Studying Dynamic Plasma Membrane Binding of TCR-CD3 Chains During Immunological Synapse Formation Using Donor-Quenching FRET and FLIM-FRET

Antigen Geometry Tunes Mast Cell Signaling Through Distinct FcεRI Aggregation and Structural Changes

Immunoreceptor tyrosine-based activation motif (ITAM)-containing Fc receptors are critical components of the innate and adaptive immune systems. FcεRI mediates the allergic response via crosslinking of IgE-bound receptors by multivalent antigens. Yet, the underlying molecular mechanisms that govern the response of FcεRI to specific antigens remain poorly understood. We compared responses induced by two antigens with distinct geometries, high valency DNP-BSA and trivalent DF3, and found unique secretion and receptor phosphorylation profiles that are due to differential recruitment of Lyn and SHIP1. To understand how these two antigens can cause such markedly different outcomes, we used direct stochastic optical reconstruction microscopy (dSTORM) super-resolution imaging combined with Bayesian Grouping of Localizations (BaGoL) analysis to compare the nanoscale characteristics of FcεRI aggregates. DF3 aggregates were found to be smaller and more densely packed than DNP-BSA aggregates. Using lifetime-based Förster resonance energy transfer (FRET) measurements, we discovered that FcεRI subunits undergo structural rearrangements upon crosslinking with either antigen, and in response to interaction with monovalent antigen presented on a supported lipid bilayer. The extent of conformational change is positively correlated with signaling efficiency. Finally, we provide evidence for forces in optimizing FcεRI signaling, such that immobilizing DF3 on a rigid surface promoted degranulation while increasing DNP-BSA flexibility lowered degranulation. These results provide a link between the physical attributes of allergens, including size, shape, valency, and flexibility, and FcεRI signaling strength. Thus, the antigen modulates mast cell outcomes by creating unique aggregate geometries that tune FcεRI conformation, phosphorylation and signaling partner recruitment.

TMEM16F mediates bystander TCR-CD3 membrane dissociation at the immunological synapse and potentiates T cell activation

Electrostatic interactions regulate many aspects of T cell receptor (TCR) activity, including enabling the dynamic binding of the TCR-associated CD3ε and CD3ζ chains to anionic lipids in the plasma membrane to prevent spontaneous phosphorylation. Substantial changes in the electrostatic potential of the plasma membrane occur at the immunological synapse, the interface between a T cell and an antigen-presenting cell. Here, we investigated how the electrostatic interactions that promote dynamic membrane binding of the TCR-CD3 cytoplasmic domains are modulated during signaling and affect T cell activation. We found that Ca2+-dependent activation of the phosphatidylserine scramblase TMEM16F, which was previously implicated in T cell activation, reduced the electrostatic potential of the plasma membrane during immunological synapse formation by locally redistributing phosphatidylserine. This, in turn, increased the dissociation of bystander TCR-CD3 cytoplasmic domains from the plasma membrane and enhanced TCR-dependent signaling and consequently T cell activation. This study establishes the molecular basis for the role of TMEM16F in bystander TCR-induced signal amplification and identifies enhancement of TMEM16F function as a potential therapeutic strategy for promoting T cell activation.

Current Methods for Detecting Cell Membrane Transient Interactions

Short-lived cell membrane complexes play a key role in regulating cell signaling and communication. Many of these complexes are formed based on low-affinity and transient interactions among various lipids and proteins. New techniques have emerged to study these previously overlooked membrane transient interactions. Exciting functions of these transient interactions have been discovered in cellular events such as immune signaling, host-pathogen interactions, and diseases such as cancer. In this review, we have summarized current experimental methods that allow us to detect and analyze short-lived cell membrane protein-protein, lipid-protein, and lipid-lipid interactions. These methods can provide useful information about the strengths, kinetics, and/or spatial patterns of membrane transient interactions. However, each method also has its own limitations. We hope this review can be used as a guideline to help the audience to choose proper approaches for studying membrane transient interactions in different membrane trafficking and cell signaling events.