Murder, Suicide, and Canibalism: the life of a cell

[Monique]

The title sounds strange, but that is what the life of a cell will eventually experience.

Cells from multicellular organisms are specialized in performing suicide, it is actually a very important mechanisms of survival of the organisms as a whole. The reason is, that programmed cell death, or apoptosis (from greek: popping off) is a very clean way for a cell to die.

A normal cell that is murdered by toxins will swell by the influx of water (osmosis) since the cell membranes become leaky. This cell will eventually explode and spill its contents over neighbouring cells, potentially causing a dangerous inflamation reaction.

A cell that performs apoptosis, will shrink and collaps on itself, since it is breaking down its cytoskeleton and concentrates its cytoplasm. The cell will form pockets that budd of the cell, these cellular vesicles will be eaten up (phagocytised) by neighbouring cells who will digest the cell and recycle it consituents.

So a cell who is killed will swell and explode, a cell who commits suicide and shrink, having an appearance as if it is boiling down to nothing. Other cells can be seen having another cell in its interior because of canibalism.

[FZ+]

Very intriguing.

Quick question: What process could have let to the evolution of such sophisticated mechanism of self-destruction? Any idea when these evolved?

[Monique]

My immediate guess would be to avoid extensive DNA damage. A cell that has a lot of damaged DNA can get out of control and start dividing rapidly without noticing the nearby cells which tell it to quite down. Such a cell is a cancer cell and will destroy the organism.

The cell has developed mechanism that detect the DNA damage and activate the process of apoptosis. Right now in all your cells there are many inactive forms of the protein needed to start the death-cycle, all they need is the go signal.

[kris]

Hi.

Apoptosis is not just a response to DNA damage (though clearly that is one stimulus).

In development, apoptosis is important in the shaping of limbs and digits (etc. etc.).
T cell attack of altered-self cells (eg. virus infected cells) induces apoptosis in those cells.
B-cells undergoing a process called somatic hypermutation (Activated B-cells gather in a germinal center and mutate their immunoglobulin (Ig or antibody) genes. A selection process occurs by which only B-cells making the highest affinity antibodies live, and the others die by apoptosis.

As for the evolution of the process, damaged (necrotized) cells are a good danger signal for the immune system and cause inflammation. This type of immunity long predates the evolution of the adaptive immune system (ie: B cells, T cells, etc.)
Apoptosis is a way of eliminating cells while not causing this type of inflammation. It didn't evolve to fix only one problem that life encounters, but as a general mechanism of taking out your trash... Which would have been necessary from the beginning.

Cheers!! :)
-K

[Monique]

Hi Kris! Welcome to the forum. But wouldn't you agree that it would be more likely for the mechanism to have evolved for one specific survival reason and then have evolved to branch out and include other pathways? For instance, you mention digit formation. This is very true, but that type of cell-cell communication mediated apoptosis must have evolved much later in the process of development.

That is why I thought that apoptosis as a response to DNA damage would be the basic underlying selective pressure.

[Monique]

I see you are a grad student working in the lab? :) What do you research, if I may ask?