Tellurium-Induced Neuropathy: Metabolic Alterations Associated with Demyelination and Remyelination in Rat Sciatic Nerve

Rats fed a diet containing 1.25% elemental tellurium initiated on postnatal day 20 undergo a transient neuropathy characterized by synchronous demyelination of peripheral nerves. In sciatic nerve, the extent of demyelination was maximal after 5 days of tellurium exposure; there was a loss of 25% of the myelin, as assayed by concentration of myelin-specific P0 protein. Tellurium-induced alterations in the metabolic capacity of Schwann cells were examined by measuring the synthesis of myelin lipids in vitro in isolated sciatic nerve segments. Exposure to tellurium resulted in an early marked decrease of approximately 50% in overall incorporation of [14C]acetate into lipids, with a preferential depression in synthesis of cerebrosides, cholesterol, and ethanolamine plasmalogens (components enriched in myelin). Most dramatically, within 1 day of initiation of tellurium exposure, there was a profound increase in [14C]acetate-derived radioactivity in squalene; 23% of incorporated label was in this intermediate of cholesterol biosynthesis, compared to less than 0.5% in controls. In association with the remyelinating phase seen after 5 days of tellurium exposure, synthesis of myelin components gradually returned to normal levels. After 30 days, metabolic and morphologic alterations were no longer apparent. We suggest that the sequence of metabolic events in sciatic nerve following tellurium treatment initially involves inhibition of the conversion of squalene to 2,3-epoxysqualene, and that this block in the cholesterol biosynthesis pathway results, either directly or indirectly, in the inhibition of the synthesis of myelin components and breakdown of myelin.     

Soluble and Participate Forms of the Organophosphorus Neuropathy Target Esterase in Hen Sciatic Nerve

Neuropathy target esterase (NTE) is the suggested "target" molecule involved in the initiation of organophosphorus-induced delayed polyneuropathy. Sciatic nerve NTE was separated into particulate (P-NTE) and soluble (S-NTE) fractions by ultracentrifugation at 100,000 g for 1 h in 0.32 M sucrose and compared with the corresponding brain extract. Total sciatic NTE activity was 80-100 nmol/min/g tissue from which 50-60% was recovered in the soluble supernatant fraction and the remaining 40-50% in the pellet fraction. About 90% of brain tissue activity (approximately 1,800 nmol/min/g tissue) was recovered as P-NTE. A similar distribution was obtained when more drastic centrifugation without sucrose was performed. P-NTE and S-NTE were distributed with the membrane and cytosolic markers assayed, respectively, glucose-6-phosphatase, Na+,K(+)-ATPase, 5'-nucleotidase, phospholipids, and lactate dehydrogenase. When the pH during the centrifugation was increased from 6.4 to 11, recovered P-NTE activity decreased from 1,750 to 118 nmol/min/g tissue for brain and from 31 to 12 nmol/min/g for sciatic nerve. However, S-NTE activity and total nonfractionated control activity were only slightly affected by the same pH treatment. The distribution pattern encountered may be better understood as representing two different proteins than an equilibrium between soluble and membrane-bound portions of a single protein, with P-NTE activity depending on a membrane factor from which it is separated through fractionation at high pH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of Nigella sativa and its Major Constituent, Thymoquinone on Sciatic Nerves in Experimental Diabetic Neuropathy

The aim of this study was designed to investigate the possible beneficial effects of Nigella sativa (NS) and thymoquinone (TQ) on histopathological changes of sciatic nerves in streptozotocin-induced diabetic rats. The rats were randomly allotted into one of four experimental groups: A (control), B (diabetic untreated), C (diabetic treated with NS) and D (diabetic treated with TQ); each group contain ten animals. B, C and D groups received streptozotocin (STZ) to induce diabetes. The rats in NS and TQ treated groups were given NS (in a dose of 400 mg/kg body weight) and TQ (50 mg/kg body weight) once a day orally by using intra-gastric intubation for 12 weeks starting 2 days after STZ injection, respectively. Blood and tissue samples were obtained for biochemical and histopathological investigation. The treatment of both NS and TQ caused a sharp decrease in the elevated serum glucose (P < 0.01, 0.05, respectively), and an increase in the lowered serum insulin concentrations (P < 0.01, 0.05, respectively), in STZ-induced diabetic rats. STZ induced a significant decrease in the area of insulin immunoreactive β-cells (P < 0.0001). NS (P < 0.001) and TQ (P < 0.01) treatment resulted in increased area of insulin immunoreactive β-cells significantly. To date, no histopathological changes of sciatic nerves in STZ induced diabetic rats by NS and TQ treatment have been reported. In this study, histologic evaluation of the tissues in diabetic animals treated with TQ and especially NS showed fewer morphologic alterations. Myelin breakdown decreased significantly after treatment with NS and TQ. The ultrastructural features of axons also showed remarkable improvement. We believe that further preclinical research into the utility of NS and TQ may indicate its usefulness as a potential treatment on peripheral neuropathy (PN) in STZ induced diabetic rats.     

Purification and Characterization of Naturally Soluble Neuropathy Target Esterase from Chicken Sciatic Nerve by HPLC and Western Blot

Abstract: Neuropathy target esterase (NTE) activity is defined operatively as the paraoxon-resistant mipafox-sensitive phenyl valerate esterase activity. A preparation containing a soluble isoform (S-NTE2) has been obtained from sciatic nerve. It was inhibited by the biotinylated organophosphorous ester S9B [1-(saligenin cyclic phospho)-9-biotinyldiaminononane] in a progressive manner showing a second-order rate constant of (3.50 ± 0.26) × 10⁶ M ⁻¹· min⁻¹ with an I₅₀ for 30 min of 6.6 ± 0.4 n M . S-NTE2 was enriched 218-fold by gel filtration followed by strong and weak anion-exchange chromatographies in HPLC. In western blots, this enriched sample showed two bands of endogenous biotinylated polypeptides after treating the blots with streptavidin-alkaline phosphatase complex. When the sample was treated with S9B, another biotinylated band was observed with a molecular mass of ∼56 kDa, which was not seen when the sample had been pretreated with mipafox before the S9B labeling. It was deduced that this band represents a polypeptide (identified as the S-NTE2 protein) that is bound by both mipafox and S9B and that should be responsible for the progressive S9B inhibition. It is possible that S-NTE2 is the target for attack by compounds that promote delayed neuropathy.     

Decreased Metabolism of Cerebrosides and Sulfatides in Rat Sciatic Nerve After Intraneural Injection of Colchicine

To obtain an understanding of the importance of the neuronal cytoskeleton in Schwann cell metabolism, an antimicrotubular agent (colchicine) was injected into the rat sciatic nerve 24 or 48 h before incubation of the nerve with labeled precursor: [35S]sulfate, [14C]galactose, or [3H]-galactose. Colchicine inhibited the incorporation of 35S radioactivity into sulfatides and, to a lesser extent, into proteins. With galactose as the radioactive precursor, synthesis of cerebrosides was reduced by colchicine injection, whereas incorporation of radioactivity into phosphatidylserine and phosphatidylcholine increased. Intraneural injection of lumicolchicine had no effect. The effects of colchicine on the metabolism of the Schwann cell are discussed in relation to its action on microtubules.     

Soluble and Particulate Organophosphorus Neuropathy Target Esterase in Brain and Sciatic Nerve of the Hen, Cat, Rat, and Chick

Considerable evidence exists suggesting that the so-called neuropathy target esterase (NTE) is involved in the mechanisms responsible for organophosphorus-induced delayed polyneuropathy (OPIDP). Earlier studies in the adult hen, the habitually employed experimental model in OPIDP, have shown that most NTE activity in the brain is centered in paniculate fractions, whereas approximately 50% of this activity in the sciatic nerve is encountered in soluble form, with the rest being paniculate NTE. In the present work, we have studied the paniculate and soluble fractional distribution of paraoxon-resistant phenylvalerate esterase activity (B activity), parabxon- and mipafox-resistant phenylvalerate esterase activity (C activity), and NTE activity (B - C) according to ultracentrifugation criteria (100,000 g for 1 h). To this effect, two sensitive (adult hen and cat) and two scarcely sensitive (rat and chick) models were used. In all four experimental models, the distribution pattern was qualitatively similar: B activity and total NTE were much greater in brain (900–2, 300 nmol/min/g of tissue) than in sciatic nerve (50–100 nmol/min/g of tissue). The proportion of soluble NTE in brain was very low (<2%), whereas its presence in sciatic nerve was substantial (30–50%). The NTE/B ratio in brain was high for the particulate fraction (>60%) and low in the soluble fraction (7–30%); in sciatic nerve the ratio was about 50% in both fractions. Slight quantitative differences were observed in terms of OPIDP sensitivity: the proportion of soluble NTE in sciatic nerve was slightly higher in the sensitive animals (hen and cat: 49 and 44%, respectively) than in the rat and chick (41 and 37%, respectively), although no differences were noted in terms of concentration (in nanomoles per minute per gram of tissue). It is concluded that the distribution pattern of the activities studied is similar in all four experimental models, with no important quantitative differences directly related to species sensitivity or age.