Vestronidase alfa: Recombinant human β-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 β-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 β-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 β-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.

Pharmacologic manipulation of lysosomal enzyme transport across the blood-brain barrier

The adult blood-brain barrier, unlike the neonatal blood-brain barrier, does not transport lysosomal enzymes into brain, making enzyme replacement therapy ineffective in treating the central nervous system symptoms of lysosomal storage diseases. However, enzyme transport can be re-induced with alpha-adrenergics. Here, we examined agents that are known to alter the blood-brain barrier transport of large molecules or to induce lysosomal enzyme transport across the blood-brain barrier ((±)epinephrine, insulin, retinoic acid, and lipopolysaccharide) in 2-week-old and adult mice. In 2-week-old adolescent mice, all these pharmacologic agents increased brain and heart uptake of phosphorylated human β-glucuronidase. In 8-week-old adult mice, manipulations with (±)epinephrine, insulin, and retinoic acid were significantly effective on uptake by brain and heart. The increased uptake of phosphorylated human β-glucuronidase was inhibited by mannose 6-phosphate for the agents (±)epinephrine and retinoic acid and by L-NG-nitroarginine methyl ester for the agent lipopolysaccharide in neonatal and adult mice. An in situ brain perfusion study revealed that retinoic acid directly modulated the transport of phosphorylated human β-glucuronidase across the blood-brain barrier. The present study indicates that there are multiple opportunities to at least transiently induce phosphorylated human β-glucuronidase transport at the adult blood-brain barrier.

Assessment of bone dysplasia by micro-CT and glycosaminoglycan levels in mouse models for mucopolysaccharidosis type I, IIIA, IVA, and VII

Mucopolysaccharidoses (MPS) are a group of lysosomal storage diseases caused by mutations in lysosomal enzymes involved in degradation of glycosaminoglycans (GAGs). Patients with MPS grow poorly and become physically disabled due to systemic bone disease. While many of the major skeletal effects in mouse models for MPS have been described, no detailed analysis that compares GAGs levels and characteristics of bone by micro-CT has been done. The aims of this study were to assess severity of bone dysplasia among four MPS mouse models (MPS I, IIIA, IVA and VII), to determine the relationship between severity of bone dysplasia and serum keratan sulfate (KS) and heparan sulfate (HS) levels in those models, and to explore the mechanism of KS elevation in MPS I, IIIA, and VII mouse models. Clinically, MPS VII mice had the most severe bone pathology; however, MPS I and IVA mice also showed skeletal pathology. MPS I and VII mice showed severe bone dysplasia, higher bone mineral density, narrowed spinal canal, and shorter sclerotic bones by micro-CT and radiographs. Serum KS and HS levels were elevated in MPS I, IIIA, and VII mice. Severity of skeletal disease displayed by micro-CT, radiographs and histopathology correlated with the level of KS elevation. We showed that elevated HS levels in MPS mouse models could inhibit N-acetylgalactosamine-6-sulfate sulfatase enzyme. These studies suggest that KS could be released from chondrocytes affected by accumulation of other GAGs and that KS could be useful as a biomarker for severity of bone dysplasia in MPS disorders.

Long circulating enzyme replacement therapy rescues bone pathology in mucopolysaccharidosis VII murine model

Mucopolysaccharidosis (MPS) type VII is a lysosomal storage disease caused by deficiency of the lysosomal enzyme β-glucuronidase (GUS), leading to accumulation of glycosaminoglycans (GAGs). Enzyme replacement therapy (ERT) effectively clears GAG storage in the viscera. Recent studies showed that a chemically modified form of GUS (PerT-GUS), which escaped clearance by mannose 6-phosphate and mannose receptors and showed prolonged circulation, reduced CNS storage more effectively than native GUS. Clearance of storage in bone has been limited due to the avascularity of the growth plate. To evaluate the effectiveness of long-circulating PerT-GUS in reducing the skeletal pathology, we treated MPS VII mice for 12 weeks beginning at 5 weeks of age with PerT-GUS or native GUS and used micro-CT, radiographs, and quantitative histopathological analysis for assessment of bones. Micro-CT findings showed PerT-GUS treated mice had a significantly lower BMD. Histopathological analysis also showed reduced storage material and a more organized growth plate in PerT-GUS treated mice compared with native GUS treated mice. Long term treatment with PerT-GUS from birth up to 57 weeks also significantly improved bone lesions demonstrated by micro-CT, radiographs and quantitative histopathological assay. In conclusion, long-circulating PerT-GUS provides a significant impact to rescue of bone lesions and CNS involvement.

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.

Mutations and polymorphisms in GUSB gene in mucopolysaccharidosis VII (Sly Syndrome)

Mucopolysaccharidosis VII (MPS VII; Sly syndrome) is an autosomal recessive disorder caused by a deficiency of beta-glucuronidase (GUS, EC 3.2.1.31; GUSB). GUS is required to degrade glycosaminoglycans (GAGs), including heparan sulfate (HS), dermatan sulfate (DS), and chondroitin-4,6-sulfate (CS). Accumulation of undegraded GAGs in lysosomes of affected tissues leads to mental retardation, short stature, hepatosplenomegaly, bone dysplasia, and hydrops fetalis. We summarize information on the 49 unique, disease-causing mutations determined so far in the GUS gene, including nine novel mutations (eight missense and one splice-site). This heterogeneity in GUS gene mutations contributes to the extensive clinical variability among patients with MPS VII. One pseudodeficiency allele, one polymorphism causing an amino acid change, and one silent variant in the coding region are also described. Among the 103 analyzed mutant alleles, missense mutations accounted for 78.6%; nonsense mutations, 12.6%; deletions, 5.8%; and splice-site mutations, 2.9%. Transitional mutations at CpG dinucleotides made up 40.8% of all the described mutations. The five most frequent mutations (accounting for 44/103 alleles) were exonic point mutations, p.L176F, p.R357X, p.P408S, p.P415L, and p.A619 V. Genotype/phenotype correlation was attempted by correlating the effects of certain missense mutations or enzyme activity and stability within phenotypes. These were in turn correlated with the location of the mutation in the tertiary structure of GUS. A total of seven murine, one feline, and one canine model of MPS VII have been characterized for phenotype and genotype.

Acidic amino acid tag enhances response to enzyme replacement in mucopolysaccharidosis type VII mice

We have tested an acidic oligopeptide-based targeting system for delivery of enzymes to tissues, especially bone and brain, in a murine mucopolysaccharidosis type VII (MPS VII) model. This strategy is based upon tagging a short peptide consisting of acidic amino acids (AAA) to N terminus of human beta-glucuronidase (GUS). The pharmacokinetics, biodistribution, and the pathological effect on MPS VII mouse after 12 weekly infusions were determined for recombinant human untagged and tagged GUS. The tagged GUS was taken up by MPS VII fibroblasts in a mannose 6-phosphate receptor-dependent manner. Intravenously injected AAA-tagged enzyme had five times more prolonged blood clearance compared with the untagged enzyme. The tagged enzyme was delivered effectively to bone, bone marrow, and brain in MPS VII mice and was effective in reversing the storage pathology. The storage in osteoblasts was cleared similarly with both enzyme types. However, cartilage showed a little response to any of the enzymes. The tagged enzyme reduced storage in cortical neurons, hippocampus, and glia cells. A highly sensitive method of tandem mass spectrometry on serum indicated that the concentration of serum dermatan sulfate and heparan sulfate in mice treated with the tagged enzyme decreased more than the untagged enzyme. These preclinical studies suggest that this AAA-based targeting system may enhance enzyme-replacement therapy.

Mannose 6-phosphate receptor-mediated transport of sulfamidase across the blood-brain barrier in the newborn mouse

Mucopolysaccharidosis type IIIA (MPS IIIA), which is a lysosomal storage disorder (LSD) caused by inherited deficiency of sulfamidase, is characterized by severe, progressive central nervous system (CNS) dysfunction. Enzyme replacement therapy (ERT) to treat CNS storage is challenging, because the access of enzymes to the brain is restricted by the blood-brain barrier (BBB). In a prior study, we found that phosphorylated beta-glucuronidase (P-GUS) could be transcytosed across the BBB in newborn mice by the mannose 6-phosphate (M6P) receptor. In order to determine whether sulfamidase can utilize this pathway, we examined brain influx and the specificity of uptake of sulfamidase after intravenous (i.v.) injection in 2-day-old and 8-week-old mice. [(131)I]Sulfamidase was transported across the BBB in neonates at rates higher than that of simultaneously injected [(125)I]albumin. In contrast, the transport of [(131)I]sulfamidase was negligible in 8-week-old mice, thereby showing that the BBB transport mechanism is developmentally downregulated. Capillary depletion revealed that 83.7% of the [(131)I]sulfamidase taken up by the brain was in the parenchyma, demonstrating transfer across the capillary wall. The uptake of [(131)I]sulfamidase into the brain was significantly reduced by co-injections of M6P and P-GUS. That is, the transport of sulfamidase into the brain parenchyma in early postnatal life is mediated by the M6P receptor, which is shared with P-GUS and is likely accessible to other M6P-containing lysosomal enzymes.

HFE association with transferrin receptor 2 increases cellular uptake of transferrin-bound iron

Mutations in either HFE or transferrin receptor 2 (TfR2) cause decreased expression of the iron regulatory hormone hepcidin and hemochromatosis. HFE and TfR2 were recently discovered to form a stable complex at the cell membrane when co-expressed in heterologous cell lines. We analyzed the functional consequences of the co-expression of these proteins using transfected TRVb cells, a Chinese hamster ovary derived cell line without endogenous HFE or transferrin receptor. The co-expression of TfR2 in TRVb cells expressing HFE led to accelerated HFE biosynthesis and late-Golgi maturation, suggesting interaction prior to cell surface localization. The co-expression of HFE in cells expressing TfR2 led to increased affinity for diferric transferrin, increased transferrin-dependent iron uptake, and relative resistance to iron chelation. These observations indicate that HFE influences the functional properties of TfR2, and suggests a model in which the interaction of these proteins might influence signal transduction to hepcidin.

Epinephrine enhances lysosomal enzyme delivery across the blood brain barrier by up-regulation of the mannose 6-phosphate receptor

Delivering therapeutic levels of lysosomal enzymes across the blood-brain barrier (BBB) has been a pivotal issue in treating CNS storage diseases, including the mucopolysaccharidoses. An inherited deficiency of beta-glucuronidase (GUS) causes mucopolysaccharidosis type VII that is characterized by increased systemic and CNS storage of glycosaminoglycans. We previously showed that the neonate uses the mannose 6-phosphate (M6P) receptor to transport phosphorylated GUS (P-GUS) across the BBB and that this transporter is lost with maturation. Induction of expression of this BBB transporter would make enzyme replacement therapy in the adult possible. Here, we tested pharmacological manipulation with epinephrine to restore functional transport of P-GUS across the adult BBB. Epinephrine (40 nmol) coinjected i.v. with (131)I-P-GUS induced the transport across the BBB in 8-week-old mice. The brain influx rate of (131)I-P-GUS (0.29 mul/g per min) returned to the level seen in neonates. Capillary depletion showed that 49% of the (131)I-P-GUS in brain was in brain parenchyma. No increases of influx rate or the vascular space for (125)I-albumin, a vascular marker, was observed with epinephrine (40 nmol), showing that enhanced passage was not caused by disruption of the BBB. Brain uptake of (131)I-P-GUS was significantly inhibited by M6P in a dose-dependent manner, whereas epinephrine failed to increase brain uptake of nonphosphorylated GUS. Thus, the effect of epinephrine on the transport of (131)I-P-GUS was ligand specific. These results indicate that epinephrine restores the M6P receptor-mediated functional transport of (131)I-P-GUS across the BBB in adults to levels seen in the neonate.

Characterization and pharmacokinetic study of recombinant human N-acetylgalactosamine-6-sulfate sulfatase

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive disorder caused by a deficiency of N-acetylgalactosamine-6-sulfate sulfatase (GALNS). The aims of this study were to establish Chinese hamster ovary (CHO) cells overexpressing recombinant human GALNS (rhGALNS) and to assess pharmacokinetics and tissue distribution of purified enzymes by using MPS IVA knock-out mouse (Galns(-/-)). The CHO-cell derived rhGALNS was purified from the media by a two-step affinity chromatography procedure. The rhGALNS was administered intravenously to 3-month-old Galns(-/-) mice at a single dose of 250U/g of body weight. The treated mice were examined by assaying the GALNS activity at baseline and up to 240min to assess clearance of the enzyme from blood circulation. The mice were sacrificed 4h after infusion of the enzyme to study the enzyme distribution in tissues. The rhGALNS was purified 1317-fold with 71% yield. The enzyme was taken up by Galns(-/-) chondrocytes (150U/mg/15h). The uptake was inhibited by mannose-6-phosphate. The enzyme activity disappeared from circulation with a half-life of 2.9min. After enzyme infusion, the enzyme was taken up and detected in multiple tissues (40.7% of total infused enzymes in liver). Twenty-four hours after a single infusion of the fluorescence-labeled enzymes into MPS IVA mice, biodistribution pattern showed the amount of tagged enzyme retained in bone, bone marrow, liver, spleen, kidney, and heart. In conclusion, we have shown that the phosphorylated rhGALNS is delivered to multiple tissues, including bone, and that it functions bioactively in Galns(-/-) chondrocytes implying a potential enzyme replacement treatment.

Expression of membrane-bound carbonic anhydrases IV, IX, and XIV in the mouse heart

Expression of membrane-bound carbonic anhydrases (CAs) of CA IV, CA IX, CA XII, and CA XIV has been investigated in the mouse heart. Western blots using microsomal membranes of wild-type hearts demonstrate a 39-, 43-, and 54-kDa band representing CA IV, CA IX, and CA XIV, respectively, but CA XII could not be detected. Expression of CA IX in the CA IV/CA XIV knockout animals was further confirmed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Cardiac cells were immunostained using anti-CA/FITC and anti-alpha-actinin/TRITC, as well as anti-CA/FITC and anti-SERCA2/TRITC. Subcellular CA localization was investigated by confocal laser scanning microscopy. CA localization in the sarcolemmal (SL) membrane was examined by double immunostaining using anti-CA/FITC and anti-MCT-1/TRITC. CAs showed a distinct distribution pattern in the sarcoplasmic reticulum (SR) membrane. CA XIV is predominantly localized in the longitudinal SR, whereas CA IX is mainly expressed in the terminal SR/t-tubular region. CA IV is present in both SR regions, whereas CA XII is not found in the SR. In the SL membrane, only CA IV and CA XIV are present. We conclude that CA IV and CA XIV are associated with the SR as well as with the SL membrane, CA IX is located in the terminal SR/t-tubular region, and CA XII is not present in the mouse heart. Therefore, the unique subcellular localization of CA IX and CA XIV in cardiac myocytes suggests different functions of both enzymes in excitation-contraction coupling.

Enzyme therapy in mannose receptor-null mucopolysaccharidosis VII mice defines roles for the mannose 6-phosphate and mannose receptors

Enzyme replacement therapy (ERT) is available for several lysosomal storage diseases. Except for Gaucher disease, for which an enzyme with exposed mannosyl residues targets mannose receptors (MR) on macrophages, ERT targets primarily the mannose 6-phosphate receptor (MPR). Most recombinant lysosomal enzymes contain oligosaccharides with both terminal mannosyl and mannose 6-phosphate residues. Effective MPR-mediated delivery may be compromised by rapid clearance of infused enzyme by the MR on fixed tissue macrophages, especially Kupffer cells. To evaluate the impact of this obstacle to ERT, we introduced the MR-null mutation onto the mucopolysaccharidosis type VII (MPS VII) background and produced doubly deficient MR-/- MPS VII mice. The availability of both MR+/+ and MR-/- mice allowed us to study the effects of eliminating the MR on MR- and MPR-mediated plasma clearance and tissue distribution of infused phosphorylated (P) and nonphosphorylated (NP) forms of human beta-glucuronidase (GUS). In MR+/+ MPS VII mice, the MR clearance system predominated at doses up to 6.4 mg/kg P-GUS. Genetically eliminating the MR slowed plasma clearance of both P- and NP-GUS and enhanced the effectiveness of P-GUS in clearing storage in kidney, bone, and retina. Saturating the MR clearance system by high doses of enzyme also improved targeting to MPR-containing tissues such as muscle, kidney, heart, and hepatocytes. Although ablating the MR clearance system genetically is not practical clinically, blocking the MR-mediated clearance system with high doses of enzyme is feasible. This approach delivers a larger fraction of enzyme to MPR-expressing tissues, thus enhancing the effectiveness of MPR-targeted ERT.

Enhancement of drug delivery to bone: characterization of human tissue-nonspecific alkaline phosphatase tagged with an acidic oligopeptide

Hypophosphatasia is caused by deficiency of activity of the tissue-nonspecific alkaline phosphatase (TNSALP), resulting in a defect of bone mineralization. Enzyme replacement therapy (ERT) with partially purified plasma enzyme was attempted but with little clinical improvement. Attaining clinical effectiveness with ERT for hypophosphatasia may require delivering functional TNSALP enzyme to bone. We tagged the C-terminal-anchorless TNSALP enzyme with an acidic oligopeptide (a six or eight residue stretch of L-Asp), and compared the biochemical properties of the purified tagged and untagged enzymes derived from Chinese hamster ovary cell lines. The specific activities of the purified enzymes tagged with the acidic oligopeptide were the same as the untagged enzyme. In vitro affinity experiments showed the tagged enzymes had 30-fold higher affinity for hydroxyapatite than the untagged enzyme. Lectin affinity chromatography for carbohydrate structure showed little difference among the three enzymes. Biodistribution pattern from single infusion of the fluorescence-labeled enzymes into mice showed delayed clearance from the plasma up to 18 h post infusion and the amount of tagged enzyme retained in bone was 4-fold greater than that of the untagged enzyme. In vitro mineralization assays with the bone marrow from a hypophosphatasia patient using each of the three enzymes in the presence of high concentrations of pyrophosphate provided evidence of bone mineralization. These results show the anchorless enzymes tagged with an acidic oligopeptide are delivered efficiently to bone and function bioactively in bone mineralization, at least in vitro. They suggest potential advantages for use of these tagged enzymes in ERT for hypophosphatasia, which should be explored.

Carbonic anhydrase XIV identified as the membrane CA in mouse retina: strong expression in Müller cells and the RPE

The presence of carbonic anhydrase (CA) activity in the neural retina has been known for several decades. CA-II, a soluble cytoplasmic isoform expressed by Müller cells and a subset of amacrine cells, was thought to be the sole source of CA activity in the neural retina. However, CA-II deficient mice retain CA activity in the neural retina, which implies that another isoform must be present in that tissue. Recently CA-XIV, an integral membrane protein, was cloned and characterized. We, therefore, sought to determine whether CA-XIV is expressed in the neural retina, and hence is responsible for the CA activity observed in CA-II null animals. Immunohistochemical analyses of histological sections from CA-II null, CA-XIV null, and control mice were performed to localize the CA-XIV isoform, as well as other known retinal markers. Immunoblotting and real-time RT-PCR analyses were also performed to test for CA-XIV expression in retina and other mouse tissues. We determined herein that CA-XIV, a approximately 45kDa membrane protein, is expressed in retina, as it is in kidney. In the retina, CA-XIV is expressed on the plasma membrane of Müller cells. CA-XIV is also found on both the apical and basal membranes of the retinal pigmented epithelium. The data presented here indicate that like CA-II, CA-XIV is highly expressed in the neural retina and, like CA-II, more specifically by the Müller cells. The cellular compartmentalization of the two isoforms in the Müller cell-one cytoplasmic and the other on the plasma membrane-suggest that the two enzymes have specific and unique functions. Future studies will be necessary to assign functions to CA-II and CA-XIV in the mouse neural retina.

Defining the pathway for Tat-mediated delivery of beta-glucuronidase in cultured cells and MPS VII mice

We used recombinant forms of human beta-glucuronidase (GUS) purified from secretions from stably transfected CHO cells to compare the native enzyme to a GUS-Tat C-terminal fusion protein containing the 11-amino-acid HIV Tat protein transduction domain for: (1) susceptibility to endocytosis by cultured cells, (2) rate of clearance following intravenous infusion, and (3) tissue distribution and effectiveness in clearing lysosomal storage following infusion in the MPS VII mouse. We found: (1) Native GUS was more efficiently taken up by cultured human fibroblasts and its endocytosis was exclusively mediated by the M6P receptor. The GUS-Tat fusion protein showed only 30-50% as much M6P-receptor-mediated uptake, but also was taken up by adsorptive endocytosis through binding of the positively charged Tat peptide to cell surface proteoglycans. (2) GUS-Tat was less rapidly cleared from the circulation in the rat (t(1/2) = 13 min vs 7 min). (3) Delivery to most tissues of the MPS VII mouse was similar, but GUS-Tat was more efficiently delivered to kidney. Histology showed that GUS-Tat more efficiently reduced storage in renal tubules, retina, and bone. These studies demonstrate that Tat modification can extend the range of tissues corrected by infused enzyme.

Development of MPS IVA mouse (Galnstm(hC79S.mC76S)slu) tolerant to human N-acetylgalactosamine-6-sulfate sulfatase

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive disease caused by N-acetylgalactosamine-6-sulfate sulfatase (GALNS) deficiency. In recent studies of enzyme replacement therapy for animal models with lysosomal storage diseases, cellular and humoral immune responses to the injected enzymes have been recognized as major impediments to effective treatment. To study the long-term effectiveness and side effects of therapies in the absence of immune responses, we have developed an MPS IVA mouse model, which has many similarities to human MPS IVA and is tolerant to human GALNS protein. We used a construct containing both a transgene (cDNA) expressing inactive human GALNS in intron 1 and an active site mutation (C76S) in adjacent exon 2 and thereby introduced both the inactive cDNA and the C76S mutation into the murine Galns by targeted mutagenesis. Affected homozygous mice have no detectable GALNS enzyme activity and accumulate glycosaminoglycans in multiple tissues including visceral organs, brain, cornea, bone, ligament and bone marrow. At 3 months, lysosomal storage is marked within hepatocytes, reticuloendothelial Kupffer cells, and cells of the sinusoidal lining of the spleen, neurons and meningeal cells. The bone storage is also obvious, with lysosomal distention in osteoblasts and osteocytes lining the cortical bone, in chondrocytes and in the sinus lining cells in bone marrow. Ubiquitous expression of the inactive human GALNS was also confirmed by western blot using the anti-GALNS monoclonal antibodies newly produced, which resulted in tolerance to immune challenge with human enzyme. The newly generated MPS IVA mouse model should provide a good model to evaluate long-term administration of enzyme replacement.

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.

Carbonic anhydrase XIV is enriched in specific membrane domains of retinal pigment epithelium, Muller cells, and astrocytes

Carbonic anhydrases (CAs) are ubiquitous enzymes important to many cell types throughout the body. They help determine levels of H(+) and HCO(-)(3) and thereby regulate intracellular and extracellular pH and volume. CA XIV, an extracellular membrane-bound CA, was recently shown to be present in brain and retina. Here, we analyze the subcellular distribution of CA XIV in retina by high-resolution immunogold cytochemistry and show that the distribution in retina (on glial cells but not neurons) is different from that reported for brain (on neurons but not glia). In addition, CA XIV is strongly expressed on retinal pigment epithelium (RPE). The specific membrane domains that express CA XIV were endfoot and nonendfoot membranes on Muller cells and astrocytes and apical and basolateral membranes of RPE. Gold particle density was highest on microvilli plasma membranes of RPE, where it was twice that of glial endfoot and Muller microvilli membranes and four times that of other glial membrane domains. Neither neurons nor capillary endothelial cells showed detectable labeling for CA XIV. This enrichment of CA XIV on specific membrane domains of glial cells and RPE suggests specialization for buffering pH and volume in retinal neurons and their surrounding extracellular spaces. We suggest that CA XIV is the target of CA inhibitors that enhance subretinal fluid absorption in macular edema. In addition, CA XIV may facilitate CO(2) removal from neural retina and modulate photoreceptor function.

Keratan sulphate levels in mucopolysaccharidoses and mucolipidoses

The mucopolysaccharidoses (MPS) is characterized by accumulation of glycosaminoglycans (GAGs), and mucolipidosis (ML) by accumulation of GAGs and sphingolipids. Each type of MPS accumulates specific GAGs. The lysosomal enzymes N-acetylgalactosamine-6-sulphate sulphatase and beta-galactosidase involve the stepwise degradation of keratan sulphate (KS). Deficiency of these enzymes results in elevation of KS levels in the body fluids and in tissues, leading to MPS IV disease. In this study, we evaluated blood and urine KS levels in types of MPS and ML other than MPS IV. Eighty-five plasma samples came from MPS I (n = 18), MPS II (n = 28), MPS III (n = 20), MPS VI (n = 3), MPS VII (n = 5) and ML (n = 11) patients while 127 urine samples came from MPS I (n = 34), MPS II (n = 34), MPS III (n = 32), MPS VI (n = 7), MPS VII (n = 9) and ML (n = 11) patients. KS levels were determined using the ELISA method. Plasma KS levels varied with age in both control and patient populations. In all age groups, the mean values of plasma KS in MPS and ML patients were significantly higher than those in the age-matched controls. Plasma KS values in four newborn patients were above the mean + 2SD of the age-matched controls (mean, 41 ng/ml). Overall, 85.9% of individual values in non-type IV MPS and ML patients were above the mean + 2SD of the age-matched controls. For urine KS levels, 24.4% of individual values in patients were above the mean + 2SD of the age-matched controls. In conclusion, KS in blood is elevated in each type of non-type IV MPS examined, in contrast to the conventional understanding. This finding suggests that measurement of KS level provides a new diagnostic biomarker in a wide variety of mucopolysaccharidoses and mucolipidoses in addition to MPS IV.

Developmentally regulated mannose 6-phosphate receptor-mediated transport of a lysosomal enzyme across the blood-brain barrier

Mucopolysaccharidosis type VII is a lysosomal storage disorder resulting from inherited deficiency of beta-glucuronidase (GUS). Mucopolysaccharidosis type VII is characterized by glycosaminoglycan storage in most tissues, including brain. In these disorders, enzyme delivery across the blood-brain barrier (BBB) is the main obstacle to correction of lysosomal storage in the CNS. Prior studies suggested mouse brain is accessible to GUS in the first 2 weeks of life but not later. To explore a possible role for the mannose 6-phosphate/insulin-like growth factor II receptor in GUS transport across the BBB in neonatal mice, we compared brain uptake of phosphorylated GUS (P-GUS) and nonphosphorylated GUS (NP-GUS) in newborn and adult mice. (131)I-P-GUS was transported across the BBB after i.v. injection in 2-day-old mice. The brain influx rate (K(in)) of (131)I-P-GUS in 2-day-old mice was 0.21 microl/g.min and decreased with age. By 7 weeks of age, transport of (131)I-P-GUS was not significant. Capillary depletion revealed that 62% of the (131)I-P-GUS in brain was in brain parenchyma in 2-day-old mice. In addition, uptake of (131)I-P-GUS into brain was significantly reduced by coinjection of unlabeled P-GUS or M6P in a dose-dependent manner. In contrast, the K(in) of (131)I-NP-GUS (0.04 microl/g.min) was significantly lower than (131)I-P-GUS in 2-day-old mice. Transcardiac brain perfusion confirmed that neither (131)I-P-GUS nor (131)I-NP-GUS crossed the BBB in adult mice. These results indicate that (131)I-P-GUS transport into brain parenchyma in early postnatal life is mediated by the mannose 6-phosphate/insulin-like growth factor II receptor. This receptor-mediated transport is not observed in adult mice.

Development and testing of new screening method for keratan sulfate in mucopolysaccharidosis IVA

Mucopolysaccharidosis IVA (MPS IVA), a progressive lysosomal storage disease, causes skeletal dysplasia through excessive storage of keratan sulfate (KS). We developed an ELISA-sandwich assay that used a MAb specific to KS. Forty-five blood and 59 urine specimens from MPS IVA patients (ages 1-65 y) were analyzed to determine whether KS concentration is a suitable marker for early diagnosis and longitudinal assessment of disease severity. Blood specimens were obtained from patients categorized as phenotypically severe (n = 36) and milder (n = 9). Urine specimens were also analyzed from patients categorized as severe (n = 56) and milder (n = 12), respectively. Blood KS levels (101-1525 ng/mL) in MPS IVA patients were two to eight times higher than those in age-matched controls (15-323 ng/mL). It was found that blood KS level varied with age and clinical severity. Blood KS levels in both MPS IVA and controls peaked between 5 and 10 y of age (mean, 776 versus 234 ng/mL, respectively). Blood levels in severe MPS IVA were 1.5 times higher than in the milder form. In contrast to blood, urine KS levels in both MPS IVA and controls peaked between 1 and 5 y (15.3 versus 0.26 mg/g creatinine), and thereafter declined with age. Urine KS level also varied with age and clinical severity, and the severe MPS IVA phenotype was associated with 6.7 times greater urine KS excretion than the milder one. These findings indicate that the new assay for blood or urine KS may be suitable for early diagnosis and longitudinal assessment of disease severity in MPS IVA.

Glycosylation-independent targeting enhances enzyme delivery to lysosomes and decreases storage in mucopolysaccharidosis type VII mice

Enzyme-replacement therapy is an established means of treating lysosomal storage diseases. Infused therapeutic enzymes are targeted to lysosomes of affected cells by interactions with cell-surface receptors that recognize carbohydrate moieties, such as mannose and mannose 6-phosphate, on the enzymes. We have tested an alternative, peptide-based targeting system for delivery of enzymes to lysosomes in a murine mucopolysaccharidosis type VII (MPS VII) model. This strategy depends on the interaction of a fragment of insulin-like growth factor II (IGF-II), with the IGF-II binding site on the bifunctional, IGF-II cation-independent mannose 6-phosphate receptor. A chimeric protein containing a portion of mature human IGF-II fused to the C terminus of human beta-glucuronidase was taken up by MPS VII fibroblasts in a mannose 6-phosphate-independent manner, and its uptake was inhibited by the addition of IGF-II. Furthermore, the tagged enzyme was delivered effectively to clinically significant tissues in MPS VII mice and was effective in reversing the storage pathology. The tagged enzyme was able to reduce storage in glomerular podocytes and osteoblasts at a dose at which untagged enzyme was much less effective. This peptide-based, glycosylation-independent lysosomal targeting system may enhance enzyme-replacement therapy for certain human lysosomal storage diseases.

Expression, Assay, and Structure of the Extracellular Domain of Murine Carbonic Anhydrase XIV

Carbonic anhydrase (CA) XIV is the most recently identified mammalian carbonic anhydrase isozyme, and its presence has been demonstrated in a number of tissues. Full-length CA XIV is a transmembrane protein composed of an extracellular catalytic domain, a single transmembrane helix, and a short intracellular polypeptide segment. The amino acid sequence identity of human CA XIV relative to the other membrane-associated isozymes (CA IV, CA IX, and CA XII) is 34-46%. We report here the expression and purification of both the full-length enzyme and a truncated, secretory form of murine CA XIV. Both forms of this isozyme are highly active, and both show an abrogation of activity in the presence of 0.2% SDS, in contrast to the behavior of murine CA IV. We also report the crystal structure of the extracellular domain of murine CA XIV at 2.8 A resolution and of an enzyme-acetazolamide complex at 2.9 A resolution. The structure shows a monomeric glycoprotein with a topology similar to that of other mammalian CA isozymes. Based on the x-ray crystallographic results, we compare and contrast known structures of membrane-associated CA isozymes to rationalize the structural elements responsible for the SDS resistance of CA IV and to discuss prospects for the design of selective inhibitors of membrane-associated CA isozymes.

Mouse model of N-acetylgalactosamine-6-sulfate sulfatase deficiency (Galns-/-) produced by targeted disruption of the gene defective in Morquio A disease

Mucopolysaccharidosis IVA is an autosomal recessive disorder caused by a deficiency of N-acetylgalactosamine-6-sulfate sulfatase (GALNS), a lysosomal enzyme required for the stepwise degradation of keratan sulfate (KS) and chondroitin-6-sulfate (C6S). To generate a model for studies of the pathophysiology and of potential therapies, we disrupted exon 2 of Galns, the homologous murine gene. Homozygous Galns-/- mice have no detectable GALNS enzyme activity and show increased urinary glycosaminoglycan (GAGs) levels. These mice accumulate GAGs in multiple tissues including liver, kidney, spleen, heart, brain and bone marrow. At 2 months old, lysosomal storage is present primarily within reticuloendothelial cells such as Kupffer cells and cells of the sinusoidal lining of the spleen. Additionally, by 12 months old, vacuolar change is observed in the visceral epithelial cells of glomeruli and cells at the base of heart valves but it is not present in parenchymal cells such as hepatocytes and renal tubular epithelial cells. In the brain, hippocampal and neocortical neurons and meningeal cells had lysosomal storage. KS and C6S were more abundant in the cytoplasm of corneal epithelial cells of Galns-/- mice compared with wild-type mice by immunohistochemistry. Radiographs revealed no change in the skeletal bones of mice up to 12 months old. Thus, targeted disruption of the murine Galns gene has produced a murine model, which shows visceral storage of GAGs but lacks the skeletal features. The complete absence of GALNS in mutant mice makes them useful for studies of pharmacokinetics and tissue targeting of recombinant GALNS designed for enzyme replacement.

Localization of carbonic anhydrase XII to the basolateral membrane of H+-secreting cells of mouse and rat kidney

Membrane-associated carbonic anhydrase (CA) has a crucial role in renal HCO(3)(-) absorption. CA activity has been localized to both luminal and basolateral membranes of the tubule epithelial cells. CA XII is a transmembrane isoenzyme that has been demonstrated in the basolateral plasma membrane of human renal, intestinal, and reproductive epithelia. The present study was designed to demonstrate the distribution of CA XII expression in the rodent kidney. A new polyclonal antibody to recombinant mouse CA XII was used in both Western blotting and immunohistochemistry. Western blotting analysis revealed a 40-45-kD polypeptide in CA XII-expressing CHO cells and isolated membranes of mouse and rat kidney. Immunofluorescence staining localized CA XII in the basolateral plasma membranes of S1 and S2 proximal tubule segments. Abundant basolateral staining of CA XII was seen in a subpopulation of cells in both cortical and medullary collecting ducts. Double immunofluorescence staining identified these cells as H(+)-secreting type A intercalated cells. The localization of CA XII in the peritubular space of proximal tubules suggests that it may play a role in renal HCO(3)(-) absorption, whereas the function of CA XII in the type A intercalated cells needs further investigation.

Production of MPS VII mouse (Gus(tm(hE540A x mE536A)Sly)) doubly tolerant to human and mouse beta-glucuronidase

Mucopolysaccharidosis VII (MPS VII, Sly syndrome) is an autosomal recessive lysosomal storage disease caused by beta-glucuronidase (GUS) deficiency. A naturally occurring mouse model of that disease has been very useful for studying experimental approaches to therapy. However, immune responses can complicate evaluation of the long-term benefits of enzyme replacement or gene therapy delivered to adult MPS VII mice. To make this model useful for studying the long-term effectiveness and side effects of experimental therapies delivered to adult mice, we developed a new MPS VII mouse model, which is tolerant to both human and murine GUS. To achieve this, we used homologous recombination to introduce simultaneously a human cDNA transgene expressing inactive human GUS into intron 9 of the murine Gus gene and a targeted active site mutation (E536A) into the adjacent exon 10. When the heterozygote products of germline transmission were bred to homozygosity, the homozygous mice expressed no GUS enzyme activity but expressed inactive human GUS protein highly and were tolerant to immune challenge with human enzyme. Expression of the mutant murine Gus gene was reduced to about 10% of normal levels, but the inactive murine GUS enzyme also conferred tolerance to murine GUS. This MPS VII mouse model should be useful to evaluate therapeutic responses in adult mice receiving repetitive doses of enzyme or mice receiving gene therapy as adults. Heterozygotes expressed only 9.5-26% of wild-type levels of murine GUS instead of the expected 50%, indicating a dominant-negative effect of the mutant enzyme monomers on the activity of GUS tetramers in different tissues. Corrective gene therapy in this model should provide high enough levels of expression of normal GUS monomers to overcome the dominant negative effect of mutant monomers on newly synthesized GUS tetramers in most tissues.

Missense models [Gustm(E536A)Sly, Gustm(E536Q)Sly, and Gustm(L175F)Sly] of murine mucopolysaccharidosis type VII produced by targeted mutagenesis

Human mucopolysaccharidosis VII (MPS VII, Sly syndrome) results from a deficiency of beta-glucuronidase (GUS) and has been associated with a wide range in severity of clinical manifestations. To study missense mutant models of murine MPS VII with phenotypes of varying severity, we used targeted mutagenesis to produce E536A and E536Q, corresponding to active-site nucleophile replacements E540A and E540Q in human GUS, and L175F, corresponding to the most common human mutation, L176F. The E536A mouse had no GUS activity in any tissue and displayed a severe phenotype like that of the originally described MPS VII mice carrying a deletion mutation (gus(mps/mps)). E536Q and L175F mice had low levels of residual activity and milder phenotypes. All three mutant MPS models showed progressive lysosomal storage in many tissues but had different rates of accumulation. The amount of urinary glycosaminoglycan excretion paralleled the clinical severity, with urinary glycosaminoglycans remarkably higher in E536A mice than in E536Q or L175F mice. Molecular analysis showed that the Gus mRNA levels were quantitatively similar in the three mutant mouse strains and normal mice. These mouse models, which mimic different clinical phenotypes of human MPS VII, should be useful in studying pathogenesis and also provide useful models for studying enzyme replacement therapy and targeted correction of missense mutations.

Regulation of transferrin-mediated iron uptake by HFE, the protein defective in hereditary hemochromatosis

The protein defective in hereditary hemochromatosis, called HFE, is similar to MHC class I-type proteins and associates with beta2-microglobulin (beta2M). Its association with beta2M was previously shown to be necessary for its stability, normal intracellular processing, and cell surface expression in transfected COS cells. Here we use stably transfected Chinese hamster ovary cell lines expressing both HFE and beta2M or HFE alone to study the effects of beta2M on the stability and maturation of the HFE protein and on the role of HFE in transferrin receptor 1 (TfR1)-mediated iron uptake. In agreement with prior studies on other cell lines, we found that overexpression of HFE, without overexpressing beta2M, resulted in a decrease in TfR1dependent iron uptake and in lower iron levels in the cells, as evidenced by ferritin and TfR1 levels measured at steady state. However, overexpression of both HFE and beta2M had the reverse effect and resulted in an increase in TfR1-dependent iron uptake and increased iron levels in the cells. The HFE-beta2M complex did not affect the affinity of TfR1 for transferrin or the internalization rate of transferrin-bound TfR1. Instead, HFE-beta2M enhanced the rate of recycling of TfR1 and resulted in an increase in the steady-state level of TfR1 at the cell surface of stably transfected cells. We propose that Chinese hamster ovary cells provide a model to explain the effect of the HFE-beta2M complex in duodenal crypt cells, where the HFE-beta2M complex appears to facilitate the uptake of transferrin-bound iron to sense the level of body iron stores. Impairment of this process in duodenal crypt cells leads them to be iron poor and to signal the differentiating enterocytes to take up iron excessively after they mature into villus cells in the duodenum of hereditary hemochromatosis patients.

Methylation patterns of the human beta-glucuronidase gene locus: boundaries of methylation and general implications for frequent point mutations at CpG dinucleotides

Methylation of CpG islands spanning promoter regions is associated with control of gene expression, although it is unclear what mechanisms define the boundaries between methylated and unmethylated regions in the genome. Methylation of genomic DNA in mammals also affects the frequency of inherited diseases by predisposing them to CpG mutations. To gain insight into these issues, we investigated patterns of cytosine methylation on almost the entire beta-glucuronidase gene (GUSB) from normal leukocyte DNAs by bisulfite genomic sequencing. We mapped the boundaries of methylation that flank the 5'- and 3'-ends of the CpG island region, and correlated methylation status with transitional mutations at CpG sites. GenBank sequence analyses showed that the CpG island of human GUSB is juxtaposed with multiple Alu repeats and also includes multiple Sp1 sites upstream and downstream of the transcription start, which has been suggested to prevent CpG islands from becoming methylated. We show that cytosine methylation is extensive across the entire gene except for CpG sites in the proximal promoter region, exon 1, and part of intron 1; the unmethylated CpG island is embedded between densely methylated flanking regions containing multiple Alu repeats; a sharp boundary separates the methylated and unmethylated regions of the 5'-flank of the CpG island, but a gradual change in methylation density over 1.0 kb is observed in the 3'-flank of the CpG island; boundaries of the 5'-end and 3'-end of the CpG island contain multiple Sp1 sites in addition to Alu repeats; methylation in both strands is symmetrical except at the boundary regions between methylated and unmethylated regions; and nonmethylation of exon 1 correlates with the absence of transitional mutations at CpG sites in exon 1.

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.

Crystal structure of the dimeric extracellular domain of human carbonic anhydrase XII, a bitopic membrane protein overexpressed in certain cancer tumor cells

Overexpression of the zinc enzyme carbonic anhydrase (CA; EC ) XII is observed in certain human cancers. This bitopic membrane protein contains an N-terminal extracellular catalytic domain, a membrane-spanning alpha-helix, and a small intracellular C-terminal domain. We have determined the three-dimensional structure of the extracellular catalytic domain of human CA XII by x-ray crystallographic methods at 1.55-A resolution. The structure reveals a prototypical CA fold; however, two CA XII domains associate to form an isologous dimer, an observation that is confirmed by studies of the enzyme in solution. The identification of signature GXXXG and GXXXS motifs in the transmembrane sequence that facilitate helix-helix association is additionally consistent with dimeric architecture. The dimer interface is situated so that the active site clefts of each monomer are clearly exposed on one face of the dimer, and the C termini are located together on the opposite face of the dimer to facilitate membrane interaction. The amino acid composition of the active-site cleft closely resembles that of the other CA isozymes in the immediate vicinity of the catalytic zinc ion, but differs in the region of the nearby alpha-helical "130's segment." The structure of the CA XII-acetazolamide complex is also reported at 1.50-A resolution, and prospects for the design of CA XII-specific inhibitors of possible chemotherapeutic value are discussed.

Active site mutant transgene confers tolerance to human beta-glucuronidase without affecting the phenotype of MPS VII mice

Mucopolysaccharidosis type VII (MPS VII; Sly syndrome) is an autosomal recessive lysosomal storage disorder due to an inherited deficiency of beta-glucuronidase. A naturally occurring mouse model for this disease was discovered at The Jackson Laboratory and shown to be due to homozygosity for a 1-bp deletion in exon 10 of the gus gene. The murine model MPS VII (gus(mps/mps)) has been very well characterized and used extensively to evaluate experimental strategies for lysosomal storage diseases, including bone marrow transplantation, enzyme replacement therapy, and gene therapy. To enhance the value of this model for enzyme and gene therapy, we produced a transgenic mouse expressing the human beta-glucuronidase cDNA with an amino acid substitution at the active site nucleophile (E540A) and bred it onto the MPS VII (gus(mps/mps)) background. We demonstrate here that the mutant mice bearing the active site mutant human transgene retain the clinical, morphological, biochemical, and histopathological characteristics of the original MPS VII (gus(mps/mps)) mouse. However, they are now tolerant to immune challenge with human beta-glucuronidase. This "tolerant MPS VII mouse model" should be useful for preclinical trials evaluating the effectiveness of enzyme and/or gene therapy with the human gene products likely to be administered to human patients with MPS VII.

Expression of membrane-associated carbonic anhydrase XIV on neurons and axons in mouse and human brain

Although long suspected from histochemical evidence for carbonic anhydrase (CA) activity on neurons and observations that CA inhibitors enhance the extracellular alkaline shifts associated with synaptic transmission, an extracellular CA in brain had not been identified. A candidate for this CA was suggested by the recent discovery of membrane CA (CA XIV) whose mRNA is expressed in mouse and human brain and in several other tissues. For immunolocalization of CA XIV in mouse and human brain, we developed two antibodies, one against a secretory form of enzymatically active recombinant mouse CA XIV, and one against a synthetic peptide corresponding to the 24 C-terminal amino acids in the human enzyme. Immunostaining for CA XIV was found on neuronal membranes and axons in both mouse and human brain. The highest expression was seen on large neuronal bodies and axons in the anterolateral part of pons and medulla oblongata. Other CA XIV-positive sites included the hippocampus, corpus callosum, cerebellar white matter and peduncles, pyramidal tract, and choroid plexus. Mouse brain also showed a positive reaction in the molecular layer of the cerebral cortex and granular cellular layer of the cerebellum. These observations make CA XIV a likely candidate for the extracellular CA postulated to have an important role in modulating excitatory synaptic transmission in brain.

Purification and kinetic analysis of recombinant CA XII, a membrane carbonic anhydrase overexpressed in certain cancers

Carbonic anhydrase XII (CA XII) is a transmembrane glycoprotein with an active extracellular CA domain that is overexpressed on cell surfaces of certain cancers. Its expression has been linked to tumor invasiveness. To characterize its catalytic properties, we purified recombinant secretory forms of wild-type and mutant CA XIIs. The catalytic properties of these enzymes in the hydration of CO(2) were measured at steady state by stopped-flow spectrophotometry and at chemical equilibrium by the exchange of (18)O between CO(2) and water determined by mass spectrometry. The catalysis of CO(2) hydration by soluble CA XII has a maximal value of k(cat)/K(m) at 34 microM(-1) small middle dots(-1), which is similar to those of the membrane-associated CA IV and to soluble CA I. The pH profiles of this catalysis and the catalyzed hydrolysis of 4-nitrophenylacetate indicate that the pK(a) of the zinc-bound water in CA XII is 7.1. His64 in CA XII acts as a proton shuttle residue, as evidenced by the reduced rate constant for proton transfer in the mutants containing the replacements His64 --> Ala and His64 --> Arg, as well as by the selective inhibition of the proton transfer step by cupric ions in wild-type CA XII. The catalytic rate of CO(2) hydration by the soluble form of CA XII is identical with that of the membrane-bound enzyme. These observations suggest a role for CA XII in CO(2)/HCO(3)(-) homeostasis in cells in which it is normally expressed. They are also compatible with a role for CA XII in acidifying the microenvironment of cancer cells in which CA XII is overexpressed, providing a mechanism for CA XII to augment tumor invasiveness and suggesting CA XII as a potential target for chemotherapeutic agents.

Mitochondrial carbonic anhydrase CA VB: differences in tissue distribution and pattern of evolution from those of CA VA suggest distinct physiological roles

A cDNA for a second mouse mitochondrial carbonic anhydrase (CA) called CA VB was identified by homology to the previously characterized murine CA V, now called CA VA. The full-length cDNA encodes a 317-aa precursor that contains a 33-aa classical mitochondrial leader sequence. Comparison of products expressed from cDNAs for murine CA VB and CA VA in COS cells revealed that both expressed active CAs that localized in mitochondria, and showed comparable activities in crude extracts and in mitochondria isolated from transfected COS cells. Northern blot analyses of total RNAs from mouse tissues and Western blot analyses of mouse tissue homogenates showed differences in tissue-specific expression between CA VB and CA VA. CA VB was readily detected in most tissues, while CA VA expression was limited to liver, skeletal muscle, and kidney. The human orthologue of murine CA VB was recently reported also. Comparison of the CA domain sequence of human CA VB with that reported here shows that the CA domains of CA VB are much more highly conserved between mouse and human (95% identity) than the CA domains of mouse and human CA VAs (78% identity). Analysis of phylogenetic relationships between these and other available human and mouse CA isozyme sequences revealed that mammalian CA VB evolved much more slowly than CA VA, accepting amino acid substitutions at least 4.5 times more slowly since each evolved from its respective human-mouse ancestral gene around 90 million years ago. Both the differences in tissue distribution and the much greater evolutionary constraints on CA VB sequences suggest that CA VB and CA VA have evolved to assume different physiological roles.

Identification of carbonic anhydrase XII as the membrane isozyme expressed in the normal human endometrial epithelium

Although previous studies demonstrated carbonic anhydrase (CA) activity in the human endometrium, the CA isozyme(s) responsible for this activity has not been established. In this report, we provide the first evidence that the CA isozyme XII, a recently identified transmembrane isozyme that is expressed in normal kidney and greatly overexpressed in some renal cancers, is present in endometrium. We show by immunohistochemistry that CA XII is expressed in the basolateral plasma membrane of epithelial cells of normal human endometrium. Expression of CA XII in uterus was confirmed by Northern blotting. Detergent-solubilized CA XII was isolated from human endometrium by inhibitor affinity chromatography and characterized by isoelectric focusing and Western blot as a polypeptide with a pI of 6.3. The high expression of CA XII in the endometrial epithelium suggests that it may be functionally linked to the pH-dependent events in spermatozoa that precede fertilization. Its basolateral location and extracellular active site could also allow it to influence the morphological changes in endometrium that occur during the menstrual cycle.

Active site residues of human beta-glucuronidase. Evidence for Glu(540) as the nucleophile and Glu(451) as the acid-base residue

Human beta-glucuronidase (hGUSB) is a member of family 2 glycosylhydrolases that cleaves beta-D-glucuronic acid residues from the nonreducing termini of glycosaminoglycans. Amino acid sequence and structural homology of hGUSB and Escherichia coli beta-galactosidase active sites led us to propose that residues Glu(451), Glu(540), and Tyr(504) in hGUSB are involved in catalysis, Glu(451) being the acid-base residue and Glu(540) the nucleophile. To test this hypothesis, we introduced mutations in these residues and determined their effects on enzymes expressed in COS cells and GUSB-deficient fibroblasts. The extremely low activity in cells expressing Glu(451), Glu(540), and Tyr(504) hGUSBs supported their roles in catalysis. For kinetic analysis, wild type and mutant enzymes were produced in baculovirus and purified to homogeneity by affinity chromatography. The k(cat)/K(m) values (mM(-1).s(-1)) of the E540A, E451A, and Y504A enzymes were 34,000-, 9100-, and 830-fold lower than that of wild type hGUSB, respectively. High concentrations of azide stimulated the activity of the E451A mutant enzyme, supporting the role of Glu(451) as the acid-base catalyst. We conclude that, like their homologues in E. coli beta-galactosidase, Glu(540) is the nucleophilic residue, Glu(451) the acid-base catalyst, and Tyr(504) is also important for catalysis, although its role is unclear. All three residues are located in the active site cavity previously determined by structural analysis of hGUSB.

The mannose 6-phosphate/insulin-like growth factor-II receptor is a substrate of type V transforming growth factor-beta receptor

The type V transforming growth factor beta (TGF-beta) receptor (TbetaR-V) is a ligand-stimulated acidotropic Ser-specific protein kinase that recognizes a motif of SXE/S(P)/D. This motif is present in the cytoplasmic domain of the mannose 6-phosphate/insulin-like growth factor-II (Man-6-P/IGF-II) receptor. We have explored the possibility that the Man-6-P/IGF-II receptor is a substrate of TbetaR-V. Purified bovine Man-6-P/IGF-II receptor was phosphorylated by purified bovine TbetaR-V in the presence of [gamma-32P]ATP and MnCl2 with an apparent Km of 130 nM. TGF-beta stimulated the phosphorylation of the Man-6-P/IGF-II receptor at 0 degrees C in mouse L cells overexpressing the Man-6-P/IGF-II receptor and in wild-type mink lung epithelial (Mv1Lu cells) metabolically labeled with [32P]orthophosphate. The in vitro and in vivo phosphorylation of the Man-6-P/IGF-II receptor occurred at the putative phosphorylation sites as revealed by phosphopeptide mapping and amino acid sequence analysis. TGF-beta stimulated Man-6-P/IGF-II receptor-mediated uptake (approximately 2-fold after 12 h treatment) of exogenous beta-glucuronidase in Mv1Lu cells and type II TGF-beta receptor (TbetaR-II)-defective mutant cells (DR26 cells) but not in type I TGF-beta receptor (TbetaR-I)-defective mutant cells (R-1B cells) and human colorectal carcinoma cells (RII-37 cells) expressing TbetaR-I and TbetaR-II but lacking TbetaR-V. These results suggest the Man-6-P/IGF-II receptor serves as an in vitro and in vivo substrate of TbetaR-V and that both TbetaR-V and TbetaR-I may play a role in mediating the TGF-beta-stimulated uptake of exogenous beta-glucuronidase.

Enzyme replacement therapy improves reproductive performance in mucopolysaccharidosis type VII mice but does not prevent postnatal losses

Mice with mucopolysaccharidosis type VII (MPS VII) are devoid of beta-glucuronidase and accumulate glycosaminoglycans in lysosomes resulting in bone dysplasia, learning disabilities, and decreased mobility. MPS VII males do not breed and, while MPS VII females occasionally mate with heterozygous males, they do not maintain their young postnatally. Heterozygous matings produce less than 25% MPS VII offspring, but until now it was unclear whether this results from prenatal or postnatal losses. The administration of recombinant beta-glucuronidase from birth significantly reduces glycosaminoglycan storage in most tissues, increases life span, and improves the animal's cognitive ability and mobility. To determine whether reproductive failure is corrected by such therapy, male and female MPS VII mice were injected with enzyme at weekly intervals from birth to 5 wk of age (6xinj). Enzyme-replaced MPS VII mice bred when mated together. The 6xinj MPS VII males mated repeatedly until they were killed 135 d postinjection. All mated 6xinj MPS VII females gave birth to two litters, but maintained few of their young. Selective loss of MPS VII offspring was observed in matings between heterozygotes. Analysis of 379 preterm fetuses from heterozygous matings showed a frequency of 24.6% MPS VII pups, indicating that the decreased number of MPS VII pups produced by mating heterozygotes results from postnatal losses. The ovaries of young adult MPS VII mice have follicles and corpora lutea, and the testes generate sperm. Results suggest that the reproductive failure in MPS VII mice is related to impaired mobility and/or impaired cognitive function, and enzyme replacement restores mating capacity.

Identification of Glu-540 as the catalytic nucleophile of human beta-glucuronidase using electrospray mass spectrometry

Human beta-glucuronidase is a member of the Family 2 glycosylhydrolases that cleaves beta-D-glucuronic acid residues from the nonreducing termini of glycosaminoglycans. The enzyme is shown to catalyze glycoside bond hydrolysis with net retention of anomeric configuration, presumably via a mechanism involving a covalent glucuronyl-enzyme intermediate. Incubation of human beta-glucuronidase with 2-deoxy-2-fluoro-beta-D-glucuronyl fluoride resulted in time-dependent inactivation of the enzyme through the accumulation of a covalent 2-deoxy-2-fluoro-alpha-D-glucuronyl-enzyme, as observed by electrospray mass spectrometry. Regeneration of the free enzyme by hydrolysis or transglycosylation and removal of excess inactivator demonstrated that the covalent intermediate was kinetically competent. Peptic digestion of the 2-deoxy-2-fluoro-alpha-D-glucuronyl-enzyme intermediate and subsequent analysis by liquid chromatography coupled with electrospray ionization triple quadrupole mass spectrometry indicated the presence of a 2-deoxy-2-fluoro-alpha-D-glucuronyl peptide. Sequence determination of the labeled peptide by tandem mass spectrometry in the daughter ion scan mode permitted the identification of Glu-540 as the catalytic nucleophile within the sequence SEYGAET.

Human carbonic anhydrase XII: cDNA cloning, expression, and chromosomal localization of a carbonic anhydrase gene that is overexpressed in some renal cell cancers

We report the cloning and characterization of a tumor-associated carbonic anhydrase (CA) that was identified in a human renal cell carcinoma (RCC) by serological expression screening with autologous antibodies. The cDNA sequence predicts a 354-amino acid polypeptide with a molecular mass of 39,448 Da that has features of a type I membrane protein. The predicted sequence includes a 29-amino acid signal sequence, a 261-amino acid CA domain, an additional short extracellular segment, a 26-amino acid hydrophobic transmembrane domain, and a hydrophilic C-terminal cytoplasmic tail of 29 amino acids that contains two potential phosphorylation sites. The extracellular CA domain shows 30-42% homology with known human CAs, contains all three Zn-binding histidine residues found in active CAs, and contains two potential sites for asparagine glycosylation. When expressed in COS cells, the cDNA produced a 43- to 44-kDa protein in membranes that had around one-sixth the CA activity of membranes from COS cells transfected with the same vector expressing bovine CA IV. We have designated this human protein CA XII. Northern blot analysis of normal tissues demonstrated a 4.5-kb transcript only in kidney and intestine. However, in 10% of patients with RCC, the CA XII transcript was expressed at much higher levels in the RCC than in surrounding normal kidney tissue. The CA XII gene was mapped by using fluorescence in situ hybridization to 15q22. CA XII is the second catalytically active membrane CA reported to be overexpressed in certain cancers. Its relationship to oncogenesis and its potential as a clinically useful tumor marker clearly merit further investigation.

Structure of human beta-glucuronidase reveals candidate lysosomal targeting and active-site motifs

The X-ray structure of the homotetrameric lysosomal acid hydrolase, human beta-glucuronidase (332,000 Mr), has been determined at 2.6 A resolution. The tetramer has approximate dihedral symmetry and each promoter consists of three structural domains with topologies similar to a jelly roll barrel, an immunoglobulin constant domain and a TIM barrel respectively. Residues 179-204 form a beta-hairpin motif similar to the putative lysosomal targeting motif of cathepsin D, supporting the view that lysosomal targeting has a structural basis. The active site of the enzyme is formed from a large cleft at the interface of two monomers. Residues Glu 451 and Glu 540 are proposed to be important for catalysis. The structure establishes a framework for understanding mutations that lead to the human genetic disease mucopolysaccharidosis VII, and for using the enzyme in anti-cancer therapy.

Enzyme replacement therapy for murine mucopolysaccharidosis type VII

Recombinant mouse beta-glucuronidase administered intravenously to newborn mice with mucopolysaccharidosis type VII (MPS VII) is rapidly cleared from the circulation and localized in many tissues. Here we determine the tissue distribution of injected enzyme and describe its effects on the histopathology in 6-wk-old MPS VII mice that received either one injection of 28,000 U recombinant beta-glucuronidase at 5 wk of age or received six injections of 28,000 U given at weekly intervals beginning at birth. These mice were compared with untreated 6-wk-old MPS VII mice. The single injection decreased lysosomal distention in the fixed tissue macrophage system. MPS VII mice that received multiple injections had 27.8, 3.5, and 3.3% of normal levels of beta-glucuronidase in liver, spleen, and kidney, respectively. Brain had detectable beta-glucuronidase, ranging from 2.0-12.1% of normal. Secondary elevations of alpha-galactosidase and beta-hexosaminidase in brain, spleen, liver, and kidney were decreased compared with untreated MPS VII mice. Although no improvement was observed in chondrocytes, glia, and some neurons, the skeleton had less clinical and pathological evidence of disease and the brain had reduced lysosomal storage in meninges and selected neuronal groups. These data show that recombinant beta-glucuronidase treatment begun in newborn MPS VII mice provides enzyme to most tissues and significantly reduces or prevents the accumulation of lysosomal storage during the first 6 wk of life. Whether therapy begun later in life can achieve this level of correction remains to be established.

C-terminal processing of human beta-glucuronidase. The propeptide is required for full expression of catalytic activity, intracellular retention, and proper phosphorylation

beta-Glucuronidase undergoes proteolytic C-terminal processing during or after its transport to lysosomes or endosomes. We determined the C-terminal processing site for human placental beta-glucuronidase to be the peptide bond between Thr633-Arg634. To evaluate the role of the 18-amino acid peptide removed in C-terminal processing, we changed the codon for Arg634 to a stop codon by site-directed mutagenesis and studied expression of the truncated mutant enzyme in COS-7 cells. An increased fraction of newly synthesized enzyme from R634Stop cDNA was secreted. Pulse-chase experiments provided no evidence for increased degradation of the intracellular R634Stop enzyme. The total amount of catalytic activity expressed from the R634Stop mutant cDNA was only half that seen with the wild type cDNA, and the Kcat of the mutant enzyme was 52% that of wild type enzyme. These results indicate that the C-terminal propeptide in the precursor is important for beta-glucuronidase to achieve maximal activity. The truncated enzyme formed hybrid tetramers in cotransfection experiments with the cDNA for rat beta-glucuronidase. There appeared to be no decrease in stability of the R634Stop enzyme, since chaotropic agents, heat treatment, and pH had similar effects on the mutant and the wild type enzymes. The uptake rate of the truncated mutant (R634Stop) enzyme by beta-glucuronidase-deficient human fibroblast cells was only 55-60% that of the wild type enzyme. Binding to the immobilized cation-independent mannose-6-phosphate receptor and measurement of the 32P-labeled phosphorylated oligosaccharides revealed that the truncated mutant enzyme was 32-34% less phosphorylated and appeared to contain proportionately more covered phosphate groups than the wild type enzyme. These results suggest that the propeptide influences the accessibility to both processing enzymes that produce the mannose-6-phosphate recognition marker on beta-glucuronidase.

Crystallization and preliminary crystallographic studies of human beta-glucuronidase

Crystals of human beta-glucuronidase have been obtained by the vapor diffusion method, using 2-methyl-2,4-pentanediol as a precipitant. The crystals belong to the orthorhombic space group P222(1), with cell dimensions a = 134.5 A, b = 95.1 A, c = 124.4 A. The unit cell contains two copies of the tetrameric enzyme. Complete native data have been collected to a resolution of 2.6 A.

The role of glycosylation and phosphorylation in the expression of active human beta-glucuronidase

Phosphorylation of mannose residues on N-linked oligosaccharide side chains of lysosomal enzymes targets them to lysosomes. We used site-directed mutagenesis to observe the effect of eliminating selective glycosylation sites from human beta-glucuronidase on enzyme sorting. Expression studies allowed us to determine which of four potential sites were glycosylated, preferentially phosphorylated, and required for catalytic activity. All four sites of the human enzyme were glycosylated, whereas in the mouse and rat enzymes, only three of four sites are used. Sites 2 and 3 were preferentially phosphorylated. Elimination of sites 2 and 3 in combination markedly decreased sorting to lysosomes and increased enzyme secretion. Each of the four glycosylation sites could be eliminated individually without drastic reduction in catalytic activity. Activity was progressively lost as combinations of two, three, and four sites were eliminated. Wild-type enzyme produced in the presence of tunicamycin was also inactive, indicating that glycosylation is required for realization of enzyme activity. However, active enzyme could be deglycosylated with only minimal loss of activity. Mutant enzyme completely lacking glycosylation did not form tetramers. Partial restoration of tetramerization was achieved by the co-expression of normal rat enzyme, provided that the normal rat enzyme supplied at least two subunits to the tetramer.

Mutational analysis of a patient with mucopolysaccharidosis type VII, and identification of pseudogenes

PCR of cDNA produced from patient fibroblasts allowed us to determine the paternal mutation in the first patient reported with beta-glucuronidase-deficiency mucopolysaccharidosis type VII (MPS VII). The G-->T transversion 1,881 bp downstream of the ATG translation initiation codon destroys an MboII restriction site and converts Trp627 to Cys (W627C). Digestion of genomic DNA PCR fragments with MboII indicated that the patient and the father were heterozygous for this missense mutation in exon 12. Failure to find cDNAs from patient RNA which did not contain this mutation suggested that the maternal mutation leads to greatly reduced synthesis or reduced stability of mRNA from the mutant allele. In order to identify the maternal mutation, it was necessary to analyze genomic sequences. This approach was complicated by the finding of multiple unprocessed pseudogenes and/or closely related genes. Using PCR with a panel of human/rodent hybrid cell lines, we found that these pseudogenes were present over chromosomes 5-7, 20, and 22 and the Y chromosome. Conditions were defined which allowed us to amplify and characterize genomic sequences for the true beta-glucuronidase gene despite this background of related sequences. The patient proved to be heterozygous for a second mutation, in which a C-->T transition introduces a termination codon (R356STOP) in exon 7. The mother was also heterozygous for this mutation. Expression of a cDNA containing the maternal mutation produced no enzyme activity, as expected. Expression of the paternal mutation in COS-7 cells produced a surprisingly high (65% of control) level of activity. However, activity was 13% of control in transiently transfected murine MPS VII cells. The level of activity of this mutant allele appears to correlate with the level of overexpression, suggesting that high concentrations of mutant monomers can drive the folding and tetramerization of mutant enzyme to produce an active and stable enzyme.

Divalent cation-dependent stimulation of ligand binding to the 46-kDa mannose 6-phosphate receptor correlates with divalent cation-dependent tetramerization

The quaternary structure and binding activity of the murine 46-kDa mannose 6-phosphate receptor (46MPR) were studied in semi-intact murine cells that overexpress the murine receptor. Chemical cross-linking studies showed that the murine 46MPR exists in monomer, dimer, and tetramer forms in membranes of overexpressing murine cells. Treatment of permeabilized cells with Mn2+ increased the tetramer form of 46MPR, and this tetramerization was reversed by removal of Mn2+. Thus, the divalent cations affected the distribution of receptor among the three forms, favoring tetramerization at the expense of dimer and monomer. Low temperature (4 degrees C) also increases the fraction present as tetramer. The binding assay results show that Mn2+ is required for the 46MPR to achieve and retain the ability to bind ligand at 37 degrees C but not at 4 degrees C. Preincubation with Mn2+ produced a 3-fold increase in Man-6-P-specific binding of beta-glucuronidase which paralleled the 3-fold increase in tetramer seen during preincubation with Mn2+. The similarity of the effects of addition and removal of Mn2+ on enzyme binding to the effects of Mn2+ on favoring tetramer formation suggests that divalent cation-dependent tetramerization of the 46MPR contributes to the stimulation of ligand binding to the 46MPR by divalent cations.