Expression Analysis of Moritella viscosa-Challenged Atlantic Salmon Identifies Disease-Responding Genes, MicroRNAs and Their Predicted Target Genes and Pathways.

Moritella viscosa is a bacterial pathogen causing winter-ulcer disease in Atlantic salmon. The lesions on affected fish lead to increased mortality, decreased fish welfare, and inferior meat quality in farmed salmon. MicroRNAs (miRNAs) are small non-coding RNAs involved in post-transcriptional regulation by guiding the miRNA-induced silencing complex to specific mRNA transcripts (target genes). The goal of this study was to identify miRNAs responding to Moritella viscosa in salmon by investigating miRNA expression in the head-kidney and the muscle/skin from lesion sites caused by the pathogen. Protein coding gene expression was investigated by microarray analysis in the same materials. Seventeen differentially expressed guide-miRNAs (gDE-miRNAs) were identified in the head-kidney, and thirty-nine in lesion sites, while the microarray analysis reproduced the differential expression signature of several thousand genes known as infection-responsive. In silico target prediction and enrichment analysis suggested that the gDE-miRNAs were predicted to target genes involved in immune responses, hemostasis, angiogenesis, stress responses, metabolism, cell growth, and apoptosis. The majority of the conserved gDE-miRNAs (e.g., miR-125, miR-132, miR-146, miR-152, miR-155, miR-223 and miR-2188) are known as infection-responsive in other vertebrates. Collectively, the findings indicate that gDE-miRNAs are important post-transcriptional gene regulators of the host response to bacterial infection.

Piscine orthoreovirus (PRV) infects Atlantic salmon erythrocytes.

Piscine orthoreovirus (PRV) belongs to the Reoviridae family and is the only known fish virus related to the Orthoreovirus genus. The virus is the causative agent of heart and skeletal muscle inflammation (HSMI), an emerging disease in farmed Atlantic salmon (Salmo salar L.). PRV is ubiquitous in farmed Atlantic salmon and high loads of PRV in the heart are consistent findings in HSMI. The mechanism by which PRV infection causes disease remains largely unknown. In this study we investigated the presence of PRV in blood and erythrocytes using an experimental cohabitation challenge model. We found that in the early phases of infection, the PRV loads in blood were significantly higher than in any other organ. Most virus was found in the erythrocyte fraction, and in individual fish more than 50% of erythrocytes were PRV-positive, as determined by flow cytometry. PRV was condensed into large cytoplasmic inclusions resembling viral factories, as demonstrated by immunofluorescence and confocal microscopy. By electron microscopy we showed that these inclusions contained reovirus-like particles. The PRV particles and inclusions also had a striking resemblance to previously reported viral inclusions described as Erythrocytic inclusion body syndrome (EIBS). We conclude that the erythrocyte is a major target cell for PRV infection. These findings provide new information about HSMI pathogenesis, and show that PRV is an important factor of viral erythrocytic inclusions.