Page 34 - Concise Pathology for Exam Preparation ( PDFDrive )

1 Cell Injury and Cell Death 19

Sequence of Biochemical Events in Apoptosis

1. Protein cleavage by proteolytic enzymes
Activation of caspases (family of cysteine proteases having a unique ability to cleave
after aspartic acid residues)

Cleavage or breakup of nuclear scaffold
Protein hydrolysis
Cleavage or breakup of cytoskeletal proteins
2. Protein cross-linkage
Activation of transglutaminases

Cross-linking of cytoplasmic proteins leading to covalently linked shrunken cells

Easy breakdown into apoptotic bodies
3. DNA condensation and breakdown
DNA breakdown into large pieces (50–300 kb)

Internucleosomal cleavage by endonucleases forming oligonucleosomes (180–200 bp)
visualized on agarose gel electrophoresis as DNA ladders
4. Recognition of dying cells by phagocytes
Flip-flop of apoptotic cell

Phosphatidylserine and thrombospondin flip on the external surface from the inner layers

Easy recognition and phagocytosis of the apoptotic cell

Mechanism of Apoptosis
Apoptosis is the end point of an energy-dependent cascade of molecular events having
four steps.
1. Initiation of apoptosis by activation of signalling pathways:
There are two main signalling pathways in apoptosis.
(a) Extrinsic/death receptor-initiated pathway (Flowchart 1.11): involves extracellular or
transmembrane signals, which may be positive (leading to initiation) or negative (op-
posing initiation). Extrinsic pathway is mainly initiated by engagement of plasma
membrane death receptors on cells. Death receptors are members of the tumour
necrosis factor (TNF)-receptor family that contains a cytoplasmic domain called
death domain because it delivers signals for apoptosis. Important death receptors
include TNFR1 and a related protein called Fas (also called CD 95; Flowchart 1.11).
The ligand for Fas is Fas ligand (Fas L) which is expressed on T cells.

Extracellular signals
• Injuries: radiation, toxins and free radicals
• Withdrawal of growth factors, hormones or cytokines
• Receptor–ligand interactions (Fas–Fas ligand, TNF–TNF receptor)

Act on intracellular regulatory molecules
OR
Directly affect targets within the cell (eg, physicochemical agents like heat, FLOWCHART 1.11. Extrinsic/
radiation, viruses and xenobiotics and glucocorticoids directly bind to death receptor-initiated path-
nuclear receptors) way.
mebooksfree.com