The biochemical basis of interactions between Glucocerebrosidase and alpha-synuclein in GBA1 mutation carriers

Functional Study of SNCA p.V15A Variant: Further Linking α-Synuclein and Glucocerebrosidase

BACKGROUND

SNCA p.V15A was reported in five families. In vitro models showed increased aggregation and seeding activity, mitochondrial damage, and apoptosis. Mutant flies had reduced flying ability and survival.

OBJECTIVES

To clinically and functionally evaluate SNCA p.V15A in a large Italian family with Parkinson's disease (PD).

METHODS

Genetic diagnosis was reached through next-generation sequencing. Pathogenicity was assessed by molecular dynamics simulation and biochemical studies on peripheral blood mononuclear cells (PBMCs).

RESULTS

Five siblings carried SNCA p.V15A; three developed bradykinetic-rigid PD in their 50s with rapid motor progression and variable cognitive impairment. A fourth sibling had isolated mood disturbance, whereas the fifth was still unaffected at age 47. The mutant protein showed decreased stability and an unstable folded structure. Proband's PBMCs showed elevated total and phosphorylated α-synuclein (α-syn) levels and significantly reduced glucocerebrosidase activity.

CONCLUSION

This study demonstrates accumulation of α-synV15A in PBMCs and strengthens the link between α-syn pathophysiology and glucocerebrosidase dysfunction. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

a-Synuclein and lipids in erythrocytes of Gaucher disease carriers and patients before and after enzyme replacement therapy

It is well established that patients with Gaucher disease, as well as carriers of the disease have an increased risk for developing Parkinson's disease. A plethora of evidence suggests that disturbed α-Synuclein homeostasis is the link between Gaucher disease and Parkinson's disease. The pathogenic mechanism linking these entities is still a topic of debate and both gain- and loss-of-function theories have been put forward, which however are not mutually exclusive. In the present study we expanded our previous studies to include not only Gaucher disease patients but also Gaucher disease carriers and Gaucher disease patients following Enzyme Replacement Therapy. In these groups we investigated α-Synuclein in red blood cell membranes in association with lipid abnormalities described in Gaucher disease. These included glucosylceramide and its species, glucosylsphingosine, glucosylcholesterol and plasmalogens. Increased oligomerization of α-Synuclein in red blood cell membranes was observed not only in Gaucher disease patients but also in carriers of the disease. There were no qualitative differences in the lipids identified in the groups studied. However, significant quantitative differences compared to controls were observed in Gaucher disease patients but not in Gaucher disease carriers. Enzyme Replacement Therapy reversed the biochemical defects and normalized α-Synuclein homeostasis, providing for the first time evidence in human subjects that such homeostatic dysregulation is reversible. Further studies investigating α-Synuclein status during the differentiation of erythroid progenitors could provide new data on the pathogenic mechanism of α-Synuclein oligomerization in this system.

Neuroimaging in Glucocerebrosidase-Associated Parkinsonism: A Systematic Review

Background

Mutations in the GBA gene cause Gaucher's disease (GD) and constitute the most frequent genetic risk factor for idiopathic Parkinson's disease (iPD). Nonmanifesting carriers of GBA mutations/variants (GBA‐NMC) constitute a potential PD preclinical population, whereas PD patients carrying some GBA mutations/variants (GBA‐PD) have a higher risk of a more aggressive disease course. Different neuroimaging techniques are emerging as potential biomarkers in PD and have been used to study GBA‐associated parkinsonism.

Objective

The aim is to critically review studies applying neuroimaging to GBA‐associated parkinsonism.

Methods

Literature search was performed using PubMed and EMBASE databases (last search February 7, 2022). Studies reporting neuroimaging findings in GBA‐PD, GD with and without parkinsonism, and GBA‐NMC were included.

Results

Thirty‐five studies were included. In longitudinal studies, GBA‐PD patients show a more aggressive disease than iPD at both structural magnetic resonance imaging and 123‐fluoropropylcarbomethoxyiodophenylnortropane single‐photon emission computed tomography. Fluorodeoxyglucose‐positron emission tomography and brain perfusion studies reported a greater cortical involvement in GBA‐PD compared to iPD. Overall, contrasting evidence is available regarding GBA‐NMC for imaging and clinical findings, although subtle differences have been reported compared with healthy controls with no mutations.

Conclusions

Although results must be interpreted with caution due to limitations of the studies, in line with previous clinical observations, GBA‐PD showed a more aggressive disease progression in neuroimaging longitudinal studies compared to iPD. Cognitive impairment, a “clinical signature” of GBA‐PD, seems to find its neuroimaging correlate in the greater cortical burden displayed by these patients as compared to iPD. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

New therapeutic approaches to Parkinson's disease targeting GBA, LRRK2 and Parkin

Parkinson's disease (PD) is defined as a complex disorder with multifactorial pathogenesis, yet a more accurate definition could be that PD is not a single entity, but rather a mixture of different diseases with similar phenotypes. Attempts to classify subtypes of PD have been made based on clinical phenotypes or biomarkers. However, the most practical approach, at least for a portion of the patients, could be to classify patients based on genes involved in PD. GBA and LRRK2 mutations are the most common genetic causes or risk factors of PD, and PRKN is the most common cause of autosomal recessive form of PD. Patients carrying variants in GBA, LRRK2 or PRKN differ in some of their clinical characteristics, pathology and biochemical parameters. Thus, these three PD-associated genes are of special interest for drug development. Existing therapeutic approaches in PD are strictly symptomatic, as numerous clinical trials aimed at modifying PD progression or providing neuroprotection have failed over the last few decades. The lack of precision medicine approach in most of these trials could be one of the reasons why they were not successful. In the current review we discuss novel therapeutic approaches targeting GBA, LRRK2 and PRKN and discuss different aspects related to these genes and clinical trials.

Gene expression with corresponding pathways analysis in Gaucher disease

Gaucher disease (GD) caused by mutation in the GBA gene has a wide spectrum of phenotypes. Besides the storage disorder, secondary alteration of various pathways occurs with modification of the expression of many genes. In our work we analysed the expression profile of genes in adult patients with type 1 GD.

METHODS

This study was an observational, cross-sectional analysis of a group of twenty patients with type 1 GD and ten healthy volunteers as a control group. First, on the group of ten persons, microarray gene analysis was performed. Afterwards, significantly regulated genes were selected, and the microarray results were confirmed by real-time PCR on the whole study group.

RESULTS

Based on the microarray results in the pathway analysis, we focused on genes related to chemokines, inflammatory processes, endocytosis, autophagy, and apoptosis. Patients with GD demonstrated up-regulation of genes related to NFkB pathway (NFkB, NKkBR SQSTM1), inflammation (IL-1b), endocytosis and autophagy (BCN1, SMAD), genes coding proteins involved in apoptosis (CASP, NFkB, BCL2) as well as genes related to proteasome degradation (PSMD2, PSMB9) and SNARE complex (SNAP, STX). Simultaneously, we showed down-regulation of genes coding proteins of chemokines and their receptors (GNB4, CCL5). The qRT-PCR results confirmed changes in expression of selected genes. Parallel microarray results showed inhibition of genes related to neurones development and survival (NTRK1) and stimulation of gene expression related to neurodegeneration and apoptosis (BCN1, IL1B).

CONCLUSIONS

The work revealed different pathway activation, especially inflammatory processes followed by autophagy and apoptosis. Our results also pay attention to new pathways leading to disorders of the functioning of the nervous tissue in patients with type 1 GD, which may lead to the development of polyneuropathy and chronic pain. These are clinical symptoms that severely decrease the quality of life in GD patients.

Glucosylceramide Associated with Gaucher Disease Forms Amyloid-like Twisted Ribbon Fibrils That Induce α-Synuclein Aggregation

A major risk factor for Gaucher's disease is loss of function mutations in the GBA1 gene that encodes lysosomal β-glucocerebrosidase, resulting in accumulation of glucosylceramide (GlcCer), a key lysosomal sphingolipid. GBA1 mutations also enhance the risk for Parkinson's disease, whose hallmark is the aggregation of α-synuclein (αSyn). However, the role of accumulated GlcCer in αSyn aggregation is not completely understood. Using various biophysical assays, we demonstrate that GlcCer self-assembles to form amyloid-like fibrillar aggregates in vitro. The GlcCer assemblies are stable in aqueous media of different pH and exhibit a twisted ribbon-like structure. Near lysosomal pH GlcCer aggregates induced αSyn aggregation and stabilized its nascent oligomers. We found that several bona fide inhibitors of proteinaceous amyloids effectively inhibited aggregation of GlcCer. This study contributes to the growing evidence of cross-talk between proteinaceous amyloids and amyloid-like aggregates of metabolites accumulated in diseases and suggests these aggregates as therapeutic targets.

Membrane Interactions of α-Synuclein Probed by Neutrons and Photons

α-Synuclein (α-syn) is a key protein in the etiology of Parkinson's disease. In a disease state, α-syn accumulates as insoluble amyloid fibrils enriched in β-sheet structure. However, in its functional state, α-syn adopts an amphipathic helix upon membrane association and plays a role in synaptic vesicle docking, fusion, and clustering. In this Account, we describe our contributions made in the past decade toward developing a molecular understanding of α-syn membrane interactions, which are crucial for function and have pathological implications. Three topics are covered: α-syn membrane binding probed by neutron reflectometry (NR), the effects of membrane on α-syn amyloid formation, and interactions of α-syn with cellular membranes.NR offers a unique perspective by providing direct measurements of protein penetration depth. By the use of segmentally deuterated α-syn generated through native chemical ligation, the spatial resolution of specific membrane-bound polypeptide regions was determined by NR. Additionally, we used NR to characterize the membrane-bound complex of α-syn and glucocerebrosidase, a lysosomal hydrolase whose mutations are a common genetic risk factor for Parkinson's disease. Although phosphatidylcholine (PC) is the most abundant lipid species in mammalian cells, interactions of PC with α-syn have been largely ignored because they are substantially weaker compared with the electrostatically driven binding of negatively charged lipids. We discovered that α-syn tubulates zwitterionic PC membranes, which is likely related to its involvement in synaptic vesicle fusion by stabilization of membrane curvature. Interestingly, PC lipid tubules inhibit amyloid formation, in contrast to anionic phosphatidylglycerol lipid tubules, which stimulate protein aggregation. We also found that membrane fluidity influences the propensity of α-synuclein amyloid formation. Most recently, we obtained direct evidence of binding of α-syn to exocytic sites on intact cellular membranes using a method called cellular unroofing. This method provides direct access to the cytosolic plasma membrane. Importantly, measurements of fluorescence lifetime distributions revealed that α-syn is more conformationally dynamic at the membrane interface than previously appreciated. This exquisite responsiveness to specific lipid composition and membrane topology is important for both its physiological and pathological functions. Collectively, our work has provided insights into the effects of the chemical nature of phospholipid headgroups on the interplay among membrane remodeling, protein structure, and α-syn amyloid formation.

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

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

Genetic causes of PD: A pathway to disease modification

The underline neuropathology of Parkinson disease is pleiomorphic and its genetic background diverse. Possibly because of this heterogeneity, no effective disease modifying therapy is available. In this paper we give an overview of the genetics of Parkinson disease and explain how this is relevant for the development of new therapies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Glucocerebrosidase: Functions in and Beyond the Lysosome

Glucocerebrosidase (GCase) is a retaining β-glucosidase with acid pH optimum metabolizing the glycosphingolipid glucosylceramide (GlcCer) to ceramide and glucose. Inherited deficiency of GCase causes the lysosomal storage disorder named Gaucher disease (GD). In GCase-deficient GD patients the accumulation of GlcCer in lysosomes of tissue macrophages is prominent. Based on the above, the key function of GCase as lysosomal hydrolase is well recognized, however it has become apparent that GCase fulfills in the human body at least one other key function beyond lysosomes. Crucially, GCase generates ceramides from GlcCer molecules in the outer part of the skin, a process essential for optimal skin barrier property and survival. This review covers the functions of GCase in and beyond lysosomes and also pays attention to the increasing insight in hitherto unexpected catalytic versatility of the enzyme.