Therapy development for diffuse axonal injury

Abstract

Diffuse axonal injury (DAI) remains a prominent feature of human traumatic brain injury (TBI) and a major player in its subsequent morbidity. The importance of this widespread axonal damage has been confirmed by multiple approaches including routine postmortem neuropathology as well as advanced imaging, which is now capable of detecting the signatures of traumatically induced axonal injury across a spectrum of traumatically brain-injured persons. Despite the increased interest in DAI and its overall implications for brain-injured patients, many questions remain about this component of TBI and its potential therapeutic targeting. To address these deficiencies and to identify future directions needed to fill critical gaps in our understanding of this component of TBI, the National Institute of Neurological Disorders and Stroke hosted a workshop in May 2011. This workshop sought to determine what is known regarding the pathogenesis of DAI in animal models of injury as well as in the human clinical setting. The workshop also addressed new tools to aid in the identification of this axonal injury while also identifying more rational therapeutic targets linked to DAI for continued preclinical investigation and, ultimately, clinical translation. This report encapsulates the oral and written components of this workshop addressing key features regarding the pathobiology of DAI, the biomechanics implicated in its initiating pathology, and those experimental animal modeling considerations that bear relevance to the biomechanical features of human TBI. Parallel considerations of alternate forms of DAI detection including, but not limited to, advanced neuroimaging, electrophysiological, biomarker, and neurobehavioral evaluations are included, together with recommendations for how these technologies can be better used and integrated for a more comprehensive appreciation of the pathobiology of DAI and its overall structural and functional implications. Lastly, the document closes with a thorough review of the targets linked to the pathogenesis of DAI, while also presenting a detailed report of those target-based therapies that have been used, to date, with a consideration of their overall implications for future preclinical discovery and subsequent translation to the clinic. Although all participants realize that various research gaps remained in our understanding and treatment of this complex component of TBI, this workshop refines these issues providing, for the first time, a comprehensive appreciation of what has been done and what critical needs remain unfulfilled.

FIG. 1.

Examples of axonal pathology in fatal cases of traumatic brain injury (TBI) as identified using amyloid precursor protein (APP) immunohistochemistry in postmortem brain tissue. Note the accumulation of APP as varicose swellings along the length of the axon and as axonal bulbs at disconnected axon terminals in (a) a 37-year-old man who died 26 h after severe TBI caused by a fall; (b) a 20-year-old man who died 2 days after severe TBI caused by an assault, and (c) an 18-year-old woman who died 21 h after severe TBI caused by a motor vehicle collision. Images were provided courtesy of Dr. William Stewart at The Glasgow TBI Archive, Glasgow, UK. Bar=100 μm.

FIG. 2.

Evolving pathophysiology of traumatic injury in myelinated axons. In this figure, we attempt, in an abbreviated fashion, to illustrate some of the key events believed to be involved in the pathobiology of traumatic axonal injury and, thereby, identify potential therapeutic targets. Although framed in the view of primary nodal involvement (A), this focus does not preclude comparable change ongoing in other regions of the axon. Panels B and C show normal axonal detail including the paranodal loops and the presence of intra-axonal mitochondria, microtubules, and neurofilaments, together with the presence of multiple axolemmal channels localized primarily to the nodal domain. Mild to moderate traumatic brain injury in panel D is observed to involve a mechanical dysregulation of the voltage sensitive sodium channels, which contribute to increased calcium influx via reversal of the sodium calcium exchanger and the opening of voltage gated calcium channels. This also impacts on the proteolysis of sodium channel inactivation that contributes further to local calcium dysregulation. Microtubular loss, neurofilament impaction, and local mitochondrial damage can follow, which, if unabated, collectively alters/impairs axonal transport illustrated in panel E. Alternatively, if these abnormalities do not progress, recovery is possible (F). When progressive, these events not only impair axonal transport but also lead to rapid intra-axonal change in the paranodal and perhaps internodal domains that elicit the collapse of the axolemma and its overlying myelin sheath to result in lobulated and disconnected axonal segments (G) that, over the next 15 min–2 h, fully detach (H). The proximal axonal segment in continuity with the cell body of origin now continues to swell from the delivery of vesicles and organelles via anterograde transport while the downstream fiber undergoes Wallerian change (I). Lastly, with the most severe forms of injury, the above identified calcium-mediated destructive cascades are further augmented by the poration of the axolemma, again primarily at the nodal region (J). The resulting calcium surge, together with potential local microtubular damage and disassembly, pose catastrophic intra-axonal change that converts anterograde to retrograde axonal transport, precluding continued axonal swelling, while the distal axonal segment fragments and disconnects (K), with Wallerian degeneration ensuing downstream (L).

FIG. 3.

Evolving pathophysiology of traumatic injury in unmyelinated axons (A). As established in the literature, the unmyelinated axon with a large axolemmal to cytoplasm ratio is easily susceptible to forces of injury. Panel B illustrates that along the axolemmal length, voltage sensitive sodium channels, sodium-calcium exchangers, and voltage sensitive calcium channels are found. In addition, note that in panel C, fine caliber unmyelinated axons that are typically less than 1 μ in diameter, contain primarily mitochondria that actually deform the axolemma, together with numerous microtubules distributed throughout the axoplasm (C and D). With mild through moderate TBI, calcium dysregulation occurs (E), together with potential mitochondrial damage and microtubular disruption, all of which lead to impaired axonal transport and axonal swelling (F). Such swelling can occur either in isolation or as multiple varicosities (G and H). Again, over a relatively brief posttraumatic timeframe, this focal interruption in calcium ion homeostasis, microtubular stability, and mitochondrial integrity can lead to focal disconnection of the axon cylinder with the segment in continuity with its cell body of origin continuing to swell because of delivery of organelles and vesicles via a disruption in axonal transport (I) with the detached distal segment undergoing Wallerian change (J).