The evolution of tachykinin/tachykinin receptor (TAC/TACR) in vertebrates and molecular identification of the TAC3/TACR3 system in zebrafish (Danio rerio).

Tachykinins are a family of peptides that are conserved from invertebrates to mammals. However, little is known about the evolutionary history of tachykinin (TAC) and tachykinin receptor (TACR) genes in vertebrates, especially in the teleost group. In the present study, five TACs and six TACRs genes were identified in the zebrafish genome. Genomic synteny analysis and phylogenetic tree analysis indicate that the increased numbers of TAC and TACR genes in vertebrates are the result of both genome duplications and local individual gene duplication. The full-length cDNA sequences encoding multiple TAC3s (TAC3a and TAC3b) and TACR3s (TACR3a1, TACR3a2 and TACR3b) were subsequently cloned from zebrafish brain samples. Sequence analysis suggested that four putative neurokinin B (NKB)-like peptides (NKBa-13, NKBa-10, NKBb-13 and NKBb-11) might be generated by the processing of two zebrafish TAC3 precursors. Tissue distribution studies in zebrafish revealed that TAC3 and TACR3 are mainly expressed in the brain regions. The biological activities of four zebrafish NKB peptides and three TACR3s were further examined using transcription reporter assays in cultured eukaryotic cells. All the synthetic NKB peptides were able to evoke the downstream signaling events of TACR3s with the exception of NKBb-11. These results indicated that the multiple TAC/TACR genes identified in vertebrates evolved from gene duplication events and that the TAC3/TACR3 systems also operate in the teleost group.