A Drosophila model of closed head traumatic brain injury

Abstract

Traumatic brain injury (TBI) is a substantial health issue worldwide, yet the mechanisms responsible for its complex spectrum of pathologies remains largely unknown. To investigate the mechanisms underlying TBI pathologies, we developed a model of TBI in Drosophila melanogaster. The model allows us to take advantage of the wealth of experimental tools available in flies. Closed head TBI was inflicted with a mechanical device that subjects flies to rapid acceleration and deceleration. Similar to humans with TBI, flies with TBI exhibited temporary incapacitation, ataxia, activation of the innate immune response, neurodegeneration, and death. Our data indicate that TBI results in death shortly after a primary injury only if the injury exceeds a certain threshold and that age and genetic background, but not sex, substantially affect this threshold. Furthermore, this threshold also appears to be dependent on the same cellular and molecular mechanisms that control normal longevity. This study demonstrates the potential of flies for providing key insights into human TBI that may ultimately provide unique opportunities for therapeutic intervention.concussion | insect | chronic traumatic encephalopathy T raumatic brain injury (TBI) is a leading cause of neurological deficits and mortality worldwide (1, 2). TBI outcomes result from primary and secondary injuries that cause cell damage and death in the brain. Primary injuries occur during the initial impact and are triggered by external mechanical forces that deform the brain, whereas secondary injuries are triggered by cellular and molecular responses that occur over time in reaction to the primary injuries. TBI outcomes are heterogeneous in the human population owing to variation in the location and strength of primary injuries as well as genetic and environmental factors that affect the severity of primary and secondary injuries. This heterogeneity is one of the most significant barriers to the development of therapeutic interventions (3-5). Therefore, research aimed at determining the genetic and environmental factors that affect the severity of primary and secondary injuries is essential for developing treatments for TBI.To investigate the underlying cellular and molecular basis of TBI, we developed a Drosophila melanogaster model. Key advantages of flies are that (i) large numbers of animals can be rapidly and inexpensively analyzed to establish causality between injuries and outcomes, (ii) many molecular and genetic tools are available to investigate the molecules and pathways that underlie injuries, and (iii) outcomes can readily be evaluated over the entire animal lifespan. Thus, flies provide unique opportunities to understand TBI.It is reasonable to expect that TBI can be modeled in flies because Drosophila has already proved to be an extremely useful model for studying other human neurodegenerative disorders (6). In fact, research in flies has already provided novel insights into neurodegeneration, memory, and sleep, all of which are affected in human TBI (6-8)...

A Method to Inflict Closed Head Traumatic Brain Injury in <em>Drosophila</em>

“…Flies subjected to strikes from the HIT device were incubated at 25°C in a vial with fly food, and the number of dead flies was counted after 24 hr. We used this approach to identify factors that affect the MI 24 and found that the MI 24 is not affected by the number of flies in a vial (10 to 60 flies was tested), the time between repetitive strikes (1 to 60 min was tested), or the sex of the fly 10 . In contrast, we found that age at the time of the primary injury and genotype did affect the MI 24 .…”

“…To some extent, all contacts are likely to cause the fly brain to ricochet and deform against the head capsule, similar to what happens to humans in falls and car crashes 9 . Accordingly, flies treated with the HIT device display phenotypes consistent with brain injury, including temporary incapacitation followed by ataxia, gradual recovery of mobility, gene expression changes in the head, and progressive neurodegeneration in the brain 10 . Thus, the HIT device makes it possible to study TBI using the enormous arsenal of experimental tools and techniques developed for flies.…”

“…On the other hand, polytrauma (traumatic injury to multiple body parts) often accompanies TBI and is thought to modulate TBI phenotypes, so the HIT device method can be used to investigate the contribution of polytrauma to TBI phenotypes. The fly TBI model also has advantages over rodent and non-human primate TBI models 10 . Large numbers of animals can be examined because flies breed rapidly (females lay ~100 eggs/day), have a short lifecycle (10 days from egg to mature adult at 25 °C), and are maintained in small vials and feed on inexpensive medium (~20¢/month/100 flies).…”

“…However, the fly TBI model can be used in many other ways to understand death as well as other consequences of TBI. For example, physical consequences of TBI can be measured using assays for parameters such as climbing or flight, and behavioral consequences of TBI can be measured using assays for parameters such as sleep and learning and memory 10,[12][13][14] . Structural effects on the brain can be determined using imaging techniques such as immunofluorescence microscopy and transmission electron microscopy.…”

The gut reaction to traumatic brain injury

“…38 This work builds upon our prior publications that describe the development and initial characterization of a fly TBI model that uses a spring-loaded device to inflict closed-head TBI in adult flies. 39,40 We found that one of the consequences of TBI is death within 24 hrs of the initial injury and that genotype plays a major role in determining the percentage of flies that die within 24 hrs, which we call the mortality index at 24 hrs (MI 24 ). 38 Through Genome-wide association study (GWAS) analysis of many wild-type fly lines, we identified single-nucleotide polymorphisms (SNPs) in three epithelial barrier-related genes, big bang (bbg), scribble (scrib), and grainyhead (grh), as being significantly associated with the MI 24 .…”