Regioselective binding of 2,5-hexanedione to high-molecular-weight rat neurofilament proteins in vitro

Elucidation of the effects of 2,5-hexandione as a metabolite of n-hexane on cognitive impairment in leptin-knockout mice (C57BL/6-Lepem1Shwl/Korl)

Exposure to n-hexane and its metabolite 2,5-hexandione (HD) is a well-known cause of neurotoxicity, particularly in the peripheral nervous system. To date, few studies have focused on the neurotoxic effects of HD on cognitive impairment. Exposure to HD and diabetes mellitus can exacerbate neurotoxicity. There are links among HD, diabetes mellitus, and cognitive impairment; however, the specific mechanisms underlying them remain unclear. Therefore, we aimed to elucidate the neurotoxic effects of HD on cognitive impairment in ob/ob (C57BL/6-Lepem1Shwl/Korl) mice. We found that HD induced cognitive impairment by altering the expression of genes (FN1, AGT, ACTA2, MYH11, MKI67, MET, CTGF, and CD44), miRNAs (mmu-miR15a-5p, mmu-miR-17-5p, and mmu-miR-29a-3p), transcription factors (transcription factor AP-2 alpha [TFAP2A], serum response factor [Srf], and paired box gene 4 [PAX4]), and signaling pathways (ERK/CERB, PI3K/AKT, GSK-3β/p-tau/amyloid-β), as well as by causing neuroinflammation (TREM1/DAP12/NF-κB), oxidative stress, and apoptosis. The prevalent use of n-hexane in various industrial applications (for instance, shoe manufacturing, printing inks, paints, and varnishes) suggests that individuals with elevated body weight and glucose levels and those employed in high-risk workplaces have greater probability of cognitive impairment. Therefore, implementing screening strategies for HD-induced cognitive dysfunction is crucial.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-024-00228-1.

Mechanisms of soft and hard electrophile toxicities

Electron-deficient chemicals (electrophiles) react with compounds that have one or more unshared valence electron pairs (nucleophiles). The resulting covalent reactions between electrophiles and nucleophiles (e.g., Michael addition, SN2 reactions) are important, not only to Organic Chemistry, but also to the fields of Molecular Biology and Toxicology. Specifically, covalent bond formation is the operational basis of many critically important cellular processes; e.g., enzyme function, neurotransmitter release, and membrane-vesicle fusion. Given this context it is understandable that these reactions are also relevant to Toxicology, since a significant number of xenobiotic chemicals are toxic electrophiles that can react with endogenous nucleophilic residues. Therefore, the purpose of this Review is to discuss electrophile-nucleophile chemistry as it pertains to cell injury and resulting organ toxicity. Our discussion will involve an introduction to the Hard and Soft, Acids and Bases (HSAB) theory of Pearson. The HSAB concept provides a framework for calculation of quantum chemical parameters that classify the electrophile and nucleophile covalent components according to their respective electronic nature (softness/hardness) and reactivity (electrophilicity/nucleophilicity). The calculated quantum indices in conjunction with corroborative in vivo, in chemico (cell free) and in vitro research can offer an illuminating approach to mechanistic discovery. Accordingly, we will provide examples that demonstrate how this approach has been used to discern mechanisms and sites of electrophile action.

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.

Toxicokinetic study of pyrrole adducts and its potential application for biological monitoring of 2,5-hexanedione subacute exposure

PURPOSE

The formation of pyrrole adducts might be responsible for peripheral nerve injury caused by n-hexane, but there is not an effective biomarker for monitoring occupational exposure of n-hexane. The current study was designed to investigate the changes of pyrrole adducts in serum and urine of rats exposed to 2,5-hexanedione (2,5-HD) and analyze the correlation between pyrrole adducts and 2,5-HD.

METHODS

Two groups of male Wistar rats (n = 8) were administered a single dose of 200 and 400 mg/kg 2,5-HD (i.p.), and another two groups (n = 8) were given daily dose of 200 and 400 mg/kg 2,5-HD (i.p.) for 5 days. Pyrrole adducts and 2,5-HD in serum and urine were determined, at different time points after dosing, using Ehrlich’s reagent and gas chromatography, respectively.

RESULTS

The levels of pyrrole adducts in serum accumulated in a time-dependant manner after repeated exposure to 2,5-HD, while pyrrole adducts in urine, and 2,5-HD in serum and urine were kept stable. The half-life times (t1/2) of 2,5-HD and pyrrole adducts in serum were 2.27 ± 0.28 and 25.3 ± 3.34 h, respectively. Furthermore, the levels of pyrrole adducts in urine were significantly correlated with the levels of 2,5-HD in serum (r = 0.736, P < 0.001) and urine (r = 0.730, P < 0.001), and the levels of pyrrole adducts in serum were correlated with the cumulative dosage of 2,5-HD (r = 0.965, P < 0.001).

CONCLUSION

The results suggested that pyrrole adducts in serum and urine might be markers of chronic exposure to n-hexane or 2,5-HD.

Role of N-acetylcysteine in protecting against 2,5-hexanedione neurotoxicity in a rat model: changes in urinary pyrroles levels and motor activity performance

The interference of N-acetylcysteine (NAC) on 2,5-hexanedione (2,5-HD) neurotoxicity was evaluated through behavioral assays and the analysis of urinary 2,5-HD, dimethylpyrrole norleucine (DMPN), and cysteine-pyrrole conjugate (DMPN NAC), by ESI-LC-MS/MS, in rats exposed to 2,5-HD and co-exposed to 2,5-HD and NAC. Wistar rats were treated with 4 doses of: 400mg 2,5-HD/kg bw (group I), 400mg 2,5-HD/kg bw+200mg NAC/kg bw (group II), 200mg NAC/kg bw (group III) and with saline (group IV). The results show a significant decrease (p<0.01) in urinary DMPN and free 2,5-HD, a significant increase (p<0.01) in DMPN NAC excretion, and a significant recovery (p<0.01) on motor activity in rats co-exposed to 2,5-HD+NAC, as compared with rats exposed to 2,5-HD alone. Taken together, our findings suggest that at the studied conditions NAC protects against 2,5-HD neurotoxicity and DMPN may be proposed as a new sensitive and specific biomarker of 2,5-HD neurotoxicity in animals treated with a toxic amount of 2,5-hexanedione.

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.

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.

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.

Association between metabolic gene polymorphisms and susceptibility to peripheral nerve damage in workers exposed ton-hexane: A preliminary study

Chronic exposure to n-hexane may result in peripheral neuropathy. 2,5-Hexanedione (2,5-HD) has been identified as a toxic metabolite of n-hexane. The CYP2E1, CYP1A1 and GST genes are involved in the formation of 2,5-hexanedione from n-hexane as well as the elimination of 2,5-HD-formed electrophile, and these genes are highly polymorphic in the general population. A nested case-control study in an industrial cohort was conducted to evaluate the associations between polymorphisms in these metabolic genes and n-hexane-induced peripheral nerve damage. The study subjects included 22 cases, who worked in a printing factory with symptoms of peripheral nerve damage, and 163 controls, who came from the same factory of cases. DNA was extracted from blood samples and genotyping was conducted for CYP2E1 Pst, CYP2E1 Dra, CYP2E1 Ins96, CYP1A1 Msp, GSTT1 null, GSTM1 null and GSTP1 105V. Unconditional logistic regression was applied to estimate the odds ratio and 95% confidence intervals. There were no significant differences between the two groups regarding age, sex, smoking and alcohol status. A significant association between Dra polymorphism and peripheral nerve damage was found. The frequency of CYP2E1 Dra homozygous mutation in the case group (18.2%) was higher than that in the control group (3.7%, p=0.015). Individuals with homozygote genotype (CC) of CYP2E1 Dra had a significantly higher risk of peripheral nerve damage compared with those with DD genotype (adjusted OR=?.58, 95% CI=1.32-23.65) after n-hexane exposure duration, sex, age, smoking and alcohol status were adjusted. No significant association was found that CYP2E1 Pst, CYP2E1 Ins96, CYP1A1 Msp, GSTT1, GSTM1, GSTP gene polymorphisms associated with the susceptibility of peripheral nerve damage. These findings suggested that CYP2E1 gene might increase the susceptibility to n-hexane-induced peripheral damage.

The toxicity of hexanedione isomers in neural and astrocytic cell lines

The metabolite 2,5-hexanedione (HD) is the cause of neurotoxicity linked with chronic n-hexane exposure. Acute exposure to high levels of 2,5-HD, have also shown toxic effects in neuronal cells and non-neuronal cells. Isomers of 2,5-HD, 2,3- and 3,4-HD, added to foodstuffs, are reported to be non-toxic. The acute cytotoxic effects of 2,5-, 2,3- and 3,4-HD were evaluated in neural (NT2.N, SK-N-SH), astrocytic (CCF-STTG1) and non-neural (NT2.D1) cell lines. All the cell lines were highly resistant to 2,5-HD (34-426mM) at 4-h exposure, although sensitivity was greatest with NT2.D1, then SK-N-SH, NT2.N and finally the CCF-STTG1 line. At 24-h exposure, cell vulnerability increased 5-10-fold. The NT2.D1 cells were again the most sensitive, followed by NT2.N, SK-N-SH and then the CCF-STTG1 cells. 2,3- and 3,4-HD (8-84mM), were significantly more toxic towards all four cell lines compared with 2,5-HD, after 4-h exposure. After 24-h exposure there was a 12-fold increase in inhibition of MTT turnover in the SK-N-SH cells and a 4-fold increase in the CCF-STTG1 cells, compared with 2,5-HD exposure. 2,3- and 3,4-HD, were significantly less toxic to the NT2.N cells than the SK-N-SH cells after 24-h exposure to the compounds, demonstrating a differing toxin vulnerability between these neural and neuroblastoma cell lines. This study indicates that these non-neuronal and neuronal cells are acutely resistant to 2,5-HD cytotoxicity, whilst the previously unreported sensitivity of all four cell lines to the 2,3- and 3,4- isomers of HD to has been shown to be significantly greater than that of 2,5-HD.

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.

Proteomic analysis of acrylamide-protein adduct formation in rat brain synaptosomes

Evidence suggests that the neurological defects (gait abnormalities, foot splay, and skeletal muscle weakness) associated with acrylamide (ACR) intoxication are mediated by impaired neurotransmission at central and peripheral synapses. ACR can form adducts with nucleophilic residues on proteins and thereby alter corresponding structure and function. To evaluate protein adduction in nerve terminals as a possible mechanism of action, recombinant N-ethylmaleimide sensitive factor (NSF) was exposed in vitro to ACR (10 micromol) and mass spectrometry (MS) was used to identify adduct sites. MS analyses demonstrated that ACR formed adducts with sulfhydryl groups on cysteine residues (carbamoylethylcysteine, or CEC) of NSF. Ex vivo incubation of whole brain synaptosomes with ACR (0.001-1.0 M) produced concentration-dependent increases in CEC that were inversely correlated to reductions in neurotransmitter release that occurred over the same neurotoxicant concentration range. In synaptosomes isolated from rats intoxicated at a higher (50 mg/kg per day x 3, 5, 8, or 11 days) or a lower (21 mg/kg per day x 14, 21, or 28 day) ACR dose rate, CEC levels increased progressively up to a moderate level of neurotoxicity. To identify protein adducts, synaptosomal proteins labeled by ex vivo 14C-ACR exposure were separated by gel electrophoresis and probed by immunoblot analysis. Results showed that NSF and the SNARE protein, SNAP-25, were tentative ACR targets. Subsequent experiments indicated that ACR exposure increased synaptosomal levels of the 7S SNARE core complex, which is consistent with inhibition of NSF, SNAP-25 function, or both. These data suggest that adduction of cysteine residues on NSF and certain SNARE proteins might be causally involved in the nerve terminal dysfunction induced by ACR.

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.

gamma-diketone central neuropathy: quantitative morphometric analysis of axons in rat spinal cord white matter regions and nerve roots

A quantitative analytical method was used to measure myelinated axon morphometric parameters (e.g., axon area, ratio of axon area/fiber area, and index of circularity) in rat nervous tissue during intoxication with 2,5-hexanedione (HD). Parameters were assessed in nerve roots (dorsal and ventral) and in ascending (gracile fasciculus and spinocerebellar tract) and descending (corticospinal and rubrospinal tracts) spinal cord white matter tracts (L4-L5) of rats intoxicated with HD at two different daily dose-rates (175 or 400 mg HD/kg/day, gavage). For each dose-rate, tissue was sampled at four neurological endpoints: unaffected, slight, moderate, and severe toxicity, as determined by gait analysis and measurements of grip strength. Results indicate that, regardless of the HD dose-rate, axon atrophy (reduced axon area) was a widespread, abundant effect that developed in concert with neurological deficits. The atrophy response occurred contemporaneously in both ascending and descending spinal tracts, which suggests that loss of caliber developed simultaneously along the proximodistal axon axis. In contrast, swollen axons were a numerically small component and were present in nerve roots and spinal tracts only during subchronic intoxication at the lower HD dose-rate (i.e., 175 mg/kg/day). Intoxication at the higher dose-rate (400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. These observations in conjunction with our previous studies of HD-induced peripheral neuropathy (Toxicol. Appl. Pharmacol. 135 (1995) 58; and Toxicol. Appl. Pharmacol. 165 (2000) 127) indicate that axon atrophy, and not axonal swelling, is a primary neuropathic phenomenon.

Evidence of zinc protection against 2,5-hexanedione neurotoxicity: correlation of neurobehavioral testing with biomarkers of excretion

Risk prevention of human exposure against n-hexane neurotoxicity is relevant towards the protective measures to be proposed in occupational toxicology. Metabolic studies have identified 2,5-hexanedione (2,5-HD) as the main neurotoxic metabolite of n-hexane, which reacts with amino groups of lysine in axonal neurofilaments forming 2,5-dimethylpyrrole adducts, which are responsible for n-hexane neurotoxicity. In the present study, we have investigated the interaction of zinc with 2,5-HD, by correlating the decrease of pyrrole derivatives excretion with changes of neurobehavioral effects. Two subchronic experiments (11 and 8 weeks of exposure) were performed in Wistar rats exposed to different doses of 2,5-HD (200, 400 mg/kg per day) and to the mixture of 2,5-HD + zinc acetate (200 + 300 mg/kg per day) and (400 + 500 mg/kg per day). The results obtained show a significant increase in the excretion of pyrroles in the groups exposed to 2,5-HD alone as compared to controls, and a significant decrease in the excretion of pyrrole derivatives in the groups of rats co-exposed to 2,5-HD + zinc acetate when compared to the rats exposed to 2,5-HD alone. These biochemical changes were immediately evident after the first day of exposure. Simultaneously, neurobehavioral testing (rearing and ambulation in open field) was performed weekly in the same groups of rats. The results demonstrated a significant decrease in neurobehavioral dysfunction in rats co-exposed to 2,5-HD and zinc acetate. At the end of the exposure period, pyrroles levels returned to control values progressively, and the recovery of the neurotoxic effects was gradually established depending on the dose of exposure. The results suggest that zinc is a potential chemo-protector against 2,5-HD neurotoxicity which was identified by neurobehavioral testing. Moreover, pyrrole derivatives are good predictive biochemical biomarkers of 2,5-HD exposure and could be used as a complementary tool to characterize its neurotoxic effects.

Early spatial memory deficit induced by 2,5-hexanedione in the rat

2,5-Hexanedione (2,5-HD), the major common neurotoxic metabolite of n-hexane and methyl n-butyl ketone, causes a delayed neuropathy with associated sensorimotor impairments. The question arises as to whether specific cognitive deficits occur even prior to changes in sensorimotor ability. The present experiments examined the effects of 2,5-HD on spatial navigation of rats in a water maze at levels/times that did not affect spontaneous exploratory motor activity in an open field holeboard apparatus. Exposure to 1% 2,5-HD in the drinking water for 2 weeks did not significantly affect escape learning, as measured by latency to find a hidden platform. However, 2,5-HD treated animals were impaired in the use of a spatial strategy during a recall test. A similar impairment in spatial memory was observed after i.p. injection of 500 mg/kg/day 2,5-HD for 4 days, in the absence of significant changes in sensorimotor ability or weight loss. Thus 2,5-HD may mediate some of the cognitive effects of hexacarbons and these changes can occur prior to the development of motor symptoms.

Interaction of zinc on biomarker responses in rats exposed to 2,5-hexanedione by two routes of exposure

The interaction of zinc(II) on the toxicokinetics of 2,5-hexanedione (2,5-HD), the ultimate toxic metabolite of n-hexane, was performed by quantifying the changes of two urinary biomarkers, free 2,5-HD and pyrrole derivatives, in rats exposed to 2,5-HD and to 2,5-HD plus zinc acetate. Eight groups of Wistar rats were exposed for 4 days (dietary and intraperitoneally) to 2,5-HD, zinc acetate and 2,5-HD plus zinc acetate and the 24 h urine was used to determine the excretion of these biomarkers. On comparing the results obtained by the two routes of exposure with different doses of 2,5-HD and zinc acetate, it was observed that there was a significant decrease (P<0.05) in the excretion of free 2,5-HD and pyrroles derivatives in rats exposed to the chemical mixture, when compared with the excretion of these biomarkers in rats exposed to 2,5-HD alone. To evaluate the mechanism of this interaction, further experiments were performed using one group of rat dietary pre-exposed to zinc acetate followed by 2,5-HD exposure. The results of our experiment suggest that zinc protect proteins of pyrrolization by coordination to amino groups, with the subsequent inhibition of protein cross-linking responsible by 2,5-HD neurotoxicity.

Effects of 2,5-hexanedione on calpain-mediated degradation of human neurofilaments in vitro

2,5-Hexanedione (2,5-HD), the neurotoxic metabolite of n-hexane, can structurally modify neurofilaments (NF) by pyrrole adduct formation and subsequent covalent cross-linking. 2,5-HD also induces accumulations of NF within the pre-terminal axon. We examined whether exposure of NF to 2,5-HD affected NF degradation. Two different models were used: (1) NF-enriched cytoskeletons isolated from human sciatic nerve were incubated with 2,5-HD in vitro and (2) differentiated human neuroblastoma cells (SK-N-SH) were exposed to 2, 5-HD in culture prior to isolation of cytoskeletal proteins. The cytoskeletal preparations were subsequently incubated with calpain II. The amount of NF-H and NF-L remaining after proteolysis was determined by SDS-PAGE and quantitative immunoblotting. NF-M proteolysis could not be quantified. Incubation of sciatic nerve cytoskeletal preparations with 2,5-HD resulted in cross-linking of all three NF proteins into high molecular weight (HMW) material with a range of molecular weights. Proteolysis of the NF-H and NF-L polypeptides was not affected by 2,5-HD-exposure. Degradation of the HMW material containing NF-H or NF-L was retarded when comparing with degradation of the NF-H and NF-L polypeptides, respectively, from control samples, but not as compared to the corresponding NF polypeptides from 2,5-HD-treated samples. Exposure of SK-N-SH cells to 2,5-HD also resulted in considerable cross-linking of NF. No differences were found between the proteolytic rates of NF-L and NF-H from exposed cells as compared with those subunits from control cells. Moreover, degradation of cross-linked NF-H was not different from monomeric NF-H. In conclusion, whether 2,5-HD affects calpain-mediated degradation of cross-linked NF proteins will depend on which model better reflects NF cross-linking as occurring in 2, 5-HD-induced axonopathy. However, with both models it was demonstrated that exposure of NF proteins to 2,5-HD without subsequent cross-linking is not adequate to inhibit NF proteolysis in vitro by added calpain.