Mao Yingying, Jin Jianguo, Daniel James L, Kunapuli Satya P
Department of Physiology, Temple University School of Medicine, Philadelphia, PA, USA.
Platelets. 2009 May;20(3):191-8. doi: 10.1080/09537100902803635.
Plasmin, a major extracellular protease, activates platelets through PAR4 receptors. Plasmin-induced full aggregation is achieved at lower concentrations (0.1 U/mL) in murine platelets as compared to human platelets (1 U/mL). In COS7 cells expressing the murine PAR4 (mPAR4) receptor, 1 U/mL plasmin caused a higher intracellular calcium mobilization than in cells expressing the human PAR4 (hPAR4) receptor. This difference was reversed when the tethered ligand sequences of mPAR4 and hPAR4 were interchanged through site-directed mutagenesis. We further investigated whether PAR3 expressed in murine platelets serves as a co-receptor for PAR4 activation by plasmin. In COS7 cells, co-expressing mPAR3 and mPAR4, plamsin produced a smaller intracellular calcium mobilization compared to cells expressing mPAR4 alone, suggesting that PAR3 might inhibit plasmin-induced PAR4 stimulation. Consistent with these results, PAR3 null murine platelets also showed a greater plasmin-induced calcium mobilization and aggregation compared to wild-type murine platelets. In conclusion, murine platelets are more sensitive to activation by plasmin than human platelets due to differences in the primary sequence of PAR4. In contrast to thrombin-dependent activation of platelets, wherein PAR3 acts as a co-receptor, mPAR3 inhibits plasmin-induced PAR4 activation.
纤溶酶是一种主要的细胞外蛋白酶，可通过PAR4受体激活血小板。与人类血小板（1 U/mL）相比，在鼠血小板中，较低浓度（0.1 U/mL）的纤溶酶就能诱导完全聚集。在表达鼠PAR4（mPAR4）受体的COS7细胞中，1 U/mL纤溶酶引起的细胞内钙动员高于表达人类PAR4（hPAR4）受体的细胞。当通过定点诱变互换mPAR4和hPAR4的拴系配体序列时，这种差异发生了逆转。我们进一步研究了鼠血小板中表达的PAR3是否作为纤溶酶激活PAR4的共受体。在共表达mPAR3和mPAR4的COS7细胞中，与单独表达mPAR4的细胞相比，纤溶酶产生的细胞内钙动员较小，这表明PAR3可能抑制纤溶酶诱导的PAR4刺激。与这些结果一致，与野生型鼠血小板相比，PAR3基因敲除的鼠血小板在纤溶酶诱导下也表现出更大的钙动员和聚集。总之，由于PAR4一级序列的差异，鼠血小板对纤溶酶激活比人类血小板更敏感。与凝血酶依赖性激活血小板（其中PAR3作为共受体）相反，mPAR3抑制纤溶酶诱导的PAR4激活。
