Electrostatic interactions: From immune receptor assembly to signaling.

Our ability to mount a long-lasting and protective immune response relies on a variety of immune receptors that enable the recognition of ongoing infections, which triggers the adaptation of a myriad of immune cells. The organization of several immune receptors, such as the T cells receptor and several natural killer cell receptors, utilizes different modules for ligand recognition and signaling. These receptors require specific recognition mechanisms between the different modules in order to ensure proper assembly and function. Once assembled, immune receptors must remain inactive in the absence of ligand to prevent the onset of unwanted immune response. Indeed, several mechanisms exist to prevent aberrant immune receptor signaling in the absence of ligand to avert the initiation of uncontrolled autoimmunity. However, once a ligand is recognized, immune receptors must rapidly and specifically engage kinases to initiate highly regulated signaling cascades that lead to the initiation of transcriptional programs that dictate the immune response. Over the last decade, compelling evidence have been presented which suggest that electrostatic interactions are critical for many aspects of immune receptor functions. In the work that follows, we present an overview of the literature that have provided evidence that illustrate how electrostatic interactions regulate immune receptor assembly, inactive state, triggering, and signaling.

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.

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

Over the last decade, advancements in the time and space resolution of microscopy technologies have enabled dissection of the molecular events involved in T cell Immunological Synapse (IS) formation. Using a combination of Förster Resonance Energy Transfer (FRET) and Fluorescence Lifetime Imagining Microscopy (FLIM), we have demonstrated dynamic plasma membrane binding by cytoplasmic domains of T cell receptor (TCR)-associated CD3 chains and other T cell transmembrane receptors. We have developed methods for imaging such membrane binding both at steady state and during receptor triggering at the IS. Plasma membrane binding by cytoplasmic domains may represent a novel mechanism for regulating the signaling function of important receptors in the immune system.