Acid beta-glucosidase: enzymology and molecular biology of Gaucher disease

Gaucher disease provides a unique window into Parkinson disease pathogenesis

An exciting development in the field of neurodegeneration is the association between the rare monogenic disorder Gaucher disease and the common complex disorder Parkinson disease (PD). Gaucher disease is a lysosomal storage disorder resulting from an inherited deficiency of the enzyme glucocerebrosidase, encoded by GBA1, which hydrolyses the glycosphingolipids glucosylceramide and glucosylsphingosine. The observation of parkinsonism in a rare subgroup of individuals with Gaucher disease first directed attention to the role of glucocerebrosidase deficiency in the pathogenesis of PD. PD occurs more frequently in people heterozygous for Gaucher GBA1 mutations, and 3-25% of people with Parkinson disease carry a GBA1 variant. However, only a small percentage of individuals with GBA1 variants develop parkinsonism, suggesting that the penetrance is low. Despite over a decade of intense research in this field, including clinical and radiological evaluations, genetic studies and investigations using model systems, the mechanism underlying GBA1-PD is still being pursued. Insights from this association have emphasized the role of lysosomal pathways in parkinsonism. Furthermore, different therapeutic strategies considered or developed for Gaucher disease can now inform drug development for PD.

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.

Glycosidase-targeting small molecules for biological and therapeutic applications

Glycosidases are ubiquitous enzymes that catalyze the hydrolysis of glycosidic linkages in oligosaccharides and glycoconjugates. These enzymes play a vital role in a wide variety of biological events, such as digestion of nutritional carbohydrates, lysosomal catabolism of glycoconjugates, and posttranslational modifications of glycoproteins. Abnormal glycosidase activities are associated with a variety of diseases, particularly cancer and lysosomal storage disorders. Owing to the physiological and pathological significance of glycosidases, the development of small molecules that target these enzymes is an active area in glycoscience and medicinal chemistry. Research efforts carried out thus far have led to the discovery of numerous glycosidase-targeting small molecules that have been utilized to elucidate biological processes as well as to develop effective chemotherapeutic agents. In this review, we describe the results of research studies reported since 2018, giving particular emphasis to the use of fluorescent probes for detection and imaging of glycosidases, activity-based probes for covalent labelling of these enzymes, glycosidase inhibitors, and glycosidase-activatable prodrugs.

LRRK2 and Parkinson's disease: from genetics to targeted therapy

LRRK2 variants are implicated in both familial and sporadic PD. LRRK2-PD has a generally benign clinical presentation and variable pathology, with inconsistent presence of Lewy bodies and marked Alzheimer's disease pathology. The mechanisms underlying LRRK2-PD are still unclear, but inflammation, vesicle trafficking, lysosomal homeostasis, and ciliogenesis have been suggested, among others. As novel therapies targeting LRRK2 are under development, understanding the role and function of LRRK2 in PD is becoming increasingly important. Here, we outline the epidemiological, pathophysiological, and clinical features of LRRK2-PD, and discuss the arising therapeutic approaches targeting LRRK2 and possible future directions for research.

Glucocerebrosidase mutations and Parkinson disease

The discovery of glucocerebrosidase (GBA1) mutations as the greatest numerical genetic risk factor for the development of Parkinson disease (PD) resulted in a paradigm shift within the research landscape. Efforts to elucidate the mechanisms behind GBA1-associated PD have highlighted shared pathways in idiopathic PD including the loss and gain-of-function hypotheses, endoplasmic reticulum stress, lipid metabolism, neuroinflammation, mitochondrial dysfunction and altered autophagy-lysosomal pathway responsible for degradation of aggregated and misfolded a-synuclein. GBA1-associated PD exhibits subtle differences in phenotype and disease progression compared to idiopathic counterparts notably an earlier age of onset, faster motor decline and greater frequency of non-motor symptoms (which also constitute a significant aspect of the prodromal phase of the disease). GBA1-targeted therapies have been developed and are being investigated in clinical trials. The most notable are Ambroxol, a small molecule chaperone, and Venglustat, a blood-brain-barrier-penetrant substrate reduction therapy agent. It is imperative that further studies clarify the aetiology of GBA1-associated PD, enabling the development of a greater abundance of targeted therapies in this new era of precision medicine.

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.

Photoswitchable inhibitors of human β-glucocerebrosidase

Light-switchable inhibitors of the enzyme β-glucocerebrosidase (GCase) have been developed by anchoring a specific azasugar to a dihydroazulene or an azobenzene responsive moiety. Their inhibitory effect towards human GCase, before and after irradiation are reported, and the effect on thermal denaturation of recombinant GCase and cytotoxicity were studied on selected candidates.

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.

The interplay between Glucocerebrosidase, α-synuclein and lipids in human models of Parkinson's disease

Mutations in the gene GBA, encoding glucocerebrosidase (GCase), are the highest genetic risk factor for Parkinson's disease (PD). GCase is a lysosomal glycoprotein responsible for the hydrolysis of glucosylceramide into glucose and ceramide. Mutations in GBA cause a decrease in GCase activity, stability and protein levels which in turn lead to the accumulation of GCase lipid substrates as well as α-synuclein (αS) in vitro and in vivo. αS is the main constituent of Lewy bodies found in the brain of PD patients and an increase in its levels was found to be associated with a decrease in GCase activity/protein levels in vitro and in vivo. In this review, we describe the reported biophysical and biochemical changes that GBA mutations can induce in GCase activity and stability as well as the current overview of the levels of GCase protein/activity, αS and lipids measured in patient-derived samples including post-mortem brains, stem cell-derived neurons, cerebrospinal fluid, blood and fibroblasts as well as in SH-SY5Y cells. In particular, we report how the levels of αS and lipids are affected by/correlated to significant changes in GCase activity/protein levels and which cellular pathways are activated or disrupted by these changes in each model. Finally, we review the current strategies used to revert the changes in the levels of GCase activity/protein, αS and lipids in the context of PD.

Enzyme kinetics and inhibition parameters of human leukocyte glucosylceramidase

Glucosylceramidase (GCase) is a lysosomal enzyme that catalyzes the cleavage of β-glucosidic linkage of glucocerebroside (GC) into glucose and ceramide; thereby, plays an essential function in the degradation of complex lipids and the turnover of cellular membranes. The growing list of 460 mutations in the gene coding for it-glucosylceramidase beta acid 1 (GBA1)-is reported to abolish its catalytic activity and decrease its enzyme stability, associating it with severe health conditions such as Gaucher disease (GD), Parkinson Disease (PD) and Dementia with Lewy bodies (DLB). Although the three-dimensional structure of wild type glucosylceramidase is elucidated, little is known about its features in human cells. Moreover, alternative sources of GCase that prove to be effective in the treatment of diseases with enzyme treatment therapies, impose the need for a simple and cost-effective procedure to study the enzyme behavior. This work, for the first time, shows a well-established, yet simple, cost- and time-efficient protocol for the study of GCase enzyme in human leukocytes by the artificial substrate p-Nitrophenyl-β-D-glucopyranoside (PNPG). Characterization of the enzyme in human leukocytes for activation parameters (optimal pH, Km, and Vmax) and enzyme inhibition was done. The results indicate that the optimum pH of GCase enzyme with PNPG is 5.0. The Km and Vmax values are 12.6mM and 333 U/mg, respectively. Gluconolactone competitively inhibits GCase, with a Ki value of 0.023 mM and IC50 of 0.047 mM. Glucose inhibition is uncompetitive with a Ki of 1.94 mM and IC50 of 55.3 mM. This is the first report for the inhibitory effect of glucose, δ-gluconolactone on human leukocyte GCase activity.

Synthesis of "All-Cis" Trihydroxypiperidines from a Carbohydrate-Derived Ketone: Hints for the Design of New β-Gal and GCase Inhibitors

Pharmacological chaperones (PCs) are small compounds able to rescue the activity of mutated lysosomal enzymes when used at subinhibitory concentrations. Nitrogen-containing glycomimetics such as aza- or iminosugars are known to behave as PCs for lysosomal storage disorders (LSDs). As part of our research into lysosomal sphingolipidoses inhibitors and looking in particular for new β-galactosidase inhibitors, we report the synthesis of a series of alkylated azasugars with a relative “all-cis” configuration at the hydroxy/amine-substituted stereocenters. The novel compounds were synthesized from a common carbohydrate-derived piperidinone intermediate 8, through reductive amination or alkylation of the derived alcohol. In addition, the reaction of ketone 8 with several lithium acetylides allowed the stereoselective synthesis of new azasugars alkylated at C-3. The activity of the new compounds towards lysosomal β-galactosidase was negligible, showing that the presence of an alkyl chain in this position is detrimental to inhibitory activity. Interestingly, 9, 10, and 12 behave as good inhibitors of lysosomal β-glucosidase (GCase) (IC₅₀ = 12, 6.4, and 60 µM, respectively). When tested on cell lines bearing the Gaucher mutation, they did not impart any enzyme rescue. However, altogether, the data included in this work give interesting hints for the design of novel inhibitors.

Neuronal Soma-Derived Degradative Lysosomes Are Continuously Delivered to Distal Axons to Maintain Local Degradation Capacity

Neurons face the challenge of maintaining cellular homeostasis through lysosomal degradation. While enzymatically active degradative lysosomes are enriched in the soma, their axonal trafficking and positioning and impact on axonal physiology remain elusive. Here, we characterized axon-targeted delivery of degradative lysosomes by applying fluorescent probes that selectively label active forms of lysosomal cathepsins D, B, L, and GCase. By time-lapse imaging of cortical neurons in microfluidic devices and standard dishes, we reveal that soma-derived degradative lysosomes rapidly influx into distal axons and target to autophagosomes and Parkinson disease-related α-synuclein cargos for local degradation. Impairing lysosome axonal delivery induces an aberrant accumulation of autophagosomes and α-synuclein cargos in distal axons. Our study demonstrates that the axon is an active compartment for local degradation and reveals fundamental aspects of axonal lysosomal delivery and maintenance. Our work establishes a foundation for investigations into axonal lysosome trafficking and functionality in neurodegenerative diseases.

Upregulating β-hexosaminidase activity in rodents prevents α-synuclein lipid associations and protects dopaminergic neurons from α-synuclein-mediated neurotoxicity

Sandhoff disease (SD) is a lysosomal storage disease, caused by loss of β-hexosaminidase (HEX) activity resulting in the accumulation of ganglioside GM2. There are shared features between SD and Parkinson's disease (PD). α-synuclein (aSYN) inclusions, the diagnostic hallmark sign of PD, are frequently found in the brain in SD patients and HEX knockout mice, and HEX activity is reduced in the substantia nigra in PD. In this study, we biochemically demonstrate that HEX deficiency in mice causes formation of high-molecular weight (HMW) aSYN and ubiquitin in the brain. As expected from HEX enzymatic function requirements, overexpression in vivo of HEXA and B combined, but not either of the subunits expressed alone, increased HEX activity as evidenced by histochemical assays. Biochemically, such HEX gene expression resulted in increased conversion of GM2 to its breakdown product GM3. In a neurodegenerative model of overexpression of aSYN in rats, increasing HEX activity by AAV6 gene transfer in the substantia nigra reduced aSYN embedding in lipid compartments and rescued dopaminergic neurons from degeneration. Overall, these data are consistent with a paradigm shift where lipid abnormalities are central to or preceding protein changes typically associated with PD.

Glucocerebrosidase (GCase) activity modulation by 2-alkyl trihydroxypiperidines: Inhibition and pharmacological chaperoning

The enzyme glucocerebrosidase (GCase) has become an important therapeutic target due to its involvement in pathological disorders consequent to enzyme deficiency, such as the lysosomal storage Gaucher disease (GD) and the neurological Parkinson disease (PD). Pharmacological chaperones (PCs) are small compounds able to stabilize enzymes when used at sub-inhibitory concentrations, thus rescuing enzyme activity. We report the stereodivergent synthesis of trihydroxypiperidines alkylated at C-2 with both configurations, by means of the stereoselective addition of Grignard reagents to a carbohydrate-derived nitrone in the presence or absence of Lewis acids. All the target compounds behave as good GCase inhibitors, with IC₅₀ in the micromolar range. Moreover, compound 11a behaves as a PC in fibroblasts derived from Gaucher patients bearing the N370/RecNcil mutation and the homozygous L444P mutation, rescuing the activity of the deficient enzyme by up to 1.9- and 1.8-fold, respectively. Rescues of 1.2-1.4-fold were also observed in wild-type fibroblasts, which is important for targeting sporadic forms of PD.

Mincle: 20 years of a versatile sensor of insults

Macrophage-inducible C-type lectin, better known as Mincle, is a member of the C-type lectin receptor family and is encoded by Clec4e. Mincle was an orphan receptor for a long time after having been discovered as a lipopolysaccharide-induced protein, yet later an adjuvant glycolipid in mycobacteria-trehalose dimycolate-was identified as a ligand. Ligands for Mincle were also found existing in bacteria, fungi and even mammals. When confronted with foreign elements, Mincle can recognize characteristic pathogen-associated molecular patterns, mostly glycolipids, from Mycobacterium tuberculosis and other pathogens, and thus induce immune responses against infection. To maintain self-homeostasis, Mincle can recognize lipid-based damage-associated molecular patterns, thereby monitoring the internal environment. The mechanism by which Mincle functions in the immune system is also becoming more clear along with the identification of its ligands. Being expressed widely on antigen-presenting cells, Mincle activation leads to the production of cytokines and chemokines, neutrophil infiltration and other inflammatory responses. Besides, Mincle can induce acquired immunity such as antigen-specific T-cell responses and antibody production as an adjuvant receptor. In this review, we will retrospectively sketch the discovery and study of Mincle, and outline some current work on this receptor.

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.

Bone manifestations in neuronopathic Gaucher disease while receiving high-dose enzyme replacement therapy

Avascular necrosis (AVN), one type of bone infarction, is a major irreversible complication of Gaucher disease (GD). In this report, two pediatric patients with GD type 3, homozygous for the L483P pathogenic variant (formerly L444P), developed AVN despite treatment on long-term, high-dose enzyme replacement therapy (ERT). ERT was initiated in both patients, who had intact spleens, shortly after diagnosis with an initial dramatic response. However, both patients exhibited AVN after 5.5 and 11 years on high-dose ERT, respectively, despite good compliance and normalized hematological findings and visceral symptoms. This report demonstrates the importance of careful, regular surveillance of the musculoskeletal system in addition to monitoring the neurological symptoms associated with neuronopathic GD. Additionally, it highlights the limitations of ERT in terms of targeting certain sanctuary sites such as bone marrow and suggests the need for new treatment modalities other than ERT monotherapy to address these limitations.

Characterization of LAMP1-labeled nondegradative lysosomal and endocytic compartments in neurons

Despite widespread distribution of LAMP1 and the heterogeneous nature of LAMP1-labeled compartments, LAMP1 is routinely used as a lysosomal marker, and LAMP1-positive organelles are often referred to as lysosomes. In this study, we use immunoelectron microscopy and confocal imaging to provide quantitative analysis of LAMP1 distribution in various autophagic and endolysosomal organelles in neurons. Our study demonstrates that a significant portion of LAMP1-labeled organelles do not contain detectable lysosomal hydrolases including cathepsins D and B and glucocerebrosidase. A bovine serum albumin-gold pulse-chase assay followed by ultrastructural analysis suggests a heterogeneity of degradative capacity in LAMP1-labeled endolysosomal organelles. Gradient fractionation displays differential distribution patterns of LAMP1/2 and cathepsins D/B in neurons. We further reveal that LAMP1 intensity in familial amyotrophic lateral sclerosis-linked motor neurons does not necessarily reflect lysosomal deficits in vivo. Our study suggests that labeling a set of lysosomal hydrolases combined with various endolysosomal markers would be more accurate than simply relying on LAMP1/2 staining to assess neuronal lysosome distribution, trafficking, and functionality under physiological and pathological conditions.

Insights into the structural biology of Gaucher disease

Gaucher disease, the most common lysosomal storage disorder, is caused by mutations in the gene encoding the acid-β-glucosidase lysosomal hydrolase enzyme that cleaves glucocerebroside into glucose and ceramide. Reduced enzyme activity and impaired structural stability arise due to >300 known disease-causing mutations. Several of these mutations have also been associated with an increased risk of Parkinson disease (PD). Since the discovery of the acid-β-glucosidase X-ray structure, there have been major advances in our understanding of the structural properties of the protein. Analysis of specific residues has provided insight into their functional and structural importance and provided insight into the pathogenesis of Gaucher disease and the contribution to PD. Disease-causing mutations are positioned throughout the acid-β-glucosidase structure, with many located far from the active site and thus retaining some enzymatic activity however, thus far no clear relationship between mutation location and disease severity has been established. Here, we review the crystal structure of acid-β-glucosidase, while highlighting important structural aspects of the protein in detail. This review discusses the structural stability of acid-β-glucosidase, which can be altered by pH and glycosylation, and explores the relationship between known Gaucher disease and PD mutations, structural stability and disease severity.

Treatment of profound thrombocytopenia in a patient with Gaucher disease type 1: Is there a role for substrate reduction therapy

The availability of three enzyme replacement therapy (ERT) drugs and two substrate reduction therapy (SRT) drugs to treat Gaucher disease provides an opportunity to tailor therapies to a patient's specific clinical concerns. However, there is a gap in the literature regarding individual drug effectiveness in treating particular symptoms and the potential benefits of combination treatment.

This report details treatment of a patient with Gaucher disease type 1 whose main clinical concern was profound thrombocytopenia (around 20 × 10⁹/L, normal range: 150–450 × 10⁹/L) with several episodes of bleeding with minimal trauma and bruises. The patient was treated with ERT at doses up to 60 units/kg weekly, with no improvement in platelet levels for 6 years. Subsequently, the patient transitioned to SRT and platelet levels increased almost two fold within the first month, and have remained stable at safe levels (30–60 × 10⁹/L) for almost 2.5 years at the time of publication.

This report demonstrates a possible therapeutic benefit of SRT in individual patients who do not meet therapeutic goals in terms of thrombocytopenia after a considerable period on first-line ERT treatment. Oral administration of SRT also improved this patient's quality of life allowing discontinuation of weekly ERT infusions, which better accommodated her demanding career and busy lifestyle.

Tricyclic Antidepressants Promote Ceramide Accumulation to Regulate Collagen Production in Human Hepatic Stellate Cells

Activation of hepatic stellate cells (HSCs) in response to injury is a key step in hepatic fibrosis, and is characterized by trans-differentiation of quiescent HSCs to HSC myofibroblasts, which secrete extracellular matrix proteins responsible for the fibrotic scar. There are currently no therapies to directly inhibit hepatic fibrosis. We developed a small molecule screen to identify compounds that inactivate human HSC myofibroblasts through the quantification of lipid droplets. We screened 1600 compounds and identified 21 small molecules that induce HSC inactivation. Four hits were tricyclic antidepressants (TCAs), and they repressed expression of pro-fibrotic factors Alpha-Actin-2 (ACTA2) and Alpha-1 Type I Collagen (COL1A1) in HSCs. RNA sequencing implicated the sphingolipid pathway as a target of the TCAs. Indeed, TCA treatment of HSCs promoted accumulation of ceramide through inhibition of acid ceramidase (aCDase). Depletion of aCDase also promoted accumulation of ceramide and was associated with reduced COL1A1 expression. Treatment with B13, an inhibitor of aCDase, reproduced the antifibrotic phenotype as did the addition of exogenous ceramide. Our results show that detection of lipid droplets provides a robust readout to screen for regulators of hepatic fibrosis and have identified a novel antifibrotic role for ceramide.

Modeling non-clinical and clinical drug tests in Gaucher disease

There is need for modeling biological systems to accelerate drug pipelines for treating metabolic diseases. The eliglustat treatment for Gaucher disease is approved by the FDA with a companion genomic test. The Transcriptome-To-Metabolome™ biosimulation technology was used to model, in silico, a standard non-clinical eliglustat test with an in vitro canine kidney cell system over-expressing a human gene; and a clinical test using human fibroblasts from control and Gaucher disease subjects. Protein homology modeling and docking studies were included to gather affinity parameters for the kinetic metabolic model. Pharmacodynamics and metabolomics analyses of the results replicated published findings and demonstrated that processing and transport of lysosomal proteins alone cannot explain the metabolic disorder. This technology shows promise for application to other diseases.

Residual enzymatic activity as a prognostic factor in patients with Gaucher disease type 1: correlation with Zimran and GAUSS-I index and the severity of bone disease

BACKGROUND

Gaucher disease (GD) is an autosomal recessive disorder produced by mutations in the glucocerebrosidase gene (GBA), causing storage of glucosylceramide in reticuloendothelial cells in multiple organs. Traditionally, the prediction of the phenotype based on the genotype has been reported to be limited.

SUBJECTS AND METHODS

We investigated the correlation between the enzymatic residual activity (ERA) and the phenotype at diagnosis of the disease in 45 GD Spanish patients (44 with type I and 1 with type III GD). The genotype involved two of the following previously expressed proteins: c.517A > C (T134P), 1%; c.721G > A (G202R), 17%; c.1090G > T (G325W), 13.9%; c.1208G > A (S364N), 4.1%; c.1226A > G (N370S), 17.8%; c.1246G > A (G377S), 17.6%; c.1289C > T (P391L), 8.5%; c.1448T > C (L444P), 3%; and c.1504C > T (R463C), 24.5%. Recombinant alleles, deletion of 55 bp in exon 9 and 84GG mutation were considered as mutations with no residual enzymatic activity.

RESULTS

The ERA showed a statistically significant correlation with chitotriosidase (P < 0.001), age (P < 0.001), spleen size (P < 0.001), 'Zimran's Severity Score Index' (P < 0.01) and the 'Gaucher Disease Severity Score Index-Type I' (P <  0.0001) at diagnosis of the disorder. Previous to any medical intervention, a comparison between the ERA and bone involvement, demonstrated a statistically significant relationship (P < 0.01) between the two variables.

CONCLUSIONS

This study data allowed us to define a new criterion for prognostic assessment of the disease at diagnosis, called Protein Severity Index, which expresses the theoretical severity of the genotype presented by patients, according to the corresponding ERA.

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.

Parkinson's disease: acid-glucocerebrosidase activity and alpha-synuclein clearance

The role of mutations in the gene GBA1 encoding the lysosomal hydrolase β-glucocerebrosidase for the development of synucleinopathies, such as Parkinson's disease and dementia with Lewy bodies, was only very recently uncovered. The knowledge obtained from the study of carriers or patients suffering from Gaucher disease (a common lysosomal storage disorder because of GBA1 mutations) is of particular importance for understanding the role of the enzyme and its catabolic pathway in the development of synucleinopathies. Decreased activity of β-glucocerebrosidase leads to lysosomal dysfunction and the accumulation of its substrate glucosylceramide and related lipid derivatives. Glucosylceramide is suggested to stabilize toxic oligomeric forms of α-synuclein that negatively influence the activity of β-glucocerebrosidase and to partially block export of newly synthesized β-glucocerebrosidase from the endoplasmic reticulum to late endocytic compartments, amplifying the pathological effects of α-synuclein and ultimately resulting in neuronal cell death. This pathogenic molecular feedback loop and most likely other factors (such as impaired endoplasmic reticulum-associated degradation, activation of the unfolded protein response and dysregulation of calcium homeostasis induced by misfolded GC mutants) are involved in shifting the cellular homeostasis from monomeric α-synuclein towards oligomeric neurotoxic and aggregated forms, which contribute to Parkinson's disease progression. From a therapeutic point of view, strategies aiming to increase either the expression, stability or delivery of the β-glucocerebrosidase to lysosomes are likely to decrease the α-synuclein burden and may be useful for an in depth evaluation at the organismal level. Lysosomes are critical for protein and lipid homeostasis. Recent research revealed that dysfunction of this organelle contributes to the development of neurodegenerative diseases such as Parkinson's disease (PD). Mutations in the lysosomal hydrolase β-glucocerebrosidase (GBA1) are a major risk factor for the development of PD and the molecular events linked to the reduced activity of GBA1 and the pathological accumulation of lipids and α-synuclein are just at the beginning to be understood. New therapeutic concepts in regards to how to increase the expression, stability, or delivery of β-glucocerebrosidase to lysosomes are currently developed. This article is part of a special issue on Parkinson disease.

Flavonol-based fluorescent indicator for determination of β-glucosidase activity

A flavonol-based ESIPT fluorescence probe for evaluation of β-glucosidase activity was synthesized and tested for sensitivity to enzymatic cleavage at different conditions.

Beyond osteogenesis imperfecta: Causes of fractures during infancy and childhood

Fractures in infancy or early childhood require prompt evaluation with consideration of accidental or non-accidental trauma as well as a large number of genetic disorders that predispose to fractures. Bone fragility has been reported in more than 100 genetic disorders, including skeletal dysplasias, inborn errors of metabolism and congenital insensitivity to pain. Most of these disorders are rare but often have distinctive clinical or radiographic findings to assist in the diagnosis. Gene sequencing is available, albeit connective tissue and skeletal dysplasia panels and biochemical studies are only helpful in a minority of cases. This article presents the clinical, radiographic, and molecular profiles of the most common heritable disorders other than osteogenesis imperfecta with increased bone fragility. In addition, the clinicians must consider non-heritable influences such as extreme prematurity, prenatal viral infection and neoplasia in the diagnostic process.

Recent advances in the development of synthetic chemical probes for glycosidase enzymes

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

Monogenic neurological disorders of sphingolipid metabolism

Sphingolipids comprise a wide variety of molecules containing a sphingoid long-chain base that can be N-acylated. These lipids are particularly abundant in the central nervous system, being membrane components of neurons as well as non-neuronal cells. Direct evidence that these brain lipids play critical functions in brain physiology is illustrated by the dramatic consequences of genetic disturbances of their metabolism. Inherited defects of both synthesis and catabolism of sphingolipids are now identified in humans. These monogenic disorders are due to mutations in the genes encoding for the enzymes that catalyze either the formation or degradation of simple sphingolipids such as ceramides, or complex sphingolipids like glycolipids. They cause varying degrees of central nervous system dysfunction, quite similarly to the neurological disorders induced in mice by gene disruption of the corresponding enzymes. Herein, the enzyme deficiencies and metabolic alterations that underlie these diseases are reviewed. Their possible pathophysiological mechanisms and the functions played by sphingolipids one can deduce from these conditions are discussed. This article is part of a Special Issue entitled Brain Lipids.

A novel GBA2 gene missense mutation in spastic ataxia

Autosomal recessive cerebellar ataxias (ARCA) encompass a heterogeneous group of rare diseases that affect the cerebellum, the spinocerebellar tract and/or the sensory tracts of the spinal cord. We investigated a consanguineous Cypriot family with spastic ataxia, aiming towards identification of the causative mutation. Family members were clinically evaluated and studied at the genetic level. Linkage analysis at marker loci spanning known ARCA genes/loci revealed linkage to the APTX locus. Thorough investigation of the APTX gene excluded any possible mutation. Whole genome linkage screening using microsatellite markers and whole genome SNP homozygosity mapping using the Affymetrix Genome-Wide Human SNP Array 6.0 enabled mapping of the disease gene/mutation in this family to Chromosome 9p21.1-p13.2. Due to the large number of candidate genes within this region, whole-exome sequencing of the proband was performed and further analysis of the obtained data focused on the mapped interval. Further investigation of the candidate variants resulted in the identification of a novel missense mutation in the GBA2 gene. GBA2 mutations have recently been associated with hereditary spastic paraplegia and ARCA with spasticity. We hereby report a novel GBA2 mutation associated with spastic ataxia and suggest that GBA2 mutations may be a relatively frequent cause of ARCA.

Biogenesis and transport of membrane domains-potential implications in brain pathologies

Lipids in biological membranes show astonishing chemical diversity, but they also show some key conserved structures in different organisms. In addition, some of their biophysical properties have been related to specific functions. In this review, we aim to discuss the role of sphingolipids- and cholesterol-rich micro- and nano-membrane domains (MD) and highlight their pivotal role in lipid-protein clustering processes, vesicle biogenesis and membrane fusion. We further review potential connections between human pathologies and defects in MD biosynthesis, recycling and homeostasis. Brain, which is second only to the adipose tissues in term of lipid abundance, is particularly affected by MD defects which are linked to neurodegenerative disorders. Finally we propose a potential connection between MD and several nutrient-related processes and envision how diet and autophagy could bring insights towards understanding the impact of global lipid homeostasis on human health and disease.

Mutations in GBA2 cause autosomal-recessive cerebellar ataxia with spasticity

Autosomal-recessive cerebellar ataxia (ARCA) comprises a large and heterogeneous group of neurodegenerative disorders with more than 20 different forms currently recognized, many of which are also associated with increased tone and some of which have limb spasticity. Gaucher disease is a lysosomal storage disease resulting from a defect in the enzyme acid β-glucosidase 1. β-glucosidase 2 is an enzyme with similar glucosylceramidase activity but to date has not been associated with a monogenic disorder. We studied four unrelated consanguineous families of Tunisian decent diagnosed with cerebellar ataxia of unknown origin. We performed homozygosity mapping and whole-exome sequencing in an attempt to identify the genetic origin of their disorder. We were able to identify mutations responsible for autosomal-recessive ataxia in these families within the gene encoding β-glucosidase 2, GBA2. Two nonsense mutations (c.363C>A [p.Tyr121(∗)] and c.1018C>T [p.Arg340(∗)]) and a substitution (c.2618G>A [p.Arg873His]) were identified, probably resulting in nonfunctional enzyme. This study suggests GBA2 mutations are a cause of recessive spastic ataxia and responsible for a form of glucosylceramide storage disease in humans.

SrfJ, a Salmonella type III secretion system effector regulated by PhoP, RcsB, and IolR

Virulence-related type III secretion systems are present in many Gram-negative bacterial pathogens. These complex devices translocate proteins, called effectors, from the bacterium into the eukaryotic host cell. Here, we identify the product of srfJ, a Salmonella enterica serovar Typhimurium gene regulated by SsrB, as a new substrate of the type III secretion system encoded by Salmonella pathogenicity island 2. The N-terminal 20-amino-acid segment of SrfJ was recognized as a functional secretion and translocation signal specific for this system. Transcription of srfJ was positively regulated by the PhoP/PhoQ system in an SsrB-dependent manner and was negatively regulated by the Rcs system in an SsrB-independent manner. A screen for regulators of an srfJ-lacZ transcriptional fusion using the T-POP transposon identified IolR, the regulator of genes involved in myo-inositol utilization, as an srfJ repressor. Our results suggest that SrfJ is synthesized both inside the host, in response to intracellular conditions, and outside the host, in myo-inositol-rich environments.

Recombinant macrophage targeted enzyme replacement therapy for Gaucher disease in India

OBJECTIVE

Gaucher disease in India has been reported only in a few case reports from India. The aim of the study was to assess the response to enzyme replacement therapy in Indian patients with Gaucher disease.

DESIGN

Retrospective analysis of patients receiving CHO-derived recombinant macrophage-targetted glucocorebrosidase.

SETTING

Five centers from India with experience in treating lysosomal storage disorders.

PATIENTS

The diagnosis of Gaucher disease was confirmed by low glucocerebrosidase levels, though it was first made on splenectomy in 8 and on bone marrow examination in 9 patients. Twenty five of 52 patients diagnosed with Gaucher disease (17 Type I, 8 mild Type III) received treatment for >6 months. Indications for treatment included symptomatic anemia, thrombo-cytopenia, organomegaly, bone disease or mild neurological symptoms leading to impairment of quality of life. Patients with significant neurological involvement were excluded. The drug infusions were given intravenously every 15 days.

MAIN OUTCOME MEASURES

Hemoglobin, platelet counts, liver and spleen volumes and growth parameters.

RESULTS

22 of the 25 children who survived were analyzed. After 6 months of treatment, the mean (range) increase in hemoglobin was 1.5 (-3.4 to 6.1) g/dL (P=0.01) and in platelet count was 32 x 10(9)/L (-98.5 x 109 to 145.5 x10(9))/L (P=0.02). The mean (range) increase in weight was 3 kg (-5.6 to 10.5) (P=0.04) and in height was 7.1 cm (0 to 26.5) (P=0.0003). Liver size decreased by a mean (range) of 38.5% (- 5.5 to 86.7) (P=0.0003) and the spleen size by 34.8% (0 to 91.7) (P=0.004). All patients had improvement in bone pains and in 2 patients, neurological symptoms improved with others remaining static.

CONCLUSIONS

This is the first reported cohort of patients in India reporting our experience with imiglucerase enzyme replacement therapy for treatment of Gaucher Disease in India.

[Molecular basis of treatment in Gaucher's disease]

In Gaucher's disease (GD) there is an imbalance in the monocyte-macrophage system between the rate of formation and degradation of glucosylceramide due to low activity of the lysosomal enzyme betaglucorebrosidase. Therapeutic approaches are aimed to reduce the synthesis of glucosylceramide by inhibiting ceramide-glucosyltransferase or increasing the degradation of glucosylceramide using a recombinant beta-glucorebrosidase or recovering the residual activity of mutant enzymes with pharmacological chaperones. Inhibitory molecules of ceramide-glucosyltransferase used mimic glucose or ceramide. All recombinant enzymes used in GD enzyme replacement therapy have similar kinetic parameters, but differ in their amino acid sequence, as well as, in the exposed mannose oligosaccharide chains. The mannose content and localization in the oligosaccharide chains are essential for cell uptake. Velaglucerase alpha have a high content of mannoses in their oligosaccharide chains.

Therapeutic strategies for Gaucher disease: miglustat (NB-DNJ) as a pharmacological chaperone for glucocerebrosidase and the different thermostability of velaglucerase alfa and imiglucerase

Gaucher disease (GD) is a disorder of glycosphingolipid metabolism caused by deficiency of lysosomal glucocerebrosidase (GlcCerase) activity, due to conformationally or functionally defective variants, resulting in progressive deposition of glycosylceramide in macrophages. The glucose analogue, N-butyldeoxynojirimycin (NB-DNJ, miglustat), is an inhibitor of the ceramide-specific glycosyltransferase, which catalyzes the first step of glycosphingolipid biosynthesis and is currently approved for the oral treatment of type 1 GD. In a previous work, we found a GlcCerase activity increase in cell cultures in the presence of NB-DNJ, which could imply that this compound is not only a substrate reducer but also a pharmacological chaperone or inhibitor for GlcCerase degradation. In this work we compare imiglucerase (the enzyme currently used for replacement therapy) and velaglucerase alfa (a novel therapeutic enzyme form) in terms of conformational stability and enzymatic activity, as well as the effect of NB-DNJ on them. The interaction between these enzymes and NB-DNJ was studied by isothermal titration calorimetry. Our results reveal that, although velaglucerase alfa and imiglucerase exhibit very similar activity profiles, velaglucerase alfa shows higher in vitro thermal stability and is less prone to aggregation/precipitation, which could be advantageous for storage and clinical administration. In addition, we show that at neutral pH NB-DNJ binds to and enhances the stability of both enzymes, while at mildly acidic lysosomal conditions it does not bind to them. These results support the potential role of NB-DNJ as a pharmacological chaperone, susceptible of being part of pharmaceutical formulation or combination therapy for GD in the future.

The enigma of the E326K mutation in acid β-glucocerebrosidase

A large number of mutations, and several polymorphisms, have been characterized in the GBA gene, encoding the lysosomal enzyme glucocerebrosidase, the activity of which is impaired in Gaucher disease. In this communication we summarize published and new data concerning biochemical characterization of the E326K amino acid change (1093G>A in the GBA1 cDNA) in tissue culture and its association with Parkinson disease, suggesting it is a disease causing mutation and not merely a polymorphism in the GBA gene.

The pathophysiology of GD - current understanding and rationale for existing and emerging therapeutic approaches

Gaucher disease is a genetic disorder of sphingolipid metabolism resulting from dysfunction of the lysosomal membrane-associated glycoprotein glucocerebrosidase (GBA) and resulting in intracellular accumulation of glucosylceramide and other glycolipids. Although the gene defect and relevant biochemical pathways have been defined, the mechanisms by which substrate accumulation causes disease manifestations are not well understood. The direct effects of a build up of substrate laden cells may account for some aspects of disease but the overall pathology is likely to be more complex with effects of stored material on a variety of intra and extra cellular functions. In this article we review the GBA gene and its protein product, with associated defects, lipid metabolism and storage, enzyme misfolding and endoplasmic reticulum stress, calcium homeostasis, oxidative stress and autophagy and at each point examine how therapies that are currently available, in clinical development or at earlier stages of basic research might address the pathological mechanisms.

Use of direct fluorescence labeling and confocal microscopy to determine the biodistribution of two protein therapeutics, Cerezyme and Ceredase

Efficient targeting of therapeutic reagents to tissues and cell types of interest is critical to achieving therapeutic efficacy and avoiding unwanted side effects due to offtarget uptake. To increase assay efficiency and reduce the number of animals used per experiment during preclinical development, we used a combination of direct fluorescence labeling and confocal microscopy to simultaneously examine the biodistribution of two therapeutic proteins, Cerezyme and Ceredase, in the same animals. We show that the fluorescent tags do not interfere with protein uptake and localization. We are able to detect Cerezyme and Ceredase in intact cells and organs and demonstrate colocalization within target cells using confocal microscopy. In addition, the relative amount of protein internalized by different cell types can be quantified using cell type-specific markers and morphometric analysis. This approach provides an easy and straightforward means of assessing the tissue and cell type-specific biodistribution of multiple protein therapeutics in target organs using a minimal number of animals.

Optimal therapy in Gaucher disease

Gaucher disease (GD), the inherited deficiency of the lysosomal enzyme glucocerebrosidase, presents with a wide range of symptoms of varying severity, and primarily affects the skeletal, hematologic and nervous systems. To date, the standard of care has included enzyme replacement therapy with imiglucerase. Although imiglucerase is highly effective in reversing the visceral and hematologic manifestations, skeletal disease is slow to respond, pulmonary involvement is relatively resistant, and the CNS involvement is not impacted. Because of the recent manufacturing and processing problems, the research and development of alternative therapeutics has become more pressing. The divergent phenotypes and the heterogeneity involving different organ systems implicates the involvement of several pathological processes that include enzyme deficiency, substrate accumulation, protein misfolding, and macrophage activation, that differ in each patient with GD. Thus, the therapy should be tailored individually in order to target multiple pathways that interplay in GD.

A new archaeal beta-glycosidase from Sulfolobus solfataricus: seeding a novel retaining beta-glycan-specific glycoside hydrolase family along with the human non-lysosomal glucosylceramidase GBA2

Carbohydrate active enzymes (CAZymes) are a large class of enzymes, which build and breakdown the complex carbohydrates of the cell. On the basis of their amino acid sequences they are classified in families and clans that show conserved catalytic mechanism, structure, and active site residues, but may vary in substrate specificity. We report here the identification and the detailed molecular characterization of a novel glycoside hydrolase encoded from the gene sso1353 of the hyperthermophilic archaeon Sulfolobus solfataricus. This enzyme hydrolyzes aryl beta-gluco- and beta-xylosides and the observation of transxylosylation reactions products demonstrates that SSO1353 operates via a retaining reaction mechanism. The catalytic nucleophile (Glu-335) was identified through trapping of the 2-deoxy-2-fluoroglucosyl enzyme intermediate and subsequent peptide mapping, while the general acid/base was identified as Asp-462 through detailed mechanistic analysis of a mutant at that position, including azide rescue experiments. SSO1353 has detectable homologs of unknown specificity among Archaea, Bacteria, and Eukarya and shows distant similarity to the non-lysosomal bile acid beta-glucosidase GBA2 also known as glucocerebrosidase. On the basis of our findings we propose that SSO1353 and its homologs are classified in a new CAZy family, named GH116, which so far includes beta-glucosidases (EC 3.2.1.21), beta-xylosidases (EC 3.2.1.37), and glucocerebrosidases (EC 3.2.1.45) as known enzyme activities.

Crystal structure of the Salmonella enterica serovar typhimurium virulence factor SrfJ, a glycoside hydrolase family enzyme

To cause infection, Salmonella enterica serovar Typhimurium uses type III secretion systems, which are encoded on two Salmonella pathogenicity islands, SPI-1 and SPI-2, the latter of which is thought to play a crucial role in bacterial proliferation in Salmonella-containing vacuoles (SCVs) after invading cells. S. Typhimurium SrfJ, located outside SPI-2, is also known to be involved in Salmonella pathogenicity and has high amino acid sequence homology with human lysosomal glucosylceramidase (GlcCerase). We present the first crystal structure of SrfJ at a resolution of 1.8 A. The overall fold of SrfJ shares high structure similarities with that of human GlcCerase, comprising two distinctive domains: a (beta/alpha)(8)-barrel catalytic domain and a beta-sandwich domain. As in human GlcCerase, the pocket-shaped active site of SrfJ is located on the C-terminal side of the barrel, and two conserved glutamic acid residues are used for the enzyme catalysis. Moreover, a glycerol-bound form of SrfJ reveals that the glucose ring moiety of the substrate might similarly bind to the enzyme as to human GlcCerase, suggesting that SrfJ might function as a glycoside hydrolase. Although some structural differences are observed between SrfJ and human GlcCerase in the substrate entrance of the active site, we speculate that, based on the high structural similarities to human GlcCerase in the overall fold and the active-site environment, SrfJ might have a GlcCerase activity and use the activity to enhance Salmonella virulence by modifying SCV membrane lipids.

Acid beta-glucosidase: insights from structural analysis and relevance to Gaucher disease therapy

In mammalian cells, glucosylceramide (GlcCer), the simplest glycosphingolipid, is hydrolyzed by the lysosomal enzyme acid beta-glucosidase (GlcCerase). In the human metabolic disorder Gaucher disease, GlcCerase activity is significantly decreased owing to one of approximately 200 mutations in the GlcCerase gene. The most common therapy for Gaucher disease is enzyme replacement therapy (ERT), in which patients are given intravenous injections of recombinant human GlcCerase; the Genzyme product Cerezyme has been used clinically for more than 15 years and is administered to approximately 4000 patients worldwide. Here we review the crystal structure of Cerezyme and other recombinant forms of GlcCerase, as well as of their complexes with covalent and non-covalent inhibitors. We also discuss the stability of Cerezyme, which can be altered by modification of its N-glycan chains with possible implications for improved ERT in Gaucher disease.