Negative regulators of platelet activation and adhesion

Abstract

Platelets are small anucleated cells that constantly patrol the cardiovascular system to preserve its integrity and prevent excessive blood loss where the vessel lining is breached. Their key challenge is to form a hemostatic plug under conditions of high shear forces. To do so, platelets have evolved a molecular machinery that enables them to sense trace amounts of signals at the site of damage and to rapidly shift from a non-adhesive to a pro-adhesive state. However, this highly efficient molecular machinery can also lead to unintended platelet activation and cause clinical complications such as thrombocytopenia and thrombosis. Thus, several checkpoints are in place to tightly control platelet activation and adhesiveness in space and time. In this review, we will discuss select negative regulators of platelet activation, which are critical to maintain patrolling platelets in a quiescent, non-adhesive state and/or to limit platelet adhesion to sites of injury.

Keywords: hemostasis; negative regulators; platelet adhesion; platelet reactivity; thrombosis.

Figure 1. Checkpoints of platelet activation

In physiological conditions the molecular mechanisms regulating platelet adhesiveness and activation are self-limiting. Several checkpoints are in place 1) to prevent unwanted activation of patrolling platelets, when the endothelial lining is biochemically and physically healthy, and 2) to limit the duration and intensity of the activation response at sites of vascular injury. This schematic figure represents platelet activation as a ladder and shows the four main checkpoints that need to be passed/climbed in order for platelets to shift from a non-adhesive patrolling state to a pro-adhesive state that ensures hemostasis. Platelets that do not make it past these checkpoints return into circulation. The regulatory elements that provide important negative feedback at these critical checkpoints are listed above each step of the activation process. The list is not exhaustive but only includes the best characterized negative regulators, which we have described in the text.

Figure 2. Negative regulators that control platelet activation and adhesiveness

The tight balance between inhibitory (red arrows) and activatory (green arrows) signaling pathways is critical to maintain patrolling platelets in a quiescent, non-adhesive state and/or to limit platelet adhesion to sites of injury. This schematic figure shows the critical signaling nodes (blue) where the positive and negative signals are integrated to finely control platelet activation and adhesiveness in space and time, most importantly at the level of 1) receptor stimulation, 2) intracellular Ca²⁺ elevation, 3) Rap1 activation, and 4) outside-in signaling. The regulatory elements that provide important negative feedback at these critical checkpoints of the activation process are labelled in red. Abbreviations: ITAMs (immune receptor tyrosine-based activation motif), ITIMs (Immunoreceptor Tyrosine-based Inhibition Motif), PECAM-1 (Platelet Endothelial Cell Adhesion Molecule-1, CD31), G6B-B (Megakaryocyte and Platelet Inhibitory Receptor), CAECAM1/2 (Carcinoembryonic antigen-related cell adhesion molecule 1/2), LILRB2 (Leukocyte immunoglobulin-like receptor subfamily B member 2), GPCRs (G protein-coupled receptors), RGS (Regulators of G-protein Signaling), TxA₂ (thromboxane A₂), PLC (phospholipase C), IP₃ (inositol 1,4,5-triphosphate), DTS (dense tubular system), Ca²⁺ (calcium ions), IP₃R (IP₃ receptor), IRAG (IP₃R-associated cGMP kinase substrate), STIM1 (Stromal interaction molecule 1), ORAI1 (Calcium Release-Activated Calcium Modulator 1), TRPC6 (Transient receptor potential cation channel, subfamily C, member 6), PMCA (plasma membrane Ca²⁺-ATPase), DAG (diacylglycerol), RAP1 (Ras-proximate-1), CALDAG-GEFI (Ca²⁺-regulated guanine nucleotide exchange factor), PKC (protein kinase C), ADP (adenosine diphosphate), PKA (protein kinase A), PKG (protein kinase G), P2Y12 (G-protein coupled purinergic receptor), RASA3 (RAS p21 protein Activator 3), RAP1GAP2 (RAP1 GTPase activating protein 2), SHIP1 (phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase 1), SHP1/2 (Src-homology 2 domain (SH2)-protein tyrosine phosphatase), JAM/A (Junctional adhesion molecule-A), ESAM (Endothelial Cell Adhesion Molecule), DOK1/2 (Docking protein 1/2), PP2A (Protein phosphatase 2A).