A recent gene finding suggests that recycling within our cells is key to all forms of MND. This story captivated many people affected by MND and our blog broke its previous record for the number of hits in one week at over 4,000. It was also linked to, as a reliable and informative piece, from a number of worldwide MND/ALS Associations and forums.
Due to the popularity of this story, Prof John Mayer from University of Nottingham will be taking you on a whirlwind tour of the recycling process within our cells. Prof Mayer is currently the chair of our Biomedical Research Advisory Panel, which ensures that we fund the most promising laboratory based research projects to investigate the causes, develop treatments and find markers of disease progression.
He’s also been pioneering the investigation of the recycling process within our cells to learn more about neurodegenerative diseases such as MND for the past 24 years. Below, Prof Mayer explains more about how he’s been involved with this story, and where this could lead us in the future:
Twenty four years ago, Prof Jim Lowe and I discovered that pile-ups of proteins in neurones in MND, and other chronic neurodegenerative diseases eg Parkinson’s disease and Alzheimer’s disease, contained ‘tagged’ proteins. We’ve been trying to understand why ever since!
We did it by detecting those tags, and ‘staining’ tissue sections from the brains and spines of patients who had died of MND in Derby and Nottingham to see if we could ‘see’ those piles of proteins in surviving neurones. We did!
From that day on, we knew that the protein recycling system must be deeply involved in neurological disease and that the system must fail or be overwhelmed in the neurones of people with MND.
How it works
Proteins can be thought of as the building blocks of our cells and all proteins are made and broken down continuously –this is called protein turnover. It is essential because faulty proteins can be made or proteins can be damaged in each cell, including neurones. Protein turnover in neurones is vital because the vast majority do not divide – once these cells are laid down we are stuck with them. Any problem protein in a neurone must be removed or it may die.
Proteins are ‘tagged’ for removal by chemically attaching a small protein to them called ubiquitin – actually chains of ubiquitins all linked to one another to create a long ‘tag’ which will be easily ‘seen’ by the machine that will destroy the tagged protein.
The machine is an enormous entity in the cell called the 26S proteasome. The tagged proteins have to be fed into large caverns in the middle of the machine for destruction, with the tags removed first to be used again. The mechanism is called the Ubiqutin Proteosome System of protein destruction in the cell, the UPS for short (and not to be confused with the ‘logistics’ company!).
It just would not do if proteins could be destroyed anywhere in the cell, like by those proteases in biological washing powders, the cell proteins would always be at risk of degradation. So, the destructive sites are hidden inside the proteasome machine, the proteins are tagged and they’re fed inside!
There are also a group of cousins of ubiquitin that transfer the tagged proteins to the proteaseome machine. These proteins have a ‘docking site’ for tags at one end and a different tag at the other end which docks to special sites on the proteasome machine. The transfer proteins seek and find tagged proteins and take them to the proteasome machine where they dock and the tagged proteins are fed in for destruction after removal of the tags.
Ubiquilin 2 is one of the carrier proteins which, when made with errors, has been found to cause MND.
Medical science is most comfortable when there is genetic proof of the importance of a process – the discovery of mistakes in ubiquilin 2 has now done this for us!
Mimicking MND by deleting ‘machine’ genes
We have used modern gene targeting in mice to demonstrate that if we deliberately deleted a gene for one of those proteins in the 26S proteasome machine conditionally in neurones in the brain, so we would not cause problems anywhere else in the body, we could ‘mimic’ different neurological diseases.
The way we did it was to target the neurones that die in Parkinson’s disease and the neurones that die in the second most common cause of dementia after Alzheimer’s disease – dementia with Lewy bodies. This was published in 2008 and our genetic approach worked!
By depleting one of the 26S proteasomes machine parts, in the section of the brain which dies in Parkinson’s disease or dementia with Lewy bodies we caused neuronal death –and there were pile ups of tagged proteins in surviving neurones – a key hallmark of disease.
Implications for the future of our MND research
The MND Association has given a pilot grant of £10,000 to Dr Lynn Bedford, who carried out this work, to see if it is possible to delete the gene in motor neurones and innervated muscles to cause MND. She is still working on this (only one pair of hands!) but the tissue sections are now ready to see if MND can be caused this way. We expect that this will be the case and we should know soon!
Keeping open minded
Research into complex disease needs open-minds and different areas of research. Genetics provide clues to familial disease, like for ubiquilin 2, but families are just a small number of people with the disease. The finding of ubiquilin 2 in pile ups with TDP-43, FUS etc shows the generality of the UPS response in MND and ubiquilin 2 will probably be in pile ups of proteins in the other disease too.
The discovery of mistakes in one gene, ubiquilin 2, whose protein product is involved in protein degradation, is fantastic to try to understand MND and other chronic neurodegenerative diseases but there is much more…
Rare mistakes in the genes for three other proteins involved in protein degradation that cause neurodegeneration have recently come to light. Mistakes in a gene called VCP cause MND and a related disease called frontotemporal dementia, mistakes in a gene called optineurin cause MND and mistakes in the p62 protein gene cause MND.
Lightning generally does not strike in the same place twice, yet alone four times! So, to have mistakes in at least four genes causing MND whose protein products are involved in protein degradation dramatically increases the likelihood that problems with this system are central to neurodegenerative disease.
For general effects in disease, researchers must have a pathway that when misbehaving or overwhelmed causes disease (not to mention to provide a therapeutic target). It is one thing to have the capability to find these genetic errors, but it is another to map out the steps in a pathway(s) that cause disease. If a pathway is identified, like through ubiquilin 2 (and the other three genes plus our other ubiquitin-related work), and in general the UPS in other neurological diseases, then I believe that this pathway should be the focus of investment to try to find a cure.
Putting my money where my mouth is
I published a review in the journal Nature Reviews Drug Discovery on the ‘druggability’ of the UPS for many unrelated diseases. Towards the end I had a ‘dream’: if the UPS could be stimulated then neurodegenerative disease could be controlled.
I could not believe it, but some time later, at the end of 2010, my friend Dan Finley and colleagues answered my dream, at least conceptually. They published in the prestigious journal Nature, work on a drug that activated 26S proteasomes to degrade proteins including some involved in neurodegeneration.
So, what are we waiting for? Answer, all the work that goes into converting an initial discovery into a novel therapeutic approach…Watch this space!
Our final thoughts
The story of the recycling process and ubiquilin 2 is indeed an exciting one that is constantly evolving to provide us with more answers as to what causes MND and how we can fight it in the future. As described by Prof Mayer, ‘the machine’, the proteasome, is normally part of a well oiled process and it is clear that if spanners are thrown into the works that the system can go terribly wrong and cause a number of neurodegenerative diseases, including MND. It will definitely be interesting to watch this research story unravel its secrets in the future.
One thing is certain though – that keeping on top of recycling is very important!