Cell Junctions — Tight, Adherens, Desmosome, and Gap Junctions

Core Explanation

Overview of the Junctional Complex

In classic polarized epithelia, the four junction types are arranged in a stereotyped apicobasal sequence:

Apical → Tight junction → Adherens junction → Desmosome → Gap junctions & hemidesmosomes (basal) → Basal

This spatial organization reflects functional logic: the tight junction creates the barrier and polarity fence; the adherens junction provides circumferential adhesion and coordinates the actin cytoskeleton; desmosomes distribute mechanical stress across the sheet; gap junctions allow metabolic and electrical communication.

Tight Junctions

Tight junctions (zonula occludens) form a continuous, belt-like seal encircling the apical perimeter of epithelial cells. They serve two primary functions:

1. Barrier function: They restrict paracellular diffusion of solutes, ions, and water between cells. This is what makes the gut epithelium selectively absorb nutrients rather than passively leaking.
2. Fence function: They maintain epithelial polarity by preventing the free diffusion of membrane proteins and lipids between the apical and basolateral domains.

Molecular Architecture

The core transmembrane proteins are the claudins — a family of ~27 tetraspan proteins in mammals, each with four transmembrane helices and two extracellular loops. The first extracellular loop contains a signature W-GLW-C-C motif and determines charge selectivity of the paracellular pore. Different claudin isoforms create barriers with different permeability properties:

Claudins polymerize into linear strands visible by freeze-fracture electron microscopy. These strands form the continuous anastomosing network that constitutes the physical barrier.

Occludin, the first-identified tight junction transmembrane protein, is not essential for strand formation (strands still form in occludin-knockout cells) but modulates barrier properties and signaling.

ZO proteins (ZO-1, ZO-2, ZO-3) are MAGUK-family cytoplasmic scaffold proteins that link claudins and occludin to the actin cytoskeleton via their PDZ domains. ZO-1/2 are essential for claudin polymerization and junction assembly.

Regulation

Tight junction permeability is dynamically regulated. Phosphorylation of claudins and occludin by kinases (PKC, PKA, Rho kinase) alters barrier properties. Proinflammatory cytokines (TNF-α, IFN-γ) increase paracellular permeability by inducing claudin redistribution and ZO-1 dissociation.

Blood–Brain Barrier

The blood–brain barrier (BBB) is a specialized tight junction system of brain capillary endothelial cells. Endothelial tight junctions — particularly claudin-5 — form the central permeability barrier that limits paracellular diffusion between blood and brain parenchyma. Claudin-5 knockout mice die at birth with a size-selective (~800 Da cutoff) BBB leak.

However, the BBB is not simply "made by tight junctions." It is a multicellular neurovascular unit — a functional assembly that includes:

• Endothelial cells: the primary permeability barrier via claudin-5/occludin tight junctions.
• Pericytes: embedded in the endothelial basement membrane; regulate tight junction expression and endothelial transcytosis rates.
• Astrocyte endfeet: ensheath capillaries; secrete factors (SHH, angiopoietin-1) that induce and maintain tight junction integrity.
• Basement membrane: the shared ECM between endothelial cells and pericytes/astrocytes.

Phenotypically, BBB endothelial cells have extremely low rates of transcytosis, lack fenestrations, and express efflux transporters (P-glycoprotein, BCRP) that actively pump lipophilic molecules back into the blood. Tight junctions are central but not sufficient — the whole neurovascular unit is required for BBB function. Pericyte deficiency alone causes BBB breakdown without primary endothelial defects.

Adherens Junctions

Adherens junctions (zonula adherens) form a continuous adhesion belt just below tight junctions. Their core components are classical cadherins (E-cadherin in epithelium, N-cadherin in neural tissue, VE-cadherin in endothelium).

Cadherins and Calcium Dependence

Classical cadherins are single-pass transmembrane glycoproteins with five extracellular cadherin (EC) repeat domains. The EC1 domain mediates homophilic trans-interaction — cadherins on one cell bind cadherins of the same type on an adjacent cell. This binding is strictly Ca²⁺-dependent: Ca²⁺ ions bind at linker regions between EC domains, rigidifying the extracellular region. Without Ca²⁺, the ectodomain becomes floppy and adhesion fails. This is why treatment with EDTA or EGTA rapidly dissociates epithelial sheets.

Catenins

The cytoplasmic tail of E-cadherin binds β-catenin (or γ-catenin/plakoglobin), which in turn binds α-catenin. α-Catenin can directly bind F-actin and also recruits actin regulators (formin-1, α-actinin, vinculin). The cadherin–catenin complex is essential for coupling adhesion to the actin cytoskeleton — without α-catenin, cadherin-mediated adhesion is non-functional.

p120-catenin binds a juxtamembrane domain of the cadherin tail and regulates cadherin stability at the surface (prevents endocytosis and degradation).

Mechanotransduction

E-cadherin adhesions are mechanosensors. Applied tension induces α-catenin to undergo a conformational change that exposes a vinculin-binding site, recruiting vinculin and reinforcing the actin linkage — analogous to talin unfolding in focal adhesions.

Desmosomes

Desmosomes (macula adherens) are spot-weld-like junctions that provide robust mechanical strength by linking the intermediate filament cytoskeletons of adjacent cells. They are particularly abundant in tissues subject to mechanical stress: skin, cardiac muscle, and the bladder urothelium.

Molecular Components

The desmosomal cadherins (desmogleins and desmocollins) have a similar EC-repeat structure to classical cadherins and are also Ca²⁺-dependent. The plaque proteins assemble into a dense, electron-dense structure that distributes mechanical load to the IF network.

Key distinction from adherens junctions: Desmosomes link to intermediate filaments (keratins in epithelia, desmin in cardiac muscle) via desmoplakin, not to actin.

Disease: Pemphigus

In pemphigus vulgaris, autoantibodies target desmoglein-3 (and Dsg1 in pemphigus foliaceus). Antibody binding directly disrupts desmosomal adhesion (steric hindrance) and triggers intracellular signaling that depletes desmosomes from the membrane. The result is acantholysis — separation of keratinocytes — producing flaccid intraepidermal blisters that can be fatal without treatment.

Gap Junctions

Gap junctions are arrays of intercellular channels (connexons) that directly connect the cytoplasm of adjacent cells, permitting the passage of ions, second messengers (cAMP, IP₃, Ca²⁺), and small metabolites (<~1.2 kDa). This creates electrical and metabolic coupling between cells.

Structure

Each connexon (hemichannel) is a hexamer of connexin subunits, arranged around a central pore. Two connexons in adjacent plasma membranes dock to form a continuous aqueous channel. Humans have 21 connexin genes, each named by molecular weight: Cx26, Cx32, Cx43, etc.

Connexons can be homomeric (six identical subunits) or heteromeric (mixed). Channels can be homotypic (identical connexons) or heterotypic (different connexons). This combinatorial diversity tunes permeability and gating properties.

Gating

Gap junction channels are gated. They close in response to:

• Elevated intracellular Ca²⁺ (protection against damage spreading from a dying cell)
• Low intracellular pH
• Transjunctional voltage differences
• Phosphorylation (PKA, PKC, MAPK, and Src can regulate Cx43)

Physiological Roles

Experimental Evidence

• Freeze-fracture EM: Revealed tight junction strands and gap junction plaques as distinct ultrastructural entities.
• Ca²⁺ switch experiments: Remove Ca²⁺ (EGTA) → cadherin adhesion fails → cells round up. Re-add Ca²⁺ → cadherins re-engage → junctions re-form. Classic demonstration of Ca²⁺ dependence.
• Dye coupling: Microinjection of Lucifer Yellow (457 Da) into one cell spreads to neighbors via gap junctions but not in Cx43/Cx32 knockout cells, confirming the connection.
• Claudin-5 knockout mouse: Claudin-5⁺/⁻ mice show size-selective BBB leakage to molecules <800 Da, demonstrating claudin-5's role as the primary size-selective paracellular barrier of the BBB.
• Cx26 mutations and deafness: Connexin 26 (GJB2) mutations account for ~50% of congenital non-syndromic sensorineural deafness cases. Cx26 is expressed in cochlear supporting cells, where it maintains K⁺ homeostasis in the endolymph through gap-junction-coupled recycling.

Disease and Clinical Connections

High-Yield Summary

• Tight junctions: claudins + occludin + ZO proteins; barrier and fence functions; BBB = endothelial TJs (claudin-5 central) + neurovascular unit (pericytes, astrocytes).
• Adherens junctions: E-cadherin → β-catenin → α-catenin → actin; Ca²⁺-dependent; circumferential belt.
• Desmosomes: desmogleins/desmocollins → plakoglobin/plakophilin → desmoplakin → intermediate filaments; point-welds for mechanical strength.
• Gap junctions: connexin hexamers (connexons); <1.2 kDa pore; electrical and metabolic coupling; gated by Ca²⁺, pH, voltage, phosphorylation.
• Cadherins (classical and desmosomal) are both Ca²⁺-dependent; the key difference is cytoskeletal linkage — actin vs. intermediate filaments.

Practice Questions

1. A patient takes EDTA-containing medication. Within hours, they develop esophageal epithelial detachment. What is the molecular mechanism?

Answer: EDTA chelates extracellular Ca²⁺. E-cadherin at adherens junctions and desmogleins/desmocollins at desmosomes are Ca²⁺-dependent adhesion molecules. Without Ca²⁺, the extracellular cadherin domains become flexible and cannot maintain trans-interaction. Adherens junctions and desmosomes disassemble, causing epithelial sheet separation (acantholysis). This also explains why standard cell culture protocols use trypsin + EDTA for passaging — EDTA alone can dissociate epithelial colonies.

2. Knock-in of claudin-2 into the distal colon epithelium (which normally has "tight" claudins like claudin-1, -3, -5) would cause what physiological phenotype?

Answer: Claudin-2 forms cation-selective, water-permeable paracellular pores. Expressing claudin-2 in the distal colon would increase paracellular Na⁺ and water flux, reducing the colon's ability to dehydrate stool. The predicted phenotype is osmotic diarrhea — water follows Na⁺ through the leaky paracellular pathway, preventing normal fecal desiccation. This is the normal function of claudin-2 in the proximal small intestine, where a leaky epithelium supports nutrient absorption by solvent drag.

3. A mouse with a cardiac-specific deletion of Cx43 survives to birth but dies at ~6 weeks of age. What is the likely cause of death, and what would an ECG show?

Answer: Cx43 is the major ventricular gap junction protein in cardiac intercalated discs. Loss of Cx43 severely slows action potential propagation between ventricular cardiomyocytes. The ECG would show a widened QRS complex (slowed ventricular depolarization). The likely cause of death is ventricular arrhythmia — slow, heterogeneous conduction creates unidirectional block and re-entrant circuits, predisposing to ventricular tachycardia/fibrillation and sudden cardiac death. This highlights the essential role of electrical coupling in cardiac function.

Common Misconceptions

"The blood–brain barrier is made by tight junctions." Overly reductionist. Tight junctions (claudin-5) are the central permeability barrier of the BBB, but the barrier is a multicellular neurovascular unit property requiring pericytes, astrocytes, and specialized endothelial transport systems. Describing the BBB as "made by" tight junctions ignores the transcellular regulation and pericyte/astrocyte contributions.

"Desmosomes and adherens junctions are basically the same." No. Adherens junctions link to actin via catenins; desmosomes link to IFs via desmoplakin. The cadherin types, plaque components, and cytoskeletal connections are entirely distinct, even though both use cadherin-family adhesion proteins.

"Gap junctions are passive pores that are always open." No. Gap junction channels are dynamically gated by intracellular Ca²⁺, pH, voltage, and phosphorylation. Their permeability is regulated, not constitutive.