Monoamine Oxidase-B Inhibition Facilitates α-Synuclein Secretion In Vitro and Delays Its Aggregation in rAAV-Based Rat Models of Parkinson's Disease.

Cell-to-cell transmission of α-synuclein (α-syn) pathology is considered to underlie the spread of neurodegeneration in Parkinson’s disease. Previous studies have demonstrated that α-syn is secreted under physiological conditions in neuronal cell lines and primary neurons. However, the molecular mechanisms that regulate extracellular α-syn secretion remain unclear. In this study, we found that inhibition of monoamine oxidase-B (MAO-B) enzymatic activity facilitated α-syn secretion in human neuroblastoma SH-SY5Y cells. Both inhibition of MAO-B by selegiline or rasagiline and siRNA-mediated knockdown of MAO-B facilitated α-syn secretion. However, TVP-1022, the S-isomer of rasagiline that is 1,000 times less active, failed to facilitate α-syn secretion. Additionally, the MAO-B inhibition-induced increase in α-syn secretion was unaffected by brefeldin A, which inhibits endoplasmic reticulum/Golgi transport, but was blocked by probenecid and glyburide, which inhibit ATP-binding cassette (ABC) transporter function. MAO-B inhibition preferentially facilitated the secretion of detergent-insoluble α-syn protein and decreased its intracellular accumulation under chloroquine-induced lysosomal dysfunction. Moreover, in a rat model (male Sprague-Dawley rats) generated by injecting recombinant adeno-associated virus (rAAV)-A53T α-syn, intracutaneous administration of selegiline delayed the striatal formation of Ser129-phosphorylated α-syn aggregates, and mitigated loss of nigrostriatal dopaminergic neurons. Selegiline also delayed α-syn aggregation and dopaminergic neuronal loss in a cell-to-cell transmission rat model (male Sprague-Dawley rats) generated by injecting rAAV-wild-type α-syn and externally inoculating α-syn fibrils into the striatum. These findings suggest that MAO-B inhibition modulates the intracellular clearance of detergent-insoluble α-syn via the ABC transporter-mediated non-classical secretion pathway, and temporarily suppresses the formation and transmission of α-syn aggregates. Significance statement The identification of a neuroprotective agent that slows or stops the progression of motor impairments is required to treat Parkinson’s disease (PD). The process of α-synuclein aggregation is thought to underlie neurodegeneration in PD. Here, we demonstrated that pharmacological inhibition or knockdown of monoamine oxidase-B (MAO-B) in SH-SY5Y cells facilitated α-synuclein secretion via a non-classical pathway involving an ATP-binding cassette transporter. MAO-B inhibition preferentially facilitated secretion of detergent-insoluble α-synuclein protein and reduced its intracellular accumulation under chloroquine-induced lysosomal dysfunction. Additionally, MAO-B inhibition by selegiline protected A53T α-synuclein-induced nigrostriatal dopaminergic neuronal loss and suppressed the formation and cell-to-cell transmission of α-synuclein aggregates in rat models. We therefore propose a new function of MAO-B inhibition that modulates α-synuclein secretion and aggregation.