Histochemically demonstrable non-specific cholinesterase as an indicator of peripheral nerve lesion in carbon disulphide-induced polyneuropathy

Effect of propionic and methylmalonic acids on the in vitro phosphorylation of intermediate filaments from cerebral cortex of rats during development

In this study we investigated the in vivo and in vitro effects of methylmalonic (MMA) and propionic acids (PA), at concentrations usually found in methylmalonic acidemia and propionic acidemia respectively, on the phosphorylation of intermediate filament proteins in cerebral cortex of rats during development. Rats of 9, 12, and 17 days were acutely injected with the acids and sacrificed 90 min after injection. The cerebral cortex was dissected, and slices were incubated with 32P-orthophosphate. The cytoskeletal fraction was extracted and the radioactivity incorporated into intermediate filament subunits was measured. In addition, cortical slices from nontreated rats of 9, 12, 15, 17, 21, and 60 days of life were incubated with the acids in the presence of 32P-orthophosphate, the cytoskeletal fraction was extracted and the radioactivity was measured. Results demonstrated that MMA and PA significantly decreased the radioactivity incorporated into intermediate filament proteins at day 12, both in vivo and in tissue slices. In contrast, PA increased the in vitro phosphorylation of the cytoskeletal proteins in slices of 21-day-old animals. It acts through PP2A and PP2B in 12-day-old rats and through PKA and PKCaMII in 21-day-old animals. We propose that alteration of cytoskeletal protein phosphorylation caused by methylmalonic and propionic acids may be related to the neurological dysfunction characteristic of propionic and methylmalonic acidemia.

In vitro phosphorylation of cytoskeletal proteins in the rat cerebral cortex is decreased by propionic acid

In the present study we demonstrate that propionic acid (PA), a metabolite that accumulates in large amounts in propionic acidemia, is able to decrease in vitro incorporation of [32P]ATP into neurofilament subunits (NF-M and NF-L) and alpha- and beta-tubulin. Considering that the endogenous phosphorylating system associated with the cytoskeletal fraction contains cAMP-dependent protein kinase (PKA), Ca2+/calmodulin protein kinase II (CaMKII), and protein phosphatase 1 (PP1), we first assayed the effect of the acid on the kinase activities by using the specific activators cAMP and Ca2+/calmodulin or the inhibitors PKAI or KN-93 for PKA and CaMKII, respectively. Results demonstrated that the acid totally inhibited the stimulatory effect of cAMP and interfered with the inhibitory effect of PKAI. In addition, PA partially prevented the stimulatory effect of Ca2+/calmodulin and interfered with the effect of KN-93. In addition, we demonstrated that PA totally inhibited in vitro dephosphorylation of neurofilament subunits and tubulins mediated by PP1 in brain slices pretreated with the acid. Taken together, these results demonstrate that PA inhibits the in vitro activities of PKA, CaMKII, and PP1 associated with the cytoskeletal fraction of the cerebral cortex of rats. This study suggests that PA at the same concentrations found in tissues from propionic acidemic children may alter phosphorylation of cytoskeletal proteins, which may contribute to the neurological dysfunction characteristic of propionic acidemia.

Methylmalonic acid reduces the in vitro phosphorylation of cytoskeletal proteins in the cerebral cortex of rats

The present work was undertaken to determine the action of methylmalonic acid (MMA), a metabolite, which accumulates in high amounts in methylmalonic acidemia, on the endogenous phosphorylating system associated with the cytoskeletal fraction proteins of cerebral cortex of young rats. We demonstrated that pre-treatment of cerebral cortex slices of young rats with 2.5 mM buffered methylmalonic acid (MMA) is effective in decreasing in vitro incorporation of [32P]ATP into neurofilament subunits (NF-M and NF-L) and alpha- and beta-tubulins. Based on the fact that this system contains cAMP-dependent protein kinase (PKA), Ca2+/calmodulin-dependent protein kinase II (CaMKII) and protein phosphatase 1 (PP1), we first tested the effect of MMA on the kinase activities by using the specific activators cAMP and Ca2+/calmodulin or the inhibitors PKAI or KN-93 for PKA and CaMKII, respectively. We observed that MMA totally inhibited the stimulatory effect of cAMP and interfered with the inhibitory effect of PKAI. In addition, the metabolite partially prevented the stimulatory effect of Ca2+/calmodulin and interfered with the effect of KN-93. Furthermore, in vitro dephosphorylation of neurofilament subunits and tubulins was totally inhibited in brain slices pre-treated with MMA. Taken together, these results suggest that MMA, at the same concentrations found in tissues of methylmalonic acidemic children, inhibits the in vitro activities of PKA, CaMKII and PP1 associated with the cytoskeletal fraction of the cerebral cortex of rats, a fact that may be involved with the pathogenesis of the neurological dysfunction characteristic of methylmalonic acidemia.

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.

Effects of chronic carbon disulfide inhalation on sensory and motor function in the rat

Chronic carbon disulfide (CS2) exposure produces debilitating motor, sensory and neuropsychiatric consequences in humans. Sensory, especially auditory, tests have been considered for indexing early intoxication. This study examines effects of chronic CS2 exposure in rat upon auditory and neuromuscular function using reflex modulation audiometry to test the feasibility of using pure tone detection thresholds as such an index. This method is sensitive to the differential effects of toxicants upon acoustic and neuromuscular functioning. Rats were tested before, during and after five or 12 weeks of 500 ppm CS2 inhalation, six h/day, five days/week. Neuromuscular integrity, reflected by baseline startle amplitude, decreased 50% after five weeks and 67% following 12 weeks of CS2 exposure; recovery to 70% of preexposure values occurred on the fourth postexposure week. Twelve weeks of CS2 inhalation had no significant effect upon acoustic thresholds. Pure tone detection thresholds, therefore, do not appear adequate to index early CS2 exposure levels in the rat, as severe neuromuscular compromise occurred at a time when acoustic thresholds remained stable.

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

The present studies were carried out to investigate the comparative roles of protein cross-linking and alteration in protein phosphorylation in the accumulation of neurofilaments due to aliphatic hexacarbons. In these studies, rats were given 2,5-hexanedione (0, 0.1, 0.25 and 1.0%) for 70 days in their drinking water. In a separate study of in vitro protein phosphorylation rats were given 1% 2,5-hexanedione for 14 days in their drinking water. Spinal cord neurofilaments were isolated and analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting using anti-neurofilament antibodies, radioimmunoassays (RIAs) of phosphorylated epitopes on neurofilament proteins and protein phosphorylation. Protein cross-linking of neurofilaments was found in all animals treated with 2,5-hexanedione including the lowest dose (0.1%) which did not produce clinical signs of intoxication. Protein phosphorylation of neurofilament proteins, as well as MAP-2 was significantly decreased upon treatment. Protein staining revealed a decreased amount of neurofilament protein and immunoblotting demonstrated neurofilament protein cross-linking in these animals. Protein staining of glial fibrillary acidic protein (GFAP) was unaltered by this treatment. RIAs of phosphorylated and non-phosphorylated epitopes of neurofilament proteins indicated that in vivo phosphorylation of these proteins was also decreased. Two-dimensional gel electrophoresis indicated a shift of the neurofilament proteins to a basic pI, indicating a dephosphorylation of neurofilament proteins. Cross-linked neurofilament proteins also exhibited a pI which was more basic than any of the individual neurofilament proteins. This report demonstrates differential effects of 2,5-hexanedione on neurofilament proteins and indicates that several mechanisms may be responsible for their accumulation.

The influence of simultaneous exposure to carbon disulfide and hydrogen sulfide on the peripheral nerve toxicity and metabolism of carbon disulfide in rats

Three groups of 10 male Sprague-Dawley rats were exposed daily, 5 days a week for 25 weeks, either to 500 ppm carbon disulfide (CS2), 50 ppm hydrogen sulfide (H2S), or to both of them as a mixture and were periodically examined for sensory and motor tail nerve conduction velocity (SNCV, MNCV). A concomitant control group of 10 rats was used. In addition, rats exposed to 500 ppm CS2, and those simultaneously exposed to 500 ppm CS2 and 50 ppm H2S, were twice examined for 24-h urine excretion of 2-thio-thiazolidine-4-carboxylic acid (TTCA) in the course of the experimental period. Simultaneous exposure to CS2 and H2S had no significant interactive effect on nerve conduction velocities. A significant time-dependent slowing down of MNCV and SNCV occurred as the result of chronic exposure to CS2, including exposure to 500 ppm CS2 and to the mixture of 500 ppm CS2 and 50 ppm H2S, but did not occur after chronic exposure to 50 ppm H2S. With combined exposure to 500 ppm CS2 and 50 ppm H2S, the quantity of TTCA excreted in 24-h urine was not significantly different from that occurring in response to CS2 exposure alone. On the basis of these results it is suggested that chronic exposure to H2S would neither influence CS2-induced peripheral nerve toxicity nor obscure the interpretation of the measurement of urinary TTCA as a biological indicator of CS2 exposure.

Cross-linking of neurofilament proteins of rat spinal cord in vivo after administration of 2,5-hexanedione

The aliphatic hexacarbons n-hexane, methyl-n-butyl ketone, and 2,5-hexanedione are known to produce a peripheral neuropathy that involves an accumulation of 10-nm neurofilaments above the nodes of Ranvier in the spinal cord and peripheral nerve. In this study, rats were treated with 0.5% 2,5-hexanedione in drinking water for 180 days, and their spinal cord neurofilaments were isolated after development of the neuropathy. Visualization by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a significant reduction in content of the neurofilament triplet proteins in treated animals and the presence of bands migrating at 138K and 260K that were not present in control animals. Analysis of the lanes using immunoblotting procedures and anti-70K, anti-160K, and anti-210K neurofilament antibodies revealed many cross-linked peptides. The 138K band cross-reacted with the anti-160K neurofilament antibody. This suggests that the 138K band is an intramolecular cross-link of the 160K neurofilament subunit. In addition to this peptide, there were numerous high-molecular-weight peptides immunoreactive with all three neurofilament protein antibodies. In addition to cross-linking, there was also a diminished amount of immunoreactive breakdown product of all three neurofilament proteins. This report demonstrates direct evidence of 2,5-hexanedione-induced cross-linking of neurofilament proteins in vivo, which maybe responsible for the accumulation of neurofilament proteins pathognomic of this neuropathy.

Carbon disulfide neuropathy in rats. A morphological and ultrastructural study of degeneration and regeneration

The aim of this study was to elucidate the site and detailed nature of peripheral nerve damage induced in the rat by chronic CS2 inhalation exposure in the light of the relationship between pathological and neurophysiological data. Adult male rats were exposed to 700 ppm of CS2 2 h/d, 5 d/week for 12 weeks and then followed-up for 18 weeks. The first alteration observed was a decrease in the nerve conduction velocity, discovered after only 3 weeks of exposure. Pathological lesions were first observed in the 10th week and consisted of a typical "giant axon" axonopathy. Obvious pathological lesions of the myelin sheaths were revealed much later, in the 3rd week after the end of exposure, when some nerve fibers were dying back. Recovery took place with the regeneration of new fibers which started in the 8th week after the end of exposure and was nearly complete in the 18th week. These findings demonstrate that CS2-induced polyneuropathy is an axonopathy very similar to that caused by other occupational neurotoxic agents like MnBK, n-hexane, and acrylamide. The timing of the pathological events within the nerve fibers suggests that the pathogenesis of the nerve lesions should probably be attributed to a primary energy failure of the axonal membrane induced by CS2.

Some aspects of the mechanisms by which industrial solvents produce neurotoxic effects

A Short Review is given of the biochemical neurotoxic mechanisms of common industrial solvents of various structures. The acute effects of organic solvents may be caused by their direct action on the nerve cell membrane and energy metabolism, whereas chronic neurotoxic effects may be explained by the formation of chemically and biologically reactive intermediates. The stability of the toxic metabolites appears to correlate with the development of neuropathy although this is structuredependent. The role of reactive intermediates is also emphasized by the potentiating effect of the induction of the cytochrome P-450 complex.

The binding of CS2 in central nervous system of control and phenobarbitone-pretreated rats

The binding of 35S- and 14C-labelled CS2 in rat central nervous system (CNS) was studied in control and phenobarbitone-pretreated rats in vivo and in vitro. Animals received CS2 through intraperitoneal injection in olive oil. Samples were taken for analysis 3 and 6 h after the injection. Sulphur atoms were bound to rat brain more highly than carbon atoms in control and phenobarbitone-pretreated rats in vivo. The phenobarbitone pretreatment increased the cerebral binding of sulphur and decreased that of carbon. Main part of the bound sulphur and carbon was detected in the trichloroacetic acid (TCA) precipitable fraction in both test groups. Pretreatment with phenobarbitone or with polychlorinated hydrocarbon (PCB) mixture did not increase significantly the binding of CS2 sulphur in brain microsomes in vitro. The present findings suggest that a considerable amount of injected CS2 is retained in the nervous system and that phenobarbitone pretreatment of test subjects may also alter brain metabolism of CS2.

Effects of acute CS2 intoxication on protein metabolism in rat brain

The effect of acute CS2 exposure on the rat brain protein metabolism was studied with control and phenobarbitone pretreated adult male rats 1, 4 and 46 h after exposure. Increased activity of acid proteinase was detected in both test groups 1 and 4 h after exposure and it was accompanied by changes in 14C-labelled leucine turnover as well as in RNA content. The changes were more conspicuous in cerebellum than in brain in both test groups while phenobarbitone pretreatment modified the brain response towards intoxication. This modification probably represents inherent effects of barbiturate on brain protein metabolism as well as altered metabolism of CS2. The activities of creatine kinase and nonspecific cholinesterase displayed only subtle changes as assayed in cerebral homogenate and serum. Thus a single acute CS2 intoxication apparently causes definitive transient changes in brain protein metabolism; serum enzyme determinations may not reflect the magnitude of these changes.

CS2 binding to rat spinal neurofilaments

The binding of CS2 sulphur to rat spinal cord neurofilaments was studied 3 and 6 h after the intraperitoneal injection of 1.3 mmol of CS2 in control and phenobarbitone pretreated rats. The binding of CS2 carbon was studied similarly 3 h after the injection. The binding of CS2 sulphur was 31% higher in the control neurofilaments than that in the phenobarbitone pretreated group. Half of the neurofilament associated CS2 sulphur was bound to protein in both groups as studied by electrophoresis. Approximately 30% of the neurofilament associated CS2 sulphur was detected 6 h after the injection of that at 3 h in both groups. All of the neurofilament associated CS2 carbon was bound to protein in control group 3 h after the injection. The binding of CS2 carbon was 4.7 times less than that of sulphur at the same time. The present data may indicate that the high binding of CS2 to the neurofilament protein contributes to the increase in neurofilaments in chronic CS2 intoxication.

High binding of CS2 sulphur in spinal cord axonal fraction

The binding of carbon disulphide sulphur was studied in the spinal cord and its axons of four control and four phenobarbitone pretreated adult rats 3 and 6 h after an intraperitoneal injection of 650 mumol of CS2 in olive oil. The binding of CS2 carbon was measured in the same fractions of two adult control rats 4.5 h after a similar administration of 1.3 mumol of the compound for reference. The specific binding of sulphur was highest in the axons of control animals 3 h after the injection while binding was 17.5% smaller in the axons of phenobarbitone treated animals. The uptake of sulphur was higher in the spinal cord homogenate of the pretreated animals in comparison to control rats. Sulphur was removed from the axonal fraction at a rate of 5.1 natoms/mg of protein/h. Phenobarbitone treatment increased the disappearance of sulphur in the control animals while the rate in the treated rats was 4.9 natoms/mg of protein/h. Phenobarbitone treatment increased the disappearance of sulphur in the spinal homogenate from the removal rate of 0.22 natoms/mg of protein/h in control animals to 0.37 natoms/mg of protein/h in treated rats. The binding of CS2 carbon was negligible 4.5 h after the injection in the axonal fraction. The present data indicate that the release and binding of CS2 sulphur may be responsible for the toxic neural manifestations in chronic CS2 poisoning.