Abstract
The following research was conducted to elucidate the evolution and expression of salmonid selenoprotein P (SelP), a selenoprotein that is unique in having multiple selenocysteine (Sec) residues, following supranutritional selenium supplementation and infection in rainbow trout. We show that in salmonids SelP is present as four paralogues and that the diversification of SelP genes during vertebrate evolution relates to whole genome duplication events. With 17 and 16 selenocysteine residues for rainbow trout (Oncorhynchus mykiss)/Atlantic salmon (Salmo salar) SelPa1 and SelPa2 proteins respectively and 1 or 2 (trout or salmon) and 4 or 3 (trout or salmon) selenocysteine residues for salmonid SelPb1 and SelPb2 proteins respectively, this is the highest number of (predicted) multiple selenocysteine containing SelP proteins reported for any vertebrate species to date. To investigate the effects of selenium form on SelP expression we added different concentrations (1 nM– 10 μM) of organic or inorganic selenium to a trout cell line (RTG-2 cells) and analysed changes in mRNA abundance. We next studied the impact of supplementation on the potential modulation of these transcripts by PAMPs and proinflammatory cytokines in RTG-2 and RTS-11 cells. These experiments revealed that selenium type influenced the responses, and that SelP gene subfunctionalisation was apparent. To get an insight into the expression patterns in vivo we conducted a feeding trial with 2 diets differing in selenium content and 5 weeks later challenged the trout with a bacterial pathogen (Aeromonas salmonicida). Four tissues were analysed for SelP paralogue expression. The results show a significant induction of SelPa1 in gills and intestine following infection in selenium supplemented fish and for SelPa2 in gills. SelPb1 was significantly reduced in head kidney of both diet groups following infection, whilst SelPb2 was significantly upregulated in skin of both diet groups post infection. Overall these findings reveal differential expression profiles for the SelPa/SelPb paralogues in trout, influenced by selenium supply, cell type/tissue and stimulant. The increase of multiple Sec containing SelP proteins in salmonids could indicate an enhanced requirement for selenium in this lineage.
Highlights
Selenium (Se) is present in two forms in eukaryotic proteins- the rare amino acid selenocysteine (Sec) and selenomethionine (SeMet)
Mouse Selenoprotein P (SelP) -/- knock-out mutants are embryonically non-lethal in contrast to other selenoprotein deletions in the mouse model, as seen with GPx4, thioredoxin reductase 1 (TRxR1), TRxR3 and the selenocysteine tRNA [7,8]
The cDNA sequences of the rainbow trout SelPa2, SelPa2-2, SelPb2 and Atlantic salmon SelPa2 and SelPb2 were deposited in Genbank under the accession numbers MH085053, MH085054, MH085056, MH085055 and MH085057, respectively
Summary
Selenium (Se) is present in two forms in eukaryotic proteins- the rare amino acid selenocysteine (Sec) and selenomethionine (SeMet). Selenoprotein P (SelP) is one of these and was first discovered in the early 1970 ́s when a protein was detected incorporating injected 75Se in rat plasma [3], that was distinct from the known plasma selenoprotein glutathione peroxidase (GPx) and accounted for 60% of plasma Se levels [4,5]. It is unique within this class of proteins in having multiple Sec residues, with human and mouse SelP both having 10 [6]. The uptake of hepatically derived SelP by other cells takes place by receptor-mediated endocytosis [12,13,14] and an N-terminal heparin binding site has been demonstrated [15]
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