Hemidesmosomes

Core Concept

Hemidesmosomes are rivet-like junctions that anchor the basal surface of epithelial cells firmly to the basal lamina, resisting mechanical shear between the epithelium and underlying connective tissue. Their molecular chain runs from extracellular laminin → integrin α6β4 → cytoplasmic linker proteins plectin and BP230 → keratin intermediate filaments. Despite the name, a hemidesmosome is not "half a desmosome" — it uses completely different molecules (integrins vs. cadherins) and connects a cell to the matrix, whereas desmosomes connect cell to cell.

Key Components

• Integrin α6β4: the hemidesmosome's core transmembrane receptor; the β4 subunit has an unusually large cytoplasmic tail that binds plectin.
• Laminin-332: the major basal-lamina ligand for α6β4.
• Plectin: a large plakin that links integrin β4 to keratin intermediate filaments.
• BP230 (BPAG1): a plakin that further anchors the keratin network at the plaque.
• Keratin intermediate filaments (K5/K14 in epidermis): the intracellular stress-bearing cables.
• BP180 (collagen XVII): a transmembrane collagen that also participates in hemidesmosome adhesion.

Mechanism

The α6β4 integrin binds laminin-332 in the basal lamina. On the cytoplasmic side, plectin and BP230 bind the β4 tail and the keratin intermediate-filament network, creating a continuous mechanical linkage: basal lamina → integrin → plaque → intermediate filaments. This distributes shear forces across the entire cytoskeleton of the cell and the tissue, giving epithelia their resistance to rubbing and stretch. Unlike focal adhesions (which link to actin and are dynamic), hemidesmosomes link to the durable intermediate-filament system and are more stable, specialized adhesions.

How It Works

1. Epithelial cells synthesize α6β4 integrin, plectin, BP230, and keratins.
2. α6β4 binds laminin-332 in the basal lamina at the cell's basal surface.
3. Plectin binds the β4 cytoplasmic tail and keratin intermediate filaments.
4. BP230 further tethers the keratin network to the plaque.
5. A continuous lamina → integrin → plaque → intermediate-filament linkage forms.
6. The adhesion resists shear and anchors the epithelium to the matrix.

Energy and Directionality

Hemidesmosome adhesion is affinity-driven — integrin–laminin binding requires no ATP — but assembling and maintaining the structure is energy-dependent (protein synthesis, transport, and intermediate-filament dynamics). The functional output is directional mechanical reinforcement: force applied to the tissue is routed through the intermediate-filament network into the matrix, rather than tearing the cell membrane. The intermediate-filament linkage provides strength under tension without the constant ATP turnover characteristic of actin-based adhesions.

Experimental Evidence

• Electron microscopy: hemidesmosomes appear as dense cytoplasmic plaques on the basal cell membrane, distinct from desmosomes.
• Knockout mice: deletion of α6, β4, or plectin causes skin blistering (epidermolysis bullosa-like) and is often lethal, proving their anchoring role.
• Human genetics: mutations in α6β4, plectin, BP180, or laminin-332 cause forms of epidermolysis bullosa.
• Autoimmunity: autoantibodies against BP180/BP230 cause bullous pemphigoid, a blistering disease.

Technique

• Transmission electron microscopy — identify the dense hemidesmosome plaque.
• Immunofluorescence — localize α6β4, plectin, BP230, and keratin at the basal surface.
• Genetic sequencing — identify mutations in epidermolysis bullosa.
• Autoantibody assays — diagnose bullous pemphigoid.
• Skin-blistering (friction) assays in mouse models — test adhesion strength.

How it works

1. Epithelial cells synthesize α6β4 integrin, plectin, BP230, and keratins.
2. α6β4 binds laminin-332 in the basal lamina at the cell's basal surface.
3. Plectin binds the β4 cytoplasmic tail and keratin intermediate filaments.
4. BP230 further tethers the keratin network to the plaque.
5. A continuous lamina → integrin → plaque → intermediate-filament linkage forms.
6. The adhesion resists shear and anchors the epithelium to the matrix.

Common confusions

• "A hemidesmosome is half of a desmosome." — No. Desmosomes are cadherin-based cell–cell junctions linking intermediate filaments; hemidesmosomes are integrin-based cell–matrix junctions. Only the appearance (half-plaque) inspired the name.
• "Hemidesmosomes use cadherins." — They use integrin α6β4.
• "Hemidesmosomes link to actin." — They link to keratin intermediate filaments (focal adhesions link to actin).
• "Hemidesmosomes and focal adhesions are the same." — Both are cell–matrix adhesions, but they use different integrins, different cytoskeletal links (IF vs. actin), and different functions (stable anchor vs. dynamic signaling).
• "Hemidesmosomes are everywhere." — They are specialized adhesions found mainly in epithelia (skin, cornea, etc.).

Quick review

• Hemidesmosome = cell–basal-lamina anchor (epithelia).
• laminin-332 → α6β4 integrin → plectin/BP230 → keratin IFs.
• Integrin-based (not cadherin); IF-linked (not actin).
• Distinct from desmosomes and focal adhesions.
• Defects → epidermolysis bullosa; autoimmunity → bullous pemphigoid.