CBNS101 Lecture 12 - The extracellular matrix

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  • Fundamentals of Cell Biology

  • Introduction to the extracellular matrix (ECM)

    • Image Credit: Riccardo Cassiani-Ingoni / Science Photo Library

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Connective Tissue and ECM Role

  • Vertebrate body primarily consists of extracellular space

  • Connective tissues fill this space, largely composed of ECM

  • Variability in ECM Amount:

    • Abundant in cartilage and bone

    • Sparse in brain and spinal cord

  • Functions of ECM:

    • Determines form and shape

    • Provides mechanical support

    • Serves as a substrate for cell adhesion and migration

    • Regulates cell signaling and differentiation

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Macromolecular Organization of ECM

  • Major components include:

    • Collagen

    • Fibronectin

    • Integrin

    • Laminin

    • Proteoglycan

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ECM Secretion

  • Secreted by cells within it:

    • Fibroblasts in soft tissues responsible for ECM secretion

    • Specialized fibroblast-like cells in bone and cartilage, each with differing compositions

    • Epithelial cells secrete the basal lamina

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Definition of ECM

  • Description:

    • A dynamic network of extracellular proteins and polysaccharides

    • Composition varies based on tissue type

  • Components:

    • Fibrous structural proteins (collagens, elastin)

    • Adhesive proteins (fibronectin, laminin)

    • Polysaccharides (glycosaminoglycans, proteoglycans)

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Properties of Collagen

  • Function:

    • Provides tensile strength to ECM

  • Prevalence:

    • Most abundant vertebrate protein (~25% by mass)

    • Extensively characterized multi-gene family (25 specific a-collagens)

  • Types of Collagen:

    • Fibrillar (Types I, II, III, V, XI): connective tissues

    • Fibril-associated (Types IX, XII): link fibrils

    • Network-forming (Types IV, VII): basement membrane support

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Structure of Collagen Fibrils

  • Description:

    • Long, stiff, triple-helical structure

    • Comprised of three winded alpha chains forming a superhelix

  • Importance of Proline and Glycine in helix formation

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Types of Collagens and Their Functions

  • Type I: Found in bones, skin, tendons, ligaments

    • Deficiency leads to severe bone defects

  • Type II: In cartilage

    • Deficiency causes cartilage issues

  • Type III: Supports skin and blood vessel structure

    • Results in fragile skin if mutated

  • Type IV: In basal lamina

    • Forms a sheet-like network

  • Unique Characteristics of Other Types:

    • Mutations lead to various diseases including myopia and fragile skin.

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Collagen Biosynthesis & Assembly

  • Collagen synthesized as procollagen with extra N- and C- terminal pro-peptides

  • Processes in ER/Golgi include:

    • Glycosylation and hydroxylation influencing H-bonding

  • Formation of a triple-stranded helix in ER

  • Cleavage of propeptides allows for self-assembly into collagen fibrils

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Strengthening of Collagen Fibrils

  • Enhanced by stacking of collagen molecules

  • Cross-linking occurs at lysine residues in non-helical regions

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Fibril-Associated Collagens

  • Generally more flexible with interrupted structures

  • Characteristics:

    • Not cleaved post-secretion and bind to ECM components

    • Connects to other collagen fibrils and additional ECM components

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Human Diseases from Collagen Defects

  • Ehlers-Danlos syndrome: Mutations in type III collagen causing skin/joint issues

  • Osteogenesis imperfecta: Caused by mutations in type I collagen, leading to bone fragility

  • Chondrodysplasia: Mutations in cartilage collagens causing deformities

  • Scurvy: Vitamin C deficiency affecting collagen modification, causing fragility in vessels and bones

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Feline Cutaneous Asthenia

  • An inherited collagen disorder similar to Ehlers-Danlos syndrome

  • Resulting condition: flexible skin, termed 'winged cat disease'

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Elastic Fibers in ECM

  • Contribute to elasticity within the ECM

  • Key proteins:

    • Elastin: provides primary elasticity

    • Fibrillin: glycoprotein assisting in elastic fiber integrity

  • Properties: Elasticity surpasses that of rubber bands

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Marfan's Syndrome

  • Mutation in fibrillin gene leading to skeletal and cardiac issues

  • Associated defects include eye problems and fragile blood vessels

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Glycosaminoglycans (GAGs) and Proteoglycans

  • Types of GAGs:

    1. Hyaluronan/hyaluronic acid

    2. Chondroitin sulfate and dermatan sulfate

    3. Heparan sulfate

    4. Keratan sulfate

  • Proteoglycans: GAGs linked to proteins providing structure and function

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Formation of GAGs and Proteoglycans

  • GAGs and proteoglycans aggregate and interact to form large complexes

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Functions of GAGs and Proteoglycans

  • Resist compression by creating large hydration spheres

  • Regulate ECM activity by interacting with proteins

  • Serve as co-receptors for signaling molecules

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Adhesive Proteins

  • Key Adhesive Proteins:

    1. Fibronectin: Multi-domain protein facilitating cell attachment

    2. Laminin: Key component of basal lamina

    3. Cell Adhesion Molecules (CAMs): Such as NCAM

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Structure of Fibronectin Dimer

  • Composed of two sub-units linked by di-sulfide bonds

  • Each sub-unit has distinct functional domains including binding sites

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Fibronectin Assembly & Cell Interaction

  • Linked to intracellular actin via integrins for structural integrity

  • Extracellular fibronectin fibrils connect within cellular spaces

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Fibronectin Assembly Mechanism

  • Process: Tension stretches fibronectin, activating binding sites for assembly

  • Ensures appropriate structural response to mechanical needs

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ECM's Role in Cell Function

  • Cell Migration: Fibronectin and laminin define pathways for cell movement

  • Cell Differentiation: ECM influences functional properties of cells (e.g. milk protein secretion)

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Cell Migration Visualization

  • Video of cells migrating on fibronectin surfaces

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Basal Lamina Organization

  • Forms flexible and thin ECM layers underlying epithelial cells

  • Composed of type IV collagen, heparin sulfate proteoglycan, and glycoproteins

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Structural Organization of Basal Lamina

  • Self-assembly properties of laminin and type IV collagen to form functional mesh

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Functions of Basal Lamina

  • Acts as a filter in kidney glomerulus; provides selectivity for macromolecules

  • Roles in tissue regeneration and cell protection

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Integrins in ECM Interaction

  • Serve as primary ECM adhesion receptors

  • Comprise different subunits with specificity for ECM components

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Integrins' Role and Functions

  • Critical for cell anchorage, signaling pathways, and cytoskeletal connections

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Integrin and Actin Cytoskeleton Interaction

  • Integrins cluster in focal adhesions connecting to actin filaments

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Integrin Clustering and Hemidesmosomes

  • Anchoring function in epithelial cells using integrin α6β4

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Integrin Regulation Mechanisms

  • Activation of integrins through binding to ligands, regulating matrix interactions and signaling pathways

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Intracellular Integrin Activation

  • Mediated by crosstalk with other pathways, including G-proteins and receptor kinases

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Examples of Integrin Function

  • Leukocyte Activation: Necessary for inflammation responses

  • Clot Formation: Role during injury response by aggregating platelets

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Influence of ECM on Cell Functions

  • ECM plays critical roles in determining cell shape and survival

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Cell Survival and ECM Correlation

  • Anchorage dependence observations: ECM level affects cell viability

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ECM's Effect on Cell Migration

  • Degradation of ECM necessary for effective cell movement

  • Important role in cancer cell migration facilitated by certain proteases

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Summary of ECM Functions

  • ECM is vital in cellular processes including structure, signaling, and support.

  • Components essential for various cellular functions, including tissue-specific roles as seen in basal lamina anatomy and interaction with integrins.

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