Enhanced activity and level of protein kinase A in the spinal cord supernatant of diisopropyl phosphorofluoridate (DFP)-treated hens. Distribution of protein kinases and phosphatases in spinal cord subcellular fractions

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.

Organophosphorus compound effects on neurotrophin receptors and intracellular signaling

Neurite outgrowth of SH-SY5Y neuroblastoma cells following the addition of spinal cord extracts from chickens exposed to a neuropathic organophosphorus (OP) compound suggests the presence of a growth factor during OP neuropathy. However, exposure of SH-SY5Y cells directly to neuropathic OP compounds results in apoptosis and/or decreased neurite outgrowth. These cellular effects may follow OP-induced interference with neurotrophin-receptor binding and/or intracellular signaling resulting from receptor binding. We hypothesized that sub-lethal concentrations of a neuropathic OP compound interferes with neurotrophin-receptor binding as well as specific intracellular signaling pathways in neuroblastoma cells which would not occur with a non-neuropathic OP compound. SH-SY5Y cells were exposed to a neuropathic OP compound (PSP; 0.01, 0.1, 1.0μM), a neuropathic OP compound with nerve growth factor (1.0μM PSP+1ng/ml NGF), a non-neuropathic OP compound (paraoxon; 100μM), and medium only for 4, 8, 24, and 48h. Western blots indicate that cells exposed to a low dose of PSP or the high dose of PSP+NGF contained the phosphorylated form of a common neurotrophin receptor (pp75) that was four times greater than that of the phosphorylated form of the high-affinity NGF receptor (pTrkA) suggesting that p75 activation may contribute to early cell death after exposure to OP compounds. Furthermore, events in signaling pathways after exposure to PSP differed from those after exposure to paraoxon, with activation of the MEK1/2 protein increasing significantly only after exposure to paraoxon. Both types of OP compounds, however, caused significant activation of Akt in the PI-3K cell-survival pathway. These results suggest that exposure to a non-neuropathic OP compound causes increased activity of the MAPK pathway whereas exposure to neuropathic OP compounds prevented upregulation of the pathway. Since this pathway is integral to neurite outgrowth and cell survival, this study has revealed molecular mechanisms implicated in neuronal response after exposure to neuropathic OP compounds.

Aggregates of cAMP-dependent kinase isoforms characterize different areas in the developing central nervous system of the chicken, Gallus gallus

The intracellular second messenger adenosine 3',5'-cyclic monophosphate (cAMP) acts mainly through cAMP-dependent protein kinases (PKA). In mammals and reptiles, the PKA regulatory isoforms (RI and RII) are differentially distributed among the various brain areas and cell types, according to the age of the animal. Since PKA distribution may be an additional marker for homologous areas, PKA regulatory subunit types RI and RII were examined in the chicken brain, a species not yet investigated. Chicken brains were examined from prehatching to adult age, by means of immunohistochemistry and biochemical characterization. Most PKA regulatory subunits were segregated in discrete non-soluble clusters that contained either RI or RII. While RII aggregates were present also in non-neuronal cells, RI aggregates were detected only in neurons of some brain areas that are mainly related to the telencephalon. They appeared later than RII aggregates; their presence and location varied during development. RI aggregates were detected first in the olfactory bulb, around embryonic day 14; within 3 days they appeared in the hyperpallium and nidopallium, where the most intense labeling was observed in the perihatching period. Fainter RI aggregates persisted up to 3 years in the olfactory bulb and nidopallium caudale. Less intense RI aggregates were present for a shorter time, from 2 weeks to 3 months, in the septal nuclei, thalamic medial nuclei, periventricular hypothalamus, optic tectum periventricular area, brainstem reticular formation and spinal cord substantia gelatinosa. RI aggregates were not detected in many brain areas including the arcopallium, striatum and cranial nerve nuclei. RII distribution showed less variation during development. From embryonic day 12, some insoluble RII aggregates were detected in the brain; however, only minor modifications were observed in positive structures once they started to harbor insoluble RII aggregates. The present results suggest that the distribution of PKA aggregates may assist in characterizing phylogenetically homologous structures of the vertebrate central nervous system and may also unravel biochemical differences among areas considered homologous.

Okadaic acid-sensitive protein phosphatases constrain phrenic long-term facilitation after sustained hypoxia

Phrenic long-term facilitation (pLTF) is a serotonin-dependent form of pattern-sensitive respiratory plasticity induced by intermittent hypoxia (IH), but not sustained hypoxia (SH). The mechanism(s) underlying pLTF pattern sensitivity are unknown. SH and IH may differentially regulate serine/threonine protein phosphatase activity, thereby inhibiting relevant protein phosphatases uniquely during IH and conferring pattern sensitivity to pLTF. We hypothesized that spinal protein phosphatase inhibition would relieve this braking action of protein phosphatases, thereby revealing pLTF after SH. Anesthetized rats received intrathecal (C4) okadaic acid (25 nm) before SH (25 min, 11% O(2)). Unlike (vehicle) control rats, SH induced a significant pLTF in okadaic acid-treated rats that was indistinguishable from rats exposed to IH (three 5 min episodes, 11% O(2)). IH and SH with okadaic acid may elicit pLTF by similar, serotonin-dependent mechanisms, because intravenous methysergide blocks pLTF in rats receiving IH or okadaic acid plus SH. Okadaic acid did not alter IH-induced pLTF. In summary, pattern sensitivity in pLTF may reflect differential regulation of okadaic acid-sensitive serine/threonine phosphatases; presumably, these phosphatases are less active during/after IH versus SH. The specific okadaic acid-sensitive phosphatase(s) constraining pLTF and their spatiotemporal dynamics during and/or after IH and SH remain to be determined.

Toxicogenomic studies of the rat brain at an early time point following acute sarin exposure

We have studied sarin-induced global gene expression patterns at an early time point (2 h: 0.5 x LD50) using Affymetrix Rat Neurobiology U34 chips and male Sprague-Dawley rats. A total of 46 genes showed statistically significant alterations from control levels. Three gene categories contained more of the altered genes than any other groups: ion channel (8 genes) and calcium channel and binding proteins (6 genes). Alterations were also found in the following gene groups: ATPases and ATP-based transporters (4), growth factors (4), G-protein-coupled receptor pathway-related molecules (3), neurotransmission and neurotransmitter transporters (3), cytoskeletal and cell adhesion molecules (2), hormones (2), mitochondria-associated proteins (2), myelin proteins (2), stress-activated molecules (2), cytokine (1), caspase (1), GABAnergic (1), glutamergic (1), immediate early gene (1), prostaglandin (1), transcription factor (1), and tyrosine phosphorylation molecule (1). Persistent alteration of the following genes also were noted: Arrb1, CaMKIIa, CaMKIId, Clcn5, IL-10, c-Kit, and Plp1, suggesting altered GPCR, kinase, channel, and cytokine pathways. Selected genes from the microarray data were further validated using relative RT-PCR. Some of those genes (GFAP, NF-H, CaMKIIa, Calm, and MBP) have been shown by other laboratories and ours, to be involved in the pathogenesis of sarin-induced pathology and organophosphate-induced delayed neurotoxicity (OPIDN). Induction of both proapoptotic (Bcl2l11, Casp6) and antiapoptotic (Bcl-X) genes, besides suppression of p21, suggest complex cell death/protection-related mechanisms operating early on. Principal component analysis (PCA) of the expression data confirmed that the changes in gene expression are a function of sarin exposure, since the control and treatment groups separated clearly. Our model (based on current and previous studies) indicates that both degenerative and regenerative pathways are activated early and contribute to the level of neurodegeneration at a later time, leading to neuro-pathological alterations.

Sarin (nerve agent GB)-induced differential expression of mRNA coding for the acetylcholinesterase gene in the rat central nervous system

We carried out a time-course study on the effects of a single intramuscular (i.m.) dose (0.5x LD(50)) of sarin (O-isopropyl methylphosphonofluoridate), also known as nerve agent GB, on the mRNA expression of acetylcholinesterase (AChE) in the brain of male Sprague-Dawley rats. Sarin inactivates the enzyme AChE which is responsible for the breakdown of the neurotransmitter acetylcholine (ACh), leading to its accumulation at ACh receptors and overstimulation of the cholinergic system. Rats were treated with 50 microg/kg of sarin (0.5x LD(50)) in 1 mL saline/kg and terminated at the following time points: 1 and 2 hr and 1, 3, and 7 days post-treatment. Control rats were treated with normal saline. Total RNA was extracted, and northern blots were hybridized with cDNA probes for AChE and 28S RNA (control). Poly-A RNA from both treated and control cortex was used for reverse transcription-polymerase chain reaction (RT-PCR)-based verification of the data from the northern blots. The results obtained indicate that a single (i.m.) dose of sarin (0.5x LD(50)) produced differential induction and persistence of AChE mRNA levels in different regions of the brain. Immediate induction of AChE transcripts was noted in the brainstem (126+/-6%), cortex (149+/-4%), midbrain (153+/-5%), and cerebellum (234+/-2%) at 1 hr. The AChE expression level, however, increased over time and remained elevated after a decline at 1 day in the previously shown more susceptible brainstem. The transcript levels remained elevated at a later time point (3 days) in the midbrain, after a dramatic decline at day 1 (110+/-2%). In the cortex, transcript levels came down to control values by day 1. The cerebellum also showed a decline of the elevated levels observed at 2 hr (275+/-2%) to control values by day 1. RT-PCR analysis of the AChE transcript at 30 min in the cortex showed an induction to 213+/-3% of the control level, confirming the expression pattern obtained by the northern blot data. The immediate induction followed by the complex pattern of the AChE mRNA time-course in the CNS may indicate that the activation of both cholinergic-related and unrelated functions of the gene plays an important role in the pathological manifestations of sarin-induced neurotoxicity.

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.