| Summary: | 碩士 === 國立臺灣大學 === 藥理學研究所 === 105 === Platelets act as an indispensable role in hemostasis. Abnormal activation of platelets may lead to formation of threatening thrombus under pathological conditions. Myocardial infarction and ischemic stroke are in high lethal rates each year. Therefore, several antiplatelet agents are indicated for preventing abnormal thrombus formation in daily life or after coronary surgery. However, current anti-platelet agents have the risk of bleeding. For safety issue, thrombus prevention with lower bleeding risk is the concerning issue of new generation antiplatelet agents. Platelet glycoprotein Ib is a key receptor in platelet adhesion, and activation during vascular injury with the immobilized vWF, especially under high shear stress. Therefore, the blockade of GPIb-mediated platelet activation would interfere more with platelet deposition in diseased arteries, than in healthy blood vessels, thus preventing thrombosis without affecting significantly physiologic hemostasis.
Recently, several snake C-type lectins (snaclecs) have been reported to inhibit platelet GPIb both in vitro and in vivo. Surprisingly, intravenous injection of these snaclecs causes thrombocytopenia in small animals. The causative reasons are still under investigation. This report focuses on agkistin, a snaclec purified from venom of China Agkistrodon acutus, inhibits GPIb-mediated platelet agglutination in vitro. However, it induces a rapid and sustained thrombocytopenia upon intravenous injection to mice. To figure out the mechanism of the induced thrombocytopenia, we evaluated the causative factors including adaptive immune system, macrophage, apoptosis, and hepatocyte etc. Our results demonstrated that agkistin could promote the platelet ingestion by hepatocytes through their Ashwell-Morell Receptor (AMR). Previous studies indicated that hepatocytes recognize and internalize desialylated platelets in several immune thrombocytopenia (ITP) patients with anti-GPIbα antibodies. Additionally, these anti-GPIbα antibodies could lead to platelet activation and loss of sialic acid on GPIbα, leading to consequent clearance by hepatocytes. However, in case of agkistin, this snaclec neither activates platelet nor causes desialylation in vitro. Further, we found that agkistin has high-binding affinity to AMR and the cleavage of GPIbα by metalloproteinase kistomin would attenuate the platelet clearance. These results indicate that both GPIbα and AMR are essential factor in the agkistin-induced thrombocytopenia. We hypothesize that agkistin might play a role as the “bridge” between GPIbα and AMR, which promotes the recognition and ingestion by hepatocytes. Nevertheless, structure-activity relationships remain to be elucidated.
Taken together, we provide a possible mechanism of hepatocyte in causing agkistin-induced thrombocytopenia in mice. These researches provide helpful information for designing novel GPIb-antagonists in minimizing the thrombocytopenia risk.
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