Mathematical modeling of heterogeneous electrophysiological responses in human β-cells

Electrical activity plays a pivotal role in glucose-stimulated insulin secretion from pancreatic β-cells. Recent findings have shown that the electrophysiological characteristics of human β-cells differ from their rodent counterparts. We show that the electrophysiological responses in human β-cells to a range of ion channels antagonists are heterogeneous. In some cells, inhibition of small-conductance potassium currents has no effect on action potential firing, while it increases the firing frequency dramatically in other cells. Sodium channel block can sometimes reduce action potential amplitude, sometimes abolish electrical activity, and in some cells even change spiking electrical activity to rapid bursting. We show that, in contrast to L-type Ca2+-channels, P/Q-type Ca2+-currents are not necessary for action potential generation, and, surprisingly, a P/Q-type Ca2+-channel antagonist even accelerates action potential firing. By including SK-channels and Ca2+ dynamics in a previous mathematical model of electrical activity in human β-cells, we investigate the heterogeneous and nonintuitive electrophysiological responses to ion channel antagonists, and use our findings to obtain insight in previously published insulin secretion measurements. Using our model we also study paracrine signals, and simulate slow oscillations by adding a glycolytic oscillatory component to the electrophysiological model. The heterogenous electrophysiological responses in human β-cells must be taken into account for a deeper understanding of the mechanisms underlying insulin secretion in health and disease, and as shown here, the interdisciplinary combination of experiments and modeling increases our understanding of human β-cell physiology.

First identification of 5,11-dideoxytetrodotoxin in marine animals, and characterization of major fragment ions of tetrodotoxin and its analogs by high resolution ESI-MS/MS

Even though tetrodotoxin (TTX) is a widespread toxin in marine and terrestrial organisms, very little is known about the biosynthetic pathway used to produce it. By describing chemical structures of natural analogs of TTX, we can start to identify some of the precursors that might be important for TTX biosynthesis. In the present study, an analog of TTX, 5,11-dideoxyTTX, was identified for the first time in natural sources, the ovary of the pufferfish and the pharynx of a flatworm (planocerid sp. 1), by comparison with totally synthesized (-)-5,11-dideoxyTTX, using high resolution ESI-LC-MS. Based on the presence of 5,11-dideoxyTTX together with a series of known deoxy analogs, 5,6, 11-trideoxyTTX, 6,11-dideoxyTTX, 11-deoxyTTX, and 5-deoxyTTX, in these animals, we predicted two routes of stepwise oxidation pathways in the late stages of biosynthesis of TTX. Furthermore, high resolution masses of the major fragment ions of TTX, 6,11-dideoxyTTX, and 5,6,11-trideoxyTTX were also measured, and their molecular formulas and structures were predicted to compare them with each other. Although both TTX and 5,6,11-trideoxyTTX give major fragment ions that are very close, m/z 162.0660 and 162.1020, respectively, they are distinguishable and predicted to be different molecular formulas. These data will be useful for identification of TTXs using high resolution LC-MS/MS.

Isolation and structural determination of the first 8-epi-type tetrodotoxin analogs from the newt, Cynops ensicauda popei, and comparison of tetrodotoxin analogs profiles of this newt and the puffer fish, Fugu poecilonotus

Identification of new tetrodotoxin (TTX) analogs from TTX-possessing animals might provide insight into its biosynthesis and metabolism. In this study, four new analogs, 8-epi-5,6,11-trideoxyTTX, 4,9-anhydro-8-epi-5,6,11-trideoxyTTX, 1-hydroxy-8-epi-5,6,11-trideoxyTTX, and 1-hydroxy-4,4a-anhydro-8-epi-5,6,11-trideoxyTTX, were isolated from the newt, Cynops ensicauda popei, and their structures were determined using spectroscopic methods. These are the first 8-epi-type analogs of TTX that have been found in a natural source. Furthermore, we examined the composition of the TTX analogs in this newt and in the ovary of the puffer fish, Fugu poecilonotus, using LC/MS. The results indicate that TTX and 11-deoxyTTX were present in both sources. However, 6-epiTTX and 8-epi-type analogs were detected only in the newt, while 5,6,11-trideoxyTTX was a specific and major analog in the puffer fish. Such considerable differences among analog compositions might reflect differences in the biosynthesis or metabolism of TTX between these animals.

The chemical synthesis of tetrodoxin: an ongoing quest

This contribution reviews all the synthetic work on tetrodotoxin that has appeared in the literature through June 2011.

On the origins and biosynthesis of tetrodotoxin

The potent neurotoxin tetrodotoxin (TTX) has been identified from taxonomically diverse marine organisms. TTX possesses a unique cage-like structure, however, its biosynthesis has yet to be elucidated. Biosynthetic studies in the TTX-producing newt Taricha torosa, and in bacterial genera, including Vibrio, have proven inconclusive. Indeed, very few studies have been performed that address the cellular production of TTX. Here we review the sources of TTX described to date and provide evidence for the biosynthesis of TTX by symbiotic microorganisms in higher taxa. Chemical and genetic based biosynthesis studies of TTX undertaken thus far are discussed and we outline approaches which may be useful for expanding upon the current body of knowledge. The complex biosynthesis of structurally similar toxins, that reveal clues into the biosynthetic pathway of TTX, is also presented.

Surface plasmon resonance biosensor for determination of tetrodotoxin: prevalidation study

A label-free surface plasmon resonance biosensor method was applied to determine tetrodotoxin (TTX) in pufferfish matrixes using an antibody inhibition assay format. A prevalidation study was conducted to demonstrate the assay performance characteristics, such as selectivity, LOD, LOQ, repeatability, reproducibility, and accuracy. Three participating laboratories reported standard curves in buffer and pufferfish matrix. A set of blind samples with TTX spiked into buffer as well as in 10% pufferfish extract were analyzed. Additionally, three blind naturally contaminated samples were analyzed, and the results were compared to those obtained using a reference method (HPLC/electrospray ionization-selected reaction monitoring-MS). The developed method was demonstrated to be capable of detecting TTX in pufferfish matrix standard samples in a broad concentration range (2-9000 ng/mL) with an LOD of 1.5 ng/mL. Between-laboratory recovery values were in the range of 51-190% with a mean of 107%, and 64-180% with a mean of 103% for TTX-spiked samples in buffer and pufferfish matrix, respectively. Between-laboratory recoveries were in the satisfactory range of 101-119% for naturally contaminated samples. This robust, rapid, and noninvasive method may serve as an attractive alternative to established methods for detection of TTX in pufferfish extracts.

The chemical and evolutionary ecology of tetrodotoxin (TTX) toxicity in terrestrial vertebrates

Tetrodotoxin (TTX) is widely distributed in marine taxa, however in terrestrial taxa it is limited to a single class of vertebrates (Amphibia). Tetrodotoxin present in the skin and eggs of TTX-bearing amphibians primarily serves as an antipredator defense and these taxa have provided excellent models for the study of the evolution and chemical ecology of TTX toxicity. The origin of TTX present in terrestrial vertebrates is controversial. In marine organisms the accepted hypothesis is that the TTX present in metazoans results from either dietary uptake of bacterially produced TTX or symbiosis with TTX producing bacteria, but this hypothesis may not be applicable to TTX-bearing amphibians. Here I review the taxonomic distribution and evolutionary ecology of TTX in amphibians with some attention to the origin of TTX present in these taxa.

Sodium channel inhibiting marine toxins

Saxitoxin (STX), tetrodotoxin (TTX) and their many chemical relatives are part of our daily lives. From killing people who eat seafood containing these toxins, to being valuable research tools unveiling the invisible structures of their pharmacological receptor, their global impact is beyond measure. The pharmacological receptor for these toxins is the voltage-gated sodium channel which transports Na ions between the exterior to the interior of cells. The two structurally divergent families of STX and TTX analogues bind at the same location on these Na channels to stop the flow of ions. This can affect nerves, muscles and biological senses of most animals. It is through these and other toxins that we have developed much of our fundamental understanding of the Na channel and its part in generating action potentials in excitable cells.

6,11-Dideoxytetrodotoxin from the puffer fish, Fugu pardalis

The presence of an unknown dideoxy analog of tetrodotoxin was suggested on the liquid chromatography/electrospray ionization-mass spectrometry mass chromatogram of the ovaries of the puffer fish, Fugu pardalis, in single ion monitoring mode to detect at m/z 288. We succeeded to isolate this analog (approximately 0.4 mg) from 200 g of the ovaries and the structure was determined as 6,11-dideoxytetrodotoxin by spectroscopic methods (high resolution-fast atom bombardment-MS and NMR spectroscopy). The discovery of the new analog is highly significant with respect to the biosynthesis or metabolism of tetrodotoxin. We also roughly determined the value of IC(50) (mice, intraperitoneal) for 6,11-dideoxytetrodotoxin as 420 microg/kg and thus it is 42 times less toxic than tetrodotoxin.

[Determination of tetrodotoxin in nassarius by solid-phase extraction, ultrafiltration and liquid chromatography-mass spectrometry]

A method for the determination and analysis of tetrodotoxin (TTX) in nassarius using liquid chromatography-mass spectrometry (LC-MS) has been developed. The homogenized sample was extracted with 0.03 mol/L acetic acid by heating in a boiling water-bath for 10 min and then was cleaned up on a Sep-Pak C18 cartridge column and filtered through a M(r) 3 000 cut-off ultrafree microcentrifuge filter. Chromatographic separation was performed on an Insertsil ODS-3 column using 30 mmol/L heptafluorobutyric acid in 1 mmol/L ammonia formate buffer-methanol (99 : 1, v/v) as mobile phase at a flow rate of 0.3 mL/min. TTX was identified by retention time in positive mode with selected ion monitoring (SIM). The results showed that TTX and its analogs can be separated with the method. The linear range was 0.01 - 10.0 mg/L and the correlation coefficients were greater than 0.995 with a detection limit of 2 microg/L (S/N = 3). The average recoveries were in the range of 72.5% - 80.4% and the relative standard deviations were 4.48% - 8.87%. TTX of 0.04 - 0.50 mg/kg was found in nassarius that was collected from Fujian Province after red tide.

Calcium block of single sodium channels: role of a pore-lining aromatic residue

Extracellular Ca(2+) ions cause a rapid block of voltage-gated sodium channels, manifest as an apparent reduction of the amplitude of single-channel currents. We examined the influence of residue Tyr-401 in the isoform rNa(V)1.4 on both single-channel conductance and Ca(2+) block. An aromatic residue at this position in the outer mouth of the pore plays a critical role in high-affinity block by the guanidinium toxin tetrodotoxin, primarily due to an electrostatic attraction between the cationic blocker and the system of pi electrons on the aromatic face. We tested whether a similar attraction between small metal cations (Na(+) and Ca(2+)) and this residue would enhance single-channel conductance or pore block, using a series of fluorinated derivatives of phenylalanine at this position. Our results show a monotonic decrease in Ca(2+) block as the aromatic ring is increasingly fluorinated, a result in accord with a cation-pi interaction between Ca(2+) and the aromatic ring. This occurred without a change of single-channel conductance, consistent with a greater electrostatic effect of the pi system on divalent than on monovalent cations. High-level quantum mechanical calculations show that Ca(2+) ions likely do not bind directly to the aromatic ring because of the substantial energetic penalty of dehydrating a Ca(2+) ion. However, the complex of a Ca(2+) ion with its inner hydration shell, Ca(2+)(H(2)O)(6), interacts electrostatically with the aromatic ring in a way that affects the local concentration of Ca(2+) ions in the extracellular vestibule.

Formation of tetracycline degradation products in chicken and pig meat under different thermal processing conditions

Tetracycline (TC) and 4-epitetracycline (4eTC) degradation, as well as anhydrotetracycline (ATC) and 4-epianhydrotetracycline (4eATC) formation, has been evaluated in thermally treated chicken breast, pig loin, and pig loin with added back-fat. Samples containing TC and 4eTC residues were submitted to microwave or boiling heating, extracted with a mixture of McIlvaine buffer/methanol (75:25), and analyzed by high-performance liquid chromatography-diode array detection on a phenyl-hexyl reverse phase chromatographic column. The formation of ATC and 4eATC, as well as of two unidentified compounds, was described for the first time in edible meat samples submitted to mild thermal treatments, similar to those applied at home to cook foods. Degradation of TC and 4eTC and formation of ATC and 4eATC versus time of treatment fitted satisfactorily a first-order kinetic. Even if the potential toxic effects of these breakdown compounds should be further investigated, their formation in cooked meat should be taken into account when maximum residue limits are established.

[Mechanisms of action of voltage-gated sodium channel ligands]

The voltage-gated sodium channels play a key role in the generation of action potential in excitable cells. Sodium channels are targeted by a number of modulating ligands. Despite numerous studies, the mechanisms of action of many ligands are still unknown. The main cause of the problem is the absence of the channel structure. Sodium channels belong to the superfamily of P-loop channels that also the data abowt includes potassium and calcium channels and the channels of ionotropic glutamate receptors. Crystallization of several potassium channels has opened a possibility to analyze the structure of other members of the superfamily using the homology modeling approach. The present study summarizes the results of several recent modelling studies of such sodium channel ligands as tetrodotoxin, batrachotoxin and local anesthetics. Comparison of available experimental data with X-ray structures of potassium channels has provided a new level of understanding of the mechanisms of action of sodium channel ligands and has allowed proposing several testable hypotheses.

Recombinant human voltage-gated skeletal muscle sodium channels are pharmacologically functional in planar lipid bilayers

Human voltage-gated sodium ion channels are major sites of action for drugs and toxins that modulate cellular excitability, and are therefore key molecular targets for ion channel research, high throughput screening for new drugs, and toxin detection. Protein suitable for these applications must be produced in a functionally active form. We report the successful use of ion metal affinity chromatography (IMAC) to purify C-terminal polyhistidine tagged human skeletal muscle voltage-gated sodium (hSkM1-HT) channels from Sf9 insect cells; hSkM1 channels were pharmacologically functional when reconstituted into liposomes and incorporated into planar bilayer lipid membranes. hSkM1-HT single channel currents activated by veratridine had a conductance of 21 pS and those activated by brevetoxin, 16 pS. Channel activity was inhibited by tetrodotoxin and saxitoxin. This protein is suitable for the development of biosensor and high throughput screening technologies.

[Rapid determination of tetrodotoxin using microwave assisted alkaline hydrolysis and spectrofluorometric analysis]

To develop a rapid, simple and sensitive method for determining the concentration of tetrodotoxin(TTX), TTX was hydrolyzed in the strong alkali solution of water mixed with isopropanol. The concentration of TTX can be indirectly analyzed by the fluorescence signal of its alkaline hydrolysis product which can be enhanced by the microwave method. The maximum excitation and emission wavelengths of the alkaline hydrolysis product of tetrodotoxin were 380 and 496 nm, respectively. The linear range of the calibration curve was 0. 1-10 micromol x L(-1) with r=0. 9991. The limit of detection was 0. 05 micromol x L(-1) , which was twenty times lower than before. A rapid, highly sensitive and accurate method was thus established. It can be used as a quantitative method for detecting TTX.

Distribution of tetrodotoxin, saxitoxin, and their analogs among tissues of the puffer fish Fugu pardalis

The anatomical distribution of tetrodotoxin (TTX), saxitoxin (STX) and their analogs (TTXs, STXs) in three female and three male specimens of the marine puffer fish Fugu pardalis from Miyagi Prefecture, 2005, Japan, were studied. 5-DeoxyTTX, 11-deoxyTTX, and 5,6,11-trideoxyTTX were quantified by liquid chromatography/mass spectrometry (LC/MS) for the first time, and other TTXs and STXs were determined by liquid chromatography-fluorescent detection (LC-FLD). As a result, 5,6,11-trideoxyTTX was found to be the major TTX analog in all tissues tested, whereas 5-deoxyTTX and 11-deoxyTTX were minor components. Especially, in female (n=3), the ratios of 5,6,11-trideoxyTTX to total of all TTX analogs (mole/mole) in ovaries (mean+/-SD, 0.42+/-0.055) were significantly larger than those in livers (0.17+/-0.025) (P<0.05). In contrary, the ratios of 4,9-anhydroTTX to total of all TTX analogs in livers (0.27+/-0.047) were significantly larger than those in ovaries (0.073+/-0.040) (P<0.01). The ratios of TTX to total of all TTX analogs were not significantly different between ovaries (0.47+/-0.078) and livers (0.55+/-0.067). In male (n=3), all these ratios were not significantly different between livers and testis. 4-S-CysteinylTTX was detected in liver, spleen, gall, and intestine in 1-6mole% of total of all TTX analogs, supporting our previous hypothesis that 4-S-cysteinylTTX is a metabolite of TTX.

Total Synthesis of (−)-5,6,11-Trideoxytetrodotoxin and Its 4-Epimer

[reaction: see text] The first total synthesis of 5,6,11-trideoxytetrodotoxin (1) and its 4-epimer were achieved. The synthesis is characterized by the stereoselective construction of the quaternary amino carbon center at C8a by an asymmetric transferring Strecker synthesis and the highly efficient conversion of cyanohydrin 4 to 1 via intramolecular cyclization reactions.

Ecological functions of tetrodotoxin in a deadly polyclad flatworm

The deadly neurotoxin tetrodotoxin (TTX) is found in a variety of animal phyla and, because of its toxicity, is most often assumed to deter predation. On the tropical Pacific island of Guam, we found an undescribed flatworm (planocerid sp. 1) that contains high levels of TTX and its analogs. Through ecological experiments, we show that TTXs do not protect these flatworms from some predators but instead are used to capture mobile prey. TTX is known to have multiple ecological functions, which has probably led to its widespread presence among prokaryotes and at least 10 metazoan phyla.

Structure-activity relationship of an alpha-toxin Bs-Tx28 from scorpion (Buthus sindicus) venom suggests a new alpha-toxin subfamily

Scorpion venoms are among the most widely known source of peptidyl neurotoxins used for callipering different ion channels, e.g., for Na(+), K(+), Ca(+) or Cl(-). An alpha-toxin (Bs-Tx28) has been purified from the venom of scorpion Buthus sindicus, a common yellow scorpion of Sindh, Pakistan. The primary structure of Bs-Tx28 was established using a combination of MALDI-TOF-MS, LC-ESI-MS, and automated Edman degradation analysis. Bs-Tx28 consists of 65 amino acid residues (7274.3+/-2Da), including eight cysteine residues, and shows very high sequence identity (82-94%) with other long-chain alpha-neurotoxins, active against receptor site-3 of mammalian (e.g., Lqq-IV and Lqh-IV from scorpions Leiurus sp.) and insect (e.g., BJalpha-IT and Od-1 from Buthotus judaicus and Odonthobuthus doriae, respectively) voltage-gated Na(+) channels. Multiple sequence alignment and phylogenetic analysis of Bs-Tx28 with other known alpha- and alpha-like toxins suggests the presence of a new and separate subfamily of scorpion alpha-toxins. Bs-Tx28 which is weakly active in both, mammals and insects (LD(50) 0.088 and 14.3microg/g, respectively), shows strong induction of the rat afferent nerve discharge in a dose-dependent fashion (EC(50)=0.01microg/mL) which was completely abolished in the presence of tetrodotoxin suggesting the binding of Bs-Tx28 to the TTX-sensitive Na(+)-channel. Three-dimensional structural features of Bs-Tx28, established by homology modeling, were compared with other known classical alpha-mammal (AaH-II), alpha-insect (Lqh-alphaIT), and alpha-like (BmK-M4) toxins and revealed subtle variations in the Nt-, Core-, and RT-CT-domains (functional domains) which constitute a "necklace-like" structure differing significantly in all alpha-toxin subfamilies. On the other hand, a high level of conservation has been observed in the conserved hydrophobic surface with the only substitution of W43 (Y43/42) and an additional hydrophobic character at position F40 (L40/A/V/G39), as compared to the other mentioned alpha-toxins. Despite major differences within the primary structure and activities of Bs-Tx28, it shares a common structural and functional motif (e.g., transRT-farCT) within the RT-CT domain which is characteristic of scorpion alpha-mammal toxins.

Tropical Marine Neurotoxins: Venoms to Drugs

Neurotoxic venoms are common among tropical marine creatures, which have specialized apparatuses for delivery of the venoms. These include jellyfish and anemones, venomous cone snails, venomous fish, stingrays, sea snakes, and venomous octopuses. Numerous toxic neuropeptides are found within these venoms, and some can discriminate between closely related intracellular targets, a characteristic that makes them useful to define cation channels and attractive for drug development. A synthetic derivative of an omega-conotoxin is now available, representing a new class of analgesics. In general, toxic marine venoms contain proteins that are heat labile, providing opportunity for therapeutic intervention following envenomation, while ingestible seafood toxins are thermostable toxins. Ingestible toxins found in the tropics include those associated with reef fish, pufferfish, and some shellfish, which serve as food-chain vectors for toxins produced by marine microorganisms.

Parallel arms races between garter snakes and newts involving tetrodotoxin as the phenotypic interface of coevolution

Parallel "arms races" involving the same or similar phenotypic interfaces allow inference about selective forces driving coevolution, as well as the importance of phylogenetic and phenotypic constraints in coevolution. Here, we report the existence of apparent parallel arms races between species pairs of garter snakes and their toxic newt prey that indicate independent evolutionary origins of a key phenotype in the interface. In at least one area of sympatry, the aquatic garter snake, Thamnophis couchii, has evolved elevated resistance to the neurotoxin tetrodotoxin (TTX), present in the newt Taricha torosa. Previous studies have shown that a distantly related garter snake, Thamnophis sirtalis, has coevolved with another newt species that possesses TTX, Taricha granulosa. Patterns of within population variation and phenotypic tradeoffs between TTX resistance and sprint speed suggest that the mechanism of resistance is similar in both species of snake, yet phylogenetic evidence indicates the independent origins of elevated resistance to TTX.

Selective detection of saxitoxin over tetrodotoxin using acridinylmethyl crown ether chemosensor

At pH 7.1, saxitoxin decomposes to produce a trace impurity that can interfere with fluorescence sensing when using irradiation wavelengths near 325 nm. The fluorophore acridine is found to be a suitable component of arylmethyl crown ether chemosensors for the fluorescent detection of saxitoxin. These sensors are selective for the detection of saxitoxin over tetrodotoxin.

Novel Conotoxins from Conus striatus and Conus kinoshitai Selectively Block TTX-Resistant Sodium Channels

The peptides isolated from venoms of predatory marine Conus snails ("conotoxins") are well-known to be highly potent and selective pharmacological agents for voltage-gated ion channels and receptors. We report the discovery of two novel TTX-resistant sodium channel blockers, mu-conotoxins SIIIA and KIIIA, from two species of cone snails. The two toxins were identified and characterized by combining molecular techniques and chemical synthesis. Both peptides inhibit TTX-resistant sodium currents in neurons of frog sympathetic and dorsal root ganglia but poorly block action potentials in frog skeletal muscle, which are mediated by TTX-sensitive sodium channels. The amino acid sequences in the C-terminal region of the two peptides and of the previously characterized mu-conotoxin SmIIIA (which also blocks TTX-resistant channels) are similar, but the three peptides differ in the length of their first N-terminal loop. We used molecular dynamics simulations to analyze how altering the number of residues in the first loop affects the overall structure of mu-conotoxins. Our results suggest that the naturally occurring truncations do not affect the conformation of the C-terminal loops. Taken together, structural and functional differences among mu-conotoxins SmIIIA, SIIIA, and KIIIA offer a unique insight into the "evolutionary engineering" of conotoxin activity.

A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes

The common garter snake (Thamnophis sirtalis) preys upon the rough-skinned newt (Taricha granulosa), which contains the neurotoxin tetrodotoxin (TTX) in the skin. TTX is toxic, large quantities are present in a newt, and highly resistant snakes have the ability to ingest multiple newts; subsequently snakes harbor significant amounts of active toxin in their own tissues after consuming a newt. Snakes harbor TTX in the liver for 1 mo or more after consuming just one newt, and at least 7 wk after consuming a diet of newts. Three weeks after eating one newt, snakes contained an average of 42 microg of TTX in the liver. This amount could severely incapacitate or kill avian predators, and mammalian predators may be negatively affected as well.