Fig. 1

 A schematic illustrating the pathways and proteins involved in the natural mechanism that prevents blood from clotting, the activated platelet response and the degradation of fibrin clots. Created with Biorender.com 

A.Natural anti-coagulation mechanism. In normal condition, nitric oxide (NO) and prostacyclin (PGI₂) secreted from endothelial cells inhibit platelets, keeping them inactive and preventing them from binding to the endothelial lining. Platelet coagulation factors are also inactivated by antithrombin III (ATIII), a protein bound to endothelial cells by a glycose immune glycan (heparin sulfate) and activated protein C (APC) in association with protein S. ATIII cleaves circulating clotting factors (F-II, F-IX, F-X) and inactivates them. Protein C is activated by thrombin (F-II) bound to thrombomodulin (TM) in the endothelial cells. APCs cleaves circulating clotting factors (F-V, F-VIII) and inactivate them

B.Platelet activation and aggregation. In response to damage to endothelial cells, circulating platelets migrate to the site of injury and bind to a protein, Von Willebrand factor (vWF), produced by endothelial cells through another platelet-surface protein glycoprotein 1b (GP1b) that activates the platelets. Activated platelets release granules containing adenosine di-phosphate (ADP) and thromboxane A2 (TXA2) which bind to their respective receptors expressed in platelets, allowing more platelets to migrate and form clusters at the site of injury, a process called ‘platelet aggregation’ through which ‘platelet-plaque’ is formed at the injury site. Platelet activation also allows the membrane translocation of CD40 ligand (CD40LG). The translocation of CD40LG seems to coincide with the release α-granule contents, including platelet-derived growth factor (PDGF), transforming growth factor beta (TGFβ) and platelet factor 4 (PF4). The surface-expressed CD40LG is cleaved and shed from the platelet surface in a time-dependent manner as sCD40LG. Platelet-endothelial interactions also promote progressive plaque, and this interaction is facilitated by rolling and adhering of activated platelets to the endothelial cell surface. Rolling of platelets to endothelial cells is mediated by platelet P-selectin glycoprotein ligand − 1 (PSGL-1) binding to endothelial cell P-selectin

C. Fibrinolytic system. The final step of the blood homeostasis system involves the natural breakdown of the blood clot (platelet-fibrin plaque). Endothelial cells express a protein tissue plasminogen activator (TPA), which converts plasminogen to plasmin. Plasmin degrades the fibrin mesh in the platelet-fibrin plaque and releases fibrinogen and D-dimer. TPA is inhibited by an endothelial plasminogen activator inhibitor called plasminogen activator inhibitor-1 (PAI-1).

D. Cascade pathways of blood coagulation system. The platelet surfaces in the platelet-fibrin clot (shown separately with broken, red-dotted lines) contain the phosphatidyl serine groups which are negatively charged. Coagulation factor XII, secreted from the liver, binds to the negatively charged surface and this contact triggers a conformational change in the XII zymogen, inducing autoactivation (contact activation). The activation leads the cascade pathway activation of XI, IX, VIII (intrinsic pathway). Tissue damage activates coagulation factors III and VII (extrinsic pathway) and, finally, both pathways activate factor X (common pathway) which leads to the conversion of prothrombin to thrombin. Thrombin converts fibrin polymer (insoluble) from fibrinogen monomers (soluble) and, with the help of XIII, fibrin finally forms a fibrin mesh to coagulate blood (with aggregated platelets at the site of injury)

E. Comparison of % change of platelet activation-related proteins in type 2 diabetes. The table shows the % change of platelet activation-related proteins in response to mild and deep hypoglycemia in control subjects (C) and in subjects with type 2 diabetes (T2D). The upward red arrows indicate a % increase, and the downward blue arrow indicates a % decrease

Hypo, hypoglycemia; 24 h-p-hypo, 24 h post-hypoglycemia; PF3, Platelet factor 3; C, control subjects; T2D, type 2 diabetes subjects