Endocytosis Flashcards
What is the purpose of the RAB system?
The RAB system is general and directs vesicle traffic throughout cell. RAB attracts motors to vesciles until they get to the correct organelle.
What are the types of endocytosis?
Fluid phase or pinocytosis- “cell drinking”
Potocytosis
Phagocytosis- “cell eating”
Receptor mediated endocytosis
Describe fluid phase endocytosis/pinocytosis
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
Describe potocytosis
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
What is transcytosis?
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
Describe phagocytosis
Ingestion of large particles via specific receptors–like bacteria & old/damaged cells
Occurs primarily in macrophages&PMNs
Not continual, larger vesicles (phagosomes:1-10m 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
What is the zipper mechanism?
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.
Describe receptor-mediated endocytosis
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
Describe endosomal trafficing
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.
How does the pinching off process work?
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.
What is an adaptin?
The receptors can link to clatherin coat known as adaptin (AP), adaptor protein between receptors cytoplasmic tail and clathrin coat
What is and describe the structure of a triskelion
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.
How does dissembly of clathrin coat actually occur?
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.
Define lysosomes
Acidic organelles that contain many acid hydrolases to degrade targeted materials
What is a primary lysosome
Primary lysosomes: newly formed, small, uniformly stained in EM
What is a secondary lysosome?
Secondary lysosomes: non-uniformly stained–> material being digested
- Phagolysomes: occur in phagocytes °rade phagocytosed materials - Autophagic vacuoles or cytolysome:degradation of cell’s organelles; difficult to digest material winds up as residual bodies
What are some examples of endocytosis in medicine
RME & Cholesterol
What do you know about RME and Cholesterol?
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
What is the structure of the primary carrier (LDL) of cholesterol
Consist of protein with lipid monolayer and in interior 1500 cholesterol esters
What are the steps to cholesterol uptake?
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
What are some effects of free cholesterol in the cell?
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)
Describe hypercholesterolemia
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
What happens when you have high cholesterol and high plasma LDL levels?
MP take up cholesterol and become foam cells forming fatty streak
How do you reduce cholesterol?
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
How does treatment with statin and/or bile acid binding resin work?
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
Describe influenza virus infection
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.
Describe HIV infection
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.
