The Coffin-Lowry Syndrome-associated Protein RSK2 Controls Neuroendocrine Secretion through the Regulation of Phospholipase D1 at the Exocytotic Sites

Phospholipase D1 produces phosphatidic acid at sites of secretory vesicle docking and fusion

Phospholipase D1 (PLD1) activity is essential for the stimulated exocytosis of secretory vesicles where it acts as a lipid-modifying enzyme to produces phosphatidic acid (PA). PLD1 localizes to the plasma membrane and secretory vesicles, and PLD1 inhibition or knockdowns reduce the rate of fusion. However, temporal data resolving when and where PLD1 and PA are required during exocytosis is lacking. In this work, PLD1 and production of PA are measured during the trafficking, docking, and fusion of secretory vesicles in PC12 cells. Using fluorescently tagged PLD1 and a PA-binding protein, cells were imaged using TIRF microscopy to monitor the presence of PLD1 and the formation of PA throughout the stages of exocytosis. Single docking and fusion events were imaged to measure the recruitment of PLD1 and the formation of PA. PLD1 is present on mobile, docking, and fusing vesicles and also colocalizes with Syx1a clusters. Treatment of cells with PLD inhibitors significantly reduces fusion, but not PLD1 localization to secretory vesicles. Inhibitors also alter the formation of PA; when PLD1 is active, PA slowly accumulates on docked vesicles. During fusion, PA is reduced in cells treated with PLD1 inhibitors, indicating that PLD1 produces PA during exocytosis.

Structure and regulation of human phospholipase D

Mammalian phospholipase D (PLD) generates phosphatidic acid, a dynamic lipid secondary messenger involved with a broad spectrum of cellular functions including but not limited to metabolism, migration, and exocytosis. As a promising pharmaceutical target, the biochemical properties of PLD have been well characterized. This has led to the recent crystal structures of human PLD1 and PLD2, the development of PLD specific pharmacological inhibitors, and the identification of cellular regulators of PLD. In this review, we discuss the PLD1 and PLD2 structures, PLD inhibition by small molecules, and the regulation of PLD activity by effector proteins and lipids.

The Coffin-Lowry syndrome-associated protein RSK2 regulates neurite outgrowth through phosphorylation of phospholipase D1 (PLD1) and synthesis of phosphatidic acid

More than 80 human X-linked genes have been associated with mental retardation and deficits in learning and memory. However, most of the identified mutations induce limited morphological alterations in brain organization and the molecular bases underlying neuronal clinical features remain elusive. We show here that neurons cultured from mice lacking ribosomal S6 kinase 2 (Rsk2), a model for the Coffin-Lowry syndrome (CLS), exhibit a significant delay in growth in a similar way to that shown by neurons cultured from phospholipase D1 (Pld1) knock-out mice. We found that gene silencing of Pld1 or Rsk2 as well as acute pharmacological inhibition of PLD1 or RSK2 in PC12 cells strongly impaired neuronal growth factor (NGF)-induced neurite outgrowth. Expression of a phosphomimetic PLD1 mutant rescued the inhibition of neurite outgrowth in PC12 cells silenced for RSK2, revealing that PLD1 is a major target for RSK2 in neurite formation. NGF-triggered RSK2-dependent phosphorylation of PLD1 led to its activation and the synthesis of phosphatidic acid at sites of neurite growth. Additionally, total internal reflection fluorescence microscopy experiments revealed that RSK2 and PLD1 positively control fusion of tetanus neurotoxin insensitive vesicle-associated membrane protein (TiVAMP)/VAMP-7 vesicles at sites of neurite outgrowth. We propose that the loss of function mutations in RSK2 that leads to CLS and neuronal deficits are related to defects in neuronal growth due to impaired RSK2-dependent PLD1 activity resulting in a reduced vesicle fusion rate and membrane supply.

The role of genetics in the establishment and maintenance of the epigenome

Epigenetic mechanisms play an important role in gene regulation during development. DNA methylation, which is probably the most important and best-studied epigenetic mechanism, can be abnormally regulated in common pathologies, but the origin of altered DNA methylation remains unknown. Recent research suggests that these epigenetic alterations could depend, at least in part, on genetic mutations or polymorphisms in DNA methyltransferases and certain genes encoding enzymes of the one-carbon metabolism pathway. Indeed, the de novo methyltransferase 3B (DNMT3B) has been recently found to be mutated in several types of cancer and in the immunodeficiency, centromeric region instability and facial anomalies syndrome (ICF), in which these mutations could be related to the loss of global DNA methylation. In addition, mutations in glycine-N-methyltransferase (GNMT) could be associated with a higher risk of hepatocellular carcinoma and liver disease due to an unbalanced S-adenosylmethionine (SAM)/S-adenosylhomocysteine (SAH) ratio, which leads to aberrant methylation reactions. Also, genetic variants of chromatin remodeling proteins and histone tail modifiers are involved in genetic disorders like α thalassemia X-linked mental retardation syndrome, CHARGE syndrome, Cockayne syndrome, Rett syndrome, systemic lupus erythematous, Rubinstein-Taybi syndrome, Coffin-Lowry syndrome, Sotos syndrome, and facioescapulohumeral syndrome, among others. Here, we review the potential genetic alterations with a possible role on epigenetic factors and discuss their contribution to human disease.

Inhibition of formyl peptide-stimulated phospholipase D activation by Fal-002-2 via blockade of the Arf6, RhoA and protein kinase C signaling pathways in rat neutrophils

Three recently developed selective phospholipase D (PLD) inhibitors N-(2-(4-(2-oxo-2,3-dihydro-1H-benzo[d]imidazol-1-yl)piperidin-1-yl)ethyl)-2-naphthamide (VU0155056), (S)-N-(1-(4-(5-chloro-2-oxo-2,3-dihydro-1H-benzo[d]imidazol-1-yl)piperidin-1-yl)propan-2-yl)-2-naphthamide (VU0155069), and N-(2-(4-oxo-1-phenyl-1,3,8-triazaspiro[4,5]decan-8-yl)ethyl)quinoline-3-carboxamide (VU0285655-1) inhibited O2 (•-) generation in formyl-Met-Leu-Phe (fMLP)-stimulated rat neutrophils. A novel 2-phenyl-4-quinolone compound 6-chloro-2-(2-chlorophenyl)-4-oxo-1,4-dihydroquinoline-3-carboxylate (Fal-002-2), which inhibited O2 (•-) generation, also reduced the fMLP- but not phorbol ester-stimulated PLD activity (IC50 16.0 ± 5.0 μM). Fal-002-2 attenuated the interaction of PLD1 with ADP-ribosylation factor (Arf) 6, Ras homology (Rho) A and protein kinase C (PKC) isoforms (α, βI, and βII), and also inhibited the membrane recruitment of Arf6 and RhoA in fMLP-stimulated neutrophils, but not in GTPγS-stimulated cell-free system. The cellular levels of GTP-bound Arf6 and GTP-bound RhoA were reduced by Fal-002-2. Fal-002-2 also attenuated the membrane recruitment of Rho-associated protein kinase 1, phosphorylation of myosin light chain 2 at Thr18/Ser19 and PLD1 at Thr147, and the interaction of Arf6 with both arfaptin 1 and phosphatidylinositol 4-phosphate 5-kinase 1A. The association between RhoA and Vav, the interaction of Vav with both Lyn and Lck, the membrane recruitment of Vav, and the phosphorylation of Vav at Tyr174, but not Src family at Tyr416, were all attenuated by Fal-002-2 in fMLP-stimulated neutrophils. These results indicate that Fal-002-2 is not a direct PLD inhibitor, but the inhibition of fMLP-stimulated PLD activity by Fal-002-2, which partly accounts for its suppression of O2 (•-) generation, is attributable to the blockade of both Arf6 and RhoA activation and attenuation of the interaction of Arf6, RhoA and PKC isoforms with PLD1 in rat neutrophils.

Emerging major synaptic signaling pathways involved in intellectual disability

Genetic causes of intellectual disability (ID) include mutations in proteins with various functions. However, many of these proteins are enriched in synapses and recent investigations point out their crucial role in the subtle regulation of synaptic activity and dendritic spine morphogenesis. Moreover, in addition to genetic data, functional and animal model studies are providing compelling evidence that supports the emerging unifying synapse-based theory for cognitive deficit. In this review, we highlight ID-related gene products involved in synaptic morphogenesis and function, with a particular focus on the emergent signaling pathways involved in synaptic plasticity whose disruption results in cognitive deficit.

Genetically encoded probes for phosphatidic acid

In addition to forming bilayers to separate cellular compartments, lipids participate in vesicular trafficking and signal transduction. Among others, phosphatidic acid (PA) is emerging as an important signaling molecule. The spatiotemporal distribution of cellular PA appears to be tightly regulated by localized synthesis and a rapid metabolism. Although PA has been long proposed as a pleiotropic bioactive lipid, when and where PA is produced in the living cells have only recently been explored using biosensors that specifically bind to PA. The probes that we have generated are composed of the PA-binding domains of either Spo20p or Raf1 directly fused to GFP. In this chapter, we will describe the expression and purification of GST-fusion proteins of these probes, and the use of phospholipid strips to validate the specificity of their interaction with PA. We will then illustrate the use of GFP-tagged probes to visualize the synthesis of PA in the neurosecretory PC12 cells and RAW 267.4 macrophages. Interestingly, the two probes show a differential distribution in these cell types, indicating that they may have different affinities for PA or recognize different pools of PA. In conclusion, the development of a broader choice of probes may be required to adequately follow the complex dynamics of PA in different cell types, in order to determine the cellular distribution of PA and its role in various cellular processes.

Role of phosphatidic acid in plant galactolipid synthesis

Phosphatidic acid (PA) is a precursor metabolite for phosphoglycerolipids and also for galactoglycerolipids, which are essential lipids for formation of plant membranes. PA has in addition a main regulatory role in a number of developmental processes notably in the response of the plant to environmental stresses. We review here the different pools of PA dispatched at different locations in the plant cell and how these pools are modified in different growth conditions, particularly during plastid membrane biogenesis and when the plant is exposed to phosphate deprivation. We analyze how these modifications can affect galactolipid synthesis by tuning the activity of MGD1 enzyme allowing a coupling of phospho- and galactolipid metabolisms. Some mechanisms are considered to explain how physicochemical properties of PA allow this lipid to act as a central internal sensor in plant physiology.

Signaling mechanisms of inhibition of phospholipase D activation by CHS-111 in formyl peptide-stimulated neutrophils

A selective phospholipase D (PLD) inhibitor 5-fluoro-2-indolyl des-chlorohalopemide (FIPI) inhibited the O(2)(-) generation and cell migration but not degranulation in formyl-Met-Leu-Phe (fMLP)-stimulated rat neutrophils. A novel benzyl indazole compound 2-benzyl-3-(4-hydroxymethylphenyl)indazole (CHS-111), which inhibited O(2)(-) generation and cell migration, also reduced the fMLP- but not phorbol ester-stimulated PLD activity (IC(50) 3.9±1.2μM). CHS-111 inhibited the interaction of PLD1 with ADP-ribosylation factor (Arf) 6 and Ras homology (Rho) A, and reduced the membrane recruitment of RhoA in fMLP-stimulated cells but not in GTPγS-stimulated cell-free system. CHS-111 reduced the cellular levels of GTP-bound RhoA, membrane recruitment of Rho-associated protein kinase 1 and the downstream myosin light chain 2 phosphorylation, and attenuated the interaction between phosphatidylinositol 4-phosphate 5-kinase (PIP5K) and Arf6, whereas it only slightly inhibited the guanine nucleotide exchange activity of human Dbs (DH/PH) protein and did not affect the arfaptin binding to Arf6. CHS-111 inhibited the interaction of RhoA with Vav, the membrane association and the phosphorylation of Vav. CHS-111 had no effect on the phosphorylation of Src family kinases (SFK) but attenuated the interaction of Vav with Lck, Hck, Fgr and Lyn. CHS-111 also inhibited the interaction of PLD1 with protein kinase C (PKC) α, βI and βII isoenzymes, and the phosphorylation of PLD1. These results indicate that inhibition of fMLP-stimulated PLD activity by CHS-111 is attributable to the blockade of RhoA activation via the interference with SFK-mediated Vav activation, attenuation of the interaction of Arf6 with PLD1 and PIP5K, and the activation of Ca(2+)-dependent PKC in rat neutrophils.

Synthesis of fusogenic lipids through activation of phospholipase D1 by GTPases and the kinase RSK2 is required for calcium-regulated exocytosis in neuroendocrine cells

Exocytosis of hormones occurs through the fusion of large dense-core secretory vesicles with the plasma membrane. This highly regulated process involves key proteins such as SNAREs (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors) and also specific lipids at the site of membrane fusion. Among the different lipids required for exocytosis, our recent observations have highlighted the crucial role of PA (phosphatidic acid) in the late stages of membrane fusion in various exocytotic events. An RNAi (RNA interference) strategy coupled with the detection of PA in living cells has pointed to plasma membrane-associated PLD1 (phospholipase D1) as the main producer of PA in response to secretagogue stimulation. We have identified several GTPases which regulate the activation level of PLD1 in neuroendocrine cells. Finally, RSK2 (ribosomal S6 kinase 2) appears to phosphorylate and regulate the activity of PLD1 in a calcium-dependent manner. Altogether our results have unravelled a complex set of regulatory pathways controlling the synthesis of fusogenic lipids at the secretory granule fusion site by PLD1.