Inhibition of calcium-stimulated ATPase in the hen brain P2 synaptosomal fraction by organophosphorus esters: relevance to delayed neuropathy

Allelochemicals of Alexandrium tamarense and its algicidal mechanism for Prorocentrum donghaiense and Heterosigma akashiwo

The effects of culture filtrate of Alexandrium tamarense on Prorocentrum donghaiense and Heterosigma akashiwo were investigated, including determination of algal density, photosynthesis, intracellular enzyme content and activity. The filtrate of A. tamarense had a stronger inhibitory effect on P. donghaiense than H. akashiwo, and the inhibitory effect decreased with higher temperature treatment of the filtrate. Instantaneous fluorescence (Ft) and maximum quantum yield of photosystem II (Fv/Fm) values of both kinds of target algae were reduced as exposed to the filtrate of A. tamarense, which proved that allelopathy could inhibit the normal operation of photosynthetic system. The increase of Malondialdehyde (MDA) content of the two kinds of target algae indicated that the cell membrane was seriously damaged by allelochemicals released by A. tamarense. The different responses of Superoxide Dismutase (SOD) and Catalase (CAT) activity in two kinds of target algae demonstrated the complexity and diversity of allelopathic mechanism. The filtrate of A. tamarense also influenced the metabolic function (ATPases) of P. donghaiense and H. akashiwo, and the influence on P. donghaiense was greater. Liquid-liquid extraction was used to extract and isolate allelochemicals from the filtrate of A. tamarense. It was found that only component I with molecular weight of 424.2573 and 434.2857 could inhibit the growth of P. donghaiense by HPLC-MS.

PNPLA6/NTE, an Evolutionary Conserved Phospholipase Linked to a Group of Complex Human Diseases

Patatin-like phospholipase domain-containing protein 6 (PNPLA6), originally called Neuropathy Target Esterase (NTE), belongs to a family of hydrolases with at least eight members in mammals. PNPLA6/NTE was first identified as a key factor in Organophosphate-induced delayed neuropathy, a degenerative syndrome that occurs after exposure to organophosphates found in pesticides and nerve agents. More recently, mutations in PNPLA6/NTE have been linked with a number of inherited diseases with diverse clinical symptoms that include spastic paraplegia, ataxia, and chorioretinal dystrophy. A conditional knockout of PNPLA6/NTE in the mouse brain results in age-related neurodegeneration, whereas a complete knockout causes lethality during embryogenesis due to defects in the development of the placenta. PNPLA6/NTE is an evolutionarily conserved protein that in Drosophila is called Swiss-Cheese (SWS). Loss of SWS in the fly also leads to locomotory defects and neuronal degeneration that progressively worsen with age. This review will describe the identification of PNPLA6/NTE, its expression pattern, and normal role in lipid homeostasis, as well as the consequences of altered NPLA6/NTE function in both model systems and patients.

Uncovering malathion (an organophosphate insecticide) action on Ca2+ signal transduction and investigating the effects of BAPTA-AM (a cell-permeant Ca2+ chelator) on protective responses in glial cells

Malathion, one of commonly used organophosphate insecticides, has a wide range of toxic actions in different models. However, the effect of this compound on Ca²⁺ homeostasis and its related cytotoxicity in glial cells is elusive. This study examined whether malathion evoked intracellular Ca²⁺ concentration ([Ca²⁺]_i) rises and established the relationship between Ca²⁺ signaling and cytotoxicity in normal human astrocytes, rat astrocytes and human glioblastoma cells. The data show that malathion induced concentration-dependent [Ca²⁺]_i rises in Gibco^® Human Astrocytes (GHA cells), but not in DI TNC1 normal rat astrocytes and DBTRG-05MG human glioblastoma cells. In GHA cells, this Ca²⁺ signal response was reduced by removing extracellular Ca²⁺. In Ca²⁺-free medium, pretreatment with the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin abolished malathion-induced [Ca²⁺]_i rises. Conversely, incubation with malathion abolished thapsigargin-induced [Ca²⁺]_i rises. Inhibition of phospholipase C (PLC) with U73122 also blocked malathion-induced [Ca²⁺]_i rises. In Ca²⁺-containing medium, malathion-induced [Ca²⁺]_i rises was inhibited by store-operated Ca²⁺ channel blockers (2-APB, econazole or SKF96365) and the protein kinase C (PKC) inhibitor GF109203X. Malathion (5-25 μM) concentration-dependently caused cytotoxicity in GHA, DI TNC1 and DBTRG-05MG cells. This cytotoxic effect was partially prevented by prechelating cytosolic Ca²⁺ with BAPTA-AM (a selective Ca²⁺ chelator) only in GHA cells. Together, in GHA but not in DI TNC1 and DBTRG-05MG cells, malathion induced [Ca²⁺]_i rises by inducing PLC-dependent Ca²⁺ release from the endoplasmic reticulum and Ca²⁺ entry via PKC-sensitive store-operated Ca²⁺ channels. Furthermore, malathion induced Ca²⁺-associated cytotoxicity, suggesting that Ca²⁺ chelating may have a protective effect on malathion-induced cytotoxicity in normal human astrocytes.

Effects of magnesium chloride on in vitro cholinesterase and ATPase poisoning by organophosphate (chlorpyrifos)

The present study investigated possible benefits of magnesium ion (as MgCl₂) in organophosphorus poisoning targeting its ability to interact with substrates and membrane enzymes. Blood samples collected from volunteered healthy adult by venepuncture into anticoagulant test tubes containing EDTA were separated into plasma and red blood cell and divided into three groups namely: normal, pesticide only (0.25‐2.0 mmol/L chlorpyrifos) and pesticide (0.25‐2.0 mmol/L chlorpyrifos) + 0.1 mol/L MgCl₂. Acetylcholinesterase, Na⁺/K⁺ATPase and Ca²⁺ATPase activities were evaluated. Results showed that Chlorpyrifos significantly (P < .05) reduced the levels of cholinesterase both in plasma and on red blood cells. Red blood cells Na⁺/K⁺ATPase and Ca²⁺ATPase were also significantly (P < .05) reduced by chlorpyrifos while MgCl₂ counteracted effects of chlorpyrifos with significant (P < .05) increase in the levels of cholinesterase, Na⁺/K⁺ATPase and Ca²⁺ATPase. We concluded that MgCl₂ neutralized effects of chlorpyrifos by promoting normal ATPase activities and inhibiting release of acetylcholine from cell.

The effect of chlorpyrifos upon ATPase activity of sarcoplasmic reticulum and biomechanics of skeleta l muscle contraction

We investigated the effect of chlorpyrifos, an organophosphate insecticide, on Ca2+,Mg2+-ATPase activity of sarcoplasmic reticulum and on contraction dynamics (force and length changes) of Rana temporaria m. tibialis anterior muscle fiber bundles. All of the used concentrations of chlorpyrifos (10-6 to 10-5 M) caused decrease of Ca2+,Mg2+-ATPase activity. The inhibition of Ca2+,Mg2+-ATPase activity by chlorpyriphos in concentrations of 10-6 M to 7.5·10-6 M is due to permeation of sarcoplasmic reticulum rather than due to direct enzyme inhibition by organophosphate insecticides. The inhibitory properties of the compound were higher at increased concentration and exposure timeframes. Chlorpyrifos in concentration range of 10-6 to 7.5·10-6 M causes changes in muscle fiber response force that were more pronounced than changes in contractile length. We demonstrated inhibition of Ca2+,Mg2+-ATPase activity caused by noncholinergic effects of chlorpyriphos. It is possible to conclude that influence of organophosphate insecticides happens not only in the neuromuscular transmission but also on the level of subcellular structures.

Neuropathy target esterase (NTE): overview and future

Neuropathy target esterase (NTE) was discovered by M.K. Johnson in his quest for the entity responsible for the striking and mysterious paralysis brought about by certain organophosphorus (OP) esters. His pioneering work on OP neuropathy led to the view that the biochemical lesion consisted of NTE that had undergone OP inhibition and aging. Indeed, nonaging NTE inhibitors failed to produce disease but protected against neuropathy from subsequently administered aging inhibitors. Thus, inhibition of NTE activity was not the culprit; rather, formation of an abnormal protein was the agent of the disorder. More recently, however, Paul Glynn and colleagues showed that whereas conventional knockout of the NTE gene was embryonic lethal, conditional knockout of central nervous system NTE produced neurodegeneration, suggesting to these authors that the absence of NTE rather than its presence in some altered form caused disease. We now know that NTE is the 6th member of a 9-protein family called patatin-like phospholipase domain-containing proteins, PNPLA1-9. Mutations in the catalytic domain of NTE (PNPLA6) are associated with a slowly developing disease akin to OP neuropathy and hereditary spastic paraplegia called NTE-related motor neuron disorder (NTE-MND). Furthermore, the NTE protein from affected individuals has altered enzymological characteristics. Moreover, closely related PNPLA7 is regulated by insulin and glucose. These seemingly disparate findings are not necessarily mutually exclusive, but we need to reconcile recent genetic findings with the historical body of toxicological data indicating that inhibition and aging of NTE are both necessary in order to produce neuropathy from exposure to certain OP compounds. Solving this mystery will be satisfying in itself, but it is also an enterprise likely to pay dividends by enhancing our understanding of the physiological and pathogenic roles of the PNPLA family of proteins in neurological health and disease, including a potential role for NTE in diabetic neuropathy.

Microtubule-associated targets in chlorpyrifos oxon hippocampal neurotoxicity

Prolonged exposure to organophosphate (OP) pesticides may produce cognitive deficits reflective of hippocampal injury in both humans and rodents. Recent work has indicated that microtubule trafficking is also adversely affected by exposure to the OP pesticide chlorpyrifos, suggesting a novel mode of OP-induced neurotoxicity. The present studies examined effects of prolonged exposure to chlorpyrifos oxon (CPO) on acetylcholinesterase (AChE) activity, immunoreactivity (IR) of microtubule-associated proteins, neuronal injury, and tubulin polymerization using in vitro organotypic slice cultures of rat hippocampus and bovine tubulin. Cultures were exposed to CPO (0.1-10 microM) in cell culture medium for 1-7 days, a regimen producing progressive reductions in AChE activity of 15-60%. Cytotoxicity (somatic uptake of the non-vital marker propidium iodide), as well as IR of alpha-tubulin and microtubule-associated protein-2 (a/b) [MAP-2], was assessed 1, 3, and 7 days after the start of CPO exposure. As early as 24 h after the start of exposure, CPO-induced deficits in MAP-2 IR were evident and progressive in each region of slice cultures at concentrations as low as 0.1 microM. CPO exposure did not alter alpha-tubulin IR at any time point. Concentration-dependent injury in the cornu ammonis (CA)1 pyramidal cell layer and to a lesser extent, CA3 and dentate cells, was evident 3 days after the start of CPO exposure (>or=0.1 microM) and was greatest after 7 days. Tubulin polymerization assays indicated that CPO (>or=0.1 microM) markedly inhibited the polymerization of purified tubulin and MAP-rich tubulin, though effects on MAP-rich tubulin were more pronounced. These data suggest that exposure to CPO produces a progressive decrease in neuronal viability that may be associated with impaired microtubule synthesis and/or function.

Comparative developmental neurotoxicity of organophosphate insecticides: effects on brain development are separable from systemic toxicity

A comparative approach to the differences between systemic toxicity and developmental neurotoxicity of organophosphates is critical to determine the degree to which multiple mechanisms of toxicity carry across different members of this class of insecticides. We contrasted neuritic outgrowth and cholinergic synaptic development in neonatal rats given different organophosphates (chlorpyrifos, diazinon, parathion) at doses spanning the threshold for impaired growth and viability. Animals were treated daily on postnatal days 1-4 by subcutaneous injection so as to bypass differences in first-pass activation to the oxon or catabolism to inactive products. Evaluations occurred on day 5. Parathion (maximum tolerated dose, 0.1 mg/kg) was far more systemically toxic than was chlorpyrifos or diazinon (maximum tolerated dose, 1-5 mg/kg). Below the maximum tolerated dose, diazinon impaired neuritic outgrowth in the forebrain and brainstem, evidenced by a deficit in the ratio of membrane protein to total protein. Diazinon also decreased choline acetyltransferase activity, a cholinergic neuronal marker, whereas it did not affect hemicholinium-3 binding to the presynaptic choline transporter, an index of cholinergic neuronal activity. There was no m(subscript)2(/subscript)-muscarinic acetylcholine receptor down-regulation, as would have occurred with chronic cholinergic hyperstimulation. The same pattern was found previously for chlorpyrifos. In contrast, parathion did not elicit any of these changes at its maximum tolerated dose. These results indicate a complete dichotomy between the systemic toxicity of organophosphates and their propensity to elicit developmental neurotoxicity. For parathion, the threshold for lethality lies below that necessary for adverse effects on brain development, whereas the opposite is true for chlorpyrifos and diazinon.

Carbofuran and malathion inhibit nucleotide hydrolysis in zebrafish (Danio rerio) brain membranes

Carbofuran and malathion are broad spectrum pesticides widely used in agricultural practice throughout the world. Toxicity of these pesticides has been correlated with their inhibitory effects on acetylcholinesterase activity. Nucleotides are extracellular signaling molecules, which trigger multiple biological effects. Studies have demonstrated the co-transmission of acetylcholine and ATP at the nerve endings. The control of neurotransmitter ATP levels is promoted by enzymes named ectonucleotidases, which include nucleoside triphosphate diphosphohydrolase (NTPDase) family and ecto-5'-nucleotidase. Since acetylcholine and ATP are co-released at the synapse and the acetylcholinesterase inhibition is an important target for pesticide action, here we verified the effect of exposure in vitro and in vivo to carbofuran and malathion on ectonucleotidase activities from brain membranes of zebrafish. To verify if carbofuran and malathion have a direct inhibitory effect on NTPDase and 5'-nucleotidase activities in brain membranes of zebrafish, we have tested in vitro concentrations of pesticides varying from 0.25 to 5 mM. Carbofuran, in vitro, inhibited ATP and ADP hydrolysis in an uncompetitive manner, but no effect was observed on AMP hydrolysis. Malathion decreased ATP and ADP hydrolysis in competitive and an uncompetitive manner, respectively, but not altered AMP hydrolysis. After exposure to carbofuran (50 and 500 microg/L) during 7 days, ADP hydrolysis was significantly decreased in both concentrations tested (by 19 and 24.5%, respectively). Malathion, at 500 microg/L, was able to inhibit ADP and AMP hydrolysis (by 28 and 58.5%, respectively). This study has shown that ectonucleotidases from brain membranes of zebrafish can be a potential target for pesticide neurotoxicity.

Effects of organophosphorus compounds on ATP production and mitochondrial integrity in cultured cells

Recent studies in vivo and in vitro suggested that mitochondrial dysfunction follows exposure to organophosphorus (OP) esters. As mitochondrial ATP production is important for cellular integrity, ATP production in the presence of OP neurotoxicants was examined in a human neuronal cell line (SH-SY5Y neuroblastoma cells) and primary dorsal root ganglia (DRG) cells isolated from chick embryos and subsequently cultured to achieve maturation with axons. These cell culture systems were chosen to evaluate toxic effects on the mitochondrial respiratory chain associated with exposure to OP compounds that do and do not cause OP-induced delayed neuropathy (OPIDN), a disorder preceded by inhibition of neurotoxic esterase (NTE). Concentration- and time-response studies were done in neuroblastoma cells exposed to phenyl saligenin phosphate (PSP) and mipafox, both compounds that readily induce delayed neuropathy in hens, or paraoxon, which does not. Phenylmethylsulfonyl fluoride (PMSF) was included as a non-neuropathic inhibitor of NTE. Purified neuronal cultures from 9 day-old chick embryo DRG were treated for 12 h with 1 microM PSP, mipafox, or paraoxon. In situ evaluation of ATP production measured by bioluminescence assay demonstrated decreased ATP concentrations both in neuroblastoma cells and chick DRG neurons treated with PSP. Mipafox decreased ATP production in DRG but not in SH-SY5Y cells. This low energy state was present at several levels of the mitochondrial respiratory chain, including Complexes I, II, III, and IV, although Complex I was the most severely affected. Paraoxon and PMSF were not effective at all complexes, and, when effective, required higher concentrations than needed for PSP. Results suggest that mitochondria are an important early target for OP compounds, with exposure resulting in depletion of ATP production. The targeting of neuronal, rather than Schwann cell mitochondria in DRG following exposure to PSP and mipafox was verified by loss of the mitochondrial-specific dye, tetramethylrhodamine, in these cells. No such loss was seen in paraoxon exposed neurons isolated from DRG or in Schwann cells treated with any of the test compounds.

Altered expression of transcripts for ?-tubulin and an unidentified gene in the spinal cord of phenyl saligenin phosphate treated hens (Gallus gallus)

Phenyl saligenin phosphate (PSP) induces a central-peripheral distal axonopathy in domestic fowl that develops 7-21 days after a single exposure. Neurotoxic esterase (NTE) is the initial molecular target for this neurotoxicity. PSP has to covalently bind to NTE and chemically "age" for induction of axonopathy. It was hypothesized that exposure to PSP results in early changes in spinal cord gene expression that do not occur with phenylmethylsulfonyl fluoride, a non-neuropathic compound that also inhibits NTE, or DMSO controls. Targeted display was used to screen approximately 15,000 gel bands. Three candidate genes were identified, but only the transcript designated P1 showed decreased expression following PSP exposure (2 mg/kg i.m.) in subsequent Northern blot and in situ hybridization experiments in samples taken <48 h after exposure. Additional experiments revealed that a approximately 2.5 kb alpha-tubulin transcript had decreased expression at 12-48 h after PSP exposure, with maximum change at 48 h (33%, p = 0.0479). A approximately 4.5 kb alpha-tubulin transcript had increased expression at 12 h (38%, p = 0.0125) and decreased expression at 48 h (28%, p = 0.0576). In situ hybridization on spinal cord revealed neuronal expression of P1 and alpha-tubulin transcripts. Decreased expression of transcripts for P1 and alpha-tubulin was present at 12 and 48 h, respectively. This decrease occurred in all neurons, not just those whose axons degenerate. Results suggest that (1) in PSP-induced OPIDN (organophosphorus-induced delayed neurotoxicity) some gene transcript expression changes are associated with initiation of axonopathy, and (2) PSP modulates spinal cord gene expression in neuronal types that do not undergo axonal degeneration.

Dioxins in commercial United States baby food

This is the first known study of dioxins, dibenzofurans, and polychlorinated biphenyls (PCBs) in commercial American bottled baby foods purchased in the United States. Dioxins, persistent chlorinated organics, are inadvertent by-products of chemical synthesis or combustion and are toxic to humans and other animals. Almost all dioxins enter the body through food consumption, specifically from food products containing animal fat. Major-brand bottled baby food containing meat was purchased at U.S. supermarkets and 12 pooled samples were analyzed for dioxins using high-resolution gas chromatography with high-resolution mass spectrometry. Low levels of dioxins were found in these products. The range was from 28 to 226 parts per quadrillion (ppq) dioxin toxic equivalents (TEQ). This is reported on a whole or wet weight (as eaten) basis. As a comparison, findings of dioxins in U.S. supermarket meat ranged from 28 to 540 ppq. Although dioxin levels are generally lower in these baby foods than in meat or poultry, the presence of dioxins in commercial baby food containing meat is cause for concern.

Biochemical changes in primary culture of skeletal muscle cells following dimethoate exposure

In order to investigate the cellular mechanism of muscular weakness in the Intermediate Myasthenia Syndrome (IMS) following acute organophosphate poisoning, we studied the cytotoxicity of dimethoate and its effects on the activity of acetylcholine esterase (AChE), Na+-K+-ATPase, succinate dehydrogenase (SDH), and Ca2+-ATPase in primary cultured skeletal muscle cells. The results showed that the activity of AChE was significantly inhibited in a dose and time-dependent manner when cells were exposed to dimethoate for 2 h, but the expression of heat-shock protein (HSP70) in muscle cells was significantly increased in a time-dependent manner following dimethoate exposure. Dimethoate can significantly increase the activity of Na+-K+-ATPase in the mitochondrial and cytoplasm fraction of muscle cells, and inhibit the activity of Ca2+-ATPase. This study suggests that the disruption of intracellular homeostasis and energy metabolism of the muscle cells may play a role in the etiology of IMS.