Cell Adhesion (Näthke)

Question 1

What simple things does multicellularity require?

Answer 1

Multicellularity requires:

• Strength within tissues
• Communication between cells of tissues to coordinate allowing the entire organism to function as a whole (Connections between cells that can relay information and transmit force = cell junctions)

Question 2

Functions of epithelial tissue?

Answer 2

Barrier:
-Regulated entry into & exit from body of water and nutrients

Protection against:

• Chemical
• Mechanical stress e.g. stretching abrasion
• Bacteria and environmental pathogens

Question 3

How do epithelia tissue layers provide protection?

Answer 3

• The Extracellular Matrix (ECM) directly bears the mechanical stresses of tension and compression
• Mechanical stress is transmitted from cell-cell by cytoskeletal filaments

Question 4

Types of Junctions:

Answer 4

• Occluding Junctions (Lateral Membrane) e.g. tight junctions
• Cell-Cell Anchoring Junctions (Lateral Membrane) e.g. desmosomes and adherens junctions
• Channel-Forming Junctions (Lateral Membrane) e.g. gap junctions
• Cell-Matrix Anchoring Junctions (Basal Membrane) e.g. hemidesmosomes

Question 5

Key components in adhesion:

Answer 5

Adhesion or extracellular matrix molecules
Adhesion molecule
Plaque/linker proteins
Signalling molecules (e.g. kinases) cytoskeleton

Question 6

Tight Junctions

Answer 6

A type of occluding junction.
Formed from 2 families of proteins called: Claudins and Occludins

Acts as a diffusion barrier.
e.g. Glucose actively transported into cell from apical surface and diffuses out of cell at basolateral membrane.
Tight junctions prevent backflow of glucose from basal side of epithelium into gut lumen.

Question 7

Anchoring Junctions

Answer 7

Cell membranes are strengthened by presence of strong membrane-spanning structures that are tethered to the cytoskeleton.
Common in tissues that are subjected to high amounts of mechanical stress (skin, muscle, heart) .

Intermediate-Filament Attachment Sites:
Desmosomes (Cell-Cell) , hemidesmosomes (Cell-Matrix)

Actin-Filament Attachment Sites:
Adherens Junctions (Cell-Cell)

Anchoring junctions have two major components:

1. Transmembrane Protein (Adhesion protein): Extracellular domain that interacts with the ECM OR with the extracellular domain of a partner-adhesion protein on a neighbouring cell. Also posses an intracellular tail that binds to an intracellular protein.
2. Intracellular Proteins: Bind to the intracellular tail of the Transmembrane Protein, to create a physical and signaling link to cell interior.

Question 8

Cadherins

Answer 8

A type of cell adhesion molecule (CAM) that is important in the formation of adherens junctions and desmosomes.
Uses His-Ala-Val sequence for binding.
Link directly to beta-catenin proteins.

Question 9

Integrins

Answer 9

Attach the cell cytoskeleton to the extracellular matrix. Important for hemidesmosomes.
Integrins transmit signals bidirectionally:
- Outside-In Signalling: Mediates cellular responses induced by ligand binding to integrins.
- Inside-Out Signalling: Activates the ligand binding function of integrins .

Attachment to the ECM via Focal Adhesions is vital for development. Focal Adhesion Kinase (FAK), loss of FAK = lethal.

Question 10

Connexins and Gap Junctions

Answer 10

Connexins assemble to form gap junctions.
The closure of these gap junctions are regulated by Calcium;
High Calcium = Shut
Low Calcium = Open

Gap junctions pass electrical signals and share small molecules.