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

Abstract

In mammals, neurokinin B (NKB) is a short peptide encoded by the gene tac3. It is involved in the brain control of reproduction by stimulating gonadotropin-releasing hormone (GnRH) neurons, mainly via kisspeptin. We investigated tac3 genes and peptides in a basal teleost, the European eel, which shows an atypical blockade of the sexual maturation at a prepubertal stage. Two tac3 paralogous genes (tac3a and tac3b) 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. Tac3 genes were identified in silico 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 tac3 genes. Duplicated tac3 genes 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 tac3b gene would have even occurred in some teleost lineages. NKB-RP peptide, encoded beside NKB by tac3 gene in actinopterygians and basal sarcopterygians, would have been lost in ancestral amniotes. Tissue distribution of eel tac3a and tac3b mRNAs showed major expression of both transcripts in the brain especially in the diencephalon, as analyzed by specific qPCRs. Human NKB has been tested in vitro on primary culture of eel pituitary cells. Human NKB dose-dependently inhibited the expression of lhβ, while having no effect on other glycoprotein hormone subunits (fshβ, tshβ, and gpα) nor on gh. Human NKB also dose-dependently inhibited the expression of GnRH receptor (gnrh-r2). The four eel peptides have been synthesized and also tested in vitro. They all inhibited the expression of both lhβ and of gnrh-r2. This reveals a potential dual inhibitory role of the four peptides encoded by the two tac3 genes in eel reproduction, exerted at the pituitary level on both luteinizing hormone and GnRH receptor.

Deciphering Direct and Indirect Effects of Neurokinin B and GnRH in the Brain-Pituitary Axis of Tilapia

“…One of the peptides that was generated by the processing of the Tac3 precursors was similar to the mammalian NKB, while the second related peptide was termed NKF due to its existence only in fish and frogs (20). Treatment with NKB peptides caused an increase in both FSH and LH synthesis and release in zebrafish (20), tilapia (23), goldfish (21), striped bass ( Morone saxatilis ) (42) and orange spotted grouper ( Epinephelus coioides ) (43) while in the European eel, human NKB was found to inhibit the expression of lhβ (22). Moreover, NKB and NKF bound and activated Tac3 receptors of zebrafish (20) and tilapia (23).…”

“…Like kisspeptin, humans carrying loss-of-function mutations in neurokinin B [NKB; also known as tachykinin 3 (Tac3)] or in NKBR also display hypogonadotropic hypogonadism (19). The first indication that NKB is widely expressed in fish species and has a stimulatory reproductive effect was shown in zebrafish (20), goldfish ( Carassius auratus ) (21), European eel ( Anguilla Anguilla ) (22) and tilapia (23). Teleost have one or two tac3 genes that encodes two mature peptides, NKB and NKB-related peptide (also called neurokinin F (NKF), and two tac3r genes (Tac3ra and Tac3rb) (20, 23).…”

Tachykinins: Neuropeptides That Are Ancient, Diverse, Widespread and Functionally Pleiotropic

“…Tac1 and Tac3 have also been identified from zebrafish, and the Tac3 prototype gene appears to have duplicated to give rise to Tac3a and Tac3b (Biran et al, 2012) as shown in Figure 4. In addition, two Tac3 have also been characterized from an eel, Anguilla anguilla , that is a basal vertebrate (Campo et al, 2018). Due to teleost-specific whole genome duplication multiple Tac3 genes were generated in teleost fish (Moriyama and Koshiba-Takeuchi, 2018).…”

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

“…Gnrh-r1a and gnrh-r1b expressions were below the threshold of detection in primary cultures of pituitary cells (Campo et al, 2018;Pasquier et al, 2018), and were thus not assayed in this study.…”