Monitoring agonist-induced phospholipase C activation in live cells by fluorescence resonance energy transfer

α-Synuclein-dependent increases in PIP5K1γ drive inositol signaling to promote neurotoxicity

Anomalous aggregation of α-synuclein (α-Syn) is a pathological hallmark of many degenerative synucleinopathies including Lewy body dementia (LBD) and Parkinson's disease (PD). Despite its strong link to disease, the precise molecular mechanisms that link α-Syn aggregation to neurodegeneration have yet to be elucidated. Here, we find that elevated α-Syn leads to an increase in the plasma membrane (PM) phosphoinositide PI(4,5)P2, which precipitates α-Syn aggregation and drives toxic increases in mitochondrial Ca2+ and reactive oxygen species leading to neuronal death. Upstream of this toxic signaling pathway is PIP5K1γ, whose abundance and localization is enhanced at the PM by α-Syn-dependent increases in ARF6. Selective inhibition of PIP5K1γ or knockout of ARF6 in neurons rescues α-Syn aggregation and cellular phenotypes of toxicity. Collectively, our data suggest that modulation of phosphoinositide metabolism may be a therapeutic target to slow neurodegeneration for PD and other related neurodegenerative disorders.

Targeting phosphoinositide signaling in cancer: relevant techniques to study lipids and novel avenues for therapeutic intervention

Phosphoinositides serve as essential players in numerous biological activities and are critical for overall cellular function. Due to their complex chemical structures, localization, and low abundance, current challenges in the phosphoinositide field include the accurate measurement and identification of specific variants, particularly those with acyl chains. Researchers are intensively developing innovative techniques and approaches to address these challenges and advance our understanding of the impact of phosphoinositide signaling on cellular biology. This article provides an overview of recent advances in the study of phosphoinositides, including mass spectrometry, lipid biosensors, and real-time activity assays using fluorometric sensors. These methodologies have proven instrumental for a comprehensive exploration of the cellular distribution and dynamics of phosphoinositides and have shed light on the growing significance of these lipids in human health and various pathological processes, including cancer. To illustrate the importance of phosphoinositide signaling in disease, this perspective also highlights the role of a family of lipid kinases named phosphatidylinositol 5-phosphate 4-kinases (PI5P4Ks), which have recently emerged as exciting therapeutic targets for cancer treatment. The ongoing exploration of phosphoinositide signaling not only deepens our understanding of cellular biology but also holds promise for novel interventions in cancer therapy.

Localization of the tubby domain, a PI(4,5)P2 biosensor, to E-Syt3-rich endoplasmic reticulum-plasma membrane junctions

The phospholipid phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] acts as a signaling lipid at the plasma membrane (PM) with pleiotropic regulatory actions on multiple cellular processes. Signaling specificity might result from spatiotemporal compartmentalization of the lipid and from combinatorial binding of PI(4,5)P2 effector proteins to additional membrane components. Here, we analyzed the spatial distribution of tubbyCT, a paradigmatic PI(4,5)P2-binding domain, in live mammalian cells by total internal reflection fluorescence (TIRF) microscopy and molecular dynamics simulations. We found that unlike other well-characterized PI(4,5)P2 recognition domains, tubbyCT segregates into distinct domains within the PM. TubbyCT enrichment occurred at contact sites between PM and endoplasmic reticulum (ER) (i.e. at ER-PM junctions) as shown by colocalization with ER-PM markers. Localization to these sites was mediated in a combinatorial manner by binding to PI(4,5)P2 and by interaction with a cytosolic domain of extended synaptotagmin 3 (E-Syt3), but not other E-Syt isoforms. Selective localization to these structures suggests that tubbyCT is a novel selective reporter for a ER-PM junctional pool of PI(4,5)P2. Finally, we found that association with ER-PM junctions is a conserved feature of tubby-like proteins (TULPs), suggesting an as-yet-unknown function of TULPs.

Chemical Tools for Lipid Cell Biology

Lipids are key components of all organisms. We are well educated in their use as fuel and their essential role to form membranes. We also know much about their biosynthesis and metabolism. We are also aware that most lipids have signaling character meaning that a change in their concentration or location constitutes a signal that helps a living cell to respond to changes in the environment or to fulfill its specific function ranging from secretion to cell division. What is much less understood is how lipids change location in cells over time and what other biomolecules they interact with at each stage of their lifetime. Due to the large number of often quite similar lipid species and the sometimes very short lifetime of signaling lipids, we need highly specific tools to manipulate and visualize lipids and lipid-protein interactions. If successfully applied, these tools provide fabulous opportunities for discovery.In this Account, I summarize the development of synthetic tools from our lab that were designed to address crucial properties that allow them to function as tools in live cell experiments. Techniques to change the concentration of lipids by adding a small molecule or by light are described and complemented by examples of biological findings made when applying the tools. This ranges from chemical dimerizer-based systems to synthetic "caged" lipid derivatives. Furthermore, I discuss the problem of locating a lipid in an intact cell. Synthetic molecular probes are described that help to unravel the lipid location and to determine their binding proteins. These location studies require in-cell lipid tagging by click chemistry, photo-cross-linking to prevent further movement and the "caging" groups to avoid premature metabolism. The combination of these many technical features in a single tool allows for the analysis of not only lipid fluxes through metabolism but also lipid transport from one membrane to another as well as revealing the lipid interactome in a cell-dependent manner. This latter point is crucial because with these multifunctional tools in combination with lipidomics we can now address differences in healthy versus diseased cells and ultimately find the changes that are essential for disease development and new therapeutics that prevent these changes.

Two-step structural changes in M3 muscarinic receptor activation rely on the coupled Gq protein cycle

G protein-coupled receptors (GPCRs) regulate diverse intracellular signaling pathways through the activation of heterotrimeric G proteins. However, the effects of the sequential activation-deactivation cycle of G protein on the conformational changes of GPCRs remains unknown. By developing a Förster resonance energy transfer (FRET) tool for human M3 muscarinic receptor (hM3R), we find that a single-receptor FRET probe can display the consecutive structural conversion of a receptor by G protein cycle. Our results reveal that the G protein activation evokes a two-step change in the hM3R structure, including the fast step mediated by G_q protein binding and the subsequent slower step mediated by the physical separation of the Gα_q and Gβγ subunits. We also find that the separated Gα_q-GTP forms a stable complex with the ligand-activated hM3R and phospholipase Cβ. In sum, the present study uncovers the real-time conformational dynamics of innate hM3R during the downstream G_q protein cycle.

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

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

Impact of Ca2+-Induced PI(4,5)P2 Clusters on PH-YFP Organization and Protein-Protein Interactions

Despite its low abundance, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a key modulator of membrane-associated signaling events in eukaryotic cells. Temporal and spatial regulation of PI(4,5)P2 concentration can achieve localized increases in the levels of this lipid, which are crucial for the activation or recruitment of peripheral proteins to the plasma membrane. The recent observation of the dramatic impact of physiological divalent cation concentrations on PI(4,5)P2 clustering, suggests that protein anchoring to the plasma membrane through PI(4,5)P2 is likely not defined solely by a simple (monomeric PI(4,5)P2)/(protein bound PI(4,5)P2) equilibrium, but instead depends on complex protein interactions with PI(4,5)P2 clusters. The insertion of PI(4,5)P2-binding proteins within these clusters can putatively modulate protein-protein interactions in the membrane, but the relevance of such effects is largely unknown. In this work, we characterized the impact of Ca2+ on the organization and protein-protein interactions of PI(4,5)P2-binding proteins. We show that, in giant unilamellar vesicles presenting PI(4,5)P2, the membrane diffusion properties of pleckstrin homology (PH) domains tagged with a yellow fluorescent protein (YFP) are affected by the presence of Ca2+, suggesting direct interactions between the protein and PI(4,5)P2 clusters. Importantly, PH-YFP is found to dimerize in the membrane in the absence of Ca2+. This oligomerization is inhibited in the presence of physiological concentrations of the divalent cation. These results confirm that cation-dependent PI(4,5)P2 clustering promotes interactions between PI(4,5)P2-binding proteins and has the potential to dramatically influence the organization and downstream interactions of PI(4,5)P2-binding proteins in the plasma membrane.

Biophysical physiology of phosphoinositide rapid dynamics and regulation in living cells

Phosphoinositide membrane lipids are ubiquitous low-abundance signaling molecules. They direct many physiological processes that involve ion channels, membrane identification, fusion of membrane vesicles, and vesicular endocytosis. Pools of these lipids are continually broken down and refilled in living cells, and the rates of some of these reactions are strongly accelerated by physiological stimuli. Recent biophysical experiments described here measure and model the kinetics and regulation of these lipid signals in intact cells. Rapid on-line monitoring of phosphoinositide metabolism is made possible by optical tools and electrophysiology. The experiments reviewed here reveal that as for other cellular second messengers, the dynamic turnover and lifetimes of membrane phosphoinositides are measured in seconds, controlling and timing rapid physiological responses, and the signaling is under strong metabolic regulation. The underlying mechanisms of this metabolic regulation remain questions for the future.

Quantitative FRET Microscopy Reveals a Crucial Role of Cytoskeleton in Promoting PI(4,5)P2 Confinement

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is an essential plasma membrane component involved in several cellular functions, including membrane trafficking and cytoskeleton organization. This function multiplicity is partially achieved through a dynamic spatiotemporal organization of PI(4,5)P2 within the membrane. Here, we use a Förster resonance energy transfer (FRET) approach to quantitatively assess the extent of PI(4,5)P2 confinement within the plasma membrane. This methodology relies on the rigorous evaluation of the dependence of absolute FRET efficiencies between pleckstrin homology domains (PHPLCδ) fused with fluorescent proteins and their average fluorescence intensity at the membrane. PI(4,5)P2 is found to be significantly compartmentalized at the plasma membrane of HeLa cells, and these clusters are not cholesterol-dependent, suggesting that membrane rafts are not involved in the formation of these nanodomains. On the other hand, upon inhibition of actin polymerization, compartmentalization of PI(4,5)P2 is almost entirely eliminated, showing that the cytoskeleton network is the critical component responsible for the formation of nanoscale PI(4,5)P2 domains in HeLa cells.

Cellular context shapes cyclic nucleotide signaling in neurons through multiple levels of integration

Intracellular signaling with cyclic nucleotides are ubiquitous signaling pathways, yet the dynamics of these signals profoundly differ in different cell types. Biosensor imaging experiments, by providing direct measurements in intact cellular environment, reveal which receptors are activated by neuromodulators and how the coincidence of different neuromodulators is integrated at various levels in the signaling cascade. Phosphodiesterases appear as one important determinant of cross-talk between different signaling pathways. Finally, analysis of signal dynamics reveal that striatal medium-sized spiny neuron obey a different logic than other brain regions such as cortex, probably in relation with the function of this brain region which efficiently detects transient dopamine.

An asymmetric junctional mechanoresponse coordinates mitotic rounding with epithelial integrity

Epithelia are continuously self-renewed, but how epithelial integrity is maintained during the morphological changes that cells undergo in mitosis is not well understood. Here, we show that as epithelial cells round up when they enter mitosis, they exert tensile forces on neighboring cells. We find that mitotic cell-cell junctions withstand these tensile forces through the mechanosensitive recruitment of the actin-binding protein vinculin to cadherin-based adhesions. Surprisingly, vinculin that is recruited to mitotic junctions originates selectively from the neighbors of mitotic cells, resulting in an asymmetric composition of cadherin junctions. Inhibition of junctional vinculin recruitment in neighbors of mitotic cells results in junctional breakage and weakened epithelial barrier. Conversely, the absence of vinculin from the cadherin complex in mitotic cells is necessary to successfully undergo mitotic rounding. Our data thus identify an asymmetric mechanoresponse at cadherin adhesions during mitosis, which is essential to maintain epithelial integrity while at the same time enable the shape changes of mitotic cells.

An Arrhythmic Mutation E7K Facilitates TRPM4 Channel Activation via Enhanced PIP2 Interaction

A Ca²⁺-activated monovalent cation-selective TRPM4 channel is abundantly expressed in the heart. Recently, a single gain-of-function mutation identified in the distal N-terminus of the human TRPM4 channel (Glu⁵ to Lys⁵; E7K) was found to be arrhythmogenic because of enhanced cell membrane expression. In this study, we conducted detailed analyses of this mutant channel from more functional aspects, in comparison with its wild type (WT). In an expression system, intracellular application of a short soluble PIP₂ (diC₈PIP₂) restored the single-channel activities of both WT and E7K, which had quickly faded after membrane excision. The potency (K_d) of diC₈PIP₂ for this recovery was stronger in E7K than its WT (1.44 vs. 2.40 μM). FRET-based PIP₂ measurements combined with the Danio rerio voltage-sensing phosphatase (DrVSP) and patch clamping revealed that lowering the endogenous PIP₂ level by DrVSP activation reduced the TRPM4 channel activity. This effect was less prominent in E7K than its WT (apparent K_d values estimated from DrVSP-mediated PIP₂ depletion: 0.97 and 1.06 μM, respectively), being associated with the differential PIP₂-mediated modulation of voltage dependence. Moreover, intracellular perfusion of short N-terminal polypeptides containing either the ‘WT’ or ‘E7K’ sequences respectively attenuated the TRPM4 channel activation at whole-cell and single-channel levels, but in both configurations, the E7K polypeptide exerted greater inhibitory effects. These results collectively suggest that N-terminal interaction with endogenous PIP₂ is essential for the TRPM4 channel to function, the extent of which may be abnormally strengthened by the E7K mutation through modulating voltage-dependent activation. The altered PIP₂ interaction may account for the arrhythmogenic potential of this mutation.

Control of Neuronal Excitability by Cell Surface Receptor Density and Phosphoinositide Metabolism

Phosphoinositides are members of a family of minor phospholipids that make up about 1% of all lipids in most cell types. Despite their low abundance they have been found to be essential regulators of neuronal activities such as action potential firing, release and re-uptake of neurotransmitters, and interaction of cytoskeletal proteins with the plasma membrane. Activation of several different neurotransmitter receptors can deplete phosphoinositide levels by more than 90% in seconds, thereby profoundly altering neuronal behavior; however, despite the physiological importance of this mechanism we still lack a profound quantitative understanding of the connection between phosphoinositide metabolism and neuronal activity. Here, we present a model that describes phosphoinositide metabolism and phosphoinositide-dependent action potential firing in sympathetic neurons. The model allows for a simulation of activation of muscarinic acetylcholine receptors and its effects on phosphoinositide levels and their regulation of action potential firing in these neurons. In this paper, we describe the characteristics of the model, its calibration to experimental data, and use the model to analyze how alterations of surface density of muscarinic acetylcholine receptors or altered activity levels of a key enzyme of phosphoinositide metabolism influence action potential firing of sympathetic neurons. In conclusion, the model provides a comprehensive framework describing the connection between muscarinic acetylcholine signaling, phosphoinositide metabolism, and action potential firing in sympathetic neurons which can be used to study the role of these signaling systems in health and disease.

Methods to study phosphoinositide regulation of ion channels

Ion channel are embedded in the lipid bilayers of biological membranes. Membrane phospholipids constitute a barrier to ion movement, and they have been considered for a long time as a passive environment for channel proteins. Membrane phospholipids, however, do not only serve as a passive amphipathic environment, but they also modulate channel activity by direct specific lipid-protein interactions. Phosphoinositides are quantitatively minor components of biological membranes, and they play roles in many cellular functions, including membrane traffic, cellular signaling and cytoskeletal organization. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] is mainly found in the inner leaflet of the plasma membrane. Its role as a potential ion channel regulator was first appreciated over two decades ago and by now this lipid is a well-established cofactor or regulator of many different ion channels. The past two decades witnessed the steady development of techniques to study ion channel regulation by phosphoinositides with progress culminating in recent cryoEM structures that allowed visualization of how PI(4,5)P₂ opens some ion channels. This chapter will provide an overview of the methods to study regulation by phosphoinositides, focusing on plasma membrane ion channels and PI(4,5)P₂.

Cellular Plasticity during Metastasis: New Insights Provided by Intravital Microscopy

Metastasis is a highly dynamic process during which cancer and microenvironmental cells undergo a cascade of events required for efficient dissemination throughout the body. During the metastatic cascade, tumor cells can change their state and behavior, a phenomenon commonly defined as cellular plasticity. To monitor cellular plasticity during metastasis, high-resolution intravital microscopy (IVM) techniques have been developed and allow us to visualize individual cells by repeated imaging in animal models. In this review, we summarize the latest technological advancements in the field of IVM and how they have been applied to monitor metastatic events. In particular, we highlight how longitudinal imaging in native tissues can provide new insights into the plastic physiological and developmental processes that are hijacked by cancer cells during metastasis.

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.

G αq Sensitizes TRPM8 to Inhibition by PI(4,5)P2 Depletion upon Receptor Activation

The cold- and menthol-sensitive transient receptor potential melastatin 8 (TRPM8) channel is important for both physiological temperature detection and cold allodynia. Activation of G-protein-coupled receptors (GPCRs) by proinflammatory mediators inhibits these channels. It was proposed that this inhibition proceeds via direct binding of G _αq to the channel. TRPM8 requires the plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂ or PIP₂] for activity. However, it was claimed that a decrease in cellular levels of this lipid upon receptor activation does not contribute to channel inhibition. Here, we show that supplementing the whole-cell patch pipette with PI(4,5)P₂ reduced inhibition of TRPM8 by activation of G_αq-coupled receptors in mouse dorsal root ganglion (DRG) neurons isolated from both sexes. Stimulating the same receptors activated phospholipase C (PLC) and decreased plasma membrane PI(4,5)P₂ levels in these neurons. PI(4,5)P₂ also reduced inhibition of TRPM8 by activation of heterologously expressed muscarinic M1 receptors. Coexpression of a constitutively active G _αq protein that does not couple to PLC inhibited TRPM8 activity, and in cells expressing this protein, decreasing PI(4,5)P₂ levels using a voltage-sensitive 5'-phosphatase induced a stronger inhibition of TRPM8 activity than in control cells. Our data indicate that, upon GPCR activation, G _αq binding reduces the apparent affinity of TRPM8 for PI(4,5)P₂ and thus sensitizes the channel to inhibition induced by decreasing PI(4,5)P₂ levels.SIGNIFICANCE STATEMENT Increased sensitivity to heat in inflammation is partially mediated by inhibition of the cold- and menthol-sensitive transient receptor potential melastatin 8 (TRPM8) ion channels. Most inflammatory mediators act via G-protein-coupled receptors that activate the phospholipase C pathway, leading to the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. How receptor activation by inflammatory mediators leads to TRPM8 inhibition is not well understood. Here, we propose that direct binding of G _αq both reduces TRPM8 activity and sensitizes the channel to inhibition by decreased levels of its cofactor, PI(4,5)P₂ Our data demonstrate the convergence of two downstream effectors of receptor activation, G _αq and PI(4,5)P₂ hydrolysis, in the regulation of TRPM8.

Optical approaches for single-cell and subcellular analysis of GPCR-G protein signaling

G protein-coupled receptors (GPCRs), G proteins, and their signaling associates are major signal transducers that control the majority of cellular signaling and regulate key biological functions including immune, neurological, cardiovascular, and metabolic processes. These pathways are targeted by over one-third of drugs on the market; however, the current understanding of their function is limited and primarily derived from cell-destructive approaches providing an ensemble of static, multi-cell information about the status and composition of molecules. Spatiotemporal behavior of molecules involved is crucial to understanding in vivo cell behaviors both in health and disease, and the advent of genetically encoded fluorescence proteins and small fluorophore-based biosensors has facilitated the mapping of dynamic signaling in cells with subcellular acuity. Since we and others have developed optogenetic methods to regulate GPCR-G protein signaling in single cells and subcellular regions using dedicated wavelengths, the desire to develop and adopt optogenetically amenable assays to measure signaling has motivated us to take a broader look at the available optical tools and approaches compatible with measuring single-cell and subcellular GPCR-G protein signaling. Here we review such key optical approaches enabling the examination of GPCR, G protein, secondary messenger, and downstream molecules such as kinase and lipid signaling in living cells. The methods reviewed employ both fluorescence and bioluminescence detection. We not only further elaborate the underlying principles of these sensors but also discuss the experimental criteria and limitations to be considered during their use in single-cell and subcellular signal mapping.

Biased signaling of Ca2+-sensing receptors in cardiac myocytes regulates GIRK channel activity

Ca²⁺-sensing receptors (CaSRs) belong to the class C of G protein-coupled receptors and are activated by extracellular Ca²⁺. CaSRs display biased G protein signaling by coupling to different classes of heterotrimeric G proteins depending on agonist and cell type. In this study we used fluorescent biosensors to directly analyze G protein coupling to CaSRs and downstream signaling in living cells. In HEK 293 cells, CaSRs displayed biased signaling: elevation of extracellular Ca²⁺ or application of the alternative agonist spermine caused activation of G_i- and G_q-proteins. Adult cardiac myocytes express endogenous CaSRs, which have been implicated in regulating Ca²⁺ signaling and contractility. Biased signaling of CaSRs has not been investigated in these cells. To evaluate efficiencies of G_i- and G_q-signaling via CaSRs in rat atrial myocytes, we measured G protein-activated K⁺ (GIRK) channels. Activation of GIRK requires binding of Gβγ subunits released from G_i proteins, whereas G_q-signaling results in inhibition of GIRK channel activity. Stimulation of CaSRs by Ca²⁺ or spermine failed to directly activate G_i and GIRK channels. When GIRK channels were pre-activated via endogenous M₂ receptors, stimulation of CaSRs caused pronounced inhibition of GIRK currents. This effect was specific to CaSR activation: GIRK current inhibition was sensitive to NPS-2143, a negative allosteric modulator of CaSRs, and abrogated by FR900359, a direct inhibitor of G_q. GIRK current inhibition was also sensitive to the PKC inhibitor chelerythrine, suggesting that following activation of CaSR and G_q, GIRK currents are modulated by PKC phosphorylation. We conclude from this data that cardiac CaSRs do not activate G_i and affect GIRK currents preferentially via the G_q/PKC pathway.

Development of Nonspecific BRET-Based Biosensors to Monitor Plasma Membrane Inositol Lipids in Living Cells

There are several difficulties to face when investigating the role of phosphoinositides. Although they are present in most organelles, their concentration is very low, sometimes undetectable with the available methods; moreover, their level can quickly change upon several external stimuli. Here we introduce a newly improved lipid sensor tool-set based on the balanced expression of luciferase-fused phosphoinositide recognizing protein domains and a Venus protein targeted to the plasma membrane, allowing us to perform Bioluminescence Resonance Energy Transfer (BRET) measurements that reflect phosphoinositide changes in a population of transiently transfected cells. This method is highly sensitive, specific, and capable of semiquantitative characterization of plasma membrane phosphoinositide changes with high temporal resolution.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

Profilin binding couples chloride intracellular channel protein CLIC4 to RhoA-mDia2 signaling and filopodium formation

Chloride intracellular channel 4 (CLIC4) is a cytosolic protein implicated in diverse actin-based processes, including integrin trafficking, cell adhesion, and tubulogenesis. CLIC4 is rapidly recruited to the plasma membrane by RhoA-activating agonists and then partly colocalizes with β1 integrins. Agonist-induced CLIC4 translocation depends on actin polymerization and requires conserved residues that make up a putative binding groove. However, the mechanism and significance of CLIC4 trafficking have been elusive. Here, we show that RhoA activation by either lysophosphatidic acid (LPA) or epidermal growth factor is necessary and sufficient for CLIC4 translocation to the plasma membrane and involves regulation by the RhoA effector mDia2, a driver of actin polymerization and filopodium formation. We found that CLIC4 binds the G-actin–binding protein profilin-1 via the same residues that are required for CLIC4 trafficking. Consistently, shRNA-induced profilin-1 silencing impaired agonist-induced CLIC4 trafficking and the formation of mDia2-dependent filopodia. Conversely, CLIC4 knockdown increased filopodium formation in an integrin-dependent manner, a phenotype rescued by wild-type CLIC4 but not by the trafficking-incompetent mutant CLIC4(C35A). Furthermore, CLIC4 accelerated LPA-induced filopodium retraction. We conclude that through profilin-1 binding, CLIC4 functions in a RhoA–mDia2–regulated signaling network to integrate cortical actin assembly and membrane protrusion. We propose that agonist-induced CLIC4 translocation provides a feedback mechanism that counteracts formin-driven filopodium formation.

Gi/o-coupled muscarinic receptors co-localize with GIRK channel for efficient channel activation

G protein-gated inwardly rectifying K+ (GIRK) channel regulates cellular excitability upon activation of Gi/o-coupled receptors. In Gi/o-coupled muscarinic M2R, the intracellular third loop (i3) is known as a key domain for Gi/o coupling, because replacement of i3 of Gq-coupled muscarinic M1R with that of M2R enables the chimeric receptor (MC9) to activate the GIRK channel. In the present study, we showed that MC9, but not M1R, co-localizes with the GIRK channel and Gαi1 by Förster resonance energy transfer (FRET) analysis. When M1R was forced to stay adjacent to the channel through ligation with short linkers, M1R activated the GIRK channel. FRET analysis further suggested that the efficacy of channel activation is correlated with the linker length between M1R and the GIRK channel. The results show that co-localization is an important factor for activating the GIRK channel. In contrast, for MC9 and M2R, the GIRK channel was activated even when they were connected by long linkers, suggesting the formation of a molecular complex even in the absence of a linker. We also observed that replacement of 13 amino acid residues at the N-terminal end of i3 of MC9 with those of M1R impaired the co-localization with the GIRK channel as well as channel activation. These results show that localization of the receptor near the GIRK channel is a key factor in efficiently activating the channel and that the N-terminal end of i3 of M2R plays an important role in co-localization.

The N-terminal homology (ENTH) domain of Epsin 1 is a sensitive reporter of physiological PI(4,5)P2 dynamics

Phospholipase Cβ (PLCβ)-induced depletion of phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P₂) transduces a plethora of signals into cellular responses. Importance and diversity of PI(4,5)P₂-dependent processes led to strong need for biosensors of physiological PI(4,5)P₂ dynamics applicable in live-cell experiments. Membrane PI(4,5)P₂ can be monitored with fluorescently-labelled phosphoinositide (PI) binding domains that associate to the membrane depending on PI(4,5)P₂ levels. The pleckstrin homology domain of PLCδ1 (PLCδ1-PH) and the C-terminus of tubby protein (tubby_CT) are two such sensors widely used to study PI(4,5)P₂ signaling. However, certain limitations apply to both: PLCδ1-PH binds cytoplasmic inositol-1,4,5-trisphosphate (IP₃) produced from PI(4,5)P₂ through PLCβ, and tubby_CT responses do not faithfully report on PLCβ-dependent PI(4,5)P₂ dynamics. In searching for an improved biosensor, we fused N-terminal homology domain of Epsin1 (ENTH) to GFP and examined use of this construct as genetically-encoded biosensor for PI(4,5)P₂ dynamics in living cells. We utilized recombinant tools to manipulate PI or G_q protein-coupled receptors (G_qPCR) to stimulate PLCβ signaling and characterized PI binding properties of ENTH-GFP with total internal reflection (TIRF) and confocal microscopy. ENTH-GFP specifically recognized membrane PI(4,5)P₂ without interacting with IP₃, as demonstrated by dialysis of cells with the messenger through a patch pipette. Utilizing Ci-VSP to titrate PI(4,5)P₂ levels, we found that ENTH-GFP had low PI(4,5)P₂ affinity. Accordingly, ENTH-GFP was highly sensitive to PLCβ-dependent PI(4,5)P₂ depletion, and in contrast to PLCδ1-PH, overexpression of ENTH-GFP did not attenuate G_qPCR signaling. Taken together, ENTH-GFP detects minute changes of PI(4,5)P₂ levels and provides an important complementation of experimentally useful reporters of PI(4,5)P₂ dynamics in physiological pathways.

Measurement of intracellular concentration of fluorescently-labeled targets in living cells

Estimations of intracellular concentrations of fluorescently-labeled molecules within living cells are very important for guidance of biological experiments and interpretation of their results. Here we propose a simple and universal approach for such estimations. The approach is based upon common knowledge that the dye fluorescence is directly proportional to its quantum yield and the number of its molecules and that a coefficient of proportionality is determined by spectral properties of the dye and optical equipment used to record fluorescent signals. If two fluorescent dyes are present in the same volume, then a ratio of their concentrations is equal to a ratio of their fluorescence multiplied by some dye- and equipment-dependent coefficient. Thus, if the coefficient and concentration of one dye is known then the concentration of another dye can be determined. Here we have demonstrated how to calculate this coefficient (called a ratio factor) and how to use it for concentration measurements of fluorescently tagged molecules. As an example of how this approach can be used, we estimated a concentration of exogenously expressed neuronal Ca2+ sensor protein, hippocalcin, tagged by a fluorescent protein in a dendritic tree of rat hippocampal neurons loaded via a patch pipette with Alexa Fluor dye of known concentration. The new approach should allow performing a fast, inexpensive and reliable quantitative analysis of fluorescently-labeled targets in different parts of living cells.

A membrane-associated, fluorogenic reporter for mammalian phospholipase C isozymes

A diverse group of cell-surface receptors, including many G protein-coupled receptors and receptor tyrosine kinases, activate phospholipase C (PLC) isozymes to hydrolyze phosphatidylinositol 4,5-bisphosphate into the second messengers diacylglycerol and 1,4,5-inositol trisphosphate. Consequently, PLCs control various cellular processes, and their aberrant regulation contributes to many diseases, including cancer, atherosclerosis, and rheumatoid arthritis. Despite the widespread importance of PLCs in human biology and disease, it has been impossible to directly monitor the real-time activation of these enzymes at membranes. To overcome this limitation, here we describe XY-69, a fluorogenic reporter that preferentially partitions into membranes and provides a selective tool for measuring the real-time activity of PLCs as either purified enzymes or in cellular lysates. Indeed, XY-69 faithfully reported the membrane-dependent activation of PLC-β3 by Gα_q Therefore, XY-69 can replace radioactive phosphatidylinositol 4,5-bisphosphate used in conventional PLC assays and will enable high-throughput screens to identify both orthosteric and allosteric PLC inhibitors. In the future, cell-permeable variants of XY-69 represent promising candidates for reporting the activation of PLCs in live cells with high spatiotemporal resolution.

Fatty-acyl chain profiles of cellular phosphoinositides

Phosphoinositides are rapidly turning-over phospholipids that play key roles in intracellular signaling and modulation of membrane effectors. Through technical refinements we have improved sensitivity in the analysis of the phosphoinositide PI, PIP, and PIP2 pools from living cells using mass spectrometry. This has permitted further resolution in phosphoinositide lipidomics from cell cultures and small samples of tissue. The technique includes butanol extraction, derivatization of the lipids, post-column infusion of sodium to stabilize formation of sodiated adducts, and electrospray ionization mass spectrometry in multiple reaction monitoring mode, achieving a detection limit of 20pg. We describe the spectrum of fatty-acyl chains in the cellular phosphoinositides. Consistent with previous work in other mammalian primary cells, the 38:4 fatty-acyl chains dominate in the phosphoinositides of the pineal gland and of superior cervical ganglia, and many additional fatty acid combinations are found at low abundance. However, Chinese hamster ovary cells and human embryonic kidney cells (tsA201) in culture have different fatty-acyl chain profiles that change with growth state. Their 38:4 lipids lose their dominance as cultures approach confluence. The method has good time resolution and follows well the depletion in <20s of both PIP2 and PIP that results from strong activation of Gq-coupled receptors. The receptor-activated phospholipase C exhibits no substrate selectivity among the various fatty-acyl chain combinations.

mScarlet: a bright monomeric red fluorescent protein for cellular imaging

We report the engineering of mScarlet, a truly monomeric red fluorescent protein with record brightness, quantum yield (70%) and fluorescence lifetime (3.9 ns). We developed mScarlet starting with a consensus synthetic template and using improved spectroscopic screening techniques; mScarlet's crystal structure reveals a planar and rigidified chromophore. mScarlet outperforms existing red fluorescent proteins as a fusion tag, and it is especially useful as a Förster resonance energy transfer (FRET) acceptor in ratiometric imaging.

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.

Contributions of protein kinases and β-arrestin to termination of protease-activated receptor 2 signaling

Systematic imaging studies and modeling reveal new details of the regulation of the G_q-coupled GPCR, protease-activated receptor 2, by phosphorylation and β-arrestin.

Activated G_q protein–coupled receptors (G_qPCRs) can be desensitized by phosphorylation and β-arrestin binding. The kinetics and individual contributions of these two mechanisms to receptor desensitization have not been fully distinguished. Here, we describe the shut off of protease-activated receptor 2 (PAR2). PAR2 activates G_q and phospholipase C (PLC) to hydrolyze phosphatidylinositol 4,5-bisphosphate (PIP₂) into diacylglycerol and inositol trisphosphate (IP₃). We used fluorescent protein–tagged optical probes to monitor several consequences of PAR2 signaling, including PIP₂ depletion and β-arrestin translocation in real time. During continuous activation of PAR2, PIP₂ was depleted transiently and then restored within a few minutes, indicating fast receptor activation followed by desensitization. Knockdown of β-arrestin 1 and 2 using siRNA diminished the desensitization, slowing PIP₂ restoration significantly and even adding a delayed secondary phase of further PIP₂ depletion. These effects of β-arrestin knockdown on PIP₂ recovery were prevented when serine/threonine phosphatases that dephosphorylate GPCRs were inhibited. Thus, PAR2 may continuously regain its activity via dephosphorylation when there is insufficient β-arrestin to trap phosphorylated receptors. Similarly, blockers of protein kinase C (PKC) and G protein–coupled receptor kinase potentiated the PIP₂ depletion. In contrast, an activator of PKC inhibited receptor activation, presumably by augmenting phosphorylation of PAR2. Our interpretations were strengthened by modeling. Simulations supported the conclusions that phosphorylation of PAR2 by protein kinases initiates receptor desensitization and that recruited β-arrestin traps the phosphorylated state of the receptor, protecting it from phosphatases. Speculative thinking suggested a sequestration of phosphatidylinositol 4-phosphate 5 kinase (PIP5K) to the plasma membrane by β-arrestin to explain why knockdown of β-arrestin led to secondary depletion of PIP₂. Indeed, artificial recruitment of PIP5K removed the secondary loss of PIP₂ completely. Altogether, our experimental and theoretical approaches demonstrate roles and dynamics of the protein kinases, β-arrestin, and PIP5K in the desensitization of PAR2.

Approaches to Study Phosphatases

Phosphatases play key roles in normal physiology and diseases. Studying phosphatases has been both essential and challenging, and the application of conventional genetic and biochemical methods has led to crucial but still limited understanding of their mechanisms, substrates, and exclusive functions within highly intricate networks. With the advances in technologies such as cellular imaging and molecular and chemical biology in terms of sensitive tools and methods, the phosphatase field has thrived in the past years and has set new insights for cell signaling studies and for therapeutic development. In this review, we give an overview of the existing interdisciplinary tools for phosphatases, give examples on how they have been applied to increase our understanding of these enzymes, and suggest how they-and other tools yet barely used in the phosphatase field-might be adapted to address future questions and challenges.

Probes for manipulating and monitoring IP3

Inositol 1,4,5-trisphosphate (IP₃) is an important second messenger produced via G-protein-coupled receptor- or receptor tyrosine kinase-mediated pathways. IP₃ levels induce Ca²⁺ release from the endoplasmic reticulum (ER) via IP₃ receptor (IP₃R) located in the ER membrane. The resultant spatiotemporal pattern of Ca²⁺ signals regulates diverse cellular functions, including fertilization, gene expression, synaptic plasticity, and cell death. Therefore, monitoring and manipulating IP₃ levels is important to elucidate not only the functions of IP₃-mediated pathways but also the encoding mechanism of IP₃R as a converter of intracellular signals from IP₃ to Ca²⁺.

Receptor Species-dependent Desensitization Controls KCNQ1/KCNE1 K+ Channels as Downstream Effectors of Gq Protein-coupled Receptors

Activation of G_q protein-coupled receptors (G_qPCRs) might induce divergent cellular responses, related to receptor-specific activation of different branches of the G_q signaling pathway. Receptor-specific desensitization provides a mechanism of effector modulation by restricting the spatiotemporal activation of signaling components downstream of G_q We quantified signaling events downstream of G_qPCR activation with FRET-based biosensors in CHO and HEK 293 cells. KCNQ1/KCNE1 channels (I_Ks) were measured as a functional readout of receptor-specific activation. Activation of muscarinic M₁ receptors (M₁-Rs) caused robust and reversible inhibition of I_Ks. In contrast, activation of α_1B-adrenergic receptors (α_1B-ARs) induced transient inhibition of I_Ks, which turned into delayed facilitation after agonist withdrawal. As a novel finding, we demonstrate that G_qPCR-specific kinetics of I_Ks modulation are determined by receptor-specific desensitization, evident at the level of Gα_q activation, phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion, and diacylglycerol production. Sustained I_Ks inhibition during M₁-R stimulation is attributed to robust membrane PIP₂ depletion, whereas the rapid desensitization of α_1B-AR delimits PIP₂ reduction and augments current activation by protein kinase C (PKC). Overexpression of Ca²⁺-independent PKCδ did not affect the time course of α_1B-AR-induced diacylglycerol formation, excluding a contribution of PKCδ to α_1B-AR desensitization. Pharmacological inhibition of Ca²⁺-dependent PKC isoforms abolished fast α_1B receptor desensitization and augmented I_Ks reduction, but did not affect I_Ks facilitation. These data indicate a contribution of Ca²⁺-dependent PKCs to α_1B-AR desensitization, whereas I_Ks facilitation is induced by Ca²⁺-independent PKC isoforms. In contrast, neither inhibition of Ca²⁺-dependent/Ca²⁺-independent isoforms nor overexpression of PKCδ induced M₁ receptor desensitization, excluding a contribution of PKC to M₁-R-induced I_Ks modulation.

Recent developments of genetically encoded optical sensors for cell biology

Optical sensors are powerful tools for live cell research as they permit to follow the location, concentration changes or activities of key cellular players such as lipids, ions and enzymes. Most of the current sensor probes are based on fluorescence which provides great spatial and temporal precision provided that high-end microscopy is used and that the timescale of the event of interest fits the response time of the sensor. Many of the sensors developed in the past 20 years are genetically encoded. There is a diversity of designs leading to simple or sometimes complicated applications for the use in live cells. Genetically encoded sensors began to emerge after the discovery of fluorescent proteins, engineering of their improved optical properties and the manipulation of their structure through application of circular permutation. In this review, we will describe a variety of genetically encoded biosensor concepts, including those for intensiometric and ratiometric sensors based on single fluorescent proteins, Forster resonance energy transfer-based sensors, sensors utilising bioluminescence, sensors using self-labelling SNAP- and CLIP-tags, and finally tetracysteine-based sensors. We focus on the newer developments and discuss the current approaches and techniques for design and application. This will demonstrate the power of using optical sensors in cell biology and will help opening the field to more systematic applications in the future.

Guidelines for the Use of Protein Domains in Acidic Phospholipid Imaging

Acidic phospholipids are minor membrane lipids but critically important for signaling events. The main acidic phospholipids are phosphatidylinositol phosphates (PIPs also known as phosphoinositides), phosphatidylserine (PS), and phosphatidic acid (PA). Acidic phospholipids are precursors of second messengers of key signaling cascades or are second messengers themselves. They regulate the localization and activation of many proteins, and are involved in virtually all membrane trafficking events. As such, it is crucial to understand the subcellular localization and dynamics of each of these lipids within the cell. Over the years, several techniques have emerged in either fixed or live cells to analyze the subcellular localization and dynamics of acidic phospholipids. In this chapter, we review one of them: the use of genetically encoded biosensors that are based on the expression of specific lipid binding domains (LBDs) fused to fluorescent proteins. We discuss how to design such sensors, including the criteria for selecting the lipid binding domains of interest and to validate them. We also emphasize the care that must be taken during data analysis as well as the main limitations and advantages of this approach.

PIP2 in pancreatic β-cells regulates voltage-gated calcium channels by a voltage-independent pathway

Phosphatidylinositol-4,5-bisphosphate (PIP₂) is a membrane phosphoinositide that regulates the activity of many ion channels. Influx of calcium primarily through voltage-gated calcium (Ca_V) channels promotes insulin secretion in pancreatic β-cells. However, whether Ca_V channels are regulated by PIP₂, as is the case for some non-insulin-secreting cells, is unknown. The purpose of this study was to investigate whether Ca_V channels are regulated by PIP₂ depletion in pancreatic β-cells through activation of a muscarinic pathway induced by oxotremorine methiodide (Oxo-M). Ca_V channel currents were recorded by the patch-clamp technique. The Ca_V current amplitude was reduced by activation of the muscarinic receptor 1 (M₁R) in the absence of kinetic changes. The Oxo-M-induced inhibition exhibited the hallmarks of voltage-independent regulation and did not involve PKC activation. A small fraction of the Oxo-M-induced Ca_V inhibition was diminished by a high concentration of Ca²⁺ chelator, whereas ≥50% of this inhibition was prevented by diC8-PIP₂ dialysis. Localization of PIP₂ in the plasma membrane was examined by transfecting INS-1 cells with PH-PLCδ1, which revealed a close temporal association between PIP₂ hydrolysis and Ca_V channel inhibition. Furthermore, the depletion of PIP₂ by a voltage-sensitive phosphatase reduced Ca_V currents in a way similar to that observed following M₁R activation. These results indicate that activation of the M₁R pathway inhibits the Ca_V channel via PIP₂ depletion by a Ca²⁺-dependent mechanism in pancreatic β- and INS-1 cells and thereby support the hypothesis that membrane phospholipids regulate ion channel activity by interacting with ion channels.

Phosphoinositide 5- and 3-phosphatase activities of a voltage-sensing phosphatase in living cells show identical voltage dependence

Voltage-sensing phosphatases (VSPs) are homologs of phosphatase and tensin homolog (PTEN), a phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2] and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] 3-phosphatase. However, VSPs have a wider range of substrates, cleaving 3-phosphate from PI(3,4)P2 and probably PI(3,4,5)P3 as well as 5-phosphate from phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and PI(3,4,5)P3 in response to membrane depolarization. Recent proposals say these reactions have differing voltage dependence. Using Förster resonance energy transfer probes specific for different PIs in living cells with zebrafish VSP, we quantitate both voltage-dependent 5- and 3-phosphatase subreactions against endogenous substrates. These activities become apparent with different voltage thresholds, voltage sensitivities, and catalytic rates. As an analytical tool, we refine a kinetic model that includes the endogenous pools of phosphoinositides, endogenous phosphatase and kinase reactions connecting them, and four exogenous voltage-dependent 5- and 3-phosphatase subreactions of VSP. We show that apparent voltage threshold differences for seeing effects of the 5- and 3-phosphatase activities in cells are not due to different intrinsic voltage dependence of these reactions. Rather, the reactions have a common voltage dependence, and apparent differences arise only because each VSP subreaction has a different absolute catalytic rate that begins to surpass the respective endogenous enzyme activities at different voltages. For zebrafish VSP, our modeling revealed that 3-phosphatase activity against PI(3,4,5)P3 is 55-fold slower than 5-phosphatase activity against PI(4,5)P2; thus, PI(4,5)P2 generated more slowly from dephosphorylating PI(3,4,5)P3 might never accumulate. When 5-phosphatase activity was counteracted by coexpression of a phosphatidylinositol 4-phosphate 5-kinase, there was accumulation of PI(4,5)P2 in parallel to PI(3,4,5)P3 dephosphorylation, emphasizing that VSPs can cleave the 3-phosphate of PI(3,4,5)P3.

Phosphoinositide signaling in somatosensory neurons

Somatosensory neurons of the dorsal root ganglia (DRG) and trigeminal ganglia (TG) are responsible for detecting thermal and tactile stimuli. They are also the primary neurons mediating pain and itch. A large number of cell surface receptors in these neurons couple to phospholipase C (PLC) enzymes leading to the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and the generation of downstream signaling molecules. These neurons also express many different ion channels, several of which are regulated by phosphoinositides. This review will summarize the knowledge on phosphoinositide signaling in DRG neurons, with special focus on effects on sensory and other ion channels.

Effects of protein crowding on membrane systems

Cellular membranes are typically decorated with a plethora of embedded and adsorbed macromolecules, e.g. proteins, that participate in numerous vital processes. With typical surface densities of 30,000 proteins per μm(2) cellular membranes are indeed crowded places that leave only few nanometers of private space for individual proteins. Here, we review recent advances in our understanding of protein crowding in membrane systems. We first give a brief overview on state-of-the-art approaches in experiment and simulation that are frequently used to study crowded membranes. After that, we review how crowding can affect diffusive transport of proteins and lipids in membrane systems. Next, we discuss lipid and protein sorting in crowded membrane systems, including effects like protein cluster formation, phase segregation, and lipid droplet formation. Subsequently, we highlight recent progress in uncovering crowding-induced conformational changes of membranes, e.g. membrane budding and vesicle formation. Finally, we give a short outlook on potential future developments in the field of crowded membrane systems. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

BRET-monitoring of the dynamic changes of inositol lipid pools in living cells reveals a PKC-dependent PtdIns4P increase upon EGF and M3 receptor activation

Deciphering many roles played by inositol lipids in signal transduction and membrane function demands experimental approaches that can detect their dynamic accumulation with subcellular accuracy and exquisite sensitivity. The former criterion is met by imaging of fluorescence biosensors in living cells, whereas the latter is facilitated by biochemical measurements from populations. Here, we introduce BRET-based biosensors able to detect rapid changes in inositol lipids in cell populations with both high sensitivity and subcellular resolution in a single, convenient assay. We demonstrate robust and sensitive measurements of PtdIns4P, PtdIns(4,5)P2 and PtdIns(3,4,5)P3 dynamics, as well as changes in cytoplasmic Ins(1,4,5)P3 levels. Measurements were made during either experimental activation of lipid degradation, or PI 3-kinase and phospholipase C mediated signal transduction. Our results reveal a previously unappreciated synthesis of PtdIns4P that accompanies moderate activation of phospholipase C signaling downstream of both EGF and muscarinic M3 receptor activation. This signaling-induced PtdIns4P synthesis relies on protein kinase C, and implicates a feedback mechanism in the control of inositol lipid metabolism during signal transduction.

Homo-FRET Based Biosensors and Their Application to Multiplexed Imaging of Signalling Events in Live Cells

Multiplexed imaging of Förster Resonance Energy Transfer (FRET)-based biosensors potentially presents a powerful approach to monitoring the spatio-temporal correlation of signalling pathways within a single live cell. Here, we discuss the potential of homo-FRET based biosensors to facilitate multiplexed imaging. We demonstrate that the homo-FRET between pleckstrin homology domains of Akt (Akt-PH) labelled with mCherry may be used to monitor 3'-phosphoinositide accumulation in live cells and show how global analysis of time resolved fluorescence anisotropy measurements can be used to quantify this accumulation. We further present multiplexed imaging readouts of calcium concentration, using fluorescence lifetime measurements of TN-L15-a CFP/YFP based hetero-FRET calcium biosensor-with 3'-phosphoinositide accumulation.

A Polybasic Plasma Membrane Binding Motif in the I-II Linker Stabilizes Voltage-gated CaV1.2 Calcium Channel Function

L-type voltage-gated Ca(2+) channels (LTCCs) regulate many physiological functions like muscle contraction, hormone secretion, gene expression, and neuronal excitability. Their activity is strictly controlled by various molecular mechanisms. The pore-forming α1-subunit comprises four repeated domains (I-IV), each connected via an intracellular linker. Here we identified a polybasic plasma membrane binding motif, consisting of four arginines, within the I-II linker of all LTCCs. The primary structure of this motif is similar to polybasic clusters known to interact with polyphosphoinositides identified in other ion channels. We used de novo molecular modeling to predict the conformation of this polybasic motif, immunofluorescence microscopy and live cell imaging to investigate the interaction with the plasma membrane, and electrophysiology to study its role for Cav1.2 channel function. According to our models, this polybasic motif of the I-II linker forms a straight α-helix, with the positive charges facing the lipid phosphates of the inner leaflet of the plasma membrane. Membrane binding of the I-II linker could be reversed after phospholipase C activation, causing polyphosphoinositide breakdown, and was accelerated by elevated intracellular Ca(2+) levels. This indicates the involvement of negatively charged phospholipids in the plasma membrane targeting of the linker. Neutralization of four arginine residues eliminated plasma membrane binding. Patch clamp recordings revealed facilitated opening of Cav1.2 channels containing these mutations, weaker inhibition by phospholipase C activation, and reduced expression of channels (as quantified by ON-gating charge) at the plasma membrane. Our data provide new evidence for a membrane binding motif within the I-II linker of LTCC α1-subunits essential for stabilizing normal Ca(2+) channel function.

Differential Regulation of Multiple Steps in Inositol 1,4,5-Trisphosphate Signaling by Protein Kinase C Shapes Hormone-stimulated Ca2+ Oscillations

How Ca(2+) oscillations are generated and fine-tuned to yield versatile downstream responses remains to be elucidated. In hepatocytes, G protein-coupled receptor-linked Ca(2+) oscillations report signal strength via frequency, whereas Ca(2+) spike amplitude and wave velocity remain constant. IP3 uncaging also triggers oscillatory Ca(2+) release, but, in contrast to hormones, Ca(2+) spike amplitude, width, and wave velocity were dependent on [IP3] and were not perturbed by phospholipase C (PLC) inhibition. These data indicate that oscillations elicited by IP3 uncaging are driven by the biphasic regulation of the IP3 receptor by Ca(2+), and, unlike hormone-dependent responses, do not require PLC. Removal of extracellular Ca(2+) did not perturb Ca(2+) oscillations elicited by IP3 uncaging, indicating that reloading of endoplasmic reticulum stores via plasma membrane Ca(2+) influx does not entrain the signal. Activation and inhibition of PKC attenuated hormone-induced Ca(2+) oscillations but had no effect on Ca(2+) increases induced by uncaging IP3. Importantly, PKC activation and inhibition differentially affected Ca(2+) spike frequencies and kinetics. PKC activation amplifies negative feedback loops at the level of G protein-coupled receptor PLC activity and/or IP3 metabolism to attenuate IP3 levels and suppress the generation of Ca(2+) oscillations. Inhibition of PKC relieves negative feedback regulation of IP3 accumulation and, thereby, shifts Ca(2+) oscillations toward sustained responses or dramatically prolonged spikes. PKC down-regulation attenuates phenylephrine-induced Ca(2+) wave velocity, whereas responses to IP3 uncaging are enhanced. The ability to assess Ca(2+) responses in the absence of PLC activity indicates that IP3 receptor modulation by PKC regulates Ca(2+) release and wave velocity.

Phosphatidylinositol-Phosphatidic Acid Exchange by Nir2 at ER-PM Contact Sites Maintains Phosphoinositide Signaling Competence

Sustained agonist-induced production of the second messengers InsP3 and diacylglycerol requires steady delivery of phosphatidylinositol (PtdIns) from its site of synthesis in the ER to the plasma membrane (PM) to maintain PtdIns(4,5)P2 levels. Similarly, phosphatidic acid (PtdOH), generated from diacylglycerol in the PM, has to reach the ER for PtdIns resynthesis. Here, we show that the Drosophila RdgB homolog, Nir2, a presumed PtdIns transfer protein, not only transfers PtdIns from the ER to the PM but also transfers PtdOH to the opposite direction at ER-PM contact sites. PtdOH delivery to the ER is impaired in Nir2-depleted cells, leading to limited PtdIns synthesis and ultimately to loss of signaling from phospholipase C-coupled receptors. These studies reveal a unique feature of Nir2, namely its ability to serve as a highly localized lipid exchanger that ensures that PtdIns synthesis is matched with PtdIns(4,5)P2 utilization so that cells maintain their signaling competence.

Depression of voltage-activated Ca2+ release in skeletal muscle by activation of a voltage-sensing phosphatase

Phosphoinositides act as signaling molecules in numerous cellular transduction processes, and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) regulates the function of several types of plasma membrane ion channels. We investigated the potential role of PtdIns(4,5)P2 in Ca(2+) homeostasis and excitation-contraction (E-C) coupling of mouse muscle fibers using in vivo expression of the voltage-sensing phosphatases (VSPs) Ciona intestinalis VSP (Ci-VSP) or Danio rerio VSP (Dr-VSP). Confocal images of enhanced green fluorescent protein-tagged Dr-VSP revealed a banded pattern consistent with VSP localization within the transverse tubule membrane. Rhod-2 Ca(2+) transients generated by 0.5-s-long voltage-clamp depolarizing pulses sufficient to elicit Ca(2+) release from the sarcoplasmic reticulum (SR) but below the range at which VSPs are activated were unaffected by the presence of the VSPs. However, in Ci-VSP-expressing fibers challenged by 5-s-long depolarizing pulses, the Ca(2+) level late in the pulse (3 s after initiation) was significantly lower at 120 mV than at 20 mV. Furthermore, Ci-VSP-expressing fibers showed a reversible depression of Ca(2+) release during trains, with the peak Ca(2+) transient being reduced by ∼30% after the application of 10 200-ms-long pulses to 100 mV. A similar depression was observed in Dr-VSP-expressing fibers. Cav1.1 Ca(2+) channel-mediated current was unaffected by Ci-VSP activation. In fibers expressing Ci-VSP and a pleckstrin homology domain fused with monomeric red fluorescent protein (PLCδ1PH-mRFP), depolarizing pulses elicited transient changes in mRFP fluorescence consistent with release of transverse tubule-bound PLCδ1PH domain into the cytosol; the voltage sensitivity of these changes was consistent with that of Ci-VSP activation, and recovery occurred with a time constant in the 10-s range. Our results indicate that the PtdIns(4,5)P2 level is tightly maintained in the transverse tubule membrane of the muscle fibers, and that VSP-induced depletion of PtdIns(4,5)P2 impairs voltage-activated Ca(2+) release from the SR. Because Ca(2+) release is thought to be independent from InsP3 signaling, the effect likely results from an interaction between PtdIns(4,5)P2 and a protein partner of the E-C coupling machinery.

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.

Reversible chemical dimerizer-induced recovery of PIP2 levels moves clathrin to the plasma membrane

Chemical dimerizers are powerful non-invasive tools for bringing molecules together inside intact cells. We recently introduced a rapidly reversible chemical dimerizer system which enables transient translocation of enzymes to and from the plasma membrane (PM). Here we have applied this system to transiently activate phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown at the PM via translocation of phosphoinositide 5-phosphatase (5Ptase). We found that the PIP2 sensor phospholipase C-δ PH domain (PLCδ-PH) is released from the PM upon addition of the reversible chemical dimerizer rCD1. By outcompeting rCD1, rapid release of the 5Ptase from the PM is followed by PIP2 recovery. This permits the observation of the PIP2-dependent clathrin assembly at the PM.