Demyelination precedes axonal loss in the transneuronal spread of human neurodegenerative disease

The spread of neurodegeneration through the human brain network is reported as underlying the progression of neurodegenerative disorders. However, the exact mechanisms remain unknown. The human visual pathway is characterized by its unique hierarchical architecture and, therefore, represents an ideal model to study trans-synaptic degeneration, in contrast to the complexity in neural connectivity of the whole brain. Here we show in two specifically selected patient cohorts, including (i) glaucoma patients with symmetrical bilateral hemifield defects respecting the horizontal meridian (n = 25, 14 females, 64.8 +/- 10.1 years; versus 13 normal controls with similar age/sex distributions); and (ii) multiple sclerosis patients without optic radiation lesions (to avoid potential effects of lesions on diffusivity measures) (n = 30, 25 females, 37.9 +/- 10.8 years; versus 20 controls), that there are measurable topographic changes in the posterior visual pathways corresponding to the primary optic nerve defects. A significant anisotropic increase of water diffusion was detected in both patient cohorts in the optic radiations, characterized by changes in perpendicular (radial) diffusivity (a measure of myelin integrity) that extended more posteriorly than those observed in parallel (axial) diffusivity (reflecting axonal integrity). In glaucoma, which is not considered a demyelinating disease, the observed increase in radial diffusivity within the optic radiations was validated by topographically linked delay of visual evoked potential latency, a functional measure of demyelination. Radial diffusivity change in the optic radiations was also associated with an asymmetrical reduction in the thickness of the calcarine cortex in glaucoma. In addition, 3 years longitudinal observation of the multiple sclerosis patient cohort revealed an anterograde increase of radial diffusivity in the anterior part of optic radiations which again was retinotopically associated with the primary damage caused by optic neuritis. Finally, in an animal model of optic nerve injury, we observed early glial activation and demyelination in the posterior visual projections, evidenced by the presence of myelin-laden macrophages. This occurred prior to the appearance of amyloid precursor protein accumulation, an indicator of disrupted fast axonal transport. This study demonstrated strong topographical spread of neurodegeneration along recognized neural projections and showed that myelin and glial pathology precedes axonal loss in the process, suggesting that the mechanism of trans-synaptic damage may be at least partially mediated by glial components at the cellular level. The findings may have broad biological and therapeutic implications for other neurodegenerative disorders.