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Monday, November 4, 2013

Knocking fly heads around for science

What do you do if you wish to study traumatic brain injury (concussion) but you can’t ethically control exactly how and when those injuries occur in humans? Create a way to give concussions to flies, that’s how. 

Unlike with human patients, you can subject populations of flies to traumatic brain injury (TBI). You can, that is, if you build the fly-concussionator. Okay, Rebeccah Katzenberger and her colleagues from the University of Wisconsin-Madison actually named their invention the ‘high-impact trauma’ (HIT) device, but I like my name choice better. In any case, the HIT is a spring-loaded contraption which smashes a vial of flies against a polyurethane pad. 

Figure 2

The HIT device was used to inflict TBI in flies. Images show the HIT device and flies before and after a strike.
(A) the HIT device with the spring deflected to 90o before a strike
(B) the HIT device immediately after a strike
(C) 60 flies in a vial before a strike
(D) after a strike.
PNAS PMID: 24127584

You may be surprised to learn that fly brains have a lot in common with human brains. They have similar structures, attach to the fly equivalent of the spinal cord and use the same neurotransmitters as we do. Fly brains are even enclosed in a hard, inelastic layer (cuticle) against which those brains can ricochet upon sudden impact. In other words, just like human brains.

After their HIT treatment, the flies showed similar behaviors and deficits to those seen in humans, namely a loss of motor ability that disappeared over the next two days. About 5% of flies died within 24 hours of each impact. Neither the number of impacts, the duration of time between impacts nor the number of flies in the vial had much effect on this mortality rate, although older flies fared worse than younger ones. There was also a strong genetic component to survival, with flies from some genetic lines being four times as likely to drop dead.

There was bad news even for the flies that survived the initial impact. Fourteen days after their HIT treatment, flies showed signs of neuro-degeneration. They also had significantly shorter lifespans, living at least ten fewer days than their unshaken cohorts. That might not seem like much, but for a creature that rarely makes it to its two month anniversary, that’s a big difference. 

Obviously, you can only take an animal model so far. For one thing, it’s hard to make a nuanced analysis of brain problems when the flies’ main health markers are being able to move around and not being dead. That would be a pretty low bar for human recovery. Still, the fly-concussionator, sorry, the HIT device, is looking like it might be a very useful tool for studying TBI.

Katzenberger RJ, Loewen CA, Wassarman DR, Petersen AJ, Ganetzky B, & Wassarman DA (2013). A Drosophila model of closed head traumatic brain injury. Proceedings of the National Academy of Sciences of the United States of America PMID: 24127584.

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