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.

Redox status of the testes and sperm of rats following exposure to 2,5-hexanedione

OBJECTIVES

Exposure to 2,5-hexanedione (2,5-HD) is well known to be associated with reproductive dysfunctions in both humans and animals. However, the role of oxidative stress in 2,5-HD-induced toxicity in testes and sperm has not yet been studied.

METHODOLOGY

The present study investigated the influence of 2,5-HD on antioxidant systems in the testes and epididymal sperm of rats following exposure to 0, 0.25, 0.5, and 1% 2,5-HD in drinking water for 21 consecutive days.

RESULTS

Administration of 0.5% 2,5-HD significantly (P < 0.05) decreased epididymis weight, whereas 1% 2,5-HD-treated rats showed significantly decreased body weight, testis, and epididymis weights compared with the control group. Exposure to 2,5-HD caused a significant dose-dependent increase in the activities of superoxide dismutase, catalase, and glutathione peroxidase in both testes and sperm compared with the control group. Moreover, 2,5-HD-exposed rats showed significant decrease in glutathione-S-transferase activity and glutathione level with concomitant significant elevation in the levels of hydrogen peroxide and malondialdehyde in both testes and sperm. Testicular and epididymal atrophy with significant, dose-dependent, decrease in epididymal sperm number, sperm motility, and viability were observed in 2,5-HD-treated rats.

CONCLUSION

2,5-HD exposure impaired testicular function and sperm characteristics by disruption of the antioxidant systems and consequently, increased oxidative stress in the treated rats.

Determination of total urinary 2,5-hexanedione in the Chinese general population

OBJECTIVE

Determination of the urinary levels of 2.5-hexanedione (2,5-HD) was performed in subjects belonging to the Chinese general population to define the reference value for this metabolite.

METHODS

Urine samples were collected from 8235 individuals (4216 men and 4019 women) from the healthy general population who had not been occupationally exposed to n-hexane or methyl-n-butyl ketone. The determination was performed by a gas chromatography mass spectrometry method using an ion-trap mass spectrometer.

RESULTS

The result showed that the urinary 2,5-HD median level was 0.159mg/L for the total samples. Males had statistically significant higher excretion of 2,5-HD in urine than females (median 0.171mg/L compared to 0.147mg/L, Z=-8.21, P<0.001). There was a statistically significant difference in urinary 2,5-HD levels among age groups. The excretion of 2,5-HD in urine was related to increasing age (r=-0.160, P<0.05). There was statistically significant difference in urinary 2,5-HD levels among people from difference provinces. The results showed that there was also a statistically significant effect in urinary 2,5-HD levels between current smokers and non-smokers.

CONCLUSION

Finding a measurable amount of 2,5-HD in urine does not mean that the level of 2,5-HD causes an adverse health effect. Biomonitoring studies on levels of urinary 2,5-HD can provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of 2,5-HD than are found in the Chinese general population. These data can also provide a foundation for scientists to make a plan for further study.

Diallyl trisulfide attenuated n-hexane induced neurotoxicity in rats by modulating P450 enzymes

Chronic exposure to n-hexane can induce serious nerve system impairments without effective preventive medicines. Diallyl trisulfide (DATS) is a garlic-derived organosulfur compound, which has been demonstrated to have many beneficial effects. The current study was designed to evaluate whether DATS could restrain n-hexane induced neurotoxicity in rats and to explore the underlying mechanisms. Rats were treated with n-hexane (3 g/kg, p.o.) and different doses of DATS (10, 20 and 30 mg/kg, p.o.) for 8 weeks. Behavioral assessment showed that DATS could inhibit n-hexane induced neurotoxicity, demonstrated by the improvement of the grip strength and decline of gait scores. Toxicokinetic analysis revealed that the C_max and AUC_0-t of 2,5-hexanedione (product of n-hexane metabolic activation) and 2,5-hexanedione protein adducts in serum were significantly declined in DATS-treated rats, and the levels of pyrrole adducts in tissues were significantly reduced. Furthermore, DATS activated CYP1A1 and inhibited n-hexane induced increased expression and activity of CYP2E1 and CYP2B1. Collectively, these findings indicated that DATS protected the rats from n-hexane-induced neurotoxicity, which might be attributed to the modulation of P450 enzymes by DATS.

Effects of 2,5-hexanedione on angiogenesis and vasculogenesis in chick embryos

n-Hexane is widely used in industry and its metabolite, 2,5-hexanedione (2,5-HD), has been implicated as a neural toxin in the developing fetus. Using the chick embryo model, we have previously revealed the neurotoxicity of 2,5-HD during development and established that high dose of 2,5-HD was embryo lethal. In view of the close linkage in biology for neurogenesis and angiogenesis, we speculated that it was most likely caused by cardiovascular dysplasia, therefore in this study, we investigated the effects of 2,5-HD on the development of the vasculature, which involves vasculogenesis and angiogenesis. Using gastrulating chick embryos as a model, we demonstrated that the hemangioblasts (precursor of hematopoietic and endothelial cells) migrated to the area opaca where they form the blood islands. However, this process was impaired when the embryos were treated with 2,5-HD, suggesting that 2,5-HD is capable of impairing vasculogenesis. To study the effect of 2,5-HD exposure on angiogenesis, we used the chick yolk-sac membrane (YSM) and chorioallantoic membrane (CAM) models. We found that, at low (0.02M) concentration, 2,5-HD stimulated angiogenesis while at higher concentrations (>0.1M) it inhibited this process. This biphasic response of angiogenesis to 2,5-HD exposure was found to be associated with altered expression of the VEGF-R, FGF-2 and angiogenin. Moreover, we also determined that 2,5-HD exposure increased the reactive oxygen species (ROS) production. In conclusion, 2,5-HD could induce dysplasia in the developing vasculature, which in turn could cause extravascular hemolysis and the embryos to die.

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.

Bone marrow mesenchymal stem cells promote remyelination in spinal cord by driving oligodendrocyte progenitor cell differentiation via TNFα/RelB-Hes1 pathway: a rat model study of 2,5-hexanedione-induced neurotoxicity

BACKGROUND

N-hexane, with its metabolite 2,5-hexanedine (HD), is an industrial hazardous material. Chronic hexane exposure causes segmental demyelination in the peripheral nerves, and high-dose intoxication may also affect central nervous system. Demyelinating conditions are difficult to treat and stem cell therapy using bone marrow mesenchymal stem cells (BMSCs) is a promising novel strategy. Our previous study found that BMSCs promoted motor function recovery in rats modeling hexane neurotoxicity. This work aimed to explore the underlying mechanisms and focused on the changes in spinal cord.

METHODS

Sprague Dawley rats were intoxicated with HD (400 mg/kg/day, i.p, for 5 weeks). A bolus of BMSCs (5 × 10⁷ cells/kg) was injected via tail vein. Demyelination and remyelination of the spinal cord before and after BMSC treatment were examined microscopically. Cultured oligodendrocyte progenitor cells (OPCs) were incubated with HD ± BMSC-derived conditional medium (BMSC-CM). OPC differentiation was studied by immunostaining and morphometric analysis. The expressional changes of Hes1, a transcription factor negatively regulating OPC-differentiation, were studied. The upstream Notch1 and TNFα/RelB pathways were studied, and some key signaling molecules were measured. The correlation between neurotrophin NGF and TNFα was also investigated. Statistical significance was evaluated using one-way ANOVA and performed using SPSS 13.0.

RESULTS

The demyelinating damage by HD and remyelination by BMSCs were evidenced by electron microscopy, LFB staining and NG2/MBP immunohistochemistry. In vitro cultured OPCs showed more differentiation after incubation with BMSC-CM. Hes1 expression was found to be significantly increased by HD and decreased by BMSC or BMSC-CM. The change of Hes1 was found, however, independent of Notch1 activation, but dependent on TNFα/RelB signaling. HD was found to increase TNFα, RelB and Hes1 expression, and BMSCs were found to have the opposite effect. Addition of recombinant TNFα to OPCs or RelB overexpression similarly caused upregulation of Hes1 expression. The secretion of NGF by BMSC and activation of NGF receptor was found important for suppression of TNFα production in OPCs.

CONCLUSIONS

Our findings demonstrated that BMSCs promote remyelination in the spinal cord of HD-exposed rats via TNFα/RelB-Hes1 pathway, providing novel insights for evaluating and further exploring the therapeutical effect of BMSCs on demyelinating neurodegenerative disease.

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.

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.

No-observed-adverse-effect level of hair pyrrole adducts in chronic n-hexane intoxication in rats

n-Hexane has been reported to induce serious peripheral neuropathy in workers. Pyrrole adducts are the unique reaction products of n-hexane in organisms and have been demonstrated to be critical to n-hexane neuropathy. Our previous studies have demonstrated that pyrrole adducts could accumulate in hair and showed high correlation with neuropathy at the end of experiments in rat models. In the present study, we examined the time course of hair pyrrole adducts and behavioral changes in rats exposed to different dosages of n-hexane in both treatment (24 weeks) and recovery phases. Our results showed: 1. After treatment, 1.0, 2.0, and 4.0 g/kg dosage groups all lost weight, but the 0.5 g/kg dosage group showed no impairment; after recovery, all impaired rats regained weight. 2. After treatment, 1.0, 2.0, and 4.0 g/kg dosage groups all showed a rise in gait scores, decreased rotarod latency, and decreased motor nerve conduction velocity, whereas the 0.5 g/kg dosage group showed no impairment; after recovery, all impaired rats were completely rehabilitated. 3. After treatment, levels of pyrrole adducts in serum, urine, and hair of experimental groups increased; after recovery, serum and urine pyrrole adducts showed no difference from the control (P > 0.05), whereas hair pyrrole adducts were significantly different from the control (P < 0.01). 4. The half-lives of serum and urine pyrrole adducts were 47.8-78.0 h and 42.7-52.9 h, while the half-life of hair pyrrole adducts was 14-24 weeks. 5. During treatment and recovery, levels of serum, urine, and hair pyrrole adducts showed high correlation with gait scores (P < 0.01), and hair pyrrole adducts had the largest partial correlation coefficient. In conclusion, hair pyrrole adducts could serve as a stable and reliable biomarker for the prevention of n-hexane intoxication. Furthermore, the no-observed-adverse-effect level of hair pyrrole adducts in rats is 275.2 ± 61.5 nmol/g protein. Further studies are required for the definition of the biological exposure limit in humans.

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.

NLRP3 inflammasome mediates 2,5-hexanedione-induced neurotoxicity through regulation of macrophage infiltration in rats

Currently, whether nod-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome activation contributes to neuropathy induced by 2,5-Hexanedione (HD), the toxic metabolite of n-hexane, remains unknown. In this study, we found that HD intoxication elevated NLRP3 expression, caspase-1 activation and interleukin-1β production in sciatic nerve of rats, indicating activation of NLRP3 inflammasome. The increased cleavage of gasdermin D (GSDMD) protein, an important mediator of pyroptosis, and axon degeneration were also observed in sciatic nerves of HD-intoxicated rats. Interestingly, glybenclamide, a widely used inhibitor of NLRP3 inflammasome, significantly reduced NLRP3 inflammasome activation, which was associated with decreased GSDMD cleavage and axon degeneration as well as improved motor performance of HD-intoxicated rats. Subsequently, we found that inhibition of NLRP3 inflammasome by glybenclamide attenuated macrophage infiltration, activation and M1 polarization in sciatic nerves of HD-intoxicated rats. Furthermore, decreased malondialdehyde (MDA) contents and increased glutathione (GSH) level and total anti-oxidative capacity were also observed in sciatic nerves of rats treated with combined glybenclamide and HD compared with HD alone group. Altogether, our findings suggest that NLRP3 inflammasome activation contributes to HD-induced neurotoxicity by enhancing macrophage infiltration and activation as well as oxidative stress, providing a novel mechanism of neuropathy induced by this neurotoxicant.

2,5-Hexanedione influences primordial follicular development in cultured neonatal mouse ovaries by interfering with the PI3K signaling pathway via miR-214-3p

The mechanisms by which 2,5-hexanedione (2,5-HD) exposure adversely affects reproduction are unclear. In the present study, whole neonatal mouse ovaries were exposed to 2,5-HD in vitro and then assessed for progesterone levels to determine the effects of this compound on ovary function. Ovarian histomorphological analyses were performed to assess the effects of 2,5-HD on follicular development, and PI3K signaling pathway was evaluated to elucidate the molecular mechanisms of 2,5-HD-mediated toxicity on follicular development. The results showed that after ovarian exposure to 2,5-HD in vitro, the percentage of secondary follicles decreased, while the progesterone levels and the percentage of unhealthy follicles increased, with oocytes identified as the target of damage. The 2,5-HD treatment significantly decreased the of the gene encoding the apoptosis-related protein caspase-8, and PI3K/AKT/FOXO3 pathway signaling was also altered. Furthermore, the effects of 2,5-HD on the gene expression of the PI3K/AKT/FOXO3 and follicular development were blocked by 740Y-P (a PI3K activator), miR-214-3p was abnormally expressed, and luciferase reporter assay results demonstrated that the 3' untranslated region of PI3K was a direct target of miR-214-3p. Overall, the results of the present study indicate that 2,5-HD exposure inhibits follicular development, and the underlying mechanism may involve interference with miR-214-3p-mediated regulation of the PI3K signaling pathway.

Pyrrole adducts mediated mitochondrial dysfunction activates SARM1-dependent axon degeneration in 2,5-hexanedione-induced neuropathy

2,5-hexanedione (HD) is the γ-diketone metabolite of industrial organic solvent n-hexane, primarily responsible for n-hexane neurotoxicity. Previous studies have shown that the formation of pyrrole adducts (PAs) is crucial for the toxic axonopathy induced by HD. However, the exact mechanism underlying PAs-induced axonal degeneration remains unclear. Recently, Sterile α and toll/interleukin 1 receptor motif-containing protein 1 (SARM1) has been identified as the central executor of axon degeneration. This study was designed to investigate the role of SARM1-mediated axon degeneration in rats exposed to HD. Furthermore, the causal relationship between PAs and SARM1-mediated axon degeneration was further explored using Sarm1 KO mice. Our findings suggest that HD causes axon degeneration and neuronal loss in animals. Mechanistic studies revealed that HD activates SARM1-dependent axonal degeneration machinery. In contrast, Sarm1 KO attenuates motor dysfunction and rescues neuron loss following HD exposure. Interestingly, the PAs formed by the binding of HD to proteins primarily accumulate on mitochondria, leading to mitochondrial dysfunction. This dysfunction serves as an upstream event in HD-induced nerve injuries. Our findings highlight the crucial role of PAs formation in the major pathological changes during n-hexane poisoning, providing a potential therapeutic target for n-hexane neuropathy.