Evidence for multiple mechanisms responsible for 2,5-hexanedione-induced neuropathy

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.

2,5-hexanedione induced apoptosis in mesenchymal stem cells from rat bone marrow via mitochondria-dependent caspase-3 pathway

2,5-hexanedione (HD) induces apoptosis of nerve cells. However,the mechanism of HD-induced apoptosis remains unknown. Mesenchymal stem cells (MSCs) are multipotential stem cells with the ability to differentiate into various cell types. This study is designed to investigate the apoptosis induced by HD in rat bone marrow MSCs (BMSCs) and the related underlying mechanisms. The fifth generation of MSCs was treated with 0, 10, 20 and 40 mM HD respectively. The viability of BMSCs was observed by MTT. Apoptosis were estimated by Hoechst 33342 staining and TUNEL assay. The disruption of mitochondrial transmembrane potential (MMP) was examined by JC-1 staining. Moreover, the expression of Bax and Bcl-2, cytochrome c release, and caspase-3 activity were determined by real-time RT-PCR, Western blot and Spectrophotometry. Our results showed that HD induced apoptosis in MSCs in a dose dependent manner. Moreover, HD downregulated the Bcl-2 expression,upregulated the Bax expression and the Bax/Bcl-2 ratio, promoted the disruption of MMP, induced the release of cytochrome c from mitochondria to cytosol, and increased the activity of caspase-3 in MSCs. These results indicate that HD induces apoptosis in MSCs and the activated mitochondria-dependent caspase-3 pathway may be involved in the HD-induced apoptosis.

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.

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.

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.

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.

Occupational exposure to n-hexane in Italy--analysis of a registry of biological monitoring

OBJECTIVE

This study assesses 2,5-hexanedione (2,5-HD) in the urine of subjects exposed to n-hexane solvent between 1991 and 1998, from details obtained from the Registry of Biological Monitoring (BM) at the Florence Local Health Unit, and its development over time.

METHODS

The Registry contains 15,925 samples from 6,650 subjects occupationally exposed to n-hexane, especially in leather (9,099 samples; 3,607 subjects) and shoe (3,865 samples; 1,938 subjects) production.

RESULTS

Over the time span studied there was a total reduction of 31.9% in urinary 2,5-HD level. The yearly decrease over the entire period was 5.4%. Dividing the 8 years into three periods: before the introduction of the new legislation for health protection in the workplace (1991-1993), during its transition (1994-1996) and after its complete enforcement (1997-1998), respectively, we observed a marked decrease in the last period. Women and young people (under 30 years) experienced significantly higher absorption levels (respectively, 7.1% and 24.4%).

CONCLUSION

The data suggest that monitoring was more frequent in subjects with higher starting values, and the greatest decrease was reported in this group. Reduction may be due to less n-hexane in the products used, better structural conditions in the factories, and the effectiveness of inspections carried out by the authority for hygiene and safety in the workplace. The results confirm the usefulness of the reporting of risk levels of exposure to industrial toxicants by routine biological monitoring.

Fast axonal transport: a site of acrylamide neurotoxicity?

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

Gamma-diketone peripheral neuropathy III. Neurofilament gene expression

Evidence suggests the morphologic hallmark of gamma-diketone neuropathy is axon atrophy and that this effect is associated with reduced neurofilament (NF) subunit protein content (Toxicol Appl Pharmacol 2000;165:141-7). To investigate the mechanism of diminished NF content, subunit (NF-L, -M and -H) gene expression was quantified in dorsal root ganglion (DRG) of slightly affected and moderately intoxicated groups of rats exposed to 2,5-hexanedione (HD) at one of three daily dosing rates (175, 250 and 400 mg/kg per day). Results show that sensory ganglia from slightly affected rats exhibited no changes in gene expression, whereas at a moderate level of neurotoxicity, each dosing protocol was associated with small but significant reductions (approximately 20%) in mean NF subunit mRNA. This was not a generalized effect on expression of cytoskeletal components in sensory ganglia since tubulin message levels were not affected. Although the observed reduction in NF gene expression might be related to diminished levels of subunit proteins in peripheral nerve, the actual contribution is likely to be minimal. The magnitude of effect was small and did not correspond to the dose-rate dependent effect of HD on respective isotype proteins. The mechanism of gamma-diketone-induced axon atrophy is unknown but might involve local changes in axonal NF phosphorylation and degradation.

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.

gamma-diketone peripheral neuropathy. II. Neurofilament subunit content

Quantitative morphometric analyses have demonstrated that axon atrophy is the primary neuropathic alteration in peripheral nerve of 2,5-hexanedione (HD)-intoxicated rats (Lehning et al., Toxicol. Appl. Pharmacol. 165, 127-140, 2000). Research suggests that axon caliber is regulated by neurofilament (NF) content and density. Therefore, as a possible mechanism of atrophy, NF subunit (NF-L, -M, and -H) proteins were quantitated in moderately affected rats intoxicated with HD at three daily dosing rates (175, 250, and 400 mg/kg/day). Analyses of subunit protein contents in proximal sciatic nerves indicated uniformly small decreases, which corresponded to minimal changes in axon area occurring in this region. In distal tibial nerve, subunit proteins were decreased substantially (40-70%) when rats were exposed to the 175 and 250 mg/kg/day doses. These reductions in NFs corresponded to significant decreases (approximately 50%) in tibial axon area induced by lower dosing rates. In contrast, 400 mg/kg/day produced similar changes in caliber but smaller reductions (18-25%) in NF-L, -M, and -H levels. This suggests that a decrement in axonal NF content is unlikely to be solely responsible for gamma-diketone-induced axon atrophy and that the corresponding mechanism probably involves additional changes in factors regulating NF density. Analysis of NF content in peripheral nerve also identified the presence of anomolous higher molecular weight NF-H proteins. However, the neurotoxicological significance of these abnormal subunits is uncertain based on their limited occurrence and inconsistent spatiotemporal expression.

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

Previous studies have shown selective binding of the neurotoxicant 2,5-hexanedione (2,5-HD) to carboxyl-terminal domains of rat neurofilament (NF) M and H proteins in vitro. The present study was designed to further localize this binding in native rat NF preparations exposed to [14C]2,5-HD. Purified M and H proteins from 2,5-HD-treated NFs were subjected to cyanogen bromide (CNBr) cleavage, and the resultant peptides were separated by Tris-tricine SDS-PAGE and electroblotted to PVDF membranes. Peptides were identified by direct sequencing of stained bands and the relative radiolabeling of each peptide was determined by comparing band intensities in fluorographed blots. For NF-M, the highest label was found in CNBr 10, a peptide corresponding to residues 678-846 at the extreme carboxyl terminus. This region of the protein includes three highly conserved lysine-containing sequences believed to be critical to its function. For NF-H, the greatest binding was localized in CNBr 7 + 8, representing an incomplete cleavage product of residues 390-810. This peptide contains essentially all of the phosphorylation sites in the carboxyl terminus of NF-H, a domain believed to control NF interactions in the axon. Only minor radiolabeling was observed in other M or H peptides. Extensive dephosphorylation of NFs prior to 2,5-HD exposure had no effect on relative adduct levels in each protein. These results provide additional support for limited and specific binding of 2,5-HD to neurofilaments and indicate that the phosphorylation state of the protein may not substantially influence this binding.

Protein-bound pyrroles in rat hair following subchronic intraperitoneal injections of 2,5-hexanedione

Studies were initiated to ascertain whether body hair could be used to develop a biological marker for chronic exposure to industrial neurotoxicants that yield the metabolite 2,5-hexanedione (2,5-HD), that is, n-hexane and methyl n-butyl ketone. Rats were injected daily with a 50 mg/kg ip dose of 2,5-HD for 45 d. At intervals, body hair and individual vibrissae were removed (under general anesthesia) and tested for the presence of pyrrole substances with p-N,N-dimethylaminobenzaldehyde (DMAB, Ehrlich's reagent). Vibrissae and body hair were stained a reddish color that was distinctly different from that observed with the hair taken from control animals. Solubilized body hair protein from the treated animals gave a positive Ehrlich's test, while that from control animals was negative. Spectral analysis of the DMAB-treated hair from experimental animals disclosed a maximum absorbance at 530 nm, which indicated the presence of pyrrole substituents. Serial analysis of individual nose hairs taken during 2,5-HD administration showed a progression with time of the region staining positively for pyrroles, thus indicating that the process can proceed in growing hair. These findings suggest the potential utility of hair as an indicator for chronic exposure to this class of industrial chemicals possessing neurotoxicity potential. This could complement urinary analysis, which is now used to confirm recent exposure.

Phosphorylation in vitro of glial fibrillary acidic protein is increased in rat hippocampus by administration of 2,5-hexanedione

Rats were treated with 2,5-hexanedione (2,5-HD) daily for 30 days. Hippocampal microslices were then incubated with [32P]phosphate and the in vitro rate of phosphorylation and the immuno-content of glial acidic fibrillary protein (GFAP) were measured. Exposure to 2,5-HD decreased by 25% the immuno-content of GFAP and increased by 35% its rate of phosphorylation, resulting in an increase of 88% in the ratio phosphorylation rate/immuno-content for this protein.

In vitro binding of [14C]2,5-hexanedione to rat neuronal cytoskeletal proteins

2,5-Hexanedione (2,5-HD) induces central-peripheral axonpathy characterized by the accumulation of 10-nm neurofilaments proximal to the nodes of Ranvier and a Wallerian-type degeneration. It has been postulated that neurofilament crosslinking may be involved in the production of this axonopathy. A potential initiating event in this neurotoxic process may be the direct binding of 2,5-HD to neurofilament and microtubule proteins. In this study, the in vitro binding of [14C]2,5-HD to neurofilament and microtubule proteins was examined. Neurofilament proteins isolated from rat spinal cord or microtubule proteins isolated from rat brain were incubated in the presence of 2,5-HD at concentrations ranging from 25 to 500 mM. Quantitative analysis of sodium dodecyl sulfate (SDS) polyacrylamide gels revealed a dose- and time-dependent binding of 2,5-HD to both neurofilament proteins and microtubule proteins. Expressed as pmol 2,5-HD bound per microgram protein, the observed relative binding was MAP2 > NF160 > NF200 > > NF68 > tubulin. These data demonstrate the direct binding of 2,5-HD to cytoskeletal proteins including both neurofilaments and microtubules.

Carbon disulfide inhalation increases Ca2+/calmodulin-dependent kinase phosphorylation of cytoskeletal proteins in the rat central nervous system

The Ca2+/calmodulin-dependent phosphorylation of neuronal cytoskeletal proteins was studied in brain supernatants prepared from rats exposed via inhalation to 600 to 800 ppm carbon disulfide (CS2) for 14 days. Exposure to CS2 resulted in increased phosphorylation of endogenous MAP-2 and exogenously added neurofilament triplet proteins. There also was an observed increase in the autophosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Slight increases in the binding of a monoclonal antibody to the alpha subunit of CaM kinase II were seen, while large increases in the binding of [125I]calmodulin to the alpha subunit of CaM kinase II also were observed. The finding of large increases in the autophosphorylation and calmodulin-binding to CaM kinase II with only slight increases in the amount of antibody-binding suggests that CS2 exposure results in increased Ca2+/calmodulin-dependent phosphorylation of proteins by inducing an increase in kinase activity.

Evidence that the monoclonal antibodies SMI-31 and SMI-34 recognize different phosphorylation-dependent epitopes of the murine high molecular mass neurofilament subunit

The monoclonal antibodies SMI-31 and SMI-34 react with phosphate-dependent epitopes of the high molecular mass (200 kDa) neurofilament protein (Hphos). Determination of whether or not these monoclonals react with different epitopes would assist in interpretation of post mortem immunocytochemical analyses in neurodegenerative disorders and in normal aging. We therefore examined the relative immunoreactivity of these antibodies against Triton-insoluble (cytoskeleton-associated) and Triton-soluble Hphos variants in NB2a/d1 neuroblastoma and post-natal mouse brain in immunoblot analysis. Densitometric analysis yielded a 'reactivity ratio' (soluble Hphos/insoluble Hphos) for each antibody. This ratio was approximately 44% and 87% less for SMI-34 than for SMI-31 in neuroblastoma and brain, respectively. These findings confirm that the SMI-34 epitope is distinct from that recognized by SMI-31, and, in these systems, is preferentially associated with the cytoskeleton.

Quantitative alterations of S-100 protein and neuron specific enolase in the rat nervous system after chronic 2,5-hexanedione exposure

The regional changes in quantities of the glial S-100 protein and the neuron specific enolase in the rat nervous system have been studied after long-term exposure to 2,5-hexanedione. The wet weights of most of the examined nervous tissues were found to be reduced, with an extensive effect seen in the brain stem. Using dot immunobinding assays, the concentrations of S-100 were found to be increased in most of the examined tissues, but unaffected in the brain stem. The total amount of S-100 per tissue was markedly reduced in the brain stem. The content of neuron specific enolase was reduced only in the brain stem. Thus the effects of 2,5-hexanedione on the nervous system varied regionally. The brain stem was severely atrophied with a reduction of neuronal as well as of glial marker proteins. Other brain regions contained increased glial cell marker proteins as signs of progressive astroglial reactions.

Quantitative and qualitative alterations of neuronal and glial intermediate filaments in rat nervous system after exposure to 2,5-hexanedione

The precise mechanism for the neurotoxicity of 2,5-hexanedione is not known, but cross-linking of neurofilament proteins has been suggested as one possibility. In this study the effects of long-term exposure to 2,5-hexanedione were studied in the rat nervous system with special reference to regional changes in the quantities of neuronal and glial intermediate filaments. Using enzyme-linked immunosorbent assays the concentrations of 68- and 200-kDa neurofilament polypeptides were shown to be reduced in all brain regions studied. Similar results were obtained in the sciatic nerve. The concentration of glial fibrillary acidic protein was decreased in the cerebellar vermis and the dorsal cerebral cortex, whereas it was increased in the spinal cord, a result suggesting a regional variation in glial sensitivity. The intermediate filaments of the exposed animals were also immunoblotted using polyclonal antisera against the various neurofilament polypeptides and glial fibrillary acidic protein. In all tissues studied, several aggregates with molecular weights higher than those of the monomeric polypeptides were demonstrated. Contrary to clinical observations, these data indicate pronounced effects in both CNS and PNS and call for further studies on CNS effects in humans.

Cytoskeletal changes induced by 2,5-hexanedione on developing human neurons in vitro

Dissociated dorsal root ganglion cells from human fetuses were exposed to 2,5-hexanedione (2,5-HD) for 2 weeks. Morphological changes induced by 2,5-HD consisted in focal neurofilament (NF)-containing enlargements preferentially located in distal, preterminal regions of unmyelinated fibers. Tangles of NF were also observed in the perikarya of nerve cells. Morphometric analysis disclosed that the cross-sectional areas of the 2,5-HD treated axons were 30% smaller than those of control axons. This alteration was associated with reduction of number of NF per unit area. These findings demonstrate that 2,5-HD treatment induces a generalized disorganization of neuronal and axonal NF responsible for focal enlargements as well as atrophic changes of unmyelinated fibers.

Neurofilament protein phosphorylation in spinal cord of experimentally diabetic rats

This study was designed to determine if the known decrease in slow axonal transport of proteins in the sciatic nerve of experimentally diabetic rats is related to altered phosphorylation of neurofilament proteins (NFPs). Rats were rendered diabetic with 50 mg/kg of streptozotocin, i.p. At 3 and 6 weeks later, NFPs were prepared from spinal cord. The in vivo phosphorylation state of NFPs was examined by using phosphate-dependent (RT97) and -independent (RMd09) antibodies against high-molecular-mass NFPs on Western blots. Neurofilament-associated kinase activity was also measured in vitro by incubation of NFPs with [32P]ATP. Phosphorylation of all three NFPs (high, medium, and low molecular mass) occurred, as confirmed by gel electrophoresis and autoradiography. At 30 min of incubation, protein-bound radioactivity in NFPs from diabetic animals was reduced to 86.7 +/- 3.4 and 54.3 +/- 19.6% of that in nondiabetic animals at 3 and 6 weeks of diabetes, respectively (p less than 0.001 and p less than 0.05, respectively). NFPs were also incubated with acid phosphatase and rephosphorylated. Results showed that the increased in vivo phosphorylation contributed to the decreased in vitro phosphorylation. Extraction of protein kinases and addition back to the NFPs revealed, in addition, a reduced activity in the diabetic animals of the protein kinases measured in vitro.

Induction of cytochrome P450 isozymes by simultaneous inhalation exposure of hens to n-hexane and methyl iso-butyl ketone (MiBK)

Chickens were exposed simultaneously to the industrial hexacarbon solvents n-hexane and methyl iso-butyl ketone (MiBK). n-Hexane has been shown to be neurotoxic in both humans and other vertebrates. While MiBK is not neurotoxic, it has been shown to greatly synergize the clinical appearance of neurotoxicity in animals exposed to both of these solvents. Groups of hens were exposed for 29 days in inhalation chambers to 1000 ppm n-hexane in combination with 10, 100, 250, 500, or 1000 ppm MiBK. Other groups received either 1000 ppm n-hexane, 1000 ppm MiBK, or ambient air and served as controls. A dose-dependent decrease in body weight and an increase in clinical effects were noted for the highest exposure groups (1000 ppm n-hexane combined with 1000, 500 or 250 ppm MiBK). There was an MiBK dose-dependent increase in cytochrome P450 content and benzphetamine N-demethylase activity, but there was no distinct pattern for ethoxyresorufin O-deethylase or cytochrome c reductase activities. Mixed-function oxidase levels and activities (cytochrome P450 content and benzphetamine N-demethylase) were elevated significantly (P less than 0.05) over controls even in the lowest MiBK group (10 ppm), although there were no clinical signs of neurotoxicity. Four different isozymes of cytochrome P450 were measured immunologically. There was a dose-dependent increase in three of the isozymes, two of which were phenobarbital inducible and one of which was induced by beta-napthoflavone. Quantitatively, the largest increase was in the PB-A isozyme, a phenobarbital-inducible isozyme which accounted for approximately 70% of the cytochrome P450 present in animals treated with MiBK. The results suggest that MiBK selectively induces cytochrome P450 isozymes leading to the metabolic activation of the weak neurotoxicant n-hexane to the potent neurotoxicant 2,5-hexanedione (2,5-HD).

Dynamics of phosphorylation and assembly of the high molecular weight neurofilament subunit in NB2a/d1 neuroblastoma

In neuronal systems thus far studied, newly synthesized neurofilament subunits rapidly associate with the Triton-insoluble cytoskeleton and subsequently undergo extensive phosphorylation. However, in the present study we demonstrate by biochemical and immunological criteria that NB2a/d1 neuroblastoma cells also contain Triton-soluble, extensively phosphorylated 200-kDa high molecular weight neurofilament subunits (NF-H). High-speed centrifugation (100,000 g) of the Triton-soluble fraction for 1 h sedimented some, but not all, soluble NF-H subunits; immunoelectron microscopic analyses of the resulting pellet indicated that a portion of the NF-H subunits in this fraction are assembled into (Triton-soluble) neurofilaments. When cells were pulse labeled for 15 min with [35S]methionine, radiolabel was first associated with the Triton-soluble 200-kDa NF-H variants. Because only extensively phosphorylated NF-H subunits migrate at 200 kDa, whereas hypophosphorylated subunits migrate instead at 160 kDa, these findings suggest that some newly synthesized subunits were phosphorylated before they polymerized. In pulse-chase analyses, radiolabeled 200-kDa NF-H migrated into the 100,000 g particulate fraction of Triton-soluble extracts before its arrival in the Triton-insoluble cytoskeleton. Undifferentiated cells, which do not possess axonal neurites and lack a significant amount of Triton-insoluble, extensively phosphorylated NF-H, contain a sizeable pool of Triton-soluble extensively phosphorylated NF-H subunits and polymers. We interpret these data to indicate that the integration of newly synthesized NF-H into the cytoskeleton occurs in a progression of distinct stages, and that assembly of NF-H into neurofilaments and integration into the Triton-insoluble cytoskeleton are not prerequisites for the incorporation of certain phosphate groups on these polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Giant axonopathy characterized by intermediate location of axonal enlargements and acceleration of neurofilament transport

It has previously been shown that 2,5-hexanedione (2,5-HD) and its 3,4-dimethyl derivative (3,4-DMHD) induce neurofilamentous accumulations at prenodal sites in distal and proximal, respectively, regions of peripheral axons. For 2,5-HD, neurofilament (NF) transport is accelerated and this is thought to be directly related to the appearance of the axonal enlargements. For 3,4-DMHD, however, the rate of NF transport cannot be assessed owing to the very proximal position of NF accumulation. In the present study, it is shown that administration to rats of 3-methyl-2,5-hexanedione, the structural 'average' of 2,5-HD and 3,4-DMHD, induces NF accumulations at midway axonal positions of the sciatic and optic systems, and results in acceleration of NF in the sections of optic axons proximal to the enlargements. These results suggest that a common mechanism underlies all gamma-diketone neuropathies, and that the proximodistal pattern of axonal enlargements represents pharmacokinetic variables rather than differences in mode of action. The neurotoxicity of gamma-diketones probably arises from pyrrolation of lysine epsilon-amino groups in crucial regions of NF or related proteins responsible for maintaining the proper supramolecular organization of the cytoskeleton. Acceleration of NF transport appears to be a common characteristic of chemically induced axonopathies, regardless of location, and this is contrary to the theory that gamma-diketone-induced NF accumulation results primarily from a progressive cross-linking of NF occurring subsequent to pyrrole formation.

In vivo inactivation of transglutaminase during the acute acrylamide toxic syndrome in the rat

The activity of liver and brain transglutaminase is rapidly lost following i.p. injection of acrylamide (50-200 mg/kg). Other enzymes investigated were not modified by the treatment, with the exception of brain enolase.

The Sertoli cell cytoskeleton: a target for toxicant-induced germ cell loss

Numerous studies in recent years have elucidated fundamental properties of axoplasmic structure, biochemistry, and function. The structural role of the cytoskeletal elements, the orientation of MTs within the axon, the phenomenon of MT-dependent transport, and the identity and direction of movement of two MT motors--kinesin and MAP-1C--have been revealed. For many years to come, researchers investigating the structure and function of the Sertoli cell cytoskeleton will be able to adapt techniques gleaned from work on the axonal cytoskeleton. Innovative thinking will be required to apply these techniques to the special circumstances of the male reproductive system; however, the underlying questions are similar. For example, knowledge of several fundamental properties of transport processes in the Sertoli cell would facilitate the toxicologic evaluation of this system. What is the orientation of MTs within the Sertoli cell cytoplasm? Are the fast-growing (+) ends of all MTs in the Sertoli cell cytoplasm directed toward the lumen? This is an important question because the direction of MT-dependent transport involving known MT motors is dependent upon the MT orientation. Which of the Sertoli cell transport pathways are MT-dependent pathways? What are the MT motors involved in these pathways? Ultrastructural examination following exposure to specific cytoskeleton-disrupting agents has highlighted the importance of AFs, IFs, and MTs in the Sertoli cell. Future research will focus on the nature of those molecules which integrate these cytoskeletal components into a dynamic whole, the regulatory systems which control this integration, and the role of an integrated cytoskeleton in Sertoli cell function and testicular homeostasis. Toxicology will be an active participant in this process of scientific discovery. The selective nervous system and testicular toxicants may be useful tools in revealing similarities in the cytoskeletal organization of these apparently disparate organ systems. By searching for common targets in the testis and nervous system, the mechanisms of action of these agents may be more easily, and more confidently, determined.