FK506-binding proteins (FKBPs) are members of the immunophilins, enzymes that assist protein folding with their peptidyl-prolyl isomerase (PPIase) activity. Some non-immunosuppressive inhibitors of these enzymes have neuroregenerative and neuroprotective properties with an unknown mechanism of action. We have previously shown that FKBPs accelerate the aggregation of ␣-synuclein (␣-SYN) in vitro and in a neuronal cell culture model for synucleinopathy. In this study we investigated whether acceleration of ␣-SYN aggregation is specific for the FKBP or even the PPIase family. Therefore, we studied the effect of several physiologically relevant PPIases, namely FKBP12, FKBP38, FKBP52, FKBP65, Pin1, and cyclophilin A, on ␣-SYN aggregation in vitro and in neuronal cell culture. Among all PPIases tested in vitro, FKBP12 accelerated ␣-SYN aggregation the most. Furthermore, only FKBP12 accelerated ␣-SYN fibril formation at subnanomolar concentrations, pointing toward an enzymatic effect. Although stable overexpression of various FKBPs enhanced the aggregation of ␣-SYN and cell death in cell culture, they were less potent than FKBP12. When FKBP38, FKBP52, and FKBP65 were overexpressed in a stable FKBP12 knockdown cell line, they could not fully restore the number of ␣-SYN inclusion-positive cells. Both in vitro and cell culture data provide strong evidence that FKBP12 is the most important PPIase modulating ␣-SYN aggregation and validate the protein as an interesting drug target for Parkinson disease.
Parkinson disease (PD)2 is the second most common neurodegenerative disorder. Accumulating evidence points to a causative role for the protein ␣-synuclein (␣-SYN) in PD (1-13), but the exact relationship between ␣-SYN aggregation and pathogenesis remains unresolved.We have recently shown that FKBP12, a member of the FK506-binding proteins (FKBPs), accelerates the aggregation of ␣-SYN in vitro (14). Furthermore, we demonstrated that FKBP12 and FKBP52 also enhance the aggregation of ␣-SYN in a neuronal cell culture model for synucleinopathy (15). FK506, a small molecule inhibitor of the FKBPs, counteracts this effect in a dose-dependent way. In addition, knockdown of FKBP12 and FKBP52 decreased the number of ␣-SYN aggregates in this cellular model and protected against cell death.FKBP12 and FKBP52 belong to the human FKBP family, of which currently 15 members have been identified. FKBPs are members of the immunophilins. These are enzymes that bind immunosuppressant drugs such as FK506 and have a peptidylprolyl cis-trans isomerase (PPIase) activity (16). They are able to accelerate the interconversion of cis-trans isomers of Xaa-Pro peptide bonds. This is an energy-demanding step in protein folding (17). Several other functions have also been assigned to the human FKBP family, such as regulation of Ca 2ϩ levels in muscles (18,19), regulation of immune activation (20), and chemotropic nerve guidance (21). In addition, it was also shown that immunophilin ligands, such as FK506, exhibit significant neuroregenerative and neuroprotec...
