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RPAI-1 was the first sialogenin whose function was identified as ADP-binding protein (KD 50 nM). applications. Keywords:Aegyptin, Ixolaris, D7-short, RPAI-1, Nitrophorin, Anophelin, Lipocalin, Tick, Mosquito, Sand fly, Ixodegrin, Ornatin, Metalloproteases, Sialogenin == 1. Hematophagy and platelet activation == Hematophagous animals have evolved various strategies to counteract their host hemostatic system and to successfully feed on blood. In evolution, it has been suggested that insects may have been initially nest parasites or predators on other insects. They may also have fed on secretions from vertebrate eyes or other mucosal membranes where access to a meal is easier and does not require piercing of the skin. Perhaps this was the first adaptation toward acquiring the ability to penetrate the skin in search of blood (Francischetti, et al., 2009,Ribeiro and Francischetti, 2003). Actually, blood-feeding may occur through two different pathways: from hemorrhagic pools that accumulate in tissues following PDCD1 skin lacerations, or through direct cannulation of venules or arterioles. For example, mosquitoes and bugs cannulate the arterioles and venules deep in the skin, sometimes reaching several millimeters. In contrast, sand fly mouthparts penetrate up to 0.5 mm into their host skin and therefore can feed only from superficial hematomas caused by laceration of capillaries. Tabanids, which have cutting scissors, can also lacerate several vessels simultaneously, potentially forming a large hematoma. Further, ticks introduce their mouthparts into the host and generate a feeding cavity from which they feed on blood (Francischetti, et al., 2009,Ribeiro and Francischetti, 2003). These animals express molecules from hematophagous salivary secretions, herein named sialogenins (from the Greeksialo, saliva;gen, origin, source; andinsfor proteins). Sialogenins target the host response to injury, modulate immune response, prevent pain, and other mechanisms associated with blood feeding. While several functions of sialogenins have been reviewed before (Francischetti, et al., 2009,Koh and Kini, 2009,Maritz-Olivier, et al., 2007,Ribeiro and Francischetti, 2003,Steen, et al., 2006), this review focuses on those that target platelet aggregation. Platelets have a central role in hemostasis. The first step in the hemostatic cascade is platelet interaction with the exposed extracellular matrix (ECM), which contains a large number of adhesive macromolecules such as collagens and fibronectin. Under conditions of high shear, such as in small arteries and arterioles, the initial tethering of platelets to the ECM is mediated by interaction between the platelet receptor glycoprotein (GP)Ib and vWF bound to collagen (Furie and Furie, 2005,Jackson and Schoenwaelder, 2003,Ruggeri, 2002,Watson, et al., 2005). GPIb to vWF binding dissociates rapidly and is insufficient to mediate stable adhesion but rather maintains the platelet in close contact with the exposed surface. This interaction allows the collagen receptor GPVI (Jandrot-Perrus, et al., 2000) to bind to collagen. This triggers the conformational change of integrins (e.g., 21) to a high-affinity state, thereby enabling them to mediate firm adhesion to collagen and also promotes SDZ 205-557 HCl the release of TXA2and ADP (Furie and Furie, 2005,Jackson SDZ 205-557 HCl and Schoenwaelder, 2003,Ruggeri, 2002,Watson, et al., 2005). ADP binds SDZ 205-557 HCl to the Gq-protein-linked P2Y1receptor on platelets, which causes a change in cell shape, mobilization of calcium, and initiation of reversible aggregation via stimulation SDZ 205-557 HCl of PLC. It also binds the Gi-linked P2Y12receptor to amplify aggregation via inhibition of adenylyl-cyclase-mediated cyclic AMP SDZ 205-557 HCl production. The resulting platelet activation leads to PI-3-kinase activation, granule secretion, and inside-out activation of integrin IIb3, which increases its affinity for fibrinogen and vWF. These ligands then bind to the receptors to form bridges between adjacent platelets, which results in outside-in signalling and aggregation (Kahner, et al., 2006,Varga-Szabo, et al., 2008). Accordingly, sustained ADP-induced platelet aggregation requires activation of both P2Y1and P2Y12receptors. TXA2 activates platelets through the TP receptors that are coupled to Gq and G12/13. While it promotes shape change, activation of this pathway is unable to induce sustained platelet aggregation in the absence of a Gi-coupled receptor agonist such.