Control of Immune Responses by Trafficking Cell Surface Proteins, Vesicles and Lipid Rafts to and from the Immunological Synapse

Abstract

Supramolecular clusters at the immunological synapse provide a mechanism for structuring complex communication networks between cells of the immune system. Regulating intra-and intercellular trafficking of proteins and lipids to and from the immunological synapse provides an additional level of complexity in determining the functional outcome of immune cell interactions. An emergent principle is that molecules requiring tightly regulated cell surface expression, e.g. negative regulators of cell activation or molecules promoting cytotoxicity, are trafficked to the immunological synapse from intracellular secretory lysosomes as required. Many molecules required for the early stages of the intercellular communication are already present at the cell surface, sometimes in lipid rafts, and are rapidly translocated laterally to the intercellular contact. Our understanding of these events critically depends on utilizing appropriate technologies for probing supramolecular recognition in live cells. Thus, we also present here a critical discussion of the technologies used to study lipid rafts and, more broadly, a map of the spatial and temporal dimensions covered by current live cell physical techniques, highlighting where advances are needed to exceed current spatial and temporal boundaries.

Clustering of Raft-Associated Proteins in the External Membrane Leaflet Modulates Internal Leaflet H-Ras Diffusion and Signaling

“…7 can give rise to a general mechanism for transbilayer modulation of intracellular signaling events. Related mechanisms were proposed for the triggering of T-cell (and B-cell) signaling by clustering external-leaflet proteins/lipids or transmembrane receptors (9,20,21,27,45,51,53). In this context, it should be noted that although there is a general agreement on the involvement of membrane subdomains in T-cell signaling, the mechanisms involved are controversial.…”

“…However, the existence of such domains in living cells has recently been debated (1,8,10,17,26,33,44,49,53,54). Cholesterol-sensitive assemblies were suggested to exist in both the external and internal leaflets of the plasma membrane (7,18,26,44,50,53,55), but the mechanisms that couple them to enable transbilayer effects on signaling remained elusive. In the current report, we employed patch/FRAP studies on the lateral diffusion of GFP-Ras isoforms, combined with functional studies on Ras signaling, to explore concomitant effects of external cross-linking of raft-associated HA proteins on these biophysical and biochemical parameters.…”

“…The differences between these views involve mainly the nature of cholesterol-dependent assemblies in resting cell membranes. The studies presented here focus not on the resting state but rather on effects triggered by the clustering of proteins associated with cholesterol-sensitive assemblies; therefore, we will refer to such assemblies throughout as "rafts," without implications to a specific model.Numerous reports provide evidence in support of such domains in both the outer and inner plasma membrane leaflets (7,18,26,44,50,53,55). The coupling between raft-like domains in different membrane leaflets has an intriguing potential for modulating transbilayer signaling; however, this issue has remained largely unexplored.…”

“…Numerous reports provide evidence in support of such domains in both the outer and inner plasma membrane leaflets (7,18,26,44,50,53,55). The coupling between raft-like domains in different membrane leaflets has an intriguing potential for modulating transbilayer signaling; however, this issue has remained largely unexplored.…”

“…The coupling between raft-like domains in different membrane leaflets has an intriguing potential for modulating transbilayer signaling; however, this issue has remained largely unexplored. Related studies were conducted mainly in T cells, where external clustering of raftassociated molecules was reported to induce intracellular signaling, interpreted to indicate a role for raft stabilization/ growth in signal transduction (9,20,21,37,45,53). However, even in these cells, which are characterized by high spatial organization at the immunological synapse, the involvement of lipid rafts in clustering-induced transbilayer signaling was recently questioned (8,15,29,54).…”