Alteration in neurofilament axonal transport in the sciatic nerve of the diisopropyl phosphorofluoridate (DFP)-treated hen

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.

Screening for novel central nervous system biomarkers in veterans with Gulf War Illness

Gulf War illness (GWI) is primarily diagnosed by symptom report; objective biomarkers are needed that distinguish those with GWI. Prior chemical exposures during deployment have been associated in epidemiologic studies with altered central nervous system functioning in veterans with GWI. Previous studies from our group have demonstrated the presence of autoantibodies to essential neuronal and glial proteins in patients with brain injury and autoantibodies have been identified as candidate objective markers that may distinguish GWI. Here, we screened the serum of 20 veterans with GWI and 10 non-veteran symptomatic (low back pain) controls for the presence of such autoantibodies using Western blot analysis against the following proteins: neurofilament triplet proteins (NFP), tubulin, microtubule associated tau proteins (Tau), microtubule associated protein-2 (MAP-2), myelin basic protein (MBP), myelin associated glycoprotein (MAG), glial fibrillary acidic protein (GFAP), calcium-calmodulin kinase II (CaMKII) and glial S-100B protein. Serum reactivity was measured as arbitrary chemiluminescence units. As a group, veterans with GWI had statistically significantly higher levels of autoantibody reactivity in all proteins examined except S-100B. Fold increase of the cases relative to controls in descending order were: CaMKII 9.27, GFAP 6.60, Tau 4.83, Tubulin 4.41, MAG 3.60, MBP 2.50, NFP 2.45, MAP-2 2.30, S-100B 1.03. These results confirm the continuing presence of neuronal injury/gliosis in these veterans and are in agreement with the recent reports indicating that 25years after the war, the health of veterans with GWI is not improving and may be getting worse. Such serum autoantibodies may prove useful as biomarkers of GWI, upon validation of the findings using larger cohorts.

Sarin (GB, O-isopropyl methylphosphonofluoridate) neurotoxicity: critical review

Sarin (GB, O-isopropyl methylphosphonofluoridate) is a potent organophosphorus (OP) nerve agent that inhibits acetylcholinesterase (AChE) irreversibly. The subsequent build-up of acetylcholine (ACh) in the central nervous system (CNS) provokes seizures and, at sufficient doses, centrally-mediated respiratory arrest. Accumulation of ACh at peripheral autonomic synapses leads to peripheral signs of intoxication and overstimulation of the muscarinic and nicotinic receptors, which is described as "cholinergic crisis" (i.e. diarrhea, sweating, salivation, miosis, bronchoconstriction). Exposure to high doses of sarin can result in tremors, seizures, and hypothermia. More seriously, build-up of ACh at neuromuscular junctions also can cause paralysis and ultimately peripherally-mediated respiratory arrest which can lead to death via respiratory failure. In addition to its primary action on the cholinergic system, sarin possesses other indirect effects. These involve the activation of several neurotransmitters including gamma-amino-butyric acid (GABA) and the alteration of other signaling systems such as ion channels, cell adhesion molecules, and inflammatory regulators. Sarin exposure is associated with symptoms of organophosphate-induced delayed neurotoxicity (OPIDN) and organophosphate-induced chronic neurotoxicity (OPICN). Moreover, sarin has been involved in toxic and immunotoxic effects as well as organophosphate-induced endocrine disruption (OPIED). The standard treatment for sarin-like nerve agent exposure is post-exposure injection of atropine, a muscarinic receptor antagonist, accompanied by an oxime, an AChE reactivator, and diazepam.

Autoantibodies to nervous system-specific proteins are elevated in sera of flight crew members: biomarkers for nervous system injury

This descriptive study reports the results of assays performed to detect circulating autoantibodies in a panel of 7 proteins associated with the nervous system (NS) in sera of 12 healthy controls and a group of 34 flight crew members including both pilots and attendants who experienced adverse effects after exposure to air emissions sourced to the ventilation system in their aircrafts and subsequently sought medical attention. The proteins selected represent various types of proteins present in nerve cells that are affected by neuronal degeneration. In the sera samples from flight crew members and healthy controls, immunoglobin (IgG) was measured using Western blotting against neurofilament triplet proteins (NFP), tubulin, microtubule-associated tau proteins (tau), microtubule-associated protein-2 (MAP-2), myelin basic protein (MBP), glial fibrillary acidic protein (GFAP), and glial S100B protein. Significant elevation in levels of circulating IgG-class autoantibodies in flight crew members was found. A symptom-free pilot was sampled before symptoms and then again afterward. This pilot developed clinical problems after flying for 45 h in 10 d. Significant increases in autoantibodies were noted to most of the tested proteins in the serum of this pilot after exposure to air emissions. The levels of autoantibodies rose with worsening of his condition compared to the serum sample collected prior to exposure. After cessation of flying for a year, this pilot's clinical condition improved, and eventually he recovered and his serum autoantibodies against nervous system proteins decreased. The case study with this pilot demonstrates a temporal relationship between exposure to air emissions, clinical condition, and level of serum autoantibodies to nervous system-specific proteins. Overall, these results suggest the possible development of neuronal injury and gliosis in flight crew members anecdotally exposed to cabin air emissions containing organophosphates. Thus, increased circulating serum autoantibodies resulting from neuronal damage may be used as biomarkers for chemical-induced CNS 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.

Early differential cell death and survival mechanisms initiate and contribute to the development of OPIDN: a study of molecular, cellular, and anatomical parameters

Organophosphorus-ester induced delayed neurotoxicity (OPIDN) is a neurodegenerative disorder characterized by ataxia progressing to paralysis with a concomitant central and peripheral, distal axonapathy. Diisopropylphosphorofluoridate (DFP) produces OPIDN in the chicken that results in mild ataxia in 7-14 days and severe paralysis as the disease progresses with a single dose. White leghorn layer hens were treated with DFP (1.7 mg/kg, sc) after prophylactic treatment with atropine (1mg/kg, sc) in normal saline and eserine (1mg/kg, sc) in dimethyl sulfoxide. Control groups were treated with vehicle propylene glycol (0.1 ml/kg, sc), atropine in normal saline and eserine in dimethyl sulfoxide. The hens were euthanized at different time points such as 1, 2, 5, 10 and 20 days, and the tissues from cerebrum, midbrain, cerebellum, brainstem and spinal cord were quickly dissected and frozen for mRNA (northern) studies. Northern blots were probed with BCL2, GADD45, beta actin, and 28S RNA to investigate their expression pattern. Another set of hens was treated for a series of time points and perfused with phosphate buffered saline and fixative for histological studies. Various staining protocols such as Hematoxylin and Eosin (H&E); Sevier-Munger; Cresyl echt Violet for Nissl substance; and Gallocynin stain for Nissl granules were used to assess various patterns of cell death and degenerative changes. Complex cell death mechanisms may be involved in the neuronal and axonal degeneration. These data indicate altered and differential mRNA expressions of BCL2 (anti apoptotic gene) and GADD45 (DNA damage inducible gene) in various tissues. Increased cell death and other degenerative changes noted in the susceptible regions (spinal cord and cerebellum) than the resistant region (cerebrum), may indicate complex molecular pathways via altered BCL2 and GADD45 gene expression, causing the homeostatic imbalance between cell survival and cell death mechanisms. Semi quantitative analysis revealed that the order of severity of damage declines from the spino-cerebellar, ventral, and dorsal tract respectively, suggesting neuroanatomical specificity. Thus, early activation of cell death and cell survival processes may play significant role in the clinical progression and syndromic clinical feature presentation of OPIDN.

Exposure to tri-o-cresyl phosphate detected in jet airplane passengers

The aircraft cabin and flight deck ventilation are supplied from partially compressed unfiltered bleed air directly from the engine. Worn or defective engine seals can result in the release of engine oil into the cabin air supply. Aircrew and passengers have complained of illness following such "fume events". Adverse health effects are hypothesized to result from exposure to tricresyl phosphate mixed esters, a chemical added to jet engine oil and hydraulic fluid for its anti-wear properties. Our goal was to develop a laboratory test for exposure to tricresyl phosphate. The assay was based on the fact that the active-site serine of butyrylcholinesterase reacts with the active metabolite of tri-o-cresyl phosphate, cresyl saligenin phosphate, to make a stable phosphorylated adduct with an added mass of 80 Da. No other organophosphorus agent makes this adduct in vivo on butyrylcholinesterase. Blood samples from jet airplane passengers were obtained 24-48 h after completing a flight. Butyrylcholinesterase was partially purified from 25 ml serum or plasma, digested with pepsin, enriched for phosphorylated peptides by binding to titanium oxide, and analyzed by mass spectrometry. Of 12 jet airplane passengers tested, 6 were positive for exposure to tri-o-cresyl phosphate that is, they had detectable amounts of the phosphorylated peptide FGEpSAGAAS. The level of exposure was very low. No more than 0.05 to 3% of plasma butyrylcholinesterase was modified. None of the subjects had toxic symptoms. Four of the positive subjects were retested 3 to 7 months following their last airplane trip and were found to be negative for phosphorylated butyrylcholinesterase. In conclusion, this is the first report of an assay that detects exposure to tri-o-cresyl phosphate in jet airplane travelers.

Mice treated with chlorpyrifos or chlorpyrifos oxon have organophosphorylated tubulin in the brain and disrupted microtubule structures, suggesting a role for tubulin in neurotoxicity associated with exposure to organophosphorus agents

Exposure to organophosphorus (OP) agents can lead to learning and memory deficits. Disruption of axonal transport has been proposed as a possible explanation. Microtubules are an essential component of axonal transport. In vitro studies have demonstrated that OP agents react with tubulin and disrupt the structure of microtubules. Our goal was to determine whether in vivo exposure affects microtubule structure. One group of mice was treated daily for 14 days with a dose of chlorpyrifos that did not significantly inhibit acetylcholinesterase. Beta-tubulin from the brains of these mice was diethoxyphosphorylated on tyrosine 281 in peptide GSQQY(281)RALTVPELTQQMFDSK. A second group of mice was treated with a single sublethal dose of chlorpyrifos oxon (CPO). Microtubules and cosedimenting proteins from the brains of these mice were visualized by atomic force microscopy nanoimaging and by Coomassie blue staining of polyacrylamide gel electrophoresis bands. Proteins in gel slices were identified by mass spectrometry. Nanoimaging showed that microtubules from control mice were decorated with many proteins, whereas microtubules from CPO-treated mice had fewer associated proteins, a result confirmed by mass spectrometry of proteins extracted from gel slices. The dimensions of microtubules from CPO-treated mice (height 8.7 +/- 3.1 nm and width 36.5 +/- 15.5 nm) were about 60% of those from control mice (height 13.6 +/- 3.6 nm and width 64.8 +/- 15.9 nm). A third group of mice was treated with six sublethal doses of CPO over 50.15 h. Mass spectrometry identified diethoxyphosphorylated serine 338 in peptide NS(338)NFVEWIPNNVK of beta-tubulin. In conclusion, microtubules from mice exposed to chlorpyrifos or to CPO have covalently modified amino acids and abnormal structure, suggesting disruption of microtubule function. Covalent binding of CPO to tubulin and to tubulin-associated proteins is a potential mechanism of neurotoxicity.

Chronic, intermittent exposure to chlorpyrifos in rats: protracted effects on axonal transport, neurotrophin receptors, cholinergic markers, and information processing

Persistent behavioral abnormalities have been commonly associated with acute organophosphate (OP) pesticide poisoning; however, relatively little is known about the consequences of chronic OP exposures that are not associated with acute cholinergic symptoms. In this study, the behavioral and neurochemical effects of chronic, intermittent, and subthreshold exposures to the OP pesticide, chlorpyrifos (CPF), were investigated. Rats were injected with CPF s.c. (dose range, 2.5-18.0 mg/kg) every other day over the course of 30 days and then were given a 2-week CPF-free washout period. In behavioral experiments conducted during the washout period, dose-dependent decrements in a water-maze hidden platform task and a prepulse inhibition procedure were observed, without significant effects on open-field activity, Rotorod performance, grip strength, or a spontaneous novel object recognition task. After washout, levels of CPF and its metabolite 3,5,6-trichloro-2-pyridinol were minimal in plasma and brain; however, cholinesterase inhibition was still detectable. Furthermore, the 18.0 mg/kg dose of CPF was associated with (brain region-dependent) decreases in nerve growth factor receptors and cholinergic proteins including the vesicular acetylcholine transporter, the high-affinity choline transporter, and the alpha(7)-nicotinic acetylcholine receptor. These deficits were accompanied by decreases in anterograde and retrograde axonal transport measured in sciatic nerves ex vivo. Thus, low-level (intermittent) exposure to CPF has persistent effects on neurotrophin receptors and cholinergic proteins, possibly through inhibition of fast axonal transport. Such neurochemical changes may lead to deficits in information processing and cognitive function.

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.

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.

Organophosphorus ester-induced chronic neurotoxicity

Organophosphorus compounds are potent neurotoxic chemicals that are widely used in medicine, industry, and agriculture. The neurotoxicity of these chemicals has been documented in accidental human poisoning, epidemiological studies, and animal models. Organophosphorus compounds have 3 distinct neurotoxic actions. The primary action is the irreversible inhibition of acetylcholinesterase, resulting in the accumulation of acetylcholine and subsequent overstimulation of the nicotinic and muscarinic acetylcholine receptors, resulting in cholinergic effects. Another action of some of these compounds, arising from single or repeated exposure, is a delayed onset of ataxia, accompanied by a Wallerian-type degeneration of the axon and myelin in the most distal portion of the longest tracts in both the central and peripheral nervous systems, and is known as organophosphorus ester-induced delayed neurotoxicity (OPIDN). In addition, since the introduction and extensive use of synthetic organophosphorus compounds in agriculture and industry half a century ago, many studies have reported long-term, persistent, chronic neurotoxicity symptoms in individuals as a result of acute exposure to high doses that cause acute cholinergic toxicity, or from long-term, low-level, subclinical doses of these chemicals. The author attempts to define the neuronal disorder that results from organophosphorus ester-induced chronic neurotoxicity (OPICN), which leads to long-term neurological and neurobehavioral deficits. Although the mechanisms of this neurodegenerative disorder have yet to be established, the sparse available data suggest that large toxic doses of organophosphorus compounds cause acute necrotic neuronal cell death in the brain, whereas sublethal or subclinical doses produce apoptotic neuronal cell death and involve oxidative stress.

Differential alteration of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA in the central nervous system of hens treated with diisopropylphosphorofluoridate (DFP)

A single dose (1.7 mg/kg, s.c.) of diisopropylphosphorofluoridate (DFP) causes organophosphorus ester-induced delayed neurotoxicity (OPIDN) in susceptible species. We studied the effects of DFP administration on the mRNA expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), an important glycolytic protein at different time points (1, 2, 5, 10 and 20 days) post-treatment. Total RNA was extracted from cerebrum, cerebellum, brainstem, midbrain, and spinal cord of the control and DFP-treated hens, and northern blots were prepared using standard protocols and hybridized with GAPDH, as well as beta-actin and 28S RNA cDNA (control) probes. There was a distinct spatial/temporal mRNA expression pattern for the different tissues studied. Non-susceptible tissue, cerebrum showed a dramatic increase in GAPDH mRNA at day 1, post-treatment and levels remained high at all time points, suggestive of protective mechanisms from the beginning. In contrast, highly susceptible tissues like brainstem, spinal cord and midbrain showed either no elevation or slight down-regulation at day 1, suggesting trauma and cell injury/cell death. Overall, there was moderate level of induction during the subsequent time points in these tissues, indicative of pathways of either recovery or degeneration. Cerebellum being the less susceptible tissue showed moderate increase initially, followed by higher induction, suggestive of rapid recovery. Our current data on GAPDH provides an important link in this complex network of molecular changes involving pathways identified by our group and others, such as nitric oxide (NO), CaM kinase-II (CaMK-II), protein kinase-A (PKA), c-fos, and phosphorylated-CREB (p-CREB) in DFP-induced OPIDN.

Early differential elevation and persistence of phosphorylated cAMP-response element binding protein (p-CREB) in the central nervous system of hens treated with diisopropyl phosphorofluoridate, an OPIDN-causing compound

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus-ester-induced delayed neurotoxicity in sensitive species. We studied the effect of single dose of DFP on the expression of phosphorylated cAMP-response element binding protein (p-CREB), which is a well known transcription factor involved in several pathways mediating different types of external stimuli. The hens were perfused with neutral buffered formalin at different time points, i.e., 0.5, 1.0, and 2.0 hrs, as well as 1, 2, 5, and 20 days after dosing. The central nervous system regions of the whole brain were dissected and 7-micron sections were stained for either p-CREB immunopositivity or with hematoxylin and eosin. Results indicated an early differential increase of p-CREB immunopositivity in susceptible regions such as cerebellum, brainstem, and midbrain within 2 hrs. These induced levels persisted upto 5 days in these tissues, although the time course of p-CREB immunopositivity was distinctly different for each region. In the cerebellum induction of p-CREB was seen in the granular layer where both the granulocytes and the glial cells showed induction. Increased immunopositivity for p-CREB in the Purkinje cells and in some basket cells of the molecular layer was noticed over time, but the induction was not as great as in the granular layer. Of all the tissues cerebellum showed the strongest intensity of immunopositivity of the cells as well as the highest (absolute) number of pCREB-positive cells. The brainstem showed a similar fluctuating pattern like the cerebellum with the highest percentage increase of the immunoreactive cells at 5 days preceded by the lowest dip in immunopositivity at 2 days. In the midbrain, there was a time-dependent increase in the immunopositivity from 0.5 hr onwards until reaching control levels at 20 days. Immunopositivity was also noted in portions of the spina medularis and spina oblongata. The cerebrum (non-susceptible tissue) of DFP-treated hens did not show much deviation from the controls. The endothelial cells of the susceptible regions showed induction at early time points, in contrast to the absence of induction in cerebrum. Spatial and temporal differences in the immunopositivity pattern indicate probable involvement of CREB-independent pathways also. Overall, the complex induction pattern of p-CREB, along with our earlier observations of the early induction of c-fos, c-jun and Protein Kinase A (PKA) as well as the induction of Calcium2+/Calmodulin dependent Protein Kinase II (CaM kinase II) at later periods, strongly suggest an activator role of CREB mediated pathways that may lead to the clinical development of delayed neurotoxicity.

Sarin causes altered time course of mRNA expression of alpha tubulin in the central nervous system of rats

Sarin induced neurotoxicity is suspected to be one of the key factors responsible for Gulf-war syndrome. We studied the effect of a single (50 microg/kg/i.m) dose of sarin (0.5 x LD50) on the mRNA expression of alpha tubulin in the central nervous system (CNS) of rats which were sacrificed at different time points i.e. 1 and 2 hrs, as well as, 1, 3 and 7 days post-treatment. Northern data collected from CNS regions indicate differential, spatial, and temporal regulation of alpha tubulin mRNA levels. Immediate induction and persistence of alpha tubulin transcripts in sarin-treated CNS suggest that sarin-induced neurotoxicity is in part mediated by the altered expression of cytoskeletal genes which may be regulated at multiple levels.

Altered time course of mRNA expression of alpha tubulin in the central nervous system of hens treated with diisopropyl phosphorofluoridate (DFP)

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus-ester induced delayed neurotoxicity (OPIDN) in the hen, human and other sensitive species. We studied the effect of single dose of DFP (1.7 mg/kg/s.c.) on the expression of alpha tubulin which is one of the major sub-unit of tubulin polymers that constitute an important constituent of cellular architecture. The hens were sacrificed at different time points i.e. 1, 2, 5, 10, and 20 days. Total RNA was extracted from the following brain regions: cerebrum, cerebellum, and brainstem as well as spinal cord. Northern blots prepared using standard protocols were hybridized with alpha tubulin as well as with beta-actin and 28S RNA cDNA (controls) probes. The results indicate a differential/spatial/temporal regulation of alpha tubulin levels which may be the result of perturbed microtubule dynamics not only in the axons but also in perikarya of neurons in the CNS of DFP treated hens. In the highly susceptible tissues like brainstem and spinal cord the initial down-regulation of mRNA levels could be attributed to DFP induced stress response resulting in inhibited cell metabolism and or cell injury/cell death. Increase in levels of mRNA at 5 days and thereafter coincided with increased tubulin transport which may be due to increased phosphorylation of tubulins in both axons and perikarya and other intraaxonal changes resulting in impaired axonal transport. DFP induced decreased rate of tubulin polymerization resulting in increased levels of free tubulin monomers may be involved in the altered alpha tubulin mRNA expression at different time points by autoregulatory circuits. Cerebellum being the less susceptible tissue showed only a moderate decline at day 2, while the alpha tubulin remained at near control levels at day 1. Delayed down-regulation may be due to the co-ordinated up or down-regulation of different sub-types of alpha and beta tubulins as well as the differential response of specialised cell types in cerebellum. Continuous overexpression of alpha tubulin in cerebrum from the beginning may be its effective protective strategy to safeguard itself from neurotoxicity. Differential expression pattern observed could be due to the differential susceptibility and variability in the rate of axonal transport of different regions besides the tubulin heterogenity of CNS. Hence our results indicte differential expression of alpha tubulin is either one of the reasons for the development of OPIDN or the result of progressive changes taking place during OPIDN.

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.

Altered expression of neurofilament subunits in diisopropyl phosphorofluoridate-treated hen spinal cord and their presence in axonal aggregations

Diisopropyl phosphorofluoridate (DFP) is an organophosphorus ester, which produces organophosphorus ester-induced delayed neuropathy (OPIDN) in hen and other sensitive species. A single dose of DFP (1.7 mg/kg, sc.) produces mild ataxia in 7-14 days in hens, which develops into severe ataxia or paralysis with the progression of disease. OPIDN is associated with axonal swellings and degeneration of axons. This study was carried out to investigate the expression of neurofilament (NF) subunits in the spinal cord of DFP-treated hens. Hens were treated with a single dose of DFP and sacrificed 1, 5, 10, and 20 days post-treatment. Western blot analysis showed increased expression of middle molecular weight neurofilament protein (NF-M), and decreased expression of high molecular weight (NF-H) and low molecular weight (NF-L) neurofilament proteins in the 2 M urea extracts of spinal cord particulate fraction. These changes were observed within 24 h of DFP administration and persisted for 10-20 days. Thus, there was increase in the stoichiometry of NF-M:NF-L in the spinal cord of DFP-treated hens. Immunoprecipitation, cross-linking, and two-dimensional polyacrylamide gel electrophoresis showed the presence of heterodimers, but not heterotetramers, in the hen spinal cord extract. Immunohistochemical staining revealed the presence of all three NF subunits in the cytoskeletal inclusions in DFP-treated hen spinal cord cross-sections. The results suggested that each NF subunit might be accumulated by a different mechanism in the axonal aggregations of DFP-treated hen.

Electrophysiological and biochemical effects of single and multiple doses of the organophosphate diazinon in the mouse

Diazinon is an organophosphorus compound (OP) widely used in pesticides. The relationship between dose of diazinon, inhibition of acetylcholinesterase, and effect on neuromuscular transmission has been studied in a mouse model. Inhibition of acetylcholinesterase activity occurred within 1 h, was maximal by 3 h and remained inhibited for at least 24 h. Blood, brain, diaphragm, and soleus acetylcholinesterase activities were differentially affected by diazinon. Brain and soleus activities were not affected by low doses. Multiple daily dosing of diazinon caused a cumulative decrease in acetylcholinesterase activity, although to a lesser extent in brain and soleus. Diazinon had no effect on the activity of neuropathy target esterase. Plasma and brain levels of diazinon peaked at 15 min after dosing and declined with a half-life of 2.5 h. Metabolic products of diazinon were cleared from the urine within 24 h. Increased miniature end-plate current half decay times occurred in a dose-dependent manner. Single doses of diazinon caused an increase in the jitter (variability of latencies) of evoked action potentials recorded in the diaphragm but did not affect end-plate potential (EPP) jitter. Multiple lower doses of diazinon caused an increase in EPP jitter after 28 days. This effect on nerve function was delayed and occurred when acetylcholinesterase activity had returned to control levels. The results indicate that diazinon produces long-term electrophysiological changes in neurotransmission following repeated dosing in the mouse. This has implications for the current use of diazinon; however, there is a need to further define the mechanism of this effect.

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.

Tau phosphorylation by diisopropyl phosphorofluoridate (DFP)-treated hen brain supernatant inhibits its binding with microtubules: role of Ca2+/Calmodulin-dependent protein kinase II in tau phosphorylation

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus ester-induced delayed neurotoxicity (OPIDN) in hen, human, and other sensitive species. This is characterized by mild ataxia, which progresses to severe ataxia or paralysis in a few days. Ultrastructurally, OPIDN is associated with the degeneration of axons in central and peripheral nervous systems. Bacterially expressed longest human tau protein (htau40) phosphorylated by DFP-treated hen brain supernatant showed a decrease in microtubule binding in a shorter time than that phosphorylated by control hen brain supernatant. The decrease in htau40-microtubule binding observed on htau40 phosphorylation by the recombinant Ca2+/calmodulin (CaM)-dependent protein kinase II (CaM kinase II) alpha-subunit showed that CaM kinase II present in brain supernatant could participate in tau phosphorylation even in the absence of Ca2+/CaM and decrease tau-microtubule binding. In addition, use of htau40 mutants, htau40m1 (Ala416) and htau40m6 (Asp416), suggested that replacement of Ser416 by neutral or acidic amino acid produced some change in htau40 conformation that caused diminished binding with microtubules phosphorylated by brain supernatant in the presence of ethylene glycol bis(beta-aminoethyl ether) N, N'tetraacetic acid (EGTA). The change in conformation produced by Ser416 phosphorylation, however, was different from that produced by mutants since only nonmutated htau40 showed a significant decrease in binding with microtubules on phosphorylation by recombinant CaM kinase II in the presence of Ca2+/CaM compared to that obtained by phosphorylation in the presence of EGTA. This study showed that enhanced Ca2+/CaM-dependent protein kinase activity in DFP-treated hen brain supernatant may cause decreased tau-microtubule binding and destabilization of microtubules and may be involved in axonal degeneration in OPIDN.