ISKNV ORF48R Functions as a New Viral Vascular Endothelial Growth Factor

Infectious spleen and kidney necrosis virus (ISKNV) causes a pandemic and serious disease in fish. Infection by ISKNV causes epidermal lesions, in which petechial haemorrhages and abdominal oedema are prominent features. ISKNV ORF48R contains a domain similar to that of the platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) families of proteins. ISKNV ORF48R showed higher similarity to the VEGFs encoded by megalocytivirus and parapoxvirus than fish and mammalian animals. We used zebrafish as a model and constructed a recombinant plasmid containing the DNA sequence of ISKNV ORF48R to study ISKNV infection. The plasmid was microinjected into zebrafish embryos at the one cell stage. Overexpression of ISKNV ORF48R gene results in pericardial oedema and dilation at the tail region of zebrafish embryos, suggesting that ISKNV ORF48R induces vascular permeability. ISKNV ORF48R is also able to stimulate a striking expression of flk1 in the zebrafish dorsal aorta and the axial vein. Furthermore, ISKNV ORF48R, while co-operating with zebrafish VEGF121, can stimulate more striking expression of flk1 than either ISKNV ORF48R or zebrafish VEGF121 alone. However, decreased expression of FLK-1 by gene knockdown results in the disappearance of pericardial oedema and dilation at the tail region of zebrafish embryos induced by overexpression of ISKNV ORF48R in the early stages of embryonic development.