Neuron-to-neuron α-synuclein propagation in vivo is independent of neuronal injury

Introduction: Interneuronal propagation of alpha-synuclein has been demonstrated in a variety of experimental models and may be involved in disease progression during the course of human synucleinopathies. The aim of this study was to assess the role that neuronal injury or, vice versa, cell integrity could have in facilitating interneuronal alpha-synuclein transfer and consequent protein spreading in an in vivo animal model. Results: Viral vectors carrying the DNA for human alpha-synuclein were injected into the rat vagus nerve to trigger protein overexpression in the medulla oblongata and consequent spreading of human alpha-synuclein toward pons, midbrain and forebrain. Two vector preparations sharing the same viral construct were manufactured using identical procedures with the exception of methods for their purification. They were also injected at concentrations that induced comparable levels of alpha-synuclein transduction/overexpression in the medulla oblongata. alpha-Synuclein load was associated with damage (at 6 weeks post injection) and death (at 12 weeks) of medullary neurons after treatment with only one of the two vector preparations. Of note, neuronal injury and degeneration was accompanied by a substantial reduction of caudo-rostral propagation of human alpha-synuclein. Conclusions: Interneuronal alpha-synuclein transfer, which underlies protein spreading from the medulla oblongata to more rostral brain regions in this rat model, is not a mere consequence of passive release from damaged or dead neurons. Neuronal injury and degeneration did not exacerbate alpha-synuclein propagation. In fact, data suggest that cell-to-cell passage of alpha-synuclein may be particularly efficient between intact, relatively healthy neurons.