Stretch-induced functional disorder of axonal transport in the cultured rat cortex neuron

Background: Diffuse axonal injury (DAI) is a one of the main causes of traumatic brain injury and caused by the impulsive stretching of neuronal axons resulting from rapid deformation of a brain. DAI is characterized by a gradual swelling of the axons which is formed by the accumulation of cellular organelles and proteins, and the swelling is the morphological hallmark of DAI pathology. In recent years, the details of the stress and strain of the whole damaged brain are becoming clear with the rapid development of the computational mechanics and finite element (FE) head models are able to give better prediction to the brain injury and to evaluate the protective safety methods with more detailed neuronal tolerance criteria. In this study, axonal injury induced by precisely controlled impulsive strain and strain rate was evaluated and the tolerance criteria for the functional disorder for each dysfunction and disruption level of axonal transport was obtained by observation of β-amyloid precursor protein (β-APP) in cultured rat cortex neuron. Methods: The uniaxial stretching device which could give various combinations of strains and strain rates to neurons was developed. The various loading conditions for neurons were verified by comparing the experimental displacement history of the substrate taken by microscopy with the FE strain distribution analysis of culturing substrate. The primary rat cortex neurons were stretched by different combinations of strains and strain rates and β-APP was immunostained at 3h after loading and observed by fluorescence microscopy. Results: The number of swellings and bulbs formed on axons by β-APP-accumulation after stretching were observed and counted by fluorescent images. The dysfunction of the axonal transport was defined as the rate of neurons that have β-APP-accumulating axonal swellings and disruption of the axonal transport was defined as the rate of neurons that have β-APP-accumulating axonal bulbs, respectively. The degree of the functional disorder of the axonal transport advanced with the increase of strain and strain rate. Conclusions: The mechanical threshold of dysfunction and disruption of axonal transport were the strain with 0.22 and the strain rate with 27 /s. The intervals between swellings on an axon are constant and do not depend on the axonal injury level nor the magnitude of the strain of the axons.