Topic 11: Apoptosis Flashcards
What is the purpose of apoptosis?
remove diseased or damaged tissue
allows tissue remodeling and maintenance
must be tightly controlled
What are some general characteristics of apoptosis?
nucleus is dismantled (nuclear envelope vesiculates, nucleus shrinks)
DNA is degraded by DNAses into ~200bp fragments
cytoplasm shrinks
mitochondria breakdown and release cytochrome c
cells release ATP and UTP into extracellular to recruit macrophages
phagocytic cells secrete inhibitory cytokines to reduce inflammation
cell cytoskeleton dismantles (causes “blebbing”)
cell vesiculates (gets dismantled into vesicles)
What are the three main steps of apoptosis?
as a cell begins to undergo apoptosis, its chromosomes condense and its cytoplasm shrinks
eventually, the nucleus becomes fragmented, its DNA is digested at regular intervals (“laddering”), the cytoplasm becomes fragmented, and the cell extends numerous blobs
ultimately the remnants of the dead cell (apoptotic bodies) are ingested by phagocytic cells
What is phosphatidylserine (PS)?
normally on intracellular leaflet
PS flips to extracellular face and acts as an “eat me” signal
What is the balance of signals in controlling apoptosis?
the balance between loss of survival signals and reception of death signals (intracellular and extracellular)
How does the withdrawal of positive signals control apoptosis?
the continued survival of most cells requires that they receive continuous stimulation from other cells and, for many, continued adhesion to the surface on which they are growing
example: anchorage dependent growth (signals via integrins
they tell the cell it is healthy and to stay alive, therefore loss of this signal should trigger apoptosis
How does the receipt of negative signals control apoptosis?
internal signal: DNA damage or oxidative stress
external signal: FasL and TNF (tumor necrosis signal) trigger apoptosis via caspase 8
What is TNF?
TNF produced during inflammation can trigger TNFR
What is FasL?
FasL (Fas ligand) binds Fas receptors and triggers apoptosis
What are the three different mechanisms by which a cell commits suicide by apoptosis?
- signals arising from within the cell
- oxidative damage (cause accumulation of reactive oxygen species, ROS = do widescale cell damage)
- receipt of death signals
What are caspases?
a family of over a dozen caspases
they are all proteases
they get their name because they cleave proteins, mostly each other, at aspartic acid (Asp) residues
What is Caspase 9?
protease that cleaves over caspases
caspases are produced as inactive precursors (pro-proteins) that require proteolytic cleavage for activation
called the initiator caspase
What is Caspase 3?
called the effector caspase
activated by caspase 9
cleaves many downstream targets: rMLC (causes cell blebbing), GRASP6S (induce Golgi fragmentation), MST1 (promotes nuclear fragmentation), and others
cleaves adhesion proteins involved in cell-cell junctions
What is the role of the mitochondria in apoptosis?
highly metabolic, involves reactions involving oxygen, can create toxic oxygen species
stores cytochrome C (component of ETC), cyt. c is also a signaling molecule that activates caspase 9
What is apoptosis triggered by internal signals, the intrinsic or mitochondrial pathway?
in a healthy cell, the outer membranes of its mitochondria display the protein Bcl-2 on their surface, Bcl-2 inhibits apoptosis
Bcl-2 mediated internal damage to the cell
cell stress (from mitochondria and p53 that monitors genome health) can trigger downstream response
sensors of cell stress –| Bcl2 –| Bax –> cyt C
Bax is in mitochondria and responds to the loss of inhibition by Bcl-2, inserts into MOM allowing release of cyt C
Apaf-1: when activated by cyt C, polymerizes into apoptosome, “wheel of death”, apoptosome cleaves and activates caspase 9
What is apoptosis triggered by external signals, the extrinsic or death receptor pathway?
FasL and TNF –> death signals
receptors for death ligands (FasLR and TNFR) are constitutively expressed in all cells
binding of death signal activates caspase 8
active caspase 8 –> target caspase 3
active caspase 3 –> apoptosis proceeds in the same manner as intrinsic
What are the steps in the receipt of death signals pathway to apoptosis?
cytotoxic T cell produces death signals
FasL binds to FasLR
activates procaspase 8 which cleaves and activates caspase 3, which triggers apoptosis
What are the steps in the loss of survival signals pathway to apoptosis?
as sensors of cell stress increase, Bcl2 is inhibited
since Bcl2 levels decrease, levels of Bax increase
as Bax levels increase so does the level of cyt. C
Apaf-1 is activated by cytc. C and polymerizes into apoptosome (wheel of death)
the apoptosome activates caspase 9, which in turn activates caspase 3, which then causes apoptosis
What is the intrinsic pathway to apoptosis?
ROS damage to DNA
this signal is received by p53 (gossip queen), which senses many forms of cell damage
from here, the pathway follows the same steps as the loss of survival signals pathway
What is the Apoptosis-Inducing Factor (AIF)?
neurons, and perhaps other cells, have another way to self-destruct that, unlike the two paths described above, does not use caspases
AIF is a protein that is normally located in the intermembrane space of the mitochondria
when the cell receives a signal telling it that is is time to die, AIF is released into cytoplasm
caspase-independent
AID is released from mito and migrates to nucleus (bind and degrade DNA)
What is the relationship between apoptosis and cancer?
some viruses associated with cancers use tricks to prevent apoptosis of the cells they have transformed
defects in apoptotic pathways contribute to immortality of cancer cells
Bcl-2 is overexpressed in B cell lymphoma, gain of function = promoted cell survival by enhancing apoptotic inhibition
What are the ways cancer cells avoid apoptosis?
upregulating Bcl-2 (oncogene)
downregulating p53 (TSG), 50% of all cancers contain a p53 mutation, most frequent mutation in all cancers
decrease Apaf-1 expression (TSG)
secreting decoy death signals (bind and block TNFR and FasLR to prevent activation of receptor)
