The L444P Gba1 mutation enhances alpha-synuclein induced loss of nigral dopaminergic neurons in mice

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.

Recalibrating the Why and Whom of Animal Models in Parkinson Disease: A Clinician's Perspective

Animal models have been used to gain pathophysiologic insights into Parkinson's disease (PD) and aid in the translational efforts of interventions with therapeutic potential in human clinical trials. However, no disease-modifying therapy for PD has successfully emerged from model predictions. These translational disappointments warrant a reappraisal of the types of preclinical questions asked of animal models. Besides the limitations of experimental designs, the one-size convergence and oversimplification yielded by a model cannot recapitulate the molecular diversity within and between PD patients. Here, we compare the strengths and pitfalls of different models, review the discrepancies between animal and human data on similar pathologic and molecular mechanisms, assess the potential of organoids as novel modeling tools, and evaluate the types of questions for which models can guide and misguide. We propose that animal models may be of greatest utility in the evaluation of molecular mechanisms, neural pathways, drug toxicity, and safety but can be unreliable or misleading when used to generate pathophysiologic hypotheses or predict therapeutic efficacy for compounds with potential neuroprotective effects in humans. To enhance the translational disease-modification potential, the modeling must reflect the biology not of a diseased population but of subtypes of diseased humans to distinguish What data are relevant and to Whom.

Advancements in Genetic and Biochemical Insights: Unraveling the Etiopathogenesis of Neurodegeneration in Parkinson's Disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative movement disorder worldwide, which is primarily characterized by motor impairments. Even though multiple hypotheses have been proposed over the decades that explain the pathogenesis of PD, presently, there are no cures or promising preventive therapies for PD. This could be attributed to the intricate pathophysiology of PD and the poorly understood molecular mechanism. To address these challenges comprehensively, a thorough disease model is imperative for a nuanced understanding of PD's underlying pathogenic mechanisms. This review offers a detailed analysis of the current state of knowledge regarding the molecular mechanisms underlying the pathogenesis of PD, with a particular emphasis on the roles played by gene-based factors in the disease's development and progression. This study includes an extensive discussion of the proteins and mutations of primary genes that are linked to PD, including α-synuclein, GBA1, LRRK2, VPS35, PINK1, DJ-1, and Parkin. Further, this review explores plausible mechanisms for DAergic neural loss, non-motor and non-dopaminergic pathologies, and the risk factors associated with PD. The present study will encourage the related research fields to understand better and analyze the current status of the biochemical mechanisms of PD, which might contribute to the design and development of efficacious and safe treatment strategies for PD in future endeavors.

Modeling familial and sporadic Parkinson's disease in small fishes

The establishment of animal models for Parkinson's disease (PD) has been challenging. Nevertheless, once established, they will serve as valuable tools for elucidating the causes and pathogenesis of PD, as well as for developing new strategies for its treatment. Following the recent discovery of a series of PD causative genes in familial cases, teleost fishes, including zebrafish and medaka, have often been used to establish genetic PD models because of their ease of breeding and gene manipulation, as well as the high conservation of gene orthologs. Some of the fish lines can recapitulate PD phenotypes, which are often more pronounced than those in rodent genetic models. In addition, a new experimental teleost fish, turquoise killifish, can be used as a sporadic PD model, because it spontaneously manifests age-dependent PD phenotypes. Several PD fish models have already made significant contributions to the discovery of novel PD pathological features, such as cytosolic leakage of mitochondrial DNA and pathogenic phosphorylation in α-synuclein. Therefore, utilizing various PD fish models with distinct degenerative phenotypes will be an effective strategy for identifying emerging facets of PD pathogenesis and therapeutic modalities.

Animal models of Parkinson's disease: bridging the gap between disease hallmarks and research questions

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by motor and non-motor symptoms. More than 200 years after its first clinical description, PD remains a serious affliction that affects a growing proportion of the population. Prevailing treatments only alleviate symptoms; there is still neither a cure that targets the neurodegenerative processes nor therapies that modify the course of the disease. Over the past decades, several animal models have been developed to study PD. Although no model precisely recapitulates the pathology, they still provide valuable information that contributes to our understanding of the disease and the limitations of our treatment options. This review comprehensively summarizes the different animal models available for Parkinson's research, with a focus on those induced by drugs, neurotoxins, pesticides, genetic alterations, α-synuclein inoculation, and viral vector injections. We highlight their characteristics and ability to reproduce PD-like phenotypes. It is essential to realize that the strengths and weaknesses of each model and the induction technique at our disposal are determined by the research question being asked. Our review, therefore, seeks to better aid researchers by ensuring a concrete discernment of classical and novel animal models in PD research.

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.

Biochemical Characteristics of iPSC-Derived Dopaminergic Neurons from N370S GBA Variant Carriers with and without Parkinson's Disease

GBA variants increase the risk of Parkinson's disease (PD) by 10 times. The GBA gene encodes the lysosomal enzyme glucocerebrosidase (GCase). The p.N370S substitution causes a violation of the enzyme conformation, which affects its stability in the cell. We studied the biochemical characteristics of dopaminergic (DA) neurons generated from induced pluripotent stem cells (iPSCs) from a PD patient with the GBA p.N370S mutation (GBA-PD), an asymptomatic GBA p.N370S carrier (GBA-carrier), and two healthy donors (control). Using liquid chromatography with tandem mass spectrometry (LC-MS/MS), we measured the activity of six lysosomal enzymes (GCase, galactocerebrosidase (GALC), alpha-glucosidase (GAA), alpha-galactosidase (GLA), sphingomyelinase (ASM), and alpha-iduronidase (IDUA)) in iPSC-derived DA neurons from the GBA-PD and GBA-carrier. DA neurons from the GBA mutation carrier demonstrated decreased GCase activity compared to the control. The decrease was not associated with any changes in GBA expression levels in DA neurons. GCase activity was more markedly decreased in the DA neurons of GBA-PD patient compared to the GBA-carrier. The amount of GCase protein was decreased only in GBA-PD neurons. Additionally, alterations in the activity of the other lysosomal enzymes (GLA and IDUA) were found in GBA-PD neurons compared to GBA-carrier and control neurons. Further study of the molecular differences between the GBA-PD and the GBA-carrier is essential to investigate whether genetic factors or external conditions are the causes of the penetrance of the p.N370S GBA variant.

Role of rodent models in advancing precision medicine for Parkinson's disease

With a current lack of disease-modifying treatments, an initiative toward implementing a precision medicine approach for treating Parkinson's disease (PD) has emerged. However, challenges remain in how to define and apply precision medicine in PD. To accomplish the goal of optimally targeted and timed treatment for each patient, preclinical research in a diverse population of rodent models will continue to be an essential part of the translational path to identify novel biomarkers for patient diagnosis and subgrouping, understand PD disease mechanisms, identify new therapeutic targets, and screen therapeutics prior to clinical testing. This review highlights the most common rodent models of PD and discusses how these models can contribute to defining and implementing precision medicine for the treatment of PD.

The GBA variant E326K is associated with alpha-synuclein aggregation and lipid droplet accumulation in human cell lines

Sequence variants or mutations in the GBA gene are numerically the most important risk factor for Parkinson disease (PD). The GBA gene encodes for the lysosomal hydrolase enzyme, glucocerebrosidase (GCase). GBA mutations often reduce GCase activity and lead to the impairment of the autophagy-lysosomal pathway, which is important in the turnover of alpha-synuclein, accumulation of which is a key pathological hallmark of PD. Although the E326K variant is one of the most common GBA variants associated with PD, there is limited understanding of its biochemical effects. We have characterized homozygous and heterozygous E326K variants in human fibroblasts. We found that E326K variants did not cause a significant loss of GCase protein or activity, endoplasmic reticulum (ER) retention or ER stress, in contrast to the L444P GBA mutation. This was confirmed in human dopaminergic SH-SY5Y neuroblastoma cell lines overexpressing GCase with either E326K or L444P protein. Despite no loss of the GCase activity, a significant increase in insoluble alpha-synuclein aggregates in E326K and L444P mutants was observed. Notably, SH-SY5Y overexpressing E326K demonstrated a significant increase in the lipid droplet number under basal conditions, which was exacerbated following treatment with the fatty acid oleic acid. Similarly, a significant increase in lipid droplet formation following lipid loading was observed in heterozygous and homozygous E326K fibroblasts. In conclusion, the work presented here demonstrates that the E326K mutation behaves differently to the common loss of function GBA mutations; however, lipid dyshomeostasis and alpha-synuclein pathology are still evident.

DNA Methylation of α-Synuclein Intron 1 Is Significantly Decreased in the Frontal Cortex of Parkinson's Individuals with GBA1 Mutations

Parkinson's disease (PD) is a common movement disorder, estimated to affect 4% of individuals by the age of 80. Mutations in the glucocerebrosidase 1 (GBA1) gene represent the most common genetic risk factor for PD, with at least 7-10% of non-Ashkenazi PD individuals carrying a GBA1 mutation (PD-GBA1). Although similar to idiopathic PD, the clinical presentation of PD-GBA1 includes a slightly younger age of onset, a higher incidence of neuropsychiatric symptoms, and a tendency to earlier, more prevalent and more significant cognitive impairment. The pathophysiological mechanisms underlying PD-GBA1 are incompletely understood, but, as in idiopathic PD, α-synuclein accumulation is thought to play a key role. It has been hypothesized that this overexpression of α-synuclein is caused by epigenetic modifications. In this paper, we analyze DNA methylation levels at 17 CpG sites located within intron 1 and the promoter of the α-synuclein (SNCA) gene in three different brain regions (frontal cortex, putamen and substantia nigra) in idiopathic PD, PD-GBA1 and elderly non-PD controls. In all three brain regions we find a tendency towards a decrease in DNA methylation within an eight CpG region of intron 1 in both idiopathic PD and PD-GBA1. The trend towards a reduction in DNA methylation was more pronounced in PD-GBA1, with a significant decrease in the frontal cortex. This suggests that PD-GBA1 and idiopathic PD have distinct epigenetic profiles, and highlights the importance of separating idiopathic PD and PD-GBA1 cases. This work also provides initial evidence that different genetic subtypes might exist within PD, each characterized by its own pathological mechanism. This may have important implications for how PD is diagnosed and treated.

Lysosomal Pathogenesis of Parkinson's Disease: Insights From LRRK2 and GBA1 Rodent Models

The discovery of mutations in LRRK2 and GBA1 that are linked to Parkinson's disease provided further evidence that autophagy and lysosome pathways are likely implicated in the pathogenic process. Their protein products are important regulators of lysosome function. LRRK2 has kinase-dependent effects on lysosome activity, autophagic efficacy and lysosomal Ca2+ signaling. Glucocerebrosidase (encoded by GBA1) is a hydrolytic enzyme contained in the lysosomes and contributes to the degradation of alpha-synuclein. PD-related mutations in LRRK2 and GBA1 slow the degradation of alpha-synuclein, thus directly implicating the dysfunction of the process in the neuropathology of Parkinson's disease. The development of genetic rodent models of LRRK2 and GBA1 provided hopes of obtaining reliable preclinical models in which to study pathogenic processes and perform drug validation studies. Here, I will review the extensive characterization of these models, their impact on understanding lysosome alterations in the course of Parkinson's disease and what novel insights have been obtained. In addition, I will discuss how these models fare with respect to the features of a "gold standard" animal models and what could be attempted in future studies to exploit LRRK2 and GBA1 rodent models in the fight against Parkinson's disease.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

Oxidative stress and synaptic dysfunction in rodent models of Parkinson's disease

Parkinson's disease (PD) is a multifactorial disorder involving a complex interplay between a variety of genetic and environmental factors. In this scenario, mitochondrial impairment and oxidative stress are widely accepted as crucial neuropathogenic mechanisms, as also evidenced by the identification of PD-associated genes that are directly involved in mitochondrial function. The concept of mitochondrial dysfunction is closely linked to that of synaptic dysfunction. Indeed, compelling evidence supports the role of mitochondria in synaptic transmission and plasticity, although many aspects have not yet been fully elucidated. Here, we will provide a brief overview of the most relevant evidence obtained in different neurotoxin-based and genetic rodent models of PD, focusing on mitochondrial impairment and synaptopathy, an early central event preceding overt nigrostriatal neurodegeneration. The identification of early deficits occurring in PD pathogenesis is crucial in view of the development of potential disease-modifying therapeutic strategies.

Genetic variations in GBA1 and LRRK2 genes: Biochemical and clinical consequences in Parkinson disease

Variants in the GBA1 and LRRK2 genes are the most common genetic risk factors associated with Parkinson disease (PD). Both genes are associated with lysosomal and autophagic pathways, with the GBA1 gene encoding for the lysosomal enzyme, glucocerebrosidase (GCase) and the LRRK2 gene encoding for the leucine-rich repeat kinase 2 enzyme. GBA1-associated PD is characterized by earlier age at onset and more severe non-motor symptoms compared to sporadic PD. Mutations in the GBA1 gene can be stratified into severe, mild and risk variants depending on the clinical presentation of disease. Both a loss- and gain- of function hypothesis has been proposed for GBA1 variants and the functional consequences associated with each variant is often linked to mutation severity. On the other hand, LRRK2-associated PD is similar to sporadic PD, but with a more benign disease course. Mutations in the LRRK2 gene occur in several structural domains and affect phosphorylation of GTPases. Biochemical studies suggest a possible convergence of GBA1 and LRRK2 pathways, with double mutant carriers showing a milder phenotype compared to GBA1-associated PD. This review compares GBA1 and LRRK2-associated PD, and highlights possible genotype-phenotype associations for GBA1 and LRRK2 separately, based on biochemical consequences of single variants.

The GBA1 D409V mutation exacerbates synuclein pathology to differing extents in two alpha-synuclein models

Heterozygous mutations in the GBA1 gene – encoding lysosomal glucocerebrosidase (GCase) – are the most common genetic risk factors for Parkinson's disease (PD). Experimental evidence suggests a correlation between decreased GCase activity and accumulation of alpha-synuclein (aSyn). To enable a better understanding of the relationship between aSyn and GCase activity, we developed and characterized two mouse models that investigate aSyn pathology in the context of reduced GCase activity. The first model used constitutive overexpression of wild-type human aSyn in the context of the homozygous GCase activity-reducing D409V mutant form of GBA1. Although increased aSyn pathology and grip strength reductions were observed in this model, the nigrostriatal system remained largely intact. The second model involved injection of aSyn preformed fibrils (PFFs) into the striatum of the homozygous GBA1 D409V knock-in mouse model. The GBA1 D409V mutation did not exacerbate the pathology induced by aSyn PFF injection. This study sheds light on the relationship between aSyn and GCase in mouse models, highlighting the impact of model design on the ability to model a relationship between these proteins in PD-related pathology.

Summary: Phenotyping data for two novel mouse models combining glucocerebrosidase (GCase) deficiencies and alpha-synuclein pathology reveal the extent to which alpha-synuclein pathology is exacerbated by GCase deficiency.

GBA Variants and Parkinson Disease: Mechanisms and Treatments

The GBA gene encodes for the lysosomal enzyme glucocerebrosidase (GCase), which maintains glycosphingolipid homeostasis. Approximately 5–15% of PD patients have mutations in the GBA gene, making it numerically the most important genetic risk factor for Parkinson disease (PD). Clinically, GBA-associated PD is identical to sporadic PD, aside from the earlier age at onset (AAO), more frequent cognitive impairment and more rapid progression. Mutations in GBA can be associated with loss- and gain-of-function mechanisms. A key hallmark of PD is the presence of intraneuronal proteinaceous inclusions named Lewy bodies, which are made up primarily of alpha-synuclein. Mutations in the GBA gene may lead to loss of GCase activity and lysosomal dysfunction, which may impair alpha-synuclein metabolism. Models of GCase deficiency demonstrate dysfunction of the autophagic-lysosomal pathway and subsequent accumulation of alpha-synuclein. This dysfunction can also lead to aberrant lipid metabolism, including the accumulation of glycosphingolipids, glucosylceramide and glucosylsphingosine. Certain mutations cause GCase to be misfolded and retained in the endoplasmic reticulum (ER), activating stress responses including the unfolded protein response (UPR), which may contribute to neurodegeneration. In addition to these mechanisms, a GCase deficiency has also been associated with mitochondrial dysfunction and neuroinflammation, which have been implicated in the pathogenesis of PD. This review discusses the pathways associated with GBA-PD and highlights potential treatments which may act to target GCase and prevent neurodegeneration.

Sphingolipid changes in Parkinson L444P GBA mutation fibroblasts promote α-synuclein aggregation

Intraneuronal accumulation of aggregated α-synuclein is a pathological hallmark of Parkinson’s disease. Therefore, mechanisms capable of promoting α-synuclein deposition bear important pathogenetic implications. Mutations of the glucocerebrosidase 1 (GBA) gene represent a prevalent Parkinson’s disease risk factor. They are associated with loss of activity of a key enzyme involved in lipid metabolism, glucocerebrosidase, supporting a mechanistic relationship between abnormal α-synuclein–lipid interactions and the development of Parkinson pathology.

In this study, the lipid membrane composition of fibroblasts isolated from control subjects, patients with idiopathic Parkinson’s disease and Parkinson's disease patients carrying the L444P GBA mutation (PD-GBA) was assayed using shotgun lipidomics.

The lipid profile of PD-GBA fibroblasts differed significantly from that of control and idiopathic Parkinson’s disease cells. It was characterized by an overall increase in sphingolipid levels. It also featured a significant increase in the proportion of ceramide, sphingomyelin and hexosylceramide molecules with shorter chain length and a decrease in the percentage of longer-chain sphingolipids. The extent of this shift was correlated to the degree of reduction of fibroblast glucocerebrosidase activity. Lipid extracts from control and PD-GBA fibroblasts were added to recombinant α-synuclein solutions. The kinetics of α-synuclein aggregation were significantly accelerated after addition of PD-GBA extracts as compared to control samples. Amyloid fibrils collected at the end of these incubations contained lipids, indicating α-synuclein–lipid co-assembly. Lipids extracted from α-synuclein fibrils were also analysed by shotgun lipidomics. Data revealed that the lipid content of these fibrils was significantly enriched by shorter-chain sphingolipids. In a final set of experiments, control and PD-GBA fibroblasts were incubated in the presence of the small molecule chaperone ambroxol. This treatment restored glucocerebrosidase activity and sphingolipid levels and composition of PD-GBA cells. It also reversed the pro-aggregation effect that lipid extracts from PD-GBA fibroblasts had on α-synuclein.

Taken together, the findings of this study indicate that the L444P GBA mutation and consequent enzymatic loss are associated with a distinctly altered membrane lipid profile that provides a biological fingerprint of this mutation in Parkinson fibroblasts. This altered lipid profile could also be an indicator of increased risk for α-synuclein aggregate pathology.

Glucocerebrosidase Mutations Cause Mitochondrial and Lysosomal Dysfunction in Parkinson’s Disease: Pathogenesis and Therapeutic Implications

Parkinson’s disease (PD) is the second most common neurodegenerative disease and is characterized by multiple motor and non-motor symptoms. Mutations in the glucocerebrosidase (GBA) gene, which encodes the lysosomal enzyme glucocerebrosidase (GCase), which hydrolyzes glucosylceramide (GlcCer) to glucose and ceramide, are the most important and common genetic PD risk factors discovered to date. Homozygous GBA mutations result in the most common lysosomal storage disorder, Gaucher’s disease (GD), which is classified according to the presence (neuronopathic types, type 2 and 3 GD) or absence (non-neuronopathic type, type 1 GD) of neurological symptoms. The clinical manifestations of PD in patients with GBA mutations are indistinguishable from those of sporadic PD at the individual level. However, accumulating data have indicated that GBA-associated PD patients exhibit a younger age of onset and a greater risk for cognitive impairment and psychiatric symptoms. The mechanisms underlying the increased risk of developing PD in GBA mutant carriers are currently unclear. Contributors to GBA-PD pathogenesis may include mitochondrial dysfunction, autophagy-lysosomal dysfunction, altered lipid homeostasis and enhanced α-synuclein aggregation. Therapeutic strategies for PD and GD targeting mutant GCase mainly include enzyme replacement, substrate reduction, gene and pharmacological small-molecule chaperones. Emerging clinical, genetic and pathogenic studies on GBA mutations and PD are making significant contributions to our understanding of PD-associated pathogenetic pathways, and further elucidating the interactions between GCase activity and neurodegeneration may improve therapeutic approaches for slowing PD progression.

A double-hit in vivo model of GBA viral microRNA-mediated downregulation and human alpha-synuclein overexpression demonstrates nigrostriatal degeneration

Preclinical and clinical studies support a strong association between mutations in the GBA1 gene that encodes beta-glucocerebrosidase (GCase) (EC 3.2.1.45; glucosylceramidase beta) and Parkinson's disease (PD). Alpha-synuclein (AS), a key player in PD pathogenesis, and GBA1 mutations may independently and synergistically cause lysosomal dysfunction and thus, embody clinically well-validated targets of the neurodegenerative disease process in PD. However, in vivo models, recapitulating pathological features of PD that can be used to dissect the nature of the complex relationship between GCase and AS on the nigrostriatal axis, the region particularly vulnerable in PD, are direly needed. To address this, we implemented a bidirectional approach in mice to examine the effects of: 1) GCase overexpression (wild-type and mutant N370S GBA) on endogenous AS levels and 2) downregulation of endogenous GCase (Gba) combined with AS overexpression. Striatal delivery of viral-mediated GCase overexpression revealed minimal effects on cortical and nigrostriatal AS tissue levels and no significant effect on dopaminergic system integrity. On the other hand, microRNA (miR)-mediated Gba1 downregulation (miR Gba), combined with virus-mediated human AS overexpression (+AS), yields decreased GCase activity in the cortex, mimicking levels seen in GBA1 heterozygous carriers (30-40%), increased astrogliosis and microgliosis, decreased striatal dopamine levels (50% compared to controls) and loss of nigral dopaminergic neurons (~33%)- effects that were all reversible with miR rescue. Most importantly, the synergistic neurodegeneration of miR Gba + AS correlated with augmented AS accumulation and extracellular release in the striatum. Collectively, our results suggest that GCase downregulation alone is not sufficient to recapitulate key pathological features of PD in vivo, but its synergistic interplay with AS, via increased AS levels and extracellular release, drives nigrostriatal neurodegeneration. Furthermore, we report a novel double-hit GBA-AS model that can be used to identify putative mechanisms driving PD pathophysiology and can be subsequently used to test novel therapeutic approaches.

Heterozygous GBA D409V and ATP13a2 mutations do not exacerbate pathological α-synuclein spread in the prodromal preformed fibrils model in young mice

Autophagic dysregulation and lysosomal impairment have been implicated in the pathogenesis of Parkinson's disease, partly due to the identification of mutations in multiple genes involved in these pathways such as GBA, SNCA, ATP13a2 (also known as PARK9), TMEM175 and LRRK2. Mutations resulting in lysosomal dysfunction are proposed to contribute to Parkinson's disease by increasing α-synuclein levels, that in turn may promote aggregation of this protein. Here, we used two different genetic models-one heterozygous for a mutated form of the GBA protein (D409V), and the other heterozygous for an ATP13a2 loss-of-function mutation, to test whether these mutations exacerbate the spread of α-synuclein pathology following injection of α-synuclein preformed fibrils in the olfactory bulb of 12-week-old mice. Contrary to our hypothesis, we found that mice harboring GBA D409V^+/- and ATP13a2^+/- mutations did not have exacerbated behavioral impairments or histopathology (α-synuclein, LAMP2, and Iba1) when compared to their wildtype littermates. This indicates that in the young mouse brain, neither the GBA D409V mutation or ATP13a2 loss-of-function mutation accelerate the spread of α-synuclein pathology. As a consequence, we postulate that these mutations increase Parkinson's disease risk only by acting in one of the initial, upstream events in the Parkinson's disease pathogenic process. Further, the mutations, and the molecular pathways they impact, appear to play a less important role once the pathogenic process has been triggered and therefore do not specifically influence α-synuclein pathology spread.

Do Not Miss the (Genetic) Diagnosis of Gaucher Syndrome: A Narrative Review on Diagnostic Clues and Management in Severe Prenatal and Perinatal-Lethal Sporadic Cases

With a growing number of proved therapies and clinical trials for many lysosomal storage disorders (LSDs), a lot of hope for many patients and families exists. However, there are sometimes cases with poor prognosis, fatal outcomes when our efforts must be directed towards a prompt and correct genetic diagnosis, which offers the only possibility of providing the family with appropriate prevention and treatment. To address this issue, in this article, we present the clinical and genetic hallmarks of the lethal form of Gaucher disease (PLGD) and discuss the potential management. We hope that this will draw attention to its specific manifestations (such as collodion-baby phenotype, ichthyosis, arthrogryposis), which differ from best-known GD complications and ensure appropriate diagnostic assessment to provide families at risk with reliable counselling and treatment to avoid the medical complication of GD.

Rodent models based on endolysosomal genes involved in Parkinson's disease

Genes associated with endolysosomal function have been recently associated with familial Parkinson's disease and described as risk factors for sporadic cases. This indicates that deficits in this pathway predispose to parkinsonism. To better understand the role of these genes in disease development, rodent models have been created by targeting genes playing a role in endolysosomal function, such as LRRK2, DNAJC6, SYNJ1, VPS35, GBA1, ATP13A2 and TMEM175. Here, we review the latest findings describing parkinsonian features in these animal models secondary to endolysosomal dysfunction. Also, we provide suggestions for further development and application of these animal models to better understand the contribution of endolysosomal dysfunction in Parkinson's disease and provide novel models for testing therapeutic approaches.

Inhibition of microglial β-glucocerebrosidase hampers the microglia-mediated antioxidant and protective response in neurons

BACKGROUND

Homozygotic mutations in the GBA gene cause Gaucher's disease; moreover, both patients and heterozygotic carriers have been associated with 20- to 30-fold increased risk of developing Parkinson's disease. In homozygosis, these mutations impair the activity of β-glucocerebrosidase, the enzyme encoded by GBA, and generate a lysosomal disorder in macrophages, which changes morphology towards an engorged phenotype, considered the hallmark of Gaucher's disease. Notwithstanding the key role of macrophages in this disease, most of the effects in the brain have been attributed to the β-glucocerebrosidase deficit in neurons, while a microglial phenotype for these mutations has never been reported.

METHODS

We applied the bioluminescence imaging technology, immunohistochemistry and gene expression analysis to investigate the consequences of microglial β-glucocerebrosidase inhibition in the brain of reporter mice, in primary neuron/microglia cocultures and in cell lines. The use of primary cells from reporter mice allowed for the first time, to discriminate in cocultures neuronal from microglial responses consequent to the β-glucocerebrosidase inhibition; results were finally confirmed by pharmacological depletion of microglia from the brain of mice.

RESULTS

Our data demonstrate the existence of a novel neuroprotective mechanism mediated by a direct microglia-to-neuron contact supported by functional actin structures. This cellular contact stimulates the nuclear factor erythroid 2-related factor 2 activity in neurons, a key signal involved in drug detoxification, redox balance, metabolism, autophagy, lysosomal biogenesis, mitochondrial dysfunctions, and neuroinflammation. The central role played by microglia in this neuronal response in vivo was proven by depletion of the lineage in the brain of reporter mice. Pharmacological inhibition of microglial β-glucocerebrosidase was proven to induce morphological changes, to turn on an anti-inflammatory/repairing pathway, and to hinder the microglia ability to activate the nuclear factor erythroid 2-related factor 2 response, thus increasing the neuronal susceptibility to neurotoxins.

CONCLUSION

This mechanism provides a possible explanation for the increased risk of neurodegeneration observed in carriers of GBA mutations and suggest novel therapeutic strategies designed to revert the microglial phenotype associated with β-glucocerebrosidase inhibition, aimed at resetting the protective microglia-to-neuron communication.

The transcription factor BCL11A defines distinct subsets of midbrain dopaminergic neurons

Midbrain dopaminergic (mDA) neurons are diverse in their projection targets, effect on behavior, and susceptibility to neurodegeneration. Little is known about the molecular mechanisms establishing this diversity during development. We show that the transcription factor BCL11A is expressed in a subset of mDA neurons in the developing and adult murine brain and in a subpopulation of pluripotent-stem-cell-derived human mDA neurons. By combining intersectional labeling and viral-mediated tracing, we demonstrate that Bcl11a-expressing mDA neurons form a highly specific subcircuit within the murine dopaminergic system. In the substantia nigra, the Bcl11a-expressing mDA subset is particularly vulnerable to neurodegeneration upon α-synuclein overexpression or oxidative stress. Inactivation of Bcl11a in murine mDA neurons increases this susceptibility further, alters the distribution of mDA neurons, and results in deficits in skilled motor behavior. In summary, BCL11A defines mDA subpopulations with highly distinctive characteristics and is required for establishing and maintaining their normal physiology.

Glucocerebrosidase deficiency promotes release of α-synuclein fibrils from cultured neurons

Mutations in the GBA gene, which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the most important genetic risk factor for Parkinson disease (PD). GCase activity is also decreased in sporadic PD brains and with normal ageing. Loss of GCase activity impairs the autophagy lysosomal pathway resulting in increased α-synuclein (α-syn) levels. Furthermore, elevated α-syn results in decreased GCase activity. Although the role of α-syn in PD remains unclear, evidence indicates that aggregated α-syn fibrils are a pathogenic species in PD, passing between neurons and inducing endogenous native α-syn to aggregate; spreading pathology through the brain. We have investigated if preformed α-syn fibrils (PFFs) impair GCase activity in mouse cortical neurons and differentiated dopaminergic cells, and whether GCase deficiency in these models increased the transfer of α-syn pathology to naïve cells. Neurons treated with PFFs induced endogenous α-syn to become insoluble and phosphorylated at Ser129 to a greater extent than monomeric α-syn-treatment. PFFs, but not monomeric α-syn, inhibited lysosomal GCase activity in these cells and induced the unfolded protein response. Neurons in which GCase was inhibited by conduritol β-epoxide did not increase the amount of insoluble monomeric α-syn or its phosphorylation status. Instead the release of α-syn fibrils from GCase deficient cells was significantly increased. Co-culture studies showed that the transfer of α-syn pathology to naïve cells was greater from GCase deficient cells. This study suggests that GCase deficiency increases the spread of α-syn pathology and likely contributes to the earlier age of onset and increased cognitive decline associated with GBA-PD.

Wild-type GBA1 increases the α-synuclein tetramer-monomer ratio, reduces lipid-rich aggregates, and attenuates motor and cognitive deficits in mice

The mechanisms responsible for brain α-synuclein (αS) dyshomeostasis, caused by Gaucher’s GBA1 mutations that increase Parkinson’s disease (PD) risk, are largely unknown. We previously showed that abrogating physiological αS tetramers by a familial PD-E46K–amplified 3K mutation produces PD-like syndrome in mice and that treatment with stearoyl-CoA desaturase inhibitors increased a portion of the αS tetramers, benefitting the motor phenotypes. Here, we show that—similar to previous findings in GBA1-mutant PD culture—GCase elevation prolonged the stabilization of wild-type and 3K mutant αS tetramers in wtGBA1–transduced mouse brains, improving lysosomal integrity and motor and cognitive phenotypes. These data help elucidating lipid modulators that impact the αS physiological state in vivo and the development of PD therapeutic approaches.

Loss-of-function mutations in acid beta-glucosidase 1 (GBA1) are among the strongest genetic risk factors for Lewy body disorders such as Parkinson’s disease (PD) and Lewy body dementia (DLB). Altered lipid metabolism in PD patient–derived neurons, carrying either GBA1 or PD αS mutations, can shift the physiological α-synuclein (αS) tetramer–monomer (T:M) equilibrium toward aggregation-prone monomers. A resultant increase in pSer129+ αS monomers provides a likely building block for αS aggregates. 3K αS mice, representing a neuropathological amplification of the E46K PD–causing mutation, have decreased αS T:M ratios and vesicle-rich αS+ aggregates in neurons, accompanied by a striking PD-like motor syndrome. We asked whether enhancing glucocerebrosidase (GCase) expression could benefit αS dyshomeostasis by delivering an adeno-associated virus (AAV)–human wild-type (wt) GBA1 vector into the brains of 3K neonates. Intracerebroventricular AAV-wtGBA1 at postnatal day 1 resulted in prominent forebrain neuronal GCase expression, sustained through 6 mo. GBA1 attenuated behavioral deficits both in working memory and fine motor performance tasks. Furthermore, wtGBA1 increased αS solubility and the T:M ratio in both 3K-GBA mice and control littermates and reduced pS129+ and lipid-rich aggregates in 3K-GBA. We observed GCase distribution in more finely dispersed lysosomes, in which there was increased GCase activity, lysosomal cathepsin D and B maturation, decreased perilipin-stabilized lipid droplets, and a normalized TFEB translocation to the nucleus, all indicative of improved lysosomal function and lipid turnover. Therefore, a prolonged increase of the αS T:M ratio by elevating GCase activity reduced the lipid- and vesicle-rich aggregates and ameliorated PD-like phenotypes in mice, further supporting lipid modulating therapies in PD.

Behavioral Phenotyping in a Murine Model of GBA1-Associated Parkinson Disease

Mutations in GBA1, the gene encoding glucocerebrosidase, are common genetic risk factors for Parkinson disease (PD). While the mechanism underlying this relationship is unclear, patients with GBA1-associated PD often have an earlier onset and faster progression than idiopathic PD. Previously, we modeled GBA1-associated PD by crossing gba haploinsufficient mice with mice overexpressing a human mutant α-synuclein transgene (SNCA^A53T), observing an earlier demise, shorter life span and faster symptom progression, although behavioral testing was not performed. To assess whether gba^+/−//SNCA^A53T mice exhibit a prodromal behavioral phenotype, we studied three cardinal PD features: olfactory discrimination, memory dysfunction, and motor function. The longitudinal performance of gba^+/−//SNCA^A53T (n = 8), SNCA^A53T (n = 9), gba^+/− (n = 10) and wildtype (n = 6) mice was evaluated between ages 8 and 23 months using the buried pellet test, novel object recognition test and the beam walk. Fifteen-month-old gba^+/−//SNCA^A53T mice showed more olfactory and motor deficits than wildtype mice. However, differences between gba^+/−//SNCA^A53T and SNCA^A53T mice generally did not reach statistical significance, possibly due to small sample sizes. Furthermore, while gba haploinsufficiency leads to a more rapid demise, this might not result in an earlier prodromal stage, and other factors, including aging, oxidative stress and epigenetics, may contribute to the more fulminant disease course.

Prodromal sensory neuropathy in Pink1-/- SNCAA53T double mutant Parkinson mice

AIMS

Parkinson's disease (PD) is frequently associated with a prodromal sensory neuropathy manifesting with sensory loss and chronic pain. We have recently shown that PD-associated sensory neuropathy in patients is associated with high levels of glucosylceramides. Here, we assessed the underlying pathology and mechanisms in Pink1^-/- SNCA^A53T double mutant mice.

METHODS

We studied nociceptive and olfactory behaviour and the neuropathology of dorsal root ganglia (DRGs), including ultrastructure, mitochondrial respiration, transcriptomes, outgrowth and calcium currents of primary neurons, and tissue ceramides and sphingolipids before the onset of a PD-like disease that spontaneously develops in Pink1^-/- SNCA^A53T double mutant mice beyond 15 months of age.

RESULTS

Similar to PD patients, Pink1^-/- SNCA^A53T mice developed a progressive prodromal sensory neuropathy with a loss of thermal sensitivity starting as early as 4 months of age. In analogy to human plasma, lipid analyses revealed an accumulation of glucosylceramides (GlcCer) in the DRGs and sciatic nerves, which was associated with pathological mitochondria, impairment of mitochondrial respiration, and deregulation of transient receptor potential channels (TRPV and TRPA) at mRNA, protein and functional levels in DRGs. Direct exposure of DRG neurons to GlcCer caused transient hyperexcitability, followed by a premature decline of the viability of sensory neurons cultures upon repeated GlcCer application.

CONCLUSIONS

The results suggest that pathological GlcCer contribute to prodromal sensory disease in PD mice via mitochondrial damage and calcium channel hyperexcitability. GlcCer-associated sensory neuron pathology might be amenable to GlcCer lowering therapeutic strategies.

Putative second hit rare genetic variants in families with seemingly GBA-associated Parkinson's disease

Rare variants in the beta-glucocerebrosidase gene (GBA1) are common genetic risk factors for alpha synucleinopathy, which often manifests clinically as GBA-associated Parkinson's disease (GBA-PD). Clinically, GBA-PD closely mimics idiopathic PD, but it may present at a younger age and often aggregates in families. Most carriers of GBA variants are, however, asymptomatic. Moreover, symptomatic PD patients without GBA variant have been reported in families with seemingly GBA-PD. These observations obscure the link between GBA variants and PD pathogenesis and point towards a role for unidentified additional genetic and/or environmental risk factors or second hits in GBA-PD. In this study, we explored whether rare genetic variants may be additional risk factors for PD in two families segregating the PD-associated GBA1 variants c.115+1G>A (ClinVar ID: 93445) and p.L444P (ClinVar ID: 4288). Our analysis identified rare genetic variants of the HSP70 co-chaperone DnaJ homolog subfamily B member 6 (DNAJB6) and lysosomal protein prosaposin (PSAP) as additional factors possibly influencing PD risk in the two families. In comparison to the wild-type proteins, variant DNAJB6 and PSAP proteins show altered functions in the context of cellular alpha-synuclein homeostasis when expressed in reporter cells. Furthermore, the segregation pattern of the rare variants in the genes encoding DNAJB6 and PSAP indicated a possible association with PD in the respective families. The occurrence of second hits or additional PD cosegregating rare variants has important implications for genetic counseling in PD families with GBA1 variant carriers and for the selection of PD patients for GBA targeted treatments.

L444P Gba1 mutation increases formation and spread of α-synuclein deposits in mice injected with mouse α-synuclein pre-formed fibrils

Parkinson disease is the most common neurodegenerative movement disorder, estimated to affect one in twenty-five individuals over the age of 80. Mutations in glucocerebrosidase 1 (GBA1) represent the most common genetic risk factor for Parkinson disease. The link between GBA1 mutations and α-synuclein accumulation, a hallmark of Parkinson disease, is not fully understood. Following our recent finding that Gba1 mutations lead to increased α-synuclein accumulation in mice, we have studied the effects of a single injection of mouse α-synuclein pre-formed fibrils into the striatum of Gba1 mice that carry a L444P knock-in mutation. We found significantly greater formation and spread of α-synuclein inclusions in Gba1-transgenic mice compared to wild-type controls. This indicates that the Gba1 L444P mutation accelerates α-synuclein pathology and spread.

Characterization of a pathogenic variant in GBA for Parkinson's disease with mild cognitive impairment patients

Parkinson's disease (PD) is the second most common neurodegenerative disease, and mild cognitive impairment (MCI) is a well-established risk factor for the development of dementia in PD. A growing body of evidence suggests that low expression of glucocerebrosidase (GBA) promotes the transmission of α-synuclein (α-Syn) interpolymers and the progression of PD. However, how GBA mutations affect the pathogenesis of PD via abnormal aggregation of α-Syn is unclear, and no clinically valid PD-MCI genetic markers have been identified. Here, we first located a GBA eQTL, rs12411216, by analysing DHS, eQTL SNP, and transcription factor binding site data using the UCSC database. Subsequently, we found that rs12411216 was significantly associated with PD-MCI (P < 0.05) in 306 PD patients by genotyping. In exploring the relationship between rs12411216 and GBA expression, the SNP was found to be associated with GBA expression in 50 PD patients through qPCR verification. In a further CRISPR/Cas9-mediated genome editing module, the SNP was identified to cause a decrease in GBA expression, weaken enzymatic activity and enhance the abnormal aggregation of α-Syn in SH-SY5Y cells. Additionally, using an electrophoretic mobility shift assay, we confirmed that the binding efficiency of transcription factor E2F4 was affected by the rs12411216 SNP. In conclusion, our results showed that rs12411216 regulated GBA expression, supporting its potential role as a PD-MCI genetic biomarker and highlighting novel mechanisms underlying Parkinson's disease.

Small Molecule Chaperones for the Treatment of Gaucher Disease and GBA1-Associated Parkinson Disease

Parkinson disease, the second most common movement disorder, is a complex neurodegenerative disorder hallmarked by the accumulation of alpha-synuclein, a neural-specific small protein associated with neuronal synapses. Mutations in the glucocerebrosidase gene (GBA1), implicated in the rare, autosomal recessive lysosomal disorder Gaucher disease, are the most common known genetic risk factor for Parkinson disease. Insights into the inverse relationship between glucocerebrosidase and alpha-synuclein have led to new therapeutic approaches for the treatment of Gaucher disease and GBA1-associated Parkinson disease. Unlike the current drugs used to treat Gaucher disease, which are highly expensive and do not cross the blood-brain-barrier, new small molecules therapies, including competitive and non-competitive chaperones that enhance glucocerebrosidase levels are being developed to overcome these limitations. Some of these include iminosugars, ambroxol, other competitive glucocerebrosidase inhibitors, and non-inhibitory chaperones or activators that do not compete for the active site. These drugs, which have been shown in different disease models to increase glucocerebrosidase activity, could have potential as a therapy for Gaucher disease and GBA1- associated Parkinson disease. Some have been demonstrated to reduce α-synuclein levels in pre-clinical studies using cell-based or animal models of GBA1-associated Parkinson disease, and may also have utility for idiopathic Parkinson disease.

Oxyphylla A Promotes Degradation of α-Synuclein for Neuroprotection via Activation of Immunoproteasome

Parkinson’s disease (PD), the second most common neurodegenerative disorder, is neuropathologically characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNc) and the presence of Lewy bodies in surviving neurons. α-synuclein (α-syn) is the major component of Lewy bodies and its deposition in neurons is critical pathological event in the pathogenesis of PD. Herein, we reported that Oxyphylla A, a novel lead compound from the fruit of Alpinia oxyphylla, significantly promoted α-syn degradation in a cellular PD model. When exploring the molecular pathways, we found that Oxyphylla A promoted α-syn degradation in a ubiquitin proteasome system (UPS)-dependent and autophagy-independent manner. We further confirmed that Oxyphylla A enhanced UPS activity by upregulating 20S subunit PSMB8 expression. A mechanism study revealed that Oxyphylla A activated the PKA/Akt/mTOR pathway to trigger PSMB8 expression and enhance UPS activity. Finally, we illustrated that Oxyphylla A alleviated the accumulation of both Triton-soluble and Triton-insoluble forms of α-syn and protected against α-syn-induced neurotoxicity in A53T α-syn transgenic mice. These findings suggest that the activation of UPS, via small molecular UPS enhancers including Oxyphylla A, may be a therapeutic strategy for intervention against PD and related diseases.

Modeling Parkinson's Disease With the Alpha-Synuclein Protein

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

Advances in Sphingolipidoses: CRISPR-Cas9 Editing as an Option for Modelling and Therapy

Sphingolipidoses are inherited genetic diseases characterized by the accumulation of glycosphingolipids. Sphingolipidoses (SP), which usually involve the loss of sphingolipid hydrolase function, are of lysosomal origin, and represent an important group of rare diseases among lysosomal storage disorders. Initial treatments consisted of enzyme replacement therapy, but, in recent decades, various therapeutic approaches have been developed. However, these commonly used treatments for SP fail to be fully effective and do not penetrate the blood-brain barrier. New approaches, such as genome editing, have great potential for both the treatment and study of sphingolipidoses. Here, we review the most recent advances in the treatment and modelling of SP through the application of CRISPR-Cas9 genome editing. CRISPR-Cas9 is currently the most widely used method for genome editing. This technique is versatile; it can be used for altering the regulation of genes involved in sphingolipid degradation and synthesis pathways, interrogating gene function, generating knock out models, or knocking in mutations. CRISPR-Cas9 genome editing is being used as an approach to disease treatment, but more frequently it is utilized to create models of disease. New CRISPR-Cas9-based tools of gene editing with diminished off-targeting effects are evolving and seem to be more promising for the correction of individual mutations. Emerging Prime results and CRISPR-Cas9 difficulties are also discussed.

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.

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.

Sensory neuropathy and nociception in rodent models of Parkinson's disease

Parkinson's disease (PD) often manifests with prodromal pain and sensory losses whose etiologies are not well understood. Multiple genetic and toxicity-based rodent models of PD partly recapitulate the histopathology and motor function deficits. Although far less studied, there is some evidence that rodents, similar to humans, develop sensory manifestations of the disease, which may precede motor disturbances and help to elucidate the underlying mechanisms of PD-associated pain at the molecular and neuron circuit levels. The present Review summarizes nociception and other sensory functions in frequently used rodent PD models within the context of the complex phenotypes. In terms of mechanisms, it appears that the acute loss of dopaminergic neurons in systemic toxicity models (MPTP, rotenone) primarily causes nociceptive hyperexcitability, presumably owing to a loss of inhibitory control, whereas genetic models primarily result in a progressive loss of heat perception, reflecting sensory fiber neuropathies. At the molecular level, neither α-synuclein deposits alone nor failure of mitophagy alone appear to be strong enough to result in axonal or synaptic pathology of nociceptive neurons that manifest at the behavioral level, and peripheral sensory loss may mask central ‘pain’ in behavioral tests. Hence, allostatic combinations or additional challenges and novel behavioral assessments are needed to better evaluate PD-associated sensory neuropathies and pain in rodents.

Summary: Rodent models of Parkinson's disease partially develop prodromal somatosensory and olfactory dysfunctions reminiscent of sensory neuropathies in patients and reveal mechanistic insight, but data are incomplete and fragmented.

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.

Neural Stem Cells of Parkinson's Disease Patients Exhibit Aberrant Mitochondrial Morphology and Functionality

Emerging evidence suggests that Parkinson's disease (PD), besides being an age-associated disorder, might also have a neurodevelopment component. Disruption of mitochondrial homeostasis has been highlighted as a crucial cofactor in its etiology. Here, we show that PD patient-specific human neuroepithelial stem cells (NESCs), carrying the LRRK2-G2019S mutation, recapitulate key mitochondrial defects previously described only in differentiated dopaminergic neurons. By combining high-content imaging approaches, 3D image analysis, and functional mitochondrial readouts we show that LRRK2-G2019S mutation causes aberrations in mitochondrial morphology and functionality compared with isogenic controls. LRRK2-G2019S NESCs display an increased number of mitochondria compared with isogenic control lines. However, these mitochondria are more fragmented and exhibit decreased membrane potential. Functional alterations in LRRK2-G2019S cultures are also accompanied by a reduced mitophagic clearance via lysosomes. These findings support the hypothesis that preceding mitochondrial developmental defects contribute to the manifestation of the PD pathology later in life.

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Mitochondrial gene expression is altered in NESCs carrying the LRRK2-G2019 mutation

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LRRK2-G2019S mutation induces alterations in mitochondrial morphology in NESCs

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Mitophagy is affected in PD-specific NESCs carrying the LRRK2-G2019S mutation

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Mitochondrial phenotypes in NESC are rescued by genetic correction of LRRK2-G2019S

Genetically engineered animal models of Parkinson's disease: From worm to rodent

Parkinson's disease (PD) is a progressive neurological disorder characterised by aberrant accumulation of insoluble proteins, including alpha-synuclein, and a loss of dopaminergic neurons in the substantia nigra. The extended neurodegeneration leads to a drop of striatal dopamine levels responsible for disabling motor and non-motor impairments. Although the causes of the disease remain unclear, it is well accepted among the scientific community that the disorder may also have a genetic component. For that reason, the number of genetically engineered animal models has greatly increased over the past two decades, ranging from invertebrates to more complex organisms such as mice and rats. This trend is growing as new genetic variants associated with the disease are discovered. The EU Joint Programme - Neurodegenerative Disease Research (JPND) has promoted the creation of an online database aiming at summarising the different features of experimental models of Parkinson's disease. This review discusses available genetic models of PD and the extent to which they adequately mirror the human pathology and reflects on future development and uses of genetically engineered experimental models for the study of PD.

Development and biochemical characterization of a mouse model of Parkinson's disease bearing defective glucocerebrosidase activity

GBA1 gene encodes for the lysosomal membrane protein glucocerebrosidase (GCase). GBA1 heterozygous mutations profoundly impair GCase activity and are currently recognized as an important risk factor for the development of Parkinson's disease (PD). Deficits in lysosomal degradation pathways may contribute to pathological α-synuclein accumulation, thereby favoring dopaminergic neuron degeneration and associated microglial activation. However, the precise mechanisms by which GCase deficiency may influence PD onset and progression remain unclear. In this work we used conduritol-β-epoxide (CBE), a potent inhibitor of GCase, to induce a partial, systemic defect of GCase activity comparable to that associated with heterozygous GBA1 mutations, in mice. Chronic (28 days) administration of CBE (50 mg/kg, i.p.) was combined with administration of a classic PD-like inducing neurotoxin, such as MPTP (30 mg/kg, i.p. for 5 days). The aim was to investigate whether a pre-existing GCase defect may influence the effects of MPTP in terms of nigrostriatal damage, microglia activation and α-synuclein accumulation. Pre-treatment with CBE had tendency to enhance MPTP-induced neurodegeneration in striatum and caused significant increase of total α-synuclein expression in substantia nigra. Microglia was remarkably activated by CBE alone, without further increases when combined with MPTP. Overall, we propose this model as an additional tool to study pathophysiological processes of PD in the presence of GCase defects.

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.

D409H GBA1 mutation accelerates the progression of pathology in A53T α-synuclein transgenic mouse model

Heterozygous mutations in glucocerebrosidase 1 (GBA1) are a major genetic risk factor for Parkinson’s disease and Dementia with Lewy bodies. Mutations in GBA1 leads to GBA1 enzyme deficiency, and GBA1-associated parkinsonism has an earlier age of onset and more progressive parkinsonism. To investigate a potential influence of GBA1 deficiency caused by mutations in GBA1 on the disease progression of PD, GBA1 mice carrying D409H knock-in mutation were crossbred with the human A53T (hA53T) α-synuclein transgenic mice. Here, we show that GBA1 enzyme activity plays a significant role in the hA53T α-synuclein induced α-synucleinopathy. The expression of D409H GBA1 markedly shortens the lifespan of hA53T α-synuclein transgenic mice. Moreover, D409H GBA1 expression exacerbates the formation of insoluble aggregates of α-synuclein, glial activation, neuronal degeneration, and motor abnormalities in the hA53T α-synuclein transgenic mice. Interestingly, the expression of D409H GBA1 results in the loss of dopaminergic neurons in the substantia nigra pars compacta of hA53T transgenic mice. Taken together, these results indicate that GBA1 deficiency due to D409H mutation affects the disease onset and course in hA53T α-synuclein transgenic mice. Therefore, strategies aimed to maintain GBA1 enzyme activity could be employed to develop an effective novel therapy for GBA1 linked-PD and related α-synucleinopathies.

Cellular and Molecular Basis of Neurodegeneration in Parkinson Disease

It has been 200 years since Parkinson disease (PD) was described by Dr. Parkinson in 1817. The disease is the second most common neurodegenerative disease characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Although the pathogenesis of PD is still unknown, the research findings from scientists are conducive to understand the pathological mechanisms. It is well accepted that both genetic and environmental factors contribute to the onset of PD. In this review, we summarize the mutations of main seven genes (α-synuclein, LRRK2, PINK1, Parkin, DJ-1, VPS35 and GBA1) linked to PD, discuss the potential mechanisms for the loss of dopaminergic neurons (dopamine metabolism, mitochondrial dysfunction, endoplasmic reticulum stress, impaired autophagy, and deregulation of immunity) in PD, and expect the development direction for treatment of PD.

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

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