Growth characteristics and morphology of Paramoeba perurans from Atlantic salmon Salmo salar L. and ballan wrasse Labrus bergylta in Norway

BACKGROUND

Paramoeba perurans is the causative agent of amoebic gill disease (AGD) in Atlantic salmon Salmo salar L. and many other farmed marine fish species worldwide. The first cases of AGD in Norway were reported in 2006, and it has subsequently become established as a significant gill disease that affects the country's salmonid aquaculture industry. Despite several decades of research on AGD, there is still a lack of knowledge of the biology of P. perurans and its interactions with its hosts and the environment.

METHODS

The growth and morphology of 10 clonal isolates of P. perurans were studied. The isolates were from farmed Atlantic salmon and ballan wrasse that had been obtained from different sites along the Norwegian coast between 2013 and 2015. The morphology and population growth patterns of these clonal amoeba isolates were examined in vitro using light microscopy and real-time reverse transcription polymerase chain reaction under a range of temperatures (4, 12, 15 and 21 °C) and salinities (20, 25, 30 and 34 ‰).

RESULTS

We found distinct morphological differences between both locomotive and floating forms of the amoeba isolates. The locomotive amoebae of the clonal isolates varied in size (area) from 453 µm² to 802 µm². There were differences in the growth patterns of the clonal amoeba isolates under similar conditions, and in their responses to variations in temperature and salinity. While most of the isolates grew well at salinities of 25-34 ‰, a significant reduction in growth was seen at 20 ‰. Most of the amoeba isolates grew well at 12 °C and 15 °C. At 4 °C, amoebae grew slower and, in contrast to the other temperatures, no extended pseudopodia could be seen in their floating form. The isolates seemed to reach a plateau phase faster at 21 °C, with a higher number of smaller, rounded amoebae.

CONCLUSIONS

The differences observed here between clonal isolates of P. perurans should be further examined in experimental in vivo challenge studies, as they may be of relevance to the virulence and proliferation potential of this amoeba on gills. Potential differences in virulence within P. perurans could have implications for management strategies for AGD.

Experimental challenge of Atlantic salmon Salmo salar using clones of Paramoeba perurans, P. pemaquidensis and Tetramitus sp

Salmon gill disease in Norway is in most cases associated with a range of different pathogens, stress and environmental factors. Paramoeba perurans and other amoebae have been isolated during such disease outbreaks. Other amoebae isolated from salmon with gill disease in Norway include P. pemaquidensis, Tetramitus sp. and Vannella sp. Here we tested the pathogenicity of the first 2 species in challenge experiments. We found that even when clonal cultures of P. pemaquidensis established an infection on the gills of salmon, it failed to cause gill disease, while Tetramitus sp. appeared to be unable to establish a lasting infection on the gills of healthy salmon. The result of the challenge with P. pemaquidensis confirms the results of similar studies performed in the USA and in Australia. Tetramitus sp. is probably a common amoeba in the marine environment, and its presence on the gills of farmed salmon may just be accidental. Based on this study, we conclude that P. perurans is the only known amoeba in marine salmon farming associated with amoebic gill disease in Norway.

Optimisation of gene expression analysis in Atlantic salmon lenses by refining sampling strategy and tissue storage

Analysis of gene expression in the lens is one of the analytical tools employed to investigate cataract formation in Atlantic salmon (Salmo salar L.). High quality RNA preparations are an essential prerequisite for gene expression analysis. The first aim of the present study was to investigate the possible effects of two methods of tissue preservation on the quality of RNA extracted from Atlantic salmon lenses. RNA was extracted from lenses either stored in RNAlater or flash-frozen in liquid nitrogen. Both tissue preservation methods yielded RNA of similarly high quality. The second aim was to examine if stress related to fish handling and the choice of anaesthesia during the sampling procedure affected gene expression in the lens. Six sampling procedures were tested on groups of sea water adapted Atlantic salmon smolt. Fish were either killed instantaneously (control group) or sampled after 30 min anaesthetised with isoeugenol, after 30 min without anaesthesia, after 120 min anaesthetised with isoeugenol, after 120 min with 15 min anaesthesia with metacaine or after 120 min without anaesthesia. The expression levels of specific genes, of special interest in the study of molecular mechanisms of cataractogenesis, were analysed in lenses by real-time RT-PCR. Fish not anaesthetised had significantly increased levels of heat shock protein 70 (HSP70) mRNA after 30 min compared to the control group. Glutathione reductase (GR) and manganese superoxide dismutase (Mn-SOD) were expressed at significantly lower levels in groups of Atlantic salmon sampled after 120 min anaesthetised with isoeugenol or metacaine, and anaesthetised with isoeugenol, metacaine or without anaesthesia, respectively. The same expression patterns were found in corresponding gill tissues for these two antioxidant genes. In conclusion, preservation in liquid nitrogen instead of RNAlater is recommended due to practical conditions in RNA extraction. A quick sampling protocol with the use of anaesthetics and not exceeding 30 min should be preferred to avoid effects of the sampling procedure on lens gene expression in Atlantic salmon.

The level of synthesis and secretion of Gaussia princeps luciferase in transfected CHO cells is heavily dependent on the choice of signal peptide

There is a great demand for the improvement of mammalian cell production systems such that they can compete economically with their prokaryotic counterparts. Of a number of parameters that need to be explored to accomplish this we have tested the effects of different signal peptides on the synthesis and secretion of Gaussia princeps luciferase in mammalian cells. A series of plasmids were transfected into CHO cells where the coding region for the marine luciferase was fused to the signal peptide coding regions derived from different sources. Both cell extracts and medium samples were analysed for luciferase activity. When the native Gaussia luciferase signal sequence in the vector was substituted by that from human interleukin-2 or albumin then the amount of active recombinant protein produced was substantially reduced, both in transiently and stably transfected cells. Western blotting showed that enzyme activity and protein levels mirrored one another. The major decrease in luciferase activity was shown not to be a result of decreased mRNA levels, indicating the involvement of a post-transcriptional event. When the coding region of human endostatin was fused to that of the Gaussia luciferase signal peptide then an elevated level of secreted endostatin was observed compared to when that of the albumin signal peptide was used. Stable transfection of HepG2 cells with the different signal peptide constructs gave essentially the same results as seen in CHO cells. The overall results indicate that the choice of signal peptide can be imperative to ensure an optimal synthesis and secretion of a recombinant protein in a mammalian cell culture system.