Module 11 - Programmed Cell Death

Question 1

regulated cell death/apoptosis

Answer 1

• normal part of the life cycle of a cell
• between 50 billion and 70 billion cells die each day due to apoptosis in the average human adult
• over the course of a year this is equivalent to the mass of an entire human
• beyond development, cells are always added to replace lost and dying cells
• a network of interacting proteins signals and regulates mitosis to ensure error free division
• number of cells in our body is at equilibrium
• cells are always being added to out bodies by cell division, but cells are also always taken away by the process of programmed cell death
• can generate too many cells (as in cancer) not only by too much cell division, but also by too little cell loss
• a network of proteins is required to signal and regulate apoptosis

Question 2

sculpting digits using apoptosis

Answer 2

• while development of a multicellular organism is associated. with high rates of cell division, cell death is equally important to many processes
• week 6 there is skin forming webbing between the digits both on the feet and the hand
• week 11, webbing in between the digits has disappeared and the digits have resolved
• in the absence of appropriate cell death, the webbing will persist in these digits

Question 3

TUNEL assay detecting DNA nicks

Answer 3

• sculpting of the digits by apoptosis is seen in developing mouse
• the assay takes advantage of the fact that the DNA in apoptotic cells contains nicks or DNA breaks
• dUPT can be incorporated into the nicks by the addition of an enzyme called terminal deoxynucleotidyl transferase

Question 4

apoptosis during frog metamorphosis

Answer 4

• as a tadpole undergoes metamorphisis to become a frog, the cells in the tadpole tail are induced to undergo apoptosis in the tail and is lost
• all the changes that occur during metamorphosis, including the induction of apoptosis in the tail, are stimulate by an increase in thyroid hormone in the blood
• a similar process of tail loss through apoptosis occurs during embryonic development in humans

Question 5

apoptosis during development of the nervous system

Answer 5

• normal brain development requires that as many as half of the neurons that are originally produced undergo apoptosis
• the cells that have not. achieved synaptic connections, or in which the connections are faulty, are going to undergo cell death
• here we have a collection of nerve cells that are originally produced in development through cell division and differentiation, but only some of these cells have made contact with a target cell
• the nerve cells which have not established appropriate connections undergo apoptosis
• this is an essential process of the development of the nervous system that matches the number of nerve cells with the number of target cells

Question 6

cellular degeneration

Answer 6

while cell death is part of the normal turnover of cells in our body, inappropriate cell death can also. lead to disease
-too much cell death may be the cause of collection of neurodegenerative disorder

Question 7

neural degeneration

Answer 7

• in Alzheimer’s disease, neurons in the hippocampus and certain regions of the cerebral cortex die
• in Huntington disease, dopamine neurons in the substantial nigra undergo apoptosis

Question 8

muscular degeneration

Answer 8

-musuclar degeneration such as in duchenne muscular distrophy is associated with inappropriate cell death

Question 9

necrosis

Answer 9

cells can. die through damage, some agent might damage the cell exterior, and the cell will undergo necrosis
-cells swell and release contents into surrounding tissues, this can lead to infection

Question 10

apoptosis or programmed cell death

Answer 10

• a regulated process
• cell commit suicide either in. response to stress or damage or as part of normal development
• disposal of cellular debris that does not damage the surrounding cells
• is contained and recycled

Question 11

apoptotic pathway

Answer 11

• has 3 steps
  1) cell execution
  2) engulfment
  3) clearance

Question 12

apoptotic cells

Answer 12

• at the cell surface, there are visible changes in cell shape
• the cell is shrinking and the membrane undergoes a process called blebing, in which protrusions or bulges appear
• inside the cell the mitochondria has lost its permeability, the nucleus is being broken down and DNA and proteins are being degraded
• the cell blebs at the surface shrink considerably in size and the cell breaks down into small contained fragments or vehicles of cellular debris
• this dynamic process not only kills the cell but also reduces all of the components into reusable materials

Question 13

ultrastructural features of apoptosis

Answer 13

• the chromatin is compact and becoming condensed
• the nuclear envelope breaks down, the contents of the nucleus are fragmented, the DNA is broken down and the proteins are degraded
• the cytoplasm also undergoes a process of condensation as cellular components aggregate
• the mitochondria becomes permeablized and mitochondrial proteins are released into the cytosol
• the cell membrane begins to. move and change shape, creating the blebs or protrusions.
• the cells fragment, creating compartment of the cell that contains the debris of the dead cell
• each of these little cell fragments will be phagocytized and the components will be recycled and rescued

Question 14

nematode for studying apoptosis

Answer 14

• 947 somatic cells as a result of cell division and apoptosis
• lineage of every cell has been traced back to the single fertilized zygotic cell
• 131 cells undergo programmed cell death or apoptosis

Question 15

screening for genes required for apoptosis

Answer 15

• developed an assay for identidying mutations in genes required for apoptosis required cell death genes
• fundamental to this assay was a mutation in a gene called ced-1
• a loss of function mutation of the ced-1 gene allows cells to undergo apoptosis, but the cells are not englufed by. phagocytosis
• genetic screen was developed to identify mutations in cell death genes that did not. create these apoptotic cells, cells that were visible in the ced-1 mutant background
• mutation in ced-1 and ced-3, no apoptotic cells observed
• ced-3 is said to be essential for apoptotic pathway as there is no. apoptosis in the absence of a functional ced-3 gene
• this has proven. to be a very valuable way for screening for mutations that affect apoptosis

Question 16

genes coding for essential apoptotic proteins

Answer 16

ced-3, ced-4, ced-9 and egl-1

• since the identification of these genes in c.elegans, the mammalian homologs have. been identified. for each of these genes
• 2 of the mammalian homologs have been implicated In the formation of human tumors as a result of the failure in the removal of cells thorough apoptosis

Question 17

apoptotic pathway in worms and mammals

Answer 17

• mammalian apoptotic pathway is seen on the right
• homologs of the Egl-1 protein are called Bid and Bim
• these are members of the BH3 family of proteins
• the BCL2 protein functions in a similar way to ced-9 and is found on the membrane of the mitochondria
• BCL-2 is controlling related proteins called bak and bax
• the CED-4 and CED-3 proteins form a complex called the caspase holoenzyme which is a protease that will target many different. proteins for degradation
• loss of function mutations in ced-4 and ced-3 prevent apoptosis
• in contrast, in a loss of function mutation in ced-9, all of the cells die
• ced-9 is an inhibitor of apoptosis that acts by inhibiting the activation of the caspase hologenzyme
• egl-1 is the signal for. apoptosis that activates. the pathway by inhibiting the inhibitor (ced-9)
• the caspase holoenzyme is called the apoptosome in the mammalian cell
• it contains. direct homologues of the c.elegans proteins Apaf -1 is similar to ced-4 and caspase 9 is similar to ced-3 as well as additional proteins including inhibitors and effectors
• in both cases, the protease activity of the caspase holoenzyme or the apoptosome leads to protein degradation and cell death

Question 18

activation of the caspase holoenzyme

Answer 18

in c.elegans, ced-9 inhibits apoptosis by binding to ced-4 dimers and keeping them inactive

• the addition of egl-1 and binding of egl-1 to ced-9, releases ced-4
• once the ced-4 is released, it joins with ced-3 to form the caspase holoenzyme
• activation of caspase leads to the degradation of cytosolic and nuclear proteins
• this is a good model for the formation of the apoptosome in mammalian cells

Question 19

role of mitochondria in apoptosis

Answer 19

the mammalian homologue of the ced-9 protein bcl-2, is anchored to the outer membrane of the mitochondria in the mammalian cell

• bcl-2 acts by altering the permeability of the outer mitochondrial membrane
• normally bcl-2 functions to maintain low permeability of the outer mitochondrial membrane
• inactivation of bcl-2 leads. to the formation of pores in the outer membrane and it is this permeabilization that is associated with apoptosis

Question 20

release of cytochrome C

Answer 20

• in a healthy cell, the outermembrane of the mitochondria carry the bcl-2 protein Ced-9
• bcl-2 inhibits apoptosis
• a loss of function mutation of ced-9 causes all cells in the c.elegans worm to die
• the apoptotic signal bad, is inactive while phosphorylated and bound to the cytosolic adapter protein 14-3-3
• signalling pathway lead to dephosphorylation of bad and release from 14-3-3
• this allows bad to bind to the mitochondrial associated bcl-2 protien
• another member of the ced-9 family is bax
• activation of bcl-2 leads to the activation of bad. through the organization of bad in the outer mitochondrial. membrane
• bax aggregates in. clusters in the membrane to create pores
• these pores increase membrane. permeability and release many mitochondrial proteins from the intermembrane space into the cytosol including a protein called cytochrome c
• cytochrome c is essential for the formation of the mammalian apoptosome
• both intrinsic cell damage including DNA damage and cellular stress and extrinsic signals can lead to this apoptotic pathway

Question 21

trophic factors

Answer 21

• inhibit apoptosis through bad
• while apoptotic signals promote bad dephosphorylation and apoptosis, trophic factors prevent apoptosis and keep the cell alive
• trophic factors initiate a kinase cascade that leads to phosphorylation of the bad protein
• however, once. those trophic factors are removed the bad protein can be phosphorylated and apoptosis can occur