## 2 Platelets in Cellular Blood Hemostasis
### Signaling Involved in Platelet Activation  
Platelet activation is a complex orchestrated process in which various intracellular biochemical reactions cause versatile changes, including the formation of pseudopodia, alterations of the plasma membrane, and release of vesicles [14]. This serves to recruit additional platelets, which attach to the first layer, forming a cross-linked white thrombus that is later solidified by the formation of fibrin [37]. The involved signaling cascades are highly interconnected and predominantly mediated by G-coupled proteins that consist of different subunits and exhibit different functions. Further, various other platelet receptors, e.g., immunoreceptor tyrosine-based activation motif (ITAM) receptors, ligand-dependent ion channels, and glycoprotein receptor complexes, contribute to their impressive ability to react within seconds after a blood vessel's endothelial wall is disrupted [38]. Several platelet receptors, including the adenosine diphosphate (ADP)-responsive purinergic receptors P2Y\({}_{12}\)[39] and P2Y\({}_{1}\)[40], the thromboxane receptor TxA\({}_{2}\)-R [41], and the thrombin protease-activated receptors PAR1/4 [42], are coupled to G\({}_{\text{q}}\)/G\({}_{\text{i}}\) proteins. G\({}_{\text{q}}\)/G\({}_{\text{i}}\)-induced activation of phospholipase C-\(\beta\) (PLC-\(\beta\)) initiates cleavage of phosphatidylinositol-4,5-bisphosphate (PIP\({}_{2}\)), a phospholipid mainly located in the inner cell membrane under the emergence of diacylglycerolcerol (DAG) and inositol triphosphate (IP\({}_{3}\)) [43]. The second messenger DAG further activates protein kinase C (PKC), followed by signaling involved in thromboxane A\({}_{2}\) synthesis, vesicle secretion, and activation of phosphodiesterase (PDE) [44]. Upon binding to IP\({}_{3}\) receptors located at the dense tubular system, IP\({}_{3}\) initiates the distribution of intracellular Ca\({}^{2+}\)[45; 46]. Besides Gq-mediated signaling, coupling of PAR1/4 to G\({}_{\text{13}}\) proteins induces activation of the small GTPase Rho, which is crucial for changes in platelet shape [42]. Rho mediates actin polymerization and increases myosin light chain kinase (MLCK) activity by inhibiting its opponent myosin light chain phosphatase (MLCP). Together with increased intracellular Ca\({}^{2+}\) level downstream PLC-\(\beta\) activation, this facilitates actin-myosin interactions [43; 47]. Integrin activation downstream phosphoinositide 3-kinase (PI3K) signaling is needed for sufficient cellular cross-linking during clot formation [48; 49]. PI3K can be activated by G\({}_{\text{i}}\) proteins [50], but also through glycoprotein VI (GPVI) and the receptor complex GPIb-IX-V [51]. GPVI is a member of the immunoglobulin (Ig) superfamily present on platelets and an established receptor for cellular cross-linking upon contact with collagen [52]. The GPIb-IX-V complex, part of the leucine-rich repeat (LRR) family, is activated upon binding of vWF and essential for platelet adhesion under high shear stress conditions. GPIV-IX-V signaling requires cooperativity of PAR1/4 and vice versa, making the interconnection of both receptors an important driver of platelet activation, especially at low thrombin concentrations [53]. Secondly, PI3K activation can cause increased formation of nitric oxide (NO) by NO synthase (NOS) via Akt [54]. Being an uncharged gas, NO can diffuse across cell membranes and plays a biphasic role in blood hemostasis [55]. Initially, it promotes platelet activation via soluble guanylate cyclase (sGC)/PKG/p38 [47; 56], whereas, as blood coagulation progresses, it acts as a suppressant to hemostasis [57]. Platelet activation is finely regulated and can be counteracted by protein kinase A (PKA) signaling. PKA is activated by cyclic adenosine monophosphate (cAMP), which emerges from the cleavage of adenosine triphosphate (ATP) by adenylate cyclase (AC) [58]. PKA mediates multiple responses, such as reducing the intracellular Ca\({}^{2+}\) concentration and subsequently decreasing platelet shape change [59].