Abstract
Amoebic gill disease (AGD) is one of the main diseases affecting Atlantic salmon (Salmo salar L.) mariculture. Hallmarks of AGD are hyperplasia of the lamellar epithelium and increased production of gill mucus. This study investigated the expression of genes involved in mucus secretion, cell cycle regulation, immunity and oxidative stress in gills using a targeted 21-gene PCR array. Gill samples were obtained from experimental and natural Neoparamoeba perurans infections, and sampling points included progressive infection stages and post-freshwater treatment. Up-regulation of genes related to mucin secretion and cell proliferation, and down-regulation of pro-inflammatory and pro-apoptotic genes were associated with AGD severity, while partial restoration of the gill homeostasis was detected post-treatment. Mucins and Th2 cytokines accoun ted for most of the variability observed between groups highlighting their key role in AGD. Two mucins (muc5, muc18) showed differential regulation upon disease. Substantial up-regulation of the secreted muc5 was detected in clinical AGD, and the membrane bound muc18 showed an opposite pattern. Th2 cytokines, il4/13a and il4/13b2, were significantly up-regulated from 2 days post-infection onwards, and changes were lesion-specific. Despite the differences between experimental and natural infections, both yielded comparable results that underline the importance of the studied genes in the respiratory organs of fish, and during AGD progression.
Highlights
Gill diseases, caused by infectious and non-infectious agents, are one of the main health challenges for marine Atlantic salmon (Salmo salar L.) aquaculture worldwide[1,2,3,4,5]
Cultured N. perurans collected from naturally infected fish (Fig. 2A) were used to infect by bath challenge naïve Atlantic salmon smolts
N. perurans was detected by real-time polymerase chain reaction (PCR) in 50% of the fish sampled at 2 dpi, and in 100% of the fish sampled at 7, 14 and 21 dpi
Summary
Gill diseases, caused by infectious and non-infectious agents, are one of the main health challenges for marine Atlantic salmon (Salmo salar L.) aquaculture worldwide[1,2,3,4,5]. Previous studies on AGD gene expression have largely focused on immune genes[23,24,25,26,27] or microarray-based transcriptome profiling[28,29,30] Based on this information, the present study was designed as a targeted approach pinpointing genes of interest belonging to pathways involved in mucus secretion, cell proliferation and apoptosis, and oxidative/cellular stress during an experimentally controlled AGD infection and a natural outbreak. The rationale for the selection of genes was supported by previous proteomic[31] studies, and known gene functions in other animal species; such as stimulation of mucin secretion (MUC5AC/MUC5B) by Th2 interleukins[32,33] and epidermal growth factor receptor (EGFR)[34,35] in mammals, or the important role of mitogen activated protein kinase (MAPK) pathways and Krüppel-like factors (KLFs) in cell cycle regulation[36,37]. The results obtained contribute to the ongoing understanding of AGD host response and pathogenesis and identify key molecules involved in gill health
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