High-density single-molecule maps reveal transient membrane receptor interactions within a dynamically varying environment

Over recent years, super-resolution and single-molecule imaging methods have delivered unprecedented details on the nanoscale organization and dynamics of individual molecules in different contexts. Yet, visualizing single-molecule processes in living cells with the required spatial and temporal resolution remains highly challenging. Here, we report on an analytical approach that extracts such information from live-cell single-molecule imaging at high-labeling densities using standard fluorescence probes. Our high-density-mapping (HiDenMap) methodology provides single-molecule nanometric localization accuracy together with millisecond temporal resolution over extended observation times, delivering multi-scale spatiotemporal data that report on the interaction of individual molecules with their dynamic environment. We validated HiDenMaps by simulations of Brownian trajectories in the presence of patterns that restrict free diffusion with different probabilities. We further generated and analyzed HiDenMaps from single-molecule images of transmembrane proteins having different interaction strengths to cortical actin, including the transmembrane receptor CD44. HiDenMaps uncovered a highly heterogenous and multi-scale spatiotemporal organization for all the proteins that interact with the actin cytoskeleton. Notably, CD44 alternated between periods of random diffusion and transient trapping, likely resulting from actin-dependent CD44 nanoclustering. Whereas receptor trapping was dynamic and lasted for hundreds of milliseconds, actin remodeling occurred at the timescale of tens of seconds, coordinating the assembly and disassembly of CD44 nanoclusters rich regions. Together, our data demonstrate the power of HiDenMaps to explore how individual molecules interact with and are organized by their environment in a dynamic fashion.Comment: 33 pages, 5 figure

Active emulsions in living cell membranes driven by contractile stresses and transbilayer coupling

The spatiotemporal organisation of proteins and lipids on the cell surface has direct functional consequences for signaling, sorting and endocytosis. Earlier studies have shown that multiple types of membrane proteins including transmembrane proteins that have cytoplasmic actin binding capacity and lipid-tethered GPI-anchored proteins (GPI-APs) form nanoscale clusters driven by active contractile flows generated by the actin cortex. To gain insight into the role of lipids in organizing membrane domains in living cells, we study the molecular interactions that promote the actively generated nanoclusters of GPI-APs and transmembrane proteins. This motivates a theoretical description, wherein a combination of active contractile stresses and transbilayer coupling drive the creation of active emulsions, mesoscale liquid ordered (lo) domains of the GPI-APs and lipids, at temperatures greater than equilibrium lipid-phase segregation. To test these ideas we use spatial imaging of homo-FRET combined with local membrane order and demonstrate that mesoscopic domains enriched in nanoclusters of GPI-APs are maintained by cortical actin activity and transbilayer interactions, and exhibit significant lipid order, consistent with predictions of the active composite model

Dynamic actin-mediated nano-scale clustering of CD44 regulates its meso-scale organization at the plasma membrane

Transmembrane adhesion receptors at the cell surface, such as CD44, are often equipped with modules to interact with the extracellular matrix (ECM) and the intracellular cytoskeletal machinery. CD44 has been recently shown to compartmentalize the membrane into domains by acting as membrane pickets, facilitating the function of signaling receptors. While spatial organization and diffusion studies of membrane proteins are usually conducted separately, here we combine observations of organization and diffusion by using high spatio-temporal resolution imaging on living cells to reveal a hierarchical organization of CD44. CD44 is present in a meso-scale meshwork pattern where it exhibits enhanced confinement and is enriched in nanoclusters of CD44 along its boundaries. This nanoclustering is orchestrated by the underlying cortical actin dynamics. Interaction with actin is mediated by specific segments of the intracellular domain. This influences the organization of the protein at the nano-scale, generating a selective requirement for formin over Arp2/3-based actin-nucleation machinery. The extracellular domain and its interaction with elements of ECM do not influence the meso-scale organization, but may serve to reposition the meshwork with respect to the ECM. Taken together, our results capture the hierarchical nature of CD44 organization at the cell surface, with active cytoskeleton-templated nanoclusters localized to a meso-scale meshwork pattern

A Van Gogh/Vangl tyrosine phosphorylation switch regulates its interaction with core Planar Cell Polarity factors Prickle and Dishevelled.

Epithelial tissues can be polarized along two axes: in addition to apical-basal polarity they are often also polarized within the plane of the epithelium, known as planar cell polarity (PCP). PCP depends upon the conserved Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl in mammals). Here, taking advantage of the complementary features of Drosophila wing and mouse skin PCP establishment, we dissect how Vang/Vangl phosphorylation on a specific conserved tyrosine residue affects its interaction with two cytoplasmic core PCP factors, Dishevelled (Dsh/Dvl1-3 in mammals) and Prickle (Pk/Pk1-3). We demonstrate that Pk and Dsh/Dvl bind to Vang/Vangl in an overlapping region centered around this tyrosine. Strikingly, Vang/Vangl phosphorylation promotes its binding to Prickle, a key effector of the Vang/Vangl complex, and inhibits its interaction with Dishevelled. Thus phosphorylation of this tyrosine appears to promote the formation of the mature Vang/Vangl-Pk complex during PCP establishment and conversely it inhibits the Vang interaction with the antagonistic effector Dishevelled. Intriguingly, the phosphorylation state of this tyrosine might thus serve as a switch between transient interactions with Dishevelled and stable formation of Vang-Pk complexes during PCP establishment

Charged amino acids interfere with Dsh binding to Vang.

Charged amino acids interfere with Dsh binding to Vang.</p

Vang point mutants affect localization of core PC factors in vivo.

Vang point mutants affect localization of core PC factors in vivo.</p

Y374 peptide and phospho-peptide binding to Pk and Dsh.

Y374 peptide and phospho-peptide binding to Pk and Dsh.</p

Pk and Dsh bind to adjacent, partially overlapping regions in the C-terminal tail of Vang.

(A) Schematic of Vang showing the previously mapped binding region of Pk and Dsh within its C-terminal tail (shaded in gray), residues 363–447 [53]. Y374 is indicated in red (this residue is equivalent to Y308 in mouse Vangl2, see alignment in Fig 1F). Note that Y374 is located within the shaded region. (B) Western blot showing binding between Myc-Pk and C-terminal truncations of Flagx3-Vang, using a series of C-terminal truncations within residues 363–447. Binding is retained up until the 1–363 truncation, defining residues 363–375 in Vang as critical for its binding to Pk. (C) Schematic of sequences of the Vang C-terminal truncations as used in (B) and (D), red box highlights amino acids required for Pk-binding. (D) Western blot showing binding between Dsh-GFP and C-terminal truncations of Flagx3-Vang. Note that binding is retained up until the 1–387 truncation and markedly reduced in the 1–375 truncation and shorter. (E) Sequence alignment showing the conservation of amino acids in the Drosophila Vang 364–387 region with mouse and human Vangl proteins. Colored asterisks highlight amino acids mutated in binding experiments shown in panels F, G and H. (F) Western blot showing binding between Myc-Pk and selected Vang-Flagx3 mutants as indicated. Colored asterisks refer to specific amino acids in sequence schematic in (E) mutated in the experiment. Note marked reduction in binding for the single mutant (Y374A) and almost complete loss of binding in the triple mutant FKYY371AAYA. (F’) Quantification, derived from 6 independent replicates, and statitistical analysis of binding differences. * pG) Western blot showing binding between Dsh-GFP and the indicated Vang-Flagx3 mutants. Note the reduction in binding follows a similar pattern to what was observed with Pk (compare to panel F), the equivalent Y residue to Y374 is detected as phosphorylated in mouse Vangl2 (see Fig 1D–1G). (G’) Quantification, combined from 6–7 replicates, and statitistical analysis of binding differences. **** p H) Western blot showing binding between Dsh-GFP and Vang-Flagx3 with the V376A mutant. Note a marked reduction in binding. V376 is at the junction to the Pk-binding region (blue asterisk in E), suggesting an overlap in binding regions between Pk and Dsh. It was the only single residue mutation in the 376–387 stretch to affect Dsh binding. (H’) Quantification, combined from 6 replicates, and statitistical analysis of binding differences. * p <0.05 as determined with Anova.</p

Specificity of Pk and Dsh binding to the Vang region 364–387.

Specificity of Pk and Dsh binding to the Vang region 364–387.</p