Apoptosis: Mechanisms and Pathways

• Apoptosis is a form of cell death where a cell self-destructs.
• The objectives are to discuss the causes, mechanisms, and morphological features of apoptosis and also to differentiate the features of apoptosis from necrosis.

Key Terms

• Apoptosis means "to fall off" in the context of cell death.
• The caspase cascade involves enzyme activation that leads to clean cellular death without any inflammation.
• Intrinsic Pathway is a mitochondrial pathway.
• Extrinsic Pathway involves death receptors like Fas and TNF receptors.
• BCL-2 is an anti-apoptotic mitochondrial protein that helps cells survive and is often overexpressed in follicular lymphoma.

Mechanisms of Apoptosis

• The intrinsic pathway is mitochondrial.
• BAX and BAK proteins (BCL-2 family) cause cytochrome C to be released from the mitochondria which then binds to Apaf-1.
• After this, the apoptosome forms, activating caspase-9, which in turn activates caspase-3, leading to apoptosis.
• The extrinsic pathway involves death receptors
• T cells use the extrinsic pathway to trigger apoptosis in infected cells, particularly in viral infections.
• Caspase-9 is cleaved into caspase-9 which activates caspase-3, causing the cell to break down.

Apoptosis vs. Necrosis (Morphological Features)

• Apoptosis morphological features involve specific nuclear and cellular changes.
• Pyknosis involves nuclear chromatin condensation.
• Karyorrhexis involves nuclear fragmentation.
• Apoptotic bodies form.
• Cell membranes will start blebbing and the cytoplasm will fragment.
• Phospholipids on the cell surface act as "Eat me" signals attracting macrophages.
• Necrosis morphological features also involve nuclear and cellular differences.
• Karyolysis involves the fading away of the nucleus.
• Cellular changes include swelling, loss of membrane integrity, and inflammation.
• Karyolysis is unique to necrosis and not present during apoptosis.
• Apoptotic bodies form cleanly, unlike the debris that is formed by necrotic cells.

Key Points of Apoptosis

• Physiologic causes include the normal aging of cells while undergoing apoptosis.
• Remnants of apoptotic bodies are seen in the colonic epithelium during pathological examination.
• Pathologic causes include viral hepatitis where apoptotic cells have pyknotic, shrunken nuclei and eosinophilic cytoplasm.
• Huntington's Disease causes an accumulation of Huntington protein that damages neurons, which then leads to neurodegeneration.

High-Yield Points

• BCL-2 is an anti-apoptotic protein.
• Follicular lymphoma overexpresses BCL-2 in order to prevent apoptosis.
• Caspase-3 is an important part of the execution phase of apoptosis, and is important in diseases such as Huntington's.
• Senile cells undergo physiologic apoptosis.
• Viral hepatitis causes a pathological form of apoptosis that leads to characteristic cell morphology.

Morphology Tips

• Apoptosis results in pyknosis, karyorrhexis, and formation of apoptotic bodies.
• Necrosis causes karyolysis, membrane rupture, and inflammation.

Quiz/Practice

• Knocking down BCL-2 will promote apoptosis.
• Chromatin condensation happens first, and then fragmentation.
• Apoptotic cells display shrunken, eosinophilic cytoplasm and fragmented nuclei (e.g., in viral hepatitis).

Recap

• Apoptosis is a controlled process of programmed cell death.
• Two major pathways exist which are the Intrinsic (mitochondrial) and Extrinsic (death receptor).
• Apoptosis leads to clean cell death without causing inflammation.
• Key morphological changes include pyknosis, karyorrhexis, and apoptotic bodies.
• BCL-2 helps prevent apoptosis.
• The membrane remains intact in apoptosis and the cell contents are packaged into apoptotic bodies, preventing inflammation.
• In contrast, necrosis causes the cell membrane to be lost, leading to cell rupture, spilling of contents, and inflammation.
• BCL-2 inhibits the release of cytochrome C from the mitochondria, which prevents the formation of apoptosomes.
• Knocking down BCL-2 will cause cytochrome C to be released as well as caspase activation, leading to damaged cells being removed.
• Liver biopsies from patients with viral hepatitis will show hepatocytes that are in apoptosis.
• Expect to see pyknosis, eosinophilic cytoplasm, and karyorrhexis.
• The intrinsic pathway is triggered by intracellular signals leading to mitochondrial changes, and the extrinsic pathway is initiated by extracellular ligands binding to death receptors.
• Caspase-3 breaks down cellular organelles and the cytoskeleton which leads to apoptotic bodies forming.
• Phospholipids externalized on the cell surface are signals for macrophages to engulf apoptotic bodies.