Phosphoinositides as key regulators of synaptic function

An overview of the synaptic vesicle lipid composition

Chemical neurotransmission is the major mechanism of neuronal communication. Neurotransmitters are released from secretory organelles, the synaptic vesicles (SVs) via exocytosis into the synaptic cleft. Fusion of SVs with the presynaptic plasma membrane is balanced by endocytosis, thus maintaining the presynaptic membrane at steady-state levels. The protein machineries responsible for exo- and endocytosis have been extensively investigated. In contrast, less is known about the role of lipids in synaptic transmission and how the lipid composition of SVs is affected by dynamic exo-endocytotic cycling. Here we summarize the current knowledge about the composition, organization, and function of SV membrane lipids. We also cover lipid biogenesis and maintenance during the synaptic vesicle cycle.

Reorganization of Ternary Lipid Mixtures of Nonphosphorylated Phosphatidylinositol Interacting with Angiomotin

Phosphatidylinositol (PI) lipids are necessary for many cellular signaling pathways of membrane associated proteins, such as angiomotin (Amot). The Amot family regulates cellular polarity, growth, and migration. Given the low concentration of PI lipids in these membranes, it is likely that such protein-membrane interactions are stabilized by lipid domains or small lipid clusters. By small-angle X-ray scattering, we show that nonphosphorylated PI lipids induce lipid demixing in ternary mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE), likely because of preferential interactions between the head groups of PE and PI. These results were obtained in the presence of buffer containing tris(hydroxymethyl)aminomethane, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, NaCl, ethylenediaminetetraacetic acid, dithiothreitol, and benzamidine at pH 8.0 that in previous work showed an ability to cause PC to phase separate but are necessary to stabilize Amot for in vitro experimentation. Collectively, this provided a framework for determining the effect of Amot on lipid organization. Using fluorescence spectroscopy, we were able to show that the association of Amot with this lipid platform causes significant reorganization of the lipid into a more homogenous structure. This reorganization mechanism could be the basis for Amot membrane association and fusogenic activity previously described in the literature and should be taken into consideration in future protein-membrane interaction studies.

proBDNF and p75NTR Control Excitability and Persistent Firing of Cortical Pyramidal Neurons

Persistent firing of entorhinal cortex (EC) pyramidal neurons is a key component of working and spatial memory. We report here that a pro-brain-derived neurotrophic factor (proBDNF)-dependent p75NTR signaling pathway plays a major role in excitability and persistent activity of pyramidal neurons in layer V of the EC. Using electrophysiological recordings, we show that proBDNF suppresses persistent firing in entorhinal slices from wild-type mice but not from p75NTR-null mice. Conversely, function-blocking proBDNF antibodies enhance excitability of pyramidal neurons and facilitate their persistent firing, and acute exposure to function-blocking p75NTR antibodies results in enhanced firing activity of pyramidal neurons. Genetic deletion of p75NTR specifically in neurons or during adulthood also induces enhanced excitability and persistent activity, indicating that the proBDNF-p75NTR signaling cascade functions within adult neurons to inhibit pyramidal activity. Phosphatidylinositol 4,5-bisphosphate (PIP2)-sensitive transient receptor potential canonical channels play a critical role in mediating persistent firing in the EC and we hypothesized that proBDNF-dependent p75NTR activation regulates PIP2 levels. Accordingly, proBDNF decreases cholinergic calcium responses in cortical neurons and affects carbachol-induced depletion of PIP2. Further, we show that the modulation of persistent firing by proBDNF relies on a p75NTR-Rac1-PI4K pathway. The hypothesis that proBDNF and p75NTR maintain network homeostasis in the adult CNS was tested in vivo and we report that p75NTR-null mice show improvements in working memory but also display an increased propensity for severe seizures. We propose that the proBDNF-p75NTR axis controls pyramidal neuron excitability and persistent activity to balance EC performance with the risk of runaway activity.

SIGNIFICANCE STATEMENT

Persistent firing of entorhinal cortex (EC) pyramidal neurons is required for working memory. We report here that pro-brain-derived neurotrophic factor (proBDNF) activates p75NTR to induce a Rac1-dependent and phosphatidylinositol 4,5-bisphosphate-dependent signaling cascade that suppresses persistent activity. Conversely, using loss-of-function approaches, we find that endogenous proBDNF or p75NTR activation strongly decreases pyramidal neuron excitability and persistent firing, suggesting that a physiological role of this proBDNF-p75NTR cascade may be to regulate working memory in vivo. Consistent with this, mice rendered null for p75NTR during adulthood show improvements in working memory but also display an increased propensity for severe seizures. We propose that by attenuating EC network performance, the proBDNF-p75NTR signaling cascade reduces the probability of epileptogenesis.

Spatial neuroproteomics using imaging mass spectrometry

The nervous system constitutes arguably the most complicated and least understood cellular network in the human body. This consequently manifests itself in the fact that the molecular bases of neurodegenerative diseases remain unknown. The limited understanding of neurobiological mechanisms relates directly to the lack of appropriate bioanalytical technologies that allow highly resolved, sensitive, specific and comprehensive molecular imaging in complex biological matrices. Imaging mass spectrometry (IMS) is an emerging technique for molecular imaging. The technique is characterized by its high chemical specificity allowing comprehensive, spatial protein and peptide profiling in situ. Imaging MS represents therefore a powerful approach for investigation of spatio-temporal protein and peptide regulations in CNS derived tissue and cells. This review aims to provide a concise overview of major developments and applications concerning imaging mass spectrometry based protein and peptide profiling in neurobiological and biomedical research. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

Synaptic vesicle endocytosis

Neurons can sustain high rates of synaptic transmission without exhausting their supply of synaptic vesicles. This property relies on a highly efficient local endocytic recycling of synaptic vesicle membranes, which can be reused for hundreds, possibly thousands, of exo-endocytic cycles. Morphological, physiological, molecular, and genetic studies over the last four decades have provided insight into the membrane traffic reactions that govern this recycling and its regulation. These studies have shown that synaptic vesicle endocytosis capitalizes on fundamental and general endocytic mechanisms but also involves neuron-specific adaptations of such mechanisms. Thus, investigations of these processes have advanced not only the field of synaptic transmission but also, more generally, the field of endocytosis. This article summarizes current information on synaptic vesicle endocytosis with an emphasis on the underlying molecular mechanisms and with a special focus on clathrin-mediated endocytosis, the predominant pathway of synaptic vesicle protein internalization.

Structural requirements of membrane phospholipids for M-type potassium channel activation and binding

M-channels are voltage-gated potassium channels that regulate cell excitability. They are heterotetrameric assemblies of Kv7.2 and Kv7.3 subunits. Their opening requires the presence of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). However, the specificity of PI(4,5)P₂ as a binding and activating ligand is unknown. Here, we tested the ability of different phosphoinositides and lipid phosphates to activate or bind to M-channel proteins. Activation of functional channels was measured in membrane patches isolated from cells coexpressing Kv7.2 and Kv7.3 subunits. Channels were activated to similar extents (maximum open probability of ∼0.8 at 0 mV) by 0.1–300 μm dioctanoyl homologs of the three endogenous phosphoinositides, PI(4)P, PI(4,5)P₂, and PI(3,4,5)P₃, with sensitivity increasing with increasing numbers of phosphates. Non-acylated inositol phosphates had no effect up to 100 μm. Channels were also activated with increasing efficacy by 1–300 μm concentrations of the monoacyl monophosphates fingolimod phosphate, sphingosine 1-phosphate, and lysophosphatidic acid but not by phosphate-free fingolimod or sphingosine or by phosphate-masked phosphatidylcholine or phosphatidylglycerol. An overlay assay confirmed that a fusion protein containing the full-length C terminus of Kv7.2 could bind to a broad range of phosphoinositides and phospholipids. A mutated Kv7.2 C-terminal construct with reduced sensitivity to PI(4,5)P showed significantly less binding to most polyphosphoinositides. We concluded that M-channels bind to, and are activated by, a wide range of lipid phosphates, with a minimum requirement for an acyl chain and a phosphate headgroup. In this, they more closely resemble inwardly rectifying Kir6.2 potassium channels than the more PI(4,5)P₂-specific Kir2 channels. Notwithstanding, the data also support the view that the main endogenous activator of M-channels is PI(4,5)P₂.

Distribution of lipids in human brain

The enormous abundance of lipid molecules in the central nervous system (CNS) suggests that their role is not limited to be structural and energetic components of cells. Over the last decades, some lipids in the CNS have been identified as intracellular signalers, while others are known to act as neuromodulators of neurotransmission through binding to specific receptors. Neurotransmitters of lipidic nature, currently known as neurolipids, are synthesized during the metabolism of phospholipid precursors present in cell membranes. Therefore, the anatomical identification of each of the different lipid species in human CNS by imaging mass spectrometry (IMS), in association with other biochemical techniques with spatial resolution, can increase our knowledge on the precise metabolic routes that synthesize these neurolipids and their localization. The present study shows the lipid distribution obtained by MALDI-TOF IMS in human frontal cortex, hippocampus, and striatal area, together with functional autoradiography of cannabinoid and LPA receptors. The combined application of these methods to postmortem human brain samples may be envisioned as critical to further understand neurological diseases, in general, and particularly, the neurodegeneration that accompanies Alzheimer's disease.

Microarray analysis of differentially regulated genes in human neuronal and epithelial cell lines upon exposure to type A botulinum neurotoxin

Among the seven serotypes (A-G), type A botulinum neurotoxin (BoNT/A) is the most prevalent etiologic agent and the most potent serotype to cause foodborne botulism, characterized by flaccid muscle paralysis. Upon ingestion, BoNT/A crosses epithelial cell barriers to reach lymphatic and circulatory systems and blocks acetylcholine release at the pre-synaptic cholinergic nerve terminals of neuromuscular junctions (NMJs) resulting in paralysis. One of the unique features of BoNT/A intoxication is its neuroparalytic longevity due to its persistent catalytic activity. The persistent presence of the toxin inside the cell can induce host cell responses. To understand the pathophysiology and host response at the cellular level, gene expression changes upon exposure of human HT-29 colon carcinoma (epithelial) and SH-SY5Y neuroblastoma cell lines to BoNT/A complex were investigated using microarray analysis. In HT-29 cells, 167 genes were up-regulated while 60 genes were down-regulated, whereas in SH-SY5Y cells about 223 genes were up-regulated and 18 genes were down-regulated. Modulation of genes and pathways involved in neuroinflammatory, ubiquitin-proteasome degradation, phosphatidylinositol, calcium signaling in SH-SY5Y cells, and genes relevant to focal adhesion, cell adhesion molecules, adherens and gap junction related pathways in HT-29 cells suggest a massive host response to BoNT/A. A clear differential response in epithelial and neuronal cells indicates that the genes affected may play a distinct role in BoNTs cellular mode of action, involving these two types of host cells.

Investigation of the H+–myo-inositol transporter (HMIT) as a neuronal regulator of phosphoinositide signalling

Phosphoinositide signalling regulates a series of important neuronal processes that are thought to be altered in mood disorders. Furthermore, mood-stabilizing drugs inhibit key enzymes that regulate phosphoinositide production and alter neuronal growth cone morphology in an inositol-reversible manner. Inositol is taken up by neurons from the extracellular fluid, presumably via membrane transporters; it can also be synthesized by the enzyme MIP-synthase (myo-inositol-1-phosphate synthase) and, in addition, it is generated by inositol phospholipid hydrolysis. The neuronal-specific HMIT (H+–myo-inositol transporter) represents a potential regulator of inositol signalling in neurons that warrants further investigation.

cGMP signalling in the mammalian brain: role in synaptic plasticity and behaviour

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays a crucial role in the control of cardiovascular and gastrointestinal homeostastis, but its effects on neuronal functions are less established. This review summarizes recent biochemical and functional data on the role of the cGMP signalling pathway in the mammalian brain, with a focus on the regulation of synaptic plasticity, learning, and other complex behaviours. Expression profiling, along with pharmacological and genetic manipulations, indicates important functions of nitric oxide (NO)-sensitive soluble guanylyl cyclases (sGCs), cGMP-dependent protein kinases (cGKs), and cGMP-regulated phosphodiesterases (PDEs) as generators, effectors, and modulators of cGMP signals in the brain, respectively. In addition, neuronal cGMP signalling can be transmitted through cyclic nucleotide-gated (CNG) or hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels. The canonical NO/sGC/cGMP/cGK pathway modulates long-term changes of synaptic activity in the hippocampus, amygdala, cerebellum, and other brain regions, and contributes to distinct forms of learning and memory, such as fear conditioning, motor adaptation, and object recognition. Behavioural studies indicate that cGMP signalling is also involved in anxiety, addiction, and the pathogenesis of depression and schizophrenia. At the molecular level, different cGK isoforms appear to mediate effects of cGMP on presynaptic transmitter release and postsynaptic functions. The cGKs have been suggested to modulate cytoskeletal organization, vesicle and AMPA receptor trafficking, and gene expression via phosphorylation of various substrates including VASP, RhoA, RGS2, hSERT, GluR1, G-substrate, and DARPP-32. These and other components of the cGMP signalling cascade may be attractive new targets for the treatment of cognitive impairment, drug abuse, and psychiatric disorders.

Ca2+-regulated pool of phosphatidylinositol-3-phosphate produced by phosphatidylinositol 3-kinase C2alpha on neurosecretory vesicles

Phosphatidylinositol-3-phosphate [PtdIns(3)P] is a key player in early endosomal trafficking and is mainly produced by class III phosphatidylinositol 3-kinase (PI3K). In neurosecretory cells, class II PI3K-C2alpha and its lipid product PtdIns(3)P have recently been shown to play a critical role during neuroexocytosis, suggesting that two distinct pools of PtdIns(3)P might coexist in these cells. However, the precise characterization of this additional pool of PtdIns(3)P remains to be established. Using a selective PtdIns(3)P probe, we have identified a novel PtdIns(3)P-positive pool localized on secretory vesicles, sensitive to PI3K-C2alpha knockdown and relatively resistant to wortmannin treatment. In neurosecretory cells, stimulation of exocytosis promoted a transient albeit large increase in PtdIns(3)P production localized on secretory vesicles sensitive to PI3K-C2alpha knockdown and expression of PI3K-C2alpha catalytically inactive mutant. Using purified chromaffin granules, we found that PtdIns(3)P production is controlled by Ca(2+). We confirmed that PtdIns(3)P production from recombinantly expressed PI3K-C2alpha is indeed regulated by Ca(2+). We provide evidence that a dynamic pool of PtdIns(3)P synthesized by PI3K-C2alpha occurs on secretory vesicles in neurosecretory cells, demonstrating that the activity of a member of the PI3K family is regulated by Ca(2+) in vitro and in living neurosecretory cells.

NO/cGMP-dependent modulation of synaptic transmission

Nitric oxide (NO) is a multifunctional messenger in the CNS that can signal both in antero- and retrograde directions across synapses. Many effects of NO are mediated through its canonical receptor, the soluble guanylyl cyclase, and the second messenger cyclic guanosine-3',5'-monophosphate (cGMP). An increase of cGMP can also arise independently of NO via activation of membrane-bound particulate guanylyl cyclases by natriuretic peptides. The classical targets of cGMP are cGMP-dependent protein kinases (cGKs), cyclic nucleotide hydrolysing phosphodiesterases, and cyclic nucleotide-gated (CNG) cation channels. The NO/cGMP/cGK signalling cascade has been linked to the modulation of transmitter release and synaptic plasticity by numerous pharmacological and genetic studies. This review focuses on the role of NO as a retrograde messenger in long-term potentiation of transmitter release in the hippocampus. Presynaptic mechanisms of NO/cGMP/cGK signalling will be discussed with recently identified potential downstream components such as CaMKII, the vasodilator-stimulated phosphoprotein, and regulators of G protein signalling. NO has further been suggested to increase transmitter release through presynaptic clustering of a-synuclein. Alternative modes of NO/cGMP signalling resulting in inhibition of transmitter release and long-term depression of synaptic activity will also be addressed, as well as anterograde NO signalling in the cerebellum. Finally, emerging evidence for cGMP signalling through CNG channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels will be discussed.

p75 neurotrophin receptor mediates neuronal cell death by activating GIRK channels through phosphatidylinositol 4,5-bisphosphate

The pan neurotrophin receptor p75(NTR) signals programmed cell death both during nervous system development and after neural trauma and disease in the adult. However, the molecular pathways by which death is mediated remain poorly understood. Here, we show that this cell death is initiated by activation of G-protein-coupled inwardly rectifying potassium (GIRK/Kir3) channels and a consequent potassium efflux. Death signals stimulated by neurotrophin-mediated cleavage of p75(NTR) activate GIRK channels through the generation and binding of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2/PIP2] to GIRK channels. Both GIRK channel activity and p75(NTR)-mediated neuronal death are inhibited by sequestration of PtdIns(4,5)P2 and application of GIRK channel inhibitors, whereas pertussis toxin treatment has no effect. Thus, p75(NTR) activates GIRK channels without the need for G(i/o)-proteins. Our results demonstrate a novel mode of activation of GIRK channels, representing an early step in the p75(NTR)-mediated cell death pathway and suggesting a function for these channels during nervous system development.

Docosahexaenoic acid-induced changes in phospholipids in cortex of young and aged rats: a lipidomic analysis

The age-related decline in cognitive function has been associated with biochemical changes that can be attenuated following n-3 polyunsaturated fatty acid treatment. Dietary supplementation with docosahexaenoic acid (DHA) has been shown to reverse age-related changes in synaptic function. Here, lipidomic analyses were undertaken to examine changes in lipid classes and phospholipid species in cortical tissue of young (2-4 months) and aged (20-22 months), control- and DHA-treated (10mg daily) rats following treatment for 8 weeks, aiming to explore the mechanism of DHA action. Dietary supplementation normalised the age-related decrease in unsaturation index, reduced the levels of arachidonic acid-containing phospholipids in both young and aged animals, and gave rise to production of new phosphatidylserine and phosphatidylinositol species. These findings suggest that DHA may mediate some of its effects through alterations in the membrane lipid composition that can consequently affect the production of pro-inflammatory mediators and signalling molecular species.

Identification of Secretory Granule Phosphatidylinositol 4,5-Bisphosphate-interacting Proteins Using an Affinity Pulldown Strategy

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) synthesis is required for calcium-dependent exocytosis in neurosecretory cells. We developed a PtdIns(4,5)P2 bead pulldown strategy combined with subcellular fractionation to identify endogenous chromaffin granule proteins that interact with PtdIns(4,5)P2. We identified two synaptotagmin isoforms, synaptotagmins 1 and 7; spectrin; alpha-adaptin; and synaptotagmin-like protein 4 (granuphilin) by mass spectrometry and Western blotting. The interaction between synaptotagmin 7 and PtdIns(4,5)P2 and its functional relevance was investigated. The 45-kDa isoform of synaptotagmin 7 was found to be highly expressed in adrenal chromaffin cells compared with PC12 cells and to mainly localize to secretory granules by subcellular fractionation, immunoisolation, and immunocytochemistry. We demonstrated that synaptotagmin 7 binds PtdIns(4,5)P2 via the C2B domain in the absence of calcium and via both the C2A and C2B domains in the presence of calcium. We mutated the polylysine stretch in synaptotagmin 7 C2B and demonstrated that this mutant domain lacks the calcium-independent PtdIns(4,5)P2 binding. Synaptotagmin 7 C2B domain inhibited catecholamine release from digitonin-permeabilized chromaffin cells, and this inhibition was abrogated with the C2B polylysine mutant. These data indicate that synaptotagmin 7 C2B-effector interactions, which occur via the polylysine stretch, including calcium-independent PtdIns(4,5)P2 binding, are important for chromaffin granule exocytosis.

Polyphosphoinositol lipids: Under-PPInning synaptic function in health and disease

Phosphoinositides (PPIn) form a unique family of lipids derived by phosphorylation of the parent compound, phosphatidylinositol. Despite being minor constituents of synaptic membranes, these lipids have exceptionally high rates of metabolic turnover and are involved with myriad aspects of pre- and post-synaptic function, from the control of the synaptic vesicle cycle to postsynaptic excitability. In this review, we outline the main synaptic processes known to be regulated by these molecules, focusing mainly but not exclusively on the major species phosphatidylinositol 4-phosphate and phosphatidylinositol (4,5)-bisphosphate. Furthermore, we discuss the enzymes responsible for their synthesis and degradation, with a view to exploring how the activity-dependent control of their enzymatic action can lead to the precise regulation of PPIn levels at the nerve terminal. Also, the modulation of synaptic PPIn turnover by drugs used for the treatment of bipolar disorder is discussed. We propose that the modulation of PPIn levels may act as a central mechanism to coordinate the cascade of synaptic events leading to neurotransmission.

Arachidonic acid potentiates exocytosis and allows neuronal SNARE complex to interact with Munc18a

Neuronal communication relies on the fusion of neurotransmitter-containing vesicles with the neuronal plasma membrane. Recent genetic studies have highlighted the critical role played by polyunsaturated fatty acids in neurotransmission, however, there is little information available about which fatty acids act on exocytosis and, more importantly, by what mechanism. We have used permeabilized chromaffin cells to screen various fatty acids of the n-3 and n-6 series for their acute effects on exocytosis. We have demonstrated that an n-6 series polyunsaturated fatty acid, arachidonic acid, potentiates secretion from intact neurosecretory cells regardless of the secretagogue used. We have shown that arachidonic acid dose dependently increases soluble NSF attachment protein receptor complex formation in chromaffin cells and bovine cortical brain extracts and that a non-hydrolysable analogue of arachidonic acid causes a similar increase in SNARE complex formation. This prompted us to examine the effect of arachidonic acid on SNARE protein interactions with Munc18a, a protein known to prevent Syntaxin1a engagement into the SNARE complex in vitro. In the presence of arachidonic acid, we show that Munc18a can interact with the neuronal SNARE complex in a dose-dependent manner. We further demonstrate that arachidonic acid directly interacts with Syntaxin1a.

[Update on the genetics of X-linked mental retardation]

Mutations in X-linked genes are likely to account for the observation that more males than females are affected with mental retardation. Causative mutations have been identified in both syndromic XLMR and in the genetically heterogeneous non-syndromic forms of XLMR, without a clear clinical phenotype other than cognitive deficit. Progress in genome analysis and the establishment of large collaborations between clinical and molecular research teams, especially the European XLMR consortium, have led to the identification of 20 non-syndromic XLMR genes and 25 syndromic XLMR genes. Given the extensive heterogeneity of non syndromic XLMR, different strategies are used for the identification of new genes: linkage analysis, studies of balanced chromosomal rearrangements (X-autosome translocations, microdeletions) and candidate genes strategies by mutation screening in regions of the X chromosome known to be involved in neuronal development and function. Delineating the monogenic causes of XLMR and their molecular and cellular consequences will provide insight into the mechanisms that are required for normal development of cognitive function in humans. Non syndromic XLMR proteins include 5 distinct classes: transmembrane receptors, small GTPases effectors or regulators, enzymes and translational regulators.

Phosphoinositide regulation of neuroexocytosis: adding to the complexity

Exocytosis of neurotransmitter containing vesicles supports neuronal communication. The importance of molecular interactions involving specific lipids has become progressively more evident and the lipid composition of both the synaptic vesicle and the pre-synaptic plasma membrane at the active zone has significant functional consequences for neurotransmitter release. Several classes of lipids have been implicated in exocytosis including polyunsaturated fatty acids and phosphoinositides. This minireview will focus on recent developments regarding the role of phosphoinositides in neurosecretion.

An emerging role for PtdIns(4,5)P2-mediated signalling in human disease

Although an established regulator of many cellular functions, the phosphoinositide phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2) appears to have evaded the attention of drug-discovery companies. An increasing number of reports have identified potential links between PtdIns(4,5)P2-mediated signalling pathways and the aetiology of many human diseases. Here, we review current knowledge of the regulation and function of PtdIns(4,5)P2 and discuss how aberrant PtdIns(4,5)P2-mediated signalling might contribute to human pathologies such as cardiac failure, bipolar disorder, channelopathies and the genetic disorder Lowe syndrome.

Phosphatidylinositol 3-kinase C2alpha is essential for ATP-dependent priming of neurosecretory granule exocytosis

Neurotransmitter release and hormonal secretion are highly regulated processes culminating in the calcium-dependent fusion of secretory vesicles with the plasma membrane. Here, we have identified a role for phosphatidylinositol 3-kinase C2alpha (PI3K-C2alpha) and its main catalytic product, PtdIns3P, in regulated exocytosis. In neuroendocrine cells, PI3K-C2alpha is present on a subpopulation of mature secretory granules. Impairment of PI3K-C2alpha function specifically inhibits the ATP-dependent priming phase of exocytosis. Overexpression of wild-type PI3K-C2alpha enhanced secretion, whereas transfection of PC12 cells with a catalytically inactive PI3K-C2alpha mutant or a 2xFYVE domain sequestering PtdIns3P abolished secretion. Based on these results, we propose that production of PtdIns3P by PI3K-C2alpha is required for acquisition of fusion competence in neurosecretion.

Neuronal calcium sensor-1 facilitates neuronal exocytosis through phosphatidylinositol 4-kinase

This work tested the theory that neuronal calcium sensor-1 (NCS-1) has effects on neurotransmitter release beyond its actions on membrane channels. We used nerve-ending preparations where membrane channels are bypassed through membrane permeabilization made by mechanical disruption or streptolysin-O. Nerve ending NCS-1 and phosphatidylinositol 4-kinase (PI4K) are largely or entirely particulate, so their concentrations in nerve endings remain constant after breaching the membrane. Exogenous, myristoylated NCS-1 stimulated nerve ending phosphatidylinositol 4-phosphate [PI(4)P] synthesis, but non-myristoylated-NCS-1 did not. The N-terminal peptide of NCS-1 interfered with PI(4)P synthesis, and with spontaneous and Ca(2+)-evoked release of both [(3)H]-norepinephrine (NA) and [(14)C]-glutamate (glu) in a concentration-dependent manner. An antibody raised against the N-terminal of NCS-1 inhibited perforated nerve ending PI(4)P synthesis, but the C-terminal antibody had no effects. Antibodies against the N- and C-termini of NCS-1 caused significant increases in mini/spontaneous/stimulation-independent release of [(3)H]-NA from perforated nerve endings, but had no effect on [(14)C]-glu release. These results support the idea that NCS-1 facilitates nerve ending neurotransmitter release and phosphoinositide production via PI4K and localizes these effects to the N-terminal of NCS-1. Combined with previous work on the regulation of channels by NCS-1, the data are consistent with the hypothesis that a NCS-1-PI4K (NP, neuropotentiator) complex may serve as an essential linker between lipid and protein metabolism to regulate membrane traffic and co-ordinate it with ion fluxes and plasticity in the nerve ending.

Lipid regulation of the synaptic vesicle cycle

Membrane vesicle cycling is orchestrated through the combined actions of proteins and lipids. At neuronal synapses, this orchestration must meet the stringent demands of speed, fidelity and sustainability of the synaptic vesicle cycle that mediates neurotransmission. Historically, the lion's share of the attention has been focused on the proteins that are involved in this cycle; but, in recent years, it has become clear that the previously unheralded plasma membrane and vesicle lipids are also key regulators of this cycle. This article reviews recent insights into the roles of lipid-modifying enzymes and lipids in the acute modulation of neurotransmission.

Electrostatic sequestration of PIP2 on phospholipid membranes by basic/aromatic regions of proteins

The basic effector domain of myristoylated alanine-rich C kinase substrate (MARCKS), a major protein kinase C substrate, binds electrostatically to acidic lipids on the inner leaflet of the plasma membrane; interaction with Ca2+/calmodulin or protein kinase C phosphorylation reverses this binding. Our working hypothesis is that the effector domain of MARCKS reversibly sequesters a significant fraction of the L-alpha-phosphatidyl-D-myo-inositol 4,5-bisphosphate (PIP2) on the plasma membrane. To test this, we utilize three techniques that measure the ability of a peptide corresponding to its effector domain, MARCKS(151-175), to sequester PIP2 in model membranes containing physiologically relevant fractions (15-30%) of the monovalent acidic lipid phosphatidylserine. First, we measure fluorescence resonance energy transfer from Bodipy-TMR-PIP2 to Texas Red MARCKS(151-175) adsorbed to large unilamellar vesicles. Second, we detect quenching of Bodipy-TMR-PIP2 in large unilamellar vesicles when unlabeled MARCKS(151-175) binds to vesicles. Third, we identify line broadening in the electron paramagnetic resonance spectra of spin-labeled PIP2 as unlabeled MARCKS(151-175) adsorbs to vesicles. Theoretical calculations (applying the Poisson-Boltzmann relation to atomic models of the peptide and bilayer) and experimental results (fluorescence resonance energy transfer and quenching at different salt concentrations) suggest that nonspecific electrostatic interactions produce this sequestration. Finally, we show that the PLC-delta1-catalyzed hydrolysis of PIP2, but not binding of its PH domain to PIP2, decreases markedly as MARCKS(151-175) sequesters most of the PIP2.

Effects of wortmannin and latrunculin A on slow endocytosis at the frog neuromuscular junction

Phosphoinositides are key regulators of synaptic vesicle cycling and endocytic traffic; the actin cytoskeleton also seems to be involved in modulating these processes. We investigated the effects of perturbing phosphoinositide signalling and actin dynamics on vesicle cycling in frog motor nerve terminals, using fluorescence and electron microscopy, and electrophysiology. Antibody staining for beta-actin revealed that actin surrounds but does not overlap with synaptic vesicle clusters. Latrunculin A, which disrupts actin filaments by binding actin monomers, and wortmannin, an inhibitor of phosphatidyl inositol-3-kinase (PI3-kinase), each disrupted the pattern of presynaptic actin staining, but not vesicle clusters in resting terminals. Latrunculin A, but not wortmannin, also reduced vesicle mobilization and exocytosis. Both drugs inhibited the stimulation-induced uptake of the styryl dye FM1-43 and blocked vesicle reformation from internalized membrane objects after tetanic stimulation. These results are consistent with a role of PI3-kinase and the actin cytoskeleton in the slow pathway of vesicle endocytosis, used primarily by reserve pool vesicles.

Synaptotagmin Interaction with the Syntaxin/SNAP-25 Dimer Is Mediated by an Evolutionarily Conserved Motif and Is Sensitive to Inositol Hexakisphosphate

Synaptotagmins are membrane proteins that possess tandem C2 domains and play an important role in regulated membrane fusion in metazoan organisms. Here we show that both synaptotagmins I and II, the two major neuronal isoforms, can interact with the syntaxin/synaptosomal-associated protein of 25 kDa (SNAP-25) dimer, the immediate precursor of the soluble NSF attachment protein receptor (SNARE) fusion complex. A stretch of basic amino acids highly conserved throughout the animal kingdom is responsible for this calcium-independent interaction. Inositol hexakisphosphate modulates synaptotagmin coupling to the syntaxin/SNAP-25 dimer, which is mirrored by changes in chromaffin cell exocytosis. Our results shed new light on the functional importance of the conserved polybasic synaptotagmin motif, suggesting that synaptotagmin interacts with the t-SNARE dimer to up-regulate the probability of SNARE-mediated membrane fusion.

Bidirectional signaling links the Abelson kinases to the platelet-derived growth factor receptor

The c-Abl nonreceptor tyrosine kinase is activated by growth factor signals such as the platelet-derived growth factor (PDGF) and functions downstream of the PDGF-beta receptor (PDGFR) to mediate biological processes such as membrane ruffling, mitogenesis, and chemotaxis. Here, we show that the related kinase Arg is activated downstream of PDGFRs in a manner dependent on Src family kinases and phospholipase C gamma1 (PLC-gamma1)-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis, as we showed previously for c-Abl. PIP2, a highly abundant phosphoinositide known to regulate cytoskeletal and membrane proteins, inhibits the tyrosine kinase activities of both Arg and c-Abl in vitro and in cells. We now demonstrate that c-Abl and Arg form inducible complexes with and are phosphorylated by the PDGFR tyrosine kinase in vitro and in vivo. Moreover, c-Abl and Arg, in turn, phosphorylate the PDGFR. We show that c-Abl and Arg exhibit nonredundant functions downstream of the activated PDGFR. Reintroduction of c-Abl into Arg-Abl double-null fibroblasts rescues the ability of PLC-gamma1 to increase PDGF-mediated chemotaxis, while reexpression of Arg fails to rescue the chemotaxis defect. These data show that, although both kinases are activated and form complexes with proteins in the PDGFR signaling pathway, only c-Abl functions downstream of PLC-gamma1 to mediate chemotaxis.

Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain

This review summarizes key aspects of tetraspanin proteins, with a focus on the functional relevance and structural features of these proteins and how they are organized into a novel type of membrane microdomain. Despite the size of the tetraspanin family and their abundance and wide distribution over many cell types, most have not been studied. However, from studies of prototype tetraspanins, information regarding functions, cell biology, and structural organization has begun to emerge. Genetic evidence points to critical roles for tetraspanins on oocytes during fertilization, in fungi during leaf invasion, in Drosophila embryos during neuromuscular synapse formation, during T and B lymphocyte activation, in brain function, and in retinal degeneration. From structure and mutagenesis studies, we are beginning to understand functional subregions within tetraspanins, as well as the levels of connections among tetraspanins and their many associated proteins. Tetraspanin-enriched microdomains (TEMs) are emerging as entities physically and functionally distinct from lipid rafts. These microdomains now provide a context in which to evaluate tetraspanins in the regulation of growth factor signaling and in the modulation of integrin-mediated post-cell adhesion events. Finally, the enrichment of tetraspanins within secreted vesicles called exosomes, coupled with hints that tetraspanins may regulate vesicle fusion and/or fission, suggests exciting new directions for future research.

Phosphatidylinositol 4,5-bisphosphate promotes both [3H]-noradrenaline and [14C]-glutamate exocytosis from nerve endings

Inhibition of phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) synthesis by phenylarsine oxide (PAO) inhibits both [3H]-noradrenaline ([3H]-NA) and [14C]-glutamate ([14C]-glu) exocytosis from streptolysin-O (SLO)-perforated synaptosomes. When PI4,5P(2) is blocked by an antibody or chelated by neomycin, neurotransmitter exocytosis again is inhibited. Also, when phosphoinositide (PI) synthesis is indirectly decreased by shunting phosphatidic acid (PA) synthesis into phosphatidylbutanol production, both [14C]-glutamate and [3H]-noradrenaline exocytosis are inhibited. All of these results indicate that PI4,5P(2) is necessary for exocytosis of both synaptic vesicles (SVs) and dense core vesicles (DCVs).

Regulated exocytosis of an H+/myo-inositol symporter at synapses and growth cones

Phosphoinositides, synthesized from myo-inositol, play a critical role in the development of growth cones and in synaptic activity. As neurons cannot synthesize inositol, they take it up from the extracellular milieu. Here, we demonstrate that, in brain and PC12 cells, the recently identified H(+)/myo-inositol symporter HMIT is present in intracellular vesicles that are distinct from synaptic and dense-core vesicles. We further show that HMIT can be triggered to appear on the cell surface following cell depolarization, activation of protein kinase C or increased intracellular calcium concentrations. HMIT cell surface expression takes place preferentially in regions of nerve growth and at varicosities and leads to increased myo-inositol uptake. The symporter is then endocytosed in a dynamin-dependent manner and becomes available for a subsequent cycle of stimulated exocytosis. HMIT is thus expressed in a vesicular compartment involved in activity-dependent regulation of myo-inositol uptake in neurons. This may be essential for sustained signaling and vesicular traffic activities in growth cones and at synapses.

Synaptojanin is recruited by endophilin to promote synaptic vesicle uncoating

We describe the isolation and characterization of Drosophila synaptojanin (synj) mutants. synj encodes a phosphatidylinositol phosphatase involved in clathrin-mediated endocytosis. We show that Synj is specifically localized to presynaptic terminals and is associated with synaptic vesicles. The electrophysiological and ultrastructural defects observed in synj mutants are strikingly similar to those found in endophilin mutants, and Synj and Endo colocalize and interact biochemically. Moreover, synj; endo double mutant synaptic terminals exhibit properties that are very similar to terminals of each single mutant, and overexpression of Endophilin can partially rescue the functional defects in partial loss-of-function synj mutants. Interestingly, Synj is mislocalized and destabilized at synapses devoid of Endophilin, suggesting that Endophilin recruits and stabilizes Synj on newly formed vesicles to promote vesicle uncoating. Our data also provide further evidence that kiss-and-run is able to maintain neurotransmitter release when synapses are not extensively challenged.

Nuclear phosphoinositides and their functions

Phosphoinositides are minor components of biological membranes, which have emerged as essential regulators of a variety of cellular processes, both on the plasma membrane and on several intracellular organelles. The versatility of these lipids stems from their ability to function either as substrates for the generation of second messengers, as membrane-anchoring sites for cytosolic proteins or as regulators of the actin cytoskeleton. Despite a vast literature demonstrating the presence of phosphoinositides in the nucleus, only recently has the function(s) of the nuclear pool of these lipids and their soluble analogues, inositol polyphosphates, started to emerge. These compounds have been shown to serve as essential co-factors for several nuclear processes, including DNA repair, transcription regulation and RNA dynamics. In this light, phosphoinositides and inositol polyphosphates might represent high turnover activity switches for nuclear complexes responsible for these processes. The regulation of these large machineries would be linked to the phosphorylation state of the inositol ring and limited temporally and spatially based on the synthesis and degradation of these molecules.

Polymorphism screening of PIP5K2A: a candidate gene for chromosome 10p-linked psychiatric disorders

Lithium is a potent noncompetitive inhibitor of inositol monophosphatases, enzymes involved in phosphoinositide (PI) and inositol phosphate metabolism. A critical component of the PI pathway is phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), which is hydrolyzed to second messengers and has a direct role in synaptic vesicle function. Interestingly, a number of genes involved in the synthesis and dephosphorylation of PtdIns(4,5)P(2) are found in regions of the genome previously mapped in bipolar disorder (BD) including 10p12, 21q22, and 22q11, among others. Some of these regions overlap with loci mapped in schizophrenia (SZ). One gene involved in PI metabolism that maps to a region of interest is 10p12-linked PIP5K2A, a member of the phosphatidylinositol 4-phosphate 5-kinase family. Polymorphism screening revealed the existence of an imperfect CT repeat polymorphism located near the exon 9-intron 9 splice donor site. A modest difference was found in the distribution of alleles from this highly polymorphic variant when bipolar and schizophrenic subjects were compared with controls; relatively rare short repeat variants were found more commonly in patients and homozygosity for a common long repeat variant was found more commonly in controls. These data suggest that the imperfect CT repeat in PIP5K2A intron 9 should be further investigated as a possible candidate allele for 10p12-linked psychiatric disorders.

Retrograde regulation of synaptic vesicle endocytosis and recycling

Sustained release of neurotransmitter depends upon the recycling of synaptic vesicles. Until now, it has been assumed that vesicle recycling is regulated by signals from the presynaptic bouton alone, but results from rat hippocampal neurons reported here indicate that this need not be the case. Fluorescence imaging and pharmacological analysis show that a nitric oxide (NO) signal generated postsynaptically can regulate endocytosis and at least one later step in synaptic vesicle recycling. The proposed retrograde pathway involves an NMDA receptor (NMDAR)-dependent postsynaptic production of NO, diffusion of NO to a presynaptic site, and a cGMP-dependent increase in presynaptic phosphatidylinositol 4,5-biphosphate (PIP2). These results indicate that the regulation of synaptic vesicle recycling may integrate a much broader range of neural activity signals than previously recognized, including postsynaptic depolarization and the activation of NMDARs at both immediate and nearby postsynaptic active zones.

Visualizing phosphoinositide signalling in single neurons gets a green light

There is now substantial evidence, from single-cell imaging, that complex patterns of release from Ca(2+) stores play an important role in regulating synaptic efficacy and plasticity. Moreover, the major mechanism of store release depends on the generation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] through the action of phospholipase(s) C on phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)], and several neurotransmitters can enhance receptor-mediated activation of this enzyme. The recent development of techniques to image real-time changes in PtdIns(4,5)P(2) hydrolysis according to generation of Ins(1,4,5)P(3) and diacylglycerol in single cells has significantly advanced our ability to investigate these signalling pathways, particularly in relation to single-cell Ca(2+) signals. This article reviews these new approaches and how they have provided novel insights into mechanisms underlying spatio-temporal Ca(2+) signals and phospholipase C activation in neurons.

Dynamics of phosphoinositides in membrane retrieval and insertion

The phosphoinositides PtdIns(4,5)P2 and PtdIns(3,4,5)P3 are concentrated in plasma membranes of eukaryotic cells, and excluded from endosomes, whereas PtdIns(3)P is formed in these latter intracellular membranes and is apparently excluded from the plasma membrane. The logic of this asymmetric disposition is now revealed by the nature of the effector proteins that selectively bind these lipids through specific modules and by the processes that they catalyze. PtdIns(3,4,5)P3 has a role in directing exocytosis, in addition to many other signaling events, whereas PtdIns(4,5)P2 directs endocytosis through its ability to anchor several coat proteins to the plasma membrane. Remarkably, the elimination of PtdIns(4,5)P2 from forming endosomes may be required for membrane fission to occur. Thus membrane insertion and retrieval can be regulated by plasma membrane concentrations of PtdIns(3,4,5)P3 and PtdIns(4,5)P2, whereas PtdIns(3)P directs the downstream trafficking and recycling of intracellular membranes through its attraction of proteins that catalyze these processes. The phosphoinositides thereby control many cell features that depend upon protein sorting, including the composition of the plasma membrane itself, which in turn determines the cell's responses to its environment.

A new link between the c-Abl tyrosine kinase and phosphoinositide signalling through PLC-gamma1

The c-Abl tyrosine (Tyr) kinase is activated after platelet-derived-growth factor receptor (PDGFR) stimulation in a manner that is partially dependent on Src kinase activity. However, the activity of Src kinases alone is not sufficient for activation of c-Abl by PDGFR. Here we show that functional phospholipase C-gamma1 (PLC-gamma1) is required for c-Abl activation by PDGFR. Decreasing cellular levels of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) by PLC-gamma1-mediated hydrolysis or dephosphorylation by an inositol polyphosphate 5-phosphatase (Inp54) results in increased Abl kinase activity. c-Abl functions downstream of PLC-gamma1, as expression of kinase-inactive c-Abl blocks PLC-gamma1-induced chemotaxis towards PDGF-BB. PLC-gamma1 and c-Abl form a complex in cells that is enhanced by PDGF stimulation. After activation, c-Abl phosphorylates PLC-gamma1 and negatively modulates its function in vivo. These findings uncover a newly discovered functional interdependence between non-receptor Tyr kinase and lipid signalling pathways.

Bulk membrane retrieval in the synaptic terminal of retinal bipolar cells

The mechanism of bulk membrane uptake at the synapse remains poorly defined, although exocytosis of synaptic vesicles is followed by compensatory membrane retrieval into both small vesicles and large cisternas or vacuoles. We investigated bulk retrieval in the presynaptic terminal of retinal bipolar cells. Fluorescence imaging of the membrane dye FM1-43 indicated that Ca2+-triggered exocytosis was followed by endocytosis into small vesicles and larger vacuoles that could be selectively labeled using large fluorescent dextrans. Disruption of actin filaments with cytochalasin D or latrunculin B inhibited the formation and transport of vacuoles, but exocytosis and endocytosis continued at normal rates. Bulk retrieval was linked to remodeling of the actin network, and both processes were inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). The regulation of F-actin dynamics by Ca2+ and PI 3-kinase therefore played an important role in compensatory endocytosis at this synapse, but this role was confined to bulk membrane uptake. Capacitance measurements demonstrated that fast endocytosis and refilling of the rapidly releasable pool of vesicles were not dependent on F-actin or PI 3-kinase activity. The basic properties of bulk membrane retrieval at this synapse were very similar to macropinocytosis described in non-neural cells. Bulk retrieval did not play an essential role in maintaining the vesicle cycle during maintained stimulation, but we suggest that it may play a role in the structural plasticity of this synaptic terminal.

Phosphatidylinositol 4-OH kinase is a downstream target of neuronal calcium sensor-1 in enhancing exocytosis in neuroendocrine cells

Neuronal calcium sensor-1 (NCS-1), the mammalian orthologue of frequenin, belongs to a family of EF-hand-containing Ca(2+) sensors. NCS-1/frequenin has been shown to enhance synaptic transmission in PC12 cells and Drosophila and Xenopus, respectively. However, the precise molecular mechanism for the enhancement of exocytosis is largely unknown. In PC12 cells, NCS-1 potentiated exocytosis evoked by ATP, an agonist to phospholipase C-linked receptors, but had no effect on depolarization-evoked release. NCS-1 also enhanced exocytosis triggered by ionomycin, a Ca(2+) ionophore that bypasses K(+) and Ca(2+) channels. Overexpression of NCS-1 caused a shift in the dose-response curve of inhibition of ATP-evoked secretion using phenylarsine oxide, an inhibitor of phosphatidylinositol 4-OH kinase (PI4K). Plasma membrane phosphatidylinositol 4,5-bisphosphate pools were increased upon NCS-1 transfection as visualized using a phospholipase C-delta pleckstrin homology domain-green fluorescent protein construct. NCS-1-transfected cell extracts displayed increased phosphatidylinositol-4-phosphate biosynthesis, indicating an increase in PI4K activity. Mutations in NCS-1 equivalent to those that abolish the interaction of recoverin, another EF-hand-containing Ca(2+) sensor, with its downstream target rhodopsin kinase, lost their ability to enhance exocytosis. Taken together, the present data indicate that NCS-1 modulates the activity of PI4K, leading to increased levels of phosphoinositides and concomitant enhancement of exocytosis.

Delayed reentry of recycling vesicles into the fusion-competent synaptic vesicle pool in synaptojanin 1 knockout mice

Synaptojanin 1 is a polyphosphoinositide phosphatase implicated in synaptic vesicle recycling. We used FM1-43 imaging and electron microscopy in cultured cortical neurons from control and synaptojanin 1 knockout mice to study how the absence of this protein affects specific steps of the synaptic vesicle cycle. Exoendocytosis after a moderate stimulus was unchanged. However, during prolonged stimulation, the regeneration of fusion-competent synaptic vesicles was severely impaired. In stimulated nerve terminals, there was a persistent accumulation of clathrin-coated vesicles and a backup of newly reformed vesicles in the cytomatrix-rich area around the synaptic vesicle cluster. These findings demonstrate that synaptojanin 1 function is needed for the progression of recycling vesicles to the functional synaptic vesicle pool.

Rac GTPase plays an essential role in exocytosis by controlling the fusion competence of release sites

The role of small GTPases of the Rho family in synaptic functions has been addressed by analyzing the effects of lethal toxin (LT) from Clostridium sordellii strain IP82 (LT82) on neurotransmitter release at evoked identified synapses in the buccal ganglion of Aplysia. LT82 is a large monoglucosyltranferase that uses UDP-glucose as cofactor and glucosylates Rac (a small GTPase related to Rho), and Ras, Ral, and Rap (three GTPases of the Ras family). Intraneuronal application of LT (50 nm) rapidly inhibits evoked acetylcholine (ACh) release as monitored electrophysiologically. Injection of the catalytic domain of the toxin similarly blocked ACh release, but not when key amino acids needed for glucosylation were mutated. Intraneuronal application of competitive nucleotide sugars that differentially prevent glucosylation of Rac- and Ras-related GTPases, and the use of a toxin variant that affects a different spectrum of small GTPases, established that glucosylation of Rac is responsible for the reduction in ACh release. To determine the quantal release parameters affected by Rac glucosylation, we developed a nonstationary analysis of the fluctuations in postsynaptic response amplitudes that was performed before and after the toxin had acted or during toxin action. The results indicate that neither the quantal size nor the average probability for release were affected by lethal toxin action. ACh release blockage by LT82 was only caused by a reduction in the number of functional release sites. This reveals that after docking of synaptic vesicles, vesicular Rac stimulates a membrane effector (or effectors) essential for the fusion competence of the exocytotic sites.

Phosphatidylinositol 3 kinase regulates synapse specificity of hippocampal long-term depression

To investigate the role of phosphatidylinositol 3 kinase (PI3 kinase) in hippocampal synaptic plasticity, we used whole-cell patch clamp recordings from rat CA1 neurons to determine the effects of PI3 kinase inhibitors on long-term depression (LTD). PI3 kinase blockade caused a loss of synapse specificity of LTD that was dependent on the co-activation of NMDA-type glutamate receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), and involved release of Ca(2+) from intracellular stores. These findings suggest that the synapse specificity of hippocampal LTD may not be an intrinsic property of this form of homosynaptic plasticity, but rather that it can be regulated by PI3 kinase.

The endocytic machinery at an interface with the actin cytoskeleton: a dynamic, hip intersection

Clathrin-mediated endocytosis is the major mechanism by which proteins and membrane lipids gain access into cells. Over the past several years, an array of proteins has been identified that define the molecular machinery regulating the formation of clathrin-coated pits and vesicles. This article focuses on how the identification of this machinery has begun to reveal a molecular basis for a link between endocytosis and the actin cytoskeleton--a link that had long been suspected to exist in mammalian cells but which had remained elusive. In particular, I discuss the relationship between actin and three components of the endocytic machinery--dynamin, HIPs (huntingtin-interacting proteins) and intersectin.

Parallel activation of phosphatidylinositol 4-kinase and phospholipase C by the extracellular calcium-sensing receptor

The calcium-sensing receptor (CaR) is a G protein-coupled receptor that regulates physiological processes including Ca(2+) metabolism, Na(+), Cl(-), K(+), and H(2)0 balance, and the growth of some epithelial cells through diverse signaling pathways. Although many effects of CaR are mediated by the heterotrimeric G proteins Galpha(q) and Galpha(i), not all signaling pathways regulated by CaR have been identified. We used human embryonic kidney (HEK)-293 cells that stably express human CaR to study the regulation of inositol lipid metabolism by CaR. The nonfunctional mutant CaR(R796W) was used as a negative control. We found that CaR regulates phosphatidylinositol (PI) 4-kinase, the first step in inositol lipid biosynthesis. In cells pretreated with to inhibit phospholipase C activation and to block the degradation of PI 4,5-bisphosphate to form [(3)H]inositol trisphosphate (IP(3)), CaR stimulated the accumulation of [(3)H]PI monophosphate (PIP). Additionally, wortmannin, an inhibitor of both PI 3-kinase and type III PI 4-kinase, blocked CaR-stimulated accumulation of [(3)H]PIP and inhibited [(3)H]IP(3) production. CaR-stimulated inositol lipid synthesis was attributable to PI 4-kinase and not PI 3-kinase because CaR did not activate Akt, a downstream target of PI 3-kinase. CaR associates with PI 4-kinase based on the findings that CaR and the 110-kDa PI 4-kinase beta can be co-immunoprecipitated with antibodies against either CaR or PI 4-kinase. The PI-4 kinase in co-immunoprecipitates with anti-CaR antibody was activated in Ca(2+)-stimulated HEK-293 cells, which stably express the wild type CaR. Pertussis toxin did not affect the formation of [(3)H]IP(3) or the rise in intracellular Ca(2+) (Handlogten, M. E., Huang, C. F., Shiraishi, N., Awata, H., and Miller, R. T. (2001) J. Biol. Chem. 276, 13941-13948). RGS4, an accelerator of GTPase activity of members of the Galpha(i) and Galpha(q) families, attenuated the CaR-stimulated PLC activation and IP(3) accumulation, which is mediated by Galpha(q), but did not inhibit CaR-stimulated [(3)H]PIP formation. In HEK-293 cells, which express wild type CaR, Rho was enriched in immune complexes co-immunoprecipitated with the anti-CaR antibody. C(3) toxin, an inhibitor of Rho, also inhibited the CaR-stimulated [(3)H]IP(3) production but did not lead to CaR-stimulated [(3)H]PIP formation, reflecting inhibition of PI 4-kinase. Taken together, our data demonstrate that CaR stimulates PI 4-kinase, the first step in inositol lipid biosynthesis conversion of PI to PI 4-P by Rho-dependent and Galpha(q)- and Galpha(i)-independent pathways.

Signals from the X: signal transduction and X-linked mental retardation

The dramatic increase in genomic information is allowing the rapid identification of genes that are altered in mental retardation (MR). It is necessary to place their resulting gene products in their cellular context to understand how they may have contributed to a patient's cognitive deficits. This review will consider signaling molecules that have been implicated in X-linked MR and the known pathways by which these proteins covey information will be delineated. The proteins discussed include four distinct classes: transmembrane receptors, guanine nucleotide related proteins, kinases, and translational regulators.

Bringing together cell biology commentary and genetics

March 27, 2002 saw the official inauguration of the latest Max Planck Institute in Germany, the Institute of Molecular Cell Biology and Genetics (MPI-CBG) in Dresden. This new institute is innovative on many levels, from the range of science it aims to cover to the potential it provides for additional integration of European science. These themes were highlighted in the symposium that accompanied the inauguration.