gamma-diketone peripheral neuropathy. I. Quality morphometric analyses of axonal atrophy and swelling

2,5-Hexanedione induced apoptosis in rat spinal cord neurons and VSC4.1 cells via the proNGF/p75NTR and JNK pathways

Increasing evidence suggests that n-hexane induces nerve injury via neuronal apoptosis induced by its active metabolite 2,5-hexanedione (HD). However, the underlying mechanism remains unknown. Studies have confirmed that pro-nerve growth factor (proNGF), a precursor of mature nerve growth factor (mNGF), might activate apoptotic signaling by binding to p75 neurotrophin receptor (p75NTR) in neurons. Therefore, we studied the mechanism of the proNGF/p75NTR pathway in HD-induced neuronal apoptosis. Sprague-Dawley (SD) rats were injected with 400 mg/kg HD once a day for 5 weeks, and VSC4.1 cells were treated with 10, 20, and 40 mM HD in vitro. Results showed that HD effectively induced neuronal apoptosis. Moreover, it up-regulated proNGF and p75NTR levels, activated c-Jun N-terminal kinase (JNK) and c-Jun, and disrupted the balance between B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax). Our findings revealed that the proNGF/p75NTR signaling pathway was involved in HD-induced neuronal apoptosis; it can serve as a theoretical basis for further exploration of the neurotoxic mechanisms of HD.

Neuroprotein Targets of γ-Diketone Metabolites of Aliphatic and Aromatic Solvents That Induce Central-Peripheral Axonopathy

Peripheral neuropathy associated with chronic occupational and deliberate overexposure to neurotoxic organic solvents results from axonal degeneration in the central and peripheral nervous system. Human and experimental studies show that axonopathy is triggered by the action of neuroprotein-reactive γ-diketone metabolites formed from exposure to certain aliphatic solvents (n-hexane, 2-hexanone) and aromatic compounds (1,2-diethylbenzene, 1,2-4-triethylbenzene, 6-acetyl-1,1,4,4-tetramethyl-7-ethyl-1,2,3,4-tetralin). Neuroprotein susceptibility is related primarily to their differential content of lysine, the ∊-amino group of which is targeted by γ-diketones. Specific neuroprotein targets have been identified, and the sequence of molecular mechanisms leading to axonal pathology has been illuminated. While occupational n-hexane neuropathy continues to be reported, lessons learned from its experimental study may have relevance to other causes of peripheral neuropathy, including those associated with aging and diabetes mellitus.

Diallyl sulfide-induced attenuation of n-hexane-induced peripheral nerve impairment is associated with metabolic inhibition of n-hexane

Chronic exposure to n-hexane could induced serious peripheral nerve impairments. It has been well documented that the metabolic activation from n-hexane to 2,5-hexanedione (2,5-HD) is vital in the pathogenesis. Diallyl sulfide (DAS) is an extract of garlic and able to block the bioactivation of xenobiotic. The current study was designed to investigate whether DAS can attenuate n-hexane induced neuropathy. Male Wistar rats were pretreated with DAS (50 or 100 mg/kg.bw) and then n-hexane (3 g/kg.bw) for 7 weeks. Behavioral performance, biomarker measurement and toxicokinetic studies were performed. Enzymatic methods and western blotting analyses were also conducted to investigate the hepatic phase I enzymes (including cytochrome P450(CYP)2E1, CYP1A1 and CYP2B1) and phase II enzymes (including glutathione S transferase theta 1 (GSTT1) and NA(D)PH dehydrogenase quinone 1 (NQO1)). The results showed that DAS improved the behavioral performance while reducing the toxic metabolite: 2,5-HD and pyrrole adducts. Besides, DAS reduced the expression of CYP2E1 with a proportional decrease in activity, which largely decreased the bioactivation of n-hexane in vivo. The results suggested that DAS decreased the toxic metabolites of n-hexane to attenuate n-hexane-induced peripheral neuropathy.

2,5-hexanedione-induced deregulation of axon-related microRNA expression in rat nerve tissues

Exposure to organic solvent in industry, including n-hexane is correlated with central-peripheral axonopathy, which is mediated by its active metabolite, 2,5-hexanedione (HD). However, the underlying mechanism is still largely unknown. Recently identified microRNAs (miRNAs) may play important roles in toxicant exposure and in the process of toxicant-induced neuropathys. To examine the role of miRNAs in HD-induced toxicity, neuropathic animal model was successfully built. miRNA microarray analysis revealed 105 differentially expressed miRNAs after HD exposure. Bioinformatics analysis showed that "Axon" and "Neurotrophin Signaling Pathway" was the top significant GO term and pathway, respectively. 7 miRNAs both related to "Axon" and "Neurotrophin Signaling Pathway" were screened out and further confirmed by Real-Time PCR. Correspondingly, the deregulation expression levels of proteins of four target genes (GSK3β, Map3k1, BDNF and MAP1B) were further confirmed via western blot, verifying the results of gene target analysis. Taken together, our results showed that the axon-related miRNAs to be associated with MAP1B or neurotrophin signal pathways changed in nerve tissues following HD exposure. These miRNAs may play important roles in HD-induced neurotoxicity.

2,5-Hexanedione mediates neuronal apoptosis through suppression of NGF via PI3K/Akt signaling in the rat sciatic nerve

Because precise mechanism for 2,5-hexanedione (HD)-induced neuronal apoptosis largely remains unknown, we explored the potential mechanisms both in vivo and in vitro Rats were intraperitoneally exposed to HD at different doses for 5 weeks, following which the expression levels of nerve growth factor (NGF), phosphorylation of Akt and Bad, dimerization of Bad and Bcl-xL, as well as the release of cytochrome c and the caspase-3 activity were measured. Moreover, these variables were also examined in vitro in HD-exposed VSC4.1 cells with or without a PI3K-specific agonist (IGF-1), and in HD-exposed VSC4.1 cells with or without a PI3K-specific inhibitor (LY294002) in the presence or absence of NGF. The data indicate that, as the concentration of HD increased, rats exhibited progressive gait abnormalities, and enhanced neuronal apoptosis in the rat sciatic nerve, compared with the results observed in the control group. Furthermore, HD significantly down-regulated NGF expression in the rat sciatic nerve. Moreover, suppression of NGF expression inhibited the phosphorylation of Akt and Bad. Meanwhile, an increase in the dimerization of Bad and Bcl-xL in mitochondria resulted in cytochrome c release and caspase-3 activation. In contrast, HD-induced apoptosis was eliminated by IGF-1. Additionally, NGF supplementation reversed the decrease in phosphorylation of Akt and Bad, as well as reversing the neuronal apoptosis in HD-exposed VSC4.1 cells. However, LY294002 blocked these effects of NGF. Collectively, our results demonstrate that mitochondrial-dependent apoptosis is induced by HD through NGF suppression via the PI3K/Akt pathway both in vivo and in vitro.

Hair pyrrole adducts serve as biomarkers for peripheral nerve impairment induced by 2,5-hexanedione and n-hexane in rats

Pyrrole adducts are specific reaction products of 2,5-hexadione (2,5-HD) in vivo and are considered highly relevant to the pathogenesis of peripheral nerve impairments after exposure to n-hexane, though the exact mechanism remains unclear. In this study, 40 male Wistar rats were randomly divided into three experimental groups and one control group, in which all rat’s hair were shaved completely at the beginning. The rats in three experimental groups were treated with 2,5-HD by gavage at dosages of 100, 200 and 300 mg/kg per day respectively, six times per week for 8 weeks. Abnormality of gait and changes in the rota-rod latency were surveilled. Pyrrole adducts in hair, urine and serum of all rats were measured at the endpoint. Results showed that the increased pyrrole adducts in hair, urine and serum accumulated in dose-response relationship. Spearman’s correlation analysis between pyrrole adducts and gait scores showed that hair pyrrole adducts were highly relevant to the gait scores. Moreover, we treated rats with n-hexane and succeed to verify the results aforesaid. Further, multiply linear regression analysis showed that hair pyrrole adducts have higher partial correlation coefficients than these in serum and urine in both 2,5-HD and n-hexane treated models. Our findings draw the conclusion that the hair pyrrole adducts might serve as a promising biomarker of n-hexane induced peripheral neuropathy.

2,5-hexanedione downregulates nerve growth factor and induces neuron apoptosis in the spinal cord of rats via inhibition of the PI3K/Akt signaling pathway

2,5-hexanedione (2,5-HD) is the main active metabolite of n-hexane and induces apoptosis in nerve tissue, however, the mechanism of which remains unclear. In the present study, neuropathic animal models were successfully constructed in rats by injecting 100, 200 and 400 mg/kg 2,5-HD intraperitoneally for 5 weeks. Rats exposed to 2,5-HD exhibited progressive gait abnormalities and slower motor neural response in a dose-dependent manner. TUNEL analysis and immunofluorescence dual labeling revealed that the spinal cord of the 2,5-HD treated rats underwent significantly more apoptosis in the cells of spinal cord than that of the control group. The neuron apoptosis index in spinal cord was 4.1%, 6.7%, 9.8% respectively in rats exposed to 100, 200 and 400 mg/kg 2,5-HD, compared with 1.1% in the control group (p < 0.05). Biochemical analysis showed that 2,5-HD exposure downregulated NGF expression in the spinal cord of the intoxicated rats; inhibited the phosphorylation of Akt and Bad, two key players in PI3K/Akt pathway downstream of NGF; increased the dimerization of Bad with Bcl-xL in the mitochondrial fraction, followed by the release of cytochrome c and activation of caspase-3 in the spinal cord of rats. In vitro study showed that the NGF expression decreased significantly in VSC4.1 cells dosed with 5.0, 10.0 mM 2,5-HD in comparison with the control group. It was also found that NGF supplement repressed the induced apoptosis, and increased p-Akt and p-Bad level in 2,5-HD treated VSC4.1 cells, which could be antagonized by PI3K kinase (the upstream member of Akt) inhibitor LY294002. Taken together, our experimental results indicate that 2,5-HD may induce apoptosis in the spinal cord of rats via downregulating NGF expression and subsequently repressing PI3K/Akt signaling pathway.

Cystamine attenuated behavioral deficiency via increasing the expression of BDNF and activating PI3K/Akt signaling in 2,5-hexanedione intoxicated rats

Organic solvent-induced neurodegeneration is a severe public health problem which has no effective prevention measures yet. Cystamine stands as a promising neuroprotective agent against many degenerative diseases. In the present study, we investigated the possible protective effects of cystamine against 2,5-hexanedione (2,5-HD) induced peripheral neuropathy. Chronic exposure to 2,5-HD (300 mg kg^-1, 6 times per week for 6 weeks) resulted in obvious peripheral nerve damage shown as the elevation of gait scores and the increase of latency in an accelerating rota-rod test. Cystamine (30 mg kg^-1 and 60 mg kg^-1) co-treatment obviously ameliorated 2,5-HD-induced impairments of the peripheral nervous system. To decipher the underlying mechanisms, we investigated the effects of cystamine on the regulation of brain-derived neurotrophic factor (BDNF) and heat shock protein-70 (Hsp70) expression and the PI3K/Akt signaling pathway. The results revealed that cystamine up-regulated the protein levels of BDNF and Hsp70, accompanied by the activation of the PI3K/Akt pathway in the spinal cord, which might account for the protection of cystamine against 2,5-HD-induced neuropathy.

Bone marrow mesenchymal stem cells attenuate 2,5-hexanedione-induced neuronal apoptosis through a NGF/AKT-dependent pathway

Growing evidence suggests that the increased neuronal apoptosis is involved in n-hexane-induced neuropathy. We have recently reported that bone marrow-mesenchymal stem cells-derived conditioned medium (BMSC-CM) attenuated 2,5-hexanedione (HD, the active metabolite of n-hexane)-induced apoptosis in PC12 cells. Here, we explored the anti-apoptotic efficacy of BMSC in vivo. HD-treated rats received BMSC by tail vein injection 5 weeks after HD intoxication. We found that in grafted rats, BMSC significantly attenuated HD-induced neuronal apoptosis in the spinal cord, which was associated with elevation of nerve growth factor (NGF). Neutralization of NGF in BMSC-CM blocked the protection against HD-induced apoptosis in VSC4.1 cells, suggesting that NGF is essential for BMSC-afforded anti-apoptosis. Mechanistically, we found that the decreased activation of Akt induced by HD was significantly recovered in the spinal cord by BMSC and in VSC4.1 cells by BMSC-CM in a TrkA-dependent manner, leading to dissociation of Bad/Bcl-xL complex in mitochondria and release of anti-apoptotic Bcl-xL. The importance of Akt was further corroborated by showing the reduced anti-apoptotic potency of BMSC in HD-intoxicated VSC4.1 cells in the presence of Akt inhibitor, MK-2206. Thus, our findings show that BMSC attenuated HD-induced neuronal apoptosis in vivo through a NGF/Akt-dependent manner, providing a novel solution against n-hexane-induced neurotoxicity.

Selective regional alterations in the content or distribution of neuronal and glial cytoskeletal proteins in brain of rats chronically exposed to 2,5-hexanedione

Hexane is used in many industrial processes and induces neurotoxic effects in the central and peripheral nervous system. Hexane is metabolized to 2,5-hexanedione, which is the neurotoxic agent. Continued exposure to hexane or 2,5-hexanedione results in loss of sensorial and motor function in arms and legs and to alterations in axonal neurofilament proteins. The effects of 2,5-hexanedione on different cytoskeletal proteins in different brain areas have not been studied in detail. The aim of this work was to study the effects of chronic exposure of rats to 2,5-hexanedione (1% in the drinking water) on tubulin, neurofilament NF-L, microtubule-associated protein MAP-2, and on glial fibrillary acidic protein (GFAP), in cerebellum, hippocampus and cerebral cortex. The amount of each protein was determined by immunoblotting and its distribution was analysed by immunohistochemistry. The results obtained show a regional selectivity in the 2,5-hexanedione effects on cytoskeletal proteins. NF-L content decreased in all brain areas. MAP-2 decreased in cerebellum and hippocampus and tubulin decreased only in cerebellum. GFAP decreased only in cerebral cortex, but its distribution was altered in cerebellum, with increased content in the granular layer and decreased content in the molecular layer. The area most affected was the cerebellum, where all the proteins analysed were altered. These cytoskeletal proteins alterations may impair the transfer of information involved in the regulation by the cerebellum of motor function and contribute to the altered motor performance in rats exposed to 2,5-hexanedione and humans exposed to hexane.

Antinociceptive effects of gentiopicroside on neuropathic pain induced by chronic constriction injury in mice: a behavioral and electrophysiological study

Gentiopicroside (Gent) is promising as an important protective secoiridoid compound against pain. The present study was designed to investigate whether administration of Gent would alleviate the expression of nociceptive behaviors and whether it would cause the relevant electrophysiological changes in a chronic constriction injury (CCI) model of neuropathic pain in mice. Gent was administered from the seventh day after surgery for 8 consecutive days. Behavioral parameters and sciatic functional index were assessed immediately before surgery and on days 7, 8, 10, 12, and 14 post-CCI, and electrophysiological activities of sciatic nerve were recorded immediately after the behavioral test on the last day. The present study has shown that administration of Gent (at a dose of 50 and 100 mg/kg) increased behavioral parameters from day 8 compared with the CCI-NS group. Electrophysiological data indicated that CCI caused a significant reduction in nerve conduction velocities in the sciatic nerves and the amplitudes of compound action potential, while Gent at a dose of 50 or 100 mg/kg caused a significant recovery of electrophysiological changes induced by CCI. Our data indicated that Gent has antinociceptive effects on neuropathic pain induced by CCI.

Exposure to 2,5-hexanedione is accompanied by ovarian and uterine oxidative stress and disruption of endocrine balance in rats

2,5-Hexanedione (2,5-HD) is an aliphatic diketone identified as the main neurotoxic metabolite of the industrial chemicals n-hexane and methyl-n-butyl ketone. Considering the dearth of information on the female reproductive toxicity effects of 2,5-HD in the literature, we assessed the potential oxidative stress mechanisms of 2,5-HD in the ovary and uterus of Wistar rats. A total of 32 female rats were randomly allotted to four groups, in which rats were exposed to 2,5-HD at doses of 0% (control), 0.25%, 0.5% and 1.0% respectively in their drinking water for 21 days. The results showed that 2,5-HD significantly increased ovarian and uterine malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels (p < 0.05). Additionally, while significant decreases in ovarian catalase, superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione-S-transferase (GST) activities occurred in all the 2,5-HD-treated groups, uterine catalase, GST, and GPx activities increased. Further, 2,5-HD increased follicle stimulating hormone, but decreased estrogen levels in all the 2,5-HD-treated groups, while prolactin increased in the 0.5, and 1.0% 2,5-HD-treated rats compared with the control (p < 0.05). Thus, these data imply that 2,5-HD exposure disrupts hormonal homeostasis and induces oxidative stress in the ovary and uterus of rats. These findings may therefore have toxicological implications in women occupationally exposed to n-hexane and methyl-n-butyl ketone.

Toxic neuropathies: Mechanistic insights based on a chemical perspective

2,5-Hexanedione (HD) and acrylamide (ACR) are considered to be prototypical among chemical toxicants that cause central-peripheral axonopathies characterized by distal axon swelling and degeneration. Because the demise of distal regions was assumed to be causally related to the onset of neurotoxicity, substantial effort was devoted to deciphering the respective mechanisms. Continued research, however, revealed that expression of the presumed hallmark morphological features was dependent upon the daily rate of toxicant exposure. Indeed, many studies reported that the corresponding axonopathic changes were late developing effects that occurred independent of behavioral and/or functional neurotoxicity. This suggested that the toxic axonopathy classification might be based on epiphenomena related to dose-rate. Therefore, the goal of this mini-review is to discuss how quantitative morphometric analyses and the establishment of dose-dependent relationships helped distinguish primary, mechanistically relevant toxicant effects from non-specific consequences. Perhaps more importantly, we will discuss how knowledge of neurotoxicant chemical nature can guide molecular-level research toward a better, more rational understanding of mechanism. Our discussion will focus on HD, the neurotoxic γ-diketone metabolite of the industrial solvents n-hexane and methyl-n-butyl ketone. Early investigations suggested that HD caused giant neurofilamentous axonal swellings and eventual degeneration in CNS and PNS. However, as our review will point out, this interpretation underwent several iterations as the understanding of γ-diketone chemistry improved and more quantitative experimental approaches were implemented. The chemical concepts and design strategies discussed in this mini-review are broadly applicable to the mechanistic studies of other chemicals (e.g., n-propyl bromine, methyl methacrylate) that cause toxic neuropathies.

Acrylamide exposure impairs blood-cerebrospinal fluid barrier function

Previous studies show that chronic acrylamide exposure leads to central and peripheral neu-ropathy. However, the underlying mechanisms remained unclear. In this study, we examined the permeability of the blood-cerebrospinal fluid barrier, and its ability to secrete transthyretin and transport leptin of rats exposed to acrylamide for 7, 14, 21 or 28 days. Transthyretin levels in cerebrospinal fluid began to decline on day 7 after acrylamide exposure. The sodium fluorescein level in cerebrospinal fluid was increased on day 14 after exposure. Evans blue concentration in cerebrospinal fluid was increased and the cerebrospinal fluid/serum leptin ratio was decreased on days 21 and 28 after exposure. In comparison, the cerebrospinal fluid/serum albumin ratio was increased on day 28 after exposure. Our findings show that acrylamide exposure damages the blood-cerebrospinal fluid barrier and impairs secretory and transport functions. These changes may underlie acrylamide-induced neurotoxicity.

Decelerated transport and its mechanism of 2,5-hexanedione on middle-molecular-weight neurofilament in rat dorsal root ganglia cells

Chronic exposure to n-hexane induces peripheral-central axonopathy, mediated by its metabolite 2,5-hexanedione (2,5-HD), in occupational workers and experimental animals, but the underlying mechanism is still unclear. In the current study, we investigated the effects of 2,5-HD on middle-molecular-weight neurofilament (NF-M) axonal transport using live-cell imaging technique in cultured rat dorsal root ganglia (DRG) cells. PA-GFP-NF-M plasmid was transfected into DRG neurons and live-cell imaging was performed to observe the slow axonal transport of NF-M. The levels of cytoskeleton and motor proteins in DRG cells were detected by Western-blot and the concentration of ATP was determined using an ATP Assay Kit. The results showed that 2,5-HD administration resulted in a decrease of NF-M axonal transport and a reduction of three neurofilament subunits levels in DRG cells. Furthermore, 2,5-HD exposure significantly decreased ATP contents and the protein levels of kinesin heavy chain (KHC). These findings indicated that 2,5-HD reduced slow axonal transport, neurofilaments cargoes, motor proteins and ATP energy in rat DRG cells, which may contribute to 2,5-HD-induced neurotoxicity.

Biological exposure indices of pyrrole adducts in serum and urine for hazard assessment of n-hexane exposure

BACKGROUND

Pyrrole adducts might be used as a biomarker for monitoring occupational exposure to n-hexane, but the Biological Exposure Indices of pyrrole adducts in serum and urine are still unknown. The current study was designed to investigate the biological exposure limit of pyrrole adducts for hazard assessment of n-hexane.

METHODS

Male Wistar rats were given daily dose of 500, 1000, 1500, 2000, 4000 mg/kg bw n-hexane by gavage for 24 weeks. The levels of pyrrole adducts in serum and urine were determined at 8, 24 hours postdose once a week. The Biological Exposure Indices was evaluated by neurological evaluation and the levels of pyrrole adducts. The difference in pyrrole adducts formation between humans and rats were estimated by using in vitro test.

RESULTS

Dose-dependent effects were observed between the doses of n-hexane and pyrrole adducts in serum and urine, and the levels of pyrrole adduct in serum and urine approached a plateau at week 4. There was a significantly negative correlation between the time to paralysis and the level of pyrrole adducts in serum and urine, while a positive correlation between gait score and levels of pyrrole adducts in serum and urine was observed. In vitro, pyrrole adducts formed in human serum was about two times more than those in rat serum at the same level of 2,5-HD.

CONCLUSION

It was concluded that the BEIs of pyrrole adducts in humans were 23.1 ± 5.91 nmol/ml in serum 8 h postdose, 11.7 ± 2.64 nmol/ml in serum 24 h postdose, 253.8 ± 36.3 nmol/ml in urine 8 h postdose and 54.6 ± 15.42 nmol/ml in urine 24 h postdose.

2,5-hexanedione altered the degradation of low-molecular-weight neurofilament in rat nerve tissues

Occupational exposure to n-hexane produces a central-peripheral distal axonopathy, which is characterized by giant axonal swellings filled with neurofilaments (NFs). To investigate the change of NFs degradation and their possible role in n-hexane neuropathy, adult male Wistar rats were administered intraperitoneally at a dosage of 400 mg/kg/day 2,5-hexanedione (2,5-HD) for 4 weeks. The time course of low-molecular-weight neurofilament (NF-L) degradation and autophagy-related protein in rat sciatic nerves and spinal cords was determined by Western blotting. The results demonstrated that the administration of 2,5-HD inhibited NF-L degradation to an undetectable level in sciatic nerves. Furthermore, a significant reduction of NF-L degradation in spinal cords was observed in the early stage of 2,5-HD exposure. In the meantime, 2,5-HD significantly decreased the level of Beclin-1, a key autophagy-regulated protein in sciatic nerves of rats while increased the level of P62, a selective substrate of autophagy degrading pathway, which indicated a dysfunctional autophagy in rat nerve tissues. Collectively, our findings suggested that the inhibition of autophagy by 2,5-HD might be responsible for the reduction of NF-L degradation in rat sciatic nerves, and involved in the pathogenesis of 2,5-HD-induced axonopathy.

Butenenitriles have low axonopathic potential in the rat

IDPN (3,3'-iminodipropionitrile) causes a neurofilamentous proximal axonopathy. This study addressed the hypothesis that the butenenitriles (allylnitrile, cis-crotononitrile and trans-crotononitrile) have an IDPN-like axonopathic potential. First, male adult rats were exposed (i.p.) to IDPN, allylnitrile, cis-crotononitrile or trans-crotononitrile at 3.25 mmol/kg/day, 0.89 mmol/kg/day, 1.79 mmol/kg/day, or 3.75 mmol/kg/day for 3 consecutive days, respectively; lumbar dorsal root ganglia were examined for axonal swelling eight days after dosing. IDPN caused axonal swelling, a few swollen axons were recorded in one trans-crotononitrile animal, and no axonal abnormalities were observed following cis-crotononitrile or allylnitrile. To further evaluate trans-crotononitrile, additional rats were given this nitrile through a 10-day i.p. dosing schedule (2.5 mmol/kg/day, 2.75 mmol/kg/day, 3.0 mmol/kg/day or 3.25 mmol/kg/day) or a 9-week drinking water exposure (12.3, 24.6 and 49.1mM, three weeks each), and examined by light and electron microscopy. Semithin sections revealed no overt swelling in axons from several locations of the nervous system after trans-crotononitrile; quantitative analysis in the L5 dorsal root ganglion showed no increase in proximal axon diameter in comparison to control animals. At the transmission electron microscopy level, pathological effects were mild; they were mostly found in the animals submitted to the 10-day dosing regimen, and did not include evidence of significant axonal swelling. Although an axonopathic potential for the three unsaturated 4-carbon nitriles cannot be excluded, the present data indicated that this potential is significantly lower than that of IDPN.

Gamma-diketone axonopathy: analyses of cytoskeletal motors and highways in CNS myelinated axons

2,5-Hexanedione (HD) intoxication is associated with axon atrophy that might be responsible for the characteristic gait abnormalities, hindlimb skeletal muscle weakness and other neurological deficits that accompany neurotoxicity. Although previous mechanistic research focused on neurofilament triplet proteins (NFL, NFM, NFH), other cytoskeletal targets are possible. Therefore, to identify potential non-NF protein targets, we characterized the effects of HD on protein-protein interactions in cosedimentation assays using microtubules and NFs prepared from spinal cord of rats intoxicated at different daily dose rates (175 and 400 mg/kg/day). Results indicate that HD did not alter the presence of alpha- or beta-tubulins in these preparations, nor were changes noted in the distribution of either anterograde (KIF1A, KIF3, KIF5) or retrograde (dynein) molecular motors. The cosedimentation of dynactin, a dynein-associated protein, also was not affected. Immunoblot analysis of microtubule-associated proteins (MAPs) in microtubule preparations revealed substantial reductions (45-80%) in MAP1A, MAP1B heavy chain, MAP2, and tau regardless of HD dose rate. MAP1B light chain content was not altered. Finally, HD intoxication did not influence native NF protein content in either preparation. As per previous research, microtubule and NF preparations were enriched in high-molecular weight NF species. However, these NF derivatives were common to both HD and control samples, suggesting a lack of pathognomonic relevance. These data indicate that, although motor proteins were not affected, HD selectively impaired MAP-microtubule binding, presumably through adduction of lysine residues that mediate such interactions. Given their critical role in cytoskeletal physiology, MAPs could represent a relevant target for the induction of gamma-diketone axonopathy.

Interactive image analysis programs for quantifying injury-induced axonal beading and microtubule disruption

Focal axonal beading and focal disruption of microtubule structure are characteristic to traumatic axonal injury. We have recently reproduced these morphological and structural changes in our in vitro model system [D. Kilinc, G. Gallo, K.A. Barbee, Mechanically induced membrane poration causes axonal beading and localized cytoskeletal damage, Exp. Neurol. 212 (2008) 422-430]. In order to measure bead formation objectively, an observer-independent quantification of beading was necessary. In addition, a quantitative measure for the extent of co-localization of axonal beads and microtubule disruptions was required to establish a causal relationship between focal cytoskeletal damage and bead formation. In this paper we describe Matlab-based, interactive image analysis programs for axonal beading quantification and co-localization analysis. Injury-induced increases in the axonal beading could be successfully detected using the bead analysis program.

Comparative covalent protein binding of 2,5-hexanedione and 3-acetyl-2,5-hexanedione in the rat

2,5-Hexanedione (HD) is the metabolite implicated in n-hexane neurotoxicity. This gamma-diketone reacts with protein lysine amines to form 2,5-dimethylpyrrole adducts. Pyrrole adduction of neurofilaments (NF) and/or other axonal proteins was proposed as a critical step in the neuropathy. While pyrrole adduction is widely accepted as necessary, subsequent pyrrole oxidation, which may result in protein cross-linking, was alternatively postulated as the critical mechanistic step. Previous studies have indicated that 3-acetyl-2,5-HD (AcHD), an analogue that forms pyrroles that do not oxidize, was not neurotoxic in rats. However, relative levels of pyrrole adduction of NF or other axonal proteins were not reported. In the present study, groups of 6 male Wistar rats were given saline, [1,6-(14)C]-HD (3 mmol/kg/d), or [5-(14)C]-AcHD (0.1 mmol/kg/d), i.p. for 21 d. HD- and AcHD-treated rats lost 10% and gained 14% body weight, respectively, compared to a 22% gain for control rats. At termination, HD- and AcHD-treated rats exhibited mean scores of 3.5 and 1.4, respectively, for hindlimb weakness (0-5 scale). Incorporation of radiolabel from HD was 27.8 +/- 3.9, 13.9 +/- 2.6, and 7.8 +/- 0.6 nmol/mg in plasma protein, purified globin, and axonal cytoskeletal proteins, respectively, compared to 0.6 +/- 0.1, 1.6 +/- 0.5, and 1.0 +/- 0.1 for AcHD. Binding of HD to the NF-L, -M, and -H subunit proteins from treated animals was 4-, 24-, and 13-fold higher, respectively, that that of AcHD, indicating differing stoichiometry and patterns of NF adduction for the two diketones. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis of globin and NF proteins did not demonstrate protein cross-linking for either diketone at the dose levels and time period examined. These results indicate that that the lack of neurotoxicity previously reported for AcHD may reflect differences in adduct levels at critical axonal target sites rather than an inability to form cross-linking adducts. Based on these data, further studies are required to fully assess the neurotoxic potency of AcHD and other non-cross-linking analogues as compared to HD.

Changes of cytoskeletal proteins in nerve tissues and serum of rats treated with 2,5-hexanedione

To investigate the mechanisms and biomarker of the neuropathy induced by 2,5-hexanedione (HD), male Wistar rats were administrated HD at dosage of 200 or 400mg/kg for 8 weeks (five-times per week). All rats were sacrificed after 8 weeks of treatment and the cerebrum cortex (CC), spinal cord (SC) and sciatic nerves (SN) were dissected, homogenized and used for the determination of cytoskeletal proteins by western blotting. The levels of neurofilaments (NFs) subunits (NF-L, NF-M and NF-H) in nerve tissues of 200 and 400mg/kg HD rats significantly decreased in both the supernatant and pellet fractions. Furthermore, significant negative correlations between NFs levels and gait abnormality were observed. As for microtubule (MT) and microfilament (MF) proteins, the levels of alpha-tubulin, beta-tubulin and beta-actin in the supernatant and pellet fraction of SN significantly decreased in 200 and 400mg/kg HD rats and correlated negatively with gait abnormality. However, the contents of MT and MF proteins in CC and SC were inconsistently affected and had no significant correlation with gait abnormality. The levels of NF-L and NF-H in serum significantly increased, while NF-M, alpha-tubulin, beta-tubulin and beta-actin contents remain unchanged. A significant positive correlation (R=0.9427, P<0.01) was observed between gait abnormality and NF-H level in serum as the intoxication went on. These findings suggested that HD intoxication resulted in a progressive decline of cytoskeletal protein contents, which might be relevant to the mechanisms of HD-induced neuropathy. NF-H was the most sensitive index, which may serve as a good indicator for neurotoxicity of n-hexane or HD.

The reversibility of neurofilaments decline induced by 2,5-hexanedione in rat nerve tissues

To investigate the reversibility of the neuropathy induced by 2,5-HD, adult male rats were administered at a dosage of 400 mg/kg/day 2,5-HD (five times per week) for 2, 4, and 8 weeks, respectively. After stopping HD exposure, half of 8-week treated animals were allowed to naturally recover for 16 weeks. The relative levels of NF-H, NF-M, and NF-L in spinal cords and sciatic nerves of rats were determined by immunoblotting during the HD neuropathy. The results showed that NFs content in nerve tissues demonstrated a progressive decline as the intoxication continued. Furthermore, after a recovery of 16 weeks, the levels of three NF subunits in spinal cords of treated rats returned to normal while those in sciatic nerves displayed an inconsistent reversal. Among them, the level of NF-H in sciatic nerves returned to normal completely, and NF-L also showed a significant improvement, whereas NF-M did not demonstrate an obvious reversal. These findings suggest that HD-induced NFs decline is at least partially irreversible within the time frame of this study, which might be associated with the incomplete recovery of neurological dysfunctions of HD-treated rats.

Axonopathy-Inducing 1,2-Diacetylbenzene Forms Adducts with Motor and Cytoskeletal Proteins Required for Axonal Transport

The aromatic hydrocarbon 1,2-diacetylbenzene (1,2-DAB) is a protein-reactive gamma-diketone metabolite of the neurotoxic solvent 1,2-diethylbenzene (1,2-DEB). The effect of neurotoxic 1,2-DAB and its non-neurotoxic isomer 1,3-DAB has been studied on motor proteins and cytoskeletal proteins of rat spinal cord (SC). For in vitro studies, SC slices were incubated with 1, 2, 5, 10 mM of DAB isomers for 30 min at 37 degrees C. For in vivo studies, rats received (i.p.) 20 mg/kg/day of 1,2-DAB or 1,3-DAB, or vehicle (2% acetone in saline), 5 days a week for 2 weeks. Spinal cord and sciatic nerve proteins were subjected to Western blotting using monoclonal mouse antibodies to NF-M, kinesin, dynein, and tau. Proteins were quantified and paired mean comparisons performed to assess concentration-dependent changes in native protein bands. In vitro, 1,2-DAB produced a concentration-dependent decrease of motor and cytoskeletal proteins. While dynein and tau appeared similarly affected by 1,2-DAB, kinesin was most affected by the toxicant. In vivo, 1,2-DAB affected motor and cytoskeletal proteins of sciatic nerves and spinal cord differentially. In general, sciatic nerve proteins were much more affected than spinal cord proteins. The results show that motor proteins that drive axonal transport anterogradely (kinesin) and retrogradely (dynein), cytoskeletal protein NF-M, which is slowly transported in the anterograde direction, and microtubule-associated protein, tau, which is involved in axonal transport, are differentially impacted by 1,2-DAB. By contrast, non-neurotoxic isomer 1,3-diacetylbenzene (1,3-DAB), had no adverse effect on neural proteins either in vitro or in vivo. 2D-Differential in gel electrophoresis (2D-DIGE) of sciatic nerves from neurotoxic 1,2-DAB and non-neurotoxic 1,3-DAB treated rats revealed 197 and 304 protein spots, respectively.

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.

Time-dependent Alteration of Cytoskeletal Proteins in Cerebral Cortex of Rat During 2,5-Hexanedione-induced Neuropathy

To investigate the mechanisms of the axonopathy induced by 2,5-hexanedione (2,5-HD), male Wistar rats were administered at a dosage of 400 mg/kg/day 2,5-HD (five times per week). The rats produced a slightly, moderately, or severely abnormal neurological changes, respectively, after 2, 4, or 8 weeks of treatment. The cerebrums were Triton-extracted and ultracentrifuged to yield a pellet fraction and a corresponding supernatant fraction. The relative levels of six cytoskeletal proteins (NF-L, NF-M, NF-H, alpha-tubulin, beta-tubulin, and beta-actin) in both fractions were determined by immunoblotting. The results showed that NFs content in HD-treated rats demonstrated a progressive decline as the intoxication of HD continued. As for microtubule proteins, the levels of alpha-tubulin and beta-tubulin demonstrated some inconsistent changes. The content of alpha-tubulin kept unchangeable, while the content of beta-tubulin increased significantly at the late stage of HD exposure. Furthermore, the content of beta-actin in both fractions remained unaffected throughout the study. These findings suggest that HD intoxication resulted in a progressive decline of NFs, which was highly correlated with the development of HD-induced neuropathy.

Expression changes of apoptotic-related proteins in nerve tissues of rats treated with allyl chloride

Allyl chloride (AC) is widely used in industries as raw material and has been reported to produce occupational peripheral neuropathies in man chronically exposure to it. Although many studies have been done addressing to it, the mechanisms still remain unclear. To elucidate the molecular mechanism of neuropathy induced by AC, we measured the contents of glutathione (GSH), Bcl-2, Bax, cytochrome c (CytC) and Caspase-3 in a time-dependent manner by biochemical and quantitative immunoblotting techniques in rats' cerebrum and spinal cord after 3, 6, 9 and 12 weeks of AC intoxication. The results showed that the levels of Bcl-2 of cerebrum and spinal cord significantly (P<0.05) decreased after 9 and 6 weeks of AC intoxication, respectively, while GSH levels decreased after 12 week. However, the levels of Bax, CytC and Caspase-3 significantly (P<0.05) increased both in cerebrum and spinal cord. Bax levels of cerebrum and spinal cord increased after 12 and 9 weeks of AC administration, respectively. The levels of CytC and Caspase-3 also went up after 9 weeks of AC treatment in cerebrum and 9, 6 weeks in spinal cord, respectively. Thus, subchronic exposure to AC affected the expressions of apoptotic-related proteins in the central nervous system (CNS) and peripheral nervous system (PNS) tissues and the time dependent changes of these indexes occurred. The regulatory mechanism of apoptosis might be involved and served as one of mechanisms of toxic neuropathy induced by AC.

Acrylamide alters cytoskeletal protein level in rat sciatic nerves

Occupational exposure and experimental intoxication with acrylamide (ACR) produce neuropathy characterized by nerve degeneration. To investigate the mechanism of ACR-induced neuropathy, male adult Wistar rats were given ACR (20, 40 mg/kg i.p. 3 days/week) for 8 weeks. Sciatic nerves were Triton-extracted and centrifuged at a high speed (100,000 x g) to yield pellet and supernatant fractions. The contents of six cytoskeletal proteins (NF-L, NF-M, NF-H, alpha-tubulin, beta-tubulin, and beta-actin) in both fractions were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. Results showed that the three neurofilament (NF) subunits (NF-L, NF-M, NF-H) in both the pellet and the supernatant fraction decreased significantly (P < 0.01) in the high-dosing group, except for NF-M in the pellet. alpha-tubulin, beta-tubulin, and beta-actin increased significantly in the supernatant (P < 0.01), whereas both alpha-tubulin and beta-tubulin decreased significantly in the pellet (P < 0.01). However, beta-actin was not altered significantly in the sciatic nerves pellet. These findings suggest that ACR altered the cytoskeletal protein level in sciatic nerve, which may be one of the molecular mechanisms of ACR-induced peripheral neuropathy.

Malondialdehyde and catalase as the serum biomarkers of allyl chloride-induced toxic neuropathy

Chronic exposure to allyl chloride (AC) is known to produce a central-peripheral distal axonopathy. To access the biomarker of exposure and elucidate the mechanism of neuropathy induced by AC, we performed a longitudinal observational study of malondialdehyde (MDA), anti-reactive oxygen species (anti-ROS), glutathione (GSH), catalase (CAT), glutathione peroxidase (GPx) and superoxide dismutase (SOD) in rats serum and sciatic nerve after 0, 3, 6, 9 and 12 weeks of AC administration. AC was administrated to Wistar rats by gavage at a single dosage of 200 mg/kg/per dose (three times per week). Rats were sacrificed after 0, 3, 6, 9 and 12 weeks of AC treatment, serum and sciatic nerves were quickly collected at 4 degrees C. The results showed that MDA levels in serum (115.4 and 126.2%) and sciatic nerve (130.5 and 145.3%) significantly increased (p<0.05) on 3rd week of AC treatment and at gait score of 2, and further changes of MDA levels were observed after 6, 9 and 12 weeks and at gait score of 3 and 4. While a decrease (p<0.05) in the activities of CAT on 6th week of AC intoxication and at gait score of 2 was observed in serum (81.2 and 72.8%) and sciatic nerve (71.7 and 70.7%). The other antioxidants also decreased in serum and sciatic nerve after 3, 6 and 9, 12 weeks' intoxication and at gait score of 2, 3 and 4. Significant (p<0.05) positive correlations were observed between serum and sciatic nerve in MDA levels (r=0.9162 and 0.9551, respectively) and CAT (r=0.9410 and 0.9557, respectively) activities as time went on and symptoms developed. Thus, AC intoxication was associated with elevation of lipid peroxidation and reduction of antioxidative status, and the time dependent changes of these indexes in Wistar rats' serum and sciatic nerve occurred. The misbalance of lipid peroxidation and antioxidation status might be one of mechanisms of toxic neuropathy induced by AC. MDA and CAT could be served as the biomarkers of AC exposure to afford the early diagnosis of AC-induced toxic neuropathy.

The Changes of Cytoskeletal Proteins in Plasma of Acrylamide-Induced Rats

Acrylamide (ACR) is a known industrial neurotoxic chemical that can induce neurodegeneration. Cytoskeletal protein aggregation is a pathological hallmark of neurodegenerative disorders. This study was an initial exploration on cytoskeletal proteins in plasma as potential biomarkers of ACR neurotoxicity. Low and high ACR groups received 20 mg/kg and 40 mg/kg ACR by intraperitoneal injection in adult Wistar rats and control group received physiological saline. Rats were all killed after 8 weeks to evaluate the levels of neurofilament(NF)-L, NF-M, NF-H, beta-actin, alpha-tubulin, beta-tubulin, tau, MAP2 proteins in plasma using both SDS-PAGE and western blotting. Compared with the control, the levels of NF-L, NF-M, NF-H, beta-actin, tau, MAP2 proteins decreased and the level of alpha-tubulin increased in high ACR group, the levels of alpha-tubulin, beta-tubulin and MAP2 increased in low ACR group. The results suggested that the changes of these proteins might be relevant to the neurotoxicity of ACR. Some of the cytoskeletal proteins in plasma might be used as marker of biological effect in ACR induced neuropathy.

Acrylamide inhibits dopamine uptake in rat striatal synaptic vesicles

Evidence suggests that acrylamide (ACR) neurotoxicity is mediated by decreased presynaptic neurotransmitter release. Defective release might involve disruption of neurotransmitter storage, and therefore, we determined the effects of in vivo and in vitro ACR exposure on 3H-dopamine (DA) transport into rat striatal synaptic vesicles. Results showed that vesicular DA uptake was decreased significantly in rats intoxicated at either 50 mg/kg/day x 5 days or 21 mg/kg/day x 21 days. ACR intoxication also was accompanied by a reduction in KCl-evoked synaptosomal DA release, although consistent changes in presynaptic membrane transport were not observed. Silver stain and immunoblot analyses suggested that reduced vesicular uptake was not due to active nerve terminal degeneration or to a reduction in the synaptic vesicle content of isolated striatal synaptosomes. Nor did the in vivo presynaptic effects of ACR involve changes in synaptosomal glutathione concentrations. In vitro exposure of striatal vesicles showed that both ACR and two sulfhydryl reagents, N-ethylmaleimide (NEM) and iodoacetic acid (IAA), produced concentration-dependent decreases in 3H-DA uptake. Although ACR was significantly less potent than either NEM or IAA, all three chemicals caused comparable maximal inhibitions of vesicular uptake. Kinetic analysis of DA uptake showed that in vitro exposure to either ACR or NEM decreased V(max) and increased K(m). Determination of radiolabel efflux from 3H-DA-loaded vesicles indicated that in vitro ACR did not affect neurotransmitter retention. These data suggest that ACR impaired neurotransmitter uptake into striatal synaptic vesicles, possibly by interacting with sulfhydryl groups on functionally relevant proteins. The resulting disruption of neurotransmitter storage might mediate defective presynaptic release.

Acrylamide-Induced Changes in the Neurofilament Protein of Rat Cerebrum Fractions

Acrylamide (ACR) is known to produce central-peripheral distal axonopathy, which is characterized by distal swellings and secondary degeneration both in experimental animals and human. Ultrastructurally, excessive accumulation of neurofilaments (NFs) in the distal swollen axon is a major pathological hallmark. However, the mechanisms of ACR axonopathy remain unknown. Twenty seven male Wistar rats were randomly divided into three groups. Lower and higher ACR groups were received 20 and 40 mg/kg ACR by i.p. injection respectively. The control group received physiological saline. All rats were sacrificed after 8 weeks of treatment and their cerebrums were dissected, homogenized and used for the determination of the NF proteins. In general, the levels of light NF (NF-L) and medium NF (NF-M) subunits increased consistently in the supernatant, whereas they decreased consistently in the pellet from rats treated with ACR. Compared to that of the control group, the levels of NF-L increased respectively by 104% and 45% (P<0.01) in the supernatant and decreased by 16% and 11% (P<0.01) in the pellet of rat cerebrums in lower and higher groups. The enhancement of NF-M was 76% and 147% (P<0.05, P<0.01) in supernatant, and the reduction was 26% and 36% (P<0.01) in pellet in lower and higher group respectively. The heavy NF (NF-H) level changed slightly. The present results suggested that the change of NF-L and NF-M levels in cerebrum might be relevant to the mechanisms of the neurofilamentous axonopathies induced by ACR.

2,5-Hexanedione induced reduction in protein content and mRNA expression of neurofilament in rat cerebral cortex

Exposure chronically to n-hexane produces central-peripheral axonopathy mediated by 2,5-hexanedione (HD). Studies have shown neurofilament (NF) subunit proteins are decreased substantially in cerebral cortices, optic axons, spinal cords, and sciatic nerves from HD-exposed rats. To deeply investigate the alterations in NF contents in HD neuropathy, the relative levels of NF-L, NF-M, and NF-H in rat cerebral cortex were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. HD was administrated to Wistar rats by intraperitoneal injection at dosage of 200 or 400mg/kg. Rats were sacrificed after 6 weeks of treatment, and cerebral cortices were dissected, homogenized, and used for the determination of NF subunit proteins. The results, except for supernatant NF-L and NF-M that could not be assayed, showed HD intoxication resulted in significant decreases by 32-67% (P<0.01) in NF subunits in both of the pellet and supernatant fractions of cerebral cortex homogenate. As an initial investigation to determine how such changes in NF proteins might occur, the gene expression of NF-L, NF-M, and NF-H subunit mRNA was quantified using reverse transcription-polymerase chain reaction (RT-PCR). Statistical analysis revealed that HD exposure caused a significant reduction in the expression of NF-L and NF-H gene (P<0.05 or P<0.01), while the levels of NF-M mRNA kept unaffected (P>0.05). These suggest that the observed reduction in NF gene expression might be related to diminished levels of subunit proteins, while the actual contribution might be uncertain. The functional significance of the reduced protein contents and the regulation of gene expression remain to be determined.

Gamma-Diketone central neuropathy: quantitative analyses of cytoskeletal components in myelinated axons of the rat rubrospinal tract

Loss of axon caliber is a primary component of gamma-diketone neuropathy [LoPachin RM, DeCaprio AP. gamma-Diketone central neuropathy: axon atrophy and the role of cytoskeletal protein adduction. Toxicol Appl Pharmacol 2004;199:20-34]. It is possible that this effect is mediated by changes in the density of cytoskeletal components and corresponding spatial relationships. To examine this possibility, morphometric methods were used to quantify the effects of 2,5-hexanedione (HD) intoxication on neurofilament-microtubule densities and nearest neighbor distances in myelinated rubrospinal axons. Rats were exposed to HD at one of two daily dose-rates (175 or 400 mg/kg per day, gavage) until a moderate level of neurotoxicity was achieved (99 or 21 days of intoxication, respectively) as determined by gait analysis and measurements of hindlimb grip strength. Results indicate that, regardless of dose-rate, HD intoxication did not cause changes in axonal neurofilament (NF) density, but did significantly increase microtubule (MT) density. No consistent alterations in interneurofilament or NF-MT distances were detected by ultrastructural morphometric analyses. These data suggest that the axon atrophy induced by HD was not mediated by major disruptions of stationary cytoskeletal organization. Recent biochemical studies of spinal cord from HD intoxicated rats showed that, although the NF protein content in the stationary cytoskeleton (polymer fraction) was not affected, the mobile subunit pool was depleted substantially [LoPachin RM, He D, Reid ML, Opanashuk LA. 2,5-Hexanedione-induced changes in the monomeric neurofilament protein content of rat spinal cord fractions. Toxicol Appl Pharmacol 2004;198:61-73]. The stability of the polymer fraction during HD intoxication is consistent with the absence of significant ultrastructural modifications noted in the present study. Together, these findings implicate loss of mobile NF proteins as the primary mechanism of axon atrophy.

2,5-Hexanedione-induced changes in the neurofilament subunit pools of rat peripheral nerve

Axon atrophy is the principle morphological feature of the peripheral neuropathy induced by 2,5-hexanedione (HD). Axon caliber is determined by a stationary neurofilamentous cytoskeleton that is maintained through dynamic interactions with mobile neurofilament (NF) subunits. To determine the effects of HD on the stationary and mobile NF pools, groups of rats were exposed to HD at dosing schedules (175 mg/kg x 101 days or 400 mg/kg x 26 days) that produced moderate levels of neurological deficits and, as assessed by previous studies, prevalent axon atrophy in peripheral nerve. Sciatic and tibial nerves from HD-intoxicated rats and their age-matched controls were triton-extracted and separated by differential centrifugation into a high-speed pellet (P1) of NF polymer and a corresponding supernatant fraction (S1), which presumably contained mobile monomer. Cytoskeletal proteins (NF-L, NF-M, NF-H and beta-tubulin) in each fraction were determined by immunoblot analysis. Results show that regardless of HD dose-rate, triton-soluble NF subunits in the supernatant fractions were significantly reduced, whereas triton-insoluble proteins in the corresponding pellets were inconsistently affected. Beta-tubulin also exhibited inconsistent fractional changes, while abnormal higher molecular weight NF proteins were detected primarily in the triton-insoluble fraction. Studies with antibodies directed against phosphorylated (RT97) and non-phosphorylated (SMI32) epitopes on NF-H did not reveal major changes in subunit phosphorylation. These results suggest that HD intoxication is primarily associated with depletion of soluble NF proteins, which could produce axon atrophy through disruption of cytoskeletal turnover and maintenance.

A new murine model of giant proximal axonopathy

The aromatic gamma-diketone 1,2-diacetylbenzene (1,2-DAB), the putative active metabolite of the organic solvent 1,2-diethylbenzene, forms blue-colored polymeric protein adducts and induces the formation of amyotrophic lateral sclerosis (ALS)-like giant, intraspinal neurofilamentous axonal swellings in Sprague Dawley rats. The pathogenetic mechanism of this neuropathy has yet to be understood. We assessed whether these pathological changes are also seen in the C57BL/6 mouse, the animal of choice for toxicogenomic studies. Mice were treated intraperitoneally with 30, 35, 50, or 70 mg/kg 1,2-DAB or its inactive isomer 1,3-DAB per day (or on alternate days) for up to 43 days. Animals treated with 30 or 35 mg/kg per day 1,2-DAB, but not with 1,3-DAB, developed muscle spasms and progressive weakness, most prominently in hind limbs. Light microscopy revealed swollen axons in spinal anterior horns and proximal ventral roots, and to a lesser extent in dorsal root ganglia of 1,2-DAB-treated animals. Ultrastructural examination of swollen axons revealed clumps of maloriented 10-nm neurofilaments. Sciatic nerves showed clustering of axonal microtubules and other organelles. These findings are qualitatively comparable to those reported in rats treated with 1,2-DAB and represent a suitable phenotype with which to explore molecular mechanisms of proximal, giant neurofilamentous axonopathy using proteomic and genomic technologies.

2,5-Hexanedione Induced Decrease in Cytoskeletal Proteins of Rat Sciatic?tibial Nerve

Exposure chronically to n-hexane produces peripheral-central axonopathy mediated by 2,5-hexanedione (HD). Previous studies have demonstrated decreases in neurofilament (NF) contents of peripheral and central nervous regions from rats intoxicated with HD, and recent analysis has demonstrated that axonal atrophy, instead of NF-filled swellings, is a specific component of morphologic alterations. To deeply investigate the alterations of cytoskeletal proteins in HD peripheral neuropathy, the relative levels of NF-L, NF-M, NF-H, alpha-tubulin, beta-tubulin and beta-actin of rat sciatic-tibial nerves were determined by SDS-PAGE and immunoblotting. HD was administrated to Wistar rats by intraperitoneal injection at dosage of 200 or 400 mg/kg/day (five-times per week). Rats were sacrificed after 6 weeks of treatment, and sciatic-tibial nerves were dissected, homogenized, and used for the determination of cytoskeletal proteins. Except for supernatant NF-L that could not be assayed, the results showed HD intoxication was associated with significant decreases in NF subunits in both of the supernatant and the pellet fractions of sciatic-tibial nerve homogenates (P < 0.01), and obvious reductions in alpha-tubulin, beta-tubulin and beta-actin only in the supernatant (P < 0.05 or P < 0.01). Among these alterations, the falls in the levels of NF subunits tended to be greater compared to those of the other cytoskeletal proteins in all HD-exposed groups, and the trend for decrements in NF-M was greater than those in the other NF subunits. Thus, HD intoxication was associated with significant declines in cytoskeletal protein contents in rat sciatic-tibial nerves, and the decreases might be related to the involvement of the peripheral axonopathy induced by HD.

Levuglandins and isolevuglandins: stealthy toxins of oxidative injury

Inspired by a reaction discovered through basic research on the chemistry of the bicyclic peroxide nucleus of the prostaglandin endoperoxide PGH2, we postulated that levulinaldehyde derivatives with prostaglandin side chains, levuglandins (LGs), and structurally isomeric analogues, isolevuglandins (iso[n]LGs), would be generated by nonenzymatic rearrangements of prostanoid and isoprostanoid endoperoxides. Two decades of subsequent studies culminated in our discoveries of the LG and isoLG pathways, branches of the cyclooxygenase and isoprostane pathways, respectively. In cells, PGH2 rearranges nonenzymatically to LGs even in the presence of enzymes that use PGH2 as a substrate. IsoLGs, also known as isoketals or neuroketals, are generated in vivo through free radical-induced autoxidation of polyunsaturated phospholipid esters. Hydrolysis occurs after rapid adduction of isoLG phospholipids to proteins. The proclivity of these reactive species to avidly bind covalently with and cross-link proteins and nucleic acids complicated the hunt for LGs and isoLGs in vivo. The extraordinary reactivity of these "stealthy toxins" underlies much, if not all, of the biological consequences of LG and isoLG generation. They interfere with protein function and are among the most potent neurotoxic products of lipid oxidation known. Because they can accumulate over the lifetimes of proteins, iso[n]LG-protein adducts represent a convenient dosimeter of oxidative stress.

2,5-Hexanedione-induced changes in the monomeric neurofilament protein content of rat spinal cord fractions

Quantitative morphometric analyses have demonstrated that axon atrophy is the primary neuropathic feature in the CNS and PNS of rats intoxicated with 2,5-hexanedione (HD). Axon caliber is maintained by the exchange of mobile neurofilament (NF) subunits with the stationary polymer and, therefore, HD might produce atrophy by disrupting cytoskeletal turnover. To evaluate this possibility, groups of rats were exposed to HD at dosing schedules (175 mg/kg x 101 days or 400 mg/kg x 26 days) that produced moderate levels of neurological deficits and prevalent axon atrophy in spinal cord white matter tracts. Lumbar spinal cord regions from HD-intoxicated rats and their age-matched controls were Triton-extracted and separated by differential fractionation into a low-speed, insoluble pellet (P1) of NF polymer and a high-speed supernatant fraction (S2), which presumably contained mobile monomer. Cytoskeletal protein contents (NF-L, -M, -H, and beta-tubulin) in each fraction were determined by immunoblot analysis. Results show that regardless of HD dose-rate, the NF polymer in P1 remained unaffected, although soluble monomer in the S2 fraction was depleted significantly (60-80% reduction). Fractional beta-tubulin contents were inconsistently affected and abnormal higher-molecular-weight NF proteins were detected in the P1 fraction only. Studies with antibodies directed against phosphorylated (RT97) and nonphosphorylated (SMI32) epitopes on NF-H and measurements of corresponding isoelectric range suggested that alterations in phosphorylation were not involved. The selective depletion of Triton-soluble protein suggested that HD adduction of NFs interfered with the dynamic interactions of the polymeric and mobile monomeric pools.

γ-Diketone neuropathy: axon atrophy and the role of cytoskeletal protein adduction

Multifocal giant neurofilamentous axonal swellings and secondary distal degeneration have been historically considered the hallmark features of gamma-diketone neuropathy. Accordingly, research conducted over the past 25 years has been directed toward discerning mechanisms of axonal swelling. However, this neuropathological convention has been challenged by recent observations that swollen axons were an exclusive product of long-term 2.5-hexanedione (HD) intoxication at lower daily dose-rates (e.g., 175 mg/kg/day); that is, higher HD dose-rates (e.g., 400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. The observation that neurological toxicity can be expressed without axonal swelling suggests that this lesion is not an important pathophysiological event. Instead, several research groups have now shown that axon atrophy is prevalent in nervous tissues of laboratory animals intoxicated over a wide range of HD dose-rates. The well-documented nerve conduction defects associated with axon atrophy, in conjunction with the temporal correspondence between this lesion and the onset of neurological deficits, strongly suggest that atrophy has pathophysiological significance. In this commentary, we present evidence that supports a pathognomonic role for axon atrophy in gamma-diketone neuropathy and suggests that the functional consequences of this lesion mediate the corresponding neurological toxicity. Previous research has demonstrated that HD interacts with proteins via formation of pyrrole adducts. We therefore discuss the possibility that this chemical process is essential to the mechanism of atrophy. Evidence presented in this review suggests that "distal axonopathy" is an inaccurate classification and future nosological schemes should be based on the apparent primacy of axon atrophy.

Reproductive toxicity of 2,5-hexanedione in male rats

Background, Aims and Method:  2,5-Hexanedione is an industrial solvent which causes peripheral neuropathy. In the current study, the effect of 2,5-hexanedione on testicular histology of adult rats as well as sperm concentration, motility, and morphology were studied by administering 100 mg, 200 mg, 400 mg/kg per day for 12 weeks. Results:  No sperm motility was observed in the 200 mg and 400 mg/kg per day treatment groups and significantly reduced motility was observed in the 100 mg/kg per day group. The morphology were also significantly reduced in the 200 mg and 400 mg/kg per day groups compared to the control group, but the sperm concentration was significantly reduced only in the 400 mg/kg per day group. Histological examination of the testes in the 400 mg/kg per day group revealed that two-thirds of the testes had Sertoli cell only syndrome, whereas in the 200 mg/kg per day group half of the testes showed maturation arrest and sperm as well as spermatids were observed in 83% of the testes. Conclusions:  In conclusion, we have shown that 2,5-hexanedione severely affected sperm motility even at low doses, whereas high doses adversely affected all the sperm parameters as well as causing testicular injury. (Reprod Med Biol 2004; 3: 59-62).