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

Lysosomal dysfunction in α-synuclein pathology: molecular mechanisms and therapeutic strategies

In orchestrating cell signaling, facilitating plasma membrane repair, supervising protein secretion, managing waste elimination, and regulating energy consumption, lysosomes are indispensable guardians that play a crucial role in preserving intracellular homeostasis. Neurons are terminally differentiated post-mitotic cells. Neuronal function and waste elimination depend on normal lysosomal function. Converging data suggest that lysosomal dysfunction is a critical event in the etiology of Parkinson's disease (PD). Mutations in Glucosylceramidase Beta 1 (GBA1) and leucine-rich repeat kinase 2 (LRRK2) confer an increased risk for the development of parkinsonism. Furthermore, lysosomal dysfunction has been observed in the affected neurons of sporadic PD (sPD) patients. Given that lysosomal hydrolases actively contribute to the breakdown of impaired organelles and misfolded proteins, any compromise in lysosomal integrity could incite abnormal accumulation of proteins, including α-synuclein, the major component of Lewy bodies in PD. Clinical observations have shown that lysosomal protein levels in cerebrospinal fluid may serve as potential biomarkers for PD diagnosis and as signs of lysosomal dysfunction. In this review, we summarize the current evidence regarding lysosomal dysfunction in PD and discuss the intimate relationship between lysosomal dysfunction and pathological α-synuclein. In addition, we discuss therapeutic strategies that target lysosomes to treat PD.

Interactions of alpha-synuclein with membranes in Parkinson's disease: Mechanisms and therapeutic strategies

Parkinson's disease (PD), the second most common neurodegenerative disease worldwide, is marked by the presence of Lewy bodies and Lewy neurites, neuronal lesions containing large amounts of the synaptic protein alpha-synuclein (αS). While the underlying mechanisms of disease progression in PD remain unclear, increasing evidence supports the importance of interactions between αS and cellular membranes in PD pathology. Therefore, understanding the αS-membrane interplay in health and disease is crucial for the development of therapeutic strategies. In this review, we (1) discuss key scenarios of pathological αS-membrane interactions; (2) present in detail therapeutic strategies explicitly reported to modify αS-membrane interactions; and (3) introduce additional therapeutic strategies that may involve aspects of interfering with αS-membrane interaction. This way, we aim to provide a holistic perspective on this important aspect of disease-modifying strategies for PD and other α-synucleinopathies.

Interaction between α-Synuclein and Bioactive Lipids: Neurodegeneration, Disease Biomarkers and Emerging Therapies

The present review provides a comprehensive examination of the intricate dynamics between α-synuclein, a protein crucially involved in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease and multiple system atrophy, and endogenously-produced bioactive lipids, which play a pivotal role in neuroinflammation and neurodegeneration. The interaction of α-synuclein with bioactive lipids is emerging as a critical factor in the development and progression of neurodegenerative and neuroinflammatory diseases, offering new insights into disease mechanisms and novel perspectives in the identification of potential biomarkers and therapeutic targets. We delve into the molecular pathways through which α-synuclein interacts with biological membranes and bioactive lipids, influencing the aggregation of α-synuclein and triggering neuroinflammatory responses, highlighting the potential of bioactive lipids as biomarkers for early disease detection and progression monitoring. Moreover, we explore innovative therapeutic strategies aimed at modulating the interaction between α-synuclein and bioactive lipids, including the development of small molecules and nutritional interventions. Finally, the review addresses the significance of the gut-to-brain axis in mediating the effects of bioactive lipids on α-synuclein pathology and discusses the role of altered gut lipid metabolism and microbiota composition in neuroinflammation and neurodegeneration. The present review aims to underscore the potential of targeting α-synuclein-lipid interactions as a multifaceted approach for the detection and treatment of neurodegenerative and neuroinflammatory diseases.

Proteomics and lipidomic analysis reveal dysregulated pathways associated with loss of sacsin

Introduction

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a rare incurable neurodegenerative disease caused by mutations in the SACS gene, which codes for sacsin, a large protein involved in protein homeostasis, mitochondrial function, cytoskeletal dynamics, autophagy, cell adhesion and vesicle trafficking. However, the pathogenic mechanisms underlying sacsin dysfunction are still largely uncharacterized, and so attempts to develop therapies are still in the early stages.

Methods

To achieve further understanding of how processes are altered by loss of sacsin, we used untargeted proteomics to compare protein profiles in ARSACS fibroblasts versus controls.

Results

Our analyses confirmed the involvement of known biological pathways and also implicated calcium and lipid homeostasis in ARSACS skin fibroblasts, a finding further verified in SH-SY5Y SACS -/- cells. Validation through mass spectrometry-based analysis and comparative quantification of lipids by LC-MS in fibroblasts revealed increased levels of ceramides coupled with a reduction of diacylglycerols.

Discussion

In addition to confirming aberrant Ca2+ homeostasis in ARSACS, this study described abnormal lipid levels associated with loss of sacsin.

Development of Midbrain Dopaminergic Neurons and the Advantage of Using hiPSCs as a Model System to Study Parkinson's Disease

Midbrain dopaminergic (mDA) neurons are significantly impaired in patients inflicted with Parkinson's disease (PD), subsequently affecting a variety of motor functions. There are four pathways through which dopamine elicits its function, namely, nigrostriatal, mesolimbic, mesocortical and tuberoinfundibular dopamine pathways. SHH and Wnt signalling pathways in association with favourable expression of a variety of genes, promotes the development and differentiation of mDA neurons in the brain. However, there is a knowledge gap regarding the complex signalling pathways involved in development of mDA neurons. hiPSC models have been acclaimed to be effective in generating complex disease phenotypes. These models mimic the microenvironment found in vivo thus ensuring maximum reliability. Further, a variety of therapeutic compounds can be screened using hiPSCs since they can be used to generate neurons that could carry an array of mutations associated with both familial and sporadic PD. Thus, culturing hiPSCs to study gene expression and dysregulation of cellular processes associated with PD can be useful in developing targeted therapies that will be a step towards halting disease progression.

Peripheral Upregulation of Parkinson's Disease-Associated Genes Encoding α-Synuclein, β-Glucocerebrosidase, and Ceramide Glucosyltransferase in Major Depression

Due to the high comorbidity of Parkinson's disease (PD) with major depressive disorder (MDD) and the involvement of sphingolipids in both conditions, we investigated the peripheral expression levels of three primarily PD-associated genes: α-synuclein (SNCA), lysosomal enzyme β-glucocerebrosidase (GBA1), and UDP-glucose ceramide glucosyltransferase (UGCG) in a sex-balanced MDD cohort. Normalized gene expression was determined by quantitative PCR in patients suffering from MDD (unmedicated n = 63, medicated n = 66) and controls (remitted MDD n = 39, healthy subjects n = 61). We observed that expression levels of SNCA (p = 0.036), GBA1 (p = 0.014), and UGCG (p = 0.0002) were higher in currently depressed patients compared to controls and remitted patients, and expression of GBA1 and UGCG decreased in medicated patients during three weeks of therapy. Additionally, in subgroups, expression was positively correlated with the severity of depression and anxiety. Furthermore, we identified correlations between the gene expression levels and PD-related laboratory parameters. Our findings suggest that SNCA, GBA1, and UGCG analysis could be instrumental in the search for biomarkers of MDD and in understanding the overlapping pathological mechanisms underlying neuro-psychiatric diseases.

Inhibition of cysteine protease cathepsin L increases the level and activity of lysosomal glucocerebrosidase

The glucocerebrosidase (GCase) encoded by the GBA1 gene hydrolyzes glucosylceramide (GluCer) to ceramide and glucose in lysosomes. Homozygous or compound heterozygous GBA1 mutations cause the lysosomal storage disease Gaucher disease (GD) due to severe loss of GCase activity. Loss-of-function variants in the GBA1 gene are also the most common genetic risk factor for Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Restoring lysosomal GCase activity represents an important therapeutic approach for GBA1-associated diseases. We hypothesized that increasing the stability of lysosomal GCase protein could correct deficient GCase activity in these conditions. However, it remains unknown how GCase stability is regulated in the lysosome. We found that cathepsin L, a lysosomal cysteine protease, cleaves GCase and regulates its stability. In support of these data, GCase protein was elevated in the brain of cathepsin L-KO mice. Chemical inhibition of cathepsin L increased both GCase levels and activity in fibroblasts from patients with GD. Importantly, inhibition of cathepsin L in dopaminergic neurons from a patient GBA1-PD led to increased GCase levels and activity as well as reduced phosphorylated α-synuclein. These results suggest that targeting cathepsin L-mediated GCase degradation represents a potential therapeutic strategy for GCase deficiency in PD and related disorders that exhibit decreased GCase activity.

Ups and downs of lysosomal pH: conflicting roles of LAMP proteins?

The acidic pH of lysosomes is critical for catabolism in eukaryotic cells and is altered in neurodegenerative disease including Alzheimer and Parkinson. Recent reports using Drosophila and mammalian cell culture systems have identified novel and, at first sight, conflicting roles for the lysosomal associated membrane proteins (LAMPs) in the regulation of the endolysosomal system.Abbreviation: AD: Alzheimer disease; LAMP: lysosomal associated membrane protein; LTR: LysoTracker; PD: Parkinson disease; TMEM175: transmembrane protein 175; V-ATPase: vacuolar-type H+-translocating ATPase.

GBA1 Variants and Parkinson's Disease: Paving the Way for Targeted Therapy

Glucosylceramidase beta 1 (GBA1) variants have attracted enormous attention as the most promising and important genetic candidates for precision medicine in Parkinson's disease (PD). A substantial correlation between GBA1 genotypes and PD phenotypes could inform the prediction of disease progression and promote the development of a preventive intervention for individuals at a higher risk of a worse disease prognosis. Moreover, the GBA1-regulated pathway provides new perspectives on the pathogenesis of PD, such as dysregulated sphingolipid metabolism, impaired protein quality control, and disrupted endoplasmic reticulum-Golgi trafficking. These perspectives have led to the development of novel disease-modifying therapies for PD targeting the GBA1-regulated pathway by repositioning treatment strategies for Gaucher's disease. This review summarizes the current hypotheses on a mechanistic link between GBA1 variants and PD and possible therapeutic options for modulating GBA1-regulated pathways in PD patients.

The endo-lysosomal system in Parkinson's disease: expanding the horizon

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease, and its prevalence is increasing with age. A wealth of genetic evidence indicates that the endo-lysosomal system is a major pathway driving PD pathogenesis with a growing number of genes encoding endo-lysosomal proteins identified as risk factors for PD, making it a promising target for therapeutic intervention. However, detailed knowledge and understanding of the molecular mechanisms linking these genes to the disease are available for only a handful of them (e.g. LRRK2, GBA1, VPS35). Taking on the challenge of studying poorly characterized genes and proteins can be daunting, due to the limited availability of tools and knowledge from previous literature. This review aims at providing a valuable source of molecular and cellular insights into the biology of lesser-studied PD-linked endo-lysosomal genes, to help and encourage researchers in filling the knowledge gap around these less popular genetic players. Specific endo-lysosomal pathways discussed range from endocytosis, sorting, and vesicular trafficking to the regulation of membrane lipids of these membrane-bound organelles and the specific enzymatic activities they contain. We also provide perspectives on future challenges that the community needs to tackle and propose approaches to move forward in our understanding of these poorly studied endo-lysosomal genes. This will help harness their potential in designing innovative and efficient treatments to ultimately re-establish neuronal homeostasis in PD but also other diseases involving endo-lysosomal dysfunction.

Disrupting the α-synuclein-ESCRT interaction with a peptide inhibitor mitigates neurodegeneration in preclinical models of Parkinson's disease

Accumulation of α-synuclein into toxic oligomers or fibrils is implicated in dopaminergic neurodegeneration in Parkinson's disease. Here we performed a high-throughput, proteome-wide peptide screen to identify protein-protein interaction inhibitors that reduce α-synuclein oligomer levels and their associated cytotoxicity. We find that the most potent peptide inhibitor disrupts the direct interaction between the C-terminal region of α-synuclein and CHarged Multivesicular body Protein 2B (CHMP2B), a component of the Endosomal Sorting Complex Required for Transport-III (ESCRT-III). We show that α-synuclein impedes endolysosomal activity via this interaction, thereby inhibiting its own degradation. Conversely, the peptide inhibitor restores endolysosomal function and thereby decreases α-synuclein levels in multiple models, including female and male human cells harboring disease-causing α-synuclein mutations. Furthermore, the peptide inhibitor protects dopaminergic neurons from α-synuclein-mediated degeneration in hermaphroditic C. elegans and preclinical Parkinson's disease models using female rats. Thus, the α-synuclein-CHMP2B interaction is a potential therapeutic target for neurodegenerative disorders.

Targeting the GBA1 pathway to slow Parkinson disease: Insights into clinical aspects, pathogenic mechanisms and new therapeutic avenues

The GBA1 gene encodes the lysosomal enzyme glucocerebrosidase (GCase), which is involved in sphingolipid metabolism. Biallelic variants in GBA1 cause Gaucher disease (GD), a lysosomal storage disorder characterised by loss of GCase activity and aberrant intracellular accumulation of GCase substrates. Carriers of GBA1 variants have an increased risk of developing Parkinson disease (PD), with odds ratio ranging from 2.2 to 30 according to variant severity. GBA1 variants which do not cause GD in homozygosis can also increase PD risk. Patients with PD carrying GBA1 variants show a more rapidly progressive phenotype compared to non-carriers, emphasising the need for disease modifying treatments targeting the GBA1 pathway. Several mechanisms secondary to GCase dysfunction are potentially responsible for the pathological changes leading to PD. Misfolded GCase proteins induce endoplasmic reticulum stress and subsequent unfolded protein response and impair the autophagy-lysosomal pathway. This results in α-synuclein accumulation and spread, and promotes neurodegenerative changes. Preclinical evidence also shows that products of GCase activity can promote accumulation of α-synuclein, however there is no convincing evidence of substrate accumulation in GBA1-PD brains. Altered lipid homeostasis secondary to loss of GCase activity could also contribute to PD pathology. Treatments that target the GBA1 pathway could reverse these pathological processes and halt/slow the progression of PD. These range from augmentation of GCase activity via GBA1 gene therapy, restoration of normal intracellular GCase trafficking via molecular chaperones, and substrate reduction therapy. This review discusses the pathways associated with GBA1-PD and related novel GBA1-targeted interventions for PD treatment.

Genetic modifiers of synucleinopathies-lessons from experimental models

α-Synuclein is a pleiotropic protein underlying a group of progressive neurodegenerative diseases, including Parkinson's disease and dementia with Lewy bodies. Together, these are known as synucleinopathies. Like all neurological diseases, understanding of disease mechanisms is hampered by the lack of access to biopsy tissues, precluding a real-time view of disease progression in the human body. This has driven researchers to devise various experimental models ranging from yeast to flies to human brain organoids, aiming to recapitulate aspects of synucleinopathies. Studies of these models have uncovered numerous genetic modifiers of α-synuclein, most of which are evolutionarily conserved. This review discusses what we have learned about disease mechanisms from these modifiers, and ways in which the study of modifiers have supported ongoing efforts to engineer disease-modifying interventions for synucleinopathies.

The Consequences of GBA Deficiency in the Autophagy-Lysosome System in Parkinson's Disease Associated with GBA

GBA gene variants were the first genetic risk factor for Parkinson's disease. GBA encodes the lysosomal enzyme glucocerebrosidase (GBA), which is involved in sphingolipid metabolism. GBA exhibits a complex physiological function that includes not only the degradation of its substrate glucosylceramide but also the metabolism of other sphingolipids and additional lipids such as cholesterol, particularly when glucocerebrosidase activity is deficient. In the context of Parkinson's disease associated with GBA, the loss of GBA activity has been associated with the accumulation of α-synuclein species. In recent years, several hypotheses have proposed alternative and complementary pathological mechanisms to explain why lysosomal enzyme mutations lead to α-synuclein accumulation and become important risk factors in Parkinson's disease etiology. Classically, loss of GBA activity has been linked to a dysfunctional autophagy-lysosome system and to a subsequent decrease in autophagy-dependent α-synuclein turnover; however, several other pathological mechanisms underlying GBA-associated parkinsonism have been proposed. This review summarizes and discusses the different hypotheses with a special focus on autophagy-dependent mechanisms, as well as autophagy-independent mechanisms, where the role of other players such as sphingolipids, cholesterol and other GBA-related proteins make important contributions to Parkinson's disease pathogenesis.

Lysosomal lipid alterations caused by glucocerebrosidase deficiency promote lysosomal dysfunction, chaperone-mediated-autophagy deficiency, and alpha-synuclein pathology

Mutations in the GBA gene that encodes the lysosomal enzyme β-glucocerebrosidase (GCase) are a major genetic risk factor for Parkinson’s disease (PD). In this study, we generated a set of differentiated and stable human dopaminergic cell lines that express the two most prevalent GBA mutations as well as GBA knockout cell lines as a in vitro disease modeling system to study the relationship between mutant GBA and the abnormal accumulation of α-synuclein. We performed a deep analysis of the consequences triggered by the presence of mutant GBA protein and the loss of GCase activity in different cellular compartments, focusing primarily on the lysosomal compartment, and analyzed in detail the lysosomal activity, composition, and integrity. The loss of GCase activity generates extensive lysosomal dysfunction, promoting the loss of activity of other lysosomal enzymes, affecting lysosomal membrane stability, promoting intralysosomal pH changes, and favoring the intralysosomal accumulation of sphingolipids and cholesterol. These local events, occurring only at a subcellular level, lead to an impairment of autophagy pathways, particularly chaperone-mediated autophagy, the main α-synuclein degradative pathway. The findings of this study highlighted the role of lysosomal function and lipid metabolism in PD and allowed us to describe a molecular mechanism to understand how mutations in GBA can contribute to an abnormal accumulation of different α-synuclein neurotoxic species in PD pathology.

Exploring the link between GBA1 mutations and Dementia with Lewy bodies, A mini-review

IMPORTANCE

Dementia with Lewy bodies (DLB) is a neurodegenerative disease linked to abnormal accumulation of phosphorylated α-synuclein. GBA1 is the gene encoding the lysosomal enzyme glucocerebrosidase (GCase), whose mutations are a risk factor of DLB.

OBJECTIVE

To report all available data exploring the association between GBA1 mutations and DLB.

EVIDENCE REVIEW

All publications focused on GCase and DLB in humans between 2003 and 2022 were identified on PubMed, Cochrane and ClinicalTrials.gov.

FINDINGS

29 studies were included and confirmed the strong association between GBA1 mutations and DLB (Odds Ratio [OR]: 8.28). GBA1 mutation carriers presented a more malignant phenotype, with earlier symptom onset, more severe motor and cognitive dysfunctions, more visual hallucinations and rapid eye movement sleep disorder. GBA1 mutations were associated with "purer" neuropathological DLB. No therapeutic recommendations exist and clinical trials targeting GCase are just starting in DLB patients.

CONCLUSIONS AND RELEVANCE

This review reports a link between GBA1 mutations and the DLB phenotype with limited evidence due to the small number of studies.

Lysosomal ceramides regulate cathepsin B-mediated processing of saposin C and glucocerebrosidase activity

Variants in multiple lysosomal enzymes increase Parkinson's disease (PD) risk, including the genes encoding glucocerebrosidase (GCase), acid sphingomyelinase (ASMase) and galactosylceramidase. Each of these enzymes generates ceramide by hydrolysis of sphingolipids in lysosomes, but the role of this common pathway in PD pathogenesis has not yet been explored. Variations in GBA1, the gene encoding GCase, are the most common genetic risk factor for PD. The lysosomal enzyme cathepsin B has recently been implicated as an important genetic modifier of disease penetrance in individuals harboring GBA1 variants, suggesting a mechanistic link between these enzymes. Here, we found that ceramide activates cathepsin B, and identified a novel role for cathepsin B in mediating prosaposin cleavage to form saposin C, the lysosomal coactivator of GCase. Interestingly, this pathway was disrupted in Parkin-linked PD models, and upon treatment with inhibitor of ASMase which resulted in decreased ceramide production. Conversely, increasing ceramide production by inhibiting acid ceramidase activity was sufficient to upregulate cathepsin B- and saposin C-mediated activation of GCase. These results highlight a mechanistic link between ceramide and cathepsin B in regulating GCase activity and suggest that targeting lysosomal ceramide or cathepsin B represents an important therapeutic strategy for activating GCase in PD and related disorders.

Lipid Metabolism Influence on Neurodegenerative Disease Progression: Is the Vehicle as Important as the Cargo?

Many neurodegenerative diseases are characterized by abnormal protein aggregates, including the two most common neurodegenerative diseases Alzheimer’s disease (AD) and Parkinson’s disease (PD). In the global search to prevent and treat diseases, most research has been focused on the early stages of the diseases, including how these pathogenic protein aggregates are initially formed. We argue, however, that an equally important aspect of disease etiology is the characteristic spread of protein aggregates throughout the nervous system, a key process in disease progression. Growing evidence suggests that both alterations in lipid metabolism and dysregulation of extracellular vesicles (EVs) accelerate the spread of protein aggregation and progression of neurodegeneration, both in neurons and potentially in surrounding glia. We will review how these two pathways are intertwined and accelerate the progression of AD and PD. Understanding how lipid metabolism, EV biogenesis, and EV uptake regulate the spread of pathogenic protein aggregation could reveal novel therapeutic targets to slow or halt neurodegenerative disease progression.

Inborn errors of metabolism: Lessons from iPSC models

The possibility of reprogramming human somatic cells to pluripotency has opened unprecedented opportunities for creating genuinely human experimental models of disease. Inborn errors of metabolism (IEMs) constitute a greatly heterogeneous class of diseases that appear, in principle, especially suited to be modeled by iPSC-based technology. Indeed, dozens of IEMs have already been modeled to some extent using patient-specific iPSCs. Here, we review the advantages and disadvantages of iPSC-based disease modeling in the context of IEMs, as well as particular challenges associated to this approach, together with solutions researchers have proposed to tackle them. We have structured this review around six lessons that we have learnt from those previous modeling efforts, and that we believe should be carefully considered by researchers wishing to embark in future iPSC-based models of IEMs.

Lipidomics Prediction of Parkinson's Disease Severity: A Machine-Learning Analysis

BACKGROUND

The role of the lipidome as a biomarker for Parkinson's disease (PD) is a relatively new field that currently only focuses on PD diagnosis.

OBJECTIVE

To identify a relevant lipidome signature for PD severity markers.

METHODS

Disease severity of 149 PD patients was assessed by the Unified Parkinson's Disease Rating Scale (UPDRS) and the Montreal Cognitive Assessment (MoCA). The lipid composition of whole blood samples was analyzed, consisting of 517 lipid species from 37 classes; these included all major classes of glycerophospholipids, sphingolipids, glycerolipids, and sterols. To handle the high number of lipids, the selection of lipid species and classes was consolidated via analysis of interrelations between lipidomics and disease severity prediction using the random forest machine-learning algorithm aided by conventional statistical methods.

RESULTS

Specific lipid classes dihydrosphingomyelin (dhSM), plasmalogen phosphatidylethanolamine (PEp), glucosylceramide (GlcCer), dihydro globotriaosylceramide (dhGB3), and to a lesser degree dihydro GM3 ganglioside (dhGM3), as well as species dhSM(20:0), PEp(38:6), PEp(42:7), GlcCer(16:0), GlcCer(24:1), dhGM3(22:0), dhGM3(16:0), and dhGB3(16:0) contribute to PD severity prediction of UPDRS III score. These, together with age, age at onset, and disease duration, also contribute to prediction of UPDRS total score. We demonstrate that certain lipid classes and species interrelate differently with the degree of severity of motor symptoms between men and women, and that predicting intermediate disease stages is more accurate than predicting less or more severe stages.

CONCLUSION

Using machine-learning algorithms and methodologies, we identified lipid signatures that enable prediction of motor severity in PD. Future studies should focus on identifying the biological mechanisms linking GlcCer, dhGB3, dhSM, and PEp with PD severity.

Ceramide Metabolism and Parkinson's Disease-Therapeutic Targets

Ceramide is a bioactive sphingolipid involved in numerous cellular processes. In addition to being the precursor of complex sphingolipids, ceramides can act as second messengers, especially when they are generated at the plasma membrane of cells. Its metabolic dysfunction may lead to or be a consequence of an underlying disease. Recent reports on transcriptomics and electrospray ionization mass spectrometry analysis have demonstrated the variation of specific levels of sphingolipids and enzymes involved in their metabolism in different neurodegenerative diseases. In the present review, we highlight the most relevant discoveries related to ceramide and neurodegeneration, with a special focus on Parkinson's disease.

Mini review - The role of Glucocerebrosidase and Progranulin as possible targets in the treatment of Parkinson's disease

As per recent reports, there is an association between glucocerebrosidase (Gcase) enzyme and Parkinson's disease (PD). In addition, certain mutations in the Gcase gene (GBA) and the progranulin (PGRN) gene are found to be linked with the imbalance in the levels of Gcase enzyme. This imbalance or decrease or impairment in Gcase activity can lead to Gaucher disease, frontotemporal lobar degeneration (FTLD), dementia, etc. Recent evidences suggest that the drugs used to treat these diseases can be used for PD. The present review has focused on the therapeutic approaches used for diseases linked with Gcase enzyme, which can be used for PD. The review also considered possible target specific novel strategies, which may help to meet the unmet needs in the treatment of PD.

iPSCs: A Preclinical Drug Research Tool for Neurological Disorders

The development and commercialization of new drugs is an articulated, lengthy, and very expensive process that proceeds through several steps, starting from target identification, screening new leading compounds for testing in preclinical studies, and subsequently in clinical trials to reach the final approval for therapeutic use. Preclinical studies are usually performed using both cell cultures and animal models, although they do not completely resume the complexity of human diseases, in particular neurodegenerative conditions. To this regard, stem cells represent a powerful tool in all steps of drug discovery. The recent advancement in induced Pluripotent Stem Cells (iPSCs) technology has opened the possibility to obtain patient-specific disease models for drug screening and development. Here, we report the use of iPSCs as a disease model for drug development in the contest of neurological disorders, including Alzheimer’s (AD) and Parkinson’s disease (PD), Amyotrophic lateral Sclerosis (ALS), and Fragile X syndrome (FRAX).

Dysregulation of mitochondria-lysosome contacts by GBA1 dysfunction in dopaminergic neuronal models of Parkinson's disease

Mitochondria-lysosome contacts are recently identified sites for mediating crosstalk between both organelles, but their role in normal and diseased human neurons remains unknown. In this study, we demonstrate that mitochondria-lysosome contacts can dynamically form in the soma, axons, and dendrites of human neurons, allowing for their bidirectional crosstalk. Parkinson's disease patient derived neurons harboring mutant GBA1 exhibited prolonged mitochondria-lysosome contacts due to defective modulation of the untethering protein TBC1D15, which mediates Rab7 GTP hydrolysis for contact untethering. This dysregulation was due to decreased GBA1 (β-glucocerebrosidase (GCase)) lysosomal enzyme activity in patient derived neurons, and could be rescued by increasing enzyme activity with a GCase modulator. These defects resulted in disrupted mitochondrial distribution and function, and could be further rescued by TBC1D15 in Parkinson's patient derived GBA1-linked neurons. Together, our work demonstrates a potential role of mitochondria-lysosome contacts as an upstream regulator of mitochondrial function and dynamics in midbrain dopaminergic neurons in GBA1-linked Parkinson's disease.

The cell biology of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder resulting from the death of dopamine neurons in the substantia nigra pars compacta. Our understanding of PD biology has been enriched by the identification of genes involved in its rare, inheritable forms, termed PARK genes. These genes encode proteins including α-syn, LRRK2, VPS35, parkin, PINK1, and DJ1, which can cause monogenetic PD when mutated. Investigating the cellular functions of these proteins has been instrumental in identifying signaling pathways that mediate pathology in PD and neuroprotective mechanisms active during homeostatic and pathological conditions. It is now evident that many PD-associated proteins perform multiple functions in PD-associated signaling pathways in neurons. Furthermore, several PARK proteins contribute to non-cell-autonomous mechanisms of neuron death, such as neuroinflammation. A comprehensive understanding of cell-autonomous and non-cell-autonomous pathways involved in PD is essential for developing therapeutics that may slow or halt its progression.

Elevated glucosylsphingosine in Gaucher disease induced pluripotent stem cell neurons deregulates lysosomal compartment through mammalian target of rapamycin complex 1

Gaucher disease (GD) is a lysosomal storage disorder caused by mutations in GBA1, the gene that encodes lysosomal β‐glucocerebrosidase (GCase). Mild mutations in GBA1 cause type 1 non‐neuronopathic GD, whereas severe mutations cause types 2 and 3 neuronopathic GD (nGD). GCase deficiency results in the accumulation of glucosylceramide (GlcCer) and glucosylsphingosine (GlcSph). GlcSph is formed by deacylation of GlcCer by the lysosomal enzyme acid ceramidase. Brains from patients with nGD have high levels of GlcSph, a lipid believed to play an important role in nGD, but the mechanisms involved remain unclear. To identify these mechanisms, we used human induced pluripotent stem cell‐derived neurons from nGD patients. We found that elevated levels of GlcSph activate mammalian target of rapamycin (mTOR) complex 1 (mTORC1), interfering with lysosomal biogenesis and autophagy, which were restored by incubation of nGD neurons with mTOR inhibitors. We also found that inhibition of acid ceramidase prevented both, mTOR hyperactivity and lysosomal dysfunction, suggesting that these alterations were caused by GlcSph accumulation in the mutant neurons. To directly determine whether GlcSph can cause mTOR hyperactivation, we incubated wild‐type neurons with exogenous GlcSph. Remarkably, GlcSph treatment recapitulated the mTOR hyperactivation and lysosomal abnormalities in mutant neurons, which were prevented by coincubation of GlcSph with mTOR inhibitors. We conclude that elevated GlcSph activates an mTORC1‐dependent pathogenic mechanism that is responsible for the lysosomal abnormalities of nGD neurons. We also identify acid ceramidase as essential to the pathogenesis of nGD, providing a new therapeutic target for treating GBA1‐associated neurodegeneration.

Genome editing in lysosomal disorders

Lysosomal disorders are a group of heterogenous diseases caused by mutations in genes that encode for lysosomal proteins. With exception of some cases, these disorders still lack both knowledge of disease pathogenesis and specific therapies. In this sense, genome editing arises as a technique that allows both the creation of specific cell lines, animal models and gene therapy protocols for these disorders. Here we explain the main applications of genome editing for lysosomal diseases, with examples based on the literature. The ability to rewrite the genome will be of extreme importance to study and potentially treat these rare disorders.

The Potential Role of CERS1 in Autophagy Through PI3K/AKT Signaling Pathway in Hypophysoma

To explore the role and mechanism of CERS1 in hypophysoma and investigate whether CERS1 overexpression can change the autophagy process of hypophysoma, and then to explore whether CERS1’s effect was regulated by the PI3K/AKT signaling pathway. Western blot and RT-PCR were used to analyze the expression or mRNA level of CERS1 at different tissues or cell lines. Afterwards, the occurrence and development of hypophysoma in vivo and in vitro, respectively, was observed by using CERS1 overexpression by lentivirus. Finally, MK-2206 and LY294002 were applied to discuss whether the role of CERS1 was regulated by the PI3K/AKT signaling pathway. Results show that the CERS1 expression and mRNA level in tumor or AtT-20 cells were decreased. CERS1 over-expressed by lentivirus could inhibit hypophysoma development in vivo and in vitro by reducing tumor volume and weight, weakening tumor proliferation and invasion, and enhancing apoptosis. In addition, shCERS1 could reverse the process. The above results indicate that CERS1 is possibly able to enhance autophagy in hypophysoma through the PI3K/AKT signaling pathway.

Patient-Derived Induced Pluripotent Stem Cell-Based Models in Parkinson's Disease for Drug Identification

Parkinson's disease (PD) is a common progressive neurodegenerative disorder characterized by loss of striatal-projecting dopaminergic neurons of the ventral forebrain, resulting in motor and cognitive deficits. Despite extensive efforts in understanding PD pathogenesis, no disease-modifying drugs exist. Recent advances in cell reprogramming technologies have facilitated the generation of patient-derived models for sporadic or familial PD and the identification of early, potentially triggering, pathological phenotypes while they provide amenable systems for drug discovery. Emerging developments highlight the enhanced potential of using more sophisticated cellular systems, including neuronal and glial co-cultures as well as three-dimensional systems that better simulate the human pathophysiology. In combination with high-throughput high-content screening technologies, these approaches open new perspectives for the identification of disease-modifying compounds. In this review, we discuss current advances and the challenges ahead in the use of patient-derived induced pluripotent stem cells for drug discovery in PD. We address new concepts implicating non-neuronal cells in disease pathogenesis and highlight the necessity for functional assays, such as calcium imaging and multi-electrode array recordings, to predict drug efficacy. Finally, we argue that artificial intelligence technologies will be pivotal for analysis of the large and complex data sets obtained, becoming game-changers in the process of drug discovery.

Lyso-glycosphingolipids: presence and consequences

Lyso-glycosphingolipids are generated in excess in glycosphingolipid storage disorders. In the course of these pathologies glycosylated sphingolipid species accumulate within lysosomes due to flaws in the respective lipid degrading machinery. Deacylation of accumulating glycosphingolipids drives the formation of lyso-glycosphingolipids. In lysosomal storage diseases such as Gaucher Disease, Fabry Disease, Krabbe disease, GM1 -and GM2 gangliosidosis, Niemann Pick type C and Metachromatic leukodystrophy massive intra-lysosomal glycosphingolipid accumulation occurs. The lysosomal enzyme acid ceramidase generates the deacylated lyso-glycosphingolipid species. This review discusses how the various lyso-glycosphingolipids are synthesized, how they may contribute to abnormal immunity in glycosphingolipid storing lysosomal diseases and what therapeutic opportunities exist.

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.

Lysosomal storage diseases: current therapies and future alternatives

Lysosomal storage disorders (LSDs) are a group of monogenic diseases characterized by progressive accumulation of undegraded substrates into the lysosome, due to mutations in genes that encode for proteins involved in normal lysosomal function. In recent years, several approaches have been explored to find effective and successful therapies, including enzyme replacement therapy, substrate reduction therapy, pharmacological chaperones, hematopoietic stem cell transplantation, and gene therapy. In the case of gene therapy, genome editing technologies have opened new horizons to accelerate the development of novel treatment alternatives for LSD patients. In this review, we discuss the current therapies for this group of disorders and present a detailed description of major genome editing technologies, as well as the most recent advances in the treatment of LSDs. We will further highlight the challenges and current bioethical debates of genome editing.

Finding pathogenic commonalities between Niemann-Pick type C and other lysosomal storage disorders: Opportunities for shared therapeutic interventions

Lysosomal storage disorders (LSDs) are diseases characterized by the accumulation of macromolecules in the late endocytic system and are caused by inherited defects in genes that encode mainly lysosomal enzymes or transmembrane lysosomal proteins. Niemann-Pick type C disease (NPCD), a LSD characterized by liver damage and progressive neurodegeneration that leads to early death, is caused by mutations in the genes encoding the NPC1 or NPC2 proteins. Both proteins are involved in the transport of cholesterol from the late endosomal compartment to the rest of the cell. Loss of function of these proteins causes primary cholesterol accumulation, and secondary accumulation of other lipids, such as sphingolipids, in lysosomes. Despite years of studying the genetic and molecular bases of NPCD and related-lysosomal disorders, the pathogenic mechanisms involved in these diseases are not fully understood. In this review we will summarize the pathogenic mechanisms described for NPCD and we will discuss their relevance for other LSDs with neurological components such as Niemann- Pick type A and Gaucher diseases. We will particularly focus on the activation of signaling pathways that may be common to these three pathologies with emphasis on how the intra-lysosomal accumulation of lipids leads to pathology, specifically to neurological impairments. We will show that although the primary lipid storage defect is different in these three LSDs, there is a similar secondary accumulation of metabolites and activation of signaling pathways that can lead to common pathogenic mechanisms. This analysis might help to delineate common pathological mechanisms and therapeutic targets for lysosomal storage diseases.

Elusive Roles of the Different Ceramidases in Human Health, Pathophysiology, and Tissue Regeneration

Ceramide and sphingosine are important interconvertible sphingolipid metabolites which govern various signaling pathways related to different aspects of cell survival and senescence. The conversion of ceramide into sphingosine is mediated by ceramidases. Altogether, five human ceramidases—named acid ceramidase, neutral ceramidase, alkaline ceramidase 1, alkaline ceramidase 2, and alkaline ceramidase 3—have been identified as having maximal activities in acidic, neutral, and alkaline environments, respectively. All five ceramidases have received increased attention for their implications in various diseases, including cancer, Alzheimer’s disease, and Farber disease. Furthermore, the potential anti-inflammatory and anti-apoptotic effects of ceramidases in host cells exposed to pathogenic bacteria and viruses have also been demonstrated. While ceramidases have been a subject of study in recent decades, our knowledge of their pathophysiology remains limited. Thus, this review provides a critical evaluation and interpretive analysis of existing literature on the role of acid, neutral, and alkaline ceramidases in relation to human health and various diseases, including cancer, neurodegenerative diseases, and infectious diseases. In addition, the essential impact of ceramidases on tissue regeneration, as well as their usefulness in enzyme replacement therapy, is also discussed.

Mutated ATP10B increases Parkinson's disease risk by compromising lysosomal glucosylceramide export

Parkinson’s disease (PD) is a progressive neurodegenerative brain disease presenting with a variety of motor and non-motor symptoms, loss of midbrain dopaminergic neurons in the substantia nigra pars compacta and the occurrence of α-synuclein-positive Lewy bodies in surviving neurons. Here, we performed whole exome sequencing in 52 early-onset PD patients and identified 3 carriers of compound heterozygous mutations in the ATP10B P4-type ATPase gene. Genetic screening of a Belgian PD and dementia with Lewy bodies (DLB) cohort identified 4 additional compound heterozygous mutation carriers (6/617 PD patients, 0.97%; 1/226 DLB patients, 0.44%). We established that ATP10B encodes a late endo-lysosomal lipid flippase that translocates the lipids glucosylceramide (GluCer) and phosphatidylcholine (PC) towards the cytosolic membrane leaflet. The PD associated ATP10B mutants are catalytically inactive and fail to provide cellular protection against the environmental PD risk factors rotenone and manganese. In isolated cortical neurons, loss of ATP10B leads to general lysosomal dysfunction and cell death. Impaired lysosomal functionality and integrity is well known to be implicated in PD pathology and linked to multiple causal PD genes and genetic risk factors. Our results indicate that recessive loss of function mutations in ATP10B increase risk for PD by disturbed lysosomal export of GluCer and PC. Both ATP10B and glucocerebrosidase 1, encoded by the PD risk gene GBA1, reduce lysosomal GluCer levels, emerging lysosomal GluCer accumulation as a potential PD driver.

Genetic perspective on the synergistic connection between vesicular transport, lysosomal and mitochondrial pathways associated with Parkinson's disease pathogenesis

Parkinson's disease (PD) and atypical parkinsonian syndromes (APS) are symptomatically characterized by parkinsonism, with the latter presenting additionally a distinctive range of atypical features. Although the majority of patients with PD and APS appear to be sporadic, genetic causes of several rare monogenic disease variants were identified. The knowledge acquired from these genetic factors indicated that defects in vesicular transport pathways, endo-lysosomal dysfunction, impaired autophagy-lysosomal protein and organelle degradation pathways, α-synuclein aggregation and mitochondrial dysfunction play key roles in PD pathogenesis. Moreover, membrane dynamics are increasingly recognized as a key player in the disease pathogenesis due lipid homeostasis alterations, associated with lysosomal dysfunction, caused by mutations in several PD and APS genes. The importance of lysosomal dysfunction and lipid homeostasis is strengthened by both genetic discoveries and clinical epidemiology of the association between parkinsonism and lysosomal storage disorders (LSDs), caused by the disruption of lysosomal biogenesis or function. A synergistic coordination between vesicular trafficking, lysosomal and mitochondria defects exist whereby mutations in PD and APS genes encoding proteins primarily involved one PD pathway are frequently associated with defects in other PD pathways as a secondary effect. Moreover, accumulating clinical and genetic observations suggest more complex inheritance patters of familial PD exist, including oligogenic and polygenic inheritance of genes in the same or interconnected PD pathways, further strengthening their synergistic connection.Here, we provide a comprehensive overview of PD and APS genes with functions in vesicular transport, lysosomal and mitochondrial pathways, and highlight functional and genetic evidence of the synergistic connection between these PD associated pathways.

Lysosomal Ceramide Metabolism Disorders: Implications in Parkinson's Disease

Ceramides are a family of bioactive lipids belonging to the class of sphingolipids. Sphingolipidoses are a group of inherited genetic diseases characterized by the unmetabolized sphingolipids and the consequent reduction of ceramide pool in lysosomes. Sphingolipidoses include several disorders as Sandhoff disease, Fabry disease, Gaucher disease, metachromatic leukodystrophy, Krabbe disease, Niemann Pick disease, Farber disease, and GM2 gangliosidosis. In sphingolipidosis, lysosomal lipid storage occurs in both the central nervous system and visceral tissues, and central nervous system pathology is a common hallmark for all of them. Parkinson’s disease, the most common neurodegenerative movement disorder, is characterized by the accumulation and aggregation of misfolded α-synuclein that seem associated to some lysosomal disorders, in particular Gaucher disease. This review provides evidence into the role of ceramide metabolism in the pathophysiology of lysosomes, highlighting the more recent findings on its involvement in Parkinson’s disease.

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.

Quality Control Strategy for CRISPR-Cas9-Based Gene Editing Complicated by a Pseudogene

CRISPR-Cas9 mediated gene editing in induced pluripotent stem cells became an efficient tool to investigate biological mechanisms underlying genetic-driven diseases while accounting for the respective genetic background. This technique relies on the targeting of a specific nucleotide sequence present in the gene of interest. Therefore, the gene editing of some genes can be complicated by non-coding pseudogenes presenting a high homology of sequence with their respective genes. Among them, GBA is raising special interest because of its implication as the most common genetic risk factor for Parkinson’s disease. In this study, we present an easy-to-use CRISPR-Cas9 gene editing strategy allowing for specific editing of point mutations in a gene without genetic alteration of its pseudogene exemplified by the correction or insertion of the common N370S mutation in GBA. A quality control strategy by combined fluorescence and PCR-based screening allows the early identification of correctly edited clones with unambiguous identification of the status of its pseudogene, GBAP1. Successful gene editing was confirmed by functional validation. Our work presents the first CRISPR-Cas9 based editing of a point mutation in GBA and paves the way for technically demanding gene engineering due to the presence of pseudogenes.

Organoid and pluripotent stem cells in Parkinson's disease modeling: an expert view on their value to drug discovery

Introduction: Parkinson's disease is a devastating neurodegenerative disorder preferentially involving loss of dopaminergic neurons in the substantia nigra, leading to typical motor symptoms. While there are still no therapeutics to modify disease course, recent work using induced pluripotent stem cell (iPSC) and 3D brain organoid models have provided further insight into Parkinson's disease pathogenesis and potential therapeutic targets.Areas covered: This review highlights the generation of iPSC neurons and neural organoids as models for studying Parkinson's disease. It further discusses the recent work using patient-derived neurons from both familial and sporadic forms of Parkinson's to study disease pathogenic phenotypes and pathways. It additionally provides an evaluation of iPSC neurons and organoid models for therapeutic development in Parkinson's.Expert opinion: The use of Parkinson's disease patient-derived neurons and organoids provides us with the exciting opportunity to directly investigate pathogenic mechanisms and test drug compounds in human neurons. Future studies will involve generating more sophisticated models of brain organoids, studying neuronal pathways using larger patient cohorts, and routinely assessing therapeutics in these models.

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.

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.

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.

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.

Emerging links between pediatric lysosomal storage diseases and adult parkinsonism

Lysosomal storage disorders comprise a clinically heterogeneous group of autosomal-recessive or X-linked genetic syndromes caused by disruption of lysosomal biogenesis or function resulting in accumulation of nondegraded substrates. Although lysosomal storage disorders are diagnosed predominantly in children, many show variable expressivity with clinical presentations possible later in life. Given the important role of lysosomes in neuronal homeostasis, neurological manifestations, including movement disorders, can accompany many lysosomal storage disorders. Over the last decade, evidence from genetics, clinical epidemiology, cell biology, and biochemistry have converged to implicate links between lysosomal storage disorders and adult-onset movement disorders. The strongest evidence comes from mutations in Glucocerebrosidase, which cause Gaucher's disease and are among the most common and potent risk factors for PD. However, recently, many additional lysosomal storage disorder genes have been similarly implicated, including SMPD1, ATP13A2, GALC, and others. Examination of these links can offer insight into pathogenesis of PD and guide development of new therapeutic strategies. We systematically review the emerging genetic links between lysosomal storage disorders and PD. © 2019 International Parkinson and Movement Disorder Society.

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.

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.

Parkinsonisms and Glucocerebrosidase Deficiency: A Comprehensive Review for Molecular and Cellular Mechanism of Glucocerebrosidase Deficiency

In the last years, lysosomal storage diseases appear as a bridge of knowledge between rare genetic inborn metabolic disorders and neurodegenerative diseases such as Parkinson’s disease (PD) or frontotemporal dementia. Epidemiological studies helped promote research in the field that continues to improve our understanding of the link between mutations in the glucocerebrosidase (GBA) gene and PD. We conducted a review of this link, highlighting the association in GBA mutation carriers and in Gaucher disease type 1 patients (GD type 1). A comprehensive review of the literature from January 2008 to December 2018 was undertaken. Relevance findings include: (1) There is a bidirectional interaction between GBA and α- synuclein in protein homeostasis regulatory pathways involving the clearance of aggregated proteins. (2) The link between GBA deficiency and PD appears not to be restricted to α–synuclein aggregates but also involves Parkin and PINK1 mutations. (3) Other factors help explain this association, including early and later endosomes and the lysosomal-associated membrane protein 2A (LAMP-2A) involved in the chaperone-mediated autophagy (CMA). (4) The best knowledge allows researchers to explore new therapeutic pathways alongside substrate reduction or enzyme replacement therapies.