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12/12/2005 3:36:03 PM
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12/7/2005 9:53:53 AM
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Tau
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Tau is a microtubule associated protein, predominantly found in the axons of neurons. Binding of tau to tubulin promotes microtubule stabilisation and polymerisation and functions to organise microtubules regulating axonal transport and enabling axonal elongation during neuronal differentiation. The transport system present in cells is analagous to a railway system, where microtubules are the railway tracks, and the interaction of tau allows them to form in the correct place, at the correct rate and at the correct time. Motor proteins such as kinesin then act as trains, to allow other cellular components to be transported around the cell appropriately.

In the adult human central nervous system, there are 6 different isoforms of tau, generated by the alternative splicing of a single gene located on chromosome 17. The isoforms differ by the presence of either zero, one or two repeats at its N-terminus and either three or four microtubule binding domains located at the C-terminus. The N-terminal repeats are generated by exons 2 and 3, and the fourth microtubule binding domain results from the inclusion of exon 10. Exon 3 is never included independently of exon 2, hence there are only six tau isoforms and not eight.

DIAGRAM HERE!

The interaction of tau with microtubules is controlled by its phosphorylation. Phosphorlation is a wide-spread mechanism that cells use to control the activity of proteins, and a certain amount of phosphorylation is necessary and entirely normal. There is negative correlation between tau phosphorylation and microtubule formation, i.e. the higher the phosphorylation state of tau the less stable microtubules are. In some cases the regulation of tau phosphorylation breaks down and tau becomes hyperphosphorylated. Hyperphosphorylated tau forms aggregates that are the hallmark of a range of neurodegenerative diseases known as the tauopathies, of which Alzheimers disease is the most well-characterised. In Alzheimers disease, aggregated hyperphosphorylated tau is mislocalised to the somatodendritic compartment of neurons forming neurofibrillary tangles.

Neurodegenerative diseases are often described as diseases of protein metabolism; however the importance of abnormal RNA processing in neurodegeneration is becoming increasingly clear. Research being carried out in the group of Dr. Jean-Marc Gallo is focused on the regulation of tau RNA processing in neurons and its impairment in neurodegeneration.

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