Single-molecule microscopy reveals plasma membrane microdomains created by protein-protein networks that exclude or trap signaling molecules in T cells

Investigations of membrane protein interactions in cells using fluorescence microscopy

The interactions between proteins in membranes govern many cellular functions. Our ability to probe for such interactions has greatly evolved in recent years due to the introduction of new fluorescence techniques. As a result, we currently have a choice of methods that can be used to assess the spatial distribution of a membrane protein, its association state, and the thermodynamic stability of the oligomers in the native milieu. These biophysical measurements have revealed new insights into important biological processes in cellular membranes.

Redistribution of the glycocalyx exposes phagocytic determinants on apoptotic cells

Phagocytes remove dead and dying cells by engaging "eat-me" ligands such as phosphatidylserine (PtdSer) on the surface of apoptotic targets. However, PtdSer is obscured by the bulky exofacial glycocalyx, which also exposes ligands that activate "don't-eat-me" receptors such as Siglecs. Clearly, unshielding the juxtamembrane "eat-me" ligands is required for the successful engulfment of apoptotic cells, but the mechanisms underlying this process have not been described. Using human and murine cells, we find that apoptosis-induced retraction and weakening of the cytoskeleton that anchors transmembrane proteins cause an inhomogeneous redistribution of the glycocalyx: actin-depleted blebs emerge, lacking the glycocalyx, while the rest of the apoptotic cell body retains sufficient actin to tether the glycocalyx in place. Thus, apoptotic blebs can be engaged by phagocytes and are targeted for engulfment. Therefore, in cells with an elaborate glycocalyx, such as mucinous cancer cells, this "don't-come-close-to-me" barrier must be removed to enable clearance by phagocytosis.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

A combined biochemical and cellular approach reveals Zn2+-dependent hetero- and homodimeric CD4 and Lck assemblies in T cells

The CD4 or CD8 co-receptors' interaction with the protein-tyrosine kinase Lck initiates the tyrosine phosphorylation cascade leading to T cell activation. A critical question is: to what extent are co-receptors and Lck coupled? Our contribution concerns Zn2+, indispensable for CD4- and CD8-Lck formation. We combined biochemical and cellular approaches to show that dynamic fluctuations of free Zn2+ in physiological ranges influence Zn(CD4)2 and Zn(CD4)(Lck) species formation and their ratio, although the same Zn(Cys)2(Cys)2 cores. Moreover, we demonstrated that the affinity of Zn2+ to CD4 and CD4-Lck species differs significantly. Increased intracellular free Zn2+ concentration in T cells causes higher CD4 partitioning in the plasma membrane. We additionally found that CD4 palmitoylation decreases the specificity of CD4-Lck formation in the reconstituted membrane model. Our findings help elucidate co-receptor-Lck coupling stoichiometry and demonstrate that intracellular free Zn2+ has a major role in the interplay between CD4 dimers and CD4-Lck assembly.

Mechanical forces amplify TCR mechanotransduction in T cell activation and function

Adoptive T cell immunotherapies, including engineered T cell receptor (eTCR) and chimeric antigen receptor (CAR) T cell immunotherapies, have shown efficacy in treating a subset of hematologic malignancies, exhibit promise in solid tumors, and have many other potential applications, such as in fibrosis, autoimmunity, and regenerative medicine. While immunoengineering has focused on designing biomaterials to present biochemical cues to manipulate T cells ex vivo and in vivo, mechanical cues that regulate their biology have been largely underappreciated. This review highlights the contributions of mechanical force to several receptor-ligand interactions critical to T cell function, with central focus on the TCR-peptide-loaded major histocompatibility complex (pMHC). We then emphasize the role of mechanical forces in (i) allosteric strengthening of the TCR-pMHC interaction in amplifying ligand discrimination during T cell antigen recognition prior to activation and (ii) T cell interactions with the extracellular matrix. We then describe approaches to design eTCRs, CARs, and biomaterials to exploit TCR mechanosensitivity in order to potentiate T cell manufacturing and function in adoptive T cell immunotherapy.

Simple visualization of submicroscopic protein clusters with a phase-separation-based fluorescent reporter

Protein clustering plays numerous roles in cell physiology and disease. However, protein oligomers can be difficult to detect because they are often too small to appear as puncta in conventional fluorescence microscopy. Here, we describe a fluorescent reporter strategy that detects protein clusters with high sensitivity called CluMPS (clusters magnified by phase separation). A CluMPS reporter detects and visually amplifies even small clusters of a binding partner, generating large, quantifiable fluorescence condensates. We use computational modeling and optogenetic clustering to demonstrate that CluMPS can detect small oligomers and behaves rationally according to key system parameters. CluMPS detected small aggregates of pathological proteins where the corresponding GFP fusions appeared diffuse. CluMPS also detected and tracked clusters of unmodified and tagged endogenous proteins, and orthogonal CluMPS probes could be multiplexed in cells. CluMPS provides a powerful yet straightforward approach to observe higher-order protein assembly in its native cellular context. A record of this paper's transparent peer review process is included in the supplemental information.

Syntaxin clusters and cholesterol affect the mobility of Syntaxin1a

Syntaxin1a (Syx1a) is essential for stimulated exocytosis in neuroendocrine cells. The vesicle docking process involves the formation of nanoscale Syx1a domains on the plasma membrane and the Syx1a clusters disintegrate during the fusion process. Syx1a nanodomains are static yet Syx1a molecules dynamically enter and leave the domains; the process by which these clusters maintain this balance is unclear. In this work, the dynamics of the Syx1a molecules is elucidated relative to the cluster position through a labeling strategy that allows both the bulk position of the Syx clusters to be visualized concurrent with the trajectories of single Syx1a molecules on the surface of PC12 cells. Single Syx1a molecules were tracked in time relative to cluster positions to decipher how Syx1a moves within a cluster and when clusters are not present. Syx1a is mobile on the plasma membrane, more mobile at the center of clusters, and less mobile near the edges of clusters; this depends on the presence of the N-terminal Habc domain and cholesterol, which are essential for proper exocytosis. Simulations of the dynamics observed at clusters support a model where clusters are maintained by a large cage (r = 100 nm) within which Syx1a remains highly mobile within the cluster (r = 50 nm). The depletion of cholesterol dramatically reduces the mobility of Syx1a within clusters and less so over the rest of the plasma membrane. This suggests that fluidity of Syx1a supramolecular clusters is needed for function.

A partial human LCK defect causes a T cell immunodeficiency with intestinal inflammation

Lymphocyte-specific protein tyrosine kinase (LCK) is essential for T cell antigen receptor (TCR)-mediated signal transduction. Here, we report two siblings homozygous for a novel LCK variant (c.1318C>T; P440S) characterized by T cell lymphopenia with skewed memory phenotype, infant-onset recurrent infections, failure to thrive, and protracted diarrhea. The patients' T cells show residual TCR signal transduction and proliferation following anti-CD3/CD28 and phytohemagglutinin (PHA) stimulation. We demonstrate in mouse models that complete (Lck-/-) versus partial (LckP440S/P440S) loss-of-function LCK causes disease with differing phenotypes. While both Lck-/- and LckP440S/P440S mice exhibit arrested thymic T cell development and profound T cell lymphopenia, only LckP440S/P440S mice show residual T cell proliferation, cytokine production, and intestinal inflammation. Furthermore, the intestinal disease in the LckP440S/P440S mice is prevented by CD4+ T cell depletion or regulatory T cell transfer. These findings demonstrate that P440S LCK spares sufficient T cell function to allow the maturation of some conventional T cells but not regulatory T cells-leading to intestinal inflammation.

The Membrane Phase Transition Gives Rise to Responsive Plasma Membrane Structure and Function

Several groups have recently reported evidence for the emergence of domains in cell plasma membranes when membrane proteins are organized by ligand binding or assembly of membrane proximal scaffolds. These domains recruit and retain components that favor the liquid-ordered phase, adding to a decades-old literature interrogating the contribution of membrane phase separation in plasma membrane organization and function. Here we propose that both past and present observations are consistent with a model in which membranes have a high compositional susceptibility, arising from their thermodynamic state in a single phase that is close to a miscibility phase transition. This rigorous framework naturally allows for both transient structure in the form of composition fluctuations and long-lived structure in the form of induced domains. In this way, the biological tuning of plasma membrane composition enables a responsive compositional landscape that facilitates and augments cellular biochemistry vital to plasma membrane functions.

Recent advances in understanding TCR signaling: a synaptic perspective

The T cell receptor is a multi-subunit complex that carries out a range of recognition tasks for multiple lymphocyte types and translates recognition into signals that regulate survival, growth, differentiation, and effector functions for innate and adaptive host defense. Recent advances include the cryo-electron microscopy-based structure of the extracellular and transmembrane components of the complex, new information about coupling to intracellular partners, lateral associations in the membrane that all add to our picture of the T cell signaling machinery, and how signal termination relates to effector function. This review endeavors to integrate structural and biochemical information through the lens of the immunological synapse- the critical interface with the antigen-presenting cell.

Two-dimensional molecular condensation in cell signaling and mechanosensing

Membraneless organelles (MLO) regulate diverse biological processes in a spatiotemporally controlled manner spanning from inside to outside of the cells. The plasma membrane (PM) at the cell surface serves as a central platform for forming multi-component signaling hubs that sense mechanical and chemical cues during physiological and pathological conditions. During signal transduction, the assembly and formation of membrane-bound MLO are dynamically tunable depending on the physicochemical properties of the surrounding environment and partitioning biomolecules. Biomechanical properties of MLO-associated membrane structures can control the microenvironment for biomolecular interactions and assembly. Lipid-protein complex interactions determine the catalytic region's assembly pattern and assembly rate and, thereby, the amplitude of activities. In this review, we will focus on how cell surface microenvironments, including membrane curvature, surface topology and tension, lipid-phase separation, and adhesion force, guide the assembly of PM-associated MLO for cell signal transductions.

Structure and mechanism of immunoreceptors: New horizons in T cell and B cell receptor biology and beyond

Immunoreceptors, also named non-catalytic tyrosine-phosphorylated receptors, are a large class of leukocyte cell-surface proteins critically involved in innate and adaptive immune responses. Their most characteristic defining feature is a shared signal transduction machinery where binding events of cell surface-anchored ligands to the small extracellular receptor domains are translated into phosphorylation of conserved tyrosine-containing cytosolic sequence motifs initiating downstream signal transduction cascades. Despite their central importance to immunology, the molecular mechanism of how ligand binding activates the receptors and results in robust intracellular signaling has remained enigmatic. Recent breakthroughs in our understanding of the architecture and triggering mechanism of immunoreceptors come from cryogenic electron microscopy studies of the B cell and T cell antigen receptors.

Dimensional Reduction for Single-Molecule Imaging of DNA and Nucleosome Condensation by Polyamines, HP1α and Ki-67

Macromolecules organize themselves into discrete membrane-less compartments. Mounting evidence has suggested that nucleosomes as well as DNA itself can undergo clustering or condensation to regulate genomic activity. Current in vitro condensation studies provide insight into the physical properties of condensates, such as surface tension and diffusion. However, methods that provide the resolution needed for complex kinetic studies of multicomponent condensation are desired. Here, we use a supported lipid bilayer platform in tandem with total internal reflection microscopy to observe the two-dimensional movement of DNA and nucleosomes at the single-molecule resolution. This dimensional reduction from three-dimensional studies allows us to observe the initial condensation events and dissolution of these early condensates in the presence of physiological condensing agents. Using polyamines, we observed that the initial condensation happens on a time scale of minutes while dissolution occurs within seconds upon charge inversion. Polyamine valency, DNA length, and GC content affect the threshold polyamine concentration for condensation. Protein-based nucleosome condensing agents, HP1α and Ki-67, have much lower threshold concentrations for condensation than charge-based condensing agents, with Ki-67 being the most effective, requiring as low as 100 pM for nucleosome condensation. In addition, we did not observe condensate dissolution even at the highest concentrations of HP1α and Ki-67 tested. We also introduce a two-color imaging scheme where nucleosomes of high density labeled in one color are used to demarcate condensate boundaries and identical nucleosomes of another color at low density can be tracked relative to the boundaries after Ki-67-mediated condensation. Our platform should enable the ultimate resolution of single molecules in condensation dynamics studies of chromatin components under defined physicochemical conditions.

TRIM24 controls induction of latent HIV-1 by stimulating transcriptional elongation

Binding of USF1/2 and TFII-I (RBF-2) at conserved sites flanking the HIV-1 LTR enhancer is essential for reactivation from latency in T cells, with TFII-I knockdown rendering the provirus insensitive to T cell signaling. We identified an interaction of TFII-I with the tripartite motif protein TRIM24, and these factors were found to be constitutively associated with the HIV-1 LTR. Similar to the effect of TFII-I depletion, loss of TRIM24 impaired reactivation of HIV-1 in response to T cell signaling. TRIM24 deficiency did not affect recruitment of RNA Pol II to the LTR promoter, but inhibited transcriptional elongation, an effect that was associated with decreased RNA Pol II CTD S2 phosphorylation and impaired recruitment of CDK9. A considerable number of genomic loci are co-occupied by TRIM24/TFII-I, and we found that TRIM24 deletion caused altered T cell immune response, an effect that is facilitated by TFII-I. These results demonstrate a role of TRIM24 for regulation of transcriptional elongation from the HIV-1 promoter, through its interaction with TFII-I, and by recruitment of P-TEFb. Furthermore, these factors co-regulate a significant proportion of genes involved in T cell immune response, consistent with tight coupling of HIV-1 transcriptional activation and T cell signaling.

Intercellular Receptor-ligand Binding: Effect of Protein-membrane Interaction

Gaining insights into the intercellular receptor-ligand binding is of great importance for understanding numerous physiological and pathological processes, and stimulating new strategies in drug design and discovery. In contrast to the in vitro protein interaction in solution, the anchored receptor and ligand molecules interact with membrane in situ, which affects the intercellular receptor-ligand binding. Here, we review theoretical, simulation and experimental works regarding the regulatory effects of protein-membrane interactions on intercellular receptor-ligand binding mainly from the following aspects: membrane fluctuations, membrane curvature, glycocalyx, and lipid raft. In addition, we discuss biomedical significances and possible research directions to advance the field and highlight the importance of understanding of coupling effects of these factors in pharmaceutical development.

Regulation of cell size and Wee1 kinase by elevated levels of the cell cycle regulatory protein kinase Cdr2

Many cell cycle regulatory proteins catalyze cell cycle progression in a concentration-dependent manner. In the fission yeast Schizosaccharomyces pombe, the protein kinase Cdr2 promotes mitotic entry by organizing cortical oligomeric nodes that lead to inhibition of Wee1, which itself inhibits the cyclin-dependent kinase Cdk1. cdr2Δ cells lack nodes and divide at increased size due to overactive Wee1, but it has not been known how increased Cdr2 levels might impact Wee1 and cell size. It also has not been clear if and how Cdr2 might regulate Wee1 in the absence of the related kinase Cdr1/Nim1. Using a tetracycline-inducible expression system, we found that a 6× increase in Cdr2 expression caused hyperphosphorylation of Wee1 and reduction in cell size even in the absence of Cdr1/Nim1. This overexpressed Cdr2 formed clusters that sequestered Wee1 adjacent to the nuclear envelope. Cdr2 mutants that disrupt either kinase activity or clustering ability failed to sequester Wee1 and to reduce cell size. We propose that Cdr2 acts as a dosage-dependent regulator of cell size by sequestering its substrate Wee1 in cytoplasmic clusters, away from Cdk1 in the nucleus. This mechanism has implications for other clustered kinases, which may act similarly by sequestering substrates.

The kinase occupancy of T cell coreceptors reconsidered

The sensitivity of the αβ T cell receptor (TCR) is enhanced by the coreceptors CD4 and CD8αβ, which are expressed primarily by cells of the helper and cytotoxic T cell lineages, respectively. The coreceptors bind to major histocompatibility complex (MHC) molecules and associate intracellularly with the Src-family kinase Lck, which catalyzes TCR phosphorylation during receptor triggering. Although coreceptor/kinase occupancy was initially believed to be high, a recent study suggested that most coreceptors exist in an Lck-free state, and that this low occupancy helps to effect TCR antigen discrimination. Here, using the same method, we found instead that the CD4/Lck interaction was stoichiometric (~100%) and that the CD8αβ/Lck interaction was substantial (~60%). We confirmed our findings in live cells using fluorescence cross-correlation spectroscopy (FCCS) to measure coreceptor/Lck codiffusion in situ. After introducing structurally guided mutations into the intracellular domain of CD4, we used FCCS to also show that stoichiometric coupling to Lck required an amphipathic α-helix present in CD4 but not CD8α. In double-positive cells expressing equal numbers of both coreceptors, but limiting amounts of kinase, CD4 outcompeted CD8αβ for Lck. In T cells, TCR signaling induced CD4/Lck oligomerization but did not affect the high levels of CD4/Lck occupancy. These findings help settle the question of kinase occupancy and suggest that the binding advantages that CD4 has over CD8 could be important when Lck levels are limiting.

Discrete LAT condensates encode antigen information from single pMHC:TCR binding events

LAT assembly into a two-dimensional protein condensate is a prominent feature of antigen discrimination by T cells. Here, we use single-molecule imaging techniques to resolve the spatial position and temporal duration of each pMHC:TCR molecular binding event while simultaneously monitoring LAT condensation at the membrane. An individual binding event is sufficient to trigger a LAT condensate, which is self-limiting, and neither its size nor lifetime is correlated with the duration of the originating pMHC:TCR binding event. Only the probability of the LAT condensate forming is related to the pMHC:TCR binding dwell time. LAT condenses abruptly, but after an extended delay from the originating binding event. A LAT mutation that facilitates phosphorylation at the PLC-γ1 recruitment site shortens the delay time to LAT condensation and alters T cell antigen specificity. These results identify a function for the LAT protein condensation phase transition in setting antigen discrimination thresholds in T cells.

Role of the membrane anchor in the regulation of Lck activity

Theoretical work suggests that collective spatiotemporal behavior of integral membrane proteins should be modulated by boundary lipids sheathing their membrane anchors. Here, we show evidence for this prediction while investigating the mechanism for maintaining a steady amount of the active form of integral membrane protein Lck kinase (Lck_A) by Lck trans-autophosphorylation regulated by the phosphatase CD45. We used super-resolution microscopy, flow cytometry, and pharmacological and genetic perturbation to gain insight into the spatiotemporal context of this process. We found that Lck_A is generated exclusively at the plasma membrane, where CD45 maintains it in a ceaseless dynamic equilibrium with its unphosphorylated precursor. Steady Lck_A shows linear dependence, after an initial threshold, over a considerable range of Lck expression levels. This behavior fits a phenomenological model of trans-autophosphorylation that becomes more efficient with increasing Lck_A. We then challenged steady Lck_A formation by genetically swapping the Lck membrane anchor with structurally divergent ones, such as that of Src or the transmembrane domains of LAT, CD4, palmitoylation-defective CD4 and CD45 that were expected to drastically modify Lck boundary lipids. We observed small but significant changes in Lck_A generation, except for the CD45 transmembrane domain that drastically reduced Lck_A due to its excessive lateral proximity to CD45. Comprehensively, Lck_A formation and maintenance can be best explained by lipid bilayer critical density fluctuations rather than liquid-ordered phase-separated nanodomains, as previously thought, with "like/unlike" boundary lipids driving dynamical proximity and remoteness of Lck with itself and with CD45.

Choosing the Probe for Single-Molecule Fluorescence Microscopy

Probe choice in single-molecule microscopy requires deeper evaluations than those adopted for less sensitive fluorescence microscopy studies. Indeed, fluorophore characteristics can alter or hide subtle phenomena observable at the single-molecule level, wasting the potential of the sophisticated instrumentation and algorithms developed for advanced single-molecule applications. There are different reasons for this, linked, e.g., to fluorophore aspecific interactions, brightness, photostability, blinking, and emission and excitation spectra. In particular, these spectra and the excitation source are interdependent, and the latter affects the autofluorescence of sample substrate, medium, and/or biological specimen. Here, we review these and other critical points for fluorophore selection in single-molecule microscopy. We also describe the possible kinds of fluorophores and the microscopy techniques based on single-molecule fluorescence. We explain the importance and impact of the various issues in fluorophore choice, and discuss how this can become more effective and decisive for increasingly demanding experiments in single- and multiple-color applications.

Interplay of receptor-ligand binding and lipid domain formation during cell adhesion

Cell adhesion involved in biological processes such as cell migration, immune responses, and cancer metastasis, is mediated by the specific binding of receptor and ligand proteins. Some of these proteins exhibit affinity for nanoscale lipid clusters in cell membranes. A key question is how these nanoscale lipid clusters influence and react to the receptor-ligand binding during cell adhesion. In this article, we review recent computational studies that shed new light on the interplay of the receptor-ligand binding and the formation of lipid domains in adhering membranes. These studies indicate that the receptor-ligand binding promotes coalescence of lipid clusters into mesoscale domains, which, in turn, enhances both the affinity and cooperativity of the receptor-ligand binding in cell-cell adhesion with mobile ligands. In contrast, in the case of cell-extracellular matrix adhesion with immobile ligands, the receptor-ligand binding and the lipid cluster coalescence can be correlated or anti-correlated, depending strongly on the ligand distribution. These findings deepen our understanding of correlations between cell adhesion and membrane heterogeneities.

Cis interactions between CD2 and its ligands on T cells are required for T cell activation

CD2 is largely described to promote T cell activation when engaged by its ligands, CD48 in mice and CD58 in humans, that are present on antigen-presenting cells (APCs). However, both CD48 and CD58 are also expressed on T cells. By generating new knockout mouse strains lacking CD2 or CD48 in the C57BL/6 background, we determined that whereas CD2 was necessary on T cells for T cell activation, its ligand CD48 was not required on APCs. Rather, CD48 was also needed on T cells. One exception was during cytotoxicity, which required CD48 on T cells and APCs. Fluorescence resonance energy transfer (FRET) studies in nonimmune cells provided evidence that cis interactions between CD2 and CD48 existed within individual cells. CD2-CD48 interactions on T cells enabled more robust T cell receptor (TCR) signals, including protein tyrosine phosphorylation. Using T cells from a CD2 knock-in mouse in which a tag was inserted at the carboxyl terminus of CD2, mass spectrometry analyses revealed that the role of CD2 in T cell activation correlated with its ability to interact with components of the TCR complex and the protein tyrosine kinase Lck. CD2-CD58 provided a similar function in human T cells. Thus, our data imply that T cell-intrinsic cis interactions of CD2 with its ligands are required for TCR signaling and T cell activation. Interactions with ligands on APCs contribute during cytotoxicity.

CD28 and chemokine receptors: Signalling amplifiers at the immunological synapse

T cells are master regulators of the immune response tuning, among others, B cells, macrophages and NK cells. To exert their functions requiring high sensibility and specificity, T cells need to integrate different stimuli from the surrounding microenvironment. A finely tuned signalling compartmentalization orchestrated in dynamic platforms is an essential requirement for the proper and efficient response of these cells to distinct triggers. During years, several studies have depicted the pivotal role of the cytoskeleton and lipid microdomains in controlling signalling compartmentalization during T cell activation and functions. Here, we discuss mechanisms responsible for signalling amplification and compartmentalization in T cell activation, focusing on the role of CD28, chemokine receptors and the actin cytoskeleton. We also take into account the detrimental effect of mutations carried by distinct signalling proteins giving rise to syndromes characterized by defects in T cell functionality.

Enhanced permeation by amphiphilic surfactant is spatially heterogenous at membrane and cell level

In the context of increased interest in permeability enhancement technologies to achieve mucosal delivery of drugs and biologics, we report our study on effects of the amphiphilic surfactant at cell membrane and cell population levels. Our results show that modulation in membrane order and fluidity initially occurs on insertion of individual surfactant molecules into the outer leaflet of membrane lipid bilayer; a process occurring at concentrations below surfactant's critical micellar concentration. The surfactant insertion, and consequent increase in membrane fluidity, are observed to be spatially heterogenous, i.e. manifested as 'patches' of increased membrane fluidity. At the cell population level, spatially heterogeneous activity of surfactant is also manifested, with certain cells displaying high permeability amongst a 'background' population. We propose that this heterogeneity is further manifested in a broad profile of intracellular and nuclear exposure levels to a model drug (doxorubicin) observed in cell population. The study points to heterogeneous nature of surfactant effects at cell membrane and cells in population levels.

Understanding immune signaling using advanced imaging techniques

Advanced imaging is key for visualizing the spatiotemporal regulation of immune signaling which is a complex process involving multiple players tightly regulated in space and time. Imaging techniques vary in their spatial resolution, spanning from nanometers to micrometers, and in their temporal resolution, ranging from microseconds to hours. In this review, we summarize state-of-the-art imaging methodologies and provide recent examples on how they helped to unravel the mysteries of immune signaling. Finally, we discuss the limitations of current technologies and share our insights on how to overcome these limitations to visualize immune signaling with unprecedented fidelity.

The T cell receptor displays lateral signal propagation involving non-engaged receptors

T cells are highly sensitive to low levels of antigen, but how this sensitivity is achieved is currently unknown. Here, we imaged proximal TCR-CD3 signal propagation with single molecule localization microscopy (SMLM) in T cells activated with nanoscale clusters of TCR stimuli. We observed the formation of large TCR-CD3 clusters that exceeded the area of the ligand clusters, and required multivalent interactions facilitated by TCR-CD3 phosphorylation for assembly. Within these clustered TCR-CD3 domains, TCR-CD3 signaling spread laterally for ∼500 nm, far beyond the activating site, via non-engaged receptors. Local receptor density determined the functional cooperativity between engaged and non-engaged receptors, but lateral signal propagation was not influenced by the genetic deletion of ZAP70. Taken together, our data demonstrates that clustered ligands induced the clustering of non-ligated TCR-CD3 into domains that cooperatively facilitate lateral signal propagation.

Phase separation in immune signalling

Immune signalling pathways convert pathogenic stimuli into cytosolic events that lead to the resolution of infection. Upon ligand engagement, immune receptors together with their downstream adaptors and effectors undergo substantial conformational changes and spatial reorganization. During this process, nanometre-to-micrometre-sized signalling clusters have been commonly observed that are believed to be hotspots for signal transduction. Because of their large size and heterogeneous composition, it remains a challenge to fully understand the mechanisms by which these signalling clusters form and their functional consequences. Recently, phase separation has emerged as a new biophysical principle for organizing biomolecules into large clusters with fluidic properties. Although the field is still in its infancy, studies of phase separation in immunology are expected to provide new perspectives for understanding immune responses. Here, we present an up-to-date view of how liquid-liquid phase separation drives the formation of signalling condensates and regulates immune signalling pathways, including those downstream of T cell receptor, B cell receptor and the innate immune receptors cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) and retinoic acid-inducible gene I protein (RIG-I). We conclude with a summary of the current challenges the field is facing and outstanding questions for future studies.

Tetraspanin-enriched microdomains: The building blocks of migrasomes

The migrasome is a newly discovered organelle of migrating cells. Migrasomes play diverse physiological roles including mitochondrial quality control, lateral transfer of material between cells, and delivery of signaling molecules to spatially defined locations. The formation of migrasomes is dependent on tetraspanins, a group of membrane proteins containing four transmembrane domains, which form membrane microdomains named tetraspanin-enriched microdomains (TEMs). In this review, we will discuss the mechanisms for migrasome biogenesis, with a focus on the role of TEMs and the organizing principles underlying the formation of TEMs.

Lipid–Protein Interactions in Plasma Membrane Organization and Function

Lipid–protein interactions in cells are involved in various biological processes, including metabolism, trafficking, signaling, host–pathogen interactions, and transmembrane transport. At the plasma membrane, lipid–protein interactions play major roles in membrane organization and function. Several membrane proteins have motifs for specific lipid binding, which modulate protein conformation and consequent function. In addition to such specific lipid–protein interactions, protein function can be regulated by the dynamic, collective behavior of lipids in membranes. Emerging analytical, biochemical, and computational technologies allow us to study the influence of specific lipid–protein interactions, as well as the collective behavior of membranes on protein function. In this article, we review the recent literature on lipid–protein interactions with a specific focus on the current state-of-the-art technologies that enable novel insights into these interactions.

Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Liquid-liquid phase separation: Orchestrating cell signaling through time and space

Cell signaling is a complex process. The faithful transduction of information into specific cellular actions depends on the synergistic effects of many regulatory molecules, nurtured by their strict spatiotemporal regulation. Over the years, we have gained copious insights into the subcellular architecture supporting this spatiotemporal control, including the roles of membrane-bound organelles and various signaling nanodomains. Recently, liquid-liquid phase separation (LLPS) has been recognized as another potentially ubiquitous framework for organizing signaling molecules with high specificity and precise spatiotemporal control in cells. Here, we review the pervasive role of LLPS in signal transduction, highlighting several key pathways that intersect with LLPS, including examples in which LLPS is controlled by signaling events. We also examine how LLPS orchestrates signaling by compartmentalizing signaling molecules, amplifying signals non-linearly, and moderating signaling dynamics. We focus on the specific molecules that drive LLPS and highlight the known functional and pathological consequences of LLPS in each pathway.

DNA origami patterning of synthetic T cell receptors reveals spatial control of the sensitivity and kinetics of signal activation

Receptor clustering plays a key role in triggering cellular activation, but the relationship between the spatial configuration of clusters and the elicitation of downstream intracellular signals remains poorly understood. We developed a DNA-origami-based system that is easily adaptable to other cellular systems and enables rich interrogation of responses to a variety of spatially defined inputs. Using a chimeric antigen receptor (CAR) T cell model system with relevance to cancer therapy, we studied signaling dynamics at single-cell resolution. We found that the spatial arrangement of receptors determines the ligand density threshold for triggering and encodes the temporal kinetics of signaling activities. We also showed that signaling sensitivity of a small cluster of high-affinity ligands is enhanced when surrounded by nonstimulating low-affinity ligands. Our results suggest that cells measure spatial arrangements of ligands, translate that information into distinct signaling dynamics, and provide insights into engineering immunotherapies.

A correlative super-resolution protocol to visualise structural underpinnings of fast second-messenger signalling in primary cell types

Nanometre-scale cellular information obtained through super-resolution microscopies are often unaccompanied by functional information, particularly transient and diffusible signals through which life is orchestrated in the nano-micrometre spatial scale. We describe a correlative imaging protocol which allows the ubiquitous intracellular second messenger, calcium (Ca2+), to be directly visualised against nanoscale patterns of the ryanodine receptor (RyR) Ca2+ channels which give rise to these Ca2+ signals in wildtype primary cells. This was achieved by combining total internal reflection fluorescence (TIRF) imaging of the elementary Ca2+ signals, with the subsequent DNA-PAINT imaging of the RyRs. We report a straightforward image analysis protocol of feature extraction and image alignment between correlative datasets and demonstrate how such data can be used to visually identify the ensembles of Ca2+ channels that are locally activated during the genesis of cytoplasmic Ca2+ signals.

Lipid-based and protein-based interactions synergize transmembrane signaling stimulated by antigen clustering of IgE receptors

Antigen (Ag) crosslinking of immunoglobulin E-receptor (IgE-FcεRI) complexes in mast cells stimulates transmembrane (TM) signaling, requiring phosphorylation of the clustered FcεRI by lipid-anchored Lyn tyrosine kinase. Previous studies showed that this stimulated coupling between Lyn and FcεRI occurs in liquid ordered (Lo)-like nanodomains of the plasma membrane and that Lyn binds directly to cytosolic segments of FcεRI that it initially phosphorylates for amplified activity. Net phosphorylation above a nonfunctional threshold is achieved in the stimulated state but not in the resting state, and current evidence supports the hypothesis that this relies on Ag crosslinking to disrupt a balance between Lyn and tyrosine phosphatase activities. However, the structural interactions that underlie the stimulation process remain poorly defined. This study evaluates the relative contributions and functional importance of different types of interactions leading to suprathreshold phosphorylation of Ag-crosslinked IgE-FcεRI in live rat basophilic leukemia mast cells. Our high-precision diffusion measurements by imaging fluorescence correlation spectroscopy on multiple structural variants of Lyn and other lipid-anchored probes confirm subtle, stimulated stabilization of the Lo-like nanodomains in the membrane inner leaflet and concomitant sharpening of segregation from liquid disordered (Ld)-like regions. With other structural variants, we determine that lipid-based interactions are essential for access by Lyn, leading to phosphorylation of and protein-based binding to clustered FcεRI. By contrast, TM tyrosine phosphatase, PTPα, is excluded from these regions due to its Ld-preference and steric exclusion of TM segments. Overall, we establish a synergy of lipid-based, protein-based, and steric interactions underlying functional TM signaling in mast cells.

Utility of TPP-manufactured biophysical restrictions to probe multiscale cellular dynamics

Biophysical restrictions regulate protein diffusion, nucleus deformation, and cell migration, which are all universal and important processes for cells to perform their biological functions. However, current technologies addressing these multiscale questions are extremely limited. Herein, through two-photon polymerization (TPP), we present the precise, low-cost, and multiscale microstructures (micro-fences) as a versatile investigating platform. With nanometer-scale printing resolution and multiscale scanning capacity, TPP is capable of generating micro-fences with sizes of 0.5–1000 μm. These micro-fences are utilized as biophysical restrictions to determine the fluidity of supported lipid bilayers (SLB), to investigate the restricted diffusion of Src family kinase protein Lck on SLB, and also to reveal the mechanical bending of cell nucleus and T cell climbing ability. Taken together, the proposed versatile and low-cost micro-fences have great potential in probing the restricted dynamics of molecules, organelles, and cells to understand the basics of physical biology.

Graphic abstract

Cholesterol-dependent plasma membrane order (Lo) is critical for antigen-specific clonal expansion of CD4+ T cells

Early “T cell activation” events are initiated within the lipid microenvironment of the plasma membrane. Role of lipid membrane order (L_o) in spatiotemporal signaling through the antigen receptor in T cells is posited but remains unclear. We have examined the role of membrane order (L_o)/disorder (L_d) in antigen specific CD4⁺ T cell activation and clonal expansion by first creating membrane disorder, and then reconstituting membrane order by inserting cholesterol into the disordered plasma membrane. Significant revival of antigen specific CD4⁺ T cell proliferative response was observed after reconstituting the disrupted membrane order with cholesterol. These reconstitution experiments illustrate Koch’s postulate by demonstrating that cholesterol-dependent membrane order (L_o) is critical for responses generated by CD4⁺ T cells and point to the importance of membrane order and lipid microenvironment in signaling through T cell membrane antigen receptors.

Dynamic Palmitoylation of the Sodium-Calcium Exchanger Modulates Its Structure, Affinity for Lipid-Ordered Domains, and Inhibition by XIP

The transmembrane sodium-calcium (Na-Ca) exchanger 1 (NCX1) regulates cytoplasmic Ca levels by facilitating electrogenic exchange of Ca for Na. Palmitoylation, the only reversible post-translational modification known to modulate NCX1 activity, controls NCX1 inactivation. Here, we show that palmitoylation of NCX1 modifies the structural arrangement of the NCX1 dimer and controls its affinity for lipid-ordered membrane domains. NCX1 palmitoylation occurs dynamically at the cell surface under the control of the enzymes zDHHC5 and APT1. We identify the position of the endogenous exchange inhibitory peptide (XIP) binding site within the NCX1 regulatory intracellular loop and demonstrate that palmitoylation controls the ability of XIP to bind this site. We also show that changes in NCX1 palmitoylation change cytosolic Ca. Our results thus demonstrate the broad molecular consequences of NCX1 palmitoylation and highlight a means to manipulate the inactivation of this ubiquitous ion transporter that could ameliorate pathologies linked to Ca overload via NCX1.

•

NCX1 is dynamically palmitoylated at the cell surface by zDHHC5 and APT1

•

Palmitoylation modifies the NCX1 dimer’s structure and affinity for lipid rafts

•

We identify the binding site of the endogenous XIP domain in NCX1’s regulatory loop

•

Palmitoylation modifies NCX1 XIP affinity and hence regulates intracellular Ca

What Can Mushroom Proteins Teach Us about Lipid Rafts?

The lipid raft hypothesis emerged as a need to explain the lateral organization and behavior of lipids in the environment of biological membranes. The idea, that lipids segregate in biological membranes to form liquid-disordered and liquid-ordered states, was faced with a challenge: to show that lipid-ordered domains, enriched in sphingomyelin and cholesterol, actually exist in vivo. A great deal of indirect evidence and the use of lipid-binding probes supported this idea, but there was a lack of tools to demonstrate the existence of such domains in living cells. A whole new toolbox had to be invented to biochemically characterize lipid rafts and to define how they are involved in several cellular functions. A potential solution came from basic biochemical experiments in the late 1970s, showing that some mushroom extracts exert hemolytic activities. These activities were later assigned to aegerolysin-based sphingomyelin/cholesterol-specific cytolytic protein complexes. Recently, six sphingomyelin/cholesterol binding proteins from different mushrooms have been identified and have provided some insight into the nature of sphingomyelin/cholesterol-rich domains in living vertebrate cells. In this review, we dissect the accumulated knowledge and introduce the mushroom lipid raft binding proteins as molecules of choice to study the dynamics and origins of these liquid-ordered domains in mammalian cells.

High-speed single-molecule imaging reveals signal transduction by induced transbilayer raft phases

Using single-molecule imaging with enhanced time resolutions down to 5 ms, we found that CD59 cluster rafts and GM1 cluster rafts were stably induced in the outer leaflet of the plasma membrane (PM), which triggered the activation of Lyn, H-Ras, and ERK and continually recruited Lyn and H-Ras right beneath them in the inner leaflet with dwell lifetimes <0.1 s. The detection was possible due to the enhanced time resolutions employed here. The recruitment depended on the PM cholesterol and saturated alkyl chains of Lyn and H-Ras, whereas it was blocked by the nonraftophilic transmembrane protein moiety and unsaturated alkyl chains linked to the inner-leaflet molecules. Because GM1 cluster rafts recruited Lyn and H-Ras as efficiently as CD59 cluster rafts, and because the protein moieties of Lyn and H-Ras were not required for the recruitment, we conclude that the transbilayer raft phases induced by the outer-leaflet stabilized rafts recruit lipid-anchored signaling molecules by lateral raft-lipid interactions and thus serve as a key signal transduction platform.

Interactions between Phase-Separated Liquids and Membrane Surfaces

Liquid-liquid phase separation has recently emerged as an important fundamental organizational phenomenon in biological settings. Most studies of biological phase separation have focused on droplets that "condense" from solution above a critical concentration, forming so-called "membraneless organelles" suspended in solution. However, membranes are ubiquitous throughout cells, and many biomolecular condensates interact with membrane surfaces. Such membrane-associated phase-separated systems range from clusters of integral or peripheral membrane proteins in the plane of the membrane to free, spherical droplets wetting membrane surfaces to droplets containing small lipid vesicles. In this review, we consider phase-separated liquids that interact with membrane surfaces and we discuss the consequences of those interactions. The physical properties of distinct liquid phases in contact with bilayers can reshape the membrane, and liquid-liquid phase separation can construct membrane-associated protein structures, modulate their function, and organize collections of lipid vesicles dynamically. We summarize the common phenomena that arise in these systems of liquid phases and membranes.

Soft Polydimethylsiloxane-Supported Lipid Bilayers for Studying T Cell Interactions

Much of what we know about the early stages of T cell activation has been obtained from studies of T cells interacting with glass-supported lipid bilayers that favor imaging but are orders of magnitude stiffer than typical cells. We developed a method for attaching lipid bilayers to polydimethylsiloxane polymer supports, producing "soft bilayers" with physiological levels of mechanical resistance (Young's modulus of 4 kPa). Comparisons of T cell behavior on soft and glass-supported bilayers revealed that whereas late stages of T cell activation are thought to be substrate-stiffness dependent, early calcium signaling was unaffected by substrate rigidity, implying that early steps in T cell receptor triggering are not mechanosensitive. The exclusion of large receptor-type phosphatases was observed on the soft bilayers, however, even though it is yet to be demonstrated at authentic cell-cell contacts. This work sets the stage for an imaging-based exploration of receptor signaling under conditions closely mimicking physiological cell-cell contact.

Microscope calibration protocol for single-molecule microscopy

Single-molecule microscopy allows for the investigation of the dynamics of individual molecules and the visualization of subcellular structures at high spatial resolution. For single-molecule imaging experiments, and particularly those that entail the acquisition of multicolor data, calibration of the microscope and its optical components therefore needs to be carried out at a high level of accuracy. We propose here a method for calibrating a microscope at the nanometer scale, in the sense of determining optical aberrations as revealed by point source localization errors on the order of nanometers. The method is based on the imaging of a standard sample to detect and evaluate the amount of geometric aberration introduced in the optical light path. To provide support for multicolor imaging, it also includes procedures for evaluating the geometric aberration caused by a dichroic filter and the axial chromatic aberration introduced by an objective lens.

Advances in understanding and in multi-disciplinary methodology used to assess lipid regulation of signalling cascades from the cancer cell plasma membrane

The lipid bilayer is a functional component of cells, forming a stable platform for the initiation of key biological processes, including cell signalling. There are distinct changes in the lipid composition of cell membranes during oncogenic transformation resulting in aberrant activation and inactivation of signalling transduction pathways. Studying the role of the cell membrane in cell signalling is challenging, since techniques are often limited to by timescale, resolution, sensitivity, and averaging. To overcome these limitations, combining 'computational', 'wet-lab' and 'semi-dry' approaches offers the best opportunity to resolving complex biological processes involved in membrane organisation. In this review, we highlight analytical tools that have been applied for the study of cell signalling initiation from the cancer cell membranes through computational microscopy, biological assays, and membrane biophysics. The cancer therapeutic potential of extracellular membrane-modulating agents, such as cholesterol-reducing agents is also discussed, as is the need for future collaborative inter-disciplinary research for studying the role of the cell membrane and its components in cancer therapy.

Regulations of T Cell Activation by Membrane and Cytoskeleton

Among various types of membrane proteins that are regulated by cytoskeleton, the T cell receptor (TCR) greatly benefits from these cellular machineries for its function. The T cell is activated by the ligation of TCR to its target agonist peptide. However, the binding affinity of the two is not very strong, while the T cell needs to discriminate agonist from many nonagonist peptides. Moreover, the strength and duration of the activation signaling need to be tuned for immunological functions. Many years of investigations revealed that dynamic acto-myosin cytoskeletons and plasma membranes in T cells facilitate such regulations by modulating the spatiotemporal distributions of proteins in plasma membranes and by applying mechanical loads on proteins. In these processes, protein dynamics in multiple scales are involved, ranging from collective molecular motions and macroscopic molecular organizations at the cell–cell interface to microscopic changes in distances between receptor and ligand molecules. In this review, details of how cytoskeletons and membranes regulate these processes are discussed, with the emphasis on how all these processes are coordinated to occur within a single cell system.

Membrane nanodomains homeostasis during propofol anesthesia as function of dosage and temperature

Some anesthetics bind and potentiate γ-aminobutyric-acid-type receptors, but no universal mechanism for general anesthesia is known. Furthermore, often encountered complications such as anesthesia induced amnesia are not understood. General anesthetics are hydrophobic molecules easily dissolving into lipid bilayers. Recently, it was shown that general anesthetics perturb phase separation in vesicles extracted from fixed cells. Unclear is whether under physiological conditions general anesthetics induce perturbation of the lipid bilayer, and whether this contributes to the transient loss of consciousness or anesthesia side effects. Here we show that propofol perturbs lipid nanodomains in the outer and inner leaflet of the plasma membrane in intact cells, affecting membrane nanodomains in a concentration dependent manner: 1 μM to 5 μM propofol destabilize nanodomains; however, propofol concentrations higher than 5 μM stabilize nanodomains with time. Stabilization occurs only at physiological temperature and in intact cells. This process requires ARP2/3 mediated actin nucleation and Myosin II activity. The rate of nanodomain stabilization is potentiated by GABA_A receptor activity. Our results show that active nanodomain homeostasis counteracts the initial disruption causing large changes in cortical actin. SIGNIFICANCE STATEMENT: General anesthesia is a routine medical procedure with few complications, yet a small number of patients experience side-effects that persist for weeks and months. Very young children are at risk for effects on brain development. Elderly patients often exhibit subsequent amnesia. Here, we show that the general anesthetic propofol perturbs the ultrastructure of the lipid bilayer of the cell membrane in intact cells. Initially propofol destabilized lipid nanodomains. However, with increasing incubation time and propofol concentration, the effect is reversed and nanodomains are further stabilized. We show that this stabilization is caused by the activation of the actin cortex under the membrane. These perturbations of membrane bilayer and cortical actin may explain how propofol affects neuronal plasticity at synapses.

A framework for understanding the functions of biomolecular condensates across scales

Biomolecular condensates are found throughout eukaryotic cells, including in the nucleus, in the cytoplasm and on membranes. They are also implicated in a wide range of cellular functions, organizing molecules that act in processes ranging from RNA metabolism to signalling to gene regulation. Early work in the field focused on identifying condensates and understanding how their physical properties and regulation arise from molecular constituents. Recent years have brought a focus on understanding condensate functions. Studies have revealed functions that span different length scales: from molecular (modulating the rates of chemical reactions) to mesoscale (organizing large structures within cells) to cellular (facilitating localization of cellular materials and homeostatic responses). In this Roadmap, we discuss representative examples of biochemical and cellular functions of biomolecular condensates from the recent literature and organize these functions into a series of non-exclusive classes across the different length scales. We conclude with a discussion of areas of current interest and challenges in the field, and thoughts about how progress may be made to further our understanding of the widespread roles of condensates in cell biology.

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.

A dynamic CD2-rich compartment at the outer edge of the immunological synapse boosts and integrates signals

The CD2-CD58 recognition system promotes adhesion and signaling and counters exhaustion in human T cells. We found that CD2 localized to the outer edge of the mature immunological synapse, with cellular or artificial APC, in a pattern we refer to as a 'CD2 corolla'. The corolla captured engaged CD28, ICOS, CD226 and SLAM-F1 co-stimulators. The corolla amplified active phosphorylated Src-family kinases (pSFK), LAT and PLC-γ over T cell receptor (TCR) alone. CD2-CD58 interactions in the corolla boosted signaling by 77% as compared with central CD2-CD58 interactions. Engaged PD-1 invaded the CD2 corolla and buffered CD2-mediated amplification of TCR signaling. CD2 numbers and motifs in its cytoplasmic tail controlled corolla formation. CD8+ tumor-infiltrating lymphocytes displayed low expression of CD2 in the majority of people with colorectal, endometrial or ovarian cancer. CD2 downregulation may attenuate antitumor T cell responses, with implications for checkpoint immunotherapies.

Coupled membrane lipid miscibility and phosphotyrosine-driven protein condensation phase transitions

Lipid miscibility phase separation has long been considered to be a central element of cell membrane organization. More recently, protein condensation phase transitions, into three-dimensional droplets or in two-dimensional lattices on membrane surfaces, have emerged as another important organizational principle within cells. Here, we reconstitute the linker for activation of T cells (LAT):growth-factor-receptor-bound protein 2 (Grb2):son of sevenless (SOS) protein condensation on the surface of giant unilamellar vesicles capable of undergoing lipid phase separations. Our results indicate that the assembly of the protein condensate on the membrane surface can drive lipid phase separation. This phase transition occurs isothermally and is governed by tyrosine phosphorylation on LAT. Furthermore, we observe that the induced lipid phase separation drives localization of the SOS substrate, K-Ras, into the LAT:Grb2:SOS protein condensate.

Gaining insight into cellular cardiac physiology using single particle tracking

Single particle tracking (SPT) is a robust technique to monitor single-molecule behaviors in living cells directly. By this approach, we can uncover the potential biological significance of particle dynamics by statistically characterizing individual molecular behaviors. SPT provides valuable information at the single-molecule level, that could be obscured by simple averaging that is inherent to conventional ensemble measurements. Here, we give a brief introduction to SPT including the commonly used optical implementations, fluorescence labeling strategies, and data analysis methods. We then focus on how SPT has been harnessed to decipher myocardial function. In this context, SPT has provided novel insight into the lateral diffusion of signal receptors and ion channels, the dynamic organization of cardiac nanodomains, subunit composition and stoichiometry of cardiac ion channels, myosin movement along actin filaments, the kinetic features of transcription factors involved in cardiac remodeling, and the intercellular communication by nanotubes. Finally, we speculate on the prospects and challenges of applying SPT to future questions regarding cellular cardiac physiology using SPT.