Apoptosis vs Necrosis

Features of Apoptosis and Necrosis. Apoptosis is Programmed Cell Death (PCD) or "cellular suicide", necrosis involves stress induced swelling and lysis.

Morphological features

	Apoptosis (image)	Necrosis (image)
Outset	Shrinking of cytoplasm, condensation of nucleus.	Swelling of cytoplasm and mitochondria.
Plasma membrane	Blebbing of plasma membrane without loss of integrity	Loss of membrane integrity
Chromatin	Aggregation of chromatin at the nuclear membrane.
Organelles	Mitochondria become leaky due to pore formation involving proteins of the bcl-2 family.	Disintegration (swelling) of organelles
Vesicles	Formation of membrane bound vesicles (apoptotic bodies)	No vesicle formation, complete lysis
Terminal	Fragmentation of cell into smaller bodies	Total cell lysis

Biochemical features

	Apoptosis	Necrosis
Regulation	Tightly regulated process involving activation and enzymatic steps.	Loss of regulation of ion homeostasis.
Energy input	Energy (ATP)-dependent (active process, does not occur at 4°C)	No energy requirement (passive process, also occurs at 4°C)
DNA	Non-random mono- and oligonucleosomal length fragmentation of DNA (Ladder pattern after agarose gel electrophoresis)	Random digestion of DNA (smear of DNA after agarose gel electrophoresis)
Timing	Prelytic DNA fragmentation	Postlytic DNA fragmentation (= late event in cell death)
Biochemical events	Release of various factors (cytochrome C, AIF) into cytoplasm by mitochondria. Activation of caspase cascade. Alterations in membrane asymmetry (translocation of phosphatidylserine from the cytoplasmic to the extracellular side of the membrane)

Physiological impact

	Apoptosis	Necrosis
Extent	Localized effect that destroys individual cells.	Affects groups of contiguous cells.
Induction	Induced by physiological stimuli (lack of growthfactors, changes in hormonal environment).	Evoked by non-physiological disturbances (complement attack, lytic viruses, hypothermia, hypoxia, ischemica, metabolic poisons)
Phagocytosis	Phagocytosis by adjacent cells or macrophages.	Phagocytosis by macrophages
Immune system	No inflammatory response.	Significant inflammatory response.

Apoptosis

Induction of Apoptosis:
Death receptor pathway: binding of ligand to death receptor forms a Death-Inducing Signalling Complex (DISC) comprising death receptor, ligand, and adaptor proteins such as FADD (Fas-Associated Death Domain). Apoptosis induced by the death receptors Fas and TNFR-1 proceeds through recruitment of FADD and caspase-8 to the receptor complex.
DR family	Ligand for death receptor (DR)
CD95 (Apo-1 or Fas)	CD-95L
TNF-R (tumour necrosis factor) – TNFR1-induced apoptosis (in contrast to Fas) is a two-step process, mediated by two sequential signaling complexes	TNFα Binding of TNF to TNFR1 leads to rapid recruitment of TRADD, RIP1 and TRAF2. Subsequently, TNFR1, TRADD and RIP1 are modified and dissociate from TNFR1. The liberated death domain (DD) of TRADD (and/or RIP1) binds to FADD, leading to recruitment of caspase-8/10 recruitment (forming complex II) and resulting in apoptosis. [s]
Mitochondrial pathway:
Bcl-2 regulators of mitochondrial permeability: Bcl-2 subfamily (Bcl-2, Bcl-xL and Bcl-w) anti-apoptotic, promote cell survival. BH1-BH4. Bax subfamily (Bax, Bak, BAD) pro-apoptotic, promote cell death. Sequence homology with BH1, BH2, and BH3 regions of Bcl-2. BH3 only subfamily (Bid) pro-apoptotic, promotes cell death. Sequence homology only in BH3 region. Bid lacks the transmembrane-spanning region.
Pro-apoptic substances (leak through PT pore comprising ANT, PBR (MBR), and VDAC (porin): AIF - Apoptosis Inducing Factor – flavoprotein that fragments DNA. Endonuclease G – degrades single stranded DNA. Smac/DIABLO - Second Mitochondrial Activator of Caspases/Direct IAP Binding protein with Low pI – pro-aptoptic activity through inhibition of IAPs (Inhibitors of Apoptosis Proteins), which are inhibitors of caspases (3, 8). Cytochrome c – component of oxidative phosporylation that drives generation of apoptotic body (apoptosome).
Components of PT pore (Mitochondrial Permeability Transition Pore):
ANT	Adenine Nucleotide Transporter – adenine nucleotide transporter of inner mitochondrial membrane.
PBR (MBR)	Mitochonrial or Peripheral Benzodiazepine Receptor – outer mitochondrial membrane.
VDAC	Voltage Dependent Anion Channel – outer mitochondrial membrane.
Apoptotic events:
Caspase function: cysteinyl aspartate-specific proteases that that cleave proteins at aspartate residues, leading to activation (caspases, ICAD) or deactivation ( PARP-1, PKB/Akt, Raf-1). Pro-apoptotic caspases are generally activated by death-receptors or damaged mitochondria and are inhibited by a number of cytoplasmic proteins including the caspase-8 homologue FLIP. Modulators of caspase activation are aberrantly expressed in pathological processes such as neurodegenerative diseases or cancer. [s]
Initiator caspases	Activated by autoactivation. Caspase-8 and caspase-9 activate effector caspases.
Effector caspases	Transactivated by initiator caspases. Caspase cascade leads to protein cleavage, ultimately leading to apoptotic cell death.
Inflammatory caspases	Inflammatory caspase-1 and caspase-5 are involved in the proteolytic activation of IL-1β, which activates the rapidly acting innate immune system. They are activated by a multiprotein complex, the inflammasome.[s]
Caspase actions:
Site	Action
Nuclear pores	Active caspase-9 damages/enlarges nuclear pores, permitting entry of caspase-3 and caspase-7
Nucleus	Caspase-3 cleaves the inhibitory subunit of ICAD, releasing CAD, which cleaves DNA between nucleosomes. ICAD is an inactive protein-CAD complex (45kDa), where CAD is Caspase-Activated DNase (DNA Fragmentation Factor, 40kDa)
Plasma membrane	Externalization of phospholipid, phosphatidyl-serine from inner to outer plasma membrane signals for phagocytosis after phagocytes adhere (CD14).
Cytoskeleton	Reorganization of cytoskeleton and disintegration into apoptosomes.