Apoptosis, or programmed cell death, is an important process by which cells self-destruct in a regulated manner. During development, apoptosis sculpted structures like fingers and toes by killing cells between them. Apoptosis also causes tissues like the tail to disappear at metamorphosis. The process is mediated by caspase proteases and regulated by Bcl-2 family proteins, with Bax and Bak activating caspases. Cells undergo apoptosis to maintain tissue homeostasis, avoid harming neighbors, and be removed by macrophages.

1. Programmed cell
death
APOPTOSIS

2.  Apoptosis helps regulate animal cell numbers
 If cells are no longer needed, they can commit
suicide by activating an intracellular death
program—a process called programmed cell death.
 In animals, by far the most common form of
programmed cell death is called apoptosis (Greek
word meaning ‘falling off,’)
 A form of cell death in which a programmed
sequence of events leads to the elimination of
cells without releasing harmful substances into
the surrounding area.

4. What purposes does this massive
cell suicide serve?
 Mouse paws—and our own hand and feet—are
sculpted by apoptosis during embryonic
development: they start out as spade like structures,
and the individual fingers and toes separate because
the cells between them die
 When a tadpole changes into a frog at
metamorphosis, the cells in the tail die, and the tail,
which is not needed in the frog, disappears

5. Cell death usually exactly balances cell
division, unless the tissue is growing or
shrinking
 Eg. If part of the liver is removed in an adult rat,
for example, liver cells proliferate to make up the
loss. Conversely, if a rat is treated with the drug
phenobarbital, which stimulates liver cell division,
the liver enlarges.
 However, when the phenobarbital treatment is
stopped, apoptosis in the liver greatly increases
until the organ has returned to its original size,
usually within a week or so. Thus, the liver is kept
at a constant size through regulation of both the
cell death rate and the cell birth rate.

6. Cell that undergoes apoptosis dies
neatly, without damaging its
neighbours.
 cytoskeleton collapses, the nuclear envelope
disassembles, and the nuclear DNA breaks up
into fragments.
 macrophages – cells engulf the apoptotic cell
before it spills its contents
 destructive and self-amplifying but also
irreversible

7. Machinery that is responsible for
apoptosis
 involves the caspase family of proteases - made
as inactive precursors called procaspases.
 apoptosis is mediated by an intracellular
proteolytic cascade.

9. Death program is regulated by the
bcl2 family of intracellular proteins
 All nucleated animal cells - inactive procaspases
lie waiting for a signal to destroy the cell.
 main proteins that regulate the activation of
procaspases are members of the Bcl2 family
 Some members of this protein family promote
procaspase activation and cell death, whereas
others inhibit these processes.
 most important death-promoting family members
are proteins called Bax and Bak.

10. Bax and Bak.
 These proteins activate procaspases indirectly, by
inducing the release of cytochrome c from
mitochondria into the cytosol.
 Cytochrome c promotes the assembly of a large,
seven-armed pinwheel-like structure that recruits
specific procaspase molecules, forming a protein
complex called an apoptosome.
 The procaspase molecules become activated within
the apoptosome, triggering a caspase cascade that
leads to apoptosis
 Bax and Bak proteins are themselves activated by
other death-promoting members of the Bcl2 family,

12. Extracellular signals to survive,
Grow, and divide
 Most of the extracellular signal molecules that influence
cell survival, cell growth, and cell division are either
soluble proteins secreted by other cells or proteins bound
to the surface of other cells or the extracellular matrix.
 Three major categories:
1. Survival factors promote cell survival, largely by
suppressing apoptosis.
2. Mitogens stimulate cell division, primarily by overcoming
the intracellular braking mechanisms that tend to block
progression through the cell cycle.
3. Growth factors stimulate cell growth (an increase in cell
size and mass) by promoting the synthesis and inhibiting
the degradation of proteins and other macromolecules.

13. Survival factors to avoid
apoptosis
 signals from other cells helps to ensure that cells
survive only when and where they are needed
 Survival factors usually act by binding to cell-surface
receptors.
 These activated receptors then turn on intracellular
signaling pathways that keep the death program
suppressed, usually by regulating members of the
Bcl2 family of proteins.
 Some survival factors, for example, increase the
production of Bcl2, a protein that suppresses
apoptosis

15. Pathways
The Extrinsic Pathway:
 In the extrinsic pathway, signal molecules known as ligands, which
are released by other cells, bind to transmembrane death receptors
on the target cell to induce apoptosis.
 For example, the immune system’s natural killer cells possess the
Fas ligand (FasL) on their surface .
 The binding of the FasL to Fas receptors (a death receptor) on the
target cell will trigger multiple receptors to aggregate together on the
surface of the target cell.
 The aggregation of these receptors recruits an adaptor protein
known as Fas-associated death domain protein (FADD) on the
cytoplasmic side of the receptors.
 FADD, in turn, recruits caspase-8, an initiator protein, to form the
death-inducing signal complex (DISC).
 Through the recruitment of caspase-8 to DISC, caspase-8 will be
activated and it is now able to directly activate caspase-3, an effector
protein, to initiate degradation of the cell.
 Active caspase-8 can also cleave BID protein to tBID, which acts as
a signal on the membrane of mitochondria to facilitate the release of
cytochrome c in the intrinsic pathway.

16. The Intrinsic Pathway:
 The intrinsic pathway is triggered by cellular stress, specifically
mitochondrial stress caused by factors such as DNA damage
and heat shock.
 Upon receiving the stress signal, the proapoptotic proteins in
the cytoplasm, BAX and BID, bind to the outer membrane of
the mitochondria to signal the release of the internal content.
 However, the signal of BAX and BID is not enough to trigger a
full release. BAK, another proapoptotic protein that resides
within the mitochondria, is also needed to fully promote the
release of cytochrome c and the intramembrane content from
the mitochondria.
 Following the release, cytochrome c forms a complex in the
cytoplasm with adenosine triphosphate (ATP), an energy
molecule, and Apaf-1, an enzyme.
 Following its formation, the complex will activate caspase-9,
an initiator protein.
 In return, the activated caspase-9 works together with the
complex of cytochrome c, ATP and Apaf-1 to form an
apoptosome, which in turn activates caspase-3, the effector
protein that initiates degradation.
 Besides the release of cytochrome c from the intramembrane