Premed · Premed · Cell Biology
Lecture 23: Cell-Cell Adhesion and Tight Junctions
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the four major types of cell junctions and their functions
- Explain the structure and function of cadherins and adherens junctions
- Describe the structure and function of desmosomes
- Explain how tight junctions create a paracellular barrier and establish cell polarity
- Describe gap junctions and their role in direct cell-cell communication
Lecture Content
I. Overview of Cell Junctions
Cell junctions are specialized structures that mediate cell-cell and cell-matrix adhesion, and they are essential for the organization and function of multicellular tissues. In polarized epithelial cells, four major types of cell-cell junctions are arranged in a stereotyped order from apical to basal. Tight junctions (zonula occludens) seal the space between cells and provide barrier function. Adherens junctions (zonula adherens) mediate strong adhesion and are linked to the actin cytoskeleton. Desmosomes (macula adherens) provide robust adhesion and are linked to intermediate filaments. Gap junctions form direct communication channels between adjacent cells. Together, the tight junctions, adherens junctions, and desmosomes form the junctional complex of polarized epithelial cells. Most cell-cell adhesion molecules are Ca2+-dependent, with cadherins being the prime example. Notable exceptions include gap junction connexins and immunoglobulin superfamily CAMs, which do not require calcium.
II. Cadherins and Adherens Junctions
Cadherins are Ca2+-dependent transmembrane adhesion molecules that play central roles in tissue organization. The classical cadherins include E-cadherin (epithelial), N-cadherin (neural and mesenchymal), P-cadherin (placenta), and VE-cadherin (vascular endothelium). Each is a single-pass transmembrane protein with an extracellular domain composed of five cadherin repeats (EC1 through EC5). Ca2+ ions bind between adjacent repeats and rigidify the extracellular domain. The EC1 domain mediates homophilic binding, in which cadherins on one cell bind the same type of cadherin on an adjacent cell. The binding mechanism involves strand swapping, where a tryptophan residue on EC1 inserts into the hydrophobic pocket of the partner EC1 domain.
The cytoplasmic domain of classical cadherins binds a set of intracellular proteins called catenins. Beta-catenin binds directly to the cadherin cytoplasmic tail, and alpha-catenin binds beta-catenin and provides the link to the actin cytoskeleton. p120-catenin binds the juxtamembrane region of the cadherin and stabilizes it at the cell surface by preventing endocytosis. The homophilic, Ca2+-dependent adhesion mediated by cadherins means that cells preferentially adhere to other cells expressing the same cadherin type, a principle demonstrated by classic cell sorting experiments. This property underlies tissue organization and boundary formation during development.
Adherens junctions (zonula adherens) form a continuous belt-like junction encircling the apical region of epithelial cells, just below the tight junctions. They are E-cadherin-based and connected to a circumferential actin belt. Alpha-catenin provides the linkage to actin filaments both directly and through vinculin. Actin dynamics and myosin II-driven contraction at adherens junctions drive morphogenetic movements such as apical constriction during gastrulation and neural tube closure. Adherens junctions are also sites of mechanosensing: when mechanical force is applied to alpha-catenin, it unfolds to expose a vinculin-binding site, reinforcing the junction.
E-cadherin functions as a tumor suppressor. Loss of E-cadherin is a hallmark of epithelial-to-mesenchymal transition (EMT) and invasion. The CDH1 gene encoding E-cadherin is a recognized tumor suppressor, and germline CDH1 mutations cause hereditary diffuse gastric cancer. Transcriptional repressors including Snail, Slug, Twist, and ZEB1 downregulate E-cadherin expression during EMT.
<image>Cadherins and adherens junctions. Panel A: E-cadherin structure — five extracellular cadherin repeats (EC1-EC5) with Ca2+ ions (orange spheres) bound between repeats, transmembrane domain, and cytoplasmic domain bound to p120-catenin (juxtamembrane), beta-catenin, and alpha-catenin linking to actin filaments (with vinculin). Two E-cadherin molecules from opposing cells engage in trans-homophilic binding via strand swapping at EC1. Panel B: Adherens junction belt — cross-section of two adjacent epithelial cells showing the continuous belt of E-cadherin-mediated adhesion near the apical surface, linked to the circumferential actin bundle with myosin II. Panel C: Cell sorting experiment — dissociated cells expressing E-cadherin (green) and N-cadherin (red) spontaneously sort into separate clusters, demonstrating homophilic specificity.</image>
III. Desmosomes
Desmosomes (macula adherens) are spot-like junctions that provide exceptionally strong adhesion, and they are especially abundant in tissues subject to mechanical stress such as the skin and cardiac muscle. Their molecular architecture includes three categories of components. Desmosomal cadherins, consisting of desmoglein (Dsg1-4) and desmocollin (Dsc1-3), engage in heterophilic interactions in the extracellular space. Different isoforms are expressed in different layers of the epidermis, with Dsg1 predominantly suprabasal and Dsg3 predominantly basal. Armadillo proteins, including plakoglobin (gamma-catenin) and plakophilin, bind the cytoplasmic tails of the desmosomal cadherins. Desmoplakin links the armadillo proteins to intermediate filaments, specifically keratins in epithelia and desmin in cardiac muscle.
Several important diseases arise from desmosome dysfunction. Pemphigus vulgaris is caused by autoantibodies against Dsg3 (and often Dsg1), which disrupt desmosomal adhesion and cause keratinocyte separation (acantholysis), leading to blistering of skin and mucous membranes. The Nikolsky sign, in which gentle rubbing detaches the epidermis, is positive. Pemphigus foliaceus involves autoantibodies directed exclusively against Dsg1, producing superficial blistering confined to the upper epidermis where Dsg1 is the sole desmoglein. Arrhythmogenic right ventricular cardiomyopathy (ARVC) results from mutations in desmosomal genes including plakoglobin, desmoplakin, plakophilin-2, Dsg2, and Dsc2. Cardiomyocyte detachment leads to fibro-fatty replacement of the myocardium, causing ventricular arrhythmias and sudden cardiac death.
IV. Tight Junctions (Zonula Occludens)
Tight junctions are the most apical junction in epithelial and endothelial cell sheets and serve two essential functions. The barrier (gate) function seals the paracellular space and controls the passage of ions, water, and solutes between cells. The fence function maintains cell polarity by preventing the diffusion of membrane proteins between the apical and basolateral membrane domains. Structurally, tight junctions consist of a network of sealing strands that form a continuous belt around each cell, with the strands composed of rows of transmembrane proteins that form paired interactions with corresponding proteins on the adjacent cell.
The principal transmembrane proteins are the claudins, a family of approximately 27 members that constitute the major structural component of tight junction strands. Different claudins confer different permeability properties. Claudin-2, for example, forms cation-selective pores that create the leaky junctions found in the proximal tubule, while claudin-16 (paracellin-1) functions as a Mg2+ channel in the thick ascending limb of Henle's loop, and mutations in its gene cause familial hypomagnesemia (FHHNC). Occludin was one of the first tight junction proteins discovered and plays a regulatory role, though it is not essential for strand formation. JAM proteins (junctional adhesion molecules) are immunoglobulin superfamily members that regulate leukocyte transmigration across endothelia.
On the cytoplasmic face, ZO-1, ZO-2, and ZO-3 (zonula occludens proteins) serve as PDZ-domain scaffolds that link claudins and occludin to the actin cytoskeleton and recruit signaling molecules and polarity proteins. Regulation of tight junction permeability is physiologically important. Cytokines such as TNF-alpha and IFN-gamma increase permeability, a process relevant to inflammatory bowel disease. Zonulin is an endogenous modulator of tight junction permeability in the intestine. Myosin light chain kinase (MLCK)-driven actin contraction can also open tight junctions. The blood-brain barrier is formed by endothelial tight junctions in brain capillaries that are exceptionally tight, with very high electrical resistance. Claudin-5 is critical for this barrier, and astrocyte end-feet and pericytes provide additional support for barrier integrity.
<image>Tight junction structure and function. Panel A: Cross-section of two adjacent epithelial cells with tight junction at the apex — network of sealing strands (anastomosing lines) composed of claudin and occludin molecules from opposing cells interacting in the intercellular space. ZO-1 on the cytoplasmic face links to the actin cytoskeleton. Panel B: Fence and gate functions — the tight junction separates the apical membrane domain (with specific proteins, shown in blue) from the basolateral domain (different proteins, shown in red), preventing lateral diffusion (fence function). It also blocks paracellular passage of solutes (gate function), with selective claudin-based pores allowing specific ions through. Panel C: Freeze-fracture electron microscopy view showing the network of tight junction strands as ridges on the P-face and complementary grooves on the E-face, illustrating the variability in strand number (more strands = tighter barrier).</image>
V. Gap Junctions
Gap junctions mediate direct cell-cell communication by forming channels that connect the cytoplasm of adjacent cells. The building blocks are connexins, four-pass transmembrane proteins with approximately 21 family members in humans. Six connexin subunits assemble into a ring with a central pore of approximately 1.5 nm diameter, forming a connexon (hemichannel). Two connexons from adjacent cells dock to create a complete gap junction channel, and channels cluster into gap junction plaques containing hundreds to thousands of channels.
Gap junction channels allow the passage of molecules smaller than approximately 1 kDa. This includes ions such as Ca2+ and K+, which provide electrical coupling between cells, and small metabolites such as cAMP, IP3, glucose, and amino acids, which provide metabolic coupling. Proteins and nucleic acids are excluded. Gap junction channels can be opened or closed through gating mechanisms. Channels close in response to low pH, high intracellular Ca2+, and phosphorylation by kinases including Src and PKC. This gating in response to cell damage prevents the spread of injury signals to healthy neighboring cells.
Gap junctions come in several types based on their subunit composition. Homomeric connexons contain six identical connexin subunits, while heteromeric connexons contain mixed connexin types. Homotypic channels consist of two identical connexons, whereas heterotypic channels are formed by two different connexons docking together.
The clinical significance of gap junctions is considerable. Mutations in connexin 26 (GJB2) are the most common cause of hereditary non-syndromic sensorineural deafness. Connexin 43 (GJA1) is the major cardiac gap junction protein, and mutations cause oculodentodigital dysplasia. Cardiac gap junctions mediate the electrical coupling between cardiomyocytes that is essential for coordinated contraction, and gap junction remodeling in heart failure contributes to arrhythmias.
VI. Selectins and the Immunoglobulin Superfamily
Selectins are Ca2+-dependent adhesion molecules with lectin domains that bind carbohydrate ligands. L-selectin is expressed on leukocytes, E-selectin on activated endothelium, and P-selectin on activated endothelium and platelets, where it is stored in Weibel-Palade bodies. Selectins mediate the initial rolling adhesion of leukocytes on the endothelium during inflammation.
Immunoglobulin superfamily CAMs (IgCAMs) include ICAM-1 and VCAM-1 on the endothelium, which bind integrins on leukocytes to mediate firm adhesion, and NCAM, which mediates Ca2+-independent homophilic binding in neural cell adhesion.
Leukocyte extravasation (diapedesis) follows a multi-step adhesion cascade. Selectin-mediated rolling is followed by chemokine-triggered activation of leukocyte integrins, then integrin-ICAM firm adhesion, and finally transmigration through the endothelium, which can occur via either a paracellular or transcellular route. Defects in this cascade cause leukocyte adhesion deficiency (LAD): mutations in integrin beta-2 cause LAD type I, while defects in selectin ligand synthesis cause LAD type II, both resulting in recurrent infections due to the inability of leukocytes to reach sites of infection.
