Junctions

Question 1

Integrins

Answer 1

1. Transmembrane proteins that tie the ECM to cell cytoskeleton
2. Integrins bind their ligands with low affinity, but are present in high numbers on the cell surface.

Question 2

Integrin Structure

Answer 2

1. Composed of two non-covalent lay associated units (alpha and beta) both are required for activity. The Alpha unit contributes to specificity of binding while beta interacts with cytoskeleton
2. Binding requires divalent cations (Mg or Mn)

Question 3

Integrin Function

Answer 3

1. No intrinsic enzymatic activity
2. Following the binding of integrin to its ligand, the tail of beta subunit binds to proteins talin and alpha actinin
3. This then initiates assembly of complex of intracellular attachment proteins that link integrins to actin filaments in cytoskeleton
4. Then leads to increases in both intracellular pH and Ca levels and Tyr phosphorylation of proteins
5. Binding also initiates assembly of complex of catalytic and regulatory proteins that control gene expression, changes in cytoskeleton and adhesion
6. Binding anchors cell to matrix at focal adhesions (bundles of actin filaments -stress fibers- attach to focal adhesions)

Question 4

Integrins and Disease

Answer 4

1. Beta2 subunit forms dimers with different alpha subunits exclusively on surface of WBC’s
2. Leukocyte adhesion deficiency - loss of beta2 which eliminates adhesion in WBC’s causing bacterial infections
3. Beta3 integrins found in variety of cells, including platelets, and bind several ECM proteins, including fibrinogen
4. Glanzmann’s disease - missing or defective beta3 so doesn’t for GPIIb/IIIa which is a receptor for fibrinogen, so platelets can’t bind to fibrinogen to initiate blood clotting, treated with platelet transfusions
5. GPIIb/IIIa antagonists used to prevent heart attacks and strokes because they block platelet aggregation and formation of clots

Question 5

After binding to its ligand, an integrins binds directly to which of the following intracellular proteins?

Answer 5

Talin

Question 6

Selectins

Answer 6

1. cell-surface carbohydrate binding proteins synthesized by endothelial cells, platelets, and WBC
2. E or P selectins bind to oligosaccharides of WBC’s, L selectins bind to endothelial cells
3. Mediate weak adhesion between WBC and endothelial cells
4. Selectins and oligosaccharides are present on both WBC’s and endothelial cells

Question 7

ICAMs

Answer 7

• intercellular adhesion molecules
• expressed on endothelial cells
• bind to integrins on surface of WBC- heterophilic binding
• mediates stronger interaction which traps WBC’s at sites of inflammation
• specific integrins expressed on WBC’s that recognize ICAM’s

Question 8

Cell adhesion in inflammation Model

Answer 8

1. WBC’s circulate through bloodstream binding weakly to endothelial cells via selectins
2. When these cells receive signal from immune system, integrins on surface are activated and bind tightly to endothelial cells lining blood vessel and migrate through endothelial cells to reach site of inflammation
3. Integrins of WBC’s often have to be activated, which allows these cells to circulate unimpeded until they are activated by appropriate stimulus

Question 9

Cadherins

Answer 9

• transmembrane glycoproteins that promote cell adhesion in vertebrate tissues by homophilic binding
• adhesion depends on presence of Ca which prevents proteolysis
• virtually all cells seem to express one or more cadherins

Question 10

Cadherin structure

Answer 10

• large extracellular region
• highly conserved cytoplasmic domain
• interacts with actin cytoskeleton

Question 11

Catenins

Answer 11

• alpha and beta catenins
• required for adhesion by cadherins
• alpha catenin links cadherins to actin cytoskeletons
• beta catenin binds to cytoplasmic domain of cadherins and links it to alpha catenin
• p120 binds to cytoplasmic domain of cadherin

Question 12

Beta catenin and cancer

Answer 12

1. Beta catenin is either associated with cadherins or goes into nucleus and binds to transcription factor Tcf-4, which can turn on genes
2. Amount of free Beta catenin regulated
3. Function depends on Wnt signal

Question 13

In differentiated cells, no Wnt signal

Answer 13

1) Beta catenin forms complex with APC protein, CK-1 and GSK-3
2) beta catenin is phosphorylase do by CK-1 and GSK-3
3) Beta catenin degraded
4) Tcf protein represses transcription of target genes

Question 14

In undifferentiated cells, Wnt present

Answer 14

1) binds to frazzled and LRP
2) cytoplasmic protein Dishevelled phosphorylated
3) Leads to phosphorylation of LRP
4) axin associates with receptor, taking CK-1 and GSK-3 with it
5) Beta catenin not degraded
6) Beta catenin translocates to nucleus, forming complex with TcF
7) Complex is transcriptional activator of target genes

Question 15

APC protein

Answer 15

• product of APC gene
• mutation in APC gene is responsible for FAP which is characterized by hundreds of polyps in colon
• polyps can lead to colon cancer
• mutations in APC can lead to colorectal tumors
• defective APC means beta catenin can’t be degraded.
• target genes include those in cell proliferation (myc, cyclin D1)
• pathway involved in melanomas, liver tumors, and prostate and breast cancer

Question 16

Which of the following gens is mutated in familial adenomatous polyposis

Answer 16

APC

Question 17

Adherents Junctions

Answer 17

1) Connects cells to each other via actin
2) Continuous belt around epithelial cells
3) cadherins—-> catenins + p120——> actin

Question 18

Desmosomes

Answer 18

1. Connect cells to each other via IFs
2. Cadherin-related proteins (Desmoglein, desmocolin) bind to intracellular proteins(plakophilin, plakoglobin, desmoplakin) that bind to IF’s
3. Cadherin-related proteins + intracellular proteins ——-> IF’s
4. Skin (pemphigus) and heart diseases

Question 19

Hemidesomsomes

Answer 19

1. Connect cells to matrix via IF’s

2. Laminin—–> alpha6beta4—–> BP230 + plectin——> IFs

Question 20

Tight Junctions

Answer 20

• important in function of epithelial and endothelial cells as barrier
• found in intestine, kidney, liver, ear, blood-brain barrier.
• network of protein strands that go around entire circumference of cell
• link cells to actin

Question 21

Tight junctions Components

Answer 21

1. Integral membrane proteins. Occludins, claudins, JAMs
2. Adapter proteins - ZO 1-3, PARs, cingulin
3. Occludin/claudin/JAM—>ZO—-> actin

Question 22

Which of the following molecules mediates the association of actin filaments with cadherins in an adherens junction?

Answer 22

Catenin

Question 23

Gap junctions

Answer 23

• found in large numbers in most animal tissues
• allow for electrical and metabolic coupling of cytoplasm between cells of single type
• Coupled cells can share small molecules but not macromolecules
• Cytoplasm of one cell directly connected to that of neighbor by aligned channels called connexons
  1. 2 connexons form gap junction
  2. Connexons are hexamers of connexins
  3. Different cells have different connexins
• Size of channel only allows molecules

Question 24

Gap Junction Diseases

Answer 24

1. Charcot-Marie-Tooth disease
   - progressive degeneration of peripheral nerves
   - leads to loss of muscle control and muscle degeneration
   - caused by mutation in connexin 32 gene in Schwann cells
   - loss of connexin thought to lead to loss of myelin
2. Deafness, skin diseases, cataracts, cardiac arrthymias

Question 25

If you mixed developing pre-nerve cells expressing only N-cadherin on their surfaces with epithelial cells expressing only E-cadherin, what result would you expect?

Answer 25

Nerve cells would adhere to one another and epithelial cells would adhere to one another

Question 26

Which of the following mediate strong interaction between WBC’s and endothelial cells during an immune response?

Answer 26

Integrins + CAM’s

Question 27

Which of the following junctions use integrins as the linker protein to attach to IFs?

Answer 27

Hemidesomosomes