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Biomedical subjects

Sacha M Dopheide

Publications and source records attributed to Sacha M Dopheide.

8 recordsLinked to original sources
PubMed2006

Identification of a 2-stage platelet aggregation process mediating shear-dependent thrombus formation.

Disturbances of blood flow at sites of atherosclerotic plaque rupture are one of the key pathogenic events promoting platelet activation and arterial thrombus formation. Shear effects of platelets have been extensively investigated in vitro; however, the mechanisms by which shear promotes platelet aggregation in vivo remain incompletely understood. By employing high-resolution imaging techniques to in vitro and in vivo thrombosis models, we demonstrate a unique mechanism initiating shear-dependent platelet aggregation involving aggregate formation between discoid platelets. These discoid platelet aggregates are initially unstable and result from the development of membrane tethers between coadhering platelets. Tether formation involves the adhesive function of GPIb/V/IX and integrin alphaIIbbeta3, and conversion of discoid platelet aggregates into stable aggregates requires released ADP. The efficiency of this process is regulated by 3 independent variables, including the reactivity of the adhesive substrate, the level of shear flow, and the platelet density at the adhesive surface. These studies identify a new mechanism initiating platelet aggregation that is critically influenced by shear, physical proximity between translocating platelets, and membrane tether formation. Moreover, they provide a model to explain how the discoid morphology of platelets facilitates the maintenance of adhesive interactions with thrombogenic surfaces under high shear stress conditions.

Adenosine Diphosphate↗
PubMed2005

Shear induces a unique series of morphological changes in translocating platelets: effects of morphology on translocation dynamics.

OBJECTIVE: The platelet glycoprotein (GP) Ib/V/IX complex plays an important role in regulating the morphology of resting platelets and can induce shape change during adhesion to immobilized von Willebrand factor (vWf). In this study we have examined the effects of fluid shear stress on GPIb-dependent changes in platelet morphology during translocation on vWf. METHODS AND RESULTS: We demonstrate that translocating platelets undergo a unique series of morphological changes in response to increasing fluid shear stress. Under moderately low shear conditions (600 s(-1)), initial shape change involved extension of membrane tethers and/or filopodia from the platelet surface. With increasing shear rate, platelets adopted a spherical morphology with numerous surface projections (1800 to 5000 s(-1)). At high wall shear rates (10000 to 20,000 s(-1)), translocating platelets retracted filopodia, developing a smooth ball-like appearance. These changes in morphology were dependent on reorganization of the actin and microtubule components of the cytoskeleton and were regulated by intracellular signaling processes linked to Src kinases. Functionally, alterations in platelet shape had a major effect on translocation dynamics in that conversion from discs to spheres resulted in a 3- to 8-fold increase in rolling velocity. CONCLUSIONS: These studies demonstrate that platelets undergo shear-specific morphological changes during surface translocation on vWf that may serve to regulate translocation dynamics under flow.

Actin Cytoskeleton↗
PubMed2005

PI 3-kinase p110beta: a new target for antithrombotic therapy.

Platelet activation at sites of vascular injury is essential for the arrest of bleeding; however, excessive platelet accumulation at regions of atherosclerotic plaque rupture can result in the development of arterial thrombi, precipitating diseases such as acute myocardial infarction and ischemic stroke. Rheological disturbances (high shear stress) have an important role in promoting arterial thrombosis by enhancing the adhesive and signaling function of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa). In this study we have defined a key role for the Type Ia phosphoinositide 3-kinase (PI3K) p110beta isoform in regulating the formation and stability of integrin alpha(IIb)beta(3) adhesion bonds, necessary for shear activation of platelets. Isoform-selective PI3K p110beta inhibitors have been developed which prevent formation of stable integrin alpha(IIb)beta(3) adhesion contacts, leading to defective platelet thrombus formation. In vivo, these inhibitors eliminate occlusive thrombus formation but do not prolong bleeding time. These studies define PI3K p110beta as an important new target for antithrombotic therapy.

Animals↗
PubMed2003

Intercellular calcium communication regulates platelet aggregation and thrombus growth.

The ability of platelets to form stable adhesion contacts with other activated platelets (platelet cohesion or aggregation) at sites of vascular injury is essential for hemostasis and thrombosis. In this study, we have examined the mechanisms regulating cytosolic calcium flux during the development of platelet-platelet adhesion contacts under the influence of flow. An examination of platelet calcium flux during platelet aggregate formation in vitro demonstrated a key role for intercellular calcium communication (ICC) in regulating the recruitment of translocating platelets into developing aggregates. We demonstrate that ICC is primarily mediated by a signaling mechanism operating between integrin alpha IIb beta 3 and the recently cloned ADP purinergic receptor P2Y12. Furthermore, we demonstrate that the efficiency by which calcium signals are propagated within platelet aggregates plays an important role in dictating the rate and extent of thrombus growth.

Adenosine Diphosphate↗
PubMed2002

Essential role for phosphoinositide 3-kinase in shear-dependent signaling between platelet glycoprotein Ib/V/IX and integrin alpha(IIb)beta(3).

Platelet adhesion and aggregation at sites of vascular injury are critically dependent on the interaction between von Willebrand factor (VWF) and 2 major platelet adhesion receptors, glycoprotein (GP) Ib/V/IX and integrin alpha(IIb)beta(3). GP Ib/V/IX binding to VWF mediates platelet tethering and translocation, whereas activation of integrin alpha(IIb)beta(3) promotes cell arrest. To date, the signaling pathways used by the VWF-GP Ib/V/IX interaction to promote activation of integrin alpha(IIb)beta(3), particularly under shear, have remained poorly defined. In this study, the potential involvement of type 1 phosphoinositide (PI) 3-kinases in this process was investigated. Results show that platelet adhesion and spreading on immobilized VWF results in a specific increase in the PI 3-kinase lipid product, PtdIns(3,4)P(2). Under static conditions, inhibiting PI 3-kinase with LY294002 or wortmannin did not prevent platelet adhesion, integrin alpha(IIb)beta(3) activation, or platelet spreading although it significantly delayed the onset of these events. In contrast, PI 3-kinase inhibition under shear dramatically reduced both platelet adhesion and spreading. Real-time analysis of intracellular calcium demonstrated that under static conditions inhibiting PI 3-kinase delayed the onset of intracellular fluxes in adherent platelets, but did not affect the final magnitude of the calcium response. However, under shear, inhibiting PI 3-kinase dramatically reduced intracellular calcium mobilization and integrin alpha(IIb)beta(3) activation, resulting in impaired thrombus growth. The studies demonstrate a shear-dependent role for PI 3-kinase in promoting platelet adhesion on immobilized VWF. Under static conditions, platelets appear to mobilize intracellular calcium through both PI 3-kinase-dependent and -independent mechanisms, whereas under shear PI 3-kinase is indispensable for VWF-induced calcium release.

Androstadienes↗
PubMed2002

Shear-dependent tether formation during platelet translocation on von Willebrand factor.

The adhesion and aggregation of platelets at sites of vascular injury is dependent on the initial binding of the GP Ib/V/IX receptor complex to immobilized von Willebrand factor (VWF). Under flow conditions, this interaction supports platelet translocation that is characteristically stop-start in nature. High resolution imaging of platelets during surface translocation on immobilized VWF revealed that thin membrane tethers (length: 0.91 microm-47.90 microm) were pulled from the surface of these cells. Membrane tethers were dynamic structures that extended from small, localized adhesion contacts under the influence of flow. Perfusion of platelets in the presence of blocking antibodies against integrin alpha(IIb)beta(3), or over isolated A1 domains, demonstrated that the VWF-GP Ib interaction was sufficient to induce membrane tether formation. The rate and extent of tether elongation was shear-dependent (shear range: 150 s(-1)-10,000 s(-1)), with mean tether length ranging from 3.23 microm to 16.55 microm, tether frequency from 2.67% to 97.33%, and tether growth rate from 0.04 microm/sec to 8.39 microm/sec. Tether formation and retraction did not require platelet activation; however, the growth rate, lifetime, and dimensions were significantly affected by the actin polymerization inhibitor, cytochalasin D, and by chelating intracellular calcium. Single-cell analysis revealed that formation of membrane tethers regulates the stop-start phases of platelet translocation on VWF, suggesting a potentially important role for this phenomenon in regulating the dynamics of the platelet-VWF interaction under flow.

Blood Platelets↗
PubMed2001

Distinct glycoprotein Ib/V/IX and integrin alpha IIbbeta 3-dependent calcium signals cooperatively regulate platelet adhesion under flow.

We have investigated the calcium signaling relationship between the two major platelet adhesion receptors, glycoprotein Ib/V/IX (GPIb/V/IX) and integrin alpha(IIb)beta(3), involved in regulating platelet adhesion on von Willebrand factor (vWf) under flow. Our studies demonstrate that GPIb engagement of immobilized vWf elicits a transient calcium spike that may function to promote reversible arrest of translocating platelets. Subsequent integrin alpha(IIb)beta(3) engagement of vWf promotes sustained calcium oscillations that are essential for the maintenance of irreversible adhesion. GPIb-induced calcium spikes appear distinct from those initiated by integrin alpha(IIb)beta(3), in that the former are exclusively mediated through release of intracellular calcium stores via a signaling mechanism independent of PI 3-kinase. In contrast, integrin alpha(IIb)beta(3)-dependent calcium flux involves a PI 3-kinase-dependent signaling mechanism linked to intracellular calcium mobilization and subsequent transmembrane calcium influx. Studies employing the caged calcium chelator (o-nitrophenyl-EGTA) demonstrate that transient calcium spikes initiate a transient phase of platelet arrest that is converted to irreversible adhesion with the development of sustained oscillatory calcium flux. These studies demonstrate the existence of a dual step calcium signaling mechanism utilized by GPIb and integrin alpha(IIb)beta(3) that serves to regulate the dynamics of platelet adhesion under flow.

Blood Platelets↗