A New Glucocerebrosidase Chaperone Reduces α-Synuclein and Glycolipid Levels in iPSC-Derived Dopaminergic Neurons from Patients with Gaucher Disease and Parkinsonism

Silencing of Glucocerebrosidase Gene in Drosophila Enhances the Aggregation of Parkinson's Disease Associated α-Synuclein Mutant A53T and Affects Locomotor Activity

Background: Mutations in glucocerebrosidase (GBA), a lysosomal enzyme are the most common genetic risk factor for developing Parkinson's disease (PD). We studied how reduced GCase activity affects α-synuclein (α-syn) and its mutants (A30P and A53T) aggregation, neurodegeneration, sleep and locomotor behavior in a fly model of PD.

Methods: We developed drosophila with GBA gene knockdown (RNAi) (with reduced GCase activity) that simultaneously expresses either wildtype (WT) or mutants such as A30P or A53T α-syn. Western blot and confocal microscopy were performed to study the α-syn aggregation and neurodegeneration in these flies. We also studied the sleep and locomotor activity of those flies using Drosophila activity monitor (DAM) system.

Results: Western blot analysis showed that GBA RNAi A53T α-syn flies (30 days old) had an increased level of Triton insoluble synuclein (that corresponds to α-syn aggregates) compared to corresponding A53T flies without GBA RNAi (control), while mRNA expression of α-syn remained unchanged. Confocal imaging of whole brain staining of 30 days old drosophila showed a statistically significant decrease in neuron numbers in PPL1 cluster in flies expressing α-syn WT, A30P and A53T in the presence GBA RNAi compared to corresponding control. Staining with conformation specific antibody for α-syn aggregates showed an increased number of neurons staining for α-syn aggregates in A53T fly brain with GBA RNAi compared to control A53T flies, thus confirming our protein analysis finding that under decreased GBA enzyme activity, mutant A53T aggregates more than the control A53T without GBA silencing. Sleep analysis revealed decreased total activity in GBA silenced flies expressing mutant A53T compared to both A53T control flies and GBA RNAi flies without synuclein expression.

Conclusion: In A53T flies with reduced GCase activity, there is increased α-syn aggregation and dopamine (DA) neuronal loss. This study demonstrates that reduced GCase activity both in the context of heterozygous GBA1 mutation associated with PD and in old age, contribute to increased aggregation of mutant α-syn A53T and exacerbates the phenotype in a fly model of PD.

Glucocerebrosidase and Parkinson Disease: Molecular, Clinical, and Therapeutic Implications

Parkinson disease (PD) is a complex neurodegenerative disease characterised by multiple motor and non-motor symptoms. In the last 20 years, more than 20 genes have been identified as causes of parkinsonism. Following the observation of higher risk of PD in patients affected by Gaucher disease, a lysosomal disorder caused by mutations in the glucocerebrosidase (GBA) gene, it was discovered that mutations in this gene constitute the single largest risk factor for development of idiopathic PD. Patients with PD and GBA mutations are clinically indistinguishable from patients with idiopathic PD, although some characteristics emerge depending on the specific mutation, such as slightly earlier onset. The molecular mechanisms which lead to this increased PD risk in GBA mutation carriers are multiple and not yet fully elucidated, they include alpha-synuclein aggregation, lysosomal-autophagy dysfunction and endoplasmic reticulum stress. Moreover, dysfunction of glucocerebrosidase has also been demonstrated in non-GBA PD, suggesting its interaction with other pathogenic mechanisms. Therefore, GBA enzyme function represents an interesting pharmacological target for PD. Cell and animal models suggest that increasing GBA enzyme activity can reduce alpha-synuclein levels. Clinical trials of ambroxol, a glucocerebrosidase chaperone, are currently ongoing in PD and PD dementia, as is a trial of substrate reduction therapy. The aim of this review is to summarise the main features of GBA-PD and discuss the implications of glucocerebrosidase modulation on PD pathogenesis.

Effects of ambroxol on the autophagy-lysosome pathway and mitochondria in primary cortical neurons

Glucocerebrosidase (GBA1) mutations are the major genetic risk factor for Parkinson's Disease (PD). The pathogenic mechanism is still unclear, but alterations in lysosomal-autophagy processes are implicated due to reduction of mutated glucocerebrosidase (GCase) in lysosomes. Wild-type GCase activity is also decreased in sporadic PD brains. Small molecule chaperones that increase lysosomal GCase activity have potential to be disease-modifying therapies for GBA1-associated and sporadic PD. Therefore we have used mouse cortical neurons to explore the effects of the chaperone ambroxol. This chaperone increased wild-type GCase mRNA, protein levels and activity, as well as increasing other lysosomal enzymes and LIMP2, the GCase transporter. Transcription factor EB (TFEB), the master regulator of the CLEAR pathway involved in lysosomal biogenesis was also increased upon ambroxol treatment. Moreover, we found macroautophagy flux blocked and exocytosis increased in neurons treated with ambroxol. We suggest that ambroxol is blocking autophagy and driving cargo towards the secretory pathway. Mitochondria content was also found to be increased by ambroxol via peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α). Our data suggest that ambroxol, besides being a GCase chaperone, also acts on other pathways, such as mitochondria, lysosomal biogenesis, and the secretory pathway.

Pathological role of lipid interaction with α-synuclein in Parkinson's disease

Alpha-synuclein (αSyn) plays a central role in the pathogenesis of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). In sporadic PD and DLB, normally harmless αSyn proteins without any mutations might gain toxic functions by unknown mechanisms. Thus, it is important to elucidate the factors promoting the toxic conversion of αSyn, towards understanding the pathogenesis of and developing disease-modifying therapies for PD and DLB. Accumulating biophysical and biochemical studies have demonstrated that αSyn interacts with lipid membrane, and the interaction influences αSyn oligomerization and aggregation. Furthermore, genetic and clinicopathological studies have revealed mutations in the glucocerebrosidase 1 (GBA1) gene, which encodes a degrading enzyme for the glycolipid glucosylceramide (GlcCer), as strong risk factors for PD and DLB, and we recently demonstrated that GlcCer promotes toxic conversion of αSyn. Moreover, pathological studies have shown the existence of αSyn pathology in lysosomal storage disorders (LSDs) patient' brain, in which glycosphingolipids (GSLs) is found to be accumulated. In this review, we focus on the lipids as a key factor for inducing wild-type (WT) αSyn toxic conversion, we summarize the knowledge about the interaction between αSyn and lipid membrane, and propose our hypothesis that aberrantly accumulated GSLs might contribute to the toxic conversion of αSyn. Identifying the trigger for toxic conversion of αSyn would open a new therapeutic road to attenuate or prevent crucial events leading to the formation of toxic αSyn.

Altered Differentiation Potential of Gaucher's Disease iPSC Neuronal Progenitors due to Wnt/β-Catenin Downregulation

Gaucher’s disease (GD) is an autosomal recessive disorder caused by mutations in the GBA1 gene, which encodes acid β-glucocerebrosidase (GCase). Severe GBA1 mutations cause neuropathology that manifests soon after birth, suggesting that GCase deficiency interferes with neuronal development. We found that neuronopathic GD induced pluripotent stem cell (iPSC)-derived neuronal progenitor cells (NPCs) exhibit developmental defects due to downregulation of canonical Wnt/β-catenin signaling and that GD iPSCs’ ability to differentiate to dopaminergic (DA) neurons was strikingly reduced due to early loss of DA progenitors. Incubation of the mutant cells with the Wnt activator CHIR99021 (CHIR) or with recombinant GCase restored Wnt/β-catenin signaling and rescued DA differentiation. We also found that GD NPCs exhibit lysosomal dysfunction, which may be involved in Wnt downregulation by mutant GCase. We conclude that neuronopathic mutations in GCase lead to neurodevelopmental abnormalities due to a critical requirement of this enzyme for canonical Wnt/β-catenin signaling at early stages of neurogenesis.

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Neuronopathic GBA1 mutations attenuate canonical Wnt signaling in iPSC-derived NPCs

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GD NPC differentiation to DA neurons impaired due to early loss of DA progenitors

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GBA1-mediated lysosomal alterations may be involved in Wnt signal downregulation

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The Wnt pathway may be a potential new therapeutic target for neuronopathic GD

Emptying the stores: lysosomal diseases and therapeutic strategies

Lysosomal storage disorders (LSDs) - designated as 'orphan' diseases - are inborn errors of metabolism caused by defects in genes that encode proteins involved in various aspects of lysosomal homeostasis. For many years, LSDs were viewed as unattractive targets for the development of therapies owing to their low prevalence. However, the development and success of the first commercial biologic therapy for an LSD - enzyme replacement therapy for type 1 Gaucher disease - coupled with regulatory incentives rapidly catalysed commercial interest in therapeutically targeting LSDs. Despite ongoing challenges, various therapeutic strategies for LSDs now exist, with many agents approved, undergoing clinical trials or in preclinical development.

Molecular mechanisms of α-synuclein and GBA1 in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative movement disorder characterized pathologically by the presence of Lewy bodies comprised of insoluble alpha (α)-synuclein. Pathological, clinical and genetic studies demonstrate that mutations in the GBA1 gene, which encodes the lysosomal enzyme glucocerebrosidase (GCase) that is deficient in Gaucher's disease, are important risk factors for the development of PD. The molecular mechanism for the association between these two diseases is not completely understood. We discuss several possible mechanisms that may lead to GBA1-related neuronal death and α-synuclein accumulation including disruptions in lipid metabolism, protein trafficking and impaired protein quality control mechanisms. Elucidating the mechanism between GCase and α-synuclein may provide insight into potential therapeutic pathways for PD and related synucleinopathies.

Neurological manifestations of lysosomal disorders and emerging therapies targeting the CNS

Lysosomal disorders have been an area of interest since intravenous enzyme replacement therapy was successfully introduced for the treatment of Gaucher's disease in the early 1990s. This treatment approach has also been developed for several other lysosomal disorders, including Fabry's disease, Pompe's disease, lysosomal acid lipase deficiency, and five types of mucopolysaccharidosis. Despite the benefits of enzyme replacement therapy, it has limitations-most importantly, its ineffectiveness in treating the neurological components of lysosomal disorders, as only a small proportion of recombinant enzymes can cross the blood-brain barrier. Development of strategies to improve drug delivery to the CNS is now the primary focus in lysosomal disorder research. This Review discusses the neurological manifestations and emerging therapies for the CNS component of these diseases. The therapies in development (which are now in phase 1 or phase 2 clinical trials) might be for specific lysosomal disorders (enzyme replacement therapy via intrathecal or intracerebroventricular routes or with fusion proteins, or gene therapy) or applicable to more than one lysosomal disorder (haemopoietic stem cell transplantation, pharmacological chaperones, substrate reduction therapy, or stop codon readthrough). The combination of early diagnosis with effective therapies should change the outlook for patients with lysosomal disorders with neurological involvement in the next 5-10 years.

Endo-lysosomal dysfunction: a converging mechanism in neurodegenerative diseases

Endo-lysosomal pathways are essential in maintaining protein homeostasis in the cell. Numerous genes in the endo-lysosomal pathways have been found to associate with neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and frontotemporal dementia (FTD). Mutations of these genes lead to dysfunction in multiple steps of the endo-lysosomal network: autophagy, endocytic trafficking and lysosomal degradation, resulting in accumulation of pathogenic proteins. Although the exact pathogenic mechanism varies for different disease-associated genes, dysfunction of the endo-lysosomal pathways represents a converging mechanism shared by these diseases. Therefore, strategies that correct or compensate for endo-lysosomal dysfunction may be promising therapeutic approaches to treat neurodegenerative diseases.

Insights into the structural biology of Gaucher disease

Gaucher disease, the most common lysosomal storage disorder, is caused by mutations in the gene encoding the acid-β-glucosidase lysosomal hydrolase enzyme that cleaves glucocerebroside into glucose and ceramide. Reduced enzyme activity and impaired structural stability arise due to >300 known disease-causing mutations. Several of these mutations have also been associated with an increased risk of Parkinson disease (PD). Since the discovery of the acid-β-glucosidase X-ray structure, there have been major advances in our understanding of the structural properties of the protein. Analysis of specific residues has provided insight into their functional and structural importance and provided insight into the pathogenesis of Gaucher disease and the contribution to PD. Disease-causing mutations are positioned throughout the acid-β-glucosidase structure, with many located far from the active site and thus retaining some enzymatic activity however, thus far no clear relationship between mutation location and disease severity has been established. Here, we review the crystal structure of acid-β-glucosidase, while highlighting important structural aspects of the protein in detail. This review discusses the structural stability of acid-β-glucosidase, which can be altered by pH and glycosylation, and explores the relationship between known Gaucher disease and PD mutations, structural stability and disease severity.

The Complicated Relationship between Gaucher Disease and Parkinsonism: Insights from a Rare Disease

The discovery of a link between mutations in GBA1, encoding the lysosomal enzyme glucocerebrosidase, and the synucleinopathies directly resulted from the clinical recognition of patients with Gaucher disease with parkinsonism. Mutations in GBA1 are now the most common known genetic risk factor for several Lewy body disorders, and an inverse relationship exists between levels of glucocerebrosidase and oligomeric α-synuclein. While the underlying mechanisms are still debated, this complicated association is shedding light on the role of lysosomes in neurodegenerative disorders, demonstrating how insights from a rare disorder can direct research into the pathogenesis and therapy of seemingly unrelated common diseases.

AAV-PHP.B-Mediated Global-Scale Expression in the Mouse Nervous System Enables GBA1 Gene Therapy for Wide Protection from Synucleinopathy

The lack of technology for direct global-scale targeting of the adult mouse nervous system has hindered research on brain processing and dysfunctions. Currently, gene transfer is normally achieved by intraparenchymal viral injections, but these injections target a restricted brain area. Herein, we demonstrated that intravenous delivery of adeno-associated virus (AAV)-PHP.B viral particles permeated and diffused throughout the neural parenchyma, targeting both the central and the peripheral nervous system in a global pattern. We then established multiple procedures of viral transduction to control gene expression or inactivate gene function exclusively in the adult nervous system and assessed the underlying behavioral effects. Building on these results, we established an effective gene therapy strategy to counteract the widespread accumulation of α-synuclein deposits throughout the forebrain in a mouse model of synucleinopathy. Transduction of A53T-SCNA transgenic mice with AAV-PHP.B-GBA1 restored physiological levels of the enzyme, reduced α-synuclein pathology, and produced significant behavioral recovery. Finally, we provided evidence that AAV-PHP.B brain penetration does not lead to evident dysfunctions in blood-brain barrier integrity or permeability. Altogether, the AAV-PHP.B viral platform enables non-invasive, widespread, and long-lasting global neural expression of therapeutic genes, such as GBA1, providing an invaluable approach to treat neurodegenerative diseases with diffuse brain pathology such as synucleinopathies.

The Role of Astrocyte Dysfunction in Parkinson's Disease Pathogenesis

Astrocytes are the most populous glial subtype and are critical for brain function. Despite this, historically there have been few studies into the role that they may have in neurodegenerative diseases, such as Parkinson's disease (PD). Recently, however, several studies have determined that genes known to have a causative role in the development of PD are expressed in astrocytes and have important roles in astrocyte function. Here, we review these recent developments and discuss their impact on our understanding of the pathophysiology of PD, and the implications that this might have for its treatment.

Cellular models as tools for the study of the role of alpha-synuclein in Parkinson's disease

Neurodegenerative diseases are highly debilitating conditions characterised primarily by progressive neuronal loss and impairment of the nervous system. Parkinson's disease (PD) is one of the most common of these disorders, affecting 1-2% of the population above the age of 65. Although the underlying mechanisms of PD have been extensively studied, we still lack a full understanding of the molecular underpinnings of the disease. Thus, the in vitro and in vivo models currently used are able to only partially recapitulate the typical phenotypes of the disease. Here, we review various cell culture models currently used to study the molecular basis of PD, with a focus on alpha-synuclein-associated molecular pathologies. We also discuss how different cell models may constitute powerful tools for high-throughput screening of molecules capable of modulating alpha-synuclein toxicity.

Induced Pluripotent Stem Cell Modeling of Gaucher's Disease: What Have We Learned?

Gaucher’s disease (GD) is the most frequently inherited lysosomal storage disease, presenting both visceral and neurologic symptoms. Mutations in acid β-glucocerebrosidase disrupt the sphingolipid catabolic pathway promoting glucosylceramide (GlcCer) accumulation in lysosomes. Current treatment options are enzyme replacement therapy (ERT) and substrate reduction therapy (SRT). However, neither of these approaches is effective in treating the neurological aspect of the disease. The use of small pharmacological compounds that act as molecular chaperones is a promising approach that is still experimental. In recent years, an association between GD and Parkinson like synucleinopathies has been discovered. Since 1992, a number of mouse models of GD have been the developed and partially reproduce phenotype of the disease. More recently, the discovery of direct reprograming has allowed the derivation of induced pluripotent stem cells (iPSc) from fibroblasts obtained from GD patients. iPSc can be expanded indefinitely in vitro and differentiated to macrophages and neurons, the main relevant cell types involved in GD. In this work, we review iPSc models of GD and summarize what we have learned from this system.

The Spectrum of Neurological Manifestations Associated with Gaucher Disease

Gaucher disease, the most common lysosomal storage disorder, is due to a deficiency in the enzyme glucocerebrosidase. This leads to the accumulation of its normal substrate, glucocerebroside, in tissue macrophages, affecting the hematological, visceral, bone and neurologic systems. Gaucher disease is classified into three broad phenotypes based upon the presence or absence of neurological involvement: type 1 (non-neuronopathic), type 2 (acute neuronopathic), and type 3 (subacute neuronopathic). Phenotypically, there is a wide spectrum of visceral and neurological manifestations. Enzyme replacement is effective in managing the visceral disease; however, treating the neurological manifestations has proved to be more challenging. This review discusses the various neurological manifestations encountered in Gaucher disease, and provides a brief overview regarding the treatment and ongoing research challenges.

Small molecules as therapeutic agents for inborn errors of metabolism

Most inborn errors of metabolism (IEM) remain without effective treatment mainly due to the incapacity of conventional therapeutic approaches to target the neurological symptomatology and to ameliorate the multisystemic involvement frequently observed in these patients. However, in recent years, the therapeutic use of small molecules has emerged as a promising approach for treating this heterogeneous group of disorders. In this review, we focus on the use of therapeutically active small molecules to treat IEM, including readthrough agents, pharmacological chaperones, proteostasis regulators, substrate inhibitors, and autophagy inducers. The small molecules reviewed herein act at different cellular levels, and this knowledge provides new tools to set up innovative treatment approaches for particular IEM. We review the molecular mechanism underlying therapeutic properties of small molecules, methodologies used to screen for these compounds, and their applicability in preclinical and clinical practice.

α-synuclein toxicity in neurodegeneration: mechanism and therapeutic strategies

Alterations in α-synuclein dosage lead to familial Parkinson's disease (PD), and its accumulation results in synucleinopathies that include PD, dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Furthermore, α-synuclein contributes to the fibrilization of amyloid-b and tau, two key proteins in Alzheimer's disease, which suggests a central role for α-synuclein toxicity in neurodegeneration. Recent studies of factors contributing to α-synuclein toxicity and its disruption of downstream cellular pathways have expanded our understanding of disease pathogenesis in synucleinopathies. In this Review, we discuss these emerging themes, including the contributions of aging, selective vulnerability and non-cell-autonomous factors such as α-synuclein cell-to-cell propagation and neuroinflammation. Finally, we summarize recent efforts toward the development of targeted therapies for PD and related synucleinopathies.

Induced pluripotent stem cell technology: a decade of progress

Since the advent of induced pluripotent stem cell (iPSC) technology a decade ago, enormous progress has been made in stem cell biology and regenerative medicine. Human iPSCs have been widely used for disease modelling, drug discovery and cell therapy development. Novel pathological mechanisms have been elucidated, new drugs originating from iPSC screens are in the pipeline and the first clinical trial using human iPSC-derived products has been initiated. In particular, the combination of human iPSC technology with recent developments in gene editing and 3D organoids makes iPSC-based platforms even more powerful in each area of their application, including precision medicine. In this Review, we discuss the progress in applications of iPSC technology that are particularly relevant to drug discovery and regenerative medicine, and consider the remaining challenges and the emerging opportunities in the field.

Therapeutic potential of autophagy-enhancing agents in Parkinson's disease

Converging evidence from genetic, pathological and experimental studies have increasingly suggested an important role for autophagy impairment in Parkinson’s Disease (PD). Genetic studies have identified mutations in genes encoding for components of the autophagy-lysosomal pathway (ALP), including glucosidase beta acid 1 (GBA1), that are associated with increased risk for developing PD. Observations in PD brain tissue suggest an aberrant regulation of autophagy associated with the aggregation of α-synuclein (α-syn). As autophagy is one of the main systems involved in the proteolytic degradation of α-syn, pharmacological enhancement of autophagy may be an attractive strategy to combat α-syn aggregation in PD. Here, we review the potential of autophagy enhancement as disease-modifying therapy in PD based on preclinical evidence. In particular, we provide an overview of the molecular regulation of autophagy and targets for pharmacological modulation within the ALP. In experimental models, beneficial effects on multiple pathological processes involved in PD, including α-syn aggregation, cell death, oxidative stress and mitochondrial dysfunction, have been demonstrated using the autophagy enhancers rapamycin and lithium. However, selectivity of these agents is limited, while upstream ALP signaling proteins are involved in many other pathways than autophagy. Broad stimulation of autophagy may therefore cause a wide spectrum of dose-dependent side-effects, suggesting that its clinical applicability is limited. However, recently developed agents selectively targeting core ALP components, including Transcription Factor EB (TFEB), lysosomes, GCase as well as chaperone-mediated autophagy regulators, exert more specific effects on molecular pathogenetic processes causing PD. To conclude, the targeted manipulation of downstream ALP components, rather than broad autophagy stimulation, may be an attractive strategy for the development of novel pharmacological therapies in PD. Further characterization of dysfunctional autophagy in different stages and molecular subtypes of PD in combination with the clinical translation of downstream autophagy regulation offers exciting new avenues for future drug development.

Gaucher disease: Progress and ongoing challenges

Over the past decades, tremendous progress has been made in the field of Gaucher disease, the inherited deficiency of the lysosomal enzyme glucocerebrosidase. Many of the colossal achievements took place during the course of the sixty-year tenure of Dr. Roscoe Brady at the National Institutes of Health. These include the recognition of the enzymatic defect involved, the isolation and characterization of the protein, the localization and characterization of the gene and its nearby pseudogene, as well as the identification of the first mutant alleles in patients. The first treatment for Gaucher disease, enzyme replacement therapy, was conceived of, developed and tested at the Clinical Center of the National Institutes of Health. Advances including recombinant production of the enzyme, the development of mouse models, pioneering gene therapy experiments, high throughput screens of small molecules and the generation of induced pluripotent stem cell models have all helped to catapult research in Gaucher disease into the twenty-first century. The appreciation that mutations in the glucocerebrosidase gene are an important risk factor for parkinsonism further expands the impact of this work. However, major challenges still remain, some of which are described here, that will provide opportunities, excitement and discovery for the next generations of Gaucher investigators.

The translational potential of human induced pluripotent stem cells for clinical neurology : The translational potential of hiPSCs in neurology

The induced pluripotent state represents a decade-old Nobel prize-winning discovery. Human-induced pluripotent stem cells (hiPSCs) are generated by the nuclear reprogramming of any somatic cell using a variety of established but evolving methods. This approach offers medical science unparalleled experimental opportunity to model an individual patient’s disease “in a dish.” HiPSCs permit developmentally rationalized directed differentiation into any cell type, which express donor cell mutation(s) at pathophysiological levels and thus hold considerable potential for disease modeling, drug discovery, and potentially cell-based therapies. This review will focus on the translational potential of hiPSCs in clinical neurology and the importance of integrating this approach with complementary model systems to increase the translational yield of preclinical testing for the benefit of patients. This strategy is particularly important given the expected increase in prevalence of neurodegenerative disease, which poses a major burden to global health over the coming decades.

Lysosomal trafficking defects link Parkinson's disease with Gaucher's disease

Lysosomal dysfunction has been implicated in multiple diseases, including lysosomal storage disorders such as Gaucher's disease, in which loss-of-function mutations in the GBA1 gene encoding the lysosomal hydrolase β-glucocerebrosidase result in lipid substrate accumulation. In Parkinson's disease, α-synuclein accumulates in Lewy bodies and neurites contributing to neuronal death. Previous clinical and genetic evidence has demonstrated an important link between Parkinson's and Gaucher's disease, as GBA1 mutations and variants increase the risk of Parkinson's and Parkinson's patients exhibit decreased β-glucocerebrosidase activity. Using human midbrain neuron cultures, we have found that loss of β-glucocerebrosidase activity promotes α-synuclein accumulation and toxicity, whereas α-synuclein accumulation further contributes to decreased lysosomal β-glucocerebrosidase activity by disrupting β-glucocerebrosidase trafficking to lysosomes. Moreover, α-synuclein accumulation disrupts trafficking of additional lysosomal hydrolases, further contributing to lysosomal dysfunction and neuronal dyshomeostasis. Importantly, promoting β-glucocerebrosidase activity reduces α-synuclein accumulation and rescues lysosomal and neuronal dysfunction, suggesting that β-glucocerebrosidase may be an important therapeutic target for advancing drug discovery in synucleinopathies including Parkinson's disease. © 2016 International Parkinson and Movement Disorder Society.