Moss - Control of Cell Numbers and Cell Size Flashcards
what is apoptosis?
programmed “cell death”
what are the morphological characteristics of apoptotic cell death:
- cells shrink, condense, and round-up
- nuclear envelope disassembles
- chromatin condenses and breaks into fragments
- cytoskeleton collapses
- plasma membrane blebs and bulges, but remains intact
- cell corpse may break into fragments
- surface of corpse changes to be recognized by neighboring cells for phagocytosis
- they are engulfed by neighboring cells before the contents spill out
- apoptotic cell vanishes without a trace
TRUE or FALSE: the plasma membrane remains intact when it blebs and bulges during apoptotic cell death
TRUE
what are apoptotic bodies?
the fragments that a cell corpse may break into during apoptotic cell death
why does the surface of the cell corpse change to be recognized for neighboring cells?
for phagocytosis
what is the following cell?
a normal white blood cell has microvilli, which are membrane covered microfilaments, extensions of the cytoskeleton
what is the following cell?
an apoptotic white blood cell with blebs
what are blebs?
regions of the membrane that are detached from the cytoskeleton
seen during apoptosis
what do the arrows in the following picture indicate?
apoptotic cells that are shrunken with condensed cytoplasm. The nuclei are condensed and fragmented. Note the lack of inflammation
micrograph of mouse pancreas
what is the difference between an apoptotic cell and a necrotic cell?
apoptosis
- an active ATP-dependent process
- mutations prevent
- completely enclosed
necrosis
- mainly passive
- no mutation can prevent
- leaks
- inflammation
what is the intrinsic pathway?
cell decides to self-kill; death stimulus; radiation, hypoxia, etc.
what is the extrinsic pathway?
a cell (killing cell), decides to initiate the death cycle in another cell (dying cell)
what is common to both the intrinsic and extrinsic pathways?
caspase cascade
what is the caspase cascade?
the initiator caspase (caspase-8, made of dimers) is inactive and must be cleaved
once cleaved the pro-caspase is active
a second initiator caspase (caspase-9) can be cleaved
both pro-caspases interact with the executioner caspase (caspase-3) and are cleaved of nuclear components, cytoskeletal components etc.
this final cleavage causes morphological changes in the nucleus and plasma membrane
the cell is engulfed and phagocytosed by another cell
what is the amplification step of the caspase cascade?
interaction of the two pro-caspases with the executioner caspase
what is a caspase made of? How is it assembled/activated?
two inactive procaspase molecules are cleaved of their pro-domains and assembled into one active caspase molecule
what are caspases?
they are proteinases that have a cysteine at their active site and cleave their target proteins at specific aspartic acid residues
what does proteolytic cleavage do to procaspases?
proteolytic cleavage of a pro-caspase rearranges the protein to form the active site
what activates initiator and executioner caspases?
proteolytic cleavage
when are initiator caspases active?
when they dimerize
when are executioner caspases activated?
when they are cleaved by their initiator caspases
what kind of proteolytic cleavage leads to apoptosis?
one involving initiator and executioner caspases
what leads to the amplification of the initial apoptotic signal?
one initiator caspase activating many executioner caspases
TRUE or FALSE: One initiator caspase can activate many executioner caspases, leading to the amplification of the initial apoptotic signal
TRUE
what are BH3-only proteins? what do they do?
they are pro-apoptotic proteins
they act as sensors of a variety of apoptotic signals
they inhibit the anti-apoptotic Bcl2 proteins, allowing the pro-apoptotic effector proteins to cluster, which causes the release of cytochrome c
some may bind the effector proteins directly to stimulate aggregation
what do pro-apoptotic BH3-only proteins inhibit? what does this allow?
They inhibit the anti-apoptotic Bcl2 proteins, allowing the pro-apoptotic effector proteins to cluster, which causes the release of cytochrome c
what are anti-apoptotic Bcl2 family proteins?
they bind to and block the activation (by preventing oligomerization) of the pro-apoptotic Bcl2 family proteins (Bax and Bak)
they act to prevent pores that release cytochrome c from forming
where are active anti-apoptotic Bcl2 Family proteins (Bcl2 and BclXL) found?
they are located on the cytosolic surface of the outer mitochondrial membrane
how do active anti-apoptotic Bcl2 Family proteins (Bcl2 and BclXL) bind and block activation of Bax and Bak?
they prevent oligomerization
what does every cell need to survive?
at least one active anti-apoptotic Bcl2 protein
how do the Bcl2 family proteins regulate apoptosis?
by controlling mitochondrial membrane permeability and the release of cytochrome c and other intermembrane proteins into the cytosol
what do active pro-apoptotic effector Bcl2 proteins (Bax and Bak) do?
they aggregate on the outer mitochondrial membrane and cause intermembrane proteins to be released
what happens to a cell without effector Bcl2 proteins?
it cannot respond to an intrinsic apoptotic signal
what does the intrinsic pathway of apoptosis involve?
mitochondria, specifically the release of cytochrome c from the intermembrane space into the cytosol
how does the intrinsic pathway lead to the release of mitochondrial proteins into the cytosol?
diverse apoptotic stimuli, that are not death receptor-mediated, initiate signaling that activate the Bcl2-family proteins Bax and Bak.
cells change their inner mitochondrial membrane, and opening a pore, loss of the mitochondrial transmembrane potential, and release of the mitochondrial proteins into the cytosol
how can apoptotic cells that have released cytochrome c be differentiated from nonapoptotic cells?
by fluorescence microscopy
stain antibody to cytochrome c
stain the antibody to the mitochondrial ATP-synthase
merge
you can see whether the cell contains the cytochrome c or not
what happens once cytochrome c leaves the cell?
cytosolic cytochrome c binds an adaptor protein (Apaf1), which heptamerizes into an apoptosome
binding of cytochrome c causes the adaptor protein to unfold partly
unfolding of adaptor protein exposes a domain that interacts with the same domain in other activated protein molecules
what happens when cytochrome c binds to the adaptor protein?
the adaptor protein partly unfolds, exposing a domain that interacts with the same domain in other activated adaptor proteins
what happens after cytochrome c binds the adaptor proteins and it unfolds?
the adaptor protein binds pro-caspase-9, an initiator caspase, and forms a hexamer
what happens after the adaptor protein binds pro-caspase-9?
the multimerization of pro-caspase 9 leads to its proteolytic cleavage and activation
this allows it to cleave executioner caspases
what is Fas?
the death receptor on a dying cell
what is the Fas ligand?
the killing cell
what is the process of the extrinsic pathway?
Trimeric Fas ligand binds trimeric Fas
death domains on the cytosolic tails of Fas bind to the intracellular death domain adaptor protein (FADD)
FADD binds initiator caspases (primarily caspase-8), forming the death-inducing complex (DISC)
DISC allows dimerization and activation of initiator caspases
initiator caspases cleave their partners
caspases are released to activate executioner caspases
apoptosis is induced
what is FADD? what does it do?
it is an intracellular death domain adaptor that binds to the initiator caspases to form the death-inducing complex after the tails of Fas bind it
what does the DISC do?
it allows dimerization and activation of initiator caspases, which cleave their partners, are released to activate executioner caspases, and induce apoptosis
what are death receptors (in general)?
they are transmembrane proteins with an extracellular ligand-binding domain, a single transmembrane domain and an intracellular death domain
they belong to the tumor necrosis factor receptor (TNFR) family
their ligands belong to the TNF family of signaling proteins
both the death ligand and the death receptor are…..
trimeric
what happens when the death ligand binds the death receptor?
binding of the ligand alters the conformation of the receptor so that it binds a death domain adaptor protein (FADD), which then recruits and activates an initiator caspase to trigger the caspase cascade and cell death
what is ALPS (autoimmune lymphoproliferative syndrome)?
it is a disease associated with dominant mutations in the death receptor Fas that include point mutations and C-terminal truncations
in individuals that are heterozygous for these mutations, lymphocytes do not die at their normal rate and accumulate
what do the intrinsic and extrinsic pathway have in common?
they are different up until the caspase cascade.
what diseases have either too much or too little apoptosis as a major factor?
heart attacks and strokes
autoimmune disease
neurodegenerative diseases
cancer
how is apoptosis a factor in heart attacks and strokes?
many cells die by necrosis due to ischemia (lack of blood supply), but some also die by apoptosis
how is apoptosis a factor in autoimmune diseases?
caused by mutations in Fas death receptor or Fas ligand, which leads to overproduction of lymphocytes and a breakdown of self-tolerance
how is apoptosis a factor in neurodegenerative diseases?
Alzheimer’s, Parkinson’s, Huntington’s diseases, and ALS
cells die due to oxidative stress, perturbed calcium homeostasis, mitochondrial dysfunction
how is apoptosis a factor in cancer?
mutations in Bcl2 lead to B-cell lymphoma; p53 is mutated or inhibited in more than half of cancers, thus blocking the signal from DNA damage to the apoptotic pathway
what is ABT-737?
a drug that binds anti-apoptotic Bcl2 family proteins, like BclXL, preventing them from blocking apoptosis
The drug binds within a long hydrophobic pocket of BclXL which is where the prop-apoptotic BH3-only proteins bind
what do survival factors do?
block apoptosis
what do mitogens do?
promote cell entry into S phase typically by overcoming intrinsic inhibition of cell cycle
what do growth factors do?
stimulate growth in cell size and mass by promoting biosynthesis and inhibiting degradation
how do cells prevent their apoptosis?
Cells may compete for limited amounts of survival factors produced by other cells to prevent apoptosis
why do cells compete for survival factors to prevent apoptosis?
to control tissue development and maintenance
what happens when there is a normal overproduction of nerve cells?
nerves must compete for a limited supply of survival factors that are secreted by the target cells that they connect to
what happens to cells if they don’t receive survival factors?
they die
the nerve cells that die must have an active default death pathway that must be inhibited by the survival signal for them to survive
how do survival factors stop apoptosis?
they inhibit the default death pathway
what can extracellular survival factors do to Bcl2?
they may stimulate transcription of anti-apoptotic Bcl2 proteins
what is the difference between a mitogen and a growth factor?
mitogens control cell division
growth factors control cell growth
their pathways are the same
what are myoblasts/satellite cells in adult muscle?
muscle stem cells
why are satellite cells in muscle activated when you are at the gym?
to add new muscle fibers to the muscle as it is damaged during exercise
what is myostatin?
a TGF Beta family member
an extracellular signaling molecule that does the opposite of mitogen
it prevents the cell from dividing and becoming muscle (prevents over proliferation)
why do muscle fibers secrete myostatin?
to inhibit proliferation and differentiation of myoblasts
what occurs in mice lacking myostatin?
they have muscles 2-3 x normal size
what occurs in “double muscled” cattle?
they have mutations in the myostatin gene
what is the executioner caspase?
caspase 3
what are the initiator caspases?
caspase - 8 (extrinsic)
caspase - 9 (intrinsic)
