Autophagy Flashcards
Function of lysosomes?
- Organelle that degrades either
- Obsolete cell components
- Material taken into cell by endocytosis or phagocytosis.
How do lysosomes function?
- Contain enzymes that digest nucleic acids, proteins, carbs, and lipids
- All work at acidic pH.
How are things shipped to the lysosome?
- Tagged with a signal protein (Mannose 6-phosphate) M6P
How to make Mannose 6-phosphate?
- Addition and initail processing of N-linked oligosaccharide precursors occur in rough ER
- M6P signal generated in cis-Golgi
- N-acetylglucosamine phosphotransferase transfers phosphorylated N-acetylglucosamine (GlcNAc) group to carbon atom 6 of one of mannose residues.
- Lysosomal protein released from phosphotransferase
- GlcNAc group removed by phosphodiesterase, leaving phosphorylated mannose residue.
Steps in traffiking of soluble lysosomal proteins from the trans-golgi and cell surface to lysosomes
- Lysosomal proteins enter clathrin-coated vesicles
- Clathrin coat is removed
- Uncoated vesicles fuse with late endosomes
- Late endosome containing dephosphorylated lysosomal proteins fuses with lysosome.
- Some M6P receptors delivered to cell surface
- Some phosphorylated lysosomal proteins secreted and retrieved by endocytosis in clathrin-coated pits
- Clathrin coat removed from vesicle
- Endocytic vesicles fuses with late endosomes.
Lysosomal Storage diseases
- Caused by absense of one or more lysosomal enzymes
- Undigested glycolipids and extracellular components accumulate in lysosomes as large inclusions
Definition of autophagy
- Regulated process by which cytosolic components and organelles are delivered to lysosomes, degraded and recycled
- Serves normal housekeeping function
- Usually doesn’t kill cell but cell can die if process deregulated
What induces autophagy?
- Environmental cues
- Starvation
- High temp
- Low O2
- Hormonal stimulation
- Intracellular stress
- Damaged organelles
- Accumulation of misfolded or aggregated proteins
- Microbial and viral infection.
3 Types of Autophagy?
Micro-, Macro-, Chaperone-mediated
What is microautophagy?
- Cytosolic components taken up directly by lysosome
- Lysosomal membrane invaginates
What is macrautophagy?
- Cytosolic components delivered to lysosome by vesicle called autophagosome
- Fuses with lysosome to form autolysosome
Macroautophagy pathway?
- Three steps to form autophagosome
- Nucleation
- Growth and completion/elongation
- Targeting and fusion/docking
What is the role of Atg5 in Autosophagosome formation?
Binds with Atg12 to form dimer, then the dimer associates with Atg16 to form the vesicle that was started by Atg8
What is the role of Atg8 in Autosophagosome formation?
- It is required for initial vesicle fusion
- Covalently linked to phosphatidylethanolamine
Regulation of Macroautophagy?
- Macroautophagy regulated at autophagosome formation
- During periods of high/sufficient nutrient levels, autophagy not activated.
- mTOR phosphorylates regulatory protein upstream of Atg8, 12, 15.
- This prevents induction of autophagy
- mTOR phosphorylates regulatory protein upstream of Atg8, 12, 15.
- During starvation or stress
- mTOR dissociates from complex
- Autophagic induction proteins activated by phosphatases.
- During periods of high/sufficient nutrient levels, autophagy not activated.
Regulation of chaperone-mediated autophagy?
Mediated by levels of LAMP-2A protein in lysosomal membrane.
Autophagy in disease
- Neurodegenerative diseases:
- Parkinsons
- Alzhiemers
- Huntington’s
- Accumulation of aberrant or misfolded proteins not removed by autophagy.
- Cancer:
- Depends on level of autophagy
- Autophagy can help tumors survive stresses such as hypoxia and nutrient deprivation
- Some cancer drugs promote lethal autophagy.
