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Adherens Junctions
Strong contact points between adjacent cells anchor them to each other, forming tissues. Such anchoring junctions are of two types – adherens junction…
The adherens junction is an anchoring junction that holds cells together in tissues such as the epithelium.
Adherens junctions are large multi-protein complexes comprising three main protein classes - the adhesive cadherins, the anchoring actin cytoskeleton, and the linker catenins .
Adherens junction assembly begins with the polymerization of actin monomers near the cell membrane to form protrusions called lamellipodia.
When lamellipodia from neighboring cells are close enough, the cadherins on their surface interact and trigger the localization of more cadherins, forming clusters.
These clusters are stabilized by beta-, alpha-, and p120 catenins, which help recruit more cadherin-catenin complexes to the growing site.
As the contact zone between the two cells expands, the alpha-catenins shift actin polymerization from branched to unbranched, and arrange them parallel to the cell membrane.
These actin filaments then associate with myosin to form contractile bundles, thus forming a mature adherens junction.
The junction links the cytoskeleton of one cell with another, held together via the extracellular interactions of cadherin molecules.
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Q1: What are the three main protein classes that make up adherens junctions?
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.
Q2: How do cadherins and catenins work together during adherens junction assembly?
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.
Q3: What is the role of actin polymerization in forming a mature adherens junction?
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.
Q4: Where do adherens junctions appear in different tissue types?
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.
Q5: How do inflammatory signals cause adherens junctions to disassemble?
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.
Q6: How do pathogenic bacteria exploit adherens junctions to infect host cells?
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.
Q7: Why are adherens junctions considered dynamic structures?
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.