Cell Junctions — Robbins, Cotran & Kumar: Pathologic Basis of Disease (11th Edition)

Overview

1. Occluding junctions (tight junctions)
2. Anchoring junctions (adherens junctions and desmosomes)
3. Communicating junctions (gap junctions)

1. Occluding Junctions (Tight Junctions)

Structure

• Form a tight meshlike network of macromolecular contacts between neighboring cells (visible en face by freeze-fracture electron microscopy).
• The transmembrane proteins involved belong to two families:
  • Claudins — the principal structural proteins of the tight junction strand
  • Tight junction-associated MARVEL proteins (TAMPs) — a tetraspan protein family that includes occludin and tricellulin

Intracellular Scaffolding

• ZO-1, ZO-2, ZO-3 (zonula occludens protein family)
• Cingulin

Key Functions

Tight junctions are dynamic structures — not static walls. They open during inflammation to permit leukocyte migration.

2. Anchoring Junctions

2a. Adherens Junctions

• Often located just below tight junctions (apically placed in the junctional complex).
• The transmembrane adhesion molecules (cadherins) are linked to intracellular actin microfilaments.
• Through this actin linkage, they can influence cell shape and motility.

Loss of the epithelial adherens junction protein E-cadherin (encoded by CDH1) explains the discohesive invasion pattern seen in:

• Diffuse-type gastric carcinoma (~50% harbor loss-of-function CDH1 mutations or E-cadherin silencing by promoter hypermethylation)
• Lobular carcinoma of the breast

2b. Desmosomes

• Located more basally than adherens junctions.
• Cadherins here are linked to intracellular intermediate filaments (e.g., keratin in epithelial cells).
• This linkage allows extracellular forces to be mechanically communicated and dissipated across multiple cells — giving epithelial sheets their tensile strength.

Pemphigus vulgaris — autoimmune disease caused by IgG autoantibodies against desmogleins (desmoglein-1 and desmoglein-3) that disrupt desmosome integrity, resulting in intraepidermal blister formation.

Arrhythmogenic right ventricular cardiomyopathy (ARVC) — caused by mutations in genes encoding desmosomal junctional proteins at the intercalated disk (e.g., plakoglobin) or proteins interacting with the desmosome.

2c. Hemidesmosomes

• A half-desmosome that connects cells to the ECM (not cell-to-cell).
• The transmembrane connector proteins are integrins (not cadherins), which attach to intermediate filaments and link the cytoskeleton to the ECM.
• Focal adhesion complexes — composed of >100 proteins — localize at hemidesmosomes.
• Their component proteins can generate intracellular signals when cells are subjected to shear stress (e.g., endothelium in the bloodstream, cardiac myocytes in a failing heart).

3. Communicating Junctions (Gap Junctions)

Structure

• Consist of a dense planar array of 1.5–2 nm pores called connexons.
• Each connexon is formed by a pair of hexamers (one hexamer contributed by each adjacent cell) of transmembrane connexin proteins.
• These pores permit passage of:
  • Ions (e.g., Ca²⁺)
  • Nucleotides
  • Sugars
  • Amino acids
  • Vitamins
  • Other small molecules (<1,000 Da)

Regulation of Permeability

Key Physiological Role — Cardiac Syncytium

Gap junctions in cardiac myocytes allow cell-to-cell calcium fluxes that permit the many cells of the myocardium to behave as a functional syncytium with coordinated waves of contraction.

Pathological Relevance

Charcot-Marie-Tooth disease (X-linked form) — linked to mutations in the GJB1 gene (encoding connexin 32 / gap junction protein beta-1), expressed in Schwann cells. Males are typically more severely affected, though female carriers can also be symptomatic.

Summary Comparison Table

Clinical Pearls from Robbins & Cotran

• E-cadherin (CDH1) loss is a hallmark of diffuse gastric carcinoma and lobular breast carcinoma — tumors show a "single-file" or non-cohesive invasive pattern.
• Desmoglein autoantibodies in pemphigus cause the epidermis to fall apart (acantholysis); the level of the split depends on which desmoglein isoform is targeted.
• Connexin 32 mutations (GJB1) cause X-linked Charcot-Marie-Tooth disease by disrupting gap junction–mediated communication in Schwann cells.
• Desmosomal protein mutations underlie ARVC, a cardiomyopathy prone to sudden cardiac death and fibro-fatty replacement of the right ventricle.

Visual Guide to Cell Junctions

Fig. 1.9 — Robbins & Cotran (Official Textbook Diagram)

This is the actual Fig. 1.9 from the textbook. It shows, from apical to basal:

• Tight junction (yellow, just below the microvilli) — sealed by claudins/TAMPs
• Adherens junction (red, linked to actin microfilaments — green)
• Desmosome (green, linked to intermediate filaments — purple/magenta)
• Gap junctions (gold/orange, lateral, scattered)
• Hemidesmosome (basal, with blue integrin anchors into the basement membrane)

Junction Proteins in Endothelial Cells (Blood-Brain Barrier)

Clearly labels the three junction types stacked on the lateral membrane of endothelial cells: Connexin (gap junction) at the top, E-cadherin (adherens junction) in the middle, and JAM/Occludin/Claudin (tight junction) at the bottom. The right panel shows how ischemia fragments tight junction proteins, increasing permeability.

Claudin Family — Functions of Tight Junction Proteins

Illustrates how claudins — the key transmembrane proteins of tight junctions — go beyond forming a simple barrier. They regulate selective permeability, barrier formation, cell migration (via MMPs), cell proliferation (via Sp1/ZONAB), and intracellular signaling (PI3K/Akt/Bcl-2 and MEK/ERK1/2 pathways).

Gap Junction / Connexin — Therapeutic & Structural Overview

Shows the connexin protein structure (four transmembrane domains, central pore) and multiple strategies to modulate gap junction function, illustrating how connexons work as the molecular machinery of gap junctions.

Quick Visual Reference: Junction Location in the Cell

What's Covered in This Summary

Topic 1: Morphology & Physiology of Bacteria (General Bacteriology)

1. Size of bacteria — from 0.1 μm (Chlamydia) to several microns
2. Prokaryote vs. eukaryote differences — full comparison table (ribosome, nucleus, wall, etc.)
3. Bacterial classification — by shape, arrangement, Gram staining
4. Bacterial structure (outside-in):
   • Capsule → Cell wall (GP vs. GN) → Peptidoglycan → Cytoplasmic membrane → Cytoplasm
   • Special features: LPS/endotoxin, teichoic acids, porins, plasmids, 70S ribosomes
5. Surface structures — Flagella (types, H antigen), Pili/fimbriae (adherence, sex pili), Endospores (resistance, destruction by autoclave)
6. Bacterial physiology — Binary fission, growth curve (4 phases), oxygen requirements, temperature, genetic exchange (transformation/transduction/conjugation)
7. Special bacteria — Mycobacteria, Mycoplasma, Chlamydia, Rickettsia, Spirochetes

Topic 2: Laboratory Diagnosis of Bacterial Infections

1. Specimen collection — Blood, CSF, urine, respiratory, wound/pus, stool, sterile fluids (volumes, techniques, transport media)
2. Microscopy — Gram stain, Acid-fast stain (ZN/Kinyoun), India ink, Albert's stain, Dark-field, Fluorescent microscopy
3. Culture methods — Types of media (simple, enriched, selective, differential, transport), incubation conditions, colony characteristics
4. Biochemical tests — Catalase, coagulase, oxidase, urease, indole, TSI, CAMP, bile solubility, PYR
5. Antibiotic susceptibility — MIC (broth dilution), Disk diffusion (Kirby-Bauer), E-test, automated systems
6. Immunological tests — Latex agglutination, ELISA, immunofluorescence, Widal, VDRL/TPHA
7. Molecular tests — PCR/NAAT, MALDI-TOF MS, 16S rRNA sequencing, FilmArray
8. Summary comparison table of all methods by speed, sensitivity, cost