Apoptosis
Apoptosis is generally considered as programmed cell death whereas necrosis
signifies unprogrammed cell death. The distinction between these was originally
based on morphological criteria; apoptotic cells shrink while necrotic cells swell.
It is not clear whether the distinction is quite so simple but there is a suite of
morphological features that characterize apoptosis.
! Apoptotic cells lose substrate attachment and become rounded.
! Cells shrink, condense their chromatin, and display membrane blebbing.
! Apoptotic cells fragment their DNA into approximately 200 base-pair
fragments.
! At the end of apoptosis, the cell is broken into multiple apoptotic bodies
that are phagocytized by neighboring cells.
Thus these morphological changes are the outward manifestations of the cell
systematically dismantling itself and packaging itself up in membrane bound
vesicles to be absorbed by neighboring cells. Because cellular contents are not
released, this occurs with little inflammation.
Apoptosis is an essential component of normal development and homeostasis
as well as being critical for eliminating diseased or damaged cells. Mutations in
key components of apoptotic pathways are lethal.
! Approximately 50% of neurons undergo apoptosis during mammalian
embryogenesis. Severe mental retardation results if the extra neurons are
not eliminated.
! In the immune system, autoreactive lymphocytes are eliminated by
apoptosis and failure in this system results in autoimmune disease.
As a safeguard against disease, every cell in our bodies expresses the
components of the apoptotic pathways and is ready for rapid self-destruction. In
fact, it has recently become clear that cells must receive the proper set of signals
to prevent apoptosis or they will self-destruct.
Caspase Cascade
The central machinery of apoptosis consists of a cascade of cysteine proteases
called caspases. There are 2 major types of caspases, initiator caspases and
effector caspases. Effector caspases are the enzymes responsible for
disassembling the cells. Substrates for effector caspases include:
1. apoptosis inhibitors (eg. Bcl2, Rb)
2. cell structures
3. other proteolytically activated enzymes
gelsolin—degrades cytoskeleton
CAD—caspase activated DNAse
Initiator caspases activate effector caspases by proteolytic cleavave of an
effector pro-caspase. The 2 most important initiator caspases are caspase8 and
caspase9. These are associated with the 2 major pathways for initiating
apoptosis. Caspase 8 is involved in receptor-mediated apoptosis while caspase
9 is involved with the mitocondrial pathway. Mice deficient for either of these
caspases usually die before birth and always within 3 days of birth. The mice
show distinct defects for each caspase, some of which include brain deformities,
heart malformations and blood overproduction.
Caspases are ubiquitously expressed; therefore every cell is poised for rapid
self-destruction.
Mitochondrial pathway
The Bcl2 family proteins are the central regulators of the mitochondrial pathway.
Bcl2 is an inhibitor of apoptosis. Bcl2 binds and inhibits a protein called Apaf.
Apaf is an activator of the initiator caspase9. Therefore,
Bcl2 --| Apaf # caspase9 # APOPTOSIS
Bcl2 is located on the cytosolic face of several membranes including the outer
mitochondrial membrane, the ER and the nuclear envelope. It is thought that
Bcl2 may somehow monitor damage in these compartments.
Bax is a protein,related to Bcl2, which inhibits Bcl2 thereby promoting apoptosis.
It is thought that binding of Bax to Bcl2 releases Apaf to then activate caspase9.
Thus, Bax --| Bcl2 --| APOPTOSIS, (net result, Bax # APOPTOSIS)
CytochromeC is also an activator of Apaf and apoptosis. CytochromeC is
important for electron transport. The presence of cytochromeC in the cytoplasm
indicates that the integrity of mitochondrial membranes has been compromised,
and therefore that cellular damage has been sustained.
Death Receptor mediated pathways
A number of cell surface receptors can induce apoptosis when activated by a
signal ligand. One of the most well known of these is tumor necrosis factor (TNF)
and the receptor TNFR. Another important one is Fas. Death receptors are
plasma membrane spanning proteins that have a conserved motif in their
cytoplasmic domain called the Death Domain. The death domain mediates
protein interactions that lead to the association and activation of the initiator
caspase, caspase8 with the receptor complex. Thus receptor mediated signaling
can activate apoptosis independent of the mitochondrial pathway.
TNF and other “death signals” can have very different effects on different cell
types. TNF can signal division in some cells, differentiation in other cells, and
apoptosis in yet other cell types. The response of a given cell depends on what
other proteins are expressed by that cell. In the case of TNF signaling, one
protein of particular importance is a transcription factor called NF-ΚB. Cells that
do not express NF-ΚB are induced to die by TNF while those that do express
NF-ΚB are not.
Sensitization of cells
Inputs from different sources can act in combination to induce apoptosis. Cells
that receive stimuli that are insufficient to induce apoptosis become more
sensitive to induction by other stimuli. For example, mild DNA damage induces a
low level of p53 expression. p53 inhibits the expression of Bcl2 and stimulates
the expression of Fas. Therefore, both the mitochondrial and receptor mediated
pathways become more sensitive to induction by other stimuli.