Aggregation of the nerve cell protein ␣-synuclein is a characteristic of the common neurodegenerative ␣-synucleinopathies like Parkinson's disease and Lewy body dementia, and it plays a direct pathogenic role as demonstrated by early onset diseases caused by missense mutations and multiplication of the ␣-synuclein gene. We investigated the existence of ␣-synuclein proaggregatory brain proteins whose dysregulation may contribute to disease progression, and we identified the brain-specific p25␣ as a candidate that preferentially binds to ␣-synuclein in its aggregated state. Functionally, purified recombinant human p25␣ strongly stimulates the aggregation of ␣-synuclein in vitro as demonstrated by thioflavin-T fluorescence and quantitative electron microscopy. p25␣ is normally only expressed in oligodendrocytes in contrast to ␣-synuclein, which is normally only expressed in neurons. This expression pattern is changed in ␣-synucleinopathies. In multiple systems atrophy, degenerating oligodendrocytes displayed accumulation of p25␣ and dystopically expressed ␣-synuclein in the glial cytoplasmic inclusions. In Parkinson's disease and Lewy body dementia, p25␣ was detectable in the neuronal Lewy body inclusions along with ␣-synuclein. The localization in ␣-synuclein-containing inclusions was verified biochemically by immunological detection in Lewy body inclusions purified from Lewy body dementia tissue and glial cytoplasmic inclusions purified from tissue from multiple systems atrophy. We suggest that p25␣ plays a pro-aggregatory role in the common neurodegenerative disorders hallmarked by ␣-synuclein aggregates.The group of ␣-synucleinopathies is dominated by the frequent neurodegenerative disorders Parkinson's disease (PD), 1 Lewy body dementia (LBD), a Lewy body variant of Alzheimer's disease, and multiple system atrophy (MSA) (1-4). Their unifying hallmark is the development of aggregates of the 140-amino acid ␣-synuclein (AS) protein, which is deposited in intracellular inclusions. The inclusions comprise the Lewy body-type of inclusions in the neuronal cell body, the Lewy neurites in axons, and the oligodendrocytic glial cytoplasmic inclusions in MSA. AS can play an active role in the degenerative processes as evidenced by studies of families with autosomal dominant early onset PD and LBD caused by missense mutations in the AS gene (5-7) and the overexpression of the wild type protein caused by gene multiplications of the AS locus (8 -10). However, the role of AS in the sporadic diseases is less clear. The frequency of AS aggregation in human diseases has given rise to extensive studies of the process in vitro. These studies have revealed that this aggregation represents a nucleation-dependent process (11). The transition from monomeric AS to filamentous aggregates is characterized by a lag phase during which a build-up of soluble nucleation-competent oligomeric AS species takes places. The rapid filament growth does not occur until these structures have reached a critical concentration. This process is stimulated...

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