Cellular effects of deoxynojirimycin analogues: inhibition of N-linked oligosaccharide processing and generation of free glucosylated oligosaccharides

In the accompanying paper [Mellor, Neville, Harvey, Platt, Dwek and Butters (2004) Biochem. J. 381, 861-866] we treated HL60 cells with N-alk(en)yl-deoxynojirimycin (DNJ) compounds to inhibit glucosphingolipid (GSL) biosynthesis and identified a number of non-GSL-derived, small, free oligosaccharides (FOS) most likely produced due to inhibition of the oligosaccharide-processing enzymes a-glucosidases I and II. When HL60 cells were treated with concentrations of N-alk(en)ylated DNJ analogues that inhibited GSL biosynthesis completely, N-butyl- and N-nonyl-DNJ inhibited endoplasmic reticulum (ER) glucosidases I and II, but octadecyl-DNJ did not, probably due to the lack of ER lumen access for this novel, long-chain derivative. Glucosidase inhibition resulted in the appearance of free Glc1-3Man structures, which is evidence of Golgi glycoprotein endomannosidase processing of oligosaccharides with retained glucose residues. Additional large FOS was also detected in cells following a 16 h treatment with N-butyl- and N-nonyl-DNJ. When these FOS structures (>30, including >20 species not present in control cells) were characterized by enzyme digests and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS, all were found to be polymannose-type oligosaccharides, of which the majority were glucosylated and had only one reducing terminal GlcNAc (N-acetylglucosamine) residue (FOS-GlcNAc1), demonstrating a cytosolic location. These results support the proposal that the increase in glucosylated FOS results from enzyme-mediated cytosolic cleavage of oligosaccharides from glycoproteins exported from the ER because of misfolding or excessive retention. Importantly, the present study characterizes the cellular properties of DNJs further and demonstrates that side-chain modifications allow selective inhibition of protein and lipid glycosylation pathways. This represents the most detailed characterization of the FOS structures arising from ER a-glucosidase inhibition to date.

Cellular effects of deoxynojirimycin analogues: uptake, retention and inhibition of glycosphingolipid biosynthesis

Deoxynojirimycin (DNJ) analogues are inhibitors of ceramide glucosyltransferase (CGT), which catalyses the first step in the glucosphingolipid (GSL) biosynthetic pathway. We have synthesized a series of DNJ analogues to study the contribution of N-alk(en)yl side chains (C4, C9 or C18) to the behaviour of these analogues in cultured HL60 cells. When cells were treated for 16 h at non-cytotoxic concentrations of inhibitor, a 40-50% decrease in GSL levels was measured by HPLC analysis of GSL-derived oligosaccharides following ceramide glycanase digestion of GSL and 2-aminobenzamide labelling of the released oligosaccharides. Using a novel technique for short-term [14C]galactose labelling of cellular GSL, we used compound inhibition of GSL biosynthesis as a marker for compound uptake into cells. Surprisingly, the uptake of all three of the DNJ analogues was extremely rapid and was not dependent upon the length of the N-alk(en)yl moiety. Compound uptake occurred in less than 1 min, as shown by the complete inhibition of GSL labelling in cells treated with all the DNJ analogues. Greatly increased cellular retention of N-cis-13-octadecenyl-DNJ was observed relative to the shorter-chain compounds, N-butyl-DNJ and N-nonyl-DNJ, as indicated by complete inhibition of CGT 24 h after removal of inhibitor from the culture medium. The present study further characterizes the properties of N-alk(en)ylated DNJs, and demonstrates that increasing the length of the side chain is a simple way of improving imino sugar retention and therefore inhibitory efficacy for CGT in cultured cells.

Novel oral treatment of Gaucher's disease with N-butyldeoxynojirimycin (OGT 918) to decrease substrate biosynthesis

BACKGROUND

Current treatment for Gaucher's disease involves administration of intravenous glucocerebrosidase to degrade glucocerebroside stored in lysosomes. Lowering the rate of biosynthesis of glucocerebroside should decrease accumulation of this substrate. We investigated the safety and efficacy of OGT 918 (N-butyldeoxynojirimycin), an inhibitor of glucosyltransferase, as a novel oral treatment for non-neuronopathic Gaucher's disease.

METHODS

We recruited, into a 1-year open-label study, 28 adults (seven with previous splenectomies) from four national Gaucher's referral clinics, who were unable or unwilling to receive enzyme treatment. We measured liver and spleen volume by computed tomography or magnetic resonance imaging at baseline and at months 6 and 12, and biochemical and haematological variables monthly, including chitotriosidase activity (a sensitive marker of Gaucher's disease activity). Patients were started on 100 mg oral OGT 918 three times daily.

FINDINGS

Baseline liver volumes were 1.1-2.7 times normal and spleen volumes 5.1-24.8 times normal. At 12 months, mean liver and spleen volumes were significantly lowered by 12% (95% CI 7.8-16.4) and 19% (14.3-23.7), respectively (each p<0.001). Haematological variables improved slightly. Mean organ volume and blood counts improved continually between 6 months and 12 months of treatment. Mean chitotriosidase concentrations fell by 16.4% over 12 months (p<.0001). Six patients withdrew because of gastrointestinal complaints (two), personal reasons (two), or severe pre-existing disease (two). The most frequent adverse effect was diarrhoea, which occurred in 79% of patients shortly after the start of treatment.

INTERPRETATION

Decrease of substrate formation by OGT 918 improves key clinical features of non-neuronopathic Gaucher's disease. The strategy justifies further trials in this and other glycosphingolipid storage disorders.

Cell-specific glycoforms of sialoadhesin and CD45 are counter-receptors for the cysteine-rich domain of the mannose receptor

We previously reported that CR-Fc, an Fc chimeric protein containing the cysteine-rich (CR) domain of the mannose receptor, binds to marginal zone metallophilic macrophages (Mo) and B cell areas in the spleen and to subcapsular sinus Mo in lymph nodes of naive mice (CR-Fc(+) cells). Several CR-Fc ligands were found in spleen and lymph node tissue lysates using ligand blots. In this paper we report the identification of two of these ligands as sialoadhesin (Sn), an Mo-specific membrane molecule, and the leukocyte common antigen, CD45. CR-Fc bound selectively to Sn purified from spleen and lymph nodes and to two low molecular weight isoforms of CD45 in a sugar-dependent manner. CR-Fc binding and non-binding forms of Sn, probably derived from CR-Fc(+) and CR-Fc(-) cells respectively, were selected from spleen lysates. Analysis of the glycan pool associated with the CR-Fc-binding form revealed the presence of charged structures resistant to sialidase, absent in the non-binding form, that could correspond to sulfated structures. These results confirm the identification of the CR region of the mannose receptor as a lectin. We also demonstrate that the same glycoprotein expressed in different cells of the same organ can display distinct sugar epitopes that determine its binding properties.

Aberrant trafficking of hepatitis B virus glycoproteins in cells in which N-glycan processing is inhibited

The role of N-glycan trimming in glycoprotein fate and function is unclear. We have recently shown that hepatitis B virus (HBV) DNA is not efficiently secreted from cells in which alpha-glucosidase mediated N-glycan trimming is inhibited. Here it is shown that, in cells in glucosidase-inhibited cells, viral DNA, accompanied by envelope and core proteins, most likely accumulate within lysosomal compartments. Pulse-chase experiments show that although the viral glycoproteins (L, M, and S) are dysfunctional, in the sense that they do not mediate virion egress and are not efficiently secreted from the cell, they all still leave the endoplasmic reticulum (ER). Surprisingly, however, the glycoproteins retained within the cell were not rapidly degraded, appearing as aggregates, enriched for L and M, with intracellular half-lives exceeding 20 h. Moreover, by 24 h after synthesis, a substantial fraction of the detained glycoproteins appeared to return to the ER, although a considerable amount was also found in the lysosomes. To our knowledge, this is the first report that shows, as a consequence of inhibiting glycosylation processing, certain glycoproteins (i) become dysfunctional and aggregate, yet still depart from the ER, and (ii) have extended rather than shortened half-lives. Taken together, these data suggest that proper intracellular routing of HBV glycoproteins requires ER glucosidase function. It is hypothesized that failure to process N-glycan causes HBV glycoproteins to aggregate and that impaired protein-protein interactions and trafficking are the result of misfolding.

Evidence that N-linked glycosylation is necessary for hepatitis B virus secretion

Human hepatitis B virus (HBV) envelopes contain three distinct glycoproteins called L, M, and S HBsAg. Each is posttranslationally modified to contain N-linked oligosaccharides. N-linked oligosaccharides, after attachment to a polypeptide backbone, are processed by enzymes within the endoplasmic reticulum (ER). There is uncertainty about what role, if any, these N glycans and their modification in the ER play in the function of the HBV envelope proteins. By treating hepatoblastoma cultures which secrete HBV (HepG 2.2.15 cells) with inhibitors of different steps of the glycosylation and glycan modifying pathway, we provide evidence that glycosylation and the first step in the processing pathway are necessary for virion, but not subviral particle, secretion. That is, using a highly sensitive immunoprecipitation/polymerase chain reaction system, enveloped HBV could not be detected in the medium of HepG2.2.15 cells incubated with tunicamycin. However, HBV subviral particle secretion was not prevented by tunicamycin. Moreover, inhibitors of alpha-glucosidase I (the first step in the glycan processing pathway) also prevented virion secretion. Inhibitors of mannose trimming (a later step) and glycolipid synthesis, did not prevent virion secretion, defining the limits of the glycosylation requirements in secretion. These results demonstrate a requirement for N-glycosylation and glucosidase processing in the secretion of virions and further distinguish between the requirements for virion and subviral particle secretion.

Glycoprotein glycosylation and the immunosuppressive effects of human pregnancy serum

Pregnancy serum contains a factor or factors which suppress T lymphocyte proliferation, although the identity of the factor(s) is still unclear. We have demonstrated that the immunosuppressive activity of pregnancy sera can be destroyed by treatment with periodate which oxidises protein-linked oligosaccharides. Similar effects have been noted with uromodulin, a potent immunosuppressive glycoprotein initially isolated from pregnancy urine. We find, however, that uromodulin is present in both pregnancy and non-pregnancy sera, and that removal of uromodulin from pregnancy serum by lectin affinity chromatography is not associated with loss of activity, ruling out this glycoprotein as the immunosuppressive factor. The possible role of protein-linked oligosaccharides of other serum glycoproteins in causing the pregnancy-related immunosuppression is discussed.

Abnormalities in the glycosylation of IgG in spouses of patients with rheumatoid arthritis. A family study

Forty-seven members of eight families with a rheumatoid proband were analysed for abnormal glycosylation of IgG. The results (%G(o) which is the percentage of oligosaccharide chains lacking galactose) were corrected for age and expressed as SD units about the mean for the normal population. Seven of 8 probands, 3/8 spouses, 3/5 RA relatives and 4/26 non-RA relatives had %G(o) values greater than 1SD above the age corrected mean for the normal control population (P less than 0.001, less than 0.01, less than 0.005 and greater than 0.5 respectively). A further 13 spouse pairs were studied. Ten of 13 probands and 8/13 spouses had %G(o) values greater than 1SD above the mean (P less than 0.001 and less than 0.001 respectively). Thus in total, a strikingly high number of unaffected spouses had high %G(o) values (11/21). IgM, IgA and IgG rheumatoid factors were studied. While RA patients' sera showed a correlation between IgM and IgA rheumatoid factors and %G(o), (IgM, r = 0.41 0.05 greater than P greater than 0.02, IgA, r = 0.36, P = 0.05), no correlation between IgG RF and %G(o) was noted in the RA patients. No correlation was found between any of the RF classes and %G(o) in spouses and non-RA relatives.

Agalactosyl IgG in inflammatory bowel disease: correlation with C-reactive protein

The proportion of oligosaccharide chains on the Fc fragment of IgG which terminate with N-acetylglucosamine (GlcNAc) rather than galactose is increased in rheumatoid arthritis and tuberculosis, and in sera from patients with Crohn's disease, probably because of decreased activity of a galactosyltransferase in B lymphocytes. We have assayed the prevalence of agalactosyl oligosaccharides on IgG in sera from 67 patients with inflammatory bowel disease (32 ulcerative colitis and 35 Crohn's disease). The prevalence of agalactosyl IgG significantly increases in the majority of Crohn's patients (19/35 patients), and correlates with the level of C-reactive protein (r = 0.79), and inversely with the concentration of serum albumin. Sera from ulcerative colitis patients show less frequent (nine of 32) and less marked rises in agalactosyl IgG, and sera with high C-reactive protein values can contain normal levels. Thus in ulcerative colitis no correlation was seen between the two assays. The diseases in which the percentage of agalactosyl IgG is raised (rheumatoid arthritis, tuberculosis, Crohn's disease and some ulcerative colitis) are characterised by simultaneous T cell mediated granulomatous tissue damage, and acute phase responses. Levels are normal in less tissue damaging granulomatous conditions, including sarcoidosis, and leprosy (except during episodes of erythema nodosum leprosum). We suggest therefore that a raised percentage of agalactosyl IgG is a correlate of a particular type of T cell mediated pathology which may be relevant to the pathogenesis of inflammatory bowel disease.

A transient rise in agalactosyl IgG correlating with free interleukin 2 receptors, during episodes of erythema nodosum leprosum

The proportion of oligosaccharide chains on the Fc fragment of IgG which terminate with N-acetylglucosamine and not galactose (%GO) has previously been shown to be raised in rheumatoid arthritis (RA), Crohn's disease (CD) and tuberculosis (Tb), but to be normal in sarcoidosis (SA), and in both lepromatous and tuberculoid leprosy. However we have now studied %GO in sequential serum samples collected from lepromatous leprosy patients undergoing episodes of erythema nodosum leprosum (ENL). During ENL %GO is transiently raised, and this rise parallels an increase in circulating interleukin 2 receptors (IL-2R). These findings confirm that changes in T cell function occur during ENL. Moreover it appears that %GO rises when there is, simultaneously, T-cell-mediated tissue damage and an acute phase response (RA, CD, Tb, ENL), but not when there is an acute phase response without major T cell involvement, or chronic T cell activity alone (SA, and tuberculoid leprosy). We suggest therefore that %GO is an indicator of a type of T cell activity with broad immunopathological implications.

Age-related galactosylation of the N-linked oligosaccharides of human serum IgG

In a study of 151 normal, healthy individuals of both sexes varying in age from 1-70 yr, it was found that the relative incidence of agalactosyl (with both outer arms terminating in N-acetylglucosamine) N-linked oligosaccharides on total serum IgG decreased from birth to a minimum (at 25 yr of age) and then increased with age. The relative incidence of digalactosyl structures varied inversely to this, and the relative incidence of monogalactosyl structures was constant. Galactosylation of the N-linked oligosaccharides of the human serum IgG of normal individuals is therefore an age-related molecular parameter. Several reports have suggested that rheumatoid arthritis is associated with a decreased galactosylation of serum IgG (3-5). The normal variation in galactosylation with age as described here allows a true assessment of disease-associated changes in this parameter, and raises the possibility that one of the lesions in rheumatoid arthritis is an accelerated aging of the immune system. In addition, heterogeneity within age groups may be due to intrinsic differences in genetic endowment, or may reflect the impact of extrinsic factors (8).