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

2,5-hexanedione induces NLRP3 inflammasome activation and neurotoxicity through NADPH oxidase-dependent pathway

Chronic exposure to n-hexane causes sensorimotor neuropathy, which is mediated by 2,5-hexanedione (HD), a toxic metabolite of n-hexane. Activation of the nucleotide-binding and oligomerization domain, leucine-rich repeat, and pyrin domain-containing 3 (NLRP3) inflammasome is involved in multiple neurodegenerative diseases. However, whether the NLRP3 inflammasome contributes to HD-induced neurotoxicity remains unclear. In this study, the effects of HD on NLRP3 inflammasome activation and the underlying mechanisms were determined by using HD-treated rat and cell culture models. Increased NLRP3 expression, caspase-1 activation and interleukin-1β production were observed in both the brain and spinal cord of HD-treated rats. Double-immunofluorescence staining showed that ASC speck formation and caspase-1 expression were mainly localized in microglia. HD-induced activation of the NLRP3 inflammasome was further mirrored in BV2 microglial cells and was associated with NADPH oxidase activation. Interestingly, inhibition of NADPH oxidase by apocynin or specific siRNAs significantly mitigated HD-induced NLRP3 inflammasome activation. Furthermore, apocynin suppressed activation of the MAPK and NF-κB signaling pathways. Blocking activation of p38-MAPK and NF-κB significantly reduced HD-induced capase-1 activation and interleukin-1β maturation, indicating a critical role of NADPH oxidase and downstream MAPK and NF-κB pathways in regulating activation of NLRP3 inflammasome, in HD-treated microglia. Finally, we found that inhibition of microglial NLRP3 inflammasome and NADPH oxidase activation abrogated HD-induced microglial activation and neurodegeneration in both SHSY5Y neuronal cells and primary cortical neuron-glia cultures. Altogether, our findings suggest that NADPH oxidase-dependent activation of microglial NLRP3 inflammasome contributes to HD-induced neurotoxicity, providing novel insight into the mechanisms of this solvent-induced neuropathy.

Taurine attenuates acrylamide-induced axonal and myelinated damage through the Akt/GSK3β-dependent pathway

Acrylamide (ACR), formed during the Maillard reaction induced by high temperature in food processing, is one of the main causes of neurodegenerative diseases. Taurine, a free intracellular β-amino acid, is characterized by many functions, including antioxidation, anti-inflammatory, and neuroprotective properties. This promotes its application in the treatment of neurodegenerative diseases. In this study, the neuroprotective effects of taurine against ACR-induced neurotoxicity and the potential underlying mechanisms were explored. Rats were intoxicated with ACR and injected with taurine in different groups for totally 2 weeks between January and July 2017. Electron microscopic analysis was used to observe the changes in tissues of the rats. Meanwhile, the levels of proteins including p-Akt, p-GSK3β, SIM312, and MBP were detected by Western blot. Furthermore, the GSK3β phosphorylation in taurine-treated dorsal root ganglion (DRG) with ACR was examined in the presence of the Akt inhibitor, MK-2206. The analysis of behavioral performances and electron micrographs indicated that taurine treatment significantly attenuated the toxic manifestations induced by ACR and stimulated the growth of axons and the medullary sheath, which was associated with the activation of the Akt/GSK3β signaling pathway. Mechanistically, it was found that taurine activated GSK3β, leading to significant recovery of the damage in ACR-induced sciatic nerves. Furthermore, MK-2206, an inhibitor of Akt, was applied in DRG cells, suggesting that taurine-induced GSK3β phosphorylation was Akt dependent. Our findings demonstrated that taurine attenuated ACR-induced neuropathy in vivo, in an Akt/GSK3β-dependent manner. This confirmed the treatment with taurine to be a novel strategy against ACR-induced neurotoxicity.

2,5-Hexanedione increases the percentage of proliferative Sox2+ cells in rat hippocampus

n-Hexane is an organic solvent widely used in industry. 2,5-Hexanedione (2,5-HD), the major neurotoxic metabolite of n-hexane, decreases the levels of neurofilaments (NFs) in neurons. Neurogenesis occurs throughout life, and the hippocampal dentate gyrus is one of two major brain areas showing neurogenesis in adulthood. In the current study, rats were intraperitoneally injected with normal saline solution or 2,5-HD five times per week for five continuous weeks. 2,5-HD was administered to the low-dose and high-dose groups at 200 and 400 mg/kg/day, respectively. Then, immunoreactive cells were counted in the hippocampal granule cell layer (GCL) and subgranular zone (SGZ). Ki67+ cells significantly decreased in the high-dose group, while the percentage of proliferative Sox2+ cells significantly increased, consistent with high hippocampal Sox2 expression. Additionally, western blotting showed that exposure to high doses of 2,5-HD led to decreased NF-L in both the cortex and hippocampus, whereas low doses led to a significant reduction in the cortex only. In conclusion, 2,5-HD increases the percentage of proliferating neural stem and progenitor (Sox2+) cells in the SGZ/GCL.

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

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

Correlation between levels of 2, 5-hexanedione and pyrrole adducts in tissues of rats exposure to n-hexane for 5-days

Background

The formation of pyrrole adducts might be responsible for peripheral nerve injury caused by n-hexane. The internal dose of pyrrole adducts would supply more information for the neurotoxicity of n-hexane. The current study was designed to investigate the tissue distributions of 2, 5-hexanedione (2,5-HD) and pyrrole adducts in rats exposed to n-hexane, and analyze the correlation between pyrrole adducts and 2,5-HD in tissues.

Methods

Male Wistar rats were given daily dose of 500,1000, 2000, 4000 mg/kg bw n-hexane by gavage for 5 days. The rats were sacrificed 24 hours after the last administration. The levels of 2, 5-hexanedione and pyrrole adducts in tissues were measured by gas chromatography and Ehrlich’s reagent, respectively. The correlations between 2, 5-hexanedione and pyrrole adducts were analyzed by linear regression

Results

Dose-dependent effects were observed between the dosage of n-hexane and 2, 5-hexanedione, and pyrrole adducts in tissues. The highest level of 2, 5-hexanedione was found in urine and the lowest in sciatic nerve, while the highest level of pyrrole adducts was seen in liver and the lowest in serum. There were significant correlations among the free 2, 5-hexanedione, total 2, 5-hexanedione and pyrrole adducts within the same tissues. Pyrrole adducts in serum showed the most significant correlation with free 2, 5-hexanedione or pyrrole adducts in tissues.

Conclusion

The findings suggested that pyrrole adducts in serum might be a better indicator for the internal dose of free 2, 5-hexanedione and pyrrole adducts in tissues.

Probing mechanisms of axonopathy. Part II: Protein targets of 2,5-hexanedione, the neurotoxic metabolite of the aliphatic solvent n-hexane

Neuroprotein changes in the spinal cord of rodents with aliphatic gamma-diketone axonopathy induced by 2,5-hexanedione (2,5-HD) are compared with those reported previously in aromatic gamma-diketone-like axonopathy induced by 1,2-diacetylbenzene (1,2-DAB). Sprague-Dawley rats were treated intraperitoneally with 500 mg/kg/day 2,5-HD, equimolar doses of 2,3-hexanedione (negative control), or an equivalent amount of saline containing 50% dimethyl sulfoxide (vehicle), 5 days a week, for 3 weeks. Analysis of the lumbosacral proteome by 2-dimensional differential in-gel electrophoresis and matrix-assisted laser desorption ionization time-of-flight/tandem mass spectrometry revealed 34 proteins markedly modified by 2,5-HD of which neurofilament triplet L, gelsolin, protein disulfide isomerase, glutathione S-transferase, nicotinamide adenine dinucleotide (reduced) dehydrogenase 1 alpha, pyruvate kinase, and fatty acid synthase were also modified by 1,2-DAB. The expression of proteins involved in maintaining the physical integrity of the cytoskeleton or controlling the redox and protein-folding mechanisms was reduced, whereas that of proteins supporting energy metabolism was mainly increased. The similarity of the neuroproteomic patterns of 2,5-HD and 1,2-DAB axonopathy suggests common biomarkers and/or mechanisms of neurotoxicity associated with exposure to their parent chemicals, namely the industrial solvents n-hexane and 1,2-diethylbenzene, respectively.