Expression of Neuropathy Target Esterase in mouse embryonic stem cells during differentiation

The phospholipase A2 superfamily as a central hub of bioactive lipids and beyond

In essence, "phospholipase A₂" (PLA₂) means a group of enzymes that release fatty acids and lysophospholipids by hydrolyzing the sn-2 position of glycerophospholipids. To date, more than 50 enzymes possessing PLA₂ or related lipid-metabolizing activities have been identified in mammals, and these are subdivided into several families in terms of their structures, catalytic mechanisms, tissue/cellular localizations, and evolutionary relationships. From a general viewpoint, the PLA₂ superfamily has mainly been implicated in signal transduction, driving the production of a wide variety of bioactive lipid mediators. However, a growing body of evidence indicates that PLA₂s also contribute to phospholipid remodeling or recycling for membrane homeostasis, fatty acid β-oxidation for energy production, and barrier lipid formation on the body surface. Accordingly, PLA₂ enzymes are considered one of the key regulators of a broad range of lipid metabolism, and perturbation of specific PLA₂-driven lipid pathways often disrupts tissue and cellular homeostasis and may be associated with a variety of diseases. This review covers current understanding of the physiological functions of the PLA₂ superfamily, focusing particularly on the two major intracellular PLA₂ families (Ca²⁺-dependent cytosolic PLA₂s and Ca²⁺-independent patatin-like PLA₂s) as well as other PLA₂ families, based on studies using gene-manipulated mice and human diseases in combination with comprehensive lipidomics.

The alteration of the expression level of neuropathy target esterase in human neuroblastoma SK-N-SH cells disrupts cellular phospholipids homeostasis

Neuropathy target esterase (NTE) has been proven to act as a lysophospholipase (LysoPLA) and phospholipase B (PLB) in mammalian cells. In this study, we took human neuroblastoma SK-N-SH cells as the research object and explored the effect of NTE on phospholipid homeostasis. The results showed that phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) levels significantly increased (> 40%), while glycerophosphocholine (GPC) decreased (below 60%) after NTE gene was knockdown in the cells (NTE < 30% of control), which were prepared by gene silencing with dsRNA-NTE. However, in the NTE-overexpressed cells (NTE > 50% of control), which were prepared by expressing recombinant catalytic domain of NTE, LPC remarkably decreased (below 80%) and GPC enhanced (> 40%). Mipafox, a neuropathic organophosphorus compound (OP), significantly inhibited NTE-LysoPLA and NTE-PLB activities (> 95-99% inhibition at 50 μM), which was accompanied with a decreased GPC level (below 40%) although no change of the PC and LPC levels was observed; while paraoxon, a non-neuropathic OP, suppresses neither the activities of NTE-phospholipases nor the levels of PC, LPC, and GPC. Thus, we concluded that both the stable up- or down-regulated expression of NTE gene and the loss of NTE-LysoPLA/PLB activities disrupts phospholipid homeostasis in the cells although the inhibition of NTE activity only decreased GPC content without altering PC and LPC levels.

PNPLA6/NTE, an Evolutionary Conserved Phospholipase Linked to a Group of Complex Human Diseases

Patatin-like phospholipase domain-containing protein 6 (PNPLA6), originally called Neuropathy Target Esterase (NTE), belongs to a family of hydrolases with at least eight members in mammals. PNPLA6/NTE was first identified as a key factor in Organophosphate-induced delayed neuropathy, a degenerative syndrome that occurs after exposure to organophosphates found in pesticides and nerve agents. More recently, mutations in PNPLA6/NTE have been linked with a number of inherited diseases with diverse clinical symptoms that include spastic paraplegia, ataxia, and chorioretinal dystrophy. A conditional knockout of PNPLA6/NTE in the mouse brain results in age-related neurodegeneration, whereas a complete knockout causes lethality during embryogenesis due to defects in the development of the placenta. PNPLA6/NTE is an evolutionarily conserved protein that in Drosophila is called Swiss-Cheese (SWS). Loss of SWS in the fly also leads to locomotory defects and neuronal degeneration that progressively worsen with age. This review will describe the identification of PNPLA6/NTE, its expression pattern, and normal role in lipid homeostasis, as well as the consequences of altered NPLA6/NTE function in both model systems and patients.

Effects of mipafox, paraoxon, chlorpyrifos and its metabolite chlorpyrifos-oxon on the expression of biomarker genes of differentiation in D3 mouse embryonic stem cells

Chlorpyrifos (CPS) is an organophosphorus compound (OP) capable of causing well-known cholinergic and delayed syndromes through the inhibition of acetylcholinesterase and Neuropathy Target Esterase (NTE), respectively. CPS is also able to induce neurodevelopmental toxicity in animals. NTE is codified by the Pnpla6 gene and plays a central role in differentiation and neurodifferentiation. We tested, in D3 mouse embryonic stem cells under differentiation, the effects of the NTE inhibition by the OPs mipafox, CPS and its main active metabolite chlorpyrifos-oxon (CPO) on the expression of genes Vegfa, Bcl2, Amot, Nes and Jun, previously reported to be under- or overexpressed after Pnpla6 silencing in this same cellular model. Mipafox did not significantly alter the expression of such genes at concentrations that significantly inhibited NTE. However, CPS and CPO at concentrations that caused NTE inhibition at similar levels to mipafox statistically and significantly altered the expression of most of these genes. Paraoxon (another OP with capability to inhibit esterases but not NTE) caused similar effects to CPS and CPO. These findings suggest that the molecular mechanism for the neurodevelopmental toxicity induced by CPS is not based on NTE inhibition, and that other unknown esterases might be potential targets of neurodevelopmental toxicity.

Genetic basis for developmental toxicity due to statin intake using embryonic stem cell differentiation model

The in utero environment is a key factor controlling the fate of the growing embryo. The deleterious effects of statins during the fetal development are still not very well understood. Data from animal studies and retrospective studies performed in pregnant women give conflicting reports. In this study, using in vitro differentiation model of embryonic stem cells, which mimic the differentiation process of the embryo, we have systematically exposed the cells to lipophilic statins, simvastatin, and atorvastatin at various doses and at critical times during differentiation. The analysis of key genes controlling the differentiation into ecto-, meso- and endodermal lineages was assessed by quantitative polymerase chain reaction. Our results show that genes of the mesodermal lineage were most sensitive to statins, leading to changes in the transcript levels of brachyury, Flk-1, Nkx2.5, and α/β-myosin heavy chain. In addition, changes to endodermal marker α-fetoprotein, along with ectodermal Nes and Neurofilament 200 kDa, imply that during early differentiation exposure to these drugs leads to altered signaling, which could translate to the congenital abnormalities seen in the heart and limbs.

Functional pathways altered after silencing Pnpla6 (the codifying gene of neuropathy target esterase) in mouse embryonic stem cells under differentiation

Neuropathy target esterase (NTE) is involved in several disorders in adult organisms and embryos. A relationship between NTE and nervous system integrity and maintenance in adult systems has been suggested. NTE-related motor neuron disease is associated with the expression of a mutant form of NTE and the inhibition and further modification of NTE by organophosphorus compounds is the trigger of a delayed neurodegenerative neuropathy. Homozygotic NTE knockout mice embryos are not viable, while heterozygotic NTE knockout mice embryos yields mice with neurological disorders, which suggest that this protein plays a critical role in embryonic development. The present study used D3 mouse embryonic stem cells with the aim of gaining mechanistic insights on the role of Pnpla6 (NTE gene encoding) in the developmental process. D3 cells were silenced by lipofectamine transfection with a specific interference RNA for Pnpla6. Silencing Pnpla6 in D3 monolayer cultures reduced NTE enzymatic activity to 50% 20 h post-treatment, while the maximum loss of Pnpla6 expression reached 80% 48 h postsilencing. Pnpla6 was silenced in embryoid bodies and 545 genes were differentially expressed regarding the control 96 h after silencing, which revealed alterations in multiple genetic pathways, such as cell motion and cell migration, vesicle regulation, and cell adhesion. These findings also allow considering that these altered pathways would impair the formation of respiratory, neural, and vascular tubes causing the deficiencies observed in the in vivo development of nervous and vascular systems. Our findings, therefore, support the previous observations made in vivo concerning lack of viability of mice embryos not expressing NTE and help to understand the biology of several neurological and developmental disorders in which NTE is involved.

Chlorpyrifos and its metabolites alter gene expression at non-cytotoxic concentrations in D3 mouse embryonic stem cells under in vitro differentiation: considerations for embryotoxic risk assessment

The effects of organophosphate insecticide chlorpyrifos (CPF) on development are currently under discussion. CPF and its metabolites, chlorpyrifos-oxon (CPO) and 3,5,6-trichloro-2-pyridinol (TClP), were more cytotoxic for D3 mouse embryonic stem cells than for differentiated fibroblasts 3T3 cells. Exposure to 10 μM CPF and TClP and 100 μM CPO for 12 h significantly altered the in vitro expression of biomarkers of differentiation in D3 cells. Similarly, exposure to 20 μM CPF and 25 μM CPO and TClP for 3 days also altered the expression of the biomarkers in the same model. These exposures caused no significant reduction in D3 viability with mild inhibition of acetylcholinesterase and neuropathy target esterase by CPF and severe inhibition by CPO. We conclude that certain in vivo exposure scenarios are possible, which cause inhibition of acetylcholinesterase but without clinical symptoms that reach high enough systemic CPF concentrations able to alter the expression of genes involved in cellular differentiation with potentially hazard effects on development. Conversely, the risk for embryotoxicity by CPO and TClP was very low because the required exposure would induce severe cholinergic syndrome.

Shortening and Improving the Embryonic Stem Cell Test through the Use of Gene Biomarkers of Differentiation

The embryonic Stem cell Test (EST) is a validated assay for testing embryotoxicity in vitro. The total duration of this protocol is 10 days, and its main end-point is based on histological determinations. It is suggested that improvements on EST must be focused toward molecular end-points and, if possible, to reduce the total assay duration. Five days of exposure of D3 cells in monolayers under spontaneous differentiation to 50 ng/mL of the strong embryotoxic 5-fluorouracil or to 75 μg/mL of the weak embryotoxic 5,5-diphenylhydeantoin caused between 20 and 74% of reductions in the expression of the following genes: Pnpla6, Afp, Hdac7, Vegfa, and Nes. The exposure to 1 mg/mL of nonembryotoxic saccharin only caused statistically significant reductions in the expression of Nes. These exposures reduced cell viability of D3 cells by 15, 28, and 34%. We applied these records to the mathematical discriminating function of the EST method to find that this approach is able to correctly predict the embryotoxicity of all three above-mentioned chemicals. Therefore, this work proposes the possibility of improve EST by reducing its total duration and by introducing gene expression as biomarker of differentiation, which might be very interesting for in vitro risk assessment embryotoxicity.

The role of molecular biology in the biomonitoring of human exposure to chemicals

Exposure to different substances in an occupational environment is of utmost concern to global agencies such as the World Health Organization and the International Labour Organization. Interest in improving work health conditions, particularly of those employees exposed to noxious chemicals, has increased considerably and has stimulated the search for new, more specific and selective tests. Recently, the field of molecular biology has been indicated as an alternative technique for monitoring personnel while evaluating work-related pathologies. Originally, occupational exposure to environmental toxicants was assessed using biochemical techniques to determine the presence of higher concentrations of toxic compounds in blood, urine, or other fluids or tissues; results were used to evaluate potential health risk. However, this approach only estimates the presence of a noxious chemical and its effects, but does not prevent or diminish the risk. Molecular biology methods have become very useful in occupational medicine to provide more accurate and opportune diagnostics. In this review, we discuss the role of the following common techniques: (1) Use of cell cultures; (2) evaluation of gene expression; (3) the “omic” sciences (genomics, transcriptomics, proteomics and metabolomics) and (4) bioinformatics. We suggest that molecular biology has many applications in occupational health where the data can be applied to general environmental conditions.

Motor neuron disease due to neuropathy target esterase mutation: enzyme analysis of fibroblasts from human subjects yields insights into pathogenesis

Recently, we identified neuropathy target esterase (NTE) mutation as the cause of an autosomal recessive motor neuron disease (NTE-MND). Subsequently, we showed that NTE-MND mutations reduced specific activity (SA) and altered inhibitory kinetics of NTE catalytic domain constructs. Recent preliminary results showed that NTE is expressed in cultured human skin fibroblasts, and others have used mutant forms of neuronal proteins expressed in fibroblasts as biomarkers of neurogenetic diseases. Therefore, the present study was carried out to test the hypothesis that NTE in cultured skin fibroblasts from NTE-MND subjects also exhibit altered enzymological properties assessed by SA and IC(50) values of mipafox (MIP) and chlorpyrifos oxon (CPO). NTE SA was reduced to 65% of control (wild-type NTE from commercially obtained fibroblasts) in homozygous M1012V fibroblasts and 59-61% of control in compound heterozygous R890H/c2946_2947InsCAGC fibroblasts. MIP IC(50) values were unaffected by the NTE mutations, but the CPO IC(50) increased 4.5-fold in homozygous M1012V fibroblasts. Interestingly, markedly reduced NTE SAs (40-43% of control) were observed in fibroblasts from asymptomatic subjects heterozygous for NTE insertion c2946_2947InsCAGC. This insertion is predicted to produce truncated NTE missing the last 235 residues of its catalytic domain. These observations confirm that NTE-MND mutations reduce NTE SA in vitro. Moreover, to the extent observations made in cultured fibroblasts may be generalized to events in the nervous system, lack of correlation between reduced fibroblast NTE SA and the occurrence of NTE-MND in NTE insertion mutation heterozygotes indicates that reduction of NTE SA alone is insufficient to cause MND.