Localized Induction of Wild-Type and Mutant Alpha-Synuclein Aggregation Reveals Propagation along Neuroanatomical Tracts

Abstract

The accumulation of alpha-synuclein (αS) inclusions is a hallmark feature of Parkinson's disease (PD) and PD-related diseases. Recently, a number of studies have demonstrated similarities between the prion protein and αS, including its ability to spread along neuroanatomical tracts throughout the central nervous system (CNS). However, there are caveats in each of these studies in which the injection routes used had the potential to result in a widespread dissemination of the αS-containing inocula, making it difficult to precisely define the mechanisms of spread. In this study, we assessed the spread of pathology following a localized induction of αS inclusions in the lumbar spinal cord following a unilateral injection in the sciatic nerve. Using this paradigm, we demonstrated the ability for αS inclusion spread and/or induction along neuroanatomical tracts within the CNS of two αS-overexpressing mouse models.Misfolded alpha-synuclein (αS) may exhibit a number of characteristics similar to those of the prion protein, including the apparent ability to spread along neuroanatomical connections. The demonstration for this mechanism of spread is largely based on the intracerebral injections of preaggregated αS seeds in mice, in which it cannot be excluded that diffuse, surgical perturbations and hematogenous spread also contribute to the propagation of pathology. For this reason, we have utilized the sciatic nerve as a route of injection to force the inoculum into the lumbar spinal cord and induce a localized site for the onset of αS inclusion pathology. Our results demonstrate that mouse αS fibrils (fibs) injected unilaterally in the sciatic nerve are efficient in inducing pathology and the onset of paralytic symptoms in both the M83 and M20 lines of αS transgenic mice. In addition, a spatiotemporal study of these injections revealed a predictable spread of pathology to brain regions whose axons synapse directly on ventral motor neurons in the spinal cord, strongly supporting axonal transport as a mechanism of spread of the αS inducing, or seeding, factor. We also revealed a relatively decreased efficiency for human αS fibs containing the E46K mutation to induce disease via this injection paradigm, supportive of recent studies demonstrating a diminished ability of this mutant αS to undergo aggregate induction. These results further demonstrate prion-like properties for αS by the ability for a progression and spread of αS inclusion pathology along neuroanatomical connections. IMPORTANCE The accumulation of alpha-synuclein (αS) inclusions is a hallmark feature of Parkinson's disease (PD) and PD-related diseases. Recently, a number of studies have demonstrated similarities between the prion protein and αS, including its ability to spread along neuroanatomical tracts throughout the central nervous system (CNS). However, there are caveats in each of these studies in which the injection routes used had the potential to result in a widespread dissemination of the αS-containing inocula, making it difficult to precisely define the mechanisms of spread. In this study, we assessed the spread of pathology following a localized induction of αS inclusions in the lumbar spinal cord following a unilateral injection in the sciatic nerve. Using this paradigm, we demonstrated the ability for αS inclusion spread and/or induction along neuroanatomical tracts within the CNS of two αS-overexpressing mouse models.