Subcellular distribution in rat liver of a novel broad-specificity (alpha 1----2, alpha 1----3 and alpha 1----6) mannosidase active on oligomannose glycans

Essential and mutually compensatory roles of {alpha}-mannosidase II and {alpha}-mannosidase IIx in N-glycan processing in vivo in mice

Many proteins synthesized through the secretory pathway receive posttranslational modifications, including N-glycosylation. alpha-Mannosidase II (MII) is a key enzyme converting precursor high-mannose-type N-glycans to matured complex-type structures. Previous studies showed that MII-null mice synthesize complex-type N-glycans, indicating the presence of an alternative pathway. Because alpha-mannosidase IIx (MX) is a candidate enzyme for this pathway, we asked whether MX functions in N-glycan processing by generating MII/MX double-null mice. Some double-nulls died between embryonic days 15.5 and 18.5, but most survived until shortly after birth and died of respiratory failure, which represents a more severe phenotype than that seen in single-nulls for either gene. Structural analysis of N-glycans revealed that double-nulls completely lack complex-type N-glycans, demonstrating a critical role for at least one of these enzymes for effective N-glycan processing. Recombinant mouse MX and MII showed identical substrate specificities toward N-glycan substrates, suggesting that MX is an isozyme of MII. Thus, either MII or MX can biochemically compensate for the deficiency of the other in vivo, and either of two is required for late embryonic and early postnatal development.

N-Glycan structure analysis using lectins and an alpha-mannosidase activity assay

Alpha-mannosidase IIx (MX) and alpha-mannosidase II (MII) are homologous enzymes whose critical roles in N-glycan processing were established in large part by analysis of the MII/MX double-knockout mouse. To analyze the structures of N-glycans synthesized in the mutant mice, we employed lectin blot and lectin histochemistry in addition to mass spectrometry analysis and two-dimensional high-performance liquid chromatography (HPLC) mapping. We also produced soluble MII and MX by transfecting mammalian cells with expression vectors and determined substrate specificity of MX. This chapter describes methods using lectins to analyze N-glycans in knockout mice and provides a protocol to assay alpha-mannosidase activity using soluble MX.

Comparing N-glycan processing in mammalian cell lines to native and engineered lepidopteran insect cell lines

In the past decades, a large number of studies in mammalian cells have revealed that processing of glycoproteins is compartmentalized into several subcellular organelles that process N-glycans to generate complex-type oligosaccharides with terminal N -acetlyneuraminic acid. Recent studies also suggested that processing of N-glycans in insect cells appear to follow a similar initial pathway but diverge at subsequent processing steps. N-glycans from insect cell lines are not usually processed to terminally sialylated complex-type structures but are instead modified to paucimannosidic or oligomannose structures. These differences in processing between insect cells and mammalian cells are due to insufficient expression of multiple processing enzymes including glycosyltransferases responsible for generating complex-type structures and metabolic enzymes involved in generating appropriate sugar nucleotides. Recent genomics studies suggest that insects themselves may include many of these complex transferases and metabolic enzymes at certain developmental stages but expression is lost or limited in most lines derived for cell culture. In addition, insect cells include an N -acetylglucosaminidase that removes a terminal N -acetylglucosamine from the N-glycan. The innermost N -acetylglucosamine residue attached to asparagine residue is also modified with alpha(1,3)-linked fucose, a potential allergenic epitope, in some insect cells. In spite of these limitations in N-glycosylation, insect cells have been widely used to express various recombinant proteins with the baculovirus expression vector system, taking advantage of their safety, ease of use, and high productivity. Recently, genetic engineering techniques have been applied successfully to insect cells in order to enable them to produce glycoproteins which include complex-type N-glycans. Modifications to insect N-glycan processing include the expression of missing glycosyltransferases and inclusion of the metabolic enzymes responsible for generating the essential donor sugar nucleotide, CMP- N -acetylneuraminic acid, required for sialylation. Inhibition of N -acetylglucosaminidase has also been applied to alter N-glycan processing in insect cells. This review summarizes current knowledge on N-glycan processing in lepidopteran insect cell lines, and recent progress in glycoengineering lepidopteran insect cells to produce glycoproteins containing complex N-glycans.

Golgi alpha-mannosidase II deficiency in vertebrate systems: implications for asparagine-linked oligosaccharide processing in mammals

The maturation of N-glycans to complex type structures on cellular and secreted proteins is essential for the roles that these structures play in cell adhesion and recognition events in metazoan organisms. Critical steps in the biosynthetic pathway leading from high mannose to complex structures include the trimming of mannose residues by processing mannosidases in the endoplasmic reticulum (ER) and Golgi complex. These exo-mannosidases comprise two separate families of enzymes that are distinguished by enzymatic characteristics and sequence similarity. Members of the Class 2 mannosidase family (glycosylhydrolase family 38) include enzymes involved in trimming reactions in N-glycan maturation in the Golgi complex (Golgi mannosidase II) as well as catabolic enzymes in lysosomes and cytosol. Studies on the biological roles of complex type N-glycans have employed a variety of strategies including the treatment of cells with glycosidase inhibitors, characterization of human patients with enzymatic defects in processing enzymes, and generation of mouse models for the enzyme deficiency by selective gene disruption approaches. Corresponding studies on Golgi mannosidase II have employed swainsonine, an alkaloid natural plant product that causes "locoism", a phenocopy of the lysosomal storage disease, alpha-mannosidosis, as a result of the additional targeting of the broad-specificity lysosomal mannosidase by this compound. The human deficiency in Golgi mannosidase II is characterized by congenital dyserythropoietic anemia with splenomegaly and various additional abnormalities and complications. Mouse models for Golgi mannosidase II deficiency recapitulate many of the pathological features of the human disease and confirm that the unexpectedly mild effects of the enzyme deficiency result from a tissue-specific and glycoprotein substrate-specific alternate pathway for synthesis of complex N-glycans. In addition, the mutant mice develop symptoms of a systemic autoimmune disorder as a consequence of the altered glycosylation. This review will discuss the biochemical features of Golgi mannosidase II and the consequences of its deficiency in mammalian systems as a model for the effects of alterations in vertebrate N-glycan maturation during development.

Insect cells encode a class II alpha-mannosidase with unique properties

Previously, we cloned and characterized an insect (Sf9) cell cDNA encoding a class II alpha-mannosidase with amino acid sequence and biochemical similarities to mammalian Golgi alpha-mannosidase II. Since then, it has been demonstrated that other mammalian class II alpha-mannosidases can participate in N-glycan processing. Thus, the present study was performed to evaluate the catalytic properties of the Sf9 class II alpha-mannosidase and to more clearly determine its relationship to mammalian Golgi alpha-mannosidase II. The results showed that the Sf9 enzyme is cobalt-dependent and can hydrolyze Man(5)GlcNAc(2) to Man(3)GlcNAc(2), but it cannot hydrolyze GlcNAcMan(5)GlcNAc(2). These data establish that the Sf9 enzyme is distinct from Golgi alpha-mannosidase II. This enzyme is not a lysosomal alpha-mannosidase because it is not active at acidic pH and it is localized in the Golgi apparatus. In fact, its sensitivity to swainsonine distinguishes the Sf9 enzyme from all other known mammalian class II alpha-mannosidases that can hydrolyze Man(5)GlcNAc(2). Based on these properties, we designated this enzyme Sf9 alpha-mannosidase III and concluded that it probably provides an alternate N-glycan processing pathway in Sf9 cells.

Overexpression of the Golgi-localized enzyme alpha-mannosidase IIx in Chinese hamster ovary cells results in the conversion of hexamannosyl-N-acetylchitobiose to tetramannosyl-N-acetylchitobiose in the N-glycan-processing pathway

Golgi alpha-mannosidase II is an enzyme that processes the intermediate oligosaccharide Gn(1)M(5)Gn(2) to Gn(1)M(3)Gn(2) during biosynthesis of N-glycans. Previously, we isolated a cDNA encoding a protein homologous to alpha-mannosidase II and designated it alpha-mannosidase IIx. Here, we show by immunocytochemistry that alpha-mannosidase IIx resides in the Golgi in HeLa cells. When coexpressed with alpha-mannosidase II, alpha-mannosidase IIx colocalizes with alpha-mannosidase II in COS cells. A protein A fusion of the catalytic domain of alpha-mannosidase IIx hydrolyzes a synthetic substrate, 4-umbelliferyl-alpha-D-mannoside, and this activity is inhibited by swainsonine. [(3)H]glucosamine-labeled Chinese hamster ovary cells overexpressing alpha-mannosidase IIx show a reduction of M(6)Gn(2) and an accumulation of M(4)Gn(2). Structural analysis identified M(4)Gn(2) to be Man alpha 1-->6(Man alpha 1-->2Man alpha 1-->3)Man beta 1-->4GlcNAc beta 1-->4GlcNAc. The results suggest that alpha-mannosidase IIx hydrolyzes two peripheral Man alpha 1-->6 and Man alpha 1-->3 residues from [(Man alpha 1-->6)(Man alpha 1-->3)Man alpha 1-->6](Man alpha 1-->2Man alpha 1-->3)Man beta 1-->4GlcNAc beta 1-->4GlcNAc, during N-glycan processing.

Congenital disorders of glycosylation: glycosylation defects in man and biological models for their study

Several inherited disorders affecting the biosynthetic pathways of N-glycans have been discovered during the past years. This review summarizes the current knowledge in this rapidly expanding field and covers the molecular bases of these disorders as well as their phenotypical consequences.

Cell surface glycoproteins undergo postbiosynthetic modification of their N-glycans by stepwise demannosylation

Primary rat hepatocytes and two hepatoma cell lines have been used to study whether high mannose-type N-glycans of plasma membrane glycoproteins may be modified by the removal of mannose residues even after transport to the cell surface. To examine glycan remodeling of cell surface glycoproteins, high mannose-type glycoforms were generated by adding the reversible mannosidase I inhibitor deoxymannojirimycin during metabolic labeling with [3H]mannose, thereby preventing further processing of high mannose-type N-glycans to complex structures. Upon transport to the cell surface, glycoproteins were additionally labeled with sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate. This strategy allowed us to follow selectively the fate of cell surface glycoproteins. Postbiosynthetic demannosylation was monitored by determining the conversion of Man8-9GlcNAc2 to smaller structures during reculture of cells in the absence of deoxymannojirimycin. The results show that high mannose-type N-glycans of selected cell surface glycoproteins are trimmed from Man8-9GlcNAc2 to Man5GlcNAc2 with Man7GlcNAc2 and Man6GlcNAc2 formed as intermediates. It could be clearly shown in MH 7777 as well as in HepG2 cells that demannosylation affects plasma membrane glycoproteins after they are routed to the cell surface. As was determined for total cell surface glycoproteins in HepG2 cells, this process occurs with a half-time of 6.7 h. By analyzing the size of high mannose-type glycans of glycoproteins isolated from the cell surface at the end of the reculture period, i.e. after trimming had occurred, we were able to demonstrate that glycoproteins carrying trimmed high mannose glycans become exposed at the cell surface. From these data we conclude that cell surface glycoproteins can be trimmed by mannosidases at sites peripheral to N-acetylglucosaminyltransferase I without further processing of their glycans to the complex form. This glycan remodeling may occur at the cell surface or during endocytosis and recycling back to the cell surface.

Alpha-mannosidase-II deficiency results in dyserythropoiesis and unveils an alternate pathway in oligosaccharide biosynthesis

Alpha-mannosidase-II (alphaM-II) catalyzes the first committed step in the biosynthesis of complex asparagine-linked (N-linked) oligosaccharides (N-glycans). Genetic deficiency of alphaM-II should abolish complex N-glycan production as reportedly does inhibition of alphaM-II by swainsonine. We find that mice lacking a functional alphaM-II gene develop a dyserythropoietic anemia concurrent with loss of erythrocyte complex N-glycans. Unexpectedly, nonerythroid cell types continued to produce complex N-glycans by an alternate pathway comprising a distinct alpha-mannosidase. These studies reveal cell-type-specific variations in N-linked oligosaccharide biosynthesis and an essential role for alphaM-II in the formation of erythroid complex N-glycans. alphaM-II deficiency elicits a phenotype in mice that correlates with human congenital dyserythropoietic anemia type II.

Intracellular disposal of incompletely folded human alpha1-antitrypsin involves release from calnexin and post-translational trimming of asparagine-linked oligosaccharides

Protection of lung elastin fibers from proteolytic destruction is compromised by inefficient secretion of incompletely folded allelic variants of human alpha1-antitrypsin from hepatocytes. Pulse-chase radiolabeling with [35S]methionine and sucrose gradient sedimentation and coimmunoprecipitation techniques were employed to investigate quality control of human alpha1-antitrypsin secretion from stably transfected mouse hepatoma cells. The secretion-incompetent variant null(Hong Kong) (Sifers, R. N., Brashears-Macatee, S., Kidd, V. J., Muensch, H., and Woo, S. L. C. (1988) J. Biol. Chem. 263, 7330-7335) cannot fold into a functional conformation and was quantitatively associated with the molecular chaperone calnexin following biosynthesis. Assembly with calnexin required cotranslational trimming of glucose from asparagine-linked oligosaccharides. Intracellular disposal of pulse-radiolabeled molecules coincided with their release from calnexin. Released monomers and intracellular disposal were nonexistent in cells chased with cycloheximide, an inhibitor of protein synthesis. Post-translational trimming of asparagine-linked oligosaccharides and intracellular disposal were abrogated by 1-deoxymannojirimycin, an inhibitor of alpha-mannosidase activity, without affecting the monomer population. The data are consistent with a recently proposed quality control model (Hammond, C., Braakman, I., and Helenius, A. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 913-917) in which intracellular disposal requires dissociation from calnexin and post-translational trimming of mannose from asparagine-linked oligosaccharides.

Intra-Golgi transport inhibition by megalomicin

Megalomicin (MGM) is a macrolide antibiotic which has been demonstrated previously to cause an anomalous glycosylation of viral proteins. Here we show that MGM produces profound alterations on Golgi morphology and function. The addition of MGM at 50 microM for 1 h caused a dilation of the Golgi detected by immunofluorescence staining for medial- and trans-Golgi markers. The effect of MGM was clearly more intense on the trans-side of the Golgi, as evidenced in electron microscope preparations. The effect on Golgi morphology was reversible and correlated with an impairment of glycoprotein processing in the trans-Golgi. Thus, although the vesicular stomatitis virus G protein was processed in the presence of MGM to an endoglycosidase H-resistant form, it was poorly sialylated. The sialylation of cellular proteins was also inhibited, resulting in cells with low level of sialylation on the cell surface. However MGM did not inhibit the activities of the galactosyl- or sialyltransferase as measured in vitro. MGM inhibited cis- to medial-, and more strongly, medial- to trans-Golgi transport of vesicular stomatitis virus G protein in an in vitro system, suggesting that the impairment in glycoprotein maturation observed in vivo is the result of intra-Golgi transport inhibition.

Properties of the active plum pox potyvirus RNA polymerase complex in defined glycerol gradient fractions

As a first step in the study of the replication of plum pox virus (PPV) RNA, an in vitro virus-specific RNA polymerase activity was characterized in a crude membrane extract (Martin and Garcia, 1991). In this study, we report the fractionation of the crude membrane extract by centrifugation in glycerol gradients. The sedimentation properties after different treatments of the crude extract and its insensitivity to micrococcal nuclease treatment suggest that the RNA polymerase activity was localized in a defined and enclosed membranous structure. Subcellular membrane characterization of the different glycerol gradient fractions indicated that PPV-specific RNA synthesis occurred in fractions enriched in endoplasmic reticulum and tonoplast vesicles.

Soluble forms of alpha-D-mannosidases from rat liver. Separation and characterization of two enzymic forms with different substrate specificities

We have previously reported the substrate specificity of the rat liver cytosolic alpha-D-mannosidase [Haeuw, J. F., Strecker, G., Wieruszeski, J. M., Montreuil, J. & Michalski, J.-C. (1991) Eur. J. Biochem. 202, 1257-1268]. Here, we report the characterization and the purification of this alpha-D-mannosidase and the presence of two soluble forms of alpha-D-mannosidases from rat liver. The cytosolic alpha-D-mannosidase was purified nearly 660-fold with 2.66% recovery to a state approaching homogeneity using: (a) (NH4)2SO4 precipitation; (b) concanavalin-A-Sepharose chromatography; (c) affinity chromatography on a cobalt-chelating Sepharose column; (d) ion-exchange (DEAE-trisacryl M) column chromatography; (e) molecular-size chromatography (Sephacryl S 200). The enzyme was eluted from the final column at an apparent molecular mass of 113 kDa. SDS/PAGE analysis yielded a major protein band at 108 kDa. Moreover, the purification allowed to distinguish two mannosidase activities with different kinetic properties. The first cytosolic activity retained on the cobalt-chelating column was optimally active at neutral pH, was activated by Co2+, was strongly inhibited by swainsonine (Ki = 3.7 microM) but not by deoxymannojirimycin and was active with p-nitrophenyl alpha-D-mannoside (Km = 0.072 mM). Man9GlcNAc was hydrolysed by the purified enzyme down to a Man5GlcNAc structure, i.e. Man(alpha 1-2)Man(alpha 1-2)Man(alpha 1-3)[Man(alpha 1-6)]Man(beta 1-4) GlcNA c, which represents the Man5 oligosaccharide chain of the dolichol pathway formed in the cytosolic compartment during the biosynthesis of N-glycosylprotein glycans. The second activity not retained on the cobalt-chelating column was optimally active at neutral pH, was inhibited by swainsonine (Ki = 28.4 microM) but not by deoxymannojirimycin and was active with p-nitrophenyl alpha-D-mannoside (Km = 0.633 mM). Man9GlcNAc was broken by this enzymic activity down to Man8GlcNAc and Man7GlcNAc structures. Similitaries with endoplasmic reticulum alpha-D-mannosidase exist and this enzyme could be the cytosolic form of the endoplasmic reticulum alpha-D-mannosidase.

Estrogen modulates expression of the glycosyltransferases that synthesize sulfated oligosaccharides on lutropin

The glycoprotein hormone lutropin (LH) bears oligosaccharides terminating with the sequence SO4-4-GalNAc beta 1,4GlcNAc beta 1,2 Man alpha. We have determined that estrogen actively modulates expression of the GalNAc- and sulfotransferases responsible for synthesis of sulfated oligosaccharides on LH alpha and beta subunits. Consequently, terminal glycosylation of LH oligosaccharides with GalNAc-4-SO4 is maintained when LH synthesis and secretion are markedly increased, as occurs during the midcycle surge and following ovariectomy. Maintenance of sulfated oligosaccharides on LH has important biologic consequences because LH circulatory half-life as well as biologic activity at the hormone receptor level are dramatically affected by glycosylation. To our knowledge, regulation of glycosyltransferase levels in response to specific stimuli has not been observed previously, further emphasizing the biologic significance of glycosylation for expression of LH bioactivity in vivo.

Purification and characterization of rat epididymal-fluid alpha-D-mannosidase: similarities to sperm plasma-membrane alpha-D-mannosidase

We have previously reported the occurrence and partial characterization of a novel alpha-D-mannosidase activity on rat sperm plasma membranes [Tulsiani, Skudlarek and Orgebin-Crist (1989) J. Cell Biol. 109, 1257-1267]. Here, we report the presence of a similar alpha-D-mannosidase activity in a soluble form in rat epididymal fluid. The soluble enzyme was purified nearly 500-fold with 9-12% recovery to a state approaching homogeneity using: (1) (NH4)2SO4 precipitation; (2) affinity chromatography on immobilized mannan and D-mannosamine; (3) ion-exchange (DE-52) column chromatography; (4) molecular-sieve chromatography. The enzyme was eluted from the final column (Sephacryl S-400) at an apparent molecular mass of 460 kDa. When resolved by SDS/PAGE (under denaturing conditions), the enzyme showed a major protein band (115 kDa) and few very minor bands. The polyclonal antibody raised against the major protein band was found to cross-react with the alpha-D-mannosidase activity present in epididymal fluid (soluble) and detergent-solubilized spermatozoa from the rat and mouse. This result suggested that the soluble and membrane-bound enzyme activities shared a common antigenic site(s). The antibody was used to characterize further the alpha-D-mannosidase activity(ies) present in the rat epididymal fluid and rat sperm plasma membranes. Data from these studies show that the two forms are similar in (a) subunit molecular mass, (b) substrate specificity and (c) inhibitory effect of several sugars. These similarities suggest that the soluble and membrane-bound alpha-D-mannosidase activities are isoforms. Immunoprecipitation studies after solubilization of the testis and epididymal particulate fraction from sexually immature rats show that the testis (but not the epididymis) contains the immunoreactive alpha-D-mannosidase activity. This result and the fact that spermatozoa from the rat rete testis show alpha-D-mannosidase activity indicate that the sperm enzyme is synthesized in the testis during spermatogenesis.