Partial characterization of neuropathy target esterase and related phenyl valerate esterases from bovine adrenal medulla

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.

An alternative in vitro method for detecting neuropathic compounds based on acetylcholinesterase inhibition and on inhibition and aging of neuropathy target esterase (NTE)

Organophosphorus-induced delayed polyneuropathy (OPIDP) is a syndrome induced by certain organophosphorus compounds (OPs) through a mechanism based on the inhibition and further modification (aging) of neuropathy target esterase (NTE). OECD guidelines for testing the capability of OPs to trigger OPIDP include two in vivo tests with hens. Activities of acetylcholinesterase and NTE found in SH-SY5Y human neuroblastoma cells were inhibited by 10 different OPs with kinetics similar to those found with chicken brain enzymes (model system for in vivo and in vitro-ex vivo assays). NTE in SH-SY5Y cells inhibited by these OPs aged and reactivated similarly to that described for hen brain NTE ex vivo. In short, we have developed an alternative methodology for predicting the capability of OPs to induce OPIDP based on the inhibition kinetics of acetylcholinesterase and NTE and on the capability of OPs to age the inhibited NTE from SH-SY5Y cell line. The results obtained always agreed with the previously reported ex vivo results with hen brain. The developed methodology correctly predicted the neuropathic potential of the tested OPs in eight cases. The in vivo-in vitro discrepancies with two of the tested compounds can be explained on the basis of differences between in vivo and in vitro biotransformation.

Recovery of neuropathy target esterase activity after inhibition with mipafox and O-hexyl O-2,5-dichlorophenyl phosphoramidate in bovine chromaffin cell cultures

Neuropathy target esterase (NTE) is a membrane protein present in various tissues whose physiological function has been recently suggested to be the maintenance of phosphatidylcholine homeostasis. Inhibition and further modification of NTE by certain organophosphorus compounds (OPs) were related to the induction of the "organophosphorus induced delayed neuropathy". Bovine chromaffin cells were cultured at 75,000cells/well in 96-well plates and exposed to 25microM mipafox or 3microM O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) for 60min. Inhibitors were removed by washing cells three times with Krebs solution. Then NTE activity was assayed at 0, 24, 48 and 120h after exposure using the Biomek 1000 workstation. Immediately after mipafox treatment NTE activity represented 3% of the control (6.7+/-1.9mU/10(6) cells). At 24, 48 and 120h after removing inhibitor, recorded activities were 33%, 42% and 111% of their respective controls (5.7+/-3.1; 5.7+/-1.9; 5.4+/-0.0mU/10(6) cells, respectively). Treatment with HDCP also displayed a time-dependent pattern of NTE recovery. As NTE inhibited by phosphoramidates is not reactivated in homogenized tissues, these results confirm a time-dependent regeneration of NTE after inhibition by neuropathic OPs.

Comparison of chromaffin cells from several animal sources for their use as an in vitro model to study the mechanism of organophosphorous toxicity

It had been observed that the chromaffin cells of bovine adrenal medulla contain high levels of neuropathy target esterase (NTE), the esterase whose inhibition and aging is associated with induction of the organophosphorous induced delayed neuropathy. In this study, total esterase and NTE activities, and their inhibition kinetics by OPs are characterized in adrenal medulla of several species in order to find the best source for chromaffin cells. Total esterase activity in membrane fraction of bovine, equine, porcine, ovine and caprine were 6100+/-840, 4200+/-270, 5000+/-120, 28800+/-3000, and 10800+/-2400mU/gtissue, respectively (mean+/-S.D., n=3-4). NTE represented around 70%, 24%, 58%, 10% and 24% of the total esterases in the same tissues, respectively. It was deduced that NTE represents between 69% and 89% of the "B-activity" (activity resistant to 40microM paraoxon) in the membrane fraction of all species. The mipafox I(50) calculated for 30-min inhibition of NTE at 37 degrees Celsius ranged between 7.4 and 12microM. These values are in the range of that for brain NTE in hen (the usual model for testing OP delayed neurotoxicity). Considering that bovine adrenal medulla contains high NTE activity, that it represents a high proportion of total activity, it is easier to dissect than adrenal medulla from equine, caprine or ovine, and is more readily available than species cited previously, and that its inhibitory properties are similar to the classical hen brain model, it is deduced that bovine adrenal medulla is the most appropriate source of chromaffin cells to study OP toxicity, with porcine as the second alternative. The kinetic properties of chromaffin cell cultures from bovine and porcine were in accordance with their properties in homogenate and subcellular fractions, and they displayed an appropriate stability and viability of the primary culture to be used in in vitro toxicological studies for both mechanistic and testing purposes.

Bovine chromaffin cell cultures as model to study organophosporus neurotoxicity

Based on the high level of phenyl valerate esterase activities, and in particular of neuropathy target esterase (NTE) found in bovine adrenal medulla, chromaffin cells culture have been proposed as an alternative model for the study of organophosphorus neurotoxicity. Organophosphorus-induced polyneuropathy is a syndrome related to the inhibition and further modification by organophosphorus compounds of NTE (a protein that displays phenyl valerate esterase activity resistant to mipafox and sensitive to paraoxon). Total phenyl valerate esterase activities found in homogenate, particulate and soluble fractions of bovine adrenal medulla were 5200+/-35, 5000+/-280 and 1700+/-260 mU/g tissue, respectively. Cultured chromaffin cells displayed a total hydrolysing activity of 41+/-5 mU/10(6) cells. Homogenates of bovine adrenal medulla displayed only about 6% of activity sensitive to paraoxon. Most of the phenyl valerate esterase activity inhibited by mipafox (a neuropathy inducing compound) was found in particulate fraction. Cultured chromaffin cells displayed kinetics of inhibition by mipafox similar to the kinetics displayed by homogenates of bovine adrenal medulla. We conclude that NTE could be assayed in this system by only using one inhibitor (mipafox) instead of two (paraoxon and mipafox). Also, the proposal is supported of using chromaffin cells as in vitro model for the study of the role of NTE and related esterases in organophosphorus-induced polyneuropathy.

A comparative study of binding sites for diisopropyl phosphorofluoridate in membrane and cytosol preparations from spinal cord and brain of hens

Biochemical events in the initiation of organophosphorus induced delayed neurotoxicity (OPIDN) are not well understood. To find new putative target(s) for OPIDN, we investigated the biochemical and pharmacological characteristics of [3H] diisopropyl phosphorofluoridate (DFP) binding to membrane and cytosol preparations from the brain and spinal cord of hens in vitro. [3H]DFP binding to both preparations was determined by the specific binding obtained by subtracting non-specific binding from total binding. The specific binding sites of [3H]DFP were found not only on membrane but also in cytosol. Kd values were higher and Bmax values were lower in cytosol than in membrane. Moreover, the Kd values in both membrane and cytosol preparations from spinal cord were lower than those of brain. The Bmax values in membrane and cytosol were similar between brain and spinal cord. The specific binding to both preparations was markedly displaced by unlabeled DFP. The specific binding of DFP to the membrane was highly or partly displaced by organophosphorus compounds (OPs) or a carbamate, respectively. However, both the OPs and the carbamate had considerably weaker blocking effects on the specific binding of DFP to cytosol. None of the compounds known to interact with neuropathy target esterase (NTE) had a strong blocking effect on the specific binding of DFP to either membrane or cytosol. These results show that the specific binding of DFP to the membrane may be binding with cholinesterase (ChE). However, cytosol, especially in spinal cord, may have DFP binding sites other than ChE and NTE.

Correlation of binding sites for diisopropyl phosphorofluoridate with cholinesterase and neuropathy target esterase in membrane and cytosol preparations from hen

To find new putative target(s) for organophosphorus induced delayed neurotoxicity (OPIDN), we investigated the biochemical and pharmacological characteristics of [3H] diisopropyl phosphorofluoridate (DFP) binding to membrane and cytosol preparations from the brain and spinal cord of hens. Specific [3H]DFP binding was determined by subtracting non-specific binding from total binding. The binding sites of [3H]DFP, an organophosphate that induces OPIDN, were found not only on membrane but also in cytosol. Reduction of subsequent ex vivo specific [3H]DFP binding by in vivo pretreatment with unlabeled DFP was found in cytosol, not membrane. The reduced binding lasted to the onset of OPIDN, especially in spinal cord. These results suggest that the specific DFP binding sites in cytosol, rather than on membrane, are the most important with regard to the initiation of OPIDN. Inhibitors of cholinesterase (ChE) and neuropathy target esterase (NTE) other than DFP did not affect specific [3H]DFP binding to either membranes or cytosol in vivo. Additionally, inhibition of the activities of these esterases by these compounds was not consistent with either the degree of inhibition of the [3H]DFP binding or a time-dependent manner of OPIDN. These results suggest that DFP binding site(s) involved in the initiation of OPIDN may be different from the active sites of ChE and NTE.

NTE soluble isoforms: new perspectives for targets of neuropathy inducers and promoters

Neural carboxylesterases can be discriminated by differential inhibition assays with organophosphorus compounds (OPs), paraoxon (O,O'-diethyl p-nitrophenyl phosphate) and mipafox (N,N'-diisopropyl phosphorodiamidofluoridate) being the ones used to discriminate esterases that should be either irrelevant or candidates as targets of the mechanism of induction of the organophosphorus-induced delayed polyneuropathy (OPIDP). The brain membrane-bound phenyl valerate esterase (PVase) defined by Dr Johnson in 1969 as neuropathy target esterase (NTE) and recently cloned by Dr Glynn and coworkers is termed here as particulate NTE due to its association to the membrane particulate fraction. It is considered as the target of OPIDP and is the activity measured in standard NTE assays and toxicity tests. Following the same operational criteria in the soluble fraction of sciatic nerve a paraoxon-resistant but mipafox-sensitive PVase activity was described and termed as S-NTE, with an apparent lower sensitivity to some inhibitors than particulate NTE. Two isoforms (S-NTE1 and S-NTE2) were subsequently separated by gel filtration chromatography. In a partly purified S-NTE2 preparation polypeptides were identified in western blots by labelling with S9B [1-(saligenin cyclic phospho)-9-biotinyldiaminononane], the same biotinylated OP used to label and isolate particulate NTE, but not with anti-particulate NTE antibodies. From sequential inhibition protocols, inhibitor washing-out and time course inhibition studies it is deduced that reversibility of inhibition is a new factor introducing a higher complexity in the identification of the esterases that could be candidates as targets of the mechanisms of induction and/or promotion of neuropathy. We have evidences that in sciatic nerve soluble fraction a high proportion (about 70%) of the activity that is inhibited by paraoxon in the usual concurrent assay is quickly reactivated after removing paraoxon and it is permanently inhibited by mipafox. Under this improved sequential paraoxon/mipafox inhibition procedure S-NTE represents about 50% of total PVases while in the usual concurrent assay it was only apparently about 1-2%. Moreover with such criteria, S-NTE2 isoform(s) represents about 97-99% of total S-NTE, and S-NTE1 is only a marginal amount probably resulting of a partial solubilization from particulate NTE. Fixed time inhibiton curves with variable mipafox concentration failed to discriminate more than one component. However kinetic behaviour of the time progressive inhibition cannot be explained by a simple model with a single exponential mathematical component, indicating that either the possibility of more than one component or a more complex mechanistic model should be considered. Consequently both particulate NTE and S-NTE assay protocols and their role in induction and promotion of neuropathies will need to be reviewed. Data published by Drs Lotti, Moretto and coworkers suggest that particulate NTE cannot be the target of promotion of axonopathies. The proposal that S-NTE2 could be such a target is suggestive and under collaborative biochemical and toxicological studies.

Bovine chromaffin cells in culture show carboxylesterase activities sensitive to organophosphorus compounds

Carboxylesterase activities are widely distributed in a great variety of tissues; however, the biological function of these enzymes remains unclear. Some organophosphorus compounds induce a neurodegenarative syndrome related to the covalent modification of a carboxylesterase known as neuropathy target esterase. We investigated the expression of neuropathy target esterase and related carboxylesterase in bovine chromaffin cells with the aim of developing a potential in vitro model for studying the cellular function of carboxylesterase enzymes and toxic effects of organophosphorus compounds. Total phenyl valerate esterase exhibited an activity of 1.27 +/- 0.19 mU/10(5) cells (SD, n = 15). From the phenyl valerate esterase paraoxon and mipafox inhibition curves the following activities have been determined: B-activity (resistant to 40 microM paraoxon), 1.05 +/- 0.08 mU/10(5) cells (n = 8); C-activity (resistant to 40 microM paraoxon plus 250 microM mipafox), 0.12 +/- 0.05 mU/10(5) cells (n = 8); and neuropathy target esterase, calculated by the difference between B- and C-activities, 0.93 +/- 0.08 mU/10(5) cells (n = 8). All of these activities increased linearly with the number of cells and time of incubation with the substrate. Most of the phenol product of the reaction was released and detected in the extracellular medium. None of the components of the reaction were shown to affect cell viability when assessed by trypan blue exclusion. The study shows that bovine chromaffin cells possess carboxylesterase activities and respond to inhibition by paraoxon and mipafox, thus facilitating the discrimination of neuropathy target esterase. In conclusion, bovine chromaffin cells are appropriate as an in vitro cell model for studying toxic effects of organophosphorus compounds.

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.

Neuropathy target esterase (NTE) and organophosphorus-induced delayed polyneuropathy (OPIDP): recent advances

The identification of neuropathy target esterase (NTE) as the site for initiation of organophosphorus-induced delayed polyneuropathy (OPIDP) has led to informative acute and chronic neurotoxicity tests (adopted by OECD and EPA), to structure/activity and in vitro/in vivo predictions, and to a sound basis for extrapolations to man. Purification of the sodium dodecyl sulphate (SDS)-denatured 155-kDa sub-unit of NTE has enabled partial sequencing and molecular biological studies. A MAb to the chicken brain sub-unit and PAbs to synthetic peptides have been raised: preliminary experiments suggest that one is effective for immunohistochemistry of frozen tissue. cDNA libraries are being screened with synthetic oligonucleotides, polymerase chain reaction (PCR)-developed primers, and with Ab in order to obtain cloned NTE. Previous studies of NTE in vivo have not revealed its normal physiological function or the route from inhibition to degeneration of axons, but the current progress in molecular biology of NTE is applicable to study of the function of normal and organophosphorus (OP)-modified NTE in cultured neural cells.

Comparative studies of two organophosphorus compounds in the mouse

A rodent model, the albino mouse, was used to investigate the in vitro and in vivo capacity of 2 organophosphate (OP) compounds, mipafox and ecothiopate, to inhibit enzymes considered to be involved in the mechanisms of OP toxicity. Mipafox and ecothiopate were chosen as model compounds because the former can produce a delayed neuropathy whereas the latter does not. Mipafox (110 mumol/kg, s.c.) inhibited brain acetylcholinesterase (AChE), neuropathy target esterase (NTE) and phenylvalerate hydrolases by 58, 64 and 65%, while diaphragm AChE and phenylvalerate hydrolases were inhibited by 66 and 80%, respectively. In contrast, ecothiopate (0.5 mumol/kg) had no effect on brain NTE or on brain or diaphragm phenylvalerate hydrolases. At the same time, diaphragm AChE was inhibited by 60% while brain AChE activity had increased by 15% of control. Mipafox was a potent inhibitor of AChE and NTE in vitro. Although ecothiopate was a highly potent anti-ChE in vitro, it had no inhibitory effect on NTE.