Sensitivity of acetylcholinesterase molecular forms to inhibition by high MgCl2 concentration

"Calcium bombs" as harbingers of synaptic pathology and their mitigation by magnesium at murine neuromuscular junctions

Excitotoxicity is thought to be an important factor in the onset and progression of amyotrophic lateral sclerosis (ALS). Evidence from human and animal studies also indicates that early signs of ALS include degeneration of motor nerve terminals at neuromuscular junctions (NMJs), before degeneration of motor neuron cell bodies. Here we used a model of excitotoxicity at NMJs in isolated mouse muscle, utilizing the organophosphorus (OP) compound omethoate, which inhibits acetylcholinesterase activity. Acute exposure to omethoate (100 μM) induced prolonged motor endplate contractures in response to brief tetanic nerve stimulation at 20-50 Hz. In some muscle fibers, Fluo-4 fluorescence showed association of these contractures with explosive increases in Ca²⁺ ("calcium bombs") localized to motor endplates. Calcium bombs were strongly and selectively mitigated by increasing Mg²⁺ concentration in the bathing medium from 1 to 5 mM. Overnight culture of nerve-muscle preparations from Wld^S mice in omethoate or other OP insecticide components and their metabolites (dimethoate, cyclohexanone, and cyclohexanol) induced degeneration of NMJs. This degeneration was also strongly mitigated by increasing [Mg²⁺] from 1 to 5 mM. Thus, equivalent increases in extracellular [Mg²⁺] mitigated both post-synaptic calcium bombs and degeneration of NMJs. The data support a link between Ca²⁺ and excitotoxicity at NMJs and suggest that elevating extracellular [Mg²⁺] could be an effective intervention in treatment of synaptic pathology induced by excitotoxic triggers.

The activity of detoxifying enzymes in the infective juveniles of Heterorhabditis bacteriophora strains: Purification and characterization of two acetylcholinesterases

The infectivity and detoxifying enzyme activities including glutathione-S-transferase (GST), acetylcholinesterase (AChE) and carboxylesterase (CaE) are investigated in the infective juveniles (IJs) of six different strains of Heterorhabditis bacteriophora as a biocontrol agent against insect pests. The specific activities ranged from 10.8-29.8 and 50-220units/mg protein for GST and AChE, respectively; and from 24.7-129 and 22.6-77.3units/mg protein for CaE as estimated by P-nitrophenyl and α-naphthyl acetates, respectively. H. bacteriophora EM2 strain has the highest infectivity and the highest enzymatic activities as well. AChE is the predominant detoxifying enzyme that might imply its major role in the detoxification of insecticide(s). The isoenzyme pattern demonstrated two major slow-moving isoforms in all EPN strains examined. Purification of two AChE isoforms, AChEAII and AChEBI, from H. bacteriophora EM2 strain is performed by ammonium sulfate precipitation, gel filtration on Sephacryl S-200 and chromatography on DEAE-Sepharose. AChEAII and AChEBII have specific activities of 1207 and 1560unit/mg protein, native molecular weights of 180 and 68kDa, and are found in dimeric and monomeric forms, respectively. Both isoforms showed optimum activity at pH8.5 and 35°C. AChEBI exhibited higher thermal stability and higher activation energy than AChEAII. The enzymatic activities of purified AChEs are completely inhibited by Hg(+2) and Ni(+2) and greatly enhanced by Mn(+2). The substrate specificity, the relative efficiency of substrates hydrolysis, substrate inhibition and inhibition by BW284C51, but not by iso-OMPA, clearly indicated that they are true AChEs; their properties are compared with those recorded for insects as target hosts for H. bacteriophora EM2.

Different sensitivities of rat skeletal muscles and brain to novel anti-cholinesterase agents, alkylammonium derivatives of 6-methyluracil (ADEMS)

BACKGROUND AND PURPOSE

The rat respiratory muscle diaphragm has markedly lower sensitivity than the locomotor muscle extensor digitorum longus (EDL) to the new acetylcholinesterase (AChE) inhibitors, alkylammonium derivatives of 6-methyluracil (ADEMS). This study evaluated several possible reasons for differing sensitivity between the diaphragm and limb muscles and between the muscles and the brain.

EXPERIMENTAL APPROACH

Increased amplitude and prolonged decay time of miniature endplate currents were used to assess anti-cholinesterase activity in muscles. In hippocampal slices, induction of synchronous network activity was used to follow cholinesterase inhibition. The inhibitor sensitivities of purified AChE from the EDL and brain were also estimated.

KEY RESULTS

The intermuscular difference in sensitivity to ADEMS is partly explained caused by a higher level of mRNA and activity of 1,3-bis[5(diethyl-o-nitrobenzylammonium)pentyl]-6-methyluracildibromide (C-547)-resistant BuChE in the diaphragm. Moreover, diaphragm AChE was more than 20 times less sensitive to C-547 than that from the EDL. Sensitivity of the EDL to C-547 dramatically decreased after treadmill exercises that increased the amount of PRiMA AChE(G4), but not ColQ AChE(A12) molecular forms. The A12 form present in muscles appeared more sensitive to C-547. The main form of AChE in brain, PRiMA AChE(G4), was apparently less sensitive because brain cholinesterase activity was almost three orders of magnitude more resistant to C-547 than that of the EDL.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that ADEMS compounds could be used for the selective inhibition of AChEs and as potential therapeutic tools.

Anticonvulsant activity of aqueous root extract of Ficus religiosa

ETHNOPHARMACOLOGICAL RELEVANCE

Ficus religiosa Linn is frequently used for the treatment of nervous disorders among Pawara tribe of the Satpuda range, India.

AIM OF THE STUDY

This study aimed to investigate the anticonvulsant activity of the aqueous aerial root extract of Ficus religiosa in chemoconvulsant-induced seizures in mice.

MATERIALS AND METHODS

The anticonvulsant activity of the extract (25, 50 and 100 mg/kg, p.o.) was investigated in strychnine-, pentylenetetrazole-, picrotoxin- and isoniazid-induced seizures in mice. Rat ileum and fundus strip preparations were used to study the effect of the extract on acetylcholine (Ach)- and serotonin (5-HT)-induced contractions, respectively.

RESULTS

The extract showed no toxicity and protected the animals in the strychnine and pentylenetetrazole tests in a dose-dependent manner. Its effect in the picrotoxin and isoniazid tests, however, was less potent. The extract also exhibited dose-dependent potentiation of Ach in rat ileum but failed to potentiate the effect of 5-HT in rat fundus strip preparation.

CONCLUSIONS

The results suggest that an orally administered aqueous root extract of Ficus religiosa has dose-dependent and potent anticonvulsant activities against strychnine- and pentylenetetrazole-induced seizures. The observed activities may be ascribed to the appreciable content of zinc and magnesium in the extract.

Eosinophil-Mediated Cholinergic Nerve Remodeling

Eosinophils are observed to localize to cholinergic nerves in a variety of inflammatory conditions such as asthma, rhinitis, eosinophilic gastroenteritis, and inflammatory bowel disease, where they are also responsible for the induction of cell signaling. We hypothesized that a consequence of eosinophil localization to cholinergic nerves would involve a neural remodeling process. Eosinophil co-culture with cholinergic IMR32 cells led to increased expression of the M2 muscarinic receptor, with this induction being mediated via an adhesion-dependent release of eosinophil proteins, including major basic protein and nerve growth factor. Studies on the promoter sequence of the M2 receptor indicated that this induction was initiated at a transcription start site 145 kb upstream of the gene-coding region. This promoter site contains binding sites for a variety of transcription factors including SP1, AP1, and AP2. Eosinophils also induced the expression of several cholinergic genes involved in the synthesis, storage, and metabolism of acetylcholine, including the enzymes choline acetyltransferase, vesicular acetylcholine transferase, and acetylcholinesterase. The observed eosinophil-induced changes in enzyme content were associated with a reduction in intracellular neural acetylcholine but an increase in choline content, suggesting increased acetylcholine turnover and a reduction in acetylcholinesterase activity, in turn suggesting reduced catabolism of acetylcholine. Together these data suggest that eosinophil localization to cholinergic nerves induces neural remodeling, promoting a cholinergic phenotype.