The neurotoxic lipopeptide kalkitoxin interacts with voltage-sensitive sodium channels in cerebellar granule neurons

The marine neurotoxin kalkitoxin, a thiazoline-containing lipid derived from the pantropical marine cyanobacterium Lyngbya majuscula, was assayed for interaction with the tetrodotoxin-sensitive, voltage-sensitive sodium channel (TTX-VSSC) in cerebellar granule neuron cultures (CGN). The naturally occurring isomer of kalkitoxin (KTx-7) blocked veratridine-induced (30 microM) neurotoxicity in a concentration-dependent manner (EC50 22.7 nM [9.5-53.9 nM, 95% confidence interval {CI}]) in CGN. Kalkitoxin was a potent inhibitor (EC50 26.1 nM [12.3-55.0 nM, 95% CI]) of the elevation of intracellular Ca2+ concentration [Ca2+](i) that accompanies exposure of CGN to veratridine. To further explore the potential interaction of KTx-7 with TTX-VSSC, we assessed the influence of KTX-7 on the binding of [3H]batrachotoxin ([3H]BTX) to neurotoxin site 2 on the TTX-VSSC. Although kalkitoxin was without effect on the basal binding of [3H]BTX to intact cerebellar granule neurons, in the presence of the positive allosteric modulator, deltamethrin, [3H]BTX binding was inhibited by KTx-7 in a concentration-dependent manner (11.9 nM [IC50=3.8-37.2 nM, 95% CI]). These results provide both direct and functional evidence for an interaction of kalkitoxin with the neuronal TTX-VSSC.

Characterization of the preferred stereochemistry for the neuropharmacologic actions of antillatoxin

Antillatoxin is a potent ichthyotoxin and cytotoxin previously discovered from the marine cyanobacterium Lyngbya majuscula. Ensuing studies of its mechanism of action showed it to activate the mammalian voltage-gated sodium channel at a pharmacological site that is distinct from any previously described. The structure of antillatoxin, initially formulated from spectroscopic information, was subsequently corrected at one stereocenter (C-4) as a result of synthesis of four different antillatoxin stereoisomers (all possible C-4 and C-5 diastereomers). In the current study these four stereoisomers, (4R,5R)-, (4S,5R)-, (4S,5S)-, and (4R,5S)-antillatoxin, were characterized in five different biological assay systems: ichthyotoxicity to goldfish, microphysiometry using cerebellar granule cells (CGCs), lactose dehydrogenase efflux from CGCs, monitoring of intracellular Ca(2+) concentrations in CGCs, and cytotoxicity to Neuro 2a cells. Across these various biological measures there was great consistency in that the natural antillatoxin (the 4R,5R-isomer) was greater than 25-fold more potent than any of the other stereoisomers. Detailed NMR studies provided a number of torsion and distance constraints that were modeled using the MM2 force field to yield predicted solution structures of the four antillatoxin stereoisomers. The macrocycle and side chain of natural (4R,5R)-antillatoxin present an overall "L-shaped" topology with an accumulation of polar substituents on the external surface of the macrocycle and a hydrogen bond between N(H)-7' and the C(O)-1 carbonyl. The decreased potency of the three non-naturally occurring antillatoxin stereoisomers is certainly a result of their dramatically altered overall molecular topologies.

Antillatoxin is a marine cyanobacterial toxin that potently activates voltage-gated sodium channels

Antillatoxin (ATX) is a lipopeptide derived from the pantropical marine cyanobacterium Lyngbya majuscula. ATX is neurotoxic in primary cultures of rat cerebellar granule cells, and this neuronal death is prevented by either N-methyl-d-aspartate (NMDA) receptor antagonists or tetrodotoxin. To further explore the potential interaction of ATX with voltage-gated sodium channels, we assessed the influence of tetrodotoxin on ATX-induced Ca2+ influx in cerebellar granule cells. The rapid increase in intracellular Ca2+ produced by ATX (100 nM) was antagonized in a concentration-dependent manner by tetrodotoxin. Additional, more direct, evidence for an interaction with voltage-gated sodium channels was derived from the ATX-induced allosteric enhancement of [3H]batrachotoxin binding to neurotoxin site 2 of the alpha subunit of the sodium channel. ATX, moreover, produced a strong synergistic stimulation of [3H]batrachotoxin binding in combination with brevetoxin, which is a ligand for neurotoxin site 5 on the voltage-gated sodium channel. Positive allosteric interactions were not observed between ATX and either alpha-scorpion toxin or the pyrethroid deltamethrin. That ATX interaction with voltage-gated sodium channels produces a gain of function was demonstrated by the concentration-dependent and tetrodotoxin-sensitive stimulation of 22Na+ influx in cerebellar granule cells exposed to ATX. Together these results demonstrate that the lipopeptide ATX is an activator of voltage-gated sodium channels. The neurotoxic actions of ATX therefore resemble those of brevetoxins that produce neural insult through depolarization-evoked Na+ load, glutamate release, relief of Mg2+ block of NMDA receptors, and Ca2+ influx.

Total synthesis of the antiviral marine natural product (-)-hennoxazole A

[structure:see text] The marine natural product hennoxazole A was synthesized by a convergent approach. The diastereoselective Mukaiyama aldol reaction with beta-alkoxy aldehyde was used to construct the tetrahydropyran segment, and the preparation of the nonconjugated triene moiety was accomplished via S(N)2 displacement of allylic bromide with vinyllithium and Takai's iodoolefination followed by palladium-catalyzed cross coupling with MeMgBr. The final steps involve an amide coupling using DEPC and oxazole synthesis via a oxidation/cyclodehydration process.

Asymmetric total synthesis of (-)-mycothiazole

[structure: see text] In this Letter we describe the first total synthesis of mycothiazole, a polyketide thiazole from a marine sponge. Key steps include our CMD oxidation for the conversion of thiazolidine 11 to thiazole 12 and the Nagao acetate aldol reaction of 5 with aldehyde 4 to construct the chiral secondary alcohol. The skipped diene was constructed by the standard Stille coupling, and the conjugated diene was synthesized by lithium(I)- and copper(I)-mediated Stille coupling.

Identification of new Cdc25 dual specificity phosphatase inhibitors in a targeted small molecule array

Dual specificity protein phosphatases (DSPases) are key regulators of signal transduction, oncogenesis and the cell cycle. Few potent or specific inhibitors of DSPases, however, are readily available for these pharmacological targets. We have used a combinatorial/parallel synthetic approach to rigidify the variable core region and modify the side chains of 4-(benzyl-(2-[2,5-diphenyl-oxazole-4-carbonyl)-amino]-ethyl)-carbamoyl)- 2-decanoylamino butyric acid (or SC-alphaalphadelta9), which is the most active element in a previously described library of phosphatase inhibitors (Rice, R. L.; Rusnak, J. M.; Yokokawa, F.; Yokokawa, S.; Messner, D. J.; Boynton, A. L.; Wipf, P.; Lazo, J. S. Biochemistry 1997, 36, 15965). Several analogues were identified as effective inhibitors of the protein tyrosine phosphatase (PTPase) PTP1B and the DSPases VHR and Cdc25B2. Two compounds, FY3-alphaalpha09 and FY21-alphaalpha09, were partial competitive inhibitors of Cdc25B2 with Ki values of 7.6+/-0.5 and 1.6+/-0.2 microM, respectively. FY21-alphaalpha09 possessed only moderate activity against PTP1B. Consistent with its in vitro anti-phosphatase activity, FY21-alphaalpha09 inhibited growth in MDA-MB-231 and MCF-7 human breast cancer cell lines. FY21-alphaalpha09 also inhibited the G2/M transition in tsFT210 cells, consistent with Cdc25B inhibition. Several architectural requirements for DSPase inhibition were revealed through modification of the side chain moieties or variable core region of the pharmacophore, which resulted in decreased compound potency. The structure of FY21-alphaalpha09 provides a useful platform from which additional potent and more highly selective phosphatase inhibitors might be generated.

Disruption of insulin-like growth factor-1 signaling and down-regulation of cdc2 by SC-alphaalphadelta9, a novel small molecule antisignaling agent identified in a targeted array library

We previously reported the generation of a library of hydrophobic oxazole-based small molecules designed as inhibitors of phosphatases involved in cellular signaling and cell cycle control. One member of the targeted array library, 4-(benzyl-(2-[(2, 5-diphenyl-oxazole-4-carbonyl)-amino]-ethyl)-carbamoyl)-2-decanoylami no butyric acid (SC-alphaalphadelta9), inhibited cell growth in the G0/G1 phase of the cell cycle. To investigate potential mechanisms for SC-alphaalphadelta9 antiproliferative activity, we have used mouse embryonic fibroblasts transformed with simian virus 40 large T antigen mouse embryonic fibroblasts as a model system for a malignant phenotype that depends on overexpression of cell cycle regulators and autocrine stimulation by insulin-like growth factor-1. Structure-activity relationship studies with SC-alphaalphadelta9 and four library congeners demonstrated that antiproliferative activity was not a result of overall hydrophobicity. Rather, SC-alphaalphadelta9 decreased insulin-like growth factor-1 receptor tyrosine phosphorylation, receptor expression, mitogen-activated protein kinase activation and levels of the cyclin-dependent kinase Cdc2. Less toxic congeners only partially affected receptor expression, receptor tyrosine phosphorylation and Cdc2 levels. Thus SC-alphaalphadelta9, which is structurally distinct from other known small molecules that decrease intracellular Cdc2 levels, has profound effects on intracellular signaling. Furthermore, SC-alphaalphadelta9, but not vanadate or okadaic acid, selectively inhibited the growth of simian virus 40 large T antigen mouse embryonic fibroblasts compared to the parental cells. These results suggest that overexpression of Cdc2 and increased dependence on insulin-like growth factor-1 autocrine stimulation are responsible for the increased sensitivity of simian virus 40 large T antigen mouse embryonic fibroblasts to SC-alphaalphadelta9. The SC-alphaalphadelta9 pharmacophore could be a useful platform for the development of novel antisignaling agents.

A targeted library of small-molecule, tyrosine, and dual-specificity phosphatase inhibitors derived from a rational core design and random side chain variation

Tyrosine phosphatases (PTPases) dephosphorylate phosphotyrosines while dual-specificity phosphatases (DSPases) dephosphorylate contiguous and semicontiguous phosphothreonine and phosphotyrosine on cyclin dependent kinases and mitogen-activated protein kinases. Consequently, PTPases and DSPases have a central role controlling signal transduction and cell cycle progression. Currently, there are few readily available potent inhibitors of PTPases or DSPases other than vanadate. Using a pharmacophore modeled on natural product inhibitors of phosphothreonine phosphatases, we generated a refined library of novel, phosphate-free, small-molecule compounds synthesized by a parallel, solid-phase combinatorial-based approach. Among the initial 18 members of this targeted diversity library, we identified several inhibitors of DSPases: Cdc25A, -B, and -C and the PTPase PTP1B. These compounds at 100 microM did not significantly inhibit the protein serine/threonine phosphatases PP1 and PP2A. Kinetic studies with two members of this library indicated competitive inhibition for Cdc25 DSPases and noncompetitive inhibition for PTP1B. Compound AC-alphaalpha69 had a Ki of approximately 10 microM for recombinant human Cdc25A, -B, and -C, and a Ki of 0.85 microM for the PTP1B. The marked differences in Cdc25 inhibition as compared to PTP1B inhibition seen with relatively modest chemical modifications in the modular side chains demonstrate the structurally demanding nature of the DSPase catalytic site distinct from the PTPase catalytic site. These results represent the first fundamental advance toward a readily modifiable pharmacophore for synthetic PTPase and DSPase inhibitors and illustrate the significant potential of a combinatorial-based strategy that supplements the rational design of a core structure by a randomized variation of peripheral substituents.