Tachykinins (TKs) constitute the largest vertebrate brain/gut peptide family. Since discovery of Substance P as a structurally unidentified vasodilatory and contractile compound in 1931, continuous and tremendous advances have been made regarding molecular and functional characterization of TKs and their receptors, revealing diverse molecular species of TK peptides with a C-terminal consensus -Phe-X-Gly-Leu-Met-NH2, not ubiquitous but wide distribution and multiple biological activities of TKs and their receptors in central and peripheral tissues, elaborate and complicated ligand-recognition and multiple functional conformation of receptors, evolutionary aspects of brain/gut peptides, and the implication of TK peptides and receptors in many disorders of current keen interest. Indeed, the tachykinergic systems are now regarded as promising targets of novel clinical agents aimed at a variety of pathological symptoms and processes such as nociception, inflammation, neurodegeneration, and neuroprotection. In this review, we present an overview of basic knowledge and a buildup of recent advances in extensive fields of the tachykinin kingdom including mammalian non-neuronal TKs, invertebrate salivary gland-specific TKs and TK-related brain/gut peptides (TKRPs). These findings shed new light on (1) the biological and biochemical significance of TKs, (2) evolutionary relationship of the structures and functions between mammalian and non-mammalian TK family peptides and receptors, and (3) the binding mode for the TK family peptides and their receptors and the resultant activation of the complexes that are essential for design and development of leading compounds.

Lignans are structurally and functionally diverse phytochemicals biosynthesized in diverse plant species and have received wide attentions as leading compounds of novel drugs for tumor treatment and healthy diets to reduce of the risks of lifestyle-related non-communicable diseases. However, the lineage-specific distribution and the low-amount of production in natural plants, some of which are endangered species, hinder the efficient and stable production of beneficial lignans. Accordingly, the development of new procedures for lignan production is of keen interest. Recent marked advances in the molecular and functional characterization of lignan biosynthetic enzymes and endogenous and exogenous factors for lignan biosynthesis have suggested new methods for the metabolic engineering of lignan biosynthesis cascades leading to the efficient, sustainable, and stable lignan production in plants, including plant cell/organ cultures. Optimization of light conditions, utilization of a wide range of elicitor treatments, and construction of transiently gene-transfected or transgenic lignan-biosynthesizing plants are mainly being attempted. This review will present the basic and latest knowledge regarding metabolic engineering of lignans based on their biosynthetic pathways and biological activities, and the perspectives in lignan production via metabolic engineering.

Lignans are a large class of secondary metabolites in plants, with numerous biological effects in mammals, including antitumor and antioxidant activities. Sesamin, the most abundant furofuran-class lignan in sesame seeds (Sesamum plants), is produced by the cytochrome P450 enzyme CYP81Q1 from the precursor lignan, pinoresinol. In contrast, Forsythia plants produce dibenzylbutyrolactone-class lignans, such as matairesinol, from pinoresinol via the catalysis of pinoresinol/lariciresinol reductase (PLR) and secoisolariciresinol dehydrogenase. Here we present the engineering of lignan biosynthesis in Forsythia cell suspension cultures for the development of an efficient production method of beneficial lignans. A suspension cell culture prepared from leaves of Forsythia koreana produced lignans, mainly pinoresinol and matairesinol glucosides, at levels comparable with that obtained from the leaves. In an attempt to increase the pinoresinol content in Forsythia, we generated a transgenic cell line overexpressing an RNA interference (RNAi) construct of PLR (PLR-RNAi). Down-regulation of PLR expression led to a complete loss of matairesinol and an accumulation of approximately 20-fold pinoresinol in its glucoside form in comparison with the non-transformant. Moreover, the Forsythia transgenic cells co-expressing CYP81Q1 and PLR-RNAi exhibited production of sesamin as well as accumulation of pinoresinol glucoside. These data suggest Forsythia cell suspension to be a promising tool for the engineering of lignan production. To the best of our knowledge, this is the first report on transgenic production of an exogenous lignan in a plant species.

Key transmembrane proteins in the innate immune system, Toll-like receptors (TLRs), have been suggested to occur in the genome of non-mammalian organisms including invertebrates. However, authentic invertebrate TLRs have been neither structurally nor functionally investigated. In this paper, we originally present the structures, localization, ligand recognition, activities, and inflammatory cytokine production of all TLRs of the ascidian Ciona intestinalis, designated as Ci-TLR1 and Ci-TLR2. The amino acid sequence of Ci-TLR1 and Ci-TLR2 were found to possess unique structural organization with moderate sequence similarity to functionally characterized vertebrate TLRs. ci-tlr1 and ci-tlr2 genes were expressed predominantly in the stomach and intestine as well as in hemocytes. Ci-TLR1 and Ci-TLR2 expressed in HEK293 cells, unlike vertebrate TLRs, were localized to both the plasma membrane and endosomes. Intriguingly, both Ci-TLR1 and Ci-TLR2 stimulate NF-κB induction in response to multiple pathogenic ligands such as double-stranded RNA, and bacterial cell wall components that are differentially recognized by respective vertebrate TLRs, revealing that Ci-TLRs recognize broader pathogen-associated molecular patterns than vertebrate TLRs. The Ci-TLR-stimulating pathogenic ligands also induced the expression of Ci-TNFα in the intestine and stomach where Ci-TLRs are expressed. These results provide evidence that the TLR-triggered innate immune systems are essentially conserved in ascidians, and that Ci-TLRs possess “hybrid” biological and immunological functions, compared with vertebrate TLRs. Moreover, it is presumed that chordate TLR ancestors also acquired the Ci-TLR-like multiple cellular localization and pathogen-associated molecular pattern recognition.

Tachykinins (TKs) are the most prevalent vertebrate brain/gut peptides. In this study, we originally identified authentic TKs and their receptor from a protochordate, Ciona intestinalis. The Ciona TK (Ci-TK) precursor, like mammalian γ-preprotachykinin A (γ-PPTA), encodes two TKs, Ci-TK-I and -II, including the -FXGLM-NH₂ vertebrate TK consensus. Mass spectrometry of the neural extract revealed the production of both Ci-TKs. Ci-TK-I contains several Substance P (SP)-typical amino acids, whereas a Thr is exceptionally located at position 4 from the C terminus of Ci-TK-II. The Ci-TK gene encodes both Ci-TKs in the same exon, indicating no alternative generation of Ci-TKs, unlike the PPTA gene. These results suggested that the alternative splicing of the PPTA gene was established during evolution of vertebrates. The only Ci-TK receptor, Ci-TK-R, was equivalently activated by Ci-TK-I, SP, and neurokinin A at physiological concentrations, whereas Ci-TK-II showed 100-fold less potent activity, indicating that the ligand selectivity of Ci-TK-R is distinct from those of vertebrate TK receptors. Ci-TK-I, like SP, also elicited the typical contraction on the guinea pig ileum. The Ci-TK gene was expressed in neurons of the brain ganglion, small cells in the intestine, and the zone 7 in the endostyle, which corresponds to the vertebrate thyroid gland. Furthermore, the Ci-TK-R mRNA was distributed in these three tissues plus the gonad. These results showed that Ci-TKs play major roles in sexual behavior and feeding in protochordates as brain/gut peptides and endocrine/paracrine molecules. Taken together, our data revealed the biochemical and structural origins of vertebrate TKs and their receptors.