The N370S (Asn370-->Ser) mutation affects the capacity of glucosylceramidase to interact with anionic phospholipid-containing membranes and saposin C

Genetic mechanism vs genetic subtypes: The example of GBA

Genetic variants in GBA, encoding the lysosomal enzyme glucocerebrosidase (GCase), are common risk factors for Parkinson's disease (PD). Genotype-phenotype studies have demonstrated that different types of GBA variants have differential effects on the phenotype. Variants could be classified as mild or severe depending on the type of Gaucher disease they cause in the biallelic state. It was shown that severe GBA variants, as compared to mild variants, are associated with higher risk of PD, earlier age at onset, and faster progression of motor and nonmotor symptoms. The observed difference in phenotype might be caused by a diversity of cellular mechanisms related to the particular variants. The lysosomal function of GCase is thought to play a significant role in the development of GBA-associated PD, and other mechanisms such as endoplasmic reticulum retention, mitochondrial dysfunction, and neuroinflammation have also been suggested. Moreover, genetic modifiers such as LRRK2, TMEM175, SNCA, and CTSB can either affect GCase activity or modulate risk and age at onset of GBA-associated PD. To achieve ideal outcomes with precision medicine, therapies will have to be tailored to individuals with specific variants, potentially in combination with known modifiers.

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

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

Glucocerebrosidase Gene Therapy Induces Alpha-Synuclein Clearance and Neuroprotection of Midbrain Dopaminergic Neurons in Mice and Macaques

Mutations in the GBA1 gene coding for glucocerebrosidase (GCase) are the main genetic risk factor for Parkinson’s disease (PD). Indeed, identifying reduced GCase activity as a common feature underlying the typical neuropathological signatures of PD—even when considering idiopathic forms of PD—has recently paved the way for designing novel strategies focused on enhancing GCase activity to reduce alpha-synuclein burden and preventing dopaminergic cell death. Here we have performed bilateral injections of a viral vector coding for the mutated form of alpha-synuclein (rAAV9-SynA53T) for disease modeling purposes, both in mice as well as in nonhuman primates (NHPs), further inducing a progressive neuronal death in the substantia nigra pars compacta (SNpc). Next, another vector coding for the GBA1 gene (rAAV9-GBA1) was unilaterally delivered in the SNpc of mice and NHPs one month after the initial insult, together with the contralateral delivery of an empty/null rAAV9 for control purposes. Obtained results showed that GCase enhancement reduced alpha-synuclein burden, leading to improved survival of dopaminergic neurons. Data reported here support using GCase gene therapy as a disease-modifying treatment for PD and related synucleinopathies, including idiopathic forms of these disorders.

Glucocerebrosidase Gene Therapy Induces Alpha-Synuclein Clearance and Neuroprotection of Midbrain Dopaminergic Neurons in Mice and Macaques

Mutations in the GBA1 gene coding for glucocerebrosidase (GCase) are the main genetic risk factor for Parkinson's disease (PD). Indeed, identifying reduced GCase activity as a common feature underlying the typical neuropathological signatures of PD-even when considering idiopathic forms of PD-has recently paved the way for designing novel strategies focused on enhancing GCase activity to reduce alpha-synuclein burden and preventing dopaminergic cell death. Here we have performed bilateral injections of a viral vector coding for the mutated form of alpha-synuclein (rAAV9-SynA53T) for disease modeling purposes, both in mice as well as in nonhuman primates (NHPs), further inducing a progressive neuronal death in the substantia nigra pars compacta (SNpc). Next, another vector coding for the GBA1 gene (rAAV9-GBA1) was unilaterally delivered in the SNpc of mice and NHPs one month after the initial insult, together with the contralateral delivery of an empty/null rAAV9 for control purposes. Obtained results showed that GCase enhancement reduced alpha-synuclein burden, leading to improved survival of dopaminergic neurons. Data reported here support using GCase gene therapy as a disease-modifying treatment for PD and related synucleinopathies, including idiopathic forms of these disorders.

Glucocerebrosidase Gene Therapy Induces Alpha-Synuclein Clearance and Neuroprotection of Midbrain Dopaminergic Neurons in Mice and Macaques

Mutations in the GBA1 gene coding for glucocerebrosidase (GCase) are the main genetic risk factor for Parkinson’s disease (PD). Indeed, identifying reduced GCase activity as a common feature underlying the typical neuropathological signatures of PD—even when considering idiopathic forms of PD—has recently paved the way for designing novel strategies focused on enhancing GCase activity to reduce alpha-synuclein burden and preventing dopaminergic cell death. Here we have performed bilateral injections of a viral vector coding for the mutated form of alpha-synuclein (rAAV9-SynA53T) for disease modeling purposes, both in mice as well as in nonhuman primates (NHPs), further inducing a progressive neuronal death in the substantia nigra pars compacta (SNpc). Next, another vector coding for the GBA1 gene (rAAV9-GBA1) was unilaterally delivered in the SNpc of mice and NHPs one month after the initial insult, together with the contralateral delivery of an empty/null rAAV9 for control purposes. Obtained results showed that GCase enhancement reduced alpha-synuclein burden, leading to improved survival of dopaminergic neurons. Data reported here support using GCase gene therapy as a disease-modifying treatment for PD and related synucleinopathies, including idiopathic forms of these disorders.

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

The discovery of genes involved in familial as well as sporadic forms of Parkinson disease (PD) constitutes an important milestone in understanding this disorder's pathophysiology and potential treatment. Among these genes, GBA1 is one of the most common and well-studied, but it is still unclear how mutations in GBA1 translate into an increased risk for developing PD. In this review, we provide an overview of the biochemical and structural relationship between GBA1 and PD to help understand the recent advances in the development of PD therapies intended to target this pathway.

Effects of glucocerebrosidase gene polymorphisms and mutations on the risk of Parkinson's disease dementia: A meta-analysis

OBJECTIVE

Exploring the impact of glucocerebrosidase gene (GBA) polymorphisms and mutations on the pathogenesis of Parkinson's disease dementia (PDD) plays an important role in the diagnosis and treatment of this disease. This meta-analysis aimed to investigate the effects of GBA polymorphisms and mutations on the risk of PDD and to identify the relationship between GBA genotype and PDD.

METHODS

A computer-based search was performed to retrieve publications from PubMed, Cochrane library, Embase, Chinese Biomedical Literature Database, China National Knowledge Infrastructure, and Wanfang databases using the search terms "glucocerebrosidase", "Parkinson's disease", and "dementia". After rigorous screening, cohort studies were included for meta-analysis.

RESULTS

The risk of PDD in GBA variant carriers was 1.94 times that in non-carriers (hazard ratio [HR], 1.94; 95% confidence interval [CI], 1.53-2.46). The risk of PDD in GBA polymorphism carriers was 1.87 times that in non-carriers (HR, 1.87; 95% CI, 1.18-2.98). The risk of PDD in GBA mutation carriers was 3.64 times that in non-carriers (HR, 3.64; 95% CI, 2.74-4.83). The risk of PDD in p.L444P variant carriers (HR, 4.81; 95% CI, 3.37-6.86) was significantly higher than that in p.N370S variant carriers (HR, 1.95; 95% CI, 1.29-1.94).

CONCLUSION

GBA polymorphisms and mutations are potential risk factors for PDD.

Glucocerebrosidase activity, cathepsin D and monomeric α-synuclein interactions in a stem cell derived neuronal model of a PD associated GBA1 mutation

The presence of GBA1 gene mutations increases risk for Parkinson's disease (PD), but the pathogenic mechanisms of GBA1 associated PD remain unknown. Given that impaired α-synuclein turnover is a hallmark of PD pathogenesis and cathepsin D is a key enzyme involved in α-synuclein degradation in neuronal cells, we have examined the relationship of glucocerebrosidase (GCase), cathepsin D and monomeric α-synuclein in human neural crest stem cell derived dopaminergic neurons. We found that normal activity of GCase is necessary for cathepsin D to perform its function of monomeric α-synuclein removal from neurons. GBA1 mutations lead to a lower level of cathepsin D protein and activity, and higher level of monomeric α-synuclein in neurons. When GBA1 mutant neurons were treated with GCase replacement or chaperone therapy; cathepsin D protein levels and activity were restored, and monomeric α-synuclein decreased. When cathepsin D was inhibited, GCase replacement failed to reduce monomeric α-synuclein levels in GBA1 mutant neurons. These data indicate that GBA1 gene mutations increase monomeric α-synuclein levels via an effect on lysosomal cathepsin D in neurons.

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Cathepsin D protein and activity decreased in GBA mutation-associated PD neurons.

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α-synuclein protein level increased in GBA mutation-associated PD neurons.

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GBA enzyme replacement treatment increased cathepsin D, decreased α-synuclein levels.

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GBA enzyme chaperone treatment increased cathepsin D, decreased α-synuclein levels.

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The influence of GBA enzyme replacement on α-synuclein mediated through cathepsin D.

Combination of acid β-glucosidase mutation and Saposin C deficiency in mice reveals Gba1 mutation dependent and tissue-specific disease phenotype

Gaucher disease is caused by mutations in GBA1 encoding acid β-glucosidase (GCase). Saposin C enhances GCase activity and protects GCase from intracellular proteolysis. Structure simulations indicated that the mutant GCases, N370S (0 S), V394L (4L) and D409V(9V)/H(9H), had altered function. To investigate the in vivo function of Gba1 mutants, mouse models were generated by backcrossing the above homozygous mutant GCase mice into Saposin C deficient (C*) mice. Without saposin C, the mutant GCase activities in the resultant mouse tissues were reduced by ~50% compared with those in the presence of Saposin C. In contrast to 9H and 4L mice that have normal histology and life span, the 9H;C* and 4L;C* mice had shorter life spans. 9H;C* mice developed significant visceral glucosylceramide (GC) and glucosylsphingosine (GS) accumulation (GC»GS) and storage macrophages, but lesser GC in the brain, compared to 4L;C* mice that presents with a severe neuronopathic phenotype and accumulated GC and GS primarily in the brain. Unlike 9V mice that developed normally for over a year, 9V;C* pups had a lethal skin defect as did 0S;C* mice resembled that of 0S mice. These variant Gaucher disease mouse models presented a mutation specific phenotype and underscored the in vivo role of Saposin C in the modulation of Gaucher disease.

Mechanism of glucocerebrosidase activation and dysfunction in Gaucher disease unraveled by molecular dynamics and deep learning

The lysosomal enzyme glucocerebrosidase-1 (GCase) catalyzes the cleavage of a major glycolipid glucosylceramide into glucose and ceramide. The absence of fully functional GCase leads to the accumulation of its lipid substrates in lysosomes, causing Gaucher disease, an autosomal recessive disorder that displays profound genotype-phenotype nonconcordance. More than 250 disease-causing mutations in GBA1, the gene encoding GCase, have been discovered, although only one of these, N370S, causes 70% of disease. Here, we have used a knowledge-based docking protocol that considers experimental data of protein-protein binding to generate a complex between GCase and its known facilitator protein saposin C (SAPC). Multiscale molecular-dynamics simulations were used to study lipid self-assembly, membrane insertion, and the dynamics of the interactions between different components of the complex. Deep learning was applied to propose a model that explains the mechanism of GCase activation, which requires SAPC. Notably, we find that conformational changes in the loops at the entrance of the substrate-binding site are stabilized by direct interactions with SAPC and that the loss of such interactions induced by N370S and another common mutation, L444P, result in destabilization of the complex and reduced GCase activation. Our findings provide an atomistic-level explanation for GCase activation and the precise mechanism through which N370S and L444P cause Gaucher disease.

Exploring genetic modifiers of Gaucher disease: The next horizon

Gaucher disease is an autosomal recessive lysosomal storage disorder resulting from mutations in the gene GBA1 that lead to a deficiency in the enzyme glucocerebrosidase. Accumulation of the enzyme's substrates, glucosylceramide and glucosylsphingosine, results in symptoms ranging from skeletal and visceral involvement to neurological manifestations. Nonetheless, there is significant variability in clinical presentations amongst patients, with limited correlation between genotype and phenotype. Contributing to this clinical variation are genetic modifiers that influence the phenotypic outcome of the disorder. In this review, we explore the role of genetic modifiers in Mendelian disorders and describe methods to facilitate their discovery. In addition, we provide examples of candidate modifiers of Gaucher disease, explore their relevance in the development of potential therapeutics, and discuss the impact of GBA1 and modifying mutations on other more common diseases like Parkinson disease. Identifying these important modulators of Gaucher phenotype may ultimately unravel the complex relationship between genotype and phenotype and lead to improved counseling and treatments.

GBA-Associated Parkinson's Disease and Other Synucleinopathies

PURPOSE OF REVIEW

GBA mutations are the most common known genetic cause of Parkinson's disease (PD). Its biological pathway may be important in idiopathic PD, since activity of the enzyme encoded by GBA, glucocerebrosidase, is reduced even among PD patients without GBA mutations. This article describes the structure and function of GBA, reviews recent literature on the clinical phenotype of GBA PD, and suggests future directions for research, counseling, and treatment.

RECENT FINDINGS

Several longitudinal studies have shown that GBA PD has faster motor and cognitive progression than idiopathic PD and that this effect is dose dependent. New evidence suggests that GBA mutations may be important in multiple system atrophy. Further, new interventional studies focusing on GBA PD are described. These studies may increase the interest of PD patients and caregivers in genetic counseling. GBA mutation status may help clinicians estimate PD progression, though mechanisms underlying GBA and synucleinopathy require further understanding.

The Role of Lipids Interacting with α-Synuclein in the Pathogenesis of Parkinson's Disease

α-synuclein is a small protein abundantly expressed in the brain and mainly located in synaptic terminals. The conversion of α-synuclein into oligomers and fibrils is the hallmark of a range of neurodegenerative disorders including Parkinson's disease and dementia with Lewy bodies. α-synuclein is disordered in solution but can adopt an α-helical conformation upon binding to lipid membranes. This lipid-protein interaction plays an important role in its proposed biological function, i.e., synaptic plasticity, but can also entail the aggregation of the protein. Both the chemical properties of the lipids and the lipid-to-protein-ratio have been reported to modulate the aggregation propensity of α-synuclein. In this review, the influence of changes in the nature and levels of lipids on the aggregation propensity of α-synuclein in vivo and in vitro will be discussed within a common general framework. In particular, while biophysical measurements and kinetic analyses of the time courses of α-synuclein aggregation in the presence of different types of lipid vesicles allow a mechanistic dissection of the influence of the lipids on α-synuclein aggregation, biological studies of cellular and animal models of Parkinson's disease allow the determination of changes in lipid levels and properties associated with the disease.

Intracellular Cholesterol Trafficking and Impact in Neurodegeneration

Cholesterol is a critical component of membrane bilayers where it plays key structural and functional roles by regulating the activity of diverse signaling platforms and pathways. Particularly enriched in brain, cholesterol homeostasis in this organ is singular with respect to other tissues and exhibits a heterogeneous regulation in distinct brain cell populations. Due to the key role of cholesterol in brain physiology and function, alterations in cholesterol homeostasis and levels have been linked to brain diseases and neurodegeneration. In the case of Alzheimer disease (AD), however, this association remains unclear with evidence indicating that either increased or decreased total brain cholesterol levels contribute to this major neurodegenerative disease. Here, rather than analyzing the role of total cholesterol levels in neurodegeneration, we focus on the contribution of intracellular cholesterol pools, particularly in endolysosomes and mitochondria through its trafficking via specialized membrane domains delineated by the contacts between endoplasmic reticulum and mitochondria, in the onset of prevalent neurodegenerative diseases such as AD, Parkinson disease, and Huntington disease as well as in lysosomal disorders like Niemann-Pick type C disease. We dissect molecular events associated with intracellular cholesterol accumulation, especially in mitochondria, an event that results in impaired mitochondrial antioxidant defense and function. A better understanding of the mechanisms involved in the distribution of cholesterol in intracellular compartments may shed light on the role of cholesterol homeostasis disruption in neurodegeneration and may pave the way for specific intervention opportunities.

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.

How α-Helical Motifs Form Functionally Diverse Lipid-Binding Compartments

Lipids are produced site-specifically in cells and then distributed nonrandomly among membranes via vesicular and nonvesicular trafficking mechanisms. The latter involves soluble amphitropic proteins extracting specific lipids from source membranes to function as molecular solubilizers that envelope their insoluble cargo before transporting it to destination sites. Lipid-binding and lipid transfer structural motifs range from multi-β-strand barrels, to β-sheet cups and baskets covered by α-helical lids, to multi-α-helical bundles and layers. Here, we focus on how α-helical proteins use amphipathic helical layering and bundling to form modular lipid-binding compartments and discuss the functional consequences. Preformed compartments generally rely on intramolecular disulfide bridging to maintain conformation (e.g., albumins, nonspecific lipid transfer proteins, saposins, nematode polyprotein allergens/antigens). Insights into nonpreformed hydrophobic compartments that expand and adapt to accommodate a lipid occupant are few and provided mostly by the three-layer, α-helical ligand-binding domain of nuclear receptors. The simple but elegant and nearly ubiquitous two-layer, α-helical glycolipid transfer protein (GLTP)-fold now further advances understanding.

Autophagic lysosome reformation dysfunction in glucocerebrosidase deficient cells: relevance to Parkinson disease

Glucocerebrosidase (GBA1) gene mutations increase the risk of Parkinson disease (PD). While the cellular mechanisms associating GBA1 mutations and PD are unknown, loss of the glucocerebrosidase enzyme (GCase) activity, inhibition of autophagy and increased α-synuclein levels have been implicated. Here we show that autophagy lysosomal reformation (ALR) is compromised in cells lacking functional GCase. ALR is a cellular process controlled by mTOR which regenerates functional lysosomes from autolysosomes formed during macroautophagy. A decrease in phopho-S6K levels, a marker of mTOR activity, was observed in models of GCase deficiency, including primary mouse neurons and the PD patient derived fibroblasts with GBA1 mutations, suggesting that ALR is compromised. Importantly Rab7, a GTPase crucial for endosome-lysosome trafficking and ALR, accumulated in GCase deficient cells, supporting the notion that lysosomal recycling is impaired. Recombinant GCase treatment reversed ALR inhibition and lysosomal dysfunction. Moreover, ALR dysfunction was accompanied by impairment of macroautophagy and chaperone-mediated autophagy, increased levels of total and phosphorylated (S129) monomeric α-synuclein, evidence of amyloid oligomers and increased α-synuclein release. Concurrently, we found increased cholesterol and altered glucosylceramide homeostasis which could compromise ALR. We propose that GCase deficiency in PD inhibits lysosomal recycling. Consequently neurons are unable to maintain the pool of mature and functional lysosomes required for the autophagic clearance of α-synuclein, leading to the accumulation and spread of pathogenic α-synuclein species in the brain. Since GCase deficiency and lysosomal dysfunction occur with ageing and sporadic PD pathology, the decrease in lysosomal reformation may be a common feature in PD.

Mitochondrial dysfunction associated with glucocerebrosidase deficiency

The lysosomal hydrolase glucocerebrosidase (GCase) is encoded for by the GBA gene. Homozygous GBA mutations cause Gaucher disease (GD), a lysosomal storage disorder. Furthermore, homozygous and heterozygous GBA mutations are numerically the greatest genetic risk factor for developing Parkinson's disease (PD), the second most common neurodegenerative disorder. The loss of GCase activity results in impairment of the autophagy‐lysosome pathway (ALP), which is required for the degradation of macromolecules and damaged organelles. Aberrant protein handling of α-synuclein by the ALP occurs in both GD and PD. α-synuclein is the principle component of Lewy bodies, a defining hallmark of PD. Mitochondrial dysfunction is also observed in both GD and PD. In this review we will describe how mitochondria are affected following loss of GCase activity. The pathogenic mechanisms leading to mitochondria dysfunction will also be discussed, focusing on the likely inhibition of the degradation of mitochondria by the ALP, also termed mitophagy. Other pathogenic cellular processes associated with GBA mutations that might contribute, such as the unfolding of GCase in the endoplasmic reticulum, calcium dysregulation and neuroinflammation will also be described. Impairment of the ALP and mitochondria dysfunction are common pathogenic themes between GD and PD and probably explain why GBA mutations increase the risk of developing PD that is very similar to sporadic forms of the disease.

The role of saposin C in Gaucher disease

Saposin C is one of four homologous proteins derived from sequential cleavage of the saposin precursor protein, prosaposin. It is an essential activator for glucocerebrosidase, the enzyme deficient in Gaucher disease. Gaucher disease is a rare autosomal recessive lysosomal storage disorder caused by mutations in the GBA gene that exhibits vast phenotypic heterogeneity, despite its designation as a "simple" Mendelian disorder. The observed phenotypic variability has led to a search for disease modifiers that can alter the Gaucher phenotype. The PSAP gene encoding saposin C is a prime candidate modifier for Gaucher disease. In humans, saposin C deficiency due to mutations in PSAP results in a Gaucher-like phenotype, despite normal in vitro glucocerebrosidase activity. Saposin C deficiency has also been shown to modify phenotype in one mouse model of Gaucher disease. The role of saposin C as an activator required for normal glucocerebrosidase function, and the consequences of saposin C deficiency are described, and are being explored as potential modifying factors in patients with Gaucher disease.

A Guided Tour of the Structural Biology of Gaucher Disease: Acid-β-Glucosidase and Saposin C

Mutations in both acid-β-glucosidase (GCase) and saposin C lead to Gaucher disease, the most common lysosomal storage disorder. The past several years have seen an explosion of structural and biochemical information for these proteins, which have provided new insight into the biology and pathogenesis of Gaucher disease, as well as opportunities for new therapeutic directions. Nearly 20 crystal structures of GCase are now available, from different heterologous sources, complexed with different ligands in the active site, in different glycosylation states, as well as one that harbors a prevalent disease-causing mutation, N370S. For saposin C, two NMR and 3 crystal structures have been solved, each with its unique snapshot. This review focuses on the details of these structures to highlight salient common and disparate features that contribute to our current state of knowledge of this complex orphan disease.

Efficient one-step chromatographic purification and functional characterization of recombinant human Saposin C

Saposin (Sap) C is a small lysosomal disulfide bridge-containing glycoprotein required for glucosylceramide (GC) hydrolysis by glucosylceramidase (GCase). Sap C deficiency causes a variant form of Gaucher disease (GD), a rare genetic disorder characterized by GC accumulation in lysosomes of monocyte/macrophage lineage. Efforts to develop fast and efficient methodologies to express and purify Sap C have been made in the last years. Here, human Sap C was expressed in a bacterial strain that greatly enhances disulfide bond formation, and the recombinant protein was purified in a single chromatographic step using an affinity tag-based protein purification system. Mass spectrometry analysis demonstrated that disulfide bridges required for Sap C stability and functionality were retained. Consistently, the recombinant protein was shown to interact with anionic phospholipids-containing vesicles, and reconstitute GCase activity in vitro. Recombinant Sap C was efficiently endocytosed by Sap C-deficient fibroblasts, and targeted to lysosomes. These findings document that the bacterially purified Sap C exerts biological properties functionally equivalent to those observed for the native protein, indicating its potential use in the development of therapeutic intervention.

Molecular basis of reduced glucosylceramidase activity in the most common Gaucher disease mutant, N370S

Gaucher disease is caused by the defective activity of the lysosomal hydrolase, glucosylceramidase. Although the x-ray structure of wild type glucosylceramidase has been resolved, little is known about the structural features of any of the >200 mutations. Various treatments for Gaucher disease are available, including enzyme replacement and chaperone therapies. The latter involves binding of competitive inhibitors at the active site to enable correct folding and transport of the mutant enzyme to the lysosome. We now use molecular dynamics, a set of structural analysis tools, and several statistical methods to determine the flexible behavior of the N370S Gaucher mutant at various pH values, with and without binding the chaperone, N-butyl-deoxynojirimycin. We focus on the effect of the chaperone on the whole protein, on the active site, and on three important structural loops, and we demonstrate how the chaperone modifies the behavior of N370S in such a way that it becomes more active at lysosomal pH. Our results suggest a mechanism whereby the binding of N-butyl-deoxynojirimycin helps target correctly folded glucosylceramidase to the lysosome, contributes to binding with saposin C, and explains the initiation of the substrate-enzyme complex. Such analysis provides a new framework for determination of the structure of other Gaucher disease mutants and suggests new approaches for rational drug design.

X-ray and biochemical analysis of N370S mutant human acid β-glucosidase

Gaucher disease is caused by mutations in the enzyme acid β-glucosidase (GCase), the most common of which is the substitution of serine for asparagine at residue 370 (N370S). To characterize the nature of this mutation, we expressed human N370S GCase in insect cells and compared the x-ray structure and biochemical properties of the purified protein with that of the recombinant human GCase (imiglucerase, Cerezyme®). The x-ray structure of N370S mutant acid β-glucosidase at acidic and neutral pH values indicates that the overall folding of the N370S mutant is identical to that of recombinant GCase. Subtle differences were observed in the conformation of a flexible loop at the active site and in the hydrogen bonding ability of aromatic residues on this loop with residue 370 and the catalytic residues Glu-235 and Glu-340. Circular dichroism spectroscopy showed a pH-dependent change in the environment of tryptophan residues in imiglucerase that is absent in N370S GCase. The mutant protein was catalytically deficient with reduced V(max) and increased K(m) values for the substrate p-nitrophenyl-β-D-glucopyranoside and reduced sensitivity to competitive inhibitors. N370S GCase was more stable to thermal denaturation and had an increased lysosomal half-life compared with imiglucerase following uptake into macrophages. The competitive inhibitor N-(n-nonyl)deoxynojirimycin increased lysosomal levels of both N370S and imiglucerase 2-3-fold by reducing lysosomal degradation. Overall, these data indicate that the N370S mutation results in a normally folded but less flexible protein with reduced catalytic activity compared with imiglucerase.

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.

Glycosylation- and phosphorylation-dependent intracellular transport of lysosomal hydrolases

Lysosomes contain more than 50 soluble hydrolases that are targeted to lysosomes in a mannose 6-phosphate (Man6P)-dependent manner. The phosphorylation of man- nose residues on high mannose-type oligosaccharides of newly synthesized lysosomal enzymes is catalyzed by two multimeric enzymes, GlcNAc-1-phosphotransferase and GlcNAc-1-phosphodiester-alpha-N-acetylglucosaminidase, allowing the binding to two distinct Man6P receptors in the Golgi apparatus. Inherited defects in the GlcNAc-1-phosphotransferase complex result in missorting and cellular loss of lysosomal enzymes, and the subsequent lysosomal dysfunction causes the lysosomal storage disorders mucolipidosis types II and III. Biosynthetic studies and the availability of Man6P receptor-deficient mouse models have provided new insights into the structural requirements for preferential binding of subsets of lysosomal enzymes to Man6P receptors as well as the identification of alternative targeting pathways.

Detection of ligand binding hot spots on protein surfaces via fragment-based methods: application to DJ-1 and glucocerebrosidase

The identification of hot spots, i.e., binding regions that contribute substantially to the free energy of ligand binding, is a critical step for structure-based drug design. Here we present the application of two fragment-based methods to the detection of hot spots for DJ-1 and glucocerebrosidase (GCase), targets for the development of therapeutics for Parkinson's and Gaucher's diseases, respectively. While the structures of these two proteins are known, binding information is lacking. In this study we employ the experimental multiple solvent crystal structures (MSCS) method and computational fragment mapping (FTMap) to identify regions suitable for the development of pharmacological chaperones for DJ-1 and GCase. Comparison of data derived via MSCS and FTMap also shows that FTMap, a computational method for the identification of fragment binding hot spots, is an accurate and robust alternative to the performance of expensive and difficult crystallographic experiments.

Lipid composition of microdomains is altered in a cell model of Gaucher disease

The formation of cholesterol and sphingolipids into specialized liquid-ordered membrane microdomains (rafts) has been proposed to function in the intracellular sorting and transport of proteins and lipids. Defined by biochemical criteria, rafts resist solubilization in nonionic detergents, enabling them to be isolated as detergent-resistant membranes (DRM). In this study, we characterized the lipid composition of DRM from a cell model of the sphingolipid storage disorder, Gaucher disease, in which the catabolism of the sphingolipid glucosylceramide (GC) is impaired. In this cell model, we showed that GC accumulated primarily in the DRM, with smaller secondary increases in ceramide, dihexosylceramide, trihexosylceramide, and phosphatidylglycerol. This suggested that not only was lipid metabolism altered as a consequence of the cells' inability to degrade GC, but this affected the DRM rather than other regions of the membrane. This increase in lipids in the DRM may be responsible for the altered lipid and protein sorting seen in Gaucher disease. Analysis of individual lipid species revealed preservation of the shorter and fully saturated fatty acid species in the DRM, suggesting that the highly ordered and tightly packed nature of the DRM is maintained.

An evolutionary and structure-based docking model for glucocerebrosidase-saposin C and glucocerebrosidase-substrate interactions - relevance for Gaucher disease

Gaucher disease, the most prevalent lysosomal storage disorder, is principally caused by malfunction of the lysosomal enzyme glucocerebrosidase (GBA), a 497-amino acid membrane glycoprotein that catalyzes the hydrolysis of glucosylceramide to ceramide and glucose in the presence of an essential 84-residue activator peptide named saposin C (SapC). Knowledge of the GBA structure, a typical (beta/alpha)(8) TIM barrel, explains the effect of few mutations, directly affecting or located near the catalytic site. To identify new regions crucial for proper GBA functionality, we analyzed the interactions of the enzyme with a second (substrate) and a third (cofactor) partner. We build 3D docking models of the GBA-SapC and the GBA-ceramide interactions, by means of methodologies that integrate both evolutive and structural information. The GBA-SapC docking model confirm the implication of three spatially closed regions of the GBA surface (TIM barrel-helix 6 and helix 7, and the Ig-like domain) in binding the SapC molecule. This model provides new basis to understand the pathogenicity of several mutations, such as the prevalent Leu444Pro, and the additive effect of Glu326Lys in the double mutant Glu326Lys-Leu444Pro. Overall, 39 positions in which amino acid changes are known to cause Gaucher disease were localized in the GBA regions identified in this work. Our model is discussed in relation to the phenotype (pathogenic effect) of these mutations, as well as to the enzymatic activity of the recombinant proteins when available. Both data fully correlates with the proposed model, which will provide a new tool to better understand Gaucher disease and to design new therapy strategies.

The stimulatory effect of phosphatidylethanolamine on N-acylphosphatidylethanolamine-hydrolyzing phospholipase D (NAPE-PLD)

N-Acylphosphatidylethanolamine (NAPE)-hydrolyzing phospholipase D (NAPE-PLD) is a membrane-bound enzyme which releases the endocannabinoid anandamide and other bioactive N-acylethanolamines from their corresponding NAPEs in animal tissues. Our previous studies showed that NAPE-PLD solubilized from the membrane is remarkably stimulated by millimolar concentrations of Ca(2+) while the membrane-bound form is much less sensitive to Ca(2+). This finding suggested that certain membrane constituents diminished the stimulatory effect of Ca(2+). In the present studies, we examined the effects of membrane fractions from COS-7 cells and brain tissue on the purified recombinant rat NAPE-PLD, and found that heat-stable membrane component(s) dose-dependently activated NAPE-PLD up to 4.8-5.0 fold. In the presence of the membrane fractions, however, the stimulatory effect of Ca(2+) on the purified NAPE-PLD was considerably reduced. When it was examined if the membrane fractions can be replaced with various pure phospholipids, phosphatidylethanolamine activated NAPE-PLD up to 3.3 fold, which was followed by decrease in the stimulatory effects of Ca(2+) and several other divalent cations. These results suggest that membrane components including phosphatidylethanolamine keep the membrane-associated form of NAPE-PLD constitutively active.

Secondary sphingolipid accumulation in a macrophage model of Gaucher disease

Glucosylceramide (GC) is a metabolic intermediate derived from the cellular turnover of membrane gangliosides and globosides. The catabolism of GC is impaired in Gaucher disease (GD) and consequently GC accumulates in affected cells leading to clinical manifestations of GD. The primary cell type affected in GD is the macrophage, and we investigated what effect excess GC has on the spatial coordination of other sphingolipids and phospholipids in a macrophage model of GD. A THP-1 macrophage model of GD was established by supplementation of the culture media with conduritol B epoxide, a specific irreversible inhibitor of acid beta-glucosidase. This cell model accumulated up to 12-fold more GC compared with untreated cells. Sub-cellular fractionation showed that, initially, the primary site of GC accumulation was the lysosome but as more GC accumulated it distributed relatively evenly across the cell and was present in all sub-cellular fractions. We also observed secondary elevations in the concentrations of ceramide, di- and trihexosylceramide and phosphatidylglycerol, which all had similar sub-cellular distributions to that of GC, initially increasing in the lysosome and then throughout the sub-cellular compartments. Our results suggest that with excess GC accumulation, the pathway trafficking GC to the lysosome becomes saturated and GC as well as other sphingolipids are shunted to other parts of the cell. The presence of these sphingolipids at non-physiological concentrations is likely to interfere with other biochemical pathways outside the lysosome, leading to cell dysfunction and ultimately pathological mechanisms apparent in GD, such as macrophage activation.

Molecular imaging of membrane interfaces reveals mode of beta-glucosidase activation by saposin C

Acid beta-glucosidase (GCase) is a soluble lysosomal enzyme responsible for the hydrolysis of glucose from glucosylceramide and requires activation by the small nonenzymatic protein saposin C (sapC) to gain access to the membrane-embedded glycosphingolipid substrate. We have used in situ atomic force microscopy (AFM) with simultaneous confocal and epifluorescence microscopies to investigate the interactions of GCase and sapC with lipid bilayers. GCase binds to sites on membranes transformed by sapC, and enzyme activity occurs at loci containing both GCase and sapC. Using FRET, we establish the presence of GCase/sapC and GCase/product contacts in the bilayer. These data support a mechanism in which sapC locally alters regions of bilayer for subsequent attack by the enzyme in stably bound protein complexes.

Isofagomine- and 2,5-anhydro-2,5-imino-D-glucitol-based glucocerebrosidase pharmacological chaperones for Gaucher disease intervention

Gaucher disease, resulting from deficient lysosomal glucocerebrosidase (GC) activity, is the most common lysosomal storage disorder. Clinically important GC mutant enzymes typically have reduced specific activity and reduced lysosomal concentration, the latter due to compromised folding and trafficking. We and others have demonstrated that pharmacological chaperones assist variant GC folding by binding to the active site, stabilizing the native conformation of GC in the neutral pH environment of the endoplasmic reticulum (ER), enabling its trafficking from the ER to the Golgi and on to the lysosome. The mutated GC fold is generally stable in the lysosome after pharmacological chaperone dissociation, owing to the low pH environment for which the fold was evolutionarily optimized and the high substrate concentration, enabling GC to hydrolyze glucosylceramide to glucose and ceramide. The hypothesis of this study was that we could combine GC pharmacological chaperone structure-activity relationships from distinct chemical series to afford potent novel chaperones comprising a carbohydrate-like substructure that binds in the active site with a hydrophobic substructure that binds in a nearby pocket. We combined isofagomine and 2,5-anhydro-2,5-imino-D-glucitol active site binding substructures with hydrophobic alkyl adamantyl amides to afford novel small molecules with enhanced ability to increase GC activity in patient-derived fibroblasts. The cellular activity of N370S and G202R GC in fibroblasts is increased by 2.5- and 7.2-fold with isofagmine-based pharmacological chaperones N-adamantanyl-4-((3R,4R,5R)-3,4-dihydroxy-5-(hydroxymethyl)piperidin-1-yl)-butanamide (3) and N-adamantanyl-4-((3R,4R,5R)-3,4-dihydroxy-5-(hydroxymethyl)piperidin-1-yl)pentanamide (4), respectively, the best enhancements observed to date.

Structure of acid beta-glucosidase with pharmacological chaperone provides insight into Gaucher disease

Gaucher disease results from mutations in the lysosomal enzyme acid beta-glucosidase (GCase). Although enzyme replacement therapy has improved the health of some affected individuals, such as those with the prevalent N370S mutation, oral treatment with pharmacological chaperones may be therapeutic in a wider range of tissue compartments by restoring sufficient activity of endogenous mutant GCase. Here we demonstrate that isofagomine (IFG, 1) binds to the GCase active site, and both increases GCase activity in cell lysates and restores lysosomal trafficking in cells containing N370S mutant GCase. We also compare the crystal structures of IFG-bound GCase at low pH with those of glycerol-bound GCase at low pH and apo-GCase at neutral pH. Our data indicate that IFG induces active GCase, which is secured by interactions with Asn370. The design of small molecules that stabilize substrate-bound conformations of mutant proteins may be a general therapeutic strategy for diseases caused by protein misfolding and mistrafficking.

Sphingolipid metabolism diseases

Human diseases caused by alterations in the metabolism of sphingolipids or glycosphingolipids are mainly disorders of the degradation of these compounds. The sphingolipidoses are a group of monogenic inherited diseases caused by defects in the system of lysosomal sphingolipid degradation, with subsequent accumulation of non-degradable storage material in one or more organs. Most sphingolipidoses are associated with high mortality. Both, the ratio of substrate influx into the lysosomes and the reduced degradative capacity can be addressed by therapeutic approaches. In addition to symptomatic treatments, the current strategies for restoration of the reduced substrate degradation within the lysosome are enzyme replacement therapy (ERT), cell-mediated therapy (CMT) including bone marrow transplantation (BMT) and cell-mediated "cross correction", gene therapy, and enzyme-enhancement therapy with chemical chaperones. The reduction of substrate influx into the lysosomes can be achieved by substrate reduction therapy. Patients suffering from the attenuated form (type 1) of Gaucher disease and from Fabry disease have been successfully treated with ERT.

The lipoprotein receptor-related protein-1 (LRP) adapter protein GULP mediates trafficking of the LRP ligand prosaposin, leading to sphingolipid and free cholesterol accumulation in late endosomes and impaired efflux

One of the conserved functional pathways linked to engulfment of apoptotic corpses involves two membrane proteins low density lipoprotein receptor-related protein-1 (LRP) and ABCA1 and the LRP adapter protein GULP. Because LRP and ABCA1 play roles in cellular lipid trafficking and efflux, here we addressed whether the third member, the LRP adapter protein GULP, also affects cellular lipid transport. Several lines of evidence show that overexpression of GULP causes glycosphingolipid and free cholesterol accumulation in the late endosome/lysosome compartment that is accompanied by down-regulation of ABCA1 and decreased efflux. Conversely, knockdown of endogenous GULP expression promoted cholesterol flux through the late endosomes and up-regulation of ABCA1, even in the context of a disease state such as Niemann-Pick Type C disease. Mechanistically, we were able to show that trafficking of the LRP ligands alpha2-macroglobulin and prosaposin, a protein cofactor necessary for glycosphingolipid degradation, are impaired in cells expressing full-length GULP protein, resulting in glycosphingolipid and free cholesterol accumulation in the late endosome/lysosome compartment. On the other hand, knockdown of endogenous GULP results in enhanced targeting of prosaposin and enhanced clearance of glycosphingolipids and cholesterol from the late endosomes. Taken together, these data reveal that GULP/LRP/ABCA1 represents a triad of molecules involved in engulfment and cellular lipid homeostasis.

Imino sugar inhibitors for treating the lysosomal glycosphingolipidoses

The inherited metabolic disorders of glycosphingolipid (GSL) metabolism are a relatively rare group of diseases that have diverse and often neurodegenerative phenotypes. Typically, a deficiency in catabolic enzyme activity leads to lysosomal storage of GSL substrates and in many diseases, several other glycoconjugates. A novel generic approach to treating these diseases has been termed substrate reduction therapy (SRT), and the discovery and development of N-alkylated imino sugars as effective and approved drugs is discussed. An understanding of the molecular mechanism for the inhibition of the key enzyme in GSL biosynthesis, ceramide glucosyltransferase (CGT) by N-alkylated imino sugars, has also lead to compound design for improvements to inhibitory potency, bioavailability, enzyme selectivity, and biological safety. Following a successful clinical evaluation of one compound, N-butyl-deoxynojirimycin [(NB-DNJ), miglustat, Zavesca], for treating type I Gaucher disease, issues regarding the significance of side effects and CNS access have been addressed as exposure of drug to patients has increased. An alternative experimental approach to treat specific glycosphingolipid (GSL) lysosomal storage diseases is to use imino sugars as molecular chaperons that assist protein folding and stability of mutant enzymes. The principles of chaperon-mediated therapy (CMT) are described, and the potential efficacy and preclinical status of imino sugars is compared with substrate reduction therapy (SRT). The increasing use of imino sugars for clinical evaluation of a group of storage diseases that are complex and often intractable disorders to treat has considerable benefit. This is particularly so given the ability of small molecules to be orally available, penetrate the central nervous system (CNS), and have well-characterized biological and pharmacological properties.