Calpain – calcium activated protease Cathepsin – lysosomal protease (active at low pH) Caspase – a protease associated with apopotosis
In a healthy cell, the outer membranes of its mitochondria display the protein Bcl-2 on their surface. Internal damage to the cell (e.g., from reactive oxygen species ) causes Bcl-2 to activate a related protein, Bax , which punches holes in the outer mitochondrial membrane, causing cytochrome c to leak out. The released cytochrome c binds to the protein Apaf-1 ("apoptotic protease activating factor-1"). Using the energy provided by ATP , these complexes aggregate to form apoptosomes . The apoptosomes bind to and activate caspase-9 . Caspase-9 is one of a family of over a dozen caspases. They are all proteases . They get their name because they cleave proteins — mostly each other — at aspartic acid ( Asp ) residues). Caspase-9 cleaves and, in so doing, activates other caspases (caspase-3 and -7). The activation of these "executioner" caspases creates an expanding cascade of proteolytic activity (rather like that in blood clotting and complement activation) which leads to digestion of structural proteins in the cytoplasm, degradation of chromosomal DNA, and phagocytosis of the cell.
Superantigens are molecules which short-circuit the immune system, resulting in massive activation of T-cells rather than the usual, carefully controlled response to foreign antigens. It is believed that they do this by binding to both the variable region of the beta-chain of the T-cell receptor (V-beta) and to MHC II molecules, cross-linking them in a non-specific way:This results in polyclonal T-cell activation rather than the usual situation where only the few clones of T-cells responsive to a particular antigen presented by the MHC II molecule are activated. The over-response of the immune system produced results in autoimmunity, as rare clones of T-cells which recognise self antigens are activated, and immune suppression, as the activated cells subsequently die or are killed by other activated T-cells. It is possible that such superantigens might also induce apoptosis (pronounced "apo-tosis"), or 'programmed cell killing' (Cohen JJ. Apoptosis. Immunol. Today 14: 126-136, 1993):
Apoptosis – Programmed Cell Death
• (True/False) In adult tissues cell death exactly balances cell division
• In apoptosis the cell destroys itself from within and avoids leakage of the cell
contents into the extracellular space. Why do you think that this occurs via a
different mechanism than in necrosis?
• What are some signals that indicate to a cell that apoptosis needs to occur?
Where do these signals come from?
• What are some cellular components involved in the apoptotic pathway?
• What is the difference between a mitogen, a growth factor, and a survival factor?
• In what phase of the cell cycle do cells exit to undergo apoptosis?
• What effects do telomeres and telomerase have on cell aging and death? If you
could turn on telomerase activity in all of our cells, would it prevent aging?
• Do the following types of cells exist in humans?
–Cells that do not grow and do not divide
–Cells that grow, but do not divide
–Cells that divide, but do not grow
–Cells that grow and divide
Death by Injury vs. Death by Suicide
(Necrosis vs. Apoptosis)
Trauma (toxic chemicals, mechanical injury, heat, hypoxia)
Loss of ability to regulate internal environment
Ca2+ influx accompanied by swelling
Alteration of protein activity
Production of toxic compounds
(activation of cyclooxygenases)
Apoptosis – Programmed Cell Death
Protective (destroy cells that are a threat)
infected with virus
turn off immune response
DNA damaged cells
What makes a cell commit suicide?
withdrawal of positive signals (growth factors, Il-2)
receipt of negative signals (increased levels of oxidants, DNA damage via X-ray
or UV light, chemotherapeutic drugs, accumulation of improperly folded
proteins, death activators such as: TNF-a, TNF-b, Fas/FasL)
Steps in apoptosis:
the decision to activate the pathway;
the actual "suicide" of the cell;
engulfment of the cell remains by specialized immune cells called phagocytes;
degradation of engulfed cell.
The actual steps in cell death require:
condensing of the cell nucleus and breaking it into pieces
condensing and fragmenting of cytoplasm into membrane bound apoptotic
breaking chromosomes into fragments containing multiple number of
nucleosomes (a nucleosome ladder)
Apoptosis Triggered via Two Pathways
Intrinsic or mitochondrial pathway
Extrinsic or death receptor pathway
• Binding of Fas by FasL
induces recruitment of FADD
to the cytoplasmic tail of Fas
• The opposite end of FADD
contains a death effector
domain (hatched boxes);
recruitment of either
procaspase-8 or c-FLIP
• Caspase-8 can cleave Bid
• truncated Bid (tBid) can
inactivate Bcl-2 in the
• This allows the escape of
cytochrome c, which clusters
with Apaf-1 and caspase-9 in
the presence of dATP to
• Smac/DIABLO is also released
from the mitochondria and
inactivates inhibitors of
• breakdown of several
cytoskeletal proteins and
degradation of the inhibitor of
Extrinsic or Death
Apoptosis is involved in
• Cancer ( via HPV, Epstein bar virus; melanoma)
• regulation of the immune system,
• organ transplants
Melanoma (the most dangerous type of skin cancer)
cells avoid apoptosis by inhibiting the expression of the
gene encoding Apaf-1.
Superantigens are molecules which short-circuit the immune system, resulting in massive
activation of T-cells rather than the usual, carefully controlled response to foreign antigens.
The over-response of the immune system produced results in autoimmunity, as rare clones
of T-cells which recognize self antigens are activated, and immune suppression, as the
activated cells subsequently die or are killed by other activated T-cells. It is possible that
such superantigens might also induce apoptosis
Anterior chamber of the eye and testes fail to elicit an immune response
because these cells produce lots of FasL, so kill immune cells when they
enter these sites.
Possible therapy by inducing production of FasL in other tissues –
lowering need for immune rejection drugs
•Cytoplasm shrinks due to
cleavage of nuclear lamins
•Chromatin is broken
down as nucleus
•Cells shrink making an
easy meal for phagocytes
•Cells undergo plasma
membrane changes –
from inner to outer leaflet
of the membrane; attracts