Cell Junctions

Front 1

Intestine order

Back 1

1. Lumen of gut
2. Epithelium (microvilli)
3. Connective tissue (fibroblasts)
4. Circular fibers (smooth muscle)
5. Longitudinal fibers (smooth muscle)
6. Connective tissue
7. Epithelium (absorb nutrients)

Front 2

Blood vessels and ducts

Back 2

1. Endothelial cells line on inside
2. Held together by cell-cell contacts
3. Impermeable barrier
4. Basement membrane supports

Front 3

Parts of epithelium

Back 3

apical=top, sides/bottom=basolateral

Front 4

Polarized cell

Back 4

epithelial cells are polarized because the apical side is different from teh basolateral

Front 5

Specialized functions of epithelia

Back 5

1. Uptake (nutrients, gases, wastes)
2. Secretion (hormones, milk)
3. Separation of functional liquids

Front 6

5 major epithelial cell junctions

Back 6

1. Tight junction
2. Adherens junction
3. Desmosome
4. Gap junction
5. Hemidesmosome

Front 7

Tight junction (function)

Back 7

seals neighboring cells together in an epithelial sheet to prevent leakage of molecules between them, very close apposition of adjacent cells, separates the apical and basolateral domains of epithelial cells (molecules different), precents diffusion of small molecules and proteins

Front 8

Adherens junction (function)

Back 8

joins an actin bundle in one cell to a similar bundle in a neighboring cell

Front 9

Desmosome (function)

Back 9

joins the intermediate filaments in one cell to those in a neighbor

Front 10

Gap junction (function)

Back 10

allows the passage of small water-soluble ions and molecules

Front 11

Hemidesmosome (function)

Back 11

anchors intermediate filaments in a cell to the basal lamina

Front 12

Interlocking tight junction network

Back 12

formed by 4-pass transmembrane proteins that make contact across the intercellular space and create a seal (claudin and occludin)

Front 13

Organization of occludin

Back 13

has 2 small extracellular loop domains that are hydrophobic, the loops stack very closely on top of each other and across to neighboring domains

Front 14

Confluence of epithelial cells

Back 14

As they reach confluence, they become impermeable due to tight junctions and other interactions

Front 15

Epithelial cells grown on porous membrane filters

Back 15

Can add things to either apical or basolateral side and can measure passage to the other side of the monolayer to measure transcytosis (tight junctions cause high resistance, electrical resistance)

Front 16

Transcytosis

Back 16

mechanism for transporting certain substances across an epithelial sheet that combines receptor-mediated endocytosis and exocytosis

Front 17

Electron-dense tracer molecules and tight junctions

Back 17

can be added to either apical or basolateral side, usually tight junctions can be shown to stop the dye from moving to the other side

Front 18

Placement of adherens junctions

Back 18

always found next to tight junctions on the lateral surface of cells (towards the basolateral side), sites of attachment to actin cytoskeleton, does not hold cells as close as tight junctions

Front 19

Ca2+ switch assay

Back 19

Allow cell juntctions to form on a cell surface and remove calcium so that adherens junctions are separated, addition of antibodies against transmembrane proteins at adherens junctions can be tested for the ability to block formation of the original polarizaed monolayer

Front 20

Cadherin protein structure

Back 20

1. Ca2+ binding modules
2. N-terminal modules confers specificity to bind to cadherin on adjacent cell
3. Has a transmembrane domain
4. Highly conserved cytoplasmic domain
5. Exists as a homodimer
6. 5 cadherin repeats

Front 21

Calcium and cadherins

Back 21

Has Ca2+ binding sites between each pair of cadherin repeat, so at too low calcium concentration, the homodimer is floppy and at greater Ca2+ levels, the cadherin dimers begin to straighten and then interact with neighbors

Front 22

Why junction connected to cytoskeleton?

Back 22

force resistance, otherwise any pulling on proteins (like tight junction proteins) will remove them right out of the plasma membrane

Front 23

Organization of adherens junction

Back 23

1. Cadherins cluster to form strong connections
2. α and β catenins link cadherins to actin (also p120)

Front 24

Cadherins and filopodia

Back 24

cadherins at the tips of filopodia make first contact with neighboring cells (E-cadherins cause epithelial cells to zipper up next to each other)

Front 25

E-cadherin

Back 25

Epithelia
Adherens junction

Front 26

N-cadherin

Back 26

Neurons, heart, skeleton muscle, lens, and fibroblasts
Adherens junctions and chemical synapses

Front 27

Cadherin specificity

Back 27

The cadherins are similar structurally, but are usually formed on different types of cells or places on cells

Front 28

Cadherins in neural tube formation

Back 28

E-cadherin expressed only in the overlying ectodern and N-cadherin only expressed in the neural tube

Front 29

Cadherin-dependent cell sorting

Back 29

Labeling two types of cells, either with different cadherins or different levels of one cadherin, shows that there is a sorting out process in which similar cells aggregate with each other

Front 30

Desmosome (structure)

Back 30

1. Thicker, stronger, denser than adherins junctions
2. Ca2+ dependent connected to IF
3. Cytoplasmic plaque is formed on the inside of the cell and is made of intracellular anchor proteins
4. IF (keratin) filaments attach to the surface of each plaque
5. Transmembrane adhesion proteins (cadherins) bind to the plaques intracellularly
6. Thicker space at desmosome junction

Front 31

Desmosome (proteins)

Back 31

1. Cadherins extracellularly interact with cadherins on the neighboring cell
2. The cytoplasmic tails of desmosomal cadherins interact with plakoglobin (gamma-catenin)
3. Plakoglobin binds to desmoplakin, which binds to the IFs
4. Desmoglein and desmocollin are the main desmosome cadherins

Front 32

Hemidesmosome (structure)

Back 32

1. Looks like a half desmosome
2. Connects cell to ECM and links intracellularly to IFs
3. α6β4 integrins
4. Bind to laminin outside and "plaque" proteins on the inside
5. Plaque proteins connect integrin cytoplasmic tails to IFs

Front 33

Epidermolysis bullosa

Back 33

severe skin blistering caused by mutations in a component of the hemidesmosome

Front 34

Anchoring junctions (Cell-Cell)

Back 34

1. Adherens junction (cadherin, cadherin in neighboring cell, actin)
2. Desmosome (cadherin, desmogleins and desmocollins in neighboring cell, IFs)

Front 35

Dye experiment to show cell communication (gap junctions)

Back 35

Inject dye into one epithelial cell and over time it will traverse into neighboring cells, then the whole sheet; shows molecular size limits for passage of different sized dyes

Front 36

Connexin

Back 36

protein of 26-40 kD that make up a connexon in a gap junction

Front 37

Connexon

Back 37

formed from 6 connexins, can be either homomeric or heteromeric

Front 38

Gap junctions

Back 38

formed when 2 connexons overlap, homotypic when homomeric connexon overlap and heterotypic when two different connexons overlap, shuts at high calcium and low pH, opens at low calcium and high pH, channel is large enough to share small metabolites/ions but not proteins

Front 39

Cell-cell adhesions with junctional adhesion mechanisms

Back 39

1. Tight junctions-nothing
2. Adherens junctions (adhesion belt)-actin
3. Desmosomes-IF
4. Gap junctions-nothing

Front 40

Cell matric adhesions with junctional adhesion mechanisms

Back 40

1. Hemidesmosomes (integrins)-IF
2. Focal adhesions (integrins)-actin

Front 41

Cell-cell adhesion with nonjunctional adhesion mechanisms

Back 41

1. Cadherins
2. Ig-like CAMs
3. Integrins
4. Selectins
(2-4=non-cadherin)

Front 42

Cell-matrix with nonjunctional adhesion mechanism

Back 42

1. Integral membrane proteoglycan
2. Integrins

Front 43

Cell Adhesion Molecules (CAMs)

Back 43

1. Transmembrane proteins
2. Extracellular domain is composed of immunoglobin repeats

Front 44

NCAM (N=neural)

Back 44

1. Homophilic, binds other NCAMs
2. Doesn't require calcium
3. Important in fine-tuning adhesive interactions in neural cells during development

Front 45

VCAM (v=vascular) & ICAM (I=intercellualr)

Back 45

1. Heterophilic (binds to integrins)
2. Ca2+-dependent (integrins)
3. Inducible expression (not always present on cell surface)
4. Blood cell express (leukocytes, lymphocytes)
5. Required for extravasation

Front 46

Selectins (function)

Back 46

interact with carbohydrates on other cell surfaces; essential for binding of leukocytes to endothelial cells during trafficking of cells out of the bloodstream to sites of injury or inflammation

Front 47

Selectins (structure)

Back 47

1. Bound to actin inside cell via anchor proteins
2. Selectin comes out of cell with a lectin domain on top of an EGF-like domain at the tip (lectins bind carbohydrates)

Front 48

Extravasation process

Back 48

Selectin-carbohydrate interactions mediate rolling (weak adhesion) and integrin-CAM interactions mediate adhesion and extravasation (strong adhesion)