Phylogeny and expression patterns of two apolipoprotein E genes in the flatfish Senegalese sole

The apolipoprotein E (ApoE) is a key component of several lipoproteins involved in lipid homeostasis. In this study, two cDNA sequences encoding ApoE (referred to as apoEa and apoEb) were characterized in the flatfish Solea senegalensis. The predicted peptides contained conserved structural blocks related with their capacity for lipid binding and lipoprotein receptor interaction. At genomic level, both genes contained five exons and four introns and they were organized into two tandem arrays with apoA-IV gene copies. The phylogenetic analysis clearly separated them into two well-supported clusters that matched with their organization in the genome of teleosts. Whole-mount in situ hybridization located the apoEa signal in the yolk syncytial layer (YSL) of lecitothrophic larval stages (0 dph) and in the anterior intestine of exotrophic larvae and benthic fish. In the case of apoEb, hybridization signals were located in the YSL, tail bud, eyes and mouth at 0 dph and in the otic vesicle, hindbrain, eyes, pharynx, mouth, heart and intestine at 1 dph. In exotrophic larvae, apoEb was ubiquitously expressed in several tissues such as taste buds, brain, mouth, nostril, gills, intestine, liver and around the neuromasts and eyes. Quantification of mRNA levels in pools of whole larvae confirmed distinct expression patterns with a significant reduction of apoEa and an increase of apoEb mRNA levels throughout larval development. Moreover, only apoEa transcripts increased in response to food supply suggesting that this paralog mostly participates in the absorption and transport of dietary lipids and the apoEb in the redistribution of endogenous lipids as well as in neural tissue regeneration.