An experimental and modelling study of injured axons

Abstract

Injury-induced retraction and beading (degeneration) of axons during traumatic brain injury (TBI) are believed to play key roles in the disintegration of the neural network and eventually lead to severe symptoms such as permanent memory loss and emotional disturbances. However, fundamental questions such as how these phenomena actually take place and what physical factors govern these processes still remain unclear. In addition, a method allowing us to quantitatively and systematically examine the injury response of axons is still lacking.

In this study, we report a combined experimental and modeling study to address these questions. Specifically, a sharp atomic force microscope (AFM) probe was used to transect axons and trigger the response in a precisely controlled manner. The retraction and beading response of axon, as well as related cytoskeletal changes insider, were then monitored with a fluorescent microscope. Interestingly, we showed that the retracting motion of a well-developed axon can be arrested by strong cell-substrate attachment. However, axon retraction was found to be re-triggered if a second transection was conducted, albeit with a lower shrinking amplitude. Furthermore, disruption of the actin cytoskeleton or cell-substrate adhesion significantly altered the retracting dynamics of injured axons. Besides retracting homogeneously, beading of transected axons was also commonly observed (mainly on the thin distal part of the axon) in our experiment. The beading wavelength, representing the averaged distance between beads, was found to correlate with the size and cytoskeleton integrity of axon, with a thinner axon or a disrupted actin cytoskeleton both leading to a shorter beading wavelength. On the theoretical side, a mathematical model was developed to explain the observed injury response of neural cells where the retracting motion was assumed to be driven by the pre-tension in the axon and progress against neuron-substrate adhesion as well as the viscous resistance of the cell. Using realistic parameters, model predictions were found to be in good agreement with our observations. Furthermore, a model that suggests axon beading originates from the shape instability of membrane, and is driven by release of the work done by axonal tension and reduction of the surface energy of membrane was also developed. The beading wavelength predicted from this theory was in good agreement with our experiments under various conditions, indicating that the essential physics has been captured by the model.

Finally, possible interplay between the Alzheimer's disease (AD) and the injury response of neural cells was examined. Interestingly, it was found that the treatment of amyloid-beta (Aβ) on neurons, a common way to mimic the progression of AD, led to greatly reduced retraction but more severe beading and fragmentation of transected axons, indicating a much weakened axonal cytoskeleton along with a significantly reduced tension has been induced by this disease.