By David N. Leff
Your typical, paradigmatic human (Homo sapiens) weighs in at, say, 150 pounds ¿ 68 kilograms. So on a seesaw, it would take some 68,000 fruit flies (Drosophila melanogaster) to even the balance.
Each of these small but informative insects tips the balance at 1 milligram. Lined up in single file, eight of them would span an inch. A minor mystery in modern life is where fruit flies come from. Set out a bowl of slightly overripe fruit, and suddenly they¿re swarming and buzzing all around it ¿ apparently emerging from nowhere.
Clinical neuropathologist Mel Feany clears up this perplexity: ¿Fruit has little visitors on its peel when it comes from the store. The flies have laid eggs on it. Those eggs hatch and new flies come out.¿ She added, ¿Eating a few fly eggs won¿t hurt you.¿
Feany herself, a molecular neuroscientist at Harvard-affiliated Brigham and Women¿s Hospital in Boston, is wrestling with a far bigger mystery in D. melanogaster. She and her colleagues have turned the ubiquitous insect into a research model of Parkinson¿s disease (PD).
¿In a broad sense,¿ Feany told BioWorld Today, ¿we want to understand the molecular mechanism that underlies neuronal death in PD. To do that we needed an animal model, an experimental organism, to work on. We chose Drosophila because we¿re particularly interested in identifying the proteins that mediate neurodegeneration in the disease. That¿s quite difficult ¿ you have to do genetics. And that¿s really hard in people, whose life span is 60, 70, 80 or more years. It¿s really hard in mice, which live about 24 months, but it¿s really easy in fruit flies, which die after 60 days. Also, flies are good for drug screening, but our motivation was really a genetic model.¿
Wages Of Old Age: Parkinson¿s, Alzheimer¿s
Second only to Alzheimer¿s disease, Parkinson¿s is the most common disorder of aging in the Western world. Its prevalence is estimated at one to two cases per thousand, and this increases tenfold with advancing years. ¿PD¿s symptoms,¿ she added, ¿the shaking and stiff, tremulous movements seen in former boxing champion Muhammad Ali, Pope John Paul II and Attorney General Janet Reno, are found in most PD patients.¿
Feany is senior author of a report in today¿s Nature, dated March 23, 2000, and titled: ¿A Drosophila model of Parkinson¿s disease.¿ To construct their transgenic insects, she recounted, ¿We took the human alpha-synuclein gene, spliced it into a circular piece of DNA ¿ a plasmid vector ¿ and injected that construct into early embryos of Drosophila. We included a marker gene that changes the insects¿ eye color from white to red, to help us pull out flies that do express alpha-synuclein.
¿A feature of neurodegenerative diseases in general, and PD in particular,¿ Feany continued, ¿is that they affect only the nervous system, not other tissues. So in our transgenic model it was very important to make sure the flies behaved the same way: Did alpha-synuclein have toxicity just in the nervous system, or did it affect all sorts of different tissues? So we used a special activator protein that works only in the brain, in neurons, not in any other tissue.
¿The overall result that we¿ve presented in our Nature paper,¿ she summed up, ¿is that we have produced an excellent model of PD, which replicates the three cardinal manifestations of the human disease: (1) locomotor dysfunction; (2) age-dependent loss of dopaminergic neurons; and (3) abnormal alpha-synuclein protein fibrillar aggregates in those neurons ¿ the so-called Lewy bodies.
¿Alpha-synuclein is a very abundant protein that is found only in the brain,¿ Feany said, ¿comprising about 1 percent of total brain protein in people and in mice. Despite the fact that we¿ve known for some years now that the protein exists, we really have no idea what its normal function is.¿ (See BioWorld Today, June 27, 1997, p. 1.)
To confirm that the PD-mimicking flies really reproduced movement disorders similar to those of the disease, the co-authors placed 40 of the insects in the bottom of a vial or culture tube about 6 inches high and 1.25 inches in diameter. ¿Regular normal flies,¿ Feany observed, ¿have a strong negative geotactic response. That is, they love to climb up. So when we tap them down to the bottom of the vial, normal flies just climb right back up to the top of the tube.
¿Our alpha-synuclein flies were abnormal, in that early on they looked and behaved just like normal flies. As they got older, they lost their ability to climb up to the top of the vial. They would start to try to walk up, and couldn¿t make it. They¿d fall back down, often flat on their backs, which is quite abnormal for flies.
¿But that¿s really quite similar to what you see in patients with PD. One of the very common features is the fact that they often fall down, and frequently have quite a bit of difficulty getting back up, because of their motor problems.
¿We tested a few other things, such as whether the transgenics flew as well as normal flies. One easy thing we did was to lay a great big piece of white paper on the ground, hold the fly about a meter up off the ground, then let it drop. Normal flies never land on the white paper. They just buzz right on off. But flies that can¿t fly very well often do land on the paper. So we tried that simple test, and we couldn¿t see any flying abnormalities in our flies.¿
Given the similarity of the transgenic PD models to the human disease, Feany suspects that ¿the biotech community is going to be very excited about the possibilities of using these flies as drug-screening organisms. They¿re small, they¿re cheap, they don¿t live very long, and they¿ll eat pretty much anything.¿
Agent Targeting Fibril Clumping In Neurons Starts Test
Feany has a collaboration with biophysical chemist Peter Lansbury, associate professor of neurology at Harvard University, and director of an NIH PD Center of Excellence. ¿We¿re starting next week,¿ Lansbury told BioWorld Today, ¿feeding the flies inhibitors to the fibrilization of alpha-synuclein. These compounds may not have the characteristics that Merck would like to see, but they answer the question that Merck wants answered.
¿We¿ve been working for about two years on PD,¿ Lansbury went on, ¿because we believe that this fibrilization process is a good therapeutic target. But what¿s nice about PD is that you can observe what appears to be the intrinsic features of PD in a fly. It¿s incredible that you can actually model it in a fly, because movement is such a basic and primeval circuitry.¿