A Serpine1-F2-PAR1 axis facilitates apoptosis and disease severity during GCRV infection in grass carp

Coagulation factors are crucial in physiological processes, including the progression of viral infections. However, the relationship between the coagulation cascade and viral hemorrhagic diseases in teleost fish remains poorly understood. In this study, grass carp coagulation factor 2 (F2) and its receptor protease-activated receptor 1(PAR1) were cloned and functionally characterized for their roles in response to grass carp reovirus (GCRV) infection. At the individual level, F2 expression was promptly upregulated during GCRV-Ⅱ infection, highlighting its involvement in the early immune response. At the cellular level, F2 overexpression enhanced viral replication, PAR1-mediated signaling, and apoptosis, whereas F2 knockdown significantly suppressed viral proliferation and reduced apoptosis. Furthermore, F2-mediated enhancement of apoptosis was shown to be dependent on PAR1 signaling. We further discovered that Serpine1, a major inhibitor of the fibrinolytic system, was strongly induced at the early stage of GCRV infection. Inhibition of Serpine1 led to reduced F2 and PAR1 expression, increased tPA and PLG levels, and suppressed viral replication. Together, these findings reveal a previously unrecognized Serpine1-F2-PAR1 signaling axis that links fibrinolysis-related gene regulation with virus-associated apoptosis and disease manifestations during GCRV infection. This study provides new insight into the interplay between coagulation-related pathways and viral pathogenesis in fish.