The experimental neuropathy in rats caused by p-bromophenylacetylurea

NTE: one target protein for different toxic syndromes with distinct mechanisms?

Epidemics of organophosphate-induced delayed neuropathy (OPIDN) have paralysed thousands of people. This syndrome of nerve axon degeneration is initiated by organophosphates which react with neuropathy target esterase (NTE). Dosing experiments with adult chickens raise the possibility that OPIDN is initiated by a gain-of-function mechanism. By contrast, loss of NTE function by mutation causes massive apoptosis in Drosophila brain. Now, Winrow et al. show that nte(-/-) mice die by mid-gestation, but nte(+/-) mice appear hyperactive and are more sensitive than wild-type mice to a fatal form of OP toxicity. Thus, different toxic syndromes may be initiated via a single target protein.

Absorption, distribution, metabolism and excretion of p-bromophenylacetylurea in the female rat

1. This study has investigated absorption, distribution, metabolism and excretion of p-bromophenylacetylurea (BPAU) in the F344 female rat. BPAU and its metabolites were determined by HPLC. 2. Following a single p.o. dose of 150 mg/kg BPAU, the absorbed fraction of dosed BPAU was 65.9% and its half-life in the blood was 9.4 h. The relative distribution of BPAU (tissue/serum ratio) at 6 h (peak time point) after a single i.p. dose of 150 mg/kg BPAU was spinal cord (4.6+/-0.2) > liver (3.7+/-0.1) > brain (2.9+/-0.1) (mean+/-SD, n = 5), and they were significantly different from each other (p < 0.05). BPAU in spinal cord reached the highest level. 3. Absorbed BPAU was metabolized in vivo into three major metabolites. N'-hydroxy-p-bromophenylacetylurea (M1) was a dominant metabolite in tissues, whereas 4-(4-bromophenyl)-3-oxapyrrolidine-2,5-dione (M2) reached a high concentration in blood. N'-methyl-p-bromophenylacetylurea (M3) was mainly found in the urine. All three metabolites were excreted via the urine and together accounted for 87% of absorbed BPAU. 4. This study provides a basic understanding of BPAU absorption, distribution, metabolism and elimination in rat.

Application of the high-performance liquid chromatographic method for separation, purification and characterisation of p-bromophenylacetylurea and its metabolites

This study presents a HPLC method for the separation and purification of p-bromophenylacetylurea (BPAU) and its metabolites. The method effectively separated and purified BPAU and its metabolites. Three metabolites of BPAU, M1, M2 and M3 were characterised by mass spectroscopy and nuclear magnetic resonance. They are named as N'-hydroxy-p-bromophenylacetylurea, 4-(4-bromophenyl)-3-oxapyrrolidine-2,5-dione and N'-methyl-p-bromophenylacetylurea, respectively. The major metabolic pathways of BPAU were proposed. The establishment of the HPLC method and characterisation of BPAU metabolites make it possible for further pharmacokinetic studies to explore the mechanism of BPAU-induced delayed neuropathy.

Neurotoxic potentiation is related to a metabolic interaction between p-bromophenylacetylurea and phenylmethanesulfonyl fluoride

This study investigated the neurotoxic potentiation and metabolic interaction between p-bromophenylacetylurea (BPAU) and phenylmethanesulfonyl fluoride (PMSF). The results showed that F344 rats given two successive daily doses of 150 mg/kg BPAU developed a moderate degree of ataxia. When rats were coadministrated a single intraperitoneal dose of 100 mg/kg PMSF either 1 day before, or 4 h or 1 day after the two daily doses of BPAU, the severity of ataxia was significantly increased. No such effect was observed when PMSF was given 4 days after BPAU, although this time point was still prior to the development of the neuropathy. The enhancement or potentiation of neuropathy by PMSF was thus seen only at times when parent BPAU was present in the target tissues. A pharmacokinetic study showed that PMSF increased the concentrations of BPAU and its metabolite, N'-hydroxy-p-bromophenylacetylurea (M1), in tissues and decreased the concentration of the metabolite 4-(4-bromophenyl)-3-oxapyrrolidine-2,5-dione (M2) in serum. This indicated that PMSF inhibited the M2 pathway and more BPAU was metabolized via the M1 pathway. This increased both BPAU and M1 levels in tissues and hence would have increased BPAU-induced neurotoxicity. We conclude that PMSF does not need to act directly on target sites to potentiate BPAU-induced neurotoxicity, since its interference with BPAU metabolism was sufficient to account for the increase in BPAU neurotoxicity. Thus a metabolic interaction underlies the neurotoxic potentiation between these two compounds rather than the target site interaction seen between PMSF and neuropathic organophosphates. This study is the first to demonstrate that interference with the metabolism of BPAU is an important aspect of the potentiation of BPAU-induced neurotoxicity.

Amyloid beta precursor protein and ubiquitin epitopes in human and experimental dystrophic axons. Ultrastructural localization

Dystrophic axons (DA) represent a major pathological feature of several neurodegenerative disorders, including infantile neuroaxonal dystrophy (INAD) and Alzheimer disease. We have previously presented evidence that amyloid beta precursor protein (BPP) and ubiquitin (Ub) are present in DA of different origin. We have now characterized the immunoreactivity of DA experimentally induced in rat by the administration of parabromophenylacetylurea (BPAU) and examined the subcellular localization of Ub and BPP in BPAU-induced DA and in DA present in subjects affected by INAD. BPAU-induced DA strongly immunoreacted with antisera to Ub and to COOH- and NH2-terminal regions of BPP. Immunoblots of DA-enriched brain regions were consistent with an increase in the amount of Ub and BPP in DA. Moreover, BPAU-induced DA immunoreacted with antibodies to PGP 9.5, a neuronal-specific Ub COOH-terminal hydrolase, and to the inducible heat shock protein 70. Antigenic characterization also indicated that the tubulovesicular membranes within DA derived largely from the smooth endoplasmic reticulum rather than from the Golgi system or the synaptic vesicles. Subcellular immunolocalization of Ub and BPP in both INAD- and BPAU-induced DA revealed that Ub and BPP colocalize in granulovesicular material in both conditions. In INAD DA intense Ub immunoreactivity was also detected in nonmembranous electron dense structures that were present only in these DA, probably because of the chronic course of INAD. Although BPP immunostaining may be related to accumulation of BPP-containing membranes in DA, Ub immunostaining is likely to result from activation of the Ub system by the neuron in the attempt to remove excessive and possibly abnormal proteins. A similar pathogenesis can be postulated for DA of Alzheimer disease.

A paradigm for examining toxicant effects on viability, structure, and axonal transport of neurons in culture

N1E.115 murine neuroblastoma cells differentiating in serum-free medium were used to develop a paradigm for testing neurotoxicity in vitro. The paradigm was designed to test the effects of toxicants on four different aspects of cell function or structure: 1. Viability as shown by the retention of cellular radiolabel (51Cr); 2. Growth and maintenance of neurites as reflected by the incidence and average length of these processes; 3. Gross structure of neurites; and 4. Velocity and flux of rapid anterograde and retrograde axonal transport as judged by video-enhanced differential interference contrast microscopy. To evaluate this paradigm, colchicine and vinblastine were used as neurotoxicants with a well-understood mechanism of action. These agents were only weakly cytotoxic according to the Cr-release assay, but were able to interfere with neurite outgrowth at nanomolar concentrations. Neurites that were elaborated in the presence of vinblastine and colchicine were often disfigured by numerous swellings packed with organelles. In established neurites, micromolar concentrations of vinblastine inhibited organellar motility with great rapidity, blocking all signs of transport within 20 min. The effect of colchicine was slower and less complete, but still impressive. We suggest that this four-part analysis represents a highly sensitive in vitro test for neurotoxicity, and a means of analyzing the relation between abnormalities of transport and structural damage of nerve cells.

Tubulomembranous lesions in p-bromophenylacetylurea neuropathy reflect local stasis of fast axonal transport: evidence from electron microscopic autoradiography

The source of the membranous materials that accumulate in distal axons of rats intoxicated with p-bromophenylacetylurea (BPAU) was studied by electron microscopy. 35S-Methionine was injected into the ventral horn of the spinal cord at 2, 14, and 35 days after injection of BPAU. Three days later, samples of the deep peroneal nerves were obtained, and autoradiographs of thin cross sections were prepared. Organellar accumulations were absent from vehicle-treated control nerves and rare in the clinically latent period after administration of BPAU. In later stages of neuropathy, approximately 20% of the myelinated axons in any specific section were swollen and packed with tubules, membranes, and mitochondria. Numerous silver grains were located over the accumulated organelles, and the coincidence was statistically significant. The results indicate a sporadic local stasis of fast-transported proteins and provide a plausible explanation for axonal damage in BPAU neuropathy.

Premature onset of fast axonal transport in bromophenylacetylurea neuropathy: an electrophoretic analysis of proteins exported into motor nerve

To test whether abnormal processing of proteins for fast axonal transport is involved in the neuropathy induced by BPAU (p-bromophenylacetylurea) we examined transport onset. [35S]Methionine was injected into the lumbar ventral horn of rats 2 weeks after BPAU, 400 mg/kg (i.p.) or vehicle. At intervals of 30-90 min consecutive 3-mm segments of the L4 and L5 ventral roots were digested for polyacrylamide gel electrophoresis and fluorography. Fast transported proteins were identified by comparison with samples from mid-thigh sciatic nerve ligated for 16 h after radiolabeling. A prominent 26 kDa band represented the earliest exported protein. It was usually absent at 30 min, but it entered the roots by 45 min. This band was consistently displaced further in BPAU nerve (n = 11) than in controls (n = 11). The mean difference was 5 +/- 0.6 mm (P less than 0.001). However, there was no difference in the apparent velocity of transport. These results imply premature onset of transport in BPAU neuropathy.

Reduced anterograde and retrograde accumulation of axonally transported phosphofructokinase in streptozotocin-diabetic rats: effects of insulin and the aldose reductase inhibitor 'Statil'

This study examined anterograde and retrograde accumulation of axonally transported 6-phosphofructokinase activity, proximal and distal to sciatic nerve constrictions, in rats with streptozotocin-induced diabetes of 4 weeks' duration. There were deficits in accumulation on both sides of the constriction in untreated diabetic rats (proximal accumulation 66% of controls, p less than 0.05; distal accumulation 32% of controls, p less than 0.01). There was also a reduction in the phosphofructokinase activity per unit length unconstricted sciatic nerve in the untreated diabetic rats (87% of controls, p less than 0.05). Treatment of an age-matched group of diabetic rats with twice-daily insulin prevented all the above changes. There were significant increases, over untreated diabetic rats, in phosphofructokinase activity accumulated at constrictions (p less than 0.01 for both proximal and distal) and in unconstricted nerve (p less than 0.05). Indeed the activities measured in insulin-treated diabetic rats were virtually identical to those of controls. Treatment of a third group of diabetic rats with the aldose reductase inhibitor 'Statil' prevented or attenuated accumulations of polyol pathway metabolites and prevented depletion of myo-inositol in the sciatic nerve. In spite of these indications of effective aldose reductase inhibition, the drug was without effect on the deficits in accumulation of activity at ligatures or unconstricted nerve levels of phosphofructokinase activity. We conclude that short-term experimental diabetes in rats induced defects in both anterograde and retrograde axonal transport of 6-phosphofructokinase activity. These defects were prevented by intensive insulin treatment but were resistant to an effective aldose reductase inhibitor, indicating a lack of involvement of polyol pathway flux in their pathogenesis.

Neurotoxicity of halogenated phenylacetylureas is linked to abnormal onset of rapid axonal transport

A structure-activity study was performed to investigate the mechanism of neurotoxicity induced in rats by treatment with p-bromophenylacetylurea (BPAU). Phenylacetylurea and 7 derivatives were tested for their ability to induce hindlimb weakness after twice weekly administration in doses of 200 mg/kg, up to a cumulative maximum of 2000 mg/kg. In this test, BPAU and its chloro- analog were about equipotent, but none of the other analogs displayed any evidence of neurotoxicity. Since BPAU toxicity was believed to involve abnormalities in rapid axonal transport, selected analogs were examined in a transport experiment. None of the compounds led to alterations in the maximal rate of rapid anterograde transport, as measured after intraspinal injections of [35S]methionine in rats treated with 400 mg/kg of toxicant, 7 days earlier. However, both BPAU and its chloro- analog caused marked shortening of the delay between isotope injection and transport onset, an effect not seen with either of the two non-neurotoxic analogs tested. It is hypothesized that the accelerated transport onset is a key step in development of the neuropathy, possibly causing organelle abnormalities that interfere with turnaround and recirculation of transported particles.

A proposal for a classification of neuropathies according to their axonal transport abnormalities

Recent studies on axonal transport in experimental neuropathy are reviewed and the following combinations of pathological changes and underlying axonal transport abnormalities are proposed for a classification of polyneuropathies. Alterations of the anterograde transport of slow component a(SCa) leads to changes of the dimensions of the axon calibre without the occurrence either of overt neuropathy or fibre loss. Thus damming of SCa in beta,beta'-iminodiproprionitrile (IDPN) intoxication results in axonal swelling in nerve roots whereas decrease of SCa leads to atrophy distal to the swellings in IDPN intoxication and in streptozotocin induced diabetes as well. Decrease in the amount of material conveyed within the anterograde fast component (aFC) leads to acute axonal degeneration including break down of axons and fibre loss. This state occurs in acute hypoglycaemia and in doxorubicin intoxication. The most frequent type of polyneuropathy, namely distal axonopathy with accumulation of axon organelles leading to distal fibre loss, is associated with decrease in amount of the retrograde fast component (rFC). The transport is impaired before the appearance of symptoms and electrophysiological signs of neuropathy develop in the intoxications induced by parabromophenylacetylurea, acrylamide and 2.5 hexanedione, and the severity of neuropathy is proportional to the rFC impairment.

Axonal transport in the motor neurons of rats with neuropathy induced by p-bromophenylacetylurea

Axonal transport was studied in sciatic motor neurons of rats with neuropathy induced by p-bromophenylacetylurea (BPAU) in dimethylsulfoxide solution. Control rats were treated with the vehicle alone. To label rapidly transported proteins, the rats received an injection of 35S-methionine into the ventral horn of the spinal cord at the L1 vertebral level. Radiolabeled protein was collected at ligatures applied on the sciatic nerve at intervals thereafter. In animals with severe motor weakness owing to treatment with BPAU, 400 mg/kg, there was evidence of increased delivery of labeled protein into the axon during the early period after isotope injection, but reduced delivery later. A dose-dependent decrease in the amount of labeled protein recirculated by retrograde axonal transport was also noted. A significant reduction in the amount of protein transported retrogradely was also detected during the latent subclinical phase of the neuropathy. The velocity of rapid anterograde transport, examined in unligated sciatic nerves, was unaffected by BPAU treatment. However, the lag time between precursor injection and the onset of transport was shorter in BPAU-treated rats than in controls. This effect was not explainable on the basis of fluctuations in core body temperature. The results are consistent with the view that disturbances of rapid anterograde and retrograde transport play a role in the peripheral neurotoxicity of BPAU. Attention is directed to the possibility that the transport disturbances and the subsequent neuropathy are related to alterations in the processing of rapidly transported membrane-limited organelles in the nerve cell bodies.

Unimpaired energy metabolism in experimental neuropathy induced by p-bromophenylacetylurea

The contribution of defective energy metabolism to the induction of neuronal pathology by p-bromophenylacetylurea (BPAU) was examined in several ways. It was found that a saturated aqueous solution of BPAU had no effect on the activity of crystalline glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or phosphofructokinase (PFK). In rats with total hindlimb paralysis from treatment with BPAU (400 mg/kg), the endogenous GAPDH and PFK of sciatic nerve showed normal activity. Endogenous enolase and nerve-specific enolase activities were likewise unaffected. Consequently, it appeared improbable that BPAU neuropathy involves impaired glycolysis. This conclusion was supported by the failure to prevent hindlimb weakness by feeding pyruvate, a substrate for the Krebs cycle. To test for interference with glycolysis at other steps, or for an impairment in oxidative phosphorylation, adenosine triphosphate (ATP) and creatine phosphate were measured. The amounts of high energy phosphates in nerves of paralyzed animals were found to be the same as in nerves of untreated and vehicle-treated controls. A similar observation was made in nerves regenerating from a crush injury. To test turnover, ATP and creatine phosphate were measured in nerves exposed to an N2 atmosphere in vitro. Since the high energy phosphates disappeared at the same rates in all groups, it was concluded that BPAU neuropathy does not alter energy utilization. In our view, BPAU neuropathy arises by a mechanism that does not depend on altered energy metabolism.

Morphometric evaluation of degenerative and regenerative changes in isoniazid-induced neuropathy

Morphometric studies of the pathologic changes were carried out on the peripheral nerves, spinal roots, and different levels of the Goll's tract in rats given isoniazid and killed 1, 2, 3, 4, 5, 6, 7, 14, and 30 days after intoxication. In teased fiber preparations, axonal degeneration was the main change present, and this was seen as early as day 2 in the peroneal and distal sural nerves. The frequency of myelinated fibers showing axonal degeneration was higher in the distal than the proximal sural nerve, and in the ventral than the dorsal root. In the group of rats killed on 5, 6, 7, and 14 days, a significant decrease of the myelinated fiber density was observed in the distal and proximal sural nerves, ventral root, and at the third cervical level of the Goll's tract. The degree of fiber degeneration was more severe in the distal than in the proximal sural nerve and in the third cervical than the fifth thoracic level of the Goll's tract. Preferential decrease of large myelinated fibers was noted in all the affected nerves. No definite abnormalities, however, were seen in nerve cells of the 6th lumbar spinal ganglia and anterior horn cells of the lumbar spinal cord on light microscopy. On 30 days, regeneration at varying degrees was discerned in all the affected nerves with significant increase of small myelinated fibers, particularly in the ventral root. The findings indicate that both centrally and peripherally directed myelinated axons are more affected in the distal than in the proximal segments while the neuronal cell bodies are spared. The spatio-temporal evolution of this pattern of change is compatible with the concept of the "dying back" process or central-peripheral distal axonopathy.

Axonal transport of enzymes and labeled proteins in experimental axonopathy induced by p-bromophenylacetylurea

Axonal transport was studied by several techniques in the sciatic nerves of adult male Sprague-Dawley rats with neuropathy induced by treatment with p-bromophenylacetylurea (BPAU) in dimethylsulfoxide solution. Control rats were treated with solvent alone. BPAU, 200 mg/kg, induced severe muscle weakness in the hindlimbs, beginning after a latent period of 1 week and progressing to near total paralysis by 2 weeks. Axonal transport of the endogenous transmitter enzymes, acetylcholinesterase, dopamine-beta-hydroxylase and choline acetyltransferase, was normal at both 2 and 15 days after administration of BPAU, as judged by the accumulation of enzyme activity above and below a set of double ligatures on the sciatic nerve. The velocity of fast anterograde transport of [35S] methionine labeled protein was also unaffected by BPAU. However, 4 abnormalities of transport were detected in BPAU- treated rats: (1) doubling of the time for initiation of fast anterograde transport after precursor injection in the dorsal root ganglion, (2) 25% fall in the velocity of slow axonal transport of [3H] leucine labeled protein, (3) 30% reduction in the proximal accumulation of fast transported labeled protein in ligated nerve, 8-30 h after injection of precursor, and (4) 50-60% reduction in distal accumulation of "early arriving" labeled protein, 8-14 h after precursor injection. The last abnormality, suggesting an impaired turnaround from anterograde to retrograde transport, was detected as soon as 2 days after BPAU administration. The turnaround abnormality was correlated with the severity of neuropathy as estimated by independent clinical scoring in the group of rats treated with 200 mg/kg of drug. However, further studies showed that turnaround was delayed even in rats treated with doses as low as 50 mg/kg, which never led to clinically evident neuropathy. Nevertheless it is proposed that the abnormalities of transport play a role, as yet undefined, in the distal axonopathy caused by BPAU.

Morphometric evaluation of primary sensory neurons in experimental p-bromophenylacetylurea intoxication

To evaluate the three-dimensional pathology of lumbar primary sensory neurons in p-bromophenylacetylurea intoxication, the number and size distribution of neurons and of myelinated fibers were evaluated at the L-6 spinal ganglion level and at proximal and distal levels of sural nerve and thoracic (proximal) and cervical (distal) levels of Goll's tract, respectively, 2 and 6 weeks after the intoxication in rats. The number and size distribution of ganglion neuron cell bodies were not significantly different between intoxicated and control rats. The distal level of sural nerve had significantly fewer large myelinated fibers than did control, and a significantly higher frequency of fibers undergoing degeneration. Proximal levels of sural nerve showed similar, but less severe changes. Similarly, the myelinated fibers of Goll's tract were significantly more affected at cervical than at thoracic level. Therefore, by morphometric criteria both centrally and peripherally directed myelinated fibers are most affected distally and less affected proximally while neuron cell bodies are not affected at all. These three-dimensional morphological changes must be taken into consideration in formulating possible mechanisms for the development of this neuropathy.

Spheroids and altered axons in the spinal gray matter of the normal cat. An electron-microscopic study

In our recent ultrastructural studies on synapses of the nucleus dorsalis, central cervical nucleus, and anterior horn of the spinal cord of the normal cat we happened to find spheroids and several types of axonal alterations. These spheroids were up to 39 micrometer in diameter. They were found in myelinated and unmyelinated terminal axons and at the node of Ranvier and showed two different types of internal structure. One type was large and composed of spirally arranged neurofilaments and mitochondria in increased quantity, although the mean population density of mitochondria was not high being 1.4/micrometer2 as compared to the normal value 2.0/micrometer2. Another type was smaller and consisted of small mitochondria and dense bodies which were increased in number: their mean population densities were 4.5/micrometer2 and 1.9/micrometer2, respectively. At present, the rare occurrence of spheroids and atypical axonal alterations makes it difficult to determine the origin of affected axons, although some of them presumably arise from primary afferents.

Factors affecting neurotoxicity by p-bromophenylacetylurea in rats

Adult rats more than 9-10 weeks old are more susceptible to neurointoxication by p-bromophenylacetylurea than young, more actively growing rats. There is no difference in sensitivity between the sexes.

Partial hepatectomy and damaging the liver with carbon tetrachloride before administering pBPAU are procedures which enhance the degree of neurotoxicity. These studies suggest that the substance is toxic per se, and that it is probably detoxicated by the liver.

Animals treated with sodium phenobarbital both before giving pBPAU and afterwards showed a slightly increased degree of neurointoxication. Pretreatment only with sodium phenobarbital before giving pBPAU did not alter the degree of neurotoxicity. This suggests that the liver microsomal enzymes induced by phenobarbital do not play a direct role in the hepatic metabolism of pBPAU.

pBPAU is not an inhibitor of acetylcholine acetyl-hydrolase (EC No. 3.1.1.7) or benzoylcholine hydrolase (EC No. 3.1.1.9).

The delayed neurotoxic effect of some organophosphorus compounds. Identification of the phosphorylation site as an esterase

1. Organophosphorus compounds that produce a delayed neurotoxic effect in hens phosphorylate a specific site in the brain soon after administration. 2. Phosphorylation of the specific site by di-isopropyl [(32)P]phosphorofluoridate in vitro is blocked by the prior addition of phenyl phenylacetate. 3. A small proportion of the total activity of hen brain hydrolysing phenyl phenylacetate in vitro was shown to be due to an enzyme different from two others previously described. 4. This enzyme is only slightly inhibited in vitro by concentrations of tetraethyl pyrophosphate and paraoxon (diethyl 4-nitrophenyl phosphate) up to 64mum and is completely inhibited by 6mum-di-isopropyl phosphorofluoridate and 128mum-mipafox. 5. It is also inhibited in vivo by effective doses of neurotoxic organophosphorus compounds but not by high doses of non-neurotoxic analogues. 6. It is deduced that the active site of this enzyme is the phosphorylation site associated with the genesis of delayed neurotoxicity.