Akt-PDK1 complex mediates epidermal growth factor-induced membrane protrusion through Ral activation

Lysosomal PIP3 revealed by genetically encoded lipid biosensors

3-Phosphoinositides (3-PIs), phosphatidylinositol (3,4) bisphosphate [PI(3,4)P2] and phosphatidylinositol (3,4,5) trisphosphate (PIP3), are important lipid second messengers in the Phosphoinositide 3-Kinase (PI3K)/Akt signaling pathway, which is crucial to cell growth and frequently dysregulated in cancer. Emerging evidence suggests these lipid second messengers may be present in membranes beyond the plasma membrane, yet their spatial regulation within other membrane compartments is not well understood. To dissect the spatial regulation of specific 3-PI species, we developed genetically encodable biosensors with selectivity for PIP3 or PI(3,4)P2. Using these biosensors, we showed that PIP3 significantly accumulated at the lysosome upon growth factor stimulation, in contrast to the conventional view that PIP3 is exclusively present in the plasma membrane. Furthermore, we showed that lysosomal PIP3 originates from the plasma membrane and relies on dynamin-dependent endocytosis for lipid internalization. Thus, PIP3 can exploit dynamic trafficking pathways to access subcellular compartments and regulate signaling in a spatially selective manner.

Multiphoton intravital microscopy of rodents

Tissues are heterogeneous with respect to cellular and non-cellular components and in the dynamic interactions between these elements. To study the behaviour and fate of individual cells in these complex tissues, intravital microscopy (IVM) techniques such as multiphoton microscopy have been developed to visualize intact and live tissues at cellular and subcellular resolution. IVM experiments have revealed unique insights into the dynamic interplay between different cell types and their local environment, and how this drives morphogenesis and homeostasis of tissues, inflammation and immune responses, and the development of various diseases. This Primer introduces researchers to IVM technologies, with a focus on multiphoton microscopy of rodents, and discusses challenges, solutions and practical tips on how to perform IVM. To illustrate the unique potential of IVM, several examples of results are highlighted. Finally, we discuss data reproducibility and how to handle big imaging data sets.

The development of 3-substituted indolin-2-one derivatives as kinase inhibitors for cancer therapy

Kinases are pivotal regulators in tumorigenesis and metastasis by modulating the expression of oncogenes and the transcription of antioncogenes directly or indirectly. Correspondingly, multifarious 3-substituted indolin-2-one derivatives as selective kinase inhibitors for cancer therapy exhibited a low nanomolar activity with prominent efficacy, superior response rate and admirable tolerability. Particularly, certain 3-substituted indolin-2-one derivatives have met the requirements for clinical trials or the pharmaceutical market. Herein, we focus on the traits of 3-substituted indolin-2-one derivatives as kinase inhibitors for cancer therapy, overview recent progress of 3-substituted indolin-2-one derivatives as kinase inhibitors for cancer therapy, analyze the selectivity for tyrosine kinases inhibitors and serine/threonine kinases inhibitors from the molecular aspects based on the molecular docking studies, summarize the structure-activity relationships (SARs) as selective kinase inhibitors and provide our perspectives for the development of 3-substituted indolin-2-one derivatives as kinase inhibitors for cancer therapy.

Akt phosphorylates insulin receptor substrate to limit PI3K-mediated PIP3 synthesis

The phosphoinositide 3-kinase (PI3K)-Akt network is tightly controlled by feedback mechanisms that regulate signal flow and ensure signal fidelity. A rapid overshoot in insulin-stimulated recruitment of Akt to the plasma membrane has previously been reported, which is indicative of negative feedback operating on acute timescales. Here, we show that Akt itself engages this negative feedback by phosphorylating insulin receptor substrate (IRS) 1 and 2 on a number of residues. Phosphorylation results in the depletion of plasma membrane-localised IRS1/2, reducing the pool available for interaction with the insulin receptor. Together these events limit plasma membrane-associated PI3K and phosphatidylinositol (3,4,5)-trisphosphate (PIP3) synthesis. We identified two Akt-dependent phosphorylation sites in IRS2 at S306 (S303 in mouse) and S577 (S573 in mouse) that are key drivers of this negative feedback. These findings establish a novel mechanism by which the kinase Akt acutely controls PIP3 abundance, through post-translational modification of the IRS scaffold.

Quantifying and visualising the nuances of cellular dynamics in vivo using intravital imaging

Intravital imaging is a powerful technology used to quantify and track dynamic changes in live cells and tissues within an intact environment. The ability to watch cell biology in real-time 'as it happens' has provided novel insight into tissue homeostasis, as well as disease initiation, progression and response to treatment. In this minireview, we highlight recent advances in the field of intravital microscopy, touching upon advances in awake versus anaesthesia-based approaches, as well as the integration of biosensors into intravital imaging. We also discuss current challenges that, in our opinion, need to be overcome to further advance the field of intravital imaging at the single-cell, subcellular and molecular resolution to reveal nuances of cell behaviour that can be targeted in complex disease settings.

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

TROP2 increases growth and metastasis of human oral squamous cell carcinoma through activation of the PI3K/Akt signaling pathway

Most malignant neoplasms of the oral cavity are oral squamous cell carcinoma (OSCC), which is a type of highly malignant tumor with a propensity for forming distant metastases. Trophoblast cell surface antigen 2 (TROP2) is a transmembrane protein that is overexpressed in several types of tumor cells, although its role and regulatory mechanism in OSCC have not been determined. The aim of the present study was to examine the effects of TROP2 in human OSCC cell lines. The present study demonstrated that TROP2 protein expression was upregulated in OSCC cell lines. Transfection of short hairpin RNA (shRNA) targeting TROP2 (sh‑TROP2) reduced cell proliferation, migration and invasion of OSCC cell lines, whereas overexpression of TROP2 increased proliferation, migration and invasion. sh‑TROP2 transfection in OSCC cell lines inhibited tumor growth in OSCC mouse models. Furthermore, TROP2 expression activated the phosphoinositide 3‑kinase (PI3K)/Akt signaling pathway in human OSCC cells. These results suggest that TROP2 induces cell growth, migration and invasion through activation of the PI3K/Akt signaling pathway in OSCC cells.

Encapsulated Optically Responsive Cell Systems: Toward Smart Implants in Biomedicine

Managing increasingly prevalent chronic diseases will require close continuous monitoring of patients. Cell-based biosensors may be used for implantable diagnostic systems to monitor health status. Cells are indeed natural sensors in the body. Functional cellular systems can be maintained in the body for long-term implantation using cell encapsulation technology. By taking advantage of recent progress in miniaturized optoelectronic systems, the genetic engineering of optically responsive cells may be combined with cell encapsulation to generate smart implantable cell-based sensing systems. In biomedical research, cell-based biosensors may be used to study cell signaling, therapeutic effects, and dosing of bioactive molecules in preclinical models. Today, a wide variety of genetically encoded fluorescent sensors have been developed for real-time imaging of living cells. Here, recent developments in genetically encoded sensors, cell encapsulation, and ultrasmall optical systems are highlighted. The integration of these components in a new generation of biosensors is creating innovative smart in vivo cell-based systems, bringing novel perspectives for biomedical research and ultimately allowing unique health monitoring applications.

Upregulation of multiple signaling pathways by Dock5 deletion in epithelial cells

Purpose

Rupture of lens cataract (RLC) is a hereditary mouse model that shows spontaneous rupture of the lens at the posterior pole at 45-100 days of age. The responsible gene for this phenotype was identified as Dock5, a guanine nucleotide exchange factor for small GTPase Rac1. This study was performed to elucidate the pathway initiating this phenotype.

Methods

We examined the RNA expression by microarray in lens epithelial cells (LECs) from wild-type and RLC mice at the pre-rupture age of 21 days. We applied the list of altered genes to an Ingenuity Pathway Analysis (IPA) to predict the pathways that are altered upon dedicator of cytokinesis-5 (Dock5) protein loss. The activation status of the predicted pathways was examined by western blotting in the cultured epithelial cells treated with a Dock5 inhibitor.

Results

The highest-scored network was "Antimicrobial Response, Inflammatory Response, Dermatological Diseases and Conditions." In that network, it is predicted that extracellular signal-regulated kinase (Erk) is activated in LECs from RLC mice. Our test confirmed that Erk was more phosphorylated in the LECs at the equator in both Dock5-knockout mice and RLC mice. In an in vitro experiment of the cultured epithelial cells, the inhibition of Dock5 activity significantly induced Erk activation. It was also confirmed that Akt (cellular homolog of murine thymoma virus akt8 oncogene, also called protein kinase B) and nuclear factor-kappa B (NFκB), predicted to be the key molecules in two other high-scoring networks by IPA, were activated upon Dock5 inhibition in the cultured epithelial cells.

Conclusions

Dock5 participates in epithelial cell maintenance by regulating gene expression.

Recent Advances in Development of Genetically Encoded Fluorescent Sensors

Genetically encoded fluorescent sensors are essential tools in modern biological research, and recent advances in fluorescent proteins (FPs) have expanded the scope of sensor design and implementation. In this review we compare different sensor platforms, including Förster resonance energy transfer (FRET) sensors, fluorescence-modulated single FP-based sensors, translocation sensors, complementation sensors, and dimerization-based sensors. We discuss elements of sensor design and engineering for each platform, including the incorporation of new types of FPs and sensor screening techniques. Finally, we summarize the wide range of sensors in the literature, exploring creative new sensor architectures suitable for different applications.

Allosteric substrate switching in a voltage-sensing lipid phosphatase

Allostery provides a critical control over enzyme activity, biasing the catalytic site between inactive and active states. We find the Ciona intestinalis voltage-sensing phosphatase (Ci-VSP), which modifies phosphoinositide signaling lipids (PIPs), to have not one but two sequential active states with distinct substrate specificities, whose occupancy is allosterically controlled by sequential conformations of the voltage sensing domain (VSD). Using fast FRET reporters of PIPs to monitor enzyme activity and voltage clamp fluorometry to monitor conformational changes in the VSD, we find that Ci-VSP switches from inactive to a PIP₃-preferring active state when the VSD undergoes an initial voltage sensing motion and then into a second PIP₂-preferring active state when the VSD activates fully. This novel 2-step allosteric control over a dual specificity enzyme enables voltage to shape PIP concentrations in time, and provides a mechanism for the complex modulation of PIP-regulated ion channels, transporters, cell motility and endo/exocytosis.

Fluorescent proteins as genetically encoded FRET biosensors in life sciences

Fluorescence- or Förster resonance energy transfer (FRET) is a measurable physical energy transfer phenomenon between appropriate chromophores, when they are in sufficient proximity, usually within 10 nm. This feature has made them incredibly useful tools for many biomedical studies on molecular interactions. Furthermore, this principle is increasingly exploited for the design of biosensors, where two chromophores are linked with a sensory domain controlling their distance and thus the degree of FRET. The versatility of these FRET-biosensors made it possible to assess a vast amount of biological variables in a fast and standardized manner, allowing not only high-throughput studies but also sub-cellular measurements of biological processes. In this review, we aim at giving an overview over the recent advances in genetically encoded, fluorescent-protein based FRET-biosensors, as these represent the largest and most vividly growing group of FRET-based sensors. For easy understanding, we are grouping them into four categories, depending on their molecular mechanism. These are based on: (a) cleavage; (b) conformational-change; (c) mechanical force and (d) changes in the micro-environment. We also address the many issues and considerations that come with the development of FRET-based biosensors, as well as the possibilities that are available to measure them.

An optogenetic system for interrogating the temporal dynamics of Akt

The dynamic activity of the serine/threonine kinase Akt is crucial for the regulation of diverse cellular functions, but the precise spatiotemporal control of its activity remains a critical issue. Herein, we present a photo-activatable Akt (PA-Akt) system based on a light-inducible protein interaction module of Arabidopsis thaliana cryptochrome2 (CRY2) and CIB1. Akt fused to CRY2phr, which is a minimal light sensitive domain of CRY2 (CRY2-Akt), is reversibly activated by light illumination in several minutes within a physiological dynamic range and specifically regulates downstream molecules and inducible biological functions. We have generated a computational model of CRY2-Akt activation that allows us to use PA-Akt to control the activity quantitatively. The system provides evidence that the temporal patterns of Akt activity are crucial for generating one of the downstream functions of the Akt-FoxO pathway; the expression of a key gene involved in muscle atrophy (Atrogin-1). The use of an optical module with computational modeling represents a general framework for interrogating the temporal dynamics of biomolecules by predictive manipulation of optogenetic modules.

Ceramide limits phosphatidylinositol-3-kinase C2β-controlled cell motility in ovarian cancer: potential of ceramide as a metastasis-suppressor lipid

Targeting cell motility, which is required for dissemination and metastasis, has therapeutic potential for ovarian cancer metastasis, and regulatory mechanisms of cell motility need to be uncovered for developing novel therapeutics. Invasive ovarian cancer cells spontaneously formed protrusions, such as lamellipodia, which are required for generating locomotive force in cell motility. Short interfering RNA screening identified class II phosphatidylinositol 3-kinase C2β (PI3KC2β) as the predominant isoform of PI3K involved in lamellipodia formation of ovarian cancer cells. The bioactive sphingolipid ceramide has emerged as an antitumorigenic lipid, and treatment with short-chain C6-ceramide decreased the number of ovarian cancer cells with PI3KC2β-driven lamellipodia. Pharmacological analysis demonstrated that long-chain ceramide regenerated from C6-ceramide through the salvage/recycling pathway, at least in part, mediated the action of C6-ceramide. Mechanistically, ceramide was revealed to interact with the PIK-catalytic domain of PI3KC2β and affect its compartmentalization, thereby suppressing PI3KC2β activation and its driven cell motility. Ceramide treatment also suppressed cell motility promoted by epithelial growth factor, which is a prometastatic factor. To examine the role of ceramide in ovarian cancer metastasis, ceramide liposomes were employed and confirmed to suppress cell motility in vitro. Ceramide liposomes had an inhibitory effect on peritoneal metastasis in a murine xenograft model of human ovarian cancer. Metastasis of PI3KC2β knocked-down cells was insensitive to treatment with ceramide liposomes, suggesting specific involvement of ceramide interaction with PI3KC2β in metastasis suppression. Our study identified ceramide as a bioactive lipid that limits PI3KC2β-governed cell motility, and ceramide is proposed to serve as a metastasis-suppressor lipid in ovarian cancer. These findings could be translated into developing ceramide-based therapy for metastatic diseases.

Modern fluorescent techniques to investigate the mechanisms of lymphocyte activation

Fluorescent proteins are promising toolsfor studying intracellular signaling processes in lymphocytes. This brief review summarizes fluorescence-based imaging techniques developed in recent years and discusses new methodological advances, such as fluorescent photoswitches, fluorescence recovery after photobleaching (FRAP), fluorescent resonance energy transfer (FRET), fluorescence lifetime imaging microscopy (FLIM), photoactivated localization microscopy (PALM), stochastic optical reconstruction microscopy (STORM), stimulated emission depletion (STED), total internal reflection fluorescence (TIRF) and other techiques. This survey also highlights recent advances in vitro imaging of live tissues, novel applications of flow cytometry with genetically modifed fluorescent proteins, and future prospects for the development of new immunological test systems based on fluorescent protein technology.

Akt signaling dynamics in individual cells

The protein kinase Akt (for which there are three isoforms) is a key intracellular mediator of many biological processes, yet knowledge of Akt signaling dynamics is limited. Here, we have constructed a fluorescent reporter molecule in a lentiviral delivery system to assess Akt kinase activity at the single cell level. The reporter, a fusion between a modified FoxO1 transcription factor and clover, a green fluorescent protein, rapidly translocates from the nucleus to the cytoplasm in response to Akt stimulation. Because of its long half-life and the intensity of clover fluorescence, the sensor provides a robust readout that can be tracked for days under a range of biological conditions. Using this reporter, we find that stimulation of Akt activity by IGF-I is encoded into stable and reproducible analog responses at the population level, but that single cell signaling outcomes are variable. This reporter, which provides a simple and dynamic measure of Akt activity, should be compatible with many cell types and experimental platforms, and thus opens the door to new insights into how Akt regulates its biological responses.

Fluorescent protein-based biosensors to visualize signal transduction beneath the plasma membrane

In response to extracellular stimuli, cells display a variety of behaviors, including proliferation, differentiation, morphological changes and migration. The analysis of the spatiotemporal regulation of signal transduction in living cells is needed for a better understanding of such behaviors, and such investigations have been greatly accelerated by the development of fluorescent protein-based biosensors. Currently, by using these biosensors a range of molecular actions, including lipid metabolism, protein activation, and ion dynamics, can be visualized in living cells. We recently reported that intracellular calcium, with its relevant downstream signaling pathways consisting of the small GTPase Ras and the lipid kinase phoshoinositide-3-kinase (PI3K), can be exploited in an efficient incorporation of influenza A viruses into host cells via endocytosis using a set of biosensors based on fluorescent proteins and the principle of Förster resonance energy transfer. Here, we focus this review on fluorescent protein-based biosensors that have been utilized in our recent research reports.

A method to control phosphoinositides and to analyze PTEN function in living cells using voltage sensitive phosphatases

Voltage sensitive phosphatases (VSPs), including engineered voltage sensitive PTEN, are excellent tools to rapidly and reversibly alter the phosphoinositide (PI) content of the plasma membrane in vivo and study the tumor suppressor PTEN. However, widespread adoption of these tools is hampered by the requirement for electrophysiological instrumentation to control the activity of VSPs. Additionally, monitoring and quantifying the PI changes in living cells requires sophisticated microscopy equipment and image analysis. Here we present methods that bypass these obstacles. First, we explore technically simple means for activation of VSPs via extracellularly applied agents or light. Secondly, we characterize methods to monitor PI(4,5)P₂ and PI(3,4,5)P₃ levels using fluorescence microscopy or photometry in conjunction with translocation or FRET based PI probes, respectively. We then demonstrate the application of these techniques by characterizing the effect of known PTEN mutations on its enzymatic activity, analyzing the effect of PTEN inhibitors, and detecting in real time rapid inhibition of protein kinase B following depletion of PI(3,4,5)P₃. Thus, we established an approach that does not only allow for rapidly manipulating and monitoring PI(4,5)P₂ and PI(3,4,5)P₃ levels in a population of cells, but also facilitates the study of PTEN mutants and pharmacological targeting in mammalian cells.

Polyphosphoinositide binding domains: Key to inositol lipid biology

Polyphosphoinositides (PPIn) are an important family of phospholipids located on the cytoplasmic leaflet of eukaryotic cell membranes. Collectively, they are critical for the regulation of many aspects of membrane homeostasis and signaling, with notable relevance to human physiology and disease. This regulation is achieved through the selective interaction of these lipids with hundreds of cellular proteins, and thus the capability to study these localized interactions is crucial to understanding their functions. In this review, we discuss current knowledge of the principle types of PPIn-protein interactions, focusing on specific lipid-binding domains. We then discuss how these domains have been re-tasked by biologists as molecular probes for these lipids in living cells. Finally, we describe how the knowledge gained with these probes, when combined with other techniques, has led to the current view of the lipids' localization and function in eukaryotes, focusing mainly on animal cells. This article is part of a Special Issue entitled Phosphoinositides.

Developments in preclinical cancer imaging: innovating the discovery of therapeutics

Integrating biological imaging into early stages of the drug discovery process can provide invaluable readouts of drug activity within complex disease settings, such as cancer. Iterating this approach from initial lead compound identification in vitro to proof-of-principle in vivo analysis represents a key challenge in the drug discovery field. By embracing more complex and informative models in drug discovery, imaging can improve the fidelity and statistical robustness of preclinical cancer studies. In this Review, we highlight how combining advanced imaging with three-dimensional systems and intravital mouse models can provide more informative and disease-relevant platforms for cancer drug discovery.

Genetically encoded fluorescent biosensors for live-cell visualization of protein phosphorylation

Fluorescence-based, genetically encodable biosensors are widely used tools for real-time analysis of biological processes. Over the last few decades, the number of available genetically encodable biosensors and the types of processes they can monitor have increased rapidly. Here, we aim to introduce the reader to general principles and practices in biosensor development and highlight ways in which biosensors can be used to illuminate outstanding questions of biological function. Specifically, we focus on sensors developed for monitoring kinase activity and use them to illustrate some common considerations for biosensor design. We describe several uses to which kinase and second-messenger biosensors have been put, and conclude with considerations for the use of biosensors once they are developed. Overall, as fluorescence-based biosensors continue to diversify and improve, we expect them to continue to be widely used as reliable and fruitful tools for gaining deeper insights into cellular and organismal function.

Prostate specific membrane antigen knockdown impairs the tumorigenicity of LNCaP prostate cancer cells by inhibiting the phosphatidylinositol 3-kinase/Akt signaling pathway

BACKGROUND

Prostate specific membrane antigen (PSMA) can facilitate the growth, migration, and invasion of the LNCaP prostate cancer cell lines, but the underlying molecular mechanisms have not yet been clearly defined. Here, we investigated whether PSMA serves as a novel regulator of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling by employing PSMA knockdown model and PI3K pharmacological inhibitor (LY294002) in LNCaP prostate cancer cells.

METHODS

PSMA knockdown had been stably established by transfecting with lentivirus-mediated siRNA in our previous study. Then, LNCaP cells were divided into interference, non-interference, and blank groups. We first testified the efficacy of PSMA knockdown in our LNCaP cell line. Then, we compared the expression of PSMA and total/activated Akt by Western blotting in the above three groups with or without LY294002 treatment. Furthermore, immunocytochemistry was performed to confirm the changes of activated Akt (p-Akt, Ser473) in groups. Besides, cell proliferation, migration, and cell cycle were measured by CCK-8 assay, Transwell analysis, and Flow cytometry respectively.

RESULTS

After PSMA knockdown, the level of p-Akt (Ser473) but not of total-Akt (Akt1/2) was significantly decreased when compared with the non-interference and blank groups. However, LY294002 administration significantly reduced the expression of p-Akt (Ser473) in all the three groups. The results of immunocytochemistry further confirmed that PSMA knockdown or LY294002 treatment was associated with p-Akt (Ser473) down-regulation. Decrease of cell proliferation, migration, and survival were also observed upon PSMA knockdown and LY294002 treatment.

CONCLUSIONS

Taken together, our results reveal that PI3K/Akt signaling pathway inhibition may serve as a novel molecular mechanism in LNCaP prostate cancer cells of PSMA knockdown and suggest that Akt (Ser473) may play a critical role as a downstream signaling target effector of PSMA in this cellular model.

Development of a FRET biosensor with high specificity for Akt

The serine/threonine kinase Akt plays a critical role in cell proliferation, survival, and tumorigenesis. As a central kinase in the phosphatidylinositol 3-kinase pathway, its activation mechanism at the plasma membrane has been well characterized. However, the subcellular Akt activity in living cells is still largely unknown. Fluorescence resonance energy transfer (FRET)-based biosensors have emerged as indispensable tools to visualize the subcellular activities of signaling molecules. In this study, we developed a highly specific FRET biosensor for Akt based on the Eevee backbone, called Eevee-iAkt. Using inhibitors targeting kinases upstream and downstream of Akt, we showed that Eevee-iAkt specifically monitors Akt activity in living cells. To visualize Akt activity at different subcellular compartments, we targeted Eevee-iAkt to raft and non-raft regions of the plasma membrane, mitochondria, and nucleus in HeLa and Cos7 cells. Interestingly, we revealed substantial differences in Akt activation between HeLa and Cos7 cells upon epidermal growth factor (EGF) stimulation: Akt was transiently activated in HeLa cells with comparable levels at the plasma membrane, cytosol, and mitochondria. In contrast, sustained and spatially localized Akt activation was observed in EGF-stimulated Cos7 cells. We found high Akt activity at the plasma membrane, low activity in the cytosol, and no detectable activity at the mitochondria and nucleus in Cos7 cells. The Eevee-iAkt biosensor was shown to be a valuable tool to study the functional relationship between subcellular Akt activation and its anti-apoptotic role in living cells.

RCP-driven α5β1 recycling suppresses Rac and promotes RhoA activity via the RacGAP1-IQGAP1 complex

Inhibition of αvβ3 or expression of mutant p53 promotes invasion into fibronectin (FN)-containing extracellular matrix (ECM) by enhancing Rab-coupling protein (RCP)-dependent recycling of α5β1 integrin. RCP and α5β1 cooperatively recruit receptor tyrosine kinases, including EGFR1, to regulate their trafficking and downstream signaling via protein kinase B (PKB)/Akt, which, in turn, promotes invasive migration. In this paper, we identify a novel PKB/Akt substrate, RacGAP1, which is phosphorylated as a consequence of RCP-dependent α5β1 trafficking. Phosphorylation of RacGAP1 promotes its recruitment to IQGAP1 at the tips of invasive pseudopods, and RacGAP1 then locally suppresses the activity of the cytoskeletal regulator Rac and promotes the activity of RhoA in this subcellular region. This Rac to RhoA switch promotes the extension of pseudopodial processes and invasive migration into FN-containing matrices, in a RhoA-dependent manner. Thus, the localized endocytic trafficking of α5β1 within the tips of invasive pseudopods elicits signals that promote the reorganization of the actin cytoskeleton, protrusion, and invasion into FN-rich ECM.

Dynamics of Akt activation during mouse embryo development: distinct subcellular patterns distinguish proliferating versus differentiating cells

Akt is a highly conserved serine-threonine protein kinase which has been implicated in a wide variety of cellular functions, from the regulation of growth and metabolism, to activation of pro-survival pathways and cell proliferation, and promotion of differentiation in specific cell types. However, very little is known about the spatial and temporal pattern of Akt activity within cells and whether this pattern changes as cells enter and proceed in their differentiation programs. To address this issue we profiled Akt activation in E8.5-E13.5 mouse embryos and in C2C12 cells. We used a commercial antibody against Akt, phosphorylated on one of its activating residues, Thr-308, and performed high resolution confocal imaging of the immunofluorescence in labeled embryos. We observe strong Akt activity during mitosis in the dermomyotome, the neuroepithelium and some mesenchymal cells. This burst of activity fills the whole cell except for heterochromatin-positive areas in the nucleus. A surge in activity during mitosis is also observed in subconfluent C2C12 cells. Later on in the differentiation programs of skeletal muscle and neural cells, derivatives of the dermomyotome and neuroepithelium, respectively, we find robust, sustained Akt activity in the cytoplasm, but not in the nucleus. Concomitantly with skeletal muscle differentiation, Akt activity becomes concentrated in the sarcomeric Z-disks whereas developing neurons maintain a uniform cytoplasmic pattern of activated Akt. Our findings reveal unprecedented cellular and subcellular details of Akt activity during mouse embryo development, which is spatially and temporally consistent with proposed functions for Akt in mitosis and myogenic and neural differentiation and/or survival. Our results thus demonstrate a subcellular change in the pattern of Akt activation when skeletal muscle and neural progenitor cells cease dividing and progress in their differentiation programs.

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.

Fluorescence resonance energy transfer microscopy as demonstrated by measuring the activation of the serine/threonine kinase Akt

This protocol describes procedures for performing fluorescence resonance energy transfer (FRET) microscopy analysis by three different methods: acceptor photobleaching, sensitized emission and spectral imaging. We also discuss anisotropy and fluorescence lifetime imaging microscopy-based FRET techniques. By using the specific example of the FRET probe Akind (Akt indicator), which is a version of Akt modified such that FRET occurs when the probe is activated by phosphorylation, indicating Akt activation. The protocol provides a detailed step-by-step description of sample preparation, image acquisition and analysis, including control samples, image corrections and the generation of quantitative FRET/CFP ratio images for both sensitized emission and spectral imaging. The sample preparation takes 2 d, equipment setup takes 2-3 h and image acquisition and analysis take 6-8 h.

Chimaerin suppresses Rac1 activation at the apical membrane to maintain the cyst structure

Epithelial organs are made of a well-polarized monolayer of epithelial cells, and their morphology is maintained strictly for their proper functions. Previously, we showed that Rac1 activation is suppressed at the apical membrane in the mature organoid, and that such spatially biased Rac1 activity is required for the polarity maintenance. Here we identify Chimaerin, a GTPase activating protein for Rac1, as a suppressor of Rac1 activity at the apical membrane. Depletion of Chimaerin causes over-activation of Rac1 at the apical membrane in the presence of hepatocyte growth factor (HGF), followed by luminal cell accumulation. Importantly, Chimaerin depletion did not inhibit extension formation at the basal membrane. These observations suggest that Chimaerin functions as the apical-specific Rac1 GAP to maintain epithelial morphology.

A phosphatidylinositol lipids system, lamellipodin, and Ena/VASP regulate dynamic morphology of multipolar migrating cells in the developing cerebral cortex

In the developing mammalian cerebral cortex, excitatory neurons are generated in the ventricular zone (VZ) and subventricular zone; these neurons migrate toward the pial surface. The neurons generated in the VZ assume a multipolar morphology and remain in a narrow region called the multipolar cell accumulation zone (MAZ) for ∼24 h, in which they extend and retract multiple processes dynamically. They eventually extend an axon tangentially and begin radial migration using a migratory mode called locomotion. Despite the potential biological importance of the process movement of multipolar cells, the molecular mechanisms remain to be elucidated. Here, we observed that the processes of mouse multipolar cells were actin rich and morphologically resembled the filopodia and lamellipodia in growth cones; thus, we focused on the actin-remodeling proteins Lamellipodin (Lpd) and Ena/vasodilator-stimulated phosphoprotein (VASP). Lpd binds to phosphatidylinositol (3,4)-bisphosphate [PI(3,4)P₂] and recruits Ena/VASP, which promotes the assembly of actin filaments, to the plasma membranes. In situ hybridization and immunohistochemistry revealed that Lpd is expressed in multipolar cells in the MAZ. The functional silencing of either Lpd or Ena/VASP decreased the number of primary processes. Immunostaining and a Förster resonance energy transfer analysis revealed the subcellular localization of PI(3,4)P₂ at the tips of the processes. A knockdown experiment and treatment with an inhibitor for Src homology 2-containing inositol phosphatase-2, a 5-phosphatase that produces PI(3,4)P₂ from phosphatidylinositol (3,4,5)-triphosphate, decreased the number of primary processes. Our observations suggest that PI(3,4)P₂, Lpd, and Ena/VASP are involved in the process movement of multipolar migrating cells.

The high and low molecular weight forms of hyaluronan have distinct effects on CD44 clustering

CD44 is a major cell surface receptor for the glycosaminoglycan hyaluronan (HA). Native high molecular weight hyaluronan (nHA) and oligosaccharides of hyaluronan (oHA) provoke distinct biological effects upon binding to CD44. Despite the importance of such interactions, however, the feature of binding with CD44 at the cell surface and the molecular basis for functional distinction between different sizes of HA is still unclear. In this study we investigated the effects of high and low molecular weight hyaluronan on CD44 clustering. For the first time, we provided direct evidence for a strong relationship between HA size and CD44 clustering in vivo. In CD44-transfected COS-7 cells, we showed that exogenous nHA stimulated CD44 clustering, which was disrupted by oHA. Moreover, naturally expressed CD44 was distributed into clusters due to abundantly expressed nHA in HK-2 cells (human renal proximal tubule cells) and BT549 cells (human breast cancer cell line) without exogenous stimulation. Our results suggest that native HA binding to CD44 selectively induces CD44 clustering, which could be inhibited by oHA. Finally, we demonstrated that HA regulates cell adhesion in a manner specifically dependent on its size. oHA promoted cell adhesion while nHA showed no effects. Our results might elucidate a molecular- and/or cellular-based mechanism for the diverse biological activities of nHA and oHA.

Activity of PLCε contributes to chemotaxis of fibroblasts towards PDGF

Cell chemotaxis, such as migration of fibroblasts towards growth factors during development and wound healing, requires precise spatial coordination of signalling events. Phosphoinositides and signalling enzymes involved in their generation and hydrolysis have been implicated in regulation of chemotaxis; however, the role and importance of specific components remain poorly understood. Here, we demonstrate that phospholipase C epsilon (PLCε) contributes to fibroblast chemotaxis towards platelet-derived growth factor (PDGF-BB). Using PLCe1 null fibroblasts we show that cells deficient in PLCε have greatly reduced directionality towards PDGF-BB without detrimental effect on their basal ability to migrate. Furthermore, we show that in intact fibroblasts, signalling events, such as activation of Rac, are spatially compromised by the absence of PLCε that affects the ability of cells to enlarge their protrusions in the direction of the chemoattractant. By further application of live cell imaging and the use of FRET-based biosensors, we show that generation of Ins(1,4,5)P(3) and recruitment of PLCε are most pronounced in protrusions responding to the PDGF-BB gradient. Furthermore, the phospholipase C activity of PLCε is critical for its role in chemotaxis, consistent with the importance of Ins(1,4,5)P(3) generation and sustained calcium responses in this process. As PLCε has extensive signalling connectivity, using transgenic fibroblasts we ruled out its activation by direct binding to Ras or Rap GTPases, and suggest instead new unexpected links for PLCε in the context of chemotaxis.

Angiotensin-II and MARCKS: a hydrogen peroxide- and RAC1-dependent signaling pathway in vascular endothelium

MARCKS is an actin-binding protein that modulates vascular endothelial cell migration and cytoskeleton signaling (Kalwa, H., and Michel, T. (2011) J. Biol. Chem. 286, 2320-2330). Angiotensin-II is a vasoactive peptide implicated in vascular physiology as well as pathophysiology; the pathways connecting angiotensin-II and cytoskeletal remodeling are incompletely understood. Here we show that MARCKS is expressed in intact arterial preparations, with prominent staining of the endothelium. In endothelial cells, angiotensin-II-promoted MARCKS phosphorylation is abrogated by PEG-catalase, implicating endogenous H(2)O(2) in the angiotensin-II response. Studies using the H(2)O(2) biosensor HyPer2 reveal that angiotensin-II promotes increases in intracellular H(2)O(2). We used a Rac1 FRET biosensor to show that angiotensin-II promotes Rac1 activation that is attenuated by PEG-catalase. siRNA-mediated Rac1 knockdown blocks angiotensin-II-stimulated MARCKS phosphorylation. Cell imaging studies using a phosphoinositide 4,5-bisphosphate (PIP(2)) biosensor revealed that angiotensin-II PIP(2) regulation depends on MARCKS and H(2)O(2). siRNA-mediated knockdown of MARCKS or Rac1 attenuates receptor-mediated activation of the tyrosine kinase c-Abl and disrupts actin fiber formation. These studies establish a critical role for H(2)O(2) in angiotensin-II signaling to the endothelial cytoskeleton in a novel pathway that is critically dependent on MARCKS, Rac1, and c-Abl.

The exocyst complex in exocytosis and cell migration

Exocytosis is a fundamental membrane trafficking event in eukaryotic cells in which membrane proteins or lipids are incorporated into the plasma membrane and vesicle contents are secreted to the exterior of the cell. The exocyst, an evolutionarily conserved octameric protein complex, plays a crucial role in the targeting of secretory vesicles to the plasma membrane during exocytosis. The exocyst has been shown to be involved in diverse cellular processes requiring polarized exocytosis such as yeast budding, epithelial polarity establishment, and neurite outgrowth. Recently, the exocyst has also been implicated in cell migration through mechanisms independent of its role in exocytosis. In this review, we will first summarize our knowledge on the exocyst complex at a molecular and structural level. Then, we will discuss the specific functions of the exocyst in exocytosis in various cell types. Finally, we will review the emerging roles of the exocyst during cell migration and tumor cell invasion.

The endosomal adaptor protein APPL1 impairs the turnover of leading edge adhesions to regulate cell migration

Cell migration is a complex process that requires the integration of signaling events that occur in distinct locations within the cell. Adaptor proteins, which can localize to different subcellular compartments, where they bring together key signaling proteins, are emerging as attractive candidates for controlling spatially coordinated processes. However, their function in regulating cell migration is not well understood. In this study, we demonstrate a novel role for the adaptor protein containing a pleckstrin-homology (PH) domain, phosphotyrosine-binding (PTB) domain, and leucine zipper motif 1 (APPL1) in regulating cell migration. APPL1 impairs migration by hindering the turnover of adhesions at the leading edge of cells. The mechanism by which APPL1 regulates migration and adhesion dynamics is by inhibiting the activity of the serine/threonine kinase Akt at the cell edge and within adhesions. In addition, APPL1 significantly decreases the tyrosine phosphorylation of Akt by the nonreceptor tyrosine kinase Src, which is critical for Akt-mediated cell migration. Thus, our results demonstrate an important new function for APPL1 in regulating cell migration and adhesion turnover through a mechanism that depends on Src and Akt. Moreover, our data further underscore the importance of adaptor proteins in modulating the flow of information through signaling pathways.

In the newborn hippocampus, neurotrophin-dependent survival requires spontaneous activity and integrin signaling

The nervous system develops through a program that first produces neurons in excess and then eliminates as many as half in a specific period of early postnatal life. Neurotrophins are widely thought to regulate neuronal survival, but this role has not been clearly defined in the CNS. Here we show that neurotrophins promote survival of young neurons by promoting spontaneous activity. Survival of hippocampal neurons in neonatal rat requires spontaneous activity that depends on the excitatory action of GABA. Neurotrophins facilitate recruitment of cultured neurons into active networks, and it is this activity, combined with integrin receptor signaling, that controls neuronal survival. In vivo, neurotrophins require integrin signaling to control neuron number. These data are the first to link the early excitatory action of GABA to the developmental death period and to assign an essential role for activity in neurotrophin-mediated survival that establishes appropriate networks.

Emerging role of Akt substrate protein AS160 in the regulation of AQP2 translocation

AS160, a novel Akt substrate of 160 kDa, contains a Rab GTPase-activating protein (GAP) domain. The present study examined the role of Akt and AS160 in aquaporin-2 (AQP2) trafficking. The main strategy was to examine the changes in AQP2 translocation in response to small interfering RNA (siRNA)-mediated AS160 knockdown in mouse cortical collecting duct cells (M-1 cells and mpkCCDc14 cells). Short-term dDAVP treatment in M-1 cells stimulated phosphorylation of Akt (S473) and AS160, which was also seen in mpkCCDc14 cells. Conversely, the phosphoinositide 3-kinase (PI3K) inhibitor LY 294002 diminished phosphorylation of Akt (S473) and AS160. Moreover, siRNA-mediated Akt1 knockdown was associated with unchanged total AS160 but decreased phospho-AS160 expression, indicating that phosphorylation of AS160 is dependent on PI3K/Akt pathways. siRNA-mediated AS160 knockdown significantly decreased total AS160 and phospho-AS160 expression. Immunocytochemistry revealed that AS160 knockdown in mpkCCDc14 cells was associated with increased AQP2 density in the plasma membrane [135 ± 3% of control mpkCCDc14 cells ( n = 65), P < 0.05, n = 64] despite the absence of dDAVP stimulation. Moreover, cell surface biotinylation assays of mpkCCDc14 cells with AS160 knockdown exhibited significantly higher AQP2 expression [150 ± 15% of control mpkCCDc14 cells ( n = 3), P < 0.05, n = 3]. Taken together, PI3K/Akt pathways mediate the dDAVP-induced AS160 phosphorylation, and AS160 knockdown is associated with higher AQP2 expression in the plasma membrane. Since AS160 contains a GAP domain leading to a decrease in the active GTP-bound form of AS160 target Rab proteins for vesicle trafficking, decreased expression of AS160 is likely to play a role in the translocation of AQP2 to the plasma membrane.

PDK1 recruitment to the SHPS-1 signaling complex enhances insulin-like growth factor-i-stimulated AKT activation and vascular smooth muscle cell survival

In vascular smooth muscle cells, exposed to hyperglycemia and insulin-like growth factor-I (IGF-I), SHPS-1 functions as a scaffold protein, and a signaling complex is assembled that leads to AKT activation. However, the underlying mechanism by which formation of this complex activates the kinase that phosphorylates AKT (Thr(308)) is unknown. Therefore, we investigated the mechanism of PDK1 recruitment to the SHPS-1 signaling complex and the consequences of disrupting PDK1 recruitment for downstream signaling. Our results show that following IGF-I stimulation, PDK1 is recruited to SHPS-1, and its recruitment is mediated by Grb2, which associates with SHPS-1 via its interaction with Pyk2, a component of the SHPS-1-associated complex. A proline-rich sequence in PDK1 bound to an Src homology 3 domain in Grb2 in response to IGF-I. Disruption of Grb2-PDK1 by expression of either a Grb2 Src homology 3 domain or a PDK1 proline to alanine mutant inhibited PDK1 recruitment to SHPS-1, leading to impaired IGF-I-stimulated AKT Thr(308) phosphorylation. Following its recruitment to SHPS-1, PDK1 was further activated via Tyr(373/376) phosphorylation, and this was required for a maximal increase in PDK1 kinase activity and AKT-mediated FOXO3a Thr(32) phosphorylation. PDK1 recruitment was also required for IGF-I to prevent apoptosis that occurred in response to hyperglycemia. Assembly of the Grb2-PDK1 complex on SHPS-1 was specific for IGF-I signaling because inhibiting PDK1 recruitment to SHPS-1 had no effect on EGF-stimulated AKT Thr(308) phosphorylation. These findings reveal a novel mechanism for recruitment of PDK1 to the SHPS-1 signaling complex, which is required for IGF-I-stimulated AKT Thr(308) phosphorylation and inhibition of apoptosis.

Heterogeneity of phosphatidic acid levels and distribution at the plasma membrane in living cells as visualized by a Föster resonance energy transfer (FRET) biosensor

Phosphatidic acid (PA) is one of the major phospholipids in the plasma membrane. Although it has been reported that PA plays key roles in cell survival and morphology, it remains unknown when and where PA is produced in the living cell. Based on the principle of Förster resonance energy transfer (FRET), we generated PA biosensor, and named Pii (phosphatidic acid indicator). In these biosensors, the lipid-binding domain of DOCK2 is sandwiched with the cyan fluorescent protein and yellow fluorescent protein and is tagged with the plasma membrane-targeting sequence of K-Ras. The addition of synthetic PA, or the activation of phospholipase D or diacylglycerol kinase at the plasma membrane, changed the level of FRET in Pii-expressing cells, demonstrating the response of Pii to PA. The biosensor also detected divergent PA content among various cell lines as well as within one cell line. Interestingly, the growth factor-induced increment in PA content correlated negatively with the basal PA content before stimulation, suggesting the presence of an upper threshold in the PA concentration at the plasma membrane. The biosensor also revealed uneven PA distribution within the cell, i.e. the basal level and growth factor-induced accumulation of PA was higher at the cell-free edges than at the cell-cell contact region. An insufficient increase in PA may account for ineffective Ras activation at areas of cell-cell contact. In conclusion, the PA biosensor Pii is a versatile tool for examining heterogeneity in the content and distribution of PA in single cells as well as among different cells.

Aberrant overexpression of the Rgl2 Ral small GTPase-specific guanine nucleotide exchange factor promotes pancreatic cancer growth through Ral-dependent and Ral-independent mechanisms

Our recent studies established essential and distinct roles for RalA and RalB small GTPase activation in K-Ras mutant pancreatic ductal adenocarcinoma (PDAC) cell line tumorigencity, invasion, and metastasis. However, the mechanism of Ral GTPase activation in PDAC has not been determined. There are four highly related mammalian RalGEFs (RalGDS, Rgl1, Rgl2, and Rgl3) that can serve as Ras effectors. Whether or not they share distinct or overlapping functions in K-Ras-mediated growth transformation has not been explored. We found that plasma membrane targeting to mimic persistent Ras activation enhanced the growth-transforming activities of RalGEFs. Unexpectedly, transforming activity did not correlate directly with total cell steady-state levels of Ral activation. Next, we observed elevated Rgl2 expression in PDAC tumor tissue and cell lines. Expression of dominant negative Ral, which blocks RalGEF function, as well as interfering RNA suppression of Rgl2, reduced PDAC cell line steady-state Ral activity, growth in soft agar, and Matrigel invasion. Surprisingly, the effect of Rgl2 on anchorage-independent growth could not be rescued by constitutively activated RalA, suggesting a novel Ral-independent function for Rgl2 in transformation. Finally, we determined that Rgl2 and RalB both localized to the leading edge, and this localization of RalB was dependent on endogenous Rgl2 expression. In summary, our observations support nonredundant roles for RalGEFs in Ras-mediated oncogenesis and a key role for Rgl2 in Ral activation and Ral-independent PDAC growth.

Salmonella-directed recruitment of new membrane to invasion foci via the host exocyst complex

Salmonella attachment to the intestinal epithelium triggers delivery of bacterial effector proteins into the host cytosol through a type III secretion system (T3SS), leading to pronounced membrane ruffling and macropinocytic uptake of attached bacteria. The tip of the T3SS is made up of two proteins, SipB and SipC, which insert into the host plasma membrane, forming a translocation pore. Both the N and C termini of SipC are exposed in the host cytosol and have been shown to directly modulate actin cytoskeleton assembly. We have identified a direct interaction between SipC and Exo70, a component of the exocyst complex, which mediates docking and fusion of exocytic vesicles with the plasma membrane. Here, we show that exocyst components coprecipitate with SipC and accumulate at sites of invasion by Salmonella typhimurium. Exocyst assembly requires activation of the small GTPase RalA, which we show is triggered during Salmonella infection by the translocated effector, SopE. Knockdown of RalA or Sec5 results in reduced membrane ruffling at sites of attachment and impairs bacterial entry into host cells. These findings suggest that S. typhimurium enhances invasion efficiency by promoting localized membrane expansion, directly through SipC-dependent recruitment of the exocyst and indirectly via SopE-dependent activation of RalA.

Visualisation of signalling in immune cells

Currently, a great number of approaches are employed in investigation of the immune system. These range from experiments in live animals and biochemical techniques to investigate whole organs or cell populations down to single cell and molecular techniques to look at dynamics in specific cell-cell interactions. It is the latter approach that this chapter focusses on. The use of Förster resonance energy transfer (FRET) techniques to probe protein-protein interactions that are involved in receptor signalling to the cytoskeleton in intact cells is now well established. Various FRET biosensors are available to visualise several critical cell processes, giving information about activity and the location of key signalling molecules. As a specific set of examples in this chapter, we have generated variants of the original Rho, Rac and Cdc42 "Raichu" probes and improved their fluorophore combination to make them suitable for FLIM. These were employed in a number of assays to determine signal dynamics in T and NK cells. Specific protocols of how to use these probes and technical notes are described.

Dynamic visualization of cellular signaling

Our understanding of cellular signaling is critically dependent on our ability to visualize and quantify specific signaling events with high spatial and temporal resolution in the cellular context. Over the past decade or so, biosensors based on fluorescent proteins and fluorescence resonance energy transfer (FRET) have emerged as one major class of fluorescent probes that are capable of tracking a variety of cellular signaling events, such as second messenger dynamics and enzyme activation/activity, in time and space. Here we review recent advances in the development of such biosensors and some biological insights revealed by these biosensors in living cells, tissue, and organisms.

Visualization of growth signal transduction cascades in living cells with genetically encoded probes based on Förster resonance energy transfer

Fluorescence probes based on the principle of Förster resonance energy transfer (FRET) have shed new light on our understanding of signal transduction cascades. Among them, unimolecular FRET probes containing fluorescence proteins are rapidly increasing in number because these genetically encoded probes can be easily loaded into living cells and allow simple acquisition of FRET images. We have developed probes for small GTPases, tyrosine kinases, serine-threonine kinases and phosphoinositides. Images obtained with these probes have revealed that membrane protrusions such as nascent lamellipodia or neurites provide an active signalling platform in the growth factor-stimulated cells.

Rapid turnover rate of phosphoinositides at the front of migrating MDCK cells

Phosphoinositides (PtdInss) play key roles in cell polarization and motility. With a series of biosensors based on Förster resonance energy transfer, we examined the distribution and metabolism of PtdInss and diacylglycerol (DAG) in stochastically migrating Madin-Darby canine kidney (MDCK) cells. The concentrations of phosphatidylinositol (4,5)-bisphosphate, phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), phosphatidylinositol (3,4)-bisphosphate, and DAG were higher at the plasma membrane in the front of the cell than at the plasma membrane of the rear of the cell. The difference in the concentrations of PtdInss was estimated to be less than twofold between the front and rear of the migrating MDCK cells. To decode the spatial activities of PtdIns metabolic enzymes from the obtained concentration maps of PtdInss, we developed a one-dimensional reaction diffusion model of PtdIns metabolism. In this model, the activities of phosphatidylinositol monophosphate 5-kinase, phosphatidylinositol 3-kinase, phospholipase C, and PIP(3) 5-phosphatases were higher at the plasma membrane of the front than at the plasma membrane of the rear of the cell. This result suggests that, although the difference in the steady-state level of PtdInss is less than twofold, PtdInss were more rapidly turned over at the front than the rear of the migrating MDCK cells.

Imaging of Rab5 activity identifies essential regulators for phagosome maturation

Efficient phagocytosis of apoptotic cells is crucial for tissue homeostasis and the immune response. Rab5 is known as a key regulator of the early endocytic pathway and we have recently shown that Rab5 is also implicated in apoptotic cell engulfment; however, the precise spatio-temporal dynamics of Rab5 activity remain unknown. Here, using a newly developed fluorescence resonance energy transfer biosensor, we describe a change in Rab5 activity during the engulfment of apoptotic thymocytes. Rab5 activity on phagosome membranes began to increase on disassembly of the actin coat encapsulating phagosomes. Rab5 activation was either continuous or repetitive for up to 10 min, but it ended before the collapse of engulfed apoptotic cells. Expression of a dominant-negative mutant of Rab5 delayed this collapse of apoptotic thymocytes, showing a role for Rab5 in phagosome maturation. Disruption of microtubules with nocodazole inhibited Rab5 activation on the phagosome membrane without perturbing the engulfment of apoptotic cells. Furthermore, we found that Gapex-5 is the guanine nucleotide exchange factor essential for Rab5 activation during the engulfment of apoptotic cells. Gapex-5 was bound to a microtubule-tip-associating protein, EB1, whose depletion inhibited Rab5 activation during phagocytosis. We therefore propose a mechanistic model in which the recruitment of Gapex-5 to phagosomes through the microtubule network induces the transient Rab5 activation.