Phosphorylation of dynamin I on Ser-795 by protein kinase C blocks its association with phospholipids

Dynamin is primed at endocytic sites for ultrafast endocytosis

Dynamin mediates fission of vesicles from the plasma membrane during endocytosis. Typically, dynamin is recruited from the cytosol to endocytic sites, requiring seconds to tens of seconds. However, ultrafast endocytosis in neurons internalizes vesicles as quickly as 50 ms during synaptic vesicle recycling. Here, we demonstrate that Dynamin 1 is pre-recruited to endocytic sites for ultrafast endocytosis. Specifically, Dynamin 1xA, a splice variant of Dynamin 1, interacts with Syndapin 1 to form molecular condensates on the plasma membrane. Single-particle tracking of Dynamin 1xA molecules confirms the liquid-like property of condensates in vivo. When Dynamin 1xA is mutated to disrupt its interaction with Syndapin 1, the condensates do not form, and consequently, ultrafast endocytosis slows down by 100-fold. Mechanistically, Syndapin 1 acts as an adaptor by binding the plasma membrane and stores Dynamin 1xA at endocytic sites. This cache bypasses the recruitment step and accelerates endocytosis at synapses.

Highly Basic Clusters in the Herpes Simplex Virus 1 Nuclear Egress Complex Drive Membrane Budding by Inducing Lipid Ordering

During replication of herpesviruses, capsids escape from the nucleus into the cytoplasm by budding at the inner nuclear membrane. This unusual process is mediated by the viral nuclear egress complex (NEC) that deforms the membrane around the capsid by oligomerizing into a hexagonal, membrane-bound scaffold. Here, we found that highly basic membrane-proximal regions (MPRs) of the NEC alter lipid order by inserting into the lipid headgroups and promote negative Gaussian curvature. We also find that the electrostatic interactions between the MPRs and the membranes are essential for membrane deformation. One of the MPRs is phosphorylated by a viral kinase during infection, and the corresponding phosphomimicking mutations block capsid nuclear egress. We show that the same phosphomimicking mutations disrupt the NEC-membrane interactions and inhibit NEC-mediated budding in vitro, providing a biophysical explanation for the in vivo phenomenon. Our data suggest that the NEC generates negative membrane curvature by both lipid ordering and protein scaffolding and that phosphorylation acts as an off switch that inhibits the membrane-budding activity of the NEC to prevent capsid-less budding. IMPORTANCE Herpesviruses are large viruses that infect nearly all vertebrates and some invertebrates and cause lifelong infections in most of the world's population. During replication, herpesviruses export their capsids from the nucleus into the cytoplasm by an unusual mechanism in which the viral nuclear egress complex (NEC) deforms the nuclear membrane around the capsid. However, how membrane deformation is achieved is unclear. Here, we show that the NEC from herpes simplex virus 1, a prototypical herpesvirus, uses clusters of positive charges to bind membranes and order membrane lipids. Reducing the positive charge or introducing negative charges weakens the membrane deforming ability of the NEC. We propose that the virus employs electrostatics to deform nuclear membrane around the capsid and can control this process by changing the NEC charge through phosphorylation. Blocking NEC-membrane interactions could be exploited as a therapeutic strategy.

Protein Kinase C and Calmodulin Serve As Calcium Sensors for Calcium-Stimulated Endocytosis at Synapses

Calcium influx triggers and facilitates endocytosis, which recycles vesicles and thus sustains synaptic transmission. Despite decades of studies, the underlying calcium sensor remained not well understood. Here, we examined two calcium binding proteins, protein kinase C (PKC) and calmodulin. Whether PKC is involved in endocytosis was unclear; whether calmodulin acts as a calcium sensor for endocytosis was neither clear, although calmodulin involvement in endocytosis had been suggested. We generated PKC (α or β-isoform) and calmodulin (calmodulin 2 gene) knock-out mice of either sex and measured endocytosis with capacitance measurements, pHluorin imaging and electron microscopy. We found that these knock-outs inhibited slow (∼10-30 s) and rapid (<∼3 s) endocytosis at large calyx-type calyces, and inhibited slow endocytosis and bulk endocytosis (forming large endosome-like structures) at small conventional hippocampal synapses, suggesting the involvement of PKC and calmodulin in three most common forms of endocytosis-the slow, rapid and bulk endocytosis. Inhibition of slow endocytosis in PKC or calmodulin 2 knock-out hippocampal synapses was rescued by overexpressing wild-type PKC or calmodulin, but not calcium-binding-deficient PKC or calmodulin mutant, respectively, suggesting that calcium stimulates endocytosis by binding with its calcium sensor PKC and calmodulin. PKC and calmodulin 2 knock-out inhibited calcium-dependent vesicle mobilization to the readily releasable pool, suggesting that PKC and calmodulin may mediate calcium-dependent facilitation of vesicle mobilization. These findings shed light on the molecular signaling link among calcium, endocytosis and vesicle mobilization that are crucial in maintaining synaptic transmission and neuronal network activity.SIGNIFICANCE STATEMENT Vesicle fusion releases neurotransmitters to mediate synaptic transmission. To sustain synaptic transmission, fused vesicles must be retrieved via endocytosis. Accumulating evidence suggests that calcium influx triggers synaptic vesicle endocytosis. However, how calcium triggers endocytosis is not well understood. Using genetic tools together with capacitance measurements, optical imaging and electron microscopy, we identified two calcium sensors, including protein kinase C (α and β isoforms) and calmodulin, for the most commonly observed forms of endocytosis: slow, rapid, and bulk. We also found that these two proteins are involved in calcium-dependent vesicle mobilization to the readily releasable pool. These results provide the molecular signaling link among calcium, endocytosis, and vesicle mobilization that are essential in sustaining synaptic transmission and neuronal network activity.

Mass spectrometry strategies to unveil modified aminophospholipids of biological interest

The biological functions of modified aminophospholipids (APL) have become a topic of interest during the last two decades, and distinct roles have been found for these biomolecules in both physiological and pathological contexts. Modifications of APL include oxidation, glycation, and adduction to electrophilic aldehydes, altogether contributing to a high structural variability of modified APL. An outstanding technique used in this challenging field is mass spectrometry (MS). MS has been widely used to unveil modified APL of biological interest, mainly when associated with soft ionization methods (electrospray and matrix-assisted laser desorption ionization) and coupled with separation techniques as liquid chromatography. This review summarizes the biological roles and the chemical mechanisms underlying APL modifications, and comprehensively reviews the current MS-based knowledge that has been gathered until now for their analysis. The interpretation of the MS data obtained by in vitro-identification studies is explained in detail. The perspective of an analytical detection of modified APL in clinical samples is explored, highlighting the fundamental role of MS in unveiling APL modifications and their relevance in pathophysiology.

Endothelial permeability, LDL deposition, and cardiovascular risk factors-a review

Early atherosclerosis features functional and structural changes in the endothelial barrier function that affect the traffic of molecules and solutes between the vessel lumen and the vascular wall. Such changes are mechanistically related to the development of atherosclerosis. Proatherogenic stimuli and cardiovascular risk factors, such as dyslipidaemias, diabetes, obesity, and smoking, all increase endothelial permeability sharing a common signalling denominator: an imbalance in the production/disposal of reactive oxygen species (ROS), broadly termed oxidative stress. Mostly as a consequence of the activation of enzymatic systems leading to ROS overproduction, proatherogenic factors lead to a pro-inflammatory status that translates in changes in gene expression and functional rearrangements, including changes in the transendothelial transport of molecules, leading to the deposition of low-density lipoproteins (LDL) and the subsequent infiltration of circulating leucocytes in the intima. In this review, we focus on such early changes in atherogenesis and on the concept that proatherogenic stimuli and risk factors for cardiovascular disease, by altering the endothelial barrier properties, co-ordinately trigger the accumulation of LDL in the intima and ultimately plaque formation.

Historical perspective: phosphatidylserine and phosphatidylethanolamine from the 1800s to the present

This article provides a historical account of the discovery, chemistry, and biochemistry of two ubiquitous phosphoglycerolipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), including the ether lipids. In addition, the article describes the biosynthetic pathways for these phospholipids and how these pathways were elucidated. Several unique functions of PS and PE in mammalian cells in addition to their ability to define physical properties of membranes are discussed. For example, the translocation of PS from the inner to the outer leaflet of the plasma membrane of cells occurs during apoptosis and during some other specific physiological processes, and this translocation is responsible for profound life-or-death events. Moreover, mitochondrial function is severely impaired when the PE content of mitochondria is reduced below a threshold level. The discovery and implications of the existence of membrane contact sites between the endoplasmic reticulum and mitochondria and their relevance for PS and PE metabolism, as well as for mitochondrial function, are also discussed. Many of the recent advances in these fields are due to the use of isotope labeling for tracing biochemical pathways. In addition, techniques for disruption of specific genes in mice are now widely used and have provided major breakthroughs in understanding the roles and metabolism of PS and PE in vivo.

Autophagy and Macropinocytosis: Keeping an Eye on the Corneal/Limbal Epithelia

Autophagy and macropinocytosis are processes that are vital for cellular homeostasis, and help cells respond to stress and take up large amounts of material, respectively. The limbal and corneal epithelia have the machinery necessary to carry out both processes; however, autophagy and macropinocytosis are relatively understudied in these two epithelia. In this Perspectives, we describe the basic principles behind macropinocytosis and autophagy, discuss how these two processes are regulated in the limbal and corneal epithelia, consider how these two processes impact on the physiology of limbal and corneal epithelia, and elaborate on areas of future research in autophagy and macropinocytosis as related to the limbal/corneal epithelia.

MicroRNAs-103/107 coordinately regulate macropinocytosis and autophagy

The miR-103/107 family is preferentially expressed in the stem cell–enriched limbal epithelium and regulates multiple characteristics associated with stem cells. Park et al. show that miR-103/107 also contribute to limbal epithelial homeostasis by suppressing macropinocytosis and preserving end-stage autophagy.

Macropinocytosis, by which cells ingest large amounts of fluid, and autophagy, the lysosome-based catabolic process, involve vesicular biogenesis (early stage) and turnover (end stage). Much is known about early-stage events; however, our understanding of how the end stages of these processes are governed is incomplete. Here we demonstrate that the microRNA-103/107(miR-103/107) family, which is preferentially expressed in the stem cell–enriched limbal epithelium, coordinately regulates aspects of both these activities. Loss of miR-103/107 causes dysregulation of macropinocytosis with the formation of large vacuoles, primarily through up-regulation of Src, Ras, and Ankfy1. Vacuole accumulation is not a malfunction of early-stage autophagy; rather, miR-103/107 ensure proper end-stage autophagy by regulating diacylglycerol/protein kinase C and cyclin-dependent kinase 5 signaling, which enables dynamin to function in vacuole clearance. Our findings unveil a key biological function for miR-103/107 in coordinately suppressing macropinocytosis and preserving end-stage autophagy, thereby contributing to maintenance of a stem cell–enriched epithelium.

Analysis of protein phosphorylation in nerve terminal reveals extensive changes in active zone proteins upon exocytosis

Neurotransmitter release is mediated by the fast, calcium-triggered fusion of synaptic vesicles with the presynaptic plasma membrane, followed by endocytosis and recycling of the membrane of synaptic vesicles. While many of the proteins governing these processes are known, their regulation is only beginning to be understood. Here we have applied quantitative phosphoproteomics to identify changes in phosphorylation status of presynaptic proteins in resting and stimulated nerve terminals isolated from the brains of Wistar rats. Using rigorous quantification, we identified 252 phosphosites that are either up- or downregulated upon triggering calcium-dependent exocytosis. Particularly pronounced were regulated changes of phosphosites within protein constituents of the presynaptic active zone, including bassoon, piccolo, and RIM1. Additionally, we have mapped kinases and phosphatases that are activated upon stimulation. Overall, our study provides a snapshot of phosphorylation changes associated with presynaptic activity and provides a foundation for further functional analysis of key phosphosites involved in presynaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.14530.001

The human nervous system contains more than a hundred billion neurons that are connected with each other via junctions called synapses. When an electrical impulse travelling along a neuron arrives at a synapse, it triggers bubble-like packages called synaptic vesicles within the neuron to merge with the neuron’s surface membrane. The contents of these vesicles – chemical messengers called neurotransmitters – are then released into the synapse and carry the signal to the next neuron.

Complex molecular machines made from many different proteins control the release of neurotransmitters. Quite a few of these proteins are regulated by the addition of phosphate groups at specific sites. However, not all of the proteins involved in the release of neurotransmitters have been studied in detail and it is largely unclear how most of them are regulated.

Now, Kohansal-Nodehi et al. have used techniques involving mass spectrometry to find out which proteins have phosphate groups added or removed in neurons that are releasing neurotransmitters. The experiments used pinched-off synapses isolated from rat brains. These structures, referred to as “synaptosomes”, lend themselves to this kind of study because they can be induced to continuously release neurotransmitters for several minutes. Kohansal-Nodehi et al. identified over 250 specific sites on proteins in the synaptosomes where phosphate groups are attached, including many on the key proteins known to operate in neurotransmitter release. Moreover, some proteins were modified at multiple sites, especially the proteins that form a scaffold to capture synaptic vesicles close to the membrane and prepare them for release. The data also revealed important clues about the enzymes that either attach or remove the phosphate groups.

Together, these findings provide new insights into the regulatory networks that control many proteins at the same time. The next challenge is to sort out which of these modifications change the interactions between the proteins that control neurotransmitter release, and to understand how these changes influence the trafficking of synaptic vesicles.

DOI:http://dx.doi.org/10.7554/eLife.14530.002

Phosphorylation of PACSIN2 by protein kinase C triggers the removal of caveolae from the plasma membrane

PACSIN2, a membrane-sculpting BAR domain protein, localizes to caveolae. Here, we found that protein kinase C (PKC) phosphorylates PACSIN2 at serine 313, thereby decreasing its membrane binding and tubulation capacities. Concomitantly, phosphorylation decreased the time span for which caveolae could be tracked at the plasma membrane (the 'tracking duration'). Analyses of the phospho-mimetic S313E mutant suggested that PACSIN2 phosphorylation was sufficient to reduce caveolar-tracking durations. Both hypotonic treatment and isotonic drug-induced PKC activation increased PACSIN2 phosphorylation at serine 313 and shortened caveolar-tracking durations. Caveolar-tracking durations were also reduced upon the expression of other membrane-binding-deficient PACSIN2 mutants or upon RNA interference (RNAi)-mediated PACSIN2 depletion, pointing to a role for PACSIN2 levels in modulating the lifetime of caveolae. Interestingly, the decrease in membrane-bound PACSIN2 was inversely correlated with the recruitment and activity of dynamin 2, a GTPase that mediates membrane scission. Furthermore, expression of EHD2, which stabilizes caveolae and binds to PACSIN2, restored the tracking durations of cells with reduced PACSIN2 levels. These findings suggest that the PACSIN2 phosphorylation decreases its membrane-binding activity, thereby decreasing its stabilizing effect on caveolae and triggering dynamin-mediated removal of caveolae.

C-reactive protein promotes atherosclerosis by increasing LDL transcytosis across endothelial cells

BACKGROUND AND PURPOSE

The retention of plasma low-density lipoprotein (LDL) particles in subendothelial space following transcytosis across the endothelium is the initial step of atherosclerosis. Whether or not C-reactive protein (CRP) can directly affect the transcytosis of LDL is not clear. Here we have examined the effect of CRP on transcytosis of LDL across endothelial cells and have explored the underlying mechanisms.

EXPERIMENTAL APPROACH

Effects of CRP on transcytosis of FITC-labelled LDL were examined with human umbilical vein endothelial cells and venous rings in vitro and, in vivo, ApoE(-/-) mice. Laser scanning confocal microscopy, immunohistochemistry and Oil Red O staining were used to assay LDL.

KEY RESULTS

CRP increased transcytosis of LDL. An NADPH oxidase inhibitor, diphenylene iodonium, and the reducing agent, dithiothreitol partly or completely blocked CRP-stimulated increase of LDL transcytosis. The PKC inhibitor, bisindolylmaleimide I and the Src kinase inhibitor, PP2, blocked the trafficking of the molecules responsible for transcytosis. Confocal imaging analysis revealed that CRP stimulated LDL uptake by endothelial cells and vessel walls. In ApoE(-/-) mice, CRP significantly promoted early changes of atherosclerosis, which were blocked by inhibitors of transcytosis.

CONCLUSIONS AND IMPLICATIONS

CRP promoted atherosclerosis by directly increasing the transcytosis of LDL across endothelial cells and increasing LDL retention in vascular walls. These actions of CRP were associated with generation of reactive oxygen species, activation of PKC and Src, and translocation of caveolar or soluble forms of the N-ethylmaleimide-sensitive factor attachment protein.

Replacement of Arg-386 with Gly in dynamin 1 middle domain reduced GTPase activity and oligomer stability in the absence of lipids

Dynamin plays an important role in membrane fission during endocytosis, and its middle domain is involved in the formation of functional oligomers. In this study, we found that replacement of Arg-386 with Gly in the middle domain region of dynamin 1 did not affect the intermolecular interactions of dynamin 1 in the presence of phosphatidylserine-liposomes. But, unexpectedly, this variant showed lower guanosine 5'-triphosphatase activity in the absence of phosphatidylserine-liposomes and enhanced monomer formation from oligomers. Our results indicate that GTPase activity in the absence of lipids is important in the dissociation of oligomer complexes, i.e., reduced basal dynamin 1 GTPase activity is associated with instability of dynamin oligomers.

Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are metabolically related membrane aminophospholipids. In mammalian cells, PS is required for targeting and function of several intracellular signaling proteins. Moreover, PS is asymmetrically distributed in the plasma membrane. Although PS is highly enriched in the cytoplasmic leaflet of plasma membranes, PS exposure on the cell surface initiates blood clotting and removal of apoptotic cells. PS is synthesized in mammalian cells by two distinct PS synthases that exchange serine for choline or ethanolamine in phosphatidylcholine (PC) or PE, respectively. Targeted disruption of each PS synthase individually in mice demonstrated that neither enzyme is required for viability whereas elimination of both synthases was embryonic lethal. Thus, mammalian cells require a threshold amount of PS. PE is synthesized in mammalian cells by four different pathways, the quantitatively most important of which are the CDP-ethanolamine pathway that produces PE in the ER, and PS decarboxylation that occurs in mitochondria. PS is made in ER membranes and is imported into mitochondria for decarboxylation to PE via a domain of the ER [mitochondria-associated membranes (MAM)] that transiently associates with mitochondria. Elimination of PS decarboxylase in mice caused mitochondrial defects and embryonic lethality. Global elimination of the CDP-ethanolamine pathway was also incompatible with mouse survival. Thus, PE made by each of these pathways has independent and necessary functions. In mammals PE is a substrate for methylation to PC in the liver, a substrate for anandamide synthesis, and supplies ethanolamine for glycosylphosphatidylinositol anchors of cell-surface signaling proteins. Thus, PS and PE participate in many previously unanticipated facets of mammalian cell biology. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Enzymatic measurement of phosphatidylserine in cultured cells

Phosphatidylserine (PS) is a quantitatively minor membrane phospholipid involved in diverse cellular functions. In this study, we developed a new fluorometric method for measuring PS using combinations of specific enzymes and Amplex Red. The calibration curve for PS measurement was linear and hyperbolic at low (0-50 µM) and high (50-1000 µM) concentrations, respectively, and the detection limit was 5 µM (50 pmol in the reaction mixture). This assay quantified PS regardless of the chain length and the number of double bonds. We applied this new method to the determination of PS content in HEK293 cells, which was validated by a recovery study and comparison with TLC-phosphorus assay. We showed that the PS content was high in sparse cells. The overexpression of PS synthase 1 elevated not only the cellular PS content but also the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) contents, suggesting the conversion of PS into PE and the enhancement of PC production. This new assay for PS measurement is simple, specific, sensitive, and high throughput, and it will be useful to clarify the metabolism and biological functions of PS.

Proper synaptic vesicle formation and neuronal network activity critically rely on syndapin I

Synaptic transmission relies on effective and accurate compensatory endocytosis. F-BAR proteins may serve as membrane curvature sensors and/or inducers and thereby support membrane remodelling processes; yet, their in vivo functions urgently await disclosure. We demonstrate that the F-BAR protein syndapin I is crucial for proper brain function. Syndapin I knockout (KO) mice suffer from seizures, a phenotype consistent with excessive hippocampal network activity. Loss of syndapin I causes defects in presynaptic membrane trafficking processes, which are especially evident under high-capacity retrieval conditions, accumulation of endocytic intermediates, loss of synaptic vesicle (SV) size control, impaired activity-dependent SV retrieval and defective synaptic activity. Detailed molecular analyses demonstrate that syndapin I plays an important role in the recruitment of all dynamin isoforms, central players in vesicle fission reactions, to the membrane. Consistently, syndapin I KO mice share phenotypes with dynamin I KO mice, whereas their seizure phenotype is very reminiscent of fitful mice expressing a mutant dynamin. Thus, syndapin I acts as pivotal membrane anchoring factor for dynamins during regeneration of SVs.

Surface expression and limited proteolysis of ADAM10 are increased by a dominant negative inhibitor of dynamin

Background

The amyloid precursor protein (APP) is cleaved by β- and γ-secretases to generate toxic amyloid β (Aβ) peptides. Alternatively, α-secretases cleave APP within the Aβ domain, precluding Aβ formation and releasing the soluble ectodomain, sAPPα. We previously showed that inhibition of the GTPase dynamin reduced APP internalization and increased release of sAPPα, apparently by prolonging the interaction between APP and α-secretases at the plasma membrane. This was accompanied by a reduction in Aβ generation. In the present study, we investigated whether surface expression of the α-secretase ADAM (a disintegrin and metalloprotease)10 is also regulated by dynamin-dependent endocytosis.

Results

Transfection of human embryonic kidney (HEK) cells stably expressing M3 muscarinic receptors with a dominant negative dynamin I mutant (dyn I K44A), increased surface expression of both immature, and mature, catalytically active forms of co-expressed ADAM10. Surface levels of ADAM10 were unaffected by activation of protein kinase C (PKC) or M3 receptors, indicating that receptor-coupled shedding of the ADAM substrate APP is unlikely to be mediated by inhibition of ADAM10 endocytosis in this cell line. Dyn I K44A strongly increased the formation of a C-terminal fragment of ADAM10, consistent with earlier reports that the ADAM10 ectodomain is itself a target for sheddases. The abundance of this fragment was increased in the presence of a γ-secretase inhibitor, but was not affected by M3 receptor activation. The dynamin mutant did not affect the distribution of ADAM10 and its C-terminal fragment between raft and non-raft membrane compartments.

Conclusions

Surface expression and limited proteolysis of ADAM10 are regulated by dynamin-dependent endocytosis, but are unaffected by activation of signaling pathways that upregulate shedding of ADAM substrates such as APP. Modulation of ADAM10 internalization could affect cellular behavior in two ways: by altering the putative signaling activity of the ADAM10 C-terminal fragment, and by regulating the biological function of ADAM10 substrates such as APP and N-cadherin.

Residues within a lipid-associated segment of the PECAM-1 cytoplasmic domain are susceptible to inducible, sequential phosphorylation

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptors inhibit cellular responsiveness to immunoreceptor tyrosine-based activation motif (ITAM)-linked receptors. Although tyrosine phosphorylation is central to the initiation of both inhibitory ITIM and stimulatory ITAM signaling, the events that regulate receptor phosphorylation are incompletely understood. Previous studies have shown that ITAM tyrosines engage in structure-inducing interactions with the plasma membrane that must be relieved for phosphorylation to occur. Whether ITIM phosphorylation is similarly regulated and the mechanisms responsible for release from plasma membrane interactions to enable phosphorylation, however, have not been defined. PECAM-1 is a dual ITIM-containing receptor that inhibits ITAM-dependent responses in hematopoietic cells. We found that the PECAM-1 cytoplasmic domain is unstructured in an aqueous environment but adopts an α-helical conformation within a localized region on interaction with lipid vesicles that mimic the plasma membrane. The lipid-interacting segment contains the C-terminal ITIM tyrosine and a serine residue that undergo activation-dependent phosphorylation. The N-terminal ITIM is excluded from the lipid-interacting segment, and its phosphorylation is secondary to phosphorylation of the membrane-interacting C-terminal ITIM. On the basis of these findings, we propose a novel model for regulation of inhibitory signaling by ITIM-containing receptors that relies on reversible plasma membrane interactions and sequential ITIM phosphorylation.

The distribution and function of phosphatidylserine in cellular membranes

Phosphatidylserine (PS) is the most abundant negatively charged phospholipid in eukaryotic membranes. PS directs the binding of proteins that bear C2 or gamma-carboxyglutamic domains and contributes to the electrostatic association of polycationic ligands with cellular membranes. Rather than being evenly distributed, PS is found preferentially in the inner leaflet of the plasma membrane and in endocytic membranes. The loss of PS asymmetry is an early indicator of apoptosis and serves as a signal to initiate blood clotting. This review discusses the determinants and functional implications of the subcellular distribution and membrane topology of PS.

The in vivo phosphorylation sites in multiple isoforms of amphiphysin I from rat brain nerve terminals

Amphiphysin I (amphI) is dephosphorylated by calcineurin during nerve terminal depolarization and synaptic vesicle endocytosis (SVE). Some amphI phosphorylation sites (phosphosites) have been identified with in vitro studies or phosphoproteomics screens. We used a multifaceted strategy including 32P tracking to identify all in vivo amphI phosphosites and determine their relative abundance and potential relevance to SVE. AmphI was extracted from 32P-labeled synaptosomes, phosphopeptides were isolated from proteolytic digests using TiO2 chromatography, and mass spectrometry revealed 13 sites: serines 250, 252, 262, 268, 272, 276, 285, 293, 496, 514, 539, and 626 and Thr-310. These were distributed into two clusters around the proline-rich domain and the C-terminal Src homology 3 domain. Hierarchical phosphorylation of Ser-262 preceded phosphorylation of Ser-268, -272, -276, and -285. Off-line HPLC separation and two-dimensional tryptic mapping of 32P-labeled amphI revealed that Thr-310, Ser-293, Ser-285, Ser-272, Ser-276, and Ser-268 contained the highest 32P incorporation and were the most stimulus-sensitive. Individually Thr-310 and Ser-293 were the most abundant phosphosites, incorporating 16 and 23% of the 32P. The multiple phosphopeptides containing Ser-268, Ser-276, Ser-272, and Ser-285 had 27% of the 32P. Evidence for a role for at least one proline-directed protein kinase and one non-proline-directed kinase was obtained. Four phosphosites predicted for non-proline-directed kinases, Ser-626, -250, -252, and -539, contained low amounts of 32P and were not depolarization-responsive. At least one alternatively spliced amphI isoform was identified in synaptosomes as being constitutively phosphorylated because it did not incorporate 32P during the 1-h labeling period. Multiple phosphosites from amphI-co-migrating synaptosomal proteins were also identified, including SGIP (Src homology 3 domain growth factor receptor-bound 2 (Grb2)-like (endophilin)-interacting protein 1), AAK1, eps15R, MAP6, alpha/beta-adducin, and HCN1. The results reveal two sets of amphI phosphosites that are either dynamically turning over or constitutively phosphorylated in nerve terminals and improve understanding of the role of individual amphI sites or phosphosite clusters in synaptic SVE.

Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are two aminophospholipids whose metabolism is interrelated. Both phospholipids are components of mammalian cell membranes and play important roles in biological processes such as apoptosis and cell signaling. PS is synthesized in mammalian cells by base-exchange reactions in which polar head groups of preexisting phospholipids are replaced by serine. PS synthase activity resides primarily on mitochondria-associated membranes and is encoded by two distinct genes. Studies in mice in which each gene has been individually disrupted are beginning to elucidate the importance of these two synthases for biological functions in intact animals. PE is made in mammalian cells by two completely independent major pathways. In one pathway, PS is converted into PE by the mitochondrial enzyme PS decarboxylase. In addition, PE is made via the CDP-ethanolamine pathway, in which the final reaction occurs on the endoplasmic reticulum and nuclear envelope. The relative importance of these two pathways of PE synthesis has been investigated in knockout mice. Elimination of either pathway is embryonically lethal, despite the normal activity of the other pathway. PE can also be generated from a base-exchange reaction and by the acylation of lyso-PE. Cellular levels of PS and PE are tightly regulated by the implementation of multiple compensatory mechanisms.

The interplay between clathrin-coated vesicles and cell signalling

Internalization of cargo proteins and lipids at the cell surface occurs in both a constitutive and signal-regulated manner through clathrin-mediated and other endocytic pathways. Clathrin-coated vesicle formation is a principal uptake route in response to signalling events. Protein-lipid and protein-protein interactions control both the targeting of signalling molecules and their binding partners to membrane compartments and the assembly of clathrin coats. An emerging aspect of membrane trafficking research is now addressing how signalling cascades and vesicle coat assembly and subsequently disassembly are integrated.

The in vivo phosphorylation sites of rat brain dynamin I

Dynamin I (dynI) is phosphorylated in synaptosomes at Ser(774) and Ser(778) by cyclin-dependent kinase 5 to regulate recruitment of syndapin I for synaptic vesicle endocytosis, and in PC12 cells on Ser(857). Hierarchical phosphorylation of Ser(774) precedes phosphorylation of Ser(778). In contrast, Thr(780) phosphorylation by cdk5 has been reported as the sole site (Tomizawa, K., Sunada, S., Lu, Y. F., Oda, Y., Kinuta, M., Ohshima, T., Saito, T., Wei, F. Y., Matsushita, M., Li, S. T., Tsutsui, K., Hisanaga, S. I., Mikoshiba, K., Takei, K., and Matsui, H. (2003) J. Cell Biol. 163, 813-824). To resolve the discrepancy and to better understand the biological roles of dynI phosphorylation, we undertook a systematic identification of all phosphorylation sites in rat brain nerve terminal dynI. Using phosphoamino acid analysis, exclusively phospho-serine residues were found. Thr(780) phosphorylation was not detectable. Mutation of Ser(774), Ser(778), and Thr(780) confirmed that Thr(780) phosphorylation is restricted to in vitro conditions. Mass spectrometry of (32)P-labeled phosphopeptides separated by two-dimensional mapping revealed seven in vivo phosphorylation sites: Ser(774), Ser(778), Ser(822), Ser(851), Ser(857), Ser(512), and Ser(347). Quantification of (32)P radiation in each phosphopeptide showed that Ser(774) and Ser(778) were the major sites (up to 69% of the total), followed by Ser(851) and Ser(857) (12%), and Ser(853) (2%). Phosphorylation of Ser(851) and Ser(857) was restricted to the long tail splice variant dynIxa and was not hierarchical. Co-purified, (32)P-labeled dynIII was phosphorylated at Ser(759), Ser(763), and Ser(853). Ser(853) is homologous to Ser(851) in dynIxa. The results identify all major and several minor phosphorylation sites in dynI and provide the first measure of their relative abundance and relative responses to depolarization. The multiple phospho-sites suggest subtle regulation of synaptic vesicle endocytosis by new protein kinases and new protein-protein interactions. The homologous dynI and dynIII phosphorylation indicates a high mechanistic similarity. The results suggest a unique role for the long splice variants of dynI and dynIII in nerve terminals.

Calcineurin interacts with KIN-29, a Ser/Thr kinase, in Caenorhabditis elegans

Calcineurin is a Ca2+/Calmodulin activated Ser/Thr phosphatase that is well conserved from yeast to human. In Caenorhabditis elegans, tax-6 and cnb-1 encode catalytic and regulatory subunits of calcineurin, respectively. We performed yeast two-hybrid screening using TAX-6 as a bait to identify calcineurin interacting proteins. KIN-29 is one of proteins that specifically interacted with TAX-6. KIN-29 is a Ser/Thr kinase previously shown to be involved in regulating gene expression of a subset of chemoreceptors in specific neurons. Both TAX-6 and KIN-29 are expressed in hypodermis, muscles, and neurons. Moreover, both calcineurin and kin-29 mutants exhibit similar phenotypes, namely small body size, small brood size, and slow growth. Here we describe specific genetic interaction between tax-6 and kin-29 in regulating body size, serotonin mediated egg laying, and chemoreceptor expression.

Opposing effects of phorbol-12-myristate-13-acetate, an activator of protein kinase C, on the signaling of structurally related human dopamine D1 and D5 receptors

The 'cross-talk' between different types of neurotransmitters through second messenger pathways represents a major regulatory mechanism in neuronal function. We investigated the effects of activation of protein kinase C (PKC) on cAMP-dependent signaling by structurally related human D1-like dopaminergic receptors. Human embryonic kidney 293 (HEK293) cells expressing D1 or D5 receptors were pretreated with phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, followed by analysis of dopamine-mediated receptor activation using whole cell cAMP assays. Unpredictably, PKC activation had completely opposite effects on D1 and D5 receptor signaling. PMA dramatically augmented agonist-evoked D1 receptor signaling, whereas constitutive and dopamine-mediated D5 receptor activation were rapidly blunted. RT-PCR and immunoblotting analyses showed that phorbol ester-regulated PKC isozymes (conventional: alpha, betaI, betaII, gamma; novel: delta, epsilon, eta, theta) and protein kinase D (PKCmicro) are expressed in HEK293 cells. PMA appears to mediate these contrasting effects through the activation of Ca2+-independent novel PKC isoforms as revealed by specific inhibitors, bisindolylmaleimide I, Gö6976, and Gö6983. The finding that cross-talk between PKC and cAMP pathways can produce such opposite outcomes following the activation of structurally similar D1-like receptor subtypes is novel and further strengthens the view that D1 and D5 receptors serve distinct functions in the mammalian nervous and endocrine systems.

Asymmetric Distribution of Phospholipids in Biomembranes

In eukaryotic cells, the biological membrane is characterized by a non-uniform distribution of membrane lipids, vertically as well as laterally. The paradigm for the vertical non-random distribution is the plasma membrane, where phosphatidylcholine (PC), sphingomyelin (SM), and glycosphingolipids are primarily located on the exoplasmic leaflet, while aminophospholipids, including phosphatidylserine (PS) and phosphatidylethanolamine (PE), are generally enriched in the cytoplasmic leaflet. Other minor phospholipids, such as phosphatidic acid and phosphatidylinositol (PI), are also enriched on the cytoplasmic face. Such asymmetrical distribution is related to each lipid regulating various biological events through interaction with other molecules. The clarification of the regulatory mechanism of the distribution and movement of membrane lipids is crucial to understanding the physiological roles of lipids. Here we focus on PS, which has been reported to be involved in apoptosis, blood coagulation and other biological phenomena, and summarize the present understanding of the dynamics of this phospholipid, including biosynthesis, metabolism, transport, and transbilayer movement. We also refer to diseases that have been reported to be related to phospholipid asymmetry.

Genetic enhancement of visual learning by activation of protein kinase C pathways in small groups of rat cortical neurons

Although learning and memory theories hypothesize that memories are encoded by specific circuits, it has proven difficult to localize learning within a cortical area. Neural network theories predict that activation of a small fraction of the neurons in a circuit can activate that circuit. Consequently, altering the physiology of a small group of neurons might potentiate a specific circuit and enhance learning, thereby localizing learning to that circuit. In this study, we activated protein kinase C (PKC) pathways in small groups of neurons in rat postrhinal (POR) cortex. We microinjected helper virus-free herpes simplex virus vectors that expressed a constitutively active PKC into POR cortex. This PKC was expressed predominantly in glutamatergic and GABAergic neurons in POR cortex. This intervention increased phosphorylation of five PKC substrates that play critical roles in neurotransmitter release (GAP-43 and dynamin) or glutamatergic neurotransmission (specific subunits of AMPA or NMDA receptors and myristoylated alanine-rich C kinase substrate). Additionally, activation of PKC pathways in cultured cortical neurons supported activation-dependent increases in release of glutamate and GABA. This intervention enhanced the learning rate and accuracy of visual object discriminations. In individual rats, the numbers of transfected neurons positively correlated with this learning. During learning, neuronal activity was increased in neurons proximal to the transfected neurons. These results demonstrate that potentiating small groups of glutamatergic and GABAergic neurons in POR cortex enhances visual object learning. More generally, these results suggest that learning can be mediated by specific cortical circuits.

Inhibition of dynamin-dependent endocytosis increases shedding of the amyloid precursor protein ectodomain and reduces generation of amyloid beta protein

Background

The amyloid precursor protein (APP) is transported via the secretory pathway to the cell surface, where it may be cleaved within its ectodomain by α-secretase, or internalized within clathrin-coated vesicles. An alternative proteolytic pathway occurs within the endocytic compartment, where the sequential action of β- and γ-secretases generates the amyloid β protein (Aβ). In this study, we investigated the effects of modulators of endocytosis on APP processing.

Results

Human embryonic kidney cells were transfected with a dominant negative mutant of dynamin I, an important mediator of clathrin-dependent endocytosis, and APP proteolysis was analyzed. Overexpression of the mutant dynamin (dyn I K44A) resulted in increased shedding of the APP ectodomain (sAPPα), accumulation of the C-terminal α-secretase product C83, and a reduction in the release of Aβ. Levels of mature APP on the cell surface were increased in cells expressing dyn I K44A, and internalization of surface-immunolabeled APP, assessed by fluorescence microscopy, was inhibited. Dynamin is a substrate for protein kinase C (PKC), and it was hypothesized that activators of PKC, which are known to stimulate α-secretase-mediated cleavage of APP, might exert their effects by inhibiting dynamin-dependent endocytosis. However, the internalization of surface-biotinylated APP was unaffected by treatment of cells with phorbol 12-myristate 13-acetate in the presence of the α-secretase inhibitor TAPI-1.

Conclusion

The results indicate that APP is internalized by a dynamin-dependent process, and suggest that alterations in the activity of proteins that mediate endocytosis might lead to significant changes in Aβ production.

Dynamin I phosphorylation and the control of synaptic vesicle endocytosis

The GTPase dynamin I is essential for synaptic vesicle endocytosis in nerve terminals. It is a nerve terminal phosphoprotein that is dephosphorylated on nerve terminal stimulation by the calcium-dependent protein phosphatase calcineurin and then rephosphorylated by cyclin-dependent kinase 5 on termination of the stimulus. Because of its unusual phosphorylation profile, the phosphorylation status of dynamin I was assumed to be inexorably linked to synaptic vesicle endocytosis; however, direct proof of this link has been elusive until very recently. This review will describe current knowledge regarding dynamin I phosphorylation in nerve terminals and how this regulates its biological function with respect to synaptic vesicle endocytosis.

A proteomic approach to identify phosphoproteins encoded by cDNA libraries

We report a method for large-scale rapid analysis of phosphoproteins in tissues or cells by combining immobilized metal affinity chromatography (IMAC) with phage display cDNA library screening. We expressed a testis cDNA library as fusion proteins on phage and, using IMAC, enriched for sequences encoding phosphoproteins. Selected clones were polymerase chain reaction amplified and sequenced. The majority of the clones sequenced (80%) encoded known proteins previously identified as phosphoproteins. Immunoblotting with phosphotyrosine antibodies confirmed that some of the selected sequences encoded tyrosine phosphorylated proteins when expressed on phage. An advantage of this method is the rapid identification of phosphoproteins encoded by a cDNA library, which can identify proteins that are potentially phosphorylated in vivo. When this method is combined with limited enzymatic digestion and tandem mass spectrometric techniques, the specific phosphorylation site in a protein can be identified. This technique can be used in proteomics studies to effectively detect phosphorylated proteins and avoid time-consuming and expensive peptide sequencing.

Externalization of phosphatidylserine during apoptosis does not specifically require either isoform of phosphatidylserine synthase

Phosphatidylserine (PtdSer) is made in mammalian cells by two PtdSer synthases, PSS1 and PSS2. In the plasma membrane PtdSer is normally localized on the inner leaflet but undergoes transbilayer movement during apoptosis and becomes exposed on the cell surface. We induced apoptosis with staurosporine in four Chinese hamster ovary (CHO) cell lines that are deficient in PSS1 and/or PSS2 to determine if PtdSer generated by either of these enzymes is required for externalization on the cell surface during apoptosis. The onset of apoptosis was confirmed by the appearance of morphological changes and DNA fragmentation while the plasma membrane remained largely intact. In all cell lines, regardless of their content of PSS1 and/or PSS2, apoptosis occurred to approximately the same extent, and within approximately the same time frame, as in parental CHO-K1 cells. The exposure of PtdSer on the cell surface was assessed by annexin V labeling and flow cytometry. Cells that were deficient in either PSS1 or PSS2, as well as cells that were deficient in both PSS1 and PSS2, externalized normal amounts of PtdSer. Our study demonstrates, that reduction of in vitro serine-exchange activity, even by 97%, does not restrict the externalization of PtdSer during apoptosis. Moreover, a normal level of expression of PSS1 and/or PSS2 is not required for generating the pool of PtdSer externalized during apoptosis.

Ca2+-dependent inhibition of NHE3 requires PKC alpha which binds to E3KARP to decrease surface NHE3 containing plasma membrane complexes

The intestinal brush border (BB) Na+/H+ exchanger isoform 3 (NHE3) is acutely inhibited by elevation in the concentration of free intracellular Ca2+ ([Ca2+]i) by the cholinergic agonist carbachol and Ca2+ ionophores in a protein kinase C (PKC)-dependent manner. We previously showed that elevating [Ca2+]i with ionomycin rapidly inhibited NHE3 activity and decreased the amount of NHE3 on the plasma membrane in a manner that depended on the presence of the PDZ domain-containing protein E3KARP (NHE3 kinase A regulatory protein, also called NHERF2). The current studies were performed in PS120 fibroblasts (NHE-null cell line) stably transfected with NHE3 and E3KARP to probe the mechanism of PKC involvement in Ca2+ regulation of NHE3. Pretreatment with the general PKC inhibitor, GF109203X prevented ionomycin inhibition of NHE3 without altering basal NHE3 activity. Similarly, the Ca2+-mediated inhibition of NHE3 activity was blocked after pretreatment with the conventional PKC inhibitor Gö-6976 and a specific PKCalpha pseudosubstrate-derived inhibitor peptide. [Ca2+]i elevation caused translocation of PKCalpha from cytosol to membrane. PKCalpha bound to the PDZ1 domain of GST-E3KARP in vitro in a Ca2+-dependent manner. PKCalpha and E3KARP coimmunoprecipitated from cell lysates; this occurred to a lesser extent at basal [Ca2+]i and was increased with ionomycin exposure. Biotinylation studies demonstrated that [Ca2+]i elevation induced oligomerization of NHE3 in total lysates and decreased the amount of plasma membrane NHE3. Treatment with PKC inhibitors did not affect the oligomerization of NHE3 but did prevent the decrease in surface amount of NHE3. These results suggest that PKCalpha is not necessary for the Ca2+-dependent formation of the NHE3 plasma membrane complex, although it is necessary for decreasing the membrane amounts of NHE3, probably by stimulating NHE3 endocytosis.

Cdk5 is essential for synaptic vesicle endocytosis

Synaptic vesicle endocytosis (SVE) is triggered by calcineurin-mediated dephosphorylation of the dephosphin proteins. SVE is maintained by the subsequent rephosphorylation of the dephosphins by unidentified protein kinases. Here, we show that cyclin-dependent kinase 5 (Cdk5) phosphorylates dynamin I on Ser 774 and Ser 778 in vitro, which are identical to its endogenous phosphorylation sites in vivo. Cdk5 antagonists and expression of dominant-negative Cdk5 block phosphorylation of dynamin I, but not of amphiphysin or AP180, in nerve terminals and inhibit SVE. Thus Cdk5 has an essential role in SVE and is the first dephosphin kinase identified in nerve terminals.

Synapsin I-associated phosphatidylinositol 3-kinase mediates synaptic vesicle delivery to the readily releasable pool

Maintaining synaptic transmission requires replenishment of docked synaptic vesicles within the readily releasable pool (RRP) from synaptic vesicle clusters in the synapsin-bound reserve pool. We show that synapsin forms a complex with phosphatidylinositol 3-kinase (PI 3-kinase) in intact nerve terminals and that synapsin-associated kinase activity increases on depolarization. Disruption of either PI 3-kinase activity or its interaction with synapsin inhibited replenishment of the RRP, but did not affect exocytosis from the RRP. Thus we conclude that a synapsin-associated PI 3-kinase activity plays a role in synaptic vesicle delivery to the RRP. This also suggests that PI 3-kinase contributes to the maintenance of synaptic transmission during periods of high activity, indicating a possible role in synaptic plasticity.

A novel endocytic pathway induced by clustering endothelial ICAM-1 or PECAM-1

Antibody conjugates directed against intercellular adhesion molecule (ICAM-1) or platelet-endothelial cell adhesion molecule (PECAM-1) have formed the basis for drug delivery vehicles that are specifically recognized and internalized by endothelial cells. There is increasing evidence that ICAM-1 and PECAM-1 may also play a role in cell scavenger functions and pathogen entry. To define the mechanisms that regulate ICAM-1 and PECAM-1 internalization, we examined the uptake of anti-PECAM-1 and anti-ICAM-1 conjugates by endothelial cells. We found that the conjugates must be multimeric, because monomeric anti-ICAM-1 and anti-PECAM-1 are not internalized. Newly internalized anti-ICAM-1 and anti-PECAM-1 conjugates did not colocalize with either clathrin or caveolin, and immunoconjugate internalization was not reduced by inhibitors of clathrin-mediated or caveolar endocytosis, suggesting that this is a novel endocytic pathway. Amiloride and protein kinase C (PKC) inhibitors, agents known to inhibit macropinocytosis, reduced the internalization of clustered ICAM-1 and PECAM-1. However, expression of dominant-negative dynamin-2 constructs inhibited uptake of clustered ICAM-1. Binding of anti-ICAM-1 conjugates stimulated the formation of actin stress fibers by human umbilical vein endothelial cells (HUVEC). Latrunculin, radicicol and Y27632 also inhibited internalization of clustered ICAM-1, suggesting that actin rearrangements requiring Src kinase and Rho kinase (ROCK) were required for internalization. Interestingly, these kinases are part of the signal transduction pathways that are activated when circulating leukocytes engage endothelial cell adhesion molecules, suggesting the possibility that CAM-mediated endocytosis is regulated using comparable signaling pathways.

Dynamin participates in focal extracellular matrix degradation by invasive cells

The degradation of extracellular matrix (ECM) by matrix metalloproteases is crucial in physiological and pathological cell invasion alike. Degradation occurs at specific sites where invasive cells make contact with the ECM via specialized plasma membrane protrusions termed invadopodia. Herein, we show that the dynamin 2 (Dyn2), a GTPase implicated in the control of actin-driven cytoskeletal remodeling events and membrane transport, is necessary for focalized matrix degradation at invadopodia. Dynamin was inhibited by using two approaches: 1) expression of dominant negative GTPase-impaired or proline-rich domain-deleted Dyn2 mutants; and 2) inhibition of the dynamin regulator calcineurin by cyclosporin A. In both cases, the number and extension of ECM degradation foci were drastically reduced. To understand the site and mechanism of dynamin action, the cellular structures devoted to ECM degradation were analyzed by correlative confocal light-electron microscopy. Invadopodia were found to be organized into a previously undescribed ECM-degradation structure consisting of a large invagination of the ventral plasma membrane surface in close spatial relationship with the Golgi complex. Dyn2 seemed to be concentrated at invadopodia.

Molecular and cell biology of phosphatidylserine and phosphatidylethanolamine metabolism

In this review, the pathways for phosphatidylserine (PS) and phosphatidylethanolamine (PE) biosynthesis, as well as the genes and proteins involved in these pathways, are described in mammalian cells, yeast, and prokaryotes. In mammalian cells, PS is synthesized by a base-exchange reaction in which phosphatidylcholine or PE is substrate for PS synthase-1 or PS synthase-2, respectively. Isolation of Chinese hamster ovary cell mutants led to the cloning of cDNAs and genes encoding these two PS synthases. In yeast and prokaryotes PS is produced by a biosynthetic pathway completely different from that in mammals: from a reaction between CDP-diacylglycerol and serine. The major route for PE synthesis in cultured cells is from the mitochondrial decarboxylation of PS. Alternatively, PE can be synthesized in the endoplasmic reticulum (ER) from the CDP-ethanolamine pathway. Genes and/or cDNAs encoding all the enzymes in these two pathways for PE synthesis have been isolated and characterized. In mammalian cells, PS is synthesized on the ER and/or mitochondria-associated membranes (MAM). PS synthase-1 and -2 are highly enriched in MAM compared to the bulk of ER. Since MAM are a region of the ER that appears to be in close juxtaposition to the mitochondrial outer membrane, it has been proposed that MAM act as a conduit for the transfer of newly synthesized PS into mitochondria. A similar pathway appears to operate in yeast. The use of yeast mutants has led to identification of genes involved in the interorganelle transport of PS and PE in yeast, but so far none of the corresponding genes in mammalian cells has been identified. PS and PE do not act solely as structural components of membranes. Several specific functions have been ascribed to these two aminophospholipids. For example, cell-surface exposure of PS during apoptosis is thought to be the signal by which apoptotic cells are recognized and phagocytosed. Translocation of PS from the inner to outer leaflet of the plasma membrane of platelets initiates the blood-clotting cascade, and PS is an important activator of several enzymes, including protein kinase C. Recently, exposure of PE on the cell surface was identified as a regulator of cytokinesis. In addition, in Escherichia coli, PE appears to be involved in the correct folding of membrane proteins; and in Drosophila, PE regulates lipid homeostasis via the sterol response element-binding protein.

Relevance of dopamine signals anchoring dynamin-2 to the plasma membrane during Na+,K+-ATPase endocytosis

Clathrin-dependent endocytosis of Na(+),K(+)-ATPase in response to dopamine regulates its catalytic activity in intact cells. Because fission of clathrin-coated pits requires dynamin, we examined the mechanisms by which dopamine receptor signals promote dynamin-2 recruitment and assembly at the site of Na(+),K(+)-ATPase endocytosis. Western blotting revealed that dopamine increased the association of dynamin-2 with the plasma membrane and with phosphatidylinositol 3-kinase. Dopamine inhibited Na(+),K(+)-ATPase activity in OK cells and in those overexpressing wild type dynamin-2 but not in cells expressing a dominant-negative mutant. Dephosphorylation of dynamin is important for its assembly. Dopamine increased protein phosphatase 2A activity and dephosphorylated dynamin-2. In cells expressing a dominant-negative mutant of protein phosphatase 2A, dopamine failed to dephosphorylate dynamin-2 and to reduce Na(+),K(+)-ATPase activity. Dynamin-2 is phosphorylated at Ser(848), and expression of the S848A mutant significantly blocked the inhibitory effect of dopamine. These results demonstrate a distinct signaling network originating from the dopamine receptor that regulates the state of dynamin-2 phosphorylation and that promotes its location (by interaction with phosphatidylinositol 3-kinase) at the site of Na(+),K(+)-ATPase endocytosis.

Analysis of tyrosine phosphorylation sites in signaling molecules by a phosphotyrosine-specific immonium ion scanning method

Signal transduction pathways involve cascades of events, such as formation of second messengers and protein complexes that alter the activities of proteins. This can ultimately lead to changes in gene expression in response to the stimuli. Reversible phosphorylation of proteins is an important mechanism for activating or inhibiting enzymes and for the assembly of multiprotein complexes. Here, we describe a mass spectrometry-based phosphotyrosine-specific immonium ion scanning (PSI scanning) method for selective detection of tyrosine-phosphorylated peptides. Once the tyrosine-phosphorylated peptides are identified, they can be directly sequenced in the same experiment to localize the phosphorylation site. We provide protocols for isolation and preparation of samples for analysis, and detailed instructions for operation of a quadrupole time-of-flight (TOF) mass spectrometer for this method. Because of its simplicity and specificity, PSI scanning is likely to become an important tool in proteomic studies of pathways involving tyrosine phosphorylation.

Src-dependent tyrosine phosphorylation regulates dynamin self-assembly and ligand-induced endocytosis of the epidermal growth factor receptor

Endocytosis of ligand-activated receptors requires dynamin-mediated GTP hydrolysis, which is regulated by dynamin self-assembly. Here, we demonstrate that phosphorylation of dynamin I by c-Src induces its self-assembly and increases its GTPase activity. Electron microscopic analyses reveal that tyrosine-phosphorylated dynamin I spontaneously self-assembles into large stacks of rings. Tyrosine 597 was identified as being phosphorylated both in vitro and in cultured cells following epidermal growth factor receptor stimulation. The replacement of tyrosine 597 with phenylalanine impairs Src kinase-induced dynamin I self-assembly and GTPase activity in vitro. Expression of Y597F dynamin I in cells attenuates agonist-driven epidermal growth factor receptor internalization. Thus, c-Src-mediated tyrosine phosphorylation is required for the function of dynamin in ligand-induced signaling receptor internalization.

Molecular constituents and phosphorylation-dependent regulation of the post-synaptic density

The post-synaptic density (PSD) contains receptors with associated signaling- and scaffolding-proteins that organize signal-transduction pathways near the post-synaptic membrane. The PSD plays an important role in synaptic plasticity, and protein phosphorylation is critical to the regulation of PSD function, including learning and memory. Recently, studies have investigated the protein constituents of the PSD and substrate proteins for various protein kinases by proteomic analysis. The present review focuses on the molecular properties of PSD proteins, and substrates of protein kinases and their regulation by phosphorylation in order to understand the role of PSD in synaptic plasticity.

Analysis of proteins and proteomes by mass spectrometry

A decade after the discovery of electrospray and matrix-assisted laser desorption ionization (MALDI), methods that finally allowed gentle ionization of large biomolecules, mass spectrometry has become a powerful tool in protein analysis and the key technology in the emerging field of proteomics. The success of mass spectrometry is driven both by innovative instrumentation designs, especially those operating on the time-of-flight or ion-trapping principles, and by large-scale biochemical strategies, which use mass spectrometry to detect the isolated proteins. Any human protein can now be identified directly from genome databases on the basis of minimal data derived by mass spectrometry. As has already happened in genomics, increased automation of sample handling, analysis, and the interpretation of results will generate an avalanche of qualitative and quantitative proteomic data. Protein-protein interactions can be analyzed directly by precipitation of a tagged bait followed by mass spectrometric identification of its binding partners. By these and similar strategies, entire protein complexes, signaling pathways, and whole organelles are being characterized. Posttranslational modifications remain difficult to analyze but are starting to yield to generic strategies.

Phosphatidylinositol 4,5-bisphosphate stimulates vesicle formation from liposomes by brain cytosol

As a step toward the elucidation of mechanisms in vesicle budding, a cell-free assay that measures cytosol-induced vesicle generation from liposomes was established. This assay then was used to explore the role of phosphoinositides in vesicle formation. Liposomes incubated with brain cytosol in the presence of ATP and GTP massively generated small vesicles, as assessed both quantitatively and qualitatively by a dynamic light-scattering assay. Both ATP and GTP were required. Vesicle formation was inhibited greatly by the immunodepletion of dynamin 1 from the cytosol, indicating a major contribution of this GTPase in this reaction and suggesting that it mimics endocytic vesicle fission. Increasing the concentration of l-alpha-phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] but not of l-alpha-phosphatidylinositol 4-monophosphate or l-alpha-phosphatidylinositol in the lipid membranes enhanced vesicle formation. Lipid analysis revealed rapid degradation of PtdIns(4,5)P2 to l-alpha-phosphatidylinositol during the incubation with the reaction reaching a maximum within 5 sec, whereas vesicle formation proceeded with a longer time course. PtdIns(4,5)P2 degradation was independent of vesicle formation and occurred also in the presence of guanosine 5'-O-(thiotriphosphate), where few vesicle formations occurred. These results suggest that PtdIns(4,5)P2 plays a critical role in the early step of vesicle formation, possibly in the recruitment of coats and fission factors to membranes.

The dephosphins: dephosphorylation by calcineurin triggers synaptic vesicle endocytosis

When nerve terminals in the brain are stimulated, a group of phosphoproteins called the dephosphins are coordinately dephosphorylated by calcineurin, the Ca(2+)-dependent protein phosphatase. Amazingly, the seven presently known dephosphins are not structurally related, yet each has been independently shown to be essential for synaptic vesicle endocytosis (SVE). Nowhere else in biology is there a similar example of the coordinated dephosphorylation of such a large group of proteins each sharing roles in the same biological response. This suggests that dephosphorylation and phosphorylation of the dephosphins is essential for SVE. Recent studies in synaptosomes have confirmed this view, with calcineurin-mediated dephosphorylation of the dephosphins essential for triggering SVE. The phosphorylation cycle of the dephosphins might regulate SVE by targeting the proteins to sites of action and by stimulating the assembly of several large essential endocytic protein complexes.