Chap. 21 Stem Cells, Cell Asymmetry, and
Cell Death
Topics
• Cell Death and Its Regulation
Goals
• Learn the basic mechanism of apoptosis
and its regulation.
• Learn the basic roles of neurotrophins
and apoptosis in wiring of the nervous
system.
• Learn the functions of caspases in
apoptosis.
Overview of Apoptosis
Apoptosis (programmed cell death) is a cell fate that is essential in
some developmental programs. Apoptosis is highly regulated. It can
be induced by withdrawal of trophic factors, which signal cells to
stay alive. Alternatively, signals (e.g., death signals like tumor
necrosis factor) trigger
apoptosis. The structural
changes that occur during
apoptosis are morphologically
distinct from changes that
occur due to cell death via
necrosis (Fig. 21.30). In
necrosis cells typically burst
and release their contents
outside. This damages
surrounding cells and can lead
to inflammation. In apoptosis,
cells shrink, condense, and
fragment without the release of
cell contents. Cell fragments
known as apoptotic bodies are
later phagocytosed. Three
types of proteins control cell
death. Killer proteins initiate
apoptosis, destruction proteins
digest DNA and other cellular
components, and engulfment
proteins are required in
phagocytosis.
Overview of Apoptosis Pathways
Apoptosis pathways discovered
by genetic studies in worms and
mammals are shown in Fig.
21.33. Proteins such as the
CED-9 and Bcl-2 homologs serve
as sensors of signals that
regulate apoptosis. In the
presence of a trophic factor,
these regulators block the
activation of adaptor proteins
(CED-4 and Apaf-1) and thereby
prevent effector proteins (the
CED-3, Casp9, and Casp3
proteases) from becoming
activated. These effectors are
enzymes known as caspases.
When activated, caspases cause
cells to undergo apoptosis by
cleaving key intracellular
substrates resulting in a cascade
of events leading to disassembly
of cellular structures and death.
Apoptosis in Motor Neuron Development
In the development of the
nervous system, motor neurons
and other neurons must
establish connections between
themselves and tissues they
innervate, such as muscle.
Typically, more neurons grow
initially and migrate towards
the target tissue than
ultimately will survive. Those
that fail to form connections to
the target tissue are selected
for apoptosis. Grafting studies
performed with embryos showed
that the formation of
connections and survival of
developing neurons depends on
the quantity of neurotrophic
factors they receive from the
target field tissue they will
innervate (Fig. 21.35). The
greater the quantity of factors
received, the higher the
percentage of cell survival.
Neurotrophins and Their Receptors
Neurotrophic factors (neurotrophins) include nerve growth factor
(NGF), neurotrophin-3 (NT-3), and brain-derived neurotrophic
factor (BDNF). The receptors for neurotrophins are RTKs named
Trks. Trk receptors reside on the cell surface of neurons and bind
to their ligands released from the tissues that they will eventually
innervate. Studies with knockout mice have established some of
the roles of neurotrophins in nervous system development (Fig.
21.36). When the genes encoding NGF or its receptor TrkA are
knocked out, mice selectively fail to form nociceptive (pain-sensing)
neurons that innervate the skin. Knockout of the genes encoding
NT-3 or its receptor TrkC inhibits the formation of propioceptive
neurons that innervate skeletal muscle fibers. These studies show
that neurotrophin signaling is required for developing neurons to
survive instead of undergoing apoptosis.
Caspase Activation in the Absence of
Trophic Factors
The mechanism by which
caspases are activated in
the absence of a trophic
factor (e.g., NGF) is
illustrated in Fig. 21.38.
In the absence of the
trophic factor, the proapoptotic protein Bad
inhibits the anti-apoptotic
protein Bcl-2. As a
result, Bcl-2 cannot
inhibit the activity of the
Bax pro-apoptotic protein.
This results in release of
cytochrome c (Cyt c) from
mitochondria. Cyt c binds
to the Apaf-1 adaptor
protein, and this triggers
procaspase 9 to undergo
activation. Caspase 9 then
cleaves and activates procaspase 3. Caspase 3 cleaves
substrates leading to alterations that result in cell death.
Caspase activation and cell death can also be triggered by
binding of death signals, such as tumor necrosis factor
(TNF) to cells (not shown).
Inhibition of Caspase Activation by
Trophic Factors
In the presence of a
trophic factor, caspases
are maintained in their
inactive pro-forms (Fig.
21.38). In neurons, NGF
causes Bad to be
phosphorylated and
inactivated via the PI-3
kinase/PKB signaling
pathway. Under these
conditions Bcl-2 can inhibit
the activity of Bax. This
prevents Cyt c release
from mitochondria and
blocks the activation of
procaspase 9 by Apaf-1.