Activation of host endogenous reverse transcriptase in response to white spot syndrome virus infection in Penaeus monodon

Abstract

It was first hypothesized in 2009 that endogenous host reverse transcriptase (eRT) arising from retrotransposon elements in arthropod genomes would be critical for defense against viral pathogens in crustaceans and insects. This was not proven until 2016 in insects. In the succeeding 20 years, insect studies have confirmed the eRT role in antiviral defenses that result in tolerated, persistent infections via viral accommodation mechanisms (VAM). In insects, eRT transcribes viral RNA fragments into viral copy DNA (vcDNA) in both linear (lvcDNA) and circular (cvcDNA) forms. These, in turn, give rise to small interfering RNA (siRNA) fragments to feed the host RNA interference (RNAi) mechanism. At the same time, some vcDNA fragments are integrated into the host genome as endogenous viral elements (EVE), that can also give rise to siRNA. Studies with shrimp have progressed more slowly but results so far have mimicked those revealed with insects. Here we describe the presence and functional activity of a 5383 bp-RT-related gene from a retrotransposon in the black tiger shrimp Penaeus monodon upon challenge with white spot syndrome virus (WSSV). It encompasses four conserved domains of protease (retropepsin), reverse transcriptase (RT-RNase H), transposase, and integrase as a single ORF of 1248 deduced amino acids. The recombinant protein of a conserved 408–amino acid RT-RNase H domain, designated as Pm-eRT was expressed and found to possess reverse transcriptase activity. During WSSV infection, Pm-eRT was upregulated in shrimp hemocytes and pleopods and DNA/RNA hybrids in hemocytes were detected. The DNA/RNA hybrid signals were translocated from the plasma membrane to the nuclear membrane in the shrimp that survived WSSV infection with low viral loads. Pharmacological inhibition of Pm-eRT activity using azidothymidine (AZT) resulted in enhanced viral replication and accelerated shrimp mortality. Collectively, these findings support those described for insects, paving the way to further work on details of VAM in shrimp with the ultimate aim of developing genetically viral-tolerant breeding stocks for shrimp cultivation.