Experimental Modelling and Molecular Mechanisms of Wallerian Degeneration in Traumatic Axonal Injury
Traumatic brain injury (TBI) is a common event that can lead to profound consequences for the individual involved, and a considerable socio-economic cost. The initial injury event triggers a series of secondary brain injury mechanisms that lead to further mortality and contribute to morbidity. One classical injury pathology is termed traumatic axonal injury (TAI), which in clinical settings produces the picture of diffuse axonal injury. TAI occurs both as a primary insult, and as a consequence of secondary mechanisms. One secondary injury mechanism that worsens TAI may be Wallerian degeneration (WD), a cell-autonomous axonal death pathway. The relationship between traumatic axonal injury and WD is poorly characterised. This thesis explores the basic mechanisms by which a physical axonal trauma can lead to WD, and how modulation of the WD pathway can affect the cellular responses to a traumatic injury. This involves the development and characterisation of in vitro and in vivo models of traumatic axonal injury. These models are then used to explore the response of cellular cultures to injury when treated with pharmacological and genetic modulators of WD. Using a primary neuronal stretch-injury system we demonstrate that rates of neurite degeneration are altered by modulators of the WD pathway but that a purported neuroprotective compound ‘P7C3-A20’ did not protect primary cultures in vivo and did not act via a WD dependent mechanism. An organotypic hippocampal slice stretch injury model was then used to demonstrate genetic rescue of cellular death, and used to assess amyloidogenic responses to injury. Next we established a TBI model using Drosophila Melanogaster, and demonstrated that a loss of function mutation in a key WD gene ‘highwire’ which controls NMNAT levels, was capable of rescuing premature death and a range of behavioral deficits after a high impact trauma. The injury caused dopaminergic neuronal loss and this was rescued by highwire mutation. Furthermore, this dopaminergic neuronal protection extended to a genetic PINK1 model of Parkinsonism. Together these results help establish WD as an important secondary injury mechanism in TBI, and provide evidence that modulation of the WD pathways can improve outcomes in various model systems. This provides a foundation for future translational research into the fields of WD and TBI.
Axon, Wallerian, Degeneration, Traumatic brain injury
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