Junctional Reorganization

Junctional Reorganization is the dynamic, cell-biological process by which the physical structures and trafficking behavior of cell-cell junctions are actively remodeled during epithelial-to-mesenchymal transition, encompassing the mechanics of junction disassembly, protein internalization and redistribution, and the conversion of residual adhesive contacts into forms compatible with a motile phenotype. It is distinguished from epithelial program suppression and epithelial marker loss by its focus on the physical, trafficking-level remodeling of junctional structures themselves — internalization, redistribution, and repurposing — rather than on transcriptional regulation of junction gene expression or the resulting steady-state marker readout.

Disassembly Mechanics of Adherens Junctions

Adherens junction disassembly during EMT proceeds through an active, regulated trafficking process rather than simple passive protein loss:

1. Endocytic Internalization of E-Cadherin — Rather than being degraded in place at the membrane, E-cadherin at destabilizing junctions is actively internalized via clathrin-mediated and caveolin-mediated endocytosis, removing it from the cell surface and reducing available adhesive contact even before transcriptional downregulation substantially reduces total protein levels.
2. Catenin Release and Redistribution — Internalization or proteolytic cleavage of E-cadherin releases the cytoplasmic catenins (β-catenin, p120-catenin, α-catenin) that normally couple it to the actin cytoskeleton; released β-catenin becomes available for nuclear translocation and participation in WNT-responsive transcription, functionally linking junctional disassembly directly to further EMT-promoting transcriptional signaling rather than being merely a structural byproduct.
3. Ubiquitination and Degradation — E-cadherin ubiquitination, mediated by E3 ubiquitin ligases including Hakai, targets internalized E-cadherin for lysosomal or proteasomal degradation, providing the mechanism by which junctional disassembly becomes a durable rather than merely transient reorganization.

Tight Junction Disassembly Kinetics

Tight junction disassembly proceeds through coordinated redistribution of claudins, occludin, and ZO-1 from the continuous apical junctional belt into discontinuous, then diffuse cytoplasmic pools, generally following a comparable but not necessarily synchronous kinetic timeline to adherens junction disassembly; partial retention of tight junction proteins after substantial adherens junction loss (or vice versa) is observed experimentally, indicating that the two junction types, while coordinately regulated, are not obligately co-dependent in their disassembly kinetics during a given EMT induction event.

Conversion of Residual Adhesive Structures

Rather than disappearing entirely, residual cell-cell contact machinery is frequently repurposed into structurally and functionally distinct forms during junctional reorganization:

• Adherens-to-Focal-Adhesion-like Transition — Components of the adherens junction adhesion machinery can be redistributed toward newly forming integrin-based focal adhesions at the cell periphery, contributing structural and signaling components to the emerging migratory adhesion apparatus rather than being entirely discarded.
• Retained Partial Junctions in Collective Invasion — In leader-follower collective invasion structures, junctional reorganization frequently proceeds only partially, retaining sufficient residual adherens junction function (often N-cadherin-based rather than E-cadherin-based) to permit mechanical force transmission across the group while still allowing the increased dynamic remodeling needed for coordinated group movement, illustrating that junctional reorganization can produce a functionally intermediate adhesive state rather than complete disassembly.
• Tricellular Junction Remodeling — At points where three or more cells meet, specialized tricellular junction structures undergo distinct remodeling dynamics that have been implicated in determining sites of initial single-cell detachment from a collectively invading front.

Diagram: Sequential Steps of Adherens Junction Reorganization

Coupling to Rho-GTPase Cytoskeletal Signaling

Junctional reorganization is mechanistically coupled to the same Rho-GTPase signaling network governing cytoskeletal reorganization during EMT: local RhoA activation at destabilizing junctions promotes actomyosin contractility that mechanically assists junction disassembly, while Rac1 activity, redistributing away from junctional maintenance toward lamellipodial protrusion at the emerging cell periphery, further shifts the cytoskeletal machinery away from junction-stabilizing configurations, meaning junctional reorganization and the establishment of migratory cytoskeletal polarity proceed as mechanistically linked, rather than independent, processes.

Experimental Assessment

Junctional reorganization is studied using live-cell imaging of fluorescently tagged E-cadherin and catenin constructs to directly visualize internalization, trafficking, and redistribution dynamics in real time, biochemical fractionation to distinguish membrane-associated, cytoplasmic, and nuclear pools of catenins at different stages of EMT induction, and super-resolution or electron microscopy to resolve the fine structural changes in residual junctional and emerging focal adhesion structures during the transition from stable epithelial contact to migratory adhesion organization.