Alpha-synuclein (α-SYN) is a key player in the pathogenesis of Parkinson's disease (PD). In pathological conditions, the protein is present in a fibrillar, aggregated form inside cytoplasmic inclusions called Lewy bodies. The second protein family prominently present in this work are the FK506 Binding Proteins (FKBP). The original link between these seemingly unrelated proteins was an accidental discovery that FKBPs could accelerate α-SYN aggregation. My PhD project set out to extensively investigate the nature and the importance of these initial findings in increasingly complex model systems for PD. Our original findings showed that FKBP12 accelerates the aggregation of α-SYN in vitro. This effect could be abrogated by FK506, a specific enzymatic inhibitor of the FKBP family. Therefore, we first wanted to confirm these results in a cell culture model for synucleinopathy. Indeed, FK506 also dose-dependently inhibited α-SYN aggregation and neuronal cell death in cell culture. Moreover,knockdown (KD) of FKBP12 or FKBP52 reduced the number of α-SYN aggregates and protected against cell death, whereas overexpression of FKBP12 or FKBP52 had the opposite effect. Due to their common enzymatic activity, the FKBPs, cyclophilins and parvulins belong to the peptidyl-prolyl isomerase (PPIase) family. To investigate if acceleration of α-SYN aggregation is specific for the FKBP or even the PPIase family, we next studied the effect of several physiologically relevant PPIases, namely FKBP12, FKBP38, FKBP52, FKBP65, Pin1, and cyclophilin A, on α-SYN aggregation in vitro and in neuronal cell culture. Among all PPIases tested in vitro, FKBP12 accelerated α-SYN aggregation the most. Furthermore, only FKBP12 accelerated α-SYN fibril formation at subnanomolar concentrations, pointing toward an enzymatic effect. Although stable overexpression of various other FKBPs enhanced the aggregation of α-SYN and cell death in cell culture, they were less potent than FKBP12. When FKBP38, FKBP52, and FKBP65 were overexpressed in a stable FKBP12 KD cell line, they could not fully restore the number of α-SYN inclusion-positive cells.Since both in vitro and cell culture data provided strong evidence that FKBP12 is the most important PPIase modulating α-SYN aggregation, we finally downregulated FKBP12 via adeno-associated viral vectors (AAV) in the brain of α-SYN transgenic mice. Stereological quantification of the number of α-SYN aggregate-positive cells, revealed that FKBP12 knockdown led to a lower number of inclusion-positive cells compared to mice injected with a control vector.All data together validate FKBP12 as a novel drug target for a causative treatment of PD.