Cell Death and Cell Renewal 17. 17 Cell Death and Cell Renewal Chapter Outline Programmed Cell Death Stem Cells and the Maintenance of Adult Tissues.

Cell Death and Cell Renewal

17

17 Cell Death and Cell Renewal

Chapter Outline

Programmed Cell Death

Stem Cells and the Maintenance of Adult Tissues

Embryonic Stem Cells and Therapeutic Cloning

Introduction

Cell death and cell proliferation are balanced throughout the life of multicellular organisms.

Animal development involves not only cell proliferation and differentiation but also cell death.

Most cell death occurs by a normal physiological process of programmed cell death.

Introduction

In adult organisms, cell death must be balanced by cell renewal.

Most tissues contain stem cells that can replace cells that have been lost.


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.


During development, programmed cell death plays a key role by eliminating unwanted cells from a variety of tissues.


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 17.1 Apoptosis


Apoptotic cells and cell fragments are recognized and phagocytosed by macrophages and neighboring cells, and are rapidly removed from tissues.

Necrotic cells swell and lyse; the contents are released into the extracellular space and cause inflammation.


Apoptotic cells express “eat me” signals, such as phosphatidylserine.

In normal cells, phosphatidylserine is restricted to the inner leaflet of the plasma membrane.

Figure 17.2 Phagocytosis of apoptotic cells


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 17.1: Identification of Genes Required for Programmed Cell Death


The genes ced-3 and ced-4 were required for developmental cell death.

A third gene, ced-9, functioned as a negative regulator of apoptosis.

These genes are the central regulators and effectors of apoptosis that are highly conserved in evolution.

Figure 17.3 Programmed cell death in C. elegans


Ced-3 is the prototype of a family of proteases known as caspases.

Caspases have cysteine (C) residues at their active sites and cleave after aspartic acid (Asp) residues in their substrate proteins.


Caspases are the ultimate executioners of programmed cell death.

They bring about the events of apoptosis by cleaving 100 different cell target proteins.

The activation of an initiator caspase starts a chain reaction of caspase activation leading to death of the cell.

Figure 17.4 Caspase targets


Ced-4 and its mammalian homolog (Apaf-1) bind to caspases and promote their activation.

In mammalian cells, caspase-9 is activated by binding to Apaf-1 in a protein complex called the apoptosome.

Cytochrome c is also required, which is released from mitochondria.

Figure 17.5 Caspase activation


ced-9 in C. elegans is closely related to a mammalian gene called bcl-2, which was first identified as an oncogene.

Bcl-2 inhibits apoptosis. Cancer cells are unable to undergo apoptosis.


Mammalian cells encode about 20 proteins related to Bcl-2, in three functional groups.

Some inhibit apoptosis, while 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 17.6 The Bcl-2 family

Figure 17.7 Regulatory interactions between Bcl-2 family members


In mammalian cells, members of the Bcl-2 family act at the mitochondria, which play a central role in controlling programmed cell death.

Cytochrome c is released from mitochondria, which triggers caspase activation in the apoptosome.

Figure 17.8 The mitochondrial pathway of apoptosis


Caspases are also regulated by a family of proteins called the IAP (inhibitor of apoptosis).

They either inhibit caspase activity or target caspases for ubiquitination and degradation in the proteasome.

Figure 17.9 Regulation of caspases by IAPs in Drosophila


Regulation of programmed cell death is mediated by signaling pathways, some acting to induce cell death and others acting to promote cell survival.

Many forms of cell stress, such as DNA damage, can trigger programmed cell death.


A major pathway leading to cell cycle arrest in response to DNA damage is mediated by the transcription factor p53.

Activation of p53 due to DNA damage can also lead to apoptosis.

Figure 17.10 Role of p53 in DNA damage-induced apoptosis


A major intracellular signaling pathway that promotes cell survival is initiated by the enzyme PI 3-kinase, which activates Akt.

Akt then phosphorylates a number of proteins that regulate apoptosis.

Figure 17.11 The PI 3-kinase pathway and cell survival


Polypeptides in the tumor necrosis factor (TNF) family signal cell death by activating cell surface receptors.

These receptors directly activate a distinct initiator caspase, caspase-8.

Figure 17.12 Cell death receptors (Part 1)

Figure 17.12 Cell death receptors (Part 2)


Programmed cell death can also occur by non-apoptotic mechanisms such as autophagy.

In normal cells, autophagy provides a mechanism for gradual turnover of the cell’s components by uptake of proteins or organelles into vesicles that fuse with lysosomes.


Autophagy can also be an alternative to apoptosis as a pathway of cell death.

Autophagic cell death does not require caspases.

It can be activated by cellular stress and provide an alternative to apoptosis when apoptosis is blocked.


Some forms of necrosis can be a programmed cellular response to stimuli such as infection or DNA damage.

Regulated necrosis may provide an alternative pathway of cell death if apoptosis does not occur.


In early development, cells proliferate rapidly, then differentiate to form the specialized cells of adult tissues and organs.

To maintain a constant number of cells in adult tissues, cell death must be balanced by cell proliferation.


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.


Some types of 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 17.13 Skin fibroblasts


Endothelial cells that line blood vessels can proliferate to form new blood vessels for repair and regrowth of damaged tissue.

Figure 17.14 Endothelial cells


Endothelial cell proliferation is triggered by vascular endothelial growth factor (VEGF), which is produced by cells that lack oxygen.

Figure 17.15 Proliferation of endothelial cells


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 17.16 Liver regeneration


Stem cells are less differentiated, self-renewing cells present in most adult tissues.

They retain the capacity to proliferate and replace differentiated cells throughout the lifetime of an animal.


The key property of stem cells:

They divide to produce one daughter cell that remains a stem cell and one that divides and differentiates.

Figure 17.17 Stem cell proliferation


Many types of cells have short life spans and must be continually replaced by proliferation of stem cells:

These include: blood cells, sperm, and epithelial cells of the skin and lining the digestive tract.


Hematopoietic (blood-forming) stem cells were the first to be identified.

There are several distinct types of blood cells with specialized functions: erythrocytes, granulocytes, macrophages, platelets, and lymphocytes; all derived from the same population of stem cells.

Figure 17.18 Formation of blood cells


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.

Figure 17.19 Renewal of the intestinal epithelium (Part 1)




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 17.20 Stem cells of the skin


Stem cells also play a role in the repair of damaged tissue.

Skeletal muscle normally has little cell turnover, but it can regenerate rapidly in response to injury or exercise.

Regeneration is mediated by proliferation of satellite cells—the stem cells of adult muscle.

Figure 17.21 Muscle satellite cells


Most adult tissues have stem cells, which reside in distinct microenvironments or niches.

Niches provide the environmental signals that maintain stem cells throughout life and control the balance between self-renewal and differentiation.


Adult stem cells have potential utility in clinical medicine.

Hematopoietic stem cell transplantation (or bone marrow transplantation) plays an important role in the treatment of a variety of cancers.

Figure 17.22 Hematopoietic stem cell transplantation


Epithelial stem cells are also used in the form of skin grafts to treat burns, wounds, and ulcers.


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 science and clinical applications.


Embryonic stem cells were first cultured from mouse embryos in 1981.

Mouse embryonic stem cells are an important experimental tool:

• They can be used to introduce altered genes into mice.

• They provide an outstanding model system for studying the molecular and cellular events associated with cell differentiation.

Figure 17.23 Culture of mammalian embryonic stem cells

Key Experiment 17.2: Culture of Embryonic Stem Cells


Human embryonic stem cell lines were first established in 1998.

Clinical transplantation therapies based on embryonic stem cells may provide the best hope for treatment of diseases such as Parkinson’s and Alzheimer’s disease, diabetes, and spinal cord injuries.


Mouse embryonic stem cells are grown in the presence of growth factor LIF, which is required to maintain the cells in their undifferentiated state.

If LIF is removed, the cells aggregate and differentiate.

Stem cells can be directed to differentiate along specific pathways by the addition of appropriate growth factors.

Figure 17.24 Differentiation of embryonic stem cells


In 1997 Ian Wilmut and colleagues cloned Dolly the sheep.

Dolly arose by a process called somatic cell nuclear transfer.

This type of cloning in mammals is a difficult and inefficient process.

Figure 17.25 Cloning by somatic cell nuclear transfer


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 17.26 Therapeutic cloning


Problems to be overcome:

The low efficiency of generating embryos by somatic cell nuclear transfer.

Ethical concerns with respect to the possibility of cloning human beings (reproductive cloning), and with respect to the destruction of embryos.


These technical and ethical difficulties may be overcome by using induced pluripotent stem cells—reprogramming somatic cells to resemble embryonic stem cells.

The action of only four key transcription factors is sufficient to reprogram adult mouse somatic cells.

Figure 17.27 Induced pluripotent stem cells


Adult human fibroblasts can be reprogrammed to pluripotency by a similar procedure.

Although problems remain, induced pluripotent stem cells may someday be used for patient-specific transplantation therapy.

Cell Death and Cell Renewal 17. 17 Cell Death and Cell Renewal Chapter Outline Programmed Cell Death Stem Cells and the Maintenance of Adult Tissues.

Documents

cell renewal1717 cell

cell proliferation

cell fragments

accidental cell death

developmental cell death

programmed cell deathnecrosis

programmed cell deathcaspases

constant cell numbers