Wednesday, 13 June 2012

Caspase: the not so friendly protein

It’s somewhat of an antithesis to say that death is essential for life, however in multicellular organisms, such as humans, without cells dying we would not live. The topic of this week’s blog is therefore a fundamental phenomenon of all cells in our body termed apoptosis, or programmed cell death. As the name implies, this form of cell death is highly controlled and essentially runs a program to cause the death of the cell. Apoptosis is controlled by a set of proteins called caspases which drive the necessary changes to kill the cell.

Hand sculpting
So why do cells need to die? There are a lot of answers to that question but I will only look at a few of these. Firstly, right back when we are a foetus we all start with webbed hands and toes (see the image). However, I assume most people no longer have such webbing, a fact which all comes down to death of the excess cells between the digits. Another example of this sculpting process can be seen when a tadpole matures to a frog (the tail isn’t chopped off by a small frog knife).

Human embryo hand at 8 weeks

Two further roles for apoptosis can be seen during infection. Viruses enter cells and use them to produce many new viruses, causing damage to the host. One of the front line defences against this is to simply kill the infected cell, which blocks replication of the virus. Also when we are infected with any pathogen an immune response is produced. This response is controlled by white blood cells, our army against the invading infectious organism. When an organism enters the number of white blood cells rapidly increases, in a sense, the army begins conscription. When the infecting organism has been removed from the body all these excess cells must be removed otherwise they could cause autoimmune disease (where they attack healthy tissue causing disease), or could continue growing in number, causing a cancer. This removal all comes down to cell death.

The final example for the importance of apoptosis I would like to look at is in development of the brain. Our brains are initially made with a huge excess of neurones. Interesting these cells (like many others) are naturally programed to die; death is the default setting. The cells in the brain only survive if they are able to make contacts to other cells (forming synapses). The cells that connect in the brain survive and produce the brain of the foetus; those that make no contacts undergo their default setting and die, so as not to clog up the brain with excess, useless neurones. Around 50% of the initially produced neurones will die because of not making any contacts. Apoptosis is therefore an essential function for life, without it we would not progress beyond being an embryo.

So now the question to ask is what causes cells to die. As I mentioned, the key players of cell death are proteins called caspases. Caspases are expressed in all cells as zymogens, a fancy word for an inactive enzyme. These zymogens sit in the cell waiting for any time there is a trigger to activate them. This can be thought of almost like a light switch, everything is there waiting to be turned on, but it needs you to press the switch to get light. In the cell there are two main pathways that lead to activation of the caspases; intrinsic and extrinsic pathways. Intrinsic signalling relies on proteins in the cell which form pores in the mitochondria (the molecular machine that produces energy in our cells). These pores cause the release of two proteins, called cytochrome C and Apaf1, which together bind a certain caspase (caspase-9) and cause its activation in the ‘apoptosome.’ The extrinsic pathway relies on a receptor on the surface of the cell binding a signal (this is the case in the immune response for killing infected cells.) When the receptor engages with its target protein a platform is set up inside the cell called DISC (death inducing signalling complex) which binds capsase-8 and activates it. While both pathways activate different caspases the effect is the same; RIP cell.

Apoptotic cell blebs being engulfed by a macrophage
With the activation of caspases our poor cell is terminal. Many changes occur to the cell and these are all controlled by the activity of caspases. Firstly the cell begins to shrink as all the proteins that keep a cell at its usual size are broken up. Over time the DNA inside the cell is chopped up into tiny fragments, rendering the cell useless. Eventually parts of the cell start breaking off in small units called blebs, until at the end a big macrophage (an immune cell) comes along and eats up the cell. This may sound like a pretty nasty way to go, shrinking down, having the DNA chopped up and then being eaten by a giant, but this process protects the rest of the organism from damage. Other forms of cell death often involve the cell bursting which causes inflammation in the surrounding area and therefore more extensive damage. Apoptosis, on the other hand, is completely silent therefore providing the most effective and safest way to kill a cell.

It fascinates me how essential cell death is for life. Without death we could not be born; it’s that fundamental. We continue to need cell death right up to our own death as it can protect from cancer, aid in clearance of infection and may help to stop autoimmunity developing. However too much death is obviously not a good thing either and is seen in many neurological disorders, so it’s a fine balancing act between the good and the bad of cell death. It’s also interesting to think of cell death in an evolutionary sense. When life began it was only as a single, isolated cell whose only purpose was to survive. As life became more complex and organisms made of multiple cells emerged, the ability to kill off a few cells to protect the organism as a whole became a huge advantage. Perhaps the evolution of this self-sacrifice was an essential step in the development of complex life itself…

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