Research at the Medical Research Council

Organophosphorus pesticide research at the MRC Toxicology Unit, Leicester: Target proteins for organophosphorus pesticides

Scientific staff: Drs. D. E. Ray, P. Glynn, Y. Li, and P. G. Richards

Some forms of organophosphorus pesticide toxicity are well defined in terms of the effects on man and the mechanism of the toxicity (such as the acute cholinergic crisis), but others are either poorly defined or controversial in nature. Our objective has been to identify the protein targets associated with these less well defined forms of toxicity, and so improve our understanding of them. Once characterised these proteins can be used to predict the toxic potential of individual organophosphates, to monitor exposure, and to understand molecular mechanisms of toxicity.

We have identified a number of nervous system proteins that are selectively targeted by specific organophosphates. We have already shown that the delayed neuropathy produced by some organophosphates is initiated by a defined action on one target protein discovered by our group: neuropathy target esterase (NTE). The biological function of NTE is now being investigated using molecular biology approaches. NTE has been purified, sequenced, an active fragment expressed in cell culture, and antibodies have been raised. Within the nervous system, the NTE protein is present only in neuronal cells, and is expressed from a very early stage of development. It is an intracellular membrane protein, and an active fragment of it can form ion channels in artificial membranes. It is structurally unrelated to other identified organophosphate targets in mammals, and is essential for normal brain development in fruit flies and mice. NTE transgenic mice are being produced to study the normal biological function of the protein, and its role in producing nerve fibre degeneration. The ability of individual organophosphates to produce an allosteric change in the NTE protein in vitro is an excellent predictor of their potential to produce delayed polyneuropathy, but not of other forms of toxicity.

Investigations of other toxic effects of organophosphates are at the earlier stage of identification of their target proteins using novel radiolabelled protein detection technology. This will be followed by characterisation of the proteins as they are identified. Several new target proteins have already been found in rat brain, some of which are sensitive enough to be affected at organophosphate dose levels producing no overt toxicity. These are being purified as a first step to identify their function, and one has already been identified as acylpeptide hydrolase, which may be the target for the cognitive enhancement effect produced by low doses of some organophosphates. We will attempt to link these new protein targets with specific effects of long-term medium-level exposure to organophosphates on brain function seen in rats by collaboration with Professor Laszlo Nagymajtenyi and co-workers in Hungary.

A better ability to understand and predict the toxicity of organophosphates is important for this very varied class, the less toxic members of which make a major contribution to human and animal health world-wide, but the more toxic of which represent a major health problem. The identification of new targets which can be linked to other more subtle forms of organophosphate toxicity will aid their investigation and clinical detection. Since it is likely that different organophosphates will have very different effects on these novel targets, it is important to better determine which compounds are active and which are benign. The therapeutic potential of organophosphates and related agents in Alzheimers disease may be extended by a better understanding of novel target processes which may help to reduce side-effects. In addition, since NTE is one of very few proteins causally linked to a human neurodegenerative condition, better understanding of its dysfunction may benefit human health generally.

References:

Dési I, & Nagymajtényi L (1999) Electrophysiological biomarkers of an organophosphorous pesticide, dichlorvos. Tox. Let. 107: 55-64.

Glynn, P, Holton, JL, Nolan, CC, Read, DJ, Brown, L, Hubbard, A and Cavanagh, JB (1998) Neuropathy Target Esterase: immunolocalisation to neuronal cell bodies and axons. Neuroscience 83:295-302.

Lush, MJ, Li, Y, Read, DJ, Willis, AC & Glynn, P (1998) Neuropathy target esterase (NTE) and a homologous Drosophila neurodegeneration-associated mutant protein contain a novel domain conserved from bacteria to man. Biochem. J. 332:1-4.

IEH (1998) Organophosphorus esters: A review of neurotoxic effects (Report SR5), Leicester, UK, Institute for Environment and Health. ISBN 1 899110 19 4.

Richards, P., Johnson, M., Ray, D. & Walker, C. (1999) Novel protein targets for organophosphorus compounds. Chem. Biol. Interact. 119-120: 503-511.

Ray, D.E. (1999) Chronic effects of low level exposure to anticholinesterases - a mechanistic review. Toxicol. Lett. 103:527-533.