Vestronidase alfa: Recombinant human ß-glucuronidase as an enzyme replacement therapy for MPS VII.

Mucopolysaccharidosis VII (MPS VII) is a rare lysosomal storage disease characterized by a deficiency in the enzyme ß-glucuronidase that has previously been successfully treated in a mouse model with enzyme replacement therapy. Here, we present the generation of a novel, highly sialylated version of recombinant human ß-glucuronidase (rhGUS), vestronidase alfa, that has high uptake, resulting in an improved enzyme replacement therapy for the treatment of patients with MPS VII. In vitro, vestronidase alfa has 10-fold more sialic acid per mole of rhGUS monomer than a prior rhGUS version (referred to as GUS 43/44) and demonstrated very high affinity at ~1 nM half maximal uptake in human MPS VII fibroblasts. Vestronidase alfa has a longer enzymatic half-life after uptake into fibroblasts compared with other enzymes used as replacement therapy for MPS (40 days vs 3 to 4 days, respectively). In pharmacokinetic and tissue distribution experiments in Sprague-Dawley rats, intravenous administration of vestronidase alfa resulted in higher serum rhGUS levels and enhanced ß-glucuronidase activity distributed to target tissues. Weekly intravenous injections of vestronidase alfa (0.1 mg/kg to 20 mg/kg) in a murine model of MPS VII demonstrated efficient enzyme delivery to all tissues, including bone and brain, as well as reduced lysosomal storage of glycosaminoglycans (GAGs) in a dose-dependent manner, resulting in increased survival after 8 weeks of treatment. Vestronidase alfa was well-tolerated and demonstrated no toxicity at concentrations that reached 5-times the proposed clinical dose. In a first-in-human phase 1/2 clinical trial, a dose-dependent reduction in urine GAG levels was sustained over 38 weeks of treatment with vestronidase alfa. Together, these results support the therapeutic potential of vestronidase alfa as an enzyme replacement therapy for patients with MPS VII.

Condoliase for treatment of lumbar disc herniation.

Lumbar disc herniation (LDH) is generally treated with a conservative therapy, and surgery is the only therapeutic option currently available for patients unresponsive to the conservative therapy. In the 1980s, chemonucleolysis with chymopapain, a protease, was widely used as the intermediate treatment between conservative therapy and surgical therapy in the Western countries. However, since chymopapain was withdrawn from the market in 2002 for non-scientific commercial reasons, chemonucleolysis has not been a therapeutic option for LDH. Condoliase (chondroitin sulfate ABC endolyase), a glycosaminoglycan-degrading enzyme, was approved by the drug regulatory authority in Japan as a newer intradiscal therapy for LDH after clinical studies conducted in Japan demonstrated efficacy and safety for patients with LDH. This review will focus on the preclinical pharmacology, pharmacokinetics, efficacy and safety of condoliase as a new option for treatment of LDH.

Pharmacokinetic and Pharmacodynamic Modeling to Optimize the Dose of Vestronidase Alfa, an Enzyme Replacement Therapy for Treatment of Patients with Mucopolysaccharidosis Type VII: Results from Three Trials.

INTRODUCTION: Mucopolysaccharidosis type VII (MPS VII, Sly Syndrome) is a progressive, debilitating, ultra-rare lysosomal storage disorder caused by the deficiency of ß-glucuronidase (GUS), an enzyme required for breakdown of glycosaminoglycans (GAGs). Vestronidase alfa, a recombinant human GUS, is an enzyme replacement therapy approved in the US and EU for the treatment of MPS VII. METHODS: The pharmacokinetics (PK) and pharmacodynamics (PD) of vestronidase alfa were evaluated in 23 adult and pediatric subjects with MPS VII enrolled in phase I-III clinical trials to optimize the clinical dosing regimen of vestronidase alfa. The serum concentration-time profiles were adequately described by a two-compartment population PK model incorporating subjects' body weight as the only significant covariate. RESULTS: Model-based simulations predicted a substantially decreased time duration of serum exposures exceeding the level of Kuptake (the in vitro determined vestronidase alfa concentration corresponding to 50% maximum rate of cellular uptake) for 4 or 8 mg/kg once every 4 weeks dosing, compared with 4 mg/kg once every other week (QOW) dosing by intravenous infusion, suggesting that given the same total monthly dose, the QOW dosing frequency should result in more efficient delivery to the GUS-deficient tissue cells, and therefore superior treatment efficacy. A standard inhibitory maximal effect model reasonably explained the observed pharmacological PD responses of reduction in urinary GAGs from pretreatment baseline, which appeared to have reached the plateau of maximal effect at the 4 mg/kg QOW dose. CONCLUSION: The modeling results, together with the clinical evidence of safety and efficacy, supported the recommended 4 mg/kg QOW dosing regimen of vestronidase alfa for pediatric and adult patients with MPS VII. CLINICAL TRIAL REGISTRATION: NCT01856218, NCT02418455, NCT02230566.

Induction of ß-glucuronidase release by cytostatic agents in small tumors.

Extracellular ß-glucuronidase (ß-GUS) in tumors has been investigated as a target enzyme for prodrug therapy. However, despite encouraging preclinical results, animal studies also indicate that the success of prodrug therapy might be limited by the insufficient prodrug-converting enzyme activity, especially in small tumors. We hypothesized that a single dose of a cytostatic drug might induce the release of ß-GUS in small tumors, resulting in increased levels of extracellular ß-GUS and consequently a higher efficacy of the prodrug treatment. Here we examine the extent of ß-GUS release in small C6 glioma tumors after a single treatment of doxorubicin (DOX), carmustine (BCNU) and tumor necrosis factor α (TNF-α) with positron emission tomography (PET) and the tracer 1-O-(4-(2-fluoroethyl-carbamoyloxymethyl)-2-nitrophenyl)-O-ß-d-glucopyronuronate, [(18)F]FEAnGA, which has been proven to be selective for extracellular ß-GUS. Induction of ß-GUS release was first investigated in cultured C6 glioma cells. In addition, a [(18)F]FEAnGA PET study was performed in C6 tumor-bearing rats 48 h after a single treatment with different cytostatics to evaluate the extent of ß-glucuronidase release. The cleavage of [(18)F]FEAnGA by ß-GUS was analyzed in tumor homogenates. The induction of tumor necrosis and leukocyte infiltration was confirmed by histochemical analysis and flow cytometry. The in vitro studies indicated that all treatments resulted in a decline of viable cells and an increase of extracellular ß-GUS activity. PET studies confirmed that ß-GUS was released in vivo and the distribution volume of the PET tracer [(18)F]FEAnGA in C6 gliomas was increased significantly by 15-70%, depending on the treatment. Histochemical analysis of the tumors indicated that carmustine and TNF-α treatment caused a larger necrotic area with the absence of infiltrating immune cells, whereas doxorubicin induced an increase in leukocyte infiltration. These results were confirmed by flow cytometry. In conclusion, the present study demonstrates that a single dose of a cytostatic agent is able to increase the release of ß-GUS. The release in ß-GUS can be monitored by [(18)F]FEAnGA PET in a noninvasive manner. This study may open the way to a two-step chemotherapy-prodrug approach, in which tumors are treated with a single dose of a cytostatic drug prior to prodrug treatment.

New strategies for enzyme replacement therapy for lysosomal storage diseases.

Enzyme replacement therapy is an established means of treating lysosomal storage diseases. Infused enzymes are normally targeted to the lysosomes of affected cells by interactions with cell-surface receptors that recognize carbohydrate moieties such as mannose and mannose 6-phosphate on the enzymes. Therefore, we have investigated alternative strategies to deliver the lysosomal enzyme beta-glucuronidase in the enzyme-deficient mucopolysaccharidosis type VII mouse model. Here we summarize our recent efforts to use nontraditional ways to deliver beta-glucuronidase. First, we used a chimeric protein of the insulin-like growth factor II (IGF-II) fused to beta-glucuronidase to deliver enzyme via the IGF-II binding site on the bifunctional IGF-II/mannose 6-phosphate receptor. Second, we used the 11-amino-acid human immunodeficiency virus (HIV) Tat domain fused to beta-glucuronidase to mediate uptake by absorptive endocytosis. Interaction with heparan sulfate on the cell surface internalizes and delivers the Tat-tagged enzyme to the lysosome via plasma membrane recycling. Third, we created a chimeric beta-glucuronidase fused to the Fc portion of human immunoglobulin G (IgG) Fc, which was transported by the neonatal Fc receptor from the maternal circulation across the placenta to sites of storage in fetal tissues. Finally, periodate treatment was used to eliminate interaction with carbohydrate receptors, creating an enzyme with increased plasma half-life, resulting in transport across the blood-brain barrier and clearance of storage in neurons. These strategies for delivering lysosomal enzymes could also be used to target nonlysosomal proteins or enzymes identified for bioremediation of other conditions.

Genetic engineering of IgG-glucuronidase fusion proteins.

beta-Glucuronidase (GUSB) is a lysosomal enzyme that could be developed as a brain therapy for Type VII Mucopolysaccharidosis. However, GUSB does not cross the blood-brain barrier (BBB). To enable BBB transport of the enzyme, human GUSB was re-engineered as a fusion protein with the chimeric monoclonal antibody (MAb) to the human insulin receptor (HIR). The HIRMAb crosses the BBB on the endogenous insulin receptor, and acts as a molecular Trojan horse to ferry into brain the GUSB. The 611 amino acid GUSB was fused to either the carboxyl or amino terminus of the heavy chain of the HIRMAb. This study illustrates the differential retention of functionality of IgG-enzyme fusion proteins depending on how the fusion protein is engineered.

Infused Fc-tagged beta-glucuronidase crosses the placenta and produces clearance of storage in utero in mucopolysaccharidosis VII mice.

Glycosaminoglycan storage begins in prenatal life in patients with mucopolysaccharidosis (MPS). In fact, prenatal hydrops is a common manifestation of MPS VII because of beta-glucuronidase (GUS) deficiency. One way to address prenatal storage might be to deliver the missing enzyme across the placenta into the fetal circulation. Maternal IgG is transported across the placenta by the neonatal Fc receptor (FcRn), which recognizes the Fc domain of IgG and mediates transcytosis from maternal to fetal circulation. We hypothesized that we could exploit this process to deliver corrective enzyme to the fetus. To test this hypothesis, the C-terminal fusion protein, GUS-Fc, was compared with native, untagged, recombinant GUS for clearance from the maternal circulation, delivery to the fetus, and reduction of lysosomal storage in offspring of MPS VII mice. We observed that GUS-Fc, infused into pregnant mothers on embryonic days 17 and 18, was transported across the placenta. Similarly infused untagged GUS was not delivered to the fetus. GUS-Fc plasma enzyme activity in newborn MPS VII mice was 1,000 times that seen after administration of untagged GUS and approximately 100 times that of untreated WT newborns. Reduced lysosomal storage in heart valves, liver, and spleen provided evidence that in utero enzyme replacement therapy with GUS-Fc targeted sites of storage in the MPS VII fetus. We hypothesize that this noninvasive approach could deliver the missing lysosomal enzyme to a fetus with any lysosomal storage disease. It might also provide a method for inducing immune tolerance to the missing enzyme or another foreign protein.

Overcoming the blood-brain barrier with high-dose enzyme replacement therapy in murine mucopolysaccharidosis VII.

Enzyme replacement therapy (ERT) effectively reverses storage in several lysosomal storage diseases. However, improvement in brain is limited by the blood-brain barrier except in the newborn period. In this study, we asked whether this barrier could be overcome by higher doses of enzyme than are used in conventional trials. We measured the distribution of recombinant human beta-glucuronidase (hGUS) and reduction in storage by weekly doses of 0.3-40 mg/kg administered i.v. over 1-13 weeks to mucopolysaccharidosis type VII mice immunotolerant to recombinant hGUS. Mice given up to 5 mg/kg enzyme weekly over 3 weeks had moderate reduction in meningeal storage but no change in neo-cortical neurons. Mice given 20-40 mg/kg three times over 1 week showed no reduction in storage in any area of the CNS except the meninges. In contrast, mice receiving 4 mg/kg per week for 13 weeks showed clearance not only in meninges but also in parietal neocortical and hippocampal neurons and glia. Mice given 20 mg/kg once weekly for 4 weeks also had decreased neuronal, glial, and meningeal storage and averaged 2.5% of wild-type hGUS activity in brain. These results indicate that therapeutic enzyme can be delivered across the blood-brain barrier in the adult mucopolysaccharidosis type VII mouse if administered at higher doses than are used in conventional ERT trials and if the larger dose of enzyme is administered over a sufficient period. These results may have important implications for ERT for lysosomal storage diseases with CNS involvement.

Developmental differences between cation-independent and cation-dependent mannose-6-phosphate receptors in rat brain at perinatal stages.

Mannose-6-phosphate receptors (MPRs) play a role in the selective transport of macromolecules bearing mannose-6-phosphate residue to lysosomes. To date, two types of MPRs have been described in most of cells and tissues: the cation-dependent (CD-MPR) and cation-independent mannose-6-phosphate receptor (CI-MPR). In order to elucidate their possible role in the central nervous system, the expression and binding properties of both MPRs were studied in rat brain along perinatal development. It was observed that the expression of CI-MPR decreases progressively from fetuses to adults, while the CD-MPR increases around the 10th day of birth, and maintains these values up to adulthood. Binding assays showed differences in the Bmax and KD values between the ages studied, and they did not correlate with the expression levels of both MPRs. Variations in lysosomal enzyme activities and expression of phosphomannosylated ligands during development correlated more with CD-MPR than with CI-MPR expression. These results suggest that both receptors play a different role in rat brain during perinatal development, being CD-MPR mostly involved in lysosome maturation.

Biodistribution, kinetics, and efficacy of highly phosphorylated and non-phosphorylated beta-glucuronidase in the murine model of mucopolysaccharidosis VII.

Enzyme replacement therapy (ERT) has been shown to be effective at reducing the accumulation of undegraded substrates in lysosomal storage diseases. Most ERT studies have been performed with recombinant proteins that are mixtures of phosphorylated and non-phosphorylated enzyme. Because different cell types use different receptors to take up phosphorylated or non-phosphorylated enzyme, it is difficult to determine which form of enzyme contributed to the clinical response. Here we compare the uptake, distribution, and efficacy of highly phosphorylated and non-phosphorylated beta-glucuronidase (GUSB) in the MPS VII mouse. Highly phosphorylated murine GUSB was efficiently taken up by a wide range of tissues. In contrast, non-phosphorylated murine GUSB was taken up primarily by tissues of the reticuloendothelial (RE) system. Although the tissue distribution was different, the half-lives of both enzymes in any particular tissue were similar. Both preparations of enzyme were capable of preventing the accumulation of lysosomal storage in cell types they targeted. An important difference in clinical efficacy emerged in that phosphorylated GUSB was more efficient than non-phosphorylated enzyme at preventing the hearing loss associated with this disease. These data suggest that both forms of enzyme contribute to the clinical responses of ERT in MPS VII mice but that enzyme preparations containing phosphorylated GUSB are more broadly effective than non-phosphorylated enzyme.

Efficient clearance of poly(ethylene glycol)-modified immunoenzyme with anti-PEG monoclonal antibody for prodrug cancer therapy.

The F(ab')(2) fragment of the anti-TAG-72 antibody, B72.3, was covalently linked to Escherichia coli-derived beta-glucuronidase that was modified with methoxypoly(ethylene glycol). The conjugate (B72.3-betaG-PEG) localized to a peak concentration in LS174T xenografts within 48 h after injection, but enzyme activity persisted in plasma such that prodrug administration had to be delayed for at least 4 days to avoid systemic prodrug activation and associated toxicity. Conjugate levels in tumors decreased to 36% of peak levels at this time. Intravenous administration of AGP3, an IgM mAb against methoxypoly(ethylene glycol), accelerated clearance of conjugate from serum and increased the tumor/blood ratio of B72. 3-betaG-PEG from 3.9 to 29.6 without significantly decreasing the accumulation of conjugate in tumors. Treatment of nude mice bearing established human colon adenocarcinoma xenografts with B72. 3-betaG-PEG followed 48 h later with AGP3 and a glucuronide prodrug of p-hydroxyaniline mustard significantly (p< or =0.0005) delayed tumor growth with minimal toxicity compared to therapy with a control conjugate or conventional chemotherapy.

Accelerated clearance of polyethylene glycol-modified proteins by anti-polyethylene glycol IgM.

Tumor therapy by the preferential activation of a prodrug at tumor cells targeted with an antibody-enzyme conjugate may allow improved treatment efficacy with reduced side effects. We examined antibody-mediated clearance of poly(ethylene glycol)-modified beta-glucuronidase (betaG-sPEG) as a method to reduce serum concentrations of enzyme and minimize systemic prodrug activation. Enzyme-linked immunosorbent assay and immunoblot analysis of two monoclonal antibodies generated by immunization of BALB/c mice with an antibody-betaG-sPEG conjugate showed that mAb 1E8 (IgG1) bound betaG and betaG-sPEG whereas mAb AGP3 (IgM) bound poly(ethylene glycol). Neither antibody affected the betaG activity. mAb 1E8 and AGP3 were modified with 36 and 208 galactose residues (1E8-36G and AGP3-208G) with retention of 72 and 48% antigen-binding activity, respectively, to target immune complexes to the asialoglycoprotein receptor on liver cells. mAb 1E8 and AGP3 cleared betaG-PEG from the circulation of mice as effectively as 1E8-36G and AGP3-208G, respectively. mAb AGP3, however, cleared betaG-sPEG more completely and rapidly than 1E8, reducing the serum concentration of betaG-sPEG by 38-fold in 8 h. AGP3 also reduced the concentration of an antibody-betaG-sPEG conjugate in blood by 280-fold in 2 h and 940-fold in 24 h. AGP3-mediated clearance did not produce obvious damage to liver, spleen, or kidney tissues. In addition, AGP3 clearance of betaG-sPEG before administration of BHAMG, a glucuronide prodrug of p-hydroxyaniline mustard, prevented toxicity associated with systemic activation of the prodrug based on mouse weight and blood cell numbers. AGP3 should be generally useful for accelerating the clearance of PEG-modified proteins as well as for improving the tumor/blood ratios of antibody-betaG-PEG conjugates for glucuronide prodrug therapy of cancer.

Poly(ethylene glycol) modification of beta-glucuronidase-antibody conjugates for solid-tumor therapy by targeted activation of glucuronide prodrugs.

Methoxypoly(ethylene glycol) (PEG) modification of Escherichia coli beta-glucuronidase (betaG) was examined as a method to improve the stability and pharmacokinetics of antibody-betaG conjugates for the targeted activation of glucuronide prodrugs at tumor cells. Introduction of 3 PEG molecules did not affect betaG activity whereas higher degrees of PEG modification produced progressively greater loss of enzymatic activity. The enzyme was found to be stable in serum regardless of PEG modification. PEG-modified betaG was coupled via a thioether bond to mAb RH1, an IgG2a antibody that binds to the surface of AS-30D hepatoma cells, to produce conjugates with 3 (RH1-betaG-3PEG), 5.2 (RH1-betaG-5PEG) or 9.8 (RH1-betaG-10PEG) PEG molecules per betaG with retention of 75%, 45% and 40% of the combined antigen-binding and enzymatic activity of the unmodified conjugate RH1-betaG. In contrast to the rapid serum clearance of RH1-betaG observed in mice, the PEG-modified conjugates displayed extended serum half-lives. RH1-betaG-3PEG and RH1-betaG-5PEG also exhibited reduced spleen uptake and greater tumor accumulation than RH1-betaG. BHAMG, the glucuronide prodrug of p-hydroxyaniline mustard (pHAM), was relatively nontoxic in vivo. Injection of 6 mg/kg or 12 mg/kg pHAM i.v. depressed white blood cell numbers by 46% and 71% whereas 80 mg/kg BHAMG reduced these levels by 22%. Although the tumor/blood ratio of RH1-betaG-5PEG was adversely affected by slow clearance from serum, combined therapy of small solid hepatoma tumors with this conjugate, followed 4 and 5 days later with i.v. injections of BHAMG, cured all of seven mice with severe combined immunodeficiency. Combined treatment with a control antibody-betaG conjugate and BHAMG delayed tumor growth and cured two of six mice while treatment with pHAM or BHAMG alone was ineffective.

A monoclonal antibody against human beta-glucuronidase for application in antibody-directed enzyme prodrug therapy.

The selectivity of anticancer agents may be improved by antibody-directed enzyme prodrug therapy (ADEPT). The immunogenicity of antibody-enzyme conjugates and the low tumor to normal tissue ratio calls for the use of a human enzyme and the development of a monoclonal antibody (MAb) against that enzyme for rapid clearance of the conjugate from the circulation. We isolated beta-glucuronidase from human liver. BALB/c mice were immunized with the roughly purified human liver beta-glucuronidase and we obtained an MAb designated 105. Immunoblotting showed reactivity with native tetrameric human beta-glucuronidase. MAb 105 neither bound to enzyme from bovine liver, rat liver, or mouse liver nor reacted with other human lysosomal enzymes. The antibody appeared to be useful to further purify human beta-glucuronidase from human liver or human placenta to homogeneity by affinity chromatography. MAb 105 did not inhibit the activity of human beta-glucuronidase. When human beta-glucuronidase was injected i.v. into BALB/c mice, the newly generated MAb 105 could indeed accelerate the clearance of the enzyme with a 50% drop in its activity within 5 min.

Tumor-selective prodrug activation by fusion protein-mediated catalysis.

A two component system, consisting of a fusion protein and an appropriate prodrug, suited to perform selective tumor therapy in vivo is presented. The fusion protein, due to its humanized carcinoembryonic antigen-specific variable region, specifically binds to carcinoembryonic antigen-expressing tumors and has an enzymatic activity comparable to that of human beta-glucuronidase. The prodrug is a nontoxic glucuronide-spacer derivative of doxorubicin decomposing to doxorubicin by enzymatic deglucuronidation. In vivo studies in nude mice bearing human carcinoembryonic antigen-expressing tumor xenografts revealed that 7 days after injection of 20 mg/kg fusion protein a high specificity ratio (> 100:1) was obtained between tumor and plasma or tumor and normal tissues. Injection of 250 mg/kg of prodrug at day 7 resulted in tumor therapeutic effects superior to those of conventional chemotherapy without any detectable toxicity. These superior therapeutic effects which were observed using established human tumor xenografts can be explained by the approximately 4-12-fold higher doxorubicin concentrations found in tumors of mice treated with fusion protein and prodrug than in those treated with the maximal tolerable dose of drug alone. The nondetectable toxicity in the animals treated with fusion protein and prodrug is probably caused by up to 5-fold lower drug concentrations in normal tissues compared to the animals treated with doxorubicin. Thus, a more tumor-selective therapy, resulting in stronger therapeutic effects and reduced toxicity seems to be possible by the appropriate use of the humanized nontoxic fusion protein and the nontoxic prodrug.

Enzyme replacement with recombinant beta-glucuronidase in the newborn mucopolysaccharidosis type VII mouse.

beta-Glucuronidase injected i.v. into newborn mucopolysaccharidosis VII mice was cleared from the circulation in less than 1 h and taken up by tissues in a distribution corresponding to the location of the mannose 6-phosphate receptor. One h after a 3.5-mg/kg beta-glucuronidase injection, beta-glucuronidase levels were equal to or greater than normal in every organ examined with the exception of the brain, where 31% normal activity was present. Enzyme was detectable histochemically in the major sites of pathology for mucopolysaccharidosis VII including bone, brain, heart, and fixed tissue macrophages. The half-life of recombinant beta-glucuronidase activity in various organs of injected mucopolysaccharidosis VII mice was 1.5 to 4.5 d. These studies show that recombinant beta-glucuronidase administered to newborn mice reaches the sites of clinically important storage in murine mucopolysaccharidosis VII.

Endocytosis and transcytosis by the capsule cell of vertebrate muscle spindles.

The vertebrate muscle spindle has been observed to be ionically and biochemically isolated from the surrounding muscle fibers by the spindle capsule. We have explored the possibility that the capsular cells are endocytically active and can transport both small molecules and macromolecules into the capsular space. Transcytosis (the endocytic transport of extracellular substances across a cell) through the capsule cell layer was examined with muscle spindles of snake, rat, and cat using fluorescent markers for fluorescence microscopy and horseradish peroxidase (HRP) and ferritin for electron microscopic examination. The fluorescent markers were actively taken up by capsule cells, making it easy to locate the spindle capsular region of spindles among extrafusal fibers by their strong fluorescence. Ferritin and HRP were used to identify the pathway of transcytosis by electron microscopy. These markers were found in endocytic vesicles of capsule cells, in the narrow space between capsule layers and in the capsular space, indicating that the marker was transferred to the capsular space by the pinocytic activity of capsule cells. Scattered cells in the capsule of cat muscle spindles appeared to take up fluorescein isothiocyanate (FITC)-coupled beta-glucuronidase by a receptor-mediated process. The uptake was sensitive to temperature and [Ca2+], and specifically inhibited by yeast mannan. By electron microscopy with dilute HRP (10 micrograms/ml) this specific uptake was by isolated cells in the interlamellar space. The functional significance of the above findings is discussed.

Receptor-mediated uptake of beta-glucuronidase into primary astrocytes and C6 glioma cells from rat brain.

The ability of both primary astrocytes from rat cerebrum and a rat C6 glioma cell line to take up lysosomal enzymes by receptor-mediated endocytosis was compared. The beta-glucuronidase secreted by 3T3 fibroblasts was purified to homogeneity by antibody affinity chromatography, iodinated and used as a typical enzyme to determine the nature of receptors involved in its uptake into glial cells. Both primary astrocytes and C6 glioma cells took up 125I-labelled enzyme in a rapid and saturable manner indicative of specific receptors, while immunostaining with an anti-mouse beta-glucuronidase antibody showed that the enzyme was distributed in a mainly punctate pattern after uptake, characteristic of that of lysosomes. Subcellular fractionation of C6 glioma cells following endocytosis revealed that the enzyme became localised in lysosomes, after first passing through an endosomal compartment. Uptake of enzyme was reduced markedly after its sugar side chains had been removed with N-glycanase, indicating that endocytosis was mediated via a carbohydrate-recognising receptor. A range of carbohydrates and glycoproteins were tested for their ability to inhibit receptor-mediated endocytosis but of these only sialic acid had a notable effect. Further evidence that endocytosis of beta-glucuronidase into primary astrocytes and C6 gliomas may be mediated via sialic acid receptors was provided by the large reduction in rate of uptake observed following removal of this sugar from the enzyme with sialidase.

In vitro fusion of endosomes following receptor-mediated endocytosis.

Receptor-mediated endocytosis and receptor recycling involve a series of intracellular membrane fusion events that appear to play an important role in the regulation of the overall rate and efficiency of the process. An endosome-endosome fusion assay is described using two ligands that (i) rapidly and efficiently enter the endosomal compartment via the macrophage mannose receptor and (ii) that mutually recognize each other. Dinitrophenol-derivatized beta-glucuronidase (DNP-beta-glucuronidase), a ligand for the mannose receptor, was endocytosed by one population of J774 E clone cells, and mannose-derivatized monoclonal anti-DNP IgG (Man-IgG) was internalized by a second set of cells. Both ligands were localized in endosomes as determined by fractionation on Percoll gradients. Incubation of vesicles prepared from the two set of cells resulted in vesicle fusion as indicated by the formation of DNP-beta-glucuronidase-Man-IgG complexes. Under standard conditions, fusion was time-, ATP-, and temperature-dependent. KCl was required for fusion. Fusion required both cytosolic- and membrane-associated proteins. N-Ethylmaleimide treatment of cytosol inhibited fusion. Proton ionophores and amines had no effect on the fusion reaction. ATP-dependent fusion was only observed between early endocytic compartments. While in the presence of a Ca2+ chelator fusion was ATP-dependent, in its absence fusion was also observed in an ATP-independent fashion.