Changes in stiffness of the optic nerve and involvement of neurofilament light chains in the course of experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis.

Multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE), are often accompanied by optic neuritis associated with neurofilament disruption. In this study, the stiffness of the optic nerve was investigated by atomic force microscopy (AFM) in mice with induced EAE in the successive phases of the disease: onset, peak, and chronic. AFM results were compared with the intensity of the main pathological processes in the optic nerve: inflammation, demyelination, and axonal loss, as well as with the density of astrocytes, assessed by quantitative histology and immunohistochemistry. Optic nerve tissue and serum levels of neurofilament light chain protein (NEFL) were also examined by immunostaining and ELISA, respectively. The stiffness of the optic nerve in EAE mice was lower than that in control and naïve animals. It increased in the onset and peak phases and sharply decreased in the chronic phase. Serum NEFL level showed similar dynamics, while tissue NEFL level decreased in the onset and peak phases, indicating a leak of NEFL from the optic nerve to body fluids. Inflammation and demyelination gradually increased to reach the maximum in the peak phase of EAE, and inflammation slightly declined in the chronic phase, while demyelination did not. The axonal loss also gradually increased and had the highest level in the chronic phase. Among these processes, demyelination and especially axonal loss most effectively decrease the stiffness of the optic nerve. NEFL level in serum can be regarded as an early indicator of EAE, as it rapidly grows in the onset phase of the disease.