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

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.

AAV-mediated VEGFA overexpression promotes angiogenesis and recovery of locomotor function following spinal cord injury via PI3K/Akt signaling

Spinal cord injury (SCI) is a disorder of the central nervous system resulting from various factors such as trauma, inflammation, tumors, and other etiologies. This condition leads to impairment in motor, sensory, and autonomic functions below the level of injury. Limitations of current therapeutic approaches prompt an investigation into therapeutic angiogenesis through persistent local expression of proangiogenic factors. Here, we investigated whether overexpression of adeno-associated virus (AAV)-mediated vascular endothelial growth factor A (VEGFA) in mouse SCI promoted locomotor function recovery, and whether the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) pathway was mechanistically involved. Three weeks before SCI, AAV-VEGFA was injected at the T10 level to induce VEGFA overexpression. Neurofunctional, histological, and biochemical assessments were done to determine tissue damage and/or recovery of neuromuscular and behavioral impairments. Daily injections of the PI3K/Akt pathway inhibitor LY294002 were made to assess a possible mechanism. AAV-VEGFA overexpression dramatically improved locomotor function and ameliorated pathological injury caused by SCI. Improved motor-evoked potentials in hindlimbs and more spinal CD31-positive microvessels were observed in AAV-VEGFA-overexpressing mice. LY294002 reduced PI3K and Akt phosphorylation levels and attenuated AAV-VEGFA-related improvements. In conclusion, sustained local AAV-mediated VEGFA overexpression in spinal cord can significantly promote angiogenesis and ameliorate locomotor impairment after SCI in a contusion mouse model through activation of the PI3K/Akt signaling pathway.

Neuritin-overexpressing transgenic mice demonstrate enhanced neuroregeneration capacity and improved spatial learning and memory recovery after ischemia-reperfusion injury

Acute ischemia-reperfusion (IR)-induced brain injury is further exacerbated by a series of slower secondary pathogenic events, including delayed apoptosis due to neurotrophic factor deficiency. Neuritin, a neurotrophic factor regulating nervous system development and plasticity, is a potential therapeutic target for treatment of IR injury. In this study, Neuritin-overexpressing transgenic (Tg) mice were produced by pronuclear injection and offspring with high overexpression used to generate a line with stable inheritance for testing the neuroprotective capacity of Neuritin against transient global ischemia (TGI). Compared to wild-type mice, transgenic mice demonstrated reduced degradation of the DNA repair factor poly [ADP-ribose] polymerase 1 (PARP 1) in the hippocampus, indicating decreased hippocampal apoptosis rate, and a greater number of surviving hippocampal neurons during the first week post-TGI. In addition, Tg mice showed increased expression of the regeneration markers NF-200, synaptophysin, and GAP-43, and improved recovery of spatial learning and memory. Our findings exhibited that the window of opportunity of neural recovery in Neuritin transgenic mice group had a tendency to move ahead after TGI, which indicated that Neuritin can be used as a potential new therapeutic strategy for improving the outcome of cerebral ischemia injury.

Acute in vitro effects on embryonic rat dorsal root ganglion (DRG) cultures by in silico predicted neurotoxic chemicals: Evaluations on cytotoxicity, neurite length, and neurophysiology

The Hard-Soft Acid and Base hypothesis can be used to predict the potential bio-reactivity (electrophilicity) of a chemical with intracellular proteins, resulting in neurotoxicity. Twelve chemicals predicted to be neurotoxic were evaluated in vitro in rat dorsal root ganglia (DRG) for effects on cytotoxicity (%LDH), neuronal structure (total neurite length/neuron, NLPN), and neurophysiology (mean firing rate, MFR). DRGs were treated acutely on days in vitro (DIV) 7 (1-100 μM) with test chemical; %LDH and NLPN were measured after 48 h. 4-cyclohexylhexanone (4-C) increased %LDH release at 50 (29%) and 100 μM (56%), citronellal (Cit) and 1-bromopropane increased %LDH at 100 μM (22% and 26%). 4-C, Cit, 2,5 Hexanedione (2,5Hex), phenylacetylaldehyde (PAA) and 2-ethylhexanal decreased mean NLPN at 48 h; 50 and 100 μM for 4-C (28% and 60%), 100 μM Cit (52%), 100 μM 2,5- Hex (37%) 100 μM PAA (41%) and 100 μM for 2-ethylhexanal (23%). Separate DRG cultures were treated on DIV 14 and changes in MFR measured. Four compounds decreased MFR at 50 or 100 μM: Acrylamide (-83%), 3,4-dichloro-1-butene (-93%), 4-C (-89%) and hexane (-79%, 50 μM). Changes in MFR and NLPN occurred in absence of cytotoxicity. While the current study showed little cytotoxicity, it gave insight to initial changes in MFR. Results provide insight for future chronic exposure experiments to evaluate 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.

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

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

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

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

2,5-Hexanedione 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 protect against n-hexane-induced neuropathy through beclin 1-independent inhibition of autophagy

Chronic exposure to n-hexane, a widely used organic solvent in industry, induces central-peripheral neuropathy, which is mediated by its active metabolite, 2,5-hexanedione (HD). We recently reported that transplantation of bone marrow-mesenchymal stem cells (BMSC) significantly ameliorated HD-induced neuronal damage and motor deficits in rats. However, the mechanisms remain unclear. Here, we reported that inhibition of HD-induced autophagy contributed to BMSC-afforded protection. BMSC transplantation significantly reduced the levels of microtubule-associated protein 1 light chain 3-II (LC3-II) and the degradation of sequestosome-1 (p62) in the spinal cord and sciatic nerve of HD-intoxicated rats. Downregulation of autophagy by BMSC was also confirmed in VSC4.1 cells exposed to HD. Moreover, inhibition of autophagy by PIK III mitigated the neurotoxic effects of HD and, meanwhile, abolished BMSC-afforded neuroprotection. Furthermore, we found that BMSC failed to interfere with Beclin 1, but promoted activation of mammalian target of rapamycin (mTOR). Unc-like kinse 1 (ULK1) was further recognized as the downstream target of mTOR responsible for BMSC-mediated inhibition of autophagy. Altogether, BMSC transplantation potently ameliorated HD-induced autophagy through beclin 1-independent activation of mTOR pathway, providing a novel insight for the therapeutic effects of BMSC against n-hexane and other environmental toxicants-induced neurotoxicity.

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.

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.

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

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

Phenylmethylsulfonyl fluoride protects against the degradation of neurofilaments in tri-ortho-cresyl phosphate (TOCP) induced delayed neuropathy

Tri-ortho-cresyl phosphate (TOCP) is an organophosphorus ester, which can cause a type of neurotoxicity known as organophosphate-induced delayed neuropathy (OPIDN). Our recent study has shown that the enhanced degradation of neurofilament (NF) in peripheral nerve of hens is an early event of TOCP-induced OPIDN (Song et al., 2009). The main objective of this investigation is to study the effect of TOCP administration on NF content and NF degradation when OPIDN is blocked by pretreatment with phenylmethylsulfonyl fluoride (PMSF). The hens were pretreated 24h earlier with PMSF and subsequently treated with a single dosage of 750 mg/kg TOCP, then sacrificed on the corresponding time points of 0, 1, 5, 10, and 21 days after dosing TOCP, respectively. The tibial nerves were dissected, homogenized, and centrifuged at 100,000 x g. The level of NF triplet protein in both pellet and supernatant fractions of tibial nerves was determined. Western blotting analysis showed a significant increase of three NF subunits in hens treated with PMSF and TOCP compared with the control. These changes were observed within 24h of PMSF administration and then followed by an obvious recovery. Furthermore, accompanied with the increase of NF content, a significant decline in NF-L degradation rate was observed in both fractions of tibial nerves. Taken together, these results demonstrated the pretreatment with PMSF could inhibit TOCP-induced NF degradation while it protected hens against the development of OPIDN, which suggested the inhibition of NF-associated protease in peripheral nerves might be an underlying protective mechanism of PMSF against OPIDN.

Neurofilament isoform alterations in the rat cerebellum following cytosine arabinoside administration

A number of neurotoxic agents could potentially exert their action by degrading or modifying cytoskeleton components like neurofilaments (NF). Cytosine arabinoside (AraC) is an anticancer drug commonly used in leukemia treatment. Its side effects include neuronal cell death in the cerebellum and severe motor coordination deficits. We have previously shown that AraC administration (400mg/kg bw) in adult rats reduced NF immunostaining in cerebellar neurons. To further delineate the susceptibility of individual NF isoforms (NF-H, NF-M, NF-L) to AraC, in the present study we used Western blot analysis to quantify their level. A significant and selective reduction of NF-H isoform was observed in the cerebellum of AraC-treated animals, compared to the controls. Administration of the antioxidant N-acetylcysteine (NAC) for a period of 14 days (prior to and during AraC treatment), which was previously shown to ameliorate the AraC-induced motor deficits in these animals, largely prevented the reduction in NF-H isoform. Given the significant role of NF proteins and particularly NF-H in maintaining structural integrity and synaptic transport, the observed loss of this isoform may be a key-target of AraC action in cerebellar neurons. Moreover, this study provides further data on the neuroprophylactic role of NAC in vivo against chemotherapy-induced toxicity.

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

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

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

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

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

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

The function of microglia, either neuroprotection or neurotoxicity, is determined by the equilibrium among factors released from activated microglia in vitro

Opposing functions of activated microglia, namely neuroprotection or neurotrophy versus neurodestruction or neurotoxicity, have been observed in a number of experimental models of neurotrauma and neurodegenerative diseases. However, the mechanism(s) involved in the determination of which function activated microglia execute under a given set of conditions still remains to be elucidated. Our current in vitro study has revealed that a neuroprotective/neurotrophic or a neurodestructive/neurotoxic microglial function may be configured by the equilibrium among various microglial factors released into the microenvironment. When NSC-34 neurons were treated with lower concentrations of lipopolysaccharide-stimulated BV-2 microglial conditioned medium (LPS-BVCM), viability of the NSC-34 neurons increased, outgrowth of neuronal processes was promoted, and the formation of 2,5-hexanedione-induced aggregates was prevented. However, when NSC-34 neurons were treated with higher concentrations of the same LPS-BVCM, neuronal viability was reduced, apoptosis was induced and outgrowth of neuronal processes was prevented. Measurement of the cytokines tumor necrotic factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and IL-6 in the LPS-BVCM has shown that the upregulation in expression for each cytokine varied both temporally and quantitatively. It is postulated that an alteration in the concentration of the LPS-BVCM might significantly affect the functional balance of microglial factors in the microenvironment with a resultant different microglial function.

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.

Carbon Disulfide-Induced Alterations of Neurofilaments and Calpains Content in Rat Spinal Cord

To investigate the mechanism of carbon disulfide-induced neuropathy, male Wistar rats were randomly divided into two experimental groups and one control group. The rats in two experimental groups were treated with carbon disulfide by gavage at dosages of 300 and 500 mg/kg/day, respectively, five times per week for 12 weeks. Spinal cords of carbon disulfide-intoxicated rats and their age-matched controls were Triton-extracted and ultracentrifuged to yield a pellet fraction of neurofilament (NF) polymer and a corresponding supernatant fraction. Then, the contents of NF triplet proteins (NF-H, NF-M, NF-L) and two calpain isoforms (m-calpain and mu-calpain) in both fractions were determined by immunoblotting. In the meantime, the mRNA levels of NF-H, NF-M, and NF-L in spinal cords were quantified using reverse transcriptase-polymerase chain reaction. Results showed that in the pellet fraction, the contents of three NF subunits in both treated groups decreased significantly except NF-L in low dose group. In the supernatant fraction, the pattern of NFs alteration varied according to dose-levels. Compared to controls, three neurofilmant subunits in the high dose group displayed significant reduction consistently. However, in the low dose group, they remained unaffected. As for calpains, the contents of mu-calpain in both fractions increased significantly regardless of carbon disulfide dose-levels. Meanwhile, m-calpain demonstrated a significant decline in the supernatant fraction, and remained unchangeable in the pellet fraction compared to the control group. Furthermore, the levels of mRNA expression of NF-H, NF-M, and NF-L genes were elevated consistently in CS(2)-treated groups. These findings suggested that carbon disulfide intoxication was associated with obvious alterations of NFs content in rat spinal cord, which might be involved in the development of carbon disulfide neurotoxicity.

Carbon Disulfide-Induced Changes in Cytoskeleton Protein Content of Rat Cerebral Cortex

To investigate the mechanism of carbon disulfide-induced neuropathy, male wistar rats were administrated by gavage at dosage of 300 or 500 mg/kg carbon disulfide, five times per week for 12 weeks. By the end of the exposure, the animals produced a slight or moderate level of neurological deficits, respectively. Cerebrums of carbon disulfide-intoxicated rats and their age-matched controls were Triton-extracted and centrifuged at a high speed (100,000 x g) to yield a pellet fraction of NF polymer and a corresponding supernatant fraction, which presumably contained mobile monomer. Then, the contents of six cytoskeletal protein (NF-L, NF-M, NF-H, alpha-tubulin, beta-tubulin, and beta-actin) in both fractions were determined by immunoblotting. Results showed that the contents of the three neurofilament subunits in the pellet and the supernatant fraction decreased significantly regardless of dose levels (P<0.01). As for microtubule proteins, in the pellet fraction of cerebrum, the levels of alpha-tubulin and beta-tubulin demonstrated some inconsistent changes. However, in the supernatant fractions, the content of alpha-tubulin and beta-tubulin increased significantly in both two dose groups (P<0.01). In comparison to neurofilament and tubulin proteins, the content of beta-actin changed less markedly, only the supernatant fraction of the high dose group displayed significant increase (P<0.01), but the others remained unaffected. These findings suggested that the changes of cytoskeleton protein contents in rat cerebrum were associated with the intoxication of carbon disulfide, which might be involved in the development of carbon disulfide neurotoxicity.

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.