Lecture 24

Question 1

What are the two types of cell interactions?

Answer 1

1. cell- cell

2. cell- (extra)cellular matrix

Question 2

Describe the key features of tissue adhesion

Hint: illustrate using epithelial and connective tissues as examples

Answer 2

Epithelial tissue: Cells are bound together into sheets (collagen), cells are anchored on the thin matrix mat of the basal lamina and the forces are born by intracellular cytoskeleton linked through adhesions.

Connective tissue is formed mainly by extracellular matrix with the cells secreting the matrix are distributed within it and forces are born through extracellular matrix

Question 3

How does epithelial tissue respond to mechanical stress?

Answer 3

Mechanical stress is transmitted from cell-cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites

Question 4

How does connective tissue respond to mechanical stress?

Answer 4

extracellular matrix directly bears mechanical stresses of tension and compression

Question 5

What are the cell junctions anchoring the lateral side?

Answer 5

adherence junctions (actin filaments) and desmosomes (intermediate filaments)

Question 6

What are the cell junctions anchoring the basal side?

Answer 6

actin linked cell matrix junctions and hemidesmosomes for intermediate filaments

Question 7

Functional role of tight junctions

Answer 7

Tight junctions fence between plasma membrane domains to seal gaps between epithelial cells

Question 8

adheren junctions

Answer 8

connects actin filament bundle between adjacent cells

Question 9

desmosome

Answer 9

connects intermediate filaments between adjacent cells

Question 10

actin-linked cell matrix junction

Answer 10

anchors actin filaments in cell to extracellular matrix

Question 11

hemidesmosome

Answer 11

anchors intermediate filaments in a cell to extracellular matrix

Question 12

What do anchoring junctions depend on?

Answer 12

transmembrane adhesion proteins

Question 13

What are the two superfamilies of anchoring junctions?

Answer 13

Cadherins (cell-cell interactions)

Integrins (cell-matrix interactions)

Question 14

Cadherins

Answer 14

Except prokaryotes, fungi, and plants cadherins form a diverse family of adhesion molecules in all animals and choanoflagellates. Cadherins need calcium for adhesion function. Anchoring of cell-cell junctions through cadherins are usually symmetrical and mediate homotypic (homophilic) adhesions.

Question 15

Velcro principle of Cadherins

Answer 15

Cadherins mediate weak interactions, but many of these interactions typically have strong adhesion. To remodel cell junctions, individual interactions are easily broken

Question 16

How do cadherins guide the organization of developing tissues?

Answer 16

Qualitative and quantitative differences in cadherin expression have different roles in mediating highly selective recognition in organization of developing tissues.

In 1950s, dissociated embryos reformed and many structures resembled the original embryo

Question 17

catenins

Answer 17

In adherens junctions, catenins link classical cadherins to actin cytoskeleton.

Question 18

Adherens junction assembly mechanism

Answer 18

1. Cell-cell contact is initiated by membrane protrusions with small cadherin and catenin clusters
2. Actin and cadherin recruitment expands junction with recruitment of more cadherins and catenins
3. Actin remodeling and myosin recruitment expands adherens junction to form contractile actin and myosin bundles

Question 19

How do adherens junction respond to forces generated by actin cytoskeleton ?

Answer 19

1. equalize actin contractility in neighboring cells
2. coordinate actin usage in neighboring cells
3. loosen adhesion after actin contraction

Question 20

In cell junctions, tissue remodeling depends on what?

Answer 20

coordination of actin-mediated contraction with cell-cell adhesion

Question 21

Diffusion through tight junctions

Answer 21

Paracellular diffusion of ions vary across epithelia

Tight junctions in small intestine 10, 000x more permeable to ions like Na+ than urinary bladder

Question 22

What are the two tight junction proteins?

Answer 22

claudin and occludin

Question 23

Claudins

Answer 23

Claudins can form ion-specific pores. The intracellular domains of claudins interact with ZO proteins. Claudins are important for structural assembly of tight junctions

Question 24

Occludins

Answer 24

limit junctional permeability

Question 25

Tricellulins

Answer 25

seals membrane where three cells meet