Desmosomes

Core Concept

Desmosomes are strong, spot-weld-like cell–cell junctions that anchor intermediate filaments (keratin in epithelia, desmin in muscle) to the plasma membrane, giving tissues the mechanical strength to withstand stretching and shearing. Their adhesion is mediated by the desmosomal cadherins — desmoglein and desmocollin — which bind homophilically between cells; inside the cell, armadillo proteins (plakoglobin, plakophilin) and the plakin desmoplakin link these cadherins to the intermediate-filament network. Desmosomes are especially abundant in skin and heart, where mechanical stress is highest.

Key Components

• Desmoglein (Dsg1–4) and desmocollin (Dsc1–3): desmosomal cadherins (Ca²⁺-dependent, homophilic).
• Plakoglobin (γ-catenin) and plakophilin: armadillo-family proteins that link cadherins to desmoplakin.
• Desmoplakin: the plakin that directly binds intermediate filaments, anchoring them to the plaque.
• Intermediate filaments (keratin K5/K14, desmin): the load-bearing cytoskeletal elements.
• Dense plaque: the electron-dense structure visible in EM at the cytoplasmic face.

Mechanism

Desmoglein and desmocollin on opposing cells engage in Ca²⁺-dependent homophilic binding across the intercellular space. Their cytoplasmic tails recruit plakoglobin and plakophilin, which in turn bind desmoplakin. Desmoplakin's C-terminus captures intermediate filaments, tethering the entire cytoskeletal network to the adhesion site. This arrangement distributes mechanical stress across the whole cell and the tissue, allowing epithelia and cardiac muscle to withstand repeated deformation without tearing.

How It Works

1. Cells express desmoglein, desmocollin, plakoglobin, plakophilin, and desmoplakin.
2. Desmosomal cadherins bind homophilically across the cell–cell gap (Ca²⁺-dependent).
3. Plakoglobin/plakophilin assemble onto the cadherin tails.
4. Desmoplakin binds the armadillo proteins.
5. Desmoplakin captures keratin (or desmin) intermediate filaments.
6. A durable cadherin → plaque → intermediate-filament linkage forms.

Energy and Directionality

Like other cadherin adhesions, desmosomal binding is Ca²⁺-dependent and affinity-driven, requiring no ATP for the binding event itself. Strength and durability come from linkage to the intermediate-filament system, which — unlike actin — provides passive, load-bearing resilience without constant ATP-driven turnover. Energy is still spent in synthesis, trafficking, and regulated remodeling. The functional direction is mechanical: forces are routed from the extracellular environment into the intermediate-filament network, protecting the cell from shear damage.

Experimental Evidence

• Electron microscopy: desmosomes show a characteristic dense midline and cytoplasmic plaques.
• Pemphigus autoantibodies: sera from pemphigus patients contain antibodies against desmogleins; injecting them causes skin blistering (acantholysis) — proof that desmogleins mediate adhesion.
• Desmoplakin knockouts: mice lacking desmoplakin die early with defects in heart and skin, showing desmosomes' essential mechanical role.
• Human genetics: mutations in desmosomal proteins cause arrhythmogenic cardiomyopathy and skin disorders (e.g., plakophilin defects).

Technique

• Transmission electron microscopy — identify the desmosome plaque and midline.
• Immunofluorescence — localize desmoglein, desmocollin, desmoplakin, and keratin.
• Autoantibody detection (ELISA/IIF) — diagnose pemphigus vulgaris/foliaceus.
• Dispase / mechanical-stress assays — test tissue adhesion strength.
• Genetic testing — identify desmosomal mutations in cardiomyopathy.

How it works

1. Cells express desmoglein, desmocollin, plakoglobin, plakophilin, and desmoplakin.
2. Desmosomal cadherins bind homophilically across the cell–cell gap (Ca²⁺-dependent).
3. Plakoglobin/plakophilin assemble onto the cadherin tails.
4. Desmoplakin binds the armadillo proteins.
5. Desmoplakin captures keratin (or desmin) intermediate filaments.
6. A durable cadherin → plaque → intermediate-filament linkage forms.

Common confusions

• "A desmosome is the same as a hemidesmosome." — No: desmosomes are cadherin-based cell–cell junctions; hemidesmosomes are integrin-based cell–matrix junctions. Both link intermediate filaments, but via different receptors and to different partners.
• "Desmosomes link to actin." — They link to intermediate filaments (adherens junctions link to actin).
• "Desmosomes and adherens junctions use the same cadherins." — Different cadherin families: desmosomal (desmoglein/desmocollin) vs. classical (E-cadherin).
• "Pemphigus is a genetic disease." — It is autoimmune (antibodies against desmogleins).
• "Desmosomes are only in skin." — They are also critical in cardiac muscle (and other epithelia).

Quick review

• Desmosomes = mechanical rivets; cadherin → plaque → intermediate filaments.
• Desmoglein + desmocollin (Ca²⁺-dependent homophilic binding).
• Plakoglobin/plakophilin → desmoplakin → keratin/desmin.
• Skin + heart; pemphigus (autoimmune) and arrhythmogenic cardiomyopathy (mutations).