Imaging phosphoinositide dynamics using GFP-tagged protein domains

MARCKS and PI(4,5)P2 reciprocally regulate actin-based dendritic spine morphology

Myristoylated, alanine-rich C-kinase substrate (MARCKS) is an F-actin and phospholipid binding protein implicated in numerous cellular activities, including the regulation of morphology in neuronal dendrites and dendritic spines. MARCKS contains a lysine-rich effector domain that mediates its binding to plasma membrane phosphatidylinositol-4,5-biphosphate (PI(4,5)P2) in a manner controlled by PKC and calcium/calmodulin. In neurons, manipulations of MARCKS concentration and membrane targeting strongly affect the numbers, shapes, and F-actin properties of dendritic spines, but the mechanisms remain unclear. Here, we tested the hypothesis that the effects of MARCKS on dendritic spine morphology are due to its capacity to regulate the availability of plasma membrane PI(4,5)P2. We observed that the concentration of free PI(4,5)P2 on the dendritic plasma membrane was inversely proportional to the concentration of MARCKS. Endogenous PI(4,5)P2 levels were increased or decreased, respectively, by acutely overexpressing either phosphatidylinositol-4-phosphate 5-kinase (PIP5K) or inositol polyphosphate 5-phosphatase (5ptase). PIP5K, like MARCKS depletion, induced severe spine shrinkage; 5ptase, like constitutively membrane-bound MARCKS, induced aberrant spine elongation. These phenotypes involved changes in actin properties driven by the F-actin severing protein cofilin. Collectively, these findings support a model in which neuronal activity regulates actin-dependent spine morphology through antagonistic interactions of MARCKS and PI(4,5)P2.

A Plethora of Functions Condensed into Tiny Phospholipids: The Story of PI4P and PI(4,5)P2

Phosphoinositides (PIs) are small, phosphorylated lipids that serve many functions in the cell. They regulate endo- and exocytosis, vesicular trafficking, actin reorganization, and cell mobility, and they act as signaling molecules. The most abundant PIs in the cell are phosphatidylinositol-4-monophosphate (PI4P) and phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂]. PI4P is mostly localized at the Golgi apparatus where it regulates the anterograde trafficking from the Golgi apparatus to the plasma membrane (PM), but it also localizes at the PM. On the other hand, the main localization site of PI(4,5)P₂ is the PM where it regulates the formation of endocytic vesicles. The levels of PIs are regulated by many kinases and phosphatases. Four main kinases phosphorylate the precursor molecule phosphatidylinositol into PI4P, divided into two classes (PI4KIIα, PI4KIIβ, PI4KIIIα, and PI4KIIIβ), and three main kinases phosphorylate PI4P to form PI(4,5)P₂ (PI4P5KIα, PI4P5KIβ, and PI4P5KIγ). In this review, we discuss the localization and function of the kinases that produce PI4P and PI(4,5)P₂, as well as the localization and function of their product molecules with an overview of tools for the detection of these PIs.

FilGAP, a GAP protein for Rac, regulates front-rear polarity and tumor cell migration through the ECM

Migrating tumor cells are characterized by a sustained front-rear asymmetry, with a front enriched in filamentous actin, which is induced by Rho small GTPase Rac. Regulation of Rac activity by its regulators should be required for effective motility. Here, we show that FilGAP, a GTPase-activating protein (GAP) for Rac, controls front-rear polarity and contributes to maintain effective tumor cell migration through the extracellular matrix (ECM). Overexpression of FilGAP in breast cancer cells induced polarized morphology and led to increased migration speed in collagen matrices, while depletion of FilGAP impaired the cell polarity and migration. FilGAP localizes to the cell front through its pleckstrin-homology (PH) domain in a phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent manner and appears to inactivate Rac at its site. We found that the affinity of PH domain to PIP3 is critically involved in the maintenance of cell polarity. Moreover, small GTPase ADP-ribosylation factor 6 (Arf6), which binds to the FilGAP PH domain, also regulates FilGAP-mediated cell polarity and migration of breast cancer cells. We propose that FilGAP regulates front-rear polarity through its PIP3 and Arf6 binding in tumor cell migration through the ECM.

Risk1, a Phosphatidylinositol 3-Kinase Effector, Promotes Rickettsia typhi Intracellular Survival

Rickettsia species are Gram-negative obligate intracellular bacteria that infect a wide range of eukaryotes and vertebrates. In particular, human body louse-borne Rickettsia prowazekii and flea-borne Rickettsia typhi have historically plagued humankind and continue to reemerge globally. The unavailability of vaccines and limited effectiveness of antibiotics late in infection place lethality rates up to 30%, highlighting the need to elucidate the mechanisms of Rickettsia pathogenicity in greater detail. Here, we characterize a new effector, Risk1, as a secreted phosphatidylinositol 3-kinase (PI3K) with unique dual class I and class III activities. Risk1 is required for host colonization, and its vacuolar phosphatidylinositol 3-phosphate generation modulates endosomal trafficking to arrest autophagosomal maturation. Collectively, Risk1 facilitates R. typhi growth by altering phosphoinositide metabolism and subverting intracellular trafficking.

To establish a habitable intracellular niche, various pathogenic bacteria secrete effectors that target intracellular trafficking and modulate phosphoinositide (PI) metabolism. Murine typhus, caused by the obligate intracellular bacterium Rickettsia typhi, remains a severe disease in humans. However, the mechanisms by which R. typhi effector molecules contribute to internalization by induced phagocytosis and subsequent phagosomal escape into the cytosol to facilitate the intracellular growth of the bacteria remain ill-defined. Here, we characterize a new molecule, Risk1, as a phosphatidylinositol 3-kinase (PI3K) secreted effector and the first bacterial secretory kinase with both class I and III PI3K activities. Inactivation of Risk1 PI3K activities reduced the phosphorylation of phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 3,4,5-trisphosphate within the host, which consequently diminished host colonization by R. typhi. During infection, Risk1 targets the Rab5-EEA1-phosphatidylinositol 3-phosphate [PI(3)P] signaling axis to promote bacterial phagosomal escape. Subsequently, R. typhi undergoes ubiquitination and induces host autophagy; however, maturation to autolysosomes is subverted to support intracellular growth. Intriguingly, only enzymatically active Risk1 binds the Beclin-1 core complex and contributes to R. typhi-induced autophagosome formation. In sum, our data suggest that Risk1, with dual class I and class III PI3K activities, alters host PI metabolism and consequently subverts intracellular trafficking to facilitate intracellular growth of R. typhi.

A PI(4,5)P2-derived "gasoline engine model" for the sustained B cell receptor activation

To efficiently initiate activation responses against rare ligands in the microenvironment, lymphocytes employ sophisticated mechanisms involving signaling amplification. Recently, a signaling amplification mechanism initiated from phosphatidylinositol (PI) 4, 5-biphosphate [PI(4,5)P2] hydrolysis and synthesis for sustained B cell activation has been reported. Antigen and B cell receptor (BCR) recognition triggered the prompt reduction of PI(4,5)P2 density within the BCR microclusters, which led to the positive feedback for the synthesis of PI(4,5)P2 outside of the BCR microclusters. At single molecule level, the diffusion of PI(4,5)P2 was slow, allowing for the maintenance of a PI(4,5)P2 density gradient between the inside and outside of the BCR microclusters and the persistent supply of PI(4,5)P2 from outside to inside of the BCR microclusters. Here, we review studies that have contributed to uncovering the molecular mechanisms of PI(4,5)P2-derived signaling amplification model. Based on these studies, we proposed a "gasoline engine model" in which the activation of B cell signaling inside the microclusters is similar to the working principle of burning gasoline within the engine chamber of a gasoline engine. We also discuss the evidences showing the potential universality of this model and future prospects.

Fluorescent Biosensors for Multiplexed Imaging of Phosphoinositide Dynamics

Phosphoinositides constitute a critical family of lipids that regulate numerous cellular processes. Phosphatidylinositol 4,5-bisphosphate (PIP₂) is arguably the most important plasma membrane phosphoinositide and is involved in regulating diverse processes. It is also the precursor of phosphatidylinositol 3,4,5-trisphosphate (PIP₃), which is critical for growth factor signaling, as well as membrane polarization and dynamics. Studying these lipids remains challenging, because of their compartmentalized activities and location-dependent signaling profiles. Here, we introduce several new genetically encoded fluorescent biosensors, including FRET-based and dimerization-dependent fluorescent protein (ddFP)-based biosensors, that enable real-time monitoring of PIP₂ levels in live cells. In addition, we developed a red fluorescent biosensor for 3-phosphoinositides that can be co-imaged with the green PIP₂ indicator. Simultaneous visualization of the dynamics of PIP₂ and 3-phosphoinositides in the same cell shows that plasma membrane PIP₃ formation upon EGF stimulation is coupled to a decrease in the local pool of PIP₂.

The role of Beclin 1 in IR-induced crosstalk between autophagy and G2/M cell cycle arrest

OBJECTIVES

Beclin 1 is a well-established core mammalian autophagy protein. Autophagy has been demonstrated to play roles in cellular responses to DNA damage, such as cell cycle regulation and apoptosis. In the present study, we investigated the exact mechanism by which Beclin 1 acts as a bridge between autophagy and cell cycle, when cells are exposed to ionizing radiation (IR).

MATERIALS AND METHODS

Western blotting and coimmunoprecipitation were performed to investigate protein expression levels and interactions. Immunofluorescence was used to monitor the localization and distribution of the indicated proteins. The levels of apoptosis and cell cycle changes were evaluated by flow cytometry. Double thymidine deoxyribonucleoside (TdR) blocking was conducted to differentiate G2 from mitotic delay. In vitro kinase assays using ATM kinase were performed to elucidate the specific phosphorylation site in Beclin 1.

RESULTS

In this study, we show that Beclin 1 knockdown reduces IR-induced autophagy. IR enhanced Beclin 1/PIK3CIII complex activity as demonstrated by the results of coimmunoprecipitation and immunofluorescence assays. An investigation to assess the possible relationship between autophagy and G2/M arrest showed that, similar to the autophagy inhibitor 3MA, Beclin 1 knockdown delayed IR-induced G2/M arrest. Furthermore, the interactions between Beclin 1 and several G2/M checkpoint-related proteins, namely, PLK1 and CDC25C, were observed to increase. In addition, we observed that both 3MA and Beclin 1 inhibition decreased IR-induced apoptosis. Regarding the potential mechanism associated with this phenomenon, we showed that IR induced the interaction between Beclin 1 and Tip60 as well as their redistribution. Furthermore, we demonstrated that Beclin 1 T57 may be a targeted phosphorylation site for ATM.

CONCLUSIONS

In the present study, we demonstrate the crucial and intricate roles of Beclin 1 in IR-induced autophagy, G2/M cell cycle arrest, and apoptosis. Additionally, Tip60 and ATM were identified as important molecular regulators of Beclin 1. Our findings show the precise mechanism of crosstalk between IR-induced autophagy and G2/M cell cycle arrest.

Insulin Induces Microtubule Stabilization and Regulates the Microtubule Plus-end Tracking Protein Network in Adipocytes

Insulin-stimulated glucose uptake is known to involve microtubules, although the function of microtubules and the microtubule-regulating proteins involved in insulin action are poorly understood. CLASP2, a plus-end tracking microtubule-associated protein (+TIP) that controls microtubule dynamics, was recently implicated as the first +TIP associated with insulin-regulated glucose uptake. Here, using protein-specific targeted quantitative phosphoproteomics within 3T3-L1 adipocytes, we discovered that insulin regulates phosphorylation of the CLASP2 network members G2L1, MARK2, CLIP2, AGAP3, and CKAP5 as well as EB1, revealing the existence of a previously unknown microtubule-associated protein system that responds to insulin. To further investigate, G2L1 interactome studies within 3T3-L1 adipocytes revealed that G2L1 coimmunoprecipitates CLASP2 and CLIP2 as well as the master integrators of +TIP assembly, the end binding (EB) proteins. Live-cell total internal reflection fluorescence microscopy in adipocytes revealed G2L1 and CLASP2 colocalize on microtubule plus-ends. We found that although insulin increases the number of CLASP2-containing plus-ends, insulin treatment simultaneously decreases CLASP2-containing plus-end velocity. In addition, we discovered that insulin stimulates redistribution of CLASP2 and G2L1 from exclusive plus-end tracking to "trailing" behind the growing tip of the microtubule. Insulin treatment increases α-tubulin Lysine 40 acetylation, a mechanism that was observed to be regulated by a counterbalance between GSK3 and mTOR, and led to microtubule stabilization. Our studies introduce insulin-stimulated microtubule stabilization and plus-end trailing of +TIPs as new modes of insulin action and reveal the likelihood that a network of microtubule-associated proteins synergize to coordinate insulin-regulated microtubule dynamics.

PI(4,5)P2 determines the threshold of mechanical force-induced B cell activation

B lymphocytes use B cell receptors (BCRs) to sense the chemical and physical features of antigens. The activation of isotype-switched IgG-BCR by mechanical force exhibits a distinct sensitivity and threshold in comparison with IgM-BCR. However, molecular mechanisms governing these differences remain to be identified. In this study, we report that the low threshold of IgG-BCR activation by mechanical force is highly dependent on tethering of the cytoplasmic tail of the IgG-BCR heavy chain (IgG-tail) to the plasma membrane. Mechanistically, we show that the positively charged residues in the IgG-tail play a crucial role by highly enriching phosphatidylinositol (4,5)-biphosphate (PI(4,5)P2) into the membrane microdomains of IgG-BCRs. Indeed, manipulating the amounts of PI(4,5)P2 within IgG-BCR membrane microdomains significantly altered the threshold and sensitivity of IgG-BCR activation. Our results reveal a lipid-dependent mechanism for determining the threshold of IgG-BCR activation by mechanical force.

Visualization of Phosphatidylinositol 3,5-Bisphosphate Dynamics by a Tandem ML1N-Based Fluorescent Protein Probe in Arabidopsis

Phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2] is a low-abundance phospholipid known to be associated with a wide variety of physiological functions in plants. However, the localization and dynamics of PI(3,5)P2 in plant cells remain largely unknown, partially due to the lack of an effective fluorescent probe. Using Arabidopsis transgenic plant expressing the PI(3,5)P2-labeling fluorescent probe (tagRFP-ML1N*2) developed based on a tandem repeat of the cytosolic phosphoinositide-interacting domain (ML1N) of the mammalian lysosomal transient receptor potential cation channel, Mucolipin 1 (TRPML1), here we show that PI(3,5)P2 is predominantly localized on the limited membranes of the FAB1- and SNX1-positive late endosomes, but rarely localized on the membranes of plant vacuoles or trans-Golgi network/early endosomes of cortical cells of the root differentiation zone. The late endosomal localization of tagRFP-ML1N*2 is reduced or abolished by pharmacological inhibition or genetic knockdown of expression of genes encoding PI(3,5)P2-synthesizing enzymes, FAB1A/B, but markedly increased with FAB1A overexpression. Notably, reactive oxygen species (ROS) significantly increase late endosomal levels of PI(3,5)P2. Thus, tandem ML1N-based PI(3,5)P2 probes can reliably monitor intracellular dynamics of PI(3,5)P2 in Arabidopsis cells with less binding activity to other endomembrane organelles.

Genetically encoded fluorescent tags

Genetically encoded fluorescent tags are protein sequences that can be fused to a protein of interest to render it fluorescent. These tags have revolutionized cell biology by allowing nearly any protein to be imaged by light microscopy at submicrometer spatial resolution and subsecond time resolution in a live cell or organism. They can also be used to measure protein abundance in thousands to millions of cells using flow cytometry. Here I provide an introduction to the different genetic tags available, including both intrinsically fluorescent proteins and proteins that derive their fluorescence from binding of either endogenous or exogenous fluorophores. I discuss their optical and biological properties and guidelines for choosing appropriate tags for an experiment. Tools for tagging nucleic acid sequences and reporter molecules that detect the presence of different biomolecules are also briefly discussed.

Quantifying lipid changes in various membrane compartments using lipid binding protein domains

One of the largest challenges in cell biology is to map the lipid composition of the membranes of various organelles and define the exact location of processes that control the synthesis and distribution of lipids between cellular compartments. The critical role of phosphoinositides, low-abundant lipids with rapid metabolism and exceptional regulatory importance in the control of almost all aspects of cellular functions created the need for tools to visualize their localizations and dynamics at the single cell level. However, there is also an increasing need for methods to determine the cellular distribution of other lipids regulatory or structural, such as diacylglycerol, phosphatidic acid, or other phospholipids and cholesterol. This review will summarize recent advances in this research field focusing on the means by which changes can be described in more quantitative terms.

Modulation of membrane phosphoinositide dynamics by the phosphatidylinositide 4-kinase activity of the Legionella LepB effector

Legionella pneumophila, the causative bacterium for Legionnaires' disease, hijacks host membrane trafficking for the maturation of the Legionella-containing vacuole (LCV). The LCV membrane mainly contains PtdIns4P, which is important for anchoring many secreted Legionella effectors onto the LCV. Here, we identify a cryptic functional domain (LepB_NTD) preceding the well-characterized RabGAP domain in the Legionella Dot/Icm type IV secretion system effector LepB. LepB_NTD alone is toxic to yeast and can disrupt the Golgi in mammalian cells. The crystal structure reveals an unexpected kinase fold and catalytic motif important for LepB_NTD function in eukaryotes. Cell biology-guided biochemical analyses uncovered a lipid kinase activity in LepB_NTD that specifically converts PtdIns3P into PtdIns(3,4)P₂. PtdIns(3,4)P₂ is efficiently hydrolysed into PtdIns4P by another Dot/Icm effector SidF that is known to possess phosphoinositide phosphatase activity. Consistently, SidF is capable of counteracting the cellular functions of LepB_NTD. Genetic analyses show a requirement for LepB kinase activity as well as lipid phosphatase activity of SidF for PtdIns4P biosynthesis on the LCV membrane. Our study identifies an unprecedented phosphatidylinositide 4-kinase activity from bacteria and highlights a sophisticated manipulation of host phosphoinositide metabolism by a bacterial pathogen.

The Growing and Glowing Toolbox of Fluorescent and Photoactive Proteins

Over the past 20 years, protein engineering has been extensively used to improve and modify the fundamental properties of fluorescent proteins (FPs) with the goal of adapting them for a fantastic range of applications. FPs have been modified by a combination of rational design, structure-based mutagenesis, and countless cycles of directed evolution (gene diversification followed by selection of clones with desired properties) that have collectively pushed the properties to photophysical and biochemical extremes. In this review, we provide both a summary of the progress that has been made during the past two decades, and a broad overview of the current state of FP development and applications in mammalian systems.

The chloride-channel blocker 9-anthracenecarboxylic acid reduces the nonlinear capacitance of prestin-associated charge movement

The basis of the extraordinary sensitivity and frequency selectivity of the cochlea is a chloride-sensitive protein called prestin which can produce an electromechanical response and which resides in the basolateral plasma membrane of outer hair cells (OHCs). The compound 9-anthracenecarboxylic acid (9-AC), an inhibitor of chloride channels, has been found to reduce the electromechanical response of the cochlea and the OHC mechanical impedance. To elucidate these 9-AC effects, the functional electromechanical status of prestin was assayed by measuring the nonlinear capacitance of OHCs from the guinea-pig cochlea and of prestin-transfected human embryonic kidney 293 (HEK 293) cells. Extracellular application of 9-AC caused reversible, dose-dependent and chloride-sensitive reduction in OHC nonlinear charge transfer, Qmax . Prestin-transfected cells also showed reversible reduction in Qmax . For OHCs, intracellular 9-AC application as well as reduced intracellular pH had no detectable effect on the reduction in Qmax by extracellularly applied 9-AC. In the prestin-transfected cells, cytosolic application of 9-AC approximately halved the blocking efficacy of extracellularly applied 9-AC. OHC inside-out patches presented the whole-cell blocking characteristics. Disruption of the cytoskeleton by preventing actin polymerization with latrunculin A or by decoupling of spectrin from actin with diamide did not affect the 9-AC-evoked reduction in Qmax . We conclude that 9-AC acts on the electromechanical transducer principally by interaction with prestin rather than acting via the cytoskeleton, chloride channels or pH. The 9-AC block presents characteristics in common with salicylate, but is almost an order of magnitude faster. 9-AC provides a new tool for elucidating the molecular dynamics of prestin function.

Phosphoinositides: minor lipids make a major impact on photoreceptor cell functions

Activation of the phosphoinositide (PI) cycle generates the second messengers that control various aspects of cellular signaling. We have previously shown that two PI cycle enzymes, type II phosphatidylinositol 5-phosphate 4-kinase (PIPK IIα) and phosphoinositide 3-kinase (PI3K), are activated through light stimulation. In our earlier studies, we measured enzyme activities, instead of directly measuring the products, due to lack of sensitive analytical techniques. Cells have very low levels of PIs, compared to other lipids, so special techniques and sensitive analytical instruments are necessary for their identification and quantification. There are also other considerations, such as different responses in different cell types, which may complicate quantification of PIs. For example, although light activated PIPK IIα, there was no increase in PI-4,5-P₂ measured by liquid chromatography–mass spectrometry (LC/MS) This discrepancy is due to the heterogeneous nature of the retina, which is composed of various cell types. In this study, we examined PI generation in situ using immunohistochemistry with specific PI antibodies. PIs were generated in specific retinal cell layers, suggesting that analyzing PIs from the total retina by LC/MS underscores the significance. This suggests that PI-specific antibodies are useful tools to study the cell-specific regulation of PIs in the retina.

An Interaction Library for the FcεRI Signaling Network

Antigen receptors play a central role in adaptive immune responses. Although the molecular networks associated with these receptors have been extensively studied, we currently lack a systems-level understanding of how combinations of non-covalent interactions and post-translational modifications are regulated during signaling to impact cellular decision-making. To fill this knowledge gap, it will be necessary to formalize and piece together information about individual molecular mechanisms to form large-scale computational models of signaling networks. To this end, we have developed an interaction library for signaling by the high-affinity IgE receptor, FcεRI. The library consists of executable rules for protein–protein and protein–lipid interactions. This library extends earlier models for FcεRI signaling and introduces new interactions that have not previously been considered in a model. Thus, this interaction library is a toolkit with which existing models can be expanded and from which new models can be built. As an example, we present models of branching pathways from the adaptor protein Lat, which influence production of the phospholipid PIP₃ at the plasma membrane and the soluble second messenger IP₃. We find that inclusion of a positive feedback loop gives rise to a bistable switch, which may ensure robust responses to stimulation above a threshold level. In addition, the library is visualized to facilitate understanding of network circuitry and identification of network motifs.

A genomewide overexpression screen identifies genes involved in the phosphatidylinositol 3-kinase pathway in the human protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and liver abscess. E. histolytica relies on motility, phagocytosis, host cell adhesion, and proteolysis of extracellular matrix for virulence. In eukaryotic cells, these processes are mediated in part by phosphatidylinositol 3-kinase (PI3K) signaling. Thus, PI3K may be critical for virulence. We utilized a functional genomics approach to identify genes whose products may operate in the PI3K pathway in E. histolytica. We treated a population of trophozoites that were overexpressing genes from a cDNA library with a near-lethal dose of the PI3K inhibitor wortmannin. This screen was based on the rationale that survivors would be overexpressing gene products that directly or indirectly function in the PI3K pathway. We sequenced the overexpressed genes in survivors and identified a cDNA encoding a Rap GTPase, a protein previously shown to participate in the PI3K pathway. This supports the validity of our approach. Genes encoding a coactosin-like protein, EhCoactosin, and a serine-rich E. histolytica protein (SREHP) were also identified. Cells overexpressing EhCoactosin or SREHP were also less sensitive to a second PI3K inhibitor, LY294002. This corroborates the link between these proteins and PI3K. Finally, a mutant cell line with an increased level of phosphatidylinositol (3,4,5)-triphosphate, the product of PI3K activity, exhibited increased expression of SREHP and EhCoactosin. This further supports the functional connection between these proteins and PI3K in E. histolytica. To our knowledge, this is the first forward-genetics screen adapted to reveal genes participating in a signal transduction pathway in this pathogen.

Genetically encoded fluorescent probe to visualize intracellular phosphatidylinositol 3,5-bisphosphate localization and dynamics

Phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2] is a low-abundance phosphoinositide presumed to be localized to endosomes and lysosomes, where it recruits cytoplasmic peripheral proteins and regulates endolysosome-localized membrane channel activity. Cells lacking PI(3,5)P2 exhibit lysosomal trafficking defects, and human mutations in the PI(3,5)P2-metabolizing enzymes cause lysosome-related diseases. The spatial and temporal dynamics of PI(3,5)P2, however, remain unclear due to the lack of a reliable detection method. Of the seven known phosphoinositides, only PI(3,5)P2 binds, in the low nanomolar range, to a cytoplasmic phosphoinositide-interacting domain (ML1N) to activate late endosome and lysosome (LEL)-localized transient receptor potential Mucolipin 1 (TRPML1) channels. Here, we report the generation and characterization of a PI(3,5)P2-specific probe, generated by the fusion of fluorescence tags to the tandem repeats of ML1N. The probe was mainly localized to the membranes of Lamp1-positive compartments, and the localization pattern was dynamically altered by either mutations in the probe, or by genetically or pharmacologically manipulating the cellular levels of PI(3,5)P2. Through the use of time-lapse live-cell imaging, we found that the localization of the PI(3,5)P2 probe was regulated by serum withdrawal/addition, undergoing rapid changes immediately before membrane fusion of two LELs. Our development of a PI(3,5)P2-specific probe may facilitate studies of both intracellular signal transduction and membrane trafficking in the endosomes and lysosomes.

Spatiotemporal dynamics of phosphorylation in lipid second messenger signaling

The plasma membrane serves as a dynamic interface that relays information received at the cell surface into the cell. Lipid second messengers coordinate signaling on this platform by recruiting and activating kinases and phosphatases. Specifically, diacylglycerol and phosphatidylinositol 3,4,5-trisphosphate activate protein kinase C and Akt, respectively, which then phosphorylate target proteins to transduce downstream signaling. This review addresses how the spatiotemporal dynamics of protein kinase C and Akt signaling can be monitored using genetically encoded reporters and provides information on how the coordination of signaling at protein scaffolds or membrane microdomains affords fidelity and specificity in phosphorylation events.

Generation of somatic electromechanical force by outer hair cells may be influenced by prestin-CASK interaction at the basal junction with the Deiter's cell

The motor protein, prestin, situated in the basolateral plasma membrane of cochlear outer hair cells (OHCs), underlies the generation of somatic, voltage-driven mechanical force, the basis for the exquisite sensitivity, frequency selectivity and dynamic range of mammalian hearing. The molecular and structural basis of the ontogenetic development of this electromechanical force has remained elusive. The present study demonstrates that this force is significantly reduced when the immature subcellular distribution of prestin found along the entire plasma membrane persists into maturity, as has been described in previous studies under hypothyroidism. This observation suggests that cochlear amplification is critically dependent on the surface expression and distribution of prestin. Searching for proteins involved in organizing the subcellular localization of prestin to the basolateral plasma membrane, we identified cochlear expression of a novel truncated prestin splice isoform named prestin 9b (Slc26A5d) that contains a putative PDZ domain-binding motif. Using prestin 9b as the bait in a yeast two-hybrid assay, we identified a calcium/calmodulin-dependent serine protein kinase (CASK) as an interaction partner of prestin. Co-immunoprecipitation assays showed that CASK and prestin 9b can interact with full-length prestin. CASK was co-localized with prestin in a membrane domain where prestin-expressing OHC membrane abuts prestin-free OHC membrane, but was absent from this area for thyroid hormone deficiency. These findings suggest that CASK and the truncated prestin splice isoform contribute to confinement of prestin to the basolateral region of the plasma membrane. By means of such an interaction, the basal junction region between the OHC and its Deiter's cell may contribute to efficient generation of somatic electromechanical force.

A role for lipid-mediated signaling in plant gravitropism

Gravitropism is a universal plant response. It is initiated by the sensing of the primary signal (mass or pressure), which is then converted into chemical signals that are transduced and propagated in a precise spatial and temporal fashion, resulting in a differential growth response. Our thesis is that membrane lipids and lipid-mediated signaling pathways play critical roles in the initial signaling and in the establishment of polarity. In this review, we highlight results from recent literature and discuss the major questions that remain unanswered.

Smaller, faster, brighter: advances in optical imaging of living plant cells

The advent of fluorescent proteins and access to modern imaging technologies have dramatically accelerated the pace of discovery in plant cell biology. Remarkable new insights into such diverse areas as plant pathogenesis, cytoskeletal dynamics, sugar transport, cell wall synthesis, secretory control, and hormone signaling have come from careful examination of living cells using advanced optical probes. New technologies, both commercially available and on the horizon, promise a continued march toward more quantitative methods for imaging and for extending the optical exploration of biological structure and activity to molecular scales. In this review, we lay out fundamental issues in imaging plant specimens and look ahead to several technological innovations in molecular tools, instrumentation, imaging methods, and specimen handling that show promise for shaping the coming era of plant cell biology.

Multiplexed visualization of dynamic signaling networks using genetically encoded fluorescent protein-based biosensors

Cells rely on a complex, interconnected network of signaling pathways to sense and interpret changes in their extracellular environment. The development of genetically encoded fluorescent protein (FP)-based biosensors has made it possible for researchers to directly observe and characterize the spatiotemporal dynamics of these intracellular signaling pathways in living cells. However, detailed information regarding the precise temporal and spatial relationships between intersecting pathways is often lost when individual signaling events are monitored in isolation. As the development of biosensor technology continues to advance, it is becoming increasingly feasible to image multiple FP-based biosensors concurrently, permitting greater insights into the intricate coordination of intracellular signaling networks by enabling parallel monitoring of distinct signaling events within the same cell. In this review, we discuss several strategies for multiplexed imaging of FP-based biosensors, while also underscoring some of the challenges associated with these techniques and highlighting additional avenues that could lead to further improvements in parallel monitoring of intracellular signaling events.

Imaging beyond the proteome

Imaging technologies developed in the early 20th century achieved contrast solely by relying on macroscopic and morphological differences between the tissues of interest and the surrounding tissues. Since then, there has been a movement toward imaging at the cellular and molecular level in order to visualize biological processes. This rapidly growing field is known as molecular imaging. In the last decade, many methodologies for imaging proteins have emerged. However, most of these approaches cannot be extended to imaging beyond the proteome. Here, we highlight some of the recently developed technologies that enable imaging of non-proteinaceous molecules in the cell: lipids, signalling molecules, inorganic ions, glycans, nucleic acids, small-molecule metabolites, and protein post-translational modifications such as phosphorylation and methylation.

Studying lipids involved in the endosomal pathway

Endosomes along the degradation pathway exhibit a multivesicular appearance and differ in their lipid compositions. Association of proteins to specific membrane lipids and presumably also lipid-lipid interactions contribute to the formation of functional membrane platforms that regulate endosome biogenesis and function. This chapter provides a brief review of the functions of endosomal lipids in the degradation pathway, a discussion of techniques that allow studying lipid-based mechanisms and a selection of step-by-step protocols for in vivo and in vitro methods commonly used to study lipid roles in endocytosis. The techniques described here have been used to elucidate the function of the late endosomal lipid lysobisphosphatidic acid and allow the monitoring of lipid distribution, levels and dynamics, as well as the characterization of lipid-binding partners.

A highly dynamic ER-derived phosphatidylinositol-synthesizing organelle supplies phosphoinositides to cellular membranes

Polyphosphoinositides are lipid signaling molecules generated from phosphatidylinositol (PtdIns) with critical roles in vesicular trafficking and signaling. It is poorly understood where PtdIns is located within cells and how it moves around between membranes. Here we identify a hitherto-unrecognized highly mobile membrane compartment as the site of PtdIns synthesis and a likely source of PtdIns of all membranes. We show that the PtdIns-synthesizing enzyme PIS associates with a rapidly moving compartment of ER origin that makes ample contacts with other membranes. In contrast, CDP-diacylglycerol synthases that provide PIS with its substrate reside in the tubular ER. Expression of a PtdIns-specific bacterial PLC generates diacylglycerol also in rapidly moving cytoplasmic objects. We propose a model in which PtdIns is synthesized in a highly mobile lipid distribution platform and is delivered to other membranes during multiple contacts by yet-to-be-defined lipid transfer mechanisms.

Treponema denticola major outer sheath protein induces actin assembly at free barbed ends by a PIP2-dependent uncapping mechanism in fibroblasts

The major outer sheath protein (Msp) of Treponema denticola perturbs actin dynamics in fibroblasts by inducing actin reorganization, including subcortical actin filament assembly, leading to defective calcium flux, diminished integrin engagement of collagen, and retarded cell migration. Yet, its mechanisms of action are unknown. We challenged Rat-2 fibroblasts with enriched native Msp. Msp activated the small GTPases Rac1, RhoA and Ras, but not Cdc42, yet only Rac1 localized to areas of actin rearrangement. We used Rac1 dominant negative transfection and chemical inhibition of phosphatidylinositol-3 kinase (PI3K) to show that even though Rac1 activation was PI3K-dependent, neither was required for Msp-induced actin rearrangement. Actin free barbed end formation (FBE) by Msp was also PI3K-independent. Immunoblotting experiments showed that gelsolin and CapZ were released from actin filaments, whereas cofilin remained in an inactive state. Msp induced phosphatidylinositol (4,5)-bisphosphate (PIP2) formation through activation of a phosphoinositide 3-phosphatase and its recruitment to areas of actin assembly at the plasma membrane. Using a PIP2 binding peptide or lipid phosphatase inhibitor, PIP2 was shown to be required for Msp-mediated actin uncapping and FBE formation. Evidently, Msp induces actin assembly in fibroblasts by production and recruitment of PIP2 and release of the capping proteins CapZ and gelsolin from actin barbed ends.

A genetically encoded tool kit for manipulating and monitoring membrane phosphatidylinositol 4,5-bisphosphate in intact cells

Background

Most ion channels are regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) in the cell membrane by diverse mechanisms. Important molecular tools to study ion channel regulation by PtdIns(4,5)P₂ in living cells have been developed in the past. These include fluorescent PH-domains as sensors for Förster resonance energy transfer (FRET), to monitor changes in plasma membrane_. For controlled and reversible depletion of PtdIns(4,5)P₂, voltage-sensing phosphoinositide phosphatases (VSD) have been demonstrated as a superior tool, since they are independent of cellular signaling pathways. Combining these methods in intact cells requires multiple transfections. We used self-cleaving viral 2A-peptide sequences for adenovirus driven expression of the PH-domain of phospholipase-Cδ1 (PLCδ1) fused to ECFP and EYFP respectively and Ciona intestinalis VSP (Ci-VSP), from a single open reading frame (ORF) in adult rat cardiac myocytes.

Methods and Results

Expression and correct targeting of ECFP-PH-PLCδ1_, EYFP-PH-PLCδ1, and Ci-VSP from a single tricistronic vector containing 2A-peptide sequences first was demonstrated in HEK293 cells by voltage-controlled FRET measurements and Western blotting. Adult rat cardiac myocytes expressed Ci-VSP and the two fluorescent PH-domains within 4 days after gene transfer using the vector integrated into an adenoviral construct. Activation of Ci-VSP by depolarization resulted in rapid changes in FRET ratio indicating depletion of PtdIns(4,5)P₂ in the plasma membrane. This was paralleled by inhibition of endogenous G protein activated K⁺ (GIRK) current. By comparing changes in FRET and current, a component of GIRK inhibition by adrenergic receptors unrelated to depletion of PtdIns(4,5)P₂ was identified.

Conclusions

Expression of a FRET sensor pair and Ci-VSP from a single ORF provides a useful approach to study regulation of ion channels by phosphoinositides in cell lines and transfection-resistant postmitotic cells. Generally, adenoviral constructs containing self-cleaving 2A-peptide sequences are highly suited for simultaneous transfer of multiple genes in adult cardiac myocytes.

PtdIns(3,4,5)P₃ is a regulator of myosin-X localization and filopodia formation

Phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P₃] is a key regulator of cell signaling that acts by recruiting proteins to the cell membrane, such as at the leading edge during cell migration. Here, we show that PtdIns (3,4,5)P₃ plays a central role in filopodia formation via the binding of myosin-X (Myo10), a potent promoter of filopodia. We found that the second pleckstrin homology domain (Myo10-PH2) of Myo10 specifically binds to PtdIns(3,4,5)P₃, and that disruption of this binding led to impairment of filopodia and partial re-localization of Myo10 to microtubule-associated Rab7-positive endosomal vesicles. Given that the localization of Myo10 was dynamically restored to filopodia upon reinstatement of PtdIns(3,4,5)P₃-binding, our results indicate that PtdIns(3,4,5)P₃ binding to the Myo10-PH2 domain is involved in Myo10 trafficking and regulation of filopodia dynamics.

The use of translocating fluorescent biosensors for real-time monitoring of GPCR-mediated signaling events

The ability to visualize the subcellular localization of proteins by labeling them with fluorescent proteins is a powerful tool in cell biology. In the G protein-coupled receptor signaling field, this technique has been utilized to examine the various aspects of receptor behavior, including activation, internalization and recycling, as well as alterations in the cellular levels of a variety of second messengers and signaling intermediates. Attaching variants of green fluorescent protein on to protein modules, which possess high affinity and selectivity for specific signaling molecules has allowed the visualization of key signaling pathway intermediates in real time, in living cells. This chapter outlines a protocol for the expression and visualization (by confocal microscopy) of such fluorescent "biosensors" and provides guidance on the analysis and interpretation of data obtained from such experiments.

Transfection of cells with DNA encoding a visible fluorescent protein-tagged lipid-binding domain

Visualization of a certain lipid species can be achieved by producing a fusion protein between a lipid-binding domain and a visible fluorescent protein (VFP). After a DNA construct for a VFP-tagged lipid-binding domain has been prepared, the desired cell line is transfected with the DNA and visualized using fluorescence microscopy, as described here. The DNA encoding a VFP-tagged lipid-binding domain is isolated from Escherichia coli, and the cells to be transfected are grown on glass-bottom dishes or on coverslips. We routinely perform transfection with commercially available reagents, although, depending on the cell line, the calcium phosphate precipitation method may provide an economic alternative.

Imaging lipids in living cells

The investigation of lipids in living cells is one of the underdeveloped areas in cell biology. Although it is possible to analyze the global lipid composition of a cell type, fractionation of the various types of membranes from cells is extraordinarily difficult, mainly because most membranes appear to be in contact with each other. Therefore, we know the lipid components, but we have a difficult time finding out their exact position, how dynamically they change location, and how rapidly they are metabolized. Imaging lipids in cells seems to be the obvious solution to the problem. The most common way to image molecules is by the artificial addition of a fluorescent tag. The use of fluorescent proteins has become the mainstay of protein imaging, but this method is, of course, not suitable for small molecules such as lipids. Unfortunately, the fluorescent tag is usually as large as the lipid and is therefore likely to have a severe influence on lipid location and metabolism. To circumvent this problem, two solutions have been developed--namely, the use of fluorescently labeled proteins that specifically recognize lipids and a chemical method to introduce the fluorescent tag inside the cell. This article describes procedures necessary to image lipids by fluorescently tagged lipid-binding domains and by labeling lipid derivatives in fixed and living cells.

Analysis of cellular phosphatidylinositol (3,4,5)-trisphosphate levels and distribution using confocal fluorescent microscopy

We have developed an immunocytochemistry method for the semiquantitative detection of phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P3) at the cell plasma membrane. This protocol combines the use of a glutathione S-transferase-tagged pleckstrin homology (PH) domain of the general phosphoinositides-1 receptor (GST-GRP1PH) with fluorescence confocal microscopy and image segmentation using cell mask software analysis. This methodology allows the analysis of PI(3,4,5)P3 subcellular distribution in resting and epidermal growth factor (EGF)-stimulated HEK293T cells and in LIM1215 (wild-type phosphoinositide 3-kinase (PI3K)) and LIM2550 (H1047R mutation in PI3K catalytic domain) colonic carcinoma cells. Formation of PI(3,4,5)P3 was observed 5min following EGF stimulation and resulted in an increase of the membrane/cytoplasm fluorescence ratio from 1.03 to 1.53 for HEK293T cells and from 2.2 to 3.3 for LIM1215 cells. Resting LIM2550 cells stained with GST-GRP1PH had an elevated membrane/cytoplasm fluorescence ratio of 9.8, suggesting constitutive PI3K activation. The increase in the membrane/cytoplasm fluorescent ratio was inhibited in a concentration-dependent manner by the PI3K inhibitor LY294002. This cellular confocal imaging assay can be used to directly assess the effects of PI3K mutations in cancer cell lines and to determine the potential specificity and effectiveness of PI3K inhibitors in cancer cells.

Phosphoinositide 3-kinase signaling in the vertebrate retina

The phosphoinositide (PI) cycle, discovered over 50 years ago by Mabel and Lowell Hokin, describes a series of biochemical reactions that occur on the inner leaflet of the plasma membrane of cells in response to receptor activation by extracellular stimuli. Studies from our laboratory have shown that the retina and rod outer segments (ROSs) have active PI metabolism. Biochemical studies revealed that the ROSs contain the enzymes necessary for phosphorylation of phosphoinositides. We showed that light stimulates various components of the PI cycle in the vertebrate ROS, including diacylglycerol kinase, PI synthetase, phosphatidylinositol phosphate kinase, phospholipase C, and phosphoinositide 3-kinase (PI3K). This article describes recent studies on the PI3K-generated PI lipid second messengers in the control and regulation of PI-binding proteins in the vertebrate retina.

Amot recognizes a juxtanuclear endocytic recycling compartment via a novel lipid binding domain

Polarity proteins promote the asymmetric organization of cells by orienting intracellular sorting mechanisms, such as protein trafficking and cytoskeletal assembly. The localization of individual polarity proteins in turn is often determined by association with factors that mediate contact with other cells or the substratum. This arrangement for the Par and Crb apical polarity complexes at the tight junction is disrupted by the adaptor protein Amot. Amot directly binds the scaffolding proteins Patj and Mupp1 and redistributes them and their binding partners from the plasma membrane to endosomes. However, the mechanism by which Amot is targeted to endosomes is unknown. Here, a novel lipid binding domain within Amot is shown to selectively bind with high affinity to membranes containing monophosphorylated phosphatidylinositols and cholesterol. With similar lipid specificity, Amot inserts into and tubulates membranes in vitro and enlarges perinuclear endosomal compartments in cells. Based on the similar distribution of Amot with cholesterol, Rab11, and Arf6, such membrane interactions are identified at juxtanuclear endocytic recycling compartments. Taken together, these findings indicate that Amot is targeted along with associated apical polarity proteins to the endocytic recycling compartment via this novel membrane binding domain.

Imaging phosphoinositide dynamics in living cells

To improve our understanding of the important roles played by inositol lipid derivatives in signalling and other cellular processes, it is crucial to measure phosphoinositide concentration changes in individual cells with high spatial and temporal resolution. A number of protein domains that interact with inositol lipids in a specific manner have been identified. Tagged with the green fluorescent protein or its colour variants, these protein modules can be used as probes to visualize various phosphoinositide species in different sub-cellular compartments. Here, we present protocols for fluorescence imaging of phosphoinositide dynamics in single living cells. Total internal reflection fluorescence microscopy is particularly powerful for time-lapse recordings of phosphoinositides in the plasma membrane. We demonstrate how this technique can be used to record phospholipase C- and PI3-kinase-induced changes in inositol lipids in insulin-secreting cells. These procedures should be applicable to studies of the spatio-temporal regulation of phosphoinositide metabolism in many types of cells.

Odorant-stimulated phosphoinositide signaling in mammalian olfactory receptor neurons

Recent evidence has revived interest in the idea that phosphoinositides (PIs) may play a role in signal transduction in mammalian olfactory receptor neurons (ORNs). To provide direct evidence that odorants indeed activate PI signaling in ORNs, we used adenoviral vectors carrying two different fluorescently tagged probes, the pleckstrin homology (PH) domains of phospholipase C delta 1 (PLC delta 1) and the general receptor of phosphoinositides (GRP1), to monitor PI activity in the dendritic knobs of ORNs in vivo. Odorants mobilized PI(4,5)P(2)/IP(3) and PI(3,4,5)P(3), the substrates and products of PLC and PI3K. We then measured odorant activation of PLC and PI3K in olfactory ciliary-enriched membranes in vitro using a phospholipid overlay assay and ELISAs. Odorants activated both PLC and PI3K in the olfactory cilia within 2s of odorant stimulation. Odorant-dependent activation of PLC and PI3K in the olfactory epithelium could be blocked by enzyme-specific inhibitors. Odorants activated PLC and PI3K with partially overlapping specificity. These results provide direct evidence that odorants indeed activate PI signaling in mammalian ORNs in a manner that is consistent with the idea that PI signaling plays a role in olfactory transduction.

Are inositol pyrophosphates signalling molecules?

The inositol polyphosphate family of small, cytosolic molecules has a prominent place in the field of cell signalling, and inositol pyrophosphates are the most recent addition to this large family. First identified in 1993, they have since been found in all eukaryotic organisms studied. The defining feature of inositol pyrophosphates is the presence of the characteristic 'high energy' pyrophosphate group, which immediately attracted interest in them as possible signalling molecules. In addition to their unique 'high energy' pyrophosphate bond, their concentration in the cell is tightly regulated with an extremely rapid turnover. This, together with the history of other inositol polyphosphates, makes it likely that they have an important role in intracellular signalling involving some basic cellular processes. This hypothesis is supported by the surprisingly wide range of cellular functions where inositol pyrophosphates seem to be involved. A seminal finding was that inositol pyrophosphates are able to directly phosphorylate pre-phosphorylated proteins, thereby identifying an entirely new post-translational protein modification, namely serine-pyrophosphorylation. Rapid progress has been made in characterising the metabolism of these molecules in the 15 years since their first identification. However, their detailed signalling role in specific cellular processes and in the context of relevant physiological cues has developed more slowly, particularly in mammalian system. We will discuss inositol pyrophosphates from the cell signalling perspective, analysing how their intracellular concentration is modulated, what their possible molecular mechanisms of action are, together with the physiological consequences of this novel form of signalling.

Spatio-temporal dynamics of phosphatidylinositol-3,4,5-trisphosphate signalling

Many effects of insulin, insulin-like growth factors and other receptor stimuli are mediated via the phospholipid second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP(3)). PIP(3) is formed by the activity of phosphoinositide 3-kinases in the plasma membrane, where it serves to recruit signalling proteins. These proteins coordinate complex events leading to changes in cell metabolism, growth, movement and survival. Over the past decade, new techniques for measurements of PIP(3) in the plasma membrane of individual living cells have markedly improved our understanding of the role of this messenger in a variety of cellular processes. This review summarises the mechanisms involved in formation and degradation of PIP(3) in insulin-responsive cells, how PIP(3) can be measured in individual cells as well as accumulating evidence that the plasma membrane PIP(3) concentration undergoes complex spatio-temporal patterns in many types of cells, with particular emphasis on autocrine insulin-induced PIP(3) oscillations in pancreatic beta-cells.

Activation and membrane binding of retinal protein kinase Balpha/Akt1 is regulated through light-dependent generation of phosphoinositides

Akt is a phospholipid-binding protein and the downstream effector of the phosphoinositide 3-kinase (PI3K) pathway. Akt has three isoforms: Akt1, Akt2, and Akt3. All of these isoforms are expressed in rod photoreceptor cells, but the individual functions of each isoform are not known. In this study, we found that light induces the activation of Akt1. The membrane binding of Akt1 to rod outer segments (ROS) is insulin receptor (IR)/PI3K-dependent as demonstrated by reduced binding of Akt1 to ROS membranes of photoreceptor-specific IR knockout mice. Membrane binding of Akt1 is mediated through its Pleckstrin homology (PH) domain. To determine whether binding of the PH domain of Akt1 to photoreceptor membranes is regulated by light, various green fluorescent protein (GFP)/Akt1-PH domain fusion proteins were expressed in rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The R25C mutant PH domain of Akt1, which does not bind phosphoinositides, failed to associate with plasma membranes in a light-dependent manner. This study suggests that light-dependent generation of phosphoinositides regulates the activation and membrane binding of Akt1 in vivo. Our results also suggest that actin cytoskeletal organization may be regulated through light-dependent generation of phosphoinositides.

Molecular pathogenesis of megalencephalic leukoencephalopathy with subcortical cysts: mutations in MLC1 cause folding defects

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare type of leukodystrophy, most often caused by mutations in the MLC1 gene. MLC1 is an oligomeric plasma membrane (PM) protein of unknown function expressed mainly in glial cells and neurons. Most disease-causing missense mutations dramatically reduced the total and PM MLC1 expression levels in Xenopus oocytes and mammalian cells. The impaired expression of the mutants was verified in primary cultures of rat astrocytes, as well as human monocytes, cell types that endogenously express MLC1, demonstrating the relevance of the tissue culture models. Using a combination of biochemical, pharmacological and imaging methods, we also demonstrated that increased endoplasmatic reticulum-associated degradation and endo-lysosomal-associated degradation can contribute to the cell surface expression defect of the mutants. Based on these results, we suggest that MLC1 mutations reduce protein levels in vivo. Since the expression defect of the mutants could be rescued by exposing the mutant-protein expressing cells to low temperature and glycerol, a chemical chaperone, we propose that MLC belongs to the class of conformational diseases. Therefore, we suggest the use of pharmacological strategies that improve MLC1 expression to treat MLC patients.

Overexpression of partner of numb induces asymmetric distribution of the PI4P 5-Kinase Skittles in mitotic sensory organ precursor cells in Drosophila

Unequal segregation of cell fate determinants at mitosis is a conserved mechanism whereby cell fate diversity can be generated during development. In Drosophila, each sensory organ precursor cell (SOP) divides asymmetrically to produce an anterior pIIb and a posterior pIIa cell. The Par6-aPKC complex localizes at the posterior pole of dividing SOPs and directs the actin-dependent localization of the cell fate determinants Numb, Partner of Numb (Pon) and Neuralized at the opposite pole. The plasma membrane lipid phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates the plasma membrane localization and activity of various proteins, including several actin regulators, thereby modulating actin-based processes. Here, we have examined the distribution of PIP2 and of the PIP2-producing kinase Skittles (Sktl) in mitotic SOPs. Our analysis indicates that both Sktl and PIP2 reporters are uniformly distributed in mitotic SOPs. In the course of this study, we have observed that overexpression of full-length Pon or its localization domain (LD) fused to the Red Fluorescent Protein (RFP::Pon^LD) results in asymmetric distribution of Sktl and PIP2 reporters in dividing SOPs. Our observation that Pon overexpression alters polar protein distribution is relevant because RFP::Pon^LD is often used as a polarity marker in dividing progenitors.

Pathogen destruction versus intracellular survival: the role of lipids as phagosomal fate determinants

Phagocytosis is a key component of the innate immune response and of the clearance of apoptotic bodies. Phagosome formation and subsequent maturation require extensive cytoskeletal rearrangement and precisely choreographed vesicular fusion and fission events. The objectives of this review are to highlight the functional importance of lipids in the phagocytic process, to discuss how pathogenic microorganisms can in some cases manipulate host lipid metabolism to either co-opt or disrupt phagosome maturation and promote their own survival, and to describe how defective phagosomal lipid metabolism can result in disease.

Real-time cross-correlation image analysis of early events in IgE receptor signaling

Signaling in mast cells and basophils is mediated through IgE and its high affinity cell surface receptor, FcepsilonRI. Crosslinking of the receptors by a cognate multivalent antigen leads to degranulation and release of mediators of the allergic immune response. Using multicolor fluorescence confocal microscopy, we probed the spatio-temporal dynamics of early events in the IgE receptor signal cascade. We monitored the recruitment of GFP-/CFP-labeled signaling proteins by acquiring sequential images with time resolution of 3 s during stimulation of RBL-2H3 mast cells with multivalent antigen. A fluorescent tag on the antigen allowed us to visualize the plasma membrane localization of crosslinked receptors, and fluorescent cholera toxin B served as a plasma membrane marker. We developed an automated image analysis scheme to quantify the recruitment of fluorescent intracellular proteins to the plasma membrane and to assess the time-dependent colocalization of these and other membrane-associated proteins with crosslinked receptors as measured by cross-correlation between the plasma membrane distributions of the two fluorophores. This automated method permits analysis of thousands of individual images from multiple experiments for each cross-correlation pair. We systematically applied this analysis to characterize stimulated interactions of IgE receptors with several signaling proteins, including the tyrosine kinases Lyn and Syk, and the adaptor protein LAT. Notably, for Syk-CFP we observed a rapid stimulated translocation to the plasma membrane but very little colocalization with aggregated receptors. Our results demonstrate the utility of this simple, automated method to monitor protein interactions quantitatively during cell signaling.