Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.

Chitosan nanoparticle formulation attenuates poly (I:C) induced innate immune responses against inactivated virus vaccine in Atlantic salmon (Salmo salar)

Many vaccine formulations, in particular vaccines based on inactivated virus, needs adjuvants to boost immunogenicity. In aquaculture, mineral and plant oil are used as adjuvant in commercial vaccines, and the advent of oil-adjuvanted vaccines was crucial to aquaculture development. Nevertheless, some of these approved vaccines display suboptimal performance in the field compared to experimental conditions. Therefore, there is a need to improve adjuvants and delivery methods for fish vaccines against viruses. We used RNA sequencing of Atlantic salmon head kidney to analyse the difference in gene expression 24 h after injection of different experimental vaccine formulations. We compared five different formulations in addition to a PBS control: inactivated virus alone (group V), soluble poly (I:C) (group P), nanoparticles containing poly (I:C) (group N), soluble poly (I:C) + inactivated virus (group PV) and finally nanoparticles containing poly (I:C) + inactivated virus (group NV). Our results showed poly (I:C)'s ability as adjuvant and its capacity influence innate immune genes expression in Atlantic salmon. Soluble poly (I:C) upregulated multiple immune related genes and was more effective compared to poly (I:C) formulated into chitosan nanoparticles (more than 10 fold increase in differentially expressed genes, DEGs). However, inclusion of inactivated ISA virus in the nanoparticle vaccine, increased the number of DEGs fivefold suggesting a synergistic effect of adjuvant and antigen. Our results indicate that the way poly (I:C) is formulated and the presence of antigen is important for the magnitude of the innate immune response in Atlantic salmon.

Kinetics of transcriptional response against poly (I:C) and infectious salmon anemia virus (ISAV) in Atlantic salmon kidney (ASK) cell line

Vaccine adjuvants induce host innate immune responses improving long-lasting adaptive immunity against vaccine antigens. In vitro models can be used to compare these responses between adjuvants and the infection targeted by the vaccine. We utilized transcriptomic profiling of an Atlantic salmon cell line to compare innate immune responses against ISAV and an experimental viral vaccine adjuvant: poly (I:C). Induction of interferon and interferon induced genes were observed after both treatments, but often with different amplitude and kinetics. Using KEGG ortholog database and available software from Bioconductor we could specify a complete bioinformatic pipeline for analysis of transcriptomic data from Atlantic salmon, a feature not previously available. We have identified important differences in the transcriptional profile of Atlantic salmon cells exposed to viral infection and a viral vaccine adjuvant candidate, poly (I:C). This report increases our knowledge of viral host-pathogen interaction in salmon and to which extent these can be mimicked by adjuvant compounds.

Interaction between dietary fatty acids and genotype on immune response in Atlantic salmon (Salmo salar) after vaccination: A transcriptome study

A pivotal matter to aquaculture is the sourcing of sustainable resources as ingredients to aquafeeds. Levels of plant delivered oils as source of fatty acids (FA) in aquafeeds have reached around 70% resulting in reduced levels of long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in salmon fillet composition. EPA and DHA can modulate inflammation and immune response, so it is crucial to understand how fish immune response is affected by low LC n-3 PUFA diet and if this diet can have a detrimental effect on vaccine response. Atlantic salmon (Salmo salar) can produce EPA/DHA from α-linolenic acid (ALA) and this endogenous capacity can be explored to develop families with higher tolerance to low LC n-3 PUFA diets. Here we analyze innate and adaptive immune response in Atlantic salmon to a commercial vaccine after being fed low levels of EPA and DHA, and we also compare three strains of salmon selected by their endogenous capacity of synthesizing LC- n-3 PUFA. A total of 2,890 differentially expressed genes (DEGs) were identified (p-value adjusted < 0.1) when comparing vaccinated fish against control non-vaccinated. Gene ontology (GO) and KEGG analysis with 442 up/downregulated genes revealed that most DEGs were both related to immune response as well as part of important immune related pathways, as "Toll-like receptor" and "Cytokine-Cytokine interaction". Adaptive response was also addressed by measuring antigen specific IgM, and titers were significantly higher than in the pre-immune fish at 62 days post-immunization. However, diet and strain had no/little effect on vaccine-specific IgM or innate immune responses. Atlantic salmon therefore display robustness in its response to vaccination even when feed low levels of LC n-3 PUFA.

Synthesis and biological evaluation of zinc chelating compounds as metallo-β-lactamase inhibitors

The syntheses of metallo-β-lactamase inhibitors comprising chelating moieties, with varying zinc affinities, and peptides partly inspired from bacterial peptide sequences, have been undertaken. The zinc chelator strength was varied using the following chelators, arranged in order of ascending binding affinity: dipicolylamine (DPA, tridentate), dipicolyl-1,2,3-triazolylmethylamine (DPTA, tetradentate) dipicolyl ethylenediamine (DPED, tetradentate) and trispicolyl ethylenediamine (TPED, pentadentate). The chosen peptides were mainly based on the known sequence of the C-terminus of the bacterial peptidoglycan precursors. Biological evaluation on clinical bacterial isolates, harbouring either the NDM-1 or VIM-2 metallo-β-lactamase, showed a clear relationship between the zinc chelator strength and restoration of meropenem activity. However, evaluation of toxicity on different cancer cell lines demonstrated a similar trend, and thus inclusion of the bacterial peptides did possess rather high toxicity towards eukaryotic cells.

Antimicrobial Activity and Cytotoxicity of Ag(I) and Au(I) Pillarplexes

The biological activity of four pillarplex compounds featuring different metals and anions was investigated. The toxicity of the compounds against four bacterial strains [Bacillus subtilis (ATCC6633), Staphylococcus aureus (ATCC6538), Escherichia coli (UVI isolate), Pseudomonas aeruginosa], one fungus (Candida albicans), and a human cell line (HepG2) was determined. Additionally, a UV-Vis titration study of the pillarplexes was carried out to check for stability depending on pH- and chloride concentration changes and evaluate the applicability in physiological media. All compounds are bioactive: the silver compounds showed higher activity against bacteria and fungi, and the corresponding gold pillarplexes were less toxic against human cells.

Synthesis and Preclinical Evaluation of TPA-Based Zinc Chelators as Metallo-β-lactamase Inhibitors

The rise of antimicrobial resistance (AMR) worldwide and the increasing spread of multi-drug-resistant organisms expressing metallo-β-lactamases (MBL) require the development of efficient and clinically available MBL inhibitors. At present, no such inhibitor is available, and research is urgently needed to advance this field. We report herein the development, synthesis, and biological evaluation of chemical compounds based on the selective zinc chelator tris-picolylamine (TPA) that can restore the bactericidal activity of Meropenem (MEM) against Pseudomonas aeruginosa and Klebsiella pneumoniae expressing carbapenemases Verona integron-encoded metallo-β-lactamase (VIM-2) and New Delhi metallo-β-lactamase 1 (NDM-1), respectively. These adjuvants were prepared via standard chemical methods and evaluated in biological assays for potentiation of MEM against bacteria and toxicity (IC₅₀) against HepG2 human liver carcinoma cells. One of the best compounds, 15, lowered the minimum inhibitory concentration (MIC) of MEM by a factor of 32-256 at 50 μM within all tested MBL-expressing clinical isolates and showed no activity toward serine carbapenemase expressing isolates. Biochemical assays with purified VIM-2 and NDM-1 and 15 resulted in inhibition kinetics with k_inact/ K_I of 12.5 min^-1 mM^-1 and 0.500 min^-1 mM^-1, respectively. The resistance frequency of 15 at 50 μM was in the range of 10^-7 to 10^-9. 15 showed good tolerance in HepG2 cells with an IC₅₀ well above 100 μM, and an in vivo study in mice showed no acute toxic effects even at a dose of 128 mg/kg.

Effects of dietary n-3 fatty acids on Toll-like receptor activation in primary leucocytes from Atlantic salmon (Salmo salar)

The shortage of the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the international markets has led to increasing substitution of fish oil by plant oils in Atlantic salmon (Salmo salar) feed and thereby reducing the EPA and DHA content in salmon. However, the minimum required levels of these fatty acids in fish diets for securing fish health are unknown. Fish were fed with 0, 1 or 2% EPA or DHA alone or in combination of both over a period, growing from 50 to 400 g. Primary head kidney leucocytes were isolated and stimulated with Toll-like receptor (TLR) ligands to determine if EPA and DHA deficiency can affect expression of important immune genes and eicosanoid production. Several genes related to viral immune response did not vary between groups. However, there was a tendency that the high-level EPA and DHA groups expressed lower levels of IL-1β in non-stimulated leucocytes. These leucocytes were also more responsive to the TLR ligands, inducing higher expression levels of IL-1β and Mx1 after stimulation. The levels of prostaglandin E2 and leukotriene B4 in serum and media from stimulated leucocytes were lower in both low and high EPA and DHA groups. In conclusion, leucocytes from low EPA and DHA groups seemed to be less responsive towards immunostimulants, like TLR ligands, indicating that low levels or absence of dietary EPA and DHA may have immunosuppressive effects.

Protective effect of a recombinant VHSV-G vaccine using poly(I:C) loaded nanoparticles as an adjuvant in zebrafish (Danio rerio) infection model

There is a constant need to increase the efficiency of vaccines in the aquaculture industry. Although several nano-based vaccine formulations have been reported, to the best of our knowledge so far only one of them have been implemented in the industry. Here we report on chitosan-poly(I:C) nanoparticles (NPs) that could be used as a non-specific adjuvant in antiviral vaccines in aquaculture. We have characterized the physical parameters of the NPs, studied the in vivo and in vitro bio-distribution of fluorescent NPs and verified NP uptake by zebrafish leucocytes. We used the zebrafish model to test the protective efficiency of the recombinant glycoprotein G (rgpG) of VHSV compared to inactivated whole virus (iV) against VHSV using NPs as an adjuvant in both formulations. In parallel we tested free poly(I:C) and rgpG (pICrgpG), and free chitosan and rgpG (CSrgpG) vaccine formulations. While the iV group (with NP adjuvant) provided the highest overall survival, all vaccine formulations with poly(I:C) provided a significant protection against VHSV; possibly through an early induction of an anti-viral state. Our results suggest that chitosan-poly(I:C) NPs are a promising adjuvant candidate for future vaccine formulations.

Short-term effect of bisphenol-a on oxidative stress responses in Atlantic salmon kidney cell line: a transcriptional study

Bisphenol A (BPA) is regularly detected in aquatic ecosystems due to increased use of products based on polycarbonate plastics and epoxy resins. It migrates from these products directly into rivers and marine waters or indirectly through effluents from wastewater treatment plants and landfilled sites. BPA can affect aquatic organisms both chronically and acutely at sensitive live stages. Despite reports indicating harmful effects of BPA, little is known about its role in oxidative stress responses in fish. In this study, we investigated the transcriptional effect of BPA (0, 1, 10, 100 μM) on an Atlantic salmon kidney (ASK) cell line for 6 h and 24 h by monitoring expression of 11 genes: elongation factor 1-alpha (ef1a), 18S ribosomal RNA (18s), gluthation (gsh), superoxide dismutase (sod), thioredoxin (txd), Salmo salar oxidative stress-responsive serine-rich 1 (oxr), glucose-regulated protein 78 (grp78), heat shock protein 70 (hsp70), sequestosome1 (p62), interleukin-1 beta (il-1beta) and toll-like receptor 8 (tlr8). In general, only the 100 μM concentration treatment altered the mRNA expression. BPA down-regulated the expression of gsh and sod genes for both exposure-times while txd gene was the only down-regulated after 6-h exposure. The up-regulation of genes in the ASK cell line exposed for 6 h was only observed in il-1beta, while the 24-h exposure resulted in the up-regulation of oxr, tlr8, hsp70, p62 and il-1beta genes. The last three genes increased several fold compared to the others. The results showed that BPA exposure at 100 μM imposed oxidative stress on the ASK cell line and longer exposure time involved transcriptional responses of immune-related genes. This may indicate the possible role of BPA-associated oxidative stress in induction of inflammatory response in this macrophage-like cell type.

Fish from Head to Tail: The 9th European Zebrafish Meeting in Oslo

The 9th European Zebrafish Meeting took place recently in Oslo (June 28-July 2, 2015). A total of 650 participants came to hear the latest research news focused on the zebrafish, Danio rerio, and to its distant evolutionary relative medaka, Oryzias latipes. The packed program included keynote and plenary talks, short oral presentations and poster sessions, workshops, and strategic discussions. The meeting was a great success and revealed dramatically how important the zebrafish in particular has become as a model system for topics, such as developmental biology, functional genomics, biomedicine, toxicology, and drug development. A new emphasis was given to its potential as a model for aquaculture, a topic of great economic interest to the host country Norway and for the future global food supply in general. Zebrafish husbandry as well as its use in teaching were also covered in separate workshops. As has become a tradition in these meetings, there was a well-attended Wellcome Trust Sanger Institute and ZFIN workshop focused on Zebrafish Genome Resources on the first day. The full EZM 2015 program with abstracts can be read and downloaded from the EZM 2015 Web site zebrafish2015.org .

Structurally diverse genes encode Tlr2 in rainbow trout: The conserved receptor cannot be stimulated by classical ligands to activate NF-κB in vitro

The mammalian toll-like receptor 2 (TLR2) is a dominant receptor for the recognition of Gram-positive bacteria. Its structure and functional properties were unknown in salmonid fish. In RT-PCR and RACE experiments, we obtained the full-length cDNA sequence encoding Tlr2 from rainbow trout (Oncorhynchus mykiss) as well as a copy of an unspliced nonsense message from a highly segmented gene. The primary structure of the encoded receptor complies with the domain structure and ligand-binding sites known from mammals and other fish species and sorts well into the evolutionary tree of teleostean Tlr2s. We retrieved a gene version encoding the receptor on a single exon (tlr2a) and also a partial sequence of a second gene variant being segmented into multiple exons (tlr2b). Surprisingly, the abundances of both transcript variants accounted only for ∼10% of all Tlr2-encoding transcripts in various tissues and cell types of healthy fish. This suggests the expression of several distinct tlr2 gene variants in rainbow trout. We expressed tlr2a in HEK-293 cells, but were unable to demonstrate its functionality through NF-κB activation. Neither synthetic lipopeptides known to stimulate mammalian TLR2 nor different bacterial challenges induced OmTLR2-mediated NF-κB activation, not in HEK-293 or in salmon CHSE-214 cells. Positive demonstration of TLR2-MYD88 interaction excluded that its functional impairment caused the failure of NF-κB activation. We discuss impaired heterodimerization with a necessary Tlr partner as one from among several alternatives to explain the dysfunction of Tlr2a in the interspecies reconstitution system of TLR signaling.

Activation of unfolded protein response pathway during infectious salmon anemia virus (ISAV) infection in vitro an in vivo

Infectious salmon anemia virus (ISAV) is a salmon pathogen causing serious outbreaks in fish farms world-wide. There is currently no effective commercially available vaccine and there is a need for better understanding of host pathogen interactions with this virus. Various strains can cause both acute and persistent infections and therefore establish a balance with the host immune responses. We have studied host responses to this infection by analyzing the main branches of the unfolded protein response (UPR) in salmon cells in vitro and in tissues from infected fish to gain a better understanding of virus-host interactions. ISAV induce the main symptoms and signaling pathways of UPR (ATF6, PERK and IRE1) without inducing translational attenuation. This may be due to concomitant induction of an important negative feedback loop via the phosphatase regulator GADD34. The host cells can therefore respond with translation of cytokine and antiviral proteins to curb or control infection.

Use of Poly(I:C) Stabilized with Chitosan As a Vaccine-Adjuvant Against Viral Hemorrhagic Septicemia Virus Infection in Zebrafish

There is an urgent need for more efficient viral vaccines in finfish aquaculture worldwide. Here, we report the use of poly(I:C) stabilized with chitosan as an adjuvant for development of better finfish vaccines. The adjuvant was co-injected with inactivated viral hemorrhagic septicemia virus (VHSV) (CSpIC+iV vaccine) in adult zebrafish and its efficiency in protection against VHSV infection was compared to a live, attenuated VHS virus vaccine (aV). Both free and stabilized poly(I:C) were strong inducers of an antiviral state, measured by transcriptional activation of the genes of viral sensors: toll-like receptors, interferons, and interferon-stimulated genes, such as MXa within 48 h after injection. Both the CSpIC+iV and the aV formulations provided a significant protection against VHSV-induced mortality. However, when plasma from survivors was tested for neutralizing antibodies in an in vitro protection assay, we could not demonstrate any protective effect. On the contrary, plasma from aV vaccinated fish enhanced cytopathic effects, indicating that antibody-dependent entry may play a role in this system. Our results show that poly(I:C) is a promising candidate as an adjuvant for fish vaccination against viral pathogens, and that the zebrafish is a promising model for aquaculture-relevant vaccination studies.

Reactive oxygen species and cytotoxicity in rainbow trout hepatocytes: effects of medium and incubation time

This study evaluated the effects of exposure medium and culture age on intracellular reactive oxygen species (ROS) development and cytotoxicity in fish hepatocytes following exposure to copper (Cu). ROS was quantified using the fluorescent probes DHR 123 and CM-H2DCFDA following exposure to Cu in Leibovitz' medium (L-15) or Tris-buffered saline (TBS). Similarly, culture age effects were investigated using 1-, 2- and 4-day-old cultured hepatocytes by exposing them to Cu in TBS. The exposure in L-15 resulted in significantly higher ROS compared to TBS using CM-H2DCFDA, but not DHR 123. The age of the primary cultures significantly affected the development of ROS for both probes. None of the exposures caused cytotoxicity in the hepatocytes. The results showed that both factors may affect responses to stressors, and suggested that the use of a simple medium such as TBS may be preferable for some applications. It is also preferable to use 1-day-old primary hepatocyte cultures.

Effects of TLR agonists and viral infection on cytokine and TLR expression in Atlantic salmon (Salmo salar)

The development of efficient and cheap vaccines against several aquatic viruses is necessary for a sustainable fish farming industry. Toll-like receptor (TLR) ligands have already been used as good adjuvants in human vaccines. With more understanding of TLR expression, function, and ligand specificity in fish, more efficient adjuvants for fish viral vaccines can be developed. In this paper, we examine all known TLRs in Atlantic salmon (Salmo salar) and demonstrate that head kidney and spleen are the main organs expressing TLRs in salmon. We also show that adherent head kidney leucocytes from salmon are able to respond to many of the known agonists for human TLRs, and that viral infection can induce up-regulation of several TLRs. These findings substantiate these receptors' role in immune responses to pathogens in salmonids making their ligands attractive as vaccine adjuvant candidates.

The use of zebrafish as a model system for development of new adjuvant strategies in finfish DNA vaccination against viruses (P6072)

Zebrafish (Danio rerio) has emerged as an important experimental system to study both vertebrate immunity and as a model system for fish immunology. Studies in this species have already revealed important knowledge about fish-pathogen interactions and fish vaccinology. Although more than 18 different Toll-like receptors (TLRs) have been genomically identified in fish, little is known about their agonists and functional role in fish immunity. We have established a NFkB-luciferase reporter system to analyse agonist profiles for several fish TLRs. In addition, after adaption of zebrafish to 15°C the protective effects of various TLR ligands alone and as adjuvants for DNA vaccination against surface proteins from cold water pathogens like viral hemorrhagic septicemia virus (VHSV) have been investigated. We also investigated the activation of zebrafish leucocytes by TLR ligands and recombinant G protein from VHSV (VHSV-G) in vitro. Our results demonstrate that poly I:C (TLR3 agonist) induce inflammatory transcripts in vitro. In vivo, one injection of poly I:C, could partially protect zebrafish against subsequent VHSV infection. The onset of mortalities was delayed by ten days (day 20 compared to day 10 in control) in the poly I:C treated fish. We will also present interesting results showing the effects of formulating plasmid DNA vaccines against VHSV-G with various TLR agonists included as adjuvants (on-going studies).

The genome sequence of Atlantic cod reveals a unique immune system

Atlantic cod (Gadus morhua) is a large, cold-adapted teleost that sustains long-standing commercial fisheries and incipient aquaculture. Here we present the genome sequence of Atlantic cod, showing evidence for complex thermal adaptations in its haemoglobin gene cluster and an unusual immune architecture compared to other sequenced vertebrates. The genome assembly was obtained exclusively by 454 sequencing of shotgun and paired-end libraries, and automated annotation identified 22,154 genes. The major histocompatibility complex (MHC) II is a conserved feature of the adaptive immune system of jawed vertebrates, but we show that Atlantic cod has lost the genes for MHC II, CD4 and invariant chain (Ii) that are essential for the function of this pathway. Nevertheless, Atlantic cod is not exceptionally susceptible to disease under natural conditions. We find a highly expanded number of MHC I genes and a unique composition of its Toll-like receptor (TLR) families. This indicates how the Atlantic cod immune system has evolved compensatory mechanisms in both adaptive and innate immunity in the absence of MHC II. These observations affect fundamental assumptions about the evolution of the adaptive immune system and its components in vertebrates.

Enhanced transfection of cell lines from Atlantic salmon through nucoleofection and antibiotic selection

Background

Cell lines from Atlantic salmon kidney have made it possible to culture and study infectious salmon anemia virus (ISAV), an aquatic orthomyxovirus affecting farmed Atlantic salmon. However, transfection of these cells using calcium phosphate precipitation or lipid-based reagents shows very low transfection efficiency. The Amaxa Nucleofector technology™ is an electroporation technique that has been shown to be efficient for gene transfer into primary cells and hard to transfect cell lines.

Findings

Here we demonstrate, enhanced transfection of the head kidney cell line, TO, from Atlantic salmon using nucleofection and subsequent flow cytometry. Depending on the plasmid promoter, TO cells could be transfected transiently with an efficiency ranging from 11.6% to 90.8% with good viability, using Amaxa's cell line nucleofector solution T and program T-20. A kill curve was performed to investigate the most potent antibiotic for selection of transformed cells, and we found that blasticidin and puromycin were the most efficient for selection of TO cells.

Conclusions

The results show that nucleofection is an efficient way of gene transfer into Atlantic salmon cells and that stably transfected cells can be selected with blasticidin or puromycin.

Validation of reference genes for quantitative RT-qPCR studies of gene expression in Atlantic cod (Gadus morhua l.) during temperature stress

Background

One important physiological response to environmental stress in animals is change in gene expression. To obtain reliable data from gene expression studies using RT-qPCR it is important to evaluate a set of possible reference genes as normalizers for expression. The expression of these candidate genes should be analyzed in the relevant tissues during normal and stressed situations. To find suitable reference genes it was crucial that the genes were stably expressed also during a situation of physiological stress. For poikilotermic animals like cod, changes in temperature are normal, but if the changes are faster than physiological compensation, the animals respond with typical stress responses. It has previously been shown that Atlantic cod show stress responses when elevation of water temperature is faster than 1 degree/day, for this reason we chose hyperthermia as stress agent for this experiment.

Findings

We here describe the expression of eight candidate reference genes from Atlantic cod (Gadus morhua l.) and their stability during thermal stress (temperature elevation of one degree C/day for 5 days). The genes investigated were: Eukaryotic elongation factor 1 alpha, ef1a; 18s ribosomal RNA; 18s, Ubiquitin conjugate protein; ubiq, cytoskeletal beta-actin; actb, major histcompatibility complex I; MHC-I light chain, beta-2 -microglobulin; b2m, cytoskeletal alpha-tubulin; tba1c, acidic ribosomal phosphoprotein; rplp1, glucose-6-phosphate dehydrogenase; g6pd. Their expression were analyzed in 6 tissues (liver, head kidney, intestine, spleen, heart and gills) from cods exposed to elevated temperature and compared to a control group. Although there were variations between tissues with respect to reference gene stability, four transcripts were more consistent than the others: ubiq, ef1a, 18s and rplp1. We therefore used these to analyze the expression of stress related genes (heat shock proteins) induced during hyperthermia. We found that both transcripts were significantly upregulated in several tissues in fish exposed to increased temperature.

Conclusion

This is the first study comparing reference genes for RT-qPCR analyses of expression during hyperthermia in Atlantic cod. ef1a, 18s, rplp1 and ubiq transcripts were found to be well suited as reference genes during these experimental conditions.

Structural and functional analysis of the hemagglutinin-esterase of infectious salmon anaemia virus

Infectious salmon anaemia virus (ISAV) is a piscine orthomyxovirus causing a serious disease in farmed Atlantic salmon (Salmo salar L.). The virus surface glycoprotein hemagglutinin-esterase (HE) is responsible for both viral attachment and release. Similarity to bovine and porcine torovirus hemagglutinin-esterase (BToV HE, PToV HE), bovine coronavirus HE (BCoV HE) and influenza C hemagglutinin-esterase-fusion (InfC HEF) proteins were exploited in a computational homology-based structure analysis of ISAV HE. The analysis resolved structural aspects of the protein and identified important features of relevance to ISAV HE activity. By recombinant expression and purification of secretory HE (recHE) proteins, receptor-binding and quantitative analyses of enzymatic activities displayed by ISAV HE molecules are presented for the first time. Three different recHE molecules were constructed: one representing a high virulent isolate, one a low virulent, while in the third a Ser(32) to Ala(32) amino acid substitution was introduced in the enzymatic catalytic site as inferred from the model. The three amino acid differences between the high and low virulent variants, of which two localized to the putative receptor-binding domain and one in the esterase domain, had no impact on receptor-binding or -release activities. In contrast, the Ser(32) amino acid substitution totally abolished enzymatic activity while receptor binding increased, as observed by agglutination of Atlantic salmon red blood cells. This demonstrates the essential role of a serine in the enzyme's catalytic site. In conclusion, structural analysis of ISAV HE in combination with selected recHE proteins gave insights into structure-function relationships and opens up for further studies aiming at dissecting molecular determinants of ISAV virulence.

In situ localisation of major histocompatibility complex class I and class II and CD8 positive cells in infectious salmon anaemia virus (ISAV)-infected Atlantic salmon

It is assumed that the mobilisation of a strong cellular immune response is important for the survival of Atlantic salmon infected with infectious salmon anaemia virus (ISAV). In this study, the characterisation of immune cell populations in tissues of non-ISAV infected Atlantic salmon and during the early viraemia of ISAV was undertaken. Immunohistochemical investigations of spleen, head kidney and gills using monoclonal antibodies against recombinant proteins from MHC I, II and CD8 were performed on tissues from Atlantic salmon collected day 17 post-challenge in a cohabitant infection model. The localisations of MHC I and II in control salmon were consistent with previous reports but this study presents novel observations on the distribution of CD8 labelled cell populations in Atlantic salmon including the description of significant mucosal populations in the gills. The distribution of MHC I, MHC II and CD8 positive cell populations differed between control salmon and cohabitant salmon in the early stages of ISAV infection. The changes in MHC I labelled cells differed between organs in ISAV cohabitants but all investigated organs showed a decreased presence of MHC II labelled cells. Together with a clustering of CD8 labelled cells in the head kidney and a reduced presence of CD8 labelled cells in the gills, these observations support the early mobilisation of cellular immunity in the response of Atlantic salmon to ISAV infection. However, differences between the present study and the findings from studies investigating immune gene mRNA expression during ISAV infection suggest that viral strategies to interfere with protein expression and circumvent the host immune response could be operative in the early response to ISAV infection.

Analysis of host- and strain-dependent cell death responses during infectious salmon anemia virus infection in vitro

Background

Infectious salmon anemia virus (ISAV) is an aquatic orthomyxovirus and the causative agent of infectious salmon anemia (ISA), a disease of great importance in the Atlantic salmon farming industry. In vitro, ISAV infection causes cytophatic effect (CPE) in cell lines from Atlantic salmon, leading to rounding and finally detachment of the cells from the substratum. In this study, we investigated the mode of cell death during in vitro ISAV infection in different Atlantic salmon cell lines, using four ISAV strains causing different mortality in vivo.

Results

The results show that caspase 3/7 activity increased during the course of infection in ASK and SHK-1 cells, infected cells showed increased surface expression of phosphatidylserine and increased PI uptake, compared to mock infected cells; and morphological alterations of the mitochondria were observed. Expression analysis of immune relevant genes revealed no correlation between in vivo mortality and expression, but good correlation in expression of interferon genes.

Conclusion

Results from this study indicate that there is both strain and cell type dependent differences in the virus-host interaction during ISAV infection. This is important to bear in mind when extrapolating in vitro findings to the in vivo situation.

Changes in fatty acids metabolism during differentiation of Atlantic salmon preadipocytes; effects of n-3 and n-9 fatty acids

Atlantic salmon (Salmo salar) preadipocytes, isolated from visceral adipose tissue, differentiate from an unspecialized fibroblast like cell type to mature adipocytes filled with lipid droplets in culture. The expression of the adipogenic gene markers peroxisome proliferated activated receptor (PPAR) alpha, lipoprotein lipase (LPL), microsomal triglyceride transfer protein (MTP), fatty acid transport protein (FATP) 1 and fatty acid binding protein (FABP) 3 increased during differentiation. In addition, we describe a novel alternatively spliced form of PPARgamma (PPARgamma short), the expression of which increased during differentiation. Eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA) lowered the triacylglycerol (TAG) accumulation in mature salmon adipocytes compared to oleic acid (18:1n-9, OA). This finding indicates that a reduced level of highly unsaturated n-3 fatty acids (HUFAs) in fish diets, when the traditional marine oil is exchanged for n-9 fatty acids (FAs) rich vegetable oils (VOs), may influence visceral fat deposition in salmonids. Moreover, major differences in the metabolism of EPA, DHA and OA at different stages during differentiation of adipocytes occur. Most of the EPA and DHA were oxidized in preadipocytes, while they were mainly stored in TAGs in mature adipocytes in contrast to OA which was primarily stored in TAGs at all stages of differentiation.

Effect of early infectious salmon anaemia virus (ISAV) infection on expression of MHC pathway genes and type I and II interferon in Atlantic salmon (Salmo salar L.) tissues

A number of viral diseases affecting teleost fish are characterized but few studies have addressed the effects of viral infection on gene expression in vivo. In this study, we investigated the effect of the early stages of infectious salmon anaemia virus (ISAV) infection on important components of the innate and adaptive immune response by monitoring expression of five genes in the MHC class I pathway, MHC class IIbeta, type I IFN-alpha, Mx, and type II IFN-gamma from cohabitant-infected Atlantic salmon tissues using quantitative real-time PCR. There was an increased expression of type I IFN-alpha in all tissues analyzed in response to infection that was proportional to viral load (relative to virus RNA levels) in gills and head kidney. Basal expression of IFN-gamma was modest or absent in all tissues, but expression was strongly induced and proportional to ISAV RNA levels in heart, spleen and head kidney. A 10-fold or higher level of virally induced IFN-alpha, in addition to significantly elevated levels of IFN-gamma, enhanced transcription of MHC class I pathway genes in heart, spleen and head kidney. In gills, the main entry site for ISAV, there was no induction of MHC class I pathway genes. MHC IIbeta and PSMB9 were not significantly induced in any tissue. Thus, by analysing various immune genes in a range of tissues from early cohabitant ISAV-infected salmon, we demonstrate that ISAV infection induced a rapid type I and II IFN response in the major infected lymphoid tissues, which was concurrent with induced expression of MHC class I pathway genes but not MHC IIbeta. This may suggest that CD8(+) T cell responses are more important than CD4(+) T cell responses during early ISAV viraemia.

Effects of dietary thia fatty acids on lipid composition, morphology and macrophage function of Atlantic salmon (Salmo salar L.) kidney

High lipid levels are being used in modern salmonid diets to promote rapid growth; however there is a limiting supply of the traditional fish oils as the fish farming industry expands. One way to utilize the lipid sources better, could be to find ways to stimulate fatty acid (FA) oxidation so that Atlantic salmon use more energy for muscle growth and less for storage in perivisceral adipose tissue. We have previously shown that dietary inclusion of the thia FA tetradecylthioacetic acid (TTA) promoted hepatic beta-oxidation and reduced total body lipid levels. However, dietary TTA also had some negative effects, leading to accumulation of sulfone and sulfoxide metabolites of TTA in the kidney and increasing mortality rates, particularly at low water temperatures. Therefore we also wish to investigate the effects of TTA on kidney function at high and low temperatures, including some immune system parameters. The production of leukotriene B4 (LTB4) and prostaglandin E2 (PGE2) immunoreactive material from exogenously added arachidonic acid in isolated head kidney macrophages was affected by both diet and temperature. The phagocytic activity in these cells was reduced by DTA in the 12 degrees C group and there was significantly higher protein degradation in head kidney macrophages at 12 degrees C compared to 5 degrees C in all dietary groups. Interestingly, the incorporation of thia FAs in the kidney was higher at 5 degrees C (0.3% TTA and 0.6% DTA) than at 12 degrees C (0.1% TTA and 0.5% DTA). Additionally, there were lower levels of saturated FAs, while higher levels of polyunsaturated FAs (PUFAs) in the kidney of TTA fed fish at 5 degrees C. We also observed temperature-independent tubular dilatation and a reduction in the density of melanomacrophages of the kidney in salmon fed TTA. Nevertheless, the mRNA expression of some immune-relevant genes in head kidney tissue was not affected by TTA-inclusion in salmon diets. In conclusion, it is clear that 0.6% TTA-inclusion in the feed leads to changes in the kidney function particularly at low water temperatures.

Cloning and expression analysis of an Atlantic salmon (Salmo salar L.) tapasin gene

Loading of the major histocompatibility complex (MHC) class I molecule with peptide is mediated by the multimeric peptide-loading complex in the ER where the glycoprotein tapasin (TAPBP) is required for stabilization of the complex and for control of peptide loading onto MHC class I. To expand our knowledge on antigen presentation genes in Atlantic salmon, we isolated a full-length salmon tapasin cDNA sequence (Sasa-TAPBP). It encoded a 443 bp amino acid sequence with two N-glycosylation sites, two conserved mammalian tapasin signature motifs, two Ig superfamily (IgSf) domains, a transmembrane (TM) domain and an ER-retention KK motif at the C-terminal end, indicative of a similar function as mammalian tapasins. We analysed the regulation of Sasa-TAPBP under immunostimulatory conditions and found an mRNA-upregulation during early infectious salmon anemia virus (ISAV) infection and poly I:C stimulation in vivo and in vitro, in line with our previous findings for other MHC class I pathway genes.

Characterization of scavenger receptor class B, type I in Atlantic salmon (Salmo salar L.)

The scavenger receptor class B, type I (SR-BI) is an important player in regulation of mammalian lipid homeostasis. We therefore wanted to study this receptor in Atlantic salmon (Salmo salar L.), which requires a diet with particular high lipid content. We have for the first time cloned and characterized SR-BI from a salmonid fish. The predicted 494 amino acid protein contained two transmembrane domains, several putative N-glycosylation sites, and showed 72% sequence identity with the predicted homolog from zebrafish. SR-BI expression was analyzed by reverse transcription Real-Time PCR in several tissues, and a high relative expression in salmon midgut was detected, which may suggest that SR-BI has a role in uptake of lipids from the diet. We also expressed a construct of salmon myc-tagged SR-BI in salmon TO cells and HeLa cells, which gave a protein of approximately 80 kDa on reducing SDS-PAGE using an antibody against the myc-epitope. Immunofluorescence microscopy analyses of the salmon SR-BI protein in transiently transfected HeLa cells revealed staining in the cell periphery and in some intracellular membranes, but not in the nucleus, which indicated that the salmon protein may be a functional membrane protein. We also observed a high degree of co-localization using an anti-peptide SR-BI antiserum. We found that 20 microg mL(-1) insulin up-regulated the SR-BI mRNA levels in primary cultures of salmon hepatocytes relative to untreated cells. Oleic acid, EPA, DHA, or dexamethasone did not affect the relative expression of SR-BI in this liver model system. In conclusion, the salmon SR-BI cDNA encoded a protein with several features common to those of mammalian species. SR-BI gene expression was high in the intestine, which leads us to propose that SR-BI may contribute to the uptake of lipids from the diet.

Expression of MHC class I pathway genes in response to infectious salmon anaemia virus in Atlantic salmon (Salmo salar L.) cells

Infectious salmon anaemia virus (ISAV) is the causative agent of an important viral disease threatening Atlantic salmon aquaculture. Although its structure and pathogenesis is well described little is known about its immunomodulatory effects on the host. Cellular immunity is critical in the host control of virus infections, an event attributable to antigen presentation through the MHC class I pathway, whose genes are transcriptionally activated by interferons (IFN) and other cytokines. In this study we analysed the regulation and kinetics of key genes in the salmon MHC class I pathway in relation to type I IFN during ISAV infection and poly I:C stimulation in the permissive Atlantic salmon kidney cell line (ASK). As measured by quantitative real-time PCR, ISAV induced an mRNA shut-off equivalent to 2.5-5.5-fold reduced levels of housekeeping genes at 7 days post infection. Relative to this shut-off (by normalising to beta-actin) transcription increased to peak levels at 2.8-fold for MHC class I, 10-fold for beta 2 microglobulin (beta 2m), 5.9-fold for the peptide transporter ABCB2, 8.8-fold for the proteasome component PSMB8 and 4.6-fold for the proteasome component PSMB9, presumably by activation of the IFN system as a 26-fold induction was observed for type I IFN-alpha. Expression of Mx protein was also induced 17-fold at peak level. Similar kinetics and activation levels of these genes were seen in poly I:C stimulated cells. We also isolated the salmon MHC class I UBA*0301 promoter and identified a conserved interferon-stimulated response element (ISRE) and GAAA-elements plus several GAS- and IRF-sites, all supporting IFN-inducible properties. In summary, we demonstrate a concerted induction of the MHC class I pathway and type I IFN by ISAV comparable to levels induced by the synthetic double-stranded RNA (dsRNA) poly I:C. Thus, unlike influenza and several other viruses ISAV does not seem to interfere with MHC and IFN expression.

Effects of 3-thia fatty acids on expression of some lipid related genes in Atlantic salmon (Salmo salar L.)

In this study, the effects of in vivo administration of 3-thia fatty acids (FAs) on lipid metabolism in muscle and liver of Atlantic salmon were investigated. Prior to analysis, the fish were kept in tanks supplied with 5 degrees C seawater for 20 weeks. The fish were fed fish meal and fish oil (FO)-based diets supplemented with either nothing (FO), or 0.3% and 0.6% of the 3-thia FAs dodecylthioacetic acid (DTA) and tetradecylthioacetic acid (TTA) respectively. The fish grew from an initial weight of 110 g to 220 g in the FO group and to approximately 160 g in the 3-thia FA groups. There was a significant higher mortality (66%) in fish fed 0.6% TTA than in fish fed the 0.3% DTA (15%) and FO diets (15%). None of the 3-thia FA diets affected the lipid content of the salmon muscle. The liver index, however, was significantly higher and the total liver fat content lower in the TTA group than in the FO group. Both DTA and TTA were incorporated into the lipid fraction of muscle and liver (0.4% to 0.9%). There were no major differences in the total FA composition of liver and muscle between the dietary groups; except for a small increase of n-3 polyunsaturated FAs (PUFAs) in liver of the DTA group. The mRNA expression of peroxisome proliferator-activated receptor (PPAR) alpha, apolipoprotein AI (ApoAI), apolipoprotein CII (ApoCII) and low-density lipoprotein receptor (LDL-R) was down-regulated in liver of the salmon fed 0.3% DTA. PPARalpha and ApoAI transcripts were also reduced in liver of salmon fed 0.6% TTA. Additionally, the hepatic lipoprotein lipase (LPL) mRNA level was 3.8 fold increased in TTA fish relative to the FO group. In muscle there were no significant changes in gene expression pattern of any of the genes investigated. This is the first report on the effects of 3-thia FAs on gene expression in Atlantic salmon.

Bile acids reduce SR-BI expression in hepatocytes by a pathway involving FXR/RXR, SHP, and LRH-1

Hepatic SR-BI mediates uptake of circulating cholesterol into liver hepatocytes where a part of the cholesterol is metabolised to bile acids. In the hepatocytes, bile acids reduce their own synthesis by a negative feedback loop to prevent toxic high levels of bile acids. Bile acid-activated FXR/RXR represses expression of CYP7A1, the rate-limiting enzyme during bile acid synthesis, by inducing the expression of SHP, which inhibits LXR/RXR and LRH-1-transactivation of CYP7A1. The present paper presents data indicating that CDCA suppresses SR-BI expression by the same pathway. As previously reported, LRH-1 induces SR-BI promoter activity. Here we show that CDCA or over-expression of SHP inhibit this transactivation. No FXR-response element was identified in the bile acid-responsive region of the SR-BI promoter (-1200bp/-937bp). However, a binding site for LRH-1 was characterised and shown to specifically bind LRH-1. The present study shows that also the SR-BI-mediated supply of cholesterol, the substrate for bile acid synthesis, is feedback regulated by bile acids.

Beta-oxidation, esterification, and secretion of radiolabeled fatty acids in cultivated Atlantic salmon skeletal muscle cells

The white muscle of Atlantic salmon metabolizes FA with different chain lengths and different saturations at different rates, but few details are available on the processes involved or the products formed. We have investigated how multinucleated muscle cells (myotubes) in culture metabolize [1-(14)C]8:0, [1-(14)C]18:1n-9, and [1-(14)C]20:5n-3. The myotubes were formed by the differentiation of isolated myosatellite cells from the white skeletal muscle of salmon fry. Almost all (98%) of the [1-(14)C]8:0 substrate was oxidized to acid-soluble products (ASP) and (14)CO2 after 48 h of incubation, whereas only approximately 50% of the [1-(14)C]18:1n-9 and [1-(14)C]20:5n-3 substrates were oxidized. However, only one cycle of beta-oxidation was measured by the method used. For all three substrates, the main ASP were acetate and a combined fraction of oxaloacetate and malate. Nearly twice as much radioactivity from the [1-(14)C]20:5n-3 substrate was found in the cellular lipids as radioactivity from [1-(14)C]18:1n-9, indicating that [1-(14)C]20:5n-3 was taken up into muscle cells more rapidly than [1-(14)C]18:1n-9. Approximately 10% of the added [1-(14)C]20:5n-3 substrate and 5% of the added [1-(14)C]18:1n-9 substrate was secreted from the muscle cells into the culture media as esterified lipids. Immunocytochemical staining showed that the cells synthesized apolipoprotein A-I. Differentiated muscle cells also expressed peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARbeta, two transcription factors that are involved in regulating beta-oxidation.

Effect of 18∶1n−9, 20∶5n−3, and 22∶6n−3 on lipid accumulation and secretion by atlantic salmon hepatocytes

We have studied the effects of dietary FA on the accumulation and secretion of [3H]glycerolipids by salmon hepatocytes in culture. Atlantic salmon were fed diets supplemented with either 100% soybean oil (SO) or 100% fish oil (FO), and grew from an initial weight of 113 +/- 5 g to a final weight of 338 +/- 19 g. Hepatocytes were isolated from both dietary groups and incubated with [3H]glycerol in an FA-free medium; a medium supplemented with 0.75 mM of one of three FA-18:1 n-9, 20:5n-3, or 22:6n-3--or a medium supplemented with 0.75 mM of the sulfur-substituted FA analog tetradecylthioacetic acid (TTA), which cannot undergo beta-oxidation. Incubations were allowed to proceed for 1, 2, 6, or 24 h. The rate of the secretion of radioactive glycerolipids with no FA added was 36% lower from hepatocytes isolated from fish fed the FO diet than it was from hepatocytes isolated from fish fed the SO diet. Hepatocytes incubated with 18:1 n-9 secreted more [3H]TAG than when incubated with no FA, whereas hepatocytes incubated with 20:5n-3 or TTA secreted less labeled TAG than when incubated with no FA. This observation was independent of the feeding group. Hepatocytes incubated with 22:6n-3 secreted the highest amounts of total [3H]glycerolipids compared with the other treatments, owing to increased secretion of phospholipids and mono- and diacylglycerols (MDG). In contrast, the same amounts of [3H]TAG were secreted from these cells as from cells incubated in an FA-free medium. The lipid-lowering effect of FO is thus independent of 22:6n-3, showing that 20:5n-3 is the FA that is responsible for the lipid-lowering effect. The ratio of TAG to MDG in lipids secreted from hepatocytes to which 20:5n-3 or TTA had been added was lower than that in lipids secreted from hepatocytes incubated with 18:1 n-9 or 22:6n-3, suggesting that the last step in TAG synthesis was inhibited. Morphometric measurements revealed that hepatocytes incubated with 20:5n-3 accumulated significantly more cellular lipid than cells treated with 18:1n-9, 22:6n-3, TTA, or no treatment. The area occupied by mitochondria was also greater in these cells. The present study shows that dietary FO reduces TAG secretion from salmon hepatocytes and that 20:5n-3 mediates this effect.

Hepatic scavenger receptor class B, type I is stimulated by peroxisome proliferator-activated receptor gamma and hepatocyte nuclear factor 4alpha

Excessive cellular cholesterol is transported to the liver by a pathway called 'reverse cholesterol transport.' Scavenger receptor class B, type I (SR-BI) mediates cholesterol uptake in the liver. Polyunsaturated fatty acids, known to activate peroxisome proliferator-activated receptor (PPAR), have been reported to increase hepatic cholesterol uptake. We found in the present study that PPARgamma induces expression of SR-BI in rat hepatocytes, liver endothelial cells, and Kupffer cells. In contrast, PPARalpha increased SR-BI levels only in hepatocytes and liver endothelial cells. PPARgamma/RXR binds to a response element between -459 and -472 bp in the human SR-BI promoter. Furthermore, hepatocyte nuclear factor 4alpha (HNF4alpha) was found to enhance PPARgamma-mediated SR-BI transcription. Thiazolidinedione (TZD)-activated PPARgamma/RXR increased hepatic SR-BI levels, which may lead to increased hepatic cholesterol uptake and less accumulation of lipids in peripheral tissues. The present results are in agreement with previous reports, indicating that specific PPARgamma-agonists (such as TZDs) protect against atherosclerosis.

An in vitro method for studying the proliferation and differentiation of Atlantic salmon preadipocytes

The aim of the present study was to develop a cell culture system for studying the proliferation and differentiation of preadipocytes isolated from Atlantic salmon adipose tissue. The expression of proliferating cell nuclear antigen (PCNA) was used as a marker for cell proliferation. The cells started to proliferate within 48 h after seeding and continued to proliferate throughout the culture period of 2 wk. Undifferentiated preadipocytes showed a fibroblast-like morphology with a homogeneous cytoplasm devoid of lipid droplets. At confluence, an exogenous lipid mixture was added to the cell cultures. The preadipocytes became larger and rounder during the subsequent days, and the cytoplasm gradually filled with lipid-rich droplets. These droplets were revealed by oil red O staining. Immunocytochemical staining showed that differentiated adipocytes expressed detectable levels of the three regulatory proteins associated with adipocyte differentiation: peroxisome proliferator-activated receptor gamma (PPARgamma), CCAAT/enhancer binding protein alpha (C/EBPalpha), and leptin. The cells also showed activity of glycerol-3-phosphate dehydrogenase (GPDH) (EC 1.1.1.8), a biochemical marker of adipocyte differentiation. The morphological and biochemical data presented here show that fish preadipocytes have properties that are similar to those of preadipocytes in mammals. We conclude therefore that salmon adipose tissue contains a sizable population of preadipocytes. Exogenous lipids promote the activation of adipose-related genes and induce the differentiation of fish preadipocytes in vitro.

Characterization of the receptor-destroying enzyme activity from infectious salmon anaemia virus

Infectious salmon anaemia virus (ISAV) infects cells via the endocytic pathway and, like many other enveloped viruses, ISAV contains a receptor-destroying enzyme. We have analysed this acetylesterase activity with respect to substrate specificity, enzyme kinetics, inhibitors, temperature and pH stability. The ISAV acetylesterase was inhibited by di-isopropyl fluorophosphate (DFP) in a dose-dependent fashion but not by other known hydrolase inhibitors, suggesting that a serine residue is part of the active site. The pH optimum of the enzyme was in the range 7.5-8.0 and the enzymatic activity was lessened at temperatures above 40 degrees C. The effect of DFP on agglutination/elution of erythrocytes by ISAV demonstrated that the acetylesterase activity is the bona fide receptor-destroying enzyme. A haemadsorption assay was used to analyse whether the esterase was active on the surface of infected cells or not.

Metabolism of oxidized and chemically modified low density lipoproteins in rainbow trout--clearance via scavenger receptors

Oxidative modifications of low density lipoprotein (LDL) convert LDL into a ligand recognized by a variety of scavenger receptors (SR) in mammals. This oxidized LDL (oxLDL) activate several cell types, and have been shown to induce expression of a variety of genes in mammals. Lipoproteins of poikilothermic animals like salmonid fishes contain high levels of polyunsaturated fatty acids susceptible to oxidative modifications. We have investigated, and found trout LDL to be susceptible to oxidation in the presence of Cu(2+). When oxidized or acetylated trout LDL was injected intravenously, the clearance rate was increased compared to that of native LDL. Modified LDL was taken up almost exclusively in the kidney, whereas native LDL was also taken up in the liver. Uptake of both oxLDL and acetylated LDL in the kidney was significantly inhibited by lipoteichoic acid (LTA) and formaldehyde treated BSA (fBSA), both of which are known ligands of SR.

The expression of scavenger receptor class B, type I (SR-BI) and caveolin-1 in parenchymal and nonparenchymal liver cells

The liver is the major site of cholesterol synthesis and metabolism, and the only substantive route for eliminating blood cholesterol. Scavenger receptor class B, type I (SR-BI) has been reported to be responsible for mediating the selective uptake of high-density lipoprotein cholesteryl esters (HDL-CE) in liver parenchymal cells (PC). We analysed the expression of SR-BI in isolated rat liver cells, and found the receptor to be highly expressed in liver PC at both the mRNA and protein levels. We also found SR-BI to be expressed in liver endothelial cells (LEC) and Kupffer cells (KC). SR-BI has not previously been reported to be present in LEC. CD36 mRNA was expressed in all three liver cell types. Since caveolin-1 appears to colocalize with SR-BI and CD36 in caveolae of several cell lines, the distribution and expression of caveolin-1 in the liver cells were investigated. Caveolin-1 was not detected in PC but was found in both LEC and KC. This led to the suggestion that caveolin-1 may be more important in the efflux of cholesterol than in the selective uptake of cholesterol in the liver.

Fluid phase endocytosis of [125I]iodixanol in rat liver parenchymal, endothelial and Kupffer cells

Endocytosis of [125I]iodixanol was studied in vivo and in vitro in rat liver cells to determine fluid phase endocytic activity in different liver cells (hepatocytes, Kupffer cells and endothelial cells). The Kupffer cells were more active in the uptake of [l25I]iodixanol than parenchymal cells or endothelial cells. Inhibition of endocytic uptake via clathrin-coated pits (by potassium depletion and hypertonic medium) reduced uptake of [125I]iodixanol much more in Kupffer cells and endothelial cells than in hepatocytes. To gain further information about the importance of clathrin-mediated fluid phase endocytosis, the expression of proteins known to be components of the endocytic machinery was investigated. Using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting, endothelial cells and Kupffer cells were found to express approximately fourfold more rab4, rab5 and rab7 than parenchymal cells, while clathrin was expressed at a higher level in endothelial cells than in Kupffer cells and hepatocytes. Using electron microscopy it was shown that liver endothelial cells contained approximately twice as many coated pits per membrane unit than the parenchymal and Kupffer cells, thus confirming the immunoblotting results concerning clathrin expression. Electron microscopy on isolated liver cells following fluid phase uptake of horseradish peroxidase (HRP) showed that HRP-containing organelles had a different morphology in the different cell types: In the liver endothelial cells HRP was in small, tubular endosomes, while in Kupffer cells HRP was mainly found in larger structures, reminiscent of macropinosomes. Parenchymal cells contained HRP in small vacuolar endosomes with a punctuated distribution. In conclusion, we find that the Kupffer cells and the endothelial cells have a higher pinocytic activity than the hepatocytes. The hepatocytes do, however, account for most of the total hepatic uptake. The fluid phase endocytosis in liver endothelial cells depends mainly on clathrin-mediated endocytosis, while the parenchymal cells have additional clathrin-independent mechanisms that may play an important role in the uptake of plasma membrane components. In the Kupffer cells the major uptake of fluid phase markers seems to take place via a macropinocytic mechanism.

Effect of soybean oil and fish oil on individual molecular species of Atlantic salmon head kidney phospholipids determined by normal-phase liquid chromatography coupled to negative ion electrospray tandem mass spectrometry

The effect of soybean oil (SO) and fish oil (FO) on the relative molecular species distribution of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS) in Atlantic salmon head kidney was studied using normal-phase liquid chromatography coupled with negative ion electrospray tandem mass spectrometry. The conformation of identity of the phospholipid species was based on retention time, the mass of the [M-H]- ([M-15]- for PC) molecular ions and the carboxylate anion fragments in the product ion spectrum. The intensity ratio of sn-1/sn-2 fragment ions increased with increasing number of double bonds in the sn-2 acyl chain but was not affected by increasing number of double bonds in the sn-1 acyl chain of the species examined. The relative distribution of the molecular species was determined by multiple reaction monitoring of the carboxylate anion fragment from the sn-1 position. A total of 68 different phospholipid species were determined in the head kidney and the largest amount was found in PE (22 species). Depending on the diet, the main species identified in the different phospholipid classes were; PC 16:0/18:1, PE 16:0/22:6, PI 18:0/20:4 and PS 16:0/22:6. The SO diet significantly increased the 18:2, 20:3 and most 20:4 containing species and significantly reduced the 14:0 and most 20:5 and 22:6 fatty acid containing species. The increase of the 20:4 and the decrease of the 20:5 and 22:6 containing species were dependent on the fatty acid combination of the species.

Initial events in infectious salmon anemia virus infection: evidence for the requirement of a low-pH step

We have investigated the initial steps in the interaction between infectious salmon anemia virus (ISAV) and cultured cells from Atlantic salmon (SHK-1 cell line). Using radioactively or fluorescently labelled viral particles we have studied the binding and fusion kinetics and the effect of pH on binding, uptake, and fusion of ISAV to SHK-1 cells and liposomes. As pH in the medium was reduced from 7.5 to 4.5, the association of virus to the cells was nearly doubled. The same effect of pH was observed when fusion between ISAV and liposomes was analyzed. In addition, the binding of ISAV to intact SHK-1 cells and to cell membrane proteins blotted onto filters was neuraminidase sensitive. However, the increased binding induced by low pH was not neuraminidase sensitive, probably reflecting activation of a fusion peptide at low pH. By using confocal fluorescence microscopy, the increased fusion of fluorescently labelled ISAV with the plasma membrane due to low pH could be demonstrated. When vacuolar pH in the cells was raised during inoculation with chloroquine or ammonium chloride, both electron and confocal microscopy showed accumulation of ISAV in endosomes and lysosomes. Production of infectious virus could be increased by lowering the extracellular pH during infection. Furthermore, chloroquine present during virus inoculation also caused a reduction in the synthesis of viral proteins in ISAV-infected cells as well as in the production of infective virus. These results indicate that ISAV binds to sialic acid residues on the cell surface and that the fusion between virus and cell membrane takes place in the acid environment of endosomes. This provides further evidence for a high degree of similarity between ISAV and influenza virus and extends the basis for the classification of this virus as a member of the Orthomyxoviridae family.

A role for scavenger receptors in phagocytosis of protein-coated particles in rainbow trout head kidney macrophages

In macrophages of higher vertebrates, Fc receptors and receptors for complement and other serum factors, are generally known to enhance the phagocytic process. In lower vertebrates like salmonid fishes, none of these or other phagocytic receptors have been thoroughly characterized. The purpose of this study was to elucidate to what extent these and other receptors are involved in the process of phagocytosis in rainbow trout (Oncorhynchus mykiss) head kidney macrophages. We used tosyl activated, paramagnetic dynabeads (2.8 microm in diameter), specifically coated with 125I labeled Atlantic salmon (Salmo salar) IgM or bovine serum albumin (BSA) as phagocytic probes. The effect of complement opsonization was also investigated by incubating the beads in serum. Our results indicate that neither the Fc- nor the complement-receptor(s) were important for phagocytosis of these beads. Our data support the idea that scavenger receptors are involved in phagocytosis in rainbow trout head kidney macrophages, as the use of a competitive scavenger receptor ligand extensively decreased degradation of the labeled protein coat on the beads.

Peroxisome proliferator activated receptors in Atlantic salmon (Salmo salar): effects on PPAR transcription and acyl-CoA oxidase activity in hepatocytes by peroxisome proliferators and fatty acids

A cDNA fragment which encodes salmon peroxisome proliferator activated receptor y (sPPARgamma) was amplified by PCR from the liver of Atlantic salmon (Salmo salar L.). The fragment was 627 bp long. The sequence of the amplified PCR product was similar to the PPARgamma of mouse and hamster. 59% of the bases were identical. Northern blot analysis of salmon liver mRNA showed that the amplified sPPARgamma fragment hybridised to three specific transcripts of lengths 1.6, 2.4 and 3.3 kb. Clofibric acid and bezafibrate, administered to salmon hepatocytes in culture, resulted in a 1.7-fold increase of the 1.6 kb sPPARgamma transcript. The activity of acyl-CoA oxidase also increased approx. 1.7-fold after administration of fibrates. These results indicate that PPAR is an important factor in mediating enzymatic response to fibrates in fish.

Differences between fluid-phase endocytosis (pinocytosis) and receptor-mediated endocytosis in isolated rat hepatocytes

To characterize possible differences between the fluid-phase endocytosis (pinocytosis) of bovine serum albumin and the receptor-mediated endocytosis of asialo-orosomucoid (AOM) in isolated rat hepatocytes, both probes were conjugated to radioiodinated tyramine-cellobiose, [125I]TC. The use of these conjugates made it possible to measure the uptake and intracellular distribution of the intact proteins as well as of their acid-soluble, membrane-impermeant degradation products. [125I]TC-albumin was taken up at a very low rate (0.5%/h) compared to [125I]TC-AOM (45%/h), suggesting that neither membrane adsorption nor membrane permeation compromised its suitability as a fluid-phase marker. Sucrose gradient analysis indicated that both probes sequentially entered light endosomes (1.11 g/ml), dense endosomes (1.14 g/ml) and lysosomes (1.18 g/ml), but [125I]TC-albumin traversed the endocytic compartments more rapidly than [125I]TC-AOM, and was partially degraded intralysosomally already after 15 min. The microtubule inhibitor, vinblastine, had a stronger inhibitory effect on the uptake and degradation of [125I]TC-AOM (80% and 95%, respectively) than on the uptake and degradation of [125I]TC-albumin (50% and 70%, respectively). In the presence of vinblastine, [125I]TC-AOM was retained both in light and dense endosomes, whereas [125I]TC-albumin was retained in dense endosomes only, suggesting that the early steps of fluid-phase endocytosis were less critically dependent on microtubular function than the early steps of receptor-mediated endocytosis. A perturbant of vacuolar pH, propylamine, inhibited the degradation of both probes strongly (75-100%), as would be expected from its lysosomotropic effect. Propylamine also inhibited endocytic uptake, with a stronger effect on [125I]TC-AOM uptake (95% inhibition) than on [125I]TC-albumin uptake (60% inhibition), probably reflecting a reduction in endosomal acidity, reduced receptor-ligand dissociation and diminished recycling of free asialoglycoprotein receptors to the cell surface in addition to a general trapping of membrane in swollen vacuoles. A protein phosphatase inhibitor, okadaic acid, strongly (80-100%) inhibited the uptake and degradation of both [125I]TC-albumin and [125I]TC-AOM. An inhibitor of lysosomal proteinases, leupeptin, strongly suppressed the degradation of both probes and moderately reduced the uptake of [125I]TC-AOM, whereas the uptake of [125I]TC-albumin was unaffected. In contrast, an inhibitor of autophagic sequestration, 3-methyladenine, reduced both the uptake and degradation of [125I]TC-albumin markedly (55% and 75%, respectively), with considerably less effect on [125I]TC-AOM (25% and 35%, respectively). As autophagy-inhibitory amino acid mixture did not share these effects, suggesting that 3-methyladenine may suppress endocytic fluid-phase uptake by an autophagy-independent mechanism. Fluid-phase and receptor-mediated endocytosis in hepatocytes thus appear to differ with respect to uptake mechanisms as well as in the kinetics by which endocytosed material traverses the endocytic-lysosomal pathway.

The expression of endosomal rab proteins correlates with endocytic rate in rat liver cells

Hepatic endocytosis is characterized by a division of labor between the different cell types with respect to endocytosis, which is mediated by receptors expressed on their cell surface. We have investigated the expression of GTPases of the rab family in rat liver parenchymal and endothelial cells. Small GTPases of the rab protein family control distinct steps of intracellular transport both in the secretory and the endocytic pathway. As controls have been employed the normal rat kidney (NRK) cell line and brain tissue, neuronal cells are known to express high levels of components of the endocytic machinery (clathrin, adaptins, dynamin, uncoating adenosine triphosphatase, etc.). Endothelial cells were found to express four to seven times more rab4, rab5, and rab7 than parenchymal cells. A similar relationship was found between the endocytic rates in the two cell types; the rate of internalization from the plasma membrane of mannose receptors in rat liver endothelial cells was 2.3 pools/min, whereas the corresponding value for internalization of the galactose receptor in parenchymal liver cell was 0.27 pools/min (comparable with the rate of transferrin internalization in NRK cells). Both immunofluorescence and subcellular fractionation experiments showed that rab5 and rab7 were associated with compartments along the endocytic pathway. Brain tissue showed a similar high expression of endocytic components (rab4, rab5, and rab7) as liver endothelial cells, whereas lower values were found in NRK cells. We also analyzed the following proteins involved in endocytosis: clathrin, alpha-adaptin, beta-adaptin, and rabaptin-5. These proteins showed the same pattern of expression as the rab proteins. In conclusion, the results obtained with liver cells corroborate the data obtained in transfected cells and support the notion that rab proteins may be involved in controlling the endocytic rate in liver cells.