Glucocerebrosidase gene therapy prevents α-synucleinopathy of midbrain dopamine neurons

Glucocerebrosidase Gene Therapy Induces Alpha-Synuclein Clearance and Neuroprotection of Midbrain Dopaminergic Neurons in Mice and Macaques

Mutations in the GBA1 gene coding for glucocerebrosidase (GCase) are the main genetic risk factor for Parkinson’s disease (PD). Indeed, identifying reduced GCase activity as a common feature underlying the typical neuropathological signatures of PD—even when considering idiopathic forms of PD—has recently paved the way for designing novel strategies focused on enhancing GCase activity to reduce alpha-synuclein burden and preventing dopaminergic cell death. Here we have performed bilateral injections of a viral vector coding for the mutated form of alpha-synuclein (rAAV9-SynA53T) for disease modeling purposes, both in mice as well as in nonhuman primates (NHPs), further inducing a progressive neuronal death in the substantia nigra pars compacta (SNpc). Next, another vector coding for the GBA1 gene (rAAV9-GBA1) was unilaterally delivered in the SNpc of mice and NHPs one month after the initial insult, together with the contralateral delivery of an empty/null rAAV9 for control purposes. Obtained results showed that GCase enhancement reduced alpha-synuclein burden, leading to improved survival of dopaminergic neurons. Data reported here support using GCase gene therapy as a disease-modifying treatment for PD and related synucleinopathies, including idiopathic forms of these disorders.

Glucocerebrosidase Gene Therapy Induces Alpha-Synuclein Clearance and Neuroprotection of Midbrain Dopaminergic Neurons in Mice and Macaques

Mutations in the GBA1 gene coding for glucocerebrosidase (GCase) are the main genetic risk factor for Parkinson's disease (PD). Indeed, identifying reduced GCase activity as a common feature underlying the typical neuropathological signatures of PD-even when considering idiopathic forms of PD-has recently paved the way for designing novel strategies focused on enhancing GCase activity to reduce alpha-synuclein burden and preventing dopaminergic cell death. Here we have performed bilateral injections of a viral vector coding for the mutated form of alpha-synuclein (rAAV9-SynA53T) for disease modeling purposes, both in mice as well as in nonhuman primates (NHPs), further inducing a progressive neuronal death in the substantia nigra pars compacta (SNpc). Next, another vector coding for the GBA1 gene (rAAV9-GBA1) was unilaterally delivered in the SNpc of mice and NHPs one month after the initial insult, together with the contralateral delivery of an empty/null rAAV9 for control purposes. Obtained results showed that GCase enhancement reduced alpha-synuclein burden, leading to improved survival of dopaminergic neurons. Data reported here support using GCase gene therapy as a disease-modifying treatment for PD and related synucleinopathies, including idiopathic forms of these disorders.

Is SGSH heterozygosity a risk factor for early-onset neurodegenerative disease?

Lysosomal dysfunction may be an important factor in the pathogenesis of neurodegenerative disorders such as Parkinson's disease (PD). Heterozygous mutations in the gene encoding the lysosomal enzyme glucocerebrosidase (GBA1) have been found in PD patients, and some but not all mutations in other lysosomal enzyme genes, for example, NPC1 and MCOLN1 have been associated with PD. We have examined the behaviour and brain structure of mice carrying a D31N mutation in the sulphamidase (Sgsh) gene which encodes a lysosomal sulphatase. Female heterozygotes and wildtype mice aged 12-, 15-, 18- and 21-months of age underwent motor phenotyping and the brain was comprehensively evaluated for disease-associated lesions. Heterozygous mice exhibited impaired performance in the negative geotaxis test when compared with wildtype mice. Whilst the brain of Sgsh heterozygotes aged up to 21-months did not exhibit any of the gross features of PD, Alzheimer's disease or the neurodegenerative lysosomal storage disorders, for example, loss of striatal dopamine, reduced GBA activity, α-synuclein-positive inclusions, perturbation of lipid synthesis, or cerebellar Purkinje cell drop-out, we noted discrete structural aberrations in the dendritic tree of cortical pyramidal neurons in 21-month old animals. The overt disease lesions and resultant phenotypic changes previously described in individuals with heterozygous mutations in lysosomal enzyme genes such as glucocerebrosidase may be enzyme dependent. By better understanding why deficiency in, or mutant forms of some but not all lysosomal proteins leads to heightened risk or earlier onset of classical neurodegenerative disorders, novel disease-causing mechanisms may be identified.

Glucocerebrosidase Gene Therapy Induces Alpha-Synuclein Clearance and Neuroprotection of Midbrain Dopaminergic Neurons in Mice and Macaques

Mutations in the GBA1 gene coding for glucocerebrosidase (GCase) are the main genetic risk factor for Parkinson’s disease (PD). Indeed, identifying reduced GCase activity as a common feature underlying the typical neuropathological signatures of PD—even when considering idiopathic forms of PD—has recently paved the way for designing novel strategies focused on enhancing GCase activity to reduce alpha-synuclein burden and preventing dopaminergic cell death. Here we have performed bilateral injections of a viral vector coding for the mutated form of alpha-synuclein (rAAV9-SynA53T) for disease modeling purposes, both in mice as well as in nonhuman primates (NHPs), further inducing a progressive neuronal death in the substantia nigra pars compacta (SNpc). Next, another vector coding for the GBA1 gene (rAAV9-GBA1) was unilaterally delivered in the SNpc of mice and NHPs one month after the initial insult, together with the contralateral delivery of an empty/null rAAV9 for control purposes. Obtained results showed that GCase enhancement reduced alpha-synuclein burden, leading to improved survival of dopaminergic neurons. Data reported here support using GCase gene therapy as a disease-modifying treatment for PD and related synucleinopathies, including idiopathic forms of these disorders.

Novel β-Glucocerebrosidase Activators That Bind to a New Pocket at a Dimer Interface and Induce Dimerization

Genetic, preclinical and clinical data link Parkinson's disease and Gaucher's disease and provide a rational entry point to disease modification therapy via enhancement of β-Glucocerebrosidase (GCase) activity. We discovered a new class of pyrrolo[2,3-b]pyrazine activators effecting both Vmax and Km. They bind to human GCase and increase substrate metabolism in the lysosome in a cellular assay. We obtained the first crystal structure for an activator and identified a novel non-inhibitory binding mode at the interface of a dimer, rationalizing the observed structure-activity relationship (SAR). The compound binds GCase inducing formation of a dimeric state at both endoplasmic reticulum (ER) and lysosomal pHs, as confirmed by analytical ultracentrifugation. Importantly, the pyrrolo[2,3-b]pyrazines have central nervous system (CNS) drug-like properties. Our findings are important for future drug discovery efforts in the field of GCase activation and provide a deeper mechanistic understanding of the requirements for enzymatic activation, pointing to the relevance of dimerization.

Novel targeted therapies for Parkinson's disease

Parkinson’s disease (PD) is the second more common neurodegenerative disease with increasing incidence worldwide associated to the population ageing. Despite increasing awareness and significant research advancements, treatment options comprise dopamine repleting, symptomatic therapies that have significantly increased quality of life and life expectancy, but no therapies that halt or reverse disease progression, which remain a great, unmet goal in PD research. Large biomarker development programs are undertaken to identify disease signatures that will improve patient selection and outcome measures in clinical trials. In this review, we summarize PD-related mechanisms that can serve as targets of therapeutic interventions aiming to slow or modify disease progression, as well as previous and ongoing clinical trials in each field, and discuss future perspectives.

Fibroblasts from idiopathic Parkinson's disease exhibit deficiency of lysosomal glucocerebrosidase activity associated with reduced levels of the trafficking receptor LIMP2

Lysosomal dysfunction is a central pathway associated with Parkinson's disease (PD) pathogenesis. Haploinsufficiency of the lysosomal hydrolase GBA (encoding glucocerebrosidase (GCase)) is one of the largest genetic risk factors for developing PD. Deficiencies in the activity of the GCase enzyme have been observed in human tissues from both genetic (harboring mutations in the GBA gene) and idiopathic forms of the disease. To understand the mechanisms behind the deficits of lysosomal GCase enzyme activity in idiopathic PD, this study utilized a large cohort of fibroblast cells from control subjects and PD patients with and without mutations in the GBA gene (N370S mutation) (control, n = 15; idiopathic PD, n = 31; PD with GBA N370S mutation, n = 6). The current data demonstrates that idiopathic PD fibroblasts devoid of any mutations in the GBA gene also exhibit reduction in lysosomal GCase activity, similar to those with the GBA N370S mutation. This reduced GCase enzyme activity in idiopathic PD cells was accompanied by decreased expression of the GBA trafficking receptor, LIMP2, and increased ER retention of the GBA protein in these cells. Importantly, in idiopathic PD fibroblasts LIMP2 protein levels correlated significantly with GCase activity, which was not the case in control subjects or in genetic PD GBA N370S cells. In conclusion, idiopathic PD fibroblasts have decreased GCase activity primarily driven by altered LIMP2-mediated transport of GBA to lysosome and the reduced GCase activity exhibited by  the genetic GBA N370S derived PD fibroblasts occurs through a different mechanism.

Protection from α-Synuclein induced dopaminergic neurodegeneration by overexpression of the mitochondrial import receptor TOM20

Dopaminergic neurons of the substantia nigra are selectively vulnerable to mitochondrial dysfunction, which is hypothesized to be an early and fundamental pathogenic mechanism in Parkinson's disease (PD). Mitochondrial function depends on the successful import of nuclear-encoded proteins, many of which are transported through the TOM20-TOM22 outer mitochondrial membrane import receptor machinery. Recent data suggests that post-translational modifications of α-synuclein promote its interaction with TOM20 at the outer mitochondrial membrane and thereby inhibit normal protein import, leading to dysfunction, and death of dopaminergic neurons. As such, preservation of mitochondrial import in the face of α-synuclein accumulation might be a strategy to prevent dopaminergic neurodegeneration, however, this is difficult to assess using current in vivo models of PD. To this end, we established an exogenous co-expression system, utilizing AAV2 vectors to overexpress human α-synuclein and TOM20, individually or together, in the adult Lewis rat substantia nigra to assess whether TOM20 overexpression attenuates α-synuclein-induced dopaminergic neurodegeneration. Twelve weeks after viral injection, we observed that AAV2-TOM20 expression was sufficient to prevent loss of nigral dopaminergic neurons caused by AAV2-αSyn overexpression. The observed TOM20-mediated dopaminergic neuron preservation appeared to be due, in part, to the rescued expression (and presumed import) of nuclear-encoded mitochondrial electron transport chain proteins that were inhibited by α-synuclein overexpression. In addition, TOM20 overexpression rescued the expression of the chaperone protein GRP75/mtHSP70/mortalin, a stress-response protein involved in α-synuclein-induced injury. Collectively, these data indicate that TOM20 expression prevents α-synuclein-induced mitochondrial dysfunction, which is sufficient to rescue dopaminergic neurons in the adult rat brain.

Cell type-specific lipid storage changes in Parkinson's disease patient brains are recapitulated by experimental glycolipid disturbance

Neurons are dependent on proper trafficking of lipids to neighboring glia for lipid exchange and disposal of potentially lipotoxic metabolites, producing distinct lipid distribution profiles among various cell types of the central nervous system. Little is known of the cellular distribution of neutral lipids in the substantia nigra (SN) of Parkinson's disease (PD) patients and its relationship to inflammatory signaling. This study aimed to determine human PD SN neutral lipid content and distribution in dopaminergic neurons, astrocytes, and microglia relative to age-matched healthy subject controls. The results show that while total neutral lipid content was unchanged relative to age-matched controls, the levels of whole SN triglycerides were correlated with inflammation-attenuating glycoprotein non-metastatic melanoma protein B (GPNMB) signaling in human PD SN. Histological localization of neutral lipids using a fluorescent probe (BODIPY) revealed that dopaminergic neurons and midbrain microglia significantly accumulated intracellular lipids in PD SN, while adjacent astrocytes had a reduced lipid load overall. This pattern was recapitulated by experimental in vivo inhibition of glucocerebrosidase activity in mice. Agents or therapies that restore lipid homeostasis among neurons, astrocytes, and microglia could potentially correct PD pathogenesis and disease progression.

Gene and Cell-Based Therapies for Parkinson's Disease: Where Are We?

Parkinson's disease (PD) is a neurodegenerative disorder that carries large health and socioeconomic burdens. Current therapies for PD are ultimately inadequate, both in terms of symptom control and in modification of disease progression. Deep brain stimulation and infusion therapies are the current mainstay for treatment of motor complications of advanced disease, but these have very significant drawbacks and offer no element of disease modification. In fact, there are currently no agents that are established to modify the course of the disease in clinical use for PD. Gene and cell therapies for PD are now being trialled in the clinic. These treatments are diverse and may have a range of niches in the management of PD. They hold great promise for improved treatment of symptoms as well as possibly slowing progression of the disease in the right patient group. Here, we review the current state of the art for these therapies and look to future strategies in this fast-moving field.

Enhancing the Activity of Glucocerebrosidase as a Treatment for Parkinson Disease

Mutations in the glucocerebrosidase (GBA1) gene are the most common genetic risk factor for Parkinson disease (PD). Homozygous or compound heterozygous GBA1 mutations cause the lysosomal storage disorder Gaucher disease (GD), characterized by deficient activity of the glucocerebrosidase enzyme (GCase). Both individuals with GD type I and heterozygous carriers of pathogenic variants of GBA1 have an increased risk of developing PD, by approximately ten- to 20-fold compared to non-carriers. GCase activity is also reduced in PD patients without GBA1 mutations, suggesting that the GCase lysosomal pathway might be involved in PD pathogenesis. Available evidence indicates that GCase can affect α-synuclein pathology in different ways. Misfolded GCase proteins are retained in the endoplasmic reticulum, altering the lysosomal trafficking of the enzyme and disrupting protein trafficking. Also, deficient GCase leads to accumulation of substrates that in turn may bind α-synuclein and promote pathological formation of aggregates. Furthermore, α-synuclein itself can lower the enzymatic activity of GCase, indicating that a bidirectional interaction exists between GCase and α-synuclein. Targeted therapies aimed at enhancing GCase activity, augmenting the trafficking of misfolded GCase proteins by small molecule chaperones, or reducing substrate accumulation, have been tested in preclinical and clinical trials. This article reviews the molecular mechanisms linking GCase to α-synuclein and discusses the therapeutic drugs that by targeting the GCase pathway can influence PD progression.

A modulator of wild-type glucocerebrosidase improves pathogenic phenotypes in dopaminergic neuronal models of Parkinson's disease

Mutations in the GBA1 gene encoding the lysosomal enzyme β-glucocerebrosidase (GCase) represent the most common risk factor for Parkinson's disease (PD). GCase has been identified as a potential therapeutic target for PD and current efforts are focused on chemical chaperones to translocate mutant GCase into lysosomes. However, for several GBA1-linked forms of PD and PD associated with mutations in LRRK2, DJ-1, and PARKIN, activating wild-type GCase represents an alternative approach. We developed a new small-molecule modulator of GCase called S-181 that increased wild-type GCase activity in iPSC-derived dopaminergic neurons from sporadic PD patients, as well as patients carrying the 84GG mutation in GBA1, or mutations in LRRK2, DJ-1, or PARKIN who had decreased GCase activity. S-181 treatment of these PD iPSC-derived dopaminergic neurons partially restored lysosomal function and lowered accumulation of oxidized dopamine, glucosylceramide and α-synuclein. Moreover, S-181 treatment of mice heterozygous for the D409V GBA1 mutation (Gba1^D409V/+ ) resulted in activation of wild-type GCase and consequent reduction of GCase lipid substrates and α-synuclein in mouse brain tissue. Our findings point to activation of wild-type GCase by small-molecule modulators as a potential therapeutic approach for treating familial and sporadic forms of PD that exhibit decreased GCase activity.

Experimental studies of mitochondrial and lysosomal function in in vitro and in vivo models relevant to Parkinson's disease genetic risk

Several studies have identified the involvement of mitochondrial and lysosomal dysfunction in Parkinson's disease (PD) pathology. In this review we discuss recent work that has identified deficits in mitophagy, mitochondrial network formation, increased sensitivity to mitochondrial stressors and alterations in proteins regulating mitochondrial fission and fusion associated with patient-derived fibroblasts harboring mutations in LRRK2 gene and from sporadic PD patient cells. We further focus on alterations of lysosomal enzymes, in particular glucocerebrosidase activity, and resultant lipid dyshomeostasis in PD and aging, in human tissue and in vivo rodent models. Future studies aimed at understanding the convergence of mitochondrial and lysosomal pathways will be of essence for the identification of unique cellular defects in PD and for the development of new treatments.

Modeling Parkinson's Disease With the Alpha-Synuclein Protein

Alpha-synuclein (α-Syn) is a key protein involved in Parkinson's disease (PD) pathology. PD is characterized by the loss of dopaminergic neuronal cells in the substantia nigra pars compacta and the abnormal accumulation and aggregation of α-Syn in the form of Lewy bodies and Lewy neurites. More precisely, the aggregation of α-Syn is associated with the dysfunctionality and degeneration of neurons in PD. Moreover, mutations in the SNCA gene, which encodes α-Syn, cause familial forms of PD and are the basis of sporadic PD risk. Given the role of the α-Syn protein in the pathology of PD, animal models that reflect the dopaminergic neuronal loss and the widespread and progressive formation of α-Syn aggregates in different areas of the brain constitute a valuable tool. Indeed, animal models of PD are important for understanding the molecular mechanisms of the disease and might contribute to the development and validation of new therapies. In the absence of animal models that faithfully reproduce human PD, in recent years, numerous animal models of PD based on α-Syn have been generated. In this review, we summarize the main features of the α-Syn pre-formed fibrils (PFFs) model and recombinant adeno-associated virus vector (rAAV) mediated α-Syn overexpression models, providing a detailed comparative analysis of both models. Here, we discuss how each model has contributed to our understanding of PD pathology and the advantages and weakness of each of them.

Glucocerebrosidase Defects as a Major Risk Factor for Parkinson's Disease

Heterozygous mutations of the GBA1 gene, encoding for lysosomal enzyme glucocerebrosidase (GCase), occur in a considerable percentage of all patients with sporadic Parkinson's disease (PD), varying between 8% and 12% across the world. Genome wide association studies have confirmed the strong correlation between PD and GBA1 mutations, pointing to this element as a major risk factor for PD, possibly the most important one after age. The pathobiological mechanisms underlying the link between a defective function of GCase and the development of PD are still unknown and are currently the focus of intense investigation in the community of pre-clinical and clinical researchers in the PD field. A major controversy regards the fact that, despite the unequivocal correlation between the presence of GBA1 mutations and the risk of developing PD, only a minority of asymptomatic carriers with GBA1 mutations convert to PD in their lifetime. GBA1 mutations reduce the enzymatic function of GCase, impairing lysosomal efficiency and the cellular ability to dispose of pathological alpha-synuclein. Changes in the cellular lipidic content resulting from the accumulation of glycosphingolipids, triggered by lysosomal dysfunction, may contribute to the pathological modification of alpha-synuclein, due to its ability to interact with cell membrane lipids. Mutant GCase can impair mitochondrial function and cause endoplasmic reticulum stress, thereby impacting on cellular energy production and proteostasis. Importantly, reduced GCase activity is associated with clear activation of microglia, a major mediator of neuroinflammatory response within the brain parenchyma, which points to neuroinflammation as a major consequence of GCase dysfunction. In this present review article, we summarize the current knowledge on the role of GBA1 mutations in PD development and their phenotypic correlations. We also discuss the potential role of the GCase pathway in the search for PD biomarkers that may enable the development of disease modifying therapies. Answering these questions will aid clinicians in offering more appropriate counseling to the patients and their caregivers and provide future directions for PD preclinical research.

Battling Neurodegenerative Diseases with Adeno-Associated Virus-Based Approaches

Neurodegenerative diseases (NDDs) are most commonly found in adults and remain essentially incurable. Gene therapy using AAV vectors is a rapidly-growing field of experimental medicine that holds promise for the treatment of NDDs. To date, the delivery of a therapeutic gene into target cells via AAV represents a major obstacle in the field. Ideally, transgenes should be delivered into the target cells specifically and efficiently, while promiscuous or off-target gene delivery should be minimized to avoid toxicity. In the pursuit of an ideal vehicle for NDD gene therapy, a broad variety of vector systems have been explored. Here we specifically outline the advantages of adeno-associated virus (AAV)-based vector systems for NDD therapy application. In contrast to many reviews on NDDs that can be found in the literature, this review is rather focused on AAV vector selection and their preclinical testing in experimental and preclinical NDD models. Preclinical and in vitro data reveal the strong potential of AAV for NDD-related diagnostics and therapeutic strategies.

Intra-CSF AAV9 and AAVrh10 Administration in Nonhuman Primates: Promising Routes and Vectors for Which Neurological Diseases?

The identification of the most efficient method for whole central nervous system targeting that is translatable to humans and the safest route of adeno-associated virus (AAV) administration is a major concern for future applications in clinics. Additionally, as many AAV serotypes were identified for gene introduction into the brain and the spinal cord, another key to human gene-therapy success is to determine the most efficient serotype. In this study, we compared lumbar intrathecal administration through catheter implantation and intracerebroventricular administration in the cynomolgus macaque. We also evaluated and compared two AAV serotypes that are currently used in clinical trials: AAV9 and AAVrh10. We demonstrated that AAV9 lumbar intrathecal delivery using a catheter achieved consistent transgene expression in the motor neurons of the spinal cord and in the neurons/glial cells of several brain regions, whereas AAV9 intracerebroventricular delivery led to a consistent transgene expression in the brain. In contrast, AAVrh10 lumbar intrathecal delivery led to rare motor neuron targeting. Finally, we found that AAV9 efficiently targets respiratory and skeletal muscles after injection into the cerebrospinal fluid (CSF), which represents an outstanding new property that can be useful for the treatment of diseases affecting both the central nervous system and muscle.

Glucocerebrosidase as a therapeutic target for Parkinson's disease

Introduction: The association between Gaucher disease, caused by the inherited deficiency of glucocerebrosidase, and Parkinson's disease was first recognized in the clinic, noting that patients with Gaucher disease and their carrier relatives had an increased incidence of Parkinson's disease. Currently, mutations in glucocerebrosidase (GBA1) are the most common genetic risk factor for Parkinson's disease and dementia with Lewy bodies, with an inverse relationship between glucocerebrosidase and α-synuclein, a key factor in Parkinson pathogenesis. The hypothesis that therapeutic enhancement of brain glucocerebrosidase levels might reduce the aggregation, accumulation or spread of α-synuclein has spurred great interest in glucocerebrosidase as a novel therapeutic target.Area covered: This article explores the potential molecular mechanisms underlying the association between GBA1 mutations and Parkinson's disease and outlines therapeutic strategies to increase brain glucocerebrosidase, including gene therapy, targeted delivery of recombinant glucocerebrosidase to the brain, small-molecule chaperones to rescue mutant glucocerebrosidase, and small-molecule modulators to activate wild-type glucocerebrosidase.Expert opinion: Although an improved understanding of the mechanistic basis for GBA1-associated parkinsonism is essential, enhancing levels of brain glucocerebrosidase may have wide therapeutic implications. While gene therapy may ultimately be effective, less expensive and invasive small-molecule non-inhibitory chaperones or activators could significantly impact the disease course.

Gut-seeded α-synuclein fibrils promote gut dysfunction and brain pathology specifically in aged mice

Parkinson's disease is a synucleinopathy that is characterized by motor dysfunction, death of midbrain dopaminergic neurons and accumulation of α-synuclein (α-Syn) aggregates. Evidence suggests that α-Syn aggregation can originate in peripheral tissues and progress to the brain via autonomic fibers. We tested this by inoculating the duodenal wall of mice with α-Syn preformed fibrils. Following inoculation, we observed gastrointestinal deficits and physiological changes to the enteric nervous system. Using the AAV-PHP.S capsid to target the lysosomal enzyme glucocerebrosidase for peripheral gene transfer, we found that α-Syn pathology is reduced due to the increased expression of this protein. Lastly, inoculation of α-Syn fibrils in aged mice, but not younger mice, resulted in progression of α-Syn histopathology to the midbrain and subsequent motor defects. Our results characterize peripheral synucleinopathy in prodromal Parkinson's disease and explore cellular mechanisms for the gut-to-brain progression of α-Syn pathology.

GBA1 mutations: Prospects for exosomal biomarkers in α-synuclein pathologies

The discovery that patients with Gaucher Disease (GD), a rare lysosomal storage disorder, were developing symptoms similar to Parkinson's disease (PD) led to investigation of the relationship between the two seemingly unrelated pathologies. GD, an autosomal recessive disorder, is the result of a biallelic mutation in the gene GBA1, which encodes for the enzyme glucocerebrosidase (GCase). Since the observation of its relation to PD, GBA1 mutations have become recognized as the most common genetic risk factor for development of synucleinopathies such as PD and dementia with Lewy bodies. Although the exact mechanism by which GBA1 mutations promote PD is unknown, current understanding suggests that impaired GCase inhibits lysosomal activity and decreases the overall ability of the cell to degrade proteins, specifically the neuronal protein α-synuclein. Decreased elimination of α-synuclein can lead to its abnormal accumulation and aggregation, an important component of PD development. Further understanding of how decreased GCase activity increases risk for α-synuclein pathology can assist with the development of clinical biomarkers for early detection of synucleinopathies, as well as promote novel treatments tailored for people with a GBA1 mutation. Historically, α-synuclein has not been a reliable biomarker for PD. However, recent research on α-synuclein content within exosomes, which are small vesicles released by cells that carry specific cellular cargo, has yielded encouraging results. Moreover, decreased GCase activity has been shown to influence exosomal contents. Exosomes have emerged as a promising new avenue for the identification of novel biomarkers and therapeutic targets aimed at improving neuronal GCase function and limiting the development of synucleinopathies.

Novel approaches to counter protein aggregation pathology in Parkinson's disease

The primary neuropathological characteristics of the Parkinsonian brain are the loss of nigral dopamine neurons and the aggregation of alpha synuclein protein. Efforts to development potentially disease-modifying treatments have largely focused on correcting these aspects of the condition. In the last decade treatments targeting protein aggregation have entered the clinical pipeline. In this chapter we provide an overview of ongoing clinical trial programs for different therapies attempting to reduce protein aggregation pathology in Parkinson's disease. We will also briefly consider various novel approaches being proposed-and being developed preclinically-to inhibit/reduce aggregated protein pathology in Parkinson's.

Stress-Induced Cellular Clearance Is Mediated by the SNARE Protein ykt6 and Disrupted by α-Synuclein

Age-related neurodegenerative disorders are characterized by a slow, persistent accumulation of aggregated proteins. Although cells can elicit physiological responses to enhance cellular clearance and counteract accumulation, it is unclear how pathogenic proteins evade this process in disease. We find that Parkinson's disease α-synuclein perturbs the physiological response to lysosomal stress by impeding the SNARE protein ykt6. Cytosolic ykt6 is normally autoinhibited by a unique farnesyl-mediated regulatory mechanism; however, during lysosomal stress, it activates and redistributes into membranes to preferentially promote hydrolase trafficking and enhance cellular clearance. α-Synuclein aberrantly binds and deactivates ykt6 in patient-derived neurons, thereby disabling the lysosomal stress response and facilitating protein accumulation. Activating ykt6 by small-molecule farnesyltransferase inhibitors restores lysosomal activity and reduces α-synuclein in patient-derived neurons and mice. Our findings indicate that α-synuclein creates a permissive environment for aggregate persistence by inhibiting regulated cellular clearance and provide a therapeutic strategy to restore protein homeostasis by harnessing SNARE activity.

Current Opinions and Consensus for Studying Tremor in Animal Models

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

Autophagic and endo-lysosomal dysfunction in neurodegenerative disease

Due to their post-mitotic state, metabolic demands and often large polarised morphology, the function and survival of neurons is dependent on an efficient cellular waste clearance system both for generation of materials for metabolic processes and removal of toxic components. It is not surprising therefore that deficits in protein clearance can tip the balance between neuronal health and death. Here we discuss how autophagy and lysosome-mediated degradation pathways are disrupted in several neurological disorders. Both genetic and cell biological evidence show the diversity and complexity of vesicular clearance dysregulation in cells, and together may ultimately suggest a unified mechanism for neuronal demise in degenerative conditions. Causative and risk-associated mutations in Alzheimer's disease, Frontotemporal Dementia, Amyotrophic Lateral Sclerosis, Parkinson's disease, Huntington's disease and others have given the field a unique mechanistic insight into protein clearance processes in neurons. Through their broad implication in neurodegenerative diseases, molecules involved in these genetic pathways, in particular those involved in autophagy, are emerging as appealing therapeutic targets for intervention in neurodegeneration.

Glucocerebrosidase activity, cathepsin D and monomeric α-synuclein interactions in a stem cell derived neuronal model of a PD associated GBA1 mutation

The presence of GBA1 gene mutations increases risk for Parkinson's disease (PD), but the pathogenic mechanisms of GBA1 associated PD remain unknown. Given that impaired α-synuclein turnover is a hallmark of PD pathogenesis and cathepsin D is a key enzyme involved in α-synuclein degradation in neuronal cells, we have examined the relationship of glucocerebrosidase (GCase), cathepsin D and monomeric α-synuclein in human neural crest stem cell derived dopaminergic neurons. We found that normal activity of GCase is necessary for cathepsin D to perform its function of monomeric α-synuclein removal from neurons. GBA1 mutations lead to a lower level of cathepsin D protein and activity, and higher level of monomeric α-synuclein in neurons. When GBA1 mutant neurons were treated with GCase replacement or chaperone therapy; cathepsin D protein levels and activity were restored, and monomeric α-synuclein decreased. When cathepsin D was inhibited, GCase replacement failed to reduce monomeric α-synuclein levels in GBA1 mutant neurons. These data indicate that GBA1 gene mutations increase monomeric α-synuclein levels via an effect on lysosomal cathepsin D in neurons.

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Cathepsin D protein and activity decreased in GBA mutation-associated PD neurons.

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α-synuclein protein level increased in GBA mutation-associated PD neurons.

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GBA enzyme replacement treatment increased cathepsin D, decreased α-synuclein levels.

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GBA enzyme chaperone treatment increased cathepsin D, decreased α-synuclein levels.

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The influence of GBA enzyme replacement on α-synuclein mediated through cathepsin D.

New Frontiers in Parkinson's Disease: From Genetics to the Clinic

The greatest unmet therapeutic need in Parkinson's disease (PD) is a treatment that slows the relentless progression of the symptoms and the neurodegenerative process. This review highlights the utility of genetics to understand the pathogenic mechanisms and develop novel therapeutic approaches for PD. The focus is on strategies provided by genetic studies: notably via the reduction and clearance of α-synuclein, inhibition of LRRK2 kinase activity, and modulation of glucocerebrosidase-related substrates. In addition, the critical role of precompetitive public-private partnerships in supporting trial design optimization, overall drug development, and regulatory approvals is illustrated. With these great advances, the promise of developing transformative therapies that halt or slow disease progression is a tangible goal.

Lipid and immune abnormalities causing age-dependent neurodegeneration and Parkinson's disease

This article describes pathogenic concepts and factors, in particular glycolipid abnormalities, that create cell dysfunction and synaptic loss in neurodegenerative diseases. By phenocopying lysosomal storage disorders, such as Gaucher disease and related disorders, age- and dose-dependent changes in glycolipid cell metabolism can lead to Parkinson's disease and related dementias. Recent results show that perturbation of sphingolipid metabolism can precede or is a part of abnormal protein handling in both genetic and idiopathic Parkinson's disease and Lewy body dementia. In aging and genetic predisposition with lipid disturbance, α-synuclein's normal vesicular and synaptic role may be detrimentally shifted toward accommodating and binding such lipids. Specific neuronal glycolipid, protein, and vesicular interactions create potential pathophysiology that is amplified by astroglial and microglial immune mechanisms resulting in neurodegeneration. This perspective provides a new logic for therapeutic interventions that do not focus on protein aggregation, but rather provides a guide to the complex biology and the common sequence of events that lead to age-dependent neurodegenerative disorders.

Preserving Lysosomal Function in the Aging Brain: Insights from Neurodegeneration

Lysosomes are acidic, membrane-bound organelles that serve as the primary catabolic compartment of the cell. They are crucial to a variety of cellular processes from nutrient storage to autophagy. Given the diversity of lysosomal functions, it is unsurprising that lysosomes are also emerging as important players in aging. Lysosomal dysfunction is implicated in several aging-related neurodegenerative diseases including Alzheimer's, Parkinson's, amyotrophic lateral sclerosis/frontotemporal dementia, and Huntington's. Although the precise role of lysosomes in the aging brain is not well-elucidated, some insight into their function has been gained from our understanding of the pathophysiology of age-dependent neurodegenerative diseases. Therapeutic strategies targeting lysosomes and autophagic machinery have already been tested in several of these diseases with promising results, suggesting that improving lysosomal function could be similarly beneficial in preserving function in the aging brain.

GBA, Gaucher Disease, and Parkinson's Disease: From Genetic to Clinic to New Therapeutic Approaches

Parkinson’s disease (PD) is the second most common degenerative disorder. Although the disease was described more than 200 years ago, its pathogenetic mechanisms have not yet been fully described. In recent years, the discovery of the association between mutations of the GBA gene (encoding for the lysosomal enzyme glucocerebrosidase) and PD facilitated a better understating of this disorder. GBA mutations are the most common genetic risk factor of the disease. However, mutations of this gene can be found in different phenotypes, such as Gaucher’s disease (GD), PD, dementia with Lewy bodies (DLB) and rapid eye movements (REM) sleep behavior disorders (RBDs). Understanding the pathogenic role of this mutation and its different manifestations is crucial for geneticists and scientists to guide their research and to select proper cohorts of patients. Moreover, knowing the implications of the GBA mutation in the context of PD and the other associated phenotypes is also important for clinicians to properly counsel their patients and to implement their care. With the present review we aim to describe the genetic, clinical, and therapeutic features related to the mutation of the GBA gene.

Acid ceramidase inhibition ameliorates α-synuclein accumulation upon loss of GBA1 function

GBA1 encodes the lysosomal enzyme β-glucocerebrosidase (GCase) which converts glucosylceramide into ceramide and glucose. Mutations in GBA1 lead to Gaucher's disease and are a major risk factor for Parkinson's disease (PD) and Dementia with Lewy bodies (DLB), synucleinopathies characterized by accumulation of intracellular α-synuclein. In this study, we examined whether decreased ceramide that is observed in GCase-deficient cells contributes to α-synuclein accumulation. We demonstrated that deficiency of GCase leads to a reduction of C18-ceramide species and altered intracellular localization of Rab8a, a small GTPase implicated in secretory autophagy, that contributed to impaired secretion of α-synuclein and accumulation of intracellular α-synuclein. This secretory defect was rescued by exogenous C18-ceramide or chemical inhibition of lysosomal enzyme acid ceramidase that converts lysosomal ceramide into sphingosine. Inhibition of acid ceramidase by carmofur resulted in increased ceramide levels and decreased glucosylsphingosine levels in GCase-deficient cells, and also reduced oxidized α-synuclein and levels of ubiquitinated proteins in GBA1-PD patient-derived dopaminergic neurons. Together, these results suggest that decreased ceramide generation via the catabolic lysosomal salvage pathway in GCase mutant cells contributes to α-synuclein accumulation, potentially due to impaired secretory autophagy. We thus propose that acid ceramidase inhibition which restores ceramide levels may be a potential therapeutic strategy to target synucleinopathies linked to GBA1 mutations including PD and DLB.

Glucocerebrosidase mutations and synucleinopathies: Toward a model of precision medicine

Glucocerebrosidase is a lysosomal enzyme. The characterization of a direct link between mutations in the gene coding for glucocerebrosidase (GBA1) with the development of Parkinson's disease and dementia with Lewy bodies has heightened interest in this enzyme. Although the mechanisms through which glucocerebrosidase regulates the homeostasis of α-synuclein remains poorly understood, the identification of reduced glucocerebrosidase activity in the brains of patients with PD and dementia with Lewy bodies has paved the way for the development of novel therapeutic strategies directed at enhancing glucocerebrosidase activity and reducing α-synuclein burden, thereby slowing down or even preventing neuronal death. Here we reviewed the current literature relating to the mechanisms underlying the cross talk between glucocerebrosidase and α-synuclein, the GBA1 mutation-associated clinical phenotypes, and ongoing therapeutic approaches targeting glucocerebrosidase. © 2018 International Parkinson and Movement Disorder Society.

Lipid-dependent deposition of alpha-synuclein and Tau on neuronal Secretogranin II-positive vesicular membranes with age

This report demonstrates insoluble alpha-synuclein (aSYN)+ aggregates in human sporadic Parkinson’s disease (PD) midbrain that are linearly correlated with loss of glucocerebrosidase (GCase) activity. To identify early protein-lipid interactions that coincide with loss of lipid homeostasis, an aging study was carried out in mice with age-dependent reductions in GCase function. The analysis identified aberrant lipid-association by aSYN and hyperphosphorylated Tau (pTau) in a specific subset of neurotransmitter-containing, Secretogranin II (SgII)+ large, dense-core vesicles (LDCVs) responsible for neurotransmission of dopamine and other monoamines. The lipid vesicle-accumulation was concurrent with loss of PSD-95 suggesting synaptic destabilization. aSYN overexpression in the absence of lipid deregulation did not recapitulate the abnormal association with SgII+ vesicles. These results show lipid-dependent changes occur with age in neuronal vesicular membrane compartments that accumulate lipid-stabilized aSYN and pTau.

CNS repurposing - Potential new uses for old drugs: Examples of screens for Alzheimer's disease, Parkinson's disease and spasticity

Drug repurposing is recently gaining increasing attention, not just from pharmaceutical companies but also from government agencies in an attempt to generate new medications to address increasing unmet medical needs in a cost effective and expedite manner. There are several approaches to identify novel indications for known drugs. Many are based on rational selection e.g. the known or a new mechanism of action of a drug. This review will focus rather on phenotypic or high content screening of compounds in models that are believed to be predictive of effectiveness of compounds irrespective of their mechanism of action. Three short cases studies of screens for Alzheimer's disease, Parkinson's disease and spasticity will be given as examples. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

α-Synuclein interacts directly but reversibly with psychosine: implications for α-synucleinopathies

Aggregation of α-synuclein, the hallmark of α-synucleinopathies such as Parkinson’s disease, occurs in various glycosphingolipidoses. Although α-synuclein aggregation correlates with deficiencies in the lysosomal degradation of glycosphingolipids (GSL), the mechanism(s) involved in this aggregation remains unclear. We previously described the aggregation of α-synuclein in Krabbe’s disease (KD), a neurodegenerative glycosphingolipidosis caused by lysosomal deficiency of galactosyl-ceramidase (GALC) and the accumulation of the GSL psychosine. Here, we used a multi-pronged approach including genetic, biophysical and biochemical techniques to determine the pathogenic contribution, reversibility, and molecular mechanism of aggregation of α-synuclein in KD. While genetic knock-out of α-synuclein reduces, but does not completely prevent, neurological signs in a mouse model of KD, genetic correction of GALC deficiency completely prevents α-synuclein aggregation. We show that psychosine forms hydrophilic clusters and binds the C-terminus of α-synuclein through its amino group and sugar moiety, suggesting that psychosine promotes an open/aggregation-prone conformation of α-synuclein. Dopamine and carbidopa reverse the structural changes of psychosine by mediating a closed/aggregation-resistant conformation of α-synuclein. Our results underscore the therapeutic potential of lysosomal correction and small molecules to reduce neuronal burden in α-synucleinopathies, and provide a mechanistic understanding of α-synuclein aggregation in glycosphingolipidoses.

Ambroxol modulates 6-Hydroxydopamine-induced temporal reduction in Glucocerebrosidase (GCase) enzymatic activity and Parkinson's disease symptoms

Reduced glucocerebrosidase (GCase) enzymatic activity is found in sporadic cases of Parkinson's disease making GCase a serious risk factor for PD. GCase gene mutations constitute a major risk factor in early-onset PD but only account for 5-10% cases. Having enough evidence for construct and face validity, 6-OHDA-induced hemiparkinson's model may be useful to assess the GCase-targeting drugs in order to have new leads for treatment of PD. Ambroxol (AMB) is reported to increase GCase activity in different brain-regions. Therefore, we investigated anti-PD like effects of AMB as well as GCase activity in striatal and nigral tissues of rats in hemiparkinson's model. AMB was given a dose of 400 mg/kg per oral twice daily and SEL used as positive control was given in the dose of 10 mg/kg per oral daily from D-4 to D-27 after 6-OHDA administration. 6-OHDA reduced GCase activity in striatal and in a progressive manner in nigral tissues. AMB and SEL attenuated 6-OHDA-induced motor impairments, dopamine (DA) depletion and GCase deficiency. AMB and SEL also ameliorated 6-OHDA-induced mitochondrial dysfunction in terms of MTT reduction, α-synuclein pathology, loss of nigral cells, and intrinsic pathway of apoptosis by modulating cytochrome-C, caspase-9, and caspase-3 expressions. The results suggest that AMB attenuated 6-OHDA-induced GCase deficiency and PD symptoms. Therefore, the regenerative effects of AMB in dopamine toxicity may be due to its effects on GCase activity and mitochondrial function. Results indicate that SEL also has regenerative effect in the 6-OHDA model. Thus, GCase enzymatic activity is likely to be involved in the development of PD symptoms, and 6-OHDA-induced hemiparkinson's model may be used to evaluate compounds targeting GCase activity for management of PD symptoms.

Glycosphingolipid levels and glucocerebrosidase activity are altered in normal aging of the mouse brain

Aging is the predominant risk factor for both genetic and sporadic Parkinson's disease (PD). The majority of PD cases are nonfamilial, and the connection between aging and PD-associated genes is not well understood. Haploinsufficiency of the GBA gene, leading to a reduction in glucocerebrosidase (GCase) activity, is one of the most common genetic risk factors for PD. Furthermore, GCase activity is also reduced in brain regions of sporadic PD patients, with a corresponding accumulation of its glycosphingolipid (GSL) substrates. Recent findings in PD patients and aging control cases, and in human PD patient induced pluripotent stem cell neurons, have shown an age-dependent reduction in GCase activity and an elevation of some GSLs. We therefore asked whether aging-induced changes to both lysosomal and nonlysosomal GCase activity and GSL homeostasis in the brain could also be reflected in other nonhuman mammalian systems. Increases in brain polyubiquitin and the lysosomal-associated membrane protein, LAMP2A, were found in 24-month-old wild-type mice compared to 1.5-month-old mice. A lipidomic analysis was performed on brains of wild-type mice of different strains between 1.5 and 24 months of age. Aging created GSL changes that are reminiscent of sporadic PD. Levels of glucosylceramide, glucosylsphingosine, lactosylceramide, and GM1a were elevated in the brain of aged mice, and levels of complex gangliosides, GD1a, GD1b, and GT1b, were reduced with age. Parallel biochemical analyses revealed a change in lipid metabolism probably mediated by lysosomal hydrolases, with reduced GCase and increased neuraminidase activity. Based on these data, we hypothesize that perturbation of GSL metabolism in the aging brain may precede or may be part of abnormal protein handling and may accelerate PD pathophysiological processes in vulnerable neurons in PD and other age-related neurodegenerative disorders.

Glucocerebrosidase Mutations in Parkinson Disease

Following the discovery of a higher than expected incidence of Parkinson Disease (PD) in Gaucher disease, a lysosomal storage disorder, mutations in the glucocerebrocidase (GBA) gene, which encodes a lysosomal enzyme involved in sphingolipid degradation were explored in the context of idiopathic PD. GBA mutations are now known to be the single largest risk factor for development of idiopathic PD. Clinically, on imaging and pharmacologically, GBA PD is almost identical to idiopathic PD, other than certain features that can be identified in the specialist research setting but not in routine clinical practice. In patients with a known GBA mutation, it is possible to monitor for prodromal signs of PD. The clinical similarity with idiopathic PD and the chance to identify PD at a pre-clinical stage provides a unique opportunity to research therapeutic options for early PD, before major irreversible neurodegeneration occurs. However, to date, the molecular mechanisms which lead to this increased PD risk in GBA mutation carriers are not fully elucidated. Experimental models to define the molecular mechanisms and test therapeutic options include cell culture, transgenic mice and other in vivo models amenable to genetic manipulation, such as drosophilia. Some key pathological pathways of interest in the context of GBA mutations include alpha synuclein aggregation, lysosomal-autophagy axis changes and endoplasmic reticulum stress. Therapeutic agents that exploit these pathways are being developed and include the small molecule chaperone Ambroxol. This review aims to summarise the main features of GBA-PD and provide insights into the pathological relevance of GBA mutations on molecular pathways and the therapeutic implications for PD resulting from investigation of the role of GBA in PD.

Molecular regulations and therapeutic targets of Gaucher disease

Gaucher disease (GD) is the most common lysosomal storage disease caused by deficiency of beta-glucocerebrosidase (GCase) resulting in lysosomal accumulation of its glycolipid substrate glucosylceramide. The activity of GCase depends on many factors such as proper folding and lysosomal localization, which are influenced by mutations in GCase encoding gene, and regulated by various GCase-binding partners including Saposin C, progranulin and heat shock proteins. In addition, proinflammatory molecules also contribute to pathogenicity of GD. In this review, we summarize the molecules that are known to be important for the pathogenesis of GD, particularly those modulating GCase lysosomal appearance and activity. In addition, small molecules that inhibit inflammatory mediators, calcium ion channels and other factors associated with GD are also described. Discovery and characterization of novel molecules that impact GD are not only important for deciphering the pathogenic mechanisms of the disease, but they also provide new targets for drug development to treat the disease.

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.

New Developments in Genetic rat models of Parkinson's Disease

Preclinical research on Parkinson's disease has relied heavily on mouse and rat animal models. Initially, PD animal models were generated primarily by chemical neurotoxins that induce acute loss of dopaminergic neurons in the substantia nigra. On the discovery of genetic mutations causally linked to PD, mice were used more than rats to generate laboratory animals bearing PD-linked mutations because mutagenesis was more difficult in rats. Recent advances in technology for mammalian genome engineering and optimization of viral expression vectors have increased the use of genetic rat models of PD. Emerging research tools include "knockout" rats with disruption of genes in which mutations have been causally linked to PD, including LRRK2, α-synuclein, Parkin, PINK1, and DJ-1. Rats have also been increasingly used for transgenic and viral-mediated overexpression of genes relevant to PD, particularly α-synuclein. It may not be realistic to obtain a single animal model that completely reproduces every feature of a human disease as complex as PD. Nevertheless, compared with mice with the same mutations, many genetic rat animal models of PD better reproduce key aspects of PD including progressive loss of dopaminergic neurons in the substantia nigra, locomotor behavior deficits, and age-dependent formation of abnormal α-synuclein protein aggregates. Here we briefly review new developments in genetic rat models of PD that may have greater potential for identifying underlying mechanisms, for discovering novel therapeutic targets, and for developing greatly needed treatments to slow or halt disease progression. © 2018 International Parkinson and Movement Disorder Society.

The lysosomal enzyme alpha-Galactosidase A is deficient in Parkinson's disease brain in association with the pathologic accumulation of alpha-synuclein

The aberrant accumulation of alpha-synuclein (α-syn) is believed to contribute to the onset and pathogenesis of Parkinson's disease (PD). The autophagy-lysosome pathway (ALP) is responsible for the high capacity clearance of α-syn. ALP dysfunction is documented in PD and pre-clinical evidence suggests that inhibiting the ALP promotes the pathological accumulation of α-syn. We previously identified the pathological accumulation of α-syn in the brains of mice deficient for the soluble lysosomal enzyme alpha-Galactosidase A (α-Gal A), a member of the glycosphingolipid metabolism pathway. In the present study, we quantified α-Gal A activity and levels of its glycosphingolipid metabolites in postmortem temporal cortex specimens from control individuals and in PD individuals staged with respect to α-syn containing Lewy body pathology. In late-state PD temporal cortex we observed significant decreases in α-Gal A activity and the 46kDa "active" species of α-Gal A as determined respectively by fluorometric activity assay and western blot analysis. These decreases in α-Gal A activity/levels correlated significantly with increased α-syn phosphorylated at serine 129 (p129S-α-syn) that was maximal in late-stage PD temporal cortex. Mass spectrometric analysis of 29 different isoforms of globotriaosylceramide (Gb3), a substrate of α-Gal A indicated no significant differences with respect to different stages of PD temporal cortex. However, significant correlations were observed between increased levels of several Gb3 isoforms and with decreased α-Gal A activity and/or increased p129S-α-syn. Deacylated Gb3 (globotriaosylsphingosine or lyso-Gb3) was also analyzed in PD brain tissue but was below the limit of detection of 20pmol/g. Analysis of other lysosomal enzymes revealed a significant decrease in activity for the lysosomal aspartic acid protease cathepsin D but not for glucocerebrosidase (GCase) or cathepsin B in late-stage PD temporal cortex. However, a significant correlation was observed between decreasing GCase activity and increasing p129S-α-syn. Together our findings indicate α-Gal A deficiency in late-stage PD brain that correlates significantly with the pathological accumulation of α-syn, and further suggest the potential for α-Gal A and its glycosphingolipid substrates as putative biomarkers for PD.