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.

α-突触核蛋白 (α-syn) 病理学的细胞间传递被认为是帕金森病 (PD) 神经变性传播的基础。先前的研究表明，α-syn 在神经元细胞系和原代神经元的生理条件下分泌。然而，调节细胞外α-syn分泌的分子机制仍不清楚。在这项研究中，我们发现抑制单胺氧化酶-B (MAO-B) 酶活性促进了人神经母细胞瘤 SH-SY5Y 细胞中的 α-syn 分泌。司来吉兰或雷沙吉兰对 MAO-B 的抑制和 siRNA 介导的 MAO-B 敲低均促进了 α-syn 分泌。然而，雷沙吉兰的 S-异构体 TVP-1022 活性低 1000 倍，未能促进 α-syn 分泌。此外，MAO-B 抑制诱导的 α-syn 分泌增加不受 brefeldin A 的影响，brefeldin A 抑制内质网 (ER)/高尔基体转运，但被丙磺舒和格列本脲阻断，抑制 ATP 结合盒 (ABC) 转运蛋白功能。MAO-B 抑制优先促进洗涤剂不溶性 α-syn 蛋白的分泌，并在氯喹诱导的溶酶体功能障碍下减少其细胞内积累。此外，在通过注射重组腺相关病毒 (rAAV)-A53T α-syn 产生的大鼠模型（雄性 Sprague Dawley 大鼠）中，司来吉兰的皮下给药延迟了 Ser129 磷酸化 α-syn 聚集体的纹状体形成，并减轻了黑质纹状体多巴胺能神经元。在通过注射 rAAV 野生型 α-syn 并将 α-syn 原纤维外部接种到纹状体中产生的细胞间传递大鼠模型（雄性 Sprague Dawley 大鼠）中，司来吉兰还延迟了 α-syn 聚集和多巴胺能神经元丢失。这些发现表明，MAO-B 抑制通过 ABC 转运蛋白介导的非经典分泌途径调节去污剂不溶性 α-syn 的细胞内清除，并暂时抑制 α-syn 聚集体的形成和传递。

重要性声明治疗帕金森病 (PD) 需要鉴定一种减缓或停止运动障碍进展的神经保护剂。α-突触核蛋白 (α-syn) 聚集过程被认为是 PD 神经变性的基础。在这里，我们证明了 SH-SY5Y 细胞中单胺氧化酶-B (MAO-B) 的药理学抑制或敲低通过涉及 ATP 结合盒 (ABC) 转运蛋白的非经典途径促进了 α-syn 分泌。MAO-B 抑制优先促进洗涤剂不溶性 α-syn 蛋白的分泌，并在氯喹诱导的溶酶体功能障碍下减少其细胞内积累。此外，司来吉兰对 MAO-B 的抑制保护了 A53T α-syn 诱导的黑质纹状体多巴胺能神经元丢失，并抑制了大鼠模型中 α-syn 聚集体的形成和细胞间传递。因此，我们提出了一种新的 MAO-B 抑制功能，可以调节 α-syn 的分泌和聚集。