Comprehensive Study Notes on Cell Adhesion and Junctions

Overview and Evolutionary Context of Cell Adhesion

• Evolutionary Origins: Cell adhesion mechanisms emerged approximately $600$ million years ago alongside the first multicellular metazoans. The basic organization established then is maintained in the modern tissues of mammals today.
• Multicellular Maintenance: Cell adhesion is fundamentally crucial for building and maintaining complex multicellular organisms.
• Cohesion Mechanisms: Cells utilize specific molecular mechanisms to adhere to one another or to the extracellular matrix (ECM). This forms an intricate network that supports tissue structures.
• Response to External Forces: Adhesive structures do more than hold cells together; they allow tissues to withstand and respond to external forces. This facilitates dynamic processes including:     * Cell movement.     * Tissue distribution during embryonic development.     * Wound healing.

Dynamic Nature of Cell Adhesion and Migration

• Migration Requirements: Migrating cells must perform a specific sequence of actions: reach, recognize, bind, and remain with their target cells, tissues, or organs.
• Biological Implications: Adhesion has critical implications in:     * Embryogenesis.     * Cancer progression and metastasis.     * Infection mechanisms.     * Transplant potential (specifically regarding stem cells).     * Various pathologies.
• Tissue Development Examples:     * Epithelial cell to basal lamina: Crucial for post-burn skin regeneration.     * Neural crest cells: Essential for the development of the nervous system in the embryo.     * Endothelium and blood vessels: Relevant to cancer progression and metastasis.

Molecular Guiding Mechanisms

• Navigation: Cells utilize molecular guiding mechanisms to reach destinations, primarily classified as:     * Chemotaxis: Movement toward a chemical stimulus.     * Chemorepulsion: Movement away from a chemical stimulus.
• Biological Joining: The two primary ways animal cells are joined together are:     1. Connective Tissues.     2. Epithelial Cells.

Cellular Adherence and Tissue Formation

• Morphogenetic Processes: The combination of cell motility and adherence allows for various morphogenetic processes.
• Recognition and Assembly: Once a migrating cell reaches its destination via chemotaxis or chemorepulsion, it must recognize the appropriate cell types and join them to assemble into a tissue.
• Cell Adhesion Molecules (CAM): Adhesion is mediated by cell surface proteins known as CAMs. These may or may not be calcium-dependent ($Ca^{2+}$).
• Sequence of Junction Formation:     1. Events Before Junction Formation: Initial adhesion occurs, followed by the organization of the cytoskeleton around the molecules mediating that adhesion.     2. Junction Formation: In epithelia, daughter cells from stem cells remain adherent to the matrix. Selective adhesions thereafter create the specific architecture of the tissue or organ.
• Lymphocyte Extravasation: The process of lymphocytes crossing the endothelium involves a specific sequence:     * Rolling: Weak initial interaction.     * Adhesion: Mediated by homing receptors and vascular addressins.     * Migration: The cell moves across the vessel wall.     * Entry: The cell enters the target tissue.

Principles of Homophilic and Heterophilic Adhesion

• Homophilic (Homotypic) Adhesion:     * Characterized by binding between identical or very similar molecules on adjacent cell surfaces.     * Predominant in cell-cell junctions mediated by cadherins.     * Essential for tissue cohesion and the formation of homogeneous tissues.
• Heterophilic (Heterotypic) Adhesion:     * Determines the bond between different types of molecules.     * Typical in cell-matrix interactions.     * Important for anchoring cells to the ECM and for cell migration.     * Allows cells to interact with a diverse extracellular environment.

Functional Classification of Cell Junctions

• Anchoring Junctions: Mechanically connect cells to neighbors or the ECM.     * Actin Filament Attachment Sites:         * Cell-cell: Adherens junctions.         * Cell-matrix: Actin-linked cell-matrix adhesions.     * Intermediate Filament Attachment Sites:         * Cell-cell: Desmosomes.         * Cell-matrix: Hemidesmosomes.
• Occluding Junctions: Seal epithelial cells together to prevent paracellular passage.     * Tight Junctions: Found in vertebrates.     * Septate Junctions: Found in invertebrates.
• Channel-Forming Junctions: Mediate transfer of chemical or electrical signals.     * Gap Junctions (Communicating Junctions): Found in animals.     * Plasmodesmata: Found in plants.
• Signal-Transducing Junctions: Mediate signal transfer through specialized contacts.     * Chemical Synapses: In the nervous system.     * Immunological Synapses: In the immune system.     * Transmembrane Ligand-Receptor Interactions: Such as Delta-Notch signaling.
• Note: Anchoring, occluding, and channel-forming junctions can all possess signaling functions alongside their structural roles.

Detailed Structure of Anchoring Junctions

• Components: Anchoring junctions consist of two main classes of proteins:     1. Transmembrane Adhesion Proteins: Possess a cytoplasmic tail attaching to intracellular anchoring proteins and an extracellular domain interacting with ligands on other cells or the matrix.     2. Intracellular Anchoring Proteins: Form a plaque on the cytoplasmic surface of the membrane to connect the junctional complex to actin or intermediate filaments.
• Signaling: Many junctions contain intracellular signaling proteins that permit the junction to transmit signals back into the cell.

Adherens Junctions: Epithelial and Non-Epithelial

• Epithelial Adherens Junctions:     * Formed by Cadherin-Catenin complexes, specifically E-cadherin.     * Uses calcium-mediated adhesion ($Ca^{2+}$).     * Forms an adhesion belt (circumferential belt) beneath tight junctions.     * Connects cortical actin filaments of interacting cells.     * Contractile Bundle: A bundle of actin filaments runs parallel to the membrane, bound via proteins like catenins, vinculin, and $\alpha$-actinin.     * Key Proteins: $\beta$-catenin, $p120$-catenin, and $\alpha$-catenin link cadherins to the actin cytoskeleton.
• Non-Epithelial Adherens Junctions:     * Found in muscle or nervous tissue; require greater dynamism.     * Utilize different cadherins, such as N-cadherin.     * Functions include transmission of mechanical and chemical signals between cells.
• Dynamics and Regulation:     * Regulated by intracellular signals, post-translational modifications (e.g., phosphorylation), and ECM remodeling enzymes (metalloproteinases).     * Epithelial-Mesenchymal Transition (EMT): Profound remodeling where adherens junctions dissociate, and cells acquire mesenchymal properties (mobility and invasiveness).

Cadherins: The Calcium-Dependent Adhesion Proteins

• General Properties: Integral membrane glycoproteins that mediate cell adhesion in the presence of $Ca^{2+}$. Calcium stabilizes the external conformation to allow for binding.
• Structure:     * Extracellular region: Contains five copies of the extracellular cadherin domain separated by flexible hinges.     * Role of $Ca^{2+}$: Ions bind near hinges to prevent bending. In the presence of $Ca^{2+}$, the structure is rigid; in its absence, the structure becomes flexible and fails to adhere.     * Cytoplasmic Domain: Binds the actin cytoskeleton via catenins.
• Classification:     1. Classical Cadherins: E-cadherin (epithelial), N-cadherin (neuronal), P-cadherin (placental).     2. Non-classical Cadherins: Includes desmosomal cadherins like desmogleins and desmocolins.
• Cadherin Switch: Transitions (e.g., E-cadherin to N-cadherin) are critical during embryogenesis, wound healing, and tumor metastasis.
• Velcro Analogy: Cadherins function like Velcro, holding cells together through lateral interactions forming linear systems that interweave between adjacent cells.

Cadherin Associated Complex (CAC)

• Functions:     * Stabilizes cell-cell adhesion by linking cadherins to the actin exoskeleton.     * Mediates intracellular signaling (cell proliferation, differentiation, survival).     * Controls cell morphology and motility by modulating actin dynamics.
• Key Components:     * $\alpha$-Catenin: Linker between cadherins and actin; regulator of signaling.     * $\beta$-Catenin: Binds cadherins and transmits signals. Can translocate to the nucleus as a transcription factor when not bound.     * $\gamma$-Catenin (Plakoglobin): Structurally similar to $\beta$-catenin; also found in desmosomes.     * $p120$-catenin: Regulates localization and stability of cadherin on the membrane.

Responses to Mechanical Force in Adherens Junctions

• Tension Sensing: Adherens junctions act as tension sensors connected to contractile bundles of actin and non-muscle myosin II.
• Mechanotransduction Mechanism:     1. Non-muscle myosin II pulls actin filaments from within the cell.     2. The resulting force unwinds a domain of $\alpha$-catenin.     3. This exposes a hidden binding site for the adaptor protein vinculin.     4. Vinculin recruits further actin, strengthening the junction through positive feedback.

Desmosomes and Intermediate Filaments

• Structure: Composed of non-classical cadherins (desmogleins and desmocolins). Intracellular adapters like desmoplakin and plakoglobin anchor these to intermediate filaments (e.g., keratin).
• Functions: Provide strong intercellular adhesion in tissues under mechanical stress (epidermis, cardiac muscle). They distribute mechanical forces across large tissue areas.
• Tissue Layers (Skin): Involve components like $Dsg1$, $Dsg3$, $Dsc1$, $Dsc3$, and $PKP1/2$ across the basal, spinous, granular layers and the stratum corneum.

Other Major Adhesion Superfamilies

• Selectins:     * $Ca^{2+}$-dependent surface glycoproteins that bind carbohydrates (lectins).     * Types: L-selectin (leukocytes), P-selectin (platelets/activated endothelial cells), E-selectin (activated endothelial cells).     * Mediate transient "rolling" adhesion during inflammation.
• Immunoglobulin Superfamily (IgSF CAMs):     * $Ca^{2+}$-independent adhesion; products of alternative splicing.     * Examples: NCAM, ICAM-1, VCAM-1, PECAM-1, L1CAM.     * Often bind long chains of sialic acid, which have a strong negative charge that weakens adhesion to favor growth/segregation.     * Crush Syndrome: Associated with mutations linked to mental retardation.
• Integrins:     * Heterodimers ($\alpha$ and $\beta$ subunits).     * Traverse the membrane to link the ECM to the cytoskeleton.     * Humans have $24$ types ($8$ genes for $\beta$, $18$ for $\alpha$).

Integrin Activation and Focal Adhesions

• Conformational States:     * Inactive: Compact structure, binding sites hidden.     * Active: Unhooked subunits, binding sites exposed for ligands and talin.
• Activation Mechanisms:     * Outside-In: Matrix ligand binding (e.g., RGD sequence in fibronectin) induces a high-affinity state.     * Inside-Out: Intracellular signals cause talin to bind the $\beta$ subunit, disengaging it from the $\alpha$ subunit and exposing extracellular sites.
• Focal Adhesions: Specialized anchor points where integrins cluster ($10-100$ times higher concentration than other receptors).     * Includes signaling proteins like FAK (Focal Adhesion Kinase).     * Talin Mechanotransduction: Tension on talin exposes hidden vinculin binding sites, recruiting more actin to increase junction resistance.

Tight Junctions and the Epithelial Barrier

• Function: Seal epithelial cells to prevent paracellular passage and lateral diffusion of membrane proteins (maintaining cell polarity).
• Proteins:     * Claudins: Main structural components.     * Occludins: Function uncertain; associate with ZO proteins (Zonula Occludens) which anchor to actin.
• Example (Enterocyte): Tight junctions prevent glucose and sodium-driven glucose transporters from diffusing between apical and basolateral surfaces, maintaining the concentration gradient.

Communicating Junctions (Gap Junctions and Plasmodesmata)

• Gap Junctions (Animals):     * Formed by connexins which assemble into connexons.     * Allow passage of ions and small water-soluble molecules ($< 800 - 1000$ Da).     * Regulated by $pH$ and $Ca^{2+}$ to protect against spreading damage.     * Nexus: A molecular complex including $c\text{-}src$, tubulin, and $ZO\text{-}1$.
• Plasmodesmata (Plants):     * Traverse rigid cellulose cell walls ($20-40$ nm diameter).     * A central desmotubule is continuous with the smooth ER.     * Permit molecules up to $800$ Da to pass through a ring of cytosol.

Pathologies of Cell Junctions

• Skin Diseases: Pemphigus is caused by desmosome dysfunction, leading to skin erosions.
• Cardiovascular: Mutations in desmosomal components cause cardiomyopathies, heart failure, and arrhythmias.
• Cancer: Loss of E-cadherin is a hallmark of tumor invasion and metastasis.
• Fibrosis: Aberrant ECM remodeling and cell-matrix junction signaling lead to fibrosis in lungs, liver, and kidneys.