Interaction of Cells with Other Cells (5) Cadherins – glycoproteins that mediate Ca 2+ - dependent...

Interaction of Cells with Other Cells (5)

• Cadherins –glycoproteins that mediate Ca2+-dependent cell-cell adhesion.

Interaction of Cells with Other Cells (6)

• Cadherins (continued)– Also involved in transmitting signals from the

ECM to the cytoplasm.– Mediate many of the changes in adhesive

contacts during embryonic development by forming epithelial-mesenchymal transition (EMT).

Cadherins and the EMT

Cadherins and the EMT

Interaction of Cells with Other Cells (8)

• Adherens Junctions and Desmosomes: Anchoring Cells to Other Cells– Adherens junctions – they form “belts” near

apical surface called junctional complex.– Cells of an adherens junction held together by

calcium-dependent linkages.

Intercellular junction complex

Intercellular junction complex

Molecular architecture of adherens junction

Interaction of Cells with Other Cells (9)

• Desmosomes – disk-shaped adhesive junctions between cells.– Found in a variety of tissues.– Contain cadherins that link the two cells

across a narrow gap.– Cadherins of desmososme shave different

domain structures: desmogelins and desmocollins.

Structure of a desmosome

Interaction of Cells with Other Cells (10)

• The Role of Cell-Adhesion Receptors in Transmembrane Signaling– The transfer of information across the plasma

membrane is transmembrane signaling.– Integrins and cadherins can transmit signals

from the extracellular environment to the cytoplasm.

Interactions involving the cell surface

Interaction of Cells with Other Cells (11)

• The binding of an integrin with its ligand can induce a responses such as changes in growth potential.

The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (1)

• Inflammation is a response to infection or injury but can be triggered inappropriately.

• Inflammatory response:– Recruitment of leukocytes to site of injury.– Neutrophils attach to P- and E-selectins.– Neutrophils start to “roll” along wall of vessel.

Movement of neutrophils during inflammation

The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (2)

• As neutrophils interact with inflamed venule endothelium, platelet activating factor (PAF) is displayed.

• PAF sends a signal to increase the binding activity of some integrins.

• Activated integrins cause neutrophils to stop rolling and adhere firmly to wall of vessel.

The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (3)

• Cancer is the result of abnormal cell proliferation.

• The spread of a tumor to other parts of the body is called metastasis.

• Metastatic cells have special cell adhesion properties:– Are less adhesive.– Are able to penetrate several barriers.– Are able to invade normal tissues.

Steps leading to metastatic spread

The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (4)

• During growth and development of a tumor there is loss of E-cadherin leading to less adhesion.

• Changes in the numbers and types of cell-adhesion molecules lead to promote metastasis.

7.4 Tight Junctions: Sealing the Extracellular Space (1)

• Tight junctions (TJs) – specialized contacts between epithelial cells.– Located at the very apical end of the

junctional complex between adjacent cells.– TJs serve as a barrier to the free diffusion of

water and solutes from the extracellular compartment.

– Some TJs are permeable to specific ions or solutes.

Tight junctions

Tight junctions

Tight Junctions: Sealing the Extracellular Space (2)

• Occludins are proteins found in TJs.

• Claudins form the major structural component of TJs, and may account for selective differences in TJ permeability.

• TJs form the blood-brain barrier.

7.5 Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (1)

• Gap junctions – sites between animal cells for intercellular communication.– Composed entirely of membrane protein

connexin.– Connexins are organized into a complex

called connexon.

Gap junctions

Gap junctions

Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (2)

• Gap-junction intercellular communication (GJIC) allows the passage of low-weight molecules.

• Gap junctions can allow integration of activities of individual cells into a functional unit.

• Compatibility differences between connexins either promote or prevent communication between different cells.

Passage of low-molecular-weight solutes through gap junctions

Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (3)

• A new type of communication has been discovered – tunneling nanotubes.– It has been observed

in cells growing in culture.

Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (4)

• Plasmodesmata are cytoplasmic channels passing through cell walls of adjacent plant cells.– Are lined by plasma membrane.– Contain a central structure, the desmotubule.– Serve as sites of cell-cell communication.

Plasmodesmata

7.6 Cell Walls (1)

• Cell walls provide plants protection against mechanical abrasion, pathogens, and osmotic stress.– The fibrous component is cellulose.– Cellulose is organized into microfibrils, which

provide rigidity to the cell wall.

The plant cell wall

Cell Walls (2)

• The matrix of the cell wall contains hemicelluloses, pectins, and proteins.

• Cell walls arise as a cell plate that forms between the plasma membranes of newly formed daughter cells.

• The walls of growing cells are primary walls and allow flexibility lacking in the thicker secondary walls of mature cells.

Synthesis of a plant cell wall