MB 207 Molecular cell biology

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

Cell junctions, cell adhesion and the extracellular matrix

• 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

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.

– 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.

Occluding junctions

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.

• 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.

Junction Transmembrane proteins

Extracellular Ligand Intracellular cytoskeletal attachment

Adherens junction Cadherin (E-cadherin)

Cadherin in neighbouring cell

Actin filaments

Desmosome Cadherin (desmoglein, desmocollin)

Desmoglein & desmocollin in neighbouring cell

Intermediate filaments

Focal adhesion Integrin ECM Actin filaments

Hemidesmosome Integrin ECM Intermediate filaments

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.

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.

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.

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.

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.

Summary of cell junctions found in vertebrate epithelial cell

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

Types of CAMs

• 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.

Cadherins

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.

• 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.

Selectins

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.

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.

• 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.

Immunoglobulin (Ig) superfamily

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

extracellular matrix

Family Ligands recognized

Stable cell junctions

Selectins Carbohydrates NoIntegrins Extracellular

matrixFocal adhesions and hemidesmosomes

Ig superfamily Integrins NoHomophilic interactions

No

Cadherins Homophilic interaction

Adherens junctions and desmosomes

Cell Adhesion Molecules

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.