Lecture Notes on ECM, Cytoskeleton, and Cell Migration

Interactions with Extracellular Matrix (ECM)

  • Epithelial Cells vs. Mesenchymal Cells

    • Epithelial: form linings in organs.
    • Mesenchymal: connect tissues through ECM.

  • Extracellular Matrix (ECM):

    • Non-cellular component in all tissues and organs, secreted by cells.
    • Provides structural and biochemical support to surrounding cells.
    • Key components:
    • Basement Membrane: Thin layer beneath epithelial sheets.
    • Stromal Matrix: Composed of proteins and carbohydrates, varies with tissue type, age, and function.

Components of Basal Laminae

  • Major Components:

    • Laminin: Large flexible molecule (3 chains).
    • Binds integrins, dystroglycan, and perlecan.
    • Collagen IV: Essential for structural stability; mutations linked to disease.
    • Nidogen/Entactin & Perlecan: Stabilize ECM but cannot bind to each other.

  • Type IV Collagen Assembly Process:

    1. Monomer Formation: Single α chain.
    2. NC1 Interactions: Initiate formation of a protomer (trimer).
    3. Dimerization: Protomer pairs up.
    4. Tetramer Formation: Dimer forms tetramer.
    5. Suprastructure Assembly: Into larger structures.

Pathological Effects of Collagen IV Mutations

  • Collagen4A1 Mutations: Lethal; associated with:

    • Small vessel disease.
    • Hemorrhagic stroke.
    • Hemolysis in retinal vessels.
    • Hanac syndrome.

  • COL4A3/A4/A5 Mutations:

    • Alport syndrome and hereditary glomerulonephritis; affects filtration.

Laminin Polymerization and Function

  • Laminin Structure:

    • Composed of three chains connected by disulfide bonds.
    • Functions via:
    • Binding of cell integrins at α-N end for stabilization and cell surface linkage.
    • Self-assembly at the beta and gamma ends for network formation in ECM.

  • Laminin-1: Essential for basement membrane assembly during development.

Collagen Dynamics

  • Fibrillar Collagens: (Types I, II, III, V, XI) are abundant and provide tensile strength.

    • Composed of three polypeptide chains forming a triple helix, with glycine in every third position.

  • Collagen Biosynthesis Steps:

    • Intracellular:
    • Alpha chain synthesis.
    • Hydroxylation of lysine and proline.
    • Triple helix formation (tropocollagen).
    • Extracellular:
    • Removal of terminal peptides.
    • Lysine conversion into reactive aldehydes for cross-linking.

  • Cross-linking:

    • Strengthens collagen fibrils; inhibition leads to fragility in tissues like skin and tendons.

Integrins and Actin Cytoskeleton

  • Integrins:

    • Transmembrane receptor proteins linking ECM and the cytoskeleton.
    • Composed of heterodimers (α & β subunits).
    • Allow signal transduction upon ligand binding to influence cellular behavior.

  • Fibronectin (FN):

    • Glycoprotein that facilitates cell adhesion and signals through integrins.
    • Critical for matrix assembly and cell migration.

Actin Cytoskeleton Overview

  • G-actin:
    • Globular form, most abundant protein in cells.
    • Polymerizes to filamentous F-actin.
  • Dynamics:
    • Polymerization involves nucleation and elongation steps.
    • Actin treadmilling maintains filament length while allowing dynamic rearrangements.

Regulation of Actin Dynamics

  • Profilin: Stabilizes ATP-actin, promotes polymerization.
  • Thymosin: Sequesters monomeric actin, preventing polymerization.
  • Arp2/3 Complex: Facilitates branching of actin filaments for network formation critical for cell movement.

Cell Migration Mechanics

  • Steps in Cell Migration:
    1. Polarization: Establishing a front-rear cell axis.
    2. Protrusion: Cellular extensions move forward.
    3. Adhesion: Integrins create physical attachments to ECM.
    4. Translocation: Cytoskeleton contracts to pull the cell body forward.
    5. De-adhesion: Trailing edge of the cell detaches from the substrate.

Mechanotransduction in Cells

  • Mechanosensitive proteins (like Filamin) translate mechanical stress into biochemical signals impacting cell behavior, including adhesion and movement.

Developmental Biology - Gastrulation and Branching Morphogenesis

  • Gastrulation: Major cellular rearrangements forming germ layers through processes like invagination and involution, crucial for establishing body plans.
  • Branching Morphogenesis: Normal development of tissues such as lungs and trachea requires interactions between epithelial and mesenchymal cells, crucial for nutrient delivery and organ formation.
  • Signaling Factors: FGF and other gradients promote directional migration during development, influencing cell shape and fate decisions.