Difference between apoptosis and necrosis
Apoptosis is a “silent” and energy-dependent form of cell death. During this process, the lymphocyte shrinks, loses its microvilli, and its membrane forms blebs while fully maintaining structural integrity. DNA degradation is strictly ordered. Ultimately, apoptotic bodies—nuclear fragments surrounded by an intact membrane—bleb off from the cell.
These bodies are immediately phagocytosed, preventing cellular contents from spilling into the tissues and causing no inflammation.
In necrosis (e.g., resulting from hypoxia or toxins), the cell swells, its membrane ruptures, and enzymes and DNA spill into the intercellular space. This is a passive process that is always accompanied by a pronounced inflammatory response.
Main pathways of apoptosis initiation
The cell death program can be triggered via two main routes that ultimately intersect.
- Mitochondrial (intrinsic) pathway. Develops when mitochondria are damaged. Pro-apoptotic proteins of the Bcl-2 family (such as Bax and Bak) form pores in the mitochondrial membrane. Cytochrome c and Apaf-1 factor are released into the cytosol. Combining with ATP, they form a multimeric complex—the apoptosome—which activates initiator caspase-9.
- Receptor (extrinsic) pathway. Mediated by membrane “death receptors” (Fas, TNF-R1, DR4/5/6) upon binding with their ligands (FasL, TNF, TRAIL). Intracellular domains of the receptors recruit adapter proteins (FADD, TRADD), forming the DISC complex. This complex converts procaspase-8 into active caspase-8.
The pathways can interact. For instance, the receptor pathway can stimulate the Bid factor, which additionally engages the mitochondrial destruction mechanism.
Effector phase and cell destruction
Both starting pathways converge on the activation of caspase-3, the principal executioner protease. It targets numerous cellular structures (up to 280 targets), making the process irreversible:
- Nuclear targets: cleavage of the CAD enzyme leads to internucleosomal DNA fragmentation. Degradation of PARP blocks DNA repair capabilities.
- Cytoplasmic targets: action on gelsolin and focal adhesion kinases disrupts the cell cytoskeleton, causing shrinkage.
The cell dies either from direct destruction of its genetic apparatus or from complete exhaustion of energy reserves consumed while attempting to repair the fragmented DNA.
Role of apoptosis in adaptive immunity
Apoptosis is the primary tool for quality control and population sizing within the immune system. It governs lymphocytes at all stages of their life cycle:
- During development: excess cells are eliminated and clones with defective assembly of antigen-recognition receptors are culled.
- During selection: T lymphocytes that fail to recognize self MHC-peptide complexes die by neglect (default death). Those that recognize them too aggressively are eliminated via activation-induced apoptosis to prevent autoimmune reactions.
- During the immune response: B lymphocytes with low affinity for the antigen perish, making way for higher-affinity clones.
- At the resting stage: following victory over an infection, spent effector cells and senescent lymphocytes are cleared to maintain tissue homeostasis.
Briefly: answers to topical questions
Which caspases are initiator caspases and which are effector caspases?
Initiator caspases include caspases 8 and 9, while effector (executioner) caspases include caspases 3, 6, and 7.
- Initiator caspases trigger the apoptotic process. Caspase-8 is activated via the receptor pathway, whereas caspase-9 is activated via the mitochondrial pathway.
- Effector caspases carry out the direct destruction of cellular structures. The main representative is caspase-3, with caspases 6 and 7 participating less frequently.
Which Bcl-2 family proteins exhibit anti-apoptotic activity?
Anti-apoptotic activity is exhibited by the following Bcl-2 family proteins: Bcl-2, Bcl-X_L, and Mcl-1.
They play a decisive role in regulating mitochondrial membrane permeability by suppressing transmembrane pore formation. These factors bind pro-apoptotic proteins (forming dimers with them) and block the mitochondrial pathway of apoptosis initiation, preventing the release of cytochrome c into the cytosol.
What autoimmune disorders develop due to lymphocyte apoptosis defects?
Defects in lymphocyte apoptosis lead to the development of:
- Autoimmune lymphoproliferative syndrome (Canale–Smith syndrome)—caused by defects in the Fas receptor, Fas ligand, or caspases 8 and 10, which disrupts receptor-mediated apoptosis and leads to autoimmune phenomena and lymphocyte hyperproliferation.
- X-linked lymphoproliferative syndrome—characterized by infectious mononucleosis, lymphoproliferation, dysgammaglobulinemia, or hypogammaglobulinemia.
- Systemic lupus erythematosus (SLE)—can develop when phagocytes excessively engulf DNA fragments, activating intracellular TLRs and triggering pathological pathways that lead to systemic autoimmune disease.
What intracellular inhibitors of apoptosis (IAP) exist?
Intracellular inhibitors of apoptosis of the IAP family (Inhibitors of apoptosis) include the BIRC family of proteins. Their overexpression is characteristic of certain tumor antigens.
How to memorize the main caspases: 9 triggers apoptosis from the inside (resembling a coiled spiral inside the cell—mitochondria), 8 works from the outside (two circles acting as receptor and ligand), while 3 is the effector, “hacking” everything in its path (like a blade).
Class and exam questions
What is the main difference between apoptosis and necrosis?
Apoptosis is an active, energy-requiring process with preservation of the cell membrane and absence of inflammation. Necrosis is passive destruction accompanied by cell rupture and a strong tissue inflammatory response.
How do macrophages recognize that a cell needs to be phagocytosed?
During apoptosis, phosphatidylserine appears on the outer surface of the membrane, whereas in a healthy cell it is hidden on the inner leaflet. This serves as an “eat me” signal for phagocyte receptors.
Why is apoptosis necessary during negative selection of lymphocytes?
It is essential for eliminating autoreactive clones. If a lymphocyte reacts too aggressively against the body's own proteins, activation-induced apoptosis is triggered, protecting against the development of autoimmune diseases.
What else to study on this topic
- Structure and functions of Bcl-2 family proteins (pro- and anti-apoptotic factors)
- Molecular structure of the apoptosome and the DISC complex
- Role of ceramide and other sphingomyelin metabolites in regulating proliferation
- Mechanism of Toll-like receptor (TLR) activation during ineffective phagocytosis of apoptotic bodies
- Contact cytolysis: perforin-granzyme mechanism of target cell destruction
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