2,5-Hexanedione and acrylamide produce reorganization of motoneuron perikarya

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

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.

Expression of Bcl-2, Bax and Caspase-3 in nerve tissues of rats chronically exposed to 2,5-hexanedione

Occupational exposure and experimental intoxication with n-hexane or its metabolite 2,5-hexanedione (HD) produce a central-peripheral neuropathy. However, the mechanism remains unknown. We hypothesized that HD affected the expression of Bcl-2, Bax and Caspase-3 in the central nervous system (CNS) and the peripheral nervous system (PNS). Male adult Wistar rats were administered by intraperitoneal injection at a dosage of 200 or 400 mg/kg HD, five days per week for 8 weeks. Samples of the cerebral cortex, cerebellum, spinal cord and sciatic nerves were collected and examined for Bcl-2, Bax and Caspase-3 expression using Western blotting. Subchronic exposure to HD resulted in significantly increased expression of both anti-apoptotic protein Bcl-2 and pro-apoptotic protein Bax and Caspase-3 in cerebral cortex and cerebellum, which exhibited a dose-dependent pattern. Though little change was detected in spinal cord, our results showed that the expression of Bcl-2, Bax and Caspase-3 was markedly enhanced in the sciatic nerves. These findings suggested that the changes of apoptosis-related protein level in rat nerve tissues were associated with the intoxication of HD, which might be involved in early molecular regulatory mechanism of apoptosis in the HD-induced neuropathy.

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.

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.

Fast axonal transport: a site of acrylamide neurotoxicity?

The cellular and molecular site and mode of action of acrylamide (ACR) leading to neurotoxicity has been investigated for four decades, without resolution. Although fast axonal transport compromise has been the central theme for several hypotheses, the results of many studies appear contradictory. Our analysis of the literature suggests that differing experimental designs and parameters of measurement are responsible for these discrepancies. Further investigation has demonstrated consistent inhibition of the quantity of bi-directional fast transport following single ACR exposures. Repeated compromise in fast anterograde transport occurs with each exposure. Modification of neurofilaments, microtubules, energy-generating metabolic enzymes and motor proteins are evaluated as potential sites of action causing the changes in fast transport. Supportive and contradictory data to the hypothesis that deficient delivery of fast-transported proteins to the axon causes, or contributes to, neurotoxicity are critically summarized. A hypothesis of ACR action is presented as a framework for future investigations.

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.

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.

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.

Phosphorylated neurofilament antigen redistribution in intercostal nerve subsequent to retrograde axonal transport of diphtheria toxin

A novel enzyme-linked immunosorbent assay technique using specific monoclonal antibodies has been used to examine the proximal-distal distribution of phosphorylated neurofilament proteins (pNF) in normal feline intercostal nerve, and to compare it with that following retrograde axonal transport of the ADP-ribosylating protein diphtheria toxin (DTX) to thoracic motoneurones. The molecular target of DTX is elongation factor 2 which resides solely in the cell body. Normal intercostal nerves exhibited significantly higher amounts of the 200-kDa pNF-H, 160-kDA pNF-M, and 68-kDA pNF-L in proximal nerve than in the distal nerve. The overall content of all three triplet pNF proteins decreased 3 days after injection of DTX, but the normal proximal-distal gradient was retained. By 8 days post DTX injection, the proximal-distal gradient had reversed, with proximal nerve starved of pNF-H and pNF-M and distal nerve showing abnormally high pNF-L content. Correlative immunocytochemistry of spinal cords from normal animals verified that pNF-H and pNF-M are confined to efferent axons in the spinal grey matter, and that motoneurones are only reactive for pNF-L. At 8 days following toxin treatment, motoneurones in the ipsilateral ventral horn were strongly immunoreactive for all pNF. Contralateral motoneurones were non-reactive. Onset of abnormal perikaryal pNF immunoreactivity at 3 days precedes onset of ultrastructural cytopathology. Together these results indicate an early deficit in transference to the axon of NF proteins synthesised prior to full toxicity, probably because of a toxin-induced failure in regulation of phosphorylation-dependent NF assembly and turnover immediately prior to entry into the proximal axon. Results are discussed in relation to diphtheritic motoneuronopathy.

Acrylamide alters neurofilament protein gene expression in rat brain

Acrylamide, a prototype neurotoxin, alters neurofilament protein (NF) gene expression in rat brain. Levels of mRNA coding for neurofilament protein subunits NF-L, NF-M, and NF-H have been determined by Northern blot analysis using 32P-labeled cDNA probes. Acrylamide given acutely (100 mg/kg, single intraperitoneal injection) causes a selective increase in NF-M mRNA (approximately 50%) compared to controls. The expression of NF-L or NF-H mRNA is not affected by acrylamide. In contrast, chronic treatment with acrylamide [0.03% (w/v) in drinking water for 4 weeks] induces a modest but significant increase (approximately 22%) in NF-L mRNA compared to controls. Levels of NF-M, and NF-H mRNA are not altered by acrylamide treatment. The expression of beta-actin mRNA, an ubiquitous protein, is not affected by either treatment regimen of acrylamide. The results of this study show that acrylamide increases the expression of mRNA for NF protein subunits in rat brain. The increase of specific mRNA for NF subunits depends on the dose, duration and route of acrylamide administration.

Neurofilament reorganisation and neurofilament antigen redistribution in spinal motoneurones following retrograde axonal transport of diphtheria toxin

Single unilateral injections of diphtheria toxin (DTX) into the external anal sphincter muscle or internal intercostal nerve of cat induced characteristic ultrastructural lesions in corresponding ipsilateral spinal motoneurones 6-8 days later. The chief neuronal lesion was a progressive disruption of Nissl body composition and organisation, which between days 8-19 post injection was accompanied by a progressive accumulation of neurofilaments in motoneuronal perikarya and dendrites. Some axons in the ipsilateral ventral horn became hypertrophied due to neurofilamentous accumulation. Related immunocytochemical investigations 6-35 days after injection of DTX revealed abnormal immunoreactivity intoxicated motoneurones for 200-kDa and 160-kDa phosphorylated neurofilament proteins, but not in contralateral motoneurones. By day 35 abnormal neurofilament immunostaining also occurred in ipsilateral and some contralateral interneurones but not contralateral motoneurones. Abnormalities of Nissl body endoplasmic reticulum, neurofilament organisation, and neurofilament protein immunostaining were identical after either intraneural and intramuscular injections of DTX, indicating abnormalities were attributable to toxicity and not injection-related axonal damage. Since DTX acts specifically in the soma to inhibit protein synthesis, neurofilament abnormalities are secondary to cytotoxicity and probably result from deficits in transference of existing partially phosphorylated neurofilaments to the axonal transport system, or axonal transport per se.

Total number and mean cell volume of neocortical neurons in rats exposed to 2,5-hexanedione with and without acetone

The toxicological effects of 2,5-hexanedione (2,5-HD) alone and combined with acetone on the number and size of neurons in the cerebral cortex of rats were evaluated with stereological techniques. Thirty rats were equally divided into three groups: One control, one receiving 0.5% 2,5-HD, and one receiving 0.5% 2,5-HD and 0.5% acetone in the drinking water for seven weeks. Unbiased estimates of the total number of neocortical neurons, as well as the mean neuronal nuclei and cell body volumes were obtained from systematically sampled 3.5-microns sections. The total number of neurons in the 2,5-HD group was significantly smaller than the control group (p less than 0.05, one-tailed t-test). Both test groups showed significant changes in the mean cell body volume: Compared with the control group, animals exposed to 2,5-HD had 11% smaller cell body volumes while animals exposed to 2,5-HD and acetone had 13% larger cell body volumes. These data represent the first unbiased estimation of mean cell volume in toxicology. We propose the nucleator method as an efficient and accurate tool for estimating quantitative changes in toxicological research.

A Method Based on the Roller Chamber Technique for Determination of CO2 Production in Cultured Cells: Effects of Acrylamide in Neuroblastoma N1E115 Cultures

The effects of acrylamide on CO2 production from [14C]-labelled glucose, pyruvate and glutamine have been studied in a mouse neuroblastoma cell line C1300, clone N1E115. A rotation metabolism chamber, permitting closed incubation of monolayer, anchorage-dependent cell cultures under good physiological conditions, was developed for making the determinations. The cells were exposed to acrylamide (0.35mM) for 14 days. The total amunt of CO2 produced from glucose and pyruvate was increased by exposure to acrylamide, whereas a slight inhibition was found in the production from glutamine. The production of lactate remained unchanged. Comparison of these results with other data obtained in our laboratory leads us to conclude that the method described is relevant for the determination of energy metabolic processes through the quantification of CO2 production. Furthermore, we assume that acrylamide causes an increased demand for energy in the cells and that this demand is met by the cells through the increased oxidative phosphorylation of glucose.

Graded inhibition of retrograde axonal transport by compression of rabbit vagus nerve

Effects of experimental compression at different pressures on retrograde axonal transport were studied in rabbit vagus nerve. Proteins in the sensory neurones were radiolabelled by injection of [3H]leucine into the nodose ganglion. Sixteen hours after labelling, a small compression chamber and/or ligatures were applied around the cervical part of the vagus nerve for 8 h. Compression of the vagus nerve at 20, 30 and 200 mm Hg pressure induced a graded inhibition of both retrograde and anterograde transport of the radiolabelled proteins. Neither retrograde nor anterograde transport was affected by the presence of the non-inflated chamber. The results indicate that compression at pressures similar to those found in human carpal tunnel syndrome can block retrograde axonal transport. The consequences of inhibition of retrograde and anterograde axonal transport for the metabolism in the nerve cell bodies are discussed.