RNA-Seq analysis of European sea bass (Dicentrarchus labrax L.) infected with nodavirus reveals powerful modulation of the stress response

Raquel Lama,Valentina Valenzuela-Muñoz,Valentina Valenzuela-Muñoz,Antonio Figueras,Beatriz Novoa,Patricia Pereiro,Patricia Pereiro,Patricia Pereiro,Cristian Gallardo-Escárate,Cristian Gallardo-Escárate,Lluis Tort

doi:10.1186/s13567-020-00784-y

Abstract

Nodavirus, or nervous necrosis virus (NNV), is the causative agent of viral encephalopathy and retinopathy (VER), a severe disease affecting numerous fish species worldwide. European sea bass, a cultured species of great economic importance, is highly susceptible to the disease. To better understand the response of this organism to NNV, we conducted RNA-Seq analysis of the brain and head kidney from experimentally infected and uninfected sea bass juveniles at 24 and 72 hours post-infection (hpi). Contrary to what was expected, we observed modest modulation of immune-related genes in the brain, the target organ of this virus, and some of these genes were even downregulated. However, genes involved in the stress response showed extremely high modulation. Accordingly, the genes encoding the enzymes implicated in the synthesis of cortisol were almost the only overexpressed genes in the head kidney at 24 hpi. This stress response was attenuated after 72 h in both tissues, and a progressive immune response against the virus was mounted. Moreover, experiments were conducted to determine how stress activation could impact NNV replication. Our results show the complex interplay between viral activity, the stress reaction and the immune response.

Highlights

European sea bass (Dicentrarchus labrax L.) is a very valuable fish species in Mediterranean countries, and it is currently one of the main cultured fish species in Europe [1]
Effects of nervous necrosis virus (NNV) challenge in juvenile sea bass European sea bass challenged with NNV began to manifest clinical signs of infection at 8 dpi; these signs mainly consisted of erratic swimming behaviours, such as spiral or whirling swimming, belly-up floating, and laying down at rest (Additional file 2)
10.2 these samples and, statistically significant differences were not observed between 24 and 72 hpi, the viral replication tended to increase with time in the brain, whereas the uninfected control samples were negative for the detection of the virus (Figure 1B)

Summary

Introduction

European sea bass (Dicentrarchus labrax L.) is a very valuable fish species in Mediterranean countries, and it is currently one of the main cultured fish species in Europe [1]. The in vivo response of European sea bass remains almost completely unexplored, and only a small number of publications have reported the modulation or involvement of immune factors in different tissues of D. labrax infected with NNV [14,15,16,17,18,19,20,21,22]. Numerous publications reported the effect of a variety of stressors in the susceptibility to diseases and their impact on different immune parameters [23], the stress response induced by pathogens remains practically unexplored in fish. Pathogens are considered as an important environmental biotic stressor for plants and animals This is the first time that RNA-Seq analysis has shown an interaction between neuroendocrine pathways and the immune system through the HPI axis during nodavirus infection

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Conclusion