INTRODUCTION: Parkinson’s disease (PD) is the second most common neurodegenerative disorder and leads to slowness of movement, tremor, rigidity, and, in the later stages of PD, cognitive impairment. Pathologically, PD is characterized by the accumulation of α-synuclein in Lewy bodies and neurites. There is degeneration of neurons throughout the nervous system, with the degeneration of dopamine neurons in the substantia nigra pars compacta leading to the major symptoms of PD. RATIONALE: In the brains of PD patients, pathologic α-synuclein seems to spread from cell to cell via self-amplification, propagation, and transmission in a stereotypical and topographical pattern among neighboring cells and/or anatomically connected brain regions. The spread or transmission of pathologic α-synuclein is emerging as a potentially important driver of PD pathogenesis. The underlying mechanisms and molecular entities responsible for the transmission of pathologic α-synuclein from cell to cell are not known, but the entry of pathologic α-synuclein into neurons is thought to occur, in part, through an active clathrin-dependent endocytic process. RESULTS: Using recombinant α-synuclein preformed fibrils (PFF) as a model system with which to study the transmission of misfolded α-synuclein from neuron to neuron, we screened a library encoding transmembrane proteins for α-synuclein-biotin PFF–binding candidates via detection with streptavidin-AP (alkaline phosphatase) staining. Three positive clones were identified that bind α-synuclein PFF and include lymphocyte-activation gene 3 (LAG3), neurexin 1β, and amyloid β precursor-like protein 1 (APLP1). Of these three transmembrane proteins, LAG3 demonstrated the highest ratio of selectivity for α-synuclein PFF over the α-synuclein monomer. α-Synuclein PFF bind to LAG3 in a saturable manner (dissociation constant = 77 nM), whereas the α-synuclein monomer does not bind to LAG3. Co-immunoprecipitation also suggests that pathological α-synuclein PFF specifically bind to LAG3. Tau PFF, β-amyloid oligomer, and β-amyloid PFF do not bind to LAG3, indicating that LAG3 is specific for α-synuclein PFF. The internalization of α-synuclein PFF involves LAG3 because deletion of LAG3 reduces the endocytosis of α-synuclein PFF. LAG3 colocalizes with the endosomal guanosine triphosphatases Rab5 and Rab7 and coendocytoses with pathologic α-synuclein. Neuron-to-neuron transmission of pathologic α-synuclein and the accompanying pathology and neurotoxicity is substantially attenuated by deletion of LAG3 or by antibodies to LAG3. The lack of LAG3 also substantially delayed α-synuclein PFF–induced loss of dopamine neurons, as well as biochemical and behavioral deficits in vivo. CONCLUSION: We discovered that pathologic α-synuclein transmission and toxicity is initiated by binding to LAG3 and that neuron-to-neuron transmission of pathological α-synuclein involves the endocytosis of exogenous α-synuclein PFF by the engagement of LAG3 on neurons. Depletion of LAG3 or antibodies to LAG3 substantially reduces the pathology set in motion by the transmission of pathologic α-synuclein. The identification of LAG3 as an α-synuclein PFF–binding protein provides a new target for developing therapeutics designed to slow the progression of PD and related α-synucleinopathies. LAG3 deletion or antibodies to LAG3 delay α-synuclein PFF transmission.: Compared with wild-type neurons, binding and endocytosis of α-synuclein PFF is dramatically reduced with antibodies to LAG3 or when LAG3 is deleted, resulting in delayed pathologic α-synuclein transmission and toxicity. Illustration credit: I-Hsun Wu

PAR promotes α-synuclein toxicity: How pathologic α-synuclein (α-syn) leads to neurodegeneration in Parkinson's disease (PD) remains poorly understood. Kam et al. studied the α-syn preformed fibril (α-syn PFF) model of sporadic PD (see the Perspective by Brundin and Wyse). They found that pathologic α-syn–activated poly(adenosine 5′-diphosphate–ribose) (PAR) polymerase–1 (PARP-1) and inhibition of PARP or knockout of PARP-1 protected mice from pathology. The generation of PAR by α-syn PFF–induced PARP-1 activation converted α-syn PFF to a strain that was 25-fold more toxic, termed PAR–α-syn PFF. An increase of PAR in the cerebrospinal fluid and evidence of PARP activation in the substantia nigra of PD patients indicates that PARP activation contributes to the pathogenesis of PD through parthanatos and conversion of α-syn to a more toxic strain. Science , this issue p. eaat8407 ; see also p. 521

Mutations in glucocerebrosidase (GBA) cause Gaucher disease (GD) and increase the risk of developing Parkinson’s disease (PD) and Dementia with Lewy Bodies (DLB). Since both genetic and environmental factors contribute to the pathogenesis of sporadic PD, we investigated the susceptibility of nigrostriatal dopamine (DA) neurons in L444P GBA heterozygous knock-in (GBA +/L444P ) mice to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a selective dopaminergic mitochondrial neurotoxin. We used GBA +/L444P mice, α-synuclein knockout (SNCA −/− ) mice at 8 months of age, and adeno-associated virus (AAV)-human GBA overexpression to investigate the rescue effect of DA neuronal loss and susceptibility by MPTP. Mitochondrial morphology and functional assay were used to identify mitochondrial defects in GBA +/L444P mice. Motor behavioral test, immunohistochemistry, and HPLC were performed to measure dopaminergic degeneration by MPTP and investigate the relationship between GBA mutation and α-synuclein. Mitochondrial immunostaining, qPCR, and Western blot were also used to study the effects of α-synuclein knockout or GBA overexpression on MPTP-induced mitochondrial defects and susceptibility. L444P GBA heterozygous mutation reduced GBA protein levels, enzymatic activity and a concomitant accumulation of α-synuclein in the midbrain of GBA +/L444P mice. Furthermore, the deficiency resulted in defects in mitochondria of cortical neurons cultured from GBA +/L444P mice. Notably, treatment with MPTP resulted in a significant loss of dopaminergic neurons and striatal dopaminergic fibers in GBA +/L444P mice compared to wild type (WT) mice. Levels of striatal DA and its metabolites were more depleted in the striatum of GBA +/L444P mice. Behavioral deficits, neuroinflammation, and mitochondrial defects were more exacerbated in GBA +/L444P mice after MPTP treatment. Importantly, MPTP induced PD-like symptoms were significantly improved by knockout of α-synuclein or augmentation of GBA via AAV5-hGBA injection in both WT and GBA +/L444P mice. Intriguingly, the degree of reduction in MPTP induced PD-like symptoms in GBA +/L444P α-synuclein (SNCA) −/− mice was nearly equal to that in SNCA −/− mice after MPTP treatment. Our results suggest that GBA deficiency due to L444P GBA heterozygous mutation and the accompanying accumulation of α-synuclein render DA neurons more susceptible to MPTP intoxication. Thus, GBA and α-synuclein play dual physiological roles in the survival of DA neurons in response to the mitochondrial dopaminergic neurotoxin, MPTP.