Progressive deficit of retrograde axonal transport is associated with the pathogenesis of di-n-butyl dichlorvos axonopathy

PNPLA6 disorders: what's in a name?

BACKGROUND

Variants in the patatin-like phospholipase domain containing 6 (PNPLA6) gene cause a broad spectrum of neurological disorders characterized by gait disturbance, visual impairment, anterior hypopituitarism, and hair anomalies. This review examines the clinical, cellular, and biochemical features found across the five PNPLA6-related diseases, with a focus on future questions to be addressed.

MATERIALS AND METHODS

A literature review was performed on published clinical reports on patients with PNPLA6 variants. Additionally, in vitro and in vivo models used to study the encoded protein, Neuropathy Target Esterase (NTE), are summarized to lend mechanistic perspective to human diseases.

RESULTS

Biallelic pathogenic PNPLA6 variants cause five systemic neurological disorders: spastic paraplegia type 39, Gordon-Holmes, Boucher-Neuhäuser, Laurence-Moon, and Oliver-McFarlane syndromes. PNPLA6 encodes NTE, an enzyme involved in maintaining phospholipid homeostasis and trafficking in the nervous system. Retinal disease presents with a unique chorioretinal dystrophy that is phenotypically similar to choroideremia and Leber congenital amaurosis. Animal and cellular models support a loss-of-function mechanism.

CONCLUSIONS

Clinicians should be aware of choroideremia-like ocular presentation in patients who also experience growth defects, motor dysfunction, and/or hair anomalies. Although NTE biochemistry is well characterized, further research on the relationship between genotype and the presence or absence of retinopathy should be explored to improve diagnosis and prognosis.

Neuropathy target esterase (NTE/PNPLA6) and organophosphorus compound-induced delayed neurotoxicity (OPIDN)

Systemic inhibition of neuropathy target esterase (NTE) with certain organophosphorus (OP) compounds produces OP compound-induced delayed neurotoxicity (OPIDN), a distal degeneration of axons in the central nervous system (CNS) and peripheral nervous system (PNS), thereby providing a powerful model for studying a spectrum of neurodegenerative diseases. Axonopathies are important medical entities in their own right, but in addition, illnesses once considered primary neuronopathies are now thought to begin with axonal degeneration. These disorders include Alzheimer's disease, Parkinson's disease, and motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Moreover, conditional knockout of NTE in the mouse CNS produces vacuolation and other degenerative changes in large neurons in the hippocampus, thalamus, and cerebellum, along with degeneration and swelling of axons in ascending and descending spinal cord tracts. In humans, NTE mutations cause a variety of neurodegenerative conditions resulting in a range of deficits including spastic paraplegia and blindness. Mutations in the Drosophila NTE orthologue SwissCheese (SWS) produce neurodegeneration characterized by vacuolization that can be partially rescued by expression of wild-type human NTE, suggesting a potential therapeutic approach for certain human neurological disorders. This chapter defines NTE and OPIDN, presents an overview of OP compounds, provides a rationale for NTE research, and traces the history of discovery of NTE and its relationship to OPIDN. It then briefly describes subsequent studies of NTE, including practical applications of the assay; aspects of its domain structure, subcellular localization, and tissue expression; abnormalities associated with NTE mutations, knockdown, and conventional or conditional knockout; and hypothetical models to help guide future research on elucidating the role of NTE in OPIDN.

Analysis of the neurotoxic effects of neuropathic organophosphorus compounds in adult zebrafish

Inhibition and aging of neuropathy target esterase (NTE) by exposure to neuropathic organophosphorus compounds (OPs) can result in OP-induced delayed neuropathy (OPIDN). In the present study we aimed to build a model of OPIDN in adult zebrafish. First, inhibition and aging of zebrafish NTE activity were characterized in the brain by using the prototypic neuropathic compounds cresyl saligenin phosphate (CBDP) and diisopropylphosphorofluoridate (DFP). Our results show that, as in other animal models, zebrafish NTE is inhibited and aged by both neuropathic OPs. Then, a neuropathic concentration inhibiting NTE activity by at least 70% for at least 24 h was selected for each compound to analyze changes in phosphatidylcholines (PCs), lysophosphatidylcholines (LPCs) and glycerolphosphocholine (GPC) profiles. In spite to the strong inhibition of the NTE activity found for both compounds, only a mild increase in the LPCs level was found after 48 h of the exposure to DFP, and no effect were observed by CBDP. Moreover, histopathological evaluation and motor function outcome analyses failed to find any neurological abnormalities in the exposed fish. Thus, our results strongly suggest that zebrafish is not a suitable species for the development of an experimental model of human OPIDN.

Disturbed phospholipid homeostasis in endoplasmic reticulum initiates tri-o-cresyl phosphate-induced delayed neurotoxicity

Tri-o-cresyl phosphate (TOCP) is a widely used organophosphorus compound, which can cause a neurodegenerative disorder, i.e., organophosphate-induced delayed neurotoxicity (OPIDN). The biochemical events in the initiation of OPIDN were not fully understood except for the essential inhibition of neuropathy target esterase (NTE). NTE, located in endoplasmic reticulum (ER), catalyzes the deacylation of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) to glycerophosphocholine (GPC). The present study aims to study the changes of ER phospholipids profile as well as levels of important intermediates of phospholipid synthesis such as diacylglycerol (DAG) and phosphatidic acid (PA) at the initiation stage of OPIDN. Hens are the most commonly used animal models of OPIDN. The spinal cord phospholipidomic profiles of hens treated by TOCP were studied by using HPLC-MS-MS. The results revealed that TOCP induced an increase of PC, LPC, and sphingomyelin (SM) levels and a decrease of GPC, phosphatidylethanolamine (PE), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), phosphatidylglycerol (PG), and phosphatidylinositol (PI) levels., Levels of DAG and PA were also decreased. Pretreatment with phenylmethylsulfonyl fluoride (PMSF) 24 h before TOCP administration prevented OPIDN and restored the TOCP-induced changes of phospholipids except GPC. Thus, the disruption of ER phospholipid homeostasis may contribute to the initiation of organophosphate-induced delayed neurotoxicity.

Potential biomarkers of dichlorvos induced neuronal injury in rats

The present study was designed to develop suitable biochemical markers of chronic dichlorvos exposure using rat as the animal model. Animals were exposed to dichlorvos (6 mg kg-1 (body weight) day-1) for 8 weeks and the activities of five potential markers were assayed. Acetylcholinesterase, assayed as an index of cholinergic function, was found to decrease in both haemolysate and brain tissue. Cytochrome oxidase, used as a marker of impaired energy metabolism, was also seen to decrease in platelets and brains of dichlorvos-treated animals. However, acid phosphatase, a lysosomal marker of tissue injury, was increased in both serum and brains of experimental animals. Chronic dichlorvos exposure also led to a decrease in the activity of glucose-6-phosphate dehydrogenase, which was assayed in brain as an index of oxidative stress. Dichlorvos administration did not affect 2', 3'-cyclic nucleotide phosphohydrolase. The present study therefore, indicates that apart from acetylcholinesterase, which is probably a non-specific marker of dichlorvos neurotoxicity, the levels of cytochrome oxidase, acid phosphatase and glucose-6-phosphate dehydrogenase may serve as useful determinants of dichlorvosinduced neuronal injury.

Calcium-dependent neutral cysteine protease and organophosphate-induced delayed neuropathy

A few organophosphorus compounds (OPs) can cause toxic neuropathy known as organophosphorus ester-induced delayed neuropathy (OPIDN). Although the incidents of OPIDN have been documented for over a century, its molecular mechanisms underlying the axonopathy are still unclear. Recently, increasing evidences suggest that proteases are closely associated with OPIDN. Herein, we have summarized the roles of calcium-dependent cysteine proteases (calpains) in OPIDN. The activation of calpains should be an early molecular event during the onset and development of OPIDN. However, the understanding of the mechanism underlying the disruption of Ca(2+) homeostasis and the activation of calpain by neurotoxic OPs is still limited. Therefore, a better understanding of molecular mechanisms that can prevent the disturbance in cellular Ca(2+) homeostasis can facilitate to establish the novel therapeutic strategies for OPIDN.

Reduction of retrograde axonal transport associated-proteins motor proteins, dynein and dynactin in the spinal cord and cerebral cortex of hens by tri-ortho-cresyl phosphate (TOCP)

Tri-ortho-cresyl phosphate (TOCP) can cause a type of neurotoxicity known as organophosphate-induced delayed neuropathy (OPIDN). The characteristic axonal swelling containing aggregations of neurofilaments, microtubules, and multivesicular vesicles is consistent with a disturbance of axonal transport. We hypothesized that there existed a disturbance of molecular motor in the pathogenesis of OPIDN. In the present study, adult hens were treated with a dosage of 750 mg/kg TOCP by gavage, or pretreated 24h earlier with phenylmethanesulfonyl fluoride (PMSF) and subsequently with TOCP, then sacrificed on the time-points of 0, 1, 5, 10, and 21 days after dosing of TOCP, respectively. The level of kinesin-1, dynein, and dynactin in spinal cords and cerebral cortexes of hens was determined. Immunoblotting analysis showed a progressive decline of dynein and dynactin in spinal cords after dosing TOCP. Furthermore, a significant reduction in dynactin and dynein was observed in cerebral cortexes at several time-points post dosing TOCP. In contrast, no significant changes of kinesin-1 were observed throughout the period of experiment. When given before TOCP administration, PMSF could inhibit TOCP-induced motor protein disruption, while it protected hens against the delayed neuropathy. In conclusion, the reduction of the motor proteins, dynein and dynactin, might be associated with the disruption of retrograde neuronal axonal transport in OPIDN.

Vacuolation of sensory ganglion neuron cytoplasm in rats with long-term exposure to organophosphates

Cytoplasmic vacuolation of sensory neurons has been reported to occur within the dorsal root ganglia in studies investigating various neuropathic conditions including the effects of neurotoxic chemicals. In this study, we investigated this lesion in adult (98-119 days old) male Long-Evans rats, after multiple exposures to two organophosphates (tri-ortho-tolyl phosphate [TOTP] and chlorpyrifos) and the modifying effects of concurrent corticosterone. Tri-ortho-tolyl phosphate was administered by gavage (75, 150, or 300 mg/kg) every other day between days 14 and 28 and between days 49 and 63, chlorpyrifos (60 mg/kg) was administered subcutaneously on days 7 and 42, and corticosterone was provided in the drinking water throughout the study at a concentration of 400 microg/mL. Although relatively uncommon, there was an increase in frequency of cytoplasmic vacuoles seen in treatment groups having multiple exposures to TOTP. They were characterized as peripherally located, single-limiting membrane-bound structures in the neuronal perikarya. There was no associated cell death, even when vacuoles were large. This is the initial report of an association of this change following exposure to neurotoxic organophosphates.

Neurobehavioral impairments, generation of oxidative stress and release of pro-apoptotic factors after chronic exposure to sulphur mustard in mouse brain

Recent global events have focused attention on the potential threat of international and domestic chemical terrorism, as well as the possibility of chemical warfare proliferation. Sulphur mustard (SM) is one of the potent chemical warfare agents (CWA), which initiates a cascade of events that converge on the redox mechanisms common to brain injury. The present study was designed to examine the effects of chronic SM exposure on neurobehavioral impairments, mitochondrial oxidative stress in male Swiss Albino mice and its role in inducing apoptotic neuronal cell death. The animals were divided into four groups (control, low, medium and high dose) of 5 animals each. Exposure to SM was given percutaneously daily for 12 weeks. The results demonstrated impairment in neurobehavioral indices viz. rota rod, passive avoidance and water maze tests in a dose dependent manner. There was a significant increase in lipid peroxidation and protein carbonyl content whereas, decrease in the activity of manganese superoxide dismutase (MnSOD), glutathione reductase and glutathione peroxidase suggesting impaired antioxidant defense system. Immunoblotting of cytochrome c, Bcl-2, Bax and activation of caspase-3 suggest induction of apoptosis in a dose dependent manner. Finally, increased p53 expression suggests that it may target the mitochondrial pathway for inducing apoptosis in response to DNA damage signals. In conclusion, chronic SM exposure may have the potential to generate oxidative stress which may trigger the release of cytochrome c as well as caspase-3 activation in neurons leading to cell death by apoptosis in a dose dependent manner which may in the end be responsible for the disruption of cognitive functions in mice.

Role of muscarinic signal transduction and CREB phosphorylation in dichlorvos-induced memory deficits in rats: an acetylcholine independent mechanism

The present study was designed to explore the alternative mechanism (other than AChE inhibition) for chronic, low-level exposure to dichlorvos, an organophosphate, in vivo. Dichlorvos, at a dose of 1.0 and 6.0 mg/kg body weight (b.wt.) for 12 weeks, showed impairment in neurobehavioral indices viz. rota rod, passive avoidance and water maze tests. Though high dose of dichlorvos had a detrimental effect on acetylcholinesterase activity, no significant inhibition was seen with low dose of dichlorvos. Western blot analysis and immunofluorescence studies showed a significant reduction in the expression of M(1), M(2) and M(3) muscarinic receptor subtypes in high dose group animals, whereas in low dose group animals only the M(2) receptor subtype was reduced significantly. Further, the signal transduction cascade linked to these receptor subtypes was affected in high dose group animals whereas in low dose group only adenylyl cyclase-linked signaling pathway was impaired. Finally, the phosphorylation of CREB, a memory enhancing transcription factor, was significantly reduced in both low dose and high dose group animals. Thus, the present study reveals the significance of M(2) muscarinic receptor linked adenylyl cyclase signaling pathway and phosphorylation of CREB in the development of neurobehavioral impairments after chronic low-level exposure to dichlorvos.

Axonal degeneration and neuropathy target esterase

This brief review summarizes recent observations which suggest a possible mechanism for organophosphate-induced delayed neuropathy (OPIDN). Neuropathy target esterase (NTE) has been shown to deacylate endoplasmic reticulum (ER) membrane phosphatidylcholine (PtdCho). Raised levels of PtdCho are present in the brains of Swiss cheese/NTE mutant Drosophila together with abnormal membrane structures, axonal and dendritic degeneration and neural cell loss. Similar vacuolated pathology is found in the brains of mice with brain-specific deletion of the NTE gene and, in old age, these mice show clinical and histopathological features of neuropathy resembling those in wild-type mice chronically dosed with tri-ortho-cresylphosphate. It is suggested that OPIDN results from the loss of NTE's phospholipase activity which in turn causes ER malfunction and perturbation of axonal transport and glial-axonal interactions.

Aberrant activation of CDK5 is involved in the pathogenesis of OPIDN

Exposure to triorthocresyl phosphate (TOCP) may result in a late neurological complication, i.e. organophosphate-induced delayed neuropathy (OPIDN). The aim of this study was to examine changes in levels of cyclin-dependent kinase 5 (CDK5) and of its activator, p35/p25, in the spinal cord of hens treated by TOCP. After exposure to a single dose of TOCP, groups of adult hens were examined in 3, 5, 7, 9, 14, and 18 days after exposure. CDK5, p35/p25 expression and distribution in the lumbar spinal cord were evaluated by immunohistochemistry and Western blotting. The hens showed signs of OPIDN around day 9 after exposure. The number of p (phosphorylated) -CDK5 and p35 positive cells increased significantly. Co-localization and mislocalization of p-CDK5 and p35/p25 was identified and became evident in neurons around the 9th day. Meanwhile, CDK5, p-CDK5, p35, p25 protein levels and p25/p35 ratio were increased, and peaked around the 9th day, then decreased. Some hens' unilateral common peroneal was treated by roscovitine 3 days after TOCP exposure. Axonal transport of these nerves was faster than of their opposite side and of those simply treated by TOCP. These findings indicate aberrant activation of CDK5 may be involved in the pathogenesis of OPIDN.

The L-Type Calcium Channel Blocker Nimodipine Mitigates Cytoskeletal Proteins Phosphorylation in Dichlorvos-Induced Delayed Neurotoxicity in Rats

The present investigation was carried out to assess the protective efficacy of nimodipine against dichlorvos-induced organophosphate induced delayed neurotoxicity (OPIDN). Single subcutaneous dose of dichlorvos (200 mg/kg body weight) led to a consistent increase in the activity of both microtubule associated protein kinases viz. Ca2+/Calmodulin-dependent and cAMP dependent protein kinases, at all post exposure intervals (day 7, 15 and 21) as compared to that of controls. Autoradiography followed by microdensitometric studies demonstrated enhanced phosphorylation of 55 kDa and 280 kDa proteins in dichlorvos-exposed animals. These two proteins were confirmed to be tubulin and microtubule associated protein-2 (MAP-2) by western blotting. The hyperphosphorylation of these two proteins was shown to interfere with the assembly of neuronal microtubules as shown by electron microscopic studies that may eventually lead to possible disruption of neuronal cytoarchtecture resulting in axonal degeneration. Administration of nimodipine along with dichlorvos brought about a significant reduction in the activities of both the kinases as well as the extent of microtubule associated protein phosphorylation. This indicates that nimodipine, a centrally acting calcium channel blocker, may contribute to the amelioration of dichlorvos induced neurotoxicity by attenuation of calcium mediated disruption of cytoskeletal proteins and hence, calcium channel blockers like nimodipine have great future as new therapeutic agents for OPIDN.

Neuropathological studies of rats following multiple exposure to tri-ortho-tolyl phosphate, chlorpyrifos and stress

Adult male Long-Evans rats were exposed to 2 neurotoxic organophosphates in a setting of chronic stress, over a 63-day period. The organophosphates were tri-ortho-tolyl phosphate (TOTP) administered in 14 gavage doses of 75, 150 or 300 mg/kg, and chlorpyrifos, given in two 60 mg/kg subcutaneous exposures. Corticosterone was added to the drinking water at 400 microg/ml, to model aspects of chronic stress. These compounds/dosages were administered individually and in combination, with appropriate controls, giving rise to 16 experimental groups. The major neuropathologic change was the presence of axonal degeneration progressing to myelinated fiber degeneration, mainly in distal regions of selected fiber tracts and peripheral nerve, seen in animals sacrificed on experimental day 63. The cervical spinal cord and medullary levels of the sensory gracile fasciculus were most prominently affected. This axonopathy/fiber degeneration was TOTP dose-related at the 300 and 150 mg/kg levels. There was association of this lesion with inhibition of the enzyme neurotoxic esterase in hippocampal tissue from TOTP-treated rats. Such an association categorizes this disease process as organophosphate ester-induced delayed neuropathy. Neither chlorpyrifos nor corticosterone appeared to contribute to the neuropathic events or the enzyme inhibition. A cohort of rats was maintained on the corticosterone dosing, but without additional exposure to TOTP or chlorpyrifos, for an additional 27 days. When these rats were examined on day 90, the nerve fiber degeneration had progressed in all experimental groups administered the 300 mg/kg dose of TOTP (lower doses were not studied at the 90-day interval), although hippocampal neurotoxic esterase had returned to control values.

Effects of organophosphorus compounds on ATP production and mitochondrial integrity in cultured cells

Recent studies in vivo and in vitro suggested that mitochondrial dysfunction follows exposure to organophosphorus (OP) esters. As mitochondrial ATP production is important for cellular integrity, ATP production in the presence of OP neurotoxicants was examined in a human neuronal cell line (SH-SY5Y neuroblastoma cells) and primary dorsal root ganglia (DRG) cells isolated from chick embryos and subsequently cultured to achieve maturation with axons. These cell culture systems were chosen to evaluate toxic effects on the mitochondrial respiratory chain associated with exposure to OP compounds that do and do not cause OP-induced delayed neuropathy (OPIDN), a disorder preceded by inhibition of neurotoxic esterase (NTE). Concentration- and time-response studies were done in neuroblastoma cells exposed to phenyl saligenin phosphate (PSP) and mipafox, both compounds that readily induce delayed neuropathy in hens, or paraoxon, which does not. Phenylmethylsulfonyl fluoride (PMSF) was included as a non-neuropathic inhibitor of NTE. Purified neuronal cultures from 9 day-old chick embryo DRG were treated for 12 h with 1 microM PSP, mipafox, or paraoxon. In situ evaluation of ATP production measured by bioluminescence assay demonstrated decreased ATP concentrations both in neuroblastoma cells and chick DRG neurons treated with PSP. Mipafox decreased ATP production in DRG but not in SH-SY5Y cells. This low energy state was present at several levels of the mitochondrial respiratory chain, including Complexes I, II, III, and IV, although Complex I was the most severely affected. Paraoxon and PMSF were not effective at all complexes, and, when effective, required higher concentrations than needed for PSP. Results suggest that mitochondria are an important early target for OP compounds, with exposure resulting in depletion of ATP production. The targeting of neuronal, rather than Schwann cell mitochondria in DRG following exposure to PSP and mipafox was verified by loss of the mitochondrial-specific dye, tetramethylrhodamine, in these cells. No such loss was seen in paraoxon exposed neurons isolated from DRG or in Schwann cells treated with any of the test compounds.

Altered expression of transcripts for ?-tubulin and an unidentified gene in the spinal cord of phenyl saligenin phosphate treated hens (Gallus gallus)

Phenyl saligenin phosphate (PSP) induces a central-peripheral distal axonopathy in domestic fowl that develops 7-21 days after a single exposure. Neurotoxic esterase (NTE) is the initial molecular target for this neurotoxicity. PSP has to covalently bind to NTE and chemically "age" for induction of axonopathy. It was hypothesized that exposure to PSP results in early changes in spinal cord gene expression that do not occur with phenylmethylsulfonyl fluoride, a non-neuropathic compound that also inhibits NTE, or DMSO controls. Targeted display was used to screen approximately 15,000 gel bands. Three candidate genes were identified, but only the transcript designated P1 showed decreased expression following PSP exposure (2 mg/kg i.m.) in subsequent Northern blot and in situ hybridization experiments in samples taken <48 h after exposure. Additional experiments revealed that a approximately 2.5 kb alpha-tubulin transcript had decreased expression at 12-48 h after PSP exposure, with maximum change at 48 h (33%, p = 0.0479). A approximately 4.5 kb alpha-tubulin transcript had increased expression at 12 h (38%, p = 0.0125) and decreased expression at 48 h (28%, p = 0.0576). In situ hybridization on spinal cord revealed neuronal expression of P1 and alpha-tubulin transcripts. Decreased expression of transcripts for P1 and alpha-tubulin was present at 12 and 48 h, respectively. This decrease occurred in all neurons, not just those whose axons degenerate. Results suggest that (1) in PSP-induced OPIDN (organophosphorus-induced delayed neurotoxicity) some gene transcript expression changes are associated with initiation of axonopathy, and (2) PSP modulates spinal cord gene expression in neuronal types that do not undergo axonal degeneration.

Quantitative structure-activity relationships predict the delayed neurotoxicity potential of a series of O-alkyl-O-methylchloroformimino phenylphosphonates

Inhibition of acetylcholinesterase (AChE) versus inhibition and aging of neuropathy target esterase (NTE) by organophosphorus (OP) compounds in vivo can give rise to distinct neurological consequences: acute cholinergic toxicity versus OP compound-induced delayed neurotoxicity (OPIDN). Previous work has shown that the relative potency of an OP compound to react with NTE versus AChE in vitro may predict its capability to produce OPIDN. The present study was conducted to evaluate further the validity of such predictions and to enhance them with quantitative structure-activity relationships (QSAR) using a homologous series of alkyl phenylphosphonates (RO)C6H5P(O)ON = CCICH3 (PhP; R = alkyl). Neuropathic potential of PhP was assessed by measuring ki(NTE)ki(AChE) ratios in vitro and comparing these with ED50 ratios in vivo. Selectivity for NTE increased with rising R-group hydrophobicity. The ki(NTE)/ki(AChE) ratios were 0.42 (methyl), 3.6 (ethyl), 15 (isopropyl), 36 (propyl), 69 (isobutyl), 105 (butyl), and 124 (pentyl). Ratios > 1 suggest the potential to produce OPIDN at doses lower than the LD50. Inhibition of NTE and AChE in hen brain in vivo was studied 24 h after i.m. injection of hens with increasing doses of methyl and butyl derivatives. Analysis of dose-response curves yielded ED50(AChE)/ED50(NTE) ratio of 0.86 for methyl PhP and 22.1 for butyl PhP. These results predict that the butyl derivative should be more neuropathic than the methyl analogue. Excellent correspondence between in vivo and in vitro predictions of neuropathic potential indicate that valid predictive QSAR models may be based on the in vitro approach. Adoption of this system would result in reducing experimental animal use, lowering costs, accelerating data production, and enabling standardization of a biochemically based risk assessment of the neuropathic potential of OP compounds.

Repeated exposures to subthreshold doses of chlorpyrifos in rats: hippocampal damage, impaired axonal transport, and deficits in spatial learning

Organophosphorus (OP) compounds are detectable in the environment for years after use and endanger many populations. Although the effects of acutely toxic doses of many OP compounds are well described, much less is known about repeated low-level exposures. The purpose of these studies was to further evaluate potential toxicological effects of the extensively used OP pesticide chlorpyrifos (CPF) in rats. CPF, across a range of subthreshold doses (i.e., for acute toxicity), reduced rearing and sniffing activity and the magnitude of weight gain over 14 days of repeated exposure. Performance in a spatial learning task was impaired after 14 days of exposure to CPF (18.0 and 25.0 mg/kg) when testing was initiated 24 h after the last injection but not after a 14-day washout. However, inhibition of both fast anterograde and retrograde axonal transport was observed for up to 20 days after exposure to 25.0 mg/kg CPF. Studies using hippocampal cultures indicated that 8 days of continuous exposure to the parent compound, CPF (> or =100 micro M), resulted in cell toxicity and death. Furthermore, a dose (2.5 mg/kg) of CPF that had no effects on weight gain or memory performance when administered 5 days per week over 38 days impaired forelimb grip strength in the later days of testing. Collectively, these results indicate that repeated exposures to subthreshold doses of CPF may lead to growth retardation, behavioral abnormalities, and muscle weakness. Some of these symptoms may be attributed to effects of the OP on axonal transport.

Promotion of organophosphate induced delayed polyneuropathy by certain esterase inhibitors

Organophosphate induced delayed polyneuropathy (OPIDP) is an axonopathy caused by single doses of some organophosphates (OPs). Other OPs, sulphonyl halides, carbamates, thiocarbamates and phosphinates do not cause axonopathy but elicit or intensify the clinical expression of OPIDP when given after a neuropathic OP (promotion of OPIDP). One enzymatic activity (M200) was identified by means of selective inhibitors in hen peripheral nerve crude homogenates. Promotion of OPIDP initiated with several OPs was found to correlate with inhibition of M200 when various promoters were given to hens. Most M200 is in the soluble fraction of peripheral nerves and was separated from other esterases by means of molecular exclusion chromatography. In a second series of experiments, inhibition of this fraction also correlated with promotion when induced by the same chemicals. Further ion exchange chromatography identified a protein (60 kDa MW): the inhibition of its enzymatic activity correlated with promotion in another series of in vivo experiments. Biochemical and structural analyses of this protein are underway. Several experiments indirectly suggest that promotion may be related to mechanisms of repair and/or compensation of peripheral nerves. These include the observation that promotion results in clinical expression of biochemical lesions that otherwise would be well compensated, that promotion is not specific because axonopathies of other etiology are also exaggerated, and that promoters are effective when given several days before the neuropathic insult. Moreover, developing animals are more resistant to promotion.

Effect of ethanol on axonal transport of cholinergic enzymes in rat sciatic nerve

The aim of this study was to investigate whether the fast axonal transport of acetylcholinesterase (AChE) and the slow transport of choline acetyltransferase (ChAT) in the sciatic nerve of the rat are affected by chronic ethanol consumption or by associated nutritional deficiency. Adult rats drank ethanol (20% [vol./vol.]) instead of water for 20 weeks. Animals that consumed an isocaloric diet, representing a nutritional deficiency, received the same amount of food as ethanol-treated rats and water with sucrose replacing ethanol isocalorically. The control group received food and water ad libitum. Axonal transport was investigated by the stop-flow ligation technique as follows. After 20 weeks, the sciatic nerve was ligated for 24 h, and accumulation of AChE and ChAT was measured above and below the ligature. No significant differences in the accumulation of both enzymes were found above the ligature. However, the accumulation of AChE transported retrogradely below the nerve ligature was reduced by 60%, but only in ethanol-treated animals. Our study results seem to indicate that, under experimental conditions, (1) neither ethanol nor associated nutritional deficiency has any effect on anterograde axonal transport of AChE and ChAT and (2) the deficit in retrograde transport of AChE is due to the direct toxic effect of ethanol and probably precedes the axonal degeneration from the most distal portions of axons toward the cell body.

Protective effect of nimodipine on dichlorvos-induced delayed neurotoxicity in rat brain(1)

The effect of dichlorvos (200 mg/kg body weight) with or without nimodipine (6 mg/kg body weight/day for 3 days, starting 1 day prior to the administration of dichlorvos) on calcium homeostasis was studied in the rat brain. The delayed neurotoxic potential of dichlorvos was assessed in terms of neuropathy target esterase (NTE) inhibition in the brain and the subsequent development of motor incoordination at 21 days post-exposure. NTE activity had recovered up to 84% at the time of clinical manifestations. No signs of motor deficit were present when nimodipine was given with dichlorvos. The administration of dichlorvos alone caused an increase in intrasynaptosomal Ca(2+) with a concomitant increase in calpain activity. These increases in calpain activity and in the levels of intracellular Ca(2+) were not observed when nimodipine was administered to rats treated with dichlorvos. Also, the inhibition of calcium ATPase following the exposure to dichlorvos was reduced when animals received nimodipine. This indicates that nimodipine, a centrally acting calcium channel blocker, may contribute to the amelioration of dichlorvos-induced neurotoxicity by attenuation of calcium-mediated disruption of cytoskeletal homeostasis, without preventing NTE inhibition.

Nerve conduction and ATP concentrations in sciatic-tibial and medial plantar nerves of hens given phenyl saligenin phosphate

To assess the relationship of nerve conduction and adenosine triphosphate (ATP) status in organophosphorus-induced delayed neuropathy (OPIDN), we evaluated both in adult hen peripheral nerves following exposure to a single 2.5 mg/kg dose of phenyl saligenin phosphate (PSP). ATP concentrations were determined at days 2, 4, 7, and 14 post-dosing, from five segments (n = 5 per group) representing the entire length of the sciatic-tibial and medial plantar nerve. Initial effects of PSP dosing were seen in the most distal segment at day 2, when a transient ATP concentration increase (388 +/- 79 pmol/ml/mg versus control value of 215 +/- 23, P < 0.05) was noted. Subsequently, ATP concentration in this distal segment returned to normal. In the most proximal nerve segment, ATP concentrations were decreased on day 7, and further decreased on day 14 post-dosing (P < 0.05). Changes in ATP concentration and nerve conduction velocity begin at post-dosing day 2, and were found prior to development of clinical neuropathy and axonopathic lesions. These results suggest that alterations in sciatic-tibial and medial plantar nerve conduction associated with sciatic-tibial and medial plantar nerve ATP concentration are early events in the development of OPIDN.

Neural development and neurodegeneration: two faces of neuropathy target esterase

Neuropathy target esterase (NTE) is an integral membrane protein in vertebrate neurons. Recent evidence suggests that NTE plays an important role in neural development, possibly via involvement in a signalling pathway between neurons and glial cells. NTE is a member of a novel protein family, represented in organisms from bacteria to man. NTE comprises an N-terminal regulatory domain (with some sequence similarity to cyclic nucleotide-binding proteins) and a C-terminal catalytic domain: the latter has three predicted transmembrane segments and requires membrane-association for activity. In vitro, NTE potently catalyses hydrolysis of phenyl valerate: however, its physiological substrate is likely to be a metabolite of a much longer chain carboxylic acid, possibly associated with cell membranes. NTE was discovered originally as the primary target for those organophosphorus esters (OPs) which cause a delayed neuropathy with degeneration of long axons in peripheral nerves and spinal cord. Paradoxically, NTE's catalytic activity appears redundant in adult vertebrates. Neuropathic OPs react covalently with NTE in a rapid two-step process which not only inhibits catalytic activity but also leaves a negatively-charged OP group attached to the active site serine. The latter event is proposed to induce a toxic gain of function in NTE. OP-modified NTE somehow engenders a "chemical transection of the axon". In turn, this leads to calcium entry, elevation of axonal calpain activity and Wallerian-type degeneration. The net damage to peripheral nerve axons is a balance between ongoing degenerative and repair processes: the latter involve serine hydrolases which can be inhibited by the same OPs used to modify NTE.

Promoters and promotion of axonopathies

Promotion is the exacerbation by certain esterase inhibitors (organophosphates, organophosphinates, sulfonyl halides, carbamates and thiocarbamates) of the clinical and morphological expression of toxic and traumatic axonopathies. Promotion is believed to interfere with mechanisms of compensation/repair of the nerves. The target of promotion is unknown but there are indications that it might be similar and/or linked to neuropathy target esterase (NTE), which is the molecular target of organophosphate-induced delayed polyneuropathy (OPIDP). OPIDP is the model axonopathy used to characterize promotion. NTE is defined as the activity resistant to paraoxon (40 microM) and sensitive to mipafox (50 microM). An esterase activity sensitive to higher concentrations (1 mM) of mipafox was identified in the nervous system homogenate, and its inhibition correlated with promotion. An activity with similar characteristics was present in the soluble fraction of peripheral nerves and could be physically separated (about 60 kDa). Identification and characterization of the target of promotion might be helpful in understanding the mechanism(s) of compensation and repair of the peripheral nervous system.

Mechanisms of toxic injury in the peripheral nervous system: neuropathologic considerations

The anatomical distribution and organization of the peripheral nervous system as well as its frequent ability to reflect neurotoxic injury make it useful for the study of nerve fiber and ganglionic lesions. Contemporary neuropathologic techniques provide sections with excellent light-microscopic resolution for use in making such assessments. The histopathologist examining such peripheral nerve samples may see several patterns of neurotoxic injury. Most common are axonopathies, conditions in which axonal alterations are noted; these axonopathies often progress toward the Wallerian-like degeneration of affected fibers. These are usually more severe in distal regions of the neurite, and they affect both peripheral and central fibers. Examples of such distal axonopathies are organophosphorous ester-induced delayed neuropathy, hexacarbon neuropathy, and p-bromophenylacetylurea intoxication. These axonopathies may have varying pathologic features and sometimes have incompletely understood toxic mechanisms. In such neuropathies with fiber degeneration, peripheral nerve axons may regenerate, which can complicate pathologic interpretation of neurotoxicity. On occasion neurotoxins elicit more severe injury in proximal regions of the fiber (not included in this review). Axonal pathology is also a feature of the neuronopathies, toxic states in which the primary injuries are found in neuronal cell bodies. This is exemplified by pyridoxine neurotoxicity, where there is sublethal or lethal damage to larger cytons in the sensory ganglia, with failure of such neurons to maintain their axons. Lastly, one may encounter myelinopathies, conditions in which the toxic effect is on the myelin-forming cell or sheath. An example of this is tellurium intoxication, where demyelination noted in young animals is coincident with toxin-induced interference of cholesterol synthesis by Schwann cells. In this paper, the above-noted examples of toxic neuropathy are discussed, with emphasis on mechanistic and morphologic considerations.

Promotion of organophosphate induced delayed polyneuropathy by certain esterase inhibitors

Certain esterase inhibitors elicit or intensify the clinical expression of various insults to axons. This phenomenon was called promotion of axonopathies because these chemicals are not additive neurotoxicants nor do they interfere with the pharmacokinetics. Characterization of promotion was carried out by using organophosphate induced delayed polyneuropathy (OPIDP) as a model. The search for a physiological explanation of promotion has the following background: (1) Promotion expresses clinically the biochemical lesions which are otherwise well compensated (such as 30/40% neuropathy target esterase (NTE) inhibition by neuropathic organophosphates). (2) Promotion is not specific because axonopathies of different origin are affected. (3) Promoters are effective when given several days before the neuropathic insult. (4) Promotion is less effective in young animals as compared with adults. (5) Promotion occurs when axons, but not necessarily the cell body, are targeted by promoters. (6) Repeated dosing with a promoter failed to produce axonopathy. Based on this evidence it is suggested that promotion might interfere with a mechanism(s) of compensation and/or repair of long axons. The target of promotion of axonopathies is thought to be similar or linked to NTE which is defined as the phenyl valerate esterase activity (PVE) in nervous tissues resistant to paraoxon and sensitive to mipafox (40 and 50 microM, pH 8.0, 20 min, respectively). Mipafox (50 microM) resistant PVEs include some activity sensitive to the promoter phenylmethane sulfonylfluoride (PMSF) but no correlation was found between its inhibition and promotion. A complete titration curve of paraoxon-resistant PVEs by mipafox (0-1 mM) dissected, besides NTE (I50 about 10 microM), another PVE with an I50 of approximately 200 microM. This enzyme was present in hen brain, spinal cord and peripheral nerve, corresponding to about 10, 20 and 30% of NTE activity, respectively, and was sensitive both in vitro and in vivo to promoters and much less so to neuropathic NTE inhibitors. By means of chromatography, other workers have identified in soluble extracts of peripheral nerves two forms of mipafox-sensitive PVEs with different molecular weights and different sensitivity to mipafox. These might correspond to NTE and to the other enzyme. Inhibition in vivo of the latter also correlated with promotion.

The role of phosphotriesterases in the detoxication of organophosphorus compounds

The enzymes that hydrolyze organophosphorus compounds are called phosphotriesterases. The presence of phosphotriesterases has been described in a variety of tissues. The physiological role of these enzymes is not known, although a clear correlation exists between the levels of phosphotriesterases and susceptibility of the species to the toxic effects of organophosphorus compounds. Thus, mammals that possess high levels of phosphotriesterases in serum and liver are more tolerant to the toxic effects of these compounds than birds and insects - these being species considered lacking of phosphotriesterases. Because most of these enzymes are not well characterized, they are usually differentiated according to their different patterns of response to activators and/ or inhibitors. Phosphotriesterases usually depend of divalent cations and therefore EDTA usually inhibits them. A peculiar EDTA-resistant phosphotriesterase has been described in serum albumin. The biotechnological and therapeutical applications of phosphotriesterases are currently subject to study.

Neuropathy target esterase: immunolocalization to neuronal cell bodies and axons

Determination of the molecular mechanisms involved in organophosphate-induced axonopathy may help to elucidate those involved in normal axonal maintenance and in other neurodegenerative conditions. In this study we aimed to define the cellular distribution of neuropathy target esterase, the primary target protein for neuropathic organophosphates. A synthetic peptide corresponding to the sequence of a proteolytic fragment of neuropathy target esterase purified from chicken brain was used to raise a rabbit antiserum designated R28. The antiserum was shown by immunoprecipitation and western blotting of brain extracts to react with a polypeptide of the expected molecular size (155,000 mol. wt); this reaction was blocked by preincubating the antiserum with the immunizing peptide. Prominent intracellular immunostaining by R28 was seen in neuronal cell bodies and, in some cases, proximal axon segments in frozen sections of chicken brain cortex, optic tectum, cerebellum, spinal cord, and dorsal root ganglia. Cells with glial morphology were not immunostained, neither were normal sciatic nerve or motor end plates. However, 8-12 h following sciatic nerve ligation, immunoreactive material was seen to accumulate both proximal and, to a lesser extent, distal to the ligature, indicating that neuropathy target esterase undergoes fast axonal transport. No gross qualitative or quantitative changes in the above pattern of neuropathy target esterase immunoreactivity were detected in tissue obtained from chickens one or three days following treatment with a neuropathic organophosphate. The presence of neuropathy target esterase in essentially all neurons indicates that the selective vulnerability of long axons to neuropathic organophosphates is dependent on factors additional to the presence of the target protein.

The role of nicotinic receptors and calcium channels in mipafox induced inhibition of catecholamine release in bovine chromaffin cells

Depolarization induced catecholamine release from chromaffin cells was decreased 28% by N,N'-diisopropyl diamido-phosphorofluoridate (mipafox), an organophosphorus compound (OP) causing neurotoxic effects, while secretion stimulated by nicotinic agonist was inhibited 65%. The reversibility of this effect and the fact that calcium-dependent secretion from digitonin-permeabilized cells was unaffected by mipafox suggest that this compound affects the ionic currents implicated in catecholamine release. Patch-clamp experiments showed that the activity of voltage-dependent calcium channels (VDCC) was inhibited 35% by mipafox being this effect reversible whereas only minor effects were detected on Na(+) and K(+) currents. Finally, we studied the effect of mipafox on nicotinic ionic currents in chromaffin cells. In this case, the OP was able to cause reversible inhibition reaching maximal effects of 50-60%. In conclusion, nicotinic receptors and VDCC should be considered as potential targets in order to understand the neurotoxicity of these chemicals.

Structure and function of tetanus and botulinum neurotoxins

Tetanus and botulinum neurotoxins are produced by Clostridia and cause the neuroparalytic syndromes of tetanus and botulism. Tetanus neurotoxin acts mainly at the CNS synapse, while the seven botulinum neurotoxins act peripherally. Clostridial neurotoxins share a similar mechanism of cell intoxication: they block the release of neurotransmitters. They are composed of two disulfide-linked polypeptide chains. The larger subunit is responsible for neurospecific binding and cell penetration. Reduction releases the smaller chain in the neuronal cytosol, where it displays its zinc-endopeptidase activity specific for protein components of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxins B, D, F and G recognize specifically VAMP/ synaptobrevin. This integral protein of the synaptic vesicle membrane is cleaved at single peptide bonds, which differ for each neurotoxin. Botulinum A, and E neurotoxins recognize and cleave specifically SNAP-25, a protein of the presynaptic membrane, at two different sites within the carboxyl-terminus. Botulinum neurotoxin type C cleaves syntaxin, another protein of the nerve plasmalemma. These results indicate that VAMP, SNAP-25 and syntaxin play a central role in neuroexocytosis. These three proteins are conserved from yeast to humans and are essential in a variety of docking and fusion events in every cell. Tetanus and botulinum neurotoxins form a new group of zinc-endopeptidases with characteristic sequence, mode of zinc coordination, mechanism of activation and target recognition. They will be of great value in the unravelling of the mechanisms of exocytosis and endocytosis, as they are in the clinical treatment of dystonias.

Triphenylphosphite neuropathy in hens

Single doses of triphenyl phosphite (TPP), a triester of trivalent phosphorus, cause ataxia and paralysis in hens. Characteristics of neurotoxicity were described as somewhat different from organophosphate induced delayed polyneuropathy (OPIDP), which is caused by triesters of pentavalent phosphorus. The onset of TPP neuropathy was reported to occur earlier than that of OPIDP (5-10 versus 7-14 days after dosing, respectively), and chromatolysis, neuronal necrosis and lesions in certain areas of the brain were found in TPP neuropathy only. Pretreatment with phenylmethanesulfonyl fluoride (PMSF) protects from OPIDP, but it either partially protected from effects of low doses or exacerbated those of higher doses of TPP. In order to account for these differences with OPIDP, it was suggested that TPP neuropathy results from the combination of two independent mechanisms of toxicity: typical OPIDP due to inhibition of neuropathy target esterase (NTE) plus a second neurotoxicity related with other target(s). We explored TPP neuropathy in the hen with attention to the phenomena of promotion and protection which are both caused by PMSF when given in combination with typical neuropathic OPs. When PMSF is given before neuropathic OPs it protects from OPIDP; when given afterwards it exaggerates OPIDP. The former effect is due to interactions with NTE, the latter to interactions with an unknown site. The time course of NTE reappearance after TPP (60 or 90 mg/kg i.v.) inhibition showed a longer half-life when compared to that after PMSF (30 mg/kg s.c.) (10-15 versus 4-6 days, respectively). The clinical signs of TPP neuropathy (60 or 90 mg/kg i.v.) were similar to those observed in OPIDP, appeared 7-12 days after treatment, correlated with more than 70% NTE inhibition/aging and were preceded by a reduction of retrograde axonal transport in sciatic nerve of hens. TPP (60 mg/kg i.v.) neuropathy was promoted by PMSF (120 mg/kg s.c.) given up to 12 days afterwards and was partially protected by PMSF (10-120 mg/kg s.c.) when given 24 h before TPP (60 or 90 mg/kg i.v.). The previously reported early onset of TPP neuropathy might be related to the higher dose used in those experiments and to the resulting more severe neuropathy. The lack of full protection might be explained by the slow kinetics of TPP, which would cause substantial NTE inhibition when PMSF effects on NTE had subsided. Since PMSF also affects the promotion site when given before initiation of neuropathy, the resulting neuropathy would then be due to both protection from and promotion of TPP effects by PMSF. No promotion by PMSF (120 mg/kg s.c.) was observed in TPP neuropathy (90 mg/kg i.v.) partially protected by PMSF (10-30 mg/kg s.c.). This might also be explained by the concurrent effects on NTE and on the promotion site obtained with PMSF pretreatment. We conclude that TPP neuropathy in the hen is likely to be the same as typical OPIDP. The unusual effects of combined treatment to hens with TPP and PMSF are explained by the prolonged pharmacokinetics of TPP and by the dual effect of PMSF i.e. protection from and promotion of OPIDP.

Promotion of peripheral axonopathies by certain esterase inhibitors

Certain esterase inhibitors were found to exacerbate the clinical signs of polyneuropathy caused by various neurotoxic compounds and to delay the recovery from nerve crush. This phenomenon is referred to as promotion of axonopathies. The molecular target of promotion has not yet been identified. However, all known promoters are also inhibitors of neuropathy target esterase (NTE), the putative target of organophosphate neuropathy, but it has been shown that the target of promotion is unlikely to be NTE. Available data suggest that promoters might affect a target and a mechanism present in the nervous system that is not activated by axonal lesions. Promotion may be important to understand the physiological mechanism of nerve damage and repair. This finding also implies a changing perspective for the risk assessment of exposures to esterase inhibitors, some of which are used as pesticides and might be promoters.

Local application of neuropathic organophosphorus compounds to hen sciatic nerve: inhibition of neuropathy target esterase and peripheral neurological impairments

Diisopropyl phosphorofluoridate (DFP), mipafox, cresylsaligenyl phosphate, and phenylsaligenyl phosphate were applied to a 1.5-cm segment of the common trunk of the sciatic nerve in adult hens. At doses of 18-182 micrograms mipafox and 9-110 micrograms DFP, inhibition of neuropathy target esterase (NTE) for the treated segment was over 80%, whereas for the adjacent distal and proximal segments inhibition was under 40%, 15 min after application. NTE was not affected in the peripheral distal terminations arising from the common sciatic nerve (peroneal branches), contralateral sciatic nerve, brain, and spinal cord. A 24-hr study suggested a displacement of the activity-free region toward more distal segments of the nerve. All animals treated with 55 and 110 micrograms DFP or 110 micrograms mipafox lost a characteristic avian retraction reflex in the treated leg 9-15 days after dosing, suggesting peripheral neurological alterations. Only hens dosed at the maximum dose in both extremities presented alterations in motility (Grade 1 or 2 on a 0-8 scale), suggesting no significant central nervous system alterations. Electron microscopy of peroneal branches showed axon swelling and accumulation of smooth endoplasmic reticulum similar to animals dosed systemically (s.c.) with 1-2 mg/kg DFP. The branches also contained granular and electron-dense materials, as well as some intraaxonal and intramyelinic vacuolization. Clinical effects were not observed in animals protected with a 30 mg/kg (s.c.) dose of phenylmethanesulphonyl fluoride. It is concluded that the peripheral neurological effects of local dosing correlate with the specific modification of NTE in a segment of sciatic nerve and that the axon is a more likely target than the perikaryon or nerve terminal in the triggering mechanism of this axonopathy.

Subacute ethanol consumption reverses p-xylene-induced decreases in axonal transport

Human exposure to organic solvents is often complicated by ethanol ingestion and the literature is replete with demonstrations of metabolic interactions between ethanol and organic solvents at a pharmacokinetic level. Because of the possible modulation of xylene toxicity by ethanol consumption, the present group of studies characterizes the effect of ethanol on the p-xylene-induced decrease in axonal transport in the rat optic system previously reported by our laboratory. Long-Evans, hooded, male rats were divided randomly into two groups: those receiving 10% ethanol in their drinking water and those receiving water only. These two groups were further subdivided into two groups which were either exposed by inhalation to 1600 ppm p-xylene for 6 h/day, 5 days/week for 8 exposure-days or were treated identically except that they were exposed to air while in the inhalation chambers. The ethanol-drinking rats were given ethanol 6 days prior to and on the days of the inhalation exposure. Immediately after removal from the inhalation chambers on the last exposure day, the animals were injected intraocularly with [35S]methionine and [3H]fucose to measure the synthesis and rapid axonal transport of proteins and glycoproteins, respectively, in the retinal ganglion cells. The animals were sacrificed 20 h later, and the amount of radioactivity in different areas of the retinal ganglion cells was determined by liquid scintillation counting. As in previous experiments, the xylene exposure group showed a significant reduction in axonal transport of proteins and glycoproteins, whereas the ethanol exposure alone produced no significant reductions in the transport of either proteins or glycoproteins. In the animals receiving both ethanol and xylene, however, the ethanol treatment prevented the decreased transport characteristic of the xylene only animals, i.e. in all areas of the optic projections the level of transport were similar to the level present in the control groups. These data suggest that the xylene-induced reduction in rapid axonal transport was reversed (or prevented) by subacute ethanol consumption.

The pathogenesis of organophosphate polyneuropathy

This review discusses the facts regarding organophosphate-induced delayed polyneuropathy (OPIDP) as they are related to its pathogenesis rather than being a comprehensive review of all available data. Neuropathy target esterase (NTE) is considered to be the molecular target for OPIDP which is affected by several esterase inhibitors. Such inhibitors are ranked according to their toxicological effects as follows: 1. Phosphates, phosphoroamidates, and phosphonates cause OPIDP when high amounts of NTE are inhibited. In most cases 70 to 80% inhibition is enough, whereas in others much more is required. 2. Phosphinates, carbamates, and sulfonyl halides cause either protection from or promotion of OPIDP when given before or after a neuropathic OP, respectively. Both effects are related to doses that inhibit NTE. Neuropathy is also caused by the combined treatment with a carbamate and a sulfonyl fluoride. The potency of a given NTE inhibitor to cause OPIDP is related to the chemistry of the residue left attached to NTE, in addition to its affinity for the enzyme. The capability of inhibited NTE to undergo the aging process distinguishes inhibitors with high from those with negligible or very low potency to cause OPIDP. Therefore, protection from neuropathic doses of effective OPs is obtained when NTE is mostly inhibited with nonageable inhibitors. Promotion of OPIDP is likely to involve another site besides NTE because it might occur when almost all NTE is affected. Promotion affects either progression or expression of OPIDP after the initial biochemical lesion on NTE. Since only NTE inhibitors have been proven to be promoters, it is possible that this site is made available after the initiation of OPIDP and that it may have biochemical properties indistinguishable from those of NTE of naïve birds. Age-related resistance to OPIDP also seems to be related to either progression or expression of OPIDP and/or to the different physiology of NTE at a given age. Previously reported resistance of rats to clinical OPIDP seems also to be age-dependent. The physiological function(s) of NTE is unknown, but some practical gains have been obtained from its identification, including OPIDP risk assessment and biomonitoring.

Phenylmethanesulfonyl fluoride elicits and intensifies the clinical expression of neuropathic insults

It has been recently reported that phenylmethanesulfonyl fluoride (PMSF) when given to hens after a neuropathic organophosphate (OP) promotes organophosphate-induced delayed polyneuropathy (OPIDP). Chicks are resistant to OPIDP despite high inhibition/aging of neuropathy target esterase (NTE), the putative target of OPIDP initiation. However, when PMSF (300 mg/kg s.c.) is given to chicks after di-butyl 2,2-dichlorovinyl phosphate (DBDCVP, 1 or 5 mg/kg s.c.), OPIDP is promoted. Inhibition/aging of at least 30% of NTE was thought to be an essential prerequisite for promotion to be elicited in adult hens. However, we observed in hens that when NTE is maximally affected (greater than 90%) by phenyl N-methyl N-benzyl carbamate (40 mg/kg i.v.), a non-ageable inhibitor of NTE, and then PMSF is given (120 mg/kg/day s.c. x 3 days) clinical signs of neuropathy become evident. Methamidophos (50 mg/kg p.o. to hens), which produces in vivo a reactivatable form of inhibited NTE, was shown either to protect from or promote OPIDP caused by DBDCVP (0.45 mg/kg s.c.), depending on the sequence of dosing. Because very high doses of methamidophos cause OPIDP, we considered this effect to be a "self-promoted" OPIDP. We concluded that NTE inhibitors might have different intrinsic activities for producing OPIDP once NTE is affected. Aging might differentiate highly neuropathic OPs, like DBDCVP, from less neuropathic OPs, like methamidophos, or from the least neuropathic carbamates, which require promotion in order for neuropathy to be expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Age sensitivity to organophosphate-induced delayed polyneuropathy. Biochemical and toxicological studies in developing chicks

Young animals are resistant to organophosphate-induced delayed polyneuropathy (OPIDP). The putative target protein in the nervous system for initiation of OPIDP in the adult hen is an enzyme called Neuropathy Target Esterase (NTE), which is dissected by selective inhibitors among nervous tissue esterases hydrolysing phenyl valerate (PV). We report here that the pool of PV-esterases sensitive to paraoxon was different in peripheral nerves of chicks as compared to that of hens while that of brain and spinal cord was not. NTE activity decreased with age in brain, spinal cord and peripheral nerve, but its sensitivity to several inhibitors remained unchanged. In the adult hen more than 70% inhibition of peripheral nerve NTE by neuropathic OPs is followed by deficit of retrograde axonal transport, axonal degeneration and paralysis. Similar NTE inhibition in 40-day-old or younger chicks however is not followed by changes in retrograde axonal transport nor by OPIDP. Chicks aged 60 to 80 days are only marginally sensitive to a single dose of DFP otherwise clearly neuropathic to hens. In vitro and in vivo phosphorylation by DFP and subsequent aging of brain NTE is similar both in chicks and in hens. The recovery of NTE activity monitored in vivo after inhibition by DFP is faster (half-life of about 3 days) in chick peripheral nerves as compared to chick brain, hen brain and hen peripheral nerve (half-life of about 5 days). It is concluded that the reduced sensitivity to OPIDP in chicks is not due to differences in OP-NTE interactions. The resistance might be explained by a more efficient repair mechanism, as suggested by the faster recovery of peripheral nerve NTE activity.

Promotion of organophosphate-induced delayed polyneuropathy by phenylmethanesulfonyl fluoride

Certain sulfonates, like phenylmethanesulfonyl fluoride (PMSF), carbamates, and phosphinates, when given prior to neuropathic doses of organophosphates such as diisopropyl phosphorofluoridate (DFP), protect hens from organophosphate-induced delayed polyneuropathy (OPIDP). Protection was related to inhibition of the putative target of OPIDP, which is called Neuropathy Target Esterase (NTE). NTE inhibition above 70-80% in the nervous system of hens followed by a molecular rearrangement called aging initiates OPIDP. PMSF and other protective chemicals inhibit NTE but OPIDP does not develop because aging cannot occur. DFP (1 mg/kg sc) inhibited NTE above 70-80% in peripheral nerve and caused OPIDP in hens. Lower doses (0.3 and 0.5 mg/kg sc) caused about 40-60% NTE inhibition and no or marginal OPIDP. Chlorpyrifos (90 mg/kg po) also caused OPIDP. When repeated (30 mg/kg sc daily for 9 days) or single (5-120 mg/kg sc) doses of PMSF were given after either DFP or chlorpyrifos, OPIDP developed in birds treated with nonneuropathic doses of DFP and was more severe in birds treated with chlorpyrifos or higher doses of DFP. PMSF increased NTE inhibition to greater than 90%. Promotion of OPIDP with a single dose of PMSF (120 mg/kg sc) was obtained in birds up to 11 days after a marginally neuropathic dose of DFP (0.5 mg/kg sc). Promotion was also obtained with phenyl N-methyl N-benzyl carbamate (40 mg/kg iv) but not with non-NTE inhibitors in vivo such as paraoxon or benzenesulfonyl fluoride when given at maximum tolerated doses. These results indicate that protection from OPIDP is only one effect of PMSF because promotion of OPIDP is also observed depending upon the sequence of dosing. Either effect is always related to the doses of PMSF, which inhibit NTE.

Protease activity in brain, nerve, and muscle of hens given neuropathy-inducing organophosphates and a calcium channel blocker

Activity of calcium-activated neutral protease (CANP or calpain), an enzyme responsible for degradation of axonal and muscle cytoskeletal elements, was determined in brain, sciatic nerve, and gastrocnemius muscle of hens given tri-ortho-tolyl phosphate (TOTP, 360 mg/kg po) or active congener phenyl saligenin phosphate (PSP, 2.5 mg/kg im) with and without a calcium channel blocker which ameliorated clinical signs of organophosphate-induced delayed neuropathy (nifedipine 1 mg/kg/day x 5). Calcium channel blocker administration was initiated 1 day prior to administration of organophosphate (OP). OP administration caused an increase in CANP activity in brain within 4 days and in sciatic nerve and gastrocnemius muscle within 2 days of administration. This increase did not occur if nifedipine was administered to PSP-treated hens. Total sciatic nerve calcium concentrations were also increased by PSP, but not until OP-treated hens were no longer being administered calcium blockers. This indicates that calcium channel blockers may contribute to amelioration of organophosphate-induced delayed neuropathy by attenuation of calcium-mediated disruption of axonal and muscle cytoskeletal homeostasis.

Retrograde axonal transport in chronic ethanol-fed and thiamine-deficient rats

Retrograde axonal transport may play an important role in the feedback regulation of protein synthesis in neuronal perikarya, and the anterograde axonal transport of protein. The "dying-back" neuropathies associated with thiamine deficiency and chronic alcoholism may arise as a consequence of altered axonal transport. We have reported alterations in fast anterograde axonal transport in rats as a result of ethanol exposure or thiamine deficiency. The present studies were undertaken to determine whether retrograde transport was also affected by these experimental treatments. One group of rats was fed a liquid diet containing 6.7% ethanol for 16 weeks. Another group of rats was made thiamine deficient with a thiamine-free diet for 4 weeks. Retrograde axonal transport was labeled by injecting the left sciatic nerve unilaterally with 3H-N-succinimidyl propionate. This compound covalently labels proteins in the nerve at the site of injection and is accumulated by retrograde axonal transport to sensory nerve cell bodies in the dorsal root ganglia and motor nerve cell bodies in the spinal cord. After 7 days, dorsal root ganglia of chronic ethanol-fed rats showed a significant 45% decrease in the amount of accumulated retrograde label compared to controls. No significant differences in accumulation were found in the spinal cords. These results suggest that the peripheral neuropathies caused by chronic alcoholism and thiamine deficiency follow different etiologies, and that motor and sensory fibers are affected differently by ethanol.

Organophosphates and delayed neuropathy--is NTE alive and well?

Neuropathy target esterase (NTE) is a membrane-bound protein with high esterase catalytic activity. The physiological function of the protein is not known and the catalytic activity is not essential to health of nerve axons. Nevertheless there is overwhelming evidence that modification of the structure of NTE by covalent binding of some organophosphorus esters initiates an irreversible polyneuropathy: this event can be monitored. The experimental evidence for this conclusion is reviewed and some conceptual objections are resolved. Studies of NTE have generated successful predictions concerning (1) prophylaxis; (2) structure-activity relationships including stereospecificity; (3) the effects of prolonged low-level administration of neurotoxicants; and (4) extrapolations from (a) NTE responses seen after low doses to enzyme and clinical effects seen after high doses, (b) from in vitro to in vivo, and (c) from hen to human responses. The relationship of initiation on NTE to subsequent events in development of neuropathy is considered. Purification of NTE is reaching the point where antibodies may be obtained for neurobiological study. No single rigid protocol can be devised for incorporation of NTE assays into toxicological evaluations. A proposed two-stage procedure requires interpretation of Stage 1 to influence the design of Stage 2.

In vivo and in vitro regional differential sensitivity of neuropathy target esterase to di-n-butyl-2,2-dichlorovinyl phosphate

Organophosphate-induced delayed polyneuropathy (OPIDP) is initiated by inhibition/aging of more than 70-75% of neuropathy target esterase (NTE). Di-n-butyl-2,2-dichlorovinyl phosphate (DBDCVP) (1 mg/kg s.c.) inhibited 96%, 86% and 83% of NTE in brain, spinal cord and peripheral nerve, respectively, and induced a typical central peripheral distal axonopathy in hens. A lower dose (0.45 mg/kg s.c.) caused 90%, 83% and 54% NTE inhibition in the same organs; by contrast, hens developed a spastic ataxia with axonal degeneration in spinal cord but not in peripheral nerve. With a dose of 0.2 mg/kg s.c., a suprathreshold inhibition of NTE was produced in brain (78%) but not in spinal cord (56%) and peripheral nerve (33%) and no morphological or clinical signs of neuropathy developed in hens. With doses up to 4.0 mg/kg s.c., acetylcholinesterase (AChE) inhibition was similar throughout the nervous system. In vitro time-course inhibition studies showed a different sensitivity to DBDCVP of NTE from peripheral nerve (ka = 5.4 x 10(6)) relative to that from spinal cord (ka = 13.9 x 10(6)) or brain (ka = 20.6 x 10(6)). In vitro I50s of DBDCVP for AChE were similar in brain, spinal cord and peripheral nerve (11-17 nM). These data support the hypothesis that the critical target for initiation of OPIDP is located in the nerve fiber, possibly in the axon and also suggest that peripheral nerve NTE has a different sensitivity to DBDCVP than the brain enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration of anterograde axonal transport in cat sciatic nerve by diisopropyl phosphorofluoridate

The effect of a neurotoxic dose of 5.0 mg/kg, s.c. diisopropyl phosphorofluoridate (DFP) on anterograde transport of 35S-methionine labeled proteins in cat peripheral nerve was studied. Seven days after dosing, after 24 h of flow there was 48% less radioactivity in the distal portion of the nerve. A lesser effect was also found at 4 and 14 days after dosing. After 10 or 14 h of flow, the height of the crest was unchanged, but the distance of the crest from the ganglia was greater in the DFP-treated animals. These experiments indicate that DFP treatment accelerates fast anterograde transport.

Axotomy-induced ornithine decarboxylase activity in the mouse dorsal root ganglion is inhibited by the vinca alkaloids

Vinca alkaloids were used to study the role of retrograde axon transport (RT) in activating neuron perikaryal repair response to nerve transection. Mouse lumbar dorsal root ganglia (DRG) (L4-L6) were excised 48 hours after unilateral transection of the sciatic nerve and ornithine decarboxylase (ODC) activity determined. ODC activity in DRG ipsilateral to nerve transection was increased 10-20 fold over contralateral values. Typical ODC activities in ipsilateral and contralateral DRG samples were 6.18 +/- 1.4 and 0.31 +/- 0.09 pmol 14CO2 released/h/3DRG, respectively. Systemic administration of single doses of either vincristine (1 mg/kg) or vinblastine (5 mg/kg) immediately prior to axotomy attenuated ODC induction in ipsilateral DRG by 39% and 47%, respectively. A direct inhibition of ODC activity in the DRG appears unlikely since only high concentrations of vinblastine (0.5-1.0 mM) were able to inhibit ODC activity in vitro. We suggest vinca alkaloids inhibit ODC induction as a consequence of disrupting retrograde axonal transport. Interruption of this intracellular communication mechanism may be etiologically linked to the the distal axon degeneration which follows repetitive exposure to vinca alkaloids and other agents that induce toxic axonal neuropathy.