Chinese Whispovirus: A chitin-binding protein is involved in White Spot Syndrome

White Spot Syndrome (WSS) is a viral disease of crustaceans. Outbreaks of WSS can be devastating to aquaculture yields and severe infections in Asian penaeid prawn farms have pushed facilities to near collapse. The etiological agent responsible for WSS is an enveloped, lytic dsDNA virus known as Whispovirus (WSSV) which causes tissue necrosis culminating in host mortality. There exists, therefore, a heavy economic incentive to investigate the pathology of WSSV and intensive research has been conducted to understand the interaction of WSSV and its host to design an effective treatment. Recent work by Li et al, (2016) has identified a chitin-binding role in a major envelope protein that the authors believe is integral to infection.

The cramped, squalid conditions of shrimp aquaculture places healthy shrimp in close proximity to the carcasses of the infected and deceased. Due to the cannibalistic nature of penaeid prawns, the authors hypothesised that ingestion may the primary route of viral infection. Large areas of the prawns’ digestive track are coated with a chitinous lining, and chitin-binding by WSSV may facilitate infection, however little is known about chitin-binding and host invasion in this virus. Therefore, the authors investigated the ability of four major envelope proteins to bind to chitin.

Recombinant proteins were expressed in transformed E. coli and incubated with chitin beads. Subsequent immunoblotting revealed that one of the proteins, VP24, exhibited chitin-binding and became a prime candidate for further investigation. Following a mutagenesis screen, the chitin-binding site of VP24 was identified as a 14 amino acid sequence and the authors explored the possibility of exploiting this region to aid in the treatment of WSS. A synthesised peptide of the same sequence blocked the ability of recombinant VP24 to bind to chitin when co-incubated with chitin beads and a subsequent in vivo experiment strikingly showed that oral inoculation of the host with the binding peptide significantly decreased the infectivity and viral load of WSSV in Whiteleg Shrimp (Litopenaeus vannamei). Such peptide inoculations could prove vital in treating WSS across the globe. 

Overall, this study employed a diverse methodology to achieve an exciting insight into the molecular interaction of the WSSV ‘infectome’. Their discovery of a chitin-binding role in VP24 and synthesis of a chitin-binding therapeutic peptide is tantalizing and, while preliminary, could play a major role in our understanding and treatment of WSS. Chitin-binding is implicated in various marine diseases (such as in various vibrioses) and therefore understanding this phenomenon may provide insights into treating other infections. 

This is, however, not the first treatment for WSS proposed by researchers and previous attempts to treat the disease have ranged from DNA vaccinations (Ning et al, 2009) to medicinal herb extracts (Citarasu et al, 2006). A previous attempt to orally inoculate shrimp with WSSV envelope proteins too reported success as far back as 2004 (Witteveldt et al, 2004) but WSS is still devastating shrimp industries. While this treatment involves a different protein and administration technique, time will have to tell as to whether this discovery can make a significant contribution to protecting shrimp aquaculture or whether it is yet another false lead. Nevertheless, this paper makes a significant advancement in our understanding of a globally and economically important pathogen and offers another step closer to an effective treatment. 

Reviewed Paper: Li, Z., Li, F., Han, Y., Xu, L., & Yang, F. (2016). VP24 is a chitin-binding protein involved in white spot syndrome virus infection. Journal of virology, 90(2), 842-850. http://jvi.asm.org/content/90/2/842.short

DNA Vaccine: Ning, J. F., Zhu, W., Xu, J. P., Zheng, C. Y., & Meng, X. L. (2009). Oral delivery of DNA vaccine encoding VP28 against white spot syndrome virus in crayfish by attenuated Salmonella typhimurium. Vaccine, 27(7), 1127-1135. http://www.sciencedirect.com/science/article/pii/S0264410X08015715

Herbal Remedy: Citarasu, T., Sivaram, V., Immanuel, G., Rout, N., & Murugan, V. (2006). Influence of selected Indian immunostimulant herbs against white spot syndrome virus (WSSV) infection in black tiger shrimp, Penaeus monodon with reference to haematological, biochemical and immunological changes. Fish & shellfish immunology, 21(4), 372-384. http://www.sciencedirect.com/science/article/pii/S1050464806000088

Previous Oral Inoculation: Witteveldt, J., Cifuentes, C. C., Vlak, J. M., & van Hulten, M. C. (2004). Protection of Penaeus monodon against white spot syndrome virus by oral vaccination. Journal of virology, 78(4), 2057-2061. http://jvi.asm.org/content/78/4/2057.short s