Packaging of chemicals in the defensive secretory glands of the sea hare Aplysia californica

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

Molecules and Mechanisms Underlying the Antimicrobial Activity of Escapin, an l-Amino Acid Oxidase from the Ink of Sea Hares

Many marine animals use chemicals to defend themselves and their eggs from predators. Beyond their ecologically relevant functions, these chemicals may also have properties that make them beneficial for humans, including biomedical and industrial applications. For example, some chemical defenses are also powerful antimicrobial or antitumor agents with relevance to human health and disease. One such chemical defense, escapin, an l-amino acid oxidase in the defensive ink of the sea hare Aplysia californica, and related proteins have been investigated for their biomedical properties. This review details our current understanding of escapin's antimicrobial activity, including the array of molecules generated by escapin's oxidation of its major substrates, l-lysine and l-arginine, and mechanisms underlying these molecules' bactericidal and bacteriostatic effects on planktonic cells and the prevention of formation and removal of bacterial biofilms. Models of escapin's effects are presented, and future directions are proposed.

Finding food: how marine invertebrates use chemical cues to track and select food

This review covers recent research on how marine invertebrates use chemical cues to find and select food.

Chemical mediation as a structuring element in marine gastropod predator-prey interactions

Some diterpenoid compounds protect the sacoglossansElysiasp. andCyerce nigricansfrom their carnivorous predator the dorid nudibranch,Gymnodorissp., unlike chemically unprotected gastropods that are consumed by this voracious nudibranch (photo Philippe Bourseiller).

Inhibition and Dispersal of Pseudomonas aeruginosa Biofilms by Combination Treatment with Escapin Intermediate Products and Hydrogen Peroxide

Escapin is an l-amino acid oxidase that acts on lysine to produce hydrogen peroxide (H2O2), ammonia, and equilibrium mixtures of several organic acids collectively called escapin intermediate products (EIP). Previous work showed that the combination of synthetic EIP and H2O2 functions synergistically as an antimicrobial toward diverse planktonic bacteria. We initiated the present study to investigate how the combination of EIP and H2O2 affected bacterial biofilms, using Pseudomonas aeruginosa as a model. Specifically, we examined concentrations of EIP and H2O2 that inhibited biofilm formation or fostered disruption of established biofilms. High-throughput assays of biofilm formation using microtiter plates and crystal violet staining showed a significant effect from pairing EIP and H2O2, resulting in inhibition of biofilm formation relative to biofilm formation in untreated controls or with EIP or H2O2 alone. Similarly, flow cell analysis and confocal laser scanning microscopy revealed that the EIP and H2O2 combination reduced the biomass of established biofilms relative to that of the controls. Area layer analysis of biofilms posttreatment indicated that disruption of biomass occurs down to the substratum. Only nanomolar to micromolar concentrations of EIP and H2O2 were required to impact biofilm formation or disruption, and these concentrations are significantly lower than those causing bactericidal effects on planktonic bacteria. Micromolar concentrations of EIP and H2O2 combined enhanced P. aeruginosa swimming motility compared to the effect of either EIP or H2O2 alone. Collectively, our results suggest that the combination of EIP and H2O2 may affect biofilms by interfering with bacterial attachment and destabilizing the biofilm matrix.

Distribution in Different Organisms of Amino Acid Oxidases with FAD or a Quinone As Cofactor and Their Role as Antimicrobial Proteins in Marine Bacteria

Amino acid oxidases (AAOs) catalyze the oxidative deamination of amino acids releasing ammonium and hydrogen peroxide. Several kinds of these enzymes have been reported. Depending on the amino acid isomer used as a substrate, it is possible to differentiate between l-amino acid oxidases and d-amino acid oxidases. Both use FAD as cofactor and oxidize the amino acid in the alpha position releasing the corresponding keto acid. Recently, a novel class of AAOs has been described that does not contain FAD as cofactor, but a quinone generated by post-translational modification of residues in the same protein. These proteins are named as LodA-like proteins, after the first member of this group described, LodA, a lysine epsilon oxidase synthesized by the marine bacterium Marinomonas mediterranea. In this review, a phylogenetic analysis of all the enzymes described with AAO activity has been performed. It is shown that it is possible to recognize different groups of these enzymes and those containing the quinone cofactor are clearly differentiated. In marine bacteria, particularly in the genus Pseudoalteromonas, most of the proteins described as antimicrobial because of their capacity to generate hydrogen peroxide belong to the group of LodA-like proteins.

Packaging and delivery of chemical weapons: a defensive trojan horse stratagem in chromodorid nudibranchs

Background

Storage of secondary metabolites with a putative defensive role occurs in the so-called mantle dermal formations (MDFs) that are located in the more exposed parts of the body of most and very likely all members of an entire family of marine mollusks, the chromodorid nudibranchs (Gastropoda: Opisthobranchia). Given that these structures usually lack a duct system, the mechanism for exudation of their contents remains unclear, as does their adaptive significance. One possible explanation could be that they are adapted so as to be preferentially attacked by predators. The nudibranchs might offer packages containing highly repugnant chemicals along with parts of their bodies to the predators, as a defensive variant of the strategic theme of the Trojan horse.

Methodology and Principal Findings

We detected, by quantitative ¹H-NMR, extremely high local concentrations of secondary metabolites in the MDFs of six species belonging to five chromodorid genera. The compounds were purified by chromatographic methods and subsequently evaluated for their feeding deterrent properties, obtaining dose-response curves. We found that only distasteful compounds are accumulated in the reservoirs at concentrations that far exceed the values corresponding to maximum deterrent activity in the feeding assays. Other basic evidence, both field and experimental, has been acquired to elucidate the kind of damage that the predators can produce on both the nudibranchs' mantles and the MDFs.

Significance

As a result of a long evolutionary process that has progressively led to the accumulation of defensive chemical weapons in localized anatomical structures, the extant chromodorid nudibranchs remain in place when molested, retracting respiratory and chemosensory organs, but offering readily accessible parts of their body to predators. When these parts are masticated or wounded by predators, breakage of the MDFs results in the release of distasteful compounds at extremely high concentration in a way that maximizes their repugnant impact.

Community ecology and the evolution of molecules of keystone significance

Molecules of keystone significance are vital in structuring ecological communities. Select bioactive compounds can cause disproportionately large effects by connecting such seemingly disparate processes as microbial loop dynamics and apex predation. Here, we develop a general theory and propose mechanisms that could lead to the evolution of keystone molecules. Introduced into a respective community by one, or only a few, autotrophic or microbial species, these compounds often originate as chemical defenses. When co-opted by resistant consumer species, however, they are used either in chemical defense against higher-order predators or as chemosensory cues that elicit courtship and mating, alarm, and predatory search. Requisite to these multifunctional properties, biosynthetic capacity evolves along with mechanisms for resistance and/or toxin storage in primary producers. Subsequently, consumers acquire resistances or tolerances, and the toxins are transferred through food webs via trophic interactions. In consumers, mechanisms eventually evolve for recognizing toxins as feeding cues and, ultimately, as signals or pheromones in chemical communication within or between species. One, or a few, active compounds can thus mediate a vast array of physiological traits, expressed differentially across many species in a given community. Through convergent evolution, molecules of keystone significance provide critical information to phylogenetically diverse species, initiate major trophic cascades, and structure communities within terrestrial, freshwater, coastal-ocean and open-ocean habitats.

Regulation of the Marinomonas mediterranea antimicrobial protein lysine oxidase by L-lysine and the sensor histidine kinase PpoS

Some Gram-negative bacteria express a novel enzyme with lysine-epsilon-oxidase (LOD) activity (EC 1.4.3.20). The oxidation of l-Lys generates, among other products, hydrogen peroxide, which confers antimicrobial properties to this kind of enzyme and has been shown to be involved in cell death during biofilm development and differentiation. In addition to LOD, the melanogenic marine bacterium Marinomonas mediterranea, which forms part of the microbiota of the marine plant Posidonia oceanica, expresses two other oxidases of biotechnological interest, a multicopper oxidase, PpoA, with laccase activity and a tyrosinase named PpoB, which is responsible for melanin synthesis. By using both lacZ fusions with the lodAB promoter and quantitative reverse transcription-PCR (qRT-PCR), this study shows that the hybrid sensor histidine kinase PpoS regulates LOD activity at the transcriptional level. Although PpoS also regulates PpoA and PpoB, in this case, the regulatory effect cannot be attributed only to a transcriptional regulation. Further studies indicate that LOD activity is induced at the posttranscriptional level by l-Lys as well as by two structurally similar compounds, l-Arg and meso-2,6-diaminopimelic acid (DAP), neither of which is a substrate of the enzyme. The inducing effect of these compounds is specific for LOD activity since PpoA and PpoB are not affected by them. This study offers, for the first time, insights into the mechanisms regulating the synthesis of the antimicrobial protein lysine-epsilon-oxidase in M. mediterranea, which could be important in the microbial colonization of the seagrass P. oceanica.

Effects of sea hare ink secretion and its escapin-generated components on a variety of predatory fishes

Sea hares, Aplysia californica, have a diversity of anti-predatory defenses. One is an actively released chemical defense: an ink secretion that is a mixture of two glandular products--ink from the ink gland and opaline from the opaline gland. The mechanisms of action of ink secretion and its components have recently been examined in detail against several predatory invertebrates. Our goal is to extend this mechanistic analysis to predatory vertebrates. Toward this end, the current study details the effects of ink, opaline, and one set of its components--the products of the reaction of escapin, an l-amino acid oxidase, with its natural substrates, L-lysine and L-arginine--on the palatability of food for five species of fishes: bluehead wrasses Thalassoma bifasciatum, señorita wrasses Oxyjulis californica, pinfish Lagodon rhomboides, mummichogs Fundulus heteroclitus, and bonnethead sharks Sphyrna tiburo. These fishes have different feeding styles, ranging from large fishes able to engulf sea hares to smaller fishes able to attack sea hares by pecking at them; and they live in a variety of habitats, including those that sea hares typically inhabit. We show that ink but not opaline significantly decreases the palatability of food for all five species, and that escapin products are mildly unpalatable to the two species of wrasses but not to the other species. These results, together with others, show that sea hare ink affects a diversity of predatory fishes, setting the stage for mechanistic studies using electrophysiological analysis of their chemosensory systems.

High-molecular weight protein toxins of marine invertebrates and their elaborate modes of action

High-molecular weight protein toxins significantly contribute to envenomations by certain marine invertebrates, e.g., jellyfish and fire corals. Toxic proteins frequently evolved from enzymes meant to be employed primarily for digestive purposes. The cellular intermediates produced by such enzymatic activity, e.g., reactive oxygen species or lysophospholipids, rapidly and effectively mediate cell death by disrupting cellular integrity. Membrane integrity may also be disrupted by pore-forming toxins that do not exert inherent enzymatic activity. When targeted to specific pharmacologically relevant sites in tissues or cells of the natural enemy or prey, toxic enzymes or pore-forming toxins even may provoke fast and severe systemic reactions. Since toxin-encoding genes constitute "hot spots" of molecular evolution, continuous variation and acquirement of new pharmacological properties are guaranteed. This also makes individual properties and specificities of complex proteinaceous venoms highly diverse and inconstant. In the present chapter we portray high-molecular weight constituents of venoms present in box jellyfish, sea anemones, sea hares, fire corals and the crown-of-thorns starfish. The focus lies on the latest achievements in the attempt to elucidate their molecular modes of action.

The chemistry of escapin: identification and quantification of the components in the complex mixture generated by an L-amino acid oxidase in the defensive secretion of the sea snail Aplysia californica

Escapin is an L-amino acid oxidase in the ink of a marine snail, the sea hare Aplysia californica, which oxidizes L-lysine (1) to produce a mixture of chemicals which is antipredatory and antimicrobial. The goal of our study was to determine the identity and relative abundance of the constituents of this mixture, using molecules generated enzymatically with escapin and also using products of organic syntheses. We examined this mixture under the natural range of pH values for ink-from approximately 5 at full strength to approximately 8 when fully diluted in sea water. The enzymatic reaction likely forms an equilibrium mixture containing the linear form alpha-keto-epsilon-aminocaproic acid (2), the cyclic imine Delta(1)-piperidine-2-carboxylic acid (3), the cyclic enamine Delta(2)-piperidine-2-carboxylic acid (4), possibly the linear enol 6-amino-2-hydroxy-hex-2-enoic acid (7), the alpha-dihydroxy acid 6-amino-2,2-dihydroxy-hexanoic acid (8), and the cyclic aminol 2-hydroxy-piperidine-2-carboxylic acid (9). Using NMR and mass spectroscopy, we show that 3 is the major component of this enzymatic product at any pH, but at more basic conditions, the equilibrium shifts to produce relatively more 4, and at acidic conditions, the equilibrium shifts to produce relatively more 2, 7, and/or 9. Studies of escapin's enzyme kinetics demonstrate that because of the high concentrations of escapin and L-lysine in the ink secretion, millimolar concentrations of 3, H(2)O(2), and ammonia are produced, and also lower concentrations of 2, 4, 7, and 9 as a result. We also show that reactions of this mixture with H(2)O(2) produce delta-aminovaleric acid (5) and delta-valerolactam (6), with 6 being the dominant component under the naturally acidic conditions of ink. Thus, the product of escapin's action on L-lysine contains an equilibrium mixture that is more complex than previously known for any L-amino acid oxidase.

Identification of potent bactericidal compounds produced by escapin, an L-amino acid oxidase in the ink of the sea hare Aplysia californica

The ink of sea hares (Aplysia californica) contains escapin, an L-amino acid oxidase that metabolizes L-lysine, thereby producing a mixture that kills microbes and deters attacking predators. This secretion contains H2O2,ammonia, and an equilibrium mixture of "escapin intermediate product" (EIP-K) that includes alpha-keto-epsilon-aminocaproic acid and several other molecules. Components of the equilibrium mixture react nonenzymatically with H2O2 to form "escapin end product" (EEP-K), which contains delta-aminovaleric acid and delta-valerolactam. The proportions of the molecules in this equilibrium mixture change with pH, and this is biologically important because the secretion is pH 5 when released but becomes pH 8 when fully diluted in seawater. The goal of the current study was to identify which molecules in this equilibrium mixture are bactericidal. We show that a mixture of H2O2 and EIP-K, but not EEP-K, at low mM concentrations is synergistically responsible for most of the bactericidal activity of the secretion against Escherichia coli, Vibrio harveyi, Staphylococcus aureus,and Pseudomonas aeruginosa. Low pH enhances the bactericidal effect, and this does not result from stress associated with low pH itself. Sequential exposure to low mM concentrations of EIP-K and H2O2, in either order, does not kill E. coli. Reaction products formed when L-arginine is substituted for L-lysine have almost no bactericidal activity. Our results favor the idea that the bactericidal activity is due to unstable intermediates of the reaction of alpha-keto-epsilon-aminocaproic acid with H2O2.

Identification of nitrogen mineralization enzymes, L-amino acid oxidases, from the ectomycorrhizal fungi Hebeloma spp. and Laccaria bicolor

Amino acids are major nitrogen sources in soils and they harbour a central position in the nitrogen metabolism of cells. We determined whether Hebeloma spp. and Laccaria bicolor expressed the enzyme L-amino acid oxidase (LAO), which catalyses the oxidative deamination of the alpha-amino group of L-amino acids. We measured LAO activities from the mycelial extracts of seven laboratory-grown fungal strains with three methods, and we measured how LAO activities were expressed in one Hebeloma sp. strain grown on four nitrogen sources. Hebeloma spp. and L. bicolor converted L-phenylalanine, but not D-phenylalanine, to hydrogen peroxide, 2-oxoacid, and ammonia, suggesting that they expressed LAO enzymes. The enzymes utilized five out of seven tested L-amino acids as substrates. LAO activities were maximal at pH 8, where Michaelis constant (Km) values were 2-5mm. The LAO of Hebeloma sp. was expressed on every nitrogen source analysed, and the activities were the highest in mycelia grown in nitrogen-rich conditions. We suggest that LAO is a mechanism for cellular amino acid catabolism in Hebeloma spp. and L. bicolor. Many soil bacteria and fungi also express LAO enzymes that have broad substrate specificities. Therefore, LAO is a potential candidate for a mechanism that catalyses nitrogen mineralization from amino acids at the ecosystem level.

Hydrogen peroxide linked to lysine oxidase activity facilitates biofilm differentiation and dispersal in several gram-negative bacteria

The marine bacterium Pseudoalteromonas tunicata produces an antibacterial and autolytic protein, AlpP, which causes death of a subpopulation of cells during biofilm formation and mediates differentiation, dispersal, and phenotypic variation among dispersal cells. The AlpP homologue (LodA) in the marine bacterium Marinomonas mediterranea was recently identified as a lysine oxidase which mediates cell death through the production of hydrogen peroxide. Here we show that AlpP in P. tunicata also acts as a lysine oxidase and that the hydrogen peroxide generated is responsible for cell death within microcolonies during biofilm development in both M. mediterranea and P. tunicata. LodA-mediated biofilm cell death is shown to be linked to the generation of phenotypic variation in growth and biofilm formation among M. mediterranea biofilm dispersal cells. Moreover, AlpP homologues also occur in several other gram-negative bacteria from diverse environments. Our results show that subpopulations of cells in microcolonies also die during biofilm formation in two of these organisms, Chromobacterium violaceum and Caulobacter crescentus. In all organisms, hydrogen peroxide was implicated in biofilm cell death, because it could be detected at the same time as the killing occurred, and the addition of catalase significantly reduced biofilm killing. In C. violaceum the AlpP-homologue was clearly linked to biofilm cell death events since an isogenic mutant (CVMUR1) does not undergo biofilm cell death. We propose that biofilm killing through hydrogen peroxide can be linked to AlpP homologue activity and plays an important role in dispersal and colonization across a range of gram-negative bacteria.

Escape by inking and secreting: marine molluscs avoid predators through a rich array of chemicals and mechanisms

Inking by marine molluscs such as sea hares, cuttlefish, squid, and octopuses is a striking behavior that is ideal for neuroecological explorations. While inking is generally thought to be used in active defense against predators, experimental evidence for this view is either scant or lacks mechanistic explanations. Does ink act through the visual or chemical modality? If inking is a chemical defense, how does it function and how does it affect the chemosensory systems of predators? Does it facilitate escape not only by acting directly on predators but also by being an alarm signal for conspecifics? This review examines these issues, within a broader context of passive and active chemical defensive secretions. It focuses on recent work on mechanisms of defense by inking in sea hares (Aplysia) and extends what we have learned about sea hares to other molluscs including the cephalopods.

Gene expression and distribution of antibacterial L-amino acid oxidase in the rockfish Sebastes schlegeli

Antibacterial factors in the epidermal mucus of fish have a potential importance in the first line of the host defense response to bacterial pathogens. We previously isolated a novel antibacterial protein termed SSAP (Sebastes schlegeli antibacterial protein) from the skin mucus of the rockfish S. schlegeli and identified it as a new member of the L-amino acid oxidase (LAO) family. In the present study, the localization of SSAP in S. schlegeli was investigated by reverse transcription (RT)-PCR, quantitative real time RT-PCR, Western blotting and measurements of LAO and antibacterial activities. SSAP mRNA was expressed dominantly in skin and gill and weakly in ovary or kidney as shown by RT-PCR and real time RT-PCR. The quantity of SSAP mRNA in skin varied among the individuals, ranging from 1.1 to 13.9 ng microg(-1) total RNA, although no relationship was found between the size of fish and gene expression. SSAP was exclusively detected in skin and gill by Western blotting using a specific anti-SSAP antiserum. In addition, the extracts of both tissues apparently showed LAO activity and antibacterial activity against Photobacterium damselae subsp. piscicida. This study demonstrates that SSAP is predominantly synthesized in skin and gill and probably functions as an antibacterial LAO in both tissues.

Molecular characterization of l-amino acid oxidase from king cobra venom

An L-amino acid oxidase from Ophiophagus hannah snake venom (Oh-LAAO) was purified by successive gel filtration, ion-exchange and heparin chromatography. Oh-LAAO did not induce platelet aggregation; however, it had potent inhibitory activity on platelet aggregation induced by ADP and U46619, but showed no effect on platelet aggregation induced by thrombin, mucetin, ristocetin and stejnulxin. By RT-PCR and 5'-RACE methods, the complete Oh-LAAO cDNA was cloned from the venom gland total RNA preparations. The cDNA sequence contains an open-reading frame (ORF) of 1476-bp, which encodes a protein of 491 amino acids comprising a signal peptide of 25 amino acids and 466-residue mature protein. The predicted protein sequence of Oh-LAAO was confirmed by N-terminal and trypsin-digested internal peptides sequencing together with peptide mass fingerprinting. cDNAs encoding for ORF of LAAOs from Bungarus fasciatus and B. multicinctus were cloned and reported in this study. In addition, partial cDNA encoding for Naja atra LAAO was also reported. Oh-LAAO shared approximately 50% protein sequence identity with other known snake venom LAAOs. Phylogenetic analysis indicated that Oh-LAAO is evolutionary distant to other snake venom LAAOs.