Effect of H-7 and Lat-B on retinal physiology

PURPOSE

To investigate the effects of H-7 and Latrunculin B (Lat-B) on retinal vascular permeability and electrophysiology at concentrations that increase outflow facility in monkeys.

METHODS

One eye of 1 rhesus and 22 cynomolgus monkeys received an intravitreal bolus injection of H-7 or Lat-B; the opposite eye received vehicle. Multifocal electroretinograms (mfERGs), and photopic and scotopic full-field electroretinograms (ffERGs, sERGs) were recorded in subsets of monkeys at baseline and at multiple time-points post-H-7 or Lat-B. Vitreous fluorophotometry (VF) and fluorescein angiography (FA) were also performed.

RESULTS

No differences between the H-7 or Lat-B treated and control eyes were found in ffERGs, mfERGs, sERGs, or in FAs in any monkey. No significant difference was found in vitreous fluorescein levels between H-7 treated or Lat-B treated vs. control eyes.

CONCLUSIONS

No effect on retinal vascular permeability or retinal electrophysiology was apparent after intravitreal administration of H-7 or Lat-B at doses that increase outflow facility and lower IOP when given intracamerally.

Spirolides isolated from Danish strains of the toxigenic dinoflagellate Alexandrium ostenfeldii

Using LC/MS methodology, spirolides were detected in two clonal isolates of Alexandrium ostenfeldii isolated from Limfjorden, Denmark. Examination of the LC/MS profiles of extracts from these Danish cultures revealed the presence of two dominant peaks representing two previously unidentified spirolide components and one minor peak identified as the previously reported desmethyl spirolide C (1). Culturing of these clonal strains, LF 37 and LF 38, of A. ostenfeldii resulted in the accumulation of sufficient cell biomass to allow for the isolation and structure elucidation of two new spirolides, 13,19-didesmethylspirolide C (2) and spirolide G (3). While 2 was found to differ from 1 only in that it contained one less methyl group, 3 was the first spirolide to be isolated that contained a 5:6:6-trispiroketal ring system. The effect of this new feature on the toxicity of 3 relative to other spirolides is presently being pursued.

Actin cytoskeleton derangement induces apoptosis in renal ischemia/reperfusion

This study evaluated whether cytoskeletal alterations during the ischemic conditions associated with kidney preservation could determine apoptosis. Cytoskeletal alterations are among the main effects of ischemia and may induce apoptosis. Rat kidneys were preserved in University of Wisconsin (UW) solution for 24 h. Some groups of animals underwent 45 min of warm ischemia (WI) to evaluate its effect on both the actin cytoskeleton and apoptosis (assessed by caspase-3 activity and TUNEL staining). Swinholide A (SwinA) and Latrunculin B (LB), two actin cytoskeleton-targeted agents, were administered to assess the effect of direct actin disruption on apoptosis. Jasplakinolide (JP), a compound that stabilizes actin filaments, was administered to evaluate the effect of actin stabilization. Apoptosis was evaluated at 3 h of ex vivo reperfusion using the isolated perfused rat kidney (IPK) model.

RESULTS

Apoptosis increased during reperfusion with WI or administration of actin disruptor agents. Administration of stabilizing agents reversed apoptosis in kidneys that had previously undergone WI or had received an actin disruptor agent.

CONCLUSION

The disruption of the actin cytoskeleton during ischemic conditions associated with kidney preservation induces apoptosis upon reperfusion through caspase-3 activation.

Effects of latrunculin-B on outflow facility and trabecular meshwork structure in human eyes

PURPOSE

To determine the effect of the F-actin-disrupting agent latrunculin-B on aqueous outflow facility and trabecular meshwork architecture in human eyes.

METHODS

After baseline facility measurement in human eye bank eyes (n = 9 pairs), one eye of each pair received anterior chamber exchange and continued perfusion with medium containing 1 microM latrunculin-B. Contralateral eyes were treated in a similar manner with vehicle. Eyes were fixed by anterior chamber exchange and perfusion with universal fixative at 8 mm Hg (corresponding to a physiologic pressure of 15 mm Hg in vivo), and outflow pathway tissues were examined by transmission and scanning electron microscopy.

RESULTS

Perfusion of eyes with 1 microM latrunculin-B caused a continuous and ongoing increase in outflow facility, resulting in a net facility difference of 64% 2 hours after drug administration (P < 0.006). Transmission electron microscopy showed subtle and focal detachment of the inner wall of Schlemm's canal, rarefaction of the juxtacanalicular tissue (JCT), and cell-cell and cell-matrix detachment. Scanning electron microscopy showed collapsed vacuoles in the inner wall of Schlemm's canal and a marked increase in the number and size of border (paracellular) pores in the inner wall.

CONCLUSIONS

Latrunculin-B increases outflow facility in postmortem human eyes. The mechanism of facility increase is most likely due to loss of mechanical integrity of the trabecular meshwork as a consequence of reduction in cell-cell and cell-matrix adhesion. The facility increase and the extent of inner wall separation from the JCT that we observed were both qualitatively similar to that reported in living monkey eyes, but the magnitude of the facility increase and morphologic changes were much less than in the living monkey. This supports the idea that inner wall separation from the JCT may modulate outflow facility.

Changes in Membrane Potential: An Early Signal Triggered by Neurologically Active Phycotoxins

Most common phycotoxin poisoning syndromes have important neurological symptoms. However, little is known of the cellular and molecular targets of many of the phycotoxins that produce those human intoxications. We explore the effect of representative toxins on the membrane potential in human neuroblastoma cells by using a fluorimetric assay. Results presented in this study demonstrate that maitotoxin, palytoxins, brevetoxins, and ciguatoxins triggered a dose-dependent membrane depolarization. Mechanisms responsible for the toxins-induced changes in membrane potential are always related to a direct action of the compounds on membrane ion fluxes. This initial screening of the phycotoxins effect is the starting point to lately develop functional methods of detection.

Azaspiracid-1 inhibits bioelectrical activity of spinal cord neuronal networks

Azaspiracid-1 (AZA-1) is a recently identified phycotoxin that accumulates in molluscs and can cause severe human intoxications. For this study, we utilized murine spinal cord and frontal cortex neuronal networks grown over 64 channel microelectrode arrays (MEAs) to gain insights into the mechanism of action of AZA-1 on neuronal cells. Extracellular recordings of spontaneous action potentials were performed by monitoring mean spike rate as an assay of the efficacy of AZA-1 to alter the bioelectrical activity of neurons in the networks. Via slow onset, AZA-1 decreased the mean spike rate of the spinal cord neurons with an IC(50) of ca. 2.1nM, followed by partial recovery of original activity when toxin was removed. Pre-treatment with the GABA(A) receptor antagonist bicuculline led to an increased response of the neuronal networks to AZA-1 exposure and resulted in an irreversible inhibition of spike rate. AZA-1 did not cause any changes in frontal cortex networks upon drug exposure. In addition, whole-cell patch clamp recordings from spinal cord neurons showed that AZA-1 had no significant effect on the voltage-gated sodium (Na(+)) or calcium (Ca(2+)) currents, suggesting that the toxin affected synaptic transmission in the neuronal networks through a mechanism independent of these voltage-gated channels.

Hepatitis B virus x gene and cyanobacterial toxins promote aflatoxin B1-induced hepatotumorigenesis in mice

AIM: To assess the combinative role of aflatoxin B₁ (AFB₁), cyanobacterial toxins (cyanotoxins), and hepatitis B virus (HBV) x gene in hepatotumorigenicity.

METHODS: One-week-old animals carrying HBV x gene and their wild-type littermates were intraperitoneally (ip) injected with either single-dose AFB₁ [6 mg/kg body weight (bw)], repeated-dose cyanotoxins (microcystin-LR or nodularin, 10 μg/kg bw once a week for 15 wk), DMSO (vehicle control) alone, or AFB₁ followed by cyanotoxins a week later, and were sacrificed at 24 and 52 wk post-treatment.

RESULTS: AFB₁ induced liver tumors in 13 of 29 (44.8%) transgenic mice at 52 wk post-treatment, significantly more frequent than in wild-type mice (13.3%). This significant difference was not shown in the 24-wk study. Compared with AFB₁ exposure alone, MC-LR and nodularin yielded approximately 3-fold and 6-fold increases in the incidence of AFB₁-induced liver tumors in wild-type animals at 24 wk, respectively. HBV x gene did not further elevate the risk associated with co-exposure to AFB₁ and cyanotoxins. With the exception of an MC-LR-dosed wild-type mouse, no liver tumor was observed in mice treated with cyanotoxins alone at 24 wk. Neither DMSO-treated transgenic mice nor their wild-type littermates had pathologic alterations relevant to hepatotumorigenesis in even up to 52 wk.

CONCLUSION: HBV x gene and nodularin promote the development of AFB₁-induced liver tumors. Co-exposure to AFB₁ and MC-LR tends to elevate the risk of liver tumors at 24 wk relative to exposure to one of them. The combinative effect of AFB₁, cyanotoxins and HBVx on hepatotumorigenesis is weak at 24 wk.

Focal adhesions are hotspots for keratin filament precursor formation

Recent studies showed that keratin filament (KF) formation originates primarily from sites close to the actin-rich cell cortex. To further characterize these sites, we performed multicolor fluorescence imaging of living cells and found drastically increased KF assembly in regions of elevated actin turnover, i.e., in lamellipodia. Abundant KF precursors (KFPs) appeared within these areas at the distal tips of actin stress fibers, moving alongside the stress fibers until their integration into the peripheral KF network. The earliest KFPs were detected next to actin-anchoring focal adhesions (FAs) and were only seen after the establishment of FAs in emerging lamellipodia. Tight spatiotemporal coupling of FAs and KFP formation were not restricted to epithelial cells, but also occurred in nonepithelial cells and cells producing mutant keratins. Finally, interference with FA formation by talin short hairpin RNA led to KFP depletion. Collectively, our results support a major regulatory function of FAs for KF assembly, thereby providing the basis for coordinated shaping of the entire cytoskeleton during cell relocation and rearrangement.

Cytotoxicity in the marine dinoflagellate Prorocentrum mexicanum from Brazil

The microscopic algae in the oceans are crucial food for filter feeding bivalve shellfish (oysters, mussels, scallops, clams, etc.) as well as for the larvae of commercially important crustaceans. Some species of microalgae have the capacity to produce potent toxins, such as saxitoxins and ciguatoxins, which may intoxicate humans. Among the marine phytoplankton, the dinoflagellates are the main toxin producers. Studies on the marine phytoplankton from the São Sebastião Channel, southeastern coast of Brazil, showed a great diversity of dinoflagellates. Some species were collected and cultured at the Marine Biology Center of the São Paulo University (USP). The polar (PEs) (aqueous) and apolar (AEs) (methylene chloride) extracts of the cultivated dinoflagellate species were tested on different stages of the sea urchin development, on mouse erythrocytes and on microfilaments organization in a neuroblastoma cell line. Prorocentrum mexicanum PE and AE induced cells anomalies and cell division inhibition of sea urchin eggs at EC50 of 78.75 microg/mL (95% CI from 32.56 to 190.50) and 22.50 microg/mL (95% CI from 2.96 to 170.80) respectively (n=3). Both AE and PE of P. mexicanum induced hemolysis with EC50 of 65.07 microg/mL (95% CI from 27.40 to 154.60) and 84.29 microg/mL (95% CI from 53.26 to 133.40 microg/mL), respectively. P. mexicanum PE was tested in immunofluorescence for actin filaments organization in neuroblastoma cultured cell.

Aurilides B and C, cancer cell toxins from a Papua New Guinea collection of the marine cyanobacterium Lyngbya majuscula

Cytotoxicity-guided fractionation of a strain of the marine cyanobacterium Lyngbya majuscula collected from Papua New Guinea led to the isolation of aurilides B (1) and C (2). The planar structures of 1 and 2 were established by spectroscopic analysis, including HR-FABMS, 1D (1)H and (13)C NMR, and 2D COSY, HSQC, HSQC-TOCSY, and HMBC spectra. The absolute configuration was determined by spectroscopic analysis and chiral HPLC analysis of acid hydrolysates of 1 and 2. Both aurilides B and C showed in vitro cytotoxicity toward NCI-H460 human lung tumor and the neuro-2a mouse neuroblastoma cell lines, with LC(50) values between 0.01 and 0.13 microM. Aurilide B (1) was evaluated in the NCI 60 cell line panel and found to exhibit a high level of cytotoxicity (the mean panel GI(50) concentration was less than 10 nM) and to be particularly active against leukemia, renal, and prostate cancer cell lines.

Ca2+ channel activating action of maitotoxin in cultured brainstem neurons

The actions of maitotoxin were studied using cultured brainstem cells and adrenal chromaffin cells. Maitotoxin induced a profound increase in the Ca2+ influx into cultured brainstem cells after a brief lag period. The maitotoxin-induced Ca2+ influx was suppressed by various voltage-dependent Ca2+ channel blockers such as Co2+, Mn2+, verapamil and diltiazem. Maitotoxin-catecholamine release in brainstem cells initiated to increase after a lag period of about 1 min and the increase continued even at 4 min after treatment, while in the adrenal chromaffin cells the release started after an about 1-min lag period to attain a maximum within first 2-min and gradually decrease thereafter. These results suggest that maitotoxin acts on Ca2+ channels to increase the Ca2+ influx, accompanied by enhancement of catecholamine release in the brainstem cells with a different temporal profile from that in the adrenal chromaffin cells.

[Ca(2+)]-dependent generation of intracellular reactive oxygen species mediates maitotoxin-induced cellular responses in human umbilical vein endothelial cells

Maitotoxin (MTX) is known as one of the most potent marine toxins involved in Ciguatera poisoning, but intracellular signaling pathways caused by MTX was not fully understood. Thus, we have investigated whether intracellular reactive oxygen species (ROS) are involved in MTX-induced cellular responses in human umbilical vein endothelial cells. MTX induced a dose-dependent increase of intracellular [Ca(2+)]. MTX stimulated the production of intracellular ROS in a dose- and time-dependent manner, which was suppressed by BAPTA-AM, an intracellular Ca(2+) che-lator. Ionomycin also elevated the ROS production in a dose-dependent manner. MTX elevated transamidation activity in a time-dependent manner and the activation was largely inhibited by transfection of tissue transglutaminase siRNA. The activation of tissue transglutaminase and ERK1/2 by MTX was sup-pressed by BAPTA-AM or ROS scavengers. In addition, MTX-induced cell death was significantly de-layed by BAPTA-AM or a ROS scavenger. These results suggest that [Ca(2+)]-dependent generation of in-tracellular ROS, at least in part, play an important role in MTX-stimulated cellular responses, such as activation of tTGase, ERK phosphorylation, and in-duction of cell death, in human umbilical vein endothelial cells.

The inhibition of CHO-K1-BH4 cell proliferation and induction of chromosomal aberrations by brevetoxins in vitro

Brevetoxins (PbTxs) are highly potent trans-syn polyether neurotoxins produced during blooms of several species of marine dinoflagellates, most notably Karenia brevis. These neurotoxins act on voltage-sensitive sodium channels prolonging the active state. During red tides, the commercial fishing and tourism industries experience millions of dollars of lost revenue. Human consumption of shellfish contaminated with PbTxs results in neurotoxic shellfish poisoning (NSP). Additionally, blooms of K. brevis are potentially responsible for adverse human health effects such as respiratory irritation and airway constriction in coastal residents. There is little information regarding the full range of potential toxic effects caused by PbTxs. Recent evidence suggests that PbTxs are genotoxic substances. The purpose of this study was to determine if PbTxs could induce chromosomal aberrations and inhibit cellular proliferation in CHO-K1-BH4 cells, and if so, could the damage be negated or reduced by the PbTx antagonist brevenal. Results from the chromosomal aberrations assay demonstrated that PbTxs are potent inducers of CHO-K1-BH4 chromosome damage. Results from the inhibition of cellular proliferation assays demonstrated that PbTxs inhibit the ability of CHO-K1-BH4 cells to proliferate, an effect which can be reduced with brevenal.

Can general anaesthesia be used for the Paralytic Shellfish Poison bioassay?

The current method for Paralytic Shellfish Poisoning (PSP) testing in shellfish is based on the mouse bioassay (MBA), which involves injecting shellfish extract into a conscious mouse, and then converting its time to death into PSP toxicity using Sommer's table. To improve animal welfare, the present study investigated the use of anaesthesia. A saxitoxin (STX) calibration study was conducted where known amounts of STX were injected into both unanaesthetised and anaesthetised mice. Death time was approximately doubled when mice were anaesthetised. Both unanaesthetised and anaesthetised animals showed a linear relationship between the inverse death time and log(STX). Based on these data, new calibration curves were developed. This study revealed that the current method employing Sommer's table underestimates toxicity by up to 50% for higher toxin levels. Subsequently, shellfish samples were tested on both unanaesthetised and anaesthetised mice. Using the new calibration curves, the numbers of samples exceeding the field closure limit were similar for unanaesthetised and anaesthetised mice, and were nearly two-fold higher than those obtained with the current method. The studies showed that the bioassay gives variable results for both unanaesthetised and anaesthetised animals. Anaesthesia forms a viable and more ethical alternative to the current bioassay, at least in the short term. A practical summary on how to conduct this method is given.

Detection of marine toxins, brevetoxin-3 and saxitoxin, in seawater using neuronal networks

There is a need for assay systems that can detect known and unanticipated neurotoxins associated with harmful algal blooms. The present work describes our attempt to monitor the presence of brevetoxin-3 (PbTx-3) and saxitoxin (STX) in a seawater matrix using the neuronal network biosensor (NNB). The NNB relies on cultured mammalian neurons grown over microelectrode arrays, where the inherent bioelectrical activity of the network manifested as extracellular action potentials can be monitored noninvasively. Spinal cord neuronal networks were prepared from embryonic mice and the mean spike rate across the network was analyzed before and during exposure to the toxins. Extracellular action potentials from the network are highly sensitive not only to purified STX and PbTx-3, but also when in combination with matrixes such as natural seawater and algal growth medium. Detection limits for STX and PbTx-3, respectively, are 0.031 and 0.33 nM in recording buffer and 0.076 and 0.48 nM in the presence of 25-fold-diluted seawater. Our results demonstrated that neuronal networks could be used for analysis of Alexandrium fundyense (STX-producer) and Karenia brevis (PbTx-producer) algal samples lysed directly in the seawater-based growth medium and appropriately diluted with HEPES-buffered recording medium. The cultured network responded by changes in mean spike rate to the presence of STX-or PbTx-producing algae but not to the samples of two non-STX and non-PbTx isolates of the same algal genera. This work provides evidence that the NNB has the capacity to rapidly detect toxins associated with cells of toxic algal species or as dissolved forms present in seawater and hasthe potential for monitoring toxin levels during harmful algal blooms.

Isolation of swinholide A and related glycosylated derivatives from two field collections of marine cyanobacteria

[structure: see text] Chemical investigation of two field collections of marine cyanobacteria has led to the discovery of two new cytotoxic natural products, ankaraholides A (2) and B (3), along with the known compound swinholide A (1). Since swinholide-type compounds were previously localized to the heterotrophic bacteria of sponges, these findings raise intriguing questions about their true metabolic source.

Cryopyrin activates the inflammasome in response to toxins and ATP

A crucial part of the innate immune response is the assembly of the inflammasome, a cytosolic complex of proteins that activates caspase-1 to process the proinflammatory cytokines interleukin (IL)-1beta and IL-18. The adaptor protein ASC is essential for inflammasome function, binding directly to caspase-1 (refs 3, 4), but the triggers of this interaction are less clear. ASC also interacts with the adaptor cryopyrin (also known as NALP3 or CIAS1). Activating mutations in cryopyrin are associated with familial cold autoinflammatory syndrome, Muckle-Wells syndrome and neonatal onset multisystem inflammatory disease, diseases that are characterized by excessive production of IL-1beta. Here we show that cryopyrin-deficient macrophages cannot activate caspase-1 in response to Toll-like receptor agonists plus ATP, the latter activating the P2X7 receptor to decrease intracellular K+ levels. The release of IL-1beta in response to nigericin, a potassium ionophore, and maitotoxin, a potent marine toxin, was also found to be dependent on cryopyrin. In contrast to Asc-/- macrophages, cells deficient in the gene encoding cryopyrin (Cias1-/-) activated caspase-1 and secreted normal levels of IL-1beta and IL-18 when infected with Gram-negative Salmonella typhimurium or Francisella tularensis. Macrophages exposed to Gram-positive Staphylococcus aureus or Listeria monocytogenes, however, required both ASC and cryopyrin to activate caspase-1 and secrete IL-1beta. Therefore, cryopyrin is essential for inflammasome activation in response to signalling pathways triggered specifically by ATP, nigericin, maitotoxin, S. aureus or L. monocytogenes.

Peptide toxins in sea anemones: structural and functional aspects

Sea anemones are a rich source of two classes of peptide toxins, sodium channel toxins and potassium channel toxins, which have been or will be useful tools for studying the structure and function of specific ion channels. Most of the known sodium channel toxins delay channel inactivation by binding to the receptor site 3 and most of the known potassium channel toxins selectively inhibit Kv1 channels. The following peptide toxins are functionally unique among the known sodium or potassium channel toxins: APETx2, which inhibits acid-sensing ion channels in sensory neurons; BDS-I and II, which show selectivity for Kv3.4 channels and APETx1, which inhibits human ether-a-go-go-related gene potassium channels. In addition, structurally novel peptide toxins, such as an epidermal growth factor (EGF)-like toxin (gigantoxin I), have also been isolated from some sea anemones although their functions remain to be clarified.

Actin polymerization regulates clathrin coat maturation during early stages of synaptic vesicle recycling at lamprey synapses

Although it is established that presynaptic actin participates in synaptic vesicle recycling at several synapses, the earliest stages at which actin polymerization is employed during this process are still unclear. To address this, we prevented actin polymerization at lamprey synapses by applying latrunculin B or swinholide A. Latrunculin and swinholide depolymerize actin by sequestering actin monomers and, in addition, swinholide can sever existing actin filaments. When injected into individual presynaptic axons of the intact spinal cord, fluorescently labeled monomeric actin rapidly incorporated in a calcium-dependent manner into a stable, filamentous actin network concentrated at endocytic zones. This pool of actin was disrupted completely by latrunculin. At stimulated synapses, specific disruption of actin polymerization with latrunculin and swinholide induced a selective increase in unconstricted clathrin-coated pits and, in the case of swinholide, an additional increase in the size of plasma membrane evaginations. These results indicate that actin polymerization participates initially in the maturation of clathrin-coated pits during early stages of synaptic vesicle recycling.

Saxitoxin, a toxic marine natural product that targets a multitude of receptors

Saxitoxin (STX) was discovered early last century and can contaminate seafood and drinking water, and over time has become an invaluable research tool and an internationally regulated chemical weapon. Among natural products, toxins obtain a unique reputation from their high affinity and selectivity for their target pharmacological receptor, which for STX has long been considered to only be the voltage gated sodium channel. In recent times however, STX has been discovered to also bind to calcium and potassium channels, neuronal nitric oxide synthase, STX metabolizing enzymes and two circulatory fluid proteins, namely a transferrin-like family of proteins and a unique protein found in the blood of pufferfish.

Protein biotoxins of military significance

There is a spectrum of several threat agents, ranging from nerve agents and mustard agents to natural substances, such as biotoxins and new, synthetic, bioactive molecules produced by the chemical industry, to the classical biological warfare agents. The new, emerging threat agents are biotoxins produced by animals, plants, fungi, and bacteria. Many types of organisms produce substances that are toxic to humans. Examples of such biotoxins are botulinum toxin, tetanus toxin, and ricin. Several bioactive molecules produced by the pharmaceutical industry can be even more toxic than are the classical chemical warfare agents. Such new agents, like the biotoxins and bioregulators, often are called mid-spectrum agents. The threat to humans from agents developed by modern chemical synthesis and by genetic engineering also must be considered, since such agents may be more toxic or more effective in causing death or incapacitation than classical warfare agents. By developing effective medical protection and treatment against the most likely chemical and mid-spectrum threat agents, the effects of such agents in a war scenario or following a terrorist attack can be reduced. Toxin-mediated diseases have made humans ill for millennia. Unfortunately, the use of biological agents as weapons of terror has now been realized, and separating naturally occurring disease from bioterroristic events has become an important public health goal. The key to timely identification of such attacks relies on education of primary care physicians, first responders, and public health officials.

Crystal structures of protein phosphatase-1 bound to motuporin and dihydromicrocystin-LA: elucidation of the mechanism of enzyme inhibition by cyanobacterial toxins

The microcystins and nodularins are tumour promoting hepatotoxins that are responsible for global adverse human health effects and wildlife fatalities in countries where drinking water supplies contain cyanobacteria. The toxins function by inhibiting broad specificity Ser/Thr protein phosphatases in the host cells, thereby disrupting signal transduction pathways. A previous crystal structure of a microcystin bound to the catalytic subunit of protein phosphatase-1 (PP-1c) showed distinct changes in the active site region when compared with protein phosphatase-1 structures bound to other toxins. We have elucidated the crystal structures of the cyanotoxins, motuporin (nodularin-V) and dihydromicrocystin-LA bound to human protein phosphatase-1c (gamma isoform). The atomic structures of these complexes reveal the structural basis for inhibition of protein phosphatases by these toxins. Comparisons of the structures of the cyanobacterial toxin:phosphatase complexes explain the biochemical mechanism by which microcystins but not nodularins permanently modify their protein phosphatase targets by covalent addition to an active site cysteine residue.

Indirect modulation by alpha7 nicotinic acetylcholine receptors of noradrenaline release in rat hippocampal slices: interaction with glutamate and GABA systems and effect of nicotine withdrawal

Nicotinic acetylcholine receptors (nAChRs) can modulate transmitter release. Striatal [(3)H]dopamine ([(3)H]DA) release is regulated by presynaptic nAChR on dopaminergic terminals and alpha7 nAChR on neighboring glutamatergic afferents. Here, we explored the role of alpha7 nAChR in the modulation of [(3)H]noradrenaline ([(3)H]NA) release from rat hippocampal slices. The nicotinic agonist anatoxin-a (AnTx) evoked monophasic [(3)H]NA release (EC(50) = 1.2 microM) that was unaffected by alpha-conotoxin-MII or dihydro-beta-erythroidine, antagonists of alpha3/alpha6beta2* and beta2* nAChR, respectively. In contrast AnTx-evoked striatal [(3)H]DA release was biphasic (EC(50) = 138.9 nM; 7.1 microM) and blocked by these antagonists. At a high AnTx concentration (25 microM), alpha7 nAChR antagonists (methyllycaconitine, alpha-conotoxin-ImI) and glutamate receptor (GluR) antagonists [kynurenic acid, 6,7-dinitroquinoxaline-2,3-dione (DNQX)] partially inhibited [(3)H]NA release. The alpha7 nAChR-selective agonist choline evoked [(3)H]NA release (E(max) = 33% of that of AnTx) that was blocked by GluR antagonists, supporting a model in which alpha7 nAChRs trigger glutamate release that subsequently stimulates [(3)H]NA release. A GABAergic component was also revealed: choline-evoked [(3)H]NA release was partially blocked by the GABA(A) receptor antagonist bicuculline, and coapplication of bicuculline and DNQX fully abolished this response. These findings support alpha7 nAChR on GABAergic neurons that can promote GABA release which, in turn, leads to [(3)H]NA release, probably by disinhibition. To investigate the impact of long-term nicotine exposure on this model, rats were exposed for 14 days to nicotine (4 mg/kg/day) with or without 3 or 7 days of withdrawal. alpha7 nAChR responses were selectively and transiently up-regulated after 3 days of withdrawal. This functional up-regulation could contribute to the withdrawal effects of nicotine.

Rho kinase regulates fragmentation and phagocytosis of apoptotic cells

During the execution phase of apoptosis, a cell undergoes cytoplasmic and nuclear changes that prepare it for death and phagocytosis. The end-point of the execution phase is condensation into a single apoptotic body or fragmentation into multiple apoptotic bodies. Fragmentation is thought to facilitate phagocytosis; however, mechanisms regulating fragmentation are unknown. An isoform of Rho kinase, ROCK-I, drives membrane blebbing through its activation of actin-myosin contraction; this raises the possibility that ROCK-I may regulate other execution phase events, such as cellular fragmentation. Here, we show that COS-7 cells fragment into a number of small apoptotic bodies during apoptosis; treating with ROCK inhibitors (Y-27632 or H-1152) prevents fragmentation. Latrunculin B and blebbistatin, drugs that interfere with actin-myosin contraction, also inhibit fragmentation. During apoptosis, ROCK-I is cleaved and activated by caspases, while ROCK-II is not activated, but rather translocates to a cytoskeletal fraction. siRNA knock-down of ROCK-I but not ROCK-II inhibits fragmentation of dying cells, consistent with ROCK-I being required for apoptotic fragmentation. Finally, cells dying in the presence of the ROCK inhibitor Y-27632 are not efficiently phagocytized. These data show that ROCK plays an essential role in fragmentation and phagocytosis of apoptotic cells.

Conformation-specific antibodies reveal distinct actin structures in the nucleus and the cytoplasm

For many years the existence of actin in the nucleus has been doubted because of the lack of phalloidin staining as well as the failure to document nuclear actin filaments by electron microscopy. More recent findings reveal actin to be a component of chromatin remodeling complexes and of the machinery involved in RNA synthesis and transport. With distinct functions for nuclear actin emerging, the quest for its conformation and oligomeric/polymeric structure in the nucleus has resumed importance. We used chemically cross-linked 'lower dimer' (LD) to generate mouse monoclonal antibodies specific for different actin conformations. One of the resulting antibodies, termed 1C7, recognizes an epitope that is buried in the F-actin filament, but is surface-exposed in G-actin as well as in the LD. In immunofluorescence studies with different cell lines, 1C7 selectively reacts with non-filamentous actin in the cytoplasm. In addition, it detects a discrete form of actin in the nucleus, which is different from the nuclear actin revealed by the previously described 2G2 [Gonsior, S.M., Platz, S., Buchmeier, S., Scheer, U., Jockusch, B.M., Hinssen, H., 1999. J. Cell Sci. 112, 797]. Upon latrunculin-induced disassembly of the filamentous cytoskeleton in Rat2 fibroblasts, we observed a perinuclear accumulation of the 1C7-reactive actin conformation. In addition, latrunculin treatment led to the assembly of phalloidin-staining actin structures in chromatin-free regions of the nucleus in these cells. Our results indicate that distinct actin conformations and/or structures are present in the nucleus and the cytoplasm of different cell types and that their distribution varies in response to external signals.