slide set 14

Question 1

epithelial tissue vs. connective tissue

Answer 1

epithelial tissue: mechanical stresses are transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites

connective tissue: extracellular matrix directly bears mechanical stresses of tension and compression

Question 2

type of cell-cell junctions

Answer 2

tight junction

adherens junction

desmosome

gap junction

Question 3

tight junction

Answer 3

seals gap between epithelial cells

Question 4

adherens junction

Answer 4

connects actin filament bundle in one cell with that in the next cell

Question 5

desmosome

Answer 5

connects intermediate filaments in one cell to those in the next cell

Question 6

gap junction

Answer 6

allows the passage of small water-soluble molecules from cell to cell

Question 7

hemidesmosome

Answer 7

anchors intermediate filaments in a cell to extracellular matrix

Question 8

tight junctions

Answer 8

made of 2 membrane proteins

claudins: essential for formation
occludins: limit permeability

extracellular domains of each bind tightly to each other

homophilic binding: the proteins bind “self”

Question 9

tight junctions prevent…

Answer 9

small molecules from passing between cells

EX: epithelial cells of small intestine, skin, provide structure to epithelium

Question 10

tight junctions and diffusion

Answer 10

tight junctions can limit membrane protein diffusion to zones within a cell’s plasma membrane

tight junctions confine transport proteins to diff regions of the plasma membrane in epithelial cells of the small intestine

Question 11

gap junctions

Answer 11

pores between cells

allows small molecules to move from one cell to another

Question 12

how to test if cells are connected by gap junctions

Answer 12

inject small dye into one cell

see if dye moves into adjacent cells

Question 13

types of gap junctions

Answer 13

channels, connexons, connexins

2 connexons form a channel

Question 14

how do cells assemble gap junctions?

Answer 14

• use a fluorescent tag that can be photoconverted from one fluorescent color to another
  
  • convert the tag on older connexins to green, the remaining connexins are labeled with a red tag
• new connexons are added to the outside edges of a gap junction

Question 15

assembling gap junctions

Answer 15

new connexons inserted into plasma membrane by exocytosis (like other integral membrane proteins) and then diffuse in the plane of the membrane until they bump into the periphery of a connexon plaque and are trapped

Question 16

anchoring junctions

Answer 16

cells adhere to each other or adhere to the extracellular matrix through anchoring junctions

a cytoskeletal filament attaches to a transmembrane protein through adaptor proteins (cytoskeletal filament can be actin or intermediate filaments)

membrane protein for cell-cell contact is a cadherin protein

membrane protein for cell-ECM is an integrin protein

Question 17

cadherin protein function

Answer 17

cell-cell contact

adherens junction

desmosome

Question 18

integrin protein function

Answer 18

cell-ECM contact

actin-linked cell-matrix junction

hemidesmosome

Question 19

cadherin protein family

Answer 19

share homology domains but also have differences among individuals

cytoplasmic tail varies among family members

extracellular cadherin domain is shared by all cadherins

(domains are fairly rigid, hinge regions in between domains are more flexible)

Question 20

cadherin structure

Answer 20

EC region of classic cadherin has 5 copies of extracellular cadherin domain separated by flexible hinge regions

Ca2+ ions bind at each hinge to prevent it from flexing

EC region is rigid and curved

to form cell-cell adhesion, cadherin at N-terminal tip of one cadherin molec binds N-terminal domain from cadherin molec on a diff cell

Question 21

how did we find out structure of cadherin

Answer 21

X-ray diffraction

Question 22

role of calcium in cadherin

Answer 22

calcium prevents hinge region from flexing

when Ca2+ is bound, there is limited flexibility and EC domain extends away from cell membrane

when Ca2+ isn’t bound, hinge regions are flexible and floppy

Question 23

cadherins are like velcro

Answer 23

each protein’s attachment to their partner is relatively weak, but many weak attachments in parallel are strong

Question 24

homophilic binding

Answer 24

individual cells bind to cells expressing the same cadherin

Question 25

homophilic binding experiment

Answer 25

1. take cells that don’t express cadherins (L Cells)
2. Transfect with a plasmid to express E-cadherin or N-cadherin and mix them together

The cells sort themselves into islands!

they sorted themselves by the type and level of cadherins they express

Question 26

cadherin expression changes during development as tissues form

Answer 26

as patterns of gene expression change the different groups of cells segregate according to the cadherins they express

Question 27

adherens junctions

Answer 27

cytoskeletal filament is actin

adaptor proteins are catenins, which bind to cytoplasmic tail of cadherin and link to actin and associated proteins (including myosin II)

Question 28

mature adherens junction formation

Answer 28

100s - 1000s of cadherins

1. membrane protrusions initiate cell-cell contact
2. actin and cadherin recruitment expands junction (recruited by Rac)
3. actin remodeling and myosin recruitment expands the adherens junction

Question 29

tension

Answer 29

myosin II and attachments generate tension

2 cells pull equally on each other

cell-cell junctions can sense increased tension and respond by strengthening their actin linkages (tension sensing is done by alpha-catenin)

Question 30

desmosomes

Answer 30

cadherins linked to intermediate filaments through adaptors

Question 31

Pemphigus

Answer 31

autoimmune disease

patients make antibodies against their own cadherins that form desmosomes

causes severe blistering of skin

keratin intermediate filament networks of adjacent cells are indirectly connected to one another through desmosomes

and to basal lamina through hemidesmosomes