Lack of enzyme activity in GBA2 mutants associated with hereditary spastic paraplegia/cerebellar ataxia (SPG46)

Hereditary spastic paraparesis type 46 (SPG46): new GBA2 variants in a large Italian case series and review of the literature

Hereditary spastic paraparesis (HSP) is a group of central nervous system diseases primarily affecting the spinal upper motor neurons, with different inheritance patterns and phenotypes. SPG46 is a rare, early-onset and autosomal recessive HSP, linked to biallelic GBA2 mutations. About thirty families have been described worldwide, with different phenotypes like complicated HSP, recessive cerebellar ataxia or Marinesco-Sjögren Syndrome. Herein, we report five SPG46 patients harbouring five novel GBA2 mutations, the largest series described in Italy so far. Probands were enrolled in five different centres and underwent neurological examination, clinical cognitive assessment, column imaging for scoliosis assessment, ophthalmologic examination, brain imaging, GBA2 activity in peripheral blood cells and genetic testing. Their phenotype was consistent with HSP, with notable features like upper gaze palsy and movement disorders. We review demographic, genetic, biochemical and clinical information from all documented cases in the existing literature, focusing on the global distribution of cases, the features of the syndrome, its variable presentation, new potential identifying features and the significance of measuring GBA2 enzyme activity.

First-in-human single-dose study of nizubaglustat, a dual inhibitor of ceramide glucosyltransferase and non-lysosomal glucosylceramidase: Safety, tolerability, pharmacokinetics, and pharmacodynamics of single ascending and multiple doses in healthy adults

Nizubaglustat is a novel, orally available, brain penetrant, potent, and selective dual inhibitor of ceramide glucosyltranferase and non-lysosomal neutral glucosylceramidase (NLGase), which is currently under development for the treatment of subjects with neurological manifestations in primary and secondary gangliosidoses. The objectives of this first-in-human study were to evaluate the safety and tolerability, pharmacokinetics, and pharmacodynamics (PD) of single oral doses of nizubaglustat after single (1, 3, and 9 mg) and multiple oral doses (9 mg once per day (QD) over 14 days) in healthy adults. Nizubaglustat was rapidly absorbed and systemic exposure was dose-proportional. Steady-state was achieved after three days of QD multiple dosing with minimal accumulation. Renal clearance accounted for around 15% of nizubaglustat elimination. Following multiple dosing, plasma concentrations of glucosylceramide (GlcCer), lactosylceramide (LacCer), and monosialodihexosylganglioside (GM3) decreased to a nadir at Day 10. PD target engagement of GCS inhibition was shown by a median decrease from baseline of plasma concentrations of GlcCer, LacCer, and GM3 ganglioside by 70%, 50%, and 48%, respectively. NLGase inhibition was also manifested by increased concentrations of GlcCer in cerebrospinal fluid from Day 1 to Day 14. Nizubaglustat was safe and well-tolerated at all doses tested. Consistent with the high selectivity, and the absence of intestinal disaccharidases inhibition, no cases of diarrhea were reported. No decreased appetite or weight loss was noted. Only treatment-emergent adverse events with preferred terms belonging to the system organ class skin and subcutaneous disorders of mild intensity were reported as drug-related in the nizubaglustat arm, in line with the pharmacological mechanism targeting glucosylceramide metabolism. Taken together, these data support QD dosing of nizubaglustat and its ongoing development in patients with primary and secondary forms of gangliosidoses.

Reaction Mechanism of Glycoside Hydrolase Family 116 Utilizes Perpendicular Protonation

Retaining glycoside hydrolases use acid/base catalysis with an enzymatic acid/base protonating the glycosidic bond oxygen to facilitate leaving-group departure alongside attack by a catalytic nucleophile to form a covalent intermediate. Generally, this acid/base protonates the oxygen laterally with respect to the sugar ring, which places the catalytic acid/base and nucleophile carboxylates within about 4.5-6.5 Å of each other. However, in glycoside hydrolase (GH) family 116, including disease-related human acid β-glucosidase 2 (GBA2), the distance between the catalytic acid/base and the nucleophile is around 8 Å (PDB: 5BVU) and the catalytic acid/base appears to be above the plane of the pyranose ring, rather than being lateral to that plane, which could have catalytic consequences. However, no structure of an enzyme-substrate complex is available for this GH family. Here, we report the structures of Thermoanaerobacterium xylanolyticum β-glucosidase (TxGH116) D593N acid/base mutant in complexes with cellobiose and laminaribiose and its catalytic mechanism. We confirm that the amide hydrogen bonding to the glycosidic oxygen is in a perpendicular rather than lateral orientation. Quantum mechanics/molecular mechanics (QM/MM) simulations of the glycosylation half-reaction in wild-type TxGH116 indicate that the substrate binds with the nonreducing glucose residue in an unusual relaxed 4C1 chair at the -1 subsite. Nevertheless, the reaction can still proceed through a 4H3 half-chair transition state, as in classical retaining β-glucosidases, as the catalytic acid D593 protonates the perpendicular electron pair. The glucose C6OH is locked in a gauche, trans orientation with respect to the C5-O5 and C4-C5 bonds to facilitate perpendicular protonation. These data imply a unique protonation trajectory in Clan-O glycoside hydrolases, which has strong implications for the design of inhibitors specific to either lateral protonators, such as human GBA1, or perpendicular protonators, such as human GBA2.

GBA1 Gene Mutations in α-Synucleinopathies-Molecular Mechanisms Underlying Pathology and Their Clinical Significance

α-Synucleinopathies comprise a group of neurodegenerative diseases characterized by altered accumulation of a protein called α-synuclein inside neurons and glial cells. This aggregation leads to the formation of intraneuronal inclusions, Lewy bodies, that constitute the hallmark of α-synuclein pathology. The most prevalent α-synucleinopathies are Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). To date, only symptomatic treatment is available for these disorders, hence new approaches to their therapy are needed. It has been observed that GBA1 mutations are one of the most impactful risk factors for developing α-synucleinopathies such as PD and DLB. Mutations in the GBA1 gene, which encodes a lysosomal hydrolase β-glucocerebrosidase (GCase), cause a reduction in GCase activity and impaired α-synuclein metabolism. The most abundant GBA1 gene mutations are N370S or N409S, L444P/L483P and E326K/E365K. The mechanisms by which GCase impacts α-synuclein aggregation are poorly understood and need to be further investigated. Here, we discuss some of the potential interactions between α-synuclein and GCase and show how GBA1 mutations may impact the course of the most prevalent α-synucleinopathies.

Clinical and genetic characterization of a Taiwanese cohort with spastic paraparesis combined with cerebellar involvement

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurodegenerative disorders clinically characterized by progressive lower-limb spasticity. Cerebellar ataxia commonly co-occurs with complicated HSPs. HSP with concurrent cerebellar ataxia has significant clinical and genetic overlaps with hereditary cerebellar ataxia (HCA) and other inherited neurological diseases, adding to the challenge of planning genetic testing for the disease. In this study, we characterized clinical features of a cohort of 24 patients (male/female: 15/9) from 22 families who presented spastic paraparesis combined with cerebellar involvement, with a median disease onset age 20.5 (range 5–53) years. Aside from the core phenotype, 18 (75%) patients had additional neuropsychiatric and systemic manifestations. A stepwise genetic testing strategy stratified by mode of inheritance, distinct neuroimaging features (e.g., thin corpus callosum), population-specific prevalence and whole-exome sequencing was utilized to investigate the genetic etiology. Causative mutations in up to 10 genes traditionally related to HSP, HCA and other neurogenetic diseases (autosomal recessive spastic ataxia of Charlevoix-Saguenay, neurodegeneration with brain iron accumulation, and progressive encephalopathy with brain atrophy and thin corpus callosum) were detected in 16 (73%) of the 22 pedigrees. Our study revealed the genetic complexity of HSP combined with cerebellar involvement. In contrast to the marked genetic diversity, the functions of the causative genes are restricted to a limited number of physiological themes. The functional overlap might reflect common underlying pathogenic mechanisms, to which the corticospinal tract and cerebellar neuron circuits may be especially vulnerable.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

Systematic Functional and Computational Analysis of Glucose-Binding Residues in Glycoside Hydrolase Family GH116

Glycoside hydrolases (GH) bind tightly to the sugar moiety at the glycosidic bond being hydrolyzed to stabilize its transition state conformation. We endeavored to assess the importance of glucose-binding residues in GH family 116 (GH116) β-glucosidases, which include human β-glucosylceramidase 2 (GBA2), by mutagenesis followed by kinetic characterization, X-ray crystallography, and ONIOM calculations on Thermoanaerobacterium xylanolyticum TxGH116, the structural model for GH116 enzymes. Mutations of residues that bind at the glucose C3OH and C4OH caused 27–196-fold increases in KM for p-nitrophenyl-β-D-glucoside, and significant decreases in the kcat, up to 5000-fold. At the C6OH binding residues, mutations of E777 decreased the kcat/KM by over 60,000-fold, while R786 mutants increased both the KM (40-fold) and kcat (2–4-fold). The crystal structures of R786A and R786K suggested a larger entrance to the active site could facilitate their faster rates. ONIOM binding energy calculations identified D452, H507, E777, and R786, along with the catalytic residues E441 and D593, as strong electrostatic contributors to glucose binding with predicted interaction energies > 15 kcal mol−1, consistent with the effects of the D452, H507, E777 and R786 mutations on enzyme kinetics. The relative importance of GH116 active site residues in substrate binding and catalysis identified in this work improves the prospects for the design of inhibitors for GBA2 and the engineering of GH116 enzymes for hydrolytic and synthetic applications.

Transcriptomic characterization of tissues from patients and subsequent pathway analyses reveal biological pathways that are implicated in spastic ataxia

BACKGROUND

Spastic ataxias (SAs) encompass a group of rare and severe neurodegenerative diseases, characterized by an overlap between ataxia and spastic paraplegia clinical features. They have been associated with pathogenic variants in a number of genes, including GBA2. This gene codes for the non-lysososomal β-glucosylceramidase, which is involved in sphingolipid metabolism through its catalytic role in the degradation of glucosylceramide. However, the mechanism by which GBA2 variants lead to the development of SA is still unclear.

METHODS

In this work, we perform next-generation RNA-sequencing (RNA-seq), in an attempt to discover differentially expressed genes (DEGs) in lymphoblastoid, fibroblast cell lines and induced pluripotent stem cell-derived neurons derived from patients with SA, homozygous for the GBA2 c.1780G > C missense variant. We further exploit DEGs in pathway analyses in order to elucidate candidate molecular mechanisms that are implicated in the development of the GBA2 gene-associated SA.

RESULTS

Our data reveal a total of 5217 genes with significantly altered expression between patient and control tested tissues. Furthermore, the most significant extracted pathways are presented and discussed for their possible role in the pathogenesis of the disease. Among them are the oxidative stress, neuroinflammation, sphingolipid signaling and metabolism, PI3K-Akt and MAPK signaling pathways.

CONCLUSIONS

Overall, our work examines for the first time the transcriptome profiles of GBA2-associated SA patients and suggests pathways and pathway synergies that could possibly have a role in SA pathogenesis. Lastly, it provides a list of DEGs and pathways that could be further validated towards the discovery of disease biomarkers.

Impact of Gba2 on neuronopathic Gaucher's disease and α-synuclein accumulation in medaka (Oryzias latipes)

Homozygous mutations in the lysosomal glucocerebrosidase gene, GBA1, cause Gaucher’s disease (GD), while heterozygous mutations in GBA1 are a strong risk factor for Parkinson’s disease (PD), whose pathological hallmark is intraneuronal α-synuclein (asyn) aggregates. We previously reported that gba1 knockout (KO) medaka exhibited glucosylceramide accumulation and neuronopathic GD phenotypes, including short lifespan, the dopaminergic and noradrenergic neuronal cell loss, microglial activation, and swimming abnormality, with asyn accumulation in the brains. A recent study reported that deletion of GBA2, non-lysosomal glucocerebrosidase, in a non-neuronopathic GD mouse model rescued its phenotypes. In the present study, we generated gba2 KO medaka and examined the effect of Gba2 deletion on the phenotypes of gba1 KO medaka. The Gba2 deletion in gba1 KO medaka resulted in the exacerbation of glucosylceramide accumulation and no improvement in neuronopathic GD pathological changes, asyn accumulation, or swimming abnormalities. Meanwhile, though gba2 KO medaka did not show any apparent phenotypes, biochemical analysis revealed asyn accumulation in the brains. gba2 KO medaka showed a trend towards an increase in sphingolipids in the brains, which is one of the possible causes of asyn accumulation. In conclusion, this study demonstrated that the deletion of Gba2 does not rescue the pathological changes or behavioral abnormalities of gba1 KO medaka, and GBA2 represents a novel factor affecting asyn accumulation in the brains.

Effect of Expression of Human Glucosylceramidase 2 Isoforms on Lipid Profiles in COS-7 Cells

Glucosylceramide (GlcCer) is a major membrane lipid and the precursor of gangliosides. GlcCer is mainly degraded by two enzymes, lysosomal acid β-glucosidase (GBA) and nonlysosomal β-glucosidase (GBA2), which may have different isoforms because of alternative splicing. To understand which GBA2 isoforms are active and how they affect glycosphingolipid levels in cells, we expressed nine human GBA2 isoforms in COS-7 cells, confirmed their expression by qRT-PCR and Western blotting, and assayed their activity to hydrolyze 4-methylumbelliferyl-β-D-glucopyranoside (4MUG) in cell extracts. Human GBA2 isoform 1 showed high activity, while the other isoforms had activity similar to the background. Comparison of sphingolipid levels by ultra-high resolution/accurate mass spectrometry (UHRAMS) analysis showed that isoform 1 overexpression increased ceramide and decreased hexosylceramide levels. Comparison of ratios of glucosylceramides to the corresponding ceramides in the extracts indicated that GBA2 isoform 1 has broad specificity for the lipid component of glucosylceramide, suggesting that only one GBA2 isoform 1 is active and affects sphingolipid levels in the cell. Our study provides new insights into how increased breakdown of GlcCer affects cellular lipid metabolic networks.

Analyzing Gene Expression Profiles from Ataxia and Spasticity Phenotypes to Reveal Spastic Ataxia Related Pathways

Spastic ataxia (SA) is a group of rare neurodegenerative diseases, characterized by mixed features of generalized ataxia and spasticity. The pathogenetic mechanisms that drive the development of the majority of these diseases remain unclear, although a number of studies have highlighted the involvement of mitochondrial and lipid metabolism, as well as calcium signaling. Our group has previously published the GBA2 c.1780G > C (p.Asp594His) missense variant as the cause of spastic ataxia in a Cypriot consanguineous family, and more recently the biochemical characterization of this variant in patients’ lymphoblastoid cell lines. GBA2 is a crucial enzyme of sphingolipid metabolism. However, it is unknown if GBA2 has additional functions and therefore additional pathways may be involved in the disease development. The current study introduces bioinformatics approaches to better understand the pathogenetic mechanisms of the disease. We analyzed publicly available human gene expression datasets of diseases presented with ‘ataxia’ or ‘spasticity’ in their clinical phenotype and we performed pathway analysis in order to: (a) search for candidate perturbed pathways of SA; and (b) evaluate the role of sphingolipid signaling pathway and sphingolipid metabolism in the disease development, through the identification of differentially expressed genes in patients compared to controls. Our results demonstrate consistent differential expression of genes that participate in the sphingolipid pathways and highlight alterations in the pathway level that might be associated with the disease phenotype. Through enrichment analysis, we discuss additional pathways that are connected to sphingolipid pathways, such as PI3K-Akt signaling, MAPK signaling, calcium signaling, and lipid and carbohydrate metabolism as the most enriched for ataxia and spasticity phenotypes.

Lipid metabolic pathways converge in motor neuron degenerative diseases

Motor neuron diseases (MNDs) encompass an extensive and heterogeneous group of upper and/or lower motor neuron degenerative disorders, in which the particular clinical outcomes stem from the specific neuronal component involved in each condition. While mutations in a large number of molecules associated with lipid metabolism are known to be implicated in MNDs, there remains a lack of clarity regarding the key functional pathways involved, and their inter-relationships. This review highlights evidence that defines defects within two specific lipid (cholesterol/oxysterol and phosphatidylethanolamine) biosynthetic cascades as being centrally involved in MND, particularly hereditary spastic paraplegia. We also identify how other MND-associated molecules may impact these cascades, in particular through impaired organellar interfacing, to propose ‘subcellular lipidome imbalance’ as a likely common pathomolecular theme in MND. Further exploration of this mechanism has the potential to identify new therapeutic targets and management strategies for modulation of disease progression in hereditary spastic paraplegias and other MNDs.

Dystonia as initial presentation of compound heterozygous GBA2 mutations: Expanding the phenotypic spectrum of SPG46

GBA2 associated spastic paraplegia type 46 (SPG46) is an autosomal-recessive disorder associated with a clinical presentation of spastic gait, muscle weakness as well as an array of clinical symptoms including pseudobulbar palsy and progressive cognitive decline. Several neurological and non-neurological symptoms are associated with GBA2 mutations. An initial presentation with dystonia has not been reported so far. We report clinical, genetic and brain imaging findings in two siblings with hereditary spastic paraparesis. One sister presented with juvenile-onset leg spasticity and progressed to spastic tetraparesis, cervical and jaw opening dystonia, pseudobulbar symptoms and dementia. The other sister initially developed cervical dystonia in adulthood followed by gait spasticity and cognitive decline in the disease course. Molecular genetic testing revealed novel compound heterozygous variants in GBA2 in both sisters. The initial presentation with cervical dystonia and the differing clinical disease progression expand the clinical phenotype of GBA2 associated SPG46.

Truncated mutants of beta-glucosidase 2 (GBA2) are localized in the mitochondrial matrix and cause mitochondrial fragmentation

The enzyme β-glucosidase 2 (GBA2) is clinically relevant because it is targeted by the drug miglustat (Zavesca^®) and because it is involved in inherited diseases. Mutations in the GBA2 gene are associated with two neurological diseases on the ataxia-spasticity spectrum, hereditary spastic paraplegia 46 (SPG46) and Marinesco-Sjögren-like syndrome (MSS). To establish how GBA2 mutations give rise to neurological pathology, we have begun to investigate mutant forms of GBA2 encoded by disease-associated GBA2 alleles. Previously, we found that five GBA2 missense mutants and five C-terminally truncated mutants lacked enzyme activity. Here we have examined the cellular locations of wild-type (WT) and mutant forms of GBA2 by confocal and electron microscopy, using transfected cells. Similar to GBA2-WT, the D594H and M510Vfs*17 GBA2 mutants were located at the plasma membrane, whereas the C-terminally truncated mutants terminating after amino acids 233 and 339 (GBA2-233 and -339) were present in the mitochondrial matrix, induced mitochondrial fragmentation and loss of mitochondrial transmembrane potential. Deletional mutagenesis indicated that residues 161–200 are critical for the mitochondrial fragmentation of GBA2-233 and -339. Considering that the mitochondrial fragmentation induced by GBA2-233 and -339 is consistently accompanied by their localization to the mitochondrial matrix, our deletional analysis raises the possibility that that GBA2 residues 161–200 harbor an internal targeting sequence for transport to the mitochondrial matrix. Altogether, our work provides new insights into the behaviour of GBA2-WT and disease-associated forms of GBA2.

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.

Lipids in the Physiopathology of Hereditary Spastic Paraplegias

Hereditary spastic paraplegias (HSP) are a group of neurodegenerative diseases sharing spasticity in lower limbs as common symptom. There is a large clinical variability in the presentation of patients, partly underlined by the large genetic heterogeneity, with more than 60 genes responsible for HSP. Despite this large heterogeneity, the proteins with known function are supposed to be involved in a limited number of cellular compartments such as shaping of the endoplasmic reticulum or endolysosomal function. Yet, it is difficult to understand why alteration of such different cellular compartments can lead to degeneration of the axons of cortical motor neurons. A common feature that has emerged over the last decade is the alteration of lipid metabolism in this group of pathologies. This was first revealed by the identification of mutations in genes encoding proteins that have or are supposed to have enzymatic activities on lipid substrates. However, it also appears that mutations in genes affecting endoplasmic reticulum, mitochondria, or endolysosome function can lead to changes in lipid distribution or metabolism. The aim of this review is to discuss the role of lipid metabolism alterations in the physiopathology of HSP, to evaluate how such alterations contribute to neurodegenerative phenotypes, and to understand how this knowledge can help develop therapeutic strategy for HSP.

A case of motor neuron involvement in Gaucher disease

Gaucher disease (GD) is a genetic disorder characterized by an accumulation of glucosylceramide in cells in the monocyte-macrophage system. We describe a case of a 33-year-old man with a previous diagnosis of type 3 GD who displayed a progressive weakening of the limbs followed by upper motor neuron involvement. A diagnosis of definite Amyotrophic Lateral Sclerosis was made. This is the first reported case of concurrent Gaucher disease and the ALS phenotype in the same patient.

Complexity of Generating Mouse Models to Study the Upper Motor Neurons: Let Us Shift Focus from Mice to Neurons

Motor neuron circuitry is one of the most elaborate circuitries in our body, which ensures voluntary and skilled movement that requires cognitive input. Therefore, both the cortex and the spinal cord are involved. The cortex has special importance for motor neuron diseases, in which initiation and modulation of voluntary movement is affected. Amyotrophic lateral sclerosis (ALS) is defined by the progressive degeneration of both the upper and lower motor neurons, whereas hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS) are characterized mainly by the loss of upper motor neurons. In an effort to reveal the cellular and molecular basis of neuronal degeneration, numerous model systems are generated, and mouse models are no exception. However, there are many different levels of complexities that need to be considered when developing mouse models. Here, we focus our attention to the upper motor neurons, which are one of the most challenging neuron populations to study. Since mice and human differ greatly at a species level, but the cells/neurons in mice and human share many common aspects of cell biology, we offer a solution by focusing our attention to the affected neurons to reveal the complexities of diseases at a cellular level and to improve translational efforts.

Autosomal recessive cerebellar ataxia with spasticity due to a rare mutation in GBA2 gene in a large consanguineous Saudi family

The nonlysosomal glucosylceramidase β2 (GBA2) gene encode an enzyme that catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Mutations in the GBA2 gene have been reported to cause hereditary spastic paraplegia, autosomal recessive cerebellar ataxia with spasticity, and Marinescu-Sjögren-Like Syndrome. In this study, we report the clinical features and genetic diagnosis of autosomal recessive cerebellar ataxia with spasticity due to a rare mutation in GBA2 gene in a large consanguineous Saudi family. We included a large consanguineous Saudi family with a presumptive clinical diagnosis of ataxia at King Abdulaziz Medical City in Jeddah, Saudi Arabia. The family included six affected individuals and four unaffected in addition to the parents. Whole exome sequencing (WES) was performed for the proband IV-5, and Sanger sequencing was used to confirm the variant in other family members. Segregation study was performed using DNA from the parents and siblings of the proband. Sequence analysis identified a homozygous variant c.2618G>A, p.(Arg873His) in GBA2 gene. The homozygous variant was identified in affected members of the family while the parents and the other four siblings were heterozygous carriers of the variant. One sibling was not available for genetic testing. The variant identified in our patients is classified as pathogenic considering the current evidence of the variant. Autosomal recessive cerebellar ataxia with spasticity is an extremely rare genetic disorder with very few cases reported in the literature. We conclude that the c.2617G>A mutation in GBA2 gene causes the loss of function with abolishment of the enzymatic activity that causes the disease. This report adds further evidence to support the pathogenicity of this variant. The patients had the classical clinical phenotype of cerebellar ataxia and spasticity consistent with previous reports in the literature.

Assay of β-glucosidase 2 (GBA2) activity using lithocholic acid β-3-O-glucoside substrate for cultured fibroblasts and glucosylceramide for brain tissue

Beta (β)-glucosidase 2 (GBA2) is deficient in a form of human spastic paraplegia due to defects in GBA2 (SPG46). GBA2 was proposed as a modifier of Gaucher disease, a lysosomal storage disease resulting from deficient β-glucosidase 1; GBA1. Current GBA2 activity assays using artificial substrates incompletely model the activity encountered in vivo. We studied GBA2 activity, using lithocholic acid β-glucoside or glucosylceramide as natural β-glucosidase substrates in murine tissues or cultured patient fibroblasts with the pathologic genotypes: Gba1-/-; Gba2-/-; GBA1-/-; GBA2+/- and found expected and unexpected deviations from normal controls.

Species-specific differences in nonlysosomal glucosylceramidase GBA2 function underlie locomotor dysfunction arising from loss-of-function mutations

The nonlysosomal glucosylceramidase β2 (GBA2) catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Mutations in the human GBA2 gene have been associated with hereditary spastic paraplegia (HSP), autosomal-recessive cerebellar ataxia (ARCA), and the Marinesco-Sjögren-like syndrome. However, the underlying molecular mechanisms are ill-defined. Here, using biochemistry, immunohistochemistry, structural modeling, and mouse genetics, we demonstrate that all but one of the spastic gait locus #46 (SPG46)-connected mutations cause a loss of GBA2 activity. We demonstrate that GBA2 proteins form oligomeric complexes and that protein-protein interactions are perturbed by some of these mutations. To study the pathogenesis of GBA2-related HSP and ARCA in vivo, we investigated GBA2-KO mice as a mammalian model system. However, these mice exhibited a high phenotypic variance and did not fully resemble the human phenotype, suggesting that mouse and human GBA2 differ in function. Whereas some GBA2-KO mice displayed a strong locomotor defect, others displayed only mild alterations of the gait pattern and no signs of cerebellar defects. On a cellular level, inhibition of GBA2 activity in isolated cerebellar neurons dramatically affected F-actin dynamics and reduced neurite outgrowth, which has been associated with the development of neurological disorders. Our results shed light on the molecular mechanism underlying the pathogenesis of GBA2-related HSP and ARCA and reveal species-specific differences in GBA2 function in vivo.

Distinguishing the differences in β-glycosylceramidase folds, dynamics, and actions informs therapeutic uses

Glycosyl hydrolases (GHs) are carbohydrate-active enzymes that hydrolyze a specific β-glycosidic bond in glycoconjugate substrates; β-glucosidases degrade glucosylceramide, a ubiquitous glycosphingolipid. GHs are grouped into structurally similar families that themselves can be grouped into clans. GH1, GH5, and GH30 glycosidases belong to clan A hydrolases with a catalytic (β/α)8 TIM barrel domain, whereas GH116 belongs to clan O with a catalytic (α/α)6 domain. In humans, GH abnormalities underlie metabolic diseases. The lysosomal enzyme glucocerebrosidase (family GH30), deficient in Gaucher disease and implicated in Parkinson disease etiology, and the cytosol-facing membrane-bound glucosylceramidase (family GH116) remove the terminal glucose from the ceramide lipid moiety. Here, we compare enzyme differences in fold, action, dynamics, and catalytic domain stabilization by binding site occupancy. We also explore other glycosidases with reported glycosylceramidase activity, including human cytosolic β-glucosidase, intestinal lactase-phlorizin hydrolase, and lysosomal galactosylceramidase. Last, we describe the successful translation of research to practice: recombinant glycosidases and glucosylceramide metabolism modulators are approved drug products (enzyme replacement therapies). Activity-based probes now facilitate the diagnosis of enzyme deficiency and screening for compounds that interact with the catalytic pocket of glycosidases. Future research may deepen the understanding of the functional variety of these enzymes and their therapeutic potential.

Biochemical Characterization of the GBA2 c.1780G>C Missense Mutation in Lymphoblastoid Cells from Patients with Spastic Ataxia

The GBA2 gene encodes the non-lysosomal glucosylceramidase (NLGase), an enzyme that catalyzes the conversion of glucosylceramide (GlcCer) to ceramide and glucose. Mutations in GBA2 have been associated with the development of neurological disorders such as autosomal recessive cerebellar ataxia, hereditary spastic paraplegia, and Marinesco-Sjogren-Like Syndrome. Our group has previously identified the GBA2 c.1780G>C [p.Asp594His] missense mutation, in a Cypriot consanguineous family with spastic ataxia. In this study, we carried out a biochemical characterization of lymphoblastoid cell lines (LCLs) derived from three patients of this family. We found that the mutation strongly reduce NLGase activity both intracellularly and at the plasma membrane level. Additionally, we observed a two-fold increase of GlcCer content in LCLs derived from patients compared to controls, with the C₁₆ lipid being the most abundant GlcCer species. Moreover, we showed that there is an apparent compensatory effect between NLGase and the lysosomal glucosylceramidase (GCase), since we found that the activity of GCase was three-fold higher in LCLs derived from patients compared to controls. We conclude that the c.1780G>C mutation results in NLGase loss of function with abolishment of the enzymatic activity and accumulation of GlcCer accompanied by a compensatory increase in GCase.

The Enigmatic Role of GBA2 in Controlling Locomotor Function

The non-lysosomal glucosylceramidase GBA2 catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Loss of GBA2 function results in accumulation of glucosylceramide. Mutations in the human GBA2 gene have been associated with hereditary spastic paraplegia (HSP) and autosomal-recessive cerebellar ataxia (ARCA). Patients suffering from these disorders exhibit impaired locomotion and neurological abnormalities. GBA2 mutations found in these patients have been proposed to impair GBA2 function. However, the molecular mechanism underlying the occurrence of mutations in the GBA2 gene and the development of locomotor dysfunction is not well-understood. In this review, we aim to summarize recent findings regarding mutations in the GBA2 gene and their impact on GBA2 function in health and disease.

Lipid Involvement in Neurodegenerative Diseases of the Motor System: Insights from Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) are a heterogeneous group of rare inherited metabolic diseases that are frequently triggered by the accumulation of lipids inside organelles of the endosomal-autophagic-lysosomal system (EALS). There is now a growing realization that disrupted lysosomal homeostasis (i.e., lysosomal cacostasis) also contributes to more common neurodegenerative disorders such as Parkinson disease (PD). Lipid deposition within the EALS may also participate in the pathogenesis of some additional neurodegenerative diseases of the motor system. Here, I will highlight the lipid abnormalities and clinical manifestations that are common to LSDs and several diseases of the motor system, including amyotrophic lateral sclerosis (ALS), atypical forms of spinal muscular atrophy, Charcot-Marie-Tooth disease (CMT), hereditary spastic paraplegia (HSP), multiple system atrophy (MSA), PD and spinocerebellar ataxia (SCA). Elucidating the underlying basis of intracellular lipid mislocalization as well as its consequences in each of these disorders will likely provide innovative targets for therapeutic research.

Inhibition of β-Glucocerebrosidase Activity Preserves Motor Unit Integrity in a Mouse Model of Amyotrophic Lateral Sclerosis

Recent metabolomic reports connect dysregulation of glycosphingolipids, particularly ceramide and glucosylceramide, to neurodegeneration and to motor unit dismantling in amyotrophic lateral sclerosis at late disease stage. We report here altered levels of gangliosides in the cerebrospinal fluid of amyotrophic lateral sclerosis patients in early disease stage. Conduritol B epoxide is an inhibitor of acid beta-glucosidase, and lowers glucosylceramide degradation. Glucosylceramide is the precursor for all of the more complex glycosphingolipids. In SOD1G86R mice, an animal model of amyotrophic lateral sclerosis, conduritol B epoxide preserved ganglioside distribution at the neuromuscular junction, delayed disease onset, improved motor function and preserved motor neurons as well as neuromuscular junctions from degeneration. Conduritol B epoxide mitigated gene dysregulation in the spinal cord and restored the expression of genes involved in signal transduction and axonal elongation. Inhibition of acid beta-glucosidase promoted faster axonal elongation in an in vitro model of neuromuscular junctions and hastened recovery after peripheral nerve injury in wild type mice. Here, we provide evidence that glycosphingolipids play an important role in muscle innervation, which degenerates in amyotrophic lateral sclerosis from the early disease stage. This is a first proof of concept study showing that modulating the catabolism of glucosylceramide may be a therapeutic target for this devastating disease.

Overcoming the divide between ataxias and spastic paraplegias: Shared phenotypes, genes, and pathways

Autosomal-dominant spinocerebellar ataxias, autosomal-recessive spinocerebellar ataxias, and hereditary spastic paraplegias have traditionally been designated in separate clinicogenetic disease classifications. This classification system still largely frames clinical thinking and genetic workup in clinical practice. Yet, with the advent of next-generation sequencing, phenotypically unbiased studies have revealed the limitations of this classification system. Various genes (eg, SPG7, SYNE1, PNPLA6) traditionally rooted in either the ataxia or hereditary spastic paraplegia classification system have now been shown to cause ataxia on the one end of the disease continuum and hereditary spastic paraplegia on the other. Other genes such as GBA2 and KIF1C were almost simultaneously published as both a hereditary spastic paraplegia and an ataxia gene. The variability and fluidity of observed phenotypes along the ataxia-spasticity spectrum warrants a rethinking of the traditional classification system. We propose to replace this divisive diagnosis-driven ataxia and hereditary spastic paraplegia classification system by a descriptive, unbiased approach of modular phenotyping. This approach is also open to expansion of the phenotype beyond ataxia and spasticity, which often occur as part of broader multisystem neuronal dysfunction. The concept of a continuous ataxia-spasticity disease spectrum is further supported by ataxias and hereditary spastic paraplegias sharing not only overlapping phenotypes and underlying genes, but also common cellular pathways and disease mechanisms. This suggests a shared vulnerability of cerebellar and corticospinal neurons for common pathophysiological processes. It might be this mechanistic overlap that drives their clinical overlap. A mechanistically inspired classification system will help to pave the way for mechanism-based strategies for drug development. © 2017 International Parkinson and Movement Disorder Society.

Bacterial β-Glucosidase Reveals the Structural and Functional Basis of Genetic Defects in Human Glucocerebrosidase 2 (GBA2)

Human glucosylcerebrosidase 2 (GBA2) of the CAZy family GH116 is responsible for the breakdown of glycosphingolipids on the cytoplasmic face of the endoplasmic reticulum and Golgi apparatus. Genetic defects in GBA2 result in spastic paraplegia and cerebellar ataxia, while cross-talk between GBA2 and GBA1 glucosylceramidases may affect Gaucher disease. Here, we report the first three-dimensional structure for any GH116 enzyme, Thermoanaerobacterium xylanolyticum TxGH116 β-glucosidase, alone and in complex with diverse ligands. These structures allow identification of the glucoside binding and active site residues, which are shown to be conserved with GBA2. Mutagenic analysis of TxGH116 and structural modeling of GBA2 provide a detailed structural and functional rationale for pathogenic missense mutations of GBA2.

Characterization of the Zebrafish Homolog of β-Glucosidase 2: A Target of the Drug Miglustat

The small-molecular compound miglustat (N-butyldeoxynojirimycin, Zavesca(®)) has been approved for clinical use in type 1 Gaucher disease and Niemann-Pick type C disease, which are disorders caused by dysfunction of the endosomal-autophagic-lysosomal system. Miglustat inhibits a number of enzymes involved in glycoconjugate and glycan metabolism, including β-glucosidase 2 (GBA2), which is exceptionally sensitive to inhibition by miglustat. GBA2 is a glucosylceramide-degrading enzyme that is located on the plasma membrane/endoplasmic reticulum, and is distinct from the lysosomal enzyme glucocerebrosidase (GBA). Various strands of evidence suggest that inhibition of GBA2 contributes to the therapeutic benefits of miglustat. To further explore the pharmacology and biology of GBA2, we investigated whether the zebrafish homolog of GBA2 has similar enzymatic properties and pharmacological sensitivities to its human counterpart. We established that zebrafish has endogenous β-glucosidase activity toward lipid- and water-soluble GBA2 substrates, which can be inhibited by miglustat, N-butyldeoxygalactonojirimycin, and conduritol B epoxide. β-Glucosidase activities with highly similar characteristics were expressed in cells transfected with the zebrafish gba2 cDNA and in cells transfected with the human GBA2 cDNA. These results provide a foundation for the use of zebrafish in screening GBA2-targeting molecules, and for wider studies investigating GBA2 biology.