Cloning of a phosphatidic acid-preferring phospholipase A1 from bovine testis

From Classical to Alternative Pathways of 2-Arachidonoylglycerol Synthesis: AlterAGs at the Crossroad of Endocannabinoid and Lysophospholipid Signaling

2-arachidonoylglycerol (2-AG) is the most abundant endocannabinoid (EC), acting as a full agonist at both CB1 and CB2 cannabinoid receptors. It is synthesized on demand in postsynaptic membranes through the sequential action of phosphoinositide-specific phospholipase Cβ1 (PLCβ1) and diacylglycerol lipase α (DAGLα), contributing to retrograde signaling upon interaction with presynaptic CB1. However, 2-AG production might also involve various combinations of PLC and DAGL isoforms, as well as additional intracellular pathways implying other enzymes and substrates. Three other alternative pathways of 2-AG synthesis rest on the extracellular cleavage of 2-arachidonoyl-lysophospholipids by three different hydrolases: glycerophosphodiesterase 3 (GDE3), lipid phosphate phosphatases (LPPs), and two members of ecto-nucleotide pyrophosphatase/phosphodiesterases (ENPP6-7). We propose the names of AlterAG-1, -2, and -3 for three pathways sharing an ectocellular localization, allowing them to convert extracellular lysophospholipid mediators into 2-AG, thus inducing typical signaling switches between various G-protein-coupled receptors (GPCRs). This implies the critical importance of the regioisomerism of both lysophospholipid (LPLs) and 2-AG, which is the object of deep analysis within this review. The precise functional roles of AlterAGs are still poorly understood and will require gene invalidation approaches, knowing that both 2-AG and its related lysophospholipids are involved in numerous aspects of physiology and pathology, including cancer, inflammation, immune defenses, obesity, bone development, neurodegeneration, or psychiatric disorders.

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.

Phosphorylation of human phospholipase A1 DDHD1 at newly identified phosphosites affects its subcellular localization

Phospholipase A1 (PLA1) hydrolyzes the fatty acids of glycerophospholipids, which are structural components of the cellular membrane. Genetic mutations in DDHD1, an intracellular PLA1, result in hereditary spastic paraplegia (HSP) in humans. However, the regulation of DDHD1 activity has not yet been elucidated in detail. In the present study, we examined the phosphorylation of DDHD1 and identified the responsible protein kinases. We performed MALDI-TOF MS/MS analysis and Phos-tag SDS-PAGE in alanine-substitution mutants in HEK293 cells and revealed multiple phosphorylation sites in human DDHD1, primarily Ser8, Ser11, Ser723, and Ser727. The treatment of cells with a protein phosphatase inhibitor induced the hyperphosphorylation of DDHD1, suggesting that multisite phosphorylation occurred not only at these major, but also at minor sites. Site-specific kinase-substrate prediction algorithms and in vitro kinase analyses indicated that cyclin-dependent kinase CDK1/cyclin A2 phosphorylated Ser8, Ser11, and Ser727 in DDHD1 with a preference for Ser11 and that CDK5/p35 also phosphorylated Ser11 and Ser727 with a preference for Ser11. In addition, casein kinase CK2α1 was found to phosphorylate Ser104, although this was not a major phosphorylation site in cultivated HEK293 cells. The evaluation of the effects of phosphorylation revealed that the phosphorylation mimic mutants S11/727E exhibit only 20% reduction in PLA1 activity. However, the phosphorylation mimics were mainly localized to focal adhesions, whereas the phosphorylation-resistant mutants S11/727A were not. This suggested that phosphorylation alters the subcellular localization of DDHD1 without greatly affecting its PLA1 activity.

DDHD1, but Not DDHD2, Suppresses Neurite Outgrowth in SH-SY5Y and PC12 Cells by Regulating Protein Transport From Recycling Endosomes

DDHD1 and DDHD2 are both intracellular phospholipases A₁ and hydrolyze phosphatidic acid in vitro. Given that phosphatidic acid participates in neurite outgrowth, we examined whether DDHD1 and DDHD2 regulate neurite outgrowth. Depletion of DDHD1 from SH-SY5Y and PC12 cells caused elongation of neurites, whereas DDHD2 depletion prevented neurite elongation. Rescue experiments demonstrated that the enzymatic activity of DDHD1 is necessary for the prevention of neurite elongation. Depletion of DDHD1 caused enlargement of early endosomes and stimulated tubulation of recycling endosomes positive for phosphatidic acid-binding proteins syndapin2 and MICAL-L1. Knockout of DDHD1 enhanced transferrin recycling from recycling endosomes to the cell surface. Our results suggest that DDHD1 negatively controls the formation of a local phosphatidic acid-rich domain in recycling endosomes that serves as a membrane source for neurite outgrowth.

Immune-related genes in tumor-specific CD4+ and CD8+ T cells in colon cancer

Background

Immune escape is an immunological mechanism underlying tumorigenesis, and T cells play an important role in this process. In this study, immune-related genes were evaluated in tumor-infiltrating CD4⁺ and CD8⁺ T cells in colon cancer.

Methods

ESTIMATE was used to calculate stromal and immune scores for tumor datasets downloaded from The Cancer Genome Atlas–Colon Cancer (COAD). Differentially expressed genes (DEGs) between samples with high and low stromal and immune scores were screened, followed by a functional enrichment analysis of the overlapping DEGs. The DEGs related to CD4⁺ and the CD8⁺ T cells were then screened. Predicted miRNA–mRNA and lncRNA–miRNA pairs were used to construct a competing endogenous RNA (ceRNA) network. Furthermore, chemical–gene interactions were predicted for genes in the ceRNA network. Kaplan–Meier survival curves were also plotted.

Results

In total, 83 stromal-related DEGs (5 up-regulated and 78 down-regulated) and 1270 immune-related DEGs (807 up-regulated and 293 down-regulated genes) were detected. The 79 overlapping DEGs were enriched for 39 biological process terms. Furthermore, 79 CD4⁺ T cell-related genes and 8 CD8⁺ T cell-related genes, such as ELK3, were screened. Additionally, ADAD1 and DLG3, related to CD4⁺ T cells, were significantly associated with the prognosis of patients with colon cancer. The chr22-38_28785274–29,006,793.1–miR-106a-5p-DDHD1 and chr22-38_28785274–29,006,793.1–miR-4319-GRHL1 axes obtained from CD4⁺ and CD8⁺ T cell-related ceRNAs were identified as candidates for further studies.

Conclusion

ELK3 is a candidate immune-related gene in colon cancer. The chr22-38_28785274–29,006,793.1–miR-106a-5p-DDHD1 and chr22-38_28785274–29,006,793.1–miR-4319-GRHL1 axes may be related to CD4⁺ and CD8⁺ T cell infiltration in colon cancer.

Lipids in the Physiopathology of Hereditary Spastic Paraplegias

Hereditary spastic paraplegias (HSP) are a group of neurodegenerative diseases sharing spasticity in lower limbs as common symptom. There is a large clinical variability in the presentation of patients, partly underlined by the large genetic heterogeneity, with more than 60 genes responsible for HSP. Despite this large heterogeneity, the proteins with known function are supposed to be involved in a limited number of cellular compartments such as shaping of the endoplasmic reticulum or endolysosomal function. Yet, it is difficult to understand why alteration of such different cellular compartments can lead to degeneration of the axons of cortical motor neurons. A common feature that has emerged over the last decade is the alteration of lipid metabolism in this group of pathologies. This was first revealed by the identification of mutations in genes encoding proteins that have or are supposed to have enzymatic activities on lipid substrates. However, it also appears that mutations in genes affecting endoplasmic reticulum, mitochondria, or endolysosome function can lead to changes in lipid distribution or metabolism. The aim of this review is to discuss the role of lipid metabolism alterations in the physiopathology of HSP, to evaluate how such alterations contribute to neurodegenerative phenotypes, and to understand how this knowledge can help develop therapeutic strategy for HSP.

Phosphatidylinositol synthesis at the endoplasmic reticulum

Phosphatidylinositol (PI) is a minor phospholipid with a characteristic fatty acid profile; it is highly enriched in stearic acid at the sn-1 position and arachidonic acid at the sn-2 position. PI is phosphorylated into seven specific derivatives, and individual species are involved in a vast array of cellular functions including signalling, membrane traffic, ion channel regulation and actin dynamics. De novo PI synthesis takes place at the endoplasmic reticulum where phosphatidic acid (PA) is converted to PI in two enzymatic steps. PA is also produced at the plasma membrane during phospholipase C signalling, where hydrolysis of phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) leads to the production of diacylglycerol which is rapidly phosphorylated to PA. This PA is transferred to the ER to be also recycled back to PI. For the synthesis of PI, CDP-diacylglycerol synthase (CDS) converts PA to the intermediate, CDP-DG, which is then used by PI synthase to make PI. The de novo synthesised PI undergoes remodelling to acquire its characteristic fatty acid profile, which is altered in p53-mutated cancer cells. In mammals, there are two CDS enzymes at the ER, CDS1 and CDS2. In this review, we summarise the de novo synthesis of PI at the ER and the enzymes involved in its subsequent remodelling to acquire its characteristic acyl chains. We discuss how CDS, the rate limiting enzymes in PI synthesis are regulated by different mechanisms. During phospholipase C signalling, the CDS1 enzyme is specifically upregulated by cFos via protein kinase C.

The Spastic Paraplegia-Associated Phospholipase DDHD1 Is a Primary Brain Phosphatidylinositol Lipase

Deleterious mutations in the serine hydrolase DDHD domain containing 1 (DDHD1) cause the SPG28 subtype of the neurological disease hereditary spastic paraplegia (HSP), which is characterized by axonal neuropathy and gait impairments. DDHD1 has been shown to display PLA1-type phospholipase activity with a preference for phosphatidic acid. However, the endogenous lipid pathways regulated by DDHD1 in vivo remain poorly understood. Here we use a combination of untargeted and targeted metabolomics to compare the lipid content of brain tissue from DDHD1+/+ and DDHD1-/- mice, revealing that DDHD1 inactivation causes a substantial decrease in the level of polyunsaturated lysophosphatidylinositol (LPI) lipids and a corresponding increase in the level of phosphatidylinositol (PI) lipids. Levels of other phospholipids were mostly unchanged, with the exception of decreases in the levels of select polyunsaturated lysophosphatidylserine (LPS) and lysophosphatidylcholine lipids and a striking remodeling of PI phosphates (e.g., PIP and PIP2) in DDHD1-/- brain tissue. Biochemical assays confirmed that DDHD1 hydrolyzes PI/PS to LPI/LPS with sn-1 selectivity and accounts for a substantial fraction of the PI/PS lipase activity in mouse brain tissue. These data indicate that DDHD1 is a principal regulator of bioactive LPI and other lysophospholipids, as well as PI phosphates, in the mammalian nervous system, pointing to a potential role for these lipid pathways in HSP.

Lipid-metabolizing serine hydrolases in the mammalian central nervous system: endocannabinoids and beyond

The metabolic serine hydrolases hydrolyze ester, amide, or thioester bonds found in broad small molecule substrates using a conserved activated serine nucleophile. The mammalian central nervous system (CNS) express a diverse repertoire of serine hydrolases that act as (phospho)lipases or lipid amidases to regulate lipid metabolism and signaling vital for normal neurocognitive function and CNS integrity. Advances in genomic DNA sequencing have provided evidence for the role of these lipid-metabolizing serine hydrolases in neurologic, psychiatric, and neurodegenerative disorders. This review briefly summarizes recent progress in understanding the biochemical and (patho)physiological roles of these lipid-metabolizing serine hydrolases in the mammalian CNS with a focus on serine hydrolases involved in the endocannabinoid system. The development and application of specific inhibitors for an individual serine hydrolase, if available, are also described. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

The phospholipase DDHD1 as a new target in colorectal cancer therapy

Background

Our previous study demonstrates that Citrus-limon derived nanovesicles are able to decrease colon cancer cell viability, and that this effect is associated with the downregulation of the intracellular phospholipase DDHD domain-containing protein 1 (DDHD1). While few studies are currently available on the contribution of DDHD1 in neurological disorders, there is no information on its role in cancer. This study investigates the role of DDHD1 in colon cancer.

Methods

DDHD1 siRNAs and an overexpression vector were transfected into colorectal cancer and normal cells to downregulate or upregulate DDHD1 expression. In vitro and in vivo assays were performed to investigate the functional role of DDHD1 in colorectal cancer cell growth. Quantitative proteomics using SWATH-MS was performed to determinate the molecular effects induced by DDHD1 silencing in colorectal cancer cells.

Results

The results indicate that DDHD1 supports colon cancer cell proliferation and survival, since its downregulation reduces in vitro colon cancer cell viability and increases apoptosis rate, without affecting normal cells. On the contrary, in vivo studies demonstrate that the xenograft tumors, derived from DDHD1-overexpressing cells, have a higher proliferation rate compared to control animals. Additionally, we found that functional categories, significantly affected by DDHD1 silencing, were specifically related to cancer phenotype and for the first time associated to DDHD1 activity.

Conclusions

In conclusion, this study provides the first evidence confirming the role of DDHD1 in cancer, providing a possibility to define a new target to design more effective therapies for colon cancer patients.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0753-z) contains supplementary material, which is available to authorized users.

Functional Contribution of the Spastic Paraplegia-Related Triglyceride Hydrolase DDHD2 to the Formation and Content of Lipid Droplets

Deleterious mutations in the serine lipase DDHD2 are a causative basis of complex hereditary spastic paraplegia (HSP, subtype SPG54) in humans. We recently found that DDHD2 is a principal triglyceride hydrolase in the central nervous system (CNS) and that genetic deletion of this enzyme in mice leads to ectopic lipid droplet (LD) accumulation in neurons throughout the brain. Nonetheless, how HSP-related mutations in DDHD2 relate to triglyceride metabolism and LD formation remains poorly understood. Here, we have characterized a set of HSP-related mutations in DDHD2 and found that they disrupt triglyceride hydrolase activity in vitro and impair the capacity of DDHD2 to protect cells from LD accumulation following exposure to free fatty acid, an outcome that was also observed with a DDHD2-selective inhibitor. We furthermore isolated and characterized LDs from brain tissue of DDHD2^-/- mice, revealing that they contain both established LD-associated proteins identified previously in other organs and CNS-enriched proteins, including several proteins with genetic links to human neurological disease. These data, taken together, indicate that the genetic inactivation of DDHD2, as caused by HSP-associated mutations, substantially perturbs lipid homeostasis and the formation and content of LDs, underscoring the importance of triglyceride metabolism for normal CNS function and the key role that DDHD2 plays in this process.

Co-Expression Network and Pathway Analyses Reveal Important Modules of miRNAs Regulating Milk Yield and Component Traits

Co-expression network analyses provide insights into the molecular interactions underlying complex traits and diseases. In this study, co-expression network analysis was performed to detect expression patterns (modules or clusters) of microRNAs (miRNAs) during lactation, and to identify miRNA regulatory mechanisms for milk yield and component traits (fat, protein, somatic cell count (SCC), lactose, and milk urea nitrogen (MUN)) via miRNA target gene enrichment analysis. miRNA expression (713 miRNAs), and milk yield and components (Fat%, Protein%, lactose, SCC, MUN) data of nine cows at each of six different time points (day 30 (D30), D70, D130, D170, D230 and D290) of an entire lactation curve were used. Four modules or clusters (GREEN, BLUE, RED and TURQUOISE) of miRNAs were identified as important for milk yield and component traits. The GREEN and BLUE modules were significantly correlated (|r| > 0.5) with milk yield and lactose, respectively. The RED and TURQUOISE modules were significantly correlated (|r| > 0.5) with both SCC and lactose. In the GREEN module, three abundantly expressed miRNAs (miR-148a, miR-186 and miR-200a) were most significantly correlated to milk yield, and are probably the most important miRNAs for this trait. DDR1 and DDHX1 are hub genes for miRNA regulatory networks controlling milk yield, while HHEX is an important transcription regulator for these networks. miR-18a, miR-221/222 cluster, and transcription factors HOXA7, and NOTCH 3 and 4, are important for the regulation of lactose. miR-142, miR-146a, and miR-EIA17-14144 (a novel miRNA), and transcription factors in the SMAD family and MYB, are important for the regulation of SCC. Important signaling pathways enriched for target genes of miRNAs of significant modules, included protein kinase A and PTEN signaling for milk yield, eNOS and Noth signaling for lactose, and TGF β, HIPPO, Wnt/β-catenin and cell cycle signaling for SCC. Relevant enriched gene ontology (GO)-terms related to milk and mammary gland traits included cell differentiation, G-protein coupled receptor activity, and intracellular signaling transduction. Overall, this study uncovered regulatory networks in which miRNAs interacted with each other to regulate lactation traits.

Higher LPA2 and LPA6 mRNA Levels in Hepatocellular Carcinoma Are Associated with Poorer Differentiation, Microvascular Invasion and Earlier Recurrence with Higher Serum Autotaxin Levels

Hepatocellular carcinoma (HCC) commonly develops in patients with liver fibrosis; in these patients, the blood levels of lysophosphatidic acid (LPA) and its generating enzyme autotaxin (ATX) increase with the liver fibrosis stage. We aimed to examine the potential relevance of ATX and LPA in HCC. Fifty-eight HCC patients who underwent surgical treatment were consecutively enrolled in the study. Among the LPA receptors in HCC, higher LPA2 mRNA levels correlated with poorer differentiation, and higher LPA6 mRNA levels correlated with microvascular invasion, which suggested a higher malignant potential of HCC with increased LPA2 and LPA6 expression. In patients with primary HCC, neither LPA2 nor LPA6 mRNA levels were associated with recurrence. However, when serum ATX levels were combined for analysis as a surrogate for plasma LPA levels, the cumulative intra-hepatic recurrence rate was higher in patients in whom both serum ATX levels and LPA2 or LPA6 mRNA levels were higher than the median. However, the mRNA level of phosphatidic acid-selective phospholipase A1ɑ, another LPA-generating enzyme, in HCC patients was not associated with pathological findings or recurrence, even in combination with the expression of LPA receptors. Higher LPA2 mRNA levels were associated with poorer differentiation, and higher LPA6 levels were associated with microvascular invasion in HCC; both became a risk factor for recurrence after surgical treatment when combined with increased serum ATX levels. ATX and LPA receptors merit consideration as therapeutic targets of HCC.

A novel frameshift mutation of DDHD1 in a Japanese patient with autosomal recessive spastic paraplegia

Spastic paraplegia (SPG) type 28 is an autosomal recessive SPG caused by mutations in the DDHD1 gene. We examined a Japanese 54-years-old male patient with autosomal recessive SPG. His parents were consanguineous. He needed a wheelchair for transfer due to spastic paraplegia. There was a history of operations for bilateral hallux valgus, thoracic ossification of the yellow ligament, bilateral carpal tunnel syndrome, bilateral ankle contracture, and lumbar spinal canal stenosis. He noticed gait disturbance at age 14. He used a cane for walking in his 40s. On neurological examination, he showed hyperreflexia, spasticity, and weakness in the lower extremities and bilateral Babinski reflexes. Urinary dysfunctions and impaired vibration sense in the lower limbs were observed. By exome sequencing analysis using Agilent SureSelect and Illumina MiSeq, we identified 17,248 homozygous nucleotide variants in the patient. Through the examination of 48 candidate genes known to be responsible for autosomal recessive SPG, we identified a novel homozygous 4-bp deletion, c.914_917delGTAA, p.Ser305Ilefs*2 in exon2 of the DDHD1 gene encoding phosphatidic acid-preferring phospholipase A1 (PA-PLA1). The mutation is expected to cause a frameshift generating a premature stop codon 3-bp downstream from the deletion. In consequence, the DDHD domain that is known to be critical for PLA1 activity is completely depleted in the mutated DDHD1 protein, predicted to be a functionally null mutation of the DDHD1 gene. By Sanger sequencing, we confirmed that both parents are heterozygous for the mutation. This variation was not detected in 474 Japanese control subjects as well as the data of the 1,000G Project. We conclude that the novel mutation in DDHD1 is the causative variant for the SPG28 patient that is the first record of the disease in Japanese population.

Defining the metabolome: size, flux, and regulation

Renewed interest in metabolic research over the last two decades has inspired an explosion of technological developments for studying metabolism. At the forefront of methodological innovation is an approach referred to as "untargeted" or "discovery" metabolomics. The experimental objective of this technique is to comprehensively measure the entire metabolome, which constitutes a largely undefined set of molecules. Given its potential comprehensive coverage, untargeted metabolomics is often the first choice of experiments for investigators pursuing a metabolic research question. It is important to recognize, however, that untargeted metabolomics may not always be the optimal experimental approach. Conventionally, untargeted metabolomics only provides information about relative differences in metabolite pool sizes. Therefore, depending on the specific scientific question at hand, a complementary approach involving stable isotopes (such as metabolic flux analysis) may be better suited to provide biological insights. Unlike untargeted metabolomics, stable-isotope methods can provide information about differences in reaction rates.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.

The hereditary spastic paraplegia-related enzyme DDHD2 is a principal brain triglyceride lipase

Complex hereditary spastic paraplegia (HSP) is a genetic disorder that causes lower limb spasticity and weakness and intellectual disability. Deleterious mutations in the poorly characterized serine hydrolase DDHD2 are a causative basis for recessive complex HSP. DDHD2 exhibits phospholipase activity in vitro, but its endogenous substrates and biochemical functions remain unknown. Here, we report the development of DDHD2(-/-) mice and a selective, in vivo-active DDHD2 inhibitor and their use in combination with mass spectrometry-based lipidomics to discover that DDHD2 regulates brain triglycerides (triacylglycerols, or TAGs). DDHD2(-/-) mice show age-dependent TAG elevations in the central nervous system, but not in several peripheral tissues. Large lipid droplets accumulated in DDHD2(-/-) brains and were localized primarily to the intracellular compartments of neurons. These metabolic changes were accompanied by impairments in motor and cognitive function. Recombinant DDHD2 displays TAG hydrolase activity, and TAGs accumulated in the brains of wild-type mice treated subchronically with a selective DDHD2 inhibitor. These findings, taken together, indicate that the central nervous system possesses a specialized pathway for metabolizing TAGs, disruption of which leads to massive lipid accumulation in neurons and complex HSP syndrome.

Hepatic transcriptome profiles differ among maturing beef heifers supplemented with inorganic, organic, or mixed (50% inorganic:50% organic) forms of dietary selenium

Selenium (Se) is an important trace mineral that, due to deficiencies in the soil in many parts of the USA, must be supplemented directly to the diet of foraging cattle. Both organic and inorganic forms of dietary Se supplements are available and commonly used, and it is known that Se form affects tissue assimilation, bioavailability, and physiological responses. However, little is known about the effects of form of dietary Se supplements on gene expression profiles, which ostensibly account for Se form-dependent physiological processes. To determine if hepatic transcriptomes of growing beef (Angus-cross) heifers (0.5 kg gain/day) was altered by form of dietary supplemental Se, none (Control), or 3 mg Se/day as inorganic Se (ISe, sodium selenite), organic (OSe, Sel-Plex®), or a blend of ISe and OSe (1.5 mg:1.5 mg, Mix) Se was fed for 168 days, and the RNA expression profiles from biopsied liver tissues was compared by microarray analysis. The relative abundance of 139 RNA transcripts was affected by Se treatment, with 86 of these with complete gene annotations. Statistical and bioinformatic analysis of the annotated RNA transcripts revealed clear differences among the four Se treatment groups in their hepatic expression profiles, including (1) solely and commonly affected transcripts; (2) Control and OSe profiles being more similar than Mix and ISe treatments; (3) distinct OSe-, Mix-, and ISe-Se treatment-induced "phenotypes" that possessed both common and unique predicted physiological capacities; and (4) expression of three microRNAs were uniquely sensitive to OSe, ISe, or Mix treatments, including increased capacity for redox potential induced by OSe and Mix Se treatments resulting from decreased expression of MiR2300b messenger RNA. These findings indicate that the form of supplemental dietary Se consumed by cattle will affect the composition of liver transcriptomes resulting, presumably, in different physiological capacities.

Hereditary spastic paraplegia: clinical-genetic characteristics and evolving molecular mechanisms

Hereditary spastic paraplegia (HSP) is a group of clinically and genetically heterogeneous neurological disorders characterized by pathophysiologic hallmark of length-dependent distal axonal degeneration of the corticospinal tracts. The prominent features of this pathological condition are progressive spasticity and weakness of the lower limbs. To date, 72 spastic gait disease-loci and 55 spastic paraplegia genes (SPGs) have been identified. All modes of inheritance (autosomal dominant, autosomal recessive, and X-linked) have been described. Recently, a late onset spastic gait disorder with maternal trait of inheritance has been reported, as well as mutations in genes not yet classified as spastic gait disease. Several cellular processes are involved in its pathogenesis, such as membrane and axonal transport, endoplasmic reticulum membrane modeling and shaping, mitochondrial function, DNA repair, autophagy, and abnormalities in lipid metabolism and myelination processes. Moreover, recent evidences have been found about the impairment of endosome membrane trafficking in vesicle formation and about the involvement of oxidative stress and mtDNA polymorphisms in the onset of the disease. Interactome networks have been postulated by bioinformatics and biological analyses of spastic paraplegia genes, which would contribute to the development of new therapeutic approaches.

Phosphatidic acid (PA)-preferring phospholipase A1 regulates mitochondrial dynamics

Recent studies have suggested that phosphatidic acid (PA), a cone-shaped phospholipid that can generate negative curvature of lipid membranes, participates in mitochondrial fusion. However, precise mechanisms underling the production and consumption of PA on the mitochondrial surface are not fully understood. Phosphatidic acid-preferring phospholipase A1 (PA-PLA1)/DDHD1 is the first identified intracellular phospholipase A1 and preferentially hydrolyzes PA in vitro. Its cellular and physiological functions have not been elucidated. In this study, we show that PA-PLA1 regulates mitochondrial dynamics. PA-PLA1, when ectopically expressed in HeLa cells, induced mitochondrial fragmentation, whereas its depletion caused mitochondrial elongation. The effects of PA-PLA1 on mitochondrial morphology appear to counteract those of MitoPLD, a mitochondrion-localized phospholipase D that produces PA from cardiolipin. Consistent with high levels of expression of PA-PLA1 in testis, PA-PLA1 knock-out mice have a defect in sperm formation. In PA-PLA1-deficient sperm, the mitochondrial structure is disorganized, and an abnormal gap structure exists between the middle and principal pieces. A flagellum is bent at that position, leading to a loss of motility. Our results suggest a possible mechanism of PA regulation of the mitochondrial membrane and demonstrate an in vivo function of PA-PLA1 in the organization of mitochondria during spermiogenesis.

The actions and metabolism of lysophosphatidylinositol, an endogenous agonist for GPR55

Lysophosphatidylinositol (LPI) is a subspecies of lysophospholipid and is assumed to be not only a degradation product of phosphatidylinositol (PI), but also a bioactive lysophospholipid mediator. However, not much attention has been directed toward LPI compared to lysophosphatidic acid (LPA), since the receptor for LPI has not been identified. During screening for an agonist for the orphan G protein coupled receptor GPR55, we identified LPI, 2-arachidonoyl LPI in particular, as an agonist for GPR55. Our efforts to identify an LPI receptor facilitated research on LPI as a lipid messenger. In addition, we also found that DDHD1, previously identified as phosphatidic acid-preferring phospholipase A1, was one of the synthesizing enzymes of 2-arachidonoyl LPI. Here, we summarized the background for discovering the LPI receptor, and the actions/metabolism of LPI. We also referred to the biosynthesis of PI, a 1-stearoyl-2-arachidonoyl species, since the molecule is the precursor of 2-arachidonoyl LPI. Furthermore, we discussed physiological and/or pathophysiological processes involving LPI and GPR55, including the relevance of LPI-GPR55 and cannabinoids, since GPR55 was previously postulated to be another cannabinoid receptor. Although there is no doubt that GPR55 is the LPI receptor, we should re-consider whether or not GPR55 is in fact another cannabinoid receptor.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Alteration of fatty-acid-metabolizing enzymes affects mitochondrial form and function in hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is considered one of the most heterogeneous groups of neurological disorders, both clinically and genetically. The disease comprises pure and complex forms that clinically include slowly progressive lower-limb spasticity resulting from degeneration of the corticospinal tract. At least 48 loci accounting for these diseases have been mapped to date, and mutations have been identified in 22 genes, most of which play a role in intracellular trafficking. Here, we identified mutations in two functionally related genes (DDHD1 and CYP2U1) in individuals with autosomal-recessive forms of HSP by using either the classical positional cloning or a combination of whole-genome linkage mapping and next-generation sequencing. Interestingly, three subjects with CYP2U1 mutations presented with a thin corpus callosum, white-matter abnormalities, and/or calcification of the basal ganglia. These genes code for two enzymes involved in fatty-acid metabolism, and we have demonstrated in human cells that the HSP pathophysiology includes alteration of mitochondrial architecture and bioenergetics with increased oxidative stress. Our combined results focus attention on lipid metabolism as a critical HSP pathway with a deleterious impact on mitochondrial bioenergetic function.

14-3-3 protein and ATRAP bind to the soluble class IIB phosphatidylinositol transfer protein RdgBβ at distinct sites

PITPs (phosphatidylinositol transfer proteins) are characterized by the presence of the PITP domain whose biochemical properties of binding and transferring PI (phosphatidylinositol) are well studied. Despite their wide-spread expression in both unicellular and multicellular organisms, they remain functionally uncharacterized. An emerging theme is that individual PITPs play highly specific roles in either membrane trafficking or signal transduction. To identify specific roles for PITPs, identification of interacting molecules would shed light on their molecular function. In the present paper, we describe binding partners for the class IIB PITP RdgBβ (retinal degeneration type Bβ). RdgBβ is a soluble PITP but is unique in that it contains a region of disorder at its C-terminus following its defining N-terminal PITP domain. The C-terminus of RdgBβ is phosphorylated at two serine residues, Ser274 and Ser299, which form a docking site for 14-3-3 proteins. Binding to 14-3-3 proteins protects RdgBβ from degradation that occurs at the proteasome after ubiquitination. In addition to binding 14-3-3, the PITP domain of RdgBβ interacts with the Ang II (angiotensin II)-associated protein ATRAP (Ang II receptor-associated protein). ATRAP is also an interacting partner for the AT1R (Ang II type 1 receptor). We present a model whereby RdgBβ functions by being recruited to the membrane by ATRAP and release of 14-3-3 from the C-terminus allows the disordered region to bind a second membrane to create a membrane bridge for lipid transfer, possibly under the control of Ang II.

Rice phospholipase A superfamily: organization, phylogenetic and expression analysis during abiotic stresses and development

Background

Phospholipase A (PLA) is an important group of enzymes responsible for phospholipid hydrolysis in lipid signaling. PLAs have been implicated in abiotic stress signaling and developmental events in various plants species. Genome-wide analysis of PLA superfamily has been carried out in dicot plant Arabidopsis. A comprehensive genome-wide analysis of PLAs has not been presented yet in crop plant rice.

Methodology/Principal Findings

A comprehensive bioinformatics analysis identified a total of 31 PLA encoding genes in the rice genome, which are divided into three classes; phospholipase A₁ (PLA₁), patatin like phospholipases (pPLA) and low molecular weight secretory phospholipase A₂ (sPLA₂) based on their sequences and phylogeny. A subset of 10 rice PLAs exhibited chromosomal duplication, emphasizing the role of duplication in the expansion of this gene family in rice. Microarray expression profiling revealed a number of PLA members expressing differentially and significantly under abiotic stresses and reproductive development. Comparative expression analysis with Arabidopsis PLAs revealed a high degree of functional conservation between the orthologs in two plant species, which also indicated the vital role of PLAs in stress signaling and plant development across different plant species. Moreover, sub-cellular localization of a few candidates suggests their differential localization and functional role in the lipid signaling.

Conclusion/Significance

The comprehensive analysis and expression profiling would provide a critical platform for the functional characterization of the candidate PLA genes in crop plants.

Plant phospholipases: an overview

Plant phospholipases can be grouped into four major types, phospholipase D, phospholipase C, phospholipase A1 (PLA(1)), and phospholipase A2 (PLA(2)), that hydrolyze glycerophospholipids at different ester bonds. Within each type, there are different families or subfamilies of enzymes that can differ in substrate specificity, cofactor requirement, and/or reaction conditions. These differences provide insights into determining the cellular function of specific phospholipases in plants, and they can be explored for different industrial applications.

Surface loops of extracellular phospholipase A(1) determine both substrate specificity and preference for lysophospholipids

Members of the pancreatic lipase family exhibit both lipase activity toward triacylglycerol and/or phospholipase A(1) (PLA(1)) activity toward certain phospholipids. Some members of the pancreatic lipase family exhibit lysophospholipase activity in addition to their lipase and PLA(1) activities. Two such enzymes, phosphatidylserine (PS)-specific PLA(1) (PS-PLA(1)) and phosphatidic acid (PA)-selective PLA(1)α (PA-PLA(1)α, also known as LIPH) specifically hydrolyze PS and PA, respectively. However, little is known about the mechanisms that determine their substrate specificities. Crystal structures of lipases and mutagenesis studies have suggested that three surface loops, namely, β5, β9, and lid, have roles in determining substrate specificity. To determine roles of these loop structures in the substrate recognition of these PLA(1) enzymes, we constructed a number of PS-PLA(1) mutants in which the three surface loops are replaced with those of PA-PLA(1)α. The results indicate that the surface loops, especially the β5 loop, of PA-PLA(1)α play important roles in the recognition of PA, whereas other structure(s) in PS-PLA(1) is responsible for PS preference. In addition, β5 loop of PS-PLA(1) has a crucial role in lysophospholipase activity toward lysophosphatidylserine. The present study revealed the critical role of lipase surface loops, especially the β5 loop, in determining substrate specificities of PLA(1) enzymes.

Lysophosphatidylinositol causes neurite retraction via GPR55, G13 and RhoA in PC12 cells

GPR55 was recently identified as a putative receptor for certain cannabinoids, and lysophosphatidylinositol (LPI). Recently, the role of cannabinoids as GPR55 agonists has been disputed by a number of reports, in part, because studies investigating GPR55 often utilized overexpression systems, such as the GPR55-overexpressing HEK293 cells, which make it difficult to deduce the physiological role of endogenous GPR55. In the present study, we found that PC12 cells, a neural model cell line, express endogenous GPR55, and by using these cells, we were able to examine the role of endogenous GPR55. Although GPR55 mRNA and protein were expressed in PC12 cells, neither CB₁ nor CB₂ mRNA was expressed in these cells. GPR55 was predominantly localized on the plasma membrane in undifferentiated PC12 cells. However, GPR55 was also localized in the growth cones or the ruffled border in differentiated PC12 cells, suggesting a potential role for GPR55 in the regulation of neurite elongation. LPI increased intracellular Ca²⁺ concentration and RhoA activity, and induced ERK1/2 phosphorylation, whereas endogenous and synthetic cannabinoids did not, thereby suggesting that cannabinoids are not GPR55 agonists. LPI also caused neurite retraction in a time-dependent manner accompanied by the loss of neurofilament light chain and redistribution of actin in PC12 cells differentiated by NGF. This LPI-induced neurite retraction was found to be G_q-independent and G₁₃-dependent. Furthermore, inactivation of RhoA function via C3 toxin and GPR55 siRNA knockdown prevented LPI-induced neurite retraction. These results suggest that LPI, and not cannabinoids, causes neurite retraction in differentiated PC12 cells via a GPR55, G₁₃ and RhoA signaling pathway.

Phospholipases A₁

Phospholipase A(1) (PLA(1)) is an enzyme that hydrolyzes phospholipids and produces 2-acyl-lysophospholipids and fatty acids. This lipolytic activity is conserved in a wide range of organisms but is carried out by a diverse set of PLA(1) enzymes. Where their function is known, PLA(1)s have been shown to act as digestive enzymes, possess central roles in membrane maintenance and remodeling, or regulate important cellular mechanisms by the production of various lysophospholipid mediators, such as lysophosphatidylserine and lysophosphatidic acid, which in turn have multiple biological functions.

Intracellular phospholipase A1 and acyltransferase, which are involved in Caenorhabditis elegans stem cell divisions, determine the sn-1 fatty acyl chain of phosphatidylinositol

Phosphatidylinositol (PI), an important constituent of membranes, contains stearic acid as the major fatty acid at the sn-1 position. This fatty acid is thought to be incorporated into PI through fatty acid remodeling by sequential deacylation and reacylation. However, the genes responsible for the reaction are unknown, and consequently, the physiological significance of the sn-1 fatty acid remains to be elucidated. Here, we identified acl-8, -9, and -10, which are closely related to each other, and ipla-1 as strong candidates for genes involved in fatty acid remodeling at the sn-1 position of PI. In both ipla-1 mutants and acl-8 acl-9 acl-10 triple mutants of Caenorhabditis elegans, the stearic acid content of PI is reduced, and asymmetric division of stem cell-like epithelial cells is defective. The defects in asymmetric division of these mutants are suppressed by a mutation of the same genes involved in intracellular retrograde transport, suggesting that ipla-1 and acl genes act in the same pathway. IPLA-1 and ACL-10 have phospholipase A(1) and acyltransferase activity, respectively, both of which recognize the sn-1 position of PI as their substrate. We propose that the sn-1 fatty acid of PI is determined by ipla-1 and acl-8, -9, -10 and crucial for asymmetric divisions.

GPR35 is a novel lysophosphatidic acid receptor

GPR35 is a rhodopsin-like G protein-coupled receptor identified in 1998. It has been reported that kynurenic acid, a tryptophan metabolite, may act as an endogenous ligand for GPR35. However, the concentrations of kynurenic acid required to elicit the cellular responses are usually high, raising the possibility that another endogenous ligand may exist. In this study, we searched for another endogenous ligand for GPR35. Finally, we found that the magnitude of the Ca(2+) response induced by 2-acyl lysophosphatidic acid in the GPR35-expressing HEK293 cells was markedly greater than that in the vector-transfected control cells. Such a difference was not apparent in the case of 1-acyl lysophosphatidic acid. 2-Acyl lysophosphatidic acid also caused the sustained activation of RhoA and the phosphorylation of extracellular signal-regulated kinase, and triggered the internalization of the GPR35 molecule. These results strongly suggest that 2-acyl lysophosphatidic acid is an endogenous ligand for GPR35.

Activity-based probes that target functional subclasses of phospholipases in proteomes

Phospholipases are a large and diverse set of enzymes that metabolize the phospholipid components of cell membranes and function in key lipid-signaling pathways. The molecular characterization of novel phospholipases would benefit from chemical probes that selectively target these enzymes on the basis of their distinct substrate specificities and catalytic properties. Here we present the synthesis and characterization of a set of activity-based protein profiling (ABPP) probes that contain key recognition and reactivity elements for targeting phospholipases of the serine hydrolase superfamily. We show that these probes accurately report on the sn-1 and sn-2 substrate specificities of phospholipases in cell and tissue proteomes, including the sn-1-selective phospholipase DDHD1 and a calcium-dependent transacylase activity implicated in endocannabinoid biosynthesis. We anticipate that these phospholipase-directed ABPP probes will facilitate the discovery of new lipid-metabolizing enzymes and provide valuable insights into their substrate preferences.

Incorporation and remodeling of phosphatidylethanolamine containing short acyl residues in yeast

Phosphatidylethanolamine (PE) is one of the essential phospholipids in the yeast Saccharomyces cerevisiae. We have previously shown that a yeast strain, the endogenous PE synthesis of which was controllable, grew in the presence of PE containing decanoyl residues (diC10PE) when PE synthesis was repressed. In this study, we investigated the fate of diC10PE, its uptake and remodeling in yeast. Deletion of the genes encoding Lem3p/Ros3p or P-type ATPases, Dnf1p and Dnf2p, impaired the growth of the mutants in the medium containing diC10PE, suggesting the involvement of these proteins in the uptake of diC10PE. Analysis of the metabolism of deuterium-labeled diC10PE by electrospray ionization tandem mass spectrometry revealed that it was rapidly converted to deuterium-labeled PEs containing C16 or C18 acyl residues. The probable intermediate PEs that contained decanoic acid and C16 or C18 fatty acids as acyl residues were also detected. In addition, a substantial amount of decanoic acid was released into the culture medium during growth in the presence of diC10PE. These results imply that diC10PE was remodeled to PEs with longer acyl residues and used as membrane components. Defects in the remodeling of diC10PE in the deletion mutants of ALE1 and SLC1, products of which were capable of acyl-transfer to the sn-2 position of lyso-phospholipids, suggested their involvement in the introduction of acyl residues to the sn-2 position of lyso-phosphatidylethanolamine in the remodeling reaction of diC10PE. Our results also suggest the presence of a mechanism to maintain the physiological length of PE acyl residues in yeast.

Intracellular phospholipase A1gamma (iPLA1gamma) is a novel factor involved in coat protein complex I- and Rab6-independent retrograde transport between the endoplasmic reticulum and the Golgi complex

The mammalian intracellular phospholipase A(1) (iPLA(1)) family consists of three members, iPLA(1)alpha/PA-PLA(1), iPLA(1)beta/p125, and iPLA(1)gamma/KIAA0725p. Although iPLA(1)beta has been implicated in organization of the ER-Golgi compartments, little is known about the physiological role of its closest paralog, iPLA(1)gamma. Here we show that iPLA(1)gamma mediates a specific retrograde membrane transport pathway between the endoplasmic reticulum (ER) and the Golgi complex. iPLA(1)gamma appeared to be localized to the cytosol, the cis-Golgi, and the ER-Golgi intermediate compartment (ERGIC). Time-lapse microscopy revealed that a population of GFP-iPLA(1)gamma was associated with transport carriers moving out from the Golgi complex. Knockdown of iPLA(1)gamma expression by RNAi did not affect the anterograde transport of VSVGts045 but dramatically delayed two types of Golgi-to-ER retrograde membrane transport; that is, transfer of the Golgi membrane into the ER in the presence of brefeldin A and delivery of cholera toxin B subunit from the Golgi complex to the ER. Notably, knockdown of iPLA(1)gamma did not impair COPI- and Rab6-dependent retrograde transports represented by ERGIC-53 recycling and ER delivery of Shiga toxin, respectively. Thus, iPLA(1)gamma is a novel membrane transport factor that contributes to a specific Golgi-to-ER retrograde pathway distinct from presently characterized COPI- and Rab6-dependent pathways.

The life cycle of the endocannabinoids: formation and inactivation

In this chapter, we summarise the current thinking about the nature of endocannabinoids. In describing the life cycle of these agents, we highlight the synthetic and catabolic enzymes suggested to be involved. For each of these, we provide a systematic analysis of information on sequence, subcellular and cellular distribution, as well as physiological and pharmacological substrates, enhancers and inhibitors, together with brief descriptions of the impact of manipulating enzyme levels through genetic mechanisms (dealt with in more detail in the chapter "Genetic Models of the Endocannabinoid System" by Monory and Lutz, this volume). In addition, we describe experiments investigating the stimulation of endocannabinoid synthesis and release in intact cell systems.

Beta-catenin asymmetry is regulated by PLA1 and retrograde traffic in C. elegans stem cell divisions

Asymmetric division is an important property of stem cells. In Caenorhabditis elegans, the Wnt/beta-catenin asymmetry pathway determines the polarity of most asymmetric divisions. The Wnt signalling components such as beta-catenin localize asymmetrically to the cortex of mother cells to produce two distinct daughter cells. However, the molecular mechanism to polarize them remains to be elucidated. Here, we demonstrate that intracellular phospholipase A(1) (PLA(1)), a poorly characterized lipid-metabolizing enzyme, controls the subcellular localizations of beta-catenin in the terminal asymmetric divisions of epithelial stem cells (seam cells). In mutants of ipla-1, a single C. elegans PLA(1) gene, cortical beta-catenin is delocalized and the asymmetry of cell-fate specification is disrupted in the asymmetric divisions. ipla-1 mutant phenotypes are rescued by expression of ipla-1 in seam cells in a catalytic activity-dependent manner. Furthermore, our genetic screen utilizing ipla-1 mutants reveals that reduction of endosome-to-Golgi retrograde transport in seam cells restores normal subcellular localization of beta-catenin to ipla-1 mutants. We propose that membrane trafficking regulated by ipla-1 provides a mechanism to control the cortical asymmetry of beta-catenin.

Mammalian Sec16/p250 plays a role in membrane traffic from the endoplasmic reticulum

Coat protein complex II (COPII)-coated vesicles/carriers, which mediate export of proteins from the endoplasmic reticulum (ER), are formed at special ER subdomains in mammals, termed ER exit sites or transitional ER. The COPII coat consists of a small GTPase, Sar1, and two protein complexes, Sec23-Sec24 and Sec13-Sec31. Sec23-Sec24 and Sec13-Sec31 appear to constitute the inner and the outermost layers of the COPII coat, respectively. We previously isolated two mammalian proteins (p125 and p250) that bind to Sec23. p125 was found to be a mammalian-specific, phospholipase A(1)-like protein that participates in the organization of ER exit sites. Here we show that p250 is encoded by the KIAA0310 clone and has sequence similarity to yeast Sec16 protein. Although KIAA0310p was found to be localized at ER exit sites, subcellular fractionation revealed its predominant presence in the cytosol. Cytosolic KIAA0310p was recruited to ER membranes in a manner dependent on Sar1. Depletion of KIAA0310p mildly caused disorganization of ER exit sites and delayed protein transport from the ER, suggesting its implication in membrane traffic out of the ER. Overexpression of KIAA0310p affected ER exit sites in a manner different from that of p125. Binding experiments suggested that KIAA0310p interacts with both the inner and the outermost layer coat complexes, whereas p125 binds principally to the inner layer complex. Our results suggest that KIAA0310p, a mammalian homologue of yeast Sec16, builds up ER exit sites in cooperation with p125 and plays a role in membrane traffic from the ER.

A novel phospholipase from Trypanosoma brucei

Phospholipase A(1) activities have been detected in most cells where they have been sought and yet their characterization lags far behind that of the phospholipases A(2), C and D. The study presented here details the first cloning and characterization of a cytosolic PLA(1) that exhibits preference for phosphatidylcholine (GPCho) substrates. Trypanosoma brucei phospholipase A(1) (TbPLA(1)) is unique from previously identified eukaryotic PLA(1) because it is evolutionarily related to bacterial secreted PLA(1). A T. brucei ancestor most likely acquired the PLA(1) from a horizontal gene transfer of a PLA(1) from Sodalis glossinidius, a bacterial endosymbiont of tsetse flies. Nano-electrospray ionization tandem mass spectrometry analysis of TbPLA(1) mutants established that the enzyme functions in vivo to synthesize lysoGPCho metabolites containing long-chain mostly polyunsaturated and highly unsaturated fatty acids. Analysis of purified mutated recombinant forms of TbPLA(1) revealed that this enzyme is a serine hydrolase whose catalytic mechanism involves a triad consisting of the amino acid residues Ser-131, His-234 and Asp-183. The TbPLA(1) homozygous null mutants generated here constitute the only PLA(1) double knockouts from any organism.