Function of oligosaccharide modification in glucocerebrosidase, a membrane-associated lysosomal hydrolase

HEPES‐buffering of bicarbonate‐containing culture medium perturbs lysosomal glucocerebrosidase activity

Glucocerebrosidase (GCase), encoded by the GBA gene, degrades the ubiquitous glycosphingolipid glucosylceramide. Inherited GCase deficiency causes Gaucher disease (GD). In addition, carriers of an abnormal GBA allele are at increased risk for Parkinson's disease. GCase undergoes extensive modification of its four N‐glycans en route to and inside the lysosome that is reflected in changes in molecular weight as detected with sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. Fluorescent activity‐based probes (ABPs) that covalently label GCase in reaction‐based manner in vivo and in vitro allow sensitive visualization of GCase molecules. Using these ABPs, we studied the life cycle of GCase in cultured fibroblasts and macrophage‐like RAW264.7 cells. Specific attention was paid to the impact of 4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid (HEPES) supplementation to bicarbonate‐buffered medium. Here, we report how HEPES‐buffered medium markedly influences processing of GCase, its lysosomal degradation, and the total cellular enzyme level. HEPES‐containing medium was also found to reduce maturation of other lysosomal enzymes (α‐glucosidase and β‐glucuronidase) in cells. The presence of HEPES in bicarbonate containing medium increases GCase activity in GD‐patient derived fibroblasts, illustrating how the supplementation of HEPES complicates the use of cultured cells for diagnosing GD.

Cross-talks among GBA mutations, glucocerebrosidase, and α-synuclein in GBA-associated Parkinson's disease and their targeted therapeutic approaches: a comprehensive review

Current therapies for Parkinson's disease (PD) are palliative, of which the levodopa/carbidopa therapy remains the primary choice but is unable to modulate the progression of neurodegeneration. Due to the complication of such a multifactorial disorder and significant limitations of the therapy, numerous genetic approaches have been proved effective in finding out genes and mechanisms implicated in this disease. Following the observation of a higher frequency of PD in Gaucher's disease (GD), a lysosomal storage condition, mutations of glycosylceramidase beta (GBA) encoding glucocerebrosidase (GCase) have been shown to be involved and have been explored in the context of PD. GBA mutations are the most common genetic risk factor of PD. Various studies have revealed the relationships between PD and GBA gene mutations, facilitating a better understanding of this disorder. Various hypotheses delineate that the pathological mutations of GBA minimize the enzymatic activity of GCase, which affects the proliferation and clearance of α-synuclein; this affects the lysosomal homeostasis, exacerbating the endoplasmic reticulum stress or encouraging the mitochondrial dysfunction. Identification of the pathological mechanisms underlying the GBA-associated parkinsonism (GBA + PD) advances our understanding of PD. This review based on current literature aims to elucidate various genetic and clinical characteristics correlated with GBA mutations and to identify the numerous pathological processes underlying GBA + PD. We also delineate the therapeutic strategies to interfere with the mutant GCase function for further improvement of the related α-synuclein-GCase crosstalks. Moreover, the various therapeutic approaches such as gene therapy, chaperone proteins, and histone deacetylase inhibitors for the treatment of GBA + PD are discussed.

Cross-talks among GBA mutations, glucocerebrosidase, and α-synuclein in GBA-associated Parkinson’s disease and their targeted therapeutic approaches: a comprehensive review

Current therapies for Parkinson’s disease (PD) are palliative, of which the levodopa/carbidopa therapy remains the primary choice but is unable to modulate the progression of neurodegeneration. Due to the complication of such a multifactorial disorder and significant limitations of the therapy, numerous genetic approaches have been proved effective in finding out genes and mechanisms implicated in this disease. Following the observation of a higher frequency of PD in Gaucher’s disease (GD), a lysosomal storage condition, mutations of glycosylceramidase beta (GBA) encoding glucocerebrosidase (GCase) have been shown to be involved and have been explored in the context of PD. GBA mutations are the most common genetic risk factor of PD. Various studies have revealed the relationships between PD and GBA gene mutations, facilitating a better understanding of this disorder. Various hypotheses delineate that the pathological mutations of GBA minimize the enzymatic activity of GCase, which affects the proliferation and clearance of α-synuclein; this affects the lysosomal homeostasis, exacerbating the endoplasmic reticulum stress or encouraging the mitochondrial dysfunction. Identification of the pathological mechanisms underlying the GBA-associated parkinsonism (GBA + PD) advances our understanding of PD. This review based on current literature aims to elucidate various genetic and clinical characteristics correlated with GBA mutations and to identify the numerous pathological processes underlying GBA + PD. We also delineate the therapeutic strategies to interfere with the mutant GCase function for further improvement of the related α-synuclein–GCase crosstalks. Moreover, the various therapeutic approaches such as gene therapy, chaperone proteins, and histone deacetylase inhibitors for the treatment of GBA + PD are discussed.

Novel defect in phosphatidylinositol 4-kinase type 2-alpha (PI4K2A) at the membrane-enzyme interface is associated with metabolic cutis laxa

Inherited cutis laxa, or inelastic, sagging skin is a genetic condition of premature and generalised connective tissue ageing, affecting various elastic components of the extracellular matrix. Several cutis laxa syndromes are inborn errors of metabolism and lead to severe neurological symptoms. In a patient with cutis laxa, a choreoathetoid movement disorder, dysmorphic features and intellectual disability we performed exome sequencing to elucidate the underlying genetic defect. We identified the amino acid substitution R275W in phosphatidylinositol 4-kinase type IIα, caused by a homozygous missense mutation in the PI4K2A gene. We used lipidomics, complexome profiling and functional studies to measure phosphatidylinositol 4-phosphate synthesis in the patient and evaluated PI4K2A deficient mice to define a novel metabolic disorder. The R275W residue, located on the surface of the protein, is involved in forming electrostatic interactions with the membrane. The catalytic activity of PI4K2A in patient fibroblasts was severely reduced and lipid mass spectrometry showed that particular acyl-chain pools of PI4P and PI(4,5)P₂ were decreased. Phosphoinositide lipids play a major role in intracellular signalling and trafficking and regulate the balance between proliferation and apoptosis. Phosphatidylinositol 4-kinases such as PI4K2A mediate the first step in the main metabolic pathway that generates PI4P, PI(4,5)P₂ and PI(3,4,5)P₃ . Although neurologic involvement is common, cutis laxa has not been reported previously in metabolic defects affecting signalling. Here we describe a patient with a complex neurological phenotype, premature ageing and a mutation in PI4K2A, illustrating the importance of this enzyme in the generation of inositol lipids with particular acylation characteristics.

Localization of active endogenous and exogenous β-glucocerebrosidase by correlative light-electron microscopy in human fibroblasts

β-Glucocerebrosidase (GBA) is the enzyme that degrades glucosylceramide in lysosomes. Defects in GBA that result in overall loss of enzymatic activity give rise to the lysosomal storage disorder Gaucher disease, which is characterized by the accumulation of glucosylceramide in tissue macrophages. Gaucher disease is currently treated by infusion of mannose receptor-targeted recombinant GBA. The recombinant GBA is thought to reach the lysosomes of macrophages, based on the impressive clinical response that is observed in Gaucher patients (type 1) receiving this enzyme replacement therapy. In this study, we used cyclophellitol-derived activity-based probes (ABPs) with a fluorescent reporter that irreversibly bind to the catalytic pocket of GBA, to visualize the active enzymes in a correlative microscopy approach. The uptake of pre-labeled recombinant enzyme was monitored by fluorescence and electron microscopy in human fibroblasts that stably expressed the mannose receptor. The endogenous active enzyme was simultaneously visualized by in situ labeling with the ABP containing an orthogonal fluorophore. This method revealed the efficient delivery of recombinant GBA to lysosomal target compartments that contained endogenous active enzyme.

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.

Activity-Based Probes for Glycosidases: Profiling and Other Applications

Glycosidases mediate the fragmentation of glycoconjugates in the body, including the vital recycling of endogenous molecules. Several inherited diseases in man concern deficiencies in lysosomal glycosidases degrading glycosphingolipids. Prominent is Gaucher disease caused by an impaired lysosomal β-glucosidase (glucocerebrosidase, GBA) and resulting in pathological lysosomal storage of glucosylceramide (glucocerebroside) in tissue macrophages. GBA is a retaining glucosidase with a characteristic glycosyl-enzyme intermediate formed during catalysis. Using the natural suicide inhibitor cyclophellitol as a lead, we developed mechanism-based irreversible inhibitors of GBA equipped with a fluorescent reporter. These reagents covalently link to the catalytic nucleophile residue of GBA and permit specific and sensitive visualization of active enzyme molecules. The amphiphilic activity-based probes (ABPs) allow in situ detection of active GBA in cells and organisms. Furthermore, they may be used to biochemically confirm the diagnosis of Gaucher disease and they might assist in screening for small compounds interacting with the catalytic pocket. While the focus of this chapter is ABPs for β-glucosidases and Gaucher disease, the described concept has meanwhile been extended to other retaining glycosidases and related disease conditions as well.

In situ visualization of glucocerebrosidase in human skin tissue: zymography versus activity-based probe labeling

Epidermal β-glucocerebrosidase (GBA1), an acid β-glucosidase normally located in lysosomes, converts (glucosyl)ceramides into ceramides, which is crucial to generate an optimal barrier function of the outermost skin layer, the stratum corneum (SC). Here we report on two developed in situ methods to localize active GBA in human epidermis: i) an optimized zymography method that is less labor intensive and visualizes enzymatic activity with higher resolution than currently reported methods using either substrate 4-methylumbelliferyl-β-D-glucopyranoside or resorufin-β-D-glucopyranoside; and ii) a novel technique to visualize active GBA1 molecules by their specific labeling with a fluorescent activity-based probe (ABP), MDW941. The latter method pro-ved to be more robust and sensitive, provided higher resolution microscopic images, and was less prone to sample preparation effects. Moreover, in contrast to the zymography substrates that react with various β-glucosidases, MDW941 specifically labeled GBA1. We demonstrate that active GBA1 in the epidermis is primarily located in the extracellular lipid matrix at the interface of the viable epidermis and the lower layers of the SC. With ABP-labeling, we observed reduced GBA1 activity in 3D-cultured skin models when supplemented with the reversible inhibitor, isofagomine, irrespective of GBA expression. This inhibition affected the SC ceramide composition: MS analysis revealed an inhibitor-dependent increase in the glucosylceramide:ceramide ratio.

Stabilization of Glucocerebrosidase by Active Site Occupancy

Glucocerebrosidase (GBA) is a lysosomal β-glucosidase that degrades glucosylceramide. Its deficiency results in Gaucher disease (GD). We examined the effects of active site occupancy of GBA on its structural stability. For this, we made use of cyclophellitol-derived activity-based probes (ABPs) that bind irreversibly to the catalytic nucleophile (E340), and for comparison, we used the potent reversible inhibitor isofagomine. We demonstrate that cyclophellitol ABPs improve the stability of GBA in vitro, as revealed by thermodynamic measurements (T_m increase by 21 °C), and introduce resistance to tryptic digestion. The stabilizing effect of cell-permeable cyclophellitol ABPs is also observed in intact cultured cells containing wild-type GBA, N370S GBA (labile in lysosomes), and L444P GBA (exhibits impaired ER folding): all show marked increases in lysosomal forms of GBA molecules upon exposure to ABPs. The same stabilization effect is observed for endogenous GBA in the liver of wild-type mice injected with cyclophellitol ABPs. Stabilization effects similar to those observed with ABPs were also noted at high concentrations of the reversible inhibitor isofagomine. In conclusion, we provide evidence that the increase in cellular levels of GBA by ABPs and by the reversible inhibitor is in part caused by their ability to stabilize GBA folding, which increases the resistance of GBA against breakdown by lysosomal proteases. These effects are more pronounced in the case of the amphiphilic ABPs, presumably due to their high lipophilic potential, which may promote further structural compactness of GBA through hydrophobic interactions. Our study provides further rationale for the design of chaperones for GBA to ameliorate Gaucher disease.

Towards broad spectrum activity-based glycosidase probes: synthesis and evaluation of deoxygenated cyclophellitol aziridines

Deoxygenated cyclophellitol aziridines enable activity-based inter-class labeling of glycosidases including LC-MS/MS identification.

Role of LIMP-2 in the intracellular trafficking of β-glucosidase in different human cellular models

Acid β-glucosidase (GCase), the enzyme deficient in Gaucher disease (GD), is transported to lysosomes by the lysosomal integral membrane protein (LIMP)-2. In humans, LIMP-2 deficiency leads to action myoclonus-renal failure (AMRF) syndrome. GD and AMRF syndrome share some clinical features. However, they are different from clinical and biochemical points of view, suggesting that the role of LIMP-2 in the targeting of GCase would be different in different tissues. Besides, the role of LIMP-2 in the uptake and trafficking of the human recombinant (hr)GCase used in the treatment of GD is unknown. Thus, we compared GCase activity and intracellular localization in immortalized lymphocytes, fibroblasts, and a neuronal model derived from multipotent adult stem cells, from a patient with AMRF syndrome, patients with GD, and control subjects. In fibroblasts and neuronlike cells, GCase targeting to the lysosomes is completely dependent on LIMP-2, whereas in blood cells, GCase is partially targeted to lysosomes by a LIMP-2-independent mechanism. Although hrGCase cellular uptake is independent of LIMP-2, its trafficking to the lysosomes is mediated by this receptor. These data provide new insights into the mechanisms involved in the intracellular trafficking of GCase and in the pathogeneses of GD and AMRF syndrome.

Mechanism-based inhibitors of glycosidases: design and applications

This article covers recent developments in the design and application of activity-based probes (ABPs) for glycosidases, with emphasis on the different enzymes involved in metabolism of glucosylceramide in humans. Described are the various catalytic reaction mechanisms employed by inverting and retaining glycosidases. An understanding of catalysis at the molecular level has stimulated the design of different types of ABPs for glycosidases. Such compounds range from (1) transition-state mimics tagged with reactive moieties, which associate with the target active site—forming covalent bonds in a relatively nonspecific manner in or near the catalytic pocket—to (2) enzyme substrates that exploit the catalytic mechanism of retaining glycosidase targets to release a highly reactive species within the active site of the enzyme, to (3) probes based on mechanism-based, covalent, and irreversible glycosidase inhibitors. Some applications in biochemical and biological research of the activity-based glycosidase probes are discussed, including specific quantitative visualization of active enzyme molecules in vitro and in vivo, and as strategies for unambiguously identifying catalytic residues in glycosidases in vitro.

Action myoclonus-renal failure syndrome: diagnostic applications of activity-based probes and lipid analysis

Lysosomal integral membrane protein-2 (LIMP2) mediates trafficking of glucocerebrosidase (GBA) to lysosomes. Deficiency of LIMP2 causes action myoclonus-renal failure syndrome (AMRF). LIMP2-deficient fibroblasts virtually lack GBA like the cells of patients with Gaucher disease (GD), a lysosomal storage disorder caused by mutations in the GBA gene. While GD is characterized by the presence of glucosylceramide-laden macrophages, AMRF patients do not show these. We studied the fate of GBA in relation to LIMP2 deficiency by employing recently designed activity-based probes labeling active GBA molecules. We demonstrate that GBA is almost absent in lysosomes of AMRF fibroblasts. However, white blood cells contain considerable amounts of residual enzyme. Consequently, AMRF patients do not acquire lipid-laden macrophages and do not show increased plasma levels of macrophage markers, such as chitotriosidase, in contrast to GD patients. We next investigated the consequences of LIMP2 deficiency with respect to plasma glycosphingolipid levels. Plasma glucosylceramide concentration was normal in the AMRF patients investigated as well as in LIMP2-deficient mice. However, a marked increase in the sphingoid base, glucosylsphingosine, was observed in AMRF patients and LIMP2-deficient mice. Our results suggest that combined measurements of chitotriosidase and glucosylsphingosine can be used for convenient differential laboratory diagnosis of GD and AMRF.

Production of active human glucocerebrosidase in seeds of Arabidopsis thaliana complex-glycan-deficient (cgl) plants

There is a clear need for efficient methods to produce protein therapeutics requiring mannose-termination for therapeutic efficacy. Here we report on a unique system for production of active human lysosomal acid β-glucosidase (glucocerebrosidase, GCase, EC 3.2.1.45) using seeds of the Arabidopsis thaliana complex-glycan-deficient (cgl) mutant, which are deficient in the activity of N-acetylglucosaminyl transferase I (EC 2.4.1.101). Gaucher disease is a prevalent lysosomal storage disease in which affected individuals inherit mutations in the gene (GBA1) encoding GCase. A gene cassette optimized for seed expression was used to generate the human enzyme in seeds of the cgl (C5) mutant, and the recombinant GCase was mainly accumulated in the apoplast. Importantly, the enzymatic properties including kinetic parameters, half-maximal inhibitory concentration of isofagomine and thermal stability of the cgl-derived GCase were comparable with those of imiglucerase, a commercially available recombinant human GCase used for enzyme replacement therapy in Gaucher patients. N-glycan structural analyses of recombinant cgl-GCase showed that the majority of the N-glycans (97%) were mannose terminated. Additional purification was required to remove ∼15% of the plant-derived recombinant GCase that possessed potentially immunogenic (xylose- and/or fucose-containing) N-glycans. Uptake of cgl-derived GCase by mouse macrophages was similar to that of imiglucerase. The cgl seed system requires no addition of foreign (non-native) amino acids to the mature recombinant GCase protein, and the dry transgenic seeds represent a stable repository of the therapeutic protein. Other strategies that may completely prevent plant-like complex N-glycans are discussed, including the use of a null cgl mutant.

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.

Inhibitor screening of pharmacological chaperones for lysosomal β-glucocerebrosidase by capillary electrophoresis

Pharmacological chaperones (PCs) represent a promising therapeutic strategy for treatment of lysosomal storage disorders based on enhanced stabilization and trafficking of mutant protein upon orthosteric and/or allosteric binding. Herein, we introduce a simple yet reliable enzyme assay using capillary electrophoresis (CE) for inhibitor screening of PCs that target the lysosomal enzyme, β-glucocerebrosidase (GCase). The rate of GCase-catalyzed hydrolysis of the synthetic substrate, 4-methylumbelliferyl-β-D: -glucopyranoside was performed using different classes of PCs by CE with UV detection under standardized conditions. The pH and surfactant dependence of inhibitor binding on recombinant GCase activity was also examined. Enzyme inhibition studies were investigated for five putative PCs including isofagomine (IFG), ambroxol, bromhexine, diltiazem, and fluphenazine. IFG was confirmed as a potent competitive inhibitor of recombinant GCase with half-maximal inhibitory concentration (IC(50)) of 47.5 ± 0.1 and 4.6 ± 1.4 nM at pH 5.2 and pH 7.2, respectively. In contrast, the four other non-carbohydrate amines were demonstrated to function as mixed-type inhibitors with high micromolar activity at neutral pH relative to acidic pH conditions reflective of the lysosome. CE offers a convenient platform for characterization of PCs as a way to accelerate the clinical translation of previously approved drugs for oral treatment of rare genetic disorders, such as Gaucher disease.

Generation of polyclonal antibodies against recombinant human glucocerebrosidase produced in Escherichia coli

Deficiency of the lysosomal glucocerebrosidase (GCR) enzyme results in Gaucher's disease, the most common inherited storage disorder. Treatment consists of enzyme replacement therapy by the administration of recombinant GCR produced in Chinese hamster ovary cells. The production of anti-GCR antibodies has already been described with placenta-derived human GCR that requires successive chromatographic procedures. Here, we report a practical and efficient method to obtain anti-GCR polyclonal antibodies against recombinant GCR produced in Escherichia coli and further purified by a single step through nickel affinity chromatography. The purified GCR was used to immunize BALB/c mice and the induction of anti-GCR antibodies was evaluated by enzyme-linked immunosorbent assay. The specificity of the antiserum was also evaluated by western blot analysis against recombinant GCR produced by COS-7 cells or against endogenous GCR of human cell lines. GCR was strongly recognized by the produced antibodies, either as cell-associated or as secreted forms. The detected molecular masses of 59-66 kDa are in accordance to the expected size for glycosylated GCR. The GCR produced in E. coli would facilitate the production of polyclonal (shown here) and monoclonal antibodies and their use in the characterization of new biosimilar recombinant GCRs coming in the near future.

Impaired lysosomal trimming of N-linked oligosaccharides leads to hyperglycosylation of native lysosomal proteins in mice with alpha-mannosidosis

Alpha-mannosidosis is caused by the genetic defect of the lysosomal alpha-d-mannosidase (LAMAN), which is involved in the breakdown of free alpha-linked mannose-containing oligosaccharides originating from glycoproteins with N-linked glycans, and thus manifests itself in an extensive storage of mannose-containing oligosaccharides. Here we demonstrate in a model of mice with alpha-mannosidosis that native lysosomal proteins exhibit elongated N-linked oligosaccharides as shown by two-dimensional difference gel electrophoresis, deglycosylation assays, and mass spectrometry. The analysis of cathepsin B-derived oligosaccharides revealed a hypermannosylation of glycoproteins in mice with alpha-mannosidosis as indicated by the predominance of extended Man3GlcNAc2 oligosaccharides. Treatment with recombinant human alpha-mannosidase partially corrected the hyperglycosylation of lysosomal proteins in vivo and in vitro. These data clearly demonstrate that LAMAN is involved not only in the lysosomal catabolism of free oligosaccharides but also in the trimming of asparagine-linked oligosaccharides on native lysosomal proteins.

A nonsense mutation in the LIMP-2 gene associated with progressive myoclonic epilepsy and nephrotic syndrome

The main clinical features of two siblings from a consanguineous marriage were progressive myoclonic epilepsy without intellectual impairment and a nephrotic syndrome with a strong accumulation of C1q in capillary loops and mesangium of kidney. The biochemical analysis of one of the patients revealed a normal beta-glucocerebrosidase activity in leukocytes, but a severe enzymatic deficiency in cultured skin fibroblasts. This deficiency suggested a defect in the intracellular sorting pathway of this enzyme. The sequence analysis of the gene encoding LIMP-2 (SCARB2), the sorting receptor for beta-glucocerebrosidase, confirmed this hypothesis. A homozygous nonsense mutation in codon 178 of SCARB2 was found in the patient, whereas her healthy parents were heterozygous for the mutation. Besides lacking immunodetectable LIMP-2, patient fibroblasts also had decreased amounts of beta-glucocerebrosidase, which was mainly located in the endoplasmic reticulum, as assessed by its sensitivity to Endo H. This is the first report of a mutation in the SCARB2 gene associated with a human disease, which, contrary to earlier proposals, shares no features with Charcot-Marie-Tooth disease both at the clinical and neurophysiological levels.

Delivery of lysosomal enzymes for therapeutic use: glucocerebrosidase as an example

Enzyme therapies for lysosomal storage diseases have developed over the past decade into the standard-of-care for affected patients. Such therapy for Gaucher disease has been the prototype, using natural source or recombinant forms of human acid beta-glucosidase (GCase). In Gaucher disease, macrophages are the repository for the pathological lipid and the target for delivery of GCase. The macrophage mannose receptor provides a Trojan horse for intracellular delivery of intravenously administered GCase (man-GCase) with mannosyl-terminated oligosaccharide chains. Passage through several hostile compartments (e.g., plasma) leads to inefficient delivery of man-GCase to macrophage lysosomes. However, regular infusions of man-GCase re-establishes health in affected patients. Similar results are being obtained in several other lysosomal storage diseases. Evolving gene and chaperone approaches provide alternative treatment strategies.

Substrate reduction therapy of glycosphingolipid storage disorders

In the last 15 years enormous progress has been made regarding therapy of type I Gaucher disease, a severely disabling disorder characterized by intralysosomal storage of glucosylceramide in tissue macrophages. Effective enzyme replacement therapy of type I Gaucher disease, based on chronic intravenous administration of mannose-terminated recombinant human glucocerebrosidase, has been available since 1990 and has been applied in several thousand patients without serious adverse effects. An alternative therapeutic approach, so-called substrate reduction therapy, is based on partial reduction of the synthesis of glucosylceramide and hence of subsequent metabolites. Oral administration of an inhibitor of glucosylceramide synthesis (N-butyldeoxynojirimycin, registered in Europe since 2002 as miglustat (Zavesca)), is effective in reversing clinical symptoms in type I Gaucher patients with mild to moderate disease manifestations. The growing long-term experience with substrate reduction therapy indicates that this treatment is also without major adverse effects. Substrate reduction therapy, in conjunction with enzyme replacement therapy, may play an important role in the future clinical management of patients suffering from type I Gaucher disease. Clinical trials are under way that should reveal the value of substrate reduction for maintenance therapy of type I Gaucher disease and for treatment of neuronopathic variants of Gaucher disease, Niemann-Pick disease type C, late-onset Tay-Sachs disease and Sandhoff disease.

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

The properties of the endolysosomal enzyme GCase (glucosylceramidase), carrying the most prevalent mutation observed in Gaucher patients, namely substitution of an asparagine residue with a serine at amino acid position 370 [N370S (Asn370-->Ser) GCase], were investigated in the present study. We previously demonstrated that Sap (saposin) C, the physiological GCase activator, promotes the association of GCase with anionic phospholipid-containing membranes, reconstituting in this way the enzyme activity. In the present study, we show that, in the presence of Sap C and membranes containing high levels of anionic phospholipids, both normal and N370S GCases are able to associate with the lipid surface and to express their activity. Conversely, when the amount of anionic phospholipids in the membrane is reduced (approximately 20% of total lipids), Sap C is still able to promote binding and activation of the normal enzyme, but not of N370S GCase. The altered interaction of the mutated enzyme with anionic phospholipid-containing membranes and Sap C was further demonstrated in Gaucher fibroblasts by confocal microscopy, which revealed poor co-localization of N370S GCase with Sap C and lysobisphosphatidic acid, the most abundant anionic phospholipid in endolysosomes. Moreover, we found that N370S Gaucher fibroblasts accumulate endolysosomal free cholesterol, a lipid that might further interfere with the interaction of the enzyme with Sap C and lysobisphosphatidic acid-containing membranes. In summary, our results show that the N370S mutation primarily affects the interaction of GCase with its physiological activators, namely Sap C and anionic phospholipid-containing membranes. We thus propose that the poor contact between N370S GCase and its activators may be responsible for the low activity of the mutant enzyme in vivo.

Biochemistry of glycosphingolipid storage disorders: implications for therapeutic intervention

The physiological importance of the degradative processes in lysosomes is revealed by the existence of at least 40 distinct inherited diseases, the so-called lysosomal storage disorders. Most of these diseases are caused by a deficiency in a single lysosomal enzyme, or essential cofactor, and result in the lysosomal accumulation of one, or sometimes several, natural compounds. The most prevalent subgroup of the lysosomal storage disorders is formed by the sphingolipidoses, inherited disorders that are characterized by excessive accumulation of one or multiple (glyco)sphingolipids. The biology of glycosphingolipids has been extensively discussed in other contributions during this symposium. This review will therefore focus in depth on (type 1) Gaucher disease, a prototypical glycosphingolipidosis. The elucidation of the primary genetic defect, being a deficiency in the lysosomal glucocerebrosidase, is described. Characterization of glucocerebrosidase at protein and gene level has subsequently opened avenues for therapeutic intervention. The development of successful enzyme replacement therapy for type 1 Gaucher disease is discussed. Attention is also paid to the alternative approach of substrate modulation using orally administered inhibitors of glucosylceramide synthesis. Novel developments about the monitoring of age of onset, progression and correction of disease are described. The remaining challenges about pathophysiology of glycosphingolipidoses are discussed in view of further improvements in therapy for these debilitating disorders.

Biosynthesis, glycosylation, and enzymatic processing in vivo of human tripeptidyl-peptidase I

Human tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal serine protease that removes tripeptides from the free N termini of small polypeptides and also shows a minor endoprotease activity. Due to various naturally occurring mutations, an inherited deficiency of TPP I activity causes a fatal lysosomal storage disorder, classic late infantile neuronal ceroid lipofuscinosis (CLN2). In the present study, we analyzed biosynthesis, glycosylation, transport, and proteolytic processing of this enzyme in stably transfected Chinese hamster ovary cells as well as maturation of the endocytosed proenzyme in CLN2 lymphoblasts, fibroblasts, and N2a cells. Human TPP I was initially identified as a single precursor polypeptide of approximately 68 kDa, which, within a few hours, was converted to the mature enzyme of approximately 48 kDa. Compounds affecting the pH of intracellular acidic compartments, those interfering with the intracellular vesicular transport as well as inhibition of the fusion between late endosomes and lysosomes by temperature block or 3-methyladenine, hampered the conversion of TPP I proenzyme into the mature form, suggesting that this process takes place in lysosomal compartments. Digestion of immunoprecipitated TPP I proenzyme with both N-glycosidase F and endoglycosidase H as well as treatment of the cells with tunicamycin reduced the molecular mass of TPP I proenzyme by approximately 10 kDa, which indicates that all five potential N-glycosylation sites in TPP I are utilized. Mature TPP I was found to be partially resistant to endo H treatment; thus, some of its N-linked oligosaccharides are of the complex/hybrid type. Analysis of the effect of various classes of protease inhibitors and mutation of the active site Ser(475) on human TPP I maturation in cultured cells demonstrated that although TPP I zymogen is capable of autoactivation in vitro, a serine protease that is sensitive to AEBSF participates in processing of the proenzyme to the mature, active form in vivo.

Intracellular transport of acid beta-glucosidase and lysosome-associated membrane proteins is affected in Gaucher's disease (G202R mutation)

Gaucher's disease (GD) is caused by an inherited deficiency of acid beta-glucosidase with storage of glucosylceramides in the lysosomes of macrophages. This study identifies a G202R mutation in the acid beta-glucosidase gene in an infant with severe neuronopathic (type 2) GD and only slightly reduced acid beta-glucosidase activity. Western blot analysis, pulse chase experiments, and the thin frozen section immunogold method were used to analyse the implications of this mutation on the pathogenesis, clinical heterogeneity and diagnostic evaluation of GD. The results show that acid beta-glucosidase persists in the patient's fibroblasts as a mannose-rich polypeptide in the endoplasmic reticulum and is not transported to the lysosomes. By contrast, high expression of the lysosome-associated membrane proteins LAMP-1 and LAMP-2, saposin C, and cathepsin D was observed in the patient's lysosomes. Immunogold labelling of the integral membrane proteins LAMP-1 and LAMP-2 increases significantly at the cell surface of Kupffer cells and fibroblasts as well as at the apical membrane of hepatocytes. In addition, LAMP-1 and LAMP-2 associate with the bilayer of stored glucosylceramide. It is concluded that defective intracellular transport of mutant acid beta-glucosidase from the endoplasmic reticulum to lysosomes leads to a more severe clinical phenotype than the residual enzyme activity may indicate. Furthermore, the detection of LAMP in the tubular bundles of undigested glucosylceramides, as well as their increased concentration at the surfaces of the affected cells, suggests that these proteins play a role in the storage or removal of substrate in GD. Intracellular targeting of acid beta-glucosidase and LAMP contributes to the broad phenotypic heterogeneity of GD.

Detection and Isolation of Gene-Corrected Cells in Gaucher Disease Via a Fluorescence-Activated Cell Sorter Assay for Lysosomal Glucocerebrosidase Activity

Gaucher disease type 1 results from the accumulation of glucocerebroside in macrophages of the reticuloendothelial system, as a consequence of a deficiency in glucocerebrosidase (GC) activity. Recent improvements in the methodologies for introducing foreign genes into bone marrow stem cells have prompted several groups to test the efficacy of gene transfer therapy as a curative treatment for Gaucher disease. Limitations of this approach include the potential for insufficient engraftment of gene-corrected cells and incomplete transduction of hematopoietic stem cells using retroviral gene transfer. Overcoming these obstacles may be critical in the case of treatment for Gaucher disease type 1, because GC transduced cells have not been shown to have a growth advantage over noncorrected cells. Here, we describe the development and application of a novel, fluorescence-activated cell sorter based assay that directly quantitates GC activity at the single cell level. In a test of this application, fibroblasts from a Gaucher patient were transduced, and high expressing cells sorted based on GC activity. Reanalysis of cultured sorted fibroblasts reveals that these cells maintain high levels of enzymatic activity, compared with the heterogeneous population from which they were sorted. The assay is sufficiently sensitive to distinguish GC activity found in Gaucher patient monocytes from that in normal controls. Furthermore, preliminary results indicate that increased GC activity can be detected in transduced, CD34+ enriched peripheral blood mononuclear cells isolated from a Gaucher patient. This method should be a useful addition to current gene therapy protocols as a means to quantitatively assess gene correction of relevant cell populations and potentially purify transduced cells for transplantation.

Synthesis, sorting, and processing into distinct isoforms of human macrophage chitotriosidase

Chitotriosidase, the human analogue of chitinases from non-vertebrate species, has recently been identified. The macrophage-derived enzyme is remarkably heterogeneous in molecular mass and isoelectric point. The synthesis and modification of the enzyme in cultured macrophages is reported. Chitotriosidase is synthesized as a 50-kDa protein with a pI of about 6.5 and 7.2. It is predominantly secreted, but in part processed into a 39-kDa form with a pI of 8.0 that accumulates in lysosomes. In the C-terminal extension of the 50-kDa chitotriosidase, sialic-acid containing O-linked glycans are present, causing its heterogeneous acidic isoelectric point. Chitotriosidase lacks N-linked glycans and the mechanism of routing to lysosomes proves to be distinct from that of soluble, N-glycosylated, lysosomal enzymes. It was observed that, in macrophages, alternative splicing generates a distinct chitotriosidase mRNA species, encoding a 40-kDa chitotriosidase that is C-terminally truncated. This enzyme is almost identical to the 39-kDa chitotriosidase formed from the 50-kDa precursor by proteolytic processing. It is concluded that the C-terminus present in the 50-kDa chitotriosidase, but absent in the 39-kDa isoform, was found to mediate tight binding to chitin. In the blood stream the secretory 50-kDa chitotriosidase occurs predominantly, whilst in tissues the 39-kDa form is also abundant. These findings are consistent with the data on the synthesis and processing of chitotriosidase in the cultured macrophage model.

Quantitative analysis of the targeting of mannose-terminal glucocerebrosidase. Predominant uptake by liver endothelial cells

Gaucher's disease is an inherited lysosomal storage disorder that is caused by a deficiency of glucocerebrosidase. The resulting accumulation of the substrate glucosylceramide in macrophages of liver, spleen, and bone marrow causes severe clinical symptoms. Gaucher's disease is treated by intravenous administration of a modified glucocerebrosidase (Alglucerase), which has exposed mannose residues to promote uptake by target macrophages. To evaluate the effectiveness of the targeting of Alglucerase, we studied the fate of the enzyme in the rat. Intravenously injected Alglucerase was rapidly cleared from the circulation (half-life 2.0 +/- 0.5 min). The liver was the main site of uptake, with 65.6 +/- 1.2% of the dose present at 10 min after injection. Smaller amounts ( < 3% of the dose) were taken up by spleen and bone marrow. Previous injection with mannan substantially increased the plasma half-life of the enzyme (14.8 +/- 3.2 min versus 1.7 +/- 0.3 min in solvent-preinjected controls) and uptake of the enzyme by liver, spleen and bone marrow was reduced by > 90%. These findings indicate that the enzyme is taken up by these organs via mannose-specific receptors. Subcellular fractionation of the liver indicated that the enzyme is internalized and transported to the lysosomes. By isolating various liver cell types after injection of the Alglucerase, it was found that endothelial cells are the main site of uptake of the enzyme: 60.8 +/- 3.4% of the total liver uptake. Parenchymal and Kupffer cells were responsible for 31.0 +/- 3.1% and 8.2 +/- 0.7% of the hepatic uptake, respectively. We conclude that Alglucerase is rapidly cleared from the circulation by mannose-specific receptors in liver, spleen, and bone marrow. However, less than 10% of the enzyme taken up by the liver is accounted for by Kupffer cells, the hepatic target cells for therapeutic intervention. It is suggested that alterations of the formulation of the therapeutic enzyme may lead to a higher uptake by Kupffer cells and other macrophages, and thus to a more (cost)effective therapy of Gaucher's disease.

Isoelectric focusing as a tool for the investigation of post-translational processing and chemical modifications of proteins

It has been demonstrated that good agreement may be observed between computed and experimental isoelectric point (pI) values when proteins of known sequence are focused under denaturing conditions on immobilized pH gradient IPG slabs, at least in the pH range 4-7.5. Hence, discrepancies between expected and found in this experimental set-up may be reliably ascribed to some kind of post-transcriptional processing, or chemical modification, having taken place in the sample. This evaluation is made easier when the comparison is set between the pI of a parent molecule and that (or those) of one to several of its derivatives as resolved in a single experiment (for instance, as a spot row in two-dimensional maps); no previous knowledge is required in these cases about the amino acid composition of the primary structure. The effects on protein surface charge are discussed in this review mainly for two biologically relevant processes, glycosylation and phosphorylation. Then, the pI shifts are analysed for some protein modifications that may occur naturally but can also be artefactually elicited, such as NH2 terminus blocking, deamidation and thiol redox reactions. Finally, carboxymethylation and carbamylation are used to exemplify chemical treatments often applied in connection with electrophoretic techniques and involving charged residues. Procedures to be applied in order to verify whether a given modification has occurred, and often relying on the focusing of a treated specimen, are detailed in each section. Numerical examples on model proteins are also discussed. As an important field of application of the above concepts may be genetic engineering, an exhaustive bibliographic list dealing with pI evaluation and structural assessment on recombinant proteins is included.

Individualised low-dose alglucerase therapy for type 1 Gaucher's disease

Previous studies have shown that enzyme supplementation therapy with alglucerase for type 1 Gaucher's disease is effective at doses of 30-130 U/kg per month. Since both the clinical presentation and the response to therapy in Gaucher's disease are highly variable, individual dosing seems indicated. This notion, as well as the high costs of alglucerase and the unknown long-term side-effects, led us to investigate the efficacy of an individualised very low dose of alglucerase. Twenty-five adults with symptomatic type 1 Gaucher's disease (thirteen splenectomised) received alglucerase 1.15 U/kg three times a week (15 U/kg per month). Every 6 months, the dose was halved, maintained, or doubled, according to the response (based on haematological variables and liver and spleen volume). After 6 months of treatment, eighteen (72%) patients had a response (seventeen moderate, one good). After 12 months (in nineteen patients) and 18 months (in seven patients), all had sustained improvement. Severe splenomegaly resulted in slower haematological responses. Our results are similar to those obtained by others with higher-dose regimens and better than a low-dose regimen of 10U/kg every 2 weeks. We conclude that very low initial doses of alglucerase, when administered frequently, are effective and cost-saving in the treatment of type 1 Gaucher's disease.

A quantitative immunoelectronmicroscopic study on soluble, membrane-associated and membrane-bound lysosomal enzymes in human intestinal epithelial cells

We have used quantitative immunoelectronmicroscopy to compare the in situ localization of acid alpha-glucosidase, lysosomal acid phosphatase, beta-hexosaminidase and glucocerebrosidase in intestinal epithelial cells of the human duodenum. Differences between these four lysosomal enzymes were observed with respect to their presence at the apical cell surface. Transport to the apical membrane seems to be a more important intracellular route for lysosomal acid phosphatase and acid alpha-glucosidase than it is for beta-hexosaminidase. The membrane associated lysosomal enzyme glucocerebrosidase is not transported to the microvilli. The studies emphasize that lysosomal enzyme transport pathways are enzyme and cell type specific.

Clinical phenotype of Gaucher disease in relation to properties of mutant glucocerebrosidase in cultured fibroblasts

We have investigated several parameters of glucocerebrosidase in cultured skin fibroblasts from patients with various clinical phenotypes of Gaucher disease. In this study no strict correlation was found between the clinical manifestations of Gaucher disease and the parameters investigated in fibroblasts. These parameters included the specific activity of the enzyme in extracts towards natural lipid and artificial substrate in the presence of different activators; the enzymic activity per unit of glucocerebrosidase protein; the rate of synthesis of the enzyme and its stability; and the post-translational processing of the enzyme. In addition, the activity in situ of glucocerebrosidase in fibroblasts was investigated using a novel method by analysis of the catabolism of NBD-glucosylceramide in cells that were loaded with bovine serum albumin-lipid complexes. Again, no complete correlation with the clinical phenotype of patients was detectable. Glucocerebrosidase in fibroblasts from most non-neuronopathic (type 1) Gaucher disease patients differs in some aspects from enzyme in cells from patients with neurological forms (types 2 and 3). The stimulation by activator protein and phospholipid is clearly more pronounced in type 1 than in types 2 and 3; the enzymic activity per unit of glucocerebrosidase protein in type 1 is severely reduced in the presence of taurocholate and the amount of glucocerebrosidase appears (near) normal in contrast to the situation in types 2 and 3 Gaucher fibroblasts. However, this distinction was not always consistent; glucocerebrosidase in fibroblasts from some type 1 Gaucher patients, particularly some South African cases, was comparable in properties to enzyme in type 2 and 3 patients.