Mounting evidence suggests that α‐synuclein, a major protein component of Lewy bodies (LB), may be responsible for initiating and spreading the pathological process in Parkinson disease (PD). Supporting this concept, intracerebral inoculation of synthetic recombinant α‐synuclein fibrils can trigger α‐synuclein pathology in mice. However, it remains uncertain whether the pathogenic effects of recombinant synthetic α‐synuclein may apply to PD‐linked pathological α‐synuclein and occur in species closer to humans.

Nigral LB‐enriched fractions containing pathological α‐synuclein were purified from postmortem PD brains by sucrose gradient fractionation and subsequently inoculated into the substantia nigra or striatum of wild‐type mice and macaque monkeys. Control animals received non‐LB fractions containing soluble α‐synuclein derived from the same nigral PD tissue.

In both mice and monkeys, intranigral or intrastriatal inoculations of PD‐derived LB extracts resulted in progressive nigrostriatal neurodegeneration starting at striatal dopaminergic terminals. No neurodegeneration was observed in animals receiving non‐LB fractions from the same patients. In LB‐injected animals, exogenous human α‐synuclein was quickly internalized within host neurons and triggered the pathological conversion of endogenous α‐synuclein. At the onset of LB‐induced degeneration, host pathological α‐synuclein diffusely accumulated within nigral neurons and anatomically interconnected regions, both anterogradely and retrogradely. LB‐induced pathogenic effects required both human α‐synuclein present in LB extracts and host expression of α‐synuclein.

α‐Synuclein species contained in PD‐derived LB are pathogenic and have the capacity to initiate a PD‐like pathological process, including intracellular and presynaptic accumulations of pathological α‐synuclein in different brain areas and slowly progressive axon‐initiated dopaminergic nigrostriatal neurodegeneration. ANN NEUROL 2014;75:351–362