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α-Synuclein (αS) is the major component of the filamentous inclusions that constitute the defining characteristic of neurodegenerative synucleinopathies, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. αS is deposited in a hyperphosphorylated and ubiquitinated form with a β-sheet-rich fibrillar structure in diseased brains. In 2008, some researchers reported that embryonic neurons transplanted into Parkinson's disease brains had Lewy body-like pathologies, suggesting that pathological αS propagates from diseased neurons to young neurons. Subsequently, a growing body of evidence supported the cell-to-cell spread of αS pathologies. Recent studies have revealed that intracerebral injection of insoluble αS into wild-type mice can induce prion-like propagation of phosphorylated αS pathology even 1 month after injection, while injection into αS-knockout mice failed to induce any pathology. We also showed that intracerebral injection of insoluble αS into adult common marmoset brains results in the spreading of abundant αS pathology. These in vivo experiments clearly indicate that insoluble αS has prion-like properties and that it propagates through neural networks. The underlying mechanisms of αS propagation are still poorly understood, but αS propagation model animals could be helpful in elucidating the pathogenetic mechanisms and developing drugs for synucleinopathies.

Accumulation of insoluble alpha-synuclein (αS) is a pathological hallmark of some progressive neurodegenerative diseases including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, collectively termed synucleinopathies. In diseased brain, αS forms β-sheet-rich amyloid fibrils and it is accumulated in neurons or glial cells. A growing body of evidence suggests that spreading of αS pathology occur by prion-like propagation mechanisms. Our study revealed that intracerebral injection of synthetic αS amyloid fibrils into wild-type mice induced prion-like propagation of αS pathology at 1 month post injection, while injection of soluble αS did not induce αS pathology. Furthermore, injection of αS amyloid fibrils into αS knockout mice failed to induce any pathologies. We also have demonstrated that intracerebral injection of αS amyloid fibrils into small primates, adult common marmosets, resulted in spreading of αS pathologies and loss of TH-positive neurons. These in vivo experiments clearly indicate that αS amyloid fibrils has prion-like properties and it propagates through neural networks. The underlying mechanisms of αS propagation are poorly understood, however, αS propagation model animals would be useful in elucidating pathogenetic mechanisms and developing disease-modifying drugs for sporadic synucleinopathies.