Endocytosis

Question 1

What is the purpose of the RAB system?

Answer 1

The RAB system is general and directs vesicle traffic throughout cell. RAB attracts motors to vesciles until they get to the correct organelle.

Question 2

What are the types of endocytosis?

Answer 2

Fluid phase or pinocytosis- “cell drinking”

Potocytosis

Phagocytosis- “cell eating”

Receptor mediated endocytosis

Question 3

Describe fluid phase endocytosis/pinocytosis

Answer 3

Constitutive-bulk uptake of solutes outside cell

Non-concentrative (don’t concentrate cells)

Endocyotic vesicles: 0.1-1.0 m in diameter

Macrophages can internalize 25% of their volume/h and 3% of surface/minute–> membrane recycling

Question 4

Describe potocytosis

Answer 4

Takes place in small flask-like invaginations called caveolae

GPI-anchored proteins & signaling molecules located in caveolae

Major structrural protein: caveolin, a cholesterol-binding protein

May take up small molecules like folate

Responsible for transcytosis

Question 5

What is transcytosis?

Answer 5

Take material across endothelial or epithelial cells.

In the intestinal lumen there is a layer of epithelial cells that bear a receptor for the FC portion of ab. That cuases endocytosis of the structure into vesciles. The vesciles move into early endosome to recycling endosome where they are exocytosed to basolateral surface of another cell to be transported to extracellular fluid.

Example is a newborn obtains immunity by tranferring mother’s abs from milk

Question 6

Describe phagocytosis

Answer 6

Ingestion of large particles via specific receptors–like bacteria & old/damaged cells

Occurs primarily in macrophages&PMNs

Not continual, larger vesicles (phagosomes:1-10m in diameter, depending on what is engulfed)

Local response of the cell surface via zipper mechanism

Phagosomes fuse with either lysosomes or Golgi vesicles bearing lysosomal hydrolases which causes degredation of what is in phagosome

Question 7

What is the zipper mechanism?

Answer 7

Zipper Mechanism: As the cell zips up around item to be phagocytosed and then is ingested completely.

Bacterium is coated with an antibacterial ab produced by the cell. Ab binds to the bacteria’s surface and extends its FC region outward. On the surface of the macrophage FC receptors that bind ab and cause the membrane to be pulled over the bacteria to be phagocytosed.

Question 8

Describe receptor-mediated endocytosis

Answer 8

Uptake occurs by specific receptors which concentrate in coated pits (concentrative)

Uptake of growth factors and nutrients(cholesterol via LDL, iron via transferrin) at low external concentrations

Coated pits invaginate to form coated vesicles

Coated vesicles take cargo inside cell

Question 9

Describe endosomal trafficing

Answer 9

There is cell surface with receptors in coated pit thats coated with clatherin or receptors that are diffused to the surface that are stuck to ligand and these receptors get stuck on the surface. Coated pit pinches off into a coated vesicle. The coat disappear and we get an early endosome where pH drops bc there is a proton pump in the membrane of the endosome that pumps in H to lower pH. This matures to CURL compartment that is responsible for uncoupling ligand from receptor. The receptor is recycled is back to the membrane and the remaining part of the CURL matures into late endosome that becomes lysosome where degredation occurs.

Question 10

How does the pinching off process work?

Answer 10

The motor protein Dynamin uses GTP (so its a GTPase) which contricts neck of vescile so it will bud from the parent membrane. Dynamin hydrolyzes GTP to GDP and pinches off neck.

Question 11

What is an adaptin?

Answer 11

The receptors can link to clatherin coat known as adaptin (AP), adaptor protein between receptors cytoplasmic tail and clathrin coat

Question 12

What is and describe the structure of a triskelion

Answer 12

A clathrin molecule. They have three heavy chains and three light chains. Thirty-six of them self assemble to make the clathrin coat structure around vesicle.

Question 13

How does dissembly of clathrin coat actually occur?

Answer 13

There is a chaperon called Hsc70 that triggers uncoating of the vesicle. When Hsc70 comes in contact with coated vesciles transkilions and AP fall away. Uncoated vescile will be early endosome.

Question 14

Define lysosomes

Answer 14

Acidic organelles that contain many acid hydrolases to degrade targeted materials

Question 15

What is a primary lysosome

Answer 15

Primary lysosomes: newly formed, small, uniformly stained in EM

Question 16

What is a secondary lysosome?

Answer 16

Secondary lysosomes: non-uniformly stained–> material being digested

- Phagolysomes: occur in phagocytes &degrade phagocytosed materials
- Autophagic vacuoles or cytolysome:degradation of cell’s organelles; difficult to digest material winds up as residual bodies

Question 17

What are some examples of endocytosis in medicine

Answer 17

RME & Cholesterol

Question 18

What do you know about RME and Cholesterol?

Answer 18

Cholesterol is required by membranes
Synthesized by cells or taken up from blood stream via low density lipoproteins (LDL)
Hi cholesterol levels in blood contribute to athersclerotic plaques in the blood vessels–> heart attack & stroke

Question 19

What is the structure of the primary carrier (LDL) of cholesterol

Answer 19

Consist of protein with lipid monolayer and in interior 1500 cholesterol esters

Question 20

What are the steps to cholesterol uptake?

Answer 20

When cells need cholesterol they synthesize LDL receptors

Cholesterol-LDL internalized by RME

Coated vesicles become endosomes when they lose their clathrin coats

LDL dissociates from LDLr because of low pH in endosome; LDL cholesterol–> lysosome & LDLr recycles–>PM

LDL degraded & cholesterol released to cytoplasm

Feedback control via HMG-CoA reductase

Question 21

What are some effects of free cholesterol in the cell?

Answer 21

Free cholesterol causes

1) HMG-CoA reductase activity to be decreased or negatively regulated
2) Increase in ACAT (acetylCoA:cholesterol acyltransferase) storage of cholesterol esters (droplets)
3) Decreases/downregulate the synthesis of LDL receptors
4) Cholesterol is converted to bile acids so there is an increase in 7alpha hydroxlyase (bile acids)

Question 22

Describe hypercholesterolemia

Answer 22

Results from defective LDL receptors

Homozygous form of familial hypercholesterolemia–> no LDL receptors–> no cholesterol uptake by cells–>heart attacks at early age

Only cure: liver transplant as liver is responsible for ~75% of cholesterol uptake

In heterozygous hypercholesterolemia, individual has fewer functional LDL receptors

Question 23

What happens when you have high cholesterol and high plasma LDL levels?

Answer 23

MP take up cholesterol and become foam cells forming fatty streak

Question 24

How do you reduce cholesterol?

Answer 24

Low cholesterol diet

Drugs which inhibit HMG-CoA reductase (statins)–>cells synthesize more LDL receptors and take up more cholesterol from blood

Common statins: lipitor, zocor, pravachol &mevacor

Drugs or agents which prevent reabsorption of bile salts by intestines. Cholesterol is metabolized to bile salts in intestine

Question 25

How does treatment with statin and/or bile acid binding resin work?

Answer 25

In order to prevent reabsorption of bile acids into liver cells one type of treatment is to have resin. The bile acid is absorbed into the instestine and it stays there. This sets up a steady state that drives cholesterol out of the blood stream into the cell. Cholesterol concentration increases within the cell and inhibits the enzyme that sythesizes chol within the cell which reduces chol within cell which makes more chol come into cell from blood stream. This reduces chol in blood stream.

Resin binds bile acid reducing pool thereby converting more chol to bile acid

Statins interfere w/ cell chol synthesis–>
cell wants chol from blood stream

Question 26

Describe influenza virus infection

Answer 26

Flu virus is a lipid envelope virus and inside is matrix protein with nucleocapsid and viral nucleic acid. In the lipid envelope is transmembrane protein (viral hemaglutinin) This protein can bind to receptors in the coated pits. The pits uptake virus via RME. In the low pH environment of endosome hemaglutinin changes confirmation and causes fusion of the viral env with endosomal env and allows release of viral na into the cell to infect the cell.

Question 27

Describe HIV infection

Answer 27

This is a lipid env virus. There is a coat protein that embeds in lipid membrane. Inside is the viral capside and reverse transcriptase. RT will change the viral RNA into form that can infect cell. Mode of entry is via direct fusion with pm. In order for virus to dock on cell surface two receptors are required. On macrophages it is a CD4 receptor plus the beta chemikin receptor (ccr5). On the t cell its CD4 and alpha chemokine receptor CXCR4.

The virus docks on pm and at nml pH fusion process occurs. NA and capsid are released into cell, RT changes it into form that can take viral genome and mediate insertion to cell genome.