). In plasma, complement proteins Factor B and Factor D coexist with the inhibitory protein FI. To investigate the effect of VWF on complement activity in the presence of both pro-activation and inhibitory complement proteins, we incubated C3, FB, FD, and FI with VWF. In the presence of FB and FD, C3 be activated and resulted in the generation of C3a and C3b. Addition of recombinant VWF put a brake on complement activation and shifted C3 toward the generation of iC3b (Figure (C)). We conclude that normal plasma VWF, function as a cofactor, prevents complement activation through steers the complement pathway toward the generation of inactive iC3b. ULVWF multimers, as are present in patients with thrombotic microangiopathy, lack an inhibitory effect on complement and permit complement activation. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Abstract 490 Atypical hemolytic uremic syndrome (aHUS) is a clinically defined thrombotic microangiopathic anemia (TMA) characterized by the symptom triad of renal failure, thrombocytopenia and microangiopathic hemolytic anemia in the absence of Shiga-toxin-producing bacteria as a triggering factor. Its pathogenesis is linked to dysregulation of the alternative pathway of the complement cascade, with loss-of-function mutations reported in complement regulators like factor H (CFH), membrane cofactor protein (MCP), factor I (CHI) and thrombomodulin (THBD), and gain-of-function mutations in complement activators like factor B (CFB) and complement component 3 (C3). In nearly 40% of patients, however, mutations are not identified in these genes raising the possibility of unrecognized contributory genetic causes. Genetic variants in ADAMTS13 (a disintegrin and metalloprotease with thrombospondin motifs) have been causally related to thrombotic thrombocytopenic purpura (TTP), another TMA characterized by the pentad of neurologic symptoms, fever, kidney failure, thrombocytopenia and microangiopathichemolytic anemia. The phenotypic similarities between aHUS and TTP can at times lead to difficulty in their clinical distinction. On this basis, we hypothesized that partial deficiency in ADAMTS13 function may coexist with abnormalities in the alternative complement pathway in patients with aHUS. To test this hypothesis, we measured ADAMTS13 functional activity in 26 patients with aHUS by fluorescence resonance energy transfer substrate-von Willebrandfactor 73mer (FRETS-VWF73) and recombinant VWF A2 domain cleavage assay. We also genotyped all patients for ADAMTS13 and alternative complement pathway genes. We transiently expressed 10 different ADAMTS13 missense variants in HEK293 cells to analyze activity and secretion of each recombinant protein. The ADAMTS13 functional activity was partially reduced (< 60%) in serum from 20 patients (77%) as measured by FRETS-VWF73 and recombinant VWF A2 cleavage assays. Genetic variants in ADAMTS13 were identified in 21 patients (81%) and included R386C, Q448E, A900V, V832M, R1060W, R7W/Q448E, A900V/Q448E, R1096H/A747V, R7W/A1033T, R7W/P618A/Q448E, R7W/P618A/A900V/Q448E, R7W/P618A/A732V/Q448E. The coexistence of both ADAMTS13 deficiency and excessive complement functional activity was present in 13 patients (50%). Activity of recombinant ADAMTS13 proteins containing the following variants was normal: R386C, Q448E, P618A, A732V, A747V, V832M, A900V, A1033T and R1060H. The R1096H variant of ADAMTS13 was associated with a reduction in functional activity by 50%. The secretion of recombinant proteins with the following variants was severely reduced: P618A, R1060H ( This study implicates ADAMTS13 in the pathogenesis of aHUS and mandates the evaluation of ADAMTS13 in aHUS patients. Defining the role of ADAMTS13 in aHUS may aid in the development of new treatments and/or assist in clarifying whether long-term treatment with currently available anti-complement therapy is required for this disease. Disclosures: Kroll: Aplagon: Membership on an entity's Board of Directors or advisory committees; Optimer: Consultancy; Leo: Honoraria, Travel Expenses, Travel Expenses Other.

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced by an integrin’s ligand-bound extracellular domain through its β subunit’s cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin αIIbβ3 is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on αIIbβ3, we examined αIIbβ3/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain and Syk co-immunoprecipitate with αIIbβ3 in resting platelets, and that 120 dynes/cm2 shear stress leads to their disassociation from αIIbβ3. Shear-induced disassociation of α-actinin and myosin heavy chain from the β3 tail is unaffected by blocking VWF binding to GpIb-IX-V but abolished by blocking VWF binding to αIIbβ3. Syk’s disassociation from β3 is inhibited when VWF binding to either GpIb-IX-V or αIIbβ3 is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine phosphorylated SLAP-130 are inhibited by blocking ligand binding to αIIbβ3 but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary (CHO) cells expressing human αIIbβ3 demonstrate calcium and shear-dependent (1 and 10 dynes/cm2) attachment to glass cover slips coated with 20 μg/ml VWF. CHO cells expressing αIIbβ3 with β3 truncated of its cytoskeletal binding domains at C-terminal residue 716 demonstrate decreased VWF-dependent adhesion and cohesion in response to 1 dyne/cm2 shear stress. These results support the hypothesis that shear stress directly modulates αIIbβ3 function, and suggest that shear-induced αIIbβ3-mediated signaling regulates platelet aggregation by directing the release of α-actinin and myosin heavy chain from the β3 tail.

von Willebrand factor (VWF) binding to platelet glycoprotein (Gp) Ib-IX-V triggers platelet activation. Under conditions of pathologically elevated arterial wall shear stress, VWF-dependent platelet adhesion is coupled to aggregation and thrombus formation principally through GpIbα-induced signaling to α IIbβ 3. To elucidate the mechanism of GpIbα signaling to α IIbβ 3, we have examined molecular interactions involving structural proteins that bind to the cytoplasmic domains of GpIbα and β 3. In CHO cells co-expressing human GpIb-IX and α IIbβ 3, the activation of α IIbβ 3 as reported by monoclonal antibody PAC-1 binding is stimulated by ristocetin (1 mg/ml) + purified human VWF(5 μg/ml). When filamin binding to the cytoplasmic domain of GpIbα is eliminated by deleting GpIbα residues 560-570, PAC-1 binding is eliminated. When human platelets in reconstituted whole blood are treated with a peptide that interferes with filamin binding to GpIbα (as reported in Blood2003;102:2122–2129), shear-dependent (1500 sec−1 shear rate or 60 dynes/cm2 shear stress) platelet deposition onto bovine type I collagen is inhibited. In washed resting platelets and platelets activated by 120 dynes/cm2 shear stress, filamin co-immunoprecipitates with both GpIbα and α IIbβ 3; only its association with β 3 is eliminated by DNaseI (1 mg/ml), demonstrating that filamin binds indirectly to α IIbβ 3 through other cytoskeletal elements. One such element is observed to be talin, which co-immunoprecipitates with filamin, α IIbβ 3 and small amounts of GpIbα in resting platelets. When platelets are sheared for two minutes at 120 dynes/cm2, talin’s DNaseI-resistant association with filamin is decreased and its DNaseI-sensitive association with α IIbβ 3 is increased. These changes are prevented when shear-dependent VWF binding to GpIb-IX-V is blocked by monoclonal antibody AK2. Shear-dependent VWF binding to GpIb-IX-V also results in the proteolysis of talin, which is considered to be one mechanism by which the N-terminal head domain of talin regulates α IIbβ 3 activation. Shear-dependent talin proteolysis is not affected by blocking VWF binding to α IIbβ 3 with a RGD peptide. These results demonstrate that the cytoplasmic domain of GpIbα transduces signals to activate α IIbβ 3 through its interactions with filamin. These signals depend only on VWF binding to GpIb-IX-V. Under pathologically elevated wall shear stress in vitro, the mechanism of signaling may be the release of talin by filamin, thus permitting the proteolysis of talin and enhancing talin’s interaction with α IIbβ 3.

The platelet glycoprotein (GP) Ib/IX/V receptor complex controls crucial steps in haemostasis and thrombosis by mediating adhesion to von Willebrand factor (VWF). Nevertheless, the downstream consequences of VWF binding to GPIb remain unclear. The GPIb/IX/V complex consists of four polypeptides GPIbα, GPIbβ, GPIX and GPV. We have shown that a palmitylated (Pal-) peptide based on the sequence of the GPIbβ cytoplasmic domain between R151 and A161 (Pal-RRLRARARARA abbreviated to RARA) increases the velocity of platelets rolling on VWF, and inhibits VWF-dependent adhesion and aggregation. The peptide RARA is delivered into platelets and does not cause any reduction in the surface expression of GPIb-IX-V. We examined the effect of RARA and control peptides (Pal-RAAAARARARA and Pal-RRLRADADADA) on GPIb/IX/V-mediated tyrosine kinase signalling induced by VWF/GPIb interactions under shear conditions using a cone and plate viscometer. Whole platelet lysate phosphotyrosine immunoblots were characterised in the presence and absence of RARA and the control peptides before and after shearing at 120 dynes/cm2 for 5 mins. We then further investigated the effect of RARA on PI-3 kinase signalling, the tyrosine kinase Src and calcium influx under the conditions described above. The platelet permeable peptide RARA alone specifically caused protein tyrosine phosphorylation under resting conditions and caused platelet microaggregate formation with a 35% reduction in single platelets as assessed by flow cytometry. In addition RARA significantly inhibited shear-induced aggregation. When tyrosine phosphorylation was examined in platelets sheared for 5 minutes at 120 dynes/cm2, we observed RARA failed to inhibit tyrosine phosphorylation despite inhibiting shear-induced aggregation. RARA caused Src kinase activation as determined by site-specific phosphorylation of Y416 on Src. Furthermore the Src inhibitor PP1 inhibited RARA induced tyrosine phosphorylation. RARA caused a Src-dependent increase in intracellular [Ca2+] and inhibited any further increase in intracellular calcium induced by thrombin. Pretreatment of platelets with the PI-3 kinase inhibitor wortmannin demonstrated that tyrosine phosphorylation induced by RARA was not PI-3 kinase dependent. These results indicate that a platelet permeable peptide corresponding to specific residues of the cytoplasmic tail of GPIbβ directly stimulates Src, and suggest that the mechanism of its inhibitory effect on VWF-mediated platelet adhesion and aggregation involves the tyrosine kinase-mediated desensitisation of a GPIb/IX/V-triggered signalling pathway. This study demonstrates a novel role for the GPIbβ cytoplasmic tail in mediating Src-dependent shear induced signalling through GPIb/VWF.

In resting platelets, the cytoplasmic domains of glycoprotein (Gp) Ibα and β3 integrin link the GpIb-IX-V and αIIbβ3 complex with adapter proteins, cytoskeletal elements and lipid and tyrosine kinases. Biochemical and pharmacological analyses of intact platelets and genetic analyses of recombinant GpIb-IX-V and αIIbβ3 in CHO cells point towards the hypothesis that a series of cytoskeletal proteins form a functional tether connecting the cytoplasmic domains of GpIbα and β3 integrin. To test the hypothesis that dynamic contractility of this tether modulates VWF-induced signaling between GpIb-IX-V and αIIbβ3, we examined how pathological shear stress affects connections between tethering elements and how altering the contractile function of the tether affects VWF-induced platelet adherence and aggregation. We observed in resting platelets that there is no co-immunoprecipitation of GpIbα and αIIbβ3. The large dimeric actin-binding protein filamin A co-immunoprecipitates with both GpIbα and β3 integrin, but its association with αIIbβ3 is inhibited by DNaseI, indicating that it binds indirectly to αIIbβ3 through cytoskeletal connections. These connections were investigated in resting platelets by examining a series of immunoprecipitates (IP) for co-precipitating proteins using immunoblotting (IB). We observed the following IP/IB pairs [(+) designates that the co-precipitation is DNaseI-sensitive]: β3/talin (+); talin/α-actinin (+); talin/vinculin (+); and talin/filamin A (−). We also observed that myosin heavy chain (MHC) and the tyrosine kinase Syk co-immunoprecipitate with αIIbβ3 in resting platelets. When washed platelets in buffer containing calcium (1 mM) and VWF (5 μg/ml) were sheared at 120 dynes/cm2 for 2 minutes, the cytoskeletal linkage separates: there was enhanced filamin A, actin and α-actinin binding to GpIbα, enhanced vinculin binding to α-actinin, enhanced talin binding to αIIbβ3, and both myosin and the tyrosine kinase Syk disassociated from the β3 tail. Inhibiting myosin contractility with the myosin light chain kinase (MLCK) inhibitor ML-9 (1 mM) inhibited the shear-induced association between talin and β3, as well as platelet aggregation in response to 120 dynes/cm2 in the cone plate viscometer and platelet-dependent thrombosis from whole blood onto type I collagen in a parallel plate flow chamber with a shear rate of 500 sec−1 (shear stress of ~ 20 dynes/cm2). The data presented support a model of shear-induced platelet aggregation in which a series of cytoskeletal proteins serve as a mechanotransducing scaffolding linking the cytoplasmic domains of GpIbα and β3 integrin. When GpIb-IX-V engages ligand under shearing forces, the force is transduced from GpIbα to filamin to actin/α-actinin to vinculin to talin to β3, and contractility driven by the activation of αIIbβ3-associated myosin enhances talin binding to β3, thereby effecting a conformation change that creates a ligand-receptive αIIbβ3. Such interactions may be regulated by cytosolic ionized calcium (which effects MLCK activation) and tyrosine kinases (αIIbβ3-associated Syk disassociates from β3 following shear). These results provide evidence that the structural scaffolding connecting GpIb-IX-V with αIIbβ3 localizes signaling elements to functionally important compartments that modulate αIIbβ3 activation and shear-induced platelet aggregation.