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 (PD).

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 knock-down of MAO-B facilitated α-syn secretion.

However, TVP-1022, the S-isomer of rasagiline that is 1000 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 (ER)/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, subcutaneous 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 (α-syn) aggregation is thought to underlie neurodegeneration in PD.

Here, we demonstrated that pharmacological inhibition or knock-down of monoamine oxidase-B (MAO-B) in SH-SY5Y cells facilitated α-syn secretion via a non-classical pathway involving an ATP-binding cassette (ABC) transporter.

MAO-B inhibition preferentially facilitated secretion of detergent-insoluble α-syn protein and reduced its intracellular accumulation under chloroquine-induced lysosomal dysfunction.

Additionally, MAO-B inhibition by selegiline protected A53T α-syn-induced nigrostriatal dopaminergic neuronal loss and suppressed the formation and cell-to-cell transmission of α-syn aggregates in rat models.

We therefore propose a new function of MAO-B inhibition that modulates α-syn secretion and aggregation.