Notes on Apoptosis: Pathways and Regulation
Apoptosis and Cell Death: Comprehensive Notes
Overview
- Two main ways cells die: necrosis and apoptosis.
- Necrosis: death due to injurious agents; typically results in inflammation.
- Apoptosis (also called programmed cell death, PCD): an intrinsic, regulated process where a single cell activates an internal death program and dies without provoking inflammation.
- Etymology: the word apoptosis comes from the Greek meaning “a falling off.”
- Distinguishing features:
- Apoptosis: cells die one at a time, in an orderly manner; no inflammation.
- Necrosis: massive death among many cells, occurs simultaneously, followed by inflammation.
Physiological roles of apoptosis (normal life cycle)
- Normal response to physiological stimuli involves development and maintenance:
- Fetal development: important for removing webbing between fingers; shaping male vs. female reproductive organs; postnatal thymus involution (shrinks after infancy).
- Embryogenesis: apoptosis enables limbs to develop correctly (prevents inappropriate webbing).
- Thymus: large in infancy, atrophies during childhood.
- Adulthood roles: menstrual shedding; elimination of autoimmune cells; removal of aging cells; turnover of intestinal crypt cells.
- Key points:
- Apoptosis is essential for normal development and tissue homeostasis.
Pathological and clinical contexts of apoptosis
- Apoptosis also occurs in disease and injury:
- Infected or injured cells (e.g., HIV, Hepatitis B).
- Cells with DNA damage (radiation, chemotherapeutic drugs, birth defects).
- Cancer therapy (chemotherapy) induces apoptosis in tumor cells.
- Transplant rejection involves apoptosis of donor or recipient cells.
- Immune system involvement:
- Cytotoxic T cells and Natural Killer (NK) cells can trigger PCD in infected cells.
- Some viruses carry “apoptosis brake” genes to inhibit infected cells from dying.
Core concepts and definitions
- Programmed Cell Death (PCD): intrinsic death program active within a cell under genetic control.
- Apoptosis vs necrosis: apoptosis is orderly, non-inflammatory; necrosis is chaotic, often inflammatory.
- Phagocytosis of apoptotic cells: apoptotic bodies are cleared by macrophages or neighboring cells; early signals include exposure of phosphatidylserine on the outer leaflet of the plasma membrane.
Triggers of apoptosis: signals that push a cell toward self-destruction
- Withdrawal of positive survival signals (pro-survival factors):
- Growth factors for neurons.
- Interleukin-2 (IL-2) for lymphocytes.
- Receipt of negative (death) signals: death ligands bind to receptors to trigger apoptosis:
- Tumor Necrosis Factor (TNF).
- Fas Ligand (FasL; CD95 ligand).
Apoptosis pathways (two main routes converge on execution)
- Extrinsic (death receptor) pathway:
- Initiated by binding of death ligands to death receptors on target cells.
- Key components: death receptors, adaptor proteins, initiator caspases (e.g., Caspase-8), executioner caspases (e.g., Caspase-3).
- Typical sequence: Death ligand binds receptor → adaptor protein recruitment → initiator caspase 8 activation → executioner caspase 3 activation → apoptosis.
- External pathway is often termed the “Extrinsic Pathway.”
- Intrinsic (mitochondrial) pathway:
- Triggered by intracellular stress (DNA damage, growth factor withdrawal, ER stress, protein misfolding, etc.).
- Key mediators: mitochondrial outer membrane permeabilization, release of cytochrome c and Apaf-1, formation of the apoptosome, activation of Caspase-9 (initiator) → Caspase-3 (executioner).
- The mitochondrial pathway is often termed the “Intrinsic Pathway.”
- Convergence: both pathways activate executioner caspases (e.g., Caspase-3) leading to cellular demolition.
- Alternative/exceptions:
- Apoptosis Inducing Factor (AIF): can drive neuronal apoptosis without caspases; caspases are not always involved in all forms of neuronal death.
Molecular players in the apoptotic machinery
- Caspases: cysteine proteases that cleave target proteins after aspartate residues; they orchestrate the apoptotic cascade by activating other caspases and dismantling cellular components.
- Caspases as executioners: they cleave ~100 different cellular targets to execute apoptosis.
- Initiator caspases (e.g., Caspase-8, Caspase-9) start the caspase cascade; executioner caspases (e.g., Caspase-3) execute.
- Regulation of caspase activity:
- Initiated by upstream signals (death receptors or mitochondrial signals) that activate caspases.
- Endonucleases:
- Fragment DNA during apoptosis, typically at histone linker regions, producing characteristic DNA laddering.
- DNA fragmentation site length approximation: about intervals.
- Phosphatidylserine externalization:
- Externalized PS on the outer leaflet acts as an “eat me” signal for phagocytes during apoptosis.
Intrinsic pathway details
- Mitochondria are central
- Involves Cytochrome C release and activation of Caspase-9 via the apoptosome.
- Bcl-2 family proteins regulate mitochondrial outer membrane permeabilization.
- Key regulators and effectors:
- Anti-apoptotic: Bcl-2, Bcl-XL, etc.; overexpression can inhibit apoptosis (example: Bcl-2 overexpression linked to certain leukemias).
- Pro-apoptotic: Bax, Bak, Bad, BMF, etc.
- p53 tumor suppressor: upregulates pro-apoptotic BCL proteins in response to DNA damage, promoting apoptosis.
- Sequence of mitochondrial pathway events:
- Stress signals lead to changes in Bcl-2 family balance → Bax/Bak activation → mitochondrial outer membrane permeabilization → Cytochrome C release → formation of the apoptosome → Caspase-9 activation → Caspase-3 activation → apoptosis.
Extrinsic pathway details
- Death receptors and ligands
- Fas (CD95) receptor and Fas Ligand (FasL).
- TNF receptor (and TNF-related ligands).
- Signaling cascade:
- Ligand binding recruits adaptor proteins (e.g., FADD) to death receptors → activation of initiator Caspase-8 → downstream caspases (e.g., Caspase-3) → apoptosis.
- Cross-talk:
- In some cells, Caspase-8 can cleave Bid, feeding into the mitochondrial pathway and amplifying apoptosis.
AIF and caspase-independent apoptosis
- AIF (Apoptosis-Inducing Factor): can mediate chromatin condensation and DNA fragmentation in caspase-independent pathways, particularly notable in neurons.
- Distinction: AIF involvement means apoptosis can proceed without caspase activation in certain contexts.
Regulation and modulators of apoptosis
- Bcl-2 family proteins (mitochondrial gatekeepers):
- Anti-apoptotic members: Bcl-2, Bcl-XL, etc.; keep mitochondria intact and prevent cytochrome C release.
- Pro-apoptotic members: Bax, Bak, Bad, BH3-only proteins; promote mitochondrial outer membrane permeabilization.
- p53: tumor suppressor that can transcriptionally upregulate pro-apoptotic BCL proteins in response to DNA damage, tipping the balance toward apoptosis.
- Protective mechanisms and inhibitors:
- Viral and cellular inhibitors of apoptosis (IAPs) and related regulators can dampen caspase activity and apoptosis.
- Immune system orchestration:
- Cytotoxic T cells and NK cells can direct infected cells to undergo PCD.
Morphology and staging of apoptosis
- Early changes:
- Cell shrinkage; formation of a tight, rounded cell body.
- Chromatin condensation and nuclear fragmentation occurs later.
- Membrane and organelle changes:
- Membrane blebbing; organelle condensation; endonucleases cleave chromosomes at ~ intervals.
- Final stages:
- Formation of apoptotic bodies; phagocytosis by macrophages or neighboring cells.
- In contrast, necrosis features:
- Breakdown of plasma membrane, organelles, and nucleus; leakage of cellular contents; inflammation.
The apoptotic process in clinical and real-world contexts
- Increased apoptosis is associated with:
- Neurodegenerative diseases (e.g., Alzheimer’s disease).
- Viral infections.
- Graft rejection scenarios.
- Decreased apoptosis is associated with:
- Autoimmune diseases (e.g., arthritis).
- Cancer (where cells evade death and continue proliferating).
- AIDS example:
- CD4$^+$ T-cells infected at a very low rate, approximately fraction $<10^{-5}$ of total cells; most cell death in AIDS is due to uninfected bystander cells rather than infected cells themselves.
- Aging and apoptosis:
- Apoptosis contributes to aging processes and tissue remodeling with age.
Numerical and molecular references worth noting (LaTeX-formatted)
- DNA fragmentation length in apoptosis: approximately intervals.
- Fraction of CD4$^+$ T-cells infected in AIDS: <10^{-5} (i.e., less than one in a hundred thousand).
- Target protein counts in caspase-mediated cleavage: around different cellular targets.
- Caspase cascade execution: cleavage of substrates leads to coordinated dismantling of the cell; executioner caspases (e.g., Caspase-3) act downstream of initiator caspases (e.g., Caspase-8, Caspase-9).
- DNA protection vs damage balance can shift toward apoptosis via p53-mediated transcriptional upregulation of pro-apoptotic BCL proteins.
Key figures and concepts mentioned (for reference)
- Figure references (conceptual):
- Apoptosis as an embryonic process enabling limb development.
- Thymus involution in infancy and its atrophy with childhood.
- Phagocytosis of apoptotic cells via exposure of phosphatidylserine and engagement with phagocytic receptors.
- Distinction between linear apoptotic signaling (extrinsic) and mitochondrial signaling (intrinsic).
- Pathway schemes:
- Extrinsic pathway: Death ligands → Death receptors → Caspase-8 → Caspase-3.
- Intrinsic pathway: Stress signals → Mitochondrial outer membrane permeabilization → Cytochrome C release → Caspase-9 → Caspase-3.
- Converging node: Caspase-3 acts as a common executioner.
Notes on the future and implications (as discussed in course material)
- The future of PCD research:
- Increased apoptosis in neurodegenerative diseases, viral infections, and graft rejection.
- Decreased apoptosis in autoimmune diseases and cancer.
Practical and ethical considerations (brief)
- Therapeutic targeting of apoptosis pathways for diseases like cancer and neurodegeneration carries potential benefits and risks, given the dual roles of apoptosis in preventing cancer and contributing to tissue degeneration.
- Understanding apoptosis helps in designing strategies to protect cells (e.g., in neuroprotection) or promote death of diseased cells (e.g., cancer therapy).
Additional resources referenced in course material
- Online resources illustrating Necrosis vs. Apoptosis and Apoptotic Pathways can be found at the provided YouTube links for visual reinforcement.
Quick recap of major points
- Apoptosis is a programmed, orderly form of cell death that prevents inflammation, while necrosis is inflammatory and chaotic.
- Apoptosis can be triggered by withdrawal of survival signals or by death signals via extrinsic and intrinsic pathways.
- The extrinsic pathway involves death receptors and Caspase-8; the intrinsic pathway involves mitochondria, Cytochrome C, Apaf-1, and Caspase-9; both converge on Caspase-3.
- Regulation is complex and involves Bcl-2 family proteins, p53, AIF, and inhibitors; immune cells can induce PCD in infected cells.
- Apoptosis plays essential roles in development and aging, with significant implications for disease and therapy.
Notation and stylistic reminders for exam prep
- Be able to distinguish morphological features: cell shrinkage, blebbing, chromatin condensation, nuclear fragmentation, apoptotic bodies, phagocytosis (apoptosis) vs membrane rupture, swelling, organelle breakdown, inflammatory leakage (necrosis).
- Memorize key players: Fas/FasL, TNF, Caspase-8, Caspase-9, Caspase-3, Cytochrome C, Apaf-1, Bax/Bak, Bcl-2/Bcl-XL, p53, AIF.
- Understand the functional outcomes: absence vs presence of inflammation; cell-by-cell death vs tissue-wide death.