Apoptosis

What is apoptosis?
Apoptosis is a type of programmed cell-death that is required for maintaining tissue homeostasis in
multi-cellular organisms. Defects in apoptosis can lead to the development of various disorders.

Apoptosis
Apoptosis also known as programmed cell death is a homeostatic mechanism that helps in the growth,
development and maintenance of multi-cellular organisms. During embryogenesis, apoptosis is required
for determining the size and shape of limbs and other tissues. Whereas, in adults, apoptosis aids in
removing damaged, stressed or infected cells. Apoptosis is not a random process. It involves a series of
molecular events triggered by various signals.

When do cells undergo apoptosis?
Cells undergo apoptosis when positive signals that stimulate cell growth such as growth factors, cell
adhesion etc. are decreased and negative signals such as infection, protein misfolding etc. that inhibit
cell growth are increased.

Hallmarks of apoptosis
Cells undergoing apoptosis exhibit certain characteristics that distinguish them from normal cells. These
characteristics are called the hallmarks of apoptosis. They exhibit morphological as well as biochemical
hallmarks.

The morphological hallmarks include:

 Cell shrinkage
 Nuclear condensation and breakage (Karyorrhexis)
 Detachment and convolution of the cell (Budding/Blebbing)
 Formation of apoptotic bodies, which are small structures which are formed from the disruption
of the apoptotic cell.




Image-1: Morphological changes in an apoptotic cell


The biochemical hallmarks include:

 DNA fragmentation
 Activation of caspases which are enzymes that cleave proteins

, Signals that trigger apoptosis
Apoptotic signals can either be internal signals such as DNA damage, oxidative stress, ischemia,
oncogenes, growth factor deprivation or external signals such as UV exposure, bacterial or viral
infections etc.

Caspases
Caspases are proteolytic enzymes that cleave their substrates at the C-terminal aspartate residue. They
play central roles in apoptosis and inflammation. Caspases are generally of three types: Initiator
caspases (Caspases 8,9,10), Executioner caspases (Caspases 3,6,7) and Inflammatory caspases (Caspases
1,4,5,11). Caspases 2,12 and 14 play various roles in cell differentiation and cell cycle. The targets of
caspases include:

 Protein kinases such as focal adhesion kinase (FAK) etc. that are crucial for cell survival
 Lamins that are found on the nuclear membrane. Cleaving lamins leads to nuclear breakdown.
 Cytoskeletal proteins such as actin, myosin, tubulin etc. Cleaving cytoskeletal proteins will lead
to cell convolution and blebbing.
 Caspase activated DNase (CAD) which cleaves DNA.

Apoptotic pathways
There are three kinds of apoptotic pathways: intrinsic pathway or mitochondrial pathway, extrinsic
pathway or death receptor pathway and perforin or granzyme pathway. They are classified based on the
triggering signals.

The intrinsic pathway or the mitochondrial pathway
The intrinsic pathway is facilitated by the members of B-cell lymphoma-2 (Bcl-2) family of proteins which
are characterized by the presence of one or more Bcl-2 homology (BH) domains. The Bcl-2 protein family
is sub-divided into three categories:

 Pro-apoptotic Bcl-2 proteins such as Bcl-2 associated X (BAX), Bcl-2 homologous killer (BAK) etc.
 Anti-apoptotic Bcl-2 proteins such as Bcl-2, B-cell lymphoma-extra-large (Bcl-XL) etc.
 Bcl-2 Homology 3 (BH3) only proteins.

The external mitochondrial membrane of healthy cells carries the anti-apoptotic protein Bcl-2. Internal
apoptotic signals such as an increase in reactive oxygen species (ROS), DNA damage etc. cause the pro-
apoptotic protein BAX which is normally displayed in the internal mitochondrial membrane to migrate to
the external membrane. Upon migration, BAX inhibits the pro-apoptotic activity of Bcl-2 and inserts
itself into the external membrane. This leads to the formation of pores in the membrane through which
a protein called Cytochrome C (Cyt c) is released. The endoplasmic reticulum (ER) releases high levels of
Calcium at this stage. The calcium ions increase mitochondrial membrane permeability and facilitate the
release of second mitochondria-derived activator of caspases (SMACs) into the cytosol. In the cytosol,
Cyt C combines with apoptotic protease activating factor-1 (Apaf-1) to form a multi-subunit complex
called Apoptosome. The Apoptosome then recruits and activates Caspase-9, the initiator caspase.
Caspase-9 then cleaves and activates Caspase-3 and Caspase-7, the executioner caspases. The
executioner caspases carry out proteolysis which will ultimately lead to cell death.