Deletion of the Glucosidase II Gene in Trypanosoma brucei Reveals Novel N-Glycosylation Mechanisms in the Biosynthesis of Variant Surface Glycoprotein

The trypanosomatids are generally aberrant in their protein N-glycosylation pathways. However, protein N-glycosylation in the African trypanosome Trypanosoma brucei, etiological agent of human African sleeping sickness, is not well understood. Here, we describe the creation of a bloodstream-form T. brucei mutant that is deficient in the endoplasmic reticulum enzyme glucosidase II. Characterization of the variant surface glycoprotein, the main glycoprotein synthesized by the parasite with two N-glycosylation sites, revealed unexpected changes in the N-glycosylation of this molecule. Structural characterization by mass spectrometry, nuclear magnetic resonance spectroscopy, and chemical and enzymatic treatments revealed that one of the two glycosylation sites was occupied by conventional oligomannose structures, whereas the other accumulated unusual structures in the form of Glcalpha1-3Manalpha1-2Manalpha1-2Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc, Glcalpha1-3Manalpha1-2Manalpha1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc, and Glcalpha1-3Manalpha1-2Manalpha1-2Manalpha1-3(Galbeta1-4GlcNAcbeta1-2Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc. The possibility that these structures might arise from Glc1Man9GlcNAc2 by unusually rapid alpha-mannosidase processing was ruled out using a mixture of alpha-mannosidase inhibitors. The results suggest that bloodstream-form T. brucei can transfer both Man9GlcNAc2 and Man5GlcNAc2 to the variant surface glycoprotein in a site-specific manner and that, unlike organisms that transfer exclusively Glc3Man9GlcNAc2, the T. brucei UDP-Glc: glycoprotein glucosyltransferase and glucosidase II enzymes can use Man5GlcNAc2 and Glc1Man5GlcNAc2, respectively, as their substrates. The ability to transfer Man5GlcNAc2 structures to N-glycosylation sites destined to become Man(4-3)GlcNAc2 or complex structures may have evolved as a mechanism to conserve dolichol-phosphate-mannose donors for glycosylphosphatidylinositol anchor biosynthesis and points to fundamental differences in the specificities of host and parasite glycosyltransferases that initiate the synthesis of complex N-glycans.

Polycystic liver disease is a disorder of cotranslational protein processing

Autosomal-dominant polycystic liver disease (PCLD) is a rare disorder that is characterized by the progressive development of fluid-filled biliary epithelial cysts in the liver. Positional cloning has identified two genes that are mutated in patients with polycystic liver disease, PRKCSH and SEC63, which encode the beta-subunit of glucosidase II and Sec63, respectively. Both proteins are components of the molecular machinery involved in the translocation, folding and quality control of newly synthesized glycoproteins in the endoplasmic reticulum. Most mutations are truncating and probably lead to a complete loss of the corresponding proteins and the defective processing of a key regulator of biliary cell growth. The finding that PCLD is caused by proteins involved in oligosaccharide processing was unexpected and implicates a new avenue for research into neocystogenesis, and might ultimately result in the identification of novel therapeutic drugs.

Cloning and characterization of the glucosidase II alpha subunit gene of Trichoderma reesei: a frameshift mutation results in the aberrant glycosylation profile of the hypercellulolytic strain Rut-C30

We describe isolation and characterization of the gene encoding the glucosidase II alpha subunit (GIIalpha) of the industrially important fungus Trichoderma reesei. This subunit is the catalytic part of the glucosidase II heterodimeric enzyme involved in the structural modification within the endoplasmic reticulum (ER) of N-linked oligosaccharides present on glycoproteins. The gene encoding GIIalpha (gls2alpha) in the hypercellulolytic strain Rut-C30 contains a frameshift mutation resulting in a truncated gene product. Based on the peculiar monoglucosylated N-glycan pattern on proteins produced by the strain, we concluded that the truncated protein can still hydrolyze the first alpha-1,3-linked glucose residue but not the innermost alpha-1,3-linked glucose residue from the Glc2Man9GlcNAc2 N-glycan ER structure. Transformation of the Rut-C30 strain with a repaired T. reesei gls2alpha gene changed the glycosylation profile significantly, decreasing the amount of monoglucosylated structures and increasing the amount of high-mannose N-glycans. Full conversion to high-mannose carbohydrates was not obtained, and this was probably due to competition between the endogenous mutant subunit and the introduced wild-type GIIalpha protein. Since glucosidase II is also involved in the ER quality control of nascent polypeptide chains, its transcriptional regulation was studied in a strain producing recombinant tissue plasminogen activator (tPA) and in cultures treated with the stress agents dithiothreitol (DTT) and brefeldin A (BFA), which are known to block protein transport and to induce the unfolded protein response. While the mRNA levels were clearly upregulated upon tPA production or BFA treatment, no such enhancement was observed after DTT addition.

Clustering of MAL genes in Hansenula polymorpha: cloning of the maltose permease gene and expression from the divergent intergenic region between the maltose permease and maltase genes

Hansenula polymorpha uses maltase to grow on maltose and sucrose. Inspection of genomic clones of H. polymorpha showed that the maltase gene HPMAL1 is clustered with genes corresponding to Saccharomyces cerevisiae maltose permeases and MAL activator genes orthologues. We sequenced the H. polymorpha maltose permease gene HPMAL2 of the cluster. The protein (582 amino acids) deduced from the HPMAL2 gene is predicted to have eleven transmembrane domains and shows 39-57% identity with yeast maltose permeases. The identity of the protein is highest with maltose permeases of Debaryomyces hansenii and Candida albicans. Expression of the HPMAL2 in a S. cerevisiae maltose permease-negative mutant CMY1050 proved functionality of the permease protein encoded by the gene. HPMAL1 and HPMAL2 genes are divergently positioned similarly to maltase and maltose permease genes in many yeasts. A two-reporter assay of the expression from the HPMAL1-HPMAL2 intergenic region showed that expression of both genes is coordinately regulated, repressed by glucose, induced by maltose, and that basal expression is higher in the direction of the permease gene.

Delayed or late-onset type II glycogenosis with globular inclusions

Three unrelated patients, one girl, one boy, and an adult female, aged 14, 11 and 41 years, respectively, at the time of biopsy, revealed lysosomal glycogen storage, autophagic vacuoles and peculiar globular inclusions of distinct ultrastructure, which were reducing but did not appear like true "reducing bodies" as described in the congenital myopathy "reducing body myopathy". All three patients had residual activity of acid alpha-glucosidase in their muscle biopsy samples. Leukocytes in the girl showed normal acid alpha-glucosidase activity, but in the boy activity was reduced. Molecular genetic analysis of the GAA gene revealed disease-causing mutations in each patient: H568L/R672W, IVS1-13T>G/G615F, and IVS1-13T>G/IVS1-13T>G. Although only one patient with such globular inclusions has been reported up to now, the three patients described here indicate that in the late-onset type of GSD II such inclusions may not be rare.

Effects of a mosquitocidal toxin on a mammalian epithelial cell line expressing its target receptor

The spread of diseases transmitted by Anopheles and Culex mosquitoes, such as malaria and West Nile fever, is a growing concern for human health. Bacillus sphaericus binary toxin (Bin) is one of the few available bioinsecticides able to control populations of these mosquitoes efficiently. We previously showed that Bin binds to Cpm1, an alpha-glucosidase located on the apical side of Culex larval midgut epithelium. We analysed the effects of Bin by expressing a construct encoding Cpm1 in the mammalian epithelial MDCK cell line. Cpm1 is targeted to the apical side of polarized MDCK, where it is anchored by glycosylphosphatidylinositol (GPI) and displays alpha-glucosidase activity. Bin bound to transfected cells and induced a non-specific current presumably related to the opening of pores. The formation of these pores may be related to the location of the toxin/receptor complex in lipid raft microdomains. Finally, Bin promoted the time-dependent appearance of intracytoplasmic vacuoles but did not drive cell lysis. Thus, the dual functionality (enzyme/toxin receptor) of Cpm1 is fully conserved in MDCK cells and Cpm1 is an essential target protein for Bin cytotoxicity in Culex mosquitoes.

Correction of glycogen storage disease type II by an adeno-associated virus vector containing a muscle-specific promoter

Glycogen storage disease type II (Pompe disease) causes death in infancy from cardiorespiratory failure due to acid alpha-glucosidase (GAA; acid maltase) deficiency. An AAV2 vector pseudotyped as AAV6 (AAV2/6 vector) transiently expressed high-level human GAA in GAA-knockout (GAA-KO) mice without reducing glycogen storage; however, in immunodeficient GAA-KO/SCID mice the AAV2/6 vector expressed high-level GAA and reduced the glycogen content of the injected muscle for 24 weeks. A CD4+/CD8+ lymphocytic infiltrate was observed in response to the AAV2/6 vector in immunocompetent GAA-KO mice. When a muscle-specific creatine kinase promoter was substituted for the CB promoter (AAV-MCKhGAApA), that AAV2/6 vector expressed high-level GAA and reduced glycogen content in immunocompetent GAA-KO mice. Muscle-restricted expression of hGAA provoked only a humoral (not cellular) immune response. Intravenous administration of a high number of particles of AAV-MCKhGAApA as AAV2/7 reduced the glycogen content of the heart and skeletal muscle and corrected individual myofibers in immunocompetent GAA-KO mice 24 weeks postinjection. In summary, persistent correction of muscle glycogen content was achieved with an AAV vector containing a muscle-specific promoter in GAA-KO mice, and this approach should be considered for muscle-targeted gene therapy in Pompe disease.

Novel catalytic mechanism of glycoside hydrolysis based on the structure of an NAD+/Mn2+ -dependent phospho-alpha-glucosidase from Bacillus subtilis

GlvA, a 6-phospho-alpha-glucosidase from Bacillus subtilis, catalyzes the hydrolysis of maltose-6'-phosphate and belongs to glycoside hydrolase family GH4. GH4 enzymes are unique in their requirement for NAD(H) and a divalent metal for activity. We have determined the crystal structure of GlvA in complex with its ligands to 2.05 A resolution. Analyses of the active site architecture, in conjunction with mechanistic studies and precedent from the nucleotide diphosphate hexose dehydratases and other systems, suggest a novel mechanism of glycoside hydrolysis by GlvA that involves both the NAD(H) and the metal.

Effects of non-contractile inclusions on mechanical performance of skeletal muscle

Glycogen storage disease II is an inherited progressive muscular disease in which the lack of functional acid 1-4 alpha-glucosidase results in the accumulation of lysosomal glycogen. In the present study, we examine the effect of these non-contractile inclusions on the mechanical performance of skeletal muscle. To this end, force developed in an isometrically contracting slice of a muscle was calculated with a finite element model. Force was calculated at several inclusion densities and distributions and compared to muscle lacking inclusions. Furthermore, ankle dorsal flexor torque was measured in situ of alpha-glucosidase null mice of 6 months of age and unaffected litter mates as was inclusion density in the dorsal flexor muscles. The calculated force loss was shown to be almost exclusively dependent on the inclusion density and less on the type of inclusion distribution. The force loss predicted by the model (6%) on the basis of measured inclusion density (3.3%) corresponded to the loss in mass-normalized strength in these mice measured in situ (7%). Therefore, we conclude that the mechanical interaction between the non-contractile inclusions and the nearby myofibrils is a key factor in the loss of force per unit muscle mass during early stages of GSD II in mice. As glycogen accumulation reaches higher levels in humans, it is highly probable that the impact of this mechanical interaction is even more severe in human skeletal muscle.

Cloning and expression of a gene for an alpha-glucosidase from Saccharomycopsis fibuligera homologous to family GH31 of yeast glucoamylases

Cloning of cDNA encoding an alpha-glucosidase from the dimorphous yeast Saccharomycopsis fibuligera and characterization of the gene product were performed. The cDNA of the putative alpha-glucosidase gene consists of 2,886 bp, which includes an open reading frame encoding a 19 amino acid signal peptide at the N-terminal end and a 944 amino acid mature protein with a predicted molecular mass of 105.4 kDa and pI value of 4.52. The deduced amino acid sequence shows a high degree of identity (70%) with two yeast glucoamylases, namely, the extracellular glucoamylase Gam from Schwanniomyces occidentalis and the cell surface glucoamylase Gca from Candida albicans. The recombinant product, synthesized in Saccharomyces cerevisiae, is localized on the cell surface and hydrolyses maltooligosaccharides exclusively without the ability to digest soluble starch, which is consistent with the specificity characteristic of alpha-glucosidase, EC. 3.2.1.20.

Multiple Sampling in Single-Cell Enzyme Assays Using CE-Laser-Induced Fluorescence to Monitor Reaction Progress

A novel method for assaying enzymes from a single cell or small cell populations is described. The key advantage of this method is the ability to repeatedly sample a single cell enzyme reaction. Whereas multiple sampling has been achieved for larger cell types with a diameter of 1 mm, we report a technique by which single cell enzyme assays of small cells (15 microm in diameter) can be repeatedly carried out. Individual cells were isolated using an in-house-built micromanipulator and placed in nanoliter-scale reaction vessels. The cells were lysed with solution containing substrate, and enzyme activity was assayed by removing 5-nL aliquots with a recently developed nanopipettor. The reaction aliquot was then analyzed using capillary electrophoresis with laser-induced fluorescence detection to quantitate enzyme activity. Sf9 cells were assayed at the single cell level and found to be highly heterogeneous with respect to alpha-glucosidase II activity. Since only 5 nL of the single cell reaction was removed, multiple sampling was possible, allowing triplicate analysis of enzyme activity for each individual cell. Multiple sampling also permitted a single cell reaction to be monitored over time. The sensitivity of this method was demonstrated in the analysis of a low-abundance enzyme, alpha1,3-N-acetylgalactosaminyltransferase, from single HT29 cells. Detecting the product of this enzyme reaction required minimizing the dilution of cellular contents. To demonstrate the potential applications of this methodology in small scale biochemical analyses, single Arabidopsis knf embryos lacking the alpha-glucosidase I encoding KNOPF gene were assayed. Mutant embryos demonstrated insignificant conversion of a triglucose substrate, as compared to wild type, confirming the deletion of alpha-glucosidase I. Embryos were simultaneously assayed for a second enzyme, beta-galactosidase, illustrating that the mutants were viable except for their lack of alpha-glucosidase I activity.

Mutations in SIN4 and RGR1 cause constitutive expression of MAL structural genes in Saccharomyces cerevisiae

Transcription of the Saccharomyces MAL structural genes is induced 40-fold by maltose and requires the MAL-activator and maltose permease. To identify additional players involved in regulating MAL gene expression, we carried out a genetic selection for MAL constitutive mutants. Strain CMY4000 containing MAL1 and integrated copies of MAL61promoter-HIS3 and MAL61promoter-lacZ reporter genes was used to select constitutive mutants. The 29 recessive mutants fall into at least three complementation groups. Group 1 and group 2 mutants exhibit pleiotropic phenotypes and represent alleles of Mediator component genes RGR1 and SIN4, respectively. The rgr1 and sin4 constitutive phenotype does not require either the MAL-activator or maltose permease, indicating that Mediator represses MAL basal expression. Further genetic analysis demonstrates that RGR1 and SIN4 work in a common pathway and each component of the Mediator Sin4 module plays a distinct role in regulating MAL gene expression. Additionally, the Swi/Snf chromatin-remodeling complex is required for full induction, suggesting a role for chromatin remodeling in the regulation of MAL gene expression. A sin4Delta mutation is unable to suppress the defects in MAL gene expression resulting from loss of the Swi/Snf complex component Snf2p. The role of the Mediator in MAL gene regulation is discussed.

Solvent isotope effects on alpha-glucosidase

The solvent kinetic isotope effects (SKIE) on the yeast alpha-glucosidase-catalyzed hydrolysis of p-nitrophenyl and methyl-d-glucopyranoside were measured at 25 degrees C. With p-nitrophenyl-D-glucopyranoside (pNPG), the dependence of k(cat)/K(m) on pH (pD) revealed an unusually large (for glycohydrolases) solvent isotope effect on the pL-independent second-order rate constant, (DOD)(k(cat)/K(m)), of 1.9 (+/-0.3). The two pK(a)s characterizing the pH profile were increased in D(2)O. The shift in pK(a2) of 0.6 units is typical of acids of comparable acidity (pK(a)=6.5), but the increase in pK(a1) (=5.7) of 0.1 unit in going from H(2)O to D(2)O is unusually small. The initial velocities show substrate inhibition (K(is)/K(m) approximately 200) with a small solvent isotope effect on the inhibition constant [(DOD)K(is)=1.1 (+/-0.2)]. The solvent equilibrium isotope effects on the K(is) for the competitive inhibitors D-glucose and alpha-methyl D-glucoside are somewhat higher [(DOD)K(i)=1.5 (+/-0.1)]. Methyl glucoside is much less reactive than pNPG, with k(cat) 230 times lower and k(cat)/K(m) 5 x 10(4) times lower. The solvent isotope effect on k(cat) for this substrate [=1.11 (+/-0. 02)] is lower than that for pNPG [=1.67 (+/-0.07)], consistent with more extensive proton transfer in the transition state for the deglucosylation step than for the glucosylation step.

Inhibition of small-intestinal sugar absorption mediated by sodium orthovanadate Na3VO4 in rats and its mechanisms

AIM: To investigate the inhibitory effects of sodium orthovanadate on small-intestinal glucose and maltose absorption in rats and its mechanism.

METHODS: Normal Wistar rats were lavaged with sodium orthovanadate (16 mg/kg, 4 mg/kg and 1 mg/kg) for 6 d. Blood glucose values were measured after fasting and 0.5, 1, 1.5 and 2 h after glucose and maltose feeding with oxidation-enzyme method. α-glucosidase was abstracted from the upper small intestine, and its activity was examined. mRNA expression of α-glucosidase and glucose-transporter 2 (GLUT2) in epithelial cells of the small intestine was observed by in situ hybridization.

RESULTS: Sodium orthovanadate could delay the increase of plasma glucose concentration after glucose and maltose loading, area under curve (AUC) in these groups was lower than that in control group. Sodium orthovanadate at dosages of 10 μmol/L, 100 μmol/L and 1000 μmol/L could suppress the activity of α-glucosidase in the small intestine of normal rats, with an inhibition rate of 68.18%, 87.22% and 91.91%, respectively. Sodium orthovanadate reduced mRNA expression of α-glucosidase and GLUT2 in epithelial cells of small intestine.

CONCLUSION: Sodium orthovanadate can reduce and delay the absorption of glucose and maltose. The mechanism may be that it can inhibit the activity and mRNA expression of α-glucosidase, as well as mRNA expression of GLUT2 in small intestine.

Mechanistic and structural analysis of a family 31 alpha-glycosidase and its glycosyl-enzyme intermediate

We have determined the first structure of a family 31 alpha-glycosidase, that of YicI from Escherichia coli, both free and trapped as a 5-fluoroxylopyranosyl-enzyme intermediate via reaction with 5-fluoro-alpha-D-xylopyranosyl fluoride. Our 2.2-A resolution structure shows an intimately associated hexamer with structural elements from several monomers converging at each of the six active sites. Our kinetic and mass spectrometry analyses verified several of the features observed in our structural data, including a covalent linkage from the carboxylate side chain of the identified nucleophile Asp(416) to C-1 of the sugar ring. Structure-based sequence comparison of YicI with the mammalian alpha-glucosidases lysosomal alpha-glucosidase and sucrase-isomaltase predicts a high level of structural similarity and provides a foundation for understanding the various mutations of these enzymes that elicit human disease.

A steady-state modeling approach to validate an in vivo mechanism of the GAL regulatory network in Saccharomyces cerevisiae

Cellular regulation is a result of complex interactions arising from DNA-protein and protein-protein binding, autoregulation, and compartmentalization and shuttling of regulatory proteins. Experiments in molecular biology have identified these mechanisms recruited by a regulatory network. Mathematical models may be used to complement the knowledge-base provided by in vitro experimental methods. Interactions identified by in vitro experiments can lead to the hypothesis of multiple candidate models explaining the in vivo mechanism. The equilibrium dissociation constants for the various interactions and the total component concentration constitute constraints on the candidate models. In this work, we identify the most plausible in vivo network by comparing the output response to the experimental data. We demonstrate the methodology using the GAL system of Saccharomyces cerevisiae for which the steady-state analysis reveals that Gal3p neither dimerizes nor shuttles between the cytoplasm and the nucleus.

Val216 decides the substrate specificity of alpha-glucosidase in Saccharomyces cerevisiae

Differences in the substrate specificity of alpha-glucosidases should be due to the differences in the substrate binding and the catalytic domains of the enzymes. To elucidate such differences of enzymes hydrolyzing alpha-1,4- and alpha-1,6-glucosidic linkages, two alpha-glucosidases, maltase and isomaltase, from Saccharomyces cerevisiae were cloned and analyzed. The cloned yeast isomaltase and maltase consisted of 589 and 584 amino acid residues, respectively. There was 72.1% sequence identity with 165 amino acid alterations between the two alpha-glucosidases. These two alpha-glucosidase genes were subcloned into the pKP1500 expression vector and expressed in Escherichia coli. The purified alpha-glucosidases showed the same substrate specificities as those of their parent native glucosidases. Chimeric enzymes constructed from isomaltase by exchanging with maltase fragments were characterized by their substrate specificities. When the consensus region II, which is one of the four regions conserved in family 13 (alpha-amylase family), is replaced with the maltase type, the chimeric enzymes alter to hydrolyze maltose. Three amino acid residues in consensus region II were different in the two alpha-glucosidases. Thus, we modified Val216, Gly217, and Ser218 of isomaltase to the maltase-type amino acids by site-directed mutagenesis. The Val216 mutant was altered to hydrolyze both maltose and isomaltose but neither the Gly217 nor the Ser218 mutant changed their substrate specificity, indicating that Val216 is an important residue discriminating the alpha-1,4- and 1,6-glucosidic linkages of substrates.

The Lec23 Chinese hamster ovary mutant is a sensitive host for detecting mutations in alpha-glucosidase I that give rise to congenital disorder of glycosylation IIb (CDG IIb)

Lec23 Chinese hamster ovary cells are defective in alpha-glucosidase I activity, which removes the distal alpha(1,2)-linked glucose residue from Glc(3)Man(9)GlcNAc(2) moieties attached to glycoproteins in the endoplasmic reticulum. Mutations in the human GCS1 gene give rise to the congenital disorder of glycosylation termed CDG IIb. Lec23 mutant cells have been shown to alter lectin binding and to synthesize predominantly oligomannosyl N-glycans on endogenous glycoproteins. A single point mutation (TCC to TTC; Ser to Phe) was identified in Lec23 Gcs1 cDNA and genomic DNA. Serine at the analogous position is highly conserved in all GCS1 gene homologues. A human GCS1 cDNA reverted the Lec23 phenotype, whereas GCS1 cDNA carrying the lec23 mutation (S440F in human) did not. By contrast, GCS1 cDNA with an R486T or F652L CDG IIb mutation gave substantial rescue of the Lec23 phenotype. Nevertheless, in vitro assays of each enzyme gave no detectable alpha-glucosidase I activity. Clearly the R486T and F652L GCS1 mutations are only mildly debilitating in an intact cell, whereas the S440F mutation largely inactivates alpha-glucosidase I both in vitro and in vivo. However, the S440F alpha-glucosidase I may have a small amount of alpha-glucosidase I activity in vivo based on the low levels of complex N-glycans in Lec23. A sensitive test for complex N-glycans showed the presence of polysialic acid on the neural cell adhesion molecule. The Lec23 Chinese hamster ovary mutant represents a sensitive host for detecting a wide range of mutations in human GCS1 that give rise to CDG IIb.

Improved efficacy of gene therapy approaches for Pompe disease using a new, immune-deficient GSD-II mouse model

Glycogen storage disease type II (GSD-II) is a lysosomal storage disorder in which the lack of human acid-alpha glucosidase (hGAA) activity results in massive accumulations of glycogen in cardiac and skeletal muscle fibers. Affected individuals die of cardiorespiratory failure secondary to the skeletal and/or cardiac muscle involvement. Recombinant hGAA enzyme replacement therapy (ERT) is currently in clinical trials and, although promising, ERT may be limited by large-scale production issues and/or the need for frequent infusions. These limitations could be circumvented or augmented by gene therapy strategies. Previous findings in our lab demonstrated that hepatic targeting of a modified adenovirus vector expressing human GAA was able to correct the glycogen accumulation in multiple affected muscles in the GAA-KO mice, by virtue of high-level, hepatic secretion of hGAA. However, although the vector persisted and expressed hGAA for 6 months in the liver, plasma hGAA was not detectable beyond 10 dpi (days postinjection), and reaccumulation of glycogen was observed. Two possibilities may have contributed to this phenomenon, the shut down of the CMV promoter and/or the onset of high levels of anti-hGAA antibodies. In order to test these and other possibilities, we have now developed an immune-deficient mouse model of GSD-II by interbreeding GAA-KO mice with severe combined immune-deficient (SCID) mice, generating double knockout, GAA-KO/SCID mice. In this new mouse model, we evaluated the efficacy of an [E1-, polymerase-] AdhGAA vector, in the absence of anti-hGAA antibody responses. After intravenous injection, GAA detection in the plasma was prolonged for at least 6 months secondary to the lack of anti-hGAA antibody production in all of the treated mice. GAA-KO/SCID mice treated with high doses of viral vector demonstrated longer durations of glycogen correction in both skeletal and cardiac muscles, relative to mice injected with lower doses of the vector. Notably, within 2 weeks of vector injection, muscle strength and coordination was normalized, and the improved muscle function persisted for at least 6 months. In summary, this new mouse model of GSD-II now makes it possible to assess the full potential for efficacy of any GAA-expressing vector (and/or ERT) contemplated for use in GSD-II gene therapy, without the negative influence that anti-hGAA antibodies entail.

Overexpression and characterization of two unknown proteins, YicI and YihQ, originated from Escherichia coli

The proteins encoded in the yicI and yihQ gene of Escherichia coli have similarities in the amino acid sequences to glycoside hydrolase family 31 enzymes, but they have not been detected as the active enzymes. The functions of the two proteins have been first clarified in this study. Recombinant YicI and YihQ produced in E. coli were purified and characterized. YicI has the activity of alpha-xylosidase. YicI existing as a hexamer shows optimal pH at 7.0 and is stable in the pH range of 4.7-10.1 with incubation for 24h at 4 degrees C and also is stable up to 47 degrees C with incubation for 15 min. The enzyme shows higher activity against alpha-xylosyl fluoride, isoprimeverose (6-O-alpha-xylopyranosyl-glucopyranose), and alpha-xyloside in xyloglucan oligosaccharides. The alpha-xylosidase catalyzes the transfer of alpha-xylosyl residue from alpha-xyloside to xylose, glucose, mannose, fructose, maltose, isomaltose, nigerose, kojibiose, sucrose, and trehalose. YihQ exhibits the hydrolysis activity against alpha-glucosyl fluoride, and so is an alpha-glucosidase, although the natural substrates, such as alpha-glucobioses, are scarcely hydrolyzed. alpha-Glucosidase has been found for the first time in E. coli.

DER7, encoding α-glucosidase I is essential for degradation of malfolded glycoproteins of the endoplasmic reticulum

Proteins entering the endoplasmic reticulum (ER) have to acquire an export-competent structure before they are delivered to their final destination. This folding process is monitored by an ER protein quality control system. Folding-incompetent conformers are eliminated via a mechanism called ER-associated protein degradation (ERAD). Genetic studies in the yeast Saccharomyces cerevisiae have revealed that carbohydrate modification plays a crucial role in these processes. Here we show that a previously isolated der mutant (der7-1) is defective in ERAD. We identify DER7 as the gene encoding N-glycan-processing alpha-glucosidase I (EC 3.2.1.106) of the ER and demonstrate that its inactivity, due to a substitution of the conserved glycine residue at position 725 by arginine (G725R) in the der7-1 mutant, leads to ER-stress.

Regulation of the Hansenula polymorpha maltase gene promoter in H. polymorpha and Saccharomyces cerevisiae1

Hansenula polymorpha is an exception among methylotrophic yeasts because it can grow on the disaccharides maltose and sucrose. We disrupted the maltase gene (HPMAL1) in H. polymorpha 201 using homologous recombination. Resulting disruptants HP201HPMAL1Delta failed to grow on maltose and sucrose, showing that maltase is essential for the growth of H. polymorpha on both disaccharides. Expression of HPMAL1 in HP201HPMAL1Delta from the truncated variants of the promoter enabled us to define the 5'-upstream region as sufficient for the induction of maltase by disaccharides and its repression by glucose. Expression of the Saccharomyces cerevisiae maltase gene MAL62 was induced by maltose and sucrose, and repressed by glucose if expressed in HP201HPMAL1Delta from its own promoter. Similarly, the HPMAL1 promoter was recognized and correctly regulated by the carbon source in a S. cerevisiae maltase-negative mutant 100-1B. Therefore we suggest that the transcriptional regulators of S. cerevisiae MAL genes (MAL activator and Mig1 repressor) can affect the expression of the H. polymorpha maltase gene, and that homologues of these proteins may exist in H. polymorpha. Using the HPMAL1 gene as a reporter in a H. polymorpha maltase disruption mutant it was shown that the strength of the HPMAL1 promoter if induced by sucrose is quite comparable to the strength of the H. polymorpha alcohol oxidase promoter under conditions of methanol induction, revealing the biotechnological potential of the HPMAL1 promoter.

Rescue of enzyme deficiency in embryonic diaphragm in a mouse model of metabolic myopathy: Pompe disease

Several human genetic diseases that affect striated muscle have been modeled by creating knockout mouse strains. However, many of these are perinatal lethal mutations that result in death from respiratory distress within hours after birth. As the diaphragm muscle does not contract until birth, the sudden increase in diaphragm activity creates permanent injury to the muscle causing it to fail to meet respiratory demands. Therefore, the impact of these mutations remains hidden throughout embryonic development and early death prevents investigators from performing detailed studies of other striated muscle groups past the neonatal stage. Glycogen storage disease type II (GSDII), caused by a deficiency in acid alpha-glucosidase (GAA), leads to lysosomal accumulation of glycogen in all cell types and abnormal myofibrillogenesis in striated muscle. Contractile function of the diaphragm muscle is severely affected in both infantile-onset and late-onset individuals, with death often resulting from respiratory failure. The knockout mouse model of GSDII survives well into adulthood despite the gradual weakening of all striated muscle groups. Using this model, we investigated the delivery of recombinant adeno-associated virus (rAAV) vectors encoding the human GAA cDNA to the developing embryo. Results indicate specific high-level transduction of diaphragm tissue, leading to activity levels up to 10-fold higher than normal and restoration of normal contractile function. Up to an estimated 50 vector copies per diploid genome were quantified in treated diaphragms. Histological glycogen staining of treated diaphragms revealed prevention of lysosomal glycogen accumulation in almost all fibers when compared with untreated controls. This method could be employed with disease models where specific rescue of the diaphragm would allow for increased survival and thus further investigation into the impact of the gene deletion on other striated muscle groups.

A case of childhood Pompe disease demonstrating phenotypic variability of p.Asp645Asn

A six-year-old child presented at 8 months of age with proximal muscle weakness and mild cardiac hypertrophy. Some alpha-glucosidase activity was detected in muscle but not in fibroblasts. As none of the two pathogenic mutations, [c.1933G>A]+[c.2702T>A] (Asp645Asn/Leu901Gln), led to detectable alpha-glucosidase activity upon expression in COS cells, the phenotype of the patient remained unexplained. A functionally comparable set of mutations, Asp645Asn/insGnt2243, was reported previously to cause classic infantile Pompe disease [Biochem Biophys Res Commun 244 (1998) 921]. We conclude that secondary genetic or environmental factors can be decisive for the phenotypic outcome of classic infantile versus childhood Pompe disease, when the acid alpha-glucosidase activity is extremely low.

Processing enzyme glucosidase II: proposed catalytic residues and developmental regulation during the ontogeny of the mouse mammary gland

Following the action of glucosidase I to clip the terminal alpha1,2-linked glucose, glucosidase II sequentially cleaves the two inner alpha1,3-linked glucose residues from the Glcalpha1,2Glcalpha1,3Glcalpha1,3Man(9)GlcNAc(2) oligosaccharide of the incipient glycoprotein as it undergoes folding and maturation. Glucosidase II belongs to family 31 glycosidases. These enzymes act by the acid-base catalytic mechanism. The cDNA of the wild-type and several mutant forms of the fusion protein of the enzyme in which mutations were introduced in the conserved motif D(564)MNE(567) were expressed in Sf9 cells, and the proteins were purified on Ni-NTA matrix. The catalytic activity of the purified proteins was determined with radioactive Glc(2)Man(9)GlcNAc(2) substrate. The results show that the aspartate and glutamate within the D(564)MNE(567) motif can serve for catalysis, most likely as the acid-base pair within the active site of the enzyme. The developmental regulation of glucosidase II was studied during the ontogeny of the mouse mammary gland for its growth and differentiation. The mRNA of both alpha and beta subunits of the enzyme, immunoreactive alpha and beta subunits, and enzyme activity were measured over the complete developmental cycle. The changes in all the parameters were consistent with similar fluctuations with several other enzymes of the N-glycosylation machinery reported earlier, reaching a three- to fourfold increase over the basal level in the virgin gland at the peak of lactation. Altogether it appears that there is a coordinated regulation of the enzymes involved in protein N-glycosylation during the development of the mouse mammary gland.