27th October 2020

Apoptosis Pathway: Intrinsic and Extrinsic Pathways

Apoptosis Pathway is the normal physiologic mechanism of each living multicellular organism. The meaning of this word in ancient Greek is “falling off”; i.e. the cells of our living body perish by a common physiologic manner known as programmed cell death or in one-word apoptosis. The apoptosis word was 1st time suggested by James Cormack who was the professor of the Greek language of Aberdeen university.

Discovery and Etymology

The sires of biochemical and cellular changes lead to this common complex cellular mechanism and help to develop new physiologic characters. The adult human body drops 50-70 billion cells each day and cell dropping number of children 25 billion/ day by an apoptotic mechanism. So, apoptosis is as familiar as breathing like phenomena of every multicellular organism.

This plays a central mechanism of multicellular development, cellular deletion of the entire structure, and maintain the number of nerves in the nervous system. Series of cellular and molecular changes take place to complete this process these are convolution, chromatin compacting as well as segregation, cytoplasmic condensation, nuclear fragmentation, bleb formation, cellular fragmentation, reduction of cellular integrity, apoptotic body formation etc.

Furthermore, morphological hallmarks along with chromatin condensation and nuclear fragmentation the cellular volume reduce simultaneously which known as pyknosis and produces pseudopods. The chromatin additional condenses until it breaks up inside a cell with an intact membrane, a feature described as karyorrhexis.

Chromatin condensation starts at the nuclear membrane periphery, form a crescent or ring-like structure. Including these cellular changes some molecular changes also occur like position changing of phosphatidylserine from the inner leaflet of the cell membrane to the outer leaflet of cell membrane, DNA and protein breakdown, vital cellular proteins breakdown as well as degradation of nuclear scaffold & cytoskeleton.

Along with these changes, cellular destruction or the apoptosis pathway is completed. In some time for some developmental context like the immune system, certain signals persuade the killing of cells on the other hand cell to commit suicide for lack of oxygen and survival signaling by apoptosis. A condition involving apoptosis in physiologic condition:

  1. Programmed cell destruction in development for sculpting of tissue.
  2. Physiologic involution like a regression of the lactating breast, shedding of the endometrium.
  • Involution of the thymus in early are.
  1. Degeneration of gill & tail of frog during metamorphosis.

Pathological condition:

  1. Anticancer drug-induced cell death in tumors.
  2. Cell death due to the induction of cytotoxic T cell, i.e. Graft versus host diseases, immune rejection.
  • CD4 cell destruction in AIDS by apoptosis pathway.
  1. Cell destruction due to the adverse conditions of the cellular environment like hypoxia, ionizing radiation, thermal injury, etc.
  2. Cellular destruction in neurodegenerative diseases like Alzheimer’s, Parkinson’s, etc.
  3. Cell death occurs in heart disease and myocardial infarction.

There has a long history behind the discovery of the apoptosis pathway. At first, in 1842 the Garman scientist Carl Vogt has described the principle of this process and after that many scientists from time to time publish their valuable information on this particular topic.

After the invention of the electron microscope, the scientist of Queensland University John Foxton Ross Kerr studied the proper cellular events of both apoptotic cell and necrotic cells and give a clear view about apoptotic cellular degradation as well as traumatic cell death, necrosis. He along with his colleagues published their experimental knowledge in 1972 at The British Journal of Cancer. Scientist Kerr was received Paul Ehrlich and Ludwig Darmstaedter Prize for their novel research in March 2000. Then another group of scientists received Noble Prize in Medicine in 2002 for their important research, identifying the genes which control the apoptosis in nematode C.elegans. These homology genes are also functioning in humans for regulating apoptosis.


Activation of Apoptosis Pathway Process

The activation of the apoptosis pathway is strictly regulated by the activating mechanism of cells. It is very crucial to know about the importance of signal transduction as well as the activity of caspase proteases. Signaling is the most vital part of any kind of cellular communication between both cases like intra & intercellular cross-talking. Without signaling, the cell can’t survive for a single moment. From cellular respiration to nutrition, digestion to gene expression each and every single function can’t be completed without signal transduction.

On the other hand, caspase is the cysteine dependent aspartate protease play and important as well as a principal role in apoptosis pathways.

Apoptotic cell degradation or program cell deaths activate by different pathways like intrinsic pathways (stimulus generation from inside of the cell), extrinsic pathway (stimuli generated from the outside or neighboring cell via ligands), and granzyme pathway (stimuli come from macrophages or NK cell).

The internal stimulus generated from the cell due to significant level of cellular damages like irreversible genetic damages, viral infection, accumulation of a large number of free radicals, oxygen leakage and huge oxidative stress, mitochondrial and ER stress, increase deposition of cytosolic Ca2+ leads the apoptotic activation of the intrinsic pathway.

Bcl 2 family member proteins play the central role in this initiation. This family protein contains different BH domains (BH1, BH2, BH3, BH4) and these proteins are simply classified into 2 groups i. proapoptotic members lead the apoptosis pathway (Bax and Bad) contain BH3, BH1, BH2 domains, ii. Antiapoptotic members who protect cells from apoptosis (Bcl x, Bcl 2) contain BH4, BH3, BH1, BH2 domain, iii. BH3 only proteins contain only the BH3 domain, this protein promotes apoptotic pathways indirectly (Bid, Bad, Puma, and Bim).

Depending upon the particular protein involvement the BH3 only protein plays their role in 2 different ways, i.e. some of them activate Bcl 2 like antiapoptotic proteins and others turn on Bax like apoptotic proteins. So, in short, this third type of Bcl 2 homology (BH) family proteins involved in the cell survival via apoptotic and anti-apoptotic proteins as well as the signal transduction processes.

Fig: Intrinsic and Extrinsic Pathway

Intrinsic Pathway

In general, in normal cells or during in presence of trophic factor the proapoptotic Bcl2 family protein like Bad inactivate by 14-3-3 protein (inhibitor of Bad) because of the stimulation of PI3 kinase activity which leads Atk kinase activation through downstream kinase cascade activity.

  • That’s why antiapoptotic proteins Bcl-2 or Bcl-XL (mitochondrial membrane-bound protein) are able to block the activation of another proapoptotic protein like Bax (translocating protein present in the cytosol) and reduce the tendency of apoptosis, help to cell survival.
  • But in the stressful condition, the pro-apoptotic proteins are activated and induce a series of event which promote the apoptosis pathway. Proapoptotic protein Bad binds with antiapoptotic protein Bcl-2 or Bcl-XL and inactive them which leads the activation of Bax.
  • Activated Bax protein translocates through the cytosol undergo a conformational change & assembled in a multisubunit protein lined channel and binds with the outer membrane of mitochondria which promotes ionic influx into mitochondria and increase mitochondrial permeability.
  • Due to ionic influx cytochrome-C (mitochondrial proteins) start to accumulate in inner mitochondrial space. The release of cytochrome-C leads irreversible cell apoptosis pathway, from this is the “point of no return”.
  • By the hydrolyzing ATPs, Cytochrome-C bind with Apaf-1 (apoptotic protease activating factor) protein leads to conformational changes and form apoptosome (wheel-like multiprotein mainly seven subunit complex) in the cytosol.
  • Through incorporating procaspase 9 into apoptosome leads auto-activation of caspase 9 which further activates caspase 3 by its proteolytic activity.
  • Caspases are the distinct group of cysteine protease that promotes the activities of early stages of apoptosis and leads to cellular destruction.

In this total pathway, the regulatory protein is Bcl-2 or Bcl-XL, adapter protein is Apaf 1 and effector protein caspase 9 and initiator caspase 9 and effector caspase 3. This is also known as the mitochondrial pathway, there has another intrinsic pathway known as the intrinsic endoplasmic reticulum pathway.  This is a very rear and less known pathway.

  • It is believed to be caspase 12 dependent and mitochondrial independent. Due to the cellular stress ER injury leads to unfolding proteins accumulation and reduction of protein synthesis in the cell, promotes dissociation of adaptor protein like TNF receptor-associated factor 2 (TNFR2) from procaspase 12, resulting activation of the cell death process.
  • In general, ER is stopped the secretory pathway wherein chaperone associated polypeptide folding and modification ensure the proteins obtain their mature conformation.
  • When the protein folding capacity of ER overwhelmed, a highly conserved unfolded protein response (UPR) signal transduction pathway is activated. A high level of damages can’t be restored and mammalian UPR finally initiates the apoptosis pathway.

Extrinsic Pathway

Extrinsic pathway ligand-mediated apoptotic pathway. Extracellular messenger proteins play a major role in the initiation of sequential cell death. The external messengers are may be viral proteins or cytokines like TNF-α, TGF-β or FAS proteins, etc which secrete from the cell due to exposure of ionizing radiation, viral infection, elevated temperature, and toxic chemical agents. Tumor necrosis factors are released from the infected cells or phagocytic cells like macrophages, monocyte, NK cells and leads ligand-mediated apoptosis pathway namely extrinsic pathway.

  • In TNF-α mediated extrinsic pathway getting started when trimeric TNF molecule bind with a transmembrane receptor TNFR1 (tumor necrosis factor receptor).
  • These TNFR1 receptors have a death domain in the cytosolic site which further associated with TNFR1 associated death domain (TRADD) which induce death signal into the cell.
  • Into cell Fas-associated death domain (FADD) is linked with 2 procaspases 8 which convert to caspase 8 by kinase activity of receptor-interacting serine/threonine-protein kinase (RIP1K), connected with TRADD.
  • The assembled complex of TNFR1, TRADD, RIP1K proteins are simultaneously activated after binding of TNF and leads the progression of cell death.
  • Activated initiator caspase 8 downstream caspases and promote apoptosis pathways.

The RIP1K protein also induces necrosis with the help of another activated protein RIP3K. assembled protein heteromer activate mixed linked kinase ligand (MLKL) via phosphorylation and allow to translocate towards the cell membrane. Activated MLKL oligomerized at the cell membrane and form channel which induce cytoplasmic leakage leads necrosis.

FAS ligand (FasL) mediated apoptotic mechanisms are similar to TNF mediated cell death except for initiation of the total process. FasL a homotrimeric transmembrane protein present in the cytotoxic T-lymphocytes and NK cells. FasL bind with Fas receptors (FasR) of targeted cell and induce apoptotic signaling via death-inducing signaling complex (DISC), which associated with FasR death domain and FADD. DISC activates caspase8 and promotes the apoptosis pathway.

3 thoughts on “Apoptosis Pathway: Intrinsic and Extrinsic Pathways

  1. It’s written in an excellent level of thinking, this will be beneficial to students of all stage starting from bachelor’s to Ph D. Biotrivia plz
    Keep uploading such information

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