Author Report for: Costa LG

Increasing interest has recently focused on determining whether several natural compounds, collectively referred to as nutraceuticals, may exert neuroprotective actions in the developing, adult, and aging nervous system. Quercetin, a polyphenol widely present in nature, has received the most attention in this regard. Several studies in vitro, in experimental animals and in humans, have provided supportive evidence for neuroprotective effects of quercetin, either against neurotoxic chemicals or in various models of neuronal injury and neurodegenerative diseases. The exact mechanisms of such protective effects remain elusive, though many hypotheses have been formulated. In addition to a possible direct antioxidant effect, quercetin may also act by stimulating cellular defenses against oxidative stress. Two such pathways include the induction of Nrf2-ARE and induction of the antioxidant/anti-inflammatory enzyme paraoxonase 2 (PON2). In addition, quercetin has been shown to activate sirtuins (SIRT1), to induce autophagy, and to act as a phytoestrogen, all mechanisms by which quercetin may provide its neuroprotection.

Paraoxonase-1 (PON1), an esterase/lactonase primarily associated with plasma high-density lipoprotein (HDL), was the first member of this family of enzymes to be characterized. Its name was derived from its ability to hydrolyze paraoxon, the toxic metabolite of the insecticide parathion. Related enzymes PON2 and PON3 were named from their evolutionary relationship with PON1. Mice with each PON gene knocked out were generated at UCLA and have been key for elucidating their roles in organophosphorus (OP) metabolism, cardiovascular disease, innate immunity, obesity, and cancer. PON1 status, determined with two-substrate analyses, reveals an individual's functional Q192R genotype and activity levels. The three-dimensional structure for a chimeric PON1 has been useful for understanding the structural properties of PON1 and for engineering PON1 as a catalytic scavenger of OP compounds. All three PONs hydrolyze microbial N-acyl homoserine lactone quorum sensing factors, quenching Pseudomonas aeruginosa's pathogenesis. All three PONs modulate oxidative stress and inflammation. PON2 is localized in the mitochondria and endoplasmic reticulum. PON2 has potent antioxidant properties and is found at 3- to 4-fold higher levels in females than males, providing increased protection against oxidative stress, as observed in primary cultures of neurons and astrocytes from female mice compared with male mice. The higher levels of PON2 in females may explain the lower frequency of neurological and cardiovascular diseases in females and the ability to identify males but not females with Parkinson's disease using a special PON1 status assay. Less is known about PON3; however, recent experiments with PON3 knockout mice show them to be susceptible to obesity, gallstone formation and atherosclerosis. Like PONs 1 and 2, PON3 also appears to modulate oxidative stress. It is localized in the endoplasmic reticulum, mitochondria and on HDL. Both PON2 and PON3 are upregulated in cancer, favoring tumor progression through mitochondrial protection against oxidative stress and apoptosis.

Paraoxonase 2 (PON2) is a member of the paraoxonase gene family also comprising PON1 and PON3. PON2 functions as a lactonase and exhibits anti-bacterial as well as antioxidant properties. At the cellular level, PON2 localizes to the mitochondrial and endoplasmic reticulum membranes where it scavenges reactive oxygen species. PON2 is of particular interest as it is the only paraoxonase expressed in brain tissue and appears to play a critical role in mitigating oxidative stress in the brain. The aim of this study was to investigate the expression of PON2 at the protein and mRNA level in the brain and liver of mice through development to identify potential age windows of susceptibility to oxidative stress, as well as to compare expression of hepatic PON2 to expression of PON1 and PON3. Overall, PON2 expression in the brain was lower in neonatal mice and increased with age up to postnatal day (PND) 21, with a significant decrease observed at PND 30 and 60. In contrast, the liver showed continuously increasing levels of PON2 with age, similar to the patterns of PON1 and PON3. PON2 protein levels were also investigated in brain samples from non-human primates, with PON2 increasing with age up to the infant stage and decreasing at the juvenile stage, mirroring the results observed in the mouse brain. These variable expression levels of PON2 suggest that neonatal and young adult animals may be more susceptible to neurological insult by oxidants due to lower levels of PON2 in the brain.

In addition to the well-established effects of air pollution on the cardiovascular and respiratory systems, emerging evidence has implicated it in inducing negative effects on the central nervous system. Diesel exhaust particulate matter (DEP), a major component of air pollution, is a complex mixture of numerous toxicants. Limited studies have shown that DEP-induced dopaminergic neuron dysfunction is mediated by microglia, the resident immune cells of the brain. Here we show that mouse microglia similarly mediate primary cerebellar granule neuron (CGN) death in vitro. While DEP (0, 25, 50, 100mug/2cm2) had no effect on CGN viability after 24h of treatment, in the presence of primary cortical microglia neuronal cell death increased by 2-3-fold after co-treatment with DEP, suggesting that microglia are important contributors to DEP-induced CGN neurotoxicity. DEP (50mug/2cm2) treatment of primary microglia for 24h resulted in morphological changes indicative of microglia activation, suggesting that DEP may induce the release of cytotoxic factors. Microglia-conditioned medium after 24h treatment with DEP, was also toxic to CGNs. DEP caused a significant increase in reactive oxygen species in microglia, however, antioxidants failed to protect neurons from DEP/microglia-induced toxicity. DEP increased mRNA levels of the pro-inflammatory cytokines IL-6 and IL1-beta, and the release of IL-6. The antibiotic minocycline (50muM) and the peroxisome proliferator-activated receptor-gamma agonist pioglitazone (50muM) attenuated DEP-induced CGN death in the co-culture system. Microglia and CGNs from male mice appeared to be somewhat more susceptible to DEP neurotoxicity than cells from female mice possibly because of lower paraoxonase-2 expression. Together, these results suggest that microglia-induced neuroinflammation may play a critical role in modulating the effect of DEP on neuronal viability. .

Chlorpyrifos oxon (CPO), the toxic metabolite of the organophosphorus (OP) insecticide chlorpyrifos, causes developmental neurotoxicity in humans and rodents. CPO is hydrolyzed by paraoxonase-1 (PON1), with protection determined by PON1 levels and the human Q192R polymorphism. To examine how the Q192R polymorphism influences fetal toxicity associated with gestational CPO exposure, we measured enzyme inhibition and fetal-brain gene expression in wild-type (PON1(+/+)), PON1-knockout (PON1(-/-)), and tgHuPON1R192 and tgHuPON1Q192 transgenic mice. Pregnant mice exposed dermally to 0, 0.50, 0.75, or 0.85 mg/kg/d CPO from gestational day (GD) 6 through 17 were sacrificed on GD18. Biomarkers of CPO exposure inhibited in maternal tissues included brain acetylcholinesterase (AChE), red blood cell acylpeptide hydrolase (APH), and plasma butyrylcholinesterase (BChE) and carboxylesterase (CES). Fetal plasma BChE was inhibited in PON1(-/-) and tgHuPON1Q192, but not PON1(+/+) or tgHuPON1R192 mice. Fetal brain AChE and plasma CES were inhibited in PON1(-/-) mice, but not in other genotypes. Weighted gene co-expression network analysis identified five gene modules based on clustering of the correlations among their fetal-brain expression values, allowing for correlation of module membership with the phenotypic data on enzyme inhibition. One module that correlated highly with maternal brain AChE activity had a large representation of homeobox genes. Gene set enrichment analysis revealed multiple gene sets affected by gestational CPO exposure in tgHuPON1Q192 but not tgHuPON1R192 mice, including gene sets involved in protein export, lipid metabolism, and neurotransmission. These data indicate that maternal PON1 status modulates the effects of repeated gestational CPO exposure on fetal-brain gene expression and on inhibition of both maternal and fetal biomarker enzymes.

Paraoxonase 2 (PON2) is a member of a gene family which also includes the more studied PON1, as well as PON3. PON2 is unique among the three PONs, as it is expressed in brain tissue. PON2 is a lactonase and displays anti-oxidant and anti-inflammatory properties. PON2 levels are highest in dopaminergic regions (e.g. striatum), are higher in astrocytes than in neurons, and are higher in brain and peripheral tissues of female mice than male mice. At the sub-cellular level, PON2 localizes primarily in mitochondria, where it scavenges superoxides. Lack of PON2 (as in PON2(-/-) mice), or lower levels of PON2 (as in male mice compared to females) increases susceptibility to oxidative stress-induced toxicity. Estradiol increases PON2 expression in vitro and in vivo, and provides neuroprotection against oxidative stress. Such neuroprotection is not present in CNS cells from PON2(-/-) mice. Similar results are also found with the polyphenol quercetin. PON2, given its cellular localization and antioxidant and anti-inflammatory actions, may represent a relevant enzyme involved in neuroprotection, and may represent a novel target for neuroprotective strategies. Its differential expression in males and females may explain gender differences in the incidence of various diseases, including neurodevelopmental, neurological, and neurodegenerative diseases.

Evidence from in vivo and epidemiological studies suggests that organophosphorus insecticides (OPs) are developmental neurotoxicants, but possible underlying mechanisms are still unclear. Astrocytes are increasingly recognized for their active role in normal neuronal development. This study sought to investigate whether the widely-used OP diazinon (DZ), and its oxygen metabolite diazoxon (DZO), would affect glial-neuronal interactions as a potential mechanism of developmental neurotoxicity. Specifically, we investigated the effects of DZ and DZO on the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons. The results show that both DZ and DZO adversely affect astrocyte function, resulting in inhibited neurite outgrowth in hippocampal neurons. This effect appears to be mediated by oxidative stress, as indicated by OP-induced increased reactive oxygen species production in astrocytes and prevention of neurite outgrowth inhibition by antioxidants. The concentrations of OPs were devoid of cytotoxicity, and cause limited acetylcholinesterase inhibition in astrocytes (18 and 25% for DZ and DZO, respectively). Among astrocytic neuritogenic factors, the most important one is the extracellular matrix protein fibronectin. DZ and DZO decreased levels of fibronectin in astrocytes, and this effect was also attenuated by antioxidants. Underscoring the importance of fibronectin in this context, adding exogenous fibronectin to the co-culture system successfully prevented inhibition of neurite outgrowth caused by DZ and DZO. These results indicate that DZ and DZO increase oxidative stress in astrocytes, and this in turn modulates astrocytic fibronectin, leading to impaired neurite outgrowth in hippocampal neurons.

Evidence demonstrating that human exposure to various organophosphorus insecticides (OPs) is associated with neurobehavioral deficits in children continues to emerge. The present study focused on diazinon (DZ) and its active oxygen metabolite, diazoxon (DZO), and explored their ability to impair neurite outgrowth in rat primary hippocampal neurons as a mechanism of developmental neurotoxicity. Both DZ and DZO (0.5-10muM) significantly inhibited neurite outgrowth in hippocampal neurons, at concentrations devoid of any cyototoxicity. These effects appeared to be mediated by oxidative stress, as they were prevented by antioxidants (melatonin, N-t-butyl-alpha-phenylnitrone, and glutathione ethyl ester). Inhibition of neurite outgrowth was observed at concentrations below those required to inhibit the catalytic activity of acetylcholinesterase. The presence of astrocytes in the culture was able to provide protection against inhibition of neurite outgrowth by DZ and DZO. Astrocytes increased neuronal glutathione (GSH) in neurons, to levels comparable to those of GSH ethyl ester. Astrocytes depleted of GSH by l-buthionine-(S,R)-sulfoximine no longer conferred protection against DZ- and DZO-induced inhibition of neurite outgrowth. The findings indicate that DZ and DZO inhibit neurite outgrowth in hippocampal neurons by mechanisms involving oxidative stress, and that these effects can be modulated by astrocytes and astrocyte-derived GSH. Oxidative stress from other chemical exposures, as well as genetic abnormalities that result in deficiencies in GSH synthesis and regulation, may render individuals more susceptible to these developmental neurotoxic effects of OPs.

Paraoxonase (PON1) is an A-esterase capable of hydrolyzing the active metabolites (oxons) of a number of organophosphorus (OP) insecticides such as parathion, diazinon and chlorpyrifos. PON1 activity is highest in liver and in plasma. Human PON1 displays two polymorphisms in the coding region (Q192R and L55M) and several polymorphisms in the promoter and the 3'-UTR regions. The Q192R polymorphism imparts differential catalytic activity toward some OP substrates, while the polymorphism at position -108 (C/T) is the major contributor of differences in the levels of PON1 expression. Both contribute to determining an individual's PON1 "status". Animal studies have shown that PON1 is an important determinant of OP toxicity. Administration of exogenous PON1 to rats or mice protects them from the toxicity of specific OPs. PON1 knockout mice display a high sensitivity to the toxicity of diazoxon and chlorpyrifos oxon, but not of paraoxon. In vitro catalytic efficiencies of purified PON192 alloforms for hydrolysis of specific oxon substrates accurately predict the degree of in vivo protection afforded by each isoform. Evidence is slowly emerging that a low PON1 status may increase susceptibility to OP toxicity in humans. Low PON1 activity may also contribute to the developmental toxicity and neurotoxicity of OPs, as shown by animal and human studies.

Quercetin is a common flavonoid polyphenol which has been shown to exert neuroprotective actions in vitro and in vivo. Though quercetin has antioxidant properties, it has been suggested that neuroprotection may be ascribed to its ability of inducing the cell's own defense mechanisms. The present study investigated whether quercetin could increase the levels of paraoxonase 2 (PON2), a mitochondrial enzyme expressed in brain cells, which has been shown to have potent antioxidant properties. PON2 protein, mRNA, and lactonase activity were highest in mouse striatal astrocytes. Quercetin increased PON2 levels, possibly by activating the JNK/AP-1 pathway. The increased PON2 levels induced by quercetin resulted in decreased oxidative stress and ensuing toxicity induced by two oxidants. The neuroprotective effect of quercetin was significantly diminished in cells from PON2 knockout mice. These findings suggest that induction of PON2 by quercetin represents an important mechanism by which this polyphenol may exert its neuroprotective action.

Paraoxonase 2 (PON2), a member of a gene family that also includes PON1 and PON3, is expressed in most tissues, including the brain. In mouse brain, PON2 levels are highest in dopaminergic areas (e.g., striatum) and are higher in astrocytes than in neurons. PON2 is primarily located in mitochondria and exerts a potent antioxidant effect, protecting mouse CNS cells against oxidative stress. The aim of this study was to characterize PON2 expression and functions in the brains of male and female mice. Levels of PON2 (protein, mRNA, and lactonase activity) were higher in brain regions and cells of female mice. Astrocytes and neurons from male mice were significantly more sensitive (by 3- to 4-fold) to oxidative stress-induced toxicity than the same cells from female mice. Glutathione levels did not differ between genders. Importantly, no significant gender differences in susceptibility to the same oxidants were seen in cells from PON2(-/-) mice. Treatment with estradiol induced a time- and concentration-dependent increase in the levels of PON2 protein and mRNA in male (4.5-fold) and female (1.8-fold) astrocytes, which was dependent on activation of estrogen receptor-alpha. In ovariectomized mice, PON2 protein and mRNA were decreased to male levels in brain regions and in liver. Estradiol protected astrocytes from wild-type mice against oxidative stress-induced neurotoxicity, but did not protect cells from PON2(-/-) mice. These results suggest that PON2 is a novel major intracellular factor that protects CNS cells against oxidative stress and confers gender-dependent susceptibility to such stress. The lower expression of PON2 in males may have broad ramifications for susceptibility to diseases involving oxidative stress, including neurodegenerative diseases.

Exposure to organophosphorus (OP) compounds can lead to serious neurological damage or death. Following bioactivation by the liver cytochromes P450, the OP metabolites produced are potent inhibitors of serine active-site enzymes including esterases, proteases and lipases. OPs may form adducts on other cellular proteins. Blood cholinesterases (ChEs) have long served as biomarkers of OP exposure in humans. However, the enzymatic assays used for biomonitoring OP exposures have several drawbacks. A more useful approach will focus on multiple biomarkers and avoid problems with the enzymatic activity assays. OP inhibitory effects result from a covalent bond with the active-site serine of the target enzymes. The serine OP adducts become irreversible following a process referred to as aging where one alkyl group dissociates over variable lengths of time depending on the OP adduct. The OP-adducted enzyme then remains in circulation until it is degraded, allowing for a longer window of detection compared with direct analysis of OPs or their metabolites. Mass spectrometry (MS) provides a very sensitive method for identification of post-translational protein modifications. MS analyses of the percentage adduction of the active-site serine of biomarker proteins such as ChEs will eliminate the need for basal activity levels of the individual and will provide for a more accurate determination of OP exposure. MS analysis of biomarker proteins also provides information about the OP that has caused inhibition. Other useful biomarker proteins include other serine hydrolases, albumin, tubulin and transferrin.

Chlorpyrifos (CPF), one of the most widely-used organophosphorus (OP) insecticides in agriculture, is degraded in the field to its oxon form, chlorpyrifos-oxon (CPO), which can represent a significant contaminant in exposures to adults and children. CPO is also responsible for the acetylcholinesterase (AChE) inhibition associated with CPF exposures; CPF is converted by liver CYP450 enzymes to CPO, which binds to and inhibits AChE and other serine active-site esterases, lipases and proteases. Young children represent a particularly susceptible population for exposure to CPF and CPO, in part because levels of the plasma enzyme, paraoxonase (PON1), which hydrolyzes CPO, are very low during early development. While a number of studies have demonstrated developmental neurotoxicity associated with CPF exposure, including effects at or below the threshold levels for AChE inhibition, it is unclear whether these effects were due directly to CPF or to its active metabolite, CPO. PON1 knockout (PON1-/-) mice, which lack PON1, represent a highly sensitive mouse model for toxicity associated with exposure to CPF or CPO. To examine the neurobehavioral consequences of CPO exposure during postnatal development, PON1-/- mice were exposed daily from PND 4 to PND 21 to CPO at 0.15, 0.18, or 0.25 mg/kg/d. A neurobehavioral test battery did not reveal significant effects of CPO on early reflex development, motor coordination, pre-pulse inhibition of startle, startle amplitude, open field behavior, or learning and memory in the contextual fear conditioning, Morris water maze, or water radial-arm maze tests. However, body weight gain and startle latency were significantly affected by exposure to 0.25 mg/kg/d CPO. Additionally, from PNDs 15-20 the mice exposed repeatedly to CPO at all three doses exhibited a dose-related transient hyperkinesis in the 20-min period following CPO administration, suggesting possible effects on catecholaminergic neurotransmission. Our previous study demonstrated wide-ranging effects of neonatal CPO exposure on gene expression in the brain and on brain AChE inhibition, and modulation of both of these effects by the PON1(Q192R) polymorphism. The current study indicates that the neurobehavioral consequences of these effects are more elusive, and suggests that alternative neurobehavioral tests might be warranted, such as tests of social interactions, age-dependent effects on learning and memory, or tests designed specifically to assess dopaminergic or noradrenergic function.

Microarray analysis was used to examine effects of repeated postnatal exposure to chlorpyrifos oxon (CPO) on gene expression in the cerebellum of genetically modified mice. The high-density lipoprotein-associated enzyme paraoxonase 1 (PON1) plays a significant role in the detoxication of CPO, which is present in exposures and generated from chlorpyrifos (CPF) in vivo following exposure. Two factors are important in modulating toxicity of CPO, the Q192R PON1 polymorphism and PON1 plasma level, which is low at birth and increases throughout postnatal development. Mice used in these studies included wild type (PON1(+/+)), PON1 knockout (PON1(-/-)), and two transgenic lines (tgHuPON1(Q192), tgHuPON1(R192)) expressing either human PON1(Q192) or PON1(R192) on the PON1(-/-) background. PON1(R192) hydrolyzes CPO more efficiently than PON1(Q192). All four genotypes exposed to CPO (0.35 or 0.50 mg/kg/day) daily from postnatal day (PND) 4 to PND 21 showed significant differences in gene expression on PND 22 compared with controls. Pathway analysis and Gene Set Analysis revealed multiple pathways and gene sets significantly affected by CPO exposure, including genes involved in mitochondrial dysfunction, oxidative stress, neurotransmission, and nervous system development. Comparison between genotypes revealed specific genes, gene sets, and pathways differentially affected between tgHuPON1(Q192) and tgHuPON1(R192) mice and between PON1(-/-) and PON1(+/+) mice following CPO exposure. Repeated CPO exposure also resulted in a dose-related decrease in brain acetylcholinesterase activity during postnatal development in PON1(-/-) and tgHuPON1(Q192) mice but not in PON1(+/+) or tgHuPON1(R192) mice. These findings indicate that PON1 status plays a critical role in modulating the effects of neonatal CPO exposure in the developing brain.

Paraoxonase 1 (PON1) is a high density lipoprotein (HDL)-associated enzyme displaying esterase and lactonase activity. PON1 hydrolyzes several organophosphorus (OP) insecticides and nerve agents, a number of exogenous and endogenous lactones, and metabolizes toxic oxidized lipids of low density lipoproteins (LDL) and HDL. As such, PON1 plays a relevant role in determining susceptibility to OP toxicity, cardiovascular diseases and several other diseases. Serum PON1 activity in a given population can vary by at least 40-fold. Most of this variation can be accounted for by genetic polymorphisms in the coding region (Q192R, L55M) and in the promoter region (T-108C). However, exogenous factors may also modulate PON1 activity and/or level of expression. This paper examines various factors that have been found to positively modulate PON1. Certain drugs (e.g. hypolipemic and anti-diabetic compounds), dietary factors (antioxidants, polyphenols), and life-style factors (moderate alcohol consumption) appear to increase PON1 activity. Given the relevance of PON1 in protecting from certain environmental exposure and from cardiovascular and other diseases, there is a need for further mechanistic, animal, and clinical research in this area, and for consideration of possible alternative strategies for increasing the levels and activity of PON1.

The aims of this study were to characterize the expression of paraoxonase 2 (PON2) in mouse brain and to assess its antioxidant properties. PON2 levels were highest in the lung, intestine, heart and liver, and lower in the brain; in all tissues, PON2 expression was higher in female than in male mice. PON2 knockout [PON2(-/-)] mice did not express any PON2, as expected. In the brain, the highest levels of PON2 were found in the substantia nigra, the nucleus accumbens and the striatum, with lower levels in the cerebral cortex, hippocampus, cerebellum and brainstem. A similar regional distribution of PON2 activity (measured by dihydrocoumarin hydrolysis) was also found. PON3 was not detected in any brain area, while PON1 was expressed at very low levels, and did not show any regional difference. PON2 levels were higher in astrocytes than in neurons isolated from all brain regions, and were highest in cells from the striatum. PON2 activity and mRNA levels followed a similar pattern. Brain PON2 levels were highest around birth, and gradually declined. Subcellular distribution experiments indicated that PON2 is primarily expressed in microsomes and in mitochondria. The toxicity in neurons and astrocytes of agents known to cause oxidative stress (DMNQ and H(2)O(2)) was higher in cells from PON2(-/-) mice than in the same cells from wild-type mice, despite similar glutathione levels. These results indicate that PON2 is expressed in the brain, and that higher levels are found in dopaminergic regions such as the striatum, suggesting that this enzyme may provide protection against oxidative stress-mediated neurotoxicity.

Most chemical exposures involve complex mixtures. The role of paraoxonase 1 (PON1) and the Q192R polymorphism in the detoxication of individual organophosphorous (OP) compounds has been well-established. The extent to which PON1 protects against a given OP is determined by its catalytic efficiency. We used a humanized transgenic mouse model of the Q192R polymorphism to demonstrate that PON1 modulates the toxicity of OP mixtures by altering the activity of another detoxication enzyme, carboxylesterase (CaE). Chlorpyrifos oxon (CPO), diazoxon (DZO), and paraoxon (PO) are potent inhibitors of CaE, both in vitro and in vivo. We hypothesized that exposure of mice to these OPs would increase their sensitivity to the CaE substrate, malaoxon (MO), and that the degree of effect would vary among PON1 genotypes if the OP was a physiologically relevant PON1 substrate. When wild-type mice were exposed dermally to CPO, DZO, or PO and then, after 4 h, to different doses of MO, the toxicity of MO was increased compared to mice that received MO alone. The potentiation of MO toxicity by CPO and DZO was higher in PON1 knockout mice, which are less able to detoxify CPO or DZO. Potentiation by CPO was higher in Q192 mice than in R192 mice due to the decreased ability of PON1(Q192) to detoxify CPO. Potentiation by DZO was similar in the Q192 and R192 mice, due to their equivalent effectiveness at detoxifying DZO. PO exposure resulted in equivalent potentiation of MO toxicity among all four genotypes. These results indicate that PON1 status modulates the ability of CaE to detoxicate OP compounds from specific mixed insecticide exposures. PON1 status can also impact the capacity to metabolize drugs or other CaE substrates following insecticide exposure.

Human paraoxonase 1 (PON1) is a high-density lipoprotein (HDL)-associated serum enzyme that exhibits a broad substrate specificity. In addition to protecting against exposure to some organophosphorus (OP) pesticides by hydrolyzing their toxic oxon metabolites, PON1 is important in protecting against vascular disease by metabolizing oxidized lipids. Recently, PON1 has also been shown to play a role in inactivating the quorum sensing factor N-(3-oxododecanoyl)-l-homoserine lactone (3OC12-HSL) of Pseudomonas aeruginosa. Native, untagged engineered recombinant human PON1 (rHuPON1) expressed in Escherichia coli and purified by conventional column chromatographic purification is stable, active, and capable of protecting PON1 knockout mice (PON1(-/-)) from exposure to high levels of the OP compound diazoxon. The bacterially derived rHuPON1 can be produced in large quantities and lacks the glycosylation of eukaryotic systems that can produce immunogenic complications when inappropriately glycosylated recombinant proteins are used as therapeutics. Previous studies have shown that the determination of PON1 status, which reveals both PON1(192) functional genotype and serum enzyme activity level, is required for a meaningful evaluation of PON1's role in risk of disease or exposure. We have developed a new two-substrate assay/analysis protocol that provides PON1 status without use of toxic OP substrates, allowing for use of this protocol in non-specialized laboratories. Factors were also determined for inter-converting rates of hydrolysis of different substrates. PON1 status also plays an important role in revealing changes in HDL-associated PON1 activities in male patients with Parkinson disease (PD). Immunolocalization studies of PONs 1, 2 and 3 in nearly all mouse tissues suggest that the functions of PONs 1 and 3 extend beyond the plasma and the HDL particle.

A transgenic mouse model of the human hPON1(Q192R) polymorphism was used to address the role of paraoxonase (PON1) in modulating toxicity associated with exposure to mixtures of organophosphorus (OP) compounds. Chlorpyrifos oxon (CPO), diazoxon (DZO), and paraoxon (PO) are potent inhibitors of carboxylesterases (CaE). We hypothesized that a prior exposure to these OPs would increase sensitivity to malaoxon (MO), a CaE substrate, and the degree of the effect would vary among PON1 genotypes if the OP was a physiologically significant PON1 substrate in vivo. CPO and DZO are detoxified by PON1. For CPO hydrolysis, hPON1(R192) has a higher catalytic efficiency than hPON1(Q192). For DZO hydrolysis, the two alloforms have nearly equal catalytic efficiencies. For PO hydrolysis, the catalytic efficiency of PON1 is too low to be physiologically relevant. When wild-type mice were exposed dermally to CPO, DZO, or PO followed 4-h later by increasing doses of MO, toxicity was increased compared to mice receiving MO alone, presumably due to CaE inhibition. Potentiation of MO toxicity by CPO and DZO was greater in PON1(-/-) mice, which have greatly reduced capacity to detoxify CPO or DZO. Potentiation by CPO was more pronounced in hPON1(Q192) mice than in hPON1(R192) mice due to the decreased efficiency of hPON1(Q192) for detoxifying CPO. Potentiation by DZO was similar in hPON1(Q192) and hPON1(R192) mice, which are equally efficient at hydrolyzing DZO. Potentiation by PO was equivalent among all four genotypes. These results indicate that PON1 status can have a major influence on CaE-mediated detoxication of OP compounds.

This review examines the large body of toxicological and epidemiological information on human exposures to chlorpyrifos, with an emphasis on the controversial potential for chlorpyrifos to induce neurodevelopmental effects at low doses. The results of this review demonstrate that the use of urinary 3,5,6-trichlorpyridinol (TCPy), a metabolite of chlorpyrifos as a biomarker of nonoccupational exposure is problematic and may overestimate nonoccupational exposures to chlorpyrifos by 10-to 20-fold because of the widespread presence of both TCPy and chlorpyrifos-methyl in the food supply. Current "background" (nonoccupational) levels of exposure to chlorpyrifos are several orders of magnitude lower than those required to inhibit plasma cholinesterase activity, which is a more sensitive target than nervous system cholinesterase. However, several in vitro studies have identified putative neurodevelopmental mechanisms that are altered at concentrations of chlorpyrifos below those that inhibit cholinesterases. Although one human cohort study reported an association between maternal and cord blood chlorpyrifos levels and several measures of neurodevelopment, two other cohort studies that utilized urinary TCPy as a surrogate for chlorpyrifos exposure did not demonstrate an association. Although the weight of the scientific evidence demonstrates that current levels of chlorpyrifos exposure will not have any adverse effects on neurodevelopment that might result from inhibition of nervous system cholinesterases, several recent studies propose alternative mechanisms. Thus, further in vivo investigation on neurodevelopment in an appropriate animal model is needed; additional epidemiological studies may be warranted if a suitable, chlorpyrifos-exposed cohort can be identified and more rigorous measures of exposure are utilized.

Organophosphates (OPs) are one of the main classes of insecticides, in use since the mid 1940s. OPs can exert significant adverse effects in non-target species including humans. Because of the phosphorylation of acetylcholinesterase, they exert primarily a cholinergic toxicity, however, some can also cause a delayed polyneuropathy. Currently debated and investigated issues in the toxicology of OPs are presented in this review. These include: 1) possible long-term effects of chronic low-level exposures; 2) genetic susceptibility to OP toxicity; 3) developmental toxicity and neurotoxicity; 4) common mechanism of action; 5) mechanisms of delayed neurotoxicity; and 6) possible additional OP targets. Continuing and recent debates, and molecular advances in these areas, and their contributions to our understanding of the toxicology of OPs are discussed.

OBJECTIVES: The Q192R polymorphism of paraoxonase (PON1) has been shown to affect hydrolysis of organophosphorus compounds. The Q192 and R192 alloforms exhibit equivalent catalytic efficiencies of hydrolysis for diazoxon, the oxon form of the pesticide (DZ). However, the R192 alloform has a higher catalytic efficiency of hydrolysis than does the Q192 alloform for chlorpyrifos oxon (CPO), the oxon form of the pesticide chlorpyrifos (CPS). The current study examined the relevance of these observations for in-vivo exposures to chlorpyrifos and chlorpyrifos oxon.
METHODS: Using a transgenic mouse model we examined the relevance of the Q192R polymorphism for exposure to CPS and CPO in vivo. Transgenic mice were generated that expressed either human PON1Q192 or PON1R192 at equivalent levels, in the absence of endogenous mouse PON1. Dose-response and time course experiments were performed on adult mice exposed dermally to CPS or CPO. Morbidity and acetylcholinesterase (AChE) activity in the brain and diaphragm were determined in the first 24 h following exposure.
RESULTS: Mice expressing PON1Q192 were significantly more sensitive to CPO, and to a lesser extent CPS, than were mice expressing PON1R192. The time course of inhibition following exposure to 1.2 mg/kg CPO revealed maximum inhibition of brain AChE at 6-12 h, with PON1R192, PON1Q192, and PON1 mice exhibiting 40, 70 and 85% inhibition, respectively, relative to control mice. The effect of PON1 removal on the dose-response curve for CPS exposure was remarkably consistent with a PBPK/PD model of CPS exposure. CONCLUSION: These results indicate that individuals expressing only the PON1Q192 allele would be more sensitive to the adverse effects of CPO or CPS exposure, especially if they are expressing a low level of plasma PON1Q192.

Paraoxonase (PON1) is a liver and plasma enzyme most studied because of its ability to hydrolyze the active metabolites of several organophosphorus insecticides. The discovery that PON1 can also metabolize oxidized phospholipids has spurred research on its possible role in coronary heart disease and atherosclerosis. Additionally, its potential roles in metabolizing pharmaceutical drugs and microbial quorum sensing factors are also being explored. PON1 displays several polymorphisms that influence both its level of expression and its catalytic activity, thus determining the rates at which a given individual will detoxify a specific insecticide, metabolize harmful oxidized lipids, and activate or inactivate specific drugs and quorum sensing factors.

Organophosphorus (OP) compounds are still among the most widely used insecticides, and their main mechanism of acute toxicity is associated with inhibition of acetylcholinesterase. Measurements of urine metabolites and of blood cholinesterase activity are established biomarkers of exposure to OPs and of early biological effects. In recent years, increasing attention has been given to biomarkers of susceptibility to OP toxicity. Here we discuss the polymorphisms of paraoxonase (PON1), a liver and serum enzyme that hydrolyzes a number of OP compounds, and its role in modulating the toxicity of OPs. We stress the importance of determining PON1 status, which encompasses the PON1192Q/R polymorphism (that affects catalytic ability toward different substrates) and PON1 levels (which are modulated in part by a C-108T polymorphism) over straight genotyping. Epidemiological studies on OP-exposed workers that include assessment of PON1 status to validate in human populations the role of PON1 as a determinant of susceptibility to OPs, as indicated by animal studies, are needed. Documentation of exposure and of early health effects would be most relevant to increase the predictive value of the test.

Paraoxonase 1 (PON1) is a serum enzyme closely associated with high density lipoprotein (HDL). PON1 hydrolyzes several organophosphorus compounds used as insecticides, as well as nerve agents; it metabolizes toxic oxidized lipids associated with both low density lipoprotein (LDL) and HDL; and it can hydrolyze a number of lactone-containing pharmaceutical compounds, inactivating some, while activating others. Serum PON1 activity in a given population can vary by 40-fold. Though most of this variation can be explained by polymorphisms in the coding region (Q192R) and the 5' regulatory region (T-108C), modulation of PON1 by a variety of other factors should be taken into account, including other polymorphisms recently discovered but not yet characterized. This paper examines the major factors (environmental chemicals, drugs, smoking, alcohol, diet, age, disease conditions) that have been shown to modulate PON1 activity in either direction. As PON1 plays a protective role in organophosphate toxicity, and, because of its antioxidant capacity, in cardiovascular disease, a better understanding of how PON1 can be modulated by environmental factors has potential toxicological and clinical consequences.

Individual differences in detoxication capacities for specific organophosphorous (OP) compounds are due largely to differences in catalytic efficiency or abundance of the HDL-associated enzyme, paraoxonase (PON1). First, we provide evidence that children less than 2 years of age represent a particularly susceptible population for OP exposure due to low abundance of PON1 and variable onset of plasma PON1 activity. Second, we describe studies examining the neurotoxic effects of chronic, low-level OP pesticide exposure in mice. PON1 knockout (PON1(-/-)) and wild-type mice were exposed chronically (PN4 to PN21) to low levels of chlorpyrifos oxon (CPO). Endpoints included cholinesterase activity, histopathology, gene expression, and behavior. Even at PN4, when PON1 levels were low in wild-type mice, PON1(-/-) mice were more sensitive to inhibition of brain cholinesterase by CPO. At PN22, and persisting as long as 4 months, chronic developmental exposure to 0.18 mg/kg/d or 0.25 mg/kg/d CPO resulted in perinuclear vacuolization of cells in a discrete area of the neocortex and irregular distribution of neurons in the cortical plate, with an increase in the number of affected cells at 0.25mg/kg/d. Third, we describe a transgenic mouse model in which human transgenes encoding either hPON1Q192 or hPON1R192 were expressed at equal levels in place of mouse PON1. The developmental onset of expression followed the mouse time course and was identical for the two transgenes, allowing these mice to be used to assess the importance of the Q192R polymorphism during development. Adult mice expressing hPON1R192 were significantly more resistant than hPON1Q192 mice to CPO toxicity. Our studies indicate that children less than 2 years old, especially those homozygous for PON1Q192, would be predicted to be particularly susceptible to CPO toxicity.

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to investigate the influence of glial cells on the neurotoxicity of two organophosphorus pesticides (OPs), chlorpyrifos and parathion. Mixed-cell aggregate cultures were treated continuously for 10 days between DIV 5 and 15. Parathion induced astrogliosis at concentration at which MAP-2 immunostaining, found here to be more sensitive than neuron-specific enzyme activities, was not affected. In contrast, chlorpyrifos induced a comparatively weak gliotic reaction, and only at concentrations at which neurons were already affected. After similar treatments, increased neurotoxicity of parathion and chlorpyrifos was found in aggregate cultures deprived of glial cells. These results suggest that glial cells provide neuroprotection against OPs toxicity. To address the question of the difference in toxicity between parathion and chlorpyrifos, the toxic effects of their leaving groups, p-nitrophenol and trichloropyridinol, were studied in mixed-cell aggregates. General cytotoxicity was more pronounced for trichloropyridinol and both compounds had similar toxic effects on neuron-specific enzyme activities. In contrast, trichloropyridinol induced a much stronger decrease in glutamine synthetase activity, the enzymatic marker of astrocytes. Trichloropyridinol may exert a toxic effect on astrocytes, compromising their neuroprotective function, thus exacerbating the neurotoxicity of chlorpyrifos. This is in line with the suggestion that glial cells may contribute to OPs neurotoxicity, and with the view that OPs may exert their neurotoxic effects through different mechanisms.

Susceptibility to organophosphorus (OP) insecticides and nerve agents is strongly influenced by genetic and developmental factors. A number of organophosphorothioate insecticides are detoxified in part via a two-step pathway involving bioactivation of the parent compound by the cytochrome P450 systems, then hydrolysis of the resulting oxygenated metabolite (oxon) by serum and liver paraoxonases (PON1). Serum PON1 has been shown to be polymorphic in human populations. The Arg192 isoform (PON1R192) of this HDL-associated protein hydrolyzes paraoxon (POX) at a high rate, while the Gln192 isoform (PON1Q192) hydrolyzes paraoxon at a low rate. The effect of the polymorphism is reversed for the hydrolysis of diazoxon (DZO), soman and particularly sarin. Phenylacetate is hydrolyzed at approximately the same rate by both PON1 isoforms and chlorpyrifos oxon (CPO) slightly faster by the PON1R192 isoform. In addition to the effect of the amino acid substitution on rates of toxicant hydrolysis, two other factors influence these rates. The expression of PON1 is developmentally regulated. Newborns have very low levels of PON1. Adult levels in rats and mice are reached at 3 weeks of age and in humans, sometime after 6 months of age. In addition, among individuals of a given genotype, there is at least a 13-fold difference in expression of PON1 that is stable over time. Dose/response experiments with normal mice injected with purified PON1 and with PON1 knockout mice have clearly demonstrated that the observed differences of in vitro rates of hydrolysis are significant in determining differential sensitivities to specific insecticides processed through the P450/PON1 pathway. Injection of purified rabbit PON1 protects mice from cholinesterase inhibition by chlorpyrifos (CPS) and CPO. Knockout mice are much more sensitive to CPO and DZO than are their PON1+/+ littermates or wild-type mice. A number of recent reports have also indicated that the PON1R192 isoform may be a risk factor for cardiovascular disease. Studies with PON1 knockout mice are also consistent with a role of PON1 in preventing vascular disease.

Human paraoxonase (PON1) is a polymorphic, high-density lipoprotein (HDL)-associated esterase that hydrolyzes the toxic metabolites of several organophosphorus (OP) insecticides and nerve agents. The activity polymorphism is determined by a Gln/Arg (Q/R) substitution at position 192. Injection of purified PON1 protects animals from OP poisoning. In the present study, we investigated the in-vivo function of PON1 for detoxifying organophosphorus insecticides in PON1-knockout mice that were challenged via dermal exposure with diazoxon, diazinon and paraoxon. PON1-knockout mice were extremely sensitive to diazoxon. Doses (2 and 4 mg/kg) that caused no cholinesterase (ChE) inhibition in wild-type mice were lethal to the knockout mice, which also showed slightly increased sensitivity to the parent compound diazinon. Surprisingly, these knockout mice did not show increased sensitivity to paraoxon. In-vitro assays indicated that the PON1R192 isoform hydrolyzed diazoxon less rapidly than did the PON1Q192 isoform. In-vivo analysis, where PON1-knockout mice received the same amount of either PON1(192) isoform via intraperitoneal (i.p.) injection 4 h prior to exposure, showed that both isoforms provided a similar degree of protection against diazoxon, while PON1R192 conferred better protection against chlorpyrifos-oxon than PON1Q192. Injection of purified rabbit PON1 or either human PON1(192) isoform did not protect PONI-knockout mice from paraoxon toxicity, nor did over-expression of the human PON1R192 transgene in wild-type mice. Kinetic analysis of the two human PON1(192) isoforms revealed that the catalytic efficiency (Vmax/Km) determines the in-vivo efficacy of PON1 for organophosphorus detoxication. The results indicate that PON1 plays a major role in the detoxication of diazoxon and chlorpyrifos oxon but not paraoxon.

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to study the maturation-dependent sensitivity of brain cells to two organophosphorus pesticides (OPs), chlorpyrifos and parathion, and to their oxon derivatives. Immature (DIV 5-15) or differentiated (DIV 25-35) brain cells were treated continuously for 10 days. Acetylcholinesterase (AChE) inhibitory potency for the OPs was compared to that of eserine (physostigmine), a reversible AChE inhibitor. Oxon derivatives were more potent AChE inhibitors than the parent compounds, and parathion was more potent than chlorpyrifos. No maturation-dependent differences for AChE inhibition were found for chlorpyrifos and eserine, whereas for parathion and paraoxon there was a tendency to be more effective in immature cultures, while the opposite was true for chlorpyrifos-oxon. Toxic effects, assessed by measuring protein content as an index of general cytotoxicity, and various enzyme activities as cell-type-specific neuronal and glial markers (ChAT and GAD, for cholinergic and GABAergic neurons, respectively, and GS and CNP, for astrocytes and oligodendrocytes, respectively) were only found at more than 70% of AChE inhibition. Immature compared to differentiated cholinergic neurons appeared to be more sensitive to OP treatments. The oxon derivates were found to be more toxic on neurons than the parent compounds, and chlorpyrifos was more toxic than parathion. Eserine was not neurotoxic. These results indicate that inhibition of AChE remains the most sensitive macromolecular target of OP exposure, since toxic effects were found at concentrations in which AChE was inhibited. Furthermore, the compound-specific reactions, the differential pattern of toxicity of OPs compared to eserine, and the higher sensitivity of immature brain cells suggest that the toxic effects and inhibition of AChE are unrelated.

In human populations, serum paraoxonase (PON1) exhibits a substrate dependent polymorphism. The Arg192 isoform hydrolyzes paraoxon rapidly but diazoxon, soman and especially sarin slowly. On the other hand, the Gln192 isoform hydrolyzes paraoxon slowly, but diazoxon, soman and sarin more rapidly than the Arg192 isoform. Our experiments with a mouse model system have convincingly shown that PON1 plays a major role in the detoxication of organophosphate (OP) compounds processed through the P450/PON1 pathway. Recent studies have also shown that PON1 plays an important role in the metabolism of oxidized lipid compounds. Currently, there is an effort underway to identify genes and polymorphisms that play an important role in 'environmental susceptibility'. The PON1 polymorphism has been cited as a prime example of such a genetic polymorphism. The advent of the polymerase chain reaction (PCR) for DNA amplification with improvements, modifications and automation has provided a very convenient way to do individual genotyping. It is tempting to set up large scale PCR analyses of populations to determine individuals at risk for environmental exposures affected by the PON1 polymorphism. In fact, a number of such studies have already been carried out in examining the relationship of the PON1 polymorphism to vascular disease. We advocate the use of a high throughput two-dimensional enzyme assay that provides both PON1 genotype and phenotype (PON1 status). The high level of variation of gene expression within each genetic class in humans, together with our animal model studies indicate that it is very important to determine PON status as opposed to PON1 genotype alone. Experiments in rats and mice have shown that injection of PON1 purified from rabbit serum by the i.v., i.p. or i.m. route, significantly increases PON1 activities in rodents' plasma. Under these conditions, the acute toxicity (assessed by the degree of acetylcholinesterase inhibition) of paraoxon and chlorpyrifos oxon is significantly decreased, compared to control animals. Protection is maximal when PON1 is administered before the OPs, but still occurs when PON1 is utilized as a post-exposure treatment. Furthermore, protection by PON1 is also provided toward the parent compound chlorpyrifos. Pon1-knockout mice display a much greater sensitivity to chlorpyrifos oxon toxicity than wild mice. However, the acute toxicity of guthion, which is not a substrate for PON1, does not differ between knockout and wild mice. These observations underline the importance of considering both genetic variability of enzyme isoform as well as enzyme level (PON1 status) and the developmental time course of appearance of PON1 in developing risk assessment models.

Exposure of mature rat cerebellar granule neurons to non-depolarizing conditions (5 mM K+) for 24 h resulted in the onset of apoptosis. NMDA, forskolin, carbachol and GABA attenuated low K+-induced toxicity, although to a different extent, with NMDA and GABA being the most effective agents. When cells were co-exposed for 24 h to ethanol, the survival promoting action of NMDA and carbachol, but not that of forskolin and GABA, was attenuated. By contrast, a 24 h cell pre-treatment with ethanol, followed by its removal prior to K+ deprivation, was ineffective towards the neurotrophic action of NMDA and carbachol. The concomitant presence of alcohol and neurotrophic factors was not required for the pro-apoptotic effect of ethanol to be manifest after a long-term alcohol exposure: inhibition of NMDA- and carbachol-mediated neurotrophism was still observed when cells were pre-exposed for 72 h to alcohol in depolarizing conditions, prior to the challenge with 5 mM K+-containing medium and the test compounds in the absence of ethanol. The present study shows that ethanol promotes apoptotic cell death of cultured cerebellar neurons by selectively inhibiting the neurotrophic effect of NMDA and carbachol, and suggests that alcohol may cause permanent changes in the control mechanisms of apoptosis: this finding may have significant implications for the in vivo toxicity of prenatal ethanol exposure on the developing cerebellum.

Several organophosphorus insecticides and nerve agents are detoxified through the cytochrome P450/paraoxonase (PON1) pathway. PON1 is an HDL-associated enzyme encoded as a 355 amino acid protein in humans. The PON1 Arg192 isoform hydrolyzes paraoxon rapidly while the Gln192 isoform hydrolyzes this compound slowly. Both isoforms hydrolyze phenylacetate and chlorpyrifos oxon at approximately the same rate. We recently found that the effect of this polymorphism is dramatically reversed for sarin hydrolysis. The PON1 Arg192 isoform has virtually no sarinase activity while the Gln192 isoform has substantial activity. The Gln192 isoform also hydrolyzes diazoxon and soman faster than the Arg192 isoform. In addition to the large differences in rates of hydrolysis observed for some OP substrates by the two PON1 isoforms, there is also a large variability in serum PON1 concentrations that is stable over time between individuals. Thus, two factors govern the PON1 status of a given individual, the PON1 genotype as well as the amount of protein expressed from each allele. A two-dimensional enzyme analysis provides an excellent assessment of an individual's PON1 status, ie. the position 192 genotype as well as phenotype, or level of serum PON1 (Nature Genet 14:334-336). Do these interindividual differences in rates of substrate hydrolysis by PON1 reflect an individual's sensitivity or resistance to OP compounds processed through the P450/PON1 pathway? Injection of purified PON1 into mice clearly demonstrates the protective effect of having high serum levels of PON1 against toxicity by chlorpyrifos oxon or chlorpyrifos. Preliminary experiments with PON1 knockout mice, on the other hand, clearly demonstrate that low PON1 levels result in dramatically increased sensitivity to chlorpyrifos oxon. Attempts to express human PON1 in mice from constructs containing either of the human PON1 cDNA sequences were unsuccessful, despite the generation of the respective transgenic mice.

Serum paraoxonase (PON1) is an esterase that is associated with high-density lipoproteins (HDLs) in the plasma; it is involved in the detoxification of organophosphate insecticides such as parathion and chlorpyrifos. PON1 may also confer protection against coronary artery disease by destroying pro-inflammatory oxidized lipids present in oxidized low-density lipoproteins (LDLs). To study the role of PON1 in vivo, we created PON1-knockout mice by gene targeting. Compared with their wild-type littermates, PON1-deficient mice were extremely sensitive to the toxic effects of chlorpyrifos oxon, the activated form of chlorpyrifos, and were more sensitive to chlorpyrifos itself. HDLs isolated from PON1-deficient mice were unable to prevent LDL oxidation in a co-cultured cell model of the artery wall, and both HDLs and LDLs isolated from PON1-knockout mice were more susceptible to oxidation by co-cultured cells than the lipoproteins from wild-type littermates. When fed on a high-fat, high-cholesterol diet, PON1-null mice were more susceptible to atherosclerosis than their wild-type littermates.

Serum paraoxonase hydrolyses the toxic metabolites of several organophosphorus insecticides, as well as the nerve agents soman and sarin. We have previously shown that elevated serum paraoxonase levels protect mice against organophosphate toxicity. In the present study, we determined the cDNA sequence and chromosomal location of the mouse paraoxonase gene, as well as its developmental expression in mice and rats. The mouse cDNA encodes a protein of 355 amino acids and shows 81% identity with the human sequence. In situ hybridization demonstrated that the mouse paraoxonase gene maps to chromosome 6, a region conserved with the paraoxonase region of chromosome 7q21-22 in humans. Serum paraoxonase activities toward three substrates, paraoxon, chlorpyrifos-oxon and diazoxon, were very low at birth and increased with age reaching adult levels at 20 days in mice and 25 days in rats. The increase of serum paraoxonase activity in developing animals correlates well with the increased resistance to organophosphate poisoning that has been reported in previous studies.

The in vitro effects of mercuric chloride (HgCl2) and methylmercury (CH3HgOH) on the M1 and M2 muscarinic receptor subtypes were investigated in rat brain cortical membranes. HgCl2 and CH3HgOH were almost equipotent in inhibiting the binding of [3H]telenzepine to M1 receptors (IC50s = 2.2 and 3.4 microM, respectively). Conversely, HgCl2 was a thirty-fold more potent inhibitor of [3H]AF-DX 384 binding to M2 sites than CH3HgOH (lC50s = 5 and 149 microM, respectively). In all cases HgCl2 showed steep and monophasic inhibition curves, whereas those of CH3HgOH were biphasic (M1) or shallow (M2). CH3HgOH-induced inhibition of both [3H]telenzepine and [3H]AF-DX 384 binding was of the competitive type, while HgCl2 caused a pronounced reduction of the Bmax value associated with a small change in affinity. CH3HgOH also decreased the affinity of the agonist carbachol for M1 and M2 receptors, while inorganic mercury had minimal effects on the carbachol dose-response curves. These results indicate that inorganic and organic mercury differ in their interaction with muscarinic receptor subtypes and that M1 receptors may represent a preferential target for their effects.

The mitogenic effect of muscarinic receptor agonists in glial cells has been characterized in rat cortical astrocytes and human 132 1N1 astrocytoma cells. The muscarinic receptor agonist carbachol caused a dose- and time-dependent increase in proliferation, as measured by [3H]thymidine incorporation. The mitogenic effect was mimicked by several muscarinic, but not nicotinic receptor agonists, and was blocked by muscarinic receptor antagonists. Reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated the presence of m2, m3 and to a lesser degree, m5 muscarinic receptor mRNA in both astrocytes and astrocytoma cells. Proliferation experiments with subtype-specific muscarinic receptor antagonists suggest that carbachol-induced proliferation is due to activation of muscarinic M3 receptors. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) also stimulated glial cell proliferation. Down-regulation of protein kinase C, or the protein kinase C antagonist 1,5-(isoquinolynsulfanyl)-2-methylpiperazine dihydrochloride (H7) blocked proliferation induced by either TPA or carbachol. Of other neurotransmitters tested, histamine caused glial cell proliferation, norepinephrine and gamma-aminobutyric acid were ineffective, while serotonin and glutamate inhibited basal or serum-stimulated proliferation.

Repeated exposure to organophosphorus (OP) insecticides results in a decrease of muscarinic acetylcholine receptors (MRs) in the central nervous system. OP-induced MR down-regulation in vivo is modeled by chronic in vitro exposure to muscarinic agonists. Many studies, both in vivo and in vitro, indicate that the treatment-induced decrease in MR number is accompanied by a decrease in the mRNA levels of specific MR subtypes. In this study, the in vivo effects of OP exposure on the mRNA levels of three MR subtypes (m1, m2, and m3) were examined in brain tissue and in peripheral mononuclear cells, which express the m3 subtype. Adult male Sprague-Dawley rats were orally administered disulfoton (2 mg/kg/day) for 14 days, and a subset of exposed animals was allowed to recover for 28 days. This treatment caused a 28% and 81% decrease, respectively, in [3H]-QNB binding and acetylcholinesterase activity in the cortex, similar to that observed in previous studies; after recovery, these levels had returned to 99% and 90%, respectively, of controls. There was a significant decrease in m1 mRNA levels in hippocampus (23%) after disulfoton treatment, while no change was observed in the cortex, corpus striatum, medulla, or cerebellum. The m2 subtype mRNA was significantly decreased in both hippocampus (24%) and medulla (19%), but not in cortex, striatum, or cerebellum. m3 mRNA levels were significantly decreased in cortex (10%), but no change was observed in hippocampus, medulla, cerebellum, or lymphocytes. After recovery, no differences in m1 or m3 mRNA levels were observed in any tissue examined, whereas the decrease in m2 mRNA in the hippocampus remained significant (29%). These results indicate that OP exposure can differentially regulate mRNA levels for MR subtypes in different brain areas, and suggest that m2 muscarinic receptors in the hippocampus are most affected by this treatment.

In this study we made use of the Reverse transcription-polymerase chain reaction to analyze the expression of mRNA for the five subtypes of muscarinic acetylcholine receptors in human blood mononuclear cells. mRNA for m3, m4 and m5 subtypes was detected, while mRNA for m1 and m2 muscarinic receptors was not found. Similar results were obtained for three different healthy human subjects studied. Interestingly, the m5 subtype was expressed at higher levels in blood mononuclear cells than in cerebral cortex. To our knowledge this is the first time that m5 muscarinic receptor mRNA has been found outside of the central nervous system.

Paraoxonase can hydrolyze paraoxon (PO), chlorpyrifos-oxon (CPO) and other organophosphates. Previous studies have indicated that the levels of serum paraoxonase can influence the toxicity of PO and CPO. In the present study we have investigated whether exogenous paraoxonase administered to mice would offer protection toward the acute toxicity of a phosphorothioate, chlorpyrifos (CPS). Paraoxonase was purified from rabbit serum and injected i.v., or i.v. plus i.p., in mice. Inhibition of acetylcholinesterase (AChE) in brain, diaphragm, plasma and red blood cells was measured as an index of CPS (100 mg/kg) toxicity. Administration of paraoxonase 30 min before CPS increased plasma enzyme activity toward CPO by 35-fold, and protected against its toxicity; protection was still present at 24 h, when enzyme activity was still 20-fold over basal. When paraoxonase was given 30 min after CPS, a significant protection against CPS toxicity was still observed, while after 3 h the protective effect was decreased. To mimic conditions of severe acute poisoning, a higher dose of CPS (150 mg/kg) was also administered. Administration of paraoxonase 30 min after this exposure abolished cholinergic signs and significantly protected against AChE inhibition. These results indicate that exogenous paraoxonase offers significant protection against CPS toxicity when administered both before and after the organophosphate, suggesting that it may be considered as a potential additional treatment of organophosphate poisoning.

The presence of muscarinic acetylcholine receptors on lymphocytes has been demonstrated by radioligand binding experiments. Although the specific subtype(s) of muscarinic acetylcholine receptors expressed in lymphocytes is still unknown, some reports suggest the presence of the m2 subtype. In this study we analyzed the expression of m2 subtype mRNA in rat mononuclear cells, B lymphocytes and T lymphocytes by Northern blot hybridization and reverse transcription-polymerase chain reaction. Positive signals for the presence of m2 mRNA were found in rat heart, brainstem, cerebral cortex, corpus striatum and hippocampus, which were used as positive controls. On the other hand, no expression of m2 was detected in lymphocytes. These results indicate that mRNA for the m2 subtype is absent in rat lymphocytes and that one or more other subtypes may be responsible for the reported results in binding experiments.

The cholinergic system plays a role in the neuroimmune network. White blood cells have been found to express muscarinic and nicotinic cholinergic receptors as well as acetylcholinesterase on their surface, and cholinergic agents have been shown to modulate immune functions. Although the presence of muscarinic receptors on white blood cells has been well documented by binding and functional studies, the subtype(s) of muscarinic receptor present on these cells is still unknown. We have previously shown the absence of m2 muscarinic receptor subtype mRNA in rat mononuclear cells (Costa et al., 1994). The expression of m1, m3, m4 and m5 mAChR mRNA was analyzed in this study in rat peripheral blood mononuclear cells by Northern blotting hybridization experiments and reverse transcription-polymerase chain reaction. Only mRNA of the m3 subtype was detected in rat mononuclear cells, at levels about 100 times lower than in brain. Traces of m4 subtype mRNA were found in lymphocytes, at levels about 10,000 times lower than the cerebral cortex, while m5 mRNA was undetectable.

Human neuroblastoma cells (line SH-SY5Y) were used to examine the interaction of single exposure to organophosphorus compounds (OPs) with muscarinic receptors. In this study, SH-SY5Y cells were exposed for 30 min to concentrations of paraoxon, diisopropyl phosphorofluoridate (DFP), phenyl saligenin cyclic phosphate (PSP), and mipafox (N,N'-diisopropyl phosphorodiamide fluoridate) that ranged between 10(-9) M and 10(-3) M (10(-2) M for mipafox). Ability to interfere with muscarinic receptor binding was determined by change in the binding of the nonspecific antagonist [3H]-N-methylscopolamine (3H-NMS). Concentrations of paraoxon > 0.5 x 10(-3) M and PSP 1 x 10(-3) M significantly inhibited the binding of a saturating concentration of 3H-NMS. Concentrations of > 10(-5) M paraoxon or PSP could significantly inhibit the binding of a half-saturating concentration of 3H-NMS. Studies using specific antagonists for muscarinic subtypes (pirenzepine for M1, AFDX-116 for M2, and 4-DAMP for M3) indicated that SH-SY5Y cells have muscarinic receptors most sensitive to the specific antagonist for the M3 subtype (IC50 of 10(-8) M for 4-DAMP compared to 2.5 x 10(-6) M and 2.7 x 10(-5) M for pirenzepine and AFDX-116, respectively). As M3 receptor stimulation results in formation of inositol phosphates from membrane phosphoinositides the capability of OPs to alter levels of inositol phosphates and agonist-stimulated increases in inositol phosphate formation was examined. Intact cells were prelabeled with [3H]myo-inositol and then incubated for 15 min with the OPs before addition of 10(-5) M to 10(-3) M carbachol. Levels of inositol phosphates were determined as the amount of aqueous soluble radiolabeled product extracted from the reaction mixture. Paraoxon and PSP, but not mipafox or DFP, decreased basal levels of inositol phosphates in a concentration-related manner. This could be overcome in cells stimulated with carbachol, a muscarinic agonist, and with sodium fluoride, which does not act at muscarinic receptors. These results indicate that certain OPs, upon acute exposure, interact with muscarinic receptors, but that they also have effects on levels of inositol phosphates that may be associated with another site of action in SH-SY5Y cells.

In rat primary cortical cultures, carbachol caused a time- and concentration-dependent increase in guanosine cyclic 3',5'-monophosphate (cGMP) levels, which was antagonized by the muscarinic antagonist atropine. Glutamate and sodium nitroprusside also caused large increases in cGMP levels, as previously reported. Two nitric oxide (NO) synthase inhibitors, L-NG-nitroarginine and L-NG-monomethylarginine, were tested for their ability to inhibit the carbachol- and the glutamate-induced cGMP formation. The cGMP response to carbachol was decreased by both compounds in a dose-dependent fashion. The effect of L-NG-nitroarginine was competitively reversed by addition of an excess of L-arginine. Similarly, the stimulatory effect of glutamate on cGMP levels was antagonized by L-NG-nitroarginine and L-NG-monomethylarginine. Hemoglobin, a scavenger of NO, also blocked the carbachol-stimulated cGMP production. These results indicate that muscarinic receptor-stimulated cGMP formation in rat cerebral cortex is mediated by NO.

Repeated exposures to organophosphorus (OP) insecticides has been shown to cause a decrease of cholinergic muscarinic receptors (mAChR) in brain and in peripheral tissues. These changes are believed to be involved in the development of tolerance to OP toxicity and may play a role in cognitive dysfunctions observed following repeated OP exposure. Recently, mAChRs identified in circulating lymphocytes have been shown to be modulated similarly to brain mAChRs following repeated OP exposure, suggesting that these peripheral cells may be useful as indicators of mAChR changes in the central nervous system. This study was designed to further investigate whether mAChRs on lymphocytes could serve as a biomarker for changes in brain mAChRs during prolonged OP exposure and during recovery from such exposure. Using the mAChR antagonist [3H]quinuclidinyl benzilate (QNB) to label mAChRs, we found that exposure to disulfoton for 14 days (2 mg/kg/day by gavage) caused a significant decrease (25-35%) in muscarinic receptors density in the cerebral cortex, hippocampus, and striatum, as well as in circulating lymphocytes. The decline of mAChR density in lymphocytes paralleled those observed in brain, particularly in cortex and hippocampus, during exposure to disulfoton; however, while brain mAChR levels recovered slowly after termination of exposure and remained significantly reduced 4 weeks after the last treatment, [3H]QNB binding in lymphocytes recovered rapidly within 1 week. Similarly, lymphocyte acetylcholinesterase (AChE) activity was significantly inhibited and correlated well with brain AChE activity during exposure, but the recovery was rapid relative to AChE activity in brain.

Human and rabbit paraoxonases/arylesterases were purified to homogeneity by chromatographic and gel electrophoretic/isofocusing procedures coupled with activity stains. N-terminal and peptide sequence analysis suggested retention of the secretion signal sequence and allowed design of oligonucleotide probes. The probes were used to isolate a 1294-bp rabbit paraoxonase cDNA clone, which, in turn, was used to isolate three human cDNA clones. Comparison of rabbit and human protein and cDNA sequences indicated a high degree of sequence conservation (approximately 85% identity) and verified that paraoxonase retains its signal sequence (except for the N-terminal Met). The rabbit cDNA encodes a protein of 359 amino acids and the human a protein of 355 amino acids. In situ hybridization demonstrated, as expected, that the paraoxonase gene maps to the long arm of human chromosome 7. Arginine at position 192 specifies high activity paraoxonase and glutamine low activity human paraoxonase. Variation in protein levels explains the variation of enzyme activity observed within a genetic class. Toxicity studies showed that raising rat plasma paraoxonase levels by i.v. administration of partially purified rabbit paraoxonase protected animals against cholinesterase inhibition by paraoxon and chlorpyrifos oxon. Protection correlated with the relative rates of hydrolysis of these two compounds.

A number of lines of evidence suggest that serum paraoxonase is protective against poisoning by organophosphorus substrates of this enzyme. Birds that have very low levels of paraoxon hydrolyzing activity in their sera are very susceptible to parathion poisoning. Rabbits, which have a sevenfold higher enzyme level compared with rats, have a fourfold higher resistance to paraoxon poisoning than rats. Rabbit paraoxonase hydrolyzes chlorpyrifos-oxon with a much higher turnover number than does rat paraoxonase, resulting in a very high resistance of rabbits to chlorpyrifos toxicity. Direct tests of paraoxonase protection have been carried out by injecting purified rabbit enzyme into rats. The protection achieved was higher for chlorpyrifos-oxon than for paraoxon, probably due to the high hydrolytic activity of the rabbit enzyme for chlorpyrifos-oxon. In humans, a substrate-dependent polymorphism of serum paraoxonase is observed, where one isoform of paraoxonase has a high turnover number for paraoxon and the other a low turnover number. Both isoforms appear to hydrolyze chlorpyrifos-oxon and phenylacetate at the same rate. Cloning and sequencing of the human paraoxonase cDNAs has elucidated the molecular basis of the polymorphism. Arginine at position 192 determines high paraoxonase activity, and glutamine at this position, low paraoxonase activity. In addition to the polymorphism, a 13-fold variation in serum enzyme levels within a given genetic class is seen. The experiments reported here demonstrate that rabbit paraoxonase injected into mice provides protection against the parent insecticide chlorpyrifos as well as the toxic oxon. These results suggest that serum paraoxonase status may serve as a biomarker for insecticide susceptibility in humans.

The developing brain is extremely sensitive to the neurotoxicity of ethanol; however, the mechanism(s) of its developmental neurotoxicity are still elusive. In the developing rat brain, ethanol exerts an age-, brain region-, and receptor-specific inhibitory effect on muscarinic receptor-stimulated phosphoinositide metabolism, which may be linked to some of the neurotoxic effects of ethanol found in children with fetal alcohol syndrome. Since some studies have suggested that the ethanol metabolite acetaldehyde may mediate, at least in part, the developmental effects of ethanol, in the present study we have examined whether acetaldehyde would inhibit carbachol-stimulated phosphoinositide metabolism in brain slices from immature rats. We also tested propionaldehyde, the corresponding aldehyde of n-propanol, another alcohol shown to cause microencephaly and to affect phosphoinositide metabolism in the developing rat. Neither acetaldehyde nor propionaldehyde, at concentrations up to 1 mM, had any inhibitory effect on this system, while the two alcohols did, as previously reported. These results suggest that ethanol itself may be the primary agent responsible for its developmental neurotoxicity.

The muscarinic agonist oxotremorine produced a concentration-dependent increase in membrane fluidity in intact viable rat splenic lymphocytes in vitro. This effect was antagonized by atropine, but only at high concentrations (1 mM), while scopolamine was ineffective. Two other muscarinic agonists, carbachol and pilocarpine, did not affect membrane fluidity in lymphocytes. The fluidizing effect of oxotremorine occurred at both 10 and 37 degrees C with a similar time-course. Oxotremorine also increased membrane fluidity in liposomes of DMPC in gel phase, although its effect was less pronounced than in lymphocytes. The data suggest that the fluidization caused by oxotremorine is primarily nonreceptor-mediated and associated with a nonspecific physicochemical effect.

Aluminum (Al) is believed to exert a primary role in the neurotoxicity associated with dialysis encephalopathy and has been suggested to be involved in a number of other neurological disorders, including Alzheimer's disease. Al, complexed with fluoride to form fluoroaluminate (AlF4-), can activate the GTP-binding (G) proteins of the adenylate cyclase and retinal cyclic GMP phosphodiesterase systems. Since an involvement of G-proteins with cerebral phosphoinositide (PtdIns) metabolism has also been suggested, in this study we investigated the interaction of the stable GTP analogue GTP(S), Al salts and NaF with this system. In rat cerebral cortical membranes, GTP(S) dose-dependently stimulated [3H]inositol phosphates ([3H]InsPs) accumulation. This effect was potentiated by carbachol and was partially prevented by the GTP-binding antagonist GDP(S), indicating that CNS muscarinic receptor activation is coupled to PtdIns hydrolysis via putative G-protein(s). GTP(S) stimulation was also inhibited by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, which is known to exert a negative feedback control on agonist-stimulated PtdIns metabolism. Both Al salts and NaF mimicked the action of GTP(S) in stimulating PtdIns turnover. Their actions were highly synergistic, suggesting that AlF4- could be the active stimulatory species. However, the stimulatory effects of AlCl3 and/or NaF were not potentiated by carbachol and were not inhibited by GDP(S) and PMA, suggesting that separate sites of action might exist for GTP(S) and AlF4-. In the nervous tissue, activation of PtdIns hydrolysis by Al (probably as AlF4-) may be mediated by activating a regulatory G-protein at a location distinct from the GTP-binding site or by a direct stimulation of phospholipase C.

The muscarinic antagonist AF-DX 384 (5,11-dihydro-11-(((2-(2-((dipropylamino)methyl)-1- piperidinyl)ethyl)amino)carbonyl)-6H-pyrido(2,3b)(1,4)-benzodiazepin+ ++-6-one methansulfonate) was used to label cholinergic muscarinic receptors of the M2 subtype in rat brain. In the brainstem [3H] AF-DX 384 labeled a single population of binding sites with Kd = 3-4 nM and Bmax = 430-610 fmol/mg of protein. The pharmacological profile of these sites was similar to that observed with the muscarinic M2 agonist [3H] AF-DX 116. Muscarinic M2 receptors were unequally distributed in rat brain regions: in brainstem, about 80% of total muscarinic receptors (measured with [3H] quinuclidinyl benzilate) were labeled by [3H] AF-DX 384, while in other brain areas they represented only a fraction of total binding. Because of its high specific binding, high affinity and specificity, [3H] AF-DX 384 represents a useful novel ligand to study M2 receptors in brain tissue.

Paraoxon and chlorpyrifos-oxon, the active metabolites of the organophosphorus insecticides parathion and chlorpyrifos, respectively, are hydrolyzed by an "A"-esterase, paraoxonase, which is present in the sera of several mammalian species. In this study, we investigated whether levels of serum paraoxonase activity in laboratory animals can influence the in vivo toxicity of paraoxon and chlorpyrifos-oxon. Paraoxonase was found to be 7-fold higher in rabbit serum than in rat serum. The dose of paraoxon required to produce similar signs of toxicity and similar degrees of cholinesterase inhibition in rats and rabbits (0.5 and 2.0 mg/kg, respectively) differed by 4-fold. Paraoxonase was then purified from rabbit serum and 8.35 units was injected in the tail veins of rats, increasing the peak hydrolytic activity of rat serum by 9-fold toward paraoxon and by 50-fold toward chlorpyrifos-oxon. The increase in serum paraoxonase/chlorpyrifos-oxonase activity was long-lasting, with a 2- and 10-fold increase, respectively, still present after 24 hr. Thirty minutes following enzyme injection, rats were challenged with an acute dose of paraoxon or chlorpyrifos-oxon given by the intravenous, intraperitoneal, dermal, or oral route. Cholinesterase activities were measured in plasma, red blood cells, brain, and diaphragm after 4 hr. Rats pretreated with paraoxonase exhibited less inhibition of cholinesterase than vehicle-treated controls following identical doses of paraoxon, particularly when the organophosphate was given iv or dermally. A very high degree of protection, particularly toward brain and diaphragm cholinesterase, was provided by paraoxonase pretreatment in animals challenged with chlorpyrifos-oxon by all routes. These results indicate that levels of serum paraoxonase activity can affect the toxicity of paraoxon and chlorpyrifos-oxon.

Memory impairment is one of the recurrent complaints of agricultural workers repeatedly exposed to organophosphorus insecticides. In an effort to establish an animal model for such behavioral effects, which would allow studying its underlying biochemical mechanism(s), in this study we evaluated spatial memory in animals following repeated organophosphate exposure. Male Long-Evans rats were given daily i.p. injections of either diisopropylfluorophosphate (DFP; 1 mg/kg/day) or disulfoton (O,O-diethyl S-[2-(ethylthio)ethyl] phosphorodithioate; 2 mg/kg/day) for 14 days. Acetylcholinesterase activity was inhibited 71-77% in the cortex, hippocampus, and striatum of rats treated with DFP, and 73-74% in those treated with disulfoton. Binding of [3H]quinuclidinyl benzilate ([3H]QNB) to cholinergic muscarinic receptors in the same brain areas was reduced 16-28% in organophosphate-treated rats. This decrease was due to a reduction in muscarinic receptor density (Bmax) with no changes in receptor affinity. At the end of the treatment rats were tested for spatial memory using the spontaneous alternation task in a T-maze. Rates of true spontaneous alternation were 64.4, 45.0, and 44.8% in animals which received corn oil, DFP, or disulfoton, respectively (P less than 0.05). These results indicate that prolonged inhibition of acetylcholinesterase caused by repeated organophosphate exposure alters spatial memory functions in rats, as well as causing a loss of muscarinic receptors. Considering the role of the cholinergic system in cognitive processes, these biochemical alterations could be related to the observed behavioral changes and may offer a potential explanation of the memory impairment reported by workers chronically exposed to organophosphates.