MB 207 Molecular cell biology

1. MB 207 Molecular cell biology Cell junctions, cell adhesion and the extracellular matrix

2. Cell junctions, cell adhesion and the extracellular matrix • Cell Junctions ¤ Occluding junctions ¤ Anchoring junctions ¤ Communicating junctions • Cell adhesion molecules ¤ Cadherins ¤ Selectins ¤ Integrins ¤ Immunoglobulin superfamily • Extracellular matrix

3. In muticellular organisms, individual cells are organized into tissues that allows multicellular organisms to adopt complex structures. → Cells must be attached to one another. • In order for individual cells to associate in precise patterns to form tissues, organs and organ systems, individual cells must be able to recognize, adhere to and communicate with each other. • These associations usually involve specialized modifications of the plasma membrane at the point where two cells come together. → cell junctions • In addition, cells also interact with extracellular materials that are crucial for tissue structure and fucntion. → extracellular matrix

4. Cell Junctions • Occurs at points of cell-cell and cell-matrix contact in all tissues. • Categorized into three functional groups: -- Occluding junctions: seal cells together to prevent even small molecules from leaking from one side of the cell to the other, e.g. tight junctions. • Anchoring junctions: mechanically attach cells to their neighbours or to the extracellular matrix, e.g. adherens junctions, focal junctions, desmosomes & hemidesmosomes. • Communicating junctions: mediate the passage of a chemical or electrical signals from one interacting cell to its partner, e.g. gap junctions.

6. Occluding junctions • Role carried by tight junctions (vertebrate). • Functions: i) Maintenance of selective permeability, separating fluids on either side that have a different chemical composition. ii) Seal neighboring cells together. • Confine the transport proteins to their appropriate membrane domain by acting as diffusion barriers. • Can be regulated to permit increase flow of solutes or water at the barriers → paracellular transport (amino acids and monosaccharides). • Structure: Composed of a branching network of sealing strands that completely encircles the apical end of each cell. → Each tight junction sealing strand is composed of a long row of transmembrane adhesion proteins (e.g. claudins & occludin) embedded in each of the 2 interacting plasma membranes.

7. Anchoring junctions • Link cells together into tissues, thereby enabling the cells to function as a unit • → Anchoring cytoskeleton to the cell surface. • → Resulting interconnected cytoskeletal network helps to maintain tissue integrity and to withstand mechanical stress. • Composed of two main classes of proteins: • i) intracellular anchor proteins • → link the junction to the appropriate cytoskeletal filaments on the cytoplasmic face of the plasma membrane. • ii) transmembrane adhesion proteins • → cytoplasmic tail that binds to one or more intracellular anchor proteins and an extracellular domain that interacts with either extracellular matrix or extracellular domains of a specific transmembrane adhesion proteins on another cell.

8. Occurs in two functionally different forms: • i) Adherens junctions and desmosomes hold cells together and formed by transmembrane adhesion proteins that belong to the cadherin family. • ii) Focal adhesions and hemidesmosomes bind cells to the extracellular matrix and are formed by transmembrane adhesion proteins of the integrin family.

9. Adherens junctions • Connect the actin of a cell to the actin of its neighbours. • Prominent in epithelial cells. • Structure: → forms a continuous belt that encircles the cell near the apical end of the lateral membrane.

10. Desmosomes • Connect the intermediate filaments of a cell to the intermediate filaments of its neighbors. • Structure: ¤ Button-like points of strong adhesion between adjacent cells in a tissue. → serves as an anchoring sites for intermediate filaments, which form a structural framework of great tensile strength.

11. Focal adhesions • Connect actin of a cell to the extracellular matrix through integrins. • The extracellular domains of transmembrane integrin proteins bind to a protein component of the extracellular matrix, while their intracellular domains bind indirectly to bundles of actin filaments via intracellular anchor proteins.

12. Hemidesmosome • Connecting intermediate filaments of a cell to extracellular matrix • Extracellular domains of integrins mediate the adhesion bind to a laminin protein in the basal lamina, while an intracellular domains binds via an anchor protein to keratin intermediate filaments.

13. Communicating junctions: Gap junctions • Region where the plasma membranes of two cells are aligned and brought into intimate contact. • → uniform narrow gap which is spanned by channel-forming proteins (connexins). • The gap is spanned by channel-forming proteins (connexins, 6 subunits), and the pore is called connexon. • Permeability of gap junctions in different cells can vary  different forms of connexins (construction of transmembrane proteins). • Individual gap junction channels do not remain continuously open. They flip between open and closed states. • The channels allow inorganic ions and other small water soluble molecules to pass directly from the cytoplasm of one cell to the cytoplasm of the other. • → solutes with molecular weights up to about 1000 daltons.

14. Summary of cell junctions found in vertebrate epithelial cell

15. Cell Adhesion Molecule (CAMs) • Are cell-surface proteins. • → adhering cells to each other and to the extracellular matrix. • Two types: • i) cell-cell adhesion molecules • ii) cell-matirx adhesion molecules • Three mechanisms by which cell-surface molecules can mediate cell-cell adhesion: • i) Homophilic binding • ii) Heterophilic binding • iii) Binding through an extracellular linker molecule

16. Types of CAMs Cadherins • Major CAMs responsible for Ca2+-dependent cell-cell adhesion. • Three types of cadherins: E-cadherin, N-cadherin and P-cadherin. • Important in initial attachment such as in development of cells, and maintaining the structure and integrity of cells • Single-pass transmembrane glycoproteins which normally function as a dimer/oligomer. • Linked to actin/intermediate filaments through intracellular anchor protein.

17. The linkage of classical cadherins to actin filaments • The cadherins are coupled indirectly to actin filaments by the anchor protein α-catenin and β-catenin. • A third intracellular protein, p120, also binds to cadherin cytoplasmic tail and regulates cadherin function.

18. Selectins • Cell-surface carbohydrate-binding proteins (lectin) that mediate Ca2+-dependent cell-cell adhesion in the bloodstream. • 3 types: ¤ L-selectin: on white blood cells ¤ P-selectin: on platelets and some endothelial cells ¤ E-selectin: on activated endothelial cells • Transmembrane protein with highly conserved lectin domain that binds to a specific oligosaccharide on another cell.

19. Integrins • Main function: Binds and mechanically fixes cells to ECM. • Additional functions: • i) serves as cell-cell adhesion molecule. • ii) serves in signal transduction. • Composed of two noncovalently associated transmembrane glycoprotein subunits, α and β. • Divalent ions dependent (Ca2+ or Mg2+) • → Influence both affinity and specificity of the binding of an integrin to its ligands.

20. The regulation of the extracellular binding activity of a cell’s integrins from within • An extracellular signal activates an intracellular signaling cascade that alters the integrin so that its extracellular binding site can now mediate cell adhesion.

21. Immunoglobulin (Ig) superfamily • Ca2+-independent cell-cell adhesion • Contain 1 or more Ig-like domains. • e.g. N-CAM (neural CAM) • Involve in fine-tuning the adhesive interaction mediated by cadherins during development and regeneration.

22. Summary of junctional and non-junctional adhesive mechanisms used by animal cells in binding to one another and to the extracellular matrix

23. Cell Adhesion Molecules

24. Extracellular Matrix (ECM) • Tissues are not made up solely of cells. • Large part of the tissue is extracellular space which is filled by intricate network of macromolecules constituting the ECM. • ECM is composed of proteins and polysaccharides that are secreted locally and assembled into an organized meshwork in close association with the surface of the cell that produced them. • Cell surface receptors bind to the extracelular matrix and anchor the cytoskeleton at cell matrix junction. • Variation in the types of matrix molecules lead to various different form of functional connective tissue i.e. bone, teeth, cornea, tendons etc.