Glucocerebrosidase deficiency in substantia nigra of parkinson disease brains

Reduced sphingolipid hydrolase activities, substrate accumulation and ganglioside decline in Parkinson's disease

Background

Haploinsufficiency in the Gaucher disease GBA gene, which encodes the lysosomal glucocerebrosidase GBA, and ageing represent major risk factors for developing Parkinson’s disease (PD). Recently, more than fifty other lysosomal storage disorder gene variants have been identified in PD, implicating lysosomal dysfunction more broadly as a key risk factor for PD. Despite the evidence of multiple lysosomal genetic risks, it remains unclear how sphingolipid hydrolase activities, other than GBA, are altered with ageing or in PD. Moreover, it is not fully known if levels of glycosphingolipid substrates for these enzymes change in vulnerable brain regions of PD. Finally, little is known about the levels of complex gangliosides in substantia nigra which may play a significant role in ageing and PD.

Methods

To study sphingolipid hydrolase activities and glycosphingolipid expression in ageing and in PD, two independent cohorts of human substantia nigra tissues were obtained. Fluorescent 4-methylumbelliferone assays were used to determine multiple enzyme activities. The lysosomal GBA and non-lysosomal GBA2 activities were distinguished using the inhibitor NB-DGJ. Sensitive and quantitative normal-phase HPLC was performed to study glycosphingolipid levels. In addition, glycosphingolipid levels in cerebrospinal fluid and serum were analysed as possible biomarkers for PD.

Results

The present study demonstrates, in two independent cohorts of human post-mortem substantia nigra, that sporadic PD is associated with deficiencies in multiple lysosomal hydrolases (e.g. α-galactosidase and β-hexosaminidase), in addition to reduced GBA and GBA2 activities and concomitant glycosphingolipid substrate accumulation. Furthermore, the data show significant reductions in levels of complex gangliosides (e.g. GM1a) in substantia nigra, CSF and serum in ageing, PD, and REM sleep behaviour disorder, which is a strong predictor of PD.

Conclusions

These findings conclusively demonstrate reductions in GBA activity in the parkinsonian midbrain, and for the first time, reductions in the activity of several other sphingolipid hydrolases. Furthermore, significant reductions were seen in complex gangliosides in PD and ageing. The diminished activities of these lysosomal hydrolases, the glycosphingolipid substrate accumulation, and the reduced levels of complex gangliosides are likely major contributors to the primary development of the pathology seen in PD and related disorders with age.

The NAD+ Precursor Nicotinamide Riboside Rescues Mitochondrial Defects and Neuronal Loss in iPSC and Fly Models of Parkinson's Disease

While mitochondrial dysfunction is emerging as key in Parkinson's disease (PD), a central question remains whether mitochondria are actual disease drivers and whether boosting mitochondrial biogenesis and function ameliorates pathology. We address these questions using patient-derived induced pluripotent stem cells and Drosophila models of GBA-related PD (GBA-PD), the most common PD genetic risk. Patient neurons display stress responses, mitochondrial demise, and changes in NAD+ metabolism. NAD+ precursors have been proposed to ameliorate age-related metabolic decline and disease. We report that increasing NAD+ via the NAD+ precursor nicotinamide riboside (NR) significantly ameliorates mitochondrial function in patient neurons. Human neurons require nicotinamide phosphoribosyltransferase (NAMPT) to maintain the NAD+ pool and utilize NRK1 to synthesize NAD+ from NAD+ precursors. Remarkably, NR prevents the age-related dopaminergic neuronal loss and motor decline in fly models of GBA-PD. Our findings suggest NR as a viable clinical avenue for neuroprotection in PD and other neurodegenerative diseases.

Role of Autophagy in Parkinson's Disease

Autophagy is an essential catabolic mechanism that delivers misfolded proteins and damaged organelles to the lysosome for degradation. Autophagy pathways include macroautophagy, chaperone-mediated autophagy and microautophagy, each involving different mechanisms of substrate delivery to lysosome. Defects of these pathways and the resulting accumulation of protein aggregates represent a common pathobiological feature of neurodegenerative disorders such as Alzheimer, Parkinson and Huntington disease. This review provides an overview of the role of autophagy in Parkinson's disease (PD) by summarizing the most relevant genetic and experimental evidence showing how this process can contribute to disease pathogenesis. Given lysosomes take part in the final step of the autophagic process, the role of lysosomal defects in the impairment of autophagy and their impact on disease will also be discussed. A glance on the role of non-neuronal autophagy in the pathogenesis of PD will be included. Moreover, we will examine novel pharmacological targets and therapeutic strategies that, by boosting autophagy, may be theoretically beneficial for PD. Special attention will be focused on natural products, such as phenolic compounds, that are receiving increasing consideration due to their potential efficacy associated with low toxicity. Although many efforts have been made to elucidate autophagic process, the development of new therapeutic interventions requires a deeper understanding of the mechanisms that may lead to autophagy defects in PD and should take into account the multifactorial nature of the disease as well as the phenotypic heterogeneity of PD patients.

Autophagic- and Lysosomal-Related Biomarkers for Parkinson's Disease: Lights and Shadows

Parkinson’s disease (PD) is a neurodegenerative disorder that currently affects 1% of the population over the age of 60 years, for which no disease-modifying treatments exist. This lack of effective treatments is related to the advanced stage of neurodegeneration existing at the time of diagnosis. Thus, the identification of early stage biomarkers is crucial. Biomarker discovery is often guided by the underlying molecular mechanisms leading to the pathology. One of the central pathways deregulated during PD, supported both by genetic and functional studies, is the autophagy-lysosomal pathway. Hence, this review presents different studies on the expression and activity of autophagic and lysosomal proteins, and their functional consequences, performed in peripheral human biospecimens. Although most biomarkers are inconsistent between studies, some of them, namely HSC70 levels in sporadic PD patients, and cathepsin D levels and glucocerebrosidase activity in PD patients carrying GBA mutations, seem to be consistent. Hence, evidence exists that the impairment of the autophagy-lysosomal pathway underlying PD pathophysiology can be detected in peripheral biosamples and further tested as potential biomarkers. However, longitudinal, stratified, and standardized analyses are needed to confirm their clinical validity and utility.

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.

Therapies to Slow, Stop, or Reverse Parkinson's Disease

Our understanding of PD pathophysiology is vastly improved compared to the situation 20 years ago. We have identified the major genetic risks for PD, we now have far more representative animal models of the disease, and we can be inspired by the early successes of others using Antisense Oligonucleotide and vaccination approaches in other neurodegenerative diseases. We also have a broad range of repurposed drugs showing the first signals of potential efficacy in the translational pipeline which are being driven forward through the various clinical trial stages. We believe we can be optimistic that the next 20 years will be a time for major breakthroughs towards the discovery of therapies that may slow, stop, or reverse PD.

LRRK2 inhibition prevents endolysosomal deficits seen in human Parkinson's disease

LRRK2 has been implicated in endolysosomal function and likely plays a central role in idiopathic Parkinson’s disease (iPD). In iPD, dopaminergic neurons within the substantia nigra are characterized by increased LRRK2 kinase activity, endolysosomal deficits, and accumulation of autophagic vesicles with incompletely degraded substrates, including α-synuclein. Although LRRK2 has been implicated in endolysosomal and autophagic function, it remains unclear whether inhibition of LRRK2 kinase activity can prevent endolysosomal deficits or reduce dopaminergic neurodegeneration. In this study, we characterized the endolysosomal and autophagic defects in surviving dopaminergic neurons of iPD patient brain tissue. We next showed that these defects could be reproduced reliably in vivo using the rotenone model of iPD. Results suggested that there was impaired endosomal maturation, resulting in lysosomal dysfunction and deficits in protein degradation. A highly selective, brain-penetrant LRRK2 kinase inhibitor not only improved apparent endosomal maturation and lysosomal function, but also prevented rotenone-induced neurodegeneration in vivo. The fact that a LRRK2 kinase inhibitor was capable of preventing the neuropathological and endolysosomal abnormalities observed in human iPD suggests that LRRK2 inhibitors may have broad therapeutic utility in iPD, not only in those who carry a LRRK2 mutation.

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.

Neuronal vulnerability in Parkinson disease: Should the focus be on axons and synaptic terminals?

While current effective therapies are available for the symptomatic control of PD, treatments to halt the progressive neurodegeneration still do not exist. Loss of dopamine neurons in the SNc and dopamine terminals in the striatum drive the motor features of PD. Multiple lines of research point to several pathways which may contribute to dopaminergic neurodegeneration. These pathways include extensive axonal arborization, mitochondrial dysfunction, dopamine's biochemical properties, abnormal protein accumulation of α-synuclein, defective autophagy and lysosomal degradation, and synaptic impairment. Thus, understanding the essential features and mechanisms of dopaminergic neuronal vulnerability is a major scientific challenge and highlights an outstanding need for fostering effective therapies against neurodegeneration in PD. This article, which arose from the Movement Disorders 2018 Conference, discusses and reviews the possible mechanisms underlying neuronal vulnerability and potential therapeutic approaches in PD. © 2019 International Parkinson and Movement Disorder Society.

Evolution of prodromal parkinsonian features in a cohort of GBA mutation-positive individuals: a 6-year longitudinal study

OBJECTIVES

GBA1 mutations are a frequent risk factor for Parkinson disease (PD). The aim of this study is to evaluate clinical features in a group of GBA1 mutation-positive individuals over a 6-year follow-up.

METHODS

This is a longitudinal study on a cohort of GBA1-positive carriers. We enrolled 31 patients with Gaucher disease type 1 (GD), 29 GBA1 heterozygous carriers (Het GBA group) and 30 controls (HC) at baseline and followed them for 6 years. We assessed baseline motor and non-motor signs of PD in all subjects using clinical questionnaires and scales (reduced Unified Multiple System Atrophy Rating Scale (UMSARS), Montreal Cognitive assessment (MoCA), University of Pennsylvania Smell Identification Test (UPSIT), REM Sleep Behavior Disorder screening questionnaire (RBDsq), Movement Disorders Society Unified Parkinson's Disease Rating Scale motor subscale (MDS-UPDRS III) and Beck Depression Inventory (BDI). We repeated these at the 6-year follow-up alongside venous blood sampling for measurement of glucocerebrosidase enzymatic activity (GCase). We explored whether the GCase activity level was altered in leucocytes of these subjects and how it was related to development of PD.

RESULTS

We observed a significant worsening in UMSARS, RBDsq, MDS-UPDRS III and BDI scores at the 6-year follow-up compared with baseline in both the GD and Het GBA groups. Intergroup comparisons showed that GD subjects had significantly worse scores in UPSIT, UMSARS, MoCA and MDS-UPDRS III than HC, while Het GBA displayed worse outcomes in UPSIT and MDS-UPDRS III compared with HC. In GBA1 mutation-positive individuals (Het GBA and GD), an UPSIT score of 23 at baseline was correlated with worse outcome at 6 years in UPSIT, MoCA, MDS-UPDRS III and BDI.

CONCLUSION

In this 6-year-long longitudinal study, GBA1 mutation-positive subjects showed a worsening in motor and non-motor prodromal PD features.

Endo-lysosomal proteins and ubiquitin CSF concentrations in Alzheimer's and Parkinson's disease

BACKGROUND

Increasing evidence implicates dysfunctional proteostasis and the involvement of the autophagic and endo-lysosomal system and the ubiquitin-proteasome system in neurodegenerative diseases. In Alzheimer's disease (AD), there is an accumulation of autophagic vacuoles within the neurons. In Parkinson's disease (PD), susceptibility has been linked to genes encoding proteins involved in autophagy and lysosomal function, as well as mutations causing lysosomal disorders. Furthermore, both diseases are characterized by the accumulation of protein aggregates.

METHODS

Proteins associated with endocytosis, lysosomal function, and the ubiquitin-proteasome system were identified in the cerebrospinal fluid (CSF) and targeted by combining solid-phase extraction and parallel reaction monitoring mass spectrometry. In total, 50 peptides from 18 proteins were quantified in three cross-sectional cohorts including AD (N = 61), PD (N = 21), prodromal AD (N = 10), stable mild cognitive impairment (N = 15), and controls (N = 68).

RESULTS

A pilot study, including subjects selected based on their AD CSF core biomarker concentrations, showed increased concentrations of several targeted proteins in subjects with core biomarker levels indicating AD pathology compared to controls. Next, in a clinically characterized cohort, lower concentrations in CSF of proteins in PD were found compared to subjects with prodromal AD. Further investigation in an additional clinical study again revealed lower concentrations in CSF of proteins in PD compared to controls and AD.

CONCLUSION

In summary, significantly different peptide CSF concentrations were identified from proteins AP2B1, C9, CTSB, CTSF, GM2A, LAMP1, LAMP2, TCN2, and ubiquitin. Proteins found to have altered concentrations in more than one study were AP2B1, CTSB, CTSF, GM2A, LAMP2, and ubiquitin. Interestingly, given the genetic implication of lysosomal function in PD, we did identify the CSF concentrations of CTSB, CTSF, GM2A, and LAMP2 to be altered. However, we also found differences in proteins associated with endocytosis (AP2B1) and the ubiquitin-proteasome system (ubiquitin). No difference in any peptide CSF concentration was found in clinically characterized subjects with AD compared to controls. In conclusion, CSF analyses of subjects with PD suggest a general lysosomal dysfunction, which resonates well with recent genetic findings, while such changes are minor or absent in AD.

Role of the endolysosomal system in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative disorders, affecting 1-1.5% of the total population. While progress has been made in understanding the neurodegenerative mechanisms that lead to cell death in late stages of PD, mechanisms for early, causal pathogenic events are still elusive. Recent developments in PD genetics increasingly point at endolysosomal (E-L) system dysfunction as the early pathomechanism and key pathway affected in PD. Clathrin-mediated synaptic endocytosis, an integral part of the neuronal E-L system, is probably the main early target as evident in auxilin, RME-8, and synaptojanin-1 mutations that cause PD. Autophagy, another important pathway in the E-L system, is crucial in maintaining proteostasis and a healthy mitochondrial pool, especially in neurons considering their inability to divide and requirement to function an entire life-time. PINK1 and Parkin mutations severely perturb autophagy of dysfunctional mitochondria (mitophagy), both in the cell body and synaptic terminals of dopaminergic neurons, leading to PD. Endolysosomal sorting and trafficking is also crucial, which is complex in multi-compartmentalized neurons. VPS35 and VPS13C mutations noted in PD target these mechanisms. Mutations in GBA comprise the most common risk factor for PD and initiate pathology by compromising lysosomal function. This is also the case for ATP13A2 mutations. Interestingly, α-synuclein and LRRK2, key proteins involved in PD, function in different steps of the E-L pathway and target their components to induce disease pathogenesis. In this review, we discuss these E-L system genes that are linked to PD and how their dysfunction results in PD pathogenesis. This article is part of the Special Issue "Synuclein".

Glucocerebrosidase and its relevance to Parkinson disease

Mutations in GBA1, the gene encoding the lysosomal enzyme glucocerebrosidase, are among the most common known genetic risk factors for the development of Parkinson disease and related synucleinopathies. A great deal is known about GBA1, as mutations in GBA1 are causal for the rare autosomal storage disorder Gaucher disease. Over the past decades, significant progress has been made in understanding the genetics and cell biology of glucocerebrosidase. A least 495 different mutations, found throughout the 11 exons of the gene are reported, including both common and rare variants. Mutations in GBA1 may lead to degradation of the protein, disruptions in lysosomal targeting and diminished performance of the enzyme in the lysosome.

Gaucher disease is phenotypically diverse and has both neuronopathic and non-neuronopathic forms. Both patients with Gaucher disease and heterozygous carriers are at increased risk of developing Parkinson disease and Dementia with Lewy Bodies, although our understanding of the mechanism for this association remains incomplete. There appears to be an inverse relationship between glucocerebrosidase and α-synuclein levels, and even patients with sporadic Parkinson disease have decreased glucocerebrosidase. Glucocerebrosidase may interact with α-synuclein to maintain basic cellular functions, or impaired glucocerebrosidase could contribute to Parkinson pathogenesis by disrupting lysosomal homeostasis, enhancing endoplasmic reticulum stress or contributing to mitochondrial impairment. However, the majority of patients with GBA1 mutations never develop parkinsonism, so clearly other risk factors play a role. Treatments for Gaucher disease have been developed that increase visceral glucocerebrosidase levels and decrease lipid storage, although they have yet to properly address the neurological defects associated with impaired glucocerebrosidase. Mouse and induced pluripotent stem cell derived models have improved our understanding of glucocerebrosidase function and the consequences of its deficiency. These models have been used to test novel therapies including chaperone proteins, histone deacetylase inhibitors, and gene therapy approaches that enhance glucocerebrosidase levels and could prove efficacious in the treatment of forms of parkinsonism. Consequently, this rare monogenic disorder, Gaucher disease, provides unique insights directly applicable to our understanding and treatment of Parkinson disease, a common and complex neurodegenerative disorder.

The interplay of aging, genetics and environmental factors in the pathogenesis of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is characterized by dopaminergic neuronal loss in the substantia nigra pars compacta and intracellular inclusions called Lewy bodies (LB). During the course of disease, misfolded α-synuclein, the major constituent of LB, spreads to different regions of the brain in a prion-like fashion, giving rise to successive non-motor and motor symptoms. Etiology is likely multifactorial, and involves interplay among aging, genetic susceptibility and environmental factors.

MAIN BODY

The prevalence of PD rises exponentially with age, and aging is associated with impairment of cellular pathways which increases susceptibility of dopaminergic neurons to cell death. However, the majority of those over the age of 80 do not have PD, thus other factors in addition to aging are needed to cause disease. Discovery of neurotoxins which can result in parkinsonism led to efforts in identifying environmental factors which may influence PD risk. Nevertheless, the causality of most environmental factors is not conclusively established, and alternative explanations such as reverse causality and recall bias cannot be excluded. The lack of geographic clusters and conjugal cases also go against environmental toxins as a major cause of PD. Rare mutations as well as common variants in genes such as SNCA, LRRK2 and GBA are associated with risk of PD, but Mendelian causes collectively only account for 5% of PD and common polymorphisms are associated with small increase in PD risk. Heritability of PD has been estimated to be around 30%. Thus, aging, genetics and environmental factors each alone is rarely sufficient to cause PD for most patients.

CONCLUSION

PD is a multifactorial disorder involving interplay of aging, genetics and environmental factors. This has implications on the development of appropriate animal models of PD which take all these factors into account. Common converging pathways likely include mitochondrial dysfunction, impaired autophagy, oxidative stress and neuroinflammation, which are associated with the accumulation and spread of misfolded α-synuclein and neurodegeneration. Understanding the mechanisms involved in the initiation and progression of PD may lead to potential therapeutic targets to prevent PD or modify its course.

Glucocerebrosidase regulators SCARB2 and TFEB are up-regulated in Lewy body disease brain

Mutations in the glucocerebrosidase (GCase) gene (GBA) and GCase deficiency are major risk factors for Lewy body diseases. Decreased GCase activity enhances alpha-synuclein aggregation and disease development. Lysosomal integral membrane protein type 2, encoded by SCARB2, binds GCase targeting it to lysosomes and transcription factor EB (Tfeb) regulates lysosomal proteostasis. Our aim was to find out if GCase deficiency in Lewy body diseases is accompanied by SCARB2 and TFEB deregulation at the transcriptional level involving alternative splicing as well. Relative mRNA expression of two SCARB2 and two TFEB transcripts was studied by real-time PCR in post-mortem brain samples of cases with pure Lewy body pathology (LBP), cases with concomitant LBP and Alzheimer disease-like pathology, and controls. TFEB expression was increased in the temporal cortex and caudate nucleus of LBP cases, and SCARB2 was differentially expressed. Female-gender associated overexpression of all transcripts was found in the caudate nucleus, and disease duration associated TFEB expression changes in the temporal cortex. SCARB2 and TFEB expression correlated negatively with GBA mRNA expression in the temporal cortex. Our findings show disease-specific deregulation of TFEB and SCARB2 expression affecting alternative promoter usage and alternative splicing in Lewy body diseases.

The role of dopamine in the pathogenesis of GBA1-linked Parkinson's disease

Our understanding of the molecular mechanisms underlying differential vulnerability of substantia nigra dopamine neurons in Parkinson's disease (PD) remains limited, and previous therapeutic efforts targeting rodent nigral neurons have not been successfully translated to humans. However, recent emergence of induced pluripotent stem cell technology has highlighted some fundamental differences between human and rodent midbrain dopamine neurons that may at least in part explain relative resistance of rodent neurons to degeneration in genetic models of PD. Using GBA1-linked PD as an example, we discuss cellular pathways that may predispose human neurons to degeneration in PD, including mitochondrial oxidant stress, elevated intracellular calcium, altered synaptic vesicle endocytosis, accumulation of oxidized dopamine and neuromelanin. Recent studies have suggested that a combination of mitochondrial oxidant stress and accumulation of oxidized dopamine contribute to dysfunction of nigral neurons in various genetic and sporadic forms of PD. We also briefly summarize the development of targeted therapies for GBA1-associated synucleinopathies and highlight that modulation of wild-type GCase activity serves as an important target for the treatment of genetic and idiopathic forms of PD and dementia with Lewy bodies.

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.

Age-related neurochemical and behavioural changes in D409V/WT GBA1 mouse: Relevance to lewy body dementia

Heterozygous mutations in GBA1, the gene which encodes the lysosomal enzyme glucocerebrosidase (GCase), are a strong genetic risk factor for the development of Lewy body dementia (LBD). Until this point however, recapitulation of the symptoms and pathology of LBD has been limited to a homozygous GBA1 mouse model which genetically and enzymatically reflects the lysosomal storage disorder Gaucher's disease. This study reports for the first time cognitive impairment by two independent behavioural tests in heterozygous GBA1 mutant mice (D409V/WT) which demonstrate significant cognitive impairment by the age of 12 months. Furthermore, reductions in GBA1 GCase enzyme activity within the brain reflects levels seen in sporadic and GBA1 mutant LBD patients. While there is no overt deposition of Lewy bodies within the hippocampus, alterations to cholinergic machinery and glial proliferation are evident, both pathological features of LBD. Interestingly, we also describe the novel finding of significantly reduced GBA2 GCase enzyme activity specifically within the hippocampus. This suggests that reduced GBA1 GCase enzyme activity dis-equilibrates the finely balanced glycosphingolipid metabolism pathway and that reductions in GBA2 GCase enzyme could contribute to the pathological and behavioural effects seen. Overall, this study presents evidence to suggest that pathological hallmarks associated with LBD specifically affecting brain regions intrinsically linked with cognition are present in the D409V/WT mice. In the absence of Lewy body deposition, the D409V/WT mice could be considered an early pre-clinical model of LBD with potential for drug discovery. Since few robust pre-clinical models of LBD currently exist, with further characterization, the mouse model described here may contribute significantly to developments in the LBD field.

Can GBA1-Associated Parkinson Disease Be Modeled in the Mouse?

Homozygous and heterozygous mutations in GBA1, the gene implicated in Gaucher disease, increase the risk and severity of Parkinson disease (PD). We evaluated the design, phenotype, strengths, and limitations of current GBA1-associated PD mouse models. Although faithful modeling of a genetic risk factor poses many challenges, the different approaches taken were successful in revealing predisposing abnormalities in heterozygotes for GBA1 mutations and demonstrating the deleterious effects of GBA1 impairment on the PD course in PD models. GBA1-PD models differ in key parameters, with no single model recapitulating all aspects of the GBA1-PD puzzle, emphasizing the importance of selecting the proper in vivo model depending on the specific molecular mechanism or potential therapy being studied.

The Link between Gaucher Disease and Parkinson's Disease Sheds Light on Old and Novel Disorders of Sphingolipid Metabolism

Sphingolipid metabolism starts with the biosynthesis of ceramide, a bioactive lipid and the backbone for the biosynthesis of complex sphingolipids such as sphingomyelin and glycosphingolipids. These are degraded back to ceramide and then to sphingosine, which enters the ceramide–sphingosine-1-phosphate signaling pathway or is further degraded. Several enzymes with multiple catalytic properties and subcellular localizations are thus involved in such metabolism. Hereditary defects of lysosomal hydrolases have been known for several years to be the cause of lysosomal storage diseases such as gangliosidoses, Gaucher disease, Niemann–Pick disease, Krabbe disease, Fabry disease, and Farber disease. More recently, many other inborn errors of sphingolipid metabolism have been recognized, involving enzymes responsible for the biosynthesis of ceramide, sphingomyelin, and glycosphingolipids. Concurrently, epidemiologic and biochemical evidence has established a link between Gaucher disease and Parkinson’s disease, showing that glucocerebrosidase variants predispose individuals to α-synuclein accumulation and neurodegeneration even in the heterozygous status. This appears to be due not only to lysosomal overload of non-degraded glucosylceramide, but to the derangement of vesicle traffic and autophagy, including mitochondrial autophagy, triggered by both sphingolipid intermediates and misfolded proteins. In this review, old and novel disorders of sphingolipid metabolism, in particular those of ganglioside biosynthesis, are evaluated in light of recent investigations of the link between Gaucher disease and Parkinson’s disease, with the aim of better understanding their pathogenic mechanisms and addressing new potential therapeutic strategies.

Lipids as Trans-Acting Effectors for α-Synuclein in the Pathogenesis of Parkinson's Disease

Aggregation of α-synuclein (αSyn) plays a central role in the pathogenesis of Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). Lewy bodies (LBs) and Lewy neurites, which consist mainly of aggregated αSyn, are widely observed in the affected regions of patient brains. Except for some familial forms of PD/DLB, most sporadic PD/DLB patients express the wild-type (WT) αSyn protein without any mutations, and the mechanisms as to how WT αSyn gains the propensity to pathologically aggregate still remains unclear. Furthermore, the mechanisms by which the same αSyn protein can cause different synucleinopathies with distinct phenotypes and pathologies, such as PD, DLB, and multiple system atrophy (MSA), still remain largely unknown. Recently, mutations in the GBA1 gene (encoding glucocerebrosidase), which are responsible for the lysosomal storage disorder Gaucher disease (GD), have been reported to be the strongest risk factor for developing sporadic PD/DLB. We previously demonstrated that glucosylceramide accumulated by GBA1 deficiency promotes the conversion of αSyn into a proteinase K-resistant conformation. Furthermore, decreased glucocerebrosidase activity has also been reported in the brains of patients with sporadic PD/DLB. Moreover, αSyn pathology has also been shown in the brains of lysosomal storage disorder patients, which show glycosphingolipid accumulation. These observations suggest the possibility that altered lipid metabolism and lipid accumulation play roles in αSyn aggregation and PD/DLB pathogenesis. Indeed, several previous studies have demonstrated that lipid interactions affect the conformation of αSyn and induces its oligomerization and aggregation. In this review, we will give an overview of the association between αSyn aggregation and lipid interactions from the viewpoints of the etiology, pathology, and genetics of PD/DLB. We also discuss the distinct species of αSyn aggregates and their association with specific types of synucleinopathies, and introduce our hypothesis that lipid interactions play a role as trans-acting effectors in producing distinct strains of αSyn fibrils.

Evolution and clustering of prodromal parkinsonian features in GBA1 carriers

Background

Five to 25% of patients with PD carry glucocerebrosidase gene mutations, and 10% to 30% of glucocerebrosidase carriers will develop PD by age 80. Stratification of PD risk in glucocerebrosidase carriers provides an opportunity to target disease‐modifying therapies.

Objective

Cross‐sectional and longitudinal survey of prodromal PD signs among glucocerebrosidase carriers.

Design

Prospective assessment of 82 glucocerebrosidase mutation carriers and 35 controls over 4 to 5 years for prodromal clinical PD features.

Results

At all time points, olfactory (measured using University of Pennsylvania Smell Identification Test) and cognitive (Montreal Cognitive Assessment) function and the International Parkinson and Movement Disorder Society UPDRS parts II and III scores were significantly worse amongst glucocerebrosidase mutation carriers. Progression to microsmia (odds ratio: 8.5; 95% confidence interval: 2.6–28.2; P < 0.05) and mild cognitive impairment (odds ratio: 4.2; 95% confidence interval: 1.1–16.6; P < 0.05) were more rapid compared to controls. Those with worse olfaction also had worse cognition (OR, 1.5; 95% CI: 0.0–2.8; P < 0.05) and depression (OR, 1.3; 95% CI: 0.6–2.8; P < 0.05). No participants reached the MDS prodromal PD diagnostic criteria before PD diagnosis. One participant developed PD. He did not fulfill the International Parkinson and Movement Disorder Society prodromal PD criteria before diagnosis.

Conclusion

Assessment of individual and clustered PD prodromal features may serve as a useful tool to identify high‐risk subjects for conversion to PD. As a result of the low conversion rate in our glucocerebrosidase mutation carriers to date, prospective validation is needed in larger cohorts to establish the profile of these features in PD convertors. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Autophagy in Synucleinopathy: The Overwhelmed and Defective Machinery

Alpha-synuclein positive-intracytoplasmic inclusions are the common denominators of the synucleinopathies present as Lewy bodies in Parkinson’s disease, dementia with Lewy bodies, or glial cytoplasmic inclusions in multiple system atrophy. These neurodegenerative diseases also exhibit cellular dyshomeostasis, such as autophagy impairment. Several decades of research have questioned the potential link between the autophagy machinery and alpha-synuclein protein toxicity in synucleinopathy and neurodegenerative processes. Here, we aimed to discuss the active participation of autophagy impairment in alpha-synuclein accumulation and propagation, as well as alpha-synuclein-independent neurodegenerative processes in the field of synucleinopathy. Therapeutic approaches targeting the restoration of autophagy have started to emerge as relevant strategies to reverse pathological features in synucleinopathies.

The Mechanisms by Which the Ketone Body D-β-Hydroxybutyrate May Improve the Multiple Cellular Pathologies of Parkinson's Disease

Parkinson's disease, a progressive neurodegenerative disorder characterized by motor and non-motor symptoms, is strongly associated with the death of dopaminergic neurons in the brain's substantia nigra. Although dopamine replacement therapy temporarily helps patients manage their motor symptoms, this current standard of care fails to address the underlying network of pathologies that contribute to the persistent death of dopaminergic neurons. Thus, new treatment approaches are needed that address the underlying pathologies and, thereby, slow or halt the progression of the actual disease. D-β-hydroxybutyrate – a ketone body produced by the liver to support brain function during periods of starvation – may provide an option. Lifestyle interventions that induce endogenous D-β-hydroxybutyrate production, such as caloric restriction and ketogenic diets, are known to increase healthspan and lifespan in animal models and are used to treat neurological disorders. The efficacy of these ketosis-inducing interventions, along with the recent development of commercially available D-β-hydroxybutyrate-based nutritional supplements, should inspire interest in the possibility that D-β-hydroxybutyrate itself exerts neuroprotective effects. This review provides a molecular model to justify the further exploration of such a possibility. Herein, we explore the cellular mechanisms by which the ketone body, D-β-hydroxybutyrate, acting both as a metabolite and as a signaling molecule, could help to prevent the development, or slow the progression of, Parkinson's disease. Specifically, the metabolism of D-β-hydroxybutyrate may help neurons replenish their depleted ATP stores and protect neurons against oxidative damage. As a G-protein-coupled receptor ligand and histone deacetylase inhibitor, D-β-hydroxybutyrate may further protect neurons against energy deficit and oxidative stress, while also decreasing damaging neuroinflammation and death by apoptosis. Restricted to the available evidence, our model relies largely upon the interpretation of data from the separate literatures on the cellular effects of D-β-hydroxybutyrate and on the pathogenesis of Parkinson's disease. Future studies are needed to reveal whether D-β-hydroxybutyrate actually has the potential to serve as an adjunctive nutritional therapy for Parkinson's disease.

Dysfunction of Cellular Proteostasis in Parkinson's Disease

Despite decades of research, current therapeutic interventions for Parkinson’s disease (PD) are insufficient as they fail to modify disease progression by ameliorating the underlying pathology. Cellular proteostasis (protein homeostasis) is an essential factor in maintaining a persistent environment for neuronal activity. Proteostasis is ensured by mechanisms including regulation of protein translation, chaperone-assisted protein folding and protein degradation pathways. It is generally accepted that deficits in proteostasis are linked to various neurodegenerative diseases including PD. While the proteasome fails to degrade large protein aggregates, particularly alpha-synuclein (α-SYN) in PD, drug-induced activation of autophagy can efficiently remove aggregates and prevent degeneration of dopaminergic (DA) neurons. Therefore, maintenance of these mechanisms is essential to preserve all cellular functions relying on a correctly folded proteome. The correlations between endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) that aims to restore proteostasis within the secretory pathway are well-established. However, while mild insults increase the activity of chaperones, prolonged cell stress, or insufficient adaptive response causes cell death. Modulating the activity of molecular chaperones, such as protein disulfide isomerase which assists refolding and contributes to the removal of unfolded proteins, and their associated pathways may offer a new approach for disease-modifying treatment. Here, we summarize some of the key concepts and emerging ideas on the relation of protein aggregation and imbalanced proteostasis with an emphasis on PD as our area of main expertise. Furthermore, we discuss recent insights into the strategies for reducing the toxic effects of protein unfolding in PD by targeting the ER UPR pathway.

Dysregulated Lipid Metabolism and Its Role in α-Synucleinopathy in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, the main pathological hallmark of which is the accumulation of α-synuclein (α-syn) and the formation of filamentous aggregates called Lewy bodies in the brainstem, limbic system, and cortical areas. Lipidomics is a newly emerging field which can provide fresh insights and new answers that will enhance our capacity for early diagnosis, tracking disease progression, predicting critical endpoints, and identifying risk in pre-symptomatic persons. In recent years, lipids have been implicated in many aspects of PD pathology. Biophysical and lipidomic studies have demonstrated that α-syn binds preferentially not only to specific lipid families but also to specific molecular species and that these lipid-protein complexes enhance its interaction with synaptic membranes, influence its oligomerization and aggregation, and interfere with the catalytic activity of cytoplasmic lipid enzymes and lysosomal lipases, thereby affecting lipid metabolism. The genetic link between aberrant lipid metabolism and PD is even more direct, with mutations in GBA and SMPD1 enhancing PD risk in humans and loss of GALC function increasing α-syn aggregation and accumulation in experimental murine models. Moreover, a number of lipidomic studies have reported PD-specific lipid alterations in both patient brains and plasma, including alterations in the lipid composition of lipid rafts in the frontal cortex. A further aspect of lipid dysregulation promoting PD pathogenesis is oxidative stress and inflammation, with proinflammatory lipid mediators such as platelet activating factors (PAFs) playing key roles in arbitrating the progressive neurodegeneration seen in PD linked to α-syn intracellular trafficking. Lastly, there are a number of genetic risk factors of PD which are involved in normal lipid metabolism and function. Genes such as PLA2G6 and SCARB2, which are involved in glycerophospholipid and sphingolipid metabolism either directly or indirectly are associated with risk of PD. This review seeks to describe these facets of metabolic lipid dysregulation as they relate to PD pathology and potential pathomechanisms involved in disease progression, while highlighting incongruous findings and gaps in knowledge that necessitate further research.

Autophagy dysfunction in peripheral blood mononuclear cells of Parkinson's disease patients

BACKGROUND

Alpha-synuclein aggregation is considered one of the main causes of Parkinson's Disease (PD). Malfunction of autophagy-lysosomal pathways is believed to be an underlying mechanism of α-synuclein aggregation. Although such malfunction has been observed in PD brains, it is unclear whether it may also occur in extraneuronal tissues.

OBJECTIVES

To assess lysosome-mediated protein degradation in cultured Peripheral Blood Mononuclear Cells (PBMCs) of PD patients and healthy controls.

METHODS

Total protein degradation in cultured PBMCs was measured by labelling the cells with ³H-leucine using pulse-chase experiments. Different inhibitors were used to measure a range of autophagic pathways.

RESULTS

Protein degradation through the main autophagic pathways is reduced in PD patients (n = 18) compared to age- and sex-matched healthy controls (n = 18), (macroautophagy, p = .018; Chaperone-Mediated autophagy, p = .04; and total lysosomal function, p = .007).

CONCLUSIONS

Lysosomal dysfunction is present in cultured PBMCs of PD patients, suggesting that it may reflect a systemic feature of the disease.

Lysosomal enzyme activities as possible CSF biomarkers of synucleinopathies

Mutations on the GBA gene, encoding for the lysosomal enzyme β-glucocerebrosidase (GCase), have been identified as the most common genetic risk factor involved in the development of Parkinson's disease (PD) and dementia with Lewy bodies (DLB), indicating a direct contribution of this enzyme to the pathogenesis of synucleinopathies. Decreased GCase activity has been observed repeatedly in brain tissues and biological fluids of both GBA mutation carrier and non-carrier PD and DLB patients, suggesting that lower GCase activity constitutes a typical feature of these disorders. Additional genetic, pathological and biochemical data on other lysosomal enzymes (e.g., Acid sphingomyelinase, Cathepsin D, α-galactosidase A and β-hexosaminidase) have further strengthened the evidence of a link between lysosomal dysfunction and synucleinopathies. A few studies have been performed for assessing the potential value of lysosomal enzyme activities in cerebrospinal fluid (CSF) as biomarkers for synucleinopathies. The reduction of GCase activity in the CSF of PD and DLB patients was validated in several of them, whereas the behaviour of other lysosomal enzyme activities was not consistently reliable among the studies. More in-depth investigations on larger cohorts, following stringent standard operating procedures should be committed to really understand the diagnostic utility of lysosomal enzymes as biomarkers for synucleinopathies. In this review, we reported the evidences of the association between the defective function of lysosomal proteins and the pathogenesis of synucleinopathies, and examined the role of lysosomal enzyme activities in CSF as reliable biomarkers for the diagnosis of PD and related neurodegenerative disorders.

Advances in the Research of Risk Factors and Prodromal Biomarkers of Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease in the world. With the advent of an aging population and improving life expectancy worldwide, the number of PD patients is expected to increase, which may lead to an urgent need for effective preventive and diagnostic strategies for PD. Although there is increasing research regarding the pathogenesis of PD, there is limited knowledge regarding the prevention of PD. Moreover, the diagnosis of PD depends on clinical criteria, which require the occurrence of bradykinesia and at least one symptom of rest tremor or rigidity. However, converging evidence from clinical, genetic, neuropathological, and imaging studies suggests the initiation of PD-specific pathology prior to the initial presentation of these classical motor clinical features by years or decades. This latent stage of neurodegeneration in PD is a particularly important stage for effective neuroprotective therapies, which might retard the progression or prevent the onset of PD. Therefore, the exploration of risk factors and premotor biomarkers is not only crucial to the early diagnosis of PD but is also helpful in the development of effective neuroprotection and health care strategies for appropriate populations at risk for PD. In this review, we searched and summarized ∼249 researches and 31 reviews focusing on the risk factors and prodromal biomarkers of PD and published in MEDLINE.

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.

Ambroxol as a novel disease-modifying treatment for Parkinson's disease dementia: protocol for a single-centre, randomized, double-blind, placebo-controlled trial

BACKGROUND

Currently there are no disease-modifying treatments for Parkinson's disease dementia (PDD), a condition linked to aggregation of the protein α-synuclein in subcortical and cortical brain areas. One of the leading genetic risk factors for Parkinson's disease is being a carrier in the gene for β-Glucocerebrosidase (GCase; gene name GBA1). Studies in cell culture and animal models have shown that raising the levels of GCase can decrease levels of α-synuclein. Ambroxol is a pharmacological chaperone for GCase and is able to raise the levels of GCase and could therefore be a disease-modifying treatment for PDD. The aims of this trial are to determine if Ambroxol is safe and well-tolerated by individuals with PDD and if Ambroxol affects cognitive, biochemical, and neuroimaging measures.

METHODS

This is a phase II, single-centre, double-blind, randomized placebo-controlled trial involving 75 individuals with mild to moderate PDD. Participants will be randomized into Ambroxol high-dose (1050 mg/day), low-dose (525 mg/day), or placebo treatment arms. Assessments will be undertaken at baseline, 6-months, and 12-months follow up times. Primary outcome measures will be the Alzheimer's disease Assessment Scale-cognitive subscale (ADAS-Cog) and the ADCS Clinician's Global Impression of Change (CGIC). Secondary measures will include the Parkinson's disease Cognitive Rating Scale, Clinical Dementia Rating, Trail Making Test, Stroop Test, Unified Parkinson's disease Rating Scale, Purdue Pegboard, Timed Up and Go, and gait kinematics. Markers of neurodegeneration will include MRI and CSF measures. Pharmacokinetics and pharmacodynamics of Ambroxol will be examined through plasma levels during dose titration phase and evaluation of GCase activity in lymphocytes.

DISCUSSION

If found effective and safe, Ambroxol will be one of the first disease-modifying treatments for PDD.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02914366, 26 Sep 2016/retrospectively registered.

Sphingolipids in the Pathogenesis of Parkinson's Disease and Parkinsonism

The pathogenic mechanisms underlying Parkinson's disease (PD)/parkinsonism affect mitochondrial and endolysosomal trafficking. The retromer is required to retrieve some proteins from endosomes to the Golgi and plasma membrane. Here, we discuss how retromer-dependent retrieval also affects ceramide metabolism. Compelling studies across PD models in Drosophila and mammalian neurons reveal a pathogenic cascade implicating retromer dysfunction and mitochondrial defects. We argue that ceramides may play a critical role in the pathobiology based on the studies of PLA2G6 and VPS35 in Drosophila mutants and human knock-down cells. In addition, pathogenic variants in many lysosomal storage disorder genes have recently been associated with PD, suggesting a potential overlap between the pathogenic mechanisms underlying these disorders. We propose that disruption of ceramide metabolism may affect endolysosomal and mitochondrial function, and plays an important role in PD/parkinsonism.

Dysregulation of the autophagic-lysosomal pathway in Gaucher and Parkinson's disease

The finding that mutations in the Gaucher's Disease (GD) gene GBA1 are a strong risk factor for Parkinson's Disease (PD) has allowed for unique insights into pathophysiology centered on disruption of the autophagic-lysosomal pathway. Protein aggregations in the form of Lewy bodies and the effects of canonical PD mutations that converge on the lysosomal degradation system suggest that neurodegeneration in PD is mediated by dysregulation of protein homeostasis. The well-characterized clinical and pathological relationship between PD and the lysosomal storage disorder GD emphasizes the importance of dysregulated protein metabolism in neurodegeneration, and one intriguing piece of this relationship is a shared phenotype of autophagic-lysosomal dysfunction in both diseases. Translational application of these findings may be accelerated by the use of midbrain dopamine neuronal models derived from induced pluripotent stem cells (iPSCs) that recapitulate several pathological features of GD and PD. In this review, we discuss evidence linking autophagic dysfunction to the pathophysiology of GD and GBA1-linked parkinsonism and focus more specifically on studies performed recently in iPSC-derived neurons.

Mitochondrial dysfunction and mitophagy defect triggered by heterozygous GBA mutations

Heterozygous mutations in GBA, the gene encoding the lysosomal enzyme glucosylceramidase beta/β-glucocerebrosidase, comprise the most common genetic risk factor for Parkinson disease (PD), but the mechanisms underlying this association remain unclear. Here, we show that in GbaL444P/WT knockin mice, the L444P heterozygous Gba mutation triggers mitochondrial dysfunction by inhibiting autophagy and mitochondrial priming, two steps critical for the selective removal of dysfunctional mitochondria by autophagy, a process known as mitophagy. In SHSY-5Y neuroblastoma cells, the overexpression of L444P GBA impeded mitochondrial priming and autophagy induction when endogenous lysosomal GBA activity remained intact. By contrast, genetic depletion of GBA inhibited lysosomal clearance of autophagic cargo. The link between heterozygous GBA mutations and impaired mitophagy was corroborated in postmortem brain tissue from PD patients carrying heterozygous GBA mutations, where we found increased mitochondrial content, mitochondria oxidative stress and impaired autophagy. Our findings thus suggest a mechanistic basis for mitochondrial dysfunction associated with GBA heterozygous mutations. Abbreviations: AMBRA1: autophagy/beclin 1 regulator 1; BECN1: beclin 1, autophagy related; BNIP3L/Nix: BCL2/adenovirus E1B interacting protein 3-like; CCCP: carbonyl cyanide 3-chloroyphenylhydrazone; CYCS: cytochrome c, somatic; DNM1L/DRP1: dynamin 1-like; ER: endoplasmic reticulum; GBA: glucosylceramidase beta; GBA-PD: Parkinson disease with heterozygous GBA mutations; GD: Gaucher disease; GFP: green fluorescent protein; LC3B: microtubule-associated protein 1 light chain 3 beta; LC3B-II: lipidated form of microtubule-associated protein 1 light chain 3 beta; MitoGreen: MitoTracker Green; MitoRed: MitoTracker Red; MMP: mitochondrial membrane potential; MTOR: mechanistic target of rapamycin kinase; MYC: MYC proto-oncogene, bHLH transcription factor; NBR1: NBR1, autophagy cargo receptor; Non-GBA-PD: Parkinson disease without GBA mutations; PD: Parkinson disease; PINK1: PTEN induced putative kinase 1; PRKN/PARK2: parkin RBR E3 ubiquitin protein ligase; RFP: red fluorescent protein; ROS: reactive oxygen species; SNCA: synuclein alpha; SQSTM1/p62: sequestosome 1; TIMM23: translocase of inner mitochondrial membrane 23; TOMM20: translocase of outer mitochondrial membrane 20; VDAC1/Porin: voltage dependent anion channel 1; WT: wild type.

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.

Neurological effects of glucocerebrosidase gene mutations

The association between Gaucher disease (GD) and Parkinson disease (PD) has been described for almost two decades. In the biallelic state (homozygous or compound heterozygous) mutations in the glucocerebrosidase gene (GBA) may cause GD, in which glucosylceramide, the sphingolipid substrate of the glucocerebrosidase enzyme (GCase), accumulates in visceral organs leading to a number of clinical phenotypes. In the biallelic or heterozygous state, GBA mutations increase the risk for PD. Mutations of the GBA allele are the most significant genetic risk factor for idiopathic PD, found in 5%–20% of idiopathic PD cases depending on ethnicity. The neurological consequences of GBA mutations are reviewed and the proposition that GBA mutations result in a disparate but connected range of clinically and pathologically related neurological features is discussed. The literature relating to the clinical, biochemical and genetic basis of GBA PD, type 1 GD and neuronopathic GD is considered highlighting commonalities and distinctions between them. The evidence for a unifying disease mechanism is considered.

Computational modelling approaches as a potential platform to understand the molecular genetics association between Parkinson's and Gaucher diseases

Gaucher's disease (GD) is a genetic disorder in which glucocerebroside accumulates in cells and specific organs. It is broadly classified into type I, type II and type III. Patients with GD are at high risk of Parkinson's disease (PD), and the clinical and pathological presentation of GD patients with PD is almost identical to idiopathic PD. Several experimental models like cell culture, animal models, and transgenic mice models were used to understand the molecular mechanism behind GD and PD association; however, such mechanism remains unclear. In this context, based on literature reports, we identified the most common mutations K198T, E326K, T369M, N370S, V394L, D409H, L444P, and R496H, in the Glucosylceramidase (GBA) protein that are known to cause GD1, and represent a risk of developing PD. However, to date, no computational analyses have designed to elucidate the potential functional role of GD mutations with increased risk of PD. The present computational pipeline allows us to understand the structural and functional significance of these GBA mutations with PD. Based on the published data, the most common and severe mutations were E326K, N370S, and L444P, which further selected for our computational analysis. PredictSNP and iStable servers predicted L444P mutant to be the most deleterious and responsible for the protein destabilization, followed by the N370S mutation. Further, we used the structural analysis and molecular dynamics approach to compare the most frequent deleterious mutations (N370S and L444P) with the mild mutation E326K. The structural analysis demonstrated that the location of E326K and N370S in the alpha helix region of the protein whereas the mutant L444P was in the starting region of the beta sheet, which might explain the predicted pathogenicity level and destabilization effect of the L444P mutant. Finally, Molecular Dynamics (MD) at 50 ns showed the highest deviation and fluctuation pattern in the L444P mutant compared to the two mutants E326K and N370S and the native protein. This was consistent with more loss of intramolecular hydrogen bonds and less compaction of the radius of gyration in the L444P mutant. The proposed study is anticipated to serve as a potential platform to understand the mechanism of the association between GD and PD, and might facilitate the process of drug discovery against both GD and PD.

The glycoprotein GPNMB is selectively elevated in the substantia nigra of Parkinson's disease patients and increases after lysosomal stress

GPNMB is a glycoprotein observed upon tissue damage and inflammation and is associated with astrocytes, microglia, and macrophages. Gene variations in GPNMB are linked with Parkinson's disease (PD) risk, and changes in protein levels of GPNMB have been found in lysosomal storage disorders, including Gaucher's disease with glucocerebrosidase (GCase) deficiency. In the current study, GPNMB increases were seen in the substantia nigra (SN) of PD patients compared to age-matched controls. Such PD patients have a decrease in GCase activity and corresponding elevation of glycosphingolipids in the SN (Rocha et al., 2015a). Interestingly, transgenic mice modelling synucleinopathy did not show GPNMB elevations or altered GCase activity levels compared to wild-type mice. However, upon CBE-induced GCase lysosomal dysfunction with elevated glycosphingolipids in wild-type mice, there were similar changes in GPNMB levels in the brain as seen in PD patient brains. These results indicate that GPNMB levels do not depend on alpha-synuclein load per se but relate directly to the lipidopathy changes induced by CBE-mediated GCase inhibition. The experimental modelling of elevating glycolipids resulted in GPNMB elevations with glial activation in several brain regions in mice. This is the first demonstration of region-specific elevations of GPNMB protein in Parkinson's disease. The presence of GPNMB in PD patient substantia nigra, the induction of GPNMB after experimental glycosphingolipid increases, but not with pure alpha-synucleinopathy, point towards the potential for primary lipid-induced degeneration in PD.

Parkinson's Disease: Basic Pathomechanisms and a Clinical Overview

PD is a common and a debilitating degenerative movement disorder. The number of patients is increasing worldwide and as yet there is no cure for the disease. The majority of existing treatments target motor symptom control. Over the last two decades the impact of the genetic contribution to PD has been appreciated. Significant discoveries have been made, which have advanced our understanding of the pathophysiological and molecular basis of PD. In this chapter we outline current knowledge of the clinical aspects of PD and the basic mechanistic understanding.

Parkinsonism in Inherited Metabolic Disorders: Key Considerations and Major Features

Parkinson's Disease (PD) is a common neurodegenerative disorder manifesting as reduced facilitation of voluntary movements. Extensive research over recent decades has expanded our insights into the pathogenesis of the disease, where PD is indicated to result from multifactorial etiological factors involving environmental contributions in genetically predisposed individuals. There has been considerable interest in the association between neurological manifestations in PD and in inherited metabolic disorders (IMDs), which are genetic disorders characterized by a deficient activity in the pathways of intermediary metabolism leading to multiple-system manifestations. In addition to the parallel in various clinical features, there is increasing evidence for the notion that genetic mutations underlying IMDs may increase the risk of PD development. This review highlights the recent advances in parkinsonism in patients with IMDs, with the primary objective to improve the understanding of the overlapping pathogenic pathways and clinical presentations in both disorders. We discuss the genetic convergence and disruptions in biochemical mechanisms which may point to clues surrounding pathogenesis-targeted treatment and other promising therapeutic strategies in the future.

The Genetics of Dementia with Lewy Bodies: Current Understanding and Future Directions

PURPOSE OF REVIEW

Dementia with Lewy bodies (DLB) is a neurodegenerative disease that can be clinically and pathologically similar to Parkinson's disease (PD) and Alzheimer's disease (AD). Current understanding of DLB genetics is insufficient and has been limited by sample size and difficulty in diagnosis. The first genome-wide association study (GWAS) in DLB was performed in 2017; a time at which the post-GWAS era has been reached in many diseases.

RECENT FINDINGS

DLB shares risk loci with AD, in the APOE E4 allele, and with PD, in variation at GBA and SNCA. Interestingly, the GWAS suggested that DLB may also have genetic risk factors that are distinct from those in AD and PD. Although off to a slow start, recent studies have reinvigorated the field of DLB genetics and these results enable us to start to have a more complete understanding of the genetic architecture of this disease.

Drug Repurposing in Parkinson's Disease

The development of an intervention to slow or halt disease progression remains the greatest unmet therapeutic need in Parkinson's disease. Given the number of failures of various novel interventions in disease-modifying clinical trials in combination with the ever-increasing costs and lengthy processes for drug development, attention is being turned to utilizing existing compounds approved for other indications as novel treatments in Parkinson's disease. Advances in rational and systemic drug repurposing have identified a number of drugs with potential benefits for Parkinson's disease pathology and offer a potentially quicker route to drug discovery. Here, we review the safety and potential efficacy of the most promising candidates repurposed as potential disease-modifying treatments for Parkinson's disease in the advanced stages of clinical testing.

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.