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Cell Death and Cell Renewal
18 Cell Death and Cell Renewal
•
Programmed Cell Death
•
Stem Cells and the Maintenance of Adult
Tissues
•
Pluripotent Stem Cells, Cellular
Reprogramming and Regenerative
Medicine
Introduction
Cell death and proliferation are balanced
throughout the life of multicellular
organisms.
Animal development involves cell
proliferation and differentiation and also
cell death.
Most cell death occurs by a normal process
of programmed cell death.
Introduction
Most tissues have stem cells that can
replace cells that have been lost.
Abnormalities of cell death are involved
in cancers, autoimmune diseases, and
neurodegenerative disorders.
The ability of stem cells to proliferate
and differentiate makes them a
promising mechanism for replacing
damaged tissues.
Programmed Cell Death
Programmed cell death is carefully
regulated.
In adults, it balances cell proliferation
and maintains constant cell numbers.
It also eliminates damaged and
potentially dangerous cells (e.g., virusinfected cells).
Programmed Cell Death
During development, programmed cell
death plays a key role by eliminating
unwanted cells from many tissues.
Examples:
• Elimination of larval tissues during
amphibian and insect
metamorphosis.
• Elimination of tissue between the
digits in the formation of fingers and
toes.
Programmed Cell Death
In development of the mammalian
nervous system, up to 50% of
developing neurons are eliminated by
programmed cell death.
Those that survive have made the correct
connections with their target cells,
which secrete growth factors that signal
cell survival by blocking the neuronal
cell death program.
Programmed Cell Death
Necrosis: Accidental cell death from acute
injury.
Apoptosis: Programmed cell death; an
active process. Characterized by:
• DNA fragmentation
• Chromatin condensation
• Fragmentation of the nucleus and cell
Figure 18.1 Apoptosis (Part 1)
Figure 18.1 Apoptosis (Part 2)
Figure 18.1 Apoptosis (Part 3)
Programmed Cell Death
Apoptotic cells and cell fragments are
recognized and phagocytosed by
macrophages and neighboring cells,
and rapidly removed from tissues.
Necrotic cells swell and lyse; the
contents are released into the
extracellular space and cause
inflammation.
Programmed Cell Death
Apoptotic cells express “eat me” signals,
such as phosphatidylserine.
In normal cells, phosphatidylserine is
restricted to the inner leaflet of the
plasma membrane.
Figure 18.2 Phagocytosis of apoptotic cells
Programmed Cell Death
Studies of C. elegans by the Robert
Horvitz lab identified three genes with
key roles in apoptosis.
C. elegans development includes the
death of 131 specific cells.
Their experiments used mutant strains in
which the cell death did not occur.
Key Experiment, Ch. 18, p. 694 (2)
Programmed Cell Death
The genes ced-3 and ced-4 are required
for developmental cell death.
A third gene, ced-9, is a negative
regulator.
These genes are the central regulators
and effectors of apoptosis that are
highly conserved in evolution.
Figure 18.3 Programmed cell death in C. elegans
Programmed Cell Death
Ced-3 is the prototype of the caspases
family of proteases.
Caspases have cysteine (C) residues at
their active sites and cleave after
aspartic acid (Asp) residues in their
substrate proteins.
Programmed Cell Death
Caspases are the ultimate executioners
of programmed cell death by cleaving
over 100 different target proteins.
Activation of an initiator caspase starts a
chain reaction of caspase activation
leading to death of the cell.
Figure 18.4 Caspase targets
Programmed Cell Death
Caspases are synthesized as inactive
precursors (procaspases) that convert to
active forms by proteolytic cleavage,
catalyzed by other caspases.
Initiator caspases are activated in response
to various signals. They then cleave and
activate effector caspases, which digest
the cellular target proteins.
Programmed Cell Death
ced-9 in C. elegans is closely related to
the mammalian gene bcl-2, which was
first identified as an oncogene.
Bcl-2 inhibits apoptosis. Cancer cells
are unable to undergo apoptosis.
Programmed Cell Death
Mammals encode about 20 proteins
related to Bcl-2, in three functional
groups.
Some inhibit apoptosis; others induce
caspase activation.
The fate of the cell is determined by the
balance of activity of proapoptotic and
antiapoptotic Bcl-2 family members.
Figure 18.5 The Bcl-2 family
Programmed Cell Death
Bax and Bak are the downstream
effectors that directly induce apoptosis.
They are inhibited by antiapoptotic Bcl-2.
BH3-only members are upstream; when
activated by cell death signals, they
antagonize the Bcl-2 family and
activate Bax and Bak.
Figure 18.6 Regulatory interactions between Bcl-2 family members
Programmed Cell Death
In mammal cells, Bcl-2 proteins act at
mitochondria, which play a central role
in controlling programmed cell death.
Bax and Bak induce release of
cytochrome c which triggers caspase
activation.
Programmed Cell Death
Caspase-9 is activated by forming a
complex with Apaf-1 and cytochrome c
in a complex called the apoptosome.
Under normal conditions, cytochrome c is
in the mitochondrial intermembrane
space; Apaf-1 and caspase-9 are in the
cytosol, so caspase-9 remains inactive.
Figure 18.7 The mitochondrial pathway of apoptosis (Part 1)
Figure 18.7 The mitochondrial pathway of apoptosis (Part 2)
Programmed Cell Death
Caspases are also regulated by the IAP
(inhibitor of apoptosis) family.
They either inhibit caspase activity or
target caspases for ubiquitylation and
degradation in a proteasome.
In Drosophila, initiator caspases are
chronically activated but held in check
by IAPs.
Figure 18.8 Regulation of caspases by IAPs in Drosophila
Programmed Cell Death
Regulation of programmed cell death is
mediated by integrated signaling
pathways; some induce cell death and
others promote cell survival.
Intrinsic pathways are activated by DNA
damage and other cell stress.
Extrinsic pathways are activated by
signals from other cells.
Programmed Cell Death
DNA damage can lead to cancer
development; it is a principal trigger of
programmed cell death.
A major pathway leading to cell cycle
arrest in response to DNA damage is
mediated by the transcription factor
p53.
Programmed Cell Death
ATM and Chk2 protein kinases
phosphorylate and stabilize p53,
resulting in rapid increases in p53
levels.
p53 then activates transcription of genes
encoding the proapoptotic BH3-only
proteins PUMA and Noxa, leading to
cell death.
Figure 18.9 Role of p53 in DNA damage-induced apoptosis
Programmed Cell Death
A major signaling pathway that promotes
cell survival is initiated by PI 3-kinase,
which phosphorylates PIP2 to form
PIP3, which activates the serine/
threonine kinase Akt.
Akt phosphorylates a number of proteins
that regulate apoptosis.
Programmed Cell Death
Phosphorylation of the BH3-only protein
Bad and FOXO transcription factors
maintain them in an inactive state.
In the absence of Akt signaling, Bad
promotes apoptosis and FOXO
stimulates transcription of another
proapoptotic BH3-only protein, Bim.
Figure 18.10 The PI 3-kinase pathway and cell survival
Programmed Cell Death
Extrinsic pathway:
Polypeptides in the tumor necrosis
factor (TNF) family are the signals.
Receptors activate an initiator caspase,
caspase-8.
Caspase-8 can cleave and activate
effector caspases and Bid, which leads
to activation of caspase-9.
Figure 18.11 Cell death receptors
Programmed Cell Death
Programmed cell death can also occur
by non-apoptotic mechanisms such as
autophagy.
In normal cells, autophagy is a
mechanism for gradual turnover of cell
components.
In starvation conditions, degradation of
components provides energy and
recycles materials.
Programmed Cell Death
Autophagy and apoptosis both eliminate
larval tissue in metamorphosis of
Drosophila.
Autophagic cell death does not require
caspases; dying cells are
characterized by an accumulation of
lysosomes.
Programmed Cell Death
Some forms of necrosis (necroptosis)
can be a programmed cell response to
stimuli such as infection or DNA
damage.
Regulated necrosis may provide an
alternative pathway of cell death if
apoptosis does not occur.
Programmed Cell Death
Stimulation of the TNF receptor leads to
cell death by necroptosis as well as
apoptosis.
Receptor interacting protein kinase-3
(RIPK3) is stimulated and MLKL is
phosphorylated.
Phosphorylated MLKL forms oligomers
that disrupt the plasma membrane,
causing cell death by necroptosis.
Figure 18.12 Necroptosis
Stem Cells and the Maintenance of Adult Tissues
In early development, cells proliferate
rapidly, then differentiate to form the
specialized cells of tissues and organs.
To maintain a constant number of cells
in adult tissues, cell death must be
balanced by cell proliferation.
Stem Cells and the Maintenance of Adult Tissues
Most differentiated cells in adult animals
are no longer capable of proliferation.
If these cells are lost they are replaced
by proliferation of cells derived from
self-renewing stem cells.
Stem Cells and the Maintenance of Adult Tissues
Some differentiated cells retain the
ability to proliferate as needed, to
repair damaged tissue throughout the
life of the organism.
Fibroblasts in connective tissue can
proliferate quickly in response to
platelet-derived growth factor (PDGF)
released at the site of a wound.
Figure 18.13 A skin fibroblast
Stem Cells and the Maintenance of Adult Tissues
Endothelial cells that line blood vessels
can proliferate to form new blood
vessels for repair and regrowth of
damaged tissue.
Proliferation is triggered by vascular
endothelial growth factor (VEGF),
which is produced by cells that lack
oxygen.
Figure 18.14 Endothelial cells
Figure 18.15 Proliferation of endothelial cells
Stem Cells and the Maintenance of Adult Tissues
The epithelial cells of some internal
organs are also able to proliferate to
replace damaged tissue.
Liver cells, normally arrested in the G0
phase of the cell cycle, are stimulated
to proliferate if large numbers of liver
cells are lost (e.g., by surgical
removal).
Figure 18.16 Liver regeneration
Stem Cells and the Maintenance of Adult Tissues
Stem cells are less differentiated, selfrenewing cells present in most adult
tissues.
They retain the capacity to proliferate
and replace differentiated cells
throughout the lifetime of an animal.
Stem Cells and the Maintenance of Adult Tissues
The key property of stem cells:
They divide to produce one daughter cell
that remains a stem cell and one
daughter cell that divides and
differentiates.
Stem cells are self-renewing and serve
as a source of differentiated cells
throughout life.
Figure 18.17 Stem cell proliferation
Stem Cells and the Maintenance of Adult Tissues
Many types of cells have short life spans
and must be continually replaced by
proliferation of stem cells:
• Blood cells, sperm, epithelial cells
of the skin and in the lining of the
digestive tract.
Stem Cells and the Maintenance of Adult Tissues
Hematopoietic (blood-forming) stem cells
were the first to be identified.
There are several types of blood cells
with specialized functions; all are
derived from the same population of
stem cells.
100 billion human blood cells are lost
every day and are continually produced
from stem cells in the bone marrow.
Figure 18.18 Formation of blood cells
Stem Cells and the Maintenance of Adult Tissues
Epithelial cells that line the intestines live
only a few days before they die by
apoptosis.
New cells are derived from the continuous
but slow division of stem cells at the
bottom of intestinal crypts.
New cells proliferate for three to four cell
divisions and then differentiate.
Figure 18.19 Renewal of the intestinal epithelium (Part 1)
Figure 18.19 Renewal of the intestinal epithelium (Part 2)
Figure 18.19 Renewal of the intestinal epithelium (Part 3)
Stem Cells and the Maintenance of Adult Tissues
Skin and hair are also renewed by stem
cells.
The epidermis, hair follicles, and
sebaceous glands are all maintained
by their own stem cells.
Figure 18.20 Stem cells of the skin (Part 1)
Figure 18.20 Stem cells of the skin (Part 2)
Stem Cells and the Maintenance of Adult Tissues
Stem cells also play a role in repair of
damaged tissue.
Skeletal muscle normally has little cell
turnover but can regenerate rapidly in
response to injury or exercise.
Satellite cells (stem cells of adult
muscle) are normally arrested in G0,
but proliferate in response to injury.
Figure 18.21 Muscle satellite cells (Part 1)
Figure 18.21 Muscle satellite cells (Part 2)
Stem Cells and the Maintenance of Adult Tissues
Most adult tissues have stem cells,
which reside in distinct
microenvironments or niches (e.g.,
intestinal crypts).
Niches provide the signals that maintain
stem cells throughout life and control
the balance between self-renewal and
differentiation.
Stem Cells and the Maintenance of Adult Tissues
Identification of stem cells and their
niches is a major challenge in stem cell
biology.
Stem cells in intestinal crypts were first
identified by Clevers et al. in 2007.
Wnt polypeptides secreted by adjacent
epithelial cells and fibroblasts control
proliferation of these stem cells.
Figure 18.22 The intestinal stem cell niche
Stem Cells and the Maintenance of Adult Tissues
Adult stem cells have potential utility in
clinical medicine.
In principal, stem cells could be used to
replace damaged tissue and treat a
variety of disorders, such as diabetes,
muscular dystrophy, Parkinson’s or
Alzheimer’s.
Stem Cells and the Maintenance of Adult Tissues
Hematopoietic stem cell
transplantation (bone marrow
transplantation) is already important
in the treatment of many cancers to
replace cells damaged by toxic
chemotherapy drugs.
Figure 18.23 Hematopoietic stem cell transplantation
Stem Cells and the Maintenance of Adult Tissues
Epithelial stem cells are used in the form
of skin grafts to treat burns, wounds,
and ulcers.
Epidermal skin cells can be cultured and
then transferred to the patient.
Because the patient’s own skin is
used, it eliminates the problem of
rejection by the immune system.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Embryonic stem cells can be grown
indefinitely as pure stem cell
populations that have pluripotency—
the capacity to develop into all of the
different types of cells in adult tissues.
Thus, there is enormous interest in
embryonic stem cells for both basic
research and clinical applications.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Embryonic stem cells were first cultured
from mouse embryos in 1981.
They can be propagated indefinitely and
are an important experimental tool:
• They can introduce altered genes into
mice.
• They provide a model system to study
cell differentiation.
Figure 18.24 Culture of mammalian embryonic stem cells
Key Experiment, Ch. 18, p. 713 (2)
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Human embryonic stem cell lines were
first established in 1998.
This raised the possibility of using
embryonic stem cells in clinical
transplantation therapies.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Mouse embryonic stem cells are grown
with growth factor LIF to maintain the
cells in the undifferentiated state.
If LIF is removed, the cells aggregate and
differentiate.
Stem cells will differentiate along specific
pathways if appropriate growth factors
are added.
Figure 18.25 Differentiation of embryonic stem cells
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Mouse and human stem cells have been
used to develop cardiomyocytes,
neurons, and insulin-producing
pancreatic β cells.
These cells have been used for
treatment of mouse models of various
diseases.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
In 1997, Ian Wilmut and colleagues
cloned Dolly the sheep.
Dolly arose by a process called somatic
cell nuclear transfer.
Other mammals have since been
cloned, but it is a difficult and inefficient
process.
Figure 18.26 Cloning by somatic cell nuclear transfer
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
In therapeutic cloning, a nucleus from
an adult human cell would be
transferred to an enucleated egg.
The resulting embryo could produce
differentiated cells for transplantation
therapy.
This would bypass the problem of tissue
rejection.
Figure 18.27 Therapeutic cloning (Part 1)
Figure 18.27 Therapeutic cloning (Part 2)
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Problems to be overcome:
• Low efficiency of generating
embryos by somatic cell nuclear
transfer (1 or 2% of embryos
survive).
• Ethical concerns with respect to the
possibility of cloning human beings
(reproductive cloning) and with
respect to the destruction of
embryos.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
These difficulties may be overcome by
using induced pluripotent stem cells:
reprogramming somatic cells to
resemble embryonic stem cells.
Only four transcription factors introduced
by retrovirus vectors are needed to
reprogram adult mouse somatic cells.
Figure 18.28 Induced pluripotent stem cells
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Adult human fibroblasts can also be
reprogrammed to pluripotency.
This provides a new route to the
derivation of pluripotent stem cells for
use in transplantation therapy.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Several combinations of transcription
factors have recently been shown to
induce pluripotency.
They activate a transcriptional program
that is also expressed in embryonic
cells.
Transcription factors Oct4, Sox2, and
Nanog play central roles in this program.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
These factors form an autoregulatory
loop, positively regulating each other’s
expression.
They also activate other genes that
maintain the pluripotent state, while
repressing genes that enable
differentiation.
Figure 18.29 Pluripotency transcriptional program
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
The positive autoregulation maintains
pluripotency while allowing the cells to
undergo differentiation.
This helps overcome major obstacles:
transcription factors originally used to
reprogram fibroblasts can act as
oncogenes, and the retroviral genes
can also cause mutations.
Pluripotent Stem Cells, Cellular Reprogramming,
and Regenerative Medicine
Transdifferentiation: reprogramming
somatic cells into other types of
differentiated cells (e.g., fibroblasts to
muscle cells or neurons).
This would bypass the need for
pluripotent stem cells.
Mouse fibroblasts have been turned into
heart muscle cells and nerve cells
using only three transcription factors.
Figure 18.30 Transdifferentiation