Apoptosis | Apoptosis in Pathological and Physiological context | Molecular pathway of apoptosis

What is Apoptosis?

Overview of Apoptosis

• Apoptosis, derived from Greek meaning "falling off," is a type of programmed cell death characterized by cell shrinkage and the formation of apoptotic bodies.

• This process involves intrinsic enzymes that degrade cellular DNA and proteins, leading to auto-destruction without causing inflammation.

Comparison with Necrosis

• Unlike necrosis, apoptosis maintains plasma membrane integrity while altering its composition to attract phagocytes for cleanup.

• Necrosis typically results in an inflammatory response due to cytoplasmic leakage, contrasting with the non-inflammatory nature of apoptosis.

Types of Apoptosis

Subtypes of Apoptosis

• Apoptosis can be classified into two main subtypes: pathological apoptosis (occurring during infections or diseases) and physiological apoptosis (essential for development).

Role in Development

• Physiological apoptosis plays a crucial role in human development, such as digit separation at birth where cells die to sculpt new structures.

• Examples include tadpole tail loss in frogs and the death of transient cell populations like Cajal-Retzius cells during brain development.

Apoptosis in Immune Response

Importance in Immunology

• In the immune system, particularly within lymph nodes, immature B cells undergo apoptosis if they exhibit low affinity for antigens, ensuring only high-affinity cells survive.

Pathological Circumstances Leading to Apoptosis

Triggers for Pathological Apoptosis

• DNA breaks can trigger apoptosis when irreparable; this prevents further damage to surrounding cells.

• Accumulation of misfolded proteins leads to endoplasmic reticulum stress and subsequent activation of apoptotic pathways, common in neurodegenerative diseases like Parkinson's.

Molecular Pathways Regulating Apoptosis

Intrinsic vs. Extrinsic Pathways

• Two primary pathways regulate apoptosis: the intrinsic pathway involving mitochondria and the extrinsic pathway relying on receptor-ligand interactions.

Extrinsic Pathway Details

• The extrinsic pathway includes receptors like Fas that bind ligands triggering initiator caspase activation leading to a cascade effect culminating in executioner caspases like caspase 3.

Intrinsic Pathway Details

Understanding Apoptosis and Its Detection

Role of Cytochrome C in Apoptosis

• Cytochrome c is crucial in the electron transport chain; its leakage into the cytoplasm indicates cellular distress, triggering apoptosis.

• The process converges at caspase-3 cleavage, which executes apoptosis by leading to DNA fragmentation and organelle destruction.

Mechanism of Viral Infection and Apoptosis

• During viral infections, specific cell types utilize class 1 MHC generation machinery to present viral proteins on their surface.

• CD8 positive cytotoxic T cells recognize these peptides, releasing granules containing perforin and granzymes that induce apoptosis.

Pathways Triggering Apoptosis

• Perforin forms pores in the target cell membrane, allowing granzymes to enter and activate Bcl-2 family regulators, further promoting cytochrome c release.

• Interactions between Fas ligand and Fas can also trigger caspase-8 activation, leading to downstream effects that culminate in apoptosis.

Detecting Apoptosis: Techniques Used by Researchers

• Immunohistochemistry against cleaved caspase-3 helps identify cells undergoing or about to undergo apoptosis based on accumulation levels.

• The TUNEL assay utilizes terminal deoxynucleotidyl transferase (TdT) to label fragmented DNA characteristic of apoptotic cells.

Advanced Detection Methods

• Anexin V/PI assay combines annexin V binding to exposed phosphatidylserine with flow cytometry for detecting early apoptotic changes.

• In healthy cells, phosphatidylserine remains internal; during apoptosis, it flips outward—this change is a key indicator detected by annexin V.

Understanding Cell Viability Through Markers

• Propidium iodide serves as a late-stage apoptosis marker; its presence indicates significant cellular damage.

• Distinguishing between viable cells, late-stage apoptosis, necrosis involves analyzing annexin V binding alongside propidium iodide uptake.

Conclusion on Apoptosis Mechanisms

• Unlike necrosis which triggers inflammation through cytokine secretion, apoptosis does not evoke such responses.

Understanding Necroptosis and Its Implications

Overview of Necroptosis

• Necroptosis is described as a hybrid form of cell death that combines characteristics of necrosis and apoptosis, indicating a highly regulated process.

• This type of cell death involves specific cytokine secretion leading to robust inflammation, which distinguishes it from traditional necrosis.

Characteristics of Necroptosis

• Key features include loss of ATP, cellular swelling, reactive oxygen species (ROS) generation, and the release of lysosomal enzymes.

• These characteristics are significant markers for identifying this particular mode of cell death in biological studies.

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