Junctional ER Organization Affects Mechanotransduction at Cadherin-Mediated Adhesions

Cell-cell junctions in focus - imaging junctional architectures and dynamics at high resolution

Studies utilizing electron microscopy and live fluorescence microscopy have significantly enhanced our understanding of the molecular mechanisms that regulate junctional dynamics during homeostasis, development and disease. To fully grasp the enormous complexity of cell-cell adhesions, it is crucial to study the nanoscale architectures of tight junctions, adherens junctions and desmosomes. It is important to integrate these junctional architectures with the membrane morphology and cellular topography in which the junctions are embedded. In this Review, we explore new insights from studies using super-resolution and volume electron microscopy into the nanoscale organization of these junctional complexes as well as the roles of the junction-associated cytoskeleton, neighboring organelles and the plasma membrane. Furthermore, we provide an overview of junction- and cytoskeletal-related biosensors and optogenetic probes that have contributed to these advances and discuss how these microscopy tools enhance our understanding of junctional dynamics across cellular environments.

The desmosome as a dynamic membrane domain

Cell junctions integrate extracellular signals with intracellular responses to polarize tissues, pattern organs, and maintain tissue architecture by promoting cell-cell adhesion and communication. In this review, we explore the mechanisms whereby the adhesive junctions, adherens junctions and desmosomes, co-assemble and then segregate into unique plasma membrane domains. In addition, we highlight emerging evidence that these junctions are spatially and functionally integrated with the endoplasmic reticulum to mediate stress sensing and calcium homeostasis. We conclude with a discussion of the role of the endoplasmic reticulum in the mechanical stress response and how disruption of these connections may cause disease.

Alternative molecular mechanisms for force transmission at adherens junctions via β-catenin-vinculin interaction

Force transmission through adherens junctions (AJs) is crucial for multicellular organization, wound healing and tissue regeneration. Recent studies shed light on the molecular mechanisms of mechanotransduction at the AJs. However, the canonical model fails to explain force transmission when essential proteins of the mechanotransduction module are mutated or missing. Here, we demonstrate that, in absence of α-catenin, β-catenin can directly and functionally interact with vinculin in its open conformation, bearing physiological forces. Furthermore, we found that β-catenin can prevent vinculin autoinhibition in the presence of α-catenin by occupying vinculin´s head-tail interaction site, thus preserving force transmission capability. Taken together, our findings suggest a multi-step force transmission process at AJs, where α-catenin and β-catenin can alternatively and cooperatively interact with vinculin. This can explain the graded responses needed to maintain tissue mechanical homeostasis and, importantly, unveils a force-bearing mechanism involving β-catenin and extended vinculin that can potentially explain the underlying process enabling collective invasion of metastatic cells lacking α-catenin.

Architecture and dynamics of a desmosome-endoplasmic reticulum complex

The endoplasmic reticulum (ER) forms a dynamic network that contacts other cellular membranes to regulate stress responses, calcium signalling and lipid transfer. Here, using high-resolution volume electron microscopy, we find that the ER forms a previously unknown association with keratin intermediate filaments and desmosomal cell-cell junctions. Peripheral ER assembles into mirror image-like arrangements at desmosomes and exhibits nanometre proximity to keratin filaments and the desmosome cytoplasmic plaque. ER tubules exhibit stable associations with desmosomes, and perturbation of desmosomes or keratin filaments alters ER organization, mobility and expression of ER stress transcripts. These findings indicate that desmosomes and the keratin cytoskeleton regulate the distribution, function and dynamics of the ER network. Overall, this study reveals a previously unknown subcellular architecture defined by the structural integration of ER tubules with an epithelial intercellular junction.