II. Post-transcription | 32. Types, mechanism and role of autophagy Flashcards
I. Autophagy
1. What is autophagy?
Autophagy (‘’eating oneself’’) is when the cell is placed under stressful conditions (starvation) and can produce nutrients from recycling macromolecules by lysosomal degradation.
I. Autophagy
2. What is the role of autophagic pathway?
- The autophagic pathway allows cytosolic proteins and organelles to be targeted by lysosome interior for degradation.
- Autophagy can remove large objects (macromolecules, large protein aggregates and whole organelles).
I. Autophagy
3. What happen if there is defect in autophagy?
Defects in autophagy may prevent cells from clearing unwanted protein aggregates and abnormal proteins, and thereby contributing to diseases ranging from infectious disorders to neurodegeneration and cancer.
II. Lysosomes
1. What is the role of Lysosomes?
role of lysosome is to be the compartment for degradation
II. Lysosomes
2. What are the features of lysosomes?
- Membrane-enclosed organelles filled with soluble hydrolytic enzymes that digest macromolecules.
- Lysosomes contain about 40 types of hydrolytic enzymes (derived from the ER). All are acid hydrolases – work best at acidic pH.
- For optimal activity, they first need to be activated by proteolytic cleavage (if not, cell will die), which also requires acidic environment.
- The lysosome provides this acidity, maintaining an interior pH of about 5.
- ATPase’s (V-type) in the lysosome membrane, uses the energy of ATP hydrolysis to pump H+ into the lysosome.
II. Lysosomes
3. What are lysosomes filled with?
Membrane-enclosed organelles filled with soluble hydrolytic enzymes that digest macromolecules.
II. Lysosomes
1. What is the role of Lysosomes?
role of lysosome is to be the compartment for degradation
II. Lysosomes
4. What do lysosomes contain?
Lysosomes contain about 40 types of hydrolytic enzymes (derived from the ER).
- All are acid hydrolases – work best at acidic pH. For optimal activity, they first need to be activated by proteolytic cleavage (if not, cell will die), which also requires acidic environment.
II. Lysosomes
5. What is the interior pH of lysosome?
- The lysosome provides this acidity, maintaining an interior pH of about 5.
- ATPase’s (V-type) in the lysosome membrane, uses the energy of ATP hydrolysis to pump H+ into the lysosome.
III. Types of autophagy
1. What can autophagy be?
1) Non-selective autophagy
2) Selective autophagy
III. Types of autophagy
2. What are the features of Non-selective autophagy?
- Non-selective autophagy: bulk portion of cytoplasm is sequestered in autophagosomes (double-membraned organelle that envelopes cytosolic materials).
- May occur in starvation conditions when external nutrients are limiting
III. Types of autophagy
3. What are the features of selective autophagy?
Selective autophagy: specific cargo is packaged into autophagosomes, tend to contain little cytosol.
- Mediates degradation of worn out/unwanted mitochondria, peroxisomes, ribosomes and ER
III. Types of autophagy
4. How is autophagy activated?
Autophagy is a regulated process, it is activated by various stresses:
1. Nutrient depletion (starvation, fasting, lack of even a single essential AA)
2. Infection (invading intracellular pathogens)
3. Proteotoxic stress (fever)
4. Organelle damage, dysfunction (mitochondria)
III. Types of autophagy
5. What is the function of autophagy?
- Survival of nutrient deprivation: oocyte, newborn, starving adult
- Turnover of damaged, old cellular components
- Tumor suppression - because it destroys cells
- Antigen presentation
=> A prime mechanism involved in longevity and having implications in age-related disease = autophagy decreases with age, contributing to accumulation of damaged macromolecules + organelles during aging (remember: lipofuscin!)
III. Types of autophagy
6. What are the 3 types of autophagy?
- Macroautophagy
- Microautophagy
- Chaperon-mediated autophagy (CMA)
III. Types of autophagy
7A. What is Macroautophagy?
- Macroautophagy is a process in which cells form double-membrane vesicles, autophagosomes, around a portion of cytoplasm (aggresomes), used primarily to eliminate damaged cell organelles or unused proteins.
- Autophagy is upregulated under physiological stress conditions, such as starvation. It acts as a buffer against starvation, because it has the capacity to recycle macromolecules for use as nutrients, liberate building materials and energy sources from cellular components
III. Types of autophagy
7B. How does macroautophagy work?
- The autophagic pathway begins with the formation of a flattened double-membrane cup- shaped structure that envelops a region of the cytosol or an entire organelle (ex: mitochondrion), forming an autophagosome.
- The autophagosomal membrane is a lipid bilayer, and its outer membrane can fuse with a lysosome.
- Amino acid permeases in the lysosomal membrane then allows for the transport of free amino acids back into the cytosol for use in synthesis of new proteins.
III. Types of autophagy
7C. What are the characteristics of macroautophagy?
- Breakdown of bulk cellular components
- Not merely proteolysis (all molecules are turned over – continuous renewal)
- Activated in early starvation (30 min – 8 hours)
- Autophagosomes are formed around aggresomes
III. Types of autophagy
7D. What is Aggresome?
- Aggresome refers to an aggregation of misfolded proteins in the cell, formed when the protein-degradation system of the cell is overwhelmed.
- Aggresome formation is a highly- regulated process, which possibly serves to organize misfolded proteins into a single location.
- Aggresomes are targeted to autophagy.
III. Types of autophagy
7E1. What are the 5 steps of Autophagosome formation and fusion?
1) Initiation
2) Vesicular nucleation
3) vesicle elongation (conjugation)
4) Fusion
5) Vesicle breakdown + degradation of cargo
III. Types of autophagy - Autophagosome formation and fusion
7E2. Describe step 1. Initiation of Autophagosome formation and fusion?
- Initiation by activation of signaling molecules
- protein kinase become activated and signal the autophagic machinery (ULK1 kinase complex)
III. Types of autophagy - Autophagosome formation and fusion
7E3. Describe step 2. Vesicular nucleation of Autophagosome formation and fusion?
ULIK1 phosphorylate Beclin1, which recruits protein that
eventually recruit PIP3 protein
=> membrane grows
III. Types of autophagy - Autophagosome formation and fusion
7E4. Describe step 3. Vesicle elongation (conjugation) of Autophagosome formation and fusion?
extension of the membrane + LC3 proteins which
bind adaptor proteins that bring the cargo inside
III. Types of autophagy - Autophagosome formation and fusion
7E4. Describe step 4. Fusion of Autophagosome formation and fusion?
transport of mature + closed autophagosome to juxtanuclear position where it fuses with a lysosome (catalyzed by SNARE proteins)
=> autolysosome
III. Types of autophagy
8A. What is Microautophagy?
- Autophagy type where the cargo (organelles or substrate protein) in cytosol are invaginated (inward folding of the lysosomal membrane) and trapped by lysosome.
- Constitutive process, but induced in response to starvation or fasting.
- Involves the direct engulfment of cytoplasmic material into the lysosome.
III. Types of autophagy
8B. What are the functions Microautophagy?
- As macroautophagy, it is necessary for nutrient recycling under starvation for the survival of the cell
- Nitrogen restriction
- One of the mechanisms of glycogen delivery to lysosomes
III. Types of autophagy
9A. What is Chaperone mediated autophagy?
Refers to the chaperon-dependent selection of soluble cytosolic proteins that are then targeted to lysosome and directly translocated across the lysosome membrane for degradation.
III. Types of autophagy
9B. What is the function of Chaperone mediated autophagy?
proteostasis + amino acid recycling
III. Types of autophagy
9C. What are the Substrates of Chaperone mediated autophagy?
KFERQ-like amino acid sequence – the recognition site for Hsc70 (heat shock cognate protein of 70kDa) which targets the substrate to the lysosomal surface and allows cytosolic chaperons to bind
III. Types of autophagy
9D. What are the steps of Chaperone mediated autophagy?
1) Recognition of substrate (KFERQ-like amino acid sequence) by the Hsc80 and formation of CMA substrate-chaperon complex in cytosol
2) This complex binds to lysosome-associated membrane protein (LAMP-2A), which acts as the receptor for this pathway
3) Upon recognition by LAMP-2A receptor, the substrate protein unfolds and is translocated across the lysosomal membrane with the help of Hsc70 chaperons
4) After translocation, the substrate proteins are rapidly degraded by lysosomal proteases
=> CMA is maximally activated in response to stressors and changes in the cellular nutritional status
