cell death and cell renewal.ppt [호환 모드]
TRANSCRIPT
Programmed Cell Death● Programmed cell death (apoptosis cell suicide) 기능● Programmed cell death (apoptosis, cell suicide) 기능
Defense – virus-infected cells undergo programmed cell deathcellular insults such as DNA damage induce programmed cell death
Development – elimination of unwanted cells- elimination of larval tissue during morphogenesisg p g- Neuron cells are produced in excess & cells in incorrect connection with target cells are eliminated- Many cells undergo programmed cell death unless they contact with neighboring cells or ECM
correctly or in the absence of appropriate growth factors
Cells that die as a result of acute injury swelland lyse, releasing their contents into theextracellular space and causing inflammation
● Th f A i● The events of Apoptosis
The chromatin decondenses
The nucleus breaks up
Ch l DNA i f t dChromosomal DNA is fragmented
The cell shrinks and breaks up into
membrane-enclosed fragments
(apoptotic bodies)(apoptotic bodies)
Apoptotic cells are efficiently recognized and removed by phagocytic cellsy p g y
The removal of apoptotic cells is mediated by the expression of so-called “eat me” signals on the cell surface
This signal include phosphatidylserine, which is normally restricted to the inner leaflet of the plasma membrane.
During apoptosis, phosphatidylserine become exposed on the cell surface (outer leaflet of the plasma membrane) by “scramblase” where it is recognized by receptors expressed by phagocytic cells
Caspases – the executers of apoptosis
Molecular mechanism of apoptosis – from studies of the development of nematode (C. elegans)
The nematode consists of 959 somatic cells – during normal nematode development, 131 cells are
eliminated by apoptosiseliminated by apoptosis
Mutants that affect apoptosis in nematode were isolated, ced-3, ced-4, ced-9
Ced-3 was first identified as a protease and is the prototype of a family of more than a dozen
proteases, known as caspases (cystein residues at their active sites – C, cleave after aspartic acid – Asp)
● Key targets of the caspases
a inhibitor of a DNase
nuclear lamins
cytoskeletal proteinscytoskeletal proteins
Caspases – act as either initiators or effectorsCaspases act as either initiators or effectors
Caspases are synthesized as inactive precursors and converted to active
forms by proteolytic cleavage, catalyzed by other caspases
The activation of an initiator caspase starts off a chain reaction leading to
activation of downstream effector caspases
The key initiator caspase – caspase-9
Apoptosome – Apaf-1 (ced-4), caspase-9, and cytochrome c
- cytochrome c is released from mitochondira by stimulicytochrome c is released from mitochondira by stimuli
that trigger apoptosis and activates apoptosome
- Then the activated caspase-9 cleaves and thus activates
d ff ddownstream effector caspases, caspase-3 and caspase-7
Central regulators of Apoptosis – Bcl-2 family
C. elegans Ced-9 = mammal Bcl-2, Bcl-2 was previously identified as an oncogene that contribute to B cell lymphomasBcl-2 was shown to inhibit apoptosis first identified connection between apoptosis and cancer
Bcl 2 family membersBcl-2 family membersAll members share conserved regions called Bcl-2 homology (BH) domain
Some are antiapoptoticOthers are proapoptoticOthers are proapoptotic
In normal cells, multidomain t ti t i l ithproapoptotic proteins complex with
antiapoptotic proteins
In response to apoptotic signals, BH3-l t ti t i ti t donly proapoptotic proteins are activated
and bind to antiapoptotic proteins, releasing multiapoptotic proteins
In mammals, regulatory apoptotic proteins act at mitochondria
1. When activated, Bax and Bak (multiproapoptic proteins)
form oligomers in the mitochondrial outer membrane
(either form pores or interact other mitochondrial outer ( p
membrane proteins), leading to release of cytochrome c
from mitochondria
2. Other mitochondrial proteins that promote apoptosis
Smac/Diablo and Omi/Htr2 are also released via Bax
and Bak oligomers and inhibits IAPs
- IAP (inhibitors of apoptosis): Members of IAP family
directly interact with caspases and suppress apoptosis y p pp p p
either by inhibiting caspase activity or by targeting
caspases for ubiquitination and degradation in the
proteasomeproteasome
Signaling pathways that regulate apoptosis
p53
One important role of apoptosis is the elimination of damaged One important role of apoptosis is the elimination of damaged
cells such as DNA damage by UV light
DNA d i d ATM d CHK t i ki hi h iDNA damage induces ATM and CHK protein kinases, which in
turn phosphorylate and stabilize p53
p53 either arrests cells by activating p21 (CKI) expression or
induces apoptosis
p53 activates the expression of BH3-only proapoptotic p p y p p p
proteins, PUMA or Noxa, leading to apoptosis
Tumor necrosis factor (TNF)
Secreted polypeptides that signal cell death
O f th b t h t i d TNF t i FOne of the best characterized TNF receptors is Fas
TNF-Fas plays important roles in immune system
- Killing target cells such as cancer cells, virus-infected cells, or
excess lymphocytes at the end of an immune response
TNF and related family members contain the cytoplasmic y y p
domain that adaptor molecules bind.
The adaptors recruit and activate caspase-8 (an initiator caspase)
Caspase 8 either activates other caspases ( 3 and 7) or cleaveCaspase-8 either activates other caspases (-3 and-7) or cleave
the proapoptotic BH3-only protein Bid, leading to Bid activation,
permeabilization of mitochondria, and activation of caspase-9
Signaling pathways that promote cell survival
A half of neuron cells die by apoptosis with the survivors having received sufficient amounts of survivalA half of neuron cells die by apoptosis, with the survivors having received sufficient amounts of survival signals from their target cells – NGF (nerve growth factor)
One of the major intracellular signaling pathways for cell survival is initiated by PI 3-kinase, leading to activation of Aktactivation of Akt
- Akt phosphorylates Bad (proapoptotic protein) 14-3-3 chaperone binds and sequester Bad
Akt inhibits FOXO which is a transcription factor to stimulate transcription of proapoptotic genes- Akt inhibits FOXO, which is a transcription factor to stimulate transcription of proapoptotic genes including Bim
Stem cells and the maintenance of adult tissue
Proliferation of differentiated cells
Fully differentiated that remains capable of proliferateFully differentiated that remains capable of proliferate
Skin fibroblast (dispersed in connective tissues where they secrete
collagen)
N ll t d i G- Normally arrested in G0
- rapidly proliferate if needed to repair damage
PDGF from blood platelets activates a receptor protein-tyrosine
kinase, stimulating both their proliferation and migration
The endothelial cells that line blood vessels
-VEGF (vascular endothelial growth factor) triggers endothelial cell
proliferation and the resulting formation of blood capillaries
The prod ction of VEGF is in t rn triggered b a lack of o gen- The production of VEGF is in turn triggered by a lack of oxygen
The epithelial cells of liver and pancreas are able to proliferate to
replace damaged tissue
Stem cells
Most fully differentiated cells in adult animals are no longer capable of cell divisionA subpopulation of less differentiated self-renewing cells called stem cells proliferate and replace the loss of differentiated cells
The key property of stem cells is that they divide to produce one daughter cell that remains a stem cell and one that divides and differentiates
Blood cells – their life spans are limited ranging from less than a day to a few monthsAll are derived from the same population of hematopoietic stem cells in bone marrow
The epithelial cells lining intestine- These cells are exposed to harsh environment and have a lifetime of only a few days
Stem cells reside at the bottom of intestinal crypts- Stem cells reside at the bottom of intestinal crypts- Stem cells give rise to a population of transit-amplifying cells- The transit-amplifying cells proliferate for 3-4 divisions and then differentiate into the three types of the colon surface epithelium (an absorptive epithelial cells and two secretory cells)
The epithelial cells of the skinThese cells are exposed to a harsh external- These cells are exposed to a harsh external
environment - The epidermis turns over every two weeks- The epidermis is a multi-layered epithelium, which p y pis maintained by epidermal stem cells resining in a single basal layer- The epidermal stem cells give rise to transit-amplifying cells, which undergo 3-6 divisions before differentiating and moving outward to the surface of the skin
The satellite cells of the Skeletal muscle cells- Skeletal muscle is composed of large multinucleated cells formed by cell fusion during y gdevelopment- Satellite cells-the stem cells of the muscle-are located beneath the basal lamina of muscle fibers
h ll i d i h- They are normally quiescent, arrested in the G0
phase, but are activated to proliferate in response to injury or exercise- Once activated they give rise to progeny thatOnce activated, they give rise to progeny that undergo several divisions and then differentiate and fuse to form new muscle fibers
Adult stem cells, embryonic stem cells and their medical applications
Adult stem cells (성체줄기세포)
Bone marrow transplantation
- The hematopoietic stem cells are among the most rapidly dividing cells of the body, so the toxic
effects of anticancer drugs on theses cells limit the effectiveness of chemotherapy in cancer treatmenteffects of anticancer drugs on theses cells limit the effectiveness of chemotherapy in cancer treatment
- The patient is treated with high doses of chemotherapy, which kill both tumor cells and the
hematopoietic cells
- This damage is then repaired by transplantation of new hematopoietic stem cells, which are obtainedThis damage is then repaired by transplantation of new hematopoietic stem cells, which are obtained
from the patient himself prior to chemotherapy or from a healthy donor
Epithelial stem cells
- A clinical application in the form of skin grafts that are used to treat patients with burns or woundspp g p
- The epidermal skin cells are in vitro cultured, made to form an epithelial sheet, and then transferred
to the patient
- This eliminates the potential complication of graft rejection by the immune systemp p g j y y
Difficulties in the use of the adult stem cells
- Isolation and culturing the appropriate stem cell populations are difficult
- Stem cells have not yet been identified in many adult tissues
Embryonic stem cells (배아줄기세포)
Embryonic stem cells can be propagate indefinitely in culture
and retain the capacity to develop into all of the different types
of cells
Mouse embryonic stem cells are grown in the presence of a
growth factor called LIF (for leukemia inhibitory factor), which
signals through the JAK/STAT pathway and is required to
maintain these cells in their undifferentiated state.
-If LIF is removed, the cells aggregate into structure that
resemble embryos and then differentiate into a wide range of
cells
- A great deal of current research is focused on the
development of culture conditions to promote the
differentiation of embryonic stem cells along specific pathways,
thereby producing populations of differentiated cells that can
be used for transplantation therapy
Somatic cell nuclear transfer
Cloning of an animal by somatic cell nuclear transfer
In 1997, Dolly arose from the nucleus of a mammary epithelial
cell that was transplanted into an unfertilized egg in place of the
normal egg nucleus – a process called somatic cell nuclear
transfer
(In the 1950s, this type of experiment was first carried out in
frogs)
Si h i i i l f f l i f d l i llSince the initial success, transfer of nuclei from adult somatic cells
into enucleated eggs has been used to create cloned offspring of
a variety of animals
O l 1 2% f b ll i i t li ff i- Only 1-2% of embryos generally give rise to live offspring
Therapeutic cloning (환자맞춤형줄기세포)
- A nucleus from an adult human cell is transferred to an
enucleated egg which is then used to produce an early embryoenucleated egg, which is then used to produce an early embryo
in culture
- Embryonic stem cells are then cultured form the cloned embryo
and used to generate appropriate types of differentiated cells forand used to generate appropriate types of differentiated cells for
transplantation therapy
- The major advantage is that the embryonic stem cells derived
by this procedure would be genetically identical to the recipientby this procedure would be genetically identical to the recipient
of the transplant – this bypasses the barrier of the immune
system in rejecting the transplanted tissue
The application of therapeutic cloning combined with gene transfer – correction of an inherited e app cat o o t e apeut c c o g co b ed t ge e t a s e co ect o o a e ted
immunodeficiency, the genetic defect from the absence of a functional gene encoding the RAG2
N l i f fib bl t f RAG2 d fi i t i t f d t l t d hi hNuclei from fibroblasts of RAG2-deficient mice were transferred to an enucleated egg, which was
used as a source of embryonic stem cells. A functional RAG2 was then introduced into embryonic
stem cells in culture, correcting the genetic defect. These corrected embryonic stem cells were
tehn differentiated into hematopoietic stem cells, and transplanted into RAG2-deficient mice,
resulting in rescue of the immunodeficiency at least partially