Kinetics of ADP-induced Human Platelet Shape Change: Apparent Positive Cooperativity

The kinetics of ADP-induced human platelet shape change have been examined. Initial velocities of platelet shape change were estimated by two methods: 1) the slope of the initial decrease in light transmission through stirred, citrated platelet-rich plasma, and 2) direct examination of platelet morphologies by phase-contrast microscopy. In both cases, a value of the Hill coefficient, n, significantly greater than 1 is obtained (2.0 +/- 0.2 and 1.8 +/- 0.2, respectively). The observed elevated value of n is not due to a substantial fraction of the ADP being platelet bound, the presence of factors in the plasma, platelet heterogeneity, of the influence of the rate of platelet shape change reversion. Our observations suggest that ADP-induced platelet shape change may be a positive cooperative or threshold type response

Time and Force Dependence of the Rupture of Glycoprotein IIb-IIIa-Fibrinogen Bonds between Latex Spheres

AbstractWe studied the shear-induced breakup of doublets of aldehyde/sulfate (A/S) latex spheres covalently linked with purified platelet GPIIb-IIIa receptor, and cross-linked by fibrinogen. Flow cytometry with fluorescein isothiocyanate-fibrinogen showed than an average of 22,500 molecules of active GPIIb-IIIa were captured per sphere, with a mean Kd=56nM for fibrinogen binding. The spheres, suspended in buffered 19% Ficoll 400 containing 120 or 240pM fibrinogen, were subjected to Couette flow in a counter-rotating cone-plate rheoscope. Doublets, formed by two-body collisions at low shear rate (G=8s−1) for ≤15min, were subjected to shear stress from 0.6 to 2.9 Nm−2, their rotations recorded until they broke up or were lost to view. Although breakup was time dependent, occurring mostly in the first 2 rotations after the onset of shear, the percentage of doublets broken up after 10 rotations were almost independent of normal hydrodynamic force, Fn: at 240 pN, 15.6, 16.0, and 17.0% broke up in the force range 70–150 pN, 150–230 pN, and 230–310 pN. Unexpectedly, at both [fibrinogen], the initial rate of breakup was highest in the lowest force range, and computer simulation using a stochastic model of breakup was unable to simulate the time course of breakup. When pre-sheared at low G for >15min, no doublets broke up within 10 rotations at 70<Fn<310 pN; it required >3min shear (>1110 rotations) at Fn=210 pN for significant breakup to occur. Other published work has shown that binding of fibrinogen to GPIIb-IIIa immobilized on plane surfaces exhibits an initial fast reversible process with relative low affinity succeeded by transformation of GPIIb-IIIa to a stable high-affinity complex. We postulate that most doublet breakups observed within 10 rotations were from a population of young doublets having low numbers of bonds, by dissociation of the initial receptor complex relatively unresponsive to force. The remaining, older doublets with GPIIb-IIIa in the high-affinity complex were not broken up in the time or range of forces studied

Disturbing Times: Medieval Pasts, Reimagined Futures

From Kehinde Wiley to W.E.B. Du Bois, from Nubia to Cuba, Willie Doherty’s terror in ancient landscapes to the violence of institutional Neo-Gothic, Reagan’s AIDS policies to Beowulf fanfiction, this richly diverse volume brings together art historians and literature scholars to articulate a more inclusive, intersectional medieval studies. It will be of interest to students working on the diaspora and migration, white settler colonialism and pogroms, Indigenous studies and decolonial methodology, slavery, genocide, and culturecide. The authors confront the often disturbing legacies of medieval studies and its current failures to own up to those, and also analyze fascist, nationalist, colonialist, anti-Semitic, and other ideologies to which the medieval has been and is yoked, collectively formulating concrete ethical choices and aims for future research and teaching. In the face of rising global fascism and related ideological mobilizations, contemporary and past, and of cultural heritage and history as weapons of symbolic and physical oppression, this volume’s chapters on Byzantium, Medieval Nubia, Old English, Hebrew, Old French, Occitan, and American and European medievalisms examine how educational institutions, museums, universities, and individuals are shaped by ethics and various ideologies in research, collecting, and teaching

Rap1 binding and a lipid-dependent helix in talin F1 domain promote integrin activation in tandem.

Rap1 GTPases bind effectors, such as RIAM, to enable talin1 to induce integrin activation. In addition, Rap1 binds directly to the talin1 F0 domain (F0); however, this interaction makes a limited contribution to integrin activation in CHO cells or platelets. Here, we show that talin1 F1 domain (F1) contains a previously undetected Rap1-binding site of similar affinity to that in F0. A structure-guided point mutant (R118E) in F1, which blocks Rap1 binding, abolishes the capacity of Rap1 to potentiate talin1-induced integrin activation. The capacity of F1 to mediate Rap1-dependent integrin activation depends on a unique loop in F1 that has a propensity to form a helix upon binding to membrane lipids. Basic membrane-facing residues of this helix are critical, as charge-reversal mutations led to dramatic suppression of talin1-dependent activation. Thus, a novel Rap1-binding site and a transient lipid-dependent helix in F1 work in tandem to enable a direct Rap1-talin1 interaction to cause integrin activation

A simplified mathematical model for thrombin generation

A new phenomenological mathematical model based directly on laboratory data for thrombin generation and having a patient-specific character is described. A set of the solved equations for cell-based models of blood coagulation that can reproduce the temporal evolution of thrombin generation is proposed; such equations are appropriate for use in computational fluid dynamic (CFD) simulations. The initial values for the reaction rates are either taken from already existing model or experimental data, or they can obtained from simple reasoning under certain assumptions; it is shown that coefficients can be adjusted in order to fit a range of different thrombin generation curves as derived from thrombin generation assays. The behaviour of the model for different platelet concentration seems to be in good agreement with reported experimental data. It is shown that the reduced set of equations used represents to a good approximation a low-order model of the detailed mechanism and thus it can represent a cost-effective and-case specific mathematical model of coagulation reactions up to thrombin generation