Stuck in FUS – the story of arginines, MND and FTD

In recent news, a number of press releases highlighted a paper published in the journal Cell, in which scientists, under the leadership of the University of Toronto’s Professor Peter St George-Hyslop, and in collaboration with University of Cambridge, described the process of how the FUS protein leads to the development of motor neurone disease (MND) and frontotemporal dementia (FTD).

MND and FTD – what is the connection?

We know that there is a link between MND and FTD, which in most part is caused by a mutation in the C9ORF72 gene, causing familial MND in around 35% cases and FTD in 25% of cases. Mistakes in the gene disrupt normal processes leading to toxic accumulation of TDP-43 protein in the neurons, and their subsequent death. There is however another protein toxic to neurons which results in the development of MND and FTD – the one that makes it slightly easier for us science writers to come up with witty titles: FUS (see one of our previous articles ‘What’s the FUS all about’).

Specifically, this gene mutation is found in about 4% of people with familial MND and around 1% with sporadic MND. The corrupted FUS gene leads to mistakes in the way FUS protein completes its main role; that is, helping to transport RNA and essential cargos along the cell (especially to the axon and dendrites – projections of the cell body). The FUS protein then gets trapped in the neuron where it forms aggregates. Interestingly, while in FTD this incorrect functioning of the FUS protein is still observed, it hasn’t yet been shown that this is due to a mutation in the FUS gene. (Read more about MND, FTD and FUS in a paper by Nolan et al. (2016).)

So what is the news?

All of the above information is a pretty established knowledge by now. So what did the new paper show? The main purpose of the study was to investigate the exact process by which the FUS protein creates clumps in the neurons and how this could potentially be prevented.

The FUS protein is able to undergo reversible state change, a so called ‘phase transition’– this allows switching from a dispersed liquid form to a droplet-like state or gel-like aggregates (also called phase separation – as the liquid separates into compartments) to accommodate to its surroundings.  This property is required for FUS to form temporary structures that take up, transport, and then release important cargos that control the efficient function and survival of synapses at the distant connecting end of neurons, the synapse.

In healthy cells, FUS can easily switch between the three states to fulfil its role to transport RNA in the solid form and release it as it turns into liquid. In neurodegenerative diseases, and specifically MND and FTD, the FUS protein can get stuck in the solid form, trapping the RNAs and form toxic aggregations within the cell. Understanding how this ‘reversible phase transition’ works is crucial to understand the disease and highlight potential therapeutic targets.

The researchers attributed the fault to an improper ‘methylation’ of the FUS protein. One of the properties of the FUS protein is that, in order to function properly, it attaches a chemical structure called the ‘methyl group’ to one of its amino acids – arginine (remember that proteins are composed of a number of amino acids that are dependent on the sequence of chemical bases in DNA). In normally-functioning FUS, the arginine amino acids making up the protein are heavily methylated.

In FTD however, the FUS protein is ‘hypomethylated’ (hypo = less than the norm) which leads to toxic accumulation of FUS in neurons. What is more, hypomethylation of only less than 5% of the FUS protein can result in triggering of the irreversible gel-like state. It has therefore been suggested that fluctuations in arginine methylation might be responsible for controlling the ability of FUS to be able to undergo reversible state change. So what can we do with these findings?

Using a number of experiments, the liquid/solid state of FUS was manipulated by using varying levels of salts. This led to the observation that reducing salt levels results in less dispersed state and increased phase separation (i.e. formation of droplets). The researchers could then observe the circumstances under which FUS will stay in the dispersed form even in the lowest salt levels. And indeed, when the affected arginine was replaced with other amino acids, or when other amino acids within FUS were reshuffled, the phase separation process was not observed (and the FUS protein stayed in the liquid form).

Next, the team investigated this using frog neurons to clearly visualise nerve axons and their endings, which is where ‘the real action’ occurs (i.e. where the electrical signal is transmitted from one neuron to another). The group mutated the FUS protein in the neurons by either reducing the number of building blocks of the protein that could be methylated, or by introducing changes in order to increase aggregation of the FUS protein.

The team demonstrated that a protein called Transportin1 acts as chaperone – assistant and guide – that prevents the FUS protein from aggregating in the neuronal nucleus. By tagging the transportin with a fluorescent marker, they could see its movement with the FUS protein attached along the axon. This showed the importance of Transportin1 to maintain the ability of FUS to transition normally and to potentially ‘rescue’ some of the aggregation of the FUS protein.

This important work shows that the protein Transportin1 and the methylation of arginine, building block of the FUS protein, play a vital role in preventing aggregation of the protein in MND and FTD. By identifying the specific changes that may stop the formation of the toxic aggregates in FUS, the race is now on to use enzymes that can modulate the amino acids in FUS and have a protective role for the protein (keep it from turning into the irreversible solid state).

How does this fall into the ‘finding a cure’ puzzle?

Year on year we see an increase in the number of significant findings that lead us closer to solving the MND cause/cure puzzle faster. The study described above is a great example of this; while it is not THE solution we are all hoping for, it provides a clearer picture of one segment of the whole MND jigsaw. Techniques that were used to find out how FUS behaves, how it can be manipulated and what can prevent its toxic aggregation, can be translated into studying different variations of MND, also caused by toxic aggregation of a protein (such as TDP-43 or SOD1).