Studies of signal transduction events using chimeras to green fluorescent protein

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

Protein kinase C: the "masters" of calcium and lipid

The coordinated and physiological behavior of living cells in an organism critically depends on their ability to interact with surrounding cells and with the extracellular space. For this, cells have to interpret incoming stimuli, correctly process the signals, and produce meaningful responses. A major part of such signaling mechanisms is the translation of incoming stimuli into intracellularly understandable signals, usually represented by second messengers or second-messenger systems. Two key second messengers, namely the calcium ion and signaling lipids, albeit extremely different in nature, play an important and often synergistic role in such signaling cascades. In this report, we will shed some light on an entire family of protein kinases, the protein kinases C, that are perfectly designed to exactly decode these two second messengers in all of their properties and convey the signaling content to downstream processes within the cell.

Membrane-surface anchoring of charged diacylglycerol-lactones correlates with biological activities

Synthetic diacylglycerol-lactones (DAG-lactones) are effective modulators of critical cellular signaling pathways, downstream of the lipophilic second messenger diacylglycerol, that activate a host of protein kinase C (PKC) isozymes and other nonkinase proteins that share similar C1 membrane-targeting domains with PKC. A fundamental determinant of the biological activity of these amphiphilic molecules is the nature of their interactions with cellular membranes. This study examines the biological properties of charged DAG-lactones exhibiting different alkyl groups attached to the heterocyclic nitrogen of an α-pyridylalkylidene chain, and particularly the relationship between membrane interactions of the substituted DAG-lactones and their respective biological activities. Our results suggest that bilayer interface localization of the N-alkyl chain in the R(2) position of the DAG-lactones inhibits translocation of PKC isoenzymes onto the cellular membrane. However, the orientation of a branched alkyl chain at the bilayer surface facilitates PKC binding and translocation. This investigation emphasizes that bilayer localization of the aromatic side residues of positively charged DAG-lactone derivatives play a central role in determining biological activity, and that this factor contributes to the diversity of biological actions of these synthetic biomimetic ligands.

Monitoring changes in membrane phosphatidylinositol 4,5-bisphosphate in living cells using a domain from the transcription factor tubby

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) is a key component in signal transduction, being a precursor to other signalling molecules and itself associated with roles in signal transduction and cell biology. Tubby is a membrane bound transcription factor whose dysfunction results in obesity in mice. It contains a domain that selectively binds PtdIns(4,5)P(2). We have investigated the use of a fluorescently tagged version of this domain to monitor changes in PtdIns(4,5)P(2) concentration in living cells and compared it to the pleckstrin homology domain of PLCdelta1. Our results show that selected mutants of this domain report receptor-mediated changes in cellular PtdIns(4,5)P(2). In contrast to the pleckstrin homology domain of PLCdelta1 it does not have a significant affinity for inositol 1,4,5-trisphosphate (IP(3)). Using a selected mutant, we examine the regulation of ATP-sensitive K(+) channels via a G(q/11)-coupled receptor. These experiments reveal a correlation between reporter translocation and the onset of current inhibition whilst the recovery of current after agonist removal is delayed when compared to the reporter. Furthermore our studies reveal the importance of Ca(2+) in determining the overall activity of phospholipase C in living cells. This probe may be valuable in examining changes in PtdIns(4,5)P(2) distinct from those of IP(3) in intact cells in a variety of physiological settings.

The PtdIns(4,5)P2 ligand itself influences the localization of PKCalpha in the plasma membrane of intact living cells

Rapamycin-triggered heterodimerization strategy is becoming an excellent tool for rapidly modifying phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P2] levels at the plasma membrane and for studying their influence in different processes. In this work, we studied the effect of modulation of the PtdIns(4,5)P2 concentration on protein kinase C (PKC) alpha membrane localization in intact living cells. We showed that an increase in the PtdIns(4,5)P2 concentration enlarges the permanence of PKCalpha in the plasma membrane when PC12 cells are stimulated with ATP, independently of the diacylglycerol generated. The depletion of this phosphoinositide decreases both the percentage of protein able to translocate to the plasma membrane and its permanence there. Our results demonstrate that the polybasic cluster located in the C2 domain of PKCalpha is responsible for this phosphoinositide-protein interaction. Furthermore, the C2 domain acts as a dominant interfering module in the neural differentiation process of PC12 cells, a fact that was also supported by the inhibitory effect obtained by knocking down PKCalpha with small interfering RNA duplexes. Taken together, these data demonstrate that PtdIns(4,5)P2 itself targets PKCalpha to the plasma membrane through the polybasic cluster located in the C2 domain, with this interaction being critical in the signaling network involved in neural differentiation.

The C2 domains of classical PKCs are specific PtdIns(4,5)P2-sensing domains with different affinities for membrane binding

C2 domains are conserved protein modules in many eukaryotic signaling proteins, including the protein kinase (PKCs). The C2 domains of classical PKCs bind to membranes in a Ca(2+)-dependent manner and thereby act as cellular Ca(2+) effectors. Recent findings suggest that the C2 domain of PKCalpha interacts specifically with phosphatidylinositols 4,5-bisphosphate (PtdIns(4,5)P(2)) through its lysine rich cluster, for which it shows higher affinity than for POPS. In this work, we compared the three C2 domains of classical PKCs. Isothermal titration calorimetry revealed that the C2 domains of PKCalpha and beta display a greater capacity to bind to PtdIns(4,5)P(2)-containing vesicles than the C2 domain of PKCgamma. Comparative studies using lipid vesicles containing both POPS and PtdIns(4,5)P(2) as ligands revealed that the domains behave as PtdIns(4,5)P(2)-binding modules rather than as POPS-binding modules, suggesting that the presence of the phosphoinositide in membranes increases the affinity of each domain. When the magnitude of PtdIns(4,5)P(2) binding was compared with that of other polyphosphate phosphatidylinositols, it was seen to be greater in both PKCbeta- and PKCgamma-C2 domains. The concentration of Ca(2+) required to bind to membranes was seen to be lower in the presence of PtdIns(4,5)P(2) for all C2 domains, especially PKCalpha. In vivo experiments using differentiated PC12 cells transfected with each C2 domain fused to ECFP and stimulated with ATP demonstrated that, at limiting intracellular concentration of Ca(2+), the three C2 domains translocate to the plasma membrane at very similar rates. However, the plasma membrane dissociation event differed in each case, PKCalpha persisting for the longest time in the plasma membrane, followed by PKCgamma and, finally, PKCbeta, which probably reflects the different levels of Ca(2+) needed by each domain and their different affinities for PtdIns(4,5)P(2).

Locating inositol 1,4,5-trisphosphate in the nucleus and neuronal dendrites with genetically encoded fluorescent indicators

Inositol 1,4,5-trisphosphate (InsP3) is a key second messenger in many cell types and also in distinct subcellular regions of single living cells; however, little is examined about the subcellular dynamics of InsP3 in a variety of cell types. We have developed fluorescent indicators to locate InsP3 dynamics in single living cells based on an intramolecular fluorescence resonance energy transfer. Our indicator has visualized InsP3 dynamics in the cytoplasm of cultured cells and even in single thin dendrites of hippocampal neurons, which has been unseen previously. We have further localized the present indicator in the nucleus and pinpointed nuclear InsP3 dynamics. The observation with our nuclear InsP3 indicator has solved a question on nuclear propagation of InsP3 from the cytoplasm and has drawn a conclusion that the nuclear InsP3 dynamics synchronously occurs with cytosolic InsP3 dynamics evoked by agonist stimulations. The present approach contributes to the understanding of when, where, and how InsP3 is generated and removed in a variety of living cells.

The C2 Domain of PKCα Is a Ca2+-dependent PtdIns(4,5)P2 Sensing Domain: A New Insight into an Old Pathway

The C2 domain is a targeting domain that responds to intracellular Ca2+ signals in classical protein kinases (PKCs) and mediates the translocation of its host protein to membranes. Recent studies have revealed a new motif in the C2 domain, named the lysine-rich cluster, that interacts with acidic phospholipids. The purpose of this work was to characterize the molecular mechanism by which PtdIns(4,5)P2 specifically interacts with this motif. Using a combination of isothermal titration calorimetry, fluorescence resonance energy transfer and time-lapse confocal microscopy, we show here that Ca2+ specifically binds to the Ca2+ -binding region, facilitating PtdIns(4,5)P2 access to the lysine-rich cluster. The magnitude of PtdIns(4,5)P2 binding is greater than in the case of other polyphosphate phosphatidylinositols. Very importantly, the residues involved in PtdIns(4,5)P2 binding are essential for the plasma membrane localization of PKCalpha when RBL-2H3 cells are stimulated through their IgE receptors. Additionally, CFP-PH and CFP-C1 domains were used as bioprobes to demonstrate the co-existence of PtdIns(4,5)P2 and diacylglycerol in the plasma membrane, and it was shown that although a fraction of PtdIns(4,5)P2 is hydrolyzed to generate diacylglycerol and IP3, an important amount still remains in the membrane where it is available to activate PKCalpha. These findings entail revision of the currently accepted model of PKCalpha recruitment to the membrane and its activation.

Specific and Stable Fluorescence Labeling of Histidine-Tagged Proteins for Dissecting Multi-Protein Complex Formation

Labeling of proteins with fluorescent dyes offers powerful means for monitoring protein interactions in vitro and in live cells. Only a few techniques for noncovalent fluorescence labeling with well-defined localization of the attached dye are currently available. Here, we present an efficient method for site-specific and stable noncovalent fluorescence labeling of histidine-tagged proteins. Different fluorophores were conjugated to a chemical recognition unit bearing three NTA moieties (tris-NTA). In contrast to the transient binding of conventional mono-NTA, the multivalent interaction of tris-NTA conjugated fluorophores with oligohistidine-tagged proteins resulted in complex lifetimes of more than an hour. The high selectivity of tris-NTA toward cumulated histidines enabled selective labeling of proteins in cell lysates and on the surface of live cells. Fluorescence labeling by tris-NTA conjugates was applied for the analysis of a ternary protein complex in solution and on surfaces. Formation of the complex and its stoichiometry was studied by analytical size exclusion chromatography and fluorescence quenching. The individual interactions were dissected on solid supports by using simultaneous mass-sensitive and multicolor fluorescence detection. Using these techniques, formation of a 1:1:1 stoichiometry by independent interactions of the receptor subunits with the ligand was shown. The incorporation of transition metal ions into the labeled proteins upon labeling with tris-NTA fluorophore conjugates provided an additional sensitive spectroscopic reporter for detecting and monitoring protein-protein interactions in real time. A broad application of these fluorescence conjugates for protein interaction analysis can be envisaged.

Role of the Lysine-rich Cluster of the C2 Domain in the Phosphatidylserine-dependent Activation of PKCα

The C2 domain of PKCalpha is a Ca(2+)-dependent membrane-targeting module involved in the plasma membrane localization of the enzyme. Recent findings have shown an additional area located in the beta3-beta4 strands, named the lysine-rich cluster, which has been demonstrated to be involved in the PtdIns(4,5)P(2)-dependent activation of the enzyme. Nevertheless, whether other anionic phospholipids can bind to this region and contribute to the regulation of the enzyme's function is not clear. To study other possible roles for this cluster, we generated double and triple mutants that substituted the lysine by alanine residues, and studied their binding and activation properties in a Ca(2+)/phosphatidylserine-dependent manner and compared them with the wild-type protein. It was found that some of the mutants exerted a constitutive activation independently of membrane binding. Furthermore, the constructs were fused to green fluorescent protein and were expressed in fibroblast cells. It was shown that none of the mutants was able to translocate to the plasma membrane, even in saturating conditions of Ca(2+) and diacylglycerol, suggesting that the interactions performed by this lysine-rich cluster are a key event in the subcellular localization of PKCalpha. Taken together, the results obtained showed that these lysine residues might be involved in two functions: one to establish an intramolecular interaction that keeps the enzyme in an inactive conformation; and the second, once the enzyme has been partially activated, to establish further interactions with diacylglycerol and/or acidic phospholipids, leading to the full activation of PKCalpha.

The simultaneous production of phosphatidic acid and diacylglycerol is essential for the translocation of protein kinase Cepsilon to the plasma membrane in RBL-2H3 cells

To evaluate the role of the C2 domain in protein kinase Cepsilon (PKCepsilon) localization and activation after stimulation of the IgE receptor in RBL-2H3 cells, we used a series of mutants located in the phospholipid binding region of the enzyme. The results obtained suggest that the interaction of the C2 domain with the phospholipids in the plasma membrane is essential for anchoring the enzyme in this cellular compartment. Furthermore, the use of specific inhibitors of the different pathways that generate both diacylglycerol and phosphatidic acid has shown that the phosphatidic acid generated via phospholipase D (PLD)-dependent pathway, in addition to the diacylglycerol generated via phosphoinosite-phospholipase C (PLC), are involved in the localization of PKCepsilon in the plasma membrane. Direct stimulation of RBL-2H3 cells with very low concentrations of permeable phosphatidic acid and diacylglycerol exerted a synergistic effect on the plasma membrane localization of PKCepsilon. Moreover, the in vitro kinase assays showed that both phosphatidic acid and diacylglycerol are essential for enzyme activation. Together, these results demonstrate that phosphatidic acid is an important and essential activator of PKCepsilon through the C2 domain and locate this isoenzyme in a new scenario where it acts as a downstream target of PLD.

High-content assays for ligand regulation of G-protein-coupled receptors

High-content assays rely on the imaging of cellular events. They can be used to monitor the activation of G-protein-coupled receptors (or other receptors), their internalization into the cell, or alterations in their amount. In addition, multiplexed assays can provide further information about the characteristics of the receptor. Recent improvements in throughput using high-content screening platforms means that such assays are now an integral element of functional analysis in the drug discovery process.

Role of the Ca2+/phosphatidylserine binding region of the C2 domain in the translocation of protein kinase Calpha to the plasma membrane

Signal transduction via protein kinase C (PKC) is closely regulated by its subcellular localization. To map the molecular determinants mediating the C2 domain-dependent translocation of PKCalpha to the plasma membrane, full-length native protein and several point mutants in the Ca(2+)/phosphatidylserine-binding site were tagged with green fluorescent protein and transiently expressed in rat basophilic leukemia cells (RBL-2H3). Substitution of several aspartate residues by asparagine completely abolished Ca(2+)-dependent membrane targeting of PKCalpha. Strikingly, these mutations enabled the mutant proteins to translocate in a diacylglycerol-dependent manner, suggesting that neutralization of charges in the Ca(2+) binding region enables the C1 domain to bind diacylglycerol. In addition, it was demonstrated that the protein residues involved in direct interactions with acidic phospholipids play differential and pivotal roles in the membrane targeting of the enzyme. These findings provide new information on how the C2 domain-dependent membrane targeting of PKCalpha occurs in the presence of physiological stimuli.

Sustained and dynamic inositol lipid metabolism inside and outside the immunological synapse

T cell activation is triggered by several hours of contact with peptide-major histocompatibility (MHC) complexes on the surface of antigen-presenting cells (APCs). The nature and location of the sustained signal transduction pathways required for T cell activation are unknown. We show here that the production of phosphatidylinositol(3,4,5)triphosphate (PIP3) was dynamically sustained for hours as T cells responded to antigen. In addition, sustained elevation of PIP3 was essential for T cell proliferation. There was PIP3 accumulation in the T cell-APC contact zone and at the antipodal pole of the cell. The immune synapse is thus not the sole site of sustained signal transduction in activated T cells.

Diheteroarylethenes as thermally stable photoswitchable acceptors in photochromic fluorescence resonance energy transfer (pcFRET)

We have employed diheteroarylethenes as acceptors for photochromic FRET (pcFRET), a technique introduced for the quantitative determination of fluorescence resonance energy transfer (FRET). In pcFRET, the fluorescent emission of the donor is modulated by cyclical transformations of a photochromic acceptor. Light induces a reversible change in the structure and, concomitantly, in the absorption properties of the acceptor. Only the closed forms of the selected diheteroarylethenes 2a and 2b have an absorption band overlapping the emission band of the donor, 1. The corresponding variation in the overlap integral (and thus critical transfer distance R(o)) between the two states provides the means for reversibly switching the process of FRET on and off, allowing direct and repeated evaluation of the relative changes in the donor fluorescence quantum yield. The diheteroarylethenes demonstrate excellent stability in aqueous media, an absence of thermal back reactions, and negligible fatigue. The equilibration of these systems after exposure to near-UV or visible light follows simple monoexponential kinetics. We developed a general conceptual scheme for such coupled photochromic-FRET reactions, allowing quantitative interpretations of the photostationary and kinetic data, from which the quantum yields for the cyclization and cycloreversion reactions of the photochromic acceptor were calculated.