Abstract
To form the egg yolk, which represents the bulk of proteins and lipids that will sustain the nutritional requirements of the future offspring, the oocytes of oviparous species actively accumulate vitellogenin (Vtg) through receptor-mediated endocytosis. The body of knowledge related to the Vtg receptor system in teleost fish relies on a few species, in particular, higher teleosts. To better understand Vtg uptake and the Vtg receptor system in the shortfinned eel, Anguilla australis, a basal teleost, diverse molecular biology techniques and bioinformatic tools were used. Using an RNAseq analysis, ovarian transcriptomes from pre-vitellogenic and early vitellogenic eels were compared to discuss possible mechanisms involved in Vtg uptake regulation, implicating cell junctions and the receptor-mediated endocytosis machinery. Then, the same transcriptomic database was used to identify several ovarian membrane receptors belonging to the low-density lipoprotein receptor family that are expressed during early development, including three putative Vtg receptors (Vtgrs, i.e., Lr8+ and Lr8- isoforms, and Lrp13). Using qPCR, the latter were all found to be highly expressed in the ovary when compared to other somatic tissues, with lr8+ transcript abundance exceeding that of the other receptors. Lastly, a human embryonic kidney cell line (HEK293) expressing the A. australis putative Vtgrs was used to carry out an in vitro binding assay with eel plasma rich in Vtg and ApoB-containing lipoproteins (i.e., low-density lipoproteins). Accordingly, while the Lr8+ and Lr8- isoforms, but not Lrp13, were able to bind Vtg, none of the receptors were able to bind ApoB-containing lipoproteins. Although further research will be required to elucidate the function of A. australis Lrp13 (the cloned sequence presented a deleterious amino acid change that may have affected its binding properties), the Lr8 isoforms, and most probably the Lr8+ isoform, may be the principal receptor/s mediating Vtg uptake in A. australis.