A novel ZAP-70 dependent FRET based biosensor reveals kinase activity at both the immunological synapse and the antisynapse

Molecular Spies in Action: Genetically Encoded Fluorescent Biosensors Light up Cellular Signals

Cellular function is controlled through intricate networks of signals, which lead to the myriad pathways governing cell fate. Fluorescent biosensors have enabled the study of these signaling pathways in living systems across temporal and spatial scales. Over the years there has been an explosion in the number of fluorescent biosensors, as they have become available for numerous targets, utilized across spectral space, and suited for various imaging techniques. To guide users through this extensive biosensor landscape, we discuss critical aspects of fluorescent proteins for consideration in biosensor development, smart tagging strategies, and the historical and recent biosensors of various types, grouped by target, and with a focus on the design and recent applications of these sensors in living systems.

Optical sensing and control of T cell signaling pathways

T cells regulate adaptive immune responses through complex signaling pathways mediated by T cell receptor (TCR). The functional domains of the TCR are combined with specific antibodies for the development of chimeric antigen receptor (CAR) T cell therapy. In this review, we first overview current understanding on the T cell signaling pathways as well as traditional methods that have been widely used for the T cell study. These methods, however, are still limited to investigating dynamic molecular events with spatiotemporal resolutions. Therefore, genetically encoded biosensors and optogenetic tools have been developed to study dynamic T cell signaling pathways in live cells. We review these cutting-edge technologies that revealed dynamic and complex molecular mechanisms at each stage of T cell signaling pathways. They have been primarily applied to the study of dynamic molecular events in TCR signaling, and they will further aid in understanding the mechanisms of CAR activation and function. Therefore, genetically encoded biosensors and optogenetic tools offer powerful tools for enhancing our understanding of signaling mechanisms in T cells and CAR-T cells.

Neo-Antigen-Reactive T Cells Immunotherapy for Colorectal Cancer: A More Personalized Cancer Therapy Approach

Colorectal cancer (CRC) is the second most common malignancy in women and the third most frequent cancer in men. Evidence has revealed that the survival of patients with metastatic CRC is very low, between one and three years. Neoantigens are known proteins encoded by mutations in tumor cells. It is theorized that recognizing neoantigens by T cells leads to T cell activation and further antitumor responses. Neoantigen-reactive T cells (NRTs) are designed against the mentioned neoantigens expressed by tumor cells. NRTs selectively kill tumor cells without damage to non-cancerous cells. Identifying patient-specific and high immunogen neoantigens is important in NRT immunotherapy of patients with CRC. However, the main challenges are the side effects and preparation of NRTs, as well as the effectiveness of these cells in vivo. This review summarized the properties of neoantigens as well as the preparation and therapeutic outcomes of NRTs for the treatment of CRC.

Neoantigen-reactive T cell: An emerging role in adoptive cellular immunotherapy

Adoptive cellular immunotherapy harnessing the intrinsic immune system for precise treatment has exhibited preliminary success against malignant tumors. As one of the emerging roles in adoptive cellular immunotherapy, neoantigen-reactive T cell (NRT) focuses on the antigens expressed only by tumor cells. It exclusively obliterates tumor and spares normal tissues, achieving more satisfying effects. However, the development of NRT immunotherapy remains in a relatively primitive stage. Current challenges include identification of NRTs and maintenance of adoptive cell efficacy in vivo. The possible side effects and other limitations of this treatment also hinder its application. Here, we present an overview of NRT immunotherapy and discuss the progress and challenges as well as the prospects in this promising field.

Integration of FRET and sequencing to engineer kinase biosensors from mammalian cell libraries

The limited sensitivity of Förster Resonance Energy Transfer (FRET) biosensors hinders their broader applications. Here, we develop an approach integrating high-throughput FRET sorting and next-generation sequencing (FRET-Seq) to identify sensitive biosensors with varying substrate sequences from large-scale libraries directly in mammalian cells, utilizing the design of self-activating FRET (saFRET) biosensor. The resulting biosensors of Fyn and ZAP70 kinases exhibit enhanced performance and enable the dynamic imaging of T-cell activation mediated by T cell receptor (TCR) or chimeric antigen receptor (CAR), revealing a highly organized ZAP70 subcellular activity pattern upon TCR but not CAR engagement. The ZAP70 biosensor elucidates the role of immunoreceptor tyrosine-based activation motif (ITAM) in affecting ZAP70 activation to regulate CAR functions. A saFRET biosensor-based high-throughput drug screening (saFRET-HTDS) assay further enables the identification of an FDA-approved cancer drug, Sunitinib, that can be repurposed to inhibit ZAP70 activity and autoimmune-disease-related T-cell activation.

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.

Bioengineering tools for probing intracellular events in T lymphocytes

T lymphocytes are the central coordinator and executor of many immune functions. The activation and function of T lymphocytes are mediated through the engagement of cell surface receptors and regulated by a myriad of intracellular signaling network. Bioengineering tools, including imaging modalities and fluorescent probes, have been developed and employed to elucidate the cellular events throughout the functional lifespan of T cells. A better understanding of these events can broaden our knowledge in the immune systems biology, as well as accelerate the development of effective diagnostics and immunotherapies. Here we review the commonly used and recently developed techniques and probes for monitoring T lymphocyte intracellular events, following the order of intracellular events in T cells from activation, signaling, metabolism to apoptosis. The techniques introduced here can be broadly applied to other immune cells and cell systems. This article is categorized under: Immune System Diseases > Molecular and Cellular Physiology Immune System Diseases > Biomedical Engineering Infectious Diseases > Biomedical Engineering.

Dynamic Mitochondrial Migratory Features Associated with Calcium Responses during T Cell Antigen Recognition

A T cell clone is able to distinguish Ags in the form of peptide-MHC complexes with high specificity and sensitivity; however, how subtle differences in peptide-MHC structures translate to distinct T cell effector functions remains unknown. We hypothesized that mitochondrial positioning and associated calcium responses play an important role in T cell Ag recognition. We engineered a microfluidic system to precisely manipulate and synchronize a large number of cell-cell pairing events, which provided simultaneous real-time signaling imaging and organelle tracking with temporal precision. In addition, we developed image-derived metrics to quantify calcium response and mitochondria movement. Using myelin proteolipid altered peptide ligands and a hybridoma T cell line derived from a mouse model of experimental autoimmune encephalomyelitis, we observed that Ag potency modulates calcium response at the single-cell level. We further developed a partial least squares regression model, which highlighted mitochondrial positioning as a strong predictor of calcium response. The model revealed T cell mitochondria sharply alter direction within minutes following exposure to agonist peptide Ag, changing from accumulation at the immunological synapse to retrograde movement toward the distal end of the T cell body. By quantifying mitochondria movement as a highly dynamic process with rapidly changing phases, our result reconciles conflicting prior reports of mitochondria positioning during T cell Ag recognition. We envision applying this pipeline of methodology to study cell interactions between other immune cell types to reveal important signaling phenomenon that is inaccessible because of data-limited experimental design.

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.

Context-dependent intravital imaging of therapeutic response using intramolecular FRET biosensors

Intravital microscopy represents a more physiologically relevant method for assessing therapeutic response. However, the movement into an in vivo setting brings with it several additional considerations, the primary being the context in which drug activity is assessed. Microenvironmental factors, such as hypoxia, pH, fibrosis, immune infiltration and stromal interactions have all been shown to have pronounced effects on drug activity in a more complex setting, which is often lost in simpler two- or three-dimensional assays. Here we present a practical guide for the application of intravital microscopy, looking at the available fluorescent reporters and their respective expression systems and analysis considerations. Moving in vivo, we also discuss the microscopy set up and methods available for overlaying microenvironmental context to the experimental readouts. This enables a smooth transition into applying higher fidelity intravital imaging to improve the drug discovery process.

Imaging Spatiotemporal Activities of ZAP-70 in Live T Cells Using a FRET-Based Biosensor

The zeta-chain-associated protein kinase 70 kDa (ZAP-70), a member of the spleen tyrosine kinase (Syk) family, plays an essential role in early T cell receptor (TCR) signaling. Defects in ZAP-70 lead to impaired thymocyte development and peripheral T cell activation. To better understand its activation dynamics and regulation, we visualized ZAP-70 activities in single live T cells with a Förster resonance energy transfer (FRET)-based biosensor, which was designed for probing kinase activities of the Syk family. We observed in Jurkat E6.1 T cells rapid and specific FRET changes following anti-CD3 stimulation and subsequent piceatannol inhibition. The initiation of ZAP-70 activation was prompt (within 10 s) and correlates with the accompanied intracellular calcium elevation, as revealed by simultaneous imaging of the biosensor and calcium. Different from the previously reported ZAP-70 activation in the immunological synapse and the opposite pole (anti-synapse), we have observed rapid and sustained ZAP-70 activation only at the synapse with superantigen-pulsed Raji B cells. Furthermore, ZAP-70 signaling was impaired by cholesterol depletion, further supporting the importance of membrane organization in TCR signaling. Together our results provide a direct characterization of the spatiotemporal features of ZAP-70 activity in real time at subcellular levels.

T cell adhesion triggers an early signaling pole distal to the immune synapse

The immunological synapse forms at the interface between a T cell and an antigen-presenting cell after foreign antigen recognition. The immunological synapse is considered to be the site where the signaling cascade leading to T lymphocyte activation is triggered. Here, we show that another signaling region can be detected before formation of the synapse at the opposite pole of the T cell. This structure appears during the first minute after the contact forms, is transient and contains all the classic components that have been previously described at the immunological synapse. Its formation is independent of antigen recognition but is driven by adhesion itself. It constitutes a reservoir of signaling molecules that are potentially ready to be sent to the immunological synapse through a microtubule-dependent pathway. The antisynapse can thus be considered as a pre-synapse that is triggered independently of antigen recognition.

Early T cell activation: integrating biochemical, structural, and biophysical cues

T cells carry out the formidable task of identifying small numbers of foreign antigenic peptides rapidly and specifically against a very noisy environmental background of endogenous self-peptides. Early steps in T cell activation have thus fascinated biologists and are among the best-studied models of cell stimulation. This remarkable process, critical in adaptive immune responses, approaches and even seems to exceed the limitations set by the physical laws ruling molecular behavior. Despite the enormous amount of information concerning the nature of molecules involved in the T cell antigen receptor (TCR) signal transduction network, and the description of the nanoscale organization and real-time analysis of T cell responses, the general principles of information gathering and processing remain incompletely understood. Here we review currently accepted key data on TCR function, discuss the limitations of current research strategies, and suggest a novel model of TCR triggering and a few promising ways of going further into the integration of available data.

Fluorescent Reporters and Biosensors for Probing the Dynamic Behavior of Protein Kinases

Probing the dynamic activities of protein kinases in real-time in living cells constitutes a major challenge that requires specific and sensitive tools tailored to meet the particular demands associated with cellular imaging. The development of genetically-encoded and synthetic fluorescent biosensors has provided means of monitoring protein kinase activities in a non-invasive fashion in their native cellular environment with high spatial and temporal resolution. Here, we review existing technologies to probe different dynamic features of protein kinases and discuss limitations where new developments are required to implement more performant tools, in particular with respect to infrared and near-infrared fluorescent probes and strategies which enable improved signal-to-noise ratio and controlled activation of probes.

Synchronizing atomic force microscopy force mode and fluorescence microscopy in real time for immune cell stimulation and activation studies

A method is presented for combining atomic force microscopy (AFM) force mode and fluorescence microscopy in order to (a) mechanically stimulate immune cells while recording the subsequent activation under the form of calcium pulses, and (b) observe the mechanical response of a cell upon photoactivation of a small G protein, namely Rac. Using commercial set-ups and a robust signal coupling the fluorescence excitation light and the cantilever bending, the applied force and activation signals were very easily synchronized. This approach allows to control the entire mechanical history of a single cell up to its activation and response down to a few hundreds of milliseconds, and can be extended with very minimal adaptations to other cellular systems where mechanotransduction is studied, using either purely mechanical stimuli or via a surface bound specific ligand.

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.

Biosensor architectures for high-fidelity reporting of cellular signaling

Understanding mechanisms of information processing in cellular signaling networks requires quantitative measurements of protein activities in living cells. Biosensors are molecular probes that have been developed to directly track the activity of specific signaling proteins and their use is revolutionizing our understanding of signal transduction. The use of biosensors relies on the assumption that their activity is linearly proportional to the activity of the signaling protein they have been engineered to track. We use mechanistic mathematical models of common biosensor architectures (single-chain FRET-based biosensors), which include both intramolecular and intermolecular reactions, to study the validity of the linearity assumption. As a result of the classic mechanism of zero-order ultrasensitivity, we find that biosensor activity can be highly nonlinear so that small changes in signaling protein activity can give rise to large changes in biosensor activity and vice versa. This nonlinearity is abolished in architectures that favor the formation of biosensor oligomers, but oligomeric biosensors produce complicated FRET states. Based on this finding, we show that high-fidelity reporting is possible when a single-chain intermolecular biosensor is used that cannot undergo intramolecular reactions and is restricted to forming dimers. We provide phase diagrams that compare various trade-offs, including observer effects, which further highlight the utility of biosensor architectures that favor intermolecular over intramolecular binding. We discuss challenges in calibrating and constructing biosensors and highlight the utility of mathematical models in designing novel probes for cellular signaling.

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.

Modulation of T cell signaling by the actin cytoskeleton

The actin cytoskeleton provides a dynamic framework to support membrane organization and cellular signaling events. The importance of actin in T cell function has long been recognized to go well beyond the maintenance of cell morphology and transport of proteins. Over the past several years, our understanding of actin in T cell activation has expanded tremendously, in part owing to the development of methods and techniques to probe the complex interplay between actin and T cell signaling. On the one hand, biochemical methods have led to the identification of many key cytoskeleton regulators and new signaling pathways, whereas, on the other, the combination of advanced imaging techniques and physical characterization tools has allowed the spatiotemporal investigation of actin in T cell signaling. All those studies have made a profound impact on our understanding of the actin cytoskeleton in T cell activation. Many previous reviews have focused on the biochemical aspects of the actin cytoskeleton. However, here we will summarize recent studies from a biophysical perspective to explain the mechanistic role of actin in modulating T cell activation. We will discuss how actin modulates T cell activation on multiple time and length scales. Specifically, we will reveal the distinct roles of the actin filaments in facilitating TCR triggering, orchestrating 'signalosome' assembly and transport, and establishing protein spatial organization in the immunological synapse.

Imaging early signaling events in T lymphocytes with fluorescent biosensors

Many recent advances in our understanding of T lymphocyte functions in adaptive immunity are derived from sophisticated imaging techniques used to visualize T lymphocyte behavior in vitro and in vivo. A current challenge is to couple such imaging techniques with methods that will allow researchers to visualize signaling phenomenon at the single-cell level. Fluorescent biosensors, either synthetic or genetically encoded, are emerging as important tools for revealing the spatio-temporal regulation of intracellular biochemical events, such as specific enzyme activities or fluctuations in metabolites. In this review, we revisit the development of fluorescent Ca(2+) sensors with which the first experiments visualizing T lymphocyte activation at the single-cell were performed, and which have since become routine tools in immunology. We then examine a number of examples of how fluorescence resonance energy transfer (FRET)-based biosensors have been developed and applied to T lymphocyte migration, adhesion and T-cell receptor (TCR)-mediated signal transduction. These include the study of small GTPases such as RhoA, Rac and Rap1, the tyrosine kinases Lck and ZAP-70, and metabolites such as cAMP and Ca(2+) . Future development and use of biosensors should allow immunologists to reconcile the vast wealth of existing biochemical data concerning T-cell functions with the power of dynamic live-cell imaging.

Application of FRET probes in the analysis of neuronal plasticity

Breakthroughs in imaging techniques and optical probes in recent years have revolutionized the field of life sciences in ways that traditional methods could never match. The spatial and temporal regulation of molecular events can now be studied with great precision. There have been several key discoveries that have made this possible. Since green fluorescent protein (GFP) was cloned in 1992, it has become the dominant tracer of proteins in living cells. Then the evolution of color variants of GFP opened the door to the application of Förster resonance energy transfer (FRET), which is now widely recognized as a powerful tool to study complicated signal transduction events and interactions between molecules. Employment of fluorescent lifetime imaging microscopy (FLIM) allows the precise detection of FRET in small subcellular structures such as dendritic spines. In this review, we provide an overview of the basic and practical aspects of FRET imaging and discuss how different FRET probes have revealed insights into the molecular mechanisms of synaptic plasticity and enabled visualization of neuronal network activity both in vitro and in vivo.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs. Early on it was recognized that Ras activation is critical for T cell development and that the RasGEFs play an important role herein. More recent work has revealed that nuances in Ras activation appear to significantly impact T cell development and selection. These nuances include distinct biochemical patterns of analog versus digital Ras activation, differences in cellular localization of Ras activation, and intricate interplays between the RasGEFs during distinct T cell developmental stages as revealed by various new mouse models. In many instances, the exact nature of these nuances in Ras activation or how these may result from fine-tuning of the RasGEFs is not understood. One large group of biomolecules critically involved in the control of RasGEFs functions are lipid second messengers. Multiple, yet distinct lipid products are generated following T cell receptor (TCR) stimulation and bind to different domains in the RasGRP and SOS RasGEFs to facilitate the activation of the membrane-anchored Ras GTPases. In this review we highlight how different lipid-based elements are generated by various enzymes downstream of the TCR and other receptors and how these dynamic and interrelated lipid products may fine-tune Ras activation by RasGEFs in developing T cells.

Microspectroscopy reveals mechanisms of lymphocyte activation

The immunological synapse (IS) regulates immune responses by integrating extracellular stimuli into intracellular signalling networks, which causes leukocyte differentiation and effector functions. The dynamic spatial organisation of molecules at the IS was initially characterised by wide-field fluorescence microscopy of cell conjugates and cells interacting with planar lipid bilayers. These methods showed stable supramolecular clusters of several microns in size, which were proposed to be responsible for sustained signalling and cell-cell adhesion. The recent emergence of microspectroscopy techniques with higher spatial and temporal resolution nonetheless reveals the complex dynamics of molecular reactions that mediate IS assembly and function. This review describes microspectroscopy-based in vitro experimental approaches for imaging the molecular dynamics at the IS, as well as their contributions and open questions in the field. It also describes experimental methods to obtain quantitative parameters of dynamic biochemical reactions in living cells, and discusses about the important role of quantitative imaging and theoretical science in our understanding of molecular mechanisms underlying lymphocyte activation.

Murine Tim-1 is excluded from the immunological synapse

The interaction between T cells and APCs bearing cognate antigen results in the formation of an immunological synapse (IS). During this process, many receptors and signaling proteins segregate to regions proximal to the synapse. This protein movement is thought to influence T cell function. However, some proteins are transported away from the IS, which is controlled in part by ERM family proteins. Tim-1 is a transmembrane protein with co-stimulatory functions that is found on many immune cells, including T cells. However, the expression pattern of Tim-1 on T cells upon activation by APCs has not been explored. Interestingly, in this study we demonstrate that the majority of Tim-1 on activated T cells is excluded from the IS. Tim-1 predominantly resides outside of the IS, and structure/function studies indicate that the cytoplasmic tail influences Tim-1 polarization. Specifically, a putative ERM binding motif (KRK 244-246) in the Tim-1 cytoplasmic tail appears necessary for proper Tim-1 localization. Furthermore, mutation of the KRK motif results in enhanced early tyrosine phosphorylation downstream of TCR/CD28 stimulation upon ectopic expression of Tim-1. Paradoxically however, the KRK motif is necessary for Tim-1 co-stimulation of NFAT/AP-1 activation and co-stimulation of cytokine production. This work reveals unexpected complexity underlying Tim-1 localization and suggests potentially novel mechanisms by which Tim-1 modulates T cell activity.

A FRET-Based Biosensor for Imaging SYK Activities in Living Cells

Spleen tyrosine kinase (SYK) is crucial to cellular functions mediated by immunoreceptors and integrins. We have developed and characterized a new genetically-encoded Förster resonance energy transfer (FRET)-based biosensor for studying the dynamics of SYK activities in living cells at a subcellular level. It contains an N-terminal ECFP, SH2 domain, a peptide derived from a SYK substrate VAV2, and a C-terminal YPet. Upon the specific phosphorylation by SYK in vitro, the biosensor substrate peptide bound to the intramolecular SH2 domain to reduce the FRET efficiency. Transfection of the biosensor did not affect activation of the endogenous SYK in host cells. Phosphorylation of the biosensor followed the same kinetics as the endogenous VAV2. Using FRET imaging and ratiometric analysis with this SYK biosensor, we visualized and quantified the realtime activation of SYK in K562 cells upon IgG Fc engagement of Fcc receptor IIA and in mouse embryonic fibroblasts upon stimulation by the platelet derived growth factor. These results demonstrate our biosensor as a powerful tool for studying cellular signaling that involves SYK.

Imaging techniques for assaying lymphocyte activation in action

Imaging techniques have greatly improved our understanding of lymphocyte activation. Technical advances in spatial and temporal resolution and new labelling tools have enabled researchers to directly observe the activation process. Consequently, research using imaging approaches to study lymphocyte activation has expanded, providing an unprecedented level of cellular and molecular detail in the field. As a result, certain models of lymphocyte activation have been verified, others have been revised and yet others have been replaced with new concepts. In this article, we review the current imaging techniques that are used to assess lymphocyte activation in different contexts, from whole animals to single molecules, and discuss the advantages and potential limitations of these methods.

Early contacts between T lymphocytes and activating surfaces

Cells continually probe their environment to adapt their behaviour. A current challenge is to determine how they analyse nearby surfaces and how they process information to take decisions. We addressed this problem by monitoring human T lymphocyte attachment to surfaces coated with activating anti-CD3 or control anti-HLA antibodies. Interference reflection microscopy allowed us to monitor cell-to-surface apposition with a few nanometre vertical resolution during the first minutes following contact. We found that (i) when a cell fell on a surface, contact extension was preceded by a lag of several tens of seconds. (ii) During this lag, vertical membrane undulations seemed to generate transient contacts with underlying surfaces. (iii) After the lag period, the contact area started increasing linearly with a rate of about 1.5 µm(2) s(-1) on activating surfaces and about 0.2 µm(2) s(-1) on control surfaces. (iv) Concomitantly with lateral surface extension, the apparent distance between cell membranes and surfaces steadily decreased. These results are consistent with the hypothesis that the cell decision to spread rapidly on activating surfaces resulted from the integration of information yielded by transient contacts with these surfaces generated by membrane undulations during a period of about 1 min.

Dynamic visualization of signal transduction in living cells: from second messengers to kinases

The study of signal transduction, or the highly regulated series of biochemical events which allow a cell to convert a given stimulus into a functional response, has seen a paradigm shift with a recent explosion in the number of genetically encoded FRET-based biosensors capable of detecting spatial and temporal regulation of various signaling events in living cells. The two classes of biosensors discussed, namely kinase activity and second messenger biosensors, utilize two fluorescent proteins (FP) suitable for FRET and convert a signaling event of interest into a conformational change in the biosensor that can be measured as a change in FRET between the two FPs. Individually, these biosensors have been used to elucidate many complex signal transduction mechanisms in various biological systems. However, it has become increasingly clear that it is often more desirable to study multiple signaling events simultaneously, allowing for precise correlation of the temporal profiles of multiple signaling molecules without the complication of cell to cell variability. With the design of spectrally distinct biosensors and new coimaging strategies, simultaneous imaging of multiple signaling events is not only possible, but has aided in mapping the intricate network of cellular signal transduction cascades. Furthermore, as aberrant signal transduction involving second messengers and kinases is implicated in numerous disease states, it is hopeful that these FRET-based biosensors and coimaging strategies can help to unravel the molecular links between altered signal transduction and certain disease states.

Do membrane undulations help cells probe the world?

Cells sense physical properties of their environment including substratum rigidity, roughness, and topography of recognition sites. The cell surface displays continuous deformations of nanometer-scale amplitude and Hz frequency. Recent results support the hypothesis that these surface undulations constitute a powerful strategy for the rapid acquisition of environmental cues: transient contact with surroundings generates forces of piconewton intensity as a result of rapid formation and dissociation of intermolecular bonds. The combination of binding and steric forces is expected to drive conformational changes and lateral reorganization of membrane biomolecules, thus generating signaling cascades. We propose that spontaneous membrane mobility shapes the initial information generated by cell-to-surface contacts, and thereby biases later consequences of these interactions.