29.9
Risposta alla tensione delle giunzioni aderenti
Le giunzioni aderenti che ancorano insieme le cellule sono complessi multiproteici che si adattano dinamicamente agli stimoli meccanici come le forze…
Tessuti come la pelle e l'epitelio intestinale subiscono costantemente forze meccaniche esterne come pizzicamento o stiramento.
Le giunzioni aderenti che ancorano insieme queste cellule epiteliali possono cambiare dinamicamente in risposta a tale tensione meccanica. Questa conversione di stimoli meccanici in cambiamenti biochimici è chiamata meccanotrasduzione.
In queste cellule, le giunzioni aderenti formano una zona continua di attaccamento, la cintura di adesione, che comprende grandi gruppi di caderine, catenine e fasci contrattili di actomiosisina.
Le alfa-catenine in queste giunzioni agiscono come proteine meccanosensoriali che cambiano conformazione in risposta alla tensione meccanica.
Quando l'aumento delle forze contrattili tira i filamenti di actina, l'alfa-catenina cambia dalla conformazione ripiegata a quella dispiegata.
Questo dispiegamento apre un sito di legame criptico alla vinculina, una proteina legante il citoscheletro che rimane nella sua forma chiusa e inattiva nel citoplasma.
Dopo aver legato l'alfa-catenina, la vinculina cambia conformazione nella sua forma aperta e attiva.
La vinculina attiva promuove il reclutamento di ulteriori filamenti di actina nel sito, rafforzando così la giunzione e distribuendo la forza attraverso le cellule.
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Q1: What is mechanotransduction at adherens junctions?
Mechanotransduction is the conversion of mechanical stimuli, such as tension or stretching, into biochemical changes at adherens junctions. Epithelial tissues like skin and intestinal epithelium constantly experience external mechanical forces. Mechanosensory proteins at these junctions detect these forces and undergo conformational changes that alter their function, allowing cells to respond dynamically to mechanical stress.
Q2: How does alpha-catenin respond to mechanical tension?
Alpha-catenin is a mechanosensory protein that changes conformation when contractile forces pull on actin filaments. Under normal conditions, alpha-catenin remains folded with its vinculin homology binding domains inaccessible. When tension increases, alpha-catenin unfolds, exposing these cryptic binding sites and enabling interaction with other proteins like vinculin to strengthen the junction.
Q3: What role does vinculin play in junction strengthening?
Vinculin is a cytoskeletal binding protein that exists in an inactive, closed form in the cytoplasm. When exposed alpha-catenin binding sites activate vinculin, it changes to an open conformation. The active vinculin recruits additional actin filaments to the junction, strengthening it and distributing mechanical force across cells more effectively.
Q4: What is the adhesion belt in epithelial cells?
The adhesion belt is a continuous zone of attachment formed by adherens junctions, comprising large clusters of cadherins, catenins, and actomyosin contractile bundles. This structure anchors epithelial cells together and provides the mechanical foundation for cells to respond to external forces like pinching or stretching while maintaining tissue integrity.
Q5: How does vinculin binding affect the actin cytoskeleton?
When vinculin binds to unfolded alpha-catenin, its open neck region becomes accessible to actin-regulating proteins like Arp2/3. This allows vinculin to recruit more actin filaments and dynamically reorganize the actin cytoskeleton in response to mechanical stress, enhancing junction stability and force distribution.
Q6: Why do adherens junctions need to adapt to mechanical forces?
Tissues like skin and intestinal epithelium experience constant external mechanical forces such as pinching and stretching. Adherens junctions must dynamically adapt to these forces to maintain tissue integrity and prevent cell separation. This adaptive response through mechanotransduction allows cells to strengthen junctions under stress and distribute forces efficiently across the tissue.
Q7: What structural changes occur in alpha-catenin during mechanotransduction?
Alpha-catenin undergoes an allosteric shift from a folded to an unfolded conformation when mechanical tension increases. This unfolding exposes three vinculin homology domains that were previously inaccessible. The conformational change allows these domains to bind vinculin and other actin-binding proteins, initiating the cascade that strengthens the junction.