Dissecting cell adhesion architecture using advanced imaging techniques

Proximity Mapping of Desmosomes Reveals a Striking Shift in Their Molecular Neighborhood Associated With Maturation

Desmosomes are multiprotein adhesion complexes that link intermediate filaments to the plasma membrane, ensuring the mechanical integrity of cells across tissues, but how they participate in the wider signaling network to exert their full function is unclear. To investigate this, we carried out protein proximity mapping using biotinylation (BioID). The combined interactomes of the essential desmosomal proteins desmocollin 2a, plakoglobin, and plakophilin 2a (Pkp2a) in Madin-Darby canine kidney epithelial cells were mapped and their differences and commonalities characterized as desmosome matured from Ca2+ dependence to the mature, Ca2+-independent, hyper-adhesive state, which predominates in tissues. Results suggest that individual desmosomal proteins have distinct roles in connecting to cellular signaling pathways and that these roles alter substantially when cells change their adhesion state. The data provide further support for a dualistic concept of desmosomes in which the properties of Pkp2a differ from those of the other, more stable proteins. This body of data provides an invaluable resource for the analysis of desmosome function.

Measuring Plasma Membrane Recycling Using Microscopic and Biochemical Approaches

The endocytic pathway has an intricate network of vesicular compartments carrying a variety of proteins referred to as cargoes. Endosomal trafficking is exclusively required to transport these cargoes through various intracellular routes for their delivery to the site of action. Among these, recycling of cargoes to the plasma membrane is a crucial pathway for the efficient functioning of the cell. Hence, endosomal cargo recycling assays are crucial to gain insight into the molecular mechanism governing recycling of the cargoes and in turn to understand their key role in maintaining cellular physiology. These assays have been efficiently utilized to study the recycling of adhesion molecules, transporters, channels, receptors, and so on to the plasma membrane. The basic methodology involves labelling of the cargo at the surface, allowing its internalization followed by direct or indirect measurement of the amount of the cargo recycled back to the plasma membrane. These microscopy-based and biochemical methods can be used as a tool to study the role of various trafficking or signaling molecules on the cell surface involved with the recycling of the membrane proteins, by altering their expression either by silencing or overexpressing the gene.

Quantitative characterization of single-cell adhesion properties by atomic force microscopy using protein-functionalized microbeads

A method was developed to characterize the adhesion properties of single cells by using protein-functionalized atomic force microscopy (AFM) probes. The quantification by force spectroscopy of the mean detachment force between cells and a gelatin-functionalized colloidal tip reveals differences in cell adhesion properties that are not within reach of a traditional bulk technique, the washing assay. In this latter method, experiments yield semiquantitative and average adhesion properties of a large population of cells. They are also limited to stringent conditions and cannot highlight disparities in adhesion in the subset of adherent cells. In contrast, this AFM-based method allows for a reproducible and quantitative investigation of the adhesive properties of individual cells in common cell culture conditions and allows for the detection of adhesive subpopulations of cells. These characteristics meet the critical requirements of many fields, such as the study of cancer cell migratory abilities.

High-resolution imaging of a cell-attached nanointerface using a gold-nanoparticle two-dimensional sheet

This paper proposes a simple, effective, non-scanning method for the visualization of a cell-attached nanointerface. The method uses localized surface plasmon resonance (LSPR) excited homogeneously on a two-dimensional (2D) self-assembled gold-nanoparticle sheet. The LSPR of the gold-nanoparticle sheet provides high-contrast interfacial images due to the confined light within a region a few tens of nanometers from the particles and the enhancement of fluorescence. Test experiments on rat basophilic leukemia (RBL-2H3) cells with fluorescence-labeled actin filaments revealed high axial and lateral resolution even under a regular epifluorescence microscope, which produced higher quality images than those captured under a total internal reflection fluorescence (TIRF) microscope. This non-scanning-type, high-resolution imaging method will be an effective tool for monitoring interfacial phenomena that exhibit relatively rapid reaction kinetics in various cellular and molecular dynamics.

Fungal adhesion protein guides community behaviors and autoinduction in a paracrine manner

Microbes live mostly in a social community rather than in a planktonic state. Such communities have complex spatiotemporal patterns that require intercellular communication to coordinate gene expression. Here, we demonstrate that Cryptococcus neoformans, a model eukaryotic pathogen, responds to an extracellular signal in constructing its colony morphology. The signal that directs this community behavior is not a molecule of low molecular weight like pheromones or quorum-sensing molecules but a secreted protein. Znf2, a master regulator of morphogenesis in Cryptococcus, is necessary and sufficient for the production of this signal protein. Cfl1, a prominent Znf2-downstream adhesion protein (adhesin), was identified to be responsible for the paracrine communication. Consistent with its role in communication, Cfl1 is highly induced during mating colony differentiation, and some of the Cfl1 proteins undergo shedding and are released from the cell wall. The released Cfl1 is enriched in the extracellular matrix and acts as an autoinduction signal to stimulate neighboring cells to phenocopy Cfl1-expressing cells via the filamentation-signaling pathway. We further demonstrate the importance of an unannotated and yet conserved domain in Cfl1's signaling activity. Although adhesion proteins have long been considered to be mediators of microbial pathogenicity and the structural components of biofilms, our work presented here provides the direct evidence supporting the signaling activation by microbial adhesion/matrix proteins.