Why platelet mechanotransduction matters for hemostasis and thrombosis

Abstract

Mechanotransduction is the ability of cells to "feel" or sense their mechanical microenvironment and integrate and convert these physical stimuli into adaptive biochemical cellular responses. This phenomenon is vital for the physiology of numerous nucleated cell types to affect their various cellular processes. As the main drivers of hemostasis and clot retraction, platelets also possess this ability to sense the dynamic mechanical microenvironments of circulation and convert those signals into biological responses integral to clot formation. Like other cell types, platelets leverage their "hands" or receptors/integrins to mechanotransduce important signals in responding to vascular injury to achieve hemostasis. The clinical relevance of cellular mechanics and mechanotransduction is imperative as pathologic alterations or aberrant mechanotransduction in platelets has been shown to lead to bleeding and thrombosis. As such, the aim of this review is to provide an overview of the most recent research related to platelet mechanotransduction, from platelet generation to platelet activation, within the hemodynamic environment and clot contraction at the site of vascular injury, thereby covering the entire "life cycle" of platelets. Additionally, we describe the key mechanoreceptors in platelets and discuss the new biophysical techniques that have enabled the field to understand how platelets sense and respond to their mechanical microenvironment via those receptors. Finally, the clinical significance and importance of continued exploration of platelet mechanotransduction have been discussed as the key to better understanding of both thrombotic and bleeding disorders lies in a more complete mechanistic understanding of platelet function by way of mechanotransduction.

Keywords: biomechanics; blood platelets; hemostasis; mechanotransduction; thrombosis.

FIGURE 1

Biophysics of shear flow and cell stiffness. Fluid flowing through a blood vessel reaches a steady state with a parabolic velocity profile. Shear rate is the change in velocity as the fluid distance from the vessel wall increases. Shear stress is a measure of how much force is acting on an object and is proportional to shear rate and fluid viscosity. Platelets aggregate with fibrin to form a network to develop a clot. The ability to control geometric constraints and endothelialize microfluidics has enabled in vitro studies that closely mimic the in vivo microenvironment. Clot stiffness changes over the course of contraction and deforms as platelets mechanically remodel the fibrin network.

FIGURE 2

Biomechanical platelet interactions with the microenvironment. When platelets are in suspension in circulation, they not only move in the direction of the flow but also rotate and are subjected to mechanical interactions with red blood cells, which “push” them to the periphery. After vascular injury occurs, the extracellular matrix (collagen, laminin, and fibronectin) is exposed, and platelets in a high-shear environment bind and tether to von Willibrand factor and bind to collagen to begin thrombus formation. Following this, platelet activation and aggregation occur with signaling of mechanical and biochemical pathways. This subsequently leads to platelet plug formation and dot formation to dramatically shrink and stabilize the thrombus.

FIGURE 3

Platelet mechanotransduction occurs in all aspects of hemostasis after injury. Glycoprotein (GP)Ib mechanosensing: GPIb binds to von Willibrand factor and leverages its mechanosensory domain to activate platelets. GPVI collagen mechanosensing: GPVI interacts with collagen to activate platelets and support stable adhesion, responding to stiffness in the mechanical microenvironment. GPIIb/IIIa fibrinogen mechanosensing: The integrin α_IIIb β₃ (GPIIb/IIIa), the most abundant receptor on platelets, interacts with fibrin(ogen) and responds to changes in the developing clot microenvironment.

FIGURE 4

Platelet mechanotransduction demonstrated using the “trigger” model of glycoprotein GPIb-IX signaling and integrin α_IIIb β₃ affinity maturation. First, mechanoreception occurs by mechanical pulling force applied by von Willibrand factor on GPIbα to unfold. Mechanoreception is demonstrated by leucine-rich repeat domain engaging with GPIbα. This leads to mechanotransmission along the GPIbα macroglycopeptide stalk and transfers this signal via mechanotransduction to induce platelet activation via the biochemical pathway (soluble-agonist dependent) and biomechanical pathway. The biomechanical pathway upregulates integrin α_IIIb β₃ from the inactive state (bent closed) to the intermediate state (extended closed). Outside-in signaling further promotes affinity maturation to the high-affinity state (extended open integrin conformation).

FIGURE 5

Aberrant hemostasis occurs when there is too little clotting (bleeding) or too much clotting thrombosis, wherein platelet dysfunction can lead to both disease processes. At the single-platelet level, decreased platelet activation or interactions with exposed extracellular matrix proteins can lead to deficiencies at the bulk clot level, causing decreases in platelet density and decreases in contraction force, leading to impaired hemostasis. Alternatively, platelet preactivation leads to enhanced recruitment, leading to thrombosis. Interestingly, both increases and decreases in bulk clot contraction force have been implicated in thrombosis. Consequently, more thorough exploration is necessary to determine the reasoning as to why in some disorders, both increased and decreased forces leads to thrombosis.