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1

What are the two main ways animal cells are bound together?

Connective tissue and epithelial tissue

2

Compare epithelial tissue and connective tissue (4 major categories).

1. Epithelial - scanty ECM, Connective - plentiful ECM
2. Epithelial - many cells, Connective - sparse cell distribution
3. Epithelial - cells tightly bound together in sheets, Connective - matrix rich in fibrous polymers
4. Epithelial - cells bear most of mechanical stress through junctional complexes attached to the cytoskeleton, Connective - matrix bears mechanical stress

3

Long-term connections between cells require complex ___.

Cell junctions

4

What are the 4 major types of connections between cells? Briefly describe their function.

1. Anchoring junctions - link cells to cells (via TM cadherins) or cells to matrices (via TM integrins)
2. Occluding/tight junctions - seal gaps between epithelial cells (involve claudins)
3. Channel-forming junctions - form passageways for small molecules and ions to pass from cell to cell (involve connexins)
4. Signal-relaying junctions - complex structures that typically involve anchorage proteins alongside proteins mediating signal transduction.

5

What are the three functions of occluding/tight junctions?

1. Seal cells together to create a permeability barrier
2. Regulate paracellular transport
3. Fence function that separates the apical and basolateral membrane domains, keeping membrane proteins and lipids in the appropriate domains

6

Provide 3 examples of the use of occluding junctions to seal cells together.

1. Intestinal epithelium use occluding junctions for transcellular glucose transport.
2. Endothelial cells use occluding junctions to prevent plasma leakage from the blood vessels
3. Brain endothelial cells use occluding junctions to form the blood-brain barrier.

7

What is paracellular transport?

Leakage of materials between cells (rather than through cells)

8

Describe the general process of transcellular glucose transport.

The apical surface of the cell (lumen of the gut) has a high glucose concentration. The sodium/glucose symporter transports glucose into the cell, which also has a high glucose concentration. Then, the glucose flows out of the cell into the blood on the basolateral surface, down its concentration gradient.

9

What is the main occluding protein and how many membrane passes does it make?

Claudin; 4

10

What are the 2 functions of anchoring junctions?

1. Stabilize cells against mechanical stress/transmit force
2. Mechanically attach cells and their cytoskeletons to their neighbors or to the ECM.

11

What are the 2 broad categories of anchoring junctions?

1. Cell-cell
2. Cell-matrix

12

What are the 2 types of cell-cell anchoring junctions?

1. Adherins junctions
2. Desmosomes

13

What are the 2 types of cell-matrix anchoring junctions?

1. Focal contacts (actin-linked cell-matrix junctions)
2. Hemidesmosomes

14

What is the adhesion protein of adherens junctions? What extracellular ligand to these bind to? Finally, what is the intracellular cytoskeletal attachment?

Adhesion protein: classical cadherin
Extracellular ligand: classical cadherin on neighboring cell
Intracellular cytoskeletal attachment: actin

15

What is the adhesion protein of desmosomes? What extracellular ligand to these bind to? Finally, what is the intracellular cytoskeletal attachment?

Adhesion protein: non-classical cadherin
Extracellular ligand: non-classical cadherin on neighboring cell
Intracellular cytoskeletal attachment: intermediate filament

16

What is the adhesion protein of focal contacts? What extracellular ligand to these bind to? Finally, what is the intracellular cytoskeletal attachment?

Adhesion protein: integrin
Extracellular ligand: extracellular matrix proteins
Intracellular cytoskeletal attachment: actin

17

What are the adhesion proteins of hemidesmosome? What extracellular ligand to these bind to? Finally, what is the intracellular cytoskeletal attachment?

Adhesion proteins: alpha6beta4 integrin, type XVII collagen
Extracellular ligand: extracellular matrix proteins
Intracellular cytoskeletal attachment: intermediate filament

18

Adherens junctions form an adhesive belt just below ___ junctions.

Tight

19

Adherens junctions join an ___ bundle in one cell to that of another cell.

Actin

20

What are cadherins?

Calcium-dependent TM adhesion molecules that mediate homotypic adhesion; they are dimers.

21

What is the adaptor/anchor protein for cadherins and what do they do?

Catenins; connect cadherins to actin

22

Desmosomes form ___ that anchor cells together via intermediate filaments.

Spot welds

23

What are the two types of desmosomal cadherins?

Desmogleins and desmocollins

24

Desmosomal cadherins mediate cell-cell ___ adhesion.

Homotypic

25

What are the adaptor/anchor proteins connecting desmosomal cadherins to the intermediate filaments?

Plakoglobin and desmoplakin

26

What is pemphigus?

An autoimmune disease where auto-antibodies against desmosomal cadherins cause loss of cell-cell adhesion and blistering.

27

What are tonofilaments?

Bundles of cytokeratin intermediate filaments

28

What do focal contacts/focal adhesions do?

Link ECM to actin filaments so that cells can "hang on" to their surroundings

29

What are integrins?

TM ECM-binding proteins that bind to actin indirectly via adaptor/anchor proteins

30

What are the adaptor/anchor proteins of integrins and what do they do?

Alpha-actinin, talin, filamin; link integrins to actin