Thromboxane A2合成酶剔除鼠功能研究

Abstract

Thromboxane A2合成酶(TXA2 synthase)所生產的TXA2具有引發血小板凝集、血管收縮及調節腎臟等功能，且在包含心肌梗塞、中風、氣喘及許多的腎臟疾病發生的過程中均扮演重要的角色。除了參與眾所周知的凝血與血栓形成外，TXA2被認為可能參與血小板生成及淋巴球的分化。為了進一步了解其生理功能，本論文以基因轉殖的方式製作TXA2合成酶基因剔除(TXAS-/-)小鼠模式。分析結果發現TXAS-/-小鼠具有正常的骨髓巨核細胞，其胸腺及脾臟的淋巴球數量也正常。但其出血時間異常且以花生四烯酸(arachidonic acid；AA)刺激TXAS-/-小鼠的血小板，血小板無法凝集且無法釋出TXB2，但是prostaglandin-E2 (PGE2)、PGD2及PGF2α的代謝卻增加。以花生四烯酸注入正常的小鼠血管內會造成小鼠血壓急速下降、心跳停止而導致死亡，但TXAS-/-小鼠或是以TP受體拮抗劑(antagonist)處理過的野生型小鼠卻不會休克。進一步實驗發現在花生四烯酸刺激下，當TP受體存在時，TXAS-/-小鼠可維持正常的血壓，但是當TP受體以拮抗劑阻斷時，血壓即下降，此結果顯示受體參與血壓的維持。此外，本論文發現TXAS缺乏會影響花生四烯酸經lipoxygenase路徑中15-HETE及LTB4的生成，顯示TXA2可調節其他發炎介體(inflammatory mediator)的生成。由目前的實驗結果可知，血小板生成及淋巴球的分化不需TXA2合成酶的參與，缺乏TXA2合成酶會有輕微的凝血缺陷，然而對花生四烯酸的刺激較無反應，且不易產生休克及死亡。TXS-/-小鼠模式將是研究與TXA2及花生四烯酸代謝相關病理現象的良好素材。

Thromboxane A2 (TXA2) has potent actions on platelet aggregation, vasoconstriction, as well as regulating renal hemodynamics. It plays a role in the pathogenesis of a number of disease states including myocardial infarction, thrombotic stroke, bronchial asthma and a variety of renal diseases. Besides its well-recognized role in hemostasis and thrombosis, thromboxane A2 synthase (TXAS) is proposed to be involved in thrombopoiesis and lymphocyte differentiation. To evaluate its various physiological roles, TXAS-deleted mice was generated by gene targeting. TXAS-/- mice had normal bone marrow megakaryocytes, blood platelet counts, and normal CD4 and CD8 lymphocyte counts in thymus and spleen. Platelets from TXAS-/- mice failed to aggregate or generate thromboxane B2 in response to arachidonic acid (AA) but produced increased PGE2, PGD2 and PGF2α. AA infusion caused a progressive drop of mean arterial pressure (MAP), cardiac arrest and death in WT mice, but did not induce shock in TXAS-/- mice, nor in WT and TXAS-/- mice treated with antagonist to the TP receptor. The TXAS-/- mice were able to maintain normal MAP upon AA insult when TP was present but were unable to do so when TP was blocked by an antagonist, suggesting a role of endoperoxide accumulation in influencing MAP. We also found that TXAS deletion can influence 15-HETE and LTB4 production from lipoxygenase pathway, suggested that TXA2 may regulate inflammatory mediator formation. We conclude that TXAS is not essential for thrombopoiesis and lymphocyte differentiation. Its deficiency causes a mild hemostatic defect and protects mice against arachidonate-induced shock and death. The TXAS-deleted mice will be valuable for investigating the roles of arachidonate metabolic shunt in various pathophysiological processes.