Quaternary and tertiary aldoxime antidotes for organophosphate exposure in a zebrafish model system

An Antidote Screening System for Organophosphorus Poisoning Using Zebrafish Larvae

Organophosphorus (OP) cholinesterase inhibitors, which include insecticides and chemical warfare nerve agents, are very potent neurotoxicants. Given that the actual treatment has several limitations, the present study provides a general method, called the zebrafish-OP-antidote test (ZOAT), and basic scientific data, to identify new antidotes that are more effective than the reference pyridinium oximes after acute OP poisoning. The reactivation capacity of a chemical compound can be measured using in vivo and ex vivo acetylcholinesterase (AChE) assays. We demonstrated that it is possible to differentiate between chemical compound protective efficacies in the central and peripheral nervous system via the visual motor response and electric field pulse motor response tests, respectively. Moreover, the ability to cross the brain-blood barrier can be estimated in a physiological context by combining an AChE assay on the head and trunk-tail fractions and the cellular and tissue localization of AChE activity in the whole-mount animal. ZOAT is an innovative method suitable for the screening and rapid identification of chemicals and mixtures used as antidote for OP poisoning. The method will make it easier to identify more effective medical countermeasures for chemical threat agents, including combinatorial therapies.

Assessing the toxicity of three "inert" herbicide safeners toward Danio rerio: Effects on embryos development

Recent studies indicated that 'inert ingredients' exert negative effects on the environment. Herbicide safeners are classed as 'inert ingredients', which increase the selectivity and detoxification of herbicides. However, little attention has been focused on the environmental behavior of herbicide safeners. AD-67 (AD), fenclorim (FM), and flurazole (FZ) are three commercially available herbicide safeners. In this study, zebrafish embryos were used as a model to investigate the potential developmental toxicity of these three safeners. The results showed that AD, FM, and FZ 96 h-LC₅₀ values were 2.52, 1.26, and 2.01 mg/L, respectively. Significant decreased body lengths were observed in embryos after 96 h of exposure to 0.2 mg/L FM and FZ. Hatching rates significantly increased at 24 h and decreased at 96 h in all treatment groups (0.02 mg/L AD, 0.2 mg/L AD, 0.02 mg/L AD, 0.2 mg/L FZ, as well as 0.01 and 0.1 mg/L FM). No significant (p > 0.05) changes in heartbeat numbers (60 s), but clearly increased malformation rates were observed in response to safeners exposure. Furthermore, embryos showed signs of oxidative stress, such as decreased activities of superoxide dismutase, altered activities of glutathione reductase and catalase and cell apoptosis. The gene transcription related to body malformation (egf, krt 17, and tbx 16) and cell apoptosis (bcl 2 and bax) changed in treated groups. These genes have been connected to potential toxicological mechanisms. These results indicate that the herbicide safeners AD, FM, and FZ induced developmental toxicities in zebrafish embryos. This study is the first report of the toxicity of safeners in the development of zebrafish embryos. The results may be useful for assessing the risk of herbicides safeners in the aquatic ecosystem.

Development of a Larval Zebrafish Model for Acute Organophosphorus Nerve Agent and Pesticide Exposure and Therapeutic Evaluation

Organophosphorus compound exposure remains a present threat through agricultural accidents, warfare, or terrorist activity. The primary mechanism of organophosphorus toxicity is through inhibition of the enzyme acetylcholinesterase, with current emergency treatment including anticholinergics, benzodiazepines, and oxime reactivators. However, a need for more effective and broadly acting countermeasures remains. This study aimed to develop larval zebrafish as a high-throughput model for evaluating novel therapeutics against acute organophosphorus exposure. Larval zebrafish at six days post-fertilization were exposed to acute concentrations of seven organophosphorus compounds and treated with one of three oximes. Lethality studies indicated similar relative toxicity to that seen in the established rodent model, with chemical warfare agents proving more lethal than organophosphorus pesticides. Additionally, the organophosphorus-specific response for oxime reactivation of acetylcholinesterase was comparable to what has been previously reported. Behavioral studies measuring the visual motor response demonstrated greater efficacy for centrally acting oxime compounds than for those that are contained to the peripheral tissue. Overall, these results support the use of this larval zebrafish model as a high-throughput screening platform for evaluating novel treatments following acute organophosphorus exposure.

Characterization and kinetic study of the brain and muscle acetylcholinesterase from Danio rerio

Acetylcholinesterase (AChE) plays an important role in the therapy of Alzheimer's disease and in the detection of pesticides such as organophosphates which are also widely used in chemical warfare. The aim of this study is the physicochemical and kinetic characterization of brain and muscle ChE from Danio rerio (Zebrafish). Optimal activity was found for brain ChE at alkaline pH 9.0 at 30 °C, and for muscle ChE at alkaline pH 8.5 at temperatures between 20 °C and 35 °C. The apparent kinetic constants, K_m^app and V_max^app, for brain ChE were determined as 0.191 ± 0.024 mM and 0.566 ± 0.028 U/mg protein, and for muscle ChE as 0.230 ± 0.030 mM and 0.677 ± 0.039 U/mg protein. Both brain and muscle ChE showed inhibition at high substrate concentrations. Brain and muscle ChE showed IC50 values for physostigmine of 0.61 μM and 0.37 μM, respectively. The ChE activity in brain was significantly inhibited by BW254c51 in all concentrations tested, but not by Iso-OMPA, while muscle ChE presented a moderate decrease (13 to 29%) in the activity values, indicating that BuChE is present.

Larvicidal, Histopathological, Antibacterial Activity of Indigenous Fungus Penicillium sp. Against Aedes aegypti L and Culex quinquefasciatus (Say) (Diptera: Culicidae) and Its Acetylcholinesterase Inhibition and Toxicity Assessment of Zebrafish (Danio rerio)

Fungal metabolites are considered to be most efficient tools to overcome the issues related to insecticide resistance and environmental pollution. The present study focus on the evaluation of the mosquito larvicidal efficacy of metabolites of seven indigenous fungal isolates (Penicillium sp. Aspergillus niger, A. flavus, A. parasiticus, Rhizopus sp. Mucor sp. and Aspergillus sp.) on the larvae of Aedes aegypti and Culex quinquefasciatus under the laboratory condition. The preliminary screening of the isolate, Penicillium sp. showed better larvicidal effect when compared to other fungi. The fungus was grown on Potato Dextrose Broth (PDB) in the laboratory (at 25°C) and maintained in the relative humidity (at 76 ± 4% for 15 days). Larvicidal potency of mycelial ethyl acetate extract (MEAE) of Penicillium sp. was performed against 1st to 4th instars larvae of Ae. aegypti and Cx. quinquefasciatus using four different concentrations (100, 200, 300, and 500 μg/ml) that showed better larval mortality values (μg/ml) of LC50 = 6.554, 5.487, 6.874, 6.892, and the LC90 = 11.486, 10.366, 12.879, 13.865 for Ae. aegypti and LC50 = 7.000, 13.943, 18.129, 25.212 and the LC90 = 12.541, 23.761, 30.923, 41.696 for Cx. quinquefasciatus. Exposure of metabolite to larvae resulted in behavior changes i.e., excitation, up and down with aggressive movement, anal papillae biting behavior. Further, the larvae treated with Penicillium sp. metabolite exhibited significant reduction in the levels of acetylcholinesterase. The 4th instar mosquito larvae treated with the 500 μg/ml mycelia extract showed severe histological damages. During the antibacterial analysis of Penicillium sp.- mycelium the maximum growth inhibition zone was recorded in Shigella dysenteriae (31.2 mm) and Klebsiella pneumoniae (31.1 mm) followed by others. In addition, to check the toxicity of Penicillium sp. MEAE against embryos of Zebrafish, a model system, using different concentrations of metabolites (1.0, 0.5, 0.125 mg/ml, 30, 3.0, and 0.5 μg/ml) and life-stage parameters were observed at 124 hpf. Furthermore, the Fourier Transformed Infrared and GCMS spectrum analysis of mycelium reflected several chemical compounds. The outcome of the study clearly shows that Penicillium sp. metabolites could serve as an ideal eco-friendly, single-step and inexpensive source for the control of Ae. aegypti and Cx. quinquefasciatus larvae.

Toxicity assessment of pyriproxyfen in vertebrate model zebrafish embryos (Danio rerio): A multi biomarker study

Pyriproxyfen (2-[1-methyl-2-(4-phenoxyphenoxy) ethoxy] pyridine) (PPF), a pyridine-based pesticide widely used to control agricultural insect pests and mosquitoes in drinking water sources. However, its ecotoxicological data is limited in aquatic vertebrates particularly in fish. Hence, the present study aimed to evaluate the adverse effect of PPF in zebrafish embryo development (Danio rerio). In order to investigate the impact of PPF, embryos were exposed to 0.16, 0.33 and 1.66 μg/mL (0.52, 1.04 and 5.2 μM, respectively) for 96 hpf and various biomarker indices such as developmental toxicity (edema formation, hyperemia, heart size and scoliosis), oxidative stress (reactive oxygen species (ROS), lipid peroxidation (LPO) and nitric oxide (NO)), antioxidant responses (superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), glutathione peroxidase (GPx) and reduced glutathione (GSH)), biochemical (lactate dehydrogenase (LDH) and acid phosphatase (AP)), neurotoxicity (acetylcholinesterase (AChE)), genotoxicity (apoptosis and DNA damage) and histopathological changes were determined. The results showed that severe developmental deformities and changes in heart rate were observed in embryos treated with highest (1.66 μg/mL) concentration than the control (P < 0.05). Heart size measurement showed that, significant change in heart size (P < 0.01) was observed in embryos of 96 hpf only at 1.66 μg/mL PPF exposure. The oxidative stress was apparent at highest test concentration (1.66 μg/mL) as reflected by the elevated ROS, LPO and NO and changes in antioxidant enzyme activities including SOD, CAT, GST and GPx (P < 0.05). Besides, GSH level and AChE activity were significantly lowered in 1.66 μg/mL PPF exposed group than the control. After 96 hpf of PPF exposure, no significant changes were found in AP activity whereas, a biphasic response was observed in the LDH activity. There was no genotoxic effect in embryos exposed to PPF at 0.16 and 0.33 μg/mL, while significant (P < 0.05) DNA damage and apoptosis were found in 1.66 μg/mL treated group. Histopathological analysis revealed that exposure to PPF at 1.66 μg/mL resulted in thinning of heart muscles, pericardial edema and hyperemia while there was no obvious changes were observed in other treatment groups. Hence, the results of the present study demonstrate that PPF could cause adverse effect on early developmental stages of zebrafish at higher concentration.

Cholinesterase reactivators and bioscavengers for pre- and post-exposure treatments of organophosphorus poisoning

Organophosphorus agents (OPs) irreversibly inhibit acetylcholinesterase (AChE) causing a major cholinergic syndrome. The medical counter-measures of OP poisoning have not evolved for the last 30 years with carbamates for pretreatment, pyridinium oximes-based AChE reactivators, antimuscarinic drugs and neuroprotective benzodiazepines for post-exposure treatment. These drugs ensure protection of peripheral nervous system and mitigate acute effects of OP lethal doses. However, they have significant limitations. Pyridostigmine and oximes do not protect/reactivate central AChE. Oximes poorly reactivate AChE inhibited by phosphoramidates. In addition, current neuroprotectants do not protect the central nervous system shortly after the onset of seizures when brain damage becomes irreversible. New therapeutic approaches for pre- and post-exposure treatments involve detoxification of OP molecules before they reach their molecular targets by administrating catalytic bioscavengers, among them phosphotriesterases are the most promising. Novel generation of broad spectrum reactivators are designed for crossing the blood-brain barrier and reactivate central AChE. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Planarian cholinesterase: in vitro characterization of an evolutionarily ancient enzyme to study organophosphorus pesticide toxicity and reactivation

The freshwater planarian Dugesia japonica has recently emerged as an animal model for developmental neurotoxicology and found to be sensitive to organophosphorus (OP) pesticides. While previous activity staining of D. japonica, which possess a discrete cholinergic nervous system, has shown acylthiocholine catalysis, it is unknown whether this is accomplished through an acetylcholinesterase (AChE), butyrylcholinesterase (BChE), or a hybrid esterase and how OP exposure affects esterase activity. Here, we show that the majority of D. japonica cholinesterase (DjChE) activity departs from conventional AChE and BChE classifications. Inhibition by classic protonable amine and quaternary reversible inhibitors (ethopropazine, donepezil, tacrine, edrophonium, BW284c51, propidium) shows that DjChE is far less sensitive to these inhibitors than human AChE, suggesting discrete differences in active center and peripheral site recognition and structures. Additionally, we find that different OPs (chlorpyrifos oxon, paraoxon, dichlorvos, diazinon oxon, malaoxon) and carbamylating agents (carbaryl, neostigmine, physostigmine, pyridostigmine) differentially inhibit DjChE activity in vitro. DjChE was most sensitive to diazinon oxon and neostigmine and least sensitive to malaoxon and carbaryl. Diazinon oxon-inhibited DjChE could be reactivated by the quaternary oxime, pralidoxime (2-PAM), and the zwitterionic oxime, RS194B, with RS194B being significantly more potent. Sodium fluoride (NaF) reactivates OP-DjChE faster than 2-PAM. As one of the most ancient true cholinesterases, DjChE provides insight into the evolution of a hybrid enzyme before the separation into distinct AChE and BChE enzymes found in higher vertebrates. The sensitivity of DjChE to OPs and capacity for reactivation validate the use of planarians for OP toxicology studies.

Profenofos induced biochemical alterations and in silico modelling of hatching enzyme, ZHE1 in zebrafish (Danio rerio) embryos

The current study was aimed to investigate the oxidative stress response in zebrafish embryos exposed to sub-lethal (LC10) and lethal (LC50) concentrations of profenofos for 96-h and in silico modelling of zebrafish hatching enzyme, ZHE1 to explain the delayed hatching. Embryos exposed to profenofos under semi-static conditions significantly diminished glutathione (GSH), superoxide dismutase (SOD) and glutathione reductase (GR) levels, but increased the activities of catalase (CAT) and glutathione S-transferase (GST) concomitantly with marked elevation in malondialdehyde (MDA) content in whole-body homogenate of the treated groups compared with control. In addition, stress protein Hsp70 expression and DNA damage were significantly increased in a concentration- dependent manner compared with controls. From the computational docking studies of ZHE1 with profenofos revealed that profenofos is binding to three amino acids, histidine 99, histidine 109 and arginine 182 at the active site of the enzyme through hydrogen bonding which may lead to inhibition of hatching.

Zebrafish as a model for acetylcholinesterase-inhibiting organophosphorus agent exposure and oxime reactivation

The current research progression efforts for investigating novel treatments for exposure to organophosphorus (OP) compounds that inhibit acetylcholinesterase (AChE), including pesticides and chemical warfare nerve agents (CWNAs), rely solely on in vitro cell assays and in vivo rodent models. The zebrafish (Danio rerio) is a popular, well-established vertebrate model in biomedical research that offers high-throughput capabilities and genetic manipulation not readily available with rodents. A number of research studies have investigated the effects of subacute developmental exposure to OP pesticides in zebrafish, observing detrimental effects on gross morphology, neuronal development, and behavior. Few studies, however, have utilized this model to evaluate treatments, such as oxime reactivators, anticholinergics, or anticonvulsants, following acute exposure. Preliminary work has investigated the effects of CWNA exposure. The results clearly demonstrated relative toxicity and oxime efficacy similar to that reported for the rodent model. This review surveys the current literature utilizing zebrafish as a model for OP exposure and highlights its potential use as a high-throughput system for evaluating AChE reactivator antidotal treatments to acute pesticide and CWNA exposure.