Neurotoxicity of glycidamide, an acrylamide metabolite, following intraperitoneal injections in rats

Acrylamide-induced damage to postsynaptic plasticity is CYP2E1 dependent in an SH-SY5Y co-culture system

Acrylamide (ACR), a neurotoxic substance, is characterized by a range of industrial and population exposures. The effects of ACR on synapses have been examined, but the regulation and molecular mechanism of key proteins related to ACR and its metabolite glycidamide (GA) have not been elucidated. In this study, we constructed two co-culture systems to mimic neurons that do not express and overexpress CYP2E1. In these co-cultures, we observed the effects and relative influence of ACR and GA on cell survival as well as synaptic structural and functional plasticity. Next, we investigated the relationship between ACR-induced nerve damage and key proteins in the postsynaptic membrane. After ACR exposure, cell death and synaptic damage were significantly worse in CYP2E1-overexpressing co-culture systems, suggesting that ACR-induced neurotoxicity may be related to metabolic efficiency (including CYP2E1 activity). Moreover, with increasing doses of ACR, the key postsynaptic membrane proteins PSD-95 expression was reduced and CaMKII and NMDAR-2B phosphorylation was increased. ACR exposure also triggered a rapid dose- and time-dependent increase in intracellular Ca²⁺, whose changes can affect the expression of the above-mentioned key proteins. In summary, we clarified the relationship between ACR exposure, neuronal damage and postsynaptic plasticity and proposed an ACR-CYP2E1-GA: Ca²⁺-PSD-95-NMDAR-Ca²⁺-CaMKII effect chain. This information will further improve the development of an alternative pathway strategy for investigating the risk posed by ACR.

Subchronic Acrylamide Exposure Activates PERK-eIF2α Signaling Pathway and Induces Synaptic Impairment in Rat Hippocampus

Acrylamide (ACR), a well-documented neurotoxicant to humans, is extensively found in starchy foods. More than 30% of the typical daily calorie intake comes from ACR-containing foods. Epidemiological and toxicological studies have found that ACR exposure is associated with mild cognitive change in men and experimental animals. However, there is limited information on the mechanisms by which ACR exposure induces memory deficits. The aberrant activation of the PKR-like ER kinase (PERK)-eukaryotic initiation factor 2α (eIF2α) signaling pathway is emerging as a major common theme in cognitive decline. The present study is designed to explore the effect of subchronic ACR exposure on the PERK signaling and the synaptic impairment to elucidate the potential mechanism of ACR-induced cognitive dysfunction in rat. ACR exposure at 5 and 10 (mg/kg)/day by gavage for 14 weeks results in gait abnormality and cognitive impairment in rats, which were accompanied by neuronal loss, glial cell proliferation, and synaptic ultrastructure damage in the hippocampus. ACR reduced the expression of phosphorylated cAMP response element-binding protein (P-CREB), brain-derived neurotrophic factor (BDNF), and synaptic vesicle proteins synapsin-1 and synaptophysin synthesis. ACR also excessively activates the PERK-eIF2α signaling, resulting in overexpression of C/EBP homologous protein (CHOP) and activating transcription factor 4 (ATF4). This work helps to propose a possible mechanism of subchronic exposure of ACR-induced neurotoxicity.

Acrylamide and its metabolite induce neurotoxicity via modulation of protein kinase C and AMP-activated protein kinase pathways

Acrylamide is known as a neurotoxicant found in commonly consumed food as well as in human body. However, the underlying mechanisms involved in neurotoxicity by acrylamide and its metabolite, glycidamide remain largely unknown. In this study, we have examined the interplay between CYP2E1, AMPK, ERK and PKC in acrylamide-induced neurotoxicity associated with autophagy in PC12 cells. Acrylamide-induced cell death was mediated by CYP2E1 expression and the activation of ERK, PKC-ɑ and PKC-δ, whereas AMPK knockdown exacerbated the acrylamide-induced neurotoxic effects. PKC-ɑ, but not PKC-δ, plays an upstream regulator of ERK and AMPK. Moreover, AMPK activation suppressed ERK, and CYP2E1 and AMPK bilaterally inhibit each other. Furthermore, acrylamide increased autophagy with impaired autophagic flux, evidenced by the increased beclin-1, LC3-II and p62 protein. Acrylamide-induced neuronal death was ameliorated by 3-methyladenine, an autophagy inhibitor, whereas neuronal death was exacerbated by chloroquine, a lysosomal inhibitor. Interestingly, PKC-δ siRNA, but not PKC-ɑ siRNA, dramatically reduced acrylamide-induced beclin-1 and LC3-II levels, whereas AMPK siRNA further increased beclin-1, LC3-II and p62 protein levels. Glycidamide, a major metabolite, mimicked acrylamide only with a higher potency. Taken together, acrylamide- and glycidamide-induced neurotoxicity may involve cytotoxic autophagy, which is mediated by interplay between PKCs and AMPK pathways.

In VitroStudies of Acrylamide Neurotoxicity in Rat Pheochromocytoma (PC12) Cells

This review discusses our studies on molecular mechanisms of acrylamide neurotoxicity by using the rat pheochromocytoma (PC12) cell line. The results showed that: a) acrylamide altered the gross morphology of PC12 cells; b) acrylamide induced neurofilament accumulation in PC12 cells; c) the effects of acrylamide on PC12 cells are consistent with its neurotoxicity in vivo; d) acrylamide stimulated neurofilament protein synthesis in PC12 cells; e) acrylamide did not act via nerve growth factor (NGF) receptor gp140trk to regulate neurofilament synthesis in PC12 cells; f) dexamethasone antagonised NGF and/or acrylamide-induced neurofilament protein synthesis and expression; and g) acrylamide differentially regulated the mRNA levels of three neurofilament subunit genes in PC12 cells. These molecular studies provide the first evidence that: a) there are distinctive and convergent signalling pathways for NGF-regulated and acrylamide-regulated neurofilament expression; b) acrylamide may differentially regulate the expression of each subunit, resulting in aberrant accumulation of neurofilament proteins; and c) there is a dexamethasone-sensitive signalling step common to NGF and acrylamide. These results could partially explain the mechanisms of neurofilament accumulation in distal axonal swellings, a pathognomonic feature of acrylamide neurotoxicity.

Hazardous effects of fried potato chips on the development of retina in albino rats

OBJECTIVE

To evaluate the hazardous effects of fried potato chips upon the retina of two developmental stages of the albino rats aged 7 and 14 days from parturition.

METHODS

PREGNANT RATS WERE ARRANGED INTO TWO GROUPS: control pregnant rats and consequently their delivered newborns until reaching 7 and 14 days old from parturition and fried potato chips group in which pregnant rats at the 6th day of gestation maintained on diet formed of fried potato chips supplied from the market mixed with standard diet at a concentration of 50% per each till 7 and 14 post-partum. Three fold integrated approaches were adopted, namely, histological, ultrastructural and proteomic analysis.

RESULTS

Histological examination of the retina of the experimental offsprings revealed many histopathological changes, including massive degeneration, vacuolization and cell loss in the ganglion cell layer, as well as general reduction in retinal size. At the ultrastructural level, the retina of experimental offsprings exhibited number of deformities, including ill differentiated and degenerated nuclear layer, malformed and vacuolated pigment epithelium with vesiculated and fragmented rough endoplasmic reticulum, degenerated outer segment of photoreceptors, as well as swollen choriocapillaris and loss of neuronal cells. Proteomic analysis of retina of the two experimental developmental stages showed variations in the expressed proteins as a result of intoxication which illustrated the adverse toxic effects of fried potato chips upon the retina.

CONCLUSIONS

It can be concluded that the effect of fried potato chips on the development of retina in rats may be due to the presence of acrylamide or its metabolite.

Decrease in the acrylamide content in canned coffee by heat treatment with the addition of cysteine

Acrylamide (AA) is classified as a Group 2A carcinogen according to the International Agency for Research on Cancer. Although coffee contains a small amount of AA, it is a popular beverage worldwide. Approximately 10 billion canned coffees are consumed each year in Japan. In this study, we investigated how to decrease AA contained in canned coffee by modifying the heat treatment used for sterilization during the manufacturing process. The AA content of both types of canned coffee (black and milk) was decreased by approximately 95% by heat treatment with adding cysteine at 121 °C for 6 min. The content was also decreased by heat treatment with dithiothreitol, although that with cystine had no effect. Therefore, it is shown that thiol groups in cysteine and dithiothreitol might play an important role in decreasing the AA content.

Antioxidant and immunostimulant effect of carica papaya linn. Aqueous extract in acrylamide intoxicated rats

INTRODUCTION

The present study was conducted to evaluate the antioxidant and immunostimulant effects of The Carica papaya fruit aqueous extract (CPF, Caricaceae) against acrylamide induced oxidative stress and improvement of Immune functions which affected by free radicals liberating acrylamide in rats.

MATERIAL AND METHODS

Sixty male wistar albino rats (195-230g) were assigned to four groups, (fifteen/group). The first group used as control group and received normal physiological saline orally daily. The second group was supplemented with acrylamide 0.05% in drinking water. The third group was gastro-gavaged with 250 mg/kg of papaya fruit extract orally on daily basis. The fourth group was supplemented with acrylamide 0.05% in drinking water and gastro-gavaged with 250 mg/kg of papaya fruit extract orally on daily basis. The chosen dose of papaya fruit extract was based on the active pharmacological dose range obtained from the orientation study earlier conducted. The experimental period was extended to forty day. At the expiration of the experimental period and night fasting, blood samples were collected from the orbital venous sinus. The sera were separated and used for determining of IgG and IgM and the stomach, liver and kidney homogenates for estimation of MDA, GSH level, SOD and CAT activity as a biomarker of lipid peroxidation and antioxidative stress.

RESULTS AND DISCUSSION

The obtained results revealed that, acrylamide caused significant increases in MDA and decrease of GSH level, SOD and CAT activity due to the oxidative stress induced by acrylamide on membrane polyunsaturated fatty acids in rat's stomach, liver and kidney while administration of CPF aqueous extract, was significantly ameliorated the increased levels of MDA and decline of GSH, SOD and CAT activity in the stomach, liver and kidney tissues caused by acrylamide toxicity. Meanwhile, CPF aqueous extract significantly increased immune functions (IgG and IgM) while acrylamide significantly decrease it specially IgG. Thus, this study suggests that acrylamide-induced oxidative stress in rats can be ameliorated by administration of CPF aqueous extract.

Structural and ultrastructural evidence of neurotoxic effects of fried potato chips on rat postnatal development

OBJECTIVE

Acrylamide (ACR), a proved rodent carcinogen and neurotoxic agent, is present in significant quantities in commonly consumed foods such as fried potato chips (FPC) and French fries, raising a health concern worldwide. We investigated and compared the neurotoxic effects of ACR and FPC on postnatal development.

METHODS

Female rats were treated with ACR (30 mg/kg of body weight), fed a diet containing approximately 30% of FPC during pregnancy, or fed a standard diet (control) and their offspring were examined.

RESULTS

Female rats treated with ACR or fed a diet containing FPC during pregnancy gave birth to litters with delayed growth and decreased body and brain weights. Light microscopic studies of the cerebellar cortex of treated animals revealed drastic decreases in Purkinje cells and internal granular layers. Different patterns of cell death were detected in Purkinje cells and neurons in the brains of pups born to treated mothers. Ultrastructural analysis of Purkinje cells revealed changes in the endoplasmic reticulum, loss of the normal arrangement of polyribosomes, swollen mitochondria with abnormally differentiated cristae, and an abnormal Golgi apparatus. The gastrocnemius muscle in the ACR and FPC groups showed extensive degeneration of myofibrils as evidenced by poorly differentiated A, H, and Z bands.

CONCLUSION

The present study reveals for the first time that rat fetal exposure to ACR, as a pure compound or from a maternal diet of FPC, causes cerebellar cortical defects and myodegeneration of the gastrocnemius muscle during the postnatal development of pups. These results warrant a systematic study of the health effects of the consumption of FPC and French fries in the general population.

Genetic polymorphisms in detoxification and DNA repair genes and susceptibility to glycidamide-induced DNA damage

Acrylamide (AA) is a probable human carcinogen formed in carbohydrate-rich foodstuffs upon heating. Glycidamide (GA), the AA metabolite formed by epoxidation, is considered the ultimate genotoxic agent. In this study, the in vitro genotoxic potential of AA and GA in human whole blood leukocytes was compared using the alkaline comet assay. Although AA did not induce significant DNA damage in the concentrations tested (up to 1000 μM), GA markedly increased the percentage of tail DNA at concentrations ≥250 μM. Further, this study addressed the role of genetic polymorphisms in key genes involved in metabolism and DNA repair pathways (BER, NER, HRR, and NHEJ) on GA-induced genotoxicity assessed by the alkaline comet assay. The results obtained suggested associations between DNA damage and polymorphisms of BER (MUTYH Gln335His and XRCC1 Gln399Arg) and NER (XPC Ala499Val) genes, either alone or in combination.

Acrylamide-induced nerve terminal damage: relevance to neurotoxic and neurodegenerative mechanisms

Acrylamide (ACR) has demonstrable neurotoxic effects in animals and humans that stem from its chemical behavior as a soft electrophilic alpha,beta-unsaturated carbonyl compound. Evidence is presented that the nerve terminal is a primary site of ACR action and that inhibition of neurotransmission mediates the development of neurological deficits. At the mechanistic level, recent proteomic, neurochemical, and kinetic data are considered, which suggest that ACR inhibits neurotransmission by disrupting presynaptic nitric oxide (NO) signaling. Nerve-terminal damage likely mediates the neurological complications that accompany the occupational exposure of humans to ACR. In addition, the proposed molecular mechanism of synaptotoxicity has substantial implications for the pathogenesis of Alzheimer's disease and other neurodegenerative conditions that involve neuronal oxidative stress and the secondary endogenous generation of acrolein and other conjugated carbonyl chemicals.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

Acrylamide, an in vivo thyroid carcinogenic agent, induces DNA damage in rat thyroid cell lines and primary cultures

Chronic treatment of rats with acrylamide induces various tumors among which thyroid tumors are the most frequent. The aim of the present study was to develop an in vitro model of acrylamide action on thyroid cells to allow the investigation of the mechanism of this tumorigenic action. The first part of the study considered as targets, characteristics of thyroid metabolism, which could explain the thyroid specificity of acrylamide action: the cAMP mitogenic effect and the important H2O2 generation by thyroid cells. However, acrylamide did not modulate H2O2 or cAMP generation in the thyroid cell models studied. No effect on thyroid cell proliferation was observed in the rat thyroid cell line FRTL5. On the other hand, as shown by the comet assay, acrylamide induced DNA damage, as the positive control H2O2 in the PC Cl3 and FRTL5 rat thyroid cell lines, as well as in thyroid cell primary cultures. The absence of effect of acrylamide on H2AX histone phosphorylation suggests that this effect does not reflect the induction of DNA double strand breaks. DNA damage leads to the generation of mutations. It is proposed that such mutations could play a role in the carcinogenic effect of acrylamide. The mechanism of this effect can now be studied in this in vitro model.

Kinetics of elimination of urinary metabolites of acrylamide in humans

Acrylamide (AM), used in the manufacture of polyacrylamide and grouting agents, is produced during the cooking of foods. Workplace exposure to AM can occur through the dermal and inhalation routes. The objective of this study was to define the kinetics of elimination of AM and its metabolites following oral and dermal administration. This is the second part of a study in which metabolites and hemoglobin adducts of AM were determined in people (Fennell et al., 2005, Toxicol. Sci. 85, 447-459). (1,2,3-(13)C(3))AM was administered in an aqueous solution orally (single dose of 0.5, 1.0, or 3.0 mg/kg) or dermally (three daily doses of 3.0 mg/kg) to sterile male volunteers. Urine samples were collected at 0-2, 2-4, 4-8, 8-16, and 16-24 h following administration orally, or at 0-2, 2-4, 4-8, 8-16, and 16-24 h following each of three daily dermal doses. (13)C(3)-AM and its metabolites in urine, (13)C(3)-glycidamide, (13)C(3)-N-acetyl-S-(3-amino-3-oxopropyl)cysteine and its S-oxide, and (13)C(3)-N-acetyl-S-(3-amino-2-hydroxy-3-oxopropyl)cysteine, were quantitated using liquid chromatography-tandem mass spectrometry. The recovered urinary metabolites accounted for 45.6, 49.9, and 39.9% of a 0.5, 1.0, and 3.0 mg/kg oral dose (0-24 h), respectively, and for 4.5% of the dose after 3 mg/kg was administered daily for 3 days dermally (0-4 days). These results indicate that after oral administration AM is rapidly absorbed and eliminated. The half-life estimated for elimination of AM in urine was 3.1-3.5 h. After dermal administration, AM uptake is slow. This study indicated that skin provides a barrier that slows the absorption of AM, and results in limited systemic availability following dermal exposure to AM.

Acrylamide Neurotoxicity: Neurological, Morhological and Molecular Endpoints in Animal Models

Acrylamide (AA) monomer is used in numerous chemical industries and is a contaminant in potato- and grain-based foods prepared at high temperatures. Although experimental animal studies have implicated carcinogenicity and reproductive toxicity as possible consequences of exposure, neurotoxicity is the only outcome identified by epidemiological studies of occupationally exposed human populations. Neurotoxicity in both humans and laboratory animals is characterized by ataxia and distal skeletal muscle weakness. Early neuropathological studies suggested that AA neurotoxicity was mediated by distal axon degeneration. However, more recent electrophysiological and quantitative morphometric analyses have identified nerve terminals as primary sites of AA action. A resulting defect in neurotransmitter release appears to be the pathophysiological basis of the developing neurotoxicity. Corresponding mechanistic research suggests that AA impairs release by adducting cysteine residues on functionally important presynaptic proteins. In this publication we provide an overview of recent advances in AA research. This includes a discussion of the cumulative nature of AA neurotoxicity and the putative sites and molecular mechanisms of action.

The Changing View of Acrylamide Neurotoxicity

Acrylamide (ACR) is a water-soluble, vinyl monomer that has multiple chemical and industrial applications: e.g., waste water management, ore processing. In addition, ACR is used extensively in molecular laboratories for gel chromatography and is present in certain foods that have been prepared at very high temperatures. Extensive studies in rodents and other laboratory animals have provided evidence that exposure to monomeric ACR causes cellular damage in both the nervous and reproductive systems, and produces tumors in certain hormonally responsive tissues. Whereas human epidemiological studies have demonstrated a significantly elevated incidence of neurotoxicity in occupationally exposed populations, such research has not, to date, revealed a corresponding increase in cancer risk. Since the announcement by a Swedish research group in April 2002 [J. Ag. Food Chem. 50 (2002) 4998] regarding the presence of ACR in potato and grain-based foods, there has been a renewed interest in the toxic actions of this chemical. Therefore, in this review, we consider the different toxic effects of ACR. The neurotoxic actions of ACR will be the focal point since neurotoxicity is a consequence of both human and laboratory animal exposure and since this area of investigation has received considerable attention over the past 30 years. As will be discussed, a growing body of evidence now indicates that the nerve terminal is a primary site of ACR action and that inhibition of corresponding membrane-fusion processes impairs neurotransmitter release and promotes eventual degeneration. The electrophilic nature of ACR suggests that this neurotoxicant adducts nucleophilic sulfhydryl groups on certain proteins that are critically involved in membrane fusion. Adduction of thiol groups also might be common to the reproductive and carcinogenic effects of ACR. A final goal of this review is to identify data gaps that retard a comprehensive understanding of ACR pathophysiological processes.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

Acrylamide axonopathy revisited

Distal swelling and eventual degeneration of axons in the CNS and PNS have been considered to be the characteristic neuropathological features of acrylamide (ACR) neuropathy. These axonopathic changes have been the basis for classifying ACR neuropathy as a central-peripheral distal axonopathy and, accordingly, research over the past 30 years has focused on the primacy of axon damage and on deciphering underlying mechanisms. However, based on accumulating evidence, we have hypothesized that nerve terminals, and not axons, are the primary site of ACR action and that compromise of corresponding function is responsible for the autonomic, sensory, and motor defects that accompany ACR intoxication (NeuroToxicology 23 (2002) 43). In this paper, we provide a review of data from a recently completed comprehensive, longitudinal silver stain study of brain and spinal cord from rats intoxicated with ACR at two different daily dosing rates, i.e., 50 mg/kg/day, ip or 21 mg/kg/day, po. Results show that, regardless of dose-rate, ACR intoxication was associated with early, progressive nerve terminal degeneration in all CNS regions and with Purkinje cell injury in cerebellum. At the lower dose-rate, initial nerve terminal argyrophilia was followed by abundant retrograde axon degeneration in white matter tracts of spinal cord, brain stem, and cerebellum. The results support and extend our nerve terminal hypothesis and suggest that Purkinje cell damage also plays a role in ACR neurotoxicity. Substantial evidence now indicates that axon degeneration is a secondary effect and is, therefore, not pathophysiologically significant. These findings have important implications for future mechanistic research, classification schemes, and assessment of neurotoxicity risk.

Nerve terminals as the primary site of acrylamide action: a hypothesis

Acrylamide (ACR) is considered to be prototypical among chemicals that cause a central-peripheral distal axonopathy. Multifocal neurofilamentous swellings and eventual degeneration of distal axon regions in the CNS and PNS have been traditionally considered the hallmark morphological features of this axonopathy. However, ACR has also been shown to produce early nerve terminal degeneration of somatosensory, somatomotor and autonomic nerve fibers under a variety of dosing conditions. Recent research from our laboratory has demonstrated that terminal degeneration precedes axonopathy during low-dose subchronic induction of neurotoxicity and occurs in the absence of axonopathy during higher-dose subacute intoxication. This relationship suggests that nerve terminal degeneration, and not axonopathy, is the primary or most important pathophysiologic lesion produced by ACR. In this hypothesis paper, we review evidence suggesting that nerve terminal degeneration is the hallmark lesion of ACR neurotoxicity, and we propose that this effect is mediated by the direct actions of ACR at nerve terminal sites. ACR is an electrophile and, therefore, sulfhydryl groups on presynaptic proteins represent rational molecular targets. Several presynaptic thiol-containing proteins (e.g. SNAP-25, NSF) are critically involved in formation of SNARE (soluble N-ethylmaleimide (NEM)-sensitive fusion protein receptor) complexes that mediate membrane fusion processes such as exocytosis and turnover of plasmalemmal proteins and other constituents. We hypothesize that ACR adduction of SNARE proteins disrupts assembly of fusion core complexes and thereby interferes with neurotransmission and presynaptic membrane turnover. General retardation of membrane turnover and accumulation of unincorporated materials could result in nerve terminal swelling and degeneration. A similar mechanism involving the long-term consequences of defective SNARE-based turnover of Na+/K(+)-ATPase and other axolemmal constituents might explain subchronic induction of axon degeneration. The ACR literature occupies a prominent position in neurotoxicology and has significantly influenced development of mechanistic hypotheses and classification schemes for neurotoxicants. Our proposal suggests a reevaluation of current classification schemes and mechanistic hypotheses that regard ACR axonopathy as a primary lesion.

Determination of acrylamide and glycidamide in rat plasma by reversed-phase high performance liquid chromatography

Acrylamide is a widely used monomer that produces peripheral neuropathy. It is metabolized to the epoxide, glycidamide, which is also considered to be neurotoxic. A new reversed-phase high-performance liquid chromatography (HPLC) method is described that permits simultaneous determination of acrylamide and glycidamide in rat plasma. Samples were deproteinized with acetonitrile and chromatography was performed using isocratic elution and UV absorption detection. The limits of detection for acrylamide and glycidamide were 0.05 and 0.25 microg/ml in plasma, respectively, and recovery of both analytes was greater than 90%. The assay was linear from 0.1 to 100 microg/ml for acrylamide and from 0.5 to 100 microg/ml for glycidamide. Variation over the range of the standard curve was less than 15%. The method was used to determine the concentration-time profiles of acrylamide and glycidamide in the plasma of acrylamide-treated rats.

Metabolism, toxicokinetics and hemoglobin adduct formation in rats following subacute and subchronic acrylamide dosing

Long-term, low-dose (subchronic) oral acrylamide (ACR) exposure produces peripheral nerve axon degeneration, whereas irreversible axon injury is not a component of short-term, higher dose (subacute) i.p. intoxication [Toxicol Appl Pharmacol 1998;151:211]. It is possible that this differential axonopathic expression is a product of exposure-dependent differences in ACR biotransformation and/or tissue distribution. Therefore, we determined the toxicokinetics and metabolism of ACR following subchronic oral (2.8 mM in drinking water for 34 days) or subacute i.p. (50 mg/kg per day for 11 days) administration to rats. Both dosing regimens produced moderate levels of behavioral neurotoxicity and, for each, ACR was rapidly absorbed from the site of administration and evenly distributed to tissues. Peak ACR plasma concentrations and tissue levels were directly related to corresponding daily dosing rates (20 or 50 mg/kg per day). During subchronic oral dosing a larger proportion (30%) of plasma ACR was converted to the epoxide metabolite glycidamide (GLY) than was observed following subacute i.p. intoxication (8%). This subchronic effect was not specifically related to changes in enzyme activities involved in GLY formation (cytochrome P450 2E1) ormetabolism (epoxide hydrolases). Both ACR and GLY formed hemoglobin adducts during subacute and subchronic dosing, the absolute quantity of which did not change as a function of neurotoxicant exposure. Compared to subacute i.p. exposure, the subchronic schedule produced approximately 30% less ACR adducts but two-fold more GLY adducts. GLY has been considered to be an active ACR metabolite and might mediate axon degeneration during subchronic ACR administration. However, corresponding peak GLY plasma concentrations were relatively low and previous studies have shown that GLY is only a weak neurotoxicant. Our study did not reveal other toxicokinetic idiosyncrasies that might be a basis for subchronic induction of irreversible axon damage. Consequently the mechanism of axon degeneration does not appear to involve route- or rate-dependent differences in metabolism or disposition.

Rate of neurotoxicant exposure determines morphologic manifestations of distal axonopathy

Exposure to a variety of agricultural, industrial, and pharmaceutical chemicals produces nerve damage classified as a central-peripheral distal axonopathy. Morphologically, this axonopathy is characterized by distal axon swellings and secondary degeneration. Over the past 25 years substantial research efforts have been devoted toward deciphering the molecular mechanisms of these presumed hallmark neuropathic features. However, recent studies suggest that axon swelling and degeneration are related to subchronic low-dose neurotoxicant exposure rates (i.e., mg toxicant/kg/day) and not to the development of neurophysiological deficits or behavioral toxicity. This suggests these phenomena are nonspecific and of uncertain pathophysiologic relevance. This possibility has significant implications for research investigating mechanisms of neurotoxicity, development of exposure biomarkers, design of risk assessment models, neurotoxicant classification schemes, and clinical diagnosis and treatment of toxic neuropathies. In this commentary we will review the evidence for the dose-related dependency of distal axonopathies and discuss how this concept might influence our current understanding of chemical-induced neurotoxicities.

Redefining toxic distal axonopathies

Nerve damage classified as a central-peripheral distal axonopathy is produced by a variety of chemicals (e.g. acrylamide, n-hexane). Historically, axon swelling and secondary degeneration have been considered the morphologic hallmarks of toxic axonopathies and substantial research has been devoted toward deciphering corresponding molecular mechanisms. However, recent studies from the author's laboratory investigating rate (mg toxicant/kg/day) and route (i.p. vs gavage) of intoxication have shown that swelling and degeneration were related to neurotoxicant dosing conditions (i.e. low-dose, subchronic exposure) and not to development of neurophysiological deficits or classic behavioral toxicity. This suggests the presumed hallmarks of distal axonopathy are epiphenomena of uncertain pathophysiologic significance. Therefore, the current definition of and chemical classification scheme for toxic distal axonopathies requires re-evaluation.

Acrylamide-regulated neurofilament expression in rat pheochromocytoma cells

Using the rat pheochromocytoma cell line (PC12), we present molecular evidence that the neurotoxicant acrylamide directly induces neurofilament gene expression, and the signaling pathways are initially distinctive from, but eventually merged into, that for nerve growth factor (NGF)-induced neurofilament expression. In PC12 cells, acrylamide increased neurofilament protein levels and synthesis. Acrylamide had no effect on the stability of neurofilament mRNAs suggesting that it directly increased neurofilament mRNA synthesis. K252a, a selective inhibitor for NGF receptor gp140trk, had no effect on acrylamide induction, but completely inhibited NGF-induced neurofilament protein synthesis. Therefore, the initial step for acrylamide signaling was distinctive from NGF. Dexamethasone reversed the effects of both NGF and acrylamide on neurofilament protein levels and synthesis indicated that there is a dexamethasone-sensitive signaling step upon which NGF and acrylamide merge, suggesting involvement of transcription-activating proteins like AP-1. These results, taken together with previous studies of transgenic mice that overexpress neurofilament genes, may partially explain the mechanisms of neurofilament accumulation in distal parts of large axons, a pathognomonic feature of acrylamide neurotoxicity in animals.

Acrylamide and carbon disulfide treatments increase the rate of rat brain tubulin polymerization

Acrylamide and carbon disulfide produce central-peripheral distal axonopathy in experimental animals and humans. The main feature of this disease is the focal swellings containing neurofilaments in distal axons, followed by nerve degeneration beyond these swellings. We studied the possible role of tubulin assembly kinetics in this disease. The rats were either administered acrylamide (50 mg/kg, ip, saline) or exposed to carbon disulfide (700 ppm, 9 h) via inhalation for 12 and 15 d, respectively. Tubulin, purified from both acrylamide-(10.37 +/- 0.3 vs 11.3 +/- 0.15) and carbon disulfide-treated (9.72 +/- 0.5 vs 11.18 +/- 0.25) rat brains showed increase in Vmax (OD/min x 10(3)) of its polymerization. However, only acrylamide treatment showed a decrease in time to Vmax, when brain supernatant was used for tubulin polymerization. In vitro addition of acrylamide (0.1-1 mM) to bovine brain tubulin also showed a decrease in time to Vmax (16-21%) of its polymerization. Carbon disulfide treatment of rats, on the other hand, showed a decrease in MAP-2 and an increase in a 120-kDa peptide concentration. The latter showed immunoreactivity with anti-MAP-2. The increase in the rate of tubulin polymerization by acrylamide and carbon disulfide treatment may alter the rate of transport of axonal constituents, including neurofilament, and contribute toward their accumulation in the focal swellings observed in this neuropathy.

Alterations in the neutral proteinase activities of central and peripheral nervous systems of acrylamide-, carbon disulfide-, or 2,5-hexanedione-treated rats

Proteinases are widespread in neuronal or nonneuronal eukaryotic cells. They are suggested to play an important role in the turnover of proteins in neuronal perikaryon and axon, and digestion of the transported cytoskeletal proteins in synaptic terminals. We examined the effect of acrylamide (50 mg/kg, ip), carbon disulfide (700 ppm, 9 h, 7 d a week), and 2,5-hexanedione (2,5-HD) (1% in drinking water) treatment of rats on mCANP (2 mM Ca2+), microCANP (0.1 mM Ca2+), and CINP (Ca(2+)-independent) activity in telencephalon + diencephalon (FB), rhombencephalon + mesencephalon (LB), spinal cord (SC), and sciatic nerve (SN). The proteinase activity was determined in the 30,000g supernatant fraction of tissues using 14C-methylated casein as the substrate. mCANP activity in FB, LB, and SC was inhibited only by acrylamide. Acrylamide or 2,5-HD treatment had no effect on microCANP and CINP activities of SN, whereas carbon disulfide enhanced microCANP after 15 d and CINP activity after 10 d. It is suggested that alteration in in vitro calpain activity shown by these chemicals may not be directly related to their neurotoxic effect. However, calpain may still be playing a role in this polyneuropathy by alteration in activity through inflow of Ca2+, release of Ca2+ from intracellular organelles, or other factors. Modification of cytoskeletal proteins making them more susceptible to proteases and the role of some other proteinase is also possible.

Biochemical markers of neurotoxicity: research strategies and epidemiological applications

A great deal of attention has been given in the past few years to the development and validation of biomarkers for non-cancer endpoints, to be used in human epidemiological studies. In this research, as they apply to the field of neurotoxicology, will be discussed. As biomarkers are often divided into indicators of exposure, effect and susceptibility, one example for each of these classes is presented. Measurements of hemoglobin adducts were developed as a way to monitor exposure to acrylamide (a peripheral neurotoxicant) in animals as well as humans, and have been successfully applied in a field study in occupationally exposed workers. Activity of monoamine oxidase B (MAO-B) in platelets was found to be inversely correlated with the levels of exposure to styrene, suggesting that this biochemical measurement may be a useful effect-related biomarker, though additional studies are needed to understand the mechanistic implications of these findings. Serum paraoxonase activity has been shown in animals to modulate the toxicity of organophosphorus insecticides. Studies in humans have lead to the identification of a genetic polymorphism for this enzyme and to define its underlying molecular basis. Epidemiological studies are needed to determine the relevance of this polymorphism in determining susceptibility to organophosphate toxicity.

In vitro calcium and calmodulin-dependent kinase-mediated phosphorylation of rat brain and spinal cord neurofilament proteins is increased by glycidamide administration

This study was carried out to determine the action of glycidamide (2,3-epoxy-1-propanamide), a neurotoxic metabolite of acrylamide, on Ca2+/calmodulin (CaM)-dependent protein kinase phosphorylation of cytoskeletal proteins. Acrylamide has been shown to increase Ca2+/CaM-dependent phosphorylation of neurofilament (NF) triplet proteins and autophosphorylation of Ca2+/CaM-dependent protein kinase II (CaM kinase II; EC 2.7.1.37). A daily intraperitoneal dose of 0.7 mmol/kg b.wt. of glycidamide or deionized water was administered to male Sprague-Dawley rats. Animals were sacrificed when signs of severe neurotoxicity became apparent at 13-16 days of treatment. Axonal floatation was used to isolate neurofilaments (NFs) and endogenous kinases from brains and spinal cords of treated and control animals. Samples isolated from brain and spinal cord of glycidamide-treated animals showed increased in vitro Ca2+/CaM-dependent phosphorylation of endogenous and exogenous NF proteins and increased autophosphorylation of CaM kinase II when compared with controls. CaM binding to the alpha, beta, and beta' subunits of CaM kinase II and antibody binding to the alpha-subunit of CaM kinase II in brain supernatant isolates was increased as a result of glycidamide treatment. These results suggest that increased Ca2+/CaM-dependent phosphorylation of cytoskeletal proteins may be involved in the pathogenesis of glycidamide-induced neurotoxicity.

Acrylamide increases in vitro calcium and calmodulin-dependent kinase-mediated phosphorylation of rat brain and spinal cord neurofilament proteins

Male Sprague-Dawley rats were administered a daily i.p. dose of 0.70 mmol/kg body weight of acrylamide, propionamide (a non-neurotoxic structural analog of acrylamide) or deionized water. Animals were sacrificed when signs of severe neurotoxicity were apparent. Neurofilaments (NFs) and endogenous kinase were isolated from the brain and spinal cord by axonal floatation. Increased in vitro Ca2+/calmodulin-dependent phosphorylation of endogenous and exogenous NF proteins and autophosphorylation of Ca2+/calmodulin protein kinase II (CaM kinase II, EC 2-7-1-37) were observed in samples from both brain and spinal cord of acrylamide-treated animals compared with controls. There was no significant difference between samples isolated from propionamide-treated animals and controls. Increased calmodulin binding to brain supernatant CaM kinase II was also observed as a result of acrylamide treatment. There was no significant difference observed in the amount of antibody binding to the alpha-subunit of brain supernatant CaM kinase II between treated or control animals. These results suggest that increased CaM kinase II-dependent phosphorylation of cytoskeletal proteins may be involved in the mechanisms of acrylamide-induced neurotoxicity.

Toxic neurofilamentous axonopathies and fast axonal transport. V. Reduced bidirectional vesicle transport in cultured neurons by acrylamide and glycidamide

Fast axonal transport deficiencies as mechanisms of action of acrylamide in producing axonal degeneration are under evaluation. The current study determines the effects of acrylamide and several analogues on the number of vesicles moving within the neurite processes of cultured rat embryonic neurons. Acrylamide produced severe, concentration-dependent (0.25-1.0 mM) and time-dependent (0-60 min) reduction in the quantity of vesicles translocated in both the anterograde and retrograde directions. Glycidamide, a potential neurotoxic metabolite of acrylamide, produced a time-dependent but not a concentration-dependent (in the 0.25-1.0 mM range) reduction in bidirectional transport. Based on inhibition at 60 min, glycidamide was estimated to be 4 times more potent than acrylamide in altering transport. Propionamide, a C1-C2 saturated nonneurotoxic acrylamide analogue, had no effect on axonal transport. While a tendency for methylene bisacrylamide (MbACR) to reduce vesicle transport was noted, at the concentration used no statistically significant differences from control were observed. The data support the correlation between toxicant-induced fast anterograde and retrograde axonal transport reductions and axonal degeneration produced by acrylamide and its analogues.