Triterpenoids in Echinoderms: Fundamental Differences in Diversity and Biosynthetic Pathways

Sea Cucumber Viscera Contains Novel Non-Holostane-Type Glycoside Toxins that Possess a Putative Chemical Defense Function

Sea cucumbers frequently expel their guts in response to predators and an aversive environment, a behavior perceived as releasing repellents involved in chemical defense mechanisms. To investigate the chemical nature of the repellent, the viscera of stressed sea cucumbers (Apostichopus japonicus) in the Yellow Sea of China were collected and chemically analyzed. Two novel non-holostane triterpene glycosides were isolated, and the chemical structures were elucidated as 3ꞵ-O-[ꞵ-D-glucopyranosyl-(1→2)-ꞵ-D-xylopyranosyl]-(20S)-hydroxylanosta-7,25-diene-18(16)-lactone (1) and 3ꞵ-O-[ꞵ-D-quinovopyranosyl-(1→2)-ꞵ-D-xylopyranosyl]-(20S)-hydroxylanosta-7,25-diene-18(16)-lactone (2) by spectroscopic and mass-spectrometric analyses, exemplifying a triterpene glycoside constituent of an oligosaccharide containing two sugar-units and a non-holostane aglycone. Zebrafish embryos were exposed to various doses of 1 and 2 from 4 to 96 hpf. Compound 1 exposure showed 96 h-LC50 41.5 µM and an increased zebrafish mortality rates in roughly in a dose- and time-dependent manner. Compound 2, with different sugar substitution, exhibited no mortality and moderate teratogenic toxicity with a 96 h-EC50 of 173.5 µM. Zebrafish embryos exhibited teratogenic effects, such as reduced hatchability and total body length. The study found that triterpene saponin from A. japonicus viscera had acute toxicity in zebrafish embryos, indicating a potential chemical defense role in the marine ecosystem.

Endocrine regulation of reproductive biology in echinoderms: An evolutionary perspective from closest marine invertebrate relatives to chordates

Echinoderms are a phylum of invertebrate deuterostomes, which contain echinoids, asteroids, holothuroids, crinoids, and ophiuroids. Echinoderms have special evolutionary position and unique characteristics, including pentamerous radial body structure, elaborate calcareous endoskeletons, and versatile water vascular system. Echinoderms exhibit extraordinarily diverse reproductive modes: asexual reproduction, sexual reproduction, sexual reversal, etc. Endocrine regulation plays important well-known roles in sex differentiation, gonadal development and maturation, gametogenesis, and reproductive behavior in vertebrates. However, the entire picture of reproductive endocrinology in echinoderms as an evolutionary model of the closest marine invertebrate relatives to chordates has not been revealed. Here, we reviewed previous and recent research progress on reproductive endocrinology in echinoderms, mainly including two sections: Sex steroids in echinoderms and neuropeptide regulation in echinoderm reproduction. This review introduces a variety of endocrine regulatory mechanisms in reproductive biology of echinoderms. It discusses the vertebrate-like sex steroids, putative steroidogenic pathway and metabolism, and reproduction-related neuropeptides. The review will provide a deeper understanding about endocrine regulatory mechanisms of gonadal development in lower deuterostomes and the application of endocrine control in economic echinoderm species in aquaculture.

A Distinct Saponin Profile Drives an Olfactory-Mediated Aggregation in the Aquacultivated Sea Cucumber Holothuria scabra

Intraspecific chemical communication between echinoderms has often been limited to prespawning aggregation. However, sea cucumber farmers have long observed year-round adult aggregation as a potential source of disease propagation and the suboptimal use of available sea pen acreage and food resources. In this study, through spatial distribution statistics, we demonstrated the significant aggregation of the aquacultivated sea cucumber Holothuria scabra both as adults in large sea-based pens and as juveniles in laboratory-based aquaria, proving that aggregation in these animals is not only observed during spawning. The role of chemical communication in aggregation was investigated using olfactory experimental assays. Our study established that the sediment that H. scabra feeds on as well as the water preconditioned by conspecifics induced positive chemotaxis in juvenile individuals. More specifically, through comparative mass spectrometry, a distinct triterpenoid saponin profile/mixture was identified to be a pheromone allowing sea cucumber intraspecific recognition and aggregation. This “attractive” profile was characterized as containing disaccharide saponins. This “attractive” aggregation-inducing saponin profile was, however, not conserved in starved individuals that were no longer attractive to other conspecifics. In summary, this study sheds new light on the pheromones in echinoderms. It highlights the complexity of the chemical signals detected by sea cucumbers and suggests a role of saponins well beyond that of a simple toxin.

Neuritogenic steroid glycosides from crown-of-thorns starfish: Possible involvement of p38 mitogen-activated protein kinase and attenuation of cognitive impairment in senescence-accelerated mice (SAMP8) by peripheral administration

Novel steroid glycosides, acanthasterosides A1, B1, and B3, have been isolated from the crown-of-thorns starfish Acanthaster planci. Acanthasterosides B1 and B3 having two separated xyloses induced neurite outgrowth as like as nerve growth factor (NGF) in the rat pheochromocytoma cell line PC12, whereas acanthasteroside A1, having one xylose, did not induce neurite outgrowth. The acanthasteroside B3 induced neuritogenesis via the significant activation of p38 mitogen-activated protein kinase after the activation of the small G-protein Cdc42 rather than via Ras-MEK-ERK pathway that is predominantly activated by NGF. Following subcutaneous administration, acanthasteroside B3 attenuated cognitive impairment of senescence-accelerated mice (SAMP8) in two different cognitive tests. Liquid chromatography-mass spectrometry-assisted quantitative analysis demonstrated that acanthasteroside B3 could be transported into the brain via the circulatory system in mice. Thus, acanthasteroside B3 (and possibly B1) are a novel class of potential drug candidates for neurodegenerative diseases.

The Isolation, Structure Elucidation and Bioactivity Study of Chilensosides A, A1, B, C, and D, Holostane Triterpene Di-, Tri- and Tetrasulfated Pentaosides from the Sea Cucumber Paracaudina chilensis (Caudinidae, Molpadida)

Five new triterpene (4,4,14-trimethylsterol) di-, tri- and tetrasulfated pentaosides, chilensosides A (1), A₁ (2), B (3), C (4), and D (5) were isolated from the Far-Eastern sea cucumber Paracaudina chilensis. The structures were established on the basis of extensive analysis of 1D and 2D NMR spectra and confirmed by HR-ESI-MS data. The structural variability of the glycosides concerned the pentasaccharide chains. Their architecture was characterized by the upper semi-chain consisting of three sugar units and the bottom semi-chain of two sugars. Carbohydrate chains of compounds 2–5 differed in the quantity and positions of sulfate groups. The interesting structural features of the glycosides were: the presence of two sulfate groups at C-4 and C-6 of the same glucose residue in the upper semi-chain of 1, 2, 4, and 5 and the sulfation at C-3 of terminal glucose residue in the bottom semi-chain of 4 that makes its further elongation impossible. Chilensoside D (5) was the sixth tetrasulfated glycoside found in sea cucumbers. The architecture of the sugar chains of chilensosides A–D (1–5), the positions of sulfation, the quantity of sulfate groups, as well as the aglycone structures, demonstrate their similarity to the glycosides of the representatives of the order Dendrochirotida, confirming the phylogenetic closeness of the orders Molpadida and Dendrochirotida. The cytotoxic activities of the compounds 1–5 against human erythrocytes and some cancer cell lines are presented. Disulfated chilensosides A₁ (2) and B (3) and trisulfated chilensoside C (4) showed significant cytotoxic activity against human cancer cells.

Structures and Biologic Activity of Chitonoidosides I, J, K, K1 and L-Triterpene Di-, Tri- and Tetrasulfated Hexaosides from the Sea Cucumber Psolus chitonoides

Five new triterpene di-, tri- and tetrasulfated hexaosides (chitonoidosides I (1), J (2), K (3), K₁ (4) and L (5)) were isolated from the Far-Eastern sea cucumber Psolus chitonoides, collected near Bering Island (Commander Islands) from a depth of 100–150 m. The structural variability of the glycosides concerned both the aglycones (with 7(8)- or 9(11)-double bonds) and carbohydrate chains differing from each other by the third sugar residue (Xyl or sulfated by C-6 Glc) and/or by the fourth—terminal in the bottom semi-chain—residue (Glc or sulfated by C-6 MeGlc) as well as by the positions of a sulfate group at C-4 or C-6 in the sixth—terminal in the upper semi-chain—residue (MeGlc). Hemolytic activities of these compounds 1–5 against human erythrocytes as well as cytotoxicity against human cancer cell lines, HeLa, DLD-1 and HL-60, were studied. The hexaosides, chitonoidosides K (3) and L (5) with four sulfate groups, were the most active against tumor cells in all the tests. Noticeably, the sulfate group at C-4 of MeGlc6 did not decrease the membranolytic effect of 5 as compared with 3, having the sulfate group at C-6 of MeGlc6. Erythrocytes were, as usual, more sensitive to the action of the studied glycosides than cancer cells, although the sensitivity of leukemia promyeloblast HL-60 cells was higher than that of other tumor cells. The glycosides 1 and 2 demonstrated some weaker action in relation to DLD-1 cells than against other tumor cell lines. Chitonoidoside K₁ (4) with a hydroxyl at C 25 of the aglycone was not active in all the tests. The metabolic network formed by the carbohydrate chains of all the glycosides isolated from P. chitonoides as well as the aglycones biosynthetic transformations during their biosynthesis are discussed and illustrated with schemes.

tRNA copy number and codon usage in the sea cucumber genome provide insights into adaptive translation for saponin biosynthesis

Genomic tRNA copy numbers determine cytoplasmic tRNA abundances, which in turn influence translation efficiency, but the underlying mechanism is not well understood. Using the sea cucumber Apostichopus japonicus as a model, we combined genomic sequence, transcriptome expression and ecological food resource data to study its codon usage adaptation. The results showed that, unlike intragenic non-coding RNAs, transfer RNAs (tRNAs) tended to be transcribed independently. This may be attributed to their specific Pol III promoters that lack transcriptional regulation, which may underlie the correlation between genomic copy number and cytoplasmic abundance of tRNAs. Moreover, codon usage optimization was mostly restrained by a gene's amino acid sequence, which might be a compromise between functionality and translation efficiency for stress responses were highly optimized for most echinoderms, while enzymes for saponin biosynthesis (LAS, CYPs and UGTs) were especially optimized in sea cucumbers, which might promote saponin synthesis as a defence strategy. The genomic tRNA content of A. japonicus was positively correlated with amino acid content in its natural food particles, which should promote its efficiency in protein synthesis. We propose that coevolution between genomic tRNA content and codon usage of sea cucumbers facilitates their saponin synthesis and survival using food resources with low nutrient content.

Triterpene Glycosides from the Far Eastern Sea Cucumber Thyonidium (=Duasmodactyla) kurilensis (Levin): The Structures, Cytotoxicities, and Biogenesis of Kurilosides A3, D1, G, H, I, I1, J, K, and K1

Nine new mono-, di-, and trisulfated triterpene penta- and hexaosides, kurilosides A₃ (1), D₁ (2), G (3), H (4), I (5), I₁ (6), J (7), K (8), and K₁ (9) and two desulfated derivatives, DS-kuriloside L (10), having a trisaccharide branched chain, and DS-kuriloside M (11), having hexa-nor-lanostane aglycone with a 7(8)-double bond, have been isolated from the Far-Eastern deep-water sea cucumber Thyonidium (=Duasmodactyla) kurilensis (Levin) and their structures were elucidated based on 2D NMR spectroscopy and HR-ESI mass-spectrometry. Five earlier unknown carbohydrate chains and two aglycones (having a 16β,(20S)-dihydroxy-fragment and a 16β-acetoxy,(20S)-hydroxy fragment) were found in these glycosides. All the glycosides 1–9 have a sulfate group at C-6 Glc, attached to C-4 Xyl1, while the positions of the other sulfate groups vary in different groups of kurilosides. The analysis of the structural features of the aglycones and the carbohydrate chains of all the glycosides of T. kurilensis showed their biogenetic relationships. Cytotoxic activities of the compounds 1–9 against mouse neuroblastoma Neuro 2a, normal epithelial JB-6 cells, and erythrocytes were studied. The highest cytotoxicity in the series was demonstrated by trisulfated hexaoside kuriloside H (4), having acetoxy-groups at C(16) and C(20), the latter one obviously compensated the absence of a side chain, essential for the membranolytic action of the glycosides. Kuriloside I₁ (6), differing from 4 in the lacking of a terminal glucose residue in the bottom semi-chain, was slightly less active. The compounds 1–3, 5, and 8 did not demonstrate cytotoxic activity due to the presence of hydroxyl groups in their aglycones.

Structures and Bioactivities of Quadrangularisosides A, A1, B, B1, B2, C, C1, D, D1-D4, and E from the Sea Cucumber Colochirus quadrangularis: The First Discovery of the Glycosides, Sulfated by C-4 of the Terminal 3-O-Methylglucose Residue. Synergetic Effect on Colony Formation of Tumor HT-29 Cells of these Glycosides with Radioactive Irradiation

Thirteen new mono-, di-, and trisulfated triterpene glycosides, quadrangularisosides A-D₄ (1-13) have been isolated from the sea cucumber Colochirus quadrangularis, which was collected in Vietnamese waters. The structures of these glycosides were established by 2D NMR spectroscopy and HR-ESI (High Resolution Electrospray Ionization) mass spectrometry. The novel carbohydrate moieties of quadrangularisosides D-D₄ (8-12), belonging to the group D, and quadrangularisoside E (13) contain three sulfate groups, with one of them occupying an unusual position-at C(4) of terminal 3-O-methylglucose residue. Quadrangularisosides A (1) and D₃ (11) as well as quadrangularisosides A₁ (2) and D₄ (12) are characterized by the new aglycones having 25-hydroperoxyl or 24-hydroperoxyl groups in their side chains, respectively. The cytotoxic activities of compounds 1-13 against mouse neuroblastoma Neuro 2a, normal epithelial JB-6 cells, erythrocytes, and human colorectal adenocarcinoma HT-29 cells were studied. All the compounds were rather strong hemolytics. The structural features that most affect the bioactivity of the glycosides are the presence of hydroperoxy groups in the side chains and the quantity of sulfate groups. The membranolytic activity of monosulfated quadrangularisosides of group A (1, 2) against Neuro 2a, JB-6 cells, and erythrocytes was relatively weak due to the availability of the hydroperoxyl group, whereas trisulfated quadrangularisosides D₃ (11) and D₄ (12) with the same aglycones as 1, 2 were the least active compounds in the series due to the combination of these two structural peculiarities. The erythrocytes were more sensitive to the action of the glycosides than Neuro 2a or JB-6 cells, but the structure-activity relationships observed for glycosides 1-13 were similar in the three cell lines investigated. The compounds 3-5, 8, and 9 effectively suppressed the cell viability of HT-29 cells. Quadrangularisosides A₁ (2), C (6), C₁ (7), and E (13) possessed strong inhibitory activity on colony formation in HT-29 cells. Due to the synergic effects of these glycosides (0.02 μM) and radioactive irradiation (1 Gy), a decreasing of number of colonies was detected. Glycosides 1, 3, and 9 enhanced the effect of radiation by about 30%.

A Seed-Specific Regulator of Triterpene Saponin Biosynthesis in Medicago truncatula

Plants produce a vast array of defense compounds to protect themselves from pathogen attack or herbivore predation. Saponins are a specific class of defense compounds comprising bioactive glycosides with a steroidal or triterpenoid aglycone backbone. The model legume Medicago truncatula synthesizes two types of saponins, hemolytic saponins and nonhemolytic soyasaponins, which accumulate as specific blends in different plant organs. Here, we report the identification of the seed-specific transcription factor TRITERPENE SAPONIN ACTIVATION REGULATOR3 (TSAR3), which controls hemolytic saponin biosynthesis in developing M. truncatula seeds. Analysis of genes that are coexpressed with TSAR3 in transcriptome data sets from developing M. truncatula seeds led to the identification of CYP88A13, a cytochrome P450 that catalyzes the C-16α hydroxylation of medicagenic acid toward zanhic acid, the final oxidation step of the hemolytic saponin biosynthesis branch in M. truncatula In addition, two uridine diphosphate glycosyltransferases, UGT73F18 and UGT73F19, which glucosylate hemolytic sapogenins at the C-3 position, were identified. The genes encoding the identified biosynthetic enzymes are present in clusters of duplicated genes in the M. truncatula genome. This appears to be a common theme among saponin biosynthesis genes, especially glycosyltransferases, and may be the driving force of the metabolic evolution of saponins.