Synucleinopathies such as dementia with Lewy bodies or Parkinson's disease are characterized by intracellular deposition of pathologically aggregated a-synuclein. The details of the molecular pathogenesis of PD and especially the conditions that lead to intracellular aggregation of a-synuclein and the role of these aggregates in cell death remain unknown. In cell free in vitro systems considerable knowledge about the aggregation processes has been gathered. In comparison, the knowledge about these aggregation processes in cells is far behind. In cells alpha-synuclein aggregates can be toxic. However, the crucial particle species responsible for decisive steps in pathogenesis such as seeding a continuing aggregation process and triggering cell death remain to be identified. In order to understand the complex nature of intracellular alpha-synuclein aggregate formation, we analyzed fluorescent particles formed by venus and alpha-synuclein-venus fusion proteins and alpha-synuclein-hemi-venus fusion proteins derived from gently lyzed cells. With these techniques we were able to identify and characterize alpha-synuclein oligomers formed in cells. Especially the use of alpha-synuclein-hemi-venus fusion proteins enabled us to identify very small alpha-synuclein oligomers with high sensitivity. Furthermore, we were able to study the molecular effect of heat shock protein 70, which is known to inhibit alpha-synuclein aggregation in cells. Heat shock protein 70 does not only influence the size of alpha-synuclein oligomers, but also their quantity. In summary, this approach based on fluorescence single particle spectroscopy, that is suited for high throughput measurements, can be used to detect and characterize intracellularly formed alpha-synuclein aggregates and characterize the effect of molecules that interfere with alpha-synuclein aggregate formation. (C) 2016 Elsevier Inc. All rights reserved.