Definition
Tachykinins are multifunctional brain/gut peptides. In mammals and insects, various isoforms play an important neuromodulatory role in the central nervous system. They are involved in the processing of sensory information and in the control of motor activities. In addition, the peptides elicit stimulatory responses on a variety of visceral muscles1.
Related Peptides
Among the numerous families of neuropeptides, which are evolutionarily the oldest neuro- transmitters, even older than acetylcholine and catecholamines, four tachykinin-like peptides seem to occupy a very important position: invertebrate tachykinin-like pnneptides, prevertebrate tachykinin-like peptides, submammalian vertebrate tachykinins, mammalian tachykinins 1.
Discovery
Schoofs et al., in 1990 first described the occurrence in insects, more precisely in extracts of brain, corpora cardiaca-corpora allata, and suboesophageal ganglion of Locusta migratoria, of five peptides, the locustatachykinins, which exhibited sequence homologies (up to 45%) with the vertebrate tachykinins, especially with amphibian and fish tachykinins. The locustatachykinins were completely inactive in all bioassay preparations used for the vertebrate tachykinins but showed a myotropic action in the insect intestine, eliciting a potent contraction of the cockroach hindgut. The prediction of Schoofs' group in their first paper that "the peptides discovered in this study may be just the first in a whole series of substances from arthropod species to be identified as tachykinin family peptides" was correct even beyond any expectation. Up to the present, as many as 20 locustatachykinin-like peptides were isolated not only from various other arthropods, but also from an echinoid worm and from mollusks 2.
Structural Characteristics
Amphibian Skin Tachykinins: The great majority of amphibian skin peptides have the classical C-terminal pentapeptide sequence: Phe-X-Gly-Leu-Met-NH2. However, important exceptions are represented by: 1) some tachykinins from the skin of the Australian frog Agalychnis callidryas, namely AC-AR1, AC-AR2, and AC-AR3 with the C-terminal pentapeptide sequence Phe-Tyr-Pro-Gly-Met-NH2 and AC-AR4 with sequence Phe-Tyr-Pro-Val-Met-NH2; and 2) hylambatin from the skin of the South-African frog Hylambates maculatus with the C-terminal pentapeptide sequence Phe-Tyr-Gly-Met-Met-NH2. It is evident that in the C-terminal pentapeptide only the Phe residue at position 5 from the C terminus and Met-NH2 are immutable
Brain and Gut Tachykinins: All of these tachykinins, with the exception of ranatachynin D, show the classical C-terminal pentapeptide Phe-X-Gly-Leu-Met-NH2. Of considerable interest is the fact that in goldfish, cod, and trout NKA-like peptides, the usual acidic Asp residue at position 7 from the C terminus, crucial for receptor NK2/NK3 selectivity, is replaced by the neutral Asn residue. NKA is present in as many as six submammalian species also by its elongated form, the ?-neuropeptides.
Mammalian Tachykinins: They are derived from two preprotachykinin genes: the PPT-A gene, which encodes the sequences of SP, NKA, and neuropeptide K and neuropeptide-?, and the PPT-B gene, which encodes the sequence of NKB. The precursor RNA from PPT-A is alternatively processed to yield three different mRNAs. The three precursor proteins from which the mRNA codes are designated a-, ß-, and ?-PPT; a-PPT, which generates SP; ß-PPT, which generates SP, NKA, and neuropeptide K; and ?-PPT, which generates SP, NKA, and neuropeptide-?. The biological significance of the alternative splicing of PPT-A is unknown. The relative proportion of a-, ß-, and ?-PPT mRNAs is markedly species dependent. Tachykinins are liberated from their precursors by the action of specific processing proteases. Typical cleavage points are Lys-Arg, Arg-Arg, and Arg-Lys doublets and the cleavage is carried out by six groups of proteolytic enzymes called convertases. COOH-terminal amidation after cleavage is generated from the precursor sequence, Gly-Leu-Met-Gly-Lys-Arg, in which Gly acts as the amide donor 1.
Mode of Action
Structurally tachykinin-related peptides have been isolated from various invertebrate species and shown to exhibit their biological activities through a G-protein-coupled receptor (GPCR) for a tachykinin-related peptide. A novel tachykinin-related peptide receptor, the urechistachykinin receptor (UTKR) from the echiuroid worm, Urechis unitinctus. The deduced UTKR precursor includes seven transmembrane domains and typical sites for mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors. A functional analysis of the UTKR expressed in Xenopus oocytes demonstrated that UTKR, like tachykinin receptors and tachykinin-related peptide receptors, activates calcium-dependent signal transduction upon binding to its endogenous ligands, urechistachykinins (Uru-TKs) I2013V and VII, which were isolated as Urechis tachykinin-related peptides from the nervous tissue of the Urechis unitinctus in our previous study. UTKR responded to all Uru-TKs equivalently, showing that UTKR possesses no selective affinity with Uru-TKs. In contrast, UTKR was not activated by substance P or an Uru-TK analog containing a C-terminal Met-NH2 instead of Arg-NH2 3.
Functions
It is beyond doubt that neuronal tachykinins play an important role in neurotransmission/neuro- modulation both in the CNS and in periphery. This is demonstrated by the overall occurrence of tachykinins in the brain and other nervous structures from the lowest invertebrates to mammals. Although important, the tachykinin peptide family represents only one of the numerous peptide and nonpeptide families involved in neurotransmission and neuromodulation. Members of these families are expressed in a variety of tissues, and very frequently a tachykinin is costored and cosecreted by the nerve endings with other peptides or biogenic amines. Moreover, the tachykinins, like all other neuropeptides, may enter in competition, positive or negative, with a number of active extraneuronal compounds originating in blood (bradykinin and angiotensin) or in compact or diffuse endocrine organs. Tachykinins, with their variable primary structure seem to be adapted to display, in the better way, their function in the different invertebrate and vertebrate phyla. In all examined species, and especially in mammals (the phylum more thoroughly studied), tachykinins elicit a spectrum of biological activity (both in the CNS and in the periphery), which may vary conspicuously in the different species and even in the various strains of single species, again strongly supporting the concept of a general, important functional significance of these peptides 1.
References
1. Severini C, Improta G, Falconieri-Erspamer G, Salvadori S, Erspamer V (2002). The Tachykinin Peptide Family. Pharmacological Reviews. 54(2):285-322.
2. Schoofs L, Hollman GM, Hayes TK, Nachman RI, De Loof A (1990). Locustatachykinin I and II, two novel insect neuropeptides with homology to peptides of the vertebrate tachykinin family. FEBS Lett., 261:397-401.
3. Kawada T, Furukawa Y, Shimizu Y, Minakata H, Nomoto K, Satake H (2002). A novel tachykinin-related peptide receptor. European Journal of Biochemistry, 269(17):4238-4246.