What is it about?

Cells, as the fundamental units within organisms, typically reside in a homeostatic state but are sensitive to environmental stimuli that initiate transmembrane signaling leading to specific cellular responses. Participation of the cell’s plasma membrane to orchestrate the protein-based interactions in signal transduction pathways remains debated but thought to involve ordered-lipid nanodomains known colloquially as “rafts.” We employed statistically robust Imaging Fluorescence Correlation Spectroscopy (ImFCS) to measure diffusion of a dozen, structurally distinctive probes and evaluate organizing features of the plasma membrane. In particular, we examined components involved in transmembrane signaling in mast cells to initiate allergic responses, caused by antigen-crosslinking of immunoglobulin E (IgE) bound to cell surface receptors (FcεRI). These ImFCS measurements allowed us to quantitatively evaluate contributions of protein-based and lipid-based interactions to initiate transmembrane signaling.

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Why is it important?

Our ImFCS measurements confirm local stabilization of ordered-lipid nanodomains (rafts), around antigen-induced IgE-FcεRI clusters, possibly by dynamic recruitment of saturated lipids. This reorganization is particularly distinctive within in the inner leaflet of the plasma membrane, which is known to be compositionally different from the outer leaflet. At the inner leaflet, a peripherally-associated tyrosine kinase (Lyn) preferentially partitions into the nanodomains, primarily through lipid-based interactions of its membrane-anchoring saturated lipid, and secondarily by protein interactions with cytoplasmic subunits of FcεRI. In contrast, a transmembrane tyrosine phosphatase (PTPα) is excluded from these domains, due both to less favorable interactions with ordered-lipid nanodomain and to steric exclusion from the clustered FcεRI. This spatial filtering to enhance FcεRI encounters with Lyn, while suppressing those with PTPα, tips the balance to net phosphorylation of FcεRI, which propagates signaling events within the cytoplasm. Precise measurements by ImFCS of antigen-stimulated changes in diffusional patterns allow subtle redistributions of Lyn and PTPα to be detected, which can be correlated with functional outcomes. These observations underscore the pivotal importance of spatial filtering mediated by organized plasma membrane heterogeneity to orchestrate critical protein-protein interactions during stimulated transmembrane signaling. The components of FcεRI signaling are analogous to those of other antigen-activated immune receptors, including T- and B-cell receptors, and similar principles are likely to underly analogous signaling events.

Perspectives

This study demonstrates how precise measurements of physical properties (diffusion), which are dictated by structural features of membrane components, lead to new mechanistic understanding of their collective participation in cell biology. We expect the principles confirmed and newly uncovered in this study are also present in other transmembrane signaling systems. We are excited about the wide applicability of ImFCS, together with complementary quantitative fluorescence microscopy approaches, to explore these principles in future investigations of functional events orchestrated by the plasma membrane.

Barbara Baird
Cornell University

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This page is a summary of: Lipid-based and protein-based interactions synergize transmembrane signaling stimulated by antigen clustering of IgE receptors, Proceedings of the National Academy of Sciences, August 2021, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2026583118.
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