Acrylamide inhibits dopamine uptake in rat striatal synaptic vesicles

Increased IP3R-3 degradation induced by acrylamide promoted Ca2+-dependent calpain activation and axon damage in rats

Occupational and environmental exposure to acrylamide (ACR) can cause selective peripheral and central nerve fiber degeneration. IP3R-3 is an important transmembrane Ca²⁺ channel on the endoplasmic reticulum (ER), previous studies have found that ACR could induce Ca²⁺-dependent calpain activation and axon injury, but the exact role of IP3R-3 in ACR neuropathy is still unclear. Here we show that ACR exposure (40mg/kg) markedly increased the ubiquitination of IP3R-3 in rat spinal cords, and promoted the degradation of IP3R-3 through the ubiquitin-proteasome pathway. Furthermore, the normal structure of ER, especially the mitochondrial associated membranes (MAMs) component, was significantly impaired in ACR neuropathy, and the ER stress pathway was activated, which indicated that the aberrant increase of cytoplasmic Ca²⁺ could be attributed the destruction of IP3R-3. Further investigation demonstrated that the proteasome inhibitor MG-132 effectively rescued the IP3R-3 loss, attenuated the intracellular Ca²⁺ increase, and reduced the axon loss of Neuron 2a (N2a) cells following ACR exposure. Moreover, the calpain inhibitor ALLN also reduced the loss of IP3R-3 and axon injury in N2a cells, but did not alleviate the Ca²⁺ increase in cytosol, supporting that the abnormal ubiquitination of IP3R-3 was the upstream of the cellular Ca²⁺ rise and axon damage in ACR neuropathy. Taken together, our results suggested that the aberrant IP3R-3 degradation played an important role in the disturbance of Ca²⁺ homeostasis and the downstream axon loss in ACR neuropathy, thus providing a potential therapeutic target for ACR neurotoxicity.

The sensitivity of the zebrafish embryo coiling assay for the detection of neurotoxicity by compounds with diverse modes of action

In the aim to determine neurotoxicity, new methods are being validated, including tests and test batteries comprising in vitro and in vivo approaches. Alternative test models such as the zebrafish (Danio rerio) embryo have received increasing attention, with minor modifications of the fish embryo toxicity test (FET; OECD TG 236) as a tool to assess behavioral endpoints related to neurotoxicity during early developmental stages. The spontaneous tail movement assay, also known as coiling assay, assesses the development of random movement into complex behavioral patterns and has proven sensitive to acetylcholine esterase inhibitors at sublethal concentrations. The present study explored the sensitivity of the assay to neurotoxicants with other modes of action (MoAs). Here, five compounds with diverse MoAs were tested at sublethal concentrations: acrylamide, carbaryl, hexachlorophene, ibuprofen, and rotenone. While carbaryl, hexachlorophene, and rotenone consistently induced severe behavioral alterations by ~ 30 h post fertilization (hpf), acrylamide and ibuprofen expressed time- and/or concentration-dependent effects. At 37-38 hpf, additional observations revealed behavioral changes during dark phases with a strict concentration-dependency. The study documented the applicability of the coiling assay to MoA-dependent behavioral alterations at sublethal concentrations, underlining its potential as a component of a neurotoxicity test battery.

Mitochondrial Localization of SARM1 in Acrylamide Intoxication Induces Mitophagy and Limits Neuropathy

Sterile α and toll/interleukin 1 receptor motif-containing protein 1 (SARM1) is the defining molecule and central executioner of programmed axon death, also known as Wallerian degeneration. SARM1 has a mitochondrial targeting sequence, and it can bind to and stabilize PTEN-induced putative kinase 1 (PINK1) for mitophagy induction, but the deletion of the mitochondrial localization sequence is found to disrupt the mitochondrial localization of SARM1 in neurons without altering its ability to promote axon degeneration after axotomy. The biological significance of SARM1 mitochondrial localization remains elusive. In this study, we observed that the pro-degeneration factor, SARM1, was upregulated in acrylamide (ACR) neuropathy, a slow, Wallerian-like, programmed axonal death process. The upregulated SARM1 accumulated on mitochondria, interfered with mitochondrial dynamics, and activated PINK1-mediated mitophagy. Importantly, rapamycin (RAPA) intervention eliminated mitochondrial accumulation of SARM1 and partly attenuated ACR neuropathy. Thus, mitochondrial localization of SARM1 may contribute to its clearance through the SARM1-PINK1 mitophagy pathway, which inhibits axonal degeneration through a negative feedback loop. The mitochondrial localization of SARM1 complements the coordinated activity of the pro-survival factor, nicotinamide mononucleotide adenyltransferase 2 (NMNAT2), and SARM1 and is part of the self-limiting molecular mechanisms underpinning programmed axon death in ACR neuropathy. Mitophagy clearance of SARM1 is complementary to the coordinated activity of NMNAT2 and SARM1 in ACR neuropathy.

Neuroprotection by melatonin against acrylamide-induced brain damage in pinealectomized rats

The current study aimed to evaluate the neuroprotective effect of exogenous melatonin against acrylamide (ACR)-induced oxidative stress and inflammatory and apoptotic responses in the brain tissues in pinealectomized rats (PINX). ACR is a toxic chemical carcinogen that occurs owing to the preparation of carbohydrate-rich foods at high temperatures or other thermal processes. The rats who underwent pinealectomy and sham pinealectomy were exposed to ACR (25 mg/kg b.w., orally) alone or with exogenous melatonin (10 mg/kg b.w., i.p.) for 21 consecutive days. Alterations of brain oxidant/antioxidant status, dopamine (DA), Brain-Derived Neurotropic Factor (BDNF) inflammatory mediator and apoptosis during exposure to ACR in pinealectomized rats were more than without pinealectomized rats. Histopathological changes were more in brain tissue of pinealectomized rats after ACR administration. Exogenous melatonin treatment in ACR -exposed rats following pinealectomy increased the activities of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT) and improved brain total antioxidant status (TAS) compared to PINX+ACR. Moreover, melatonin suppressed lipid peroxidation, inflammatory pathways and apoptosis in ACR-intoxicated brain tissues. In addition, after exposure to ACR on pinealectomized rats, melatonin treatment ameliorated BDNF and DA levels in brain tissues. Furthermore, exogenous melatonin intervention in ACR-intoxicated rats significantly rescued the architecture of neuronal tissues. In summary, the present study, for the first time, suggested that exogenous melatonin treatment could reduce oxidative damage by increasing the activities of antioxidant enzymes, inhibiting lipid peroxidation and inflammation, and improving histopathological alterations in the brain tissue of pinealectomized rats after ACR administration.

Chronic acrylamide exposure resulted in dopaminergic neuron loss, neuroinflammation and motor impairment in rats

Acrylamide (ACR) as a by-product of Maillard reaction is widely present in food. Although ACR is known to exhibit neurotoxicity, most studies about ACR neurotoxicity are currently short-term high-dose providing limited reference value for human exposure. The present study aims to determine the effects of chronic ACR exposure on dopaminergic neurons in rat nigra and the potential mechanism from the perspective of NLRP3 inflammasome-mediated neuroinflammation. The SD rats were maintained on treated drinking water providing dosages of 0, 0.5, or 5 mg/kg/day ACR for 12 months. ACR exposure caused motor dysfunction in rats, which was associated with dopaminergic neuron loss, α-Synuclein (α-Syn) accumulation and decreased brain-derived neurotrophic factor (BDNF) in nigra. ACR activated microglia by increasing Iba-1⁺, Iba-1⁺CD68⁺ positive cells and the percentage of ameboid-shaped ones in rat nigra. ACR markedly upregulated the protein levels of NLRP3 inflammasome constituents NLRP3 and caspase-1 and inflammatory cytokine IL-1β. ACR chronic exposure increased the risk of Parkinson's disease (PD) like dopaminergic neuron depletion in nigra potentially through NLRP3 inflammasome-mediated neuroinflammtion.

The involvement of oxidative stress, neuronal lesions, neurotransmission impairment, and neuroinflammation in acrylamide-induced neurotoxicity in C57/BL6 mice

Acrylamide (ACR) is a typical environmental contaminant, presenting potential health hazards that have been attracting increasing attention. Its neurotoxicity is known to cause significant damage to health. However, the mechanisms of ACR-induced neurotoxicity require further clarification. This study uses a mouse model to explore how ACR-induced oxidative stress, neuronal lesions, neurotransmission impairment, and neuroinflammation mutually contribute to neurotoxicity. A distinct increase in the cellular reactive oxygen species (ROS) levels, malondialdehyde (MDA), and 8-hydroxy-2-deoxyguanosine (8-OHdG) content and a significant decrease in the glutathione (GSH) content after ACR exposure were indicative of oxidative stress. Moreover, ACR caused neurological defects associated with gait abnormality and neuronal loss while suppressing the acetylcholine (ACh) and dopamine (DA) levels and increasing the protein expression of α-synuclein (α-syn), further inhibiting cholinergic and dopaminergic neuronal function. Additionally, ACR treatment caused an inflammatory response via nuclear factor-kappa B (NF-κB) activation and increased the protein expression of NOD-like receptor protein-3 (NLRP3), consequently activating the NLRP3 inflammasome constituents, including cysteinyl aspartate specific proteinase 1 (Caspase-1), apoptosis-associated speck-like protein containing CARD (ASC), N domain gasdermin D (N-GSDMD), interleukin-1β (IL-1β), and IL-18. The results revealed the underlying molecular mechanism of ACR-induced neurotoxicity via oxidative stress, neurotransmission impairment, and neuroinflammation-related signal cascade. This information will further improve the development of an alternative pathway strategy for investigating the risk posed by ACR. The hypothetical mechanism of ACR-induced neurotoxicity in vivo.

The Mechanism of Acrylamide-Induced Neurotoxicity: Current Status and Future Perspectives

Acrylamide (ACR), a potential neurotoxin, is produced by the Maillard reaction between reducing sugars and free amino acids during food processing. Over the past decade, the neurotoxicity of ACR has caused increasing concern, prompting many related studies. This review summarized the relevant literature published in recent years and discussed the exposure to occupational, environmental, and daily ACR contamination in food. Moreover, ACR metabolism and the potential mechanism of ACR-induced neurotoxicity were discussed, with particular focus on the axonal degeneration of the nervous system, nerve cell apoptosis, oxidative stress, inflammatory response, and gut-brain axis homeostasis. Additionally, the limitations of existing knowledge, as well as new perspectives, were examined, specifically regarding the connection between the neurotoxicity caused by ACR and neurodegenerative diseases, NOD-like receptor protein 3 (NLRP3) inflammasome-related neuroinflammation, and microbiota-gut-brain axis signaling. This review might provide systematic information for developing an alternative pathway approach to assess ACR risk.

The Possible Protective Role of Dark Chocolate Against Acrylamide Neurotoxicity in Weaning Rats Cerebellum

Acrylamide (ACR) is selective neurotoxicity, could be found in foods processed by high temperature. This work aimed to evaluate the protective role of the dark chocolate (DC) against cerebellar neurotoxicity induced by subchronic ACR exposure in recently weaned rat pups and to propose it as protective supplement against dietary ACR hazards. Eighteen weaning pups were used in the current study and divided into three groups, six rats in each group; group 1 (control group), group 2 (ACR group), and group 3 (ACR + DC group). The pups were sacrificed after 21 days and the cerebellums were removed for light microscope using H&E stain, ultrastructural study, morphometric analysis of the neurons count, biochemical analysis of oxidant and antioxidant markers and real-time quantitative PCR to evaluate the nuclear receptor subfamily 4, group A, member 2 (Nr4a2) gene expression. Pups with ACR consumption showed signs of neuronal degeneration and reduced Nr4a2 expression. On the other hand, pups with ACR + DC consumption showed relative signs of neuronal restoration and enhanced Nr4a2 expression. In conclusion, DC can be used as effective supplement to decrease the dietary ACR cerebellar neuronal risks.

Comprehensive analysis of metabolic changes in rats exposed to acrylamide

Acrylamide (ACR) is a widely used environmentally hazardous compound that is known to be neurotoxic, genotoxic, carcinogenic, and reproductive toxicity. It is widely present in soil, water, agents used in chemical industries, and food. It can be distributed to all organs and tissues, and can cause damage to various human systems and those of other animals. Previous metabolomics studies have mainly focused on metabolites in serum and urine, but have lacked comprehensive analysis of major organs and tissues. In the current study, a gas chromatography-massspectrometry method was used to investigate mechanisms underlying organ toxicity, in an effort to identify potentially sensitive biomarkers in the main target tissues of rats after ACR exposure. Male Sprague-Dawley rats were assigned to two groups; a control group and a group treated with 20 mg kg^-1 ACR intragastrically for 6 weeks. Metabolite changes in the two groups were statistically analyzed. The respective numbers of altered metabolites in the hippocampus, cortex, kidney, serum, heart, liver, and kidney fat were 21, 21, 17, 5, 15, 14, and 6. There were 14 metabolic pathways related to amino acid, fatty acid, purine, and energy metabolism, revealing that the toxic mechanism of ACR may involve oxidative stress, inflammation, and amino acid metabolism and energy disorders.

Effect of long-term exposure to acrylamide on endoplasmic reticulum stress and autophagy in rat cerebellum

Acrylamide (ACR) is a widely used chemical compound that has neurotoxicity in human, but whether ACR could impair the cerebellum and the related mechanism were still unknown. This study aimed to observe the changes in behavioral performance and cerebellar morphology caused by chronic ACR exposure, and to evaluate its influence on apoptosis, endoplasmic reticulum stress (ERS) and autophagy. Rats were treated with 0, 0.5 and 5 mg/kg ACR by drinking water for 12 months. Results showed that 5 mg/kg ACR treatment damaged the gait, balance ability, hindlimb muscle strength and motor coordination ability of rats. The results of hematoxylin and eosin and Nissl staining indicated that ACR impaired the structures of all three layers of the cerebellum, especially the Purkinje cell layer, showing abnormal morphology with nucleus condensation and pyknosis. Accumulation of autophagosomes, dilated endoplasmic reticulum and swollen mitochondria were observed in neurons under transmission electron microscopy. The enhanced apoptotic rates and the increased Bax expression indicated the elevated level of apoptosis. The results of Western blot showed that ACR treatment elevated protein levels of Beclin1, LC3-II/LC3-I, p-PERK/t-PERK, ATF4 and CHOP, indicating the initiation of autophagy, the activation of PERK pathway in ERS. This work helps to demonstrate the ACR neurotoxicity on cerebellum under chronic treatment and its underlying mechanism.

Protective effects of selenium on acrylamide-induced neurotoxicity and hepatotoxicity in rats

Objective(s):

Acrylamide (ACR), has wide uses in different industries. ACR induced several toxicities including neurotoxicity and hepatotoxicity. The probable protective effects of selenium on ACR-induced neurotoxicity and hepatotoxicity in rats were evaluated.

Materials and Methods:

Male Wistar rats were studied for 11 days in 8 groups: 1. Control, 2. ACR (50 mg/kg, IP), 3-5. ACR+ selenium (0.2, 0.4, 0.6 mg/kg, IP), 6. ACR+ the most effective dose of selenium (0.6 mg/kg, IP) three days after ACR administration, 7. ACR+ vitamin E (200 mg/kg IP, every other day) 8. Selenium (0.6 mg/kg IP). Finally, behavioral tests were done. The levels of malondialdehyde (MDA), glutathione (GSH), Bcl-2, Bax and caspase 3 proteins in liver and cerebral cortex tissues were measured. Also, the amount of albumin, total protein, alanine transaminase (ALT) and aspartate transaminase (AST) enzymes were determined in serum.

Results:

ACR caused the severe motor impairment, increased MDA level and decreased GSH content, enhanced Bax/Bcl-2 ratio and caspase 3 proteins in brain and liver tissues. Besides, the level of AST was elevated while the total serum protein and albumin levels were decreased. Administration of selenium (0.6 mg/kg) (from the first day of the experiment and the third day) significantly recovered locomotor disorders, increased GSH content, and reduced MDA level. Also, selenium decreased Bax/Bcl-2 ratio and caspase 3 levels in brain and liver tissues.

Conclusion:

The oxidative stress and apoptosis pathways have important roles in neurotoxicity and hepatotoxicity of ACR. Selenium significantly reduced ACR-induced toxicity through inhibition of oxidative stress and apoptosis.

Analysis of silymarin-modulating effects against acrylamide-induced cerebellar damage in male rats: Biochemical and pathological markers

BACKGROUND

Acrylamide (ACR) is a well-proven neurotoxin and potential food carcinogen in humans and rodent models. Silymarin (SIL) is a flavonoid mixture isolated from seeds, leaves, and fruits of Silymarin marianum (milk thistle) that possesses a free-radical scavenging effect.

OBJECTIVE

In this work, the primary focus was to investigate the efficacy of SIL to mitigate ACR-induced subacute neurotoxic effects and oxidative changes in rat cerebellum.

METHODS

Adult male rats were treated intraperitoneally with ACR (50 mg/kg) with or without SIL (160 mg/kg). The neuropathology and biochemical parameters viz. lipid peroxidation (measured as levels of malondialdehyde or MDA), catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), serotonin (5-hydroxytryptamine; 5-HT), dopamine (DA), and cathepsin D (CTSD) in the cerebellum have been evaluated.

RESULTS

The data showed that ACR induced redox disruptions as measured by increased MDA levels and inhibition of CAT, SOD, and GPx antioxidant enzyme activities. Besides, cerebellar monoamine neurotransmitters, 5-HT and DA, were depleted in ACR-treated rats. Furthermore, ACR administration caused a significant elevation of CTSD activity, indicating that ACR could trigger apoptosis or apoptosis-like death. At the tissue level, cerebellar cortex sections from ACR-treated animals were characterized by severe neuronal damage. The administration of SIL to ACR-treated rats remarkably alleviated all the aforementioned ACR-induced effects.

CONCLUSION

SIL has a potent therapeutic effect against ACR-induced cerebellar neurotoxicity in experimental rats via the attenuation of oxidative/antioxidative responses and the inhibition of CTSD-activity.

Minocycline protects against acrylamide-induced neurotoxicity and testicular damage in Sprague-Dawley rats

This study investigated the protective effects of minocycline against acrylamide (ACR)-induced neurotoxicity and testicular damage in Sprague-Dawley rats. Forty rats were divided into five groups (eight rats each). Group I received saline (0.5 mL/rat) daily for 10 days and served as the untreated control group. Group II received ACR (30 mg/kg body weight (b.w.)) daily for 10 days. Group III received ACR (30 mg/kg b.w.) daily for 10 days and subsequently minocycline (60 mg/kg b.w.) for five days. Group IV received ACR (30 mg/kg b.w.) daily for 10 days followed by saline for five days and served as the control group for the ACR-minocycline-treated group. Group V received minocycline (60 mg/kg b.w.) for five days. All treatments were administered orally. Rats in group I and V showed normal locomotor behavior and normal histology of the brain and testes. Administration of ACR (Group II and IV) resulted in weight loss and gait abnormalities. Furthermore, neuronal degeneration in the hippocampus and cerebellum and degeneration of the seminiferous tubular epithelium with formation of spermatid giant cells were observed. Ultrastructurally, ACR specifically damaged spermatogonia and spermatocytes. Acrylamide was also seen to cause a significant increase of malondialdehyde levels in the brain and testes. Treatment of ACR-administered rats with minocycline (Group III) significantly alleviated the loss of body weight and improved locomotor function. Minocycline also ameliorated neuronal degeneration and seminiferous tubular damage and decreased malondialdehyde concentrations. In conclusion, minocycline protects against neurotoxic effects of acrylamide and seminiferous tubular damage. Decreasing lipid peroxidation by minocycline might play a role in such protection.

Glutathione S-transferase is a good biomarker in acrylamide induced neurotoxicity and genotoxicity

Glutathione S-transferases (GSTs) are major defence enzymes of the antioxidant enzymatic system. Cytosolic GSTs are more involved in the detoxification than mitochondrial and microsomal GSTs. GSTs are localized in the cerebellum and hippocampus of the rat brain. Acrylamide (AC) is a well assessed neurotoxin of both animals and humans and it produces skeletal muscle weakness and ataxia. AC is extensively used in several industries such as cosmetic, paper, textile, in ore processing, as soil conditioners, flocculants for waste water treatment and it is present in daily consumed food products, like potato chips, French fries, bread, breakfast cereals and beverages like coffee; it is detected on tobacco smoking. GST acts as a biomarker in response to acrylamide. AC can interact with DNA and therefore generate mutations. In rats, low level expression of glutathione S-trasferase (GST) decreases both memory and life span. The major aim of this review is to provide better information on the antioxidant role of GST against AC induced neurotoxicity and genotoxicity.

Dietary Acrylamide and the Risks of Developing Cancer: Facts to Ponder

Acrylamide (AA) is a water soluble white crystalline solid commonly used in industries. It was listed as an industrial chemical with potential carcinogenic properties. However to date, AA was used to produce polyacrylamide polymer, which was widely used as a coagulant in water treatment; additives during papermaking; grouting material for dams, tunnels, and other underground building constructions. AA in food could be formed during high-temperature cooking via several mechanisms, i.e., formation via acrylic acid which may be derived from the degradation of lipid, carbohydrates, or free amino acids; formation via the dehydration/decarboxylation of organic acids (malic acid, lactic acid, and citric acid); and direct formation from amino acids. The big debate is whether this compound is toxic to human beings or not. In the present review, we discuss the formation of AA in food products, its consumption, and possible link to the development of any cancers. We discuss the body enzymatic influence on AA and mechanism of action of AA on hormone, calcium signaling pathways, and cytoskeletal filaments. We also highlight the deleterious effects of AA on nervous system, reproductive system, immune system, and the liver. The present and future mitigation strategies are also discussed. The present review on AA may be beneficial for researchers, food industry, and also medical personnel.

Aluminium and Acrylamide Disrupt Cerebellum Redox States, Cholinergic Function and Membrane-Bound ATPase in Adult Rats and Their Offspring

Accumulation of aluminium and acrylamide in food is a major source of human exposure. Their adverse effects are well documented, but there is no information about the health problems arising from their combined exposure. The aim of the present study was to examine the possible neurotoxic effects after co-exposure of pregnant and lactating rats to aluminium and acrylamide in order to evaluate redox state, cholinergic function and membrane-bound ATPases in the cerebellum of adult rats and their progeny. Pregnant female rats have received aluminium (50 mg/kg body weight) via drinking water and acrylamide (20 mg/kg body weight) by gavage, either individually or in combination from the 14th day of pregnancy until day 14 after delivery. Exposure to these toxicants provoked an increase in malondialdehyde (MDA) and advanced oxidation protein product (AOPP) levels and a decrease in SOD, CAT, GPx, Na⁺K⁺-ATPase, Mg²⁺-ATPase and AChE activities in the cerebellum of mothers and their suckling pups. A reduction in GSH, NPSH and vitamin C levels was also observed. These changes were confirmed by histological results. Interestingly, co-exposure to these toxicants exhibited synergism based on physical and biochemical variables in the cerebellum of mothers and their progeny.

Transcriptomics analysis and hormonal changes of male and female neonatal rats treated chronically with a low dose of acrylamide in their drinking water

Acrylamide is known to produce follicular cell tumors of the thyroid in rats. RccHan Wistar rats were exposed in utero to a carcinogenic dose of acrylamide (3 mg/Kg bw/day) from gestation day 6 to delivery and then through their drinking water to postnatal day 35. In order to identify potential mechanisms of carcinogenesis in the thyroid glands, we used a transcriptomics approach. Thyroid glands were collected from male pups at 10 PM and female pups at 10 AM or 10 PM in order to establish whether active exposure to acrylamide influenced gene expression patterns or pathways that could be related to carcinogenesis. While all animals exposed to acrylamide showed changes in expected target pathways related to carcinogenesis such as DNA repair, DNA replication, chromosome segregation, among others; animals that were sacrificed while actively drinking acrylamide-laced water during their active period at night showed increased changes in pathways related to oxidative stress, detoxification pathways, metabolism, and activation of checkpoint pathways, among others. In addition, thyroid hormones, triiodothyronine (T3) and thyroxine (T4), were increased in acrylamide-treated rats sampled at night, but not in quiescent animals when compared to controls. The data clearly indicate that time of day for sample collection is critical to identifying molecular pathways that are altered by the exposures. These results suggest that carcinogenesis in the thyroids of acrylamide treated rats may ensue from several different mechanisms such as hormonal changes and oxidative stress and not only from direct genotoxicity, as has been assumed to date.

Farnesol quells oxidative stress, reactive gliosis and inflammation during acrylamide-induced neurotoxicity: Behavioral and biochemical evidence

Acrylamide (ACR) is an industrial pollutant, to which humans are exposed through chemicals associated with day to day human life and contributes to neurological disorders. The role of reactive gliosis upon toxic insults remains paradoxical, and the immunomodulatory events during ACR intoxication remain obscure. In view of this, the present study investigated ACR-induced (20mg/kgb.wt for 4weeks) neurodegeneration in the context of oxidative stress and associated inflammatory events and the ability of farnesol, a sesquiterpene, to mitigate reactive gliosis in the brain of Swiss albino mice. Farnesol supplementation (100mg/kgb.wt.) showed a marked improvement in gait performance, neuromuscular function and fine motor coordination and attenuated ACR-induced diminution in glutathione (GSH) with parallel reduction in lipid peroxidation (LPO), protein carbonyls, hydroxide, hydroperoxide and nitrite levels. Farnesol treatment significantly ameliorated ACR-mediated histological aberrations and reactive gliosis by downregulating Glial fibrillary acidic protein (GFAP) and Ionizsed calcium-binding adapter molecule-1 ​(Iba-1) in the cortex, hippocampus and striatum. Further, ACR stimulated increase in levels of pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β) and inducible form of nitric oxide synthase (iNOS) were considerably decreased by farnesol. In conclusion, our findings indicate that farnesol exerts neuroprotective efficacy during ACR-induced neuropathology by suppressing reactive gliosis and associated inflammatory events.

Reactions of electrophiles with nucleophilic thiolate sites: relevance to pathophysiological mechanisms and remediation

Electrophiles are electron-deficient species that form covalent bonds with electron-rich nucleophiles. In biological systems, reversible electrophile-nucleophile interactions mediate basal cytophysiological functions (e.g. enzyme regulation through S-nitrosylation), whereas irreversible electrophilic adduction of cellular macromolecules is involved in pathogenic processes that underlie many disease and injury states. The nucleophiles most often targeted by electrophiles are side chains on protein amino acids (e.g. Cys, His, and Lys) and aromatic nitrogen sites on DNA bases (e.g. guanine N7). The sulfhydryl thiol (RSH) side chain of cysteine residues is a weak nucleophile that can be ionized in specific conditions to a more reactive nucleophilic thiolate (RS(-)). This review will focus on electrophile interactions with cysteine thiolates and the pathophysiological consequences that result from irreversible electrophile modification of this anionic sulfur. According to the Hard and Soft, Acids and Bases (HSAB) theory of Pearson, electrophiles and nucleophiles can be classified as either soft or hard depending on their relative polarizability. HSAB theory suggests that electrophiles will preferentially and more rapidly form covalent adducts with nucleophiles of comparable softness or hardness. Application of HSAB principles, in conjunction with in vitro and proteomic studies, have indicated that soft electrophiles of broad chemical classes selectively form covalent Michael-type adducts with soft, highly reactive cysteine thiolate nucleophiles. Therefore, these electrophiles exhibit a common mechanism of cytotoxicity. As we will discuss, this level of detailed mechanistic understanding is a necessary prerequisite for the rational development of effective prevention and treatment strategies for electrophile-based pathogenic states.

The effect of lipoic acid on acrylamide-induced neuropathy in rats with reference to biochemical, hematological, and behavioral alterations

CONTEXT

Acrylamide (ACR) is a well-known neurotoxicant and carcinogenic agent which poses a greater risk for human and animal health.

OBJECTIVE

The present study evaluates the beneficial effects of α-lipoic acid (LA) on ACR-induced neuropathy.

MATERIALS AND METHODS

A total of 40 male rats were divided into four groups: a placebo group; LA-treated group, administered orally 1% (w/w) LA mixed with diet; ACR-treated group, given 0.05% (w/v) ACR dissolved in drinking water; and LA + ACR-treated group, given LA 1% 7 d before and along with ACR 0.05% for 21 d. After 28 d, blood samples were collected, the rats were decapitated, and the tissues were excised for the measurement of brain biomarkers, antioxidant status, and hematological analysis. Also, the gait score of rats was evaluated.

RESULTS

ACR-exposed rats exhibited abnormal gait deficits with significant (p < 0.05) decline in acetylcholine esterase (AChE) and creatine kinase in serum and brain tissues, respectively. However, the lactate dehydrogenase activity was increased in serum by 123%, although it decreased in brain tissues by -74%. ACR significantly (p < 0.05) increased the malondialdehyde level by 273% with subsequent depletion of glutathione S-transferase (GST), glutathione peroxidase (GPx), and glutathione reductase (GR) activities and reduced the glutathione (GSH) level in brain tissue. Interestingly, LA significantly (p < 0.05) improved brain enzymatic biomarkers, attenuated lipid peroxidation (LPO), and increased antioxidant activities compared with the ACR-treated group.

DISCUSSION AND CONCLUSION

These results suggested that LA may have a role in the management of ACR-induced oxidative stress in brain tissues through its antioxidant activity, attenuation of LPO, and improvement of brain biomarkers.

Acrylamide increases dopamine levels by affecting dopamine transport and metabolism related genes in the striatal dopaminergic system

Dopaminergic system dysfunction is proved to be a possible mechanism in acrylamide (ACR) -induced neurotoxicity. The neurotransmitter dopamine (DA) has an increasingly important role in the dopaminergic system. Thus, the goal of this study is to evaluate effects of ACR on dopamine and its metabolite levels, dopamine transport and metabolic gene expression in dopaminergic neurons. Male Sprague-Dawley (SD) rats were dosed orally with ACR at 0 (saline), 20, 30, and 40 mg/kg/day for 20 days. Splayed hind limbs, reduced tail flick time and abnormal gait which preceded other neurologic parameters were observed in the above rats. ACR significantly increased dopamine levels, decreased 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA) contents in an area dependent manner in rat striatum. Immunohistochemical staining of the striatum revealed that the number of tyrosine hydroxylase (TH) positive cells significantly increased, while monoamine oxidase (MAO) positive cells were drastically reduced, which was consistent with changes in their mRNA and protein expressions. In addition, dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) expression levels were both down-regulated in the striatum. These results suggest that dopamine levels increase significantly in response to ACR, presumably due to changes in the dopamine transport and metabolism related genes expression in the striatal dopaminergic neurons.

Acrylamide induces locomotor defects and degeneration of dopamine neurons in Caenorhabditis elegans

Acrylamide can form in foods during the cooking process and cause multiple adverse effects. However, the neurotoxicity and mechanisms of acrylamide have not been fully elucidated. In Caenorhabditis elegans, we showed that 48 h exposure to 10-625 mg l(-1) acrylamide resulted in a significant decline in locomotor frequency of body bending, head thrashing and pharynx pumping. In addition, acrylamide exposure reduced crawling speeds and changed angles of body bending. It indicates that acrylamide induces locomotor defects, along with parkinsonian-like movement impairment, including bradykinesia and hypokinesia. Acrylamide also affected chemotaxis plasticity and reduced learning ability. Using transgenic nematodes, we found that acrylamide induced downexpression of P(dat-1) and led to the degeneration of dopaminergic neurons. Moreover, the enhanced expression of unc-54, encoding a subunit of α-synuclein was found. It illustrates that acrylamide is efficient in inducing crucial parkinsonian pathology, including dopaminergic damage and α-synuclein aggregation. These findings suggest the acrylamide-induced locomotor defects and neurotoxicity are associated with Parkinson's disease.

Interaction of aldehydes derived from lipid peroxidation and membrane proteins

A great variety of compounds are formed during lipid peroxidation of polyunsaturated fatty acids of membrane phospholipids. Among them, bioactive aldehydes, such as 4-hydroxyalkenals, malondialdehyde (MDA) and acrolein, have received particular attention since they have been considered as toxic messengers that can propagate and amplify oxidative injury. In the 4-hydroxyalkenal class, 4-hydroxy-2-nonenal (HNE) is the most intensively studied aldehyde, in relation not only to its toxic function, but also to its physiological role. Indeed, HNE can be found at low concentrations in human tissues and plasma and participates in the control of biological processes, such as signal transduction, cell proliferation, and differentiation. Moreover, at low doses, HNE exerts an anti-cancer effect, by inhibiting cell proliferation, angiogenesis, cell adhesion and by inducing differentiation and/or apoptosis in various tumor cell lines. It is very likely that a substantial fraction of the effects observed in cellular responses, induced by HNE and related aldehydes, be mediated by their interaction with proteins, resulting in the formation of covalent adducts or in the modulation of their expression and/or activity. In this review we focus on membrane proteins affected by lipid peroxidation-derived aldehydes, under physiological and pathological conditions.

Neurotoxicity of acrylamide in exposed workers

Acrylamide (ACR) is a water-soluble chemical used in different industrial and laboratory processes. ACR monomer is neurotoxic in humans and laboratory animals. Subchronic exposure to this chemical causes neuropathies, hands and feet numbness, gait abnormalities, muscle weakness, ataxia, skin and in some cases, cerebellar alterations. ACR neurotoxicity involves mostly the peripheral but also the central nervous system, because of damage to the nerve terminal through membrane fusion mechanisms and tubulovescicular alterations. Nevertheless, the exact action mechanism is not completely elucidated. In this paper we have reviewed the current literature on its neurotoxicity connected to work-related ACR exposure. We have analyzed not only the different pathogenetic hypotheses focusing on possible neuropathological targets, but also the critical behavior of ACR poisoning. In addition we have evaluated the ACR-exposed workers case studies. Despite all the amount of work which have being carried out on this topic more studies are necessary to fully understand the pathogenetic mechanisms, in order to propose suitable therapies.

Thermally induced process-related contaminants: the example of acrolein and the comparison with acrylamide: opinion of the Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG)

α,β-Unsaturated aliphatic carbonyl compounds are naturally widespread in food, but are also formed during the thermal treatment of food. This applies, for example, to the genotoxic carcinogen acrylamide (AA), but also to acrolein (AC), the simplest α,β-unsaturated aldehyde. First observations indicate that human exposure to AC may be higher than the exposure to AA. The DFG Senate Commission on Food Safety therefore compared data on AC and AA available in the scientific literature, evaluating current knowledge on formation, occurrence, exposure, metabolism, biological effects, toxicity, and carcinogenicity and defined knowledge gaps as well as research needs in an opinion on November 19, 2012, in German. The English version was agreed on April 17, 2013.

Protein targets of acrylamide adduct formation in cultured rat dopaminergic cells

Acrylamide (ACR) is an electrophilic unsaturated carbonyl derivative that produces neurotoxicity by forming irreversible Michael-type adducts with nucleophilic sulfhydryl thiolate groups on cysteine residues of neuronal proteins. Identifying specific proteins targeted by ACR can lead to a better mechanistic understanding of the corresponding neurotoxicity. Therefore, in the present study, the ACR-adducted proteome in exposed primary immortalized mesencephalic dopaminergic cells (N27) was determined using tandem mass spectrometry (LTQ-Orbitrap). N27 cells were characterized based on the presumed involvement of CNS dopaminergic damage in ACR neurotoxicity. Shotgun proteomics identified a total of 15,243 peptides in N27 cells of which 103 unique peptides exhibited ACR-adducted Cys groups. These peptides were derived from 100 individual proteins and therefore ~0.7% of the N27 cell proteome was adducted. Proteins that contained ACR adducts on multiple peptides included annexin A1 and pleckstrin homology domain-containing family M member 1. Sub-network enrichment analyses indicated that ACR-adducted proteins were involved in processes associated with neuron toxicity, diabetes, inflammation, nerve degeneration and atherosclerosis. These results provide detailed information regarding the ACR-adducted proteome in a dopaminergic cell line. The catalog of affected proteins indicates the molecular sites of ACR action and the respective roles of these proteins in cellular processes can offer insight into the corresponding neurotoxic mechanism.

Molecular mechanism of acrylamide neurotoxicity: lessons learned from organic chemistry

BACKGROUND

Acrylamide (ACR) produces cumulative neurotoxicity in exposed humans and laboratory animals through a direct inhibitory effect on presynaptic function.

OBJECTIVES

In this review, we delineate how knowledge of chemistry provided an unprecedented understanding of the ACR neurotoxic mechanism. We also show how application of the hard and soft, acids and bases (HSAB) theory led to the recognition that the α,β-unsaturated carbonyl structure of ACR is a soft electrophile that preferentially forms covalent bonds with soft nucleophiles.

METHODS

In vivo proteomic and in chemico studies demonstrated that ACR formed covalent adducts with highly nucleophilic cysteine thiolate groups located within active sites of presynaptic proteins. Additional research showed that resulting protein inactivation disrupted nerve terminal processes and impaired neurotransmission.

DISCUSSION

ACR is a type-2 alkene, a chemical class that includes structurally related electrophilic environmental pollutants (e.g., acrolein) and endogenous mediators of cellular oxidative stress (e.g., 4-hydroxy-2-nonenal). Members of this chemical family produce toxicity via a common molecular mechanism. Although individual environmental concentrations might not be toxicologically relevant, exposure to an ambient mixture of type-2 alkene pollutants could pose a significant risk to human health. Furthermore, environmentally derived type-2 alkenes might act synergistically with endogenously generated unsaturated aldehydes to amplify cellular damage and thereby accelerate human disease/injury processes that involve oxidative stress.

CONCLUSIONS

These possibilities have substantial implications for environmental risk assessment and were realized through an understanding of ACR adduct chemistry. The approach delineated here can be broadly applied because many toxicants of different chemical classes are electrophiles that produce toxicity by interacting with cellular proteins.

Neuroprotective efficacy of eugenol and isoeugenol in acrylamide-induced neuropathy in rats: behavioral and biochemical evidence

The primary objective of this investigation was to assess the neuroprotective efficacy of spice active principles namely Eugenol (Eug) and isoeugenol (IE) in an acrylamide (ACR) neuropathy model in rats. In the present study, ACR administration (50 mg/kg bw, i.p. 3 times/week) for 5 weeks to growing rats caused typical symptoms of neuropathy. We found that treatment of ACR rats with spice active principles (10 mg/kg bw, for 5 weeks) caused marked improvement in gait score and responses in a battery of behavioral tests. Terminally, both spice active principles markedly attenuated ACR-induced markers of oxidative stress viz., reactive oxygen species (ROS), malondialdehyde (MDA) and nitric oxide (NO) in sciatic nerve (SN) as well as brain regions (cortex Ct, cerebellum Cb). Treatment with Eug restored the reduced glutathione levels in SN and brain regions. Interestingly, both spice active principles effectively diminished ACR-induced elevation in cytosolic calcium levels and acetylcholinesterase activity in SN and Ct. Further, the diminished activity of ATPase among ACR rats was enhanced in SN and restored in brain regions. Furthermore, Eug treatment significantly offset ACR-induced depletion in dopamine levels in brain regions. Collectively our findings suggest the propensity of these spice active principles to attenuate ACR-induced neuropathy. Further studies are necessary to understand the precise molecular mechanism/s by which these spice active principles attenuate neuropathy. Nevertheless, our data clearly demonstrate the beneficial effects of spice active principles in ACR-induced neuropathy in rats and suggest their possible therapeutic usage as an adjuvant in the management of other forms of neuropathy in humans.

Evidence of acrylamide induced oxidative stress and neurotoxicity in Drosophila melanogaster - its amelioration with spice active enrichment: relevance to neuropathy

Acrylamide (ACR) intoxication in its monomeric form leads to neuronal damage in both experimental animals and humans. Oxidative stress is one of the principle mechanisms related to the neurotoxicity of ACR exposure. Hence, the present study aimed to recapitulate the potential of ACR to cause oxidative stress and neurotoxic effects in Drosophila melanogaster. Exposure of adult male flies (Oregon K strain) to ACR (1-10 mM, 7 d) in the diet resulted in a concentration and time dependent mortality, while the survivors exhibited significant locomotor deficits. Further, ACR exposure (1-5 mM, 3 d) caused robust oxidative stress as evidenced by markedly elevated levels of reactive oxygen species and hypdroperoxides in head/body regions. Enhanced lipid peroxidation, perturbations in the activities of antioxidant enzymes accompanied with depletion of reduced glutathione levels in head region at high concentrations suggested induction of oxidative stress. Further, marked diminution in the activities of complexes I-III, Succinic dehydrogenase, with concomitant reduction in MTT suggested the propensity of ACR to impair mitochondrial function. Furthermore, ACR-induced neurotoxic effects were discernible in terms of diminished ATPase activity, enhanced activity of acetylcholinesterase and dopamine depletion. In a satellite study, employing a co-exposure paradigm, we tested the propensity of spice actives namely eugenol (EU) and isoeugenol (IE) to ameliorate ACR-induced neurotoxicity. EU/IE enriched diet offered marked protection against ACR-induced mortality, locomotor dysfunctions and oxidative stress. Furthermore, the spice actives prevented the depletion of reduced GSH levels, maintained the activity of AChE enzyme and dopamine levels in head region. Collectively, these findings clearly demonstrate that ACR induced neurotoxicity in Drosophila may be mediated through oxidative stress mechanisms and the potential of spice actives to abrogate the condition. These data suggest that Drosophila may serve as a suitable model to understand the possible mechanism/s associated with ACR associated neuropathy.

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.

Application of the Hard and Soft, Acids and Bases (HSAB) theory to toxicant--target interactions

Many chemical toxicants and/or their active metabolites are electrophiles that cause cell injury by forming covalent bonds with nucleophilic targets on biological macromolecules. Covalent reactions between nucleophilic and electrophilic reagents are, however, discriminatory since there is a significant degree of selectivity associated with these interactions. Over the course of the past few decades, the theory of Hard and Soft, Acids and Bases (HSAB) has proven to be a useful tool in predicting the outcome of such reactions. This concept utilizes the inherent electronic characteristic of polarizability to define, for example, reacting electrophiles and nucleophiles as either hard or soft. These HSAB definitions have been successfully applied to chemical-induced toxicity in biological systems. Thus, according to this principle, a toxic electrophile reacts preferentially with biological targets of similar hardness or softness. The soft/hard classification of a xenobiotic electrophile has obvious utility in discerning plausible biological targets and molecular mechanisms of toxicity. The purpose of this perspective is to discuss the HSAB theory of electrophiles and nucleophiles within a toxicological framework. In principle, covalent bond formation can be described by using the properties of their outermost or frontier orbitals. Because these orbital energies for most chemicals can be calculated using quantum mechanical models, it is possible to quantify the relative softness (σ) or hardness (η) of electrophiles or nucleophiles and to subsequently convert this information into useful indices of reactivity. This atomic level information can provide insight into the design of corroborative laboratory research and thereby help investigators discern corresponding molecular sites and mechanisms of toxicant action. The use of HSAB parameters has also been instrumental in the development and identification of potential nucleophilic cytoprotectants that can scavenge toxic electrophiles. Clearly, the difficult task of delineating molecular sites and mechanisms of toxicant action can be facilitated by the application of this quantitative approach.

Critical review of dose-response options for F344 rat mammary tumors for acrylamide - additional insights based on mode of action

Previous risk assessment reviews analyzed the potential for dietary acrylamide to increase breast cancer risk. Here, we critically review acrylamide animal bioassay data on mammary tumors for human relevance. We applied a systematic evaluation using reasonable standards of scientific certainty and a systematic weight of evidence (WOE) approach to evaluate several hypothesized modes of action (MOA), including (1) genotoxicity related to glycidamide formation and oxidative stress, (2) endocrine effects due to age-related hyperprolactinemia or secondary to neurotoxicity, and (3) epigenetic effects. We conclude that the appropriate approach for low-dose extrapolation of the rat mammary tumors can be narrowed to two options: (1) linear low-dose extrapolation (i.e., based on a MOA of mutagenicity from direct DNA interaction) from a point of departure (POD) for the combined incidence of adenomas and adenocarcinomas, since these tumor types are related; or (2) non-linear extrapolation, using uncertainty factors to estimate a Reference Dose (RfD) from a POD for tumor promotion derived using the combined fibroadenoma, adenoma and adenocarcinoma data. Non-linear extrapolation is used in the latter approach because these combined tumor types are unlikely to be exclusively caused by mutagenicity. Comparison of the WOE for each alternative MOA indicates that a non-linear approach (option 2) is more appropriate for evaluation of acrylamide-induced mammary tumors; a linear approach (option 1) is shown for comparison.

Expression analysis of hepatic mitochondria-related genes in mice exposed to acrylamide and glycidamide

Acrylamide (AA) is an industrial chemical that has been extensively investigated for central nervous system (CNS), reproductive, and genetic toxicity. However, AA effects on the liver, a major organ of drug metabolism, have not been adequately explored. In addition, the role of mitochondria in AA-mediated toxicity is still unclear. Changes in expression levels of genes associated with hepatic mitochondrial function of male transgenic Big Blue (BB) mice administered 500 mg/L AA or an equimolar concentration (600 mg/L) of its reactive metabolite glycidamide (GA) in drinking water for 3 and 4 wk, respectively, were examined. Transcriptional profiling of 542 mitochondria-related genes indicated a significant downregulation of genes associated with the 3-beta-hydroxysteroid dehydrogenase family in AA- and GA-treated mice, suggesting a possible role of both chemicals in altering hepatic steroid metabolism in BB mice. In addition, genes associated with lipid metabolism were altered by both treatments. Interestingly, only the parental compound (AA) significantly induced expression levels of genes associated with oxidative phosphorylation, in particular ATP synthase, which correlated with elevated ATP levels, indicating an increased energy demand in liver during AA exposure. Acrylamide-treated mice also showed significantly higher activity of glutathione S-transferase in association with decreased levels of reduced glutathione (GSH), which may imply an enhanced rate of conjugation of AA with GSH in liver. These results suggest different hepatic mechanisms of action of AA and GA and provide important insights into the involvement of mitochondria during their exposures.

Molecular mechanism of glyceraldehyde-3-phosphate dehydrogenase inactivation by α,β-unsaturated carbonyl derivatives

α,β-Unsaturated carbonyls make up an important class of chemicals involved in environmental toxicity and disease processes. Whereas adduction of cysteine residues on proteins is a well-documented reaction of these chemicals, such a generic effect cannot explain the molecular mechanism of cytotoxicity. Instead, more detailed information is needed regarding the possible specificity and kinetics of cysteine targeting and the quantitative relationship between adduct burden and protein dysfunction. To address these data gaps, we incubated purified human glyceraldehyde-3-phosphate dehydrogenase (GAPDH) with acrylamide (ACR), acrolein, or methylvinyl ketone (MVK). Results show that these α,β-unsaturated carbonyl toxicants inhibited GAPDH activity in a concentration- and time-dependent manner. The rank order of enzyme inhibition (K(I)) (i.e., ACR ≪ MVK < acrolein) was related to the calculated electrophilic reactivity of each compound and to the corresponding kinetics of cysteine adduct formation. Tandem mass spectrometry revealed that adduct formation was selective at lower concentrations; i.e., ACR preferentially formed adducts with Cys152 (residues 146-162). At higher concentrations, ACR also formed adducts with Cys156 and Cys247 (residues 235-248). Adduct formation at Cys152 was correlated to enzyme inhibition, which is consistent with the regulatory role of this residue in enzyme function and its location within the GAPDH active site. Further analyses indicated that Cys152 was present in a pK(a)-lowering microenvironment (pK(a) = 6.03), and at physiological pH, the corresponding sulfhydryl group exists in the highly reactive nucleophilic thiolate state. These data suggest a general cytotoxic mechanism in which electrophilic α,β-unsaturated carbonyls selectively form adducts with reactive nucleophilic cysteine residues specifically associated with the active sites of proteins. These specialized cysteine residues are toxicologically relevant molecular targets, because chemical derivatization causes loss of protein function.

Role of the Nrf2-ARE pathway in acrylamide neurotoxicity

Acrylamide (ACR) intoxication is associated with selective nerve terminal damage in the central and peripheral nervous systems. As a soft electrophile, ACR could form adducts with nucleophilic sulfhydryl groups on cysteine residues of kelch-like erythroid cell-derived protein with CNS homology-associated protein 1 (Keap1) leading to dissociation of the transcription factor, nuclear factor erythroid 2-related factor 2 (Nrf2). Nrf2 activation of the antioxidant-responsive element (ARE) and subsequent upregulated gene expression of phase II detoxification enzymes and anitoxidant proteins might provide protection in neuronal regions with transcriptional capabilities (e.g., cell body). In contrast, non-transcriptional cell regions (axons, nerve terminals) might be vulnerable to electrophile-induced damage. To test this possibility, immunoblot analysis was used to measure protein products of Nrf2-activated ARE genes in nerve terminals and in cytosolic/nuclear factions of neuronal cell bodies isolated from rats intoxicated at two different ACR dose-rates; i.e., 50mg/kg/d×10 days, 21mg/kg/d×38 days. To detect possible differences in cell-specific induction, the cytoprotective response to ACR intoxication was determined in hepatic cells. Results show that control brain and hepatic cell fractions exhibited distinct subcellular distributions of Nrf2, Keap1 and several ARE protein products. ACR intoxication, however, did not alter the levels of these proteins in synaptosomal, brain cytoplasm or liver cell fractions. These data indicate that ACR was an insufficient electrophilic signal for ARE induction in all subcellular fractions tested. Because a cytoprotective response was not induced in any fraction, nerve terminal vulnerability to ACR cannot be ascribed to the absence of transcription-based defense mechanisms in this neuronal region.

β-dicarbonyl enolates: a new class of neuroprotectants

Curcumin, phloretin and structurally related phytopolyphenols have well-described neuroprotective properties that appear to be at least partially mediated by 1,3-dicarbonyl enol substructures that form nucleophilic enolates. Based on their structural similarities, we tested the hypothesis that enolates of simple 1,3-dicarbonyl compounds such as acetylacetone might also possess neuroprotective actions. Our results show that the β-diketones, particularly 2-acetylcyclopentanone, protected rat striatal synaptosomes and a neuronal cell line from thiol loss and toxicity induced by acrolein, an electrophilic α,β-unsaturated aldehyde. The 1,3-dicarbonyl compounds also provided substantial cytoprotection against toxicity induced by hydrogen peroxide in a cellular model of oxidative stress. Initial chemical characterization in cell-free systems indicated that the 1,3-dicarbonyl compounds acted as surrogate nucleophilic targets that slowed the rate of sulfhydryl loss caused by acrolein. Although the selected 1,3-dicarbonyl congeners did not scavenge free radicals, metal ion chelation was a significant property of both acetylacetone and 2-acetylcyclopentanone. Our data suggest that the 1,3-dicarbonyl enols represent a new class of neuroprotectants that scavenge electrophilic metal ions and unsaturated aldehydes through their nucleophilic enolate forms. As such, these enols might be rational candidates for treatment of acute or chronic neurodegenerative conditions that have oxidative stress as a common molecular etiology.

Molecular mechanisms of 4-hydroxy-2-nonenal and acrolein toxicity: nucleophilic targets and adduct formation

Acrolein and 4-hydroxy-2-nonenal (HNE) are byproducts of lipid peroxidation and are thought to play central roles in various traumatic injuries and disease states that involve cellular oxidative stress, for example, spinal cord trauma, diabetes, and Alzheimer's disease. In this review, we will discuss the chemical attributes of acrolein and HNE that determine their toxicities. Specifically, these aldehydes are classified as type 2 alkenes and are characterized by an alpha,beta-unsaturated carbonyl structure. This structure is a conjugated system that contains mobile pi-electrons. The carbonyl oxygen atom is electronegative and can promote the withdrawal of mobile electron density from the beta-carbon atom causing regional electron deficiency. On the basis of this type of electron polarizability, both acrolein and HNE are considered to be soft electrophiles that preferentially form 1,4-Michael type adducts with soft nucleophiles. Proteomic, quantum mechanical, and kinetic data will be presented, indicating that cysteine sulfhydryl groups are the primary soft nucleophilic targets of acrolein and HNE. This is in contrast to nitrogen groups on harder biological nucleophiles such as lysine or histidine residues. The toxicological outcome of adduct formation is not only dependent upon residue selectivity but also the importance of the targeted amino acid in protein function or structure. In attempting to discern the toxicological significance of a given adduct, we will consider the normal roles of cysteine, lysine, and histidine residues in proteins and the relative merits of corresponding adducts in the manifestations of diseases or toxic states. Understanding the molecular actions of acrolein and HNE could provide insight into many pathogenic conditions that involve initial cellular oxidative stress and could, thereby, offer new efficacious avenues of pharmacological defense.

Synaptosomal toxicity and nucleophilic targets of 4-hydroxy-2-nonenal

4-Hydroxy-2-nonenal (HNE) is an aldehyde by-product of lipid peroxidation that is presumed to play a primary role in certain neuropathogenic states (e.g., Alzheimer disease, spinal cord trauma). Although the molecular mechanism of neurotoxicity is unknown, proteomic analyses (e.g., tandem mass spectrometry) have demonstrated that this soft electrophile preferentially forms Michael-type adducts with cysteine sulfhydryl groups. In this study, we characterized HNE synaptosomal toxicity and evaluated the role of putative nucleophilic amino acid targets. Results show that HNE exposure of striatal synaptosomes inhibited (3)H-dopamine membrane transport and vesicular storage. These concentration-dependent effects corresponded to parallel decreases in synaptosomal sulfhydryl content. Calculations of quantum mechanical parameters (softness, electrophilicity) that describe the interactions of an electrophile with its nucleophilic target indicated that the relative softness of HNE was directly related to both the second-order rate constant (k(2)) for sulfhydryl adduct formation and corresponding neurotoxic potency (IC(50)). Computation of additional quantum mechanical parameters that reflect the relative propensity of a nucleophile to interact with a given electrophile (chemical potential, nucleophilicity) indicated that the sulfhydryl thiolate state was the HNE target. In support of this, we showed that the rate of adduct formation was related to pH and that N-acetyl-L-cysteine, but not N-acetyl-L-lysine or beta-alanyl-L-histidine, reduced in vitro HNE neurotoxicity. These data suggest that, like other type 2 alkenes, HNE produces nerve terminal toxicity by forming adducts with sulfhydryl thiolates on proteins involved in neurotransmission.

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.

Postnatal manganese exposure alters dopamine transporter function in adult rats: Potential impact on nonassociative and associative processes

In the present study, we examined whether exposing rats to a high-dose regimen of manganese chloride (Mn) during the postnatal period would depress presynaptic dopamine functioning and alter nonassociative and associative behaviors. To this end, rats were given oral supplements of Mn (750 microg/day) on postnatal days (PD) 1-21. On PD 90, dopamine transporter (DAT) immunoreactivity and [3H]dopamine uptake were assayed in the striatum and nucleus accumbens, while in vivo microdialysis was used to measure dopamine efflux in the same brain regions. The effects of postnatal Mn exposure on nigrostriatal functioning were evaluated by assessing rotorod performance and amphetamine-induced stereotypy in adulthood. In terms of associative processes, both cocaine-induced conditioned place preference (CPP) and sucrose-reinforced operant responding were examined. Results showed that postnatal Mn exposure caused persistent declines in DAT protein expression and [3H]dopamine uptake in the striatum and nucleus accumbens, as well as long-term reductions in striatal dopamine efflux. Rotorod performance did not differ according to exposure condition, however Mn-exposed rats did exhibit substantially more amphetamine-induced stereotypy than vehicle controls. Mn exposure did not alter performance on any aspect of the CPP task (preference, extinction, or reinstatement testing), nor did Mn affect progressive ratio responding (a measure of motivation). Interestingly, acquisition of a fixed ratio task was impaired in Mn-exposed rats, suggesting a deficit in procedural learning. In sum, these results indicate that postnatal Mn exposure causes persistent declines in various indices of presynaptic dopaminergic functioning. Mn-induced alterations in striatal functioning may have long-term impact on associative and nonassociative behavior.

Mechanisms by which amphetamine redistributes dopamine out of vesicles: a computational study

Computer simulations of dopamine (DA) and amphetamine interactions associated with dopaminergic storage vesicles were developed in order to better explain how amphetamine causes redistribution of DA out of the vesicles. In the model, DA can be transported into vesicles via the vesicular monoamine transporter. Amphetamine competitively inhibits DA uptake either as a substrate for the transporter or by interference with DA binding to the transporter. Both of the amines can passively diffuse across the membrane in both directions, but only the neutral species can cross the membrane in this manner. The abundance of neutral and positive moieties of the amines is governed by the Henderson-Hasselbalch equation. The model reproduces experimentally observed steady-state DA levels in vesicles, vesicles emptying faster after a change of pH inside the vesicle than after a change in access of DA for the vesicular monoamine transporter, and the impact of amphetamine on DA uptake and efflux in a variety of experimental paradigms. The simulations indicate that a small percentage of DA is constantly diffusing out of vesicles and is replaced by DA entering the vesicle by the vesicular monoamine transporter. Low doses of amphetamine cause DA redistribution out of vesicles primarily by inhibiting DA storage while an enhancement of efflux rates as a result of a change in vesicle pH is added at higher concentrations of amphetamine.