Identification and stable expression of vitellogenin receptor through vitellogenesis in the European eel.

In teleosts, vitellogenin (Vtg) is a phospholipoglycoprotein synthesized by the liver, released into the blood circulation and incorporated into the oocytes via endocytosis mediated by the Vtg receptor (VTGR) to form the yolk granules. The VTGR is crucial for oocyte growth in egg-laying animals but is also present in non-oviparous vertebrates, such as human. The VTGR belongs to the low-density lipoprotein receptor superfamily (LDLR) and is also named very-low-density lipoprotein receptor (VLDLR). In this study, we identified and phylogenetically positioned the VTGR of a basal teleost, the European eel, Anguilla anguilla. We developed quantitative real-time PCR (qRT-PCR) and investigated the tissue distribution of vtgr transcripts. We compared by qRT-PCR the ovarian expression levels of vtgr in juvenile yellow eels and pre-pubertal silver eels. We also analyzed the regulation of ovarian vtgr expression throughout vitellogenesis in experimentally matured eels. The Vtg plasma level was measured by homologous ELISA experimental maturation. Our in silico search and phylogenetical analysis revealed a single vtgr in the European eel, orthologous to other vertebrate vtgr. The qRT-PCR studies revealed that vtgr is mainly expressed in the ovary and also detected in various other tissues such as brain, pituitary, gill, fat, heart, and testis, suggesting some extra-ovarian functions of VTGR. We showed that vtgr is expressed in ovaries of juvenile yellow eels with no higher expression in pre-pubertal silver eels nor in experimentally matured eels. This suggests that vtgr transcription already occurs during early pre-vitellogenesis of immature eels and is not further activated in vitellogenic oocytes. European eel Vtg plasma level increased throughout experimental maturation in agreement with previous studies. Taken together, these results suggest that vtgr transcript levels may not be a limiting step for the uptake of Vtg by the oocyte in the European eel.

Gonadotropin-inhibitory hormone in teleosts: New insights from a basal representative, the eel.

Since its discovery in birds, gonadotropin-inhibitory hormone (GnIH) has triggered investigation in the other groups of vertebrates. In the present study, we have identified a single gnih gene in the European eel (Anguilla anguilla), a representative species of a basal group of teleosts (Elopomorphs). We have also retrieved a single gnih gene in Osteoglossomorphs, as well as in more recently emerged teleosts, Clupeocephala. Phylogeny and synteny analyses allowed us to infer that one of the two gnih paralogs emerged from the teleost-specific whole genome duplication (TWGD or 3R), would have been lost shortly after the 3R, before the emergence of the basal groups of teleosts. This led to the presence of a single gnih in extant teleosts as in other vertebrates. Two gnih paralogs were still found in some teleost species, such as in salmonids, but resulting from the additional whole genome duplication that specifically occurred in this lineage (4R). Eel gnih was mostly expressed in the diencephalon part of the brain, as analyzed by quantitative real-time PCR. Cloning of eel gnih cDNA confirmed that the sequence of the GnIH precursor encoded three putative mature GnIH peptides (aaGnIH-1, aaGnIH-2 and aaGnIH-3), which were synthesized and tested for their direct effects on eel pituitary cells in vitro. Eel GnIH peptides inhibited the expression of gonadotropin subunits (lhß, fshß, and common a-subunit) as well as of GnRH receptor (gnrh-r2), with no effect on tshß and gh expression. The inhibitory effect of GnIH peptides on gonadotropic function in a basal teleost is in agreement with an ancestral inhibitory role of GnIH in the neuroendocrine control of reproduction in vertebrates.