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We have identified and characterized a cDNA from the brain tissue of the highly commercial marine fish species, the gilthead sea bream (Sparus aurata), which encodes a 496 amino acid residue protein sharing the organizational and structural features of the mammalian prion proteins. Tissue mRNA expression analyses revealed the presence of this transcript in various tissues of the gilthead sea bream including the brain, the spleen, and the heart. Sequence comparison and phylogenetic analysis showed the gilthead sea bream protein to be the homologue of one of the long form prion proteins identified from the model fish species Takifugu rubripes and Danio rerio. Unique to this fish prion protein is an extended Gly-Tyr-Pro-rich region, a structural feature that apparently resulted from multiple duplications of a core motif. The presence of this feature in other seemingly unrelated proteins suggests the involvement of common mechanism(s) in its formation and infers possible evolutionary trends related to its function.

Understanding the control of piscine fatty acid metabolism is important for determining the nutritional requirements of fish, and hence for the production of optimal aquaculture diets. The regulation and expression of carnitine palmitoyltransferase 1 (CPT1; EC No 2.3.1.21) are critical processes in the control of fatty acid metabolism, and here we report a cDNA from gilthead sea bream (Sparus aurata) which encodes a protein with high identity to vertebrate CPT1. This sea bream CPT1 mRNA is predominantly expressed in skeletal and cardiac muscle, with little expression in other tissues. Phylogenetic analysis of other vertebrate CPT1 sequences show that fish genomes contain a single gene related to mammalian CPT1B, and a further two multi-gene families related to mammalian CPT1A. Genes related to mammalian CPT1C are absent in fish. Therefore, based on both functional and evolutionary orthology to mammalian CPT1B, the sea bream CPT1 reported here is a CPT1B isoform. Sea bream CPT1B mRNA expression progressively decreases in heart and muscle up to 12 h after last feeding, but returns to initial, non-fasted levels after 72 h. In contrast, in liver non-fasted expression is low, but strongly increases at 24 and 72 h after last feeding. In white muscle and liver, CPT1B mRNA expression is highly correlated with the expression of peroxisomal proliferator-activated receptor β (PPARβ). Thus fatty acid metabolism by CPT1B and its control by PPARs are similar in fish and mammals, but multiple genes for CPT1A-like proteins in fish also suggest different and more complex pathways of lipid utilisation than in mammals.

The cloning and characterization of cDNAs and genes encoding three peroxisome proliferator-activated receptor (PPAR) isotypes from two species of marine fish, the plaice (Pleuronectes platessa) and the gilthead sea bream (Sparus aurata), are reported for the first time. Although differences in the genomic organization of the fish PPAR genes compared with their mammalian counterparts are evident, sequence alignments and phylogenetic comparisons show the fish genes to be homologs of mammalian PPARα, PPARβ/δ, and PPARγ. Like their mammalian homologs, fish PPARs bind to a variety of natural PPAR response elements (PPREs) present in the promoters of mammalian or piscine genes. In contrast, the mRNA expression pattern of PPARs in the two fish species differs from that observed in other vertebrates. Thus, PPARγ is expressed more widely in fish tissues than in mammals, whereas PPARα and β are expressed similarly in profile to mammals. Furthermore, nutritional status strongly influences the expression of all three PPAR isotypes in liver, whereas it has no effect on PPAR expression in intestinal and adipose tissues. Fish PPARα and β exhibit an activation profile similar to that of the mammalian PPAR in response to a variety of activators/ligands, whereas PPARγ is not activated by mammalian PPARγ-specific ligands. Amino acid residues shown to be critical for ligand binding in mammalian PPARs are not conserved in fish PPARγ and therefore, together with the distinct tissue expression profile of this receptor, suggest potential differences in the function of PPARγ in fish compared with mammals.