Protein domains, catalytic activity, and subcellular distribution of neuropathy target esterase in Mammalian cells

PNPLA6 disorders: what's in a name?

BACKGROUND

Variants in the patatin-like phospholipase domain containing 6 (PNPLA6) gene cause a broad spectrum of neurological disorders characterized by gait disturbance, visual impairment, anterior hypopituitarism, and hair anomalies. This review examines the clinical, cellular, and biochemical features found across the five PNPLA6-related diseases, with a focus on future questions to be addressed.

MATERIALS AND METHODS

A literature review was performed on published clinical reports on patients with PNPLA6 variants. Additionally, in vitro and in vivo models used to study the encoded protein, Neuropathy Target Esterase (NTE), are summarized to lend mechanistic perspective to human diseases.

RESULTS

Biallelic pathogenic PNPLA6 variants cause five systemic neurological disorders: spastic paraplegia type 39, Gordon-Holmes, Boucher-Neuhäuser, Laurence-Moon, and Oliver-McFarlane syndromes. PNPLA6 encodes NTE, an enzyme involved in maintaining phospholipid homeostasis and trafficking in the nervous system. Retinal disease presents with a unique chorioretinal dystrophy that is phenotypically similar to choroideremia and Leber congenital amaurosis. Animal and cellular models support a loss-of-function mechanism.

CONCLUSIONS

Clinicians should be aware of choroideremia-like ocular presentation in patients who also experience growth defects, motor dysfunction, and/or hair anomalies. Although NTE biochemistry is well characterized, further research on the relationship between genotype and the presence or absence of retinopathy should be explored to improve diagnosis and prognosis.

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.

Multiple neurological effects associated with exposure to organophosphorus pesticides in man

This article reviews available data regarding the possible association of organophosphorus (OP) pesticides with neurological disorders such as dementia, attention deficit hyperactivity disorder, neurodevelopment, autism, cognitive development, Parkinson's disease and chronic organophosphate-induced neuropsychiatric disorder. These effects mainly develop after repeated (chronic) human exposure to low doses of OP. In addition, three well defined neurotoxic effects in humans caused by single doses of OP compounds are discussed. Those effects are the cholinergic syndrome, the intermediate syndrome and organophosphate-induced delayed polyneuropathy. Usually, the poisoning can be avoided by an improved administrative control, limited access to OP pesticides, efficient measures of personal protection and education of OP pesticide applicators and medical staff.

Insulin, dibutyryl-cAMP, and glucose modulate expression of patatin-like domain containing protein 7 in cultured human myotubes

Expression of patatin-like phospholipase domain containing protein 7 (PNPLA7), also known as neuropathy target esterase-related esterase (NRE), a lysophospholipase, increases with fasting and decreases with feeding in mouse skeletal muscle, indicating it is regulated by insulin, counterregulatory hormones, such as glucocorticoids and catecholamines, and/or nutrients. In cultured mouse adipocytes insulin reduces Pnpla7 expression, underscoring the possibility that insulin regulates PNPLA7 in skeletal muscle. The first aim of this study was to establish whether PNPLA7 is functionally expressed in cultured human skeletal muscle cells. The second aim was to determine whether PNPLA7 is regulated by insulin, glucocorticoids, cAMP/protein kinase A pathway, and/or glucose. Cultured human skeletal muscle cells expressed PNPLA7 mRNA and protein. Gene silencing of PNPLA7 in myoblasts reduced the phosphorylation of 70 kDa ribosomal protein S6 kinase and ribosomal protein S6 as well as the abundance of α1-subunit of Na⁺,K⁺-ATPase and acetyl-CoA carboxylase, indirectly suggesting that PNPLA7 is functionally important. In myotubes, insulin suppressed PNPLA7 mRNA at 1 g/L glucose, but not at low (0.5 g/L) or high (4.5 g/L) concentrations. Treatment with synthetic glucocorticoid dexamethasone and activator of adenylyl cyclase forskolin had no effect on PNPLA7 regardless of glucose concentration, while dibutyryl-cAMP, a cell-permeable cAMP analogue, suppressed PNPLA7 mRNA at 4.5 g/L glucose. The abundance of PNPLA7 protein correlated inversely with the glucose concentrations. Collectively, our results highlight that PNPLA7 in human myotubes is regulated by metabolic signals, implicating a role for PNPLA7 in skeletal muscle energy metabolism.

The Catalytic Domain of Neuropathy Target Esterase Influences Lipid Droplet Biogenesis and Lipid Metabolism in Human Neuroblastoma Cells

As an endoplasmic reticulum (ER)-anchored phospholipase, neuropathy target esterase (NTE) catalyzes the deacylation of lysophosphatidylcholine (LPC) and phosphatidylcholine (PC). The catalytic domain of NTE (NEST) exhibits comparable activity to NTE and binds to lipid droplets (LD). In the current study, the nucleotide monophosphate (cNMP)-binding domains (CBDs) were firstly demonstrated not to be essential for the ER-targeting of NTE, but to be involved in the normal ER distribution and localization to LD. NEST was associated with LD surface and influenced LD formation in human neuroblastoma cells. Overexpression of NEST enhances triacylglycerol (TG) accumulation upon oleic acid loading. Quantitative targeted lipidomic analysis shows that overexpression of NEST does not alter diacylglycerol levels but reduces free fatty acids content. NEST not only lowered levels of LPC and acyl-LPC, but not PC or alkyl-PC, but also widely altered levels of other lipid metabolites. Qualitative PCR indicates that the increase in levels of TG is due to the expression of diacylglycerol acyltransferase 1 gene by NEST overexpression. Thus, NTE may broadly regulate lipid metabolism to play roles in LD biogenesis in cells.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

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.

IntEResting structures: formation and applications of organized smooth endoplasmic reticulum in plant cells

The endoplasmic reticulum (ER) is an organelle with remarkable plasticity, capable of rapidly changing its structure to accommodate different functions based on intra- and extracellular cues. One of the ER structures observed in plants is known as "organized smooth endoplasmic reticulum" (OSER), consisting of symmetrically stacked ER membrane arrays. In plants, these structures were first described in certain specialized tissues, e.g. the sieve elements of the phloem, and more recently in transgenic plants overexpressing ER membrane resident proteins. To date, much of the investigation of OSER focused on yeast and animal cells but research into plant OSER has started to grow. In this update, we give a succinct overview of research into the OSER phenomenon in plant cells with case studies highlighting both native and synthetic occurrences of OSER. We also assess the primary driving forces that trigger the formation of OSER, collating evidence from the literature to compare two competing theories for the origin of OSER: that OSER formation is initiated by oligomerizing protein accumulation in the ER membrane or that OSER is the result of ER membrane proliferation. This has long been a source of controversy in the field and here we suggest a way to integrate arguments from both sides into a single unifying theory. Finally, we discuss the potential biotechnological uses of OSER as a tool for the nascent plant synthetic biology field with possible applications as a synthetic microdomain for metabolic engineering and as an extensive membrane surface for synthetic chemistry or protein accumulation.

Interaction of the Lysophospholipase PNPLA7 with Lipid Droplets through the Catalytic Region

Mammalian patatin-like phospholipase domain containing proteins (PNPLAs) play critical roles in triglyceride hydrolysis, phospholipids metabolism, and lipid droplet (LD) homeostasis. PNPLA7 is a lysophosphatidylcholine hydrolase anchored on the endoplasmic reticulum which associates with LDs through its catalytic region (PNPLA7-C) in response to increased cyclic nucleotide levels. However, the interaction of PNPLA7 with LDs through its catalytic region is unknown. Herein, we demonstrate that PNPLA7-C localizes to the mature LDs ex vivo and also colocalizes with pre-existing LDs. Localization of PNPLA7-C with LDs induces LDs clustering via non-enzymatic intermolecular associations, while PNPLA7 alone does not induce LD clustering. Residues 742-1016 contains four putative transmembrane domains which act as a LD targeting motif and are required for the localization of PNPLA7-C to LDs. Furthermore, the N-terminal flanking region of the LD targeting motif, residues 681-741, contributes to the LD targeting, whereas the C-terminal flanking region (1169-1326) has an anti-LD targeting effect. Interestingly, the LD targeting motif does not exhibit lysophosphatidylcholine hydrolase activity even though it associates with LDs phospholipid membranes. These findings characterize the specific functional domains of PNPLA7 mediating subcellular positioning and interactions with LDs, as wells as providing critical insights into the structure of this evolutionarily conserved phospholipid-metabolizing enzyme family.

Neuropathy target esterase (NTE/PNPLA6) and organophosphorus compound-induced delayed neurotoxicity (OPIDN)

Systemic inhibition of neuropathy target esterase (NTE) with certain organophosphorus (OP) compounds produces OP compound-induced delayed neurotoxicity (OPIDN), a distal degeneration of axons in the central nervous system (CNS) and peripheral nervous system (PNS), thereby providing a powerful model for studying a spectrum of neurodegenerative diseases. Axonopathies are important medical entities in their own right, but in addition, illnesses once considered primary neuronopathies are now thought to begin with axonal degeneration. These disorders include Alzheimer's disease, Parkinson's disease, and motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Moreover, conditional knockout of NTE in the mouse CNS produces vacuolation and other degenerative changes in large neurons in the hippocampus, thalamus, and cerebellum, along with degeneration and swelling of axons in ascending and descending spinal cord tracts. In humans, NTE mutations cause a variety of neurodegenerative conditions resulting in a range of deficits including spastic paraplegia and blindness. Mutations in the Drosophila NTE orthologue SwissCheese (SWS) produce neurodegeneration characterized by vacuolization that can be partially rescued by expression of wild-type human NTE, suggesting a potential therapeutic approach for certain human neurological disorders. This chapter defines NTE and OPIDN, presents an overview of OP compounds, provides a rationale for NTE research, and traces the history of discovery of NTE and its relationship to OPIDN. It then briefly describes subsequent studies of NTE, including practical applications of the assay; aspects of its domain structure, subcellular localization, and tissue expression; abnormalities associated with NTE mutations, knockdown, and conventional or conditional knockout; and hypothetical models to help guide future research on elucidating the role of NTE in OPIDN.

Characterization of the Interaction of Neuropathy Target Esterase with the Endoplasmic Reticulum and Lipid Droplets

Neuropathy target esterase (NTE) is an endoplasmic reticulum (ER)-localized phospholipase that deacylates phosphatidylcholine (PC) and lysophosphatidylcholine (LPC). Loss-of-function mutations in the human NTE gene have been associated with a spectrum of neurodegenerative disorders such as hereditary spastic paraplegia, ataxia and chorioretinal dystrophy. Despite this, little is known about structure–function relationships between NTE protein domains, enzymatic activity and the interaction with cellular organelles. In the current study we show that the C-terminal region of NTE forms a catalytically active domain that exhibits high affinity for lipid droplets (LDs), cellular storage organelles for triacylglycerol (TAG), which have been recently implicated in the progression of neurodegenerative diseases. Ectopic expression of the C domain in cultured cells decreases cellular PC, elevates TAG and induces LD clustering. LD interactions of NTE are inhibited by default by a non-enzymatic regulatory (R) region with three putative nucleotide monophosphate binding sites. Together with a N-terminal TMD the R region promotes proper distribution of the catalytic C-terminal region to the ER network. Taken together, our data indicate that NTE may exhibit dynamic interactions with the ER and LDs depending on the interplay of its functional regions. Mutations that disrupt this interplay may contribute to NTE-associated disorders by affecting NTE positioning.

The phospholipase PNPLA7 functions as a lysophosphatidylcholine hydrolase and interacts with lipid droplets through its catalytic domain

Mammalian patatin-like phospholipase domain–containing proteins (PNPLAs) are lipid-metabolizing enzymes with essential roles in energy metabolism, skin barrier development, and brain function. A detailed annotation of enzymatic activities and structure–function relationships remains an important prerequisite to understand PNPLA functions in (patho-)physiology, for example, in disorders such as neutral lipid storage disease, non-alcoholic fatty liver disease, and neurodegenerative syndromes. In this study, we characterized the structural features controlling the subcellular localization and enzymatic activity of PNPLA7, a poorly annotated phospholipase linked to insulin signaling and energy metabolism. We show that PNPLA7 is an endoplasmic reticulum (ER) transmembrane protein that specifically promotes hydrolysis of lysophosphatidylcholine in mammalian cells. We found that transmembrane and regulatory domains in the PNPLA7 N-terminal region cooperate to regulate ER targeting but are dispensable for substrate hydrolysis. Enzymatic activity is instead mediated by the C-terminal domain, which maintains full catalytic competence even in the absence of N-terminal regions. Upon elevated fatty acid flux, the catalytic domain targets cellular lipid droplets and promotes interactions of PNPLA7 with these organelles in response to increased cAMP levels. We conclude that PNPLA7 acts as an ER-anchored lysophosphatidylcholine hydrolase that is composed of specific functional domains mediating catalytic activity, subcellular positioning, and interactions with cellular organelles. Our study provides critical structural insights into an evolutionarily conserved class of phospholipid-metabolizing enzymes.

NTE/PNPLA6 is expressed in mature Schwann cells and is required for glial ensheathment of Remak fibers

Neuropathy target esterase (NTE) or patatin-like phospholipase domain containing 6 (PNPLA6) was first linked with a neuropathy occurring after organophosphate poisoning and was later also found to cause complex syndromes when mutated, which can include mental retardation, spastic paraplegia, ataxia, and blindness. NTE/PNPLA6 is widely expressed in neurons but experiments with its Drosophila orthologue Swiss-cheese (SWS) suggested that it may also have glial functions. Investigating whether NTE/PNPLA6 is expressed in glia, we found that NTE/PNPLA6 is expressed by Schwann cells in the sciatic nerve of adult mice with the most prominent expression in nonmyelinating Schwann cells. Within Schwann cells, NTE/PNPLA6 is enriched at the Schmidt-Lanterman incisures and around the nucleus. When analyzing postnatal expression patterns, we did not detect NTE/PNPLA6 in promyelinating Schwann cells, while weak expression was detectable at postnatal day 5 in Schwann cells and increased with their maturation. Interestingly, NTE/PNPLA6 levels were upregulated after nerve crush and localized to ovoids forming along the nerve fibers. Using a GFAP-based knock-out of NTE/PNPLA6, we detected an incomplete ensheathment of Remak fibers whereas myelination did not appear to be affected. These results suggest that NTE/PNPLA6 is involved in the maturation of nonmyelinating Schwann cells during development and de-/remyelination after neuronal injury. Since Schwann cells play an important role in maintaining axonal viability and function, it is therefore likely that changes in Schwann cells contribute to the locomotory deficits and neuropathy observed in patients carrying mutations in NTE.

Disturbed phospholipid homeostasis in endoplasmic reticulum initiates tri-o-cresyl phosphate-induced delayed neurotoxicity

Tri-o-cresyl phosphate (TOCP) is a widely used organophosphorus compound, which can cause a neurodegenerative disorder, i.e., organophosphate-induced delayed neurotoxicity (OPIDN). The biochemical events in the initiation of OPIDN were not fully understood except for the essential inhibition of neuropathy target esterase (NTE). NTE, located in endoplasmic reticulum (ER), catalyzes the deacylation of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) to glycerophosphocholine (GPC). The present study aims to study the changes of ER phospholipids profile as well as levels of important intermediates of phospholipid synthesis such as diacylglycerol (DAG) and phosphatidic acid (PA) at the initiation stage of OPIDN. Hens are the most commonly used animal models of OPIDN. The spinal cord phospholipidomic profiles of hens treated by TOCP were studied by using HPLC-MS-MS. The results revealed that TOCP induced an increase of PC, LPC, and sphingomyelin (SM) levels and a decrease of GPC, phosphatidylethanolamine (PE), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), phosphatidylglycerol (PG), and phosphatidylinositol (PI) levels., Levels of DAG and PA were also decreased. Pretreatment with phenylmethylsulfonyl fluoride (PMSF) 24 h before TOCP administration prevented OPIDN and restored the TOCP-induced changes of phospholipids except GPC. Thus, the disruption of ER phospholipid homeostasis may contribute to the initiation of organophosphate-induced delayed neurotoxicity.

The destruction box is involved in the degradation of the NTE family proteins by the proteasome

Neuropathy target esterase (NTE) and NTE-related esterase (NRE) are endoplasmic reticulum (ER) membrane-anchored proteins belonging to the NTE protein family. NTE and NRE are degraded by macroautophagy and by the ubiquitin-proteasome pathway. However, the regulation of NTE and NRE by proteasome has not been well understood. Western blotting showed that the deletion of the regulatory region of NTE and NRE led to protein accumulation compared with that of the corresponding wild-type proteins. Further, deletion and site-directed mutagenesis experiments demonstrated that the destruction (D) box was required for the proteasomal degradation of NTE and NRE. However, unlike the deletion of the regulatory region, the deletion of the D box did not affect the subcellular localisation of NTE or NRE or disrupt the ER. Moreover, the deletion of the D box or the regulatory region of NTE has similar inhibitory effects on cell growth, which are greater than those produced by the full-length NTE. Here, for the first time, we show that the D box is involved in the regulation of NTE family proteins by the proteasome but not in their subcellular localisation. In addition, these results suggest that the NTE overexpression-mediated inhibition of cell growth is related to active protein levels but not to its ER disruption effect.

Neuropathy Target Esterase Is Degraded by the Ubiquitin-Proteasome Pathway with ARA54 as the Ubiquitin Ligase

Neuropathy target esterase (NTE) is an endoplasmic reticulum membrane-associated phospholipase B, which is essential for embryonic and nervous system development. However, the regulation of NTE at the protein level had not been thoroughly investigated. Our previous study showed that NTE was degraded not only by the macroautophagy-lysosome pathway but also by the ubiquitin-proteasome pathway. Here we further reveal that androgen receptor-associated protein 54 (ARA54) regulated the ubiquitin-proteasome degradation of NTE. We find that deletion of the regulatory domain of NTE, which possesses a putative destruction box and thus is essential for its degradation by the proteasome, prevented its degradation by the proteasome. In addition, we demonstrate that ARA54, which has a RING finger domain and E3 ligase activity, interacts directly with NTE. Overexpression of ARA54 downregulates the protein level of NTE, and knockdown of ARA54 inhibits the degradation of NTE. The mutation in the RING domain of ARA54 blocks the degradation of NTE by ARA54, which indicates that the RING domain is essential for ARA54's E3 activity. These findings suggest that ARA54 acts as the ubiquitin ligase to regulate the ubiquitin-proteasome degradation of NTE.

Neuropathy target esterase impairments cause Oliver-McFarlane and Laurence-Moon syndromes

BACKGROUND

Oliver-McFarlane syndrome is characterised by trichomegaly, congenital hypopituitarism and retinal degeneration with choroidal atrophy. Laurence-Moon syndrome presents similarly, though with progressive spinocerebellar ataxia and spastic paraplegia and without trichomegaly. Both recessively inherited disorders have no known genetic cause.

METHODS

Whole-exome sequencing was performed to identify the genetic causes of these disorders. Mutations were functionally validated in zebrafish pnpla6 morphants. Embryonic expression was evaluated via in situ hybridisation in human embryonic sections. Human neurohistopathology was performed to characterise cerebellar degeneration. Enzymatic activities were measured in patient-derived fibroblast cell lines.

RESULTS

Eight mutations in six families with Oliver-McFarlane or Laurence-Moon syndrome were identified in the PNPLA6 gene, which encodes neuropathy target esterase (NTE). PNPLA6 expression was found in the developing human eye, pituitary and brain. In zebrafish, the pnpla6 curly-tailed morphant phenotype was fully rescued by wild-type human PNPLA6 mRNA and not by mutation-harbouring mRNAs. NTE enzymatic activity was significantly reduced in fibroblast cells derived from individuals with Oliver-McFarlane syndrome. Intriguingly, adult brain histology from a patient with highly overlapping features of Oliver-McFarlane and Laurence-Moon syndromes revealed extensive cerebellar degeneration and atrophy.

CONCLUSIONS

Previously, PNPLA6 mutations have been associated with spastic paraplegia type 39, Gordon-Holmes syndrome and Boucher-Neuhäuser syndromes. Discovery of these additional PNPLA6-opathies further elucidates a spectrum of neurodevelopmental and neurodegenerative disorders associated with NTE impairment and suggests a unifying mechanism with diagnostic and prognostic importance.

Association of sick building syndrome with neuropathy target esterase (NTE) activity in Japanese

Sick building syndrome (SBS) is a set of several clinically recognizable symptoms reported by occupants of a building without a clear cause. Neuropathy target esterase (NTE) is a membrane bound serine esterase and its reaction with organophosphates (OPs) can lead to OP-induced delayed neuropathy (OPIDN) and nerve axon degeneration. The aim of our study was to determine whether there was a difference in NTE activity in the peripheral blood mononuclear cells (PBMCs) of Japanese patients with SBS and healthy controls and whether PNPLA6 (alias NTE) gene polymorphisms were associated with SBS. We found that the enzymatic activity of NTE was significantly higher (P < 0.0005) in SBS patients compared with controls. Moreover, population with an AA genotype of a single nucleotide polymorphism (SNP), rs480208, in intron 21 of the PNPLA6 gene strongly reduced the activity of NTE. Fifty-eight SNP markers within the PNPLA6 gene were tested for association in a case-control study of 188 affected individuals and 401 age-matched controls. Only one SNP, rs480208, was statistically different in genotype distribution (P = 0.005) and allele frequency (P = 0.006) between the cases and controls (uncorrected for testing multiple SNP sites), but these were not significant by multiple corrections. The findings of the association between the enzymatic activity of NTE and SBS in Japanese show for the first time that NTE activity might be involved with SBS.

CREB is required for cAMP/PKA signals upregulating neuropathy target esterase expression

Neuropathy target esterase (NTE), which has been proposed as the primary target of organophosphorus compounds that cause delayed neuropathy with degeneration of nerve axons, is expressed primarily in neural cells but is also detected in non-neural cells. However, little is known about the regulation of NTE gene in cells. We found that a cyclic-AMP (cAMP)-response element (CRE) exists in the 5' flanking sequence of NTE gene in HeLa cells, which implies that NTE may be regulated by the transcription factor cAMP-response element-binding protein (CREB). In the study, knockdown of CREB decreased the protein and mRNA levels of NTE and inhibited the upregulation by cAMP/PKA signaling. Moreover, we observed that knockdown of CREB significantly decreased luciferase activity of the NTE gene promoter, while it had no effect on that of the CREB binding sites of mutated NTE gene promoter and truncated NTE gene promoter lacking the CREB binding site. cAMP/PKA signals could increase NTE reporter gene activity, while knockdown of CREB inhibited the increase. We found that the transcription factor CREB can bind to the promoter sequence of NTE by chromatin immunoprecipitation. In conclusion, we provided evidence that CREB is required for cAMP/PKA signals upregulating NTE expression in HeLa cells.

Protein adducts as biomarkers of exposure to organophosphorus compounds

Exposure to organophosphorus (OP) compounds can lead to serious neurological damage or death. Following bioactivation by the liver cytochromes P450, the OP metabolites produced are potent inhibitors of serine active-site enzymes including esterases, proteases and lipases. OPs may form adducts on other cellular proteins. Blood cholinesterases (ChEs) have long served as biomarkers of OP exposure in humans. However, the enzymatic assays used for biomonitoring OP exposures have several drawbacks. A more useful approach will focus on multiple biomarkers and avoid problems with the enzymatic activity assays. OP inhibitory effects result from a covalent bond with the active-site serine of the target enzymes. The serine OP adducts become irreversible following a process referred to as aging where one alkyl group dissociates over variable lengths of time depending on the OP adduct. The OP-adducted enzyme then remains in circulation until it is degraded, allowing for a longer window of detection compared with direct analysis of OPs or their metabolites. Mass spectrometry (MS) provides a very sensitive method for identification of post-translational protein modifications. MS analyses of the percentage adduction of the active-site serine of biomarker proteins such as ChEs will eliminate the need for basal activity levels of the individual and will provide for a more accurate determination of OP exposure. MS analysis of biomarker proteins also provides information about the OP that has caused inhibition. Other useful biomarker proteins include other serine hydrolases, albumin, tubulin and transferrin.

Neuropathy target esterase (NTE): overview and future

Neuropathy target esterase (NTE) was discovered by M.K. Johnson in his quest for the entity responsible for the striking and mysterious paralysis brought about by certain organophosphorus (OP) esters. His pioneering work on OP neuropathy led to the view that the biochemical lesion consisted of NTE that had undergone OP inhibition and aging. Indeed, nonaging NTE inhibitors failed to produce disease but protected against neuropathy from subsequently administered aging inhibitors. Thus, inhibition of NTE activity was not the culprit; rather, formation of an abnormal protein was the agent of the disorder. More recently, however, Paul Glynn and colleagues showed that whereas conventional knockout of the NTE gene was embryonic lethal, conditional knockout of central nervous system NTE produced neurodegeneration, suggesting to these authors that the absence of NTE rather than its presence in some altered form caused disease. We now know that NTE is the 6th member of a 9-protein family called patatin-like phospholipase domain-containing proteins, PNPLA1-9. Mutations in the catalytic domain of NTE (PNPLA6) are associated with a slowly developing disease akin to OP neuropathy and hereditary spastic paraplegia called NTE-related motor neuron disorder (NTE-MND). Furthermore, the NTE protein from affected individuals has altered enzymological characteristics. Moreover, closely related PNPLA7 is regulated by insulin and glucose. These seemingly disparate findings are not necessarily mutually exclusive, but we need to reconcile recent genetic findings with the historical body of toxicological data indicating that inhibition and aging of NTE are both necessary in order to produce neuropathy from exposure to certain OP compounds. Solving this mystery will be satisfying in itself, but it is also an enterprise likely to pay dividends by enhancing our understanding of the physiological and pathogenic roles of the PNPLA family of proteins in neurological health and disease, including a potential role for NTE in diabetic neuropathy.

Neuronal phospholipid deacylation is essential for axonal and synaptic integrity

Recessively-inherited deficiency in the catalytic activity of calcium-independent phospholipase A2-beta (iPLA2β) and neuropathy target esterase (NTE) causes infantile neuroaxonal dystrophy and hereditary spastic paraplegia, respectively. Thus, these two related phospholipases have non-redundant functions that are essential for structural integrity of synapses and axons. Both enzymes are expressed in essentially all neurons and also have independent roles in glia. iPLA2β liberates sn-2 fatty acid and lysophospholipids from diacyl-phospholipids. Ca(2+)-calmodulin tonically-inhibits iPLA2β, but this can be alleviated by oleoyl-CoA. Together with fatty acyl-CoA-mediated conversion of lysophospholipid to diacyl-phospholipid this may regulate sn-2 fatty acyl composition of phospholipids. In the nervous system, iPLA2β is especially important for the turnover of polyunsaturated fatty acid-associated phospholipid at synapses. More information is required on the interplay between iPLA2β and iPLA2-gamma in deacylation of neuronal mitochondrial phospholipids. NTE reduces levels of phosphatidylcholine (PtdCho) by degrading it to glycerophosphocholine and two free fatty acids. The substrate for NTE may be nascent PtdCho complexed with a phospholipid-binding protein. Protein kinase A-mediated phosphorylation enhances PtdCho synthesis and may allow PtdCho accumulation by coordinate inhibition of NTE activity. NTE operates primarily at the endoplasmic reticulum in neuronal soma but is also present in axons. NTE-mediated PtdCho homeostasis facilitates membrane trafficking and this appears most critical for the integrity of axon terminals in the spinal cord and hippocampus. For maintenance of peripheral nerve axons, iPLA2β activity may be able to compensate for NTE-deficiency but not vice-versa. Whether agonists acting at neuronal receptors modulate the activity of either enzyme remains to be determined. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Identification of two novel splicing variants of murine NTE-related esterase

NTE-related esterase (NRE) is an insulin-regulated lysophospholipase with homology to neuropathy target esterase (NTE), which plays a role in energy metabolism. Here, we reported two alternative splicing variants of the murine NRE (mNRE) gene, termed mNREV1 and mNREV2. Genomic organization analysis indicated that 5' splice site of mNRE intron 33 was changed in both mNREV1 and mNREV2, and mNRE exon 21 was deleted in mNREV2. mNREV1 had the same protein domains with mNRE, while mNREV2 lacked the patatin domain in the C-terminal catalytic region. Green fluorescent protein-mNREV1 or mNREV2 fusion proteins localized to the endoplasmic reticulum. mNREV1 and mNRE exhibited equal hydrolytic activity to the substrate phenyl valerate, whereas mNREV2 did not have any catalytic activity. The expression profiles of mNRE and its splicing isoforms in white adipose tissue, cardiac muscle, skeletal muscle, and testis tissues were further analyzed by real time quantitative-PCR in fed and fasted states, which indicated that the major isoform of mNRE mRNA generated switched from mNREV2 to mNREV1 during fasting. Thus there was a nutritional regulation of mNRE expression at the mRNA levels via alternative splicing.

Toxicity of Organophosphates and Carbamates

Organophosphate (OP) and carbamate (CM) compounds are commonly used as insecticides around the world. Some of them are extremely toxic to non-target species, including mammals. OP and CM insecticides are acetylcholinesterase (AChE) inhibitors and are commonly referred to as anticholinesterase agents. In addition to their cholinergic mechanisms, these insecticides exert toxicity through non-cholinergic mechanisms, thereby affecting several vital organs and body systems. The brain and skeletal muscles are the major target organs. Cardiovascular, respiratory and immune systems are also affected. There are similarities and differences between and among the toxicity profiles of OPs and CMs. This is due in part to variability in the interaction of each OP or CM with target and non-target receptors, enzymes and proteins. Treatment of CM poisoning rests with atropine, while the treatment of OP poisoning includes atropine in combination with an oxime.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Influence of lysophospholipid hydrolysis by the catalytic domain of neuropathy target esterase on the fluidity of bilayer lipid membranes

Neuropathy target esterase (NTE) is an integral membrane protein localized in the endoplasmic reticulum in neurons. Irreversible inhibition of NTE by certain organophosphorus compounds produces a paralysis known as organophosphorus compound-induced delayed neuropathy. In vitro, NTE has phospholipase/lysophospholipase activity that hydrolyses exogenously added single-chain lysophospholipids in preference to dual-chain phospholipids, and NTE mutations have been associated with motor neuron disease. NTE's physiological role is not well understood, although recent studies suggest that it may control the cytotoxic accumulation of lysophospholipids in membranes. We used the NTE catalytic domain (NEST) to hydrolyze palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (p-lysoPC) to palmitic acid in bilayer membranes comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and the fluorophore 1-oleoyl-2-[12-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC). Translational diffusion coefficients (D(L)) in supported bilayer membranes were measured by fluorescence recovery after pattern photobleaching (FRAPP). The average D(L) for DOPC/p-lysoPC membranes without NEST was 2.44 microm(2)s(-1)+/-0.09; the D(L) for DOPC/p-lysoPC membranes containing NEST and diisopropylphosphorofluoridate, an inhibitor, was nearly identical at 2.45+/-0.08. By contrast, the D(L) for membranes comprising NEST, DOPC, and p-lysoPC was 2.28+/-0.07, significantly different from the system with inhibited NEST, due to NEST hydrolysis. Likewise, a system without NEST containing the amount of palmitic acid that would have been produced by NEST hydrolysis of p-lysoPC was identical at 2.26+/-0.06. These results indicate that NTE's catalytic activity can alter membrane fluidity.

The role of cell cycle-dependent neuropathy target esterase in cell proliferation

Neuropathy target esterase (NTE) is a novel phospholipase B and plays a role in phospholipid homeostasis. Although over-expression of NTE inhibits cell division, the role of NTE in cell proliferation is still unknown. In the current study, we firstly used synchronous HeLa cells to study the expression profile of NTE during the cell cycle. NTE protein and activity are regulated during the cell cycle with highest level at G1 and lowest at G2/M phase. However, NTE mRNA levels are constant during the cell cycle. The role of NTE in cell proliferation was investigated by short hairpin RNA (shRNA) to suppress the expression of NTE. Knockdown of NTE significant down-regulated of NTE expression and reduced the glycerophosphocholine level. However, suppression of NTE did not affect phosphatidylcholine content or cell cycle progression. In addition, NTE was demonstrated to be degraded by the ubiquitin-proteasome pathway. These results suggested for the first time that NTE is a cell cycle-dependent protein, but is not essential for cell proliferation, and the ubiquitin-mediated proteolysis may be involved in the regulation of NTE during the cell cycle.

Protein domains, catalytic activity, and subcellular distribution of mouse NTE-related esterase

A mammalian family of lipid hydrolases, designated "patatin-like phospholipase domain containing (PNPLA)" recently has attracted attention. NTE-related esterase (NRE) as a member of PNPLA is an insulin-regulated lysophospholipase with homology to neuropathy target esterase (NTE). Mouse NRE (mNRE) has a predicted amino-terminal transmembrane region (TM), a putative regulatory (R) domain, and a hydrophobic catalytic (C) domain. In the current study, we described the expression of green fluorescent protein (GFP)-tagged constructs of mNRE and mutant proteins lacking the specific protein domains. Esterase assays indicated that neither the TM nor R-domain was essential for mNRE esterase activity, but the TM significantly contributed to its activity. Subcellular distribution showed that mNRE was anchored in ER via its TM domain and that its C-domain was associated with ER. Furthermore, experiments involving proteinase treatment revealed that most of mNRE molecule was exposed on the cytoplasmic face of ER membranes. Collectively, our results for the first time revealed the protein domains, catalytic activity, and subcellular location of mNRE and a simplified model for mNRE was proposed.

Neuropathy target esterase is required for adult vertebrate axon maintenance

The enzyme neuropathy target esterase (NTE) is present in neurons and deacylates the major membrane phospholipid, phosphatidylcholine (PtdCho). Mutation of the NTE gene or poisoning by neuropathic organophosphates--chemical inhibitors of NTE--causes distal degeneration of long spinal axons in humans. However, analogous neuropathological changes have not been reported in nestin-cre:NTEfl/fl mice with NTE-deficient neural tissue. Furthermore, altered PtdCho homeostasis has not been detected in NTE-deficient vertebrates. Here, we describe distal degeneration of the longest spinal axons in approximately 3-week-old nestin-cre:NTEfl/fl mice and in adult C57BL/6J mice after acute dosing with a neuropathic organophosphate: in both groups early degenerative lesions were followed by swellings comprising accumulated axoplasmic material. In mice dosed acutely with organophosphate, maximal numbers of lesions, in the longest spinal sensory axon tract, were attained within days and were preceded by a transient rise in neural PtdCho. In nestin-cre:NTEfl/fl mice, sustained elevation of PtdCho over many months was accompanied by progressive degeneration and massive swelling of axons in sensory and motor spinal tracts and by increasing hindlimb dysfunction. Axonal lesion distribution closely resembled that in hereditary spastic paraplegia (HSP). The importance of defective membrane trafficking in HSP and the association of NTE with the endoplasmic reticulum--the starting point for the constitutive secretory pathway and transport of neuronal materials into axons--prompted investigation for a role of NTE in secretion. Cultured NTE-deficient neurons displayed modestly impaired secretion, consistent with neuronal viability and damage in vivo initially restricted to distal parts of the longest axons.

What's new about osmotic regulation of glycerophosphocholine

Glycerophosphocholine is an abundant renal medullary organic osmolyte that protects renal medullary cells from the high interstitial concentrations of NaCl and urea to which they are normally exposed. We consider the metabolism of glycerophosphocholine, its osmotic regulation, and the recently discovered molecular identity of the enzymes that osmoregulate its abundance.

Effect of inhibition of neuropathy target esterase in mouse nervous tissues in vitro on phosphatidylcholine and lysophosphatidylcholine homeostasis

Neuropathy target esterase has been shown to be a lysophospholipase in mouse. The authors investigate the effect of neuropathy target esterase inhibition in mouse nervous tissues in vitro on the homeostasis of phosphatidylcholine and lysophosphatidylcholine by treating the homogenates with tri-ortho-cresyl phosphate, paraoxon, paraoxon plus mipafox, and phenylmethylsulfonyl fluoride. The activity of neuropathy target esterase is significantly inhibited by phenylmethylsulfonyl fluoride and paraoxon plus mipafox but not by paraoxon alone. Tri-ortho-cresyl phosphate slightly but significantly inhibits neuropathy target esterase activity in brain. The levels of phosphatidylcholine and lysophosphatidylcholine in all 3 nervous tissues are not obviously altered after treatment with tri-ortho-cresyl phosphate, paraoxon, or paraoxon plus mipafox. However, phosphatidylcholine and lysophosphatidylcholine levels are clearly enhanced by phenylmethylsulfonyl fluoride. It is concluded that inhibition of neuropathy target esterase in mouse nervous tissues is not enough to disrupt the homeostasis of phosphatidylcholine and lysophosphatidylcholine and that the upregulation by phenylmethylsulfonyl fluoride may be the consequence of combined inhibition of neuropathy target esterase and other phospholipases.

The homeostasis of phosphatidylcholine and lysophosphatidylcholine in nervous tissues of mice was not disrupted after administration of tri-o-cresyl phosphate

Neuropathy target esterase (NTE) is proven to act as a lysophospholipase (LysoPLA) in mice and phospholipase B (PLB) in cultured mammalian cells. In sensitive species, organophosphate (OP)-induced delayed neurotoxicity is initiated when NTE is inhibited by > 70% and then aged. It is hypothesized that homeostasis of phosphatidylcholine (PC) and/or lysophosphatidylcholine (LPC) in mice might be disrupted by the OPs since NTE and other phospholipases could be inhibited. To test this hypothesis, we treated mice using tri-o-cresyl phosphate (TOCP), which can inhibit and age NTE. Phenylmethylsulfonyl fluoride (PMSF), which inhibits NTE but cannot age, was used as a negative control. Effects on activity of NTE, LysoPLA, and PLB, the levels of PC, LPC, and glycerophosphocholine (GPC), and the aging of NTE in the brain, spinal cord, and sciatic nerve were examined. The results showed that the activities of NTE, NTE-LysoPLA, LysoPLA, NTE-PLB, and PLB were significantly inhibited in both TOCP- and PMSF-treated mice, and the inhibition of NTE and NTE-LysoPLA or NTE-PLB showed a high correlation coefficient. The NTE inhibited by TOCP was of the aged type, while nearly all NTE inhibited by PMSF was of the unaged type. Although the GPC level was remarkedly decreased, no significant change of PC and LPC levels was observed. However, the inhibition of these enzymes in mice by TOCP exhibited different characteristics from the TOCP-treated hens that we previously reported, which indicates that these enzymes were inhibited and then recovered more rapidly in mice than in hens. All results suggest that PC and LPC homeostasis was not disrupted in mice after exposure to TOCP. Differences in inhibition of NTE, LysoPLA, and PLB activities by TOCP between mice and hens may elucidate why these two species display different signs after exposure to the same neuropathic OPs.

Identification and characterization of chicken neuropathy target esterase

Neuropathy target esterase (NTE) was proposed as the initial target during the process of organophosphate-induced delayed neuropathy (OPIDN) and adult hens are the animal model of OPIDN. However, little has been known about the sequence and characteristics of chicken NTE. Here, we firstly identified the full length cDNA of chicken NTE (cNTE), which contained an open reading frame of 3966 nucleotides encoding 1321 amino acids. Chicken NTE had two distinct regions, one was the regulatory domain (cNTER) and the other was the catalytic domain (cNEST). Over-expression of cNTER in mammalian cells did not show any NTE activity, whereas cNEST had NTE activity. Cells expressing cNTER tagged with green fluorescent protein (GFP) showed accumulation of cNTER-GFP in an endoplasmic reticulum-like localization pattern. In addition, macroautophagy and the proteasome pathways were found to be involved in the degradation of cNTER, but not cNEST. These results first showed that cNTE was an ER-anchored protein and degraded by macroautophagy as well as the proteasome.

Swiss cheese et allii, some of the first neurodegenerative mutants isolated in Drosophila

Drosophila has only recently become a model organism to study progressive neurodegeneration, mainly using transgenic flies expressing human disease genes. However, classical forward genetics isolating and characterizing fly mutants that show characteristic features of progressive neurodegeneration can also provide a useful tool to get insights into the mechanisms of neurodegeneration. Interestingly, the first such mutants have been already isolated in the 1970s, and this review focuses on the description of four such mutants originally isolated by Martin Heisenberg.

Swiss Cheese, a protein involved in progressive neurodegeneration, acts as a noncanonical regulatory subunit for PKA-C3

The Drosophila Swiss Cheese (SWS) protein and its vertebrate ortholog Neuropathy Target Esterase (NTE) are required for neuronal survival and glial integrity. In humans, NTE is the target of organophosphorous compounds which cause a paralyzing axonal degeneration and recently mutations in NTE have been shown to cause a Hereditary Spastic Paraplegia called NTE-related Motor-Neuron Disorder. SWS and NTE are concentrated in the endoplasmic reticulum and both have been shown to have an esterase function against an artificial substrate. However, the functional mechanisms and the pathways in which SWS/NTE are involved in are still widely unknown. Here, we show that SWS interacts specifically with the C3 catalytic subunit of cAMP activated protein kinase (PKA-C3), which together with orthologs in mouse (Pkare) and human (PrKX) forms a novel class of catalytic subunits of unknown function. This interaction requires a domain of SWS which shows homology to regulatory subunits of PKA and, like conventional regulatory subunits, the binding of SWS to the PKA-C3 inhibits its function. Consistent with this result, expression of additional PKA-C3 induces degeneration and enhances the neurodegenerative phenotype in sws mutants. We also show that the complex formation with the membrane-bound SWS tethers PKA-C3 to membranes. We therefore propose a model in which SWS acts as a noncanonical subunit for PKA-C3, whereby the complex formation regulates the localization and kinase activity of PKA-C3, and that disruption of this regulation can induce neurodegeneration.

Degradation of neuropathy target esterase by the macroautophagic lysosomal pathway

AIMS

Neuropathy target esterase (NTE) was proposed as the initial target during the process of organophosphate-induced delayed neuropathy (OPIDN) in humans and some sensitive animals. NTE was recently identified as a novel phospholipase B that is anchored to the cytoplasmic side of the endoplasmic reticulum. However, little is known about the degradation of NTE. In this study, we have investigated the role of the macroautophagic-lysosomal pathway in NTE degradation in neuronal and non-neuronal cells.

MAIN METHODS

Macroautophagy inhibitors and activators were used to interrupt the lysosomal pathway, and NTE protein level was followed using western blotting analysis. A fluorescent microscopy assay was used to determine the co-localization of NTE and lysosomes.

KEY FINDINGS

Western blotting analysis showed that the macroautophagy inhibitors 3-methyladenine and ammonium chloride increased the levels of a heterologously expressed NTE-GFP fusion protein as well as endogenous NTE. Starvation had the opposite effect. The role of macroautophagy in NTE degradation was further supported by the co-localization of exogenous NTE with lysosomes in starved COS7 cells. Furthermore, the contribution of NTE activity and protein domains to the degradation of NTE by macroautophagy was investigated, showing that both the transmembrane and regulatory domains played a role in the degradation of NTE and that the catalytic domain, and thus NTE activity, was not involved.

SIGNIFICANCE

Our findings clearly demonstrate, for the first time, that the macroautophagy/lysosome pathway plays a role in controlling NTE quantity, providing a further understanding of the function of NTE.

The homeostasis of phosphatidylcholine and lysophosphatidylcholine was not disrupted during tri-o-cresyl phosphate-induced delayed neurotoxicity in hens

Little is known regarding early biochemical events in organophosphate-induced delayed neurotoxicity (OPIDN) except for the essential inhibition of neuropathy target esterase (NTE). We hypothesized that the homeostasis of lysophosphatidylcholine (LPC) and/or phosphatidylcholine (PC) in nervous tissues might be disrupted after exposure to the organophosphates (OP) which participates in the progression of OPIDN because new clues to possible mechanisms of OPIDN have recently been discovered that NTE acts as lysophospholipase (LysoPLA) in mice and phospholipase B (PLB) in cultured mammalian cells. To bioassay for such phospholipids, we induced OPIDN in hens using tri-o-cresyl phosphate (TOCP) as an inducer with phenylmethylsulfonyl fluoride (PMSF) as a negative control; and the effects on the activities of NTE, LysoPLA and PLB, the levels of PC, LPC, and glycerophosphocholine (GPC), and the aging of NTE enzyme in the brain, spinal cord, and sciatic nerves were examined. The results demonstrated that the activities of NTE, NTE-LysoPLA, LysoPLA, NTE-PLB and PLB were significantly inhibited in both TOCP- and PMSF-treated hens. The inhibition of NTE and NTE-LysoPLA or NTE-PLB showed a high correlation coefficient in the nervous tissues. Moreover, the NTE inhibited by TOCP was of the aged type, while nearly all of the NTE inhibited by PMSF was of the unaged type. No significant change in PC or LPC levels was observed, while the GPC level was significantly decreased. However, there is no relationship found between the GPC level and the delayed symptoms or aging of NTE. All results suggested that LPC and/or PC homeostasis disruption may not be a mechanism for OPIDN because the PC and LPC homeostasis was not disrupted after exposure to the neuropathic OP, although NTE, LysoPLA, and PLB were significantly inhibited and the GPC level was remarkably decreased.

GDPD5 is a glycerophosphocholine phosphodiesterase that osmotically regulates the osmoprotective organic osmolyte GPC

Glycerophosphocholine (GPC) is an abundant osmoprotective renal medullary organic osmolyte. We previously found that its synthesis from phosphatidylcholine is catalyzed by tonicity-regulated activity of the phospholipase B, neuropathy target esterase. We also found that its degradation is catalyzed by glycerophosphocholine phosphodiesterase (GPC-PDE) activity and that elevating osmolality from 300 to 500 mosmol/kg by adding NaCl or urea, inhibits GPC-PDE activity, which contributes to the resultant increase of GPC. In the present studies we identify GDPD5 (glycerophosphodiester phosphodiesterase domain containing 5) as a GPC-PDE that is rapidly inhibited by high NaCl or urea. Recombinant GDPD5 colocalizes with neuropathy target esterase in the perinuclear region of HEK293 cells, and immuno-precipitated recombinant GDPD5 degrades GPC in vitro. The in vitro activity is reduced when the cells from which the GDPD5 is immuno-precipitated have been exposed to high NaCl or urea. In addition, high NaCl decreases mRNA abundance of GDPD5 via an increase of its degradation rate, although high urea does not. At 300 mosmol/kg siRNA knockdown of GDPD5 increases GPC in mouse inner medullary collecting duct-3 cells, and over expression of recombinant GDPD5 increases cellular GPC-PDE activity, accompanied by decreased GPC. We conclude that GDPD5 is a GPC-PDE that contributes to osmotic regulation of cellular GPC.

Identification and characterization of a splice variant of the catalytic domain of mouse NTE-related esterase

Patatin-domain containing proteins constitute a large family of enzymes including known lipases that hydrolyze triglycerides, diglycerides, and phospholipids, some of which still remain to be characterized. One of those is NTE-related esterase (NRE), which exhibits sequence and domain homology to neuropathy target esterase (NTE). A splice variant of the catalytic domain of mouse NRE (mNRECV) was identified in multiple adult tissues, including brain, kidney, liver and testis. Genomic organization showed that mNRECV gene lacked the 22nd exon of mouse NRE and the 14th exon termination site of mNRECV was behind of 5 bp with the comparison of the 34th exon of mNRE gene. Over-expression of mNREC and mNRECV in mammalian cells showed that they had similar phenyl valerate esterase activities, but different from human NTE esterase domain. Subcellular distribution of an enhanced green fluorescent protein-mNRECV fusion protein was mainly observed to colocalize with endoplasmic reticulum in the juxtanuclear area and a little in cytoplasm. Moreover, autophagy/lysosome pathway was found to be involved in the degradation of mNRECV protein by inhibition and induction of autophagy, as well as co-location of mNRECV-EGFP with lysosomes. The high identity between mNRECV and mNREC suggested that mouse NRE has similar characteristics.

Axonal degeneration and neuropathy target esterase

This brief review summarizes recent observations which suggest a possible mechanism for organophosphate-induced delayed neuropathy (OPIDN). Neuropathy target esterase (NTE) has been shown to deacylate endoplasmic reticulum (ER) membrane phosphatidylcholine (PtdCho). Raised levels of PtdCho are present in the brains of Swiss cheese/NTE mutant Drosophila together with abnormal membrane structures, axonal and dendritic degeneration and neural cell loss. Similar vacuolated pathology is found in the brains of mice with brain-specific deletion of the NTE gene and, in old age, these mice show clinical and histopathological features of neuropathy resembling those in wild-type mice chronically dosed with tri-ortho-cresylphosphate. It is suggested that OPIDN results from the loss of NTE's phospholipase activity which in turn causes ER malfunction and perturbation of axonal transport and glial-axonal interactions.