Cucumis sativus extract elicits chloride secretion by stimulation of the intestinal TMEM16A ion channel

CONTEXT

Cucumber (Cucumis sativus Linn. [Cucurbitaceae]) is widely known for its purgative, antidiabetic, antioxidant, and anticancer therapeutic potential. However, its effect on gastrointestinal (GI) disease is unrecognised.

OBJECTIVE

This study investigated the effect of C. sativus fruit extract (CCE) on intestinal chloride secretion, motility, and motor function, and the role of TMEM16A chloride channels.

MATERIALS AND METHODS

CCE extracts were obtained from commercially available cucumber. Active fractions were then purified by HPLC and analysed by high resolution mass spectrometry. The effect of CCE on intestinal chloride secretion was investigated in human colonic T84 cells, ex vivo mouse intestinal tissue using an Ussing chamber, and the two-electrode voltage-clamp technique to record calcium sensitive TMEM16A chloride currents in Xenopus laevis oocytes. In vivo, intestinal motility was investigated using the loperamide-induced C57BL/6 constipation mouse model. Ex vivo contractility of mouse colonic smooth muscles was assessed by isometric force measurements.

RESULTS

CCE increased the short-circuit current (ΔIsc 34.47 ± µA/cm²) and apical membrane chloride conductance (ΔI_Cl 95 ± 8.1 µA/cm²) in intestinal epithelial cells. The effect was dose-dependent, with an EC₅₀ value of 0.06 µg/mL. CCE stimulated the endogenous TMEM16A-induced Cl^- current in Xenopus laevis oocytes. Moreover, CCE increased the contractility of smooth muscle in mouse colonic tissue and enhanced small bowel transit in CCE treated mice compared to loperamide controls. Mass spectrometry suggested a cucurbitacin-like analogue with a mass of 512.07 g/mol underlying the bioactivity of CCE.

CONCLUSION

A cucurbitacin-like analog present in CCE activates TMEM16A channels, which may have therapeutic potential in cystic fibrosis and intestinal hypodynamic disorders.

Anti-Inflammatory Activity of Glycolipids and a Polyunsaturated Fatty Acid Methyl Ester Isolated from the Marine Dinoflagellate Karenia mikimotoi

A new monogalactosyldiacylglycerol (MGDG), a known monogalactosylmonoacylglycerol (MGMG) and a known polyunsaturated fatty acid methyl ester (PUFAME) were isolated from the marine dinoflagellate Karenia mikimotoi. The planar structure of the glycolipids was elucidated using mass spectroscopy (MS) and nuclear magnetic resonance (NMR) analyses and comparisons to the known glycolipid to confirm its structure. The MGDG was characterized as 3-O-β-D-galactopyranosyl-1-O-3,6,9,12,15-octadecapentaenoyl-2-O-tetradecanoylglycerol 1. The MGMG and PUFAME were characterized as (2S)-3-O-β-D-galactopyranosyl-1-O-3,6,9,12,15-octadecapentaenoylglycerol 2 and Methyl (3Z,6Z,9Z,12Z,15Z)-octadeca-3,6,9,12,15-pentaenoate 3, respectively. The isolation of the PUFAME strongly supports the polyunsaturated fatty acid (PUFA) fragment of these glycolipids. The relative configuration of the sugar was deduced by comparisons of ³J_HH values and proton chemical shifts with those of known glycolipids. All isolated compounds MGDG, MGMG and PUFAME 1-3 were evaluated for their antimicrobial and anti-inflammatory activity. All compounds modulated macrophage responses, with compound 3 exhibiting the greatest anti-inflammatory activity.

Uptake and localization of fluorescently-labeled Karenia brevis metabolites in non-toxic marine microbial taxa

Brevetoxin (PbTx) is a neurotoxic secondary metabolite of the dinoflagellate Karenia brevis. We used a novel, fluorescent BODIPY-labeled conjugate of brevetoxin congener PbTx-2 (B-PbTx) to track absorption of the metabolite into a variety of marine microbes. The labeled toxin was taken up and brightly fluoresced in lipid-rich regions of several marine microbes including diatoms and coccolithophores. The microzooplankton (20-200 μm) tintinnid ciliate Favella sp. and the rotifer Brachionus rotundiformis also took up B-PbTx. Uptake and intracellular fluorescence of B-PbTx was weak or undetectable in phytoplankton species representative of dinoflagellates, cryptophytes, and cyanobacteria over the same (4 h) time course. The cellular fate of two additional BODIPY-conjugated K. brevis associated secondary metabolites, brevenal (B-Bn) and brevisin (B-Bs), were examined in all the species tested. All taxa exhibited minimal or undetectable fluorescence when exposed to the former conjugate, while most brightly fluoresced when treated with the latter. This is the first study to observe the uptake of fluorescently-tagged brevetoxin conjugates in non-toxic phytoplankton and zooplankton taxa, demonstrating their potential in investigating whether marine microbes can serve as a significant biological sink for algal toxins. The highly variable uptake of B-PbTx observed among taxa suggests some may play a more significant role than others in vectoring lipophilic toxins in the marine environment.

Interaction of a dinoflagellate neurotoxin with voltage-activated ion channels in a marine diatom

Background

The potent neurotoxins produced by the harmful algal bloom species Karenia brevis are activators of sodium voltage-gated channels (VGC) in animals, resulting in altered channel kinetics and membrane hyperexcitability. Recent biophysical and genomic evidence supports widespread presence of homologous sodium (Na⁺) and calcium (Ca²⁺) permeable VGCs in unicellular algae, including marine phytoplankton. We therefore hypothesized that VGCs of these phytoplankton may be an allelopathic target for waterborne neurotoxins produced by K. brevis blooms that could lead to ion channel dysfunction and disruption of signaling in a similar manner to animal Na⁺ VGCs.

Methods

We examined the interaction of brevetoxin-3 (PbTx-3), a K. brevis neurotoxin, with the Na⁺/Ca²⁺ VGC of the non-toxic diatom Odontella sinensis using electrophysiology. Single electrode current- and voltage- clamp recordings from O. sinensis in the presence of PbTx-3 were used to examine the toxin’s effect on voltage gated Na⁺/Ca²⁺ currents. In silico analysis was used to identify the putative PbTx binding site in the diatoms. We identified Na⁺/Ca²⁺ VCG homologs from the transcriptomes and genomes of 12 diatoms, including three transcripts from O. sinensis and aligned them with site-5 of Na⁺ VGCs, previously identified as the PbTx binding site in animals.

Results

Up to 1 µM PbTx had no effect on diatom resting membrane potential or membrane excitability. The kinetics of fast inward Na⁺/Ca²⁺ currents that underlie diatom action potentials were also unaffected. However, the peak inward current was inhibited by 33%, delayed outward current was inhibited by 25%, and reversal potential of the currents shifted positive, indicating a change in permeability of the underlying channels. Sequence analysis showed a lack of conservation of the PbTx binding site in diatom VGC homologs, many of which share molecular features more similar to single-domain bacterial Na⁺/Ca²⁺ VGCs than the 4-domain eukaryote channels.

Discussion

Although membrane excitability and the kinetics of action potential currents were unaffected, the permeation of the channels underlying the diatom action potential was significantly altered in the presence of PbTx-3. However, at environmentally relevant concentrations the effects of PbTx- on diatom voltage activated currents and interference of cell signaling through this pathway may be limited. The relative insensitivity of phytoplankton VGCs may be due to divergence of site-5 (the putative PbTx binding site), and in some cases, such as O. sinensis, resistance to toxin effects may be because of evolutionary loss of the 4-domain eukaryote channel, while retaining a single domain bacterial-like VGC that can substitute in the generation of fast action potentials.

Glycoconjugated Site-Selective DNA-Methylating Agent Targeting Glucose Transporters on Glioma Cells

DNA-alkylating drugs continue to remain an important weapon in the arsenal against cancers. However, they typically suffer from several shortcomings because of the indiscriminate DNA damage that they cause and their inability to specifically target cancer cells. We have developed a strategy for overcoming the deficiencies in current DNA-alkylating chemotherapy drugs by designing a site-specific DNA-methylating agent that can target cancer cells because of its selective uptake via glucose transporters, which are overexpressed in most cancers. The design features of the molecule, its synthesis, its reactivity with DNA, and its toxicity in human glioblastoma cells are reported here. In this molecule, a glucosamine unit, which can facilitate uptake via glucose transporters, is conjugated to one end of a bispyrrole triamide unit, which is known to bind to the minor groove of DNA at A/T-rich regions. A methyl sulfonate moiety is tethered to the other end of the bispyrrole unit to serve as a DNA-methylating agent. This molecule produces exclusively N3-methyladenine adducts upon reaction with DNA and is an order of magnitude more toxic to treatment resistant human glioblastoma cells than streptozotocin is, a Food and Drug Administration-approved, glycoconjugated DNA-methylating drug. Cellular uptake studies using a fluorescent analogue of our molecule provide evidence of uptake via glucose transporters and localization within the nucleus of cells. These results demonstrate the feasibility of our strategy for developing more potent anticancer chemotherapeutics, while minimizing common side effects resulting from off-target damage.

Fluorescent Receptor Binding Assay for Detecting Ciguatoxins in Fish

Ciguatera fish poisoning is an illness suffered by > 50,000 people yearly after consumption of fish containing ciguatoxins (CTXs). One of the current methodologies to detect ciguatoxins in fish is a radiolabeled receptor binding assay (RBA_(R)). However, the license requirements and regulations pertaining to radioisotope utilization can limit the applicability of the RBA_(R) in certain labs. A fluorescence based receptor binding assay (RBA_(F)) was developed to provide an alternative method of screening fish samples for CTXs in facilities not certified to use radioisotopes. The new assay is based on competition binding between CTXs and fluorescently labeled brevetoxin-2 (BODIPY^®- PbTx-2) for voltage-gated sodium channel receptors at site 5 instead of a radiolabeled brevetoxin. Responses were linear in fish tissues spiked from 0.1 to 1.0 ppb with Pacific ciguatoxin-3C (P-CTX-3C) with a detection limit of 0.075 ppb. Carribean ciguatoxins were confirmed in Caribbean fish by LC-MS/MS analysis of the regional biomarker (C-CTX-1). Fish (N = 61) of six different species were screened using the RBA_(F). Results for corresponding samples analyzed using the neuroblastoma cell-based assay (CBA-N2a) correlated well (R² = 0.71) with those of the RBA_(F), given the low levels of CTX present in positive fish. Data analyses also showed the resulting toxicity levels of P-CTX-3C equivalents determined by CBA-N2a were consistently lower than the RBA_(F) affinities expressed as % binding equivalents, indicating that a given amount of toxin bound to the site 5 receptors translates into corresponding lower cytotoxicity. Consequently, the RBA_(F), which takes approximately two hours to perform, provides a generous estimate relative to the widely used CBA-N2a which requires 2.5 days to complete. Other RBA_(F) advantages include the long-term (> 5 years) stability of the BODIPY^®- PbTx-2 and having similar results as the commonly used RBA_(R). The RBA_(F) is cost-effective, allows high sample throughput, and is well-suited for routine CTX monitoring programs.

Development of a fluorescence assay for the characterization of brevenal binding to rat brain synaptosomes

The marine dinoflagellate Karenia brevis produces a family of neurotoxins known as brevetoxins. Brevetoxins elicit their effects by binding to and activating voltage-sensitive sodium channels (VSSCs) in cell membranes. K. brevis also produces brevenal, a brevetoxin antagonist, which is able to inhibit and/or negate many of the detrimental effects of brevetoxins. Brevenal binding to VSSCs has yet to be fully characterized, in part due to the difficulty and expense of current techniques. In this study, we have developed a novel fluorescence binding assay for the brevenal binding site. Several fluorescent compounds were conjugated to brevenal to assess their effects on brevenal binding. The assay was validated against the radioligand assay for the brevenal binding site and yielded comparable equilibrium inhibition constants. The fluorescence-based assay was shown to be quicker and far less expensive and did not generate radioactive waste or need facilities for handling radioactive materials. In-depth studies using the brevenal conjugates showed that, while brevenal conjugates do bind to a binding site in the VSSC protein complex, they are not displaced by known VSSC site specific ligands. As such, brevenal elicits its action through a novel mechanism and/or currently unknown receptor site on VSSCs.

A new cytotoxicity assay for brevetoxins using fluorescence microscopy

Brevetoxins are a family of ladder-framed polyether toxins produced during blooms of the marine dinoflagellate, Karenia brevis. Consumption of shellfish or finfish exposed to brevetoxins can lead to the development of neurotoxic shellfish poisoning. The toxic effects of brevetoxins are believed to be due to the activation of voltage-sensitive sodium channels in cell membranes. The traditional cytotoxicity assay for detection of brevetoxins uses the Neuro-2A cell line, which must first be treated with the neurotoxins, ouabain and veratridine, in order to become sensitive to brevetoxins. In this study, we demonstrate several drawbacks of the Neuro-2A assay, which include variability for the EC₅₀ values for brevetoxin and non-linear triphasic dose response curves. Ouabain/veratridine-treated Neuro-2A cells do not show a typical sigmoidal dose response curve in response to brevetoxin, but rather, have a polynomial shaped curve, which makes calculating EC₅₀ values highly variable. We describe a new fluorescence live cell imaging model, which allows for accurate calculation of cytotoxicity via nuclear staining and additional measurement of other viability parameters depending on which aspect of the cell is stained. In addition, the SJCRH30 cell line shows promise as an alternative to Neuro-2A cells for testing brevetoxins without the need for ouabain and veratridine.

Development and utilization of a fluorescence-based receptor-binding assay for the site 5 voltage-sensitive sodium channel ligands brevetoxin and ciguatoxin

Brevetoxins are a family of ladder-frame polyether toxins produced during blooms of the marine dinoflagellate Karenia brevis. Consumption of fish exposed to K. brevis blooms can lead to the development of neurotoxic shellfish poisoning. The toxic effects of brevetoxins are due to activation of voltage-sensitive sodium channels (VSSCs) in cell membranes. Binding of toxins has historically been measured using a radioligand competition assay that is fraught with difficulty. In this study, we developed a novel fluorescence-based binding assay for the brevetoxin receptor. Several fluorophores were conjugated to polyether brevetoxin-2 and used as the labeled ligand. Brevetoxin analogs were able to compete for binding with the fluorescent ligands. This assay was qualified against the standard radioligand receptor assay for the brevetoxin receptor. Furthermore, the fluorescence-based assay was used to determine relative concentrations of toxins in raw extracts of K. brevis culture, and to determine ciguatoxin affinity to site 5 of VSSCs. The fluorescence-based assay was quicker, safer, and far less expensive. As such, this assay can be used to replace the current radioligand assay and will be a vital tool for future experiments examining the binding affinity of various ligands for site 5 on sodium channels.

Brevenal, a brevetoxin antagonist from Karenia brevis, binds to a previously unreported site on mammalian sodium channels

Brevetoxins are a family of ladder-frame polyether toxins produced by the marine dinoflagellate Karenia brevis. During blooms of K. brevis, inhalation of brevetoxins aerosolized by wind and wave action can lead to asthma-like symptoms in persons at the beach. Consumption of either shellfish or finfish contaminated by K. brevis blooms can lead to the development of neurotoxic shellfish poisoning. The toxic effects of brevetoxins are due to binding at a defined site on, and subsequent activation of, voltage-sensitive sodium channels (VSSCs) in cell membranes (site 5). In addition to brevetoxins, K. brevis produces several other ladder-frame compounds. One of these compounds, brevenal, has been shown to antagonize the effects of brevetoxin. In an effort to further characterize to effects of brevenal, a radioactive analog ([³H]-brevenol) was produced by reducing the side-chain terminal aldehyde moiety of brevenal to an alcohol using tritiated sodium borohydride. A K_D of 67 nM and B_max of 7.1 pmol/mg protein were obtained for [³H]-brevenol in rat brain synaptosomes, suggesting a 1:1 matching with VSSCs. Brevenal and brevenol competed for [³H]-brevenol binding with K_i values of 75 nM and 56 nM, respectively. However, although both brevenal and brevenol can inhibit brevetoxin binding, brevetoxin was completely ineffective at competition for [³H]-brevenol binding. After examining other site-specific compounds, it was determined that [³H]-brevenol binds to a site that is distinct from the other known sites including the brevetoxin site (site 5) although some interaction with site 5 is apparent.

Development of a competitive fluorescence-based synaptosome binding assay for brevetoxins

Brevetoxins are a family of ladder-frame polyether toxins produced during blooms of the marine dinoflagellate Karenia brevis. Inhalation of brevetoxins aerosolized by wind and wave action can lead to asthma-like symptoms in beach goers. Consumption of either shellfish or finfish exposed to K. brevis blooms can lead to the development of neurotoxic shellfish poisoning. The toxic effects of brevetoxins are due to activation of voltage-sensitive sodium channels (VSSCs) in cell membranes. Binding of brevetoxin analogs and competitors to site 5 on these channels has historically been measured using a radioligand competition assay that is fraught with difficulty, including slow analysis time, production of radioactive waste, and cumbersome and expensive methods associated with the generation of radioactive labeled ligands. In this study, we describe the development of a novel fluorescent synaptosome binding assay for the brevetoxin receptor. BODIPY(®)-conjugated to PbTx-2 was used as the labeled ligand. The BODIPY(®)-PbTx-2 conjugate was found to displace [(3)H]-PbTx-3 from its binding site on VSSCs on rat brain synaptosomes with an equilibrium inhibition constant of 0.11 nM. We have shown that brevetoxin A and B analogs are all able to compete for binding with the fluorescent ligand. Most importantly, this assay was validated against the current site 5 receptor binding assay standard, the radioligand receptor assay for the brevetoxin receptor using [(3)H]-PbTx-3 as the labeled ligand. The fluorescence based assay yielded equilibrium inhibition constants comparable to the radioligand assay for all brevetoxin analogs. The fluorescence based assay was quicker, far less expensive, and did not generate radioactive waste or need radioactive facilities. As such, this fluorescence-based assay can be used to replace the current radioligand assay for site 5 on voltage-sensitive sodium channels and will be a vital tool for future experiments examining the binding affinity of various ligands for site 5 on sodium channels.

Compositional changes in neurotoxins and their oxidative derivatives from the dinoflagellate, Karenia brevis, in seawater and marine aerosol

The harmful alga, Karenia brevis, produces a suite of polyether neurotoxins, brevetoxins or PbTx, that cause marine animal mortality and neurotoxic shellfish poisoning (NSP). A characteristic of K. brevis blooms is associated airborne toxins that result in severe respiratory problems. This study was undertaken to determine the composition of aerosolized brevetoxins and oxidative derivatives to which beachgoers are exposed during a K. brevis bloom. The suite of brevetoxins and derivatives in seawater is comprised of intra-cellular (IC) and extra-cellular (EC) compounds. We hypothesized that aerosolized compounds are generated primarily from EC, hydrophobic compounds in seawater by bubble-mediated transport. Thus the composition of aerosolized brevetoxins and derivatives, to which beachgoers are exposed, would reflect the EC composition of the source matrix (the local surf zone). Brevetoxins were extracted from water collected along the shore and from marine aerosols along Siesta Beach and Lido Beach in Sarasota, FL, USA, during K. brevis blooms. Water samples were further processed into IC and EC components. The primary brevetoxins observed in water and air included PbTx-1, -2, -3, -PbTx-2-carboxylic acid, and brevenal. Oxidation and/or hydrolysis products of PbTx-1, -2, -3 and -7 were also found in EC water and in aerosol, but not IC.

Review of Florida Red Tide and Human Health Effects

This paper reviews the literature describing research performed over the past decade on the known and possible exposures and human health effects associated with Florida red tides. These harmful algal blooms are caused by the dinoflagellate, Karenia brevis, and similar organisms, all of which produce a suite of natural toxins known as brevetoxins. Florida red tide research has benefited from a consistently funded, long term research program, that has allowed an interdisciplinary team of researchers to focus their attention on this specific environmental issue-one that is critically important to Gulf of Mexico and other coastal communities. This long-term interdisciplinary approach has allowed the team to engage the local community, identify measures to protect public health, take emerging technologies into the field, forge advances in natural products chemistry, and develop a valuable pharmaceutical product. The Review includes a brief discussion of the Florida red tide organisms and their toxins, and then focuses on the effects of these toxins on animals and humans, including how these effects predict what we might expect to see in exposed people.

Brevetoxin inhalation alters the pulmonary response to influenza A in the male F344 rat

Epidemiological studies demonstrated that the number of emergency-room visits for respiratory indications increases during periods of Florida Red Tides. The purpose of this study was to examine whether or not repeated brevetoxin inhalation, as may occur during a Florida Red Tide, affects pulmonary responses to influenza A. Male F344 rats were divided into four groups: (1) sham aerosol/no influenza; (2) sham aerosol/influenza; (3) brevetoxin/no influenza; and (4) brevetoxin/influenza. Animals were exposed by nose-only inhalation to vehicle or 50 μg brevetoxin-3/m3, 2 h/d for 12 d. On d 6 of aerosol exposure, groups 2 and 4 were administered 10,000 plaque-forming units of influenza A, strain HKX-31 (H3N2), by intratracheal instillation. Subgroups were euthanized at 2, 4, and 7 d post influenza treatment. Lungs were evaluated for viral load, cytokine content, and histopathologic changes. Influenza virus was cleared from the lungs over the 7-d period; however, there was significantly more virus remaining in the group 4 lungs compared to group 2. Influenza virus significantly increased interleukins-1α and -6 and monocyte chemotactic protein-1 in lung; brevetoxin exposure significantly enhanced the influenza-induced response. At 7 d, the severity of perivascular and peribronchiolar inflammatory cell infiltrates was greatest in group 4. Bronchiolitis persisted, with low incidence and severity, only in group 4 at d 7. These results suggest that repeated inhalation exposure to brevetoxin may delay virus particle clearance and recovery from influenza A infection in the rat lung.

Absolute configuration of brevisamide and brevisin: confirmation of a universal biosynthetic process for Karenia brevis polyethers

The discovery of brevisin, the first example of an "interrupted" polycyclic ether, obtained from the dinoflagellate Karenia brevis, posed some important questions regarding the mechanism of the cyclization process. Consequently, we have established absolute configurations of brevisin and its related metabolite brevisamide using a modified Mosher's esterification method. For brevisin, analysis was carried out on both the 31-monokis- and the 10,31-bis-MTPA esters. The results suggest that both metabolites, like other polyethers from K. brevis, result from polyepoxide precursors with uniform (S, S) configurations for all epoxides and provide further support for a universal stereochemical model for dinoflagellate polyether formation.

Characterization of tamulamides A and B, polyethers isolated from the marine dinoflagellate Karenia brevis

Florida red tides occur as the result of blooms of the marine dinoflagellate Karenia brevis. K. brevis is known to produce several families of fused polyether ladder compounds. The most notable compounds are the brevetoxins, potent neurotoxins that activate mammalian sodium channels. Additional fused polyether ladder compounds produced by K. brevis include brevenal, brevisin, and hemibrevetoxin B, all with varying affinities for the same binding site on voltage-sensitive sodium channels. The structure elucidation and biological activity of two additional fused polyether ladder compounds containing seven fused ether rings will be described in this paper. Tamulamide A (MW = 638.30) and tamulamide B (MW = 624.29) were isolated from K. brevis cultures, and their structures elucidated using a combination of NMR spectroscopy and high-resolution mass spectrometry. Tamulamides A and B were both found to compete with tritiated brevetoxin-3 ([(3)H]-PbTx-3) for its binding site on rat brain synaptosomes. However, unlike the brevetoxins, tamulamides A and B showed no toxicity to fish at doses up to 200 nM and did not cause significant bronchoconstriction in sheep pulmonary assays.

UV and solar TiO(2) photocatalysis of brevetoxins (PbTxs)

Karenia brevis, the harmful alga associated with red tide, produces brevetoxins (PbTxs). Exposure to these toxins can have a negative impact on marine wildlife and serious human health consequences. The elimination of PbTxs is critical to protect the marine environment and human health. TiO(2) photocatalysis under 350 nm and solar irradiation leads to significant degradation of PbTxs via first order kinetics. ELISA results demonstrate TiO(2) photocatalysis leads to a significant decrease in the bioactivity of PbTxs as a function of treatment time. Experiments conducted in the presence of synthetic seawater, humic material and a hydroxyl scavenger showed decreased degradation. PbTxs are highly hydrophobic and partition to organic microlayer on the ocean surface. Acetonitrile was employed to probe the influence of an organic media on the TiO(2) photocatalysis of PbTxs. Our results indicate TiO(2) photocatalysis may be applicable for the degradation of PbTxs.

Brevisin: an aberrant polycyclic ether structure from the dinoflagellate Karenia brevis and its implications for polyether assembly

Brevisin is an unprecedented polycyclic ether isolated from the dinoflagellate Karenia brevis, an organism well-known to produce complex polycyclic ethers. The structure of brevisin was determined by detailed analyses of MS and 2D NMR spectra and is remarkable in that it consists of two separate fused polyether ring assemblies linked by a methylene group. One of the polycyclic moieties contains a conjugated aldehyde side chain similar to that recently observed in other K. brevis metabolites, though the "interrupted" polyether structure of brevisin is novel and provides further insight into the biogenesis of such fused-ring polyether systems. On the basis of the unusual structure of brevisin, principles underlying the initiation of polyether assemblies are proposed. Brevisin was found to inhibit the binding of [(3)H]-PbTx-3 to its binding site on the voltage-sensitive sodium channels in rat brain synaptosomes.

Brevenal inhibits pacific ciguatoxin-1B-induced neurosecretion from bovine chromaffin cells

Ciguatoxins and brevetoxins are neurotoxic cyclic polyether compounds produced by dinoflagellates, which are responsible for ciguatera and neurotoxic shellfish poisoning (NSP) respectively. Recently, brevenal, a natural compound was found to specifically inhibit brevetoxin action and to have a beneficial effect in NSP. Considering that brevetoxin and ciguatoxin specifically activate voltage-sensitive Na⁺ channels through the same binding site, brevenal has therefore a good potential for the treatment of ciguatera. Pacific ciguatoxin-1B (P-CTX-1B) activates voltage-sensitive Na⁺ channels and promotes an increase in neurotransmitter release believed to underpin the symptoms associated with ciguatera. However, the mechanism through which slow Na⁺ influx promotes neurosecretion is not fully understood. In the present study, we used chromaffin cells as a model to reconstitute the sequence of events culminating in ciguatoxin-evoked neurosecretion. We show that P-CTX-1B induces a tetrodotoxin-sensitive rise in intracellular Na⁺, closely followed by an increase in cytosolic Ca²⁺ responsible for promoting SNARE-dependent catecholamine secretion. Our results reveal that brevenal and β-naphtoyl-brevetoxin prevent P-CTX-1B secretagogue activity without affecting nicotine or barium-induced catecholamine secretion. Brevenal is therefore a potent inhibitor of ciguatoxin-induced neurotoxic effect and a potential treatment for ciguatera.

Brevisamide: an unprecedented monocyclic ether alkaloid from the dinoflagellate Karenia brevis that provides a potential model for ladder-frame initiation

The dinoflagellate Karenia brevis is known for the production of brevetoxins, a family of polycyclic ether toxins, as well as their antagonist brevenal. Further examination of organic extracts of K. brevis has uncovered yet another unprecedented cyclic ether alkaloid named brevisamide. This report describes the structure elucidation of brevisamide based on detailed MS and NMR spectral analysis, and the importance of this new compound in shedding light on the biogenesis of fused polyethers is discussed.

Total synthesis, structure revision, and absolute configuration of (-)-brevenal

Total synthesis of structure 1 originally proposed for brevenal, a nontoxic polycyclic ether natural product isolated from the Florida red tide dinoflagellate, Karenia brevis, was accomplished. The key features of the synthesis involved (i) convergent assembly of the pentacyclic polyether skeleton based on our developed Suzuki-Miyaura coupling chemistry and (ii) stereoselective construction of the multi-substituted (E,E)-dienal side chain by using copper(I) thiophen-2-carboxylate (CuTC)-promoted modified Stille coupling. The disparity of NMR spectra between the synthetic material and the natural product required a revision of the proposed structure. Detailed spectroscopic comparison of synthetic 1 with natural brevenal, coupled with the postulated biosynthetic pathway for marine polyether natural products, suggested that the natural product was most likely represented by 2, the C26 epimer of the proposed structure 1. The revised structure was finally validated by completing the first total synthesis of (-)-2, which also unambiguously established the absolute configuration of the natural product.

The effect of brevenal on brevetoxin-induced DNA damage in human lymphocytes

Brevenal is a nontoxic short-chain trans-syn polyether that competes with brevetoxin (PbTx) for the active site on voltage-sensitive sodium channels. The PbTxs are highly potent polyether toxins produced during blooms of several species of marine dinoflagellates, most notably Karenia brevis. Blooms of K. brevis have been associated with massive fish kills, marine mammal poisoning, and are potentially responsible for adverse human health effects such as respiratory irritation and airway constriction in beach-goers. Additionally, the consumption of shellfish contaminated with PbTxs results in neurotoxic shellfish poisoning (NSP). The purpose of the present study was to determine whether PbTx could induce DNA damage in a human cell type, the lymphocyte, and if so, whether the damage could be antagonized or ameliorated by brevenal, a brevetoxin antagonist. The DNA damage may occur through both endogenous and exogenous physiological and pathophysiological processes. Unrepaired or erroneously repaired DNA damage may result in gene mutation, chromosome aberration, and modulation of gene regulation, which have been associated with immunotoxicity and carcinogenesis. A single-cell gel electrophoresis assay, or comet assay, was used to determine and compare DNA damage following various treatments. The data were expressed as tail moments, which is the percentage of DNA in the tail multiplied by the length between the center of the head and center of the tail (in arbitrary units). The negative control tail moment was 29.2 (SE=+/-0.9), whereas the positive control (hydrogen peroxide) was 72.1 (1.5) and solvent (ethanol) was 24.2 (2.1). The PbTx-2 (from Sigma, St. Louis, MO, USA), 10(-8) M was 41.3 (3.6), PbTx-9 (Sigma), 10(-8) M was 57.0 (5.3), PbTx-2 (from University of North Carolina at Wilmington, UNCW), 10(-8) M was 49.4 (9.9), and PbTx-3 (UNCW), 10(-8) M was 64.0 (6.4). 1.0 microg/ml brevenal applied 1 h before the PbTxs protected the lymphocytes from DNA damage; PbTx-2 (Sigma), 31.3 (2.1); PbTx-9 (Sigma), 35.5 (2.9); PbTx-2 (UNCW), 33.9 (1.4); PbTx-3 (UNCW), 34.9 (1.25). The tail moment for 1.0 mug/ml brevenal alone was 30.8 (2.6). The results indicate that extensive genotoxic damage is induced by PbTx-2 and 9 (Sigma), and PbTx-2 and 3 (UNCW) in normal human lymphocytes, which is fully antagonized by brevenal. This suggests that the immune systems of individuals exposed to PbTx during harmful algal bloom (HAB) events may be at risk.

Natural and derivative brevetoxins: historical background, multiplicity, and effects

Symptoms consistent with inhalation toxicity have long been associated with Florida red tides, and various causal agents have been proposed. Research since 1981 has centered on a group of naturally occurring trans-fused cyclic polyether compounds called brevetoxins that are produced by a marine dinoflagellate known as Karenia brevis. Numerous individual brevetoxins have been identified from cultures as well as from natural bloom events. A spectrum of brevetoxin derivatives produced by chemical modification of the natural toxins has been prepared to examine the effects of functional group modification on physiologic activity. Certain structural features of natural and synthetic derivatives of brevetoxin appear to ascribe specific physiologic consequences to each toxin. Differential physiologic effects have been documented with many of the natural toxins and derivatives, reinforcing the hypothesis that metabolism or modification of toxin structures modulates both the specific toxicity (lethality on a per milligram basis) and potentially the molecular mechanism(s) of action. A series of naturally occurring fused-ring polyether compounds with fewer rings than brevetoxin, known as brevenals, exhibit antagonistic properties and counteract the effects of the brevetoxins in neuronal and pulmonary model systems. Taken together, the inhalation toxicity of Florida red tides would appear to depend on the amount of each toxin present, as well as on the spectrum of molecular activities elicited by each toxin. Toxicity in a bloom is diminished by the amount brevenal present.

Inhalation toxicity of brevetoxin 3 in rats exposed for twenty-two days

Brevetoxins are potent neurotoxins produced by the marine dinoflagellate Karenia brevis. Exposure to brevetoxins may occur during a K. brevis red tide when the compounds become aerosolized by wind and surf. This study assessed possible adverse health effects associated with inhalation exposure to brevetoxin 3, one of the major brevetoxins produced by K. brevis and present in aerosols collected along beaches affected by red tide. Male F344 rats were exposed to brevetoxin 3 at 0, 37, and 237 microg/m3 by nose-only inhalation 2 hr/day, 5 days/week for up to 22 exposure days. Estimated deposited brevetoxin 3 doses were 0.9 and 5.8 microg/kg/day for the low- and high-dose groups, respectively. Body weights of the high-dose group were significantly below control values. There were no clinical signs of toxicity. Terminal body weights of both low- and high-dose-group rats were significantly below control values. Minimal alveolar macrophage hyperplasia was observed in three of six and six of six of the low- and high-dose groups, respectively. No histopathologic lesions were observed in the nose, brain, liver, or bone marrow of any group. Reticulocyte numbers in whole blood were significantly increased in the high-dose group, and mean corpuscular volume showed a significant decreasing trend with increasing exposure concentration. Humoral-mediated immunity was suppressed in brevetoxin-exposed rats as indicated by significant reduction in splenic plaque-forming cells in both low- and high-dose-group rats compared with controls. Results indicate that the immune system is the primary target for toxicity in rats after repeated inhalation exposure to relatively high concentrations of brevetoxins.

Effects of inhaled brevetoxins in allergic airways: toxin-allergen interactions and pharmacologic intervention

During a Florida red tide, brevetoxins produced by the dinoflagellate Karenia brevis become aerosolized and cause airway symptoms in humans, especially in those with pre-existing airway disease (e.g., asthma). To understand these toxin-induced airway effects, we used sheep with airway hypersensitivity to Ascaris suum antigen as a surrogate for asthmatic patients and studied changes in pulmonary airflow resistance (R(L) after inhalation challenge with lysed cultures of K. brevis (crude brevetoxins). Studies were done without and with clinically available drugs to determine which might prevent/reverse these effects. Crude brevetoxins (20 breaths at 100 pg/mL; n = 5) increased R(L) 128 +/- 6% (mean +/- SE) over baseline. This bronchoconstriction was significantly reduced (% inhibition) after pretreatment with the glucocorticosteroid budesonide (49%), the beta(2) adrenergic agent albuterol (71%), the anticholinergic agent atropine (58%), and the histamine H1-antagonist diphenhydramine (47%). The protection afforded by atropine and diphenhydramine suggests that both cholinergic (vagal) and H1-mediated pathways contribute to the bronchoconstriction. The response to cutaneous toxin injection was also histamine mediated. Thus, the airway and skin data support the hypothesis that toxin activates mast cells in vivo. Albuterol given immediately after toxin challenge rapidly reversed the bronchoconstriction. Toxin inhalation increased airway kinins, and the response to inhaled toxin was enhanced after allergen challenge. Both factors could contribute to the increased sensitivity of asthmatic patients to toxin exposure. We conclude that K. brevis aerosols are potent airway constrictors. Clinically available drugs may be used to prevent or provide therapeutic relief for affected individuals.

Airway responses to aerosolized brevetoxins in an animal model of asthma

Florida red tide brevetoxins are sodium channel neurotoxins produced by the dinoflagellate Karenia brevis. When aerosolized, the toxin causes airway symptoms in normal individuals and patients with airway disease, but systematic exposures to define the pulmonary consequences and putative mechanisms are lacking. Here we report the effects of airway challenges with lysed cultures of Karenia brevis (crude brevetoxin), pure brevetoxin-2, brevetoxin-3, and brevetoxin-tbm (brevetoxin-2 minus the side chain) on pulmonary resistance and tracheal mucus velocity, a marker of mucociliary clearance, in allergic and nonallergic sheep. Picogram concentrations of toxin caused bronchoconstriction in both groups of sheep. Brevetoxin-tbm was the least potent, indicating the importance of the side chain for maximum effect. Both histamine H(1)- and cholinergic-mediated pathways contributed to the bronchoconstriction. A synthetic antagonist, beta-naphthoyl-brevetoxin-3, and brevenal, a natural antagonist, inhibited the bronchoconstriction. Only crude brevetoxin and brevetoxin-3 decreased tracheal mucus velocity; both antagonists prevented this. More importantly, picomolar concentrations of the antagonists alone improved tracheal mucus velocity to the degree seen with mM concentrations of the sodium channel blocker amiloride. Thus, Karenia brevis, in addition to producing toxins that adversely affect the airways, may be a source of agents for treating mucociliary dysfunction.

A new polyether ladder compound produced by the dinoflagellate Karenia brevis

A new ladder-frame polyether compound containing five fused ether rings was isolated from laboratory cultures of the marine dinoflagellate Karenia brevis. This compound, named brevenal, and its dimethyl acetal derivative both competitively displace brevetoxin from its binding site in rat brain synaptosomes. Significantly, these compounds are also nontoxic to fish and antagonize the toxic effects of brevetoxins in fish. The structure and biological activity of brevenal, as well as the dimethyl acetal derivative, are described in this paper.

Brevenal is a natural inhibitor of brevetoxin action in sodium channel receptor binding assays

1. Florida red tides produce profound neurotoxicity that is evidenced by massive fish kills, neurotoxic shellfish poisoning, and respiratory distress. Red tides vary in potency, potency that is not totally governed by toxin concentration. The purpose of the study was to understand the variable potency of red tides by evaluating the potential for other natural pharmacological agents which could modulate or otherwise reduce the potency of these lethal environmental events. 2. A synaptosome binding preparation with 3-fold higher specific brevetoxin binding was developed to detect small changes in toxin binding in the presence of potential antagonists. Rodent brain labeled in vitro with tritiated brevetoxin shows high specific binding in the cerebellum as evidenced by autoradiography. Synaptosome binding assays employing cerebellum-derived synaptosomes illustrate 3-fold increased specific binding. 3. A new polyether natural product from Florida's red tide dinoflagellate Karenia brevis, has been isolated and characterized. Brevenal, as the nontoxic natural product is known, competes with tritiated brevetoxin for site 5 associated with the voltage-sensitive sodium channel (VSSC). Brevenal displacement of specific brevetoxin binding is purely competitive in nature. 4. Brevenal, obtained from either laboratory cultures or field collections during a red tide, protects fish from the neurotoxic effects of brevetoxin exposure. 5. Brevenal may serve as a model compound for the development of therapeutics to prevent or reverse intoxication in red tide exposures.

Florida Red Tide: Inhalation Toxicity of Karenia brevis Extract in Rats

Brevetoxins are neurotoxins produced by the marine dinoflagellate Karenia brevis. Histopathologic examination of marine mammals dying following repeated exposure of brevetoxins during red tide events suggests that the respiratory tract, nervous, hematopoietic, and immune systems are potential targets for toxicity in repeatedly exposed individuals. The purpose of this experiment was to evaluate the effects of repeated inhalation of K. brevis extract on these potential target systems in rats. Male Sprague-Dawley rats were exposed four hours/day, five days/week for up to four weeks to target concentrations of 200 and 1000 μg/L K. brevis extract (approximately 50 and 200 μg/L brevetoxin-like compounds; positive neurotoxicity in a fish bioassay). Control rats were sham exposed to air. Immunohistochemical staining of pulmonary macrophages indicated deposition of brevetoxin-like compound within the lung. However, exposure resulted in no clinical signs of toxicity or behavioral changes. There were no adverse effects on hematology or serum chemistry. No histopathological changes were observed in the nose, lung, liver, kidneys, lymph nodes, spleen, or brain of exposed rats. Immune suppression was suggested by reduced responses of spleen cells in the IgM-specific antibody-forming plaque cell response assay and reduced responses of lymphocytes to mitogen stimulation in vitro. Differences between responses observed in rats in this study and those observed in manatees may be a function of dose or species differences in sensitivity.

Brevetoxin Degradation and By-Product Formation via Natural Sunlight

We investigated the effects of solar radiation on brevetoxin (PbTx2). Our findings suggest that natural sunlight mediates brevetoxin (PbTx2) degradation and results in brevetoxin by-product formation via photochemical processes.

Toxicogenomic effects of marine brevetoxins in liver and brain of mouse

Although the polyether brevetoxins (PbTx's) produced by Karenia brevis (the organism responsible for blooms of the Florida red tide) are known to exert their acute toxic effects through ion-channel mediated pathways in neural tissue, prior studies have also demonstrated that at least one form of the toxin (PbTx-6) is bound avidly by the aryl hydrocarbon receptor (AhR). Since AhR binding of a prototypical ligand such as dioxin is the first step in a cascade pathway producing major changes in gene expression, we reasoned that PbTx-6 might produce similar genomic-wide changes in expression. Mice were injected i.p. with sub-lethal doses of PbTx's (either 1.5 or 3 mg/g body weight of PbTx-6; or 0.15 mg/g body weight of PbTx-2, a toxin not avidly bound by the AhR), and liver and brain tissues were sampled at 8, 24 and 72 h and RNA was isolated. Changes in gene-specific RNA levels were assessed using commercially available mouse cDNA arrays (Incyte) containing >9600 array elements, including many elements from AhR-mediated genes. Histopathology of the two organs was also assessed. We observed minor histopathological effects and a total of only 29 significant (>2.0-fold) changes in gene expression, most of which occurred in the liver, and most of which could be attributable to an 'acute phase' inflammatory response. These results argue against the hypothesis that PbTx-6 acts via a classic AhR-mediated mechanism to evoke gene expression changes. However, given the avidity with which PbTx-6 binds to the AhR, these findings have important implications for how PbTx's may act in concert with other toxicants that are sensed by the AhR.

New fish-killing alga in coastal Delaware produces neurotoxins

Ten fish mortality events, involving primarily Atlantic menhaden, occurred from early July through September 2000 in several bays and creeks in Delaware, USA. Two events involved large mortalities estimated at 1-2.5 million fish in Bald Eagle Creek, Rehoboth Bay. Samples from Indian Inlet (Bethany Beach), open to the Atlantic, as well as from an enclosed area of massive fish kills at nearby Bald Eagle Creek and Torque Canal were collected and sent to our laboratory for analysis. Microscopic examination of samples from the fish kill site revealed the presence of a single-cell Raphidophyte alga Chattonella cf. verruculosa at a maximum density of 1.04 x 10(7) cells/L. Naturally occurring brevetoxins were also detected in the bloom samples. Besides the Chattonella species, no other known brevetoxin-producing phytoplankton were present. Chromatographic, immunochemical, and spectroscopic analyses confirmed the presence of brevetoxin PbTx-2, and PbTx-3 and -9 were confirmed by chromatographic and immunochemical analyses. This is the first confirmed report in the United States of brevetoxins associated with an indigenous bloom in temperate Atlantic estuarine waters and of C. cf. verruculosa as a resident toxic organism implicated in fish kills in this area. The bloom of Chattonella continued throughout September and eventually declined in October. By the end of October C. cf. verruculosa was no longer seen, nor was toxin measurable in the surface waters. The results affirm that to avoid deleterious impacts on human and ecosystem health, increased monitoring is needed for brevetoxins and organism(s) producing them, even in areas previously thought to be unaffected.

The effects of haloperidol and clozapine on extracellular GABA levels in the prefrontal cortex of the rat: an in vivo microdialysis study

Recent electrophysiological and pharmacological data indicate that dopamine enhances the activity of interneurons in the prefrontal cortex (PFC) and induces the release of GABA from these cells. We used in vivo microdialysis to examine the effects of two dopamine receptor antagonists on GABA release in the prefrontal cortex of awake, freely moving rats. Depolarization accomplished by local perfusion of potassium chloride or veratradine markedly increased extracellular GABA levels in the PFC. In contrast, local perfusion of TTX reduced extracellular GABA levels in the PFC. These data indicate that extracellular GABA is derived in part from neurons, and that extracellular levels of the inhibitory amino acid are impulse dependent. The acute administration of haloperidol weakly but significantly decreased extracellular GABA levels in the PFC; no effect of haloperidol on striatal extracellular GABA levels was observed. Systemic administration of the atypical antipsychotic drug clozapine markedly reduced extracellular GABA levels in the PFC, but did not alter striatal GABA levels. Thus, release of GABA from interneurons in the PFC is inhibited by two antipsychotic drugs. These data may suggest that different D2-like dopamine receptors are localized to pyramidal and nonpyramidal neurons in the cortex.

Modulation of extracellular gamma-aminobutyric acid in the ventral pallidum using in vivo microdialysis

Intracranial microdialysis was used to investigate the origin of extracellular gamma-aminobutyric acid (GABA) in the ventral pallidum. Changes in basal GABA levels in response to membrane depolarizers, ion-channel blockers, and receptor agonists were determined. Antagonism of Ca2+ fluxes with high Mg2+ in a Ca(2+)-free perfusion buffer decreased GABA levels by up to 30%. Inhibition of voltage-dependent Na+ channels by the addition of tetrodotoxin also significantly decreased basal extracellular GABA concentrations by up to 45%, and blockade of Ca2+ and Na+ channels with verapamil reduced extracellular GABA by as much as 30%. The addition of either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, produced a 40% reduction in extracellular GABA. GABA release was stimulated by high K+ and the addition of veratridine to increase Na+ influx. High K(+)-induced release was predominantly Ca(2+)-dependent, whereas the effect of veratridine was potentiated in the absence of extracellular Ca2+. Both high K(+)- and veratridine-induced elevations in extracellular GABA were inhibited by baclofen, whereas only veratridine-induced release was antagonized by muscimol. These results demonstrate that at least 50% of basal extracellular GABA in the ventral pallidum is derived from Ca(2+)- or Na(+)-dependent mechanisms. They also suggest that Na(+)-dependent release of GABA via reversal of the uptake carrier can be shown in vivo.