Neurokinin Bs and neurokinin B receptors in zebrafish-potential role in controlling fish reproduction.

The endocrine regulation of vertebrate reproduction is achieved by the coordinated actions of several peptide neurohormones, tachykinin among them. To study the evolutionary conservation and physiological functions of neurokinin B (NKB), we identified tachykinin (tac) and tac receptor (NKBR) genes from many fish species, and cloned two cDNA forms from zebrafish. Phylogenetic analysis showed that piscine Tac3s and mammalian neurokinin genes arise from one lineage. High identity was found among different fish species in the region encoding the NKB; all shared the common C-terminal sequence. Although the piscine Tac3 gene encodes for two putative tachykinin peptides, the mammalian ortholog encodes for only one. The second fish putative peptide, referred to as neurokinin F (NKF), is unique and found to be conserved among the fish species when tested in silico. tac3a was expressed asymmetrically in the habenula of embryos, whereas in adults zebrafish tac3a-expressing neurons were localized in specific brain nuclei that are known to be involved in reproduction. Zebrafish tac3a mRNA levels gradually increased during the first few weeks of life and peaked at pubescence. Estrogen treatment of prepubertal fish elicited increases in tac3a, kiss1, kiss2, and kiss1ra expression. The synthetic zebrafish peptides (NKBa, NKBb, and NKF) activated Tac3 receptors via both PKC/Ca(2+) and PKA/cAMP signal-transduction pathways in vitro. Moreover, a single intraperitoneal injection of NKBa and NKF significantly increased leuteinizing hormone levels in mature female zebrafish. These results suggest that the NKB/NKBR system may participate in neuroendocrine control of fish reproduction.

Differential mRNA splicing and precursor processing of neurokinin B in neuroendocrine tissues.

The tachykinin neurokinin B which is encoded on the tachykinin 3 precursor, has prominent roles in both neuronal and endocrine systems, yet little is known about its evolution, potential splice variants and the manner in which it is processed. Here, we deduce the diversity within the vertebrate tachykinin 3 precursors, and identify novel tachykinin 3 splice variants and precursors. A total of 35 different tachykinin 3 precursors were identified in mammals, birds and reptiles. Nine additional alternatively spliced tachykinin 3 mRNA transcripts were also discovered in humans leading to the formation of three tachykinin 3 precursors (named alpha, beta and gamma tachykinin 3), but no novel tachykinin. gamma tachykinin 3, albeit rarer, was not found to encode neurokinin B. Differential processing of the tachykinin 3 precursor in the human placenta leads to the formation of potential NH2-terminally extended forms of neurokinin B. Moreover, we found increased proteolytic cleavage of the tachykinin 3 precursor during the pregnancy syndrome of pre-eclampsia. We have established neurokinin B to be an evolutionarily conserved peptide, nonetheless the significance of the three different tachykinin 3 precursors is not clear, but could represent an evolutionarily redundant splicing mechanism once employed by an ancestral gene that encoded two tachykinins. Our results indicate that differential mRNA splicing and precursor processing is likely to play an important role in differentiating the actions of the tachykinin 3 gene products in both neuronal and endocrine tissues.

The troubled story of tachykinins and neurokinins.

A family of peptides that shares a common C-terminal sequence (Phe-X-Gly-Leu-MetNH2) exists in mammalian and non-mammalian species. In mammals, three of these peptides (substance P, neurokinin A and neurokinin B) satisfy the criteria to be considered as neurotransmitters either in the central, peripheral or enteric nervous systems. In addition, multiple receptors for these peptides, which belong to the superfamily of G-protein-coupled receptors, exist. These receptors have distinct pharmacological features and selective agonists and antagonists are available for studying their functional roles. The latest update on nomenclature of these peptides and their receptors, which dates back to 1986, agreed to use the terms tachykinins and tachykinin NK1, NK2 and NK3 receptors. This 'nomenclature mismatch' has generated confusion that urges experts in the field of tachykinin research to provide a revised nomenclature.