AUTHOR=Campo Aurora , Lafont Anne-Gaëlle , Lefranc Benjamin , Leprince Jérôme , Tostivint Hervé , Kamech Nédia , Dufour Sylvie , Rousseau Karine 

TITLE=Tachykinin-3 Genes and Peptides Characterized in a Basal Teleost, the European Eel: Evolutionary Perspective and Pituitary Role

JOURNAL=Frontiers in Endocrinology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2018.00304

DOI=10.3389/fendo.2018.00304

ISSN=1664-2392

ABSTRACT=<p>In mammals, neurokinin B (NKB) is a short peptide encoded by the gene <italic>tac3</italic>. It is involved in the brain control of reproduction by stimulating gonadotropin-releasing hormone (GnRH) neurons, mainly <italic>via</italic> kisspeptin. We investigated <italic>tac3</italic> genes and peptides in a basal teleost, the European eel, which shows an atypical blockade of the sexual maturation at a prepubertal stage. Two <italic>tac3</italic> paralogous genes (<italic>tac3a</italic> and <italic>tac3b</italic>) were identified in the eel genome, each encoding two peptides (NKBa or b and NKB-related peptide NKB-RPa or b). Amino acid sequence of eel NKBa is identical to human NKB, and the three others are novel peptide sequences. The four eel peptides present the characteristic C-terminal tachykinin sequence, as well as a similar alpha helix 3D structure. <italic>Tac3</italic> genes were identified <italic>in silico</italic> in 52 species of vertebrates, and a phylogeny analysis was performed on the predicted TAC3 pre-pro-peptide sequences. A synteny analysis was also done to further assess the evolutionary history of <italic>tac3</italic> genes. Duplicated <italic>tac3 g</italic>enes in teleosts likely result from the teleost-specific whole genome duplication (3R). Among teleosts, TAC3b precursor sequences are more divergent than TAC3a, and a loss of <italic>tac3b</italic> gene would have even occurred in some teleost lineages. NKB-RP peptide, encoded beside NKB by <italic>tac3</italic> gene in actinopterygians and basal sarcopterygians, would have been lost in ancestral amniotes. Tissue distribution of eel <italic>tac3a</italic> and <italic>tac3b</italic> mRNAs showed major expression of both transcripts in the brain especially in the diencephalon, as analyzed by specific qPCRs. Human NKB has been tested <italic>in vitro</italic> on primary culture of eel pituitary cells. Human NKB dose-dependently inhibited the expression of <italic>lhβ</italic>, while having no effect on other glycoprotein hormone subunits (<italic>fshβ, tshβ</italic>, and <italic>gpα</italic>) nor on <italic>gh</italic>. Human NKB also dose-dependently inhibited the expression of GnRH receptor (<italic>gnrh-</italic>r2). The four eel peptides have been synthesized and also tested <italic>in vitro</italic>. They all inhibited the expression of both <italic>lhβ</italic> and of <italic>gnrh-r2</italic>. This reveals a potential dual inhibitory role of the four peptides encoded by the two <italic>tac3</italic> genes in eel reproduction, exerted at the pituitary level on both luteinizing hormone and GnRH receptor.</p>