Alpha-Synuclein oligomerization and aggregation: All models are useful but only if we know what they model: This is the reply to a comment "Alpha-synuclein oligomerization and aggregation: A model will always be a model" on the original article "Monitoring alpha-synuclein oligomerization and aggregation using bimolecular fluorescence complementation assays: What you see is not always what you get". The articles are accompanied by a Preface "How good are cellular models?".

Alpha-synuclein oligomerization is one of the early events on the pathway to Lewy body formation. Therefore, interfering with this process holds tremendous potential for developing therapies that block α-Syn pathology formation and toxicity. The development of robust and reliable cellular models of alpha-synuclein oligomerization is one important step toward achieving this goal. Unlike α-Syn fibrils, which can be detected and labeled using multiple tools and validated antibodies, α-Syn oligomers are very difficult to differentiate from soluble monomeric α-Syn in cells. This has led to increased reliance on fusing fluorescent proteins or fragments thereof to α-Syn to develop assays and cellular models to investigate α-Syn oligomerization. We recently presented results that highlight the limitation of one of these assays, the α-Syn Bimolecular Fluorescence (BIFC) assay (Frey et al. 2020b). Our findings underscored the critical importance of characterizing and validating cellular models before their use in mechanistic studies or drug discovery studies. In this commentary, I present my response to Dr Tiago Outeiro's recent commentary on this work, expand on our previous discussions on the BIFC assay, and propose an integrated approach for the development characterization, validation, and improvements of cellular models of α-Syn oligomerization and aggregation. Having access to multiple well-characterized and validated cellular models is essential not only for advancing our understanding of the biology of α-Syn and PD but also to identify novel therapeutic targets and drugs that could be successfully developed into treatments for PD and synucleinopathies. The more reliable the models, the faster we are likely to achieve these goals.