Phosphoinositide 3-kinase C2alpha links clathrin to microtubule-dependent movement

Phosphoinositide 3-kinase (PI3K) classes: From cell signaling to endocytic recycling and autophagy

Lipid signaling is defined as any biological signaling action in which a lipid messenger binds to a protein target, converting its effects to specific cellular responses. In this complex biological pathway, the family of phosphoinositide 3-kinase (PI3K) represents a pivotal role and affects many aspects of cellular biology from cell survival, proliferation, and migration to endocytosis, intracellular trafficking, metabolism, and autophagy. While yeasts have a single isoform of phosphoinositide 3-kinase (PI3K), mammals possess eight PI3K types divided into three classes. The class I PI3Ks have set the stage to widen research interest in the field of cancer biology. The aberrant activation of class I PI3Ks has been identified in 30-50% of human tumors, and activating mutations in PIK3CA is one of the most frequent oncogenes in human cancer. In addition to indirect participation in cell signaling, class II and III PI3Ks primarily regulate vesicle trafficking. Class III PI3Ks are also responsible for autophagosome formation and autophagy flux. The current review aims to discuss the original data obtained from international research laboratories on the latest discoveries regarding PI3Ks-mediated cell biological processes. Also, we unravel the mechanisms by which pools of the same phosphoinositides (PIs) derived from different PI3K types act differently.

Control of phosphatidylinositol-3-kinase signaling by nanoscale membrane compartmentalization

Phosphatidylinositol-3-kinases (PI3Ks) are lipid kinases that produce 3-phosphorylated derivatives of phosphatidylinositol upon activation by various cues. These 3-phosphorylated lipids bind to various protein effectors to control many cellular functions. Lipid phosphatases such as phosphatase and tensin homolog (PTEN) terminate PI3K-derived signals and are critical to ensure appropriate signaling outcomes. Many lines of evidence indicate that PI3Ks and PTEN, as well as some specific lipid effectors are highly compartmentalized, either in plasma membrane nanodomains or in endosomal compartments. We examine the evidence for specific recruitment of PI3Ks, PTEN, and other related enzymes to membrane nanodomains and endocytic compartments. We then examine the hypothesis that scaffolding of the sources (PI3Ks), terminators (PTEN), and effectors of these lipid signals with a common plasma membrane nanodomain may achieve highly localized lipid signaling and ensure selective activation of specific effectors. This highlights the importance of spatial regulation of PI3K signaling in various physiological and disease contexts.

Identification of Podocyte Cargo Proteins by Proteomic Analysis of Clathrin-Coated Vesicles

Background

Clathrin-mediated endocytosis (CME) plays a fundamental role in podocyte health. Genetic ablation of genes implicated in CME has been shown to cause severe proteinuria and foot process effacement in mice. However, little is known about the cargo of clathrin-coated vesicles (CCVs) in podocytes. The goal of this study was to isolate CCVs from podocytes and identify their cargo by proteomic analysis.

Methods

Glomeruli isolated from Podocin-Cre Rosa-DTRflox mouse kidneys were seeded and treated with diphtheria toxin to obtain pure primary podocyte cultures. CCVs were isolated by differential gradient ultracentrifugation, and enrichment of CCVs was assessed by immunoblotting and electron microscopy (EM). Liquid chromatography-mass spectrometry (LC-MS) was performed for proteomic analysis. Proteins with higher abundance than transferrin receptor protein 1 were evaluated for CCV cargo potential against previously published literature. Immunofluorescence staining of identified cargo proteins and CCVs was performed in podocytes for further verification.

Results

Immunoblotting for multiple protein markers of CME revealed enrichment in the CCV fraction. Enrichment of CCVs among other small vesicles was observed via EM. Proteomics yielded a total of >1200 significant proteins. Multiple-step data analysis revealed 36 CCV-associated proteins, of which 10 represent novel, highly abundant cargo proteins in podocytes. Colocalization of cargo proteins and CCVs on immunostaining was observed.

Conclusions

Our identification of podocyte CCV cargo proteins helps to elucidate the importance of endocytic trafficking for podocyte health and maintenance of the glomerular environment.

CLIC4/Arf6 Pathway

RATIONALE

Increased expression of CLIC4 (chloride intracellular channel 4) is a feature of endothelial dysfunction in pulmonary arterial hypertension, but its role in disease pathology is not fully understood.

OBJECTIVE

To identify CLIC4 effectors and evaluate strategies targeting CLIC4 signaling in pulmonary hypertension.

METHODS AND RESULTS

Proteomic analysis of CLIC4-interacting proteins in human pulmonary artery endothelial cells identified regulators of endosomal trafficking, including Arf6 (ADP ribosylation factor 6) GTPase activating proteins and clathrin, while CLIC4 overexpression affected protein regulators of vesicular trafficking, lysosomal function, and inflammation. CLIC4 reduced BMPRII (bone morphogenetic protein receptor II) expression and signaling as a result of Arf6-mediated reduction in gyrating clathrin and increased lysosomal targeting of the receptor. BMPRII expression was restored by Arf6 siRNA, Arf inhibitor Sec7 inhibitor H3 (SecinH3), and inhibitors of clathrin-mediated endocytosis but was unaffected by chloride channel inhibitor, indanyloxyacetic acid 94 or Arf1 siRNA. The effects of CLIC4 on NF-κB (nuclear factor-kappa B), HIF (hypoxia-inducible factor), and angiogenic response were prevented by Arf6 siRNA and SecinH3. Sugen/hypoxia mice and monocrotaline rats showed elevated expression of CLIC4, activation of Arf6 and NF-κB, and reduced expression of BMPRII in the lung. These changes were established early during disease development. Lung endothelium-targeted delivery of CLIC4 siRNA or treatment with SecinH3 attenuated the disease, reduced CLIC4/Arf activation, and restored BMPRII expression in the lung. Endothelial colony-forming cells from idiopathic pulmonary hypertensive patients showed upregulation of CLIC4 expression and Arf6 activity, suggesting potential importance of this pathway in the human condition.

CONCLUSIONS

Arf6 is a novel effector of CLIC4 and a new therapeutic target in pulmonary hypertension.

A unique role for clathrin light chain A in cell spreading and migration

Clathrin heavy chain is the structural component of the clathrin triskelion, but unique functions for the two distinct and highly conserved clathrin light chains (CLCa and CLCb, also known as CLTA and CLTB, respectively) have been elusive. Here, we show that following detachment and replating, CLCa is uniquely responsible for promoting efficient cell spreading and migration. Selective depletion of CLCa, but not of CLCb, reduced the initial phase of isotropic spreading of HeLa, H1299 and HEK293 cells by 60-80% compared to siRNA controls, and wound closure and motility by ∼50%. Surface levels of β1-integrins were unaffected by CLCa depletion. However, CLCa was required for effective targeting of FAK (also known as PTK2) and paxillin to the adherent surface of spreading cells, for integrin-mediated activation of Src, FAK and paxillin, and for maturation of focal adhesions, but not their microtubule-based turnover. Depletion of CLCa also blocked the interaction of clathrin with the nucleation-promoting factor WAVE complex, and altered actin distribution. Furthermore, preferential recruitment of CLCa to budding protrusions was also observed. These results comprise the first identification of CLCa-specific functions, with implications for normal and neoplastic integrin-based signaling and cell migration.

The class II phosphoinositide 3-kinases PI3K-C2α and PI3K-C2β differentially regulate clathrin-dependent pinocytosis in human vascular endothelial cells

Pinocytosis is an important fundamental cellular process that is used by the cell to transport fluid and solutes. Phosphoinositide 3-kinases (PI3Ks) regulate a diverse array of dynamic membrane events. However, it is not well-understood which PI3K isoforms are involved in specific mechanisms of pinocytosis. We performed knockdown studies of endogenous PI3K isoforms and clathrin heavy chain (CHC) mediated by small interfering RNA (siRNA). The results demonstrated that the class II PI3K PI3K-C2α and PI3K-C2β, but not the class I or III PI3K, were required for pinocytosis, based on an evaluation of fluorescein-5-isothiocyanate (FITC)-dextran uptake in endothelial cells. Pinocytosis was partially dependent on both clathrin and dynamin, and both PI3K-C2α and PI3K-C2β were required for clathrin-mediated-but not clathrin-non-mediated-FITC-dextran uptake at the step leading up to its delivery to early endosomes. Both PI3K-C2α and PI3K-C2β were co-localized with clathrin-coated pits and vesicles. However, PI3K-C2β, but not PI3K-C2α, was highly co-localized with actin filament-associated clathrin-coated structures and required for actin filament formation at the clathrin-coated structures. These results indicate that PI3K-C2α and PI3K-C2β play differential, indispensable roles in clathrin-mediated pinocytosis.

Neuroprotective Drug for Nerve Trauma Revealed Using Artificial Intelligence

Here we used a systems biology approach and artificial intelligence to identify a neuroprotective agent for the treatment of peripheral nerve root avulsion. Based on accumulated knowledge of the neurodegenerative and neuroprotective processes that occur in motoneurons after root avulsion, we built up protein networks and converted them into mathematical models. Unbiased proteomic data from our preclinical models were used for machine learning algorithms and for restrictions to be imposed on mathematical solutions. Solutions allowed us to identify combinations of repurposed drugs as potential neuroprotective agents and we validated them in our preclinical models. The best one, NeuroHeal, neuroprotected motoneurons, exerted anti-inflammatory properties and promoted functional locomotor recovery. NeuroHeal endorsed the activation of Sirtuin 1, which was essential for its neuroprotective effect. These results support the value of network-centric approaches for drug discovery and demonstrate the efficacy of NeuroHeal as adjuvant treatment with surgical repair for nervous system trauma.

PI3K-C2α knockdown decreases autophagy and maturation of endocytic vesicles

Phosphoinositide 3-kinase (PI3K) family members are involved in diverse cellular fates including cell growth, proliferation, and survival. While many molecular details are known about the Class I and III PI3Ks, less is known about the Class II PI3Ks. To explore the function of all eight PI3K isoforms in autophagy, we knock down each gene individually and measure autophagy. We find a significant decrease in autophagy following siRNA-mediated PIK3C2A (encoding the Class 2 PI3K, PI3K-C2α) knockdown. This defective autophagy is rescued by exogenous PI3K-C2α, but not kinase-dead PI3K-C2α. Using confocal microscopy, we probe for markers of endocytosis and autophagy, revealing that PI3K-C2α colocalizes with markers of endocytosis. Though endocytic uptake is intact, as demonstrated by transferrin labeling, PIK3C2A knockdown results in vesicle accumulation at the recycling endosome. We isolate distinct membrane sources and observe that PI3K-C2α interacts with markers of endocytosis and autophagy, notably ATG9. Knockdown of either PIK3C2A or ATG9A/B, but not PI3KC3, results in an accumulation of transferrin-positive clathrin coated vesicles and RAB11-positive vesicles at the recycling endosome. Taken together, these results support a role for PI3K-C2α in the proper maturation of endosomes, and suggest that PI3K-C2α may be a critical node connecting the endocytic and autophagic pathways.

PI3K class II α regulates δ-opioid receptor export from the trans-Golgi network

The interplay between signaling and trafficking by G protein-coupled receptors (GPCRs) has focused mainly on endocytic trafficking. Whether and how surface delivery of newly synthesized GPCRs is regulated by extracellular signals is less understood. Here we define a signaling-regulated checkpoint at the trans-Golgi network (TGN) that controls the surface delivery of the delta opioid receptor (δR). In PC12 cells, inhibition of phosphoinositide-3 kinase (PI3K) activity blocked export of newly synthesized δR from the Golgi and delivery to the cell surface, similar to treatment with nerve growth factor (NGF). Depletion of class II phosphoinositide-3 kinase α (PI3K C2A), but not inhibition of class I PI3K, blocked δR export to comparable levels and attenuated δR-mediated cAMP inhibition. NGF treatment displaced PI3K C2A from the Golgi and optogenetic recruitment of the PI3K C2A kinase domain to the TGN-induced δR export downstream of NGF. Of importance, PI3K C2A expression promotes export of endogenous δR in primary trigeminal ganglion neurons. Taken together, our results identify PI3K C2A as being required and sufficient for δR export and surface delivery in neuronal cells and suggest that it could be a key modulator of a novel Golgi export checkpoint that coordinates GPCR delivery to the surface.

Diacylglycerol Guides the Hopping of Clathrin-Coated Pits along Microtubules for Exo-Endocytosis Coupling

Many receptor-mediated endocytic processes are mediated by constitutive budding of clathrin-coated pits (CCPs) at spatially randomized sites before slowly pinching off from the plasma membrane (60-100 s). In contrast, clathrin-mediated endocytosis (CME) coupled with regulated exocytosis in excitable cells occurs at peri-exocytic sites shortly after vesicle fusion (∼10 s). The molecular mechanism underlying this spatiotemporal coupling remains elusive. We show that coupled endocytosis makes use of pre-formed CCPs, which hop to nascent fusion sites nearby following vesicle exocytosis. A dynamic cortical microtubular network, anchored at the cell surface by the cytoplasmic linker-associated protein on microtubules and the LL5β/ELKS complex on the plasma membrane, provides the track for CCP hopping. Local diacylglycerol gradients generated upon exocytosis guide the direction of hopping. Overall, the CCP-cytoskeleton-lipid interaction demonstrated here mediates exocytosis-coupled fast recycling of both plasma membrane and vesicular proteins, and it is required for the sustained exocytosis during repetitive stimulations.

Transferrin: Endocytosis and Cell Signaling in Parasitic Protozoa

Iron is the fourth most abundant element on Earth and the most abundant metal in the human body. This element is crucial for life because almost all organisms need iron for several biological activities. This is the case with pathogenic organisms, which are at the vanguard in the battle with the human host for iron. The latest regulates Fe concentration through several iron-containing proteins, such as transferrin. The transferrin receptor transports iron to each cell that needs it and maintains it away from pathogens. Parasites have developed several strategies to obtain iron as the expression of specific transferrin receptors localized on plasma membrane, internalized through endocytosis. Signal transduction pathways related to the activation of the receptor have functional importance in proliferation. The study of transferrin receptors and other proteins with action in the signaling networks is important because these proteins could be used as therapeutic targets due to their specificity or to differences with the human counterpart. In this work, we describe proteins that participate in signal transduction processes, especially those that involve transferrin endocytosis, and we compare these processes with those found in T. brucei, T. cruzi, Leishmania spp., and E. histolytica parasites.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Arf6 regulation of Gyrating-clathrin

'Gyrating-' or 'G'-clathrin are coated endocytic structures located near peripheral sorting endosomes (SEs), which exhibit highly dynamic but localized movements when visualized by live-cell microscopy. They have been implicated in recycling of transferrin from the sorting endosome directly to the cell surface, but there is no information about their formation or regulation. We show here that G-clathrin comprise a minority of clathrin-coated structures in the cell periphery and are brefeldin A (BFA)-resistant. Arf6-GTP substantially increases G-clathrin levels, probably by lengthening coated bud lifetimes as suggested by photobleaching and photoactivation results, and an Arf6(Q67L)-GTP mutant bearing an internal GFP tag can be directly visualized in G-clathrin structures in live cells. Upon siRNA-mediated depletion of Arf6 or expression of Arf6(T27N), G-clathrin levels rise and are primarily Arf1-dependent, yet still BFA-resistant. However, BFA-sensitive increased G-clathrin levels are observed upon acute incubation with cytohesin inhibitor SecinH3, indicating a shift in GEF usage. Depletion of both Arf6 and Arf1 abolishes G-clathrin, and results in partial inhibition of fast transferrin recycling consistent with the latter's participation in this pathway. Collectively, these results demonstrate that the dynamics of G-clathrin primarily requires completion of the Arf6 guanine nucleotide cycle, but can be regulated by multiple Arf and GEF proteins, reflecting both overlapping mechanisms operative in their regulation and the complexity of processes involved in endosomal sorting.

PI3Ks-drug targets in inflammation and cancer

Phosphoinositide 3-kinases (PI3Ks) control cell growth, proliferation, cell survival, metabolic activity, vesicular trafficking, degranulation, and migration. Through these processes, PI3Ks modulate vital physiology. When over-activated in disease, PI3K promotes tumor growth, angiogenesis, metastasis or excessive immune cell activation in inflammation, allergy and autoimmunity. This chapter will introduce molecular activation and signaling of PI3Ks, and connections to target of rapamycin (TOR) and PI3K-related protein kinases (PIKKs). The focus will be on class I PI3Ks, and extend into current developments to exploit mechanistic knowledge for therapy.

Motion analysis of live objects by super-resolution fluorescence microscopy

Motion analysis plays an important role in studing activities or behaviors of live objects in medicine, biotechnology, chemistry, physics, spectroscopy, nanotechnology, enzymology, and biological engineering. This paper briefly reviews the developments in this area mostly in the recent three years, especially for cellular analysis in fluorescence microscopy. The topic has received much attention with the increasing demands in biomedical applications. The tasks of motion analysis include detection and tracking of objects, as well as analysis of motion behavior, living activity, events, motion statistics, and so forth. In the last decades, hundreds of papers have been published in this research topic. They cover a wide area, such as investigation of cell, cancer, virus, sperm, microbe, karyogram, and so forth. These contributions are summarized in this review. Developed methods and practical examples are also introduced. The review is useful to people in the related field for easy referral of the state of the art.

Regulation of purinergic signaling in biliary epithelial cells by exocytosis of SLC17A9-dependent ATP-enriched vesicles

ATP in bile is a potent secretogogue, stimulating biliary epithelial cell (BEC) secretion through binding apical purinergic receptors. In response to mechanosensitive stimuli, BECs release ATP into bile, although the cellular basis of ATP release is unknown. The aims of this study in human and mouse BECs were to determine whether ATP release occurs via exocytosis of ATP-enriched vesicles and to elucidate the potential role of the vesicular nucleotide transporter SLC17A9 in purinergic signaling. Dynamic, multiscale, live cell imaging (confocal and total internal reflection fluorescence microscopy and a luminescence detection system with a high sensitivity charge-coupled device camera) was utilized to detect vesicular ATP release from cell populations, single cells, and the submembrane space of a single cell. In response to increases in cell volume, BECs release ATP, which was dependent on intact microtubules and vesicular trafficking pathways. ATP release occurred as stochastic point source bursts of luminescence consistent with exocytic events. Parallel studies identified ATP-enriched vesicles ranging in size from 0.4 to 1 μm that underwent fusion and release in response to increases in cell volume in a protein kinase C-dependent manner. Present in all models, SLC17A9 contributed to ATP vesicle formation and regulated ATP release. The findings are consistent with the existence of an SLC17A9-dependent ATP-enriched vesicular pool in biliary epithelium that undergoes regulated exocytosis to initiate purinergic signaling.

Drosophila Mtm and class II PI3K coregulate a PI(3)P pool with cortical and endolysosomal functions

Turnover of endosomal PI(3)P by mtm maintains endolysosomal homeostasis and cortical remodeling in Drosophila hemocytes during migration.

Reversible phosphoinositide phosphorylation provides a dynamic membrane code that balances opposing cell functions. However, in vivo regulatory relationships between specific kinases, phosphatases, and phosphoinositide subpools are not clear. We identified myotubularin (mtm), a Drosophila melanogaster MTM1/MTMR2 phosphoinositide phosphatase, as necessary and sufficient for immune cell protrusion formation and recruitment to wounds. Mtm-mediated turnover of endosomal phosphatidylinositol 3-phosphate (PI(3)P) pools generated by both class II and III phosphatidylinositol 3-kinases (Pi3K68D and Vps34, respectively) is needed to down-regulate membrane influx, promote efflux, and maintain endolysosomal homeostasis. Endocytosis, but not endolysosomal size, contributes to cortical remodeling by mtm function. We propose that Mtm-dependent regulation of an endosomal PI(3)P pool has separable consequences for endolysosomal homeostasis and cortical remodeling. Pi3K68D depletion (but not Vps34) rescues protrusion and distribution defects in mtm-deficient immune cells and restores functions in other tissues essential for viability. The broad interactions between mtm and class II Pi3K68D suggest a novel strategy for rebalancing PI(3)P-mediated cell functions in MTM-related human disease.

Bicaudal-D binds clathrin heavy chain to promote its transport and augments synaptic vesicle recycling

Cargo transport by microtubule-based motors is essential for cell organisation and function. The Bicaudal-D (BicD) protein participates in the transport of a subset of cargoes by the minus-end-directed motor dynein, although the full extent of its functions is unclear. In this study, we report that in Drosophila zygotic BicD function is only obligatory in the nervous system. Clathrin heavy chain (Chc), a major constituent of coated pits and vesicles, is the most abundant protein co-precipitated with BicD from head extracts. BicD binds Chc directly and interacts genetically with components of the pathway for clathrin-mediated membrane trafficking. Directed transport and subcellular localisation of Chc is strongly perturbed in BicD mutant presynaptic boutons. Functional assays show that BicD and dynein are essential for the maintenance of normal levels of neurotransmission specifically during high-frequency electrical stimulation and that this is associated with a reduced rate of recycling of internalised synaptic membrane. Our results implicate BicD as a new player in clathrin-associated trafficking processes and show a novel requirement for microtubule-based motor transport in the synaptic vesicle cycle.

Mammalian phosphoinositide kinases and phosphatases

Phosphoinositides are lipids that are present in the cytoplasmic leaflet of a cell's plasma and internal membranes and play pivotal roles in the regulation of a wide variety of cellular processes. Phosphoinositides are molecularly diverse due to variable phosphorylation of the hydroxyl groups of their inositol rings. The rapid and reversible configuration of the seven known phosphoinositide species is controlled by a battery of phosphoinositide kinases and phosphoinositide phosphatases, which are thus critical for phosphoinositide isomer-specific localization and functions. Significantly, a given phosphoinositide generated by different isozymes of these phosphoinositide kinases and phosphatases can have different biological effects. In mammals, close to 50 genes encode the phosphoinositide kinases and phosphoinositide phosphatases that regulate phosphoinositide metabolism and thus allow cells to respond rapidly and effectively to ever-changing environmental cues. Understanding the distinct and overlapping functions of these phosphoinositide-metabolizing enzymes is important for our knowledge of both normal human physiology and the growing list of human diseases whose etiologies involve these proteins. This review summarizes the structural and biological properties of all the known mammalian phosphoinositide kinases and phosphoinositide phosphatases, as well as their associations with human disorders.

Identification of novel cellular targets for therapeutic intervention against Ebola virus infection by siRNA screening

While much progress has been made in developing drugs against a few prominent viruses such as HIV, few examples exist for emerging infectious agents. In some cases broad spectrum anti-viral drugs, such as ribavirin, are effective, but for some groups of viruses, these show little efficacy in animal models. Traditional methods focus on screening small molecule libraries to identify drugs that target virus factors, with the intention that side-effects to the host can be minimized. However, this greatly limits potential drug targets and virus genes can rapidly mutate to avoid drug action. Recent advances in siRNA gene targeting technologies have provided a powerful tool to specifically target and suppress the expression of cell genes. Since viruses are completely dependent upon host cell proteins for propagation, siRNA screening promises to reveal novel cell proteins and signaling pathways that may be viable targets for drug therapy regimens. Here we used an siRNA screening approach to identify gene products that play critical roles in Ebola virus infection. By gene cluster analysis, proteins in phosphatidylinositol-3-kinase and calcium/calmodulin kinase related networks were identified as important for Zaire Ebola virus infection and prioritized for further evaluation. Key roles of each were confirmed by testing available drugs specific for members of each pathway. Interestingly, both sets of proteins are also important in cancer and subject to intense investigation. Thus development of new drugs against these cancer targets may also prove useful in combating Ebola virus.

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.

Gyrating clathrin: highly dynamic clathrin structures involved in rapid receptor recycling

We report here detection of novel intracellular clathrin-coated structures revealed by continuous high-speed imaging of cells expressing green fluorescent protein fusion proteins. These structures, which we operationally term 'gyrating clathrin' (G-clathrin), are characterized by localized but extremely rapid movement, leading to the hypothesis that they are coated buds on waving membrane tubules. G-clathrin structures have structurally and functionally distinct features. They lack detectable adaptor proteins AP-1 and AP-2 but contain GGA1 [Golgi-localized, gamma-ear-containing, Arf (ADP-ribosylation factor)-binding protein] as well as the cation-dependent mannose-6-phosphate receptor. While they accumulate internalized transferrin (Tf), they do not contain detectable levels of cargos targeted for the late endosome/lysosome pathway such as EGF and dextran. Pulse-chase studies indicate that Tf appears in G-clathrin structures in the cell periphery after sorting endosomes (SEs), but before filling of the perinuclear endocytic recycling compartment. Furthermore, the inhibitors LY294002 and wortmannin, which inhibit direct recycling of Tf from SEs to the plasma membrane, also block its appearance in G-clathrin. These observations suggest that peripheral G-clathrin contributes to rapid recycling, a kinetically defined compartment that has largely eluded structural identification. More generally, the rapid continuous live cell imaging reported here reveals new aspects of membrane trafficking.

Transcriptional response of dermal fibroblasts in direct current electric fields

During the course of normal wound healing, fibroblasts at the wound edge are exposed to electric fields (EFs) ranging from 40 to 200 mV/mm. Various forms of EFs influence fibroblast migration, proliferation, and protein synthesis. Thus, EFs may contribute to fibroblast activation during wound repair. To elucidate the role of EFs during the normal progression of healing, this study compares gene expression in normal adult dermal fibroblasts exposed to a 100 mV/mm EF for 1 h to non-stimulated controls. Significantly increased expression of 162 transcripts and decreased expression of 302 transcripts was detected using microarrays, with 126 transcripts above the level of 1.4-fold increases or decreases compared to the controls. Above the level of twofold, only 11 genes were significantly increased or decreased compared to controls. Many of these significantly regulated genes are associated with wound repair through the processes of matrix production, cellular signaling, and growth. Activity within specific cellular signaling pathways is noted, including TGF-beta, G-proteins, and inhibition of apoptosis. In addition, RT-PCR analysis of the expression of KLF6, FN1, RGS2, and JMJD1C over continued stimulation and at different field strengths suggests that there are specific windows of field characteristics for maximum induction of these genes. EFs thus appear to have an important role in controlling fibroblast activity in the process of wound healing.

Expression profiles are different in carbon ion-irradiated normal human fibroblasts and their bystander cells

Evidence has accumulated that ionizing radiation induces biological effects in non-irradiated bystander cells having received signals from directly irradiated cells; however, energetic heavy ion-induced bystander response is incompletely characterized. Here we performed microarray analysis of irradiated and bystander fibroblasts in confluent cultures. To see the effects in bystander cells, each of 1, 5 and 25 sites was targeted with 10 particles of carbon ions (18.3 MeV/u, 103 keV/microm) using microbeams, where particles traversed 0.00026, 0.0013 and 0.0066% of cells, respectively. diated cells, cultures were exposed to 10% survival dose (D), 0.1D and 0.01D of corresponding broadbeams (108 keV/microm). Irrespective of the target numbers (1, 5 or 25 sites) and the time (2 or 6h postirradiation), similar expression changes were observed in bystander cells. Among 874 probes that showed more than 1.5-fold changes in bystander cells, 25% were upregulated and the remainder downregulated. These included genes related to cell communication (PIK3C2A, GNA13, FN1, ANXA1 and IL1RAP), stress response (RAD23B, ATF4 and EIF2AK4) and cell cycle (MYCN, RBBP4 and NEUROG1). Pathway analysis revealed serial bystander activation of G protein/PI-3 kinase pathways. Instead, genes related to cell cycle or death (CDKN1A, GADD45A, NOTCH1 and BCL2L1), and cell communication (IL1B, TCF7 and ID1) were upregulated in irradiated cells, but not in bystander cells. Our results indicate different expression profiles in irradiated and bystander cells, and imply that intercellular signaling between irradiated and bystander cells activate intracellular signaling, leading to the transcriptional stress response in bystander cells.