Lecture 1: Cell Death in Evolution and Development Flashcards
What are the functions of cell death?
• Infected cells. • Shaping of bodies. • Removing damaged/aged cells. • Preventing cancer. Normal regulated cell death is beneficial. Inappropriate activation can cause diseases like strokes and neurodegenerative disease.
What are caspases?
Caspases are cysteine proteases involved in apoptosis.
• Caspases cleave after asp residues.
• Caspases need to be activated by cleavage of pro-caspases. Some of them can self-cleave and autoactivated.
• There are two different types of caspase: initiator caspases and effector caspases. The distinction isn’t absolute.
• Initiator caspases can be activated in a variety of ways.
• Caspases can be activated intrinsically using triggers like DNA damage or they can be activated from outside the cell using extrinsic activation by ligands.
What is the role of caspase 6?
Caspase 6 is involved in axon pruning.
• Mammalian neurones initially make multiple axonal branches, which are then pruned to create a simpler structure.
• Pruning occurs by limited sub-cellular apoptosis.
• RNAi knockdown of caspase 6 results in retention of branches.
• Knockdown in caspase 3 results in survival of the cell body.
• Caspase 6 primarily targets cytoskeletal proteins such as microtubule-associated proteins.
• Inappropriate caspase 6 activity may cause Alzheimer disease neurodegeneration.
Why is C. elegans useful for studying cell death?
- Simple apoptotic mechanism. Only one caspase gene (mammals have 15).
- Dispensable. Mutants with no cell death are still viable.
- In vivo system. Don’t have to worry about tissue culture.
- During somatic development, 1090 cells are born and 131 die, to leave 959.
- For females, around half of the germline ovum cells are killed.
How do Ced genes work? How did we work this out?
Ced genes were discovered through mutation studies.
• Ced-1 gives a phenotype of persisting dead cells as unengulfed corpses. We mutagenize and screen for an absence of cell corpses. This gives the mutants ced-3 and ced-4.
• Ced-3 (null) mutants show organisms where all cell deaths fail to occur.
• Ced-4 (null) mutants show organisms where all deaths fail to occur as well. It encodes for an Apaf-1 homolog (Apoptosis activating factor).
• Ced-9 is a GOF mutation where all cell deaths fail to occur. It is dominant.
• A null mutation of Ced-9 means that most (probably all) cells die in the embryo).
• A ced-9(null); ced-3 (null) double mutant is viable. All cells survive.
• Ced-9 encodes for a homolog of Bcl-2.
• Ced-9 mutant shows that almost all cells are permanently poised for suicide.
• Ced-3 over expression can compensate for a lack of ced-4 but not vice versa.
• Ced-9 is regulated by egl-1.
• Egl-1 null mutations mean that all cell deaths fail to occur.
• Ced-9 binds to the Ced-4 dimer to stop it from working.
• EGL-1 displaces it.
• Ced-4 binds to Ced-3, which is lying dormant on the nuclear membrane. Ced-4 activates it.
How does tra-1 work?
Tra-1 is involved in opposite sex determination.
• Tra-1 activity directs female development.
• Tra-1 inactivity directs male development.
• A LOF tra-1 mutation will lead to male phenotype even if the genotype is XX.
• A GOF tra-1 mutation will lead to the female phenotype even if the genotype is XO.
• TRA-1A is a protein which has 5 zinc-finger motifs.
• It represses male-specific functions such as spermatogenesis.
• It activates female-specific function such as oogenesis.
How can egl-1 be controlled by Tra-1?
Egl-1 is involved in sexual dimorphism in HSN neurons.
• TRA-1 A represses egl-1 coding in WT hermaphrodites.
• In WT males, TRA-1A is absent. Egl-1 is transcribed and the HSNs die.
• In egl-1 GOF for hermaphrodites, the TRA-1A is active but a mutant site means that it cannot bind. Egl-1 is transcribed and HSNs die.
How does germline cell death occur? What happens if it is blocked?
Around 50% of all maturing female germ cells undergo apoptosis.
• After irradiation it works via the egl-1 pathway.
• X-rays activate CEP-1 (p53) which activates egl-1.
• It is unknown how it occurs homeostatically.
• However, if the apoptosis is blocked, it reduces the fertility of the organism.
• Ced-3 (caspase null) hermaphrodites have low fertility.
How does cell death occur in Drosophila? Give an example.
- There are several caspases, not just CED-3.
- They are either initiator (Dronc) or effector (Drice) caspases.
- Mitochondria don’t play a large role.
- An excess of progenitor cells is created and then they are killed by apoptosis. Failure of apoptosis means excess cells and overactive apoptosis means insufficient cells.
How is cell death regulated in Drosophila?
Major regulation occurs through DIAP (Drosophila inhibitors of apoptosis).
• IAPs are present in mammals, not C. elegans.
• They can bind directly to initiator and effector caspases.
• DIAP1 blocks the action of caspases and targets them for proteolysis.
• DIAP1 causes ubiquitin mediated proteolysis of Dronc.
• DIAPs are inhibited by pro-apoptotic factors like Grim, Reaper and Morgue. They are specific to insects. They use ubiquitin mediated proteolysis of DIAP1.
• Morgue may act directly as a ubiquitin conjugase.
• Reaper and grim are mainly controlled transcriptionally.
How can development be controlled independently of egl-1?
• However, developmental cell death can also occur independently of egl-1. Tail spike cell death is triggered by direct transcriptional upregulation of ced-3.
