Functional role of T-cell receptor nanoclusters in signal initiation and antigen discrimination.

Antigen recognition by the T-cell receptor (TCR) is a hallmark of the adaptive immune system. When the TCR engages a peptide bound to the restricting major histocompatibility complex molecule (pMHC), it transmits a signal via the associated CD3 complex. How the extracellular antigen recognition event leads to intracellular phosphorylation remains unclear. Here, we used single-molecule localization microscopy to quantify the organization of TCR-CD3 complexes into nanoscale clusters and to distinguish between triggered and nontriggered TCR-CD3 complexes. We found that only TCR-CD3 complexes in dense clusters were phosphorylated and associated with downstream signaling proteins, demonstrating that the molecular density within clusters dictates signal initiation. Moreover, both pMHC dose and TCR-pMHC affinity determined the density of TCR-CD3 clusters, which scaled with overall phosphorylation levels. Thus, TCR-CD3 clustering translates antigen recognition by the TCR into signal initiation by the CD3 complex, and the formation of dense signaling-competent clusters is a process of antigen discrimination.

The aPKC/Par3/Par6 Polarity Complex and Membrane Order Are Functionally Interdependent in Epithelia During Vertebrate Organogenesis.

The differential distribution of lipids between apical and basolateral membranes is necessary for many epithelial cell functions, but how this characteristic membrane organization is integrated within the polarity network during ductal organ development is poorly understood. Here we quantified membrane order in the gut, kidney and liver ductal epithelia in zebrafish larvae at 3-11 days post fertilization (dpf) with Laurdan 2-photon microscopy. We then applied a combination of Laurdan imaging, antisense knock-down and analysis of polarity markers to understand the relationship between membrane order and apical-basal polarity. We found a reciprocal relationship between membrane order and the cell polarity network. Reducing membrane condensation by exogenously added oxysterol or depletion of cholesterol reduced apical targeting of the polarity protein, aPKC. Conversely, using morpholino knock down in zebrafish, we found that membrane order was dependent upon the Crb3 and Par3 polarity protein expression in ductal epithelia. Hence our data suggest that the biophysical property of membrane lipid packing is a regulatory element in apical basal polarity.

The myelin proteolipid plasmolipin forms oligomers and induces liquid-ordered membranes in the Golgi complex.

Myelin comprises a compactly stacked massive surface area of protein-poor thick membrane that insulates axons to allow fast signal propagation. Increasing levels of the myelin protein plasmolipin (PLLP) were correlated with post-natal myelination; however, its function is unknown. Here, the intracellular localization and dynamics of PLLP were characterized in primary glial and cultured cells using fluorescently labeled PLLP and antibodies against PLLP. PLLP localized to and recycled between the plasma membrane and the Golgi complex. In the Golgi complex, PLLP forms oligomers based on fluorescence resonance energy transfer (FRET) analyses. PLLP oligomers blocked Golgi to plasma membrane transport of the secretory protein vesicular stomatitis virus G protein (VSVG), but not of a VSVG mutant with an elongated transmembrane domain. Laurdan staining analysis showed that this block is associated with PLLP-induced proliferation of liquid-ordered membranes. These findings show the capacity of PLLP to assemble potential myelin membrane precursor domains at the Golgi complex through its oligomerization and ability to attract liquid-ordered lipids. These data support a model in which PLLP functions in myelin biogenesis through organization of myelin liquid-ordered membranes in the Golgi complex.

Galectin-3 drives glycosphingolipid-dependent biogenesis of clathrin-independent carriers.

Several cell surface molecules including signalling receptors are internalized by clathrin-independent endocytosis. How this process is initiated, how cargo proteins are sorted and membranes are bent remains unknown. Here, we found that a carbohydrate-binding protein, galectin-3 (Gal3), triggered the glycosphingolipid (GSL)-dependent biogenesis of a morphologically distinct class of endocytic structures, termed clathrin-independent carriers (CLICs). Super-resolution and reconstitution studies showed that Gal3 required GSLs for clustering and membrane bending. Gal3 interacted with a defined set of cargo proteins. Cellular uptake of the CLIC cargo CD44 was dependent on Gal3, GSLs and branched N-glycosylation. Endocytosis of ß1-integrin was also reliant on Gal3. Analysis of different galectins revealed a distinct profile of cargoes and uptake structures, suggesting the existence of different CLIC populations. We conclude that Gal3 functionally integrates carbohydrate specificity on cargo proteins with the capacity of GSLs to drive clathrin-independent plasma membrane bending as a first step of CLIC biogenesis.

Do signalling endosomes play a role in T cell activation?

Signalling endosomes represent a general mechanism for modulating and compartmentalizing cell signalling, which is achieved by delineating specific spatial environments and connecting the plasma membrane with intracellular events. The molecular composition of vesicles, together with their targeting mechanisms and endocytic routes, contributes to the outcome of signalling pathways that are initiated either at the plasma membrane or within endosomes themselves. In T cell signalling, it is now accepted that the spatial distribution of signalling proteins is central to T cell activation not only at the immunological synapse, but also in endosomes travelling to and from the plasma membrane. In addition, there is a global rearrangement of the endosome machinery upon T cell activation, and emerging experimental evidence suggests that vesicles in T cells contain key T cell signalling proteins. We review the various mechanisms by which endosomes contribute to signalling pathways and consider whether signalling endosomes play a role in T cell signalling.

The integration of signaling and the spatial organization of the T cell synapse.

Engagement of the T cell antigen receptor (TCR) triggers signaling pathways that lead to T cell selection, differentiation and clonal expansion. Superimposed onto the biochemical network is a spatial organization that describes individual receptor molecules, dimers, oligomers and higher order structures. Here we discuss recent findings and new concepts that may regulate TCR organization in naïve and memory T cells. A key question that has emerged is how antigen-TCR interactions encode spatial information to direct T cell activation and differentiation. Single molecule super-resolution microscopy may become an important tool in decoding receptor organization at the molecular level.

Phagocytosis of IgG-coated polystyrene beads by macrophages induces and requires high membrane order.

The biochemical composition and biophysical properties of cell membranes are hypothesized to affect cellular processes such as phagocytosis. Here, we examined the plasma membranes of murine macrophage cell lines during the early stages of uptake of immunoglobulin G (IgG)-coated polystyrene particles. We found that the plasma membrane undergoes rapid actin-independent condensation to form highly ordered phagosomal membranes, the biophysical hallmark of lipid rafts. Surprisingly, these membranes are depleted of cholesterol and enriched in sphingomyelin and ceramide. Inhibition of sphingomyelinase activity impairs membrane condensation, F-actin accumulation at phagocytic cups and particle uptake. Switching phagosomal membranes to a cholesterol-rich environment had no effect on membrane condensation and the rate of phagocytosis. In contrast, preventing membrane condensation with the oxysterol 7-ketocholesterol, even in the presence of ceramide, blocked F-actin dissociation from nascent phagosomes and particle uptake. In conclusion, our results suggest that ordered membranes function to co-ordinate F-actin remodelling and that the biophysical properties of phagosomal membranes are essential for phagocytosis.

Assembly and mobility of exon-exon junction complexes in living cells.

The exon-exon junction complex (EJC) forms via association of proteins during splicing of mRNA in a defined manner. Its organization provides a link between biogenesis, nuclear export, and translation of the transcripts. The EJC proteins accumulate in nuclear speckles alongside most other splicing-related factors. We followed the establishment of the EJC on mRNA by investigating the mobility and interactions of a representative set of EJC factors in vivo using a complementary analysis with different fluorescence fluctuation microscopy techniques. Our observations are compatible with cotranscriptional binding of the EJC protein UAP56 confirming that it is involved in the initial phase of EJC formation. RNPS1, REF/Aly, Y14/Magoh, and NXF1 showed a reduction in their nuclear mobility when complexed with RNA. They interacted with nuclear speckles, in which both transiently and long-term immobilized factors were identified. The location- and RNA-dependent differences in the mobility between factors of the so-called outer shell and inner core of the EJC suggest a hypothetical model, in which mRNA is retained in speckles when EJC outer-shell factors are missing.