Substrate specificity of diacylglycerol kinase-epsilon and the phosphatidylinositol cycle

Maternal Docosahexaenoic Acid Supplementation Alters Maternal and Fetal Docosahexaenoic Acid Status and Placenta Phospholipids in Pregnancies Complicated by High Body Mass Index

INTRODUCTION

An optimal fetal supply of docosahexaenoic acid (DHA) is critical for normal brain development. The relationship between maternal DHA intake and DHA delivery to the fetus is complex and is dependent on placental handling of DHA. Little data exist on placental DHA levels in pregnancies supplemented with the recommended dose of 200 mg/d. Our objective was to determine how prenatal DHA at the recommended 200 mg/d impacts maternal, placental, and fetal DHA status in both normal-weight and high-BMI women compared to women taking no supplements.

METHODS

Maternal blood, placenta, and cord blood were collected from 30 healthy pregnant women (BMI 18.9-43.26 kg/m2) giving birth at term. Red blood cells (RBCs) and villous tissue were isolated, and lipids were extracted to determine DHA content by LC-MS/MS. Data were analyzed by supplement group (0 vs. 200 mg/d) and maternal BMI (normal weight or high BMI) using two-way ANOVA. We measured maternal choline levels in maternal and cord plasma samples.

RESULTS

Supplementation with 200 mg/d DHA significantly increased (p < 0.05) maternal and cord RBC DHA content only in pregnancies complicated by high BMI. We did not find any impact of choline levels on maternal or cord RBC phospholipids. There were no significant differences in total placental DHA content by supplementation or maternal BMI (p > 0.05). Placental levels of phosphatidylinositol (PI) and phosphatidic acid containing DHA species were higher (p < 0.05) in high-BMI women without DHA supplementation compared to both normal-BMI and high-BMI women taking DHA supplements.

CONCLUSION

Maternal DHA supplementation at recommended doses cord increased RBC DHA content only in pregnancies complicated by higher BMI. Surprisingly, we found that obesity was related to an increase in placental PI and phosphatidic acid species, which was ameliorated by DHA supplementation. Phosphatidic acid activates placental mTOR, which regulates amino acid transport and may explain previous findings of the impact of DHA on placental function. Current recommendations for DHA supplementation may not be achieving the goal of improving fetal DHA levels in normal-weight women.

The LCLAT1/LYCAT acyltransferase is required for EGF-mediated phosphatidylinositol-3,4,5-trisphosphate generation and Akt signaling

Receptor tyrosine kinases such as EGF receptor (EGFR) stimulate phosphoinositide 3 kinases to convert phosphatidylinositol-4,5-bisphosophate [PtdIns(4,5)P2] into phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3]. PtdIns(3,4,5)P3 then remodels actin and gene expression, and boosts cell survival and proliferation. PtdIns(3,4,5)P3 partly achieves these functions by triggering activation of the kinase Akt, which phosphorylates targets like Tsc2 and GSK3β. Consequently, unchecked upregulation of PtdIns(3,4,5)P3-Akt signaling promotes tumor progression. Interestingly, 50-70% of PtdIns and PtdInsPs have stearate and arachidonate at sn-1 and sn-2 positions of glycerol, respectively, forming a species known as 38:4-PtdIns/PtdInsPs. LCLAT1 and MBOAT7 acyltransferases partly enrich PtdIns in this acyl format. We previously showed that disruption of LCLAT1 lowered PtdIns(4,5)P2 levels and perturbed endocytosis and endocytic trafficking. However, the role of LCLAT1 in receptor tyrosine kinase and PtdIns(3,4,5)P3 signaling was not explored. Here, we show that LCLAT1 silencing in MDA-MB-231 and ARPE-19 cells abated the levels of PtdIns(3,4,5)P3 in response to EGF signaling. Importantly, LCLAT1-silenced cells were also impaired for EGF-driven and insulin-driven Akt activation and downstream signaling. Thus, our work provides first evidence that the LCLAT1 acyltransferase is required for receptor tyrosine kinase signaling.

New Insights into the Neuromyogenic Spectrum of a Gain of Function Mutation in SPTLC1

Serine palmitoyltransferase long chain base subunit 1 (SPTLC1) encodes a serine palmitoyltransferase (SPT) resident in the endoplasmic reticulum (ER). Pathological SPTLC1 variants cause a form of hereditary sensory and autonomic neuropathy (HSAN1A), and have recently been linked to unrestrained sphingoid base synthesis, causing a monogenic form of amyotrophic lateral sclerosis (ALS). It was postulated that the phenotypes associated with dominant variants in SPTLC1 may represent a continuum between neuropathy and ALS in some cases, complicated by additional symptoms such as cognitive impairment. A biochemical explanation for this clinical observation does not exist. By performing proteomic profiling on immortalized lymphoblastoid cells derived from one patient harbouring an alanine to serine amino acid substitution at position 20, we identified a subset of dysregulated proteins playing significant roles in neuronal homeostasis and might have a potential impact on the manifestation of symptoms. Notably, the identified p.(A20S)-SPTLC1 variant is associated with decrease of transcript and protein level. Moreover, we describe associated muscle pathology findings, including signs of mild inflammation accompanied by dysregulation of respective markers on both the protein and transcript levels. By performing coherent anti-Stokes Raman scattering microscopy, presence of protein and lipid aggregates could be excluded.

Membrane shape as determinant of protein properties

Membrane lipids play a role in the modulation of a variety of biological processes. This is often achieved through fine-tuned changes in membrane physical and chemical properties. While some membrane physical properties (e.g., curvature, lipid domains, fluidity) have received increased scientific attention over the years, only recently has membrane shape emerged as an active modulator of protein properties. Biological membranes are mostly found organized into a lipid bilayer arrangement, in which the spontaneous shape is an intrinsically flat, planar morphology (in relation to the size of proteins). However, it is known that many cells and organelles have non-planar morphologies. In addition, perturbations in membrane morphology occur in a variety of biological processes. Recent studies have shown that membrane shape can modulate a variety of biological processes by determining protein properties. While membrane shape generation modulates proteins via changes in membrane mechanical properties, membrane shape recognition regulates proteins by providing the optimal surface for interaction. Hence, membranes have evolved an elegant mechanism to couple mesoscopic perturbations to molecular properties and vice-versa. In this review, the regulation of the enzymatic properties of two isoforms of mammalian diacylglycerol kinase, which play important roles in cellular signal transductions, will be used to exemplify the recent advancements in the field of membrane shape recognition, as well as future challenges and perspectives.

Reprogramming fatty acyl specificity of lipid kinases via C1 domain engineering

C1 domains are lipid-binding modules that regulate membrane activation of kinases, nucleotide exchange factors and other C1-containing proteins to trigger signal transduction. Despite annotation of typical C1 domains as diacylglycerol (DAG) and phorbol ester sensors, the function of atypical counterparts remains ill-defined. Here, we assign a key role for atypical C1 domains in mediating DAG fatty acyl specificity of diacylglycerol kinases (DGKs) in live cells. Activity-based proteomics mapped C1 probe binding as a principal differentiator of type 1 DGK active sites that combined with global metabolomics revealed a role for C1s in lipid substrate recognition. Protein engineering by C1 domain swapping demonstrated that exchange of typical and atypical C1s is functionally tolerated and can directly program DAG fatty acyl specificity of type 1 DGKs. Collectively, we describe a protein engineering strategy for studying metabolic specificity of lipid kinases to assign a role for atypical C1 domains in cell metabolism.

Roles of DGKs in neurons: Postsynaptic functions?

Diacylglycerol kinases (DGKs) contribute to an important part of intracellular signaling because, in addition to reducing diacylglycerol levels, they generate phosphatidic acid (PtdOH) Recent research has led to the discovery of ten mammalian DGK isoforms, all of which are found in the mammalian brain. Many of these isoforms have studied functions within the brain, while others lack such understanding in regards to neuronal roles, regulation, and structural dynamics. However, while previously a neuronal function for DGKθ was unknown, it was recently found that DGKθ is required for the regulation of synaptic vesicle endocytosis and work is currently being conducted to elucidate the mechanism behind this regulation. Here we will review some of the roles of all mammalian DGKs and hypothesize additional roles. We will address the topic of redundancy among the ten DGK isoforms and discuss the possibility that DGKθ, among other DGKs, may have unstudied postsynaptic functions. We also hypothesize that in addition to DGKθ's presynaptic endocytic role, DGKθ might also regulate the endocytosis of AMPA receptors and other postsynaptic membrane proteins.

Diacylglycerol kinases: Relationship to other lipid kinases

Lipid kinases regulate a wide variety of cellular functions and have emerged as one the most promising targets for drug design. Diacylglycerol kinases (DGKs) are a family of enzymes that catalyze the ATP-dependent phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PtdOH). Despite the critical role in lipid biosynthesis, both DAG and PtdOH have been shown as bioactive lipids mediating a number of signaling pathways. Although there is increasing recognition of their role in signaling systems, our understanding of the key enzyme which regulate the balance of these two lipid messages remain limited. Solved structures provide a wealth of information for understanding the function and regulation of these enzymes. Solving the structures of mammalian DGKs by traditional NMR and X-ray crystallography approaches have been challenging and so far, there are still no three-dimensional structures of these DGKs. Despite this, some insights may be gained by examining the similarities and differences between prokaryotic DGKs and other mammalian lipid kinases. This review focuses on summarizing and comparing the structure of prokaryotic and mammalian DGKs as well as two other lipid kinases: sphingosine kinase and phosphatidylinositol-3-kinase. How these known lipid kinases structures relate to mammalian DGKs will also be discussed.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

Endothelial cells: source, barrier, and target of defensive mediators

Endothelium is strategically located at the interface between blood and interstitial tissues, placing thus endothelial cell as a key player in vascular homeostasis. Endothelial cells are in a dynamic equilibrium with their environment and constitute concomitantly a source, a barrier, and a target of defensive mediators. This review will discuss the recent advances in our understanding of the complex crosstalk between the endothelium, the complement system and the hemostasis in health and in disease. The first part will provide a general introduction on endothelial cells heterogeneity and on the physiologic role of the complement and hemostatic systems. The second part will analyze the interplay between complement, hemostasis and endothelial cells in physiological conditions and their alterations in diseases. Particular focus will be made on the prototypes of thrombotic microangiopathic disorders, resulting from complement or hemostasis dysregulation-mediated endothelial damage: atypical hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. Novel aspects of the pathophysiology of the thrombotic microangiopathies will be discussed.

Decreased 16:0/20:4-phosphatidylinositol level in the post-mortem prefrontal cortex of elderly patients with schizophrenia

The etiology of schizophrenia includes phospholipid abnormalities. Phospholipids are bioactive substances essential for brain function. To analyze differences in the quantity and types of phospholipids present in the brain tissue of patients with schizophrenia, we performed a global analysis of phospholipids in multiple brain samples using liquid chromatography electrospray ionization mass/mass spectrometry (LC-ESI/MS/MS) and imaging mass spectrometry (IMS). We found significantly decreased 16:0/20:4-phosphatidylinositol (PI) levels in the prefrontal cortex (PFC) in the brains from patients with schizophrenia in the LC-ESI/MS/MS, and that the 16:0/20:4-PI in grey matter was most prominently diminished according to the IMS experiments. Previous reports investigating PI pathology of schizophrenia did not identify differences in the sn-1 and sn-2 fatty acyl chains. This study is the first to clear the fatty acid composition of PI in brains from patients with schizophrenia. Alteration in the characteristic fatty acid composition of PI may also affect neuronal function, and could play a role in the etiology of schizophrenia. Although further studies are necessary to understand the role of reduced 16:0/20:4-PI levels within the prefrontal cortex in the etiology of schizophrenia, our results provide insight into the development of a novel therapy for the clinical treatment of schizophrenia.

Diacylglycerol kinase ε deficiency preserves glucose tolerance and modulates lipid metabolism in obese mice

Diacylglycerol kinases (DGKs) catalyze the phosphorylation and conversion of diacylglycerol (DAG) into phosphatidic acid. DGK isozymes have unique primary structures, expression patterns, subcellular localizations, regulatory mechanisms, and DAG preferences. DGKε has a hydrophobic segment that promotes its attachment to membranes and shows substrate specificity for DAG with an arachidonoyl acyl chain in the sn-2 position of the substrate. We determined the role of DGKε in the regulation of energy and glucose homeostasis in relation to diet-induced insulin resistance and obesity using DGKε-KO and wild-type mice. Lipidomic analysis revealed elevated unsaturated and saturated DAG species in skeletal muscle of DGKε KO mice, which was paradoxically associated with increased glucose tolerance. Although skeletal muscle insulin sensitivity was unaltered, whole-body respiratory exchange ratio was reduced, and abundance of mitochondrial markers was increased, indicating a greater reliance on fat oxidation and intracellular lipid metabolism in DGKε KO mice. Thus, the increased intracellular lipids in skeletal muscle from DGKε KO mice may undergo rapid turnover because of increased mitochondrial function and lipid oxidation, rather than storage, which in turn may preserve insulin sensitivity. In conclusion, DGKε plays a role in glucose and energy homeostasis by modulating lipid metabolism in skeletal muscle.

Arachidonoyl-Specific Diacylglycerol Kinase ε and the Endoplasmic Reticulum

The endoplasmic reticulum (ER) comprises an interconnected membrane network, which is made up of lipid bilayer and associated proteins. This organelle plays a central role in the protein synthesis and sorting. In addition, it represents the synthetic machinery of phospholipids, the major constituents of the biological membrane. In this process, phosphatidic acid (PA) serves as a precursor of all phospholipids, suggesting that PA synthetic activity is closely associated with the ER function. One enzyme responsible for PA synthesis is diacylglycerol kinase (DGK) that phosphorylates diacylglycerol (DG) to PA. DGK is composed of a family of enzymes with distinct features assigned to each isozyme in terms of structure, enzymology, and subcellular localization. Of DGKs, DGKε uniquely exhibits substrate specificity toward arachidonate-containing DG and is shown to reside in the ER. Arachidonic acid, a precursor of bioactive eicosanoids, is usually acylated at the sn-2 position of phospholipids, being especially enriched in phosphoinositide. In this review, we focus on arachidonoyl-specific DGKε with respect to the historical context, molecular basis of the substrate specificity and ER-targeting, and functional implications in the ER.

Diacylglycerol Kinase-ε: Properties and Biological Roles

In mammals there are at least 10 isoforms of diacylglycerol kinases (DGK). All catalyze the phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PA). Among DGK isoforms, DGKε has several unique features. It is the only DGK isoform with specificity for a particular species of DAG, i.e., 1-stearoyl-2-arachidonoyl glycerol. The smallest of all known DGK isoforms, DGKε, is also the only DGK devoid of a regulatory domain. DGKε is the only DGK isoform that has a hydrophobic segment that is predicted to form a transmembrane helix. As the only membrane-bound, constitutively active DGK isoform with exquisite specificity for particular molecular species of DAG, the functional overlap between DGKε and other DGKs is predicted to be minimal. DGKε exhibits specificity for DAG containing the same acyl chains as those found in the lipid intermediates of the phosphatidylinositol-cycle. It has also been shown that DGKε affects the acyl chain composition of phosphatidylinositol in whole cells. It is thus likely that DGKε is responsible for catalyzing one step in the phosphatidylinositol-cycle. Steps of this cycle take place in both the plasma membrane and the endoplasmic reticulum membrane. DGKε is likely present in both of these membranes. DGKε is the only DGK isoform that is associated with a human disease. Indeed, recessive loss-of-function mutations in DGKε cause atypical hemolytic-uremic syndrome (aHUS). This condition is characterized by thrombosis in the small vessels of the kidney. It causes acute renal insufficiency in infancy and most patients develop end-stage renal failure before adulthood. Disease pathophysiology is poorly understood and there is no therapy. There are also data suggesting that DGKε may play a role in epilepsy and Huntington disease. Thus, DGKε has many unique molecular and biochemical properties when compared to all other DGK isoforms. DGKε homologs also contain a number of conserved sequence features that are distinctive characteristics of either the rodents or specific groups of primate homologs. How cells, tissues and organisms harness DGKε's catalytic prowess remains unclear. The discovery of DGKε's role in causing aHUS will hopefully boost efforts to unravel the mechanisms by which DGKε dysfunction causes disease.

Diacylglycerol Kinase-ε: Properties and Biological Roles

In mammals there are at least 10 isoforms of diacylglycerol kinases (DGK). All catalyze the phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PA). Among DGK isoforms, DGKε has several unique features. It is the only DGK isoform with specificity for a particular species of DAG, i.e., 1-stearoyl-2-arachidonoyl glycerol. The smallest of all known DGK isoforms, DGKε, is also the only DGK devoid of a regulatory domain. DGKε is the only DGK isoform that has a hydrophobic segment that is predicted to form a transmembrane helix. As the only membrane-bound, constitutively active DGK isoform with exquisite specificity for particular molecular species of DAG, the functional overlap between DGKε and other DGKs is predicted to be minimal. DGKε exhibits specificity for DAG containing the same acyl chains as those found in the lipid intermediates of the phosphatidylinositol-cycle. It has also been shown that DGKε affects the acyl chain composition of phosphatidylinositol in whole cells. It is thus likely that DGKε is responsible for catalyzing one step in the phosphatidylinositol-cycle. Steps of this cycle take place in both the plasma membrane and the endoplasmic reticulum membrane. DGKε is likely present in both of these membranes. DGKε is the only DGK isoform that is associated with a human disease. Indeed, recessive loss-of-function mutations in DGKε cause atypical hemolytic-uremic syndrome (aHUS). This condition is characterized by thrombosis in the small vessels of the kidney. It causes acute renal insufficiency in infancy and most patients develop end-stage renal failure before adulthood. Disease pathophysiology is poorly understood and there is no therapy. There are also data suggesting that DGKε may play a role in epilepsy and Huntington disease. Thus, DGKε has many unique molecular and biochemical properties when compared to all other DGK isoforms. DGKε homologs also contain a number of conserved sequence features that are distinctive characteristics of either the rodents or specific groups of primate homologs. How cells, tissues and organisms harness DGKε's catalytic prowess remains unclear. The discovery of DGKε's role in causing aHUS will hopefully boost efforts to unravel the mechanisms by which DGKε dysfunction causes disease.

Analysis of diacylglycerols by ultra performance liquid chromatography-quadrupole time-of-flight mass spectrometry: Double bond location and isomers separation

Diacylglycerols (DAGs) are important lipid intermediates and have been implicated in human diseases. Isomerism complicates their mass spectrometric analysis; in particular, it is difficult to identify fatty acid substituents and locate the double bond positions in unsaturated DAGs. We have developed an analytical strategy using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF MS) in conjunction with dimethyl disulfide (DMDS) derivatization and collision cross-section (CCS) measurement to characterize DAGs in biological samples. The method employs non-aqueous reversed-phase chromatographic separation and profile collision energy (CE) mode for MS(E) and MS/MS analyses. Three types of fragment ions were produced simultaneously. Hydrocarbon ions (m/z 50-200) obtained at high CE helped to distinguish unsaturated and saturated DAGs rapidly. Neutral loss ions and acylium ions (m/z 300-400) produced at low CE were used to identify fatty acid substituents. Informative methyl thioalkane fragment ions were used to locate the double bonds of unsaturated DAGs. Mono-methylthio derivatives were formed mainly by the reaction of DAGs with DMDS, where methyl thiol underwent addition to the first double bond farthest from the ester terminus of unsaturated fatty acid chains. The addition of CCS values maximized the separation of isomeric DAG species and improved the confidence of DAG identification. Fourteen DAGs were identified in mouse myotube cells based on accurate masses, characteristic fragment ions, DMDS derivatization, and CCS values.

Diacylglycerol kinases in membrane trafficking

Diacylglycerol kinases (DGKs) belong to a family of cytosolic kinases that regulate the phosphorylation of diacylglycerol (DAG), converting it into phosphatidic acid (PA). There are 10 known mammalian DGK isoforms, each with a different tissue distribution and substrate specificity. These differences allow regulation of cellular responses by fine-tuning the delicate balance of cellular DAG and PA. DGK isoforms are best characterized as mediators of signal transduction and immune function. However, since recent studies reveal that DAG and PA are also involved in the regulation of endocytic trafficking, it is therefore anticipated that DGKs also plays an important role in membrane trafficking. In this review, we summarize the literature discussing the role of DGK isoforms at different stages of endocytic trafficking, including endocytosis, exocytosis, endocytic recycling, and transport from/to the Golgi apparatus. Overall, these studies contribute to our understanding of the involvement of PA and DAG in endocytic trafficking, an area of research that is drawing increasing attention in recent years.

Podocyte dysfunction in atypical haemolytic uraemic syndrome

Genetic or autoimmune defects that lead to dysregulation of the alternative pathway of complement have been associated with the development of atypical haemolytic uraemic syndrome (aHUS), which is characterized by thrombocytopenia, haemolytic anaemia and acute kidney injury. The relationship between aHUS, podocyte dysfunction and the resultant proteinuria has not been adequately investigated. However, the report of mutations in diacylglycerol kinase ε (DGKE) as a cause of recessive infantile aHUS characterized by proteinuria, highlighted podocyte dysfunction as a potential complication of aHUS. DGKE deficiency was originally thought to trigger aHUS through pathogenetic mechanisms distinct from complement dysregulation; however, emerging findings suggest an interplay between DGKE and complement systems. Podocyte dysfunction with nephrotic-range proteinuria can also occur in forms of aHUS associated with genetic or autoimmune complement dysregulation without evidence of DGKE mutations. Furthermore, proteinuric glomerulonephritides can be complicated by aHUS, possibly as a consequence of podocyte dysfunction inducing endothelial injury and prothrombotic abnormalities.

Loss of DGKε induces endothelial cell activation and death independently of complement activation

Atypical hemolytic uremic syndrome (aHUS) is classically described to result from a dysregulation of the complement alternative pathway, leading to glomerular endothelial cell (EC) damage and thrombosis. However, recent findings in families with aHUS of mutations in the DGKE gene, which is not an integral component of the complement cascade, led us to consider other pathophysiologic mechanisms for this disease. Here, we demonstrate that loss of DGKε expression/activity in EC induces an increase in ICAM-1 and tissue factor expression through the upregulation of p38-MAPK-mediated signals, thus highlighting a proinflammatory and prothrombotic phenotype of DGKε-deficient ECs. More interestingly, DGKE silencing also increases EC apoptosis and impairs EC migration and angiogenesis in vitro, suggesting that DGKE loss-of-function mutations impair EC repair and angiogenesis in vivo. Conversely, DGKE knockdown moderately decreases the expression of the complement inhibitory protein MCP on quiescent EC, but does not induce complement deposition on their surface in vitro. Collectively, our data strongly suggest that in DGKE-associated aHUS patients, thrombotic microangiopathy results from impaired EC proliferation and angiogenesis rather than complement-mediated EC lesions. Our study expands the current knowledge of aHUS mechanisms and has implications for the treatment of patients with isolated DGKE mutations.

Distinct properties of the two isoforms of CDP-diacylglycerol synthase

CDP-diacylglycerol synthases (CDS) are critical enzymes that catalyze the formation of CDP-diacylglycerol (CDP-DAG) from phosphatidic acid (PA). Here we show in vitro that the two isoforms of human CDS, CDS1 and CDS2, show different acyl chain specificities for its lipid substrate. CDS2 is selective for the acyl chains at the sn-1 and sn-2 positions, the most preferred species being 1-stearoyl-2-arachidonoyl-sn-phosphatidic acid. CDS1, conversely, shows no particular substrate specificity, displaying similar activities for almost all substrates tested. Additionally, we show that inhibition of CDS2 by phosphatidylinositol is also acyl chain-dependent, with the strongest inhibition seen with the 1-stearoyl-2-arachidonoyl species. CDS1 shows no acyl chain-dependent inhibition. Both CDS1 and CDS2 are inhibited by their anionic phospholipid end products, with phosphatidylinositol-(4,5)-bisphosphate showing the strongest inhibition. Our results indicate that CDS1 and CDS2 could create different CDP-DAG pools that may serve to enrich different phospholipid species with specific acyl chains.

Phosphatidylinositol-4-phosphate 5-kinase isoforms exhibit acyl chain selectivity for both substrate and lipid activator

Phosphatidylinositol 4,5-bisphosphate is mostly produced in the cell by phosphatidylinositol-4-phosphate 5-kinases (PIP5K) and has a crucial role in numerous signaling events. Here we demonstrate that in vitro all three isoforms of PIP5K, α, β, and γ, discriminate among substrates with different acyl chains for both the substrates phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol (PtdIns) although to different extents, with isoform γ being the most selective. Fully saturated dipalmitoyl-PtdIns4P was a poor substrate for all three isoforms, but both the 1-stearoyl-2-arachidonoyl and the 1-stearoyl-2-oleoyl forms of PtdIns4P were good substrates. V(max) was greater for the 1-stearoyl-2-arachidonoyl form compared with the 1-stearoyl-2-oleoyl form, although for PIP5Kβ the difference was small. For the α and γ isoforms, K(m) was much lower for 1-stearoyl-2-oleoyl PtdIns4P, making this lipid the better substrate of the two under most conditions. Activation of PIP5K by phosphatidic acid is also acyl chain-dependent. Species of phosphatidic acid with two unsaturated acyl chains are much better activators of PIP5K than those containing one saturated and one unsaturated acyl chain. PtdIns is a poor substrate for PIP5K, but it also shows acyl chain selectivity. Curiously, there is no acyl chain discrimination among species of phosphatidic acid in the activation of the phosphorylation of PtdIns. Together, our findings indicate that PIP5K isoforms α, β, and γ act selectively on substrates and activators with different acyl chains. This could be a tightly regulated mechanism of producing physiologically active unsaturated phosphatidylinositol 4,5-bisphosphate species in the cell.

Catalytic activity and acyl-chain selectivity of diacylglycerol kinase ɛ are modulated by residues in and near the lipoxygenase-like motif

Diacylglycerol kinase (DGK) ɛ plays an important role in the resynthesis of phosphatidylinositol by mediating the phosphorylation of diacylglycerol to phosphatidic acid. DGKɛ is unique among mammalian DGK isoforms in that it is the only one that shows acyl-chain selectivity, preferring diacylglycerols with an sn-2 arachidonoyl group. The region responsible for this arachidonoyl specificity is the lipoxygenase (LOX)-like motif found in the accessory domain, adjacent to DGKɛ's catalytic site. Many mutations within the LOX-like motif result in a loss of enzyme activity. However, the few mutants that retain significant activity exhibit some decrease in selectivity for the arachidonoyl chain. In the present work, we have explored mutations in a region adjacent to the LOX-like motif, which is also contained within the same hydrophobic segment of the protein. This adjacent region also contains a cholesterol recognition/interaction amino acid consensus motif. Being outside of the LOX-like motif, this region likely has less direct contact with the substrate, and more activity is retained with mutations. This has allowed us to probe in more detail the relationship between this region of the protein and substrate specificity. We demonstrate that this cholesterol recognition/interaction amino acid consensus domain also plays a role in acyl-chain selectivity. Despite the high degree of conservation of the amino acid sequence in this region of the protein, certain mutations result in proteins with higher activity than the wild-type protein. These mutations also result in a selective gain of acyl-chain preferences for diacylglycerols with different acyl-chain profiles. In addition to the LOX-like motif, adjacent residues also contribute to selectivity for diacylglycerols with specific acyl-chain compositions, such as those found in the phosphatidylinositol cycle.