Lecture 22 - Cell adhesion

Question 1

Epithelia

Answer 1

Sheets of tightly bound cells in epithelial tissues

Question 2

What does apical-basal polarity result from?

Answer 2

the differential distribution of phospholipids, protein complexes, and cytoskeletal components

Question 3

Junctions connect cells at the ______ region

Answer 3

lateral

Question 4

Cells in the epithelium

Answer 4

attached to each other directly by cell-cell junctions, where cytoskeletal filaments are anchored, transmitting stresses across the interiors of the cells

Question 5

What are the three types of junctions that are vey important?

Answer 5

1. tight junctions
2. adherens junctions
3. gap junctions

Question 6

Adherens junctions associate with the ______ ________

Answer 6

actin cytoskeleton

• junctions must be anchored to actin

Question 7

What is the main component of the adherent junctions?

Answer 7

cadherin/catenin

Question 8

Where are cadherin and catenin located in the cell?

Answer 8

cadherin: transmembrane protein
catenin: cytoplasmic protein

Question 9

Cadherin-cadherin binding is _____ dependent

Answer 9

Ca2+

*each domain of cadherin (5) binds to Ca

Question 10

What do all of the cadherin superfamily members contain?

Answer 10

they all have extracellular portions containing multiple copies of the extracellular cadherin domain

Question 11

How do cadherin domains bind to other cadherin domains?

Answer 11

each cadherin domain forms a more-or-less rigid unit, joined to the next cadherin domain by a hinge
- Ca2+ ions bind to each hinge and prevents it from flexing
- When Ca2+ is removed, the hinges flex, and the structure becomes floppy => folds in on itself

Question 12

Typical cadherins vs atypical cadherins

Answer 12

typical: homophilic (bind to same cadherins)
atypical: heterophilic (bind to different cadherins)

Question 13

What do cadherins mediate? What does this enable?

Answer 13

they mediate highly selective recognition which enables cells of a similar type to stick together and to stay segregated from other types of cells

Question 14

How do cadherins play a role in sorting cells?

Answer 14

cells are sorted by the type ofo cadherin and amounts of cadherins

Question 15

Assembly of strong cell-cell adhesions requires….

Answer 15

changes in the actin cytoskeleton

Question 16

How do cadherins change the actin cytoskeleton?

Answer 16

they generate local signals to inhibit the GTPase Rho and activate the GTPase Rac

Question 17

What do catenins do?

Answer 17

they link classical cadherins to the actin cytoskeleton

Question 18

What are 2 examples of atypical cadherins? Why are they heterophilic?

Answer 18

Fat and Dachsous => heterophilic because of their role in planar cell polarity

Question 19

What does the barrier function of epithelia require?

Answer 19

that adjacent cells be sealed together by tight junctions so molecules cannot leak freely across the cell sheet

Question 20

How can tight junctions be visualized? What do they look like?

Answer 20

by freeze-fracture electron microscopy; seen as a branching network of sealing strands that completely encircles the apical end of each cell

Question 21

What are the main transmembrane proteins in tight junctions?

Answer 21

claudins and related proteins occludins

Question 22

What do claudins do?

Answer 22

they form selective channels allowing specific ions to cross the tight-junctional barrier, from one extracellular space to another
- they form SMALL pores

Question 23

Septate junctions

Answer 23

‘tight junctions’ in insects
- organized differently, but have the same function
- also made of claudins

Question 24

What are gap junctions and what is their structure?

Answer 24

• they are a pore that connects 2 cells, allowing transport of molecules between the cells
• they are clusters of channels that join 2 cells together and consist of building blocks of 2 connexons or hemichannels (one coming from each cell)

Question 25

What are connexons/ hemichannels formed from?

Answer 25

each one is formed from a complex of 6 connexin proteins

Question 26

Vertebrate vs invertebrate gap junctions

Answer 26

vertebrate gap junctions = connexins
invertebrate gap junctions = innexins

Question 27

What do the pores in gap junctions allow to pass?

Answer 27

exchange of inorganic ions and small molecules, but not of macromolecules

connexin and innexin form similar sized pores