Programmed Cell Death

Cell death happens frequently in normal tissue and is of two types
i. Necrosis
ii. Apoptosis
When cells die of injury, then it typically swell and burst. They spill their content all over their neighbouring cells causing a potentially damaging inflammatory response; this process is known as necrosis. In contrast, a cell that undergoes apoptosis dies neatly, without damaging its neighbours. Development in multicellular animals is largely controlled by the programmed cell death e.g., C. elegans development involve total of 1090 genes, but 131 are destined to die. There are total of three different waves of programmed cell death the first wave is observed in embryos, the second wave during the second larva stage, and the third wave occurs in the adult germline. The two important killer proteins that are essential for the programme cell death in C. elegans are ced-4 (cell death gene) and ced-3. In the cell that is undergoing apoptosis, BH-3 inhibits ced-9 from inhibiting ced-4. ced-9 is homologous to Bcl-2 and protects the cell from cell death. In nematodes, ced-9 gene appears to be a binary switch that regulates cell survival and apoptosis.

Pathways of Apoptosis
Programmed cell death or apoptosis is a method of removing cells without spilling their bioactive contents into the surrounding. It involves a molecular pathway which culminates in the activation of caspases.
i. Intrinsic Pathway It is also known as mitochondrial or BCL-2 regulated pathway. This pathway is activated by various developmental clues, viral infection, DNA damage and growth factor deprivation. It is strictly controlled by BCL-2 family proteins. This pathway predominantly activates caspase 9 but in certain cells it can also proceed in its absence or its activator Apaf 1.
ii. Extrinsic Pathway It is also known as death receptor pathway as it is triggered by the death receptors. The death receptors are the member of tumor necrosis receptor family, e.g., Fas or TNF receptor-1 that contains an intracellular death domain. Through the adaptor protein FADD (Fas-Associated Death Domain) can activate and recruit caspase 8 at the cell surface.
This causes the further activation of downstream caspases (e.g. caspase-3, caspase-6 or caspase-7) without the involvement of the BCL-2 family. The extrinsic pathway can intersect intrinsic pathway as seen in some cells. It is done through caspase 8 cleavage mediated activation of BH3-only protein BID.
The C terminal truncated form of BID (t-BID) translocates to mitochondria and promotes further caspase activation (Caspase 9 and effector caspases 3, 6 and 7) through intrinsic pathway. These causes nucleus to condense, the cell to shrink and to display on its surface signal of engulfment by other, phagocytic cells. It is often initiated by a withdrawal of growth factors from the cell, but can sometimes be an active response to a signal.

Apoptosis Associated Gene or Protein
• Caspase family They are the key enzyme for the process of apoptosis. Their activity depends on the nuclear affinity of cysteine residue. They are called cysteine dependent aspartate specific proteases as they always cut off the substrate at the site post aspartate.
i. Interleukin-1-converting enzyme (ICE)
It is the homologous gene of nematode ced-3. It can cleave the precursor of IL-1. In human cells, there are total of 11 proteins found. They are divided into ICE sub-group (participates in inflammation) and ced-3 family (participates in apoptosis). The ced-3 family is further divided into two, namely initiators (caspase-8 and csapase-9) and executioners (caspase-3, 6 and 7).

Apoptotic protease activating factor-1 (Apaf-1) It plays an important role in apoptosis in mitochondrion. It contains three domains.
i. C terminal domain To activate Apaf-1, it can bind to cytochrome c.
ii. Ced-4 homologous domain It can bind to ATP/dATP.
iii. Caspase recruitment domain (CARD) It can enrich caspase-9.
• B-cell lymphoma/leukemia-2 (Bcl-2) family It is the apoptosis suppressor gene. Bcl-2 family is mainly located on mitochondrion membrane. Bcl-2, Bcl-xl, Bcl-w and Mcl-1 are anti-apoptotic Bcl-2 members. Whereas Bax, Bak, Bad, Bid and Bim are pro-apoptotic Bcl-2 members.
i. Fas It is a member of TNF receptor family. Also known as APO-1/ CD95. The combination of Fas and Fas ligand can cause the activation of the caspase to start the apoptosis of the target cell.
ii. p53 It is a cancer suppressor gene and checks the DNA replication at G phase. In case of DNA damage, p53 inhibits the DNA cycle till DNA is repaired else the cell will be introduced to apoptosis.
iii. C-myc As a transcription inhibitor it can activate apoptotic and proliferation genes. With Bcl-2 and GF it promotes proliferation, otherwise apoptosis