The fly wing is not only translucent and a site of lengthy nerve fibers that can be easily observed, but it can also be cut to cause injury without killing the fly. That way, the researchers can follow the animal's response to nerve injury for weeks.
Using various reagents to manipulate the fly's genetic traits, the team confirmed that the cut wing nerve underwent Wallerian degeneration. They then tested versions of Nmnat and another protein called WldS, all of which had previously been shown to protect nerves from degeneration, to see if any of these might stop the process. All significantly delayed neurodegeneration. Even a form of Nmnat that hadn't worked in other animal models suppressed degeneration, although to a lesser extent.
"That indicates that our assay is really sensitive," Bonini said. "This sensitivity could help us identify genes that have moderate although important functionality at protecting against nerve degeneration."
Their investigations into the wing nerve also showed that the degenerating axon "died back," fragmenting first from the axon terminals, the side farthest from the nerve cell bodya pattern similar to what has been seen in other disorders.
Doing more genetic tinkering, the researchers showed that when the animal's own Nmnat was depleted, the nerves fragmented in the same way as if the axon was physically cut. And when Nmnat and the other "rescue" proteins were added back to these genetically modified flies, they were able to block degeneration, highlighting that Nmnat is critical to maintaining healthy axons.
In a final set of experiments, the biologists sought to narrow where in the nerve cells Nmnat might be working. They focused on mitochondria, the powerhouses of cells. When they created a genetic line of flies that blocked mitochondria from entering the axon fibers, the nerve tract degenerated, again, in a dyin
|Contact: Katherine Unger Baillie|
University of Pennsylvania