Purification and characterization of a novel broad-specificity (alpha 1----2, alpha 1----3 and alpha 1----6) mannosidase from rat liver

High-mannose glycan-dependent epitopes are frequently targeted in broad neutralizing antibody responses during human immunodeficiency virus type 1 infection

Broad and potent neutralizing antibody (BNAb) responses are rare in people infected by human immunodeficiency virus type 1 (HIV-1). Clearly defining the nature of BNAb epitopes on HIV-1 envelope glycoproteins (Envs) targeted in vivo is critical for future directions of anti-HIV-1 vaccine development. Conventional techniques are successful in defining neutralizing epitopes in a small number of individual subjects but fail in studying large groups of subjects. Two independent methods were employed to investigate the nature of NAb epitopes targeted in 9 subjects, identified by the NIAID Center for HIV/AIDS Vaccine Immunology (CHAVI) 001 and 008 clinical teams, known to make a strong BNAb response. Neutralizing activity from 8/9 subjects was enhanced by enriching high-mannose N-linked glycan (HM-glycan) of HIV-1 glycoproteins on neutralization target viruses and was sensitive to specific glycan deletion mutations of HIV-1 glycoproteins, indicating that HM-glycan-dependent epitopes are targeted by BNAb responses in these subjects. This discovery adds to accumulating evidence supporting the hypothesis that glycans are important targets on HIV-1 glycoproteins for BNAb responses in vivo, providing an important lead for future directions in developing NAb-based anti-HIV-1 vaccines.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

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.

Structural diversity of cytosolic free oligosaccharides in the human hepatoma cell line, HepG2

It is thought that free oligosaccharides in the cytosol are an outcome of quality control of glycoproteins by endoplasmic reticulum-associated degradation (ERAD). Although considerable amounts of free oligosaccharides accumulate in the cytosol, where they presumably have some function, detailed analyses of their structures have not yet been carried out. We isolated 21 oligosaccharides from the cytosolic fraction of HepG2 cells and analyzed their structures by the two-dimensional high-performance liquid chromatography (HPLC) sugar-mapping method. Sixteen novel oligosaccharides were identified in the cytosol in this study. All had a single N-acetylglucosamine at their reducing-end cores and could be expressed as (Man)n (GlcNAc)1. No free oligosaccharide with N,N'-diacetylchitobiose was detected in the cytosolic fraction of HepG2 cells. This suggested that endo-beta-N-acetylglucosaminidase was a key enzyme in the production of cytosolic free oligosaccharides. The 21 oligosaccharides were classified into three series--series 1: oligosaccharides processed from Manalpha1-2Manalpha1-6 (Manalpha1-2Manalpha1-3)Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3) Manbeta1-4GlcNAc (M9A') and Manalpha1-2Manalpha1-6(Manalpha1-3) Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc (M8A') by digestion with cytosolic alpha-mannosidase; series 2: oligosaccharides processed with Golgi alpha-mannosidases in addition to endoplasmic reticulum (ER) and cytosolic alpha-mannosidases; and series 3: glucosylated oligosaccharides produced from Glc1Man9GlcNAc1 by hydrolysis with cytosolic alpha-mannosidase. The presence of the series "2" oligosaccharides suggests that some of the misfolded glycoproteins had been processed in pre-cis-Golgi vesicles and/or the Golgi apparatus. When the cells were treated with swainsonine to inhibit cytosolic alpha-mannosidase, the amounts of M9A' and M8A' increased remarkably, suggesting that these oligosaccharides were translocated into the cytosol. Four oligosaccharides of series "2" also increased. In contrast, there were obvious reductions in Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc (M5B'), the end product from M9A' by digestion with cytosolic alpha-mannosidase, and Manalpha1-6(Manalpha1- 2Manalpha1-3)Manbeta1-4GlcNAc, derived from series "2" oligosaccharides by digestion with cytosolic alpha-mannosidase. Our data suggest that (1) some of the cytosolic oligosaccharides had been processed with Golgi alpha-mannosidases, (2) the major oligosaccharides translocated from the ER were M9A' and M8A', and (3) M5B' and Glc1M5B' were maintained at relatively high concentrations in the cytosol.

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.

Unique metal dependency of cytosolic alpha-mannosidase from Thermotoga maritima, a hyperthermophilic bacterium

A putative cytosolic alpha-mannosidase gene from a hyperthermophilic marine bacterium Thermotoga maritima was cloned and expressed in Escherichia coli. The purified recombinant enzyme appeared to be a homodimer of a 110-kDa subunit. The enzyme showed metal-dependent ability to hydrolyze p-nitrophenyl-alpha-D-mannopyranoside. In the absence of a metal, the enzyme was inactive. Cobalt and cadmium supported high activity (60 U/mg at 70 degrees C), while the activity with zinc and chromium was poor. Cobalt (0.8 mol) bound to 1 mol monomer with a K(d) of 70 microM. The optimum pH and temperature were 6.0 and 80 degrees C, respectively. The activity was inhibited by swainsonine, but not by 1-deoxymannojirimycin, which is in agreement with the features of cytosolic alpha-mannosidase.

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.

Fucosylated hybrid-type N-glycans on the secreted human epidermal growth factor receptor from swainsonine-treated A431 cells

N-Glycans linked to the human secreted form of epidermal growth factor receptor were isolated from A431 cells after swainsonine treatment. Analysis of the oligosaccharides by (1)H NMR spectroscopy and mass spectrometry shows the presence of oligomannose- and (alpha2-3)-sialylated hybrid-type glycans. The major hybrid-type oligosaccharide chains are fucosylated at the Asn-bound GlcNAc residue. Smaller amounts of the hybrid-type structures are also fucosylated at peripheral GlcNAc residues, constituting the sialyl-Le(x) antigen. No complex-type glycans are found, suggesting the absence of alpha-mannosidase III. An assay for alpha-mannosidase III on the A431 cells in the absence and presence of 6 microM swainsonine shows that Man(5)GlcNAc(2) is not converted into Man(3)GlcNAc(2), thereby confirming that these cells do not contain alpha-mannosidase III activity.

Cloning and expression of a specific human alpha 1,2-mannosidase that trims Man9GlcNAc2 to Man8GlcNAc2 isomer B during N-glycan biosynthesis

We report the isolation of a novel human cDNA encoding a type II membrane protein of 79.5 kDa with amino acid sequence similarity to Class I alpha 1,2-mannosidases. The catalytic domain of the enzyme was expressed as a secreted protein in Pichia pastoris. The recombinant enzyme removes a single mannose residue from Man9GlcNAc and [1H]-NMR analysis indicates that the only product is Man8GlcNAc isomer B, the form lacking the middle-arm terminal alpha 1,2-mannose. Calcium is required for enzyme activity and both 1-deoxymannojirimycin and kifunensine inhibit the human alpha 1,2-mannosidase. The properties and specificity of this human alpha 1,2-mannosidase are identical to the endoplasmic reticulum alpha 1,2-mannosidase from Saccharomyces cerevisiae and differ from those of previously cloned Golgi alpha 1,2-mannosidases that remove up to four mannose residues from Man9GlcNAc2 during N-glycan maturation. Northern blot analysis showed that all human tissues examined express variable amounts of a 3 kb transcript. This highly specific alpha 1,2-mannosidase is likely to be involved in glycoprotein quality control since there is increasing evidence that trimming of Man9GlcNAc2 to Man8GlcNAc2 isomer B in yeast cells is important to target misfolded glycoproteins for degradation.

Engineering N-glycosylation pathways in the baculovirus-insect cell system

The inability to produce eukaryotic glycoproteins with complex N-linked glycans is a major limitation of the baculovirus-insect cell expression system. Recent studies have demonstrated that metabolic engineering can be used to extend the glycoprotein processing capabilities of lepidopteran insect cells. This approach is being used to develop new baculovirus-insect cell expression systems that can produce more authentic recombinant glycoproteins and obtain new information on insect N-glycosylation pathways.

Molecular cloning, chromosomal mapping and tissue-specific expression of a novel human alpha1,2-mannosidase gene involved in N-glycan maturation

Class I alpha1,2-mannosidases play an essential role in the elaboration of complex and hybrid N -glycans in mammalian cells. Using degenerate primers based on amino acid sequences conserved in all members of this enzyme family for RT-PCR, two distinct PCR products were obtained from placenta and lymphocyte cDNAs. One of these was related to the previously cloned human and murine alpha1, 2-mannosidase IA whereas the other was very similar to murine alpha1, 2-mannosidase IB. Northern blot analysis of human tissues with these two alpha1,2-mannosidase probes revealed very different patterns of tissue-specific expression. Similar tissue-specific expression of alpha1,2-mannosidase IA and IB was also observed on Northern blots of adult mouse tissues. A human placenta cDNA library was screened and PCR of brain, placenta, and lymphocyte cDNAs was performed in order to isolate the human alpha1,2-mannosidase IB cDNA. This cDNA encodes a type II membrane protein of 73 kDa that is 94% identical in amino acid sequence to the murine alpha1,2-mannosidase IB (Herscovics et al., 1994, J. Biol. Chem., 269, 9864-9871). A truncated soluble form of the human alpha1,2-mannosidase IB lacking its N -terminal transmembrane domain was expressed as a secreted protein in Pichia pastoris . The recombinant enzyme was incubated with [3H]Man9GlcNAc and [3H]Man8GlcNAc (isomer B), and high performance liquid chromatography analysis of the products showed that [3H]Man9GlcNAc was readily converted to [3H]Man6GlcNAc and much more slowly to [3H]Man5GlcNAc, whereas [3H]Man8GlcNAc was rapidly trimmed to [3H]Man5GlcNAc. The human alpha1,2-mannosidase IB gene was isolated from a P1 human genomic library and shown to be at least 60 kb in size and to contain at least 13 exons. The gene was localized by fluorescence in situ hybridization to human chromosome 1p13, a region that undergoes many aberrations in various types of human cancers. These results show that there are at least two Class I alpha1,2-mannosidases in the human and murine genomes with very distinct transcriptional regulation in different tissues.

alpha-Mannosidase from Trichoderma reesei participates in the postsecretory deglycosylation of glycoproteins

The 160 kDa alpha-mannosidase (E.C. 3.2.1.24) isolated from culture filtrate of Trichoderma reesei has wide aglycon specificity but cleaves the alpha1 --> 2 and alpha1 --> 3 mannosidic bonds with higher rate than alpha1 --> 6 bond and slowly hydrolyses yeast mannan and 1,6-alpha-mannan. The specific activity of the enzyme and rate constant in the reaction with p-nitrophenyl-alpha-D-mannopyranoside were 0.15 U/mg and 1.62 x 10(-4) microM/min/microg, respectively, at optimal pH 6.5. We have found that in vitro enzyme is able to cleave off 30% of total alpha-mannopyranosyl residues from N- and O-linked glycans of secreted glycoproteins. The activity of the alpha-mannosidase toward glycoproteins in vivo was studied comparing the structures of O- and N-linked glycans of glycoproteins isolated from the cultures growing with and without 1-deoxymannojirimycin, an inhibitor of alpha-mannosidases. Difference in structures of these glycans may be explained by postsecretory deglycosylation catalysed by the alpha-mannosidase.

Defective galactosylation of serum transferrin in galactosemia

The glycosylation of serum transferrin from galactosemic patients with a deficiency of galactose-1-phosphate uridyl transferase (EC 2. 7.7 12) is abnormal but becomes normal after treatment with a galactose-free diet. To understand the structural and biochemical basis of the abnormal glycosylation, transferrin was purified from the serum of untreated and treated galactosemic patients and normal controls and the N-linked glycans analyzed by HPLC. The glycans from normal transferrin consisted predominantly (86%) of the disialylated biantennary complex type. The glycans from untreated galactosemic patients were more heterogeneous and contained four major truncated glycans in addition to a smaller amount (13%) of the disialylated biantennary complex type. The truncated glycans were deficient in galactose and sialic acid and their structures were consistent with a decrease in galactosyltransferase activity in hepatocytes, the probable cells of origin of the transferrin. This is postulated to be due to direct inhibition of the galactosyltransferase activity by the accumulated galactose-1-phosphate or to an effect on the formation of UDP-galactose, the donor substrate in the reaction. After treatment the proportion of the truncated glycans decreased and the proportion of the disialylated biantennary complex type increased, returning almost but never completely to normal, even after prolonged treatment in some cases. There was no clear relationship between the length of treatment and the normalization of glycosylation and the level of galactose-1-phosphate in red blood cells, the usual parameter for monitoring the treatment of galactosemics. It is suggested that the persistence of abnormally glycosylated proteins may contribute to the long-term complications in galactosemia.

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.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

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.

Characterisation of the phosphorylcholine-containing N-linked oligosaccharides in the excretory-secretory 62 kDa glycoprotein of Acanthocheilonema viteae

The major excretory-secretory product of the rodent filarial nematode Acanthocheilonema viteae is a 62 kDa glycoprotein (ES-62), which has phosphorylcholine, attached to the N-linked carbohydrates. In this paper, we describe structural studies of N-glycans released from ES-62 by peptide N-glycosidase F. Three major classes of N-glycan structures were observed: high mannose type structures; those which had been fully trimmed to the trimannosyl core and were sub-stoichiometrically fucosylated; and those with a trimannosyl core, with and without core fucosylation, carrying between one and four additional N-acetylglucosamine resides. Of the three classes of glycans, only the last was found to be substituted with detectable levels of phosphorylcholine. The implications of these results with respect to the probable glycosylation pathways operating in A. viteae are discussed.

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.

Functional identification of three major phosphoproteins in endocytic fractions from rat liver. A comparative in vivo and in vitro study

Liver plasma membranes originating from the sinusoidal, lateral and canalicular domains and 'early' and 'late' endosomes were prepared from rats injected with [32P]orthophosphate. The phosphorylated polypeptides in these subcellular fractions, resolved by gel electrophoresis, were analysed and compared with those obtained by in vitro phosphorylation of the fractions by endogenous protein kinases. The polypeptides phosphorylated in vitro were different in plasma membranes, endosomes and lysosomes. Three of the major phosphoproteins in the endocytic membranes were shown to be the polymeric immunoglobulin receptor, the beta subunit of the insulin receptor and the 550-kDa low-density-lipoprotein-receptor-related protein (LRP). An additional 35-kDa polypeptide of unknown function was a major phosphorylated component and thus emerges as a candidate marker protein of hepatic endosomes. Phosphoserine was shown to be the major amino acid phosphorylated in vitro in the phosphoproteins of endocytic membranes. The subcellular distribution in liver tissue of protein kinase activity was also investigated and activity shown to be recovered mainly in blood-sinusoidal and lateral plasma membranes; bile canalicular plasma membranes and endosomes contained low protein kinase activities. The results show that receptor phosphorylation is an 'early' event in endocytosis and the trafficking of ligands that is sustained especially in early endosomes in liver, and emphasizes the biochemical and thus functional distinctiveness of the plasma membrane and the endosomal and lysosomal compartments with regard to their population of phosphorylated proteins.

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.

Rapid desensitization of neonatal rat liver beta-adrenergic receptors. A role for beta-adrenergic receptor kinase

Exposure of beta-adrenergic receptors (BAR) to agonists often leads to a rapid loss of receptor responsiveness. The proposed mechanisms of such rapid receptor desensitization include receptor phosphorylation by either cAMP-dependent protein kinase or the specific beta-adrenergic receptor kinase (BARK), leading to functional uncoupling from adenylyl cyclase and sequestration of the receptors away from the cell surface. To evaluate the physiological role of such mechanisms, we have investigated whether rapid regulation of BAR occurs in the neonatal rat liver immediately after birth, a physiological situation characterized by a dramatic but transient increase in plasma catecholamines. We have detected a rapid, transient uncoupling of liver plasma membrane BARs from adenylyl cyclase (corresponding to a desensitization of approximately 45%) within the first minutes of extrauterine life, followed by a transient sequestration of approximately 40% of the BARs away from the plasma membrane. In agreement with such pattern of desensitization, we have detected (by enzymatic and immunological assays) rapid changes in BARK specific activity in different neonatal rat liver subcellular fractions that take place within the same time frame of BAR uncoupling and sequestration. Our results provide new evidence on the potential role of BAR desensitization mechanisms in vivo and suggest that they are involved in modulating catecholamines actions at the moment of birth. Furthermore, our data indicate that in addition to its suggested role as a rapid modulator of adrenergic receptor function at synapse, rapid BARK-mediated receptor regulation may have functional relevance in other tissues in response to high circulating or local levels of agonists.

Comparative study of high-mannose-type oligosaccharides in membrane glycoproteins of rat hepatocytes and different rat hepatoma cell lines

A comparative study was undertaken to characterize the oligosaccharides released by endo-beta-N-acetylglucosaminidase H (endo H) from the membrane glycoproteins of rat hepatocytes and three different Morris hepatoma cell lines (NA-MH 7777, HTC and MH1C1). It is shown that the membrane glycoproteins of hepatocytes and hepatoma cells contain markedly different quantities and forms of high-mannose-type carbohydrate chains. After radiolabelling of the cells with D-[2-3H]mannose, in the absence and presence of 1 mM 1,5-dideoxy-1,5-imino-D-mannitol (1-deoxymannojirimycin), high-mannose-type oligosaccharides were released from delipidated membrane glycoproteins by enzymic digestion with endo H. The carbohydrate chains were converted to their corresponding oligosaccharide alditols by reduction with sodium borohydride, then further analysed by HPLC using an APS-2 Hypersil column. In the absence of 1-deoxymannojirimycin, up to 10% of the radiolabelled oligosaccharides were released by endo H-treatment of the membrane glycoprotein fraction from rat hepatocytes. In contrast, the quantity of radiolabelled high-mannose-type carbohydrate chains released by endo H-treatment from tumour-cell membrane glycoproteins of hepatoma cell lines NA-MH 7777 (31.5%). MH1C1-MH 7795 (37.2%) and HTC-MH 7288c (48%) was increased up to fivefold. The formation of higher-mannosylated structures after oligosaccharide analysis was observed in all hepatoma cell lines, with Man8GlcNAcOH as the major component, whereas in hepatocytes Man5GlcNAcOH was the predominant high-mannose-type structure. In contrast, in the presence of the Golgi alpha-D-mannosidase I inhibitor, 1-deoxymannojirimycin, no significant differences were observed between the distribution of high-mannose-type oligosaccharides in the membrane glycoproteins of hepatocytes and hepatoma cells. However, in the presence of this inhibitor, the proportion of radiolabelled glycans sensitive to deglycosylation by endo H was greatly increased (> 85%) in all the cell lines investigated, the predominant structures being Man8-9-GlcNAcOH. This study shows that an increased content of high-mannose-type sugar chains is a general characteristic of membrane-bound glycoproteins for malignant transformed hepatocytes.

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.

Beta-glucosidase activity towards a bile acid glucoside in human liver

Human liver contains hydrolytic activity toward 3 beta-glucosido-chenodeoxycholic acid. This beta-glucosidase activity, localized predominantly in the microsomal fraction, was optimally active in the presence of divalent metal ions close to pH 5.0 and was inhibited by EDTA. Kinetic parameters and other catalytic properties of hydrolytic activity towards 3 beta-glucosido-chenodeoxycholic acid from human liver microsomes are described.

Swainsonine is a useful tool to monitor the intracellular traffic of N-linked glycoproteins as a function of the state of enterocytic differentiation of HT-29 cells

After treatment with swainsonine, an inhibitor of both lysosomal alpha-mannosidase and Golgi alpha-mannosidase-II activities, analysis of [3H]mannose-labeled glycans showed that HT-29 cells, derived from a human colonic adenocarcinoma, displayed distinct patterns of N-glycan expression, depending upon their state of enterocytic differentiation. In differentiated HT-29 cells hybrid-type chains were detected, whereas undifferentiated HT-29 cells accumulated high-mannose-type oligosaccharide, despite our demonstration of Golgi alpha-mannosidase-II activity in both cell populations. Pulse/chase experiments carried out in the presence of swainsonine revealed that the persistence of high-mannose-type chains in undifferentiated HT-29 cells was the result of the stabilization of glycoproteins substituted with these glycans. These data suggest that in undifferentiated HT-29 cells, glycoproteins with high-mannose-type oligosaccharides are delivered to a degradative compartment containing swainsonine-sensitive alpha-mannosidase(s), whereas in differentiated HT-29 cells glycoproteins enter a compartment in which alpha-mannosidase II (Golgi apparatus) is present. Thus, this apparent dual effect of swainsonine on N-glycan trimming may reflect differences in the intracellular traffic of glycoproteins as a function of the state of enterocytic differentiation of HT-29 cells.

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

Recently, the purification to homogeneity was reported of a novel broad-specificity alpha-mannosidase from rat liver microsomal membranes [P. Bonay and R. C. Hughes (1991) Eur. J. Biochem. 197, 229-238]. The enzyme catalyzed the ordered removal of alpha 1----2-, alpha 1----3- and alpha 1----6-linked mannose residues from MannGlcNAc oligosaccharide substrates where n = 4-9. We now show by subcellular fractionation and immunocytochemistry that the novel mannosidase is present in the endoplasmic reticulum, Golgi apparatus and endosomes. Enzyme activity is enriched in a heavy Golgi membrane fraction and to lesser extent in an intermediate density Golgi membrane fraction containing GlcNAc transferase I activity and in a 'late' endosomal fraction. Low levels of enzyme activity were detectable in endoplasmic reticulum membranes and in 'early' endosomes but not in receptor-enriched and ligand-free endosomes. Assays of enzymic activity using Golgi membrane fractions in the absence and presence of Triton X-100 showed that the active site of the enzyme faces the lumen of the membrane vesicles. Antibodies directed against the purified mannosidase showed no immunological cross-reaction to known endoplasmic reticulum and Golgi mannosidases. Conversely, the purified mannosidase was not recognized by antibodies directed against endoplasmic reticulum mannosidase nor Golgi mannosidase IA.