Peripheral B lymphocyte beta1,3-galactosyltransferase and chaperone expression in immunoglobulin A nephropathy

Do the mutations of C1GALT1C1 gene play important roles in the genetic susceptibility to Chinese IgA nephropathy?

Background

The deficiency of β1,3 galactose in hinge region of IgA1 molecule played a pivotal role in pathogenesis of IgA nephropathy (IgAN). Cosmc, encoded by C1GALT1C1 gene, was indispensable to β1,3 galactosylation of IgA1. We designed a serial study to investigate the relationship between the mutations of C1GALT1C1 gene and the genetic susceptibility to IgAN.

Methods

Nine hundred and thirty-eight subjects, including 661 patients with IgAN and 277 healthy controls were enrolled in the study. Firstly, single nucleotide polymorphisms (SNPs) in the promoter region of C1GALT1C1 gene were screened. Then the c.-347-190G>A was analyzed by PCR-restriction fragment length polymorphism (PCR-RFLP) for further case-control association analysis. Secondly the somatic mutations of DNAs from peripheral blood B lymphocytes were detected in 15 patients and 7 normal controls.

Results

No significant association was observed between the different alleles or genotypes of c.-347-190G>A and IgAN. The patients with different genotypes of C1GALT1C1 gene did not significantly associate with clinical manifestations, including hematuria, proteinuria, and serum creatinine of patients with IgAN. There was no somatic mutation detected in total 202 clones of 22 individuals.

Conclusion

The c.-347-190G>A polymorphism and the somatic mutation of encoding region of C1GALT1C1 gene were not significantly related to the genetic susceptibility to IgAN in Northern Chinese population.

Association of cell surface mucins with galectin-3 contributes to the ocular surface epithelial barrier

Maintenance of an intact mucosal barrier is critical to preventing damage to and infection of wet-surfaced epithelia. The mechanism of defense has been the subject of much investigation, and there is evidence now implicating O-glycosylated mucins on the epithelial cell surface. Here we investigate a new role for the carbohydrate-binding protein galectin-3 in stabilizing mucosal barriers through its interaction with mucins on the apical glycocalyx. Using the surface of the eye as a model system, we found that galectin-3 colocalized with two distinct membrane-associated mucins, MUC1 and MUC16, on the apical surface of epithelial cells and that both mucins bound to galectin-3 affinity columns in a galactose-dependent manner. Abrogation of the mucin-galectin interaction in four different mucosal epithelial cell types using competitive carbohydrate inhibitors of galectin binding, beta-lactose and modified citrus pectin, resulted in decreased levels of galectin-3 on the cell surface with concomitant loss of barrier function, as indicated by increased permeability to rose bengal diagnostic dye. Similarly, down-regulation of mucin O-glycosylation using a stable tetracycline-inducible RNA interfering system to knockdown c1galt1 (T-synthase), a critical galactosyltransferase required for the synthesis of core 1 O-glycans, resulted in decreased cell surface O-glycosylation, reduced cell surface galectin-3, and increased epithelial permeability. Taken together, these results suggest that galectin-3 plays a key role in maintaining mucosal barrier function through carbohydrate-dependent interactions with cell surface mucins.

Aberrantly glycosylated IgA1 in IgA nephropathy patients is recognized by IgG antibodies with restricted heterogeneity

IgA nephropathy (IgAN) is characterized by circulating immune complexes composed of galactose-deficient IgA1 and a glycan-specific IgG antibody. These immune complexes deposit in the glomerular mesangium and induce the mesangioproliferative glomerulonephritis characteristic of IgAN. To define the precise specificities and molecular properties of the IgG antibodies, we generated EBV-immortalized IgG-secreting lymphocytes from patients with IgAN and found that the secreted IgG formed complexes with galactose-deficient IgA1 in a glycan-dependent manner. We cloned and sequenced the heavy- and light-chain antigen-binding domains of IgG specific for galactose-deficient IgA1 and identified an A to S substitution in the complementarity-determining region 3 of the variable region of the gene encoding the IgG heavy chain in IgAN patients. Furthermore, site-directed mutagenesis that reverted the residue to alanine reduced the binding of recombinant IgG to galactose-deficient IgA1. Finally, we developed a dot-blot assay for the glycan-specific IgG antibody that differentiated patients with IgAN from healthy and disease controls with 88% specificity and 95% sensitivity and found that elevated levels of this antibody in the sera of patients with IgAN correlated with proteinuria. Collectively, these findings indicate that glycan-specific antibodies are associated with the development of IgAN and may represent a disease-specific marker and potential therapeutic target.

Activity of alpha2,6-sialyltransferase and its gene expression in peripheral B lymphocytes in patients with IgA nephropathy

It is known that aberrant sialylation of IgA1 is involved in the pathogenesis of IgA nephropathy (IgAN). We hypothesize that aberrant sialylation of serum IgA1 may result from changes in the activity of alpha2,6-sialyltransferase (alpha2,6-ST) or expression of its coding gene ST6GALNAC2 in peripheral B lymphocytes. Sixty patients with IgAN and 20 healthy controls were enrolled. Peripheral B lymphocytes were isolated by CD-19-positive magnetic beads. The expression level of ST6GALNAC2 was quantitatively analysed by real-time reverse-transcriptase polymerase chain reaction (PCR). Serum IgA1 and sialylation levels were detected by enzyme-linked immunosorbent assay (ELISA) and specific lectin-binding ELISA. Activity of alpha2,6-ST was measured by specific lectin-binding ELISA. Expression of ST6GALNAC2 in B peripheral lymphocytes was significantly lower in patients with IgAN than that in normal controls (3.7 +/- 2.2 versus 6.3 +/- 2.3, P = 0.016); alpha2,6-ST activity in B lymphocytes was correlated positively with the level of alpha2,6-sialic acid in serum IgA1 in patients (n = 42) and controls (n = 12) (r = 0.37, P = 0.007). However, alpha2,6-ST activity did not differ between patients with IgAN and controls (1.19 +/- 1.43 versus 1.06 +/- 1.17, P > 0.05). These data suggested that reduced sialylation of serum IgA1 may result from decreased expression of ST6GALNAC2. The factors affecting activity of alpha2,6-ST in the sialylation of IgA1 need to be further investigated.

No evidence for a role of cosmc-chaperone mutations in European IgA nephropathy patients

BACKGROUND

Altered IgA1 galactosylation is involved in the pathogenesis of IgA nephropathy (IgAN). The galactosyltransferase core-1 beta3-galactosyltransferase-1 (C1GALT1) and its chaperone cosmc are specifically required for O-galactosylation of the IgA1 hinge region. Mutations in the cosmc gene result in a secondary loss of function of C1GALT1 with subsequent undergalactosylation of glycoproteins. Mosaic mutations of cosmc have been shown to result in autoimmune disease. We hypothesized that cosmc mutations might contribute to the altered IgA1 galactosylation in IgAN patients.

METHODS

We studied cosmc gene sequences in genomic DNA obtained from male patients with biopsy-proven sporadic (n = 33) and familial IgAN (n = 6 patients from different families). To account for a potential mosaicism we sequenced cosmc in 10 different peripheral blood mononuclear cell DNA clones of every patient. To specifically assess potential mosaic mutations in IgA-producing cells, cosmc mutations were also analysed in DNA isolated from CD20+ B-lymphocytes from three male IgAN patients.

RESULTS

Despite our extensive genomic analysis, the data revealed no functionally relevant cosmc gene variants in sporadic or familial IgAN cases. A cosmc gene polymorphism, rs17261572, was identified in these IgAN patients in a similar frequency as previously reported in healthy adults. A functional consequence of this polymorphism has not yet been determined.

CONCLUSION

Although decreased C1GALT1 activity has been implicated in the IgAN pathogenesis and cosmc chaperone mutations can cause autoimmune disease, our data provide no evidence for a relevant role of cosmc gene mutations in European patients with sporadic or familial IgAN.

IgA1-secreting cell lines from patients with IgA nephropathy produce aberrantly glycosylated IgA1

Aberrant glycosylation of IgA1 plays an essential role in the pathogenesis of IgA nephropathy. This abnormality is manifested by a deficiency of galactose in the hinge-region O-linked glycans of IgA1. Biosynthesis of these glycans occurs in a stepwise fashion beginning with the addition of N-acetylgalactosamine by the enzyme N-acetylgalactosaminyltransferase 2 and continuing with the addition of either galactose by beta1,3-galactosyltransferase or a terminal sialic acid by a N-acetylgalactosamine-specific alpha2,6-sialyltransferase. To identify the molecular basis for the aberrant IgA glycosylation, we established EBV-immortalized IgA1-producing cells from peripheral blood cells of patients with IgA nephropathy. The secreted IgA1 was mostly polymeric and had galactose-deficient O-linked glycans, characterized by a terminal or sialylated N-acetylgalactosamine. As controls, we showed that EBV-immortalized cells from patients with lupus nephritis and healthy individuals did not produce IgA with the defective galactosylation pattern. Analysis of the biosynthetic pathways in cloned EBV-immortalized cells from patients with IgA nephropathy indicated a decrease in beta1,3-galactosyltransferase activity and an increase in N-acetylgalactosamine-specific alpha2,6-sialyltransferase activity. Also, expression of beta1,3-galactosyltransferase was significantly lower, and that of N-acetylgalactosamine-specific alpha2,6-sialyltransferase was significantly higher than the expression of these genes in the control cells. Thus, our data suggest that premature sialylation likely contributes to the aberrant IgA1 glycosylation in IgA nephropathy and may represent a new therapeutic target.

B-cell O-galactosyltransferase activity, and expression of O-glycosylation genes in bone marrow in IgA nephropathy

In IgA nephropathy (IgAN), pathogenic IgA1 is likely derived from bone marrow (BM) cells and exhibits reduced O-galactosylation. Defective O-galactosylation may arise from the compromised expression or function of the enzyme beta-galactosyltransferase and/or its molecular chaperone (Cosmc). We measured B-cell O-galactosylation activity and the relative gene expression of beta-galactosyltransferase and Cosmc in peripheral blood and BM taken from patients with IgAN and controls. O-galactosylation activity was measured in peripheral and BM B cells by the incorporation of radiolabeled galactose into an asialo-mucin acceptor. Gene expression of beta-galactosyltransferase and Cosmc was measured by real-time PCR and related to that of the enzyme GalNAc-T2 (UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase-2), which synthesizes the core O-glycan. Neither the B-cell O-galactosylation activity nor the gene expression of the enzyme or chaperone was different between patients and controls. However, the relationships between the O-glycosylation of serum IgA1, galactosylation activity, and beta-galactosyltransferase gene expression showed different patterns in IgAN and controls. In IgAN, O-galactosylation activity correlated with beta-galactosyltransferase gene expression, but not with IgA1 O-glycosylation, suggesting that factors other than the availability of beta-galactosyltransferase or Cosmc are responsible for altered IgA1 O-glycosylation.

The pathogenic role of IgA1 O-linked glycosylation in the pathogenesis of IgA nephropathy

Numerous abnormalities of the IgA immune system have been reported in IgAN but the most consistent finding remains aberrant IgA1 O-linked glycosylation of the IgA1 hinge region. The defect comprises reduced galactosylation of O-linked N-acetylgalactosamine residues with or without changes in the terminal sialylation of the O-linked sugars. Aberrant O-galactosylation has been found in serum IgA1, in IgA1 isolated from tonsillar lymphocytes, and in IgA1 eluted from mesangial deposits. There is evidence that changes in IgA1 O-galactosylation lead to IgA immune complex formation and mesangial IgA deposition. Mesangial cells exposed to these IgA immune complexes proliferate and adopt a pro-inflammatory phenotype; they secrete cytokines, chemokines, growth factors and extracellular matrix components promoting glomerular inflammation and glomerulosclerosis. Recent evidence suggests that the control of IgA1 O-glycosylation is linked to class switching from IgD to IgA1 synthesis and that the pattern of IgA1 O-glycosylation may be programmed at the time of initial antigen encounter. IgA1 glycosylation varies between systemic and mucosal sites and the association of aberrant IgA1 galactosylation with low affinity, polymeric IgA1 antibodies against mucosal antigens suggests undergalactosylated IgA1 may in fact be a mucosal glycoform of IgA1. Although suited to the mucosal compartment, when these IgA1 glycoforms enter the systemic circulation in appreciable quantities they deposit in the mesangium and trigger glomerular inflammation. This review will discuss the evidence for the role of IgA1 O-glycosylation in the pathogenesis of IgAN and propose an explanation for the presence of aberrantly O-glycosylated IgA1 in the circulation of patients with IgAN.

Immunocytochemical analysis for intracellular dynamics of C1GalT associated with molecular chaperone, Cosmc

The core 1 structure Galbeta1-3GalNAcalpha1-Ser/Thr (T antigen), the major constituent of O-glycan core structure, is synthesized by cooperation of core 1 synthase (C1GalT) and its specific molecular chaperone, Cosmc. The chaperone function of Cosmc has been well investigated biochemically. In this study, we established monoclonal antibodies specifically recognizing either C1GalT or Cosmc, respectively, and investigated the sub-cellular localization of each protein to elucidate how they cooperate to synthesize the core 1 structure. A sequential immunocytochemical analysis of the human colon cancer cell line, LSB, demonstrated different localization of two proteins. C1GalT was localized in Golgi apparatus, while Cosmc was localized in endoplasmic reticulum. In contrast, the LSC cells, which do not have core 1 synthase activity due to a missense mutation in the Cosmc gene, did not express the C1GalT protein. Although the treatment with a proteasome inhibitor, lactacystin, of LSC cells resulted in the increased expression of C1GalT protein, the distribution of C1GalT was not in Golgi apparatus as seen in LSB cells. On the contrary, overexpression of Cosmc but not C1GalT lead to precise localization of C1GalT protein, which distributed in Golgi apparatus and recovered the core 1 synthase activity in LSC cells. These results suggest that the intracellular dynamics of C1GalT is controlled by its specific molecular chaperon, Cosmc, in association with core 1 synthase activity.

Immunopathogenesis of IgAN

The defining hallmark of IgA nephropathy (IgAN) is deposition of polymeric IgA1 in the glomerular mesangium accompanied by a mesangial proliferative glomerulonephritis. The mechanisms involved in mesangial polymeric IgA1 deposition and the initiation of inflammatory glomerular injury remain unclear. This lack of a complete understanding of the pathogenesis of IgAN has meant that there is still no treatment known to modify mesangial deposition of IgA. Increasing evidence, however, supports the importance of IgA-containing immune complex formation as a pivotal factor driving mesangial IgA deposition and triggering of glomerular injury. A number of potentially important changes to the IgA1 molecule have been identified in IgAN, which may contribute to immune complex formation. These changes suggest that the polymeric IgA1 that deposits in IgA nephropathy is derived from mucosally primed plasma cells. The presence of this IgA in the circulation reflects displacement of mucosal B lineage cells to systemic sites and may be the result of mishoming of lymphocytes trafficking along the mucosa-bone marrow axis.

O-glycosylation of serum IgA1 antibodies against mucosal and systemic antigens in IgA nephropathy

In IgA nephropathy (IgAN), serum IgA1 with abnormal O-glycosylation deposits in the glomerular mesangium. The underlying mechanism of this IgA1 O-glycosylation abnormality is poorly understood, but recent evidence argues against a generic defect in B cell glycosyltransferases, suggesting that only a subpopulation of IgA1-committed B cells are affected. For investigation of whether the site of antigen encounter influences IgA1 O-glycosylation, the O-glycosylation of serum IgA1 antibodies against a systemic antigen, tetanus toxoid (TT), and a mucosal antigen, Helicobacter pylori (HP), was studied in patients with IgAN and control subjects. Serum IgA1 was purified from cohorts of patients with IgAN and control subjects with HP infection and after systemic TT immunization. The IgA1 samples were applied to HP- and TT-coated immunoplates to immobilize specific antibodies, and IgA1 O-glycosylation profiles were assessed by binding of the O-glycan-specific lectin Vicia villosa using a modified ELISA technique. Although total serum IgA1 had raised lectin binding in IgAN, the O-glycosylation of the specific IgA1 antibodies to TT and HP did not differ between patients and control subjects. In both groups, IgA1 anti-HP had higher lectin binding than IgA1 anti-TT. This study demonstrates that IgA1 O-glycosylation normally varies in different immune responses and that patients produce the full spectrum of IgA1 O-glycoforms. IgA1 with high lectin binding was produced in response to mucosal HP infection in all subjects. The raised circulating level of this type of IgA1 in IgAN is likely to be a consequence of abnormal systemic responses to mucosally encountered antigens rather than a fundamental defect in B cell O-glycosylation pathways.