Partial interruption of axonal transport due to microtubule breakage accounts for the formation of periodic varicosities after traumatic axonal injury

Abstract

Due to their viscoelastic nature, white matter axons are susceptible to damage by high strain rates produced during traumatic brain injury (TBI). Indeed, diffuse axonal injury (DAI) is one of the most common features of TBI, characterized by the hallmark pathological profiles of axonal bulbs at disconnected terminal ends of axons and periodic swellings along axons, known as “varicosities.” Although transport interruption underlies axonal bulb formation, it is unclear how varicosities arise, with multiple sites accumulating transported materials along one axon. Recently, axonal microtubules have been found to physically break during dynamic stretch-injury of cortical axons in vitro. Here, the same in vitro model was used in parallel with histopathological analyses of human brains acquired acutely following TBI to examine the potential role of mechanical microtubule damage in varicosity formation post-trauma. Transmission electron microscopy (TEM) following in vitro stretch-injury revealed periodic breaks of individual microtubules along axons that regionally corresponded with undulations in axon morphology. However, typically less than a third of microtubules were broken in any region of an axon. Within hours, these sites of microtubule breaks evolved into periodic swellings. This suggests axonal transport may be halted along one broken microtubule, yet can proceed through the same region via other intact microtubules. Similar axonal undulations and varicosities were observed following TBI in humans, suggesting primary microtubule failure may also be a feature of DAI. These data indicate a novel mechanism of mechanical microtubule damage leading to partial transport interruption and varicosity formation in traumatic axonal injury.

Therapy Development for Diffuse Axonal Injury

“…3,8,21,[27][28][29][30] In contrast, a series of swellings along otherwise intact axons represent ''axonal varicosities.'' 31 This common pathological profile found in acute TBI may reflect a process whereby transport is only partially interrupted at multiple sites along the axon's length, producing the string-of-beads appearance, in contrast with presumed complete interruption at one site forming singular axonal bulbs. 3,11,24,31 Typically in humans, reactive axonal swellings can be recognized within the first hours post-injury as large focal axonal swellings 10-20 lm in diameter that expand in size over 24-48 h reaching up to approximately 50 lm.…”

“…31 This common pathological profile found in acute TBI may reflect a process whereby transport is only partially interrupted at multiple sites along the axon's length, producing the string-of-beads appearance, in contrast with presumed complete interruption at one site forming singular axonal bulbs. 3,11,24,31 Typically in humans, reactive axonal swellings can be recognized within the first hours post-injury as large focal axonal swellings 10-20 lm in diameter that expand in size over 24-48 h reaching up to approximately 50 lm. 1,3,8,15,18,25,32 During this time, both axonal bulbs and varicosities are often observed together in various ratios, presumably reflecting the different rates of swelling and progression toward disconnection in individual injured axons.…”

“…During normal daily activities, axons are supple and can accommodate substantial mechanical deformation, such as stretching. 8,31,58,61 Indeed, under quasistatic loading conditions, axons can easily be stretched for over 100% of their original length and spring back without any evidence of structural or functional failure. 61 Constituents of axons, however, are thought to become brittle under very rapid mechanical loading conditions, predisposing vulnerable structures in the axolemma and/or axoplasm to mechanical damage, typically occurring at discrete points along the axon's length.…”

Cognitive and behavioral deficits arising from neurodegeneration and traumatic brain injury: a model for the underlying role of focal axonal swellings in neuronal networks with plasticity

“…FAS can lead to a remarkable 30-fold increase in axon diameter [29,30]. Additionally, recent works demonstrate that critical changes in axonal morphology can impair the underlying spike train propagation responsible for encoding information in neural activity.…”

“…But even in mild TBI, injured axons undergo changes culminating in FAS [6,7,[25][26][27][28]. FAS can lead to dramatic changes in axon diameters, potentially interrupting axonal transport [29,30] and/or significantly impairing the underlying spike train propagation responsible for encoding information in neural activity [8][9][10]31].…”