A role for phospholipase D1 in neurotransmitter release

Lipid remodeling in acrosome exocytosis: unraveling key players in the human sperm

It has long been thought that exocytosis was driven exclusively by well-studied fusion proteins. Some decades ago, the role of lipids became evident and escalated interest in the field. Our laboratory chose a particular cell to face this issue: the human sperm. What makes this cell special? Sperm, as terminal cells, are characterized by their scarcity of organelles and the complete absence of transcriptional and translational activities. They are specialized for a singular membrane fusion occurrence: the exocytosis of the acrosome. This unique trait makes them invaluable for the study of exocytosis in isolation. We will discuss the lipids' role in human sperm acrosome exocytosis from various perspectives, with a primary emphasis on our contributions to the field. Sperm cells have a unique lipid composition, very rare and not observed in many cell types, comprising a high content of plasmalogens, long-chain, and very-long-chain polyunsaturated fatty acids that are particular constituents of some sphingolipids. This review endeavors to unravel the impact of membrane lipid composition on the proper functioning of the exocytic pathway in human sperm and how this lipid dynamic influences its fertilizing capability. Evidence from our and other laboratories allowed unveiling the role and importance of multiple lipids that drive exocytosis. This review highlights the role of cholesterol, diacylglycerol, and particular phospholipids like phosphatidic acid, phosphatidylinositol 4,5-bisphosphate, and sphingolipids in driving sperm acrosome exocytosis. Furthermore, we provide a comprehensive overview of the factors and enzymes that regulate lipid turnover during the exocytic course. A more thorough grasp of the role played by lipids transferred from sperm can provide insights into certain causes of male infertility. It may lead to enhancements in diagnosing infertility and techniques like assisted reproductive technology (ART).

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.

The DDHD2-STXBP1 interaction mediates long-term memory via generation of saturated free fatty acids

The phospholipid and free fatty acid (FFA) composition of neuronal membranes plays a crucial role in learning and memory, but the mechanisms through which neuronal activity affects the brain's lipid landscape remain largely unexplored. The levels of saturated FFAs, particularly of myristic acid (C14:0), strongly increase during neuronal stimulation and memory acquisition, suggesting the involvement of phospholipase A1 (PLA1) activity in synaptic plasticity. Here, we show that genetic ablation of the PLA1 isoform DDHD2 in mice dramatically reduces saturated FFA responses to memory acquisition across the brain. Furthermore, DDHD2 loss also decreases memory performance in reward-based learning and spatial memory models prior to the development of neuromuscular deficits that mirror human spastic paraplegia. Via pulldown-mass spectrometry analyses, we find that DDHD2 binds to the key synaptic protein STXBP1. Using STXBP1/2 knockout neurosecretory cells and a haploinsufficient STXBP1+/- mouse model of human early infantile encephalopathy associated with intellectual disability and motor dysfunction, we show that STXBP1 controls targeting of DDHD2 to the plasma membrane and generation of saturated FFAs in the brain. These findings suggest key roles for DDHD2 and STXBP1 in lipid metabolism and in the processes of synaptic plasticity, learning, and memory.

Calcium dysregulation combined with mitochondrial failure and electrophysiological maturity converge in Parkinson's iPSC-dopamine neurons

Parkinson's disease (PD) is characterized by a progressive deterioration of motor and cognitive functions. Although death of dopamine neurons is the hallmark pathology of PD, this is a late-stage disease process preceded by neuronal dysfunction. Here we describe early physiological perturbations in patient-derived induced pluripotent stem cell (iPSC)-dopamine neurons carrying the GBA-N370S mutation, a strong genetic risk factor for PD. GBA-N370S iPSC-dopamine neurons show an early and persistent calcium dysregulation notably at the mitochondria, followed by reduced mitochondrial membrane potential and oxygen consumption rate, indicating mitochondrial failure. With increased neuronal maturity, we observed decreased synaptic function in PD iPSC-dopamine neurons, consistent with the requirement for ATP and calcium to support the increase in electrophysiological activity over time. Our work demonstrates that calcium dyshomeostasis and mitochondrial failure impair the higher electrophysiological activity of mature neurons and may underlie the vulnerability of dopamine neurons in PD.

Phospholipase D1 Attenuation Therapeutics Promotes Resilience against Synaptotoxicity in 12-Month-Old 3xTg-AD Mouse Model of Progressive Neurodegeneration

Abrogating synaptotoxicity in age-related neurodegenerative disorders is an extremely promising area of research with significant neurotherapeutic implications in tauopathies including Alzheimer's disease (AD). Our studies using human clinical samples and mouse models demonstrated that aberrantly elevated phospholipase D1 (PLD1) is associated with amyloid beta (Aβ) and tau-driven synaptic dysfunction and underlying memory deficits. While knocking out the lipolytic PLD1 gene is not detrimental to survival across species, elevated expression is implicated in cancer, cardiovascular conditions and neuropathologies, leading to the successful development of well-tolerated mammalian PLD isoform-specific small molecule inhibitors. Here, we address the importance of PLD1 attenuation, achieved using repeated 1 mg/kg of VU0155069 (VU01) intraperitoneally every alternate day for a month in 3xTg-AD mice beginning only from ~11 months of age (with greater influence of tau-driven insults) compared to age-matched vehicle (0.9% saline)-injected siblings. A multimodal approach involving behavior, electrophysiology and biochemistry corroborate the impact of this pre-clinical therapeutic intervention. VU01 proved efficacious in preventing in later stage AD-like cognitive decline affecting perirhinal cortex-, hippocampal- and amygdala-dependent behaviors. Glutamate-dependent HFS-LTP and LFS-LTD improved. Dendritic spine morphology showed the preservation of mushroom and filamentous spine characteristics. Differential PLD1 immunofluorescence and co-localization with Aβ were noted.

Lipids and Secretory Vesicle Exocytosis

In recent years, the number of studies implicating lipids in the regulation of synaptic vesicle exocytosis has risen considerably. It has become increasingly clear that lipids such as phosphoinositides, lysophospholipids, cholesterol, arachidonic acid and myristic acid play critical regulatory roles in the processes leading up to exocytosis. Lipids may affect membrane fusion reactions by altering the physical properties of the membrane, recruiting key regulatory proteins, concentrating proteins into exocytic "hotspots" or by modulating protein functions allosterically. Discrete changes in phosphoinositides concentration are involved in multiple trafficking events including exocytosis and endocytosis. Lipid-modifying enzymes such as the DDHD2 isoform of phospholipase A1 were recently shown to contribute to memory acquisition via dynamic modifications of the brain lipid landscape. Considering the increasing reports on neurodegenerative disorders associated with aberrant intracellular trafficking, an improved understanding of the control of lipid pathways is physiologically and clinically significant and will afford unique insights into mechanisms and therapeutic methods for neurodegenerative diseases. Consequently, this chapter will discuss the different classes of lipids, phospholipase enzymes, the evidence linking them to synaptic neurotransmitter release and how they act to regulate key steps in the multi-step process leading to neuronal communication and memory acquisition.

Differential expression patterns of phospholipase D isoforms 1 and 2 in the mammalian brain and retina

Phosphatidic acid is a key signaling molecule heavily implicated in exocytosis due to its protein-binding partners and propensity to induce negative membrane curvature. One phosphatidic acid-producing enzyme, phospholipase D (PLD), has also been implicated in neurotransmission. Unfortunately, due to the unreliability of reagents, there has been confusion in the literature regarding the expression of PLD isoforms in the mammalian brain which has hampered our understanding of their functional roles in neurons. To address this, we generated epitope-tagged PLD1 and PLD2 knockin mice using CRISPR/Cas9. Using these mice, we show that PLD1 and PLD2 are both localized at synapses by adulthood, with PLD2 expression being considerably higher in glial cells and PLD1 expression predominating in neurons. Interestingly, we observed that only PLD1 is expressed in the mouse retina, where it is found in the synaptic plexiform layers. These data provide critical information regarding the localization and potential role of PLDs in the central nervous system.

The Normal Human Adult Hypothalamus Proteomic Landscape: Rise of Neuroproteomics in Biological Psychiatry and Systems Biology

The human hypothalamus is central to the regulation of neuroendocrine and neurovegetative systems, as well as modulation of chronobiology and behavioral aspects in human health and disease. Surprisingly, a deep proteomic analysis of the normal human hypothalamic proteome has been missing for such an important organ so far. In this study, we delineated the human hypothalamus proteome using a high-resolution mass spectrometry approach which resulted in the identification of 5349 proteins, while a multiple post-translational modification (PTM) search identified 191 additional proteins, which were missed in the first search. A proteogenomic analysis resulted in the discovery of multiple novel protein-coding regions as we identified proteins from noncoding regions (pseudogenes) and proteins translated from short open reading frames that can be missed using the traditional pipeline of prediction of protein-coding genes as a part of genome annotation. We also identified several PTMs of hypothalamic proteins that may be required for normal hypothalamic functions. Moreover, we observed an enrichment of proteins pertaining to autophagy and adult neurogenesis in the proteome data. We believe that the hypothalamic proteome reported herein would help to decipher the molecular basis for the diverse range of physiological functions attributed to it, as well as its role in neurological and psychiatric diseases. Extensive proteomic profiling of the hypothalamic nuclei would further elaborate on the role and functional characterization of several hypothalamus-specific proteins and pathways to inform future research and clinical discoveries in biological psychiatry, neurology, and system biology.

CtBP1-Mediated Membrane Fission Contributes to Effective Recycling of Synaptic Vesicles

Compensatory endocytosis of released synaptic vesicles (SVs) relies on coordinated signaling at the lipid-protein interface. Here, we address the synaptic function of C-terminal binding protein 1 (CtBP1), a ubiquitous regulator of gene expression and membrane trafficking in cultured hippocampal neurons. In the absence of CtBP1, synapses form in greater density and show changes in SV distribution and size. The increased basal neurotransmission and enhanced synaptic depression could be attributed to a higher vesicular release probability and a smaller fraction of release-competent SVs, respectively. Rescue experiments with specifically targeted constructs indicate that, while synaptogenesis and release probability are controlled by nuclear CtBP1, the efficient recycling of SVs relies on its synaptic expression. The ability of presynaptic CtBP1 to facilitate compensatory endocytosis depends on its membrane-fission activity and the activation of the lipid-metabolizing enzyme PLD1. Thus, CtBP1 regulates SV recycling by promoting a permissive lipid environment for compensatory endocytosis.

Phosphatidic acid: Mono- and poly-unsaturated forms regulate distinct stages of neuroendocrine exocytosis

Lipids have emerged as important actors in an ever-growing number of key functions in cell biology over the last few years. Among them, glycerophospholipids are major constituents of cellular membranes. Because of their amphiphilic nature, phospholipids form lipid bilayers that are particularly useful to isolate cellular content from the extracellular medium, but also to define intracellular compartments. Interestingly, phospholipids come in different flavors based on their fatty acyl chain composition. Indeed, lipidomic analyses have revealed the presence in cellular membranes of up to 50 different species of an individual class of phospholipid, opening the possibility of multiple functions for a single class of phospholipid. In this review we will focus on phosphatidic acid (PA), the simplest phospholipid, that plays both structural and signaling functions. Among the numerous roles that have been attributed to PA, a key regulatory role in secretion has been proposed in different cell models. We review here the evidences that support the idea that mono- and poly-unsaturated PA control distinct steps in hormone secretion from neuroendocrine cells.

Gene Expression Profile at the Motor Endplate of the Neuromuscular Junction of Fast-Twitch Muscle

The neuromuscular junction (NMJ) is a prototypic chemical synapse between the spinal motor neuron and the motor endplate. Gene expression profiles of the motor endplate are not fully elucidated. Collagen Q (ColQ) is a collagenic tail subunit of asymmetric forms of acetylcholinesterase and is driven by two distinct promoters. pColQ1 is active throughout the slow-twitch muscle, whereas pColQ1a is active at the motor endplate of fast-twitch muscle. We made a transgenic mouse line that expresses nuclear localization signal (NLS)-attached Cre recombinase under the control of pColQ1a (pColQ1a-Cre mouse). RiboTag mouse expresses an HA-tagged ribosomal subunit, RPL22, in cells expressing Cre recombinase. We generated pColQ1a-Cre:RiboTag mouse, and confirmed that HA-tagged RPL22 was enriched at the NMJ of tibialis anterior (TA) muscle. Next, we confirmed that Chrne and Musk that are specifically expressed at the NMJ were indeed enriched in HA-immunoprecipitated (IP) RNA, whereas Sox10 and S100b, markers for Schwann cells, and Icam1, a marker for vascular endothelial cells, and Pax3, a marker for muscle satellite cells, were scarcely detected. Gene set enrichment analysis (GSEA) of RNA-seq data showed that “phosphatidylinositol signaling system” and “extracellular matrix receptor interaction” were enriched at the motor endplate. Subsequent analysis revealed that genes encoding diacylglycerol kinases, phosphatidylinositol kinases, phospholipases, integrins, and laminins were enriched at the motor endplate. We first characterized the gene expression profile under translation at the motor endplate of TA muscle using the RiboTag technique. We expect that our gene expression profiling will help elucidate molecular mechanisms of the development, maintenance, and pathology of the NMJ.

Interplay Between SNX27 and DAG Metabolism in the Control of Trafficking and Signaling at the IS

Recognition of antigens displayed on the surface of an antigen-presenting cell (APC) by T-cell receptors (TCR) of a T lymphocyte leads to the formation of a specialized contact between both cells named the immune synapse (IS). This highly organized structure ensures cell–cell communication and sustained T-cell activation. An essential lipid regulating T-cell activation is diacylglycerol (DAG), which accumulates at the cell–cell interface and mediates recruitment and activation of proteins involved in signaling and polarization. Formation of the IS requires rearrangement of the cytoskeleton, translocation of the microtubule-organizing center (MTOC) and vesicular compartments, and reorganization of signaling and adhesion molecules within the cell–cell junction. Among the multiple players involved in this polarized intracellular trafficking, we find sorting nexin 27 (SNX27). This protein translocates to the T cell–APC interface upon TCR activation, and it is suggested to facilitate the transport of cargoes toward this structure. Furthermore, its interaction with diacylglycerol kinase ζ (DGKζ), a negative regulator of DAG, sustains the precise modulation of this lipid and, thus, facilitates IS organization and signaling. Here, we review the role of SNX27, DAG metabolism, and their interplay in the control of T-cell activation and establishment of the IS.

Phospholipases in neuronal function: A role in learning and memory?

Despite the human brain being made of nearly 60% fat, the vast majority of studies on the mechanisms of neuronal communication which underpin cognition, memory and learning, primarily focus on proteins and/or (epi)genetic mechanisms. Phospholipids are the main component of all cellular membranes and function as substrates for numerous phospholipid-modifying enzymes, including phospholipases, which release free fatty acids (FFAs) and other lipid metabolites that can alter the intrinsic properties of the membranes, recruit and activate critical proteins, and act as lipid signalling molecules. Here, we will review brain specific phospholipases, their roles in membrane remodelling, neuronal function, learning and memory, as well as their disease implications. In particular, we will highlight key roles of unsaturated FFAs, particularly arachidonic acid, in neurotransmitter release, neuroinflammation and memory. In light of recent findings, we will also discuss the emerging role of phospholipase A₁ and the creation of saturated FFAs in the brain.

Phosphatidic acid metabolism regulates neuroendocrine secretion but is not under the direct control of lipins

Phosphatidic acid (PA) produced by phospholipase D1 has been shown to contribute to secretory vesicle exocytosis in a large number of cell models. Among various hypotheses, PA may contribute to recruit and/or activate at the exocytotic site a set of proteins from the molecular machinery dedicated to secretion, but also directly influence membrane curvature thereby favoring membrane rearrangements required for membrane fusion. The release of informative molecules by regulated exocytosis is a tightly controlled process. It is thus expected that PA produced to trigger membrane fusion should be rapidly metabolized and converted in a lipid that does not present similar characteristics. PA-phosphatases of the lipin family are possible candidates as they convert PA into diacylglycerol. We show here that lipin 1 and lipin 2 are expressed in neuroendocrine cells where they are cytosolic, but also partially associated with the endoplasmic reticulum. Silencing of lipin 1 or 2 did not affect significantly either basal or evoked secretion from PC12 cells, suggesting that it is unlikely that conversion of PA into a secondary lipid by lipins might represent a regulatory step in exocytosis in neurosecretory cells. However, in agreement with a model in which PA-metabolism could contribute to prevent entering into exocytosis of additional secretory vesicles, ectopic expression of lipin1B-GFP in bovine chromaffin cells reduced the number of exocytotic events as revealed by carbon fiber amperometry recording. Furthermore, individual spike parameters reflecting fusion pore dynamics were also modified by lipin1B-GFP, suggesting that a tight control of PA levels represents an important regulatory step of the number and kinetic of exocytotic events.

Role of Phospholipase D-Derived Phosphatidic Acid in Regulated Exocytosis and Neurological Disease

Lipids play a vital role in numerous cellular functions starting from a structural role as major constituents of membranes to acting as signaling intracellular or extracellular entities. Accordingly, it has been known for decades that lipids, especially those coming from diet, are important to maintain normal physiological functions and good health. On the other side, the exact molecular nature of these beneficial or deleterious lipids, as well as their precise mode of action, is only starting to be unraveled. This recent improvement in our knowledge is largely resulting from novel pharmacological, molecular, cellular, and genetic tools to study lipids in vitro and in vivo. Among these important lipids, phosphatidic acid plays a unique and central role in a great variety of cellular functions. This review will focus on the proposed functions of phosphatidic acid generated by phospholipase D in the last steps of regulated exocytosis with a specific emphasis on hormonal and neurotransmitter release and its potential impact on different neurological diseases.

Mechanism of membrane fusion: protein-protein interaction and beyond

Membrane fusion is a universal event in all living organism. It is at the heart of intracellular organelle biogenesis and membrane traffic processes such as endocytosis and exocytosis, and is also used by enveloped viruses to enter hosting cells. Regarding the cellular mechanisms underlying membrane fusion, pioneering studies by Randy Schekman, James Rothman, Thomas C. Südhof and their colleagues have demonstrated the function of specific proteins and protein-protein interactions as essential fusogenic factor to initiate membrane fusion. Since then, function of lipids and protein-lipid interaction has also been identified as important players in membrane fusion. Based on that NSF (NEM-sensitive factor where NEM stands for N-ethyl-maleimide) and acyl-CoA are required for the membrane fusion of transporting vesicles with Golgi cisternae, it is further suggested that the transfer of the acyl chain to a molecule(s) is essential for membrane fusion. Among the previously identified fusogens, phosphatidic acid (PA) is found as an acyl chain recipient. Functionally, acylation of PA is required for tethering the membranes of Rab5a vesicles and early endosomes together during membrane fusion. As certain threshold of proximity between the donor and acceptor membrane is required to initiate membrane fusion, fusogenic factors beyond protein-protein and protein-lipid interaction need to be identified.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Suppressing aberrant phospholipase D1 signaling in 3xTg Alzheimer's disease mouse model promotes synaptic resilience

Current approaches in treatment of Alzheimer's disease (AD) is focused on early stages of cognitive decline. Identifying therapeutic targets that promote synaptic resilience during early stages may prevent progressive memory deficits by preserving memory mechanisms. We recently reported that the inducible isoform of phospholipase D (PLD1) was significantly increased in synaptosomes from post-mortem AD brains compared to age-matched controls. Using mouse models, we reported that the aberrantly elevated neuronal PLD1 is key for oligomeric amyloid driven synaptic dysfunction and underlying memory deficits. Here, we demonstrate that chronic inhibition using a well-tolerated PLD1 specific small molecule inhibitor is sufficient to prevent the progression of synaptic dysfunction during early stages in the 3xTg-AD mouse model. Firstly, we report prevention of cognitive decline in the inhibitor-treated group using novel object recognition (NOR) and fear conditioning (FC). Secondly, we provide electrophysiological assessment of better synaptic function in the inhibitor-treated group. Lastly, using Golgi staining, we report that preservation of dendritic spine integrity as one of the mechanisms underlying the action of the small molecule inhibitor. Collectively, these studies provide evidence for inhibition of PLD1 as a potential therapeutic strategy in preventing progression of cognitive decline associated with AD and related dementia.

Phosphatidic acid in membrane rearrangements

Phosphatidic acid (PA) is the simplest cellular glycerophospholipid characterized by unique biophysical properties: a small headgroup; negative charge; and a phosphomonoester group. Upon interaction with lysine or arginine, PA charge increases from -1 to -2 and this change stabilizes protein-lipid interactions. The biochemical properties of PA also allow interactions with lipids in several subcellular compartments. Based on this feature, PA is involved in the regulation and amplification of many cellular signalling pathways and functions, as well as in membrane rearrangements. Thereby, PA can influence membrane fusion and fission through four main mechanisms: it is a substrate for enzymes producing lipids (lysophosphatidic acid and diacylglycerol) that are involved in fission or fusion; it contributes to membrane rearrangements by generating negative membrane curvature; it interacts with proteins required for membrane fusion and fission; and it activates enzymes whose products are involved in membrane rearrangements. Here, we discuss the biophysical properties of PA in the context of the above four roles of PA in membrane fusion and fission.

Regulation of Membrane Turnover by Phosphatidic Acid: Cellular Functions and Disease Implications

Phosphatidic acid (PA) is a simple glycerophospholipid with a well-established role as an intermediate in phospholipid biosynthesis. In addition to its role in lipid biosynthesis, PA has been proposed to act as a signaling molecule that modulates several aspects of cell biology including membrane transport. PA can be generated in eukaryotic cells by several enzymes whose activity is regulated in the context of signal transduction and enzymes that can metabolize PA thus terminating its signaling activity have also been described. Further, several studies have identified PA binding proteins and changes in their activity are proposed to be mediators of the signaling activity of this lipid. Together these enzymes and proteins constitute a PA signaling toolkit that mediates the signaling functions of PA in cells. Recently, a number of novel genetic models for the analysis of PA function in vivo and analytical methods to quantify PA levels in cells have been developed and promise to enhance our understanding of PA functions. Studies of several elements of the PA signaling toolkit in a single cell type have been performed and are presented to provide a perspective on our understanding of the biochemical and functional organization of pools of PA in a eukaryotic cell. Finally, we also provide a perspective on the potential role of PA in human disease, synthesizing studies from model organisms, human disease genetics and analysis using recently developed PLD inhibitors.

Phosphatidic Acid: From Pleiotropic Functions to Neuronal Pathology

Among the cellular lipids, phosphatidic acid (PA) is a peculiar one as it is at the same time a key building block of phospholipid synthesis and a major lipid second messenger conveying signaling information. The latter is thought to largely occur through the ability of PA to recruit and/or activate specific proteins in restricted compartments and within those only at defined submembrane areas. Furthermore, with its cone-shaped geometry PA locally changes membrane topology and may thus be a key player in membrane trafficking events, especially in membrane fusion and fission steps, where lipid remodeling is believed to be crucial. These pleiotropic cellular functions of PA, including phospholipid synthesis and homeostasis together with important signaling activity, imply that perturbations of PA metabolism could lead to serious pathological conditions. In this mini-review article, after outlining the main cellular functions of PA, we highlight the different neurological diseases that could, at least in part, be attributed to an alteration in PA synthesis and/or catabolism.

Synaptic dysfunction in amygdala in intellectual disorder models

The amygdala is a part of the limbic circuit that has been extensively studied in terms of synaptic connectivity, plasticity and cellular organization since decades (Ehrlich et al., 2009; Ledoux, 2000; Maren, 2001). Amygdala sub-nuclei, including lateral, basolateral and central amygdala appear now as "hubs" providing in parallel and in series neuronal processing enabling the animal to elicit freezing or escaping behavior in response to external threats. In rodents, these behaviors are easily observed and quantified following associative fear conditioning. Thus, studies on amygdala circuit in association with threat/fear behavior became very popular in laboratories and are often used among other behavioral tests to evaluate learning abilities of mouse models for various neuropsychiatric conditions including genetically encoded intellectual disabilities (ID). Yet, more than 100 human X-linked genes - and several hundreds of autosomal genes - have been associated with ID in humans. These mutations introduced in mice can generate social deficits, anxiety dysregulations and fear learning impairments (McNaughton et al., 2008; Houbaert et al., 2013; Jayachandran et al., 2014; Zhang et al., 2015). Noteworthy, a significant proportion of the coded ID gene products are synaptic proteins. It is postulated that the loss of function of these proteins could destabilize neuronal circuits by global changes of the balance between inhibitory and excitatory drives onto neurons. However, whereas amygdala related behavioral deficits are commonly observed in ID models, the role of most of these ID-genes in synaptic function and plasticity in the amygdala are only sparsely studied. We will here discuss some of the concepts that emerged from amygdala-targeted studies examining the role of syndromic and non-syndromic ID genes in fear-related behaviors and/or synaptic function. Along describing these cases, we will discuss how synaptic deficits observed in amygdala circuits could impact memory formation and expression of conditioned fear.

Elevated phospholipase D isoform 1 in Alzheimer's disease patients' hippocampus: Relevance to synaptic dysfunction and memory deficits

Introduction

Phospholipase D (PLD), a lipolytic enzyme that breaks down membrane phospholipids, is also involved in signaling mechanisms downstream of seven transmembrane receptors. Abnormally elevated levels of PLD activity are well-established in Alzheimer's disease (AD), implicating the two isoforms of mammalian phosphatidylcholine cleaving PLD (PC-PLD1 and PC-PLD2). Therefore, we took a systematic approach of investigating isoform-specific expression in human synaptosomes and further investigated the possibility of therapeutic intervention using preclinical studies.

Methods

Synaptosomal Western blot analyses on the postmortem human hippocampus, temporal cortex, and frontal cortex of AD patient brains/age-matched controls and the 3XTg-AD mice hippocampus (mouse model with overexpression of human amyloid precursor protein, presenilin-1 gene, and microtubule-associated protein tau causing neuropathology progressing comparable to that in human AD patients) were used to detect the levels of neuronal PLD1 expression. Mouse hippocampal long-term potentiation of PLD1-dependent changes was studied using pharmacological approaches in ex vivo slice preparations from wild-type and transgenic mouse models. Finally, PLD1-dependent changes in novel object recognition memory were assessed following PLD1 inhibition.

Results

We observed elevated synaptosomal PLD1 in the hippocampus/temporal cortex from postmortem tissues of AD patients compared to age-matched controls and age-dependent hippocampal PLD1 increases in 3XTg-AD mice. PLD1 inhibition blocked effects of oligomeric amyloid β or toxic oligomeric tau species on high-frequency stimulation long-term potentiation and novel object recognition deficits in wild-type mice. Finally, PLD1 inhibition blocked long-term potentiation deficits normally observed in aging 3XTg-AD mice.

Discussion

Using human studies, we propose a novel role for PLD1-dependent signaling as a critical mechanism underlying oligomer-driven synaptic dysfunction and consequent memory disruption in AD. We, further, provide the first set of preclinical studies toward future therapeutics targeting PLD1 in slowing down/stopping the progression of AD-related memory deficits as a complementary approach to immunoscavenging clinical trials that are currently in progress.

The Dual Function of the Polybasic Juxtamembrane Region of Syntaxin 1A in Clamping Spontaneous Release and Stimulating Ca2+-Triggered Release in Neuroendocrine Cells

The exact function of the polybasic juxtamembrane region (5RK) of the plasma membrane neuronal SNARE, syntaxin 1A (Syx), in vesicle exocytosis, although widely studied, is currently not clear. Here, we addressed the role of 5RK in Ca²⁺-triggered release, using our Syx-based intramolecular fluorescence resonance energy transfer (FRET) probe, which previously allowed us to resolve a depolarization-induced Ca²⁺-dependent close-to-open transition (CDO) of Syx that occurs concomitant with evoked release, both in PC12 cells and hippocampal neurons and was abolished upon charge neutralization of 5RK. First, using dynamic FRET analysis in PC12 cells, we show that CDO occurs following assembly of SNARE complexes that include the vesicular SNARE, synaptobrevin 2, and that the participation of 5RK in CDO goes beyond its participation in the final zippering of the complex, because mutations of residues adjacent to 5RK, believed to be crucial for final zippering, do not abolish this transition. In addition, we show that CDO is contingent on membrane phosphatidylinositol 4,5-bisphosphate (PIP2), which is fundamental for maintaining regulated exocytosis, as depletion of membranal PIP2 abolishes CDO. Prompted by these results, which underscore a potentially significant role of 5RK in exocytosis, we next amperometrically analyzed catecholamine release from PC12 cells, revealing that charge neutralization of 5RK promotes spontaneous and inhibits Ca²⁺-triggered release events. Namely, 5RK acts as a fusion clamp, making release dependent on stimulation by Ca²⁺SIGNIFICANCE STATEMENT Syntaxin 1A (Syx) is a central protein component of the SNARE complex, which underlies neurotransmitter release. Although widely studied in relation to its participation in SNARE complex formation and its interaction with phosphoinositides, the function of Syx's polybasic juxtamembrane region (5RK) remains unclear. Previously, we showed that a conformational transition of Syx, related to calcium-triggered release, reported by a Syx-based FRET probe, is abolished upon charge neutralization of 5RK (5RK/A). Here we show that this conformational transition is dependent on phosphatidylinositol 4,5-bisphosphate (PIP2) and is related to SNARE complex formation. Subsequently, we show that the 5RK/A mutation enhances spontaneous release and inhibits calcium-triggered release in neuroendocrine cells, indicating a previously unrecognized role of 5RK in neurotransmitter release.

Phosphatidic acid-producing enzymes regulating the synaptic vesicle cycle: Role for PLD?

In cortical and hippocampal neurons of the mammalian brain, the synaptic vesicle cycle is a series of steps that tightly regulate exo- and endocytosis of vesicles. Many proteins contribute to this regulation, but lipids have recently emerged as critical regulators as well. Of all the many lipid signaling molecules, phosphatidic acid is important to the physical processes of membrane fusion. Therefore, the lipid-metabolizing enzymes that produce phosphatidic acid are vital to the regulation of the cycle. Our lab is particularly interested in the potential regulatory mechanisms and neuronal roles of two phosphatidic acid-producing enzymes: diacylglycerol kinase theta (DGKθ) and phospholipase D (PLD). We recently discovered a regulatory role of DGKθ on evoked endocytosis (Goldschmidt et al., 2016). In addition to this enzyme, studies implicate PLD1 in neurotransmission, although its precise role is of some debate. Altogether, the production of phosphatidic acid by these enzymes offer an interesting and novel pathway for the regulation of the synaptic vesicle cycle.

PLD1 promotes dendritic spine development by inhibiting ADAM10-mediated N-cadherin cleavage

Synapses are the basic units of information transmission, processing and integration in the nervous system. Dysfunction of the synaptic development has been recognized as one of the main reasons for mental dementia and psychiatric diseases such as Alzheimer’s disease and autism. However, the underlying mechanisms of the synapse formation are far from clear. Here we report that phospholipase D1 (PLD1) promotes the development of dendritic spines in hippocampal neurons. We found that overexpressing PLD1 increases both the density and the area of dendritic spines. On the contrary, loss of function of PLD1, including overexpression of the catalytically-inactive PLD1 (PLD1ci) or knocking down PLD1 by siRNAs, leads to reduction in the spine density and the spine area. Moreover, we found that PLD1 promotes the dendritic spine development via regulating the membrane level of N-cadherin. Further studies showed that the regulation of surface N-cadherin by PLD1 is related with the cleavage of N-cadherin by a member of the disintegrin and metalloprotease family-ADAM10. Taking together, our results indicate a positive role of PLD1 in synaptogenesis by inhibiting the ADAM10 mediated N-cadherin cleavage and provide new therapeutic clues for some neurological diseases.

Phosphatidic acid and neurotransmission

Lipids play a vital role in the health and functioning of neurons and interest in the physiological role of neuronal lipids is certainly increasing. One neuronal function in which neuronal lipids appears to play key roles in neurotransmission. Our understanding of the role of lipids in the synaptic vesicle cycle and neurotransmitter release is becoming increasingly more important. Much of the initial research in this area has highlighted the major roles played by the phosphoinositides (PtdIns), diacylglycerol (DAG), and phosphatidic acid (PtdOH). Of these, PtdOH has not received as much attention as the other lipids although its role and metabolism appears to be extremely important. This lipid has been shown to play a role in modulating both exocytosis and endocytosis although its precise role in either process is not well defined. The currently evidence suggest this lipid likely participates in key processes by altering membrane architecture necessary for membrane fusion, mediating the penetration of membrane proteins, serving as a precursor for other important SV cycling lipids, or activating essential enzymes. In this review, we address the sources of PtdOH, the enzymes involved in its production, the regulation of these enzymes, and its potential roles in neurotransmission in the central nervous system.

A central role for phosphatidic acid as a lipid mediator of regulated exocytosis in apicomplexa

Lipids are commonly known for the structural roles they play, however, the specific contribution of different lipid classes to wide-ranging signalling pathways is progressively being unravelled. Signalling lipids and their associated effector proteins are emerging as significant contributors to a vast array of effector functions within cells, including essential processes such as membrane fusion and vesicle exocytosis. Many phospholipids have signalling capacity, however, this review will focus on phosphatidic acid (PA) and the enzymes implicated in its production from diacylglycerol (DAG) and phosphatidylcholine (PC): DGK and PLD respectively. PA is a negatively charged, cone-shaped lipid identified as a key mediator in specific membrane fusion and vesicle exocytosis events in a variety of mammalian cells, and has recently been implicated in specialised secretory organelle exocytosis in apicomplexan parasites. This review summarises the recent work implicating a role for PA regulation in exocytosis in various cell types. We will discuss how these signalling events are linked to pathogenesis in the phylum Apicomplexa.

Amygdala-Hippocampal Phospholipase D (PLD) Signaling As Novel Mechanism of Cocaine-Environment Maladaptive Conditioned Responses

BACKGROUND

Drug-environment associative memory mechanisms and the resulting conditioned behaviors are key contributors in relapse to cocaine dependence. Recently, we reported rat amygdala phospholipase D as a key convergent downstream signaling partner in the expression of cocaine-conditioned behaviors mediated by glutamatergic and dopaminergic pathways. In the present study, 1 of the 2 known upstream serotonergic targets of phospholipase D, the serotonin (5-hydroxytryptamine) 2C receptor, was investigated for its role in recruiting phospholipase D signaling in cocaine-conditioned behaviors altered in the rat amygdala and dorsal hippocampus.

METHODS

Using Western-blot analysis, amygdala phospholipase D phosphorylation and total expression of phospholipase D/5-hydroxytryptamine 2C receptor were observed in early (Day-1) and late (Day-14) withdrawal (cocaine-free) states among male Sprague-Dawley rats subjected to 7-day cocaine-conditioned hyperactivity training. Functional studies were conducted using Chinese Hamster Ovary cells with stably transfected human unedited isoform of 5-hydroxytryptamine 2C receptor.

RESULTS

Phosphorylation of phospholipase D isoforms was altered in the Day-1 group of cocaine-conditioned animals, while increased amygdala and decreased dorsal hippocampus phospholipase D/5-hydroxytryptamine 2C receptor protein expression were observed in the Day-14 cocaine-conditioned rats. Functional cellular studies established that increased p phospholipase D is a mechanistic response to 5-HT2CR activation and provided the first evidence of a biased agonism by specific 5-hydroxytryptamine 2C receptor agonist, WAY163909 in phospholipase D phosphorylation 2, but not phospholipase D phosphorylation 1 activation.

CONCLUSIONS

Phospholipase D signaling, activated by dopaminergic, glutamatergic, and serotonergic signaling, can be a common downstream element recruited in associative memory mechanisms altered by cocaine, where increased expression in amygdala and decreased expression in dorsal hippocampus may result in altered anxiety states and increased locomotor responses, respectively.

Astrocyte-derived phosphatidic acid promotes dendritic branching

Astrocytes play critical roles in neural circuit formation and function. Recent studies have revealed several secreted and contact-mediated signals from astrocytes which are essential for neurite outgrowth and synapse formation. However, the mechanisms underlying the regulation of dendritic branching by astrocytes remain elusive. Phospholipase D1 (PLD1), which catalyzes the hydrolysis of phosphatidylcholine (PC) to generate phosphatidic acid (PA) and choline, has been implicated in the regulation of neurite outgrowth. Here we showed that knockdown of PLD1 selectively in astrocytes reduced dendritic branching of neurons in neuron-glia mixed culture. Further studies from sandwich-like cocultures and astrocyte conditioned medium suggested that astrocyte PLD1 regulated dendritic branching through secreted signals. We later demonstrated that PA was the key mediator for astrocyte PLD1 to regulate dendritic branching. Moreover, PA itself was sufficient to promote dendritic branching of neurons. Lastly, we showed that PA could activate protein kinase A (PKA) in neurons and promote dendritic branching through PKA signaling. Taken together, our results demonstrate that astrocyte PLD1 and its lipid product PA are essential regulators of dendritic branching in neurons. These results may provide new insight into mechanisms underlying how astrocytes regulate dendrite growth of neurons.

Fear potentiated startle increases phospholipase D (PLD) expression/activity and PLD-linked metabotropic glutamate receptor mediated post-tetanic potentiation in rat amygdala

Long-term memory (LTM) of fear stores activity dependent modifications that include changes in amygdala signaling. Previously, we identified an enhanced probability of release of glutamate mediated signaling to be important in rat fear potentiated startle (FPS), a well-established translational behavioral measure of fear. Here, we investigated short- and long-term synaptic plasticity in FPS involving metabotropic glutamate receptors (mGluRs) and associated downstream proteomic changes in the thalamic-lateral amygdala pathway (Th-LA). Aldolase A, an inhibitor of phospholipase D (PLD), expression was reduced, concurrent with significantly elevated PLD protein expression. Blocking the PLD-mGluR signaling significantly reduced PLD activity. While transmitter release probability increased in FPS, PLD-mGluR agonist and antagonist actions were occluded. In the unpaired group (UNP), blocking the PLD-mGluR increased while activating the receptor decreased transmitter release probability, consistent with decreased synaptic potentials during tetanic stimulation. FPS Post-tetanic potentiation (PTP) immediately following long-term potentiation (LTP) induction was significantly increased. Blocking PLD-mGluR signaling prevented PTP and reduced cumulative PTP probability but not LTP maintenance in both groups. These effects are similar to those mediated through mGluR7, which is co-immunoprecipitated with PLD in FPS. Lastly, blocking mGluR-PLD in the rat amygdala was sufficient to prevent behavioral expression of fear memory. Thus, our study in the Th-LA pathway provides the first evidence for PLD as an important target of mGluR signaling in amygdala fear-associated memory. Importantly, the PLD-mGluR provides a novel therapeutic target for treating maladaptive fear memories in posttraumatic stress and anxiety disorders.

The Role of Phospholipase D in Regulated Exocytosis

There are a diversity of interpretations concerning the possible roles of phospholipase D and its biologically active product phosphatidic acid in the late, Ca(2+)-triggered steps of regulated exocytosis. To quantitatively address functional and molecular aspects of the involvement of phospholipase D-derived phosphatidic acid in regulated exocytosis, we used an array of phospholipase D inhibitors for ex vivo and in vitro treatments of sea urchin eggs and isolated cortices and cortical vesicles, respectively, to study late steps of exocytosis, including docking/priming and fusion. The experiments with fluorescent phosphatidylcholine reveal a low level of phospholipase D activity associated with cortical vesicles but a significantly higher activity on the plasma membrane. The effects of phospholipase D activity and its product phosphatidic acid on the Ca(2+) sensitivity and rate of fusion correlate with modulatory upstream roles in docking and priming rather than to direct effects on fusion per se.

KV 10.1 opposes activity-dependent increase in Ca²⁺ influx into the presynaptic terminal of the parallel fibre-Purkinje cell synapse

KEY POINTS

Voltage-gated KV 10.1 potassium channels are widely expressed in the mammalian brain but their function remains poorly understood. We report that KV 10.1 is enriched in the presynaptic terminals and does not take part in somatic action potentials. In parallel fibre synapses in the cerebellar cortex, we find that KV 10.1 regulates Ca(2+) influx and neurotransmitter release during repetitive high-frequency activity. Our results describe the physiological role of mammalian KV 10.1 for the first time and help understand the fine-tuning of synaptic transmission. The voltage-gated potassium channel KV 10.1 (Eag1) is widely expressed in the mammalian brain, but its physiological function is not yet understood. Previous studies revealed highest expression levels in hippocampus and cerebellum and suggested a synaptic localization of the channel. The distinct activation kinetics of KV 10.1 indicate a role during repetitive activity of the cell. Here, we confirm the synaptic localization of KV 10.1 both biochemically and functionally and that the channel is sufficiently fast at physiological temperature to take part in repolarization of the action potential (AP). We studied the role of the channel in cerebellar physiology using patch clamp and two-photon Ca(2+) imaging in KV 10.1-deficient and wild-type mice. The excitability and action potential waveform recorded at granule cell somata was unchanged, while Ca(2+) influx into axonal boutons was enhanced in mutants in response to stimulation with three APs, but not after a single AP. Furthermore, mutants exhibited a frequency-dependent increase in facilitation at the parallel fibre-Purkinje cell synapse at high firing rates. We propose that KV 10.1 acts as a modulator of local AP shape specifically during high-frequency burst firing when other potassium channels suffer cumulative inactivation.

Diacylglycerol, phosphatidic acid, and their metabolic enzymes in synaptic vesicle recycling

The synaptic vesicle (SV) cycle includes exocytosis of vesicles loaded with a neurotransmitter such as glutamate, coordinated recovery of SVs by endocytosis, refilling of vesicles, and subsequent release of the refilled vesicles from the presynaptic bouton. SV exocytosis is tightly linked with endocytosis, and variations in the number of vesicles, and/or defects in the refilling of SVs, will affect the amount of neurotransmitter available for release (Sudhof, 2004). There is increasing interest in the roles synaptic vesicle lipids and lipid metabolizing enzymes play in this recycling. Initial emphasis was placed on the role of polyphosphoinositides in SV cycling as outlined in a number of reviews (Lim and Wenk, 2009; Martin, 2012; Puchkov and Haucke, 2013; Rohrbough and Broadie, 2005). Other lipids are now recognized to also play critical roles. For example, PLD1 (Humeau et al., 2001; Rohrbough and Broadie, 2005) and some DGKs (Miller et al., 1999; Nurrish et al., 1999) play roles in neurotransmission which is consistent with the critical roles for phosphatidic acid (PtdOH) and diacylglycerol (DAG) in the regulation of SV exo/endocytosis (Cremona et al., 1999; Exton, 1994; Huttner and Schmidt, 2000; Lim and Wenk, 2009; Puchkov and Haucke, 2013; Rohrbough and Broadie, 2005). PLD generates phosphatidic acid by catalyzing the hydrolysis of phosphatidylcholine (PtdCho) and in some systems this PtdOH is de-phosphorylated to generate DAG. In contrast, DGK catalyzes the phosphorylation of DAG thereby converting it into PtdOH. While both enzymes are poised to regulate the levels of DAG and PtdOH, therefore, they both lead to the generation of PtdOH and could have opposite effects on DAG levels. This is particularly important for SV cycling as PtdOH and DAG are both needed for evoked exocytosis (Lim and Wenk, 2009; Puchkov and Haucke, 2013; Rohrbough and Broadie, 2005). Two lipids and their involved metabolic enzymes, two sphingolipids have also been implicated in exocytosis: sphingosine (Camoletto et al., 2009; Chan et al., 2012; Chan and Sieburth, 2012; Darios et al., 2009; Kanno et al., 2010; Rohrbough et al., 2004) and sphingosine-1-phosphate (Chan, Hu, 2012; Chan and Sieburth, 2012; Kanno et al., 2010). Finally a number of reports have focused on the somewhat less well studies roles of sphingolipids and cholesterol in SV cycling. In this report, we review the recent understanding of the roles PLDs, DGKs, and DAG lipases, as well as sphingolipids and cholesterol play in synaptic vesicle cycling.

Rho GTPases, phosphoinositides, and actin: a tripartite framework for efficient vesicular trafficking

Rho GTPases are well known regulators of the actin cytoskeleton that act by binding and activating actin nucleators. They are therefore involved in many actin-based processes, including cell migration, cell polarity, and membrane trafficking. With the identification of phosphoinositide kinases and phosphatases as potential binding partners or effectors, Rho GTPases also appear to participate in the regulation of phosphoinositide metabolism. Since both actin dynamics and phosphoinositide turnover affect the efficiency and the fidelity of vesicle transport between cell compartments, Rho GTPases have emerged as critical players in membrane trafficking. Rho GTPase activity, actin remodeling, and phosphoinositide metabolism need to be coordinated in both space and time to ensure the progression of vesicles along membrane trafficking pathways. Although most molecular pathways are still unclear, in this review, we will highlight recent advances made in our understanding of how Rho-dependent signaling pathways organize actin dynamics and phosphoinositides and how phosphoinositides potentially provide negative feedback to Rho GTPases during endocytosis, exocytosis and membrane exchange between intracellular compartments.

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 function of α-synuclein

Human genetics has indicated a causal role for the protein α-synuclein in the pathogenesis of familial Parkinson's disease (PD), and the aggregation of synuclein in essentially all patients with PD suggests a central role for this protein in the sporadic disorder. Indeed, the accumulation of misfolded α-synuclein now defines multiple forms of neural degeneration. Like many of the proteins that accumulate in other neurodegenerative disorders, however, the normal function of synuclein remains poorly understood. In this article, we review the role of synuclein at the nerve terminal and in membrane remodeling. We also consider the prion-like propagation of misfolded synuclein as a mechanism for the spread of degeneration through the neuraxis.

Phospholipase D-mediated hypersensitivity at central synapses is associated with abnormal behaviours and pain sensitivity in rats exposed to prenatal stress

Adverse events at critical stages of development can lead to lasting dysfunction in the central nervous system (CNS). To seek potential underlying changes in synaptic function, we used a newly developed protocol to measure alterations in receptor-mediated Ca(2+) fluorescence responses of synaptoneurosomes, freshly isolated from selected regions of the CNS concerned with emotionality and pain processing. We compared adult male controls and offspring of rats exposed to social stress in late pregnancy (prenatal stress, PS), which showed programmed behavioural changes indicating anxiety, anhedonia and pain hypersensitivity. We found corresponding increases, in PS rats compared with normal controls, in responsiveness of synaptoneurosomes from frontal cortex to a glutamate receptor (GluR) agonist, and from spinal cord to activators of nociceptive afferents. Through a combined pharmacological and biochemical strategy, we found evidence for a role of phospholipase D1 (PLD1)-mediated signalling, that may involve 5-HT2A receptor (5-HT2AR) activation, at both levels of the nervous system. These changes might participate in underpinning the enduring alterations in behaviour induced by PS.

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.

Molecular mechanisms of fMLP-induced superoxide generation and degranulation in mouse neutrophils

In this manuscript, involvement of PLD in fMLP-induced superoxide generation and degranulation were re-investigated using PLD(-/-) neutrophils, and the molecular mechanisms of these neutrophil functions were examined. Neither PLD1 nor PLD2 is involved in these fMLP-induced neutrophil functions. The results obtained in this study provide evidence that cPKC plays an important role in fMLP-induced superoxide generation. On the other hand, Ca(2+)-dependent signaling pathway and cPKC seem to be involved in degranulation.

Molecular mechanisms of N-formyl-methionyl-leucyl-phenylalanine-induced superoxide generation and degranulation in mouse neutrophils: phospholipase D is dispensable

Phospholipase D (PLD), which produces the lipid messenger phosphatidic acid (PA), has been implicated in superoxide generation and degranulation in neutrophils. The basis for this conclusion is the observation that primary alcohols, which interfere with PLD-catalyzed PA production, inhibit these neutrophil functions. However, off-target effects of primary alcohols cannot be totally excluded. Here, we generated PLD(-/-) mice in order to reevaluate the involvement of PLD in and investigate the molecular mechanisms of these neutrophil functions. Surprisingly, N-formyl-methionyl-leucyl-phenylalanine (fMLP) induced these functions in PLD(-/-) neutrophils, and these functions were suppressed by ethanol. These results indicate that PLD is dispensable for these neutrophil functions and that ethanol nonspecifically inhibits them, warning against the use of primary alcohols as specific inhibitors of PLD-mediated PA formation. The calcium ionophore ionomycin and the membrane-permeative compound 1-oleoyl-2-acetyl-sn-glycerol (OADG) synergistically induced superoxide generation. On the other hand, ionomycin alone induced degranulation, which was further augmented by OADG. These results demonstrate that conventional protein kinase C (cPKC) is crucial for superoxide generation, and a Ca(2+)-dependent signaling pathway(s) and cPKC are involved in degranulation in mouse neutrophils.

PLD1 Negatively Regulates Dendritic Branching

Neurons have characteristic dendritic arborization patterns that contribute to information processing. One essential component of dendritic arborization is the formation of a specific number of branches. Although intracellular pathways promoting dendritic growth and branching are being elucidated, the mechanisms that negatively regulate the branching of dendrites remain enigmatic. In this study, using gain-of-function and loss-of-function studies, we show that phospholipase D1 (PLD1) acts as a negative regulator of dendritic branching in cultured hippocampal neurons from embryonic day 18 rat embryos. Overexpression of wild-type PLD1 (WT-PLD1) decreases the complexity of dendrites, whereas knockdown or inhibition of PLD1 increases dendritic branching. We further demonstrated that PLD1 acts downstream of RhoA, one of the small Rho GTPases, to suppress dendritic branching. The restriction of dendritic branching by constitutively active RhoA (V14-RhoA) can be partially rescued by knockdown of PLD1. Moreover, the inhibition of dendritic branching by V14-RhoA and WT-PLD1 can be partially ameliorated by reducing the level of phosphatidic acid (PA), which is the enzymatic product of PLD1. Together, these results suggest that RhoA-PLD1-PA may represent a novel signaling pathway in the restriction of dendritic branching and may thus provide insight into the mechanisms of dendritic morphogenesis.

Diacylglycerol stimulates acrosomal exocytosis by feeding into a PKC- and PLD1-dependent positive loop that continuously supplies phosphatidylinositol 4,5-bisphosphate

Acrosomal exocytosis involves a massive fusion between the outer acrosomal and the plasma membranes of the spermatozoon triggered by stimuli that open calcium channels at the plasma membrane. Diacylglycerol has been implicated in the activation of these calcium channels. Here we report that this lipid promotes the efflux of intraacrosomal calcium and triggers exocytosis in permeabilized human sperm, implying that diacylglycerol activates events downstream of the opening of plasma membrane channels. Furthermore, we show that calcium and diacylglycerol converge in a signaling pathway leading to the production of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Addition of diacylglycerol promotes the PKC-dependent activation of PLD1. Rescue experiments adding phosphatidic acid or PIP(2) and direct measurement of lipid production suggest that both PKC and PLD1 promote PIP(2) synthesis. Inhibition of different steps of the pathway was reverted by adenophostin, an agonist of IP(3)-sensitive calcium channels, indicating that PIP(2) is necessary to keep these channels opened. However, phosphatidic acid, PIP(2), or adenophostin could not trigger exocytosis by themselves, indicating that diacylglycerol must also activate another factor. We found that diacylglycerol and phorbol ester stimulate the accumulation of the GTP-bound form of Rab3A. Together our results indicate that diacylglycerol promotes acrosomal exocytosis by i) maintaining high levels of IP(3) - an effect that depends on a positive feedback loop leading to the production of PIP(2) - and ii) stimulating the activation of Rab3A, which in turn initiates a cascade of protein interactions leading to the assembly of SNARE complexes and membrane fusion.

Adaptation of granule cell to Purkinje cell synapses to high-frequency transmission

The mossy fiber (MF)-granule cell (GC) pathway conveys multiple modalities of information to the cerebellar cortex, converging on Purkinje cells (PC), the sole output of the cerebellar cortex. Recent in vivo experiments have shown that activity in GCs varies from tonic firing at a few hertz to phasic bursts >500 Hz. However, the responses of parallel fiber (PF)-PC synapses to this wide range of input frequencies are unknown, and there is controversy regarding several frequency-related parameters of transmission at this synapse. We performed recordings of unitary synapses and combined variance-mean analysis with a carefully adapted extracellular stimulation method in young and adult rats. We show that, although the probability of release at individual sites is low at physiological calcium concentration, PF-PC synapses release one or more vesicles with a probability of 0.44 at 1.5 mm [Ca(2+)](e). Paired-pulse facilitation was observed over a wide range of frequencies; it renders burst inputs particularly effective and reproducible. These properties are primarily independent of synaptic weight and age. Furthermore, we show that the PF-PC synapse is able to sustain transmission at very high frequencies for tens of stimuli, as a result of accelerated vesicle replenishment and an apparent recruitment of release site vesicles, which appears to be a central mechanism of paired-pulse facilitation at this synapse. These properties ensure that PF-PC synapses possess a dynamic range enabling the temporal code of MF inputs to be transmitted reliably to the PC.