Strategies of Intracellular Parasites

Intracellular parasites have a difficult life. On the one hand, they need to utilise the resources of the host cell, which will ultimately cause it damage, however on the other they must necessarily keep the host cell alive so that they can live off it. This is particularly difficult when your host cell is a eukaryote cell in a multicellular organism, because multicellular organisms can't afford to have one cell behaving oddly. If the cell starts to notice any differences in its behaviour, it promptly commits suicide, meaning that in order for the parasite to survive it has to find ways of preventing the cell from killing itself when it notices things going wrong.

Chlamydia is a particularly well studied human intracellular bacteria, and is best known for being sexually transmitted and featuring in posters on student welfare notice boards in probably every university in the UK:

If it's worth talking about, it's worth blogging about!

The reason Chlamydia is so worth talking about is two-fold, firstly because it's a bacterium rather than a virus and can therefore be (relatively) easily treated with antibiotics. Secondly, because you don't always know you have it and therefore it's worth getting tested even if you don't appear to have any symptoms. The bacteria can live perfectly happily within your cells replicating away without the body noticing, and if left untreated can lead to quite serious problems, such as blindness, pelvic inflammation or sterility.

In order to remain replicating inside the cells unnoticed for so long, Chlamydia have to prevent the cells from either destroying them or committing suicide before they manage to replicate. One of the ways they can do this is by degrading the cell proteins involved in cell signalling pathways. An example is the serine protease (i.e an enzyme that breaks down proteins) CPAF which is secreted by the infective chlamydia particles and (among other things) breaks down the protein HIF-1 which is used to trigger the cell suicide response to low oxygen levels. They can also break down proteins which would potentially be involved in the immune responses to the damaged cell, such as NF-KB, which helps activate the innate immune system inflammatory response.

Quite how the chlamydia causes the host cell protein degradation is still a little unclear. They may use the common viral strategy of modifying proteins to make them more susceptible to being picked up by the cellular degradation machinery (although there is no biochemical evidence for this as yet) or alternatively it might activate protein degradation pathways that are usually silenced in uninfected cells. Analysis of the chlamydia genome shows several predicted proteases, so it may be possible that rather than using host proteases the bacteria is degrading specific proteins with its own protease enzymes. The bacterial protease CPAF (mentioned above) has been crystallised and the crystal structure shows the potential for several different substrates (i.e it could potentially degrade many different proteins) so it might play in important part in this process.

The use of degradation proteins also creates a potential target for therapeutics. If that sentence sounds familiar it's because I've written it several times before and will probably write it several times in the future. "New targets for therapeutics" is pretty much THE standard excuse for studying anything related to bacteria. From a more selfish and less funding-motivated standpoint, it also provides exciting new information about how bacterial and eukaryotic cells interact, and how parasites that live inside cells can control host signalling pathways to their own advantage.

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Zhong G (2009). Killing me softly: chlamydial use of proteolysis for evading host defenses. Trends in microbiology, 17 (10), 467-74 PMID: 19765998

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