Cell Adhesion and Junctional Complexes Study Guide

Learning Outcomes for Cell Adhesion and Junctional Complexes

• The primary goals of the lecture are to explain:
    - How cells adhere to one another via cell-cell adhesion mechanisms.
    - How cells adhere to the extracellular matrix (ECM) through cell-matrix adhesion.
    - The specific composition of the extracellular matrix.

Classification and Characterization of the Extracellular Matrix (ECM)

• The extracellular matrix is broadly categorized into two types based on cell density and organization:
    - Epithelium: Characterized by tightly packed cells that bind directly to one another.
    - Mesenchyme: Characterized by sparse cell distribution within the matrix.

• Two distinct forms of ECM organization:

  • Basement Membrane (Basal Lamina): A $2D$ sheet on which epithelial cells reside, secreted by epithelial cells. It underlies epithelia and surrounds certain non-epithelial cells.

    • Composed of laminin, collagen IV, nidogen and perlecan

  • Fibrillar Matrix: A $3D$ matrix composed of various fibers. Cells such as fibroblasts are buried within this matrix. It is deposited and constantly re-modelled by its embedded cells.

    • composed of collagen I, Fibronectin, elastin, proteoglycans

Overview of Junctional Complexes

• There are six primary types of junctional complexes used in cell-cell and cell-matrix interactions

  • A. Tight Junction: Forms a barrier close to the apical surface.

  • B. Adherens Junction: Linked to the actin cytoskeleton.

  • C. Desmosome: Cell-cell junction linked to intermediate filaments.

  • D. Gap Junction: Pores for chemical and electrical coupling.

  • E. Focal Adhesion: Cell-matrix junction linked to actin.

  • F. Hemidesmosome: Cell-matrix junction linked to intermediate filaments.

Desmosomes and Hemidesmosomes

• Terminology:

  • Desmosome: Derived from the Greek desmos meaning "bond," "fastening," or "chain," and soma meaning "body."

    • maintain cell to cell adhesions very strongly via desmosomal cadherins (desmoglein and desmocollin)

    • cadherins are bound to cytoskeletal proteins and intermediate filaments via desmoplakin

    • connecting intermediate filaments in neighboring cells

  

  • Hemidesmosome: Derived from the Greek hēmi- meaning "half."

    • found in epithelial cells

    • major component is alpha-6 beta-4 integrin

    • transduces signals from ECM to the interior of the cell

    • intracellular domain of integrins are linked to keratin IF via plectins (anchor proteins) which are anchored to the basal lamina

• Morphology and Structure:
    - Desmosomes appear under an electron microscope as pairs of dark, disk-like or button-like structures at cell-cell contacts.
    - Hemidesmosomes appear as "half-desmosomes" located at the cell-ECM interface.

• Scale and Dimensions:
    - A single hemidesmosome is approximately $100\,nm$ wide.
    - A desmosome is approximately $50\,nm$ high.
    - An average cell is approximately $20\,\v5m$ wide.

• Intracellular Connections:
    - Both are attached intracellularly to intermediate filaments (IFs), specifically Keratin IFs in certain cells.

• Detailed Anatomy of the Hemidesmosome and Associated Structures:
    - Keratin IFs
    - Inner plaque and Outer plaque
    - Plasma membrane
    - Sub-basal dense plate
    - Anchoring filaments
    - Anchoring fibrils
    - Layers: Lamina lucida (LL) and Lamina densa (LD).
    - EBS (Epidermolysis Bullosa Simplex), JEB (Junctional Epidermolysis Bullosa), and DEB (Dystrophic Epidermolysis Bullosa) are associated with different structural defects in these layers.

Clinical Implications of Mechanical Support and Adhesion

• Intermediate filaments provide essential support against mechanical stress. Failure in these systems leads to severe pathologies:
    - Epidermolysis Bullosa: Caused by mutations in intermediate filaments or desmosomal factors, leading to fragile skin and blistering.
    - Pemphigus: An autoimmune condition where the body produces autoantibodies against desmosomes, causing loss of cell-cell adhesion.

Tight Junctions (TJs)

• Localization: Found close to the apical surface of epithelial cells, forming a continuous ring around the cell.

• Function: They turn epithelia into barriers by preventing the passage of molecules between cells.

  • important in cell polarity, because they act as permeability barriers and restrict macromolecular transport between cells

• Molecular Components: Primarily formed by the proteins claudin and occludin.

• Experimental Evidence (Claudin knockout in mice):
    - In control mice, dye injected under the skin is contained.
    - In Claudin knockout mice, purple dye spreads into upper layers of the skin.
    - Claudin knockout pups experience rapid weight loss and death due to evaporation of water through the skin.

• Reference: Furuse et al. 2002, J Cell Biol. 156:1099-111.

Gap Junctions (GJs)

• Function: They couple cells via permeable pores known as "fenestrations."

• Capabilities:
    - Allow small molecules and ions to pass directly between cells.
    - Facilitate chemical and electrical coupling.
    - Ions which maintain the membrane potential of neurons can pass through gap junctions

• Localization: Dispersed in the lateral membrane between the apical and basal surfaces.

• Components: connexons - which connect to form channels and can open and close, important in heart muscle (contraction signal) and neuronal synapses

Adherens Junctions (AJs)

• Linkage: Found in all epithelial and endothelial cells but also maintains cell-to-cell adhesions via cadherins and connects actin networks of neighboring cells.

  • Cadherins are a family of transmembrane proteins which form calcium-dependent homodimers with other cadherins. Link actin via anchor proteins (catenins, actinin and vinculin)

• Structure: Often referred to as a "double headband" arrangement below the tight junctions.

• Function: The actin-myosin complex associated with AJs can mediate contractile forces, which facilitates the bending of epithelia.

• Binding Mechanism: Mediated by homophilic binding of E-cadherins, which is strictly dependent on calcium ions ($Ca^{2+}$).

Focal Adhesions

• Function: Anchor the cell to the ECM with multi-molecular assembly.

• Mediator: Uses integrins to bridge the intracellular actin to the basal lamina/ECM fibers. Main receptor family, heterodimeric transmembrane receptor family which recognizes motifs in ECM ligands

• Dynamics: Focal adhesions are transient; they form and disassemble rapidly during cell movement (cell migration).

  

Cadherins and the Discovery of Cell Adhesion Molecules (CAMs)

• Masatoshi Takeichi: Pioneered the discovery of cadherins (Kyoto University/Carnegie Institution) by re-investigating the dissociation of amphibian embryos (originally studied by Johannes Holtfreter).

• Chemical Mechanism: EDTA is used to sequester calcium ($Ca^{2+}$). Without calcium, cadherins cannot maintain adhesion.

• Calcium Dependency Thresholds:
    - Adhesion occurs at concentration > 1\,mM\,Ca^{2+} .
    - Dissociation occurs at concentration < 0.05\,mM\,Ca^{2+} .

• Classical Cadherins and their Locations:
    - E-cadherin: Many epithelia.
    - N-cadherin: Neurons, heart, skeletal muscle, lens, and fibroblasts.
    - P-cadherin: Placenta, epidermis, breast epithelium.
    - VE-cadherin: Endothelial cells.

Additional Adhesion Systems and Disease Links

• Other Junctions/Mechanisms:
    - Selectin-based adhesion: Responsible for "leukocyte rolling" of white blood cells in vessels.
    - Neuronal synapse: Specialized adhesion for signal transmission.
    - Plasmodesma: The plant equivalent of gap junctions.
    - Septate junctions: The invertebrate equivalent of tight junctions.

• Cancer Metastasis:
    - Metastasis is intricately linked to changes in cell adhesion.
    - Process: Primary tumor formation ("seed") → Intravasation (entering vessels) → Dissemination → Arrest in cerebral capillary bed → Extravasation (exiting vessels) → Colonization and proliferation in target organs (e.g., Brain parenchyma/"soil").