3. APOPTOSISAPOPTOSIS
Apoptosis is a form cell death designed to
eliminate un wanted host cells through activation
of a co ordinated internally programmed series of
events effected by dedicated set of gene products.
Apoptosis is a form cell death designed to
eliminate un wanted host cells through activation
of a co ordinated internally programmed series of
events effected by dedicated set of gene products.
9. Apoptosis in pathologic situations
Apoptosis eliminates cells that are genetically altered or
injured beyond repair without eliciting a severe host reaction,
thus keeping the damage as contained as possible.
13. • Activation of several members of cystine
family called caspases.can be divided into two
groups initiator and executioner.
• Initiator include caspases 8 and 9.
• Executioners including caspases 3 and 6.
• DNA breakdown by calcium and magnesium
dependent endonucleases into fragments
whose sizes are multiples of 180 units to 200
base pairs.fragments may be visualized by
electrophoresis. As DNA ladder.
• Dead cells are recognized by phogocytes by
the help of phospholipid such as
phosphatidylserine from inner leaflet to outer
leaflet of membrane.
14.
15. • Intrinsic (mitochondrial)pathway of apoptosis
• This pathway is result of increased
mitochondrial permeability and release of pro
apoptotic molecules into cytoplasm.
• Release of mitochondrial proteins is controlled
by pro and anti apoptotic members of Bcl
family of proteins.
• Bcl-2,Bcl-x and Mcl-1 are anti apoptotic
proteins they inhibit apoptosis.
16. • When cells are deprived of survival singals or
their DNA is damaged or misfolded proteins
induces ER stress,sensors of damage or stress
are activated.
• These sensors are also members of Bcl
family.they include proteins called Bim,Bid,and
Bad.
• The sensors activate two critical effectors
Bax,Bak
17. EXTRINSIC PATHWAY
• Death receptors are members of TNF receptor
family .
• Best known death receptors are type 1 TNF
receptor and related protein called Fas
(CD95).
• The machenism of apoptosis induced by these
death receptor expressed on many cell types.
• Fas is expressed on T cells that recognize self
antigens and on some cytotoxic T
lymphocytes.
18. • When FasL binds to Fas,three or more
molecules of Fas are brought togather and
there cytoplasmic death domains form a
binding site for a adapter protein that also
contains a death domain and is called FADD.
• FADD than activated the inactive form of pro
caspase-8 to active form of caspase-8.
• This pathway of apoptosis can be inhibited by
a protein called FLIP.
19. • After intiating pathways the executioner
phase is activated,executioner caspases
iclude caspase 3 and 6.
• These caspases once activated,cleave an
inhibitor of cytoplasmic DNase,thus DNase
enzyme activated,this Dnase enzyme break
down of DNA into nucleosome.
• Caspaes degraded structural component of
nuclear matrix & thus promote fragmentation
of nuclei.
20. REMOVAL OF DEAD CELLS
• In healthy cells phosphatidylserine is present
on inner leaflet of plasma membrane but in
apoptotic cells this phospholipid flips out and
is expressed on outer layer where it is
recognized by several macrophage
receptors.which help in removal of death
cells.
During development many cells are produced in excess which eventually undergo programmed cell death
A particularly instructive example for the implication of programmed cell death in animal development is the formation of free and independent digits by massive cell death in the interdigital mesenchymal tissue [Zuzarte-Luis, 2002]. Other examples are the development of the brain, during which half of the neurons that are initially created will die in later stages when the adult brain is formed [Hutchins, 1998] and the development of the reproductive organs [Meier, 2000]. Also cells of an adult organism constantly undergo physiological cell death which must be balanced with proliferation in order to maintain homeostasis in terms of constant cell numbers. The majority of the developing lymphocytes die either during genetic rearrangement events in the formation of the antigen receptor, during negative selection or in the periphery, thereby tightly controlling the pool of highly efficient and functional but not self-reactive immune cells and at the same time keeping lymphocyte numbers relatively constant [Rathmell, 2002].
Taken together, apoptotic processes are of widespread biological significance, being involved in e.g. development, differentiation, proliferation/homoeostasis, regulation and function of the immune system and in the removal of defect and therefore harmful cells. Thus, dysfunction or dysregulation of the apoptotic program is implicated in a variety of pathological conditions. Defects in apoptosis can result in cancer, autoimmune diseases and spreading of viral infections, while neurodegenerative disorders, AIDS and ischaemic diseases are caused or enhanced by excessive apoptosis [Fadeel, 1999a].
Role of mitochondria in apoptosis
Mitochondria play an important role in the regulation of cell death. They contain many pro-apoptotic proteins such as Apoptosis Inducing Factor (AIF), Smac/DIABLO and cytochrome C. These factors are released from the mitochondria following the formation of a pore in the mitochondrial membrane called the Permeability Transition pore, or PT pore. These pores are thought to form through the action of the pro-apoptotic members of the bcl-2 family of proteins, which in turn are activated by apoptotic signals such as cell stress, free radical damage or growth factor deprivation. Mitochondria also play an important role in amplifying the apoptotic signalling from the death receptors, with receptor recruited caspase 8 activating the pro-apoptotic bcl-2 protein, Bid.
Role of Bcl-2 proteins
The bcl-2 proteins are a family of proteins involved in the response to apoptosis. Some of these proteins (such as bcl-2 and bcl-XL) are anti-apoptotic, while others (such as Bad, Bax or Bid) are pro-apoptotic. The sensitivity of cells to apoptotic stimuli can depend on the balance of pro- and anti-apoptotic bcl-2 proteins. When there is an excess of pro-apoptotic proteins the cells are more sensitive to apoptosis, when there is an excess of anti-apoptotic proteins the cells will tend to be more resistant. An excess of pro-apoptotic bcl-2 proteins at the surface of the mitochondria is thought to be important in the formation of the PT pore.
An animation illustrating the general principles is shown below.
The pro-apoptotic bcl-2 proteins are often found in the cytosol where they act as sensors of cellular damage or stress. Following cellular stress they relocate to the surface of the mitochondria where the anti-apoptotic proteins are located. This interaction between pro- and anti-apoptotic proteins disrupts the normal function of the anti-apoptotic bcl-2 proteins and can lead to the formation of pores in the mitochondria and the release of cytochrome C and other pro-apoptotic molecules from the intermembrane space. This in turn leads to the formation of the apoptosome and the activation of the caspase cascade.
The release of cytochrome C from the mitochondria is a particularly important event in the induction of apoptosis. Once cytochrome C has been released into the cytosol it is able to interact with a protein called Apaf-1. This leads to the recruitment of pro-caspase 9 into a multi-protein complex with cytochrome C and Apaf-1 called the apoptosome. Formation of the apoptosome leads to activation of caspase 9 and the induction of apoptosis.
The role of mitochondria in the induction of apoptosis is summarised in the figure below.
Apoptosis can be triggered by various stimuli from outside or inside the cell, e.g. by ligation of cell surface receptors, by DNA damage as a cause of defects in DNA repair mechanisms, treatment with cytotoxic drugs or irradiation, by a lack of survival signals, contradictory cell cycle signalling or by developmental death signals. Death signals of such diverse origin nevertheless appear to eventually activate a common cell death machinery leading to the characteristic features of apoptotic cell death.