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

Serine/Threonine Protein Phosphatases 1 and 2A in Lung Endothelial Barrier Regulation

Vascular barrier dysfunction is characterized by increased permeability and inflammation of endothelial cells (ECs), which are prominent features of acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and sepsis, and a major complication of the SARS-CoV-2 infection and COVID-19. Functional impairment of the EC barrier and accompanying inflammation arises due to microbial toxins and from white blood cells of the lung as part of a defensive action against pathogens, ischemia-reperfusion or blood product transfusions, and aspiration syndromes-based injury. A loss of barrier function results in the excessive movement of fluid and macromolecules from the vasculature into the interstitium and alveolae resulting in pulmonary edema and collapse of the architecture and function of the lungs, and eventually culminates in respiratory failure. Therefore, EC barrier integrity, which is heavily dependent on cytoskeletal elements (mainly actin filaments, microtubules (MTs), cell-matrix focal adhesions, and intercellular junctions) to maintain cellular contacts, is a critical requirement for the preservation of lung function. EC cytoskeletal remodeling is regulated, at least in part, by Ser/Thr phosphorylation/dephosphorylation of key cytoskeletal proteins. While a large body of literature describes the role of phosphorylation of cytoskeletal proteins on Ser/Thr residues in the context of EC barrier regulation, the role of Ser/Thr dephosphorylation catalyzed by Ser/Thr protein phosphatases (PPases) in EC barrier regulation is less documented. Ser/Thr PPases have been proposed to act as a counter-regulatory mechanism that preserves the EC barrier and opposes EC contraction. Despite the importance of PPases, our knowledge of the catalytic and regulatory subunits involved, as well as their cellular targets, is limited and under-appreciated. Therefore, the goal of this review is to discuss the role of Ser/Thr PPases in the regulation of lung EC cytoskeleton and permeability with special emphasis on the role of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) as major mammalian Ser/Thr PPases. Importantly, we integrate the role of PPases with the structural dynamics of the cytoskeleton and signaling cascades that regulate endothelial cell permeability and inflammation.

Rabbit Antidiethoxyphosphotyrosine Antibody, Made by Single B Cell Cloning, Detects Chlorpyrifos Oxon-Modified Proteins in Cultured Cells and Immunopurifies Modified Peptides for Mass Spectrometry

Chronic low-dose exposure to organophosphorus pesticides is associated with the risk of neurodegenerative disease. The mechanism of neurotoxicity is independent of acetylcholinesterase inhibition. Adducts on tyrosine, lysine, threonine, and serine can occur after exposure to organophosphorus pesticides, the most stable being adducts on tyrosine. Rabbit monoclonal 1C6 to diethoxyphosphate-modified tyrosine (depY) was created by single B cell cloning. The amino acid sequence and binding constant (K_d 3.2 × 10^–8 M) were determined. Cultured human neuroblastoma SH-SY5Y and mouse neuroblastoma N2a cells incubated with a subcytotoxic dose of 10 μM chlorpyrifos oxon contained depY-modified proteins detected by monoclonal 1C6 on Western blots. depY-labeled peptides from tryptic digests of cell lysates were immunopurified by binding to immobilized 1C6. Peptides released with 50% acetonitrile and 1% formic acid were analyzed by liquid chromatography tandem mass spectrometry (LC-MS/MS) on an Orbitrap Fusion Lumos mass spectrometer. Protein Prospector database searches identified 51 peptides modified on tyrosine by diethoxyphosphate in SH-SY5Y cell lysate and 73 diethoxyphosphate-modified peptides in N2a cell lysate. Adducts appeared most frequently on the cytoskeleton proteins tubulin, actin, and vimentin. It was concluded that rabbit monoclonal 1C6 can be useful for studies that aim to understand the mechanism of neurotoxicity resulting from low-dose exposure to organophosphorus pesticides.

Chlorpyrifos oxon promotes tubulin aggregation via isopeptide cross-linking between diethoxyphospho-Lys and Glu or Asp: Implications for neurotoxicity

Exposure to organophosphorus toxicants (OP) can have chronic adverse effects that are not explained by inhibition of acetylcholinesterase, the cause of acute OP toxicity. We therefore hypothesized that OP-induced chronic illness is initiated by the formation of organophosphorus adducts on lysine residues in proteins, followed by protein cross-linking and aggregation. Here, Western blots revealed that exposure to the OP chlorpyrifos oxon converted porcine tubulin from its original 55-kDa mass to high-molecular-weight aggregates. Liquid chromatography–tandem MS analysis of trypsin-digested samples identified several diethoxyphospho-lysine residues in the OP-treated tubulin. Using a search approach based on the Batch Tag program, we identified cross-linked peptides and found that these chemically activated lysines reacted with acidic amino acid residues creating γ-glutamyl-ϵ-lysine or aspartyl-ϵ-lysine isopeptide bonds between β- and α-tubulin. Of note, these cross-linked tubulin molecules accounted for the high-molecular-weight aggregates. To the best of our knowledge, this is the first report indicating that chlorpyrifos oxon–exposed tubulin protein forms intermolecular cross-links with other tubulin molecules, resulting in high-molecular-weight protein aggregates. It is tempting to speculate that chronic illness from OP exposure may be explained by a mechanism that starts with OP adduct formation on protein lysines followed by protein cross-linking. We further speculate that OP-modified or cross-linked tubulin can impair axonal transport, reduce neuron connections, and result in neurotoxicity.

Neural autoantibodies in patients with neurological symptoms and histories of chemical/mold exposures

A number of studies have linked exposures to industrial and household chemicals and biological toxins to increased risk of autoimmunity in general and elevated levels of autoantibodies to neural antigens specifically. Elevated neural autoantibodies are biomarkers for many diseases such as multiple sclerosis and Parkinson's disease. Our study reports levels of six types of neural autoantibodies in a group of 24 toxicant-exposed patients. The patients were exposed to a variety of toxicants including contaminated drinking water (four patients), building water/mold damage (eight patients), pesticides (four patients), and other assorted toxic chemicals (eight patients). Levels of all six neural autoantibodies were significantly elevated in most patients and in the patient group at large, with mean antibody levels for the 24 chemically exposed patients (relative to a healthy control population), in descending order: 475% for tau proteins, 391% for microtubule associated proteins-2, 334% for neurofilament proteins (NFP), 302% for myelin basic protein, 299% for glial fibrillary acidic proteins, and 225% for tubulin. Tau protein autoantibodies were significantly elevated in the patient groups with peripheral neuropathy, muscle and joint pain, asthma, and chemical sensitivity. Autoantibodies to tubulin were significantly higher in the chemical sensitivity and asthma patients, autoantibodies to NFP were significantly higher in the patients with sleep apnea, whereas S-100B autoantibodies were significantly increased in patients with muscle/joint pain, asthma, and apnea/insomnia. In patients exposed to environmental toxicants, measurements of autoantibodies may be useful for prevention, diagnosis, and treatment. This study adds to the scientific literature the ability of a broad spectrum of environmental triggers adversely affecting the nervous system through the process of autoimmunity, which may explain the increasing incidence of neurodegenerative diseases.

Study of the chlorpyrifos neurotoxicity using neural differentiation of adipose tissue-derived stem cells

Chlorpyrifos (CPF) is the most commonly used organophosphorus insecticide which causes neurodevelopmental toxicity. So far, animals have been used as ideal models for neurotoxicity studies, but working with animals is very expensive, laborious, and ethically challenging. This has encouraged researchers to seek alternatives. During recent years, several studies have reported successful differentiation of embryonic and adult stem cells to neurons. This has provided an excellent model for neurotoxicologic studies. In this study, neural differentiation of mouse adipose tissue-derived stem cells (ADSCs) was used as an in vitro model for investigation of CPF neurotoxicity. For this purpose, mouse ADSCs were cultured in a medium containing knockout serum replacement and were treated with different concentrations of CPF at several stages of differentiation. Cytotoxic effect of CPF and the expression of neuron-specific genes and proteins were studied in the differentiating ADSCs. Furthermore, the activity of acetylcholinesterase was assessed by Ellman assay at different stages of differentiation. This study showed that up to 500 μM CPF did not alter viability of the undifferentiated ADSCs, whereas viability of the differentiating cells decreased with 500 μM CPF. CPF upregulated the expression of some neuron-specific genes and seemed to decrease the number of β-tubulin III and MAP2 proteins-expressing cells. There was no detectable acetylcholine esterase activity in differentiated ADSCs. In summary, it was shown that CPF treatment can decrease the viability of ADSC-derived neurons and dysregulate the expression of some neuronal markers through acetylcholinesterase-independent mechanisms. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1510-1519, 2016.

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.

Cytoskeletal mechanisms regulating vascular endothelial barrier function in response to acute lung injury

Endothelial cells (EC) form a semi-permeable barrier between the interior space of blood vessels and the underlying tissues. In acute lung injury (ALI) the EC barrier is weakened leading to increased vascular permeability. It is widely accepted that EC barrier integrity is critically dependent upon intact cytoskeletal structure and cell junctions. Edemagenic agonists, like thrombin or endotoxin lipopolysaccharide (LPS), induced cytoskeletal rearrangement, and EC contractile responses leading to disruption of intercellular contacts and EC permeability increase. The highly clinically-relevant cytoskeletal mechanisms of EC barrier dysfunction are currently under intense investigation and will be described and discussed in the current review.

Environmental factors in the development and progression of late-onset Alzheimer's disease

Late-onset Alzheimer's disease (LOAD) is an age-related neurodegenerative disorder characterized by gradual loss of synapses and neurons, but its pathogenesis remains to be clarified. Neurons live in an environment constituted by neurons themselves and glial cells. In this review, we propose that the neuronal degeneration in the AD brain is partially caused by diverse environmental factors. We first discuss various environmental stresses and the corresponding responses at different levels. Then we propose some mechanisms underlying the specific pathological changes, in particular, hypothalamic-pituitary adrenal axis dysfunction at the systemic level; cerebrovascular dysfunction, metal toxicity, glial activation, and Aβ toxicity at the intercellular level; and kinase-phosphatase imbalance and epigenetic modification at the intracellular level. Finally, we discuss the possibility of developing new strategies for the prevention and treatment of LOAD from the perspective of environmental stress. We conclude that environmental factors play a significant role in the development of LOAD through multiple pathological mechanisms.

Control of vascular permeability by atrial natriuretic peptide via a GEF-H1-dependent mechanism

Microtubule (MT) dynamics is involved in a variety of cell functions, including control of the endothelial cell (EC) barrier. Release of Rho-specific nucleotide exchange factor GEF-H1 from microtubules activates the Rho pathway of EC permeability. In turn, pathologic vascular leak can be prevented by treatment with atrial natriuretic peptide (ANP). This study investigated a novel mechanism of vascular barrier protection by ANP via modulation of GEF-H1 function. In pulmonary ECs, ANP suppressed thrombin-induced disassembly of peripheral MT and attenuated Rho signaling and cell retraction. ANP effects were mediated by the Rac1 GTPase effector PAK1. Activation of Rac1-PAK1 promoted PAK1 interaction with the Rho activator GEF-H1, inducing phosphorylation of total and MT-bound GEF-H1 and leading to attenuation of Rho-dependent actin remodeling. In vivo, ANP attenuated lung injury caused by excessive mechanical ventilation and TRAP peptide (TRAP/HTV), which was further exacerbated in ANP(-/-) mice. The protective effects of ANP against TRAP/HTV-induced lung injury were linked to the increased pool of stabilized MT and inactivation of Rho signaling via ANP-induced, PAK1-dependent inhibitory phosphorylation of GEF-H1. This study demonstrates a novel protective mechanism of ANP against pathologic hyperpermeability and suggests a novel pharmacological intervention for the prevention of increased vascular leak via PAK1-dependent modulation of GEF-H1 activity.

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.

Oxidative stress contributes to lung injury and barrier dysfunction via microtubule destabilization

Oxidative stress is an important part of host innate immune response to foreign pathogens, such as bacterial LPS, but excessive activation of redox signaling may lead to pathologic endothelial cell (EC) activation and barrier dysfunction. Microtubules (MTs) play an important role in agonist-induced regulation of vascular endothelial permeability, but their impact in modulation of inflammation and EC barrier has not been yet investigated. This study examined the effects of LPS-induced oxidative stress on MT dynamics and the involvement of MTs in the LPS-induced mechanisms of Rho activation, EC permeability, and lung injury. LPS treatment of pulmonary vascular EC induced elevation of reactive oxygen species (ROS) and caused oxidative stress associated with EC hyperpermeability, cytoskeletal remodeling, and formation of paracellular gaps, as well as activation of Rho, p38 stress kinase, and NF-κB signaling, the hallmarks of endothelial barrier dysfunction. LPS also triggered ROS-dependent disassembly of the MT network, leading to activation of MT-dependent signaling. Stabilization of MTs with epothilone B, or inhibition of MT-associated guanine nucleotide exchange factor (GEF)-H1 activity by silencing RNA-mediated knockdown, suppressed LPS-induced EC barrier dysfunction in vitro, and attenuated vascular leak and lung inflammation in vivo. LPS disruptive effects were linked to activation of Rho signaling caused by LPS-induced MT disassembly and release of Rho-specific GEF-H1 from MTs. These studies demonstrate, for the first time, the mechanism of ROS-induced Rho activation via destabilization of MTs and GEF-H1-dependent activation of Rho signaling, leading to pulmonary EC barrier dysfunction and exacerbation of LPS-induced inflammation.

Identification of CaMKII phosphorylation sites in Connexin43 by high-resolution mass spectrometry

Connexin43 (Cx43) is a major cardiac gap junction channel protein required for normal electrical and contractile activity. Gap junction channel assembly, function, and turnover are regulated by phosphorylation under both normal and disease conditions. The carboxyl terminus (CT) of Cx43 contains numerous amino acid residues that are phosphorylated by protein kinases. However, our knowledge of the specific residues and kinases involved is incomplete. The objective of this study was to identify amino acid residues in the Cx43-CT that are targets of the multifunctional protein kinase, Ca(2+)/calmodulin protein kinase II (CaMKII), an enzyme known to play critical roles in Ca(2+) homeostasis, transcription, apoptosis, and ischemic heart disease. We subjected fusion protein containing the Cx43-CT to phosphorylation by CaMKII in vitro, digestion with Lys-C and trypsin followed by enrichment for phosphorylated peptides using TiO(2), and analysis in an LTQ XL Orbitrap with collision-induced dissociation and electron transfer dissociation. We deduced the sites of modification by interpreting tandem spectra from these "orthogonal" methods of gas phase peptide fragmentation. We have identified 15 serine residues, including one novel site, in the Cx43-CT that are phosphorylated by CaMKII, the activity of which may be important in regulating Cx43 in normal and diseased hearts.

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.

Regulation of vascular endothelial cell barrier function and cytoskeleton structure by protein phosphatases of the PPP family

Reversible phosphorylation of cytoskeletal and cytoskeleton-associated proteins is a significant element of endothelial barrier function regulation. Therefore, understanding the mechanisms of phosphorylation/dephosphorylation of endothelial cell cytoskeletal proteins is vital to the treatment of severe lung disorders such as high permeability pulmonary edema. In vivo, there is a controlled balance between the activities of protein kinases and phosphatases. Due to various external or internal signals, this balance may be shifted. The actual balances at a given time alter the phosphorylation level of certain proteins with appropriate physiological consequences. The latest information about the structure and regulation of different types of Ser/Thr protein phosphatases participating in the regulation of endothelial cytoskeletal organization and barrier function will be reviewed here.

Role of protein phosphatase 2A in the regulation of endothelial cell cytoskeleton structure

Our recently published data suggested the involvement of protein phosphatase 2A (PP2A) in endothelial cell (EC) barrier regulation (Tar et al. [2004] J Cell Biochem 92:534-546). In order to further elucidate the role of PP2A in the regulation of EC cytoskeleton and permeability, PP2A catalytic (PP2Ac) and A regulatory (PP2Aa) subunits were cloned and human pulmonary arterial EC (HPAEC) were transfected with PP2A mammalian expression constructs or infected with PP2A recombinant adenoviruses. Immunostaining of PP2Ac or of PP2Aa + c overexpressing HPAEC indicated actin cytoskeleton rearrangement. PP2A overexpression hindered or at least dramatically reduced thrombin- or nocodazole-induced F-actin stress fiber formation and microtubule (MT) dissolution. Accordingly, it also attenuated thrombin- or nocodazole-induced decrease in transendothelial electrical resistance indicative of barrier protection. Inhibition of PP2A by okadaic acid abolished its effect on agonist-induced changes in EC cytoskeleton; this indicates a critical role of PP2A activity in EC cytoskeletal maintenance. The overexpression of PP2A significantly attenuated thrombin- or nocodazole-induced phosphorylation of HSP27 and tau, two cytoskeletal proteins, which potentially could be involved in agonist-induced cytoskeletal rearrangement and in the increase of permeability. PP2A-mediated dephosphorylation of HSP27 and tau correlated with PP2A-induced preservation of EC cytoskeleton and barrier maintenance. Collectively, our observations clearly demonstrate the crucial role of PP2A in EC barrier protection.

Gene expression profiles of the rat brain both immediately and 3 months following acute sarin exposure

We have studied sarin-induced global gene expression patterns at an early time point (15 min; 0.5xLD50) and a later time point (3 months; 1xLD50) using Affymetrix: Rat Neurobiology U34 chips in male, Sprague-Dawley rats and have identified a total of 65 (early) and 38 (late) genes showing statistically significant alterations from control levels at 15 min and 3 months, respectively. At the early time point, those that are classified as ion channel, cytoskeletal and cell adhesion molecules, in addition to neuropeptides and their receptors predominated over all other groups. The other groups included: cholinergic signaling, calcium channel and binding proteins, transporters, chemokines, GABAnergic, glutamatergic, aspartate, catecholaminergic, nitric oxide synthase, purinergic, and serotonergic signaling molecules. At the late time point, genes that are classified as calcium channel and binding proteins, cytoskeletal and cell adhesion molecules and GABAnergic signaling molecules were most prominent. Seven molecules (Ania-9, Arrb-1, CX-3C, Gabab-1d, Nos-2a, Nrxn-1b, PDE2) were identified that showed altered persistent expression in both time points. Selected genes from each of these time points were further validated using semi quantitative RT-PCR approaches. Some of the genes that were identified in the present study have been shown to be involved in organophosphate-induced neurotoxicity by both other groups as well as ours. Principal component analysis (PCA) of the expression data from both time points was used for comparative analysis of the gene expression, which indicated that the changes in gene expression were a function of dose and time of euthanasia after the treatment. Our model also predicts that besides dose and duration of post-treatment period, age and possibly other factors may be playing important roles in the regulation of pathways, leading to the neurotoxicity.

Novel role of microtubules in thrombin-induced endothelial barrier dysfunction

Disturbances in endothelial cell (EC) barrier regulation are critically dependent upon rearrangements of EC actin cytoskeleton. However, the role of microtubule (MT) network in the regulation of EC permeability is not well understood. We examined involvement of MT remodeling in thrombin-induced EC permeability and explored MT regulation by heterotrimeric G12/13 proteins and by small GTPase Rho. Thrombin induced phosphorylation of MT regulatory protein tau at Ser409 and Ser262 and peripheral MT disassembly, which was linked to increased EC permeability. MT stabilization by taxol attenuated thrombin-induced permeability, actin remodeling, and paracellular gap formation and diminished thrombin-induced activation of Rho and Rho-kinase. Expression of activated Galpha12/13 subunits involved in thrombin-mediated signaling or their effector p115RhoGEF involved in Rho activation caused MT disassembly, whereas p115RhoGEF-specific negative regulator RGS preserved MT from thrombin-induced disassembly. Consistent with these results, expression of activated RhoA and Rho-kinase induced MT disassembly. Conversely, thrombin-induced disassembly of peripheral MT network was attenuated by expression of dominant negative RhoA and Rho-kinase mutants or by pharmacological inhibition of Rho-kinase. Collectively, our data demonstrate for the first time a critical involvement of MT disassembly in thrombin-induced EC barrier dysfunction and indicate G-protein-dependent mechanisms of thrombin-induced MT alteration.

Phosphatase 2A is involved in endothelial cell microtubule remodeling and barrier regulation

We have recently shown that microtubule (MT) inhibitor, nocodazole (2-5 microM) significantly increases endothelial cells (EC) actomyosin contraction and permeability indicating the importance of MT in maintaining the EC barrier (Verin et al. [2001]: Cell Mol Physiol 281:L565-L574). Okadaic acid (OA, 2-5 nM), a powerful inhibitor of protein phosphatase 2A (PP2A), significantly potentiates the effect of submaximal concentrations of nocodazole (50-200 nM) on transendothelial electrical resistance (TER) suggesting the involvement of PP2A activity in the MT-mediated EC barrier regulation. Immunofluorescent staining of EC revealed that in control cells PP2A distributes in a pattern similar to MT. Consistent with these results, we demonstrated that significant amounts of PP2A were present in MT-enriched EC fractions indicating tight association of PP2A with MT in endothelium. Treatment of EC with OA leads to disappearance of MT-like PP2A staining suggesting dissociation of PP2A from the MT network. Next, we examined the effect of PP2A inhibition on phosphorylation status of MT-associated protein tau, which in its unphosphorylated form promotes MT assembly. OA caused significant increases in tau phosphorylation confirming that tau is a substrate for PP2A in endothelium. Immunofluorescent experiments demonstrated that the OA-induced increases in tau phosphorylation strongly correlated with translocation of phospho-tau to cell periphery and disassembly of peripheral MT. These results suggest the involvement of PP2A-mediated tau dephosphorylation in alteration of EC MT structure and highlight the potential importance of PP2A in the regulation of EC the MT cytoskeleton and barrier function.

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.

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.

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.

Possible role of enhanced microtubule phosphorylation in dichlorvos induced delayed neurotoxicity in rat

The effect of a single subcutaneous dose of 200 mg/kg body weight dichlorvos on neuronal microtubule phosphorylation has been studied in rat following the development of organophosphate induced delayed neurotoxicity (OPIDN). Microtubule associated Ca2+/calmodulin dependent as well as cAMP dependent protein kinases were assayed. Dichlorvos administration led to a consistent increase in the activity of both the kinases at all post exposure intervals (7th, 15th and 21st day) as compared to that of controls. After in vitro phosphorylation using [gamma-32P]ATP, various proteins were resolved on one-dimensional 8% SDS-PAGE, stained with Coomassie Blue and autoradiographed. The amount of 32P incorporated was quantified by microdensitometry. Dichlorvos enhanced the phosphorylation of 55- and 280-kDa proteins. These two proteins were identified as tubulin and microtubule associated protein-2 (MAP-2) by immunoblotting. This study showed that dichlorvos induced hyperphosphorylation of tubulin and MAP-2 which in turn destabilizes microtubule assembly, and may ultimately result in axonal degeneration leading to dichlorvos induced delayed neurotoxicity.

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.

Early differential induction of C-jun in the central nervous system of hens treated with diisopropylphosphorofluoridate (DFP)

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus-ester induced delayed neurotoxicity (OPIDN) in the hen, human and other sensitive species. We studied the effect of a single dose of DFP (1.7 mg/kg/sc) on the expression of c-jun, which is one of the heterodimerizing ITFs (Inducible Transcriptional Factors) of the AP-1 family. The hens were sacrificed at different time points ie 0.25, .0.50, 1 and 2 hrs. Total RNA was extracted from the following brain regions: cerebrum, cerebellum, brainstem, midbrain and as well as spinal cord. Northern blots prepared using standard protocols were hybridized with c-jun as well as b-actin and 18S RNA cDNA (control) probes. The results indicate differential regulation of c-jun levels which may be due to the activation of both cholinergic and non-cholinergic pathways of CNS, besides changing roles of c-jun (as mediator of degeneration or regeneration) depending on heterodimerization with other ITFs. In the highly susceptible tissues like brainstem and spinal cord c-jun transcript levels increased at 15 minutes and continued to increase gradually till it reached the maximum at 2 hrs. Overall spinal cord showed the maximum levels of c-jun induction (207%) at 2 hrs time point of all the CNS tissues. The enhancement of cholinergic transmisson by the inhibition of cholinestrase may be responsible for the gradual increase mediated by neural and vascular factors. In contrast, less susceptible tissue, cerebellum showed almost immediate induction to high level of (179%) at 15 minutes and the levels stayed more or less the same until it peaked to 185% at 2 hrs. Relatively low abundance of cholinergic neurons and high number of sensitized specialized cell types like Bergman glia and Purkinje cells may be responsible for the immediate higher induction. Non-susceptible tissue cerebrum did not show any changes in the c-jun levels. In midbrain the induction pattern was very similar to that of brainstem. This differential induction pattern of c-jun encomposing the differences in the quantity and time course was directly proportionate to the degree of susceptibility and cellular heterogeneity of different regions of CNS. The significant increase in c-jun levels along with our earlier observation on the increased c-fos levels indicate that AP-1 family of genes may be one of the IEGs involved in the long term changes which eventually lead to OPIDN.

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.