Signaling mechanisms and the role of caspases
Signal transmission to initiate apoptosis is carried out via cell membrane receptors or through direct action on intracellular structures. Signals are divided into two groups:
- Negative signals (block apoptosis): transmitted by hormones and growth factors.
- Positive signals (induce apoptosis): delivered via Fas ligands, tumor necrosis factor alpha (TNF-α), and transforming growth factor beta.
Some pro-apoptotic factors act directly on organelles. For example, glucocorticoids cause cell death by interacting with specific receptors on the nuclear membrane.
The central link in the pathogenesis is the activation of caspases. These are cysteine proteases that cleave proteins specifically at aspartic acid residues. The enzymes of the cascade are divided into controllers and integrators.
Their mechanism of action includes:
- Activation of the caspases themselves.
- Cleavage of numerous targets (cytoskeleton and cell matrix proteins). The main blow falls on the nucleus and nuclear proteins involved in transcription, replication, and DNA repair.
- Culmination: activation of nucleases and enzymes that destroy nucleoproteins.
A specific example is the action of caspase-3: it converts an inactive cytoplasmic DNase into an active form, which causes internucleosomal cleavage of nuclear DNA.
Intrinsic (mitochondrial) pathway
This mechanism is implemented through changes in the permeability of mitochondrial membranes. Inductors include the absence of growth factors and trophic hormones, as well as exposure to DNA-damaging agents.
The most important trigger of apoptosis is cytochrome c. Normally it is located in the mitochondria, but upon release into the cytosol, it triggers the cell death process. Membrane permeability is controlled by the Bcl-2 protein family, which includes more than 20 proteins. Their balance determines the fate of the cell:
- Apoptosis promoters (Bax, Bak, Bad): embed into the mitochondrial membrane, form channels, and ensure the release of cytochrome c into the cytosol.
- Apoptosis inhibitors (Bcl-2): regulate outer membrane permeability, preventing cytochrome c leakage.
If pro-apoptotic factors prevail, the released cytochrome c interacts with the Apaf-1 factor (protease activator, analog of the nematode ced-9 gene). The result is the induction of a proteolytic cascade that determines cell death.
Extrinsic (death receptor) pathway
Triggered via surface molecules — death receptors. Most of them belong to the TNF receptor superfamily and contain a conserved intracellular «death domain» responsible for interaction with other proteins. The prototypes are TNF receptor type I and Fas receptors (CD95).
Activation mechanism using Fas as an example:
- Membrane protein ligand (FasL) is expressed predominantly on activated T lymphocytes.
- T cells recognize targets that have Fas receptors.
- FasL causes cross-linking of Fas receptor molecules.
- Adapter proteins are activated, which bind and turn on caspase-8.
The extrinsic pathway has important physiological significance: it is necessary for eliminating autoreactive lymphocytes and destroying target cells by cytotoxic T lymphocytes (CTLs). This pathway is blocked by cellular FLIP proteins (caspase antagonists).
Interestingly, the pathways can intersect. In a number of cells, caspase-8 cleaves the Bid protein (a pro-apoptotic member of the Bcl-2 family). This supports the mitochondrial pathway, and the combined activation deals a fatal blow to the cell.
Genetics, elimination, and pathology
Genetic regulation of the process is strictly ordered. Genes and their protein products are divided into 4 groups: transmitting the signal from the membrane, controlling and integrating, executing apoptosis, and regulating phagocytosis.
The final stage is clearance (elimination) of apoptotic bodies. Immediate phagocytosis of these bodies by neighboring cells and macrophages occurs. The main consequence of this is the blocking of the inflammatory response, since the cells are removed before their contents are released. Alternative pathways for the removal of bodies include glandular secretions, as well as lymph and blood flow.
There are 3 pathological variants of regulatory disorders:
- Excessive apoptosis (exceeds the rate of proliferation).
- Insufficient apoptosis (reduced relative to proliferation).
- Incomplete apoptosis (associated with the absence or impairment of phagocytosis of apoptotic bodies).
Briefly: answers to topical questions
Which specific adapter proteins link death receptors to procaspase-8?
Death receptors link to procaspase-8 via the adapter proteins FADD and TRADD.
- FADD (Fas-associated death domain) is associated with Fas, DR4, and DR5 receptors. It recognizes death domains within procaspase-8, binds to it, and causes its activation into caspase-8.
- TRADD (TNF-receptor death domain) is associated with the TNFRI receptor. It acts similarly, but exerts its effect indirectly through interaction with FADD.
As a result of these interactions, DISC (Death-inducing signaling complex) molecular complexes are formed, triggering the caspase cascade.
What role does the p53 protein play in triggering the mitochondrial pathway of apoptosis?
The p53 protein initiates the mitochondrial pathway of apoptosis upon irreparable DNA damage. It acts as a pro-apoptotic factor that triggers the expression of apoptosis genes and the synthesis of corresponding promoter proteins. The accumulation of p53 leads to the stimulation of sensors that activate pro-apoptotic Bax and Bak proteins (the Bcl-2 family). These proteins embed into the mitochondrial membrane, forming channels for the release of cytochrome c into the cytosol, which ultimately triggers the caspase cascade. When p53 is mutated or absent, apoptosis is not triggered, leading to the survival of cells with damaged DNA.
What morphological changes occur in the cell at different stages of apoptosis?
The morphogenesis of apoptosis includes sequential changes in the nucleus, cytoplasm, and the formation of specific bodies.
- Chromatin condensation and margination — chromatin gathers into hemispheres and clumps, the nucleus becomes indented and fragments, and osmiophilic bodies appear.
- Cell shrinkage — condensation of the cytoplasm and intracellular organelles occurs due to dehydration and protein cross-linking. Organelles remain intact, and ribosomes aggregate into semi-crystalline structures.
- Formation of apoptotic bodies — the cell disintegrates into membrane-surrounded fragments containing parts of the nucleus and tightly packed organelles.
- Phagocytosis — engulfment of apoptotic bodies by macrophages or neighboring cells without the development of an inflammatory response.
By means of what methods and markers is apoptosis detected in histological specimens?
To detect apoptosis in histological specimens, methods based on the detection of specific DNA, enzyme, and ultrastructural changes in cells are used.
- TUNEL assay — an in situ uridine-labeling method to detect single-stranded breaks in cleaved DNA. It reveals cells with condensed chromatin, pyknotic nuclei, and apoptotic bodies.
- Immunohistochemical methods — used as markers to detect the activity of certain types of caspases.
- Electron microscopy — the most reliable method for ultrastructural verification, allowing the assessment of organelle preservation, chromatin condensation, and the integrity of apoptotic body membranes.
At the light-optical level, identifying apoptotic bodies is difficult due to their rapid in vivo phagocytosis.
To avoid confusing Bcl-2 family proteins, pay attention to the letter "a" in the name: Bax, Bak, Bad stand for attack (pro-apoptotic factors that puncture mitochondria). Meanwhile, classical Bcl-2 is an inhibitor protecting the cell.
Class and exam questions
What is the main enzyme of apoptosis?
Caspases play a key role — cysteine proteases that cleave proteins at aspartic acid residues. They destroy the cytoskeleton and matrix and activate nucleases to cleave DNA.
What is the difference between the extrinsic and intrinsic pathways of apoptosis?
The intrinsic (mitochondrial) pathway is triggered from inside the cell via the release of cytochrome c. The extrinsic pathway is activated via surface "death receptors" (e.g., Fas) upon contact with external ligands.
Why does apoptosis not cause inflammation?
Apoptotic bodies undergo immediate phagocytosis by macrophages and neighboring cells. The contents of the dead cell do not have time to enter the extracellular environment, thereby completely blocking the inflammatory response.
What else to study on this topic
- Mechanism of action of caspase-3 and internucleosomal DNA cleavage
- Interaction of cytochrome c with the Apaf-1 factor in the intrinsic pathway
- Tumor necrosis factor (TNF) receptor family and the structure of the "death domain"
- Genetic classification: 4 groups of genes regulating the phases of apoptosis
- Pathological variants: causes of excessive, insufficient, and incomplete apoptosis
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