Nucleus as Targets of RNA Virus Proteins and Storehouse for Nervous Necrosis Virus Particles Accumulation

Nervous necrosis virus (NNV) is the causative agent of viral nervous necrosis disease in piscine species worldwide. As a positive-sense RNA virus, NNV performs its replication, transcription, translation and assembly in cytoplasm of host cell. However, our immunocytochemistry results show that viral B2 and coat protein translocate into nucleolus in early infection stage. The viral factory vesicles containing NNV genomes RNA1 and RNA2 as well as coat protein were also observed migrating toward perinuclear compartment for virion assembly. Transmission electron microscopy (TEM) thin sections revealed viral particles arranged in ordered arrays or circular lines inside the vesicles surrounding the perinuclear area. These observations suggested that multiple vesicles fused with each other and joined into remodeled microtubule organizing center (MTOC). Unexpectedly, during late infection stage, we also observed the NNV particles containing vesicles being transported into the nucleus by crossing the nuclear membrane. The accumulation of these virion arrays within the nucleus, encapsulated by multiple membranes and nuclear matrix, may provide a more stable storage environment that ultimately facilitates viral transmission through prey/predator food chain. In summary, our results reveal that the evolutionarily ancient RNA virus such as NNV utilizes host nucleus to benefit their propagation and transmission.