Cell Death Flashcards
1
Q
Why do cells die?
A
- Remove non-functional/damaged cells
- Remove excess cells
- Severe, irreparable damage
2
Q
Why remove non-functional/damaged cells?
A
Quality control, prevent damage spreading, recycle components
3
Q
Why remove excess cells?
A
Homeostasis, development (hands/feet, neural circuits, immune system)
4
Q
Causes of death by severe, irreparable damage?
A
Injury, infection, cancer, loss of blood supply
5
Q
Describe programmed cell death
A
Physiological, regulated, beneficial to organism
Apoptosis or autophagy
6
Q
Describe necrosis
A
Pathophysiological, unregulated, detrimental to organism
7
Q
Describe necrosis
A
- Induced by insults to the cell that
cannot be repaired (overwhelm
homeostatic mechanisms): - Acute energy depletion
- Excessive ROS damage
- Extreme environmental
conditions - ATP-independent
- Cells swell, chromatin gets
digested, organelle membranes
disrupted - Cells lyse and spill contents into
surrounding area - Presence of hydrolytic enzymes
(from lysosomes) can damage
neighbouring cells - Leads to inflammation due to
release of proinflammatory
cytokines
8
Q
Describe apoptosis
A
- ATP-dependent
- Cells shrink
- The cytoskeleton collapses
- Organelles fragment
- Cell membrane blebs into
apoptotic bodies - Cell membrane becomes
chemically altered such that it is
recognised by neighbouring
cells and macrophages
9
Q
What are the triggers of apoptosis
A
Intrinsic: DNA damage, endoplasmic reticulum stess, reactive oxygen species overload, mictotubular alterations, mitotic defects
Extrinsic: pro-death signals from neigbouring cells, growth factor withdrawal, loss of pro-surviving signals from neighbouring cells
10
Q
Descrive the apoptosis intrinsic cascade
A
- Response to intracellular
damage/injury - Leads to changes in the activation of a
class of proteins – Bcl2 family
proteins - Some family members are proapoptotic, others are anti-apoptotic
- Exist in the cell as hetero-dimers in
various combinations
11
Q
Describe the intrinsic cascade in the absence and presence of an apoptotic signal
A
- In the absence of an apoptotic signal, the anti-apoptotic Bcl2 proteins bind to, and inhibit pro-apoptotic BH123proteins present on the surface of the outer mitochondrial membrane
- In the presence of a signal, proapoptotic BH3-only proteins are
activated, binding the anti-apoptotic
Bcl2 proteins - Can no longer bind the BH123
proteins, which aggregate and
activate each other - Mitochondrial outer membrane
permeabilization (MOMP) – ‘point ofno-return
12
Q
Describe the intrinsic cascade (cont)
A
- MOMP releases proteins from
intermembrane space into the
cytosol, including cytochrome c - This binds apoptotic protease
activating factor (Apaf1), causing
oligomerisation into the
Apoptosome - Allows recruitment and
activation of pro-caspase-9 - Initiates caspase cascade
13
Q
Describe caspases
A
- A specific class of proteases are
responsible for mediating apoptosis - They have a Cys residue in their active
site, and cleave targets at an Asp
residue – hence C-Asp-ases - Usually synthesised as pro-caspases,
which are inactive - Dimerisation leads directly to
cleavage - Leads to formation of large and small
subunits which re-associate to form
an active hetero-tetramer - Cleavage is mediated by upstream
(initiator) caspases (2,8,9,10), which
then activate executioner caspases
(3,6,7). - Allows amplification of initial signal
- IT IS IRREVERSIBLE
14
Q
Describe the apoptosis extrinsic cascade
A
- Certain pro-death extracellular signaling proteins (TNF, Fas) can bind TM
proteins (death receptors) - Promotes lipid raft fusion and large scale clustering of the death receptors
- A conformational change in the intracellular domains of the receptors reveals a
“death domain” - Able to recruit the DISC (Death Inducing Signalling Complex – containing FADD,
TRADD and a caspase, typically caspase 8/10) - Apoptosis can be initiated
- Absence of pro-survival extracellular signaling proteins (growth factors, Shh)
binding to TM proteins (dependence receptors) - Leading to caspase activation and initiation of apoptosis
15
Q
Describe the links between PCD pathways
A
- The BH3-only proteins Bid, Bim and Puma, can inhibit all Bcl-2 family
members and are thus the most potent activators of apoptosis - In some cells, the extrinsic pathway
(initiated at the PM) recruits the intrinsic
pathway to amplify the cascade - Caspase-8 cleaves a BH3 protein (Bid)
into a truncated form (t-Bid) - t-Bid is now able to bind to, and inhibit,
Bcl2 proteins - BH3-only proteins can also provide a link
between cell stimuli and apoptosis - Cells deprived of cell growth signals can
activate the stress activated MAP kinase,
Jnk, leading to the transcription of BH3-
only protein Bim
16
Q
Describe inhibitors of apoptosis
A
- IAPs - originally identified in insect viruses
(baculoviruses), preventing infected cells
from apoptosing - Now known that they are present in most
animal cells - IAPs can bind and inhibit activated
caspases - BUT – cells also possess anti-IAPs
- Located in the mitochondria
intermembrane space (like cytochrome c) - Upon activation of BH123 proteins, they
are released into the cytosol, binding to,
and inactivating the IAPs - Apoptosis can now proceed
17
Q
Describe survival factors
A
- Survival factors (e.g. growth
factors) bind to cell-surface
receptors, which activate
pathways that suppress
intrinsic apoptosis in one of 3
ways:
– Stimulating transcription
of anti-apoptotic Bcl-2
proteins
– Phosphorylation of proapoptotic proteins, such
as Bad, stopping their
binding and inhibition of
Bcl-2
– Phosphorylation and
inhibition of anti-IAPs
18
Q
Describe the cellular consequences of apoptosis in regards to the cytoskeleton
A
- Apoptotic blebbing requires
assembly of a cortical actin ring
that undergoes Myosin II-dependent
contraction - Brought about by caspase cleavage
of Rho Kinase (ROCK) - Intermediate filaments (e.g.
cytokeratin) cleaved by caspase - MTs are dismantled early in
apoptosis but re-appear as
extensive, novel arrays late - Possibly due to modifications to
tubulin itself (caspases can cleave
the C-terminus) - Together, this remodelling allows
packaging of material and its
membrane-bound fragmentation
into apoptotic bodies that can be
readily engulfed by phagocytes
19
Q
Describe the celullar consequences of apoptosis in terms of membranes
A
- Mitochondria fragment early in apoptosis
(before caspase activation) – promotes
cytochrome c release - ER membrane loses its tubulo-reticular
organisation and reforms into cortical sheets
and vesicles - Ca stores released, contributing to further
apoptotic events - Apoptotic cells fail to maintain the polarity of
phosphatidylserine in the plasma membrane,
allowing its exposure to the outside of the cell - This is the recognition signal for phagocytes
(‘eat-me’)
20
Q
Describe cellular consequences of apoptosis in terms of chromatin
A
- Chromatin undergoes a phase change
from a heterogeneous, genetically active
network to an inert, highly condensed
form - Lamin B is cleaved by caspases,
rendering the nucleus permeable to
proteins that are normally excluded - DNA is cleaved into fragments of distinct
sizes - DNA fragmentation factor (DFF) is an
endonuclease that cleaves between the
nucleosomes - Is present in normal cells as an inactive
heterodimer - Activation of caspase-3 causes cleavage
of one, and subsequent release of the
other, to cut the DNA
21
Q
Describe the key take-home messages of apoptosis
A
- Programmed (regulated) cell death is important for cell
physiology - Non-regulated cell death (necrosis) is a pathological process
- Apoptosis can be triggered by internal or external signals
- Apoptosis proceeds via a signalling cascade leading to
caspase activation - Apoptosis is tightly regulated to prevent initiation only under
the right conditions - Activation of caspases in apoptosis results in a host of cellular
changes to drive cell death
22
Q
Describe autophagy
A
- When organelles wear out, their
constituent parts are broken down
and recycled - Can also be used as a source of
energy for starving cells - Double membrane structures
(phagophores) containing TM Atg
proteins sequester cytosol and
organelles - Fuse and mature to form
autophagosomes - The recruitment of lipidated LC3
protein from the cytosol to the
autophagosome membrane allows
them to recognise and fuse with the
lysosome, where contents are
broken down by acid proteases
23
Q
Describe selective autophagy
A
- Selective autophagy: specific
autophagic degradation of an
organelle without cell death – e.g.
mitophagy, pexophagy, ERphagy, ribophagy, aggrephagy…
24
Q
How are dysfunctional mitochondria labelled?
A
Using PINK1 and Parkin, which ubiquitin then binds to
25
Q
Describe excess cell death and disease
A
- Excess cell death can lead to disease
- Many diseases or insults lead to necrosis (liver damage, heart disease, pathogens)
- Neurodegeneration:
– Too much cell death will lead to loss
of neuron connectivity
– Increased apoptosis and autophagy,
due to accumulation of aggregated,
insoluble protein fibres
26
Q
Describe issues with too little cell death
A
- Cancer:
– Lack of autophagy leads to
accumulation of faulty proteins, which
may trigger inappropriate/faulty cell
division
– Lack of apoptosis leads to too many
cells and tumour formation – cancer
cells escape normal control
mechanisms
– Bcl-2 mutations have been identified in
a common lymphocyte cancer in
humans. Over-production of Bcl-2
inhibits apoptosis
– Tumour suppressor p53 normally
promotes apoptosis in response to DNA
damage. Loss of p53 causes cancer
cells to survive and proliferate
27
Q
Describe the take home messages of autophagy and disease
A
- Autophagy recycles cellular components by a process of
‘self-eating’ - Selective autophagy (e.g. mitophagy) allows degradation
of damaged components without cell death - Dysregulation of cell death can lead to disease
