Recently, we demonstrated that soluble 30-50 nmsized annular a-synuclein oligomers are released by mild detergent treatment from glial cytoplasmic inclusions (GCIs) purified from multiple system atrophy brain tissue (Pountney et al., J. Neurochem. 90:502, 2004). Dynamic antibody recognition imaging using a specific anti-a-synuclein antibody confirmed that the annular structures were positive for a-synuclein. This showed that pathological a-synucleinopathy aggregates can be a source of annular a-synuclein species. In contrast to pathological a- synuclein, recombinant a-synuclein yielded only spherical oligomers after detergent treatment, indicating a greater propensity of the pathological protein to form stable annular oligomers. In vitro, we found that Ca2+ binding to monomeric a-synuclein, specifically amongst a range of different metal ions, induced the rapid formation of annular oligomers (Lowe et al., Protein Sci., 13:3245, 2004). Hence, a- synuclein speciation may also be influenced by the intracytoplasmic Ca2+ concentration. We also showed that annular a-synuclein oligomers can nucleate filament formation. We hypothesize that soluble a-synuclein annular oligomers may be cytotoxic species, either by interacting with cell membranes or components of the ubiquitin proteasome system. The equilibrium between a-synuclein species may be influenced by intracellular Ca2+ status, interaction with lipid vesicles or other factors.