LvPPAE2 induced by WSV056 confers host defense against WSSV in Litopenaeus vannamei

Proteomics analysis reveals a critical role for the WSSV immediate-early protein IE1 in modulating the host prophenoloxidase system

White spot syndrome virus (WSSV) is a large enveloped double-stranded DNA virus that is a major impediment for shrimp aquaculture worldwide. So far, the mechanisms of WSSV-host interactions are ill-defined. Recent studies have revealed that IE1, an immediate-early protein encoded by WSSV, is a multifunctional modulator implicated in virus-host interactions. In this study, the biological functions of IE1 were further explored by identifying its interacting proteins using GST-pull down and mass spectrometry analysis. A total of 361 host proteins that potentially bind to IE1 were identified. Bioinformatics analysis revealed that the identified IE1-interacting proteins were key molecules involved in various signaling pathways such as prophenoloxidase (proPO) system, PI3K-AKT, MAPK, Focal adhesion, and cell cycle. Among these, the regulatory role of IE1 in the shrimp proPO system was further studied. The Co-immunoprecipitation (Co-IP) results confirmed that IE1 interacted with the Ig-like domain of Penaeus vannamei proPO or proPO-like proteins (proPO1/2 and hemocyanin). In addition, we found that in vivo RNAi mediated knockdown of IE1 reduced the viral genes expression and viral loads, as well as caused an increase in the PO activity of hemocytes during infection, whereas recombinant IE1 protein could inhibit the PO activity in a dose-dependent manner. Finally, our result demonstrated that WSSV could suppress the PO activity of hemocytes at the early infection stage. Collectively, our current data indicate that IE1 is a novel viral regulator that negatively modulates the shrimp proPO system, which provide additional insights into the biological functions of IE1 during WSSV infection.

WSV056 Inhibits Shrimp Nitric Oxide Synthase Activity by Downregulating Litopenaeus vannamei Sepiapterin Reductase to Promote White Spot Syndrome Virus Replication

Sepiapterin reductase (Spr) plays an essential role in the biosynthesis of tetrahydrobiopterin (BH4), a key cofactor of multiple enzymes involved in various physiological and immune processes. Suppression of Spr could result in BH4 deficiency-caused diseases in human and murine models. However, information on the biological function of Spr in invertebrates is limited. In this study, two Sprs (CG12116 and Sptr) from Drosophila melanogaster were found to be downregulated in transgenic flies overexpressing white spot syndrome virus (WSSV) immediate-early protein WSV056. CG12116 and Sptr exerted an inhibitory effect on the replication of the Drosophila C virus. A Litopenaeus vannamei Spr (LvSpr) exhibiting similarity of 64.1–67.5% and 57.3–62.2% to that of invertebrate and vertebrate Sprs, respectively, were cloned. L. vannamei challenged with WSSV revealed a significant decrease in LvSpr transcription and Spr activity in hemocytes. In addition, the BH4 co-factored nitric oxide synthase (Nos) activity in shrimp hemocytes was reduced in WSSV-infected and LvSpr knockdown shrimp, suggesting WSSV probably inhibits the LvNos activity through LvSpr downregulation to limit the production of nitric oxide (NO). Knockdown of LvSpr and LvNos caused the reduction in NO level in hemocytes and the increase of viral copy numbers in WSSV-infected shrimp. Supplementation of NO donor DETA/NO or double gene knockdown of WSV056 + LvSpr and WSV056 + LvNos recovered the NO production, whereas the WSSV copy numbers were decreased. Altogether, the findings demonstrated that LvSpr and LvNos could potentially inhibit WSSV. In turn, the virus has evolved to attenuate NO production via LvSpr suppression by WSV056, allowing evasion of host antiviral response to ensure efficient replication.