As described, when the cell receives stress signals from other cells, extrinsic pathway comes into play.

TNF PATH

TNF-alpha is a cytokine produced by macrophages and is the major extrinsic mediator of apoptosis. Tumor necrosis factor (TNF-alpha) binds to its receptor TNFR1, leading to the activation of caspases.

Fas PATH

The Fas receptor is a transmembrane protein of the TNF family which binds the Fas ligand (FasL). This interaction between Fas receptor and Fas L leads to the activation of caspase.

For example, apoptosis removes the activated T lymphocytes after the infection has been cleared. T cells produce a surface receptor FAS. FAS production increases during the infection, and after a few days, the activated T lymphocytes begin to produce FAS ligand. FAS binding to FAS ligand on the same or different cells triggers apoptosis through caspases activation.

Genetic Mitochondrial Apoptosis Regulation and Cytochrome C Role 

When the cell is stressed from inside, the role of the Bcl-2 family of proteins that regulate the permeability of mitochondria in response to apoptotic signals come into play.

Bcl-2 Family of Genes

Located on chromosome 18, these are anti-apoptotic genes in that they produce protein Bcl-2. Bcl-2 binds to and inhibits APAF-1, thus preventing the release of cytochrome c from the mitochondria. Cytochrome c is present between inner and outer mitochondrial membranes. Once released it binds with APAF-1 and activates procaspase 9.

TP53 Suppressor Gene

This gene encodes a protein that regulates the cell cycle and causes tumor suppression. If the DNA is damaged, for example, by ionizing radiation, chemotherapeutic agents, or hypoxia, TP53 arrests the cell in the G1 phase of cell cycle and prevents the proliferation of cells with damaged DNA and DNA repair. But if the DNA damage is too great, then it will promote apoptosis by activating BAX apoptosis genes. BAX genes products inactivate the BCL 2 anti-apoptosis gene.

In all the normal tissues of multicellular organisms, cell proliferation and cell death are balanced. This normal cell death, vital for the normal development and health of cells, is called apoptosis and involves the following pathways. All the pathways involve activation of caspases as the final tool.

Intrinsic Pathway (Mitochondrial Pathway)

It is activated when the cell is stressed from the inside due to an array of factors like DNA damage from x-ray exposure or UV light exposure; chemotherapeutic agents; hypoxia; accumulation of misfolded proteins inside the cell as, for example, Alzheimer disease, Parkinson disease, or Huntington disease; and many others. When the cell is stressed, there is leakage of cytochrome c from the intermembrane space of mitochondria into the cytosol which leads to the activation of caspases 9. Bcl-2 and TP53 family of genes regulate this pathway.

Extrinsic Pathway

This pathway is triggered when the cell receives death signals from the other cell(s). The extrinsic pathway is receptor-linked, and the ligands from the other cells bind to these death receptors on the cell surface, leading to the activation of apoptosis. This involves the following cell surface receptors and respective ligands, ultimately leading to activation of caspase 8:

• Fas (CD95) receptor activated by FasL (Fas ligand)
• TNFR activated by cytokine TNF

Cytotoxic CD8+ T-Cell Mediated Pathway

CD8+ T cells secrete perforins that create holes in the target cells. Next CD8+ T cells secrete granzymes which enter the target cells through these holes and activate caspases.

Caspases

Caspases are a group of enzymes that are protease in nature. They are the primary effectors of apoptotic response. They are divided into the following 2 types:

Initiator Caspases

The initiator caspases are 2,8.9.10,11,12, and effector caspases include caspases 3,6,7. Caspases exist in the cell in an inactive form and require proteolytic cleavage to an active form.

Effector Caspases

The activated initiator caspases cause the activation of effector caspases. These active effector caspases bring about the cleavage of several proteins in the cell that leads to cell death and ultimately phagocytosis and removal of the cell debris.

Of all the caspases, the most frequently activated caspase is caspase 3 which catalyzes the cleavage of major cellular proteins and condensation of chromatin. Caspase also activates DNAse enzymes that cause fragmentation of DNA followed by internucleosomal fragmentation.

During Embryogenesis (for normal development)[7][8][9][10]

The formation of the digits during embryogenesis in the fetus occurs by the apoptosis of interdigital tissues.

Loss of Mullerian structures in a male fetus by Mullerian inhibitory factor synthesized by Sertoli cells.

During Menstrual Cycle

The sloughing off the inner lining of the uterus (the endometrium) after withdrawal of estrogen and progesterone in the menstrual cycle.

Required to Destroy Dangerous Cells (for organism well being)

• Virus infected cells: Cytotoxic T cells kill the virus-infected cells by apoptosis.
• Cells with DNA damage: Cells whose DNA is damaged by radiation exposure or chemotherapeutic agents are arrested in the G1 phase of the cell cycle for repair by p53 activation. P53 is a tumor suppressor gene. P53 mutation inhibits apoptosis thus leading to the survival of abnormal cells and development of carcinomas.
• Autoreactive T cells: Autoreactive T cells in the thymus are killed by apoptosis.

Required for Healthy Immune System

Apoptosis is required for the development and maintenance of a healthy immune system. When B and T lymphocytes are first produced, they are tested to see if they react against any of the body’s own “self” components. Cells that react are killed by apoptosis. If these cells are not removed, then self-reactive cells may be released into the body which can attack tissues and cause autoimmune conditions.

Apoptosis is required to turn off the immune system after the offending pathogen is cleared from the body; for example, removal of acute inflammatory cells such as neutrophils from healing sites.

Also, Destruction of B and T lymphocytes by corticosteroids occurs by apoptosis.

Removal of Misfolded Proteins

It occurs by apoptosis; for example, amyloid, proteins in prion-related disease.

Too little or too much apoptosis can have serious clinical implications such as the following:

Tumorigenesis

A decrease in apoptosis leads to increased cell-survival rates, leading to the development of cancers.

1. In follicular lymphoma, there is a translocation of BCL2 gene from chromosome 18 to chromosome 14. This leads to excessive transcription of increased levels of BCL2 which causes over inhibition of APAF-1 and thus inactivation of caspases and apoptosis, leading to follicular lymphoma.

2. Mutation or deletion of p53 genes increases the chances of developing tumor dramatically, as the cells with damaged DNA will continue to divide uncontrolled. Chemicals, radiations, and viruses can damage P53.

   People with Li-Fraumeni syndrome have only one functional copy of p53, so they are more likely to develop a tumor in early adulthood.

Autoimmune Diseases

A decrease in apoptosis of self-reactive immune cells can lead to the development of autoimmune diseases like rheumatoid arthritis, systemic lupus erythematosus (SLE), autoimmune lymphoproliferative syndrome, and others.

Neurodegenerative Diseases

Cell death has also been implicated in many neurodegenerative disorders. Both necrosis and apoptosis occur in an acute neurologic disease like acute ischemic syndrome. In chronic neurodegenerative disorders neuronal cell death mainly by apoptosis has been implicated in examples like Parkinson disease, Alzheimer disease, and Huntington disease.

Myocardial Infarction

Necrosis was long considered the sole cause of myocardial infarction, but recent studies have shown that apoptosis also occurs mainly during the reperfusion phase after the acute infarction, leading to further myocardial damage.