A Uniform Shear Assay for Human Platelet and Cell Surface Receptors via Cone-plate Viscometry

PLATELET FUNCTION IN TRAUMA: IS CURRENT TECHNOLOGY IN FUNCTION TESTING MISSING THE MARK IN INJURED PATIENTS?

ABSTRACT

Platelets are subcellular anucleate components of blood primarily responsible for initiating and maintaining hemostasis. After injury to a blood vessel, platelets can be activated via several pathways, resulting in changed shape, adherence to the injury site, aggregation to form a plug, degranulation to initiate activation in other nearby platelets, and acceleration of thrombin formation to convert fibrinogen to fibrin before contracting to strengthen the clot. Platelet function assays use agonists to induce and measure one or more of these processes to identify alterations in platelet function that increase the likelihood of bleeding or thrombotic events. In severe trauma, these assays have revealed that platelet dysfunction is strongly associated with poor clinical outcomes. However, to date, the mechanism(s) causing clinically significant platelet dysfunction remain poorly understood. We review the pros, cons, and evidence for use of many of the popular assays in trauma, discuss limitations of their use in this patient population, and present approaches that can be taken to develop improved functional assays capable of elucidating mechanisms of trauma-induced platelet dysfunction. Platelet dysfunction in trauma has been associated with need for transfusions and mortality; however, most of the current platelet function assays were not designed for evaluating trauma patients, and there are limited data regarding their use in this population. New or improved functional assays will help define the mechanisms by which platelet dysfunction occurs, as well as help optimize future treatment.

DNA Origami-Platelet Adducts: Nanoconstruct Binding without Platelet Activation

Objective. Platelets are small, mechanosensitive blood cells responsible for maintaining vascular integrity and activatable on demand to limit bleeding and facilitate thrombosis. While circulating in the blood, platelets are exposed to a range of mechanical and chemical stimuli, with the platelet membrane being the primary interface and transducer of outside-in signaling. Sensing and modulating these interface signals would be useful to study mechanochemical interactions; yet, to date, no methods have been defined to attach adducts for sensor fabrication to platelets without triggering platelet activation. We hypothesized that DNA origami, and methods for its attachment, could be optimized to enable nonactivating instrumentation of the platelet membrane. Approach and Results. We designed and fabricated multivalent DNA origami nanotile constructs to investigate nanotile hybridization to membrane-embedded single-stranded DNA-tetraethylene glycol cholesteryl linkers. Two hybridization protocols were developed and validated (Methods I and II) for rendering high-density binding of DNA origami nanotiles to human platelets. Using quantitative flow cytometry, we showed that DNA origami binding efficacy was significantly improved when the number of binding overhangs was increased from two to six. However, no additional binding benefit was observed when increasing the number of nanotile overhangs further to 12. Using flow cytometry and transmission electron microscopy, we verified that hybridization with DNA origami constructs did not cause alterations in the platelet morphology, activation, aggregation, or generation of platelet-derived microparticles. Conclusions. Herein, we demonstrate that platelets can be successfully instrumented with DNA origami constructs with no or minimal effect on the platelet morphology and function. Our protocol allows for efficient high-density binding of DNA origami to platelets using low quantities of the DNA material to label a large number of platelets in a timely manner. Nonactivating platelet-nanotile adducts afford a path for advancing the development of DNA origami nanoconstructs for cell-adherent mechanosensing and therapeutic agent delivery.

Differential regulation of the platelet GPIb-IX complex by anti-GPIbβ antibodies

BACKGROUND

Platelets' initial recognition of endothelial damage proceeds through the interaction between collagen, plasma von Willebrand factor (VWF), and the platelet glycoprotein (GP)Ib-IX complex (CD42). The GPIb-IX complex consists of one GPIbα, one GPIX, and two GPIbβ subunits. Once platelets are immobilized to the subendothelial matrix, shear generated by blood flow unfolds a membrane-proximal mechanosensory domain (MSD) in GPIbα, exposing a conserved trigger sequence and activating the receptor. Currently, GPIbα appears to solely facilitate ligand-induced activation because it contains both the MSD and the binding sites for all known ligands to GPIb-IX. Despite being positioned directly adjacent to the MSD, the roles of GPIbβ and GPIX in signal transduction remain murky.

OBJECTIVES

To characterize a novel rat monoclonal antibody 3G6 that binds GPIbβ.

METHODS

Effects of 3G6 on activation of GPIb-IX are characterized in platelets and Chinese hamster ovary cells expressing GPIb-IX (CHO-Ib-IX) and compared with those of an inhibitory anti-GPIbβ antibody, RAM.1.

RESULTS

Both RAM.1 and 3G6 bind to purified GPIbβ and GPIb-IX with high affinity. 3G6 potentiates GPIb-IX-associated filopodia formation in platelets or CHO-Ib-IX when they adhere VWF or antibodies against the ligand-binding domain (LBD) of GPIbα. Pretreatment with 3G6 also increased anti-LBD antibody-induced GPIb-IX activation. Conversely, RAM.1 inhibits nearly all GPIb-IX-related signaling in platelets and CHO-Ib-IX cells.

CONCLUSIONS

These data represent the first report of a positive modulator of GPIb-IX activation. The divergent modulatory effects of 3G6 and RAM.1, both targeting GPIbβ, strongly suggest that changes in the conformation of GPIbβ underlie outside-in activation via GPIb-IX.