Loss of Swiss cheese/neuropathy target esterase activity causes disruption of phosphatidylcholine homeostasis and neuronal and glial death in adult Drosophila

Unraveling the link between neuropathy target esterase NTE/SWS, lysosomal storage diseases, inflammation, abnormal fatty acid metabolism, and leaky brain barrier

Mutations in Drosophila Swiss cheese (SWS) gene or its vertebrate orthologue neuropathy target esterase (NTE) lead to progressive neuronal degeneration in flies and humans. Despite its enzymatic function as a phospholipase is well established, the molecular mechanism responsible for maintaining nervous system integrity remains unclear. In this study, we found that NTE/SWS is present in surface glia that forms the blood-brain barrier (BBB) and that NTE/SWS is important to maintain its structure and permeability. Importantly, BBB glia-specific expression of Drosophila NTE/SWS or human NTE in the sws mutant background fully rescues surface glial organization and partially restores BBB integrity, suggesting a conserved function of NTE/SWS. Interestingly, sws mutant glia showed abnormal organization of plasma membrane domains and tight junction rafts accompanied by the accumulation of lipid droplets, lysosomes, and multilamellar bodies. Since the observed cellular phenotypes closely resemble the characteristics described in a group of metabolic disorders known as lysosomal storage diseases (LSDs), our data established a novel connection between NTE/SWS and these conditions. We found that mutants with defective BBB exhibit elevated levels of fatty acids, which are precursors of eicosanoids and are involved in the inflammatory response. Also, as a consequence of a permeable BBB, several innate immunity factors are upregulated in an age-dependent manner, while BBB glia-specific expression of NTE/SWS normalizes inflammatory response. Treatment with anti-inflammatory agents prevents the abnormal architecture of the BBB, suggesting that inflammation contributes to the maintenance of a healthy brain barrier. Considering the link between a malfunctioning BBB and various neurodegenerative diseases, gaining a deeper understanding of the molecular mechanisms causing inflammation due to a defective BBB could help to promote the use of anti-inflammatory therapies for age-related neurodegeneration.

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 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 Drosophila blood-brain barrier emerges as a model for understanding human brain diseases

The accurate regulation of the microenvironment within the nervous system is one of the key features characterizing complex organisms. To this end, neural tissue has to be physically separated from circulation, but at the same time, mechanisms must be in place to allow controlled transport of nutrients and macromolecules into and out of the brain. These roles are executed by cells of the blood-brain barrier (BBB) found at the interface of circulation and neural tissue. BBB dysfunction is observed in several neurological diseases in human. Although this can be considered as a consequence of diseases, strong evidence supports the notion that BBB dysfunction can promote the progression of brain disorders. In this review, we compile the recent evidence describing the contribution of the Drosophila BBB to the further understanding of brain disease features in human patients. We discuss the function of the Drosophila BBB during infection and inflammation, drug clearance and addictions, sleep, chronic neurodegenerative disorders and epilepsy. In summary, this evidence suggests that the fruit fly, Drosophila melanogaster, can be successfully employed as a model to disentangle mechanisms underlying human diseases.

The Lysophospholipase PNPLA7 Controls Hepatic Choline and Methionine Metabolism

The in vivo roles of lysophospholipase, which cleaves a fatty acyl ester of lysophospholipid, remained unclear. Recently, we have unraveled a previously unrecognized physiological role of the lysophospholipase PNPLA7, a member of the Ca2+-independent phospholipase A2 (iPLA2) family, as a key regulator of the production of glycerophosphocholine (GPC), a precursor of endogenous choline, whose methyl groups are preferentially fluxed into the methionine cycle in the liver. PNPLA7 deficiency in mice markedly decreases hepatic GPC, choline, and several metabolites related to choline/methionine metabolism, leading to various symptoms reminiscent of methionine shortage. Overall metabolic alterations in the liver of Pnpla7-null mice in vivo largely recapitulate those in methionine-deprived hepatocytes in vitro. Reduction of the methyl donor S-adenosylmethionine (SAM) after methionine deprivation decreases the methylation of the PNPLA7 gene promoter, relieves PNPLA7 expression, and thereby increases GPC and choline levels, likely as a compensatory adaptation. In line with the view that SAM prevents the development of liver cancer, the expression of PNPLA7, as well as several enzymes in the choline/methionine metabolism, is reduced in human hepatocellular carcinoma. These findings uncover an unexplored role of a lysophospholipase in hepatic phospholipid catabolism coupled with choline/methionine metabolism.

Hepatic phosphatidylcholine catabolism driven by PNPLA7 and PNPLA8 supplies endogenous choline to replenish the methionine cycle with methyl groups

Choline supplies methyl groups for regeneration of methionine and the methyl donor S-adenosylmethionine in the liver. Here, we report that the catabolism of membrane phosphatidylcholine (PC) into water-soluble glycerophosphocholine (GPC) by the phospholipase/lysophospholipase PNPLA8-PNPLA7 axis enables endogenous choline stored in hepatic PC to be utilized in methyl metabolism. PNPLA7-deficient mice show marked decreases in hepatic GPC, choline, and several metabolites related to the methionine cycle, accompanied by various signs of methionine insufficiency, including growth retardation, hypoglycemia, hypolipidemia, increased energy consumption, reduced adiposity, increased fibroblast growth factor 21 (FGF21), and an altered histone/DNA methylation landscape. Moreover, PNPLA8-deficient mice recapitulate most of these phenotypes. In contrast to wild-type mice fed a methionine/choline-deficient diet, both knockout strains display decreased hepatic triglyceride, likely via reductions of lipogenesis and GPC-derived glycerol flux. Collectively, our findings highlight the biological importance of phospholipid catabolism driven by PNPLA8/PNPLA7 in methyl group flux and triglyceride synthesis in the liver.

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.

Placental Therapeutic Targets and Nanodelivery Systems

Pregnancy complications affect millions of women each year. Some of these diseases have high morbidity and mortality such as preeclampsia. At present, there is no safe and effective treatment for pregnancy complications, so it is still a difficult clinical problem. As many pregnancy complications are closely related to placental dysplasia, placenta-specific therapy, as an important method, is expected to be a safe, effective, and specific therapeutic strategy. This review explains in detail the placenta physiological structure, characteristics, and action mechanism of some biomolecules and signaling pathways that play roles in normal development and disorders of the development of the placenta, and how to use these biomolecules as therapeutic targets when the placenta disorder causes disease, combining the latest progress in the field of nanodelivery systems, so as to lay a foundation for the development of placenta-specific therapy of pregnancy complications.

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.

Current Knowledge on Mammalian Phospholipase A1, Brief History, Structures, Biochemical and Pathophysiological Roles

Phospholipase A₁ (PLA₁) is an enzyme that cleaves an ester bond at the sn-1 position of glycerophospholipids, producing a free fatty acid and a lysophospholipid. PLA₁ activities have been detected both extracellularly and intracellularly, which are well conserved in higher eukaryotes, including fish and mammals. All extracellular PLA₁s belong to the lipase family. In addition to PLA₁ activity, most mammalian extracellular PLA₁s exhibit lipase activity to hydrolyze triacylglycerol, cleaving the fatty acid and contributing to its absorption into the intestinal tract and tissues. Some extracellular PLA₁s exhibit PLA₁ activities specific to phosphatidic acid (PA) or phosphatidylserine (PS) and serve to produce lysophospholipid mediators such as lysophosphatidic acid (LPA) and lysophosphatidylserine (LysoPS). A high level of PLA₁ activity has been detected in the cytosol fractions, where PA-PLA₁/DDHD1/iPLA₁ was responsible for the activity. Many homologs of PA-PLA₁ and PLA₂ have been shown to exhibit PLA₁ activity. Although much has been learned about the pathophysiological roles of PLA₁ molecules through studies of knockout mice and human genetic diseases, many questions regarding their biochemical properties, including their genuine in vivo substrate, remain elusive.

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.

Drosophila Lysophospholipase Gene swiss cheese Is Required for Survival and Reproduction

Simple Summary

Biological evolution implies fitness of newly evolved organisms that have inherent adaptive traits because of mutations in genes. However, most mutations are detrimental, and they spoil the organism’s life, its survival and its ability to leave progeny. Some genes are extremely vital for an organism, and therefore, they tend to save their structure and do not mutate or do it very composedly. That is the case of the gene encoding PNPLA6 lysophospholipase domain that evolved in bacteria, and evolution obliged it to save its function in higher animals. In mammals, complete dysfunction of such a gene is lethal because of its high importance in placenta for early embryo development. Why is it conserved in other species, for instance insects, that have no placenta? Here we studied the role of the PNPLA6-encoding gene named swiss cheese in Drosophila melanogaster fitness. We have found that its dysfunction results in premature death of specimens and their inability to leave enough progeny. Thus, we provide the first evidence for significance of the gene that encodes the lysophospholipase enzyme in fitness of insects.

Abstract

Drosophila melanogaster is one of the most famous insects in biological research. It is widely used to analyse functions of different genes. The phosphatidylcholine lysophospholipase gene swiss cheese was initially shown to be important in the fruit fly nervous system. However, the role of this gene in non-nervous cell types has not been elucidated yet, and the evolutional explanation for the conservation of its function remains elusive. In this study, we analyse expression pattern and some aspects of the role of the swiss cheese gene in the fitness of Drosophila melanogaster. We describe the spatiotemporal expression of swiss cheese throughout the fly development and analyse the survival and productivity of swiss cheese mutants. We found swiss cheese to be expressed in salivary glands, midgut, Malpighian tubes, adipocytes, and male reproductive system. Dysfunction of swiss cheese results in severe pupae and imago lethality and decline of fertility, which is impressive in males. The latter is accompanied with abnormalities of male locomotor activity and courtship behaviour, accumulation of lipid droplets in testis cyst cells and decrease in spermatozoa motility. These results suggest that normal swiss cheese is important for Drosophila melanogaster fitness due to its necessity for both specimen survival and their reproductive success.

Mass Spectrometry Imaging Deciphers Dysregulated Lipid Metabolism in the Human Hippocampus Affected by Temporal Lobe Epilepsy

Temporal lobe epilepsy (TLE) is the most prevalent form of human epilepsy, often accompanied by neurodegeneration in the hippocampus. Like other neurological diseases, TLE is expected to disrupt lipid homeostasis. However, the lipid architecture of the human TLE brain is relatively understudied, and the molecular mechanism of epileptogenesis is poorly understood. We performed desorption electrospray ionization mass spectrometry imaging of 39 fresh frozen surgical specimens of the human hippocampus to investigate lipid profiles in TLE with hippocampal sclerosis (n = 14) and control (non-TLE; n = 25) groups. In contrast to several previous studies on animal models of epilepsy, we report reduced expression of various important lipids, notably phosphatidylcholine (PC) and phosphatidylethanolamine (PE), in the human TLE hippocampus. In addition, metabolic pathway analysis suggested the possible dysregulation of the Kennedy pathway in TLE, resulting in striking reductions of PC and PE levels. This revelation opens up opportunities to further investigate the associated molecular mechanisms and possible therapeutic targets for TLE.

Loss of swiss cheese in Neurons Contributes to Neurodegeneration with Mitochondria Abnormalities, Reactive Oxygen Species Acceleration and Accumulation of Lipid Droplets in Drosophila Brain

Various neurodegenerative disorders are associated with human NTE/PNPLA6 dysfunction. Mechanisms of neuropathogenesis in these diseases are far from clearly elucidated. Hereditary spastic paraplegia belongs to a type of neurodegeneration associated with NTE/PNLPLA6 and is implicated in neuron death. In this study, we used Drosophila melanogaster to investigate the consequences of neuronal knockdown of swiss cheese (sws)-the evolutionarily conserved ortholog of human NTE/PNPLA6-in vivo. Adult flies with the knockdown show longevity decline, locomotor and memory deficits, severe neurodegeneration progression in the brain, reactive oxygen species level acceleration, mitochondria abnormalities and lipid droplet accumulation. Our results suggest that SWS/NTE/PNPLA6 dysfunction in neurons induces oxidative stress and lipid metabolism alterations, involving mitochondria dynamics and lipid droplet turnover in neurodegeneration pathogenesis. We propose that there is a complex mechanism in neurological diseases such as hereditary spastic paraplegia, which includes a stress reaction, engaging mitochondria, lipid droplets and endoplasmic reticulum interplay.

Morpho-Functional Consequences of Swiss Cheese Knockdown in Glia of Drosophila melanogaster

Glia are crucial for the normal development and functioning of the nervous system in many animals. Insects are widely used for studies of glia genetics and physiology. Drosophila melanogaster surface glia (perineurial and subperineurial) form a blood–brain barrier in the central nervous system and blood–nerve barrier in the peripheral nervous system. Under the subperineurial glia layer, in the cortical region of the central nervous system, cortex glia encapsulate neuronal cell bodies, whilst in the peripheral nervous system, wrapping glia ensheath axons of peripheral nerves. Here, we show that the expression of the evolutionarily conserved swiss cheese gene is important in several types of glia. swiss cheese knockdown in subperineurial glia leads to morphological abnormalities of these cells. We found that the number of subperineurial glia nuclei is reduced under swiss cheese knockdown, possibly due to apoptosis. In addition, the downregulation of swiss cheese in wrapping glia causes a loss of its integrity. We reveal transcriptome changes under swiss cheese knockdown in subperineurial glia and in cortex + wrapping glia and show that the downregulation of swiss cheese in these types of glia provokes reactive oxygen species acceleration. These results are accompanied by a decline in animal mobility measured by the negative geotaxis performance assay.

Regulation of glial size by eicosapentaenoic acid through a novel Golgi apparatus mechanism

Coordination of cell growth is essential for the development of the brain, but the molecular mechanisms underlying the regulation of glial and neuronal size are poorly understood. To investigate the mechanisms involved in glial size regulation, we used Caenorhabditis elegans amphid sheath (AMsh) glia as a model and show that a conserved cis-Golgi membrane protein eas-1/GOLT1B negatively regulates glial growth. We found that eas-1 inhibits a conserved E3 ubiquitin ligase rnf-145/RNF145, which, in turn, promotes nuclear activation of sbp-1/ SREBP, a key regulator of sterol and fatty acid synthesis, to restrict cell growth. At early developmental stages, rnf-145 in the cis-Golgi network inhibits sbp-1 activation to promote the growth of glia, and when animals reach the adult stage, this inhibition is released through an eas-1-dependent shuttling of rnf-145 from the cis-Golgi to the trans-Golgi network to stop glial growth. Furthermore, we identified long-chain polyunsaturated fatty acids (LC-PUFAs), especially eicosapentaenoic acid (EPA), as downstream products of the eas-1-rnf-145-sbp-1 pathway that functions to prevent the overgrowth of glia. Together, our findings reveal a novel and potentially conserved mechanism underlying glial size control.

The molecular mechanisms underlying the regulation of glial and neuronal size are poorly understood. This study in nematodes reveals eicosapentaenoic acid as the downstream product of a pathway that functions to prevent the overgrowth of glia, suggesting a novel and potentially conserved mechanism underlying glial size control.

Repeated low-dose exposures to sarin disrupted the homeostasis of phospholipid and sphingolipid metabolism in guinea pig hippocampus

Repeated low-level exposure to sarin results to hippocampus dysfunction. Metabonomics involves a holistic analysis of a set of metabolites in an organism in the search for a relationship between these metabolites and physiological or pathological changes. The objective of the present study was to evaluate the effects of repeated exposure to low-level sarin on the metabonomics in hippocampus of a guinea pig model. Guinea pigs were divided randomly into control and sarin treated groups (n = 14). Guinea pigs in the control group received saline; while the sarin-treated group received 0.4×LD₅₀ (16.8 μg/kg) sarin. Daily injections (a total of 14 days) were administered sc between the shoulder blades in a volume of 1.0 ml/kg body weight. At the end of the final injection, 6 animals in each group were chosen for Morris water maze test. The rest guinea pigs (n = 8 for each group) were sacrificed by decapitation, and hippocampus were dissected for analysis. Compared with the control-group, the escape latency in sarin-group was significantly (p < 0.05) longer while the crossing times were significantly decreased in the Morris water task (p < 0.05). Sarin inhibited activities of acetylcholinesterase (AChE) and neuropathy target esterase (NTE) in hippocampus. The AChE activity of hippocampus from sarin-treated groups is equivalent to 59.9 ± 6.4 %, and the NTE activity of hippocampus from sarin-groups is equivalent to 78.1 ± 8.3 % of that from control-group. Metabolites were identified and validated. A total of 14 variables were selected as potential biomarkers. Phospholipids [phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylinositol (PI), Lysophosphatidylethanolamine (LysoPE or LPE)] and sphingolipids (SPs) [sphinganine (SA), phytosphingosine (PSO) and sphinganine-1-phosphate (SA1P)] were clearly modified. In conclusion, repeated low-dose exposures to sarin disrupted the homeostasis of phospholipid and sphingolipid metabolism in guinea pig hippocampus and may lead to a neuronal-specific function disorders. Identified metabolites such as SA1P need to be studied more deeply on their biological function that against sarin lesions. In future research, we should pay more attention to characterize the physiological roles of lipid metabolism enzymes as well as their involvement in pathologies induced by repeated low-level sarin exposure.

Novel variants in PNPLA6 causing syndromic retinal dystrophy

PNPLA6-related disorders include several phenotypes, such as Boucher-Neuhäuser syndrome, Gordon Holmes syndrome, spastic paraplegia, photoreceptor degeneration, Oliver-McFarlane syndrome and Laurence-Moon syndrome. In this study, detailed clinical evaluations and genetic testing were performed in five (4 Chinese and 1 Caucasian/Chinese) syndromic retinal dystrophy patients. Genotype-phenotype correlations were analyzed based on review of the literatures of previously published PNPLA6-related cases. The mean age of patients and at first visit were 20.8 years (11, 12, 25, 28, 28) and 14.2 years (4, 7, 11, 24, 25), respectively. They all presented with severe chorioretinal dystrophy and profoundly decreased vision. The best corrected visual acuity (BCVA) ranged from 20/200 to 20/2000. Systemic manifestations included cerebellar ataxia, hypogonadotropic hypogonadism and hair anomalies. Six novel and three reported pathogenic variants in PNPLA6 (NM_001166111) were identified. The genotypes of the five cases are: c.3134C > T (p.Ser1045Leu) and c.3846+1G > A, c.3547C > T (p.Arg1183Trp) and c.1841+3A > G, c.3436G > A (p.Ala1146Thr) and c.2212-10A > G, c.3436G > A (p.Ala1146Thr) and c.2266C > T (p.Gln756*), c.1238_1239insC (p.Leu414Serfs*28) and c.3130A > G (p.Thr1044Ala). RT-PCR confirmed that the splicing variants indeed led to abnormal splicing. Missense variants p.Thr1044Ala, p.Ser1045Leu, p.Ala1146Thr, p.Arg1183Trp and c.3846+1G > A are located in Patatin-like phospholipase (Pat) domain. In conclusion, we report the phenotypes in five patients with PNPLA6 associated syndromic retinal dystrophy with variable systemic involvement and typical choroideremia-like fundus changes. Ocular manifestations may be the first and the only findings for years. All of our patients carried one severe deleterious variant (stop-gain or splicing variant) and one milder variant (missense variant). Retinal involvement was significantly correlated with severe deleterious variants and variants in Pat domain.

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.

Disease-Associated PNPLA6 Mutations Maintain Partial Functions When Analyzed in Drosophila

Mutations in patatin-like phospholipase domain-containing protein 6 (PNPLA6) have been linked with a number of inherited diseases with clinical symptoms that include spastic paraplegia, ataxia, and chorioretinal dystrophy. PNPLA6 is an evolutionary conserved protein whose ortholog in Drosophila is Swiss-Cheese (SWS). Both proteins are phospholipases hydrolyzing lysophosphatidylcholine (LPC) and phosphatidylcholine (PC). Consequently, loss of SWS/PNPLA6 in flies and mice increases both lipids and leads to locomotion deficits and neurodegeneration. PNPLA6 knock-out mice are embryonic lethal, and a mutation creating an early stop codon in human PNPLA6 has only been identified in compound heterozygote patients. In contrast, disease-causing point mutations are found in homozygous patients, with some localized in the phospholipase domain while others are in a region that contains several cNMP binding sites. To investigate how different mutations affect the function of PNPLA6 in an in vivo model, we expressed them in the Drosophila sws1 null mutant. Expressing wild-type PNPLA6 suppressed the locomotion and degenerative phenotypes in sws 1 and restored lipid levels, confirming that the human protein can replace fly SWS. In contrast, none of the mutant proteins restored lipid levels, although they suppressed the behavioral and degenerative phenotypes, at least in early stages. These results show that these mutant forms of PNPLA6 retain some biological function, indicating that disruption of lipid homeostasis is only part of the pathogenic mechanism. Furthermore, our finding that mutations in the cNMP binding sites prevented the restoration of normal lipid levels supports previous evidence that cNMP regulates the phospholipase activity of PNPLA6.

ER responses play a key role in Swiss-Cheese/Neuropathy Target Esterase-associated neurodegeneration

Swiss Cheese (SWS) is the Drosophila orthologue of Neuropathy Target Esterase (NTE), a phospholipase that when mutated has been shown to cause a spectrum of disorders in humans that range from intellectual disabilities to ataxia. Loss of SWS in Drosophila also causes locomotion deficits, age-dependent neurodegeneration, and an increase in lysophosphatidylcholine (LPC) and phosphatidylcholine (PC). SWS is localized to the Endoplasmic Reticulum (ER), and recently, it has been shown that perturbing the membrane lipid composition of the ER can lead to the activation of ER stress responses through the inhibition of the Sarco/Endoplasmic Reticulum Ca²⁺ ATPase (SERCA). To investigate whether ER stress induction occurs in NTE-associated disorders, we used the fly sws null mutant as a model. sws flies showed an activated ER stress response as determined by elevated levels of the chaperone GRP78 and by increased splicing of XBP, an ER transcription factor that activates transcriptional ER stress responses. To address whether ER stress plays a role in the degenerative and behavioral phenotypes detected in sws¹, we overexpressed XBP1, or treated the flies with tauroursodeoxycholic acid (TUDCA), a chemical known to attenuate ER stress-mediated cell death. Both manipulations suppressed the locomotor deficits and neurodegeneration of sws¹. In addition, sws¹ flies showed reduced SERCA levels and expressing additional SERCA also suppressed the sws¹-related phenotypes. This suggests that the disruption in lipid compositions and its effect on SERCA are inducing ER stress, aimed to ameliorate the deleterious effects of sws¹. This includes the effects on lipid composition because XBP1 and SERCA expression also reduced the LPC levels in sws¹. Promoting cytoprotective ER stress pathways may therefore provide a therapeutic approach to alleviate the neurodegeneration and motor symptoms seen in NTE-associated disorders.

Analysis of the neurotoxic effects of neuropathic organophosphorus compounds in adult zebrafish

Inhibition and aging of neuropathy target esterase (NTE) by exposure to neuropathic organophosphorus compounds (OPs) can result in OP-induced delayed neuropathy (OPIDN). In the present study we aimed to build a model of OPIDN in adult zebrafish. First, inhibition and aging of zebrafish NTE activity were characterized in the brain by using the prototypic neuropathic compounds cresyl saligenin phosphate (CBDP) and diisopropylphosphorofluoridate (DFP). Our results show that, as in other animal models, zebrafish NTE is inhibited and aged by both neuropathic OPs. Then, a neuropathic concentration inhibiting NTE activity by at least 70% for at least 24 h was selected for each compound to analyze changes in phosphatidylcholines (PCs), lysophosphatidylcholines (LPCs) and glycerolphosphocholine (GPC) profiles. In spite to the strong inhibition of the NTE activity found for both compounds, only a mild increase in the LPCs level was found after 48 h of the exposure to DFP, and no effect were observed by CBDP. Moreover, histopathological evaluation and motor function outcome analyses failed to find any neurological abnormalities in the exposed fish. Thus, our results strongly suggest that zebrafish is not a suitable species for the development of an experimental model of human OPIDN.

Glial lipid droplets and neurodegeneration in a Drosophila model of complex I deficiency

Mitochondrial defects associated with respiratory chain complex I deficiency lead to heterogeneous fatal syndromes. While the role of NDUFS8, an essential subunit of the core assembly of the complex I, is established in mitochondrial diseases, the mechanisms underlying neuropathology are poorly understood. We developed a Drosophila model of NDUFS8 deficiency by knocking down the expression of its fly homologue in neurons or in glial cells. Downregulating ND23 in neurons resulted in shortened lifespan, and decreased locomotion. Although total brain ATP levels were decreased, histological analysis did not reveal any signs of neurodegeneration except for photoreceptors of the retina. Interestingly, ND23 deficiency-associated phenotypes were rescued by overexpressing the glucose transporter hGluT3 demonstrating that boosting glucose metabolism in neurons was sufficient to bypass altered mitochondrial functions and to confer neuroprotection. We then analyzed the consequences of ND23 knockdown in glial cells. In contrast to neuronal knockdown, loss of ND23 in glia did not lead to significant behavioral defects nor to reduced lifespan, but induced brain degeneration, as visualized by numerous vacuoles found all over the nervous tissue. This phenotype was accompanied by the massive accumulation of lipid droplets at the cortex-neuropile boundaries, suggesting an alteration of lipid metabolism in glia. These results demonstrate that complex I deficiency triggers metabolic alterations both in neurons and glial cells which may contribute to the neuropathology.

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.

Lipid metabolism in Rhodnius prolixus: Lessons from the genome

The kissing bug Rhodnius prolixus is both an important vector of Chagas' disease and an interesting model for investigation into the field of physiology, including lipid metabolism. The publication of this insect genome will bring a huge amount of new molecular biology data to be used in future experiments. Although this work represents a promising scenario, a preliminary analysis of the sequence data is necessary to identify and annotate the genes involved in lipid metabolism. Here, we used bioinformatics tools and gene expression analysis to explore genes from different genes families and pathways, including genes for fat breakdown, as lipases and phospholipases, and enzymes from β-oxidation, fatty acid metabolism, and acyl-CoA and glycerolipid synthesis. The R. prolixus genome encodes 31 putative lipase genes, including 21 neutral lipases and 5 acid lipases. The expression profiles of some of these genes were analyzed. We were able to identify nine phospholipase A2 genes. A variety of gene families that participate in fatty acid synthesis and modification were studied, including fatty acid synthase, elongase, desaturase and reductase. Concerning the synthesis of glycerolipids, we found a second isoform of glycerol-3-phosphate acyltransferase that was ubiquitously expressed throughout the organs. Finally, all genes involved in fatty acid β-oxidation were identified, but not a long-chain acyl-CoA dehydrogenase. These results provide fundamental data to be used in future research on insect lipid metabolism and its possible relevance to Chagas' disease transmission.

A review of neurotoxicity of microcystins

Cyanobacterial blooms-produced microcystins are secondary metabolites which can accumulate in the food chain and contaminate water, thus posing a potential threat to the health of aquatic animals and even humans. Microcystin toxicity affects not only the liver but also the other organs, i.e., the brain. The serious neurotoxicity effects caused by microcystins then lead to various symptoms. This review focuses on the neurotoxicity of microcystins. Microcystins can cross blood-brain barrier with the transport of Oatps/OATPs, causing neurostructural, functional, and behavioral changes. In this review, potential uptake mechanisms and neurotoxicity mechanisms are summarized, including neurotransmissions, neurochannels, signal transduction, oxidative stress, and cytoskeleton disruption. However, further researches are needed for detailed studies on signaling pathways and the downstream pathways of neurotoxicity of microcystins.

Neurodegeneration in a Drosophila model of adrenoleukodystrophy: the roles of the Bubblegum and Double bubble acyl-CoA synthetases

Debilitating neurodegenerative conditions with metabolic origins affect millions of individuals worldwide. Still, for most of these neurometabolic disorders there are neither cures nor disease-modifying therapies, and novel animal models are needed for elucidation of disease pathology and identification of potential therapeutic agents. To date, metabolic neurodegenerative disease has been modeled in animals with only limited success, in part because existing models constitute analyses of single mutants and have thus overlooked potential redundancy within metabolic gene pathways associated with disease. Here, we present the first analysis of a very-long-chain acyl-CoA synthetase (ACS) double mutant. We show that the Drosophila bubblegum(bgm) and double bubble(dbb) genes have overlapping functions, and that the consequences of double knockout of both bubblegum and double bubble in the fly brain are profound, affecting behavior and brain morphology, and providing the best paradigm to date for an animal model of adrenoleukodystrophy (ALD), a fatal childhood neurodegenerative disease associated with the accumulation of very-long-chain fatty acids. Using this more fully penetrant model of disease to interrogate brain morphology at the level of electron microscopy, we show that dysregulation of fatty acid metabolism via disruption of ACS function in vivois causal of neurodegenerative pathologies that are evident in both neuronal cells and their supporting cell populations, and leads ultimately to lytic cell death in affected areas of the brain. Finally, in an extension of our model system to the study of human disease, we describe our identification of an individual with leukodystrophy who harbors a rare mutation in SLC27a6(encoding a very-long-chain ACS), a human homolog of bgm and dbb.

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.

Delayed Induction of Human NTE (PNPLA6) Rescues Neurodegeneration and Mobility Defects of Drosophila swiss cheese (sws) Mutants

Human PNPLA6 gene encodes Neuropathy Target Esterase protein (NTE). PNPLA6 gene mutations cause hereditary spastic paraplegia (SPG39 HSP), Gordon-Holmes syndrome, Boucher-Neuhäuser syndromes, Laurence-Moon syndrome, and Oliver-McFarlane syndrome. Mutations in the Drosophila NTE homolog swiss cheese (sws) cause early-onset, progressive behavioral defects and neurodegeneration characterized by vacuole formation. We investigated sws⁵ flies and show for the first time that this allele causes progressive vacuolar formation in the brain and progressive deterioration of negative geotaxis speed and endurance. We demonstrate that inducible, neuron-specific expression of full-length human wildtype NTE reduces vacuole formation and substantially rescues mobility. Indeed, neuron-specific expression of wildtype human NTE is capable of rescuing mobility defects after 10 days of adult life at 29°C, when significant degeneration has already occurred, and significantly extends longevity of mutants at 25°C. These results raise the exciting possibility that late induction of NTE function may reduce or ameliorate neurodegeneration in humans even after symptoms begin. In addition, these results highlight the utility of negative geotaxis endurance as a new assay for longitudinal tracking of degenerative phenotypes in Drosophila.

Glial expression of Swiss cheese (SWS), the Drosophila orthologue of neuropathy target esterase (NTE), is required for neuronal ensheathment and function

Mutations in Drosophila Swiss cheese (SWS) or its vertebrate orthologue neuropathy target esterase (NTE), respectively, cause progressive neuronal degeneration in Drosophila and mice and a complex syndrome in humans that includes mental retardation, spastic paraplegia and blindness. SWS and NTE are widely expressed in neurons but can also be found in glia; however, their function in glia has, until now, remained unknown. We have used a knockdown approach to specifically address SWS function in glia and to probe for resulting neuronal dysfunctions. This revealed that loss of SWS in pseudocartridge glia causes the formation of multi-layered glial whorls in the lamina cortex, the first optic neuropil. This phenotype was rescued by the expression of SWS or NTE, suggesting that the glial function is conserved in the vertebrate protein. SWS was also found to be required for the glial wrapping of neurons by ensheathing glia, and its loss in glia caused axonal damage. We also detected severe locomotion deficits in glial sws-knockdown flies, which occurred as early as 2 days after eclosion and increased further with age. Utilizing the giant fibre system to test for underlying functional neuronal defects showed that the response latency to a stimulus was unchanged in knockdown flies compared to controls, but the reliability with which the neurons responded to increasing frequencies was reduced. This shows that the loss of SWS in glia impairs neuronal function, strongly suggesting that the loss of glial SWS plays an important role in the phenotypes observed in the sws mutant. It is therefore likely that changes in glia also contribute to the pathology observed in humans that carry mutations in NTE.

Specialized Cortex Glial Cells Accumulate Lipid Droplets in Drosophila melanogaster

Lipid droplets (LDs) are common organelles of the majority of eukaryotic cell types. Their biological significance has been extensively studied in mammalian liver cells and white adipose tissue. Although the central nervous system contains the highest relative amount and the largest number of different lipid species, neither the spatial nor the temporal distribution of LDs has been described. In this study, we used the brain of the fruitfly, Drosophila melanogaster, to investigate the neuroanatomy of LDs. We demonstrated that LDs are exclusively localised in glial cells but not in neurons in the larval nervous system. We showed that the brain’s LD pool, rather than being constant, changes dynamically during development and reaches its highest value at the beginning of metamorphosis. LDs are particularly enriched in cortex glial cells located close to the brain surface. These specialized superficial cortex glial cells contain the highest amount of LDs among glial cell types and encapsulate neuroblasts and their daughter cells. Superficial cortex glial cells, combined with subperineurial glial cells, express the Drosophila fatty acid binding protein (Dfabp), as we have demonstrated through light- and electron microscopic immunocytochemistry. To the best of our best knowledge this is the first study that describes LD neuroanatomy in the Drosophila larval brain.

Drosophila spermatid individualization is sensitive to temperature and fatty acid metabolism

Fatty acids are precursors of potent lipid signaling molecules. They are stored in membrane phospholipids and released by phospholipase A₂ (PLA₂). Lysophospholipid acyltransferases (ATs) oppose PLA₂ by re-esterifying fatty acids into phospholipids, in a biochemical pathway known as the Lands Cycle. Drosophila Lands Cycle ATs oys and nes, as well as 7 predicted PLA₂ genes, are expressed in the male reproductive tract. Oys and Nes are required for spermatid individualization. Individualization, which occurs after terminal differentiation, invests each spermatid in its own plasma membrane and removes the bulk of the cytoplasmic contents. We developed a quantitative assay to measure individualization defects. We demonstrate that individualization is sensitive to temperature and age but not to diet. Mutation of the cyclooxygenase Pxt, which metabolizes fatty acids to prostaglandins, also leads to individualization defects. In contrast, modulating phospholipid levels by mutation of the phosphatidylcholine lipase Swiss cheese (Sws) or the ethanolamine kinase Easily shocked (Eas) does not perturb individualization, nor does Sws overexpression. Our results suggest that fatty acid derived signals such as prostaglandins, whose abundance is regulated by the Lands Cycle, are important regulators of spermatogenesis.

Mutations in PNPLA6 are linked to photoreceptor degeneration and various forms of childhood blindness

Blindness due to retinal degeneration affects millions of people worldwide, but many disease-causing mutations remain unknown. PNPLA6 encodes the patatin-like phospholipase domain containing protein 6, also known as neuropathy target esterase (NTE), which is the target of toxic organophosphates that induce human paralysis due to severe axonopathy of large neurons. Mutations in PNPLA6 also cause human spastic paraplegia characterized by motor neuron degeneration. Here we identify PNPLA6 mutations in childhood blindness in seven families with retinal degeneration, including Leber congenital amaurosis and Oliver McFarlane syndrome. PNPLA6 localizes mostly at the inner segment plasma membrane in photoreceptors and mutations in Drosophila PNPLA6 lead to photoreceptor cell death. We also report that lysophosphatidylcholine and lysophosphatidic acid levels are elevated in mutant Drosophila. These findings show a role for PNPLA6 in photoreceptor survival and identify phospholipid metabolism as a potential therapeutic target for some forms of blindness.

Loss-of-function mutations in PNPLA6 encoding neuropathy target esterase underlie pubertal failure and neurological deficits in Gordon Holmes syndrome

CONTEXT

Gordon Holmes syndrome (GHS) is characterized by cerebellar ataxia/atrophy and normosmic hypogonadotropic hypogonadism (nHH). The underlying pathophysiology of this combined neurodegeneration and nHH remains unknown.

OBJECTIVE

We aimed to provide insight into the disease mechanism in GHS.

METHODS

We studied a cohort of 6 multiplex families with GHS through autozygosity mapping and whole-exome sequencing.

RESULTS

We identified 6 patients from 3 independent families carrying loss-of-function mutations in PNPLA6, which encodes neuropathy target esterase (NTE), a lysophospholipase that maintains intracellular phospholipid homeostasis by converting lysophosphatidylcholine to glycerophosphocholine. Wild-type PNPLA6, but not PNPLA6 bearing these mutations, rescued a well-established Drosophila neurodegenerative phenotype caused by the absence of sws, the fly ortholog of mammalian PNPLA6. Inhibition of NTE activity in the LβT2 gonadotrope cell line diminished LH response to GnRH by reducing GnRH-stimulated LH exocytosis, without affecting GnRH receptor signaling or LHβ synthesis.

CONCLUSION

These results suggest that NTE-dependent alteration of phospholipid homeostasis in GHS causes both neurodegeneration and impaired LH release from pituitary gonadotropes, leading to nHH.

Silencing of drpr leads to muscle and brain degeneration in adult Drosophila

Mutations in the gene encoding the single transmembrane receptor multiple epidermal growth factor-like domain 10 (MEGF10) cause an autosomal recessive congenital muscle disease in humans. Although mammalian MEGF10 is expressed in the central nervous system as well as in skeletal muscle, patients carrying mutations in MEGF10 do not show symptoms of central nervous system dysfunction. drpr is the sole Drosophila homolog of the human genes MEGF10, MEGF11, and MEGF12 (JEDI, PEAR). The functional domains of MEGF10 and drpr bear striking similarities, and residues affected by MEGF10 mutations in humans are conserved in drpr. Our analysis of drpr mutant flies revealed muscle degeneration with fiber size variability and vacuolization, as well as reduced motor performance, features that have been observed in human MEGF10 myopathy. Vacuolization was also seen in the brain. Tissue-specific RNAi experiments demonstrated that drpr deficiency in muscle, but not in the brain, leads to locomotor defects. The histological and behavioral abnormalities seen in the affected flies set the stage for further studies examining the signaling pathway modulated by MEGF10/Drpr in muscle, as well as assessing the effects of genetic and/or pharmacological manipulations on the observed muscle defects. In addition, the absence of functional redundancy for Drpr in Drosophila may help elucidate whether paralogs of MEGF10 in humans (eg, MEGF11) contribute to maintaining wild-type function in the human brain.

Impairment of Drosophila orthologs of the human orphan protein C19orf12 induces bang sensitivity and neurodegeneration

Mutations in the orphan gene C19orf12 were identified as a genetic cause in a subgroup of patients with NBIA, a neurodegenerative disorder characterized by deposits of iron in the basal ganglia. C19orf12 was shown to be localized in mitochondria, however, nothing is known about its activity and no functional link exists to the clinical phenotype of the patients. This situation led us to investigate the effects of C19orf12 down-regulation in the model organism Drosophila melanogaster. Two genes are present in D. melanogaster, which are orthologs of C19orf12, CG3740 and CG11671. Here we provide evidence that transgenic flies with impaired C19orf12 homologs reflect the neurodegenerative phenotype and represent a valid tool to further analyze the pathomechanism in C19orf12-associated NBIA.

Organophosphate-induced changes in the PKA regulatory function of Swiss Cheese/NTE lead to behavioral deficits and neurodegeneration

Organophosphate-induced delayed neuropathy (OPIDN) is a Wallerian-type axonopathy that occurs weeks after exposure to certain organophosphates (OPs). OPs have been shown to bind to Neuropathy Target Esterase (NTE), thereby inhibiting its enzymatic activity. However, only OPs that also induce the so-called aging reaction cause OPIDN. This reaction results in the release and possible transfer of a side group from the bound OP to NTE and it has been suggested that this induces an unknown toxic function of NTE. To further investigate the mechanisms of aging OPs, we used Drosophila, which expresses a functionally conserved orthologue of NTE named Swiss Cheese (SWS). Treating flies with the organophosporous compound tri-ortho-cresyl phosphate (TOCP) resulted in behavioral deficits and neurodegeneration two weeks after exposure, symptoms similar to the delayed effects observed in other models. In addition, we found that primary neurons showed signs of axonal degeneration within an hour after treatment. Surprisingly, increasing the levels of SWS, and thereby its enzymatic activity after exposure, did not ameliorate these phenotypes. In contrast, reducing SWS levels protected from TOCP-induced degeneration and behavioral deficits but did not affect the axonopathy observed in cell culture. Besides its enzymatic activity as a phospholipase, SWS also acts as regulatory PKA subunit, binding and inhibiting the C3 catalytic subunit. Measuring PKA activity in TOCP treated flies revealed a significant decrease that was also confirmed in treated rat hippocampal neurons. Flies expressing additional PKA-C3 were protected from the behavioral and degenerative phenotypes caused by TOCP exposure whereas primary neurons were not. In addition, knocking-down PKA-C3 caused similar behavioral and degenerative phenotypes as TOCP treatment. We therefore propose a model in which OP-modified SWS cannot release PKA-C3 and that the resulting loss of PKA-C3 activity plays a crucial role in developing the delayed symptoms of OPIDN but not in the acute toxicity.

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.

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.