Enhanced macrophage uptake of synthetically glycosylated human placental beta-glucocerebrosidase

Glycosylation shapes the efficacy and safety of diverse protein, gene and cell therapies

Over recent decades, therapeutic proteins have had widespread success in treating a myriad of diseases. Glycosylation, a near universal feature of this class of drugs, is a critical quality attribute that significantly influences the physical properties, safety profile and biological activity of therapeutic proteins. Optimizing protein glycosylation, therefore, offers an important avenue to developing more efficacious therapies. In this review, we discuss specific examples of how variations in glycan structure and glycoengineering impacts the stability, safety, and clinical efficacy of protein-based drugs that are already in the market as well as those that are still in preclinical development. We also highlight the impact of glycosylation on next generation biologics such as T cell-based cancer therapy and gene therapy.

Gaucher disease: achievements and prospects

Gaucher disease (GD) is the most common lysosomal storage disorder, resulting from a deficiency in the activity of a lysosomal enzyme glucocerebrosidase, which is involved in the catabolism of sphingolipids. The phenomenal progress in understanding the pathogenesis and development of specific therapy of this disease over the past 60 years dramatically changed the clinical phenotype of GD, turning a severe progressive disorder into an asymptomatic metabolic defect. The evolution of the understanding of GD associated with fundamental discoveries in the field of cell biology, biochemistry and genetics may be of interest to a wide audience as a model of the effective work of the scientific community in the treatment of rare metabolic pathology.

Genetic glycoengineering in mammalian cells

Advances in nuclease-based gene-editing technologies have enabled precise, stable, and systematic genetic engineering of glycosylation capacities in mammalian cells, opening up a plethora of opportunities for studying the glycome and exploiting glycans in biomedicine. Glycoengineering using chemical, enzymatic, and genetic approaches has a long history, and precise gene editing provides a nearly unlimited playground for stable engineering of glycosylation in mammalian cells to explore and dissect the glycome and its many biological functions. Genetic engineering of glycosylation in cells also brings studies of the glycome to the single cell level and opens up wider use and integration of data in traditional omics workflows in cell biology. The last few years have seen new applications of glycoengineering in mammalian cells with perspectives for wider use in basic and applied glycosciences, and these have already led to discoveries of functions of glycans and improved designs of glycoprotein therapeutics. Here, we review the current state of the art of genetic glycoengineering in mammalian cells and highlight emerging opportunities.

[Enzyme replacement therapy in adult patients with type I Gaucher disease]

Enzyme replacement therapy (ERT) is the standard for the treatment of Gaucher disease (GD). A lifelong intravenous administration of a recombinant analogue of human glucocerebrosidase compensates for the functional deficiency of its own enzyme. The use of ERT has changed the clinical phenotype of GD, a severe progressive disease has been turned into the status of an asymptomatic metabolic defect. At the same time, a reduced dosing ERT regimen applied in Gaucher patients who had achieved therapeutic goals has not yet been developed.

Production of recombinant human acid β-glucosidase with high mannose-type N-glycans in rice gnt1 mutant for potential treatment of Gaucher disease

Gaucher disease is an inherited metabolic disease caused by genetic acid β -glucosidase (GBA) deficiency and is currently treated by enzyme replacement therapy. For uptake into macrophages, GBA needs to carry terminal mannose residues on their N-glycans. Knockout mutant rice of N-acetylglucosaminyltransferase-I (gnt1) have a disrupted N-glycan processing pathway and produce only glycoproteins with high mannose residues. In this study, we introduced a gene encoding recombinant human GBA into both wild-type rice (WT) and rice gnt1 calli. Target gene integration and mRNA expression were confirmed by genomic DNA PCR and Northern blotting, respectively. Secreted rhGBAs in culture media from cell lines originating from both WT (WT-GBA) and rice gnt1 (gnt1-GBA) were detected by Western blotting. Each rhGBA was purified by affinity and ion exchange chromatography. In vitro catalytic activity of purified rhGBA was comparable to commercial Chinese hamster ovary cell-derived rhGBA. N-glycans were isolated from WT-GBA and gnt1-GBA and analyzed by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The amounts of high mannose-type N-glycans were highly elevated in gnt1-GBA (100%) compared to WT-GBA (1%).

Investigations on therapeutic glucocerebrosidases through paired detection with fluorescent activity-based probes

Deficiency of glucocerebrosidase (GBA) causes Gaucher disease (GD). In the common non-neuronopathic GD type I variant, glucosylceramide accumulates primarily in the lysosomes of visceral macrophages. Supplementing storage cells with lacking enzyme is accomplished via chronic intravenous administration of recombinant GBA containing mannose-terminated N-linked glycans, mediating the selective uptake by macrophages expressing mannose-binding lectin(s). Two recombinant GBA preparations with distinct N-linked glycans are registered in Europe for treatment of type I GD: imiglucerase (Genzyme), contains predominantly Man(3) glycans, and velaglucerase (Shire PLC) Man(9) glycans. Activity-based probes (ABPs) enable fluorescent labeling of recombinant GBA preparations through their covalent attachment to the catalytic nucleophile E340 of GBA. We comparatively studied binding and uptake of ABP-labeled imiglucerase and velaglucerase in isolated dendritic cells, cultured human macrophages and living mice, through simultaneous detection of different GBAs by paired measurements. Uptake of ABP-labeled rGBAs by dendritic cells was comparable, as well as the bio-distribution following equimolar intravenous administration to mice. ABP-labeled rGBAs were recovered largely in liver, white-blood cells, bone marrow and spleen. Lungs, brain and skin, affected tissues in severe GD types II and III, were only poorly supplemented. Small, but significant differences were noted in binding and uptake of rGBAs in cultured human macrophages, in the absence and presence of mannan. Mannan-competed binding and uptake were largest for velaglucerase, when determined with single enzymes or as equimolar mixtures of both enzymes. Vice versa, imiglucerase showed more prominent binding and uptake not competed by mannan. Uptake of recombinant GBAs by cultured macrophages seems to involve multiple receptors, including several mannose-binding lectins. Differences among cells from different donors (n = 12) were noted, but the same trends were always observed. Our study suggests that further insight in targeting and efficacy of enzyme therapy of individual Gaucher patients could be obtained by the use of recombinant GBA, trace-labeled with an ABP, preferably equipped with an infrared fluorophore or other reporter tag suitable for in vivo imaging.

Sequential Double "Clicks" toward Structurally Well-Defined Heterogeneous N-Glycoclusters: The Importance of Cluster Heterogeneity on Pattern Recognition In Vivo

Structurally well-defined heterogeneous N-glycoclusters are prepared on albumin via a double click procedure. The number of glycan molecules present, in addition to the spatial arrangement of glycans in the heterogeneous glycoclusters, plays an important role in the in vivo kinetics and organ-selective accumulation through glycan pattern recognition mechanisms.

Exploring the glycan interaction in vivo: Future prospects of neo-glycoproteins for diagnostics

Herein the biodistributions and in vivo kinetics of chemically prepared neoglycoproteins are reviewed. Chemical methods can be used to conjugate various mono- and oligosaccharides onto a protein surface. The kinetics and organ-specific accumulation profiles of these glycoconjugates, which are introduced through intravenous injections, have been analyzed using conventional dissection studies as well as noninvasive methods such as single photon emission computed tomography, positron emission tomography and fluorescence imaging. These studies suggest that glycan-dependent protein distribution kinetics may be useful for pharmacological and diagnostic applications.

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.

Taliglucerase alfa: an enzyme replacement therapy using plant cell expression technology

Gaucher disease (GD) is a rare, genetic lysosomal storage disorder caused by functional defects of acid β-glucosidase that results in multiple organ dysfunction. Glycosylation of recombinant acid human β-glucosidase and exposure of terminal mannose residues are critical to the success of enzyme replacement therapy (ERT) for the treatment of visceral and hematologic manifestations in GD. Three commercially available ERT products for treatment of GD type 1 (GD1) include imiglucerase, velaglucerase alfa, and taliglucerase alfa. Imiglucerase and velaglucerase alfa are produced in different mammalian cell systems and require production glycosylation modifications to expose terminal α-mannose residues, which are needed for mannose receptor-mediated uptake by target macrophages. Such modifications add to production costs. Taliglucerase alfa is a plant cell-expressed acid β-glucosidase approved in the United States and other countries for ERT in adults with GD1. A plant-based expression system, using carrot root cell cultures, was developed for production of taliglucerase alfa and does not require additional processing for postproduction glycosidic modifications. Clinical trials have demonstrated that taliglucerase alfa is efficacious, with a well-established safety profile in adult, ERT-naïve patients with symptomatic GD1, and for such patients previously treated with imiglucerase. These included significant improvements in organomegaly and hematologic parameters as early as 6months, and maintenance of achieved therapeutic values in previously treated patients. Ongoing clinical trials will further characterize the long-term efficacy and safety of taliglucerase alfa in more diverse patient populations, and may help to guide clinical decisions for achieving optimal outcomes for patients with GD1.

Comparative study on mannose 6-phosphate residue contents of recombinant lysosomal enzymes

As most recombinant lysosomal enzymes are incorporated into cells via mannose 6-phosphate (M6P) receptors, the M6P content is important for effective enzyme replacement therapy (ERT) for lysosomal diseases. However, there have been no comprehensive reports of the M6P contents of lysosomal enzymes. We developed an M6P assay method comprising three steps, i.e., acid hydrolysis of glycoproteins, derivatization of M6P, and high-performance liquid chromatography, and determined the M6P contents of six recombinant lysosomal enzymes now available for ERT and one in the process of development. The assay is easy, specific, and reproducible. The results of the comparative study revealed that the M6P contents of agalsidase alfa, agalsidase beta, modified α-N-acetylgalactosaminidase, alglucosidase alfa, laronidase, idursulfase, and imiglucerase are 2.1, 2.9, 5.9, 0.7, 2.5, 3.2, and <0.3 mol/mol enzyme, respectively. The results were correlated with those of the biochemical analyses previously performed and that of the binding assay of exposed M6P of the enzymes with the domain 9 of the cation-independent M6P receptor. This assay method is useful for comparison of the M6P contents of recombinant lysosomal enzymes for ERT.

Enzyme replacement therapy for lysosomal diseases: lessons from 20 years of experience and remaining challenges

In 1964, Christian de Duve first suggested that enzyme replacement might prove therapeutic for lysosomal storage diseases (LSDs). Early efforts identified the major obstacles, including the inability to produce large quantities of the normal enzymes, the lack of animal models for proof-of-concept studies, and the potentially harmful immune responses to the "foreign" normal enzymes. Subsequently, the identification of receptor-mediated targeting of lysosomal enzymes, the cloning and overexpression of human lysosomal genes, and the generation of murine models markedly facilitated the development of enzyme replacement therapy (ERT). However, ERT did not become a reality until the early 1990s, when its safety and effectiveness were demonstrated for the treatment of type 1 Gaucher disease. Today, ERT is approved for six LSDs, and clinical trials with recombinant human enzymes are ongoing in several others. Here, we review the lessons learned from 20 years of experience, with an emphasis on the general principles for effective ERT and the remaining challenges.

Engineering the yeast Yarrowia lipolytica for the production of therapeutic proteins homogeneously glycosylated with Man₈GlcNAc₂ and Man₅GlcNAc₂

Background

Protein-based therapeutics represent the fastest growing class of compounds in the pharmaceutical industry. This has created an increasing demand for powerful expression systems. Yeast systems are widely used, convenient and cost-effective. Yarrowia lipolytica is a suitable host that is generally regarded as safe (GRAS). Yeasts, however, modify their glycoproteins with heterogeneous glycans containing mainly mannoses, which complicates downstream processing and often interferes with protein function in man. Our aim was to glyco-engineer Y. lipolytica to abolish the heterogeneous, yeast-specific glycosylation and to obtain homogeneous human high-mannose type glycosylation.

Results

We engineered Y. lipolytica to produce homogeneous human-type terminal-mannose glycosylated proteins, i.e. glycosylated with Man₈GlcNAc₂ or Man₅GlcNAc₂. First, we inactivated the yeast-specific Golgi α-1,6-mannosyltransferases YlOch1p and YlMnn9p; the former inactivation yielded a strain producing homogeneous Man₈GlcNAc₂ glycoproteins. We tested this strain by expressing glucocerebrosidase and found that the hypermannosylation-related heterogeneity was eliminated. Furthermore, detailed analysis of N-glycans showed that YlOch1p and YlMnn9p, despite some initial uncertainty about their function, are most likely the α-1,6-mannosyltransferases responsible for the addition of the first and second mannose residue, respectively, to the glycan backbone. Second, introduction of an ER-retained α-1,2-mannosidase yielded a strain producing proteins homogeneously glycosylated with Man₅GlcNAc₂. The use of the endogenous LIP2pre signal sequence and codon optimization greatly improved the efficiency of this enzyme.

Conclusions

We generated a Y. lipolytica expression platform for the production of heterologous glycoproteins that are homogenously glycosylated with either Man₈GlcNAc₂ or Man₅GlcNAc₂ N-glycans. This platform expands the utility of Y. lipolytica as a heterologous expression host and makes it possible to produce glycoproteins with homogeneously glycosylated N-glycans of the human high-mannose-type, which greatly broadens the application scope of these glycoproteins.

Structural aspects of therapeutic enzymes to treat metabolic disorders

Protein therapeutics represents a niche subset of pharmacological agents that is rapidly gaining importance in medicine. In addition to the exceptional specificity that is characteristic of protein therapeutics, several classes of proteins have also been effectively utilized for treatment of conditions that would otherwise lack effective pharmacotherapeutic options. A particularly striking class of protein therapeutics is exogenous enzymes administered for replacement therapy in patients afflicted with metabolic disorders. To date, at least 11 enzymes have either been approved for use, or are in clinical trials for the treatment of selected inherited metabolic disorders. With the recent advancement in structural biology, a significantly larger amount of structural information for several of these enzymes is now available. This article is an overview of the correlation between structural perturbations of these enzymes with the clinical presentation of the respective metabolic conditions, as well as a discussion of the relevant structural modification strategies engaged in improving these enzymes for replacement therapies.

Production of glucocerebrosidase with terminal mannose glycans for enzyme replacement therapy of Gaucher's disease using a plant cell system

Gaucher's disease, a lysosomal storage disorder caused by mutations in the gene encoding glucocerebrosidase (GCD), is currently treated by enzyme replacement therapy using recombinant GCD (Cerezyme) expressed in Chinese hamster ovary (CHO) cells. As complex glycans in mammalian cells do not terminate in mannose residues, which are essential for the biological uptake of GCD via macrophage mannose receptors in human patients with Gaucher's disease, an in vitro glycan modification is required in order to expose the mannose residues on the glycans of Cerezyme. In this report, the production of a recombinant human GCD in a carrot cell suspension culture is described. The recombinant plant-derived GCD (prGCD) is targeted to the storage vacuoles, using a plant-specific C-terminal sorting signal. Notably, the recombinant human GCD expressed in the carrot cells naturally contains terminal mannose residues on its complex glycans, apparently as a result of the activity of a special vacuolar enzyme that modifies complex glycans. Hence, the plant-produced recombinant human GCD does not require exposure of mannose residues in vitro, which is a requirement for the production of Cerezyme. prGCD also displays a level of biological activity similar to that of Cerezyme produced in CHO cells, as well as a highly homologous high-resolution three-dimensional structure, determined by X-ray crystallography. A single-dose toxicity study with prGCD in mice demonstrated the absence of treatment-related adverse reactions or clinical findings, indicating the potential safety of prGCD. prGCD is currently undergoing clinical studies, and may offer a new and alternative therapeutic option for Gaucher's disease.

One-pot carbanionic synthesis of P1,P2-diglycosyl, P1,P1,P2-triglycosyl, and P1,P1,P2,P2-tetraribosyl methylenediphosphonates

Novel lithiated carbanions derived from ethyl glycosyl- and diglycosyl methylphosphonates were used in a direct and convenient synthesis of P1,P2-diglycosyl, P1,P1,P2-triglycosyl, and P1,P1,P2,P2-tetraribosyl methylenediphosphonates involving a one-pot methylidenediphosphonylation of sugars.

Feasibility of gene therapy in Gaucher disease using an adeno-associated virus vector

Gaucher disease, one of the common lysosomal storage disorders, is caused by a deficiency of glucocerebrosidase (GC). We investigated gene transfer using recombinant adeno-associated viral (rAAV) vectors containing human GC cDNA driven by the human elongation factor 1-alpha promoter. This rAAV vector mediated efficient expression of human GC in human Gaucher fibroblasts. GC activities were increased from 2.8 to 3.4 times in normal fibroblast and from 1.9 to 4.6 times in Gaucher fibroblasts, and these increases in GC activity were maintained over 20 weeks. Intravenous administration of vectors via the hepatic portal vein and tail vein of wild-type mice resulted in efficient transduction into the tissues. GC activities of the liver, spleen, and lung in transduced mice were increased significantly up to two fold at 6 weeks after transduction. Significantly increased GC activities persisted over 20 weeks. Therefore, rAAV vector-mediated gene transfer may provide a therapeutic approach for the treatment of Gaucher disease.

Expression and secretion of human glucocerebrosidase mediated by recombinant lentivirus vectors in vitro and in vivo: implications for gene therapy of Gaucher disease

Gaucher disease is a lysosomal storage disorder resulting from a deficiency of glucocerebrosidase (GC). In this study, we showed that vascular and hepatic delivery of a HIV-1-based lentivirus vector encoding human GC cDNA produced therapeutic levels of GC protein. A high level of expression of GC was produced in cultured fibroblasts derived from patients with Gaucher disease by transducing the cells with recombinant lentivirus vectors. GC secreted by transduced fibroblasts was taken up by adjacent GC-deficient cells by endocytosis. Intraportal administration of lenti-EF-GC viral vector resulted in efficient transduction and expression of the GC. Vascular delivery of vector resulted in high levels of GC expression in mice that persisted in most organs over the four months. No significant abnormalities were found attributable to recombinant lentivirus vectors in any of the tissues examined. This study represents an initial step toward gene transfer using recombinant lentivirus vectors for treatment of Gaucher disease.

Enzyme replacement and enhancement therapies for lysosomal diseases

Although first suggested by de Duve in 1964, enzyme replacement therapy (ERT) for lysosomal storage diseases did not become a reality until the early 1990s when its safety and effectiveness were demonstrated in type 1 Gaucher disease. Today, ERT is a reality for Gaucher disease, Fabry disease and mucopolysaccharidosis type I (MPS I), and clinical trials with recombinant human enzymes are ongoing in Pompe disease, MPS II and MPS VI, and are about to begin in Neimann-Pick B disease. In addition to ERT, enzyme enhancement therapy (EET) offers a novel therapeutic strategy to increase the residual function of mutant proteins. EET employs small molecules as 'pharmacological chaperones' to rescue misfolded and/or unstable mutant enzymes or proteins that have residual function. EET also offers the possibility of treating neurodegenerative lysosomal disorders since these small therapeutic molecules may cross the blood-brain barrier. The current status of ERT and the prospects for EET for lysosomal storage diseases are reviewed.

Enzyme therapy for lysosomal storage disease: principles, practice, and prospects

Over the past three decades, enzyme therapy for lysosomal storage diseases has moved from an academic pursuit to direct delivery of effective clinical care for affected patients and families. This success is based on understanding the complexities of lysosomal biogenesis, lysosomal hydrolase sorting and hydrolytic requirements, and the target sites of pathology of these diseases. This article reviews these concepts and their application to the treatment of affected patients with Gaucher disease, Fabry disease, and mucopolysaccharidosis I. The principles, progress, and practice in these diseases provide prototypes for expansion of enzyme therapy to a growing set of these diseases.

Enzyme replacement therapy in the mouse model of Pompe disease

Deficiency of acid alpha-glucosidase (GAA) results in widespread cellular deposition of lysosomal glycogen manifesting as myopathy and cardiomyopathy. When GAA-/- mice were treated with rhGAA (20 mg/kg/week for up to 5 months), skeletal muscle cells took up little enzyme compared to liver and heart. Glycogen reduction was less than 50%, and some fibers showed little or no glycogen clearance. A dose of 100 mg/kg/week resulted in approximately 75% glycogen clearance in skeletal muscle. The enzyme reduced cardiac glycogen to undetectable levels at either dose. Skeletal muscle fibers with residual glycogen showed immunoreactivity for LAMP-1/LAMP-2, indicating that undigested glycogen remained in proliferating lysosomes. Glycogen clearance was more pronounced in type 1 fibers, and histochemical analysis suggested an increased mannose-6-phosphate receptor immunoreactivity in these fibers. Differential transport of enzyme into lysosomes may explain the strikingly uneven pattern of glycogen removal. Autophagic vacuoles, a feature of both the mouse model and the human disease, persisted despite glycogen clearance. In some groups a modest glycogen reduction was accompanied by improved muscle strength. These studies suggest that enzyme replacement therapy, although at much higher doses than in other lysosomal diseases, has the potential to reverse cardiac pathology and to reduce the glycogen level in skeletal muscle.

Enzyme replacement therapy: conception, chaos and culmination

Soon after the enzymatic defects in Gaucher disease and in Niemann-Pick disease were discovered, enzyme replacement or enzyme supplementation was proposed as specific treatment for patients with these and related metabolic storage disorders. While relatively straightforward in concept, successful implementation of this approach required many years of intensive effort to bring it to fruition. Procedures were eventually developed to produce sufficient quantities of the requisite enzymes for clinical trials and to target therapeutic enzymes to lipid-storing cells. These achievements led to the development of effective enzyme replacement therapy for patients with Gaucher disease and for Fabry disease. These demonstrations provide strong incentive for the application of this strategy for the treatment of many human disorders of metabolism.

Enzyme replacement and enhancement therapies: lessons from lysosomal disorders

The past decade has witnessed remarkable advances in our ability to treat inherited metabolic disorders, especially the lysosomal storage diseases, a group of more than 40 disorders, each of which is caused by the deficiency of a lysosomal enzyme or protein. During the past few years, both enzyme replacement and enhancement therapies have been developed to treat these disorders. This review discusses the successes and shortcomings of these therapeutic strategies, and the contributions that they have made to treating lysosomal storage diseases.

Demonstration of feasibility of in vivo gene therapy for Gaucher disease using a chemically induced mouse model

Progress towards developing gene therapy for Gaucher disease has been hindered by the lack of an animal model. Here we describe a mouse model of Gaucher disease which has a chemically induced deficiency of glucocerebrosidase and that accumulates elevated levels of glucosylceramide (GL-1) in the lysosomes of Kupffer cells. Administration of mannose-terminated glucocerebrosidase (Cerezyme) resulted in the reduction of GL-1 levels in the livers of these animals. Gene transduction of hepatocytes with a plasmid DNA vector encoding human glucocerebrosidase (pGZB-GC) generated high-level expression and secretion of the enzyme into systemic circulation with consequent normalization of Kupffer cell GL-1 levels. This suggested that the de novo synthesized and unmodified enzyme produced by hepatocyte transduction was also capable of being delivered to the cells that are primarily affected in Gaucher disease. Immunolocalization studies also revealed that preferential transduction and expression of human glucocerebrosidase in the Kupffer cells with subsequent reduction in the GL-1 levels could be attained with a low dose of a recombinant adenoviral vector encoding the human enzyme (Ad2/CMV-GC). This observation raises the possibility of gene therapy for Gaucher disease that involves directly transducing the affected histiocytes using recombinant adenoviral vectors. Together, these data demonstrate the potential for use of in vivo gene therapy vectors for treating Gaucher disease.

Receptor mediated binding of two glycosylation forms of N-acetylgalactosamine-4-sulphatase

The lysosomal storage disorders are a group of inherited metabolic diseases each characterised by a relative or absolute deficiency of one or more of the lysosomal proteins involved in the hydrolysis of glycoconjugates or in the transport of the resulting product. Enzyme replacement therapies are under consideration for a number of these disorders and are based on the in vitro observation that cells from affected patients can be corrected by addition of exogenous enzyme. In this study, two glycosylation variants of the lysosomal enzyme N-acetylgalactosamine-4-sulphatase (4S) (the deficiency of which causes Mucopolysaccharidosis (MPS) type VI, (Maroteaux-Lamy syndrome) were made by expression of 4S cDNA in both wild type chinese hamster ovary (CHO-K1), and Lec1 (N-acetylglucosaminyltransferase I deficient CHO-K1) cells. Differences in the glycosylation pattern of the two enzyme forms were demonstrated with endoglycosidase H and N-glycosidase F digestions. The receptor mediated binding of these two forms of 4S to two cell types, human skin fibroblasts and rat alveolar macrophages, was then analysed. We have shown that both enzyme forms bind to the mannose-6-phosphate receptor on human skin fibroblasts with equal affinity demonstrating that the degree of phosphorylation of mannose residues in the two forms is similar. However, using rat alveolar macrophages, we found that the binding/uptake of the two enzymes differs considerably. These results show that differences in glycosylation of lysosomal enzymes can be an important factor in altering enzyme uptake by different cell types. Thus, producing carbohydrate modification variants in this way may be useful for altering the distribution of exogenous enzyme in vivo.

Gaucher's disease: past, present and future

A patient with what is now known as Gaucher's disease was first described by P.C.E. Gaucher in 1882. Fifty years later, Aghion reported that patients with this condition accumulated a sphingoglycolipid called glucocerebroside. Considerably more time was required for the demonstration by Brady and co-workers in 1964 that Gaucher's disease was due to reduced activity of a beta-glucosidase called glucocerebrosidase. This information provided the basis for the development of reliable diagnostic tests, detection of most of the carriers of this disorder and the prenatal diagnosis of this condition. Evidence was presented in 1990 and 1991 indicating the highly beneficial effects of enzyme replacement therapy in patients with Gaucher's disease. Gene therapy for Gaucher's disease was initiated in 1995. While little indication of success was obtained in this inaugural attempt, it is expected that improvements in this technology will provide a permanent cure for patients with this disorder.

Gaucher's disease in pregnancy

Gaucher's disease is an autosomal recessive lysosomal storage disease, resulting from a deficiency of the enzyme glucocerebrosidase, important for the physiologic recycling of cell membrane lipids. The clinical symptoms and disease presentations of Gaucher's disease are heterogeneous, including hepatosplenomegaly, bone "crisis" and fracture, anemia, thrombocytopenia and in some forms, rapid neurological decompensation. Similarly, the genetic variability of Gaucher's disease is diverse, and in some aspects affects phenotypic expression. Type 1 Gaucher's disease, however, usually present with less severe symptoms, at more advanced age, and is particularly amenable to enzyme replacement therapy with alglucerase. In type 1 patients with Gaucher's disease reproductive age is commonly reached and childbearing frequently desired with need for appropriate prenatal diagnosis, counseling and careful obstetrical surveillance. Although pregnancy concurrent with Gaucher's disease has been reported in the medical literature, only one small series of alglucerase treated Gaucher's disease during pregnancy exists. Without treatment, pregnancy concurrent with Gaucher's disease has several risks including an increased severity of anemia and thrombocytopenia that can potentiate postpartum bleeding, significant increases in organomegaly and possibly an increased spontaneous abortion rate. It is yet to be shown whether alglucerase reduces the risk of these complications during pregnancy and whether its use has any adverse effect on fetal development.

Enzyme replacement and gene therapy for Gaucher's disease

The lipid storage disorders have long been considered primary candidates for enzyme replacement therapy. This goal has been achieved with a remarkable degree of success in Gaucher's disease. Among the accomplishments that were important to obtain clinical benefit were the development of a large-scale procedure to purify human placental glucocerebrosidase and a method to target this enzyme to lipid-storing macrophages through glycoform modification. In addition, the effectiveness of recombinantly produced macrophage-targeted glucocerebrosidase has recently been demonstrated. Because macrophages originate from stem cells in the bone marrow, ex vivo transduction of these cells with retroviral vectors containing the cDNA for human glucocerebrosidase is being explored for the genetic therapy of Gaucher's disease.

Receptor mediated glycotargeting

Glycotargeting relies on carrier molecules possessing carbohydrates that are recognized and internalized by cell surface mammalian lectins. Numerous types of glycotargeting vehicles have been designed based on the covalent attachment of saccharides to proteins, polymers and other aglycones. These carriers have found their major applications in antiviral therapy, immunoactivation, enzyme replacement therapy and gene therapy. This review compared different types of glycotargeting agents and the lectins which have been successfully targeted to treat both model and human diseases. It may be concluded that the discovery of new mammalian lectins which endocytose their ligands will lead to the rapid development of new glycotargeting agents founded on the principles of carbohydrate-protein interactions.

Modifying exogenous glucocerebrosidase for effective replacement therapy in Gaucher disease

Important therapeutic principles were established in developing effective enzyme replacement therapy for patients with Gaucher disease. The background and sequence of the investigations that led to effective delivery of exogenous glucocerebrosidase to the lipid-storing macrophages in patients with Gaucher disease are described. The principle of targeting the intravenously injected enzyme to the mannose lectin on the surface of these cells by engineering the glycoform of the enzyme is a useful model of an essential requirement for effective enzyme therapy. Similar strategies are expected to be effective for the treatment of a number of hereditary metabolic disorders of humans.

Creating the costliest orphan. The Orphan Drug Act in the development of Ceredase

The FDA recently approved Ceredase, a new treatment for Gaucher's disease, under the provisions of the Orphan Drug Act. Ceredase is unusually expensive, but there are no satisfactory alternative therapies. It appears likely that Ceredase would not have become available without the protection of the Orphan Drug Act, but its expense and the lack of information about its long-term effects on health raise questions about whether the ODA provides appropriate incentives to develop cost-effective technologies.

Replacement therapy for inherited enzyme deficiency--macrophage-targeted glucocerebrosidase for Gaucher's disease

BACKGROUND AND METHODS

Gaucher's disease, the most prevalent of the sphingolipid storage disorders, is caused by a deficiency of the enzyme glucocerebrosidase (glucosylceramidase). Enzyme replacement was proposed as a therapeutic strategy for this disorder in 1966. To assess the clinical effectiveness of this approach, we infused macrophage-targeted human placental glucocerebrosidase (60 IU per kilogram of body weight every 2 weeks for 9 to 12 months) into 12 patients with type 1 Gaucher's disease who had intact spleens. The frequency of infusions was increased to once a week in two patients (children) during part of the trial because they had clinically aggressive disease.

RESULTS

The hemoglobin concentration increased in all 12 patients, and the platelet count in 7. Serum acid phosphatase activity decreased in 10 patients during the trial, and the plasma glucocerebroside level in 9. Splenic volume decreased in all patients after six months of treatment, and hepatic volume in five. Early signs of skeletal improvements were seen in three patients. The enzyme infusions were well tolerated, and no antibody to the exogenous enzyme developed.

CONCLUSIONS

Intravenous administration of macrophage-targeted glucocerebrosidase produces objective clinical improvement in patients with type 1 Gaucher's disease. The hematologic and visceral responses to enzyme replacement develop more rapidly than the skeletal response.

Influence of the galactosyl ligand structure on the interaction of galactosylated liposomes with mouse peritoneal macrophages

Liposomes bearing at their surface mono- and triantennary galactosyl ligands were prepared and their interaction with the galactose receptor of mouse peritoneal macrophages studied. Triantennary structures were synthesized by coupling derivatives of 1-thio-beta-D-galactose to the amino groups of lysyl-lysine dipeptide. Galactosylated liposomes were obtained either by synthesis of neo-galactolipids followed by their incorporation into the vesicles or by neo-galactosylation of performed liposomes by reaction between thiol-functionalized galactosyl ligands and vesicles bearing maleimido groups. The interaction of the galactosylated liposomes with the macrophage lectin was remarkably sensitive to the topology of the ligands, i.e., a spacer-arm length about 3 nm was necessary and, in contrast to results obtained with the galactose receptor of other cells, the triantennary structure did not provide additional binding. Related to the strategy of drug delivery with targeted liposomes, these results indicate that lectins from different cells might possibly be distinguished by using multiantennary ligands having optimal geometries.

Gaucher disease

Gaucher disease is a glycolipid storage disorder characterized by accumulation of glucocerebroside in the liver, spleen, and bones, and caused by a deficiency of glucocerebrosidase. Glucocerebrosidase cDNA has been cloned and sequenced, and much has been learned about the synthesis and processing of this enzyme. Inherited as an autosomal recessive disorder, Gaucher Disease is relatively common among Ashkenazi Jews. In its most common form, designated Type 1 or adult type of Gaucher disease, the central nervous system is spared. Several organ systems may be involved, including not only the hematopoietic tissues and bones, but also the lungs. Diagnosis can be achieved without marrow examination by estimating the glucocerebrosidase (beta-glucosidase) activity of the peripheral blood leukocytes. Currently available conventional therapy is purely symptomatic in nature, including orthopedic procedures and splenectomy. On an experimental basis, splenectomy may be partial instead of total. Because the disease is due to an abnormality of the monocyte-macrophage system, cells that arise from the hematopoietic stem cell, and because the central nervous system is spared it has been considered a very suitable candidate for experimental therapeutic intervention. Bone marrow transplantation has been attended with limited success, enzyme therapy has not yet been successful, and studies utilizing gene transfer are underway.

Glucocerebrosidase deficiency and lysosomal storage of glucocerebroside induced in cultured macrophages

A cell culture model stimulating the genetic deficiency of glucocerebrosidase has been developed, utilizing macrophages and conduritol B epoxide (CBE), the specific irreversible inhibitor of the enzyme. Rat peritoneal macrophage glucocerebrosidase was completely inhibited when cells were treated with 10 microM CBE for 16 h or 100 microM CBE for 2 h. The t1/2 of inactivation was 30 min at 10 microM concentration. When cells were washed free of CBE, the enzyme activity reappeared linearly with time, reaching 50% of control activity 48 h after removal of the inhibitor. CBE-treated macrophages have normal phagocytic activity toward [3H]glycine-coupled latex beads and a normal number of mannose receptors. CBE was found to have no effect on other lysosomal enzymes. When [14C]glucocerebroside, encapsulated in multilamellar liposomes with alpha-D-mannopyranoside covalently coupled to the surface, was fed to glucocerebrosidase-depleted macrophages, the radiolabelled glycolipid accumulated and was undegraded. Subcellular fractionation on a Percoll density gradient demonstrated that the stored glucocerebroside in the CBE-treated macrophages was localized in lysosomes.

Lectin-specific targeting of lysosomal enzymes to reticuloendothelial cells

The principles and methods used for enzymatic modification of the carbohydrate portion of glucocerebrosidase are similar to those performed by Ashwell and Morell, Stahl, and others. It is difficult to explain the lack of uptake of native enzyme through binding of the high-mannose type glycopeptide to Man/GlcNAc receptors since approximately 20% of the total oligosaccharides on the native enzyme are high mannose type. Possibly a requirement for multiple sites of attachment to the receptor is not met by a single high-mannose type oligosaccharide per molecule. Alternatively, the presence of complex type oligosaccharides on this enzyme, demonstrated by structural studies, may mask the mannose site and thus account for the poor uptake of native enzyme. The ability to successfully deglycosylate any protein or enzyme in order to specifically target a selected cell type requires that there be (1) an available source of pure enzyme; (2) specific exoglycosidases of high specific activity available either commercially or relatively easily purified; (3) chemical or biochemical means available for the testing of the product, preferably at each step; and (4) a means of separating the glycosidases used from the desired enzyme product. The characteristic and unique accumulation of glucocerebroside only in cells of the monocyte- histiocyte series, makes Gaucher's disease an excellent prototype for the study of enzyme replacement therapy. The principles demonstrated for the enzymatic deglycosylation of glucocerebrosidase may be applied to the cell-specific delivery of other glycoproteins as well. Other lysosomal diseases in which storage occurs in multiple cell types may be ameliorated by administration of macrophage-directed enzymes if, by so doing, storage material can be digested during the normal phagocytic turnover of senescent cells. Consideration of the kinetics of degradation and the structural features affecting the stability of enzymes in vivo are prerequisites to improving the bioengineering of targeted lysosomal enzymes. Animal and culture models have been developed for the study of glucocerebrosidase delivery to specific cell types and substrate degradation. Other studies have progressed toward a definition not only of the receptors at the plasma membrane involved in the internalization of exogenous enzyme, but also of internal receptors or properties of the lysosome responsible for intracellular protein trafficking. A complete understanding of the forces acting to direct endogenous or exogenously supplied enzyme to a given subcellular organelle will require a synthesis of experimental results from all areas of glycoprotein research.

Targeting of synthetically glycosylated human placental glucocerebrosidase

Human placental beta-glucocerebrosidase modified by covalent attachment of N2-(N2, N6-bis [3-(alpha-D-mannopyranosylthio)propionyl]-L- lysyl)-N6-[3-(alpha-D-mannopyranosylthio)propionyl]-L-lysine was administered to rats by intravenous injection. Comparison of enzyme distribution in isolated liver cell populations indicates an increase in enzyme-specific activity of 18-fold in nonparenchymal cells and only 1.5-fold to hepatocytes compared to uninjected control animals. This macrophage-specific delivery of an active lysosomal enzyme has potential for application in enzyme replacement trials.