Snapshot Issue 58 August 2009
All organisms need other organisms to survive. Flowers need bees. Frogs need flies. Humans need fruit. And viruses need us. As an example, poliovirus invades human cells where it can use what it needs to replicate and multiply. In doing so, the virus not only damages the host cell but also interferes with its activity. However, before a virus stands a chance of invading a cell, let alone propagating inside it, something has to let it in. For poliovirus, that something is a protein receptor, known as the poliovirus receptor. These receptors are found on the surface of cells and are specifically recognised by poliovirus, which docks to them and then finds a way to wriggle inside the cell.
Once an organism has been infected, poliovirus multiplies in the epithelial cells of the intestine, from where it spills into the bloodstream and finds its way into the host’s central nervous system, where it invades the nerve cells that control our muscles. And because its sole aim in life is to multiply, it will do so, thereby killing off the muscle cells one by one and subsequently causing muscle paralysis and the crippled postures many polio victims have to cope with for a lifetime.
Viruses are hugely imaginative and have devised cunning strategies to invade cells. Poliovirus uses a protein receptor, known as the ‘poliovirus receptor’. The poliovirus receptor is found in different types of cells and is usually involved in the immune response. Poliovirus, however, has devised a way to outsmart the receptor and uses it to dock itself to the cell, thanks to a specific domain on the receptor which slips into the small ‘canyons’ on the virus’s capsid. Once bound, poliovirus can either enter the cell as a whole, or inject its DNA into the cell’s cytoplasm. Both actions result in viral propagation.
Thanks to poliovirus, much about the mechanisms involved in viral infection has been understood, which makes it all the easier to develop vaccines. Today though, scientists are worried that the eradication of polio – which dates back to the 1960s – may stunt further polio-related research because of the loss of funding. This would be a shame both for fundamental research and for polio victims for whom it has taken a lifetime to accept their lame limbs and who – decades after their first encounter with the virus – are reminded of its ugliness, with the development of novel symptoms such as joint pains, exhaustion and muscular waste. Ailments which have been neatly stashed into a bag called ‘post polio syndrome’ and which, for polio victims, just feels like a stab in the back.
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