Cell Adhesion and Cell Junctions

Fundamentals of Cytology and Histology

• Cells: Defined as the fundamental structural and functional units of all life.

• Tissues: Defined as groups of similar cells that work together to carry out a specific function. Representative examples include:     * Epithelial tissue.     * Muscle tissue.     * Nervous tissue.

• Cytology: The scientific study of cells.

• Histology: The scientific study of tissues.

Tissue Organisation in Animals: Epithelium vs. Mesenchyme

• Epithelium (Epithelial Tissue):     * Characterized by tightly packed cells.     * Functions as a mechanical barrier (tissue lining) and a chemical barrier.     * Relies heavily on close cell-cell contacts.     * Polarity: Possesses distinct surfaces:         * Apical Surface: Faces the "outside" or the lumen.         * Basal Surface: Faces towards the basal lamina and underlying connective tissue.     * Structure: Sheets of cells held together by junctions with one another and the basal lamina.     * Morphological Types and Examples:         * Simple: Found in the gut.         * Cuboidal: Found in the kidney.         * Stratified: Found in the skin.         * Columnar: Found in the gut.         * Squamous: Found in the alveoli.

• Mesenchyme (Connective Tissue):     * Characterized by loosely packed cells.     * Cells are surrounded by an Extracellular Matrix (ECM).     * Features very little contact between individual cells.     * Primary Characteristic: Allows for extensive cell movement.

Epithelial-Mesenchymal Transition (EMT) and Plasticity

• Tissue Organization Flexibility: Tissue organization is not static; cells can transition between states.     * Epithelial-Mesenchymal Transition (EMT): The process where epithelial cells lose their polarity and cell-cell adhesion to become mesenchymal cells.     * Mesenchymal-Epithelial Transition (MET): The process where mesenchymal cells transition to form epithelia.

• Biological Importance of EMT/MET:     * Embryonic Development: Essential for processes like gastrulation and neural crest evasion.     * Inflammation: Plays a role in fibrosis.     * Cancer: Heavily involved in metastasis (the spread of cancer cells).

Principle Functions and Categories of Cell Junctions

• Key Elements of Epithelia:     * Specialized cell-cell contacts.     * Links with the basal lamina (ECM).     * Apical-basal polarity.

• Functional Categories:     1. Occluding Junctions: Specifically Tight Junctions. They seal cells together into sheets to form an impermeable barrier.     2. Communicating Junctions: Specifically Gap Junctions. They allow for the exchange of chemical or electrical information between adjacent cells.     3. Anchoring Junctions: Includes Adherens junctions, Desmosomes, Focal adhesions, and Hemidesmosomes. They attach cells and their cytoskeleton to other cells or the ECM, providing mechanical support.

Comprehensive Matrix of Animal Cell Junctions

• Tight Junction:     * Connection: Cell-cell.     * Molecular Components: Claudin, occludin.     * Function: Sealing the extracellular space.     * Association: Zona Occludens (ZO), cytoskeleton.     * Intercellular Space: None.

• Gap Junction:     * Connection: Cell-cell channel.     * Molecular Components: Connexins.     * Function: Cell coupling.     * Association: Connexons in register.     * Intercellular Space: $2-3\,nm$.

• Adherens Junction:     * Connection: Cell-cell adhesion.     * Molecular Components: Cadherins (homophilic).     * Function: Strengthening of epithelia and contact between connective tissue cells.     * Association: Catenin, actin filaments.     * Intercellular Space: $20-25\,nm$.

• Desmosome:     * Connection: Cell-cell adhesion.     * Molecular Components: Cadherins (homophilic).     * Function: Strengthening of epithelia.     * Association: Catenin/plakin, intermediate filaments.     * Intercellular Space: $25-35\,nm$.

• Focal Adhesion:     * Connection: Cell-ECM adhesion.     * Molecular Components: Integrins ($\alpha_5$)-fibronectin.     * Function: Contact to ECM.     * Association: Talin/vinculin, actin filaments.     * Intercellular Space: $20-25\,nm$.

• Hemidesmosome:     * Connection: Cell-basal lamina adhesion.     * Molecular Components: Integrins ($\alpha_6$)-laminin and $BPAG2$.     * Function: Anchoring to the basal lamina.     * Association: Plakin, intermediate filaments.     * Intercellular Space: $25-35\,nm$.

Occluding Junctions: Tight Junctions

• Structure and Visualization:     * Forms a seal between cells to prevent the movement of fluids and molecules across the epithelium.     * Can be visualized experimentally by injecting dye into the intracellular space.     * Molecular Basis: Interaction between transmembrane proteins, primarily claudins and occludin.     * Cytoskeletal Link: Connects to the cytoskeleton via zona occludens (ZO) proteins.

• Function and Polarity:     * Restrict paracellular movement (movement between cells).     * Enforce transcellular passage (controlled movement through the cell).     * Restrict the movement of membrane proteins between the basal and apical membranes, maintaining distinct membrane properties.

• Case Study: The Blood-Brain Barrier (BBB):     * Formed by the interaction between endothelial cells and astrocyte end feet.     * Controls the passage of water and small molecules (e.g., glucose, amino acids).     * Acts as an effective barrier for most drugs.     * Structural Basis: Very tight junctions between brain endothelial cells that lack fenestrae (pores).     * Special Proteins: Includes Claudin-3.

Communicating Junctions: Gap Junctions

• General Properties:     * Found in most epithelia and many other tissues.     * Maintain a very narrow gap of $2-4\,nm$ between membranes.     * Can be regulated: Can exist in open or closed states to control signal passage.

• Function and Molecular Structure:     * Create hydrophilic channels between cells.     * Allow exchange of ions and small molecules (crucial for coordinated actions like the heartbeat).     * Visualization: Can be visualized by injecting a small, hydrophilic dye into a single cell.     * Connexons: Formed by two hemichannels (connexons) in adjacent cells.     * Connexin: Each connexon consists of $6$ connexin molecules.     * Genetics: Encoded by a whole family of connexin ($CX$) genes.

Adhesive Cell-Cell Contacts and Transmembrane Molecules

• Two Main Modes of Adhesion:     1. Homophilic Interactions: Occur between similar molecules on adjacent cells.     2. Heterophilic Interactions: Involve different molecules on adjacent cells.

• Transmembrane Cell Adhesion Molecule (CAM) Families:     * Cadherins.     * Immunoglobulin CAMs.     * Integrins.     * Selectins.     * These molecules function as cell surface transmembrane receptors.

• Process Example: Leukocyte Extravasation:     * Rolling Adhesion: Selectins binding to glycoproteins. $E$-selectins and $P$-selectins on endothelial cells recognize leukocytes.     * Activation: Mediated by chemokines and their specific receptors.     * Tight Adhesion: Facilitated by integrins and $ICAMs$.     * Diapedesis: Facilitated by $PECAMs$.

Cadherins and Adhesion Mechanisms

• Cadherin Properties:     * Large transmembrane glycoproteins.     * Linked internally to the actin cytoskeleton.     * Calcium Dependency: Mediate adhesion only in the presence of $Ca^{2+}$ ions.

• Experimental Observations:     * Non-adhesive cells can be forced to aggregate by expressing cadherins.     * Defects in cadherin function disrupt embryonic development, proving selective cell adhesion is vital for tissue organization.

• Homophilic Adhesion Demonstration:     * Expressing two different cadherins (e.g., $P$-cadherin and $E$-cadherin) in a mixed population results in segregation.     * Cells expressing $P$-cadherin preferentially adhere to each other.     * Cells expressing $E$-cadherin preferentially adhere to each other.     * Even in mixed aggregates, populations remain well-separated, helping keep cells in the correct anatomical locations.

Anchoring Junctions: Detailed Epithelial Structure

• Adherens Junctions:     * Cadherins link to actin microfilaments.     * Adhesion Belt: Contractile bundles of actin microfilaments form a continuous band around the cell.     * Linkage: Actin is connected to cadherins via adapter proteins called catenins.     * Network: Creates an actin network across the entire epithelium that can be contractile.

• Desmosomes:     * Utilize special cadherin-like molecules: Desmoglein and Desmocollin.     * Internal Linkage: Cadherins link to plakoglobin (a catenin-like molecule), which links to the plakin desmoplakin.     * Cytoskeletal Link: Connects to intermediate filaments.     * Function: Provides great tensile strength to the tissue via dense plaques.

Cell Contacts with the Extracellular Matrix (ECM)

• Integrins: The primary cell surface receptors binding to adhesive glycoproteins in the ECM.     * Heterodimers consisting of $\alpha$ and $\beta$ subunits.     * $\alpha$ Subunit: Mainly responsible for binding specificity.     * Propeller Region: The extracellular region containing the binding site for ECM molecules.     * Recognition Sequence: Common sequence is Arg-Gly-Asp (RGD), found in molecules like fibronectin.     * Intracellular Domain: Links to the cytoskeleton via adapters.

• Focal Adhesions:     * Common in mesenchymal cells in connective tissue.     * Integrins (specifically $\alpha_5\beta_1$) link the ECM (fibronectin) to the actin cytoskeleton.     * Required Adapters: Talin, and actin-binding proteins vinculin and $\alpha$-actinin.     * Involvement: Crucial for cell movement.

• Hemidesmosomes:     * Contact points between epithelial cells and the basal lamina.     * Integrins (mainly $\alpha_6\beta_4$) bind to laminin and Type IV collagen in the basal lamina.     * Cytoskeletal Link: Linked to intermediate filaments (e.g., keratin) by plakin proteins, forming a plaque.     * Appearance: Superficially resembles half a desmosome.     * Clinical Significance: Defects in laminin-binding integrins ($\alpha_6\beta_4$), laminin $5$, or collagen $VII$ lead to Junctional Epidermolysis Bullosa (skin blistering).

Questions & Discussion

• Q1: Adherens junctions are specialized cell junctions that form by linking the __________ to transmembrane proteins known as __________.     * Answer: b) actin cytoskeleton . . . cadherins.

• Q2: Which of the following cell junctions is involved with anchoring the extracellular matrix to the cell? (I. Focal Adhesion, II. Hemidesmosome, III. Desmosome, IV. Adherent Junction)     * Answer: b) I and II (Focal adhesions and Hemidesmosomes).

• Q3: Which of the following statements is NOT TRUE about gap junctions?     * Answer: c) Gap junctions prevent molecules and ions from traveling between cells in the extracellular space. (This is the function of tight junctions).

• Q4: Which of the following proteins forms channels that permit electrical communication between cells across gap junctions?     * Answer: c) Connexin.

• Q5: Which cellular junction will be most useful in preventing the movement of material between cell membranes?     * Answer: a) Tight junctions.

• Q6: Which type of cellular junction is prominent in cardiac myocytes and allows for a unified contraction of the heart? (Hint: ions need to flow rapidly between them).     * Answer: c) Gap junctions.