Giunzioni aderenti

Adherens junctions comprise three main protein classes: adhesive cadherins that mediate cell-to-cell contact, anchoring actin cytoskeleton that provides structural support, and linker catenins that connect cadherins to the actin filaments. Together, these proteins form a multi-protein complex that holds cells together in tissues like the epithelium.

When lamellipodia from neighboring cells approach, cadherins on their surfaces interact and trigger localization of more cadherins into clusters. Beta-, alpha-, and p120 catenins then stabilize these clusters by linking cadherins to actin filaments. This recruitment of cadherin-catenin complexes creates the foundation for a mature junction that connects the cytoskeletons of adjacent cells.

Adherens junction assembly begins with actin monomer polymerization near the cell membrane, forming lamellipodia. As the contact zone expands, alpha-catenins shift actin polymerization from branched to unbranched filaments arranged parallel to the membrane. These actin filaments then associate with myosin to form contractile bundles, completing the mature junction structure.

In epithelial and endothelial tissues, adherens junctions form a continuous zone of adhesion called the adhesion belt. In heart muscle, they appear as clusters that link cells together to produce coordinated muscle contraction. This distribution reflects the different structural and functional requirements of various tissues.

During an immune response, inflammatory molecules like histamines and signaling proteins like vascular endothelial growth factor trigger phosphorylation and internalization of cadherins on endothelial cell surfaces. This leads to adherens junction disassembly and increased endothelial permeability, creating gaps that allow immune cells and molecules to pass through the blood vessel barrier.

Bacteria like Helicobacter pylori produce proteases that cleave cadherin extracellular domains, disrupting junctions and creating intercellular gaps for entry. Listeria monocytogenes hijacks E-cadherins to recruit catenins, triggering actin polymerization that forms membrane protrusions engulfing the bacterium in induced phagocytosis, allowing entry into nonphagocytic cells.

Adherens junctions are dynamically regulated in response to specific stimuli, undergoing assembly and disassembly as needed. During immune responses, inflammatory signals trigger junction breakdown to allow immune cell passage. Their maintenance and breakdown are controlled processes that adapt to physiological demands, making them essential for both barrier function and cellular communication.