Antibody-aided clearance of extracellular α-synuclein prevents cell-to-cell aggregate transmission.

Abnormal deposition and intercellular propagation of α-synuclein plays a central role in the pathogenesis of disorders such as Parkinson's Disease (PD) and dementia with Lewy bodies (DLB). Previous studies demonstrated that immunization against α-synuclein resulted in reduced α-synuclein accumulation and synaptic loss in a transgenic (tg) mouse model, highlighting the potential for immunotherapy. However, the mechanism by which immunization prevents synucleinopathy-associated deficits remains unknown. Here, we show that antibodies against α-synuclein specifically target and aid in clearance of extracellular α-synuclein proteins by microglia, thereby preventing their actions on neighboring cells. Antibody-assisted clearance occurs mainly in microglia through the Fcγ receptor, and not in neuronal cells or astrocytes. Stereotaxic administration of antibody into the brains of α-synuclein tg mice prevented neuron-to-astroglia transmission of α-synuclein and led to increased localization of α-synuclein and the antibody in microglia. Furthermore, passive immunization with α-synuclein antibody reduced neuronal and glial accumulation of α-synuclein and ameliorated neurodegeneration and behavioral deficits associated with α-synuclein overexpression. These findings provide an underlying mechanistic basis for immunotherapy for PD/DLB and suggest extracellular forms of α-synuclein as potential therapeutic targets.

Glucocerebrosidase, a new player changing the old rules in Lewy body diseases.

Mutations in the gene encoding glucocerebrosidase (GBA1) cause Gaucher disease (GD), a lysosomal storage disease with recessive inheritance. Glucocerebrosidase (GCase) is a lysosomal lipid hydrolase that digests glycolipid substrates, such as glucosylceramide and glucosylsphingosine. GBA1 mutations have been implicated in Lewy body diseases (LBDs), such as Parkinson's disease and dementia with Lewy bodies. Parkinsonism occurs more frequently in certain types of GD, and GBA1 mutation carriers are more likely to have LBDs than non-carriers. Furthermore, GCase is often found in Lewy bodies, which are composed of α-synuclein fibrils as well as a variety of proteins and vesicles. In this review, we discuss potential mechanisms of action of GBA1 mutations in LBDs with particular emphasis on α-synuclein aggregation by reviewing the current literature on the role of GCase in lysosomal functions and glycolipid metabolism.

Loss of glucocerebrosidase 1 activity causes lysosomal dysfunction and α-synuclein aggregation.

Lysosomal dysfunction is a common pathological feature of neurodegenerative diseases. GTP-binding protein type A1 (GBA1) encodes ß-glucocerebrosidase 1 (GCase 1), a lysosomal hydrolase. Homozygous mutations in GBA1 cause Gaucher disease, the most common lysosomal storage disease, while heterozygous mutations are strong risk factors for Parkinson's disease. However, whether loss of GCase 1 activity is sufficient for lysosomal dysfunction has not been clearly determined. Here, we generated human neuroblastoma cell lines with nonsense mutations in the GBA1 gene using zinc-finger nucleases. Depending on the site of mutation, GCase 1 activity was lost or maintained. The cell line with GCase 1 deficiency showed indications of lysosomal dysfunction, such as accumulation of lysosomal substrates, reduced dextran degradation and accumulation of enlarged vacuolar structures. In contrast, the cell line with C-terminal truncation of GCase 1 but with intact GCase 1 activity showed normal lysosomal function. When α-synuclein was overexpressed, accumulation and secretion of insoluble aggregates increased in cells with GCase 1 deficiency but did not change in mutant cells with normal GCase 1 activity. These results demonstrate that loss of GCase 1 activity is sufficient to cause lysosomal dysfunction and accumulation of α-synuclein aggregates.

Glucocerebrosidase depletion enhances cell-to-cell transmission of α-synuclein.

Deposition of α-synuclein aggregates occurs widely in the central and peripheral nervous systems in Parkinson's disease (PD). Although recent evidence has suggested that cell-to-cell transmission of α-synuclein aggregates is associated with the progression of PD, the mechanism by which α-synuclein aggregates spread remains undefined. Here, we show that α-synuclein aggregates are transmitted from cell to cell through a cycle involving uptake of external aggregates, co-aggregation with endogenous α-synuclein and exocytosis of the co-aggregates. Moreover, we find that glucocerebrosidase depletion, which has previously been strongly associated with PD and increased cognitive impairment, promotes propagation of α-synuclein aggregates. These studies define how α-synuclein aggregates spread among neuronal cells and may provide an explanation for how glucocerebrosidase mutations increase the risk of developing PD and other synucleinopathies.