GCRV-II major outer capsid protein VP4 promotes cell apoptosis by VDAC2-mediated calcium pathway facilitation

Grass Carp Reovirus (GCRV) is widely concerned because of its widespread prevalence and high mortality to grass carp (Ctenopharyngodon idellus). Viral protein 4 (VP4) is an important major outer capsid protein of GCRV and is involved in the regulation of cell cycle and cycle of GCRV replication. However, the elaborate function of VP4 remains to be explicated. To understand its function, we screened the transcriptome of Ctenopharyngodon idellus kidney (CIK) cells transfected with VP4 and found that VP4 may be involved in regulation of ion transmembrane transporter activity and calcium signaling pathway. It was observed through transmission electron microscopy and confocal microscopy. VP4 causes endoplasmic reticulum (ER) stress and leads to abnormal Calcium ions (Ca²⁺) concentration in cells. Also, VP4 promoted the loss of mitochondrial membrane potential, which allowed a large amount of Ca²⁺ to enter mitochondria and led to mitochondrial damage and apoptosis. In this transcriptome, we found that voltage-dependent anion channel 2 (VDAC2) was significantly upregulated. Moreover, the results also showed that the expression of C.idellus voltage-dependent anion channel 2 (CiVDAC2) and the degree of cell apoptosis were increased along with the increase of VP4 transfection. In contrast, knockdown of CiVDAC2 can reduce the concentration of Ca²⁺ and the occurrence of apoptosis caused by VP4 transfection. In conclusion, the results demonstrated that VP4 can induce cell apoptosis through VDAC2-mediated calcium pathway facilitation. This study provides some insights for the prevention and treatment of GCRV infection in grass carp.