I usually travel to London two to three times a month for meetings and lab visits. If I’ve got any length of spare time, I head for what I call my ‘London office’ – aka the British Library. It’s close to Euston station, it’s free (!) it has a nice café for informal meetings and it has copies of all the latest textbooks and major research journals.
The way in which a cell turns its genetic instructions into the protein ‘building blocks’ it needs to function and survive is sometimes compared to a library.
Our ‘cellular library’ is located in the nucleus of each and every cell in our bodies. The library bookshelves are the chromosomes upon which the books containing all the important information (the DNA) is written in the language we know as the genetic code. The human genome is sometimes called ‘the book of life’ for good reason.
However, the British Library isn’t a lending library. You aren’t allowed to take the books out of the building…..and there are a couple of burly looking gentlemen by the front door to provide additional discouragement.
Instead, you have to photocopy the articles you want and carry the copied sheets out. The cell’s equivalent of these photocopied sheets of genetic instructions are the lesser known RNA molecules, which are carried out of the nucleus to the parts of the cell where the ‘protein-making factories’ are found.
There are numerous proteins that act as couriers, ferrying RNA out of the nucleus, including the RNA-binding proteins TDP-43 and FUS, both of which are known to play an important role in motor neuron health.
Once these proteins have delivered their RNA to the right part of the cell, they head back to the nucleus for their next ‘pick up’.
Which brings us back to the British Library. The other day I had a couple of hours between meetings, so decided to pop in….. but found myself at the end of a long queue snaking across the concourse outside.
There was a new exhibition: Harry Potter: A History of Magic, which was proving popular with the tourists and our aforementioned burly doormen were busy doing bag checks on everyone going in (it is a major and busy public building, so can’t be too careful these days). But I wasn’t in particularly patient mood, so after 10 minutes of not moving, I gave up and headed for Costa Coffee instead (other coffee outlets are available).
Protein ‘mislocalisation’: what’s the FUS?
With ALS, the same problem seems to happen with TDP-43 and FUS in the motor neurons. They seem to struggle to get back into the nucleus and as a result seem to drift off to other parts of the neuron where they form clumps or ‘aggregates’. How and why this happens is not really understood.
Several presentations on the first day of the Symposium provided insight into what might be going wrong.
In the first biomedical session of the day, Prof Luc Dupuis (Strasbourg) showed that when human FUS mutations are introduced to mice, but missing a key component called the nuclear location sequence, the FUS protein mislocalises in exactly the same way as is found in human disease, leading to motor neuron death. He also found that parts of the mouse brain associated with frontotemporal dementia in humans was also affected, which may well explain some the changes in behaviour he observed in the mice.
He and his team demonstrated that the damage caused by mislocalised FUS wasn’t simply down to having less FUS protein to do the job, but was specifically linked to it being in the wrong place. This mouse model looks to be a very valuable tool in working out why this is happening and as a way of testing potential treatments.
Dr Dorothee Dormann (Munich) took us further into the mechanistic detail FUS, highlighting a particular process that seems to go wrong. She showed that another protein called ‘transportin’ which normally helps shepherd FUS back into the nucleus, struggles to do so if the FUS protein is misshapen (as happens in familial ALS cases where the FUS gene is mutated). It’s almost as if the transportin protein can’t recognise the FUS protein. Dr Dormann then went on to show that this lack of recognition and resultant aggregation of FUS protein is associated with the loss of a chemical process called arginine methylation.
Whilst FUS protein is clearly a major contributor to the disease process in the rare forms of familial ALS with FUS mutations, the importance of FUS protein in sporadic ALS is much less clear. It is a very different story with the TDP-43 protein, where cellular mis-accumulation and aggregation of the protein is a classical pathological hallmark in over 95% of all cases of ALS. It is hoped that better understanding of FUS mechanisms will improve understanding of TDP-43, given that both proteins seem to have similar roles.
C9orf72 joins the story
A number of talks discussed the detailed processes of how proteins shuttle in and out of the nucleus. Prof Ludo van den Bosch (Leuven) gave an excellent overview of what we know so far about these ‘nucleocytoplasmic transport defects’ and the stepwise order of events that seem to occur. He also introduced the most common known cause of ALS, the C9orf72 mutation, into the story, as this seems to have a powerful effect on the transport process.
Using C9orf72 models of ALS, Dr Jonathan Grima (Johns Hopkins) drilled down into the events going on at the ‘library door’.
The doorways in and out of the nucleus are structures known as nuclear pore complexes and there are around 2,000 different varieties, made up of different members of a family of proteins known as nucleoporins and – a bit like doors at Hogwarts – can move around, disappear or simply decide not to open!
Dr Grima showed that as we age, these nuclear pore complexes are not able to function quite as well as they should and this is exacerbated not only in C9orf72 ALS but also in sporadic ALS. He suggested that this may be a common defect, occurring early in the disease process.
Getting out of the library
Back at the British Library – when I was queuing to get in, I could see that the problem was being compounded by folk trying to get out of the building. Dr Grima showed some data indicating that the transport back into the nucleus might be altered by controlling the amount of transport activity going out of the nucleus.
This idea of export of molecules out of the nucleus was taken up by Dr Lydia Castelli (Sheffield) and colleagues, who wondered whether stopping the transport of C9orf72 RNA molecules out of the nucleus might limit the cellular damage. They have found that by reducing the activity of one particular protein called SRSF1, they can protect against the damaging effects caused by the C9orf72 mutation, as it seems to specifically stop the C9orf72 RNA molecules from leaving the nucleus. This work has so far only been carried out in a fruit fly model, but the team is planning to confirm this in other models.
The story continues….
There are still many unanswered questions about how (and when) nucleocytoplasmic transport deficits impact on motor neuron health and function, but given the fact that TDP-43, FUS and C9orf72 – three of the major known causes of ALS – are bound into the story, it does look like it plays an important pivotal role in the disease.
It would be tempting to talk about these as the three ‘deathly hallows’ of ALS, but that’s probably stretching the Harry Potter analogy too far…and anyway, this blog feels like it’s getting as long as the Deathly Hallows books.
Maybe I should have divided it into two parts…?