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.