Insight into Tachykinin-Related Peptides, Their Receptors, and Invertebrate Tachykinins: A review

Abstract

Tachykinins (TKs) constitute the largest vertebrate neuropeptide family with multifunctions in central and peripheral tissues. In several invertebrate species, two types of structurally related peptides, 'tachykinin-related peptides (TKRPs)' and 'invertebrate tachykinins (inv-TKs)' have been identified. TKRPs, isolated from the nerve and/or gut tissues, contain the common C-terminal sequence -Phe-X-Gly-Y-Arg-NH(2) (X and Y are variable) analogous to the vertebrate TK consensus -Phe-X-Gly-Leu-Met-NH(2), and exhibit vertebrate TK-like contractile activity on invertebrate gut tissues. Inv-TKs have been shown to be present exclusively in the salivary gland of several species, to share vertebrate TK consensus motif, and to possess TK-like potencies on vertebrate, not invertebrate tissues. However, the functional and evolutionary relevance of TKRPs and inv-TKs to vertebrate TKs remains to be understood. Recent studies have revealed that TKRP precursors dramatically differ from vertebrate preprotachykinins in structural organization and that TKRP receptors share structural and functional properties with vertebrate TK receptors. Moreover, the C-terminal arginine in TKRPs has been shown to play an essential role in discriminating their receptors from vertebrate TK receptors. Such recent marked progress is expected to enhance further investigation of biological roles of TKRPs. This review provides an overview of the basic findings obtained previously and a buildup of new knowledge regarding TKRPs and inv-TKs. We also compare TKRPs and inv-TKs to vertebrate TKs with regard to evolutionary relationships in structure and function among these structurally related peptides.

Tachykinin and Tachykinin Receptor of an Ascidian, Ciona intestinalis

“…However, only two peptides, GnRH and cionin (CCK/gastrin-related peptide), have so far been identified from ascidians (43)(44)(45), although the presence of TK-like compounds in ascidians was implied by previous immunohistochemical analyses and radioimmunoassays for several ascidian tissues (24 -28). Furthermore, TKRPs, but not TKs, have been found in invertebrates, which complicates the evolutionary aspect of the TK family (7,20). In this study, we showed the structure, activity, and localization of a TK peptide, Ci-TK, and its receptor, Ci-TK-R, of an ascidian, C. intestinalis.…”

“…Contractile Activities of Ci-TKs on Gut Tissues-TKs are known to induce a characteristic contraction of gut tissues (1,7,19,20). Nevertheless, Ci-TKs had no contractile effect on the Ciona gut (data not shown), most likely due to the absence of intestinal muscular tissues in the ascidian gut (29).…”

“…In protostomes, two types of TK-like peptides, invertebrate TK and TK-related peptides (TKRPs), have so far been identified. Peptides of the former group, containing the identical C-terminal TK consensus motif, are expressed exclusively in the salivary gland and are devoid of any activity on the cognate tissues, indicating that the TK-like peptides are not functional counterparts of vertebrate TKs (9,19,20). TKRPs exert a TK-like contractile activity, and the expression of the TKRP gene is observed in the central nervous system (20).…”

“…Peptides of the former group, containing the identical C-terminal TK consensus motif, are expressed exclusively in the salivary gland and are devoid of any activity on the cognate tissues, indicating that the TK-like peptides are not functional counterparts of vertebrate TKs (9,19,20). TKRPs exert a TK-like contractile activity, and the expression of the TKRP gene is observed in the central nervous system (20). However, they contain the analogous -FX 1 (G/A)X 2 R-NH 2 consensus, and TKRP precursors encode multiple TKRP sequences (20 -23), which are totally distinct from those of vertebrate TKs.…”

Ascidians: New Model Organisms for Reproductive Endocrinology

“…In protostomes, TK-related peptides (TKRPs) exert a TK-like contractile activity and the expression of the tkrp gene is observed in the central nervous system (Satake et al, 2003;Satake & Kawada, 2006a). However, they contain the analogous Phe-Xaa1-(Gly/Ala)-Xaa2-Arg-NH 2 consensus (Table 1), and TKRP precursors encode multiple TKRP sequences (Satake et al, 2003;Satake and Kawada, 2006a), which are totally distinct from those of vertebrate TKs. These findings suggest that tkrp genes and tk genes diverged in distinct evolutionary lineages.…”