SEB Bulletin March 2006
Using Drosophila to study neurodegenerative diseases is yielding fruitful results...
You might think that something as large and complex as the human brain would require a similarly large and complex animal to model its functioning or, in the case of neurodegenerative diseases, its dysfunctioning. If this is the case you will be surprised to discover that, although some scientists are using mammals to this effect, the opposite is true in other labs: In fact, a small and simple animal - the fruit fly (Drosophila)1 - is being studied by a number of scientists who are making significant progress in understanding the mechanisms behind these diseases of the human central nervous system. Not only that, some of the positive results of drug trials on these flies are being transferred directly to the clinical developmental stages and being employed on patients in hospitals to help alleviate some of their symptoms2.
“Using fruit flies to study diseases of the brain is not as unusual as it seems”, explains Dr Amrit Mudher3 (Southampton University) who receives substantial research funding from the Alzheimer's Society4. “Seventy percent of the genes and proteins implicated in neurodegenerative diseases in humans have counterparts (called orthologs) in flies and since the fly's genome has been fully sequenced we know a lot about its genetic information and have generated many genetic tools to study a wide range of diseases. Not only this, the flies only have 4 chromosomes which make them experimentally attractive models to use. In practical terms the flies have a short life span (only around 4-6 weeks) which makes them ideal models for getting quick results about age-related diseases - compare that with rats where we would have to wait months to see effects that we can see in days with flies. On top of all this, they're also cheap to keep!”
Dr Mudher is one of many groups of scientists investigating a number of neurodegenerative diseases including Alzheimer's, Parkinson's, Huntington's and Muscular Sclerosis. All of these diseases are a result of a breakdown in the brain's ability to transmit messages and are characterised by being largely age-related. Scientists share the vast array of mutant flies which they have generated over the years and which they have access to via central Fly Housing Centres, the two main ones being based in Cambridge (UK) and Bloomington5 in the US. “Mutants are made in different ways”, Dr Mudher explains. “We can knock out a gene in the fly, over-express a gene, or introduce a mutant human gene into the genome of the fly.”
Both Dr Mudher's group in Southampton and another group in Cambridge headed up by Dr Damian Crowther6 are investigating the mechanisms which cause Alzheimer's Disease7. “Two microscopic abnormalities are observed with Alzheimer's”, Dr Crowther explains. “One is the secretion and aggregation of Amyloid b-peptides, the other is the formation of huge 'tangles' made up of Tau proteins which form rope-like structures and accumulate inside the nerve and interfere with its proper functioning.” Both amyloid b- peptides and Tau proteins occur normally in nerve cells but in people with Alzheimer's Disease something goes wrong at the molecular level which causes them to become abnormal. The actual cause of the disease (and of others like it) is a hotly contested issue but Dr Crowther supports the hypothesis that amyloid b- peptides, which are normally secreted from cells as a toxic by-product, start to form aggregates which then cause the neurones to start accumulating abnormal Tau proteins which, in turn, form the tangles which lead to the symptoms of the disease.
To investigate this further, Dr. Crowther's group have inserted a gene for amyloid b- peptides into fruit flies so that they produce copious amounts and display the symptoms of Alzheimer's Disease. Then, by making random changes in their DNA they are screening for those flies which display less severe (or no) symptoms of the disease which would suggest that they are somehow countering the effects of the increases toxic amyloid b- peptides 8. By employing drugs, supplied by biotechnology companies, they are also testing to see if they are able to interfere with the aggregation process. Similarly, Dr Mudher's group have shown that flies that express copious amounts of Tau protein also display symptoms of disease and that treatment with Lithium Chloride can counter these effects of disease in some situations. These are both examples where drugs, which have already undergone rigorous toxicology trials, are being sped through to the patient to ease their symptoms.
Dr Zdenek Berger (The Mayo Clinic) has shown that lithium decreases toxicity of other proteins that aggregate, including mutant huntingtin, which causes Huntington's disease9, and a different aggregate-protein with expanded polyalanines. Therefore, lithium may have broad therapeutical application against different diseases. Dr Berger has also investigated effects of expanded polyalanines in vivo in flies. Expansions of polyalanine repeats cause nine different diseases. In addition, proteins containing long polyalanine stretches are present in Huntington's disease, as a result of the frameshifting of the original CAG/polyglutamine protein. Expression of long polyalanine repeats in flies leads to toxicity and aggregation, similar to that observed in the human disease caused by these polyalanine expansions.
Surprisingly, low levels of long polyalanines not only do not exert toxic effects, but actually exhibit protective properties. Long polyalanines can decrease toxicity of mutant huntingtin at low levels, raising the possibility that the polyalanine frameshift products observed in Huntington's disease may be protective. “Our data therefore suggest that proteins which are prone to aggregate and cause disease also have protective properties and this has important consequences for the way we view the role of such 'toxic' proteins”, says Dr Berger.
Parkinson's Disease10 is the second most common neurodegenerative disease after Alzheimer's and Dr Alex Whitworth (University of Sheffield) is using Drosophila to study some of the genes which are known to cause the disease in humans. Similar to Alzheimer's, neurodegeneration in typical Parkinson's is also associated with protein aggregates, called Lewy Bodies, but it is unclear whether they cause or contribute to the disease. Although this disease is rarely inherited, there are enough families showing inheritance patterns for scientists to have discovered the genes which are implicated in Parkinson's. “Four out of these five genes have counter-parts in the Drosophila genome”, says Dr Whitworth. “By mutating these genes we can mimic the symptoms of the disease and study the biological processes being affected. By targeting the expression in the eye of the fly we can observe very easily whether the eye is being disarranged and it gives us the potential to apply drugs to investigate their efficacy in treating the disease in humans. Furthermore, other biological processes affected in our fly models, which are not normally associated with the disease, can be enormously informative about the normal function of these genes and give important clues as to what goes wrong to cause the disease”.
Dr Tracey Newman11 (University of Southampton) is studying Multiple Sclerosis12 where it is the electrical signal (rather than the chemical signal - as with Alzheimer's and other neurodegenerative diseases) which is disrupted by the action of inflammatory cells which invade the brain and spinal cord. In this case it is the damage caused by the inflammatory cells that will cause the axons, which carry the electrical signal for distances up to one metre, to break so that the 'message' is halted and no longer reaches its target. Dr Newman, working with Professor David Shepherd and her co-workers, has developed a specialised system to injure the axons in fruit flies in their lab and she explains the advantages of using flies as their experimental system: “Because there are a large number of mutant and transgenic flies in the public domain which have various alterations in pathways that may be involved in the events which occur after injury to the axon we can study them in our model injury system to see what they are affecting”. They aim to use the model to investigate axonal damage in Multiple Sclerosis as well as other diseases of the central nervous system.
Other much rarer diseases are known as lysosomal storage diseases (LSDs) which are a collection of about 40 human genetic diseases that affect the function of the lysosome (the rubbish bin of the cell). The vast majority of these diseases are neurodegenerative and include Tay-Sachs13 and Batten Disease14 amongst many others. In the LSDs the lysosome accumulates material over time and for the majority of the LSDs, the defects in the lysosome lead to neurodegeneration and death in childhood. Much remains to be understood about the LSDs: Why is the nervous system so sensitive to lysosomal dysfunction? Why do different LSDs lead to clinically different diseases?
To answer some of these questions Dr Sean Sweeney15 (University of York) has turned to the powerful genetics available in the Drosophila system. “When I worked in San Francisco, I identified a mutant (called spinster) that shows all of the phenotypic [disease] characteristics of an LSD”, Dr Sweeney says. “In addition, the spinster mutant has massively overgrown synapses [points of contact between nerve cells and their targets] and I was able to show that the overgrowth of the synapse in the spinster mutant was most likely due to a misregulation of growth factors that usually act to govern appropriate synaptic growth. This suggested that a defect often seen in LSDs, excess neuronal growth, might be due to the failure of LSD compromised cells to degrade growth signals at the correct time. The failure to regulate synapse size correctly may be a major contributory factor to the loss of nervous system function observed in the LSDs”. Dr Sweeney indicates that, because of their rarity, these diseases have not, so far, attracted a great deal of interest from pharmaceutical companies, as with those mentioned previously. However he cites a key clinical hallmark which remains unaddressed - the typical build up in the lysosomes of a substance called Lipofuscin which is a common feature of LSDs is also seen in those over 70 and so could be implicated in the ageing process.
This field of research where fruit flies are being used as models to study age-related diseases is relatively very young and in the early stages of development. Like the flies themselves, it's definitely an area of rapid growth - but it will most certainly be longer lived! The contributors in this article are speaking at the SEB Annual Meeting 200616 in Canterbury and their talks will be published in a SEB book series shortly after the meeting.
Sarah Blackford
External Affairs officer
Further reading
1. www.ceolas.org/fly/
2. http://devcell.bio.uci.edu/NEWS%20Publications/marsh
%20bioessays_review.pdf
3. www.sbs.soton.ac.uk/staff/am/am.htm
4. www.alzheimers.org.uk/
5. http://flystocks.bio.indiana.edu/
6. http://www.surgicalsieve.com/id68.htm
7. www.alzforum.org/
8. www.admin.cam.ac.uk/news/press/dpp/2005032101
9. www.hda.org.uk/
10. www.pdtrials.org/parkinsons_info.php
11. www.mssociety.org.uk/research/research_explained
/whos_who/dr_tracey_newman.html
12. www.nmss.org/
13. http://www.ninds.nih.gov/disorders/taysachs
/taysachs.htm
14. http://www.bdsra.org/
15. www.york.ac.uk/depts/biol/staff/sts.htm
16. www.sebiology.org.uk/Meetings/pageview
.asp?S=2&mid=9
