Die Drosophila-Mutante loe: Neurodegeneration und Cholesterin

Projektdetails:

Thematik: Grundlagenforschung
Förderstatus: abgeschlossen
Art der Förderung: Standard Projekt
Institution: Universität Würzburg, Theodor-Boveri-Institut
Projektleiter: Dr. Doris Kretzschmar
Laufzeit: 01. November 2000 - 31. Oktober 2001
Fördersumme: 30.397,00 Euro
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Projektbeschreibung

Im Gehirn von Alzheimer-Patienten werden auch Veränderungen im Lipid- und Cholesterin-Haushalt festgestellt. Zum Beispiel ist das Apolipoprotein-E4 der größte Risikofaktor für die sporadisch auftretenden Alzheimer-Fälle. Dieses Protein ist für den Transport von Zellmembran-Lipiden und Cholesterin zwischen einzelnen Nervenzellen zuständig. Es ist bisher noch nicht bekannt, wie Veränderungen im Lipid- und Cholesterin-Haushalt zum Fortschreiten der Alzheimer-Erkrankung beitragen.

In ersten Untersuchungen konnte Dr. Doris Kretzschmar von der Universität Würzburg zeigen, dass die Fruchtfliege Drosophila ein geeignetes Tiermodell darstellt, um Gene zu identifizieren, die an neurodegenerativen Prozessen beteiligt sind. In ihrem aktuellen Forschungsprojekt wird Dr. Kretzschmar eine Mutante der Drosophila, genannt löchrig (loe), untersuchen. Diese Drosophila Mutante zeigt Degenerationen im Gehirn auf, die vergleichbar auch bei Alzheimer-Patienten auftreten. Die weitere Erforschung der Zusammenhänge zwischen dem Cholesterin-Haushalt und dem Absterben von Nervenzellen in der Drosophila Mutante loe wird das Grundlagenwissen über die Funktion des Cholesterin bei neurodegenerativen Prozessen, wie sie bei der Alzheimer-Krankheit auftreten, erweitern.

Abschlussbericht

It has been shown that simple organisms like the fruitfly Drosophila melanogaster or the worm Caenorhabditis elegans can be used as model systems to study fundamental biological mechanisms. Recently these organisms have also been used to investigate the processes that lead to neurodegeneration and cause diseases like Alzheimer's disease. One of the proteins, known to be involved in Alzheimer's disease, is the Amyloid Precursor Protein (APP). The processing of this protein leads to the production of the Aß peptide which is a key component of the amyloid plaques found in Alzheimer patients.

We have isolated a Drosophila mutant, called löchrig (loe) that shows age-dependent neurodegeneration and a reduced life span. The Loe protein is part of a complex, called AMP activated protein kinase (AMPK), which plays a key role in the regulation of the cholesterol household. Testing the cholesterol content in the brain of loe mutants we found a dramatic reduction of cholesterol ester, the main storage form of cholesterol. This is interesting because recent publications have shown an involvement of cholesterol in the cleavage of the Aß peptide from APP.

We have therefore investigated a possible connection between Loe and Appl, the fly counterpart of the human Amyloid Precursor Protein. We could show that loe interacts with Appl because flies which are mutant for both genes show a much more severe degeneration than loe alone. Flies only mutant for APPL, like mice missing APP, show no neurodegeneration. In addition we have investigated whether the processing of APPL is influenced by loe. In wild-type flies APPL exists in two forms, the full-length protein and the cleaved soluble form. In loe flies the amount of the soluble form is decreased while flies making additional Loe show more of the soluble form. These results show that the processing of APPL is indeed affected by loe. We are currently investigating a possible influence of loe on APP processing. In case APP is affected, we can use loe as a model system to study the connection between AMPK, cholesterol household and APP processing.

A key enzyme in the cholesterol houshold, called HMG-CoA reductase, has been shown to play a role in Alzheimer's disease. Drugs which reduce the activity of HMG-CoA reductase decrease the risk for Alzheimer's disease. We have tested these drugs in loe flies and found an improvement of the degeneration. Similar results were achieved when mutants in HMG-CoA reductase were crossed to loe flies. This interaction confirms a connection between both proteins and it has been shown in tissues outside the brain that the AMPK complex, of which loe is part of, regulates HMG-CoA reductase. Interestingly there exists a significant difference between human and insect HMG-CoA reductase. The human protein is responsible for the production of cholesterol, insects, however, can not produce cholesterol but take it up from their food supply. Because our results show an effect of this enzyme in insects we suggest that the influence on neurodegeneration is not mediated through cholesterol but rather through cholesterol ester. This might also be the case in humans.

Characterizing the loe gene we found that several forms of Ioe exist but only one is affected by the mutation. This form, called LoeI, can be found in the brain and has a unique structure and function. The other Loe proteins cannot substitute loel. Very recently, brain specific subunits of AMPK have been isolated and it will be interesting whether they have a specific function in the nervous system like loel. So far only loe shows that AMPK is involved in neurodegeneration and processing of an APP protein. Therefore it offers a unique opportunity to study the possible role of AMPK in the pathogenesis of Alzheimer's disease.

Wissenschaftliche Publikationen auf Basis des geförderten Projekts

Tschäpe, J.-A., Bettencourt da Cruz, A., and Kretzschmar, D. (2003). Progressive neurodegeneration in Drosophila: a model system. J Neural Transm Suppl., 65:51-62.

Tschäpe, J.-A., Hammerschmied, C., Mühlig-Versen, M., Athenstaedt, K., Daum, G., Kretzschmar, D. (2002). The neurodegeneration mutant löchrig interferes with cholesteril homeostasis and Appl processing. The EMBO Journal, 21(23):6367-6376.

Kretzschmar D. and Pflugfelder, G.O. (2001) Glia in development, function, and neurodegeneration of the adult insect brain. Brain research Bulletin, 57:121-131.

Moser, M., Stempfl, T., Li, Y., Glynn, P., Büttner, R. und Kretzschmar, D. (2000). Cloning and expression of the murine sws/NTE gene. Mechanisms of Development, Vol. 90(2):279-282.

Kretzschmar, D., Poeck,B., Roth, H., Ernst, R., Keller, A., Porsch, M., Strauss, R. and  Pflugfelder, G.O. (2000). Defective pigment granule biogenesis and abberrant behavior caused by mutations in the Drosophila AP-3ß adaptin gene ruby. Genetics, Vol. 155(1):213-223.


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