Caspases and Apoptosis

• Apoptosis involves molecular mechanisms with similarities across animal cells
• Caspases, a family of proteolytic proteins, are key components
• Caspases feature a cysteine group at the active site
• Target proteins are cleaved at specific aspartic acid points

Caspase Types and Activation

• Some caspases are involved in inflammation, others in apoptosis
• Initiator caspases start the caspase cascade to activate executioner caspases
• Executioner caspases break down proteins in the nuclear envelope and cytosol, leading to cell death
• Initiator procaspases possess a long prodomain with a caspase recruitment domain (CARD)
• CARDs enable assembly into activation complexes with adaptor proteins upon receiving an apoptotic signal
• Proximity within the complex activates initiator procaspases
• This leads to their cleavage, as well as the cleavage and activation of executioner procaspases
• The result is an irreversible proteolytic caspase cascade

Initiator Caspases

• Initiator procaspases (e.g., caspase 8, 9) are inactive soluble monomers
• They are activated by proximity to other procaspases
• An apoptotic signal triggers assembly of an adapter-protein complex, bringing two inactive monomers together
• Two monodimers form the activated caspase dimer

Executioner Caspases

• Executioner procaspases (e.g., caspase 3, 6, 7) are inactive soluble dimers
• Initiator caspases activate executioner caspases
• Active executioner caspases catalyze protein cleavage throughout the cell
• The cell undergoes apoptosis and is then engulfed and digested by neighboring cells

Caspase Cascade

• Each initiator caspase activates many copies of executioner caspases
• Activated executioner caspases catalyze protein cleavage, leading to cell death via apoptosis
• The process self-amplifies, becomes irreversible, and is termed the caspase cascade

Extrinsic Apoptosis Pathway

• Extracellular signal proteins binding to cell-surface receptors trigger the extrinsic pathways
• Fas ligand on a lymphocyte binds to the Fas receptor on a target cell
• The death domain on the cytosolic tail of the Fas death receptor activates and binds to FADD (Fas death domain) adaptor proteins in the cytosol
• The activated death effector domain in the tail of FADD recruits and binds to procaspase 8 or 10 (initiator caspases) in the cytosol
• Fas death receptor + FADD + procaspase 8 or 10 form a DISC (Death Inducing Signaling Complex)
• Initiator procaspases (8 or 10) are cleaved and activated
• Activated initiator caspases then activate executioner caspases, starting a caspase cascade and resulting in cell death

Intrinsic Apoptosis Pathway

• Stressed and damaged cells induce apoptosis, often due to injury, DNA damage, or lack of extracellular survival signals
• The intrinsic pathway is triggered by the release of proteins from the mitochondrial intermembrane space into the cytosol
• Cytochrome-c, normally part of the electron-transport chain within the mitochondrion, has a different function in the cytosol

Role of Bcl-2 Family

• In some cells, the extrinsic pathway recruits the intrinsic pathway to complete apoptosis by activating Bcl-2 family proteins
• Some Bcl-2 family members (e.g., BclXl) inhibit apoptosis by blocking cytochrome-c release from mitochondria
• Others (e.g., Bax + Bak) promote cytochrome-c release in response to a pro-apoptotic stimulus, leading to cell death
• Pro- and anti-apoptotic proteins can bind and inhibit each other's function

Cytochrome-C and Apoptosome Formation

• Cytochrome-c is released into the cytosol, binds to, and activates Apaf-1 (apoptotic protease activating factor -1)
• Bound dATP is hydrolyzed to dADP
• dADP is replaced by dATP, inducing seven activated Apaf-1 molecules to form a wheel-like structure called the apoptosome
• The CARD (caspase recruitment domain) regions of activated Apaf-1 molecules recruit and bind to procaspase protein (procaspase-9)
• Procaspase-9 molecules clump together within the apoptosome and activate each other to form caspase 9 proteins (initiator caspase)
• Activated caspase-9 molecules recruit and activate executioner procaspases to induce apoptosis

Comparison of Extrinsic and Intrinsic Pathways

• Apoptotic Trigger:
• Extrinsic: Fas ligand on killer lymphocyte plasma membrane binds to and activates Fas receptor on cell plasma membrane
• Intrinsic: Cytochrome-C released from mitochondria into cytoplasm of cell
• Adapter Protein:
• Extrinsic: FADD adapter protein
• Intrinsic: Apaf-1
• Initiator procaspase dimers
• Initiator caspase dimers
• Executioner procaspase dimers
• Activated executioner caspase dimers