Big lessons from little worms

Apoptosis

worm Image from Mingxue Cui

The worms are transparent

Worms are transparent <1000 cells

One can follow the individual lineage of each of the cells

Deciphering the cell lineage of C. elegans

Deciphering the cell lineage of C. elegans

Cell death is also a “cell fate”

Programmed cell death

Specific cells are dying in a programmed manner

Programmed cell death (PCD)The mechanism: apoptosis

Also associated with the fight of our body against cancer tumors

Is this an active process?

We know there is apoptosis

We know it is important in development and possibly in cancer

We don’t know whether it is an active processWe don’t know of any genes involved

Let alone, a pathway (if exists)

What do we do?

Mutagenesis

Forward Genetics: random mutations

They found the guy who gets rid of the body

Next, they found the murderer

ced-1ced-3

ced-3 acts to promote cell death

Is it murder or suicide?

ced-1ced-3

Mutant analysisThe gene is mapped and sequenced

Next step: does it have human homologes or is this just a worm-specific pathway?

Is it a worm-specific gene or do we have human homologs?

Apoptosis is carried out by proteins. It is an active process, not a passive death

- The ced-3 gene encodes a protease, conserved in evolution

- It kills cells by degrading their proteins

Now we also know that human caspase is involved in apoptosis

A new mutation was isolated: ced-9

ced-9(-): ectopic cell death (cells that normally live, undergo apoptosis)

ced-1ced-3

More and more mutants are isolated

Epistasis analysis and biochemical studies allow the scientists to build a pathway

Genetic control of programmed cell death

CED-4No death signal

CED-4 is inactive

Death signal

CED-9

Activates CED-3 caspase for the killing

CED-9CED-4

ced-9 Cell deathced-3ced-4

Gain of function mutations in bcl-2 were associated with human cancers (bcl= b cell

lymphoma resulting from a translocation event)

Our body uses the same apoptotic mechanisms used in PCD, for killing “bad” cells

Identify mutants in worms (with the simplest assay) and find human homologs

Bob Horvitz

Lineage and programmed

cell deathBob Horvitz John Sulston Sydney Brenner

Physiology and Medicine 2002

Nobel Prize

- Pro-apoptotic signals open up the channels, allowing cyt C to be released to the cytoplasm

A simplified representation of apoptosis

NoxaProapoptotic(BH3-only)

Proapoptotic(multidomain)

Bax

Bcl-2

Cytochrome C

Apaf

Procaspase

Caspase

Mitochondria

All the regulators are from the same protein family (containing BH domains)

Antiapoptotic