Cowpea Chlorotic Mottle Virus virus-like particles as DNA or RNA delivery vehicle to fish cells

Cowpea Chlorotic Mottle Virus (CCMV) is an icosahedral plant virus composed of 180 monomeric units of the capsid protein. Virions encapsulate the viral RNA genome, which can be removed from the virion by changing pH conditions, after which the capsid proteins can be reversibly assembled to form empty CCMV virus-like particles (VLPs), or VLPs that have encapsulated either heterologous DNA or RNA. Earlier research has shown that CCMV VLPs are suitable as delivery vehicles for heterologous DNA/RNA molecules into mammalian cells, leading to subsequent expression of their encoded genes. In this study, we evaluated the potential of CCMV VLPs to encapsulate nucleic acids and transfer them to fish cells in vitro, and to zebrafish larvae and juveniles in vivo. VLPs containing pVax-GFP plasmid (3690 bp) were stable for up to 5 weeks in buffer at pH 7.2 and pH 5, whereas VLPs containing GFP-mRNA (1168 nt) were stable for up to 3 weeks, but only when stored in a buffer at pH 4.5. When administered directly to the culture medium, VLPs were readily taken up by fish as well as mammalian cell lines but could not unpack and transfer nucleic acids to the cells. VLPs delivered by transfection however, could transfer nucleic acids to the cells, leading to reporter gene expression. When VLPs were administered directly to the culture medium along with the cell-penetrating peptide dfTAT, only VLPs containing pVax-GFP in pH 7.2 buffer were able to transfer DNA to mammalian but not fish cells. In vivo however, upon intramuscular injection, the same VLPs were able to transfer plasmid DNA to myocytes of zebrafish larvae and juveniles as efficiently as when injecting naked plasmid DNA. Altogether, these findings provide the foundation for future studies on the use of CCMV VLPs as delivery vehicles for DNA or RNA to fish cells.