Acrylamide increases in vitro calcium and calmodulin-dependent kinase-mediated phosphorylation of rat brain and spinal cord neurofilament proteins

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.

Impact of Acrylamide on Calcium Signaling and Cytoskeletal Filaments in Testes From F344 Rat

Acrylamide (AA) at high exposure levels is neurotoxic, induces testicular toxicity, and increases dominant lethal mutations in rats. RNA-sequencing in testes was used to identify differentially expressed genes (DEG), explore AA-induced pathway perturbations that could contribute to AA-induced testicular toxicity and then used to derive a benchmark dose (BMD). Male F344/DuCrl rats were administered 0.0, 0.5, 1.5, 3.0, 6.0, or 12.0 mg AA/kg bw/d in drinking water for 5, 15, or 31 days. The experimental design used exposure levels that spanned and exceeded the exposure levels used in the rat dominant lethal, 2-generation reproductive toxicology, and cancer bioassays. The time of sample collection was based on previous studies that developed gene expression-based BMD. At 12.0 mg/kg, there were 38, 33, and 65 DEG ( P value <.005; fold change >1.5) in the testes after 5, 15, or 31 days of exposure, respectively. At 31 days, there was a dose-dependent increase in the number of DEG, and at 12.0 mg/kg/d the top three functional clusters affected by AA exposure were actin filament organization, response to calcium ion, and regulation of cell proliferation. The BMD lower 95% confidence limit using DEG ranged from 1.8 to 6.8 mg/kg compared to a no-observed-adverse-effect-level of 2.0 mg/kg/d for male reproductive toxicity. These results are consistent with the known effects of AA on calcium signaling and cytoskeletal actin filaments leading to neurotoxicity and suggest that AA can cause rat dominant lethal mutations by these same mechanisms leading to impaired chromosome segregation during cell division.

Neuroprotective Effect of Calpeptin on Acrylamide-Induced Neuropathy in Rats

Acrylamide (ACR) is a vinyl monomer with established human neurotoxic effects, which is characterized by the accumulation of neurofilaments (NFs) in the distal swellings of large axons in peripheral and central nervous systems. However, the mechanisms of neurotoxicity remain unclear. The objective is to investigate the neuroprotective effect of calpeptin (CP) on ACR-induced neuropathy and its mechanism. Female adult Wistar rats were randomly divided into four groups (control, CP, ACR, and ACR + CP group). Control group received 0.9 % saline, ACR and ACR + CP groups received 30 mg/kg ACR by intraperitoneal injection. In addition, CP and ACR + CP groups also received 200 µg/kg CP. Gait analysis and hind limb splay were measured weekly to analyze neurobehavioral changes. The calpain activity and the changes of NFs protein levels in spinal cord are determined. Compared with control group, body weight of rats in ACR group decreased by 11.3 % (P < 0.01), while in ACR + CP group body weight increased significantly by 8.3 % (P < 0.01) compared with ACR group by the end of the 4th week; gait score of rats in both ACR and ACR + CP groups increased significantly by 167 % and 100 % (P < 0.01) compared with control group, while it decreased significantly by 25.1 % (P < 0.01) in ACR + CP group compared with ACR group; the distance of hind limb splay in both ACR and ACR + CP groups increased by 76.7 % and 49.5 % (P < 0.01) compared with control group, while it decreased by 15.4 % (P < 0.01) in ACR + CP group compared with ACR group; calpain activity of spinal cord at ACR and ACR + CP groups increased significantly by 14.9 % and 10.0 % (P < 0.01) compared with control group, while it decreased 4.2 % (P < 0.01) in ACR + CP group compared with ACR group; compared with control group, the levels of light NF (NF-L), medium NF (NF-M) and heavy NF (NF-H) subunits increased by 81.2 %, 263.6 % and 22.6 % (P < 0.01) in the supernatant of ACR group in spinal cord tissue and increased by 28.4 %, 96.6 % and 10.6 % (P < 0.01) in ACR + CP group, while the levels of NF-L, NF-M and NF-H subunits decreased by 29.1 %, 45.9 % and 9.8 % (P < 0.01) in ACR + CP group compared with ACR group. The present results suggested that CP can relieve ACR neuropathy by decrease calpain activity and NFs degradation. The changes of calpain activity and NFs may be one of the mechanisms of ACR-induced neuropathy.

Acrylamide neurotoxicity

Acrylamide, a food contaminant, belongs to a large class of structurally similar toxic chemicals, 'type-2 alkenes', to which humans are widely exposed. Besides, occupational exposure to acrylamide has received wide attention through the last decades. It is classified as a neurotoxin and there are three important hypothesis considering acrylamide neurotoxicity: inhibition of kinesin-based fast axonal transport, alteration of neurotransmitter levels, and direct inhibition of neurotransmission. While many researchers believe that exposure of humans to relatively low levels of acrylamide in the diet will not result in clinical neuropathy, some neurotoxicologists are concerned about the potential for its cumulative neurotoxicity. It has been shown in several studies that the same neurotoxic effects can be observed at low and high doses of acrylamide, with the low doses simply requiring longer exposures. This review is focused on the neurotoxicity of acrylamide and its possible outcomes.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on differentiating mouse N2a neuroblastoma cells

The effect of the neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) was investigated in mouse N2a neuroblastoma cells, induced to differentiate by serum withdrawal and addition of dibutyryl cyclic AMP, over a 24-h period. Addition of MPTP (10 microM) during differentiation caused a change in cell morphology characterised by an inhibition of axon outgrowth, in the absence of cell death. Biochemical characterisation by western blotting revealed that MPTP had no significant effects on the levels of actin, alpha-tubulin, or total heavy-chain neurofilament (NF-H). However, NF-H phosphorylation appeared to increase following MPTP treatment when blots were probed with the phosphorylation state-specific antibodies RMd09 and Ta51. In addition, indirect immunofluorescence analysis revealed an accumulation of phosphorylated NF-H in the cell perikaryon, suggesting that altered NF-H distribution was associated with the observed effects of MPTP on cell morphology. These changes may represent a useful in vitro marker of MPTP neurotoxicity within a simple differentiating neuronal cell model system.

Activation of NMDA receptors and Ca2+/calmodulin-dependent protein kinase participate in phosphorylation of neurofilaments induced by protein kinase C

Aberrant phosphorylation of neurofilaments, similar to that occurring in various motor neuron diseases, is produced in cultured motor neurons by activation of protein kinase C (PKC). Following exposure to synthetic diacylglycerol, persistent change in the phosphorylation state of C-terminal domains of neurofilament proteins was detected by increased perikaryal immunoreactivity with the antibody SMI34; this antibody recognizes NF-M/NF-H when C-terminal KSP repeat domains are highly phosphorylated. SMI34 labeling of perikarya and dendrites was prevented by pretreatment with either the NMDA receptor antagonist APV or by the Ca2+/calmodulin-dependent protein kinase (CaMK) inhibitor KN-62, but not by antagonists of AMPA/kainate or metabotropic glutamate receptors or by inhibitors of arachidonic acid metabolic pathways. Thus, activation of PKC may induce neurofilament phosphorylation in motor neurons by acting in cooperation with stimulation of NMDA receptors and activation of CaMK. These mechanisms may be relevant to motor neuron disease and other neuronal injuries in which increased PKC activity has been measured.

Acrylamide and carbon disulfide treatments increase the rate of rat brain tubulin polymerization

Acrylamide and carbon disulfide produce central-peripheral distal axonopathy in experimental animals and humans. The main feature of this disease is the focal swellings containing neurofilaments in distal axons, followed by nerve degeneration beyond these swellings. We studied the possible role of tubulin assembly kinetics in this disease. The rats were either administered acrylamide (50 mg/kg, ip, saline) or exposed to carbon disulfide (700 ppm, 9 h) via inhalation for 12 and 15 d, respectively. Tubulin, purified from both acrylamide-(10.37 +/- 0.3 vs 11.3 +/- 0.15) and carbon disulfide-treated (9.72 +/- 0.5 vs 11.18 +/- 0.25) rat brains showed increase in Vmax (OD/min x 10(3)) of its polymerization. However, only acrylamide treatment showed a decrease in time to Vmax, when brain supernatant was used for tubulin polymerization. In vitro addition of acrylamide (0.1-1 mM) to bovine brain tubulin also showed a decrease in time to Vmax (16-21%) of its polymerization. Carbon disulfide treatment of rats, on the other hand, showed a decrease in MAP-2 and an increase in a 120-kDa peptide concentration. The latter showed immunoreactivity with anti-MAP-2. The increase in the rate of tubulin polymerization by acrylamide and carbon disulfide treatment may alter the rate of transport of axonal constituents, including neurofilament, and contribute toward their accumulation in the focal swellings observed in this neuropathy.

Modulation of phosphorylation of neuronal cytoskeletal proteins by neuronal depolarization

1. The neuronal cytoskeletal protein tau and the carboxy tails of cytoskeletal proteins neurofilament-M (NF-M) and neurofilament-H (NF-H) are phosphorylated on serine residues by the cyclin-dependent kinase cdk-5. 2. In aggregating neuronal-glial cultures we show that veratridine-mediated cation influx causes dephosphorylation of tau, NF-M and NF-H. Dephosphorylation was blocked specifically by cyclosporine A but not by okadiac acid at concentrations up to 200 nM. 3. These results suggest that veratridine-triggered cation influx causes activation of PP-2B (calcineurin) leading to dephosphorylation of these cytoskeletal proteins.

Alterations in the neutral proteinase activities of central and peripheral nervous systems of acrylamide-, carbon disulfide-, or 2,5-hexanedione-treated rats

Proteinases are widespread in neuronal or nonneuronal eukaryotic cells. They are suggested to play an important role in the turnover of proteins in neuronal perikaryon and axon, and digestion of the transported cytoskeletal proteins in synaptic terminals. We examined the effect of acrylamide (50 mg/kg, ip), carbon disulfide (700 ppm, 9 h, 7 d a week), and 2,5-hexanedione (2,5-HD) (1% in drinking water) treatment of rats on mCANP (2 mM Ca2+), microCANP (0.1 mM Ca2+), and CINP (Ca(2+)-independent) activity in telencephalon + diencephalon (FB), rhombencephalon + mesencephalon (LB), spinal cord (SC), and sciatic nerve (SN). The proteinase activity was determined in the 30,000g supernatant fraction of tissues using 14C-methylated casein as the substrate. mCANP activity in FB, LB, and SC was inhibited only by acrylamide. Acrylamide or 2,5-HD treatment had no effect on microCANP and CINP activities of SN, whereas carbon disulfide enhanced microCANP after 15 d and CINP activity after 10 d. It is suggested that alteration in in vitro calpain activity shown by these chemicals may not be directly related to their neurotoxic effect. However, calpain may still be playing a role in this polyneuropathy by alteration in activity through inflow of Ca2+, release of Ca2+ from intracellular organelles, or other factors. Modification of cytoskeletal proteins making them more susceptible to proteases and the role of some other proteinase is also possible.

Neurofilament phosphorylation and [125I]calmodulin binding by Ca2+/calmodulin-dependent protein kinase in the brain subcellular fractions of diisopropyl phosphorofluoridate (DFP)-treated hen

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus ester-induced delayed neurotoxicity (OPIDN) in humans and sensitive animal species, e.g., adult chicken. The chickens were sacrificed 18 days after a single dose of DFP (1.7 mg/kg, s.c.), which produced severe ataxia or paralysis in 10-14 days. We studied Ca2+/calmodulin-dependent in vitro neurofilament phosphorylation by the brain subcellular fractions of control and DFP-treated hens. There was enhanced phosphorylation of all three NF subunits by the brain supernatant of treated hens. This was accompanied by enhanced autophosphorylation of both Ca2+/CaM-dependent protein kinase II (CaM-kinase II) subunits and increased calmodulin binding using either 125I-CaM or biotinylated calmodulin to only alpha subunit without concomitant increase in the amount of this enzyme. This enhanced phosphorylation of neurofilament subunits was completely and partially inhibited by mastoparan and KN-62, respectively. There was no alteration in the distribution of CaM-kinase II activity in treated hens and the activity was not related to its concentration in different subcellular fractions. The difference in 125I-CaM binding to CaM-kinase II alpha subunit in the brain supernatants of control and DFP-treated hens was not altered by its phosphorylation or dephosphorylation. The increased CaM-kinase II activity in the soluble fraction of DFP-treated hen brain may be involved in the aberrant phosphorylation of axonal neurofilaments, and thus play a role in OPIDN.

In vitro calcium and calmodulin-dependent kinase-mediated phosphorylation of rat brain and spinal cord neurofilament proteins is increased by glycidamide administration

This study was carried out to determine the action of glycidamide (2,3-epoxy-1-propanamide), a neurotoxic metabolite of acrylamide, on Ca2+/calmodulin (CaM)-dependent protein kinase phosphorylation of cytoskeletal proteins. Acrylamide has been shown to increase Ca2+/CaM-dependent phosphorylation of neurofilament (NF) triplet proteins and autophosphorylation of Ca2+/CaM-dependent protein kinase II (CaM kinase II; EC 2.7.1.37). A daily intraperitoneal dose of 0.7 mmol/kg b.wt. of glycidamide or deionized water was administered to male Sprague-Dawley rats. Animals were sacrificed when signs of severe neurotoxicity became apparent at 13-16 days of treatment. Axonal floatation was used to isolate neurofilaments (NFs) and endogenous kinases from brains and spinal cords of treated and control animals. Samples isolated from brain and spinal cord of glycidamide-treated animals showed increased in vitro Ca2+/CaM-dependent phosphorylation of endogenous and exogenous NF proteins and increased autophosphorylation of CaM kinase II when compared with controls. CaM binding to the alpha, beta, and beta' subunits of CaM kinase II and antibody binding to the alpha-subunit of CaM kinase II in brain supernatant isolates was increased as a result of glycidamide treatment. These results suggest that increased Ca2+/CaM-dependent phosphorylation of cytoskeletal proteins may be involved in the pathogenesis of glycidamide-induced neurotoxicity.