Cell Junctions, Cell Adhesion and the Extracellular Matrix

Question 1

What are lamellipodium and how do they move?

Answer 1

They are web like structures of actin filaments and the substratum allows for traction and movement.

Question 2

Describe assembly and disassembly of actin for migration.

Answer 2

Net filament assembly occurs at leading edge and net filament disassembly occurs behind leading edge.

Question 3

How do gtpases regulate the cytoskeleton?

Answer 3

Chemoattractants bind to receptor, cause events in cells that lead to movement.
Rac promotes polymerization of actin at front, Rho dominates actin-myosin contraction to pull back end forward.

Question 4

describe the extracellular matrix-

Answer 4

Organized network of proteins and polysaccharides made by the cells with which it associates
• Regulates activity and shape of cells.
• Absorbs mechanical stress

Question 5

What do fibroblasts do?

Answer 5

make proteins and sugars that provide cushioning in ECM.

Question 6

What Cellular Processes are Regulated by the ECM? •

Answer 6

Cell polarity • Cell metabolism • Protein organization in PM • Cell survival • Cell Proliferation • Cell Differentiation • Cell migration

Question 7

What are glycosaminoglycans?

Answer 7

composed of repeating disaccharide units, most linked to a protein core.

Question 8

Describe proteoglycans

Answer 8

GAGs linked to protein core, created hydrated gel that resists compression, important in connective tissues, bind and regulate activity of other secreted molecules.

Question 9

What does collagen do?

Answer 9

major component of ECM that forms fibrils, triple helix structure.

Question 10

Describe collagen assembly.

Answer 10

Occurs extracellularly, fibroblasts influence collagen organization and secrete collagen.

Question 11

What are fibronectins?

Answer 11

Glycoprotein that helps cells attach to collagen in the matrix and binds integrins on the cell surface.

Question 12

What are integrins?

Answer 12

Receptors for ECM proteins, can be activated from inside or outside the cell.

Involved in cell-matrix and cell-cell adhesion

2 Subunits: α and β • Intracellular portion binds proteins that link receptor to the cytoskeleton.

Question 13

What is the basal lamina?

Answer 13

Basement membrane, thin, tough sheet that underlies epithelial cells, maintains architecture of body, detachment of epidermis from dermis can cause skin blistering.

Includes integrin and lamina, which interact with each other.

Question 14

What are the types of cell junctions?

Answer 14

Anchoring junctions, occluding (tight) junctions, and channel forming junctions

Question 15

Describe tight junctions-

Answer 15

seals gap between epithelial cells.

Question 16

Describe the cell-cell anchoring junctions:

Answer 16

adherens junctions - connect actin filament bundle in one cell with that in the next cell, transmembrane proteins are classical cadherins
desmosome- connects intermediate filaments in one cell to those in next cell, transmembrane proteins are nonclassical cadherins

Question 17

Describe the channel forming junction:

Answer 17

gap junction - allows passage of small water soluble molecules from cell to cell.

Question 18

Describe the cell matrix anchoring junctions -

Answer 18

actin linked cell matrix junction: anchors actin filaments in cell to extracellular matrix, transmembrane protein is integrin

hemidesmosome: anchors intermediate filaments in a cell to ECM, transmembrane protein is integrin and collagen,

Question 19

What is the function of cadherin and how do they work?

Answer 19

Cell-cell adhesion, hold cells together, attaches to actin and intermediate filaments in adherens junctions and desmosomes, respectively.

Ca2+ binds hinge region, prevents flexing, allows for adhesion
• Interactions between many cadherins occur at junctions

Question 20

How do cadherins link to the cytoskeleton?

Answer 20

Intracellular anchor proteins link cadherin to cytoskeleton.

Question 21

How do cadherins bind?

Answer 21

They bind homophilically more frequently, binding the same type of molecule.

Question 22

Describe cell adhesion and cell sorting:

Answer 22

Cells with similar origins associate
• Cells expressing similar levels of cadherin preferentially associate
• Cells expressing the same type of cell adhesion molecules will preferentially associate.

Question 23

Describe assembly of adherens junctions:

Answer 23

membrane protrusions initiate cell-cell contact, actin and cadherin recruitment expands junction, actin remodeling and myosin recruitment expands adherens junction

Question 24

What does the adhesion belt do?

Answer 24

Allows coordinated movement of epithelial cells as sheets.

Question 25

Describe desmosomes:

Answer 25

Cadherin family adhesion proteins, association with intermediate filaments and provide mechanical strength.

Question 26

What are focal adhesions?

Answer 26

Actin-linked cell matrix adhesions, integrin binds ECM and is linked intracellularly to actin filaments, unstable, temporary structures.

Question 27

Describe hemidesmosomes

Answer 27

• Integrin anchors cells to the ECM and is coupled to

intermediate filaments intracellularly.

Question 28

Describe tight junctions -

Answer 28

functions as a seal and a fence by preventing movement of nutrients and small molecules and membrane proteins.

Question 29

Describe tight junction structure:

Answer 29

Occludins and Claudins associate to form sealing strands • Multiple strands make up tight junction

Question 30

What are gap junctions -

Answer 30

Direct passageways between cells • Allows exchange of small molecules • Inserted into membrane via exocytosis

Question 31

What are the functions of gap junctions?

Answer 31

Electrical coupling for rapid spreading of action potentials. • Sharing of small molecules so cells coordinate their actions. • Switch between open and closed states