Acrylamide inhibits dopamine uptake in rat striatal synaptic vesicles

Molecular mechanisms of the conjugated alpha,beta-unsaturated carbonyl derivatives: relevance to neurotoxicity and neurodegenerative diseases

Conjugated alpha,beta-unsaturated carbonyl derivatives such acrylamide, acrolein, and 4-hydroxy-2-nonenal (HNE) are members of a large class of chemicals known as the type-2 alkenes. Human exposure through diet, occupation, and pollution is pervasive and has been linked to toxicity in most major organs. Evidence suggests that these soft electrophiles produce toxicity by a common mechanism involving the formation of Michael-type adducts with nucleophilic sulfhydryl groups. In this commentary, the adduct chemistry of the alpha,beta-unsaturated carbonyls and possible protein targets will be reviewed. We also consider how differences in electrophilic reactivity among the type-2 alkenes impact corresponding toxicokinetics and toxicological expression. Whereas these concepts have mechanistic implications for the general toxicity of type-2 alkenes, this commentary will focus on the ability of these chemicals to produce presynaptic damage via protein adduct formation. Given the ubiquitous environmental presence of the conjugated alkenes, discussions of molecular mechanisms and possible neurotoxicological risks could be important. Understanding the neurotoxicodynamic of the type-2 alkenes might also provide mechanistic insight into neurodegenerative conditions where neuronal oxidative stress and presynaptic dysfunction are presumed initiating events. This is particularly germane to a recent proposal that lipid peroxidation and the subsequent liberation of acrolein and HNE in oxidatively stressed neurons mediate synaptotoxicity in brains of Alzheimer's disease patients. This endogenous neuropathogenic process could be accelerated by environmental type-2 alkene exposure because common nerve terminal proteins are targeted by alpha,beta-unsaturated carbonyl derivatives. Thus, the protein adduct chemistry of the conjugated type-2 alkenes offers a mechanistic explanation for the environmental toxicity induced by these chemicals and might provide insight into the pathogenesis of certain human neurodegenerative diseases.

Effects of chronic low-dose acrylamide exposure on progressive ratio performance in adolescent rats

Acrylamide (ACR) is a neurotoxicant known to produce peripheral neuropathy in rats and humans, but little is known of its potential for producing cognitive or motivational alterations. Chronic exposure to low doses of ACR as a food contaminant is known to occur widely in humans. This research evaluated the effects of daily ACR exposure on food-motivated behavior, with exposures beginning prenatally on gestation day 6 and continuing through approximately postnatal day (PND) 85. Plug-positive Fischer 344 dams (9-10 per dose) were gavaged daily with 0, 0.1, 0.3, 1.0 or 5.0mg/kg/day ACR. On PNDs 1-22, pups were gavaged with the same dose their dam had received. On PND 22, pups were weaned and pair-housed with a same-sex littermate and ACR exposure continued at 0, 1, 3, 10 and 50ppm via drinking water. One male and one female pup per litter were tested in an operant chamber under a progressive ratio (PR) schedule of food reinforcement from approximately 6 to 12 weeks of age. Results over 6 weeks of testing indicated a significant treatment effect of ACR on number of reinforcers earned, with Tukey HSD post hoc tests revealing significantly fewer reinforcers earned in the 5.0mg/kg/day dose group than in controls. A significant effect of ACR on response rate was also observed, with the Tukey HSD post hoc tests revealing a significantly lower response rate in the 5.0mg/kg/day group than in controls. No effects of ACR were observed on post-reinforcement pause. These data suggest that daily ACR exposure at 5.0mg/kg/day can produce measurable decrements on aspects of food-motivated behavior.

Proteomic analysis of rat striatal synaptosomes during acrylamide intoxication at a low dose rate

We have hypothesized that acrylamide (ACR) intoxication causes cumulative nerve terminal damage by forming adducts with nucleophilic cysteine sulfhydryl groups on critical presynaptic proteins. To determine the cumulative effects of ACR on the cysteine-containing proteome of nerve terminal, we employed cleavable isotope-coded affinity tagging (ICAT) and liquid chromatography-tandem mass spectrometry. ICAT analysis uses a sulfhydryl-specific tag to identify and quantitate cysteine-containing proteins. Synaptosomes were prepared from striatum of ACR-intoxicated rats (21 mg/kg/day x 7, 14, or 21 days) and their age-matched controls. The synaptosomal proteins of each experimental group were labeled with either light (12C9--control) or heavy (13C9--ACR) ICAT reagent. Results show that ACR intoxication caused a progressive reduction in the ICAT labeling of many nerve terminal proteins. A label-free mass spectrometric approach (multidimensional protein identification) was used to show that the observed reductions in ICAT incorporation were not due to general changes in protein abundance and that ACR formed adducts with cysteine residues on peptides which also exhibited reduced ICAT incorporation. The decrease in labeling was temporally correlated to the development of neurological toxicity and confirmed previous findings that cysteine adducts of ACR accumulate as a function of exposure. The accumulation of adduct is consistent with the cumulative neurotoxicity induced by ACR and suggests that cysteine adduct formation is a necessary neuropathogenic step. Furthermore, our analyses identified specific proteins (e.g., v-ATPase, dopamine transporter, N-ethylmaleimide-sensitive factor) that were progressively and significantly adducted by ACR and might, therefore, be neurotoxicologically relevant targets.

Structure-Toxicity Analysis of Type-2 Alkenes: In Vitro Neurotoxicity

Acrylamide (ACR) is a conjugated type-2 alkene that produces synaptic toxicity presumably by sulfhydryl adduction. The alpha,beta-unsaturated carbonyl of ACR is a soft electrophile and, therefore, adduction of nucleophilic thiol groups could occur through a conjugate (Michael) addition reaction. To address the mechanism of thiol adduct formation and corresponding neurotoxicological importance, we defined structure-toxicity relationships among a series of conjugated type-2 alkenes (1 microM-10mM), which included acrolein and methylvinyl ketone. Results show that exposure of rat striatal synaptosomes to these chemicals produced parallel, concentration-dependent neurotoxic effects that were correlated to loss of free sulfhydryl groups. Although differences in relative potency were evident, all conjugated analogs tested were equiefficacious with respect to maximal neurotoxicity achieved. In contrast, nonconjugated alkene or aldehyde congeners did not cause synaptosomal dysfunction or sulfhydryl loss. Acrolein and other alpha,beta-unsaturated carbonyls are bifunctional (electrophilic reactivity at the C-1 and C-3 positions) and could produce in vitro neurotoxicity by forming protein cross-links rather than thiol monoadducts. Immunoblot analysis detected slower migrating, presumably derivatized, synaptosomal proteins only at very high acrolein concentrations (>or= 25 mM). Exposure of synaptosomes to high concentrations of ACR (1M), N-ethylmaleimide (10mM), and methyl vinyl ketone (MVK) (100mM) did not alter the gel migration of synaptosomal proteins. Furthermore, hydralazine (1mM), which blocks the formation of protein cross-links, did not affect in vitro acrolein neurotoxicity. Thus, type-2-conjugated alkenes produced synaptosomal toxicity that was linked to a loss of thiol content. This is consistent with our hypothesis that the mechanism of ACR neurotoxicity involves formation of Michael adducts with protein sulfhydryl groups.