Autophagy and Apoptosis Flashcards
Neurodegeneration Main Cause
• Neurodegeneration cell death is associated with accumulation of proteins that become toxic and impair cellular quality control mechanisms, leading to apoptosis
Possible Quality Control Processes in the Cell
o Ubiquitin-proteasome system – 1st degradative cellular mechanism
Short lived proteins go to proteasome because they’re marked by poly-ubiquitination
Proteasome breaks down the proteins into peptides and individual amino acids
o Autophagy-lysosome system
Lysosome degrades proteins that are or are not ubiquitinated; degrades other organelles
Ubiqutin-Proteasome System
o Ubiquitin (Ub) Enzyme Cascade – ATP is needed for activation
Ub-activating enzyme (E1)
Ub-conjugating enzyme (E2) enzyme acts as mediator in transferring ubiquitin to E3 enzyme or to target protein
Ub-ligase E3 recognizes substrates and facilitates the covalent attachment of Ub
o Some E3 enzymes contain both HECT and RING others contain only one
o Potential source of pathology if mutates or lose function
• HECT domain E3s – Homologous to E6-AP C Terminus
o Binds with ubiquitin from the E2 BEFORE transferring it to the substrate
• RING domain E3s – Really Interesting New Gene
o Acts as molecular scaffold, brings ubiquitin-E2 and substrate in close proximity for bond formation; doesn’t directly attach to it itself
Types of Ubiquitination
o Cascade is responsible for covalent attachment of multiple ubiquitin molecules to specific target proteins, leading to their degradation and/or modification
o Polyubiquitin chains can be synthesized by adding ubiquitin to lysine residues of the previous ubiquitin attached to a target protein; LYSINE REQUIRED TO ADD UBIQUITIN
o 7 lysine residues on ubiquitin itself contribute to chain formation for cell signaling
K27 – linked to inflammation
K48 and K63 involved in degradation in proteasome or autophagy
Forms and Functions of Ubiquitination
o Mono-Ub: endocytosis, endosomal sorting, histone regulation, DNA repair, virus budding, nuclear export
o Multi-Ub (multiple Ub attached directly to multiple lysines): endocytosis
o Poly-Ub (one long straight strand attached to 1 lysine): DNA repair, endocytosis, activation of protein kinases
o Poly-Ub (one long branched strand attached to 1 lysine): proteasomal degradation
Deubiquitinating Enzymes
– group of proteases that cleave ubiquitin from proteins and enzymes in the ubiquitination pathway; nearly 100 DUB genes classified into main classes
o Cysteine proteases compromise:
Ubiquitin-specific proteases (USPs)
Ubiquitin C-terminal hydrolases (UCHs) – important therapeutic agents
Machado-Josephin domain proteases (MJDs) – involved in ataxia
Ovarian tumour proteases (OTU) – involved in ovarian tumor
o Metalloproteases include:
Jab1/Mov34/Mpr1 and Pad1 N-terminal+ (MPN+)
Autophagy
– cell’s major degradative pathway for proteins that are ubiquitinated and do NOT go to proteasome or proteins that are not ubiquitinated
o Signaled via starvation, growth-factor deprivation, & oxygen storage metabolic stress signaling events autophagy
Ionizing radiation and chemotherapy signaling events autophagy
Buildup of cytoplasmic material portion of ER will break off and start to envelope forming phagopore phagopore matures into autophagosome autophagosome forms and fuses with lysosome autolysosome that degrades cytoplasmic material
Types of Autophagy
o Microautophagy – involve organelles and soluble proteins that are uptaken by lysosome and then degraded
o Macroautophagy – involves phagopore autophagosome fusing to lysosome
o Chaperone-mediated autophagy – involves a chaperone protein recognizing ubiquitination of proteins, binds to it, and drags it into lysosome for degradation
Molecular Mechanism of Autophagy
o Signal recruits ATG (5, 12, 16) molecules that bind and form LC3 LC3 helps phagosome form enclosed double membrane autophagosome fusion of autophagosome + lysosome (ATG release) breakdown and recycling (including LC3) formation of autolysosome (degradation of aggregate-prone proteins
Clinical Correlation: Autophagy
o Excess autophagosomes are present in neurodegenerative disease
Types of Macroautophagy
depends on the stimuli that mediate their activation or on the molecular mechanisms involved in autophagy activation and execution
o Molecular mechansims are mTor dependent/independent or Beclin dependent/independent
mTor dependent – mTor is inhibited and macroatophagy occurs
Beclin – cause maturation of autophagosome
Apoptosis
– when coping mechanisms (ubiquitin-proteasome & autophagy-lysosome) have failed
o Intrinsic and extrinsic pathways become aware of excess damage
o Activation of caspase 3 is point of no return and apoptosis will occur
o Apoptosis mechanism: blebbing of membrane, cell shrinkage, and then cell death
Summary of Possible Mechanisms of Protein Degradation or Clearance
o Ubiquitination of protein proteasome
o Formation of autophagosome fusion with lysosome
o Exosomal/exocytosis of lysosome contents or induction of cell death
Can be rerouted and re-uptake via endosome/endocytosis
Can be uptake by glial cells and activate inflammation
o Cell-cell propagation – induce a cell to transfer contents (pathogenic proteins) to another cell
Possible Defects that may underlie macroautophagy malfunctioning in neurodegenerative disorders
o Reduced autophagy induction
o Enhanced autophagy repression
o Altered cargo recognition
o Inefficient autophagosome/lysosome fusion
o Inefficient degradation of the autophagic cargo in lysosomes
Clinical Correlation: Diseases and Mechanism Impaired
o Parkinson’s Disease – chaperone-mediate autophagy & macroautophagy
o Huntington’s – macroautophagy
o Frontotemporal Dementia – macroautophagy & endocytosis
o Prion Disease – macroautophagy & endocytosis
o Amyotrophic Lateral Sclerosis (ALS) – macroautophagy & endocytosis
o Spinal Muscular Atrophy - proteasome
