| dc.description.abstract | Platelets are the major players in haemostasis. They respond to the binding of a broad selection of agonists and adhesive proteins, after which they aggregate and form a haemostatic plug.
Upon vessel wall injury, collagen in the sub endothelium becomes exposed to the bloodstream,
which leads to the binding of the platelet receptor GPVI to collagen. Binding of collagen to GPVI on platelets activates them. This activation involves activation of PLCγ, PKC and PI3K. Activated platelets can spread and form pseudopods on their surface, which facilitate binding and aggregation of platelets.
Binding of an agonist to its receptor on the platelet membrane activates platelets. Most of the platelet agonists transduce signals via seven transmembrane domain receptors that are coupled to heterotrimeric G-proteins. Upon stimulation of the heterotrimeric receptor, the α-subunit is released from the βγ-complex. The liberal subunits are second messengers and may exert either a stimulatory or inhibitory role, depending on the receptor they were bound to. A major effector that is activated via a heterotrimeric receptor is PLC. PLC cleaves PIP2 in the membrane to form DAG and IP3 . DAG remains in the membrane, but IP3 enters the cytosol and binds to the IP3-receptor on the ER. This results in the release of Ca2+ from the ER, and subsequently an increase in the cytosolic calcium concentration. The presence of both DAG and Ca2+ leads to the activation of calcium-dependent isoforms of PKC. PKC phosphorylates its downstream targets, which finally results in granule secretion, activation of ion-exchangers and regulation of the integrin αIIbβ3 affinity .
Vessel injury also initiates the coagulation cascade, resulting in the conversion of prothrombin into thrombin. Thrombin converts fibrinogen into fibrin, which in turn stabilizes the thrombus. Thrombin is in addition to collagen a second platelet agonist. One of its receptors is PAR-1. This receptor is cleaved by thrombin and the new amino acid terminus acts as a tethered ligand, which auto-activates the PAR-1 receptor. This platelet activation results in activation of PLCß.
Platelet activation via GPVI and PAR-1 results in platelet granule release. Platelets contain dense granules, α-granules and lysosomes. The dense granules contain nucleotides (e.g. ADP). ADP can activate platelets via two receptors, P2Y1 and P2Y12. Stimulation of P2Y12 with ADP leads to activation of PLCβ and PKC.
The final result of all platelet stimulations is the release of granule contents and the activation of GPIIb-IIIa. These processes are strictly regulated. Granule secretion is mediated by the SNARE 1 complex, formed by three membrane associated proteins: the syntaxins, the vesicle-associated membrane proteins (VAMPs) and the SNAP family proteins. Rab GTPases are involved in granule secretion also, they regulate alpha granule release.
GPIIb-IIIa activation involves the binding of talin and kindlins to the integrin tails and the subsequent conformational change from a low to a high affinity state for ligand binding. | |
