Extracellular Matrix and Cell Adhesion

Extracellular Matrix (ECM)

• Definition: Diverse collection of molecules selected by cells in proximity, influencing cellular behavior.
• Critical Functions:
    - Cell Adhesion: Establishes connection between cells.
    - Cell-to-Cell Specificity: Ability for one cell to recognize another cell type.
    - Barrier/Filter: Provides selective permeability, represented by the basal lamina.
    - Promotion of Differentiation: Vital for the specialization of cells.
    - Strengthening Structures: Essential in forming robust structures like bones and tendons.
    - Activation of Receptors: Triggers integrin receptors involved in signaling.
    - Hydration Shell: Charged surfaces bind to hydration shells that can carry growth factors.
    - Tumor Microenvironment: Modulates interactions and characteristics of tumors.
    - Medical Applications: Components utilized in therapies, particularly involving stem cells.

Stem Cells and Medical Applications

• Stem Cell Therapy Example: Treatment aimed at blindness; various trials still ongoing.
• Development of Organoids: Miniature, simplified organs grown in vitro, facilitated by ECM components.
• Experimental Products:
    - Synvisc: An ECM product used as a cushion.
    - EpiDerm: Used to determine appropriate ECM for various applications.
• Specific Experiments:
    - Type 1 Collagen Experiments: Dry film resulted in partial cell adhesion; type 1 collagen gel improved differentiation, while adding a FCS inhibitor induced full differentiation.
    - Nano 3D System: A system enabling spontaneous generation of organoids by substrate-dependent cells, excluding blood cells.
    - Classic Sponge Experiment: Used India ink-stained sponges, separated by cheesecloth, allowing cell reassociation to study cell behavior post-separation.

Cell Adhesion Molecules (CAMs)

• Types of Cell Adhesion:
    - Homophilic: Binds similar cell types.
    - Heterophilic: Binds different cell types.
    - Multi-adhesive: Two cells bind using the same receptor or different components that permit adhesion.
• Cadherins:
    - Types include E-cadherin (epithelial), N-cadherin (neural), P-cadherin (placental).
    - Dependence on Calcium (Ca²⁺): Cadherins require Ca²⁺ for binding and function.
• Cis and Trans Binding: Describes the binding dynamics of cadherins in cell compactness.
• Experiments Demonstrating Cadherins Function:
    - L cells, which do not adhere to a substrate, were induced to express cadherin genes, leading to adhesion when Ca²⁺ was present.

Immunoglobulin Superfamily

• Definition: Contains immunoglobulin-like molecules involved in cell adhesion.
• Examples:
    - N-CAM (Neural Cell Adhesion Molecule): Homophilic interactions play a role in neural development.
    - Dysregulation in Cancer: Altered expression associated with various cancers.

ECM Forms and Diseases

• ECM Structural Variability:
    - Loosely Ordered ECM: Seen with skeletal myoblasts and interstitial spaces filled with fluid.
    - Basal Lamina Composition: Acts as a basement membrane supporting epithelial cells.
    - Loose Connective Tissue: Contains fibroblasts, elastin, and collagen, found in skin’s dermis and corneal stroma.
    - Dense Connective Tissue: Comprises structures like bones and tendons.
• Cornea Structure:
    - Layers: Surface epithelial layer (regenerative), stroma layer, Descemet's membrane (non-regenerative).
    - Osmotic Balance: Corneal health relies on endothelial cells; defects can lead to cataracts.

Collagen Overview

• Importance: Most abundant animal protein, essential for structural integrity, resists stretching.
• Structure:
    - Triple Helix Configuration: Composed of a repeating sequence with glycine as every third amino acid (e.g., Gly-Pro-Met-Gly).
    - Varietal Makeup: Over 12 different types of collagen exist in organisms.
• Synthesis:
    - Production begins in the rough endoplasmic reticulum (RER) with pre-sequence proteins (N-propeptides and C-propeptides).
    - Initial modifications involve proline hydroxylation, reliant on Vitamin C.
    - Final assembly requires heat shock protein 47 (HSP 47) to aid correct alignment into procollagen, which is then secreted out of the cell.
    - Enzymatic Action: Lysyl oxidase catalyzes covalent cross-linking in the ECM to form stable structures.

Health Implications of Collagen Synthesis

• Scurvy: Results from Vitamin C deficiency, affecting collagen production; leads to symptoms like bleeding.
• Glycosaminoglycans (GAGs):
    - Structure: Long, rigid molecules with repeating disaccharide units, bind cations and influence osmotic pressure, yielding a gel-like consistency.
    - Example: Hyaluronan, which aids in cellular diapedesis and is central to various FDA-approved fillers for skin rejuvenation.

Proteoglycans and Integrins

• Proteoglycans: Consist of proteins bound to glycosaminoglycans, e.g., Syndecan, which can bind growth factors.
• Fibronectin: A multi-adhesive protein that can link various ECM molecules together, aiding in cell attachment and migration.
• Integrin Structure and Function:
    - Composed of alpha and beta subunits, integrins bind to ECM components and mediate cellular responses.
    - Active in processes like extravasation of leukocytes during inflammation.

Integrin Receptors and Clinical Applications

• Interactions with Endothelial Cells: Activated integrins facilitate the exit of leukocytes from blood vessels during immune response.
• Diseases Associated with Integrin Dysfunction:
    - Leukocyte Adhesion Deficiency: Caused by defective β2 integrins results in insufficient immune response.
    - Duchenne Muscular Dystrophy: Related to defective laminin interactions with the ECM.
    - Ehlers-Danlos Syndrome: A group of disorders caused by abnormalities in ECM components, leading to various phenotypes.
    - Osteogenesis Imperfecta: Characterized by collagen synthesis defects, leading to brittle bones, with type 1 being mild and type 2 often fatal at birth.